Title of Invention

PROCESS AND APPARATUS FOR PRODUCING POLYESTER COPYESTER AND POLYCARBONATES

Abstract

Device for the continuous production of polyesters, copolyesters or polycarbonates by esterification of dicarboxylic acids or dicarboxylic acid esters and diols or by reesterification of dialkyl carbonates or diaryl carbonates with bisphenols in at least one reaction stage (2,4,22,24,25,60), prepolycondensation of the esterification or reesterification product in at least one reaction stage (12,33,66) and polycondensation of the prepolycondensation product in at least one reaction stage (17,38,39,44,68), in at least one of the reaction stages a product stream supplied being divisible into at least two partial streams before or within the reaction stage and the partial streams being conductible completely or partially separately from each other through the reaction stage (12,33,44,60) which, being a cascade reactor or a cage reactor, is designed with at least two spatially separate sections divided into partial chambers through which sequential separate flow is possible characterized in that a reaction stage, conducting the partial streams, to the prepolycondensation or polycondensation (33,44) is designed either as an impeller disc reactor with two separate mirror- symmetrically identical reaction chambers or that a reaction stage, conducting the partial streams, to prepolycondensation or reesterification (12,60) is designed as a multiplate reactor with plates regulated in parallel (10, 11, 58, 59) for conducting the partial streams.

Full Text

Process and Apparatus for Producing Polyesters, Copolyesters and Polycarbonates Description This invention relates to a process and an apparatus for producing polyesters, copolyesters and polycarbonates by esterification of dicarboxylic acids or dicarboxylic acid esters and diols or by transesterification of dialkyl carbonates or diaryl carbonates with bisphenols in at least one reaction stage, prepolycondensation of the esterification or transesterification product in at least one reaction stage, and polycondensation of the prepolycondensation product in at least one reaction stage. For the continuous production of polyethylene terephthalate (PET) and the copolyesters thereof, terephthalic acid (PTA) or dimethyl terephthalate (DMT) and ethylene glycol (EG) as well as possibly further comonoraers are used as starting substances PTA is mixed with EG and a catalyst solution to form a paste and is charged to a first reaction stage for esterification, in which reaction stage esterification is effected at atmospheric or superatmospheric pressure by removing water. When DMT is used, the DMT melt and the catalyst together with the EG are supplied to a first reaction stage for transesterification, in which reaction stage the reaction is performed at atmospheric pressure by removing methanol (MeOH). The substances removed, together with distilled EG, are supplied to a rectification column for recovering EG. The EG recovered is again used for esterification or for producing the paste. The product stream of the esterification/transesterification is supplied to a reaction stage for prepolycondensation, which in general is performed in a vacuum. The product stream of the prepolycondensation is introduced into a reaction stage for polycondensation. The polyester melt obtained is processed directly to obtain fibers or chips. In the process of producing PET, esterification is effected in two reaction stages which constitute stirred tanks. With plant capacities up to 400 t/day, prepolycondensation is performed in a vertical cascade reactor with bottom stirrer, and with plant capacities up to 900 t/day in two polycondensation stages of a first reaction stage constituting a stirred tank and in a succeeding horizontal cascade reactor. For polycondensation, there is likewise used a horizontal cascade reactor. These horizontal cascade reactors include chambers at the bottom and a stirrer equipped with vertical perforated disks or ring disks on a horizontal shaft for the purpose of producing a defined surface. The plant comprising two esterification stages, two prepolycondensation stages and one polycondensation stage, of which the first three reaction stages constitute stirred tanks and the last two reaction stages constitute horizontal cascade reactors, provides for a high stability and flexibility of the PET production and in addition offers the best opportunities for increasing the plant capacity, which involves, however, a considerable increase in the expenses for equipment and building (Schumann, Heinz-Dieter: Polyester producing plants: principles and technology. Landsberg/Lech: Verlag Moderne Industrie, 1996, pp. 27 to 33). In a plant comprising two stirred tanks for esterification, one story-type reactor for prepolycondensation and one horizontal cascade reactor for polycondensation, a comparable stability and flexibility of the polyester production is obtained with relatively reduced effort, but it is disadvantageous that the dimensions of the reactors of the prepolycondensation and polycondensation stages are increased because of increased vapor volumes, and the admissible transport dimensions are already achieved with mean plant capacities. Proceeding from the prior art described above, it is the object of the present invention to provide a process and an apparatus for performing the process, which provide for a substantial increase of the capacities of the plants for producing PET, which are formed of four reaction stages, and alternatively a transfer of large plants comprising five reaction stages to four reaction stages without increasing the vapor load and/or the increased risk of the entrainment of product by the vapors during the prepolycondensation or polycondensation. Furthermore, in the production of polybutylene terephthalate (PBT) from PTA and 1,4-butanediol (BDO), a critical vapor load should already be avoided in the esterification stage operated in a vacuum. The solution of this object consists in that in at least one of the reaction stages the product stream supplied is divided in at least two partial streams before or inside the reaction stage, and the partial streams are passed through the reaction stage wholly or partly separate from each other. In accordance with the further aspect of the invention, the partial streams, preferably in equal amounts, are passed through the reaction stage so as to converge up to a common product outlet, or are supplied in the direction of a common product outlet parallel to each other in portions of the reaction stage, wherein the partial streams in the reaction stage are combined when reaching the outlet at the latest and the amount of product streams introduced into and discharged from the reaction stages is controlled. An inventive alternative consists in that in the reaction stage for the prepolycondensation or polycondensation, the partial streams are supplied to separate outlets so as to diverge in opposite directions. For performing the process, a cascade reactor or cage reactor is provided in accordance with the embodiment of the invention, which reactor has at least two physically separate portions divided in partial spaces to be flown through separately one after the other. As horizontal cascade reactor there is preferably used a stirred reactor with perforated or ring disks. In the case of converging partial streams, the inlet for the partial streams is each provided at the end faces, and the outlet for the product stream formed of the partial streams is provided in the middle region of the stirred reactor. In the case of a product line branching or a product differentiation by different viscosities it is also expedient to provide the inlet for the product stream supplied for forming the partial streams in the middle region and accordingly the outlet of the partial streams at each of the ends of the stirred reactor. Instead of a stirred reactor, the use of a vertical story-type reactor is also possible, in which the inlets for the product stream are disposed in the top region and the outlet for the product stream of the combined partial streams is disposed in the bottom, and at least in the upper portion there are provided stories connected in parallel for separately guiding the partial streams and a succeeding story or a bottom space for combining the partial streams. A story-type reactor can be used such that inlet and outlet are connected via a product recirculation line disposed outside the story-type reactor, in which recirculation line an inlet for the product, a heating for the product and elements for dividing the product stream in partial streams are integrated. In the drawing, several embodiments are represented in the form of process flow diagrams, which will be described in detail below. Fig. 1 shows a plant comprising four reaction stages for producing PET, in which a paste- like mixture of PTA and EG together with catalyst solution is continuously supplied via line (1) to the first reaction stage (2) constituting a stirred reactor, in which a temperature of 260°C and a pressure of 1600 mbar (abs) exist. Via line (3), the product stream discharged from the first reaction stage (2) is charged to the second reaction stage (4), which constitutes a stirred reactor and has a temperature of 263 °C and a pressure of 1080 mbar (abs). Via lines (5, 6), the vapors formed during esterification are discharged to a rectification column not shown here and decomposed in the same to form water and EG. The EG obtained is directly recirculated to the reaction stages (2, 4) and/or again used for producing the paste-like mixture The product stream having a degree of esterification of 97 % is discharged from the reaction stage (4) via line (7) and divided in two partial product streams of equal amounts, which for the purpose of prepolycondensation are introduced via lines (8, 9) into two identical input stories (10, 11) of a vertical story-type reactor (12) with stirred bottom space (13) at a pressure of 15 mbar (abs), flow over succeeding stories (14, 15) to the bottom space (13) and are again combined in the same. Via line (16), the product stream flowing out of the story-type reactor (12) is charged to the front of a stirred reactor (17) with ring disks for polycondensation, and the finished polymer is discharged at the back thereof via line (18). Via line (19) of the story-type reactor (12) and via line (20) of the stirred reactor (17), the respectively required operating vacuum is applied. To avoid the entrainment of droplets, the stories (10, 11, 14, 15) of the story-type reactor (12) may be divided in chambers and be provided with droplet separators. With the plant concept described above, critical vapor loads can be avoided even with high flow rates. Another aspect of the process of the invention is shown in Fig. 2. For esterification, the paste-like mixture consisting of PTA and EG together with catalyst solution is supplied via line (21) to the first reaction stage (22) constituting a stirred tank, and via line (23) the reaction product is supplied to the second reaction stage (24) comprising a stirred tank, in which there is integrated a partial vacuum stage (25) with a pressure of 550 mbar (abs). Via lines (26, 27, 28), the vapors obtained during esterification are supplied to a rectification column not shown here and separated into water and EG, wherein the vapors flowing out of the partial vacuum stage (25) at a temperature of 267°C are condensed by means of a gas jet pump (29), which is operated with the vapors of the first reaction stage (22) as motive steam. The product stream withdrawn from the partial vacuum stage (25) via line (30) is divided in two partial product streams of equal amounts, of which the one partial product stream is charged to the front of a stirred reactor (33) with perforated disks via line (31), and the other partial product stream is charged to the back of said reactor via line (32) for the purpose of prepolycondensation, the partial streams flowing axially from the outside to the inside through two separate, mirror-symmetrically identical reaction spaces The product streams are combined in the middle plane of the stirred reactor (33), and the product having a temperature of 274°C is discharged via line (34). Via line (35), the required vacuum is generated, and at the same time the vapors formed during prepolycondensation are discharged for the purpose of recovering EG. The product stream discharged via line (34) is divided in two partial streams, which are each charged to a stirred reactor (38, 39) for polycondensation via line (36) and line (37), respectively. The polycondensation product, which has a temperature of 282°C, is each discharged from the stirred reactors (38, 39) via lines (40, 41). The vacuum required in the stirred reactors is applied via lines (42, 43). Two reactors on the one hand serve to maximize the entire plant capacity and on the other hand serve the polymer diversification as well as a decentralized product distribution during direct spinning. According to Fig. 2a it is also possible to charge the partial stream flowing off via line (36) at the front and the partial stream flowing off via line (37) at the back of a stirred reactor (44) with perforated disks and ring disks and supply the partial streams axially from the outside to the inside through two separate, mirror-symmetrically identical reaction spaces. The product stream formed of the combined partial streams is discharged in the middle plane of the stirred reactor via line (45). Via line (46), the necessary vaccum is produced and the vapors formed during polycondensation are discharged. Another modification of the process in accordance with Fig. 2 is shown in Fig 2b. Accordingly, from the product stream of prepolycondensated PET discharged via line (34), amorphous or partially crystalline chips can first be produced in a granulating system (47), via line (48) said chips can be supplied to a crystallizer system (49) with a temperature of 210°C, and via line (50) can subsequently be charged to a solid-state polycondensation in an inert gas stream at temperatures up to 230°C in a vertical tubular reactor (51) known as SSP reactor (SSP = Solid State Polycondensation). The finished chips are discharged from the SSP reactor via line (52). For the continuous production of polybutylene terephthalate (PBT), PTA and butanediol (BDO) are mixed and, as shown in Fig. 3, the paste-like mixture is fed via line (53) into line (54) with heat exchanger (55) mounted therein, and is divided in two partial product streams of equal amounts, which via line (56) and line (57) are then introduced into the separate input stories (58 and 59, respectively) disposed in parallel of a story-type reactor (60) for esterification at a temperature of 245°C and a pressure of 400 mbar (abs). The separately guided partial product streams are combined in the intermediate story (61) and discharged from the bottom space (62) via line (63). Upon withdrawal of the esterification product via line (64) and introduction of the raw materials via line (65), a major amount of this product stream is recirculated to the top of the story-type reactor (60) in line (54). Via line (65), the remainder of the product stream discharged from the story-type reactor (60) is supplied to the reaction stage (66), in which prepolycondensation is effected at a temperature of 240°C and a pressure of 20 mbar (abs). For polycondensation, the prepolycondensation product is supplied via line (67) to a stirred reactor (68), from which the finished PBT is discharged via line (69). The mixture of water, tetrahydrofurane (THF) and BDO, which was formed during esterification, is treated in a rectification column, THF and water as top product are recirculated to a THF recovery, and BDO as bottom product is again charged to the story- type reactor (60). The vapors from the prepolycondensation stage (66) and from the stirred reactor (68) are sucked off via lines (70, 71) to separate vacuum systems and for the purpose of recirculating BDO are subjected to a partial condensation. WE CLAIM: 1. Device for the continuous production of polyesters, copolyesters or polycarbonates by esterification of dicarboxylic acids or dicarboxylic acid esters and diols or by reesterification of dialkyl carbonates or diaryl carbonates with bisphenols in at least one reaction stage (2,4,22,24,25,60), prepolycondensation of the esterification or reesterification product in at least one reaction stage (12,33,66) and polycondensation of the prepolycondensation product in at least one reaction stage (17,38,39,44,68), in at least one of the reaction stages a product stream supplied being divisible into at least two partial streams before or within the reaction stage and the partial streams being conductible completely or partially separately from each other through the reaction stage (12,33,44,60) which, being a cascade reactor or a cage reactor, is designed with at least two spatially separate sections divided into partial chambers through which sequential separate flow is possible characterized in that a reaction stage, conducting the partial streams, to the prepolycondensation or polycondensation (33,44) is designed either as an impeller disc reactor with two separate mirror-symmetrically identical reaction chambers or that a reaction stage, conducting the partial streams, to prepolycondensation or reesterification (12,60) is designed as a multiplate reactor with plates regulated in parallel (10, 11, 58, 59) for conducting the partial streams. 2. Device as claimed in claim 1, wherein the cascade reactor is designed as a horizontal cascade reactor and is an impeller disc reactor (35) with perforated or annular discs. 3. Device as claimed in claim 2, wherein in the case of partial streams flowing towards each other, the inlet for the partial streams are arranged at the fronts and the outlet for the product stream formed from the partial streams in the central area of the impeller disc reactor (35), respectively. 4. Device as claimed in claim 2, wherein in the case of partial streams flowing in the opposite direction from each other, the inlet for the product stream supplied to form the partial streams are arranged in the central section and the outlet for the partial streams at the ends of the impeller disc reactor (35), respectively. 5. Device as claimed in claim 1, wherein the reaction stage (12, 33, 44, 60) is designed as a plate reactor (12, 60) in the case of which at least two inlets (8,9,56,57) for the product stream are arranged in the head area and the outlet for the product stream of the joined partial streams in the bottom and at least in the upper section of plates controlled in parallel (10,11,58,59) for conducting the partial streams and a subsequent plate (14,15,61) or a bottom chamber (13,62) are arranged for combining the partial streams. 6. Device as claimed in claim 5, wherein the inlet and outlet are connected via a recycling facility for the product discharged, regulated outside the plate reactor (60), by lines (63, 64,54) with elements contained therein for dividing the product stream in line (54) towards separate inlets (58, 59). 7. Device as claimed in one of claims 1 to 6 wherein esterification stages consisting of two stirred reactors (2,4), a prepolycondensation stage consisting of a plate reactor (12) and a polycondensation stage consisting of an impeller disc reactor (17). 8. Device as claimed in one of claims 1 to 6, wherein esterification stages consisting of two stirred reactors (22,24) and a vacuum stage (25), a prepolycondensation stage consisting of an impeller disc reactor (33) and a polycondensation stage consisting of an impeller disc reactor (38, 39). 9. Device as claimed in one of claims 1 to 6, wherein preesterification stages consisting of two stirred reactors (22,24) and a vacuum stage (25), a prepolycondensation stage consisting of an impeller disc reactor (33), a granulating system (47), a crystalliser system (49) and a solids polycondensation stage consisting of a vertical tubular reactor (51). 10. Process for the continuous production of polyesters, copolyesters or polycarbonates by esterification of dicarboxylic acids or dicarboxylic acid esters and diols or by reesterification of dialkyl carbonates or diaryl carbonates with bisphenols in at least one reaction stage (2, 4, 22, 24, 25, 60), prepolycondensation of the esterification or reesterification product in at least one reaction stage (12,33,66) and polycondensation of the prepolycondensation product in at least one reaction stage (17,38,39,44,68) using a device as claimed in claim 1 to 9, wherein the product stream supplied, either in a reaction stage, designed as an impeller disc reactor, to the prepolycondensation or polycondensation (33,44), is passed, in the form of at least two partial streams, through two separate, mirror-symmetrically identical reaction chambers or the product stream supplied is passed in the form of at least two partial streams, in a reaction stage, designed as plate reactor, to the polycondensation or reesterification (12, 60), through plates controlled in parallel (10, 11, 58,59). 11. Process as claimed in claim 10, wherein the partial streams are conducted through the reaction stage (12, 33,44,60) in a manner flowing towards each other up to a joint product discharge, the partial streams being combined in the reaction stage no later than at the outlet and the quantity of the product streams introduced into or discharged from the reaction stages being regulated. 12. Process as claimed in one of claims 10 and 11, wherein the partial streams are conducted in the reaction stage (12, 33, 44, 60) in sections parallel to each other in the direction of a joint product discharge, the partial streams being combined in the reaction stage no later than at the outlet and the quantity of the product streams introduced into or discharged from the reaction stages being regulated. 13. Process as claimed in claim 11 or 12, wherein the partial streams are conducted in quantities of equal size through the reaction stage (12, 33, 44, 60). 14. Process as claimed in claim 10, wherein the partial streams are conducted in the reaction stage (12,33,44,60) for the prepolycondensation or polycondensation in a manner flowing away from each other in the opposite direction to separate outlets. 15. Device as claimed in one of claims 1 to 6, wherein esterification stages consisting of two stirred reactors (22, 24) and a vacuum stage (25), a prepolycondensation stage consisting of a plate reactor (12, 60) and a poly condensation stage consisting of an impeller disc reactor (38, 39). 16. Device as claimed in one of claims 1 to 6, wherein esterification stages consisting of two stirred reactors (22,24) and a vacuum stage (25), a prepolycondensation stage consisting of a plate reactor (12, 60), a granulating system (47), a crystalliser system (49) and a solids polycondensation stage consisting of a vertical tubular reactor (51). Device for the continuous production of polyesters, copolyesters or polycarbonates by esterification of dicarboxylic acids or dicarboxylic acid esters and diols or by reesterification of dialkyl carbonates or diaryl carbonates with bisphenols in at least one reaction stage (2,4,22,24,25,60), prepolycondensation of the esterification or reesterification product in at least one reaction stage (12,33,66) and polycondensation of the prepolycondensation product in at least one reaction stage (17,38,39,44,68), in at least one of the reaction stages a product stream supplied being divisible into at least two partial streams before or within the reaction stage and the partial streams being conductible completely or partially separately from each other through the reaction stage (12,33,44,60) which, being a cascade reactor or a cage reactor, is designed with at least two spatially separate sections divided into partial chambers through which sequential separate flow is possible characterized in that a reaction stage, conducting the partial streams, to the prepolycondensation or polycondensation (33,44) is designed either as an impeller disc reactor with two separate mirror- symmetrically identical reaction chambers or that a reaction stage, conducting the partial streams, to prepolycondensation or reesterification (12,60) is designed as a multiplate reactor with plates regulated in parallel (10, 11, 58, 59) for conducting the partial streams.

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