Method for preparing caprolactam by gas-phase rearrangement

Abstract

This invention provides a method for preparing caprolactam via a gas-phase rearrangement process. The method involves vaporizing a reaction feedstock including cyclohexanone oxime and then performing a Beckmann gas-phase rearrangement reaction in a fixed-bed reactor to obtain caprolactam. The absolute pressure of the entire reaction system for this Beckmann gas-phase rearrangement reaction is 1–80 kPaA. This method of preparing caprolactam via a negative pressure reaction reduces the vaporization temperature of cyclohexanone oxime, improves the stability of the vaporization process, and thus enhances the selectivity of caprolactam. Furthermore, the negative pressure reaction avoids the use of a carrier gas, thereby reducing the energy consumption for purification during carrier gas recycling and improving the economic efficiency of the gas-phase rearrangement reaction. Simultaneously, the negative pressure system reduces the difficulty of caprolactam desorption, lowers the gas-phase rearrangement reaction temperature, reduces the energy consumption of the gas-phase rearrangement reaction, and increases the overall product yield, thereby improving economic benefits and facilitating industrialization.

Description

Technical Field

[0001] This invention relates to the field of caprolactam preparation technology, and more specifically, to a method for preparing caprolactam by gas-phase rearrangement. Background Technology

[0002] Caprolactam is an important organic chemical raw material. As a polyamide chip mainly used in the production of nylon fibers and engineering plastics, caprolactam is currently produced primarily through two methods: the liquid-phase Beckmann rearrangement using fuming sulfuric acid as a catalyst and the gas-phase Beckmann rearrangement using an MFI-structured catalyst. The liquid-phase rearrangement method suffers from equipment corrosion due to the highly corrosive nature of fuming sulfuric acid; moreover, each ton of caprolactam produces approximately 1.5–1.8 tons of low-value ammonium sulfate as a byproduct. In contrast, the gas-phase Beckmann rearrangement, utilizing an MFI-structured catalyst, does not produce ammonium sulfate as a byproduct, making it more in line with green and environmentally friendly production requirements, and thus has attracted widespread attention.

[0003] In the process of preparing caprolactam via gas-phase Beckmann rearrangement, cyclohexanone oxime is first vaporized in a vaporizer before entering the reactor for reaction. Because cyclohexanone oxime is a highly heat-sensitive substance, it is prone to side reactions such as condensation and decomposition during vaporization, leading to coking. This results in feed loss, blockage of catalyst pores, and ultimately catalyst deactivation.

[0004] Chinese patent application publication number CN103055526B discloses a method for evaporating cyclohexanone oxime. This method utilizes a rising film evaporator at 0.1–0.3 MPa. The heating medium exchanges heat with the feed liquid on the other side, causing partial vaporization. The feed liquid and vapor flow upwards to a flash chamber for vapor-liquid separation. Cyclohexanone oxime vapor is obtained at the top of the evaporation chamber, while the unevaporated vapor returns to the bottom of the rising film evaporator via a circulation pipe for further evaporation. This evaporation method requires controlling the vaporization rate of cyclohexanone oxime. An excessively high vaporization rate can lead to dry walls, coking, and carbonization on the evaporator surface, affecting the stable operation of the device and reducing the vaporization yield of cyclohexanone oxime. An excessively low vaporization rate results in excessive material circulation, prolonged heating of the material, and a higher risk of side reactions.

[0005] In addition, while typical gas-phase rearrangement reactions can be carried out at 100°C, the desorption of caprolactam from the molecular sieve requires a temperature increase to 300–400°C. The higher the acidity of the catalyst, the higher the temperature at which caprolactam is desorbed. If caprolactam is not desorbed in time, side reactions are likely to occur, leading to a decrease in selectivity.

[0006] The literature "Study on the B2O3 / TiO2-ZrO2-catalyzed gas-phase Beckmann rearrangement of cyclohexanone oxime III. Effect of reaction conditions" indicates that under normal pressure, when the reaction temperature is ≤300℃, the selectivity of caprolactam increases slightly with increasing reaction temperature. This is because at low temperatures, the product caprolactam is not easily removed from the catalyst surface, thus increasing the possibility of further polymerization and decomposition side reactions at acidic sites. Therefore, the catalyst is prone to rapid deactivation in the rearrangement reaction at low temperatures. When the reaction temperature is ≥300℃, the selectivity of caprolactam decreases. When the reaction temperature is 350℃, the selectivity of caprolactam has decreased to 79.8%, indicating that side reactions are more likely to occur at high temperatures, which is unfavorable for the production of caprolactam.

[0007] The cyclohexanone oxime gas-phase Beckmann rearrangement reaction is generally carried out at a temperature of 350–400 °C. As the temperature increases during the reaction, the carbon content on the catalyst surface also increases, leading to a shortened single-pass catalyst life. Therefore, catalyst regeneration is required. Since the carbon deposits on the catalyst surface at high temperatures are mainly "hard carbon," regeneration needs to be carried out at 400–500 °C. The regeneration process is prone to catalyst sintering, thereby reducing catalyst life and the effectiveness of the gas-phase rearrangement reaction. Summary of the Invention

[0008] The main objective of this invention is to provide a method for preparing caprolactam by gas-phase rearrangement, thereby solving the problems of poor selectivity, high energy consumption, short catalyst lifetime, and performance degradation after catalyst regeneration in existing caprolactam preparation methods.

[0009] To achieve the above objectives, according to one aspect of the present invention, a method for preparing caprolactam by gas-phase rearrangement is provided, comprising vaporizing a reaction feedstock including cyclohexanone oxime and then carrying out a Beckmann gas-phase rearrangement reaction in a fixed-bed reactor to obtain caprolactam, wherein the absolute pressure of the entire reaction system of the Beckmann gas-phase rearrangement reaction is 1 to 80 kPaA.

[0010] Furthermore, the absolute pressure of the entire reaction system described above is 10–50 kPaA.

[0011] Furthermore, the vaporization temperature is 100–160°C, preferably 110–150°C.

[0012] Furthermore, the temperature of the above-mentioned gas-phase rearrangement reaction is 200–300°C, and preferably 240–280°C.

[0013] Furthermore, the mass concentration of the above-mentioned cyclohexanone oxime is 10-70 wt%, preferably 25-50 wt%.

[0014] Furthermore, the weight hourly space velocity (WHSV) of the aforementioned cyclohexanone oxime is 0.1–10 h⁻¹. -1 The preferred weight hourly space velocity (WHSV) of cyclohexanone oxime is 0.5–5 h⁻¹. -1 .

[0015] Furthermore, the catalyst for the above-mentioned gas-phase rearrangement reaction is an MFI structure molecular sieve catalyst, preferably selected from any one or more of all-silica molecular sieves, titanium-silica molecular sieves, ZSM-5 molecular sieves, and alkali-modified β molecular sieves; preferably, the regeneration temperature of the MFI structure molecular sieve catalyst is 250-350℃, more preferably 260-300℃; preferably, the service life of the regenerated MFI structure molecular sieve catalyst is greater than 1500h.

[0016] Furthermore, the height-to-diameter ratio of the catalyst packing is 5 to 100:1, preferably 10 to 50:1.

[0017] Furthermore, the above-mentioned reaction raw materials also include an organic solvent, preferably a C1 to C6 saturated fatty alcohol, and preferably selected from any one or more of methanol, ethanol, propanol, and cyclohexanol.

[0018] Furthermore, the vaporization is performed using a vaporizer, preferably selected from any one or more of falling film evaporators, rising film evaporators, spray evaporators, atomizing vaporizers, and scraped film evaporators.

[0019] The above-mentioned method for preparing caprolactam via negative pressure reaction, applying the technical solution of this invention, reduces the vaporization temperature of cyclohexanone oxime and improves the stability of the vaporization process, thereby enhancing the selectivity of caprolactam. Furthermore, the negative pressure reaction avoids the use of carrier gas, reducing energy consumption during refining in the carrier gas recycling process and improving the economics of the gas-phase rearrangement reaction. The lower gas-phase rearrangement reaction temperature results in catalyst deactivation primarily through tar; compared to carbon-degraded catalysts, the catalyst regeneration temperature of this application is lower, resulting in stable performance and a longer service life after regeneration. Simultaneously, the negative pressure system reduces the difficulty of caprolactam desorption, thereby lowering the gas-phase rearrangement reaction temperature, reducing energy consumption, and increasing overall product yield, thus improving economic efficiency and facilitating industrialization. Detailed Implementation

[0020] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0021] As analyzed in the background section, existing methods for preparing caprolactam suffer from poor selectivity, high energy consumption, short catalyst life, and performance degradation after catalyst regeneration. To address these issues, this invention provides a method for preparing caprolactam via gas-phase rearrangement.

[0022] In a typical embodiment of this application, a method for preparing caprolactam by gas-phase rearrangement is provided, comprising: vaporizing a reaction feedstock including cyclohexanone oxime and then carrying out a Beckmann gas-phase rearrangement reaction in a fixed-bed reactor to obtain caprolactam, wherein the absolute pressure of the entire reaction system for the Beckmann gas-phase rearrangement reaction is 1 to 80 kPaA.

[0023] The above method for preparing caprolactam via negative pressure reaction lowers the vaporization temperature of cyclohexanone oxime, improves the stability of the vaporization process, and thus enhances the selectivity of caprolactam. Furthermore, the negative pressure reaction avoids the use of carrier gas, reducing energy consumption during refining and improving the economics of the gas-phase rearrangement reaction. The lower gas-phase rearrangement reaction temperature results in catalyst deactivation primarily through tar; compared to carbon-degraded catalysts, the catalyst regeneration temperature is lower, resulting in stable performance and a longer lifespan after regeneration. Simultaneously, the negative pressure system reduces the difficulty of caprolactam desorption, thereby lowering the gas-phase rearrangement reaction temperature, reducing energy consumption, and increasing overall product yield, thus improving economic efficiency and facilitating industrialization.

[0024] To further provide a suitable negative pressure environment for the reaction system, so as to reduce the vaporization temperature of cyclohexanone oxime and the difficulty of caprolactam desorption, thereby increasing the overall product yield, the absolute pressure of the entire reaction system is preferably 10-50 kPaA, such as 10 kPaA, 20 kPaA, 30 kPaA, 40 kPaA or 50 kPaA.

[0025] The negative pressure reaction conditions of this application help to lower the vaporization temperature of cyclohexanone oxime, improve the stability of the vaporization process of cyclohexanone oxime, and thus improve the selectivity of caprolactam. Preferably, the vaporization temperature is 100-160°C, such as 100°C, 110°C, 120°C, 130°C, 140°C, 150°C or 160°C, and more preferably 110-150°C, such as 110°C, 120°C, 130°C, 140°C or 150°C, which is beneficial to balance the stability of the vaporization process and the efficiency of vaporization.

[0026] Too high or too low a temperature in the gas-phase rearrangement reaction is not conducive to improving the selectivity of caprolactam. The preferred temperature for the gas-phase rearrangement reaction is 200-300℃, and the preferred temperature is 240-280℃, which is conducive to improving the selectivity of caprolactam while taking into account the efficiency of the gas-phase rearrangement reaction.

[0027] In one embodiment of this application, the mass concentration of the cyclohexanone oxime is 10-70 wt%, preferably 25-50 wt%.

[0028] A low concentration of cyclohexanone oxime helps improve the vaporization effect, while a high concentration is detrimental to the stability of the vaporization process, and a low concentration results in low vaporization efficiency, affecting the overall efficiency of the reaction. The optimal concentration of cyclohexanone oxime is beneficial for better balancing the stability and efficiency of the vaporization process.

[0029] To further improve the efficiency and effectiveness of the gas-phase rearrangement reaction, the weight hourly space velocity (WHSV) of the above-mentioned cyclohexanone oxime is preferably 0.1–10 h⁻¹. -1 The preferred weight hourly space velocity (WHSV) of cyclohexanone oxime is 0.5–5 h⁻¹. -1 .

[0030] In one embodiment of this application, the catalyst for the above-mentioned gas-phase rearrangement reaction is an MFI structured molecular sieve catalyst. Preferably, the MFI structured molecular sieve catalyst is selected from any one or more of all-silica molecular sieves, titanium-silica molecular sieves, ZSM-5 molecular sieves, and alkali-modified β molecular sieves. Preferably, the regeneration temperature of the MFI structured molecular sieve catalyst is 250-350°C, more preferably 260-300°C. Preferably, the single-pass service life of the regenerated MFI structured molecular sieve catalyst is greater than 1500 hours.

[0031] Utilizing MFI-structured catalysts for the Beckmann gas-phase rearrangement reaction eliminates the corrosion defects caused by the byproduct ammonium sulfate, making the reaction process more environmentally friendly and economical. Furthermore, the aforementioned types of MFI-structured molecular sieve catalysts further enhance the reaction efficiency of the above-mentioned reaction system. Preferably, air regeneration of the deactivated catalyst is employed, and the optimized regeneration temperature helps improve the stability of the regenerated catalyst and extend its service life.

[0032] Preferably, the height-to-diameter ratio of the catalyst loading is 5 to 100:1, and more preferably 10 to 50:1, which helps to improve the catalytic effect of the catalyst on the above reaction system.

[0033] In one embodiment of this application, the above-mentioned reaction raw materials further include an organic solvent, preferably a C1 to C6 saturated fatty alcohol, and preferably selected from any one or more of methanol, ethanol, propanol, and cyclohexanol.

[0034] The preferred organic solvents described above not only dissolve cyclohexanone oxime but also help improve the efficiency of the vaporization process.

[0035] In some embodiments of this application, a vaporizer is used for vaporization. Preferably, the vaporizer is selected from any one or more of falling film evaporators, rising film evaporators, spray evaporators, atomizing vaporizers, and scraped film evaporators, so as to more flexibly vaporize cyclohexanone oxime according to actual needs and maintain the stability of the vaporization process as much as possible.

[0036] The beneficial effects of this application will be explained below with reference to specific embodiments and comparative examples.

[0037] Example 1

[0038] A 25 wt% cyclohexanone oxime-methanol solution was vaporized at 110 °C using a falling film evaporator and then fed into a fixed-bed reactor containing 70 g of an all-silica molecular sieve catalyst (with a single-pass lifetime of 1700 h). The height-to-diameter ratio of the all-silica molecular sieve catalyst was 20:1. The absolute pressure of the reaction system was 10 kPaA, and the weight hourly space velocity (WHSV) of cyclohexanone oxime was 0.5 h⁻¹. -1 The reaction temperature was 250℃, yielding a caprolactam product system. Gas chromatography-mass spectrometry (GC-MS) with internal standard quantitative analysis showed a conversion rate of 99.98% for cyclohexanone oxime and a selectivity of 97.85% for caprolactam. Using a regenerated catalyst at 300℃, the conversion rate of cyclohexanone oxime was 99.97%, and the selectivity for caprolactam was 97.79%.

[0039] Example 2

[0040] The difference from Example 1 is that the concentration of the cyclohexanone oxime-methanol solution is 50 wt%, and the final product system of caprolactam is obtained.

[0041] Example 3

[0042] The difference from Example 1 is that the concentration of the cyclohexanone oxime-methanol solution is 10 wt%, and the final product system of caprolactam is obtained.

[0043] Example 4

[0044] The difference from Example 1 is that the concentration of the cyclohexanone oxime-methanol solution is 70 wt%, and the final product system of caprolactam is obtained.

[0045] Example 5

[0046] The difference from Example 1 is that the vaporization temperature of the cyclohexanone oxime solution was 150°C, and the caprolactam product system was finally obtained.

[0047] Example 6

[0048] The difference from Example 1 is that the vaporization temperature of the cyclohexanone oxime solution was 100°C, and the final product system of caprolactam was obtained.

[0049] Example 7

[0050] The difference from Example 1 is that the vaporization temperature of the cyclohexanone oxime solution was 160°C, and the final product system of caprolactam was obtained.

[0051] Example 8

[0052] The difference from Example 1 is that the weight hourly space velocity of cyclohexanone oxime is 2 h⁻¹. -1 Finally, the caprolactam product system was obtained.

[0053] Example 9

[0054] The difference from Example 1 is that the weight hourly space velocity of cyclohexanone oxime is 5 h⁻¹. -1 Finally, the caprolactam product system was obtained.

[0055] Example 10

[0056] The difference from Example 1 is that the weight hourly space velocity of cyclohexanone oxime is 0.1 h⁻¹. -1 Finally, the caprolactam product system was obtained.

[0057] Example 11

[0058] The difference from Example 1 is that the weight hourly space velocity of cyclohexanone oxime is 10 h⁻¹. -1 Finally, the caprolactam product system was obtained.

[0059] Example 12

[0060] The difference from Example 1 is that the gas-phase rearrangement reaction temperature is 280°C, and the final product system of caprolactam is obtained.

[0061] Example 13

[0062] The difference from Example 1 is that the gas-phase rearrangement reaction temperature was 240°C, and the final product system of caprolactam was obtained.

[0063] Example 14

[0064] The difference from Example 1 is that the gas-phase rearrangement reaction temperature is 200°C, and the final product system of caprolactam is obtained.

[0065] Example 15

[0066] The difference from Example 1 is that the gas-phase rearrangement reaction temperature is 300°C, and the final product system of caprolactam is obtained.

[0067] Example 16

[0068] The difference from Example 1 is that the absolute pressure of the gas-phase rearrangement reaction system is 40 kPaA, and the caprolactam product system is finally obtained.

[0069] Example 17

[0070] The difference from Example 1 is that the absolute pressure of the gas-phase rearrangement reaction system is 50 kPaA, and the caprolactam product system is finally obtained.

[0071] Example 18

[0072] The difference from Example 1 is that the absolute pressure of the gas-phase rearrangement reaction system is 80 kPaA, and the caprolactam product system is finally obtained.

[0073] Example 19

[0074] The difference from Example 1 is that the absolute pressure of the gas-phase rearrangement reaction system is 1 kPaA, and the caprolactam product system is finally obtained.

[0075] Example 20

[0076] The difference from Example 1 is that the solvent for the cyclohexanone oxime solution is ethanol, and the final product system of caprolactam is obtained.

[0077] Example 21

[0078] The difference from Example 1 is that the MFI structure molecular sieve catalyst is ZSM-5 molecular sieve, and the final product system of caprolactam is obtained.

[0079] Example 22

[0080] A 25 wt% cyclohexanone oxime-methanol solution was vaporized at 110 °C via an atomizer and then fed into a fixed-bed reactor containing 100 g of an all-silica molecular sieve catalyst with a height-to-diameter ratio of 30:1. The absolute pressure of the reaction system was 10 kPaA, and the weight hourly space velocity (WHSV) of the cyclohexanone oxime was 0.5 h⁻¹. -1 The reaction temperature was 250℃, and the caprolactam product system was obtained.

[0081] Example 23

[0082] The difference from Example 1 is that the height-to-diameter ratio of the all-silica molecular sieve catalyst is 5:1, resulting in a caprolactam product system.

[0083] Example 24

[0084] The difference from Example 1 is that the height-to-diameter ratio of the all-silica molecular sieve catalyst is 100:1, resulting in a caprolactam product system.

[0085] Example 25

[0086] The difference from Example 1 is that the height-to-diameter ratio of the all-silica molecular sieve catalyst is 50:1, resulting in a caprolactam product system.

[0087] Example 26

[0088] The difference from Example 1 is that the height-to-diameter ratio of the all-silica molecular sieve catalyst is 10:1, resulting in a caprolactam product system.

[0089] Example 27

[0090] The difference from Implementation 1 is that the catalyst regeneration temperature is 260℃, resulting in a caprolactam product system.

[0091] Example 28

[0092] The difference from Example 1 is that the catalyst regeneration temperature is 350°C, resulting in a caprolactam product system.

[0093] Example 29

[0094] The difference from Implementation 1 is that the catalyst regeneration temperature is 250℃, resulting in a caprolactam product system.

[0095] Comparative Example 1

[0096] The difference from Example 1 is that the reaction system was at atmospheric pressure, resulting in a caprolactam product system.

[0097] The caprolactam product systems obtained in Examples 1 to 29 and Comparative Example 1 were analyzed using gas chromatography with internal standard quantification. The results of cyclohexanone oxime (CHO) conversion, caprolactam (CPL) selectivity, catalyst and regenerated catalyst lifespan are shown in Table 1 below.

[0098] Table 1

[0099]

[0100]

[0101] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0102] The above method for preparing caprolactam via negative pressure reaction lowers the vaporization temperature of cyclohexanone oxime, improves the stability of the vaporization process, and thus enhances the selectivity of caprolactam. Furthermore, the negative pressure reaction avoids the use of carrier gas, reducing energy consumption during refining and improving the economics of the gas-phase rearrangement reaction. The lower gas-phase rearrangement reaction temperature results in catalyst deactivation primarily through tar; compared to carbon-degraded catalysts, the catalyst regeneration temperature is lower, resulting in stable performance and a longer lifespan after regeneration. Simultaneously, the negative pressure system reduces the difficulty of caprolactam desorption, thereby lowering the gas-phase rearrangement reaction temperature, reducing energy consumption, and increasing overall product yield, thus improving economic efficiency and facilitating industrialization.

[0103] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing caprolactam by gas-phase rearrangement, comprising: Caprolactam is obtained by vaporizing the reaction feedstock, which includes cyclohexanone oxime, and then carrying out the Beckmann gas-phase rearrangement reaction in a fixed-bed reactor. The characteristic of the reaction system is that the absolute pressure of the entire Beckmann gas-phase rearrangement reaction system is 10~50 kPa. The vaporization temperature is 110~150℃; the mass concentration of the cyclohexanone oxime is 25~50wt%; and the weight hourly space velocity of the cyclohexanone oxime is 0.5~5h. -1 The temperature of the gas-phase rearrangement reaction is 240~280℃; the catalyst for the gas-phase rearrangement reaction is an MFI structure molecular sieve catalyst; the height-to-diameter ratio of the catalyst packing is 10~50:

1.

2. The method according to claim 1, characterized in that, The MFI structured molecular sieve catalyst is selected from any one or more of the following: all-silica molecular sieve, titanium-silica molecular sieve, and ZSM-5 molecular sieve.

3. The method according to claim 1, characterized in that, The regeneration temperature of the MFI structured molecular sieve catalyst is 250~350℃.

4. The method according to claim 1, characterized in that, The regeneration temperature of the MFI structured molecular sieve catalyst is 260~300℃.

5. The method according to claim 1, characterized in that, The service life of the MFI structured molecular sieve catalyst is greater than 1500 hours.

6. The method according to any one of claims 1 to 5, characterized in that, The reaction raw materials also include organic solvents.

7. The method according to claim 6, characterized in that, The organic solvent is a C1-C6 saturated fatty alcohol.

8. The method according to claim 6, characterized in that, The organic solvent is selected from any one or more of methanol, ethanol, propanol, and cyclohexanol.

9. The method according to any one of claims 1 to 5, characterized in that, The vaporization is performed using a vaporizer.

10. The method according to claim 9, characterized in that, The vaporizer is selected from any one or more of the following: falling film evaporator, rising film evaporator, spray evaporator, atomizing vaporizer, and scraped film evaporator.

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