Purification method of caprolactam

Abstract

The present invention relates to the field of caprolactam production, and discloses a method for purifying caprolactam. The method includes: countercurrently contacting a slurry containing caprolactam crystals and mother liquor with a washing solvent in a washer, wherein the slurry is fed into the washer from the upper part of the washer, and the washing solvent is fed into the washer from the lower part of the washer; and a dense phase bed of caprolactam crystals and a dilute phase bed of caprolactam crystals are sequentially arranged from top to bottom in the washer. By using the method of the present invention, a high-purity caprolactam product can be obtained with a relatively small amount of washing solvent.

Description

Technical Field

[0001] The present invention relates to the field of caprolactam production, and particularly to a method for purifying caprolactam. Background Art

[0002] Caprolactam is one of the important raw materials for synthetic fibers and synthetic resins, and is mainly used for manufacturing polyamide fibers (nylon 6), resins, films, etc. Known methods for producing caprolactam include the liquid-phase Beckmann rearrangement method of cyclohexanone oxime using fuming sulfuric acid as a catalyst, the gas-phase Beckmann rearrangement method of cyclohexanone oxime using solid zeolite as a catalyst, and methods such as depolymerization of waste polymers. The gas-phase Beckmann rearrangement reaction of cyclohexanone oxime on a solid acid catalyst is a new process for realizing sulfur-free ammonium caprolactam production, which has no problems such as equipment corrosion and environmental pollution, and the separation and purification of the product will also be greatly simplified. Therefore, the new process of the sulfur-free ammonium gas-phase Beckmann rearrangement reaction has received great attention from the industry.

[0003] However, the caprolactam obtained by these methods contains various impurities. As is well known, caprolactam is used as a raw material for preparing polyamide, and it is required that the caprolactam product used for preparing polyamide and further manufacturing synthetic fibers and synthetic resins has very high quality. Impurities at the μg / g level will affect the subsequent polymerization reaction of caprolactam and are not easy to form filaments. Therefore, various separation and purification methods are used to obtain crude caprolactam, and then various refining methods are used to finally obtain high-purity caprolactam, so that high-purity caprolactam can be used to manufacture products such as synthetic fibers, synthetic resins, and films. Known purification methods for the gas-phase rearrangement product caprolactam include distillation, crystallization, hydrogenation, etc. US2813858 discloses a method for refining caprolactam, which is to add a certain amount of water or an organic hydrocarbon solvent with a melting point lower than that of caprolactam to crude caprolactam, and purify caprolactam through repeated processes such as crystallization, centrifugal separation, and washing.

[0004] US3966712 discloses a method for purifying crude caprolactam prepared by a gas-phase rearrangement method, which is to add polar solvents such as tetrahydrofuran and isopropanol to crude caprolactam, and purify caprolactam by cooling crystallization, filtration and washing, and alkali addition distillation.

[0005] CN1332158A discloses another method for purifying crude caprolactam, which is to pour a low-temperature aliphatic hydrocarbon solvent and crude caprolactam into a container together, mix the two and crystallize, and then purify the crude caprolactam through solid-liquid separation, crystal washing and other processes.

[0006] CN101070298A discloses another method for purifying crude caprolactam, which is to crystallize crude caprolactam in an ether solution, and then purify the crude caprolactam through processes such as filtration and washing, and hydrogenation.

[0007] Another purification method of crude caprolactam is disclosed in CN101070299A. In this method, crude caprolactam is crystallized in a halogenated hydrocarbon solution, and then purified through processes such as filtration and washing, and hydrogenation.

[0008] CN104072419B discloses a method for purifying caprolactam. In this method, crude caprolactam is purified through processes such as crystallization, water extraction, hydrogenation, and evaporation and dehydration.

[0009] Although the above methods have good effects on purifying caprolactam, they all face the problems of solid-liquid separation of crystals and mother liquor and crystal washing and filtration in actual industrial applications. Using existing solid-liquid separation methods such as centrifugal filtration, pressure filtration, and vacuum filtration not only involves cumbersome processes, large investments, and low washing efficiency, but also the caprolactam in the mother liquor is likely to precipitate crystal scars on the wall of the filtration equipment during operation, resulting in difficult operation. Especially for caprolactam, due to its inherent characteristics, the crystals formed are in a flaky structure, with poor strength and easy to break, making solid-liquid separation and washing and filtration more difficult, thus limiting the wide application of the crystallization method. CN104072419B uses a non-solid-liquid separation method to purify caprolactam, but it requires a relatively high proportion of solvent as a cost to obtain caprolactam of better quality, and the operating cost is relatively high. Summary of the Invention

[0010] The object of the present invention is to overcome the above problems existing in the prior art, and provide a method for purifying caprolactam, which can effectively solve the problem of separation of crystals and solvents and obtain high-quality caprolactam.

[0011] CN104072419B discloses a method for purifying caprolactam. This method includes crystallization, water extraction, hydrogenation, and evaporation and dehydration processes, without solid-liquid separation, and can produce high-quality caprolactam products. However, the washing solvent and the crystallization mother liquor are prone to backmixing in the washer, and the washing solvent is prone to directly short-circuit discharge from the bottom of the washer. A small amount of washing solvent cannot fully wash the surface of the caprolactam crystals, and more washing solvent is required to obtain high-purity caprolactam products. Therefore, the inventors of the present invention continue to study the method for purifying caprolactam, hoping to obtain higher-quality caprolactam with less washing solvent.

[0012] To achieve the above object, the present invention provides a method for purifying caprolactam, which includes:

[0013] Countercurrently contacting a slurry containing caprolactam crystals and crystallization mother liquor with a washing solvent in a washer, wherein the slurry is fed into the washer from the upper part of the washer, and the washing solvent is fed into the washer from the lower part of the washer;

[0014] Wherein, a caprolactam crystal dense phase bed layer and a caprolactam crystal dilute phase bed layer are sequentially arranged in the scrubber from top to bottom.

[0015] In the present invention, the slurry containing caprolactam crystals and crystallization mother liquor is directly contacted with the washing solvent in a countercurrent manner, and a caprolactam crystal dense phase bed layer and a caprolactam crystal dilute phase bed layer are formed in the scrubber, effectively solving the problem of crystal and solvent separation and improving the purity of the caprolactam product. The inventors of the present invention further found through research that the purity of caprolactam can be further improved by controlling the height ratio of the caprolactam crystal dense phase bed layer and the caprolactam crystal dilute phase bed layer. According to a particularly preferred embodiment of the present invention, the feeding speed of the crystal product is controlled to form a caprolactam crystal dense phase bed layer with a certain bed particle density in the upper region of the scrubber, and a caprolactam crystal dilute phase bed layer with a certain bed particle density in the lower region, and by controlling the dosage and inflow rate of the washing solvent, the high quality of the caprolactam product can be significantly improved.

[0016] The purification method of the present invention has the following beneficial effects:

[0017] (1) By using the method of the present invention, it is no longer necessary to perform solid-liquid separation on the precipitated caprolactam crystals, simplifying the technological process of caprolactam purification, and at the same time avoiding the precipitation of crystal scars on the wall of the filtration equipment. The method of the present invention effectively solves the problem of crystal and solvent separation and also improves the purity of caprolactam.

[0018] (2) The extinction value of the caprolactam product prepared by using the method of the present invention is less than 0.05, the chromaticity value is less than 0.004, the volatile base content is less than 0.3 mmol / kg, and the PMs value is higher than 28000 s; the purity of the caprolactam product is higher than 99.99%, and the yield is higher than 99%. Description of the Drawings

[0019] Figure 1 is a schematic diagram of a scrubber with a valve provided in a specific embodiment of the method provided by the present invention;

[0020] Figure 2 is a schematic diagram of a scrubber with a partition provided in a specific embodiment of the method provided by the present invention.

[0021] Description of the Reference Numerals

[0022] a--------Slurry inlet b--------Washing solvent inlet

[0023] c--------Mother liquor outlet d--------Caprolactam (water) solution outlet

[0024] e--------Circulating water inlet f--------Heating tube

[0025] g ------ valve h ------ partition Detailed implementation mode

[0026] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0027] In the present invention, the pressure refers to gauge pressure.

[0028] In the present invention, "the dilute-phase bed layer of caprolactam crystals" and "the dilute-phase bed layer" can be interchanged, "the dense-phase bed layer of caprolactam crystals" and "the dense-phase bed layer" can be interchanged, "the crystal slurry containing caprolactam crystals and crystallization mother liquor" and "the crystal slurry" can be interchanged, and "caprolactam crystals" and "crystals" can be interchanged.

[0029] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by terms such as "upper", "lower", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation to the present invention. In addition, "inner, outer" refers to the inner and outer relative to the contour of each component itself.

[0030] In the present invention, the "purity of caprolactam" refers to the percentage content of caprolactam in the sum of the weights of caprolactam and impurities, where the impurities do not include the washing solvent and the crystallization solvent in the mother liquor.

[0031] In the present invention, the "particle density" refers to the bulk density of crystal particles. Specifically, it refers to the weight of caprolactam crystals per unit volume.

[0032] The present invention provides a method for purifying caprolactam, and the method includes:

[0033] Countercurrently contacting a crystal slurry containing caprolactam crystals and crystallization mother liquor with a washing solvent in a washer, wherein the crystal slurry is fed into the washer from the upper part of the washer, and the washing solvent is fed into the washer from the lower part of the washer;

[0034] Wherein, a dense-phase bed layer of caprolactam crystals and a dilute-phase bed layer of caprolactam crystals are sequentially arranged in the washer from top to bottom.

[0035] In the present invention, "countercurrent contact" means that the slurry containing caprolactam crystals and crystallization mother liquor enters the scrubber from the slurry inlet. The precipitated crystals move downward under the action of gravity to form a dilute (dense) phase bed of caprolactam crystals in the scrubber. At the same time, the washing solvent enters the scrubber from the washing solvent inlet. A part of the washing solvent flows upward through the dilute (dense) phase bed of caprolactam crystals to displace the crystallization mother liquor. This part of the washing solvent and the crystallization mother liquor are discharged together from the mother liquor outlet, and the remaining washing solvent is carried out together with the crystals. Combined with Figure 1 To further illustrate the "countercurrent contact" of the present invention, the scrubber includes a slurry inlet a, a washing solvent inlet b, a mother liquor outlet c, a caprolactam (water) solution outlet d, a circulating water inlet e, a heating tube f, and a valve g. The slurry enters the scrubber from the slurry inlet a, and the washing solvent enters the scrubber from the washing solvent inlet b. The slurry and the washing solvent are washed countercurrently in the scrubber. The mother liquor in the slurry flows out from the mother liquor outlet c. The precipitated crystals move downward under the action of gravity to form a dense phase bed of caprolactam crystals above the valve g and a dilute phase bed of caprolactam crystals below the valve g. The circulating water introduced through the circulating water inlet e is dissolved under the action of the heating tube f to obtain a caprolactam solution, and the caprolactam solution is discharged from the caprolactam (water) solution outlet d.

[0036] In the present invention, in order to describe the positions where the slurry and the washing solvent enter the scrubber, the descriptions of "upper part" and "lower part" are introduced. The "upper part" and "lower part" are relative and do not refer to specific position points or regions. For example, the top of the scrubber can be called the "upper part" of the bottom of the scrubber. On the contrary, the bottom of the scrubber can be called the "lower part" of the top of the scrubber. For another example, the position at the mid-height of the scrubber can be called the "lower part" of the top of the scrubber and can also be called the "upper part" of the bottom of the scrubber. At the same time, the "upper part" and "lower part" can be adjacent regions. For example, the region between the top of the scrubber and the mid-height of the scrubber can be called the "upper part" of the region between the bottom of the scrubber and the mid-height of the scrubber. On the contrary, the region between the bottom of the scrubber and the mid-height of the scrubber can be called the "lower part" of the region between the top of the scrubber and the mid-height of the scrubber. The "upper part" and "lower part" can also be non-adjacent regions. For example, the region between the top of the scrubber and the one-third height of the scrubber can be called the "upper part" of the region between the bottom of the scrubber and the mid-height of the scrubber. On the contrary, the region between the bottom of the scrubber and the mid-height of the scrubber can be called the "lower part" of the region between the top of the scrubber and the one-third height of the scrubber. In the present invention, it is generally said that the dense phase bed of caprolactam crystals is located above the dilute phase bed of caprolactam crystals. Correspondingly, the dilute phase bed of caprolactam crystals is located below the dense phase bed of caprolactam crystals.

[0037] According to the present invention, the content of caprolactam in the crystal slurry can be selected within a relatively wide range. However, in order to obtain high-purity caprolactam crystals, preferably, in the crystal slurry, the mass content of caprolactam is 10-90%, more preferably 20-70%. The present invention does not particularly limit the source of the crystal slurry, which can be obtained by crystallizing a crude caprolactam product that does not meet industrial requirements in the presence of a crystallization solvent and then under crystallization conditions, or by adding a small amount of caprolactam crystals to the crystallization mother liquor. Preferably, the crystal slurry of the present invention is obtained by evaporation crystallization or cooling crystallization, that is, dissolving a crude caprolactam product with a lower purity in a crystallization solvent and then performing evaporation crystallization or cooling crystallization. The purity of the crude caprolactam product is 98-99.8 w%.

[0038] According to the present invention, preferably, the bed particle density of the dense phase bed of caprolactam crystals is 400-1000 kg / m 3 , preferably 500-900 kg / m 3 .

[0039] According to the present invention, preferably, the bed particle density of the dilute phase bed of caprolactam crystals is 100-500 kg / m 3 , preferably 100-400 kg / m 3 .

[0040] According to the present invention, the feeding speed of the crystals can be selected within a relatively wide range. Preferably, as long as the bed particle density of the dense phase bed and / or the dilute phase bed can meet the above range.

[0041] In the present invention, there is no particular limitation on the formation method of the dilute phase bed of caprolactam crystals and the dense phase bed of caprolactam crystals in the scrubber. As long as the above characteristics can be satisfied, the object of the present invention can be achieved. For example, before startup, caprolactam crystals can be loaded into the scrubber to form a dilute phase bed of caprolactam crystals or a dense phase bed of caprolactam crystals; it can also be formed during the operation of the method. Specifically, the crystal slurry can be directly added from the upper part of the scrubber, and the washing solvent can be directly added from the lower part of the scrubber, so that the crystal slurry and the washing solvent are in countercurrent contact during the washing process to precipitate crystals, and then by controlling the feeding speed of the crystals in the scrubber, a dilute phase bed and a dense phase bed can be formed in the scrubber.

[0042] According to the present invention, preferably, a partition board is provided inside the scrubber (such as Figure 2a partition or a valve as shown in the figure, which is used to reduce the flow cross-sectional area of caprolactam to form the dense-phase bed layer and the dilute-phase bed layer of caprolactam crystals. The present invention has no particular limitation on the setting manner of the partition or the valve, as long as the above object can be achieved. Preferably, the partition or the valve is located on the demarcation line between the dense-phase bed layer and the dilute-phase bed layer of caprolactam crystals in the scrubber.

[0043] According to the present invention, preferably, based on the total height of the dense-phase bed layer and the dilute-phase bed layer of caprolactam crystals, the height of the dense-phase bed layer of caprolactam crystals is 50-80%, and the height of the dilute-phase bed layer of caprolactam crystals is 20-50%; more preferably, based on the total height of the dense-phase bed layer and the dilute-phase bed layer of caprolactam crystals, the height of the dense-phase bed layer of caprolactam crystals is 50-70%, and the height of the dilute-phase bed layer of caprolactam crystals is 30-50%.

[0044] According to the present invention, preferably, the purity of the caprolactam crystals in the dense-phase bed layer and the dilute-phase bed layer of caprolactam crystals is independently 98.0-99.9%.

[0045] According to the present invention, preferably, the feeding rate of the washing solvent is 0.001-0.2 m / s, and more preferably 0.005-0.15 m / s; adopting this preferred embodiment is more conducive to cleaning the crystal surface, and the upward-flowing washing solvent entrains as few crystals as possible.

[0046] According to the present invention, the amount of the washing solvent can be selected within a relatively wide range. However, in order to reduce the discharge amount of waste liquid and lower the production cost on the premise of ensuring the washing effect, preferably, the amount of the washing solvent is 0.1-10 times, preferably 0.2-5 times, and more preferably 0.2-2 times the mass of the caprolactam crystals in the crystal slurry.

[0047] According to the present invention, the residence time of the caprolactam crystals inside the scrubber can be selected within a relatively wide range. However, in the preferred case, considering the purity, yield of the crystals and the amount of the washing solvent comprehensively, preferably, the residence time of the caprolactam crystals inside the scrubber is 400-600 s.

[0048] According to the present invention, the washing solvent can be a solvent commonly used for caprolactam crystallization. However, in order to further improve the purity of caprolactam crystals, the washing efficiency, and the yield of caprolactam, preferably, the washing solvent is selected from at least one of halogenated hydrocarbons, ethers, and alkanes having 6 to 12 carbon atoms; more preferably, the halogenated hydrocarbon is at least one of 1-chloropropane, 2-chloropropane, n-butyl chloride, 2-chlorobutane, isobutyl chloride, tert-butyl chloride, n-propyl bromide, isopropyl bromide, 1-bromobutane, and 2-bromobutane; the ether is at least one of methyl ethyl ether, diethyl ether, n-propyl ether, isopropyl ether, n-butyl ether, ethyl butyl ether, ethylene glycol dimethyl ether, vinyl ether, methyl tert-butyl ether, and ethyl tert-butyl ether; the alkanes having 6 to 12 carbon atoms have a boiling point of 60 to 180 °C (for example, n-heptane, n-hexane, isopentane, n-octane, n-nonane, methylhexane, isohexane, neohexane, isoheptane, isooctane, isononane), preferably 60 to 130 °C (for example, n-pentane, n-hexane, isopentane).

[0049] According to the present invention, preferably, the method further includes agitating the dense phase bed of caprolactam crystals; adopting this preferred embodiment is more conducive to the movement of the crystal bed on the one hand and can more effectively wash the crystal surface on the other hand.

[0050] According to the present invention, preferably, a motor is used to drive a paddle or a baffle to agitate the dense phase bed of caprolactam crystals.

[0051] According to the present invention, preferably, an overflow port is provided at the upper part of the washer, and the washing mother liquor flows out of the washer from the overflow port. Among them, a part of the upward-flowing washing solvent also flows out from the overflow port.

[0052] According to the present invention, preferably, the method further includes subjecting the caprolactam crystals obtained by washing and a part of the solvent to aqueous phase dissolution and stratification to obtain an aqueous caprolactam solution.

[0053] According to the present invention, in order to further improve the purity and yield of the caprolactam product, preferably, the amount of the aqueous phase is 0.05 to 5 times, preferably 0.1 to 1 times, the mass of the caprolactam crystals obtained by washing.

[0054] According to the present invention, preferably, the aqueous phase dissolution and stratification is dissolution at room temperature or heating dissolution, the temperature of the room temperature dissolution is 10 to 30 °C, and the temperature of the heating dissolution is 30 to 110 °C.

[0055] According to the present invention, preferably, the method further includes subjecting the aqueous caprolactam solution to aqueous phase hydrogenation after optionally concentrating and / or diluting. Whether the aqueous caprolactam solution is concentrated and / or diluted and the specific degree of concentration and / or dilution depend on the subsequent aqueous phase hydrogenation conditions.

[0056] According to the present invention, preferably, the conditions for the aqueous-phase hydrogenation include: the mass ratio of caprolactam to water is 0.05 - 4:1, the temperature is 50 - 100 °C, the pressure is 0.2 - 2.0 MPa, and the mass ratio of the catalyst to caprolactam is 0.001 - 0.05:1.

[0057] According to the present invention, preferably, the catalyst used for the aqueous-phase hydrogenation is a nickel-containing catalyst and / or a palladium-containing catalyst. The catalyst used for the aqueous-phase hydrogenation can be directly purchased or prepared by a method commonly used in the art. For the preparation of the nickel-containing catalyst, reference can be made to CN1272490A and CN1272491A, and for the preparation of the palladium-containing catalyst, reference can be made to CN102430406A.

[0058] According to the present invention, preferably, the method may further include heating and dissolving the caprolactam crystals obtained by washing and a part of the solvent to obtain a caprolactam solution, and the heating and dissolving temperature is 50 - 110 °C.

[0059] According to the present invention, preferably, the method further includes: subjecting the caprolactam solution obtained by heating and dissolving to melt hydrogenation, the pressure of the melt hydrogenation is 0.2 - 2.0 MPa, the temperature is 70 - 120 °C, the amount of hydrogen used is 0.05 - 0.5% of the mass of caprolactam, and the mass ratio of the catalyst to caprolactam is 0.001 - 0.05:1.

[0060] According to the present invention, preferably, the catalyst used for the melt hydrogenation is a nickel-containing catalyst and / or a palladium-containing catalyst. The catalyst used for the melt hydrogenation can be directly purchased or prepared by a method commonly used in the art. For the preparation of the nickel-containing catalyst, reference can be made to CN1272490A and CN1272491A, and for the preparation of the palladium-containing catalyst, reference can be made to CN102430406A.

[0061] In the following examples, the following test methods are used to evaluate the quality of the prepared caprolactam product:

[0062] (1) Purity of caprolactam

[0063] Using a capillary column Innowax 60m and a gas chromatograph 7890GC to analyze the purity and impurity content of caprolactam, and the minimum detection limit of the chromatograph is 1 μg / g.

[0064] (2) Potassium permanganate absorption value of ε-caprolactam

[0065] Pour 3.000 grams of caprolactam into a 100 ml colorimetric tube, dilute it to the mark with distilled water, shake well, place it in a constant temperature water bath at 20 °C, add 1 ml of 0.01 N potassium permanganate solution to the colorimetric tube, shake immediately, and start the stopwatch at the same time. When the color of the sample solution in the colorimetric tube is the same as that of the standard colorimetric solution (take 3 grams of primary standard Co(NO 3 )·6H 2 O and 12 mg of primary standard K 2 Cr 2 O 7 dissolve in water, dilute to 1 liter, and shake well), stop the stopwatch, and record the time consumed (in seconds), which is the potassium permanganate absorption value.

[0066] (3) Volatile base (V.B)

[0067] In an alkaline medium, distill out the alkaline low molecular impurities in the sample, absorb them with a known amount of hydrochloric acid solution, and titrate the excess hydrochloric acid with a standard sodium hydroxide solution. The determination value of the volatile base is the number of moles of acid consumption per kilogram of the sample. The calculation formula is as follows:

[0068] V.B (mmol / kg) = [(V 0 -V) × C NaOH / M] × 1000

[0069] Where: V 0 is the volume of the standard NaOH solution consumed in the blank test, in ml;

[0070] V is the volume of the standard NaOH solution consumed by the sample, in ml;

[0071] C NaOH is the accurate concentration of the standard NaOH solution, in mol / L;

[0072] M is the sample mass, in g.

[0073] (4) Extinction value E (at a wavelength of 290 nm)

[0074] Weigh 50 grams of the sample in a 300 ml conical flask, add 50 ml of distilled water, shake well to completely dissolve the sample, and let it stand for 10 minutes. Using a spectrophotometer, at a wavelength of 290 nm, detect the extinction value of the 50% sample relative to distilled water.

[0075] The present invention will be described in detail below through examples.

[0076] Example 1

[0077] The preparation method of the slurry containing caprolactam crystals and crystallization mother liquor is to dissolve the crude caprolactam in a crystallization solvent and then carry out cooling crystallization. The mass content of caprolactam in the slurry is 30%.

[0078] (1) The purification of the slurry is carried out in a scrubber. 200 g of the slurry is slowly fed into the scrubber from the upper part of the scrubber by a pump, and 45 g of the washing solvent n-heptane is fed into the scrubber from the lower part of the scrubber. The feeding rate of the washing solvent is 0.01 m / s, so that the slurry and the washing solvent are in countercurrent contact in the scrubber to wash the caprolactam crystals. The feeding speed of the crystals generated during the washing process is controlled by a valve, a dense phase bed of caprolactam crystals is formed in the upper part of the scrubber, and a dilute phase bed of caprolactam crystals is formed in the lower part of the scrubber. A partition is arranged between the dense phase bed of caprolactam crystals and the dilute phase bed of caprolactam crystals. The height ratio of the dense phase bed to the dilute phase bed is 6:4, and the height of the dilute phase bed is 40% of the height of the scrubber. The particle density in the dense phase bed is 900 kg / m 3 , and the particle density in the dilute phase bed is 300 kg / m 3 , and at the same time, the residence time of the caprolactam crystals in the scrubber is controlled to be 600 s. The purity of the caprolactam crystals obtained by washing is 99.88%.

[0079] (2) At 30 °C, the washed lactam crystals are dissolved in the aqueous phase and separated into an aqueous caprolactam solution. The amount of the aqueous phase used is 0.6 times the mass of the caprolactam crystals obtained by washing.

[0080] (3) After the aqueous caprolactam solution is concentrated, it is contacted with a palladium-containing catalyst (the content of palladium in the catalyst is 3 wt%) to carry out aqueous phase hydrogenation to obtain a caprolactam product. The conditions for the aqueous phase hydrogenation include: the mass ratio of caprolactam to water used is 1.67:1, the temperature is 70 °C, the pressure is 0.8 MPa, and the mass ratio of the catalyst to caprolactam is 0.02:1.

[0081] Example 2

[0082] The preparation method of the slurry containing caprolactam crystals and crystallization mother liquor is to dissolve the crude caprolactam in a crystallization solvent and then carry out cooling crystallization. The mass content of caprolactam in the slurry is 35%.

[0083] (1) The purification of the crystal slurry is carried out in a scrubber. 200 g of the crystal slurry is slowly pumped into the scrubber from the upper part of the scrubber, and 50 g of the washing solvent isopropyl ether is fed into the scrubber from the lower part of the scrubber. The feeding rate of the washing solvent is 0.02 m / s, and the crystal slurry and the washing solvent are brought into countercurrent contact in the scrubber to wash the caprolactam crystals. The feeding rate of the crystals formed during the washing process is controlled by a valve, a dense phase bed of caprolactam crystals is formed in the upper part of the scrubber, and a dilute phase bed of caprolactam crystals is formed in the lower part of the scrubber. A partition is provided between the dense phase bed and the dilute phase bed of caprolactam crystals. The height ratio of the dense phase bed to the dilute phase bed is 5:5, and the height of the dilute phase bed is 50% of the height of the scrubber. The particle density in the dense phase bed is 850 kg / m 3 , and the particle density in the dilute phase bed is 400 kg / m 3 . At the same time, the residence time of the caprolactam crystals in the scrubber is controlled to be 480 s. The purity of the caprolactam crystals obtained by washing is 99.9%.

[0084] (2) At 30 °C, the caprolactam crystals obtained by washing are dissolved in the aqueous phase and separated into layers to obtain an aqueous caprolactam solution. The amount of the aqueous phase used is 0.2 times the mass of the caprolactam crystals obtained by washing.

[0085] (3) After the aqueous caprolactam solution is concentrated, it is contacted with a palladium-containing catalyst (the content of palladium in the catalyst is 3 wt%) for aqueous phase hydrogenation to obtain a caprolactam product. The conditions for the aqueous phase hydrogenation include: the mass ratio of caprolactam to water used is 4:1, the temperature is 80 °C, the pressure is 1 MPa, and the mass ratio of the catalyst to caprolactam is 0.02:1.

[0086] Example 3

[0087] The preparation method of the crystal slurry containing caprolactam crystals and crystallization mother liquor is to dissolve the crude caprolactam in a crystallization solvent and then carry out cooling crystallization. The mass content of caprolactam in the crystal slurry is 40%.

[0088] (1) The purification of the crystal slurry is carried out in a scrubber. 300 g of the crystal slurry is slowly pumped into the scrubber from the upper part of the scrubber, and 80 g of the washing solvent n-octane is fed into the scrubber from the lower part of the scrubber. The feeding rate of the washing solvent is 0.005 m / s, and the crystal slurry and the washing solvent are brought into countercurrent contact in the scrubber to wash the caprolactam crystals. The feeding rate of the crystals formed during the washing process is controlled by a valve, a dense phase bed of caprolactam crystals is formed in the upper part of the scrubber, and a dilute phase bed of caprolactam crystals is formed in the lower part of the scrubber. A partition is provided between the dense phase bed and the dilute phase bed of caprolactam crystals. The height ratio of the dense phase bed to the dilute phase bed is 7:3, and the height of the dilute phase bed is 30% of the height of the scrubber. The particle density in the dense phase bed is 900 kg / m 3, the particle density in the dilute-phase bed is 350 kg / m 3 , and at the same time, the residence time of the caprolactam crystals in the scrubber is controlled to be 480 s. The purity of the caprolactam crystals obtained by washing is 99.87%.

[0089] (2) At 30 °C, the caprolactam crystals obtained by washing are dissolved in the aqueous phase and separated into layers to obtain an aqueous caprolactam solution, and the amount of the aqueous phase used is 1 time the mass of the caprolactam crystals obtained by washing.

[0090] (3) After the aqueous caprolactam solution is concentrated, it is contacted with a palladium-containing catalyst (the content of palladium in the catalyst is 2 wt%) for aqueous-phase hydrogenation to obtain a caprolactam product. The conditions for aqueous-phase hydrogenation include: the mass ratio of caprolactam to water is 1:1, the temperature is 80 °C, the pressure is 1.5 MPa, and the mass ratio of the catalyst to caprolactam is 0.03:1.

[0091] Example 4

[0092] The method for preparing the slurry containing caprolactam crystals and crystallization mother liquor is to dissolve the crude caprolactam in a crystallization solvent and then carry out cooling crystallization. The mass content of caprolactam in the slurry is 50%.

[0093] (1) The purification of the slurry is carried out in a scrubber. 300 g of the slurry is slowly pumped into the scrubber from the upper part of the scrubber, and 50 g of the washing solvent n-heptane is fed into the scrubber from the lower part of the scrubber. The feeding rate of the washing solvent is 0.008 m / s, so that the slurry and the washing solvent are in countercurrent contact in the scrubber to wash the caprolactam crystals. The feeding rate of the crystals generated during the washing process is controlled by a valve, a dense-phase bed of caprolactam crystals is formed in the upper part of the scrubber, a dilute-phase bed of caprolactam crystals is formed in the lower part of the scrubber, a partition is provided between the dense-phase bed and the dilute-phase bed of caprolactam crystals, the height ratio of the dense-phase bed to the dilute-phase bed is 8:2, the height of the dilute-phase bed is 20% of the height of the scrubber, and the particle density in the dense-phase bed is 950 kg / m 3 , the particle density in the dilute-phase bed is 500 kg / m 3 , and at the same time, the residence time of the caprolactam crystals in the scrubber is controlled to be 600 s. The purity of the caprolactam crystals obtained by washing is 99.85%.

[0094] (2) At 30 °C, the caprolactam crystals obtained by washing are dissolved in the aqueous phase and separated into layers to obtain an aqueous caprolactam solution, and the amount of the aqueous phase used is 2 times the mass of the caprolactam crystals obtained by washing.

[0095] (3) After the aqueous solution of caprolactam is concentrated, it is then contacted with a palladium-containing catalyst (the palladium content in the catalyst is 2 wt%) for aqueous-phase hydrogenation to obtain a caprolactam product. The conditions for aqueous-phase hydrogenation include: the mass ratio of caprolactam to water is 1:2, the temperature is 60 °C, the pressure is 0.5 MPa, and the mass ratio of the catalyst to caprolactam is 0.005:1.

[0096] Test example

[0097] The caprolactam products prepared in the above examples and comparative examples were analyzed, and the results are shown in Table 1.

[0098] Table 1

[0099]

[0100] It can be seen from the results in Table 1 that the caprolactam obtained by using Examples 1-4 of the present invention can meet the requirements of national standard superior products, and has the advantages of less washing solvent consumption and simple operation.

[0101] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the technical concept scope of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for purifying caprolactam, the method comprises: countercurrently contacting a slurry containing caprolactam crystals and mother liquor with a washing solvent in a washer, wherein the slurry is fed into the washer from the upper part of the washer, and the washing solvent is fed into the washer from the lower part of the washer; wherein, a caprolactam crystal dense phase bed layer and a caprolactam crystal dilute phase bed layer are sequentially arranged in the washer from top to bottom; The bed particle density of the caprolactam crystal dense phase bed is 500 - 900 kg / m 3 ; And / or, the bed particle density of the dilute-phase bed of caprolactam crystals is 100 - 400 kg / m 3 .

2. The method according to claim 1, wherein, based on the total height of the caprolactam crystal dense phase bed layer and the caprolactam crystal dilute phase bed layer, the height of the caprolactam crystal dense phase bed layer is 50 - 80%, and the height of the caprolactam crystal dilute phase bed layer is 20 - 50%.

3. The method according to claim 1, wherein, based on the total height of the caprolactam crystal dense phase bed layer and the caprolactam crystal dilute phase bed layer, the height of the caprolactam crystal dense phase bed layer is 50 - 70%, and the height of the caprolactam crystal dilute phase bed layer is 30 - 50%.

4. The method according to any one of claims 1 - 3, wherein, the purity of the caprolactam crystals in the caprolactam crystal dense phase bed layer and the caprolactam crystal dilute phase bed layer is independently 98.0 - 99.9%.

5. The method according to any one of claims 1 - 3, wherein, in the slurry, the mass content of caprolactam is 10 - 90%.

6. The method according to any one of claims 1 - 3, wherein, in the slurry, the mass content of caprolactam is 20 - 70%.

7. The method according to any one of claims 1 - 3, wherein, the feeding rate of the washing solvent is 0.001 - 0.2 m / s.

8. The method according to claim 7, wherein, the feeding rate of the washing solvent is 0.005 - 0.15 m / s.

9. The method according to any one of claims 1 - 3, wherein, the dosage of the washing solvent is 0.1 - 10 times the mass of the caprolactam crystals in the slurry.

10. The method according to claim 9, wherein, the dosage of the washing solvent is 0.2 - 5 times the mass of the caprolactam crystals in the slurry.

11. The method according to claim 10, wherein, the dosage of the washing solvent is 0.2 - 2 times the mass of the caprolactam crystals in the slurry.

12. The method according to any one of claims 1 - 3, wherein, the washing solvent is selected from at least one of halogenated hydrocarbons, ethers, and alkanes with 6 - 12 carbon atoms.

13. The method according to claim 12, wherein, the halogenated hydrocarbon is at least one of 1 - chloropropane, 2 - chloropropane, n - chlorobutane, 2 - chlorobutane, isochlorobutane, tert - chlorobutane, n - bromopropane, isobromopropane, 1 - bromobutane, and 2 - bromobutane; the ether is at least one of methyl ethyl ether, diethyl ether, n - propyl ether, isopropyl ether, n - butyl ether, ethyl butyl ether, ethylene glycol dimethyl ether, vinyl ether, methyl tert - butyl ether, and ethyl tert - butyl ether; the alkane with 6 - 12 carbon atoms has a boiling point of 60 - 180 °C.

14. The method according to any one of claims 1 - 3, wherein, the method further includes agitating the dense phase bed of caprolactam crystals.

15. The method according to claim 14, wherein, a motor drives a paddle or a baffle to agitate the dense phase bed of caprolactam crystals.

16. The method according to any one of claims 1 - 3, wherein, an overflow port is provided at the upper part of the scrubber, and the scrubbing mother liquor flows out of the scrubber from the overflow port.

17. The method according to any one of claims 1 - 3, wherein, a baffle or a valve is arranged inside the scrubber, and the baffle or the valve is used to reduce the cross - sectional area of the caprolactam flow to form the dense phase bed of caprolactam crystals and the dilute phase bed of caprolactam crystals.

18. The method according to any one of claims 1 - 3, wherein, the method further includes dissolving and layering the caprolactam crystals obtained by washing and a part of the solvent in an aqueous phase to obtain an aqueous caprolactam solution.

19. The method according to claim 18, wherein, the amount of the aqueous phase is 0.05 - 5 times the mass of the caprolactam crystals obtained by washing.

20. The method according to claim 19, wherein, the amount of the aqueous phase is 0.1 - 1 times the mass of the caprolactam crystals obtained by washing.

21. The method according to claim 18, wherein, the aqueous phase dissolution and layering is normal - temperature dissolution or heating dissolution, the temperature of the normal - temperature dissolution is 10 - 30°C, and the temperature of the heating dissolution is 30 - 110°C.

22. The method according to claim 18, wherein, the method further includes subjecting the aqueous caprolactam solution to aqueous - phase hydrogenation after optionally concentrating and / or diluting.

23. The method according to claim 22, wherein, the conditions of the aqueous - phase hydrogenation include: the mass ratio of caprolactam to water is 0.05 - 4:1, the temperature is 50 - 100°C, the pressure is 0.2 - 2.0 MPa, and the mass ratio of the catalyst to caprolactam is 0.001 - 0.05:

1.

24. The method according to claim 22, wherein, the catalyst used for the aqueous - phase hydrogenation is a nickel - containing catalyst and / or a palladium - containing catalyst.

25. The method according to any one of claims 1 - 3, wherein, the method further includes heating and dissolving the caprolactam crystals obtained by washing and a part of the solvent to obtain a caprolactam solution, and the heating and dissolving temperature is 50 - 110°C.

26. The method according to claim 25, wherein, the method further includes: subjecting the caprolactam solution obtained by heating and dissolving to molten - state hydrogenation, the pressure of the molten - state hydrogenation is 0.2 - 2.0 MPa, the temperature is 70 - 120°C, the amount of hydrogen used is 0.05 - 0.5% of the mass of caprolactam, and the mass ratio of the catalyst to caprolactam is 0.001 - 0.05:

1.

27. The method according to claim 26, wherein, the catalyst used for the molten - state hydrogenation is a nickel - containing catalyst and / or a palladium - containing catalyst.

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