CN112159357B - Refining method of caprolactam - Google Patents

Refining method of caprolactam Download PDF

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CN112159357B
CN112159357B CN202011104894.0A CN202011104894A CN112159357B CN 112159357 B CN112159357 B CN 112159357B CN 202011104894 A CN202011104894 A CN 202011104894A CN 112159357 B CN112159357 B CN 112159357B
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caprolactam
crystallization
temperature
solvent
washing
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CN112159357A (en
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程时标
王韩
张新平
马得佳
胡小康
张忠光
唐席
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Zhejiang Hengyi Petrochemical Research Institute Co Ltd
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Zhejiang Hengyi Petrochemical Research Institute Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07D—HETEROCYCLIC COMPOUNDS
    • C07D223/00—Heterocyclic compounds containing seven-membered rings having one nitrogen atom as the only ring hetero atom
    • C07D223/02—Heterocyclic compounds containing seven-membered rings having one nitrogen atom as the only ring hetero atom not condensed with other rings
    • C07D223/06—Heterocyclic compounds containing seven-membered rings having one nitrogen atom as the only ring hetero atom not condensed with other rings with hetero atoms or with carbon atoms having three bonds to hetero atoms with at the most one bond to halogen, e.g. ester or nitrile radicals, directly attached to ring carbon atoms
    • C07D223/08—Oxygen atoms
    • C07D223/10—Oxygen atoms attached in position 2
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07D—HETEROCYCLIC COMPOUNDS
    • C07D201/00—Preparation, separation, purification or stabilisation of unsubstituted lactams
    • C07D201/02—Preparation of lactams
    • C07D201/04—Preparation of lactams from or via oximes by Beckmann rearrangement
    • C07D201/06—Preparation of lactams from or via oximes by Beckmann rearrangement from ketones by simultaneous oxime formation and rearrangement
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07D—HETEROCYCLIC COMPOUNDS
    • C07D201/00—Preparation, separation, purification or stabilisation of unsubstituted lactams
    • C07D201/16—Separation or purification
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07B—GENERAL METHODS OF ORGANIC CHEMISTRY; APPARATUS THEREFOR
    • C07B2200/00—Indexing scheme relating to specific properties of organic compounds
    • C07B2200/13—Crystalline forms, e.g. polymorphs
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P20/00—Technologies relating to chemical industry
    • Y02P20/50—Improvements relating to the production of bulk chemicals
    • Y02P20/52—Improvements relating to the production of bulk chemicals using catalysts, e.g. selective catalysts

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  • Organic Chemistry (AREA)
  • Other In-Based Heterocyclic Compounds (AREA)
  • Low-Molecular Organic Synthesis Reactions Using Catalysts (AREA)

Abstract

The invention relates to the technical field of caprolactam production, in particular to a caprolactam refining method, which comprises the following steps: (1) The cyclohexanone oxime gas-phase Beckmann rearrangement reaction product is subjected to solvent recovery, dehydration, light component removal and heavy component removal treatment to obtain crude caprolactam with caprolactam content not less than 99 wt%; (2) Performing first evaporation crystallization on the crude caprolactam in the presence of an evaporation crystallization solvent to obtain first caprolactam crystals and a first crystallization mother solution; (3) Performing first washing on the first caprolactam crystal to obtain a second caprolactam crystal; (4) Optionally concentrating the first crystallization mother liquor and then performing second crystallization to obtain a third caprolactam crystal and a second crystallization mother liquor; (5) Subjecting the third caprolactam crystals to a second washing; (6) subjecting the second caprolactam crystals to a hydrogenation reaction. The caprolactam prepared by the refining method has higher yield and higher quality.

Description

Refining method of caprolactam
Technical Field
The invention relates to the technical field of caprolactam production, in particular to a caprolactam refining method.
Background
Caprolactam is mainly used for manufacturing polyamide fibers (nylon 6), resins, films and the like, and is one of important raw materials of synthetic fibers and synthetic resins. At present, the industrial production of caprolactam mainly comprises a cyclohexane oxidation method, a benzene partial hydrogenation method, a photonitrosation method and the like, wherein 90% of the processes are subjected to the Beckmann rearrangement of cyclohexanone oxime. The preparation of caprolactam from cyclohexanone oxime mainly adopts a liquid-phase Beckmann rearrangement process, and part (such as Japanese Sumitomo) adopts a new gas-phase Beckmann rearrangement process of cyclohexanone oxime.
The liquid-phase Beckmann rearrangement is that Beckmann rearrangement reaction is carried out under the catalysis of fuming sulfuric acid, and then the Beckmann rearrangement reaction is neutralized with ammonia, so that caprolactam and ammonium sulfate are obtained. The process has longer industrialization time, stable product quality and mature technology, and is the most widely used caprolactam production process in the world at present; however, this process has disadvantages such as corrosion of equipment, environmental pollution, and the like, and produces a large amount of ammonium sulfate as a byproduct.
The new technology for realizing the non-sulphurated and green caprolactam by adopting the gas-phase Beckmann rearrangement reaction of the cyclohexanone oxime under the condition of a solid acid catalyst has the problems of no equipment corrosion, no environmental pollution and the like, and the separation and purification of products are greatly simplified, so that the new technology of the gas-phase Beckmann rearrangement reaction is greatly focused and favored by the industry personnel. However, the vapor Beckmann rearrangement reaction is also relatively active due to high reaction temperature, the thermal stability of cyclohexanone oxime and caprolactam is poor, various side reactions of cracking, hydrolysis, oxidation, thermal condensation, alcoholysis, hydrogenation, dehydrogenation and Mannich reaction are often accompanied, various byproducts are generated, forty kinds of byproducts are obtained altogether, more than tens of undetermined byproducts are still obtained, and the selectivity of the vapor rearrangement reaction is only about 96.5%. The crude caprolactam obtained after distillation, typically having a purity of about 98-99.6%, contains about 0.4-2% of other impurities.
Caprolactam is known as a raw material for preparing polyamides, has high quality requirements on caprolactam products for preparing polyamides and further producing synthetic fibers and synthetic resins, and μg/g-level impurities are easy to oxidize or change chromaticity, influence subsequent polymerization of caprolactam and are not easy to form filaments. Therefore, various separation and purification processes are adopted to obtain crude caprolactam products, and various refining processes are adopted to finally obtain high-purity caprolactam, so that the high-purity caprolactam can be used for manufacturing products such as synthetic fibers, synthetic resins, films and the like.
However, the separation and purification methods using extraction, distillation, and ion exchange do not sufficiently remove impurities having chemical properties similar to those of caprolactam or byproducts having boiling points close to those of caprolactam. But the hydrogenation method is an effective means, and the absorption value of potassium permanganate in the product can be effectively improved through hydrogenation reaction. However, in the current purification process, common separation and purification methods such as distillation, rectification, extraction, ion exchange, adsorption and hydrogenation, or the combination of the existing single means or the existing multiple means cannot ensure that the obtained product can meet the purity of caprolactam required by industry.
The preparation of high purity chemicals by crystallization is one of the oldest and effective separation methods, polymer grade caprolactam is a heat sensitive material, and low impurity content is required, and separation and purification by crystallization has attracted extensive attention from large caprolactam production companies. The processes related to crystallization, including water, organic solvent crystallization and solvent-free crystallization, which have small particles and serious scale formation, are developed successively in Bayer, dutch DSM, switzerland INVENT, sumitomo, etc., and the development thereof is hindered.
Japanese sumitomo corporation in china has several patent applications related to the crystallization and purification of caprolactam, for example CN1332158A discloses a process for the preparation of caprolactam comprising the steps of: (i) Pouring molten crude caprolactam and a solvent comprising an aliphatic hydrocarbon at a temperature lower than the temperature of the crude caprolactam into a vessel, and mixing the caprolactam and the solvent to obtain a first slurry comprising crystallized caprolactam and, (ii) subjecting the slurry to solid-liquid separation to obtain caprolactam and a first liquid phase. CN1263091a discloses a process for purifying caprolactam, which comprises the steps of: crystallizing caprolactam in a hydrocarbon solution comprising crude caprolactam and contacting the crystallized caprolactam with hydrogen in the presence of a hydrogenation catalyst. CN102781913a discloses a process for producing high-quality caprolactam, comprising a refining step of crystallizing and precipitating caprolactam from crude caprolactam obtained by mixing an organic solvent and gas phase rearrangement, and obtaining high-quality caprolactam and a crystallization mother liquor by solid-liquid separation; and a recovery procedure, wherein the recovered mother liquor is evaporated and crystallized to obtain caprolactam crystals and crystallized mother liquor.
Although the above method purifies crude caprolactam to a certain extent, the chromaticity of caprolactam is not ideal, and the method provided by the prior art cannot have high yield and high quality of caprolactam.
Disclosure of Invention
The invention aims to solve the problems that the quality of caprolactam is to be further improved and the yield and the quality cannot be achieved simultaneously in the prior art, and provides a caprolactam refining method.
In order to achieve the above object, the present invention provides a method for refining caprolactam, comprising the steps of:
(1) The cyclohexanone oxime gas-phase Beckmann rearrangement reaction product is subjected to solvent recovery, dehydration, light component removal and heavy component removal treatment to obtain crude caprolactam with caprolactam content not less than 99 wt%;
(2) In the presence of an evaporative crystallization solvent, performing first evaporative crystallization on the crude caprolactam, and performing first solid-liquid separation to obtain first caprolactam crystals and a first crystallization mother solution;
(3) Performing first washing on the first caprolactam crystal to obtain a second caprolactam crystal and a first washing solution;
(4) Optionally concentrating the first crystallization mother liquor, then performing second crystallization, and performing second solid-liquid separation to obtain a third caprolactam crystal and a second crystallization mother liquor;
(5) Performing second washing on the third caprolactam crystal to obtain fourth caprolactam and a second washing solution;
(6) Carrying out hydrogenation reaction on the second caprolactam crystal obtained in the step (3);
wherein the evaporative crystallization solvent in the step (2) contains a solvent A and ethanol, the solubility of caprolactam in the solvent A is below 5 wt% at 20 ℃, and the ethanol accounts for below 2 wt% of the total amount of the evaporative crystallization solvent.
The inventors of the present invention found during the course of the study that although it is known that caprolactam has a very high solubility in ethanol, in a mutually soluble state, caprolactam crystals are hardly precipitated in ethanol, so that ethanol is normally not usable as a crystallization solvent for caprolactam; however, the inventors found that adding a small amount (less than 2 wt%) of ethanol to the evaporative crystallization solvent in the first evaporative crystallization process, and combining the above specific solvent a and the above specific purification method, can make the impurities more soluble in the evaporative crystallization solvent, is beneficial to the removal of the impurities, has less influence on the yield of caprolactam, is very beneficial to the improvement of the product quality of caprolactam, and in particular has remarkable effect of improving the chromaticity of caprolactam; when the ethanol content is more than 2% by weight of the evaporated crystallization solvent, the yield of caprolactam will be reduced by 3-5% per 1% by weight of ethanol added, and thus the addition of ethanol in an amount of more than 2% by weight will seriously affect the yield of caprolactam product, and it becomes industrially impossible.
By adopting the specific refining method, the invention not only can ensure the yield of caprolactam to be more than 98.5 percent, but also can ensure the quality of high-grade caprolactam, and the quality of caprolactam products is excellent, and the invention has the effect of a modifier because ethanol is contained in an evaporative crystallization solvent, so that the scarring phenomenon in a crystallizer is effectively relieved.
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FIG. 1 is a flow chart of a preferred embodiment of the present invention;
fig. 2 is a flow chart of another preferred embodiment of the present invention.
Detailed Description
The endpoints and any values of the ranges disclosed herein are not limited to the precise range or value, and are understood to encompass values approaching those ranges or values. For numerical ranges, one or more new numerical ranges may be found between the endpoints of each range, between the endpoint of each range and the individual point value, and between the individual point value, in combination with each other, and are to be considered as specifically disclosed herein.
As described above, the present invention provides a method for refining caprolactam, as shown in FIG. 1, comprising the steps of:
(1) The cyclohexanone oxime gas-phase Beckmann rearrangement reaction product is subjected to solvent recovery, dehydration, light component removal and heavy component removal treatment to obtain crude caprolactam with caprolactam content not less than 99 wt%;
(2) In the presence of an evaporative crystallization solvent, performing first evaporative crystallization on the crude caprolactam, and performing first solid-liquid separation to obtain first caprolactam crystals and a first crystallization mother solution;
(3) Performing first washing on the first caprolactam crystal to obtain a second caprolactam crystal and a first washing solution;
(4) Optionally concentrating the first crystallization mother liquor, then performing second crystallization, performing second solid-liquid separation,
obtaining a third caprolactam crystal and a second crystallization mother liquor;
(5) Performing second washing on the third caprolactam crystal to obtain fourth caprolactam and a second washing solution;
(6) Carrying out hydrogenation reaction on the second caprolactam crystal obtained in the step (3);
wherein the evaporative crystallization solvent in the step (2) contains a solvent A and ethanol, the solubility of caprolactam in the solvent A is below 5 wt% at 20 ℃, and the ethanol accounts for below 2 wt% of the total amount of the evaporative crystallization solvent.
The equilibrium relationship between solids and solutions is generally expressed in terms of the solubility of the solids in the solvent. In the present invention, the solubility refers to the amount of caprolactam in a solution when the solvent and caprolactam reach (physical) solid-liquid phase equilibrium at a specific temperature, i.e. a saturated solution is formed, which may also be called the solvency.
The inventor of the invention researches and discovers that after the cyclohexanone oxime gas-phase Beckmann rearrangement reaction product is subjected to specific solvent recovery, dehydration, light component removal and heavy component removal treatment, the obtained crude caprolactam with specific caprolactam content is used as a first crystallization raw material, and is matched with the ethanol with the specific content added in crystallization, thereby being beneficial to better removing byproducts affecting chromaticity and further improving the quality and yield of caprolactam. In the prior art, only crude caprolactam after heavy component removal is generally adopted as a crystallization raw material, and the caprolactam obtained by the method has relatively low yield and quality and high energy consumption.
Preferably, in step (1), the caprolactam content in the crude caprolactam is not less than 99.2 wt%.
The method and the equipment for recovering, dehydrating, removing light components and removing heavy components of the solvent are not limited, and can be the existing method and equipment in the field as long as the corresponding purpose can be achieved; for example, the apparatus may employ a rotary evaporator, a dehydration column, a light component removal column, a heavy component removal column. The conditions of the dehydration tower, the light component removal tower and the heavy component removal tower are not limited, and the dehydration tower, the light component removal tower and the heavy component removal tower are facilitated; preferably, the conditions of the dehydration tower include: the pressure at the top of the tower is 4-40kPa, more preferably 10-40kPa, still more preferably 15-25kPa, the temperature at the top of the tower is 70-100 ℃, more preferably 70-75 ℃, the total reflux at the top of the tower is 130-135 ℃, and the water content at the bottom of the tower is less than 1 wt%. Preferably, the conditions of the light ends removal column include: the temperature of the tower top is 80-85 ℃, the temperature of the tower bottom is 145-150 ℃, the pressure of the tower top is 1-2kPa, the reflux ratio of the tower top is 1:10-30, and the light component content of the tower bottom is less than 0.2 weight percent. Preferably, the conditions of the weight removal tower include: the temperature of the tower top is 130-135 ℃, the temperature of the tower bottom is 153-156 ℃, the pressure of the tower top is 1-2kPa, the internal reflux is carried out, the caprolactam content of the tower bottom is below 90 wt%, and the purity of caprolactam at the outlet of the tower top reaches 99-99.7 wt%.
The inventor further researches that the ethanol contained in the evaporative crystallization solvent can ensure the removal of impurities, particularly colored impurities, and the solvent A can ensure the yield of caprolactam, so that the ethanol solvent with a specific weight ratio is used without consideration due to the fact that the ethanol and caprolactam have larger dissolving capacity, and the ethanol solvent and the solvent A are matched for use, so that a better refining effect is achieved; among them, ethanol ensures purity, quality and chromaticity of CPL product, and solvent A (low price, easy availability, high purity) ensures crystal form (i.e. morphology) and CPL yield. Preferably, in the evaporative crystallization solvent, the ratio of ethanol to the evaporative crystallization solvent is 2% by weight or less, and the ratio of solvent a to the evaporative crystallization solvent is 98% by weight or more. In addition, the invention ensures that the first evaporation crystallization and the second crystallization are carried out in the presence of ethanol, thereby being more beneficial to preventing the occurrence of oil precipitation and improving the product quality of caprolactam.
According to the present invention, preferably, ethanol is present in the evaporative crystallization solvent in an amount of 0.5 to 2 wt%, more preferably 1 to 2 wt%, based on the total amount of the evaporative crystallization solvent. By adopting the preferable scheme, the addition of the ethanol can improve the crystallization temperature, inhibit and delay the phenomenon of oil precipitation, thereby being more beneficial to improving the yield and quality of caprolactam. In the prior art, ethanol is not added, oil precipitation is easy to occur, and the product quality of caprolactam is seriously affected, because a large amount of impurities are enriched on caprolactam crystals when recrystallization occurs after oil precipitation.
The invention has no limitation on the gas phase Beckmann rearrangement reaction, and is only beneficial to improving the yield and quality of caprolactam; preferably, the gas phase Beckmann rearrangement reaction is carried out in the presence of a molecular sieve catalyst of MFI structure.
In the present invention, preferably, the molecular sieve catalyst of MFI structure contains a silicon molecular sieve having MFI topology structure and a binder; the content of the molecular sieve in the catalyst is 50-95 wt% based on the dry weight of the catalyst, and the content of the binder in terms of oxide is 5-50 wt%;
the molecular sieve contains metal elements, and ions of the metal elements have Lewis acid characteristics; the content of the metal element in the molecular sieve is 5-100 mug/g based on the total amount of the molecular sieve.
In the present invention, the ion of the metal element having Lewis acid property means that the ion of the metal element can accept an electron pair.
The content of metal elements in the MFI topological structure silicon molecular sieve is extremely trace, and it can be judged that trace metal elements exist in a molecular sieve framework in the form of metal ions.
Preferably, in the silicon molecular sieve with the MFI topological structure, the metal element exists on the molecular sieve framework in the form of metal cations.
In the present invention, the content of the metal element was measured using an ICP inductively coupled plasma atomic emission spectrometer 7000DV (perkin elmer) company in the united states, under the following test conditions: the molecular sieve is dissolved by HF acid or aqua regia to thoroughly dissolve silicon oxide and metal oxide in the sample, and the metal ion content is measured in the water solution.
The invention has wider selection range of the content of the silicon element and the oxygen element in the molecular sieve, and in a specific preferred embodiment, the sum of the content of the silicon element, the content of the oxygen element and the content of the metal element in the molecular sieve is 100 percent by taking the total amount of the molecular sieve as the reference.
According to a preferred embodiment of the present invention, the content of the metal element in the molecular sieve is 6 to 90. Mu.g/g, preferably 30 to 80. Mu.g/g, based on the total amount of the molecular sieve. Specifically, for example, it may be 30. Mu.g/g, 35. Mu.g/g, 40. Mu.g/g, 45. Mu.g/g, 50. Mu.g/g, 55. Mu.g/g, 60. Mu.g/g, 70. Mu.g/g, 75. Mu.g/g, 80. Mu.g/g, or any value in a range constituted by any two of these values. In this preferred embodiment, the catalyst has better catalytic performance, which is more beneficial to improving the conversion rate of cyclohexanone oxime and the selectivity of caprolactam. In the invention, the excessive content of the metal element can enhance the Lewis acid characteristic of the molecular sieve, induce unnecessary side reaction and be unfavorable for improving the selectivity of caprolactam; and the less content of the metal element is unfavorable for prolonging the service life of the catalyst and improving the stability.
In the present invention, a metal element having Lewis acid character of ions is used in the present invention, and preferably, the metal element is at least one selected from the group consisting of transition metal elements, group IIIA and group IVA elements. Preferably, the transition metal element is selected from at least one of group IB, group IIB, group IVB, group VB, group VIB, group VIIB and group VIII metal elements.
According to a preferred embodiment of the present invention, the metal element is selected from at least one of Al, ga, ge, ce, ag, co, ni, cu, zn, mn, pd, pt, cr, fe, au, ru, rh, pt, rh, ti, zr, V, mo and W.
Still further preferably, the metal element has an ion valence of +3 and/or an ion valence of +4. The inventors of the present invention found during the course of the study that the use of a metal element having an ionic valence of +3 and/or an ionic valence of +4 is more advantageous for the metal element to enter the molecular sieve framework and for the charge balance.
According to the present invention, the metal element is further preferably at least one of Fe, al, ga, ge, cr, ti, zr and Ce element. In this preferred embodiment, it is more advantageous to increase the performance of the catalyst, thereby increasing the conversion of cyclohexanone oxime and the selectivity of caprolactam.
According to a preferred embodiment of the invention, the binder is silica.
The preparation method, the particle size, the shape and the loading of the catalyst are wide in selection range, and a person skilled in the art can select the catalyst appropriately according to a specific reactor for carrying out the cyclohexanone oxime gas-phase Beckmann rearrangement reaction. For example, the cyclohexanone oxime gas-phase Beckmann rearrangement reaction may be carried out in any one of a fixed-bed reactor, a radial moving-bed reactor, a single-stage and multistage fluidized-bed reactor, a reactor in which a fluidized bed is combined with a fixed bed, and a reactor in which a fluidized bed is combined with a moving bed. Preferably, the cyclohexanone oxime gas-phase Beckmann rearrangement reaction is carried out in a fixed-bed reactor, and the catalyst is in a shape of bar (obtainable by extrusion molding) or sphere (obtainable by rotational molding) and has a diameter of 1-3mm. Preferably, the cyclohexanone oxime gas phase Beckmann rearrangement reaction is carried out in a moving bed reactor, the catalyst is spherical, and the particle size of the catalyst is 0.5-3mm, preferably 0.8-2.5mm (obtained by rotary forming). Preferably, the cyclohexanone oxime gas phase Beckmann rearrangement reaction is carried out in a fluidized bed reactor, the catalyst is spherical, and the particle size of the catalyst is 20-200 μm, preferably 40-150 μm (which can be obtained by spray forming). The amount of catalyst to be charged in each reactor and the shape of the catalyst to be charged are not particularly limited in the present invention, and the shape of the catalyst to be charged may be appropriately selected depending on the kind of the reactor.
The method for preparing the catalyst used for carrying out the vapor phase Beckmann rearrangement reaction is not limited, as long as the specific catalyst can be prepared. In a preferred embodiment of the present invention, the catalyst used is spray-formed, and specifically, the preparation method of the catalyst comprises the following steps:
(a-1) mixing ethyl orthosilicate, ethanol, a metal source, tetrapropylammonium hydroxide, and water to obtain a colloidal mixture; wherein SiO is used as 2 The mole ratio of the tetraethoxysilane, the ethanol, the tetrapropylammonium hydroxide and the water is 1: (4-25): (0.06-0.45): (6-100); in SiO form 2 The weight ratio of the tetraethoxysilane to the metal source calculated by metal element is (10000-200000): 1, a step of;
(a-2) subjecting the colloidal mixture to two-stage variable temperature ethanol-hydrothermal system crystallization, the conditions of the two-stage variable temperature ethanol-hydrothermal system crystallization comprising: crystallizing at 40-78deg.C for 0.5-5 days, and crystallizing at 80-130deg.C for 0.5-5 days;
(a-3) concentrating the crystallized mother liquor obtained in the step (a-2) to obtain molecular sieve slurry;
(a-4) mixing the molecular sieve slurry with a binder and pulping to obtain molecular sieve-binder slurry; carrying out spray forming on the molecular sieve-binder slurry, and then roasting;
(a-5) contacting the product obtained by the calcination of step (a-4) with an alkaline buffer solution of a nitrogen-containing compound, followed by drying;
ions of the metal element in the metal source have Lewis acid characteristics.
In the present invention, the molar ratio and the weight ratio of the materials in the catalyst preparation process refer to the molar ratio and the weight ratio of the amount of the materials in the feed (charge) unless otherwise specified.
According to a preferred embodiment of the present invention, the catalyst provided by the present invention is prepared by a method which does not involve the addition of an organic amine. In this preferred embodiment, the catalyst performs better. In the invention, tetrapropylammonium hydroxide can be used as an organic base and can also be used as a template agent without adding organic amine. In the present invention, the organic amine refers to at least one of aliphatic amine compounds, and for example, may be at least one of mono-n-propylamine, di-n-propylamine, tri-n-propylamine, ethylamine, n-butylamine, ethylenediamine and hexamethylenediamine.
Preferably, the catalyst containing the molecular sieve with a specific structure is obtained by adopting a specific silicon source, a specific metal source and a specific organic template agent and matching ethanol under the condition of specific dosage, and the catalyst has better catalytic performance. The catalyst is particularly suitable for the cyclohexanone oxime gas-phase Beckmann rearrangement reaction, and is more beneficial to improving the economy of the whole process.
According to a preferred embodiment of the invention, siO 2 The mole ratio of the tetraethoxysilane, the ethanol, the tetrapropylammonium hydroxide and the water is 1: (4-15): (0.06-0.3): (15-50), further preferably 1: (6-14): (0.1-0.25): (20-40). In this preferred embodiment, the catalyst produced has better catalytic performance.
According to a preferred embodiment of the invention, siO 2 The weight ratio of the tetraethoxysilane to the metal source calculated by metal element is (10000-100000): 1, more preferably (15000-50000): 1. in this preferred embodiment, the amount of metal incorporated into the molecular sieve framework is more desirable to enhance the catalytic performance of the catalyst.
According to the method provided by the invention, the metal elements in the metal source are selected as described above, and are not described herein.
The metal source of the present invention is a wide range of choices, and is a compound containing various metal elements capable of providing the above metal elements, and the compound containing metal elements is preferably soluble. In the present invention, the term soluble means capable of being dissolved directly in a solvent, or in the presence of a cosolvent, preferably water.
According to the present invention, preferably, the metal source is selected from at least one of nitrate of a metal, chloride of a metal, sulfate of a metal, acetate of a metal, and ester metal compound. In a specific embodiment, the ester metal compound is tetraethyl titanate and/or tetrabutyl titanate.
According to the present invention, preferably, when the metal is an Al element, the metal aluminum source may also be a compound in the form of alumina such as SB powder, V250, pseudo-boehmite, or the like.
According to a preferred embodiment of the invention, the metal source is preferably Fe (NO 3 ) 3 、Ni(NO 3 ) 2 Tetrabutyl titanate, pd (NO) 3 ) 2 、Ce(NO 3 ) 4 、Al(NO 3 ) 3 、Cu(NO 3 ) 2 、ZrOCl 2 、Ga(NO 3 ) 3 、H 2 PtCl 6 And Cr (NO) 3 ) 3 At least one of them is more preferably Fe (NO 3 ) 3 Tetrabutyl titanate, al (NO) 3 ) 3 、Ga(NO 3 ) 3 And Cr (NO) 3 ) 3 At least one of them. The metal source may or may not contain crystal water, and the present invention is not particularly limited thereto.
The order of mixing in the step (a-1) is not particularly limited as long as the colloidal mixture can be obtained, and any two of them may be mixed first and then mixed with the remaining substances, or any three of them may be mixed first and then mixed with the remaining substances. Preferably, gel formation during the addition is avoided, and excessive liquid phase temperature rise during the addition is also prevented.
According to the present invention, preferably, the mixing of step (a-1) comprises: ethanol and tetrapropylammonium hydroxide were mixed, then ethyl orthosilicate was added, followed by water and a metal source.
The invention has a wide range of specific operational options for the mixing, which according to a preferred embodiment of the invention is carried out under stirring. In the present invention, the stirring time is not particularly limited, so long as the colloidal mixture can be obtained. For example, stirring may be carried out at room temperature (25 ℃) for 2 to 6 hours.
According to a preferred embodiment of the present invention, the two-stage temperature-variable ethanol-hydrothermal system crystallization conditions include: crystallizing at 50-80deg.C for 1-1.5 days, and crystallizing at 100-120deg.C for 1-3 days. Under the preferential mode, the utilization rate of crystallization raw materials is further improved under the specific crystallization condition, and the prepared catalyst containing the molecular sieve has better catalytic performance. In the present invention, the two-stage temperature-variable ethanol-hydrothermal system crystallization is preferably performed under a closed system under autogenous pressure, for example, in a closed reaction vessel.
According to the invention, the crystallization mother liquor preferably has a pH of more than 11, preferably not less than 13, for example 13-14.
In the present invention, the ethanol-water system crystallization means that the crystallization is performed under saturated vapor pressure at a specific temperature in the presence of alcohol and water.
The concentration mode of the step (a-3) is selected in a wide range, so long as the purpose of increasing the solid content of the molecular sieve slurry can be achieved.
According to the present invention, preferably, before said concentrating, step (a-3) further comprises: the crystallization mother liquor is washed until the pH of the washing water of the crystallization product is below 9.4, preferably below 9.2, for example pH 8.5-9.2. The washing method of the present invention is not particularly limited, and various washing methods currently used in the art may be used, and the washing agent used in the washing process of the present invention is not particularly limited, and may be, for example, water.
According to a preferred embodiment of the invention, the crystallization mother liquor is washed with water at 20-80 ℃.
According to a preferred embodiment of the invention, the washing and concentration of the molecular sieve is achieved by means of membrane filtration, for example by means of a six-tube membrane. Specific operations are well known to those skilled in the art and will not be described in detail herein.
According to the method provided by the invention, preferably, the method further comprises: before the concentration of step (a-3), if further comprising washing, preferably before washing, the crystallization mother liquor is subjected to ethanol removal. In the present invention, in industry, since ethanol contains organic oxygen, which is discharged into waste water, there is an environmental problem, and thus an ethanol removal operation is required.
The condition selection range of the ethanol removing method is wide, and the purpose of removing the ethanol is that, preferably, the condition of removing the ethanol comprises: the temperature is 50-90 ℃, preferably 60-90 ℃; the time is 1-24 hours, preferably 1-12 hours. Specifically, the reaction kettle is opened after the temperature of the reaction kettle is reduced to the operable temperature, and the reaction kettle is raised to 50-90 ℃ so that the ethanol is evaporated. In the invention, in the ethanol expelling operation, water can be added into the reaction kettle to maintain the liquid level of the reaction kettle, thereby being beneficial to improving the ethanol expelling efficiency.
In the present invention, the solid content of the molecular sieve slurry is selected to be wide, preferably, the solid content of the molecular sieve slurry in the step (a-3) is 15 to 40% by weight, and preferably, 20 to 35% by weight. In this preferred case, it is more advantageous to improve the performance of the catalyst produced.
According to the invention, preferably, in step (a-4), the solids content of the molecular sieve-binder slurry is from 10 to 40 wt%, preferably from 10 to 35 wt%. In this preferred case, the spray formation is more favored, resulting in a lower attrition index of the catalyst.
Preferably, according to the present invention, the molecular sieve in dry basis and the SiO in the molecular sieve-binder slurry 2 The weight ratio of the binder is 1: (0.05-1), preferably 1: (0.4-0.8), more preferably 1: (0.55-0.7). In this preferred case, the catalyst has better performance, which is more advantageous for improving the conversion of cyclohexanone oxime and the selectivity of caprolactam.
According to the method provided by the invention, preferably, the binder is a precursor of silicon oxide. The invention has wider selection range for the precursor of the silicon oxide, and can be converted into the silicon oxide by subsequent roasting. Preferably, the precursor of the silicon oxide is silica sol and/or white carbon black, and more preferably silica sol. The silica sol and the white carbon black can be obtained through commercial purchase.
According to the present invention, preferably, in the silica sol, siO 2 The content is 20 to 45% by weight, preferably 30 to 40% by weight.
According to the invention, the silica sol may also contain sodium ions, the content of the sodium ions is selected in a wide range, and preferably the content of the sodium ions is not higher than 1000 mug/g. In this preferred case, it is more advantageous to improve the performance of the catalyst.
The spray forming of the present invention is well known in the art. Preferably, the spray-forming conditions are such that the particles obtained by the spray-forming have a particle size of 20 to 200. Mu.m, preferably 40 to 150. Mu.m.
According to a preferred embodiment of the present invention, the spray forming conditions include: the inlet temperature is 180-240 ℃, preferably 200-220 ℃; the outlet temperature is 80-120 ℃, preferably 90-105 ℃. In this preferred embodiment, the catalyst has better performance, thereby being more advantageous for improving the conversion of cyclohexanone oxime and the selectivity of caprolactam.
According to the present invention, preferably, the conditions of the firing include: the temperature is 200-600deg.C, preferably 250-550deg.C, and the time is 1-20 hr, preferably 2-18 hr.
According to the invention, preferably, the firing may employ a staged firing, in particular, for example, the firing may include stage 1) and stage 2); the conditions of stage 1) include: the temperature is 200-400 ℃ and the time is 2-10h; the conditions of stage 2) include: the temperature is 400-600 ℃ and the time is 2-15h. And (5) feeding.
According to a preferred embodiment of the present invention, the basic buffer solution of the nitrogen-containing compound contains an ammonium salt and a base.
The solvent of the alkaline buffer solution of the nitrogen-containing compound is selected in a wide range, and water is preferable.
In the present invention, the ammonium salt is preferably ammonium nitrate and/or ammonium acetate.
According to the present invention, preferably, the base is selected from at least one of ammonia, tetramethylammonium hydroxide, tetraethylammonium hydroxide and tetrapropylammonium hydroxide, preferably ammonia.
According to a preferred embodiment of the invention, the ammonium salt is present in an amount of 0.1 to 20 wt.%, preferably 0.5 to 15 wt.%; the alkali content is 5-30 wt%, preferably 10-28 wt%.
According to the present invention, the alkaline buffer solution of the nitrogen-containing compound preferably has a pH of 8.5 to 13.5, preferably 10 to 12, more preferably 11 to 11.5.
The amount of the basic buffer solution of the nitrogen-containing compound used in the present invention is selected to be wide in the range, and preferably, the amount of the basic buffer solution of the nitrogen-containing compound used is 500 to 1500 parts by weight, preferably, 700 to 1200 parts by weight, with respect to 100 parts by weight of the baked product on a dry basis.
According to the present invention, preferably, the contacting conditions include: the temperature is 50-120deg.C, preferably 70-100deg.C; the pressure is 0.5-10kg/cm 2 Preferably 1.5-4kg/cm 2 The method comprises the steps of carrying out a first treatment on the surface of the The time is 0.1-5 hours, preferably 1-3 hours. In the present invention, the contacting is preferably performed under stirring conditions. The stirring speed is not particularly limited in the present invention, and those skilled in the art can appropriately select the stirring speed according to the actual situation.
According to the method provided by the invention, the contacting process can be repeatedly operated. The number of repetitions is not particularly limited in the present invention, and may be determined according to the effect of the contact in order to make the catalyst better in performance, and may be repeated 1 to 3 times, for example.
In the present invention, the conditions for drying the product obtained by contacting the calcined product with the basic buffer solution of the nitrogen-containing compound are not particularly limited, and may be any means known in the art as long as the solvent is removed, and the drying method includes, but is not limited to, natural drying, heat drying, and air-blast drying, and specifically, for example, the drying temperature may be 100 to 120 ℃ and the drying time may be 2 to 36 hours.
According to the present invention, preferably, the step (a-5) may further include: and (3) before the drying, sequentially filtering and washing the material obtained after the roasting product obtained in the step (a-4) is contacted with the alkaline buffer solution of the nitrogen-containing compound. The detergent used in the washing process is not particularly limited, and may be, for example, water. In particular, the washing process may include: washing until the pH of the filtrate is 9-10.5.
In the present invention, the particle size of the catalyst and the preparation method can be freely selected by those skilled in the art according to the type of the reactor, for example, preferably, when the catalyst is carried out in a moving bed reactor, the catalyst may be obtained by rotational molding to obtain a catalyst having a particle size of mm, and the preparation method may include: after the steps (a-1) and (a-2) are adopted, the crystallization mother liquor obtained in the step (a-2) is filtered and dried in sequence to obtain molecular sieve raw powder; pulverizing the molecular sieve raw powder, mixing with a binder, and then performing rotational molding to obtain spherical particles; the spherical particles are baked, contacted with an alkaline buffer solution of a nitrogen-containing compound, and then dried. The process conditions in the preparation method may be referred to as conventional conditions in the art, and the present invention is not limited thereto.
The inventors of the present invention have found during the course of the studies that the use of the catalyst of the above specific structure in the above specific process can improve the selectivity of caprolactam and the conversion of cyclohexanone oxime.
The invention has no limitation on the conditions of the gas-phase Beckmann rearrangement reaction, and is only beneficial to improving the yield and quality of caprolactam; preferably, the conditions of the vapor phase beckmann rearrangement reaction include: the temperature is 320-450 ℃ and the pressure is 0.05-0.5MPa. The weight hourly space velocity of cyclohexanone oxime is not more than 10h -1 Preferably 1 to 10 hours -1 More preferably 2 to 6 hours -1 。
According to the present invention, the solvent of the reaction may be any solvent existing in the art, such as an alcohol; preferably, the solvent for the cyclohexanone oxime vapor phase Beckmann rearrangement reaction is ethanol. The inventors of the present invention found in the course of the research that, since the gas phase beckmann rearrangement reaction product has a wide variety of impurities, a deep color and a poor chromaticity, it is difficult to completely decolorize by adopting the normal crystallization process, caprolactam needs to be distilled out from the top of the column by the subsequent distillation method, and the colored substances remain at the bottom of the column, even if so, the chromaticity of caprolactam is sometimes still not ideal, and thus, the quality of caprolactam product will be seriously affected finally. The solvent of the cyclohexanone oxime gas-phase Beckmann rearrangement reaction adopts ethanol, so that the selectivity of caprolactam is higher, the types and the content of byproducts are fewer, the improvement of the yield and the quality of caprolactam is facilitated, and the specific refining method of the invention in the corresponding crystallization solvent containing a specific small amount of ethanol can well solve the problems and further improve the quality and the yield of caprolactam products.
According to a preferred embodiment of the invention, the crude caprolactam is obtained by: in the presence of a molecular sieve catalyst with an MFI structure, the temperature is 320-420 ℃, the pressure is 0.05-0.5MPa, and the weight hourly space velocity of cyclohexanone oxime is 1-10h -1 Ethanol is used as a reaction solvent, and in the presence of carrier gas, the mixture is reacted in a fixed bed reactor (the weight hourly space velocity of cyclohexanone oxime is 1h -1 The following), a moving bed reactor (weight hourly space velocity of cyclohexanone oxime at 1h -1 Below), a fluidized bed reactor (weight hourly space velocity of cyclohexanone oxime at 5h -1 Left and right), fluidized bed reactor + fixed bed reactor (weight hourly space velocity of cyclohexanone oxime is 5 h) -1 Left and right), fluidized bed reactor + moving bed reactor (weight hourly space velocity of cyclohexanone oxime is 5 h) -1 Left and right), and the like, and then carrying out solvent recovery, removing a small amount of water in the reaction product, removing light component impurities with a boiling point lower than that of caprolactam, and removing heavy component impurities with a boiling point higher than that of caprolactam, thereby obtaining crude caprolactam, wherein the caprolactam content is not less than 99 weight percent. In the present invention, the carrier gas may be various gases which do not react with the cyclohexanone oxime and the solvent under the condition of the vapor phase Beckmann rearrangement reaction, and may be any of the existing gases Inert gas, preferably nitrogen.
The crude caprolactam is not particularly limited in the present invention, as long as the crude caprolactam product obtained from the vapor phase Beckmann rearrangement of cyclohexanone oxime can be used in the present invention. Preferably, said crude caprolactam further comprises at least one of cyclohexene, cyclohexadiene, acetonitrile, ethyl-acrylonitrile, propionitrile, ethoxy-cyclohexene, butyronitrile, ethyl-valeronitrile, cyclopentanone, ethylcyclopentanone, valeronitrile, ethylpyridine, ethyl hexenoate, ethoxy-1, 3-cyclohexadiene, ethoxy-1, 4-cyclohexadiene, cyclohexanone, hexanitrile, cyanocyclopentane, ethyl-epsilon-caprolactone imide, ethyl-cyclohexanone, 5-cyano-1-pentene, ethoxy-cyclohexanone, cyclohexenone, cyclohexanol, phenol, bicyclo [3.1.0] -pentanone-2, N-diethyl-aniline, N-ethyl-aniline, ethyl-aniline, N-hexanamide, N-valeramide, valerolactam, N-ethyl-caprolactam, 1,2,3,4,5,6,7, 8-octahydroacridine, 1,2,3,4,5,6,7, 8-octahydrophenazine, decahydrophenazine, 1,3,4, 5-tetrahydroazepine-2, 6, 5-tetralin-6, 5-tetrahydronaphthacene-2, 6, 5-tetralin and 1,3, 5-tetralin and 3, 5-tetralin. The inventors of the present invention have found that the yield and quality of caprolactam can be further improved by using this preferred embodiment, and that in the prior art (for example, japanese patent application), crude caprolactam, in which a large number of impurity types are present and which is largely different from the aforementioned impurity types in the crude caprolactam of the present invention, is used as a crystallization raw material by using methanol as a reaction solvent and only removing heavy components, is difficult to separate and purify.
In the present invention, the crude caprolactam may further contain other undetermined impurities, wherein the sum of the caprolactam and the impurities satisfies 100%, and the content of each impurity in the crude caprolactam is not limited in the present invention.
In the present invention, the method for obtaining the crude caprolactam of the above specific composition is not limited, and it is preferable to prepare the crude caprolactam of the above specific composition by using a solution in which the solvent of the vapor phase Beckmann rearrangement reaction of cyclohexanone oxime is ethanol. The adoption of the preferable scheme ensures that the selectivity of the caprolactam is higher, and is more beneficial to improving the yield and quality of the caprolactam.
According to the invention, the content of the components in the above-mentioned crude caprolactam can be selected within wide limits, preferably the crude caprolactam has a caprolactam content of from 99 to 99.7% by weight, based on the total amount of the crude caprolactam, and a content of from 0.01 to 0.1% by weight of 1,2,3,4,5,6,7, 8-octahydroacridine, 1,2,3,4,5,6,7, 8-octahydrophenazine, 5,6,7, 8-tetrahydro-2-naphthylamine and 1,3,4, 5-tetrahydrocarbazole and 1,3,4, 5-tetrahydro-2H-azepin-2-one and isomers thereof. With this preferred scheme, 1,2,3,4,5,6,7, 8-octahydroacridine, 1,2,3,4,5,6,7, 8-octahydrophenazine, 5,6,7, 8-tetrahydro-2-naphthylamine and 1,3,4, 5-tetrahydrocarbazole, and 1,3,4, 5-tetrahydro-2H-azepin-2-one and isomers thereof can be further converted to caprolactam by subsequent hydrogenation, thereby further increasing the yield of caprolactam.
In the present invention, preferably, as shown in fig. 1 or fig. 2, in step (2), a person skilled in the art may mix and dissolve the crude caprolactam with the evaporative crystallization solvent according to actual needs, and perform the first evaporative crystallization after completely dissolving.
The invention has wider optional range for the solvent A, and is only beneficial to improving the yield and quality of caprolactam; preferably, the solvent A is at least one selected from halogenated hydrocarbons, ethers and alkanes having 6 to 12 carbon atoms. Under the preferred scheme, the quality and the yield of caprolactam can be further improved. The inventor of the present invention found that although caprolactam has a low solubility in alkanes, the preferred embodiment of the present invention can further improve the yield of caprolactam, and has the advantages of low price, safety, availability, proper boiling point and low energy consumption for recovering solvents.
According to the present invention, preferably, the halogenated hydrocarbon is at least one of 1-chloropropane, 2-chloropropane, chloro-n-butane, 2-chlorobutane, chloroisobutane, chloro-sec-butane, chloro-tert-butane, n-bromopropane, bromoisopropyl, 1-bromobutane and 2-bromobutane.
According to the present invention, the ether is preferably an ether having 2 to 6 carbon atoms, and more preferably at least one of methylether, diethyl ether, n-propyl ether, isopropyl ether, n-butyl ether, ethylbutyl ether, ethyleneglycol dimethyl ether, vinyl ether, methyl t-butyl ether, and ethyl t-butyl ether.
More preferably, the ether is isopropyl ether. The inventor further found that isopropyl ether is particularly suitable as a crystallization solvent for caprolactam under the preferred scheme, and particularly suitable for evaporative crystallization, because isopropyl ether has a low boiling point, and can be easily recovered and reused (only by vacuumizing), so that the caprolactam product has good quality, and the yield of caprolactam can be ensured by combining the specific first and second crystallization methods.
According to the present invention, the alkane is not limited in kind, and may be a linear aliphatic hydrocarbon, a branched aliphatic hydrocarbon, or a cycloalkane; preferably, the alkane with the carbon number of 6-12 is selected from at least one of n-hexane, n-heptane, n-octane, n-nonane, methyl hexane, isohexane, neohexane, isoheptane, isooctane, isononane, cyclohexane, methylcyclopentane and methylcyclohexane.
Preferably, the alkane having 6 to 12 carbon atoms has a boiling point of 60 to 180 ℃, preferably 70 to 130 ℃.
According to the present invention, preferably, the solvent a is selected from at least one of isopropyl ether, n-heptane, isooctane, sec-butane chloride, and cyclohexane. By adopting the preferable scheme, the quality and the yield of caprolactam are improved more favorably. Based on the preferred scheme, the inventor of the present invention further researches that the solubility of caprolactam in n-heptane is particularly small, so that the crystallization yield is particularly high, but oil precipitation phenomenon occurs when corresponding crystallization is carried out, the integral quality of caprolactam is seriously influenced, and meanwhile, the solubility of impurities in n-heptane is also particularly small, so that the impurity removing capability is not strong as isopropyl ether; and isopropyl ether has low boiling point, strong volatility, wide explosion range, high saturated water content, high caprolactam and impurity dissolving capacity and better product quality. Further, the isopropyl ether and the n-heptane are mixed, and the isopropyl ether and the ethanol are mixed to obtain more ideal use results, so that the product quality of caprolactam can be ensured, the yield of caprolactam crystallization can be ensured, and meanwhile, no oil precipitation phenomenon exists.
In a preferred embodiment of the present invention, the evaporative crystallization solvent is selected from the group consisting of ethers and ethanol. By adopting the preferable scheme, the advantages of removing impurities, removing colored substances and improving the chromaticity of caprolactam of the corresponding crystallization solvent can be fully exerted, so that the quality and the yield of caprolactam are further improved. The inventors have further found that, with the preferred embodiment of the invention, the addition of a specific amount (less than 2% by weight) of ethanol brings the higher saturated water in the ether out by azeotropic distillation, preventing oil precipitation. However, the high saturated water content in the ether in the prior art causes a series of problems, such as free water in the crystallization system, lower yield of the crystallized product and poorer quality of the product.
The invention has wider selection range of the dosage of the evaporating crystallization solvent, and is only beneficial to improving the yield and quality of caprolactam; preferably, in step (2), the evaporative crystallization solvent is used in an amount such that the caprolactam solids content in the mixture of crude caprolactam and evaporative crystallization solvent is below 35 wt%, more preferably 25-33 wt%.
According to the invention, preferably, in step (4), the first crystallization mother liquor is concentrated such that the resulting product has a caprolactam solids content of 15% by weight or more, more preferably 18% by weight or more, most preferably 18-25% by weight.
In the invention, the first evaporation crystallization is carried out under the vacuum condition, so that the used equipment is simple and easy to operate; in the prior art, falling crystallization (for example, CN104024221a, CN104011017 a) is performed under an atmospheric pressure closed system, cooling equipment is required, and the upper part of the crystallizer wall is scarred due to splashing of the feed. In the invention, the vacuum degree refers to absolute vacuum degree, and can be regulated and controlled by controlling the operation pressure.
According to the present invention, preferably, in the step (2), the conditions of the first evaporative crystallization include: the final temperature is 10-65deg.C, preferably 30-60deg.C, more preferably 30-40deg.C; the vacuum degree is 5 to 80kPa, preferably 10 to 60kPa, more preferably 20 to 30kPa.
In the present invention, the second crystals may be second evaporative crystals or second isothermal crystals.
According to the present invention, the final temperature of the second crystal may be the same as or different from the final temperature of the first evaporative crystal; preferably, the final temperature of the second crystals is not higher than the final temperature of the first evaporative crystals, more preferably, the final temperature of the second crystals is 5-20 ℃, preferably 10-20 ℃ lower than the final temperature of the first evaporative crystals. With this preferred embodiment, the yield of caprolactam can be further improved.
In the present invention, when the evaporative crystallization method is adopted, the final temperature refers to the final temperature at the time of stopping the experiment corresponding to the evaporative crystallization, and when the isothermal crystallization method is adopted, the final temperature refers to the temperature corresponding to the isothermal crystallization. For example, the final temperature of the first evaporative crystallization refers to the final temperature at which the experiment of the first evaporative crystallization is stopped.
In the invention, in the process of the first evaporative crystallization or the second crystallization (namely the second constant temperature crystallization or the second evaporative crystallization), after reaching the final temperature, the experiment is stopped, and then generally, the person skilled in the art can optionally stay at the final temperature for a period of time according to the actual requirement, so that the crystallization process is more thorough, the crystallization and dissolution processes are perfected, and the crystal quality and the product yield are further improved; preferably, it will remain at the final temperature for 30-100min, more preferably 30-60min.
In the present invention, preferably, in the process of the first evaporative crystallization or the second crystallization (i.e. the second constant temperature crystallization or the second evaporative crystallization), after the crude caprolactam is thoroughly dissolved in the corresponding crystallization solvent, the crude caprolactam can be cooled once to the final temperature, or can be gradually cooled for a plurality of times, preferably, the latter is gradually cooled, and finally reaches the final temperature, the experiment is stopped, and then the person skilled in the art can stay for a certain time according to the actual requirement.
In a preferred embodiment of the present invention, in step (4), the second crystallization is a second isothermal crystallization. Preferably, the conditions of the second isothermal crystallization include: the temperature is 5 to 50 ℃, more preferably 10 to 40 ℃, still more preferably 10 to 20 ℃.
In another preferred embodiment of the present invention, in step (4), the second crystallization is a second evaporative crystallization. In the present invention, the second evaporative crystallization is performed under vacuum. Preferably, the vacuum degree of the second evaporative crystallization is not higher than that of the first evaporative crystallization. More preferably, the vacuum level of the second evaporative crystallization is 5 to 20kPa, more preferably 5 to 15kPa lower than the vacuum level of the first evaporative crystallization. In the present invention, the vacuum degree herein means a final vacuum degree, and a person skilled in the art may perform one-time depressurization or multi-stage stepwise depressurization according to actual needs in the process from the initial vacuum degree to the final vacuum degree, and the latter is preferable.
The conditions for the respective crystallization are not particularly limited in the present invention, and preferably the temperature of the solution (i.e., the respective crystallization solvent) or melt during the crystallization is not higher than the melting point of caprolactam (70 ℃), and is preferably between-10℃and the melting point of caprolactam, especially between 10℃and the melting point of caprolactam.
According to the present invention, preferably, in step (4), the conditions for the second evaporative crystallization include: the final temperature is 5-60deg.C, preferably 10-50deg.C, more preferably 15-25deg.C; the vacuum degree is 0 to 70kPa, preferably 5 to 50kPa, more preferably 10 to 20kPa.
In the present invention, preferably, in the first evaporation crystallization or the second evaporation crystallization process, after the crude caprolactam is thoroughly dissolved in the corresponding crystallization solvent, cooling may be performed first, then vacuumizing is performed, corresponding evaporation crystallization is performed, evaporation amount is controlled, solvent is uniformly evaporated, as the solvent is volatilized continuously, the liquid phase temperature in the crystallization kettle is reduced along with the continuous evaporation, then stepwise cooling is performed, and when stepwise cooling is performed, the operation pressure is gradually reduced (through vacuumizing control), until the solution temperature in the crystallizer reaches the final temperature, the experiment is stopped, and after a certain time (i.e. the maintaining time, preferably the maintaining time is 30-100 min), the corresponding caprolactam crystal alkane solution is obtained. The invention has no limitation on the amplitude of the stepwise cooling or depressurization, and the invention is not limited as long as the improvement of the yield and the quality of caprolactam is facilitated; in general, the operating pressure is determined on the basis of the saturated vapor pressure of the crystallization solvent, for example, in the case of pure isopropyl ether as crystallization solvent, the saturated vapor pressure at 50℃is 387.6mbar, and the operating pressure is operated at a saturated vapor pressure above 50℃ (for example, 410mbar and 460mbar are all possible); generally, the crystallization temperature and the operation pressure are both set in a matched manner, and the operation pressure is correspondingly adjusted every 3-5 ℃ lower.
In the step (2) or the step (4) of the invention, in the first evaporation crystallization or the second crystallization process, the solvent is continuously volatilized and reduced under the corresponding temperature and vacuum, and caprolactam is separated out from the solvent and continuously grows.
In the present invention, the equipment used for the first evaporative crystallization and the second evaporative crystallization is not limited, and the equipment used for the first evaporative crystallization may be, for example, a vacuum adiabatic crystallizer or an adiabatic glass crystallization kettle, and the equipment used for the second crystallization may be a vacuum adiabatic crystallizer, an adiabatic glass crystallization kettle or a thermostatic crystallizer.
According to the present invention, preferably, the temperature of the first washing in step (3) is not lower than the final temperature of the first evaporative crystallization, and preferably the difference between the temperature of the first washing and the final temperature of the first evaporative crystallization is 0 to 2 ℃. The inventors have found that at a temperature above the final temperature of the first evaporative crystallization, some solvent will be present in the caprolactam crystals, whereas at a temperature not above the final temperature of the first evaporative crystallization, less crystals will precipitate.
According to the invention, the amount of the washing solvent used for the first washing can be selected in a wide range, so long as the improvement of the yield of caprolactam is facilitated; preferably, the ratio by weight of the washing solvent used for the first washing to the first caprolactam crystals is from 0.5 to 1.5:1.
According to the present invention, preferably, the temperature of the second washing in step (5) is not lower than the final temperature of the second crystals, and preferably the difference between the temperature of the second washing and the final temperature of the second crystals is 0 to 2 ℃.
According to the invention, the amount of the washing solvent used for the second washing can be selected in a wide range, so long as the improvement of the yield of caprolactam is facilitated; preferably, the weight ratio of the amount of the washing solvent used for the second washing to the third caprolactam crystals is from 0.5 to 1.5:1.
in the present invention, the kind of the washing solvent is not limited as long as the purpose of washing out as much impurities as possible can be achieved; preferably, the washing solvents used for the first washing and the second washing are each independently the same as the evaporative crystallization solvent. By adopting the preferable scheme, on one hand, the washing solvent is more convenient to store and use, and on the other hand, the washing solvent can be directly used as a crystallization solvent after being recovered.
According to the present invention, preferably, as shown in fig. 1, the method further comprises: the first wash liquid is recycled to provide at least part of the evaporative crystallization solvent and/or the wash solvent used for the second wash. By adopting the preferred scheme, the characteristics of low impurity content and low caprolactam content in the first washing liquid obtained by the first washing are fully utilized, the solvent utilization rate is further improved, and the raw material consumption is saved.
The hydrogenation conditions are not limited in the present invention, and may be any hydrogenation conditions existing in the art, and those skilled in the art may freely select the hydrogenation conditions according to actual needs, and preferably, the hydrogenation conditions in the step (6) include: the temperature is 50-150 ℃, the pressure is 0.2-2MPa, and the hydrogen is used in an amount of 0.01-0.25 mol relative to 1 mol of the second caprolactam crystal. In the hydrogenation reaction of the invention, unreacted hydrogen can be recycled. In the present invention, the pressure means absolute pressure unless otherwise specified.
According to the present invention, preferably, the hydrogenation reaction is carried out in the presence of water, in the presence of a hydrogenation catalyst.
In the present invention, preferably, as shown in fig. 1, step (6) includes: the second caprolactam crystal is dissolved with water (or water and hydrogenation catalyst) and then the hydrogenation reaction is carried out.
According to the invention, the water is preferably used in an amount of 10 to 200 parts by weight, preferably 10 to 40 parts by weight, relative to 100 parts by weight of the second caprolactam crystals during the hydrogenation reaction. By adopting the preferable scheme, the method is beneficial to reducing the wastewater discharge and the yield, and simultaneously saves the energy consumption; in the prior art, 30 weight percent of caprolactam and 70 weight percent of water are adopted, the discharge amount of waste water is large, the energy consumption is high, and the loss of caprolactam is easy to cause.
The invention has no limitation on the types of the hydrogenation catalysts, as long as the hydrogenation of impurities in the third caprolactam crystal can be realized; preferably, the hydrogenation catalyst is selected from at least one of a nickel-based catalyst, a palladium-based catalyst and a platinum-based catalyst, and preferably is a nickel-based catalyst. The nickel-based catalyst is preferably an amorphous nickel catalyst.
In the invention, after the hydrogenation reaction, a person skilled in the art can carry out evaporation, dehydration and distillation on a product obtained by the hydrogenation reaction according to actual requirements to obtain a caprolactam product. The invention has no limitation on the evaporation, dehydration and distillation, and is only beneficial to obtaining caprolactam products with high yield and high quality; preferably, the evaporation, dehydration and distillation comprises evaporation, dehydration, triple effect evaporation and distillation, and finally higher-quality caprolactam is obtained; the conditions for evaporation and dehydration can be freely selected by those skilled in the art according to actual requirements, and the present invention is not limited thereto. The invention has no limitation on the evaporation, dehydration and distillation equipment, and is only beneficial to obtaining caprolactam products with high yield and high quality; for example, a rotary evaporator.
In the invention, by carrying out the hydrogenation reaction of the step (6), on one hand, tetrahydroazepine-2-one, isomers thereof and the like which are difficult to sufficiently remove in the crystallization process can be converted into caprolactam, thereby further improving the purity of the finally prepared caprolactam; on the other hand, the potassium permanganate absorption value of the caprolactam product can be effectively improved.
In the present invention, the hydrogenation reaction may be operated batchwise or continuously. The invention has no limitation on the dosage of the hydrogenation catalyst, and can be the prior art, and the person skilled in the art can freely select the hydrogenation catalyst according to the actual requirements. When the hydrogenation reaction is a batch operation, the hydrogenation reaction time may be from 0.5 to 3 hours, more preferably 1-2 hours; the hydrogenation catalyst is used in an amount of 0.001 to 0.02 parts by weight, more preferably 0.003 to 0.01 parts by weight, relative to 100 parts by weight of the third caprolactam crystal; in the existing large industrial device, the concentration of the hydrogenation catalyst is about 50-100 mug/g, the hydrogenation effect can be ensured by adopting the dosage of the hydrogenation catalyst, and the dosage of the hydrogenation catalyst is amplified in equal proportion along with the content of caprolactam. When the hydrogenation reaction is a continuous operation (e.g., a fixed bed process), the caprolactam may have a mass space velocity of from 0.5 to 30 hours -1 。
The reactor for carrying out the hydrogenation reaction is not particularly limited in the present invention, and may be freely selected according to the need by those skilled in the art, and for example, a magnetically stabilized bed reactor, a fixed bed reactor or a slurry bed reactor, preferably a slurry bed reactor, may be employed, wherein the hydrogenation reaction of caprolactam or an aqueous caprolactam solution in a molten state may be selected.
In order to further increase the yield of caprolactam, the process preferably further comprises: recycling said fourth caprolactam crystals to step (2) and mixing with said crude caprolactam for said first evaporative crystallization.
According to a preferred embodiment of the present invention, as shown in fig. 1, the method for refining caprolactam comprises the steps of:
(1) The cyclohexanone oxime gas-phase Beckmann rearrangement reaction product is subjected to solvent recovery, dehydration, light component removal and heavy component removal treatment to obtain crude caprolactam with caprolactam content not less than 99 wt%;
the solvent of the cyclohexanone oxime gas-phase Beckmann rearrangement reaction is ethanol;
(2) In the presence of an evaporative crystallization solvent, performing first evaporative crystallization on the crude caprolactam, and performing first solid-liquid separation to obtain first caprolactam crystals and a first crystallization mother solution;
The amount of the evaporative crystallization solvent is such that the caprolactam solids content in the mixture of crude caprolactam and evaporative crystallization solvent is below 35% by weight, further preferably 25-33% by weight;
the conditions of the first evaporative crystallization include: the final temperature is 10-65deg.C, preferably 30-60deg.C, more preferably 30-40deg.C; the vacuum degree is 5 to 80kPa, preferably 10 to 60kPa, more preferably 20 to 30kPa;
(3) Performing first washing on the first caprolactam crystal to obtain a second caprolactam crystal and a first washing solution;
the temperature of the first washing is not lower than the final temperature of the first evaporative crystallization, and preferably the difference between the temperature of the first washing and the final temperature of the first evaporative crystallization is 0-2 ℃;
(4) Optionally concentrating the first crystallization mother liquor, then performing second crystallization, and performing second solid-liquid separation to obtain a third caprolactam crystal and a second crystallization mother liquor;
concentrating the first crystallization mother liquor to obtain a product with a caprolactam solid content of more than 15 wt%, more preferably more than 18 wt%;
the final temperature of the second crystallization is 5-20 ℃ lower than that of the first evaporation crystallization, preferably 10-20 ℃ lower;
(5) Performing second washing on the third caprolactam crystal to obtain fourth caprolactam and a second washing solution;
The temperature of the second washing is not lower than the final temperature of the second crystal, preferably the difference between the temperature of the second washing and the final temperature of the second crystal is 0-2 ℃;
(6) Carrying out hydrogenation reaction on the second caprolactam crystal obtained in the step (3);
wherein the evaporative crystallization solvent in the step (2) contains a solvent A and ethanol, the solubility of caprolactam in the solvent A is below 5 wt% at 20 ℃, and the ethanol accounts for below 2 wt% of the total amount of the evaporative crystallization solvent.
In a preferred embodiment of the present invention, the second crystallization is a second evaporative crystallization, as shown in fig. 2, and the method further comprises the steps of:
(7) Optionally concentrating the second crystallization mother liquor, then carrying out constant-temperature crystallization, and then carrying out third solid-liquid separation to obtain fifth caprolactam crystals and third crystallization mother liquor;
(8) And performing third washing on the fifth caprolactam crystal to obtain a sixth caprolactam crystal and a third washing solution.
In a preferred embodiment of the present invention, the temperature of the thermostatically-crystallized in step (7) is lower than the final temperature of the second crystal, preferably the temperature of the thermostatically-crystallized is 5-20 ℃ lower than the final temperature of the second crystal. By adopting the preferable scheme of the invention, the product quality and the yield of caprolactam are improved more favorably.
In the present invention, if the caprolactam content in the third crystallization mother liquor is higher, the person skilled in the art may perform the constant temperature crystallization again on the third crystallization mother liquor in the step (7) according to the actual requirement, so as to recover caprolactam in the third crystallization mother liquor as much as possible, or take the measures of adjusting the final temperature of the constant temperature crystallization in the step (7) to be lower, and the crystallization residence time to be longer, etc. so as to recover more caprolactam.
According to the present invention, preferably, the conditions for the constant temperature crystallization include: the temperature is 5 to 50 ℃, more preferably 10 to 40 ℃, still more preferably 10 to 20 ℃. In the invention, the operating pressure of the constant temperature crystallization can be adjusted according to actual requirements by a person skilled in the art, and the crystallization temperature and the operating pressure are generally matched, so that the invention is not limited. In the constant temperature crystallization process of the present invention, one skilled in the art can optionally lengthen the residence time of the constant temperature crystallization according to actual requirements, thereby ensuring that the crystal particles are larger for separation, which is not limited by the present invention; preferably, the residence time (i.e., the hold time) is from 20 to 45 minutes.
In the present invention, the apparatus used for the isothermal crystallization (or the second isothermal crystallization) is not limited, and those skilled in the art can freely select according to actual needs, and for example, a isothermal crystallizer may be used.
According to the present invention, preferably, in step (7), the thermostatically crystallizing is performed in the presence of a thermostatically crystallizing solvent containing a solvent A and ethanol.
Further preferably, in the thermostatically crystallizing solvent, ethanol accounts for 2% by weight or less of the total amount of the thermostatically crystallizing solvent, more preferably 0.5 to 2% by weight, and still more preferably 1 to 2% by weight. In the present invention, when the solvent in the product obtained by optionally concentrating the second crystallization mother liquor satisfies the composition of the above-mentioned constant temperature crystallization solvent, no solvent a or ethanol need to be introduced in step (7); when the solvent in the product obtained by optionally concentrating the second crystallization mother liquor does not satisfy the composition of the above-mentioned thermostatically crystallizing solvent, it is necessary to introduce the solvent a and/or ethanol in step (7) so that the thermostatically crystallization is performed under the thermostatically crystallizing solvent of the above-mentioned specific composition. The optional range of the solvent a is the same as that of the solvent a in the aforementioned evaporative crystallization solvent, and will not be described here.
Preferably, in step (7), the second crystallization mother liquor is concentrated such that the caprolactam solids content of the resulting product is 15-30 wt.%. In step (7) of the present invention, ethanol may be added or not added before concentration according to actual needs, so as to satisfy the aforementioned solid content of caprolactam in the range of 15 to 30% by weight, and to satisfy the constant temperature crystallization in the presence of a constant temperature crystallization solvent containing a specific content of ethanol.
According to the present invention, it is preferable that the temperature of the third washing in the step (8) is not lower than the temperature of the thermostatically-crystallized, and the difference between the temperature of the third washing and the temperature of the thermostatically-crystallized is preferably 0 to 2 ℃.
According to the invention, preferably, the ratio by weight of the amount of the washing solvent used for the third washing to the fifth caprolactam crystals is from 0.5 to 1.5:1.
in the present invention, the type of the washing solvent used for the third washing is not limited as long as the purpose of washing out as much impurities as possible can be achieved; preferably, the washing solvent used for the third washing is the same as the evaporative crystallization solvent. By adopting the preferred scheme, on one hand, the method is more convenient to store and use, is simple and convenient, is economical and applicable, and on the other hand, the method has strong universality, and can be directly used as a crystallization solvent after the washing solvent is recovered.
According to the invention, preferably, the method further comprises: recycling said first wash liquor and/or second wash liquor to provide at least part of the wash solvent used for said third wash.
According to the invention, preferably, the method further comprises: and mixing the third washing liquid with the second crystallization mother liquid to perform the constant-temperature crystallization. In this preferred embodiment, the caprolactam yield can be further improved.
In the present invention, the first washing, the second washing and the third washing may be performed one or more times according to actual needs by those skilled in the art. In the invention, in the first washing, the second washing and the third washing, a person skilled in the art can adopt any conventional solid-liquid separation mode to carry out solid-liquid separation on the washed mixture to obtain corresponding caprolactam and corresponding washing liquid; for example, as shown in fig. 1 or fig. 2, after the first washing in step (3), solid-liquid separation is performed to obtain second caprolactam crystals and a first washing liquid.
In the method of the present invention, preferably, if only the second-stage crystallization is employed, the first-stage is the aforementioned first evaporative crystallization, and the second-stage (i.e., the crystallization performed on the first crystallization mother liquor) is the aforementioned second isothermal crystallization; if a third stage crystallization is required, the first stage is the aforementioned first evaporative crystallization, the second stage is the aforementioned second evaporative crystallization (i.e., crystallization performed on the first crystallization mother liquor), and the third stage is the constant temperature crystallization (i.e., crystallization performed on the second crystallization mother liquor) described in step (7).
In the present invention, the first solid-liquid separation in the step (2), the first washing in the step (3) and the solid-liquid separation corresponding to the first washing, the second solid-liquid separation in the step (4), the second washing in the step (5) and the solid-liquid separation corresponding to the second washing, and the third solid-liquid separation in the step (7), the third washing in the step (8) and the solid-liquid separation corresponding to the third washing may be separately performed by two devices (for example, on two devices of a washing tank and a centrifuge), or may be performed simultaneously by one device (for example, a filter press or a countercurrent washing device) in a countercurrent washing manner, and the present invention is not limited thereto; preferably has a filter press device with two functions of solid-liquid separation and washing.
The solid-liquid separation mode is not limited, and can be any existing solid-liquid separation mode, for example, centrifugal separation or filtering separation can be adopted, a push rod centrifuge can be adopted for the centrifugal separation, one-step or multi-step operation can be adopted, and a sieve plate conveying centrifuge or a screw conveying centrifuge (decanter) can also be adopted; the filtration may be accomplished by suction filters (which may be operated batchwise or continuously, optionally equipped with agitators) or belt filters.
In the present invention, it is preferable to employ concentration in step (4) and step (7), and the present invention is not limited to the manner of concentration and the apparatus employed, and the person skilled in the art can freely select according to actual demands, for example, concentration may be performed in a solvent recovery column by distillation or evaporation. The invention has no limitation on the condition of concentration (namely, solvent recovery), and the person skilled in the art can freely select the condition according to the actual requirement, so long as the concentration and the solvent recovery are facilitated; for example, when the evaporative crystallization solvent is isopropyl ether, the first solvent recovery column (i.e., the concentration column) may be an isopropyl ether solvent recovery column, the column top pressure is normal pressure, the column top temperature is 70 ℃, the column bottom temperature is 90 ℃, and the feed temperature is 71 ℃; the second solvent recovery tower can be a de-ethering tower, the tower top pressure is normal pressure, the tower top temperature is 84-90 ℃, and the tower bottom temperature is 100-105 ℃. The other crystallization solvents and washing solvents, which are different depending on their boiling points, were operated at normal pressure.
In order to further increase the yield of caprolactam, the process preferably further comprises: recycling said fourth caprolactam and said sixth caprolactam crystals to step (2) to be mixed with said crude caprolactam for said first evaporative crystallization.
According to the invention, preferably, the method further comprises: recovering the solvent in the third crystallization mother liquor. The present invention is not limited to the recovery method as long as the recovery can be achieved; for example, the recovery may be performed using a recovery column, the top outlet of which is provided with a recovery solvent, and the bottom outlet of which is provided with a residue of the lower heavy component, which may be discharged or incinerated according to actual demands by those skilled in the art.
In the present invention, the seeding may be added or not freely selected by those skilled in the art according to the need during the first evaporative crystallization, the second crystallization and optionally the isothermal crystallization, which is not limited in the present invention.
In the present invention, the person skilled in the art may perform one or more corresponding crystallization according to the actual requirement (for example, the first evaporation crystallization is performed continuously for multiple times or once), however, by adopting the method provided by the present invention, a good effect can be achieved by performing one corresponding crystallization operation, so that one corresponding crystallization is preferably adopted in the method.
In step (2), step (4) or step (7) of the present invention, since a part of the solvent remains on the surface of the corresponding caprolactam crystals obtained after the corresponding solid-liquid separation, the solvent can be recovered by a stripping method as needed, optionally by a person skilled in the art.
According to a preferred embodiment of the present invention, as shown in fig. 2, the method for refining caprolactam comprises the steps of:
(1) The cyclohexanone oxime gas-phase Beckmann rearrangement reaction product is subjected to solvent recovery, dehydration, light component removal and heavy component removal treatment to obtain crude caprolactam with caprolactam content not less than 99 wt%;
the solvent of the cyclohexanone oxime gas-phase Beckmann rearrangement reaction is ethanol;
(2) In the presence of an evaporative crystallization solvent, performing first evaporative crystallization on the crude caprolactam, and performing first solid-liquid separation to obtain first caprolactam crystals and a first crystallization mother solution;
the amount of the evaporative crystallization solvent is such that the caprolactam solids content in the mixture of crude caprolactam and evaporative crystallization solvent is below 35% by weight, further preferably 25-33% by weight;
the conditions of the first evaporative crystallization include: the final temperature is 10-65deg.C, preferably 30-60deg.C, more preferably 30-40deg.C; the vacuum degree is 5 to 80kPa, preferably 10 to 60kPa, more preferably 20 to 30kPa;
(3) Performing first washing on the first caprolactam crystal to obtain a second caprolactam crystal and a first washing solution;
the temperature of the first washing is not lower than the final temperature of the first evaporative crystallization, and preferably the difference between the temperature of the first washing and the final temperature of the first evaporative crystallization is 0-2 ℃;
(4) Optionally concentrating the first crystallization mother liquor, then performing second crystallization, and performing second solid-liquid separation to obtain a third caprolactam crystal and a second crystallization mother liquor;
concentrating the first crystallization mother liquor to obtain a product with a caprolactam solid content of more than 15 wt%, more preferably more than 18 wt%;
the final temperature of the second crystallization is 5-20 ℃ lower than that of the first evaporation crystallization, preferably 10-20 ℃ lower;
(5) Performing second washing on the third caprolactam crystal to obtain fourth caprolactam and a second washing solution;
the temperature of the second washing is not lower than the final temperature of the second crystal, preferably the difference between the temperature of the second washing and the final temperature of the second crystal is 0-2 ℃;
(6) Carrying out hydrogenation reaction on the second caprolactam crystal obtained in the step (3);
(7) Optionally concentrating the second crystallization mother liquor, then carrying out constant-temperature crystallization, and then carrying out third solid-liquid separation to obtain fifth caprolactam crystals and third crystallization mother liquor;
Concentrating the second crystallization mother liquor to obtain a product with a caprolactam solid content of 15-30 wt%;
the temperature of the constant temperature crystallization is lower than the final temperature of the second crystallization, preferably the temperature of the constant temperature crystallization is 5-20 ℃ lower than the final temperature of the second crystallization;
(8) Performing third washing on the fifth caprolactam crystal to obtain a sixth caprolactam crystal and a third washing solution;
wherein the evaporative crystallization solvent in the step (2) contains a solvent A and ethanol, the solubility of caprolactam in the solvent A is below 5 wt% at 20 ℃, and the ethanol accounts for below 2 wt% of the total amount of the evaporative crystallization solvent;
the method further comprises the steps of: mixing the third washing liquid and the second crystallization mother liquid to perform the constant-temperature crystallization;
the method further comprises the steps of: recycling said fourth caprolactam and said sixth caprolactam crystals to step (2) to be mixed with said crude caprolactam for said first evaporative crystallization.
In the refining method, at least secondary crystallization is adopted to ensure that the caprolactam yield is more than 98.5 percent; the quality of the caprolactam is higher, and the caprolactam of industrial quality products can be obtained, specifically, the absorption value (PM) value of the caprolactam is larger than 10000s, the extinction value (at the wavelength of 290 nm) of the caprolactam is not larger than 0.040, the volatile alkali value is not larger than 0.40mmol/kg, the chromaticity value is not larger than 2, the acidity is not larger than 0.05mmol/kg, and the alkalinity is not larger than 0.05mmol/kg, so that the caprolactam completely meets the requirements of industrial quality products; however, caprolactam obtained by the method in the prior art cannot have both yield and quality, and especially the quality of caprolactam is far from industrial quality.
The present invention will be described in detail by examples. In the following examples and preparations, the materials involved are commercially available unless otherwise specified.
In the following preparation examples, the metal element content was measured using an ICP inductively coupled plasma atomic emission spectrometer 7000DV (PE (perkin elmer)) company in the united states under the following test conditions: dissolving a molecular sieve by using HF acid or aqua regia to thoroughly dissolve silicon oxide and metal oxide in a sample, and measuring the content of metal ions in an aqueous solution;
the external specific surface area and BET specific surface area of the molecular sieve were measured by an automatic adsorption apparatus of the type Micromeritics ASAP-2460 in U.S. Pat. No. 5,000 under the following conditions: n (N) 2 As an adsorbate, the adsorption temperature is-196.15 ℃ (liquid nitrogen temperature), and the constant temperature degassing is carried out for 6 hours at the temperature of 1.3Pa and 300 ℃;
the X-ray diffraction spectrum was recorded by a Miniflex model 600 diffractometer, japan, under the following test conditions: cu target K alpha radiation, ni optical filter, tube voltage 40kV and tube current 40mA;
the prepared sample is analyzed by adopting a field emission scanning electron microscope of model S-4800 of Hitachi corporation;
the particle size and the particle size distribution of the catalyst obtained by spray forming are measured by a 2000E type laser particle size analyzer of Dandong Baite instruments, the test method is wet test, water is used as a medium, and the mass concentration of a sample is as follows: 0.5% -2%, the scanning speed is 2000 times/second;
The spray forming was carried out in a spray forming apparatus manufactured by the tin-free Tianyang spray drying equipment company, model LT-300.
In the following examples, the following test methods were used to evaluate the parameters related to caprolactam crystals and caprolactam products prepared:
(1) Purity of epsilon-caprolactam
The purity of epsilon-caprolactam was measured by gas chromatography, 7890GC, innovax 60m capillary column, and a minimum detection limit of 0.1. Mu.g/g.
(2) Potassium permanganate absorption (PM) of caprolactam
Pouring 3.000 g of epsilon-caprolactam into a 100mL colorimetric tube, adding distilled water to dilute the solution to 100mL, shaking the solution uniformly, placing the solution into a constant-temperature water bath at 20.0 ℃, adding 1mL of potassium permanganate solution with the concentration of 0.01N into the colorimetric tube, shaking the solution uniformly immediately, starting a stopwatch at the same time, and taking 3.000 g of high-grade pure Co (NO 3 ) 2 ·6H 2 O and 12 mg of high-grade pure K 2 Cr 2 O 7 Dissolving in water, diluting to 1 liter, shaking to obtain standard colorimetric solution), stopping the stopwatch when the colors of the standard colorimetric solution are the same, and recording the time (calculated in seconds) consumed, namely the absorption value of potassium permanganate.
(3) Volatile Base (VB)
In alkaline medium, the alkaline low molecular impurities in the sample are distilled out, absorbed by a known amount of hydrochloric acid solution, and excessive hydrochloric acid is dripped back by a sodium hydroxide standard solution. The number of moles of acid consumed per kg of sample was used as a measure of volatile base. The calculation formula is as follows:
VB(mmol/kg)＝[(V 0 -V)×C NaOH /M]×1000
Wherein: v (V) 0 The volume of the NaOH standard solution consumed by the blank test is in mL;
v is the volume of NaOH standard solution consumed by the sample, and the unit is mL;
C NaOH the unit is mol/L which is the accurate concentration of NaOH standard solution;
m is the mass of the sample in g.
(4) Extinction value E (at 290nm wavelength)
In a 300mL Erlenmeyer flask, 50 g of the sample was weighed, 50mL of distilled water was added, and the sample was shaken well to dissolve completely and allowed to stand for 10 minutes. The extinction of a sample having a concentration of 50% by weight with respect to distilled water was measured using a spectrophotometer at a wavelength of 290 nm.
(5) Chromaticity value
In a 300mL Erlenmeyer flask, 50 g of the sample was weighed, 50mL of distilled water was added, and the sample was shaken well to dissolve completely and allowed to stand for 10 minutes. The absorbance of the sample at a concentration of 50% with respect to distilled water was measured at a wavelength of 390nm using a spectrophotometer.
(6) PH value
Dissolving caprolactam sample in water, and titrating the free acid or the free base in the sample by using hydrochloric acid or sodium hydroxide standard solution with methyl red-methylene blue as an indicator. The calculation formula is as follows:
acidity (mmol/kg) = (v×c) HCl )/M×1000
Basicity (mmol/kg) = (v×c) NaOH )/M×1000
Wherein: v is the volume of standard solution consumed by the sample, and the unit is mL;
C HCl The unit is mol/L which is the accurate concentration of NaOH standard solution;
C NaOH the unit is mol/L which is the accurate concentration of NaOH standard solution;
m is the mass of the sample in g.
Preparation example 1
This preparation is intended to illustrate the preparation of the crude caprolactam of the present invention.
1. Preparation of molecular sieve catalysts of MFI structure (spray forming)
(1) 810kg of ethanol with a content of 95% by weight and 305kg of tetrapropylammonium hydroxide aqueous solution with a content of 22.5% by weight are respectively added to 2M 3 In a stainless steel reactor, stirring, then 347kg of ethyl orthosilicate was added, stirring was continued, then 325kg of water and 58.39 g of Al (NO 3 ) 3 ·9H 2 O, continuously stirring for 4 hours at normal temperature to obtain a colloid mixture; wherein SiO is used as 2 Ethyl orthosilicate: ethanol: tetrapropylammonium hydroxide: the molar ratio of water is 1:14:0.2:20, a step of; in SiO form 2 The weight ratio of the tetraethoxysilane to the metal source calculated as metal element is 23700:1, a step of;
(2) And (3) crystallizing the colloid mixture by an ethanol-hydrothermal system, wherein the crystallization conditions comprise: crystallizing at 80deg.C for 1 day, and crystallizing at 100deg.C for 2 days; obtaining crystallization mother liquor, wherein the pH value is 13.68;
(3) Evaporating ethanol from the obtained crystallization mother liquor at 88 ℃ for 7 hours (water is continuously added in the middle, the material is maintained at a certain liquid level, and the ethanol solution containing water is recovered for standby); then washing and concentrating with 50nm six-tube membrane, washing with 40-60deg.C water, and washing water consumption of 6.0M 3 The pH value of the washing water for washing the crystallized product reaches 9.1. Washing and concentrating to obtain 436kg molecular sieve slurry with 24.5 wt% solid content;
drying a small amount of the molecular sieve slurry at 120 ℃ for 20 hours, and roasting at 550 ℃ for 6 hours to obtain the molecular sieve with the metal element content of 41.4 mug/g and the BET specific surface area of 425m 2 Per gram, external specific surface 41m 2 /g；
The X-ray diffraction (XRD) spectrum of the molecular sieve is consistent with the MFI structure standard XRD spectrum characteristics recorded in Microporous Materials, vol 22, p637, 1998, which shows that the molecular sieve has an MFI crystal structure;
transmission electron microscope pictures show that the grain size of the MFI topological structure molecular sieve is uniform, and the grain size is 0.1-0.2 mu m;
(4) Mixing part of molecular sieve slurry obtained in the step (3) with 304kg of alkaline silica sol with the content of 30 weight percent (pH value is 9.5, sodium ion content)324ppm of SiO 2 40% by weight of SiO obtained after calcination 2 Surface area of 225m 2 /g), wherein the molecular sieve is mixed with SiO on a dry basis 2 The weight ratio of the alkaline silica sol is 50:50; adding 60kg of water, uniformly stirring, and pulping to obtain molecular sieve-binder slurry with the solid content of 22.5 wt%; then the mixture is sent into a spray forming device for spray forming, and the inlet temperature and the outlet temperature are 205 ℃ and 100 ℃ respectively. Then send into 3M 3 Roasting in a heated shuttle furnace at 280 deg.C, 400 deg.C and 480 deg.C for 2 hr and finally at 550 deg.C for 12 hr to obtain 181.5kg microsphere molecular sieve, wherein the content of MFI topology structure silicon molecular sieve containing very trace Lewis acid characteristic metal ion is 50 wt% and SiO 2 The binder content was 50% by weight;
95g of the microsphere molecular sieve and 950g of an alkaline buffer solution of a nitrogen compound (the alkaline buffer solution of the nitrogen compound is a mixed solution of ammonia water and an ammonium acetate aqueous solution, the pH value is 11.39, wherein the content of the ammonia water is 26 weight percent, the content of ammonium acetate in the ammonium acetate aqueous solution is 7.5 weight percent, and the weight ratio of the ammonia water to the ammonium acetate aqueous solution is 3:2) are added into a 2000ml stainless steel reaction kettle, and the temperature is 85 ℃ and the weight ratio of the ammonia water to the ammonium acetate aqueous solution is 2.6kg/cm 2 Stirring under pressure for 2 hours at constant temperature, filtering, drying at 90 ℃ for 12 hours, repeating the treatment once under the same conditions, filtering, washing until the pH of the filtered clear liquid is about 9, and drying at 120 ℃ for 24 hours to obtain a catalyst S2;
the particle size of the catalyst is concentrated between 75 and 150 mu m, and the abrasion index K is 1.4.
2. Preparation of molecular sieve catalysts of MFI structure (rotational moulding)
(1) 810kg of ethanol with a content of 95% by weight and 305kg of tetrapropylammonium hydroxide aqueous solution with a content of 22.5% by weight are respectively added to 2M 3 In a stainless steel reactor, 347kg of ethyl orthosilicate was added with stirring, after stirring for 30 minutes, 325kg of water and 58.39 g of Al (NO 3 ) 3 ·9H 2 O, stirring continuously for 4 hours at normal temperature to form a colloid mixture; wherein SiO is used as 2 Ethyl orthosilicate: ethanol: tetrapropyl hydroxideAmmonium: the molar ratio of water is 1:14:0.2:20, a step of; in SiO form 2 The weight ratio of the tetraethoxysilane to the metal source calculated as metal element is 23700:1, a step of;
(2) And (3) crystallizing the colloid mixture by an ethanol-hydrothermal system, wherein the crystallization conditions comprise: crystallizing for 1 day at 80 ℃ and then crystallizing for 2 days at 100 ℃ to obtain crystallization mother liquor;
(3) Evaporating ethanol from the crystallized mother liquor at 88 ℃ for 7 hours (water is continuously added in the middle, the material is maintained at a certain liquid level, and the aqueous ethanol solution is recovered for standby); then washing and filtering in turn, and drying at 120 ℃ for 24 hours to obtain about 131.6kg of molecular sieve raw powder;
taking a proper amount of the molecular sieve raw powder, roasting at 550 ℃ for 6 hours to obtain a molecular sieve sample, wherein the content of metal elements is 41.8ppm, and the BET specific surface area is 425m 2 /g, external specific surface 44m 2 The X-ray diffraction (XRD) spectrum of the product is consistent with the MFI structure standard XRD spectrum characteristics recorded in Microporous Materials, vol 22, p637, 1998, which shows that the molecular sieve has an MFI crystal structure;
As can be seen from a scanning electron microscope photograph, the MFI topological structure molecular sieve has uniform grain size and the grain diameter is 0.1-0.2 mu m;
(4) Pulverizing molecular sieve raw powder, taking 2kg of powder sample which is sieved to 100-1000 meshes, placing into a rotary table forming machine, wherein the diameter of a rotary table of the rotary table forming machine (sugar coating machine, model BY-1200 of Tiantai pharmaceutical machinery plant, jiangsu Taizhou) is 1.2m, the depth of the rotary table is 450mm, the dip angle of the rotary table is 50 degrees, and the rotary speed of the rotary table is set to 30rpm. Spraying 1.4kg of deionized water to obtain first spherical particles with the particle size of about 0.2-0.8 mm;
100kg of the powder was further sieved to 200-800 mesh powder and 20kg of an alkaline silica sol (sodium ion content of 543ppm, siO) 2 Content 30 wt.%) according to 5:1, adding 30kg of water, uniformly mixing and re-crushing, taking particles smaller than 30 meshes, adding 150kg of particles into the rotary table forming machine with the first spherical particles at a constant speed, and finishing the adding within 300 min; sieving with 12 mesh and 9 mesh sieve to obtain about 100kg of spherical particles with particle diameter of 1.5-2 mm;
(5) Blowing the 100kg spherical particles obtained in the above way at 45 ℃, supplementing a very small amount of water in the middle of the process for many times, tightening for 2 hours, drying for 24 hours at 120 ℃, and roasting for 10 hours at 550 ℃ to obtain 73.3kg of a roasted product with 93% molecular sieve content;
45kg of the above calcined product was added to 450kg of an alkaline buffer solution (the alkaline buffer solution is a mixture of aqueous ammonia and an aqueous ammonium nitrate solution, wherein the content of aqueous ammonia is 26% by weight, the content of ammonium nitrate in the aqueous ammonium nitrate solution is 7.5% by weight, the weight ratio of aqueous ammonia to the aqueous ammonium nitrate solution is 3:2, and the pH value of the alkaline buffer solution is 11.35) to 1M 3 In a pressurized reaction kettle, at 100 ℃ and 3.3kg/cm 2 Stirring for 1.5 hours under pressure, and then washing, filtering and drying to obtain a catalyst S2;
the particle size of the catalyst is 1.4-1.8mm, and the crushing strength sigma is 33N/particle.
3. Preparation of crude caprolactam
Preparing crude caprolactam by gas phase Beckmann rearrangement reaction in the presence of water and a catalyst by using cyclohexanone oxime, ethanol and nitrogen as raw materials, wherein the gas phase Beckmann rearrangement reaction of the cyclohexanone oxime is carried out in a self-made fixed fluidized bed reaction device (the diameter of the upper section of a vertical 316L stainless steel reactor is 20cm, the diameter of the lower section of the reactor is 10cm, the lengths of the upper section and the lower section are 60cm and 80cm respectively), and the fixed fluidized bed reaction device is filled with the prepared catalyst S1 with the catalyst loading amount of 300 g; the vertical 316L stainless steel fixed fluidized bed reaction device is communicated with the catalyst regeneration reactor and is used for recycling the catalyst flowing out of the fixed fluidized bed reaction device after regeneration, and the catalyst regeneration reactor has the structure that: diameter 15cm, length 160cm, catalyst loading 550 g. The conditions of the fixed fluidized bed reaction device are as follows: the reaction pressure is 0.1MPa, the reaction temperature is 380 ℃, the material is fed in a gas atomization spraying mode, the temperature of a vaporizer is controlled to be 190 ℃, the heat preservation of a pipeline is kept at 250 ℃, and the WHSV of cyclohexanone oxime is 5h -1 Nitrogen flow rate of 0.8M 3 And/h, the concentration of cyclohexanone oxime in cyclohexanone oxime and ethanol is 35% by weight, and the water content is 0.4% by weight relative to the total amount of water, ethanol and cyclohexanone oxime.
The reaction outlet of the fixed fluidized bed reaction device is connected to a continuous flow fixed bed reactor. The conditions for the continuous flow fixed bed reactor were: the reactor had an inner diameter of 28 mm, and was filled with the spherical catalyst S2 prepared as described above, with a catalyst loading of 80g; bed height: 30cm; reaction pressure: 0.1MPa; the reaction temperature is 380-390 ℃; the weight space velocity (WHSV) of cyclohexanone oxime was 0.5h -1 . Less than 5% by weight of unconverted cyclohexanone oxime in the product obtained in the stationary fluidized bed is converted to approximately 100% in the continuous flow fixed bed reactor.
And (3) running for 60 hours, performing water cooling on the obtained Beckmann rearrangement reaction product, then performing secondary cooling continuously through ethylene glycol solution circulation at the temperature of minus 10 ℃, and collecting the reaction product to obtain an ethanol solution mixture containing caprolactam.
1.6kg of the above mixture was taken, and solvent (ethanol) was recovered by a rotary evaporator to obtain 618.5g of a crude caprolactam product containing impurities having a higher boiling point than caprolactam and impurities having a lower boiling point than caprolactam, and the crude caprolactam product was analyzed to find that the main composition thereof was: 95.8% by weight of caprolactam, as well as other impurities. The sample analysis was performed on an Agilent 6890 gas chromatograph (hydrogen flame ion detector, PEG20M capillary chromatography column, column length 50M).
Carrying out dehydration, light impurity removal and heavy impurity removal treatment on the caprolactam crude product: and (3) carrying out reduced pressure distillation (16 trays and inverted triangle metal filler) on the crude caprolactam product, heating the crude caprolactam product from room temperature (20 ℃) to 100 ℃ for 30min at a speed of 2 ℃/min under the condition that the pressure (absolute pressure) is 4.5kPa, carrying out total reflux on the top of the tower, then raising the temperature to 120 ℃ at a speed of 2 ℃/min, directly reducing the pressure to 3.0kPa, carrying out reflux ratio on the top of the tower to 1:30 until no light component is distilled, and stopping the distillation experiment after caprolactam steam is seen to appear in a condensing column and is solidified. A new sample receiving bottle is switched, then a condensation column is filled with hot water at 75 ℃ to continue distillation, and 520g of caprolactam solid (i.e. crude caprolactam) is collected in the sample receiving bottle. Chromatographic analysis of its main composition: 99.48% by weight of caprolactam, 165. Mu.g/g of cyclohexanone oxime, 120. Mu.g/g of 1,2,3,4,5,6,7, 8-octahydroacridine, 434. Mu.g/g of 1,2,3,4,5,6,7, 8-octahydrophenazine, 389. Mu.g/g of 1,3,4, 5-tetrahydro-2H-azepin-2-one and its isomers, 30. Mu.g/g of 5,6,7, 8-tetrahydro-2-naphthylamine, 10. Mu.g/g of 1,2,3, 4-tetrahydrocarbazole and other undefined impurities.
Preparation example 2
The procedure of preparation 1 was followed and corresponding tests were carried out, except that methanol was used as the reaction solvent in the same amount, and the other procedures were the same as in preparation 1.
Sample analysis was performed on the crude caprolactam obtained, which was found to have the main composition: 99.5% by weight of caprolactam, 260. Mu.g/g of cyclohexanone oxime, 1200. Mu.g/g of cyclohexenone, 1600. Mu.g/g of N-methyl-caprolactam, 430. Mu.g/g of 1,2,3,4,5,6,7, 8-octahydrophenazine, 200. Mu.g/g of 1,3,4, 5-tetrahydro-2H-azepin-2-one and isomers thereof, 1200. Mu.g/g of decahydro phenazine, 80. Mu.g/g of 5,6,7, 8-tetrahydro-2-naphthylamine, 100. Mu.g/g of 1,2,3, 4-tetrahydrocarbazole and other undetermined impurities.
The purification method of the present invention will be described below by way of examples. In the following examples, the composition of the mixed solvent except ethanol is solvent a, and the solvent a in the following examples satisfies: the solubility of caprolactam in solvent a is below 5 wt% at 20 ℃.
Example 1
The refining method adopted in the embodiment is as follows:
(1) First evaporation crystallization: fully mixing the crude caprolactam (400 parts by weight) prepared in preparation example 1, 9 parts by weight of ethanol and 891 parts by weight of isopropyl ether mixed solvent (namely, evaporated crystallization solvent, 900 parts by weight) in an adiabatic glass crystallization kettle (namely, a first evaporation crystallizer), thoroughly dissolving the crude caprolactam in the mixed solvent at 62 ℃, slowly reducing the temperature in the crystallization kettle to 60 ℃, starting vacuumizing the crystallization kettle, performing evaporation crystallization, controlling the operation pressure to 65kPa, controlling the evaporation amount, uniformly evaporating the solvent, regulating the operation pressure along with continuous volatilization of the solvent, reducing the liquid phase temperature in the crystallization kettle along with the continuous volatilization of the solvent, and controlling the operation pressure to 38kPa when the solution temperature in the crystallization kettle is reduced to 45 ℃; when the temperature of the kettle is reduced to 44 ℃,2 weight percent of pure caprolactam with 20-30 meshes is added into the solution to serve as seed crystals, caprolactam crystals are separated out at 46 ℃, the solvent is continuously pumped out, the operating pressure is continuously adjusted according to actual conditions, when the temperature of the solution in the crystallization kettle is reduced to 35 ℃, the operating pressure is controlled to be 26kPa (namely the final vacuum degree or the final pressure) by vacuumizing, the solvent is continuously pumped out, when the temperature of the solution in the crystallizer is reduced to 30 ℃ (namely the final temperature), the vacuumizing is stopped, and the experiment is stopped after 45min (namely the residence time) is maintained at 30 ℃.
(2) First solid-liquid separation, first washing: feeding the caprolactam slurry (1300 parts by weight) obtained in the step (1) from the crystallization kettle into a centrifuge (namely a first solid-liquid separation device and a first washing device) for first solid-liquid separation to obtain first caprolactam crystals and first crystallization mother liquor. The solid phase (i.e., the first caprolactam crystal) obtained in step (2) was washed with the above-defined mixed solvent (340 parts by weight; temperature same as the final temperature of the first evaporative crystallization) composed of the same components in the same proportions, and then solid-liquid separation was continued to obtain caprolactam crystals (i.e., the second caprolactam crystal, 340 parts by weight) and a liquid phase (i.e., the first washing solution, 1300 parts by weight).
(3) And (3) second evaporation and crystallization: pouring the first crystallization mother liquor (the caprolactam content is 10% by weight) obtained by the first solid-liquid separation into a crystallization kettle (namely a second evaporation crystallizer) before crystals are not precipitated, concentrating the first crystallization mother liquor at 30-35kPa after stirring and stabilizing until the concentration of caprolactam is 18% by weight, adding seed crystals (the seed crystals are the same as those in the first evaporation crystallizer), starting to precipitate caprolactam crystals, stopping vacuumizing, controlling the operation pressure at 19kPa (namely the final vacuum degree or the final pressure), maintaining the temperature of the interlayer water bath at 40 ℃ for 30min, then reducing the temperature to 20 ℃ at a cooling rate of 0.2-0.5 ℃/min, namely the final temperature, maintaining the temperature at 20 ℃ for 30min, then sending the mixture into a centrifuge (namely a second solid-liquid separation device and a second washing device) for second solid-liquid separation, respectively obtaining third caprolactam crystals and second crystals, washing the third caprolactam crystals under the same washing conditions as the solid-liquid phase obtained by the solid-liquid separation after the first evaporation crystallizer in the embodiment, and carrying out the second solid-liquid separation at the same second washing temperature as the second evaporation crystallizer, and carrying out the second solid-liquid separation again at the same temperature as the second evaporation crystallizer, obtaining caprolactam crystals.
(4) And (3) crystallizing at constant temperature: concentrating the second crystallization mother liquor (the caprolactam content is 9% by weight) obtained by the second solid-liquid separation at 30 ℃ under 20-25kPa until the caprolactam content in the second crystallization mother liquor reaches 18% by weight, adding seed crystals (the seed crystals in the first evaporation crystallization), separating out caprolactam crystals, stopping vacuumizing, maintaining the sandwich water bath at 30 ℃ for 30min, then cooling to 10 ℃ at a cooling rate of 0.2-0.5 ℃/min, maintaining the temperature at 10 ℃ for 30min for crystallization, performing third solid-liquid separation in a centrifuge (namely a third solid-liquid separation device and a third washing device) to obtain fifth caprolactam crystals and third crystallization mother liquor, performing third washing on the fifth caprolactam crystals, wherein the washing conditions are the same as those of the solid phase obtained by the solid-liquid separation after the first evaporation crystallization, the third washing temperature is the same as that of the constant-temperature crystallization, and performing solid-liquid separation to obtain sixth caprolactam crystals and third washing liquid. And mixing the third washing liquid with the second crystallization mother liquid to perform the constant-temperature crystallization.
The purity of the fourth caprolactam obtained in the step (3) reaches 99.92%, and the purity of the sixth caprolactam crystal obtained in the step (4) reaches more than 99.6%.
The overall caprolactam yield was 98.7%.
(5) Hydrogenation reaction: 150g of the caprolactam crystal (namely, a second caprolactam crystal with the purity of 99.9934 wt%) is added into a 500mL reaction kettle (namely, a hydrogenation reactor), 37.5g of water is added, 1.5g of amorphous nickel hydrogenation catalyst (the industrial grade is SRNA-4 and manufactured by China petrochemical catalyst, long-chain Kagaku Co., ltd.) is added, the temperature is heated to about 75 ℃, then hydrogen is introduced, the flow rate of the hydrogen is controlled at 100mL/min, the reaction pressure is maintained at 0.7MPa, and the aqueous solution of the caprolactam crystal is contacted with the hydrogen for reaction for 1 hour. Then, dehydration was carried out by evaporation on a rotary evaporator (-0.09 MPa,80 ℃ C.), and distillation was carried out under reduced pressure under about 1mmHg to obtain 130g of caprolactam product, and then the distillation was stopped. The caprolactam product quality obtained by analysis, the purity of the caprolactam is 99.9975 percent, the PM value is 39600s, the VB is 0.050mmol/kg, the E value is 0.025, the chromaticity value is 0, and the alkalinity is 0.05mmol/kg.
Example 2
The preparation of crude caprolactam employed in this example was carried out in accordance with the method of preparation example 1, except that the dehydration, light impurity removal, heavy impurity removal treatments employed were different from those of preparation example 1, and the other were the same as those of preparation example 1; the embodiment adopts the following dehydration, light impurity removal and heavy impurity removal treatment methods: the crude caprolactam product of preparation example 1 was distilled under reduced pressure (16 trays and inverted triangle metal packing), the pressure (absolute pressure) was 5kPa, the temperature was heated from room temperature (20 ℃) to 90 ℃ for 40min at 3 ℃/min, no reflux was provided at the top of the column, the temperature was raised to 120 ℃ at the heating rate of 3 ℃/min, the pressure was directly reduced to 2.9kPa, the reflux ratio at the top of the column was 1:40, until no light component was distilled, at this time caprolactam vapor was seen to appear in the condensing column, and solidified, and the distillation experiment was stopped. A new sample receiving bottle was switched, then the condensation column was charged with hot water at 75 ℃ and distillation was continued, and 355g of caprolactam solid (i.e. crude caprolactam) was collected in the sample receiving bottle. Chromatographic analysis of its main composition: 99.36% by weight of caprolactam, 100. Mu.g/g of 5-cyano-1-pentene, 200. Mu.g/g of cyclohexanone oxime, 220. Mu.g/g of cyclohexenone, 460. Mu.g/g of 1,2,3,4,5,6,7, 8-octahydrophenazine, 520. Mu.g/g of 1,3,4, 5-tetrahydro-2H-azepin-2-one and isomers thereof and 480. Mu.g/g of decahydro phenazine, 30. Mu.g/g of 5,6,7, 8-tetrahydro-2-naphthylamine, 10. Mu.g/g of 1,2,3, 4-tetrahydrocarbazole and other undetermined impurities.
The crude caprolactam prepared in this example was refined as follows:
(1) First evaporation crystallization: in an adiabatic glass crystallization kettle (namely, a first evaporation crystallizer), fully mixing the crude caprolactam (300 parts by weight) prepared in the embodiment and the mixed solvent (900 parts by weight) of 18 parts by weight of ethanol and 882 parts by weight of n-heptane, thoroughly dissolving the crude caprolactam in the mixed solvent at 65 ℃, slowly reducing the temperature in the crystallization kettle to 61 ℃, adding 2% by weight of pure caprolactam with 20-30 meshes as seed crystals into the solution, then starting vacuumizing the crystallization kettle for evaporation crystallization, controlling the operation pressure to be 50kPa, controlling the evaporation amount to uniformly evaporate the solvent, continuously evaporating the solvent, reducing the liquid phase temperature in the crystallization kettle along with the continuous volatilization of the solvent, starting to precipitate caprolactam crystals when the kettle temperature is reduced to 58 ℃, continuously adjusting the operation pressure, controlling the operation pressure to be 20kPa (namely, the final vacuum degree or the final pressure) when the solution temperature in the crystallization kettle is reduced to 45 ℃, stopping vacuumizing when the solution temperature in the crystallization kettle is reduced to 40 ℃ (namely, the final temperature), and stopping the experiment at 40 ℃ for 60 minutes.
(2) First solid-liquid separation, first washing: feeding the caprolactam slurry (1200 parts by weight) obtained in the step (1) from the crystallizer into a centrifuge (namely a first solid-liquid separation device and a first washing device) for first solid-liquid separation to obtain first caprolactam crystals and a first crystallization mother liquor. The solid phase (first caprolactam crystal) obtained was washed with the mixed solvent (250 parts by weight; temperature same as the final temperature of the first evaporative crystallization) defined in the above step (1) and then solid-liquid separation was continued to obtain caprolactam crystals (i.e., second caprolactam crystals, 247 parts by weight) and a liquid phase (i.e., first washing solution, 1200 parts by weight).
(3) Second evaporative crystallization and second solid-liquid separation, second washing, and constant temperature crystallization: the above-mentioned first crystallization mother liquor was subjected to second evaporation crystallization and second solid-liquid separation, second washing, and constant temperature crystallization in this order according to the methods of steps (3) - (5) in example 1. The overall caprolactam yield was 98.8%.
(4) Hydrogenation reaction: 150g of caprolactam crystal (namely a second caprolactam crystal with the purity of 99.9926 wt%) is added into a 500mL reaction kettle (namely a hydrogenation reactor), 37.5g of water is added, 1.5g of amorphous nickel hydrogenation catalyst (the industrial brand is SRNA-4 and manufactured by China petrochemical catalyst, kagaku Co., ltd.) is added, the temperature is heated to about 75 ℃, then hydrogen is introduced, the flow rate of the hydrogen is controlled at 100mL/min, the reaction pressure is maintained at 0.7MPa, and the aqueous solution of the caprolactam crystal is contacted with the hydrogen for reaction for 1 hour. Then n-octane (-0.09 MPa,100 ℃) is removed by evaporation on a rotary evaporator, and then distillation is carried out under reduced pressure under the condition of about 1mmHg, 130g of caprolactam product is obtained, and distillation is stopped.
The caprolactam product quality obtained by analysis is 99.9953%, the purity of caprolactam is 36000s, the PM value is 0.064mmol/kg, the E value is 0.035, the chromaticity value is 1, and the alkalinity is 0.046mmol/kg.
Example 3
The crude caprolactam employed in this example was prepared in the same manner as in example 2.
(1) First evaporation crystallization: in an adiabatic glass crystallization kettle (namely a first evaporation crystallizer), fully mixing crude caprolactam (300 parts by weight) and a mixed solvent of 9 parts by weight of ethanol, 223 parts by weight of cyclohexane and 668 parts by weight of n-heptane (total 900 parts by weight of cyclohexane and n-heptane=1:3), thoroughly dissolving the crude caprolactam in the mixed solvent at 65 ℃, slowly reducing the temperature in the crystallization kettle to 61 ℃, adding 2% by weight of 20-30 mesh pure caprolactam seed crystal into the solution, vacuumizing the inside of the crystallization kettle for evaporation crystallization, controlling the evaporation amount to be 50kPa, uniformly evaporating the solvent, continuously volatilizing the solvent, continuously reducing the liquid phase temperature in the crystallization kettle, starting to precipitate caprolactam crystals when the kettle temperature is reduced to 58 ℃, continuously adjusting the operation pressure, controlling the operation pressure to be 20kPa (namely the final vacuum degree or pressure) when the temperature of the solution in the crystallization kettle is reduced to 45 ℃, continuously extracting the solvent, stopping the vacuum pumping when the temperature of the solution in the crystallization kettle is reduced to 40 ℃ (namely the final temperature), and stopping the experiment at 40 ℃ for 60 min.
(2) First solid-liquid separation, first washing: feeding the caprolactam slurry (1200 parts by weight) obtained in the step (1) from the crystallizer into a centrifuge (namely a first solid-liquid separation device and a first washing device) for first solid-liquid separation to obtain first caprolactam crystals and a first crystallization mother liquor. The solid phase (first caprolactam crystal) obtained was washed with the mixed solvent (240 parts by weight; temperature same as the final temperature of the first evaporative crystallization) of the same components in the same proportions defined in the above step (1), and then solid-liquid separation was continued to obtain caprolactam crystals (i.e., second caprolactam crystals, 245 parts by weight) and a liquid phase (i.e., first washing liquid, 1200 parts by weight).
(3) Second evaporative crystallization and second solid-liquid separation, second washing, and constant temperature crystallization: the above-mentioned first crystallization mother liquor was subjected to second evaporation crystallization and second solid-liquid separation, second washing, and constant temperature crystallization in this order according to the methods of steps (3) - (5) in example 1. The overall caprolactam yield was 99.0%.
(4) Hydrogenation reaction: 150g of caprolactam crystal (namely a second caprolactam crystal with the purity of 99.9920 wt%) is added into a 500mL reaction kettle (namely a hydrogenation reactor), 150g of water is added, 1.5g of amorphous nickel hydrogenation catalyst (with the industrial grade of SRNA-4 and manufactured by Kagaku chemical catalyst, kagaku Co., ltd.) is added, the temperature is heated to about 75 ℃, then hydrogen is introduced, the flow rate of the hydrogen is controlled at 100mL/min, the reaction pressure is maintained at 0.7MPa, and the aqueous solution of the caprolactam crystal is contacted with the hydrogen for reaction for 1 hour. Then, dehydration was carried out by evaporation on a rotary evaporator (-0.09 MPa,80 ℃ C.), and distillation was carried out under reduced pressure under about 1mmHg to obtain 130g of caprolactam product, and then the distillation was stopped.
The caprolactam product quality obtained by analysis is 99.9951% in purity, 36000s in PM value, 0.058mmol/kg in VB, 0.040 in E value, 0 in chromaticity value and 0.035mmol/kg in alkalinity.
Example 4
The crude caprolactam employed in this example was prepared in the same manner as in example 2.
(1) First evaporation crystallization: in an adiabatic glass crystallization kettle (namely a first evaporation crystallizer), fully mixing 300 parts by weight of crude caprolactam and 900 parts by weight of mixed solvent (including 14 parts by weight of ethanol and 886 parts by weight of chlorosec-butane, thoroughly dissolving the crude caprolactam in the solvent at 60 ℃, slowly reducing the temperature in the crystallization kettle to 60 ℃, vacuumizing the crystallization kettle to perform evaporation crystallization, controlling the operation pressure to be 50kPa, uniformly evaporating the solvent, and controlling the operation pressure to be 40kPa when the temperature of the solution in the crystallization kettle is reduced to 45 ℃ along with the continuous volatilization of the solvent; when the temperature of the kettle is reduced to 42 ℃, adding 2 weight percent of pure caprolactam with 20-30 meshes into the solution as seed crystals, starting to precipitate caprolactam crystals at 41 ℃, continuously pumping out the solvent, adjusting continuous operating pressure according to actual conditions, controlling the operating pressure to 22kPa (final vacuum degree or final pressure) when the temperature of the solution in the crystallizer is reduced to 35 ℃, continuously pumping out the solvent, stopping vacuumizing when the temperature of the solution in the crystallizer is reduced to 30 ℃ (final temperature), maintaining for 45min at 30 ℃, and stopping the experiment.
(2) First solid-liquid separation, first washing: feeding the caprolactam slurry (1200 parts by weight) obtained in the step (1) from the crystallizer into a centrifuge (namely a first solid-liquid separation device and a first washing device) for first solid-liquid separation to obtain first caprolactam crystals and a first crystallization mother liquor. The solid phase (first caprolactam crystal) obtained was washed with the mixed solvent (220 parts by weight; washing temperature and final temperature of the first evaporative crystallization) of the same components in the same proportions defined in the above step (1), and then solid-liquid separation was continued to obtain caprolactam crystals (i.e., second caprolactam crystals, 223 parts by weight) and a liquid phase (i.e., first washing liquid, 1200 parts by weight).
(3) Second evaporative crystallization and second solid-liquid separation, second washing, and constant temperature crystallization: the above-mentioned first crystallization mother liquor was subjected to second evaporation crystallization and second solid-liquid separation, second washing, and constant temperature crystallization in this order according to the methods of steps (3) - (5) in example 1. The overall caprolactam yield was 98.6%.
(4) Hydrogenation reaction: 150g of caprolactam crystals, i.e. the second caprolactam crystals, are subjected to the hydrogenation reaction as described in example 3. Distillation was stopped after 140g of caprolactam product was obtained. The caprolactam product quality obtained by analysis is 99.9946% in purity, 36000s in PM value, 0.055mmol/kg in VB, 0.037 in E value, 0 in chromaticity value and 0.038mmol/kg in alkalinity.
Example 5
The procedure of example 1 was followed, except that the same amount of crude caprolactam prepared in preparation example 2 was used in place of the crude caprolactam prepared in preparation example 1 in the first evaporative crystallization process of step (1), except that the procedure was the same as in example 1.
The caprolactam product yield is 98.6%, the caprolactam product quality is analyzed, the purity of the caprolactam is 99.9954%, the PM value is 36000s, the VB is 0.053mmol/kg, the E value is 0.034, the chromaticity value is 0, and the alkalinity is 0.036mmol/kg.
Example 6
The procedure of example 1 was followed, except that in step (4), the second crystallization mother liquor was concentrated, and then ethanol was added so that the caprolactam concentration was 10.5% by weight and the content of ethanol in the thermostatically crystallized solvent was 1% by weight, and then the thermostatically crystallization was further carried out, in the same manner as in example 1.
The caprolactam product yield was 99.3%, the caprolactam product quality was analyzed, the caprolactam purity was 99.9964%, the PM value was 39600s, the VB was 0.050mmol/kg, the E value was 0.034, the chroma value was 1, and the alkalinity was 0.03mmol/kg.
Example 7
The procedure of example 1 was followed, except that the step (4) of thermostatically crystallizing was not performed, and the procedure was the same as in example 1. The overall caprolactam yield was 98.5%.
The quality of caprolactam product obtained by hydrogenation reaction was analyzed, the purity of caprolactam was 99.9975%, the PM value was 39600s, VB was 0.050mmol/kg, E value was 0.025, the chroma value was 0, and the alkalinity was 0.05mmol/kg.
Example 8
The procedure of example 1 was followed, except that the fourth caprolactam obtained in step (3) and the sixth caprolactam crystals obtained in step (4) were recycled for mixing with crude caprolactam, and the first evaporative crystallization was carried out together, otherwise as in example 1.
The caprolactam product yield is 99.0%, the caprolactam product quality is analyzed, the caprolactam purity is 99.9978%, the PM value is 43200s, the VB is 0.046mmol/kg, the E value is 0.020, the chromaticity value is 0, and the alkalinity is 0.040mmol/kg.
Example 9
The procedure of example 1 was followed, except that the second evaporative crystallization of step (3) was not performed, but the first crystallization mother liquor (caprolactam content: 10% by weight) obtained by the first solid-liquid separation in step (2) was directly subjected to the isothermal crystallization of step (4) (this isothermal crystallization is the second isothermal crystallization) instead of the second crystallization mother liquor, which was the same as in example 1.
The caprolactam product yield was 98.9%, the caprolactam product quality was analyzed, the caprolactam purity was 99.9957%, the PM value was 31600s, the VB was 0.063mmol/kg, the E value was 0.033, the chroma value was 1, and the alkalinity was 0.06mmol/kg.
By adopting the method provided by the invention, the caprolactam has high purity and reaches the quality product on the premise of ensuring the higher yield of the caprolactam.
The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the scope of the technical idea of the invention, a number of simple variants of the technical solution of the invention are possible, including combinations of the individual technical features in any other suitable way, which simple variants and combinations should likewise be regarded as being disclosed by the invention, all falling within the scope of protection of the invention.

Claims (25)

1. A process for refining caprolactam, the process comprising the steps of:
(1) The cyclohexanone oxime gas-phase Beckmann rearrangement reaction product is subjected to solvent recovery, dehydration, light component removal and heavy component removal treatment to obtain crude caprolactam with caprolactam content not less than 99 wt%; the solvent of the cyclohexanone oxime gas-phase Beckmann rearrangement reaction is ethanol;
(2) In the presence of an evaporative crystallization solvent, performing first evaporative crystallization on the crude caprolactam, and performing first solid-liquid separation to obtain first caprolactam crystals and a first crystallization mother solution; the evaporative crystallization solvent is used in an amount such that the caprolactam solids content in the mixture of crude caprolactam and evaporative crystallization solvent is 35% by weight or less;
(3) Performing first washing on the first caprolactam crystal to obtain a second caprolactam crystal and a first washing solution;
(4) Optionally concentrating the first crystallization mother liquor, then performing second crystallization, and performing second solid-liquid separation to obtain a third caprolactam crystal and a second crystallization mother liquor;
(5) Performing second washing on the third caprolactam crystal to obtain fourth caprolactam and a second washing solution;
(6) Carrying out hydrogenation reaction on the second caprolactam crystal obtained in the step (3);
wherein the evaporative crystallization solvent in the step (2) contains a solvent A and ethanol, the solubility of caprolactam in the solvent A is below 5 wt% at 20 ℃, and the ethanol accounts for 1-2 wt% of the total amount of the evaporative crystallization solvent; the solvent A is isopropyl ether;
the gas-phase Beckmann rearrangement reaction is carried out in the presence of a molecular sieve catalyst with an MFI structure; the molecular sieve catalyst with the MFI structure comprises a silicon molecular sieve with the MFI topological structure and a binder, wherein the molecular sieve contains metal elements, the metal elements are at least one of Fe, al, ga, cr, ti and Zr, and the content of the metal elements in the molecular sieve is 5-100 mug/g based on the total amount of the molecular sieve.
2. The process of claim 1, wherein in step (1), the caprolactam content of the crude caprolactam is not less than 99.2 wt%.
3. The process of claim 1 wherein the crude caprolactam further comprises cyclohexene, cyclohexadiene, acetonitrile, ethyl-acrylonitrile, propionitrile, ethoxy-cyclohexene, butyronitrile, ethyl-valeronitrile, cyclopentanone, ethylcyclopentanone, valeronitrile, ethylpyridine, ethyl hexenoate, ethoxy-1, 3-cyclohexadiene, ethoxy-1, 4-cyclohexadiene, cyclohexanone, capronitrile, cyanocyclopentane, ethyl-epsilon-caprolactone imide, ethyl-cyclohexanone, 5-cyano-1-pentene, ethoxy-cyclohexanone, cyclohexenone, cyclohexanol, phenol, bicyclo [3.1.0 ]]Pentanone-2, N-diethyl-aniline, N-ethyl-aniline, ethyl-aniline, N-hexanamide, N-pentanamide, valerolactam, N-ethyl-caprolactam, 1,2,3,4,5,6,7, 8-octahydroacridine, 1,2,3,4,5,6,7, 8-octahydrophenazine, decahydrophenazine, 1,3,4, 5-tetrahydro-2H-azepin-2-one and its isomer, at least one of 5,6,7, 8-tetrahydro-2-naphthylamine and 1,3,4, 5-tetrahydrocarbazole.
4. A method according to claim 3, wherein the coarse fraction Caprolactam comprises 99 to 99.7% by weight, based on the total amount of the crude caprolactam, of 1,2,3,4,5,6,7, 8-octahydroacridine, 1,2,3,4,5,6,7, 8-octahydrophenazine, 5,6,7, 8-tetrahydro-2-naphthylamine and 1,3,4, 5-tetrahydrocarbazole and 1,3,4, 5-tetrahydro-2H-azepin-2-one and its isomer content is 0.01-0.1 wt%.
5. The process according to any one of claims 1 to 4, wherein the evaporative crystallization solvent is used in an amount such that the caprolactam solids content in the mixture of crude caprolactam and evaporative crystallization solvent is from 25 to 33% by weight.
6. The method according to any one of claims 1 to 4, wherein in step (2), the conditions for the first evaporative crystallization include: the final temperature is 10-65 ℃; the vacuum degree is 5-80kPa;
and/or, in the step (4), the final temperature of the second crystallization is 5-20 ℃ lower than the final temperature of the first evaporation crystallization;
and/or, in the step (4), concentrating the first crystallization mother liquor to obtain a product with the solid content of caprolactam of more than 15 wt%.
7. The method of claim 6, wherein in step (2), the conditions of the first evaporative crystallization include: the final temperature is 30-60 ℃; the vacuum degree is 10-60kPa;
And/or, in the step (4), the final temperature of the second crystallization is 10-20 ℃ lower than the final temperature of the first evaporation crystallization.
8. The method of claim 7, wherein in step (2), the conditions of the first evaporative crystallization include: the final temperature is 30-40 ℃; the vacuum degree is 20-30kPa.
9. The method of any one of claims 1-4, 7-8, wherein the second crystallization is a second isothermal crystallization or a second evaporative crystallization, and wherein the conditions of the second isothermal crystallization comprise: the temperature is 5-50 ℃;
and/or the vacuum degree of the second evaporative crystallization is 5-20kPa lower than that of the first evaporative crystallization;
and/or, the conditions of the second evaporative crystallization include: the final temperature is 5-60 ℃; the vacuum degree is 0-70kPa.
10. The method of claim 9, the conditions of the second thermostatically crystallizing comprising: the temperature is 10-40 ℃;
and/or the vacuum degree of the second evaporative crystallization is 5-15kPa lower than that of the first evaporative crystallization;
and/or, the conditions of the second evaporative crystallization include: the final temperature is 10-50 ℃; the vacuum degree is 5-50kPa.
11. The method of claim 10, the conditions of the second thermostatically crystallizing comprising: the temperature is 10-20 ℃;
and/or, the conditions of the second evaporative crystallization include: the final temperature is 15-25 ℃; the vacuum degree is 10-20kPa.
12. The method of any one of claims 1-4, 7-8, 10-11, wherein the temperature of the first wash in step (3) is not less than the final temperature of the first evaporative crystallization;
and/or the ratio by weight of the washing solvent used for the first washing to the first caprolactam crystals is from 0.5 to 1.5:1, a step of;
and/or, the temperature of the second washing in step (5) is not lower than the final temperature of the second crystallization;
and/or the ratio by weight of the amount of the washing solvent used for the second washing in step (5) to the weight of the third caprolactam crystals is from 0.5 to 1.5:1, a step of;
and/or, the washing solvents used for the first washing and the second washing are each independently the same as the evaporating crystallization solvent;
and/or, the method further comprises: the first wash liquid is recycled to provide at least part of the evaporative crystallization solvent and/or the wash solvent used for the second wash.
13. The method of claim 12, wherein the difference between the temperature of the first wash and the final temperature of the first evaporative crystallization in step (3) is 0-2 ℃;
and/or the difference between the temperature of the second wash in step (5) and the final temperature of the second crystallization is 0-2 ℃.
14. The process of any one of claims 1-4, 7-8, 10-11, and 13, wherein the hydrogenation conditions of step (6) comprise: the temperature is 50-150 ℃, the pressure is 0.2-2MPa, and the hydrogen is used in an amount of 0.01-0.25 mol relative to 1 mol of the second caprolactam crystal.
15. The process of claim 14, wherein the hydrogenation reaction is carried out in the presence of water, in the presence of a hydrogenation catalyst.
16. The process of claim 15, wherein the water is used in an amount of 10 to 200 parts by weight relative to 100 parts by weight of the second caprolactam crystals;
and/or the hydrogenation catalyst is selected from at least one of nickel-based catalyst, palladium-based catalyst and platinum-based catalyst.
17. The process of claim 16, wherein the water is used in an amount of 10 to 40 parts by weight relative to 100 parts by weight of the second caprolactam crystals;
and/or the hydrogenation catalyst is a nickel-based catalyst.
18. The method of any one of claims 1-4, 7-8, 10-11, 13, 15-17, wherein the second crystallization is a second evaporative crystallization, the method further comprising the steps of:
(7) Optionally concentrating the second crystallization mother liquor, then carrying out constant-temperature crystallization, and then carrying out third solid-liquid separation to obtain fifth caprolactam crystals and third crystallization mother liquor;
(8) And performing third washing on the fifth caprolactam crystal to obtain a sixth caprolactam crystal and a third washing solution.
19. The process of claim 18, wherein in step (7), the second crystallization mother liquor is concentrated such that the caprolactam solids content of the resulting product is 15-30 wt%;
and/or, the temperature of the thermostatically-crystallized in step (7) is lower than the final temperature of the second crystallization;
and/or, the conditions of the isothermal crystallization include: the temperature is 5-50 ℃.
20. The method of claim 19, wherein the temperature of the thermostatically-crystallized in step (7) is 5-20 ℃ lower than the final temperature of the second crystal;
and/or, the conditions of the isothermal crystallization include: the temperature is 10-40 ℃.
21. The method of claim 20, wherein the conditions of the thermostatically crystallizing in step (7) comprise: the temperature is 10-20 ℃.
22. The method according to claim 21, wherein in step (7), the thermostatically crystallization is performed in the presence of a thermostatically crystallization solvent containing solvent a and ethanol.
23. The method of claim 18, wherein the temperature of the third wash in step (8) is not less than the temperature of the isothermal crystallization;
And/or the weight ratio of the amount of the washing solvent used for the third washing to the fifth caprolactam crystals is 0.5-1.5:1, a step of;
and/or the washing solvent used for the third washing is the same as the evaporative crystallization solvent;
and/or, the method further comprises: recycling the first wash liquor and/or the second wash liquor to provide at least part of the wash solvent used for the third wash;
and/or, the method further comprises: and mixing the third washing liquid with the second crystallization mother liquid to perform the constant-temperature crystallization.
24. The method of claim 23, wherein the difference between the temperature of the third wash and the temperature of the thermostatted crystallization in step (8) is 0-2 ℃.
25. The method of claim 18, wherein the method further comprises: recycling said fourth caprolactam and said sixth caprolactam crystals to step (2) to mix with said crude caprolactam for said first evaporative crystallization;
and/or, the method further comprises: recovering the solvent in the third crystallization mother liquor.
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