Method for recycling lactide in process of producing polylactic acid by polymerizing lactide
During the process of polylactic acid production in lactide polymerization, the separation and purification of lactide is used to separate and purify lactide using a combination technology of preheater and purification tower, which solves the problem of lactide recycling, improves product purity and recovery rate, and reduces the Energy consumption is of great significance.
Patent Information
- Application Number
- CN202411361062.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-09-27
- Publication Date
- 2025-01-03
AI Technical Summary
The prior art is difficult to effectively recycle and utilize lactide during the process of polylactic acid production of lactide polymerization, resulting in waste of raw materials and high energy consumption.
After heating with a preheater, the crude polylactic acid product is introduced into the delight tower, and the gas phase on the top of the tower enters the purification tower. The lactide is separated and purified through the tube-type heat exchange equipment and the liquid phase distributor, and the circulation cooler and tower are used. The heat exchanger of the kettle medium is carried out to achieve the recovery and purification of lactide.
The purity of the polymer is improved, so that the product purity is no less than 99.5%, and the lactide recovery rate is as high as 95%, which significantly reduces operating costs and meets the purity requirements of the lactide raw material of the polymerization process.
Smart Images
Figure CN119241908A_ABST
Abstract
Description
Technical Field
[0001] The invention belongs to the field of production of degradable biological products, and in particular relates to a method for recycling lactide in the process of generating polylactic acid by polymerization of lactide. Background Art
[0002] Polylactic acid (PLA) is a new type of biodegradable material. It is a polyester polymer obtained by polymerization of lactic acid as the main raw material. It has good biocompatibility and biodegradability, which makes it widely used in many fields. Its advantages include biodegradability. After use, it can be completely degraded by microorganisms in nature, and finally generate carbon dioxide and water, which does not pollute the environment; it has good mechanical and physical properties and is widely used in the medical field; it has good gloss and transparency, as well as good tensile strength and ductility; it does not release toxic gases when incinerated, and it is highly safe. The application areas of polylactic acid include packaging, placement, medical treatment, 3D printing and other fields, including packaging bags, disposable infusion equipment, surgical sutures, 3D printing materials, electronics and automotive accessories.
[0003] The crude product obtained by polymerizing lactide to generate polylactic acid contains some unreacted lactide components. The presence of lactide not only affects the quality of the polylactic acid product, but also causes a waste of raw materials. Therefore, it is very important to separate and recycle the lactide in the crude product and purify the lactide as a raw material for continued production and use. At present, the purification methods of lactide mainly include distillation and recrystallization technology. For example, CN100586921C, CN104710401B and CN102625802B use distillation technology to purify lactide, but the equipment investment and operating energy consumption of distillation technology are relatively high. At the same time, the temperature difference between the bottom of the distillation process and the top of the tower is large. For lactide that is relatively sensitive to temperature, distillation is not the best purification solution.
[0004] Therefore, it is of great significance to develop a separation and purification method that can meet the requirements of low energy consumption, small temperature fluctuation, and the purity of lactide raw materials required by the polymerization process. Summary of the invention
[0005] The purpose of the present invention is to provide a method for recycling lactide in the process of polymerizing lactide to produce polylactic acid, in view of the problem that the prior art cannot well meet the problem of recycling raw materials under polymerization production process conditions.
[0006] Specifically, the present invention provides a method for recycling lactide in the process of polymerizing lactide to produce polylactic acid:
[0007] (1) The crude polylactic acid product obtained by polymerization of lactide is heated by preheater E-01 and then introduced into lightness removal tower T-01 for treatment. The crude product obtained at the bottom of the tower after treatment is taken out from the bottom of the tower for further processing, and the gas phase obtained at the top of the tower enters purification tower T-02;
[0008] (2) The purification tower T-02 is composed of three parts, the upper part is a shell-and-tube heat exchanger, the middle part is a liquid phase distributor, and the lower part is a lactide collecting tank. The gas phase from the top of the lightness removal tower T-01 enters from the middle part of the purification tower T-02 and flows upward through the liquid phase distributor to make convection contact with the lactide. Part of the gas phase is converted into liquid phase and enters the lactide collecting tank. It is cooled by the circulating pump P-02 at the bottom of the tower through the circulating cooler E-02 and returns to the middle part of the purification tower T-02 to dissolve and wash the gas phase. The gas phase that is not absorbed continues to flow to the upper part of the purification tower T-02 to the shell-and-tube heat exchanger. The shell-and-tube heat exchanger The thermal equipment solidifies the lactide in the gas phase on the outer wall of the heat exchange tube. After the solidification treatment is completed, it enters the melting stage, cuts off the cold source heat exchange medium, and uses the tower bottom material to heat the heat exchange medium to melt the solidified lactide. The liquid lactide flows to the tower bottom of the purification tower T-02, and is sent to a part of the polymerization reaction front end lactide raw material tank through the circulation pump P-02 to continue the polymerization reaction, and a part flows to the circulation cooler E-02; the gas phase enters the circulation treatment tower T-03 from the top of the purification tower T-02 for treatment, and then enters the vacuum unit C-01 from the top of the circulation treatment tower T-03, and is finally discharged safely.
[0009] Wherein, in step (1), the lactide content in the crude polylactic acid product does not exceed 20%, and the temperature is 150°C to 200°C.
[0010] In step (1), the crude polylactic acid product is heated to 210-235° C. in a preheater (E-01).
[0011] Preferably, the operating conditions of the lightness removal tower are: operating pressure of -0.95 bar (G) to -1.0 bar (G), and operating temperature of 50-120°C.
[0012] The shell-and-tube heat exchange equipment is a shell-and-tube falling film crystallizer, wherein the operating conditions of the shell-and-tube falling film crystallizer in the solidification and sweating stage are: the operating conditions in the solidification stage are: 60-75°C; the operating conditions in the melting stage are: 110-130°C.
[0013] The cooling rate in the solidification stage is 0.2-0.8°C / min, and the heating rate in the melting stage is 0.5-2°C / min.
[0014] The circulating cooler (E-02) uses external cold heat exchange medium to circulate and cool the lactide solution in the lactide collecting tank, and the temperature of the cold heat exchange medium is 55-75°C.
[0015] Specifically, the circulating cooler (E-02) reduces the temperature of the lactide solution to less than 120°C.
[0016] Preferably, in the melting stage, the high-temperature lactide in the lactide collecting tank is used as a heat source in the melting stage at the top of the purification tower (T-02) to heat the heat exchange medium, and the heated heat exchange medium is used to melt the solidified lactide.
[0017] The heat exchange medium meets the temperature operating range of 50-140°C. The heat exchange medium is introduced into the tube side of the shell-and-tube heat exchange equipment, and the shell side is the material (lactide), which is solidified on the tube from the steam state. During the solidification stage, the heat exchange medium temperature is controlled to be 50-75°C, and during the melting stage, the heat exchange medium temperature is controlled to be 115-140°C.
[0018] The invention has the following beneficial effects: the crude product of the lactide polymerization reaction utilizes the residual heat of the reaction and the heating of the preheater to separate the light component lactide under a slightly negative pressure condition, which not only improves the purity of the polymer, so that the product purity is not less than 99.5% (far exceeding the 99.0% of industrial-grade polylactic acid), but also the lactide recovery rate is as high as 95%, and the recovered lactide can be directly used as a polymerization raw material to continue to participate in the reaction. In this process, the tower bottom medium is used as a high-temperature heat source for heat exchange, and the solid lactide at the top of the tower is melted, reducing the consumption of additional energy. Therefore, the invention solves the problem of lactide recycling in the polymerization process, and significantly reduces the operating cost, which is of great significance to industrial production. BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Figure 1 The process flow chart of the implementation scheme of Example 1, wherein E-01 is a crude product preheater, T-01 is a crude product light removal tower, P-01 is a lactide delivery pump, T-02 is a lactide purification tower, E-02 is a lactide circulation cooler, P-02 is a lactide circulation pump, T-03 is a circulation treatment tower, P-03 is a waste liquid delivery pump, and C-01 is a vacuum system;
[0020] Figure 2 This is a process flow chart of the implementation scheme of Example 3. DETAILED DESCRIPTION
[0021] The technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods, but the protection scope of the present invention is not limited to the embodiments described below.
[0022] Example 1
[0023] The crude product flow rate is 1000kg / h, the lactide content is 5.5%, the temperature is 180℃, it is heated to 220℃ by a preheater, and sent to a light-removal tower, the operating temperature is 215℃, the operating pressure is -0.996bar(G), in the light-removal tower, the polymer flows to the tower bottom, wherein the lactide content is about 0.25%, and the polymer purity is as high as 99.5%, and then the product is sent to the downstream processing section by a product delivery pump, wherein the lactide component enters the purification tower in a gaseous state through the top of the tower, and the gas phase flows upward to convectively contact with the liquid lactide (in the first operation, pure lactide is directly injected into the collecting tank, and the liquid lactide obtained in the tower bottom is subsequently used) and part of it is condensed. The gas phase lactide is converted into liquid phase, cooled to 120℃ by the lactide circulation pump through the cooler, and returned to the middle of the tower to dissolve and wash the gas phase. The unabsorbed air and lactide mixed gas continue to flow to the upper part of the tower to the tubular falling film crystallization section. The tube pass is fed with a 60℃ heat exchange medium. The lactide in the mixed gas solidifies on the outer wall of the heat exchange tube. The heat exchange medium is controlled by a thermometer to achieve a solidification cooling rate of about 0.6℃ / min. After the solidification treatment is completed, it enters the melting stage, cuts off the cold source heat exchange medium, and uses the 120℃ material in the tower bottom to heat the heat exchange medium to 110℃. The 110℃ heat exchange medium melts the solidified lactide, and the liquid lactide flows to the tower bottom. A very small amount of lactide and air are discharged from the top of the tower and enter the middle of the circulation treatment tower. The heating rate in the melting stage is controlled at 1℃ / min. The gas phase after washing with alkali solution is vented, and the tower bottom liquid phase circulates and washes the gas in the tower through the circulation pump. The recycling process of this system not only improves the purity of the polymer to 99.5% (far exceeding the 99.0% of industrial-grade polylactic acid), but also achieves a lactide recovery rate of 95%. The recovered lactide can be directly used as a raw material for polymerization after simple treatment. In this process, the tower bottom medium is used as a high-temperature heat source for heat exchange to melt the solid lactide at the top of the tower, reducing the consumption of additional energy.
[0024] Example 2
[0025] The operation steps of this embodiment are the same as those of Example 1, except that the components of the raw product are changed for processing. The details are as follows: the raw product flow rate is 1000 kg / h, the lactide content is 8.5%, the temperature is 180° C., and the product is heated to 220° C. in a preheater and fed into a light-removal tower. The subsequent steps are the same as those of Example 1. The final light-removal tower polymer has a lactide content of 0.3%, and a product purity of 99.5% (far exceeding 99.0% of industrial-grade polylactic acid), and a lactide recovery rate of up to 95.5%. The recovered lactide can be directly used as a raw material for the polymerization reaction and continue to participate in the reaction.
[0026] Example 3
[0027] The operation steps of this embodiment are the same as those of embodiment 1. The crude product flow rate is 1000 kg / h, the lactide content is 5.5%, and the temperature is 180°C. The difference is that a direct heat source is used in the melting stage of the purification tower. The specific process is detailed in the attached Figure 2 The treatment results are the same as those in Example 1. The final polymer lactide content in the light-removal tower is 0.25%, the polymer purity is as high as 99.5%, the product purity is 99.5% (far exceeding the 99.0% of industrial-grade polylactic acid), and the lactide recovery rate is as high as 95%. To achieve the same treatment effect, 5688 kJ / h of high-temperature heat exchange medium is required.
[0028] By comparing Example 1 with Example 2, it can be found that the set of devices can be used to better handle the fluctuation of the upstream product components to ensure the purpose of product quality and high recovery rate. By comparing Example 1 with Example 3, it can be found that the use of the purification tower kettle medium preheating as a heat source in the melting stage saves the use of 5688kJ / h of energy. The method of the present invention has a certain operational flexibility for the treatment of the crude product, and the effect of separation and purification can be controlled by appropriately changing the preheating temperature, the solidification melting temperature and the operating time. The recovery rate and the purity of the product can be controlled within a certain range.
[0029] The above is a description of a lactide recycling system and device in a polylactic acid synthesis process provided by the present invention. For professional and technical personnel in this industry, according to the ideas of the embodiments of the present invention, there may be changes or improvements in the specific implementation methods and application scopes. Therefore, the content of this embodiment should not be regarded as a limitation of the present invention.
Claims
1. A method for recycling lactide in the process of polymerizing lactide to produce polylactic acid, characterized in that: The steps include: (1) The crude polylactic acid product obtained by polymerization of lactide is heated by a preheater (E-01) and then introduced into a light removal tower (T-01) for treatment. The crude product obtained at the bottom of the tower is taken out from the bottom of the tower for further processing, and the gas phase obtained at the top of the tower enters a purification tower (T-02); (2) The purification tower (T-02) is composed of three parts, the upper part is a shell-and-tube heat exchanger, the middle part is a liquid phase distributor, and the lower part is a lactide collecting tank. The gas phase from the top of the light removal tower (T-01) enters the middle part of the purification tower (T-02) and flows upward through the liquid phase distributor to make convection contact with the lactide collecting tank. Part of the gas phase is converted into liquid phase and enters the lactide collecting tank. It is cooled by the circulating pump (P-02) at the bottom of the tower through the circulating cooler (E-02) and returns to the middle part of the purification tower (T-02) to dissolve and wash the gas phase. The gas phase that is not absorbed continues to flow to the upper part of the purification tower (T-02) to the shell-and-tube heat exchanger. The shell-and-tube heat exchanger The thermal equipment solidifies the lactide in the gas phase on the outer wall of the heat exchange tube. After the solidification process is completed, it enters the melting stage, cuts off the cold source heat exchange medium, and uses the tower bottom material to heat the heat exchange medium to melt the solidified lactide. The liquid lactide flows to the tower bottom of the purification tower (T-02), and is sent to a part of the polymerization reaction front end lactide raw material tank through the circulation pump (P-02) to continue the polymerization reaction, and a part flows to the circulation cooler (E-02); the gas phase enters the circulation treatment tower (T-03) from the top of the purification tower (T-02) and is treated. After that, it enters the vacuum unit (C-01) from the top of the circulation treatment tower (T-03) and is finally discharged safely.
2. The lactide recovery and utilization method according to claim 1, characterized in that: In step (1), the lactide content in the crude polylactic acid product does not exceed 20%, and the temperature is 150°C to 200°C.
3. The lactide recycling method according to claim 1, characterized in that: In step (1), the crude polylactic acid product is heated to 210-235°C in a preheater (E-01).
4. The method for recycling lactide according to claim 1, characterized in that: The operating conditions of the lightness removal tower are: operating pressure is -0.95bar (G) ~ -1.0 bar (G), and the operating temperature is 50-120℃.
5. The method for recycling lactide according to claim 1, characterized in that: The shell-and-tube heat exchange equipment is a shell-and-tube falling film crystallizer, wherein the operating conditions of the shell-and-tube falling film crystallizer in the solidification and sweating stage are: the operating conditions in the solidification stage are: 60-75°C; the operating conditions in the melting stage are: 110~130°C.
6. The method for recycling lactide according to claim 4, characterized in that: The cooling rate in the solidification stage is 0.2~0.8℃ / min, and the heating rate in the melting stage is 0.5-2℃ / min.
7. The lactide recovery and utilization method according to claim 1, characterized in that: The circulating cooler (E-02) uses a heat exchange medium to circulate and cool the lactide solution in the lactide collecting tank, and the temperature of the heat exchange medium is 55-75°C.
8. The method for recycling lactide according to claim 7, characterized in that: The circulating cooler (E-02) reduces the temperature of the lactide solution to less than 120°C.
9. The method for recycling lactide according to claim 1, characterized in that: In the melting stage, the high-temperature lactide in the lactide collecting tank is used as a heat source in the melting stage at the top of the purification tower (T-02) to heat the heat exchange medium, and the heated heat exchange medium is used to melt the solidified lactide.
10. The method for recycling lactide according to claim 7, characterized in that: The heat exchange medium meets the temperature operating range of 50-140°C. The heat exchange medium is introduced into the tube of the shell-and-tube heat exchange equipment. In the solidification stage, the heat exchange medium temperature is controlled to be 50-75°C. In the melting stage, the heat exchange medium temperature is controlled to be 115-140°C.