Purification method of caprolactam distillation residual liquid

Abstract

The invention discloses a purification method of caprolactam distillation residual liquid. The purification method comprises the following steps: (1) adding a solvent into the caprolactam distillation residual liquid; wherein the solvent is selected from at least one of water, alcohols and hydrocarbons; the addition amount of the solvent is 3-20wt% of the caprolactam distillation residual liquid; (2) carrying out cooling crystallization in an inert atmosphere to obtain a caprolactam crude product; (3) sequentially carrying out first-stage heating sweating and second-stage heating sweating on the caprolactam crude product to obtain a caprolactam crystal; wherein the temperature of the second-stage heating sweating is higher than that of the first-stage heating sweating. According to the method disclosed by the invention, the crystallization yield is greatly improved, the crystallization product reaches the commercially available index, and the production cost is greatly reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of caprolactam purification, and particularly to a method for purifying caprolactam distillation residue. Background Art

[0002] Caprolactam appears as white powder or crystals and has an oily feel. It is an important organic chemical raw material and is widely used in the production fields of nylon-6 fiber and engineering plastics. Currently, the production of caprolactam usually adopts the cyclohexanone oxime Beckmann liquid-phase rearrangement process. In this process, fuming sulfuric acid is used as a catalyst for rearrangement. After rearrangement, caprolactam finished products are obtained through processes such as neutralization, extraction, alkali washing, ion exchange, catalytic hydrogenation, and distillation. Some impurities are difficult to be completely removed in the refining step, thus forming residues during the distillation process. Approximately 10-15% of the distillation residues generated in the distillation section are returned to the refining system for re-refining. These distillation residues contain 97-99% of caprolactam and some impurities, including oligomers, epoxides, hydroxides, etc. Due to the continuous enrichment of these impurities in the refining system, it not only affects the performance and quality of its subsequent processed products, but also increases the impurity removal pressure of the refining system and shortens the maintenance interval.

[0003] Regarding the recycling of caprolactam distillation residues, existing methods usually have problems such as complex processes, high costs, and low recycling efficiency. For example, extraction and purification require multiple steps of separation and purification operations, involving a large amount of chemical reagents and complex equipment, resulting in an increase in recycling costs. Approximately 10-15% of the distillation residues generated in the existing technology in the distillation section currently need to be returned to the refining system for re-refining, which not only causes impurity enrichment in the refining system and increases the refining load, thereby affecting product quality, but also consumes a large amount of steam. At the same time, due to the complexity and interaction of impurities, existing recycling methods often have difficulty achieving efficient recycling of caprolactam, making the purity and quality of the recycled products unable to meet the requirements of industrial production.

[0004] CN108658863A discloses a method for purifying caprolactam using the melt crystallization method. The crude caprolactam is added to a melt crystallizer, the temperature is raised to completely melt the caprolactam solid, and then the temperature is adjusted and maintained at a certain level. Cooling crystallization operation is carried out at a certain cooling rate. When the temperature is cooled to the final crystallization temperature, it is maintained for a period of time to achieve the purpose of sufficient crystallization. The liquid discharge valve is opened to discharge the crystallization mother liquor until all the mother liquor is drained. Then, heating and sweating operation is carried out at a certain heating rate to discharge the sweat. When the temperature is raised to the final sweating temperature, it is maintained for a period of time to drain the sweat completely. The temperature is raised to melt all the crystal layers and discharge them, and caprolactam crystals with a lower absorbance can be obtained. The crystallization yield of this method is low, and the improvement of product quality indicators is single, only improving the extinction value data.

[0005] In summary, the existing caprolactam distillation residue treatment methods have many shortcomings and cannot achieve the dual goals of effective resource utilization and environmental protection. Therefore, developing an innovative, efficient, economical and environmentally friendly caprolactam distillation residue treatment method to achieve high-purity recovery of caprolactam and effective removal of impurities has become an important technical problem that needs to be solved in the current caprolactam production field. Summary of the invention

[0006] The purpose of the present invention is to overcome the problems of low yield and single product quality index improvement in the prior art, and to provide a method for purifying caprolactam distillation residue. The method utilizes melt crystallization to purify caprolactam distillation residue, and by adding an appropriate amount of solvent to the caprolactam distillation residue to assist crystallization, the crystallization yield is greatly improved, and the crystallized product reaches the commercial index. Compared with the existing process, the production cost is greatly reduced.

[0007] In order to achieve the above object, the present invention provides a method for purifying caprolactam distillation residue, the purification method comprising the following steps:

[0008] (1) adding a solvent to the caprolactam distillation residue; wherein the solvent is selected from at least one of water, alcohols and hydrocarbons; and the amount of the solvent added is 3-20 wt % of the caprolactam distillation residue;

[0009] (2) under an inert atmosphere, performing cooling crystallization to obtain a crude caprolactam product;

[0010] (3) subjecting the crude caprolactam product to primary temperature rising sweating and secondary temperature rising sweating in sequence to obtain caprolactam crystals; wherein the temperature of the secondary temperature rising sweating is higher than the temperature of the primary temperature rising sweating.

[0011] Through the above technical solution, the beneficial technical effects achieved by the present invention are as follows:

[0012] (1) The present invention utilizes melt crystallization to purify caprolactam distillation residue. By adding an appropriate amount of solvent to the caprolactam distillation residue to assist crystallization, the crystallization yield is greatly improved, the crystallized product reaches the commercial index, and the production cost is greatly reduced.

[0013] (2) The caprolactam obtained by the method of the present invention has a high purity, which can reach more than 99%; and a high yield, which can reach 75-85%. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a schematic diagram of a melt crystallization process provided by the present invention;

[0015] Figure 2 It is a schematic diagram of a melt crystallization process provided by the present invention. DETAILED DESCRIPTION

[0016] The endpoints and any values within the ranges disclosed herein are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0017] The present invention provides a method for purifying caprolactam distillation residue, and the purification method includes the following steps:

[0018] (1) Adding a solvent to the caprolactam distillation residue; wherein, the solvent is selected from at least one of water, alcohols and hydrocarbons; the addition amount of the solvent is 3-20 wt% of the caprolactam distillation residue;

[0019] (2) Cooling and crystallizing under an inert atmosphere to obtain a crude caprolactam product;

[0020] (3) Subjecting the crude caprolactam product to primary temperature-rising sweating and secondary temperature-rising sweating in sequence to obtain caprolactam crystals; wherein, the temperature of the secondary temperature-rising sweating is higher than that of the primary temperature-rising sweating.

[0021] According to the present invention, the inert atmosphere refers to an environment composed of inert gases (such as argon, nitrogen, etc.), and a nitrogen atmosphere is preferred.

[0022] According to the present invention, the caprolactam distillation residue refers to the residue at the bottom of the distillation column generated during the distillation process in the refining section during the production of caprolactam by the cyclohexanone oxime Beckmann liquid-phase rearrangement process.

[0023] According to a preferred embodiment of the present invention, the purification method further includes: melting the caprolactam crystals obtained in step (3), and then performing nitrogen sealing.

[0024] According to the present invention, melting is for detaching the product from the crystallizer for collection, and nitrogen sealing of the collected product prevents oxidation and deterioration.

[0025] According to the present invention, the addition amount of the solvent is 3-20 wt% of the caprolactam distillation residue, such as 3 wt%, 5 wt%, 8 wt%, 10 wt%, 12 wt%, 15 wt%, 18 wt%, 20 wt%, and any value within the range composed of any two numerical values.

[0026] According to a preferred embodiment of the present invention, the addition amount of the solvent is 5-15 wt% of the caprolactam distillation residue, and more preferably 8-12 wt%.

[0027] According to a preferred embodiment of the present invention, the alcohols are selected from at least one of methanol, ethanol, ethylene glycol, and isopropanol.

[0028] According to a preferred embodiment of the present invention, the hydrocarbons are selected from at least one of cyclopentane, cyclohexane, benzene, n-heptane, and n-octane.

[0029] According to the present invention, water can remove water-soluble impurities in the caprolactam distillation residue, such as sodium hydroxide, aniline impurities, etc. Alcohols and hydrocarbons can remove oil-soluble impurities in the caprolactam distillation residue, such as 6-aminocaproic acid, oligomers and other impurities.

[0030] According to a preferred embodiment of the present invention, the solvent is selected from at least one of water, methanol, and cyclohexane.

[0031] According to the present invention, the solvent can be a mixture. For example, a mixture of water and methanol in any weight ratio, including but not limited to: the weight ratio of water to methanol is 1:1 - 2:1, and the weight ratio of water to cyclohexane is 1:1 - 1:3.

[0032] According to the present invention, the solvent content can significantly affect the purity and yield of caprolactam crystals. By controlling the solvent and its proportion, the purity and yield of caprolactam can be improved.

[0033] According to a preferred embodiment of the present invention, the temperature of the caprolactam distillation residue is 95 - 105 °C.

[0034] According to a preferred embodiment of the present invention, the step of cooling crystallization includes: cooling at a cooling rate of 0.01 - 0.1 °C / min to 15 - 90 °C and maintaining for 30 - 120 min.

[0035] According to a preferred embodiment of the present invention, the step of primary heating and sweating includes: heating at a heating rate of 0.01 - 0.1 °C / min to 15 - 75 °C, maintaining for 30 - 120 min, and discharging the sweating liquid.

[0036] According to a preferred embodiment of the present invention, the step of secondary heating and sweating includes: heating at a heating rate of 0.01 - 0.1 °C / min to 16 - 78 °C, maintaining for 30 - 120 min, and discharging the sweating liquid.

[0037] According to the present invention, in addition to controlling the solvent and its proportion, the purity and yield of caprolactam can be further improved by controlling the temperature and the sweating process.

[0038] According to the present invention, as the sweating process progresses, the purity of caprolactam gradually increases, and the purity of the secondary sweating liquid is generally higher than that of the primary sweating liquid.

[0039] According to a preferred embodiment of the present invention, the purification method further includes: (4) heating at a heating rate of 1 - 5 °C / min to 18 - 65 °C to melt the caprolactam crystals obtained in step (3), and then performing nitrogen sealing.

[0040] According to a preferred embodiment of the present invention, the purification method further includes: mixing the crystallization mother liquor after cooling crystallization and the sweating liquid discharged from the first-stage heating sweating and the second-stage heating sweating, and repeating steps (2) to (4); then using the obtained caprolactam product as the raw material for step (1).

[0041] According to a particularly preferred embodiment of the present invention, a method for purifying caprolactam distillation residue is as Figure 1 shown, and includes the following steps:

[0042] (1) The device extracts caprolactam distillation residue at 95 - 105 °C, and adds a solvent accounting for 3 - 20 wt% of the caprolactam distillation residue, including but not limited to: water, alcohol, hydrocarbons, etc.;

[0043] (2) Under a nitrogen atmosphere, cool at a cooling rate of 0.01 - 0.1 °C / min to make the temperature in the melt crystallizer drop to 15 - 90 °C and maintain for 30 - 120 min, so that the caprolactam distillation residue containing the solvent fully crystallizes in the melt crystallizer; after the caprolactam distillation residue in the melt crystallizer fully crystallizes, open the liquid discharge valve at the bottom of the crystallizer, and keep it for 30 - 60 min to completely discharge the crystallization mother liquor;

[0044] (3) With the bottom liquid discharge valve open, heat the crude caprolactam product obtained in step (2) at a heating rate of 0.01 - 0.1 °C / min to 15.5 - 50.5 °C for the first-stage sweating for 30 - 120 min, continuously discharging the sweating liquid; then heat at a heating rate of 0.01 - 0.1 °C / min to 16.5 - 51.5 °C for the second-stage sweating for 30 - 120 min, continuously discharging the sweating liquid. After the sweating ends, high-purity caprolactam crystals are obtained;

[0045] (4) Heat at a heating rate of 1 - 5 °C / min to 18 - 70 °C to melt the caprolactam in the melt crystallizer and discharge it from the bottom liquid discharge valve, obtaining high-purity caprolactam and quickly performing nitrogen sealing, with a yield of about 75 - 85%;

[0046] (5) Enrich and mix the crystallization mother liquor in step (2) and the sweating liquid discharged in step (3), return 80 - 90% as the raw material for step (1), and discharge the rest for incineration.

[0047] The present invention will be described in detail below through examples, but the protection scope of the present invention is not limited to the following description.

[0048] In the following examples and comparative examples, those without specific conditions mentioned were carried out according to conventional conditions or conditions recommended by the manufacturer. For reagents or instruments without the manufacturer indicated, they were all conventional products that could be obtained through commercial channels.

[0049] Purity and product quality of caprolactam: Determination was carried out according to "GBT13254 - 2017 Caprolactam for industrial use".

[0050] Example 1

[0051] (1) Using the process schematic diagrams as shown in Figure 1 and Figure 2 , the caprolactam distillation residue taken out from the device was at 95 °C, and water accounting for 5 wt% of the caprolactam distillation residue was added;

[0052] (2) Under a nitrogen atmosphere, the temperature was decreased at a rate of 0.05 °C / min until the temperature in the melt crystallizer dropped to 52 °C and was maintained for 120 min, so that the caprolactam distillation residue containing the solvent was fully crystallized in the melt crystallizer; after the caprolactam distillation residue was fully crystallized in the melt crystallizer, the drain valve at the bottom of the crystallizer was opened, and it was kept for 60 min to drain all the mother liquor;

[0053] (3) With the drain valve at the bottom open, the crude caprolactam obtained in step (2) was heated at a rate of 0.05 °C / min to 53.5 °C for the first - stage sweating for 120 min, continuously discharging the sweating liquid, and then heated at a rate of 0.01 / min to 54.5 °C for the second - stage sweating for 120 min, continuously discharging the sweating liquid. After the sweating was completed, high - purity caprolactam crystals were obtained;

[0054] (4) Heated at a rate of 2 °C / min to 70 °C to melt the caprolactam in the melt crystallizer and discharge it from the drain valve at the bottom, obtaining high - purity caprolactam and quickly nitrogen - sealing it. The yield of caprolactam was 85%;

[0055] (5) Enriched and mixed the mother liquor in step (2) and the sweating liquid discharged in step (3), and 85% was returned as the raw material for step (1), and the rest was discharged for incineration.

[0056] During the crystallization process, the purity of each component and the product quality are shown in Table 1.

[0057] Table 1

[0058]

[0059] Example 2

[0060] (1) Using the process schematic diagrams as shown in Figure 1 and Figure 2The process flow diagram shown has the caprolactam distillation residue withdrawn from the device at 100 °C, and cyclohexane accounting for 10% of the caprolactam distillation residue is added;

[0061] (2) Under a nitrogen atmosphere, the temperature is decreased at a rate of 0.05 °C / min, so that the temperature inside the melt crystallizer is decreased to 67 °C and maintained for 80 min, enabling the caprolactam distillation residue containing the solvent to fully crystallize in the melt crystallizer; after the caprolactam distillation residue has fully crystallized in the melt crystallizer, the drain valve at the bottom of the crystallizer is opened, and it is kept for 50 min to completely discharge the mother liquor of the crystallization;

[0062] (3) With the drain valve at the bottom open, the crude caprolactam obtained in step (2) is heated to 67.5 °C at a heating rate of 0.05 °C / min for primary sweating for 100 min, continuously discharging the sweating liquid, and then heated to 68.5 °C at a heating rate of 0.1 / min for secondary sweating for 100 min, continuously discharging the sweating liquid. After the sweating is completed, high-purity caprolactam crystals are obtained;

[0063] (4) It is heated to 80 °C at a heating rate of 5 °C / min to melt the caprolactam in the melt crystallizer and discharge it from the drain valve at the bottom, obtaining high-purity caprolactam and quickly sealing it with nitrogen. The yield of caprolactam is 83%;

[0064] (5) The mother liquor of the crystallization in step (2) and the sweating liquid discharged in step (3) are enriched and mixed, and 80% is returned as the raw material for step (1), and the rest is discharged for incineration.

[0065] During the crystallization process, the purity of each component and the product quality are shown in Table 2.

[0066] Table 2

[0067]

[0068] Example 3

[0069] (1) Using the process flow diagram as shown in Figure 1 and Figure 2 The caprolactam distillation residue withdrawn from the device is at 105 °C, and water accounting for 15 wt% of the caprolactam distillation residue is added;

[0070] (2) Under a nitrogen atmosphere, the temperature is decreased at a rate of 0.1 °C / min, so that the temperature inside the melt crystallizer is decreased to 22 °C and maintained for 30 min, enabling the caprolactam distillation residue containing the solvent to fully crystallize in the melt crystallizer; after the caprolactam distillation residue has fully crystallized in the melt crystallizer, the drain valve at the bottom of the crystallizer is opened, and it is kept for 30 min to completely discharge the mother liquor of the crystallization;

[0071] (3) With the bottom liquid discharge valve open, heat the crude caprolactam product obtained in step (2) at a heating rate of 0.1 °C / min to 23.5 °C for primary sweating for 30 min, continuously discharging the sweating liquid, and then heat it to 24.5 °C at a heating rate of 0.05 / min for secondary sweating for 30 min, continuously discharging the sweating liquid. After the sweating is completed, high-purity caprolactam crystals are obtained;

[0072] (4) Heat it to 80 °C at a heating rate of 5 °C / min to melt the caprolactam in the melt crystallizer and discharge it from the bottom liquid discharge valve to obtain high-purity caprolactam and quickly seal it with nitrogen. The yield of caprolactam is 57%;

[0073] (5) Enrich and mix the crystallization mother liquor in step (2) and the sweating liquid discharged in step (3), and return 90% as the raw material for step (1), and discharge the rest for incineration.

[0074] During the crystallization process, the purity of each component and the product quality are shown in Table 3.

[0075] Table 3

[0076]

[0077] Example 4

[0078] According to the method of Example 1, the difference is only that in step (1), 2 wt% of water and 3 wt% of methanol based on the caprolactam distillation residue are added. The specific steps are as follows:

[0079] (1) Adopt the process schematic diagrams as shown in Figure 1 and Figure 2 . The device extracts the caprolactam distillation residue at 95 °C, and adds 2 wt% of water and 3 wt% of methanol based on the caprolactam distillation residue;

[0080] (2) Under a nitrogen atmosphere, cool it at a cooling rate of 0.05 °C / min to lower the temperature in the melt crystallizer to 52 °C and maintain it for 120 min to fully crystallize the caprolactam distillation residue containing the solvent in the melt crystallizer; after the caprolactam distillation residue in the melt crystallizer is fully crystallized, open the bottom liquid discharge valve of the crystallizer and keep it for 60 min to completely discharge the crystallization mother liquor;

[0081] (3) With the bottom liquid discharge valve open, heat the crude caprolactam product obtained in step (2) at a heating rate of 0.05 °C / min to 53.5 °C for primary sweating for 120 min, continuously discharging the sweating liquid, and then heat it to 54.5 °C at a heating rate of 0.01 / min for secondary sweating for 120 min, continuously discharging the sweating liquid. After the sweating is completed, high-purity caprolactam crystals are obtained;

[0082] (4) Heat it at a heating rate of 2 °C / min to 80 °C to melt the caprolactam in the melt crystallizer and discharge it from the bottom drain valve, obtaining high-purity caprolactam and quickly sealing it with nitrogen. The yield of caprolactam is 86.3%;

[0083] (5) Enrich and mix the crystallization mother liquor in step (2) and the sweating liquid discharged in step (3), and return 85% as the raw material for step (1), and discharge the rest for incineration.

[0084] During the crystallization process, the purity of each component and the product quality are shown in Table 4.

[0085] Table 4

[0086]

[0087] Example 5

[0088] According to the method of Example 1, the difference is only that in step (1), water accounting for 10 wt% of the caprolactam distillation residue and cyclohexane accounting for 5 wt% of the caprolactam distillation residue are added. The specific steps are as follows:

[0089] (1) Adopt the process schematic diagram as shown in Figure 1 and Figure 2 , the device extracts the caprolactam distillation residue at 95 °C and adds cyclohexane accounting for 5 wt% of the caprolactam distillation residue;

[0090] (2) Under a nitrogen atmosphere, cool it at a cooling rate of 0.05 °C / min to lower the temperature in the melt crystallizer to 30 °C and maintain it for 120 min, so that the caprolactam distillation residue containing the solvent fully crystallizes in the melt crystallizer; after the caprolactam distillation residue in the melt crystallizer fully crystallizes, open the drain valve at the bottom of the crystallizer and keep it for 60 min to discharge all the crystallization mother liquor;

[0091] (3) With the bottom drain valve open, heat the crude caprolactam product obtained in step (2) to 31.5 °C at a heating rate of 0.05 °C / min for primary sweating for 120 min, continuously discharge the sweating liquid, and then heat it to 32.5 °C at a heating rate of 0.01 / min for secondary sweating for 120 min, continuously discharge the sweating liquid. After the sweating is completed, obtain high-purity caprolactam crystals;

[0092] (4) Heat it at a heating rate of 2 °C / min to 80 °C to melt the caprolactam in the melt crystallizer and discharge it from the bottom drain valve, obtaining high-purity caprolactam and quickly sealing it with nitrogen. The yield of caprolactam is 62.1%;

[0093] (5) Enrich and mix the crystallization mother liquor from step (2) and the sweating liquid discharged from step (3), return 85% as the raw material for step (1), and discharge the remaining for incineration.

[0094] The purity of each component and the product quality during the crystallization process are shown in Table 5.

[0095] Table 5

[0096]

[0097] Example 6

[0098] According to the method of Example 1, the difference is only that 20 wt% of water based on the caprolactam distillation residue is added in step (1). The specific steps are as follows:

[0099] (1) Using the process schematic diagrams as shown in Figure 1 and Figure 2 , the caprolactam distillation residue is taken out from the device at 95 °C, and 20 wt% of water based on the caprolactam distillation residue is added;

[0100] (2) Under a nitrogen atmosphere, cool at a cooling rate of 0.05 °C / min to lower the temperature in the melt crystallizer to 14 °C and maintain it for 120 min, so that the caprolactam distillation residue containing the solvent fully crystallizes in the melt crystallizer; after the caprolactam distillation residue in the melt crystallizer is fully crystallized, open the drain valve at the bottom of the crystallizer and keep it for 60 min to drain all the crystallization mother liquor;

[0101] (3) With the bottom drain valve open, heat the crude caprolactam product obtained in step (2) to 14.5 °C at a heating rate of 0.05 °C / min for primary sweating for 120 min, continuously discharge the sweating liquid, and then heat it to 15 °C at a heating rate of 0.01 / min for secondary sweating for 120 min, continuously discharge the sweating liquid. After the sweating is completed, high-purity caprolactam crystals are obtained;

[0102] (4) Heat to 70 °C at a heating rate of 2 °C / min to melt the caprolactam in the melt crystallizer and discharge it from the bottom drain valve to obtain high-purity caprolactam and quickly seal it with nitrogen. The yield of caprolactam is 42%;

[0103] (5) Enrich and mix the crystallization mother liquor from step (2) and the sweating liquid discharged from step (3), return 85% as the raw material for step (1), and discharge the remaining for incineration.

[0104] The purity of each component and the product quality during the crystallization process are shown in Table 6.

[0105] Table 6

[0106]

[0107] Example 7

[0108] According to the method of Example 1, the difference is only that 3 wt% of water is added to the caprolactam distillation residue in step (1). The specific steps are as follows:

[0109] (1) Using the process schematic diagram as shown in Figure 1 and Figure 2 , the caprolactam distillation residue taken out from the device is at 95°C, and 3 wt% of water based on the caprolactam distillation residue is added;

[0110] (2) Under a nitrogen atmosphere, the temperature is decreased at a rate of 0.05°C / min until the temperature in the melt crystallizer drops to 60°C and is maintained for 120 min, so that the caprolactam distillation residue containing the solvent fully crystallizes in the melt crystallizer; after the caprolactam distillation residue fully crystallizes in the melt crystallizer, the liquid discharge valve at the bottom of the crystallizer is opened, and it is maintained for 60 min to completely discharge the mother liquor;

[0111] (3) With the bottom liquid discharge valve open, the crude caprolactam product obtained in step (2) is heated to 60.5°C at a rate of 0.05°C / min for the first stage of sweating for 120 min, continuously discharging the sweating liquid, and then heated to 61°C at a rate of 0.01 / min for the second stage of sweating for 120 min, continuously discharging the sweating liquid. After the sweating is completed, high-purity caprolactam crystals are obtained;

[0112] (4) Heating to 70°C at a rate of 2°C / min to melt the caprolactam in the melt crystallizer and discharging it from the bottom liquid discharge valve to obtain high-purity caprolactam and quickly nitrogen seal it. The yield of caprolactam is 89%;

[0113] (5) Enrich and mix the mother liquor in step (2) and the sweating liquid discharged in step (3), return 85% as the raw material for step (1), and discharge the rest for incineration.

[0114] During the crystallization process, the purity of each component and the product quality are shown in Table 7.

[0115] Table 7

[0116]

[0117] Comparative Example 1

[0118] According to the method of Example 1, the difference is only that in step (1), water is not added to the caprolactam distillation residue. The specific steps are as follows:

[0119] (1) Using the process schematic diagram as shown in Figure 1 and Figure 2The process flow diagram shown has the caprolactam distillation residue drawn from the device at 95°C;

[0120] (2) Under a nitrogen atmosphere, cool at a rate of 0.05°C / min to lower the temperature in the melt crystallizer to 65°C and maintain for 120 min, so that the caprolactam distillation residue containing the solvent fully crystallizes in the melt crystallizer; after the caprolactam distillation residue has fully crystallized in the melt crystallizer, open the drain valve at the bottom of the crystallizer and keep it open for 60 min to completely discharge the mother liquor;

[0121] (3) With the bottom drain valve open, heat the crude caprolactam product obtained in step (2) at a rate of 0.05°C / min to 66.5°C for primary sweating for 120 min, continuously discharging the sweat liquid, and then heat at a rate of 0.01 / min to 67.5°C for secondary sweating for 120 min, continuously discharging the sweat liquid. After the sweating is completed, high-purity caprolactam crystals are obtained;

[0122] (4) Heat at a rate of 2°C / min to 80°C to melt the caprolactam in the melt crystallizer and discharge it from the bottom drain valve to obtain high-purity caprolactam and quickly seal it with nitrogen. The yield of caprolactam is 90%;

[0123] (5) After enriching and mixing the sweat liquid and the mother liquor, repeat steps (2) to (4), and use the obtained caprolactam product as the raw material for step (1).

[0124] During the crystallization process, the purity of each component and the product quality are shown in Table 8.

[0125] Table 8

[0126]

[0127] Comparative Example 2

[0128] (1) Adopt the process flow diagram shown in Figure 1 and Figure 2 The caprolactam distillation residue drawn from the device is at 95°C, and water accounting for 25 wt% of the caprolactam distillation residue is added;

[0129] (2) Under a nitrogen atmosphere, cool at a rate of 0.05°C / min to lower the temperature in the melt crystallizer to 5°C and maintain for 120 min, so that the caprolactam distillation residue containing the solvent fully crystallizes in the melt crystallizer; after the caprolactam distillation residue has fully crystallized in the melt crystallizer, open the drain valve at the bottom of the crystallizer and keep it open for 60 min to completely discharge the mother liquor;

[0130] (3) With the bottom liquid discharge valve open, heat the crude caprolactam product obtained in step (2) at a heating rate of 0.05 °C / min to 3.5 °C for the first sweating for 120 min, continuously discharging the sweating liquid, and then heat it at a heating rate of 0.01 / min to 4.5 °C for the second sweating for 120 min, continuously discharging the sweating liquid. After the sweating is completed, high-purity caprolactam crystals are obtained;

[0131] (4) Heat it at a heating rate of 2 °C / min to 70 °C to melt the caprolactam in the melt crystallizer and discharge it from the bottom liquid discharge valve to obtain high-purity caprolactam and quickly seal it with nitrogen. The yield of caprolactam is 62%;

[0132] (5) After enriching and mixing the sweating liquid and mother liquor, repeat steps (2) to (4), and use the obtained caprolactam product as the raw material for step (1).

[0133] During the crystallization process, the purity of each component and the product quality are shown in Table 9.

[0134] Table 9

[0135]

[0136] By comparing the data in Tables 1-9, it can be seen that the caprolactam crystals obtained in Examples 1-7 have good product quality. Among them, the caprolactam products obtained in Examples 1-5 all meet the excellent product standards of the technical requirements for industrial caprolactam, and the caprolactam product obtained in Example 7 meets the first-class product standards of the technical requirements for industrial caprolactam. The methods of Examples 1-7 have a high yield on the premise of ensuring the product quality of caprolactam crystals. In particular, the yields of Examples 1, 2, 4, and 7 all reach more than 80%.

[0137] In Comparative Example 1, water was not added to the caprolactam distillation residue, and the purity of the obtained caprolactam product was lower than 99%. The indexes such as chromaticity and alkalinity also did not meet the technical requirements for industrial caprolactam, so it was a non-conforming product. In Comparative Example 2, 25 wt% of water was added to the caprolactam distillation residue. Although the yield and purity of the product were high and it also met the excellent product standards of the technical requirements for industrial caprolactam, in step (2) of this method, it was necessary to cool to 5 °C to crystallize, consuming a large amount of chilled water. After calculation, the treatment cost per ton was about 30 yuan higher than that of Example 1, and the cost was too high and not suitable for industrial production.

[0138] As can be seen from the above examples and comparative examples, by adding solvents in different proportions, the crystallization process can be adjusted, thereby affecting the purity and yield of the final product. In the mixed solvent, water can remove water-soluble impurities in the caprolactam distillation residue, such as sodium hydroxide, aniline impurities, etc., thereby reducing the chromaticity, alkalinity, and volatile base indexes in the product; alcohols and hydrocarbons can remove oil-soluble impurities in the caprolactam distillation residue, such as 6-aminocaproic acid, oligomers, etc., thereby reducing the extinction value, PAN value, and volatile base in the product. In melt crystallization, it is necessary to comprehensively balance the relationship between yield, product quality, and cost. If too much solvent is added, the product quality can be guaranteed, but the yield and energy consumption will increase significantly. If too little solvent is added, impurities will be coated in the product crystals, affecting the product quality.

[0139] The present invention can effectively improve the purity of caprolactam by precisely controlling the temperature, solvent ratio, and sweating process. The yield is also affected by various factors, including the starting temperature, solvent ratio, sweating temperature, etc. Too high or too low temperature and solvent ratio may lead to a decrease in yield.

[0140] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the technical concept scope of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for purifying caprolactam distillation residue, characterized in that: The purification method comprises the following steps: (1) adding a solvent to the caprolactam distillation residue; wherein the solvent is selected from at least one of water, alcohols and hydrocarbons; and the amount of the solvent added is 3-20 wt % of the caprolactam distillation residue; (2) under an inert atmosphere, performing cooling crystallization to obtain a crude caprolactam product; (3) subjecting the crude caprolactam product to primary temperature rising sweating and secondary temperature rising sweating in sequence to obtain caprolactam crystals; wherein the temperature of the secondary temperature rising sweating is higher than the temperature of the primary temperature rising sweating.

2. The purification method according to claim 1, wherein The amount of the solvent added is 5-15 wt % of the caprolactam distillation residue.

3. The purification method according to claim 1 or 2, wherein: The alcohol is selected from at least one of methanol, ethanol, ethylene glycol and isopropanol; The hydrocarbon is selected from at least one of cyclopentane, cyclohexane, benzene, n-heptane and n-octane.

4. The purification method according to claim 3, wherein: The solvent is selected from one or more of water, methanol and cyclohexane.

5. The purification method according to any one of claims 1 to 4, wherein: The temperature of the caprolactam distillation residue is 95-105°C.

6. The purification method according to any one of claims 1 to 5, wherein: The step of cooling and crystallizing comprises: cooling to 15-90° C. at a cooling rate of 0.01-0.1° C. / min, and maintaining for 30-120 minutes.

7. The purification method according to any one of claims 1 to 6, wherein: The first-level temperature rise and sweating step includes: heating to 15-75°C at a heating rate of 0.01-0.1°C / min, maintaining for 30-120 minutes, and discharging sweat liquid.

8. The purification method according to any one of claims 1 to 7, wherein: The second stage temperature rise and sweating step includes: heating to 16-78°C at a heating rate of 0.01-0.1°C / min, maintaining for 30-120 minutes, and discharging sweat liquid.

9. The purification method according to any one of claims 1 to 8, wherein: The purification method further comprises: (4) heating the temperature to 18-65° C. at a heating rate of 1-5° C. / min to melt the caprolactam crystals obtained in step (3), and then nitrogen sealing.

10. The purification method according to claim 9, wherein: The purification method further comprises: mixing the crystallization mother liquor after cooling crystallization and the sweating liquid discharged from the first-stage heating sweating and the second-stage heating sweating, repeating steps (2) to (4); and then using the obtained caprolactam product as the raw material of step (1).

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