Chemical Engineering’s
Solids Processing Guidebook Volume 1
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Topics Include: • Silo Design and Selection • Design Safer Solids Processing Plants • The Do’s and Don’ts Of Chute Design • Don’t Fall For Common Misconceptions • Will Mass Flow Solve All Your Segregation Problems? • Pneumatic Conveying— Before Stepping the Line, Look Into Air Extraction • Guidelines for Solids Storage, Feeding and Conveying • Cover Story: Move Difficult-to-Handle Bulk Materials with Flexible Screw Conveyors • Tank Coatings: Covering the Basics of Selection and Specification • Using Bins & Silos To Heat or Cool Bulk Solids • Vibratory Feeders And Conveyors: Useful Selection Tips • Designing and Operating Gravity Dryers • Understanding Bends In Pneumatic Conveying Systems • Selecting A Conveyor Using Inserts to Address Solids Flow Problems • Hopper Inserts for Improved Solids Flow
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Silo Design and Selection Materials, construction methods, installation considerations and other design criteria are presented here Keith McGuire, Columbian TecTank
There are a variety of reasons to consider the purchase of a new tank or silo1. As organizations grow, so does the need for increased storage capacity. Meanwhile, an old tank may become corroded or worn and require replacing. Once it’s determined that a new tank is needed, there are a variety of choices to consider. Whatever the end purpose, the chemical process industries (CPI) require tanks that are safe and reliable, will suit their volume of storage needed, fit in available space, be maintenance free, last for years to come, and of course, suit the budget. Today’s advanced technology has eliminated the idea that “a tank is a tank.” The best tank and silo manufacturers are able to design and engineer storage vessels to meet the exact needs of a facility, suit the specific properties of a product, and make loading and unloading easy. These units are designed to last for decades and to be erected quickly, without exceeding reasonable cost. Once you understand how the features offered by tank manufacturers complement the properties and needs of your product to be stored, you can assuredly purchase the best tank for your budget and receive a tank that will have a long, maintenance-free life.
Material and construction type Whether the tank you need is for liquid or dry bulk use, a variety of construction materials is available. The most common construction materials in use today include concrete, aluminum, carbon steel, stainless steel, and in some cases, fiberglass and plastic. The material you choose for tank construction will most likely depend on the volume of material to be stored. Field-welded and concrete tanks are often used to store an exceptionally large volume of product. Fiberglass and plastic tanks are used on occasion for small volumes of storage. Carbon and stainless steel as well as aluminum bolted tanks may be used for both dry and liquid storage and accommodate a wide range of volume and materials to be stored. If you choose a carbon or stainless steel tank, be sure to ask the manufacturer if the construction materials have been tested and certified. Some tank manufacturers employ materials of “unknown origin” or use “commercial quality” steel, which does not necessarily provide the most strength nor meet the highest safety standards. The steel should be tested for its composition. Poor-quality steel may produce a less expensive tank, but it runs the risk of splitting or collapsing. If the vendor you’re considering uses anything less than certified steel, you might reconsider the vendor. In construction, concrete tanks require around-the-clock pouring to ensure their cure. Concrete naturally cracks and shrinks when curing. Factory-welded tanks often are used for small volumes of storage and are transported by truck to the site and erected. Field-welded tanks are erected onsite, and may store large volumes of product, but they require features and coatings to be applied in the field post-erection, which may add time to the project. Bolted tanks and silos are erected onsite with wrench and gasket, often providing erection in a fraction of the time it takes to erect welded and concrete tanks. Also, the factory-applied coatings and factory-installed features of bolted tanks translate to less manpower and less time needed for installation and hook up onsite. Thankfully, the old days of riveted tanks are gone. They have been replaced by stronger, more durable and technology-advanced tanks of concrete, welded and bolted design. Today’s most advanced bolted tanks offer a flanged-panel design, which when bolted together, offer a built-in “rib” which provides stiffness and strength at multiple levels of height. To achieve this same rigidity, flat panel tanks (such as welded) must provide extra steel to add stiffness. Field-welded tanks must be field cut after erection to accommodate piping and factory-equipment connections. Bolted tanks may be pre-engineered during manufacture prior to receiving their factory-applied coating to avoid cutting and damaging the coating post-erection. 6
Carbon steel, stainless steel and aluminum tanks and silos employ a variety of erection methods. For example, glass- and epoxy-coated tanks typically require the use of a jack-and-crane system. These tanks are erected from the top down. The roof and top ring of the tank are lifted above the base using a jack system, and additional rings are added to the bottom. For large tanks, cranes may also be used. For bolted tanks, however, it is sufficiently safe and more affordable to use scaffolding and build each ring on top of another, employing simple brackets and air wrenches.
The importance of proper design When it comes to tanks and silos, proper design makes all the difference. The first step in ensuring proper design of your tank or silo is understanding the characteristics of the product to be stored and the parameters required of the storage vessel. All materials have their own characteristics - whether dry foods, plastics, chemicals, wood products, wastewater, petrochemicals or potable water. Be sure to ask the following questions in the process: What is your volume of storage needed? Is your product free-flowing or non-free flowing? What is your product’s abrasiveness? Will your product require high-pressure storage, or low- to no-pressure storage? Determining the parameters for your tank or silo also will ensure you achieve proper design, and help identify the features that will provide the best flow, expandability, ease of use and meet the unique needs of the product to be stored. Parameters include the volume of product to be stored, available space, budget, required features and industry standards. First, if you are not familiar with your product’s flow requirement, then the product should be flow tested. This is an imperative step in choosing a dry-bulk tank. Flow testing will determine the proper degree for the hopper and hopper outlet and will help identify requirements for the flow or blending equipment needed to ensure proper flow. Designer flowability testing is available through reputable tank manufacturers.
Design criteria Volume and dimensions. Will the tank or silo you’re considering be required to fit into a tight space or will it be allowed unlimited erection space? A smaller placement space will require the tank to be of smaller diameter and taller height. Larger available space will allow the tank to be of wider diameter and shorter height. Both designs tall and thin as well as wide and short - can work well to store a variety of materials when properly designed. In this context, you must always consider important design criteria such as wind loads, seismic conditions and roof loads (such as snow loads) while designing the tank. Tanks of wider diameter and shorter height are better suited for areas that experience high seismic activity and higher wind loads. Expansion and moving needs. Would you consider expanding or moving your tank or silo in the future? If so, consider building an expandable or moveable tank now. Concrete and field-welded tanks are not moveable, and concrete is not expandable. Traditionally, bolted tanks are the best choice for moving and expanding because of their installation and expansion with just hardware and gasket. When you think of expanding a tank, think upwards. Expansion is a consideration that must be given before the construction of the initial tank. Tanks are expandable only if the foundation and lower half of the initial tank are built with the expansion in mind. The foundation must accommodate the weight of a larger tank and the increased volume of product to be stored.
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Expandable tanks are ideal for facilities that may require additional vertical storage capacity down the road, but would not have the additional space to install another tank. Depending on the size of the tank and the expansion, the actual downtime for expanding a bolted tank may be less than two weeks. Also, consider that the vertical expansion of an existing tank versus building a second tank is tremendously more cost effective. So start small today - build what you need - but consider building an initial foundation and tank that would accommodate future expansion. Mixing and blending needs. If the product to be stored will require mixing and blending, this should be a particularly important consideration in the design of the tank. Mixing and blending are common necessities for plastics and food products. Only the proper mechanics - such as proper blenders - will ensure that the output is a consistent homogeneous mixture. Be sure to consider the latest blending equipment available as well as the flowability of the individual products and the blended mixture. Component installed tanks. If your industry or product calls for specific components or parts to be placed inside the tank, consider a new option that some manufacturers offer, which is the installation of the custom components at the factory - prior to the tank’s transfer to the field and erection. Installing components at the factory rather than at the field - cuts down on time and cost for field installation. Common factory installed components include bin activators, blending tubes, mixers, plumbing, wiring, lights and flow devices. Industrial use versus commercial storage. Another consideration for tank design is the frequency of filling and emptying. Will your tank be subjected to loading and unloading daily, monthly or annually? Corrugated tanks are often used for long-term storage, for filling and emptying a few times each year. Smooth-wall, bolted tanks, along with concrete and field-welded tanks, may be filled and emptied frequently. Another consideration for frequent filling and emptying will be the need for a cone-hopper bottom versus a flat-bottom tank. Coatings. One of the most important considerations when purchasing a tank or silo is the coating. The most technologically advanced coatings today provide exceptional corrosion and abrasion resistance. Examples of coatings include epoxy, which will bend without tearing and breaking; glass fused-to-steel; high heat coatings; cold weather coatings and coatings designed to withstand high levels of acidity. The highest quality coatings are found on bolted and factory-welded tanks that receive their coatings at the factory prior to shipping. Many coatings are electrostatically applied in environmentally controlled facilities and then thermally cured. The result is a durable, controlled coating on all pieces, which are then transferred to the field and erected. Field-welded and concrete tanks are erected and receive their coatings onsite. They may need to undergo a chemical process to protect their coating while the tank is being erected (i.e., the heat from welding may damage the coating). If the tank receives its coating in the field, keep in mind that weather conditions such as rain, humidity, dust and temperature all will have an effect on the curing and quality of the coating application, including its resilience and protection offered. If field-applied coatings become too thick, they must be reapplied. In addition, weather may affect the timetable for installation and hook up of field-coated tanks.
Available features Most tank manufacturers offer a variety of custom features to suit the specific needs of every product and industry. Custom features include, for instance, winding stairways, access platforms, guardrails, safety ladders, drivethrough designs (suitable for both trucks and railcars), elevated support structures, custom components, stacks, filter flanges, level indicator openings and nozzles, manways and pressure relief devices. Feature options will vary from manufacturer to manufacturer, so identify the features that are most beneficial for your storage needs and 8
request those from the manufacturer.
Considerations for liquid tanks Liquid storage most commonly includes potable water and municipal or industrial wastewater, liquid for use in fire protection systems, for oil or petrochemical storage, and storage of all industrial liquids. Liquid tanks may be built as treatment tanks for aerobic and anaerobic processes. They may be built for use as equalization tanks in onsite wastewater treatment facilities. The most important considerations for liquid tanks buyers are the quality of coatings to prevent corrosion for years to come, the tight seal of the tank, and the maintenance expectation.
Safety and performance The most common reason for a dry bulk tank to fail can be traced to product behavior inside the tank. Understanding the behavior of a product inside a tank is paramount to ensuring safety. Failure most often occurs when companies store a product inside a tank that was not properly designed to accommodate the product. For example, some grains, meals and other products are sticky and may clog inside a tank that was not properly designed for them. A tank designed for low to no pressure storage may fail when the tank operating system induces high positive or negative pressures. Some materials when stored in a tank with incorrect geometry may form voids in the tank. Personnel have been known to pound on the tanks to relieve the voids, risking serious injury or even death when a massive drop in product occurs. When changing product in a tank, be sure to have the new product flow tested and the tank design reviewed by an expert to determine if the tank is eligible to provide storage for the new product, or what changes need to be made to the tank.
Vendor questions When interviewing tank manufacturers, the more thorough are your questions will provide a more well-rounded view of the product the manufacturer ultimately will install at your facility. • Does the manufacturer use only certified materials in the construction of their tanks? • Does the manufacture use only fasteners, gaskets and sealants that have been tested and inspected by second-party inspectors? • Does the manufacturer comply with API 12B standards? • Consider the experience of the tank manufacturer. How long has the company been in business? What is the experience of its engineers and design team? What is the company’s volume of tanks sold, into which markets, and in what countries? • Does the manufacturer use subcontractors in the design, manufacture, delivery and erection of the tank? Who are the subcontractors and what are their qualifications? The best-quality tank manufacturers instead offer in-house licensed engineers and experienced detailer and design teams that focus solely on the design of tanks. They know tanks and they know the characteristics of the products to be stored. They are familiar with individual industry standards. Beware of manufacturers that subcontract several processes except the actual manufacture of the tanks. The quality of the tank you receive from the manufacturer will rely on the quality of the subcontractors employed and their knowledge of product and industry standards 9
• The most reputable tank manufacturers constantly work to improve quality of their tanks, so be sure to inquire whether the tank manufacturer utilizes a quality improvement process and if they offer a quality assurance team that will follow up with questions or concerns. In addition, be sure to ask for details about the manufacturer’s customer service program and follow up • If the manufacturer has pre-engineered openings in the tank, or if custom components have been installed at the factory, will the manufacturer provide blueprints for the tank? • Ask the manufacturer about the team that will erect your tank onsite. Make sure the erection team is certified and qualified to erect the tank, as opposed to being a local contractor which may not be familiar with the erection of tanks • Ask the manufacturer if you are welcome to inspect your tank during all parts of the manufacturing process. Even if you do not plan to be onsite, it is good to know the manufacturer has enough confidence in its product to be transparent in its process • Consider tank manufacturers with third party accreditations, such as the ISO 9001 Quality Certification. Also, check the company’s references and ask to speak with previous customers about their tank installations. If you do not take time to research a company’s accreditations, then know that you buy at your own risk for the quality of the tank manufactured
Replaceability of parts A great quality tank will last for years and years. But sometimes select parts wear out or coatings may be damaged. Concrete tanks often require frequent maintenance. In the case of a concrete, factory welded, fiberglass or plastic tank, worn parts may require tank replacement. Some damaged coatings may be effectively repaired in the field (example: glass-fused-to-steel), but often field-welded tanks will need to be recoated more than once during their total life cycle, and blasting and re-application of a coating is expensive. In the case of panel design tanks (such as bolted tanks), the company need only remove the damaged panel and replace it with a new panel. Be sure to inquire with the manufacturer about commonly replaced parts and the cost of maintenance.
Cost Once you’ve identified the specific parameters of the tank or silo needed, it will be easy to approach tank manufacturers and ensure the cost estimates you receive are comparable - apples to apples. Be sure to analyze the total life cycle cost of each tank. In addition to initial construction cost, what would be expected cost of expansion? What is average maintenance cost over the total life of the tank? Will reapplication of coatings be necessary in the future, and if so, at what cost? Again, ask about the cost of frequently replaced parts on the tank. Finally, be sure to understand the quality guarantee offered by each manufacturer. Most companies will provide such an analysis of life expectancy and cost, but an example of life cycle cost analysis for epoxy coated and glass-fused-to-steel coated tanks may be found at www.tanks.com.
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Design Safer Solids Processing Plants This approach defines a systematic framework and points toward relevant sources in the public domain Shrikant Dhodapkar, Konanur Manjunath & Pradeep Jain, The Dow Chemical Company
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Chemicals, plastics, pharmaceutical, food, agricultural chemicals, coal and bio-chemical industries have a wide array of processes where the raw materials, intermediates and products are in particulate form. Hazards associated with particulate solids are unique, and in most cases depend on the particle size. The purpose of this paper is to present a systematic framework for designing particulate processes with appropriate pointers towards relevant sources in the public domain. We hope to raise safety awareness among chemical engineers dealing with particulate solids. It is not possible to design and operate a process safely and reliably unless the equipment suppliers, design team and plant personnel are aware of the potential hazards. Hazard awareness should include material properties, process design and operational issues. The readers are strongly encouraged to read relevant references cited in this article.
Background Through dedicated efforts of the larger process safety community, a tremendous body of useful technical information has been compiled in various handbooks and publications. These guidelines are based on current state of knowledge and vast industrial experience in handling and processing particulate materials. A flowchart has been developed to guide the reader through the process Each step is discussed in the supporting text. The overall design process is an iterative process. Hazards due to the material and due to the process must be considered independently and then jointly to assess the risk. What makes solids or particulate processes challenging is the lack of generic databases of material properties. Most of the properties have to be measured in the laboratory. Determination of suitable tests and understanding the limitation of the test result is critical.
Assess hazard potential The hazards associated with particulate process can be broadly categorized per the following: • Material (particulate) related • Mechanical — rotating equipment, pinch points • Thermal — potential of burns • Environmental — noise, dustiness, pollution or waste streams During handling, manufacturing, storage and transport of particulate solids, four generic chemical hazards related to material properties can be identified: • Combustibility or flammability • Reactivity • Toxicity • Instability 12
An in-depth discussion of these hazards can be found in the National Fire Protection Association’s NFPA 704, NFPA 654 and various publications from the Center for Chemical Process Safety (CCPS). Many important concepts and issues are addressed here. A two-step approach to assessment of particulate hazards is recommended. A preliminary assessment is based on NFPA 704 rating, UN guidelines and material safety data sheets (MSDSs). The final assessment must include laboratory data and larger scale testing. Combustibility or flammability. Information on combustibility is often lacking in MSDSs. Even when the data are reported, failure to report test method and material-specific information (such as, moisture, particle size distribution) makes the data unusable. Some guidance can be found in Flammability Rating as described in NFPA 704. Handbooks written by Eckhoff, Barknecht and Babrauskas are excellent sources for ignition and combustibility of wide variety of materials. According to NFPA 654, combustible dust is a combustible particulate solid that presents a fire and deflagration hazard when suspended in air or some other oxidizing medium over a range of concentration, regardless of particle size or shape. Dusts traditionally have been defined as materials 420 microns or smaller (capable of passing through a U.S. No. 40 standard sieve). The exceptions are fibers with high aspect ratio and agglomerates of fine particles formed by electrostatic attraction. Combustible particulate solids, on the other hand, include fines, chips, chunks, flakes, fibers and mixtures of these. Upon handling, these can attrit and generate combustible dust. A deflagration is propagation of a combustion zone at a velocity that is less than the speed of sound in the unreacted medium, whereas explosion is bursting or rupture of an enclosure or a container due to the development of internal pressure from a deflagration. For a dust explosion to occur, the following requirements must be met: • Fuel — The dust must be combustible • Oxygen and dispersion —Dust must be dispersed in air or an oxidant at or exceeding minimum explosible concentration (MEC) • Process confinement • Ignition source with sufficient energy (such as an electric spark, electrostatic discharge, flame, hot surface or glowing embers) Historical data-show that the probability of dust explosion can be varied, depending on the unit operation. While explosions in dust collectors are the most common, other process units should also be designed with particulate safety in mind. To determine potential of deflagration of a combustible dust, one must use actual test data. Depending on the process and operating conditions, some or all of the following tests may be used: 1. Particle size distribution 2. Moisture content (as received) 13
3. Electrical volume resistivity 4. Charge relaxation time 5. Chargeability 6. Minimum ignition energy (MIE) 7. Minimum explosible concentration (MEC) 8. Limiting oxidant concentration (LOC) to prevent ignition 9. Maximum explosion pressure at optimum concentration 10. Maximum rate of pressure rise at optimum concentration 11. KSt (normalized rate of pressure rise) 12. Dust cloud ignition temperature 13. Layer ignition temperature These properties should be measured according to ASTM standards. In addition, a number of consensus and other government standards have been prepared and universally practiced in the industry. Reactivity. Preliminary evaluation of reactivity hazards can be done from MSDSs and chemical incompatibility charts provided by NOAA/EPA Chemical Reactivity Worksheet. There are specific NFPA guidelines for various materials. Bretherick’s Handbook and CCPS Guidelines on reactivity hazard evaluation are excellent resources. Toxicity. Inhalation of fine particles (typically 0.2 to 7 µm) poses respiratory hazard since the particles can get lodged in the lungs. Other potential hazards include ingestion, dermal contact and eye contact. It is also important to evaluate the carcinogenicity and allergenic properties of particulate solids. Instability. Instability refers to material’s susceptibility to release energy either through decomposition or polymerization. NFPA 704 provides classification 0 to 4 in increasing order of instability hazard. Many U.S.-based companies include this information in the MSDSs.
Measure relevant properties Particulate hazards are related to particle size, size distribution, dispersion, moisture content and chemical nature. All the generic hazards stated earlier (combustibility, reactivity, instability and toxicity) increase with decreasing particle size. Unlike liquids, where general conclusions can be drawn from published data, for particulate solids one must generate material specific data. The following parameters are important in this context: • Particle size and size distribution
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• Shape • Specific surface area • Bulk Density • Fluidization characteristics • Dispersibility • Abrasiveness • Friability and hardness • Moisture content • Electrical volume resistivity
Define the operating window One must consider not only steady state conditions but also include startup, shutdown and transgression scenarios in defining the operating window. Perform sensitivity analysis on control variables to estimate the variability in operating conditions. Consider situations corresponding to emergency shut-down and loss of utilities (electric power, steam, water and nitrogen).
Assess severity of process hazards A detail analysis and how-to guide can be found in the CCPS publication “Guidelines for Hazard Evaluation Procedures — Second Edition”. A brief snapshot of various available approaches is listed below. Safety review: This is intended to identify plant conditions or operating procedures that could lead to significant property damage, injury or environmental impact. This is also known as Process Safety Review, Design Review or Loss Prevention Review. Checklists: Provide a written list of items or procedural steps written by a group of experienced experts drawing upon information from various codes and standards. The level of detail can vary depending on the process under investigation. This is an excellent tool to get inexperienced team members familiarized with the process. Relative ranking: It is a strategy for comparing and ranking the attributes of various process options to determine whether they warrant further investigation. Preliminary hazard evaluation: It focuses on the hazardous materials and major process areas in the plant. It covers raw materials, intermediates and final products, plant equipment, operating conditions, operational procedures and facility layout. What-if analysis: It is a brainstorming (hence relatively unstructured) approach where a group of experience subject matter experts generate list of questions and discuss about possible undesired and unintended vents.
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What-if/checklist analysis: This approach combines the systematic approach of checklists with rather unstructured approach of brainstorming. Hazard and operability analysis (HAZOP): This technique identifies and evaluates safety hazards and potential operability problems in a process plant. Failure mode and effects analysis (FMEA): This approach tabulates the failure modes of equipment and their effects on a system or process. Fault tree analysis (FT): It is a deductive technique which focuses on one particular event or failure and provides a method for determining the causes. Event tree analysis (ET): This analysis considers the responses of safety systems and operators to the event and determines the potential outcome using a graphical approach. Cause-consequence analysis (CCA): It is a blend of fault tree analysis and event tree analysis. Human reliability analysis (HRA): It is a systematic evaluation of factors that influence the performance of operators, maintenance staff and other plant personnel. To prepare for process hazard analysis, the following information should be gathered: • Material-specific hazard data (such as flammability, reactivity, toxicity and instability) • Process and instrumentation diagrams and index flowsheets of the process • Instrumentation: Purpose (diagnostics, safety or process control) and list of critical instruments • Control logic, including interlock logic and safety shutdown sequences • Equipment specifications and maintenance history (if preexisting) • Applicable codes, material of construction, chemical compatibility and corrosion tolerances for each equipment • Electrical area classification and requirements • Steady state operating conditions and their range for each of the unit operations • Effect of deviation of control variables on the system response, along with the response strategy — This is procedural and computer driven • Hazard analysis of each unit operation and interconnect unit operations where applicable • Methodology to handle equipment failure or loss of utilities (steam, nitrogen, water and electric power) • Industrial hygiene and occupational safety requirements • Analysis of environmental impact of the process 16
Refer to the detailed check list presented in CCPS publication for process hazard analysis for solids processes. It should be noted that each company has its own guidelines and exhaustive list of questions to evaluate process hazards. General tips • Take a system-wide approach. Processes have interconnected unit operations. Sudden change in process conditions can have a dominoes effect in an interconnected system. • It is important to perform sensitivity analysis on the basic assumptions of process parameters to ensure that conclusions are still valid for the entire range.
Identify measures to reduce hazard severity Process safety system measures or design solutions can be grouped into the following three categories: 1. Inherently safer processes / passive 2. Active 3. Procedural Inherently safer processes refer to selection of alternate process conditions or materials that are less hazardous. A common example would be the use of water as a solvent in paint formulation as compared to an organic solvent. For particulate processes, particles with larger particle size can be produced through agglomeration. However, the feasibility of this approach depends on the chemistry of the process and final application of the product. Other ideas include minimizing hazardous material in the process, designing the process with less severe conditions (temperature and pressure) and simplifying process configuration. Passive systems rely on reducing the frequency and consequences of an incident through process or equipment design, and without the use of any active device. For instance, a vessel can be designed to contain the maximum pressure during a deflagration. These approaches are most robust amongst all approaches because they do not require intervention. Active safety systems are designed to monitor a hazard and react to prevent an incident or minimize its consequences. They are also known as Engineering Controls. Safety valves rupture disks, sprinkler systems, safety interlocks, automatic shutdown, process control system and check valves are common examples of active systems. Since their effectiveness depends on reliable operation, these systems are often installed with redundancy and accompanied with a preventive or routine maintenance program. Procedural safety measures (administrative controls) refer to a set of instructions that require action from personnel to avoid an incident or minimize its consequences. The human factor inherent in this approach makes it least reliable. Standard operating procedures (SOP), safety rules, preventive maintenance, emergency response and management of change are typical examples. To determine the process safety measures, one must first identify all the failure scenarios for each unit operation. All the safety measures associated with each unit operation should then be listed. Consider the consequences of each failure scenario on connected unit operations, and check if it leads to additional failures. For example, dust explosion in a dryer can damage the downstream dust collector. Prioritize the measures by 17
first selecting the most common passive measures, followed by the active measures and then the procedural measures. One must keep track of process economics while adding each layer of protection.
Analyze and assess process risk For any unit operation involving solids or a combination of solids with vapors and liquids, an analysis of risk using tools such as layer of protection analysis (LOPA) is recommended. A team of people must be in place to carry out LOPA, representing the facility, process safety and the unit operation. In order to identify the potential for risk, the team must understand the process and be familiar with the operating conditions so that a potential scenario can be identified. There may be several such potential scenarios for a given unit operation and the team must focus on each of these scenarios and provide independent, auditable and measurable layers of protection. The independent layer is required for a particular scenario and does not depend on other scenarios. Its effectiveness must be focused on stopping the scenario, and in order to do so a measurement layer must be in place. LOPA is very effective in analyzing the situation and designing a safer plant. In order to conduct LOPA, following information is essential: • Data on the four generic chemical hazards (reactivity, flammability, toxicity and instability • Knowledge of various unit operations • Knowledge of plant problems • Knowledge to fix and troubleshoot problems
Engineering to minimize risk Various engineering approaches are possible to minimize risk in a process: • Process equipment design (use passive measures) • Select alternate unit operations • Optimize process configuration or sequence • Use different chemistry or route • Select less severe process conditions • Use raw materials and intermediate which are less hazardous
Process design It is recommended that the following issues be included while evaluating design options: • Noise generation (especially for air movers) • Energy efficiency (especially for dryers) 18
• Capital intensity (capital investment / production rate) • Process layout (minimize material transfer distances and use gravity whenever possible) • Waste stream management • Environmental impact of the process While writing equipment specification, one must pay close attention to the code requirements in country where the process will be installed. Instrumentation: Identify critical instruments and provide redundancy or have an on-line spare. Include sufficient instrumentation to aid process diagnostics. Any process design change must undergo full review to understand unintended consequences.
Installation Make sure that delivered equipment is inspected and tested. Provide detailed guidelines for on-site fabrication (such as welding), construction and testing.
Maintenance There are two types of routine maintenance: preventive and predictive. The preventive maintenance schedule draws upon recommendations from the manufacturer, repair history, design life and mean time between failure data. This type of maintenance can be planned and conducted safely in controlled conditions. Predictive maintenance relies on real-time data in put to anticipate the need for maintenance before a failure can occur.
Summary In this paper, we have proposed a logical approach towards designing safer solids processing plants. Particulate materials behave differently from liquids and gases, and it is essential to appreciate these differences. Acquiring relevant and reliable characterization data, evaluating process hazards systematically, selecting effective safety measures (design solutions) and leveraging extensive information available in the public domain are essential for designing safer solids processing plants.
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The Do’s and Don’ts of Chute Design These rules of thumb help ensure reliable flow of solids in applications where mechanical conveyors are not feasible Joseph Marinelli
The need to transfer bulk-solids materials from the outlet of a bin or from a conveyor, to a process, truck, or another bin is ubiquitous throughout the chemical process industries (CPI). Transferring material using equipment designed to mechanically convey chemicals works quite well, especially for long distances; however, for short distances, it can be prohibitively expensive and maintenance-intensive. As an alternative, chutes are used instead of expensive conveyors to transfer solids short distances. Merriam Webster describes a chute as: “an inclined plane, sloping channel, or passage down or through which things may pass.” A chute is simply a pipe or trough that is sized properly and at the correct slope angle to ensure sliding of the material to be transferred. Below is a list of do’s and don’ts to ensure reliable chute flow. A chute test can be run in the laboratory to determine the minimum chute angle required to ensure reliable flow through your chutes. As with all testing, simulation of actual handling conditions is critical. Conditions such as moisture, particle size, temperature, representative-chute surface, all come into to play when evaluating your chute designs. Don’t: Drop your material from high heights. Chutes are different from hoppers when handling chemicals and other bulk solids. When a material flows in a hopper, it is under constant pressure acting normal to the hopper walls. In a chute, solids undergo impact such as falling from a conveyor, mixer or bin onto the chute. This drop causes the product to impact the chute creating the potential for the product not to slide after impact if the chute is not steep or smooth enough. Specific relationships of these factors are discussed below. Do: Design for your material’s impact pressure. A chute must be steep enough and smooth enough to ensure sliding along its entire length. The impact of material on the chute is extremely important. Whether the material is dropping in free fall from a bin or being transferred from another chute, impact pressures and the velocities necessary to keep material moving along a chute are critical. Don’t: Guess at your chute angles. Just as 70 deg. is not the “magic” angle for mass flow in a conical hopper, 45 deg. is not the standard angle for chute slope. Not all materials are the same! Flow properties can vary as a function of moisture content, particle size, temperature and other factors. Do: Run tests to determine chute flow properties. Tests are run in the laboratory to measure the critical chute angle resulting from impact pressure. This information can then be used to develop the minimum chute angle at the point at which it impacts the chute. The test involves placing a sample of solid on a surface representative of the surface to be used as the chute. A load is applied to the sample (using weights) and removed after a short time (say 20 s) to simulate impact pressure. The weight is removed and the surface is then tilted until the sample begins to slide. That angle of slide (chute friction angle, fʹ) is then recorded as degrees from horizontal. This test is repeated several times to ensure accuracy and under a range of loads to simulate a range of impact pressures. A safety factor of about 5 to 10 deg. is added to the results. Don’t: Allow the material to cascade uncontrolled from the chute. Your material will flow uncontrolled through the chute if material velocities are too great or your chute slope too steep. This will cause wear problems, flooding, dusting, and so on. Don’t: Allow the product to buildup in the chute. Again, be aware of the solids velocity in the chute. If the velocity is too slow, the material will hang up and build up, eventually choking the chute. Do: Control material velocity. A stream of particles that impacts a chute has a velocity, V2 that is relative to its initial velocity V1. If V2 is zero, the material will not slide, remaining stagnant causing buildup and flow stoppage. 22
Don’t: Use a rectangular (flat surface) configuration with sticky materials. Although a rectangular chute configuration is common and easy to fabricate, wet, sticky, or cohesive solids will tend to build up in the corners, eventually effecting chute throughput. Do: Use circular chute configurations for wet, sticky or cohesive solids to minimize material catching in corners. Do: Use a spiral let-down chute to minimize particle attrition. A spiral let down chute is used to control particle attrition. Say you are transferring adipic acid. This material typically has fines to 1/4-in. particles and is very sensitive to particle attrition. If it is handled too roughly, the particles will break creating more fines, which are very cohesive. A spiral let down chute will gently lower the adipic acid particles to the bin or pile below, thereby minimizing particle attrition. The spirals are designed to minimize particle velocity and reduce impact.
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Don’t Fall for Common Misconceptions Identify them, understand them and challenge them S. Dhodapkar,The Dow Chemical Co. L. Bates,Ajax Equip. & G. Klinzing,U. of Pitts
Myths and misconceptions become part of accepted practice if repeated often enough. They are propagated by lack of fundamental particle-technology knowledge and by certain commercial interests. The list of presented here is by no means exhaustive. We have, however, made an attempt to cover a broad range of topics to highlight the widespread nature of these misconceptions. They are broken down by operation, which are ordered alphabetically for easy reference. We intend to engage the reader’s inquisitiveness with this list, and hope that practitioners of solids processing technology will learn to challenge established practices and not always accept them as a given facts.
Drying Misconception: The required drying time, when a contact dryer is used, does not depend on the equipment size. Reality: The required residence time to reach desired moisture content in a contact dryer (with heated walls) depends on the size of the equipment. The ratio of volume of material to the surface area of heated walls determines the drying time. This ratio increases as the dryer size increases. Therefore, residence time required in full scale dryers is longer than measured during pilot testing. One must pay close attention to such details during scaleup. Misconception: The performance of fluid bed dryers can be improved by increasing the gas flowrate. Reality: This is not always the case. For some products that require long drying time, lowering the gas velocity will improve the dryer performance. By lowering the gas flow, the residence time of material in the dryer can be increased. This will improve dryer performance.
Electrostatics and dust explosion Misconception: It is possible to drain the static charge from polymer pellets stored in a box or a container by inserting a grounding rod into the container or grounding the metal container. Reality: Static charge on insulating material can not be drained instantly even if the container is grounded or a grounding rod is inserted. The charge decay relies on surface resistivity of the polymer. Misconception: Wrapping a grounding wire on a PVC hose is an effective method to discharge the static charge in a pneumatic conveying system. Reality: The grounding wire simply prevents the operator from experiencing the effect of static charge by directing the force field and grounding the surface charges. The material being conveying will remain charged. This is not a good practice. A grounding wire can also produce spark when an ungrounded human being touches the hose. Using a charge dissipative (conductive) hose is recommended. Misconception: Dust explosion characteristics (minimum ignition energy, Kst) are material-specific. Reality: Additionally, they strongly depend on the particle size. The dust explosion potential increases as the particle size gets smaller. The fines fraction (less than 63 µm) is typically tested for dust-explosion potential. In most cases, particles larger than 420 µm have low dust-explosion potential (per NFPA). Misconception: Dust explosion in process equipment is the primary reason for loss and destruction.
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Reality: Secondary dust explosions triggered after the primary explosion due to re-entrainment of settled dust in process area will cause greater damage. Good housekeeping and well engineered systems minimize the likelihood of such losses.
Feeding and metering Misconception: The performance of various feeders can be compared by their accuracy specification. Reality: The accuracy of a feeder is often quoted as percentage of full scale value. It is more appropriate, however, to compare absolute values. It is equally important to understand the time-average basis for accuracy specifications. Feeder accuracy decreases and feed fluctuations increase at smaller time scales. The time basis must be determined by process requirements. Misconception: Vent piping on a rotary valve is meant for venting leakage air. Therefore, the piping can be routed in a configuration similar to that of an air line. Reality: The vent stream contains significant amount of entrained material. It must be designed as if it were a pneumatic conveying system; otherwise, the line may plug. Misconception: A feed screw delivers a volumetric amount that is proportional to the pitch. Reality: The amount moved forward by a flooded screw depends on the contact friction between the material and the face of the screw flight. An increase in screw pitch does not necessairly result in a proportional increase in feed rate. Misconception: A circular casing surrounding a short pitch screw will restrain material from “flushing”. Reality: If a fine product is dilated to a fluidized condition, it can only be restrained by a positive restriction to the flow channel, such as a rotary valve, and even then some leakage may occur through even small clearances.
Fluidization Misconception: The fluidization characteristics of a bulk material only depend on average particle size and difference in particle density and fluid density. Reality: Fluidization behavior is also affected by particle-size distribution and nature of particle surface (moisture, stickiness, electrostatics).
Gas-solid separation Misconception: Each cyclone design can be assigned a unique “efficiency” designation regardless of the application. Reality: Overall efficiency of a cyclone depends on grade (or fractional) efficiency of the cyclone and the particle size distribution of incoming dust. The grade efficiency is a function of cyclone design and operating parameters (gas flowrate, gass-solid properties, solid loading). It can also be affected by other factors, such as air leakage from the bottom or presence of bends at the inlet and outlet. Typically, the grade efficiency decreases with particle size. Industrial cyclones have low grade efficiency below 3-5 µm. 27
“High efficiency” cyclone designs rated at “99.9% efficiency” for coarse particles may separate only a fraction of incoming particulate if the incoming dust is extremely fine. Misconception: Frequent back pulsing of dust collectors keeps the bags clean and improves the “performance” of a dust collector. Reality: For most filter media, the efficiency is lowest when the media is clean and there is no cake on the surface. The best way to run a dust collector is to back-pulse it as needed (based on pressure drop). That will achieve the highest overall efficiency and longest possible bag life.
Mixing and blending Misconception: More residence time in a mixer results in better mixing. Reality: The mixture quality reaches an asymptotic limit after certain duration. The quality of mixture depends on the mixer design and compatibility between the mixing mechanism in the mixer and the mixture. Extended mixing action may cause attrition or fines generation, which then segregates and results in poor mixture quality. Misconception: Solids mixers can be scaled up by applying scaling laws similar to liquid mixers. Reality: Scaleup of solids mixers is much more complex. Mixer scaleup will depend on the type of mixer and mechanism of mixing. Various approaches (constant tip speed, Froude number, geometric, kinematic and dynamic similarity) have been proposed throughout the literature.
Particle characterization Misconception: It is acceptable to overlay the particle-size distributions obtained from various particle size analyzers on the same plot for comparison as long as they are converted to the same type of distribution (number, surface or volume/mass). Reality: There are many different ways to describe the ¡®size’ of a particle (volume diameter, projected area diameter, Stokes diameter etc.). Various particle size analyzers use different physical measurements to extract the particle size information. They are not always comparable or transformable from one to the other. Stick with one instrument for all measurements in a given process to avoid confusion. Different instruments also handle nonspherical particles differently since there is an inherent assumption of sphericity. Misconception: Mass-median particle diameter (particle size corresponding to 50% on cumulative mass distribution) is the best way to represent a particle size distribution. Reality: The selection of appropriate “representative” particle diameter for a distribution depends on the application. For instance, mass median diameter is not sensitive to the presence of fines. The behavior of a bulk material in fluidization, hopper flow and pneumatic conveying applications, however, is strongly influenced by presence of fines. Surface-volume mean diameter is more appropriate than mass median diameter for these applications. It is defined as the diameter of a sphere having the same volume as the particle of average surface area for the mixture. Other mean diameters are surface mean diameter and volume mean diameters.
Pneumatic conveying 28
Misconception: Pickup velocity and saltation velocity are fundamental properties of a material. Reality: First of all, let us address the definitions of pickup velocity and saltation velocity. Pickup velocity is defined as the superficial gas velocity required to entrain particles from a stationary layer on the bottom of the pipe. Saltation velocity is defined as the minimum superficial gas velocity required to keep particles in suspension for a horizontal gas-solid flow. From a design perspective, the saltation velocity determines minimum gas velocity required at the pickup location. Saltation velocity depends on particle size, particle density, gas density, pipe size and solids loading. Higher velocity is required for larger line size and higher loading. Misconception: Pneumatic conveying lines can be routed much like utility (air or steam) lines in the plant. Reality: Good design practice for pneumatic conveying systems requires sufficient straight run after pickup, minimization of bends and directional changes, avoidance of back-to-back bends and avoidance of inclined lines. Misconception: Increasing air flowrate (or providing more “oomph” to the system) will increase conveying capacity. Reality: Increasing air flowrate in dilute phase systems will decrease conveying capacity (for the same overall pressure drop). For dense phase systems, the effect is opposite. Transition from the dense-phase region to the dilute-phase region can result in unstable flow for some materials. Increased air flow will always result in higher power consumption. Misconception: Dense-phase conveying is achieved with high conveying pressure. Reality: Dense-phase conveying implies “non-suspension” flow in the conveying line. The material is not fully entrained or suspended in the gas stream. While dense phase conveying systems typically run at higher pressures, a dilute phase that is long and/or highly loaded can also require higher conveying pressure. Misconception: Injecting air in the conveying line at various intervals (with “boosters”) will result in a better dense-phase system. Reality: The need to inject air along the conveying line depends on the bulk material’s permeability and air retention characteristics. Materials with high permeability (such as pellets) and high air retention (such as cement) do not require secondary air injection. Excess air injection can result in high gas velocity (dilute phase conditions) at the end of the line.
Screening and classification Misconception: Mesh size is sufficient to specify a screen. Reality: For wire-mesh screens, mesh size determines the number of apertures per lineal inch. One must subtract the thickness of wire to determine the opening available for screening. As the wire diameter increases, the actual opening and percent open area available for screening decreases. Misconception: The cut size is determined by aperture opening alone. Reality: The cut size is a function of aperture opening (size and shape), particle shape, screener loading and screener motion. The grade efficiency of a screener is not a step function but an S-shaped curve for non-spherical 29
particles.
Segregation Misconception: Mixtures with components of different sizes, shape and density will always segregate in a process. Reality: In addition to material properties, the severity of segregation depends on operating conditions, process configuration and the scale of operation. For example, a mixture may segregate while loading a 100-ft-tall by 15-ft-dia. silo, although it may not segregate while loading a feeder hopper that is 4-ft tall by 2-ft dia. Misconception: Large particles will always sink to the bottom. Reality: Upon vibrating a mixture of particles, the large particles (regardless of density) will rise to the top surface. This is known as the “Brazil Nut Effect”. The container geometry and relative size differences in the mixture also play a role in this phenomenon. Misconception: Mass-flow hoppers will always correct segregation problems in the bin. Reality: If the segregation in the bin is such that layers of different components are created (such as alternate bands of coarse and fines), then mass-flow design (see CE, Apr. 2006, pp. 40-43) will not correct the segregation problem. The coarse and fine fraction will discharge in alternate sequence. On the other hand, mass-flow design is very effective in alleviating radial segregation (variability across the cross-section) problem. During operation, it is essential to maintain the level of material at a minimum of one bin diameter above the cone section. Otherwise, the velocity gradient in the converging section will initially deliver a portion of “rich” material from the central region and finally discharge product that was deposited around the periphery of the stored surface. Misconception: Large particles always go to the outside of a pile. Reality: When falling into a mass of dilated (fluidized) bulk, large particles can be captured by “impact penetration” and thereby express fine particles to the outer region.
Solid-liquid separation Misconception: Higher filtration pressure drop will result in higher filtration throughput. Reality: Higher pressure drop will not increase the throughput if the cake or the filter medium is compressible. Higher pressure drop causes reduction in porosity and retards the flow. Misconception: As long as the media can capture the particle, operating flowrate is not important. Reality: Operating the filter at lower flowrate will result in better capture of particles. Higher flowrate reduces the chances of particles being caught by the media. Misconception: The longer one can run a filter (between cleaning cycles), the higher the throughput. Reality: Filtration rates decrease with time. It is important to analyze the entire filtration cycle to determine whether the extended filtration time (at lower filtration rates) is better or worse than higher filtration rates for shorter duration (due to more frequent cleaning).
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Misconception: Filter aid helps to increase filtration rate - more is better. Reality: While filter aid increases filtration rate, it also decreases the actual amount of native solid removed in each cycle. An optimal amount must be determined based on filtration rate improvement and additional cycles that are required to achieve the same capacity. Misconception: All filters with the same “micron rating” will have the same performance. Reality: There are no universal standards for specification of micron rating of filter media. Each vendor tests and specifies its medium/media differently. One must pay close attention to the details of the test method. Moreover, the grade efficiency (true performance) of the unit depends on type of filter (such as cartridge, bags and metal screen), material of construction, fabrication and application parameters (such as particle characteristics, operating temperature and liquid viscosity). Bench scale testing is highly recommended when there is no prior experience.
Storage Misconception: A suitable cone angle for bin design (for mass flow) is the same as angle of repose or angle of internal friction or angle of slide. Reality: Cone angles required for mass flow must be calculated from Jenike’s theory using wall friction data and angle of internal friction from shear test. Angle of repose should not be used for bin design except for estimating its capacity. Misconception: Stress at the outlet of a large silo (say, 150 m3) is much higher than that of a small silo (say 15 m3) with same outlet dimensions. Reality: For mass-flow design, the stresses in the vicinity of the outlet are largely independent of the overall size of the silo. This does not hold true for funnel flow silos. Misconception: Mass-flow pattern gives a true FIFO (first in/ first out) or “plug flow” as observed by uniform draw down of the top surface. Reality: Broadly speaking, this is true. However, there is significant mixing in the cone section due to velocity gradients. The residence-time distribution depends on the hopper geometry and velocity profile in the hopper. Steeper and smoother hoppers will result in narrower residence-time distribution. Misconception: Hoppers should be designed first, and then a feeder selected to suit the process. Reality: Hoppers and feeders are integral units with interacting properties. The first design decision is to select the appropriate flow regime for the product. This will allow the wall angle to be determined for the form of hopper selected on the basis of many factors. A minimum outlet size is required for reliable flow and varies according to the maximum flowrate wanted. Site circumstances, fabrication considerations and a host of other factors influence the ultimate determination of hopper shape and feeder selection, which boil down to the designer’s judgment based on experience. Misconception: Moisture makes flow more difficult. Reality: Generally, the flow potential of a bulk material deteriorates with moisture up to a critical value, above 31
which flow behavior eases because the excess fluid reduces surface tension and behaves more like a lubricant. Typically, a value of 10 to 12% moisture represents the worst flow condition of a fine, granular solid such a ground coal. Similar behavior has been observed for soft elastomeric and highly frictional pellets. Misconception: Reliable stream flow can only be achieved by means of mass-flow design. Reality: Expanded flow (funnel-flow silo with mass-flow cone adapter) can be a viable option, especially when the bin can be emptied periodically. Misconception: Vibrations always help to discharge material from a silo. Reality: Vibrations can result in compaction and may adversely affect flow. Misconception: Chutes designed with 45-deg. inclined surfaces will provide adequate service. Reality: There is a minimum angle required for proper chute flow that depends upon wall friction and surface adhesion. Allowance must be made for impact and changes of direction. The capacity of a chute drops dramatically when its inclined surface is near the critical inclination. Misconception: Full chutes transfer more than partially loaded chutes. Reality: Chutes filled over the whole cross section have lower capacity than those operating at 50% fill level in the pipe. Choked (full) chutes are also prone to arching or bridging. Misconception:Smoother walls always result in lower wall friction angle. Some lining materials have universal low friction values with all bulk materials. Reality: Friction between bulk material and a surface is a complex phenomenon. Wall friction is a relationship between a given bulk material and a specific contact surface. It is necessary to measure wall friction for each pair. Often times, smooth surfaces can result in higher friction and “low-friction” surfaces may exhibit slip-stick behavior. A surface that has low friction with one bulk solid, compared with an alternative contact surface may have higher friction with another material. Ultra-high-molecular-weight-polyethylene surfaces, for example, exhibit low friction with damp materials due to their hydrophobic nature, but are prone to scouring and high friction with fine, hard particles. Misconception: Lining hopper walls with low-friction material will improve flow. Reality: Not always. Low-friction liners on vertical walls and at large spans can increase overpressures, and make the bulk material stronger in the crucial outlet region.
Summary The misconceptions discussed in this paper are merely the tip of the proverbial iceberg. The chances of success in a solids handling/processing project will be improved if the user is aware of misconceptions and avoids the pitfalls.
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Will Mass Flow Solve All Your Segregation Problems? When materials are prone to fluidizing and sifting segregation, the best solutions involve flow pattern with a twist
Joseph Marinelli, Consultant
Most chemical processes must produce products that are uniformly mixed. There have been many articles written about preventing particle segregation during bulk solids flow; and ensuring a mass flow pattern is a common suggestion. But mass flow alone is not necessarily the answer to all segregation problems that can occur in silos and tanks. We will focus on the two major mechanisms of segregation; namely, sifting and fluidization segregation.
Determine the segregation mechanism The first step in solving segregation is to determine what type of segregation is occurring in your vessel. The two most common types of segregation are sifting and fluidization segregation. The first step is to know your material’s flow properties. The next step is to identify the mechanism that is causing your segregation and provide reliable geometries and solutions that will result in segregation-free flow. Laboratory tests can be run to identify how a solid will flow. Many of these tests follow an ASTM standard that uses the Jenike method of testing your materials. The results of material testing will help you determine what type of segregation you have to overcome so that you can design a silo or hopper to store and discharge your product reliably. Always keep in mind that the feeder design is as critical as the vessel design. Sifting segregation is the most common means for particles to separate and it occurs when small, fine particles move through large, coarser particles. Suppose you have a material that has a range of particle sizes. If you form a pile with this material, the fines will concentrate in the center under the fill point, while the coarse particles roll or slide to the outside. The particles segregate in a horizontal or side-to-side pattern. In order for sifting to occur, your material must exhibit the following: •Be free flowing •Have a range of particle sizes •Have some fairly large (> 100 mesh) particles •Have some means of interparticle motion (such as forming a pile) Fluidization segregation. As a vessel is being filled, say by a pneumatic conveying system, the coarse, heavier particles are driven to the bottom, while the fine, light material tends to settle on top. This is due to the fact that fine particles tend to remain airborne longer because they are less permeable to air than coarse or heavier particles. The effect creates a vertical or top-to-bottom segregation pattern.
Funnel flow vs. mass flow There are two types of flow patterns that can develop in a silo or tank; funnel flow and mass flow. Each of these segregation mechanisms can affect your process in different ways. Funnel flow occurs when some of the material in a silo moves while the rest remains stagnant. In this case, the walls of the hopper section of the silo are not steep or smooth enough to overcome the friction that develops between them and your bulk solid. Funnel flow can result in ratholes, decreased live or usable capacity, stagnant material that can cake or spoil, and segregation problems and can ultimately cause structural failure. The first material that enters your silo — other than the small amount of material directly over the outlet — is usually the last material to discharge. This is known as a first-in, last-out flow sequence. 34
As you load your product into its silo, perhaps it segregates by the sifting mechanism. Remember, this means that the fines are in the center and coarse particles at the walls. If the silo or tank exhibits a funnel flow pattern (some material moves while most remains stagnant, likely at the walls) at some point in your process, you will discharge all fine particles or all coarse particles, which will affect your overall process negatively. The reason this flow pattern occurs is that a preferential flow channel forms, usually above the outlet, causing the center (fine particles) to empty first. Mass flow, by contrast, occurs when all the material in your vessel is in motion whenever you discharge product. The hopper walls are steep and smooth enough to overcome the friction that develops between the wall surface and bulk solid, ensuring flow along them. Remember, because all the material is moving, stable ratholes cannot form, making mass flow suitable for cohesive solids, powders, materials that degrade or spoil, and, especially, solids that segregate. Mass flow exhibits a first-in-first-out flow sequence, meaning that the first material that enters the silo is the first to exit.
Mass flow and sifting-segregation effects Using a mass flow pattern will not only prevent the flow problems typically experienced by funnel flow (ratholing, flushing, etc.) but it will minimize the effects of sifting segregation. Because of the first-in-first-out type flow sequence, the side-to-side segregated particles will be remixed as they discharge from the outlet. One thing to keep in mind about mass flow is that as you empty your silo and the head of material reaches the hopper section, a velocity gradient is created, causing the center to empty faster than the material at the walls. The flow pattern is still considered mass flow; however, your product may segregate slightly due to this velocity difference.
Mass flow and fluidization segregation Consider, for instance, that a typical approach to filling silos is to use a pneumatic conveying system to transfer product from trucks, railcars, and other conveyances. Typically, a pneumatic conveying line enters the silo in the top, center. The coarser, heavier particles are driven to the bottom, while the finer, lighter particles remain airborne and settle on top. Perhaps you have designed your vessel for mass flow (all the material moves whenever any is withdrawn — flow at the walls). If your product segregates top-to-bottom, and you are discharging from your vessel to bulk bags or boxes, most of the containers will be filled with coarse particles, while the last few containers will contain all fines. This will obviously affect your product quality and costs. Although in many handling situations, a mass flow pattern is preferred, in this case, mass flow does not solve the segregation problem. Fortunately, there is a way to correct this problem. Simply change the way the product enters the silo from top, center to entering tangentially somewhere near the top. As the material enters from the side, it swirls around the vessel walls and segregates side-to-side. And, as noted above, a side-to-side (sifting) segregation problem is corrected by using mass flow. Meanwhile, another approach is to use a specially designed letdown chute. Such a device, although used mostly to avoid particle attrition, can be used to minimize fluidization segregation by forcing the material to remain in contact along the chute. As the particles flow along the chute their velocities are slow enough to prevent separation of fines and coarse particles by fluidization.
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Summary There are other methods to resolving segregation issues not listed here. Keep in mind that in order to address a segregation problem, the first step is to know your material’s flow properties. The next step is to identify the mechanism that is causing your segregation and provide reliable geometries and solutions that will result in segregation-free flow.
Pneumatic Conveying — Before Stepping the Line, Look Into Air Extraction Successively larger diameters downstream can raise problems. Physical removal of air is an attractive alternative David Mills Pneumatic Conveying Consultant
In pneumatic conveying systems that operate under high positive pressure, it is common wisdom that as the downstream end is approached, the diameters of the pipeline segments should be larger. Air (or other conveying gas) is compressible, so if a single-diameter pipeline is instead used, the gas velocity will reach very high values. If the conveyed material is abrasive, the consequence can be severe erosive wear, particularly at the pipeline bends; if the material being conveyed is instead friable, considerable particle degradation can result. This lowering of the velocity profile through the pipeline can also improve the overall system performance in other respects. The system has a free air flowrate of 2,000 ft3/min, and, because it is conveying material in dilute-phase suspension flow, its recommended minimum value of conveying-air velocity is about 3,200 ft/min, which is a typical average value for granular materials. The inlet pressure of the air is 45 psig. A 5-in.-diameter pipeline would be needed upstream for these conditions. Unfortunately, however, complications can arise associated with pipelines employing successively larger-diameter segments. In the first place, if the positions of the step-ups along the length of a pipeline are not correctly located and the conveying air velocity falls to too low a value, the pipeline could block. (Further contributing to this complication is the fact that pipelines are available only in specific, standard diameters.) Another complication is that it is hard to purge material clear from the pipeline at the end of a conveying run, particularly with low-velocity dense phase conveying, because there is a limit on the minimum value of velocity that can be used for conveying a material. The ideal solution would be to use a tapered pipeline, so that there would be no change in velocity along the entire length of the line. This, of course, is not feasible, but the concept does provide a model of what the stepped pipeline is trying to achieve.
Air extraction A viable alternative to stepping the pipeline is to physically extract air from it at various points along its length. The risk of a too-low air velocity disappears, because the amount of air extracted can be controlled; furthermore, numerous extraction points instead of a single one can be utilized. And if the air extraction is simply interrupted during the purging phase, it will be possible to purge the pipeline very quickly. Of course, a particular advantage of air extraction arises with respect to the numerous already-operating highpressure conveying systems worldwide that, for whatever reason, were designed with only a single-diameter line. Revamping such a system into a larger-diameter one, let alone into one with successively larger-diameter steps, would be costly and time-consuming. If air extraction is instead adopted, there is no need to change the diameter of the pipeline at all.
Illuminating the options For a fuller grasp of the air extraction option, it is illuminating to compare pipeline performance for three alternatives: a single-diameter pipeline, a stepped pipeline with two diameter-stepup points, and a pipeline employing air extraction at two points. Such a comparison is enhanced if the conveyed material is capable of being conveyed in dense phase, and if two cases are considered. For the first case, we stipulate a long-distance pipeline of 4,000 ft, in which it will only be possible to convey the material in dilute-phase suspension flow. For the second case, we specify a relatively short pipeline, of 650 ft, in which it should be possible to convey the material in dense-phase flow at low velocity. In both cases, an air supply pressure of 45 psig is employed, and the material conveyed is ordinary portland cement. We employ a first-approximation method of analysis. It includes a determination of step location in the case of stepped pipelines, and of air off-take data (as well as control-system guidance) in the case of the air extraction 38
system.
Pipeline analysis For each of the two cases, (with the three alternatives considered in each case), the pipeline analysis is made in terms of the pressure drop. For a given material flowrate, and, for each section of pipeline, DP is evaluated for each of these separate pressure-drop components: that of the conveyed material through the straight pipeline sections and bends, that of the air alone, and the pressure drop due to acceleration. The quick, first-approximation solution approach employed in this article lends itself to feasibility studies, as well as to reasonable comparative studies. In all six situations studied, the pipelines are of 8-in. diameter at the inlet. For the stepped pipelines, the two successive expansions are to diameters of 10 and then 12 in. Discharge is to atmospheric pressure at 14.7 psia, and a uniform temperature of 70°F has been taken throughout. Straight pipeline sections: Pressure-drop data for the conveying of the material through straight pipelines. This information is presented in terms of a pressure gradient for a horizontal pipeline, so the length of straight horizontal line simply needs to be multiplied by the appropriate pressure-gradient value. For flow vertically upward, it is suggested that the length of vertical line be doubled and added to the length of horizontal line. This is in terms of a 4-in. pipeline, so the material flowrates have to be scaled in proportion to pipe section area to locate the operating point. Pipeline bends: Data for material conveyed through 90-deg bends. In this case, the pressure loss per bend is given in terms of an equivalent length of straight horizontal pipeline, and is based on the value of the conveying line inlet air velocity. This length can be added to the equivalent length in the same way as for the vertical sections of straight pipeline. Air-only pressure drop: Determining this pressure drop (as well as, subsequently, the acceleration pressure drop) requires a determination of the conveying-air velocity. The minimum value of conveying air velocity is given as a function of the solids loading ratio, for ordinary portland cement. The solids loading ratio, f, is defined as the ratio of the mass flowrate of the material conveyed to the mass flowrate of the air used and is a dimensionless quantity: However, since the blowers and compressors used in pneumatic conveying are generally specified in terms of free-air flowrate, V•0, in ft3/min, that quantity will be used in preference. As a safety margin, it is generally recommended that the actual conveying-air velocity employed be about 20% greater than the minimum value. This margin is applied at the conveying line inlet, at both changes in pipeline diameter, and at the air extraction points. As we are using a first-approximation method, a value of pipeline friction coefficient of 0.0045 can be taken for most pipelines. It is further recommended that mean values of both air velocity, C, and density, r, be used in the evaluation of the air-only pressure drop for each pipeline section. Pressure and velocity profiles can be quickly evaluated using the models presented below so that air density and velocity values can be determined. The bends and the pipeline fittings can generally be disregarded with respect to the air only pressure drop. In a first-approximation solution for most stepped-pipeline or air-extraction systems, this pressure drop need only be considered for the first section of pipeline (where the material becomes accelerated from rest to its terminal velocity). The conveying air velocities at the ends of each section of pipeline are usually very similar to each 39
other. Pressure and velocity profiles: These profiles for a given pipeline system can generally be evaluated without consideration of the material being conveyed. That for the air extraction system will be very similar, for (as noted above) two air extraction points will be considered, so there will be three pipeline sections (separated from each other by the extraction points) but without a change of diameter. Case 1: 4,000-ft pipeline The total horizontal pipeline length for this case is 4,000 ft. We further assume that there are three 30-ft segments of vertical lift, as well as a total of six 90-deg bends, two in each section. Even with a 45-psi pressure drop and the material being cement, only dilute-phase suspension flow will be possible, in any of the three pipelines considered. For dilute-phase conveying (and the resulting low solids-loading ratio) the recommended value of conveying-line inlet air velocity for ordinary portland cement to be about 2,400 ft/min after the 20% safety margin has been applied, the free-air flowrate can be evaluated as 3,340 ft3/min. Thus m•a,1 = 15,330 lb/h, and this will be the case for each of the three 4,000-ft pipelines considered. The single-diameter pipeline: An estimate of the material flowrate is required to start the calculation process; we select a value of 32,000 lb/h. With the above air flowrate, this gives a solids loading ratio of about 2.1, which constitutes very dilute-phase conveying. The conveying-line exit air velocity in this case, is 9,765 ft/min. Adding these three elements together gives a total of 44.7 psi. This sum is close enough to the actual value of 45 psi for a first-approximation solution so the material flowrate will be approximately 32,000 lb/h. (If the difference in pressures had instead been significant, it would have been necessary to assume a new value of material flowrate and repeat the calculation from the beginning.) The stepped pipeline: Since the minimum conveying-air velocity for the material is 2,400 ft/min, the pressures at the first and second steps, based on this same minimum velocity value, can be evaluated. This approach gives p2-3 = 38.3 psia or 23.6 psig, and p4-5 = 26.6 psia or 11.9 psig. In turn, the pressure-drop values for each section of pipeline emerge as follows: Dp1-2 = 21.4 psi, Dp3-4 = 11.7 psi and Dp5-6 = 11.9 psi. The imbalance in pressure-drop values is due in part to the pipeline diameters chosen; however, standard or available pipeline sizes need to be selected. Maximum value of the conveying air velocity is about 4,340 ft/min, compared with 9,765 ft/min for the equivalent single-diameter pipeline. An initial estimate of material flowrate is now required in order to start the calculation process; this estimate is taken as 92,000 lb/h. This gives a solids loading ratio of 6.0, which is low enough to correspond to dilute-phase conveying. The material flowrates need to be scaled for each pipe diameter section, and so these become 23,000, 14,720 and 10,222 lb/h for the 8-, 10- and 12-in.-diameter sections of pipeline, respectively. These flowrates give pressure gradients of 1.35, 0.87 and 0.60 psi per 100 ft. The bend loss is 37 ft/bend. The air-only pressure-drop values for each section can be evaluated as above, with appropriate mean values of air density and velocity; and the acceleration pressure drop, applied only to the first section, comes to 0.6 psi. The pressure drop for the first section of pipeline, of 8-in. diameter, therefore, comes to: Dp1-2 = [0.0135(L1-2 + 60 + 74) + 0.00191(L1-2 + 30) + 0.6]= 21.4 psi; 40
from this equation, L1-2 is found to be 1,229 ft. Similarly for the 10-in.-diameter section: Dp3-4 = [0.0087(L3-4 + 60 + 74) + 0.00092(L3-4 + 30)]= 11.7 psi, from which L3-4 equals 1,092 ft, and again for the 12-in.-diameter section: Dp5-6 = [0.0060(L5-6 + 60 + 74) + 0.00064(L5-6 + 30)] = 11.9 psi, which gives L5-6 = 1,668 ft. This gives a total horizontal pipeline length of 3,989 ft, which is sufficiently close to the actual length of 4,000 ft without need of further iteration. The material flowrate through the stepped pipeline is, therefore, 92,000 lb/h, compared with 32,000 lb/h through the single-diameter pipeline. The lengths of the individual sections of pipeline are approximately as indicated above. Air extraction: Because there is no stepping of the pipeline, and because the amount of air extracted can be controlled, air extraction can in practice take place at any convenient point or points along the length of the pipeline, or at the points considered necessary to maximize conveying performance. But for this example, to allow a direct comparison with the stepped pipeline described above, air will be extracted at the same two pressure points (38.3 and 26.6 psia) where the increases in diameter take place. Sufficient air, therefore, needs to be extracted from Points 2 and 4 to bring the conveying air velocity at Points 3 and 5 back to 2,400 ft/min in the constant-diameter 8-in. pipeline. It has already been established, above, that an air flowrate of 3,340 ft3/min is required at the start of the pipeline (m•a1= 15,330 lb/h). V•03 = 2,140 ft3/min and V•05 = 1,485 ft3/min. This means that 1,200 ft3/ min (namely, 3,340 minus 2,140) of air will have to be extracted at the first off-take and a further 655 ft3/min (2,140 minus 1,485) at the second. An estimate also has to be made for the material flowrate; and this is taken as 60,000 lb/h. This estimate gives solids loading ratios of 3.9, 6.1 and 8.8 for the three sections of the pipeline. The pressure gradient through the pipeline is now approximately uniform at 0.88 psi per 100 ft. The pressure drop for the first section of 8-in.-diameter pipeline, therefore, comes to: Dp1-2 = [0.0088(L1-2 + 60 + 74) + 0.00191(L1-2 + 30) + 0.4] = 21.4 psi, which gives L1-2 = 1,845 ft. Similarly for the second section of 8 in diameter pipeline: Dp3-4 = [0.0088(L3-4 + 60 + 74) + 0.00115(L3-4 + 30)] = 11.7 psi, from which L3-4 = 1,054 ft; and again for the last section of 8-in.-diameter pipeline: Dp5-6 = [0.0088(L5-6 + 60 + 74) + 0.00096(L5-6 + 30)] = 11.9 psi, from which L5-6 = 1,095 ft. This gives a total horizontal pipeline length of 3,994 ft which is sufficiently close to the actual length of 4,000 ft without need of further iteration. The material flowrate through the constant-diameter pipeline with air extrac41
tion is, thus, 60,000 lb/h, compared with 32,000 lb/h for the single-diameter pipeline without air extraction. Summary of results for 4,000-ft pipeline: With pressure drop values additionally expressed as a percentage of the total to illustrate both the absolute proportions and differences between the three cases considered. The main difference between the pipelines can clearly be seen as the very significant reduction in the air-only pressure drop for both the stepped pipeline and the air extraction system. This saving in air-only pressure drop has become harnessed to convey additional material, and the result has been a near tripling in conveying capacity for the stepped pipeline and a doubling in capacity for the air extraction system. In this respect, the stepped pipeline gives the best performance of the three alternatives, because all the conveying air is used all the time. In the air-extraction method, by contrast, the energy remaining in the air extracted from the pipeline is lost from the system (in order to possess the advantage of a constant diameter for the pipeline); a doubling in conveying capability is nevertheless achieved. When the stepped-pipeline and the air-extraction alternatives are viewed side by side, the pressure and velocity profiles show some interesting differences. With the stepped pipeline, for instance, there is a gradual but significant decrease in pressure gradient with increase in pipeline diameter (the absolute value of the slope of the pressure profile decreases), whereas for the air extraction system the pressure gradient (the slope of the pressure profile) remains almost constant along the length of the pipeline. With the stepped pipeline the velocity gradient remains approximately constant with increase in pipeline diameter, whereas with air extraction the velocity gradient increases with each section along the pipeline. This is partly due to the increase in solids loading ratio with each section of the air extraction system. With the stepped pipeline the last section of pipeline is the longest, whereas with the air extraction system it is the first section that is the longest and the last section the shortest. Step location for the stepped sections of pipeline and the location of the extractions points in the common diameter pipeline, therefore, are very different. As can be expected for so long a pipeline, the pressure drop elements for both the bends and the acceleration of the material are relatively very small in proportion, as are those for the vertical lift elements of the pipeline. These proportions, however, are different with shorter pipelines, and with pipelines having different routings. With such a very high proportion of the total pressure drop being attributed to the air-only value in the case of the single diameter pipeline, it would generally be recommended that a more accurate model than be used to evaluate this component of the pressure drop; for one thing, the mean values of conveying air velocity and air density are no longer sufficiently accurate, so an integrated form of the equation should be used. The Reynolds number for the flow should also be evaluated and a more accurate value of the pipeline friction coefficient determined. Bends and pipeline fittings would also need to be included for greater accuracy.
Case 2: 650-ft pipeline The total horizontal pipeline length for this second case is 650 ft, and it will be assumed that there are additionally three 20-ft sections of vertical lift. As in Case 1 with the long pipeline, we assume a total of six 90-deg bends, with two in each section. Because the pressure drop is again 45 psi, and because of the fine particle size and the consequent air-retention capabilities of portland cement, low-velocity dense-phase conveying should be possible, to differing extents, in each of the pipeline cases considered. Single-diameter pipeline: An estimate of the material flowrate is required to start the calculation process; we select 250,000 lb/h. A conveying-line inlet-air velocity has also to be selected, and this is taken to be 2,000 ft/min. This gives an air flowrate of 2,780 ft3/min, which leads to a solids loading ratio of about 20. This is an acceptable 42
combination, so the first element of iteration is satisfied. The conveying-line exit-air velocity in this case is 8,130 ft/min. The air-only pressure drop comes to 2.1 psi, and the acceleration pressure drop comes to 3.1 psi. Adding these three elements together gives a total of 44.8 psi. This is close enough to the actual value of 45 psi for a first-approximation solution, so the material flowrate will be approximately 250,000 lb/h. (If the difference were instead too great, a new value of material flowrate would have to be used, and the calculation repeated from the very beginning.) Stepped pipeline: For the single diameter pipeline just considered, the conveying conditions were such that a conveying-line inlet-air velocity of 2,000 ft/min could be employed. The performance with a stepped pipeline is likely to be significantly better, so it is likely that a much lower velocity could be employed. For a direct basis of comparison, however, the stepped-pipeline performance will be evaluated with the same conveying-line inlet-air velocity of 2,000 ft/min. With this same minimum value at each step, p2-3 = 38.3 and p4-5 = 26.6 psia for V•0 = 2,780 ft3/min. These pressures, in turn, set the pressure-drop values as Dp1-2 = 21.4, Dp3-4 = 11.7 and Dp5-6 = 11.9 psi. These values are the same as those for the 4,000-ft pipeline, inasmuch as there is no difference in either the air supply pressure or the pipeline diameters between the two cases. By use of the process outlined above for the 4,000-ft pipeline, the pressure-drop elements for each of the pipeline sections can be equated to the values given above and the material flowrate determined as the value required for the total length of the pipeline to come sufficiently close to 650 ft. By this process, the material flowrate is evaluated as 400,000 lb/h, in contrast with 250,000 lb/h for the single-diameter pipeline. The individual lengths emerge as 179, 173 and 298 ft for the 8-, 10- and 12- in.-diameter sections of pipeline, respectively. Air extraction. To again allow a direct comparison with the stepped-pipeline alternative, air is extracted from the pipeline at the same two pressure points as before. Given the same minimum conveying-air velocity of 2,000 ft/ min, the free-air flowrate required is again 2,780 ft3/min, but only in the upstream portion of the pipeline. The air flowrates required at Points 3 and 5, come to 1,785 and 1,240 ft3/min respectively, and consequently the air to be extracted at Points 2 and 4 comes to 995 and 545 ft3/min. The calculation process is similar to that outlined above, and the material flow that results is 270,000 lb/h. With this much shorter pipeline, the air-only pressure drop for the single-diameter alternative has changed significantly: for the 4,000-ft pipeline it amounted to about 52% of the total, whereas in this case its contribution is only about 5%. There is, therefore, not the scope for a dramatic increase in conveying capability. There is about an 8% increase with the air extraction system. And in both cases (stepped and air extraction), there is a marked improvement in the velocity profile through the pipeline, which should significantly reduce problems of erosive wear and particle degradation. The differences that were highlighted above between the stepped pipeline and the air extraction system, with regard to pressure and velocity profiles and pipeline-section lengths for the 4,000-ft pipeline, are repeated with this much shorter pipeline. The acceleration pressure drop has increased by a factor of about four, but it is still a small component of the total. For each alternative there is also an approximately fourfold increase in pressure drop due to the bends, and this contribution is quite significant in this case. The loss at each bend is approximately 3% of the total pressure drop, so if the number of bends were to increase above the total of six considered, there would be a corresponding reduction in conveying capability for the given conveying system. By the same token, a performance improvement would result if the number of bends could be reduced below six. With the increase in material flowrate for the stepped pipeline, the solids loading ratio has increased to about 31, and at this value the conveying line inlet air velocity could be reduced by a further 300 ft/min, down to 1,700 ft/ 43
min. For the air-extraction alternative, there is little change from the single-diameter pipeline for the first section of the pipeline; but for subsequent sections, there is a marked increase in solids loading ratio, and progressively lower conveying air velocities could be employed. Although any further improvement in material flowrate would be likely to be marginal, there would be a marked lowering of the velocity profile through the pipeline, and this could be of significant benefit if a very abrasive or a very friable material had to be conveyed. The concept of air extraction thus suggests an interesting possibility: it could be feasible to design a pipeline in which the material is conveyed in dilute phase and high velocity in the upstream portions, but in dense phase and low velocity toward the downstream end. This sequence, of course, is the complete reversal of what can happen in some long-established air-addition and booster systems, where the material is conveyed in dense phase and low velocity at the start of the pipeline but is conveyed in dilute phase and high velocity by the end of the pipeline.
AIR Flowrate CONTROL If air extraction is adopted, the amount of air to be removed from the pipeline must be accurately controlled, and must have the capability of being varied. Nozzles and orifice plates are commonly used for the metering and control of air flowrate. For this application, however, choked-flow nozzles (described below) seem particularly attractive.
Nozzle behavior For the single-phase flow of fluids through nozzles, the theory is well established. And for a common gas such as air, the flow behavior follows many of the equations already presented in this article. Nozzles are either of the convergent-divergent type or convergent only. Both types restrict the flow by means of a short throat section having a reduced diameter. A peculiarity of the expansion of the flow of a fluid through a nozzle under sonic-flow conditions is that for a given upstream pressure, p1, as the downstream pressure, p2, reduces, the pressure at the throat, pt, will not reduce proportionately. Instead, the pressure at the throat will reduce to a fixed proportion of the inlet pressure, and will remain unaffeced by any further reduction of the downstream pressure. Under these conditions, the nozzle is said to be choked. When critical flow conditions exist, the velocity at the throat will be equal to the local sonic velocity. The air mass-flowrate through a nozzle is a maximum under choked flow conditions, and no reduction of the downstream pressure, below the critical throat pressure, will result in any change in that mass flowrate. It can be shown that the ratio between the throat pressure and the supply or inlet pressure is given by: For convergent nozzles, however, the range of operation is limited to downstream pressures less than 52.8% of the upstream pressure; that is, below the critical pressure ratio. With convergent-divergent nozzles, this range can be extended significantly, and for a well-made nozzle it can be as high as 90% of the upstream pressure, with little deviation from the predicted flowrate. For a range of air supply pressures, a typical relationship between nozzle throat diameter, air supply pressure and air flowrate is given.
Nozzle selection When it comes to discharging air from a pipeline section, the air in most cases is likely to be discharged to atmo44
spheric pressure. If the ambient air is at the standard sea-level atmospheric pressure of 14.7 psia, a convergentdivergent nozzle can be expected to work reliably at pipeline pressures down to about 16.4 psia (1.7 psig). This is a much lower pressure than is likely to be required in practice, and very much lower than either of the offtake pressures considered in the example cases above. With a convergent nozzle, on the other hand, operation would be limited to pipeline pressures above about 27.9 psi absolute (13.2 psig), and this limitation would rule out their use for the second air offtake in the cases considered above. For most applications, therefore, a convergent-divergent nozzle is to be recommended. It can be seen that a fine control of air flowrate discharged can be obtained by means of a change in either air pressure or throat diameter. Since variation of pressure is not likely to be possible in this application, a nozzle having a variable throat diameter is recommended.
Offtake sections At each air extraction point, a section of porous pipe is perhaps the best means of extracting the air without loss of the conveyed material. For a material such as cement, it would probably be necessary to provide a short pulse of high pressure air to keep the inner surface free of dust, in much the same way that reverse air jets are used to clean filters online. Should the pipeline need to be purged of material at the end of a conveying run, the air extraction can be switched off, possibly with a separate valve.
SUMMING IT UP If high-pressure air is used to convey a material, the very high velocities that would inevitably be experienced in conventional single-diameter pipelines can be avoided by either stepping the pipeline to a larger diameter along the length of the pipeline or by extracting air from the pipeline at various points along its length. In addition to obtaining very much lower velocity profiles, it is also possible that an improvement in conveying performance will be achieved. The degree of improvement appears to increase with an increase in pipeline length; and for very long pipelines, it could be very significant, particularly with the stepped pipeline. A particular problem with stepped pipelines is that the pipeline generally has to be designed around the use of standard available pipeline diameter sizes and this does impose a limit on the number of steps that can be used. Another problem with them is that of purging material from the pipeline, particularly if a material is conveyed in dense phase and hence at low velocity. With the air extraction system, there is no theoretical limit to the number of extraction points, so more-uniform velocity profiles can be achieved. Because one can control the amount of air extracted, there is also less vulnerability to design error. And whereas stepped pipelines can rarely be used to convey a different material, or even a different grade of the same material, lines employing air extraction should be able to handle different materials if fine-tuning is applied. Keep in mind, however, that the velocity and pressure profiles through a pipeline with air extraction will be very different from those for stepped-diameter pipelines. These differences arise because the concentration of the conveyed material increases at every air offtake point along the length of the pipeline. With an increase in solids loading ratio along the length of the pipeline, however, the possibility arises for conveying a material, such as cement, in dense phase and at low velocity towards the end of the pipeline even if doing so was not possible at the start. The method of analysis presented in this article is straightforward to use. And although estimates have to be 45
made of the material flowrates, and of the conveying air velocities in the case of dense-phase flows, there are no simultaneous equations to solve and the analysis proceeds in a logical step by step process. Material flowrates and pipeline-section lengths alike can be evaluated. It must be emphasized, however, that the results presented here by way of illustration relate only to the material considered (portland cement). Different material will perform differently; and, indeed, there may also be significant differences between different grades of the same material.
46
Guidelines for Solids Storage, Feeding and Conveying These observations and recommendations — reflecting decades of experience — can help operators avoid a lot of headaches and heartache when it comes to handling bulk solids
Shrikant Dhodapkar The Dow Chemical Co. (U.S.), Lyn Bates Ajax Equipment Ltd. (U.K.), George Klinzing University of Pittsburgh (U.S.), Peter Wypych Un
Successful startup and operation of solids-handling processes depend on myriad small yet fateful decisions that are made during the various phases of the project. Many of these are not quantitative but qualitative in nature, and are largely based on experience and broad-based knowledge of solids handling. It is just as important to know what not to do as it is to know what to do. Since many engineers receive very little formal training on solids handling, their first instinct is to draw an analogy between solids-handling systems and fluids-based systems. This should be avoided. The behavior of solids (powders, pellets and granules) is often counter-intuitive to the behavior of pure fluids. The guidelines presented here draw upon the collective experience of the authors and highlight key issues that need particular attention. These recommendations are by no means exhaustive, but merely provide a starting point for a thoughtful discussion.
STORAGE Material Testing and Evaluation Get a representative and traceable sample. Make sure that the sample is stable over time. If large variability is observed, understand the source (nature and range) of the variability, and collect multiple samples to establish the bounds on physical properties. Proper documentation of sample source and history is helpful in troubleshooting if the hopper does not perform as expected. Variations in product condition arise for various reasons, most of which fall into the three general classes: uniformity, consistency and stability. Materials that initially have uniform composition may change during handling; for instance, if segregation occurs, the material may demonstrate different bulk properties. Meanwhile, supply inconsistencies may result from seasonal or process variations, or the need to secure materials from alternative sources. The product may also change with time, temperature, attrition, or through natural degradation or other mechanisms. All these changes must be considered in combination, taking into account the complete range of operating and ambient conditions to which the material may be subjected. Use shear testing for evaluating material flowability and wall friction characteristics. The Jenike flow-factor tester and Schulze Rotational Shear Tester (RST) are typically used for such measurements. Shear testing is the domain of specialists, but experienced personnel can usually make an assessment of the degree to which such testing may be necessary. Critical arching dimensions, stable rathole diameter, hopper angle required for mass flow and feeder load can be calculated using the proven theories of Jenike and other researchers. A very important note: Sometimes the material’s angle of repose is incorrectly used to calculate bin design parameters. The angle of repose should only be used to estimate the volume of material in the bin. Perform a wall-friction test on a representative wall sample obtained from the equipment fabricator. For example, a mill-finish aluminum sample from one fabricator may differ significantly from an aluminum sample from another vendor. If the material being handled is highly abrasive, consider the effect of wear on wall friction. Some wall materials become more polished (smooth) with wear while others might become rougher. Similarly, the effect of potential corrosion and oxidation must be taken into account. Be sure to measure the appropriate bulk density. This critical measurement should reflect the circumstances of the application. For instance, a rapidly filled hopper may require the holding capacity to be based on a “loose poured” density, while one filled slowly may be better assessed using a “settled” density value. On the other hand, “aerated” bulk density should be used if aeration devices are used as flow aids. Care should be taken when dealing with a product that exhibits a wide range of density values according to how it is handled. 48
Selection, Specification and Design Establish the range of moisture content, additive content and operating temperature. The design basis should reflect the full range of conditions in which a material is going to be handled and not just “typical” operating conditions. For outdoor applications, consider the consequences of temperature cycling and whether there may be condensation in the headspace (silo weeping) or on walls that may be in contact with the material. Include discharge rate expectations in your design criteria. The discharge rate of a powder can be severely limited by its lower permeability as compared to a granular material. Test the material for flowability changes resulting from consolidation over time. Try to anticipate problems that may arise from extended periods of storage, such as those occurring during normal operation, and from plant shutdowns, production cycles or weekends. Choose mass flow mode. A bin is set to operate in mass flow mode when all material in the bin is in motion while the material is being discharged. When this does not occur, the bin is operating in funnel flow mode. In general, a mass flow pattern results in a smaller required outlet size, more reliable flow for cohesive materials and some re-mixing of segregated materials. The first-in, first-out nature of mass flow also helps to prevent fluidizable powders from flushing straight through the hopper. It should be noted that the first-in, first-out flow pattern in mass flow can only be achieved if there is at least one diameter head of material in the cylindrical section of the hopper. This is a requirement to get a uniform draw of material during discharge. Mass flow hoppers are typically taller than funnel flow hoppers, due to a steeper cone angle. There are applications where a self-cleaning funnel flow design will provide comparable duty at lower cost; therefore, careful evaluation of all factors is essential. Be aware that choice of hopper shape is a multi-factor decision The decision largely hinges on whether the flow benefits of plane flow, which occurs in a V-shaped flow channel, are justified over the poorer flow behavior associated with a conical or pyramid construction. Flat walls and rectangular cross-sections allow easy construction and maximize holding capacity but generally have the worst flow characteristics and pressure capacity. Cones contain internal pressures well, but require simultaneous flow convergence in two directions at 90 deg to each other. By comparison, plane flow allows the use of less-steep walls and permits flow through smaller outlets, but normally requires the use of a well-designed feeder to extract product from an extended slot. Such a feeder must extract product from the total cross-sectional area of the outlet for mass flow applications. The critical arching dimension in a plane-flow channel is half that of the diameter of a circular outlet, provided the slot length is greater than three times its width. For example, a 12-in.-wide slot that is 3 ft long is equivalent in flow terms to a 24-in.-dia. outlet and the walls can be less steep (about 10 deg shallower). Remember, flow aids can also be used to encourage flow for hard-to-handle materials, but getting the wall slope wrong is far more difficult to correct. In retrofit situations, have structural considerations re-evaluated by a trained solids-handling expert. For example, when a funnel flow bin or silo design is converted to operate in the mass flow pattern, the cylinder-cone transition may have to be reinforced to withstand highly localized peak stresses. On the other hand, if a silo that was designed for mass flow is operated with a more cohesive material that forms large voids and ratholes, the additional dynamic loads, due to erratic flow, must be taken into account. 49
Don’t overlook the value of close cooperation and collaboration with the supplier or vendor. To ensure success, it is important to have agreement on the following issues upfront: • Definition of operating window, along with extreme bounds of operating conditions and material properties • Sampling techniques and definition of representative sample (including handling instructions) • Test methods to be used for product evaluation and eventual performance evaluation • Expectations on performance validation and financial terms associated with it • Degree of freedom available to the vendor to optimize total cost of the project and share the reduction in cost with the purchaser
Installation, Operation and Performance Keep the wall surface smooth. Surface impediments on the hopper wall such as weld splatters, offset flanges protruding gaskets, poke holes and badly installed liners prevent the material from sliding along the wall. Remember the humorous adage: “Protruding lips sink powder ships” Avoid using a slide gate to throttle the flow because this will result in preferential flow at the outlet. This preferential flow problem can be alleviated by using a vertical transition piece (H > 1.5 D) at the outlet. Slide gates should only be used in the open or closed position. The inner diameter of the bottom flange can be slightly oversized to avoid any lips or protrusions in the flow path. Avoid asymmetric stresses. Do not cut into the walls of a hopper to create additional discharge outlets. In most cases, they will disrupt the mass flow pattern and create high wall stresses. Protect the wall finish and liners of the cone from the weather to prevent oxidation (rust) and corrosion. The internal surface of the hopper must be protected from fabrication until the time it is installed in the process. Make sure that all the protective liners are removed before commissioning the hopper. Avoid welding on cones with liners and wall coatings since the heat can cause delamination. Avoid improper selection and installation of a feeder, which can destroy the flow pattern in a well-designed mass-flow hopper. Reliable flow out of a mass-flow hopper requires the material to be discharged across the whole cross-section of the outlet. For example, a screw feeder with progressively increasing pitch offers a proven option for drawing material across the entire outlet.
FEEDERS (PROCESS AND ADDITIVE)
Material Testing and Evaluation Conduct trials under realistic conditions. Materials that are sensitive to heat and moisture may appear to be wellbehaved in air-conditioned labs. This can produce false confidence based on test results. 50
Establish feeding accuracy requirements. It is important to know whether feed rate control or totalized weight is required by the process. The time basis for calculation of feed rate accuracy must be established. Make sure that acceptable accuracy is achieved at the lowest feed rate. Run extended trials (1–7 days) on new materials or critical applications. Many problems do not appear in shortduration (< 1 day) tests. Consider renting a unit for longer-term pilot-scale testing. Be sure to test representative materials. Often times, offline tests are done on materials that do not reflect flow properties of actual material in process. Such tests may be misleading.
Selection, Specification and Design Define operational requirements. Does the process require the material to be fed at a target rate (e.g., in a continuous blending operation), or to deliver a target totalized weight in a batch operation (e.g., in a packaging operation)? Choose between gravimetric and volumetric feeders. Gravimetric feeders are recommended when feed rate uniformity of better than ±2% is required, especially for sample collection times less than 30 seconds. These feeders are also required for feeding fine cohesive powders with erratic flow, or powders with unpredictable bulk density that are prone to aeration. Be sure to design the feeder and hopper as an integral unit. The feeder is not merely a “discharger;” therefore, reliable flow out of the hopper is essential for proper feeder operation. A feeder is designed to modulate the flowrate of the material that is reliably fed into it. Material discharge must take place over the entire outlet for a mass flow hopper to work For some hard-to-handle products (such as those that are cohesive, caking, fibrous, stringy, time consolidation sensitive or hygroscopic), a discharger may have to be installed upstream of the feeder to ensure reliable flow. Plan for sufficient flight tip clearances. Make sure that flight tip clearances for screw feeders are large enough to avoid particle trapping or wedging, which can result in material degradation. On the other hand, rotary feeders require the clearances to be less than the particle size to avoid smearing inside the housing. Determine whether the feeder is required to provide positive protection against uncontrolled flow that might result from flooding or flushing of fine powders. For example, a screw feeder will not restrain a fluidized powder from flushing out of a hopper. If the product is prone to flushing, install a valve on the discharge port. The valve should be interlocked to be open only when the screw is running, and should have an emergency override switch to close it. Consider system-integration issues that may arise when feeder electronics are integrated with plant network and process computers. The ability to seamlessly integrate a feeder control package with the plant network and share data and diagnostic information will result in smooth operation. Be aware that rotary and screw feeders are prone to pulsations in feed rate. Pulsations can be minimized in a number of ways: • Rotary feeders: •Use a reduced-capacity rotor and ncrease the number of vanes 51
• Screw feeders: • Install wire mesh at the nozzle outlet • Provide a slot (1.5 times the pitch) on the side of the nozzle • Install a wedge at the outlet over which the material must flow • Reduce the pitch near the outlet • Use a twin-screw configuration
Feeder selection considerations The following issues must be considered during feeder selection: 1. Material properties a. Particle size distribution and maximum lump size b. Flowability and cohesion c. Air-retention characteristics and their effect on bulk density d. Flooding or flushing characteristics e. Abrasiveness f. Friability g. Sensitivity to temperature 2. Operational requirements a. Process temperature and humidity conditions b. Feed rate (typical, maximum, turndown ratio requirements) c. Feed rate uniformity versus accuracy and sampling requirements d. Volumetric versus gravimetric feeding e. Need for dust control and containment f. Sealing requirements against pressure g. Ease of cleaning (frequency of cleaning and acceptable effort necessary)
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h. Sanitary requirements (for instance, in food and pharmaceutical applications) i. Ability to handle unexpected materials (such as tramp metal or nuts and bolts) j. Headroom constraints k. Vibrations in the process area and isolation techniques required l. Controller interface protocol and control system integration with existing plant network and process computer m. Data and parameters that need to be exchanged, namely, instantaneous flowrate, setpoint, totalized weight, bulk density, alarms, system reset and PID control parameters n. The location of the controller box vis-à-vis the feeder
Installation, Operation and Performance Always confirm that the outlet will not be blocked or obstructed or admit a reverse gas flow. Unvented rotary valves often pass back air by leakage, and via returning vane pockets. With no means of escape, this air can inhibit flow from the hopper. Allow a gas-bypass route to ensure that the material will flow freely into the feeder. Fit a torque limiter or a level detector to isolate the drive if there is any prospect of backup from downstream equipment. Verify and minimize the starting load on the feeder. It is not uncommon for the starting load to be five times the operating load. During initial filling conditions, a peak stress field is generated in the bin where large loads are transmitted to the feeder. With flow initiation, an arched stress field is developed in the hopper where most of the load is supported by the hopper walls (further discussion on arched stress fields can be found in Jenike). The arched stress field is retained even if the flow is temporarily stopped. Feeder loads during startup can be significantly reduced by creating an arched stress field; for example, by withdrawing small amount of material during filling. Leaving the hopper partially full before refilling will also achieve the same result. Provide a slide gate (or some other type of shutoff valve) above a feeder. This is especially important for situations in which the feeder fails and must be removed for maintenance or repairs when the overlying silo or bin is full of materials. Make sure that the seals are adequately purged to prevent fine powder from entering and destroying the seal. Seal and bearings must be inspected and maintained on periodic basis to avoid premature failure.
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Feeders for Pneumatic Conveying Systems Material Testing and Evaluation Feeders for pneumatic conveying systems are rarely tested on a standalone basis. Rather, their effectiveness is typically tested while conducting conveying trials. The following issues must be looked into during testing: 1. The effect of gas leakage or aeration on the bulk density of product. Reduced bulk density can result in lower feeder capacity. 2. The abrasiveness of material and its impact on feeder performance 3. Stickiness of material and any resulting reduction in feeder capacity 4. Ease of cleaning and unplugging
Selection, Specification and Design Give careful consideration during the feeder selection process. Feeder selection depends on the mode of operation (pressure versus vacuum), operating pressure, mode of conveying (batch versus continuous), space availability and material properties (such as abrasiveness and cohesion). Match feeders with the characteristics of air movers. For instance, feeders that create large pulsations in the feed rate should not be used in systems that use fans as air movers. The interaction between pressure variations resulting from feed rate fluctuations and fan characteristics can result in an unstable system.
Installation, Operation and Performance Design a proper venting system for rotary airlock feeders in positive-pressure applications. The upward leakage of air through clearances and returning pockets can impede material flow and may reduce bulk density, which can result in capacity limitations. The vent always contains entrained solids and therefore must be designed as a pneumatic conveying system. Consider the effect of pipe and bolting stresses. Feeders connected to piping system (chutes) that are exposed to large variations in temperatures either due to process conditions or diurnal variations may experience high stresses. This can affect the operational clearances and cause jamming or excessive leakage to occur. Consider relative expansion that may result from hot product and cold ambient conditions. Relative expansion of feeder internals due to a hot product can cause interference fit and result in a feeder jam. The problem can be alleviated by heat tracing the feeder. Make sure that the feeder is appropriately designed for quick clean out for sanitary applications. Provide the necessary working space in the process area. Proper interfacing of feeders with the conveying line is essential. The feeders are attached to the conveying line with a feed shoe or a feed box. A good feed shoe design provides for rapid removal of material from the bottom 54
of the feeder, minimal reduction in local gas velocity at the feed point, and minimal recirculation patterns.
Pneumatic Conveying – Dilute Phase Material Testing and Evaluation For materials for which there is no prior pneumatic conveying experience, and for which no related data are available, pilot or full scale testing is recommended. Take the following issues into consideration: • Conduct tests at material temperatures and moisture levels that are comparable to the process conditions • Conduct tests on off-grade materials (not just the prime product) to ensure process reliability across a range of material properties • Consider the effect of attrition on conveying characteristics if the material is being recirculated during the test • If the conveying line is getting coated (by materials buildup) during the test, the data will exhibit scatter and may not be reliable
Selection, Specification and Design In the project scope, clearly define current conveying-rate requirements and plant layout, along with plans for future expansions or potential rate enhancements due to improvements in process technology. This should include potential changes in the product slate due to improvements in process technology. Developing the project scope should be a collaborative effort between research, manufacturing and engineering personnel and the outside vendor. Allow a ‘reasonable’ horizontal conveying length (15 to 20 pipe diameters), before the first bend to allow the bulk material to accelerate. Use of flexible hose at the pickup should be kept to minimum. Excessive length of flexible hose, often in the form of a coil, is the worst pickup pipe configuration. Inability to provide proper configuration at the pickup will result in plugging condition at gas velocities higher than saltation velocity Consider stepping conveying lines (increasing the pipe diameter) to prevent excessive velocity at the end of the line. Be sure to maintain the minimum required Froude number at the step location, otherwise the material will settle out of the suspension (saltation). Properly stepped systems result in more efficient systems with lower degradation and wear. Using ISO pipes or tubes allows for more choices in diameters. Be aware that proper venting of the rotary airlock and feeder in a positive-pressure system is critical for reliable operation. Upward leakage of air into the feed hopper can result in reduced discharge rates or an unsteady feed rate. This issue can be addressed by using proper design of a vent system, either using a body vent or a disengagement hopper. The vent system should be designed much akin to a conveying system with sufficient gas flow and minimal bends. Be aware that product damage and wear at piping bends is very material-dependent. A blind tee piping configuration often has a lot of merit, but may also cause greater pressure drop compared to long-radius bends. Mitered elbows may offer a good compromise in some cases. 55
Remember that minimum conveying velocity is a function of conveying rate. Make sure that the gas velocity at the pickup is greater than the saltation velocity at the highest solids flowrate. Safety margin must be allowed for non-optimal line configuration at the pickup (such as insufficient acceleration lengths, back-to-back bends, and so on).
Installation, Operation and Performance Avoid conveying line layout with bends or elbows placed back to back. This will inevitably cause excessive pressure drop and premature line plugging. Be aware that conveying lines should not be routed like utility lines (i.e., those carrying compressed gas or steam), which follow the contours of a building. When designing solids-handling systems, minimizing the number of bends or directional changes will result in higher capacity, lower degradation, less erosive wear and more reliable flow. Consider electrostatic effects. Electrostatic effects in pneumatic conveying systems can be minimized by increasing the relative humidity of the conveying air to more than 70%. Remember that ‘more air’ can mean ‘less transfer capacity’ in dilute-phase systems. At constant conveying pressure, increasing gas velocity results in reduced conveying capacity. Install sufficient ports or couplings in the systems for pressure measurement during troubleshooting. Pressure measurement is a convenient way to measure the pulse of a conveying system. Take time to generate baseline data for an existing conveying system. It will come in handy for future troubleshooting. Always verify installation in the field. The technical guidelines of system designers and engineers, even if welldocumented, can be undone by contractors installing a system. Common mistakes, such as those listed here, can cause significant delay in successful startup: • Incorrect rotational direction of rotary airlock/feeder or air movers • Conveying lines with directional internal treatment (e.g., shot-peening) that are installed backwards • Poor alignment of flanges or improper seating of gaskets (which can result in air leakage or material degradation) • Specialized bends or elbows (such as blind tee configurations) that are installed backwards • No allowance for pipe flexing (which can result in high bolting stresses on the components) Always verify the control logic and the sequence of valve operation in the field. Follow these recommendations to avoid problems: • For complex systems, run simulations before startup to understand unintended consequences. A software code is easier to mend than broken hardware during a startup • Failure to include sufficient time delay between various steps may cause problems. For example, a large slide valve may take as much as 30 seconds to close fully. 56
• If control loops are being used for feed rate control, tune the parameters and provide upper limits to prevent overshooting. This might result in overfeeding solids into a conveying line resulting in plugged condition. During startup, verify process instrumentation with field measurements. Make sure that calibration parameters, conversion factors and units are correctly entered, calculated and communicated to the process computers. If possible, use local gages for verification. More guidelines on troubleshooting pneumatic conveying system can be found in Mills, and Dhodapkar and Jacob.
Pneumatic Conveying – Dense Phase Material Testing and Evaluation Conduct pilot-scale or full-scale testing on representative materials, especially for new or different products where no prior experience or data are available. Benchtop characterization tests (to determine such characteristics as wall friction, permeability and deaeration tendencies) are helpful for preliminary screening of a material’s suitability for dense-phase conveying. Dense-phase conveying performance can be sensitive to variations in material properties (such as particle size, size distribution, shape, density, moisture and cohesion).
Selection, Specification and Design Ensure that the choice of a flow mode (and system) is based on the product properties (rather than on some imprecise definition or misleading solids-loading assumptions), and that the selected or supplied flow mode is confirmed during commissioning. The term “dense phase” is often misused in practice. For example, many so-called “dense phase” systems are found to be operating in dilute phase (or suspension flow). And, many researchers and designers define “dense phase” as a flow mode with solids loading (mass of solids/mass of gas) greater than 10 or 15. Many different types of dense-phase conveyors have been developed over the past few decades to take advantage of certain product properties (such as air retention, deaeration, permeability, cohesion, and particle-size distribution). In most cases, “dense-phase” can simply be considered as some form of non-suspension flow that occurs at some time at any section along the pipeline. Using solids loading as an indicator of flow mode can be misleading (for instance, solids loading is a mass concentration parameter that depends on the mass or density of the particles; some dilute-phase systems are operating at a solid loading rate greater than 40, while some dense-phase systems operate at a solid loading rate less than 10). For materials that do not have a natural tendency for conventional dense- phase conveying, consider specialized systems with controlled and regulated gas injection or bypass pipeline technology Ensure that proper densephase flow is actually achieved during commissioning. Using a conventional or “off-the-shelf ” pipeline, not all materials can be conveyed reliably in dense phase. Some materials can be conveyed in single- or multi-slug/plug mode, some in fluidized moving-bed type flow, while others can only be conveyed in dilute phase. Not selecting the right flow mode for a particular material, or the right operating condition for a given flow mode, can result in excessive pressure spikes, system shutdown, unstable vibrations and pipeline blockages. Define the minimum and maximum conveying rates in a process at the outset. Compared with dilute-phase con57
veying, the dense phase regime can be more limiting and sensitive to variations in air flow and conveying rate. For some materials, a reduction in solids flowrate can shift the operating point into the unstable zone, thereby causing severe instability (evidenced as line vibrations and pressure spikes). Ensure proper venting at the feeder to avoid feeding problems that may result from gas blowback. Feeder gas leakage can be a significant fraction (up to 50%) of total gas consumption. The gas leakage at the feeder (especially through rotary valves) must be considered in design calculations and compensated appropriately. Use a gas-flow control system for multiproduct and multi-destination systems, to ensure that the operating point is maintained within the stable operating zone. Also, ensure that the gas-flow control system provides a constant gas mass flowrate for the full range of operating pressures and pressure fluctuations. Numerous control logic schemes are available from various vendors or can be designed by reputable consultants. For systems with high pressure drop (7 psi or 50 kPa and higher), consider stepping the line diameter to reduce the velocity and maintain dense-phase conditions. In dense-phase systems, gas expansion can be significant between the feed point and destination. This will result in a corresponding increase in gas velocity, and a possible transition from dense- to dilute-phase flow along the conveying pipeline. Remember that selection of optimal feeder is critical. Improper feeder selection can result in unreliable operation and high maintenance costs.
Installation, Operation and Performance Make sure that the operating point (gas flowrate, solids flowrate) falls well within the bounds of plug stability at all locations in the system and for all pipeline configurations (if applicable). Coarse or granular materials that can be conveyed in dense phase (low-velocity slug-flow mode) exhibit an unstable operating zone in between (highvelocity) dilute-phase and (low velocity) dense-phase conveying. The dense-phase regime is bound by a highvelocity (unstable zone) boundary and a low-velocity (blockage) boundary. A proper purge-control sequence may need to be designed and tested to avoid unnecessary product degradation or pipeline blockage. To purge a dense-phase line during cleaning, a controlled increase in gas velocity may be required. The dust collector must be designed to handle the peak gas flow rates. Work closely with an experienced vendor to design and install proper pipeline supports, to prevent excessive deflection and line movement and reduce the prospect of fatigue failure. The motion of slugs and stresses generated within the conveying line during directional changes (such as those that occur at bends or diverter valves) results in significantly higher stresses on pipe supports as compared to dilute-phase systems.
In summary The guidelines offered here provide a good starting point for designing a reliable bulk solids handling system involving selection, installation and operation of a silo, feeder and conveying system. Each new project offers unique challenges, especially with ongoing innovations in polymer science and fine (even nanoscale) particle production. The secret to success lies in understanding the fundamental concepts of solids handling, using material characteristics (real data) for designing, matching hardware performance specifications with the process requirements and paying attention to details during design and implementation.
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Cover Story: Move Difficult-to-Handle Bulk Materials with Flexible Screw Conveyors David Boger, Flexicon Corp.
Convey tough bulk materials that tend to pack, cake, smear, break apart or fluidize, and prevent separation of blended productsFlexible screw conveyors are suitable for use with most bulk materials, from sub-micron powders to large pellets, both free-flowing and non-free-flowing. They are capable of conveying bulk materials at any angle, moving over or under obstructions and through small holes in walls or ceilings These conveyors also have the advantage of simple construction, low space requirements, reliability of operation and favorable economics. Although alternate conveying methods may be preferable depending on the application parameters of a project, this article focuses specifically on flexible screw conveyors for materials that are problematic to convey.
Difficult-to-convey materials When engineering a flexible screw conveyor for difficult-to-handle materials, it is necessary to establish the materials’ physical characteristics, flow properties, temperature, moisture content, inherent hazards and allowable degree of degradation, as well as the material source and destination, conveying rate, distance, cleaning requirements, plant layout and economics. Flexible screw conveyors are appropriate for use with: • Cohesive materials • Ultra-fine particles • Fragile or friable materials • Abrasive materials • Materials that fluidize • Blended products of disparate particle sizes and bulk properties A caveat for the plant engineer: the flow characteristics of a bulk material being conveyed under unique circumstances cannot always be predicted with sufficient accuracy to ensure successful performance. In these cases, the importance of simulating plant conditions with a full-size conveyor in a test facility is extremely important. Efficient flow of a bulk material through any bulk-material-handling system is generally a function of the material’s physical properties, but it can also be affected by external factors, such as ambient moisture and temperature levels, as well as the design of the equipment in which it is contained. Although certain material parameters, such as the “angle of repose”, may be determined by evaluating a material sample in a laboratory, these controlledcondition tests are not necessarily predictive of flow behavior in full production-scale systems. When dealing with large volumes of materials under varying conditions, a bulk material’s flowability cannot be determined by physical characteristics alone, such as bulk density, particle size and shape, compressibility or cohesive strength. Therefore, when designing a flexible screw conveyor, the engineer must consider not only the material’s physical properties and flow characteristics, but also how these characteristics will be affected by actual conditions in the plant and the design of the equipment: • Is the material free-flowing, semi-free-flowing or non-free-flowing? Has the equipment been designed with proper flow-promotion devices and hopper geometry? • Is it hygroscopic? How much moisture is in the plant environment? • Does it tend to pack, cake or smear? 60
• Do the particles interlock or mat? • Is the product degradable or breakable, such that use or value is impacted? • Is it abrasive? • Is it a blend of various types and sizes of particles that should remain homogeneous during conveying? • Does it bridge or dome in storage vessels, or is it prone to the formation of “ratholes”? • Does it tend to aerate or fluidize when being handled? With the answers to these practical questions, as well as testing in a full-scale system if required, the performance of a conveying system for a specific bulk material in a unique plant environment can be predicted.
Screw geometry Geometry of the flexible screw is critical to performance. Screws vary from round wires that produce relatively high radial forces, to flat screws that generate comparatively greater directional force This difference in the manner in which the forces are distributed within the conveyor allows system performance to be optimized based on the properties of a given material. For example, due to its greater directional force, a flat design is better suited than a round design for lighter powders that tend to fluidize. Variants of these two basic screw geometries are also available. For example, flat screws with beveled outer edges distribute the forces inside the conveyor in a slightly different manner than a nonbeveled design. This variant can allow efficient transfer of materials that may cause problems with other designs. Another variant sometimes employed with high bulk-density materials is a heavy-duty version of one of the basic screw types. Materials of construction and finish levels are specific to application, with screws constructed of spring steel or stainless steel, and tubes of stainless steel or polymer.
Equipment and systems Flexible screw conveyors are frequently integrated into systems with accessories for feeding and discharging bulk materials. These might include: bulk bag dischargers or manual bag-dump stations with dust collection; feed hoppers with or without flow promotion devices, such as pneumatic vibrators or mechanical agitators; weigh batching systems for precise control of feed; discharge equipment, such as bulk-bag fillers; and control systems. Feed-hopper design is critical when specifying a conveying system for materials with poor flow characteristics, as the throughput capacity of any conveyor is limited to the rate at which material will flow down to the pick-up area of the conveyor. The shear stress created by gravitational forces and flow-promotion devices must be high enough to overcome static cohesive forces between the solid particles. If not, some particles in the vessel will remain stationary and the result will be “rat holing” or “bridging” The resulting restriction of flow may limit downstream processes because of insufficient feed, or cause flooding of the bin if material enters faster than it can exit. Problems caused by rat holing include loss of effective surge capacity in the feed hopper, reduced system throughput and additional time required for an operator to manually clean the static product out of the hopper, if necessary. The main problem caused by bridging (also known as arching or doming) is that once the bridge 61
forms, material flow essentially ceases, requiring a process shutdown while material is removed. Feed hoppers for materials that may rat hole or bridge should be designed with proper geometry and sufficiently steep walls to promote flow. They may incorporate devices such as vibrators or air fluidizers to dislodge material from hopper walls, or mechanical agitators to promote flow.
Cohesive materials Sticking, packing, caking and smearing are the result of particle binding. This can be caused by chemical reactions, partial melting, binder hardening or crystallization of dissolved substances; adhesion/cohesion of particles joined together from mechanical deformation; attractive forces, such as electrostatic or magnetic pull; interlocking forces resulting from irregular particle shapes; and moisture, oil or fat content. Moisture is particularly problematic for hygroscopic materials, such as magnesium chloride. As water is absorbed from the surrounding atmosphere, relatively free-flowing materials can begin to agglomerate. In extreme cases, large volumes of these types of materials can solidify, creating large masses of material that can impede flow or immobilize moving equipment components. Since flexible screw conveyors are totally enclosed, temperature and moisture levels of the product can be maintained. Upstream and downstream equipment such as bulkbag fillers, bulk-bag unloaders, feed hoppers, screeners, blenders and discharge vessels, can also have an airtight design. In addition, materials with high fat content, such as cake mixes, and materials such as zinc oxide and titanium dioxide, which are cohesive and compressible by nature, are generally non-free-flowing, making them good candidates for flexible screw conveyors. An example from the paint-and-coatings industry demonstrates the design of a mobile conveyor system for cohesive materials. A flexible screw conveyor transports a mixture of five materials, including calcium carbonate, titanium dioxide powder, two semi-free flowing talcs and a free-flowing resin for a producer of aftermarket autobody paints. The materials are particularly difficult to convey because of disparate bulk densities of 16 – 46 lb/ft 3 and flow characteristics ranging from free-flowing to non-free-flowing. The company converted from manual dumping of bags to a 3-m long, 45-deg angle, portable flexible-screw conveyor mounted on a cart with an integral feed hopper and dust collector. The specially engineered screw design allows the system to function across the wide range of materials. The feed hopper has been designed with steep walls and other beneficial geometric features. Flow-promotion devices combined with proper flow angles prevent bridging by directing the material toward the back wall and down into the conveyor. Conveyor interface adapters have vertical walls to keep material flowing. Feed testing on full-size equipment was integral to the success of the design.
Ultra-fine particles Mechanical conveyors have an advantage over pneumatic conveying for light or dusty materials, because fine particles can make it difficult to keep the filters operational in filter receivers. Some fine materials tend to fluidize; for example, fumed silica (synthetic amorphous silicon dioxide) is light and feathery with a bulk density of only 2.5 – 3 lb/ft 3 and a very small particle size of 0.2 – 0.3 µm It is not only prone to dusting, but can also fluidize, taking on some characteristics of a liquid, making it a particularly difficult material to convey. A properly designed screw with flat flight surfaces and some modifications will lift particles by restricting the material’s ability to fluidize. Bag-dumping stations for such fine materials should be equipped with internal dust collectors, including cartridge filters and pulse-jet filter cleaning. 62
Many pigments are comprised of particles under 5 µm, and although the bulk densities may range, materials such as titanium dioxide, iron oxide and carbon black all have a tendency to pack. In order to prevent a conveyor from seizing with such materials, the ideal conveyor screw should have a geometry that distributes the forces inside the conveyor to minimize compression. Flexible screw conveyors can reduce fluidization and aeration of light bulk materials by employing proper design elements. For example, diatomaceous earth (DE), a dry, dusty material consisting of irregularly shaped 5 – 25 µm particles, with a typical bulk density of 10 – 16 lb/ft 3, has a tendency to bridge and rathole in feed hoppers and to fluidize during transport. Flexible screw conveyors for such materials are generally designed to combat aeration with a wide, flat spiral screw to provide a wider carrying surface with positive forward force and minimal radial force.
Fragile and friable materials Testing is particularly important for fragile or friable particles that must be conveyed without breakage. The self-centering action of the rotating flexible screw can maintain ample clearance between the screw and the tube walls to eliminate or minimize product damage.
Abrasive materials Flexible screw conveyors are appropriate for abrasive materials, primarily due to the ease of maintenance resulting from a design that utilizes no internal bearings and only one moving component that contacts material. For example, anhydrous borax is abrasive, yet light and fluffy with a bulk density of 47.6 lb/ft 3 and a 74-µm particle size. A flexible screw conveyor with a heavy-duty, flat-wire screw can stand up to the abrasiveness of the product, since the flat conveying surface minimizes the radial force to reduce friction and wearing of the conveyor wall. If necessary, the flexible screw can be removed for inspection or replacement with minimal downtime.
Diverse mixtures A properly engineered flexible screw conveyor can prevent separation of blends throughout the length of the conveyor, regardless of differences in flow characteristics, bulk density or particle size, whereas pneumatic conveyors or other types of mechanical conveyors may cause separation of mixtures during transport. For example, a major spice company has over 8,000 different recipes, each consisting of a mixture of 1 – 25 ingredients, with particle sizes ranging from 150 µm to 6.4 mm. The company tried a pneumatic conveyor, which caused blended products to separate. A bucket conveyor and a rigid auger conveyor both proved difficult to clean. The company found that flexible screw conveyors did not separate blends or damage the fragile spices, and now it operates 15 flexible screw conveyors, all running daily A removable clean-out cap at the intake of each tube allows reversal of the screw to fully evacuate the tube for ease of cleaning. In conclusion, flexible screw conveyors are particularly suited for transporting of materials that are cohesive, dusty, friable and abrasive, as well as materials that fluidize and blends prone to separate. Conveying such disparate materials efficiently, however, requires engineering of each flexible screw system according to specific application requirements, and running the actual material to be conveyed on full-size test equipment at the rates anticipated during production.
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Tank Coatings: Covering the Basics of Selection and Specification Good quality coatings can make all the difference where corrosion and chemical compatibility are concerned Peter Vodak, CST Industries/Columbian TecTank
The interior surfaces of your storage tanks are constantly at risk. If you walk past a tank often without giving thought to what is happening on the inside, it is because you trust the interior coating to protect not only the storage vessel, but also to protect the product being stored from any corrosion that might occur on the tank. Without a doubt, corrosion is the principal enemy of a storage vessel. It shortens a tank’s lifespan incrementally and increases the possibility of contamination. Dry bulk products are often abrasive to the tank walls, requiring a strong coating to resist scratches and gouges, which expose the steel to corrosion. In addition, liquids are particularly aggressive, so the coating found on the interior of the tank is essential when it comes to protecting a tank against the harmful effects of corrosion. While the old adage that “a tank is a tank” is no longer true, tanks do often share the same materials of construction. Tanks and silos are made from a variety of materials, including aluminum, stainless steel, plastic, fiberglass and most commonly, carbon steel. The tank material choice is typically driven by cost and compatibility with the stored product. Carbon steel is the material of choice in many cases because if properly coated, it combines strength and strong performance at a reasonable cost. Nearly all tanks are coated on the interior to protect the tank as well as the product being stored. Corrosion occurs when the tank coating fails and the product is exposed to the tank wall. Unprotected steel can begin to corrode in a matter of hours. With aggressive liquids, holes through a tank sidewall or floor could develop in a matter of months. All materials, whether dry bulk chemicals, potable water, wastewater or liquid chemicals, will react differently to the storage vessel and have individual and specific storage requirements. Chemicals will require a tank coating that strongly resists aggressive products, such as acids or bases. Dry bulk materials require hard, abrasion resistant coatings that are slick, to ensure that all material passes out of the tank leaving minimal residue. Liquids require a coating that withstands immersion for long periods of time. Choosing a tank with a high-quality coating that has been tested and proven to withstand the materials that it will hold means less maintenance over time and less concern for you each time you pass by.
Variety in coatings Tank manufacturers are often defined by their coatings. The quality of a coating and its resistance to corrosion determines the lifespan and quality of the tank. Many different coatings are found on the market today, including epoxy, glass fused-to-steel, high-heat coatings, cold-weather coatings and coatings designed to withstand high levels of acidity or bases. The choice of one coating over another should be made depending on the product to be stored. Each product has its own requirements and specifications. For example, wastewater and wastewater sludges will require a coating that is designed to withstand the aggressive effects of ever-present fatty acids and hydrogen sulfide. Epoxy coatings are very common in the tank industry. They are generally tough and chemical resistant, with excellent corrosion resistance. Epoxy coatings can be applied electrostatically either as a powder or a liquid. On the exterior, epoxy coatings are typically combined with polyurethane topcoats to provide protection against environmental elements. Glass-fused-to-steel coatings are applied by spraying panels with a water-based slurry of ground glass particles in water. The water is then removed by drying prior to firing. The coated sheets are sent through a furnace at 1,500°F, which chemically fuses the glass coating to the steel. The finished glass coating is impermeable to liquids and unaffected by solvents. 66
Many other types of coatings also exist, and each has its advantages and disadvantages. Some, such as inorganic zincs, are intended to provide sacrificial corrosion resistance on exterior surfaces. Others, such as silicones, are intended for high-heat applications where high temperatures are expected. Regardless of the type of coating used, it is critical that the coating be suited for its intended storage application.
Coating application process While the market today offers a variety of coatings, the true differences lie in the application process itself. Coatings in general may have great selling points and use advanced technology, but if the coating is not applied correctly, the tank wall will be left exposed and vulnerable to corrosion. The highest quality application is done at the factory under environmentally controlled circumstances to ensure the most consistent application. Taken one step further, the coatings should also be thermally cured at the factory. Some manufacturers apply the coatings in the factory and then allow them to air-dry and cure with ambient heat, which exposes the cure to environmental factors such as dust and humidity. Other manufacturers outsource the coating process altogether. The optimal coating solution is one that is both applied and thermally cured in controlled factory conditions, before the tank is shipped and erected in the field. Factory welded and bolted tanks often offer this feature. Field welded and concrete tanks usually receive their coatings onsite once the tank has been erected. These tanks may need to undergo a chemical process to protect the coating while the tank is being erected (for instance, heat from welding may damage the coating). For this type of application, you should ensure that adequate quality control measures are listed in the specification, and consider third party inspections. Be aware that once a tank has been erected in the field, there are often areas of the tank that are extremely difficult to sand blast or fully prepare for field coating. Even “missing the smallest spots” leaves exposed areas open to corrosion. Also, the thickness of coatings applied in the field cannot be controlled as closely as is possible under ideal factory conditions, which may result in uneven application. Finally, weather and the environment — dust, humidity, temperature and wind — will affect the quality and timeliness of the curing process in the field. The coating-application process involves three steps, each of which helps define the quality of the coating and the effectiveness of its resistance to corrosion. Surface preparation: Surface preparation is the first and most critical step in the application process. It involves both cleaning the surface and achieving a profile on the steel. Regardless of how good the coating is, it must have a clean surface with an adequate blast profile to perform properly, in the same way that the most advanced skyscraper must have a solid foundation to rest upon. The most common method of surface preparation is abrasive blasting. Different abrasives can be used, including steel grit or shot, garnet, coal slag, and even items such as walnut shells. The type and size of grit should be chosen in accordance with the coating manufacturer’s recommendations. Abrasive blasting may either be done automatically (typically by a centrifugal blast machine) or manually (air blasting). In the case of air blasting, it is critical that the air used for blasting is dry and free of contaminants. The surface should be inspected prior to coating application to ensure that the proper cleanliness and profile have been achieved. Coating application: Coatings can be applied either automatically or manually. When coating large areas, liquid coatings are typically applied using either airless or air-assisted airless spray guns. Airless guns give a higher production rate while air-assisted airless guns use less paint and provide a smoother finish. In some cases, electrostatic versions can be used to increase transfer efficiency of the paint by applying a charge to the atomized paint, 67
which is then drawn to the part to be coated, although in this case, the part must be grounded. Rollers can be used if regulatory requirements or environmental concerns prohibit the use of spray guns. Powder coatings are applied using spray guns that apply a charge to the powder particles when they are atomized in a similar way to how a liquid electrostatic system works. Again, the part must be grounded. Powder systems have the advantage of wrapping extremely well on exposed edges. It is important that the coating thickness be measured throughout the application process to ensure that the manufacturer’s recommended thickness is achieved. If the coating is too thin, the steel may not be adequately covered and pinpoint rusting can soon occur. If the coating is excessively thick, it may crack from internal stresses or a loss of flexibility. Curing: Curing involves the process of transforming the coating from a liquid or powder to a fully set film. Many coatings cure by polymerization, wherein a chemical reaction occurs. For these types of coatings, curing is not the same as drying, which involves the evaporation of solvents or water from a liquid coating. A coating can be dry to the touch without being fully cured. Generally the rate of reaction (and thus, cure) for these types of coatings can be sped up by increasing the temperature. There are three main types of cure for polymerization coatings: oxidative, chemical and thermal. Oxidative coatings (such as oil-based house paints) cure by reacting with the oxygen in air. These coatings tend to have lesser chemical resistance and in most cases are not used on storage tanks. Chemical reaction coatings typically involve either reaction with moisture in the air or reaction between two or more components. Epoxies are typical coatings that cure by chemical reaction. In this case, two components are mixed together, which react and then cure. The rate of chemical reaction for these coatings types can typically be sped up by increasing the temperature. Thermal coatings require a high-temperature bake cycle to achieve cure. These are typically high-performing systems and must be applied and cured in a factory environment. Advantage of powders: Powder coatings have an electrostatic charge put on the powder particles while they are being atomized. This charge draws it to the grounded part. The advantage of electrostatic application is that the charged particles are drawn preferentially to the thinnest areas, which results in a more uniform coating. They are also drawn to edges that are typically difficult to coat by other means.
Performance testing To ensure the quality of the coating meets the manufacturer’s specifications, coatings are generally tested extensively by tank manufacturers before they are introduced to the marketplace. Common performance tests include the following: ultraviolet (UV) exposure, corrosion resistance and chemical resistance. UV exposure: Coatings will typically lose gloss and their color will change over time when exposed to UV light. This property is most critical for exterior topcoats that are constantly exposed to sunlight. UV-exposure testing measures a coating’s color and gloss retention when exposed to sunlight. Both natural sunlight exposure and accelerated versions that use either concentrated sunlight or simulated sunlight are commonly tested. This test measures how the coating’s color and gloss change over time. Corrosion resistance: Coatings are tested for resistance to corrosion by exposure to water or salt solutions in a humid or spray environment. For many years, the standard ASTM B117 salt-spray test was used, but recently it 68
has fallen out of favor because results did not always correlate well with those seen in the field. More recently the ASTM D5894 cyclic-corrosion test with its modified salt solution has gained favor and is considered more realistic. Regardless of the test chosen, the goal is to assess the degree of rusting and the amount of undercutting seen from a line scribed through the coating. Strong-performing coatings will show no surface rusting and very little to no undercutting, even when exposed for thousands of hours. Chemical resistance: The chemical resistance of coatings is tested by immersing a coated sample in the liquid to which it will be exposed. The temperature may be elevated to accelerate a chemical attack. This test looks for blistering or dissolution of the coating. In general, if a coating will fail a chemical-resistance test, it will tend to happen fairly quickly (within a week). For all of these tests, accelerated versions may be used, which will give a faster indication of how the coating is likely to perform in a given environment. However, it is important to keep in mind that accelerated tests generally involve some level of trade-off. Therefore, look to the tank supplier to provide a history of successful projects in similar applications.
Other considerations If you would ever consider moving or relocating a tank, think about the impact this will have on the coating. Tanks featuring factory-applied coatings are usually moveable without a follow up recoat, as panels are individually coated and easily taken apart. Coatings applied in the field often set up as a sheet and will tear or break if the pieces are moved. Also consider whether you plan to expand your tank. A bolted tank with factory coating would allow for adding rings (think upward) without affecting the coating on the original tank. A tank with a field-applied coating would require recoating after the expansion is completed.
Safety factors Coatings also play a role in safety. Some coatings are designed for storing a wide variety of products, while others may react poorly with certain products. Manufacturers should provide the results of testing of the coating with a particular product to ensure the coating is suitable for the specific application. It is unsafe to use an old storage tank for a product for which it was not designed and tested. If you are considering recycling a used tank for storing a new product (such as using a dry bulk-storage tank for liquid storage, or a potable-water storage tank for wastewater), be sure to first have the tank evaluated for safety, flow and corrosion and chemical resistance. Incorrect use of a tank could result in severe corrosion, premature failure of the coating, harm to the product or serious injury to personnel. Keep in mind that special applications require specific product approvals for tank coatings. For example, if you plan to use a tank for potable water storage in the U.S., the tank coating must be NSF Standard 61 certified. Meanwhile, in the same country, dry food applications require the tank coating to have U.S. Food and Drug Administration (FDA) compliance under the Code of Federal Regulations (21 CFR 175.300). Verification of these approvals can be obtained from the tank manufacturer or coating supplier.
Inspection and maintenance Tanks should be inspected both inside and out once a year to make sure they are in good shape. Any corroded areas should be properly prepared and touched up with a suitable maintenance coating. A tank that is neglected can result in product contamination, extensive maintenance and recoating, safety hazard and significant down69
time. Maintenance coatings are typically used to repair larger areas or even an entire tank. Some maintenance coatings require the existing coating to be fully removed through a process of blasting or waterjetting, while other maintenance coatings are designed to be applied over an existing coating or even a rusted surface. Contact the tank manufacturer for recommendations, and make sure that the chosen maintenance coating is compatible with the stored product and environment. One common rule of thumb is that a tank should be considered for repainting when it shows rust on 1% of its surface. This doesn’t sound like a lot, but in reality represents a fairly large area. For example, a 30-ft dia. tank that is 20 ft tall has an area of 1,885 ft 2. A 1% portion of this area is just under 19 ft 2.
Costs of corrosion Corrosion significantly shortens the life of a tank, so a good quality coating from the beginning often means less maintenance and less chance of the need to recoat the tank. A high quality coating that offers proven corrosion resistance may have a higher up-front cost, but lower lifecycle maintenance and recoating requirements make it the most economical choice for the life of the tank.
Vendor selection Review the history of the tank manufacturer and coating to be used. Tank vendors should provide testing data and case histories of how the coating performed against corrosion with a product like the one to be stored. Other considerations include how long the company has been fabricating tanks, whether it uses state-of-the-art coatings, if its coatings are factory applied and thermally cured, what its quality control measures are, what its volume sold is, and into what countries and markets it has sold. Also consider tank manufacturers with third-party accreditation, such as the ISO Quality Certification.
Using Bins & Silos To Heat or Cool Bulk Solids Modified storage vessels offer several advantages over other methods of heat transfer to and from solids
Greg Mehos and Brian Pittenger Jenike & Johanson, Inc.
Vessels for handling bulk solids, such as silos and bins, are frequently used to heat or cool bulk materials in cases where a slow rate of change or long residence time is required. Often, the temperature of a bulk-solids stream must be raised, for example to bring about a chemical reaction in a subsequent processing step. Alternatively, a reduction in temperature is sometimes needed, for example to allow safe handling downstream. Vessels used to alter the temperature of a bulk material are typically adaptations of conventional bins and silos, which have been modified to achieve the desired process activity. Compared to other methods for adjusting the temperature of solids streams, such as heated or water-cooled screws and discs; fluid-bed processors; and belt, rotary and tray ovens, the use of modified storage vessels offers numerous advantages including the following: • The capital cost of a modified silo or bin can dwarf that of a fluid-bed processing unit or tempering screws • Modified bins and silos have no mechanical agitators or other moving parts. This eliminates the maintenance costs associated with such devices • Adapted vessels provide greater heat fluxes than most other technologies, thereby allowing a greater range of heating or cooling Two methods are generally used to alter the temperature of bulk solids in storage vessels: heat exchanger plates provide indirect heating or cooling, while gas injection supplies direct heating or cooling. In an indirect heating or cooling scheme, heat exchanger plates are installed vertically in the straight wall section of a bin or silo. A heat-transfer medium passes inside the plates, and the bulk material passes through spaces between the plates. When heating is intended, steam, tempered water, or a heat-transfer fluid is used. When cooling is required, water or glycol is circulated. Several banks of plates can be used in series to improve the thermal efficiency of the heat exchanger system. When direct heating or cooling is used to modulate the solids temperature, air or nitrogen is injected towards the bottom in the converging wall section or cylinder of the bin or silo. The gas may be introduced using an inverted conical insert or via screens or nozzles along the perimeter (gas distribution is discussed later in more detail). A heated gas stream is used when an increase in solids temperature is desired, whereas gas is usually fed at ambient or chilled temperature when cooling is required. When designing a bin or silo to heat or cool solids, the following requirements must be met: 1. Adequate heat transfer — The solids stream discharged from the bin or silo must be at the desired temperature 2. Uniform velocity profiles — To allow homogeneous heating or cooling of the bulk-solids stream, the velocity profiles of the solids and, when applicable, gas streams should be uniform 3. Optimal gas distribution — When direct heating or cooling is employed, the gas-injection method must ensure contact throughout the moving bed of solids 4. Proper outlet size — The outlet of the bin or silo must be large enough to prevent an obstruction-to-flow from developing and allow discharge of solids at the required rate This paper discusses these design considerations, focusing on design criteria for achieving the solids and gas flow patterns for optimum performance. 72
Heat transfer Typically, the temperatures of incoming process streams are known for a given system. The vessel must then be designed to provide a target outlet temperature. This can be accomplished by solving the appropriate energybalance equations. Models that describe two-phase systems can be unwieldy, especially if heating or cooling results in a change in phase of one or more of the components involved. Simpler models, which assume uniform velocity profiles for the gas and solids streams, no evaporation or condensation, and no temperature gradients in the radial direction, can give approximate solutions that can be used for screening or preliminary design purposes. If the temperature of the solids and gas streams in the silo or bin are assumed constant, the steady-state energybalance equations for the gas and bulk-solid phases are typically written as: In the absence of experimental data, heat-transfer coefficients for the solid particle and wall are usually estimated from correlations that relate Nusselt, Reynolds, and Prandtl numbers. The energy-balance equations given above assume that only sensible heat is transferred, or in other words, there are no components that evaporate from the solids stream or condense from the gas stream. Otherwise, a material balance on each component would need to be included in the analysis, and the energy-balance equations would have to be modified to include a term that describes latent heat. One should be mindful that latent heat cannot be ignored if a change in phase is anticipated. In the case of direct heating or cooling, the energy-balance equations can be integrated numerically in a straightforward fashion, provided that the gas- and solids-stream velocity profiles are uniform, since the solids and gas stream temperatures are both known at the top of the bin or silo. For direct heating, in which the gas and solids streams pass countercurrently, however, a boundary value problem exists: the inlet solids temperature is known at the top of the vessel, whereas the temperature of the gas stream is known only at the point where the gas is introduced. The solids velocity profile in the cylinder section can usually be considered to be uniform if the bin section is designed to provide mass flow (where the entire bed of solids, including material along the walls, is in motion). Requirements for mass flow are discussed later in this paper. How quickly the gas velocity profile becomes uniform depends on how and where the gas is introduced and the permeability of the gas through the bulk solid. Solids flow properties must be known in order to design bins or silos for uniform flow of gas and solids streams.
Flow properties The flow properties needed to design a vessel for heating or cooling solids consist of: • Flow function — relationship between the cohesive strength of the bulk material and the consolidation pressure • Compressibility — relationship between bulk density and consolidation pressure • Angle of internal friction — related to friction between solid particles • Wall friction angle — related to friction between bulk solid and wall material • Permeability — relationship between gas volumetric flowrate through a bed of solids and its pressure drop 73
The flow function is used to determine the outlet size required to prevent an obstruction-to-flow from developing. This is accomplished by specifying an outlet dimension that ensures that the stresses imparted on the bulk solid are greater than its cohesive strength, which the solid develops as a result of its consolidation. The angles of internal friction and wall friction are used to determine the hopper angle necessary for mass flow. Permeability results are used to determine the limiting discharge rate of solids from the bin or silo, as well as the amount of gas that can be injected before causing fluidization. When measuring the flow properties of bulk solids, it is important that tests are conducted at the expected process conditions. Invariably, the temperature of the bulk-solids stream at the outlet of the silo or hopper is different from its inlet temperature. Properties such as cohesive strength and wall friction can be strong functions of temperature, as well as moisture content.
Mass flow The flow pattern inside a bin or silo has a great influence on the vessel’s ability to heat or cool the solids stream. There are two primary flow patterns that can occur in a bin or silo: mass flow and funnel flow. In mass flow, the entire bed of solids is in motion when material is discharged from the outlet. This behavior eliminates stagnant regions in the vessel and affords a first-in, first-out flow sequence, which provides a more uniform velocity profile. A uniform velocity profile helps provide homogenous tempering of the solids. In funnel flow, an active flow channel forms above the outlet, with stagnant material remaining at the periphery. This pattern leads to a non-uniform velocity profile inside the vessel, a reduction in solids residence time, and swings in product temperature if the amount of material in a silo or bin is increased or decreased. These temperature swings become especially severe when the vessels are allowed to empty and material near the walls sloughs into the emptying channel. Therefore, vessels used to heat or cool bulk solids should be designed for mass flow. The first step in achieving mass flow is to ensure that the converging walls are steep enough and have friction low enough to allow the bulk material to flow along them. This is accomplished by first testing the bulk-solid material to measure wall friction and then calculating the minimum hopper angle that will allow mass flow. Wall friction is measured by the method described in ASTM D-6128. The test is performed using a test apparatus in which a sample of bulk material is placed inside a retaining ring on a flat coupon of wall material. Various normal loads are then applied to the material. Material in the ring is forced to slide along the stationary wall material, and the resulting shear force is measured as a function of the applied normal force. After a number of values have been recorded, the wall yield locus is constructed by plotting shear force against normal force. The angle of wall friction, ?’, is the angle that is formed when a line is drawn from the origin to a point on the wall yield locus that corresponds to a specified pressure of interest. Design charts originally developed by Jenike provide allowable hopper angles for mass flow, given values of the wall friction angle. Values of the allowable hopper angle, ?C (measured from vertical), are on the ordinate, and values of the wall friction angle are on the abscissa. Any combination of ?’ and ?C that falls within the mass-flow region of the chart will provide mass flow. Designing right to the limit of the mass-flow region is not recommended for conical bins. Notice that there is an area on the chart that lies between the funnel-flow and mass-flow regions. In actuality, this represents a margin of safety to account for slight differences in material flow properties and wall surfaces. If the combination of wall friction angle and hopper angle lies too close to the funnel-flow line, a switch to funnel flow can occur. Hence, a 4–5 deg. margin of safety is employed with respect to the mass-flow boundary. 74
The uniformity of the solids velocity in a vessel depends on how close the hopper angle is to the mass-flow boundary. Note that as the hopper angle is steepened, the velocity profile in the hopper section becomes more uniform. In a mass-flow hopper, the velocity differences diminish in the cylinder section and the solids velocity becomes nearly uniform unless the solids level is very low. It is not necessarily sufficient to set a design goal of simply achieving mass flow. It is certainly a minimum requirement, but the solids velocity profile should be verified to ensure that the range of velocities over the cross section of the vessel is within the desired specification for residence time. In some cases, a non-uniform velocity profile may even be preferred. Some bulk solids gain cohesive strength if they remain at rest, and hence for these types of materials, interparticle motion, which is the result of velocity gradients, may be desired.
Outlet size The size of the outlet must be large enough to prevent an obstruction-to-flow from developing. There are two primary types of flow obstructions that can occur with bulk solids: particle interlocking and cohesive arching. Interlocking must be considered when handling large particles. A rule of thumb is that a circular outlet should be sized 6 to 8 times the largest particle size to prevent interlocking. Cohesive arching results whenever the cohesive strength of the bulk material, which develops as a result of its consolidation in a bin, is greater than the stresses imparted onto it at the outlet. The cohesive strength of a bulk solid is a function of consolidation pressure. The relationship between strength and pressure is called the flow function. It can be determined using the method described in ASTM D-6128, where a direct shear tester is used to measure the shear strength of a material under varying consolidation pressures. Once a material’s flow function has been determined, the minimum outlet diameter that will prevent cohesive arching can be calculated using the bin’s flow factor. Jenike showed that the ratio of the consolidating pressure (?1) to the stress on the arch has a constant value and defined the flow factor, ff as: where –?1 is the stress acting on the abutments of the arch. The flow factor is a function of the internal friction of the bulk solid, the hopper angle, and the angle of wall friction. Values of the flow factor can be found in charts provided in Jenike. Since for a given material and bin, the flow factor is a constant, a plot of the arch stress against consolidation pressure is a straight line passing through the origin. Superimposing the material’s flow function on the same graph allows the cohesive strength and arch stress to be compared. There are three possibilities: • The flow function lies below the flow factor and the two curves do not intersect. When this is the case, the stress imparted on the arch is always greater than the material’s cohesive strength, and there is no minimum outlet dimension • The flow function lies above the flow factor and the curves do not intersect. In this case, the bulk solid will not flow due to gravity alone • The flow function and flow factor intersect. At the point where the two lines intersect, the arch stress is equal to the strength of the bulk solid. The value of the stress or strength is equal to the critical stress, ?critical. The minimum outlet diameter to prevent a cohesive arch from developing in a cone can then be calculated from: 75
where the function H(?C) is given by Jenike. The term (1-?)?p g is equal to the material’s bulk density, ?.
Gas distribution Silos and bins have been used to heat or cool bulk solids by gas injection. The key is to distribute the gas evenly throughout the cylinder section of the bin or silo. Mass flow is essential for uniform gas distribution, since stagnant material will have much lower permeability than material in the flow channel, and non-uniform gas exposure will result. Localized fluidization, which causes flow instabilities, should also be avoided. The gas-injection rate will depend on the permeability of the gas through the bulk solid. Gas is often introduced underneath the inverted cone, directly into the material through the free surface that forms. Gas may also be introduced through screens around the perimeter of the inverted cone and/or through the outer hopper section. Sometimes a simple conical hopper is used without an insert. Gas is introduced through screens around the perimeter of the hopper section, either at one or multiple levels, or along the entire length of the cone. This is more commonly used for pellets than powders. The Kerscher-Goodyear design, incorporates an insert in the conical hopper section. Gas is introduced through a screen on the upper half of the insert, along with a screen around the perimeter of the vessel at the hopper/cylinder transition point. The design described in the Weber-Bergwerksverband patent features a “glide body,” or insert, in the hopper section of the vessel. The insert consists of two parts: an inner bullet-shaped insert and an outer converging section. The converging section forms an annular region, through which gas is introduced. Gas travels through the outermost annular region as well as through the center of the vessel. Improperly designed vessels can, at times, exhibit flow-related problems due to non-uniform velocity profiles of the bulk solids and/or gas. The purpose of an inverted-cone-type of insert is usually to achieve mass flow and to provide a means for gas introduction near the vessel’s centerline. Unfortunately, many designs of this type achieve these goals only marginally, if at all. If inserts are placed in a funnel-flow vessel, their size and location are critical in determining the expansion (if any) of the flow channel. An insert that is too small or too large, or too high or too low, relative to some critical size and position, will expand the flow channel very little, if at all. Even if optimally placed, achieving full mass flow is doubtful, particularly since such inserts have little effect on the flow pattern below their base. This situation can cause the bulk solid’s exposure time to the gas to be non-uniform, which can severely impact uniformity of heating or cooling. In addition, if the gas is introduced at a point where the solid’s stresses are low, localized fluidization is likely to occur. Also, if screens are used, they may blind over time. Both of these phenomena cause the gas introduction to be non-uniform. An improved approach is to use a cone-in-cone design, which can have either an open interior or a closed top. If the dimensions and material of construction of the inner cone are properly chosen, mass flow can be achieved in the vessel. In fact, the hopper angle required for mass flow can be twice as large as would be required if an inner cone were not present. This results in a substantial saving of headroom compared to other mass-flow designs. In addition, a cone-in-cone design allows control of the solids velocity pattern in a way that is impossible with other designs. Installation of a cone-in-cone can result in a dramatic improvement in the uniformity of the gas distribution. 76
Cross-beams used to support a cone-in-cone design can also double as a gas introduction plenum. This is a better arrangement than injecting gas only into the center and through the cylinder walls, since the area of gas introduction can be as large as necessary (within reason, of course) to keep the gas velocity low enough, and thereby prevent localized fluidization. In addition, the characteristic dimension for gas introduction (used to calculate the cylinder height above which the gas distribution can be considered uniform) is further reduced. The height at which the gas velocity is considered uniform is typically on the order of one vessel diameter if gas is injected through or near the cylinder walls or one half the diameter if the gas is injected near the centerline and near the cylinder walls. If crossbeams are used, most of the cylinder section is available for solids heating or cooling.
Conclusions Silos or bins modified with heat exchangers or gas-injection systems are often used to heat or cool bulk solids. Contact with heat exchanger plates or a countercurrent stream of gas allows the temperature of bulk-solids streams to be raised or lowered in an efficient manner. Care must be taken, however, to avoid flow problems by first measuring the flow properties of the bulk material, and then using the results to design a vessel that avoids obstructions-to-flow and allows mass flow with a uniform solids-velocity profile.
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Vibratory Feeders And Conveyors: Useful Selection Tips Ongoing advances in technology have helped these systems overcome key shortcomings of earlier designs Rob Yandrick, Eriez Vibratory Products
The need to move, convey and feed materials exists in virtually all segments of the chemical process industries (CPI). Vibratory feeding and conveying equipment has been used in the CPI for several decades to efficiently move both fine and coarse materials that tend to pack, cake, smear, break apart or fluidize. For instance, in many CPI facilities, vibratory feeders are used to meter precise amounts of product from hoppers, and to gently deposit them on a conveyor or a packaging machine. Vibratory equipment is also used for many types of screening applications, including size separating, scalping or removing oversized product and conglomerates, and for dust or fines removal, as well.
Key components In most process applications, materials must be conveyed repeatedly from their point of arrival throughout successive operations within the plant. Feeders, which are often turned on and off repeatedly as needed, are typically placed throughout a plant to maintain and control the flow of product, as it moves into the next stage of the process. Vibratory feeders throw the material up and forward, so that it drops to the surface at a point further down the tray, at a pre-determined displacement or amplitude. The number of times per minute that this repeats is the unit’s frequency. A third variable is the angle of deflection, meaning how high the product is thrown compared to its horizontal movement. Each vibratory feeder or conveyor is designed with a different amplitude, frequency and angle of deflection to move different materials at specific desired rates. The design is based on many factors, including the nature of the material being processed, the flowrate of the process, the nature of the environment, the need to start and stop (cycle) the process, the cost to operate the equipment and the likelihood of repairs. Most of today’s feeding and conveying systems are built to accommodate one of the following four applications: • Standard-design vibratory feeders: Ideal for denser materials, such as those greater than 100 mesh in size and heavier than 10 lb/ft 3, or for applications where significant material load may be applied to the feeder • High-speed feeders: Excellent choice for applications where fast travel speeds and frequent on/off cycling are needed. These are ideally suited for packaging • High-stroke, low-frequency feeders and conveyors: These are ideal for handling loose or powdery materials • High-stroke mechanical conveyors: Well-suited for high capacity applications where longer lengths or intermediate discharge gates are required Vibratory feeders and conveyors have undergone numerous design changes and upgrades that today enhance their role in process applications. For instance, the latest equipment offers increased energy savings, more precise control over material flow, easier maintenance, a variety of options, better technical support, and, in some cases, faster delivery of product for your plant. The newest developments, discussed below, are helping chemical process plants to improve product purity, reduce energy costs and equipment maintenance expenses, and streamline manufacturing operations.
HD electromagnetic feeders
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Electromagnetic feeders have long been a popular, trouble-free way to meter and convey bulk materials and powders. However, they have never been the most effective way to feed fine powders, or leafy and fluffy materials. Today, advanced feeders — called high-deflection (HD) electromagnetic feeders — are helping to address this shortcoming. Modern HD electromagnetic feeders offer feed rates of up to 80 ft/min (24 m/min). Materials from – 50 mesh to – 400 mesh may tend to fluidize and flush (which is to wash out of the feeder or conveyor tray) on traditional electromagnetic feeders. But after years of research, today’s HD electromagnetic feeders offer relatively high deflection — up to 3/16-in. (4.8 mm) — with lower frequency (30 cycles per minute) to handle finer and loose products, such as long-stranded chopped fiberglass, talc or flour. Newer HD feeders also offer increased throughput for greater flexibility in packaging and other difficult applications.
Energy-saving drives All vibratory equipment features a drive system with an a.c or d.c. power source, mounted either from above or below to produce a consistent vibratory force. Many newer feeder and conveyer systems use has an electromagnetic drive, which creates the same vibratory force with reduced power consumption. Electromagnetic drives in both a.c. and d.c. mode use magnets to energize the vibratory motion. Each has its advantages, with a.c. units providing high precision with lower operating costs, and d.c. models typically being less expensive. The a.c.-electromagnetic-drive systems typically consume up to 65% less energy compared to their d.c. counterparts. These units have no sliding or rotating parts to wear out, and require very little power to operate. Conversely, d.c.-operated electromagnetic drives produce the same vibratory action as a.c. units, but are not as energy efficient. Today’s d.c. drives use a less-efficient “attract and release” system where half of the sine wave is eliminated and the d.c.-pulsed power delivery is much less linear. The half-wave design feeders only energize the electromagnet drive with half the a.c. sine wave. Alternatively, an a.c.-operated drive using the full sine wave delivers power to the unit proportionately to the voltage increase.
Advanced mechanical drives Mechanical drives are still used in a variety of feeding and conveying applications. In some cases they create the vibratory motion of the tray using a direct mechanical linkage (push rod). In others they use out-of-balance weights to initiate motion in the tray and then amplify the resulting vibration through a set of springs. Among the newer mechanical drive units for conveyors are those that use strictly horizontal vibrating motion. Such a drive uses a set of weights powered by a motor that is alternately “in phase” and “out of phase,” to create a slow motion in one direction and a fast motion in the opposite direction. This action causes the tray to slide beneath the material. Such a device is particularly useful for handling fragile materials that may be damaged from the normal vibratory motion.
Limitless tray designs Today, a nearly limitless array of feeder trays is available, in terms of configuration, shape, length and width. 81
Feeder trays can be flat, curved, v-channel, tubular or custom-designed, depending upon the process application and material being conveyed. Trays are typically fabricated from mild steel or stainless steel. The latter is often used in food and pharmaceutical applications, while the former is for general-purpose process operations. Trays can also be lined with abrasion-resistant steel, stainless, urethane, rubber and other coatings. Trays can be designed for fast removal and cleanout to avoid cross contamination and minimize downtime. Many have quick-release clamps that allow the tray and cover to be removed without tools, easing overall maintenance requirements.
Over-deflection monitoring State-of-the-art vibratory feeders offer a variety of monitoring devices that allow operators to keep close tabs on equipment operation and prevent any potential equipment damage. The over-deflection monitor is one such device. This monitor detects changes in tray deflection resulting from material accumulation on the tray surface. As material accumulates, it adds weight to the tray, affecting performance and possibly damaging the feeder. The monitor can alert the operator or provide automatic shutdown so the tray can be cleaned to improve performance and reduce costly downtime. The over-deflection monitor uses a vibration transducer mounted to the tray, and a comparator amplifier, which monitors changes in the tray deflection. The vibration transducer sends a constant signal to the comparator amplifier. If an upset occurs and the tray becomes overloaded, the amplifier trips a relay to shut down the feeder or set off an alarm.
Spring systems Springs are an integral part of the feeding system process, because they amplify the vibration from the drive to the tray, causing the conveyed material to move. Just as today’s trays come in a variety of configurations, the springs on today’s vibratory feeders and conveyors also come in a variety of materials, sizes and configurations. Fiberglass springs are the most popular for light- and medium-duty applications. For instance, small electromagnetic feeders, light- to medium-duty conveyors, and most high-precision vibratory equipment use fiberglass leaf springs as their primary spring action material. Steel coil springs are commonly used on heavy-duty and high-temperature applications. These coils are effective in ambient temperatures up to 300°F. Dense rubber springs are typically used on heavy-duty feeders and conveyors, to provide stability and motion control between the drive and tray. However, the use of rubber springs is limited to applications whose maximum temperature s 120°F.
Solid-state control units The ability to adjust flowrates from tvibratory feeders continues to evolve, in parallel with other upgrades in feeders and conveyors. For instance, the latest solid-state control units operate feeders with improved precision, and 82
can easily adjust the flow of material using a manually adjusted potentiometer, or automatically, through a user’s analog signal from a programmable logic controller. Feedrates can vary from 0 to 100%, thanks to the use of control potentiometers or digital keypads that allow users to precisely lock in control settings, and “smart” transducers mounted on the tray. These are wired to the controller, to provide constant feedback of the tray’s amplitude. The constant-feedrate system allows plant operators to set the feedrate at the beginning, and the control system automatically compensates for the changes in head load, material buildup on the tray or any voltage fluctuations during the production run.
Technical support During system evaluation, material samples of various density and different equipment configurations should be tested with the original equipment manufacturer (OEM), to identify the optimum piece of vibratory and conveying equipment. Such pre-testing virtually eliminates the potential for installing an under- or oversized piece of equipment for the job at hand. Vibratory solutions have long been a preferred means to meter and convey materials. Thanks to the ongoing advances in technology discussed here — coupled with the sanitary construction, ease of cleaning and ow maintenance — these systems are more reliable than ever for CPI operators, especially those handling fragile or hardto-convey materials.
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Designing and Operating Gravity Dryers Properly designed, bulk solids bins or silos offer numerous advantages in slow, diffusion-limited drying operations Greg J. Mehos, Jenike & Johanson
Evaporation of moisture or volatile organic compounds (VOCs) from bulk solids usually takes place in two stages: one in which the drying rate is constant, and the second in which the rate decreases over time. Devolatilization during the first stage is rapid and can be readily accomplished in flash, spray, or fluidized bed dryers. In the second stage, however, the VOC or moisture removal rate is diffusion limited, and several hours of residence time are often necessary. To meet low moisture or VOC targets, two unit operations may be required. An economical way to provide the extended residence time required for slow, diffusion-limited drying is to use a bin or silo that has been modified to allow injection of a sweeping gas and, in some cases, to provide heating. These unit operations have a variety of names, including gravity dryers, purge or conditioning columns, moving bed columns, and silo or bin dryers. Relative to other methods — such as heated screws, paddles or disks, fluidized bed processors and tray dryers — modified bins or silos offer the following advantages: 1. The capital cost of a gravity dryer is generally much lower than that of a fluid-bed processing unit or of heated screws. 2. Gravity dryers have no mechanical agitators or other moving parts, eliminating the maintenance costs associated with such devices. 3. Gravity dryers provide a longer residence time than most other technologies do. 4. Gravity dryers also provide storage or surge capacity, which is desirable when interruptions in production take place. Gas, usually air or nitrogen, is injected via a distributor typically located near the cylinder-hopper junction. When polyolefins are processed, steam is also introduced to neutralize the catalyst. The gas passes upward, stripping volatile components from the solids, which flow downward. A rotary valve or other feeding device at the hopper outlet modulates the solids discharge rate. The keys to successful design and operation of a gravity dryer include the following: • Uniform solids flow. A non-uniform solids-velocity profile will impact the quality of the final product since exposure time of the solids to the gas will be variable. In the worst case, there may be no motion along the walls, and the solids will only flow in a channel above the vessel’s outlet. The residence time of the solids will be dramatically less than intended and may not allow the desired degree of volatiles removal • Uniform gas flow. The distribution system used to inject the gas must result in a constant gas velocity throughout the cross-section of the dryer. Channeling of the gas will not only cause the gas to bypass an appreciable portion of the solids, but it may cause flow instabilities • Non-zero solids stress. Unless the region where the gas is introduced is properly designed, the local solids stress can approach zero. In other words, the solid particles in the vicinity of the gas distributor may become fluidized due to high gas velocities. As a consequence, instabilities can occur throughout much of the vessel • Sufficient gas injection rate. The composition of the gas stream inside the dryer will vary with position inside the vessel. As volatiles are stripped from the solids stream, the concentration of volatile components in the gas stream will increase as it travels upstream in the vessel. If the gas injection rate is too low, the driving force for devolatilization may vanish in the upper portion of the vessel 86
• Adequate residence time. The volume of the gravity dryer must be large enough to provide the residence time necessary for the volatiles or moisture targets to be met. A residence time on the order of hours may be required when drying is diffusion limited Gravity dryers that function as desired are those whose designs are based on the fundamental flow properties of bulk solids and the diffusional and phase-equilibrium properties of the volatile species. Obtaining purposeful data is critical. At a minimum, the following properties should be obtained: • Cohesive strength. Used to determine outlet dimensions that prevent a cohesive arch from developing • Wall friction. Needed to calculate hopper angles that ensure flow of the bulk material along the vessel walls at an appropriate velocity to maintain overall residence-time-distribution goals • Compressibility. Provides the relationship between solids pressure and bulk density • Permeability. Used to specify operating conditions that circumvent flooding, to determine outlet dimensions that allow the desired solids discharge rate, and to determine the gas pressure profile in the column • Phase equilibria. Used to calculate the minimum gas-injection rate • Kinetic information. Used to calculate the required solids residence time. Modified bins or silos can also be used to remove undesirable components of a gas stream. Here, a zeolite or other suitable bulk material flows downward in the vessel and selectively adsorbs unwanted species from a countercurrent gas stream. Although the focus of this paper is devolatilization, the same concepts can be applied to moving bed adsorbers.
Achieving mass flow In general, there are two types of flow patterns in a vessel: funnel flow and mass flow. In funnel flow, an active flow channel forms above the outlet, with stagnant material remaining at the periphery. This pattern leads to a non-uniform velocity profile inside the vessel, a dramatic reduction in solids residence time, and gas flowing preferentially in the central channel due to the decreased permeability of the moving solids. Funnel flow occurs when the walls of the hopper section of the vessel are not steep enough or its friction is not low enough for the bulk material to flow along them. The preferred flow pattern in a gravity dryer is mass flow. In mass flow, the entire bed of solids is in motion when material is discharged from the outlet. This behavior eliminates stagnant regions in the vessel, which provides a more uniform velocity profile. In addition, mass flow minimizes the transition time during grade changes when more than one product is processed in the vessel. Hence, in order for a gravity dryer to operate properly, mass flow is vital. The first step in designing vessels for mass flow is to measure the wall friction between the bulk material and the wall material. Wall friction is measured by a method described in ASTM D-6128. A sample of bulk material is placed inside a retaining ring on a coupon of wall material, and various normal loads are applied. Material in the ring is forced to slide on the stationary coupon, and the resulting shear force is measured as a function of the applied normal force. The wall yield locus is constructed by plotting shear force against normal force. Design charts originally developed by Jenike provide allowable hopper angles for mass flow given values of the 87
wall friction angle (?’). The angle of wall friction is the angle that is formed when a line is drawn from the origin to a point on the wall yield locus. Designing right to the limit of the mass flow region is not recommended for conical hoppers. In actuality, this region represents a margin of safety to account for slight differences in material flow properties and wall surfaces. If the combination of wall friction angle and hopper angle lies too close to the funnel-flow line, a switch to funnel flow can occur. Hence, a 4 – 5-deg. margin of safety is used with respect to the mass flow boundary. The uniformity of the solids velocity in a vessel depends on how close the hopper angle is to the mass flow boundary. As the hopper angle is steepened, the velocity profile in the hopper section becomes more uniform. In a mass flow hopper, the velocity differences diminish in the cylinder section and the solids velocity becomes nearly uniform unless the solids level is very low.
Outlet size The outlet of the dryer must be large enough to prevent a flow obstruction from developing. If the cohesive strength of the bulk material that develops as a result of its consolidation in a vessel is greater than the stresses imparted onto it at the outlet, a cohesive arch will develop. The cohesive strength of a bulk solid is a function of consolidation pressure and is determined by the method described in ASTM D-6128, where a direct shear tester is used to measure the shear strength of a material under varying consolidation pressures. A sample of powder is placed in a split cell and then pre-sheared (consolidated by exerting a normal compacting load and then shearing it until the measured shear stress is steady). Next, the shear step is conducted, in which the vertical load is replaced with a smaller load, and the sample is again sheared until it fails. The pre-shear and shear steps are repeated for a number of normal stresses, and a yield locus is then determined by plotting the failure shear stress against normal stress. From the yield locus, the major consolidation pressure and cohesive strength are determined. By conducting the test over a range of consolidations, the relationship between consolidation pressure and the cohesive strength of the bulk material can be determined. The relationship between strength and pressure is called the flow function. The greater the material’s cohesive strength, the greater will be the tendency for the formation of a stable obstruction, such as an arch or dome at the vessel outlet. A stable cohesive arch is formed when the strength of the bulk solid is greater than the stresses acting upon it. Jenike showed that the magnitude of these stresses depends on a material’s bulk density, the outlet dimension, and the geometry of the hopper. The stresses acting to overcome a cohesive arch and cause flow are described by a hopper’s flow factor ( ff), which is a function of the internal friction of the bulk solid, the hopper angle, and the wall friction. The flow factor can be obtained from design charts given by Jenike or formulas given by Arnold and McLean. By comparing the flow factor and flow function and ensuring that the stresses at the outlet are greater than a critical value, the minimum opening size required to prevent a cohesive arch from forming can be calculated. Since the flow factor is a constant for a given material and bin, a plot of the arch stress against consolidation pressure is a straight line through the origin. Superimposing the material’s flow function on the same graph allows the cohesive strength and arch stress to be compared. There are three possible results of such a comparison: 1. The flow function lies below the flow factor and the two curves do not intersect. When this is the case, the stress imparted on the arch is always greater than the material’s cohesive strength, and there is no minimum outlet dimension. 2. The flow function lies above the flow factor and the curves do not intersect. The bulk solid will not flow 88
due to gravity alone. 3. The flow function and flow factor intersect. At the point where the two lines intersect, the arch stress is equal to the strength of the bulk solid. The value of the stress or strength is equal to the critical stress ? critical. 4. Where the function H(? c) is approximately equal to 2 and is given by Jenike, and ? is the material’s weight bulk density. 5. While an outlet diameter greater than the minimum will prevent cohesive arching, it may not necessarily be large enough to allow the desired discharge rate when fine powders are processed. The maximum flowrate of a fine powder can in fact be several orders of magnitude lower than that of coarser materials. Two-phase flow effects are significant due to the movement of interstitial gas as the powder compresses or expands during flow. A limiting condition occurs when the compaction in the cylinder section forces too much gas out through the material top surface. When the bulk material dilates in the converging section of the vessel, a slight vacuum forms, resulting in gas counter flow through the outlet. At a critical solids discharge rate, the solids contact pressure drops to zero, and efforts to exceed this limiting discharge rate will result in erratic flow. For fine powders, permeability testing is recommended to determine the outlet size required to achieve the desired discharge rate. Calculation of limiting flowrates is described by Johanson. 6. Gas distribution 7. Uniform distribution of the stripping gas is essential. Otherwise, the bulk solid’s exposure time to the gas can be non-uniform, severely impacting uniformity of purging. If the distributor is not properly designed, the gas may bypass a significant portion of the solids. If introduced at a point where the solid’s stresses are low, the gas is likely to cause localized fluidization of the solids. 8. Localized fluidization frequently occurs when gas is introduced at high velocities via nozzles or perforated plates, which can lead to flow instabilities propagating throughout the dryer. If gas is introduced through screens in the converging section of the vessel, only a relatively small amount of gas can be injected in the lower portion of the cone, since at higher injection rates, the minimum fluidization velocity will be readily exceeded due to its small cross-sectional area. 9. To avoid fluidization, a gas distributor may be accompanied by an inverted conical insert. Gas is introduced underneath the inverted cone, directly into the material through the free surface that forms. Gas may also be introduced through screens around the perimeter of the inverted cone or through the outer hopper section or both. If properly designed, an insert will expand the flow channel and eliminate stagnant regions of bulk solids in the vessel. 10. An improperly designed gas distributor, however, can cause non-uniformities to occur in gas or solids flow or to result in unstable flow. An insert that is too small or too large, or is placed too high or too low relative to some critical size and position, will expand the flow channel very little, if at all. Even if optimally located, an inverted conical insert is unlikely to convert a funnel flow pattern to mass flow, since simple inverted cones have little effect on the flow pattern below their base. Notable improvements in gas uniformity can be achieved by injecting gas via an annulus and a set of crossbeams located at the intersection of the cone and cylinder. By properly sizing the annulus and crossbeam components of the distributor, high gas injection rates can be achieved with gas velocities low enough to prevent localized fluidization. The crossbeams can also be used to support a conical insert. By properly choosing the dimensions and 89
material of construction of the inner cone, mass flow can be achieved with a hopper angle that is twice as large as would be required if an inner cone were not present. This results in a substantial saving of headroom compared to mass flow designs without inserts.
Minimum gas-injection rate As the solids travel countercurrent to the stripping gas inside the dryer, the driving force for mass transfer of the volatile species is not constant, since the compositions of both the solids and gas streams vary along the vessel height. For a specified solids feedrate, a minimum required gas injection rate exists. If an injection rate less than the minimum is used, the driving force will vanish in a portion of the column, and the desired level of devolatilization cannot be reached. To determine the minimum gas requirements, a relationship is needed to describe the phase equilibrium between the solids and gas phases. In the case of polymers, the equilibrium relationship can be determined from Flory-Huggins theory. Flory-Huggins parameters are tabulated for several solvents and polymers. In the absence of published data, a bulk material with a known volatiles concentration can be placed in a sealed container, and the vapor phase composition can be determined by gas chromatography or other analytical methods. The minimum required gas injection rate is determined by plotting the equilibrium line and an operating line on the same graph. The operating line is determined as follows. First, the target volatiles level of the solids and the concentration of volatile species in the inlet gas are located on the graph. Next, a gas injection rate is assumed, and points on the operating line are calculated using a material balance. If the gas and solids streams are lean in volatiles, the solids and gas rates are nearly constant throughout the cylinder, and the slope of the operating line is constant and equal to the ratio of the solids-to-gas, mass flowrates. The horizontal axis represents the solids volatiles content; the vertical axis denotes the volatiles content of the gas stream. Note that for a constant solids feedrate, the slope of the operating line increases as the gas injection rate is reduced. The minimum gas-injection rate is such that the concentration of the volatile component in the gas leaving the column is in equilibrium with the solids feedstream. The gas injection rate should be set safely above the minimum, and the column diameter must be sized to ensure stable operation of the dryer.
Cylinder diameter There is a trade-off between gas injection rate and required solids-residence time. Higher gas rates allow a shorter residence time, since at high rates, the volatiles in the gas phase are more dilute, the driving force for mass transfer is greater, and hence drying rates are higher. At high gas flows, however, vessels with greater diameters are required, since otherwise, unacceptably high gas velocities will result, causing fluidization and unstable operation. The cross sectional area of the column must be large enough to prevent the solids stresses from approaching too close to zero. Solids stress and gas pressure profiles can be determined by an analysis given by Johanson. Note that there is a significant increase in the solids stress at the cylinder-hopper junction, and therefore, a load analysis should be performed during the structural design of the dryer. The interstitial gas pressure increases and solids stress decreases with increasing injection rate. The gas-pressure and solids-stress profiles depend on the bulk solid’s permeability and compressibility and the feedrates of the solids and sweeping gas. As the gas injection rate is increased, the particle-to-particle contact stresses decrease. The reduction in solids stress is most severe where the gas is injected into the vessel. The minimum solids stress is plotted against gas 90
injection rate. The analysis illustrates the importance of properly sizing the vessel, as the solids stress decreases with increasing gas rate. If solids stress is low enough, the solids may become fluidized, resulting in severe flow instabilities throughout the vessel. Note that the gas rate in the purging section of the vessel (the cylinder) does not necessarily equal the gas injection rate. Usually, the outlet of the dryer is equipped with a rotary valve or other feeding device. Depending on the pressure downstream of the feeder and the pressure buildup that results from injecting gas into the column, additional gas may leak through the vessel outlet or a fraction of the injected gas may flow cocurrently with the solids stream out the outlet. This addition or reduction of gas flow in the cylinder must be taken into account when determining gas injection rate and solids residence time requirements.
Required residence time The required residence time, which directly determines the cylinder height, depends on the relative rates of the bulk solid and gas streams, the phase behavior of the volatile species, and the local rate of mass transfer of the species. To describe the transport of a trace volatile species in a particle, the diffusion equation written in spherical coordinates is used: Where x is the concentration (wt.%) volatile species in the solid, r is the radial coordinate, t denotes time, and D eff is the effective diffusivity. The initial and boundary conditions are: where R P is the Sauter mean-particle radius, x 0 is the initial concentration of the volatile component and x s is the surface concentration. The first boundary condition signifies symmetry of the intraparticle volatile-component concentration profile. The second boundary condition describes equilibrium at the interface of the solid and gas and assumes that the devolatilization process is diffusion limited (that is, there is negligible resistance to mass transfer in the gas phase). For batch stripping, where the conditions at the particle surface are constant, the diffusion equation has an analytical solution, which can be integrated to give the average concentration of the volatile component, x, as a function of time: In the case of a gravity dryer, however, where the gas passes countercurrently to the solids stream, the surface volatiles content is not constant. The concentration of volatiles in the vapor stream, and hence the equilibrium concentration at the particle surface, varies with axial position in the column. In most cases, having a low volatiles level in the gas stream exiting the dryer is undesirable, since recovering volatiles from a lean gas stream can be difficult. The diffusion equation must therefore be solved numerically, using an overall mass balance to track the volatiles level in the gas stream. One should note that not all particles are spherical or can be approximated as spheres. Powders have many shapes, and engineering judgment must be used to assess the results of analyses that assume a spherical symmetry. To determine the required residence time, the diffusion and mass-balance equations are solved iteratively. The gas injection rate is specified, one that is safely greater than the minimum, and a dryer diameter that ensures that the superficial gas velocity is low enough to prevent fluidization is determined. Next, an estimate of the required bed height is made, and the diffusion and mass-balance equations are solved to determine the volatiles content of the solids leaving the dryer. Adjustments in the bed height are then made until solving the system of equations gives the target solids-volatiles content at the outlet. Usually, additional height is specified to allow surge capability. 91
In the analysis, the Sauter mean radius is used. Sauter mean radius is defined as the radius of a sphere that has the same volume-to-surface-area ratio as a particle of interest. Because the average volatiles concentration of a solid particle is based on its volume while devolatilization takes place at its surface, the Sauter mean is appropriate for tackling transport phenomena problems. The temperature of the solids is usually set by upstream process conditions, such as the temperature of the bulk solids leaving the flash dryer, fluidized dryer or other process unit. The gas may be preheated, but considering its small thermal mass compared to that of the bulk solids stream, the gas and solids temperatures are usually approximately equal in most of the cylinder. If temperatures are expected to vary greatly inside the dryer, the solids- and gas-phase temperature profiles can be estimated using a procedure described by Munjal and Kao . When specifying the height of the cylinder section, the designer should account for the length required for the injected gas to become uniform. The height at which the gas velocity is considered uniform is typically on the order of either one vessel diameter, if gas is injected through or near the cylinder walls, or one half the diameter, if the gas injected near the centerline and near the cylinder walls. If crossbeams are used, most of the cylinder section is available for solids purging. Finding an appropriate value for the effective diffusivity may be challenging. Using published values of diffusion coefficients and adjusting them by accounting for porosity and tortuosity will not necessarily give results that predict reality. Although the analytical solution to the diffusion equation in most cases cannot be used to calculate the required residence time in a continuous purge vessel, it can be used to determine the effective diffusivity from batch stripping data. Gas is passed through a fluidized bed or a thin, fixed layer of bulk material, and the volatiles concentration of the solids is measured over time. A least-squares fit of the data to the analytical solution to the diffusion equation provides the diffusion coefficient used in the design of continuous dryers. If 25 metric tons per hour (m.t./h) solids are fed into a gravity dryer having a 3-m-dia., 20-m-tall cylinder, a 10 kg/min gas injection rate is necessary to achieve a 99% reduction in volatile component level. If a greater degree of devolatilization is desired, a dryer with a larger diameter may be required to keep gas velocities low enough to prevent fluidization and to provide additional residence time. The height of the cylinder may also need to be increased to provide the required residence time.
Summary Silos or bins used to handle bulk solids can also be used for devolatilization provided that they are properly designed. The vessel must allow uniform flow of the solids and gas, and the vessel dimensions must ensure flow instabilities do not occur due to high gas velocities. Gas must be injected at a rate high enough to provide a driving force for purging throughout the vessel, and the volume of the vessel must be large enough to afford the necessary residence time. Obtaining fundamental, bulk-solid flow properties — including cohesive strength, wall friction, compressibility, and permeability, along with phase equilibrium and kinetic data — is necessary to ensure that the gravity dryer will operate as desired.
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Understanding Bends In Pneumatic Conveying Systems Despite their apparent simplicity, bends are often poorly understood and unless properly designed, they are potentially problematic Shrikant Dhodapkar, The Dow Chemical Co.; Paul Solt, Pneumatic Conveying Consultants; George Klinzing, University of Pittsburgh
Pneumatic conveying of bulk solids has been successfully practiced — in industries as diverse as chemical, agricultural, pharmaceutical, plastics, food, mineral processing, cement and power generation — for more than a century. Pneumatic conveying provides advantages over mechanical conveying systems in many applications, including those that require complex routing, multiple source-destination combinations and product containment. Pneumatic conveying transfer lines are often routed over pipe racks and around large process equipment, giving process operators great layout flexibility. Such design flexibility is made possible by the use of bends (such as elbows and sweeps, discussed below) between straight sections (both horizontal or vertical), which enable convenient change of direction in the flow of the conveyed solids. However, among all the components of a pneumatic conveying system, bends — despite their apparent simplicity — are probably the least understood and most potentially problematic for process operators. Findings from various research studies are often not consistent, and often times public findings do not match field experience. The importance of bends in any pneumatic conveying assembly cannot be overstated since — if not properly selected and designed — they can contribute significantly to overall pressure drop, product attrition (degradation) and system maintenance (due to erosive wear). Historically, a basic long-radius bend has been the bend of choice for designers of pneumatic conveying systems, for a variety of reasons: • Long-radius bends provide the most gradual change in direction for solids, and hence are most similar to a straight section of piping • The angle of impact on the pipe wall is relatively small, which helps to minimize the risk of attrition or erosion • For lack of other experience, to maintain the status quo Years of field experience and a variety of studies conducted to troubleshoot common problems — such as line plugging, excessive product attrition (degradation), unacceptably high bend wear and higher-than-expected pressure drop — clearly indicate that the flow through bends in pneumatic piping is very complex. One should refrain from generalizing the findings until the underlying physics are well understood. This complexity is exacerbated when innovative designs are introduced to address existing issues with commonradius bends (also discussed below). Today, most of the data still resides with vendors and there is a need for fair, unbiased and technically sound comparative evaluation. The purpose of this article is to summarize the key concepts, outline key metrics used to evaluate bend performance, and provide guidance for their selection. The discussion is limited to dilute-phase conveying.
Bend Designs Bends are installed in a pneumatic conveying system wherever a change in direction is required along the conveying route. They can be broadly classified into three major categories: 1. Common-radius bends (including elbows, short-radius, long-radius and long-sweep bends) 2. Common fittings (including tee bends, mitered bends and elbows) 94
3. Specialized bends and innovative designs (such as the Gamma Bend, Hammertek Smart Elbow, Pellbow, wearback designs, and lined bends, which are described below)
Common-radius bends Common-radius bends are made by bending standard tubes or pipes. The radius of curvature ( R B) may range from 1 to 24 D (where D is the diameter of the tube or pipe). Common-radius bends can be loosely classified as follows: Elbow: R B /D = 1 to 2.5 Short radius: R B /D = 3 to 7 Long radius: R B /D = 8 to 14 Long sweep: R B /D = 15 to 24 These bends are available in a wide range of materials of construction and thicknesses, similar to the straight section of pipe (tangent) that is provided on either side of the curved section. The conveyed material may undergo multiple impacts with the pipe wall, or may slide along the outer radius, depending on material properties, solids loading (defined as mass of solids/mass of air) and gas velocity. Bend wear and material attrition commonly occur at the impact zones.
Common fittings The most commonly used fitting to accomplish a change in flow direction is a blind tee bend. In this design, one of the outlets is plugged thereby allowing conveyed solids to accumulate in the pocket. The benefit of this design is that the accumulated pocket of material cushions the impact of the incoming material, significantly reducing the potential for wear and product attrition. The extent of accumulation in the pocket will depend on the orientation of the bend, solids loading, gas velocity and material properties (such as particle size and cohesiveness). However, in a tee bend, the conveyed solids lose most of their momentum during the impact and thus must be reaccelerated downstream of the bend. As a result, pressure drop across a blind tee can be as much as three times that of a long-radius bend.
Specialized bends Today, a variety of specialized designs are available to control flow within the bend, in order to minimize attrition and wear. This is often achieved by creating a self-cleaning or replenishing pocket or layer of material, upon which the incoming stream impinges. Wear inside the piping is minimized by redirecting the gas-solid suspension away from typical wear points. Several of the most commonly used specialized bends are discussed below. Proprietary designs. The Gamma Bend from Coperion (coperion.com). Its innovative design relies on creating particle-particle impact in the impact zone and prevents sliding motion of particles along the outer radius to minimize particle smearing, so it is especially effective in preventing the formation of streamers (also known as floss or angel hair) in polymer pellets. A minimum solids loading is required to ensure accumulation of material in the impact zone. In the absence of this layer, the particles will directly impact the target plate within the bend 95
and may result in both particle attrition and pipe erosion. (A typical recommendation for minimum solids loading is 5, but it depends on the bulk density of the product.) Gamma Bends are typically fabricated from stainless steel, and provide a very tight bend radius ( R B/ D = 4 to 6). The pressure drop is higher (20 – 30%) than that experienced by a typical short-radius bend ( R B/ D = 3 to 7). The Pellbow Bend from Pelletron Corp. (pelletroncorp.com). It is similar to a short-radius bend but has an expanded pocket. The pocket is meant to accumulate a small amount of solids at the primary impact location so that most of the impact is between particles themselves. To ensure adequate accumulation of material in this pocket, the minimum recommended solids loading is typically 3, but it depends on bulk density and particle size. According to the vendor, pressure drop will be slight higher than that experienced by a short-radius bend. A wide range of materials of construction are available. The Vortice-Ell Smart Elbow from Rotaval (rotaval.co.uk) and the Hammertek Smart Elbow from Hammertek Corp. (hammertek.com) are similar in design. Both have a bulbous extension on the heel. Depending on the orientation and inlet gas velocity, the incoming material will either fill the chamber or circulate within the chamber before exiting. In either case, it results in significant reduction in wear and attrition of material. It is available in 45- and 90-deg. designs and in various materials of construction. Wearback designs. There are two major types of wearback elbow designs: 1. Elbows equipped with a wear plate with a sacrificial and replaceable back plate: o The replaceable back plate is made from hardened material, typically with Brinell hardness greater than 400 (similar to that of Ni) o Models are typically available with short-radius designs ( R B/ D = 2 to 6) and multiple angles 22.5, 45, 60 and 90 deg. o Segmented designs are available, which allows for partial replacement of the elbow body o This design is commonly used in the flyash industry 2. Tube-in-tube (pipe-in-pipe) type: o The space between the inner and outer casings can be left unfilled or filled with concrete or porcelain or another abrasion-resistant material o For the unfilled design, once the inner core wears out, the product fills the cavity. Thereafter, the material impacts on a packed bed, which continuously gets replenished. This design is not suitable for abrasive products that tend to degrade, or where cross-contamination is a concern o For the filled design, once the inner core wears out, the abrasion-resistant filling provides a longer bend life compared with many regular bends Bends with liners. Bends with abrasion-resistant liners are used for highly abrasive products. A wide range of proprietary lining materials are available. Examples include high-density alumina ceramics, zirconium corundum, hardened cast iron, silicon carbide and tungsten carbide. The presence of a liner also extends the upper limit of the operating temperature for the bend component.
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Bend Performance A variety of metrics are helpful in evaluating bend performance in pneumatic conveying systems, including the following: 1. Pressure drop related to the bend 2. Attrition or product degradation 3. Wear, erosion or bend life
Pressure drop Single-phase flow of a fluid through a bend (or any component causing directional change) will result in additional pressure drop. This behavior is well understood and reported. The pressure drop in a bend depends on the ratio of bend radius to pipe diameter ( R B/ D), the gas velocity ( U g) and the internal roughness ( k) of the pipe. When a two-phase, gas-solid suspension undergoes a directional change, such as in a bend, the bend naturally acts as a segregator or separator of the two phases. Due to the centrifugal forces acting on the particles, they are concentrated along the outer wall of the bend. For instance, in the case of fine coal, an unusual phenomenon of roping (the formation of concentrated strands) is observed. Depending on material properties, solids loading, gas velocity and pipe-wall interactions, the particles may have multiple impacts within the body of the bend. As a result of particle-particle and particle-wall impacts and the friction along the pipe wall, the particles exit the body of the bend at a velocity that is lower than their steady-state velocity. The particles must re-accelerate to their steady-state velocity after they exit the bend. The steady-state velocity for horizontal flow of a gas-solids suspension is typically in the range of 70 – 90% of the gas velocity. The energy required for reacceleration manifests itself as additional pressure loss after the bend, and the extent of the pressure drop depends on the extent to which the solids have been slowed during the transit. Simply put, the pressure drop due to a bend in gas-solid flow is due to the combination of frictional loss in the bend itself plus the energy required to reaccelerate the solids back to the steady state velocity. It should be noted that the friction coefficient within the bend will be different than the corresponding friction coefficient in an adjacent straight section. Meanwhile, additional losses due to static head (such as in horizontal-vertical and vertical-horizontal orientation) are usually minor but must also be considered. The pressure drop in a bend is most accurately quantified if the static pressures along the conveying line are measured before and after the bend location. The static pressure decreases linearly in the straight section preceding the bend. The pressure gradient increases in the body of the bend and continues to be non-linear even after the flow exits the bend. It may take considerable distance downstream of the bend (up to 15 – 20 ft; 5 – 6 m) for the flow to reach steady state pressure and for the gradient to become linear again The pressure drop incurred by a bend can be correctly estimated by extrapolating (in the upstream direction) the linear pressure gradient downstream of the bend until the imaginary outlet of the pipe bend. By comparison, if two pressure taps are placed just across the body of the bend at locations C and D, an incorrect estimation of pressure drop would be made. This is a common mistake that leads to much confusion in the literature. Calculation of bend pressure drop (EEUA). A simple approach to estimate the pressure drop resulting from standard radius bends was proposed in “EEUA Handbook”. The bend coefficient ( B) can be estimated by regression 97
using actual data. Where ∆P B = Total pressure drop due to a radius bend B = Bend loss coefficient µ = Solids loading (mass of solids / mass of conveying gas) ? g = Gas density at bend location U g = Superficial gas velocity at bend location Equivalent-length approach. An alternate approach to represent the pressure drop due to a bend is to quote an equivalent length of straight section that would result in the same pressure drop as the bend in question. The total effect of bends on system pressure drop can be estimated by multiplying the number of bends by equivalent length, and adding it to the total length of straight sections (horizontal and vertical). An equivalent length of 20 ft (6 m) is a good first guess. This approach is practical and easy but difficult to generalize for new materials. Qualitative comparison of bend pressure drop. Combining published data and practical experience, we have compiled a ranking for various types of available bends based on pressure drop characteristics. Note that in certain instances, the difference in pressure drop between tee bends and short-radius elbows can be insignificant, as some studies have suggested. Also, excess pressure drop in long-sweep bends may be attributed to their greater overall physical length. It is important to consider the pressure drop contribution of the bends in the perspective of the overall system pressure drop. The total contribution of bends to the overall system pressure drop will depend on the number of bends per unit length. If their contribution is relatively small, then replacing one type of bend with another will make little difference to the overall pressure drop (or on the conveying capacity). One must then select the bends based on other attributes. Despite numerous studies on bends and the presence of large amounts of operating data, there is still confusion and disagreement on pressure drop that is attributable to various bend geometries. Reasons for such confusion include the following: • The techniques for measurement and data analysis are not standardized. Some studies use the static pressure profile approach described above, while others estimate pressure drop due to bends by swapping one bend type with the other • It is not possible to critically evaluate all the studies since details are not always available • Most studies are done on systems with multiple bends and fail to account for the effect of location and interaction between the bends due to insufficient straight sections between them • It is difficult to generalize the results since individual studies often focus on few materials and limited range of operating conditions (such as solids loading, gas velocity, orientation) • Large-scale test data sets are very few. Most studies are conducted on pilot-scale systems 98
Attrition or particle degradation The attrition or degradation of materials during pneumatic conveying is a significant concern to the industry. Attrition generally refers to the formation of “unwanted” fractions or species in the conveyed material, which may adversely affect its value. Attrition or product degradation can manifest itself in various ways: • Change in particle size and shape distribution • Surface abrasion of particles resulting in a loss of gloss • Degradation of product due to impact heating • Smearing on the wall, which can result in cross-contamination • Undesirable loss of surface coating or additives Generation of fines due to breakage, chipping or surface abrasion can also create downstream processing issues, such as dusting, poor flowability and increased caking tendency. It may also lead to increased potential for dust explosion or increased exposure to respirable dust. During pneumatic transport of bulk solids, particles undergo multiple impacts on the pipe wall, especially at the bends. The key parameters affecting particle attrition during pneumatic conveying are summarized below.
Process-related factors: • Mode of conveying (dense versus dilute phase) • Gas velocity or particle velocity • Solids loading (or concentration) • Temperature of gas and solids (coupled with material properties) • Conveying distance • Materials of construction of straight pipeline sections and bends • Surface finish of pipeline and bends • Number of bends (frequent change in direction) • Bend geometry and flow pattern at the bend Material-related factors: • Particle size 99
• Particle shape • Particle strength or modulus or Vicker’s hardness • Elasticity of particles • Breakage function of material
Attrition and degradation issues impact bend performance in several ways: • Attrition in tee bends will be low if the primary mechanism of breakage is particle fracture due to impact loading. In tee bends, the particles are essentially impacting on a loose bed of accumulated material, which acts like a cushion. However, if the process conditions do not result in the formation of a suitable bed (for instance, the stream velocity is too high, or solids loading is too low), then particle attrition can still be significant • Attrition in short-radius bends or elbows is generally high due to impact on the bend wall • Attrition in long-radius bends or long-sweep bends can be high if chipping or surface abrasions are primary mechanisms. Multiple impacts or ricocheting inside the bend can aggravate the problem • Attrition in specialized transition designs, such as the Gamma Bend, or Pellbow (discussed above), tends to be low, as long as material accumulation occurs in the transition cavity. Overall performance will depend on the orientation of the bend The specific definition of attrition varies with the application and the product being conveyed. For agricultural products, attrition may refer to damaged or split grains, whereas for polymer pellets, attrition often manifests itself as polymer dust, chips or streamers during conveying. Based on our experience, we recommend the following measures to mitigate attrition in existing pneumatic conveying systems: • Reduce conveying velocity or increase the solids-loading ratio • Reduce the number of bends by simplifying the line layout wherever possible • Replace bends with designs that are less prone to attrition
Bend wear and erosion Each time the particles impact the pipe and bend walls, energy is transferred to the point of impact. Depending on the comparative strength of particle and wall materials, either the particle is damaged (attrition) or the pipe/ bend wears out. There are numerous ways to quantify and analyze the wear data. For instance, in research studies, wear may be characterized by erosion rate (total mass of bend eroded), specific erosion rate (mass of bend eroded per unit of mass of conveyed material), penetration rate (depth of penetration per unit mass of conveyed material) and bend life (time required to lose containment).
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While the conclusions reached depend on the applied metric, there is general agreement that the major factors associated with erosion in bends are as follows: Bend geometry: This affects the number and location of impact zones. Orientation: Orientation affects the location of impact zones. Flow pattern inside bend: This determines the penetration rate and uniformity of wear. Material of construction (hardness): Erosion rate is inversely proportional to the hardness of bend material. Particle hardness: Erosion rate is proportional to particle hardness. Particle size and shape: Three phenomena are noteworthy: • Specific erosion rate increases with particle size until a critical particle size, then the rate does not change • Bend failure due to penetration occurs faster with smaller particles • Angular particles will increase erosion rate Conveying velocity: The specific erosion rate is a strong function of gas velocity ( U g 2.5 to U g 4). Particle concentration: Significant reduction in specific erosion rate occurs at higher particle concentrations (due to greater cushioning effect). From a wear standpoint, bends can be classified into three groups: • Class I (most resistant to erosion): Blind tee, Vortice Ell or Hammertek Smart Elbow, Pellbow, radius bends with abrasion-resistant liners, wearback designs • Class II (medium resistance to erosion): Mitered bend, Gamma Bend, long sweep • Class III (very susceptible to erosion): Common-radius bends (short and long) It should be noted that significant wear can sometimes be observed in the straight section downstream (up to 10 pipe diameters) of a bend depending on the flow pattern within the bend.
Bend Location Regardless of the type of conveying system (pressure or vacuum) or the mode of conveying (dense or dilute phase), the pressure always decreases from pickup location to destination. As dictated by the Ideal Gas Law, the gas velocity will proportionally increase from pick up location to the destination. Therefore, any bends located toward the end of the conveying system will experience velocities (gas and particle) that are higher than those closer to the pickup location. Since pressure drop, attrition and erosion are all strongly affected by gas and particle velocity, bends that are of similar geometry but located toward the end of the system will incur higher pressure loss, and thus will experience greater attrition and wear. It should be noted that the solids loading (mass of solids/mass of air) in the entire 101
system remains constant, and does not depend on the location.
Pressure versus vacuum mode The increase in gas velocity (from pick up to destination) is greater when the system is operating in pull mode (vacuum system) versus push mode (pressure system). As can be seen, the velocity at the exit for a vacuum system is 42% higher than that for a pressure system. Therefore, a higher level of attrition and wear can be expected in a vacuum system, as compared to that expected in a pressure system with similar layout and overall pressure drop. Pressure system (push mode): Conveying pressure = 8 psig (55.1 kPa gage) Pick up velocity = 4,000 ft/min (20.3 m/s) Pressure in the destination receiver = 0.05 psig (0.35 kPa gage) Velocity at the exit = 6,177 ft/min (31.4 m/s) Vacuum system (pull mode): Conveying pressure = 0 psig = 14.7 psia (101.3 kPa abs) Pick up velocity = 4,000 ft/min (20.3 m/s) Pressure in the destination receiver = – 8 psig = 6.7 psia (46.2 kPa abs) Velocity at exit = 8,776 ft/min (44.6 m/s)
Selection of Bends The following key issues must be considered while selecting bends for pneumatic conveying applications: Type of conveying: Dilute versus dense phase Product characteristics: • Particle size and shape • Particle hardness (erosive wear) • Attrition or fines generation • Cohesiveness / stickiness Process requirements: 102
• Free of cross-contamination • Minimization of pressure drop or power consumption • Layout constraints • Consequences of wear or material leakage to environment • Minimize fines generation or product degradation • Materials of construction • Minimize downtime (frequency of replacement) Industry-specific practices: Consider, for instance, that the use of a smooth radius bend with polyolefin pellets can result in formation of streamers. The purchase cost of a bend and its geometry (which affects the layout of the process) has a direct impact on the cost of any pneumatic conveying project. It is prudent to consider the long term cost of ownership of a bend. For instance, a low-cost bend that results in product degradation or higher energy cost due to increase pressure drop will be more expensive in the long run.
Installation Guidance By following these recommendations, process operators can minimize problems associated with bends in pneumatic conveying systems. • Minimize the number of bends in the transfer system • Do not install a long-radius bend (horizontal to vertical) within 20 ft (6 m) of the pick up location • Back-to-back bends are not advisable. Avoid three bends in close proximity, if possible • More bends toward the end of the transfer will increase pressure drop, erosion and attrition. Consider directional changes earlier in the layout, if possible. Consider stepping up the line size, if the pressure ratio permits, to minimize the velocity toward the end of the system • Misaligned bends will increase attrition and wear • Install critical bends such that they can be easily serviced (accessible and replaceable) • Consider insulating pipe and bends when noise is an issue (especially indoors) or select appropriate type of bends. For outdoor installations, insulation can reduce the tendency of the material (such as plastic pellets) to smear inside the bend • Pay close attention to the direction of flow in specialized bends during installation
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Final thoughts Bends are a critical aspect of any pneumatic conveying system layout, and proper selection is a critical aspect of system design and operation. Improper selection of bends can result in conveying capacity limitations (due to excessive pressure drop), high product degradation/attrition, and high wear rates, which can create additional maintenance, safety and environmental issues. Optimal longterm cost of ownership can be achieved if the product characteristics and process constraints are more appropriately matched. A thorough evaluation often reveals that specialized bends may not be the best option. Available information on pipe bends in the open literature can be confusing, and these findings often conflict with field experience. Industry need to continue studying various aspects of pneumatic flow using modern tools for flow visualization and computational fluid dynamics for modeling. Edited by Suzanne Shelley
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Selecting A Conveyor Characteristics of flexible screw, aero-mechanical, vacuum and pneumatic conveyors are discussed here Michel Podevyn, Spiroflow Systems, Inc.
Conveyor selection is always defined by the larger project that it is meant to serve. First, there is a broad range of performance factors that must be met. Meanwhile, health, safety and environmental considerations usually outweigh the costs involved. After that, it comes down to the usual commercial considerations of price and delivery. This article covers the main parameters, benefits and disadvantages of four different types of conveyors, namely flexible screw, aero-mechanical, vacuum and pneumatic. Meanwhile, the task of working with vendors — both after and often during the choice in conveyor type has been made — is presented in the box below.
Flexible screw conveyors Flexible screw conveyors are often the simplest and lowest cost solution for transferring a variety of materials from Point A to Point B at rates of up to 40 ton/h over distances of up to 65 ft. If greater conveying distances are required, multiple systems can be linked together. Flexible screw conveyors consist of a special heat-treated and tempered carbon or stainless-steel spiral that rotates within an ultra-high-molecular-weight polyethylene (UHMWPE), food-grade tube. This type of conveyor is well suited for powdered, granular or flaked materials with a bulk density up to 150 lb/ft 3. The term flexible means that this type of conveyor can be curved to some extent, depending on its diameter. This provides the user with the flexibility to route the conveyor around obstacles anywhere between the inlet and outlet. Normally, using only one continuous curve is recommended. For most applications, the spiral itself has a round cross section, but a flat or profiled version can also be used for fine, cohesive or easily smeared materials. Flexible screw conveyors are designed to operate when full of material; running empty will lead to excessive noise and wear. Having a head of material in the feed hopper is desirable since it helps with the elevation of material upon startup of the conveyor. The main benefit of the flexible screw conveyor is its inherent simplicity, which translates into low initial cost, quick installation and low maintenance. For the needs of pharmaceutical, food and dairy applications, specific models are available that can be safely stripped down in minutes for thorough cleaning. Wear is only a problem with abrasive products; operating lifetime with other materials is almost indefinite. Tubes and spirals can be easily replaced. One of the latest developments is abrasion-resistant rubber tubes for such applications as aggregates, sand, cement and glass cullet. Since this type of conveyor should always operate while full, it is not recommended for transferring pre-weighed batches to a receiver. Flexible screw conveyors are best used to deliver product from storage or a bag tip station to a weigh hopper or a vessel with a high level switch. For example, it is ideal for maintaining a constant head of material in packing machine hoppers by gently filling to the high level control rather than dumping pulsed batches. Because the in-flight product is constant, flexible spiral conveyors will give very accurate, highly repeatable batches if controlled by a simple time switch. Although flexible spiral conveyors need to run full of product, they can be emptied at the end of a batch operation or at the end of a shift by removing an end bung and running the conveyor in reverse at a reduced speed if necessary. 106
Aero-mechanical conveyors The second type of conveyor, the aero-mechanical conveyor, is ideal for total transfer of products at distances from 10 to 85 ft at rates of up to 120 ton/h. An alternative and more descriptive name of the AMC is a rope and disk conveyor. This is because the AMC consists of several evenly spaced polyurethane disks attached to a wire rope. The rope and disks travel in a continuous loop fashion at a consistently high speed within parallel steel tubes. At each end, there are enclosed housings. Within these housings, the rope assembly runs from one tube to the other around specially designed sprockets. One of these sprockets drives the rope and disks while the other sprocket provides tension to the rope. The high speed of the disks produces an air stream that fluidizes and entrains the product in airflow until it is centrifugally ejected at the outlet. This method of conveying facilitates capacities up to 120 ton/h with low energy requirements, minimal product degradation and virtually no separation of mixtures. AMCs effectively operate as mechanical vacuum conveyors and should not be confused with drag-link type conveyors. Drag-link conveyors are slow-moving, heavy duty devices in which cast iron disks are often linked with rods or chains and where the product is scraped along inside the tube. Over the years, the AMC has proven to be a cost efficient method of conveying materials, dust-free and without the need for filtration. The AMC offers total batch transfer, contaminant free delivery and operation at any angle (including vertical) without any loss of capacity. For easy cleaning, AMCs can also be supplied with access panels. Besides straight-line operation, AMCs are available in a multitude of “round the corner” configurations. Other than free flowing powders, such as acrylics, flour and carbon black, AMCs can also convey difficult materials, such as titanium dioxide. They also do not have any problem conveying granules, flakes or chips. A major benefit of this type of conveyor is that degradation of the material is almost negligible. This is because an AMC creates a moving current of air in which the bulk solid is carried similar to the mechanism of a vacuum or pneumatic system. However, unlike vacuum or pneumatic systems, the AMC has a very important advantage in that it does not need a cyclone or filter to separate the product from the air. This not only saves in capital cost but also reduces maintenance and eliminates environmental issues since the air carrying the material is recycled and not released at the outlet. The material is separated from the air that carries it, and the unloaded air current is directed down into the return section of the tube where it is retained in the tube circuit. An AMC should always be started empty and fed at a controlled rate. With free-flowing products, a simple slide gate may be all that is required in terms of additional equipment. In other cases, a controlled feed device, such as a rotary valve or flexible screw conveyor, should be used. One disadvantage of an AMC is that maintenance can range from moderate to high. The rope tension needs to be adjusted regularly during the all-important startup period and then checked periodically. Rope life depends upon the length of the conveyor, the number of starts and stops, solids loading and whether routine inspection and tensioning is properly performed. Despite these drawbacks, properly maintained rope and disk assemblies on arduous duties have been known to last 14 years and longer. The effort, worry and cost of this regular maintenance can be avoided by selecting an AMC with an integral automatic rope-tension monitoring and adjustment system. 107
Vacuum conveyors The third conveyor type, vacuum conveyors, is the obvious choice where products need to be conveyed over longer distances and torturous routes. Vacuum conveying is usually restricted to throughputs of around 10 ton/h at distances over 330 ft. A vacuum conveyor uses the negative air pressure to convey materials through an enclosed pipeline. It provides a solution for users requiring a system that is easy to route, has few moving parts, is dust tight in operation and empties a product leaving minimum residue. Since the air is sucked-in, vacuum conveyors are the preferred choice for toxic or otherwise hazardous materials in the event of accidental damage to the conveying tubes because the escape of product to the atmosphere is minimized. Either an exhauster or a side-channel, high-efficiency fan located at the receiving end of the system, provides the motive force. For low capacity conveying air-powered, venturi systems are ideal. Venturi systems offer low capital cost and are not as expensive to operate as many people have been led to believe. Vacuum systems are normally the only conveying choice for customers that want to withdraw material out of bags or other open top containers such as kegs and drums. Vacuum systems are also ideal for applications with multiple inlets. Reverse-jet, self-cleaning filters clean the conveying air and return the air to the atmosphere after use. Thistype of filter reduces maintenance and minimizes product loss.
Pneumatic conveyors Pneumatic conveyors are probably the most versatile of all conveying systems, with the main negative aspect being high cost. There is virtually no limitation on capacity, product type, distance or routing. Lean phase systems (where the ratio of product to air is low) can move mountains of product. Dense phase or plug flow systems move “slugs” of product at lower speeds with minimal degradation. Positive-pressure, pneumatic conveying is generally used to convey materials from a single source to one or multiple destinations. Pneumatic conveying systems are normally the luxury of big league applications, such as the rapid discharging of road and rail tankers into silos and the transfer of product from silos to large-scale production processes. Capacities of up to 100 ton/h are not unusual. The two main disadvantages for pneumatic conveyors are the relatively high initial installation cost and the amount of filtration required. As with vacuum conveyors, self-cleaning reverse-jet filters are a big help in reducing maintenance. Maintenance is required to make sure these systems are free of leaks to ensure optimum efficiency and, above all, to avoid the associated health and environmental issues that are associated with leaks.
Other types of conveyors Beside these four types of conveyors, there are several additional types available. These include the following: • Rigid screw conveyors: Beware of the seals and bearings, which come into contact with the bulk solid material being conveyed • Bucket elevators: This type of conveyor is ideal for the most delicate products but generally not for those 108
that are dusty • Flat belt conveyors: This type of conveyor is mainly used in quarries and mines • Vibratory feeders: Vibratory feeders are ideal if only very short conveying distances are required • Air slides: Air slides are fine for dense materials that only require downhill conveying In some applications, a mix of different conveyor types is appropriate. For example, short “easy to clean” flexible screw conveyors are often used to provide a long distance, AMC with a consistent in-feed of material. Edited by Rebekkah Marshall
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Using Inserts to Address Solids Flow Problems It might seem counter intuitive at first, but inserting an obstacle in the flow path often results in improved flow characteristics Lyn Bates, Ajax Equipment, Ltd.; Shrikant Dhodapkar, The Dow Chemical Co.; George Klinzing, University of Pittsburgh
Storage of bulk materials in hoppers, bins and silos is ubiquitous in the chemical process industries (CPI), and so are the challenges that go along with it. Flow stoppage (bridging or arching), variability in mixture composition (de-mixing or segregation), erratic flow, uncontrolled flow (flooding), discharge-rate limitations and structural failures are not uncommon. While the impact of vessel geometry is relatively well-publicized in this context, the benefits that inserts offer — both to new and existing units — are not. Developed in the 1960s, Andrew Jenike’s powder test procedure and design methodology, which is based primarily on a steep cone and smooth wall, continue to provide scientifically determined solutions for securing reliable flow. It is not always feasible, however, to design and install storage units with this geometry because of space constraints, cost or the need to retrofit an existing facility for a new application. And then there are materials with variable flow properties that create a moving target. The suggestion that flow can be improved by inserting an obstacle in the flow path, or by using a wall surface with higher resistance to slip, might seem counter-intuitive at first. Closer investigation reveals, however, that overall flow regimes are formed by how the individual particles in bulk solids respond to the local forces at contact points. These forces determine whether a particle will move or not, in what direction and whether or not it remains intact. Inserts address flow on this local level.
Background The term insert embraces any static fitting on the inside of a bulk storage container, including liners and modifications that alter the internal space of a vessel. From the first step of deciding whether an insert would be helpful in a given application, it is critical to have an understanding of the patterns of flow regimes, stress systems and the behavior characteristics of loose solids. The performance of inserts depends on the hopper geometry, feeder type (or discharge control), and ambient and operating conditions. Therefore, design and selection of inserts requires an overall systems approach and must be based on the measured values of relevant bulk-material properties. This short review is meant to increase the reader’s awareness of the varied benefits inserts can provide and to provide an overview of the types, purpose, principles and attributes. Detailed design guidelines and calculation procedures for inserts are beyond the scope of this article. It must be emphasized that inserts are not a magic cure-all for grossly deficient designs. Selection and detailed design require special expertise, and therefore remain mostly in the domain of specialists. In fact, most insert concepts and designs have been developed and introduced by equipment vendors, rather than arising from fundamental research. Patents, registered designs and trademarks protect the intellectual property of some units and operating details of proprietary techniques are not in the public domain. The scope of technical publications on this subject is also limited, so the information and the design data on applications are somewhat restricted. Much work still needs to be done in developing a systematic selection criterion and in calculating stresses on inserts during filling and flow conditions.
Ensuring gravity flow Gravity flow occurs when the bulk material is deformed to the shape of the flow channel by stresses generated from the loss of potential energy of the system. Energy is lost in the form of friction, either by sliding on the container walls or by the internal friction when the flow channel boundary is within a bed of static product. The material will not flow unless the stresses generated in the flow channel due to gravity are greater than the local yield strength of the bulk material. There is often a large source of potential energy within a stored mass, but, all 112
too often it exists in the wrong place, ultimately compacting the underlying bulk to a stronger condition rather than deforming the bulk to initiate and maintain the flow. There are three basic ways to improve the potential for gravity flow: 1. Minimize the development of strength in the bulk material 2. Modify the flow channel to generate adequate stresses that will deform the bulk 3. Apply external forces on bulk material to assist gravity flow The first stage of the hopper design process is to determine the form of flow regime appropriate to the physical nature of the bulk material and circumstances of the application. Hopper constructions most often follow the simple, conventional shapes of a cone, pyramid (convergence from four directions) or V-shaped wedge form (convergence from two parallel planes). A hopper of simple shape often meets the requirements of the flow channel, however, al•be included within this comprehensive approach. Bin inserts are fitted for many functional reasons, so appreciation of purpose plays a big part in proper selection. Some insert designs are simple and easy to implement, while others require professional expertise to select and design. In all cases, care is required to ensure that adverse consequences, whether performance or safety related, are avoided. Obstructions to flow can suffer far greater loads than their size may suggest. Therefore, the structural integrity of installations must be verified. Small storage containers present special difficulties because the characteristics of bulk materials do not scale down, but structural integrity is generally less sensitive for small containers.
Minimize strength development The strength of particulate solids is dependent upon many physical properties of the constituent particles. However, in contrast to ‘solid’ solids, bulk particulates do not have a unique strength value, but exhibit a variable condition that depends, among other things, on how close the particles are packed together and the confining forces acting on the system. Bulk density is a basic measure of particle packing. Energy during so-called bed flow, which takes place in a parallel flow channel (cylindrical section), is absorbed by wall friction and compaction of the bulk. Over one-hundred years ago, Janssen’s research showed that the vertical load (stress) transmitted through a vertical bed reaches a limiting value at a depth dependent on wall friction and the hopper cross section. Underlying material carries no further increase in compacting pressure since wall friction supports additional bed depth. An optimum wall surface will have high friction on the parallel walls by liners or wall protrusions and low friction on the converging walls. Alternatively, to increase the boundary drag in flow regions of large cross section, without obstructing bulk movement, vertical ribs or cross plates can be employed. Impact of the fill stream onto previously deposited bed of material can cause compaction. An insert to slow, diffuse or deflect the flow path, to prevent the impact load on the sensitive flow region immediately above the outlet point, will reduce forces that compact the material. A more radical approach is to fit a cross of inverted V-beam (or V-cross) inserts at the transition point between the parallel and converging sections. When compared to the inverted cone, V-cross inserts have the advantage of possessing high beam strength to carry the superimposed load. Provided the remaining flow areas are larger than the critical arching dimension and flow takes place over the whole area, V-cross inserts usefully reduce overpressures. 113
With any inserts that offer multiple flow paths, it is prudent to fit vertical crossed ribs above the outlet to enforce flow from all sections and prevent the development of preferential flow channels Another method of reducing compacting pressures due to material level in the bin or silo is to install conical inserts or slip resisting fittings at various elevations along the vertical wall. This design results in transference of vertical stresses from the material to the silo wall. Soft, elastic or fibrous materials, such as cork and rubber granules, strands of plastic used for carpets, machine chippings and detergent powders, tend to be sensitive to compacting pressures. One method that has been used successfully for cork granules employs layers of coarse grids with such openness and spacing that the contents exert only a trivial pressure on the layer below, but the granules dribble through each grid To prevent undue compaction forces arising in large hoppers storing pressure sensitive granules, arrays of inverted V-shaped plates can be suspended from the roof of a silo or hopper by chains. If the development of preferential flow channels is likely, arrays of inverted V-shaped tents can used instead. In both cases, the flexibility of suspension avoids the formation of stable supports around a flow channel, as the insert will tend to move toward a region of differentially reduced pressure, as from a static bed to a live flow channel, thereby favoring flow to develop in the previously static region. Clearly, the supports need to be appropriately designed to account for loads on the insert during filling and flow.
Modify the flow channel Planar versus conical flow.The stress needed to deform a given solid also depends on the form of flow channel. Planar flow has various advantages over a conical flow. Flow in conical flow channels, termed ‘radial flow’, causes circumferential strain (π/2 ? the radial strain). Material in a V-shaped hopper deforms less by comparison, so it will flow down less-steep walls and through smaller outlets. A stable ‘rathole’ can form in a circular flow channel by virtue of the hoop strength of the material. On the other hand, a V-shaped hopper with a fully live outlet cannot sustain a rathole, provided the slot length exceeds three times its width, and the flow takes place along the full length of the slot. A circular hole has to be twice the width of a V-hopper slot for an arch to collapse and enable gravity flow. The performance of a feeder on a slot outlet is crucial to efficient hopper operation. By modifying the flow channel, an insert can make the shape of convergence more favorable for flow. An inverted cone alters the flow channel from a radial flow form to a type of annular V-shaped hopper that has a greater deforming capacity. Johanson describes the correct placement of inverted cones in his classic paper. The bullet type insert forms a two-stage flow channel of this form. The upper section has a diverging cone. The lower converging part provides a boundary for an annular, V form of flow channel that reduces the rate of convergence of the bulk. A pyramid-shaped hopper of non-mass-flow construction is also prone to forming a rathole. Fitting the hopper with a mass flow section from the outlet to a dimension greater than the critical rathole size improves flow potential and avoids rathole prospects. Extending this flow channel by walls with steeper slope provides total discharge and secures maximum storage capacity. A ‘cone-in-cone’ construction creates two flow channels. The central portion is a conventional mass-flow cone with steep walls. The outer region forms an annular, V-shaped flow channel, in which the material deforms easier than in a cone. The contents of the outer annulus slip on the wall and deform for flow against a much shallower 114
outer wall inclination than would be the case for the original cone. A limitation is that the two separate flow channels each require an outlet size greater than the critical arching span. Since the width of each outer channel (as an elongated slot) is half the diameter required for a cone, the total outlet diameter for the system has to be twice that of a normal cone. An inverted cone fitted above the outlet opposes central extraction. These inserts may be used to increase the rate of discharge, expand the flow path or induce mass flow. The stability of the annular strain acting in a conical flow can be undermined by preferentially extracting a segment of this circular region. The tube insert design, which is attributed to Lyn Bates, creates a skewed form of an annular, V-shaped flow channel in which central discharge is obstructed and a sheltered channel of preferential extraction destroys the continuity of the peripheral body of material . The shielded region fails more readily in flow and preferentially draws material from under the eccentrically extended insert. The essential feature of tube inserts is that they weaken the circumferential stress field in a conical hopper by redirecting the extraction of material from peripheral regions of the cross section. Obstructing flow in the center favors flow down from the shielded section under the insert, which allows the remaining cross section to converge in a skewed manner like the closing of an incomplete ring. Devoid of circular continuity, the residual horseshoe shape of material bends inwards, failing in tension on the periphery, and flows to the region of reduced pressure under the insert. This principle has been extended with multiple members stimulating flow from outer regions of a conical chamber. Overcoming friction. A common method employed to develop mass flow in marginal cases is to fit wall liners with lower wall friction than the original walls. Shedder plates at the hip, providing a smooth transition from the vertical to the converging wall, enhance slip by the equivalent of 2–3 deg. of extra hopper-wall steepness. Reducing dynamic overpressures. Many failures in shallow bottom silos can be attributed to difficulty in predicting dynamic overpressures during funnel flow. Dynamic overpressures occur at the location of effective transition where the switch occurs from cylindrical to arched stress field. By inserting a tube, which is open at both ends and has discharge ports at the bottom, the silo is effectively divided into two halves. The overpressures on the wall can be significantly reduced with the use of such anti-dynamic tube inserts. The diameter and height of the tube depends on the material flow properties. Promoting material dilation. A situation that gives rise to extremely high shear resistance is the initiation of shear failure of a firm granular product in confined conditions. While the structure of such a bed will reorder and pack down relatively easily under applied vibration, it offers strong resistance to any reduction in volume. For example, granular sugar or salt in a storage hopper will settle to a structural packing array whereby the grains overlap. To form a shear plane these particles must either separate or fracture. For the crystals to move apart in a confined bed, the remaining bed must compact, to which a granular structure offers exceptionally strong resistance, as it can only compacted by way of granule fracture or re-ordering of the packing to closer particle proximity. Inverted V inserts placed over a feeder screw in a hopper outlet provide a continuous void and allow the bed to expand with little resistance. Once flow commences, the product in motion dilates to ease continuation of the feeding process. Preventing caking.Caking is a time-, environment- and pressure-related process. Inserts can reduce contact pressures, as described above and influence the time that a bulk material stays in a static structural condition. A disturbance to particle packing will disrupt incipient crystal bridge formations and embryonic sintering and fusions that would hold together the bulk as a firm agglomerated mass or as clusters. Mass flow prevents stored 115
regions remaining static for an indeterminate time, but will not upset the product in a parallel body section of a mass flow hopper until it travels to reach the converging section. A design with converging/diverging wall profile can be used to distort such beds. Intermittent flow obstructions can cause regular cross section deformations that dislocate particle orientations during flow, an event that negates the growth of caking bonds within the mass. This process is caused to repeat at more frequent intervals and be more effective at preventing caking if the stored contents are recycled through the hopper, even if intermittently or at a very low rate. Reducing segregation effects.Segregation can be countered in non-mass-flow hoppers by diffusing the fill distribution, extracting material from multiple regions of storage, or both, thereby diluting the effect of any local concentration of fractions. A feed stream with a horizontal component, as from a belt conveyor, aggravates segregation. If the stream is funneled to a feed tube that impinges on a shallow inverted cone, it will spill uniformly at a large diameter, giving a more homogeneous ridge fill. Care must be given to avoid circumferential bias. Also note that product in the sensitive flow region above the outlet can be compacted by the impact of a concentrated feed stream. Diverting the flow or spreading it over a wide area by an insert will avoid or reduce this effect.
Apply external forces Vibration is often used in association with inserts to stimulate flow. Fitting a vibrator only to the outside wall of a hopper can have adverse, as well as beneficial effects because the energy input is not usually directed to the location of greatest need for flow. More effective is a flat bar extended inwards from a wall-mounted vibrator, tuned to resonate at the natural frequency of the applied vibration, can transmit vibration to sensitive flow regions. Vibrated inserts of this type have the triple effect of shielding the outlet from overpressures, redirecting the flow channel to a more efficient pattern for deformation and oscillating at the tip to break arches. Flushing, flooding or slurping are synonymous terms associated with excessive dilation (aeration) of the bulk material. A rotary vibrator on a frame with hanging rods that vibrate at a natural frequency will accelerate the de-aeration process. The rods resonate and whirl, to form vertical holes through the bed to generate ‘volcanoes’ of spurting powder and air on the surface by air escaping from lower regions. The technique may be combined with limited air injection to sustain an easy-flow state of the bulk material. The air content progressively decays as the state of the bulk material approaches, and retains, a non-fluidized, stable flow condition. A surface with a negative inclination acts also as an inverted sedimentation plate for rising gas. Solids fall away from the outer wall of a cone-in-cone insert; the underside of bullet type inserts are commonly used to provide uniform gas percolation in gas contact-bed processing. The radial arms, inner periphery of the insert and a negative step in the outer casing, employed as areas shielded from the flow path, allow an unobstructed entry for the gas.
Insert Selection Various aspects need to be considered while selecting an appropriate insert for a given application. Application objective. What are the performance objectives of the insert installation? Optimal insert design is meant to balance the relative importance of these objectives. Retrofit design versus new hopper. Retrofit situation might limit the choices due to space constraints, structural integrity of the vessel, ability to install the insert in the field and support design. Complexity of fabrication and total cost. The complexity of design and total installed cost must be balanced 116
with other alternatives available to solve the flow problem. Static inserts versus externally activated inserts. Static inserts are preferred since they are cheaper to maintain and operate. However, performance differentiation for cohesive, sticky and rubbery materials might justify externally activated inserts. Cleanability and product variability. Inserts can create cross-contamination problems for a multi-product facility. It is important to understand the potential of plugging and material accumulation during unsteady state operation, startup and production that is off specification. Acceptable time and effort required to clean the internals is a critical factor in selection. Effect on feeder. What desirable and undesirable effect does the insert have on the feed system? Patents. Many concepts and ideas are patented. Appropriate royalties must be paid to the inventor (for a list of innovative patents, click here). Commercial design. If a commercial design of the insert is available, it can be implemented much faster than designing one from the first principles.
Summary We have presented a brief overview of an extensive but largely under-developed technology of bin inserts. Application of fundamental concepts of powder mechanics and good engineering practices, driven by ingenuity, are key to future innovations in this field. Edited by Rebekkah Marshall
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Hopper Inserts for Improved Solids Flow
Storing solid materials in hoppers, bins and silos is ubiquitous in the chemical process industries (CPI), as are challenges associated with dispensing them. In many cases, using vessels with steep cones and smooth walls is enough to provide reliable flow. However, it is not always feasible to design and install hoppers with the optimal geometry, and many solids have difficult or variable flow properties. In those cases, a hopper insert may improve flow. Inserts can be defined as any static fitting mounted onto the inside of a bulk storage container to alter the internal space of the vessel. The performance of inserts depends on hopper geometry, feeder type (or discharge control), physical properties of the particulate solids and on ambient and operating conditions, so insert selection requires an overall systems approach and should be based on the measured values of bulk solid properties.
When to consider inserts Despite seeming somewhat counterintuitive, inserting an obstacle in the flow path or using a wall surface with higher resistance to slip can provide a crucial difference that allows reliable flow by influencing the local forces experienced by individual particles within bulk solids. Common reasons for installing inserts include the following: • Reduce segregation or particle attrition at the inlet region • Reduce inertial compaction by inflow and minimize dust generation at inlet • Promote or sustain gravity flow at outlet • Secure flow through smaller outlets • Increase flowrates • Secure mass flow with less-steep wall inclinations • Reduce segregation • Blend contents on discharge • Reduce compaction pressures and accelerate de-aeration inside hopper • Counteract “caking” tendency
Insert effects Gravity flow occurs when the bulk material is deformed to the shape of the flow channel by stresses generated from the loss of potential energy of the system. Energy is lost in the form of friction, either by sliding on the container walls or by internal friction when the flow-channel boundary is within a bed of static product. Bulk solid will not flow unless the stresses generated in the flow channel due to gravity are greater than the local yield strength of the material. Inserts act to improve the probability of gravity flow in one of three ways: by minimizing the development of strength in the bulk material; modifying the flow channel to generate stresses adequate to deform the bulk; and 120
applying external forces. For example, impact of the fill stream onto a previously deposited bed of material can cause compaction. An insert to slow, diffuse, or deflect the flow path, in order to prevent the impact load on the sensitive flow region immediately above the outlet point, will reduce the forces that compact the material.
Insert types Flow-channel modifiers. Modifying solids’ flow channels can make the shape of convergence more favorable for flow. A cone-in-cone insert ) creates two flow channels — a central portion where steep walls promote mass flow, and an outer region that is an annular, V-shaped flow channel in which material deforms easier than in a cone. An inverted cone insert changes the flow channel from a radial flow form into a type of annular V-shaped form that has greater deforming capacity. Bullet-type inserts form a two-stage flow channel in the same form. All can increase the rate of discharge, expand the flow path or induce mass flow. Inverted V-beam. Fitting a cross of inverted V-beam at the transition point between the parallel and converging sections of a vessel usually reduces overpressures, provided that the remaining flow areas are large enough to prevent arching. Slip-resisting fittings. Another approach to achieving the same transference of vertical stress is using slip-resistant fittings that ring the inside of the vessel at various elevations Layers of coarse grids. For some materials that are sensitive to compaction, such as soft, elastic or fibrous materials, layers of coarse grids can help avoid undue compaction forces . With the correct grid openness and intergrid spacing, the vessel contents exert only a small pressure on the layer below, but the granules dribble through each grid. Inverted V-plates. In large hoppers storing pressure-sensitive granules, arrays of inverted V-shaped plates can be suspended from the roof of a silo or hopper. The flexibility of suspension avoids the formation of stable supports around a flow channel, as the insert will tend to move toward a region of differentially reduced pressure, as from a static bed to a live flow channel, thereby favoring flow to develop in the previously static region. Tube inserts. The essential feature of tube inserts is that they weaken the circumferential stress in a conical hopper by redirecting the extraction of material from peripheral regions of the cross-section Obstructing flow in the center favors flowdown from the shielded section under the insert, which allows the remaining cross-section to converge in a skewed manner. Wall liners. Liners with lower friction than the original walls provide a smoother transition from the vertical part of the hopper to the converging part. Inlet distributor. Diffusing the fill distribution of a hopper can reduce the effects of local concentration of the material . Vibrated inserts. A flat bar extended inward into the hopper from a wall-mounted vibrator that is tuned to resonate at the natural frequency of the applied vibration can transmit vibration to the sensitive flow regions. A rotary vibrator on a frame with hanging rods that vibrate at a natural frequency will accelerate de-aeration. Material for this “Facts at your Fingertips” was adapted from the following article: Bates, L., Dhodapkar, S. and Klingzing, G., Using Inserts to Address Solids Flow Problems, Chem. Eng., July 2010, pp. 32–37.
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