JP4405829B2 - Method for recovering methylene chloride - Google Patents

Description

  The present invention relates to a method for recovering methylene chloride, and more particularly to a method for recovering methylene chloride economically and efficiently by distilling and purifying hydrous methylene chloride discharged from a polycarbonate polymerization step by interfacial polycondensation.

  Polycarbonate is a polymer excellent in transparency, heat resistance, and impact resistance, and is currently widely used in the industrial field as an engineering plastic. The polycarbonate can be produced by directly reacting an aromatic dihydroxy compound such as bisphenol A with phosgene (interfacial polycondensation method), or by melting an aromatic dihydroxy compound such as bisphenol A and a carbonic diester such as diphenyl carbonate. A method of transesterification in the state (melting method) is generally known, but the interfacial polycondensation method is still the mainstream because of problems such as quality.

  In the method for producing polycarbonate by the interfacial polycondensation method, phosgene or a phosgene derivative is reacted in the presence of an alkaline aqueous solution containing an aromatic dihydroxy compound and an inert organic solvent. As the inert organic solvent used in this reaction, various solvents have been proposed, but methylene chloride is mainly used industrially from the viewpoints of solubility and handleability.

Since the methylene chloride comes into contact with the alkaline aqueous solution as described above, the methylene chloride containing water is discharged from the polymerization step, and the discharged methylene chloride is recovered and reused.
This methylene chloride is supplemented with heptane to facilitate crystallization of the polycarbonate. The saturated concentration of water in methylene chloride is about 2000 ppm by mass (20 ° C.)
If a third component is present that is readily soluble in methylene chloride and has a lower solubility in water than in methylene chloride, the solubility of water will be reduced and water will be liberated and separated from the methylene chloride. Since the separated water has a lower density than methylene chloride, an aqueous layer is formed on top of the methylene chloride layer.

For this reason, when a separation tank with the contents extracted from the bottom is used, the ratio of the water layer in the tank gradually increases, and a large amount of water (methylene chloride solution containing water) is supplied to the next process. May be. For this reason, for example, when methylene chloride is purified using a distillation apparatus, there is a fear that the expected purity of methylene chloride may not be obtained due to fluctuations in operation due to a sudden change in raw material composition.
Therefore, the operation of removing the free water in the upper layer of the separation tank is required about once / day. For example, when extracted to the outside by a nozzle provided on the side of the separation tank, about 20% by volume of methylene chloride is added together. In some cases, the processing was difficult. As a method of separating the water layer and methylene chloride to eliminate these recovery losses, it is possible to separate and discharge water by a method of separating with a centrifuge or a method of separating with a coalescer, etc. Is expensive.
In addition, methylene chloride is dissolved in the discharged water, but these methods have a problem in recovering methylene chloride having a solubility or lower in principle.

Furthermore, a method using a dehydration tower (for example, refer to Patent Document 1) and a method for adsorbing moisture using zeolite and dewatering are known (for example, refer to Patent Document 2). These methods have the problem that the dehydration ability is reduced due to the gradual deterioration of the adsorbent such as zeolite and the methylene chloride is insufficiently dehydrated, the exchange of the adsorbent such as zeolite is complicated and the productivity is poor. It was.
However, since methylene chloride may contain halogen, it is desired to recover and reuse it as much as possible due to the recent increase in interest in environmental problems.

JP-A-9-290127 JP 2000-225316 A

  The present invention has been made under such circumstances, and by preventing sudden fluctuations in the composition of the raw material supplied to the distillation apparatus, including the methylene chloride contained in the aqueous layer, the recovery loss is very small and stable. Another object of the present invention is to provide a method for recovering purified methylene chloride that can be economically reused in the interfacial polycondensation step of polycarbonate.

As a result of intensive research to achieve the above object, the present inventors have found that a third component that is readily soluble in methylene chloride and has a lower solubility in water than methylene chloride is contained in methylene chloride containing water. in addition, by reducing the solubility of water of water was separated, prior to distillation a mixture of methylene chloride and a third component to the respective components, the separated aqueous layer, the methylene chloride contained in the mixed solution It found that the object can be achieved by re-mixing in response to the amount. The present invention has been completed based on such findings.
That is, the present invention
(1) In the production of polycarbonate by interfacial polycondensation, when water-containing methylene chloride discharged from the polymerization process is purified by distillation, (A) the water-containing methylene chloride is readily soluble in methylene chloride. A step of separating the aqueous layer liberated by adding a third component whose solubility in water is lower than that for methylene chloride from the mixed solution of methylene chloride and the third component, and (B) the aqueous layer was separated. Before distilling the mixed liquid of methylene chloride and the third component into the respective components, the aqueous layer separated in the step (A) is mixed again in accordance with the amount of methylene chloride contained in the mixed liquid. that the step of applying,
Furthermore, in the step (A), a balance line is provided between a separation tank for separating the liberated aqueous layer from the mixed liquid of methylene chloride and the third component, and a storage tank for the separated aqueous layer. method for recovering methylene chloride to, and (2) a method of recovering methylene chloride of (1) a third component heptane,
To provide.

According to the methylene chloride recovery method of the present invention, the following effects can be achieved.
(1) Abrupt fluctuations in the raw material composition supplied to the distillation apparatus can be prevented, and methylene chloride can be purified stably.
(2) Since all the methylene chloride contained in the aqueous layer is also recovered, the recovery loss of methylene chloride accompanying the extraction of the aqueous layer can be prevented. Accordingly, an expensive separation device such as a centrifuge is not required.
(3) Even when the separation operation of methylene chloride and water is carried out by a centrifugal separator or the like, methylene chloride having a concentration below the saturation solubility is contained in the water, resulting in a recovery loss. The recovery loss is zero.

According to the present invention, in the production of polycarbonate by interfacial polycondensation, when water-containing methylene chloride discharged from the polymerization step is distilled and purified to recover methylene chloride, first, in step (A), with respect to the water-containing methylene chloride, It is necessary to separate the free aqueous layer from the mixture of methylene chloride and the third component by adding a third component that is readily soluble in methylene chloride and has a lower water solubility than in methylene chloride.
The third component is not particularly limited as long as it is readily soluble in methylene chloride and has a lower solubility in water than in methylene chloride. For example, alkanes such as pentane, hexane, heptane, and octane, benzene And aromatic compounds such as toluene and xylene.
Among them, heptane having the above characteristics and used for facilitating crystallization of polycarbonate is particularly preferable.
The mixing ratio of the third component with respect to methylene chloride is usually preferably 5 to 80% by mass, more preferably 5 to 50% by mass, and still more preferably 10 to 30% by mass.

Next, in the step (B), the amount of the aqueous layer separated in the step (A) before or after distilling and separating the mixed solution of methylene chloride and the third component from which the aqueous layer has been separated into respective components. Mix again correspondingly.
The amount of the aqueous layer remixed in the mixed solution of methylene chloride and the third component is supplied to the distillation column at a flow rate that is almost saturated with respect to the amount of methylene chloride contained in the mixed solution. Continuous purification is possible without extracting the aqueous layer to the outside. Purified methylene chloride dissolves almost saturated water and is reused as an inert organic solvent in the production of polycarbonate by interfacial polycondensation.
The mixed liquid of methylene chloride and the third component is separated into a column bottom liquid containing methylene chloride and the third component at the top of the column by a distillation treatment using a distillation column. Since the column bottom liquid contains impurities derived from the polycarbonate polymerization step in addition to the third component, the purified third component is recovered by distillation treatment and reused in the step (A).

In the present invention, the remixing of the mixed solution of methylene chloride and the third component and the aqueous layer in the step (B) needs to be performed before or after the mixed solution of methylene chloride and the third component is separated into the respective components by distillation. However, before and after the supply pump to the distillation column, the supply pressure is lower in front of the supply pump, so that the discharge pressure of the free water recovery pump can be low and stable supply can be achieved. It is preferable to mix before distilling.
Furthermore, in the step (A), if necessary, a balance line is provided between the separation tank for separating the separated aqueous layer from the mixed liquid of methylene chloride and the third component and the separated water layer storage tank. Is preferred. By doing so, the liquid level of the water tank is always kept the same as the level of the separation tank, so that the liquid in the tank can be prevented from running out, and the supply pump can cause cavitation. Can be prevented.

Hereinafter, the present invention will be described in detail with reference to FIG.
FIG. 1 is a schematic view of a methylene chloride distillation purification facility showing a conventional example.
FIG. 2 is a schematic view of a methylene chloride distillation purification facility showing one embodiment of the present invention. 1 is a separation tank, 2 is a supply pump, 3 is a distillation tower, 4 is a condenser, 5 is a distillation tower receiving tank, 6 is a reboiler for heating the distillation tower, 7 is a vent line, 8 is a water tank, and 9 is drainage. A pump, 10 is a recovery pump, and 11 is a balance line.

First, from the upper part of the separation tank 1, a mixed solution of a third component which is easily soluble in hydrous methylene chloride and methylene chloride discharged from the polymerization step of polycarbonate and whose water solubility is lower with respect to methylene chloride is introduced. The Addition of the third component reduces the solubility of water in methylene chloride and liberates water.
Since the density of water is smaller than that of methylene chloride, the water is separated into two layers, ie, an aqueous layer (free water) in the upper layer of the separation tank 1 and a mixed layer of methylene chloride and the third component in the lower layer.
Next, preferably, an air-driven plunger pump is used as the drainage pump 9, and free water in the upper layer is extracted from the separation tank 1 to the storage tank 8. In addition, the extraction of the free water by the drainage pump 9 may be continuous or intermittent. The separation tank 1 and the water storage tank 8 are arranged substantially in parallel, the gas layer portions of the separation tank 1 and the water storage tank 8 are connected by a vent line 7 to equalize the pressure, and the bottoms of the separation tank 1 and the storage tank 8 are connected. By connecting with the balance line 11, the liquid level of the water tank 8 is always kept the same as that of the separation tank 1, so that the liquid in the tank 8 can be prevented from running out.

Subsequently, the aqueous layer is supplied from the storage tank 8 to the suction side of the supply pump 2 using the recovery pump 10. The discharge amount of the recovery pump 10 is desirably a constant amount in order to reduce fluctuations in the operation of the distillation tower.
The recovery pump 10 may be an air driven plunger pump similar to the drainage pump 9. The flow rate of the aqueous layer by the recovery pump 10 includes the amount of methylene chloride supplied to the separation tank 1 and the amount of water contained in the mixed liquid of methylene chloride and the third component. The amount of water is supplied so that it is almost saturated.

As a result of the above operation, the amount of free water in the liquid separation tank 1 is kept substantially constant, so that the operation fluctuation of the distillation column 3 does not occur. The distillation temperature for separating methylene chloride from the third component is 40 to 60 ° C. at atmospheric pressure at the top and 100 to 120 ° C. at the bottom, and the vapor from the top of the distillation column 3 is condensed in the condenser 4 to form a condensate. Is received in the distillation column receiving tank 5. In the distillation column receiving tank 5, the separation of free water usually does not occur and a uniform liquid layer is formed.
On the other hand, the third component is extracted as a bottom liquid of the distillation column 3 and further purified by distillation, and reused in the step (A).

  EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited at all by these examples.

Comparative Example 1
Distillation purification of hydrous methylene chloride discharged from the polycarbonate polymerization step by interfacial polycondensation was performed using the methylene chloride distillation purification facility shown in FIG. Reference numeral 12 denotes a water reservoir, and the other reference numerals are the same as those in FIG.
A liquid mixture (saturated water content 1000 mass ppm) of water-containing methylene chloride (saturated water content 2000 mass ppm) and heptane at a temperature of 20 ° C. discharged from the polymerization step of the polycarbonate in the separation tank 1 The oil layer continuously supplied from the top and extracted from the bottom of the separation tank 1 was supplied to the distillation tower 3 via the supply pump 2. Methylene chloride (containing a small amount of water) was distilled from the top of the distillation column 3, and a column bottom liquid containing heptane was extracted from the bottom.
The distillation conditions were 42 ° C. at normal pressure at the top and 109 ° C. at the bottom.
On the other hand, the water layer released and accumulated in the separation tank 1 was extracted from the nozzle installed on the side surface of the separation tank 1 to the storage tank 12 for the water layer. The discharged liquid contained about 20% by mass of methylene chloride and heptane in addition to the aqueous layer, and the aqueous layer alone could not be discharged.

Example 1
Distillation purification of hydrous methylene chloride discharged from the polycarbonate polymerization step by interfacial polycondensation was performed using the methylene chloride distillation purification equipment shown in FIG.
A liquid mixture (saturated water content 1000 mass ppm) of water-containing methylene chloride (saturated water content 2000 mass ppm) at a temperature of 20 ° C. and a heptane mass ratio 8: 2 discharged from the polymerization process of polycarbonate is separated into a separation tank 1. It was continuously fed from the top of the. The water layer liberated in the separation tank 1 is continuously extracted to the water layer storage tank 9 by using a drainage pump 9 comprising an air driven plunger pump, and from the storage tank 8 from the air driven plunger pump. The recovery pump 10 was continuously supplied to the suction side of the supply pump 2. The flow rate of the aqueous layer of the recovery pump 10 was set such that the water concentration in the methylene chloride was about 2000 ppm from the amount of methylene chloride supplied to the separation tank 1.
On the other hand, the oil layer separated in the separation tank 1 is extracted from the bottom of the separation tank 1 while being automatically controlled so that the interface between the water layer and the oil layer in the separation tank 1 becomes a substantially constant value. Was continuously fed to the distillation column 3.
In the distillation column 3, methylene chloride (containing a small amount of water) was distilled from the top, and a column bottom liquid containing heptane was extracted from the bottom.
The distillation conditions were 42 ° C. at normal pressure at the top and 109 ° C. at the bottom.
By the above operation, the amount of free water in the separation tank 1 was kept almost constant, and the operation fluctuation of the distillation tower did not occur. Further, the above from the top of the distillation column 3 was condensed by the condenser 4, and the condensate was received in the distillation column receiving tank 5. However, free water was not separated in the distillation column receiving tank 5, and a uniform liquid layer was formed. there were.

Comparative Example 2
The operation was performed in the same manner as in Example 1 except that the balance line 11 was not installed in FIG. When free water in the separation tank 1 was extracted by the drainage pump 9 and decreased, the gas-liquid interface became below the extraction nozzle, and the amount of drainage by the drainage pump 9 became zero. On the other hand, since the flow rate of the recovery pump 10 was kept constant, the amount of liquid in the water layer storage tank became zero, and the gas layer was supplied to the suction side of the supply pump 2 through the recovery pump 10. As a result, the supply pump 2 caused cavitation, and the operation of the distillation tower 3 became unstable.

Industrial applicability

  Methylene chloride has properties such as low boiling point, nonflammability, and excellent permeability, and is used in a wide range of fields such as detergents, extractants, and organic synthesis reaction solvents. These fields are particularly useful when high purity methylene chloride is required.

It is the schematic of the methylene chloride and distillation purification equipment which show a prior art example. It is the schematic of the methylene chloride and distillation purification equipment which show one embodiment of this invention.

Explanation of symbols

1 Separation tank 2 Supply pump 3 Distillation tower 4 Condenser 5 Distillation tower receiving tank 6 Reboiler for heating the distillation tower 7 Vent line 8 Water layer receiving tank 9 Drain pump 10 Recovery pump 11 Balance line 12 Water layer receiving tank (b)

Claims (2)

In the production of polycarbonate by interfacial polycondensation, when water-containing methylene chloride discharged from the polymerization step is purified by distillation, (A) the water-containing methylene chloride is readily soluble in methylene chloride and water Separating the aqueous layer liberated by adding a third component having a lower solubility in methylene chloride from the mixture of methylene chloride and the third component; and (B) methylene chloride from which the aqueous layer has been separated; Before distilling and separating the mixed solution with the third component into each component, the step of mixing the aqueous layer separated in the step (A) again in accordance with the amount of methylene chloride contained in the mixed solution Applying ,
Furthermore, in the step (A), a balance line is provided between a separation tank for separating the liberated aqueous layer from the mixed liquid of methylene chloride and the third component, and a storage tank for the separated aqueous layer. To recover methylene chloride.   The method for recovering methylene chloride according to claim 1, wherein the third component is heptane.

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