Lactide melt crystallization process system and method
By using a multi-stage melting and stepwise crystallization process to control the temperature difference and the temperature of the heat transfer medium, the problems of cumbersome and energy-intensive existing lactide purification processes have been solved, achieving efficient and low-cost lactide purification that meets the requirements of polylactic acid production.
Patent Information
- Application Number
- CN202511508158.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-10-22
- Publication Date
- 2026-02-13
AI Technical Summary
Existing lactide purification processes are cumbersome, resulting in significant lactide loss, making it difficult to achieve the required purity for polymerization. Furthermore, solvent recovery is challenging, leading to high production costs and poor performance of polylactic acid.
A multi-stage melting and stepwise crystallization process is adopted. By controlling the temperature difference and the temperature of the heat transfer medium, the crystallization and sweating process of lactide is realized, forming a layered falling film layer and separating impurities. The operation is optimized by utilizing a self-circulation system, reducing equipment and energy consumption.
It improves the production efficiency and purity of lactide, reduces energy consumption and operating costs, simplifies the operation process, and obtains high-purity lactide for polylactic acid production.
Smart Images
Figure CN121513487A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The application belongs to the technical field of lactide crystallization production, and particularly relates to a lactide melt crystallization process system and method. BACKGROUND
[0002] With the enhancement of people's environmental protection consciousness, green chemical new materials are more and more concerned by the society. As a kind of biodegradable bio-based polymer material, polylactic acid (PLA) is outstanding among many environmental protection materials due to its excellent physical and chemical properties. There are two routes for the synthesis of polylactic acid: one is direct polymerization method using lactic acid as raw material, and the other is to obtain low molecular weight polylactic acid by lactic acid polycondensation, to obtain lactide by depolymerization of the polycondensate, and to obtain polylactic acid by ring-opening polymerization of the purified lactide. So far, the polylactic acid synthesized by direct polycondensation method has low molecular weight, resulting in low polymer strength and easy decomposition. For a long time, people have paid more attention to the route of ring-opening polymerization of lactide. As an intermediate product of lactic acid for synthesizing polylactic acid, the production cost and purity of L-lactide directly affect the cost and performance of polylactic acid product. Therefore, how to obtain high-purity lactide in an economic and reasonable way is a key technical means in the production process of polylactic acid.
[0003] At present, the common processes for purifying lactide include solvent recrystallization, rectification, extraction and water extraction. These processes are relatively complicated, lactide is greatly lost, the purity of lactide is difficult to meet the requirements of polymerization, a large amount of solvent is consumed, and the solvent is difficult to recover. Therefore, it is necessary to improve the lactide separation process, extract lactide according to optical properties, and thus obtain polylactic acid products with good mechanical properties and degradation performance. SUMMARY
[0004] In view of the deficiencies of the prior art, the application provides a lactide melt crystallization process system and method, which can meet the requirements of high-purity lactide separation and purification for polylactic acid production through a multi-stage melt step crystallization process.
[0005] The specific technical scheme of the application is as follows: A lactide melt crystallization process system, comprising a melt crystallizer, a heat exchanger, an intermediate storage tank, a mother liquor tank, a sweat tank and a finished product tank, a liquid inlet end and a liquid outlet end of the melt crystallizer are connected with the mother liquor tank, the sweat tank and the finished product tank through pipelines respectively, a shell side heat exchange inlet of the melt crystallizer is connected with a tube side outlet of the heat exchanger through a pipeline, a shell side heat exchange outlet of the melt crystallizer is connected with an inlet of the intermediate storage tank through a pipeline, a bottom outlet of the intermediate storage tank is connected with a tube side inlet of the heat exchanger, a shell side inlet of the heat exchanger is communicated with a cold and hot medium pipeline, and a shell side outlet of the heat exchanger is communicated with a cold and hot medium pipeline.
[0006] The mother liquor tank is also connected with a mother liquor pump, the sweat tank is also connected with a sweat pump, the finished product tank is also connected with a finished product pump, and a circulating pump is arranged between the bottom outlet of the intermediate storage tank and the tube side inlet of the heat exchanger.
[0007] The melt crystallizer is provided with a feed liquid overflow port, and the feed liquid overflow port is connected with the finished product tank through a pipeline.
[0008] The melt crystallizer is also provided with a heat transfer medium overflow port, and the heat transfer medium overflow port is connected with the intermediate storage tank through a pipeline.
[0009] The melt crystallizer, the heat exchanger, the intermediate storage tank and the circulating pump constitute a self-circulation system, and the heat transfer medium in the self-circulation system is heated or cooled through the heat exchanger.
[0010] A lactide melt crystallization process method based on the above system, comprising the following steps: Step one, crystallization: Firstly, the temperature of the whole melt crystallizer and pipeline is maintained at 98-110 DEG C by controlling the heat transfer medium, the liquid phase lactide crude product is filled into the finished product tank, when the liquid level of the finished product tank reaches the predetermined value, the filling is completed, the finished product pump is started, the raw material is sent into the top of the melt crystallizer by the pump, and is distributed into each crystallization pipe by the distribution device, the cooling medium is cooled on the outer wall of the crystallization pipe, and the temperature difference between the material and the heat transfer medium is accurately controlled to be 5-10 DEG C, with the cooling, the lactide forms a laminar falling film layer in the crystallization pipe, and the uncrystallized material is discharged into the mother liquor tank for storage, when the liquid level in the mother liquor tank is reduced to 1 / 4-1 / 5 of the initial amount, the crystallization process is completed. Step two, sweating: After the crystallization process is completed, the temperature of the pipe outer heat transfer fluid is increased, the sweating temperature is controlled to be 55-95 DEG C, the impurities in the crystal layer are first melted due to their low melting point, and the sweating liquid is discharged into the sweating tank. When the sweat is 5%-10% of the initial amount, the sweating process is completed. Step three, all melting: after the sweating process is completed, the temperature of the heat transfer fluid is further increased, and the temperature is controlled to be 98-110 DEG C, so that the lactide crystals attached to the inner wall are all melted and discharged into the product tank, and whether the above operation is repeated is determined according to the product pure product.
[0011] The present application has the following advantages: 1. The present application provides a lactide melt crystallization process system and method, which is simple to operate, controllable for the crystallization and sweating processes, uniform and stable for the crystallization and sweating processes, greatly improves the production yield and production efficiency, and optimizes the operation environment.
[0012] 2. The system is compact, and does not need filtration because of non-suspension crystallization. Moreover, there is almost no running equipment. Therefore, the operation is reliable, the maintenance is convenient, and the investment and operation cost are low.
[0013] 3. The system has almost no transmission stirring device, and the vaporization heat of the general system is 2-3 times of the melting heat. In addition, the high reflux ratio and heat loss of rectification are considered, so that the energy consumption of the crystallization technology is only 10-30% of that of the conventional rectification separation method, and energy consumption is saved. BRIEF DESCRIPTION OF DRAWINGS
[0014] Figure 1 The figure is a schematic diagram of the lactide melt crystallization process system.
[0015] In the figure, 1 is a melt crystallizer, 2 is a heat exchanger, 3 is an intermediate storage tank, 4 is a mother liquor tank, 5 is a sweat tank, 6 is a product tank, 7 is a raw material pump, 8 is a mother liquor pump, 9 is a sweat pump, 10 is a product pump, and 11 is a circulating pump. DETAILED DESCRIPTION
[0016] The application will be described in detail below in combination with specific embodiments.
[0017] The application discloses a lactide melt crystallization process system, which is based on the following equipment: Referring to Figure 1 , the system comprises a melt crystallizer 1, a heat exchanger 2, an intermediate storage tank 3, a mother liquor tank 4, a sweat tank 5, and a product tank 6. The melt crystallizer comprises a feed liquid inlet, a feed liquid outlet, a shell side heat exchange inlet, a shell side heat exchange outlet, a feed liquid overflow port, and a heat transfer medium overflow port.
[0018] The feed liquid inlet and the feed liquid outlet are connected to the mother liquor tank 4 and the sweat tank 5 respectively through pipelines, the shell side heat exchange inlet is connected to the outlet of the tube side of the heat exchanger through a pipeline, the shell side heat exchange outlet is connected to the inlet of the intermediate storage tank 3 through a pipeline, the bottom outlet of the intermediate storage tank 3 is connected to the inlet of the tube side of the heat exchanger, and a circulating pump 11 is arranged between the two, the shell side inlet end of the heat exchanger 2 is connected to a cold and hot medium pipeline, and the shell side outlet end of the heat exchanger 2 is connected to a cold and hot medium pipeline.
[0019] The feed liquid overflow port is connected to the product tank 6 through a pipeline, and the heat transfer medium overflow port is connected to the intermediate storage tank 3 through a pipeline.
[0020] The mother liquor tank 4 is further connected to a mother liquor pump 8, the sweat tank 5 is further connected to a sweat pump 9, and the product tank 6 is further connected to a product pump 10.
[0021] Based on the above system, the application provides a lactide melt crystallization process method, and the specific steps are as follows: First, the temperature of the whole melt crystallizer and the pipeline is maintained at 98-110°C by controlling the heat exchange medium, and the product tank is filled with liquid crude lactide. When the liquid level of the product tank reaches the predetermined value, the charging is completed. The product pump is started, and the raw material is fed into the top of the crystallizer by the pump and distributed into each crystallization tube by the distribution device. The cooling medium is cooled on the outer wall of the crystallization tube, and the temperature difference between the material and the heat exchange medium is accurately controlled at 5-10°C. As the cooling proceeds, the lactide forms a laminar falling film layer in the crystallization tube, and the uncrystallized material is discharged into the mother liquor tank for storage. When the liquid level in the mother liquor tank is reduced to 1 / 4-1 / 5 of the initial amount, the crystallization process is completed.
[0022] After the completion of the crystallization process, the temperature of the heat transfer fluid outside the tube is increased, and the sweating temperature is controlled at 55-95°C. The impurities in the crystal layer melt first because of their lower melting points, and the sweat is discharged into the sweating tank. When the sweat is 5-10% of the initial amount, the sweating process is completed. After the completion of the sweating process, the temperature of the heat transfer fluid is further increased, and the temperature is controlled at 98-110°C to melt the lactide crystals attached to the inner wall and discharge them into the product tank. Thus, a refining cycle is completed, and high-purity lactide can be obtained by repeating the above crystallization and refining process.
[0023] Example 1 The low-purity lactide raw material used in this example contains 88% lactide, in addition to 0.4% moisture, 4.2% meso-lactide, 3.2% lactic acid, and 4.2% other impurities (lactic acid oligomers, etc.). First, the temperature of the whole melt crystallizer and the pipeline is maintained at 110°C by controlling the heat exchange medium, and the product tank is filled with liquid crude lactide. When the liquid level of the product tank reaches the predetermined value, the charging is completed. The product pump is started, and the raw material is fed into the top of the melt crystallizer by the pump and distributed into each crystallization tube by the distribution device. The cooling medium is cooled on the outer wall of the crystallization tube, and the temperature difference between the material and the heat exchange medium is accurately controlled at 8°C. As the cooling proceeds, the lactide forms a laminar falling film layer in the crystallization tube, and the uncrystallized material is discharged into the mother liquor tank for storage. When the liquid level in the mother liquor tank is reduced to 1 / 5 of the initial amount, the crystallization process is completed. After the completion of the crystallization process, the temperature of the heat transfer fluid outside the tube is increased, and the sweating temperature is controlled at 90°C. The impurities in the crystal layer melt first because of their lower melting points, and the sweat is discharged into the sweating tank. When the sweat is 10% of the initial amount, the sweating process is completed. After the completion of the sweating process, the temperature of the heat transfer fluid is further increased, and the temperature is controlled at 105°C to melt the lactide crystals attached to the inner wall and discharge them into the product tank. Thus, a refining cycle is completed, and high-purity lactide can be obtained by repeating the above crystallization and refining process. The high-purity lactide content is 99.91%.
[0024] Example 2 The low purity lactide used in this example has a lactide content of 91%, in addition to 0.4% moisture, 3.2% meso-lactide, 1.8% lactic acid, and 3.6% other impurities (lactic acid oligomers, etc.). The entire melt crystallizer and piping are first maintained at a temperature of 105°C by controlling the heat exchange medium, and the product tank is filled with liquid lactide crude. When the liquid level in the product tank reaches the predetermined value, the charging is complete. The product pump is started, the raw material is fed into the top of the melt crystallizer by the pump, and is distributed into each crystallization tube by the distribution device. The cooling medium is cooled on the outer wall of the crystallization tube, and the temperature difference between the material and the heat exchange medium is accurately controlled at 10°C. As the cooling proceeds, the lactide forms a laminar falling film in the crystallization tube, and the uncrystallized material is discharged into the mother liquor tank for storage. When the liquid level in the mother liquor tank is reduced to 1 / 5 of the initial amount, the crystallization process is complete. After the crystallization process is complete, the temperature of the heat transfer fluid outside the tube is increased, and the sweating temperature is controlled at 95°C. The impurities in the crystal layer melt first because of their lower melting points, and the sweat is discharged into the sweating tank. When the sweat is 10% of the initial amount, the sweating process is complete. After the sweating process is complete, the temperature of the heat transfer fluid is further increased, and is controlled at 110°C, so that the lactide crystals adhering to the inner wall are completely melted and are discharged into the product tank. In this way, a purification cycle is completed, and by repeating the above crystallization and purification process, high purity lactide can be obtained. The high purity lactide has a content of 99.90%.
Claims
1. A lactide melt crystallization process system, characterized in that: The system includes a melt crystallizer (1), a heat exchanger (2), an intermediate storage tank (3), a mother liquor tank (4), a sweat tank (5), and a finished product tank (6). The inlet and outlet of the melt crystallizer (1) are connected to the mother liquor tank (4), the sweat tank (5), and the finished product tank (6) respectively via pipelines. The shell-side heat exchange inlet of the melt crystallizer (1) is connected to the tube-side outlet of the heat exchanger (2) via a pipeline. The shell-side heat exchange outlet of the melt crystallizer (1) is connected to the inlet of the intermediate storage tank (3) via a pipeline. The bottom outlet of the intermediate storage tank (3) is connected to the tube-side inlet of the heat exchanger (2). The shell-side inlet of the heat exchanger (2) is connected to a hot and cold medium pipeline, and the shell-side outlet of the heat exchanger (2) is connected to a hot and cold medium pipeline.
2. The lactide melt crystallization process system according to claim 1, characterized in that: The mother liquor tank (4) is also connected to a mother liquor pump (8), the sweat tank (5) is also connected to a sweat pump (9), the finished product tank (6) is also connected to a finished product pump (10), and a circulation pump (11) is provided between the bottom outlet of the intermediate storage tank (3) and the tube inlet of the heat exchanger (2).
3. The lactide melt crystallization process system according to claim 2, characterized in that: The melt crystallizer (1) is provided with a liquid overflow port, and the liquid overflow port is connected to the finished product tank (6) through a pipeline.
4. The lactide melt crystallization process system according to claim 3, characterized in that: The melt crystallizer (1) is also provided with a heat transfer medium overflow port, and the heat transfer medium overflow port is connected to the intermediate storage tank (3) through a pipeline.
5. The lactide melt crystallization process system according to claim 4, characterized in that: The melt crystallizer (1), heat exchanger (2), intermediate storage tank (3) and circulation pump (11) form a self-circulating system. The heat exchange medium in the self-circulating system is heated or cooled by the heat exchanger (2).
6. A method for melt crystallization of lactide based on the system described in any one of claims 1-5, characterized in that, Includes the following steps: Step 1: Crystallization First, the temperature of the entire melt crystallizer and pipeline is maintained at 98℃-110℃ by controlling the heat exchange medium. Liquid phase crude lactide is filled into the finished product tank. When the liquid level in the finished product tank reaches the predetermined value, the loading is completed. The finished product pump is started, and the pump sends the raw material to the top of the melt crystallizer. The distribution device distributes it into each crystallization tube. The cooling medium cools the outer wall of the crystallization tube. The temperature difference between the material and the heat exchange medium is precisely controlled at 5℃-10℃. As cooling proceeds, lactide forms a layered falling film layer in the crystallization tube, and the uncrystallized material is discharged into the mother liquor tank for storage. When the liquid level in the mother liquor tank drops to 1 / 4 to 1 / 5 of the initial amount, the crystallization process ends. Step 2, Inducing Sweating: After the crystallization process is completed, the temperature of the heat transfer fluid outside the pipe is increased, and the sweating temperature is controlled at 55℃-95℃. The impurities in the crystal layer melt first because of their low melting point, and the sweating liquid is discharged into the sweating tank. When the sweating liquid is 5% to 10% of the initial amount, the sweating process ends. Step 3: Complete melting: After the sweating process is completed, the temperature of the heat transfer fluid is further increased to 98℃-110℃, so that all the lactide crystals attached to the inner wall are melted and discharged into the product tank. Whether to repeat the above operation depends on the purity of the product.