Final Project Report Plastic Bottle Manufacture
PTA Manufacture. Pack Bottles used in bewerage industry!...
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.
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.
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
1.1 1.2 1.3 1.4 1.5
7 12 13 15
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
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
APPENDIX A APPENDIX B APPENDIX C APPENDIX D APPENDIX E
68 72 78 92 95
16 25 29
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
Statistics For Production Capacity of Plastics Materials
Engro Asahi Chemical
Pak Polymer Industries
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.
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
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
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
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
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 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
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 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
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