Purification method for crude caprolactam prepared by gas phase Beckmann rearrangement

Abstract

The present invention relates to the field of caprolactam production, and discloses a purification method for preparing crude caprolactam by gas-phase Beckmann rearrangement, comprising the following steps: (1) removing solvent, water and light impurities from a mixed solution containing caprolactam prepared by gas-phase Beckmann rearrangement to obtain crude caprolactam; (2) mixing the crude caprolactam with a crystallization solvent for primary crystallization to obtain a primary crystallization product and a primary crystallization mother liquor; (3) mixing the primary crystallization product with the crystallization solvent for secondary crystallization to obtain a secondary crystallization product and a secondary crystallization mother liquor, and returning the secondary crystallization mother liquor to step (2) as at least part of the primary crystallization solvent; (4) subjecting the secondary crystallization product to hydrogenation reaction. The purification method not only omits the step of removing heavy impurities, but also reduces the quality requirements for crude caprolactam prepared by gas-phase Beckmann rearrangement, and can obtain high-quality caprolactam without significantly increasing the consumption of circulating solvent.

Description

Technical Field

[0001] The invention relates to the field of caprolactam production, and in particular to a purification method for preparing crude caprolactam through a gas phase Beckmann rearrangement method. Background Art

[0002] Caprolactam is widely used in the synthesis of plastics, fibers and other materials, and is an important organic monomer. Traditional caprolactam is often prepared by liquid-phase Beckmann rearrangement reaction, but the liquid-phase Beckmann rearrangement product contains sulfuric acid, so a large amount of ammonia is required for neutralization. This process will generate a large amount of low-value-added ammonium sulfate, and the large amount of sulfuric acid used has a great disadvantage for the green environmental protection of equipment and process flow. In order to avoid the use of sulfuric acid in the rearrangement process, people have developed a variety of new processes, among which the gas-phase rearrangement method is the most promising one.

[0003] Since the composition of the reaction products in the gas-phase rearrangement process is different from that in the liquid-phase rearrangement process, the traditional liquid-phase rearrangement process purification process is not suitable for the purification of the gas-phase rearrangement product. Therefore, in order to industrialize the gas-phase rearrangement process, it is urgent to develop a caprolactam purification process suitable for the gas-phase rearrangement product.

[0004] Sumitomo Corporation of Japan first proposed a refining process for gas-phase rearrangement products. In CN103508954B, CN103420912B, CN103420913B, and CN103420885 B, steps for purifying caprolactam by crystallization were proposed. In the relevant patents of Sumitomo Corporation, the gas-phase rearrangement product needs to go through the steps of desolvation, dehydration, light removal, and heavy removal before entering the crystallization unit. The product entering the crystallization unit is hydrogenated and distilled to obtain the final caprolactam product. However, in some patents, caprolactam is repeatedly distilled in the process, resulting in a large loss of energy, and it is necessary to prevent the caprolactam from deteriorating at high temperatures.

[0005] At present, in the existing patent literature at home and abroad, although there are many different process flows that can achieve good results, caprolactam basically needs to go through a deweighting tower and a final flash distillation process. This repeated distillation process increases the energy consumption of the purification process. In addition, once the quality of the rearrangement product is poor or there is a large amount of recycled crude caprolactam, the quality of the crystal product will be reduced.

[0006] Therefore, if a caprolactam purification process is provided to reduce energy consumption and solve problems such as reduced crystal purity due to fluctuations in the rearrangement reaction process and the use of refluxed crude caprolactam, it will have good practical significance and application prospects to achieve more stable acquisition of high-quality caprolactam products. Summary of the invention

[0007] The purpose of the present invention is to overcome the problems of reduced crystal purity caused by the high energy consumption of weight removal, easy deterioration of caprolactam, high quality requirements for rearrangement reaction products, and recycle of crude caprolactam in the prior art, and to provide a purification method for crude caprolactam prepared by gas-phase Beckmann rearrangement. The purification method not only omits the weight removal step, reduces energy consumption and the risk of caprolactam deterioration during weight removal, but also reduces the quality requirements for crude caprolactam prepared by gas-phase Beckmann rearrangement. Even if the quality of crude caprolactam prepared by gas-phase Beckmann rearrangement fluctuates, high-quality caprolactam can still be obtained.

[0008] In order to solve the above problems, although the prior art attempts to adopt the method of secondary crystallization, that is, a fresh solvent is used for the first crystallization to obtain the first crystallization product and the first crystallization mother liquor, and then the first crystallization product is subjected to the second crystallization using a fresh solvent to obtain the second crystallization product and the second crystallization mother liquor, the second crystallization product is transferred to the next production stage, and the first crystallization mother liquor and the second crystallization mother liquor are transferred to the residual liquid recovery stage. The inventors of the present invention have found in practice that the existing secondary crystallization method uses a large amount of fresh solvent for each crystallization, which will lead to a large increase in the circulating solvent, and the second crystallization mother liquor needs to be treated as a residual liquid, thereby increasing the energy consumption of subsequent treatment, and the caprolactam in the second crystallization mother liquor in the subsequent treatment is concentrated or crystallized and then applied, which will also lead to a decrease in the quality of the purified product. Based on this, the present invention will further overcome the above technical problems, and adopt the purification method of the present invention, although the secondary crystallization method is also adopted, the solvent used does not significantly increase the consumption of the circulating solvent, and after the secondary crystallization mother liquor is applied, high-quality caprolactam can still be obtained.

[0009] In order to achieve the above-mentioned object, the present invention provides a purification method for preparing crude caprolactam by gas-phase Beckmann rearrangement method, comprising the following steps: (1) removing solvent, dehydration and light impurity from a mixed solution containing caprolactam prepared by gas-phase Beckmann rearrangement method to obtain crude caprolactam; (2) mixing the crude caprolactam with a crystallization solvent for primary crystallization to obtain a primary crystallization product and a primary crystallization mother liquor; (3) mixing the primary crystallization product with the crystallization solvent for secondary crystallization to obtain a secondary crystallization product and a secondary crystallization mother liquor, and returning the secondary crystallization mother liquor to step (2) as at least part of the primary crystallization solvent; and (4) subjecting the secondary crystallization product to hydrogenation reaction.

[0010] Through the above technical solution, the present invention does not need to carry out the step of de-weighting, and also applies the secondary crystallization mother liquor, but still can obtain high-quality caprolactam. Moreover, the present invention adopts the secondary crystallization method, which reduces the quality requirements for the crude caprolactam prepared by the gas phase Beckmann rearrangement method, and even if the rearrangement reaction quality fluctuates, high-quality products can still be obtained. Although the present invention adopts the secondary crystallization method, it still does not significantly increase the amount of crystallization solvent.

[0011] According to the above technical scheme, the beneficial effects of the purification method of the present invention are:

[0012] (1) The deweighting tower is omitted, which reduces the energy consumption of the equipment and reduces the possibility of deterioration of caprolactam due to the heating process of deweighting distillation;

[0013] (2) It can effectively deal with the problems such as the quality fluctuation of the crude product obtained in the rearrangement reaction process and the reduction of the purity of the final product crystals caused by the application of the refluxed crude caprolactam;

[0014] (3) After two crystallization refining processes, the crude caprolactam has a high product purity, but does not significantly increase the consumption of circulating solvent. Compared with the conventional secondary crystallization method, the energy consumption of treating the secondary crystallization mother liquor is reduced.

[0015] The purification method of the present invention is applied to purify the crude caprolactam product to obtain a secondary crystallization product; and then through hydrogenation reaction, a superior caprolactam product can be obtained. The method provided by the present invention has the characteristics of high yield, long-term stable operation, and continuous and efficient. In addition, the potassium permanganate absorption value (PM) of the caprolactam obtained by the method provided by the present invention is greater than 20000s or more, the extinction value (at 290nm wavelength) of the caprolactam is 0.1 or less, the volatile base value is 0.3mmol / kg or less, the chromaticity value is 2 or less, and the basicity is 0.1mmol / kg or less, which fully meets the requirements of industrial superior products. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a process flow chart in a specific embodiment of the present invention;

[0017] Figure 2 It is a process flow chart in a preferred specific embodiment of the present invention. DETAILED DESCRIPTION

[0018] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0019] The inventor of the present invention found in the research process that the prior art caprolactam is crystallized after desolvation, dehydration, light removal (light impurity removal), and heavy removal (heavy impurity removal), and the particularly high temperature required weight removal process will bring some problems, because the weight removal needs to be carried out under low vacuum and high temperature, and long-term high-temperature distillation can cause caprolactam polymerization, and the impurity itself and the impurity and caprolactam will also further react, resulting in more impurities. Although the purity of the crude caprolactam obtained after weight removal rises, due to the generation of multiple impurities and even unqualified impurities in the weight removal process, impurities are not easy to remove in the subsequent processing process, making it difficult to improve the quality of the final product. Not only that, in the process of mechanical application, impurities that are difficult to remove continue to accumulate, often making it difficult to ensure product quality as production proceeds.

[0020] The present invention provides a purification method for preparing a crude caprolactam product by a gas-phase Beckmann rearrangement method, comprising the following steps: (1) removing solvent, dehydration and light impurity from a mixed solution containing caprolactam prepared by the gas-phase Beckmann rearrangement method to obtain crude caprolactam; (2) mixing the crude caprolactam with a crystallization solvent for primary crystallization to obtain a primary crystallization product and a primary crystallization mother liquor; (3) mixing the primary crystallization product with the crystallization solvent for secondary crystallization to obtain a secondary crystallization product and a secondary crystallization mother liquor, and returning the secondary crystallization mother liquor to step (2) as at least part of the primary crystallization solvent; and (4) subjecting the secondary crystallization product to a hydrogenation reaction.

[0021] Through the above technical solution, the present invention does not need to carry out the step of de-weighting, and also applies the secondary crystallization mother liquor, but still can obtain high-quality caprolactam. Moreover, the present invention adopts the secondary crystallization method, which reduces the quality requirements for the crude caprolactam prepared by the gas phase Beckmann rearrangement method, and even if the rearrangement reaction quality fluctuates, high-quality products can still be obtained. Although the present invention adopts the secondary crystallization method, it still does not significantly increase the amount of crystallization solvent.

[0022] According to the present invention, the crystallization solvent can be selected in a variety of ways. Preferably, the crystallization solvent can be azeotropic with water and is not soluble in water. The crystallization solvents selected within this range can achieve the technical effects of the present invention well. Preferably, at an absolute pressure of 5 kPa and at the boiling point temperature of the crystallization solvent, the mass percentage solubility of caprolactam in the crystallization solvent is less than 20% by weight; the selection of the crystallization solvent can be limited by the above conditions and verified by conventional tests. According to the above conditions, the boiling point of the crystallization solvent at normal pressure is 40-180°C, and it is preferably selected from one or more of ethers, alkanes, cycloalkanes, halogenated hydrocarbons and aromatic hydrocarbons.

[0023] According to the present invention, in order to further improve the purity of the crystal product and further reduce the quality requirements for the crude product, for the crystallization solvent, preferably, the ether is selected from saturated aliphatic monoether, preferably at least one of isopropyl ether, ethyl butyl ether, methyl ethyl ether, ethyl ether, n-propyl ether, n-butyl ether, ethyl butyl ether, methyl tert-butyl ether and ethyl tert-butyl ether. The cycloalkane contains 5-8 carbon atoms, preferably at least one selected from cyclohexane, cyclopentane, methylcyclopentane and methylcyclohexane; the halogenated hydrocarbon is selected from at least one of 1-chloropropane, 2-chloropropane, 1-chloro-n-butane, 2-chlorobutane, 1-chlorodimethylpropane, 2-chlorodimethylpropane, 1-n-bromopropane, 2-bromopropane, 1-bromobutane and 2-bromobutane; the aromatic hydrocarbon is selected from at least one of benzene, toluene, xylene and trimethylbenzene.

[0024] According to the present invention, the conditions for the primary crystallization can be adjusted within a wide range. Preferably, the conditions for the primary crystallization include: first dissolving the crude caprolactam in a crystallization solvent, and then performing a primary crystallization. Preferably, the mass concentration of caprolactam in the obtained caprolactam solution is 15%-45%. Preferably, the crystallization process in step (2) is generally operated between 10°C and 60°C, and the specific operating temperature needs to be determined according to the solubility of caprolactam in the selected solvent. More preferably, it can be between 25°C and 58°C.

[0025] In step (2), the primary crystallization process can be crystallized by cooling a saturated solution prepared at a high temperature, or an adiabatic flash continuous crystallization process can be used, or a reduced pressure evaporation crystallization process can be used. The preferred primary crystallization method is evaporation crystallization, and a suitable crystallization method can be preferably selected according to the solubility curve of caprolactam in the solvent. Preferably, adiabatic flash or reduced pressure evaporation crystallization is adopted. If the solubility changes significantly with temperature, then an adiabatic flash continuous crystallization method is preferably used, otherwise, a reduced pressure evaporation crystallization process is preferably used. In this preferred embodiment, the impurities in the secondary crystallization mother liquor and the crude caprolactam can be further removed to finally obtain a higher quality product.

[0026] According to the present invention, the crystallization process used in step (2) can be single-stage or multi-stage, but preferably no more than five stages, more preferably three-stage continuous crystallization. When a multi-stage continuous crystallization process is used, in order to make the particle size of the crystal product large and uniform, it is generally preferred that the crystals of the same quality are precipitated in each stage of the crystallizer.

[0027] In the method provided by the present invention, preferably, step (2) is to mix the upstream crude caprolactam with the mother liquor obtained from the secondary crystallization and part of the organic solvent to a desired concentration, and then perform a primary crystallization. The crystallization process is preferably a continuous crystallization process.

[0028] According to the present invention, the mass percentage concentration of the caprolactam solution entering the primary crystallization unit in step (2) is preferably between 15% and 45%. Different feed concentrations are preferred according to different crystallization methods. For adiabatic flash crystallization, the preferred feed concentration is between 25% and 40%. For reduced pressure evaporation crystallization, the preferred feed concentration should be less than 20%.

[0029] In a preferred embodiment of the present invention, the primary crystallization method is adiabatic flash evaporation, and the mass concentration of caprolactam in the caprolactam solution during feeding is 15%-35%; the conditions of adiabatic flash evaporation include: feed temperature of 40-80°C, operating temperature of 15-60°C, and operating pressure of 3-100kPa. In this preferred embodiment, impurities in the secondary crystallization mother liquor and crude caprolactam can be further removed, and finally a higher quality product can be obtained.

[0030] In another preferred embodiment of the present invention, the primary crystallization method is reduced pressure evaporation crystallization, the mass concentration of caprolactam in the caprolactam solution is 15%-20%; the conditions of reduced pressure evaporation crystallization include: operating temperature of 15-45°C, operating pressure of 3-100kPa. In this preferred embodiment, impurities in the secondary crystallization mother solution and crude caprolactam can be further removed, and finally a higher quality product can be obtained.

[0031] According to the present invention, the yield of the primary crystallization process in step (2) can be adjusted within a wide range, and preferably the yield of the primary crystallization product is not less than 70% by weight, which can further reduce the energy consumption of the overall crystallization process. Preferably, the yield of the primary crystallization product is 75-95% by weight.

[0032] According to the present invention, the yield of the primary crystallization product is calculated by dividing the mass of caprolactam crystals in the primary crystallization output by the mass of caprolactam in the primary crystallization feed and multiplying by 100%.

[0033] According to the present invention, the solid content (mass of crystals) of the slurry obtained at the end of the crystallization process used in step (2) should not be higher than 55%, that is, the content of caprolactam crystals in the mixture obtained at the end of the primary crystallization is not higher than 55% by weight; if it is higher than this value, the secondary mother liquor can be repeatedly injected into the final stage crystallizer to reduce the solid content in the crystallizer of this stage, so that the crystallization process can be made more uniform, and the inclusion of impurities in the primary crystallization process can be reduced, thereby reducing the content of impurities in the primary crystallization product. According to the present invention, the slurry of the crystallization discharge in step (2) can be separated from the primary crystallization product and the primary mother liquor by centrifugation, thickening or countercurrent washing. Preferably, the countercurrent washing method is used, which can continuously obtain caprolactam slurry, and avoids the scaling problem caused by solvent volatilization during liquid-solid separation, and also avoids the problem of solid transportation. The obtained slurry can be directly heated and dissolved in a stirred tank.

[0034] According to the present invention, the crystallizer for primary crystallization in step (2) can be selected from a conventional stirred tank crystallizer, a DTB crystallizer, and an OSLO crystallizer, and the present invention is not limited thereto. In order to prevent the wall scaling problem caused by liquid foam entrainment on the wall of the crystallization kettle during the continuous crystallization process, it is preferred to add an insulating layer or a jacket insulation layer on the wall of the crystallizer, or to use a solvent collected by partial evaporation and condensation to spray on the inner wall of the crystallizer to reduce the probability of scaling.

[0035] According to the present invention, the secondary crystallization process in step (3) can use the same crystallization method as the primary crystallization process, or a different crystallization method. Generally speaking, the impurity content of the secondary crystallization process is relatively low, so the more energy-saving adiabatic flash crystallization method is preferred, and the feed concentration of the secondary crystallization process can be appropriately increased, and the secondary crystallization feed concentration can be preferably 35% to 40%. In this way, the yield can be further improved to further reduce energy consumption. More preferably, the conditions of adiabatic flash evaporation include: feed temperature of 40-80°C, operating temperature of 15-60°C, and operating pressure of 3-100kPa.

[0036] The discharge of the secondary crystallization process in step (3) can also be optionally separated from the secondary mother liquor by centrifugation, thickening or countercurrent washing similar to the primary crystallization. Preferably, water is directly introduced into the bottom of the countercurrent washer to dissolve the washed crystal product. This method can greatly save the number of equipment and avoid the transportation of solids.

[0037] According to the present invention, preferably, the process further comprises: concentrating the primary crystallization mother liquor to obtain a concentrated primary crystallization mother liquor and recovering the solvent; recrystallizing the concentrated primary crystallization mother liquor to obtain a crude caprolactam product and a residual mother liquor of the mother liquor crystallization; and returning the crude caprolactam product to step (2). In this way, the yield of caprolactam can be further improved.

[0038] According to the present invention, it is preferred to recover the solvent in the mother liquor remaining after the mother liquor crystallization, and the recovered solvent is returned to step (2) or step (3) as the crystallization solvent, thereby further reducing the usage of the circulating solvent.

[0039] According to the present invention, preferably, the concentration method is distillation concentration or multi-effect evaporation; further preferably, a distillation tower is used for concentration, and further preferably, the reflux ratio is 0-2. Still further preferably, the primary crystallization mother liquor is treated in the following manner: using a double-effect distillation with pre-pressurization or post-pressurization. In this preferred embodiment, impurities in the primary mother liquor can be further removed, and a high-quality caprolactam product can be obtained on the basis of further improving the product yield, and the quality stability of the product can be further improved.

[0040] According to the present invention, the recovered solvent in the crystallization stage of step (2) and / or the crystallization stage of step (3) is returned to the mixing stage in step (2) and / or step (3). In this way, the utilization rate of the solvent can be further improved and the consumption of the circulating solvent can be further reduced.

[0041] The present invention also has no limitation on the crystallizer used for the crystallization, which may be a cooling crystallizer, an evaporative crystallizer, a vacuum crystallizer, and may include at least one of a forced external circulation crystallizer, an Oslo crystallizer, an FC crystallizer, a DTB crystallizer, a DP crystallizer, and a Messo turbulent crystallizer.

[0042] In one embodiment, the method provided according to the present invention further comprises collecting the caprolactam product by evaporation and / or reduced pressure distillation after the hydrogenation reaction, thereby obtaining a caprolactam product with a higher potassium permanganate absorption value, a smaller volatile base value and an extinction value.

[0043] According to the present invention, the hydrogenation method can adopt the conventional hydrogenation method in the art. In a preferred embodiment of the present invention, Figure 1 As shown, preferably, the method of hydrogenating the secondary crystallization product includes: a) mixing the secondary crystallization product with water to obtain a caprolactam aqueous solution; b) removing the residual crystallization solvent from the caprolactam aqueous solution by distillation to obtain a caprolactam aqueous solution after the solvent is removed and a recovered solvent; c) contacting the caprolactam aqueous solution after the solvent is removed with hydrogen in the presence of a hydrogenation catalyst to obtain a caprolactam solution; d) dehydrating the caprolactam solution to obtain a caprolactam product. In this preferred embodiment, the quality of the product can be further improved, the quality requirements for crude caprolactam can be further reduced, and the stability of production quality can be improved.

[0044] According to the present invention, preferably, the caprolactam aqueous solution needs to remove the residual trace organic solvent before entering the hydrogenation unit to further reduce the possible influence of the trace solvent on the quality of the hydrogenated product. Preferably, the residual organic solvent in the caprolactam aqueous solution is removed to below 5ppm. The organic solvent in the caprolactam aqueous solution can be removed by azeotropy. Water and caprolactam form an azeotrope, which is then condensed and separated at the top of the tower, the aqueous phase returns to the tower, and the organic phase is extracted.

[0045] According to the present invention, the dehydration method can adopt the conventional dehydration method in the art. Preferably, in step d), distillation dehydration is preferably carried out under negative pressure conditions. This is to further prevent the self-polymerization of caprolactam under high temperature and water conditions, which may lead to a decrease in product quality, thereby further improving the quality of the product. It is further preferred to control the temperature of the caprolactam below 130°C during the dehydration process.

[0046] According to the present invention, preferably, the recovered solvent in step b) is first dehydrated and then the recovered solvent after dehydration is returned to the mixing stage in step (2) and / or step (3). According to the present invention, if step a) is a mixture of slurry and water to dissolve caprolactam, then the organic phase remaining after removing the caprolactam aqueous solution needs to be dehydrated before returning to the primary crystallization unit. In order to further improve product quality, further reduce the effect of moisture content on the solubility of caprolactam, and further reduce the adverse effect of residual trace moisture in the organic phase on the control of the crystallization process, it is preferred to enter the primary crystallization unit after dehydration, and preferably remove the moisture content in the recovered solvent to below 10ppm.

[0047] The present invention has no particular limitation on the specific implementation of the hydrogenation reaction, and it can be carried out according to conventional technical means in the art. The hydrogenation reaction can be carried out in the presence of water or in a molten state, and the present invention has no particular limitation on this. Preferably, the hydrogenation reaction is carried out in the presence of water. According to the present invention, in step a), the amount of water used is 0.1-2.5 parts by weight relative to 1 part by weight of caprolactam in the secondary crystallization product.

[0048] According to the present invention, the hydrogenation process in step c) is to remove the unsaturated impurities remaining in the crude caprolactam. The hydrogenation catalyst can be a conventional catalyst in the art, such as an amorphous nickel catalyst, a Raney nickel catalyst, etc.

[0049] According to a preferred embodiment of the present invention, the hydrogenation reaction is carried out in the presence of a hydrogenation catalyst. Preferably, the hydrogenation catalyst is selected from at least one of a nickel-based catalyst, a palladium-based catalyst and a platinum-based catalyst. The hydrogenation catalyst can be obtained commercially or prepared by itself, and the present invention has no particular limitation thereto. Preferably, the hydrogenation catalyst is a nickel-based catalyst and / or a palladium-based catalyst.

[0050] In a preferred embodiment of the present invention, the nickel-based catalyst is an amorphous nickel catalyst. The amorphous nickel catalyst can be found in CN1272490A and CN1272491A, for example.

[0051] According to the present invention, the palladium catalyst has a wide selection range. Preferably, the palladium catalyst includes a carrier and palladium and rare earth oxides loaded on the carrier. The carrier can be at least one of activated carbon, silicon oxide, titanium dioxide and aluminum oxide, and the rare earth oxide can be an oxide of lanthanum and / or cerium. Preferably, the carrier is activated carbon. The present invention has a wide selection range for the content of palladium and rare earth oxide in the palladium catalyst. Preferably, based on the total amount of the palladium catalyst, the content of palladium is 0.1-5% by weight, and the content of rare earth oxide is 0.2-10% by weight. Specifically, the preparation of the palladium catalyst can be, for example, referred to CN102430406A.

[0052] According to the present invention, the hydrogenation conditions may adopt conventional conditions in the art. In order to further make the caprolactam product have a higher potassium permanganate absorption value and purity and a smaller extinction value, preferably, the conditions of the hydrogenation reaction include: a temperature of 65-180° C., a pressure of 0.5-5 MPa, and a hydrogen flow rate of 0.1-2 L / min.

[0053] According to the present invention, the time of the hydrogenation reaction can be adjusted within a wide range. Preferably, when the hydrogenation reaction is intermittent operation, the time of the hydrogenation reaction can be 0.5-3 hours, more preferably 1-2 hours. When the hydrogenation reaction is continuous operation (for example, fixed bed process), the mass space velocity of caprolactam can be 0.5-30h -1 .

[0054] The present invention has no particular limitation on the type of reactor for carrying out the hydrogenation reaction, and a magnetically stabilized bed reactor, a fixed bed reactor or a slurry bed reactor may be used, and a fixed bed reactor is preferred.

[0055] According to the present invention, the crude caprolactam can be obtained by distilling the reaction mixture of the Beckmann rearrangement reaction to recover the reaction solvent, dehydrating and removing the light. The distillation method can be carried out under normal pressure and reduced pressure conditions, and this technology is well known to those skilled in the art and will not be described in detail here.

[0056] According to the present invention, the crude caprolactam after light removal is directly added to the crystallization solvent for crystallization, and the present invention does not include the process of removing heavy impurities. It is understood that the present invention does not exclude a small amount of heavy impurities being carried out in the above process.

[0057] According to the present invention, the reaction solvent used for the Beckmann rearrangement reaction of cyclohexanone oxime is an alcohol, preferably, the reaction solvent is an alcohol containing 1-6 carbon atoms, preferably an alcohol with a boiling point below 100° C., such as methanol, ethanol, n-propanol, isopropanol, etc., and more preferably at least one of methanol, ethanol and isopropanol. In this preferred embodiment, higher quality caprolactam can be obtained with a higher yield.

[0058] According to the present invention, preferably, the Beckmann rearrangement reaction is a gas phase Beckmann rearrangement reaction. The gas phase Beckmann rearrangement reaction of the present invention can be carried out according to conventional techniques in the art, and the present invention has no particular limitation thereto. For example, the gas phase Beckmann rearrangement can be carried out in the presence of a molecular sieve catalyst of an MFI structure, in the presence of a carrier gas and a reaction solvent, to react gas phase cyclohexanone oxime. For the conditions of the gas phase Beckmann rearrangement reaction, conventional reaction conditions in the art can be adopted. Preferably, in the gas phase Beckmann rearrangement reaction, the amount of the reaction solvent used is 40-80% by weight, preferably 50-70% by weight, based on the total weight of the reaction raw materials and the reaction solvent.

[0059] According to the present invention, in the gas-phase Beckmann rearrangement reaction, the reaction raw material refers to a mixture of a low-carbon alcohol such as methanol, ethanol or isopropanol and cyclohexanone oxime as a solvent, and the weight ratio of the solvent to cyclohexanone oxime is 1:1-3:1.

[0060] Preferably, the conditions for the gas-phase Beckmann rearrangement reaction may include: a temperature of 320-450° C., preferably 370-400° C.; a pressure of 0.05-0.5 MPa, preferably 0.1-0.3 MPa; a weight hourly space velocity of cyclohexanone oxime of 0.1-5 h -1 The carrier gas may be any gas that does not react with cyclohexanone oxime and the solvent under the conditions of the gas-phase Beckmann rearrangement reaction, and the carrier gas may be, for example, nitrogen and an inert gas.

[0061] Unless otherwise specified, the pressure and partial pressure in the present invention refer to absolute pressure. The present invention does not have high requirements on the purity of crude caprolactam. Even if the quality fluctuates, high-quality caprolactam products can be obtained. Preferably, the purity of the crude caprolactam obtained in step (1) is 97-99.9% by weight; further preferably, the crude caprolactam contains caprolactam, 5-cyano-1-pentene, cyclohexenone, cyclohexanone oxime, octahydrophenazine, decahydrophenazine and tetrahydroazepine-2-one and / or isomers of tetrahydroazepine-2-one, and based on the total weight of the crude caprolactam product, the purity of caprolactam is 97-99.9% by weight. The content of the present invention is 99-99.9% by weight, the content of 5-cyano-1-pentene is 0.001-0.1% by weight, the content of cyclohexenone is 0.001-0.1% by weight, the content of cyclohexanone oxime is 0.001-0.3% by weight, the content of octahydrophenazine is 0.001-0.1% by weight, the total content of tetrahydroazepine-2-one and / or isomers of tetrahydroazepine-2-one is 0.001-0.1% by weight, the content of decahydrophenazine is 0.001-0.1% by weight, and the remainder is unidentified impurities. The sum of the mass contents of caprolactam and impurities is 100%.

[0062] In some of the examples below, crude caprolactam with a purity of 99% is used for verification. Conventional gas-phase Beckmann rearrangement reaction can generally achieve the above purity after desolvation, dehydration, and light removal. It should be noted here that although the purity of crude products with a purity of 99% and premium products seems to be similar, as mentioned above, crude caprolactam contains not only conventional impurities, but also many uncharacterized impurities that are difficult to remove. It is not easy to obtain premium products from crude products with a purity of 99%, which has always been a difficult problem for those skilled in the art to study and overcome.

[0063] The caprolactam crystallization process provided by the present invention is described below with reference to the accompanying drawings, but the present invention is not limited thereto.

[0064] In a preferred embodiment of the present invention, Figure 2As shown, the gas phase rearrangement product is subjected to desolventization, dehydration and light impurity removal to obtain crude caprolactam, and the crude caprolactam is mixed with the circulating solvent and the secondary mother liquor to the desired concentration, and then enters the primary crystallization process. The crude crystals obtained in the primary crystallization process are mixed with the solvent recovered by the primary crystallization condensation and the solvent recovered by the secondary crystallization process to the desired concentration, and then enter the secondary crystallization process. The secondary crystallization process obtains the secondary crystallization mother liquor and returns to the primary crystallization process, and the crystal product is obtained and sent to the dissolution unit. The primary mother liquor obtained by the primary crystallization is recovered by concentrated distillation, and is concentrated to the temperature required for the mother liquor crystallization. The concentrated mother liquor then enters the mother liquor crystallization unit, and the mother liquor crystallization unit obtains crude caprolactam and returns to the primary crystallization unit. The mother liquor obtained by the mother liquor crystallization enters the solvent recovery unit, recovers the remaining solvent, and discharges the residual liquid. Although the present invention adopts secondary crystallization, the circulating solvent of the entire process is not greatly increased, but the requirements and dependence on the quality of the crude product are reduced, and a superior product can be obtained, the quality of the product is improved, and the product quality stability during the production process is improved.

[0065] According to the present invention, in continuous production, the total crystallization yield is calculated as: the total weight of caprolactam transferred to the hydrogenation stage / the total weight of crude caprolactam initially fed into the crystallization stage×100%.

[0066] The following test methods were used to evaluate the quality of the prepared caprolactam products in the following examples:

[0067] (1) Purity of caprolactam

[0068] The purity and impurity content of caprolactam were analyzed using a capillary column Innowax60m and a gas chromatograph 7890GC. The minimum chromatographic detection limit was 1 μg / g.

[0069] (2) Potassium permanganate absorption value of ε-caprolactam

[0070] Pour 3.000 g of caprolactam into a 100 ml colorimetric tube, add distilled water to dilute to the scale, shake well, put it in a constant temperature water bath at 20°C, add 1 ml of 0.01N potassium permanganate solution into the colorimetric tube, shake well immediately, and start the stopwatch at the same time. Stop the stopwatch when the color of the sample solution in the colorimetric tube is the same as the color of the standard colorimetric solution (dissolve 3 g of high-grade pure Co(NO3)·6H2O and 12 mg of high-grade pure K2Cr2O7 in water, dilute to 1 liter, and shake well), and record the time consumed (in seconds), which is the potassium permanganate absorption value.

[0071] (3) Volatile base (VB)

[0072] In an alkaline medium, the alkaline low molecular impurities in the sample are distilled out and absorbed with a known amount of hydrochloric acid solution. The excess hydrochloric acid is back-titrated with a sodium hydroxide standard solution. The number of moles of acid consumed per kilogram of sample is used as the measured value of volatile alkali. The calculation formula is as follows:

[0073] VB(mmol / kg)=[(V 0 -V)×C NaOH / M]×1000

[0074] Where: V 0 is the volume of NaOH standard solution consumed in the blank test, in ml;

[0075] V is the volume of NaOH standard solution consumed by the sample, in ml;

[0076] C NaOH is the exact concentration of the NaOH standard solution, in mol / L;

[0077] M is the sample mass in g.

[0078] (4) Extinction value E (at 290nm wavelength)

[0079] In a 300ml conical flask, weigh 50g of sample, add 50ml of distilled water, shake well to dissolve the sample completely, and let it stand for 10 minutes. Use a spectrophotometer to detect the extinction value of the sample with a concentration of 50% relative to distilled water at a wavelength of 290nm.

[0080] The present invention will be described in detail below through examples.

[0081] Example 1

[0082] (1) The product of gas phase rearrangement (containing 45% by weight of methanol, 42% by weight of caprolactam, 1.5% by weight of water, and the rest being impurities) is subjected to desolventization, dehydration, and light removal to obtain crude caprolactam with a purity of 99%. The crude caprolactam contains 0.0015% of 5-cyano-1-pentene, 0.2% of N-methylcaprolactam, 0.03% of n-valeramide, 0.04% of octahydrophenazine, 0.06% of decahydrophenazine, and the remainder is unidentified impurities.

[0083] (2) After the crude caprolactam is mixed with isopropyl ether, a caprolactam solution having a mass concentration of 30% and a temperature of 50.8° C. is prepared (after the cycle starts, the crude caprolactam is mixed with the crude caprolactam obtained by the primary mother liquor crystallization, and then mixed with isopropyl ether and the secondary crystallization mother liquor to obtain the target caprolactam solution), and then the caprolactam solution is continuously injected into a primary crystallization unit, which uses a first-level adiabatic flash continuous crystallization operation, sets the operating temperature at 35° C., the operating pressure at 27 kPa, and the residence time at 240 min to perform primary crystallization, at which time the primary crystallization yield is about 85%. The condensed solvent collected during the primary crystallization process is recycled.

[0084] (3) After the crystal slurry produced by the primary crystallization is centrifuged and washed, it is mixed and dissolved with isopropyl ether again, and is also prepared into a caprolactam solution with a concentration of 30% and a temperature of 50°C, and then enters the secondary crystallization unit. The operating conditions of the secondary crystallization unit are consistent with those of the primary crystallization unit. After the secondary crystallization unit is stable, the secondary crystallization crystal slurry is collected, and the caprolactam crystal product is obtained by centrifugation and washing. The obtained secondary crystallization mother liquor is all used to prepare the feed for the primary crystallization, that is, a caprolactam solution with a mass concentration of 30% is prepared for the primary crystallization when the isopropyl ether is supplemented. The condensed solvent collected during the secondary crystallization process is recycled.

[0085] (4) Mix and dissolve the caprolactam crystalline product with an equal mass of water to obtain a caprolactam aqueous solution. The caprolactam aqueous solution is reacted with hydrogen in a stirred tank reactor at 75°C and a pressure of 0.7MPa under the condition of an amorphous nickel catalyst (industrial grade SRNA-4, produced by Sinopec Catalyst Changling Branch) (the mass ratio of the caprolactam aqueous solution to the amorphous nickel catalyst is controlled to be 50-100), and the hydrogen flow rate is controlled to be 0.1L / min. The reaction is carried out for 1h at a reaction residence time, and then a caprolactam aqueous solution is obtained. The obtained caprolactam aqueous solution is distilled and dehydrated under 5kPa, and then the heavy impurities are removed by distillation under 2kPa to obtain a caprolactam product. During this process, the system temperature is controlled to be below 130°C.

[0086] (5) Collect the mother liquor obtained from the first crystallization, recover the solvent in the mother liquor by double-effect evaporation, and concentrate the mother liquor to 45.7%. The concentrated mother liquor is passed into the mother liquor crystallization kettle, and crystallized by adiabatic flash evaporation. The final temperature is 30°C, and the mother liquor crystallization yield is 94.5%. The crystals obtained from the mother liquor crystallization are centrifuged and washed, and the obtained crude caprolactam is completely returned to the first crystallization process of step (2). The remaining mother liquor of the first mother liquor crystallization is distilled under a pressure of 25 kPa to recover the solvent therein. The impurities after the mother liquor solvent is recovered are discharged from the system. The solvent collected by the mother liquor crystallization, double-effect distillation and solvent recovery unit is recycled.

[0087] According to the above method, the production was continued for 100 hours, and the obtained caprolactam product was analyzed. The results are shown in Table 1.

[0088] According to calculation, the total yield of the caprolactam product in the crystallization process (including step (2), step (3) and step (5)) is above 99% by weight. The total yield is calculated as follows: the total weight of the caprolactam transferred to the hydrogenation stage of step (4) / the total weight of the crude caprolactam initially fed in the crystallization stage of step (2) × 100%.

[0089] Comparative Example 1

[0090] Caprolactam was purified according to the method of Example 1, except that step (3) was not performed. The other steps were the same as Example 1.

[0091] According to the above method, the production was continued for 100 hours, and the obtained caprolactam product was analyzed. The results are shown in Table 1.

[0092] Comparative Example 2

[0093] Caprolactam is purified according to the method of Example 1, except that the secondary crystallization mother liquor is collected in step (3) and not returned to the first crystallization, and the secondary crystallization mother liquor is concentrated and crystallized in the manner of step (5) in Example 1, and the crude caprolactam crystallized from the secondary mother liquor is returned to step (2), and the rest is the same as Example 1.

[0094] According to the above method, the production was continued for 100 hours, and the obtained caprolactam product was analyzed. The results are shown in Table 1.

[0095] As can be seen from Table 1, although the quality of the product obtained in Comparative Example 2 is improved compared with Comparative Example 1, the parameters such as purity, PM value, extinction value, volatile base value and chromaticity value of the obtained caprolactam are obviously worse than those in Example 1.

[0096] Moreover, in the continuous production in Comparative Example 2, the steps of concentrating and crystallizing the secondary mother liquor are added, which greatly increases the energy consumption compared with Example 1. At the same time, step (2) in Comparative Example 2 also requires more fresh solvent in the continuous production compared with Example 1 because of the need to prepare the target crystallization solution.

[0097] Example 2

[0098] (1) treating the product of gas phase rearrangement in step (1) of Example 1 to obtain the same crude caprolactam;

[0099] (2) The crude caprolactam is mixed with isopropyl ether to prepare a caprolactam solution with a mass concentration of 15% and a temperature of 40° C. (after the cycle starts, the crude caprolactam is mixed with the crude caprolactam obtained by the primary mother liquor crystallization, and then mixed with isopropyl ether and the secondary crystallization mother liquor to obtain the target caprolactam solution), and then the caprolactam solution is continuously injected into the primary crystallization unit. The primary crystallization unit uses a continuous evaporation crystallization operation, sets the operating temperature at 40° C., the operating pressure at 31 kPa, and the residence time at 240 min to perform primary crystallization, and controls the heating rate to maintain the yield of the primary crystallization at 85%.

[0100] (3) After the crystal slurry produced by the primary crystallization is centrifuged and washed, it is mixed and dissolved with isopropyl ether again to prepare a caprolactam solution with a concentration of 30% and a temperature of 54°C, and then enters the secondary crystallization unit. The secondary crystallization unit uses a first-level adiabatic flash continuous crystallization operation, sets the operating temperature at 35°C, the operating pressure at 27kPa, and the residence time at 240min for secondary crystallization. At this time, the secondary crystallization yield is 85%. After the secondary crystallization unit is stable, the secondary crystallization slurry is collected and the caprolactam crystal product is obtained by centrifugation and washing. The obtained secondary crystallization mother liquor is all used to prepare the feed for the primary crystallization, that is, a caprolactam solution with a mass concentration of 15% is prepared for the primary crystallization when the isopropyl ether is supplemented.

[0101] (4) Same as step (4) in Example 1.

[0102] (5) Same as step (5) in Example 1.

[0103] According to the above method, the production was continued for 100 hours, and the obtained caprolactam product was analyzed. The results are shown in Table 1.

[0104] Example 3

[0105] (1) using the same method as in Example 1 to obtain crude caprolactam;

[0106] (2) The crude caprolactam product and cyclohexane are configured to form a caprolactam solution with a mass fraction of 29% at 67°C (after the cycle starts, the crude caprolactam is mixed with the crude caprolactam obtained by the first mother liquor crystallization, and then mixed with cyclohexane and the secondary crystallization mother liquor to obtain the target caprolactam solution). Then, it is continuously injected into the first-stage caprolactam crystallizer. After the material in the first-stage crystallizer reaches the set value, the discharge of the first-stage crystallizer is opened, and the solution is continuously injected into the second-stage crystallizer. After the material in the second-stage crystallizer reaches the set value, the vacuum degree of each stage of the crystallizer is adjusted, and the absolute pressure of the first-stage crystallizer is adjusted to 35kPa. At this time, the temperature in the crystallizer is 52°C. The absolute pressure of the second-stage crystallizer is adjusted to 18kPa. At this time, the temperature in the crystallizer is 35°C. The caprolactam solution is continuously injected, and the caprolactam slurry is continuously extracted, and the residence time of each stage of the crystallizer is maintained at 2.25h. At this time, the total yield of the entire crystallization process is about 89%. After the continuous crystallization state is maintained for several tens of hours, part of the slurry is collected, and the obtained caprolactam crystals are centrifuged and washed.

[0107] (3) The crude crystals produced by the primary crystallization are then mixed and dissolved with cyclohexane to prepare a solution with a concentration of 29% caprolactam and a temperature of 67°C, and then enter the secondary crystallization unit. The secondary crystallization unit uses a first-level adiabatic flash operation, and the operating temperature is set to 35°C and the absolute pressure is 18kPa. After the secondary crystallization unit is stable, the secondary crystal slurry is collected and the caprolactam crystal product is obtained by centrifugation and washing. The obtained secondary crystallization mother liquor is all used to prepare the feed for the primary crystallization. The secondary crystallization mother liquor is all returned to the primary crystallization unit.

[0108] (4) Same as step (4) in Example 1.

[0109] (5) Same as step (5) in Example 1.

[0110] According to the above method, the production was continued for 100 hours, and the obtained caprolactam product was analyzed. The results are shown in Table 1.

[0111] Example 4

[0112] Caprolactam was purified according to the method of Example 1, except that, before the hydrogenation of the caprolactam aqueous solution, the following method was used instead of the method for preparing the caprolactam aqueous solution in Example 1:

[0113] The secondary crystallized slurry is counter-washed with isopropyl ether of the same mass as the crystal to obtain a caprolactam crystal product slurry. The slurry is directly mixed with 0.22 parts by mass of water to dissolve the caprolactam crystals therein to obtain a caprolactam aqueous solution with a concentration of 80%. The aqueous solution is subjected to azeotropic distillation, which uses normal pressure, a tower top temperature of 84°C, a tower bottom temperature of 114°C, and a total reflux of the water phase after phase separation at the tower top, and the total extraction of the solvent phase at the tower top.

[0114] According to the above method, the production was continued for 100 hours, and the obtained caprolactam product was analyzed. The results are shown in Table 1.

[0115] Example 5

[0116] Caprolactam was purified according to the method of Example 1, except that the impurity removal conditions in step (1) were adjusted so that the crude caprolactam product contained 98.7 wt % of caprolactam, 261 μg / g of 5-cyano-1-pentene, 245 μg / g of cyclohexanone oxime, 1488 μg / g of cyclohexenone, 421 μg / g of octahydrophenazine, 189 μg / g of tetrahydroazepine-2-one and its isomers, 1134 μg / g of decahydrophenazine and other unidentified impurities.

[0117] According to the above method, the production was continued for 100 hours, and the obtained caprolactam product was analyzed. The results are shown in Table 1.

[0118] Example 6

[0119] Caprolactam was purified according to the method of Example 1, except that step (2) was replaced by the following method:

[0120] (2) Crystallization purification: The crude caprolactam obtained in step (1) is mixed with isopropyl ether in a mass ratio of 1:1 to prepare a solution, which is heated to 54° C. to completely dissolve the crude caprolactam product, and then the temperature is lowered to 15° C. to precipitate caprolactam crystals. The obtained slurry is centrifuged to obtain primary crystals and primary crystallization mother liquor.

[0121] According to the above method, the production was continued for 100 hours, and the obtained caprolactam product was analyzed. The results are shown in Table 1.

[0122] Example 7

[0123] Caprolactam was purified according to the method of Example 1, except that 1-chloropropane was used as the crystallization solvent instead of isopropyl ether.

[0124] According to the above method, the production was continued for 100 hours, and the obtained caprolactam product was analyzed. The results are shown in Table 1.

[0125] Example 8

[0126] Caprolactam was purified according to the method of Example 1, except that the crystallization solvent was replaced by n-heptane instead of isopropyl ether.

[0127] According to the above method, the production was continued for 100 hours, and the obtained caprolactam product was analyzed. The results are shown in Table 1.

[0128] Example 9

[0129] Caprolactam was purified according to the method of Example 1, except that the crystallization solvent was toluene instead of isopropyl ether.

[0130] According to the above method, the production was continued for 100 hours, and the obtained caprolactam product was analyzed. The results are shown in Table 1.

[0131] Example 10

[0132] Caprolactam was purified according to the method of Example 1, except that a palladium catalyst was used as the catalyst, the mass ratio of the caprolactam aqueous solution to the palladium catalyst was controlled to be 50-100, the temperature of the hydrogenation reaction was 75° C., and the pressure was 0.7 MPa.

[0133] The preparation method of the palladium catalyst is as follows: Treatment of the activated carbon carrier: Take 104g of 4-10 mesh coconut shell granular activated carbon and put it into a 500mL beaker, a) soak and stir with 300mL water for 30min, wash and filter, b) then soak and stir with 200mL water for 30min, wash and filter; repeat the above steps a) and b). Then use 200mL of 0.5N nitric acid, gently stir and soak for 60min, wash and filter. Wash with distilled water to remove nitric acid until the pH is about 7, dry at 100-105°C for 6 hours, and obtain 100g of treated coconut shell granular activated carbon for use. Prepare palladium-rare earth aqueous solution: weigh 1.28g of palladium nitrate Pd(NO 3 ) 2 ·2H 2 O (Fw = 266.5) and 6.5 g of cerium nitrate Ce (NO 3 ) 3 6H 2 O (Fw = 434) was dissolved in 120 g of water to obtain a palladium-rare earth aqueous solution.

[0134] Catalyst precursor: Pour the palladium-rare earth aqueous solution into the above-mentioned treated coconut shell granular activated carbon, keep the temperature at 50°C, and immerse for 6 hours (shake once every 30 minutes); then move it into a rotary evaporator, heat it to 70°C, and remove water by rotary evaporation to obtain a catalyst precursor.

[0135] Reduction treatment: The catalyst precursor was dried in an oven at 100°C for 10 h and then calcined at 200°C for 4 h. Before use, the catalyst precursor was heated to 90°C with H 2Reduction 2h, H 2 The flow rate was 4 mL / min per gram of catalyst. A palladium catalyst was obtained, wherein the content of Pd was 0.5% by weight, and CeO 2 The content is 2.5% by weight, and the carrier is activated carbon.

[0136] Embodiment 11

[0137] Caprolactam was purified according to the method of Example 1, except that step (2) and step (3) were replaced by the following method:

[0138] (2) Crystallization purification: The crude caprolactam obtained in step (1) is mixed with isopropyl ether in a mass ratio of 1:1 to prepare a solution (after the cycle starts, the crude caprolactam is mixed with the crude caprolactam obtained by the primary mother liquor crystallization, and then mixed with isopropyl ether and the secondary crystallization mother liquor to obtain the target caprolactam solution), heated to 54°C to completely dissolve the crude caprolactam product, and then the temperature is lowered to 15°C to precipitate caprolactam crystals. The obtained slurry is centrifuged to obtain primary crystallization crystals and primary crystallization mother liquor.

[0139] (3) Prepare a solution by mixing the primary crystals with isopropyl ether in a mass ratio of 1:1, heat to 54°C to completely dissolve the crude caprolactam product, then lower the temperature to 15°C to precipitate caprolactam crystals. Centrifuge the obtained slurry to obtain secondary crystals and secondary crystallization mother liquor. The obtained secondary crystallization mother liquor is used to prepare the feed for primary crystallization, that is, prepare a caprolactam solution with a mass concentration of 50% for primary crystallization with isopropyl ether added.

[0140] According to the above method, the production was continued for 100 hours, and the obtained caprolactam product was analyzed. The results are shown in Table 1.

[0141] Table 1

[0142]

[0143]

[0144] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for purifying crude caprolactam by gas phase Beckmann rearrangement, characterized in that: The following steps are involved: (1) removing solvent, water and light impurities from a mixed solution containing caprolactam prepared by a gas-phase Beckmann rearrangement method to obtain crude caprolactam; the purity of the crude caprolactam is 97-99.9% by weight; (2) mixing the crude caprolactam with a crystallization solvent for primary crystallization to obtain a primary crystallization product and a primary crystallization mother liquor; the crystallization solvent can azeotrope with water but is not soluble in water; and the primary crystallization is carried out by evaporative crystallization; At an absolute pressure of 5 kPa and at the boiling point temperature of the crystallization solvent, the mass percentage solubility of caprolactam in the crystallization solvent is less than 20% by weight; the boiling point of the crystallization solvent at normal pressure is 40-180° C.; the crystallization solvent is selected from one of ethers, alkanes, cycloalkanes, halogenated hydrocarbons and aromatic hydrocarbons; (3) mixing the primary crystallization product with the crystallization solvent for secondary crystallization to obtain a secondary crystallization product and a secondary crystallization mother liquor, and returning the secondary crystallization mother liquor to step (2) as at least part of the primary crystallization solvent; The secondary crystallization method is adiabatic flash evaporation or reduced pressure evaporation crystallization; (4) The secondary crystallization product is subjected to hydrogenation reaction.

2. The purification method according to claim 1, wherein The ether is a saturated aliphatic monovalent ether; The alkane contains 6 to 12 carbon atoms; The cycloalkane contains 5 to 8 carbon atoms; The halogenated hydrocarbon is selected from at least one of 1-chloropropane, 2-chloropropane, 1-chloro-n-butane, 2-chlorobutane, 1-chlorodimethylpropane, 2-chlorodimethylpropane, 1-n-bromopropane, 2-bromopropane, 1-bromobutane and 2-bromobutane; The aromatic hydrocarbon is selected from at least one of benzene, toluene, xylene and trimethylbenzene.

3. The purification method according to claim 2, wherein The ether is at least one of isopropyl ether, ethyl butyl ether, methyl ethyl ether, ethyl ether, n-propyl ether, n-butyl ether, ethyl butyl ether, methyl tert-butyl ether and ethyl tert-butyl ether; The alkane is selected from at least one of n-heptane, n-hexane, isopentane, n-octane, n-nonane, methylhexane, isohexane, neohexane, isoheptane, isooctane and isononane; The cycloalkane is selected from at least one of cyclohexane, cyclopentane, methylcyclopentane and methylcyclohexane.

4. The purification method according to any one of claims 1 to 3, wherein The conditions for primary crystallization include: Firstly, the crude caprolactam is dissolved in a crystallization solvent and then crystallized once.

5. The purification method according to claim 4, wherein The mass concentration of caprolactam in the obtained caprolactam solution is 15%-45%.

6. The purification method according to claim 4, wherein The temperature of the primary crystallization is 10-60°C.

7. The purification method according to claim 6, wherein The temperature of the primary crystallization is 25°C-58°C.

8. The purification method according to any one of claims 1 to 3, wherein: The primary crystallization method is adiabatic flash evaporation or reduced pressure evaporation crystallization.

9. The purification method according to any one of claims 1 to 3, wherein: The primary crystallization method is multi-stage continuous crystallization.

10. The purification method according to claim 9, wherein The number of stages of multi-stage continuous crystallization shall not exceed 5.

11. The purification method according to claim 10, wherein The primary crystallization method is three-stage continuous crystallization.

12. The purification method according to claim 8, wherein The primary crystallization method is adiabatic flash evaporation, and the mass concentration of caprolactam in the feed caprolactam solution is 15%-35%.

13. The purification method according to claim 12, wherein: The conditions of adiabatic flash evaporation include: feed temperature of 40-80°C, operating temperature of 15-60°C, and operating pressure of 3-100 kPa.

14. The purification method according to claim 8, wherein The primary crystallization method is reduced pressure evaporation crystallization, and the mass concentration of caprolactam in the feed caprolactam solution is 15%-20%.

15. The purification method according to claim 14, wherein The conditions for reduced pressure evaporation crystallization include: an operating temperature of 15-45° C. and an operating pressure of 3-100 kPa.

16. The purification method according to any one of claims 1 to 3, wherein: The content of caprolactam crystals in the mixture obtained at the end point of the primary crystallization is not higher than 55% by weight.

17. The purification method according to any one of claims 1 to 3, wherein: The yield of the primary crystallization product is not less than 70% by weight.

18. The purification method according to claim 17, wherein: The yield of the primary crystallization product is 75-95% by weight.

19. The purification method according to any one of claims 1 to 3, wherein: The primary crystallization product is separated from the primary crystallization mother liquor by centrifugation, thickening or countercurrent washing.

20. The purification method according to claim 19, wherein The primary crystallization product is separated from the primary crystallization mother liquor by countercurrent washing.

21. The purification method according to any one of claims 1 to 3, wherein: The secondary crystallization method is adiabatic flash evaporation.

22. The purification method according to claim 21, wherein In the secondary crystallization, the mass concentration of caprolactam in the feed caprolactam solution is 35%-45%.

23. The purification method according to claim 21, wherein The conditions of adiabatic flash evaporation include: feed temperature of 40-80°C, operating temperature of 15-60°C, and operating pressure of 3-100 kPa.

24. The purification method according to any one of claims 1 to 3, wherein: Also includes: Concentrating the primary crystallization mother liquor to obtain a concentrated primary crystallization mother liquor and recovering the solvent; Recrystallizing the concentrated primary crystallization mother liquor to obtain a crude caprolactam product and a residual mother liquor of the mother liquor crystallization; The crude caprolactam product is returned to step (2).

25. The purification method according to claim 24, wherein The method further comprises: recovering the solvent in the mother liquor remaining after the mother liquor is crystallized, and returning the recovered solvent to step (2) or step (3) as the crystallization solvent.

26. The purification method according to claim 25, wherein The concentration method is distillation concentration or multiple-effect evaporation.

27. The purification method according to claim 26, wherein The distillation column is used for concentration.

28. The purification method according to claim 27, wherein The reflux ratio of the distillation column is 0-2.

29. The purification method according to any one of claims 1 to 3, wherein: The recovered solvent in the crystallization stage of step (2) and / or the crystallization stage of step (3) is returned to the mixing stage in step (2) and / or step (3).

30. The purification method according to any one of claims 1 to 3, wherein: Methods for hydrogenating the secondary crystallization product include: a) mixing the secondary crystallization product with water to obtain a caprolactam aqueous solution; b) removing residual crystallization solvent from the caprolactam aqueous solution by distillation to obtain the caprolactam aqueous solution after solvent removal and recovered solvent; c) contacting the caprolactam aqueous solution after the solvent is removed with hydrogen in the presence of a hydrogenation catalyst to obtain a caprolactam solution; d) dehydrating the caprolactam solution to obtain a caprolactam product.

31. The purification method according to claim 30, wherein In step b), the water content in the caprolactam after the solvent is removed is below 5 ppm.

32. The purification method according to claim 30, wherein: In step d), distillation and dehydration are carried out under negative pressure conditions.

33. The purification method according to claim 30, wherein In step d), the temperature of caprolactam is controlled below 130° C. during the dehydration process.

34. The purification method according to claim 30, wherein: The recovered solvent in step b) is first dehydrated and then returned to the mixing stage in step (2) and / or step (3).

35. The purification method according to claim 30, wherein: In step a), the amount of water used is 0.1-2.5 parts by weight relative to 1 part by weight of caprolactam in the secondary crystallization product.

36. The purification method according to claim 30, wherein In step c), the hydrogenation catalyst is selected from at least one of a nickel-based catalyst, a palladium-based catalyst and a platinum-based catalyst; The conditions of the hydrogenation reaction include: temperature of 65-180° C., pressure of 0.5-5 MPa, and hydrogen flow rate of 0.1-2 L / min.

37. The purification method according to claim 1, wherein The crude caprolactam contains caprolactam, 5-cyano-1-pentene, cyclohexenone, cyclohexanone oxime, octahydrophenazine, decahydrophenazine and tetrahydroazepine-2-one and / or isomers of tetrahydroazepine-2-one, and based on the total weight of the crude caprolactam product, the content of caprolactam is 99-99.9% by weight, the content of 5-cyano-1-pentene is 0.001-0.1% by weight, the content of cyclohexenone is 0.001-0.1% by weight, the content of cyclohexanone oxime is 0.001-0.3% by weight, the content of octahydrophenazine is 0.001-0.1% by weight, the total content of tetrahydroazepine-2-one and / or isomers of tetrahydroazepine-2-one is 0.001-0.1% by weight, and the content of decahydrophenazine is 0.001-0.1% by weight.

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