FM Global - Chemical Reactors and Reactions

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FM Global Property Loss Prevention Data Sheets

January 2002 Revised January 2003 Page 1 of 63

CHEMICAL REACTORS AND REACTIONS

Table of Contents Page 1.0 SCOPE ................................................................................................................................................... 3 1.1 Changes ............................................................................................................................................ 3 2.0 LOSS PREVENTION RECOMMENDATIONS ....................................................................................... 3 2.1 Recommendations for Process Safety ............................................................................................ 3 2.2 Recommendations on Chemical Reactivity ..................................................................................... 5 2.3 Recommendations for General Chemical Processing .................................................................... 6 2.4 Recommendations for Reactor Design and Operation ................................................................... 8 2.5 Recommendations for Chemical Process Risk Assessment ......................................................... 11 2.6 Recommendations for Common Reaction Hazards ...................................................................... 11 2.6.1 Unstable Materials ............................................................................................................... 11 2.6.2 Pyrophoric Materials ............................................................................................................ 12 2.6.3 Water-reactive Materials ...................................................................................................... 12 3.0 SUPPORT FOR RECOMMENDATIONS ............................................................................................. 14 3.1 Physical and Chemical Properties ................................................................................................. 14 3.2 Chemical Functional Groups that Deserve Special Attention due to their Structural Elements ........................................................................................................................................ 20 3.3 Interaction Matrix ........................................................................................................................... 21 3.4 Chemical Hazard Analysis ............................................................................................................. 22 3.5 Chemical Thermodynamics ........................................................................................................... 24 3.6 Chemical Reaction Types .............................................................................................................. 26 3.6.1 Addition ................................................................................................................................ 26 3.6.2 Alkylation ............................................................................................................................. 26 3.6.3 Amination ............................................................................................................................. 27 3.6.4 Aromatization ....................................................................................................................... 28 3.6.5 Biochemistry: Biocatalysis, Bioconversion, Biotechnology and Biotransformation ............. 28 3.6.6 Calcination ........................................................................................................................... 28 3.6.7 Combustion ......................................................................................................................... 29 3.6.8 Condensation ...................................................................................................................... 29 3.6.9 Double Decomposition ........................................................................................................ 30 3.6.10 Electrolysis ........................................................................................................................ 30 3.6.11 Esterification ...................................................................................................................... 31 3.6.12 Fermentation ..................................................................................................................... 31 3.6.13 Fuel Gas Processes .......................................................................................................... 32 3.6.14 Grignard Reactions ........................................................................................................... 32 3.6.15 Halogenation ..................................................................................................................... 32 3.6.16 Hydrogenation ................................................................................................................... 33 3.6.17 Hydration, Hydrolysis and Saponification ......................................................................... 34 3.6.18 Isomerization and Stereoisomers ...................................................................................... 35 3.6.19 Neutralization ..................................................................................................................... 35 3.6.20 Nitration ............................................................................................................................. 36 3.6.21 Organometallic Compounds .............................................................................................. 36 3.6.22 Oxidation ........................................................................................................................... 37 3.6.23 Photochemical ................................................................................................................... 39 3.6.24 Polymerization ................................................................................................................... 39 3.6.25 Pyrolysis & Cracking ......................................................................................................... 41 3.6.26 Reduction .......................................................................................................................... 42

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Chemical Reactors and Reactions FM Global Property Loss Prevention Data Sheets

3.6.27 Reforming .......................................................................................................................... 43 3.6.28 Saponification — See Hydration, Hydrolysis and Saponification ..................................... 43 3.6.29 Silicon, Silane, Silicone, and Siloxane .............................................................................. 43 3.6.30 Sulfonation ......................................................................................................................... 44 3.7 Water-Reactive Materials .............................................................................................................. 44 3.8 Reaction Quenching Methods ....................................................................................................... 45 3.9 Unstable Materials and Explosion Hazards due to Uncontrolled Chemical Reactivity ................. 45 3.10 Experimental Screening and Thermal Hazard Analysis .............................................................. 47 3.11 Scale-up Effects ........................................................................................................................... 48 3.12 Chemical Risk Assessment and Management ............................................................................ 48 3.13 General Chemical Processing ..................................................................................................... 48 3.14 Reactor Selection ........................................................................................................................ 49 3.15 Reactor Aging and Corrosion Resistance ................................................................................... 53 3.16 Illustrative Losses ........................................................................................................................ 54 4.0 REFERENCES ..................................................................................................................................... 56 4.1 FM Global ...................................................................................................................................... 56 4.2 NFPA .............................................................................................................................................. 57 4.3 Other .............................................................................................................................................. 57 APPENDIX A GLOSSARY OF TERMS ..................................................................................................... 58 A.1 Acronyms and Abbreviations ........................................................................................................ 58 A.2 Definitions ...................................................................................................................................... 59 A.3 Chemical Functional Groups ......................................................................................................... 61 APPENDIX B BIBLIOGRAPHY ................................................................................................................. 62

List of Figures Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig.

1. Reactive chemical and chemical reaction hazard evaluation. ........................................................ 15 2. Vapor pressure-temperature properties of a pure material (ethylene oxide). ................................. 16 3. Pressure-temperature properties of a typical pure material. ........................................................... 17 4. Effects of pressure on the flammability of natural gas ..................................................................... 17 5. Effects of temperature and pressure on flammability and autoignition temperature. ..................... 18 6. Effects of temperature on common salt solubility. .......................................................................... 19 7. Example of an in-progress interaction matrix for a generic chemical process. .............................. 22 8. Enthalpy diagram of an exothermic reaction. .................................................................................. 26 9. Types of polymerization reactions. .................................................................................................. 39 10. Types of Explosions. ..................................................................................................................... 45 11. Thermal Explosion Heat-Temperature Graph. ................................................................................ 46 12. Deflagration versus Detonation Pressure-Time Graph. ................................................................ 47 13. Pfaudler CSTR. .............................................................................................................................. 50 14. Braun bioreactor. ........................................................................................................................... 51 15. Plug Flow Reactor. ........................................................................................................................ 52 16. Membrane Reactor. ....................................................................................................................... 52 17. Applicability of Materials in Oxidizing and Reducing Acids. .......................................................... 55

List of Tables Table Table Table Table

1. 2. 3. 4.

Common Types of Reactions ......................................................................................................... 13 Estimated Energy of a Confined Gas ............................................................................................ 20 Unstable Structure Decomposition Energies ................................................................................ 21 Enthalpy of Decomposition, CART values, and relative hazard rankings for selected compounds ..................................................................................................................... 24 Table 5. Advantages and Disadvantages of Various Reactor Types .......................................................... 51

©2003 Factory Mutual Insurance Company. All rights reserved.

Chemical Reactors and Reactions

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1.0 SCOPE This data sheet describes the general hazards and concepts associated with chemical reactions and chemical processing. The chemical reaction section emphasizes hazard evaluation throughout product development. The chemical reactor section includes process safety management concepts and new technology. This data sheet does not apply to boilers, chemical coating processes, chemical dipping, chemical mixing, cryogenic extractions and separations, high energy material (explosive and rocket motor) manufacturing processes, radioactive material processing, solvent extraction, solvent recovery, vulcanization, and waste treatment. 1.1 Changes January 2003. Editorial changes were done for this revision. Section 3.16 was revised. Data Sheet 7-46 has been completely rewritten. Major revisions are as follows: 1. The recommendation section has been expanded and segmented to highlight recommendations dealing with an inherently safer design philosophy, chemical hazard analysis, process hazard analysis, and reactor selection. Reference is made to other data sheets rather than repeating information contained elsewhere. 2. The chemical reaction section has been streamlined and includes the information previously located in Data Sheet 7-43/17-2, Loss Prevention in Chemical Plants (February 1974 edition). 3. A section on photochemical reactions has been added. 4. A section on chemical properties and thermodynamics has been added under Support for Recommendations. 2.0 LOSS PREVENTION RECOMMENDATIONS 2.1 Recommendations for Process Safety 2.1.1 Conduct chemical process safety evaluations, analysis, and assessments from discovery, throughout development, and into production. 2.1.2 Consider the potential energy of the chemicals involved, the reaction rates, the process equipment and the equipment configuration in all chemical process safety evaluations. 2.1.3 Use the inherently safer design concepts of intensification, substitution, attenuation, and limitation throughout new product and process development to avoid hazards, thereby reducing the need for mitigation. The greatest opportunity to develop an inherently safer chemical process occurs early in the research and development process, generally prior to selecting the final synthesis route. See Data Sheet 7-43/17-2, for details on these techniques. 2.1.4 Provide protection/mitigation measures when hazards cannot be eliminated by inherent safety concepts. Passive measures should be used wherever possible. Active protection measures should be used where passive protection is not adequate. Procedural measures alone should be avoided and are generally not acceptable protection. 2.1.5 Use a formal management of change (MOC) procedure to evaluate and control all proposed chemical process procedural and equipment changes that are not in like kind. 2.1.6 Update as-built process piping and instrumentation drawings (P&IDs) as part of the MOC procedure. Have these drawings readily available for use by operation, maintenance, and emergency response personnel, and Process Hazard Analysis (PHA) revalidation teams. 2.1.7 Have standard operating procedures (SOPs) for all processes readily available. 2.1.7.1 To be effective, be sure that SOPs are clear and concise to read, and comprehensive. 2.1.7.2 In addition to normal operating procedures, SOPs should address the following situations:


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a) The margin of safety between normal operating conditions (temperature, pressure, concentration, etc.) and those that might initiate a runaway reaction, b) Potential consequences of operating outside the normal boundaries, c) Actions to be taken in the event of an identified upset condition. Include guidance on when to stop scheduled additions, initiate quenching, initiate dumping, or add inhibitor, as appropriate. d) The addition of rework materials. Rework may have a significant concentration of undesired materials, including unstable intermediates. 2.1.7.3 Control the distribution of official copies of SOPs by a management system that includes approval and distribution data. 2.1.7.4 Circulate only the latest revision of the SOPs. Document the removal and destruction of superseded SOPs. Advise operators of all new and revised SOPs. 2.1.7.5 If the SOPs cannot be followed, use the management of change system to request and receive approval for a deviation. 2.1.8 Ensure that operators use process instruction sheets, or batch sheets. Review completed sheets and then maintain them on file. As a minimum, ensure that process instruction sheets include the following: a) A pre-operational check of the reactor and associated equipment confirming that it is clean and empty. b) Instructions to conduct an integrity check of internal components, such as the agitator or heat transfer system. c) A pre-operational verification that all critical utilities are available to meet process requirements. d) Verification of process equipment configuration. This is especially important where different products are campaigned in the same equipment. Unused feed lines and equipment should be blanked or physically disconnected. e) Verification that reactants have been analyzed for chemical identity and purity and are within specifications. f) A summary of the safe limits of temperature, pressure, chemical concentration, liquid levels, and flow rates as listed in the current SOPs. 2.1.9 Train process operators on relevant chemical and process safety issues on a periodic basis. Since chemical process accidents continue to occur despite routine chemical safety and process hazards training, behavior-based safety (why people behave the way they do) may be valuable. Providing abnormal situation management systems (intelligent control systems that are used to assist operators in determining what may be going wrong and what may be done) may also be valuable. 2.1.10 Use reliability engineering to maintain a preventive or predictive maintenance program, as a minimum, covering the process safety management (PSM) mechanical and equipment integrity element. 2.1.11 Implement a preventive or predictive maintenance program for all instrumentation systems, electrical equipment, grounding and bonding systems, ground fault indication systems, and lightning protection systems in accordance with Data Sheet 7-45, Instrumentation and Control in Safety Applications; Data Sheet 5-8, Static Electricity; Data Sheet 5-10, Protective Grounding for Electrical Power Systems and Equipment; Data Sheet 5-11, Lightning and Surge Protection for Electrical Systems; Data Sheet 5-20, Electrical Testing and Data Sheet 5-23, Emergency and Stand-By Power Systems. 2.1.12 Implement inventory management systems where hazardous materials are present. Include information on composition, compatibility, location of storage/processing vessel, and quantity. The goal should be a design with minimum inventories. 2.1.13 Specifically address hazardous chemical reactivity potential in all emergency response plans and keep them readily available for reference.



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2.2 Recommendations on Chemical Reactivity 2.2.1 Conduct chemical hazard assessments for all new processes and whenever significant changes are made to existing processes so that inherent safety and inherently safer design concepts may be included in the design of new chemical processing plants. In addition, conduct chemical hazard assessments for existing processes whenever process knowledge and documentation is lacking. 2.2.2 Obtain preliminary estimates of the hazardous chemical reactivity potential using theoretical screening methods. As a minimum, prepare an interaction matrix and conduct a Chemical Hazard Analysis. Some information may need to be determined experimentally. 2.2.3 Calculate theoretical reaction energies and computed adiabatic reaction temperature (CART) and compare them with the values for known hazardous compounds. Oxygen balance calculations may be used for organic nitrates and nitro compounds. 2.2.4 Determine chemical reaction rates and kinetics. Preliminary estimates can be obtained by using kinetic rate equations. 2.2.5 Conduct the following reactivity screening tests, as applicable, on raw materials, catalysts, intermediates, products, by-products, unintended products, solvents, inhibitors, quenchers, decomposition products, and cleaning products: a) Pyrophoric properties b) Water reactivity c) Oxidizing properties d) Solid (dust), liquid, and/or vapor flammability properties e) Common contaminant reactivity (e.g., rust, heat transfer fluid, scrubber solutions) f) Mechanical sensitivity (mechanical impact and friction) g) Thermal sensitivity h) Self-reactivity 2.2.6 The results of the hazardous chemical reactivity screening tests determine to what extent detailed thermal stability, runaway reaction, and gas evolution testing is needed. The choice of test equipment for a detailed thermal stability study will depend on the size of sample available, the required temperature range, whether onset temperature and overall kinetic information is needed, mixing needs, and whether special materials of construction are needed. Screening tests may include reaction calorimetry, adiabatic calorimetry, and temperature ramp screening using accelerating rate calorimetry (ARC), reactive system screening tool (RSST), isoperibolic calorimetry, isothermal storage tests, and adiabatic storage tests (AST). See Data Sheet 7-49/12-65, Emergency Venting of Vessels, for details on ARC and RSST techniques. 2.2.7 Evaluate measures to inhibit uncontrolled chemical reactions. Common measures include adding an inhibitor, neutralization, quenching with water or another diluent, or dumping the contents into another vessel that contains a quench liquid. Carefully select the inhibitor or quench material and thoroughly understand the inhibition reaction. Include the appropriate concentration and rate of addition of the inhibitors in the SOPs. Ensure that a reliable means of material addition or dumping is provided, as appropriate. 2.2.8 Avoid use of unstable raw materials or intermediates if at all possible. Where unavoidable, carefully control unstable raw material feed so that concentrations are kept low and the material is consumed as rapidly as it is added. Do not allow unstable intermediates to accumulate or be isolated. Give special attention to the handling of rework material. 2.2.9 Evaluate use of the proposed chemicals and synthesis route to optimize safety before proceeding with process and equipment design.


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2.3 Recommendations for General Chemical Processing 2.3.1 Consider the results of the Chemical Hazard Analysis in the chemical process and equipment design. 2.3.2 For all chemical processes, determine the worst case scenario. In general, the worst case scenario should be the worst possible combination of events that could realistically occur. As a minimum, consider all single events as well as a single event that occurs while an independent component is not available (e.g., out-of-service, awaiting repair) as credible. 2.3.3 During the chemical process safety evaluations consider credible upset conditions that are listed below. Analyze each step not only for its own safety, but also for its effect on the safety of other steps. • Mischarge of Reactants: overcharge of monomer, undercharge of limiting reagent, excess catalyst, wrong catalyst, wrong sequence of addition or inadvertent addition. • Mass Load Upset/Composition & Concentration: accumulation of unreacted materials, non-uniform distribution of gas, settling of solids, phase separation, foaming, or use of re-work. • Contamination of raw materials or equipment: water, rust, chemical residue, leaking heat transfer fluid, or cleaning products. • Abnormal Temperatures: reaction rate, side reactions, etc. • Abnormal Pressures: reaction rate, side reaction, etc. • Loss of Inerting Media. • Loss of Agitation: electric or hydraulic. • Inadequate and/or Loss of Cooling: jacket, heat exchanger and condenser. • Equipment failures (e.g., plugged lines, stuck pressure relief devices, failed valves, inaccurate instrumentation readings) caused by highly viscous materials or high freezing point materials. • Backflow of materials. 2.3.4 Consider the potential loss of site and process utilities during the process safety evaluation including the following: • Electrical power • Steam • Compressed air • Instrument air • Raw water • Process water, deionized water (DI), and/or other special water treatment systems • Cooling media (e.g., chilled water, brine) and any associated equipment including cooling towers • Heating media • Natural gas • Inerting gas • Vacuum • Area humidity, ventilation, heat and air conditioning • Area containment and drainage • Waste treatment/land fills • Environmental systems (e.g., scrubbers, fume incinerators) • Instruments, controls, and information/data systems • Fire protection water



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2.3.5 Consider less hazardous chemical processing alternatives. Evaluate these alternatives for process safety using recognized Process Hazard Analysis (PHA) methodologies and best industry practices. Use the results of these evaluations in the manufacturing process development. Preliminary PHA methodologies may include Hazard and Operability (HAZOP) studies, Failure Mode and Effect Analysis (FEMA), Fault Tree Analysis (FTA), Checklist, and What-if. FM Global Data Sheet 7-43/17-2, Loss Prevention in Chemical Plants, provides information on these techniques. 2.3.6 Address scale-up effects systematically during process development and optimization. 2.3.6.1 Keep the Chemical Hazard Analysis and Process Hazard Analysis current as scale-up progresses. 2.3.6.2 Consider the following variables as the scale is increased. • Surface-to-volume ratio effects on concentration and temperature gradients: Heat removal is a concern since the minimum temperature for a runaway reaction is not absolute, but will depend on process conditions and scale, and is linked to the rate of heat loss from the system. • Vessel shape effects on agitation and flow patterns: Excessive agitation and stagnation zones are both concerns. Ignitability of flammable materials will change with flow rates. • Vessel size effects on mixing capacity and flow stability: Pockets of reactive and/or flammable mixtures may result and are a concern. Vapor space composition may vary and also is a concern. • Vapor space composition variations and potential side reactions in the vapor space. • Materials of construction changes: Different contamination levels are a concern. • Raw material specifications: Bulk material quality may differ from lab material quality. Impurities are a concern. • Mode of operation changes: Different residence time distributions are a concern. • Viscosity changes affecting adequate emergency relief venting (plugging vents). • Isolation and storage of intermediates. • Use of recycled materials. • Waste handling. 2.3.7 Provide critical site utilities from two separate sources wherever possible. Arrange supply and distribution reliably. 2.3.8 Supply critical process utilities from completely redundant systems, reliably arranged. Where more than two systems are provided, a completely redundant in-line spare should be provided (n+1 design basis). Provide a sufficient number of accessible isolation and shutoff valves. Critical process utilities are those utilities identified during a process hazard analysis as those whose loss would either result in an unacceptable process condition or a significant interruption to production. 2.3.9 Do not locate critical site and process utility lines in an area where they may be exposed by a process upset. 2.3.10 Provide emergency power to permit the safe shutdown of chemical processes in the event of an electrical power failure. Size the emergency power generator to handle critical equipment loads, which may include the following: agitators; vacuum pumps and associated equipment; heat transfer fluid systems, cooling systems including cooling tower fans; transfer pumps; instrument air compressors; instrumentation and control equipment; and fire protection systems. 2.3.11 Arrange compressed air systems reliably. Design compressed air systems intended for multiple services to prevent contaminants from entering the instrument air system. Arrange air system automatic valves to fail-safe. 2.3.12 Arrange heat transfer systems safely and reliably: a) Provide alarms and interlocks for organic and synthetic heat transfer systems in accordance with Data Sheet 7-99, Heat Transfer by Organic and Synthetic Fluids. For water-based heat transfer systems (e.g., steam), provide separate excess temperature interlocks and pressure relief devices as a minimum;


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b) Arrange automatic valves to fail-safe; c) Use a compatible heat transfer fluid in place of water whenever water-reactive materials are present or are generated by a chemical process; d) Install strainers in cooling water systems to collect any debris that could block flow; e) Use antifouling agents and corrosion inhibitors appropriate for the system. 2.3.13 Maintain an inert gas purge where the vapor space above a liquid phase can contain an explosive mixture. This may be done by inerting on startup and shutdown if the equipment passes from an air atmosphere to a flammable atmosphere and back, and by continuous inerting if an explosive mixture may be generated during operation. Provide suitable instrumentation to facilitate proper inerting. In some cases explosion suppression systems may be applicable and advisable. See Data Sheet 7-59, Inerting and Purging of Tanks, Process Vessels and Equipment. 2.3.14 Arrange vacuum systems reliably with automatic valves that fail safe, generally closed. Provide vacuum pump seal water systems with a back-up water supply, where appropriate. Arrange vacuum pump seal systems using flammable or combustible liquids in accordance with Data Sheet 7-32, Flammable Liquid Operations. 2.3.15 Conduct performance maintenance tests on critical process utilities under load conditions as applicable. The test procedure should indicate normal performance specifications/ranges and the point at which action should be taken to address performance problems. 2.4 Recommendations for Reactor Design and Operation 2.4.1 Design and construct reactors in accordance with applicable codes, standards, and state and local laws and regulations. Base design pressures, temperatures, and corrosion allowances on the most severe combination of conditions anticipated. For pressure vessels the minimum standard of construction should be the most recent version of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section VIII and Data Sheet 12-2, Pressure Vessels. 2.4.2 Review and evaluate the following factors, at a minimum, when selecting reactor vessel materials of construction: a) Compatibility with all process materials. b) Adequate corrosion resistance. This can be achieved with proper alloy selection, liners, double walled vessels or corrosion inhibitors. Monolithic construction is preferred due to the serious mechanical integrity issues associated with the use of materials with different coefficients of expansion (e.g., vessels with sacrificial linings consumed in use and/or special or exotic materials). c) The effects of pressure and temperature cycling. d) The effect of operating outside the normal operating range including startup, shutdown and emergency venting. 2.4.3 Select reactor size and type to minimize hazardous material inventories. 2.4.4 Provide adequate emergency pressure relief for all chemical reactors as follows: a) Determine the size, location, materials of construction and arrangement of emergency pressure relief devices for each process in accordance with Data Sheet 7-49/12-65, Emergency Venting of Vessels. b) If potential blockage of pressure relief devices is a concern, conduct periodic visual inspections, with the frequency based on the properties of the materials, and implement a scheduled replacement program. c) If solid accumulation is a concern, rupture disks may be used alone or installed below a pressure relief valve; heat tracing may be installed around the device and associated piping; or an inert gas sweep may be provided near the inlet to the relief device. d) Route the discharge of emergency pressure relief vents to a point where ignition of escaping vapors or liquids will not seriously expose the equipment or structure. Shared vent system designs should be based on the worst case, multiple, simultaneous release and may result in incompatible materials being discharged at the same time.



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e) The diameter of the emergency pressure relief discharge piping should be no smaller than the diameter of the pressure relief device to ensure that the full relieving capacity is unrestricted. f) Brace emergency pressure relief lines to withstand the thrust encountered with full relieving flow. Any flexible couplings should meet the same design criteria as the fixed relief lines. g) Provide adequate emergency pressure relief for reactor jackets, heat exchangers, and associated distillation columns. For reactor/reflux condenser configurations, separate emergency pressure relief should be provided for each piece of equipment. h) Where adequate pressure relief is impractical, provide substantial isolation (by distance and barricades). Examples include high pressure chemical reactions; strongly exothermic reactions where the reaction can generate pressure at a rate beyond the practical capacity of relief devices; chemical reactions where significant quantities of unstable chemicals are used or produced; and chemical processes using large volume equipment. See Data Sheet 7-44/17-3, Spacing of Facilities in Outdoor Chemical Plants, for details. 2.4.5 Design reactors to withstand full vacuum or provide vacuum breakers. Size vacuum breakers for the greater of the fill or discharge flow rates. Alternatively, consider one or more of the following: a) Inert gas pressure control may be used to minimize vacuum effects. b) Provide insulation for equipment in geographical areas where rapid ambient temperature changes may cause vacuum conditions. c) Equip cold liquid feed lines with temperature and heater alarms and interlocks if the reactor is not designed for full vacuum or if an adequately sized vacuum breaker is not provided. Sudden discharge of cold liquid into the vessel can cause condensation and an unexpected vacuum condition. d) Limit the liquid/gas discharge rate from the reactor by orifice size or pump rate interlocks if the reactor is not designed for full vacuum or if an adequately sized vacuum breaker is not provided. Pump power supplies should be interlocked to reactor pressure. 2.4.6 Design agitation equipment for reliable service with consideration of the following factors: a) Design the agitator to withstand maximum process temperatures and pressures; b) Design the agitator to function under all foreseeable viscosity conditions; c) Preferably, the agitator should be of variable speed design with shaft speed limited. Shaft speed and load should be monitored; d) Securely attach agitator components according to manufacturer’s guidelines; e) Provide adequate cooling for shaft seals and bearings. Hot bearings and seals are potential ignition sources. Provide vibration or temperature sensors where this is a concern; f) Use double or tandem mechanical seals with inert seal fluid when seal failure could release flammable or other hazardous materials, particularly hydrogen. Provide low level alarms on seal fluid reservoirs; g) Consider using process additives to improve agitation. These could include emulsifying or de-emulsifying agents to control phase separation, or diluents to control viscosity increases. 2.4.7 Select reactor heat transfer equipment taking material properties into consideration as well as process requirements. Where a runaway reaction may occur due to high temperature at an unwetted internal heating surface, limit the temperature of the heat transfer fluid. Methods to limit temperature could include split heating/cooling systems, an external heating system with process recirculation or a sparging system. 2.4.8 Arrange raw material charging systems to minimize potential mistakes and problems. Elevate feed vessels above reactors to minimize the potential for back-feed into the charge tank. 2.4.8.1 Do not premix reactive raw materials. Do not use the same weigh or feed tank for charging more than one reactive raw material. Charge lines should have no cross connections to other reactive raw materials. Where overcharging a raw material could create a hazard, size the feed tank to hold only the required amount. Do not use a weigh or feed tank that requires multiple fills to charge the full amount of a reactant, as this arrangement is prone to operator error.


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2.4.8.2 Provide overfill protection. Use independent pump timers where raw materials are fed by metering pumps. Level monitoring devices and independent high level interlocks are advisable. 2.4.8.3 Arrange automatic valves in reactive raw material charge lines to fail closed, or as determined by a process hazard analysis. Automatic valves in solvent charge lines should fail-safe. 2.4.8.4 Add flammable liquids using non-splash methods (e.g., charge nonflammables first then add flammables via a sub-surface inlet). 2.4.8.5 Add combustible solids using closed charging systems under an inert atmosphere. In no case should an open manhole be used to ‘‘shake’’ powdered materials into the reactor, as this invites static charge buildup and loss of inert atmosphere in the reactor head space. 2.4.9 Design pipe, flanges, couplings, fittings, valves, gaskets, sight glasses, and other components in accordance with Data Sheet 7-32, Flammable Liquid Operations. 2.4.9.1 Keep reactor pipe connections to a minimum, and design the piping system to the greatest extent possible to make incorrect assembly impossible. As a minimum, arrange piping so that one chemical cannot back up into the supply piping for another incompatible chemical. 2.4.9.2 Be sure that valve and valve seat materials are compatible with process materials. 2.4.9.3 Valves should not trap materials. 2.4.9.4 Locate emergency block valves on or as close as possible to the reactor containing hazardous materials. Arrange these valves to fail closed, or as determined by a process hazard analysis. 2.4.9.5 Do not permit open ended piping on operating equipment, including sample lines. Provide all dead end piping with a screwed or bolted, flanged cap and remove all permanently idled piping. Install blanks close to the source of temporary idled piping to minimize fluid filled piping. 2.4.10 Provide permanent grounding and bonding in accordance with Data Sheet 5-8, Static Electricity. 2.4.11 Size vapor lines and condensers for the worst credible case scenario as determined by a process hazard analysis. 2.4.12 Do not allow utility connections to exceed the pressure rating of the reactor and/or jacket. Provide pressure regulators and pressure relief devices with pressure alarms and interlocks arranged to isolate the utility system whenever utility pressure could create a hazard. 2.4.13 Implement a preventive maintenance program, and preferably a predictive maintenance program, for chemical reaction system vessels and piping that includes the following activities as appropriate: a) Take initial (ultrasonic) thickness readings on all important reaction system vessels and piping during commissioning to serve as a baseline for the mechanical integrity (MI) program. Take similar readings one to two years later, and one to two years after that to establish a corrosion/erosion rate for use in calculating vessel half-life. Select test points after taking erosive forces (e.g., near agitator or bottom), high flow areas (e.g., near nozzles), and chemical reactivity areas (e.g., liquid/vapor interfaces) into consideration. Conduct reactor thickness tests at least annually, where corrosion is a concern. b) Inspect welds that are prone to physical stress (e.g., at head nozzles and agitator mounts) and chemical corrosion (e.g., liquid/vapor interface) visually and using a non-destructive examination technique (e.g., liquid penetrant or magnetic particle) during the first scheduled process shutdown to determine if there is any previously undetected manufacturing defect. c) Conduct continuity tests on glass-lined and Teflon-lined vessels regularly. Provide continuous continuity testing devices on vessels where needed, as determined by process hazard analysis and product value. d) Conduct external visual inspections of reactors at least annually. Check the condition of external insulation and corrosion resistant coverings if provided. Check for evidence of leakage. Verify structural attachments and vessel connections are in good condition. Check for cuts, dents, distortion, or other evidence of deterioration. Check riveted vessels for rivet shank corrosion; where corrosion is suspected, use a nondestructive examination technique (e.g., spot radiography).



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e) Conduct internal inspections of reactors, as a minimum, according to the National Board Inspection Code (RB-3237) and API 510 Section 6.3. Use more restrictive jurisdictional inspection requirements, as the minimum, where applicable. Use more restrictive inspection requirements where warranted by unique or unusual conditions due to the possibility of non-uniform degradation (non-continuous service, operational service history, change in process chemistry, severe environment, etc.). f) Conduct external visual inspections of reactor agitators at least annually. Check to see that agitator blades are securely attached and the shaft is in good condition as part of every internal inspection. g) Calibrate and functionally test flow meters, temperature sensors, pressure sensors, and other instrumentation at least annually. h) When high viscosity (sticky) materials are being handled, inspect vacuum breakers and pressure relief devices frequently to determine that the operating mechanisms have not become stuck together. Conduct the inspections at least monthly until sufficient information is available to establish a predictive maintenance program. i) When reactors are steam-cleaned, inspect the vacuum breakers monthly. j) Inspect points of potential flammable gas leakage from equipment on a regular basis with the frequency based on the gas pressure and previous operating history. Correct leaks promptly and properly. Perform all repairs to pressure vessels in accordance with applicable codes, use certified personnel, and update equipment maintenance records. k) Where equipment operates with flammable gases or vapors above their upper flammable limit (UFL), but must be converted to an air atmosphere for maintenance, use inerting or another approved procedure to avoid passing through the explosive range. A less desirable alternative would be to design the equipment to withstand the pressures developed by an internal vapor-air explosion. 2.4.14 Arrange all process safety interlocks to be independent of the basic process control system. Follow the installation criteria in Data Sheet 7-45, Instrumentation and Control in Safety Applications, if programmable logic devices are part of the safety interlock system. 2.5 Recommendations for Chemical Process Risk Assessment 2.5.1 Prior to installation of a process with chemical reactions in an existing or new building or area, conduct a complete review of the process and site risk levels (consequence and probability). Where risk levels of the process do not meet predetermined acceptability criteria, take mitigation steps or relocate the process elsewhere. See Data Sheet 7-43/17-2, Loss Prevention in Chemical Plants, for additional guidance on risk assessment and management. 2.6 Recommendations for Common Reaction Hazards Chemical reactions and processes may be simple or complex. Chemical reactions and processes may present very little hazard or be extremely hazardous. Because of the large number of possible chemical reactions and processes, it is impossible to provide specific guidance for every possible scenario. However, most chemical reactions can be classified into a limited number of categories. The most common types of reactions, relative energies, and common critical processing parameters are summarized in Table 1. Additional information on specific reaction types may be found in the Support for Recommendations section. 2.6.1 Unstable Materials Where highly reactive or unstable materials are present in a reaction, design and operate the reactor so that these chemicals will be present in the smallest quantity possible: a) Use low-volume equipment to limit the quantity of highly reactive or unstable material present. b) Operate the reactor under conditions such that the highly reactive or unstable chemical is used up in the process as quickly as it is introduced. c) If the highly reactive or unstable chemical is produced in the reaction, conduct the reaction so that it is removed as quickly as it is produced. d) Maintain an excess of inert solvent or inhibitor so that the concentration of the highly reactive or unstable chemical is at a safe level.


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2.6.2 Pyrophoric Materials Where pyrophoric materials are present in a reaction, design and operate the reactor so that these chemicals will be present in the smallest quantity possible: a) Design process equipment to prevent the accumulation of pyrophoric materials inside the equipment. Where some accumulation is unavoidable, design equipment so that the pyrophors may be safely and easily removed on a periodic basis. b) Design process equipment to withstand deflagration pressures. Where this is not practical, provide explosion suppression systems. c) Use closed feed systems. d) Use low-volume equipment to limit the quantity of pyrophoric material present. e) Provide interlocks to prevent opening of the equipment during processing. f) Provide equipment isolation valves. g) Provide oxygen detection interlocked to an emergency purge and isolation system. 2.6.3 Water-reactive Materials Evaluate the storage, handling, and processing of water-reactive materials during Chemical and Process Hazard Analyses: a) Store water-reactive materials in a small, detached building of noncombustible construction. b) Transport water-reactive materials in watertight containers. c) Limit quantities of water-reactive materials in manufacturing areas to one shift usage. d) Do not use water or steam for heating or cooling, equipment cleaning or as barrier fluid in mechanical seals. e) Dry compressed air and inerting gas streams prior to use. f) Check raw materials to be sure that they are dry. g) Arrange scrubbers, if provided, to prevent backflow to equipment processing water-reactive materials. h) Install automatic sprinkler protection in areas where significant quantities of flammable liquids will be present.


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Table 1. Common Types of Reactions

Type Alkylation Amination

Energy Moderately to Highly Exothermic Endothermic to Highly Exothermic

Aromatization

Endothermic to Moderately Exothermic

Potentially Critical Processing Parameters 1, 2, 6 1-5 12 (diazo) 1-4

Calcination Condensation Double Decomposition Electrolysis Esterification (organic acids) Fermentation Halogenation

Endothermic Moderately Exothermic Endothermic to Mildly Exothermic

1, 6 (offgas) 1-5 6, 7, 8

NH3 decomposition potential

Endothermic Mildly Exothermic

5, 6, 7, 9 1, 2, 5

pH, electrical variables Moisture, contaminants

Mildly Exothermic Endothermic to Highly Exothermic

Hydration Hydrogenation Hydrolysis Isomerization Neutralization Nitration

Mildly Exothermic Moderately Exothermic Mildly Exothermic Mildly Exothermic Mildly Exothermic Highly Exothermic

1 5, 8, 11 (some), 12 (some) 1, 2 1, 2, 3, 5, 6, 7 1, 2 1, 2, 3 1, 2 1-6, 8, 12

Organometallic Oxidation

Highly Exothermic Endothermic to Moderately Exothermic

Polymerization Pyrolysis and Cracking Reduction Reforming Substitution Sulfonation

Mildly to Moderately Exothermic Endothermic

1-5, 8, 10, 12 1, 3, 4, 5, 11 (some) 1-7, 12 (some) 1, 2, 8

Endothermic to Mildly Exothermic Endothermic to Moderately Exothermic Endothermic to Mildly Exothermic Mildly Exothermic

1, 1, 1, 1,

Notes Excess reagent may be needed

Dumping/Suppressant may be needed

Excluding acetylene production Includes enzymes

Contamination, Dumping may be needed, Detonation potential

Viscosity issues

2, 10 (some) 3, 4, 5 2 2, 5

Processing Parameters: 1. Temperature 2. Pressure 3. Agitation

4. Cooling/Heating 5. Addition Rate 6. Concentration

7. Flammable gases (LEL detection) 8. Inerting 9. Liquid Level

10. Water reactive 11. Reactive metals 12. Critical that adequate and reliable process control be provided


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3.0 SUPPORT FOR RECOMMENDATIONS Identifying reactive chemical hazards and chemical reaction hazards requires 1) knowledge of the physical and chemical properties of the reactants, intermediates, and products at process conditions and 2) an understanding of the reaction rate and the reaction mechanism. This is a multi-step process as shown in Figure 1. 3.1 Physical and Chemical Properties All the pertinent physical and chemical properties of chemical materials should be evaluated in relation to the hazards of fire, explosion, toxicity, and corrosion. These properties should include thermal stability, shock sensitivity, vapor pressure, flash point, fire point, boiling point, ignition temperature, flammability range, solubility, and reactivity characteristics (e.g., water reactivity and oxidizing potential). Information can be found in available references or determined by testing. However, since physical property data is frequently listed at ambient conditions, this data could be misleading. It is important to understand the physical and chemical properties of the chemicals at actual reaction conditions. Vapor pressure and temperature have a direct relationship. A vapor pressure-temperature curve for ethylene oxide with the atmospheric boiling point and critical point indicated, is shown in Figure 2. Beyond the critical point is the fluid region, where the pure material is in neither the gas nor the liquid phase. A generalized pressure-temperature curve of a pure material is shown in Figure 3. An increase in pressure raises the flash point. Conversely, a decrease in pressure lowers the flash point. An increase in pressure also will widen the flammability limits; the lower flammability limit (LFL) decreases and the upper flammability limit (UFL) increases (Fig. 4). (Note: LEL & UEL, Lower & Upper explosive limit are often used interchangeably with LFL & UFL.) A decrease in pressure to about one-half atmosphere has minimal effect. Further decreases in pressure narrow the flammable range and it may essentially disappear. An increase in pressure will decrease the minimum autoignition temperature and the minimum oxygen concentration for combustion (MOC). An increase in temperature will similarly decrease the LFL, increase the UFL, decrease the minimum autoignition temperature, and decrease the MOC. Figure 5 shows this for two pressures P1 and P2, where P1


Fig. 1. Reactive chemical and chemical reaction hazard evaluation.


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Fig. 2. Vapor pressure-temperature properties of a pure material (ethylene oxide) Note: From the Handbook of Compressed Gasses, Compressed Gas Association, Van Nostrand Reinhold, 1990.



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Fig. 3. Pressure-temperature properties of a typical pure material Note: Smith, J.M. and Van Ness, H.C. Introduction to Chemical Engineering Thermodynamics, McGraw-Hill 1975.

Fig. 4. Effects of pressure on the flammability of natural gas Note: Zabetakis, Michael Fire and Explosion Hazards at Temperature and Pressure Extremes, from the AIChE Fundamentals of Fire & Explosion Hazards Course Workbook.


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Fig. 5. Effects of temperature and pressure on flammability and autoignition temperature Note: Zabetakis, Michael Fire and Explosion Hazards at Temperature and Pressure Extremes, from the AIChE Fundamentals of Fire & Explosion Hazards Course Workbook



Fig. 6. Effects of temperature on common salt solubility


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An increase in temperature and/or pressure may significantly increase the potential energy of chemical materials. As an example, the potential energy of a confined gas can be estimated by using the isothermal expansion equation (see Data Sheet 7-0, Causes and Effects of Fires and Explosions) to the energy of TNT as shown in Table 2. Table 2. Estimated Energy of a Confined Gas Gas Pressure (psig) 10 100 1,000 10,000

TNT Equivalent (lb) Per cubic foot of gas 0.001 0.02 0.42 6.53

3.2 Chemical Functional Groups that Deserve Special Attention due to their Structural Elements There are a tremendous number of chemicals that have been identified to date. Fortunately, classification systems have been developed. Chemicals are classified as either organic (carbon based) or inorganic (non-carbon based). Organic chemicals are further classified by functional group into subclasses and families. Compounds within the following families deserve special attention due to their structural elements. 1. Alkenes (double-bonded hydrocarbon): R-CH = CH-R These include vinyl acetate, vinyl benzene (styrene), and vinyl chloride. Hazards include uncontrolled polymerization and formation of unstable peroxides. 2. Carbonyl compounds: -C = O These include acetaldehyde and butyraldehyde. Hazards include rapid oxidation or reduction, uncontrolled polymerization, and formation of unstable peroxides. Note: aldehydes (RCHO) and ketones (R1R2CO) are both included in the carbonyl compound group. The hydrogen atom versus organic group attached to the carbonyl group makes a significant difference in their reactivity. Aldehydes are easily oxidized, whereas ketones are oxidized only with difficulty. Aldehydes more readily undergo nucleophilic addition, the characteristic reaction of carbonyl compounds, than do ketones. | | | | 3. Conjugated unsaturated compounds: - C = C - C = C - (two or more double bonds) These include acrylonitrile and 1,3 butadiene. Hazards include uncontrolled polymerization and formation of unstable peroxides. | | -C-C\ / O These include ethylene oxide and propylene oxide. Hazards include explosive decomposition and contaminant catalyzed uncontrolled polymerization.

4. Epoxides (oxirane ring):

5. Alkynes (triple-bonded hydrocarbons): R1-C≡C-R2, Haloacetylene derivatives: -C≡C-X These include acetylene and propargyl alcohol. Hazards include explosive decomposition. 6. Peroxides: R-O-O-R, Alkylperoxides: C-O-O-H, and Metal peroxides: -O-O-M These include benzoyl peroxide and >60% hydrogen peroxide. Hazards include self-accelerating decomposition initiated at low temperatures. 7. Nitrated organic compounds: Nitro-Alkanes: R-NO2 These include nitromethane, dinitrotoluene, and trinitrotoluene (TNT). Hazards include highly exothermic reaction with inorganic acids and instability. Azo compounds: R-N = N-R These include p-hydroxyazobenzene. Hazards include explosive decomposition.


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Diazo compounds: R = N = N These include o-toluenediazonium chloride (salt). Hazards include explosive decomposition/detonation. Azides: -N3 These include sodium azide. Hazards include explosive decomposition. Some other families of compounds that have been known to undergo violent or explosive decompositions are as follows (Note: this is not an all-inclusive list). Aci-nitro salts Acyl or alkyl nitrates and nitrites Aminachromium peroxo complexes Alkyl and ammonium perchlorates Aminemetal and nitrogenous base oxosalts Chlorite salts Diazeno Diazirines Difluoroamino compounds Halogen azides and oxides Hydrazinium salts Hydroxylammonium salts Hypohalites Metal acetylides, fulminates and N-metal derivatives Nitroso, N-Nitroso, and N-Nitro compounds Perchloryl compounds Perchloric acid Polynitro alkyl and aryl compounds Tetrazoles Teiazenes A qualitative analysis of unstable structures can be made by comparing the molar energy of decomposition (U) of some well-known unstable compounds (Table 3). Table 3. Unstable Structure Decomposition Energies1 Compounds (# tested) Aromatic nitro (30) Aromatic nitroso (4) Oxime (5) Isocyanate (4) Aromatic azo (5) Hydrazo Aromatic diazonium (5) Peroxide (20) Epoxide (4) Alkene (6)

Molecular Structure C-NO2 C-N = O C = N-OH C-N = C = O C-N = N-C C-NH-NH-C C-N2+ C-O-OCH-CH-O (ring) C=C

-U (kJ/mol) range 220-410 90-290 110-170 50-55 100-180 65-80 130-165 200-340 65-100 40-90

Bretherick has identified several structures as susceptible to autoxidation (see section 3.6.2.2, Oxidation). These structures include acetals, allyl compounds, cumene, dienes, ethers, isopropyl compounds, styrene, and vinyl compounds. 3.3 Interaction Matrix The interaction matrix is a tool for understanding potential reactions between materials. A typical matrix will list all the chemical raw materials, catalysts, solvents, potential contaminants, materials of construction, process utilities, a human factor, and any other pertinent factors on the axis (Fig. 7). Process utilities are generally listed on only one axis, as utility interactions are outside the scope of process development (these

1

Bretherick, L. Reactive Chemical Hazards: An Overview, International Symposium on Preventing Major Chemical Accidents, 1987


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are usually addressed later on during process HAZOPs). Interaction of three or more components is generally handled by listing combinations as separate entries. Each interaction is then considered and the answer documented. Documentation should include notes on the anticipated interactions, specific references, and previous incidents. An interaction matrix is best prepared by a chemist or chemical engineer, then circulated to others to fill in open blocks, modification, and review. There may be interactions with unknown consequences that require further research and/or experimentation.

Fig. 7. Example of an in-progress interaction matrix for a generic chemical process.

3.4 Chemical Hazard Analysis Chemical Hazard Analysis (CHA) is derived from Hazard and Operability Study (HAZOP) methodologies and can be considered a precursor to a Process Hazard Analysis (PHA). They are similar in that the seven basic HAZOP guidewords are used: no, more, less, part of, reverse, as well as, and other. They are different in that the CHA assumes the proposed chemical reaction is basically safe when conducted as specified (should be confirmed) and focuses on the consequence of operating outside of the specifications. The



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consequences are then considered and potential hazards documented for reference. There may be unknown consequences that require further research and/or experimentation. The CHA is used for reference during future PHAs. Essentially all materials are unstable above certain temperatures and will thermally decompose. Thermal decompositions may be exothermic or endothermic. Exothermic decompositions are usually irreversible and frequently explosive. Organic compounds that are known to decompose before melting include azides, diazo compounds, nitramines, oxygen-containing salts and metal styphnates. Decomposition characteristics of energetic materials can be significantly different from those of the same chemical when combined with a solvent,2 and different solvents may have different effects on the decomposition temperature and rate. Solids that decompose without melting usually generate gaseous products. Particle size and aging affect the decomposition rate. Age may result in crystallization of the solid surface. Endothermic decompositions are usually reversible and are typified by hydrate, hydroxide, and carbonate decompositions. For example, a substance may have several hydrates depending on the partial pressure of water vapor. Ferric chloride, FeCl2 combines with 4, 5, 7, or 12 molecules of water. The dehydration activation energy is nearly the same as the reaction enthalpy. Oxygen balance is an analytic tool based on the difference between the oxygen content of the chemical compound and that required to fully oxidize the elements of the compound. Materials and processes approaching zero oxygen balance have the greatest heat release potential and are the most energetic. Oxygen balance calculations may be used for organic nitrates and nitro compounds. However, there is no correlation between oxygen balance and general self-reactivity. Improper application of the oxygen balance criterion can result in incorrect hazard classifications. The ASTM Computer Program for Chemical Thermodynamics and Energy Release Evaluation (CHETAH) is an analytic tool used to determine the maximum enthalpy of decomposition. It is based on molecular structure-reactivity relationships. Its major limitation is that it can predict the reactivity of organic compounds only, not inorganic compounds. A substance should be considered energetic and potentially hazardous if any of the theoretical methods indicate hazardous thermal properties or if the experimental enthalpy of decomposition in the absence of oxygen is over 50-70 cal/g (∼200-300 J/g). Note that this range is highly dependent on the process conditions and does not pertain to substances that produce significant quantities of gas. A substance should be considered as having a deflagration potential if the experimental enthalpy of decomposition in the absence of oxygen is greater than 250 cal/g (∼1,000 J/g)3. A substance should be considered as having a detonation potential if the experimental enthalpy of decomposition in the absence of oxygen is greater than 700 cal/g (2,900-3,000 J/g)4. Selected enthalpy of decomposition (∆Hr) values are listed in Table 4. The calculated adiabatic reaction temperature (CART) also provides some indication of a compound’s potential hazard. Known explosive compounds have CART values higher than 1500 K. Selected values are also listed in Table 4.

2

Evon, S. E., Chervin, S., Bodman, G.T., and Torres, A. J., Can Solvent Choices Enhance Both Process Safety and Efficiency, Process Safety Progress (Vol.18, No.1) 3 ∆Hd of 250 cal/g is the value adoptedby the Center for Chemical Process Safety (CCPS). Values of 170 to 300 cal/g can be found in the literature 4 ∆Hd of 700 cal/g is the value adopted by the CCPS. Several exceptions are known. Ammonium nitrate, azides, and organic peroxides have ∆Hd of less than 475 cal/g


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Table 4. Enthalpy of Decomposition, CART values, and relative hazard rankings for selected compounds 5. Compound Acetone Acetylene Acrylic acid Ammonia Benzoyl peroxide Dinitrotoluene Di-t-butyl peroxide Ethyl ether Ethyl hydroperoxide Ethylene Ethylene oxide Furan Maleic anhydride Mercury fulminate Methane Mononitrotoluene Nitrogen trichloride Nitroguanidine Octane Phthalic anhydride RDX Silver azide Trinitrotoluene Toluene

Formula C3H6O C2H2 C3H4O2 NH3 C14H10O4 C7H6N2O4 C8H18O2 C4H10O C2H5O2 C2H4 C2H4O C4H4O C4H2O3 Hg(ONC)2 CH4 C7H7NO2 NCl3 CH4N4O2 C8H18 C8H4O3 C3H6N6O6 AgN3 C7H5N3O6 C7H8

∆Hr (kJ/g) -1.72 -10.13 -2.18 2.72 -0.70 -5.27 -0.65 -1.92 -1.38 -4.18 -2.59 -3.60 -2.43 -2.09 0.00 -4.23 -1.92 -3.77 -1.13 -1.80 -6.78 -2.05 -5.73 -2.18

CART (K) 706 2824 789 972 1511 847 723 1058 1253 1009 995 901 5300 298 104 1930 1840 552 933 2935 >4000 2066 810

Hazard Index N E N N E E E N E N N N N E N N E E N N E E E N

N-no known unconfined explosion hazard E-unconfined explosion hazard

No single tool should be depended upon in a chemical hazard analysis. This is obvious if the maximum enthalpy of decomposition and the CART data in Table 4 are compared. Toluene has no known selfreactivity hazard, but is in the moderate hazard category based on its enthalpy of decomposition value. Mononitrotoluene has no vigorous self-reactivity hazard, but is in the high hazard category based on its enthalpy of decomposition value. Neither the enthalpy of decomposition nor CART values adequately predicts the hazards presented by organic peroxides. 3.5 Chemical Thermodynamics All chemical reactions involve energy changes in order to transform reactants into the products. The heat or enthalpy of formation provides information on the thermochemical stability of the elements that are involved in the subject reaction. A positive enthalpy of formation value provides a warning of a potential hazard. A negative enthalpy of formation value is of no potential use from a hazard evaluation perspective. The heat or enthalpy of reaction provides information on the amount of stored chemical energy. For a given chemical reaction, the total amount of energy released or required is a constant. The rate at which that energy is released is not a constant. The enthalpy of reaction provides some guidance as to the potential hazard, but the rate of energy release is more important than the amount of energy released. It is the rate of the reaction that differentiates normal chemical reactions from uncontrollable chemical reactions. Reaction rate studies can be conducted to determine reaction rates. They vary in complexity relative to the chemical reaction complexity. For simple reactions, the rate expression can be derived from the stoichiometric equation. For complex reactions, there may be several reaction steps proceeding consecutively, concurrently, or reversibly. For complex reactions, the rate expression is usually based on the rate expression for a similar chemical reaction and compared to the experimental data to determine its validity.

5 Shanley, E.S. and Melham, G.A. On the Estimation of Hazard Potential for Chemical Substances, International Symposium on RunawayReactions and Pressure Relief Design, 1995



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Basic factors affecting chemical reaction rates are the frequency of the molecular collisions, the reaction activation energy and the reaction activation entropy. Practically, this means temperature, pressure, concentration and material purity. Intuitively, the greater the frequency of collisions, the faster the reaction rates. The lower the activation energy, the faster the reaction rates. The less specific the orientation required for the reaction to proceed, the faster the reaction rates. Activation energy generally is not affected by temperature (within moderate ranges). It is dependent on pressure and is affected by the presence of a catalyst. Activation entropy is affected by the presence of a catalyst. Catalysts can be thought of as forming intermediate complexes with some of the reactants. The intermediate complex then reacts with the other reactants to form the desired product and regenerate the catalyst. Catalyzed reactions have lower activation energy and/or activation entropy than the uncatalyzed reactions. And the catalyzed reaction may favor one reaction mechanism over others. Catalytic reactions are classified as homogeneous or heterogeneous. In a homogeneous catalytic reaction, the reaction occurs in one phase and the reaction rate depends on the catalyst concentration. Common examples are enzymes, proteins that catalyze biochemical reactions. In a heterogeneous catalytic reaction, the catalyst is present in a different phase from the reaction mixture. Usually the catalyst is a solid, the reaction occurs at the solid surface, and the reaction rate depends on the surface area. A negative catalyst or inhibitor may be used to stop a reaction. In a complex reaction, the reaction rate may also be affected by chemical and physical interactions. Practically, this means fluid dynamic properties, flow pattern, and interfacial surface area. Reaction rate studies need to take all these factors into consideration. In an endothermic reaction, the overall energy change is positive (more heat is added than given off). Endothermic reactions generally proceed with difficulty and are slow except at elevated temperatures. However, if an endothermic reaction temperature is raised too high, exothermic decompositions and side reactions may occur. The products of the endothermic reactions are typically highly unsaturated, contain high concentrations of nitrogen, or nitrogen-halogen bonds. They may have an inherent tendency to spontaneously decompose. Comparison of the positive energy of activation for the decomposition reaction to the negative energy of the decomposition reaction will indicate if a self-sustained decomposition reaction is likely. In an exothermic reaction, the overall energy change is negative. Exothermic reactions generally proceed readily at ambient temperatures. The assumption that heat addition to a reactor indicates endothermic reactions is frequently not correct. The minimum energy of activation is an important reaction rate characteristic that must be overcome. It is usually necessary to add heat to overcome the energy of activation of both exothermic and endothermic reactions (Fig. 8). The assumption that heat given off by a process indicates an exothermic reaction may also be false. When two or more substances are mixed to form a solution, a heat of mixing is associated with the process even though no chemical reaction takes places. Phase changes involving pure substances will exhibit latent heat effects even though no chemical reaction takes place. The rate that energy is released depends on several factors, with temperature being the most significant. In Arrhenius’ Rate Law equation, the rate of a reaction increases exponentially with temperature. Practically, reaction rates will double or triple for every 10°C increase in temperature. Since the rate of reaction increases exponentially with temperature, while vessel heat losses to the surroundings only increases linearly with temperature, there is a temperature above which an exothermic reaction cannot be controlled simply by cooling. This is why temperature control of exothermic reactions is important. In many cases, the rate of reaction can be limited by the addition rate of the reactants or by an inhibitor to prevent excessive reaction rates.


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Fig. 8. Enthalpy diagram of an exothermic reaction.

3.6 Chemical Reaction Types Common types of reactions are described and examples are provided. 3.6.1 Addition Addition reactions are reactions in which two molecules combine to form a single molecule of product. These reactions are best described by the functional group of the reagent; see alkylation, amination, halogenation, hydration, hydrogenation, organometallic compounds, oxidation, polymerization, sulfonation, etc. 3.6.2 Alkylation Alkylation is the addition or insertion of an alkyl group (CnH2n+1) into a molecule. Alkylation reactions are diverse in nature and include carbon-carbon, carbon-nitrogen, and carbon-oxygen reactions. Friedel-Crafts reactions, aluminum chloride or similar -RX catalysts in the presence of a Lewis acid, are primarily carboncarbon alkylations. Quaternization is an example of carbon-nitrogen alkylation. Williamson reactions are carbon-oxygen alkylations. Grignard reactions and many other organometallic reactions are also alkylation reactions, but are better considered separately; see organometallic compounds. There is no inclusive, universal method of conducting alkylations. Carbon-oxygen alkylations can use acids or bases as catalysts. Temperatures in this class of alkylation reaction vary widely, ranging from ambient to 750°F (400°C) for processes conducted in the vapor phase. Pressures also vary considerably. In some processes, pressure is applied to keep the volatile reactants liquid. In other cases, high pressure is applied to promote completion of the reaction. Pressures can be in excess of 1000 psi (7 MPa).


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Alkylation of saturated hydrocarbons is done catalytically. In addition to –RX catalysts, hydrofluoric, sulfuric, and phosphoric acids are also used. With catalysts, temperatures of alkylation reactions vary considerably depending upon the reactants and the catalyst used, but most range from 0° to 150°F (- 18° to 65°C). Pressures are normally only high enough to maintain the reactants in the liquid state. Alkylation products include ethyl benzene, cumene, detergents, dyes, ethers, glycols, and synthetic lubricants. Process Chemistry a) Substitution of hydrogen attached to a carbon atom with an acid catalyst: + CH(CH3)3 acid (CH3)2CHCH2C(CH3)3 CH(CH2)(CH3)2 isobutylene isobutane → 2,2,4-trimethylpentane (iso-octane) b) Addition of an alkyl group onto a benzene ring to produce a larger aromatic hydrocarbon: C6H6 + CH3CHCH2 → C6H5CH2(CH3)2 benzene propylene cumene c) Substitution of the hydrogen attached to a nitrogen atom: + 2CH3OH → C6H5N(CH3)2 + C2H5NH2 aniline methanol dimethylaniline

H2O water

d) Substitution of the hydrogen attached to an oxygen atom: C2H5OH + (CH2)O → C2H5OC2H4OH ethanol ethylene oxide Cellosolve Equipment Alkylation reactors are normally constructed of steel. Where a highly corrosive catalyst or reagent is used, a corrosion-resistant lining must be used. Special attention should be given to moving parts and instruments to protect them from corrosion. Pumps, valves, and instruments are often made from special alloys such as Monel. Large-scale alkylations are generally continuous processes. Smaller-scale alkylations may be performed in batch reactors. Because of pressure requirements, an autoclave is usually used. The autoclave may have agitation and may need heating, cooling, pressure relief and instrumentation. 3.6.3 Amination Amination is the replacement of another functional group with –NH2 or –NH3 in an unsaturated compound or by the reduction of –NO2 to –NH2 with hydrogen. Amination by ammonolysis is the replacement of constituents in a compound such as -CI, -OH, -SO3H, -NO2, and -O- with -NH2 to form an amine. It can also involve the addition of ammonia to an unsaturated compound. Amination by reduction is the preparation of amines by the reduction of an -NO2 or a similar oxidized nitrogen component of a compound to an -NH2 group. The process usually involves reaction of a material with ammonia in water solution, but also can involve reaction with liquid or gaseous anhydrous ammonia. For reduction processes using hydrogen, see hydrogenation. Aqueous ammonolysis are usually performed without catalysts. Temperatures and pressures vary, but pressures are usually high. For example, the chlorobenzene process below operates at 355-430°F (180220°C) and 950 psi (6.5 MPa). Ammonolysis using liquid anhydrous ammonia is less common than aqueous solutions since higher pressures must be used to maintain the liquid state. Process pressures of 3000 psi (21 MPa) are not unusual. Ammonolysis using gaseous anhydrous ammonia is usually performed in the presence of a catalyst. Process Chemistry a) Ammonolysis of ethylene oxide produces a mixture of monoethanolamine, diethanolamine, and triethanolamine: (CH2)2O + NH3 (aq) → NH2(CH2)OH + NH((CH2)2 OH)2 + N((CH2)2 OH)3 ethylene ammonia monoethanolamine diethanolamine triethanolamine


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b) Ammonolysis of chlorobenzene produces aniline: C6HCl + 2NH3 (aq) → C6H5NH2 chlorobenzene ammonia aniline

+

NH4Cl ammonium chloride

c) Ammonolysis of carbon dioxide produces ammonium carbonate as an intermediate, which must be dehydrated to produce urea. The overall reaction is: CO2 + 2NH3 (l) → (NH2)2CO + H2O carbon dioxide ammonia urea water d) Ammonolysis of methanol with an aluminum oxide catalyst to produce methylamines: CH3OH + NH3 (g) → NH2CH3 + NH(CH3)2 + N(CH3)3 + H2O methanol ammonia methylamine dimethylamine trimethylamine water Equipment Aminations by ammonolysis are usually carried out most economically at elevated temperatures and pressures. For pressures up to 800 psi (5.6 MPa), autoclaves are normally used as both batch and continuous reactors. Reactions at pressures above 800 psi (5.6 MPa) are usually carried out in tubular reactors. Vessels may be solid-walled or built up of successive layers. Small diameter tubes are commonly employed where the reaction is rapid or the heat of reaction must be removed rapidly. Some gas-phase reactions are carried out continuously in reactors that are essentially shell-and-tube heat exchangers. 3.6.4 Aromatization Aromatization is the conversion of aliphatic compounds to aromatic compounds (benzene ring derivatives). These reactions involve the rearrangement of the atoms in an organic molecule without changing the number of carbon atoms. Operations are typically conducted at elevated temperatures and pressures in the presence of a catalyst. Conversion is primarily by catalytic reforming; see reforming. Process Chemistry CH3CH2CH2CH2CH2CH3 n-heptane

catalyst →

C6H5CH3 toluene

+

4H2 hydrogen

Equipment In large-scale processes, continuous reactors are used. In small-scale processes, batch reactors are used. 3.6.5 Biochemistry: Biocatalysis, Bioconversion, Biotechnology and Biotransformation Biocatalysis, bioconversion, biotechnology, and biotransformation are all names for the use of biological substances in a biochemical reaction. Biochemical reactions are classified as fermentation reactions because of their use of biological substances. This is true despite the fact that the actual reaction mechanism may be an addition, amination, dehalogenation, epoxidation, esterification, hydrolysis, hydroxylation, polymerization, or reduction. See fermentation for process equipment and hazard descriptions. Biotechnology increasingly refers to processes using recombinant DNA technology, which is commonly used in the agricultural, chemical and pharmaceutical industries. 3.6.6 Calcination Calcination is the heating of a material to a high temperature to cause it to lose moisture or another volatile component. It is most common in the cement, lime, gypsum, and soda ash manufacturing. Process Chemistry a) Calcination of lime at 2200 to 2370°F (1200 to 1300°C): CaCO3 → CaO + CO2 calcium carbonate calcium oxide carbon dioxide b) Calcination of gypsum at 250 to 375°F (120 to 190°C): → CaSO4 + 2H2O CaSO4z2H2O gypsum anhydrite gypsum water


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c) Sodium bicarbonate is converted to sodium carbonate (soda ash) at 350 to 440°F (175 to 225°C): 2NaHCO3 → Na2CO3 + CO2 + H2O sodium bicarbonate sodium carbonate carbon dioxide water d) Calciners are also used in heating petroleum coke to 2550°F (1400°C) to drive off flammable volatile materials before converting the coke to graphite. Equipment Large steel kilns are used for calcining. Horizontally rotating kilns are most common. Vertical kilns are often used for lumpy material. Kilns may be heated by gas, oil, pulverized coal, waste fuels/gases, or electricity. Additional information on kilns is provided in Data Sheet 6-17/13-20, Rotary Kilns and Dryers. Additional information on off-gas treatment is provided in Data Sheet 6-11, Fume Incinerators and Data Sheet 7-2, Waste Solvent Recovery. 3.6.7 Combustion Combustion is the reaction of a solid, liquid, or gaseous hydrocarbon with air to produce carbon dioxide, water, and heat. The principal objective normally is to produce heat; see Data Sheet 6-0/12-1, Elements of Industrial Heating Equipment and the appropriate data sheet on heating equipment. Combustion reactions are also used to generate gas for use as an inerting agent; see Data Sheet 6-9, Industrial Ovens and Dryers and Data Sheet 6-10, Process Furnaces. If the combustion is intentionally incomplete (producing carbon monoxide or other unburned combustion products) or if other elements such as sulfur are burned, then the reactions are better considered separately; see oxidation. 3.6.8 Condensation Condensation is the joining together of two or more molecules by two carbon atoms, and at the same time splitting off a small molecule such as water or hydrochloric acid (HCl). Condensation reactions are diverse in nature. They may be catalyzed by bases or acids, or conducted at elevated temperatures. Aldol condensations are an example of an aldehyde or ketone condensation. Claisen, Dieckmann, Doebner, Knoevenagel, and Perkin condensations are closely related to the Aldol condensation. Condensation reaction conditions vary, but they are usually conducted at moderately elevated temperatures and near atmospheric pressures, with or without catalysts. Some condensation reactions are also alkylation or polymerization reactions; see the appropriate reactions for additional information. Condensation reactions are frequently used to produce the active ingredients in flavorings, perfumes, dyes, and pharmaceuticals. They are also used to produce heterocyclic compounds such as imidazoles, pyrazoles, quinalines, and thiazoles. Process Chemistry a) Aldol condensation using sodium hydroxide or sodium ethoxide as the catalyst: C6H5CHO + CH3CHO → C6H5CH = CHCHO + H2O benzaldehyde acetaldehyde cinnamaldehyde water b) Condensation using sulfuric acid as the catalyst at 250°F (120°C): C6H4(CO)2O + 2C6H5OH → C6H4C(C6H4OH)2COO + phthalic anhydride phenol phenolphthalein c) Condensation at a higher temperature, 560°F (293°C): CH3CH2CH2CH3 + S → C4H4S + n-butane sulfur thiophene

H2O water

H2S hydrogen sulfide

Equipment Most condensation reactions take place in stirred batch reactors, with corrosion resistant linings when necessary. Equipment may be constructed of stainless steel or special alloys (Hastelloy, Inconel, Monel, Alloy B, Alloy C, etc.) to improve corrosion resistance.


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3.6.9 Double Decomposition Double decomposition reactions involve an exchange of one or more constituents, between two reacting molecules, usually in water. An inorganic salt generally exchanges a constituent with an acid, base, or another salt. Some double decomposition reactions are also halogenation, esterification, or condensation reactions; see the appropriate reactions for additional information. These reactions are common in water softening and the manufacture of sodium, potassium, calcium, and magnesium salts, inorganic acids, and pigments. Process Chemistry a) Double decomposition of two inorganic salts: + KCl → NaCl + NaNO3 sodium nitrate potassium chloride sodium chloride b) Double decomposition of an inorganic salt and an acid: → Na2O4 + 2NaCl + H2SO4 sodium chloride sulfuric acid sodium sulfate c) Double decomposition of an inorganic salt and a (NH4)2S2O8 + 2NaOH → 2NH4OH + ammonium sodium ammonium persulfate hydroxide hydroxide

KNO3 potassium nitrate

2HCl hydrochloric acid

base: Na2S2O8 + NH3 + sodium ammonia persulfate

H2O water

Equipment These reactions are conducted in steel reactors or ordinary tanks. The reactor may also be used for separation, as a still, or as a crystallizer. Special linings or materials may be used for corrosion resistance. 3.6.10 Electrolysis Electrolysis is the separation of ions by means of electric current. The ionic compound may be in a water solution, in the form of a molten salt, or present as the anode. Highly reactive products or by-products can be generated. The most common electrolytic process in a water solution is the electrolysis of sodium chloride in diaphragm cells. Chlorine is evolved at the anode, and sodium reacts with the water to generate hydrogen at the cathode. This process is described in more detail in Data Sheet 7-34, Electrolytic Chlorine Processes. Fluorine is also produced commercially using an electrolytic process. Electrolysis is frequently used to extract or purify metals. As a general rule, metals and/or hydrogen are discharged at the cathode (-). Non-metals and/or oxygen are discharged at the anode (+). Process Chemistry a) Electrolysis of water, using a dilute sodium hydroxide solution to improve conductivity, in diaphragm cells to produce hydrogen and oxygen: H2O → H2 + O2 water hydrogen oxygen b) Electrolysis of anhydrous hydrofluoric acid in potassium bifluoride, at medium temperature and operating currents >6000A to produce fluorine. HF + KF . HF → KF . 2HF → F2 Hydrofluoric acid potassium biflouride electrolyte fluorine c) Electrolysis of ammonium sulfate or sulfuric acid produces ammonium persulfate or persulfuric acid, respectively: 2H2SO4 → H2S2O8 + H2 sulfuric acid persulfuric acid hydrogen d) Aluminum is produced by electrolysis of alumina dissolved in molten cryolite (Na3AlF6). The oxygen reacts with the carbon anode to form carbon dioxide: 2Al2O3 + 3C → 4Al + 3CO2 Alumina carbon aluminum carbon dioxide


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Equipment Electrolytic cells are normally of steel. They may be lined with carbon, fire brick, or rubber. They may also be constructed partly of concrete or plastic. Electrodes may be copper, platinum, carbon, or other metals. 3.6.11 Esterification Esters are organic compounds corresponding in structure to salts in inorganic chemistry. Esters are most often prepared by reacting an acid and an alcohol. Acids can also be converted into their esters using acid chlorides. This section covers only esters of organic acids. Where the acid is a strong inorganic acid such as nitric, sulfuric, or hydrochloric acid, the reaction may also be classified as a nitration, sulfonation, or halogenation. Esterifications involving strong inorganic acids are better considered separately; see nitration, sulfonation or halogenation reactions, as appropriate. Esterification of an organic acid and an alcohol is an equilibrium reaction. This reversibility is a disadvantage; in order to make the reaction go to completion, the equilibrium must be shifted by either removing one of the products or by using an excess of one of the reactants. It has the advantage, though, of being a single step route. Esterification via the acid chloride route is essentially irreversible, but has multiple steps. Operations are conducted at moderate temperatures and pressures. Transesterification is the replacement of one alcohol with another alcohol in an ester. Many esters are used in fragrances, flavors, or food additives because of their pleasant odor. They are also used widely as solvents, particularly in lacquers. Process Chemistry a) Esterification of an alcohol with an organic acid to form an acetate: + CH3COOH → C6H5CH2OCOCH3 + C6H5CH2OH benzyl alcohol acetic acid benzyl acetate

H2O water

b) Esterification via the acid chloride route. Thionyl chloride (SOCl2) is the catalyst for the first step. Ethanol is used as the solvent for the second step: (CH3)3CCOOH + (CH3)3CCOCl → (CH3)3CCOOC2H5 trimethylacetic acid acid chloride ethyl trimethylacetate c) A mixture of methyl esters and glycerol is prepared by reacting a glyceride (fat) with methanol, catalyzed using either an acid or base (transesterification). Equipment Esterification reactions between an alcohol and an organic acid are usually conducted in stirred batch reactors with a distillation column attached to the reactor. 3.6.12 Fermentation Fermentation was originally defined as the anaerobic (without oxygen) metabolism of organic compounds by microorganisms (yeast, bacteria, algae, molds, and protozoa) or their enzymes. This definition has been expanded over time to also include aerobic (with oxygen) microbial processes. Fermentation processes are classified based on whether they are catalyzed by microorganisms (microbial) or by enzymes (enzymatic). The key distinction between these two types of fermentations is that in microbial fermentation, the catalytic agent reproduces itself as part of the microbe’s metabolism; the desired product could be a cell waste product or a cell component. In enzymatic fermentation, the catalytic agent does not reproduce. This difference is significant because it influences the addition of reactants and equipment selection. Fermentations are typically aqueous and in some aerobic fermentation reactions supplemental oxygen may be added. Ammonia may be used to regulate pH and as a nitrogen source for the microbe. Operational temperatures are typically a little above ambient. Operational pressures obviously vary depending on whether it is an anaerobic or aerobic fermentation. Fermentation is used to produce active pharmaceutical ingredients (antibiotics, monoclonal antibodies, proteins, and vitamins) and in the fine chemical and chemical intermediate industries to replace chemical synthesis steps and/or as an alternative to petrochemical routes.


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Fermentation is most commonly used to produce alcoholic beverages by the enzymatic breakdown of carbohydrates to simple sugars with subsequent yeast metabolism to produce ethanol. Fermentation processes are also used to produce foodstuffs (tea, yogurt, sauerkraut, and pickles), citric acid, lactic acid, and to purify organic wastes (sewage). Process Chemistry a) Anaerobic fermentation of sugar cane or starch, by yeast, to produce ethanol and fusel oil (mixture of primary alcohols): C6H12O6 → CH3COCOOH + CO2 N → NCH3CHO → 2C2H5OH + fusel oil + CO2 glucose pyruvic acetaldehyde ethanol carbon dioxide b) Aerobic fermentation of molasses, C6H12O6/water + Ca (OH)2 + glucose solution calcium hydroxide

by the mold Aspergillus niger, to citric acid: H2SO4 + O2 → C6H8O7 + Ca SO4 sulfuric oxygen citric gypsum acid acid

+

CO2 carbon dioxide

Equipment Most fermenters are stirred tank reactors constructed of stainless steel. Toroidal fermenters (ring-shaped) may be used where air mixing is part of the process. Tower fermenters may be used to retain heavily coagulated cells. Most fermenters are equipped with sparger systems with large openings to avoid clogging by microbial growth. Mechanical anti-foaming devices may be provided, and are usually accompanied by surface active antifoam agent delivery systems. Fermenters operate in batch or semi-batch mode and are as large as 50,000 gal (187,000 liters). A batch may take a few days or up to three weeks to complete. Downstream processing equipment is usually sized to operate continuously, supplied from multiple fermenters. Utility reliability is critical throughout the fermentation process. Large quantities of electric power, chilled water, and compressed air are needed to achieve optimum yield from a batch. See the appropriate data sheets for additional utility equipment information and safeguards. Additional information on fermentation processes is provided in Data Sheet 7-32, Flammable Liquid Operations; Data Sheet 7-47, Physical Operations in Chemical Plants and Data Sheet 7-74, Distilleries. Additional information on oxygen use is provided in Data Sheet 7-50, Compressed Gases in Cylinders and Data Sheet 7-52/17-13, Oxygen. Additional information on ammonia use is provided in Data Sheet 7-13, Mechanical Refrigeration and Data Sheet 7-50, Compressed Gases in Cylinders. 3.6.13 Fuel Gas Processes See Oxidation. 3.6.14 Grignard Reactions See Organometallic Compounds. 3.6.15 Halogenation Halogenation is the process of introducing halogen atoms (fluorine, chlorine, bromine, or iodine) into an organic molecule. This may be done by addition to an unsaturated bond or by replacement of an -H, -OH, or -SO3 H group. Halogenation reactions are conducted at moderate temperatures and pressures. Reactions are often initiated by a light source within the reactor as an alternative to operating at higher temperatures. The reactivity of the halogen atoms varies a great deal. Fluorinations usually proceed so vigorously that even in the dark and at room temperature, reactions must be carefully controlled. Chlorinations and brominations proceed at relatively reasonable rates. Iodinations can be difficult to initiate, and proceed at relatively slow rates.



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Process Chemistry a) Halogen addition to a double bond: phosgene (highly poisonous gas): → COCl2 CO + Cl2 carbon monixide chlorine phosgene b) Halogen replacement of hydrogen: chlorination of methane to produce any of the four degrees of chlorinated methanes. The reaction is usually carried out in the vapor phase with an excess of hydrocarbon at a temperature of 650 to 700°F (345 to 370°C). The products are recycled to get the degree of chlorination desired: CH4 + Cl2 → CH3Cl + CH2Cl2 + CHCl3 + CCl4 + HCl methane chlorine methyl methylene chloroform carbon hydrochloric chloride chloride tetrachloride acid c) Halogen replacement: Because fluorine is violently reactive with many compounds, fluorination is often conducted by first chlorinating the compound and then substituting the fluorine for chlorine: CCl4 + 2HF → CCl2F2 + 2HCl carbon hydrofluoric dichlorohydrochloric tetrachloride acid difluoromethane acid Equipment Halogenations are normally carried out in reactors designed for the corrosive characteristics of the chemicals present. They may be glass or ceramic lined. If the gases involved are dry, steel may be used. Continuous tubular reactors, with thin tubes for good heat transfer, are preferred for the exothermic reactions. 3.6.16 Hydrogenation Hydrogenation is the addition of hydrogen atoms to both sides of a double or triple bond, such as C = C, C = O, or C = N, usually through the use of hydrogen gas and a catalyst. The reaction takes place on the surface of the catalyst, and localized high temperatures will exist on the catalyst surface. In the absence of a catalyst, hydrogenations usually proceed at a very slow rate, even at high temperatures. Hydrogenation reactions can be conducted at low to very high pressure. The most common use of hydrogenation is the conversion of unsaturated animal and vegetable fats and oils into more highly or completely saturated oils. Hydrogenation not only changes the physical properties of fats, but also the chemical properties. Because of the presence of the catalysts, unsaturated compounds may undergo isomerization as well as hydrogenation. Hydrogenations are also is used in many petrochemical processes, in the production of synthetic natural gas, and in the preparation of amines. Process Chemistry a) Oil hydrogenation at 250 to 300°F (120 to 150°C) and 50 to 250 psi (350 to 1750 kPa) with a nickel catalyst. (C17H31COOO)3C3H5 + 3H2 → (C17H33COO)3C3H5 linolein hydrogen olein b) Hydrogenation of carbon monoxide to form methanol. This process is conducted at 570°F (300°C) and 4500 psi (31 MPa). The catalyst is silver or copper, with oxides of zinc, chromium, manganese, or aluminum. CO + 2H2 → CH3OH carbon monoxide hydrogen methanol c) Amination and hydrogenation. The reaction of an unsaturated carbonyl compound with ammonia and hydrogen. CH2 = CHCHO + NH3 + 2H2 → CH3CH2CH2NH2 + H2O acrolein ammonia hydrogen n-propylamine water d) Amination by reduction. C6H5NO2 + 3H2 → nitrobenzene hydrogen

C6H5NH2 + 2H2O aniline water


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Equipment Hydrogenations are usually conducted under pressure in steel reactors. Since hydrogen gas attacks carbon steels, the vessel is normally of an alloy resistant to hydrogen or has a corrosion resistant lining. Since the hydrogen molecule is small, it has a tendency to leak through packings, valves and fittings more than other gases. Care should be taken to see that these are tight, particularly when operating at high pressures and when conducted indoors. Possible points of hydrogen leakage from equipment should be regularly checked. Leaks should be promptly corrected. Approved hydrogen gas analyzers should be provided in indoor hydrogenation process areas, arranged to sound an alarm at a minimum detectable concentration (not more than 2%). Additional information on hydrogen use is provided in Data Sheet 7-91, Hydrogen. 3.6.17 Hydration, Hydrolysis and Saponification Hydration is the addition of water to a compound. Hydrolysis is the reaction of a compound with water. The water effects a double decomposition, with the hydrogen ion (H+) going to one component and the hydroxyl ion (OH-) going to the other. The general formula is XY + H2O → HY + XOH. Inorganic hydrolysis generally involves water alone. Organic hydrolysis involves acidic solutions, alkaline solutions, or enzymes. Saponification is the hydrolysis of fat to form a fatty acid and glycerine. Since water is not soluble in fats, the reaction takes place on the interface between the liquids. A sulfoaromatic-emulsifying compound is usually added to increase the interface area to promote the reaction. In many hydration and hydrolysis reactions, catalysts or high temperatures and pressures must be used for the reaction to proceed at a reasonable speed. Hydration is used in the manufacturing of alcohols from alkenes, glycols from oxides, and acetaldehyde. Hydrolysis is used to manufacture soaps, high purity carboxylic acids, and sucrose derivatives. Process Chemistry a) Hydration of ethylene to ethanol, using a phosphoric acid catalyst at 570°F (300°C) and 1000 psi (7 MPa). C2H4 + H2O → C2H5OH ethylene water ethanol b) Hydration of ethylene oxide to ethylene glycol. (CH2)2O + H2O → (CH2OH)2 ethylene oxide water ethylene glycol c) Hydration of calcium carbide to acetylene (See Data Sheet 7-51, Acetylene for additional information). CaC2 + H2O → C2H2 + Ca(OH)2 carbide water acetylene lime d) Hydrolysis of a glyceride in a caustic solution to glycerol and soap. e) Saponification of glyceryl tristearate to stearic acid and glycerine. → 3C17H35COOH + C3H5(OH)3 C3H5(C18H35O2)3 + 3H2O glyceryl tristearate water stearic acid glycerine Equipment Materials of construction vary depending on whether the process is carried out in acid or alkaline solution. Alkaline materials can usually be handled in steel vessels. With acids, special corrosion resistant materials or linings usually must be used.


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3.6.18 Isomerization and Stereoisomers Isomerization reactions involve the rearrangement of the atoms in an organic molecule without changing the molecular formula. For example, ethyl alcohol and methyl ether are isomers (C2 H6 O). Isomerization can be a simple rearrangement, such as a change from a straight chain to a branched molecule or a relocation of a double bond within the molecule that produces a significant change in physical and chemical properties. Isomerization can also result in a very small change that produces only subtle differences in spatial orientation. Stereoisomers are isomers that are different from each other only in their three dimensional orientation. They are classified by appearance. Enantiomers are mirror-image stereoisomers. They have identical physical properties, except for the direction of rotation of the plane of polarized light, and identical chemical properties, except toward optically active reagents. Diastereomers are stereoisomers that are not mirror-image isomers. Diastereomers have different physical properties and similar, but not identical chemical properties. Stereoisomers can also be further classified by how they are formed. Isomerization reactions are initiated either by heat or light. Isomerization reactions are used in petroleum refining and in the preparation of many biopharmaceutical active ingredients. Some optically active compounds are obtained from natural sources. From the naturally occurring compounds, other optically active compounds can be made. Process Chemistry a) Isomerization and chlorination of n-butane CH3CH2CH2CH3 + Cl2 → CH3CH2CHClCH3 + CH3CH2CH2CH2Cl n-butane chlorine sec-butyl chloride n-butyl chloride b) Aldohexose stereoisomers (C6H12O6): Sixteen stereoisomers including (+)-glucose, (-)-glucose, (+)-mannose, and (+)-galactose. Equipment Process equipment should be specified depending on the properties of the materials involved. Special linings or materials may be needed for corrosion resistance. 3.6.19 Neutralization Neutralization is a reaction between an acid and a base to produce a salt. Reactions normally take place in aqueous solutions at or near ambient conditions. Neutralization is used widely in fertilizer manufacturing, inorganic salt processes, water treating, and in soap manufacturing. Process Chemistry a) Acid reacts with a base to form a salt; see Data Sheet 7-89, Ammonium Nitrate for additional information. + NH3 → NH4NO3 HNO3 nitric acid ammonia ammonium nitrate b) Acid reacts with a metal to form a salt and hydrogen + 2HCl + Mg → MgCl2 hydrochloric acid magnesium magnesium chloride c) Metal carbonate reacts with acid to form a salt and water + 2HNO3 → 2NaNO3 + Na2CO3 sodium carbonate nitric acid sodium nitrate

H2 hydrogen

CO2 + carbon dioxide

H2O water

Equipment Neutralization is often done in ordinary tanks or in closed reactors or absorbers. Materials should be able to resist the corrosive acids or bases present.


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3.6.20 Nitration Nitration is the introduction of one or more nitro groups (-NO2) into a compound, normally by replacement of a hydrogen atom. Nitrations of aromatic and alcohol compounds are typically performed by using nitric acid, often in the presence of sulfuric acid. If the hydrogen in the -OH group of an alcohol is replaced, the reaction is also an esterification. Nitration reactions are very sensitive to temperature. If the temperature is too low, the nitrating agent may not react as it is introduced and may accumulate. When the temperature later rises, the nitrating agent may react all at once. If the temperature is too high, side reactions such as oxidation or decomposition may result and become violent. Reaction temperatures range from 30°F (0°C) to 750°F (400°C). Pressures range from atmospheric to about 100 psi (700 kPa). Nitration is common in the manufacture of high-energy materials, dyes, perfumes, pharmaceutical intermediates, herbicides, and fungicides. Process Chemistry a) Nitration of benzene to produce nitrobenzene (aromatic nitration). C6H6 + HNO3 → C6H5NO2 + H2O benzene nitric acid nitrobenzene water b) Nitration of glycerine to produce nitroglycerine (alcohol nitration). C3H5(OH)3 + 3HNO3 → C3H5(ONO2)3 + 3H2O glycerine nitric acid nitroglycerine water c) Nitration of propane produces a variety of nitrated products (alkane nitration). CH3CH2CH3 + HNO3 → CH3CH2CH2NO2 (25%) + CH3CHNO2CH3 (40%) + propane nitric acid 1-nitropropane 2-nitropropane CH3CH2NO2 (10%) + nitroethane

CH3NO2 + nitromethane

CO2 carbon dioxide

+

H2O water

Equipment Nitration of alkanes is normally performed as a vapor phase reaction in a high-pressure continuous reactor (NO2+ ions do not react readily with alkanes). The acid and the material to be nitrated are usually not soluble in one another and form two non-miscible layers. Vigorous agitation or tubular flow is needed to bring the reactants in contact and promote the reaction.

3.6.21 Organometallic Compounds Organometallic compounds are simply a combination of an organic molecule and a metal. Grignard reagents, –RMgX, are probably the best known of these compounds. Others include organocadmium reagents, lithium dialkylcopper reagents (Corey-House synthesis), and organozinc compounds (Simmons-Smith reaction and Reformatsky reaction). Organolithium compounds, RLi, resemble Grignard reagents in their preparation and reactions, except that they are even more reactive. Organosodium compounds are so reactive that their use is limited to the synthesis of symmetrical alkanes (Wurtz reaction). Reactions involving these compounds can also be classified as alkylation, condensation, or reduction reactions, but they are covered separately because of the unique hazards they present. Process Chemistry a) Grignard reagent preparation: Grignard reagents can be purchased for laboratory or small-scale commercial use, but usually are prepared as part of the process. Typically, magnesium chips or turnings (Mg) are charged into a vessel after it has been purged with nitrogen. Ethyl ether or tetrahydrofuran (THF) is pumped in and an organic halide (RX) is added. This forms an alkymagnesium halide of the general formula RMgX. The reaction is highly exothermic and the temperature must be controlled by limiting the rate of addition of the organic halide, making sure that it reacts as it is added, and by providing cooling to the reactor jacket.


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C6H5Br bromobenzene

+

Mg magnesium

→

Page 37

C6H5MgBr phenyl-magnesium bromide

b) Grignard reactions: Grignard reagents react with a wide variety of compounds. Phenylmagnesium bromide reacts with ethylene oxide, followed by treatment with an acid to produce phenylethanol (a compound with a rose-like odor widely used in perfumes). C6H5MgBr + (CH2)2O → C6H4CH2CH2OMgBr ethylene oxide intermediate C6H5CH2CH2OMgBr

+

HCl → hydrochloric acid

C6H5CH2CH2OH phenylethanol

+

MgClBr magnesium chlorobromide

c) Grignard reactions: Grignard reagent reaction with an organic or inorganic halide. CH3CH2MgCl + C6H5Cl → C6H5CH2CH3 + MgCl2 ethyl magnesium chlorobenzene ethyl benzene magnesium chloride chloride d) Grignard reactions: Grignard reagents react violently with water to produce a flammable liquid or gas. CH3CH2CH2CH2MgCl + H2O → CH3CH2CH2CH3 + MgOHCl n-butylmagnesium chloride water n-butane (mixture of magnesium hydroxide and chloride) Equipment Agitated, semi-batch type reactors constructed of carbon steel are frequently used. Nitrogen blanketing is provided both for safety reasons and to prevent depletion of the reagent by reaction with oxygen and moisture. 3.6.22 Oxidation Oxidation is any process that increases the proportion of oxygen or acid forming elements in a compound. Oxygen is usually added to an organic molecule, but electrons may be lost or hydrogen removed. Oxidation also includes the combination of oxygen with inorganic chemicals, such as sulfur, phosphorus, and metals. In this data sheet, oxidation differs from combustion only in that the reaction is stopped enroute to carbon dioxide and water. Organic oxidation processes can take place either in the liquid or vapor phase. In vapor phase oxidation, an excess of the material to be oxidized is vaporized and mixed with air or oxygen. The reaction is prevented from going to completion by using a low oxygen concentration and controlling the temperature. A catalyst is used to promote the reaction at the low temperature. In liquid phase oxidation, air or oxygen in bubbled through the liquid solution with a catalyst. Liquid phase oxidations are performed when a higher temperature may cause the molecule to break up. Common oxidizing agents are oxygen, hydrogen peroxide (H2O2), potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7), chromic (VI) acid (CrO3), dilute nitric acid (HNO3) and numerous chlorine derivatives. The chlorine derivatives include potassium perchlorate (KClO4), sodium chlorate (NaClO3), perchloric acid (HClO4), chlorus acid (HClO2), sodium chlorite (NaClO2), and chlorine dioxide (ClO2). Tollens’ reagent contains a silver ammonia ion, Ag(NH3)2+, and is used to oxidize aldehydes and aldoses in an alkaline solution. Fehling’s solution and Benedict’s solution, containing a complexed cupric ion, are also used to oxidize aldoses and ketoses. Ozonolysis is another form of oxidation. It is the addition of ozone (O3) to a double bond to form an ozonide. This is usually followed by hydrolysis of the ozonide to yield the cleavage products, consisting of aldehydes and ketones. Formaldehyde can be produced from isoprene via ozonolysis. Cycloalkenes and cycloalkynes do not cleave, but simply open up into a six carbon molecule containing two aldehyde groups. Oxidation is common in fuel gas processes, such as purification of natural gas and the production of fuel gases from coal. Oxidation of primary alcohols is used to produce aldehydes and carboxylic acids. Oxidation of secondary alcohols is used to produce ketones. Oxidation of alkylbenzenes is used to produce carboxylic acids. Upon treatment with periodic acid (HIO4), compounds containing two or more –OH or = O groups attached to adjacent carbons undergo oxidation with cleavage of the carbon-carbon bond. A common application of this is the oxidative cleavage of carbohydrates into sugar derivatives. There are also many biological processes involving oxidation reactions.


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Inorganic oxidation is most common in the metallurgical refining (mining) industry. It is used to produce pure metals from metal minerals, typically gold, copper, nickel and zinc. Oxygen selectively reacts with undesired metallic sulfides (pyrite), iron and other metals to free up the desired valuable metals. This may be done pyrometallurgically in smelters or roasters in the presence of high concentration oxygen to produce gaseous sulfur dioxide, or hydrometallurgically in autoclaves to produce liquid sulfates. While smelters and roasters operate at high temperature, but low (atmospheric) pressure, autoclaves operate at high temperatures and pressures. Process Chemistry a) Oxidation of coal: fuel gases are produced by the reaction between carbon and steam or oxygen. Oxygen is obtained from air in the production of producer gas or low BTU gas. Pure oxygen produces medium BTU gas. C + H2O → H2 + CO or CO2 carbon water hydrogen carbon carbon monoxide dioxide C + O2 → CO or CO2 carbon oxygen carbon carbon monoxide dioxide b) Oxidation of methane at flame temperature (1500°F [815°C]), and at 850°F (454°C) with a nickel catalyst. CH4 + 2O2 → CO2 + 2H2O + heat (complete combustion) methane oxygen carbon dioxide water + O2 6CH4 methane oxygen CH4 + methane

H 2O water

→ →

2C2H2 acetylene

+

2CO + carbon monoxide

CO + carbon monoxide

2H2 hydrogen

3H2 hydrogen

c) Oxidation of an alkene using permanganate + KMnO4 → (CH3)2CO CH3CHC(CH3)2 2-methyl-2-butene potassium acetone permanganate

+

CH3COOH acetic acid

d) Oxidation of sulfur to form sulfuric acid. In the first and second steps, sulfur is oxidized. Step one is performed in an ordinary furnace. Step two is performed in a converter at a lower temperature, 1065°F (575°C), using a platinum or vanadium catalyst. In the third step, the oxide is hydrated with water to produce the acid. S + O2 → SO2 sulfur oxygen sulfur dioxide O2 → SO3 oxygen sulfur trioxide

2SO2 sulfur dioxide

+

+ SO3 sulfur trioxide

H2O water

→

H2SO4 sulfuric acid

Equipment The greatest problem in the design of organic oxidation reactors is the removal of heat in vapor-phase reactors. Removal of heat is essential to prevent destruction of apparatus, catalyst, or raw material. Maintenance of temperature at the proper level is necessary to ensure the correct rate of degree of oxidation. Another problem in the design of oxidation reactors is the possible introduction of liquid into vapor-phase reactors; the liquid may flash resulting in overpressurization. In liquid-phase reactors, the temperature is usually low, and the rate of heat generation can be readily controlled by the rate of introduction of air or other oxidizing agent. Good mixing of reactants is important. Autoclaves used in metallurgical refining often constructed of special corrosion resistant alloys such as titanium. Materials should be able to resist the corrosive materials present. Additional information on oxygen use is provided in Data Sheet 7-52/17-13, Oxygen.



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Additional information on liquid and solid oxidizing materials is provided in Data Sheet 7-82N, Storage of Liquid and Solid Oxidizing Materials. 3.6.23 Photochemical This type of reaction is outside the common classification systems. Photochemical reactions proceed in the presence of light; the rate is dependent on the intensity of incident radiation. The reactions, themselves, are otherwise classified. a) Halogenation using UV light → CH3CH2Cl CH3CH3 + Cl2 ethane chlorine chloroethane b) Isomerization of 1,3-butadiene to cyclobutene CH2 = CHCH = CH2 → CH2CH = CHCH2 (cyclic compound) 1,3-butadiene cyclobutene 3.6.24 Polymerization Polymerization is the joining together of small molecules, known as monomers, to form a much larger molecule, known as a macromolecule or polymer. There are many ways to form polymers as shown (Fig. 9). The catalyst used in addition polymerization may be an organic peroxide, an inorganic peroxide, an acid, a base, or an organometallic compound.

Fig. 9. Types of polymerization reactions.

Addition polymerization can be conducted via a homogeneous system (need only one phase for the reaction to proceed) or a heterogeneous system (need at least two phases for the reaction to proceed). In bulk polymerization, the monomer, polymer, and catalyst are the only materials in the reactor. Because of the high concentration of reactants, it is the most difficult polymerization to control. The heat given off is difficult to remove, particularly, as the polymerization progresses and the mix increases in viscosity. Bulk polymerization is often done in two stages. In the first stage, the monomer is only partly polymerized into a resin. In the second stage, the reaction is completed by molding or extruding the resin. This may be by molding into


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sheets, rods, or tubes (acrylic polymers); spraying onto forms and curing (fiberglass reinforced polyester polymers); or extruding into sheets or films. Bulk polymerization has the advantage of being free of solvents and impurities. In solution polymerization, the monomer is dissolved in a solvent. Because of the dilution of the monomer, the reaction is easier to control and the viscosity is reduced. This facilitates heat removal and temperature control. However, the product is usually of a lower molecular weight, and the solvent must be separated from the polymer after the reaction is completed. In emulsion polymerization, monomer particles are suspended in a water emulsion using soap or another emulsifying agent. The monomers are converted to a colloidal suspension of polymer particles, forming latex. The water absorbs the heat of reaction and keeps the reaction mass fluid to simplify agitation. This process is common in the polymerization of vinyl chloride. The disadvantage of this process is that the addition of emulsifying or stabilizing agents makes it difficult to prepare a pure or clear polymer. In suspension or pearl polymerization, the monomer droplets are usually suspended in water without an emulsifying agent. However, solvents can be used (hexane is used in an older polypropylene process). The droplets are larger (0.1 to 1 mm) than in the emulsion process. The catalyst is dissolved in the water. Polymers usually form as small beads that are easily filtered out, washed, and dried to form molding powders. In both emulsion and suspension polymerization, if agitation can be maintained, absorption of heat by the water or solvent readily controls the reaction. However, if agitation is lost or the emulsifying agent is inadequate, the monomer will concentrate and uncontrolled bulk polymerization would ensue. Therefore, reliable agitation is vital as well as relief venting equal to that provided for bulk polymerization processes. Condensation polymerization can be carried out in an aqueous or a solvent medium, or the reactants may be liquids. The reaction is usually interrupted while the polymers are still soluble or fusible. A curing process completes the polymerization to the final product. The catalyst used in condensation polymerization is usually an acid or base. Since water or a similar condensation product is produced and must be removed, the process is normally easier to control than addition polymerization. Copolymers are polymers that contain two or more different monomers. Copolymerization processes are similar to polymerization processes, except that the relative concentrations and reactivities of the monomers become important process parameters. Copolymerization is used to produce materials with varying properties. Synthetic polymers include elastomers, fibers, and plastics. Elastomers have the elasticity characteristics of rubber. Elastomeric Materials, published by the International Plastics Selector, identifies over 20 different generic types of synthetic rubber including acrylic, butadiene, butadiene/styrene/vinyl pyridine, butyl, ethylene-propylene, halogenated, nitrile, styrene-butadiene and urethane rubbers. Fibers are thread-like. Synthetic fibers include polyamides (Nylon 66), polyesters (Dacron, Terylene, Vycron), polyacrylonitriles (Orlon, Acrilon), polyurethanes (Spandex, Vycra), and isotatic polypropylene. Plastic characteristics depend on the molecular structure. Plastics are classified as thermoplastics or thermosetting. Thermoplastic or thermosoftening polymers are linear and branched polymers that are basically crystalline. On heating, these polymers soften. Addition polymerization is commonly used to produce thermoplastics including acid-catalyzed phenyl-formaldehyde, polyethylene, polystyrene, and polyvinyl chloride. Thermosetting polymers are highly cross-linked and form rigid, but irregular three-dimensional structures. On heating, these polymers may actually become harder due to the formation of additional cross-links. Condensation polymerization is commonly used to produce thermosetting resins including alkaline-catalyzed phenyl-formaldehyde and melamine. Process Chemistry 1. Addition polymerization of ethylene to polyethylene. a) Bulk polymerization of ethylene to produce low density polyethylene (LDPE, 0.910-0.940 g/cm3) at highpressure using small quantities of oxygen, an organic peroxide, or another strong oxidizer as the catalyst. The ethylene gas is compressed up to 60,000 psi (420 MPa) at up to 660°F (350°C). Note that at very high pressures ethylene can be decomposed by shock, and by an excess of catalyst.



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b) Solution polymerization of ethylene to produce high density polyethylene (HDPE, 0.941-0.970 g/cm3), at low-pressure using an organic or an inorganic peroxide as a catalyst. The ethylene is dissolved in an inert hydrocarbon. The solvent helps to absorb the heat of reaction. If the temperature rises, the ethylene is driven out of solution, reducing its concentration and automatically slowing the reaction. CH2 = CH2 → (-CH2 CH2-)n ethylene polyethylene 2. Condensation copolymerization of adipic acid and hexamethylenediamine to Nylon 66: + HOOC(CH2)4 COOH → -NH(CH2)6NHCO(CH2)4CO- + H2O H2N(CH2)6NH2 hexamethylenediamine adipic acid nylon water 3. Condensation copolymerization of phenol or substituted phenol and an aldehyde to prepare phenolic resins, such as phenol-formaldehyde. The type of catalyst used (caustic or acid), the ratio of the reactants, and the reaction conditions (time and temperature) determine the properties of the resin and the reaction hazards. Formaldehyde–to-phenol ratios of 0.5 to 0.8 are used with an acid catalyst to produce nolovaks. Novolaks are thermoplastic and react with cross-linking substances to give the resin desired properties; they are generally solids. Novolaks are produced in both continuous and batch processes. Molten phenol is usually charged into the reactor, followed by a precise amount of acid catalyst. The formaldehyde solution is then introduced in a slow, continuous or stepwise addition. Formaldehyde–to-phenol ratios of 1.0 to 3.0 are used with a caustic catalyst to produce resoles. Resoles are generally of lower molecular weight and may be liquid or solid. Most resoles are produced in a multi-step batch process. Acid catalyzed phenol-formaldehyde reactions are historically easier to control than caustic catalyzed reactions. C6H5OH + HCHO → C6H4OH(CH2OH) + C6H5OH → C6H5OH-CH2-C6H5OH → polymer phenol formaldehyde o-hydroxymethyl phenol intermediate Equipment Batch, semi-batch or continuous reactors are used depending on the type of polymerization. Polystyrene is prepared in batch and semi-batch reactors. Polyethylene is typically prepared in tubular flow reactors; these reactors may be 1-1⁄4 miles (2 km) long by 2.5 inches (6.4 cm) ID or 20 ft (6.1 m) high by 9 in. (22.9 cm) ID with 4.5 in. (11.4 cm) thick walls. Polyvinylchloride is prepared in large batch reactors. Additional information on organic peroxides is provided in Data Sheet 7-80, Organic Peroxides. 3.6.25 Pyrolysis & Cracking Pyrolysis is the decomposition of large molecules into smaller ones by heat. Coal may be pyrolized to form coke, ammonia, and light and heavy oils. Cracking is the pyrolysis of alkanes, particularly petroleum. In thermal cracking, alkanes are simply heated to a high temperature. Large alkanes are converted to smaller alkanes, alkenes, and hydrogen. Cracking of natural gas is conducted at temperatures up to 3000°F (1650°C). In steam cracking, alkanes are diluted with steam, briefly heated to 700-900°F (370-480°C), then rapidly cooled. This method is used to produce many hydrocarbon chemicals, including ethylene, propylene, butadiene, isoprene, and cyclopentadiene. Hydrocracking is conducted in the presence of hydrogen at high pressure and lower temperatures, i.e., 250-450°F (120-230°C). In catalytic cracking, higher boiling petroleum fractions are contacted with a catalyst at moderate temperatures and pressures, i.e., 900°F (480°C) and 8-20 psi (50-140 kPa). This method produces alkanes with the highly branched structures desired in fuels.


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Process Chemistry 1. Pyrolysis of natural gas (methane) in a continuous furnace to produce industrial carbon and hydrogen. + 2H2 CH4 → C methane carbon hydrogen If the gas is heated in an electric arc, acetylene may be formed. By rapid quenching, its decomposition can be prevented: 2CH4 → C2H2 + 3H2 methane acetylene hydrogen 2. Catalytic cracking of butane to form 1,3-butadiene (used in synthetic rubber). CH3CH2CH2CH3 → CH3CH = CHCH3 + CH3CH2CH = CH2 → CH2 = CHCH = CH2 n-butane 2-butene 1-butene 1,3-butadiene Equipment Pyrolysis and cracking equipment is very large and is designed for the specific application. 3.6.26 Reduction Historically, a reduction reaction occurred when oxygen was removed from a molecule. However, reduction is really any process where the number of electrons in the chemical substance increases. In addition to removal of oxygen, reduction reactions also include removal of halogens, sulfur or ammonia as well as the addition of hydrogen to a metal or the generation of hydrogen. Note: the oxidation state of hydrogen is +1 in all its compounds except for those with metals, where it is commonly –1. Reduction reactions are frequently thought of as the reverse of oxidation reactions. Redox reactions occur when reduction and oxidation reactions occur simultaneously. Standard redox or electrode potentials can be used to determine simple reduction potentials. Process conditions vary widely, depending whether organic or inorganic materials are involved. Among the most common organic reduction processes are the manufacture of amines from nitro compounds. These generally involve the generation of hydrogen and subsequent addition of hydrogen to the compound. Hydrazine, diborane, sodium hydride, or hydrogen may be used in organic reduction reactions. The most common inorganic reduction processes are the removal of oxygen from inorganic chemicals, such as the reduction of metal oxides to pure metals such as metallic sodium, aluminum or magnesium. Process Chemistry 1. Reduction of nitrobenzene to aniline by iron, using an acid ferrous chloride catalyst. 4C6H5NO2 + 9Fe + 4H2O → 4C6H5NH2 + 3FE3O4 nitrobenzene iron water aniline iron oxide 2. Reduction of calcium phosphate to prepare phosphorus with carbon monoxide as a by-product. 2P + 5CO Ca(PO4)2 + 3SiO2 + 5C → 3CaSiO3 + calcium sand coke calcium phosphorus carbon phosphate silicate monoxide 3. Iron extraction from haematite in a blast furnace. a) Coke is oxidized to form carbon dioxide C (s) + O2 (g) → CO2 (g) carbon oxygen carbon dioxide b) Carbon dioxide is reduced to carbon monoxide CO2 (g) + C (s) → 2CO (g) carbon dioxide carbon carbon monoxide c) Iron oxide is reduced to iron (with impurities) by carbon monoxide. Fe2O3 (s) + 3CO (g) → 2Fe (s) + 3CO2 (g) iron oxide carbon monoxide iron carbon dioxide Equipment Inorganic reduction is often done in furnaces. ©2003 Factory Mutual Insurance Company. All rights reserved.


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Organic reductions are normally carried out in ordinary steel reactors, suitably designed for the corrosive conditions that may be present. 3.6.27 Reforming Catalytic reforming may result in dehydrogenation, cyclization, and isomerization of alkanes into cycloalkanes and aromatic hydrocarbons. The most common commercial application of these reactions is in ammonia synthesis and petroleum refining. Process Chemistry 1. Reforming of methylcyclohexane to produce toluene, using a catalyst at 560°F (293°C), 300 psi (2.1 MPa) (dehydrogenation). → C6H5CH3 + 3H2 C6H11CH3 methylcyclohexane toluene hydrogen 2. Reforming of 1,3-dichloropropane to produce cyclopropane, using a catalyst and a salt (cyclization). → intermediate → (CH2)3 CH2Cl CH2CH2Cl 1,3-dichloropropane cyclopropane Equipment Ammonia synthesis and catalytic steam hydrocarbon reformers are detailed in Data Sheet 7-94/12-22, Ammonia Synthesis Units and Data Sheet 7-72/12-10, Catalytic Steam-Hydrocarbon Reformers. 3.6.28 Saponification — See Hydration, Hydrolysis and Saponification 3.6.29 Silicon, Silane, Silicone, and Siloxane Silicon (Si) is an element that is a component of many different minerals. It should not be confused with inert silica (SiO2) or silicates (M2O • mSiO2 • nH2O where M is an alkali metal and m and n are the number of moles of SiO2 and H2O, respectively, per mole of M2O) from which it is derived. Silicon is used extensively in the semiconductor and nonferrous metal industries. A silane is a compound containing a hydrogen-silicon bond and is also known as a silicon hydride. Silane (SiH4) is the simplest hydride. Thousands of inorganic silane and organosilane compounds exist. Inorganic silanes undergo many different types of reactions including oxidations and halogenations. Organosilanes also undergo a variety of chemical reactions including additions, aminations, hydrolysis, oxidations, and photolysis. The addition of organosilanes to olefins is called hydrosilylation. Silicone, RnSiO(4-n)/2m,is a synthetic polymer where n = 1-3 and m≤2. A silicone has a repeating silicon-oxygen backbone and has organic groups attached to a significant number of the silicon atoms by silicon-carbon bonds. Preparation of silicone involves a number of steps. Simply, a silane monomer is hydrolyzed to siloxane, the siloxane is hydrolyzed to an intermediate, and then the intermediate is rearranged, reduced, polymerized or copolymerized. The final product could be a fluid, a resin, or an elastomer. Silicone elastomers are further categorized by their cure-system chemistry, e.g., room-temperature-vulcanizing (RTV) or heat-cured. Silicones have several unusual properties that make them commercially important. Silicones are relatively stable and inert. They are resistant to weathering, have good dielectric strength, and have a low surface tension. Silicone fluids are used as hydraulic fluids, lubricants, surfactants, and for waterproofing. Silicone resins are used in adhesives, electrical insulations, laminates, paint, and varnishes. RTV elastomers are used in adhesives, electrical insulations, gaskets, glazings, and sealants. Heat-cured elastomers are used in belts, calendering rollers, fabric coatings, hoses, and penetration seals. Process Chemistry 1. Chlorination of silicon using methyl chloride in the presence of a copper catalyst and at elevated temperatures to produce methylchlorosilanes. CH3Cl + Si → CH3SiCl3 + (CH3)2SiCl2 + (CH3)3SiCl methyl silicon methyltrichlorosilane dimethyldichlorosilane trimethylchlorosilane chloride complex


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2. Chlorination of silicon, ferrosilicon, or calcium silicide in the presence of a copper catalyst to produce trichlorosilane. HCl + Si → HSiCl3 hydrogen chloride silicon complex trichlorosilane 3. Reduction of hydrochloric acid and silane to produce chlorosilane, using an aluminum chloride catalyst. Hydrogen is generated. HCl (g) + SiH4 (g) → SiH3Cl (g) + H2 hydrochloric acid silane chlorosilane hydrogen Equipment Because of the wide range of silicon, silane, silicone, and siloxane processes, equipment must be designed for the specific application taking into consideration material characteristics and process conditions. 3.6.30 Sulfonation Sulfonation is the process by which the sulfonic acid group, HSO3, is added to a carbon in an organic compound. Normally, this is done by reacting a compound with SO3, H2SO4 , or oleum (SO3 dissolved in H2SO4). SO3 is the most vigorous sulfonating agent, but it has a tendency to initiate side reactions with materials that are easily sulfonated. Therefore, the sulfonating agent used is generally no stronger than necessary. Most sulfonation reactions are readily reversible. Sulfonation is one of the reactions used in dissolving lignin in the processing of sulfite pulp in paper mills. It is also used in the manufacture of detergents. Process Chemistry 1. Sulfonation of phenol to o-phenolsulfonic acid at 15 to 20°F (-9 to -7°C) and p-phenolsulfonic acid at 100°F (38°C). C6H5OH + H2SO4 → C6H4(OH)(SO3H) phenol sulfuric acid phenolsulfonic acid 2. Sulfonation of an alcohol to a sulfate. n-C11H23CH2OH + H2SO4 → n-C11H23CH2OSO3H lauryl alcohol sulfuric acid lauryl hydrogen sulfate

+

NaOH → sodium hydroxide

C11H23CH2OSO3-Na+ sodium lauryl sulfate

Equipment Sulfonations may be conducted in ordinary batch reactors equipped with agitators, heat transfer systems and condensers. They may also be conducted in continuous tube reactors with tube-bundle reactors (about 100 one inch (2.5 cm) diameter tubes inside a water-cooled vessel) and falling film reactors (single 30 in. (72.2 cm) ID tube) being common. Equipment should be constructed of materials able to withstand concentrated sulfuric acid, such as glass-lined steel. 3.7 Water-Reactive Materials Water-reactive materials are labeled as such because they may undergo hazardous hydrolysis or reduction-oxidation (redox) reactions with water. Most water-reactive materials undergo exothermic hydrolysis reactions. It is not the type of reaction, but the rate of the reaction and the characteristics of the reaction products that cause concern. The rapid reaction rate results in significant heat generation and frequently gaseous products that are flammable and/or toxic. Examples include aluminum carbide and water reacting to form methane and calcium carbide and water reacting to form acetylene (see Hydrolysis reactions). Certain elements, such as phosphorus and sodium, undergo redox reactions. The element is oxidized by water, the reducing agent. Again, the rapid reaction rate results in significant heat generation and frequently the generation of gaseous reaction products that are flammable and/or toxic. For example, sodium metal in water produces hydrogen gas that is ignited by the heat of the reaction.



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3.8 Reaction Quenching Methods When application of emergency cooling won’t stop a runaway reaction, quenching the reaction is usually the next action taken. This can be done by adding an inhibitor, rapidly adding a cold liquid, and/or dumping the reactor contents. Inhibitors work by counteracting the catalyst or by complexing with the free radicals. For example, hydroquinone (an antioxidant) is used to inhibit some vinyl monomer polymerizations. Cold liquids act by decreasing the temperature of the reactor contents. The cold liquid must be compatible with the reactor contents and introduced at a sufficient pressure and flow rate to be effective (need to overcome runaway reaction pressure and not flash away). The reactor must be large enough to contain the combined volume or provided with overflow to a safe location. The thermal stress caused by quickly adding cold liquid to the equipment should be considered. The potential for boil-over should also be considered. Examples include gravity addition of a diluent and high-pressure waterspray inside the reactor. Dumping acts by rapidly discharging the reactor contents into another vessel. Bottom outlet discharge may be via gravity or pressurization of the reactor. It is critical that the dumping be completed before the runaway reaction compromises the reactor. The receiving vessel may be precharged with an inhibitor or a cold liquid and should be provided with a vent line. 3.9 Unstable Materials and Explosion Hazards due to Uncontrolled Chemical Reactivity Uncontrolled chemical reactivity has resulted in numerous explosions and has caused some of the most severe losses in the chemical processing industries. Explosions cause direct damage, and may also cause ensuing fires while disabling protective equipment (see Data Sheet 7-0, Causes and Effects of Fires and Explosions, for additional information). The type of explosion depends on the energy of the material, its reaction kinetics, the mode of ignition, and the nature of the confinement (Fig. 10).

Fig. 10. Types of Explosions.


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Chemical explosions are those that involve a chemical reaction. Chemical explosions manifest themselves as flammable vapor-air explosions and as dust explosions. Physical explosions are those that result from physical failure of a container. This is usually due to overpressurization. It may also result from a defect or weakened spot in the container, causing it to fail under normal pressures. Thermal explosions are a result of an exothermic reaction occurring under conditions of confinement with inadequate means of removing the heat of reaction (Fig. 11). Such reactions can accelerate to the point where high-pressure gases are generated, the container ruptures, and an explosion ensues. Thermal explosions may occur when: 1) two chemicals accumulate in the absence of a catalyst that is later added to initiate the reaction, 2) cooling is interrupted during a chemical reaction, or 3) two chemicals that react on contact are layered, then mixed rapidly. In a thermal explosion, no reaction front is present; it is therefore called a homogeneous explosion.

Fig. 11. Thermal Explosion Heat-Temperature Graph. Note: O’Brien, G.J., et al. Thermal Stability Hazards Analysis, Chemical Engineering Progress, January 1982

Deflagrations are chemical explosions that propagate, by heat conduction, at subsonic velocities; velocities may vary from slow (1 mm/min) to fast (1,000 m/sec). Deflagrations can be successfully vented. Deflagration pressures in closed equipment can reach eight to ten times the initial absolute pressure. Even higher pressures may occur in compartmented or interconnected equipment due to pressure piling effects. A deflagration may accelerate to a detonation. This is known as a deflagration to detonation transition (DDT). Detonations are chemical explosions that propagate, via a shock wave, at supersonic velocities; velocities may range from 3,280 to 19,700 ft/sec (1,000 to 6,000 m/sec). Detonations proceed so quickly that they cannot be successfully vented. Detonation pressures can reach 30 times the initial absolute pressure with reflected pressure even higher. Figure 12 is a schematic representation of the differences in pressure-time history of deflagrations and detonations.



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Both deflagrations and detonations have a reaction front that separates reacted and unreacted material; they are therefore called heterogeneous explosions.

Fig. 12. Deflagration versus Detonation Pressure-Time Graph.

3.10 Experimental Screening and Thermal Hazard Analysis Mechanical sensitivity is divided into sensitivity to impact (shock) and sensitivity to friction. Shock sensitive materials react exothermically when subjected to a pressure pulse and condensed-phase detonations may occur. Materials that do not exhibit an exotherm during thermal stability testing are presumed not to be shock sensitive. Methods include the drop weight, confinement cap, card gap, and adiabatic compression tests. The drop weight test is conducted by dropping a weight on a sample in a metal cup. The weight and height can be varied to provide a qualitative measure of the sample’s susceptibility to decompose upon impact. This test should be applied to any material known or suspected to contain unstable chemical functional groups. The confinement cap test is conducted using a blasting cap to determine sensitivity. The card gap test uses a shock wave generated by an explosive charge to determine whether a propagating reaction occurs. The adiabatic compression test is conducted by rapidly applying high pressure to a liquid in a U-shaped metal tube, then forcing hot compressed gas into the liquid. This test was designed to simulate water hammer and sloshing effects during transportation. Sensitivity to friction tests may be conducted on solids and semi-solids by placing a sample between two rough surfaces, then rubbing the surfaces together. Methods include the ABL, BAM, and rotary friction test. Thermal stability is determined using reaction and adiabatic calorimetry. The adiabatic temperature rise is a good measure of the thermal hazard of a chemical system since it is the maximum possible temperature increase. Reaction calorimetry techniques include differential scanning calorimetry (DSC), mixing cell calorimetry (MCC), and PHI-TEC I™. Adiabatic calorimetry techniques include accelerating rate calorimetry (ARC), the DIERS vent-sizing package (VSP) and PHI-TEC II™. See Data Sheet 7-49/12-65, Emergency Venting of Vessels for instrument details.


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DSC scans are generally the first screening tests performed. These tests determine the temperature and enthalpy of exothermic reactions. A total heat release of as little as 200-300 J/g could present a runaway reaction potential, depending on the process environment. ARC scans help determine the reaction kinetics. ARC scans will also measure the rate of pressure increase, providing information on the generation of volatile materials. The Mettler RCl™ adiabatic reaction calorimeter permits measurement of the heat generation rate as a function of time and the heat of reaction. Heat transport data, between a reaction mixture and the heating or cooling media, is also measured. VSP tests are frequently used to determine the worst credible case scenario. The Reactive System Screening Tool (RSST) and the Advanced Reactive System Screening Tool (ARSST) are calorimeters that can be used to determine the potential for runaway reactions, and measure the rate of temperature and pressure rise for gassy reactions. The temperature and pressure rises are used to determine the energy and gas release rates. The ARSST has increased temperature ramps to simulate fire exposure, a heat-wait-search operation mode with increased onset detection sensitivity, and an isothermal operation at elevated temperature mode. 3.11 Scale-up Effects Chemical reactions frequently behave very differently when conducted in large-scale commercial equipment rather than in small-scale laboratory equipment. Problems observed on a commercial scale, but not necessarily on a lab scale, include non-uniform distribution of gas, settling of solids, phase separation, foaming and localized heating. In many cases, the effect of these variables can be calculated or modeled. In other cases, only carefully designed pilot plant studies can reliably provide needed chemical kinetic, self-heating, mass transfer and heat transfer information. Specially designed reactors may be used for this work (autoclaves, high-heat-flux reactors, microreactors, etc.). Optimizing product yields involves changing process procedures (addition sequences, addition times, hold times, etc.), process conditions (concentrations, temperature profiles, pressure profiles, etc.), and/or process equipment. Obviously, this will affect the reaction and its rate; it should be carefully studied before implementing in large-scale equipment. 3.12 Chemical Risk Assessment and Management Consequence screening usually consists of a blast or explosion evaluation and a structural assessment of the process structure or building and the surrounding structures or buildings. The blast evaluation should analyze all potential explosion sources for magnitude and duration. These sources should including vapor cloud explosions (VCEs), condensed phase reactions, uncontrolled chemical reactions, boiling liquid expanding vapor explosions (BLEVEs), pressure vessel ruptures, and other physical explosions as applicable. The structural assessment should determine the overpressure resistance of the process structures and/or buildings within the significant blast or explosion overpressure ring area. Consequence screening results can then be used in risk management. 3.13 General Chemical Processing The chemical processing industry can be broken down into two manufacturing categories: commodity and fine chemicals. Commodity chemicals are those produced and delivered in bulk and frequently used in the manufacture of other products. The most widely produced chemicals vary from year to year, but usually include the following: sulfuric acid, nitrogen, oxygen, ethylene, lime, ammonia, phosphoric acid, sodium hydroxide, propylene and chlorine. Fine chemicals production is on a much smaller scale and includes active pharmaceutical ingredients. Regardless of the type of process and production levels, the inherently safer design concepts of intensification, substitution, attenuation, and limitation should be applied. See Data Sheet 7-43/17-2, Loss Prevention in Chemical Plants for details.



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3.14 Reactor Selection The selection of the type of reactor for a chemical process is based on many factors including mode of operation, material phases, chemical hazards, reaction kinetics, desired production rate, economics and process safety. Reactors are designed for batch, semi-batch or continuous operation. Batch processes are primarily associated with small production rates, long reaction times, and selective chemistry requirements. Continuous processes are associated with high production rates and short reaction times. In batch reactors, material composition and heat generation rates change over time. In continuous reactors, material composition and heat generation remains constant unless process conditions are changed. A batch reactor is an agitated vessel in which the reactants are all added at the start of the reaction and the reaction products are removed after a fixed time. Agitation is usually provided by internal impellers, gas bubbles, or pumped recirculation. Temperature control is usually provided by jackets, reflux condensers, and/or pumped recirculation through an exchanger. A semi-batch reactor is similar to a batch reactor, except that some of the reactants are fed intermittently or continuously into the vessel. This arrangement is commonly used when feed rates need to be controlled. Exothermic reactions can be slowed down and endothermic rates maintained by limiting the concentrations of some of the reactants. This arrangement is also used to increase product through-put in cases where the product volume is less than the reactant volume in the vessel. A continuous reactor is a vessel in which the reactants are being continuously added at one point and the products are being continuously removed at another point. Reactor selection also depends on the phase of the materials being processed. There are vapor phase reactors, liquid phase reactors, solid phase reactors, gas/liquid reactors, and gas/liquid/solid reactors. Catalysts may be retained in the vessel or separated out downstream. Beds may be fixed, moving, or fluidized. Gas/liquid reactors include tray tower, packed tower, falling liquid film, spray tower, bubble tower, venturi mixer, static in-line mixer, tubular flow, looped, and stirred tank. Liquid/liquid reactors include tray tower, packed tower, spray tower, rotating disk contactor, and stirred tank. Gas/liquid/solid reactors include trickle bed, flooded fixed bed, fluidized bed, bubble columns, slurry, and stirred tank. Solid reactors include horizontal rotary kilns, vertical kilns, multiple hearth, fluidized bed, and vertical moving bed (blast furnace). The bottoms of these reactors may be cone, slope, or dish shaped. The two most common reactor types are continuous-flow stirred tank reactors (CSTR) and tubular flow reactors (TFR). Continuous Stirred Tank Reactors (CSTR) are continuously stirred to maintain uniform concentrations within the reactor. They are adaptable to either batch or continuous operation and may be used in series. They are best suited for small or medium production rates. They can be used for a wide range of pressures and temperatures. Agitation is furnished by stirrer blades, or by forced circulation using external pumps. Jacketed walls, internal coils, external heat exchangers, direct fire, or electric resistance heaters can be used to provide heat transfer. If the reaction occurs with evolution of vapors, reflux condensers can be used for cooling. CSTRs are generally cylindrical vessels, installed vertically (Fig. 13). Horizontal reactors are used for processing of slurries where greater liquid surface is desired, where the boiling point rise due to hydrostatic heat is a concern, where headroom is limited, or if the material is very viscous. Bioreactors are CSTRs used in biochemical and biotechnology applications (Fig.14). High heat flux process reactors are a relatively new application for CSTRs that employ a cryogenic heat transfer system. Tubular flow reactors (TFRs) have continuous concentration gradients in the direction of flow as the reactants are continuously fed in one end and the products are continuously removed from the other end. They are used in continuous operations with medium to high production rates. TFRs may have several pipes or tubes in parallel. Individual tubes are jacketed or shell-and-tube construction is used when auxiliary heat transfer is needed; with shell-and-tube construction, the reactants may be on either the shell or the tube side. The reactant side may be filled with solid particles, either catalytic or inert, to improve heat transfer by increased turbulence or to improve interphase contact. Both horizontal and vertical orientations are common. Tubular reactors are also known as plug flow, slug flow, and piston flow reactors. Plug flow reactors (PFRs) are large-diameter tubular flow reactors with packing or trays that approach plug flow behavior (Fig. 15).


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Membrane reactors are plug-flow reactors that contain an internal cylinder of porous material, the membrane (Fig. 16). The membrane is a barrier that allows only certain components to pass through; selectivity is based on pore diameter. Catalytic membrane reactors combine reaction with separation to increase conversion and are used for catalyzed reactions such as dehydrogenations. Two types of catalytic membrane reactors are commercially available: the inert membrane reactor with catalyst on the feed side (IMRCF) and the catalytic membrane reactor (CMR). The IMRCF has an inert membrane that contains the catalyst. The CMR membrane is either coated with or made out of a material containing the catalyst. Tube furnaces are a type of tubular flow reactor. They consist of a combustion chamber lined with refractory with tubes mounted on the walls and ceiling and sometimes on the floor. In the radiant section, the tubes are in direct view of the flames. In the convection section, tubes serve to preheat the charge, to maintain the reaction temperature attained in the radiant section, and/or to recover heat by preheating combustion air or generating steam. Microreactors are also a type of tubular flow reactor. Micrometer-scale channels and chambers are used to conduct reactions. Because of the very high surface to volume ratio, good control of extremely exothermic reactions is possible. Custom microreactors are being developed for continuous processes and may be used in parallel to feed downstream equipment. Microreactors, at this time, are best suited for conducting gasphase reactions with very small production rates (lb/day). They can be used over a wide range of pressures and temperatures. The hazard is minimal due to the small quantities of material present.

Fig. 13. Pfaudler CSTR.



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Fig. 14. Braun bioreactor.

Table 5. Advantages and Disadvantages of Various Reactor Types Reactor Type Batch

Semi-batch CSTR

TFR

Advantages Agitation

Agitation Addition rates controllable Agitation Addition rates controllable Stationary conditions

Low inventory High throughput rate Stationary Conditions

Disadvantages Large inventory Low throughput rate Heat transfer problems Cycling effects Potential accumulation of reactants Precipitation problems Large inventory Low throughput rate Heat transfer problems Precipitation problems Potential for hot spots Agitation only if in-line mixers are available Inflexible


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Fig. 15. Plug Flow Reactor.

Fig. 16. Membrane Reactor.



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3.15 Reactor Aging and Corrosion Resistance Accelerated aging and corrosion can result in unexpected equipment failure. Inspections alone may not be an adequate predictor of reliability. It is important to understand the factors that influence corrosion, the effect corrosion may have on the chemical process, and the effect corrosion may have on the reliability of the process equipment. Changes in process chemistry, even relatively minor ones, may significantly change corrosion resistance. Frequent start-ups and shutdowns accelerate aging and may cause corrosion fatigue. Corrosion may be localized. Pitting is a form of very localized corrosion. Crevice corrosion occurs within or adjacent to a crevice. Oxygen-Concentration corrosion frequently occurs under gaskets. Galvanic corrosion occurs when dissimilar metals are used in contact with each other and are exposed to an electrically conducting solution. Intergranular corrosion occurs when there is selective corrosion in the grain boundaries of a metal or alloy without a significant attack on the grains; this is unusual in that the loss of strength and ductility is not proportional to the amount of metal lost due to the corrosion. Stress corrosion occurs from either internal stress or externally applied stress and can result in cracking. Liquid-metal corrosion occurs when liquid metal attacks metal grain boundaries, potentially resulting in catastrophic failure. Erosion is a form of corrosion associated with material flow. Cavitation corrosion occurs when bubbles collapse on a surface. Fretting corrosion occurs when metal pieces slide over one another. Hydrogen embrittlement occurs when a significant amount of hydrogen is present at elevated temperatures; the National Association of Corrosion Engineers has published charts detailing recommended operating limits for various hydrogen partial pressures, temperatures and steel alloys. Corrosion may also be structural. Graphite corrosion usually involves gray cast iron, or carbon steel heated above 455°F (235°C) for prolonged periods and occurs when the metal is converted into corrosion products. Dealloying corrosion occurs when only one alloy is attacked by corrosion; the most common type is dezincification of brass. Biological corrosion occurs due to metabolic activity of microorganism; microbiologically induced corrosion (MIC) is one type of biological corrosion that occurs when the microorganism attaches to a metal. As a rule-of-thumb, one volume of steel yields twenty volumes of corrosion. This accounts for what is known as rust-jacking, a process whereby rust forces surfaces apart. The following conditions may increase corrosion: • Chlorides • Water: presence or absence • pH • High Temperature • High Flow Rates • Low Oxygen Concentration • Oxidizing Agents • Environmental Conditions: internal and external • Equipment Stress • Equipment Arrangement The presence of chlorides increases localized corrosion problems. Pitting of aluminum and stainless steel in aqueous solutions containing chlorides is an example. Inhibitors can be used to prevent pitting. Chlorides may be present in either process materials or utilities. Water sometimes contributes to corrosion. The presence of water may increase the corrosiveness of non-aqueous compounds. The absence of water may increase the corrosiveness of some organic halides. pH effects the stability of the oxide films on metal alloys. Significant changes in pH may change the corrosivity of some chemicals, particularly the halides.


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High temperature and hot spots increase corrosion rates. Temperature also indirectly affects corrosion rates by impacting the solubility of air (oxidizer) and material phases. High flow rates, or velocity, can increase abrasion and friction resulting in erosion. Using harder materials, reducing flow rates, and minimizing directional changes can minimize erosion. Oxidizing agents may accelerate or retard corrosion depending on the characteristics of the metal. Reduced oxygen concentrations accelerate corrosion. Environmental conditions, both internal and external, affect corrosion rates. Essentially every metal alloy has environmental conditions that produce stress corrosion cracking. Steel cracking from caustic is an example. External chloride corrosion may result from external insulation materials, chemical spills, sea mist, and road salt. Reference Data Sheet 12-2, Pressure Vessels. Equipment stress may be caused during fabrication, by unbalanced cooling from high temperature or by external modifications (rivets, bolts, etc.). Equipment arrangement may also affect corrosion. Galvanized steel should not be welded to stainless steel unless the galvanizing is completely removed from the area. Glass-lined reactors are commonly supplied with a conventional jacket or with a half-pipe jacket constructed by welding a half-pipe coil around the outside of the tank. The conventional jacket is less susceptible to crevice corrosion and provides better thermal shock protection. Improper agitation and pumping can generate an excessive amount of bubbling that results in mechanical damage. Friction caused by metal pieces sliding over one another, if not properly lubricated, wears away metal surfaces. The resistance of materials to similar environments and process conditions is a good screening tool. However, materials selected should be evaluated under proposed conditions whenever possible. Supercritical process conditions are particularly challenging due to accelerated corrosion from the high temperatures and pressures, and shifting of reaction mechanisms. The general applicability of materials in oxidizing and reducing acids is shown in Figure 17. Chromic, nitric, and >70% sulfuric acid are common oxidizing acids. Formic, hydrochloric, phosphoric, and <70% sulfuric are common reducing acids. Nonmetals, such as plastics, may exhibit deterioration similar to corrosion. However, this is physiochemical deterioration rather than corrosion. If non-repairable materials of construction are used (e.g., graphite), or if the materials of construction are exotic or special (long lead times), then it is prudent to order complete sets of spare components such as tube bundles and wear parts. This will ensure shorter down times and help protect customer marketshare. 3.16 Illustrative Losses Chemical processing and related industries continue to experience incidents involving process chemistries and process equipment. Subsequent investigations of these incidents confirmed that damage could have been prevented or at least significantly minimized had inherently safer design concepts been used, the chemical hazards been completely understood, and/or effective process safety management programs been in place.



Fig. 17. Applicability of Materials in Oxidizing and Reducing Acids Illustration courtesy of Te-Lin Yu Consultancy


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4.0 REFERENCES 4.1 FM Global Data Sheet 5-8, Static Electricity. Data Sheet 5-10, Protective Grounding for Electric Power Systems and Equipment. Data Sheet 5-11, Lightning and Surge Protection for Electrical Systems. Data Sheet 5-20, Electrical Testing. Data Sheet 5-23, Emergency and Stand-By Power Systems. Data Sheet 6-0/12-1, Elements of Industrial Heating Equipment. Data Sheet 6-9, Industrial Ovens and Dryers. Data Sheet 6-10, Process Furnaces. Data Sheet 6-11, Fume Incinerators. Data Sheet 6-17/13-20, Rotary Kilns and Dryers. Data Sheet 7-0, Causes and Effects of Fires and Explosions. Data Sheet 7-2, Waste Solvent Recovery. Data Sheet 7-13, Mechanical Refrigeration. Data Sheet 7-32, Flammable Liquid Operations. Data Sheet 7-34, Electrolytic Chlorine Processes. Data Sheet 7-43/17-2, Loss Prevention in Chemical Plants. Data Sheet 7-44/17-3, Spacing of Facilities in Outdoor Chemical Plants. Data Sheet 7-45, Instrumentation and Control in Safety Applications. Data Sheet 7-47, Physical Operations in Chemical Plants. Data Sheet 7-49/12-65, Emergency Venting of Vessels. Data Sheet 7-50, Compressed Gases in Cylinders. Data Sheet 7-51, Acetylene. Data Sheet 7-52/17-13, Oxygen. Data Sheet 7-59, Inerting and Purging of Equipment. Data Sheet 7-72/12-10, Catalytic Steam-Hydrocarbon Reformers. Data Sheet 7-74, Distilleries. Data Sheet 7-80, Organic Peroxides. Data Sheet 7-82N, Storage of Liquid/Solid Oxidizing Materials. Data Sheet 7-89, Ammonium Nitrateand Mixed Fertilizers Containing Ammonium Nitrate. Data Sheet 7-91, Hydrogen. Data Sheet 7-94/12-22, Ammonia Synthesis Units. Data Sheet 7-99/12-19, Heat Transfer by Organic and Synthetic Fluids. Data Sheet 12-2, Pressure Vessels.



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4.2 NFPA National Fire Protection Association (NFPA): 30: Flammable and Combustible Liquids 30B: Manufacture and Storage of Aerosol Products 35: Manufacture of Organic Coatings 49: Hazardous Chemicals Data 70: National Electrical Code (NEC) 269: Standard Test for Developing Toxic Potency Data for Use in Fire Hazard Modeling 325: Guide to Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids 329: Handling Releases of Flammable and Combustible Liquids and Gases 430: Storage of Liquid and Solid Oxidizers 432: Storage of Organic Peroxide Formulations 481: Production, Processing, Handling, and Storage of Titanium 482: Production, Processing, Handling, and Storage of Zirconium 485: Storage, Handling, Processing, and Use of Lithium Metal 491: Guide to Hazardous Chemical Reactions 654: Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids 655: Prevention of Sulfur Fires and Explosions 4.3 Other Codes, Regulations and Standards Worldwide, there are numerous codes, regulations, and standards in effect today dealing with the chemical process industries. A list of well-known codes, regulations and standards, as well as organizations that develop standards, is provided below. European and UK: EC Framework Directive from Article 118A of the Treaty of Rome, 1989 Revised EC Directive issued as Seveso II (96/82/EC), 1996 UK Control of Industrial Major Accident Hazards (CIMAH), 1984 UK Control of Major Accident Hazards (COMAH), 1999 UK Management of Health and Safety at Work Regulations, 1992 (under the 1974 Health and Safety at Work Act) USA: American Institute of Chemical Engineers (AIChE): CCPS Guidelines for Hazard Evaluation Procedures CCPS Guidelines for Chemical Process Quantitative Risk Analysis CCPS Guidelines for Technical Management of Chemical Process CCPS Guidelines for Pressure Relief and Emergency Handling Systems CCPS Guidelines for Safe Automation of Chemical Processes CCPS Guidelines for Chemical Reactivity Evaluation and Applications to Process Design CCPS Guidelines for Preventing Human Error in Process Safety CCPS Guidelines for Process Safety Fundamentals for General Plant Operations CCPS Guidelines for Engineering Design for Process Safety CCPS Guidelines for Implementing Process Safety Management Systems CCPS Guidelines for Safe Process Operations and Maintenance CCPS Guidelines for Process Safety in Batch Reaction Systems CCPS Process Equipment Reliability Database CCPS Inherently Safer Processes: A Life Cycle Approach American Society of Mechanical Engineers (ASME): Boiler and Pressure Vessel Code Section VIII American Petroleum Institute (API): RP 12R1: Setting, Maintenance, Inspection, Operation, and Repair of Tanks in Production Service RP 500: Recommended Practice for Classification of Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2


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RP 500: Recommended Practice for Classification of Electrical Installations at Petroleum Facilities Classified as Class I, Zone 0, Zone 1, and Zone 2. RP 520/1: Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries RP 750: Management of Process Hazards RP 752: Management of Hazards Associated with Location of Process Plant Buildings RP 920: Prevention of Brittle Fracture of Pressure Vessels RP 2003: Protection Against Ignitions Arising out of Static, Lightning, and Stray Currents Std 510: Pressure Vessel Inspection Code Instrument Society of America (ISA) — S84.01, Application of Safety Instrumented Systems for the Process Industries National Association of Corrosion Engineers (NACE): numerous Standards and technical publications US Occupational Safety and Health Administration (OSHA) — Process Safety Management (PSM) 29 CFR 1910.119. US Environmental Protection Agency (EPA) — Risk Management Program (RMP) 40 CFR 68 APPENDIX A GLOSSARY OF TERMS A.1 Acronyms and Abbreviations AIChE — American Institute of Chemical Engineers AIT — autoignition temperature ARC — accelerating rate calorimetry ARSST — advanced reactive system screening tool ASME — American Society of Mechanical Engineers AST — adiabatic storage test ASTM — American Society for Testing and Materials BLEVE — boiling liquid, expanding vapor explosion CARAT — Chemical Accident Risk Assessment Thesaurus CART — calculated adiabatic reaction temperature CCPS — Center for Chemical Process Safety CHA — chemical hazard analysis CHETAH — Chemical Thermodynamics and Energy Release Evaluation (ASTM) CMR — catalytic membrane reactor CODATA — International Council for Science Committee of Data for Science and Technology CSTR — continuous stirred tank reactor DDT — deflagration to detonation transition DI — deionized DIERS — Design Institute for Emergency Relief Systems (AIChE) DIPPR — Design Institute for Physical Property Data (AIChE) DSC — differential scanning calorimetry DTA — differential thermal analysis FEMA — failure mode and effect analysis FTA — fault tree analysis HAZOP — hazard and operability study IMRCF — inert membrane reactor with catalyst on the feed side IUPAC — International Union of Pure and Applied Chemistry LEL — lower explosive limit LFL — lower flammable limit LOC — Limiting Oxygen Concentration MCC — mixing cell calorimetry MIC — microbial induced corrosion MOC — management of change MOCC — minimum oxygen concentration for combustion NACE — National Association of Corrosion Engineers NFPA — National Fire Protection Association NIST — National Institute of Standards and Technology OD — outer diameter



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PFR — plug flow reactor P&ID — process and instrumentation diagram PHA — process hazard analysis PSM — process safety management REITP2 — Revised Program for Evaluation of Incompatibility from Thermodynamic Properties RSST — reactive system screening tool SIL — safety interlock level SOP — standard operating procedure TFR — tubular flow reactor UEL — upper explosive limit UFL — upper flammable limit VCE — vapor cloud explosion VSP — vent sizing package A.2 Definitions Activation Energy (Ea): the critical energy needed for a reaction to occur. Activation Entropy (Sj): the relative position or orientation of the molecules needed for a reaction to occur. Adiabatic: a condition where no heat is exchanged between a system and its surroundings. Anion: an atom or molecule with a negative charge. Anode: the negative electrode at which oxidation occurs. Arrhenius Equation: k = Ae-Ea/RT

where k is the rate constant A is the frequency factor Ea is the activation energy R is the gas constant T is temperature

Autoignition temperature (AIT): the minimum temperature required to initiate self-sustained combustion in air, independent of the ignition source. For straight chain hydrocarbons, increasing the chain length decreases the AIT. Autoxidation: self-heating via slow oxidation. Boiling Point Elevation: an increase in the boiling point of a solution, proportional to the concentration of the solute. Catalyst: a substance that increases the rate of a reaction, but is recovered unchanged at the end of the reaction. Cathode: the positive electrode at which reduction occurs. Chemical hazard assessment: formal process for identifying and quantifying reactive chemical hazards. Condensed Phase Explosion: an explosion of a liquid or solid. Corrosion: the loss of metal due to chemical or electrochemical attack. Critical Point: the highest temperature and pressure at which a pure material can exist at vapor-liquid equilibrium. Diastereomers: molecules that are chemically similar, but physically different. They are not isomers. Enthalpy (∆H): the heat content of a substance or the heat of reaction. Compounds that release energy when formed usually have a negative enthalpy. Entropy (∆S): the degree of disorder in a chemical system. Reaction systems that result in greater disorder have positive entropy. Enantiomers: isomers that are mirror images of each other. They have identical physical and chemical properties except toward optically active reagents. Failure Modes and Effects Analysis (FMEA): a hazard identification technique where all known failure modes are considered and potential undesired outcomes are detailed.


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Fault Tree Analysis: a hazard frequency estimation based on a logic model of the failure mechanisms of a system. Gas Laws: Ideal: PV = nRT, where P is pressure V is volume n is the number of moles of material R is the gas constant T is temperature Avogadro: V1 N2 = V2N1 where P, T are constant Boyle: P1V1 = P2V2 where n, T are constant (volume varies inversely with pressure) Charles: T1V2 = V2T1 where n, P are constant (volume varies directly with temperature) Dalton: Ptotal = p1 + p2 + … where T, V are constant (total pressure equals sum of partial pressures) Flammable Limits: minimum and maximum concentrations of a flammable vapor or gas/air mixture that will propagate a flame (flash) when ignited. The currently accepted test method for determining flammability limits is ASTM E 681. Note: lower flammable limit (LFL) and upper flammable limit (UFL) are often used interchangeably with lower explosive limit (LEL) and upper explosive limit (UEL). Gibb’s Free Energy: a thermodynamic quantity measuring reactivity by combining enthalpy and entropy. ∆G = ∆H - T∆S Inhibitor: a substance that retards a chemical reaction, usually by affecting the required activation energy. Inhibitors are also commonly called negative catalysts or reaction poisons. Hazard Analysis: the systematic identification of chemical or physical characteristics and/or processing conditions and/or operating conditions that could lead to undesired events. Hazard and Operability Study (HAZOP): a method used to identify potential process hazards and operating problems using guidewords. Heterogeneous Equilibrium: equilibrium involving more than one phase. Homogeneous Equilibrium: chemical equilibrium established in one phase. Isoperibolic calorimetry: a system where the controlling external temperature is constant; external temperature is compared to internal temperature to determine onset temperature. Isothermal: a system where the (internal and external) temperature is constant; internal temperature changes are quickly manifested as pressure and volume changes. Le Chatelier’s principle: when a system is disturbed, it adjusts to minimize the disturbance. Limiting Oxygen Concentration (LOC): the concentration of oxidant below which flame propagation can not occur. For most hydrocarbons where oxygen is the oxidant and nitrogen is the diluent, the LOC is 9-11% oxygen, by volume. Carbon dioxide is soluble in many liquids and will react with many alkalines. Therefore, its use in inerting is limited. Where carbon dioxide is the diluent, the LOC is about 13% oxygen, by volume, due to its higher specific heat. Metallocenes: metallocenes are single-site catalysts that provide greater control over molecular chain length and structure. They are used in polyolefin production. Minimum Oxygen Concentration for Combustion (MOCC): see Limiting Oxygen Concentration. Mitigation: reduction of risk through action. Onset Temperature: temperature at which a self sustaining chemical reaction can occur. Oxidant: any material that can react with a fuel producing combustion. It is also a substance that is responsible for the oxidation of another substance. Oxygen is the most common oxidant. Oxide: a compound of oxygen and another element. Oxime: a derivative of ammonia, C = NOH



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Oxirane: another name for an epoxide ring. Racemic modification: an equal mixture of enantiomers. It is optically inactive. Reductant: a substance that is responsible for the reduction of another substance. Risk: the probability that an event will occur times the magnitude of the consequence. Risk Analysis: a qualitative or quantitative estimate of risk. Risk Assessment: use of risk analysis results to make business decisions. Runaway reaction: an uncontrollable, accelerating reaction rate which results in rapid increases in temperature and pressure. The heat generated exceeds the heat removed by the cooling system and/or vaporization. Salt: any substance that yields ions other than H+ and OH-; usually a solid with metallic and non-metallic elements. Silane or silicon hydride: general formula SinH2n+2. Analogous to saturated hydrocarbons in structure. Solubility: the ability or tendency of one substance to blend uniformly with another, e.g., solid in liquid, liquid in liquid, gas in liquid, gas in gas. For liquids and solids, it is the upper concentration limit of the solute in the solvent. For gases, it is ratio of the gas in the solution/solvent to the gas above the solution/solvent. Solute: the substance that is dissolved in a solution. Solvent: see Solubility. Spontaneous ignition: see autoxidation. Stereoisomers: isomers that are different from each other only in the way they are oriented. Vapor pressure: the pressure exerted when a solid or liquid is in equilibrium with its own vapor. The higher the vapor pressure at standard temperature and pressure, the easier a liquid will evaporate. Worst Credible Case: an event or combination of events that could result in a runaway reaction that would create the largest or fastest pressure excursion. Zeolite: a member of a family of minerals called tectosilicates. There are 40 known naturally occurring zeolites and more than 150 synthetic zeolites. These inorganic materials are frequently used as a catalyst in environmental applications (reduction of NOx, VOC removal) and are seeing increased use in the chemical processing industries. A.3 Chemical Functional Groups -C-C-C=C-C≡C-C=C=C-C=C-C=CAr-R

alkane alkene alkyne allene diene arene

RCHO R1R2C=O -C=C-C=O

aldehyde ketone unsaturated carbonyl

R-OH Ar-OH RC=O RR’C=O -COOH RCOOH R(COOH)2

alcohol phenol acyl carbonyl carboxyl carboxylic acid dicarboxylic acid


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R-O-R RC=OOR’ -C-C\ / O R-O-O-R

ether ester epoxide

R-X Ar-X RC=OCl

alkyl halides aryl halides acid chloride

R-NH2 R2NH R3N R-NO2 RN=C=O RC=ONH2 (RC=O)2NH C=NR R4NX ArN≡NX

1°amine 2° amine 3° amine nitrate isocyanate amide imide imine quaternary ammonium salt diazonium salt

2 3 5 5 5 6

fused aromatic rings fused aromatic rings member ring containing member ring containing member ring containing member ring containing

peroxide

naphthalene anthracene or phenanthrene pyrrole furan thiopene pyridine

N O S N

Miscellaneous: Isotatic — compound with all groups on one side of the chain Saturated — compound only having single bonds Unsaturated — compound having double or triple bonds Ar — a benzene ring R — a hydrocarbon X — a halogen APPENDIX B BIBLIOGRAPHY Baron, G. R. Hazards Caused by Trace Substances, International Conference on Hazard Identification and Risk Analysis Barton, J. A. and Nolan, P. F. Hazards X. Process Safety in Fine and Specialty Chemicals, The Institute of Chemical Engineers Symposium No. 115, 1989, Rugby, UK. Bretherick, L. Reactive Chemical Hazards: An Overview, International Symposium on Preventing Major Chemical Accidents, 1987 Guidelines for Chemical Reactivity Evaluation and Application to Process Design, Center for Chemical Process Safety, AIChE, New York, NY 1995 Guidelines for Preventing Human Error in Process Safety, Center for Chemical Process Safety, AIChE, New York, NY 1994 Guidelines for Process Safety in Batch Reaction Systems, Center for Chemical Process Safety, AIChE, New York, NY 1999 Donahue, E. Identifying Hazardous Chemical Reactivity, Journal of Thermal Analysis, Vol. 49 (1997), pp. 1609-1616.



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Farr, Jim Chemical Hazards Produced by Water-Reactive Substances, CAMEO Today, National Safety Council, January/February 1996. Gifford, C. Usborne Essential Chemistry, EDC Publishing, Tulsa, OK 1992 Jones, D. W. et al. Rely on a Risk-Based Approach for Resource-Effective Facility Siting, Chemical Engineering Progress, May 1999. Keltz, T. Process Plants: A Handbook for Inherently Safer Design, Taylor & Francis, Philadelphia, PA 1998 Leggett, D. and Singh, J. Process Improvements from Incident Data, Process Safety Progress, Vol. 19, No. 1 (2000), 13-18 McCoy, Michael Biocatalysis Grows for Drug Synthesis, Chemical & Engineering News, January 1999 Melham, G. A. and Shanley, E. S. On the Estimation of Hazard Potential for Chemical Substances, 30th AIChE Loss Prevention Symposium Mosley, D.W. et al. Screen Reactive Chemical Hazards Early in Process Development, Chemical Engineering Progress, November 2000 Noronha, John Why DIERS Technology Should be Used in Risk Assessment, Process Safety Progress, Vol.18, No.2. O’Brien, Gerald J. et al. Thermal Stability Hazards Analysis, Chemical Engineering Progress, January 1982 Perry, Robert H. Perry’s Chemical Engineers’ Handbook, 1999 CD-ROM Rogers, R. The Systematic Assessment of Chemical Reaction Hazards, International Symposium on Runaway Reactions, 1989 Shanley, E. S. and Melham, G. A. On the Estimation of Hazard Potential for Chemical Substances, International Symposium on Runaway Reactions and Pressure Relief Devices, 1995. Yau, Te-Lin Selecting Performance Alloys, Chemical Processing, October 1999. Yoshida, T. Safety of Reactive Chemicals, Elsevier Science, 1987


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