A method for purifying caprolactam

Abstract

This invention relates to a method for purifying caprolactam, comprising the steps of pretreatment, evaporation crystallization, and caprolactam purification. Compared with existing industrial methods, this invention exhibits significant advantages in reducing energy consumption, enhancing impurity removal capabilities, and improving product quality, particularly in reducing the content of volatile alkalis.

Description

Technical Field

[0001] This invention belongs to the field of chemical engineering, and specifically relates to a method for purifying caprolactam. Background Technology

[0002] Caprolactam (C6H11NO) is an important organic chemical raw material, mainly used in the production of polyamide 6 (commonly known as nylon-6), which is widely used in synthetic fibers, engineering plastics, and plastic films. At room temperature and pressure, it is a white solid. It readily undergoes polymerization reactions when heated and is soluble in solvents such as water, petroleum hydrocarbons, cyclohexene, benzene, methanol, chloroethanol, and diethyl ether. Its boiling point is 270℃ at normal pressure, and its freezing point is 68.9℃.

[0003] Currently, the mainstream production technology for caprolactam is to obtain it from cyclohexanone oxime through a rearrangement reaction. Because the overall process of caprolactam production is relatively complex and the crude product contains a wide variety of impurities, a series of separation and purification steps are required to obtain premium caprolactam.

[0004] The current industrial purification method for preparing caprolactam using cyclohexanone ammonium oxime followed by the Beckmann rearrangement generally involves the following steps: The reaction solution obtained after the Beckmann rearrangement is first neutralized with ammonia, then the ammonium sulfate is removed using an ammonium sulfate crystallizer. The resulting amide oil undergoes benzene extraction, water back-extraction, ion exchange, hydrogenation, dehydration, and removal of light and heavy components to finally obtain the superior grade of caprolactam. The entire separation and purification process is relatively complex, impurities can easily accumulate in the system leading to unstable product quality in later stages, and the process is lengthy and energy-intensive.

[0005] Patent CN115677582A describes a multi-stage adiabatic flash crystallization method for refining caprolactam. The raw material used in this method is crude solid caprolactam (99% purity). It requires a specific solvent to prepare a solution of a specific concentration before crystallization can begin. Moreover, this crystallization method still has problems such as excessively fast crystallization speed and difficulty in controlling supersaturation. Therefore, the caprolactam obtained by crystallization still needs to be hydrogenated and de-lightened and de-heavyened components to obtain a superior grade.

[0006] Furthermore, in the field of synthetic fibers that may come into contact with the skin, it is still desirable to minimize the content of volatile alkalis in caprolactam raw materials for people with specific allergies. Summary of the Invention

[0007] The present invention aims to provide an improved method for refining caprolactam, which uses crystallization as the main means to significantly improve the efficiency of impurity removal, especially to reduce the content of volatile alkali, improve product quality, and reduce energy consumption.

[0008] According to the present invention, a method for purifying caprolactam is provided, the method comprising the following steps:

[0009] (1) Preprocessing

[0010] The raw material benzene-hexane solution is first washed with an alkaline solution to obtain alkaline-washed benzene-hexane solution.

[0011] Then, the benzene-hexane solution after alkali washing is distilled to preliminarily remove benzene, so as to obtain a preliminarily benzene-removed benzene-hexane solution.

[0012] The alkaline solution is a NaOH or KOH solution, with a concentration of 0.1%-10% by mass, preferably 0.5%-5%, more preferably 0.8%-1.2%, and the mass ratio of the alkaline solution to the benzene / hexane solution is 1:20-1:200, preferably 1:50-1:150, more preferably 1:100-1:150.

[0013] (2) Evaporation crystallization

[0014] The benzene-hexylene solution obtained in step (1) after preliminary benzene removal is subjected to isothermal evaporation and crystallization. The crystallized material is then separated (e.g., by centrifugation) to obtain solid caprolactam crystals and liquid mother liquor. A portion of the mother liquor is returned to this step for isothermal evaporation and crystallization for recrystallization.

[0015] The crystallization temperature is 10℃-40℃, preferably 15℃-35℃, and more preferably 20℃-35℃.

[0016] (3) Caprolactam purification

[0017] The caprolactam crystals obtained in step (2) are subjected to solvent removal, for example, by vacuum drying or stripping. The desolventized caprolactam is then subjected to light and heavy weight removal to obtain superior caprolactam.

[0018] The methods for removing light and heavy substances are vacuum flash evaporation or vacuum distillation, and the theoretical plate number of the distillation column is 1-10.

[0019] According to the present invention, the method further includes the following steps:

[0020] (4) Antisolvent crystallization

[0021] A portion of the mother liquor from step (2) is subjected to vacuum evaporation to remove benzene, resulting in a caprolactam mixture with a lower benzene content of 0.1%-20%.

[0022] In this process, the mass ratio of the primary mother liquor from the isothermal evaporation and recrystallization in step (2) to the primary mother liquor entering step (4) is 1:1 to 9:1.

[0023] Then, a poor solvent for caprolactam is added to the caprolactam mixture with a lower benzene content to perform anti-solvent crystallization.

[0024] The poor solvents for caprolactam have low solubility for caprolactam and are miscible with benzene, preferably one or more selected from n-hexane, n-heptane, n-octane, and cyclohexane.

[0025] The intermediate material obtained by antisolvent crystallization was centrifuged to obtain caprolactam crystal A and secondary mother liquor in liquid phase. Caprolactam crystal A was returned to the feed in step (2) for isothermal evaporation and crystallization, and then purified by recrystallization.

[0026] The secondary mother liquor is then processed in an optional step (5).

[0027] Optional (5) Secondary mother liquor evaporation and crystallization

[0028] The secondary mother liquor obtained in step (4) is subjected to constant temperature evaporation and crystallization at a temperature of 20℃-70℃, preferably 40℃-50℃.

[0029] The suspension obtained by crystallization is centrifuged to obtain solid caprolactam crystal B and liquid mother liquor. This caprolactam crystal B is returned to step (2) for recrystallization and purification.

[0030] The mother liquor from the three treatments is then processed in an optional step (6).

[0031] And optional (6) solvent recovery

[0032] The mother liquor obtained in step (5) is fed into the solvent recovery tower to obtain solvent and residue.

[0033] Among them, the crystallization yield of step (5) can reach 80%-98%.

[0034] This method employs crystallization, particularly tertiary crystallization, to replace the steps of water back-extraction, ion exchange, hydrogenation, and dehydration in current industrial technologies. This shortens the overall refining process of caprolactam and eliminates the consumption of large amounts of water, ion exchange resin, and hydrogenation catalyst, making it more efficient and environmentally friendly.

[0035] In step (2) of this method, by appropriately recycling the mother liquor, the crystallization yield can reach over 80%, and the distilled benzene can be returned to the benzene extraction section for direct recycling, reducing the energy consumption of the benzene refining step. The remaining less than 20% of CPL can be recovered by using a small amount of solvent in steps (4) and (5) to recover more than 95% of the caprolactam in the mother liquor, and the solvent can be recycled in step (6). The total yield of the entire separation process can reach over 99%. Due to the reduction of a large amount of water evaporation energy consumption, the overall separation energy consumption cost is significantly lower than that of current industrial technology. According to simulation estimates, the comprehensive energy consumption cost of separating and refining 1 ton of caprolactam by the current industrial method is about 600 yuan, while the comprehensive energy consumption of separating and refining 1 ton of caprolactam by this method is about 400 yuan.

[0036] Current industrial methods, through extraction, can only remove impurities insoluble in benzene or water. Furthermore, due to dissolution equilibrium, these impurities cannot be completely removed, leading to their accumulation over long-term production. This can negatively impact product quality in the later stages of a production cycle. As is well known, crystals are formed by the orderly arrangement of identical molecules (atoms or ions) according to a specific pattern. Therefore, crystals exhibit strong exclusivity, and crystallization operations possess a high capacity for impurity removal. This method, which primarily uses crystallization to purify caprolactam, ensures a higher quality and more stable product throughout long-term production.

[0037] Moreover, the inventors unexpectedly discovered that, through the method of the present invention, caprolactam products with lower volatile base content can be obtained simply by crystallization without the need for a special removal of sulfate and ammonium ions through an ion exchange step.

[0038] According to one embodiment of the present invention, wherein,

[0039] In step (1), the concentration of benzene in the raw material benzene-hexane solution is 60%-90%.

[0040] The concentration of benzene in the benzene-hexane solution after preliminary benzene removal is 35%-80%.

[0041] According to one embodiment of the present invention, wherein,

[0042] The distillation temperature in step (1) is 20 to 50°C, the condensation temperature is -10 to 0°C, and the pressure is 0.1 to 20 kPa.

[0043] Preferably, the distillation temperature in step (1) is 30 to 40°C, the condensation temperature is -10 to 0°C, and the pressure is 10-15 kPa.

[0044] According to one embodiment of the present invention, wherein,

[0045] The operating pressure of the evaporator crystallizer in step (2) is 8-12 kPa, the condensation temperature is -10 to 0°C, and the mass ratio of the part that returns to the isothermal evaporation crystallization in this step for recrystallization to the part that enters step (4) is 1:1 to 4:1.

[0046] According to one embodiment of the present invention, wherein,

[0047] In step (3), solvent removal is carried out by vacuum drying, wherein the pressure of vacuum drying is 20-60 kPa and the temperature is 30-60 °C; the removal of light and heavy components is carried out by vacuum distillation, wherein the operating pressure at the top of the column is 0.1-10 kPa and the temperature at the bottom of the column is 110-150 °C.

[0048] Preferably, in step (3), the removal of light and heavy components is carried out by vacuum distillation, with the top operating pressure of the column being 1-3 kPa and the bottom temperature being 128-132°C.

[0049] According to yet another embodiment of the present invention, wherein,

[0050] In step (4), the mass ratio of the caprolactam mixture to the unsuitable solvent is 2:1 to 1:3.

[0051] The single-pass crystallization yield of caprolactam in step (4) is 70%-90%.

[0052] Beneficial effects

[0053] Compared with existing industrial methods, this invention exhibits significant advantages in reducing energy consumption, enhancing impurity removal capabilities, and improving product quality, particularly in reducing the content of volatile alkalis. Attached Figure Description

[0054] Figure 1 This is a schematic diagram of the process flow of the present invention. Detailed Implementation

[0055] Example 1

[0056] In the early stages of the production cycle, benzene-caprolactam solution was obtained from the benzene extraction section of the caprolactam production line as raw material. Analysis showed that the benzene content was 77.8%, the caprolactam content was 22%, and the total impurity content was about 0.2%.

[0057] (1) Preprocessing:

[0058] The benzene-hexane solution is subjected to alkaline washing using NaOH at a concentration of 1% and a washing temperature of 40°C. The ratio of alkaline solution to benzene-hexane solvent is 1:150. The alkaline solution is pumped into the upper part of the alkaline washing tower and comes into countercurrent contact with the benzene-hexane solution entering from the lower part of the tower, thus completing the alkaline washing process. The alkaline-washed benzene-hexane solution then proceeds to the preliminary benzene removal process.

[0059] After alkali washing, the benzene-caprolactam solution was subjected to vacuum distillation to preliminarily concentrate CPL. The distillation temperature was 35℃, the condensation temperature was -5℃, and the pressure was 12KPa. The benzene content in the distilled benzene-caprolactam solution was 49%, and the caprolactam content was 51%. Then, it entered the evaporator crystallizer.

[0060] (2) Evaporation crystallization

[0061] The evaporator crystallizer is heated to 32℃, operated at 10KPa, condensed at -5℃, and stirred at 150rpm. The crystallized material is then fed into a centrifuge to obtain high-purity CPL crystals and primary mother liquor. 75% of the primary mother liquor is returned to the feed position for further crystallization, and 25% of the primary mother liquor is fed into step (4).

[0062] (3) Caprolactam purification

[0063] The crystals obtained in step 2 were first vacuum dried at a pressure of 30 kPa and a temperature of 40 °C to remove residual benzene. Then, they were distilled under reduced pressure to remove light and heavy components. The operating pressure at the top of the column was 1 kPa and the temperature at the bottom of the column was 130 °C. The side stream near the top of the column was collected to obtain CPL premium grade. The chromatographic analysis results showed that the purity of caprolactam was 99.999%.

[0064] (4) Antisolvent crystallization

[0065] In step 2, a mother liquor is obtained. 25% of it is first subjected to vacuum distillation to remove benzene. The concentration of caprolactam after benzene removal is 90%, and the benzene content is 10%.

[0066] After benzene removal, caprolactam enters the antisolvent crystallizer. The other feed solvent is n-heptane, with a caprolactam to n-heptane feed ratio of 1:1.5 and a residence time of approximately 1.5 hours. The resulting mixture from crystallization enters a centrifuge, and the obtained CPL crystals are returned to the feed point of step 2 for recrystallization and purification. The resulting mother liquor is the secondary mother liquor, which enters the next processing step.

[0067] (5) Evaporation and crystallization of mother liquor

[0068] The secondary mother liquor obtained in step 4 enters the evaporator crystallizer, with a jacket heating temperature of 40°C, an operating pressure of 80 mbar, and a stirring speed of 200 rpm. The crystallized material enters the centrifuge, and the obtained CPL crystals are returned to the feed port of step 2 for recrystallization and purification. The resulting tertiary mother liquor enters the next processing step.

[0069] (6) Solvent recovery

[0070] The mother liquor obtained in step 5 is used to recover the solvent in a distillation column. The solvent collected from the top of the column is reused, and the bottom product is treated as solid waste.

[0071] The final yield of caprolactam was 99.5% throughout the entire process.

[0072] Example 2

[0073] One month before maintenance in the later stages of the production cycle, a benzene-caprolactam solution obtained from the benzene extraction section of the caprolactam production line was analyzed and found to contain 77% benzene, 21% caprolactam, and 2% total impurities.

[0074] (1) Preprocessing

[0075] The benzene-hexane solution is subjected to alkaline washing using NaOH at a concentration of 1% and a washing temperature of 45°C. The ratio of alkaline solution to benzene-hexane solvent is 1:110. The alkaline solution is pumped into the upper part of the alkaline washing tower and comes into countercurrent contact with the benzene-hexane solution entering from the lower part of the tower, thus completing the alkaline washing process. The alkaline-washed benzene-hexane solution then proceeds to the preliminary benzene removal process.

[0076] After alkali washing, the benzene-caprolactam solution was subjected to vacuum distillation to preliminarily concentrate CPL. The distillation temperature was 35℃, the condensation temperature was -5℃, and the pressure was 12Kpa. The benzene content in the distilled benzene-caprolactam solution was 55%, and the caprolactam content was 45%. Then, it entered the evaporator crystallizer.

[0077] (2) Evaporation crystallization

[0078] After benzene removal and concentration, the benzene-hexylene solution is mixed with CPL crystals obtained from antisolvent crystallization and mother liquor evaporation crystallization. After complete mixing and dissolution, the benzene content is 48% and the caprolactam content is 52%. The mixed material then enters the evaporation crystallizer.

[0079] The evaporator crystallizer is heated to 25℃, operated at 9KPa, condensed at -5℃, and stirred at 150rpm. The crystallized material is then fed into a centrifuge to obtain high-purity CPL crystals and primary mother liquor. 60% of the primary mother liquor is returned to the feed position for further crystallization, and 40% of the primary mother liquor is fed into step (4).

[0080] (3) Caprolactam purification

[0081] The crystals obtained in step 2 are first vacuum dried at a pressure of 50 kPa and a temperature of 50 °C to remove residual benzene. Then, they are distilled under reduced pressure to remove light and heavy components. The operating pressure at the top of the column is 1 kPa and the temperature at the bottom of the column is 130 °C. The side stream near the top of the column is collected to obtain CPL premium grade. Chromatographic analysis shows that the purity of caprolactam is 99.999%.

[0082] (4) Antisolvent crystallization

[0083] In step 2, a mother liquor is obtained. 40% of it is first subjected to vacuum distillation to remove benzene. The concentration of caprolactam after benzene removal is 96%, and the benzene content is 4%.

[0084] After benzene removal, the caprolactam enters the antisolvent crystallizer. The other feed solvent is a mixed solvent of n-heptane and benzene obtained from solvent recovery, with n-heptane content of 97% and benzene content of 3%. The feed ratio of caprolactam to n-heptane is 1:1.5, and the residence time is approximately 1.5 hours. The resulting mixture from crystallization enters a centrifuge. The obtained CPL crystals are returned to the feed point of step 2 for recrystallization and purification. The resulting mother liquor is the secondary mother liquor, which enters the next processing step.

[0085] (5) Evaporation and crystallization of secondary mother liquor

[0086] The secondary mother liquor obtained in step 4 enters the evaporator crystallizer, with a jacket heating temperature of 45°C, an operating pressure of 85 mbar, and a stirring speed of 200 rpm. The crystallized material enters the centrifuge, and the obtained CPL crystals are returned to the feed port of step 2 for recrystallization and purification. The resulting tertiary mother liquor enters the next processing step.

[0087] (6) Solvent recovery

[0088] The mother liquor obtained in step 5 is used to recover the solvent in a distillation column. The solvent collected from the top of the column is reused in the anti-solvent crystallization step, and the bottom product is treated as waste solids.

[0089] The final yield of caprolactam was 99.2% throughout the entire process.

[0090] Comparative Example 1

[0091] One month before maintenance in the later stages of the production cycle, benzene-caprolactam extract was obtained from the benzene extraction section of the caprolactam production line as raw material. Analysis showed that it contained 77% benzene, 21% caprolactam, and 2% impurities. It was then processed according to current industrial technology.

[0092] (1) Alkali washing

[0093] The benzene-hexane solution is subjected to alkaline washing using NaOH at a concentration of 1% and a washing temperature of 40°C. The ratio of alkaline solution to benzene-hexane solvent is 1:130. The alkaline solution is pumped into the upper part of the alkaline washing tower and comes into countercurrent contact with the benzene-hexane solution entering from the lower part of the tower, thus completing the alkaline washing process. The alkaline-washed benzene-hexane solution then proceeds to the water back-extraction process.

[0094] (2) Water back-extraction

[0095] The benzene-caprolactam solution, after being washed with alkali, is subjected to countercurrent extraction with the condensate from process steam. The benzene-caprolactam solution enters from the bottom of the extraction tower, while benzene overflows from the top. Water enters from the top, and the caprolactam solution flows out from the bottom. The back-extraction temperature is 45°C, the feed volume ratio of benzene-caprolactam solution to water is 2:1, and the caprolactam concentration in the caprolactam solution is 30%. The benzene obtained from the top of the tower enters the benzene refining process, and the caprolactam solution obtained from the bottom of the tower enters the ion exchange process.

[0096] (3) Benzene refining

[0097] The benzene obtained from the water back-extraction column is purified by distillation. The benzene collected from the top of the column is returned to the benzene extraction process for recycling, while a small amount of residual benzene in the bottom of the column is incinerated. The distillation operation is carried out at atmospheric pressure, and the distillation column has a theoretical number of 10 plates.

[0098] (4) Ion exchange

[0099] The aqueous solution flows through an ion exchange tower containing anion exchange resin and cation exchange resin to remove sulfate and ammonium ions. The ion exchange temperature is 40°C.

[0100] (5) Fixed-bed hydrogenation

[0101] The hexane solution after passing through the ion exchange tower enters the fixed-bed hydrogenation reactor to remove unsaturated impurities. The hydrogenation temperature is 80℃ and the pressure is 0.6MPa. The hydrogenated hexane solution then enters the triple-effect evaporation process.

[0102] (6) Triple-effect evaporation

[0103] The hydrogenated aqueous solution, which contains 70% water, is dehydrated by triple-effect evaporation to reduce the water content to 10% before entering the flash evaporation process.

[0104] (7) Flash evaporation

[0105] After triple-effect evaporation, the caprolactam-containing aqueous solution containing 90% caprolactam is flash-distilled to reduce the water content to below 0.1%, and then enters a distillation column for further purification.

[0106] (8) Distillation to remove light and heavy components

[0107] After dehydration, caprolactam enters a distillation column for dehydrogenation and deweighting. The pressure at the top of the column is 0.5 kPa and the temperature at the top of the column is 108°C. Qualified caprolactam product is collected from the side stream. Analysis shows that the purity of caprolactam is 99.989%.

[0108] The final yield of caprolactam in the entire separation process was 99%.

[0109] The national standard analysis results of the caprolactam products obtained from the examples and comparative examples are as follows:

[0110]

[0111] As can be seen from Examples 1-2, high-quality products can be obtained when using the method of the present invention, regardless of the impurity content.

[0112] As can be seen from Example 2 and Comparative Example 1, when using the same raw materials with high impurity content, the method of the present invention has significant advantages over the conventional method of Comparative Example 1, and can produce products of higher quality.

[0113] Surprisingly, the method of this invention allows for the production of caprolactam products with lower volatile base content through a simple crystallization process, without the need for a specific removal of sulfate and ammonium ions via an ion exchange step.

[0114] It is worth mentioning that if this method is used for the purification of caprolactam throughout the entire production cycle, no impurities will accumulate, the amount of impurities in the benzene-caprolactam solution will not increase, and the production process will be more stable.

Claims

1. A method for purifying caprolactam, the method comprising the following steps: (1) Preprocessing The raw material benzene-hexane solution is first washed with an alkaline solution to obtain alkaline-washed benzene-hexane solution. Then, the benzene-hexane solution after alkali washing is distilled to preliminarily remove benzene, so as to obtain a preliminarily benzene-removed benzene-hexane solution. The alkaline solution is a NaOH or KOH solution, with a concentration of 0.1%-10% by mass, and the mass ratio of alkaline solution to benzene / hexane solution is 1:20-1:

200. (2) Evaporation crystallization The benzene-hexylene liquid obtained in step (1) after preliminary benzene removal is subjected to isothermal evaporation and crystallization. The crystallized material is separated to obtain solid caprolactam crystals and liquid mother liquor. A portion of the mother liquor is returned to this step for isothermal evaporation and crystallization for recrystallization. The crystallization temperature is 10℃-40℃. (3) Caprolactam purification The caprolactam crystals obtained in step (2) are subjected to solvent removal. The desolventized caprolactam is then subjected to light and heavy element removal to obtain superior grade caprolactam. The methods for removing light and heavy substances are vacuum flash evaporation or vacuum distillation, and the theoretical plate number of the distillation column is 1-10.

2. The method according to claim 1, wherein, The alkaline solution mentioned in step (1) is a NaOH solution with a concentration of 0.5%-5% by mass, and the mass ratio of the alkaline solution to the benzene solution is 1:50-1:

150. The crystallization temperature in step (2) is 15℃-35℃.

3. The method according to claim 1, wherein, The alkaline solution mentioned in step (1) is a NaOH solution with a concentration of 0.8%-1.2% by mass, and the mass ratio of the alkaline solution to the benzene solution is 1:100-1:

150. The crystallization temperature in step (2) is 20℃-35℃.

4. The method according to claim 1, wherein, In step (1), the concentration of benzene in the raw material benzene-hexane solution is 60%-90%. The concentration of benzene in the benzene-hexane solution after preliminary benzene removal is 35%-80%.

5. The method according to claim 1, wherein, The distillation temperature in step (1) is 20 to 50°C, the condensation temperature is -10 to 0°C, and the pressure is 0.1 to 20 kPa.

6. The method according to claim 1, wherein, The operating pressure of the evaporator crystallizer in step (2) is 8-12 kPa, the condensation temperature is -10 to 0°C, and the mass ratio of the part that returns to the isothermal evaporation crystallization in this step for recrystallization to the part that enters step (4) is 1:1 to 4:

1.

7. The method according to claim 1, wherein, In step (3), solvent removal is carried out by vacuum drying, wherein the pressure of vacuum drying is 20-60 kPa and the temperature is 30-60 °C; the removal of light and heavy components is carried out by vacuum distillation, wherein the operating pressure at the top of the column is 0.1-10 kPa and the temperature at the bottom of the column is 110-150 °C.

8. The method according to claim 1, further comprising the following steps: (4) Antisolvent crystallization A portion of the mother liquor from step (2) is subjected to vacuum evaporation to remove benzene, resulting in a caprolactam mixture with a lower benzene content of 0.1%-20%. In this process, the mass ratio of the primary mother liquor from the isothermal evaporation and recrystallization in step (2) to the primary mother liquor entering step (4) is 1:1 to 9:

1. Then, a poor solvent for caprolactam is added to the caprolactam mixture with a lower benzene content to perform anti-solvent crystallization. The poor solvents for caprolactam have low solubility for caprolactam and are miscible with benzene. The intermediate material obtained by antisolvent crystallization was centrifuged to obtain caprolactam crystal A and secondary mother liquor in liquid phase. Caprolactam crystal A was returned to the feed in step (2) for isothermal evaporation and crystallization, and then purified by recrystallization. The secondary mother liquor is then processed in an optional step (5). Optional (5) Secondary mother liquor evaporation and crystallization The secondary mother liquor obtained in step (4) is subjected to isothermal evaporation and crystallization at a crystallization temperature of 20℃-70℃. The suspension obtained from crystallization is centrifuged to separate the solid phase of caprolactam crystal B and the liquid phase of the mother liquor. This caprolactam crystal B is returned to step (2) for recrystallization and purification. The mother liquor from the three treatments is then processed in an optional step (6). And optional (6) solvent recovery The mother liquor obtained in step (5) is fed into the solvent recovery tower to obtain solvent and residue.

9. The method according to claim 8, wherein, In step (4), The unsuitable solvent for the caprolactam is one or more selected from n-hexane, n-heptane, n-octane, and cyclohexane. In step (5), The secondary mother liquor obtained in step (4) is subjected to constant temperature evaporation and crystallization at a temperature of 40℃-50℃.

10. The method according to claim 8, wherein, In step (4), the mass ratio of the caprolactam mixture to the unsuitable solvent is 2:1 to 1:3.

Images