A method and device for preparing lactide by continuous coupling

Abstract

The present invention relates to a method for preparing lactide by continuous coupling, comprising: (1) mixing lactic acid oligomers with a depolymerization catalyst, preheating the mixture, and continuously conveying the mixture to a depolymerization reactor for reaction, wherein the light fraction after the reaction enters the bottom of a distillation system; and (2) after treatment in the distillation system, the resulting bottom liquid phase fraction is returned to the depolymerization reactor, and the gaseous lactide is condensed to obtain a lactide product. The present invention couples the depolymerization reactor with the distillation system for continuous reaction, thereby reducing the probability of lactide racemization and substrate coking and carbonization, and improving the product quality and yield of the entire synthesis process.

Description

Technical Field

[0001] The present invention belongs to the technical field of degradable materials, and in particular relates to a method and device for preparing lactide by continuous coupling. Background Art

[0002] Polylactic acid (PLA), with its excellent mechanical strength, biocompatibility, biodegradability, and bioabsorbability, is a green polymer material with broad application prospects and broad applications. With the continuous maturity of PLA production technology and the continued expansion of its application markets, the production of PLA via the non-food crop → lactic acid → PLA route has entered a period of industrial development. Natureworks, a US company, is currently the world's largest PLA producer, with a production capacity of 140,000 tons, including 150,000 tons of L-lactide. Global production capacity of L-lactide and D-lactide is projected to exceed 500,000 tons by 2020. Currently, high-molecular-weight PLA produced domestically and internationally is mostly derived through the ring-opening polymerization of lactide. Therefore, the key to high-quality PLA synthesis technology lies not only in the polymerization process itself but also in the purity and quality of the L-lactide raw material.

[0003] Pure L-lactide has characteristics such as high boiling point, high freezing point, heat sensitivity, and easy water absorption and ring-opening reaction. However, crude L-lactide prepared from L-lactic acid is generally light yellow or darker in color and is very prone to coking and carbonization, which seriously restricts the production of lactide.

[0004] CN107522687A discloses a method for synthesizing lactide from lactic acid using a phosphotungstic heteropoly acid catalyst. The method uses a distillation device to purify lactide. This method has the advantage of eliminating problems such as reagent recovery and environmental pollution, making it a green synthesis process. However, the high temperature of the distillation device can easily cause problems such as thermal polymerization, racemization, discoloration, and coking of the materials during the preparation process.

[0005] CN111424059A discloses a method and system for producing high-yield, optically pure lactide by preparing lactic acid using biofermentation technology. This method utilizes a falling-film reactor and offers advantages such as a fully integrated process from raw material to product, high yield, and high optical purity. However, according to a literature report (Jiang Shouying. Research on a New Green Purification Process for L-Lactide [D]. Tianjin University, 2009), the process of ring-opening and dehydrating lactide to obtain polylactic acid generally requires the intermediate lactide to have a purity of over 99%. This patent lacks a subsequent purification step, and the resulting crude lactide cannot meet the requirements for subsequent polylactic acid production.

[0006] CN211384959U discloses a reaction system for preparing lactide from lactic acid. The reaction system comprises: a lactic acid oligomerization reactor, a first reaction distillation column, a lactide synthesis reactor, and a second reaction distillation column, connected in sequence. The first reaction distillation column includes a first reactor reboiler, a purification section, and a deep oligomerization and dehydration reaction section arranged from top to bottom. A first feed port is provided in the column section between the purification section and the deep oligomerization and dehydration reaction section for passing the product from the lactic acid oligomerization reactor into the first reaction distillation column. The reaction system provided by this utility model optimizes the reaction route by integrating distillation with deep synthesis reactions, improving reaction separation efficiency, and thereby increasing product quality and yield. It also reduces equipment costs and space requirements. However, the first and second reaction distillation columns in this patent are both equipped with bottom reboilers, which not only increases energy consumption but also significantly increases the probability of thermal polymerization, racemization, coking, and carbonization of the materials during the process.

[0007] CN112679465A discloses a method for producing lactide by coupled reactive distillation. The method comprises the following steps: 1) dehydrating the aqueous lactic acid solution, and then preliminarily polymerizing the lactic acid to obtain lactic acid oligomers P1; 2) further polymerizing P1 to obtain lactic acid oligomers P2 of the desired molecular weight for depolymerization; and 3) depolymerizing P2 to obtain lactide and defocused oil. The dehydration of the lactic acid in step 1) and the preliminarily polymerizing reaction are coupled in reactive distillation column I; the further polymerization reaction in step 2) and the depolymerization reaction of the lactic acid oligomers in step 3) are coupled in reactive distillation column II. This method couples the lactic acid prepolymerization and depolymerization reactions in the same reactive distillation column, significantly simplifying the process and equipment for the two-step lactic acid production process, thereby reducing energy consumption and production costs for the entire lactide production plant. However, coupling the lactic acid prepolymerization and depolymerization reactions into the same reaction distillation tower increases the residence time of lactide at high temperature. Too long a residence time at high temperature increases the degree of racemization and makes thermal polymerization more likely to occur, thereby reducing the yield and purity of the product. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention provides a method and apparatus for preparing lactide through continuous coupling. This method couples a depolymerization reactor with a distillation system for continuous reaction, reducing the chances of lactide racemization and substrate coking and carbonization, thereby improving product quality and yield throughout the entire synthesis process.

[0009] The present invention provides a method for preparing lactide by continuous coupling, comprising the following steps:

[0010] (1) Lactic acid oligomers are mixed with a depolymerization catalyst and preheated, and then continuously transported to a depolymerization reactor for reaction. After the reaction, the light component enters the bottom of the distillation system;

[0011] (2) After being treated by the distillation system, the bottom liquid phase components are returned to the depolymerization reactor, and the gas phase lactide is condensed to obtain the lactide product.

[0012] In the above method, the molecular weight of the lactic acid oligomer described in step (1) is 800-4000, preferably 1200-2800. Lactic acid oligomers are generally prepared from L-lactic acid or D-lactic acid through a dehydration and polycondensation process. The dehydration process is to remove free water from the lactic acid, and can be carried out under normal pressure or reduced pressure. The polycondensation process has a reaction temperature of 140-170°C, a reaction time of 0.5-4.0 hours, and a vacuum degree of 1000-2000 Pa.

[0013] In the above method, the depolymerization catalyst in step (1) is a catalyst that can efficiently catalyze the depolymerization reaction of lactic acid oligomers, such as at least one of an alkali metal compound, a tin compound catalyst, etc., specifically at least one of stannous octoate, calcium oxide, SnO, SnCl2, etc.

[0014] In the above method, the amount of the depolymerization catalyst in step (1) is 0.5%-10% of the weight of the lactic acid oligomers, preferably 1%-5%.

[0015] In the above method, the preheating temperature in step (1) is 100-160°C, and the preheating time is 0.5-5h.

[0016] In the above method, the depolymerization reactor in step (1) is a falling film reactor, which can be any one of a thin film evaporator, a shell and tube evaporator, and other types of film evaporators.

[0017] In the above method, the depolymerization reaction temperature in step (1) is 170-220°C, the vacuum degree is 200-1500 Pa, and the reaction time is 1-5 hours.

[0018] In the above method, a protonated solvent is further added to the material in the depolymerization reactor in step (1), preferably at least one of diamines and diols having 12 to 18 carbon atoms, such as dodecanediamine, tetradecanediamine, hexadecanediamine, tetradecanediol, and hexadecanediol. The amount of the protonated solvent is 0.1% to 10% of the weight of the lactic acid oligomers, preferably 0.5% to 5%.

[0019] In the above method, the protonated solvent in step (1) is heated to a molten state before being added, and the melting temperature is 80-160°C, preferably 100-160°C.

[0020] In the above method, a gaseous crude lactide outlet is provided above the depolymerization reactor in step (1), and the outlet is directly connected to the bottom of the distillation system. The falling film reactor serves as a reboiler at the bottom of the distillation tower while carrying out the depolymerization reaction.

[0021] In the above method, the distillation system in step (2) includes at least one distillation tower, which is a packed tower or a plate tower with high gas-liquid separation efficiency and has 5-30 plates. The top vacuum of the distillation tower is 200-1000 Pa, and the bottom temperature is 140-180°C.

[0022] In the above method, the liquid phase components at the bottom of the distillation tower in step (2) are refluxed to the depolymerization reactor for further reaction to improve the preparation efficiency and yield of the entire process. The lactide is extracted from the top of the distillation tower.

[0023] In the above method, the product yield of the distillation purification process in step (2) is not less than 90.0%.

[0024] In the above method, the product yield of the entire process from lactic acid oligomers to the final lactide product can reach more than 85.0%, and the chemical purity and optical purity of the product can reach more than 98.0%, wherein the m-lactide content is not higher than 2.0%, the L-lactic acid content is not higher than 1.0%, and the content of lactic acid dimers and trimers is not higher than 0.8%.

[0025] The present invention also provides an apparatus for the above-mentioned continuous coupled method for preparing lactide, which mainly includes a depolymerization reaction system, a distillation system, and a condensation system. The depolymerization reaction system mainly includes a depolymerization reactor for reacting lactic acid oligomers with a depolymerization catalyst, and light components after the reaction enter from the bottom of the distillation system; the distillation system includes at least one distillation tower. After the light components produced by the depolymerization reaction are processed by the distillation system, the liquid components produced are returned to the depolymerization reaction system, and the gaseous lactide is passed through the condensation system to obtain a lactide product.

[0026] In the present invention, the depolymerization reactor is a falling film reactor, which can be any one of a thin film evaporator, a shell and tube evaporator, and other types of film evaporators.

[0027] In the present invention, the distillation tower adopts a packed tower or a plate tower with high gas-liquid separation efficiency, and the number of plates is 5-30.

[0028] Compared with the prior art, the beneficial effects of the present invention are embodied in:

[0029] (1) The falling film depolymerization reactor is coupled with distillation purification. The light components produced by the falling film depolymerization reactor are directly fed into the bottom of the distillation system, and the liquid components produced are returned to the depolymerization reactor, forming a depolymerization-distillation circulation loop. This reduces the probability of thermal polymerization, racemization, coking, and carbonization of lactide during the synthesis and distillation process, and improves the product yield and quality of lactide in the entire synthesis process.

[0030] (2) The falling film depolymerization reactor-distillation system circulation process shortens the residence time of lactide at high temperature and reduces the probability of lactide racemization at high temperature, which can effectively reduce the burden of subsequent refining; it also avoids the use of a bottom reboiler and reduces the distillation cost.

[0031] (3) Adding a specific protonated solvent during the depolymerization process can effectively reduce the degree of racemization in the lactide synthesis, solving the problem that the catalyst is very likely to cause lactide racemization in the lactide synthesis process despite the appropriate reaction rate. m - Lactide content not exceeding 2.0%. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic flow chart of the method for continuously preparing lactide according to the present invention;

[0033] Among them, Ⅰ-raw material storage tank; Ⅱ-falling film depolymerization reactor; Ⅲ-distillation tower; Ⅳ-condenser; 1-feed pipe; 2-light components of depolymerization reaction; 3-liquid phase components produced by distillation; 4-inert gas; 5-lactide product. DETAILED DESCRIPTION

[0034] The following examples further illustrate the lactide preparation method and its effects. The examples are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operating processes, but the scope of protection of the present invention is not limited to the following examples.

[0035] The experimental methods in the following examples, unless otherwise specified, are all conventional methods in the art. The experimental materials used in the following examples, unless otherwise specified, can all be purchased from biochemical reagent stores.

[0036] The lactic acid used in the embodiment of the present invention is heat-resistant grade L-lactic acid with a lactic acid content of 88% or more and an optical purity of not less than 99.0%.

[0037] This study used a Malvern Viscotek OMNISEC GPC / SEC gel permeation chromatography (GPC / SEC) instrument to analyze the molecular weight of lactic acid oligomers. Traditional calibration methods were used, with polystyrene (PS) as the internal standard. A T3000 column (300 mm × 8.0 mm) was used. The column temperature was 40°C, the flow rate was 1.0 mL / min, the sample concentration was 2-5 mg / mL, and the single injection volume was 500 μL.

[0038] The present invention uses an Agilent high-performance liquid chromatograph to analyze the chemical purity of lactide, L-lactic acid, dimer, and trimer content. A UV detector is used, phosphoric acid and acetonitrile are used as the mobile phase, and the chromatographic column model is a ZORBAX SB-Aq column with a length of 250 mm, an inner diameter of 4.6 mm, and a filler particle size of 5 μm. The detection wavelength is 200 nm, the column temperature is 40°C, the flow rate is 1 mL / min, and the injection volume is 5 μL.

[0039] The present invention adopts an Agilent gas chromatograph to analyze the content of different optical isomers of lactide, selects a CYCLOSIL-B model chromatographic column, a vaporizer temperature of 250°C, a detector temperature of 280°C, a hydrogen flame ionization detector, and a column temperature program: an initial temperature of 100°C, maintained for 5 min, then heated to 140°C at a rate of 4°C / min, maintained for 7 min, and heated to 200°C at a rate of 8°C / min, maintained for 20 min. The carrier gas flow rate of N2 is 1.4 mL / min, the hydrogen flow rate is 30 mL / min, the air flow rate is 400 mL / min, and the injection volume is 0.5 μL.

[0040] The yield Y of the lactide purification process and the product yield Y of the entire preparation and purification process 总 The calculation formula is as follows:

[0041]

[0042] in, m 0 is the mass of crude lactide, y 0 is the purity of L-lactide in crude lactide, m is the mass of lactide product, and M is the mass of lactide that a certain amount of lactic acid oligomers can theoretically be converted into, that is, the mass of lactic acid oligomers.

[0043] The specific rotation of the sample was analyzed using a WZZ-2S automatic polarimeter to characterize the optical purity of the sample. The specific rotation of pure L-lactide was -278, and the specific rotation of pure D-lactide was +278. m -The specific rotation of lactide is 0, and the optical purity of the sample X is calculated as follows;

[0044]

[0045] Among them, α 纯物质represents the specific rotation of pure lactide, α 被测样品 Indicates the specific optical rotation of the substance being measured.

[0046] The embodiment of the present invention is as follows Figure 1 The process is as follows: first, lactic acid oligomers and depolymerization catalysts are mixed in proportion and preheated in raw material storage tank I, and continuously transported to falling film depolymerization reactor II for depolymerization reaction. After the reaction, light components 2 enter from the bottom of distillation tower III, and liquid components 3 produced by distillation are returned to depolymerization reactor II. The lactide gas phase at the top of the distillation tower is condensed in condenser IV to obtain lactide product 5.

[0047] Example 1

[0048] Preparation of lactic acid oligomers: (1) Dehydration of lactic acid: Take 2000g of L-lactic acid (lactic acid content of about 88.0%) and add it to a four-necked flask with a stirring system. Use a vacuum circulating water pump to maintain the system pressure at about 60kPa. Start heating under vacuum and gradually heat to 110-120℃. Dehydrate for 2h. At this time, the free water in the reaction system is slowly evaporated out of the reaction system. (2) Preparation of lactic acid oligomers: After the free water in the system is almost completely removed, increase the vacuum degree of the system and slowly reduce the pressure of the system to about 2.0kPa. The temperature of the feed liquid is gradually raised to 140℃ and reacted for 0.5h. At this time, a condensation reaction occurs between the lactic acid molecules and the water generated by the reaction in the system is evaporated out of the system to obtain a lactic acid oligomer with a molecular weight of 1200.

[0049] Use attached Figure 1 The following process flow is used to prepare lactide: 1000g of the aforementioned lactic acid oligomers and 20g of stannous octoate are mixed in a raw material storage tank and preheated at 100°C with stirring for 0.5h. The falling film depolymerization reactor is replaced with argon three times and then evacuated. The mixture in the raw material storage tank is continuously transferred to the falling film depolymerization reactor. The depolymerization reaction conditions are: reaction temperature 170°C, vacuum 1500Pa, and reaction time 5h. The crude lactide gas from the depolymerization reactor enters the bottom of a distillation tower with 5 stages, a top vacuum of 500Pa, and a bottom temperature of 140°C. The liquid phase generated at the bottom of the tower is returned to the depolymerization reactor, and the lactide product is discharged from the top of the distillation tower after condensation in a condenser.

[0050] Analysis showed that the product yield from the distillation and purification process was 93.8%, with m-lactide content at 1.85%, L-lactic acid at 1.0%, and dimer and trimer content at 0.75%. The overall lactide synthesis process yielded 85.8%.

[0051] Example 2

[0052] Preparation of lactic acid oligomers: (1) Dehydration of lactic acid: Take 2000g of L-lactic acid (lactic acid content of about 88.0% and optical purity of 99.2%) and add it to a four-necked flask with a stirring system. Use a vacuum circulating water pump to maintain the system pressure at about 50kPa. Start heating under vacuum and gradually heat to 110-120℃. Dehydrate for 2h. At this time, the free water in the reaction system is slowly evaporated out of the reaction system. (2) Preparation of lactic acid oligomers: After the free water in the system is almost completely removed, increase the vacuum degree of the system and slowly reduce the pressure of the system to about 1.5kPa. The temperature of the feed liquid is gradually raised to 160℃ and reacted for 3h. At this time, a condensation reaction occurs between the lactic acid molecules and the water generated by the reaction in the system is evaporated out of the system to obtain a lactic acid oligomer with a molecular weight of 2200.

[0053] Use attached Figure 1 The following process flow is used to prepare lactide: 1000g of the aforementioned lactic acid oligomers and 50g of stannous octoate are mixed in a raw material storage tank and preheated at 120°C with stirring for 3 hours. The falling film depolymerization reactor is replaced with argon three times and then evacuated. The mixture in the raw material storage tank is continuously transferred to the falling film depolymerization reactor. The depolymerization reaction conditions are: reaction temperature of 200°C, vacuum of 300 Pa, and reaction time of 3 hours. The crude lactide in the gas phase from the depolymerization reactor enters the bottom of a distillation tower with 10 plates, a vacuum of 300 Pa at the top of the tower, and a bottom temperature of 150°C. The liquid phase produced at the bottom of the tower is returned to the depolymerization reactor, and the lactide product is discharged from the top of the distillation tower after condensation in a condenser.

[0054] Analysis showed that the product yield from the distillation and purification process was 95.5%, with m-lactide content at 1.62%, L-lactic acid at 0.92%, and dimer and trimer content at 0.65%. The overall lactide synthesis process yielded 86.7%.

[0055] Example 3

[0056] Preparation of lactic acid oligomers: (1) Dehydration of lactic acid: Take 2000g of L-lactic acid (lactic acid content of about 88.0% and optical purity of 99.2%) and add it to a four-necked flask with a stirring system. Use a vacuum circulating water pump to maintain the system pressure at about 50kPa. Start heating under vacuum and gradually heat to 110-120℃. Dehydrate for 3h. At this time, the free water in the reaction system is slowly evaporated out of the reaction system. (2) Preparation of lactic acid oligomers: After the free water in the system is almost completely removed, increase the vacuum degree of the system and slowly reduce the pressure of the system to about 1.0kPa. The temperature of the liquid is gradually raised to 170℃ and reacted for 4h. At this time, a condensation reaction occurs between the lactic acid molecules and the water generated by the reaction in the system is evaporated out of the system to obtain a lactic acid oligomer with a molecular weight of 2780.

[0057] Use attached Figure 1 The following process flow is used to prepare lactide: 1000g of the aforementioned lactic acid oligomers and 100g of stannous octoate are mixed in a raw material storage tank and preheated at 140°C with stirring for 5 hours. The falling film depolymerization reactor is replaced with argon three times and then evacuated. The mixture in the raw material storage tank is continuously transferred to the falling film depolymerization reactor. The depolymerization reaction conditions are: reaction temperature of 220°C, vacuum of 200 Pa, and reaction time of 1 hour. The crude lactide in the gas phase from the depolymerization reactor enters the bottom of a distillation tower with 30 plates, a vacuum of 200 Pa at the top of the tower, and a bottom temperature of 160°C. The liquid phase produced at the bottom of the tower is returned to the depolymerization reactor, and the lactide product is discharged from the top of the distillation tower after condensation in a condenser.

[0058] Analysis showed that the product yield from the distillation and purification process was 97.8%, with m-lactide content at 1.15%, L-lactic acid at 0.37%, and dimer and trimer content at 0.58%. The overall lactide synthesis process yielded 89.2%.

[0059] Example 4

[0060] Same as Example 1, except that SnO is used as the depolymerization catalyst.

[0061] Analysis showed that the product yield from the distillation and purification process was 92.1%, with m-lactide content at 1.9%, L-lactic acid at 0.93%, and dimer and trimer content at 0.72%. The overall lactide synthesis process yielded 85.3%.

[0062] Example 5

[0063] Same as Example 1, except that calcium oxide is used as the depolymerization catalyst.

[0064] Analysis showed that the product yield from the distillation and purification process was 91.5%, with m-lactide content at 1.95%, L-lactic acid at 0.95%, and dimer and trimer content at 0.78%. The overall lactide synthesis process yielded 85.0%.

[0065] Example 6

[0066] Same as Example 1, except that 5 g of dodecanediamine was added during the depolymerization process. Analysis showed that the product yield from the distillation purification process was 94.8%, with an m-lactide content of 0.65%, an L-lactic acid content of 0.87%, and a dimer and trimer content of 0.74%. The total lactide synthesis yield reached 86.7%.

[0067] Example 7

[0068] Same as Example 1, except that 5 g of hexadecanediamine was added simultaneously during the depolymerization process. Analysis showed that the product yield from the distillation purification process was 95.2%, with an m-lactide content of 0.83%, an L-lactic acid content of 0.89%, and a dimer and trimer content of 0.77%. The total lactide synthesis yield reached 87.2%.

[0069] Example 8

[0070] Same as Example 1, except that 5 g of tetradecanediol was added during the depolymerization process. Analysis showed that the product yield from the distillation purification process was 95.5%, with an m-lactide content of 1.0%, an L-lactic acid content of 0.92%, and a dimer and trimer content of 0.68%. The total lactide synthesis yield reached 87.5%.

[0071] Comparative Example 1

[0072] The same method as Example 1 was used, except that a conventional reactor was used instead of a falling film reactor for the depolymerization process. Analysis showed that the product yield during the distillation and purification process was 72%, with an m-lactide content of 10.6%, an L-lactic acid content of 6.2%, and a dimer and trimer content of 5.4%. The total lactide synthesis process yield was 68.3%.

[0073] Comparative Example 2

[0074] This process was the same as Example 1, except that the liquid fraction produced by distillation was not returned to the depolymerization reactor. Analysis showed that the product yield from the distillation purification process was 71%, with an m-lactide content of 12.8%, an L-lactic acid content of 5.4%, and a dimer and trimer content of 6.1%. The total lactide synthesis process yield was 66.8%.

[0075] Comparative Example 3

[0076] This process was the same as Example 1, except that the lactic acid oligomers and depolymerization catalyst were not pre-mixed and preheated, but were directly transferred to the depolymerization reactor. Analysis showed that the product yield during the distillation and purification process was 74%, with an m-lactide content of 7.8%, an L-lactic acid content of 4.5%, and a dimer and trimer content of 4.6%. The total lactide synthesis process yield was 72.7%.

Claims

1. A method for preparing lactide by continuous coupling, characterized in that The following steps are involved: (1) Lactic acid oligomers are mixed with a depolymerization catalyst and preheated, and then continuously transported to a depolymerization reactor for reaction, and the light components after the reaction enter the bottom of the distillation system; the preheating temperature is 100-160°C; the molecular weight of the lactic acid oligomers is 800-4000; the depolymerization catalyst is at least one of stannous octoate, calcium oxide, SnO, and SnCl2; the depolymerization reaction temperature is 170-220°C, the vacuum degree is 200-1500 Pa, and the reaction time is 1-5 hours; a protonated solvent is simultaneously added to the material in the depolymerization reactor, specifically at least one of diols and diamines having 12-18 carbon atoms; (2) After being treated by the distillation system, the bottom liquid phase components are returned to the depolymerization reactor, and the gas phase lactide is condensed to obtain the lactide product.

2. The method according to claim 1, wherein: The molecular weight of the lactic acid oligomer described in step (1) is 1200-2800.

3. The method according to claim 1 or 2, characterized in that: The lactic acid oligomer in step (1) is prepared from L-lactic acid or D-lactic acid by dehydration and polycondensation processes; the dehydration process is to remove free water from the lactic acid, using normal pressure or reduced pressure; the polycondensation process has a reaction temperature of 140-170°C, a reaction time of 0.5-4.0h, and a vacuum degree of 1000-2000Pa.

4. The method according to claim 1, wherein: The amount of the depolymerization catalyst used in step (1) is 0.5%-10% of the mass of the lactic acid oligomers.

5. The method according to claim 4, characterized in that: The amount of the depolymerization catalyst used in step (1) is 1%-5% of the mass of the lactic acid oligomers.

6. The method according to claim 1, wherein: The preheating time in step (1) is 0.5-5h.

7. The method according to claim 1, wherein: The depolymerization reactor in step (1) is a falling film reactor, including any one of a thin film evaporator and a shell and tube evaporator.

8. The method according to claim 1, wherein: The protonated solvent is at least one of dodecanediamine, tetradecanediamine, hexadecanediamine, tetradecanediol, and hexadecanediol.

9. The method according to claim 1 or 8, characterized in that: The amount of the protonated solvent used is 0.1%-10% of the amount of the lactic acid oligomer.

10. The method according to claim 9, characterized in that: The amount of protonated solvent used is 0.5%-5% of the weight of lactic acid oligomers.

11. The method according to claim 1 or 8, characterized in that: The protonated solvent is heated to a molten state before being added, and the melting temperature is 80-160°C.

12. The method according to claim 11, wherein: The protonated solvent is heated to a molten state before being added, and the melting temperature is 100-160°C.

13. The method according to claim 1, wherein: The distillation system in step (2) includes at least one distillation tower, which is a packed tower or a plate tower with high gas-liquid separation efficiency, and the number of plates is 5-30.

14. The method according to claim 13, wherein: The vacuum degree at the top of the distillation tower is 200-1000 Pa, and the temperature at the bottom of the tower is 140-180°C.

15. The method according to claim 1, wherein: The product yield of the distillation purification process in step (2) is not less than 90.0%.

16. The method according to claim 1, wherein: From lactic acid oligomers to the final lactide product, the product yield of the entire process reaches more than 85.0%, and the chemical purity and optical purity reach more than 98.0%. The m-lactide content is not higher than 2.0%, the L-lactic acid content is not higher than 1.0%, and the lactic acid dimer and trimer content is not higher than 0.8%.

Images