A method for recycling caprolactam to synthesize N-vinyl caprolactam

Abstract

The present invention provides a method for recycling caprolactam to synthesize N-vinyl caprolactam, which belongs to the field of organic chemical synthesis, and comprises the following steps: (1) caprolactam and KOH are mixed and put into a reactor to react with nitrogen and acetylene introduced, and after the synthesis reaction is completed, the reaction solution is transferred to a first flash reactor for reduced pressure flash evaporation, the flash liquid is introduced into a distillation reactor, and the flash residue is transferred to a cracking reactor; (2) N-vinyl caprolactam is received through distillation of the distillation reactor, the distillation residue is transferred to a second flash reactor, the caprolactam obtained by flash recovery and purification is transferred to the synthesis reactor in step (1) for continued use, and the flash residue is transferred to a cracking reactor; (3) the flash residue in steps (1) and (2) is heated and cracked, and the caprolactam obtained after the cracking product is separated is transferred to the reactor in step (1) for use. The method of the present invention solves the technical problems of complex process, high cost and low raw material utilization rate for preparing N-vinyl caprolactam in the related art.

Description

Technical Field

[0001] The invention relates to the technical field of synthesis of N-vinyl amide, and in particular to a method for synthesizing N-vinyl caprolactam by recycling caprolactam. Background Art

[0002] N-vinylcaprolactam (NVCL for short) is a vinyl seven-membered hybrid compound. Due to the presence of active vinyl groups in its molecules, it becomes an important organic intermediate and an important heat-sensitive material with good polymerization properties. It can achieve self-homopolymerization and copolymerization with other monomers, and has a wide range of uses.

[0003] At present, there are many methods for synthesizing N-vinyl caprolactam, among which the most reported is the acetylene liquid phase synthesis method. Many earlier invention patents in Germany, the United States, Russia and other countries all choose to use different organic solvents, such as toluene, petroleum ether, tetrahydrofuran, dioxybutane, etc., or add vinyl ether, 18-crown ether-6 and other catalysts. Domestic literature reports that earlier researchers Feng Shubo (Synthesis of N-vinyl caprolactam. Fine Chemicals, 2008, 26 (6): 610-613) and others used 18-crown ether-6 as a catalyst for acetylene synthesis, and the conversion rate did not exceed 60%. Han Qingrong (Improvement of the Synthesis Process of N-vinyl caprolactam and Product Purification Chemical Reagents. 2010, 32 (9): 859-861) and others used the method of direct distillation of the reaction liquid, washing of the crude product with water, crystallization filtration, and vacuum distillation of the filter cake during the synthesis process, with a yield of 66%; after the filtrate was recovered and decompressed and split, the cumulative yield reached about 80% through recycling. Zhao Kun (Synthesis and Amplification of N-vinyl Caprolactam. Journal of Lanzhou University of Technology. 2013, 39(2): 62~65) et al. used toluene and potassium tert-butoxide to prepare potassium caprolactam, and then introduced acetylene to synthesize N-vinyl caprolactam with a maximum yield of 75%. Shi Haifeng (Study on Reaction Selectivity of Synthesis of N-vinyl Caprolactam by Non-solvent Method. Guangdong Chemistry. 2021, 45(5): 56~60) et al. used potassium tert-butoxide as a catalyst and 18-crown ether-6 as a co-catalyst to synthesize N-vinyl caprolactam. Domestic patent CN106946778B uses a one-pot method to synthesize vinyl caprolactam, and then purifies it by polar solvent recrystallization and reduced pressure distillation. The process is relatively complicated and introduces polar solvents. Patent CN117700361A uses a relatively expensive composite catalyst system and introduces an organic solvent, n-butyl ether, to improve the conversion rate of the product.

[0004] As can be seen from the above domestic and foreign studies and public reports, the synthesis process mostly uses high-cost catalysts and co-catalysts such as potassium tert-butoxide and 18-crown ether-6, and introduces various organic solvents, which are prone to product impurity pollution and solvent separation, and the problem of unreacted caprolactam reuse is not solved. In the patent CN102532025B, it is pointed out that vinyl caprolactam is synthesized by acetylene method under the action of catalyst, and the steps of rectification, crude steaming, refining, recovery, etc. are carried out to realize the recycling of unreacted raw material caprolactam; but the organic solvent ethyl acetate is introduced, combined with water washing, and then separated by distillation, the recycling process is complicated and cumbersome.

[0005] Therefore, the prior art needs to be further developed. Summary of the invention

[0006] The purpose of the present invention is to overcome the above technical deficiencies and provide a method for recycling caprolactam to synthesize N-vinyl caprolactam, so as to solve the technical problems of complex process, high cost and low raw material utilization rate in the preparation of N-vinyl caprolactam in the related art.

[0007] In order to achieve the above technical purpose, the present invention adopts the following technical scheme: Provided is a method for recycling caprolactam to synthesize N-vinyl caprolactam, comprising the following steps: (1) caprolactam and KOH are mixed and put into a synthesis reactor to react with nitrogen and acetylene, and after the synthesis reaction is completed, the reaction liquid is transferred to a first flash reactor, and the flash liquid is introduced into a distillation reactor through reduced pressure flash evaporation, and the flash residue is transferred to a cracking reactor; (2) N-vinyl caprolactam is obtained by distillation in the distillation reactor, and the distillation residue in the distillation reactor is transferred to a second flash reactor, and the flash evaporation recovery is carried out. The pure caprolactam obtained is transferred to the synthesis reactor in step (1) for continued use, and the flash residue is transferred to the cracking reactor; (3) the flash residues in step (1) and step (2) are heated and cracked in the cracking reactor, and the caprolactam obtained after the cracking product is separated is transferred to the synthesis reactor in step (1) for use; the cracking end point is determined by the pressure in the cracking reactor, the pressure determination value P of the cracking end point is ≥0.4Mpa, the heating cracking temperature is 250~450℃, and the caprolactam and KOH are mixed according to the mass ratio, wherein KOH accounts for 0.05%~2% of the total mass.

[0008] Furthermore, the flash residual liquid transferred into the cracking kettle is combined in multiple batches before starting the cracking reaction.

[0009] Furthermore, the cracking reaction start-up condition is that the residual liquid volume reaches 1 / 2 of the cracking kettle volume.

[0010] Furthermore, the purity of caprolactam obtained by flash evaporation recovery and purification is above 95%.

[0011] Furthermore, the flash recovery in step (2) is vacuum distillation, and all evaporated components are withdrawn for recovery.

[0012] Beneficial effects:

[0013] 1. The method for recycling caprolactam to synthesize N-vinyl caprolactam of the present invention does not require the introduction of other organic solvents, nor does it require the use of expensive catalysts and co-catalysts, and the synthesis process is simple and low in cost.

[0014] 2. The method for recycling caprolactam to synthesize N-vinyl caprolactam of the present invention has a high recycling rate and saves raw materials.

[0015] 3. The method of recycling caprolactam to synthesize N-vinyl caprolactam of the present invention reduces material and energy consumption, reduces the amount of solid waste, and is easy to apply industrially. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The present invention is a flow chart of recycling caprolactam to synthesize N-vinyl caprolactam. DETAILED DESCRIPTION

[0017] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present application.

[0018] A method for recycling caprolactam to synthesize N-vinyl caprolactam, see Figure 1, comprising the following steps: (1) mixing caprolactam and KOH and putting them into a synthesis reactor to react with nitrogen and acetylene introduced therein, after the synthesis reaction is completed, transferring the reaction liquid into a first flash reactor, introducing the flash liquid into a distillation reactor by reduced pressure flash evaporation, and transferring the flash residue into a cracking reactor; (2) receiving N-vinyl caprolactam through distillation in the distillation reactor, transferring the distillation residue in the distillation reactor into a second flash reactor, transferring the caprolactam obtained through flash recovery and purification into the synthesis reactor in step (1) for continued use, and transferring the flash residue into a cracking reactor; (3) heating and cracking the flash residues in steps (1) and (2) in a cracking reactor, and transferring the caprolactam obtained after separation of the cracked products into the synthesis reactor in step (1) for use; determining the cracking end point by the pressure in the cracking reactor, wherein the pressure determination value P of the cracking end point reactor is ≥ 0.4 MPa, the heating cracking temperature is 250-450°C, and the caprolactam and KOH are mixed according to a mass ratio, wherein KOH accounts for 0.05%-2% of the total mass. The method effectively recovers and purifies unreacted caprolactam through flash evaporation and cracking steps, so that it can be directly returned to the synthesis reaction for continued use. This not only improves the utilization rate of raw materials, but also significantly reduces material waste in the production process.

[0019] The flash evaporation residue transferred to the cracking kettle is combined in multiple batches before starting the cracking reaction, which can reduce the number of times the cracking reaction is started, thereby reducing energy consumption. When the volume of the residual liquid is less than half of the volume of the cracking kettle, it is equivalent to a small amount of dosage in the cracking process, resulting in a low pressure at the end of the reaction. If the dosage is too small, the heating area of ​​the material contact is small, the cracking is slow, and the cracking time will be extended. Therefore, when the residual liquid volume reaches 1 / 2 of the volume of the cracking kettle, the cracking reaction is started to ensure that there are enough materials in the cracking kettle for reaction, thereby improving the cracking efficiency. This helps to speed up the reaction speed, shorten the production cycle, and improve the overall production efficiency. When the pressure in the kettle reaches or exceeds 0.4Mpa, it indicates that the cracking reaction has ended, and subsequent operations such as stopping heating and separating products can be performed, which helps to ensure the stability and controllability of the cracking reaction. If the pressure in the kettle is less than 0.4MPa, the cracking is insufficient, and there are more cracking products, which increases the system pressure after gasification. The higher the pressure, the more small molecule cracking products there are, indicating that the cracking is basically over. If the cracking continues, the cracking will be deteriorated without carbonization.

[0020] The present invention is further described in detail below in conjunction with embodiments.

[0021] Example 1

[0022] A total of 150 kg of caprolactam and KOH (caprolactam and KOH are mixed according to a mass ratio, wherein the mass of KOH is between 75 g and 3000 g) are added to a stainless steel synthesis reactor. After nitrogen replacement three times, vacuum is evacuated while heating and stirring are carried out, and vacuum is evacuated and kept at a constant temperature for 2 to 3 hours. After the constant temperature is reached, acetylene is introduced at a certain pressure, and the temperature is controlled to react continuously. After the reaction is completed, the material is transferred to a flash kettle while hot, and flashed and separated directly. The flash liquid is introduced into a distillation kettle for direct distillation, and the flash residue is transferred to a small cracking kettle while hot.

[0023] The distillation produced 119.4 kg of finished N-vinyl caprolactam. The distillation residue was transferred to a flash evaporator for further flash evaporation. The residue was reddish brown and weighed about 18.7 kg. The caprolactam content detected by GC was 97.2%.

[0024] The residual liquid after flash evaporation of the reaction liquid was directly heated to 250°C in the cracking kettle. The pressure in the cracking kettle gradually increased. The temperature continued to rise slowly and gradually reached 450°C. The pressure in the cracking kettle reached 0.41MPa. The temperature was kept constant and the pressure in the kettle remained basically unchanged. The cracking was judged to be completed, and samples were taken for testing. The valve of the cracking kettle separation tower was opened to slowly draw out the volatile components and exclude the gas phase to obtain 7.2kg of yellow cracking liquid. The GC detection content was 95.6% of caprolactam. About 3.1kg of semi-solid residue was shoveled out.

[0025] The cleavage solution was diluted with solvent and then detected by GC-MS, which showed that it contained 1-pentenenitrile, capronitrile, 6-aminocapronitrile, caprolactam, N-(5-cyanopentyl)butyramide, N-(5-cyanopentyl)hexanamide, 6-acetylamino-N-(6-cyanopentyl)hexanamide, 6-butyramido-N-(5-cyanopentyl)hexanamide, and N-(5-cyanopentyl)-6-hexanamide.

[0026] Example 2

[0027] 18.7 kg of caprolactam recovered from the distillation residue and 7.2 kg of caprolactam recovered from cracking in Example 1 were combined and put into the synthesis reactor, and 124 kg of fresh caprolactam and KOH were added, and the operation was continued according to the process of Example 1.

[0028] Among them, 117.2 kg of N-vinyl caprolactam was obtained as the finished distillation product; after flash evaporation of the distillation residue, 19.2 kg of residual liquid was obtained at the bottom of the kettle, and the caprolactam content was 95.3% detected by GC; 7.5 kg of yellow pyrolysis liquid was obtained by cracking, with a content of 96.1%, and 3.9 kg of solid residue was removed.

[0029] Comparative Example 1

[0030] Comparative Example 1 is a traditional method for preparing N-vinyl caprolactam. A total of 150 kg of caprolactam and KOH of the same mass and proportion are put into a stainless steel synthesis reactor. After nitrogen replacement three times, vacuum is evacuated while heating and stirring, and vacuum is continuously evacuated and kept at a constant temperature for 2 to 3 hours. After the constant temperature is completed, acetylene is introduced at a certain pressure, and the temperature is controlled to react continuously. After the reaction is completed, the material is transferred to a flash reactor while hot, and flash separation is directly performed. The flash liquid is introduced into a distillation reactor for direct distillation, and 119 kg of finished N-vinyl caprolactam is obtained by distillation. In addition, 11 kg and 19 kg of viscous dark brown residual liquid are discharged from the flash reactor and the distillation reactor, respectively, and a total of 30 kg is treated as hazardous waste.

[0031] The following is a comparison of the raw material input amounts of Example 1 and Comparative Example 1, as shown in Table 1.

[0032] Table 1 Comparison of raw material usage between Example 1 and Comparative Example 1

[0033]

[0034] As shown in Table 1, the amount of finished product obtained in Example 1 and Comparative Example 1 is similar. The difference is that the conventional method in Comparative Example 1 removes about 30 kg of solid residue from the flash kettle and the distillation kettle; while in this scheme, through the recycling operation in Example 1, not only 25.9 kg of raw materials with a purity higher than 95% are recovered from the solid residue (that is, the actual amount of raw materials used in this reaction is 124.1 kg), but also the final solid residue is reduced to 7 kg, saving raw materials and reducing the amount of solid waste.

[0035] Depend on Figure 1 Combining the above Examples 1 and 2, it can be seen that the present invention recycles caprolactam to synthesize N-vinyl caprolactam, has a high recycling rate and a simple process, and the purity of the caprolactam obtained by flash evaporation recovery and purification is above 95%. Compared with other methods for recycle raw material caprolactam to synthesize N-vinyl caprolactam, no expensive catalysts and additives are used, and no other organic solvents are introduced, which is suitable for industrial promotion.

[0036] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0037] Optionally, the specific examples in this embodiment may refer to the examples described in the above embodiments, and this embodiment will not be described in detail here.

[0038] The serial numbers of the above-mentioned embodiments of the present application are for description only and do not represent the advantages or disadvantages of the embodiments.

[0039] In the above embodiments of the present application, the description of each embodiment has its own emphasis. For parts that are not described in detail in a certain embodiment, please refer to the relevant description of other embodiments.

[0040] The above is only a preferred implementation of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present application. These improvements and modifications should also be regarded as the scope of protection of the present application.

Claims

1. A method for recycling caprolactam to synthesize N-vinyl caprolactam, characterized in that: The following steps are involved: (1) caprolactam and KOH are mixed and put into a synthesis reactor to react with nitrogen and acetylene. After the synthesis reaction is completed, the reaction liquid is transferred to the first flash reactor, and the flash liquid is introduced into the distillation reactor through reduced pressure flash evaporation, and the flash residue is transferred to the cracking reactor; (2) After distillation in a distillation kettle, N-vinyl caprolactam is obtained, and the distillation residue in the distillation kettle is transferred to a second flash kettle. The caprolactam obtained by flash recovery and purification is transferred to the synthesis reactor in step (1) for continued use, and the flash residue is transferred to a cracking kettle; (3) heating and cracking the flash residues in step (1) and step (2) in a cracking reactor, and transferring the caprolactam obtained after separation of the cracked products into the synthesis reactor in step (1) for use; The cracking end point is determined by the pressure in the cracking kettle, and the pressure determination value in the cracking end point kettle is P ≥ 0.4Mpa; The heating cracking temperature is 250~450℃; Caprolactam and KOH are mixed according to a mass ratio, wherein KOH accounts for 0.05% to 2% of the total mass; The flash evaporation residue transferred into the cracking kettle is combined in multiple batches before starting the cracking reaction; The cracking reaction start-up condition is that the residual liquid volume reaches 1 / 2 of the cracking kettle volume.

2. The method for recycling caprolactam to synthesize N-vinyl caprolactam according to claim 1, characterized in that: The purity of caprolactam obtained by flash evaporation recovery and purification is above 95%.

3. The method for recycling caprolactam to synthesize N-vinyl caprolactam according to claim 1, characterized in that: The flash recovery in step (2) is vacuum distillation, and all evaporated components are withdrawn for recovery.

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