PROCESS PLAN OF CONTINUOUS MELTPHASE POLYETHYLENE TEREPHTHALATE (PET) PRODUCTION PLANT
A Project Report Presented to Mr. Nasiruddin Shaikh (Project Instructor)
In Partial Fulfillment of Requirement for the Degree of Bachelors of Chemical Technology
By Hassan Niaz M. Umair Farooque Madiha Ismail Khan Surrayya Shafuq Siddiqui January 2009
DEPARTMENT OF CHEMICAL TECHNOLOGY
UNIVERSITY OF KARACHI
LETTER OF TRANSMITTAL
Plan a system for the Melt-Phase Polyethylene Terephthalate (PET) production, provided that the viability report has already been specified signifying the suitability of the plan. The plan report must include the Block Flow Diagram of the production facility, Generalized Process Description of the system and justification by Energy and Mass Balance, Process Flow Diagram and Detailed Designing of a single unit (equipment) in conjunction with the Preliminary Piping and Instrument Diagram. The details must also present the reaction mechanisms and catalyst selection. The report is submitted in partial fulfillment of the Degree of Bachelors of Chemical Technology, as the final year project report. The report is intended to deal with specified information of a continuous PET manufacturing process. The information of the variables on the optimum design, which has been used to turn out the aim, had been provided by the project instructor.
SUMMARY
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Synthesized polymers are used increasingly in our daily life, and industrial applications have contributed to their expansion. They are replacing metals in different walks of life because of their distinctive properties. Current studies involve new methods of polymer manufacturing, their reaction mechanism, factors that influence their properties, new methods to improve the product quality and process cost minimization. Polyethylene terephthalate (PET) is used for melt-spun polyester fibers, films, injection molded parts, and a multitude of plastic objects such as soft-drink bottles. As a widely used and fast growing polymer, economical production of PET is of great importance. Pursuant to the goal of economical and efficient production of PET, this report models the PET formation, and its process conditions. The manufacturing process plan is modeled using two-stage process, i.e. esterification followed by poly condensation to produced PET by PTA Process (which includes Ethylene Glycol & Pure Terephthalic Acid as raw materials). The model takes into account the product degree of polymerization (DP) and diethylene glycol content (DEG). The production unit is designed to produce 100 tonnes / day Bottle grade Polyethylene Terephtalate (PET). The processes that form a part of our design are shown in the Block Flow Diagram and the Process Flow Diagram shows the process scheme to simplify the visualization of process plant that we design. The design preferred for the project plan is the three reactors continuous process in series: one esterifier reactor & two polymerization reactors. The operating temperatures for the three reactors in the series are 258, 270°C and 280°C, respectively. The reactor type specifications are CSTR, CSTR and DRR respectively. The operating pressure for the first reactor is relatively low (0.88barr, guage), for the next two reactors in series are 15mbarr and 1.5mbarr, absolute, respectively. The volumes of the first two reactors in the series are relatively large suggesting that large volume reactors tend to reduce the effect of volume level fluctuations on the product DP. Molar ratio of EG to TPA is 1.2. The polymerization catalysts, which are incorporated with the EG feed stream in to the mixer, comprises of a system that includes about: 300ppm of Antimony Trioxide, Sb3O2, 40ppm of Zinc or Cobalt, and 50ppm of at least one of Magnesium or Manganese. The Recovery Unit consists of a multi-stage distillation column, which rectifies the vapors from the process reactors. The column removes 3
water and other volatile reaction by-products, including acetaldehyde. Excess ethylene glycol is recovered and recycled to the plant (usually to the paste tank & esterifier). Finally, the report furnishes the inclusive sizing calculations, done for the mixing vessel of EG & PTA and the process and instrumentation plan has also been illustrated for the same equipment. All the necessary parameters and proportions are designed in accordance with the specified provisions.
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ACKNOWLEDGEMENT
At first, we are highly grateful to ALLAH Almighty, by the grace of Whom, we have been able to complete this report. We would like to thank to all those people whose valuable guidance & co-operation made our group enable to complete the assigned task especially to Mr. Zubair (Sr.deputy manager, NOVATEX pvt.ltd), Mr.Haroon (Sr. deputy NOVATEX pvt.ltd) and to our respected madam Shagufta Aslam. We wish to express our sincere gratitude to our Instructor, Mr. Nasiruddin Shaikh, for his invaluable knowledge, guidance, and support and for many helpful discussions on this endeavor. HassanNiaz, Umair Farooque, Madiha Ismail & Surrayya Siddiqui
TABLE OF CONTENTS 5
S.N O.
CONTENT
PAGE
1.
GENERAL INTRODUCTION
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1.1 1.2 1.3 1.4 1.5
7 12 13 15
1.6 1.7
HISTORICAL & ECONOMICAL PERSPECTIVE IMPORTANCE OF PET REPROCESSING OF PET PROJECT OBJECTIVES INDUSTRIAL PRODUCTION OF POLYETHYLENE TERPHTHALATE REACTIONS CHEMISTRY CATALYST & OTHER ADDITIVES FOR PET SYNTHESIS
2.
DISCUSSIONS ON PROJECT
34
2.1
PTA PROCESS FOR PET PRODUCTION
34
3.
FINAL DESIGN CONFIGURATION
43
3.1 3.2 3.3 3.4 3.5 3.6 3.7
PROCESS CONFIGURATION PROCESS FLOW DIAGRAM PROCESS EQUIPMENTS MATERIAL BALANCE FOR THE PET PROCESS DESIGN ENERGY BALANCE FOR THE PROCESS SIZING OF MIXER FOR THE FEED P & ID OF THE SIZED MIXER
43 45 47 51 54 57 66
APPENDICES
67
APPENDIX A APPENDIX B APPENDIX C APPENDIX D APPENDIX E
68 72 78 92 95
BIBLIOGRAPHY
100
16 25 29
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CHAPTER 01 GENERAL INTRODUCTION
1.1. HISTORICAL & ECONOMICAL PERSPECTIVES
Polymers have changed dramatically many aspects of human life since the launch of their commercial mass production in the beginning of the last century. 235 million tons of synthetic polymers were consumed worldwide in 2003 by important economic sectors such as electro- and electronic industry, packaging industry, building and construction, and automobile industry among others. They are replacing metals in different walks of life because of their distinctive properties. Current studies involve new methods of polymer manufacturing, their reaction mechanism, factors that influence their properties, new methods to improve the product quality and process cost minimization. Due to wide range of their uses, the demand of PET bottles is increasing as most of the food manufactures are converting the packaging of their products to PET bottles. These 7
bottles/containers are mainly used for the packaging of mineral water, carbonated beverages, edible oil, household food containers, detergents, paints, lubricating oils, feeding bottles for babies and many other items. As PET bottles provide better packaging, and have a lower cost than the bottles made from glass and other materials, different businesses in beverage, food and non-food industry are gradually shifting towards PET bottles. The PET resin has superior properties; they are attractive, pure and safe. The low permeability of PET to oxygen, carbon dioxide and water means that it protects and maintains the integrity of products giving a good shelf life. It also has good chemical resistance. PET bottles have the advantage of being lightweight, one-tenth the weight of an equivalent glass pack. Thus, PET bottles reduce shipping costs, and because of the material in the wall is thinner, shelf utilization is improved by 25 per cent on volume as compared to glass. High strength, low weight PET bottles can be stacked as high as glass. The other benefits are no leakage, design flexibility; containers can have all shapes, sizes, neck finish designs and colors and are recyclable. PET is made from the same three elements (carbon, oxygen, and hydrogen) as paper, and contains no toxic substances. When burned, it produces carbon dioxide gas and water, leaving no toxic residues. Being recyclable is the most important factor of success of business of PET bottles. It consumes less energy and produces less pollution than glass or metal packaging. Due to completely recyclable material, in many European and Latin American countries, PET bottles are refilled and used over and over again. In the US, more than 600,000,000 pounds of PET bottles are recycled annually.
PET BUSINESS IN PAKISTAN
Pakistan, since its independence in 1947, has been able to transform itself to a large extent, from a completely agrarian economy to a fairly developed techno-industrial base. Besides textiles, Pakistan’s exports are largely manufactured items such as consumer durables and engineering products. However, it is also a fact that Pakistan has not been able to realize its potential due to internal and external compulsions and thus it lags behind many developing countries of the world.
Pakistan Plastics Industry Statistics
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Statistics For Production Capacity of Plastics Materials
Gatron Industries
Engro Asahi Chemical
Pak Polymer Industries
Dyno Limited
Total Production
PET 200,000 M.Tons perYear PVC 100,000 M.Tons per Year PS 36.000 M.Tons per Year UF 34,000 M.Tons per Year 370,000 M.Tons per years
Status of Plastic Products Industry Year 2006
Local Consumption of Plastics Local Production of Plastics Imports of Plastics Exports of Plastics Total Industrial Units o Organized Sector o SMEs
470,000 M.Tons per Year 370,000 M.Tons per Year 180,000 M.Tons per Year 58,000 M.Tons per Year 6,000 700 5, 300 475, 000 Water Coolers. Hot Pots etc.
Manpower Engaged
Major Exportable Items Local Consumption of Recycled Materials 150,000 M. Tons per Year. No. of Recycled Units More than 400. 50,000. Direct/ indirect Labor
Because of the high demand of PET bottles in Pakistan, there has been an increase in small manufacturing units of PET bottles in the main cities of Pakistan. Besides the two major players of the industry, there are around 10 small and medium units working in 9
Lahore only. These units manufacture a variety of products, ranging from bottles for mineral water to packaging for pesticides. To meet the growing demand in future a project with a production capacity of 5.82 million bottles per year can be set up. The estimated cost of the project may be about Rs6 million. Market size: The majority of the businesses in Pakistan are converting from the bottle containers of traditional material to plastic substitutes for packaging of their products. The domestic manufacturing of plastic products is growing at 10 to 15 per cent annually. Carbonated beverage, edible oil and mineral water industries are good examples for the increasing demand of PET bottles. Within five years, the share of PET bottles has grown from 2 to 3 per cent to 18 to 20 per cent in the carbonated beverage market. While with every passing day, new industries are shifting to PET bottling because of lower cost and better preservation of their product. The bottles can be manufactured in the sizes of 0.5, 1.5 and 5litre or according to the customer specifications. However, the production of 1.5litre bottles may be the highest, as they are used in the beverage and mineral water industries, which are the two largest consumers of PET bottles. Along with this, the production of 0.5litre bottles is also increasing because of the increasing usage in mineral water bottles. The production of 5litre might be lower than the other two because they are mostly used in edible oil packaging. Innovations that continuously improve products and processes are the strongest driver of profitable growth and sustained competitive strength. Because of the enormous competitive pressure and shorter product lifecycles, it is essential to come up with more and more new and innovative developments despite declining returns. Nevertheless the situation on the PET market will, in the medium term, lead to a process of consolidation. Self-reliance instead of self-sufficiency is the bottom line of Pakistan’s industrial policy. Its direction is defined by the twin considerations of import-substitution and exportorientation. Value-addition is a national priority to improve our position on the value chain. That is why more investment is required in technology transfer. Pakistan’s investment space is vast. Imperatives of the investment continuum e.g. economic interest of the country and the financial interest of the individual investors are the key considerations. There is a kind of an organic link between the national economic interest on the one hand and the individual’s financial interest on the other. Sustainability of this linkage is the key to a win-win situation. This is being achieved by completely freeing the Government from the upfront controls and regulatory overhang, which it had instituted on investment over the years. Trade and industry is no more being controlled by the Government. The private sector is now in the drivers’ seat. The Government is trying to put it on the high road of development. Approach is fast track. The policy focus is shifting to the provision of the following requirements; namely: o Adequate policy framework
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o o o o o o o o o
Simplified operating procedures Strong support mechanisms Easy access to capital Upgrading technologies Enhanced productivity Reliable quality control Enhanced management skills Well-trained manpower Improved marketing skills.
Thus a reliable investment environment is being developed. The strategic preference is massive change instead of marginal one. Value-addition is our national priority for increasing national wealth. This requires upgrading of technology and capacity building in design development for improving our position on the value chain. There is therefore an immense scope of cooperation and technology tie-ups for cost-effective comanufacturing of automotive vehicles in Pakistan for domestic and export requirements.
1.2. IMPORTANCE OF PET
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The PET resin has superior properties; they are attractive, pure and safe. The low permeability of PET to oxygen, carbon dioxide and water means that it protects and maintains the integrity of products giving a good shelf life. It also has good chemical resistance i.e. it makes a good gas and fair moisture barrier, as well as a good barrier to alcohol (requires additional "Barrier" treatment) and solvents. And since they are excellent barrier materials, they are widely used for soft drinks. It is strong and impactresistant. It is naturally colorless with high transparency. When produced as a thin film (often known by the trade name Mylar), PET is often coated with aluminium to reduce its permeability, and to make it reflective and opaque. When filled with glass particles or fibers, it becomes significantly stiffer and more durable. This glass-filled plastic, in a semi-crystalline formulation, is sold under the tradename Rynite, Arnite, Hostadur& Crastin. PET use has reduced the size of the waste stream because it has replaced heavier steel and glass containers. Because, as PET bottles provide better packaging, and have a lower cost than the bottles made from glass and other materials, different businesses in beverage, food and non-food industry are gradually shifting towards PET bottles. PET can be semi-rigid to rigid, depending on its thickness, and is very lightweight. Due to wide range of their uses, the demand of PET bottles is increasing as most of the food manufactures are converting the packaging of their products to PET bottles. These bottles/containers are mainly used for the packaging of mineral water, carbonated beverages, edible oil, detergents, paints, lubricating oils, feeding bottles for babies, household food containers such as in the product of salad dressing, fruit juices, peanut butter and milk and also used as film in oven trays, sheeting for cups and food trays, oven trays and many other items. Moreover, recycled PET can be used for clothing and carpet fiber, and fiberfill for stuffing articles such as pillows. It can also be used to make new bottles for non-food products such as cleaning products.
1.3. REPROCESSING OF PET
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Because PET is an “engineered” resin, it is more expensive than commodity resins, such as high-density polyethylene (HDPE). For the same reason, PET usually is one of the more highly valued plastic recyclables. PET is fully recyclable where facilities exist. It is given the recycling code 1. Postconsumer recycled PET (PCR PET) can be used for clothing and carpet fiber, and fiberfill for stuffing articles such as pillows. Recycled PET can be used to make new bottles for non-food products such as cleaning products. To make food and beverage containers out of PCR PET, it must pass through approved processes to ensure it has no contaminants, and it must retain enough of the original properties to meet the final quality requirements. At the first sight recycling could minimize the amount of solid waste, and easy solution to environmental problems. But PET recycling offers a potential to reduce in fossil fuel consumptions, because to produce the origin PET needs fossil fuel, so that, PET recycling could assist to solve energy crisis in the future. PET recycling also is to be able to lengthen the life expectancy of the landfills. The other benefit of PET recycling is to generate income for unskilled people and labor force. As the demand for PET in the market exceeded supply of the raw material, and the high cost of virgin PET (VPET) created a strong demand for RPET. It can be said that the cost of virgin PET was the driving force behind the development of the recycling industry, rather than government legislation for waste minimization. There are three main methods used to recycle PET and they are broken down into mechanical, thermal and chemical processes. Post consumer bottles are particularly suited to the mechanical recycling route, producing resins with properties, which approach virgin resin specifications. The process of recycling of PET involves the first stage as a combination of mechanical techniques and washing to remove waste and solid contaminants. The next stage involves the removal of PVC contamination with X-ray detectors followed by optical detectors for HDPE and PP. At this stage, the automatic sortation systems should have removed surface contamination and other polymeric materials and the remaining pure PET stream would be ground into flake, washed, further purified by sink/float separation and centrifuges (to eliminate labels and caps) and then be rinsed and dried. This is the type of recycling process clean and decontaminates post consumer PET bottles to produce RPET resin. The conventional route taken to convert flake into fibres involves re-granulating and drying the flake, melt spinning the fibres, directly followed by processing into a yarn or non-woven fabric. One drawback, however, to the known recycling techniques is that of being able to recover what is known as in the industry as “clear” PET. Instead much-colored PET is recovered. The colored material contains dark streaks or specs caused by decomposition 13
of glue and other foreign material upon melting of PET (such as when processed for pellets for further use). The colored PET contains glue, which was employed to adhere labels and bases to the containers. ‘Colored’ PET has more limited use than ‘clear’ PET that can be refabricated into products, which do not contain dark colors. Clear PET can be used to make fibers for clothing, insulation fiberfill, fish line, fabrics and other similar products.
1.4. PROJECT OBJECTIVES
This report intends to deal with specified information of a continuous polymer manufacturing process, namely polyethylene terephthalate (PET). A production unit to
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produce 100 tonnes / day Bottle grade Polyethylene Terephtalate (PET) has been designed using two-stage process, i.e. estrification followed by poly condensation to produce PET. The information of the following variables on the optimum design, which has been used to turn out the aim, had been provided by the project instructor.
Starting Raw Materials available 1. Ethylene Glycol (EG) 2. Purified Terephthalic acid (PTA)
Available Utilities 1. Cooling Water
Supply Temperature = 90ºF, Return Temperature = 115ºF, Operating Pressure =60 psig 2. Instrument Air, Operating Temperature = 110ºF, Operating Pressure = 120 psig, Dew point = 40ºF 3. Saturated steam @ 150 psig and 300 psig 4. Superheated steam @ 50 psig and 600ºF
Product Specifications 1. Density = 86.4 lbs/ft3 2. Water Absorption for 24 hours = 0.10% (max) 3. Specific Gravity = 1.38 gram/cc 4. Tensile Strength at break = 11500 psi (min) 5. Melting point = 490ºF 6. Glass Temperature = 167ºF 7. Intrinsic Viscosity > 0.76 dl/g
1.5. INDUSTRIAL PRODUCTION OF POLYETHYLENE TEREPHTHALATE
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REVIEW OF PREVIOUS WORK A considerable amount of research in the area of modeling polymerization reactors and reactions for the manufacture of polyethylene terephthalate (PET) has been reported in the open literature. Polyethylene terephthalate (PET) is one of the widely applied polymers. From annual production viewpoint, PET is in the second rank among synthetic polymers equally with polypropylene. This is due to its excellent balance of properties such as impact strength, resistance to creep under pressure, low permeability to carbon dioxide, high melting point, thermal and hydrolytic stability and high clarity. In view of the previous classification, PET is a thermoplastic polymer, which is produced by stepgrowth polycondensation polyreaction under evolution of condensates such as water, methanol or ethylene glycol (EG). PET is produced in two steps by one of two ways, called the DMT and the PTA processes, or the transesterfication and direct esterification routes, respectively. Modern plants are based on the PTA process and further; they incorporate direct product formation (fibres and filaments, films) by extruding the melt from the final polycondensation reactor. Both processes can produce low- and high-viscosity PET. Intrinsic viscosity is determined by the high polymerizer operating conditions of: (1) vacuum level, (2) temperature, (3) residence time, and (4) agitation (mechanical design).
Contrasting the DMT & PTA Processes PET resins are produced commercially from ethylene glycol (EG) and either dimethyl terephthalate (DMT) or terephthalic acid (TPA). DMT and TPA are solids. DMT has a melting point of 140°C (284°F), while TPA sublimes (goes directly from the solid phase to the gaseous phase). Both processes first produce the intermediate bis-(2hydroxyethyl)-terephthalate (BHET) monomer and either methanol (DMT process) or water (TPA process). The BHET monomer is then polymerized under reduced pressure with heat and catalyst to produce PET resins.
DMT Process First we shall contrast the DMT and the PTA processes. The main difference is the starting material. The older process used: o Dimethyl Terephthalate (DMT) and 16
o Ethylene Glycol (EG) as starting materials. This was because of the non-availability of terephthalic acid of sufficient purity in the early years of polyester production. In the DMT process, in the first step, DMT is trans-esterified with ethylene glycol (EG) to produce an intermediate called diethylene glycol terephthalate (DGT) plus a small amount of low oligomers. The reaction byproduct is methanol and this is distilled off.
The DGT is alternatively called bis hydroxy ethyl terephthalate or BHET in the literature. Manganese (II) acetate or zinc (II) acetate is typically used for this transesterification step, these being the best catalysts for this reaction. In the second step, the DGT is heated to about 280°C under high vacuum to carry out melt-phase polycondensation. The principal volatile eliminated is EG. For the second step in the DMT route, the catalyst from the first step (zinc or manganese) is sequestered or deactivated with phosphoric acid and another catalyst for polycondensation, most commonly antimony triacetate or antimony trioxide is added. This is because zinc and manganese are considered poor polycondensation catalysts. The literature indicates that the reactivity of metals for the polycondensation reaction (second step) follows the trend Ti>Sn>Sb>Ge>Mn>Zn. Moreover, for the first step, namely the transesterification of DMT with EG, the catalytic activity trend follows the reverse order, with zinc being amongst the most active. For the polycondensation reaction, Sb compounds are commercially established (compared with Sn and Ti) because the resulting polymer has the most favorable balance of properties. Note, in a usual operation, it is possible to go from step 1 to step 2 without isolating the DGT. However, if desired, the DGT and oligomers formed in step 1 can be isolated and used later for melt polycondensation (step 2). The DMT route is economically unfavorable because of the involvement of methanol and the additional step needed to produce DMT from terephthalic acid and methanol. The production of methanol in the DMT process creates the need for methanol recovery and purification operations. In addition, this methanol can produce major VOC emissions. To avoid the need to recover and purify the methanol and to eliminate the potential VOC emissions, newer plants tend to use the TPA process.
PTA Process 17
The newer industrial method uses: o o
Purified Terephthalic Acid (PTA) & Ethylene Glycol (EG)
PTA is used instead of DMT and so it is called the PTA process. The metal content of the PTA polymer is less than the DMT polymer, as only one catalyst (for polycondensation) is used for step 2, and hence the thermal stability of the polymer is higher. The PTA route to PET is made up of two steps. The first is the esterification of terephthalic acid with ethylene glycol (EG) to convert to prepolymer that contains bishydroxyethyl terephthalate (BHET) and short chain oligomers.
The esterification is not complete, and some acid end-groups remain in the prepolymer. The esterification by-product water is removed via a column system. The second reaction step is polycondensation, in which mainly the following transesterification reaction.
as well as the following esterification reaction
lead to step-growth polymerization in the melt phase. The reversible nature of the reactions demands that the condensates ethylene glycol (EG) and water are removed from the melt efficiently by using high vacuum. Figure shows a typical continuous process scheme of the melt phase polycondensation of PET. The first esterification reactor and the second esterification reactors are a series of 18
stirred tank reactors to convert TPA to BHET and oligomeric PET at temperatures of about 280oC. Because the melt viscosity remains relatively low, the EG and water condensation products formed during the process can evaporate efficiently. When the molecular weight increases further, the melt viscosity of PET becomes so high that bubble formation is hindered even under the applied vacuum, and EG and water have to diffuse out. Hence it is critical to reduce the diffusion path at the following reaction stage in order to improve removal of EG and water. This is accomplished by feeding the melt into a disk ring reactor, that creates thin and renewable film of the polymer melt, thus significantly increasing the available surface area, and decreasing the diffusion path for condensate removal. Several reviews have looked at the physical and engineering aspects of the melt polymerization of PET. At the end of the reaction, the melt is either directly spun into fibers, or extruded into 2-4 mm thick strands that solidify due to the cooling and are cut into somewhat cylindrical chips for future processing.
Figure: A typical industrial process for PET production
PTA PROCESS AMENDMENTS The challenge for production companies has been to increase capacity and to consequently lower the manufacturing cost per unit of PET. The challenge for
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engineering companies has been to lower the cost of capital for new plants with higher capacities required. The more typical PET plant capacity was 240 tonnes per day.
Zimmer AG and Mitsubishi Chemtex used to dominate the Chinese market with plants in the range of 250 / 300 tonnes per day. In the brief span of six years, China is now dominated by domestic technologies with 600 tonnes per day lines.
Lurgi Zimmer has eliminated the SSP Process & developed a direct process for making the PET bottle preforms without the SSP step. It is based on an integrated process that produces a high viscosity melt from which the chips can be fed directly to the preform unit. DuPont, in alliance with Fluor Daniel, has developed the “NG3 process” which is claimed to reduce the number of steps from six to four and to lower capital costs by 40% and overall manufacturing costs by 10-15%. Designed to produce PET resins for the bottle resin market, the process employs a pre-polymerisation step that allows the melt phase to operate under positive pressure, eliminating the need for a vacuum system. The particle formation steps simultaneously form and crystallise the low molecular weight intermediate pellets. The approach used eliminates the finisher in the conventional melt process and one crystallization stage in the SSP step. Eastman has developed the “IntegRex process” which integrates the PX-toPTA and PTA-to-PET processes. Eliminating steps, such as the hydrogenation in the PTA process and solid stating step, as well as in-process storage stages, save costs. The process was commercialized in 2007 in a plant in South Carolina that was claimed to have three times the capacity in half the footprint of conventional PET technology. Synetix has developed “a new titanium catalyst” that can replace antimonybased catalysts and works in both the esterification and polycondensation processes. In batch systems, the Synetix catalyst is claimed to effectively increase plant capacity by 15%. Rather than eliminating the solid-state process, M&G has developed a completely new process called “EasyUPTM”, with several splendid returns. EasyUPTM technology allows solid stating in very large incremental units while providing better quality due to the very tight control of residence time distribution and the virtual absence of dust. The horizontal configuration of the reactor substantially reduces the erection costs.
Solid State Polycondensation Even with the disk reactors, it is difficult to obtain PET of number average molar mass Mn greater than 20,000 g/mole (intrinsic viscosity, IV ~ 0.6 dL/g). This is because of the relatively high viscosity of the melt, which reduces the mass transfer rates for removal of EG, and water and the chemical degradation accompanying the higher temperature needed to reduce the viscosity and the long residence time needed to obtain the high molecular weight. The PET produced from melt polymerization is directly used primarily 20
as textile material for clothing etc. where higher molecular weight is not necessary. Applications such as bottles and industrial fibers demand higher molecular weight PET, which is generally achieved by post-polymerization of the PET chips produced by melt polymerization. The current industrial practice for post polymerization of PET is the solid state polymerization (SSP). It is a conventional method used to increase the molecular weight of poly(ethylene terephthalate) (PET) in order to become more suitable for applications as carbonated soft drink bottles, etc. The chemical reactions taking place during SSP are the same as those in the melt polymerization except that the SSP takes place in the solid state. Solid -state polymerization of poly (ethylene terephthalate) (PET) is carried out by heating the low molecular weight prepolymer at temperatures below its melting point but above its glass transition temperature. Post condensation occurs and the condensation byproducts can be removed by applying vacuum or inert gas. Polymers obtained usually have high molecular weight, low carboxyl and acetaldehyde content, and can be used for beverage bottle or industrial yarns. Chemical reactions involved in the solid-state polymerization are transesterification, esterification, as well as the diffusion of byproducts. Overall reaction rate was governed by the molecular weight, carboxyl content of prepolymer, crystallinity, particle size, reaction temperature, and time. Prepolymer for solid state polymerization should have intrinsic viscosity 0.4 dL/g or more, density 1.38 g/mL, and minimum dimension 3 mm or less. The reaction temperature could be 200-250�C. Polycondensation progresses through chain end reactions in the amorphous phase of the semicrystalline polymer, which in most cases is in the form of flakes (mean diameter>1.0 mm) or powder (mean diameter<100 μm); reaction by-products are removed by application of vacuum or through convection caused by passing an inert gas. Polymerization at 240-245 degree C for 3-5 h can raise the intrinsic viscosity to 0.72 dL/g and carboxyl content less than 20 meq/kg. Appropriate reaction conditions are subject to the properties of prepolymers and the design of reactors. Reactor used for solid-state polymerization could be vacuum dryer type or stationary bed. The former is suitable for a small capacity and is run batch wise. The latter is a continuous process and is economically feasible for large -scale production. The advantages of SSP include low operating temperatures, which restrain side reactions and thermal degradation of the product, while requiring inexpensive equipment, and uncomplicated and environmentally sound procedures. Disadvantages of SSP focus on low reaction rates, compared to melt phase polymerization, and possible solid particle processability problems arising from sintering. Technical Information o Process: continuous SSP process
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Capacities: 10 to 60 metric T/D Process Environment: Nitrogen Atmosphere NPU Type: Catalytic Oxidation plus Molecular Sieve Drying Feed Source: flakes or pellets Feed IV: 0.50 to 0.80 dl/g R-PET Product IV: 0.60 to > 1.0 dl/g R-PET Product [AA]: less than 1 ppm Applications: fiber, strapping, sheet, engineered resin, industrial yarn, automotive parts, packaging materials, bottles o Options: can be used with existing extrusion systems at the plant or provided with new extrusion system for industrial waste or bottle to bottle recycle applications o o o o o o o o
Benefits The SSP-R Process offers a number of benefits to PET producers and PET recyclers: Highest Product Quality- Chips or flakes are processed in an inert N2 atmosphere to prevent oxidative degradation of polymer. Product has excellent resin color, low [AA] and low [COOH]. R-PET product quality is equivalent to virgin PET product. Flexibility- Both post-consumer and industrial waste can be processed, can be used for solid stating materials that have been recycled by either physical or chemical means, can accept chips from various commercially available extrusion systems for bottle to bottle recycle applications and can also be used to process virgin amorphous PET. There is no limitation to feed or product IVs because the process is easily adjustable. Low Cost- Simple three-step processing scheme with minimal equipment results in low investment and operating costs. Use of patented low nitrogen to solids ratio and NPU ensures low consumption of utilities. Continuous SSP Process requires less personnel and utilities vs. batch-type process, also resulting in low operating cost. Reliability- simple processing scheme and equipment design result in robust, trouble-free operation. Maintenance is simple and infrequent. No Environmental Hazards- The patented NPU safely and efficiently converts all hydrocarbon waste to CO2 and H2O, using efficient catalysts and molecular sieves. There are no hydrocarbon emissions.
SSP Process Description Solid-state polymerization (SSP) is used to build up the intrinsic viscosity required by certain applications such as soft drink bottle and tire cord. All unit operations run
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between the polymer’s Tg (glass transition temperature, 69 °C) and Tm (melting-point temperature, 265 °C). Precrystallizer Amorphous feed chips are introduced into the SSP plant from storage or directly from the melt phase plant (continuous polymerization process) and subsequently fed to the precrystallizer (a pair of precrystallizers). The precrystallizer is a high efficiency multizone fluid bed heat exchanger, which heats & de-dusts the incoming PET chips and increases the crystallinity. The use of nitrogen affords high flexibility in the selection of process temperature and eliminates the possibility of chip color change. Crystallizer The crystallized chips are then fed to the crystallizer, which completes (perfects) the crystallization, under process conditions optimized to the behavior of the feed polymer. Crystallization is performed in a moist nitrogen environment, to reduce AA in the product. The crystallized polymer drops in at the top of the reactor and travels downward. Nitrogen flows up through the reactor. SSP Polycondensation Reactor The crystallized chips are then fed by gravity to the moving-bed polycondensation reactor. The polycondensation reaction achieves the desired intrinsic viscosity (IV). Byproducts from the post polycondensation (SSP) reaction, such as AA, ethylene glycol, and oligomers, are removed using a nitrogen carrier gas. The mass flow SSP reactor uses a patented low gas-to solids ratio for optimum process performance. Cooling Section The chips exit the SSP reactor and flow to the cooling section to perform the final cooling and de-dusting of the polymer chips. Product chips exiting the cooling section are ready for injection molding, bagging, or spinning. Nitrogen Purification Unit (NPU) The entire process is performed in an inert nitrogen atmosphere to ensure production of the best quality chips. NPU purifies the recirculating nitrogen gas, and a catalytic reactor converts the organic impurities from the SSP reactor to carbon dioxide and water — the only waste materials from the entire SSP plant. Both the catalyst and molecular sieves are designed to minimize consumption of utilities and promote optimum process conditions. The purified nitrogen contains <30ppm ethylene glycol, <6ppm water, and <100ppm oxygen.
Amorphous Chips Feed
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Pre-crystallization in Fluidized Bed
Closed N2 Loop
Crystallization In Plug Flow Crystallizer
Nitrogen Purification Unit (NPU)
Reactor
Cooling
Upgraded Chips Product
SSP Process With chips flowing via gravity through the system, flow scheme has very low operating costs.
NOTE: Solid State Polymerization Process is merely defined here just to furnish the entire process of production of bottle grade PET chips. Otherwise this process’ designing is not built-in in the subsequent sections.
1.6. REACTIONS CHEMISTRY
SIDE REACTIONS
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Apart from the main reactions involved in the PET manufacturing process, several side reactions occur leading to the formation of undesired byproducts, the presence of which may adversely affect the final product properties. The important reactions involved in the manufacture of PET are polycondensation, esterification, ester interchange, diethylene glycol formation, acetaldehyde formation and vinyl end group formation. The components produced due to side reactions include acid ends, diethylene glycol (DEG) in free and associated form, acetaldehyde and vinyl end groups. The rate of formation of these side products depends on the operating conditions in the PET manufacturing process. The presence of DEG in the polymer chain disturbs the regularity of the polymer chain and influences the rate and level of crystallization and the properties of the polymers. The melting point of PET decreases if DEG is incorporated in the polymer. It is therefore important' to keep DEG formation during PET manufacture within certain limits. Another important side reaction in PET synthesis is degradation of the repeat units, which may lead to a drop in the molecular weight of the polymer and an accumulation of acid and vinyl end groups. Acetaldehyde is another component, which is not desirable in the final polymer product. It is generated in melt processing (over 260oC), which can be produced by the degradation of ethylene glycol or by the degradation of the PET polymer chain. In addition, cyclic oligomers, e.g., trimer and tetramers of terephthalic acid and ethylene glycol, also may occur in minor amounts. The continued removal of ethylene glycol as it forms in the polycondensation reaction will generally reduce the formation of these by-products.
MAIN PROCESS REACTIONS & SPECIES To produce PET, the direct reaction method of a diacid with a diol is by and large utilized (i.e., terephthalic acid is reacted with ethylene glycol as shown below).
REACTION STAGES
The reaction takes place in two main stages: a pre-polymerization stage and the actual polymerization. In the first stage, before polymerization happens, you get a fairly simple ester formed between the acid and two molecules of ethane-1,2-diol.
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In the polymerisation stage, this is heated to a temperature of about 260°C and at a low pressure. A catalyst is needed - there are several possibilities, discussed in the next section, including antimony compounds like antimony(III) oxide. The polyester forms and half of the ethane-1,2-diol is regenerated. This is removed and recycled.
It has been assumed that the reactivity of the functional groups is not dependent on the length of the polymer chain. The functional groups, which are modeled here, are hydroxy ethyl ethoxy ester end group (EO), acid end group (Ee), internal ethyl diester group (ZG), internal ethyl ethoxy diester group (ZD) and hydroxy ethyl ethoxy diester group (ED). A detailed description of all the reactions, species involved, can be found in Appendix A. Figure gives details of the different components involved and their chemical formulas.
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The list of the reactions which are considered are:
Poly-condensation 1) EG + EO ↔ ZG + FG 2) EG + ED ↔ ZO + FD 3) ED + EO ↔ ZD + FG 4) ED + ED ↔ ZD + FD Esterification 5) EC + FG ↔ EG + FC 6) EC + FD↔ED + FC 7) EC + EO↔ZG + FC 27
8) EC + ED↔ ZD + FC
Ester interchange 9) ED + FG↔EO + FD Side reactions 10) EG + EG↔ED + EC 11) EG + FG↔FD + EC 12) EG + ZG↔ZD + EC 13) ZG + FG↔ED + EC
1.7. CATALYST & OTHER ADDITIVES FOR PET SYNTHESIS
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The polymerization catalyst employed in the continuous process is generally added prior to, at the start of, or during the polycondensation stage as long as it is provided sufficiently early in the polycondensation stage to facilitate polycondensation of the monomer to yield polyethylene terephthalate. The preferred catalyst system is generally employed and supplied in a form that is soluble in the polymer melt to enable the catalyst to be uniformly distributed throughout the polymer melt. It is aimed to substantially increase the polymerization rate while producing a polyethylene terephthalate polymer, which has high clarity (if dulling agents are not added) and is virtually colorless. So, such a catalyst system is created, which are effective in producing colorless PET having high clarity and that can be added at anytime before the beginning of or during the polycondensation step. In both the batch and the continuous processes, a high activity catalyst is often employed to increase the rate of polymerization thus increasing the throughput of the resulting PET polyester. The high activity catalysts, which are used in the polymerization of PET polyester, can be basic, neutral or acidic, and are often metal catalysts. The polymerization catalysts that are preferably used in the polycondensation reaction are metals. Specific examples of appropriate polyester catalysts include germanium compounds, titanium compounds, antimony compounds, zinc compounds, cadmium compounds, manganese compounds, magnesium compounds, cobalt compounds, silicon compounds, tin compounds, lead compounds, aluminum compounds, and other similar compounds. Preferred catalysts for polyester bottle resin, for example, include germanium compounds such as germanium dioxide, antimony compounds such as antimony trioxide, cobalt compounds such as cobalt acetate, titanium compounds such as titanium tetrachloride, zinc compounds such as zinc acetate, manganese compounds such as manganese acetate and silicon compounds such as methyl silicate and other organic silicates. Primarily, the traditional polymerization catalysts used in the formation of PET from both TA and DMT contains Antimony which have the best all-round properties amongst PET catalysts during melt polymerization, leading to high productivity and polymers with good thermal stability. The most common of the Antimony-containing catalysts is Antimony Trioxide, Sb2O3. However, a catalyst system specific for producing PET by the TA process includes: (1) (2) (3)
Antimony, Cobalt and/or Zinc, and At least one of Zinc, Magnesium, Manganese or Calcium.
The preferred catalyst systems include manganese, cobalt, and antimony; or zinc, cobalt, and antimony; or manganese, zinc, and antimony. These catalyst systems, when used in 29
the most effective amounts, increase the polymerization rate thereby reducing the polymerization time by approximately at least one-third, and in some cases up to one-half of the time otherwise required under control conditions. The system comprises PET having from about: o 150 ppm to about 650 ppm Antimony, o 5 ppm to about 60 ppm of at least one of Zinc and Cobalt, & o 10 ppm to about 150 ppm of at least one of Zinc, Magnesium, Manganese or Calcium. The first metal catalyst is employed in a range of from 5 ppm to about 60 ppm, preferably from 10 ppm to 40 ppm based on the theoretical polymer yield (100 percent conversion). The second metal catalyst is employed in a range of from 10 ppm to about 150 ppm, preferably from 20 ppm to 50 ppm based on the theoretical polymer yield. Antimony is employed in the range of from about 150 ppm to about 650 ppm, preferably from 250 ppm to 400 ppm based on the theoretical yield of the polymer. The amounts of catalysts added are generally the same as what generally carries through to the product produced. Some of the catalysts may volatilize and escape with the off gas from the reaction. The actual polymer yield may be less than the theoretical polymer yield. When stating that the catalyst system can be added at any time before or prior to polycondensation, it is intended to include the addition of one or more of the catalyst metals in the terephthalic acid, glycol or other feedstock materials. For example, all the catalyst metals could be added into the terephthalic acid feedstream in a continuous process, or some of the catalyst metals in the terephthalic acid feedstream and the remainder in the glycol feedstream, or the catalyst system could be added in with other additives like coloring agents. Accordingly, as the terephthalic acid and ethylene glycol are reacted at least some of the catalyst system could already be present. Generally, in continuous process, no catalyst system is employed in the direct esterification step. However, the polycondensation catalyst, generally an antimony catalyst as in the batch process, may be introduced into the first vessel with the raw materials (i.e., present during the direct esterification stage) or into a vessel further along in the process prior to or during polycondensation but after the direct esterification stage is completed. The metallic catalyst system, in addition to antimony, includes a first metallic catalyst of cobalt, zinc, or a mixture of these, and a second metallic catalyst of at least one of manganese, zinc, calcium and magnesium. It is theorized that cobalt, which is not a particularly effective metallic catalyst when combined with antimony, may make any of zinc, manganese, calcium, or magnesium more active when employed therewith. Likewise, it is theorized that zinc, as a substitute for the cobalt may also make manganese, calcium or magnesium more active, if combined with any of them. The simplest catalyst system comprises zinc and antimony.
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The amount of catalyst added refers to the "amount of metal in the catalyst itself". Thus, if 300ppm of antimony were employed, for example, it would not matter if antimony trioxide or antimony acetate were employed, so long as the actual amount of antimony metal present is 300ppm. It appears that the overall increase in rate provided by the catalyst system is the additive effect of two catalytic mechanisms. The first mechanism is the effect of antimony as a coordination catalyst for the oligomers and polymers formed during the direct esterification of terephthalic acid and ethylene glycol. The second mechanism appears to be the effect of a metallic catalyst upon the acid catalyzed polymerization of the oligomers and polymers. Alternative to Antimony Catalyst---Titanium The vast majority of PET produced today is made with antimony. However, it would seem that the tide is beginning to change and more and more companies are realizing the benefits of titanium as a catalyst for the production of PET bottles, fibres and films. At the recent PET Strategies 2006 conference in Atlanta, Jim Bruening of Wellman Inc. presented a paper on the production of polyester resin for bottles using titanium catalysts. Wellman is a world leader in the production of PET resin for bottles. Mr. Bruening stated that Wellman have chosen to make titanium catalysts their chosen strategic technology platform, and he believes other companies in the market will follow suit. Wellman also launched their new grade of resin specifically for carbonated soft drinks and bottled water, Ti842, at the conference. This grade will compliment their existing offering for hot-fill beverage bottles, the titanium catalyzed PET resin, Ti818. Traditional reluctance to use titanium was due in part to colour issues and a lower activity in the solid-state polymerization (SSP) process step. However, Mr Bruening presented data showing that Wellman had surmounted these problems and were thus able to achieve the full benefits of the titanium catalyst. In hot-fill applications, titanium PET was more resistant to shrinkage than the antimony-catalyzed bottle and Mr Bruening said that the titanium catalyst opened the door to light-weighing hot-fill bottles because of superior strength. Titanium also gives a higher clarity bottle and this has seen the adoption of the titanium-catalyzed resin for critical applications such as white grape juice bottles. In carbonated soft drink (CSD) applications, Wellman's new resin grade Ti842, also demonstrated significant advantages over the traditional antimony catalyzed resin. It showed reduction in cycle times of 5% to 10% during the injection moulding process. This corresponds to an increase in output and efficiency for the producers of bottle preforms. A 25%-30% drop in acetaldehyde content was also observed in the Ti842.
STABILIZER
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Although polymerization catalysts such as antimony trioxide result in the increased production of PET, these same polymerization catalysts will eventually begin to catalyze or encourage the degradation of the polymer formed in the condensation reaction. Such degradation of the PET polymer results in the formation of acetaldehyde and the discoloration or yellowing of the PET polyester. Acetaldehyde formation is an objectionable result of degradation, especially in the food and beverage industry, because it can adversely affect the taste of the bottled product, even when present in very small amounts. That is, once the polycondensation reaction essentially reaches completion, the polymerization catalyst begins to degrade the polymer forming acetaldehyde and causing discoloration or yellowing of the polyethylene terephthalate. In an attempt to reduce the degradation and discoloration of the PET polyester, stabilizing compounds are used to sequester ("cool") the catalyst thus reducing its effectiveness. A stabilizer containing phosphorous, is therefore added to the polymer melt to deactivate and stabilize the polymerization catalyst to prevent degradation and discoloration of the polyester. The stabilizer is added to the substantially entirely polymerized polymer melt at or after the end of the polycondensation reaction but prior to polymer processing, i.e., chipping, fiber spinning, film extrusion, and the like. The preferred method of introducing the stabilizer into the polymer melt at the end of polymerization is to inject or pump the stabilizer into the polymer melt at or after the end of the polycondensation reaction. The stabilizer is preferably added in liquid form. Accordingly; liquid stabilizers can be added directly, and solid stabilizers such as ULTRANOX® 626 are typically either melted or suspended in an inert liquid carrier prior to their addition to the polymer melt. Because the stabilizer is added at the end of the polymerization process, it can be added in its pure form without negatively affecting the properties of the polymer melt. In addition, uniform blending of the stabilizer and the polymer melt can be accomplished by mechanical blending such as passing the melt through pumps, conventional static mixers, and passing the melt through filtration elements to quickly deactivate the polymerization catalyst and thus prevent degradation and discoloration of the PET polyester. The stabilizer may also be added after polymerization when the polymer melt is extruded by using a screw extruder or similar means. The late addition of the stabilizer to the polymer melt prevents the stabilizer from inhibiting ("cooling") the polymerization catalyst during the polycondensation reaction thus increasing the productivity or throughput of the continuous polyethylene terephthalate process. Furthermore, because the stabilizer is added prior to polymer processing, the stabilizer can adequately prevent discoloration and degradation of the PET polyester. Alternatively, late addition of the stabilizer can increase the thermal stability of the polyester without adversely affecting the throughput or productivity of the polyester.
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Although adding a stabilizer to the polymer melt in a batch reactor is a relatively simple process, numerous problems arise if the stabilizers are added in the continuous production of polyethylene terephthalate. For instance, while early addition of the stabilizer prevents discoloration and degradation of the polyester, it also causes reduced production throughput (i.e., decreases polycondensation reaction rates). Moreover, such stabilizer is typically dissolved in ethylene glycol, the addition of which further slows the polymerization process. Consequently, early addition of the stabilizer in the polymerization process requires an undesirable choice between production throughput and thermal stability of the polymer. As used herein, "thermal stability" refers to a low rate of acetaldehyde generation, low discoloration, and retention of molecular weight following subsequent heat treatment or other processing. Late addition of the stabilizer (e. g., after the polymerization process during polymer processing) may provide insufficient opportunity for the stabilizer to fully blend with the polymer. Consequently, the stabilizer may not prevent degradation and discoloration of the polyester. In addition, adding stabilizer during polymer processing is inconvenient and does not provide economies of scale. Generally, a thermal stabilizer which is nonreactive with the polymer and which possesses low residual moisture will be used to deactivate the polymerization catalyst. The most commonly used stabilizers contain phosphorous, typically in the form of phosphates and phosphites. The phosphorous-containing stabilizers were first employed in batch processes to prevent degradation and discoloration of the PET polyester. The step of adding the phosphorous-containing stabilizer, with a phosphorous content of from about 25 to about 150 ppm, comprises adding a stabilizer selected from the group consisting of phosphorous, polyphosphoric acid; phosphoric acid; organophosphorus compounds, organophosphates, organophosphites, and organophosphonates; orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, tripolyphosphoric acid, phosphorous acid, hypophosphorous acid, phosphorous-containing aliphatic organic carboxylic acid salts; bismuth phosphate; monoammonium phosphate, diammonium phosphate, monammonium phosphorite; salts of phosphoric acid esters having at least one free alcoholic hydroxyl group, sodium beta-glycerophosphate, calcium betaglycerophosphate; phosphotungstic acid, ammonium phosphotungstate, sodium phosphotungstate; tertiary phosphines, tripropylphosphine, triphenylphosphine, ethylphenyltolylphosphine; quaternary phosphonium compounds, triphenylmethylphosphonium iodide, triphenylbenzylphosphonium chloride, and quaternary phosphonium compounds.
CHAPTER 02 33
DISCUSSIONS ON PROJECT
2.1. PTA PROCESS FOR PET PRODUCTION As stated in the previous chapter, this report intends to deal with specified information of a continuous PET manufacturing process and a production unit to produce 100 tonnes / day Bottle grade Polyethylene Terephtalate (PET) has been designed using two-stage process, i.e. estrification followed by poly condensation to produce PET. (The information of the variables on the optimum design, which has been used to turn out the aim, had been provided by the project instructor). Polymerization plants are made up of four main process units: • Paste preparation unit • Reaction unit • Vacuum generation unit • Distillation unit Several modifications have been made to the cost model in order to more accurately reflect actual plant costs. Although many process configurations are found in the polyester industry for polyethylene terephthalate production, they all involve a series of three or more reactors. The most efficient melt-phase method employed is a typical fivecontinuous reactor process in series: two esterification reactors, two pre-polymerization reactors, and one high-viscosity reactor, which is also called finisher.
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However, the trend is nowadays to reduce the number of reactors in the process as this saves in terms of investment to throughput ratio and maintenance cost. In addition, the compactness of the design allows savings in civil works and steel structures. Several plant-engineering companies have claimed new technologies with reduced number of reactors like Zimmer AG (3 reactors), ESPREE® (2 reactors).
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The design preferred for this project plan is the three reactors continuous process in series: one esterifier reactor & two polymerization reactors. The operating temperatures for the three reactors in the series are 258, 270°C and 280°C, respectively. The reactor type specifications are CSTR, CSTR and DRR respectively. The operating pressure for the first reactor is relatively low (0.88barr, guage), for the next two reactors in series are 15mbarr and 1.5mbarr, absolute, respectively. The volumes of the first two reactors in the series are relatively large suggesting that large volume reactors tend to reduce the effect of volume level fluctuations on the product DP. Molar ratio of EG to TPA is 1.2.
PASTE PREPARATION
Raw materials are brought on site and stored. Terephthalic acid, in powder form, may be stored in silos. The ethylene glycol is stored in tanks. The terephthalic acid and ethylene glycol, containing catalysts, are mixed in a tank to form a paste. Combining these materials into a paste is a simple means of introducing them to the process, allowing more accurate control of the feed rates to the esterification vessels (a portion of the paste is, sometimes, recycled to the mix tank. This recycled paste and feed rates of TPA and ethylene glycol are used to maintain an optimum paste density or weight percent of terephthalic acid).
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Vacuum System
EG Recovery Unit
EG PTA Mixing (Paste) Tank
ESTERIFICATION REACTORS
POLYCONDENSATION REACTORS
Additives
Amorphous PET Chips
Cooling or Quenching
SSP Process Unit NPU
Precrystallizer
Crystallizer
Crystalline PET Chips
Reactor
Chip Cooling
BLOCK FLOW DIAGRAM FOR THE PROCESS
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The polymerization catalysts that are incorporated with the EG feed stream in to the mixer comprises of a system that includes about: o 300ppm of Antimony Trioxide, Sb3O2, o 40ppm of Zinc or Cobalt, and o 50ppm of at least one of Magnesium or Manganese. (NOTE: These values are selected simply for the concern project plan) Different additives (like a color toner e.g. Blue or Red Toner) are also added along with the catalysts, depending upon the polymer product requirements or specifications.
ESTERIFICATION REACTORS:
The paste from the mix tanks is fed to an esterification vessel (referred to as esterifier, or ester exchange reactor). In the esterification stage, the terephthalic acid and the ethylene glycol react to form low molecular weight oligomers and water. In general, a continuous feed of raw materials is used employing a molar ratio of ethylene glycol to terephthalic acid of from 1 to 1.6 (in the present case, sustained at 1.2). The resulting paste is fed to the CSTR, which is known as an esterification reactor. Typically, the estrifier is operated at a pressure of 1-8bar and a temperature of 240-290 degree C for 1 to 5 hours; but the reactor is maintained at a pressure of 0.88barr & a temperature of 258 degree C for the specified designing presently. The reaction is typically un-catalyzed and forms low molecular weight oligomers and water. The vapor from esterification reactor is rectified in a multi-stage distillation column. The column removes water and other volatile reaction by-products, including acetaldehyde. Excess ethylene glycol is recovered and returned to the paste tank and the esterification reactor. The recycle rate to esterification reactor can be manipulated to control the local monomer ratio. One thing which is worth mentioning here is that since esterification reaction occurs from the beginning to the end of PET synthesis, it is an equilibrium reaction and, thus removal of the condensed water is necessary to minimize the hydrolysis of the formed ester groups. Phosphoric acid stabilizer is also introduced into the esterification reactor or prior to the pre-polymerization, to about 5-100ppm, to reduce the degradation and discoloration of the PET polyester and to cool the catalyst, which encourages the degradation of the polymer formed in the condensation reaction.
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POLYMERIZATION REACTORS:
The third reactor, or low polymerizer (LP), is typically composed of a simple CSTR. The low polymerizer (LP) operates at a medium vacuum pressure (50-500 Torr), it is maintained at 15mbarr gauge pressure for the scheme plan. This stage strips off most of the excess ethylene glycol and water remaining in the polymer. In most plants, the polymer intrinsic viscosity in the low polymerizer is below 0.2 dl/g, and the LP behaves ideally. At higher viscosity levels, the low polymerizer becomes increasing mass-transfer limited. As the polymer melt is fed into successive vessels, the molecular weight and thus the intrinsic viscosity of the polymer melt increases. The temperature of each vessel is generally increased and the pressure decreased to allow greater polymerization in each successive vessel. The final DRR (disk-ring reactor) reactor, known as the high polymerizer (HP), is operated at lower absolute pressures, often as low as 1.5mbarr. As with the low polymerizer, each of the polymerization vessels communicates with a flash vessel and is typically agitated to facilitate the removal of ethylene glycol, as EG removal is the rate-determining step of the polycondensation reaction, thus enabling the polycondensation reaction to go to completion. Therefore, these reactors are stirred tank reactors with unusual ratio of diameter/height to provide a large gas-liquid interface. Disk-ring reactor contains a number of annular disks attached to a rotating shaft. Polymer flows through holes cut into the disks. As the disks rotate they generate a surface film, which enhances the evaporation rate. Due to the high viscosity of the polymer, the performance of the finishing reactors is limited by the liquid-vapor mass transfer rate. This makes the reactor performance a function of the shaft rotation rate, as well as the temperature, pressure, and throughput.
A side view of a rotating disc reactor (left), and a cross-section of one perforated disc (right)
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The number of DRRs typically does not exceed two. All reactors have a vacuum system, which condenses each reactor's vapor product. The purpose of the vacuum system is to remove the volatile byproducts of the polymerization reaction. The retention time in the polymerization vessels and the feed rate of the ethylene glycol and terephthalic acid into the continuous process are determined in part based on the target molecular weight of the PET polyester. Because the molecular weight can be readily determined based on the intrinsic viscosity of the polymer melt, the intrinsic viscosity of the polymer melt is generally used to determine the feed rate of the reactants and the retention time in the polymerization vessels. Long residence times lead to thermal degradation of the polymer and cause black spots in the transparent final products. Equipment for each reactor includes the vessel, agitator, agitator motor, heating jacket, heat exchanger and pump (CSTR only), gear pump, and a vacuum system. Figure provides a schematic of a CSTR with auxiliary equipment. The CSTR heat exchanger is necessary to provide sufficient heat for the process feed (which enters at a lower temperature) and the vaporization of the volatile components. With the DRRs, the vapor outlet flows are much less than with the CSTRs. The vacuum system consists of jets and a spray condenser with a heat exchanger and recirculating pump.
The vapor product from each reactor enters the spray condenser. Condensate is continuously replaced with ethylene glycol to reduce the volatility of the spray fluid. The
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vapor product of the spray condenser feeds into the jet ejectors. The product of the jets combines with the liquid product from the condenser and enters the refining system. All reactors share a single refining system. It usually consists of a ten-stage distillation column with a feed heat exchanger, reboiler, and condenser. A single Dowtherm heating system provides for each reactor's heating duty. Further depiction on the vapor recovery process is described in the following paragraphs.
COOLING & CHIPS FORMATION Although this selective report case do not covers the overall process of bottle grade PET chips formation, since it only takes account of the production to the melt polymer making, it would be helpful to be acquainted with the succeeding progression stages of Melt Phase PET. Thus, once the polymer melt exits the polycondensation stage, typically from the high polymerizer, it is generally filtered and then extruded into polyester sheets, filaments, or pellets. Preferably, the polymer melt is extruded shortly after exiting the polycondensation stage and typically is extruded immediately after exiting the polycondensation stage. Once the PET polyester is extruded it is quenched, preferably in a water trough, to quickly decrease its temperature thus solidifying it. The solidified PET polyester is formed into pellets or cut into chips for storage and handling purposes. The pellets or chips may be subjected to solid-state polymerization (SSP) to increase the molecular weight of the polyester. The polyester chips are then re-melted and re-extruded to form items such as bottles, filaments, or other applications. It should be noted that because the melting and extruding steps in the formation of the PET polyester are performed at elevated temperatures of at least greater than 260°C, it is important that the PET polyester is thermally stable and does not degrade or discolor as a result of temperature increases. Therefore, it is crucial that the stabilizer adequately blend with the polymer melt to deactivate the polymerization catalyst.
EG RECOVERY Spent glycol from the PET process typically contains 90-99% EG, 1-5% DEG, 1-10% low molecular weight terephthalate oligomers, .01-.5% antimony, 2-5% water, a trace of other metallic cations, and a trace of phosphates, acetates and other anions as well as other contaminants. Glycol released in the polymerization process and any excess or unreacted glycol is drawn into a contact spray condenser (scrubber) countercurrent to a spent ethylene glycol spray. (At one facility, both the low and high polymerizer spray condensers have four spray nozzles, with rods to clear blockage by solidified polymer. Care is taken to ensure
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that the spray pattern and flow are maintained.) Recovered glycol is pumped to a central glycol recovery unit, a distillation column. Vacuum on the reactors is maintained by a series of jets with barometric intercondensers. A three-stage ejector system is used selectively for the evacuation of both the polymerizers. Generally, at some plants, a two-stage ejector system with a barometric intercondenser is used to evacuate the low polymerizer. The condensate from the intercondensers and the last jets is discharged to an open recirculating water system, which includes an open trough (referred to as a "hot well") and cooling tower. The recirculation system supplies cooling water to the intercondensers. Vapors from the spray condenser off the high polymerizers are also drawn through a jet ejector system. One facility uses a five-jet system. After the first three ejectors, there is a barometric intercondenser. Another barometric intercondenser is located between the fourth and fifth ejectors. The ejectors discharge to the cooling water hot well. The stream exiting the vacuum system is sent either to a cooling tower where the water is recirculated through the vacuum system, or to a wastewater treatment plant (once-through system). Vacuum pumps can be installed as an alternative to the last two ejectors. These pumps are installed as part of an energy conservation program and are used at the operator’s discretion. But if vacuum is lost, or is insufficient in the low or high polymerizers, offspecification product results. So, each process line must have a dual vacuum system. One five-stage ejector/vacuum pump system must be maintained as a standby for each industrial filament (high-viscosity) process line. The staple (low-viscosity) lines can have a standby ejector system, but with only one vacuum pump per process line. Nevertheless, steam ejectors recover faster from a slug of liquid carryover than do vacuum pumps, but the spare system must be used in the production of either high- or low-viscosity PET.
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CHAPTER 03 FINAL DESIGN CONFIGURATIONS
3.1. PROCESS CONFIGURATION
The preferred model of the process consists of series of continuous stirred tank reactors (CSTR) and disc ring reactors (DRR) with accessories attached to each reactor to handle the downstream products. The selection of the type of reactor (CSTR/DRR) is made depending upon the degree of polymerization of the product coming out of the reactor. 43
A DRR is selected if the product degree of polymerization is greater then 25. The feed to the first reactor consists of TPA and ethylene glycol. The molar ratio of TPA to ethylene glycol in the feed to the first reactor is 1:2. Overheads from reactors are sent to individual spray condensers. The condensate from the spray condensers is combined and refined. A multistage jet ejector is used to create vacuum in the reactor. A heat exchanger is used to cool the circulating fluids of the condenser. Ethylene glycol is added as fresh feed to the spray condenser to lower the vapor pressure of the condensables. A single distillation column is used for all the reactors in the process to separate the products leaving the jets and the spray condenser. Figure given below shows a brief description of the reactor and the accessories.
Key for the figure Identifier 1 2 3 4 5 6 7 8 9
Description Feed Reactor Spray Condenser Jet Ejector Heat Exchanger To Distillation Column (Recovery Unit) Make Up Feed of EG EG Feed to Ejector System Product to next Reactor in series
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3.2. PROCESS FLOW DIAGRAM The flow diagram, to show the sequence of equipments and unit operations in the overall process, and to simplify visualization of the manufacturing procedures, is represented and illustrated in the subsequent pages.
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E
P
This design relates to a process for the continuous production of polyethylene terephthalate (PET), wherein pure terephthalic acid is esterified with ethylene glycol, then pre-polycondensated and subsequently polycondensated, the vapors formed being rectified, the liquid phase substantially consisting of ethylene glycol being recirculated to the production process, and the vapor phase containing EG and water (lower boiling components are neglected) being condensed. In detail, terephthalic acid is mixed with ethylene glycol, with the mole ratio of 1.2, and a catalyst (together with the additives like color toner, stabilizer or preservative), to obtain a paste and is then supplied to the esterification vessel by way of a pump. Esterification is effected at 0.88bar pressure (via removing water) & at temperature of 258 degree C. The esterified liquid mixture is then passed to the pre-polycondensation, which is affected under a vacuum, at 15mbarr gauge pressure. The product is next pumped through a filter to the DRR. Polycondensation is performed in DRR at an elevated temperature and under an increased vacuum, at 1.5mbarr. The produced PET polymer melt is finally cleaned by means of a filter and then sent for additional processing of pellet formation. Here, the vacuum system consists of jets and a spray condenser with a heat exchanger and recirculating pump. The vapors are rectified, the ethylene glycol obtained as bottom product is re-circulated to the esterification stage (and the lower-boiling components, if exists, are discharged as top product and condensed). The vapor product from polymerization reactors enters the spray condensers. Condensate is continuously replaced with ethylene glycol to reduce the volatility of the spray fluid. The vapor product of the spray condenser feeds into the jet ejectors whose condensate product goes into the mother vessel. The liquid product from the condenser gets refined in a ten-stage distillation column recovery system that consists of a feed heat exchanger, and a condenser. The ethylene glycol obtained as bottom product is sent to the mother vessel from where it is recirculated to the esterifier and the mixing vessel.
46
3.3. PROCESS EQUIPMENTS
The chief equipments used for the projected plan of PET production, shown in the process flow diagram, are enlisted underneath.
LIST OF EQUIPMENTS
-
Paste preparation tank-- Continuous Stirred Tank Esterification reactor-- Continuous Stirred Tank Column Poly-condensation reactor-I-- Continuous Stirred Tank Final Poly-condensation reactor-- Disc Ring Reactor Vacuum system Pre-polymer filter--Candle Type Filter Polymer filter-- Candle Type Filter Mother vessel of spent Ethylene Glycol Different types of condensers
LIST OF PUMPS
The selection of pumps, used in the process, is mentioned in the chart given below and their general working descriptions are briefly defined in the subsequent paragraphs. Screw pump (type of positive displacement The pump after paste preparation tank and pumps) in front of esterification tank is screw pump. Gear pumps (type of positive displacement Gear pumps are installed before and after pumps) the DRR. Centrifugal pumps Rests of the pumps in the diagram are centrifugal pumps. A power pump is a positive displacement machine consisting of one or more cylinders, each containing a piston or plunger. The pistons or plungers are driven through slider-crank mechanisms and a crankshaft from an external source. The capacity of a given pump is governed by the rotational speed of the crankshaft. Unlike a centrifugal pump, a power pump does not develop pressure; it only produces a flow of fluid. The downstream process or piping system produces a resistance to this flow, thereby generating pressure in the piping system and 47
discharge portion of the pump. The flow fluctuates at a rate proportional to the pump speed and number of cylinders. The amplitude of the fluctuations is a function of the number of cylinders. In general, the greater the number of cylinders, the lower the amplitude of the flow variations at a specific rpm. All power pumps are capable of operating over a wide range of speeds, thereby making it possible to produce a variable capacity when coupled to a variable speed drive. Each pump has maximum suction and discharge pressure limits that, when combined with its maximum speed, determine the pump’s power rating. The pump can be applied to power conditions that are less than its maximum rating but at a slight decrease in mechanical efficiency. The power pump is a positive displacement device. When operating, it will continue to deliver flow independent of the pressure in the discharge piping system. Unlike a centrifugal pump, a power pump will not “deadhead” or “go back on its curve” in response to increasing discharge pressure. When this pressure exceeds the design limits of the pump, mechanical failure—often catastrophic—will result. For this reason, all piping systems incorporating power pumps must have discharge pressure relief devices to limit the pressure in the piping system and avoid pump failure. These devices must be located between the discharge connection on the pump and the first isolation valve in the piping system. It is recognized that the proper pump metallurgy for the pumped fluid must be used, but due to the vast number of possible fluids, selection of pump metallurgy is outside the scope of this section. For basic power pump selection, it is only necessary to understand that the NPSH available from the suction system must be sufficiently above the NPSH required by the pump to operate properly. The pump after paste preparation tank and in front of esterification tank is screw pump. Introduction, advantages and disadvantages are given below: SCREW PUMPS Screw pumps are a special type of rotary positive displacement pump in which the flow through the pumping elements is truly axial. The liquid is carried between screw threads on one or more rotors and is displaced axially as the screws rotate and mesh. In all other rotary pumps, the liquid is forced to travel circumferentially, thus giving the screw pump with its unique axial flow pattern and low internal velocities a number of advantages in many applications where liquid agitation or churning is objectionable. The applications of screw pumps cover a diversified range of markets including navy, marine, and utilities fuel oil services; marine cargo; industrial oil burners; lubricating oil services; chemical processes; petroleum and crude oil industries; power hydraulics for navy and machine tools; and many others. The screw pump can handle liquids in a range of viscosities, from molasses to gasoline, as well as synthetic liquids in a pressure range from 50 to 5000 lb/in2 (3.5 to 350 bar) and flows up to 8000 gal/min (1820 m3/h). Because of the 48
relatively low inertia of their rotating parts, screw pumps are capable of operating at higher speeds than other rotary or reciprocating pumps of comparable displacement. Some turbine-attached lubricating oil pumps operate at 10,000 rpm and even higher. Screw pumps, like other rotary positive displacement pumps, are self-priming and have a delivery flow characteristic, which is essentially independent of pressure, provided there is sufficient viscosity in the liquid being pumped. Screw pumps are generally classified into single- or multiple-rotor types. The latter is further divided into timed and un-timed categories. The single-screw or progressive cavity pump has a rotor thread that is eccentric to the axis of rotation and meshes with internal threads of the stator (rotor housing or body). Alternatively, the stator is made to wobble along the pump centerline. Multiple-screw pumps are available in a variety of configurations and designs. All employ one driven rotor in a mesh and one or more sealing rotors. Several manufacturers have two basic configurations available: single-end and double-end construction, of which the latter is the better known. ADVANTAGES OF SCREW PUMP:
As with every pump type, certain advantages and disadvantages can be found in a screw pump design. These should be recognized when selecting the best pump for a particular application. The advantages of a screw pump design are as follows: • A wide range of flows and pressures • A wide range of liquids and viscosities • High speed capability, allowing the freedom of driver selection • Low internal velocities • Self-priming, with good suction characteristics • A high tolerance for entrained air and other gases Multiple-screw double-end arrangement. • Low velocities for minimum churning or foaming • Low mechanical vibration, pulsation-free flow, and quiet operation • A rugged, compact design that is easy to install and maintain • High tolerance to contamination in comparison with other rotary pumps DISADVANTAGES OF SCREW PUMP:
• A relatively high cost because of close tolerances and running clearances • Performance characteristics sensitive to viscosity changes • High pressure capability requires long pumping elements
49
GEAR PUMPS Gear pumps are installed before and after the DRR. Gear pump are positive displacement rotary pumps. The Hydraulic Institute defines them as mechanisms consisting of a casing with closely fitted gears that provide a means for conveying a fluid. Their principle motion is rotating, rather than reciprocating, and they displace a finite volume of fluid with each shaft revolution. When describing them, the general term fluid is used, rather than the more restrictive liquid. Fluid, in this case, is understood to include not only true liquids, but mixtures of liquids, gases, vapors, slurries, and solids in suspension as well. CENTRIFUGAL PUMPS A centrifugal pump is a rotating machine in which flow and pressure are generated dynamically. The inlet is not walled off from the outlet, as is the case with positive displacement pumps, whether they are reciprocating or rotary in configuration. Rather, a centrifugal pump delivers useful energy to the fluid or “pumpage” largely through velocity changes that occur as this fluid flows through the impeller and the associated fixed passageways of the pump; that is, it is a “roto-dynamic” pump. All impeller pumps are roto-dynamic, including those with radial-flow, mixed-flow, and axial-flow impellers: the term “centrifugal pump” tends to encompass all roto-dynamic pumps. Although the actual flow patterns within a centrifugal pump are three-dimensional and unsteady in varying degrees, it is fairly easy, on a one-dimensional, steady-flow basis, to make the connection between the basic energy transfer and performance relationships and the geometry or what is commonly termed the “hydraulic design” (more properly the “fluid dynamical design”) of impellers and stators or stationary passageways of these machines. In fact, disciplined one-dimensional thinking and analysis enables one to deduce pump operational characteristics (for example, power and head versus flow rate) at both the optimum or design conditions and off-design conditions. This enables the designer and the user to judge whether a pump and the fluid system in which it is installed will operate smoothly or with instabilities. The user should then be able to understand the offerings of a pump manufacturer, and the designer should be able to provide a machine that optimally fits the user’s requirements. The complexities of the flow in a centrifugal pump command attention when the energy level or power input for a given size becomes relatively large. Fluid phenomena such as recirculation, cavitation, and pressure pulsations become important; “hydraulic” and mechanical interactions—involving stress, vibration, rotor dynamics, and the associated design approaches, as well as the materials used—become critical; and operational limits must be understood and respected.
50
3.4. MATERIAL BALANCE FOR THE PET PROCESS DESIGN
SPECIFIED INFORMATION: o PRODUCTION RATE----100 tonnes / day Bottle grade Polyethylene Terephtalate (PET) o RAW MATERIALS: 1. Ethylene Glycol (EG), in liquid. 2. Purified Terephthalic acid (PTA), in powder form.
OVERALL MASS BALANCE FOR 100TPD PRODUCTION PLANT Table, represented below, shows the overall material balance of the selected PET process unit. The comprehensive calculations are worked out in Appendix B.
INPUT
OUTPUT
Kg/hr
Kg/hr
SPECIES
51
MIXER PTA EG
3587.06 1607.694
5194.754
TOTAL
5194.754
5194.754
836.0011 2443.695 -
4855.465 413.8032 73.31086 688.1762
TOTAL PRE-POLYCONDENSATION
6030.755
6030.755
PTA BHET (produced) TOTAL BHET EG CONSUMED H2O (produced) PET BHET (un-reacted) EG (produced) TOTAL EG EG left PET BHET
413.8032 633.1688 5488.634 309.106 89.74046 3966.993 240.6327 1281.008 996.8378 -
19.93676 3966.993 240.6327
5294.204
4227.563
976.9011 89.74046 -
195.3802 781.5209 89.74046
1066.642
1066.642
240.6327 19.93676 3966.993
72.85181 4093.82
4227.563
4166.671
60.89114
59.67332
ESTERIFICATION MAKE UP EG (SEG) TOTAL EG IN BHET PTA EG (un-reacted) WATER
TOTAL
CONDENSER: EG VAPORS H2O VAPORS EG CONDENSATE H2O CONDENSATE TOTAL POLYCONDENSATION BHET (to be reacted) EG TOTAL PET TOTAL CONDENSER: EG
52
JET SYSTEM EG
196.598
196.598
EG RECOVERY COLUMN LIQUID FEED EG H2O
871.2613 781.5209 89.74046
VAPOR FEED EG EVAPORATED H2O
736.5514 48.3752 688.1762
TOTAL H2O TOTAL EG
777.9166 829.896
TOTAL
1607.813
DISTILLATE & REFLUX H2O EG
777.9166 27.06556
BOTTOM EG
802.8305 TOTAL
1607.813
MOTHER VESSEL COLUMN JET SYSTEM DRR CONDENSER TOTAL EG
802.8305 196.598 59.67332 1059.102
836.0011
53
3.5. ENERGY BALANCE OF THE PROCESS
SPECIES
INPUT KJ/hr
OUTPUT KJ/hr
EG PTA
2.46E+05 3.84E+04
1.64E+05 1.99E+05
TOTAL
2.84E+05
3.63E+05
SEG EG PTA BHET H2O
2.26E+05 1.64E+05 1.99E+05 -
1.14E+05 2.90E+05 3.31E+06 2.02E+06
TOTAL
5.88E+05
5.74E+06
2.81E+04 2.90E+05 3.31E+06 3.63E+06
1.80E+06 1.74E+05 2.66E+05 4.96E+06 7.21E+06
BHET PET EG
1.74E+05 4.96E+06 2.38E+04
5.52E+04 5.38E+06 1.13E+05
TOTAL
5.16E+06
5.55E+06
MIXER
ESTERIFICATION
PRE-POLYCONDENSATION EG PTA BHET H2O PET TOTAL POLYCONDENSATION
54
COLUMN
Liquid
Vapor
TOP
BOTTOM
2.02E+06 -
5.89E+05
H2O EG REFLUX H2O
4.74E+04 2.02E+06 5.80E+03 8.63E+04 1.47E+05
TOTAL
2.31E+06
2.61E+06
MOTHER VESSEL EG (JET) at 100oC
6.32E+04
-
EG (DRR) at 33oC
1.91E+03
-
EG (COLUMN) at 180oC
5.70E+05
-
TOTAL
6.35E+05
3.40E+05
-
55
HEAT EXCHANGERS
cooling water Distillate
cooling water PP Cond
cooling water DRR Cond
Temperature o C 32 46 100 70 32 46 95 40 10 46 150 33
COLUMN CONDENSER Temperature Cp K 305.15 75.4 319.15 75.49 373.15 75.9 343.15 75.55 PP HEAT EXCHANGER 305.15 75.4 319.15 75.49 368.15 144.1 313.15 131.7 DRR HEAT EXCHANGER 283.15 74 319.15 75.49 423.15 187.0 306.15 151.7
H
Q
1084.323
m*1084.3
42397.1
3664601
1084.3
m*1084.3
11794.46
1669978
3139.534
m*3139.5
32695.17
77487.55
1084.323
m*1084.3
9508.3
34214.51
1084.3
m*1084.3
12560.6
351318.7
1084.3
m*1084.3
4208.0
47424.3
m (Kmol/hr) 3379.6
1540.1
24.7
MIXER HEAT EXCHANGER
cooling water SEG
32 46 100 62
305.15 319.15 373.15 335.15
75.4 75.49 172.78 164
31.6
JET HEAT EXCHANGER 1
cooling water EG
32 46 100 54
305.15 319.15 373.15 327.15
75.4 75.5 172.6 158.4
324.0
JET HEAT EXCHANGER 2
cooling water EG
32 46 100 85
305.15 319.15 373.15 358.15
75.4 75.5 172.6 168.1
43.7
56
3.6. SIZING OF THE MIXER FOR FEED PASTE FORMATION
As described in the preceding chapters that a slurry of EG and TPA are mixed in a stirred tank vessel equipped with a stirrer for viscous fluids (e.g. intermig) to prepare a paste with molar ratio of EG & TPA of 1:2. When considering any mixing application it is important to realize that the optimum solution will depend on a variety of different factors. And it is crucial to specify precisely the right system configuration to best serve the total needs of a particular mixing operation. Mixing Mechanisms Mixing is achieved by a number of different mechanisms, as summarized in the following table. The most important mechanism will vary for any given application, and a given process may rely on any or all of these mechanisms. In order to arrive at an optimum mixer design a detailed understanding of the various mechanisms involved and their importance in achieving the process result is required.
Convection
Macro-mixing
Induced by pumping action of the impeller, Fluid moves through the different parts of vessel, preventing stratification.
Caused by turbulent flow a wide range of vortices. Smallest in the impeller region where dissipation is the highest. Separates bulk of fluid into smaller elements
Laminar shear
Below the scale of macro mixing fluid elements are further dispersed by laminar shearing. Elements are stretched, distorted and folded.
Micro-mixing
Final smallest scale mixing. Diffusion of reactants takes place and is driven only by concentration gradient. Takes place on scale smaller than any eddy size.
By far the most common of mixing applications are those which rely upon flow to achieve the required process result. These applications are often referred to as, 'Flow 57
Controlled Applications', and include such applications as Blending, Solids Suspension and Heat Transfer. For the reason that polymerization is one of the typical blending applications thus only this function is considered here.
Blending / Homogenization of Miscible Liquids Blending involves the mixing of two or more miscible liquids to achieve a uniform mixture throughout the entire volume of the tank, usually within a specified period of time. It is important to note that the blending of liquids having widely varying density and/or viscosity, as in the case of EG & TPA, requires special attention and may require longer blend times to ensure the liquids are mixed.
Information Required for Mixer Selection o o o o o o
Viscosity Density Pressure & Temperature Blend Time Volume (s) Any specific process requirement
Optimum Mixer Selection Criteria A full and accurate specification of the mixing vessel, the process parameters, and the required mixer performance is the first crucial step to arrive at an optimum mixing operation. Some of the many other variables that can affect mixer performance and so need to be considered in arriving at the optimum mixer design are noted on these pages. IMPELLERS
Impeller Type The function of any mixing impeller is to convert the rotational energy of the mixer shaft into the correct combination of flow, shear and turbulence to achieve the required process result.
58
As no one-impeller design is capable of providing optimum performance under every process condition, optimum process performance is dependent upon selecting an impeller design that has the specific characteristics required by a given process. Number of Impellers The use of a single impeller is the usual preferred option on the basis of cost. However, changes in the ratio of liquid level (Z) to vessel diameter (T) can have an adverse effect on the flow patterns generated within the vessel. This can result in the need to consider the use of multiple impellers in order to achieve an economic solution. Z/T ratio alone is not the only consideration when determining the number of impellers required. Multiple impellers may also need to be considered for other reasons including, when high viscosity fluids are involved, for mixing at low level during filling and emptying or where draw down from the liquid surface is a requirement. Impeller Positioning Whether utilizing a single or a multiple impeller configuration, the positioning of the impellers within the process fluid can have a significant effect on the overall process performance. Incorrect positioning can lead to staged flow patterns, poor dispersion of additives and impellers being out of the liquid at crucial stages of the process. D/T Ratio The ratio of mixing impeller diameter (D) to vessel diameter (T) has a very significant effect on the performance of most fluid mixers and the optimum D/T is a function of both process conditions and process requirements. Normally the optimum D/T will be in the range 0.2 < D/T < 0.5. Some special applications however, sometimes operate outside this range. Bottom Clearance The impeller bottom clearance (C/T ratio) can also have a very significant effect on the overall performance of a mixer, affecting both power draw and pumping efficiency. The optimum C/T ratio is essentially dependent upon impeller type but can also be affected by process conditions. Normally, the optimum C/T will be in the range: 0.1 < C/T < 0.3. Hydrofoils however can operate at much higher levels, up to C/T = 0.5 or more.
VESSEL DESIGN
59
Vessel Geometry When designing a vessel for mixer duty it is important to understand the role that tank geometry plays in determining the final mixer design. Poor aspect ratios and or inappropriate bottom shapes can both result in increase mixer cost and in certain circumstances make it impossible to optimize the mixer design. Aspect Ratio It is generally accepted that the ideal aspect ratio for most mixing tanks is one where the liquid depth (Z) is equal to the tank diameter (T) as this allows for the optimum number of impellers, optimum power input and optimum power distribution. In practice, the optimum Z/T will be in the range 0.9 < Z/T ^ 1.2 as this does not significantly effect mixer design. Bottom Shape Tank bottom shape can have a significant effect on the flow patterns generated within the mixing vessel and hence the mixers ability to achieve optimum process performance. Normally a dish-bottom tank is the preferred bottom shape. However, flat-bottoms and shallow cones (less than 15°) can be used for many processes without any particular problem. In the case of flat-bottomed tanks mixer performance can often be significantly improved by the introduction of corner fillets. In general deep cones should be avoided especially where the requirement is solids suspension. Baffles The importance of proper tank baffling in obtaining optimum mixer performance should not be underestimated. In a correctly baffled tank the mixer develops the fluid regime required to achieve the optimum process result. An incorrectly baffled tank on the other hand can lead to poor mixer performance and may even result in the mixer not being able to achieve the process result for which it was designed. It is normally desirable to set the baffles off the wall and off the bottom of the tank to prevent solids or fluids from stagnating at those points. The optimum baffling arrangement however, will vary from process to process and is dependant upon a variety of factors including, vessel geometry, vessel internals, specific power, the required surface effects, and viscosity. Mixing Intensity
60
Within the process industries in general it has become convenient to characterize the level of mixing required for a given application in terms of agitation intensity. This practice has led to the introduction of terms like mild or vigorous agitation. Whilst such general terms are convenient they can mean different things to different people and so need quantifying if they are to be of any practical use. The following table gives a general overview of the various degrees of agitation in common use throughout the industry.
61
MIXING EQUIPMENT SPECIFICATIONS The proposed designing specifications of the tank meant for preparing the paste of ethylene glycol and pure terephthalic acid of the desired consistency, with the mole ratio of 1.2, are evaluated below. The inclusive calculations are worked out in Appendix D.
ENTITY Mixing Temperature: Mixing Time: Viscosity of Paste: (EG & PTA) Density of Paste: Mixer Capacity: Vessel Height: Vessel Diameter: Liquid Level: Level above agitator: Selected Impeller: Impeller Diameter: No. of baffles: Baffle width: Baffle Clearance: Impeller bottom clearance: Impeller Blade width: Blade length: Impeller rotational speed: (Gear drive) Power consumption by Impeller:
PROJECTED DIMENSIONS 50oC 2hr 4.8827 * 10-3 Pa.S (or 6.88224 cP) 1217.553 kg/m3 15m3 4.24m 2.12m 2.417m 1.57m 6 pitched-blade turbine type 0.707m 4 0.212m 0.0212m 0.707m 0.1413m 0.1768m 5 rps (300rpm) 46.882 hp
62
Nomenclature: Where, D = Impeller Diameter C = Impeller off Bottom Clearance N = Impeller speed Z = liquid Depth T = Vessel Diameter
As tabulated in the table, in the mixing vessel, 6 pitched-blade turbine type axial flow impeller is selected as an agitator (impeller in which the blade makes an angle of less than 90° with the plane of rotation). Pitched-blade turbines are used on top-entering agitator shafts instead of propellers when a high axial circulation rate is desired, that is the cause it is chosen in the case of EG & TPA paste formation. Another reason is that it is effective for rapid submergence of floating particulate solids, near the upper surface of liquid in a vessel.
63
PITCHED-BLADE TURBINE
If further efficient mixing is required then two impellers can be mounted on the same shaft as shown in figure given below, having the same dimensions as mentioned above, with the distance (S) equal to the diameter of the impeller between them. But in such condition, it will be required to ensure that the upper impeller remain immersed, at all time, in the paste during agitation.
DUAL IMPELLER MIXER & DRIVE
64
FLOW PATTERN IN MIXER
In fluid agitation, the direction as well as the magnitude of the velocity is essential. The directions of the velocity vectors throughout an agitated vessel are referred to as the flow pattern. Since the velocity distribution is constant in the viscous and turbulent ranges, the flow pattern in an agitated vessel is fixed. The use of vertical sidewall baffles destroys the rotary and swirling motion in the tank (reduces the vortex formation). The axial-flow turbine (shown in figure) actually gives a flow coming off the impeller of approximately 45°, and therefore has a re-circulation pattern coming back into the impeller at the hub region of the blades.
Typical flow pattern in a baffled tank
65
3.7. PROCESS & INSTRUMENTATION DIAGRAM OF THE MIXER
We are basically concerned with the level of the mixer and it can be lowered then specified level so it can be controlled by the flow rate of ethylene glycol. Since flow rate of spent ethylene glycol can’t be maintained therefore the mole fraction of ethylene glycol is maintained by fresh one. The process and instrumentation plan, for the mixer sized, is illustrated underneath showing all the crucial appliances.
F F
Y
T
F
I C
L
I C
M
L
T
P&ID OF THE EG & PTA MIXER
66
APPENDICES
67
APPENDIX A
Reactions and Species
The following section details the reactions and components involved in the PET manufacturing process. For this study, PET formation is modeled with a system of second order reactions. The model includes eight components. Table A.l lists the component's name, type, molecular weight, and the component's symbol. Table A.I: Species Notation and Molecular Weights
68
Each reaction has the following form A + B → C + D. Although most of the reactions are reversible, all are treated as forward reactions (reversible reactions are treated as two forward reactions with the second reaction's component list inverted).
Polymerization Reactions In the polycondensation reaction, two hydroxy ethyl ester end groups react to yield an internal ethyl diester and a free glycol. 1) EG + EG ↔ ZG + FG If one of the hydroxy ethyl ester end groups is replaced by a hydroxy ethyl ethoxy ester end group, the reaction yields an internal ethyl diester and a free diethylene glycol or a free glycol and an internal ethyl ethoxy diester.
69
2) EG + ED ↔ ZG + FD 3) ED + EG ↔ ZD + FG If both of the reactants are hydroxy ethyl ethoxy ester end groups, the reaction yields an internal ethyl ethoxy diester and a free diethylene glycol. 4) ED + ED ↔ ZD + FD The kinetic parameters involved in reactions (1), (2), (3) and (4) are assumed to be identical since the functional groups where the reaction occurs are the sarne. In the esterification reaction a hydroxy ethyl ester group or a free glycol reacts with an acid end. The primary example of this type of reaction is an acid end reacting with free ethylene glycol yielding a hydroxy ethyl ester end group and water. 5) EC + FG ↔ EG + FC If the free glycol from the above reaction is replaced by a free diethylene glycol the reaction produces water and a hydroxy ethyl ethoxy ester end group. 6) EC + FD ↔ ED + FC The kinetic parameters of this reaction are the same as for reaction (5) since the functional groups involved in the reaction are the same. The esterification reaction between an acid end and a hydroxy ethyl ester end yields an internal ethyl diester and water. 7) EC + EG ↔ ZG + FC If a hydroxy ethyl ethoxy ester replaces the hydroxy ethyl ester in the above reaction it results in formation of an internal ethyl ethoxy diester and water. 8) EC + ED ↔ ZD + FC Reactions (7) and (8) can be assumed to have same kinetic parameters since the functional groups involved are the same. If a hydroxy ethyl ethoxy end reacts with a free glycol, a hydroxy ethyl ester end and a free diethylene glycol are formed. 9) ED + FG ↔ EG + FD The most important side reactions involved in PET manufacturing are DEG formation, both in the free and associated forms. The following reactions come under this category. 10) EG + EG ↔ ED + EC If one EG group is substituted by a free glycol the following reaction is observed. 70
11 ) EG + FG ↔ FD + EC Diethylene glycol contained species can also be formed by the following reactions. 12) EG + ZG ↔ ZD + EC 13) ZG + FG ↔ ED + EC It should be noted here that reactions (11) and (12) have the same kinetic parameters as reaction (10), but since free glycol has a functionality of two, the rate constant is multiplied by a factor of two. The rate constant for reaction (13) is the same as that for reaction (10). To account for the number of sites available for the occurrence of the reaction, the rate constant is Inultiplied by a factor of four.
71
APPENDIX B
Material Balance MIXER: By the help of following Equation we may calculate the amount of Raw Materials required to produce 100 TPD of Poly (ethylene Terephthalate) which is taken as the basis for calculation:
By the stoichiometry, for the production of 100TPD PET we are required 21.6088 Kmol/hr (3587.06 Kg/hr) of PTA and since 1.2-mol ratio of EG: PTA is maintained so 25.9305 Kmol/hr (1607.694 Kg/hr) of EG is required. It forms 70:30 wt% of PTA: EG. ESTERIFICATION REATOR: It taken as bases that 52% of Ethylene Glycol is added so as to maintain 1.8-mol ratio of EG. So the amount of EG is 39.41 Kmol/hr (2443 Kg/hr). From the equation it is shown that 2 moles of EG is required to produce 1 mol of PET but we are taken 1.8-mol ratio and hence EG is Limiting reactant. It is also taken as the basis that 97% conversion is taken place in the Esterifier reactor. BHET Produced
=
PTA remains un-reacted
= 0.0888 21.61 + 0.912 21.61 0.03 = 2.49Kmol/hr
EG remains un-reacted
= 0.03
Kmol/hr
39.41 = 1.18 Kmol/hr
(4855 Kg/hr)
(414Kg/hr) (73.31 Kg/hr)
72
= 0.97 39.41 = 38.23 Kmol/hr = 1.18 - 0.78 = 0.402 Kmol/hr
H2O Produced TOTAL EG OUT
(688 Kg/hr) (25 Kg/hr)
To Column: Since at high temperature all of the H2O and assumed to be 2mol % of EG is evaporated as Esterifier is operating at 258oC. EG evaporated
=
Kmol/hr (48.37 Kg/hr)
H2O
= 38.23 Kmol/hr (688 Kg/hr)
PRE-POLYCONDENSATION REACTOR: It is required to calculate first the amount of conversion of BHET into PET. It is assumed that about 0.27 IV (intrinsic viscosity) is maintained after Pre-Polycondensation unit. By the help of formula:
It is obtained by calculation that the conversion is 96 %. Since BHET converts into PET with the production of EG and this product EG is used to convert PTA. Both of these reactions are simultaneously occurring thus it is difficult to calculate them, so we first assumed the production of BHET and then conversion of BHET into PET is presumed. It is also assumed that conversion of PTA is 100% with no PTA left un-reacted.
BHET Produced TOTAL BHET EG Produced EG Consumed TOTAL EG PET produced H2O Produced
= PTA converted = 2.493+19.116 = 0.96 21.608 = 2 2.49 = 20.66-4.98 = EG produced = EG consumed
=2.493 kmol/hr (633 Kg/hr) = 21.6 Kmol/hr (5489 Kg/hr) = 20.66 Kmol/hr (1281 Kg/hr) = 4.98 Kmol/hr (309 Kg/hr) = 16.07 Kmol/hr (996 Kg/hr) = 20.66 Kmol/hr (3966 Kg/hr) = 4.98 Kmol/hr (90 Kg/hr)
73
EG OUT
= 0.32 Kmol/hr
= 16.07-15.76
(19.94 Kg/hr)
To Condenser: It is assumed that at high vacuum about 98% of EG and 100% of H2O has been removed by evaporation, it is needed to remove as high as we could to prevent the probability of reversible reaction. EG evaporated H2O
= 0.98
16.07
= 15.756 Kmol/hr (976 Kg/hr) = 4.98 Kmol/hr (90 Kg/hr)
POLYCONDENSATION REACTOR (D.R.R): It is a special type of horizontal reactor, supplied with disc ring that provides the maximum surface area that helps in termination and growth of chain. It is required to calculate first the amount of conversion of BHET into PET. It is assumed that about 0.61 IV (intrinsic viscosity) is maintained after Pre-Polycondensation unit. By the help of formula:
It is obtained by calculation that the conversion is 98.7 %. BHET to be reacted EG Produced TOTAL EG BHET un-reacted Kg/hr) TOTAL PET
= (0.9867-0.96) 21.6088 = 0.66 Kmol/hr (168 Kg/hr) = BHET reacted = 0.66 Kmol/hr (41 Kg/hr) = 0.66 + 0.32 = 0.98 Kmol/hr (61 Kg/hr) = 0.947 – 0.66 = 0.286 Kmol/hr (72.85 = 20.66 + 0.66
= 21.32 Kmol/hr (4094
Kg/hr) TOTAL PRODUCT PRODUCED
= PET+BHET un-reacted =73 + 4094
74
= 4167 Kg/hr
In Tons per Day: PRODUCT =
= 100 TPD
To Condenser: It is assumed that at high vacuum about 100% of EG has been removed by evaporation; it is needed to remove as high as we could to prevent the probability of reversible reaction. EG evaporated = 0.98 Kmol/hr (61 Kg/hr) DRR CONDENSER: It is assumed that 98% of EG is evaporated. EG Condensed EG Uncondensed
= 0.98 0.98 = 0.98 - 0.96
= 0.96 Kmol/hr (60 Kg/hr) = 0.02 Kmol/hr (1.22 Kg/hr)
PP CONDENSER: It is assumed that 80% of total vapors are condensed, which constitutes 80% of EG condensed vapors whereas 100% of H2O condensed vapors. Vapor Condensed: EG H2O
= 0.8
15.76
= 12.61 Kmol/hr (781 Kg/hr) = 4.98 Kmol/hr (89.7 Kg/hr)
Uncondensed Vapor: EG
= 15.76 - 12.61
=
3.15 Kmol/hr
(195 Kg/hr)
JET SYSTEM: Since three-stage jet system has been used, along with vacuum pump so as to condense the uncondensed vapors from both PP and DRR
75
condensers. It is assumed to be 100% efficient and all the vapors has been condensed. Uncondensed EG = 0.02 + 3.15 = 3.17 Kmol/hr (196 Kg/hr)
DISTILLATION COLUMN: FEED IN: Feed enters into the column at different flash points as a mixture of vapors and liquid. Liquid feed is used to manipulate the temperature of column and throttle accordingly. LIQUID FEED: All of the Liquid feed is taken from PP Condenser, therefore EG = 12.6 Kmol/hr (781 Kg/hr) H2O = 4.98 Kmol/hr (89.74 Kg/hr) VAPOR FEED: All of the Vapor feed is taken from Esterifier reactor, therefore EG = 0.78 Kmol/hr (48.37 Kg/hr) H2O = 38.23 Kmol/hr (688 Kg/hr) TOP OUT: It is assumed that 99 mol% of H2O is obtained as the top product. H2O
= 38.23 + 4.98
= 43.22 kmol/hr (778 Kg/hr)
EG
=
= 0.44 Kmol/hr (27 Kg/hr)
TOTAL TOP OUT
= 778+27
= 804 Kg/hr
H2O
=
= 21.61 Kmol/hr (389 Kg/hr)
EG
=
= 0.22 Kmol/hr(13.53 Kg/hr)
REFLUX: Reflux ratio is taken to be 1.
76
DISTILLATE: H2O
=
= 21.61 Kmol/hr (389 Kg/hr)
EG
=
= 0.22 Kmol/hr(13.53 Kg/hr)
BOTTOM: 100% of EG is taken as Bottom Product. EG = 13.38 - 0.436
= 12.95 Kmol/hr (803 Kg/hr)
MOTHER VESSEL:
Column EG Jet System EG DRR Condenser EG TOTAL EG IN EG to Esterifier EG to Mixer
= 12.95 + 3.17 + 0.96 = 17.08 - 13.48
= 12.95 Kmol/hr (803 Kg/hr) = 3.17 Kmol/hr (196 Kg/hr) = 0.96 Kmol/hr (60 Kg/hr) = 17.08 Kmol/hr(1060 Kg/hr) = 13.48 Kmol/hr (836 Kg/hr) = 3.6 Kmol/hr (223 Kg/hr)
77
APPENDIX C
Energy Balance HEAT CAPACITIES: The heat capacity is the amount of heat required to raise the temperature of an object or substance one degree. The basic equations used for the calculation of Heat Capacities are 1. FOR LIQUID:
2. FOR GASES:
The values of A, B, C, D and E are obtained from several sources and graphs including DIPPR 8.0 for liquid and gases except monomers and polymer (PET). Cp at 25 o C (KJ/Kmol)
ENTITY
A
B
C
D
E
Ethylene Glycol Liquid Ethylene Glycol Vapor Water Liquid Water Vapor Terephthalic Acid Poly(ethylene Terephthalate) BHET
3.55E+04 8.20E+04 2.76E+05 3.34E+04 -2.60E+04
4.36E+02 1.27E+05 -2.09E+03 2.68E+04 6.34E+02
-1.84E-01 1.69E+03 8.13E+00 2.61E+03 0.00E+00
0.00E+00 9.29E+04 -1.41E-02 8.89E+03 0.00E+00
0.00E+00 -7.54E+02 9.37E-06 1.17E+03 0.00E+00
1.49E+02
9.00E-01
5.00E-03
0.00E+00
0.00E+00
0.00E+00
4.59E+02
Cp’ = 454.3 KJ/KmolK
7.53E+01 1.63E+02
Cp=454*(0.64+0.0012*T)
4.53E+02
HEAT OF VAPORIZATION: 78
The enthalpy of vaporization, (symbol ΔvH), also known as the heat of vaporization or heat of evaporation, is the energy required to transform a given quantity of a substance into a gas. It is often measured at the normal boiling point of a substance, although tabulated values are usually corrected to 298 K: the correction is small, and is often smaller than the uncertainty in the measured value.
By using the formula;
ENTITY Ethylene Glycol Water
Hla 1.95E+04 1.35E+04
Hlb 6.45E+02 6.47E+02
H (KJ/Kmol) 4.97E+04 4.08E+04
ALL Heat of Vaporization calculated at their boiling point.
BASIS: • Reference temperature is considered to be 25 oC (298.15 K) •
Heat capacities for the saturated vapor is calculated by adding heat capacities of liquid, latent heat of vaporization and heat capacities at particular temperature.
•
H can be calculated by using following formula;
TEMPERATURES: ENTITY ESTERIFICATION PREPOLYCONDENSATION POLYCONDENSATION DRR Reference Boiling point EG Boiling point Water PTA IN EG IN OUT SEG Liquid feed Vapor feed Reflux
TEMPERATURE (oC)
TEMPERATURE (K)
258 270 280 25 197.6 100 MIXER 30 62 50 ESTERIFICATION 100 COLUMN 40 258 70
531.15 543.15 553.15 298.15 470.75 373.15 303.15 335.15 323.15 373.15 313.15 531.15 343.15
79
Top out Bottom out Q recycle Q cond Q uncond Q recycle Q cond Q uncond
100 180 PP CONDENSER 40 95 210 DRR CONDENSER 33 150 210
373.15 453.15 313.15 368.15 483.15 306.15 423.15 483.15
MIXER: FEED IN: ETHYLENE GLYCOL:
PURE TEREPHTHALIC ACID:
TOTAL IN =
= 2.84
105 KJ/hr
MIXER OUT: ETHYLENE GLYCOL:
PURE TEREPHTHALIC ACID:
TOTAL OUT =
= 3.63
105 KJ/hr
ESTERIFICATION REACTOR: FEED IN: SPENT ETHYLENE GLYCOL(SEG):
80
= 5.88
TOTAL IN =
105 KJ/hr
ESTERIFICATION OUT: To Pre-Polycondensation: ETHYLENE GLYCOL:
PURE TEREPHTHALIC ACID:
BHET:
TOTAL OUT =
+
+
=
KJ/hr
To Column: ETHYLENE GLYCOL:
WATER:
81
TOTAL OUT = 2.02 106+8.63 104 = 2.11
106 KJ/hr = 5.74
TOTAL OUT OF ESTERIFIER =
106 KJ/hr
HEAT OF REACTION: AMOUNT OF HEAT REQUIRED: = TOTAL OUT – TOTAL IN – HEAT OF REACTION = 5.74 106-5.88 105-(-2.87 105)= 5.43 106KJ/hr It is assumed that 30% of heat is supplied by the liquid dowtherm while 70% of heat is supplied by vapor dowtherm. Mass Flow of Liquid DOWTHERM:
Mass Flow of Vapor DOWTHERM:
PRE-POLYMERIZATION: FEED IN: ESTERIFIER PRODUCT =
KJ/hr
PRODUCT OUT: To DRR: Poly(ethylene Terephthalate):
82
BHET:
ETHYLENE GLYCOL:
TOTAL OUT=
= 5.16
106 KJ/hr
To Condenser: ETHYLENE GLYCOL:
WATER:
TOTAL OUT = 1.78 106+2.66 105 = 2.04
106 KJ/hr
TOTAL OUT OF PP =
= 7.21
106 KJ/hr
HEAT OF REACTION:
83
AMOUNT OF HEAT REQUIRED: = TOTAL OUT – TOTAL IN – HEAT OF REACTION – HEAT OF POLYMERIZATION =7.21 106 -3.63 106-(-3.74 104)-0= 3.62
106KJ/hr
Mass Flow of Vapor DOWTHERM:
POLYCONDENSATION (DRR): FEED IN: PRE-POLYMERIZER PRODUCT = 5.16
106 KJ/hr
MELT PHASE PRODUCT: Poly(ethylene Terephthalate):
BHET:
TOTAL OUT =
= 5.44
106 KJ/hr
To Condenser: ETHYLENE GLYCOL:
84
= 5.55
TOTAL OUT OF DRR =
106 KJ/hr
AMOUNT OF HEAT REQUIRED: = TOTAL OUT – TOTAL IN – HEAT OF POLYMERIZATION =5.55 106 -5.16 106-0= 3.90
105KJ/hr
Mass Flow of Vapor DOWTHERM:
COLUMN: LIQUID FEED: ETHYLENE GLYCOL:
WATER:
TOTAL IN =
= 5.32
104KJ/hr
VAPOR FEED: ETHYLENE GLYCOL:
85
WATER:
TOTAL IN = 2.02 106+8.63 104 = 2.11
106 KJ/hr
REFLUX: WATER:
TOTAL IN = 2.11 106+5.32 104+7.36 104 = 2.31
106KJ/hr
TOP OUT: It is assumed to be 100% water and neglecting ethylene glycol in the top product. WATER:
BOTTOM OUT: ETHYLENE GLYCOL:
TOTAL OUT = 2.02 106+5.89 105 = 2.61
106KJ/hr
AMOUNT OF HEAT REQUIRED: = TOTAL OUT – TOTAL IN 86
=2.61 106 -2.31 106= 2.99
105KJ/hr
Mass Flow of Liquid DOWTHERM:
PRE-POLYMERIZATION CONDENSER:
Q recycle: ETHYLENE GLYCOL:
WATER:
Q cond: ETHYLENE GLYCOL:
WATER:
Q uncond: ETHYLENE GLYCOL:
87
By the equation:
POLYMERIZATION CONDENSER:
Q recycle: ETHYLENE GLYCOL:
Q cond: ETHYLENE GLYCOL:
Q uncond: ETHYLENE GLYCOL:
By the equation:
88
MOTHER VESSEL: EHTYLENE GLYCOL IN: From JET SYSTEM:
From DRR VESSEL:
From COLUMN:
TOTAL IN =
= 6.35
105KJ/hr
EXIT TEMPERATURE:
ETHYLENE GLYCOL OUT:
JET SYSTEM: ENTITY Qf Qcond Quncond Qrecycle Qf Qcond Quncond Qrecycle
TEMPERATURE (oC) STAGE 1st 210 80 210 54 STAGE 2nd 210 100 210 85
TEMPERATURE (K) 483.15 353.15 483.15 327.15 483.15 373.15 483.15 358.15
89
Qf Qcond Quncond Qrecycle
STAGE 3rd 210 120 210 100
483.15 393.15 483.15 373.15
STAGE 1st:
Q recycle: ETHYLENE GLYCOL:
Q cond: ETHYLENE GLYCOL:
Q uncond: ETHYLENE GLYCOL: By the equation:
STAGE 2nd:
Q recycle: ETHYLENE GLYCOL:
90
Q cond: ETHYLENE GLYCOL:
Q uncond: ETHYLENE GLYCOL: By the equation:
STAGE 3rd:
Q recycle: ETHYLENE GLYCOL:
Q cond: ETHYLENE GLYCOL:
Q uncond: ETHYLENE GLYCOL: By the equation:
91
APPENDIX D Balances on Heat Exchangers COLUMN CONDENSER: TEMPERATURES: COOLING WATER DISTILLATE
IN 32 100
OUT 46 70
COOLING WATER: IN: OUT:
DISTILLATE: IN: OUT:
SINCE, ENERGY IN = ENERGY OUT 92
PREPOLYMERIZATION HEAT EXCHANGER: TEMPERATURES: IN 32 95
COOLING WATER PP COND
OUT 46 40
COOLING WATER: IN: OUT:
PP COND: IN: ETHYLENE GLYCOL: WATER:
OUT: ETHYLENE GLYCOL: WATER:
SINCE, ENERGY IN = ENERGY OUT
DRR HEAT EXCHANGER: TEMPERATURES: IN
OUT 93
COOLING WATER DRR COND
10 150
46 33
COOLING WATER: IN: OUT:
DRR COND: IN: OUT:
SINCE, ENERGY IN = ENERGY OUT
MIXER HEAT EXCHANGER: TEMPERATURES: COOLING WATER SPENT ETHYLENE GLYCOL
IN 32 100
OUT 46 62
COOLING WATER: IN: OUT:
SPENT ETHYLENE GLYCOL: IN: OUT: 94
SINCE, ENERGY IN = ENERGY OUT
NOTE: By the specification, it is defined that the supply water is available at 32 oC and return water having the temperature of 46 oC.
APPENDIX E
Mixing Equipment Sizing
Information Required for Mixer Selection
1. Mixer Feed Data o Mixing Temperature: 50oC o Mixing Time: 2hr
COMPONE NT
MOLE FRACTIONS x
DENSITY, ρ Kg/m3
VISCOSITY, µ Pa. S
EG
0.70
1092.218
0.0069724
TPA
0.30
1509.9995
6.6733*10-6
2. Paste Properties at 50 oC o Paste Density:
95
⇒ρ
T
= (x x ρ )EG + (x x ρ )PTA = 0.70 x 1092.218 + 0.30 x 1509.9995 = 12.17.553 kg/m3 o Paste Viscosity: ⇒µ
T
= (x x µ )EG + (x x µ )PTA = 0.70 x 0.0069724 + 0.30 x 6.6733E-6) = 4.8827E-3 Pa.S
3. Fluid Characteristics in Mixing Vessel: o Flow Rates: ⇒ Total Mass Flow Rate (out) = m = 5194.74 kg/hr ⇒ Volumetric Flow Rate = Q = m/ρ T = 5194.74 / 1217.553 = 4.2665 m3/hr o Fluid Volume: ⇒ V = Q x t = 4.2665 x 2 = 8.5331 m3 or 2275.49 gallons ∴ 1 m3 = 266.667 gallons
Sizing Calculations
1. Vessel Dimensions: o Total Mixer Capacity: Since, 60% of the total vessel volume is filled with the fluid so the total volume of the vessel will be estimated as: ⇒ Total Mixer Capacity = 8.5331 / 0.6 = 14.22 m3 ≈ 15 m3 o Vessel Height, Diameter & Volume:
REALTIONS
HEIGHT (m)
DIAMETER (m)
VOLUME (m3)
96
H=3D H=2D H=D
5.56 4.24 2.67
1.853 2.12 2.67
15.00 14.97 14.95
Selected Dimensions: - Height, H = 4.24 m - Diameter, D = 2.12m 2. Liquid Levels in the Vessel: o
Height of the Liquid in Vessel:
As, 60% of total volume is paste filled having volume = 8.5331 m3 So, the height of the liquid in vessel is: ⇒ Height / Level of Liquid = h = Volume/π r2 = 8.5331 / (3.142x2.122/4) = 2.417m Since, 65% of paste is above agitator & 35% is below it. ⇒ Level above agitator = 0.65 x 2.417 = 1.57m ⇒ Level below agitator = 0.35 x 8.5331 = 2.987m3 3. Impeller Selection: 6 pitched-blade turbine type axial flow impeller is selected as an agitator (impeller in which the blade makes an angle of less than 90° with the plane of rotation), as a topentering agitator shafts. Four baffles are estimated as optimum along with the turbine type impeller for effective mixing (also because the Reynolds number of the paste is greater then 2000). 4. Proportions of Tank: At H = 4.24m & D = 2.12m. o
Impeller Diameter: ⇒ Da/D = 1/3 or, Da = 1/3 x 2.12 = 0.707m
o Baffle Width: (10% of Tank diameter) ⇒ J/D = 1/10
97
or,
J = 2.12 /10 = 0.212m
o Clearance between Baffles & Vessel wall: (0.1-0.15J) ⇒ Ca = 0.1 J = 0.1x 0.212 Ca = 0.0212m o
Impeller Height from Bottom: ⇒ C/Da = 1 or, C = Da = 0.707m
o
Impeller Blade Width: ⇒ W = D/15 or, W = 2.12/15 = 0.1413m
o
Blade Length: ⇒ L/Da = 1/4 L = Da/4 = 0.707 / 4 = 0.1768m
5. Fluid Flow Characteristics: o
o
Velocity of Fluid in Tank: ⇒ V = Q / A = 4.2665 / 3.5304 ∴ Area = π r2 = 3.5304 m2 = 1.20852 m/sec Reynold Number: ⇒ Re = D x v x ρ T / µ T = (2.12 x 1.20852 x 1217.553) / 4.8227E-3 = 6.4683E5
6. Impeller Rotational Speed: ⇒ Re = ( Da2 n ρ T )/ µ T or, ⇒ n = (Re µ T)/ (Da2 ρ T) = (6.4683E5 x 4.8227E-3)/(0.7072 x 1217.553) = 5.13 rps ≈ 5rps (300rpm) 7. Power Consumption by Impeller: Estimating the power consumption for any agitator is essential for design. The power consumed by an agitator depends on fluid density, viscosity, vessel dimensions, internal attachments, position of the impeller, its shape, rotational speed, and impeller diameter by
98
plots of power number (gcP/ρ n3Da5) versus Reynolds number (Da2nρ /µ ). Typical correlation lines for frequently used impellers operating in Newtonian liquids in baffled cylindrical vessels are presented in the Figure A.
Figure A : Impeller power correlations: curve 1, six-blade turbine, Da /Wi = 5, with six blades, four baffles, each DT /12; curve 2, vertical blade, open turbine with six straight blades, Da /Wi = 8, four baffles each DT /12 curve 3, 45° pitched-blade turbine with six blades, Da /Wi = 8 four baffles, each DT/12; curve 4, propeller, pitch equal to 2Da, four baffles, each 0.1DT, also same propeller in angular off-center position with no baffles; curve 5, propeller, pitch equal to Da, four baffles each 0.1DT, also same propeller in angular off-center position with no baffles. Da = impeller diameter, DT = tank diameter, gc = gravitational conversion factor, N = impeller rotational speed, P = power transmitted by impeller shaft, Wi = impeller blade height, µ= viscosity of stirred liquid, and ρ= density of stirred mixture. Any set of consistent units may be used, but N must be rotations (rather than radians per unit time. In the SI system, gc is dimensionless and unity. [Curves 4 and 5 from Rushton, Costich, and Everett, Chem. Eng. Prog., 46, 395, 467 (1950), by permission; curves 2 and 3 from Bates, Fondy, and Corpstein, Ind. Eng. Chem Process Des. Dev., 2, 310 (1963), by permission of the copyright owner, the American Chemical Society.]
Thus, the empirical relation for calculating power consumption by an impeller can be written as: ⇒ P = Np ρ
T
n3 Da5
The value of Power number (Np) is taken is 1.3 for 6-pitched blade turbine, from the graph of impeller power correlations. So, the power consumed by the selected turbine is: P = 1.3 x 1217.553 x 53 x 0.7075
99
= 34.95 kWatt or 46.882 hp Watt
∴ 1 hp = 745.48
BIBLIOGRAPHY
o Robert H. Perry & Don W. Green, “Perry’s Chemical Engineering Handbook”, 7th Edition, Volume 2, Mc-Graw Hill International Edition.
o Pattalachinti, R.K., Modeling and Optimization of Continuous Melt-Phase Polyethylene Terephthalate Process. Ohio University Master's Thesis, 1994.
o Timothy J. Calmeyn, Optimization of Melt-Phase Polyethylene Terephthalate Manufacturing Process. Ohio University Master's Thesis, 1995.
o Faissal-Ali El-Toufaili, Catalytic and Mechanistic Studies of Polyethylene Terephthalate Synthesis. Project Report, 2006.
o UOP Sinco’s SSP English Brochure, Solid State Polycondensation Process, 2005. o David A. Tremblay, Using Simulation Technology to Improve Profitability In the Polymer Industry, Aspen Technology Inc., 1999.
o PET
Facts, National Association for PET Container www.napcor.com/toolbox/funfacts.html accessed July 2000.
Resources.
o United States Patents: 5008230, 5898058, 5166311, & 6864345. www.uspatent.com o Other References: -
International Search Reports and Written Opinions.
-
http://en.wikipedia.org/wiki/Polymer
-
http://www.designboom.com/contemporary/petbottles.html
- http://www.kenplas.com/
100
- http://www.plasticsinfo.org/beveragebottles/faq.html - http://www.petresin.org/pet_facts.asp
101
