PROCESS FOR CONDITIONING AND REUSING SALT-CONTAINING PROCESS WATER

Abstract

The invention relates to an integrated process for conditioning process water (1) from the production (I) of polycarbonate, which process water contains at least catalyst residues and/or organic impurities and sodium chloride, and subsequently utilizing the process water (1) in a subsequent sodium chloride electrolysis (V).

Claims

1.-18. (canceled)

19. An integrated process for workup of process water containing at least catalyst residue and/or organic impurities and sodium chloride from the production of polycarbonate, in particular of diaryl carbonates or of polycarbonate by the solution polymerization process, and subsequent processing of the process water in a downstream sodium chloride electrolysis, comprising at least the steps of: a) production of phosgene by reaction of chlorine with carbon monoxide, then either b1) reaction of the phosgene formed in step a) with at least one bisphenol in the presence of sodium hydroxide solution and optionally catalyst to afford a polycarbonate as the target product and a sodium chloride-containing aqueous solution, or b2) transesterification of one or more bisphenols with one or more diaryl carbonates to afford the oligo/polycarbonate and the monophenol, isolation/separation of the polycarbonate and the monophenol, reaction of the monophenol in the presence of sodium hydroxide solution and of catalyst with phosgene from step a) and separation of the products aqueous sodium chloride solution, polycarbonate as the target product and diaryl carbonate, wherein the diaryl carbonate is preferably reused in the initial transesterification, c) separation of the aqueous sodium chloride-containing solution obtained in step b1) or b2) from solvent residues and/or optionally catalyst residues, in particular by stripping the solution with steam, then adjustment of the prepurified solution to a pH of not more than 8 and subsequent purification (II) of the prepurified NaCl solution with adsorbents, in particular with activated carbon, d) electrochemical oxidation of at least a portion of the sodium chloride-containing solution obtained from step c) to form chlorine, sodium hydroxide solution and optionally hydrogen, e) wherein at least a portion of the chlorine produced in step d) is recycled into the production of phosgene in step a) and/or f) optionally at least a portion of the alkali metal hydroxide solution produced in step d) is recycled into the production of polycarbonate in step b1), wherein following the purification (II) of the sodium chloride-containing solution with adsorbents in step c) the purified NaCl-containing solution is in an additional step c1) subjected to a nanofiltration, wherein the NaCl-containing solution is resolved into a highly purified NaCl solution (8) as permeate and an NaCl-containing concentrate comprising organic and inorganic impurities, the highly purified NaCl solution is sent to the electrochemical oxidation d) and the concentrate is worked up or discarded as desired.

20. The process as claimed in claim 19, wherein the electrochemical oxidation d) of at least a portion of the highly purified sodium chloride-containing solution obtained from the nanofiltration c1) to afford chlorine and sodium hydroxide solution is carried out in a membrane electrolysis using an oxygen-consuming electrode as cathode.

21. The process as claimed in claim 19, wherein the nanofiltration c1) is performed at a temperature of from 10° C. to 45° C.

22. The process as claimed in claim 19, wherein the nanofiltration c1) is performed using a nanofiltration membrane having a separation limit (MWCO) of 150-300 Da.

23. The process as claimed in claim 19, wherein the nanofiltration c1) is performed using a nanofiltration membrane having a separation layer based on piperazinamide.

24. The process as claimed in claim 19, wherein the nanofiltration c1) is performed with a prepurified aqueous NaCl solution having an NaCl concentration in the range of from 4% to 20% by weight.

25. The process as claimed in claim 19, wherein the nanofiltration c1) is performed at a pressure of from 5 to 50 bar.

26. The process as claimed in claim 19, wherein the retention of the nanofiltration membrane for ammonium compounds and salts thereof is in each case independently at least 70%.

27. The process as claimed in claim 19, wherein in the nanofiltration c1) at least 50% of the sodium chloride present in the prepurified NaCl solution before the nanofiltration c1) is retained in the permeate.

28. The process as claimed in claim 19, wherein the membrane used for the nanofiltration c1) has a retention of sodium chloride of not more than 10%.

29. The process as claimed in claim 19, wherein in the purification c) the sodium chloride-containing solution is before the adsorption adjusted to a pH of not more than 8.

30. The process as claimed in claim 19, wherein the permeate flow through the membrane during the nanofiltration (IV) is from 15 to 40 L/(hm.sup.2).

31. The process as claimed in claim 19, wherein before the electrolysis d), the highly purified sodium chloride-containing solution obtained from step c1) is brought to an NaCl concentration of at least 23% by weight.

32. The process as claimed in claim 19, wherein bisphenols in the polycarbonate production (I) dihydroxyaryl compounds of formula (2)
HO—Z—OH  (2), in which Z is an aromatic radical which has 6 to 30 carbon atoms and may comprise one or more aromatic rings, may be substituted and may comprise aliphatic or cycloaliphatic radicals or alkylaryls or heteroatoms as bridging elements, are employed in the reaction b).

33. The process as claimed in claim 19, wherein the bisphenol employed in step b) is selected from the group consisting of dihydroxybiphenyls, bis(hydroxyphenyl)alkanes, bis(hydroxyphenyl)cycloalkanes, bis(hydroxyphenyl)sulfides, bis(hydroxyphenyl)ethers, and ring-alkylated and ring-halogenated thereof.

34. The process as claimed in claim 19, wherein the concentrate obtained in the nanofiltration c1), which contains sodium chloride solution and catalyst residues, is sent to a workup g) in which ionic and nonionic catalyst residues are separated from the concentrated sodium chloride solution using a cation exchange resin and/or the concentrate from c1) is purified using activated carbon and the purified concentrate is optionally sent for reuse for the electrochemical oxidation d).

35. The process as claimed in claim 19, wherein the activated carbon for the adsorption used in step c) and/or in step g) is activated carbon based on pyrolyzed coconut shells.

36. The process as claimed in claim 35, wherein the purified concentrated sodium chloride solution obtained in step g) is additionally reacted in the electrochemical oxidation d).

Description

[0112] The invention is hereinbelow more particularly elucidated with reference to the FIGURES by the examples, which do not, however, constitute any limitation of the invention.

[0113] FIG. 1 shows a schematic representation of the process according to the invention for purification of the process water from polycarbonate production by prepurification via activated carbon, removal of carbonate by stripping and nanofiltration.

[0114] FIG. 1

[0115] The definitions of the reference numerals in the FIGURE are as follows: [0116] I Polycarbonate production (generation of process water) [0117] II Prepurification of the process water using activated carbon [0118] III Removal of carbonate by stripping [0119] IV Nanofiltration [0120] V Brine circuit for chloralkali electrolysis [0121] VI Optional concentrate purification via activated carbon/cation exchanger [0122] VII Optional concentrate purification via ion exchanger [0123] 1 Process water from polycarbonate production pH 12-14 [0124] 2 Hydrochloric acid for adjusting pH to 7-8 [0125] 3 Process water prepurified via activated carbon [0126] 4 Hydrochloric acid for adjusting pH to 2-4 [0127] 5 Carbon dioxide from the stripping column [0128] 6 Prepurified and stripped process water [0129] 7 Sodium hydroxide solution for adjusting pH to 6-8 [0130] 8 Permeate from nanofiltration [0131] 9 Solid NaCl [0132] 10 Purified and concentrated process water [0133] 11 Concentrate from nanofiltration [0134] 12 Optionally purified concentrate (organics removal) [0135] 13 Optionally purified concentrate (inorganics removal)

EXAMPLES

[0136] General Description of Workup of Process Water

[0137] The diphenyl carbonate (DPC) process water I having a TOC content of about 20-100 mg/L, a concentration of ammonium compounds and salts thereof of 0.5-5 mg/L, an NaCl content of 15% to 20% by weight, a carbonate content up to 10 g/L and a pH of 12-14 is initially adjusted with HCl (2) to a pH of less than 8 and sent to the activated carbon purification II. The resulting stream 3 has a concentration of phenols, phenol derivatives and bisphenol A of not more than 2 mg/L. For the optional removal of carbonate by stripping III the process water 3 is adjusted to pH 2-4 with HCl4. The stripped process water 6 having a carbonate concentration of less than 50 mg/L is adjusted to pH 5-8 using sodium hydroxide solution 7 and fed to the nanofiltration IV. In the nanofiltration a concentration factor is established such that at least 50% by weight of the sodium chloride present in the prepurified NaCl solution before the nanofiltration (100% by weight) is retained in the permeate 8. The concentration of ammonium compounds and salts thereof is reduced by at least 90%. The purified permeate 8 may be topped up with solid NaCl 9 until saturation (about 25% by weight) (stream 10) and supplied to the brine circuit of the chloralkali electrolysis V. The concentrate 11 enriched with ammonium compounds and salts thereof and also polyvalent ions may be discarded. Concentrate 11 may optionally be worked up via the additional activated carbon purification/cation exchanger VI and ion exchanger VII and likewise supplied to the brine circuit V.

Example 1

[0138] Four solution batches BV1-BV4 having compositions as reported in table 1 were produced and supplied to the plant as feed stream. The conductivity of the feed was about 110 mS/cm. The test cell was equipped with a GE DK type nanofiltration membrane having an area of about 130 cm.sup.2. The feed was supplied with a volume flow of 500 ml/min. A constant permeate flow of 500 ml/h was generated. The pressure development on the concentrate side was registered. The concentrate was recycled until a volumetric concentration of about 4 was achieved This means for example that 100 L of feed generates 25 L of concentrate and 75 L of permeate.

[0139] The collected permeate and concentrate were then analyzed. The values are reported in table 1.

TABLE-US-00001 TABLE 1 Concentrate Permeate Experimental Monochloromethyl- conductivity conductivity Volumetric duration Pressure ethylpiperidinium Components pH [mS/cm] [mS/cm] concentration [h] [bar] chloride retention [%] BV1 7% by wt. NaCl 7 120 109 3.5 71 27 — BV2 7% by wt. NaCl 3 123 106 3.67 50.5 36 — BV3 7% by wt. NaCl + 3.8 ppm 7 121 109 4.1 74 29 94 monochloromethylethylpiperidinium chloride BV4 7% by wt. NaCl + 3.4 ppm 3 126 109 3 51 36 94 monochloromethylethylpiperidinium chloride

[0140] As is apparent from table 1 the GE DK membrane achieved a retention of monochloromethylethylpiperidinium chloride of 94% at both pH 3 and pH 7. Blocking of the membrane was not observed. The increased operating pressure at pH 3 is due to the properties of the membrane.

Example 2 (Comparison; No Prepurification with Activated Carbon)

[0141] Three solution batches BV5-BV7 having compositions as reported in table 2 were produced and a procedure analogous to example 1 was followed. In contrast to the experiment in example 1 the pressure in the test cell underwent a continuous marked increase, thus precluding stable operation of the cell. Experiment BV6 was aborted prematurely since the maximum allowable operating pressure of the membrane of not more than 41 bar had already been achieved after 15 hours. The collected permeate and concentrate were then analyzed. The values are reported in table 2.

TABLE-US-00002 TABLE 2 Concentrate Permeate Experimental conductivity conductivity duration Pressure Components pH [mS/cm] [mS/cm] Concentration [h] [bar] BV5 7% NaCl + 5 ppm 7 125 102 4.3 58 45 bisphenol A BV6 7% NaCl + 5 ppm 3 132 100 4.7 15 55 bisphenol A BV7 7% NaCl + 5 ppm 9 126 106 4.2 44 46 bisphenol A

[0142] As is apparent from the example even small concentrations of bisphenol A result in blocking of the membrane and prepurification of the process water, for example via activated carbon, is therefore necessary.

Example 3

[0143] A doped solution consisting of sodium chloride (130 mS/cm) and ethylpiperidine (EPP) (20 mg/L) was produced and supplied to the plant as feed at a volume flow of 500 ml/min. Three different nanofiltration membranes, GE DK, NF 270 Dow Filmtec and TR 60 Ropur, were tested at pH 3.2 and pH 6.8 (with an area of about 130 cm.sup.2). A constant permeate flow of 500 ml/h was generated. The values are reported in table 3.

TABLE-US-00003 TABLE 3 EPP EPP NaCl retention at retention at retention pH 3.2 pH 6.8 Pressure Components [%] [%] [%] [bar] BV8 NF 270 Dow ~0 95 71 5 Filmtec BV9 GE DK 8 96 100 20 BV10 TR 60 ~0 48 20 5

[0144] As is apparent from table 3 the retention of the membranes is in some cases strongly dependent on pH and membrane properties.

Example 4

[0145] Real process water (reaction and washing water combined) from polycarbonate production having a conductivity of about 100 mS/cm and a TOC value of 40 mg/L was adjusted to pH 7 using hydrochloric acid and supplied to the plant as feed. The concentration of monochloromethylethylpiperidinium chloride was about 5 mg/l. The investigation was carried out with the GE-DK membrane in recirculation mode (permeate and concentrate were returned). The feed pressure was 40 bar. A flow of 29 L/(hm.sup.2) was initially established and the retention for NaCl and TOC was measured at 31% and 58% respectively. A volumetric concentration by a factor of four was then performed. This means that 1.5 L of permeate was generated from 2 L of feed solution. The conductivity of the concentrate rose to a value of 131 mS/cm and the flow fell to 15 L/(hm.sup.2) at a constant TOC retention of about 56%. This was followed by a twister analysis (qualitative trace analysis) of the feed, permeate and concentrate. The values are reported in table 4. Unfortunately, a quantitative analysis was not possible in the salt solution. Characterization is therefore via the qualitative terms: large amount, moderate amount, small amount based on the relative peak areas of gas chromatograms of the samples. The monochloromethylethylpiperidinium chloride content in the concentrate and permeate was also measured: the concentrate contained 13.8 mg/L, the permeate 0.2 mg/L.

TABLE-US-00004 TABLE 4 Component Feed Concentrate Permeate Ethylpiperidine Moderate amount Large amount Traces Phenol Small amount Small amount Small amount Bisphenol A Large amount Large amount Large amount Isopropylphenol Traces Traces Traces Butylphenol Small amount Small amount Small amount

[0146] As is apparent from the example the TOC reduction in the permeate was achieved only through retention of ethylpiperidine and monochloromethylethylpiperidinium chloride. Other components passed through the membrane unhindered. The retention of the membrane for NaCl increased from 8% to 31% compared to a doped NaCl solution, thus adversely affecting overall performance. The permeate flow was also well below the values of the doped solution despite a higher pressure being employed. This is attributed to the presence of bisphenol A (see example 2).

Example 5

[0147] Real process water (reaction and washing water combined) from polycarbonate production after prepurification with activated carbon having a conductivity of about 190 mS/cm and a TOC value of 3.1 mg/L was supplied to the plant as feed at pH 7. The concentration of monochloromethylethylpiperidinium chloride in the feed was about 0.7 mg/l. The investigation was carried out with the GE DK membrane. The feed pressure was 35 bar. A concentration by a factor of four was performed. This means that 1.5 L of permeate was generated from 2 L of feed solution. An average flow of 35 L/(hm.sup.2) was established. The conductivity of the concentrate rose to 200 mS/cm. The average conductivity of the permeate was 185 mS/cm. The monochloromethylethylpiperidinium chloride content in the permeate was then measured at 0.037 mg/L. This corresponds to a retention of monochloromethylethylpiperidinium chloride of about 95%. Adverse effects such as flow reduction or retention deterioration were not observed.