High temperature solvent rearrangement combined with caprolactam aqueous solution pre-evaporation apparatus and method

Abstract

The present application relates to fine chemical technology field, disclose a kind of high-temperature solvent rearrangement and caprolactam aqueous solution pre-evaporation combined device and method, including rearrangement reaction tank and pre-evaporation tower, pre-evaporation tower below is equipped with reboiler, rearrangement reaction tank gas phase outlet is connected to reboiler heat exchange after again leading to cyclohexane treatment process;Caprolactam aqueous solution is introduced into the feed inlet of pre-evaporation tower, gas phase outlet is connected secondary steam pipeline, liquid phase outlet is connected to caprolactam aqueous solution evaporation system;Rearrangement reaction tank below is equipped with discharge port, it is connected to the rotation position ester curing process.The device of the present application produces, by improving the reaction temperature of solvent rearrangement stage, it can directly use its reaction heat to subsequent process, for the energy saving and cost reduction of caprolactam production, reduce cost has important significance.

Description

Technical Field

[0001] This invention relates to the field of fine chemical technology, specifically to a combined apparatus and method for high-temperature solvent rearrangement and caprolactam aqueous solution pre-evaporation. Background Technology

[0002] In the industrial production of caprolactam, the solvent rearrangement reaction of alkyl cyclohexanone oxime with fuming sulfuric acid (Beckmann rearrangement) is one of the core steps. Currently, this reaction is usually operated at 90-110°C. Although the reaction is exothermic, the heat of reaction is difficult to utilize directly due to the low temperature. A large amount of circulating cooling water is required to remove it as waste heat, resulting in a serious waste of energy.

[0003] Furthermore, the inventors' investigation revealed that the ripening process of the transposition ester after the rearrangement reaction requires heating the material to 130-150°C, and the pre-evaporation process of the caprolactam aqueous solution also consumes a large amount of live steam to concentrate the caprolactam aqueous solution. The entire process exhibits an unreasonable pattern of first consuming energy for cooling and then for heating, resulting in low energy efficiency. Some factories employ technologies such as mechanical vapor recompression (MVR) to recover low-grade heat, which increases significant equipment investment and electricity consumption. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a combined apparatus and method for high-temperature solvent rearrangement and pre-evaporation of caprolactam aqueous solution. By increasing the reaction temperature in the solvent rearrangement stage, the heat of reaction can be directly utilized for subsequent processes, which is of great significance for energy saving, consumption reduction, and cost reduction in caprolactam production.

[0005] The technical solution of this invention is a combined device for high-temperature solvent rearrangement and caprolactam aqueous solution pre-evaporation, characterized in that it includes a rearrangement reaction tank and a pre-evaporation tower. A reboiler is provided below the pre-evaporation tower. The gas phase outlet of the rearrangement reaction tank is connected to the reboiler for heat exchange via a pipeline and then led out to the alkane processing step. The inlet of the pre-evaporation tower is fed with caprolactam aqueous solution, the gas phase outlet is connected to a secondary steam pipeline, and the liquid phase outlet leads out concentrated caprolactam aqueous solution. The rearrangement reaction tank is provided with a discharge port below it, which is equipped with a rearrangement liquid circulation pipeline and a rearrangement liquid outlet line.

[0006] Furthermore, the rearrangement reaction vessel has a feed inlet on its side wall, and the discharge outlet below it is connected to the feed inlet of the rearrangement reaction vessel via an acid oxime mixer, a circulating pump, a heat exchanger, and a rearrangement nozzle. The acid oxime mixer is also connected to a nicotinic acid feed pipe, and the rearrangement nozzle is also connected to an alkane and cyclohexanone oxime feed pipe. A rearrangement liquid outlet line is provided between the circulating pump and the heat exchanger.

[0007] Furthermore, an MVR evaporator is also provided between the gas phase outlet of the rearrangement reactor and the reboiler.

[0008] Furthermore, the rearrangement reaction vessel is equipped with internal components at the top, which include packing material, sieve plates, demisters, membrane tubes, or baffles.

[0009] The present invention also relates to a method for producing the product using the combined high-temperature solvent rearrangement and caprolactam aqueous solution pre-evaporation apparatus, comprising the following steps: S1. The alkane solution of cyclohexanone oxime and fuming sulfuric acid are introduced into the rearrangement reaction vessel, and the Beckmann rearrangement reaction is carried out under controlled temperature of 58~200℃ and pressure of 0.2~0.7MPa (A). S2. The heat of rearrangement reaction evaporates the alkanes in the reaction system to form alkane vapor. The alkane vapor is sent to a high-efficiency reboiler to heat the caprolactam aqueous solution in the pre-evaporation tower. After heat exchange and condensation, the alkane vapor is sent to the next process. The secondary steam generated in the pre-evaporation tower is drawn out as a heat source for other processes. The concentrate at the bottom of the pre-evaporation tower is drawn out to the caprolactam aqueous solution evaporation process for concentration. S3. The material in the rearrangement reaction tank is drawn out through the discharge port and then circulated and drawn out as needed.

[0010] The present invention also relates to the alkane in S1 being a C5-C8 straight-chain alkane, a branched-chain alkane, or a cycloalkane; preferably cyclohexane. This invention also relates to a reaction temperature of 150-180°C, preferably 170°C, when the alkane in S1 is cyclohexane; a pressure of 0.5-0.7 MPa (A), preferably 0.7 MPa (A); and a temperature in the rearrangement reactor when the alkane is n-pentane. 58 At ℃ and a reaction pressure of 0.2 MPa, the vapor phase outlet of the rearrangement reaction tank is heated and pressurized by an MVR evaporator before entering the reboiler for heat exchange.

[0011] The present invention also relates to the addition of caprolactam aqueous solution to the S2 pre-evaporation tower with a concentration of 20-70 wt% and the concentration of concentrate drawn from the bottom with a concentration of 25-90 wt%. The present invention also relates to controlling the molar ratio of fuming sulfuric acid and cyclohexanone oxime in the system to be 1.0~2.0, preferably 1.5~1.9, and more preferably 1.8 when fuming sulfuric acid and cyclohexanone oxime are added in S3. Furthermore, the heat exchange medium in the heat exchanger of S3 is water, and its outlet temperature is controlled below 100℃.

[0012] The present invention has the following beneficial effects: This invention increases the temperature of the rearrangement reaction, enabling the heat of reaction to evaporate the solvent. The resulting alkane vapor heat is used for the pre-evaporation of the caprolactam aqueous solution. The alkane vapor is cooled by heat exchange in a high-efficiency reboiler, condensing into a liquid for subsequent processing. The heat released from the condensation of the alkane vapor is used for the evaporation of the caprolactam aqueous solution, generating secondary steam, which can be used in other processes such as CPL distillation and benzene distillation. Concentration of the caprolactam aqueous solution does not require pre-evaporation using external steam. Furthermore, by increasing the rearrangement reaction temperature, the temperature of the reactants remaining after solvent evaporation is reduced to 120-150°C, which precisely meets the feed requirements of the subsequent rearrangement ripening process, thus saving the heating steam or hot water required for the ripening process.

[0013] The apparatus and method provided by this invention upgrade the low-grade reaction heat that needs to be removed by cooling water in traditional processes into high-grade heat energy that can drive evaporation by increasing the reaction temperature. This energy can be directly used in the pre-evaporation process of caprolactam aqueous solution, which has high heat consumption. It can produce a large amount of secondary steam as a byproduct and save a lot of cooling water.

[0014] In order to control the temperature inside the rearrangement reactor, a heat exchanger is installed on the circulation pipeline. The temperature range of the material entering the rearrangement reactor is adjusted through the heat exchanger. At the same time, hot water can be produced for heating in the plant area, realizing the deep cascade utilization of thermal energy and the high stability of system operation.

[0015] The top of the rearrangement reaction vessel is equipped with internal components, which can be packing, sieve plates, demisters, membrane tubes, or baffles. These components are used to prevent gaseous alkanes from carrying rearrangement products and acid mist, thereby improving the reaction yield and protecting downstream equipment. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the combined device for high-temperature solvent rearrangement and caprolactam aqueous solution pre-evaporation provided by the present invention. Detailed Implementation

[0017] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, all raw materials and reagents used are commercially available.

[0018] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are only for illustrating the present invention and should not be regarded as limiting the scope of the present invention.

[0019] Example 1 This invention provides a combined apparatus for high-temperature solvent rearrangement and caprolactam aqueous solution pre-evaporation, comprising a rearrangement reaction tank 1 and a pre-evaporation tower 2. A reboiler 3 is located below the pre-evaporation tower. The gas phase outlet of the rearrangement reaction tank is connected to the reboiler for heat exchange and then led out to the alkane processing step. The inlet of the pre-evaporation tower is fed with caprolactam aqueous solution, the gas phase outlet is connected to a secondary steam pipeline, and the liquid phase outlet leads out concentrated caprolactam aqueous solution, which can enter the evaporation system for further concentration. The rearrangement reaction tank has an outlet at the bottom, which is equipped with a rearrangement liquid circulation pipeline and a rearrangement liquid outlet line. Typically, about 80% of the rearrangement liquid is circulated, and about 20% is collected as product by a pump. After the rearrangement liquid is drawn out, it is necessary to replenish the raw materials such as nicotinic acid and cyclohexanone oxime cyclohexane solution in a timely manner.

[0020] In some embodiments, the rearrangement reaction tank has a feed inlet on its side wall, and the discharge outlet below it is connected to the feed inlet of the rearrangement reaction tank via an acid oxime mixer 4, a circulating pump 5, a heat exchanger 6, and a rearrangement nozzle 7. The acid oxime mixer is also connected to a nicotinic acid feed pipe, and the rearrangement nozzle is also connected to an alkane and cyclohexanone oxime feed pipe. A rearrangement liquid outlet line is provided between the circulating pump and the heat exchanger.

[0021] Furthermore, an MVR evaporator is also provided between the gas phase outlet of the rearrangement reactor and the reboiler.

[0022] In a further preferred embodiment, the rearrangement reaction vessel is equipped with internal components at the top, including packing material, sieve plates, demisters, membrane tubes, or baffles. Baffles are the most preferred option to reduce the entrainment of rearrangement liquid and nicotinic acid during evaporation.

[0023] Example 2 The method for production using the above-mentioned high-temperature solvent rearrangement and caprolactam aqueous solution pre-evaporation combined device includes the following steps: S1. Cyclohexane solution of cyclohexanone oxime and fuming sulfuric acid are introduced into a rearrangement reactor. The temperature is controlled at 170℃, the pressure at 0.5MPa (A), and the acid-oxime molar ratio is 1.75. The Beckmann rearrangement reaction is carried out with a reaction cycle ratio (circulating liquid / rearrangement produced liquid mass ratio) of 26. Nicotinic acid and cyclohexanone oxime cyclohexane solution are added during the circulation process. The residence time of the materials in the rearrangement reactor is controlled at 6 minutes. The reaction is rapid and complete, and the conversion rate of the rearrangement liquid reaches more than 99.9%.

[0024] S2. The heat of rearrangement reaction evaporates the alkanes in the reaction system to form alkane vapor at a temperature of 160°C. The alkane vapor is sent to a high-efficiency reboiler to heat the 30% caprolactam aqueous solution in the pre-evaporation tower. The 155°C secondary steam generated in the pre-evaporation tower is led out to the downstream CPL distillation process as a heat source. After about 70% of the heat of reaction is carried away by the alkane vapor, the temperature of the transposition ester in the circulation pipeline drops to 140°C and directly enters the rearrangement ripening process without the need for additional steam.

[0025] The alkane vapor is condensed after heat exchange to form a cyclohexane solution; the concentrate at the bottom of the pre-evaporation tower has a concentration of 50%, which is drawn out to the caprolactam aqueous solution evaporation process for concentration.

[0026] In this embodiment, the heat of rearrangement reaction is utilized efficiently, the live steam consumption of the caprolactam aqueous solution pre-evaporation process is reduced by about 90%, the cooling water consumption is reduced by about 70%, the steam consumption of the curing process is completely saved, and the overall energy consumption of the device is reduced by more than 35%.

[0027] Example 3 Based on Example 2, the reaction temperature was adjusted to 180°C, the pressure to 0.7 MPa(A), the acid-oxime ratio to 1.9, and the recycle ratio to 30. The higher temperature and pressure further accelerated the reaction rate, reducing the material residence time to approximately 4 minutes. The generated alkane vapor temperature reached 175°C. This high-temperature vapor was introduced into a high-efficiency reboiler to evaporate a 40% caprolactam aqueous solution, producing secondary steam of a higher grade at 168°C, which is more suitable for distillation processes requiring higher temperature heat sources. After heat removal, the transposition ester temperature was 145°C. This approach is suitable for applications requiring maximum secondary steam grade and yield.

[0028] Example 4 Based on Example 2, the reaction temperature was controlled at 150°C, the pressure at 0.3 MPa (A), the acid-oxime ratio at 1.6, and the recycle ratio at 22. Under these conditions, the reaction was mild, and the alkane vapor temperature was 150°C.

[0029] The steam is used to heat a 28% caprolactam aqueous solution to produce secondary steam at 130°C. Although the steam grade is slightly lower, it can still be used as a follow-up effect in multi-effect evaporation or as another low-temperature heat source. This embodiment operates under mild conditions, has relatively low equipment requirements, and utilizes a spare heat exchanger on the circulation pipeline to additionally generate 90°C hot water for plant heating, achieving deep cascade utilization of thermal energy and high system operational stability.

[0030] Example 5 Similar reaction conditions to Example 2 (170°C, 0.5 MPa(A)) were employed, but the acid-oxime ratio was strictly controlled at a low level of 1.6 to reduce the concentration of free sulfuric acid in the system, thereby mitigating equipment corrosion and the production of ammonium sulfate byproducts. A combination of a high-efficiency demister and baffles was installed at the top of the rearrangement reactor to minimize the entrainment of sulfur trioxide and sulfuric acid droplets by alkane vapors, protecting subsequent equipment such as the high-efficiency reboiler. The alkane vapors generated during the reaction were used to evaporate a 35% caprolactam aqueous solution. This scheme, while ensuring efficient heat recovery, particularly emphasizes the long-term safe and stable operation of the system, making it suitable for production facilities with extremely high requirements for equipment lifespan and continuous operating time.

[0031] Example 6 Same as Example 2, but the cyclohexane is replaced with n-pentane, and an MVR evaporator is installed between the gas phase outlet of the rearrangement tank and the reboiler.

[0032] The specific production method includes the following steps: S1. A pentane solution of cyclohexanone oxime and fuming sulfuric acid are introduced into a rearrangement reactor. The temperature is controlled at 58°C, the pressure at 0.2 MPa (A), and the acid-oxime molar ratio is 1.9. The Beckmann rearrangement reaction is carried out with a reaction cycle ratio of 30. Nicotinic acid and a pentane solution of cyclohexanone oxime are added during the cycle. The residence time of the materials in the rearrangement reactor is controlled at 4 minutes. The reaction is rapid and complete, and the conversion rate of the rearrangement liquid reaches more than 99.9%.

[0033] S2. The heat of rearrangement reaction evaporates the n-pentane in the reaction system to form steam at a temperature of 58°C. The alkane steam is pressurized to 3 MPa (A) and 190°C in an MVR evaporator and sent to a high-efficiency reboiler to heat the 30% caprolactam aqueous solution in the pre-evaporation tower. The 155°C secondary steam generated in the pre-evaporation tower is led out to the downstream CPL distillation process as a heat source. After about 70% of the heat of reaction is carried away by the alkane steam, the temperature of the transposition ester in the circulation pipeline is 160°C, and it enters the rearrangement ripening process.

[0034] After heat exchange and condensation, the alkane vapor becomes a n-pentane solution; the concentrate at the bottom of the pre-evaporation tower has a concentration of 50%, which is drawn out to the caprolactam aqueous solution evaporation process for concentration.

[0035] In this embodiment, n-pentane has a boiling point of only 36°C and can be vaporized in large quantities at 58°C under normal pressure. The n-pentane scheme saves 25% of MVR electricity, reduces overall energy consumption by 38%, and has near-zero vapor consumption during pre-evaporation.

[0036] Comparative Example 1 Based on Example 2, the reaction temperature was controlled at 110°C, with other adjustments remaining unchanged. This did not affect the rearrangement conversion rate, but due to the low reaction temperature, cyclohexane could not vaporize significantly, and the heat of the rearrangement reaction could not be carried away by alkane evaporation and recovered as steam. There was insufficient alkane steam to supply the efficient reboiler, requiring the pre-evaporation tower to use entirely external live steam to heat the caprolactam aqueous solution. Furthermore, the rearrangement reaction tank required circulating cooling water to remove heat, making it impossible to eliminate or significantly reduce cooling water consumption. Simultaneously, the temperature of the transposition ester collected from the bottom of the rearrangement tank was only about 110–120°C, which did not meet the subsequent ripening process's feed requirement of 130–150°C, necessitating additional heating steam. Overall, the energy consumption was comparable to existing conventional processes, with no significant improvement observed.

[0037] Comparative Example 2 Based on Example 2, the operating pressure was controlled at 0.2 MPa (A), with other adjustments remaining unchanged. The excessively low pressure resulted in insufficient increase in the cyclohexane saturation temperature within the system. Although alkane vapor was still generated, its partial pressure, density, and heat-carrying capacity per unit mass were low, leading to less heat of reaction carried out under the same circulation rate compared to Example 2. The high-efficiency reboiler suffered from insufficient heat exchange, resulting in a 40%–50% decrease in secondary steam production in the pre-evaporation tower. Additional live steam was still required for the pre-evaporation of the caprolactam aqueous solution. Furthermore, the increased gas velocity at low pressure exacerbated the tendency for acid mist and rearrangement entrainment. Even with a demister at the top, slight acid corrosion and scaling still occurred in the reboiler and subsequent alkane treatment equipment, resulting in lower long-term operational stability compared to Example 2. Although the rearrangement conversion rate reached over 99.5%, the overall energy-saving effect and equipment reliability were significantly lower than in Example 2.

[0038] Comparative Example 3 Based on Example 2, the acid-oxime ratio was controlled at 1:1, with other adjustments remaining unchanged. Insufficient free sulfuric acid concentration in the system resulted in a lack of adequate protonation catalytic environment for the Beckmann rearrangement. Local fluctuations in acid concentration in micro-regions led to a decrease in the rearrangement conversion rate to 95.8%, accompanied by the formation of a small amount of side reactions and impurities such as azo compounds, affecting the purity of the caprolactam product. Furthermore, insufficient acid slightly increased the viscosity of the rearrangement solution, worsening circulation and heat exchange conditions, and increasing the fluctuation in the outlet temperature control of the circulating heat exchanger. Although some of the reaction heat can be recovered from the alkane vapor, the sacrifice in product quality and conversion rate makes it unsuitable for industrial applications.

[0039] The above embodiments describe preferred embodiments of the present invention, but the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other way. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. A combined apparatus for high-temperature solvent rearrangement and caprolactam aqueous solution pre-evaporation, characterized in that, It includes a rearrangement reactor and a pre-evaporation tower. A reboiler is installed below the pre-evaporation tower. The gas phase outlet of the rearrangement reactor is connected to the reboiler for heat exchange and then led out to the alkane processing process. The feed inlet of the pre-evaporation tower is fed with caprolactam aqueous solution. The gas phase outlet is connected to a secondary steam pipeline, and the liquid phase outlet leads out concentrated caprolactam aqueous solution. The rearrangement reactor has a discharge port at the bottom, which is equipped with a rearrangement liquid circulation pipeline and a rearrangement liquid lead-out line.

2. The apparatus according to claim 1, characterized in that: The rearrangement reaction vessel has a feed inlet on its side wall, and the discharge outlet below it is connected to the feed inlet of the rearrangement reaction vessel via an acid oxime mixer, a circulating pump, a heat exchanger, and a rearrangement nozzle. The acid oxime mixer is also connected to a nicotinic acid feed pipe, and the rearrangement nozzle is also connected to an alkane and cyclohexanone oxime feed pipe. A rearrangement liquid outlet line is provided between the circulating pump and the heat exchanger.

3. The apparatus according to claim 1, characterized in that: An MVR evaporator is also installed between the gas phase outlet of the rearrangement reactor and the reboiler.

4. The apparatus according to claim 1, characterized in that: The rearrangement reaction vessel has internal components located at the top, including packing material, sieve plates, demisters, membrane tubes, or baffles.

5. A method for production using the combined apparatus of high-temperature solvent rearrangement and caprolactam aqueous solution pre-evaporation as described in any one of claims 1 to 4, characterized in that, Includes the following steps: S1. The alkane solution of cyclohexanone oxime and fuming sulfuric acid are introduced into the rearrangement reaction vessel, and the Beckmann rearrangement reaction is carried out under controlled temperature of 58~200℃ and pressure of 0.2~0.7MPa (A). S2. The heat of rearrangement reaction evaporates the alkanes in the reaction system to form alkane vapor. The alkane vapor is sent to a high-efficiency reboiler to heat the caprolactam aqueous solution in the pre-evaporation tower. After heat exchange and condensation, the alkane vapor is sent to the next process. The secondary steam generated in the pre-evaporation tower is drawn out as a heat source for other processes. The concentrate at the bottom of the pre-evaporation tower is drawn out to the caprolactam aqueous solution evaporation process for concentration. S3. The material in the rearrangement reaction tank is drawn out through the discharge port and then circulated and drawn out as needed.

6. The method according to claim 5, characterized in that: The alkane in S1 is a C5-C8 straight-chain alkane, a branched-chain alkane, or a cycloalkanes; cyclohexane is preferred.

7. The method according to claim 6, characterized in that: When the alkane in S1 is cyclohexane, the reaction temperature is 150~180℃, preferably 170℃; the pressure is 0.5~0.7 MPa(A), preferably 0.7 MPa(A); when the alkane is n-pentane, the temperature in the rearrangement reactor is 58℃, the reaction pressure is 0.2 MPa, and the vapor phase outlet of the rearrangement reactor is heated and pressurized by the MVR evaporator before entering the reboiler for heat exchange.

8. The method according to claim 5, characterized in that: The concentration of caprolactam aqueous solution added to the S2 pre-evaporation tower is 20~70wt%, and the concentration of the concentrate drawn from the bottom is 25~90wt%.

9. The method according to claim 5, characterized in that: When fuming sulfuric acid and cyclohexanone oxime are added to S3, the molar ratio of the two in the system is controlled to be 1.0~2.0, preferably 1.5~1.9, and more preferably 1.

8.

10. The method according to any one of claims 5 to 9, characterized in that: The heat exchange medium in the S3 heat exchanger is water, and its outlet temperature is controlled below 100℃.

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