WO2008100165A1

WO2008100165A1 - Process for producing bisphenol a

Info

Publication number: WO2008100165A1
Application number: PCT/PL2008/000016
Authority: WO
Inventors: Boguslaw Tkacz; Teresa Rdesinska-Cwik; Kamil Kulesza; Andrzej Krueger; Stanislaw Matyja; Maria Majchrzak; Wojciech Balcerowiak; Bozena Banas; Alina Iwanejko; Jerzy Jasienkiewicz
Original assignee: Instytut Ciezkiej Syntezy Organicznej Blachownia
Priority date: 2007-02-14
Filing date: 2008-02-14
Publication date: 2008-08-21
Also published as: CN101636371B; PL381757A1; DE112008000300T5; PL210812B1; CN101636371A; UA103301C2; RU2451663C2; RU2009134199A

Abstract

The invention relates to producing bisplienol A from phenol and acetone by way of catalytic condensation in the presence of promoted, sulfonated ion- exchange resins, with using fractional crystallization to isolate the p,p'-BPA isomer. The condensation reaction of acetone and phenol is carried out in a multi¬ stage reaction system with the interstage control of reaction temperature and acetone concentration as well as with the control of water concentration before the last stage of the reaction system. A part of post-crystallization liquors from solvent crystallizations is recycled to the last reactor. Water and acetone are evaporated from the post-reaction mixture which, subsequently, is mixed with the effluent solution from fractional crystallization dissolved in the isomerizate obtained in the process of isomerization of a part of the stream of concentrated liquors after solvent crystallization, the resulting stream is subjected to solvent crystallization, the isolated bisphenol A/phenol adduct is subjected to thermal decomposition, and the resulting raw bisphenol A is subjected to fractional crystallization.

Description

Process for producing bisphenol A Technical Field

The present invention relates to a process for producing bisphenol A, a raw material for producing plastics, in particular polycarbonates.

Background Art

Bisphenol A (BPA), 2,2-bis-(4-hydroxyphenyl)propane, named also the p,p'-BPA isomer, is used for producing plastics, in particular polycarbonates, their blends and epoxy resins, as well as, but in minor quantities, for producing phenoplasts, unsaturated polyester resins, polysulfones, polyetherimide and polyacrylic resins as well as polyuretanes and additions for plastics, inter alia fiameretardants, for example tetrabromobisphenol A and phenylphosphates of BPA and thermal stabilizers for polyvinyl chloride.

In the course of developing the technology of BPA, the possibility of its production by reacting allene, methylacetylene, 2-isopropenylphenol, 4- isopropenylphenol and isopropenyl acetate with phenol, as well as by rearrangement reacting of cumene hydroxide and its reaction with phenol, was investigated. No one of these reactions could compete with processes for producing BPA by way of a condensation reaction of phenol with acetone. Many homogeneous catalysers were investigated, inter alia BF₃ and its adducts with H₃PO₄, (C₂Hs)₂O, HCOOH, CH₃COOH, C₂H₅COOH, CaF₂, as well as HCl with BF₃, AlCl₃, SnCl₄, SbCl₅, SnF₄ and SbF₃ and many other catalytic systems. However, the process for producing BPA by way of the condensation of acetone with phenol in the presence of sulfuric acid or hydrogen chloride as catalysts was used in industrial practice. An alternative process for catalyzing the reaction of obtaining BPA is the use of strongly acidic ion-exchange resins (cation exchangers) which met larger and larger approbation, and finally displaced methods using H₂SO₄ or HCl as catalysts for condensation reactions. It was recommended also the use of sulfonated copolymers of styrene and divinylbenzene (DVB) and sulfonated phenol-formaldehyde resins, the first of the mentioned catalyst types being broadly used at present on an industrial scale. Initially, cation exchangers of considerable cross-linking degree were used, and next, gel cation exchangers of lower cross-linking degree were started to be used (2-4 cg/g ofDVB).

The second important aspect associated with the development of ion exchanger catalysts was the use of so called promoters which are thioorganic compounds having a thiol group or being capable of its formation in condensation reaction conditions. Numerous tests for using homogeneous promoters were taken up. However, their use caused problems associated with obtaining a product of high quality. For this reason, at present, more and more popular are ion-exchange resins modified with thioorganic compounds in such a manner which provides their attachment with a sulfonic group, and to this end amine groups or heterocyclic compounds comprising nitrogen atom(s) of basic character are used mostly.

One embodiment used for a synthesis unit includes a multi-stage system of flow reactors with a fixed-bed catalyst, connected in series through heat exchangers which enable to maintain an assumed temperature range in successive reactors, individual embodiments differing each other in supplying acetone and post-crystallization liquors resulted from successive stages of the BPA isolation and purification, hi the case of such embodiments for a reaction system, are known and described inter alia in the US-Patent 6414199 advantages resulting from dividing the stream of acetone to be supplied into reactors connected in series, which advantages consist, first of all, in increasing the catalyst lifetime and selectivity of a condensation reaction, which results from a better temperature distribution in the catalyst bed as well as from more possibilities of controlling the course of a BPA synthesis reaction.

Numerous methods were tested or used for isolating bisphenol A, including distilling off at least a part of phenol, water, and acetone (mostly under reduced pressure), vacuum distillation of bisphenol A itself or its crystallization as a bisphenol A/phenol adduct, separation of adduct crystals by filtration, and next decomposition of this adduct by vacuum distilling off phenol and steam- or nitrogen-stripping or crystallization from various solvents. A process has been patented, consisting in melt crystallization of a raw bisphenol A obtained by way of distillation isolation of a non-reacted acetone, water, and phenol from condensation reaction products. Numerous examples of combining the mentioned methods were described, and in the document WO 0035847 a suspension crystallization of the BP A/phenol adduct, its separation from a mother liquor by filtration, decomposition of adduct by vacuum distilling off phenol and carrying out fractional crystallization of the raw BPA obtained in such a way were represented.

Advantages are known which result from recycling post-crystallization liquors, and in the case of recycling to the crystallization stage or concentration stage before crystallization the advantage consists in recycling a non-reacted phenol and a non-crystallized BPA which decreases significantly a raw material consumption. An additional advantage resulting from recycling the mother liquor to the synthesis stage consists in a limitation of formation of 2-(2- hydroxyphenyl)-2-(4-hydroxyphenyl)propane (so called o,p'-BPA isomer) as then the system approximates more closely the state of equilibrium of the p,p'-BPA and o,p'-BPA isomers.

In processes in which the use of a multi-stage system of flow reactors with a fixed-bed of cation-exchangers is provided it appears the possibility of recycling the mother liquor by several methods so that it supplies the first stage of condensation reaction or successive stages for carrying out this reaction. The most advantage regarding the limitation of forming the o,p'-BPA isomer consists in recycling the mother liquor to the first stage of synthesis, that's why the patent literature includes, first of all, the embodiments consisting in recycling a part of the mother liquor to successive stages of synthesis as an optional possibility only, without indication of resulting advantages (US-Patent 6858759).

Recycling the mother liquor does not solve fully the problem of byproducts as they are cumulated, and one method for solving this problem lies in discharging a part of the post-reaction stream from the process with the aim to maintain their concentration at an acceptable level. Even, when phenol is distilled from this stream, so this indispensable process affects undesirably economical factors of the process for producing BPA. To the methods of decreasing the range of this inadvantageous phenomenon, described in a patent literature, belongs an isomerization reaction of o,p'-BPA to p,p'-BPA in which reaction the fact is used up that after crystallization of the BP A/phenol adduct the concentration of the o,p'-BPA isomer is higher than that in an equilibrium state, and that the process of catalytic decomposition occurs under the influence of acidic (WO 0040531) or basic (PL 181992) catalysts. A process of catalytic decomposition of by-products is considered in the patent literature as an introduction to the process of a BPA synthesis from products of this decomposition, carried out most frequently in the presence of macroporous ion-exchange resins as catalysts. The isomerization reaction allows to convert o,p'-BPA into p,p'-BPA as well as, according to some reports, also trisphenols, first of all, 2,4-bis-[2-(4-hydroxyphenyi)- isopropyl]phenol (JP 08333290) appearing among them in most considerable quantities. The most kinds of by-products are not converted in isomerisation reaction conditions. However, their total contribution to products resulting during the condensation reaction of acetone and phenol in the presence of modified ion- exchange catalysts is not considerable. In consideration of accumulation of byproducts in process streams it is indispensable to carry away a part of the process stream. In spite of an inconsiderable quantity of by-products, an economical effect is observable as in a stream leaving the process, the p,p'-BPA content will be considerably greater than the total amount of by-products leaving the process. More effective in this respect is, however, a process of catalytic decomposition in combination with re-synthezing BPA in macroporous cation exchangers, hi the case of recycling all volatile decomposition products, new kinds of by-products will appear which render difficult obtaining bisphenol A of high quality.

Disclosure of Invention

A purpose of the invention was the development of a process for producing bisphenol A of clear and stable colour in a liquid state, which is characterized by good efficiency and selectivity. It has been found unexpectedly that recycling a mother liquor to the last synthesis stage, as compared with a variant in which its recycling to the reactor of the first stage, allows both to limit the number of by-products other than the o,p'-

BPA isomer and to obtain a higher degree of conversion, at the same total catalyst contact time, with the cation exchanger catalyst.

Li a process in which advantages of recycling the mother liquid to the third stage of condensation reaction are used up, and in the case of using the fractionation of products from catalytic decomposition and recycling only phenol to the process, the indices of a raw material consumption are not satisfying in view of the fact that the o,p'-BPA isomer, which is formed in such a technological variant in considerable quantities, forms in conditions of catalytic decomposition a considerable amount of isopropenylphenols and their oligomers which are discharged from the process. For this reason the process for producing bisphenol A according to the invention uses the isomerization of o,p'-BPA into p,p'-BPA and recycling the isomerizate to crystallization, as well as a catalytic decomposition of the stream of post-crystallization liquors and fractioning the decomposition products, and next recycling to the process phenol only resulting from the decomposition process. It allows to avoid problems associated with the new kinds of by-products, and at the same time to ensure high indices of the conversion of a raw material into bisphenol A.

The essence of the process of the invention consists in the fact that the condensation reaction of acetone and phenol is carried out in a multi-stage reaction system with an interstage control of the reaction temperature and acetone concentration as well as a control of water concentration before the last stage of the reaction system by recycling a part of post-crystallization liquors from a solvent crystallization to the stream directed to the last reactor, while metering in a continuous manner to the system of reactors containing a catalyst the reaction mixture comprising acetone and phenol. Next, water, acetone and a part of phenol are evaporated from a post-reaction mixture which is mixed with an effluent solution from fractional crystallization dissolved in the isomerizate obtained in the process of isomerizing a part of the stream of concentrated liquors after solvent crystallization, which process is carried out with using a macroporous sulfonated ion-exchange resin, in the hydrogen form and of a pore diameter not less than 20 nm, as a catalyst, the resulting stream is subject to solvent crystallization in which the bisphenol A/phenol adduct is isolated, which adduct is subject to thermal decomposition to give a mixture of bisphenol A isomers, phenol and by-products comprising p,p'-bisphenol A in an amount not less than 90 cg/g, the mixture is directed to fractional crystallization in which the pure bisphenol A is obtained, whereas phenol recovered from technological streams in the process of a distillation concentration, thermal decomposition of the bisphenol A/phenol adduct and rectification with simultaneous degradation of phenol derivatives comprised in a part of the stream of concentrated post-crystallization liquors, is recycled to the multi-stage reaction system.

It is preferred when the reaction mixture comprising acetone, phenol and products of their condensation is contacted with a catalyst in a 2-5-stage reaction system at a temperature 323-348 K, moreover the process parameters are selected in such a way that the molecular ratio of o,p'-BPA to p,p'-BPA isomers at the exit of the first reaction stage is not more than 5/100 and the molecular ratio of o,p'- BPA to p,p'-BPA isomers at the entry to the last reaction stage is not less than 7/100. Preferably, proportions of charge streams to the multi-stage reaction system are selected in such a way that the molecular ratio of water to acetone in the reaction mixture comprising phenol, acetone, water, BPA-isomers and byproducts, which is contacted with a catalyst at a temperature 323-348 K, is not more than 0,5 at the entry to the first reaction stage and not more than 1.2 at the last reaction stage of the multi-stage reaction system.

Preferably, the mixture of distilled phenols recovered from technological streams as a result of concentrating a post-reaction mixture from the multi-stage reaction system and from post-crystallization liquors as well as from a thermal decomposition of the BPA/phenol adduct and from rectification with simultaneous catalytic degradation of phenol derivatives, is added to the fresh phenol and directed to the first stage of the multi-stage reaction system. It is preferred when a part of the post-crystallization liquor stream from crystallization of the BPA/phenol adduct, recycled to the multi-stage reaction system, is mixed at the entry to the last reaction stage with a post-reaction mixture from the reaction stage before last in a proportion from 1:1 to 3 : 1. Preferably, the post-crystallization liquors from crystallization of the

BPA/phenol adduct are divided into two streams, whereby the larger stream consisting not more than 95 cg/g of the exit stream of liquors is directed to the last stage of the multi-stage reaction system, whereas the second stream is concentrated by distilling off a part of phenol so that a mass fraction of bisphenol A in this stream is not less than 12 cg/g, the concentrated stream being divided into two parts, one part of which is rectified with simultaneous catalytic degradation of phenol derivatives, and the second part of post-crystallization liquors is directed to isomerization.

Preferably, concentration of the post-reaction mixture from the multi-stage reaction system is carried out in such a way that the water content in post- crystallization liquors from crystallization of the BPA/phenol adduct is not more than 0.4 cg/g.

It is preferred when crystallization of the adduct is carried out so that the content of the p,p'-BPA isomer in a filtrate is not more than 12 cg/g, and the ratio of o,p'-BPA to p,p'-BPA isomers is not less than 10/100.

Preferably, the rectification with simultaneous degradation of phenol derivatives from a part of the stream of concentrated post-crystallization liquors is carried out in the presence of strong inorganic bases KOH or NaOH, at a temperature at least 443 K under reduced pressure not higher than 150 mm Hg, rectification parameters being selected so that phenol resulting from this process and recycled to the multi-stage reaction system comprises not more than 0.05 cg/g of isopropenylphenol (IPP).

Preferably, not more than 85 cg/g of the stream of concentrated post- crystallization liquors are isomerized in the presence of a macroporous, sulphonated ion-exchange catalyst at a temperature 328-353 K with a liquid hourly space velocity (LHSV) of 0.2-5 m³/(m³h), the process of isomerization being carried out so that an increment of the total by-products resulting from isomerization is not more than 0.2 cg/g.

Preferably, the effluent from fractional crystallization, enriched in the o,p'- BPA isomer, is dissolved in the isomerizate at a temperature not lower than 353 K in a proportion from 1 :5 to 1 :20.

Mode for Carrying out the Invention

Example 1

The synthesis of bisphenol A is carried out in a Ill-stage reaction system with the ion-exchanger catalyst Amberlyst A-131 in which 20.2 cmole/mole of sulfonic groups are modified with 2,2-dimethylthiazolidine. The condition of syntheses as well as compositions of solutions obtained in individual reaction stages are shown in Tables 1-4. Designations used in these tables are as follows: Ac - acetone, H₂O - water, PhOH - phenol, p,p'-BPA - 2,2-bis-(4-hydroxy- phenyl)propane, o,p'-BPA - 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane, BPR - by-products, other than o,p'-BPA, from the condensation reaction of acetone and phenol. A LHSV is defined as a number of volumetric units of liquids flowing for one hour through the catalyst unit in a state which is assumed by the catalyst in conditions existing in a reactor.

Table 2. Synthesis conditions in the reaction stage II

Table 3. The result of combining the streams of post-crystallization liquors with a post-reaction solution from the stage II

Table 4. Synthesis conditions in the reaction stage III

LHSV, Reaction Composition of reaction solution (cg/g) m³/(m³h) temperature (⁰C)

Reactor Reactor Solution Ac H₂O PhOH P,P'- o,p'- BPR entry exit component BPA BPA

0.80 57 69 entry 2.70 0.74 84.69 10.56 0.97 0.34 exit 1.06 1.25 79.39 17.00 0.95 0.35

Molecular ratio of water to acetone ; at the entry to the reaction stage III: 0 88 mole/mole

Molecular ratio of isomers o,p'-BPA to p,p'-BPA at the exit from the reaction stage III: 0. 09 mole/mole At the reaction stage I the solution of acetone (2.72 cg/g) in phenol is contacted with an ion-exchanger catalyst. As a result of running the reactions the temperature of a reacting solution is elevated from 51 ⁰C at the reactor entry to 65 ⁰C at the reactor exit, whereas the content of the p,p'-isomer increases by 7.02 cg/g (Table 1). Subsequently, the post-reaction solution from the stage I is cooled in a membrane heat exchanger to a temperature 55 ⁰C, and then to the solution is added a portion of acetone in such an amount so that its concentration is 2.54 cg/g (Table 2).

At the reaction stage II the solution of acetone, phenol and their condensation products is contacted again with the ion-exchanger catalyst as a result of which the solution temperature increases from 55 ⁰C to 66 ⁰C, whereas the content of the p,p'-BPA isomer increases to the level of 12.5 cg/g (composition of the post-reaction mixture from the synthesis stage II is shown in Table 2). Subsequently, the post-reaction mixture from the reaction stage II is mixed with post-crystallization liquors from solvent crystallization of the BP A/phenol adduct in a proportion 1.0:1.3. The stream of post-crystallization liquors, which is used for mixing with the post-reaction solution from the stage II, consists 74 cg/g of the total amount of liquors from the solvent crystallization. The compositions of solutions before and after mixing are shown in Table 3.

The homogeneous solution of post-crystallization liquors and the solution after the synthesis stage II are cooled to a temperature 57 ⁰C, and, subsequently, acetone is added up to the content of 2.7 cg/g in this stream, and it is contacted with the ion-exchanger catalyst in the synthesis stage III. As a result of acetone and phenol condensation at the synthesis stage III, the temperature of a reacting solution increases from 57 ⁰C to 69 ⁰C, and the solution of composition shown in Table 4 is obtained at the reactor exit. The solution leaving the multi-stage reaction system trickles through a 100 mesh filtration gauze and is concentrated by evaporation of water, acetone and a part of phenol at a temperature 125-130 ⁰C under reduced pressure 50 mm Hg. The stream compositions are shown in Table 5. Table 5. Concentration of the reaction solution from the synthesis stage III

Crystallization of the BP A/phenol adduct from a phenol solution is carried out in an agitated crystallizer. A homogeneous solution of bisphenol A of composition shown in Table 6 is placed in a crystallizer equipped with a mechanical agitator and an electronically regulated cooling system which enables to lower a temperature with a set rate.

Table 6. The streams used for obtaining a solution for solvent crystallization and its composition

The temperature of BPA-solution is decreased with the rate 5 °C/h from 80 ⁰C to 55 ⁰C and with the rate of 1 °C/h from 55 ⁰C to 50 ⁰C. The crystallized BPA/phenol adduct is isolated from liquors by filtration at a temperature 50 ⁰C, using a vacuum filter and without washing adduct crystals. Post-crystallization liquors of composition shown in Table 6 are recycled to the process according to previously given methodics. The BP A/phenol adduct obtained as a result of described operations is melted at a temperature 120 ⁰C and subjected to thermal decomposition. The adduct decomposition is carried out in a falling film evaporator at a temperature 165 ⁰C under reduced pressure 20 mm Hg, and, subsequently, the raw bisphenol A of phenol content more than 1.0 cg/g is additionally steam-stripped at a temperature 170 ⁰C under pressure 15 mm Hg with the aim to decrease a phenol content less than 0.5 cg/g.

A part of post-crystallization liquors in an amount of 24 cg/g of the total mass of liquors is concentrated by distillation at temperatures 125-130 ⁰C under pressure 25 mm Hg. The stream of liquors is concentrated to the content 15 cg/g of the p,p'-PBA isomer in the solution. The compositions of post-crystallization liquor streams before and after concentration are shown in Table 7.

Table 7. The composition of post-crystallization liquors before and after concentration

The concentrated post-crystallization liquors are divided into two parts, the stream of 72.5 cg/g being directed to isomerization, whereas the stream of 27.5 cg/g is degraded catalytically with simultaneous rectification of the degradation products of phenol derivatives. The catalytic decomposition of phenol derivatives is carried out in a reactive rectification column at a temperature 190 ⁰C, under pressure 120 mm Hg, in the presence of 0.1 cg/g of sodium hydroxide (NaOH). The distillate from the rectification column is re-distilled to obtain, finally, phenol of 4-isopropenylphenol (PIPH) content less than 0.01 cg/g. The distilled phenol from the catalytic decomposition of phenol derivatives is recycled to the reaction stage I. A part of the stream of concentrated liquors in an amount of 72.5 cg/g is isomerized at a temperature 65 ⁰C. The concentrated liquors of composition shown in Table 7 are contacted with the macroporous catalyst LEWATIT K2649, in the hydrogen form, of an average pore diameter 65 nm, and with LHSV of 0.5 m³/(m³h).

The effluent from fractional crystallization of a raw bisphenol A is dissolved in the isomerizate, at a temperature 87 ⁰C, in a proportion 1:12.7, and next, the resulting solution is mixed with a concentrated post-reaction mixture from the post-reaction stage III. The compositions of individual streams are shown in Table 6.

Purification of bisphenol A is carried out by way of fractional crystallization to give a final product of polycarbonate purity and high thermal stability. Characteristics of bisphenol A are shown in Table 8.

Table 8. Characteristics of the resulting bisphenol A

Claims

1. A process for producing bisphenol A from phenol and acetone by way of catalytic condensation, in the presence of promoted, sulfonated ion- exchange resins, with the use of fractional crystallization for isolating the p,p'- BPA isomer, wherein the condensation reaction of acetone and phenol is carried out in a multi-stage reaction system with the interstage control of reaction temperature and acetone concentration and with the control of water concentration before the last stage of the reaction system by way of recycling a part of post- crystallization liquors from solvent crystallization to the stream directed to the last reactor, continuously metering to the reactor system comprising catalyst a reaction mixture comprising acetone and phenol, and in next step, water, acetone and a part of phenol are evaporated from the reaction mixture which is mixed with an effluent solution from fractional crystallization dissolved in an isomerizate obtained in the isomerization process of a part of the stream of concentrated liquors after solvent crystallization, isomerization is carried out with using a macroporous, sulfonated ion-exchange resin, in the hydrogen form, of a pore diameter not less than 20 nm, as a catalyst, the resulting stream is subjected to solvent crystallization in which an adduct of bisphenol A and phenol is isolated, which adduct is subjected to thermal decomposition to obtain a mixture of bisphenol A isomers, phenol and by-products, where the p,p' -bisphenol A isomer content is not less than 90 cg/g, the mixture is directed to fractional crystallization in which the pure bisphenol A is obtained, whereas phenol recovered from technological streams in a process for concentrating by way of distillation, thermal decomposition of the bisphenol A/phenol adduct and from rectification with simultaneous degradation of phenol derivatives included in a part of the stream of concentrated post-crystallization liquors, is recycled to the multi-stage reaction system.

2. The process for producing bisphenol A as claimed in claim 1, wherein the reaction mixture comprising acetone, phenol and their condensation products is contacted with a catalyst in a 2-5-stage reaction system at a temperature 323-348 K, process parameters being selected so that at the exit from the reaction stage I the molecular ratio of o,p'-BPA to p,p'-BPA isomers is not more than 5/100, and at the entry to the last reaction stage the molecular ratio of o,p'-BPA to p,p'-BPA isomers is not less than 7/100.

3. The process for producing bisphenol A as claimed in claim 1, wherein the proportions of charge streams to the multi-stage reaction system are selected so that the molecular ratio of water to acetone in the reaction mixture comprising phenol, acetone, water, BPA isomers and by-products and contacting with a catalyst at a temperature 323-348 K, is not more than 0.5 at the entry of the reaction stage I, and not more than 1.2 at the last reaction stage of the multi-stage reaction system.

4. The process for producing bisphenol A as claimed in claim 1, wherein the mixture of distilled phenols recovered from technological streams as a result of concentrating a post-reaction mixture from the multi-stage reaction system and post-crystallization liquors as well as thermal decomposition of the BP A/phenol adduct and from rectification with simultaneous catalytic decomposition of phenol derivatives, is added to the fresh phenol and directed to the stage I of the multi-stage reaction system.

5. The process for producing bisphenol A as claimed in claim 1, wherein a part of the stream of post-crystallization liquors from crystallization of the BP A/phenol adduct, recycled to the multi-stage reaction system, is mixed at the entry to the last reaction stage with the post-reaction mixture from the reaction stage before last in a proportion 1 : 1 to 3 : 1.

6. The process for producing bisphenol A as claimed in claim 1, wherein the post-crystallization liquors from crystallization of the BP A/phenol adduct are divided into two streams, the greater one constituting not more than 95 cg/g of the exit stream of liquors being directed to the last stage of the multi-stage reaction system, whereas the second one is subjected to concentration by way of distilling off a part of phenol so that the mass fraction of bisphenol A in this stream is not less than 12 cg/g, and subsequently, the concentrated stream is divided into two parts, one of which is rectified with simultaneous catalytic degradation of phenol derivatives, and the second part of concentrated post- crystallization liquors is directed to isomerization.

7. The process for producing bisphenol A as claimed in claim 1, wherein the concentration of a post-reaction mixture from the multi-stage reaction system is carried out so that the water content in post-crystallization liquors from crystallization of the BP A/phenol adduct is not more than 0.4 cg/g.

8. The process for producing bisphenol A as claimed in claim 1, wherein the adduct crystallization is carried out so that the p,p'-BPA isomer content in a filtrate is not more than 12 cg/g, and the ratio of o,p'-BPA to p,p'- BPA isomers is not less than 10/100.

9. The process for producing bisphenol A as claimed in claim 1, wherein the rectification with simultaneous degradation of phenol derivatives from a part of the stream of concentrated post-crystallization liquors is carried out in the presence of strong inorganic bases KOH or NaOH at a temperature at least 443 K under reduced pressure not higher than 150 mm Hg, and rectification parameters being selected so that phenol obtained in this process and recycled to the multi-stage reaction system comprises not more than 0.05 cg/g of isopropenylphenol (IPP).

10. The process for producing bisphenol A as claimed in claim 1, wherein not more than 85 cg/g of the stream of concentrated post-crystallization liquors is isomerized in the presence of a macroporous, sulfonated ion-exchange catalyst, at a temperature 328-353 K, with a liquid hourly space velocity of 0.2-5 m³/(m³h), whereby the isomerization process is carried out so that the increment of total by-products as a result of isomerization is not more than 0.2 cg/g.

11. The process for producing bisphenol A as claimed in claim 1, wherein the effluent from fractional crystallization, enriched in the o,p'-BPA isomer, is disolved in the isomerizate at a temperature not lower than 353 K in a proportion from 1:5 to 1:20.