CN115677582A - Continuous flash evaporation crystallization refining method of caprolactam and production method of caprolactam - Google Patents
Continuous flash evaporation crystallization refining method of caprolactam and production method of caprolactam Download PDFInfo
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- CN115677582A CN115677582A CN202110859288.8A CN202110859288A CN115677582A CN 115677582 A CN115677582 A CN 115677582A CN 202110859288 A CN202110859288 A CN 202110859288A CN 115677582 A CN115677582 A CN 115677582A
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- 238000002425 crystallisation Methods 0.000 title claims abstract description 222
- 230000008025 crystallization Effects 0.000 title claims abstract description 220
- 238000000034 method Methods 0.000 title claims abstract description 106
- 238000007670 refining Methods 0.000 title claims abstract description 47
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Abstract
The invention relates to the field of caprolactam production, and discloses a continuous flash evaporation crystallization refining method of caprolactam and a caprolactam production method. The refining method has the characteristics of high purity, high efficiency, high yield, continuous and efficient operation of equipment for a long time and the like of crystal products, and the caprolactam production method combined with the refining method not only can obtain high-quality caprolactam products, but also has the advantages of high efficiency, high yield, continuous and efficient operation of equipment for a long time and the like.
Description
Technical Field
The invention relates to the field of caprolactam production, in particular to a continuous flash evaporation crystallization refining method of caprolactam and a production method of caprolactam.
Background
Caprolactam is an important chemical raw material, is widely applied to materials such as synthetic fibers, engineering resins and the like, and is a common component for preparing daily consumer products and industrial products. Since caprolactam monomer is required to produce the above products by polymerization, the purity index of caprolactam product is high, and various technologies for refining caprolactam have been proposed in order to obtain the caprolactam product which reaches the standard.
Because the crystal structure is compact, impurities are not easy to insert into crystal lattices, and a crystallization mode is used for purifying caprolactam in many caprolactam refining processes. However, there are three problems to be solved in the crystallization refining process of caprolactam: (1) the purity of the crystal product; (2) stability of the long-term operation of the equipment; (3) yield of crystallization process.
CN103508954B, CN103420912B, CN103420913B and CN103420885B all describe crystallization methods using seed crystals, and such methods have strong operation flexibility of crystallization, and the quality of the crystal product can reach the standard by suitable control means. However, the mode described in these patent documents is actually a batch crystallization process, which takes a long time for one crystallization cycle and has a low yield. For a flow with high requirement on yield, the equipment quantity and the occupied area required by the crystallization method are large, and the operation requirement of workers is also high.
The continuous crystallization technology is applied to the industrial refining of caprolactam by Sumitomo corporation in Japan, and the crystallization technologies in patent documents CN104024221B, CN104011017B, CN100354266C and CN1220680C belong to the continuous crystallization process, and the crystallization yield is high. However, the crystal patterns described in the patent literature of Sumitomo corporation all have one feature: the solution or the melt of caprolactam is directly mixed with a low-temperature solvent, namely, the crystallization is realized by adopting a chilling mode. The crystallization proceeds very rapidly, the temperature of the hot caprolactam solvent/solution being suddenly lowered to the set temperature, the total crystallization time not exceeding 1.5h. However, the purity of the crystals obtained by this method is not high, and there is a serious problem of fouling of the surface of the crystallizer, which makes it difficult to stably operate the crystallization process for a long period of time.
In conclusion, the prior art at home and abroad has lower yield of the crystallization refined caprolactam, larger required equipment quantity and occupied area and higher operation requirement of workers; or the surface of the crystallizer has the problem of scaling and is not suitable for long-term stable operation. At present, a method for refining caprolactam through crystallization does not exist, and the method has the characteristics of high efficiency, high yield, long-term operation stability and the like on the basis of ensuring the quality of crystal products.
Disclosure of Invention
The invention aims to overcome the problems in the prior art, and provides a caprolactam crystallization refining method and a caprolactam production method, wherein the refining method has the continuous and efficient characteristics of high purity, high efficiency, high yield, stable long-term operation and the like of a crystal product, and the caprolactam production method combined with the refining method not only can obtain a high-quality caprolactam product, but also has the continuous and efficient advantages of high efficiency, high yield, stable long-term operation and the like.
In order to achieve the above object, the present invention provides, in a first aspect, a process for refining caprolactam by crystallization, which comprises continuously feeding a solution of a crude caprolactam product and carrying out crystallization under adiabatic flash conditions, wherein the feed concentration and crystallization conditions are controlled such that crystallization is carried out in a metastable zone of caprolactam.
In a second aspect, the present invention provides a process for producing caprolactam, which comprises refining a crude caprolactam product by the crystallization refining process according to the first aspect to obtain crystalline crystals; and (3) carrying out hydrogenation reaction on the crystalline crystals.
The inventor of the present invention has found through research that, in the prior art, for example, sumitomo company adopts a chilling crystallization mode, which has the disadvantages of low crystal purity, easy equipment scaling, etc., and related to the crystallization mode, the crystallization mode is too rapid and is not favorable for regular crystal growth, and too rapid crystallization causes more impurities to be occluded in caprolactam crystals crystallized out, so that the purity of crystal products is relatively low, and the crystallization process is too rapid, which causes more severe scaling problems on the surface of the crystallizer.
The invention adopts the mode of adiabatic flash evaporation and combines the control of the concentration of caprolactam in the caprolactam mixed solution in the metastable zone range, thus not only obtaining crystals with higher purity, but also avoiding scaling in the crystallizer, and having the advantage of high efficiency in the continuous crystallization mode. The crystal purification method of the present invention has the above-described advantages because: under the condition of adiabatic flash evaporation, the supersaturated surface of the crystallization mode is positioned at a gas-liquid interface, so that the scaling probability of the wall surface of the crystallizer can be effectively reduced, and the stable operation time of the crystallization process is improved; under the condition that the concentration of caprolactam in the caprolactam mixed solution is controlled in the metastable zone range, the crystal can grow naturally, the grain diameter of the crystal can be improved, and the adsorption of impurities is reduced, so that the purity of the crystal is improved, and moreover, the crystal with larger grain diameter is more beneficial to being separated from the mother solution, so that the efficiency of downstream operation can be improved. The inventor of the present invention further researches and discovers that if the caprolactam concentration in the liquid is higher than the metastable zone, the nucleation of the crystal in the crystallization process is severe, which affects the crystal granularity of the product, obtains more fine crystal grains, and causes severe scaling in the crystallization kettle, which is not favorable for the long-term stable operation of the equipment. Therefore, the concentration in the crystallizer needs to be strictly controlled in the metastable zone.
The crystallization purification method of the present invention has the following advantageous effects:
(1) The purity of the crystal product is higher; when a downstream hydrogenation unit and a downstream flash evaporation unit are matched, the crystal product can easily realize the superior product index in the national standard GB/T13254-2017. The purity of the final product can reach more than 99.993%.
(2) The scale can be effectively avoided, the equipment can be stably operated, and the long-term continuous operation of the equipment can be ensured.
(3) The crystallization process can meet the requirements of higher yield, and the number of equipment and equipment floor space required to achieve the same yield is much less than that of a batch crystallization process.
(4) The crystal product has larger and uniform granularity, can effectively improve the operation efficiency of downstream solid-liquid separation or countercurrent washing, and reduces the yield loss caused by small crystal granularity.
(5) The crystallization process is stable in state, the process is easy to control, and the problem of batch quality difference caused by the operation experience problem of workers is avoided.
Refining a caprolactam crude product by using the crystallization refining method to obtain a crystalline crystal; then a superior caprolactam product can be obtained through hydrogenation reaction. The production method provided by the invention has the characteristics of high efficiency, high yield, stable long-term operation and the like, and is continuous and efficient. In addition, the caprolactam potassium permanganate absorption value (PM) value obtained by the method provided by the invention is greater than 20000s or more, the extinction value (at 290nm wavelength) of the caprolactam is 0.1 or less, the volatile alkali value is 0.3mmol/kg or less, the chromatic value is 2 or less, and the alkalinity is 0.1mmol/kg or less, thus completely meeting the requirements of industrial high-grade products.
Drawings
FIG. 1 is a schematic representation of a saturated solubility curve and a supersaturated solubility curve of caprolactam in a solvent;
FIG. 2 is a schematic diagram of a multi-stage continuous adiabatic flash crystallization process.
Detailed Description
The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value, and these ranges or values should be understood to encompass values close to these ranges or values. For ranges of values, between the endpoints of each of the ranges and the individual points, and between the individual points may be combined with each other to give one or more new ranges of values, and these ranges of values should be considered as specifically disclosed herein.
In a first aspect, the present invention provides a process for refining caprolactam by crystallization, which comprises continuously feeding a solution of a crude caprolactam product and carrying out crystallization under adiabatic flash conditions, wherein the concentration of the feed and the crystallization conditions are controlled such that crystallization is carried out in a metastable zone of caprolactam.
According to the invention, adiabatic flash evaporation refers to flash evaporation performed under the condition of no specific heat exchange with the outside, and is an isenthalpic process, the pressure is reduced, the solution is vaporized to absorb heat, and only the heat of the solution is absorbed due to the adiabatic process, so that the temperature of the system is reduced. Adiabatic flash crystallization is particularly advantageous for caprolactam, since solvent evaporation occurs at the gas-liquid interface, and thus the concentration is highest at the gas-liquid interface, and the wall interface at the gas-liquid interface is small, so that the adiabatic flash process can effectively avoid the scaling of the wall. The region between the saturated and supersaturated solubility curves is the metastable region and fig. 1 shows the saturated and supersaturated solubility curves of caprolactam in a solvent as determined in the laboratory. The inventor of the invention finds that crystal nuclei generated by flash evaporation at a gas-liquid interface can grow slowly in a solution main body by controlling the concentration of caprolactam in a caprolactam mixed solution in a metastable zone range, namely the concentration is controlled between a saturated solubility curve and a supersaturated solubility curve, so that crystals with certain particle size and high purity can be obtained. On the contrary, if the concentration of the solution is higher than the metastable zone, a large amount of nucleation is generated in the solution body, and the problems of excessive fine crystal, more intercrystalline occluded impurities and low crystal purity are caused.
According to the invention, the mixed liquor of the caprolactam crude product in step (1) of the invention is a saturated or unsaturated solution of caprolactam. Preferably, in order to save energy consumption, the caprolactam crude product mixed solution in the step (1) of the invention is a saturated solution of caprolactam. In the preferred embodiment, the invention can obtain self-growing crystals in the range of the metastable zone by controlling the concentration of caprolactam without additional supplementary crystal seeds, is more favorable for obtaining large-particle, uniform and high-purity crystal products, and can further reduce the generation of crystal scale on the wall of the crystallizer, thereby further increasing the stable running time of continuous crystallization refining.
According to the invention, the crystallization solvent can be chosen from a wide variety of solvents, for example conventional crystallization solvents which may be caprolactam, for example one or more solvents selected from the group consisting of alcohols, ethers, alkanes, cycloalkanes, halogenated hydrocarbons and aromatic hydrocarbons. According to the invention, in order to further improve the purity, crystallization efficiency and crystallization yield of the crystal product and further reduce the energy consumption, the solvent in the caprolactam crude product solution preferably satisfies the following conditions: at 5kPa absolute and at a temperature corresponding to the boiling point of the solvent, the mass percent solubility of caprolactam in the solvent is less than 20 wt%; further preferably, the solvent has a boiling point of 40 to 140 ℃ at normal atmospheric pressure; preferably one or more selected from the group consisting of ethers, alkanes, cycloalkanes, halogenated hydrocarbons and aromatic hydrocarbons having a boiling point at normal atmospheric pressure of 40 to 140 ℃. The choice of crystallization solvent can be determined by the above-mentioned conditions, and the choice can be verified by routine experimentation. In this case, if the solubility of the end point is less than 20 wt%, the enrichment amount of impurities in the mother liquor is not excessively high when the target yield is reached, so that the purity of the crystals can be improved.
According to the invention, in this process, the solvent may be a single or a mixture of solvents. Preferably, the solvent in the caprolactam mixed solution is selected from one or more of ethers, alkanes, cycloalkanes, halogenated hydrocarbons and aromatic hydrocarbons which meet the above conditions; preferably, the ether is a saturated aliphatic monoether, preferably at least one of isopropyl ether, ethyl butyl ether, methyl ethyl ether, n-propyl ether, n-butyl ether, ethyl butyl ether, methyl tert-butyl ether and ethyl tert-butyl ether, and more preferably is isopropyl ether and/or ethyl butyl ether; the alkane contains 6 to 12 carbon atoms, preferably at least one selected from the group consisting of n-heptane, n-hexane, isopentane, n-octane, n-nonane, methylhexane, isohexane, neohexane, isoheptane, isooctane, and isononane, more preferably at least one selected from the group consisting of n-pentane, n-hexane, and isopentane; the cycloalkane contains 5 to 8 carbon atoms, preferably at least one selected from cyclohexane, cyclopentane, methylcyclopentane and methylcyclohexane, more preferably cyclohexane and/or cyclopentane; the halogenated hydrocarbon is selected from at least one of 1-chloropropane, 2-chloropropane, 1-chloro n-butane, 2-chlorobutane, 1-chloro dimethyl propane, 2-chloro dimethyl propane, 1-n-bromopropane, 2-bromopropane, 1-bromobutane and 2-bromobutane; the aromatic hydrocarbon is selected from at least one of benzene, toluene, xylene and trimethylbenzene, and more preferably benzene and/or toluene.
According to the invention, in order to further improve the purity of the crystal product, the crystallization efficiency, the crystallization yield and the continuous crystallization stable operation time, the solvent in the caprolactam crude product mixed solution is preferably selected from one or more of isopropyl ether, n-heptane and cyclohexane.
According to the invention, the concentration of the feed is chosen within a wide range, preferably from 15 to 60% by weight, preferably from 20 to 50% by weight.
According to the present invention, the conditions for crystallization are selected from a wide range, and preferably, the conditions for crystallization include: the crystallization temperature is 10-60 ℃, the crystallization pressure is 3-100kPa, preferably 3-60kPa, the total residence time of the crystallization is 0.5-10h, preferably the residence time of the material in each crystallization stage is 0.5-5h in multi-stage crystallization. Under the condition, the method is not only beneficial to the precipitation of caprolactam crystals, but also further beneficial to the dissolution of impurities in the solvent, thereby further being beneficial to the purification in the crystallization process.
According to the invention, the number of crystallizer stages in the continuous crystallization process can be single-stage or multi-stage.
According to the invention, it is preferred that in the case of a single-stage crystallization the feed temperature is 50-60 ℃, the crystallization temperature is 20-40 ℃, the crystallization pressure is 10-35kPa, and the total residence time of the crystallization is 1-4h.
According to the present invention, preferably, the evaporative crystallization is a multistage continuous crystallization, preferably not more than 5 stages, and more preferably 3 stages. In the present invention, as shown in FIG. 2, each stage of crystallization is performed by adiabatic flash evaporation. According to the invention, the increase of the number of the crystallizer stages is more beneficial to obtaining crystals with more uniform granularity and better quality, but the equipment and operation cost is increased.
In a preferred embodiment of the present invention, in the case of multistage continuous crystallization, the difference between the feed temperature of the two adjacent stages of evaporative crystallization and the crystallization temperature in the corresponding crystallization stage is not more than 30 ℃, preferably not more than 20 ℃, and more preferably from 1 to 15 ℃. In the preferred embodiment, the purity and the crystallization yield of the crystal product can be improved, the precipitation of the crystals on the wall of the device can be further reduced, and the generation of crystal scale is reduced, so that the stable operation time of continuous crystallization is further improved.
The inventor of the present invention has found that if the temperature of the caprolactam solution/magma fed is higher than the temperature in the crystallizer by more than 30 ℃, the tube opening flash evaporation of the feed will be more intense, the requirement for stable operation of the equipment is higher, and the intense flash evaporation will also cause the crystal nucleation to be intensified, and the product quality will be reduced. More preferably the temperature difference may be within 20 ℃. The inlet temperature difference of the first stage crystallizer is particularly important and can be more preferably within 15 ℃. In the above preferred embodiment, when the caprolactam solution/crystal slurry of the previous stage enters the crystallizer with a certain negative pressure, part of the solvent is flashed off due to a relatively high temperature, and due to cooling, not only is the precipitation or growth of crystals facilitated, but also the precipitation or growth process can be more stable, and the caprolactam solution/crystal slurry has higher efficiency on the basis of better product quality.
The control of the crystallization temperature difference at each stage can be realized by adjusting the pressure of crystallization at each stage.
According to the invention, the target temperature of each stage of crystallization can be selected within a wide range, in the case of multistage continuous crystallization, the feed temperature of the first stage of crystallization is 50-75 ℃, preferably 50-65 ℃, the crystallization temperature is 40-55 ℃, the crystallization pressure is 30-60Pa, the difference between the feed temperature of two adjacent stages of crystallization and the crystallization temperature in the corresponding crystallization stage is not more than 30 ℃, and the difference between the pressure of two adjacent stages of crystallization is not more than 35kpa. Preferably, in the case of multistage continuous crystallization, the difference between the temperature of the upper stage crystallization and the temperature of the lower stage crystallization is 1 to 20 ℃ and the difference between the pressure of the upper stage crystallization and the pressure of the lower stage crystallization is 1 to 25kPa ℃. Under the preferred embodiment, the flash evaporation process and the crystallization process at each stage are more stable, the crystal product purity, the crystallization efficiency and the crystallization yield are further improved, and the stable operation time of continuous crystallization is further improved.
In a more preferred embodiment of the present invention, the crystallization refining process comprises three stages of crystallization, and the conditions of the first stage of crystallization comprise: the feeding temperature is 50-60 ℃, the crystallization temperature is 47-53 ℃, the crystallization pressure is 35-55kPa, and the material retention time is 0.5-1.5h; the conditions for the second stage of crystallization include: the crystallization temperature is 40-50 ℃, the crystallization pressure is 35-55kPa, and the material retention time is 0.5-1.5h; the conditions for the third stage of crystallization include: the crystallization temperature is 20-35 ℃, the crystallization pressure is 15-30kPa, and the material retention time is 0.5-1.5h.
The inventor of the invention finds out through research that most of impurities in the crystallization process are adsorbed on the surface of crystals, the specific surface area of fine crystals is large, the impurities are easier to adsorb, the particle size of the crystals is required to reach a certain size in order to obtain a product with higher purity, and the residence time of crystal mush directly influences the concentration of caprolactam in a crystallization kettle and the particle size of caprolactam crystals. The inventor of the present invention found through research that in a general continuous crystallization process, the single-stage yield fluctuation is more than 30%, and the fluctuation is unfavorable for the crystallization process, and can directly cause the crystallization process to be dominated by growth to nucleation, and further cause the particle size distribution to be extremely dispersed, and influence the quality of the product. Thus, in a preferred embodiment of the invention, the yield of a single stage in each crystallizer is controlled to fluctuate within ± 20% of the average yield or target design yield of a single stage in the crystallizer, i.e. the ratio of the yield of caprolactam crystals in the resulting slurry of crystallization stages to the average yield in the crystallization stages is from 0.8 to 1.2. Since the first crystallizer is a nucleation process, the stability requirement of the first crystallizer is more severe, and it is further preferable that the yield fluctuation of the first crystallizer is within ± 15% of the single-stage average yield or the target design yield, i.e., the ratio of the yield of caprolactam crystals in the crystal slurry obtained in each crystallization stage to the average yield in each crystallization stage is 0.85 to 1.15. Further preferably, in the case of multiple stages of continuous crystallization, the control target of the caprolactam crystal yield of each stage of crystallization is consistent, that is, in each stage of crystallization, when the caprolactam crystal yield (the increase of the crystal content) of the stage approaches the control target, the pulling of the crystal slurry of the stage is started.
According to the invention, in the case of multistage continuous crystallization, the yield of caprolactam crystals of each stage is selected within a wide range, preferably the caprolactam yield in the first stage crystallizer should not be higher than 50% of the total yield of the continuous crystallization unit. When a plurality of stages of continuous crystallization are used, if the amount of precipitation of a certain stage is too large, the size of the crystal of the stage becomes small, which is disadvantageous to the growth of the crystal. Therefore, the content of discharged crystal slurry in each stage of crystallizer needs to be optimized, and generally, crystals with equal mass are preferably precipitated at each stage; preferably, crystals of equal quality are precipitated in each crystallization stage, i.e. the yield increases are of the same magnitude.
According to the invention, the caprolactam crystal content of the crystal mush comprising the second stage crystallization and the subsequent crystallization stages is preferably not higher than 35% by weight; in the preferred embodiment, the crystal slurry is more uniform in the solid content range, the crystals are suspended in the solution, and the generation of crystal scale can be reduced.
According to the invention, the content of caprolactam crystals in the metastable zone controlled crystal slurry is preferably controlled by: adjusting the frequency of the withdrawn magma and/or injecting a solvent or caprolactam solution into the crystallization mixture, preferably the caprolactam solution from the mother liquor separated from the withdrawn magma. This can enhance the stirring effect. Similarly, the viscosity of the caprolactam solution should preferably not be higher than 250cP (centipoise) in order to enhance the mixing effect of the magma in the crystallizer.
According to the present invention, the total residence time of the continuous crystallization process can be adjusted within a wide range, and in order to reduce the running cost on the basis of increasing the particle size and purity of the crystals, it is preferable that the total residence time of the material in the crystallization process is 0.5 to 10 hours.
According to the invention, the residence time of the material in the crystallization stages can be adjusted within a wide range. Further preferably, the residence time of the material in each crystallization stage is between 1 and 4h.
The present invention preferably employs a vacuum type crystallizer, and the present invention is not particularly limited with respect to the specific structure of the crystallizer. Preferably, a heat insulating layer is added on the wall surface of the crystal connecting device.
In a preferred embodiment of the present invention, in order to reduce the accumulation of the crystal scale and increase the continuous crystallization stable operation time, it is preferable that the method further comprises a step of cleaning the crystal scale growing on the wall of the crystallizer in such a manner that the inner wall surface of the crystallizer is sprayed with a part of the organic solvent to wash down the crystal scale and/or the inner wall surface of the crystal scale is cleaned into the solution by a stirring rod. The above embodiments can avoid the accumulation of scale on crystallizer equipment and increase the stable operation time of the equipment.
The source of the crude caprolactam product is not particularly limited in the invention, and the crude caprolactam product can be a crude caprolactam product which contains impurities in the field and does not meet the industrial requirements. In the invention, the caprolactam crude product can be obtained by performing Beckmann rearrangement reaction on cyclohexanone oxime. In the present invention, the step and the conditions for subjecting the cyclohexanone oxime to the beckmann rearrangement reaction may be performed according to the conventional techniques in the art, and the present invention is not particularly limited thereto. The crude caprolactam product can be obtained by carrying out gas-phase Beckmann rearrangement reaction on cyclohexanone oxime or liquid-phase Beckmann rearrangement reaction on cyclohexanone oxime.
The gas phase Beckmann rearrangement reaction of the present invention can be carried out according to the conventional techniques in the art, and the present invention is not particularly limited thereto, and for example, the gas phase Beckmann rearrangement reaction can be carried out in the presence of a catalystReacting gaseous cyclohexanone oxime in the presence of a molecular sieve catalyst of MFI structure and a carrier gas and a solvent. The conditions of the gas phase beckmann rearrangement reaction may include: the temperature is 320-450 ℃, preferably 370-400 ℃; the pressure is 0.05-0.5MPa, preferably 0.1-0.3MPa; the weight hourly space velocity of the cyclohexanone-oxime is 0.1-5h -1 . The solvent is a conventional solvent for the kremen rearrangement reaction such as alcohol, aromatic hydrocarbon, etc. The carrier gas may be various gases that do not react with the cyclohexanone oxime and the solvent under the gas phase beckmann rearrangement reaction conditions, and may be, for example, nitrogen gas as well as an inert gas. Both the pressure and the partial pressure refer to absolute pressures.
According to the present invention, the pressures are absolute pressures, unless otherwise specified.
According to the invention, the caprolactam crude product is preferably obtained by: and (3) carrying out gas phase Beckmann rearrangement on the cyclohexanone oxime to obtain a mixture, and distilling under reduced pressure to remove impurities with a boiling point lower than that of caprolactam to obtain a crude caprolactam product.
According to the invention, the caprolactam raw product preferably has a purity of 97 to 99.9% by weight.
In this case, the crude caprolactam product comprises caprolactam, 5-cyano-1-pentene, cyclohexenone, cyclohexanone oxime, octahydrophenazine, decahydrophenazine and isomers of tetrahydro-azepin-2-one and/or tetrahydro-azepin-2-one, and the caprolactam content is 97 to 99.9% by weight, preferably 99 to 99.9% by weight, the 5-cyano-1-pentene content is 0.001 to 0.1% by weight, the cyclohexenone content is 0.001 to 0.1% by weight, the cyclohexanone oxime content is 0.001 to 0.3% by weight, the octahydrophenazine content is 0.001 to 0.1% by weight, the total tetrahydro-azepin-2-one and/or tetrahydro-azepin-2-one isomers content is 0.001 to 0.1% by weight, and the decahydrophenazine content is 0.001 to 0.1% by weight, based on the total weight of the crude caprolactam product.
The present invention may further comprise a step of separating the crystal crystals obtained after the crystallization from the mother liquor, preferably, the crystal crystals obtained after the crystallization are separated from the mother liquor by filtration and/or centrifugation.
The centrifugation may use a pusher centrifuge, which may be operated in one or more steps. Sieve plate transport centrifuges or screw transport centrifuges (decanters) are also suitable for the invention. The filtration can be accomplished by suction filters (which can be operated batchwise or continuously, optionally equipped with stirrers) or belt filters.
In the process provided by the present invention, during and or after the solid-liquid separation, additional process steps may be provided to increase the purity of the crystals or crystal cake, i.e. a solvent washing step. Preferably, the process provided according to the present invention may further comprise washing the crystalline crystals separated after crystallization to further obtain a caprolactam product of higher purity. In order to further reduce the content of impurities adhering to the surface of the crystals, it is preferable to provide a washing leg at the outlet of the last-stage crystallizer, and clean solvent is introduced into the crystallizer from the lower inlet of the washing leg to wash the discharged slurry in countercurrent.
The above steps of separating the crystallized crystals from the mother liquor and washing can be performed integrally, for example, by countercurrent washing, and specifically, refer to CN104059019B, CN104059018B, and CN104072419B. The advantage of the countercurrent washing is that the washing of the crystals is also completed in the process of separating the mother liquor, which is more beneficial to the implementation of industrialization.
The purpose of washing is mainly to wash off impurities attached to the surface of the caprolactam crystal and improve the purity of the crystal. The washing may be carried out one or more times, and the present invention has no particular requirement for the selection of the washing solvent, and may be at least one of linear alkanes, cycloalkanes, ethers and halogenated hydrocarbons having a boiling range of less than 150 ℃, for example, n-hexane, n-heptane, n-octane, n-nonane, methylhexane, isohexane, neohexane, isoheptane, isooctane, isononane, cyclohexane, isopropylether, methyl tert-butyl ether, diethyl ether, n-butyl chloride and the like. The washing may be carried out at room temperature (25 ℃).
The second aspect of the present invention provides a process for producing caprolactam, which comprises refining a crude caprolactam product by the crystallization refining process according to the first aspect to obtain crystalline crystals; and (4) carrying out hydrogenation reaction on the crystallized crystals.
According to the caprolactam production method provided by the invention, through the hydrogenation reaction of the crystallized crystals, on one hand, impurities which are difficult to be sufficiently removed in the crystallization process, such as tetrahydroazepine-2-ketone and isomers thereof, can be converted into caprolactam, so that the purity of the finally prepared caprolactam is further improved; on the other hand, the potassium permanganate absorption value of the caprolactam product can be effectively improved. The present invention is not particularly limited to the specific embodiment of the hydrogenation reaction, and the hydrogenation reaction can be carried out according to the conventional techniques in the art. The hydrogenation reaction may be carried out in the presence of water or in a molten state, and the present invention is not particularly limited thereto. According to a preferred embodiment of the present invention, the water may be used in an amount of 10 to 70 parts by weight, preferably 20 to 50 parts by weight, relative to 100 parts by weight of the crystalline crystals.
According to a preferred embodiment of the present invention, the hydrogenation reaction is carried out in the presence of a hydrogenation catalyst. Preferably, the hydrogenation catalyst is selected from at least one of a nickel-based catalyst, a palladium-based catalyst, and a platinum-based catalyst. The hydrogenation catalyst may be obtained commercially or by itself, and is not particularly limited in the present invention. Preferably, the hydrogenation catalyst is a nickel-based catalyst and/or a palladium-based catalyst.
In a preferred embodiment of the present invention, the nickel-based catalyst is an amorphous nickel catalyst. The amorphous nickel catalysts can be found, for example, in CN1272490A and CN1272491A.
According to the present invention, the selection range of the palladium-based catalyst is wide, and preferably, the palladium-based catalyst includes a carrier and palladium and a rare earth oxide supported on the carrier. The carrier may be at least one of activated carbon, silica, titania and alumina, and the rare earth oxide may be an oxide of lanthanum and/or cerium. Preferably, the support is activated carbon. The invention has wide selection range of the contents of palladium and rare earth oxide in the palladium catalyst, and preferably, the content of palladium is 0.1 to 5 weight percent and the content of rare earth oxide is 0.2 to 10 weight percent based on the total amount of the palladium catalyst. Specifically, the preparation of the palladium catalyst can be found in CN102430406A.
According to the present invention, the conditions of the hydrogenation reaction can be adjusted within a wide range, and hydrogenation conditions conventional in the art can also be employed. In order to further enable the caprolactam product to have higher potassium permanganate absorption value and purity and smaller extinction value, preferably, the hydrogenation reaction conditions comprise: the temperature is 50-150 ℃, and more preferably 60-90 ℃; the pressure is 0.3-2MPa, more preferably 0.5-1.5MPa; the flow rate of hydrogen is 0.1 to 2L/min, more preferably 0.3 to 1.5L/min. The pressure refers to absolute pressure.
The time for the hydrogenation reaction can be adjusted within a wide range, and preferably, when the hydrogenation reaction is a batch operation, the time for the hydrogenation reaction can be 0.5 to 3 hours, more preferably 1 to 2 hours. When the hydrogenation reaction is operated continuously (e.g., fixed bed process), the mass space velocity of caprolactam may be in the range of 0.5 to 30 hours -1 。
The reactor type for carrying out the hydrogenation reaction in the present invention is not particularly limited, and a magnetically stabilized bed reactor, a fixed bed reactor or a slurry bed reactor may be used, and a fixed bed reactor is preferred.
In one embodiment, the process provided according to the present invention further comprises collecting the caprolactam product after the hydrogenation reaction by evaporation and/or distillation under reduced pressure, thereby obtaining a caprolactam product having a higher potassium permanganate absorption value, a lower volatile base number, and an extinction value.
In the following examples:
the residence time refers to the time it takes to fill an empty crystallizer with solution at a set flow rate until the solution reaches a set volume.
The calculation method of the total yield of caprolactam after continuous crystallization comprises the following steps: the mass flow rate of caprolactam in the feed is divided by the mass flow rate of caprolactam crystals in the last crystallization run.
The caprolactam yield of each crystallization stage in the continuous crystallization process was calculated as: the difference in mass flow rate of caprolactam crystals fed to and discharged from the stage crystallizer is divided by the mass flow rate of caprolactam (mass of crystals + mass flow rate of caprolactam in solution) in the feed to the stage crystallizer.
The solid content in the discharged crystal slurry refers to the ratio of the mass of the crystals in the crystal slurry to the total mass of the solution.
The quality of the caprolactam product prepared was evaluated in the following examples using the following test methods:
(1) Purity of caprolactam
The purity and impurity content of caprolactam are analyzed by a capillary column Innovax 60m and a gas chromatography 7890GC, and the lowest detection limit of the chromatography is 1 mu g/g.
(2) Absorption value of potassium permanganate of epsilon-caprolactam
Pouring 3.000 g of caprolactam into a colorimetric cylinder of 100ml, adding distilled water to dilute to a scale, shaking up, putting into a thermostatic water bath of 20 ℃, adding 1ml of potassium permanganate solution with the concentration of 0.01N into the colorimetric cylinder, shaking up immediately, starting a stopwatch at the same time, and when the color of the sample solution in the colorimetric cylinder is matched with that of a standard colorimetric solution (taking 3 g of superior pure Co (NO) (NO is 3 g) 3 )·6H 2 O and 12 mg of Uper grade K 2 Cr 2 O 7 Dissolved in water, diluted to 1 liter, shaken up) and stopped the stopwatch when the colors are the same, and the time (in seconds) consumed is recorded, namely the potassium permanganate absorption value.
(3) Volatile base (V.B)
In an alkaline medium, the alkaline low molecular impurities in the sample are distilled out, absorbed by a known amount of hydrochloric acid solution, and the excess hydrochloric acid is back-dripped with a sodium hydroxide standard solution. The volatile base is determined in moles of acid consumed per kilogram of sample. The calculation formula is as follows:
V.B(mmol/kg)＝[(V 0 -V)×C NaOH /M]×1000
in the formula: v 0 The volume of NaOH standard solution consumed for 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 concentration is the accurate concentration of NaOH standard solution, and the unit is mol/L;
m is the sample mass in g.
(4) Extinction value E (at 290nm wavelength)
In a 300ml Erlenmeyer flask, 50 g of the sample is weighed, 50ml of distilled water is added, shaking is carried out to completely dissolve the sample, and the mixture is allowed to stand for 10 minutes. The extinction value of the sample at a concentration of 50% with respect to distilled water was measured at a wavelength of 290nm using a spectrophotometer.
The present invention will be described in detail below by way of examples.
Example 1
According to the method described in example 1 of CN102452983B, the cyclohexanone oxime undergoes a gas phase beckmann rearrangement reaction, and undergoes a solvent-stripping process to obtain a crude caprolactam product, wherein the purity of the crude caprolactam product is 99.3 wt%, and the crude caprolactam product contains impurities such as cyclohexanone oxime, octahydrophenazine, N-methylcaprolactam, etc., and the content of 5-cyano-1-pentene is 0.01 wt%, the content of cyclohexenone is 0.001 wt%, the content of cyclohexanone oxime is 0.12 wt%, the content of octahydrophenazine is 0.07 wt%, the total content of isomers of tetraazepine-2-one and/or tetraazepine-2-one is 0.06 wt%, the content of decahydrophenazine is 0.03 wt%, and other impurities are undefined.
The crude caprolactam product was first mixed with isopropyl ether (which was found to have a solubility of 1.6% by weight in isopropyl ether at a boiling temperature of the solution corresponding to an absolute pressure of 5 kPa) to give a caprolactam solution with a mass fraction of 29% at 53 ℃. Then continuously injecting into a primary caprolactam crystallizer, starting a vacuum device of the equipment, and flashing off 30% of the solvent under adiabatic conditions, wherein the temperature of the crystallization system is continuously reduced to 35 ℃ of the temperature of the crystallizer, and the absolute pressure of the crystallizer is 27kPa;
keeping the residence time of the crystals at 240min, continuously extracting caprolactam crystal slurry at the speed of 10g/min, maintaining the continuous feeding and continuous discharging state of the crystallizer, measuring the concentration of caprolactam by sampling and maintaining the caprolactam concentration at 8.6 wt% (located in a metastable zone), maintaining the pressure and the temperature in the crystallizer at 27kPa and 35 ℃, and stirring at 200rpm. The solids content in the discharge was 33% by weight.
The continuous crystallization product of caprolactam can be obtained after centrifuging and washing the crystal mush of continuous crystallization. The crystallized product was subjected to sieve analysis, and the mass fraction of crystals having a particle size of less than 30 mesh was 15%. The purity of the crystal product was 99.98% by chromatographic analysis of the crystal product.
The caprolactam yield in this example was 85%. After 2 days of continuous stable operation, the amount of scale formed on the inner wall of the crystallizer and on the stirring rods was 1.3kg/m 2 。
Dissolving the crystal product in water, carrying out hydrogenation reaction under the conditions of 0.7MPa, 75 ℃ and 0.2L/min hydrogen flow rate and taking amorphous nickel as a catalyst, dehydrating and flashing the reaction product to obtain a caprolactam product, and analyzing the caprolactam product to obtain: the extinction value is 0.031 (better than the caprolactam index of the high-class product in the national standard GB/T13254-2017); the chroma value is 2 (superior to the index of superior product caprolactam in national standard GB/T13254-2017); the content of the volatile alkali is 0.023mmol/kg (superior to the index of superior caprolactam product in the national standard GB/T13254-2017); PMs value is larger than 28000s (better than the index of superior product caprolactam in national standard GB/T13254-2017); the purity of caprolactam product was 99.995%.
Comparative example 1
The same crude caprolactam product as in example 1 was subjected to crystallization refining using a continuous crystallization protocol in direct cooling:
the crude caprolactam is maintained at 75 ℃ and isopropyl ether is maintained at-15 ℃,
firstly, injecting a certain mass of saturated solution into a crystallizer to prevent caprolactam feed from scaling, then continuously injecting caprolactam in a molten state at 75 ℃ into the crystallizer, simultaneously continuously injecting isopropyl ether at-15 ℃ into the crystallizer, maintaining the temperature of the crystallizer at 35 ℃, the pressure at normal pressure, and controlling the mass flow ratio of the isopropyl ether to the caprolactam to be 2:1, performing crystallization by adopting a quenching mode; the crystal residence time in the crystallizer was kept at 240min. While continuously injecting caprolactam and isopropyl ether, caprolactam magma was continuously withdrawn at a rate of 10 g/min. After the apparatus had been in steady operation for 48 hours, the caprolactam crystals obtained were centrifuged and washed. The centrifugation and washing steps were in accordance with the method described in example 1. The product is sieved and analyzed, and the mass fraction of the crystals with the particle size smaller than 30 meshes is 19 percent. The purity of the crystal product was 99.91% by chromatographic analysis of the crystal product.
The caprolactam crystal yield was 81%. After continuous stable operation for 48h, the amount of scale formed on the inner wall of the crystallizer and on the stirring rods was 1.2kg/m 2 。
The crystalline product was dissolved in water and subjected to hydrogenation, dehydration and flash evaporation according to the method described in example 1 to obtain a caprolactam product which was analysed to obtain: the extinction value is 0.06 (which is inferior to the index of caprolactam which is a high-grade product in the national standard GB/T13254-2017); the chroma value is 2 (superior to the index of caprolactam of high-grade products in national standard GB/T13254-2017); the content of the volatile alkali is 0.04mmol/kg (superior to the index of caprolactam which is a high-grade product in the national standard GB/T13254-2017); the PMs value is more than 20000s (is superior to the caprolactam index of the superior product in the national standard GB/T13254-2017, but is lower than that of the example 1); the caprolactam product purity was 99.990% lower than the corresponding index for example 1.
Example 2
The same crude caprolactam as in example 1 was subjected to crystallization refining in the following continuous crystallization scheme:
firstly, the mixture is mixed with isopropyl ether to prepare caprolactam solution with the mass fraction of 29 percent at 53 ℃. Then continuously injecting the solution into a primary caprolactam crystallizer, adjusting the absolute pressure of the primary crystallizer to 43.2kPa, controlling the temperature in the crystallizer to be 48.5 ℃, controlling the residence time of the primary caprolactam crystallizer to be 1.5h, sampling to obtain a caprolactam concentration which is maintained at 25.1 wt% (located in a metastable zone), starting the discharge of the primary crystallizer when the volume of the crystal mush in the primary crystallizer reaches a set value (the content of crystals in the crystal mush is estimated to be 10 wt%), continuously injecting the solution into a secondary crystallizer, adjusting the absolute pressure of the secondary crystallizer to be 40.2kPa, controlling the temperature in the crystallizer to be 46.3 ℃, controlling the residence time of the primary caprolactam to be 1.5h, sampling to determine that the concentration of caprolactam is maintained at 16.7 wt% (located in a metastable zone), when the volume of crystal mush in the crystallizer reaches a set value (the content of crystals in the crystal mush is estimated to be 20 wt%), starting the discharge of the second crystallizer, wherein the content of the crystals in the crystal mush is 20.7 wt%, continuously injecting the solution into the third crystallizer, adjusting the absolute pressure of the third crystallizer to be 26.9kPa, controlling the temperature in the crystallizer to be 35.7 ℃, controlling the residence time of the stage to be 1.5h, sampling to determine that the concentration of caprolactam is maintained at 8.5 wt% (located in the metastable zone), and when the volume of the crystal mush in the crystallizer reaches a set value (the content of the crystals in the crystal mush is estimated to be 30 wt%), wherein the content of the crystals in the crystal mush is 32.5 wt%, and then taking out the crystal mush. In the process, caprolactam solution is continuously injected into each crystallizer, and caprolactam crystal slurry is continuously extracted. At this time, the total yield of the whole crystallization process is 85%, and the yield of each stage of crystallizer is consistent in a steady state.
Collecting part of crystal slurry, centrifuging and washing the obtained caprolactam crystal, and analyzing the quality of the crystal product. And (4) screening and analyzing the product to obtain the crystal with the particle size of less than 30 meshes, wherein the mass fraction of the crystal is 11%. The purity of the crystal product was 99.98% by chromatographic analysis of the crystal product.
After 2 days of continuous stable operation, the amount of scale formed on the inner wall of the crystallizer and on the stirring rods was 0.6kg/m 2 。
To further examine the product quality of this example, the same procedure as in example 1 was used, the crystalline product obtained in this example was also subjected to hydrogenation and flash distillation, and the resulting caprolactam product was analyzed to obtain: the extinction value is 0.02 (superior to the index of superior product caprolactam in national standard GB/T13254-2017); the chromatic value is 1 (superior to the index of superior product caprolactam in national standard GB/T13254-2017); the content of the volatile alkali is 0.01mmol/kg (which is superior to the index of caprolactam which is a high-grade product in the national standard GB/T13254-2017); the PMs value is more than 30000s (better than the caprolactam index of high-class products in national standard GB/T13254-2017); the purity of caprolactam product is 99.996%.
Example 3
The same caprolactam crude product as in example 1 was subjected to crystallization refining in the following continuous crystallization scheme:
firstly, the mixture is mixed with isopropyl ether to prepare caprolactam solution with the mass fraction of 29 percent at 53 ℃. And then continuously injecting the solution into a primary caprolactam crystallizer, adjusting the absolute pressure of the primary crystallizer to 42.1kPa, controlling the temperature in the crystallizer to 47.2 ℃, keeping the retention time of the primary caprolactam crystallizer for 1.5h, sampling to obtain a caprolactam concentration of 21.0 wt% (located in a metastable zone), when the volume of the crystal slurry in the primary crystallizer reaches a set value (the content of crystals in the crystal slurry is estimated to be 15 wt%), starting the discharge of the primary crystallizer, controlling the retention time of the primary caprolactam to 1.5h, continuously injecting the solution into a secondary crystallizer, adjusting the absolute pressure of the secondary crystallizer to 26.9kPa, controlling the temperature in the crystallizer to 35.7 ℃, sampling to obtain a caprolactam concentration of 8.52 wt% (located in the metastable zone), and when the volume of the crystal slurry in the secondary crystallizer reaches a set value (the content of crystals in the crystal slurry is estimated to be 30 wt%), collecting the crystal slurry. In the process, caprolactam solution is continuously injected into each stage of crystallizer, and caprolactam crystal mush is continuously extracted. At this time, the total yield of the whole crystallization process was 85%, and the yields at all stages were consistent at steady state.
Collecting part of crystal slurry, centrifuging and washing the obtained caprolactam crystal, and analyzing the quality of the crystal product. The product is screened and analyzed, and the mass fraction of the crystals with the particle size of less than 30 meshes is 13.3 percent. The purity of the crystal product was 99.98% by chromatographic analysis of the crystal product.
After 2 days of continuous stable operation, the amount of scale formed on the inner wall of the crystallizer and on the stirring bar was 0.85kg/m 2 。
To further examine the product quality of this example, the same procedure as in example 1 was used, the crystalline product obtained in this example was also subjected to hydrogenation and flash distillation, and the resulting caprolactam product was analyzed to obtain: the extinction value is 0.025 (better than the caprolactam index of the high-grade product in the national standard GB/T13254-2017); the chromatic value is 1 (superior to the index of superior product caprolactam in national standard GB/T13254-2017); the content of the volatile alkali is 0.01mmol/kg (which is superior to the index of caprolactam which is a high-grade product in the national standard GB/T13254-2017); the PMs value is more than 30000s (better than the caprolactam index of high-class products in national standard GB/T13254-2017); the purity of caprolactam product is 99.996%.
Example 4
The same caprolactam crude product as in example 1 was subjected to crystallization refining in the following continuous crystallization scheme:
the first stage crystallization was carried out in the same manner as in example 3 except that the absolute pressure of the second stage crystallization was 10kPa, and the temperature in the second stage crystallizer was 15 ℃ at this time (the difference between the stages was more than 30 ℃ at this time), and in this process, the residence time of each stage was controlled in the same manner as in example 3.
The crystals in the caprolactam crystal slurry obtained were centrifuged and washed as in example 3 and the quality of the crystal product was analyzed. The product is screened and analyzed, and the mass fraction of the crystals with the particle size of less than 30 meshes is 14 percent. The purity of the crystal product obtained by chromatographic analysis of the crystal product was 99.96%, and the yield in the crystallization process was 96.2%.
After 2 days of continuous stable operation, the amount of scale formed on the inner wall of the crystallizer and on the stirring rods was 1.1kg/m 2 。
To further examine the quality of the product obtained in this example, the same procedure as in example 1 was used, the crystalline product obtained in this example was also subjected to hydrogenation and flash distillation, and the caprolactam product obtained was analyzed to obtain: the extinction value is 0.04 (which is equal to the index of caprolactam which is a superior product in national standard GB/T13254-2017, but is inferior to the result of example 2); the chroma value is 5 (better than the index of a high-grade caprolactam product in the national standard GB/T13254-2017, but inferior to the result of the example 2); the content of the volatile alkali is 0.01mmol/kg (which is superior to the index of caprolactam which is a high-grade product in the national standard GB/T13254-2017); the PMs value is more than 24000s (better than the caprolactam index of a high-grade product in the national standard GB/T13254-2017); the purity of caprolactam product was 99.995%.
Example 5
The same crude caprolactam product as in example 1 was subjected to crystallization refining in the following continuous crystallization protocol, except that the crystallization solvent was cyclohexane (mass percent solubility of caprolactam in the solvent at 5kPa absolute and at the boiling temperature of isopropyl ether was 1.2 wt%), and the specific crystallization refining process was controlled as follows:
firstly, mixing a caprolactam crude product with cyclohexane to prepare a caprolactam solution with the mass fraction of 29 percent at 67 ℃. The absolute pressure in the first crystallizer was then adjusted to 35kPa in a continuous feed into the primary caprolactam crystallizer, at which time the temperature in the crystallizer was 52 ℃. The absolute pressure in the second stage crystallizer was adjusted to 18kPa, at which time the temperature in the crystallizer was 35 ℃. Continuously injecting a caprolactam solution, continuously extracting caprolactam crystal slurry, and maintaining the residence time of each stage of crystallizer at 2.25h. Other crystallization control conditions were the same as in example 3, and the total yield of the whole crystallization process was 89%.
After 2 days of continuous stable operation, the amount of scale formed on the inner wall of the crystallizer and on the stirring bar was 0.8kg/m 2 。
After maintaining the continuous crystallization state for 48 hours, the produced magma was collected, and the obtained caprolactam crystals were centrifuged and washed to analyze the quality of the crystal product. The product is screened and analyzed, and the mass fraction of the crystal with the grain size of less than 30 meshes is 13 percent. The purity of the crystal product was 99.96% by chromatographic analysis of the crystal product.
In order to further examine the product quality of this example, the same procedure as in example 1 was adopted, and the crystal product obtained in this example was subjected to hydrogenation and flash distillation, and the analysis of the obtained caprolactam product gave: the extinction value is 0.02 (superior to the caprolactam index of the high-grade product in the national standard GB/T13254-2017); the chroma value is 1 (superior to the index of caprolactam of high-grade products in national standard GB/T13254-2017); the content of volatile alkali is 0.02mmol/kg (superior to the index of superior product caprolactam in the national standard GB/T13254-2017); the PMs value is more than 27000s (better than the caprolactam index of high-grade products in the national standard GB/T13254-2017); the purity of caprolactam product is 99.996%.
Comparative example 2
The crude caprolactam was purified as in example 1, except that in the first stage crystallization, the residence time in the first stage crystallizer was changed to 0.24h, and the concentration of caprolactam in the unstable zone was measured.
After continuous stable operation for 5 hours, the scale formation on the inner wall of the crystallizer and the stirring rod is too large to carry out experiments. See table 1. The product is sieved and analyzed, and the mass fraction of the crystals with the particle size smaller than 30 meshes is 33 percent. The purity of the crystal product was 99.85% by chromatographic analysis of the crystal product.
The same procedure as in example 1 was followed to hydrogenate and flash treat the obtained crystalline product and to analyze the obtained caprolactam product to obtain: the extinction value is 0.07 (which is inferior to the index of caprolactam which is a high-grade product in the national standard GB/T13254-2017); the chroma value is 2 (superior to the index of caprolactam of high-grade products in national standard GB/T13254-2017); the content of the volatile alkali is 0.02mmol/kg (which is superior to the index of caprolactam which is a high-grade product in the national standard GB/T13254-2017); the PMs value is more than 20000s (is superior to the caprolactam index of the superior product in the national standard GB/T13254-2017, but is lower than that of the example 1); the caprolactam product purity was 99.98% below the corresponding index of example 1.
Example 6
The same crude caprolactam as in example 1 was subjected to crystallization refining in the following continuous crystallization scheme:
firstly, the caprolactam solution with the mass fraction of 22.4 percent at 73 ℃ is prepared by mixing with isopropyl ether. And then continuously injecting the solution into a primary caprolactam crystallizer, adjusting the absolute pressure of the primary crystallizer to 39kPa, controlling the temperature in the crystallizer to be 46 ℃, controlling the residence time of the stage to be 1.5h, sampling to obtain a caprolactam concentration which is maintained at 18.2 wt% (located in a metastable zone), when the volume of the crystal slurry in the primary crystallizer reaches a set value (the content of crystals in the crystal slurry is estimated to be 15 wt%), starting the discharge of the primary crystallizer, wherein the content of crystals in the crystal slurry is 14.5 wt%, continuously injecting the solution into a secondary crystallizer, adjusting the absolute pressure of the secondary crystallizer to be 13.4kPa, wherein the temperature in the crystallizer is 20 ℃, controlling the residence time of the stage to be 1.5h, sampling to obtain a caprolactam concentration which is maintained at 3.6 wt% (located in the metastable zone), and taking out the crystal slurry when the volume of crystals in the crystal slurry in the primary crystallizer reaches a set value (the content of crystals in the crystal slurry is estimated to be 30 wt%). In the process, caprolactam solution is continuously injected into each stage of crystallizer, and caprolactam crystal mush is continuously extracted. At this time, the total yield of the whole crystallization process was 94.4%, and the crystallization yields at all stages were consistent at steady state.
Collecting the extracted crystal mush, centrifuging and washing the obtained caprolactam crystal, and analyzing the quality of the crystal product. The product is screened and analyzed, and the mass fraction of the crystals with the particle size of less than 30 meshes is 16 percent. The purity of the crystal product was 99.97% by chromatographic analysis of the crystal product.
After 2 days of continuous stable operation, the amount of scale formed on the inner wall of the crystallizer and on the stirring bar was 0.9kg/m 2 。
To further examine the product quality of this example, the same procedure as in example 1 was used, the crystalline product obtained in this example was also subjected to hydrogenation and flash distillation, and the resulting caprolactam product was analyzed to obtain: the extinction value is 0.038 (superior to the index of superior caprolactam in national standard GB/T13254-2017); the chroma value is 3 (superior to the index of superior product caprolactam in national standard GB/T13254-2017); the content of the volatile alkali is 0.01mmol/kg (which is superior to the index of caprolactam which is a high-grade product in the national standard GB/T13254-2017); the PMs value is more than 30000s (superior to the index of superior product caprolactam in national standard GB/T13254-2017); the purity of caprolactam product is 99.994%.
Example 7
The same crude caprolactam as in example 1 was subjected to crystallization refining in the following continuous crystallization scheme:
firstly, the mixture is mixed with isopropyl ether to prepare caprolactam solution with the mass fraction of 29 percent at 53 ℃. Then, in a continuous injection primary caprolactam crystallizer, the absolute pressure of the primary crystallizer is adjusted to 41.2kPa, the temperature in the crystallizer is 47.4 ℃, the retention time of the primary crystallizer is controlled to be 1.5h, the caprolactam concentration is kept at 21.0 wt% (located in a metastable zone) by sampling, the yield of the primary crystallizer is 42.5% in a steady state, when the volume of crystal slurry in the primary crystallizer reaches a set value (the content of crystals in the crystal slurry is estimated to be 15 wt%), the discharge of the primary crystallizer is started, the content of the crystals in the crystal slurry is 14.4 wt%, the solution is continuously injected into the secondary crystallizer, the absolute pressure of the secondary crystallizer is adjusted to be 40.2kPa, the temperature in the crystallizer is 45.3 ℃, the retention time of the primary crystallizer is controlled to be 1.5h, when the concentration of caprolactam is measured by sampling and maintained at 17.1 wt% (located in a metastable zone), when the volume of crystal mush in the crystallizer reaches a set value (the content of crystals in the crystal mush is estimated to be 20 wt%), starting the discharge of the second crystallizer when the yield of single-stage caprolactam is 21.25%, wherein the content of crystals in the crystal mush is 20.7 wt%, continuously injecting the solution into the third crystallizer, adjusting the absolute pressure of the third crystallizer to be 26.9kPa, wherein the temperature in the crystallizer is 35.7 ℃, controlling the residence time of the stage to be 1.5h, measuring by sampling, the concentration of caprolactam is maintained at 8.5 wt% (located in the metastable zone), and when the volume of crystal mush in the crystallizer reaches a set value (the content of crystals in the crystal mush is estimated to be 30 wt%), wherein the content of crystals in the crystal mush is 32.5 wt%, and crystal mush is collected. In the process, a caprolactam solution is continuously injected into the crystallizer, caprolactam crystal mush is continuously extracted, and the retention time of each stage of crystallizer is maintained at 1.5h. At this time, the total yield of the whole crystallization process was 85%, and at the steady state, the first-stage crystallization yield was 42.5%, and the remaining two-stage crystallization yields were 21.25%.
Collecting part of crystal slurry, centrifuging and washing the obtained caprolactam crystal, and analyzing the quality of the crystal product. The product is screened and analyzed, and the mass fraction of the crystals with the particle size of less than 30 meshes is 14.2 percent. The purity of the crystal product was 99.98% by chromatographic analysis of the crystal product.
After 2 days of continuous stable operation, the amount of scale formed on the inner walls of the crystallizer and on the stirring bars was 0.75kg/m 2 。
To further examine the quality of the product obtained in this example, the same procedure as in example 1 was used, the crystalline product obtained in this example was also subjected to hydrogenation and flash distillation, and the caprolactam product obtained was analyzed to obtain: the extinction value is 0.034 (superior to the index of superior caprolactam in national standard GB/T13254-2017); the chroma value is 2 (superior to the index of caprolactam of high-grade products in national standard GB/T13254-2017); the content of the volatile alkali is 0.02mmol/kg (which is superior to the index of caprolactam which is a high-grade product in the national standard GB/T13254-2017); the PMs value is more than 30000s (better than the caprolactam index of high-class products in national standard GB/T13254-2017); the purity of caprolactam product is 99.996%.
Example 8
The same caprolactam crude product as in example 1 was subjected to crystallization refining in the following continuous crystallization scheme:
first, the mixture was mixed with isopropyl ether to prepare a caprolactam solution having a mass fraction of 45.7% at 86.5 ℃ (in this case, a pressurized state). This solution was mixed with a 27 ℃ saturated solution of caprolactam in isopropyl ether in a ratio of 1:0.83 was mixed to give a 27.5% strength caprolactam solution at 59.4 ℃. And then continuously injecting the solution into a primary caprolactam crystallizer, adjusting the absolute pressure of the primary crystallizer to 40.2kPa, controlling the temperature in the crystallizer to 47.0 ℃, keeping the residence time of the primary crystallizer for 1.5h, sampling and measuring the concentration of caprolactam to be maintained at 20.2 wt% (located in a metastable zone), when the volume of crystal slurry in the primary crystallizer reaches a set value (the content of crystals in the estimated crystal slurry is 15 wt%), starting the discharging of the primary crystallizer, controlling the residence time of the primary crystallizer to be 1.5h, keeping the concentration of caprolactam in the sampled and measured to be 6.2 wt% (located in the metastable zone), continuously injecting the solution into a secondary crystallizer, adjusting the absolute pressure of the secondary crystallizer to be 20.4kPa, controlling the temperature in the crystallizer to be 30 ℃, and sampling and measuring the volume of the crystal slurry in the secondary crystallizer to be maintained at the set value (the content of crystals in the estimated crystal slurry is 30 wt%). In the process, caprolactam solution is continuously injected into each stage of crystallizer, and caprolactam crystal mush is continuously extracted. At this time, the total yield of the whole crystallization process is 90%, the crystallization yields of all levels are consistent in a steady state, and the solid content in the discharged crystal slurry is 35%.
After 2 days of continuous stable operation, the amount of scale formed on the inner wall of the crystallizer and on the stirring bar was 0.9kg/m 2 . Collecting part of crystal slurry, centrifuging and washing the obtained caprolactam crystal, and analyzing the quality of the crystal product. The product is screened and analyzed, and the mass fraction of the crystals with the particle size of less than 30 meshes is 14.5 percent. The purity of the crystal product was 99.92% by chromatographic analysis of the crystal product.
The magma was collected, 1 part of the magma was washed counter-currently with 0.23 part of isopropyl ether, and 78% by weight of the overflow mother liquor obtained was returned to be mixed with caprolactam solution at 86.5 ℃.
To further examine the product quality of this example, the same procedure as in example 1 was used, the crystalline product obtained in this example was also subjected to hydrogenation and flash distillation, and the resulting caprolactam product was analyzed to obtain: the extinction value is 0.035 (better than the caprolactam index of the high-class product in the national standard GB/T13254-2017); the chroma value is 3 (superior to the index of superior product caprolactam in national standard GB/T13254-2017); the content of the volatile alkali is 0.03mmol/kg (which is superior to the index of high-grade caprolactam in the national standard GB/T13254-2017); the PMs value is more than 30000s (better than the caprolactam index of high-class products in national standard GB/T13254-2017); the purity of caprolactam product is 99.993%.
Example 9
A crude caprolactam product was purified by the method of example 3, except that the crude caprolactam product had a purity of 98.9% and contained impurities such as cyclohexanone oxime, octahydrophenazine, and N-methylcaprolactam, and had a 5-cyano-1-pentene content of 0.01 wt%, a cyclohexenone content of 0.015 wt%, a cyclohexanone oxime content of 0.04 wt%, an octahydrophenazine content of 0.02 wt%, a total tetrahydroazepine-2-one and/or tetrahydro-azepin-2-one isomer content of 0.022 wt%, a decahydrophenazine content of 0.013 wt%, and other undecided impurities.
After 2 days of continuous stable operation, the amount of fouling on the inner walls of the crystallizer and on the stirring rods was calculated. See table 1. Collecting part of crystal slurry, centrifuging and washing the obtained caprolactam crystal, and analyzing the quality of the crystal product. The product was analyzed by sieving to obtain crystals with a particle size of less than 30 mesh at 14.5% by weight. The purity of the crystal product was 99.95% by chromatographic analysis of the crystal product.
The same procedure as in example 1 was followed to hydrogenate and flash treat the obtained crystalline product and to analyze the obtained caprolactam product to obtain: the purity of the caprolactam product is that the extinction value is 0.031 (better than the index of high-grade caprolactam in the national standard GB/T13254-2017); the chroma value is 2 (superior to the index of superior product caprolactam in national standard GB/T13254-2017); the content of the volatile alkali is 0.012mmol/kg (which is superior to the caprolactam index of a high-grade product in the national standard GB/T13254-2017); the PMs value is more than 30000s (better than the caprolactam index of high-class products in national standard GB/T13254-2017); the purity of caprolactam product was 99.995%.
TABLE 1
It can be seen from the results in table 1 that the examples 2, 3 and 7 according to the invention have significantly better results. The refining method has the characteristics of high purity, high efficiency, high yield, long-term operation stability and the like of crystal products, is continuous and efficient, can obtain high-quality caprolactam products by combining the caprolactam production method of the refining method, and has the advantages of high efficiency, high yield, long-term operation stability and the like.
The preferred embodiments of the present invention have been described above in detail, but the present invention is not limited thereto. Within the scope of the technical idea of the invention, many simple modifications can be made to the technical solution of the invention, including various technical features being combined in any other suitable way, and these simple modifications and combinations should also be regarded as the disclosure of the invention, and all fall within the scope of the invention.
Claims (14)
1. A process for refining caprolactam by crystallization, which comprises continuously feeding a solution of a crude caprolactam product and carrying out crystallization under adiabatic flash conditions, wherein the concentration of the feed and the crystallization conditions are controlled such that crystallization is carried out in a metastable zone of caprolactam.
2. The crystallization refining method according to claim 1, wherein the solvent in the caprolactam crude product solution satisfies the following condition:
at 5kPa absolute pressure and at a temperature corresponding to the boiling point of the solvent, the mass percent solubility of caprolactam in the solvent is less than 20 wt%;
preferably, the solvent has a boiling point of 40 to 140 ℃ at normal atmospheric pressure; preferably one or more selected from the group consisting of ethers, alkanes, cycloalkanes, halogenated hydrocarbons and aromatic hydrocarbons having a boiling point of 40 to 140 ℃ at normal atmospheric pressure.
3. The crystal refining method of claim 2, wherein the ether is a saturated aliphatic monohydric ether, preferably at least one of isopropyl ether, ethyl butyl ether, methyl ethyl ether, n-propyl ether, n-butyl ether, ethyl butyl ether, methyl tert-butyl ether, and ethyl tert-butyl ether;
the alkane contains 6 to 12 carbon atoms, and is preferably at least one selected from the group consisting of n-heptane, n-hexane, isopentane, n-octane, n-nonane, methylhexane, isohexane, neohexane, isoheptane, isooctane, and isononane;
the cycloalkane contains 5 to 8 carbon atoms, and is preferably at least one selected from the group consisting of cyclohexane, cyclopentane, methylcyclopentane, and methylcyclohexane;
the halogenated hydrocarbon is selected from at least one of 1-chloropropane, 2-chloropropane, 1-n-butyl chloride, 2-chlorobutane, 1-chlorodimethylpropane, 2-chlorodimethylpropane, 1-n-bromopropane, 2-bromopropane, 1-bromobutane and 2-bromobutane;
the aromatic hydrocarbon is selected from at least one of benzene, toluene, xylene and trimethylbenzene;
preferably, the solvent in the crude caprolactam solution is selected from at least one of isopropyl ether, n-heptane and cyclohexane.
4. A crystal refining process according to any one of claims 1-3, wherein the caprolactam concentration in the crude caprolactam solution at the time of feeding is 15-60 wt.%, preferably 20-50 wt.%;
and/or, the crystallization conditions include: the crystallization temperature is 10-60 deg.C, the crystallization pressure is 3-100kPa, preferably 3-60kPa, and the total residence time of the crystals is 0.5-10h.
5. The crystal refining method according to any one of claims 1 to 4, wherein the crystallization is single-stage crystallization,
preferably, in the case of a single-stage crystallization, the feed temperature is from 50 to 60 ℃, the crystallization temperature is from 20 to 40 ℃, the crystallization pressure is from 10 to 35kPa, and the total residence time of the crystallization is from 1 to 4h.
6. A crystallization refining method according to any one of claims 1 to 4, wherein the crystallization is a multistage continuous crystallization, preferably not more than 5 stages, and more preferably 3 stages.
7. The crystallization refining method according to claim 6, wherein, in the case of multistage continuous crystallization, the first-stage crystallization feed temperature is 50 to 75 ℃, the crystallization temperature is 40 to 55 ℃, and the crystallization pressure is 30 to 60kPa, the difference between the feed temperature of the adjacent two-stage crystallization and the crystallization temperature in the corresponding crystallization stage is not more than 30 ℃, and the difference between the crystallization pressures of the adjacent two-stage crystallization is not more than 35kPa; the total residence time of the crystals is 1-10h; preferably, the residence time of the materials in each stage of crystallization is 0.5-3h;
preferably, the difference between the temperature of the upper stage crystallization and the temperature of the lower stage crystallization is 1 to 20 ℃ and the difference between the pressure of the upper stage crystallization and the pressure of the lower stage crystallization is 1 to 25kPa.
8. A crystallization refining process according to claim 6 or 7, wherein the ratio of the yield of caprolactam crystals in the crystal slurry obtained in each crystallization stage to the average yield in each crystallization stage is from 0.8 to 1.2; preferably 0.85 to 1.15.
9. A crystallization refining process according to any one of claims 6 to 8, wherein the caprolactam crystal content in the crystal slurry comprising the second and subsequent crystallization stages is not higher than 35% by weight;
preferably, the content of caprolactam crystals in the magma is controlled by: adjusting the frequency of the withdrawn magma and/or injecting a solvent or caprolactam solution into the crystallization mixture, preferably the caprolactam solution from the mother liquor separated from the withdrawn magma.
10. A crystallization refining process according to any one of claims 1-9, wherein the caprolactam crude product is obtained by: distilling a mixture obtained by carrying out gas phase Beckmann rearrangement reaction on cyclohexanone oxime to remove impurities with a boiling point lower than that of caprolactam, thereby obtaining a caprolactam crude product;
preferably, the purity of the caprolactam crude product is 97 to 99.9 wt.%.
11. A process for producing caprolactam, which comprises refining a crude caprolactam product by the crystallization refining process according to any one of claims 1 to 10 to obtain crystalline crystals; and (4) carrying out hydrogenation reaction on the crystallized crystals.
12. The production method according to claim 11, wherein the hydrogenation reaction is carried out in the presence of a hydrogenation catalyst selected from at least one of nickel-based catalysts, palladium-based catalysts, and platinum-based catalysts.
13. The production method according to claim 12, wherein the hydrogenation catalyst is a nickel-based catalyst and/or a palladium-based catalyst;
preferably, the nickel-based catalyst is an amorphous nickel catalyst;
preferably, the palladium catalyst comprises a carrier and palladium and rare earth oxide loaded on the carrier, preferably, the carrier is activated carbon, and the rare earth oxide is lanthanum oxide and/or cerium oxide;
more preferably, the palladium content is 0.1 to 5 wt% and the rare earth oxide content is 0.2 to 10 wt% based on the total amount of the palladium-based catalyst.
14. The production method according to any one of claims 11 to 13, wherein the conditions of the hydrogenation reaction include: the temperature is 50-150 ℃, the pressure is 0.3-2MPa, and the flow of hydrogen is 0.1-2L/min.
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| CN108358823A (en) * | 2018-04-20 | 2018-08-03 | 陕西宏元化工技术有限公司 | A method of with organic solvent crystal refining caprolactam |
| CN109721537A (en) * | 2017-10-30 | 2019-05-07 | 中国石油化工股份有限公司 | A kind of refining methd of caprolactam |
| CN115108956A (en) * | 2021-03-22 | 2022-09-27 | 中国石油化工股份有限公司 | Purification method for preparing caprolactam crude product by gas phase Beckmann rearrangement method |
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| CN109721537A (en) * | 2017-10-30 | 2019-05-07 | 中国石油化工股份有限公司 | A kind of refining methd of caprolactam |
| CN108358823A (en) * | 2018-04-20 | 2018-08-03 | 陕西宏元化工技术有限公司 | A method of with organic solvent crystal refining caprolactam |
| CN115108956A (en) * | 2021-03-22 | 2022-09-27 | 中国石油化工股份有限公司 | Purification method for preparing caprolactam crude product by gas phase Beckmann rearrangement method |
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