Method for separating caprolactam and 6-aminocapronitrile at low temperature

Abstract

The invention relates to a method for low-temperature separation of caprolactam and 6-aminocapronitrile, and aims to recover caprolactam from a reaction light component removal liquid for ring-opening synthesis of 6-aminocapronitrile from caprolactam. The method specifically comprises the following steps: carrying out gradient cooling treatment on a raw material solution containing caprolactam and 6-aminocapronitrile to obtain a caprolactam crude product crystal and a mother solution containing 6-aminocapronitrile, carrying out high-vacuum low-temperature refining on the caprolactam crude product, effectively avoiding the use of a leaching solvent, obtaining refined caprolactam, and recovering high-purity 6-aminocapronitrile from the mother solution. Gradient crystallization and high-vacuum low-temperature refining separation technologies are used, high-quality recovery of the raw material caprolactam is achieved, the yield of the caprolactam is larger than 91%, the purity of the caprolactam is larger than 96%, meanwhile, the purity of 6-aminocapronitrile recovered from the mother liquor reaches 99%, and the problems that due to traditional vacuum refining separation, heat-sensitive substances are likely to be decomposed, and the purity of 6-aminocapronitrile is higher than 99% are effectively solved. And the operation safety is effectively improved.

Description

Technical Field

[0001] This invention relates to the field of organic separation technology, specifically to a method for separating a mixture of caprolactam and 6-aminohexanonitrile using low-temperature phase change differences. Background Technology

[0002] The mainstream methods for synthesizing hexamethylenediamine currently include the adiponitrile hydrogenation method, the butadiene method, the adipic acid method, and the caprolactam method. Given the sufficient domestic caprolactam production capacity and the mature technology for recovering caprolactam from nylon 6 and substandard nylon 6, the technology for producing hexamethylenediamine via ring-opening of 6-aminohexanonitrile and further hydrogenation using the caprolactam method has been developed and industrialized. Its advantages are: firstly, compared to the mainstream adiponitrile hydrogenation method, the raw material caprolactam has a certain production capacity in China, ensuring ample supply; secondly, it offers high selectivity, allowing for the directional breaking of chemical bonds to generate the target product through catalyst optimization, such as supported metal catalysts, with byproduct production controlled below 5%; and thirdly, it offers environmental advantages, avoiding the use of highly toxic cyanide compared to the butadiene method, effectively reducing wastewater volume, and the wastewater has a lower organic content, making it easier to treat.

[0003] Based on existing industrial caprolactam processes, caprolactam is synthesized into hexamethylenediamine through ring-opening and hydrogenation under catalytic conditions. The ring-opening reaction solution contains a certain amount of light components, the raw material caprolactam, the ring-opening product 6-aminohexanonitrile, and heavy components. The hydrogenation of 6-aminohexanonitrile to prepare hexamethylenediamine under catalytic conditions requires a certain purity (content > 99%). Research indicates that current reports show a conversion rate of 50-85% for the caprolactam process. Incomplete conversion of the raw material and the urgent need to recover the caprolactam are problems. Traditional separation and purification methods use vacuum distillation, which is lengthy and cumbersome. Furthermore, due to the presence of side reactions in the ring-opening reaction, a high number of trays is required in the separation column to achieve the required purity of the intermediate and recovered raw material, resulting in high equipment investment. Additionally, high-temperature distillation leads to high energy consumption and is prone to decomposition of heat-sensitive substances. Moreover, solvent extraction methods suffer from difficulties in solvent recovery and environmental pollution.

[0004] Therefore, it is still necessary to explore green separation processes. Summary of the Invention

[0005] This invention aims to provide a continuous reaction solution for the ring-opening synthesis of 6-aminohexanonitrile from caprolactam after a light-weight component removal pretreatment, and to recover the caprolactam feedstock. The reaction solution after light-weight component removal pretreatment mainly contains 12-15% caprolactam, 79-84% 6-aminohexanonitrile, and 4-6% heavy components. After gradient cooling separation and recovery of caprolactam, the final mother liquor contains over 99% 6-aminohexanonitrile, which can be directly used for subsequent hydrogenation synthesis of hexamethylenediamine.

[0006] This invention provides a method for the low-temperature separation of caprolactam and 6-aminohexanonitrile. A feed solution containing both caprolactam and 6-aminohexanonitrile is subjected to gradient cooling treatment to obtain crude caprolactam crystals and a mother liquor containing 6-aminohexanonitrile. The crude caprolactam crystals are then purified under high vacuum and low temperature conditions to remove entrained impurities and avoid the use of rinsing solvents, yielding caprolactam. The gradient cooling treatment is carried out in a multi-stage crystallizer, which has at least three stages. Increasing the number of stages appropriately improves the purity of the final mother liquor containing 6-aminohexanonitrile; preferably, 3 to 5 stages are used.

[0007] In one or more specific embodiments, the multi-stage crystallizer is a three-stage crystallizer, including a first-stage crystallizer, a second-stage crystallizer, and a third-stage crystallizer. The gradient cooling is set according to the crystallization process of the substance, which improves the stability of the crystallization process and the yield of the target product.

[0008] In one or more specific embodiments, the cooling rate of the first-stage crystallizer is 0.1~3℃ / 5min, preferably 0.1~1.5℃ / 5min, the final temperature is 2~15℃, preferably 5~15℃, and the holding time is 0.2~4h, preferably 0.5~3h.

[0009] In one or more specific embodiments, the cooling rate of the second-stage crystallizer is 2~6℃ / h, preferably 2~4℃ / h, the final temperature is -5~5℃, preferably -4~3℃, and the holding time is 1~7h, preferably 1.5~4h.

[0010] In one or more specific embodiments, the cooling rate of the third-stage crystallizer is 2~15℃ / h, preferably 4~12℃ / h, the final temperature is -18~-5℃, preferably -12~-5℃, and the holding time is 0.5~4h, preferably 1~3h.

[0011] The raw material solution is cooled in the first-stage crystallizer to obtain a primary precipitate and a primary mother liquor. The primary precipitate consists of heavy components and a small amount of caprolactam solid. The primary precipitate and the primary mother liquor are separated. The primary mother liquor is cooled in the second-stage crystallizer to obtain a secondary precipitate and a secondary mother liquor. The secondary precipitate consists of a large amount of caprolactam solid. The secondary precipitate and the secondary mother liquor are separated. The secondary mother liquor is cooled in the third-stage crystallizer to obtain a tertiary precipitate and a tertiary mother liquor. The tertiary precipitate consists of a small amount of caprolactam solid. The tertiary precipitate and the tertiary mother liquor are separated. The tertiary precipitate and the tertiary mother liquor are cooled in the crystallizer to obtain a tertiary precipitate and a tertiary mother liquor. The tertiary precipitate and the tertiary mother liquor are separated. The secondary and tertiary precipitates are collected and purified under vacuum to obtain caprolactam. 6-aminohexanonitrile is mainly present in the tertiary mother liquor.

[0012] The primary precipitates are mainly recombinant precipitates, the secondary precipitates are mainly caprolactam primary precipitates, and the tertiary precipitates are mainly caprolactam secondary precipitates.

[0013] In one or more specific embodiments, the separation method for the primary precipitate includes, but is not limited to, centrifugation, sedimentation, pressure filtration, vacuum filtration and membrane separation, with centrifugation or sedimentation being preferred.

[0014] In one or more specific embodiments, the separation method for secondary precipitates includes, but is not limited to, centrifugation, filtration, and sludge removal.

[0015] In one or more specific embodiments, a co-solvent is added to the first-stage crystallizer or the second-stage crystallizer to increase the solubility of the substance during crystallization, reduce the viscosity of the system, and effectively increase the yield of caprolactam. The co-solvent is selected from one or more of ethylene glycol, a mixture of ethylene glycol and water, and N,N-dimethylacetamide.

[0016] In one or more specific embodiments, the co-solvent is a mixture of ethylene glycol and water, with an ethylene glycol to water ratio of 95~75:5~25. Adding water to ethylene glycol as a co-solvent induces the crystallization temperature of the ethylene glycol-water system to approach the crystallization temperature of caprolactam. With adjusted stirring, ethylene glycol tends to dissolve towards 6-aminohexanonitrile, while water tends to dissolve towards caprolactam. Therefore, adding an appropriate amount of the ethylene glycol-water mixed solvent system during the cooling process can simultaneously achieve the dual benefits of promoting crystallization and promoting co-solubility.

[0017] In one or more specific embodiments, the amount of co-solvent is 5-40% of the total amount of the raw material solution and the co-solvent. Too much co-solvent will lead to further loss of caprolactam, while too little will not achieve the promoting effect.

[0018] The co-solvent effect in this invention tends to be positive, that is, it increases the solubility of caprolactam, further slowly reduces the temperature to recover caprolactam, and improves the recovery rate.

[0019] The addition of a co-solvent is optional. Using a co-solvent can effectively improve the quality of recovered caprolactam, including crystal form, appearance, and color. If a co-solvent is used, its recovery includes evaporation recovery and crystallization recovery. Crystallization recovery of the co-solvent is preferred as it is easier to apply.

[0020] In one or more specific embodiments, a fourth-stage crystallizer is added for low-temperature crystallization to separate the co-solvent. The third-stage mother liquor is cooled in the fourth-stage crystallizer to obtain co-solvent solid crystals and the fourth-stage mother liquor, in which 6-aminohexanonitrile is present, with a purity of over 99%.

[0021] Without a co-solvent, the 6-aminohexanonitrile content in the third-stage mother liquor reaches over 99%, but the recovered caprolactam exhibits significant agglomeration during crystallization, resulting in severe impurity entrainment and increased purification energy consumption. When a co-solvent is used, the third-stage mother liquor comprises both 6-aminohexanonitrile and the co-solvent. Since the co-solvent has a lower melting point than 6-aminohexanonitrile, only one crystallizer is needed. Through cooling crystallization, the co-solvent separates from the 6-aminohexanonitrile in solid crystal form, effectively improving the system viscosity, mitigating the caprolactam agglomeration problem during crystallization, and reducing impurity entrainment.

[0022] In one or more specific embodiments, a primary crystal nucleus is added to the second-stage crystallizer. The primary crystal nucleus is selected from one or more of caprolactam powder, silica particles, kaolin, mica sheets, and surface-modified PMMA, with caprolactam powder being preferred.

[0023] In one or more specific embodiments, the amount of crystal nuclei added at one time is 0.02 to 1% of the feed mass flow rate.

[0024] In one or more specific embodiments, a secondary crystal nucleus is added to the third-stage crystallizer. The secondary crystal nucleus is selected from one or more of caprolactam powder, silica particles, kaolin, and mica sheets, preferably one or two of caprolactam powder and mica sheets.

[0025] In one or more specific embodiments, the amount of secondary nuclei added is 0.03 to 0.3% of the feed mass flow rate.

[0026] In one or more specific embodiments, the feed temperature of the first-stage crystallizer is ≤5℃, and the feed temperature of the second-stage crystallizer is ≤-5℃.

[0027] In one or more specific embodiments, the multi-stage crystallizer is selected from one or more combinations of jacketed stirred crystallizer, continuous fluidized bed crystallizer, multi-stage countercurrent crystallizer, and vacuum cooled crystallizer.

[0028] In one or more specific embodiments, after combining the primary and secondary caprolactam precipitates, the mixture is placed in a vacuum purification apparatus and slowly heated to remove residual 6-aminohexanonitrile under certain vacuum and temperature conditions.

[0029] In one or more specific embodiments, the vacuum purification apparatus includes one or more reaction vessels and a vacuum light-weight removal tower. The basic structure includes a reaction vessel, a vacuum light-weight removal tower, and a tail gas absorption tower. The operation under the aforementioned low-temperature, high-vacuum conditions effectively avoids the decomposition of heat-sensitive substances and also avoids the use of rinsing solvents, resulting in high-purity caprolactam.

[0030] In one or more specific embodiments, the heating rate of the vacuum light removal tower is 5~50℃ / h, preferably 20~40℃ / h, the final temperature is 82~100℃, the vacuum degree is ≤1.2Kpa, and the vacuum degree is required to change positively with temperature.

[0031] In one or more specific embodiments, the feed to the refining unit column should preferably be a bottom feed.

[0032] In one or more specific embodiments, the multi-stage crystallizer is equipped with a reflux system, in which the mother liquor of at least one stage crystallizer is refluxed or returned to the next stage crystallizer, and the reflux flow rate is 15-60% of the feed flow rate, preferably 15-45%.

[0033] In one or more specific embodiments, the feed solution is a reaction solution for the ring-opening synthesis of 6-aminohexanonitrile from caprolactam, and is subjected to a light component removal pretreatment; the feed solution contains 12-15% caprolactam, 79-84% 6-aminohexanonitrile and 4-6% heavy components.

[0034] In one or more specific embodiments, the yield of caprolactam reaches 91% or more, and the purity of caprolactam reaches 96% or more, preferably 99% or more.

[0035] Compared with the prior art, the present invention has the following advantages:

[0036] (1) The innovative use of low temperature technology effectively avoids the decomposition of heat-sensitive substances that may be caused by high temperature decoking and vacuum refining separation, thus effectively improving operational safety;

[0037] (2) Using a multi-stage crystallizer combination, multi-stage countercurrent crystallization can be designed to achieve continuous production, multi-stage separation and recovery of caprolactam, and the purity of 6-aminohexanonitrile in the mother liquor after crystallization and separation is greater than 99%, which can be directly used for hydrogenation to prepare hexamethylenediamine;

[0038] (3) The coupling vacuum separation technology effectively avoids the use of rinsing solvent, avoids the influence of introducing new impurities, reduces the cumbersome operation of rinsing solvent recovery and reuse, and the purity of recovered caprolactam reaches more than 96%, with the best reaching more than 99%, and the yield of caprolactam reaches more than 91%.

[0039] (4) Innovative use of co-solvents reduces viscosity during crystallization, effectively increases crystallization rate, and reduces impurity entrainment;

[0040] (5) Using low temperature technology and vacuum separation technology, there is no waste gas or waste liquid. The generated heavy component waste solids can be further recycled, making it green and environmentally friendly. Attached Figure Description

[0041] Figure 1 This is a process flow diagram of one embodiment of the present invention.

[0042] Figure 2 This is a process flow diagram of another embodiment of the present invention. Detailed Implementation

[0043] In this document, all features defined by numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0044] The present invention will now be described in detail with reference to specific embodiments, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0045] Figure 1 This invention discloses a low-temperature separation process for caprolactam and 6-aminohexanonitrile. The reaction solution from the ring-opening synthesis of 6-aminohexanonitrile from caprolactam undergoes a light component removal pretreatment to obtain a light component-removed liquid. This liquid enters a multi-stage cooler and is cooled in the first-stage cooling crystallizer. After primary crystallization, the heavy components and a small amount of caprolactam solid precipitate are collected at the bottom. The resulting primary mother liquor is cooled in the second-stage cooling crystallizer. After secondary crystallization, a large amount of caprolactam solid precipitates. The resulting secondary mother liquor is cooled in the third-stage cooling crystallizer. After tertiary crystallization, a small amount of caprolactam solid precipitates. The resulting tertiary mother liquor contains 6-aminohexanonitrile with a purity greater than 99%. The caprolactam solids recovered from the second and third-stage crystallizers are combined and placed in a vacuum purification device. Under controlled vacuum and temperature conditions, residual 6-aminohexanonitrile is removed through vacuum treatment, yielding caprolactam with a purity of over 96%.

[0046] Figure 2This invention provides another low-temperature separation process for caprolactam and 6-aminohexanonitrile. The reaction solution from the ring-opening synthesis of 6-aminohexanonitrile from caprolactam undergoes a light-weight component removal pretreatment to obtain a light-weight component removal liquid. This liquid enters a multi-stage cooler and is cooled in the first-stage cooling crystallizer. After primary crystallization, the heavy components and a small amount of caprolactam solid precipitate are collected at the bottom. The resulting primary mother liquor is cooled in the second-stage cooling crystallizer. A co-solvent (with a melting point lower than 6-aminohexanonitrile) is added to the second-stage cooling crystallizer. After secondary crystallization, a large amount of caprolactam solid precipitates. The resulting secondary mother liquor is cooled in the third-stage cooling crystallizer. After tertiary crystallization, a small amount of caprolactam solid precipitates. The resulting tertiary mother liquor is cooled and crystallized in the fourth-stage cooling crystallizer. The co-solvent solid is recovered, and the resulting fourth-stage mother liquor contains 6-aminohexanonitrile with a purity greater than 99%. When the 6-aminohexanonitrile content in the co-solvent reaches 5% or more, a distillation column (not shown) is used. Under certain pressure and temperature conditions, the co-solvent is purified, with 6-aminohexanonitrile collected at the top and purified ethylene glycol at the bottom. The caprolactam solids recovered from the second and third stage crystallizers are combined and placed in a vacuum purification device. Under controlled vacuum and temperature conditions, residual 6-aminohexanonitrile is removed through vacuum treatment, yielding caprolactam with a purity of over 96%.

[0047] All examples and comparative examples used the same composition of the light-removed liquid. The composition of the light-removed liquid, as determined by GC gas chromatography normalization, was: 4.2% heavy components, 12.4% caprolactam, and 83.4% 6-aminohexanonitrile.

[0048] The yield is calculated using the following formula:

[0049] Recombinant fraction yield = (Recombinant fraction in primary precipitate / Recombinant fraction in feedstock) * 100%;

[0050] Caprolactam yield = (Amount of caprolactam recovered under vacuum / Amount of caprolactam in raw material) * 100%;

[0051] 6-Aminohexanonitrile yield = [(weight of mother liquor * content of 6-aminohexanonitrile + weight gain of spray absorption liquid) / weight of 6-aminohexanonitrile in raw material] * 100%.

[0052] Examples 1-11 are examples of the low-temperature separation of caprolactam and 6-aminohexanonitrile of the present invention.

[0053] Example 1

[0054] A three-stage cooling crystallizer and Figure 1 The production process flow.

[0055] The first, second, and third stage crystallizers are all plate-cooled crystallizers. The first and second stage crystallizers are connected in series with overflow. A filter is installed between the first and second stage crystallizers. The first stage crystallizer has self-reflux, and the third stage crystallizer has reflux between the second stage crystallizer. The refrigerant for each stage crystallizer is supplied separately.

[0056] The light-removed liquid was pumped into the first-stage crystallizer at a flow rate of 5 kg / h and a feed temperature of 20°C, for a total feed of 100 kg. The cooling rate of the first-stage crystallizer was 0.8°C / 5 min, with an endpoint temperature of 5-8°C. The residence time of the light-removed liquid was 2.5 h, and the reflux flow rate was 3 kg / h. The primary precipitate in the first-stage crystallizer was filtered using a filter screen, and 4.98 kg of the primary precipitate was collected. The primary mother liquor was then fed into the second-stage crystallizer.

[0057] 250g of caprolactam powder was added to the second-stage crystallizer as crystal nuclei in a single batch. The cooling rate of the second-stage crystallizer was 0.2℃ / 5min, with an endpoint temperature of -3~1℃. The feed temperature of the first-stage mother liquor was below 5℃, and the residence time of the first-stage mother liquor was 3.5h. The secondary precipitate obtained in the second-stage crystallizer was filtered using a centrifuge, yielding 14.04kg of secondary precipitate. The collected secondary mother liquor was pumped into the third-stage crystallizer at a rate of 5kg / h.

[0058] The cooling rate of the third-stage crystallizer is 0.3℃ / 5min, with an endpoint temperature of -10~-5℃. The feed temperature of the secondary mother liquor is 0℃, and the residence time of the secondary mother liquor is 2h. The third-stage crystallizer is configured to reflux to the second-stage crystallizer at a flow rate of 2kg / h. The tertiary precipitate in the third-stage crystallizer is filtered using a filter screen to obtain 0.42kg of tertiary precipitate, and the tertiary mother liquor is collected.

[0059] The secondary and tertiary precipitates are mixed and added to a distillation column. The column bottom temperature is controlled to not exceed 100℃ and the vacuum degree is 1.2KPa. 6-aminohexanonitrile is removed by vacuum. A spray absorption column is added to the tail gas as an absorption solvent to recover 6-aminohexanonitrile from the tail gas.

[0060] The composition of each precipitate was determined by testing and is as follows:

[0061] Primary precipitates: 80.05% heavy components, 14.93% caprolactam, and 5.02% 6-aminohexanonitrile;

[0062] Secondary precipitates: 0.60% heavy components, 78.61% caprolactam, and 20.79% 6-aminohexanonitrile;

[0063] Tertiary precipitates: 2.37% heavy components, 59.37% caprolactam, and 39.26% 6-aminohexanonitrile.

[0064] The purity of caprolactam obtained by distillation was 99.06%.

[0065] The purity of 6-aminohexanonitrile in the third-grade mother liquor was 99.394%.

[0066] The yields of each component were calculated, with the yield of the heavy component being 95%, caprolactam yield being 91%, and 6-aminohexanonitrile yield being 96%.

[0067] Example 2

[0068] A four-stage cooling crystallizer and Figure 2 The production process flow.

[0069] The first, second, third, and fourth stage crystallizers are all plate-cooled crystallizers. The first and second stage crystallizers are connected in series with overflow. A filter screen is installed between the first and second stage crystallizers. The first stage crystallizer is equipped with self-recirculation, and the third and second stage crystallizers are equipped with recirculation. The refrigerant for each stage crystallizer is supplied separately.

[0070] The light-removed liquid was pumped into the first-stage crystallizer at a flow rate of 5 kg / h and a feed temperature of 20°C, for a total feed of 100 kg. The cooling rate of the first-stage crystallizer was 3°C / 5 min, with an endpoint temperature of 5-8°C. The residence time of the light-removed liquid was 2.5 h, and the reflux flow rate was 3 kg / h. The primary precipitate in the first-stage crystallizer was filtered using a filter screen, and the primary mother liquor entered the second-stage crystallizer.

[0071] 500g of caprolactam powder was added at once to the second-stage crystallizer as crystal nuclei. Ethylene glycol, a co-solvent, was continuously added to the second-stage crystallizer at a rate of 0.5kg / h, accounting for approximately 10%. The cooling rate of the second-stage crystallizer was 0.3℃ / 5min, with an endpoint temperature of -3~1℃. The feed temperature of the first-stage mother liquor was below 8℃, and the residence time of the first-stage mother liquor was 3.5h. The secondary precipitate obtained in the second-stage crystallizer was filtered using a centrifuge, and the collected secondary mother liquor was pumped into the third-stage crystallizer at a rate of 5kg / h.

[0072] 50g of mica flakes were added to the third-stage crystallizer in a single batch. The cooling rate of the third-stage crystallizer was 0.35℃ / 5min, with an endpoint temperature of -10~-5℃. The feed temperature of the secondary mother liquor was 0℃, and the residence time of the secondary mother liquor was 2h. The third-stage crystallizer was configured to reflux to the second-stage crystallizer at a flow rate of 2kg / h. The tertiary precipitate in the third-stage crystallizer was filtered using a filter screen, and the tertiary mother liquor was collected.

[0073] The third-stage mother liquor enters the fourth-stage crystallizer. The cooling rate of the fourth-stage crystallizer is 0.5℃ / 5min, the final temperature is -16~-12℃, and the residence time of the third-stage mother liquor is 1.5h, yielding a co-solvent solid and the fourth-stage mother liquor. The purity of 6-aminohexanonitrile in the fourth-stage mother liquor is greater than 99%.

[0074] The secondary and tertiary precipitates are mixed and added to a distillation column. The temperature of the column bottom is controlled to not exceed 100°C. 6-aminohexanonitrile is removed under vacuum. A spray absorption column is added to the tail gas to absorb 6-aminohexanonitrile as the absorption solvent, and 6-aminohexanonitrile in the tail gas is recovered.

[0075] The cosolvent recovered from the fourth stage can be reused across multiple batches. When the 6-aminohexanonitrile content in the cosolvent reaches 5% or higher, a distillation column is used. Under controlled conditions of 1.2 kPa, the bottom temperature is controlled to ≤60℃, and the reflux ratio is controlled to 0.5. The 6-aminohexanonitrile is collected from the top of the column and recycled to the fourth-stage crystallizer, thus recovering the 6-aminohexanonitrile entrained in the cosolvent. The bottom of the column contains purified cosolvent ethylene glycol with a purity of up to 99%, which is recycled. The yield and quality of caprolactam are shown in Table 1.

[0076] Examples 3-5

[0077] The difference from Example 2 is that the co-solvent is adjusted to be a mixed solution of ethylene glycol and water, wherein the proportion of water in the co-solvent is 5%, 10%, and 30%, respectively. Crystal nuclei are not added to the second-stage crystallizer and the third-stage crystallizer. Other operations are the same as in Example 2.

[0078] The yield and quality of caprolactam are shown in Table 1.

[0079] Table 1. Yield and quality of caprolactam in Examples 2-5

[0080] It is evident that using hexanediol can effectively improve the recovery rate and quality of caprolactam. Using a mixed solution of hexanediol and water can further enhance the recovery rate and quality of the recovered caprolactam. However, as the water content in the co-solvent increases, the caprolactam yield initially rises and then falls. This is because the addition of a large amount of water causes caprolactam to cascade (resulting in large agglomerates), resulting in the loss of a significant amount of 6-aminohexanonitrile. Consequently, both the purity of the caprolactam and the yield of 6-aminohexanonitrile decrease. Therefore, it is best to control the mass ratio of the ethylene glycol-water mixed co-solvent to 5-25% to reduce the addition of crystal nuclei and simultaneously improve the quality of the target product.

[0081] Examples 6-9

[0082] Based on Example 1, the cooling rate of the second-stage crystallizer was changed to 1℃ / h, 3℃ / h, 5℃ / h, and 10℃ / h, while the holding time remained constant. The yield and quality of caprolactam are shown in Table 2.

[0083] Table 2. Yields and quality of caprolactam in Examples 6-9

[0084] It is evident that crystallization at low rates is beneficial for improving the quality and yield of caprolactam, but it requires higher energy consumption and longer residence time. Excessive cooling can lead to a decrease in the purity of caprolactam.

[0085] Comparative Example 1

[0086] Based on Example 1, the purification of caprolactam was not carried out under vacuum. The yield and quality of caprolactam are shown in Table 3.

[0087] Table 3. Yield and quality of caprolactam in Comparative Example 1

[0088] Comparative Example 2

[0089] 100 kg of the de-coking liquid was taken at a feed rate of 5 kg / h and continuously processed through decoking, primary distillation, and secondary distillation to separate caprolactam and 6-aminohexanonitrile. Decoking, primary distillation, and secondary distillation were carried out in different distillation columns to extract the heavy components, 6-aminohexanonitrile, and caprolactam, respectively.

[0090] The temperature of the bottom of the coke removal tower is 180℃. 5.48 kg of material was collected from the bottom of the coke removal tower, which contained 65.21% heavy components, 27.18% caprolactam, and 7.62% 6-aminohexanonitrile.

[0091] The bottom temperature of the primary distillation column is 170℃. The top product is 6-aminohexanonitrile, containing 0.51% heavy components, 0.15% caprolactam, and 99.34% 6-aminohexanonitrile.

[0092] The bottom temperature of the secondary distillation column is 190℃. Caprolactam is collected from the top of the column, containing 1.84% heavy components, 94.51% caprolactam, and 3.65% 6-aminohexanonitrile.

[0093] In total, the yield of the heavy components was 104.86%, the yield of caprolactam was 87%, and the yield of 6-aminohexanonitrile was 98%. Due to the possible decomposition and coking of heat-sensitive substances during the high-temperature refining process, the yield of the heavy components was abnormal, and the yield of caprolactam was relatively low.

[0094] The above examples and comparative examples demonstrate that the low-temperature phase change differential separation method for efficiently separating a mixture of caprolactam and 6-aminohexanonitrile provided by this invention is superior to traditional distillation methods. By combining low-temperature separation and vacuum separation technologies, high-quality recovery of the raw material caprolactam is achieved, while ensuring a 99% purity of 6-aminohexanonitrile in the mother liquor. This invention, through a green low-temperature phase change crystallization separation method, effectively avoids the decomposition of heat-sensitive substances caused by high temperatures. The crystallization device is integrable, reflux is adjustable, the equipment occupies a small area, requires less investment, has good sustainability, the co-solvent can be recycled and reused, the operating temperature is low, and it is safer and more stable.

Claims

1. A process for the cryogenic separation of caprolactam and 6-aminocapronitrile, characterized in that, The raw material liquid containing caprolactam and 6-aminocapronitrile is subjected to gradient cooling treatment to obtain caprolactam crude product crystals and mother liquor containing 6-aminocapronitrile, and the caprolactam crude product crystals are subjected to high-vacuum low-temperature refining to obtain caprolactam; the gradient cooling treatment is carried out in a multi-stage crystallizer, and the number of stages of the multi-stage crystallizer is at least 3 stages, preferably 3-5 stages.

2. The process for cryogenic separation of caprolactam and 6-aminocapronitrile according to claim 1, characterized in that, The number of stages of the multi-stage crystallizer is 3 stages, wherein: The cooling rate of the first-stage crystallizer is 0.1-3 ℃ / 5 min, preferably 0.1-1.5 ℃ / 5 min, the end-point temperature is 2-15 ℃, preferably 5-15 ℃, and the holding residence time is 0.2-4 h, preferably 0.5-3 h; The cooling rate of the second-stage crystallizer is 2-6 ℃ / h, preferably 2-4 ℃ / h, the end-point temperature is -5-5 ℃, preferably -4-3 ℃, and the holding residence time is 1-7 h, preferably 1.5-4 h; The cooling rate of the third-stage crystallizer is 2-15 ℃ / h, preferably 4-12 ℃ / h, the end-point temperature is -18--5 ℃, preferably -12--5 ℃, and the holding residence time is 0.5-4 h, preferably 1-3 h; The raw material liquid is subjected to cooling treatment in the first-stage crystallizer to obtain first-stage precipitates and first-stage mother liquor, and the first-stage precipitates and first-stage mother liquor are separated; the first-stage mother liquor is subjected to cooling treatment in the second-stage crystallizer to obtain second-stage precipitates and second-stage mother liquor, and the second-stage precipitates and second-stage mother liquor are separated; the second-stage mother liquor is subjected to cooling treatment in the third-stage crystallizer to obtain third-stage precipitates and third-stage mother liquor, and the third-stage precipitates and third-stage mother liquor are separated, the second-stage precipitates and third-stage precipitates are collected, and subjected to vacuum refining to obtain caprolactam, and 6-aminocapronitrile mainly exists in the third-stage mother liquor.

3. The method for low-temperature separation of caprolactam and 6-aminocapronitrile according to claim 2, characterized in that, A cosolvent is added to the first-stage crystallizer or the second-stage crystallizer, the cosolvent is selected from one or more of ethylene glycol, a mixture of ethylene glycol and water, and N,N-dimethylacetamide, and the amount of the cosolvent is 5-40% of the total amount of the raw material liquid and the cosolvent; preferably, the cosolvent is a mixture of ethylene glycol and water, and the ratio of ethylene glycol to water is 95-75:5-25.

4. The process for cryogenic separation of caprolactam and 6-aminocapronitrile according to claim 3, characterized in that, A fourth-stage crystallizer is added, and the third-stage mother liquor is subjected to cooling treatment in the fourth-stage crystallizer to obtain cosolvent crystals and fourth-stage mother liquor, and 6-aminocapronitrile mainly exists in the fourth-stage mother liquor.

5. The method for low-temperature separation of caprolactam and 6-aminocapronitrile according to claim 2, characterized in that, Primary crystal nuclei are added to the second-stage crystallizer, the primary crystal nuclei are selected from one or more of caprolactam powder, silica particles, kaolin, mica flakes, and surface-modified PMMA, and preferably caprolactam powder; The addition amount of the primary crystal nuclei is 0.02-1% of the mass flow rate of the feed.

6. The method for low-temperature separation of caprolactam and 6-aminocapronitrile according to claim 2, characterized in that, The secondary crystal nucleus is added in the third stage crystallizer, and the secondary crystal nucleus is selected from one or more of caprolactam powder, silica particles, kaolin, and mica sheet, preferably one or both of caprolactam powder and mica sheet. The amount of the secondary crystal nucleus added is 0.03-0.3% of the mass flow rate of the feed.

7. The process for cryogenic separation of caprolactam and 6-aminocapronitrile according to claim 2, characterized in that, The feed temperature of the first stage crystallizer is ≤5°C, and the feed temperature of the second stage crystallizer is ≤-5°C.

8. The process for cryogenic separation of caprolactam and 6-aminocapronitrile according to claim 2, characterized in that, The multi-stage crystallizer is selected from a combination of one or more of a jacketed stirred crystallizer, a continuous fluidized bed crystallizer, a multi-stage countercurrent crystallizer, and a vacuum cooling crystallizer.

9. The method for low-temperature separation of caprolactam and 6-aminocapronitrile according to claim 2, characterized in that, The vacuum refining is performed in a device under high vacuum and low temperature, and the vacuum refining device comprises a material mixing kettle and a vacuum light component removal tower. The temperature rising rate of the vacuum light component removal tower is 5-50°C / h, preferably 20-40°C / h, and the terminal temperature is 82-100°C, and the vacuum degree is ≤1.2 KPa.

10. The process for cryogenic separation of caprolactam and 6-aminocapronitrile according to claim 2, characterized in that, The multi-stage crystallizer is provided with reflux, and the mother liquor of at least one stage of the crystallizer is self-refluxed or refluxed to the upper stage of the crystallizer, and the reflux flow rate is 15-60% of the feed flow rate, preferably 15-45%.

11. The method for low-temperature separation of caprolactam and 6-aminocapronitrile according to claim 1, characterized in that, The raw material liquid is a reaction liquid for synthesizing 6-aminocapronitrile from caprolactam by ring opening, and is pretreated by removing light components. The raw material liquid contains 12-15% of caprolactam, 79-84% of 6-aminocapronitrile, and 4-6% of heavy components.

12. The process for cryogenic separation of caprolactam and 6-aminocapronitrile according to claim 1, characterized in that, The yield of the caprolactam is greater than 91%, and the purity of the caprolactam is greater than 96%, preferably greater than 99%.

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