JP4983041B2 - Caprolactam purification method - Google Patents

Description

  The present invention relates to a method for purifying caprolactam. Specifically, the present invention relates to a method for purifying caprolactam in which crude caprolactam obtained by the rearrangement reaction of cyclohexanone oxime is distilled to separate low boiling components.

Caprolactam is useful as a raw material for polyamide, and is usually produced industrially by subjecting cyclohexanone oxime to Beckmann rearrangement reaction. The crude caprolactam obtained by this reaction contains raw material cyclohexanone oxime, cyclohexanone, water, other low-boiling components and components having higher boiling points than caprolactam.
As a method for purifying crude caprolactam, the low boiling point component is distilled by distillation under reduced pressure in the first distillation column, the low boiling point component is condensed in a two-stage condenser, and the caprolactam containing the high boiling point component is taken out from the bottom of the column. The condensate is distilled in the second distillation column to separate the high-boiling components, and caprolactam is recovered, or the first distillation column is distilled under reduced pressure to distill the low-boiling components and caprolactam, thereby condensing in two stages. A method is known in which a high boiling point component is taken out from the bottom of the column by condensing in a vessel, a condensate is distilled in a second distillation column, a low boiling point component is distilled off, and caprolactam is taken out from the bottom of the column (Patent Literature). 1). Since cyclohexanone oxime and caprolactam are thermally unstable, distillation is usually carried out at an operating pressure of about 0.2 to 5 kPa and a column bottom temperature of about 160 ° C. or lower.

However, in the conventional method, since cyclohexanone oxime has a high freezing point of about 90 ° C., it has the possibility of precipitating in the condenser, reducing the thermal efficiency, and further causing clogging. In order to sufficiently condense the low boiling point component, it is necessary to lower the cooling temperature, and equipment such as brine cooling is required, which is not economical.
JP 2005-510562 A

  The purpose of the present invention is to suppress the precipitation of cyclohexanone oxime, etc. in the condenser, to suppress the reduction in thermal efficiency and to blockage, and to condense and separate low boiling components from crude caprolactam economically without the need for equipment such as brine cooling. An object of the present invention is to provide a method for purifying caprolactam.

  In order to solve such a problem, the present inventors diligently distilled crude caprolactam to study a method for purifying caprolactam to separate low-boiling components, and as a result, first condensed the gas distilled from the distillation tower under reduced pressure. In the condenser, the condensation is performed at 70 to 100 ° C., and the uncondensed gas in the first condenser is condensed in the second condenser by bringing the condensed liquid in the second condenser into contact with the cooled liquid. By condensing the gas with a third condenser via a dry vacuum pump, the precipitation of caprolactam, cyclohexanone oxime, etc. in the condenser is suppressed, the thermal efficiency is reduced and blocked, and brine cooling is performed. It has been found that low boiling point components can be condensed and separated economically, and the present invention has been completed.

  That is, the present invention relates to a method for distilling and purifying crude caprolactam obtained by a rearrangement reaction of cyclohexanone oxime, wherein a distillation column, a first condenser, a second condenser, a dry vacuum pump and a third condenser are sequentially communicated. Placed and sucked with a dry vacuum pump to depressurize the distillation tower, the first condenser, and the second condenser, feed the crude caprolactam to the distillation tower and distill, and separate caprolactam containing high-boiling components from the bottom of the tower. A liquid obtained by condensing the gas distilled from the distillation tower at a temperature of 70 to 100 ° C. in the first condenser, cooling the uncondensed gas in the first condenser, and the condensate in the second condenser in the second condenser. The cap is characterized in that the low-boiling components in the crude caprolactam are separated by condensing and condensing the uncondensed gas in the second condenser with the third condenser via a dry vacuum pump. It is a method for purifying a lactam.

  The method of the present invention suppresses precipitation of cyclohexanone oxime and the like in the condenser, suppresses thermal efficiency reduction and blockage, and condenses and separates low-boiling components from crude caprolactam economically without the need for equipment such as brine cooling. Thus, caprolactam can be purified.

The crude caprolactam obtained by the Beckmann rearrangement reaction of cyclohexanone oxime, in addition to the product caprolactam (hereinafter sometimes simply referred to as lactam), is a raw material cyclohexanone oxime (hereinafter sometimes simply referred to as oxime). , By-produced cyclohexanone, water, other low-boiling components and high-boiling components.
The content of each component in the crude lactam varies depending on the conditions of the Beckmann rearrangement reaction, the activity of the catalyst used at that time, the presence or absence of washing of the reaction product, etc., but the lactam is about 92 to 99% by weight, cyclohexanone oxime. Is about 0.0 to 1% by weight, cyclohexanone is about 0.1 to 0.3% by weight, water is about 0.0 to 0.3% by weight, and other low-boiling components are about 0.5 to 1.5%. The high boiling point component is about 0.3 to 0.7% by weight.

FIG. 1 is a schematic PFD (process flow diagram) showing an example of an embodiment of the present invention.
The crude lactam 21 is fed to the distillation column 1 and distilled, the gas is treated from the top of the column to separate low boiling components from the lactam, and the lactam 22 containing high boiling components is removed from the bottom of the column.
Examples of the distillation column used for the purification of lactam include a packed column and a plate column, but a packed column is preferably used because of a small pressure loss.

The distillation column 1 is heated by heating the column bottom liquid with the reboiler 3 and circulating with the circulation pump 2, and a part of the column bottom liquid is taken out as a lactam containing a high boiling point component.
Since oximes and lactams are thermally unstable, the distillation column usually has a pressure of about 0.2-5 kPa, preferably about 0.5-2 kPa and a bottom temperature of about 100-170 ° C., preferably The operation is performed at 130 to 160 ° C. and the column top temperature is about 90 to 140 ° C., preferably about 110 to 130 ° C.

The gas distilled from the top of the distillation column contains lactam, oxime, cyclohexanone, water, other low boiling point components, and higher boiling point components than lactam.
This gas is condensed in the first condenser 5, the second condenser 9 including the cooler 12, the dry vacuum pump 7, and the third condenser 8 which are arranged in order. Alternatively, a fourth condenser 6 is further arranged between the second condenser 9 and the dry vacuum pump 7, and the first condenser 5, the second condenser 9, the fourth condenser 6, the dry vacuum pump 7, and It is condensed by the third condenser 8.

  A multitubular indirect heat exchanger is usually used as the first condenser, and the gas from the top of the column is condensed at about 70 to 100 ° C. through hot water of about 70 to 80 ° C. as the cooling medium. Cooling to less than about 70 ° C. is not preferable because lactam and oxime are deposited. In the first condenser, a condensate mainly composed of lower boiling point components than lactam, oxime and lactam can be obtained. This condensate is totally refluxed into the distillation column.

The uncondensed gas in the first condenser is condensed by bringing the condensate in the second condenser into direct contact with the cooled liquid in the second condenser. The condensation in the second condenser is performed by cooling to about 10 to 50 ° C, preferably about 10 to 30 ° C. When the temperature is less than about 10 ° C., the solubility is decreased, and lactam and oxime are likely to be precipitated.
As the second condenser, a packed tower, a wet wall tower, a multi-tube indirect heat exchanger of a system in which a condensate is sprayed on the cooling surface, or the like is used.
The condensate in the second condenser is cooled to a temperature lower than the condensing temperature in the second condenser and is circulated to the second condenser. The condensate in the second condenser is usually cooled using a multi-tube indirect heat exchanger, and the condensate is cooled to about 10 to 40 ° C. through cooling water of about 5 to 30 ° C. as a cooling medium. .
In the second condenser, components having a lower boiling point than lactam, lactam, oxime and the like are mainly condensed. The condensate increasing by condensation is taken out, and if necessary, supplied to another distillation column and distilled to recover lactam and oxime. A part of the condensate may be refluxed to the distillation column.

Uncondensed gas in the second condenser is condensed in the third condenser via a dry vacuum pump. Although it varies depending on the magnitude of the resistance of the condenser and piping and the load, the inlet pressure of the dry vacuum pump is about 0.2 to 5.0 kPa, preferably about 0.5 to 2.0 kPa, and the outlet pressure is about 1. Operate at 0.7-6.5 kPa, preferably about 1.8-3 kPa.
Root type, claw type, turbo type, screw type, scroll type, etc. are used as the dry vacuum pump.
As the third condenser, a multi-tube indirect heat exchanger is usually used, and a coolant from about 5 to 30 ° C. is passed as a cooling medium, and a gas from a dry vacuum pump is about 5 to 30 ° C., preferably about Condensate at 10-20 ° C. By condensing on the outlet side, which is higher in pressure than cooling and condensing on the inlet side of the dry vacuum pump, it is possible to condense at a higher temperature than on the inlet side, thereby eliminating the need for brine cooling equipment and the like. In the third condenser, a condensate mainly composed of a low-boiling component and water is obtained from the lactam.

The uncondensed gas in the second condenser is further condensed by the fourth condenser 6 by arranging the fourth condenser 6 between the second condenser 9 and the dry vacuum pump 7, and then via the dry vacuum pump. The method of condensing with the third condenser is preferable because the load and dirt after the fourth condenser can be reduced.
As the fourth condenser, a multi-tube indirect heat exchanger is usually used, and condensation is performed through cooling water at about 5 to 20 ° C. as a cooling medium.

  By the above method, 99% or more of the low-boiling components contained in the crude caprolactam can be condensed, and most of the uncondensed gas can be converted to water, and at that time, it can be reduced without using equipment such as brine cooling. Boiling components can be condensed and separated, and further, caprolactam can be purified continuously for a long time without lowering thermal efficiency and blocking due to precipitation of cyclohexanone oxime or the like in the condenser.

  EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not restrict | limited to the following Example.

Example 1
The crude caprolactam was purified using a purification apparatus similar to the schematic PFD shown in FIG.
As shown in FIG. 1, a distillation column 1, a first condenser 5, a second condenser 9, a fourth condenser 6, a dry vacuum pump 7, and a third condenser 8 are arranged in communication with each other in this order.
The distillation column 1 was adjusted to an average pressure of about 1.4 kPa with a dry vacuum pump 7 and crude caprolactam 21 (lactam: 92 to 99% by weight, oxime: 0.0 to 1% by weight, cyclohexanone: 0.1 to 0) 3 wt%, water: 0.0-0.3 wt%, low boiling point component: 0.5-1.5 wt%, high boiling point component: 0.3-0.7 wt%) The bottom liquid was circulated with a circulation pump 2 and heated with a reboiler 3 to perform distillation. The column top temperature was about 125 ° C. on average and the column bottom temperature was about 147 ° C. on average.

The crude caprolactam 21 supplied with the lactam 22 containing a high-boiling component was taken out from the bottom of the column at a ratio of 98 to 99% by weight.
The gas distilled from the top of the column was condensed at an average of about 95 ° C. by flowing warm water of an average of about 70 ° C. through the first condenser 5. The entire amount of the condensate was refluxed to the distillation column.
The uncondensed gas of the first condenser was cooled by the second condenser 9 and brought into contact with the cooled condensate of the second condenser and condensed. The condensate of the second condenser was circulated by the circulation pump 11 through the condensate tank 10 and cooled to an average of about 25 ° C. by the cooler 12. The condensate was circulated by the circulation pump 11 through the condensate tank 10. The crude caprolactam 21 to which the condensate 23 containing a low-boiling component and lactam, oxime and the like was supplied was taken out at a ratio of 1 to 1.5% by weight.

The uncondensed gas of the second condensate was condensed in the fourth condenser 6 through which cooling water having an average of about 10 ° C. was passed. The condensate was combined with the circulating liquid of the second condenser in the condensate tank.
The uncondensed gas in the fourth condenser is increased from an average of about 1.4 kPa to an average of about 2.7 kPa by a dry vacuum pump, and condensed at an average of about 14 ° C. in the third condenser 8 through which cooling water has an average of about 10 ° C. I let you. The crude caprolactam 21 supplied with the low-boiling component and the condensate 24 mainly composed of water was taken out at a ratio of 0.5 to 1% by weight.

  Distillation was carried out continuously for about 6 months, but there was no trouble such as extreme reduction in thermal efficiency or blockage. In addition, about 99.9% by weight of the low boiling point components could be condensed and separated.

Comparative Example 1
The crude caprolactam was purified using a purification facility in which a distillation tower, a first condenser, a second condenser, a third condenser, and a steam ejector were sequentially connected. All the 1st-3rd condensers used the multi-tube type indirect heat exchanger.
A crude caprolactam similar to that used in Example 1 was fed to the distillation column, the distillate gas was hot water of about 70 ° C. for the first and second condensers, and the cooling water of about 5 ° C. for the third condenser. And condensed sequentially.
However, since lactam precipitated in the third condenser and showed a tendency to block, the cooling water was stopped from passing through the third condenser and heating with steam was performed. Therefore, a lot of uncondensed gas flowed into the vacuum system.

1 is a schematic PFD showing an example of an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Distillation tower 2 Circulation pump 3 Reboiler 5 1st condenser 6 4th condenser 7 Dry type vacuum pump 8 3rd condenser 9 2nd condenser 10 Condensate tank 11 Circulation pump 12 Cooler 21 Crude lactam 22 High boiling point component Containing lactam 23 Condensate 24 Condensate 25 Uncondensed gas

Claims (4)

  In the method for distilling and purifying crude caprolactam obtained by the rearrangement reaction of cyclohexanone oxime, a distillation column, a first condenser, a second condenser, a dry vacuum pump and a third condenser are sequentially connected to each other to form a dry vacuum. The distillation column, the first condenser, and the second condenser are depressurized by suction with a pump, and crude caprolactam is supplied to the distillation column for distillation, and caprolactam containing high-boiling components is separated from the bottom of the column and distilled from the distillation column. The gas to be discharged is condensed at a temperature of 70 to 100 ° C. in the first condenser, and the non-condensed gas in the first condenser is condensed by bringing the condensed liquid of the second condenser into contact with the cooled liquid in the second condenser. Wherein the low-boiling components in the crude caprolactam are separated by condensing the uncondensed gas in the second condenser in the third condenser via a dry vacuum pump. .   The method for purifying caprolactam according to claim 1, wherein the condensate in the first condenser is totally refluxed to the distillation column.   The method for purifying caprolactam according to claim 1 or 2, wherein the condensation in the second condenser is carried out at 10 to 50 ° C and the condensation in the third condenser is carried out at 5 to 30 ° C. The uncondensed gas in the second condenser is condensed in the fourth condenser, and the uncondensed gas in the fourth condenser is condensed in the third condenser via a dry vacuum pump. The method for purifying caprolactam according to any one of the above.

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