A coupling process method for the rectification and melt crystallization of mononitrotoluene

Abstract

A coupling process method for the rectification and melt crystallization of mononitrotoluene according to the present invention comprises the following steps: (1) The crude mononitrotoluene from the nitration process enters the rectification tower system; (2) The overhead of the rectification tower extracts the ortho-nitrotoluene raw material mainly containing ortho-nitro and meta-nitro, and the bottom of the tower extracts the crude crystallization raw material of 88.00% - 99.50% para-nitro; (3) The crude para-nitro crystallization raw material is transported to the crystallizer, and high-purity para-nitro finished products are produced through process controls such as crystallization, sweating, and melting; (4) After the crystallization is completed, the unfrozen mother liquor and the sweat generated during sweating are all returned to the rectification system, and after being mixed with the crude mononitrotoluene, they enter the rectification tower system. The present invention combines the rectification and crystallization processes, reduces the energy consumption while shortening the production cycle, and the optimization of the crystallization sweating process further reduces the energy consumption and improves the crystal yield.

Description

Technical Field

[0001] The present invention relates to the technical field of chemical production, and in particular to a coupling process method for the rectification and melt crystallization of mononitrotoluene. Background Art

[0002] Mononitrotoluene includes three isomers: o-nitrotoluene (commonly known as o-nitro), m-nitrotoluene (commonly known as m-nitro), and p-nitrotoluene (commonly known as p-nitro). They have a wide range of applications in industries such as medicine, pesticides, dyes, rubber additives, and new materials. Before use, mononitrotoluene needs to be separated and purified. Rectification is a process widely used for the separation of organic substances. However, the traditional method (separation by negative pressure rectification method) has a large separation difficulty and requires a rectification column with high separation efficiency. By using a high reflux ratio, products that meet market requirements can be obtained, but the quality is unstable and the energy consumption is large. Therefore, after rectification, a melt crystallization process is generally used for supplementation to reduce the separation difficulty and improve the product quality, which can maximize the compensation for the process and operation defects of the negative pressure rectification method. However, the separate crystallization process, which is batch production, has defects such as a long production cycle, low production capacity, and complex equipment.

[0003] The existing Chinese patent document with the publication number CN102126958B records "a device and method for coupling rectification and crystallization to prepare high-purity m-(p)-nitrotoluene". It uses a separation method that couples a vacuum rectification column and a crystallizer to prepare high-purity m-(p)-nitrotoluene. It can easily obtain p-nitrotoluene and m-nitrotoluene with a purity content of more than 99.9%, and the mother liquor is recycled with almost no loss. However, in actual production, although the purity of the separated product is as high as more than 99.9%, the crystallization efficiency is relatively low, and the crystal yield is only about 75%. That is to say, about 25% will circulate back to the rectification column with the mother liquor and the sweat generated during sweating, which will cause a large amount of energy consumption for the rectification and crystallization processes, and the energy consumption is relatively large. Summary of the Invention

[0004] In order to solve the technical problems of low crystallization efficiency and large energy consumption existing in the prior art, the present invention provides a coupling process method for the rectification and melt crystallization of mononitrotoluene. The specific technical solutions of the present invention are as follows.

[0005] A coupling process method for the rectification and melt crystallization of mononitrotoluene according to the present invention includes the following steps:

[0006] (1) The crude mononitrotoluene from the nitration process enters the rectification column system;

[0007] (2) The o-nitrotoluene raw material mainly containing o-nitro and m-nitro is taken out from the top of the rectification column, and the crude crystallization raw material containing 88.00% - 99.50% p-nitro is taken out from the bottom of the column;

[0008] (3) The raw material of crude p-nitrobenzoic acid crystals is transported to the crystallizer, and high-purity p-nitrobenzoic acid products are produced through process controls such as crystallization, sweating, and melting;

[0009] (4) After crystallization, the unfrozen mother liquor and the sweat generated during sweating are all returned to the rectification system, and after being mixed with the crude mononitrotoluene, they enter the rectification tower system;

[0010] Among them, in the crystallizer in the step (3), there are tube bundles inside. The processes of crystallization, sweating, and melting are indirectly heated by controlling the temperature of the heat exchange medium. A large-flow and low-lift circulation pump is used to keep the heat exchange medium circulating. A heat exchanger is installed at the outlet of the circulation pump to exchange heat for the heat exchange medium. The crystallization cooling process is operated in the way of gradient cooling, constant temperature, and then rapid cooling. The sweating process is operated in the way of rapid heating, constant temperature, gradient heating, and heat preservation.

[0011] In the step (3), during the crystallization process, the cooling rate of the heat exchange medium is controlled not to exceed 1 °C / h. When crystals precipitate in the crystallizer in the step (3), it is kept at a constant temperature for 2 h to 4 h. After the constant temperature, the heat exchange medium is rapidly cooled to (30 - 35) °C, and the unfrozen material is returned to the rectification system through the discharge pipeline e.

[0012] In the step (3), during the sweating process, the heat exchanger 4 is heated, indirectly heating the heat exchange medium in the crystallizer, and quickly raising the temperature of the crystallizer to the constant temperature. It is kept at a constant temperature for 2 h to 4 h, and then the heating rate of the heat exchange medium is controlled not to exceed 1 °C / h, and it is heated to 51 °C and kept warm for 2 h.

[0013] As a further technical solution, when crystals precipitate in the crystallizer in the step (3), it is kept at a constant temperature for 3 h.

[0014] As a further technical solution, during the sweating process in the step (3), it is kept at a constant temperature for 3 h.

[0015] The beneficial effects of the present invention: Through the novel coupling process method of mononitrotoluene rectification and melt crystallization, the present invention combines the rectification and crystallization processes, shortening the production cycle while reducing energy consumption. In the present invention, the processes of crystallization, sweating, and melting in the crystallizer are indirectly heated by controlling the temperature of the heat exchange medium. The crystallization cooling process is operated in the way of gradient cooling, constant temperature, and then rapid cooling. The sweating process is operated in the way of rapid heating, constant temperature, gradient heating, and heat preservation, improving the crystal yield and further reducing energy consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a schematic structural diagram of the device system used in the present invention;

[0017] In the figure: 1 - rectifying column; 2 - transfer pump; 3 - crystallizer; 4 - heat exchanger; 5 - circulation pump; a - feed pipeline for mononitrotoluene raw material; b - o-nitrotoluene raw material pipeline; c - transfer pipeline; d - p-nitrotoluene finished product transfer pipeline; e - discharging pipeline. Specific embodiments

[0018] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Improvements can be made to the disclosed content of the present invention simultaneously in terms of materials, methods and reaction conditions. All such improvements shall fall within the concept and protection scope of the present invention. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0019] As Figure 1 shown, the device system used in a coupling process method for mononitrotoluene rectification and melt crystallization according to the present invention includes five parts: a rectifying column 1, a transfer pump 2, a crystallizer 3, a heat exchanger 4 and a circulation pump 5. The middle of the rectifying column 1 is connected to the feed pipeline a for crude mononitrotoluene. A transfer pump 2 is provided on the bottom pipeline of the column to return a part of the bottom product for reflux and connect a part to the inlet of the crystallizer 3. There are two pipelines at the outlet of the crystallizer 3. One is connected to the feed pipeline a for crude mononitrotoluene, and the other is for transporting the p-nitrotoluene finished product. A circulation pipeline is installed in the internal heat exchange tubes of the crystallizer 3, and a circulation pump 5 is installed on the pipeline. The outlet pipeline of the circulation pump 5 is installed with a heat exchanger 4.

[0020] A coupling process method for mononitrotoluene rectification and melt crystallization according to the present invention includes the following steps:

[0021] (1) The crude mononitrotoluene from the nitration process enters the rectifying column system;

[0022] (2) The o-nitrotoluene raw material mainly containing o-nitrotoluene and m-nitrotoluene is taken out from the top of the rectifying column, and the crystallization raw material containing 88.00% - 99.50% p-nitrotoluene is taken out from the bottom of the column;

[0023] (3) The crude p-nitrotoluene crystallization raw material is transported to the crystallizer, and high-purity p-nitrotoluene finished product is produced through process controls such as crystallization, sweating and melting; in the specific implementation of this step, there are tubes inside the crystallizer. The processes of crystallization, sweating and melting are indirectly heated by controlling the temperature of the heat exchange medium. A large-flow and low-lift circulation pump is used to keep the heat exchange medium circulating. A heat exchanger is provided at the outlet of the circulation pump to exchange heat for the heat exchange medium. The crystallization cooling process is operated in the way of gradient cooling, constant temperature and then rapid cooling. The sweating process is operated in the way of rapid heating, constant temperature, gradient heating and heat preservation;

[0024] (4) After crystallization, the un-solidified mother liquor and the sweat produced by sweating are all returned to the rectifying system, mixed with the crude mononitrotoluene and then enter the rectifying column system.

[0025] The following is further illustrated with specific embodiments:

[0026] Embodiment 1:

[0027] The rectifying column 1 transports the crude mononitrotoluene to the inside of the rectifying column through the mononitrotoluene raw material feed pipeline a. The overhead product is sent to the next process through the o-nitrotoluene raw material pipeline b. The bottom controls the crude p-nitrotoluene crystallization raw material with a p-nitrotoluene content of 88.00% - 99.50%. The crystallization raw material enters the crystallizer 3 through the transfer pump 2 via the transfer pipeline c.

[0028] After the crystallizer 3 is filled, the heat exchanger 4 is cooled to indirectly cool the heat exchange medium in the crystallizer, and the cooling rate of the heat exchange medium is controlled not to exceed 1 °C / h. When crystals precipitate in the crystallizer, keep it at a constant temperature for 2 h. After the constant temperature, quickly cool the heat exchange medium to (30 - 35) °C, and return the unfrozen material to the rectification system through the discharge pipeline e. Then heat the heat exchanger 4 to indirectly heat the heat exchange medium in the crystallizer, quickly raise the temperature of the crystallizer to the constant temperature, keep it at a constant temperature for 2 h, and then control the heating rate of the heat exchange medium not to exceed 1 °C / h, and heat it to 51 °C and keep it warm for 2 h. During this period, the sweat continues to be returned to the rectification system through the discharge pipeline e, and then the material in the crystallizer is melted, and the p-nitrotoluene finished product is transported through the p-nitrotoluene finished product transfer pipeline.

[0029] After detection and analysis, in Embodiment 1, the product purity is 99.9%, and the crystal yield is 90.5%. Compared with the prior art, the product purity is the same at 99.9%, and the crystal yield is greater than 75% of the prior art.

[0030] Embodiment 2:

[0031] The rectifying column 1 transports the crude mononitrotoluene to the inside of the rectifying column through the mononitrotoluene raw material feed pipeline a. The overhead product is sent to the next process through the o-nitrotoluene raw material pipeline b. The bottom controls the crude p-nitrotoluene crystallization raw material with a p-nitrotoluene content of 88.00% - 99.50%. The crystallization raw material enters the crystallizer 3 through the transfer pump 2 via the transfer pipeline c.

[0032] After the crystallizer 3 is filled, the heat exchanger 4 is cooled to indirectly cool the heat exchange medium in the crystallizer, and the cooling rate of the heat exchange medium is controlled not to exceed 1 °C / h. When crystals precipitate in the crystallizer, keep it at a constant temperature for 3 h. After the constant temperature, quickly cool the heat exchange medium to (30 - 35) °C, and return the unfrozen material to the rectification system through the discharge pipeline e. Then heat the heat exchanger 4 to indirectly heat the heat exchange medium in the crystallizer, quickly raise the temperature of the crystallizer to the constant temperature, keep it at a constant temperature for 3 h, and then control the heating rate of the heat exchange medium not to exceed 1 °C / h, and heat it to 51 °C and keep it warm for 2 h. During this period, the sweat continues to be returned to the rectification system through the discharge pipeline e, and then the material in the crystallizer is melted, and the p-nitrotoluene finished product is transported through the p-nitrotoluene finished product transfer pipeline.

[0033] After detection and analysis, in Example 2, the purity is 99.9%, and the crystal yield is 93%. Compared with the prior art, the product purity is the same at 99.9%, and the crystal yield is greater than 75% of the prior art.

[0034] Example 3:

[0035] The rectifying column 1 transports the crude mononitrotoluene to the rectifying column through the feed pipeline a for mononitrotoluene raw materials. The overhead product is sent to the next process through the o-nitrotoluene raw material pipeline b. The bottom of the column controls the crude p-nitrotoluene crystal raw material with a p-nitrotoluene content of 88.00% - 99.50%. The crystal raw material enters the crystallizer 3 through the transfer pump 2 via the transfer pipeline c.

[0036] After the crystallizer 3 is filled, the heat exchanger 4 is cooled to indirectly cool the heat exchange medium in the crystallizer, and the cooling rate of the heat exchange medium is controlled not to exceed 1 °C / h. When crystals precipitate in the crystallizer, keep it at a constant temperature for 4 h. After that, quickly cool the heat exchange medium to (30 - 35) °C, and return the unfrozen material to the rectification system through the discharge pipeline e. Then, heat the heat exchanger 4 to indirectly heat the heat exchange medium in the crystallizer, quickly raise the temperature of the crystallizer to the constant temperature, keep it at a constant temperature for 4 h, and then control the heating rate of the heat exchange medium not to exceed 1 °C / h, and heat it to 51 °C and keep it warm for 2 h. During this period, the sweat continues to be returned to the rectification system through the discharge pipeline e, and then the material in the crystallizer is melted, and the p-nitrotoluene product is transported through the p-nitrotoluene product pipeline.

[0037] After detection and analysis, in Example 3, the purity is 99.9%, and the crystal yield is 92%. Compared with the prior art, the product purity is the same at 99.9%, and the crystal yield is greater than 75% of the prior art.

[0038] From the above new coupling process method and examples, it can be seen that in the present invention, the crystallization, sweating, and melting processes of the crystallizer are indirectly heated by controlling the temperature of the heat exchange medium. The crystallization cooling process is operated in a way of gradient cooling, constant temperature, and then rapid cooling. The sweating process is operated in a way of rapid heating, constant temperature, gradient heating, and heat preservation, which improves the crystal yield. The crystal yield is as high as over 90%, which has been greatly improved compared with the crystal yield of 75% in the prior art. While reducing the raw material circulation, it further reduces the energy consumption.

[0039] The above has described in detail the preferred specific embodiments and examples of the present invention with reference to the drawings. However, the present invention is not limited to the above embodiments and examples. Within the scope of knowledge possessed by those skilled in the art, various changes or equivalent replacements can be made without departing from the concept of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application belong to the scope protected by the present invention.

Claims

1. A coupling process method for the rectification and melt crystallization of mononitrotoluene, comprising the following steps: (1) The crude mononitrotoluene from the nitration process enters the rectification tower system; (2) The overhead product of the rectification tower is the o-nitrotoluene raw material mainly containing o-nitro and m-nitro, and the bottom product is the crude crystallization raw material containing 88.00% - 99.50% p-nitro; (3) The crude p-nitro crystallization raw material is transported to the crystallizer, and high-purity p-nitro finished product is produced through process controls such as crystallization, sweating, and melting; (4) The mother liquor that has not solidified after crystallization and the sweat produced during sweating are all returned to the rectification system, and after mixing with the crude mononitrotoluene, they enter the rectification tower system; It is characterized in that: In the step (3), there are tube bundles inside the crystallizer. The crystallization, sweating, and melting processes are indirectly heated by controlling the temperature of the heat exchange medium. A large-flow and low-lift circulating pump is used to keep the heat exchange medium circulating. A heat exchanger is installed at the outlet of the circulating pump to exchange heat for the heat exchange medium. The crystallization and cooling process is operated in the way of gradient cooling, constant temperature, and then rapid cooling. The sweating process is operated in the way of rapid heating, constant temperature, gradient heating, and heat preservation. In the step (3), the cooling rate of the heat exchange medium is controlled not to exceed 1 °C / h during the crystallization process. When crystals precipitate in the crystallizer, keep it at a constant temperature for 2 h to 4 h. After the constant temperature, quickly cool the heat exchange medium to (30 - 35) °C, and return the unfrozen material to the rectification system through the discharge pipeline e. In the step (3), during the sweating process, heat the heat exchanger 4 to indirectly heat the heat exchange medium in the crystallizer, quickly raise the temperature of the crystallizer to the constant temperature, keep it at a constant temperature for 2 h to 4 h, and then control the heating rate of the heat exchange medium not to exceed 1 °C / h, and heat it to 51 °C and keep it at a constant temperature for 2 h.

2. The coupling process method for the rectification and melt crystallization of mononitrotoluene according to claim 1, characterized in that: When crystals precipitate in the crystallizer in the step (3), keep it at a constant temperature for 3 h.

3. The coupling process method for the rectification and melt crystallization of mononitrotoluene according to claim 1, characterized in that: During the sweating process in the step (3), keep it at a constant temperature for 3 h.

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