Method for continuously crystallizing trimellitic anhydride into anhydride

Abstract

The invention relates to a method for continuously crystallizing trimellitic anhydride into anhydride. Pressing the intermediate products trimellitic acid and acetic acid into a buffer intermediate kettle; the materials flow into a bottom feeding port of the first flash heat exchanger from the bottom, and acetic acid is removed through flash evaporation under the vacuum effect; the materials are further heated through a second flash heat exchanger, a small amount of residual acetic acid is further removed under the vacuum effect, and the materials are promoted to be preliminarily dehydrated into anhydride; the materials enter a third flash heat exchanger for further dehydration; the materials continue to enter a fourth flash heat exchanger, and the remaining small amount of trimellitic acid is further subjected to deep reaction; after the reaction is completed, the mixture enters a light component removal tower, by-products which are not subjected to the oxidation reaction completely are removed, and a trimellitic anhydride crude product is extracted from a tower kettle; the method has the advantages that the materials are continuously fed and heated in the acetic acid removal and anhydride dehydration processes, the energy consumption is low, the system efficiency is high, the acetic acid is continuously removed, the anhydride is further dehydrated, and the reaction efficiency is improved while the product quality is ensured.

Description

Technical Field

[0001] This invention relates to the field of trimellitic anhydride preparation technology, and specifically to a method for continuous crystallization of trimellitic anhydride into anhydride. Background Technology

[0002] Trimeric trioxide (TMA), or simply trioxide, is an environmentally friendly plasticizer used in PVC products. It significantly improves heat resistance, cold resistance, and electrical insulation properties, and is suitable for automotive cables, medical devices, building materials, and children's toys.

[0003] Currently, trimellitic acid is produced by liquid-phase oxidation of 1,2,4-trimethylbenzene as a raw material and acetic acid as a solvent, under the action of cobalt / manganese salt catalysts and bromide promoters (such as tetrabromoethane) with the introduction of air. After removing acetic acid and light components, trimellitic acid is crystallized and finally dehydrated to form anhydride to obtain the product. The advantage of the crystallization tower with stirring evaporation to form anhydride is that the efficiency is significantly improved. The disadvantage is that the equipment structure is complex and prone to corrosion in high-temperature environments, resulting in a high maintenance rate. At the same time, mechanical stirring has a certain impact on the crystallization effect of anhydride. The main methods for crystallizing trimellitic acid into anhydride are intermittent heating and evaporation of acetic acid in the crystallization vessel followed by further dehydration to form anhydride, or stirring evaporation and crystallization in the crystallization tower followed by dehydration to form anhydride. The advantage of intermittent heating and evaporation in the crystallization vessel is that the operation is simple and the cost is the lowest. The disadvantage is that the heating time is long and there are many side reactions, resulting in low efficiency.

[0004] Rapid and uniform removal of acetic acid and uniform dehydration to anhydride at high temperatures are crucial prerequisites for the production of high-quality trimellitic anhydride. To achieve this, the materials need to be heated within a short reaction time while avoiding localized overheating that could lead to a large-scale reverse reaction. By controlling the feed rate of the reactants and the heat supply (heat transfer oil), tube heating under negative pressure can provide uniform heat. However, due to the poor flowability of the materials, the possibility of blockage and dead zones must be considered. Given these factors, further improvements in the efficiency and quality of the trimellitic anhydride crystallization process are necessary. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a continuous method for crystallizing trimellitic anhydride into anhydride, which solves the problems of long heating time, poor material flowability leading to increased side reactions, complex structure of tower-type stirred evaporation and dehydration anhydride forming equipment, easy corrosion in high temperature environment leading to high maintenance rate, and mechanical stirring having a certain impact on product quality in the existing process.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows: the continuous crystallization method of trimellitic anhydride into anhydride is achieved by a crystallization anhydride system; The crystallization anhydride system includes a buffer intermediate vessel, a first flash heat exchanger, a second flash heat exchanger, a third flash heat exchanger, a fourth flash heat exchanger, and a light-weight removal tower, arranged in descending order of installation height. The method for continuous crystallization of trimellitic anhydride into anhydride includes the following steps: S1. Triphenyltrimethylamic acid and acetic acid, the intermediate products after the oxidation reaction of trimesin in the oxidation reactor, are pressed into a buffer intermediate reactor under high pressure. The content of trimesin is 25-45%, the content of acetic acid is 55-75%, the material temperature is 200-210℃, and the pressure is ≤-0.08MPa. S2. The material in the buffer intermediate vessel flows into the bottom inlet of the first flash heat exchanger from the bottom. The material enters the first flash heat exchanger from the bottom inlet and is heated by the tubes. Under vacuum, the acetic acid is flashed away at the outlet of the first flash heat exchanger, removing 85-95% of the acetic acid. During this process, the material is rapidly heated to 215-225℃. The pressure of the first flash heat exchanger is ≤-0.08MPa. S3. Under the action of gravity, the material enters the bottom inlet of the second flash heat exchanger from the outlet of the first flash heat exchanger. It is then further heated by the second flash heat exchanger. Under the action of vacuum, the residual small amount of acetic acid is further removed and the material is initially dehydrated to form anhydride. Acetic acid removal ≥99%, anhydride formation reaction 5-10%. During this process, the material is rapidly heated to 230-240℃, and the pressure of the second flash heat exchanger is ≤-0.08MPa. S4. The material that has been initially dehydrated and formed anhydride further enters the third flash heat exchanger. In this heat exchanger, the reacting material is further dehydrated. Most of the trimellitic acid is dehydrated and reacted to form trimellitic anhydride. The anhydride formation reaction is 80-85%. During this process, the material is rapidly heated to 245-255℃. The pressure of the third flash heat exchanger is ≤-0.08MPa. S5. The material continues to enter the fourth flash heat exchanger. The remaining small amount of trimellitic acid undergoes further deep reaction. The material completes dehydration to form anhydride. The anhydride formation reaction is 95-98%. During this process, the material temperature rises to 260-270℃. The pressure of the fourth flash heat exchanger is ≤-0.08MPa. S6. After the material completes the reaction, it enters the light-removal tower. During this process, the material is at 260-270℃ and the pressure is ≤-0.098MPa. The byproducts of the incomplete oxidation reaction are removed, and crude trimellitic anhydride is collected in the bottom of the tower.

[0007] Furthermore, the top of the buffer intermediate vessel is connected to a feed pipe and negative pressure discharge lines for water and acetic acid. The bottom of the buffer intermediate vessel is provided with a material line a communicating with the bottom of the first flash heat exchanger. A lower valve for controlling the material flow rate is installed on material line a. The top of the first flash heat exchanger is connected to the negative pressure discharge lines for water and acetic acid. The upper outlet of the first flash heat exchanger is connected to the bottom of the second flash heat exchanger via material line b. The top of the second flash heat exchanger is connected to the negative pressure discharge lines for water and acetic acid. The upper outlet of the second flash heat exchanger is connected to the bottom of the third flash heat exchanger via material pipeline c. The top of the third flash heat exchanger is connected to the negative pressure discharge pipelines for water and acetic acid. The upper outlet of the third flash heat exchanger is connected to the bottom of the fourth flash heat exchanger via material pipeline d. The top of the fourth flash heat exchanger is connected to the negative pressure discharge pipelines for water and acetic acid. The upper outlet of the fourth flash heat exchanger is connected to the lower side of the light component removal tower via material pipeline e. The top of the light component removal tower is connected to the light component negative pressure discharge pipeline. A heat transfer oil line a is provided on one side of the fourth flash heat exchanger. A heat transfer oil regulating valve for controlling the heating amount is installed on the heat transfer oil line a. A heat transfer oil line b is connected between the upper side of the fourth flash heat exchanger and the lower side of the third flash heat exchanger. A heat transfer oil line c is connected between the upper side of the third flash heat exchanger and the lower side of the second flash heat exchanger. A heat transfer oil line d is connected between the upper side of the second flash heat exchanger and the lower side of the first flash heat exchanger. A heat transfer oil line e is connected between the upper side of the first flash heat exchanger and the bottom of the buffer intermediate vessel.

[0008] Furthermore, the buffer intermediate vessel has a jacketed heating and insulation structure. The feed and discharge surfaces of the tubes in the first, second, third, and fourth flash heat exchangers are inclined, with the tubes on the discharge surface protruding above the top of the tube opening. The material pipelines a, b, c, d, and e have an inclined design, which is beneficial for material flow. The flow direction of the heat transfer oil in the heat transfer oil pipelines a, b, c, d, and e is opposite to the flow direction of the material pipelines a, b, c, and d. As the material temperature gradually increases, the heat transfer oil temperature gradually decreases, resulting in a lower overall reaction temperature and reducing the generation of side reactions.

[0009] Furthermore, the heat exchange tubes of the first and third flash heat exchangers are 60% the length of the heat exchange tubes of the second and fourth flash heat exchangers, and the outlets of the heat exchange tubes of the first, second, third, and fourth flash heat exchangers are 2-5 cm higher than the tube sheet plane.

[0010] Furthermore, the heating medium for the first flash heat exchanger, the second flash heat exchanger, the third flash heat exchanger, and the fourth flash heat exchanger is 290°C heat transfer oil.

[0011] Furthermore, the discharge ports of the first, second, third, and fourth flash heat exchangers are angled discharge ports, installed on the side bottom of the discharge surface, and all of the first, second, third, and fourth flash heat exchangers are non-uniform perforated tube heat exchangers.

[0012] The advantages of this invention are: 1. The material is continuously fed and heated during the process of removing acetic acid and dehydrating to form anhydride. Compared with the traditional intermittent heating, the energy consumption is low and the system efficiency is high. This allows trimellitic acid after the oxidation of trimellitic acid to continuously remove acetic acid and further dehydrate to form anhydride, thereby improving the reaction efficiency while ensuring the quality of the product and meeting the production needs in a balanced manner. 2. A non-uniform perforated tube heat exchanger is adopted. The heating medium from the first flash heat exchanger to the fourth flash heat exchanger is 290℃ heat transfer oil. The heat transfer oil and the material flow direction are set in countercurrent. The material temperature gradually rises while the heat transfer oil temperature gradually decreases, resulting in a lower overall reaction temperature and reducing the generation of side reactions. This process can significantly improve the efficiency and forward conversion rate of acetic acid removal and dehydration to anhydride, avoid asynchronous reactions and side reactions caused by uneven heating, and help reduce material and energy losses. Compared with the current domestic process, it has the advantages of rapid dehydration to anhydride, high heat exchange efficiency, and good product quality. 3. Gravity transfer is used from the buffer intermediate reactor instead of mechanical conveying and forced discharge by stirring. The reaction proceeds gradually between the buffer intermediate reactor and the multi-stage flash heat exchanger. The reaction progress is regulated by the intermediate reactor bottom valve and the heat transfer oil regulating valve, resulting in more stable quality of trimellitic anhydride. 4. The feed and discharge surfaces of each stage of flash heat exchanger are inclined, which ensures uniform heating time of materials while improving material flowability. The protruding pipe design eliminates the problem of material clogging the discharge port. The overall equipment is easy to operate and maintain, has good corrosion resistance, and is conducive to long-term continuous operation. 5. Acetic acid and water released from the material are rapidly removed through the top pipeline under vacuum, reducing side reactions and reaction time, further improving efficiency, and completing the deacidification crystallization and dehydration to anhydride reaction in one go. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the structure of the present invention. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The following embodiments will enable those skilled in the art to more fully understand this invention, but do not limit the invention to the scope of the described embodiments.

[0015] like Figure 1 As shown, the specific embodiment adopts the following technical solution: The continuous crystallization of trimellitic anhydride into anhydride is achieved through a crystallization anhydride system, which includes a buffer intermediate vessel 1, a first flash heat exchanger 2, a second flash heat exchanger 3, a third flash heat exchanger 4, a fourth flash heat exchanger 5, and a light-weight removal tower 6, with the installation height decreasing sequentially.

[0016] The top of the buffer intermediate vessel 1 is connected to a feed pipe and a negative pressure discharge pipeline 11 for water and acetic acid. The bottom of the buffer intermediate vessel 1 is provided with a material pipeline a91 that communicates with the bottom of the first flash heat exchanger 2. A lower expansion valve 7 for controlling the material flow rate is installed on the material pipeline a91. The top of the first flash heat exchanger 2 is connected to the negative pressure discharge pipeline 11 for water and acetic acid. The upper outlet of the first flash heat exchanger 2 is connected to the bottom of the second flash heat exchanger 3 through a material pipeline b92. The top of the second flash heat exchanger 3 is connected to the negative pressure discharge pipeline 11 for water and acetic acid. The upper outlet of the second flash heat exchanger 3 is connected to the third flash heat exchanger 3 through a material pipeline c93. The bottom of the evaporator heat exchanger 4 is connected, the top of the third flash heat exchanger 4 is connected to the negative pressure discharge pipeline 11 for water and acetic acid, the upper discharge port of the third flash heat exchanger 4 is connected to the bottom of the fourth flash heat exchanger 5 through the material pipeline d94, the top of the fourth flash heat exchanger 5 is connected to the negative pressure discharge pipeline 11 for water and acetic acid, the upper discharge port of the fourth flash heat exchanger 5 is connected to the lower side of the light component removal tower 6 through the material pipeline e95, the top of the light component removal tower 6 is connected to the light component negative pressure discharge pipeline 12, the buffer intermediate kettle 1 and the first flash heat exchanger 2, the second flash heat exchanger 3, the third flash heat exchanger 4 and the fourth flash heat exchanger 5 are all operating under negative pressure.

[0017] A heat transfer oil pipeline a101 is provided on one side of the fourth flash heat exchanger 5. A heat transfer oil regulating valve 8 for controlling the heating amount is installed on the heat transfer oil pipeline a101. A heat transfer oil pipeline b102 is connected between the upper side of the fourth flash heat exchanger 5 and the lower side of the third flash heat exchanger 4. A heat transfer oil pipeline c103 is connected between the upper side of the third flash heat exchanger 4 and the lower side of the second flash heat exchanger 3. A heat transfer oil pipeline d104 is connected between the upper side of the second flash heat exchanger 3 and the lower side of the first flash heat exchanger 2. A heat transfer oil pipeline e105 is connected between the upper side of the first flash heat exchanger 2 and the bottom of the buffer intermediate vessel 1.

[0018] The buffer intermediate vessel 1 has a jacketed heating and insulation structure. The feed and discharge surfaces of the tubes in the first flash heat exchanger 2, the second flash heat exchanger 3, the third flash heat exchanger 4, and the fourth flash heat exchanger 5 are set at an angle, and the tubes on the discharge surface protrude from the top of the tube opening. The material pipelines a91, b92, c93, d94, and e95 have an inclined design, which is conducive to material flow. The flow direction of the heat transfer oil pipelines a101, b102, c103, d104, and e105 is opposite to the flow direction of the material pipelines a91, b92, c93, and d94. As the material temperature gradually rises, the heat transfer oil temperature gradually decreases, resulting in a lower overall reaction temperature and reducing the generation of side reactions.

[0019] The heat exchange tubes of the first flash heat exchanger 2 and the third flash heat exchanger 4 are 60% the length of the heat exchange tubes of the second flash heat exchanger 3 and the fourth flash heat exchanger 5. The outlet of the heat exchange tubes of the first flash heat exchanger 2, the second flash heat exchanger 3, the third flash heat exchanger 4, and the fourth flash heat exchanger 5 is 2-5 cm higher than the tube sheet plane.

[0020] The heating medium for the first flash heat exchanger 2, the second flash heat exchanger 3, the third flash heat exchanger 4, and the fourth flash heat exchanger 5 is 290℃ heat transfer oil.

[0021] The discharge ports of the first flash heat exchanger 2, the second flash heat exchanger 3, the third flash heat exchanger 4, and the fourth flash heat exchanger 5 are angled discharge ports, installed on the side and bottom of the discharge surface, and the first flash heat exchanger 2, the second flash heat exchanger 3, the third flash heat exchanger 4, and the fourth flash heat exchanger 5 are all non-uniform orifice tube heat exchangers.

[0022] The continuous crystallization method for trimellitic anhydride to anhydride includes the following steps: S1. Triphenyltrimethylamic acid and acetic acid, the intermediate products of the oxidation reaction of trimesin in the oxidation reactor, are pressed into the buffer intermediate reactor 1 under high pressure. The content of trimesin is 25-45% and the content of acetic acid is 55-75%. The material temperature is maintained at 200-210℃ and the pressure is ≤-0.08MPa.

[0023] S2. The material in the buffer intermediate vessel 1 flows into the first flash heat exchanger 2 from the bottom. The intermediate vessel lower expansion valve 7 of the buffer intermediate vessel 1 controls the material feed rate and adjusts the material flow into the bottom feed port of the first flash heat exchanger 1. The material enters from the bottom feed port of the first flash heat exchanger 2 and is heated by the tube shell. Under vacuum, acetic acid is flash-evaporated and removed at the outlet surface of the first flash heat exchanger 2, removing 85-95% of the acetic acid. During this process, the material rapidly heats up to 215-225℃. The pressure of the first flash heat exchanger 2 is ≤-0.08MPa. The feed surface and the outlet surface are set at an incline. The outlet of the heat exchange tube shell is installed 2-5 cm higher than the tube sheet plane to ensure that the material heating time is consistent and to reduce the possibility of material clogging the outlet. The outlet of the first flash heat exchanger 2 is an angled outlet installed on the side of the outlet surface facing the bottom, which is conducive to the material flowing to the outlet of the first flash heat exchanger 2 under the action of gravity.

[0024] S3. Under the action of gravity, the material enters the bottom inlet of the second flash heat exchanger 3 from the outlet of the first flash heat exchanger 2. After being further heated by the second flash heat exchanger 3, the residual small amount of acetic acid is further removed under vacuum and the material is initially dehydrated to form anhydride. Acetic acid removal ≥99% and anhydride formation reaction 5-10% occur. During this process, the material is rapidly heated to 230-240℃. The pressure of the second flash heat exchanger 3 is ≤-0.08MPa. The heat exchange tubes of the second flash heat exchanger 3 are 60% longer than those of the first flash heat exchanger 2, but the structure is the same.

[0025] S4. The material that has been initially dehydrated and formed anhydride further enters the third flash heat exchanger 4. In this heat exchanger, the reacting material is further dehydrated. Most of the trimellitic acid is dehydrated and reacted to form trimellitic anhydride, with an anhydride formation rate of 80-85%. During this process, the material is rapidly heated to 245-255℃. The pressure of the third flash heat exchanger 4 is ≤-0.08MPa. The structure of the third flash heat exchanger 4 is the same as that of the first flash heat exchanger 2.

[0026] S5. The material continues to enter the fourth flash heat exchanger 5. The remaining small amount of trimellitic acid undergoes further deep reaction. The material is basically dehydrated and forms anhydride. The anhydride formation reaction is 95-98%. During this process, the material temperature rises to 260-270℃. The pressure of the fourth flash heat exchanger 5 is ≤-0.08MPa. The structure of the fourth flash heat exchanger 5 is the same as that of the second flash heat exchanger 3.

[0027] S6. After the material has basically completed the reaction, it enters the light residue removal tower 6. During this process, the material is at 260-270℃ and the pressure is ≤-0.098MPa. The by-products of the incomplete oxidation reaction are removed, and crude trimellitic anhydride is collected in the bottom of the tower.

[0028] The heat transfer oil and material flow are set in a countercurrent direction. As the material temperature gradually rises, the heat transfer oil temperature gradually decreases, resulting in a lower overall reaction temperature and reducing the generation of side reactions. The material is continuously fed and heated during the removal of acetic acid and dehydration to anhydride. Compared with traditional intermittent heating, the energy consumption is lower and the system efficiency is higher. This allows trimellitic acid after the oxidation of trimellitene to continuously remove acetic acid and further dehydrate to anhydride, improving reaction efficiency while ensuring product quality and meeting production needs in a balanced manner.

[0029] Gravity transfer is used starting from the intermediate buffer reactor 1, without mechanical conveying or forced discharge by stirring. The reaction proceeds gradually between the intermediate buffer reactor 1 and the multi-stage flash heat exchanger. The reaction progress is regulated by the intermediate reactor lower expansion valve 7 and the heat transfer oil regulating valve 8, resulting in more stable quality of trimellitic anhydride.

[0030] Acetic acid and water released from the material are rapidly removed through the top pipeline under vacuum, reducing side reactions and reaction time, further improving efficiency, and completing the deacidification crystallization and dehydration to anhydride reaction in one go.

[0031] The foregoing has shown and described the basic principles and main features of the present invention, as well as its advantages. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for continuous crystallization of trimellitic anhydride into anhydride, characterized in that: The continuous crystallization method of trimellitic anhydride into anhydride is achieved through an anhydride crystallization system. The crystallization anhydride system includes a buffer intermediate vessel, a first flash heat exchanger, a second flash heat exchanger, a third flash heat exchanger, a fourth flash heat exchanger, and a light-weight removal tower, arranged in descending order of installation height. The method for continuous crystallization of trimellitic anhydride into anhydride includes the following steps: S1. Triphenyltrimethylamic acid and acetic acid, the intermediate products after the oxidation reaction of trimesin in the oxidation reactor, are pressed into a buffer intermediate reactor under high pressure. The content of trimesin is 25-45%, the content of acetic acid is 55-75%, the material temperature is 200-210℃, and the pressure is ≤-0.08MPa. S2. The material in the buffer intermediate vessel flows into the bottom inlet of the first flash heat exchanger from the bottom. The material enters the first flash heat exchanger from the bottom inlet and is heated by the tubes. Under vacuum, the acetic acid is flashed away at the outlet of the first flash heat exchanger, removing 85-95% of the acetic acid. During this process, the material is rapidly heated to 215-225℃. The pressure of the first flash heat exchanger is ≤-0.08MPa. S3. Under the action of gravity, the material enters the bottom inlet of the second flash heat exchanger from the outlet of the first flash heat exchanger. It is then further heated by the second flash heat exchanger. Under the action of vacuum, the residual small amount of acetic acid is further removed and the material is initially dehydrated to form anhydride. Acetic acid removal ≥99%, anhydride formation reaction 5-10%. During this process, the material is rapidly heated to 230-240℃, and the pressure of the second flash heat exchanger is ≤-0.08MPa. S4. The material that has been initially dehydrated and formed anhydride further enters the third flash heat exchanger. In this heat exchanger, the reacting material is further dehydrated. Most of the trimellitic acid is dehydrated and reacted to form trimellitic anhydride. The anhydride formation reaction is 80-85%. During this process, the material is rapidly heated to 245-255℃. The pressure of the third flash heat exchanger is ≤-0.08MPa. S5. The material continues to enter the fourth flash heat exchanger. The remaining small amount of trimellitic acid undergoes further deep reaction. The material completes dehydration to form anhydride. The anhydride formation reaction is 95-98%. During this process, the material temperature rises to 260-270℃. The pressure of the fourth flash heat exchanger is ≤-0.08MPa. S6. After the material completes the reaction, it enters the light-removal tower. During this process, the material is at 260-270℃ and the pressure is ≤-0.098MPa. The byproducts of the incomplete oxidation reaction are removed, and crude trimellitic anhydride is collected in the bottom of the tower.

2. The method for continuous crystallization of trimellitic anhydride into anhydride according to claim 1, characterized in that: The top of the buffer intermediate vessel is connected to a feed pipe and a negative pressure discharge pipe for water and acetic acid. The bottom of the buffer intermediate vessel is connected to a material pipeline a, which communicates with the bottom of the first flash heat exchanger. A lower valve for controlling the material flow rate is installed on material pipeline a. The top of the first flash heat exchanger is connected to the negative pressure discharge pipe for water and acetic acid. The upper outlet of the first flash heat exchanger is connected to the bottom of the second flash heat exchanger via material pipeline b. The top of the second flash heat exchanger is connected to the negative pressure discharge pipe for water and acetic acid. The upper outlet of the flash heat exchanger is connected to the bottom of the third flash heat exchanger via material pipeline c. The top of the third flash heat exchanger is connected to the negative pressure discharge pipelines for water and acetic acid. The upper outlet of the third flash heat exchanger is connected to the bottom of the fourth flash heat exchanger via material pipeline d. The top of the fourth flash heat exchanger is connected to the negative pressure discharge pipelines for water and acetic acid. The upper outlet of the fourth flash heat exchanger is connected to the lower side of the light component removal tower via material pipeline e. The top of the light component removal tower is connected to the light component negative pressure discharge pipeline. A heat transfer oil line a is provided on one side of the fourth flash heat exchanger. A heat transfer oil regulating valve for controlling the heating amount is installed on the heat transfer oil line a. A heat transfer oil line b is connected between the upper side of the fourth flash heat exchanger and the lower side of the third flash heat exchanger. A heat transfer oil line c is connected between the upper side of the third flash heat exchanger and the lower side of the second flash heat exchanger. A heat transfer oil line d is connected between the upper side of the second flash heat exchanger and the lower side of the first flash heat exchanger. A heat transfer oil line e is connected between the upper side of the first flash heat exchanger and the bottom of the buffer intermediate vessel.

3. The method for continuous crystallization of trimellitic anhydride into anhydride according to claim 2, characterized in that: The buffer intermediate vessel has a jacketed heating and insulation structure. The feed and discharge surfaces of the tubes in the first, second, third, and fourth flash heat exchangers are inclined, and the tubes on the discharge surface protrude beyond the top of the tube opening. The material pipelines a, b, c, d, and e have an inclined design. The flow direction of the heat transfer oil in the heat transfer oil pipelines a, b, c, d, and e is opposite to the flow direction of the material in the material pipelines a, b, c, and d.

4. The method for continuous crystallization of trimellitic anhydride into anhydride according to claim 1, characterized in that: The heat exchange tubes of the first and third flash heat exchangers are 60% the length of the heat exchange tubes of the second and fourth flash heat exchangers. The outlets of the heat exchange tubes of the first, second, third, and fourth flash heat exchangers are 2-5 cm higher than the tube sheet plane.

5. The method for continuous crystallization of trimellitic anhydride into anhydride according to claim 1, characterized in that: The heating medium for the first, second, third, and fourth flash heat exchangers is 290°C heat transfer oil.

6. The method for continuous crystallization of trimellitic anhydride into anhydride according to claim 1, characterized in that: The discharge ports of the first, second, third, and fourth flash heat exchangers are angled discharge ports, installed on the side and bottom of the discharge surface, and all of the first, second, third, and fourth flash heat exchangers are non-uniform perforated tube heat exchangers.

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