Lactam purification process method and purification process device

Abstract

Provided in the present invention are a lactam purification process method and purification process device. Oxime and a catalyst are subjected to a rearrangement reaction, and lactam obtained after the rearrangement reaction is purified. The method comprises the following steps: extracting lactam by using an extraction solvent to obtain a lactam solution from which the catalyst is separated, subjecting the obtained lactam solution to alkali washing and water washing to obtain a lactam solution which has been subjected to alkali washing and water washing, subjecting the lactam solution obtained again to a hydrogenation reaction, subjecting the hydrogenated lactam solution to crystallization separation to obtain a lactam product crystal, and then treating the obtained lactam product crystal by means of solvent washing or rectification or distillation or crystallization, etc. so as to obtain lactam having a purity greater than or equal to 99.9%.

Description

A purification process and apparatus for lactams Technical Field

[0001] This invention belongs to the field of fine chemical technology and relates to the purification process and apparatus for lactams. Background Technology

[0002] Lactams are important monomers for the preparation of nylon materials. Among them, cyclododecyl lactam, also known as laurolactam, with the chemical formula C12H23NO and CAS number 947-04-6, is a monomer for the preparation of nylon 12.

[0003] Cyclododecanoxim undergoes a Beckmann rearrangement in the presence of concentrated sulfuric acid, a strongly acidic catalyst, to prepare a mixture of cyclododecanoic acid and concentrated sulfuric acid. The mixture is then purified to obtain high-purity cyclododecanoic acid. The Beckmann rearrangement of cyclododecanoic acid to cyclododecanoic acid inevitably produces small amounts of impurities, resulting in a complex composition of the post-reaction mixture containing cyclododecanoic acid. Some impurities are unsaturated impurities with properties very similar to cyclododecanoic acid. The post-reaction mixture includes concentrated sulfuric acid catalyst, and light impurities such as cyclododecane, cyclododecene, cyclododecone, cyclododecyl alcohol, cyclododecone oxime, dodeconitrile, undecyl carbon light components, dodecenoic acid-12-lactam and other polycyclic compounds, or heavy impurities such as 12-24 carbon organic acids. The presence of impurities significantly affects the quality of the cyclododecanoic acid product and also has adverse effects on subsequent polymerization and other applications. Some impurities are difficult to separate using conventional distillation extraction methods, or the separation process is energy-intensive.

[0004] The mixture to be separated contains a strong acid catalyst, mixed salts, and light component impurities such as cyclododecane, cyclododecene, cyclododecanone, cyclododecyl alcohol, cyclododecanone oxime, dodeconitrile, and undecanoic acid impurities; heavy component impurities include bicyclic or tricyclic compounds with double bonds and polycyclic structures, such as dodecenoic acid-12-lactam and 12-24 carbon organic acids. See the table below for details of specific substances:

[0005] Component Name Molecular Formula Molecular Weight Room Temperature Phase Standard Boiling Point (°C) Melting Point (°C) Classification Undecane Component Unknown Light Component Cyclododecane C12H24 168.32 Solid 244 59~61 Light Component Cyclododecene C12H22 166.3 Liquid 232~2459 Light Component Cyclododecone C12H22O 182.3 Solid 276.5 259 Light Component Cyclododecyl alcohol C12H24O 184.32 Solid 272.7~27877 Light Component Cyclododecone oxime C12H23NO 197.32 Solid 140-143 (3 Torr) 133~133.5 Light Component Dodeconitrile C12H23N 181.32 Liquid 2524 Light Component Impurities such as bicyclic or tricyclic compounds with double bonds or bridged ring structures Before purification, the boiling point is close to that of lactams 348~355 Component Unknown Unsaturated impurities Heavy components, 12-24 carbon organic acid composition, unknown acidic impurities, heavy components

[0006] The separation process of cyclododecanoic acid has the following problems:

[0007] 1. The components have relatively high boiling points, at least above 240℃, and operation under high vacuum is required;

[0008] 2. The reaction product contains unsaturated impurity components with boiling points close to or even azeotropic to those of cyclododecyl lactam, and with similar solubility, which are difficult to separate by conventional distillation or extraction operations.

[0009] 3. Cyclododecanoic acid is thermosensitive and can undergo side reactions, decomposition, and deterioration at higher temperatures, leading to an increase in the amount of light and heavy components, a decrease in product quality, and a reduction in recovery rate. Summary of the Invention

[0010] In view of this, the present invention aims to propose a purification process and apparatus for lactams, in order to solve the problems of difficult separation, low separation purity, high energy consumption and low recovery rate of lactams caused by the complex and unclear impurity components in the reaction products and the similar boiling points and solubilities of the impurity components.

[0011] On one hand, this application claims a purification process for a lactam, wherein the lactam is a compound soluble in a selected solvent and capable of crystallization, the purification process comprising:

[0012] S1: Preparation process, preparing lactam through rearrangement reaction and separating the catalyst to obtain the first mixture;

[0013] S2: Washing process, wherein the first mixture is subjected to alkaline washing and water washing to obtain a second mixture;

[0014] S3: Hydrogenation process, the second mixture is subjected to a hydrogenation reaction to convert unsaturated impurities with properties similar to lactams into saturated impurities, and a third mixture is obtained;

[0015] S4: Crystallization process, crystallizing the third mixture to obtain crude lactam product;

[0016] S5: Refining process, wherein the crude lactam product is refined by at least one of solvent washing, distillation, simmering or crystallization to obtain the lactam product.

[0017] In the preferred technical solution of the above purification process, the rearrangement reaction mentioned in the preparation process in step S1 is carried out in a two-stage or three-stage jet impact micro-mixing homogeneous reactor to accelerate mass transfer and achieve rapid micro-mixing. The mixing rate is shortened to the millisecond level, which can effectively avoid the rearrangement reaction from being carried out in a non-uniform state, thereby effectively avoiding the problem of side reactions caused by the local concentration of reactants, greatly reducing the types and contents of impurities in the reaction products, and greatly simplifying the subsequent purification work.

[0018] Further, in step S1, the catalyst separation is performed by extraction separation. The extraction solvent is at least one of a saturated monohydric alcohol with 4 to 12 carbon atoms. Preferably, the extraction solvent includes at least one of n-butanol, pentanol, isoamyl alcohol, 1-hexanol, 2-hexanol, heptanol, isohepanol, n-octanol, sec-octanol, butanol, isooctanol, and dodecanol. Long-chain saturated monohydric alcohols have advantages such as low freezing point, good chemical stability, high flash point, low fire hazard, narrow explosion limits, and good safety. As a solvent, long-chain saturated monohydric alcohols have the advantages of high solubility for the target product, large solubility variation with temperature, wide metastable region, and a large difference between the solubility of impurity components and the solubility of the target product. Long-chain saturated monohydric alcohols have moderate interfacial tension and density difference with water, which facilitates extraction and phase separation operations. When using long-chain saturated monohydric alcohols as polar solvents, no other additives are needed to effectively change the surface charge of the crystal particles, thereby effectively alleviating the problem of crystal agglomeration of lactams during crystallization. Furthermore, the mass ratio of lactam to extraction solvent is 1.8:1 to 0.3:1. As a specific implementation, taking cyclododecanoic acid as an example, in step S1, cyclododecanoic acid is extracted using an extraction solvent, with the mass ratio of extraction solvent to cyclododecanoic acid to solvent being 1.8:1 to 0.3:1. The selected extractant makes the solution system less prone to aggregation even under high concentration and gentle stirring during crystallization, eliminating the need for expensive additives, reducing the amount of circulating solvent, and lowering energy consumption. Moreover, since the solubility of cyclododecanoic acid in the selected long-chain saturated monohydric alcohol solvent varies greatly with temperature, more than 91-95.5 wt% of lactam in the feed solution can be separated in the crystallization process, resulting in high separation efficiency and low energy consumption.

[0019] In the preferred embodiment of the above purification process, the washing step S2 involves first performing an alkaline wash followed by a water wash. The residual acidic catalyst and its salts, as well as the organic acid impurities produced as a reaction byproduct, are removed by alkaline washing with an inorganic alkaline aqueous solution and water washing. The acid is neutralized into salts with an inorganic alkali, increasing its water solubility and facilitating removal from the lactam solution in the oil phase. Preferably, the inorganic alkaline aqueous solution is at least one of NaOH, Na2CO3, NaHCO3, KOH, K2CO3, and KHCO3 aqueous solutions, and more preferably an aqueous solution of NaOH.

[0020] Specifically, in the washing process, the first mixture obtained in step S1 is first washed with an alkaline solution until the pH is approximately 6-8 to remove impurities. At this point, acidic byproducts, acid catalysts, and most of the strong acid-weak base salts have been removed. Then, a water wash is performed to reduce the conductivity of the alkaline-washed first mixture to ≤20 μS / cm, and the ammonium sulfate and alkali contents are reduced to below 10 ppm. Further, both the alkaline washing and water washing are carried out under atmospheric pressure to 5 barg positive pressure, and the operating temperature is 60-160°C. The detergent for the alkaline washing is an aqueous solution of 1-15 wt% inorganic alkali; preferably, the detergent for the alkaline washing is an aqueous solution of 1-15 wt% NaOH.

[0021] Furthermore, the washing process in step S2 is carried out in a washing device, the lower part of which is an alkaline washing section and the upper part is a water washing section; preferably, the washing device is a plate extraction tower or a packed extraction tower or a hybrid of plate and packed; preferably, the washing device is a small-hole sieve plate tower, and the number of tower plates in the washing device is 6 to 20 layers of sieve plates; the spacing between the tower plates in the washing device is 300 to 800 mm, wherein the 1 to 2# tower plates are composite tower plates, the bottom surface is provided with oleophilic wire mesh packing, and the top of the tower of the washing device is provided with two layers of coalescing packing, the two layers of coalescing packing being oleophilic packing and hydrophilic packing respectively.

[0022] In the preferred embodiment of the above purification process, the hydrogenation step S3 is performed after the alkaline washing and water washing steps and before the crystallization step S4. The advantages of this arrangement are that alkaline washing and water washing can remove acidic catalysts and extend the lifespan of the hydrogenation catalyst; hydrogenation in solution has the advantage of lower temperature and milder operating conditions; and before crystallization, the hydrogenation reaction eliminates unsaturated bonds such as C=O and C=C, and the resulting saturated substances have a solubility difference with lactams, facilitating the removal of saturated impurities through crystallization in step S4.

[0023] Preferably, the hydrogenation step S3 is operated under positive pressure, with reaction conditions of a temperature of 40~130℃ and a hydrogen pressure of 0.3~1.2 MPa.G. Further, the hydrogenation reaction is carried out in a fixed-bed trickle hydrogenation reactor or a two-stage slurry-bed hydrogenation reactor. The fixed-bed trickle hydrogenation reactor is filled with fixed-bed catalyst particles, while the two-stage slurry-bed hydrogenation reactor is filled with slurry-bed catalyst powder.

[0024] In the preferred embodiment of the above purification process, after the preliminary hydrogenation purification step, the solubility of lactam and saturated light component impurities in long-chain saturated monohydric alcohols differs greatly. Crystallization can be used to separate lactam from the light component impurities. For step S4, the crystallization process, it is a multi-stage cooling crystallization process with 2 to 8 stages. Preferably, the multi-stage cooling crystallization process is operated under atmospheric pressure, positive pressure, or negative pressure. Preferably, the operating pressure of the multi-stage cooling crystallization process is atmospheric pressure to 1 barg, and the operating temperature is 160 to 0°C. Further, the multi-stage cooling crystallization process is carried out at 100 to 20°C. Taking cyclododecyl lactam as an example, the purity of the second lactam product obtained after the first crystallization and solvent washing can reach 99 wt%, which can be used as a polymerization raw material to produce nylon dodecyl polymer products with lower color requirements.

[0025] Regarding the multi-stage crystallization process, it should be noted that in one possible implementation, the multi-stage crystallization process can be achieved by using multiple crystallizers in series. Each stage of the series crystallizers uses an FC, DT, or DP type crystallizer equipped with a guide tube and a stirrer or external circulation pump. Preferably, the cooler is an external forced circulation heat exchanger. Surface cooling crystallizers are used to cool the solution, causing the lactam product to precipitate from the solution and form crystalline solids, rather than evaporation crystallization or vacuum cooling crystallization methods. In another possible implementation, the multi-stage crystallization process can also use a cooling plate crystallizer with multiple compartments. The cooling plate crystallizer is equivalent to a compact multi-stage series cooling crystallizer. The cooling plate crystallizer can be a horizontal structure or a vertical tank structure. The tank is divided into 3 to 12 compartments by fixed hollow cooling elements with openings. Each compartment is equivalent to an independent crystallizer, and each compartment is equipped with a rotating disk with a scraper. A cooling shaft runs through all compartments, and the rotating disk is mounted on the cooling shaft, rotating slowly at a speed of 10 to 50 r / min. The slow rotation of the rotating disk gently stirs the crystal slurry. Use scrapers to keep the cooling surfaces clean to prevent scaling. The cooling medium flows from the cooling shaft through each cooling element, flowing counter-currently to the lactam solution being cooled, thus lowering the solution temperature and minimizing the temperature difference between each stage, reducing supersaturation. The crystal slurry can flow from one compartment to the next through openings in the cooling elements, eliminating the need for transfer pumps between crystallizers, and the solid content of the slurry increases progressively with each stage. By employing a multi-stage cooling crystallization method, excessive saturation of the lactam solution to be purified can be avoided, thereby ensuring the effective crystallization and purification of lactam.

[0026] In the preferred embodiment of the above purification process, for the refining step S5, the crude lactam product obtained in step S4 is refined by using at least one of solvent washing, distillation, or crystallization to obtain the lactam product.

[0027] Taking cyclododecyl lactam as an example, as an optional implementation method, the cyclododecyl lactam crystals separated by crystallization are washed with the same pure solvent as that in the feed solution to remove impurities carried by the mother liquor. The lactam product washed with the solvent is designated as the second lactam product. The purity of the cyclododecyl lactam solid is >99wt%, which can be used as a polymerization raw material to produce nylon dodecyl polymer products with lower color requirements.

[0028] As another optional implementation, the refining process in step S5 includes a melting process and a light component removal process. The crude lactam product and / or the second lactam product, which have not been solvent-washed, are fed into the melting process for melting treatment to obtain a melt. The melt is then sent to a light component removal device to remove light component impurities and solvent. The top of the light component removal device yields a third lactam product with a purity >98 wt%, which can be used as a polymerization raw material to produce nylon dodecyl polymer products with low mechanical strength requirements. The solvent removed from the top of the light component removal device is returned to the preparation process. Further, the refining process in step S5, after the melting and light component removal processes, also includes a distillation process: the material from the bottom of the light component removal device is sent to a still for distillation treatment to remove heavy component impurities. The top of the still yields a fourth lactam product, and the bottom of the still yields a fifth lactam product. The purity of the fourth lactam product is higher than that of the fifth lactam product. Preferably, the solvent removed in the distillation process is returned to the preparation process. The fourth lactam product is one of the high-purity products produced by this purification process, with a purity >99.9 wt%, color <5 Hazen, basicity ≤0.2 mmol / Kg, and bridged ring impurity content ≤50 mg / Kg. It can be used to manufacture polymer products with extremely high requirements for color, corrosion resistance, flexibility, water absorption, low-temperature resistance, and impact strength, such as sports equipment and automotive parts. The fifth lactam product contains cyclododecyl lactam with a purity >98 wt%, which can be used as a polymerization raw material to produce polymer products with less stringent color requirements.

[0029] Specifically, the light-light removal process is a distillation process, which is carried out under vacuum, with an operating pressure of 0.3 kPaA-3 kPaA and an operating temperature of 170-215°C; preferably, the distillation column used in this distillation process has 5-30 theoretical plates; more preferably, the heater of the distillation column is a short heating tube rising film heater, with a tube length of 0.6-2.5 m, preferably 1-2 m.

[0030] Furthermore, the distillation process is operated under vacuum, with an operating pressure of 0.3 kPaA-3 kPaA and an operating temperature of 170-215°C; preferably, the distillation process employs multi-stage evaporators connected in series, with the number of evaporator stages ranging from 1 to 6, preferably 2 to 4; preferably, the heater of the evaporator is a short-tube rising film heater; preferably, the tube length of the heater of the evaporator is 0.6-2.5 m, preferably 1-2 m.

[0031] In addition to the two optional implementation methods described above, this application also provides another optional implementation method for the refining process in step S5, including a melting process and a melt crystallization process: the crude lactam product and / or the first lactam product that has not been solvent washed are fed into the melting process for melting treatment to obtain a melt. The melt is then sent to a melt crystallizer for melt crystallization treatment. The melt crystallizer sequentially produces a sixth lactam product and a seventh lactam product in batches, wherein the purity of the seventh lactam product is higher than that of the sixth lactam product. The purity of cyclododecyl lactam in the sixth lactam product is >98wt%, which can be used as a polymerization raw material to produce polymer products with low color requirements. The seventh lactam product is the second high-purity product produced by this purification process, with a purity >99.99wt%, color <5 Hazen, basicity ≤0.2 mmol / Kg, and bridged ring impurity content ≤50 mg / Kg. It can be used to manufacture polymer products with extremely high requirements for color, corrosion resistance, flexibility, water absorption, low temperature resistance, impact strength, etc., such as sports equipment and automotive parts.

[0032] Specifically, the operating temperature of the melt crystallization process is near the melting point of the lactam, with an operating temperature of 100~180℃, preferably 120~160℃; the operating pressure is positive pressure, negative pressure, or atmospheric pressure, preferably atmospheric pressure; the reflux ratio is 0.1~10, preferably 0.5~5. The melt crystallizer is any one of a layer crystallizer or a suspension crystallizer; preferably any one of a plate crystallizer, a falling film crystallizer, a belt crystallizer, or a twin-screw (KCP) tower crystallizer.

[0033] Preferably, in various optional embodiments of step S5 above, an alkaline substance is added during the melting process to melt the crude lactam product and / or the first lactam product that has not been washed with solvent, thereby obtaining a melt; preferably, the alkaline substance in the melting process is an inorganic alkali; preferably, during the melting process, at least one of 5~2000ppm NaOH, KOH, and K2CO3 is added.

[0034] Regarding the refining process in step S5, it should be noted that those skilled in the art can choose any of the above methods to refine the lactam according to the actual situation, or they can use multiple methods to refine the lactam. In the case of using multiple methods, the multiple methods can be carried out simultaneously or in any order. Those skilled in the art can set the method according to the actual situation.

[0035] Furthermore, in the preferred embodiment of the above-mentioned lactam purification process, the purification process further includes: S6: Mother liquor refining step, in which a portion of the crystals separated in the crystallization step S4 is sent to a mother liquor distillation unit to remove impurity components from the mother liquor, preventing impurities from accumulating in the system during circulation. A first lactam product with a purity >98wt% is obtained in the bottom section of the mother liquor distillation unit, which can be used as a polymerization raw material for producing engineering plastics with low color requirements. In addition, the product obtained in the bottom section of the mother liquor distillation unit can be used as the first lactam product, or the first lactam product can be further refined using any of the optional refining methods in step S5 described above.

[0036] Furthermore, most of the mother liquor separated in the crystallization step S4 carries the uncrystallized cyclododecanoic acid back to the preparation step S1 for recycling, which improves the yield and reduces energy consumption. Specifically, 90-99.5 wt% of the mother liquor from the crystallization step S4 is returned to the preparation step S1, and 0.5-10 wt% (preferably 1-3 wt%) of the mother liquor is sent to the mother liquor distillation unit.

[0037] Regarding step S6, those skilled in the art will understand that the specific ratio between the preparation step S1 and the crystallization step S4 is not entirely limiting. Those skilled in the art can set the ratio according to the specific type of lactam and its impurities being separated. Of course, changes to the above ratio do not deviate from the scope of protection of this application. Furthermore, the mother liquor purification step S6 is a distillation operation, performed under vacuum, with an operating pressure of 0.5~10 kPaA and an operating temperature of 100~200℃; the theoretical plate number of the distillation column is 5~30.

[0038] Furthermore, regarding the lactam in the above-mentioned preferred technical solutions, it should be noted that in this application, the lactam compound is preferably a lactam with 6-20 carbon atoms, more preferably with 6, 7, 8, 9, 10, 11, or 12 carbon atoms, and even more preferably with 12 carbon atoms, i.e., cyclododecanoic lactam. Nylon 12 elastomers prepared from cyclododecanoic lactam are widely used in defense, electronics, communications, machinery, food, and other industries due to their extremely excellent properties, and have a very broad application prospect. Therefore, this application uses cyclododecanoic lactam as an example to illustrate the purification process and apparatus. Those skilled in the art should understand that, even if the number of carbon atoms of the lactam changes when using the purification process or apparatus of this application, it still does not deviate from the basic principles of this application, and therefore all will fall within the protection scope of this application.

[0039] On the other hand, this application also claims a lactam purification process apparatus capable of performing the aforementioned lactam purification process, as shown in Figure 1. The lactam purification process apparatus includes a rearrangement reaction unit R1, a washing unit T1, a hydrogenation reaction unit R2, a cooling crystallizer V1, and a mother liquor purification unit T2. The rearrangement reaction unit R1 is connected to the bottom portion of the washing unit T1, and the top portion of the washing unit T1 is connected to the hydrogenation reaction unit R2.

[0040] The rearrangement reaction products in the rearrangement reaction device R1 are extracted with solvent to remove the catalyst. The material obtained after extraction of the rearrangement reaction products enters the bottom of the washing device T1 through the rearrangement reaction product material channel 1. The alkaline and water washed material at the top of the washing device T1 enters the downstream hydrogenation reaction device R2 through the alkaline and water washed material channel 2.

[0041] Furthermore, the washing device T1 has an alkaline washing section at the bottom and a water washing section at the top. The washing device T1 is equipped with a pure water channel and an alkaline solution channel. The pure water channel is connected to the water washing section of the washing device T1, providing pure water for the water washing operation in step S2. The alkaline solution channel is connected to the alkaline washing section of the washing device T1, providing an inorganic alkaline solution for the alkaline washing operation in step S2. Furthermore, the inlet of the pure water channel on the washing device T1 is positioned higher than the inlet of the alkaline solution channel. Waste liquid 3 after water washing and waste liquid 4 after alkaline washing are discharged from the washing device T1. Preferably, the washing device T1 is a plate extraction tower, a packed extraction tower, or a hybrid of plate and packed tower; preferably, the washing device T1 is a small-hole sieve plate tower, and the number of plates in the washing device T1 is 6 to 20 sieve plates; the spacing between the plates in the washing device T1 is 300 to 800 mm, wherein the 1st and 2nd trays are composite trays, with oleophilic wire mesh packing on the bottom surface, and two layers of coalescing packing are provided on the top of the washing device, the two layers of coalescing packing being oleophilic packing and hydrophilic packing respectively.

[0042] Furthermore, the bottom part of the hydrogenation reaction unit R2 is connected to the circulating cooling part of the cooling crystallizer V1, the mother liquor outlet end of the cooling crystallizer V1 is connected to the mother liquor purification unit T2 and the rearrangement reaction unit R1, and the extractant outlet end of the mother liquor purification unit T2 is connected to the rearrangement reaction unit R1.

[0043] The material obtained by hydrogenation enters the cooling crystallizer V1 through the hydrogenation material channel 5. Most of the mother liquor discharged from the crystallization is recycled back to the rearrangement reaction process for extraction through the first crystallization mother liquor channel 8. A small portion of the mother liquor discharged from the crystallization is sent to the mother liquor refining unit T2 through the second crystallization mother liquor channel 6. The purified clean solvent stream 7 obtained from the top of the mother liquor refining unit T2 is used for washing the crystals and is eventually returned to the rearrangement reaction unit R1 for extraction.

[0044] Regarding the cooling crystallizer V1, it should be noted that it can be composed of multiple crystallizers connected in series, or it can be a cooling plate crystallizer with multiple compartments. When the cooling crystallizer V1 consists of multiple crystallizers connected in series, each stage of the series crystallizer is selected from FC, DT, or DP type crystallizers equipped with a guide tube and a stirrer or external circulation pump; preferably, the cooler uses an external forced circulation heat exchanger; the solution is cooled by a surface cooling crystallizer, causing the lactam product to precipitate from the solution and form a crystalline solid, rather than by evaporation crystallization or vacuum cooling crystallization. When the cooling crystallizer V1 is a cooling plate crystallizer with multiple compartments, the cooling plate crystallizer is equivalent to a compact multi-stage series cooling crystallizer. The cooling plate crystallizer can be a horizontal structure or a vertical tank structure. The tank is divided into 3 to 12 compartments by fixed hollow cooling elements with openings. Each compartment is equivalent to an independent crystallizer, and each compartment is equipped with a rotating disc with a scraper. A cooling shaft runs through all compartments, with a turntable mounted on it. The turntable rotates slowly at 10-50 rpm, gently agitating the crystal slurry. Scrapers are used to keep the cooling surfaces clean to prevent scaling. The cooling medium flows from the cooling shaft through each cooling element, flowing counter-currently to the lactam solution being cooled, thus lowering the solution temperature and minimizing the temperature difference between each stage, reducing supersaturation. The crystal slurry can flow from one compartment to the next through openings in the cooling elements, eliminating the need for transfer pumps between crystallizers, and allowing the solids content of the slurry to increase progressively.

[0045] Referring again to Figure 1, the lactam purification process apparatus of the present invention further includes a melting tank V2, a light-weight removal device T3, a distillation apparatus V3, and a melt crystallizer V4 for further purification of the lactam product. The bottom portion of the cooling crystallizer V1 and the bottom portion of the mother liquor purification device T2 are connected to the melting tank V2. A portion of the bottom product of the mother liquor purification device T2 can be used as a first lactam product discharge device, and the other portion is sent to the melting tank V2 for further purification through the first feed stream 10. The solid crystals separated from the cooling crystallizer V1 are washed through the solid crystallization channel 9 and then enter the melting tank V2; the melt obtained in the melting tank V2 enters the downstream light-weight removal device T3 or the downstream melt crystallizer V4.

[0046] The melting tank V2 is connected to the light component removal unit T3. The melting tank V2 enters the light component removal unit T3 through the second feed stream 11. The extractant from the top fraction of the light component removal unit T3 is returned to the rearrangement reaction unit R1 through the third feed stream 13. The third lactam product separated at the top is sent out of the device. Preferably, the lactam without light components separated at the bottom of the light component removal unit T3 can be further fed into the downstream still V3 through the fifth feed stream 14. The fourth lactam product is obtained at the top of the still V3. The extractant separated at the bottom of the still V3 is returned to the rearrangement reaction unit R1 through the sixth feed stream 15. The fifth lactam product separated at the bottom is sent out of the device.

[0047] Preferably, the distillation column of the light component removal unit T3 has 5 to 30 theoretical plates, and the heater of the distillation column is a short-tube rising film heater with a tube length of 0.6 to 2.5 m, preferably 1 to 2 m. The distiller V3 is composed of multiple evaporators connected in series, with the number of evaporator stages ranging from 1 to 6, preferably 2 to 4; preferably, the heater of the evaporator is a short-tube rising film heater; preferably, the tube length of the heater of the evaporator is 0.6 to 2.5 m, preferably 1 to 2 m.

[0048] Furthermore, the melting tank V2 is also connected to the melting crystallizer V4. Specifically, the melting tank V2 enters the melting crystallizer V4 through the fourth feed stream 12, and the melting crystallizer V4 sequentially produces the sixth lactam product and the seventh lactam product, wherein the purity of the seventh lactam product is higher than that of the sixth lactam product.

[0049] Preferably, the melt crystallizer V4 is any one of a layer crystallizer or a suspension crystallizer; more preferably, it is any one of a plate crystallizer, a falling film crystallizer, a belt crystallizer, or a twin-screw (KCP) tower crystallizer.

[0050] It should be noted that, in the description of this preferred embodiment, unless otherwise explicitly specified and limited, the terms "connected," "linked," and "connected" should be interpreted broadly. For example, they can refer to a direct connection, an indirect connection through an intermediate medium, or a connection within two elements. Therefore, they should not be construed as limiting the present invention. Furthermore, the terms "first," "second," "third," "fourth," "fifth," and "sixth" are used for descriptive purposes only. Those skilled in the art can understand the specific meaning of these terms in the present invention according to the specific circumstances.

[0051] Compared with existing technologies, the purification process and apparatus for lactams described in this invention have the following advantages:

[0052] 1. Except for the final light component removal device T3 and the distillation apparatus V3, the other equipment in this invention is basically operated at atmospheric pressure and low temperature. The equipment cost is low, the operating conditions are mild, the temperature is much lower than the temperature sensitive to heat, and there are few by-products and a high recovery rate.

[0053] 2. The rearrangement reaction of this invention adopts a specially designed jet impact micro-mixing homogeneous reactor, which accelerates mass transfer and achieves rapid micro-mixing. The mixing rate is shortened to the millisecond level, which effectively avoids the reaction from taking place in a non-uniform state, avoids the local concentration of reactants, avoids a large number of side reactions, and greatly improves the quality of the target product lactam.

[0054] 3. The solvent used in the purification and separation process of this invention is a long-chain saturated monohydric alcohol or a mixture thereof. This solvent has the advantages of a low freezing point, low fire hazard, high solubility for the target product, large solubility variation with temperature, and a large difference between the solubility of impurity components and the solubility of the target product. Long-chain saturated monohydric alcohols are polar solvents; compared to solvents mentioned in other patents, they do not require the addition of other additives, thus changing the surface charge of the crystallized particles and effectively reducing crystal agglomeration of lactams during cooling crystallization. Long-chain saturated monohydric alcohols are chemically stable, not easily deteriorated, and can be recycled.

[0055] 4. Before separation, this invention employs purification steps such as alkali washing and water washing in an extraction tower, followed by hydrogenation in a fixed-bed trickle reactor or a slurry bed reactor. These steps are simple, involve low temperature and pressure, operate under mild conditions, require minimal equipment investment, and have low energy consumption. The results are significant, greatly improving the quality of the lactam product obtained from subsequent separation.

[0056] 5. This invention employs multi-stage cooling crystallization to separate lactam products and solvents. Compared to distillation separation, energy consumption is significantly reduced, saving a large amount of steam used for solvent evaporation. Cooling crystallization does not consume high-grade steam, only low-grade circulating water, and the energy consumption of the crystallization method is only 1 / 10 of that of the distillation method.

[0057] 6. The solvents used in this invention, as well as the separated light and heavy components, are all recycled or recycled after purification by distillation, which improves the utilization rate of raw materials and reduces the amount of by-products.

[0058] 7. The distillation and evaporation of lactam utilizes a specially designed short-tube rising film heater, which is more suitable for high-temperature heating and evaporation, requiring less equipment investment. The special short tube effectively prevents heat-sensitive products from deteriorating, discoloring, coking, and scaling caused by dry tubes. Combined with a single-pass small-volume reboiler, it shortens the residence time in the high-temperature zone and improves product quality.

[0059] 8. This invention effectively separates cyclododecanoic acid products from complex byproduct impurities such as unsaturated impurities with similar properties that are difficult to separate, compounds with bridged ring structures, and organic acids with 12 to 24 carbons through a series of reactions, purification, crystallization, distillation, and evaporation. The purity of the cyclododecanoic acid product is ≥99.9%, the potassium permanganate (PM) value is >10000s, the extinction at 291nm is 0.05~0.099, the differential transmittance is <10%, the content of bridged ring impurities is ≤50 mg / Kg, the color is <5 Hazen, and the alkalinity is ≤0.2 mmol / Kg. Attached Figure Description

[0060] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0061] Figure 1 is a schematic diagram of the lactam purification process apparatus of the present invention.

[0062] Explanation of reference numerals in the attached figures:

[0063] 1. Rearrangement reaction product material channel; 2. Material channel after alkali washing and water washing; 3. Waste liquid channel after water washing; 4. Waste liquid channel after alkali washing; 5. Hydrogenation material channel; 6. Second crystallization mother liquor channel; 7. Clean solvent flow; 8. First crystallization mother liquor channel; 9. Solid crystallization channel; 10. First flow; 11. Second flow; 12. Fourth flow; 13. Third flow; 14. Fifth flow; 15. Sixth flow; R1. Rearrangement reaction device; T1. Washing device; R2. Hydrogenation reaction device; V1. Cooling crystallizer; T2. Mother liquor purification device; V2. Melting tank; T3. Light weight removal device; V3. Distillation apparatus; V4. Melting crystallizer. Detailed Implementation

[0064] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0065] Given the excellent properties and wide range of applications of cyclododecanoic acid, the purification process of the lactam in this invention will be further explained below using cyclododecanoic acid as an example: Example 1

[0066] Using the above method and apparatus, before washing and hydrogenation, concentrated sulfuric acid is used as a catalyst. The composition of the solution after extraction with a long-chain saturated monohydric alcohol by solvent after the cyclododecanoic oxime rearrangement reaction of the cyclododecanoic oxime is as follows.

[0067] Substances: Cyclododecyl lactam, Solvent: Sulfuric acid + Ammonium sulfate, Water; Light component, Heavy component (wt%): 50.81~51.46%, 46.8~47.4%, 0.8~1.1%, 0.5~0.85%, 0.05%~0.09%

[0068] At this point, the conductivity of the solution is extremely high, exceeding the range; the potassium permanganate (PM) value is low, <2000s, which will lead to an increase in the visible color of the subsequent polymer; the extinction of the distilled cyclododecanoic acid at 291nm is >0.3; the transmittance difference is >40%, indicating that the content of impurities with double bonds is too high, exceeding the acceptable threshold of 15ppm. Example 2

[0069] Using the above methods and apparatus, the composition of the cyclododecanoic acid solution obtained in Example 1 after being washed with alkali and water in washing device T1 and purified in two steps by hydrogenation reaction device R2 is as follows.

[0070] Substances: Cyclododecyl lactam, Solvent: Sulfuric acid + Ammonium sulfate, Water; Light component, Heavy component; Composition (wt%): 51.92%, 47.17%, -0.80%, ~0.05%, ~0.06%

[0071] The main operating parameters of the washing unit and the hydrogenation reactor are as follows:

[0072] Washing unit, hydrogenation reactor (fixed bed), top pressure 0~10.3~1.2 barg, operating temperature 60~100℃, 40~130℃, other operating parameters: NaOH aqueous solution concentration 3~15wt%, volume hourly space velocity 1~10h. -1 Other equipment parameters: number of trays 6-20; nickel catalyst supported on alumina.

[0073] The conductivity of the purified solution decreased to 5-20 μS / cm; the potassium permanganate (PM) value was >10000s; the extinction of the cyclododecanoic acid obtained by distillation of the purified solution at 291 nm decreased to 0.05-0.099; the transmittance difference decreased to 4.1-4.86%, which is less than the required transmittance difference of 25-35%, indicating that the content of impurities with double bonds decreased and did not exceed the acceptable threshold of 10-15 ppm. Example 3

[0074] Using the above method and apparatus, the solution obtained after hydrogenation in hydrogenation reactor R2, containing 30-52 wt% cyclododecyl lactam and 48-70% long-chain saturated monohydric alcohol solvent, and containing trace amounts of light and heavy component impurities, enters a multi-stage cooling crystallizer V1. The temperature is gradually cooled from 90-160°C to 0-20°C, and the pressure is maintained at atmospheric pressure to 1 barg. After centrifugation, the mother liquor is returned to the rearrangement reaction (rearrangement reactor R1) for recycling. The cyclododecyl lactam crystals obtained by centrifugation are washed with a long-chain saturated monohydric alcohol at room temperature.

[0075] After cooling and crystallization, 91–95.5 wt% of lactam can crystallize from the solution. The energy consumption of crystallization separation is only 1 / 10 of that of distillation separation. Furthermore, there is no thermal decomposition of lactam during the crystallization process. Compared to the approximately 0.05 wt% thermal decomposition and deterioration caused by the distillation process, the crystallization method improves the yield and reduces the impurity content.

[0076] After crystallization, washing, and separation, the wet cyclododecanoic acid crystals were dried, and the composition of the cyclododecanoic acid was 99.85~99.90 wt%, with light component impurities being largely removed. This purity meets product standards and downstream application requirements, and the product can be sold externally. Example 4

[0077] Using the above method and apparatus, the wet cyclododecyl lactam crystals obtained by crystallization contain 1-3 wt% long-chain saturated monohydric alcohol solvent. Due to the presence of trace amounts of heavy component impurities, the molten cyclododecyl lactam product has a color of approximately 10 Hazen.

[0078] After further refining through melting in melting tank V2, removing light components in light component removal device T3, and distillation in distiller V3, the obtained high-purity cyclododecyl lactam product has a purity ≥99.9wt%, bridged ring impurity content ≤50 mg / Kg, color <5 Hazen, and alkalinity ≤0.2 mmol / Kg.

[0079] The main operating parameters of the light-weight removal unit T3 and the still V3 are as follows:

[0080] T3V3 column top temperature 175~215°C 170~212°C column bottom temperature 195~230°C 170~212°C operating pressure kPa 0.5~30.3~2 Example 5

[0081] Using the above method and apparatus, the wet cyclododecanoic acid crystals obtained after crystallization contain trace impurities. The molten cyclododecanoic acid product has a color of about 10 Hazen. Using the solid layer crystallization method, applying a falling film melt crystallizer or a static plate crystallizer, the high-purity cyclododecanoic acid product obtained has a purity ≥99.99wt%, a bridged ring impurity content ≤50 mg / Kg, a color <5 Hazen, and an alkalinity ≤0.2 mmol / Kg.

[0082] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A process for the purification of a lactam, characterized in that: The lactam is a compound dissolved in a selected solvent and capable of crystallization, and the purification process comprises: S1: a preparation process, preparing a lactam through a rearrangement reaction and separating a catalyst to obtain a first mixture; S2: a washing process, washing the first mixture with alkali and water to obtain a second mixture; S3: a hydrogenation process, hydrogenating the second mixture to convert unsaturated impurities similar to the lactam into saturated impurities to obtain a third mixture; S4: a crystallization process, crystallizing the third mixture to obtain a lactam crude product; S5: a refining process, refining the lactam crude product in at least one of solvent washing, rectification, distillation or crystallization to obtain a lactam product.

2. A process for purification of a lactam according to claim 1, characterized in that: The purification process further comprises: S6: a mother liquor refining process, refining at least part of the mother liquor of the crystallization process to obtain a first lactam product; Preferably, at least part of the mother liquor of the crystallization process and / or at least part of the mother liquor of the mother liquor refining process is returned to the preparation process; Preferably, 90-99.5wt% of the mother liquor of the crystallization process is returned to the preparation process.

3. A process for the purification of a lactam according to claim 2, characterized in that, The refining process in step S5 comprises a melting process and a light-removing process; The lactam crude product subjected to solvent washing is referred to as a second lactam product; The lactam crude product not subjected to solvent washing and / or the first lactam product and / or the second lactam product is subjected to a melting process to obtain a melt, and the melt is subjected to a light-removing device to remove light-component impurities and solvent, and a third lactam product is obtained at the top of the light-removing device; Preferably, the solvent removed in the light-removing process is returned to the preparation process.

4. A process for the purification of a lactam according to claim 3, characterized in that, The refining process in step S5 further comprises a distillation process; The column still material of the light-removing device is subjected to a distillation process in a distiller to remove heavy-component impurities, and a fourth lactam product is obtained at the top of the distiller, and a fifth lactam product is obtained at the bottom of the distiller, wherein the purity of the fourth lactam product is higher than that of the fifth lactam product; Preferably, the solvent removed in the distillation process is returned to the preparation process.

5. A process for purification of a lactam according to claim 2, wherein, The refining process in step S5 comprises a melting process and a melt crystallization process; The lactam crude product subjected to solvent washing is referred to as a second lactam product; The lactam crude product not subjected to solvent washing and / or the first lactam product and / or the second lactam product is subjected to a melting process to obtain a melt, and the melt is subjected to a melt crystallizer to obtain a sixth lactam product and a seventh lactam product in batches, wherein the purity of the seventh lactam product is higher than that of the sixth lactam product.

6. A process for purification of a lactam according to claim 1, characterized in that, The rearrangement reaction is carried out in a two-stage or three-stage jet impingement micro-mixing homogeneous reactor; And / or, the catalyst separation in step S1 is extraction separation, and the extraction solvent is at least one saturated monohydric alcohol with 4-12 carbon atoms; Preferably, the mass ratio of the lactam to the extraction solvent is 1.8:1-0.3:

1. Preferably, the extraction solvent includes at least one of n-butanol, pentanol, isoamyl alcohol, 1-hexanol, 2-hexanol, heptanol, isoheptanol, n-octanol, sec-octanol, butanol, isooctanol, and dodecanol.

7. A process for purification of a lactam as claimed in claim 1, wherein: The washing process described in step S2 is to first perform alkaline washing and then water washing. The alkaline washing washes the first mixture to pH≈6~8, and the water washing reduces the conductivity of the first mixture after alkaline washing to ≤20μS / cm, and the ammonium sulfate content and alkali content are reduced to below 10ppm. Preferably, both alkaline washing and water washing are performed under atmospheric pressure to 5 barg positive pressure, and the operating temperature is 60 to 160°C. Preferably, the detergent used for alkaline washing is an aqueous solution of an inorganic alkali; Preferably, the detergent for alkaline washing is an aqueous solution of 1-15 wt% inorganic alkali; Preferably, the detergent for alkaline washing is at least one of the following aqueous solutions: NaOH, Na2CO3, NaHCO3, KOH, K2CO3, and KHCO3. Preferably, the detergent for alkaline washing is a 1-15 wt% NaOH aqueous solution; Preferably, the washing process is carried out in a washing tower, which is provided with 6 to 20 layers of sieve trays with a tray spacing of 300 to 800 mm.

8. A process for the purification of cyclododecanone according to claim 1, characterized in that: Step S3, the hydrogenation process, is operated under positive pressure, with reaction conditions of temperature 40~130℃ and hydrogen pressure 0.3~1.2MPa.G.

9. A process for purification of a lactam according to claim 1, characterized in that: The crystallization process described in step S4 is a multi-stage cooling crystallization process, with 2 to 8 stages; Preferably, the multi-stage cooling crystallization process is operated under normal pressure, positive pressure, or negative pressure; Preferably, the operating pressure of the multi-stage cooling crystallization process is atmospheric pressure to 1 barg, and the operating temperature of the multi-stage cooling crystallization process is 160 to 0°C; More preferably, the multi-stage cooling crystallization process is carried out at 100~20°C.

10. A process for purification of a lactam according to claim 3, characterized in that, The light component removal process is a distillation process, which is carried out under vacuum at an operating pressure of 0.3 kPaA-3 kPaA and an operating temperature of 170-215°C. Preferably, the distillation column in the distillation process has 5-30 theoretical plates. More preferably, the heater of the distillation column is a short-tube rising film heater, and the tube length of the heater is 0.6-2.5 m, preferably 1-2 m. And / or, the distillation process is operated under vacuum, with an operating pressure of 0.3 kPaA-3 kPaA and an operating temperature of 170-215°C; preferably, the distillation process uses multi-stage evaporators connected in series, with the number of evaporator stages ranging from 1 to 6, preferably 2 to 4; preferably, the heater of the evaporator is a short-tube rising film heater; preferably, the tube length of the heater of the evaporator is 0.6-2.5 m, preferably 1-2 m.

11. A process for purification of a lactam as claimed in claim 5, wherein, The mother liquor refining process in step S6 is a distillation operation, performed under vacuum at an operating pressure of 0.5~10 kPaA and an operating temperature of 100~200℃; the theoretical plate number of the distillation column is 5~30; and / or The operating temperature of the melt crystallization process is near the melting point of the lactam, the operating temperature is 100-180℃, preferably 120-160℃, the operating pressure is positive pressure, negative pressure, normal pressure, preferably normal pressure; the reflux ratio is 0.1-10, preferably 0.5-5.

12. A process for the purification of a lactam according to any one of claims 3 to 5, characterized in that, The melt process is carried out on the crude lactam product without solvent washing and / or the first lactam product and / or the second lactam product by adding a basic substance in the melt process to obtain a melt; Preferably, the basic substance in the melt process is an inorganic base. Preferably, in the melt process, at least one of 5-2000ppm NaOH, KOH, K2CO3 is added during melting.

13. A process for the purification of a lactam according to any one of claims 1 to 11, characterized in that, The lactam is cyclododecanolactam.

14. A process apparatus for purification of a lactam, characterized by The lactam purification process device can perform the lactam purification process method of any one of claims 1-13.

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