A process system for continuously and efficiently producing polymeric grade lactide

Abstract

The application provides a process system for continuously and efficiently producing polymerization-grade lactide, and the specific production steps of the polymerization-grade lactide are as follows: S1: pure lactic acid and stannous chloride are sent into a lactic acid dehydration reaction system to continuously dehydrate, so as to obtain lactic acid oligomers; S2: the lactic acid oligomers are continuously sent into a depolymerization and purification reaction system, a part of the generated crude lactide is cooled and then reflows into a purification tower, and another part is extracted into a crude product storage tank; light components are collected through a light component condenser on the top of the purification tower and then discharged through a reflux ratio controller; heavy components are refluxed into the bottom of the purification tower, and the generated residues are discharged through a residue discharge port on the side of a circulating pump; and S3: the crude lactide is sent into a screw filter through a scraper crystallizer, so as to obtain polymerization-grade lactide and mother liquor. According to the application, the optical purity of the lactide is more than 99%, the residence time of the intermediate material lactide is shortened in the whole production process, the occurrence of side reactions is reduced, and the yield of the lactide is greatly improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of degradable material preparation, and particularly relates to a process system for continuously and efficiently producing polymeric-grade lactide. BACKGROUND

[0002] New materials are the basis of modern scientific and technological development, and degradable plastics are emerging new plastic materials. Polylactic acid (PLA) is a kind of thermoplastic polymer with high gloss and high modulus, and can be completely biodegraded. Polylactic acid has good physical and mechanical properties, and can replace traditional general-purpose materials to some extent as raw materials for degradable products. There are two methods for preparing polylactic acid, namely, lactide ring-opening polymerization method (also known as two-step method) and direct polycondensation method (also known as one-step method). The two-step method is the most widely used method for preparing polylactic acid at present. In the two-step method, lactic acid is first dehydrated to form an oligomer, then depolymerized to form lactide, and then ring-opening polymerization is performed to obtain PLA. The key technologies are the purification of lactide, the control of energy consumption and the improvement of production capacity. Lactide, as an intermediate product of degradable materials, has high thermal sensitivity. On the one hand, there is a side reaction of isomerization between the three isomers L-lactide, Meso-lactide and D-lactide due to the optical activity. On the other hand, lactide itself has a tendency to polymerize, especially in the presence of a large amount of acidic substances in the crude product, which can easily polymerize at high temperatures to form low molecular weight lactic acid oligomers, greatly reducing the yield and increasing the difficulty of purification.

[0003] The purification of lactide is the most critical step in the entire two-step process. Only lactide with high purity can be used to synthesize PLA with high molecular weight and good physical properties. The two-step method involves a purification step of lactide, and the purification of lactide is usually carried out by multi-stage rectification, such as patent CN201010180379.0, which uses a three-stage rectification process to purify lactide. However, the repeated heating and cooling process results in high energy consumption and the production of by-products during the multi-stage high-temperature heating process, which affects the yield of the product. For example, patent CN202110070644.8 uses two-stage rectification (first remove light components such as water and free acid, and then remove heavy components such as oligomers) to coarsely purify lactide, and then performs static melt crystallization or solvent recrystallization for further purification. The process is complex, energy-consuming, and has side reactions, and cannot achieve continuous production. Patent CN115010696A provides a method for continuously preparing and purifying lactide, but the method uses fractional distillation and crystallization operation, and the intermittent operation of the crystallization temperature-raising-temperature-maintaining process results in a low yield of only 75%, which is difficult to meet the needs of industrial production.

[0004] Therefore, there is an urgent need for a continuous, efficient, low-energy, and high-purity production process of lactide, which can produce high-molecular-weight and chemically controllable, mechanically good PLA, while reducing the difficulty of the process technology and the production cost. SUMMARY

[0005] The technical problem to be solved is to provide a continuous and efficient production process of polymer-grade lactide according to the characteristics of high heat sensitivity of lactide, which can reduce energy consumption, shorten residence time, reduce the occurrence of side reactions, improve reaction conversion rate, and continuously obtain lactide with optical purity of more than 99%, thereby meeting the technical requirements of polymer-grade lactide.

[0006] The technical scheme is a continuous and efficient production system of polymer-grade lactide, which comprises:

[0007] A lactic acid dehydration reaction system comprising a lactic acid dehydration reactor 1 and a metering pump 2, which is used to generate lactic acid oligomers;

[0008] A depolymerization and purification reaction system comprising a falling film reactor 3, a purification tower 4, a circulating pump 5, a product condenser 6, a light component condenser 7, a reflux ratio controller 8, and a vacuum system 12, which is used to decompose the raw material into gaseous crude lactide, light components, heavy components, and residues;

[0009] A polymer-grade lactide separation system comprising a crude product storage tank 9, a scraper crystallizer 10, and a screw filter 11, which is used to separate and purify the polymer-grade lactide and the mother liquor.

[0010] Preferably, the lactic acid dehydration reaction system comprises a lactic acid dehydration reactor and a metering pump, and the lactic acid dehydration reactor 1 is connected to the metering pump 2.

[0011] Preferably, the depolymerization and purification reaction system comprises a falling film reactor 3, a purification tower 4, a circulating pump 5, a product condenser 6, a light component condenser 7, a reflux ratio controller 8, and a vacuum system 12, the metering pump 2 in the lactic acid dehydration reaction system is connected to the purification tower 4, the bottom of the purification tower 4 is connected to the circulating pump 5 for continuous feeding, the circulating pump 5 is connected to the falling film reactor 3, one side of the circulating pump 5 is connected to a depolymerization residue discharge port, the falling film reactor 3 is connected to the purification tower 4, the top of the purification tower 4 is connected to the light component condenser 7, the light component condenser 7 is connected to the reflux ratio controller 8 and the vacuum system 12, one side of the reflux ratio controller 8 is connected to the purification tower 4, and the other side is used to discharge the light component substances collected by the light component condenser 7, and the packing section of the purification tower 4 is connected to the product condenser 6.

[0012] Preferably, the polymeric-grade lactide separation system comprises a crude product storage tank 9, a wiped crystallizer 10 and a screw filter 11, the product condenser 6 in the depolymerization purification reaction system is connected to the crude product storage tank 9, the crude product storage tank 9 is connected to the wiped crystallizer 10, the wiped crystallizer 10 is connected to the screw filter 11 for separating the crystals and the mother liquor, the screw filter 11 is connected to the purification column 4 through a pipeline to re-feed the filtered mother liquor back to the purification column 4, and the produced polymeric-grade lactide is extracted from the bottom of the screw filter 11.

[0013] A preparation process for continuously and efficiently producing polymeric-grade lactide, characterized in that the specific production steps are as follows:

[0014] S1: feeding the raw material pure lactic acid and the catalyst stannous chloride into a lactic acid dehydration reaction system for continuous dehydration to obtain lactic acid oligomers;

[0015] S2: continuously feeding the obtained lactic acid oligomers into a depolymerization purification reaction system, the lactic acid oligomers first pass through the metering pump 2 in the lactic acid dehydration reaction system and enter the purification column 4, and then are transported to the falling film reactor 3 through the circulating pump 5, the gas-phase crude lactide generated in the falling film reactor 3 enters the packing section of the purification column 4 to generate crude lactide, light components, heavy components and residues, wherein the generated crude lactide is cooled by the product condenser 6, part of which is re-flowed into the purification column 4, and part of which is extracted to the crude product storage tank 9. The light components are collected by the light component condenser 7 at the top of the purification column 4 and then discharged through the reflux ratio controller 8. The heavy components are re-flowed to the bottom of the purification column 4 and re-enter the depolymerization purification reaction system for further depolymerization reaction to obtain lactide. At the same time, the generated residues are discharged through the residue discharge port on one side of the circulating pump 5;

[0016] S3: feeding the crude lactide in the crude product storage tank 9 into the wiped crystallizer 10 to generate crystals and mother liquor, and finally feeding the crystals and the mother liquor into the screw filter 11 to obtain polymeric-grade lactide and mother liquor, the polymeric-grade lactide is extracted through the screw filter 11, and the mother liquor is re-fed back to the depolymerization purification reaction system for further depolymerization purification.

[0017] Preferably, the temperature of the lactic acid dehydration reactor 1 is 100-115°C, the pressure is 400 kPaA, and the flow rate of the metering pump 2 is 2226.5 kg / h.

[0018] Preferably, the operating pressure of the falling film reactor 3 is 300-5000 PaA, and the reaction temperature is 150-230°C.

[0019] Preferably, the pressure of the purification column 4 is in the range of 500-7000 PaA, and the top temperature is 100-150°C; the bottom heat medium and the top cooling medium are conventional utilities.

[0020] The temperature of the crude product storage tank 9 is preferably 100-110 DEG C, the pressure is 2 kPaG, and the tank is sealed with nitrogen. The feed quantity of the wiped crystallizer 10 is preferably 2154.7 kg / h, the pressure is 0-0.5 MPaG, the temperature is 50-100 DEG C, and the solid content in the wiped crystallizer 10 can be controlled to 5%-85%.

[0021] Advantages:

[0022] 1. The device and process can continuously produce polymerization-grade lactide, the key equipment is a depolymerization and purification reactor, a wiped crystallizer and a screw filter, and the optical purity of the lactide obtained is above 99%, which meets the technical requirements of polymerization-grade lactide. The whole process system is continuously operated, the rectification process uses the superheated lactide produced by cracking as raw material and energy, realizes double recycling of materials and energy, reduces energy consumption by 30%, and the yield is increased by about 10% compared with the conventional method.

[0023] 2. The whole production process has a short residence time, the residence time of the intermediate material lactide is shortened, the occurrence of side reactions is reduced, and the yield of lactide is greatly improved. At the same time, the discharge of by-products is reduced, the processing efficiency of the whole production line is improved, and the discharge and treatment cost of three wastes is reduced. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 It is a process flow diagram, in which the lactic acid dehydration reactor 1, the metering pump 2, the falling film reactor 3, the purification tower 4, the circulating pump 5, the product condenser 6, the light component condenser 7, the reflux ratio controller 8, the crude product storage tank 9, the wiped crystallizer 10, the screw filter 11 and the vacuum system 12 are shown. DETAILED DESCRIPTION

[0025] The application will be further described in conjunction with the drawings and examples. The following examples are used to explain the application, but the application is not limited to the following examples:

[0026] A system for continuously and efficiently producing polymerization-grade lactide, which comprises:

[0027] A lactic acid dehydration reaction system, which comprises a lactic acid dehydration reactor 1 and a metering pump 2, for generating lactic acid oligomers;

[0028] A depolymerization and purification reaction system, which comprises a falling film reactor 3, a purification tower 4, a circulating pump 5, a product condenser 6, a light component condenser 7, a reflux ratio controller 8 and a vacuum system 12, for decomposing the raw material into gaseous crude lactide, light components, heavy components and residues;

[0029] The polymerization-grade lactide separation system comprises a crude product storage tank 9, a scraped surface crystallizer 10 and a screw filter 11, and the product condenser 6 in the depolymerization purification reaction system is connected to the crude product storage tank 9, the crude product storage tank 9 is connected to the scraped surface crystallizer 10, the scraped surface crystallizer 10 is connected to the screw filter 11 for separating the crystal and the mother liquor, the screw filter 11 is connected to the purification tower 4 through a pipeline to re-feed the filtered mother liquor into the purification tower 4, and the produced polymerization-grade lactide is extracted through the bottom of the screw filter 11.

[0030] The lactic acid dehydration reaction system comprises a lactic acid dehydration reactor 1 and a metering pump 2, the lactic acid dehydration reactor 1 is connected to the metering pump 2. The depolymerization purification reaction system comprises a falling film reactor 3, a purification tower 4, a circulating pump 5, a product condenser 6, a light component condenser 7, a reflux ratio controller 8 and a vacuum system 12, the metering pump 2 in the lactic acid dehydration reaction system is connected to the purification tower 4, the bottom of the purification tower 4 is connected to the circulating pump 5 for continuous feeding, the circulating pump 5 is connected to the falling film reactor 3, one side of the circulating pump 5 is connected to a depolymerization residue discharge port, the falling film reactor 3 is connected to the purification tower 4, the top of the purification tower 4 is connected to the light component condenser 7, the light component condenser 7 is connected to the reflux ratio controller 8 and the vacuum system 12, one side of the reflux ratio controller 8 is connected to the purification tower 4, and the other side is used for discharging the light component substance collected through the light component condenser 7, and the packing section of the purification tower 4 is connected to the product condenser 6.

[0031] The polymerization-grade lactide separation system comprises a crude product storage tank 9, a scraped surface crystallizer 10 and a screw filter 11, and the product condenser 6 in the depolymerization purification reaction system is connected to the crude product storage tank 9, the crude product storage tank 9 is connected to the scraped surface crystallizer 10, the scraped surface crystallizer 10 is connected to the screw filter 11 for separating the crystal and the mother liquor, the screw filter 11 is connected to the purification tower 4 through a pipeline to re-feed the filtered mother liquor into the purification tower 4, and the produced polymerization-grade lactide is extracted through the bottom of the screw filter 11.

[0032] Example 1

[0033] A process system for continuously and efficiently producing polymerization-grade lactide, and the specific production steps are as follows:

[0034] S1: The temperature of the lactic acid dehydration reactor 1 in the lactic acid dehydration reaction system is 100℃, the pressure is 400KPaA, and the flow rate of the metering pump 2 is 2226.5kg / h, and the raw material pure lactic acid and the feed amount of 0.5% stannous chloride are sent into the lactic acid dehydration reaction system for continuous dehydration to obtain lactic acid oligomers with an average molecular weight of 1600 and a temperature of 180℃;

[0035] S2: In the depolymerization and purification reaction system, the temperature of falling film reactor 3 is set to 190°C, and the pressure is set to 1.5 kPaA. The overhead temperature of purification column 4 is set to 128°C, and the pressure is set to 500 PaA. The overhead take-off is set to 27 kg / h of oligomer feed, and the depolymerization residue take-off is set to 13.5 kg / h of oligomer feed. The reflux ratio controller 8 is set to a reflux ratio of 1.5, and the product condenser 6 is set to a temperature of 102°C. The obtained lactic acid oligomer is continuously fed into the depolymerization and purification reaction system at a speed of 900 kg / h. The lactic acid oligomer first passes through the metering pump 2 in the lactic acid dehydration reaction system and enters the purification column 4, and then is transported to the falling film reactor 3 by the circulating pump 5. The generated gas phase crude lactide in the falling film reactor 3 enters the packing section of the purification column 4 to generate crude lactide, light components, heavy components, and residues. The generated crude lactide is cooled by the product condenser 6, a part of which is reflowed into the purification column 4, and a part of which is taken out to the crude product storage tank 9, with a take-off amount of 859.5 kg / h. The light components are collected by the light component condenser 7 at the top of the purification column 4 and are discharged through the reflux ratio controller 8. The heavy components are reflowed to the bottom of the purification column 4 and are returned to the depolymerization and purification reaction system for further depolymerization reaction to obtain lactide. At the same time, the generated residues are discharged through the residue discharge port on one side of the circulating pump 5, with a residue take-off amount of 13.5 kg / h of oligomer feed. The L-lactide content in the crude lactide reaches 98%, the Meso-lactide content is 1.7%, the light component content is 0.1%, and the heavy component content is 0.2%;

[0036] S3: The temperature of the crude product storage tank 9 is controlled to be 100°C, and the pressure is controlled to be 2 KPaG. The feed amount of the scraped crystalizer 10 is set to 2154.7 kg / h, the temperature is set to 78°C, and the pressure is set to 0.2 MPaG. The crude lactide in the crude product storage tank 9 is transported into the scraped crystalizer 10 to generate 645 kg / h of crystals and mother liquor. Finally, the crystals and mother liquor are sent to the screw filter 11 to obtain polymerization grade lactide and mother liquor. The polymerization grade lactide is taken out through the screw filter 11, and the mother liquor is returned to the depolymerization and purification reaction system for further depolymerization and purification. The L-lactide content is 99.4%, the Meso-lactide content is 0.5%, the remaining component content is 0.1%, and the water content is 168 ppm, which meets the requirements of polymerization grade lactide. The mother liquor is discharged at a rate of 114.5 kg / h.

[0037] Example 2

[0038] A continuous and efficient process system for producing polymerization grade lactide is provided, and the specific production steps are as follows:

[0039] S1: The temperature of the lactic acid dehydration reactor 1 in the lactic acid dehydration reaction system is 115°C, the pressure is 400 KPaA, and the flow rate of the metering pump 2 is 2226.5 kg / h. The raw material pure lactic acid and 0.5% of stannous chloride by feed amount are fed into the lactic acid dehydration reaction system for continuous dehydration to obtain lactic acid oligomers with an average molecular weight of 1600 and a temperature of 180°C.

[0040] S2: In the depolymerization purification reaction system, the temperature of the falling film reactor 3 is set to 230°C, the pressure is 5 kPaA, the top temperature of the purification column 4 is set to 150°C, the pressure is 7 KPaA, the top yield is 32.5 kg / h of the oligomer feed amount, the depolymerization residue yield is 18 kg / h of the oligomer feed amount, the reflux ratio controller 8 is set to a reflux ratio of 1.5, and the temperature of the product condenser 6 is set to 102°C. The obtained lactic acid oligomers are continuously fed into the depolymerization purification reaction system at a speed of 900 kg / h. The lactic acid oligomers first pass through the metering pump 2 in the lactic acid dehydration reaction system and enter the purification column 4, and then are transported to the falling film reactor 3 through the circulating pump 5. The gas phase crude propiolactone generated in the falling film reactor 3 enters the packing section of the purification column 4 to generate crude propiolactone, light components, heavy components, and residues. The crude propiolactone generated is cooled by the product condenser 6, part of which is reflowed into the purification column 4, and part of which is discharged to the crude product storage tank 9 with a yield of 894 kg / h. The light components are collected by the light component condenser 7 at the top of the purification column 4 and are discharged through the reflux ratio controller 8. The heavy components are reflowed to the bottom of the purification column 4 and are returned to the depolymerization purification reaction system for further depolymerization reaction to obtain propiolactone. At the same time, the generated residues are discharged through the residue discharge port on one side of the circulating pump 5, and the residue yield is 16.3 kg / h of the oligomer feed amount. The L-lactide content in the crude propiolactone is 98.2%, the Meso-lactide content is 1.5%, the light component content is 0.1%, and the heavy component content is 0.2%;

[0041] S3: The temperature of the crude product storage tank 9 is controlled at 110°C and the pressure is 2 KPaG. The feed amount of the scraped plate crystallizer 10 is set to 2154.7 kg / h, the temperature is 100°C, and the pressure is 0.5 MPaG. The crude propiolactone in the crude product storage tank 9 is transported into the scraped plate crystallizer 10 to generate 685 kg / h of crystals and mother liquor. Finally, the crystals and mother liquor are fed into the screw filter 11 to obtain polymerization grade propiolactone and mother liquor. The polymerization grade propiolactone is discharged from the screw filter 11, and the mother liquor is returned to the depolymerization purification reaction system for further depolymerization purification. The L-lactide content is 99.3%, the Meso-lactide content is 0.6%, the remaining component content is 0.1%, and the water content is 176 ppm, which meets the requirements of polymerization grade propiolactone. The mother liquor is discharged at a rate of 179.4 kg / h.

[0042] Example 3

[0043] The difference between this embodiment and embodiment 1 is that, on the basis of the normal operation of embodiment 1, the mother liquor discharged in embodiment 1 is continuously returned to the depolymerization purification reaction system.

[0044] S1: In the depolymerization and purification reaction system, the temperature of falling film reactor 3 was set at 190°C and the pressure was 1.5 kPaA. The top temperature of purification tower 4 was set at 128°C and the pressure was 500 PaA. The top extraction rate was 30.5 kg / h of the oligomer feed rate, and the depolymerization residue extraction rate was 16.8 kg / h of the oligomer feed rate. The reflux ratio of reflux ratio controller 8 was set at 1.5, and the temperature of product condenser 6 was controlled at 100°C. The mother liquor from Example 1 was continuously fed into purification tower 4 of the depolymerization and purification reaction system at a rate of 900 kg / h and then transported to falling film reactor 3 via circulation pump 5. The vapor-phase crude lactide produced in falling film reactor 3 entered the packing section of purification tower 4 to produce crude lactide, light components, heavy components, and residue. After cooling in product condenser 6, the crude lactide was partially refluxed into purification tower 4 and partially extracted into crude product storage tank 9 at a rate of 967 kg / h. The light fraction is collected by the light fraction condenser 7 at the top of purification tower 4 and discharged through the reflux ratio controller 8. The heavy fraction is refluxed to the bottom of purification tower 4 and returned to the depolymerization purification reaction system for further depolymerization to produce lactide. Simultaneously, the residue is discharged through the residue outlet on the side of the circulation pump 5. The residue output is 16.8 kg / h of the oligomer feed rate. The crude lactide contains 97.8% L-lactide and 1.9% Meso-lactide; the light fraction content is 0.1%; and the heavy fraction content is 0.2%.

[0045] S2: The temperature of crude product storage tank 9 was controlled at 100°C and the pressure at 2 kPaG. The feed rate to scraper crystallizer 10 was set at 2154.7 kg / h, the temperature at 78°C, and the pressure at 0.2 MPaG. The crude lactide from crude product storage tank 9 was fed into scraper crystallizer 10, generating 765 kg / h of crystals and mother liquor. The crystals and mother liquor were then fed into screw filter 11 to produce polymerization-grade lactide and mother liquor. The polymerization-grade lactide was removed from screw filter 11, and the mother liquor was returned to the depolymerization and purification reaction system for further depolymerization and purification. The lactide content was 99.2% L-lactide, 0.7% Meso-lactide, 0.1% other components, and 180 ppm water, meeting the requirements for polymerization-grade lactide. 202 kg / h of mother liquor was discharged.

[0046] Obviously, the above embodiments are merely example for clearly illustrating but not limitation to the embodiments. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, all the embodiments need not and can not be enumerated. The obvious changes or variations derived from the above description are still within the protection scope of the present application.

Claims

1. A system for continuously and efficiently producing polymer-grade lactide, characterized in that: The system comprises: A lactic acid dehydration reaction system, comprising a lactic acid dehydration reactor and a metering pump, wherein the lactic acid dehydration reactor is connected to the metering pump and is used to generate lactic acid oligomers; A depolymerization and purification reaction system comprising a falling film reactor, a purification tower, a circulation pump, a product condenser, a light component condenser, a reflux ratio controller, and a vacuum system for decomposing the raw material into gaseous crude lactide, light components, heavy components, and residue; A polymer-grade lactide separation system, comprising a crude product storage tank, a scraper crystallizer, and a screw filter, for separating purified polymer-grade lactide from mother liquor; The metering pump in the lactic acid dehydration reaction system is connected to a purification tower, the bottom of the purification tower is connected to a circulation pump for continuous feeding, the circulation pump is connected to a falling film reactor, wherein one side of the circulation pump is connected to a depolymerization residue discharge port, the falling film reactor is connected to the purification tower, the top of the purification tower is connected to a light component condenser, the light component condenser is connected to a reflux ratio controller and a vacuum system, one side of the reflux ratio controller is connected to the purification tower, and the other side is used to discharge the light component substances collected by the light component condenser, and the packing section of the purification tower is connected to the product condenser; The product condenser in the depolymerization purification reaction system is connected to a crude product storage tank, which is connected to a scraper crystallizer. The scraper crystallizer is connected to a screw filter for separating crystals and mother liquor. The screw filter is connected to a purification tower via a pipeline, and the filtered mother liquor is transported back to the purification tower. The produced polymerization-grade lactide is extracted from the bottom of the screw filter.

2. A process for continuously and efficiently producing polymer-grade lactide, based on the system for continuously and efficiently producing polymer-grade lactide according to claim 1, characterized in that: The specific production steps are as follows: S1: feeding the raw material pure lactic acid and the catalyst stannous chloride into a lactic acid dehydration reaction system for continuous dehydration to obtain lactic acid oligomers; S2: The obtained lactic acid oligomers are continuously fed into a depolymerization and purification reaction system. The lactic acid oligomers first enter the purification tower through a metering pump in the lactic acid dehydration reaction system and are then transported to a falling film reactor by a circulation pump. The vapor-phase crude lactide generated in the falling film reactor enters the packing section of the purification tower to generate crude lactide, light components, heavy components, and residue. The crude lactide generated is cooled in a product condenser, with a portion recirculated into the purification tower and a portion withdrawn to a crude product storage tank. The light fraction is collected by the light fraction condenser at the top of the purification tower and then discharged through the reflux ratio controller; the heavy fraction is refluxed to the bottom of the purification tower and returned to the depolymerization purification reaction system for further depolymerization reaction to obtain lactide; the residue generated is discharged through the residue discharge port on one side of the circulation pump; S3: The crude lactide in the crude product storage tank is transported to the scraper crystallizer to generate crystals and mother liquor. Finally, the crystals and mother liquor are sent to the screw filter to obtain polymerization-grade lactide and mother liquor. The polymerization-grade lactide is extracted through the screw filter, and the mother liquor returns to the depolymerization and purification reaction system for further depolymerization and purification.

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