Recycling method of caprolactam distillation residual liquid

Abstract

The invention provides a recycling method of caprolactam distillation residual liquid. The method comprises the following steps: uniformly mixing caprolactam distillation raffinate with desalted water, and carrying out vacuum filtration through a rotary drum to obtain a caprolactam aqueous solution; carrying out reduced pressure distillation on the caprolactam aqueous solution to obtain a caprolactam crude product; and sequentially adding the solvent A and the solvent B into the caprolactam crude product, and carrying out solid-liquid separation and drying to obtain the caprolactam. According to the process disclosed by the invention, caprolactam in the caprolactam distillation residual liquid is crystallized in manners of drum vacuum filtration, reduced pressure distillation and solvent crystallization; compared with a traditional production process, the method has the advantages that the high-purity caprolactam crystal can be efficiently recovered, and the caprolactam distillation residual liquid does not need to be recycled to the original caprolactam production process, so that the impurities are not enriched, and the stable quality of the caprolactam product in the original process is ensured.

Description

Technical Field

[0001] The invention relates to a method for treating industrial waste liquid, in particular to a method for recycling caprolactam distillation residue, and belongs to the technical field of waste liquid recycling. Background Art

[0002] Caprolactam (CPL) is an important organic chemical raw material. Its main use is to generate polyamide chips (usually called nylon-6 chips or nylon-6 chips) through polymerization, which can be further processed into nylon fibers, engineering plastics, plastic films, and a small amount is used to produce lysine, etc. About 90% of polyamide is used to produce synthetic fibers, namely capron, and 10% is used as plastics for manufacturing gears, bearings, pipes, medical equipment, electrical and insulating materials, etc.

[0003] At present, about 90% of the world's caprolactam is prepared from cyclohexanone oxime through the Beckmann rearrangement reaction. Due to the long and complex process flow of the caprolactam production route and the large number of raw materials used, the synthetic crude caprolactam contains a variety of impurities, about 100 kinds of impurities. Although the impurity content is low, it seriously affects the quality of the finished caprolactam, especially the production stability of downstream products.

[0004] The existing caprolactam refining and purification process is as follows: the Beckmann rearrangement reactants are neutralized with ammonia and crystallized with ammonium sulfate to obtain amide oil, the amide oil is extracted with benzene to remove water-soluble impurities, and then the organic impurities soluble in benzene and insoluble in water are removed by water back extraction, ion exchange is used to remove impurities, and unsaturated organic matter is converted into saturated organic matter by hydrogenation, and finally a three-effect evaporation and distillation are performed to obtain a caprolactam liquid product. The distillation residue with a caprolactam content of 85-95% remaining in the bottom of the tower after distillation is returned to the amide oil for reuse. Such repeated cycles will not only reduce the efficiency of the extraction and distillation and other refining processes, but also form impurity accumulation and enrichment, affecting the quality of the final product, which has become a problem that needs to be solved in the current caprolactam production.

[0005] Chinese patent (CN 110092754A) discloses a method for purifying caprolactam, which comprises the following steps: subjecting the caprolactam distillation residue to vacuum distillation, cooling crystallization, centrifugal separation and other processes to obtain a solid caprolactam product. The process of the present invention crystallizes the caprolactam in the caprolactam distillation residue by vacuum distillation and cooling crystallization. However, the invention does not remove the water-insoluble substances in the heavy residue that will be crystallized together with the caprolactam during the crystallization process.

[0006] Chinese patent (CN 106943775B) discloses a device and method for continuous separation of solid and liquid phases in heavy residual liquid of caprolactam distillation, the device includes a settler, one side of which is provided with a feed port, the other side is provided with an overflow port, and the bottom is provided with a slag discharge port; the overflow port is connected to the clarified liquid intermediate tank, and the slag discharge port is connected to the residue clarification tank; the clarified liquid intermediate tank is connected to the caprolactam process through a pipeline and a pump, and the clarified liquid intermediate tank is also connected to the caprolactam process water pipeline. It uses caprolactam process water to wash and dilute the clarified liquid and the residue, thereby realizing the continuous separation of caprolactam and oligomers in the heavy residual liquid of caprolactam distillation and the recycling of caprolactam process water. Although the invention uses the water-soluble process to remove some impurities that are insoluble in water, impurities that are soluble in water are not removed.

[0007] Chinese patent (CN 109821265A) discloses a caprolactam crude residual liquid distillation device and process, the device includes a rising film evaporator and a short-path evaporator; a heating tube is provided at the bottom of the rising film evaporator; a feed port is provided at the bottom of the heating tube; a demister is provided at the upper part of the rising film evaporator; a condenser is provided above the demister; the top of the rising film evaporator is connected to a vacuum system; a discharge port is provided at the lower part of the condenser; a discharge port is provided at the lower part of the rising film evaporator and is connected to the feed port at the top of the short-path evaporator; a scraper is provided in the short-path evaporator; a condenser is provided on the inner side of the scraper; a collecting plate is provided on the outer side of the condenser; a discharge port is provided at the bottom of the collecting plate; a condenser inlet and outlet are provided at the bottom of the short-path evaporator; a discharge port is provided at the bottom of the short-path evaporator; the top of the short-path evaporator is connected to the vacuum system; and the short-path evaporator is wrapped with a heating jacket. However, the invention does not remove oligomers in advance, resulting in insufficient molecular weight and purity of caprolactam in the obtained product.

[0008] Chinese patent (CN 211215478U) relates to a caprolactam de-heaving distillation device, which is mainly composed of a distiller, a heavy phase addition inlet, a circulation pipeline, a circulation pump, a residual liquid tank, a residual liquid reserve tank, a residual liquid delivery pump, a caprolactam collection tank, and a condenser. Among them, the distiller forms a closed loop with the residual liquid tank and the residual liquid reserve tank through the circulation pipeline, the gas phase outlet at the top of the distiller is connected to the caprolactam collection tank through a pipeline provided with a condenser, and the residual liquid discharged from the bottom of the residual liquid tank and the residual liquid reserve tank is discharged through the residual liquid discharge outlet and the residual liquid pump. This invention is equivalent to distilling the heavy residual liquid again, and the caprolactam deterioration loss is relatively large during the process. Summary of the invention

[0009] In view of the problems existing in the prior art, the first object of the present invention is to provide a method for recycling caprolactam distillation residue, which effectively removes oligomers and solid impurities through rotary drum vacuum filtration, and utilizes high temperature difference solvent crystallization to reduce caprolactam loss and prevent device scarring, thereby greatly improving not only the quality and purity of caprolactam, but also the recovery rate of caprolactam.

[0010] To achieve the above technical purpose, the present invention provides a method for recycling caprolactam distillation residue, wherein the caprolactam distillation residue is uniformly mixed with desalted water, and the mixture is vacuum filtered through a rotary drum to obtain a caprolactam aqueous solution; the caprolactam aqueous solution is decompressed and distilled to obtain a crude caprolactam product; solvent A and solvent B are added to the crude caprolactam product in sequence, and the crude caprolactam product is obtained after solid-liquid separation and drying; the caprolactam is easily soluble in solvent A and slightly soluble in solvent B; the temperature difference between the solvent A and the solvent B when the crude caprolactam product is added is ≥50°C. Solvent A is a good solvent for caprolactam, in which caprolactam is easily soluble; solvent B is a slightly soluble solvent for caprolactam, and the solubility of caprolactam is low. When solvent B is added to solvent A, the solubility of caprolactam in the mixed solvent is reduced; at the same time, the addition of solvent B reduces the system temperature and the solubility of the solute in the solvent, so that caprolactam is more easily crystallized, and impurities with greater solubility will remain in the mixed solvent, thereby achieving the effect of crystallization and purification.

[0011] As a preferred solution, the caprolactam distillation residue is the distillation residue produced during the extraction, stripping, crosslinking, hydrogenation, evaporation and distillation process of the Beckmann rearrangement amide oil, and the caprolactam content thereof is 85-95%.

[0012] As a preferred solution, the rotary drum vacuum filtration obtains a solidified product consisting of caprolactam oligomers and impurities and a caprolactam aqueous solution; and the solidified product is treated by incineration.

[0013] As a preferred solution, the caprolactam content in the caprolactam aqueous solution is 60-80%.

[0014] If the water content of the 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 will easily form agglomerates, resulting in high operating pressure, which is not conducive to subsequent drum separation.

[0015] As a preferred solution, the temperature difference between the solvent A and the solvent B when added to 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, the water contains more caprolactam, and the crystallization effect is poor; if the temperature difference exceeds 100° C., it will exceed the boiling point of the solvent, which is not conducive to the crystallization operation.

[0016] As a preferred solution, the process parameters of the rotary drum vacuum filtration are: the rotary drum speed is 0.1-3 r / min, the vacuum degree is 20-50 kPa, and the mesh size of the rotary drum filter paper is 300-600.

[0017] As a preferred solution, the process parameters of the rotary drum vacuum filtration are: the rotary drum speed is 0.5-2 r / min, the vacuum degree is 20-40 kPa, and the mesh size of the rotary drum filter paper is 400-500.

[0018] The oligomers produced by the polycondensation of caprolactam are mostly in powder form, which are easy to form a bridging effect on the filter holes, and the solidified impurities are accumulated once in the filter element, thereby reducing the filtration efficiency and affecting the filtration results. The present invention adopts rotary drum vacuum filtration to timely remove the filter cake formed by the oligomers and impurities, thereby improving the filtration efficiency and the purity of the caprolactam aqueous solution.

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

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

[0021] As a preferred solution, 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, and the caprolactam will dissolve in the crystallization water, forming stratification, and the crystallization effect will be poor.

[0022] As a preferred solution, the process parameters of the reduced pressure distillation are: the top vacuum degree is 70-90 kPa, the bottom temperature is 100-130° C., and the number of plates is 20-40.

[0023] As a preferred solution, the process parameters of the reduced pressure distillation are: the top vacuum is 75-85 kPa, the bottom temperature is 110-120°C, and the number of plates is 20-40. Since caprolactam will condense or decompose when heated, the reduced pressure distillation operation is adopted.

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

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

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

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

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

[0029] As a preferred solution, the centrifugal separation parameter is: 4000-5000r / min.

[0030] As a preferred solution, the caprolactam crystals are dried to obtain a solid caprolactam product, and the centrifuged mother liquor is treated by incineration.

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

[0032] (1) Drum vacuum filtration

[0033] The caprolactam distillation residue is mixed with desalted water, and filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities produced by caprolactam polycondensation to obtain a caprolactam aqueous solution;

[0034] (2) Vacuum distillation

[0035] The solid-free caprolactam aqueous solution obtained in step (1) is subjected to reduced pressure distillation for dehydration, and an aqueous solution containing caprolactam is obtained at the top of the tower, and the caprolactam content is ≤5%; and a crude caprolactam product with a molten purity of more than 99% is obtained at the bottom of the tower;

[0036] (3) Solvent crystallization

[0037] Adding solvent A to the crude caprolactam product obtained in step (2) to dissolve it, and then adding solvent B 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 centrifugal mother liquor, and the caprolactam crystals are dried to obtain a solid caprolactam product.

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

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

[0042] 2) In the technical solution provided by the present invention, the thermal sensitivity of caprolactam is utilized to co-crystallize caprolactam through solvents A and B with a high temperature difference, thereby effectively removing polymer impurities in the product and solving the scarring problems of the crystallizer wall, stirring paddle, gas-liquid interface, and pipeline components in the traditional crystallization process. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1Schematic diagram of the rotary drum vacuum filter used in Examples 1 to 3 of the present invention;

[0044] Figure 2 Schematic diagram of the process flow of Examples 1 to 3 of the present invention. DETAILED DESCRIPTION

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

[0046] Example 1

[0047] (1) Drum vacuum filtration

[0048] The caprolactam distillation residue is mixed with desalted water to form a 60% caprolactam aqueous solution, which is filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities to obtain a caprolactam aqueous solution free of solids; the rotary drum speed is 0.1r / min, the vacuum degree is 20kPa, and the rotary drum filter paper mesh number is 300. The caprolactam oligomers and other solid impurities after rotary drum filtration are incinerated.

[0049] (2) Dehydration by vacuum distillation

[0050] The solid-free caprolactam aqueous solution obtained in step (1) is subjected to reduced pressure distillation for dehydration, and the pressure at the top of the dehydration tower is controlled at 70 kPa (vacuum degree), the temperature at the bottom of the tower is controlled at 100° C., and the number of tower plates is 20. The top of the tower obtains an aqueous solution containing caprolactam, and the caprolactam content is ≤5%; the bottom of the tower obtains a crude caprolactam product with a molten purity of more than 99% and a water content of 1500 ppm. The caprolactam aqueous solution collected at the top of the tower is sent to the benzene extraction section of the original production process to be mixed with the crude caprolactam.

[0051] (3) Solvent crystallization

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

[0053] (4) Solid-liquid separation

[0054] The caprolactam slurry obtained in step (3) is subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor, and the centrifugal separation speed is 4000r / min. The caprolactam crystals are dried to obtain a solid caprolactam product, and the centrifugal mother liquor is incinerated.

[0055] Example 2

[0056] (1) Drum vacuum filtration

[0057] The caprolactam distillation residue is mixed with desalted water to form a 60% caprolactam aqueous solution, which is filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities to obtain a caprolactam aqueous solution free of solids; the rotary drum speed is 0.5r / min, the vacuum degree is 30kPa, and the mesh number of the rotary drum filter paper is 600. The caprolactam oligomers and other solid impurities after rotary drum filtration are incinerated.

[0058] (2) Dehydration by vacuum distillation

[0059] The solid-free caprolactam aqueous solution obtained in step (1) is subjected to reduced pressure distillation for dehydration, and the pressure at the top of the dehydration tower is controlled at 70 kPa (vacuum degree), the temperature at the bottom of the tower is controlled at 110° C., and the number of tower plates is 40. The top of the tower obtains an aqueous solution containing caprolactam, and the caprolactam content is ≤5%; the bottom of the tower obtains a crude caprolactam product with a molten purity of more than 99% and a water content of 1000 ppm. The caprolactam aqueous solution collected at the top of the tower is sent to the benzene extraction section of the original production process to be mixed with the crude caprolactam.

[0060] (3) Solvent crystallization

[0061] Firstly add 60° C. solvent A to the crude caprolactam product obtained in the bottom of the tower in step (2), and then add -20° C. solvent B, wherein A is methylcyclohexene and B is n-nonane, and the mass ratio of solvents A:B is 1:10; and obtain a slurry containing caprolactam crystals.

[0062] (4) Solid-liquid separation

[0063] The caprolactam slurry obtained in step (3) is subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor, and the centrifugal separation speed is 4500r / min. The caprolactam crystals are dried to obtain a solid caprolactam product, and the centrifugal mother liquor is incinerated.

[0064] Example 3

[0065] (1) Drum vacuum filtration

[0066] The caprolactam distillation residue is mixed with desalted water to form an 80% caprolactam aqueous solution, which is filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities to obtain a caprolactam aqueous solution free of solids; the rotary drum speed is 3r / min, the vacuum degree is 50kPa, and the rotary drum filter paper mesh number is 500. The caprolactam oligomers and other solid impurities after rotary drum filtration are incinerated.

[0067] (2) Dehydration by vacuum distillation

[0068] The solid-free caprolactam aqueous solution obtained in step (1) is subjected to reduced pressure distillation for dehydration, and the pressure at the top of the dehydration tower is controlled at 90 kPa (vacuum degree), the temperature at the bottom of the tower is controlled at 130° C., and the number of tower plates is 30. The top of the tower obtains an aqueous solution containing caprolactam, and the caprolactam content is ≤5%; the bottom of the tower obtains a crude caprolactam product with a molten purity of more than 99% and a water content of 500 ppm. The caprolactam aqueous solution collected at the top of the tower is sent to the benzene extraction section of the original production process to be mixed with the crude caprolactam.

[0069] (3) Solvent crystallization

[0070] Firstly add 80° C. solvent A to the crude caprolactam product obtained in the bottom of the tower in step (2), and then add -10° C. solvent B, wherein A is cyclohexene and B is n-octane, and the mass ratio of solvents A:B is 1:20; and obtain a slurry containing caprolactam crystals.

[0071] (4) Solid-liquid separation

[0072] The caprolactam slurry obtained in step (3) is subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor, and the centrifugal separation speed is 5000r / min. The caprolactam crystals are dried to obtain a solid caprolactam product, and the centrifugal mother liquor is incinerated.

[0073] Comparative Example 1

[0074] 1) Drum vacuum filtration

[0075] The caprolactam distillation residue is mixed with desalted water to form a 60% caprolactam aqueous solution, which is filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities to obtain a caprolactam aqueous solution free of solids; the rotary drum speed is 2r / min, the vacuum degree is 50kPa, and the rotary drum filter paper mesh number is 500. The caprolactam oligomers and other solid impurities after rotary drum filtration are incinerated.

[0076] (2) Dehydration by vacuum distillation

[0077] The solid-free caprolactam aqueous solution obtained in step (1) is subjected to reduced pressure distillation for dehydration, and the pressure at the top of the dehydration tower is controlled at 80 kPa (vacuum degree), the temperature at the bottom of the tower is controlled at 130° C., and the number of plates is controlled at 30. An aqueous solution containing caprolactam is obtained at the top of the tower, and a crude caprolactam product with a molten purity of more than 99% is obtained at the bottom of the tower. The caprolactam aqueous solution collected at the top of the tower is sent to the benzene extraction section of the original production process and mixed with the crude caprolactam.

[0078] (3) Solvent crystallization

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

[0080] (4) Solid-liquid separation

[0081] The caprolactam slurry obtained in step (3) is subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor, and the centrifugal separation speed is 4000r / min. The caprolactam crystals are dried to obtain a solid caprolactam product, and the centrifugal mother liquor is incinerated.

[0082] Comparative Example 2

[0083] (1) Drum vacuum filtration

[0084] The caprolactam distillation residue is mixed with desalted water to form an 85% caprolactam aqueous solution, which is filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities to obtain a caprolactam aqueous solution free of solids; the rotary drum speed is 2r / min, the vacuum degree is 50kPa, and the rotary drum filter paper mesh number is 500. The caprolactam oligomers and other solid impurities after rotary drum filtration are incinerated.

[0085] (2) Dehydration by vacuum distillation

[0086] The solid-free caprolactam aqueous solution obtained in step (1) is subjected to reduced pressure distillation for dehydration, and the pressure at the top of the dehydration tower is controlled at 80 kPa (vacuum degree), the temperature at the bottom of the tower is controlled at 90° C., and the number of plates is controlled at 40. An aqueous solution containing caprolactam is obtained at the top of the tower, and a molten caprolactam crude product is obtained at the bottom of the tower. The caprolactam aqueous solution collected at the top of the tower is sent to the benzene extraction section of the original production process and mixed with the crude caprolactam.

[0087] (3) Solvent crystallization

[0088] Firstly add 70° C. solvent A to the crude caprolactam product obtained in the bottom of the tower in step (2), and then add -10° C. solvent B, wherein A is methylcyclohexene and B is n-octane, and the mass ratio of the solvents is A:B=1:10; and obtain a slurry containing caprolactam crystals.

[0089] (4) Solid-liquid separation

[0090] The caprolactam slurry obtained in step (3) is subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor, and the centrifugal separation speed is 5000r / min. The caprolactam crystals are dried to obtain a solid caprolactam product, and the centrifugal mother liquor is incinerated.

[0091] Comparative Example 3

[0092] (1) Drum vacuum filtration

[0093] The caprolactam distillation residue is mixed with desalted water to form a 60% caprolactam aqueous solution, which is filtered through a rotary drum vacuum filter to remove oligomers and other solid impurities to obtain a caprolactam aqueous solution free of solids; the rotary drum speed is 2r / min, the vacuum degree is 50kPa, and the rotary drum filter paper mesh number is 500. The caprolactam oligomers and other solid impurities after rotary drum filtration are incinerated.

[0094] (2) Dehydration by vacuum distillation

[0095] The solid-free caprolactam aqueous solution obtained in step (1) is subjected to reduced pressure distillation for dehydration, and the pressure at the top of the dehydration tower is controlled at 80 kPa (vacuum degree), the temperature at the bottom of the tower is controlled at 130°C, and the number of plates is 20. The top of the tower obtains an aqueous solution containing caprolactam, and the caprolactam content is ≤5%; the bottom of the tower obtains a crude caprolactam product with a molten purity of more than 99% and a water content of 1000 ppm. The caprolactam aqueous solution collected at the top of the tower is sent to the benzene extraction section of the original production process to be mixed with the crude caprolactam.

[0096] (3) Solvent crystallization

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

[0098] (4) Solid-liquid separation

[0099] The caprolactam slurry obtained in step (3) is subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor, and the centrifugal separation speed is 4500r / min. The caprolactam crystals are dried to obtain a solid caprolactam product, and the centrifugal mother liquor is incinerated.

[0100] Comparative Example 4

[0101] (1) Filter press filtration

[0102] The caprolactam distillation residue is mixed with desalted water to form a 70% caprolactam aqueous solution, which is filtered through a filter press. The caprolactam oligomers and other solid impurities after the filter press are incinerated.

[0103] (2) Dehydration by vacuum distillation

[0104] The solid-free caprolactam aqueous solution obtained in step (1) is subjected to reduced pressure distillation for dehydration, and the pressure at the top of the dehydration tower is controlled at 80 kPa (vacuum degree), the temperature at the bottom of the tower is controlled at 130° C., and the number of plates is controlled at 30. An aqueous solution containing caprolactam is obtained at the top of the tower, and a crude caprolactam product with a molten purity of more than 99% is obtained at the bottom of the tower. The caprolactam aqueous solution collected at the top of the tower is sent to the benzene extraction section of the original production process and mixed with the crude caprolactam.

[0105] (3) Solvent crystallization

[0106] Firstly add 70° C. solvent A to the crude caprolactam product obtained in the bottom of the tower in step (2), and then add -10° C. solvent B, wherein A is methylcyclohexene and B is n-octane, and the mass ratio of the solvents is A:B=1:10; and obtain a slurry containing caprolactam crystals.

[0107] (4) Solid-liquid separation

[0108] The caprolactam slurry obtained in step (3) is subjected to solid-liquid separation to obtain caprolactam crystals and centrifugal mother liquor, and the centrifugal separation speed is 4000r / min. The caprolactam crystals are dried to obtain a solid caprolactam product, and the centrifugal mother liquor is incinerated.

[0109] The products obtained in Examples 1 to 3 and Comparative Examples 1 to 4 were tested for relevant parameters 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 GB / T 13254-2017 "Caprolactam for Industrial Use" Index Parameters

[0111]

[0112] Table 2 is the test data of the products obtained in Examples 1 to 3 and Comparative Examples 1 to 4

[0113]

[0114]

Claims

1. A method for recycling caprolactam distillation residue, characterized in that: The caprolactam distillation residue is mixed evenly with the desalted water, and the mixture is filtered by a rotary drum vacuum to obtain a caprolactam aqueous solution; the caprolactam aqueous solution is distilled under reduced pressure to obtain a crude caprolactam product; solvent A and solvent B are added to the crude caprolactam product in sequence, and the mixture is separated into solid and liquid and dried to obtain the product; The caprolactam is easily soluble in solvent A and slightly soluble in solvent B; The temperature difference between the solvent A and the solvent B when added to the crude caprolactam product is ≥50°C.

2. A method for recycling caprolactam distillation residue according to claim 1, characterized in that: The caprolactam distillation residue is the distillation residue produced in the process of extraction, stripping, cross-linking, hydrogenation, evaporation and distillation in the Beckmann rearrangement amide oil, and the 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 obtains a solidified product consisting of caprolactam oligomers and impurities and a caprolactam aqueous solution; the solidified product is treated by incineration.

4. A method for recycling caprolactam distillation residue according to claim 1 or 3, characterized in that: The caprolactam content in the caprolactam aqueous solution is 60-80%; the temperature difference between the solvent A and the solvent B when they are added into the crude caprolactam product is 50-100°C.

5. The method for recycling caprolactam distillation residue according to claim 1, characterized in that: The process parameters of the rotary drum vacuum filtration are: a rotary drum speed of 0.1 to 3 r / min, a vacuum degree of 20 to 50 kPa, and a rotary drum filter paper mesh number of 300 to 600.

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

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

8. The method for recycling caprolactam distillation residue according to claim 6, characterized in that: The process parameters of the reduced pressure distillation are: the vacuum degree at the top of the tower is 70-90 kPa, the temperature at the bottom of the tower is 100-130° C., and the number of tower plates is 20-40.

9. The method for recycling caprolactam distillation residue according to claim 1, characterized in that: The solvent A is cyclohexene and / or methylcyclohexene; the solvent B is n-octane and / or n-nonane.

10. The method for recycling caprolactam distillation residue according to claim 1 or 9, characterized in that: The temperature of the solvent A is 60-80° C., the temperature of the solvent B is -20° C.-0° C.; the mass ratio of the solvent A to the high-temperature solvent B is 1:1-20.

11. The method for recycling caprolactam distillation residue according to claim 1, characterized in that: The solid-liquid separation method is centrifugal separation to obtain caprolactam crystals and centrifugal mother liquor; the parameter of the centrifugal separation is: 4000-5000r / min.

12. The method for recycling caprolactam distillation residue according to claim 11, characterized in that: The caprolactam crystals are dried to obtain a solid caprolactam product, and the centrifuged mother liquor is treated by incineration.

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