Method for recycling of caprolactam distillation residue

Abstract

The application provides a recycling method of caprolactam distillation residue. The caprolactam distillation residue is mixed with desalted water uniformly, and then subjected to drum vacuum filtration to obtain a caprolactam aqueous solution; the caprolactam aqueous solution is subjected to reduced pressure distillation to obtain a caprolactam crude product; solvent A and solvent B are added to the caprolactam crude product in sequence, and then subjected to solid-liquid separation and drying to obtain the product. The caprolactam in the caprolactam distillation residue is crystallized out through drum vacuum filtration, reduced pressure distillation and solvent crystallization; compared with the traditional production process, the caprolactam crystal with high purity can be efficiently recycled, and the caprolactam distillation residue does not need to be returned to the original caprolactam production process, so that the impurities are not enriched, and the quality of the caprolactam product in the original process is stable.

Description

Technical Field

[0001] This invention relates to a method for treating industrial waste liquid, specifically a method for recycling caprolactam distillation residue, belonging to the field of waste liquid recycling technology. Background Technology

[0002] Caprolactam (CPL) is an important organic chemical raw material, primarily used to polymerize polyamide chips (commonly known as nylon-6 chips), which can be further processed into nylon fibers, engineering plastics, and plastic films. A small amount is used in the production of lysine, etc. Approximately 90% of polyamide is used to produce synthetic fibers, namely caprolactam, and 10% is used in plastics for manufacturing gears, bearings, pipes, medical devices, and electrical and insulating materials.

[0003] Currently, approximately 90% of the world's caprolactam is prepared from cyclohexanone oxime via the Beckmann rearrangement reaction. Due to the long and complex process and the large number of raw materials used in this caprolactam production route, the synthesized crude caprolactam contains a wide variety of impurities, approximately over 100. Although the impurity content is relatively low, it significantly affects the quality of the finished caprolactam, particularly impacting the production stability of downstream products.

[0004] The existing caprolactam refining and purification process involves: the Beckmann rearrangement reactants are neutralized with ammonia and crystallized with ammonium sulfate to obtain amide oil. The amide oil is then extracted with benzene to remove water-soluble impurities, followed by water back-extraction to remove organic impurities soluble in benzene but insoluble in water. Ion exchange is used for further impurity removal, and hydrogenation is applied to convert unsaturated organic compounds into saturated organic compounds. Finally, triple-effect evaporation and distillation are performed to obtain liquid caprolactam. The distillate residue with a caprolactam content of 85-95% is returned to the amide oil for reuse. This repeated cycle not only reduces the efficiency of the extraction and distillation refining processes but also leads to the accumulation and enrichment of impurities, affecting the quality of the final product. This has become a pressing problem in caprolactam production that needs to be solved.

[0005] Chinese Patent (CN 110092754A) discloses a method for purifying caprolactam, which includes the following steps: subjecting the caprolactam distillation residue to vacuum distillation, cooling crystallization, and centrifugation to obtain a solid caprolactam product. This invention uses vacuum distillation and cooling crystallization to crystallize caprolactam from the distillation residue. However, this invention does not remove water-insoluble substances that crystallize together with caprolactam during the crystallization process from the heavy residue.

[0006] Chinese Patent (CN 106943775B) discloses an apparatus and method for continuous solid-liquid phase separation in the heavy residue of caprolactam distillation. The apparatus includes a settling tank with an inlet on one side, an overflow outlet on the other, and a slag discharge outlet at the bottom. The overflow outlet is connected to a clarified liquid intermediate tank, and the slag discharge outlet is connected to a residue clarification tank. The clarified liquid intermediate tank is connected to the caprolactam process via pipes and a pump, and is also connected to a caprolactam process water pipeline. By using caprolactam process water to wash and dilute the clarified liquid and residue, continuous separation of caprolactam and oligomers in the heavy residue of caprolactam distillation and recycling of the caprolactam process water are achieved. Although this invention removes some water-insoluble impurities through a water-soluble process, water-soluble impurities are not removed.

[0007] Chinese patent (CN 109821265A) discloses a distillation apparatus and process for crude caprolactam residue. The apparatus includes a rising film evaporator and a short-path evaporator. A heating tube is located at the bottom of the rising film evaporator; a feed inlet is located at the bottom of the heating tube; a demister is located in the upper part of the rising film evaporator; a condenser is located above the demister; the top of the rising film evaporator is connected to a vacuum system; a discharge port is located at the bottom of the condenser; the discharge port at the bottom of the rising film evaporator is connected to the feed inlet at the top of the short-path evaporator; a scraper is located in the short-path evaporator; a condenser tube is located inside the scraper; a collecting plate is located outside the condenser tube; a discharge port is located at the bottom of the collecting plate; a condenser tube inlet and outlet are located at the bottom of the short-path evaporator; a discharge port is located at the bottom of the short-path evaporator; the top of the short-path evaporator is connected to a vacuum system; and the short-path evaporator is encased in a heating jacket. However, this invention does not remove oligomers beforehand, resulting in insufficient molecular weight and purity of caprolactam in the obtained product.

[0008] Chinese patent (CN 211215478U) relates to a caprolactam de-redistillation apparatus, which mainly consists of a distiller, a heavy phase inlet, a circulation pipeline, a circulation pump, a residual liquid tank, a residual liquid reserve tank, a residual liquid transfer pump, a caprolactam collection tank, and a condenser. The distiller forms a closed loop with the residual liquid tank and the residual liquid reserve tank via the circulation pipeline. The vapor outlet at the top of the distiller is connected to the caprolactam collection tank via a pipe equipped with a condenser. The residual liquid discharged from the bottom of the residual liquid tank and the residual liquid reserve tank is discharged through the residual liquid outlet and the residual liquid pump. This invention is equivalent to re-distilling the heavy residual liquid, during which a significant amount of caprolactam is lost due to deterioration. Summary of the Invention

[0009] To address the problems existing in the prior art, the first objective of this invention is to provide a method for recycling caprolactam distillation residue. This method effectively removes oligomers and solid impurities through rotary drum vacuum filtration and utilizes high-temperature differential solvent crystallization to reduce caprolactam loss and prevent device scaling. This not only significantly improves the quality and purity of caprolactam but also greatly increases the recovery rate of caprolactam.

[0010] To achieve the above technical objectives, this invention provides a method for recovering and utilizing caprolactam distillation residue. The caprolactam distillation residue is mixed uniformly with demineralized water and filtered under vacuum in a rotary drum to obtain an aqueous caprolactam solution. The aqueous caprolactam solution is then distilled under reduced pressure to obtain a crude caprolactam product. Solvent A and solvent B are added to the crude caprolactam product in sequence, followed by solid-liquid separation and drying to obtain the final product. The caprolactam is readily soluble in solvent A and slightly soluble in solvent B. The temperature difference between solvent A and solvent B when added to the crude caprolactam product is ≥50°C. Solvent A is a good solvent for caprolactam, in which it is readily soluble. Solvent B is a slightly soluble solvent for caprolactam, with low solubility. When solvent B is added to solvent A, the solubility of caprolactam in the mixed solvent decreases. Simultaneously, the addition of solvent B lowers the system temperature, further reducing the solubility of the solute in the solvent, making it easier for caprolactam to crystallize. Furthermore, impurities with higher solubility remain in the mixed solvent, achieving a crystallization and purification effect.

[0011] As a preferred embodiment, the caprolactam distillation residue is the distillation residue produced during the extraction, back-extraction, ion exchange, hydrogenation, evaporation and distillation processes in the production of amide oil via the Beckmann rearrangement, and its caprolactam content is 85-95%.

[0012] As a preferred embodiment, the rotary drum vacuum filtration yields a solidified product composed of caprolactam oligomers and impurities, and an aqueous solution of caprolactam; the solidified product is then incinerated.

[0013] As a preferred embodiment, the caprolactam aqueous solution contains 60-80% caprolactam.

[0014] If the water content of caprolactam aqueous solution is too high, the water consumption will increase and the production cost will be high; if the water content is too low, the oligomers will not be separated cleanly, and they will easily form agglomerates. The operating pressure will be high, which is not conducive to subsequent drum separation.

[0015] As a preferred embodiment, the temperature difference between solvent A and solvent B when adding the crude caprolactam product is 50–100°C. If the temperature difference between the high and low temperature solvents is less than 50°C, the cooling temperature cannot be reached, resulting in a higher caprolactam content in the water and poor crystallization. If the temperature difference exceeds 100°C, the solvent boiling point will be exceeded, which is detrimental to the crystallization process.

[0016] As a preferred embodiment, the process parameters for the rotary drum vacuum filtration are: a rotary drum rotation speed of 0.1–3 r / min, a vacuum degree of 20–50 kPa, and a filter paper mesh count of 300–600.

[0017] As a preferred embodiment, the process parameters for the rotary drum vacuum filtration are: a drum rotation speed of 0.5–2 r / min, a vacuum degree of 20–40 kPa, and a filter paper mesh count of 400–500.

[0018] The oligomers produced by caprolactam condensation are mostly in powder form, which easily form a bridging effect on the filter pores, solidifying the accumulation of impurities in the filter element, thereby reducing filtration efficiency and affecting filtration results. The present invention uses rotary drum vacuum filtration to remove the filter cake formed by oligomers and impurities in a timely manner, thereby improving filtration efficiency and increasing the purity of caprolactam aqueous solution.

[0019] As a preferred embodiment, the vacuum distillation process is carried out in a distillation column, with an aqueous solution containing ≤5% caprolactam obtained at the top of the column and a crude caprolactam product with a molten purity ≥99% obtained at the bottom of the column.

[0020] As a preferred embodiment, the aqueous solution obtained at the top of the column with a caprolactam content of ≤5% is sent to the benzene extraction process for recovery.

[0021] As a preferred embodiment, the water content of the crude caprolactam product is ≤1500ppm. More preferably, the water content of the crude caprolactam product is ≤800ppm; most preferably, the water content of the crude caprolactam product is ≤500ppm. If the water content of the crude caprolactam product is too high, free water is easily generated in the subsequent crystallization process. The caprolactam will dissolve in the water of crystallization, forming layers and resulting in poor crystallization.

[0022] As a preferred embodiment, the process parameters for vacuum distillation are: a top vacuum of 70–90 kPa, a bottom temperature of 100–130 °C, and 20–40 trays.

[0023] As a preferred embodiment, the process parameters for vacuum distillation are: a top vacuum of 75–85 kPa, a bottom temperature of 110–120 °C, and 20–40 trays. Since caprolactam undergoes condensation or decomposition upon heating, vacuum distillation is employed.

[0024] As a preferred embodiment, solvent A is cyclohexene and / or methylcyclohexene.

[0025] As a preferred embodiment, solvent B is n-octane and / or n-nonane.

[0026] As a preferred embodiment, the temperature of solvent A is 60–80°C, and the temperature of solvent B is -20°C to 0°C. More preferably, the temperature of solvent A is 65–75°C, and the temperature of solvent B is -15°C to -5°C.

[0027] As a preferred embodiment, the mass ratio of solvent A to solvent B is 1:1 to 20.

[0028] As a preferred embodiment, the solid-liquid separation method is centrifugation to obtain caprolactam crystals and centrifugal mother liquor.

[0029] As a preferred embodiment, the centrifugal separation parameters are 4000–5000 r / min.

[0030] As a preferred embodiment, the caprolactam crystals are dried to obtain a solid caprolactam product, and the mother liquor from centrifugation is incinerated.

[0031] This invention also provides a detailed method for recycling caprolactam distillation residue, comprising:

[0032] (1) Rotary drum vacuum filtration

[0033] The caprolactam distillation residue was mixed with demineralized water and filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities generated by caprolactam condensation, thus obtaining an aqueous caprolactam solution.

[0034] (2) Vacuum distillation

[0035] The caprolactam aqueous solution without solids obtained in step (1) is subjected to vacuum distillation to dehydrate the water. The top of the column yields an aqueous solution containing caprolactam with a caprolactam content of ≤5%. The bottom of the column yields a crude caprolactam product with a molten purity of ≥99%.

[0036] (3) Solvent crystallization

[0037] Solvent A was added to the crude caprolactam product obtained in step (2) to dissolve it, and then solvent B was added to cool and crystallize it to obtain a slurry containing caprolactam crystals.

[0038] (4) Solid-liquid separation

[0039] The caprolactam crystals obtained in step (3) are centrifuged to obtain caprolactam crystals and centrifugation mother liquor. After drying the caprolactam crystals, solid caprolactam product is obtained.

[0040] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0041] 1) The recycling method provided by the present invention effectively removes oligomers and solid impurities through rotary drum vacuum filtration, and uses high temperature difference solvent crystallization to reduce caprolactam loss and prevent device scaling. This not only greatly improves the quality and purity of caprolactam, but also significantly increases the recovery rate of caprolactam.

[0042] 2) In the technical solution provided by the present invention, the heat sensitivity of caprolactam is utilized to crystallize caprolactam by solvents A and B with a high temperature difference, effectively removing polymerization impurities from the product, and solving the scaling problems of crystallizer walls, stirring paddles, gas-liquid interfaces, and pipeline components in traditional crystallization processes. Attached Figure Description

[0043] Figure 1This is a schematic diagram of the rotary drum vacuum filter used in Embodiments 1 to 3 of the present invention;

[0044] Figure 2 This is a schematic diagram of the process flow for Embodiments 1 to 3 of the present invention. Detailed Implementation

[0045] The present invention will be further described below with reference to the embodiments. The processes and methods not described in detail in the following embodiments are conventional methods known in the art.

[0046] Example 1

[0047] (1) Rotary drum vacuum filtration

[0048] The caprolactam distillation residue was mixed with demineralized water to form a 60% caprolactam aqueous solution. This solution was then filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities, yielding a solid-free caprolactam aqueous solution. The drum rotation speed was 0.1 r / min, the vacuum degree was 20 kPa, and the filter paper mesh size was 300. The caprolactam oligomers and other solid impurities remaining after drum filtration were incinerated.

[0049] (2) Dehydration by vacuum distillation

[0050] The caprolactam aqueous solution without solids obtained in step (1) was subjected to vacuum distillation for dehydration. The pressure at the top of the dehydration column was controlled at 70 kPa (vacuum), the temperature at the bottom of the column was 100°C, and the number of trays was 20. An aqueous solution containing caprolactam was obtained at the top of the column, with a caprolactam content ≤5%; a crude caprolactam product with a molten purity of over 99% and a water content of 1500 ppm was obtained at the bottom of the column. The caprolactam aqueous solution collected at the top of the column was sent to the benzene extraction section of the original production process to be mixed with the crude caprolactam.

[0051] (3) Solvent crystallization

[0052] First, add solvent A at 70°C to the crude caprolactam product obtained in step (2) in the tower, and then add solvent B at 0°C. A is cyclohexene and B is n-octane. The mass ratio of the solvents A:B = 1:1, and a slurry containing caprolactam crystals is obtained.

[0053] (4) Solid-liquid separation

[0054] The caprolactam slurry obtained in step (3) was subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor. The centrifugation speed was 4000 r / min. After drying, the caprolactam crystals were used to obtain solid caprolactam product, and the centrifugal mother liquor was incinerated.

[0055] Example 2

[0056] (1) Rotary drum vacuum filtration

[0057] The caprolactam distillation residue was mixed with demineralized water to form a 60% caprolactam aqueous solution. This solution was then filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities, yielding a solid-free caprolactam aqueous solution. The drum speed was 0.5 r / min, the vacuum degree was 30 kPa, and the filter paper mesh size was 600. The caprolactam oligomers and other solid impurities remaining after drum filtration were incinerated.

[0058] (2) Dehydration by vacuum distillation

[0059] The caprolactam aqueous solution without solids obtained in step (1) was subjected to vacuum distillation for dehydration. The pressure at the top of the dehydration column was controlled at 70 kPa (vacuum), the temperature at the bottom of the column was 110°C, and the number of trays was 40. An aqueous solution containing caprolactam was obtained at the top of the column, with a caprolactam content ≤5%; a crude caprolactam product with a molten purity of over 99% and a water content of 1000 ppm was obtained at the bottom of the column. The caprolactam aqueous solution collected at the top of the column was sent to the benzene extraction section of the original production process to be mixed with the crude caprolactam.

[0060] (3) Solvent crystallization

[0061] First, add solvent A at 60℃ to the crude caprolactam product obtained in step (2) in the tower, and then add solvent B at -20℃. A is methylcyclohexene and B is n-nonane. The mass ratio of solvent A:B = 1:10. A slurry containing caprolactam crystals is obtained.

[0062] (4) Solid-liquid separation

[0063] The caprolactam slurry obtained in step (3) was subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor. The centrifugation speed was 4500 r / min. After drying, the caprolactam crystals were used to obtain solid caprolactam product, and the centrifugal mother liquor was incinerated.

[0064] Example 3

[0065] (1) Rotary drum vacuum filtration

[0066] The caprolactam distillation residue was mixed with demineralized water to form an 80% caprolactam aqueous solution. This solution was then filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities, yielding a solid-free caprolactam aqueous solution. The drum speed was 3 r / min, the vacuum degree was 50 kPa, and the filter paper mesh size was 500. The caprolactam oligomers and other solid impurities remaining after drum filtration were incinerated.

[0067] (2) Dehydration by vacuum distillation

[0068] The caprolactam aqueous solution without solids obtained in step (1) was subjected to vacuum distillation for dehydration. The pressure at the top of the dehydration column was controlled at 90 kPa (vacuum), the temperature at the bottom of the column was 130°C, and the number of trays was 30. An aqueous solution containing caprolactam was obtained at the top of the column, with a caprolactam content ≤5%; a crude caprolactam product with a molten purity of over 99% and a water content of 500 ppm was obtained at the bottom of the column. The caprolactam aqueous solution collected at the top of the column was sent to the benzene extraction section of the original production process to be mixed with the crude caprolactam.

[0069] (3) Solvent crystallization

[0070] First, add solvent A at 80℃ to the crude caprolactam product obtained in step (2) in the tower, and then add solvent B at -10℃. A is cyclohexene and B is n-octane. The mass ratio of the solvents A:B = 1:20. A slurry containing caprolactam crystals is obtained.

[0071] (4) Solid-liquid separation

[0072] The caprolactam slurry obtained in step (3) was subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor. The centrifugation speed was 5000 r / min. After drying, the caprolactam crystals were used to obtain solid caprolactam product, and the centrifugal mother liquor was incinerated.

[0073] Comparative Example 1

[0074] 1) Rotary drum vacuum filtration

[0075] The caprolactam distillation residue was mixed with demineralized water to form a 60% caprolactam aqueous solution. This solution was then filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities, yielding a solid-free caprolactam aqueous solution. The drum speed was 2 r / min, the vacuum degree was 50 kPa, and the filter paper mesh size was 500. The caprolactam oligomers and other solid impurities remaining after drum filtration were incinerated.

[0076] (2) Dehydration by vacuum distillation

[0077] The caprolactam aqueous solution without solids obtained in step (1) was subjected to vacuum distillation for dehydration. The pressure at the top of the dehydration column was controlled at 80 kPa (vacuum), the temperature at the bottom of the column was 130°C, and the number of trays was 30. An aqueous solution containing caprolactam was obtained at the top of the column, and crude caprolactam with a molten purity of over 99% was obtained at the bottom of the column. The caprolactam aqueous solution collected at the top of the column was sent to the benzene extraction section of the original production process to be mixed with the crude caprolactam.

[0078] (3) Solvent crystallization

[0079] First, add solvent A at 70°C to the crude caprolactam product obtained in step (2) in the tower, and then add solvent B at 60°C. A is cyclohexene and B is n-nonane. The mass ratio of the solvents A:B = 1:10. A slurry containing caprolactam crystals is obtained.

[0080] (4) Solid-liquid separation

[0081] The caprolactam slurry obtained in step (3) was subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor. The centrifugation speed was 4000 r / min. After drying, the caprolactam crystals were used to obtain solid caprolactam product, and the centrifugal mother liquor was incinerated.

[0082] Comparative Example 2

[0083] (1) Rotary drum vacuum filtration

[0084] The caprolactam distillation residue was mixed with demineralized water to form an 85% caprolactam aqueous solution. This solution was then filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities, yielding a solid-free caprolactam aqueous solution. The drum speed was 2 r / min, the vacuum degree was 50 kPa, and the filter paper mesh size was 500. The caprolactam oligomers and other solid impurities remaining after drum filtration were incinerated.

[0085] (2) Dehydration by vacuum distillation

[0086] The caprolactam aqueous solution without solids obtained in step (1) was subjected to vacuum distillation for dehydration. The pressure at the top of the dehydration column was controlled at 80 kPa (vacuum), the temperature at the bottom of the column was 90°C, and the number of trays was 40. An aqueous solution containing caprolactam was obtained at the top of the column, and molten crude caprolactam was obtained at the bottom of the column. The caprolactam aqueous solution collected at the top of the column was sent to the benzene extraction section of the original production process to be mixed with the crude caprolactam.

[0087] (3) Solvent crystallization

[0088] First, add solvent A at 70℃ to the crude caprolactam product obtained in step (2) in the tower, and then add solvent B at -10℃. A is methylcyclohexene and B is n-octane. The mass ratio of solvent A:B = 1:10. A slurry containing caprolactam crystals is obtained.

[0089] (4) Solid-liquid separation

[0090] The caprolactam slurry obtained in step (3) was subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor. The centrifugation speed was 5000 r / min. After drying, the caprolactam crystals were used to obtain solid caprolactam product, and the centrifugal mother liquor was incinerated.

[0091] Comparative Example 3

[0092] (1) Rotary drum vacuum filtration

[0093] The caprolactam distillation residue was mixed with demineralized water to form a 60% caprolactam aqueous solution. This solution was then filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities, yielding a solid-free caprolactam aqueous solution. The drum speed was 2 r / min, the vacuum degree was 50 kPa, and the filter paper mesh size was 500. The caprolactam oligomers and other solid impurities remaining after drum filtration were incinerated.

[0094] (2) Dehydration by vacuum distillation

[0095] The caprolactam aqueous solution without solids obtained in step (1) was subjected to vacuum distillation for dehydration. The pressure at the top of the dehydration column was controlled at 80 kPa (vacuum), the temperature at the bottom of the column was controlled at 130°C, and the column had 20 plates. An aqueous solution containing caprolactam was obtained at the top of the column, with a caprolactam content ≤5%; a crude caprolactam product with a molten purity of over 99% and a water content of 1000 ppm was obtained at the bottom of the column. The caprolactam aqueous solution collected at the top of the column was sent to the benzene extraction section of the original production process to be mixed with the crude caprolactam.

[0096] (3) Solvent crystallization

[0097] First, add solvent A at 70℃ to the crude caprolactam product obtained in step (2) in the tower, and then add solvent B at -10℃. A is cyclohexene and B is n-nonane. The mass ratio of the solvents A:B = 1:25; and a slurry containing caprolactam crystals is obtained.

[0098] (4) Solid-liquid separation

[0099] The caprolactam slurry obtained in step (3) was subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor. The centrifugation speed was 4500 r / min. After drying, the caprolactam crystals were used to obtain solid caprolactam product, and the centrifugal mother liquor was incinerated.

[0100] Comparative Example 4

[0101] (1) Pressure filtration

[0102] The caprolactam distillation residue was mixed with demineralized water to form a 70% caprolactam aqueous solution, which was then filtered through a filter press. The caprolactam oligomers and other solid impurities remaining after filtration were incinerated.

[0103] (2) Dehydration by vacuum distillation

[0104] The caprolactam aqueous solution without solids obtained in step (1) was subjected to vacuum distillation for dehydration. The pressure at the top of the dehydration column was controlled at 80 kPa (vacuum), the temperature at the bottom of the column was 130°C, and the number of trays was 30. An aqueous solution containing caprolactam was obtained at the top of the column, and crude caprolactam with a molten purity of over 99% was obtained at the bottom of the column. The caprolactam aqueous solution collected at the top of the column was sent to the benzene extraction section of the original production process to be mixed with the crude caprolactam.

[0105] (3) Solvent crystallization

[0106] First, add solvent A at 70℃ to the crude caprolactam product obtained in step (2) in the tower, and then add solvent B at -10℃. A is methylcyclohexene and B is n-octane. The mass ratio of solvent A:B = 1:10. A slurry containing caprolactam crystals is obtained.

[0107] (4) Solid-liquid separation

[0108] The caprolactam slurry obtained in step (3) was subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor. The centrifugation speed was 4000 r / min. After drying, the caprolactam crystals were used to obtain solid caprolactam product, and the centrifugal mother liquor was incinerated.

[0109] The products obtained in Examples 1-3 and Comparative Examples 1-4 were tested according to the national standard GB / T 13254-2017 "Industrial Caprolactam". The index parameters of the national standard are shown in Table 1, and the test results are shown in Table 2.

[0110] Table 1. Specifications of GB / T 13254-2017 "Industrial Caprolactam"

[0111]

[0112] Table 2 shows the detection data of the products obtained in Examples 1-3 and Comparative Examples 1-4.

[0113]

[0114]

Claims

1. A method for recovering and utilizing caprolactam distillation residue, characterized in that: The caprolactam distillation residue was mixed evenly with demineralized water and filtered under vacuum in a rotary drum to obtain an aqueous caprolactam solution. The caprolactam solution was then distilled under reduced pressure to obtain a crude caprolactam product. Solvent A and solvent B were added to the crude caprolactam product in sequence, and after solid-liquid separation and drying, the final product was obtained. The temperature of solvent A is 60~80℃, and the temperature of solvent B is -20℃~0℃; the mass ratio of solvent A to solvent B is 1:1~20. The caprolactam aqueous solution contains 60-80% caprolactam; the process parameters for vacuum distillation are: a top vacuum of 70-90 kPa, a bottom temperature of 100-130°C, and 20-40 trays. Solvent A is cyclohexene and / or methylcyclohexene; solvent B is n-octane and / or n-nonane.

2. The method for recycling caprolactam distillation residue according to claim 1, characterized in that: The caprolactam distillation residue is the distillation residue produced during the extraction, back-extraction, ion exchange, hydrogenation, evaporation and distillation processes in the production of amide oil by the Beckmann rearrangement, and its caprolactam content is 85-95%.

3. The method for recycling caprolactam distillation residue according to claim 1, characterized in that: The rotary drum vacuum filtration yields a solidified product composed of caprolactam oligomers and impurities, and an aqueous solution of caprolactam; the solidified product is then incinerated.

4. The method for recycling caprolactam distillation residue according to claim 1, characterized in that: The process parameters for the rotary drum vacuum filtration are as follows: the drum rotation speed is 0.1~3 r / min, the vacuum degree is 20~50 kPa, and the filter paper mesh size is 300~600.

5. The method for recycling caprolactam distillation residue according to claim 1, characterized in that: The vacuum distillation process is carried out in a distillation column, with an aqueous solution containing ≤5% caprolactam obtained at the top of the column and a crude caprolactam product with a molten purity ≥99% obtained at the bottom of the column.

6. The method for recovering and utilizing caprolactam distillation residue according to claim 5, characterized in that: The aqueous solution obtained at the top of the column with a caprolactam content of ≤5% is sent to the benzene extraction process for recovery; the water content of the crude caprolactam product is ≤1500ppm.

7. The method for recovering and utilizing caprolactam distillation residue according to claim 1, characterized in that: The solid-liquid separation method is centrifugation to obtain caprolactam crystals and centrifugal mother liquor; the parameters of the centrifugation are 4000~5000 r / min.

8. The method for recycling caprolactam distillation residue according to claim 7, characterized in that: The caprolactam crystals were dried to obtain solid caprolactam product, and the mother liquor from centrifugation was incinerated.

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