CN114539153A - Epsilon-caprolactam purification device and purification method - Google Patents

Abstract

An epsilon-caprolactam crude product crystallization device and a crystallization method are characterized in that the method comprises the step of contacting a hydrocarbon solution containing epsilon-caprolactam crude product with seed crystals, wherein the contacting treatment is carried out in a metastable zone of the hydrocarbon solution containing epsilon-caprolactam crude product, the hydrocarbon is one or more selected from straight chain aliphatic hydrocarbon, branched chain aliphatic hydrocarbon and cyclic aliphatic hydrocarbon, and the seed crystals are caprolactam particles with 2-80 meshes. The method can obtain crystals with complete crystal forms and large particles, more than 90% of the particles are in a particle interval of 0.5-3 mm, and industrial separation is facilitated.

Description

Epsilon-caprolactam purification device and purification method

Technical Field

0001 the invention relates to a crystallization method of epsilon-caprolactam crude products and a preparation method of caprolactam.

Background

0002 epsilon-caprolactam is one of the important raw materials of synthetic fibers and synthetic resins, and is mainly used for manufacturing polyamide fibers (nylon 6), resins, films and the like. At present, the method for industrially producing caprolactam adopts fuming sulfuric acid as a catalyst and a solvent, and cyclohexanone oxime undergoes a liquid phase Beckmann rearrangement reaction. The process has the defects of equipment corrosion, environmental pollution, poor economic benefit and the like, and produces a large amount of ammonium sulfate as a byproduct.

0003 the gas phase Beckmann rearrangement reaction of cyclohexanone oxime on solid acid catalyst is a new process for realizing no ammonification of epsilon-caprolactam, and has the problems of no equipment corrosion, no environmental pollution and the like, and the separation and purification of the product are greatly simplified, so the new process for the gas phase Beckmann rearrangement reaction without ammonium sulfate is greatly concerned by the industry people.

0004 in order to develop a solid acid catalyst suitable for gas phase beckmann rearrangement reaction, researchers at home and abroad have conducted a great deal of research on two main catalysts such as oxide (composite oxide) and zeolite molecular sieve, and both can prepare epsilon-caprolactam.

0005 however, these processes result in epsilon caprolactam which contains a variety of impurities. It is known that epsilon-caprolactam, a raw material used in the preparation of polyamides, requires a high quality product of epsilon-caprolactam for the preparation of polyamides and further for the manufacture of synthetic fibers and synthetic resins, and impurities of the microgram/gram grade affect the subsequent polymerization of epsilon-caprolactam and are not prone to filament formation. Therefore, various separation and purification methods are used to obtain crude epsilon-caprolactam, and then various refining methods are used to finally obtain high-purity epsilon-caprolactam, so that the high-purity epsilon-caprolactam can be used for manufacturing products such as synthetic fibers, synthetic resins, films and the like.

0006 the preparation of high purity chemicals by crystallization is one of the oldest and effective separation methods, polymer grade CPL is a heat sensitive substance and requires low impurity content, and the separation and purification by crystallization has attracted extensive attention from various caprolactam production companies. The refining process of caprolactam related to crystallization is developed successively in Bayer, INVENT, DSM and Sumitomo in Germany, and the crystallization process includes water, organic solvent crystallization and solvent-free crystallization, and the product of the solvent-free crystallization has small particle and serious scaling, so that the continuous operation is difficult in industry and the development of the crystallization is hindered.

0007 CN 101070298A and CN 101070299a disclose a method for refining epsilon-caprolactam by separation and purification, which comprises the step of crystallizing epsilon-caprolactam in an ether solution or a halogenated hydrocarbon solution containing crude epsilon-caprolactam. Distilling an alcohol solution of epsilon-caprolactam obtained by gas phase Beckmann rearrangement reaction to remove alcohol, low boiling point impurities and high boiling point impurities to obtain crude epsilon-caprolactam; dissolving crude epsilon-caprolactam in a molten state in ether or halogenated hydrocarbon, cooling and crystallizing, carrying out centrifugal separation, washing and separating by a solvent to obtain epsilon-caprolactam crystals, and contacting epsilon-caprolactam with hydrogen in the presence of a hydrogenation catalyst to carry out hydrogenation reaction so as to obtain an epsilon-caprolactam product with the extinction value, the volatile alkali value and the potassium permanganate absorption value of the epsilon-caprolactam meeting the requirements of industrial products.

0008 CN1332158A reports a process for the isolation and purification of crude caprolactam obtained by a gas phase beckmann rearrangement reaction, which process comprises: distillation, recrystallization, hydrofining and other important processes.

0009 the above process has the common problems that the obtained caprolactam has small particle (less than 600 μm), the solid-liquid separation is difficult, especially the solid-liquid transportation, solid-liquid separation and other difficulties exist in the engineering amplification process, and the continuous production of caprolactam is affected.

Disclosure of Invention

0010 it is an object of the present invention to provide a new crystallization method of crude epsilon-caprolactam which is simple, effective, capable of obtaining large and uniform particles, and suitable for industrial application.

0011 Another object of the present invention is to provide a process for the preparation of caprolactam which meets the requirements of industrial products on the basis of a novel crystallization process.

0012 based on a lot of experiments, the inventors of the present invention have unexpectedly found that when a small amount of caprolactam particles is put into the hydrocarbon solution of crude caprolactam in the metastable zone during the crystallization operation of the crystallization process, caprolactam crystals with larger particle size and complete crystal form can be obtained at a certain temperature, which is beneficial to industrial separation. The present invention has been accomplished based on this.

0013 accordingly, the present invention provides a process for crystallizing a crude product of epsilon-caprolactam, which process is characterized in that it comprises the step of treating a seed crystal by contacting said seed crystal with a hydrocarbon solution of a crude product of epsilon-caprolactam in a metastable zone, wherein said hydrocarbon is selected from one or more of a straight chain aliphatic hydrocarbon, a branched chain aliphatic hydrocarbon and a cyclic aliphatic hydrocarbon, and said seed crystal is caprolactam particles of 2 to 80 mesh.

0014 the invention also provides a process for preparing caprolactam, which comprises distilling the mixture of epsilon-caprolactam obtained by Beckmann rearrangement reaction and reaction solvent to obtain crude epsilon-caprolactam, crystallizing, washing the crystallized solvent, separating to obtain epsilon-caprolactam crystals, contacting epsilon-caprolactam with hydrogen in the presence of hydrogenation catalyst, and finally obtaining finished epsilon-caprolactam, wherein the crystallizing step comprises contacting a seed crystal with a hydrocarbon solution of crude epsilon-caprolactam in a metastable zone, wherein the hydrocarbon is selected from one or more of straight chain aliphatic hydrocarbon, branched chain aliphatic hydrocarbon and cyclic aliphatic hydrocarbon, and the seed crystal is caprolactam particles with 2-80 meshes.

0015 the crystallization method of the crude product of epsilon-caprolactam provided by the invention can obtain crystals with complete crystal forms and large particles, and more than 90% of the particles in the obtained crystals are in a particle interval of 0.5-3 mm, thereby being particularly beneficial to industrial separation. The caprolactam preparation method provided by the invention can obtain caprolactam finished products with the purity of more than 99.995%, and meets the requirements of industrial products.

Drawings

0016 FIG. 1 is a schematic representation of a saturated solubility curve and a supersaturated solubility curve of caprolactam in a hydrocarbon solvent.

0017 FIG. 2 is a particle size distribution diagram in which curve 1 is a particle size distribution curve of the epsilon caprolactam crystal particles obtained in comparative example 1 and curve 2 is a particle size distribution curve of the epsilon caprolactam crystal particles obtained in example 1.

Detailed Description

0018 the present invention provides a process for crystallizing crude epsilon-caprolactam, which comprises the step of treating a seed crystal in contact with a hydrocarbon solution of crude epsilon-caprolactam in a metastable zone, wherein said hydrocarbon is one or more selected from the group consisting of linear aliphatic hydrocarbons, branched aliphatic hydrocarbons and cyclic aliphatic hydrocarbons, and said seed crystal is caprolactam particles of 2 to 80 mesh.

0019 in more detail, the present invention provides a process for crystallizing epsilon-caprolactam which comprises dissolving a crude epsilon-caprolactam in said hydrocarbon to obtain a hydrocarbon solution of the crude epsilon-caprolactam; contacting the seed crystal with a hydrocarbon solution of an epsilon-caprolactam crude product in a temperature range of 45-62 ℃ in a metastable zone for constant-temperature crystallization to obtain caprolactam crystal particles, and then carrying out solid-liquid separation to recover caprolactam crystals.

0020 the first fraction after distillation of the organic phase has a different chemical composition than the organic phase defined in step a). Typically, the first fraction contains a higher percentage of compounds having a boiling point lower than epsilon caprolactam and a lower percentage of epsilon caprolactam and compounds having a boiling point higher than epsilon caprolactam, relative to the organic phase defined in step a). The purpose of the distillation of step a) is to remove the organic solvent. Thus, the first fraction of the organic phase contains mainly organic solvent.

0021 the remaining organic phase containing epsilon caprolactam is also considered to be the distillation bottoms. Due to the distillation, the chemical composition is different from the organic phase defined in step a). In general, it contains a higher percentage of epsilon caprolactam and compounds having a boiling point higher than epsilon caprolactam and a lower percentage of compounds having a boiling point lower than epsilon caprolactam than the organic phase of step a).

0022 typically, the aqueous phase containing epsilon caprolactam also contains impurities. Typically, these impurities have a higher boiling point than epsilon-caprolactam. Further purification is usually required. For example, it is fed to a fourth distillation zone for distillation and epsilon-caprolactam is withdrawn as an overhead. The remaining phase thus usually consists of components having a boiling point higher than epsilon-caprolactam.

0023 typically, the process further comprises subjecting the aqueous phase containing epsilon caprolactam to hydrotreating after step d) or step e). This means that the treatment can hydrogenate unsaturated impurities. Hydrotreating is typically applied using techniques well known in the art.

0024 drying the aqueous phase containing epsilon caprolactam as a result of the distillation in step f). Drying is not necessarily thorough. Typically less than 0.2 wt.% water remains after step f). Preferably, less than 0.1 wt.% water remains after step f).

0025 in the present invention, in the collecting step, the mixed liquid is supplied in a circumferential direction of the container.

0026 according to this method, there is no need for a stirring paddle for stirring the inside of the vessel, and scale formation on the stirring paddle can be eliminated.

0027 in the present invention, at least a part of the inner wall of the container in which the evaporation crystallization is performed is heated to a temperature higher than the operation temperature of the evaporation crystallization.

0028 in the present invention, in the above evaporation crystallization, the evaporation load of the solvent per unit area of the liquid surface is 100kg/m²More than hour and 2000kg/m²Less than hour.

0029 in the method of the invention, the seed crystal is caprolactam particle, usually preferably caprolactam particle with purity of 99.99 wt%, and its mesh number is 2-80 mesh. The term "mesh number" as used herein means how many holes are arranged in total in a length of 1 inch (25.4 mm), i.e., how many meshes are. For example, 10 mesh means that 10 holes are arranged in a total in a length of 1 inch; while 40 mesh represents a total of 40 holes in a 1 inch length, 80 mesh represents a total of 80 holes in a 1 inch length. The caprolactam particles of 2-80 meshes mean caprolactam particles which can leak through meshes of 2 meshes but can not leak through meshes of 80 meshes.

0030 2.0kg of the above method obtained crude epsilon caprolactam, dissolved into 2.0kg of isopropyl ether to prepare a concentration of 50 weight% solution, added into a volume of 10L three-necked bottle as the base material, and the epsilon caprolactam prepared into a concentration of 30 weight% caprolactam isopropyl ether solution, and the solution into 1kg/h rate, in the rate of 350r/min stirring, temperature maintained at 20 degrees C, pressure maintained at 0.008MPa, decompression evaporation crystallization for 60 minutes. Discharging the slurry after crystal precipitation at the speed of 1kg/h, centrifugally separating the discharged slurry to obtain crystals and mother liquor, recovering the mother liquor, sending the obtained crystals into a container with a stirrer, washing by using isopropyl ether as a washing solvent, centrifugally separating again to obtain epsilon-caprolactam crystals with the purity of 99.94%, recovering a washing liquid, wherein the yield of the epsilon-caprolactam crystals is 90%, the PM value is 120s, the V.B value is 0.25mmol/kg, and the E value is 0.1602. After the continuous crystallization operation for 100 hours, the inner wall of the three-necked flask was observed to be free from the occurrence of the scale formation. In the present invention, the yield of the epsilon caprolactam crystals is calculated based on the weight of all the epsilon caprolactam crystals obtained in a certain period of time/the weight of the caprolactam raw material charged in a three-necked flask as 100%.

0031 in the process of the present invention, the hydrocarbon is selected from the group consisting of a mixture of one or more of linear aliphatic hydrocarbons, branched aliphatic hydrocarbons and cyclic aliphatic hydrocarbons. The hydrocarbon has a boiling range of 30-150 ℃; more preferably, the hydrocarbon solution has a boiling range of 50-100 ℃; more preferably, the hydrocarbon solution has a boiling range of 60 to 90 ℃. On the premise of satisfying the boiling range of the hydrocarbon, further, the linear aliphatic hydrocarbon is selected from linear aliphatic hydrocarbons having 6 to 12 carbon atoms, the branched aliphatic hydrocarbon is selected from branched aliphatic hydrocarbons having 6 to 12 carbon atoms, and the cyclic aliphatic hydrocarbon is selected from cyclic aliphatic hydrocarbons having 6 to 12 carbon atoms. Further, the linear aliphatic hydrocarbon may be selected from, but not limited to, n-hexane, n-heptane, n-octane, n-nonane, n-decane, etc., the branched aliphatic hydrocarbon may be selected from, but not limited to, methyl hexane, isooctane, etc., and the cyclic aliphatic hydrocarbon may be selected from, but not limited to, cyclohexane, methyl cyclopentane, methyl cyclohexane, etc.

0032 in the process of the present invention, the step of contacting said seed crystals with a hydrocarbon solution of a crude epsilon-caprolactam in a metastable zone may be carried out once or more than once.

0033 in the method provided by the invention, the seed crystal is caprolactam particles with 10-80 meshes, and the addition amount of the seed crystal is 1-15 wt% of the epsilon-caprolactam crude product. The mesh number of the seed crystal can be 10-20 meshes, also can be 20-40 meshes, also can be 30-60 meshes, or also can be 40-80 meshes. The weight ratio of the seed crystals to the crude caprolactam product varies depending on the mesh size of the seed crystals added. For example, in the 40-60 mesh crystal seed, it accounts for 1-5 wt%, preferably 2-4 wt% of epsilon-caprolactam crude product; the 20-40 mesh crystal seed accounts for 5-10 wt%, preferably 6-9 wt% of the epsilon-caprolactam crude product; the seed crystal with the particle size of less than 20 meshes accounts for 10-15 wt%, preferably 11-14 wt% of the crude product of the epsilon-caprolactam.

0034 in the crystallization step, the process of adding the seed crystal to the hydrocarbon solution of crude epsilon-caprolactam in the metastable zone can be carried out once or for many times.

0035 in the crystallization step, cooling crystallization, evaporative crystallization, vacuum adiabatic crystallization, etc. can be considered as the crystallization method; the crystallization apparatus may employ a general-purpose crystallizer such as an FC type crystallizer, an Oslo type crystallizer, a DTB type crystallizer, a DP type crystallizer, a Messo type crystallizer, or the like.

0036 the process for producing caprolactam of the present invention further comprises a step of hydrogenating epsilon-caprolactam by contacting with hydrogen in the presence of a hydrogenation catalyst. It is known that separation and purification methods using extraction, distillation, and ion exchange cannot sufficiently remove impurities having a chemical property similar to that of epsilon-caprolactam or by-products having a boiling point close to that of epsilon-caprolactam. In this case, the method of hydrogenation is an effective means. On one hand, tetrahydroazepine-2-ketone and structural isomers thereof can be converted into epsilon-caprolactam through hydrogenation reaction; on the other hand, the potassium permanganate absorption value in the product can be effectively improved through hydrogenation reaction. The hydrofining of caprolactam can adopt aqueous solution hydrogenation, also can adopt molten caprolactam to hydrogenate; the reactor form of caprolactam hydrofining is not particularly limited, such as a magnetically stabilized bed reactor, a fixed bed reactor or a slurry bed reactor, wherein the fixed bed reactor can be selected from molten caprolactam or caprolactam water solution for hydrofining; the catalyst may be a nickel-based catalyst or a noble metal palladium-based catalyst. The hydrogenation is carried out under a pressure of 2 to 15atm at a temperature of 80 to 150 ℃.

0037 example 1

The cyclohexanone oxime gas phase Beckmann rearrangement reaction is carried out in a 80ml fixed bed reaction device, the inner diameter of the reactor is 28mm, the loading amount of a molecular sieve catalyst with a high silica-alumina ratio MFI structure is 9.45g (phi 1.8mm strip catalyst), the reaction pressure is 0.1MPa, the reaction temperature of a catalyst bed layer is 365-385 ℃, the carrier gas flow is 3.0L/gcat/hr, the cyclohexanone oxime WHSV is 2h-1, and the partial pressure range of the mixed materials is as follows: 5.5kPa to 11.6kPa cyclohexanone oxime, 36.9kPa to 70.6kPa methanol (solvent), and 19.4kPa to 52.6kPa nitrogen (carrier gas). And (3) circularly cooling and collecting the reaction product through an ethylene glycol solution at the temperature of-5 ℃ to obtain the reaction product containing epsilon-caprolactam.

0038 the reaction mixture is first distilled by simple distillation to remove methanol, low boiling impurities and high boiling impurities, and crude epsilon-caprolactam is finally obtained. The crude epsilon caprolactam was analyzed on an Agilent model 6890 gas chromatograph (hydrogen flame ion detector, PEG20M capillary column, column length 50 m) and consisted essentially of: 99.2% of epsilon-caprolactam.

0039 130g of crude epsilon-caprolactam obtained by the above simple distillation method was taken and charged into a 500ml three-necked flask, and 130g of a mixture of cyclohexane and n-heptane (cyclohexane: n-heptane = 1: 3) was added thereto, heated to 60 to 70 ℃, and stirred for 10 minutes to completely dissolve epsilon-caprolactam in the solvent. Continuously stirring and cooling the mixture, cooling the mixture from 70 ℃ to about 57 ℃, adding 2.5g of caprolactam crystals with 16-40 meshes of screen meshes into a three-neck flask, maintaining the temperature at 50-57 ℃ for 15 minutes, and keeping the stirring speed unchanged to generate large-particle caprolactam; cooling is carried out while stirring until the temperature reaches about 43 ℃, large particles of caprolactam are completely separated out, and the particle size distribution curve of the obtained crystals is shown as a curve 2 of figure 2. The stirring was stopped, the three-necked flask was taken out, and centrifuged to obtain 105.4g of 99.96% caprolactam crystals and centrifuged mother ether in a yield of 81%. The centrifugal mother liquor solvent can be recycled. 100g of caprolactam crystals were returned to a 500ml three-necked flask, and 100g of a mixture of cyclohexane and n-heptane (cyclohexane: n-heptane = 1: 3) was added thereto, and the mixture was stirred and washed at room temperature for 10 minutes, and then subjected to centrifugal separation to obtain 99.985% of epsilon-caprolactam crystals and a mixture of cyclohexane and n-heptane with a yield of about 94%. The solvent mixture of cyclohexane and n-heptane is recovered. The obtained epsilon-caprolactam has a PM value of 160s and an E value of < 0.5.

0040 hydrogenation reaction: 100g of caprolactam crystal with the concentration of 99.985 percent which is washed by a mixture of cyclohexane and normal heptane is added into a 500ml reaction kettle, 250g of water is added, 0.5g of amorphous nickel hydrogenation catalyst (the industrial brand is SRNA-4, produced by Jian petrochemical company Limited in Hunan) is added, the temperature is heated to about 90 ℃, hydrogen is introduced, the flow rate of the hydrogen is controlled to be 0.6L/min, the reaction pressure is maintained at 7atm, and the crystallized epsilon-caprolactam water solution is contacted with the hydrogen for reaction for 1 hour. Triple effect evaporation and reduced pressure distillation under about 1mmHg to obtain caprolactam product. The quality of the obtained caprolactam product is analyzed, the PM value is 42000s, the E value is less than 0.03, and the caprolactam product with the purity of more than 99.995 percent meets the requirements of industrial products.

0041 comparative example 1

This comparative example illustrates the effect of no seed on the crystal size distribution under the same crystallization conditions.

0042 the conditions were the same as in example 1 except that no seed crystals were added. The obtained caprolactam product has the quality that the PM value is 46000s, and the E value is less than 0.03. The product has no problem in quality, but has fine particles and no strength, and is difficult to be subjected to solid-liquid separation industrially.

0043 the crystal size distribution curve of the resulting product is shown in curve 1 of figure 2.

0044 As can be seen from the crystal size distribution curve of FIG. 2, the difference in the crystal size distribution obtained with and without seeding is significant. In comparative example 1, the most probable distribution of crystal sizes after crystallization without seeding was at 600 μm, whereas in example 1, the most probable distribution of crystal sizes after crystallization with seeding was at 900 μm, the crystal particles increased significantly, the most probable distribution increased by 300 μm, with large crystal particles approaching 2000 μm. The crystal seeds are added in the metastable zone to control the crystallization process in a low supersaturation region, which is favorable for controlling the number of crystal nuclei and is more favorable for the growth of crystals.

0045 comparative example 2

The conditions were the same as in example 1, and the addition of seeds of 80 mesh or more, since the seed particles were too fine, they were quickly dissolved, as in the case of the results without seed addition.

0046 example 2

The mixture containing epsilon-caprolactam and benzene is fed to a distillation column a'. The mixture obtained after extraction of aqueous crude epsilon caprolactam in benzene is a mixture comprising epsilon caprolactam and benzene, wherein the aqueous crude epsilon caprolactam is prepared by a beckmann rearrangement reaction of cyclohexanone oxime in the presence of oleum and is neutralized with aqueous ammonia. Impurity profile according to the method. The weight ratio of benzene to epsilon-caprolactam in the mixture of epsilon-caprolactam and benzene entering the distillation column was 4.4: 1. The distillation column a' comprises 12 equilibrium stages. The overhead gas from distillation column a 'is condensed in condenser B' where a mixture of an aqueous phase and an organic (benzene) phase is formed and discharged as a mixture through line 12. The mixture is separated in separator C' and the organic (benzene) phase leaves the phase separator via line 13 while the aqueous phase leaves via line 14. Cooling water is used as a condensing agent of the condenser B'. The organic (benzene) phase fraction withdrawn in line 13 is fed as reflux to the top of the distillation column a' via line 15. The distillation column a 'is equipped with a steam driven reboiler D'. The mixture comprising epsilon caprolactam and benzene is passed from distillation column A' via line 16 to distillation column A ". The weight ratio of benzene to epsilon caprolactam in the mixture comprising epsilon caprolactam and benzene is about 1: 8.5.

0047 distillation column a "comprises 12 equilibrium stages. The overhead gas enters condenser B "via line 17, where a mixture of an aqueous phase and an organic (benzene) phase 18 is formed. In separator C "the mixture is separated into an organic (benzene) phase removable via line 19 and an aqueous phase removable via line 20. Cooling water is used as a refrigerant for the condenser B ". The aqueous phase enters the top of the distillation column a "via line 21 as reflux. The water of the aqueous phase consists of line 20 removing the separated aqueous phase and additional deionized water.

0048 distillation column a "was equipped with a steam driven reboiler D". The mixture comprising epsilon-caprolactam and water leaves the distillation column a "via line 22. The weight ratio of water to epsilon-caprolactam in the mixture of epsilon-caprolactam and water was 1: 11.5. The mass fraction of benzene in the mixture of epsilon-caprolactam and water in line 22 is negligible (less than 0.01 wt.%).

0049 the pressure at the top of both column A and column A "was 100 kPa.

0050 example 3

As described in example 1, 70g of crude epsilon-caprolactam obtained by the above simple distillation method was taken and charged in a 500ml three-necked flask, and 260g of a mixture of cyclohexane and n-heptane (cyclohexane: n-heptane = 1: 3) was further added thereto, and the mixture was heated to 60 to 70 ℃ and stirred for 10 minutes to completely dissolve epsilon-caprolactam in the solvent. Continuously stirring and cooling the mixture, cooling the mixture from 70 ℃ to about 53.5 ℃, adding 8.0g of caprolactam crystals with 16-10 meshes of screen meshes into a three-neck flask, maintaining the temperature at 54-60 ℃ for 15 minutes, and keeping the stirring speed unchanged to generate large-particle caprolactam; and continuously stirring and cooling to about 42 ℃, completely separating out large-particle caprolactam, and enabling the obtained crystal particle size distribution curve to have the characteristic of curve 2 in figure 2. The stirring is stopped, the three-neck flask is taken out, and the three-neck flask is centrifugally separated to obtain 59g of 99.96 percent caprolactam crystal and centrifugal mother liquor, wherein the yield reaches about 76 percent. The mixture of the mother liquid cyclohexane and the normal heptane can be recovered and reused. 50g of caprolactam crystals were returned to a 500ml three-necked flask, and 65g of a mixture of cyclohexane and n-heptane (cyclohexane: n-heptane = 1: 3) was added thereto, and the mixture was stirred and washed at room temperature for 10 minutes, followed by centrifugal separation to obtain 99.99% of epsilon-caprolactam crystals and a washing mother liquor, and the yield was about 92%. The solvent is washed with a mixture of cyclohexane and n-heptane for recovery. The obtained epsilon-caprolactam has a PM value of 240s and an E value of < 0.6.

0051 hydrogenation reaction: taking 40g of 99.99 percent caprolactam crystal washed by a mixture of cyclohexane and n-heptane, adding 100g of water into a 200ml reaction kettle, adding 0.2g of amorphous nickel hydrogenation catalyst (SRNA-4, produced by Jian petrochemical corporation, Hunan), heating to about 90 ℃, introducing hydrogen, controlling the flow of hydrogen at 0.6L/min, maintaining the reaction pressure at 7atm, and contacting the crystallized epsilon-caprolactam water solution with the hydrogen for reaction for 1 hour. Triple effect evaporation and reduced pressure distillation under about 1mmHg to obtain caprolactam product. The quality of the obtained caprolactam product is analyzed, the PM value is 46000s, the E value is less than 0.03, and the caprolactam product with the purity of more than 99.995 percent meets the requirements of industrial products.

Claims (10)

1. A crystallization method of epsilon-caprolactam crude product is characterized in that: the method comprises the step of contacting a hydrocarbon solution containing epsilon-caprolactam crude products with seed crystals, wherein the contacting treatment is carried out in a metastable zone of the hydrocarbon solution containing the epsilon-caprolactam crude products, the hydrocarbon is selected from one or more of straight chain aliphatic hydrocarbon, branched chain aliphatic hydrocarbon and cyclic aliphatic hydrocarbon, and the seed crystals are caprolactam particles with 2-80 meshes.

2. The method of claim 1, wherein: dissolving epsilon-caprolactam crude product in the hydrocarbon to obtain a hydrocarbon solution of the epsilon-caprolactam crude product; and contacting the seed crystal with a hydrocarbon solution of an epsilon-caprolactam crude product in a temperature range of 45-62 ℃ in a metastable zone for constant-temperature crystallization to obtain caprolactam crystal particles, and then carrying out solid-liquid separation to recover caprolactam crystals.

3. The method according to claim 1 or 2, characterized in that: in the hydrocarbon solution of the epsilon-caprolactam crude product, the weight ratio of the epsilon-caprolactam crude product to the hydrocarbon is 1: 1 to 10.

4. The method of claim 3, wherein: the remaining organic phase containing epsilon-caprolactam contains 50 to 96 wt.% epsilon-caprolactam.

5. The method of claim 4, wherein: the weight ratio of the epsilon-caprolactam crude product to the hydrocarbon is 1: 2 to 5.

6. The method of claim 1, wherein: the crude product of the epsilon-caprolactam is obtained by carrying out gas phase Beckmann rearrangement reaction on cyclohexanone-oxime and a zeolite molecular sieve catalyst with an MFI topological structure and distilling a reaction solvent.

7. The method of claim 1, wherein: the conditions of the hydrogenation reaction include: the temperature is 80-150 ℃, the pressure is 0.2-1.5MPa, the time is 0.5-3 hours, and the weight ratio of the epsilon-caprolactam crystal to the hydrogenation catalyst is 1-100: 1.

8. The method according to claim 1 or 2, characterized in that: the liquid crude epsilon caprolactam is: molten crude epsilon caprolactam; a mixed solution of molten crude epsilon-caprolactam and aliphatic hydrocarbon; or a mixed solution of molten crude epsilon-caprolactam and a mixed solvent mixed with an aliphatic hydrocarbon and an organic solvent having higher polarity than the aliphatic hydrocarbon.

9. The method according to claim 1 or 2, characterized in that: the amount of organic phase of the reflux is at least 7 wt.% of the first fraction of organic phase removed in step b).

10. The method of claim 1, wherein: the recovery process further includes: and a step of discharging a part of the evaporative crystallization mother liquid from the container, and supplying the part of the evaporative crystallization mother liquid to the container again, thereby circulating the evaporative crystallization mother liquid outside the container, wherein the amount of the evaporative crystallization mother liquid discharged to the outside of the container is 10 times or more and 10000 times or less with respect to the evaporation amount of the solvent evaporated from the evaporative crystallization mother liquid in the evaporative crystallization.

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