Title of Invention

PROCESS FOR THE PREPARATION OF POLYESTER

Abstract

The present invention provides a process for the preparation of polyethylene terepthalate (PET) resin in a three-reaqctor system. In the present process, an esterifier, a pre-polymerization reactor and a polycondensation reactor (autoclave) are used for producing an improved polyethylene terepthalate (PET). PTAA/DMT and MEG are charges into the esterifier/(ester-interchange reactor) and the reaction is carried out under pre-determined pressure and temperature conditions. About 70-90% of the originally intended esterification reaction is completed in the esterifier and the balance 10-30% is transferred to the pre-polymer reactor. Then post-esterification followed by pre-polymerization is carried out by operating under low pressure. After the desired pre-polymer is obtained, the pre-polymer melt is transferred to the polycondensation/ autoclave reactor for final polymerization to obtain the desired final product PET resin of improved quality.

Full Text

PROCESS FOR PRODUCING POLYETHYLENE TEREPHTHALATE (PET) Technical Field The present invention relates to a process for the preparation of an improved polyethylene terepthalate (PET) in a three-reactor system comprising an esterifier reactor, a pre-polymerization reactor and a polycondensation reactor, from a feed stock of Pure Terepthalic Acid (PTA) or Dimethyl Terephthalate (DMT) and Mono ethylene glycol (MEG). Background art The conventional and widely practiced method for the preparation of polyethylene terepthalate (PET) in batch lines is by making use of a two-reactor system viz. an esterification reactor and the polycondensation reactor or the autoclave. PTA and mono ethylene glycol (MEG) are charged at appropriate mole ratios in the esterifier/esterinterchange reactor to get a monomer/oligomer. The monomer/oligomer is then transferred under nitrogen pressure to the polycondensation reactor, where the monomer/oligomer is polymerized at high temperature and very low pressure to the desired molecular weight and taken out under nitrogen pressure as granules or chips with the help of a casting system cornprising a granulator. The temperature profile and their timings are very different in this two-reactor system when compared to the three-reactor system as claimed in this patent application.(Refer Table-2). There are known processes with 3-reactor system. In conventional processes, an addition of 3^^ reactor could achieve a capacity improvement to the extent of 25-30%. However, the conventional process of modified three-reactor system could not achieve >25-30% capacity improvement the concept of postesterification and prepolycondensation in the prepolyreactor and installing a gear pump below the polyreactor for melt circulation and transfer have not been incorporated in the prior art processes. The end product (PET) obtained under conventional methods suffered from quality deficiencies such as unsatisfactory color, inconsistency and variation in quality, higher Acetaldehyde (AA) content and significant deterioration in quality during casting of the finished product, apart from restricted/lower productivity. Objects of the present invention The primary object of the present invention is to provide a process for the preparation of an improved polyethylene terepthalate (PET), by adopting a 3-reactor system comprising an esterifier, pre-polymerizing reactor and a polycondensation reactor (autoclave), from a feed stock of Pure Terepthalic Acid (PTA) and Mono ethylene glycol (MEG) in the presence of additives. An object of the present invention is to provide a process to distribute process time of esterification and polymerization among the three reactors thereby allowing the esterifier and the autoclave to become free in a shorter time to receive the feed stock for the next batch thereby enhancing the productivity. Another object of the present invention is to provide a process to achieve an improvement in the polymer quality of PET by reducing the time of exposure of the polymer meh to high final process temperature to prevent the coloration due to degradation of polymer and unwanted byproducts. Still another object of the present invention is to provide a process to obtain the final product polyethylene terepthalate (PET) with an improved Intrinsic Viscosity (IV) by effective polymer melt surface renewal using the circulating pump of the polycondensation reactor. It is also an object of the present invention to provide a process to prepare polyethylene terepthalate (PET) with improved thermal stability, reduced acetaldehyde content (AA) and diethylene glycol (DEG) levels. Yet another object of the present invention is to provide a process to increase the product polyethylene terepthalate (PET) capacity by over 80%, improve polymer filterability and achieve higher viscosity. Summary of the invention The present invention relates to a process for the preparation of polyethylene terepthalate (PET) in a three-reactor system. In the present process, an esterifier, a pre-polymerization reactor and a polycondensation reactor (autoclave) are used for producing an improved polyethylene terepthalate (PET). PTA/DMT and MEG are charged to the esterifier/ ester-interchange reactor) and the reaction is carried out under pre-determined pressure and temperature conditions. About 70-90% of the originally intended esterification reaction is completed in the esterifier and the balance 10-30% is transferred to the pre-polymer reactor. Then pre-polymerization is carried out by operating under low pressure. After the desired pre-polymer is obtained, the pre-polymer melt is transferred to the autoclave/polycondensation reactor for final polymerization to obtain the desired final product PET. In a 2-reactor system when polymer is extruded under nitrogen pressure the melt is stationary at high temperature and degrades due to thermal degradation reactions which occur at higher pressure and cause Intrinsic Viscosity (IV) drop with time as casting is in progress. Brief description of the diagram Fig 1 depicts a schematic expression of the process steps of the present invention, wherein a 3-reactor system is adopted. Detailed description of the invention The embodiments of the present invention are explained by referring to Fig 1. Fig 1 is a schematic expression of the process of the present invention, wherein a 3-reactor system is utilized to obtain an improved PET. The process steps of the present invention are performed with a 3-reactor system, said system comprising a water collection vessel (1), disposed to collect the water coming out as a byproduct, said water is further diverted to the effluent treatment (2) plant (the component (2) is not shown in this figure) before disposal. An esterifier reactor (3) forming part of the 3- reactor system of the present invention is disposed in flow communication with the water collection vessel (1) on one side and pre-polymerisation reactor (5) on the other side. The pre-polymerisation reactor (5) is equipped with a coarse monomer filter (4). In the present process Pure Terepthalic Acid (PTA) and Mono ethylene glycol (MEG) (as necessary either DMT or PTA is used in the reaction) are charged into the esterifier reactor (3) at a mole ratio in the range of about 1.04-1,115 and the esterification is carried out under pre-determined pressure in the range of about 1.0-5.0 bar absolute and temperature in the range of about 235-255^C. About 70-90% of the originally intended esterification reaction is completed in the esterifier reactor (3). The esterified product from the esterifier reactor (3) is transferred to the pre-polymerization reactor (5) under nitrogen pressure of 2.0- 4.5 bar absolute or using a pump through a coarse monomer filter (4). In the pre-polymerization reactor (5) the remaining 10-30% of the esterification is completed under pressure 1.0-5.0 bar absolute. Then pre-polymerization is carried out by operating under low pressure 10-100 mbar. As a result of incorporation of the pre-polymerization reactor (5) between the esterifier (3) and polycondensation reactor (13), (which is described below) a controlled moderate temperature of about 240-292°C is adopted by taking into consideration the cycle time and quality advantage. The pre-polymerizer reactor (5) is also provided with a melt-circulating pump (6), to help in efficient mixing and speeding up of the reaction. The water coming out during post-esterification from the pre-polymerizer reactor (5) is diverted to the effluent treatment plant (8) (not shown in this figure) through a capacity vessel (7) disposed to collect processed water. The ethylene glycol coming out during the pre-polymerization is separately collected for recycling. An Ethylene Glycol (EG) spray condenser unit (9) is connected to the pre-polymerizer reactor (5) for spraying cold MEG to condense the glycol vapour generated from the pre-polymerizer reactor (5) into liquid MEG efficiently to avoid vapor load to vacuum pump (11). An immersion vessel (10) disposed as a collection vessel for the effluent MEG with a barometric leg for keeping the whole system under vacuum. The Vacuum pump (11) is a mechanical device disposed to reduce the pressure and create vacuum in the spray condenser system and in the pre-polymerizer reactor (5). After the desired pre-polymer Intrinsic Viscosity (IV) (as provided in Table 2) is achieved, the pre-polymer melt is transferred to the polycondensation reactor /autoclave (13) under vacuum by a polymer circulation gear pump (14). A 20-micron filter (12) is disposed between the prepolyreactor and the autoclave to remove any impurities in the pre-polymer melt. In the polycondensation reactor /autoclave (13) the final polymerization to the desired Intrinsic Viscosity (IV) is achieved with a pre-determined temperature and vacuum profile. An agitator (13A) is disposed in polycondensation reactor/autoclave (13) to continuously re-circulate the polymer melt in conjunction with a polymer melt pump (14) to facilitate polymer melt surface renewal thereby resulting in faster reaction process. The polymer melt surface renewal is necessary, since as a result of building up of the molecular weight resulting in tremendous increase in viscosity, a further increase in molecular weight depends on the effective removal of the effluent MEG, which is controlled by a diffusion process. Since, the polymer melt is highly viscous the reaction glycol which gets released as the polymerization proceeds has to come out deep from the viscous melt which is a diffusion controlled process and its rate will depend on the diffusion coefficient. Hence, more fresh molten surfaces are exposed from the bottom to the top, which is augmented by the circulating pump (14) in addition to the agitator (13A). The circulation pump (14) enhances heat transfer and surface renewal. A Glycol condenser (17) is again a condensing unit, a shell and tube heat exchanger, which condenses the MEG vapor coming out of the polycondensation reactor/autoclave (13) and converts it into liquid glycol byproduct by condensation and collected in a seal pot (18) for reuse. A vacuum ejector (19) is a device comprising multi stage ejectors to reduce pressure and create fine vacuum in the polycondensation reactor/autoclave (13). The finished product PET is taken out of the polycondensation reactor/autoclave (13) for conversion to chips with under water granulator (16) keeping both the recirculation and vacuum active to obtain in an improved quality of the final product PET. The underwater strand granulator (16) is used to take the polymer melt out through a die as strands which are cut into granules or chips using this device. An extrusion valve disposed on the under water granulator to control the extrusion. In the present invention, the total cycle (process) time for the process of esterification and polymerization normally encountered in the two reactor system is distributed amongst the three reactors thereby allowing overall the esterifier reactor (3) and the polycondensation reactor/autoclave (13) to become free in a shorter cycle time to receive the feed stock for the next batch and reduced polymer exposure at higher temperature resulting in improved plant capacity and product quality. The polycondensation reactor/autoclave (13) is also provided with a gear pump (14), for the recirculation of polymer melts, and further to improve melt surface renewal, thereby enabling the finished product to be cast without breaking the vacuum thereby avoiding polymer degradation. In another embodiment of the present invention, a process wherein the increase in the production capacity of about >80%, improved quality with regard to filterability, thermal stability and product consistency are achieved by (i) carrying out post esterification and pre-polymerization in the pre-polymerization reactor, (ii) installing a gear pump below the pre-polymerization reactor for melt circulation and transfer, and (iii) modified process conditions to utilize effecfively the overall cycle fime, and In another embodiment of the present invention, conventional additives selected from Isophthalic Acid (IPA), catalysts, heat stabilizers, fast reheat material; toners are used as per the need of end use application. The present invention is exemplified by referring to PTA route process. But DMT route process can also take place in such 3-reactor system with some variation in catalysts and process conditions in esterification reactor. Distinguishing features of the process steps of the present invention vis-a-vis prior art process are provided in the following Tables 1 & 2 S.No I PARAMETER I CONVENTIONAL I PROCESS OF THE PROCESS PRESENT INVENTION time to reach the fine vacuum in the poly reactor is considerably low resulting in lower thermal degradation of the product 9. Pre-polymer reactor Not Applicable 285-292 end temperature, ^C 10. Prepoly Melt Under Not Applicable Pre-polymer melt Circulation circulation for speedy reaction ] 1. Product Filtration After Esterification Product filtration after pre- polymerisation, by means of a coarse filter between the esterifier and pre-poly reactor and a 20 micron fine filter between pre-poly reactor and autoclave thereby increasing the filtration efficiency 12. Autoclave Cycle 140-160 85-105 Time, Minutes ly Autoclave End 295-305 290-294 Temperature, °C 14. Time to achieve the 42 minutes 12 minutes maximum As the temperature is Polymerisation already increased in the Temperature pre-poly reactor to > 285°C, final temperature is achieved much earlier in the autoclave, resulting in faster poly reaction and less degradation Quality parameters of the product PET as obtained by adopting process conditions of the present invention are compared with PET as obtained conventionally and the results thus obtained are tabulated in the Table 2. Advantages of the present invention 1. The production capacity of the end product is increased by over 80% by adopting the process of the present invention. 2. The process of the present invention facilitates reduction in residence time of polymer melt phase at high temperatures which results in improved polymer quality. 3. The effective melt surface renewal in polymerization reactor reduces the cycle time of the present process. 4. The PET obtained from the process of the present invention has an improved thermal stability with reduced acetaldehyde (AA) and DEG levels. 5. The product obtained from the process of the present invention has a consistent product quality with no adverse L, a, and b colour values and without IV drop during casting (granulation). 6. Better chips quality is advantageous in the SSP reaction, since size of the chips are uniform and consistent thereby providing uniformity and consistency in SSP reaction. 7. The process of the present invention is capable of producing higher IV polymer due to the lower cycle time in the Autoclave. 8. A higher product quality for PET, PBT, PTT, PETG etc. and their copolymer blends can be achieved by 3-reactor system process of the present invention.

Documents:

511-che-2004 abstract.pdf

511-che-2004 claims granted.pdf

511-che-2004 description (complete) granted.pdf

511-che-2004 form 1.pdf

511-che-2004 form 13.pdf

511-che-2004 form 3.pdf

511-che-2004 form 5.pdf

511-che-2004-abstract.pdf

511-che-2004-claims.pdf

511-che-2004-correspondnece-others.pdf

511-che-2004-description(complete).pdf

511-che-2004-description(provisional).pdf

511-che-2004-drawings.pdf

511-che-2004-form 1.pdf

511-che-2004-form 13.pdf

511-che-2004-form 26.pdf

511-che-2004-form 3.pdf

511-che-2004-form 5.pdf