PROCESS PLAN OF CONTINUOUS
MELT-PHASE 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
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.
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
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.
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
S.N O. CONTENT PAGE 1. 1.1 1.2 1.3 1.4 1.5 1.6 1.7 2. 2.1 3. 3.1 3.2 3.3 3.4 3.5 3.6 3.7 GENERAL INTRODUCTION
HISTORICAL & ECONOMICAL PERSPECTIVE IMPORTANCE OF PET
REPROCESSING OF PET PROJECT OBJECTIVES
INDUSTRIAL PRODUCTION OF POLYETHYLENE TERPHTHALATE
REACTIONS CHEMISTRY
CATALYST & OTHER ADDITIVES FOR PET SYNTHESIS
DISCUSSIONS ON PROJECT
PTA PROCESS FOR PET PRODUCTION
FINAL DESIGN CONFIGURATION
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
APPENDICES APPENDIX A APPENDIX B APPENDIX C APPENDIX D APPENDIX E BIBLIOGRAPHY 7 7 12 13 15 16 25 29 34 34 43 43 45 47 51 54 57 66 67 68 72 78 92 95 100
C
HAPTER
01
G
ENERAL
I
NTRODUCTION
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
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.
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
Manpower Engaged Major Exportable Items
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.
Local Consumption of Recycled Materials No. of Recycled Units
Direct/ indirect Labor
150,000 M. Tons per Year.
More than 400. 50,000.
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
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 export-orientation. 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 Simplified operating procedures o Strong support mechanisms o Easy access to capital o Upgrading technologies o Enhanced productivity o Reliable quality control o Enhanced management skills o Well-trained manpower o 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 co-manufacturing of automotive vehicles in Pakistan for domestic and export requirements.
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 impact-resistant. 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.
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. Post-consumer 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
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
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
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 step-growth 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-(2-hydroxyethyl)-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 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.
The newer industrial method uses:
o Purified Terephthalic Acid (PTA) &
o 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
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
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
andMitsubishi 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-to-PTA and PX-to-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 antimony-based 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
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 Capacities: 10 to 60 metric T/D
o Process Environment: Nitrogen Atmosphere
o NPU Type: Catalytic Oxidation plus Molecular Sieve Drying o Feed Source: flakes or pellets
o Feed IV: 0.50 to 0.80 dl/g
o R-PET Product IV: 0.60 to > 1.0 dl/g o R-PET Product [AA]: less than 1 ppm
o 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
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
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 multi-zone 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). By-products 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
Upgraded Chips Product
SSP Process
With chips flowing via gravity through the system, flow scheme has very low operating costs.
N
OTE: 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 Nitrogen Purification Unit (NPU) Crystallization In Plug Flow Crystallizer Reactor Cooling Closed N2 Loop Pre-crystallization in Fluidized BedApart 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.
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.
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+
FC8) 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+
EC1.7. CATALYST & OTHER ADDITIVES
FOR PET SYNTHESIS
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) Antimony,
(2) Cobalt and/or Zinc, and
(3) At least one of Zinc, Magnesium, Manganese or Calcium.
The preferred catalyst systems include manganese, cobalt, and antimony; or zinc, cobalt,
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.
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 pre-forms. A 25%-30% drop in acetaldehyde content was also observed in the Ti842.
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.
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 glycerophosphate, calcium beta-glycerophosphate; phosphotungstic acid, ammonium phosphotungstate, sodium phosphotungstate; tertiary phosphines, tripropylphosphine, triphenylphosphine, ethylphenyltolylphosphine; quaternary phosphonium compounds, triphenylmethylphosphonium iodide, triphenylbenzylphosphonium chloride, and quaternary phosphonium compounds.
C
HAPTER
02
D
ISCUSSIONS
O
N
P
ROJECT
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
five-continuous reactor process in series: two esterification reactors, two pre-polymerization reactors, and one high-viscosity reactor, which is also called finisher.
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).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).
BLOCK FLOW DIAGRAM FOR THE PROCESS Mixing (Paste) Tank EG PTA Additives ESTERIFICATION
REACTORS POLYCONDENSATIONREACTORS EG Recovery Unit Vacuum System Cooling or Quenching Amorphous PET Chips
Precrystallizer Crystallizer Reactor
Chip Cooling Crystalline PET Chips NPU SSP Process Unit
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.
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.
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
