HK1150590A - Process for the preparation of diaryl carbonate - Google Patents

Description

Process for producing diaryl carbonate

The invention relates to a method for producing diaryl carbonates in combination with the electrolysis of the resulting alkali chloride-containing process waste water. The process according to the invention in particular makes it possible to better utilize the alkali metal chloride-containing solution obtained in the production of diaryl carbonates in electrolysis.

The production of diaryl carbonates is typically carried out in a continuous process by the production of phosgene and subsequent reaction of monophenols with phosgene at the interface in an inert solvent in the presence of a basic nitrogen catalyst and a base (Alkali).

The production of diaryl carbonates, for example by the phase interface method, is described in principle in the literature, see for example Chemistry and Physics of Polycarbonates, Polymer Reviews, H.Schnell, volume 9, John Wiley and Sons, Inc. (1964), page 50/51.

U.S. Pat. No. 6, 4016190 describes A process for the production of diaryl carbonates which is carried out at temperatures > 65 ℃. In this method, the pH is set lower (pH8-9) first and higher (10-11) then.

Process optimization and product separation by improving mixing and maintaining narrow temperature and pH profiles are described in EP 1219589 a1, EP 1216981 a2, EP 1216982 a2 and EP 784048 a 1.

However, in these known processes, the high residual phenol values in the waste water of these processes pollute the environment and cause more troublesome waste water problems for sewage treatment plants, requiring complex purification operations. For example, WO 03/070639a1 describes the removal of organic impurities in wastewater by extraction with dichloromethane.

In general, the alkali chloride-containing solution, preferably the sodium chloride-containing solution, is separated from the solvent and the organic residue and must subsequently be disposed of.

However, it is also known that the purification of sodium chloride-containing waste water can be carried out by ozonolysis according to EP 1200359A 1 or US-A-6340736 and is then suitable for use in sodium chloride electrolysis. The drawback of ozonolysis is that the process is cost intensive.

According to EP 541114 a2, the sodium chloride-containing waste water stream is evaporated to complete removal of water and the remaining salts containing organic impurities are subjected to a thermal treatment, thereby destroying the organic components. Infrared radiation is particularly preferably used here. The disadvantage of this process is that the water must be evaporated completely, which makes the process uneconomical to carry out.

According to WO 03/70639 a1, DPC production wastewater is purified by extraction and then fed to sodium chloride electrolysis. However, with this method, only up to 26% of sodium chloride can be reused from DPC production wastewater, since the water introduced into the electrolysis with the wastewater would cause a water balance imbalance in sodium chloride electrolysis at higher usage.

The alkali chloride-containing solution, preferably sodium chloride-containing solution, obtained in the production of DPC typically has an alkali chloride content, preferably a sodium chloride content, of 13 to 17 wt.%. In this way, all alkali metal chlorides present in the solution cannot be reused. At an alkali chloride concentration of 17 wt.%, only about 23% of sodium chloride can be successfully utilized from a sodium chloride-containing solution in a standard alkali chloride electrolysis, preferably a standard sodium chloride electrolysis, using a commercially available ion exchange membrane exhibiting a water transport of 3.5mol water/mol sodium. Even with saturated sodium chloride solutions having concentrations of up to about 25% by weight, only a recycling of the sodium chloride contained in the 38% sodium chloride-containing solution is achieved. The complete reuse of solutions containing alkali chlorides has not been known so far. According to WO 01/38419 a1, a sodium chloride-containing solution can be evaporated thermally to supply a highly concentrated sodium chloride solution to an electrolytic cell. However, the evaporation is energy intensive and costly.

Based on the above prior art, it is an object of the present invention to provide a method for producing diaryl carbonate, which is capable of producing a product with high purity and high yield, while making it possible to reduce environmental pollution and/or wastewater problems in a sewage treatment plant by maximally reusing alkali metal chloride from an alkali metal chloride-containing process wastewater solution generated from the production of diaryl carbonate.

In particular, in the recycling, it is necessary to take into account that the reaction from the alkali metal chloride, preferably sodium chloride, to chlorine and the lye, preferably aqueous sodium hydroxide solution, and optionally hydrogen, should be carried out with a minimum of energy consumption, thus likewise saving energy.

It has surprisingly been found that in the production of diaryl carbonates by reacting monophenols with phosgene in an inert solvent in the presence of a base and optionally a basic catalyst, an improved utilization of the alkali metal chloride-containing solution resulting from the production of diaryl carbonates can be achieved in chlorine-alkali electrolysis when the alkali metal chloride-containing solution resulting from the production of diaryl carbonates has an alkali metal chloride content of 18 to 25% by weight, based on the total weight of the alkali metal chloride-containing solution. The alkali chloride content of the alkali chloride-containing solution obtained in the diaryl carbonate production, which is from 18 to 25% by weight, can be obtained according to the invention firstly by: in the reaction of a monophenol and phosgene in the presence of a base, the monophenol and an alkali metal containing base are used in amounts such that the sodium phenolate content in a solution formed by the alkali metal containing base and the monophenol is within a specifically selected range, wherein the specified range is preferably from 31 to 40% by weight sodium phenolate based on the total weight of the solution. Additionally or alternatively, concentration of the metal chloride containing solution obtained in the diaryl carbonate production to an alkali metal chloride content of 18 to 25% by weight can likewise be achieved, for example, by recycling at least a portion of the alkali metal chloride containing solution obtained in the diaryl carbonate production to the diaryl carbonate production instead of any water which is contemplated for use.

This is surprising because an increase in sodium phenolate content in the starting solution for diaryl carbonate production results in an increase in energy build-up in the subsequent exothermic formation of diaryl carbonate, resulting in more by-product formation. In addition, when the sodium chloride-containing solution is recycled to the diaryl carbonate production, for example, by replacing any water which is expected to be used, the solubility of phenol decreases due to the presence of alkali metal chloride in the raw material solution, and a corresponding precipitation inevitably occurs. Surprisingly, in the range of from 18 to 25% by weight of alkali chloride content, based on the total weight of the alkali chloride-containing solution selected according to the invention, the above-mentioned phenomenon is not observed. In contrast, the process of the present invention unexpectedly provides the potential to recycle the alkali metal chloride component of diaryl carbonate production reaction wastewater for reuse by electrolysis of significantly more of it.

The invention relates to a method for producing diaryl carbonates and for treating at least a part of the alkali chloride-containing solution formed in a downstream alkali chloride electrolysis, comprising the following steps:

a) producing phosgene by reacting chlorine with carbon monoxide, and

b) reacting the phosgene formed in step a) with at least one monophenol in the presence of a base and optionally a basic catalyst to provide a diaryl carbonate and a solution comprising an alkali metal chloride, and

c) separating off and working up the diaryl carbonate formed in step b), and

d) separating off solvent residues and optionally catalyst residues from the alkali chloride-containing solution remaining in step c), in particular by stripping the solution with steam and treating it with an adsorbent, in particular with activated carbon, wherein the alkali chloride-containing solution is adjusted to a pH of less than or equal to 8 before the treatment with the adsorbent, and

e) electrochemically oxidizing at least a portion of the alkali chloride-containing solution from d) to form chlorine, lye and optionally hydrogen, and

f) recycling at least a part of the chlorine produced in step e) to the phosgene production of step a), and/or

g) Recycling at least a part of the lye produced in step e) to the diaryl carbonate production of step b),

characterized in that the alkali chloride-containing solution produced in step b) has an alkali chloride content of 18 to 25% by weight, based on the total weight of the alkali chloride-containing solution, and/or at least a part of the alkali chloride-containing solution produced in step d) is recycled into step b).

The elevated alkali chloride content of 18 to 25% by weight, based on the total weight of the alkali chloride-containing solution, required for the electrolysis in the process according to the invention can be obtained in the following manner: in step b), the base is an alkali metal-containing base, preferably a sodium-containing base, and the alkali metal-containing base and the monophenol are used in such an amount that the sodium phenolate content of the solution formed from the alkali metal base and the monophenol, based on the total weight of the solution, is from 31 to 40% by weight sodium phenolate, and/or at least a part of the alkali metal chloride-containing solution present in step d) is recycled into step b). In this case, it is preferred that at most 80% by weight, preferably at most 50% by weight, of the alkali metal chloride-containing solution produced in step d) is recycled into step b).

In the case where at least a part of the alkali metal chloride-containing solution produced in step d) is recycled to step b), in a preferred embodiment of the process according to the invention the remaining alkali metal chloride-containing solution of step c) can be used in step d), which remaining alkali metal chloride-containing solution is also at least partly combined with the wash water of the diaryl carbonate work-up in step c). This measure provides the additional advantage that the entire disposal of the aqueous phase can be avoided.

Preferably, in step b), the sodium phenolate content of the solution of 31 to 40% by weight of sodium phenolate containing alkali metal base with monophenol is obtained by reacting monophenol with 14 to 21% by weight of a concentration of alkali metal containing base, in particular sodium containing base.

Monophenols particularly suitable for the novel process are phenols of the formula (I)

Wherein

R is hydrogen, halogen or branched or unbranched C₁-C₉Alkyl or alkoxycarbonyl.

Within the scope of the invention, C₁-C₉Alkyl is, for example and preferably, methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, neopentyl, 1-ethylpropyl, cyclohexyl, cyclopentyl, n-hexyl, n-heptyl, n-octyl, isooctyl, n-nonyl, isononyl.

Within the scope of the present invention, halogen is, for example and preferably, fluorine, chlorine, bromine or iodine, preferably chlorine and bromine.

Thus, preferably suitable monophenols are phenol, alkylphenols such as cresol, p-tert-butylphenol, p-cumylphenol, p-n-octylphenol, p-isooctylphenol, p-n-nonylphenol and p-isononylphenol, halophenols such as p-chlorophenol, 2, 4-dichlorophenol, p-bromophenol and 2, 4, 6-tribromophenol or methyl salicylate. Phenol is particularly preferred.

Suitable bases for the reaction of the monophenols with phosgene are, for example, alkali metal salts, preferably alkali metal hydroxides, such as Na, K or Li hydroxides, particularly preferably aqueous sodium hydroxide solution. The base is preferably used in the process according to the invention in the form of a 14-21% strength by weight aqueous solution.

The reaction in step b) can be accelerated by basic catalysts such as tertiary amines, N-alkylpiperidines or onium salts. Nitrogen-containing catalysts are preferred. Tributylamine, triethylamine and N-ethylpiperidine are particularly preferred.

The basic catalysts which can be used may be open-chain or cyclic, triethylamine and ethylpiperidine being particularly preferred. The catalyst is preferably used in the process according to the invention in the form of a1 to 55% strength by weight solution.

Within the scope of the present invention, onium salts are preferably used to denote, for example, NR₄Compounds of X, where the radicals R, independently of one another, may be H and/or alkyl and/or aryl, and X is an anion, such as chloride, bromide or iodide.

Phosgene can be used in step b) in liquid, gaseous or dissolved form in an inert solvent.

In the process of the invention, in step b), the inert organic solvent which can preferably be used is preferably an aromatic solvent, a halogenated (more preferably chlorinated) aliphatic or aromatic solvent, or a mixture of these solvents. Such as toluene, dichloromethane, various dichloroethane and chloropropane compounds, chlorobenzene and chlorotoluene or mixtures thereof. Methylene chloride is particularly preferred.

The diaryl carbonates produced in step b) of the process according to the invention are preferably those of the general formula (II)

Wherein R has the meaning indicated in formula (I). The process of the present invention is particularly preferred for the production of diphenyl carbonate.

The reaction process of step b) is preferably carried out continuously, particularly preferably in the form of a plug flow without substantial back-mixingThe process is carried out. This can be achieved, for example, in a tubular reactor. The mixing of the two phases (aqueous and organic) can be carried out, for example, by means of internal tube baffles, static mixers and/or pumps.

The reaction of step b) is particularly preferably carried out in two stages.

In the first stage of the preferred process, the reaction is initiated by mixing together the starting materials phosgene, at least one inert organic solvent which preferably initially acts as a solvent for the phosgene, and a monophenol which has preferably been dissolved beforehand in a solution of the base (derder Base), preferably in lye (Alkalilauge). The residence time in the first stage is typically in the range from 2 seconds to 300 seconds, particularly preferably in the range from 4 seconds to 200 seconds. The pH value of the first stage is preferably adjusted by the ratio of alkali (preferably lye) to monophenol/phosgene in such a way that the pH is in the range from 11.0 to 12.0, preferably from 11.2 to 11.8, particularly preferably from 11.4 to 11.6. The reaction temperature in the first stage is preferably kept below 40 ℃ by cooling, particularly preferably 35 ℃ or less.

In the second stage of the preferred process, the reaction to produce diaryl carbonate is completed. In a preferred process, the residence time is from 1 minute to 2 hours, preferably from 2 minutes to 1 hour, particularly preferably from 3 minutes to 30 minutes. The second phase of the preferred process is controlled by continuously monitoring the pH, which is preferably measured on-line in a continuous process by methods known in principle, and by adjusting the pH accordingly by adding lye. The amount of alkali, preferably lye, supplied is set in particular in such a way that the pH of the reaction mixture in the second process stage is in the range from 7.5 to 10.5, preferably from 8 to 9.5, particularly preferably from 8.2 to 9.3. The reaction temperature in the second stage is maintained by cooling at preferably below 50 ℃, particularly preferably below 40 ℃, very particularly preferably 35 ℃ or less. The alkali metal chloride-containing solution produced in step b) thus has a temperature of less than 50 ℃, particularly preferably less than 40 ℃, very particularly preferably 35 ℃ or less, at the completion of the reaction for producing diaryl carbonate.

The general parameters and/or explanations cited in the present application or in the preferred ranges can also be combined with one another, i.e. between the respective ranges and preferred ranges, as required.

In a preferred embodiment of the process according to the invention, phosgene is used in step b) in a molar ratio of from 1: 2 to 1: 2.2 relative to the monophenol. The inert organic solvent is added in such a form that the diaryl carbonate is present after the reaction as a 5 to 50% strength by weight solution, preferably a 20 to 45% strength by weight solution, based on the organic phase.

The catalyst optionally used is preferably added in an amount of from 0.0001mol to 0.1mol, based on the monophenol used.

After reaction b), in step c), the diaryl carbonate-containing organic phase is preferably generally washed with an aqueous liquid and, after each washing operation, separated as far as possible from the aqueous phase. As washing liquid, for separating the catalyst, use is made of an aqueous liquid, e.g. dilute mineral acids such as HCl or H₃PO₄And demineralized water was used for further purification. HCl or H in the washing liquid₃PO₄The concentration of (b) may preferably be 0.5 to 1.0% by weight. The organic phase is for example and preferably washed twice.

The phase separation device which can be used to separate the washing liquid from the organic phase is a separation vessel, a phase separator, a centrifuge or a coalescer or a combination of these devices, the principle of which is known.

Regardless of the solvent yet to be separated, it provides an unexpectedly high diaryl carbonate purity of > 99.85%.

After diaryl carbonate synthesis, the diaryl carbonate may be isolated as a solution in an inert organic solvent used in its manufacture, such as methylene chloride.

To obtain a high purity diaryl carbonate, the solvent is preferably evaporated. The evaporation may be carried out in a plurality of evaporator stages. For example, it is carried out by one or more distillation columns in series in which the solvent is separated from the diaryl carbonate.

The purification stage c) can be carried out continuously, for example, in such a way that the bottom temperature during the distillation is between 150 ℃ and 310 ℃, preferably between 160 ℃ and 230 ℃. The pressure used for carrying out the distillation is in particular from 1 to 1000 mbar, preferably from 5 to 100 mbar.

The diaryl carbonate thus purified is characterized by its particularly high purity (> 99.95% according to GC analysis) and excellent transesterification reaction properties, thus making it suitable for the subsequent production of polycarbonates of excellent quality. The use of diaryl carbonates for the production of aromatic oligo/polycarbonates by the melt transesterification process is known from the literature and is described, for example, in EncyclopediA of Polymer Science, Vol.10 (1969), Chemistry and Physics of polycarbonates, Polymer Reviews, H.Schnell, Vol.9, John Wiley and sons, Inc. (1964) or U.S. Pat. No. A-5340905.

In the case of the separation described in step c), the aqueous alkali chloride-containing solution remaining after the phase separation (hereinafter also referred to as alkali chloride-containing waste water) is advantageously freed from readily volatile organic impurities such as organic solvent residues used in the synthesis and, optionally, catalyst residues by distillation or steam stripping. In this case, an alkali chloride-containing solution having a high content of dissolved alkali chloride of from 18 to 25% by weight, based on the total weight of the alkali chloride-containing solution, remains, which further comprises from 0.3 to 1.5% by weight of dissolved alkali carbonate, based on the total weight of the alkali chloride-containing solution. These alkali metal carbonates are formed, for example, by the hydrolysis of phosgene as a side reaction in the production of diaryl carbonates. In addition, the alkali chloride-containing wastewater is contaminated with organic compounds such as phenols (e.g., unsubstituted phenols, alkylphenols).

The pre-purified alkali chloride-containing wastewater is then treated with an adsorbent, preferably activated carbon. The alkali chloride-containing wastewater is adjusted to a pH of less than or equal to 8, preferably a pH of less than 7, prior to treatment with the adsorbent.

In process step d), this pH drop is preferably effected by addition of a protic acid. More preferably, in process step d), the pH drop is carried out using hydrochloric acid or gaseous hydrogen chloride. The use of cheaper sulfuric acid is in principle conceivable, but this is undesirable in the present process, since it leads to the formation of sodium sulfate in the pH drop, which sodium sulfate accumulates in the anolyte circuit in the subsequent electrolysis. Since, for example, according to the manufacturer's instructions, the ion-exchange membrane must only be operated in an anolyte having at most a specific sodium sulfate concentration, more anolyte has to be discharged than when hydrochloric acid or hydrogen chloride is used, and the reaction product is advantageously the desired sodium chloride as well.

Subsequently, at least a part of the alkali chloride-containing waste water of step d) is electrochemically oxidized, preferably in the form of chlor-alkali electrolysis with the formation of chlorine, lye and optionally hydrogen.

Chlor-alkali electrolysis is described in more detail below. The following description is based on the exemplary description of sodium chloride electrolysis, since in the process, as previously described, in principle any alkali metal chloride (especially LiCl, NaCl, KCl) can be used. However, since it is preferred to use sodium chloride or aqueous sodium hydroxide solution in the preceding stages of the process of the invention, the electrolysis of sodium chloride is likewise preferred in a preferred embodiment of the process of the invention.

For example, for the electrolysis of sodium chloride-containing solutions, use is generally made, for example, of membrane electrolysis (cf. Peter Schmittinger, CHLORINE, Wiley-VCH Verlag, 2000). In this case, a two-part electrolytic cell is used, which consists of an anode chamber with an anode and a cathode chamber with a cathode. The anode chamber and the cathode chamber are separated by an ion exchange membrane. A sodium chloride-containing solution, which generally has a sodium chloride concentration of more than 300g/l, is introduced into the anode compartment. In thatAt the anode, the chloride ions are oxidized to chlorine gas, which is conducted away from the chamber together with the consumed sodium chloride-containing solution. Sodium ions migrate through the ion exchange membrane into the cathode chamber under the action of an electric field. During this migration, depending on the membrane, 3.5-4.5mol of water are accompanied per mol of sodium. This results in a depletion of water in the anolyte. On the cathode side, compared to the anolyte, water is consumed as a result of the electrolysis of water into hydroxide ions and hydrogen. The water entering the catholyte together with the sodium ions is sufficient to maintain a sodium hydroxide concentration at the outlet of 31-32% by weight-at the inlet of 30% and a current density of 4kA/m²In the case of (1). In the cathode compartment, water is electrochemically reduced while hydroxide ions and hydrogen gas are formed.

Alternatively, as the cathode, a gas diffusion electrode may also be used, in which oxygen is reacted with electrons to provide hydroxide ions, without hydrogen gas being formed. The hydroxide ions form an aqueous sodium hydroxide solution together with sodium ions that pass through the ion exchange membrane into the cathode compartment. Usually, an aqueous solution of sodium hydroxide having a concentration of 30% by weight is supplied into the cathode chamber, and the aqueous solution of sodium hydroxide having a concentration of 31 to 32% by weight is removed. The aim is to achieve as high a concentration of aqueous sodium hydroxide solution as possible, since usually aqueous sodium hydroxide solution is stored or transported in the form of 50% strength by weight lye. However, commercially available membranes are generally not stable to alkali solutions with concentrations above 32% by weight, so that the aqueous sodium hydroxide solution produced must subsequently be concentrated by thermal evaporation.

In the case of sodium chloride electrolysis according to the process of the invention, additional water is introduced into the anolyte by using sodium chloride-containing waste water, but only the water itself is discharged through the membrane into the catholyte. If more water is introduced through the sodium chloride containing solution than is fed to the catholyte, the anolyte will be depleted of sodium chloride and the electrolysis can no longer be run continuously. At very low sodium chloride concentrations, side reactions occur that generate oxygen.

To economically supply the maximum amount of sodium chloride-containing solution to sodium chloride electrolysis, it may be effective to increase the water transport through the membrane. This can be done by selecting A suitable membrane, as described in US-A-4025405. The effect of the increased water transport is that the otherwise usual water addition for maintaining the lye concentration can be dispensed with.

According to US-A-3773634, electrolysis can be carried out with A high water transport through the membrane, when A lye concentration of 31-43% by weight and A sodium chloride concentration of 120-250g/l are employed.

The drawback of both of the above processes is their low current efficiency.

According to the process of the invention, the separation c) of the sodium chloride-containing waste water is carried out by phase separation. Subsequently, in step d), the solvent and optionally the catalyst used are removed, in particular by steam stripping, and may also be removed after pH adjustment by treatment with an adsorbent. Thereafter, the alkali chloride-containing wastewater may be directly supplied to the electrolysis e).

In a preferred embodiment of the process according to the invention, water can additionally be drawn off from the alkali chloride-containing waste water by a concentration process in order to increase the alkali chloride concentration. Thus, a method having the following features is preferred: prior to the electrolysis of e), the alkali chloride-containing solution obtained from d) is concentrated by a membrane distillation process or reverse osmosis. However, in contrast to the process of the present invention, which only found alkali chloride-containing waste water having an alkali chloride content of at most 17% by weight, based on the total weight of the alkali chloride-containing solution, in the production of diaryl carbonate, the process of the present invention requires significantly less water to be drawn off by the concentration process in order to obtain the same alkali chloride concentration.

In this case, for example, reverse osmosis can be used, or membrane distillation or membrane contactors (see MELIN; RAUTENBACH, Membrane Processes; SPRINGER, BERLIN, 2003) are particularly preferably used. By combining the cell operation of the present invention with the concentration process, up to 100% of the sodium chloride can theoretically be recovered from the wastewater.

In a further preferred embodiment of the process according to the invention, the alkali metal chloride-containing waste water of the diaryl carbonate production is concentrated by solid alkali metal chloride and fed to alkali metal chloride electrolysis. By this preferred embodiment of the process according to the invention, more than 50% of the alkali metal chloride can be recovered and thus reused from the waste water of diaryl carbonate production. However, this indicates that the chlorine and lye are not all used in the production of diaryl carbonate. However, in the process of the present invention, significantly less solid alkali metal chloride needs to be added for concentration than in the process of alkali metal chloride-containing wastewater having only an alkali metal chloride content of at most 17% by weight based on the total weight of the alkali metal chloride-containing solution in the production of diaryl carbonate.

The process of the invention can also be carried out using alkali chloride electrolysis, in which no hydrogen is produced at the cathode, but the cathode is replaced by a gas diffusion electrode at which oxygen is reduced to hydroxide ions. If, for example, in a complex, hydrogen is not required to participate in the chemical reaction, the product hydrogen which is forced to form can be avoided. This has the advantage that energy can be saved during electrolysis due to the lower electrolysis voltage when using gas diffusion electrodes.

The alkali chloride electrolysis is generally operated in such a way that the alkali chloride concentration in the alkali chloride solution output from the electrolysis cell is 100-280g/l sodium chloride and/or in such a way that the lye concentration output from the electrolysis cell is 13-33% by weight. Concentrations that enable the cell to operate at lower voltages are particularly preferred. For this purpose, the concentration of the alkali chloride solution discharged from the cell should preferably be 110-220g/l alkali chloride and/or the concentration of the lye discharged from the cell should be 20-30% by weight. In a preferred embodiment of the process according to the invention, the alkali chloride electrolysis is operated in such a way that the alkali chloride solution output from the cell has a concentration of less than 200g/l NaCl. In parallel to this, the concentration of lye output from the cell may be less than 30% by weight.

The water transport through the membrane depends not only on the operating parameters but also on the type of membrane used. According to the process of the invention, preference is given to using ion-exchange membranes which, under the sodium chloride and lye concentrations according to the invention, are able to transport more than 4.5mol of water per mol of sodium through the membrane.

In this case, the current density is calculated on the basis of the membrane area, in particular 2 to 6kA/m². It is particularly preferred to use an anode having a relatively large surface area. Anodes having a relatively large surface area are those whose physical surface area is significantly larger than the geometric surface area, i.e., the projected surface area. Anodes with a large surface area are for example electrodes with a foam-like or felt-like structure. Thus, on the anode, an extremely high electrode surface area is provided and the local current density is greatly reduced. The surface area of the anode is preferably such that the local current density is below 3kA/m based on the physical surface area of the electrode²The method of (1). The higher the surface area and the lower the local current density, the lower the concentration of alkali chloride in the brine that can be selected and the higher the proportion of alkali chloride that can be reused from the wastewater.

The pH of the alkali chloride-containing waste water prior to electrolysis e) is preferably below 7, particularly preferably from 0.5 to 6. The pH adjustment can be carried out by adding a protic acid, preferably hydrochloric acid or gaseous hydrogen chloride.

The ion-exchange membrane used for the electrolysis should preferably exhibit more than 4.0mol H per mol sodium₂Water transport of O/mol sodium, particularly preferably 5.5 to 6.5mol H₂O/mol sodium.

The process is preferably carried out in such a way that the electrolysis e) is operated at a temperature of from 70 to 100 c, preferably from 80 to 95 c.

The electrolysis is carried out at a pressure of 1 to 1.4 bar, preferably 1.1 to 1.2 bar absolute. The pressure conditions between the anode and cathode compartments are in particular selected in such a way that the cathode compartment pressure is higher than the anode compartment pressure. In a particularly preferred process, the pressure difference between the cathode compartment and the anode compartment should be 20 to 150 mbar, preferably 30 to 100 mbar.

Special anodic coatings can also be used at low alkali chloride concentrations. In particular, the anodic coating may comprise, in addition to ruthenium oxide, further noble metal components of transition groups 7 and 8 of the periodic table of the elements. For example, the anode coating may be doped with a palladium component. Likewise, diamond based coatings may also be used.

The sodium chloride-containing solution (sodium chloride-containing waste water) obtained from diphenyl carbonate production (DPC production) typically according to known methods has a sodium chloride content of at most 17 wt.%, provided that it is the reaction waste water. If the reaction waste water is additionally combined with the wash water resulting from the aftertreatment of step c), the NaCl concentration is, for example, only about 13% by weight. If the chlorine and the aqueous sodium hydroxide solution supplied by electrolysis are all used for DPC production, wastewater containing only a small amount of sodium chloride is used in the electrolysis. For example, using conventional ion exchange membranes and standard sodium chloride electrolysis operating parameters, only a maximum of 26% sodium chloride in 17 wt.% concentration of sodium chloride-containing DPC wastewater can be used. Standard operating parameters for NaCl electrolysis are the ideal values at the outlet of 200-240g/l brine concentration and 31-32 wt% NaOH concentration. Thus, the sodium chloride produced has not been able to be fully reused until now. In addition, concentration by thermal evaporation of water is currently not economical, since sodium chloride is a very cheap product.

Now, if all the chlorine and aqueous sodium hydroxide solution supplied by the electrolysis of sodium chloride is used for the production of DPC, significantly more than 26% of sodium chloride can be reused from waste water with a NaCl content of 18-25% by the method of the present invention. Typically, sodium chloride electrolysis is carried out in a chemical complex with a plurality of chlorine consumers, such that only part of the consumers provide a sodium chloride-containing solution for reuse. In addition, when the aqueous sodium hydroxide solution and chlorine gas supplied by the electrolysis of sodium chloride are not completely used for the production of diaryl carbonate, the proportion of sodium chloride that can be regenerated from the waste water is further increased.

By the process of the present invention, more than 26% of sodium chloride can be recovered from the waste water, compared to the prior art (WO 03/70639 a1) which can use up to 26% of the sodium chloride present in DPC production waste water in NaCl electrolysis.

The following examples are intended to illustrate the invention without limiting it.

Examples

The following examples demonstrate the process of the present invention using sodium chloride-containing wastewater produced in the production of diphenyl carbonate (DPC production).

Example 1

Addition of enriched sodium chloride-containing reaction wastewater to sodium chloride electrolysis-addition of 22.5% strength by weight sodium chloride solution resulting from DPC production to sodium chloride electrolysis

a) Separation of wastewater from DPC production

In a vertical cooled tubular reactor, a mixture of 145.2kg/h of 14.5% strength aqueous sodium hydroxide solution, prepared by diluting 65.8kg/h of 32% strength aqueous sodium hydroxide solution with 79.4kg/h of demineralized water (deionized water), and 48.3kg/h of phenol was mixed continuously with 27.5kg/h of phosgene (8 mol% excess based on phenol) and 86.2kg/h of methylene chloride solution. The reaction mixture was cooled to a temperature of 33 ℃ and after a mean residence time of 15 seconds, a pH of 11.5 was measured. Then, in the second stage of the process, 5.4kg/h of 50% strength NaOH were added to the reaction mixture, so that after a further residence time of 5 minutes, the pH of the second reaction stage was 8.5. In the case of continuously operated reactions, the metering fluctuations that occur are overcome by adjusting the respective NaOH metering. In the second stage of the process, the reaction mixture is continuously mixed by passing it through a tube having a constriction. After the addition of NaOH again, the reaction temperature was set to 30 ℃ by cooling. After separation of the organic phase from the aqueous phase (reaction wastewater), the DPC solution was washed with 0.6% strength hydrochloric acid and water. After removal of the solvent, diphenyl carbonate of 99.9% purity was obtained. The reaction waste water is combined with the wash phase and the solvent residues and catalyst are removed by stripping with steam. After neutralization with hydrochloric acid and activated carbon treatment, the process wastewater contained 13.2 wt% NaCl and less than 2ppm phenol.

It can be recycled to the production process without further purification.

b) Sodium chloride enrichment by recycling the waste water from a) to the waste water of DPC production

DPC production was carried out as described in example 1, with the difference that instead of 79.4kg/h of demineralized water, a mixture of 37.9kg/h of demineralized water and 47.7kg/h of the waste water from example 1a) was continuously used.

After separation of the organic phase from the aqueous phase (reaction waste water), the organic phase was washed with 0.6% strength hydrochloric acid and water. After removal of the solvent, diphenyl carbonate of 99.9% purity was obtained. The reaction waste water was freed of solvent residues and catalyst by stripping with steam without prior combination with the aqueous phase. After neutralization with hydrochloric acid and activated carbon treatment, the reaction waste water contained 22.5% by weight NaCl and less than 2ppm phenol.

c) Electrochemical oxidation of the reaction waste water obtained from b)

For example by having a diameter of 0.01m²The electrolysis was carried out in a laboratory cell with anode surface area. The current density is 4kA/m²The cathode side exit temperature was 88 ℃ and the anode side exit temperature was 89 ℃. An electrolytic cell with standard anode and cathode coatings of DENORA, germany, was used. The ion exchange membrane used was Nafion 982WX from DuPont. The electrolytic voltage was 3.02V. The sodium chloride-containing solution was circulated through the anode compartment by pumping at a mass flow rate of 0.96 kg/h. The solution concentration supplied to the anode compartment was 25 wt% NaCl. From the anode chamber, a 20 wt% NaCl solution was discharged. 0.14kg/h of 22.5% by weight reaction waste water from the diphenyl carbonate production in step b) and 0.0543kg/h of solid sodium chloride were added to the NaCl solution discharged from the anode compartment. The solution is then fed again into the anode compartment. The water transport through the membrane was 3.8mol water/mol sodium.

On the cathode side, an aqueous sodium hydroxide solution was pumped at a mass flow rate of 0.849 kg/h. The concentration of the aqueous sodium hydroxide solution supplied to the cathode side was 30 wt% NaOH, and the aqueous sodium hydroxide solution discharged from the cathode side had a NaOH concentration of 32.1%. 0.186kg/h of 32.1% strength lye are discharged from the volume flow, 0.057kg/h of water are added to the remaining material and recirculated to the cathode assembly.

35.7% of the reacted sodium chloride originates from the reaction waste water of DPC production.

Example 2

Addition of enriched sodium chloride-containing production wastewater to sodium chloride electrolysis-addition of 18.0% strength by weight sodium chloride solution resulting from DPC production to sodium chloride electrolysis

a) Sodium chloride concentration of process wastewater by wastewater recycling to the process

The procedure was carried out as described in example 1b), with the difference that the reaction waste water was combined with the wash phase and subsequently the solvent residues and the catalyst were removed by stripping with steam. After neutralization with hydrochloric acid and activated carbon treatment, the process wastewater contained 18.0 wt.% NaCl and less than 2ppm of organic impurities.

Which can be fed without purification to the electrochemical oxidation to form chlorine.

b) Electrochemical oxidation of the enriched process wastewater obtained from a)

For example by having a diameter of 0.01m²The electrolysis was carried out in a laboratory cell with anode surface area. The current density is 4kA/m²The cathode side exit temperature was 88 ℃ and the anode side exit temperature was 89 ℃. An electrolytic cell with standard anode and cathode coatings of DENORA, germany, was used. The ion exchange membrane used was Nafion 982WX from DuPont. The electrolytic voltage was 3.02V. The sodium chloride-containing solution was circulated through the anode compartment by pumping at a mass flow rate of 0.96 kg/h. The solution concentration supplied to the anode compartment was 25 wt% NaCl. From the anode chamber, a 20 wt% NaCl solution was discharged. 0.133kg/h of 18% by weight of production waste water from the production of diphenyl carbonate in step a) and 0.062kg/h of solid sodium chloride were added to the anodeThe NaCl solution was drained from the chamber. The solution is then fed again into the anode compartment. The water transport through the membrane was 3.8mol water/mol sodium.

On the cathode side, an aqueous sodium hydroxide solution was circulated by pumping at a mass flow rate of 0.849 kg/h. The concentration of the aqueous sodium hydroxide solution supplied to the cathode side was 30 wt% NaOH, and the aqueous sodium hydroxide solution discharged from the cathode side had a NaOH concentration of 32.2%. 0.185kg/h of 32.2% strength lye are discharged from the volume flow, 0.057kg/h of water are added to the remaining material and recirculated to the cathode assembly.

27.0% of the reacted sodium chloride originates from DPC production wastewater.

Example 3

Addition of enriched sodium chloride-containing reaction wastewater to sodium chloride electrolysis-addition of 22.3% strength by weight sodium chloride solution resulting from DPC production to sodium chloride electrolysis

a) Sodium chloride enrichment by reaction wastewater with increased sodium phenolate concentration in DPC production

The procedure was carried out as described in example 1a), with the difference that instead of 79.4kg/h only 54.6kg/h demineralized water was used. After separation of the organic phase from the aqueous phase (reaction waste water), the organic phase was washed with 0.6% strength hydrochloric acid and water. After removal of the solvent, diphenyl carbonate of 99.9% purity was obtained. The reaction waste water was stripped of solvent residues and catalyst by steam stripping without prior mixing with the aqueous phase. After neutralization of the activated carbon with hydrochloric acid, the reaction waste water contained 22.3% by weight NaCl and less than 2ppm phenol.

b) Electrochemical oxidation of the enriched reaction waste water obtained from a)

For example by having a diameter of 0.01m²The electrolysis was carried out in a laboratory cell with anode surface area. The current density is 4kA/m²The cathode side exit temperature was 88 ℃ and the anode side exit temperature was 89 ℃. Use of a standard anode with DENORA in Germany andcathode coated electrolytic cells. The ion exchange membrane used was Nafion 982WX from DuPont. The electrolytic voltage was 3.02V. The sodium chloride-containing solution was circulated through the anode compartment by pumping at a mass flow rate of 0.96 kg/h. The solution concentration supplied to the anode compartment was 25 wt% NaCl. From the anode chamber, a 20 wt% NaCl solution was discharged. 0.14kg/h of 22.3% by weight reaction waste water from the diphenyl carbonate production in step a) and 0.0546kg/h of solid sodium chloride were added to the NaCl solution discharged from the anode compartment. The solution is then fed again into the anode compartment. The water transport through the membrane was 3.8mol water/mol sodium.

On the cathode side, an aqueous sodium hydroxide solution was circulated by pumping at a mass flow rate of 0.849 kg/h. The concentration of the aqueous sodium hydroxide solution supplied to the cathode side was 30 wt% NaOH, and the aqueous sodium hydroxide solution discharged from the cathode side had a NaOH concentration of 32.2%. 0.186kg/h of 32.2% strength lye are discharged from the volume flow, 0.057kg/h of water are added to the remaining material and recirculated to the cathode assembly.

35.3% of the reacted sodium chloride originates from the DPC reaction waste water.

Example 4 (comparative)

Sodium chloride-containing reaction wastewater addition to sodium chloride electrolysis-sodium chloride solution produced by DPC production at a concentration of 17% by weight was added

a) Separation of wastewater from DPC production

In a vertical cooled tubular reactor, a mixture of 145.2kg/h of 14.5% strength aqueous sodium hydroxide solution, prepared by diluting 65.8kg/h of 32% strength aqueous sodium hydroxide solution with 79.4kg/h of demineralized water, and 48.3kg/h of phenol was mixed continuously with 27.5kg/h of phosgene (8 mol% excess based on phenol) and 86.2kg/h of methylene chloride solution. The reaction mixture was cooled to a temperature of 33 ℃ and after a mean residence time of 15 seconds, a pH of 11.5 was measured. Then, in the second stage of the process, 5.4kg/h of 50% strength NaOH were added to the reaction mixture, so that after a further residence time of 5 minutes, the pH of the second reaction stage was 8.5. In the case of continuously operated reactions, the metering fluctuations that occur are overcome by adjusting the respective NaOH metering. In the second stage of the process, the reaction mixture is continuously mixed by passing it through a tube having a constriction. After the addition of NaOH again, the reaction temperature was set to 30 ℃ by cooling. After separation of the organic phase from the aqueous phase (reaction wastewater), the DPC solution was washed with 0.6% strength hydrochloric acid and water. After removal of the solvent, diphenyl carbonate of 99.9% purity was obtained. The reaction waste water is combined with washing and the solvent residues and catalyst are removed by stripping with steam. After neutralization with hydrochloric acid and after-treatment with activated carbon, the process wastewater contained 17% by weight of NaCl and less than 2ppm of phenol.

It can be recycled without further purification to the production process.

b) Electrochemical oxidation of the reaction waste water obtained from a)

For example by having a diameter of 0.01m²The electrolysis was carried out in a laboratory cell with anode surface area. The current density is 4kA/m²The cathode side exit temperature was 88 ℃ and the anode side exit temperature was 89 ℃. An electrolytic cell with standard anode and cathode coatings of DENORA, germany, was used. The ion exchange membrane used was Nafion 982WX from DuPont. The electrolytic voltage was 3.02V. The sodium chloride-containing solution was circulated through the anode compartment by means of a pump at a mass flow rate of 0.98 kg/h. The solution concentration supplied to the anode compartment was 25 wt% NaCl. From the anode chamber, a 20 wt% NaCl solution was discharged. 0.121kg/h of 17% by weight reaction waste water from the production of diphenyl carbonate according to example 4a) and 0.0653kg/h of solid sodium chloride were added to the NaCl solution discharged from the anode compartment. The solution is then fed again into the anode compartment. The water transport through the membrane was 3.5mol water/mol sodium.

On the cathode side, an aqueous sodium hydroxide solution was circulated by pumping at a mass flow rate of 1.107 kg/h. The concentration of the aqueous sodium hydroxide solution supplied to the cathode side was 30 wt% NaOH, and the aqueous sodium hydroxide solution discharged from the cathode side had a NaOH concentration of 32%. 0.188kg/h of 32% strength lye are discharged from the volume flow, 0.0664kg/h of water are added to the remaining material and recycled into the cathode section.

Only 23.3% of the reacted sodium chloride was derived from the DPC reaction waste water.

The preceding examples 1 to 3 according to the invention show, in comparison with the comparative example, that by concentrating the sodium chloride-containing waste water, not only the proportion of sodium chloride originating from the production of DPC and converted during the subsequent electrolysis is significantly increased, but also the amount of sodium chloride which needs to be additionally added in solid form before the electrolysis can be significantly reduced. This results in a significantly better utilization of sodium chloride in the waste water and a smaller amount of salt-contaminated process waste water to be disposed of.

Claims (10)

1. A process for producing diaryl carbonate and treating at least a portion of the resulting alkali chloride-containing solution in downstream alkali chloride electrolysis comprising the steps of:

a) producing phosgene by reacting chlorine with carbon monoxide, and

b) reacting the phosgene formed in step a) with at least one monophenol in the presence of a base and optionally a basic catalyst to provide a diaryl carbonate and a solution comprising an alkali metal chloride, and

c) separating off and working up the diaryl carbonate formed in step b), and

d) separating off solvent residues and optionally catalyst residues from the alkali chloride-containing solution remaining in step c), in particular by stripping the solution with steam and treating it with an adsorbent, in particular with activated carbon, wherein the alkali chloride-containing solution is adjusted to a pH of less than or equal to 8 before the treatment with the adsorbent, and

e) electrochemically oxidizing at least a portion of the alkali chloride-containing solution from d) to form chlorine, lye and optionally hydrogen, and

f) recycling at least a part of the chlorine produced in step e) to the phosgene production of step a), and/or

g) Recycling at least a part of the lye produced in step e) to the diaryl carbonate production of step b),

characterized in that the alkali chloride-containing solution produced in step b) has an alkali chloride content of 18 to 25% by weight, based on the total weight of the alkali chloride-containing solution, and/or at least a part of the alkali chloride-containing solution produced in step d) is recycled into step b).

2. The process according to claim 1, characterized in that, in step b), an alkali metal-containing base, preferably a sodium-containing base, is used as base, and the alkali metal-containing base is used with a monophenol in such an amount that the sodium phenolate content in the solution formed from the alkali metal-containing base and the monophenol is from 31 to 40% by weight, based on the total weight of the solution.

3. Process according to claims 1 and 2, characterized in that up to 80 wt.%, preferably up to 50 wt.%, of the alkali metal chloride-containing solution produced in step d) is recycled to step b).

4. The process according to at least one of claims 1 to 3, characterized in that the electrochemical oxidation of at least a part of the alkali chloride-containing solution obtained in d) is carried out using a gas diffusion electrode as cathode to provide chlorine gas and an aqueous sodium hydroxide solution.

5. Process according to one of claims 1 to 4, characterized in that at least a part of the purified alkali chloride-containing solution obtained in d) is added to the brine cycle of the membrane electrolysis to produce chlorine, aqueous sodium hydroxide solution and optionally hydrogen.

6. The process according to any of claims 1 to 5, characterized in that in electrolysis e) additional alkali chloride is added to the alkali chloride containing solution to increase the alkali chloride concentration.

7. Process according to one of claims 1 to 6, characterized in that the solution containing alkali chloride is adjusted to a pH of less than 7 before the treatment with adsorbent in step d), in particular in the case of hydrochloric acid or hydrogen chloride.

8. Process according to one of claims 1 to 7, characterized in that, in the electrolysis e), an ion-exchange membrane is used which delivers more than 4mol H per mol of water of sodium₂O/mol sodium, preferably 5.5 to 6.5mol H₂O/mol sodium.

9. Process according to one of claims 1 to 8, characterized in that in stage b) the monophenol is phenol, C₁-C₉Alkyl phenols, halogenated phenols; wherein, the C₁-C₉Alkylphenols are in particular cresol, p-tert-butylphenol, p-cumylphenol, p-n-octylphenol, p-isooctylphenol, p-n-nonylphenol and p-isononylphenol; the halophenols are in particular p-chlorophenol, 2, 4-dichlorophenol, p-bromophenol and 2, 4, 6-tribromophenol; and the monophenol is particularly preferably phenol.

10. Method according to one of claims 1 to 9, characterized in that in electrolysis e) an anode with a surface area higher than the membrane surface area is used in the cell.