Method for treating condensates form polycodensates

Abstract

The invention relates to a method for treating vapours and condensates which arise during the production of polycondensates from bisphenols or high-value phenols by esterification and/or transesterification with alkyl or aryl esters of at least bivalent organic or inorganic acids. According to the invention, the treatment is carried out in a plurality of condensates and/or distillation columns which are connected in a step-by-step manner behind each other, each distillation column comprising a connected condensate. The dew point and the pressure in each condensate are adjusted such that in each step, monomers, oligomers or decomposition and transformation products are removed.

Description

The invention relates to a method of treating vapors and condensates that are produced during the production of polycondensates from bisphenols or polyhydric phenols by esterification and/or transesterification with alkyl and/or aryl esters of at least divalent organic or inorganic acids.

It is known that polycarbonates, polyarylates, and copolymers of polyethylene terephthalate, polypropylene terephthalate, or polybutylene terephthalate can be prepared by esterification and/or transesterification of alkyl or aryl esters of organic or inorganic, at least divalent acids with bisphenols and subsequent interfacial polycondensation or melt polycondensation. These polycondensates are known as engineering plastics for their outstanding properties. The water or other vapors released during condensation contain, apart from the main cleavage product from the transesterification or esterification that is preferably phenol, additional monomers, oligomers, or products that are produced through thermal decomposition or rearrangements. The decomposition or rearrangement products formed in the reaction contaminate the monomers and oligomers also still present in the vapors and make it impossible to return the monomers and oligomers unpurified to the polycondensation process when products that are not discolored and that do not sufficiently meet rheological and mechanical quality requirements are to be produced. The cleavage products produced by the polycondensation, such as phenols, alcohols, and water, are also so contaminated by the mentioned decomposition and rearrangement products that they cannot be easily reused. The reaction products arising during thermal decomposition interfere with the reutilization of phenols present in the vapors, for example, for the production of bisphenol A, diphenyl carbonate, triphenyl borate, or for any other phenyl ester of an organic or inorganic acid.

For this reason, the object arose to develop a method of treating vapors and condensates that are produced during the production of the above-described polycondensates, the method making it possible, on the one hand, to recover the entrained monomers and to return them to the polycondensation and, on the other, to produce the cleavage products arising in the polycondensation, primarily phenol, in such a pure form that they can be reused for other reactions without deterioration in the product quality.

A method has now been found for treating vapors and condensates that are produced during the production of polycondensates from bisphenols or polyhydric phenols by esterification or transesterification with alkyl or aryl esters of at least divalent organic or inorganic acids, the method in which the treatment occurs in several succeeding cascaded condensers and/or distillation columns with respective connected condensers, where in each condenser the dew point and pressure are established such that in the individual stages the specific monomers, oligomers, or the decomposition and rearrangement products are separated.

This method enables the return of useful materials to the process, is especially economical and cost-effective, avoids environmental pollution by chemicals, and utilizes the inevitably forming by-products to generate energy. This method can be used especially well for the regeneration of phenol-containing vapors and condensates and for the recovery of monomers, as they arise, for example, in the production of polycarbonates, polyarylates, or in the melt-phase polycondensation of polymers and copolymers, such as polyethylene terephthalate, polypropylene terephthalate, or polybutylene terephthalate with diphenols and bisphenols or polyhydric phenols by esterification or transesterification with alkyl or aryl esters of organic or inorganic, at least divalent acids and/or the acids themselves.

The above-described polycondensates are known as engineering plastics with outstanding properties and special fields of application. Their production occurs either by interfacial polycondensation as in the case of polycarbonate or by means of melt polycondensation in the direct polycondensation method from dicarboxylic acids or dialcohols or diphenols or by transesterification processes from the corresponding acid esters. In the melt condensation for the production of polycarbonates and polyarylates, aromatic dihydroxy compounds, for example, bis(4-hydroxyphenyl)alkanes, particularly bisphenol A, are transesterified with diphenyl carbonate or terephthalic diphenolate in the presence of catalysts with cleavage of phenols, oligomerized, and finally polymerized in multiple stages under a progressive vacuum. Methods of this type are described in the German patent publications DE-B-1 495 730 [U.S. Pat. No. 3,535,280] and DE-C-2 334 852 [U.S. Pat. No. 3,888,826]. In addition, the international patent application WO 2002/044244 [U.S. Pat. No. 6,838,543] describes a method by which polycarbonates are prepared by reaction of a monomeric carbonate component with at least one diphenol or dialcohol in the presence of a transesterification catalyst; here, the melted components are mixed with the transesterification catalyst and a product is produced that is polycondensed. For the polycondensation, the transesterification product is passed through a preliminary reactor, at least one intermediate reactor, and a final reactor, the reactors being connected in series and having a substantially horizontally driven shaft with mixing elements attached thereto. Care is taken that a melt residence time of 5 minutes to 2 hours is maintained in the preliminary reactor and final reactor, the temperatures in the preliminary reactor are maintained within the range of from 220 to 300° C., and the pressure in the preliminary reactor is maintained in the range of from 100 to 800 mbar and in the final reactor in the range of from 0.1 to 50 mbar.

If the operating conditions and process stages described in WO 2002/044244 are now applied to the production of polycarbonates, polyarylates, and other polymers or copolymers that were prepared from bisphenol A or other polyhydric phenols with at least divalent acids or the phenol-containing esters thereof, then different phenol-containing vapor and condensate compositions form depending on the residence time, catalyst, pressure, and temperature. Thus, for example, in a first stage primarily cleavage products from the esterification or transesterification arise such as water, phenols, or alcohols that still contain minor amounts of other monomers forming the polymer. Apart from the conventional cleavage products, depending on the selected reaction conditions, rearrangements and secondary reactions of the monomer compounds can also occur in this case, primarily of the polyhydric phenols and especially the bisphenols.

Thus, for example, it is known from U.S. Pat. No. 4,294,994 that during treatment of bisphenol A with acids, formation of isopropenylphenol and its polymers, dihydroxyindanes, dihydroxyspirobisindanes, alkyldistilbestrols, trishydroxyphenyls, and polyhydroxyaryls occurs. These by-products because of their high molecular weight have high melting and boiling points and escape only under drastic reaction conditions, i.e. at high temperatures and low pressure. They cause, inter alia, discoloration and crosslinking in the polycondensate and disrupt the molecular structure. Other substances, such as, for example the monomers, also have high boiling points, but in the case of the raw materials employed for the production of engineering plastics these are far below those of the above-described secondary reaction products, so that they can be separated by fractional methods both from the high boilers and from the low boilers.

The method of the invention will now be described in greater detail using the example of the reaction of diphenyl carbonate with bisphenol A in the presence of a catalyst, known for this reaction, according to FIG. 1 and FIG. 2.

The monomers, diphenyl carbonate 1 and bisphenol A 2, introduced into a reactor 4, are reacted in the presence of a catalyst 3, so that cleavage products 5, primarily phenol, accumulate in a large amount at the beginning of the reaction even at a low temperature and high pressure. The distillate forming hereby is conveyed to a condenser 6 whose temperature is kept above the dew point of the cleavage products and that of monomers 1 and 2, for example at a temperature above 200° C. and at 400 mbar. In this way, high-boiling products 7 of the secondary reactions and present in vapors 5 of the first stage, such as spiroidanes and indanes, can be separated out. The vapors continuing onward then pass through a rectification column 8 in which the lowest boiling cleavage products are stripped off at the top, but monomers 1 and 2 are drained off at different trays and returned to the first reaction stage 4.

The product collected in the bottom of column 8, with high boilers 30, which correspond substantially to the products of the secondary reaction, is combined with the bottom product 7 from the first condensation [6] and conveyed to a collection tank 24.

The low-boiling cleavage product 10 is condensed in a condenser 11 and taken to a collection tank 28. The first compressor stage 12 that, as indicated in FIG. 1, may consist of a mechanical blower 12, but also a jet exhaust, serves for generating the necessary low pressure. The condensates accumulating in the compression stage 12 in condenser 29 proceed to a collection tank 27.

In the second reaction section (reactor 14, supplied with product 13 from reactor 4), the amount of low-boiling cleavage products 15 is even lower. In this stage 14, however, because of the higher product viscosity, more drastic measures, such as a higher temperature and lower pressure than in stage 1, are necessary to drive the reaction or the chain growth forward, thus, for example, 270 to 300° C. and 100 hPa to 10 hPa. Because of the higher thermal loading, the products from secondary reactions appear increasingly in the vapor gas stream 16; as a result, the amount of this product type, accumulating in condenser 6′, is also greater than in previous stage 4. The condensate 7, accumulating in 6′, proceeds to a collection tank 24. The amount of monomers 1 and 2 in the cleavage products has become so low that a separation is no longer worthwhile and everything is condensed in condenser 8′, before the lowest-boiling vapors are compressed in a compressor 16. The vapors condensed in condenser 17 from the compressor 16 are combined with the stream 10 and taken to the collection tank 28. Condensate 9 is supplied to the rectification stage 8 to increase the yield of monomers 1 and 2 and the cleavage products 10.

In the third reaction section (reactor 19, supplied with product 18 from reactor 14), the lowest amount of cleavage products 20 accumulates. Here, however, because of the highest product viscosity, the most drastic measures, such as the highest temperature and lowest pressure, are necessary to drive the reaction or the chain growth forward, thus, for example, 280 to 350° C. and 10 hPa to 0.1 hPa. Because of the high thermal loading, the products in the secondary reaction occupy a large portion in the vapor gas stream 20; as a result, the amount accumulating in condenser 6″ is also much higher than in the previous stages. The condensate 7 accumulating in 6″ goes to collection tank 24. The amount of monomers 1 and 2 in the cleavage products has become so low that a separation is no longer worthwhile and, for this reason, everything is condensed in condenser 8″. The vapors condensed in condenser 22 from compression 21 are supplied as compressor condensate 22 to the collection tank 26. The decision is made on the remaining amount of condensate 9′ after analysis in quality control (QC) whether it can still be supplied to rectification 8 to increase the yield of monomers 1 and 2 and cleavage products 10, or is taken to collection tank 25 for special treatment according to FIG. 2.

The products collected in the tanks 24 to 28 are the results of a coarse fractionation or preliminary fractionation by selective choosing of the reaction and/or condensation conditions. They represent the first stage of an overall process that leads to an optimal and economic utilization of the monomers and cleavage products. The product from collection tank 24 has a high concentration of by-products, such as, for example, trisphenols, polymeric isopropenylphenols, dihydroxyindanes, dihydroxyspirobisindanes, alkyldistilbestrols, and polyhydroxyaryls, and only a minor effort is required to recover reusable products such as the monomers or cleavage product. This is achieved by another rectification 31 to increase the yield of monomers and cleavage products, before the high-boiling and discolored bottom product 32 is subjected to a thermal recovery. Or otherwise disposed of. The content of monomers 1 and 2 and low-boiling cleavage product 10 is less than 1% by weight in the outgoing product 32.

Vapors 33 containing useful materials go to the top and proceed to rectification 34, where they are rectified with product 9 and/or 9′ that stems from the middle fraction collected in collection tank 25. In this case, a fraction each of monomers 1 and 2 is obtained from the lower trays; after passing quality control (QC), these are again taken either directly after purification by crystallization 41, 43 and/or zone-melt process 42, 44 as pure monomer 1 and/or 2 to the first stage with reactor 4. The bottoms of stage 34 contain only high boilers, which proceed to incineration 32.

Product 35 escaping to the top of column 34 is rich in low-boiling cleavage products and enters the middle portion of column 36, into which the main portion of the cleavage product from collection tanks 27 and 28 is also introduced. Pure cleavage product 37, passing to the top, is recovered, which after condensation 39 is returned via return tank 40 and quality control (QC) either to column 36 or as the end product 45 is used for further reuse, for example, for the preparation of acid esters or of bisphenol A with acetone. The recovered product 45 meets the quality criteria of a product from the monomer synthesis. Quality control (QC) decides about the remaining bottom product 38 that is either again allocated to column 34 or to incineration 32 as waste.

LIST OF REFERENCE CHARACTERS

• 1 monomer A diphenyl carbonate
• 2 monomer B bisphenol A
• 3 catalyst
• 4 first reactor stage (first reaction of monomers)
• 5 vapors (cleavage products, oligomers, monomers)
• 6 high-boiler condensation first stage
• 6′ high-boiler condensation second stage
• 6″ high-boiler condensation third stage
• 7 condensate (highest boiling points)
• 8 rectification first stage
• 8′ condensation second stage
• 8″ condensation third stage
• 9 middle condensate fraction
• 9′ middle condensate fraction, not suitable for return to 8
• 10 condensate (lowest boiling point), cleavage product condensate of monomer A and/or monomer B
• 11 condenser
• 12 compressor (1^st pressure stage, highest pressure)
• 13 product with a short chain length (1 to 10)
• 14 second reactor stage (precondensation, medium chain length)
• 15 vapors (cleavage products, oligomers, monomers)
• 16 compressor (2^nd pressure stage, reduced pressure)
• 17 gas cooler/compressor-condenser
• 18 product with a medium chain length (5 to 50)
• 19 third reactor stage (polycondensation, long chain length)
• 20 vapors (cleavage products, oligomers, monomers)
• 21 compressor (3^rd pressure stage, lowest pressure)
• 22 gas cooler/compressor-condenser
• 23 end product chain length (50 to 300)
• 24 collection tank condensate highest boilers
• 25 collection tank condensate high boilers
• 26 collection tank compressor condensate, lowest pressure
• 27 collection tank compressor condensate, highest pressure
• 28 condensate collection tank compressor condensate, 1^st compressor stage
• condenser, 1^st compressor stage
• 30 bottom product rectification
• 31 evaporator for products with highest boiling point
• 32 waste for incineration
• 33 evaporation product
• 34 rectification for products with middle boiling point
• 35 evaporation product
• 36 cleavage product rectification
• 37 pure cleavage product for further processing
• 38 bottom product, after analysis waste 32 or back to 34
• 39 cleavage product cooler
• 40 collection tank cleavage products
• 41 fractional crystallization monomer 1
• 42 zone-melt purification method monomer 1
• 43 fractional crystallization monomer 2
• 44 zone-melt purification method monomer 2
• 45 pure cleavage product for further reaction
• QC analysis site, quality control, and branching

Claims

1. A method of treating vapors and condensates that are produced during the production of polycondensates from bisphenols or polyhydric phenols by esterification of acids and/or transesterification with alkyl and/or aryl esters of at least divalent organic or inorganic acids in a multiple stage polymerization with progressing vacuum wherein the preparation of the vapors and condensates from each stage occurs in several succeeding cascaded condensation steps and distillation columns with respective attached condensers, the dew point and pressure being established in each condenser such that in each stage specific monomers, oligomers, or decomposition and rearrangement products are separated and collected separately.

2. The method according to claim 1 wherein the method is used for treating vapors and condensates that are produced during the production of polymers or copolymers based on diphenols, bisphenols, or phenyl esters.

3. The method according to claim 1 wherein for the condensation of the distillate recovered in a preparation step, the dew point, temperature, and pressure are selected such that in at least one condenser predominantly the cleavage products or the monomers arising in the esterification and/or transesterification, are separated and collected in a special collection tank.

4. The method according to claim 1 wherein for the condensation of the distillate recovered in a preparation step, the dew point, temperature, and pressure are selected such that in at least one condenser predominantly the oligomers arising in the preliminary condensation, are separated and collected in a special collection tank.

5. The method according to claim 1 wherein for the condensation of the distillate recovered in a preparation step, the dew point, temperature, and pressure are selected such that in at least one condenser predominantly the reaction products arising during the polycondensation by thermal decomposition or rearrangement are separated and collected in a special collection tank.

6. The method according to claim 1 wherein the vapors and condensates are rectified in at least one distillation stage.

7. The method according to claim 1 wherein the product mixtures collected in the collection tanks are analyzed and then purified by at least one fine distillation and/or at least one crystallization and supplied taken to the polycondensation.

8. The method according to claim 1 through 6 wherein waste products are incinerated.