JP2001226297A - Method for recovering 1,2-dichloroethane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To economically advantageously recover 1,2-dichloroethane (EDC) by efficiently separating and removing low-boiling component such as chloroprene and 1,1-dichloroethane, benzene and trichloroethylene out of the EDC from a distillate at the top of a separation column for low-boiling materials of an unpyrolyzed EDC in pyrolysis of EDC. SOLUTION: This method comprises distilling and separating components containing chloroprene and having <=70 deg.C boiling points under a normal pressure at the first distillation column, then feeding bottoms of the first distillation column to the second distillation column installed at the latter step of the first distillation column and distilling the bottoms, removing benzene and trichloroethylene by distilling from the top of the column and recovering EDC from the bottom of the column when pyrolyzing EDC and recovering the EDC from the distillate at the top of the separation column for the low-boiling materials of the unpyrolyzed EDC after separating generated hydrogen chloride and vinyl chloride monomer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for recovering 1,2-dichloroethane. More specifically,
The present invention relates to a method for recovering 1,2-dichloroethane from a top distillate obtained by distilling undecomposed 1,2-dichloroethane in a thermal decomposition of 1,2-dichloroethane in a low-boiling matter separation column.

[0002]

2. Description of the Related Art 1,2-Dichloroethane (hereinafter referred to as EDC)
Is thermally decomposed to produce a vinyl chloride monomer, which is industrially implemented on a large scale. In the thermal decomposition reaction of EDC, when the decomposition rate of EDC is increased, the by-products of chloroprene and benzene increase, the selectivity of vinyl chloride monomer decreases, and the coking rate increases. For this reason, the decomposition rate of EDC is usually 50 to 65%.

[0003] In the thermal decomposition reaction of EDC, many by-products such as 1,1-dichloroethane, chloroprene, 1-chlorobutadiene and benzene are produced. In addition, in undecomposed EDC, in addition to these by-products, chloroform produced as a by-product in the oxychlorination step of ethylene, contained in the EDC fed to the pyrolysis furnace and remained as it was in the pyrolysis step,
Contains carbon tetrachloride, trichloroethylene and the like.

[0004] The undecomposed EDC in the thermal decomposition reaction is separated from the produced hydrogen chloride and vinyl chloride monomer.
The components having a lower boiling point are separated by distillation in a low-boiling substance separation column, and EDC
After the components having a higher boiling point are separated by distillation in a high-boiling substance separation column, the components are supplied to a pyrolysis furnace and reused. Benzene and trichloroethylene contained in the undecomposed EDC remain in a considerable proportion at the bottom of the low-boiling matter separation column, but other low-boiling components are distilled off at the top and removed. Accordingly, the distillate contains a large amount of low-boiling components, but the main component is E.
DC, which usually contains 40 to 60% by weight of EDC.

[0005] Since the EDC is contained in such a high concentration in the distillate at the top of the low-boiling matter separation column, methods for recovering EDC from the distillate have been studied. A proposal has been made. Chloroprene, chloroform, benzene, trichloroethylene and the like contained in the distillate are inhibitors in the thermal decomposition reaction of EDC,
In order to recover EDC from the distillate and reuse it as a pyrolysis raw material, it is necessary to separate and remove these impurities from EDC at a high removal rate.

[0006] Among the impurities contained in the distillate, chloroprene has a very high polymerizability and causes a blockage of a distillation column during distillation. Further, benzene and trichloroethylene have boiling points close to those of EDC, and are difficult to separate by distillation.

Hitherto, as a method for solving such a problem, there has been proposed a method of chlorinating chloroprene, benzene, or trichloroethylene in EDC to increase the boiling point, followed by distillation and separation. (Japanese Patent Publication No. 42-19444, Japanese Patent Publication No. 47968/1992, Japanese Patent Application Laid-Open No. 4-225929, etc.) However, this method requires a large amount of chlorine, and requires a large number of steps and increases equipment costs. There is a problem.

Japanese Patent Publication No. 46-22003 proposes a method of reacting coexisting chloroprenes with benzene using an aluminum chloride catalyst and separating and removing the same. However, this method has a drawback that the conversion of benzene in the reaction is low, and is unsatisfactory as a method for removing benzene.

[0009] US Patent No. At 4,145,367,
A method for hydrogenating and removing chloroprene in EDC has been proposed. In this method, it is necessary to make the reactor corrosion-resistant because hydrogen chloride is generated in the reaction, and there is a problem that equipment costs are increased.

US Pat. At 4,333,799,
A method of extractive distillation using a high boiling chlorinated hydrocarbon solvent such as tetrachloroethylene has been proposed. This method has a feature that benzene and trichloroethylene in EDC can be separated and removed by a distillation operation, but requires a large amount of a solvent, and consumes a large amount of energy for solvent recovery. There is a problem in terms of economy.

In the prior art, as described above, it was difficult to separate and remove chloroprene, benzene, trichloroethylene and the like more efficiently and economically than EDC, and to recover EDC.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to provide an ED
The low boiling components such as chloroprene and 1,1-dichloroethane, and benzene and trichloroethylene are removed from the top distillate of the low-boiling matter separation column of undecomposed EDC in the thermal decomposition of C by EDC.
An object of the present invention is to provide a method of recovering EDC economically and advantageously by separating and removing it more efficiently.

[0013]

Means for Solving the Problems The present inventors have developed an undegraded E
As a result of diligent studies on a method of recovering EDC from the top distillate of the low-boiling DC separation column, the present invention was completed. That is, the present invention provides an EDC for separating and removing impurities in the distillate from the EDC by two-stage distillation using two distillation columns, a first distillation column and a second distillation column provided at a subsequent stage. It is a collection method. In the first distillation column, low-boiling components containing chloroprene and having a boiling point of 70 ° C. or less at normal pressure are separated by distillation at a low temperature, and then the bottom liquid of the first distillation column is provided in the second distillation column provided. Characterized in that benzene and trichloroethylene are distilled off from the top of the column to remove them.
This is a method for recovering DC.

[0014] The present inventors have intensively studied a method of recovering EDC by only a distillation operation because it is necessary to reduce the number of steps and reduce equipment costs from an economic viewpoint. In the method of recovering EDC by a distillation operation, suppression of polymerization of chloroprene is a major problem. The low-boiling matter separation column of the undecomposed EDC of the vinyl chloride monomer plant has a top temperature of about 80 ° C., and in order to avoid blockage of the distillation column due to polymerization of chloroprene, the overhead of the low-boiling matter separation column is removed. At present, the operation is performed with the chloroprene concentration in the liquid kept at 5% by weight or less.

The present inventors have studied the effects of concentration, temperature, atmosphere, etc. on the polymerization rate of chloroprene. As a result, it was found that the polymerization rate of chloroprene increases in proportion to the concentration of chloroprene and strongly depends on temperature. I found it. The activation energy of the polymerization was about 15 kcal / mol, and it was found that the polymerization rate became about 1/3 when the temperature was lowered by 20 ° C. Based on these results, the present inventors did not carry out distillation in one distillation column, but carried out two-stage distillation using two distillation columns, whereby the top temperature of the first distillation column was reduced. Of chloroprene can be suppressed, and the polymerization of chloroprene can be suppressed.

In the first distillation column, the concentration of chloroprene at the top of the column rises to two to three times that of the feed liquid. Usually, the concentration of chloroprene in the feed liquid of the first distillation column is 4 to 5% by weight, and the concentration of chloroprene in the distillate of the first distillation column is 8 to 12% by weight. Although the polymerization rate of chloroprene increases in proportion to the concentration, the polymerization rate strongly depends on the temperature. When the temperature is lowered by 20 ° C., the polymerization rate becomes about 1/3. Therefore, if the temperature at the top of the first distillation column is 60 ° C. or less, the polymerization of chloroprene is suppressed to about the same or lower than the current polymerization rate of chloroprene at the top of the low-boiling matter separation column of undecomposed EDC. Is done. Furthermore, by performing distillation under a nitrogen atmosphere and / or by adding a polymerization inhibitor, it is possible to suppress the polymerization of chloroprene to a level that does not substantially cause a problem. Low-boiling components having a temperature of 70 ° C. or lower can be separated by distillation.
Further, in the second distillation column, water is added to the bottom liquid of the first distillation column, followed by distillation, whereby benzene and trichloroethylene contained therein are distilled off at the top of the column by azeotropic distillation with water. It is possible to separate and remove at a higher removal rate.

Hereinafter, the present invention will be described in detail.

The distillation column used in the present invention is shown in FIG. This figure is a flow sheet showing the process of recovering EDC from the low-boiling matter separation tower top distillate of undecomposed EDC in the thermal decomposition of EDC in the present invention.

In the present invention, the mixture used as the raw material is
This is a top distillate obtained by pyrolyzing EDC and separating the generated hydrogen chloride and vinyl chloride monomer, and then distilling undecomposed EDC in a low-boiling matter separation column. The top distillate contains 1,1-dichloroethane, chloroprene, 1-chlorobutadiene, dichloroethylene, chloroform, carbon tetrachloride, etc., as well as benzene and trichloroethylene, and the concentration of EDC is usually It is 40 to 60% by weight.
The overhead distillate usually contains 2 to 5% by weight of chloroprene, 1 to 5% by weight of benzene, and 1 to 5% by weight of trichloroethylene.
-5% by weight. Further, a top distillate obtained by distilling EDC produced by direct chlorination or oxychlorination in a low-boiling matter separation column can be added to the feed liquid and used as a raw material.

The first distillation column does not require a large number of plates, and the number of theoretical plates is usually 10 to 30, preferably 15
Use ~ 20 distillation columns. The reflux ratio does not need to be very high and is usually 1 to 5, preferably 2
~ 4. 1, contained in the feed liquid in this distillation column
1-dichloroethane, chloroprene, 1-chlorobutadiene, dichloroethylene, chloroform, etc., having a boiling point of 70
Low-boiling components having a temperature of not higher than 0 ° C are distilled off from the top of the tower to remove them. The chloroprene concentration in the feed liquid of the first distillation column is usually 3
To 5% by weight, and the chloroprene concentration in the distillate at the top of the distillation column varies depending on the liquid composition, but is usually 5 to 12% by weight. In order to suppress the polymerization of chloroprene in the first distillation column, the top temperature needs to be 70 ° C. or less,
It is preferably at most 65 ° C, particularly preferably at most 60 ° C.

In order to suppress the polymerization of chloroprene, it is preferable to carry out distillation under a nitrogen atmosphere and / or by adding a polymerization inhibitor. As a method of adding the polymerization inhibitor,
Nitrogen monoxide gas is mixed with nitrogen gas and added to the gas phase, or 2, 6 or 2 is added to the feed liquid and / or reflux liquid of the distillation column.
-A method of adding di-tert-butyl paracresol, tert-butyl catechol and the like. The amount of the polymerization inhibitor to be added is usually about 0.05 to 5% by weight, preferably 0.1 to 1% by weight, based on the feed liquid or the reflux liquid. The removal rate of EDC to the bottom of the first distillation column is usually 95% or more, and the loss rate of EDC to the top of the column is 5% or less.

The bottom product of the first distillation column is fed to the next second distillation column, and benzene and trichloroethylene in the feed solution are separated by distillation from the top. The concentration of benzene in the feed solution is usually 2 to 8% by weight, and the concentration of trichlorethylene is usually 2 to 6% by weight. The concentration of EDC in the feed solution is usually 75 to 95% by weight. First
The bottoms of the distillation column may be directly fed to the second distillation column for distillation, but preferably, distillation is performed by adding water as an azeotropic agent to the bottoms of the first distillation column. This results in high benzene and trichloroethylene removal rates. The amount of water added as an azeotropic agent basically corresponds to the azeotropic composition of benzene and water, trichloroethylene and water, and EDC and water. The azeotropic composition of benzene and water is 91.2% by weight of benzene and 8.8% by weight of water, and the azeotropic composition of trichlorethylene and water is 93.0% by weight of trichloroethylene, 7.0% by weight of water, EDC and water. The azeotropic composition is EDC9
It is 1.9% by weight and water is 8.1% by weight. Therefore, the amount of water to be added is preferably in the range of 0 to 0.6 in terms of weight ratio with respect to the total amount of benzene and trichloroethylene in the feed solution. If the added amount of water is too large, the steam basic unit of distillation becomes large and it is not efficient.

According to the present invention, benzene and trichloroethylene can be distilled off and removed from the top of the second distillation column, and EDC can be recovered from the bottom of the column. As the second distillation column, a distillation column having a theoretical plate number of usually 10 to 70 plates, preferably 20 to 60 plates is used. Also, the reflux ratio is usually based on weight,
The range is 5 to 60, preferably 10 to 50, particularly preferably 20 to 40. Increasing the number of distillation columns and the reflux ratio improves the removal rate of benzene and trichloroethylene, but increases the equipment cost and the unit steam consumption in distillation, so that the above range is preferable. The feed position (feed stage) of the raw material to the second distillation column is preferably fed from a stage near the center. Further, it is preferable that water added as an azeotropic agent is fed from the same stage together with the bottoms of the first distillation column. The recovered EDC obtained as the bottom product of the second distillation column can be used as it is or after removing water, as a raw material for thermal decomposition.

[0024]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

Example 1 As the first distillation column, an Oldershaw distillation column having an inner diameter of 32 mm and an actual number of plates of 20 was used. To this distillation column, the top distillate of the low-boiling matter separation column of undecomposed EDC after separating hydrogen chloride and vinyl chloride monomer produced by thermally decomposing EDC was fed. 560.4 g /
The feed solution was fed at the rate of Hr. The composition of the feed solution was as shown in Table 1, and the feed solution temperature was 60 ° C. and the reflux ratio was 4. Distillation was performed while blowing nitrogen gas at a rate of about 30 ml / min from the bottom of the column. The top temperature of the distillation column is 5
The temperature of the kettle was 9.6 ° C and the temperature of the kettle was 82.3 ° C. The amount of distillate at the top of the column is 225.8 g / Hr, and the amount of distillate at the bottom of the column is 334.
It was 6 g / Hr, and the distillate and bottoms compositions were as shown in Table 1. EDC removal rate to the bottom of the first distillation column (%) ((EDC amount in bottoms / EDC in feed solution)
Amount) x 100) was 95.6%. Next, the bottom product of the first distillation column was fed to the second distillation column to perform distillation. As the second distillation column, an Oldershaw distillation column having an inner diameter of 32 mm and an actual number of stages of 80 was used. From the central 40 stages of this distillation column, the bottom product of the first distillation column was 101.4 g / H
Feeding was performed at a rate of r to perform continuous distillation. The feed liquid temperature was 80 ° C., the reflux ratio was 40, the overhead temperature was 82.2 ° C., the distillate amount was 26.8 g / Hr, the kettle liquid temperature was 88.2 ° C., and the bottom liquid amount was 74.6 g / Hr. The bottom composition was as shown in Table 1. Benzene removal rate in the second distillation column (%)
((The amount of benzene in the feed liquid−the amount of benzene in the bottom liquid) / the amount of benzene in the feed liquid × 100) is 91.6.
%, Removal rate of trichlorethylene (%) ((amount of trichlorethylene in feed liquid−amount of trichlorethylene in bottom liquid) / amount of trichlorethylene in feed liquid × 10
0) was 98.8%, and EDC was added to the bottom of the second distillation column.
Was 85.2%. From this, the recovery ratio (%) of EDC through the first distillation column and the second distillation column (the removal ratio of EDC in the first distillation column (%) × the removal ratio of EDC in the second distillation column (%)) / 100) was 81.4%.

[0026]

[Table 1]

Example 2 As the first distillation column, an Oldershaw distillation column having an inner diameter of 32 mm and an actual number of plates of 20 was used. To this distillation column, the top distillate of the low-boiling matter separation column of undecomposed EDC after separating hydrogen chloride and vinyl chloride monomer produced by thermally decomposing EDC was fed. Feed liquid from the middle 10 stages of the distillation column to 5
It was fed at a rate of 46.0 g / Hr. The composition of the feed solution was as shown in Table 2, and the feed solution temperature was 60 ° C. and the reflux ratio was 4. Distillation was performed while blowing nitrogen gas at a rate of about 30 ml / min from the bottom of the column. The top temperature of the distillation column is 5
The temperature of the kettle was 9.6 ° C and the temperature of the kettle was 82.3 ° C. The amount of distillate at the top of the column is 217.4 g / Hr, and the amount of bottoms discharged at the bottom is 328.
It was 4 g / Hr, and the distillate and bottoms compositions were as shown in Table 2. The removal rate of EDC to the bottom of the first distillation column was 96.4%. Next, water was added to the bottoms of the first distillation column, and this was fed to the second distillation column to perform distillation. As the second distillation column, an Oldershaw distillation column having an inner diameter of 32 mm and an actual number of stages of 80 was used. 102.0 g / Hr of bottom product from the first distillation column was fed from the central 40 stages of the distillation column.
, And water was fed from the same 40 stages at a rate of 2.9 g / Hr to perform continuous distillation. The amount of water added is 0.30 in terms of weight ratio to the total amount of benzene and trichloroethylene in the liquid fed to the second distillation column. Feed liquid temperature 70 ° C, reflux ratio 20
At a top temperature of 69.2 ° C., a distillate amount of 29.8 g / Hr,
The kettle liquid temperature was 88.2 ° C., the bottom liquid amount was 75.0 g / Hr, and the bottom liquid composition was as shown in Table 2. The removal rate of benzene in the second distillation column was 93.4%, the removal rate of trichloroethylene was 99.2%, and the removal rate of EDC to the bottom of the second distillation column was 85.5%. Was. From this, the first
The recovery of EDC through the distillation column and the second distillation column was 82.2.
4%.

[0028]

[Table 2]

[0029]

According to the present invention, low boiling components such as chloroprene, 1,1-dichloroethane, and chloroform are removed from the top distillate of a low-boiling matter separation column for undecomposed EDC obtained by thermal decomposition of EDC. And trichloroethylene can be efficiently separated and removed to obtain high-purity EDC. The present invention suppresses the polymerization of chloroprene in the first distillation column, distills and separates low-boiling components having a boiling point of 70 ° C. or lower, and distills the bottoms of the first distillation column in the second distillation column. Benzene and trichloroethylene are removed to remove high purity E from the bottom of the column.
This is for recovering DC.

In the two-stage distillation separation method of the present invention, the number of steps is small by only a distillation operation, EDC can be recovered economically, and the recovered EDC can be used as a raw material for pyrolysis of EDC. Therefore, it is an industrially very advantageous method of recovering EDC.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the ED from the top distillate of a low-boiling matter separation column of undecomposed EDC in the pyrolysis of EDC according to the method of the present invention.
4 is a flow sheet showing a recovery step of C.

[Explanation of symbols]

C-1 First distillation column C-2 Second distillation column E-1 First distillation column condenser E-2 Second distillation column condenser Low-boiling matter separation tower top distillate of undecomposed EDC (first distillation column feed Liquid) First distillation column bottom liquid (second distillation column feed liquid) First distillation column distillate First distillation column reflux liquid Water Second distillation column bottom liquid (recovered EDC) Second distillation column distillate 2 Distillation column reflux

Claims (4)

[Claims] 1. A 1,2-dichloroethane is thermally decomposed, and 1,2-dichloroethane is separated from a vinyl chloride monomer. In recovering 2-dichloroethane, components having a boiling point of 70 ° C. or less at normal pressure, including chloroprene, are separated by distillation in a first distillation column, and then the bottoms of the first distillation column are subjected to the first distillation column. It is fed to a second distillation column provided in the latter stage of the distillation column to perform distillation, and benzene and trichloroethylene are distilled off from the top of the column to be removed, and 1,2-dichloroethane is recovered from the bottom of the column. A method for recovering 1,2-dichloroethane. 2. The method according to claim 1, wherein water is added to the bottom of the first distillation column in the second distillation column, and benzene and trichloroethylene are removed by azeotropic distillation. Of 1,2-dichloroethane. 3. The 1,2-dichloroethane of claim 1 or 2, wherein the first distillation column is subjected to distillation at a top temperature of 65 ° C. or lower to remove chloroprene from the top. Collection method. 4. The 1,2-dichloroethane according to claim 1, wherein the distillation is carried out in a first distillation column under a nitrogen atmosphere and / or by adding a polymerization inhibitor. Collection method.