Method for purifying caprolactam

Abstract

The present invention relates to the field of caprolactam production, and discloses a method for purifying caprolactam. The method comprises the following steps: (1) feeding a slurry containing caprolactam crystals and crystallization mother liquor into a scrubber from the upper part of the scrubber, and feeding a washing solvent into the scrubber from the lower part of the scrubber, and the slurry and the washing solvent are in countercurrent contact in the scrubber to wash the caprolactam crystals; a dilute phase bed layer of caprolactam crystals is provided in the scrubber; (2) heating and dissolving the caprolactam crystals and part of the solvent obtained by the washing or performing aqueous phase dissolution and layering. 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 caprolactam production methods 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 production of caprolactam, 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 gas-phase Beckmann rearrangement reaction has attracted 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 polyamides, and it is required that the caprolactam product used for preparing polyamides and further manufacturing synthetic fibers and synthetic resins has a 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 of 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 a polar solvent such as tetrahydrofuran and isopropanol to crude caprolactam, and purify caprolactam by cooling crystallization, filtration and washing, and alkali 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, 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. Crude caprolactam is purified through processes such as crystallization, water extraction, hydrogenation, and evaporation for dehydration.

[0009] Although the above methods have good effects on purifying caprolactam, in actual industrial applications, they all face the problems of solid-liquid separation of crystals and mother liquor, as well as crystal washing and filtration. Using existing solid-liquid separation methods such as centrifugal filtration, pressure filtration, and vacuum filtration not only involves cumbersome processes, high investment, and low washing efficiency, but also the caprolactam in the mother liquor is prone to crystallize on the filter device wall during operation, resulting in difficult operation. Especially for caprolactam, due to its inherent characteristics, the crystals formed are in flake 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 needs to use a relatively high proportion of solvent at the cost of obtaining caprolactam with better quality, and the operating cost is relatively high. Summary of the Invention

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

[0011] CN104072419B discloses a method for purifying caprolactam. This method includes processes such as crystallization, water extraction, hydrogenation, and evaporation for dehydration, without solid-liquid separation, and can produce high-quality caprolactam products. However, in this method, the washing solvent and the crystallization mother liquor are prone to backmix in the washer, and the washing solvent is prone to directly short-circuit and discharge from the bottom of the washer. A small amount of washing solvent cannot fully wash the surface of 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 purification method of caprolactam, in order to obtain higher-quality caprolactam with less washing solvent.

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

[0013] (1) Feed the crystal slurry containing caprolactam crystals and crystallization mother liquor into the washer from the upper part of the washer, and feed the washing solvent into the washer from the lower part of the washer. The crystal slurry and the washing solvent are in countercurrent contact in the washer to wash the caprolactam crystals; a dilute-phase bed layer of caprolactam crystals is provided in the washer.

[0014] (2) Heat and dissolve the caprolactam crystals obtained from the washing and part of the solvent, or perform aqueous phase dissolution and layering.

[0015] In the present invention, the slurry containing caprolactam crystals and crystallization mother liquor is directly in countercurrent contact with the washing solvent, and a dilute phase bed of caprolactam crystals is formed in the washer, effectively solving the problem of separating crystals and solvents and improving the purity of caprolactam products. The inventors of the present invention further found through research that by controlling the feeding speed of the crystal product to form two beds with different crystal particle densities in the washer, the purity of caprolactam can be further improved. According to a particularly preferred embodiment of the present invention, control the feeding speed of the crystal product to form a dilute phase bed of caprolactam crystals in the upper region of the washer and a dense phase bed of caprolactam crystals in the lower region, control the residence time of the crystals in the washer, and at the same time make the flow rates of the washing solvent in the two beds different, which can significantly improve the performance of caprolactam products.

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

[0017] (1) Using the method of the present invention, there is no need to perform solid-liquid separation on the precipitated caprolactam crystals, which simplifies the process flow of caprolactam purification, and at the same time avoids the precipitation of crystal scars on the wall of the filtration equipment. The method of the present invention effectively solves the problem of separating crystals and solvents, and at the same time 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 30000 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 It is a schematic diagram of a washer with a dilute phase bed of caprolactam crystals provided by a specific embodiment of the method provided by the present invention;

[0020] Figure 2 It is a schematic diagram of a washer with a dilute phase bed of caprolactam crystals and a dense phase bed of caprolactam crystals provided by 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--------Deionized water inlet f--------Heating tube

[0025] g--------Baffle plate 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 the 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, and 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 and outer" refer 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 the weight of caprolactam crystals per unit volume.

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

[0033] (1) Feed the crystal slurry containing caprolactam crystals and crystallization mother liquor into the scrubber from the upper part of the scrubber, and feed the washing solvent into the scrubber from the lower part of the scrubber. The crystal slurry and the washing solvent are in countercurrent contact in the scrubber to wash the caprolactam crystals; a dilute-phase bed layer of caprolactam crystals is provided in the scrubber;

[0034] (2) Heat and dissolve or perform aqueous phase dissolution and layering on the caprolactam crystals and part of the solvent obtained by the washing.

[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, and 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, and 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. The remaining washing solvent is part of the solvent in step (2). 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 deionized water inlet e, and a heating tube f. 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, and the precipitated crystals move downward under the action of gravity to form a dilute phase bed of caprolactam crystals in the scrubber. The deionized water introduced from the circulating water inlet e is mixed with the crystals and dissolved under the action of the heating tube f to obtain a caprolactam aqueous solution, and the caprolactam aqueous solution is discharged from the caprolactam (water) solution outlet d.

[0036] In the present invention, in order to illustrate the positions where the slurry and the washing solvent enter the scrubber and the positional relationship between the dilute phase bed and the dense phase bed in 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.

[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 can be obtained 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 wt%.

[0038] 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 with 6-12 carbon atoms; more preferably, the halogenated hydrocarbon is at least one of 1-chloropropane, 2-chloropropane, n-chlorobutane, 2-chlorobutane, chloro-isobutane, tert-chlorobutane, n-bromopropane, iso-bromopropane, 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 boiling point of the alkane with 6-12 carbon atoms is 60-180 °C (for example, n-heptane, n-hexane, isopentane, n-octane, n-nonane, methylhexane, isohexane, neohexane, isoheptane, isooctane, isononane), preferably 60-130 °C (for example, n-pentane, n-hexane, isopentane).

[0039] 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 while ensuring the washing effect, preferably, the amount of the washing solvent is 0.1-10 times the mass of caprolactam crystals in the crystal slurry, more preferably 0.2-5 times, and further preferably 0.2-2 times.

[0040] According to the present invention, the residence time of the caprolactam crystals inside the washer has a relatively wide selection range. However, in the preferred case, considering the purity, yield of the crystals, and the amount of the washing solvent comprehensively, preferably, in step (1), the residence time of the caprolactam crystals inside the washer is 5-60 s, more preferably 10-40 s.

[0041] According to the present invention, preferably, the height of the dilute-phase bed of caprolactam crystals accounts for 60-80% of the height of the scrubber. If the proportion of the height of the dilute-phase bed of caprolactam crystals is too large, such as more than 85%, it will cause backmixing of the washing solvent in the dilute-phase bed, resulting in a poor washing effect; if the proportion of the height of the dilute-phase bed of caprolactam crystals is too small, such as less than 55%, it will cause a reduction in the washing height of the solvent in the dilute-phase bed, and the crystal purity will not meet the requirements; therefore, limiting the height of the dilute-phase bed of caprolactam crystals within the above range can reduce solvent backmixing, thereby meeting the requirements of crystal purity while reducing the amount of solvent used.

[0042] According to the present invention, preferably, the bed particle density of the dilute-phase bed of caprolactam crystals is 100-500 kg / m 3 , more preferably 100-400 kg / m 3 .

[0043] According to the present invention, preferably, the flow rate of the washing solvent in the dilute-phase bed of caprolactam crystals is 0.001-0.02 m / s, preferably 0.005-0.015 m / s. Adopting this preferred embodiment is more conducive to obtaining crystals with high purity, reducing the loss rate of small crystals, and improving the caprolactam yield during the washing process.

[0044] According to the present invention, preferably, the purity of the caprolactam crystals in the dilute-phase bed of caprolactam crystals is 98-99%.

[0045] The inventors found during the research process that forming a dilute-phase bed and a dense-phase bed from top to bottom in the scrubber can further improve the purity of caprolactam crystals. Preferably, a dense-phase bed of caprolactam crystals is provided below the dilute-phase bed of caprolactam crystals.

[0046] According to the present invention, preferably, based on the total height of the dilute-phase bed of caprolactam crystals and the dense-phase bed of caprolactam crystals, the height of the dilute-phase bed of caprolactam crystals is 30-50%, and the height of the dense-phase bed of caprolactam crystals is 50-70%; more preferably, based on the total height of the dilute-phase bed of caprolactam crystals and the dense-phase bed of caprolactam crystals, the height of the dilute-phase bed of caprolactam crystals is 40-50%, and the height of the dense-phase bed of caprolactam crystals is 50-60%.

[0047] According to the present invention, preferably, the bed particle density of the dense-phase bed of caprolactam crystals is 400-1000 kg / m 3 , more preferably 500-900 kg / m 3 .

[0048] According to the present invention, the discharging rate of the dense phase bed can be selected within a relatively wide range. Preferably, as long as the bed particle density of the dense phase bed can meet the above range.

[0049] In the present invention, there is no particular limitation on the formation method of the dilute phase bed 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 or a dense phase bed of caprolactam crystals; it can also be formed during the operation of the method. Specifically, crystal slurry can be directly added from the upper part of the scrubber, and 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 washing to precipitate crystals. Then, by controlling the discharging rate of crystals in the scrubber, a dilute phase bed and a dense phase bed can be formed in the scrubber. For example, controlling the discharging rate of crystals in the upper region of the scrubber to be faster will form a dilute phase bed in the upper region, and controlling the discharging rate of crystals in the lower region of the scrubber to be slower will form a dense phase bed in the lower region; inclined plates or partition plates can also be provided in the lower part of the scrubber, which is more conducive to forming a dense phase bed in the lower part of the scrubber.

[0050] According to the present invention, preferably, the flow rate of the washing solvent in the dense phase bed of caprolactam crystals is 0.01 - 0.3 m / s, preferably 0.02 - 0.2 m / s.

[0051] According to the present invention, preferably, a partition plate (as Figure 2 shown) is provided inside the scrubber. The partition plate is used to reduce the flow cross-sectional area of caprolactam crystals. Adopting this preferred embodiment is more conducive to forming a dense phase bed in the scrubber. The present invention has no particular limitation on the setting method of the partition plate, as long as the above object can be achieved. Preferably, the partition plate is located on the side wall of the scrubber.

[0052] According to the present invention, in step (2), the caprolactam crystals obtained by washing are formed into a liquid phase. It can be in the form of heating and dissolving, or water phase can be introduced for water phase dissolution and layering.

[0053] According to the present invention, preferably, the purity of the caprolactam crystals in the dense phase bed of caprolactam crystals is 99 - 99.9%.

[0054] According to the present invention, in order to further improve the purity and yield of the caprolactam product, preferably, the temperature of the heating and dissolving is 50 - 110 °C.

[0055] According to the present invention, there is no particular limitation on the heating and dissolving method, as long as the temperature requirement for heating and melting can be satisfied. Preferably, the heating and dissolving adopts sleeve heating or electric heating.

[0056] According to the present invention, preferably, the method further comprises: subjecting the caprolactam solution obtained by heating and dissolving to melt hydrogenation, wherein 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.

[0057] According to the present invention, preferably, the catalyst used in the melt hydrogenation is a nickel-containing catalyst and / or a palladium-containing catalyst. The catalyst used in the melt hydrogenation can be directly purchased or prepared according to the methods 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, in order to further improve the purity and yield of the caprolactam product, preferably, in the water phase dissolution and stratification process, the amount of the water phase is 0.05 - 5 times, preferably 0.1 - 1 times, the mass of the caprolactam crystals obtained by washing.

[0059] According to the present invention, preferably, the water phase dissolution and stratification is normal temperature water phase dissolution or heated water phase dissolution. The temperature of the normal temperature water phase dissolution is 10 - 30 °C, and the temperature of the heated water phase dissolution is 30 - 110 °C.

[0060] According to the present invention, preferably, the method further comprises: subjecting the caprolactam aqueous solution obtained by water phase dissolution and stratification to optional concentration and / or dilution, and then performing aqueous phase hydrogenation. Whether the caprolactam aqueous solution is subjected to concentration and / or dilution and the specific degree of concentration and / or dilution depend on the subsequent aqueous phase hydrogenation conditions.

[0061] More preferably, the conditions of the aqueous phase hydrogenation include: the mass ratio of caprolactam to water used is 0.3 - 9:1, 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.

[0062] According to the present invention, preferably, the catalyst used in the aqueous phase hydrogenation is a nickel-containing catalyst and / or a palladium-containing catalyst. The catalyst used in the aqueous phase hydrogenation can be directly purchased or prepared according to the methods 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.

[0063] In the following examples, the following test methods are used to evaluate the quality of the prepared caprolactam product:

[0064] (1) Purity of Caprolactam

[0065] Use a capillary column Innowax 60m and a gas chromatograph 7890GC to analyze the purity and impurity content of caprolactam. The minimum detection limit of the chromatograph is 1 μg / g.

[0066] (2) Potassium Permanganate Absorbance Value of ε-Caprolactam

[0067] 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 0.01N 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 (dissolve 3 grams of primary standard Co(NO3)·6H2O and 12 milligrams of primary standard K2Cr2O7 in water, dilute to 1 liter, and shake well), stop the stopwatch and record the time consumed (in seconds), which is the potassium permanganate absorbance value.

[0068] (3) Volatile Base (V.B)

[0069] In an alkaline medium, distill 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:

[0070] V.B (mmol / kg) = [(V0 - V) × C NaOH / M] × 1000

[0071] Where: V0 is the volume of the standard NaOH solution consumed in the blank test, in ml;

[0072] V is the volume of the standard NaOH solution consumed by the sample, in ml;

[0073] C NaOH is the accurate concentration of the standard NaOH solution, in mol / L;

[0074] M is the mass of the sample, in g.

[0075] (4) Extinction Value E (at a wavelength of 290 nm)

[0076] 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. Use a spectrophotometer to detect the extinction value of the 50% sample relative to distilled water at a wavelength of 290 nm.

[0077] The present invention will be described in detail below through examples.

[0078] Example 1

[0079] 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%.

[0080] (1) The purification of the slurry is carried out in a scrubber. A baffle is provided at the lower part of the scrubber. 200 g of the slurry is slowly and continuously 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, 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, and a dilute-phase bed layer of caprolactam crystals is formed at the upper part of the scrubber. The particle density in the dilute-phase bed layer is 300 kg / m 3 , and a dense-phase bed layer of caprolactam crystals is formed at the lower part of the scrubber. The particle density in the dense-phase bed layer is 800 kg / m 3 , the height ratio of the dilute-phase bed layer to the dense-phase bed layer is 1:1, and the height of the dilute-phase bed layer is 50% of the height of the scrubber; at the same time, the flow rate of the washing solvent in the dilute-phase bed layer is controlled to be 0.006 m / s, and the flow rate in the dense-phase bed layer is 0.1 m / s, so that the caprolactam crystals stay in the scrubber for 20 s. The purity of the caprolactam crystals obtained by washing is 99.9%.

[0081] (2) The caprolactam crystals are heated and dissolved by using a double-pipe heater to obtain a caprolactam solution. Circulating water with a temperature of 55 °C is passed through the double-pipe.

[0082] (3) Then, the caprolactam solution is subjected to catalytic molten hydrogenation to obtain a caprolactam product. The pressure of the molten hydrogenation is 1.0 MPa, the temperature is 90 °C, the hydrogen consumption is 0.1% of the mass of caprolactam, the catalyst is a palladium-containing catalyst (the palladium content in the catalyst is 3 wt%), and the mass ratio of the catalyst to caprolactam is 0.005:1.

[0083] Example 2

[0084] 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 flash crystallization. The mass content of caprolactam in the slurry is 35%.

[0085] (1) The purification of the slurry is carried out in a scrubber. A baffle is provided at the lower part of the scrubber. 200 g of the slurry is slowly and continuously fed into the scrubber from the upper part of the scrubber by a pump, and 50 g of the washing solvent isopropyl ether is fed into the scrubber from the lower part of the scrubber, 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, and a dilute-phase bed layer of caprolactam crystals is formed at the upper part of the scrubber. The particle density in the dilute-phase bed layer is 400 kg / m 3, a dense phase bed of caprolactam crystals is formed at the lower part of the scrubber, and the particle density in the dense phase bed is 900 kg / m 3 , the height ratio of the dilute phase bed to the dense phase bed is 4:6, and the height of the dilute phase bed is 40% of the height of the scrubber; meanwhile, the flow rate of the washing solvent in the dilute phase bed is controlled to be 0.008 m / s, and the flow rate in the dense phase bed is 0.02 m / s, so that the residence time of the caprolactam crystals inside the scrubber is 15 s. The purity of the caprolactam crystals obtained by washing is 99.93%.

[0086] (2) At normal temperature (about 25 °C), the washed lactam crystals are dissolved in the aqueous phase and stratified to obtain an aqueous caprolactam solution, and the amount of the aqueous phase used is 0.2 times the mass of the caprolactam crystals obtained by washing.

[0087] (3) After the aqueous caprolactam solution is concentrated, it is contacted with a palladium-containing catalyst (the palladium content 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.

[0088] Example 3

[0089] 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 flash crystallization. The mass content of caprolactam in the slurry is 40%.

[0090] (1) The purification of the slurry is carried out in a scrubber. A partition is provided at the lower part of the scrubber. 300 g of the slurry is slowly fed into the scrubber from the upper part of the scrubber by a pump, and 80 g of the washing solvent n-octane is fed into the scrubber from the lower part of the scrubber, 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, and a dilute phase bed of caprolactam crystals is formed at the upper part of the scrubber, and the particle density in the dilute phase bed is 100 kg / m 3 , a dense phase bed of caprolactam crystals is formed at the lower part of the scrubber, and the particle density in the dense phase bed is 500 kg / m 3 , the height ratio of the dilute phase bed to the dense phase bed is 1:1, and the height of the dilute phase bed is 50% of the height of the scrubber; meanwhile, the flow rate of the washing solvent in the dilute phase bed is controlled to be 0.005 m / s, and the flow rate in the dense phase bed is 0.02 m / s, so that the residence time of the caprolactam crystals inside the scrubber is 25 s. The purity of the caprolactam crystals obtained by washing is 99.9%.

[0091] (2) The caprolactam crystals are heated and dissolved by electric heating to obtain a caprolactam solution, and the heating temperature is 50 °C.

[0092] (3) Then, the caprolactam solution is subjected to catalytic molten hydrogenation to obtain a caprolactam product. The pressure of the molten hydrogenation is 0.2 MPa, the temperature is 120 °C, the hydrogen dosage is 0.5% of the mass of caprolactam, the catalyst is a nickel-containing catalyst (the nickel content in the catalyst is 60 wt%), and the mass ratio of the catalyst to caprolactam is 0.01:1.

[0093] Example 4

[0094] 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 perform flash crystallization. The mass content of caprolactam in the slurry is 35%.

[0095] (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 30 g of the washing solvent n-heptane is continuously fed into the scrubber from the lower part of the scrubber, 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, and a dilute phase bed of caprolactam crystals is formed at the upper part of the scrubber. The particle density in the dilute phase bed is controlled to be 300 kg / m 3 , the height of the dilute phase bed is 70% of the height of the scrubber. At the same time, the flow rate of the washing solvent in the dilute phase bed is controlled to be 0.008 m / s, so that the residence time of the caprolactam crystals in the scrubber is 30 s. The purity of the washed caprolactam crystals is 99.85%.

[0096] (2) The caprolactam crystals are heated and dissolved by using a double-pipe heater to obtain a caprolactam solution. Circulating water with a temperature of 60 °C is passed through the double-pipe.

[0097] (3) Then, the caprolactam solution is subjected to catalytic molten hydrogenation to obtain a caprolactam product. The pressure of the molten hydrogenation is 0.7 MPa, the temperature is 70 °C, the hydrogen dosage is 0.2% of the mass of caprolactam, the catalyst is a palladium-containing catalyst (the palladium content in the catalyst is 3 wt%), and the mass ratio of the catalyst to caprolactam is 0.02:1.

[0098] Test Example

[0099] The caprolactam products prepared in the above examples and comparative examples are analyzed, and the results are shown in Table 1.

[0100] Table 1

[0101]

[0102] 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 premium products, and has the advantages of less washing solvent consumption and simple operation.

[0103] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept 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 purification method of caprolactam, the method comprising the following steps: (1) Feeding a slurry containing caprolactam crystals and crystallization mother liquor into a scrubber from the upper part of the scrubber, and feeding a washing solvent into the scrubber from the lower part of the scrubber. The slurry and the washing solvent are in countercurrent contact in the scrubber to wash the caprolactam crystals; a dilute-phase bed of caprolactam crystals is provided in the scrubber; a dense-phase bed of caprolactam crystals is provided below the dilute-phase bed of caprolactam crystals; The bed particle density of the caprolactam crystal dense phase bed is 500 - 900 kg / m 3 ; The bed particle density of the dilute-phase bed of the caprolactam crystals is 100 - 400 kg / m 3 ; (2) Heating and dissolving or performing aqueous phase dissolution and stratification on the caprolactam crystals and a part of the solvent obtained by the washing.

2. The purification method according to claim 1, wherein, The height of the dilute-phase bed of caprolactam crystals accounts for 60-80% of the height of the scrubber.

3. The purification method according to claim 1, wherein, The purity of the caprolactam crystals in the dilute-phase bed of caprolactam crystals is 98-99%.

4. The purification method according to claim 1, wherein, Based on the total height of the dilute-phase bed of caprolactam crystals and the dense-phase bed of caprolactam crystals, the height of the dilute-phase bed of caprolactam crystals is 30-50%, and the height of the dense-phase bed of caprolactam crystals is 50-70%.

5. The purification method according to claim 4, wherein, Based on the total height of the dilute-phase bed of caprolactam crystals and the dense-phase bed of caprolactam crystals, the height of the dilute-phase bed of caprolactam crystals is 40-50%, and the height of the dense-phase bed of caprolactam crystals is 50-60%.

6. The purification method according to claim 1, wherein, The purity of the caprolactam crystals in the dense-phase bed of caprolactam crystals is 99-99.9%.

7. The purification method according to any one of claims 1-6, wherein, In the slurry, the mass content of caprolactam is 10-90%.

8. The purification method according to claim 7, wherein In the slurry, the mass content of caprolactam is 20-70%.

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

10. The purification method according to claim 9, wherein, The halogenated hydrocarbon is at least one of 1-chloropropane, 2-chloropropane, chloron-butane, 2-chlorobutane, chloro-isobutane, chlorotert-butane, n-bromopropane, bromo-isopropane, 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 boiling point of the alkane having 6-12 carbon atoms is 60-180 °C.

11. The purification method according to any one of claims 1-6, wherein, The flow rate of the washing solvent in the dilute-phase bed of caprolactam crystals is 0.001-0.02 m / s; and / or, the flow rate of the washing solvent in the dense-phase bed of caprolactam crystals is 0.01-0.3 m / s.

12. The purification method according to claim 11, wherein, The flow rate of the washing solvent in the dilute-phase bed of caprolactam crystals is 0.005-0.015 m / s; and / or, the flow rate of the washing solvent in the dense-phase bed of caprolactam crystals is 0.02-0.2 m / s.

13. The purification method according to any one of claims 1-6, wherein the amount of the washing solvent is 0.1-10 times the mass of the caprolactam crystals in the crystal slurry.

14. The purification method according to claim 13, wherein the amount of the washing solvent is 0.2-5 times the mass of the caprolactam crystals in the crystal slurry.

15. The purification method according to claim 14, wherein the amount of the washing solvent is 0.2-2 times the mass of the caprolactam crystals in the crystal slurry.

16. The purification method according to any one of claims 1-6, wherein, In step (1), the residence time of the caprolactam crystals inside the washer is 5-60 s.

17. The purification method according to claim 16, wherein, In step (1), the residence time of the caprolactam crystals inside the washer is 10-40 s.

18. The purification method according to any one of claims 1-6, wherein, A partition is provided inside the washer.

19. The purification method according to any one of claims 1-6, wherein, The temperature of the heating and dissolution is 50-110 °C.

20. The purification method according to any one of claims 1-6, wherein the heating and dissolution is carried out by using a jacket heating or an electric heating.

21. The purification method according to any one of claims 1-6, wherein, The method further comprises: subjecting the caprolactam solution obtained by heating and dissolution to melt hydrogenation, wherein 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.

22. The purification method according to claim 21, wherein the catalyst used for the melt hydrogenation is a nickel-containing catalyst and / or a palladium-containing catalyst.

23. The purification method according to any one of claims 1-6, wherein, During the water phase dissolution and stratification process, the amount of the water phase is 0.05-5 times the mass of the caprolactam crystals obtained by washing.

24. The purification method according to claim 23, wherein, During the water phase dissolution and stratification process, the amount of the water phase is 0.1-1 times the mass of the caprolactam crystals obtained by washing.

25. The purification method according to any one of claims 1-6, wherein the water phase dissolution and stratification is normal temperature water phase dissolution or heated water phase dissolution, the temperature of the normal temperature water phase dissolution is 10-30 °C, and the temperature of the heated water phase dissolution is 30-110 °C.

26. The purification method according to any one of claims 1-6, wherein, The method further comprises: subjecting the aqueous caprolactam solution obtained by water phase dissolution and stratification to optional concentration and / or dilution, and then carrying out water phase hydrogenation.

27. The purification method according to claim 26, wherein the conditions of the water phase hydrogenation include: the mass ratio of caprolactam to water used is 0.3-9:1, 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.

28. The purification method according to claim 26, wherein the catalyst used for the water phase hydrogenation is a nickel-containing catalyst and / or a palladium-containing catalyst.

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