A method for preparing lactide from recycled polylactic acid by chemical depolymerization

Abstract

This invention discloses a method for preparing lactide from polylactic acid through chemical depolymerization, belonging to the field of polylactic acid waste chemical recycling and reuse technology. The method includes raw material pretreatment, oligomer preparation, reactive distillation, product purification, and carrier recycling. Polylactic acid waste, crushed and dried to a water content of <200 ppm, is melt-shear-mixed with a catalyst and a high-boiling-point polyol with a boiling point greater than 200 ppm at atmospheric pressure at 160-190°C. Subsequently, an alcoholysis chain transfer reaction is carried out at 160-200°C for 10-30 minutes, controlling the product to have a Da content of 1000-3000. The melt is then vacuum distilled at 180-230°C and <5 kPa, and the vapor phase is condensed and melt-crystallized to obtain lactide. The bottom residue rich in polyol is refluxed for recycling. This invention allows subsequent cyclization and depolymerization reactions to be carried out under relatively mild temperature conditions, thereby effectively reducing energy consumption and minimizing racemization and carbonization side reactions caused by high temperatures.

Description

Technical Field

[0001] This invention relates to the field of chemical recycling and reuse of polylactic acid waste, specifically a method for preparing lactide by chemically depolymerizing and recovering polylactic acid. Background Technology

[0002] The chemical recycling of polylactic acid to prepare lactide is a core link in realizing the closed-loop recycling and high-value utilization of bio-based materials. Its recycling efficiency and product quality directly determine the cost and benefits of the circular economy. The process covers key steps such as raw material pretreatment, melt depolymerization, vacuum distillation and product purification, aiming to convert waste polymers into high-purity monomers through chain breaking reactions. Existing technologies mainly rely on high-temperature random thermal cracking or simple catalytic degradation, lacking precise control over the molecular weight of reaction intermediates. This results in high viscosity of the reaction system, large mass transfer resistance, and low gas-liquid interface renewal efficiency, making it difficult to achieve rapid escape of lactide. At the same time, due to the uncontrollable reaction path and local overheating, racemization side reactions and severe coking are easily triggered, leading to a decrease in the optical purity of the product and poor equipment stability. In addition, existing processes are unable to break the chemical equilibrium limitation and lack a recycling mechanism for active components, resulting in the waste of catalysts and raw materials, making it difficult to meet the requirements of large-scale continuous, low-energy consumption and high-purity production. Therefore, a solution is urgently needed to address the problems existing in the current technology.

[0003] The information disclosed in the background section above is only intended to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing lactide by chemically depolymerizing and recovering polylactic acid, so as to solve the problems mentioned in the background art.

[0005] The technical solution of the present invention includes: raw material pretreatment and melt mixing to prepare polylactic acid oligomer melt, vacuum reactive distillation, product separation and purification, and active carrier recycling; The raw material pretreatment and melt mixing involve crushing and drying the polylactic acid waste to control the moisture content to less than 200 ppm. Subsequently, the dried polylactic acid waste, catalyst, and other materials with a boiling point greater than [missing information] at atmospheric pressure are [missing information]. The high-boiling-point polyol chain transfer agent is fed into the melt mixing unit and melt-shear mixed at 160-190℃ to obtain a mixture. The preparation of polylactic acid oligomer melt involves feeding the mixture into a chain transfer reactor and carrying out an alcoholysis chain transfer reaction at 160-200°C, controlling the residence time to be 10-30 minutes to degrade the polylactic acid waste and control the number average molecular weight of the reaction products. With a Da of 1000-3000, polylactic acid oligomer melt was obtained; The vacuum reactive distillation continuously feeds the polylactic acid oligomer melt into a vacuum reactive distillation column, where it undergoes a cyclization and depolymerization reaction at a temperature of 180-230°C and an absolute pressure of less than 5 kPa. The gas phase component is then vaporized and collected, and the residue in the column bottom is a bottom residue liquid rich in high-boiling-point polyol chain transfer agent. The product is separated and purified by condensing the gas phase component through a condensation system to obtain crude lactide. The crude lactide is then melt-crystallized to remove impurities, yielding the lactide product. The active carrier is recycled by using a booster pump to return the residue discharged from the vacuum reactive distillation column to the raw material pretreatment and melt mixing step or the step of preparing polylactic acid oligomer melt for reuse.

[0006] Optionally, the high-boiling-point polyol chain transfer agent has a normal-pressure boiling point greater than 280°C; The high-boiling-point polyol chain transfer agent is selected from glycerol, pentaerythritol, diglycerol, and other compounds with a number average molecular weight greater than [missing information]. At least one of polyethylene glycol; The catalyst is selected from at least one of stannous octoate, zinc lactate, and aluminum lactate.

[0007] Optionally, in the raw material pretreatment and melt mixing, the amount of catalyst added is 0.01%-0.5% of the mass of polylactic acid waste; The amount of the high-boiling-point polyol chain transfer agent added is equal to the mass of the polylactic acid waste. To ensure the number average molecular weight of the reaction products Controlled .

[0008] Optionally, the melt mixing unit employs a twin-screw extruder or a melt pump with a static mixer; The chain transfer reactor is a tubular reactor, and a static mixing element is installed inside the reactor.

[0009] Optionally, in the vacuum reactive distillation, the vacuum reactive distillation column is a scraped film evaporator or a falling film evaporator; The absolute pressure is controlled between 0.1 and 1 kPa; The temperature of the cyclization depolymerization reaction is controlled at 200-220℃.

[0010] Optionally, in the product separation and purification process, the condensation temperature of the condensation system is controlled at 70-90℃; During the melting and crystallization process, moisture, free acid, and racemic impurities in crude lactide are removed through programmed cooling crystallization and sweating operations.

[0011] Optionally, in the active carrier recycling process, the mass recycling ratio of the residual liquid in the reactor is a certain percentage of the total feed mass. ; The residual liquid in the reactor is pressurized by a gear pump before reflux and directly injected into the inlet of the chain transfer reactor to merge with the fresh mixture.

[0012] This invention provides an improved method for preparing lactide by chemically depolymerizing and recovering polylactic acid, which has the following improvements and advantages compared with the prior art: 1. This invention introduces a high-boiling-point polyol chain transfer agent and carries out an alcoholysis chain transfer reaction in a melt mixing unit and a chain transfer reactor to directionally degrade high-viscosity polylactic acid waste into polylactic acid oligomer melts with a number average molecular weight between 1000-3000 Da. This pretreatment significantly reduces the viscosity of the reaction system and optimizes the rheological properties, enabling the subsequent cyclization and depolymerization reaction to proceed under relatively mild temperature conditions, thereby effectively reducing energy consumption and minimizing racemization and carbonization side reactions caused by high temperatures. 2. This invention employs a vacuum reactive distillation process, utilizing high-boiling-point polyols with an atmospheric boiling point greater than 280℃ as the reaction medium and carrier. These polyols are retained in the reboiler under vacuum depolymerization conditions, while the generated lactide is continuously vaporized and extracted as a gaseous component. This coupled reaction and separation operation mode not only utilizes Le Chatelier's principle to break the chemical equilibrium and promote the reaction towards lactide production, but also ensures the stability of the liquid level in the reaction system and the continuous activity of the catalyst, thereby significantly improving the yield of crude lactide. 3. The present invention designs an active carrier recycling step, in which the residue liquid discharged from the vacuum reactive distillation column, rich in high-boiling-point polyol chain transfer agent and catalyst, is directly returned to the raw material pretreatment or oligomer preparation step via a booster pump; this technical feature realizes the closed-loop recycling of expensive catalysts and reaction media, significantly reduces the amount of fresh catalyst and chain transfer agent added, lowers production costs, and reduces the emission of industrial waste. 4. This invention processes the crude lactide obtained by condensation using melt crystallization technology. By using programmed cooling crystallization and sweating operations, residual moisture, free acid and racemic impurities in the crude lactide are precisely removed. Compared with traditional solvent recrystallization, this method avoids the introduction and residual risks of organic solvents and can stably prepare high-purity lactide products that meet the requirements of polymerization. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. Example 1:

[0014] A method for preparing lactide from polylactic acid through chemical depolymerization includes: raw material pretreatment and melt mixing to prepare polylactic acid oligomer melt, vacuum reactive distillation, product separation and purification, and recycling of the active carrier; the raw material pretreatment and melt mixing involves crushing and drying the polylactic acid waste to control the moisture content to less than 200 ppm, followed by the drying of the polylactic acid waste, catalyst, and a material with a boiling point greater than 200 ppm at atmospheric pressure. The high-boiling-point polyol chain transfer agent is fed into the melt mixing unit and melt-shear mixed at 160-190℃ to obtain a mixture. In this embodiment, the raw material pretreatment and melt mixing steps aim to construct a homogeneous reaction precursor system; industrially recycled polylactic acid waste is selected, crushed and sent to a drying system, and the moisture content is strictly controlled at 100 ppm. This extremely low moisture content index is to prevent uncontrollable hydrolysis reactions from occurring in non-target stages and to avoid disordered decrease in molecular weight; subsequently, the dried polylactic acid waste, catalyst and high-boiling-point polyol chain transfer agent are transported to the melt mixing unit; The melt mixing unit employs a twin-screw extruder, utilizing its strong shear force to achieve molecular-level mixing of high-viscosity melts. Process parameters are set to perform melt shear mixing at 160°C, a temperature slightly higher than the melting point of polylactic acid (PLA), ensuring material flowability while avoiding thermal degradation caused by high temperatures. The catalyst is selected from stannous octoate, added at 0.01% of the PLA waste mass; this low addition level aims to reduce costs and the subsequent purification load. The high-boiling-point polyol chain transfer agent is selected from glycerol, utilizing its high-density hydroxyl structure as initiation sites; its addition amount is 0.5% of the PLA waste mass. Based on this, in order to verify the applicability of the process parameter range, this embodiment examined the variation of melt shear mixing temperature in the range of 160-190℃ while keeping other conditions constant. The results showed that when the temperature was 190℃, the material flowability was optimal and no obvious yellowing was detected. When the temperature was 160℃, although the shear heat was slightly higher, uniform mixing could still be achieved. This confirms that the pretreatment requirements can be met in the range of 160-190℃. Polylactic acid oligomer melt was prepared by feeding the mixture into a chain transfer reactor and carrying out an alcoholysis chain transfer reaction at 160-200℃, controlling the residence time to 10-30 min to degrade the polylactic acid waste and control the number average molecular weight of the reaction products. With a Da of 1000-3000, polylactic acid oligomer melt was obtained; In this embodiment, the above mixture is fed into a chain transfer reactor and subjected to an alcoholysis chain transfer reaction under mild conditions at 160°C, with a residence time controlled at 30 min. The core of this step lies in controlled degradation, where the hydroxyl groups of glycerol attack the polylactic acid ester bonds to undergo transesterification, breaking the long chains and precisely controlling the number-average molecular weight of the reaction products. 3000Da; The selection of this molecular weight value retains an appropriate melt strength, while shortening the polymer chain length to a length that facilitates intramolecular back-ringing, resulting in a uniform polylactic acid oligomer melt, laying a rheological foundation for subsequent efficient depolymerization; the parameter range of the alcoholysis chain transfer reaction was further verified: at a fixed temperature of 160℃, the residence time was adjusted within 10-30 min. Experimental data show that when the dwell time is 10 minutes, The reaction rate is approximately 2800 Da, which is acceptable; however, when the residence time is extended to 30 minutes... The reaction rate stabilized at around 3000 Da; meanwhile, with a fixed residence time, increasing the temperature to 200℃ significantly accelerated the reaction rate. The value dropped to around 1200 Da; the above comparison verified that a wide window operation of 160-200℃ and 10-30min can effectively cover [the affected area]. The target range is 1000-3000Da; Vacuum reactive distillation involves continuously feeding polylactic acid oligomer melt into a vacuum reactive distillation column, where a cyclization and depolymerization reaction is carried out at a temperature of 180-230℃ and an absolute pressure of less than 5kPa. The gas phase component is then vaporized and collected, and the residue in the column bottom is a bottom residue liquid rich in high-boiling-point polyol chain transfer agent. In this embodiment, polylactic acid oligomer melt is continuously fed into a vacuum reactive distillation column; cyclization depolymerization reaction is carried out under deep vacuum conditions at a temperature of 200°C and an absolute pressure controlled at 0.1 kPa; under these thermodynamic conditions, the terminal hydroxyl groups of the oligomer rapidly attack the ester bond to generate lactide, and immediately vaporize and collect the gas phase component. This reaction-separation coupling mechanism breaks the chemical equilibrium and promotes the reaction to move towards the formation of lactide; The residue in the distillation column is a liquid rich in high-boiling-point polyol chain transfer agents. Since glycerol's boiling point is much higher than the reaction temperature, it remains in the liquid phase, acting as a stable reaction carrier. For the vacuum reactive distillation step, the parameter boundaries of temperature (180-230℃) and pressure (<5kPa) were investigated. The results show that at 180℃ and 0.1kPa, although the reaction rate decreases by about 15% compared to 200℃, the optical purity of the product increases to 99.8%. At 230℃ and 4.9kPa, production capacity is maximized without significant coking. This indicates that the process range can flexibly adapt to different purity and yield requirements. The product is separated and purified by condensing the gaseous component through a condensation system to obtain crude lactide. The crude lactide is then subjected to melt crystallization to remove impurities, yielding the lactide product. The condensation temperature of the condensation system is controlled at 70-90℃. During the melt crystallization process, water, free acid, and racemic impurities in the crude lactide are removed through programmed cooling crystallization and sweating operations. In this embodiment, the gaseous component is condensed through a condensation system with the condensation temperature controlled at 70℃ to obtain crude lactide. Crude lactide is purified by melt crystallization to remove impurities, specifically through programmed cooling crystallization and sweating operations, with the cooling rate controlled at [value missing]. , down to Crystallization then occurs, followed by heating to... Perform sweating operation By utilizing the difference in melting points between impurities and the main product, moisture, free acid, and racemic impurities in crude lactide are removed to obtain high-purity L-lactide product. The active carrier is recycled, and the residue discharged from the vacuum reactive distillation column is returned to the raw material pretreatment and melt mixing step or the polylactic acid oligomer melt preparation step via a booster pump for recycling; the recirculation ratio of the residue is 10%-15% of the total feed mass; the residue is pressurized by a gear pump before recirculation and directly injected into the inlet of the chain transfer reactor to merge with the fresh mixture; In this embodiment, the residue discharged from the vacuum reactive distillation column is refluxed via a booster pump; the reflux ratio of the residue is set to 10% of the total feed mass; the residue is pressurized by a gear pump before reflux and directly injected into the inlet of the chain transfer reactor to merge with the fresh mixture; since the residue is enriched with unreacted active catalyst and glycerol chain transfer agent, this cycle not only realizes the reuse of catalyst, but also reduces the energy consumption of melting the new material by utilizing the enthalpy of the residue, thus forming a closed-loop active carrier system; This embodiment, as the preferred implementation, achieves a balance between high yield and high purity through the combination of glycerol and stannous octoate at a relatively low temperature and deep vacuum. Specifically, the specific ratio of 0.5% glycerol to 0.01% stannous octoate in this embodiment, combined with... Although the yield was slightly lower than that of Example 2, the control of the molecular weight of 3000 Da effectively avoided small molecule impurities caused by excessive chain scission, thereby maintaining extremely high optical purity and low coking rate, and verifying the process strategy of maintaining melt stability through long-chain oligomers under mild conditions. Example 2:

[0015] The high-boiling-point polyol chain transfer agent has a normal-pressure boiling point greater than 280℃; the high-boiling-point polyol chain transfer agent is selected from at least one of glycerol, pentaerythritol, diglycerol, and polyethylene glycol; the catalyst is selected from at least one of stannous octoate, zinc lactate, and aluminum lactate. This embodiment mainly adjusts the process parameters to adapt to the characteristics of waste from different sources. In the raw material pretreatment and melt mixing steps, the polylactic acid waste is dried to control the moisture content to 140 ppm. The catalyst is selected from zinc lactate, and its addition amount is 0.2% of the mass of polylactic acid waste. Zinc lactate has good environmental friendliness. The high-boiling-point polyol chain transfer agent is selected from pentaerythritol, and its addition amount is 1.5% of the mass of polylactic acid waste. The high-boiling-point polyol chain transfer agent has a normal pressure boiling point greater than 280℃, and its tetrafunctional structure makes its degradation efficiency higher and can provide more reactive sites. The melt mixing unit uses a twin-screw extruder or a melt pump with a static mixer; the chain transfer reactor is a tubular reactor with a static mixing element inside. In this embodiment, the material is conveyed to the melt mixing unit, where a melt pump equipped with a static mixer performs melt shear mixing at 175°C. The mixture then enters the chain transfer reactor and reacts at 180°C, with a residence time controlled at 20 minutes. By adjusting the chain transfer agent ratio and reaction conditions, the number-average molecular weight of the reaction products is controlled. The value is 2000 Da; this molecular weight is the ideal range for balancing fluidity and cyclization reactivity. The presence of static mixing elements ensures the uniformity of radial temperature and concentration, avoiding racemization caused by local overheating. In vacuum reactive distillation, the vacuum reactive distillation column adopts a scraped film evaporator or a falling film evaporator; the absolute pressure is controlled at 0.1-1 kPa; and the temperature of the cyclization and depolymerization reaction is controlled at 200-220℃. In this embodiment, the polylactic acid oligomer melt enters a vacuum reactive distillation column; the reaction is carried out under the conditions of cyclization depolymerization at a temperature controlled at 210°C and an absolute pressure controlled at 0.5 kPa; the moderately increased temperature and pressure help to increase the production rate, while the gas phase components are vaporized and collected, and the residue in the column bottom is a bottom liquid rich in high-boiling polyol chain transfer agent; in the product separation and purification step, the condensation temperature of the condensation system is controlled at 80°C; In the active carrier recycling step, the refluxing ratio of the residue in the reactor is 12% of the total feed mass. This embodiment demonstrates the process robustness when using tetrafunctional pentaerythritol as a carrier, which is particularly suitable for scenarios with high production rate requirements. Data correlation analysis shows that this embodiment achieved the highest lactide yield and the lowest coking rate, mainly attributed to the tetrafunctional structure of pentaerythritol and its compatibility with... Matching for 2000Da: The star-shaped topology of the oligomer effectively reduces the entanglement viscosity of the melt and significantly improves the surface renewal efficiency of the liquid film in the scraped film evaporator, allowing lactide molecules to escape from the liquid phase at a faster rate, thereby reducing the residence time of the material on the high-temperature wall and suppressing coking to the maximum extent. To verify the rationality of the parameter range, the following comparative tests were conducted: keeping other conditions consistent, the depolymerization temperatures of the cyclization process were set to 200℃, 210℃, and 220℃, respectively. The data showed that the lactide formation rate at 220℃ was about 40% higher than that at 200℃, and the racemization rate only increased by 0.1% due to the shorter residence time. At the same time, by adjusting the absolute pressure within the range of 0.1-1 kPa, it was found that the condensation and collection efficiency was higher at 1 kPa, while the lactide residue in the reactor liquid was less at 0.1 kPa. This confirms that the combined range of 200-220℃ and 0.1-1 kPa can achieve efficient and stable depolymerization. Example 3:

[0016] In the raw material pretreatment and melt mixing, the amount of catalyst added is 0.01%-0.5% of the mass of polylactic acid waste; the amount of the high-boiling-point polyol chain transfer agent added is [missing information - likely a percentage] of the mass of polylactic acid waste. To ensure the number average molecular weight of the reaction products Controlled ; This embodiment aims to verify the process performance under high temperature and short residence time conditions. In the raw material pretreatment and melt mixing steps, the moisture content of the dried polylactic acid waste is controlled to be 180 ppm. In the melt mixing unit, melt shear mixing is performed at 190°C. The catalyst is selected from aluminum lactate and added at 0.5% of the mass of the polylactic acid waste. This catalyst has high stereoselectivity at high temperature and can effectively suppress racemization. The high-boiling-point polyol chain transfer agent is selected from diglycerol and added at 3.0% of the mass of the polylactic acid waste. The high-boiling-point polyol chain transfer agent has a normal-pressure boiling point greater than 280℃, and the good solubility of diglycerol helps to homogenize the system. Gradient experiments were conducted to verify the addition range. Under fixed process conditions, the catalyst addition was adjusted between 0.01% and 0.5%. It was found that even at a low addition of 0.01%, the predetermined conversion rate could be achieved with an extended reaction time. At an addition of 0.5%, the reaction was completed rapidly. Meanwhile, by varying the addition amount of high-boiling-point polyol chain transfer agent between 0.5% and 3.0%, the results showed that an addition amount of 0.5% corresponded to a product... Approximately 2900 Da, high melt strength; 3.0% addition corresponds to product Approximately 1000 Da, with extremely low viscosity; this series of comparisons confirms the correlation between the addition amount range and the target molecular weight. A good linear regulation relationship exists between 1000-3000 Da; in the step of preparing polylactic acid oligomer melt, the material undergoes alcoholysis chain transfer reaction in a chain transfer reactor at a high temperature of 200℃, with a controlled residence time of 10 min; the rapid reaction rate enables the rapid degradation of polylactic acid waste and controls the number average molecular weight of the reaction products. It is 1000Da; Oligomers at this molecular weight have extremely low viscosity, making them readily spread into films in subsequent steps. In the vacuum reactive distillation step, a falling film evaporator is used in the vacuum reactive distillation column, utilizing gravity film formation, which is suitable for low-viscosity materials. The cyclization and depolymerization reaction is carried out under conditions where the temperature is controlled at 220℃ and the absolute pressure at 1 kPa. In product separation and purification, the condensation temperature of the condensation system is controlled at 90℃. In the active carrier circulation, the reflux ratio of the residue in the reactor is 15% of the total feed mass. This embodiment, by combining a high concentration of chain transfer agent with a high-temperature, rapid-response strategy, significantly shortens the process cycle, making it suitable for large-scale continuous production; specifically, the high addition amount of diglycerol will... The viscosity rapidly decreased to 1000 Da, forming a reaction melt with extremely low viscosity. Combined with the high temperature of 220°C, a high reaction rate was achieved. Although the high temperature resulted in a slightly higher coking rate than in Example 2, the high stereoselectivity of the aluminum lactate catalyst effectively offset the risk of racemization caused by the high temperature, ensuring that the optical purity of the product remained at a high level. Example 4:

[0017] In this embodiment, polyethylene glycol (PEG) was used as a chain transfer agent to verify the effect of the flexible segment carrier. During the raw material pretreatment and melt mixing steps, the water content was controlled to be less than 120 ppm. In the melt mixing unit, mixing was carried out at 180°C. The catalyst was selected from stannous octoate, and the addition amount was 0.1% of the mass of the polylactic acid waste. The high-boiling-point polyol chain transfer agent was selected from PEG, and the addition amount was 1.0% of the mass of the polylactic acid waste. The high-boiling-point polyol chain transfer agent has a normal-pressure boiling point greater than 280°C. The flexible segments of PEG help improve melt flowability and reduce the surface tension of the system. In the step of preparing polylactic acid oligomer melt, the reaction is carried out in a chain transfer reactor at 190°C with a residence time controlled at 15 min; the number average molecular weight of the reaction product is controlled. A polylactic acid oligomer melt with a Da of 1500 was obtained. In the vacuum reactive distillation step, a scraped-film evaporator was used in the distillation column. The cyclization depolymerization reaction was carried out at a temperature controlled at 215℃ and an absolute pressure controlled at 0.8 kPa. In the product separation and purification step, the condensation temperature of the condensation system was controlled at 75℃. In the active carrier recycling step, the reflux ratio of the residue was 13% of the total feed mass. This embodiment demonstrates that introducing flexible polyether segments can effectively improve the processing performance of high-viscosity polyester waste, especially exhibiting excellent wettability when processing waste containing complex fillers. Example 5:

[0018] This embodiment verifies the synergistic effect of a specific mixed catalyst system; in the raw material pretreatment and melt mixing steps, the water content is controlled to be less than 180 ppm; in the melt mixing unit, mixing is carried out at 185°C; the catalyst is selected from a mixture of stannous octoate and aluminum lactate, and the addition amount is 0.3% of the mass of polylactic acid waste; the bimetallic catalytic system aims to achieve complementary advantages by utilizing the high activity of stannous octoate and the high selectivity of aluminum lactate; The high-boiling-point polyol chain transfer agent was selected from glycerol, and the addition amount was 2.0% of the polylactic acid waste mass. In the step of preparing polylactic acid oligomer melt, the reaction was carried out in a chain transfer reactor at 170°C, and the residence time was controlled to be 25 min. The number average molecular weight of the reaction product was controlled. The Da value was 2500 to obtain polylactic acid oligomer melt; in the vacuum reactive distillation step, the vacuum reactive distillation column adopted a scraped film evaporator reactor; The reaction was carried out at a temperature of 205°C and an absolute pressure of 0.3 kPa. In the product separation and purification step, the condensation temperature of the condensation system was controlled at 85°C. In the active carrier recycling step, the reflux ratio of the residue in the reactor was 14% of the total feed mass. The results of this embodiment show that the mixed catalyst system effectively suppressed racemic side reactions and improved the optical purity of the product while maintaining a high reaction rate.

[0019] Comparative Example 1: This comparative example is basically the same as Example 2, except that: no high-boiling-point polyol chain transfer agent was added in the raw material pretreatment and melt mixing steps; the polylactic acid waste was directly subjected to random thermal pyrolysis at high temperature; and due to the lack of chain transfer agent regulation, the number-average molecular weight of the reaction products could not be controlled. In the optimization range of 1000-3000 Da, the reaction system exhibits high viscosity, deteriorates heat transfer, and lacks carrier circulation; this comparative example is used to verify the dual necessity of high-boiling-point polyols as both chemical scissors and liquid-phase carriers.

[0020] Comparative Example 2: This comparative example is basically the same as Example 2, except that the absolute pressure is controlled at 10 kPa in the vacuum reactive distillation step. Due to the excessive pressure, lactide cannot be vaporized and the gas phase components cannot be extracted in time, which leads to the obstruction of chemical equilibrium and the product staying in the reactor for too long, resulting in secondary polymerization or carbonization. This comparative example is used to verify the necessity of a deep vacuum environment to break the thermodynamic equilibrium and drive the reaction forward.

[0021] Comparative Example 3: This comparative example is basically the same as Example 2, except that: no active carrier circulation step was set; the residue discharged from the vacuum reactive distillation column was directly discharged as waste and was not recycled; this comparative example is used to reveal the key role of active carrier circulation technology in reducing material consumption, energy consumption and maintaining the steady-state concentration of catalytic active centers.

[0022] Comparative Example 4: This comparative example is basically the same as Example 2, except that in the step of preparing polylactic acid oligomer melt, the reaction conditions are controlled to ensure that the number average molecular weight of the reaction product is... The molecular weight is 6000 Da; the higher molecular weight leads to an increase in melt viscosity, which is not conducive to liquid film renewal and mass transfer in vacuum reactive distillation; this comparative example is used to verify the effect of oligomer molecular weight range of 1000-3000 Da on the matching of mass transfer efficiency and reaction rate. Verification experiment: Performance and process stability tests of lactide products: In order to verify the effects of the embodiments and comparative examples of the present invention, performance tests were conducted on the lactide products prepared in the above examples. Test Standards and Methods: Lactide Single-Pass Yield %: Based on material balance calculations; the mass of crude lactide produced by the condensation system per unit time is collected and divided by the mass of lactide theoretically generated from the complete conversion of polylactic acid waste; this indicator reflects the depth and efficiency of the reaction depolymerization. Optical purity L-isomer %: determined by high performance liquid chromatography; a chiral column was used with n-hexane / isopropanol as the mobile phase and a detection wavelength of 254 nm; the higher the L-isomer content, the lower the degree of racemization during the reaction process and the better the product quality; Residual coking rate (%): After 48 hours of continuous operation, feed was stopped, and the concentration of residual coking product in the distillation column bottom liquid was measured. The mass fraction of black spots or carbides insoluble in chloroform retained by microporous membrane filtration; this indicator characterizes whether there is local overheating or material dead zone in the process, and reflects the long-term stability of the process; all tests are repeated three times and the average value is taken, and the data is processed to one decimal place; Table 1 Performance test data of Examples 1-5 and Comparative Examples 1-4 As shown in Table 1, Examples 1-5 of this invention, by introducing a high-boiling-point polyol chain transfer agent and precisely controlling the number-average molecular weight of the reaction products, achieve the desired results. With a Da range of 1000-3000, the combination of vacuum reactive distillation and active carrier recycling technology significantly improved the yield and optical purity of lactide, while keeping the coking rate at an extremely low level. Specifically, comparing Example 2 with Comparative Example 1, it can be seen that the lack of chain transfer agent leads to random thermal cracking, resulting in a significant decrease in yield and damage to optical purity, as well as severe coking. This is because random cracking requires higher activation energy, leading to uncontrollable reaction temperature and the generation of a large number of non-cyclizable linear short chains or cross-linked carbides. However, after introducing pentaerythritol in Example 2, the degradation process was locked within a specific molecular weight range through the chain scission mechanism initiated by hydroxyl groups, achieving directional depolymerization under mild conditions. Comparing Example 2 and Comparative Example 2, it can be seen that a vacuum environment with an absolute pressure of less than 5 kPa is crucial for driving the cyclization depolymerization reaction to equilibrium; when the pressure rises to 10 kPa, product vaporization is hindered, the residence time of lactide in the liquid phase is prolonged, and the reverse ring-opening polymerization reaction or thermal degradation is very likely to occur, resulting in a sharp drop in yield to 45.2%; comparing Example 2 and Comparative Example 4, it can be seen that controlling the oligomer molecular weight within 1000-3000 Da is key to ensuring efficient mass transfer and reaction; when At a viscosity as high as 6000 Da, the melt viscosity is too high, making it difficult to form a turbulent liquid film in the scraped film evaporator. This leads to a sharp increase in mass transfer resistance, preventing lactide molecules from escaping the liquid surface in time and reducing the yield to 78.6%. In summary, this invention solves the problems of high energy consumption, low product purity, and difficulty in continuous production of polylactic acid recovery in the prior art by organically combining molecular weight control, deep vacuum reactive distillation, and carrier recycling.

[0023] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing lactide by chemically depolymerizing and recovering polylactic acid, characterized in that, include: Raw material pretreatment and melt mixing are used to prepare polylactic acid oligomer melt, followed by vacuum reactive distillation, product separation and purification, and recycling of the active carrier. The raw material pretreatment and melt mixing involve crushing and drying the polylactic acid waste to control the moisture content to less than 200 ppm. Subsequently, the dried polylactic acid waste, catalyst, and other materials with a boiling point greater than [missing information] at atmospheric pressure are [missing information]. The high-boiling-point polyol chain transfer agent is fed into the melt mixing unit and melt-shear mixed at 160-190℃ to obtain a mixture. The preparation of polylactic acid oligomer melt involves feeding the mixture into a chain transfer reactor and carrying out an alcoholysis chain transfer reaction at 160-200°C, controlling the residence time to be 10-30 minutes to degrade the polylactic acid waste and control the number average molecular weight of the reaction products. With a Da of 1000-3000, polylactic acid oligomer melt was obtained; The vacuum reactive distillation continuously feeds the polylactic acid oligomer melt into a vacuum reactive distillation column, where it undergoes a cyclization and depolymerization reaction at a temperature of 180-230°C and an absolute pressure of less than 5 kPa. The gas phase component is then vaporized and collected, and the residue in the column bottom is a bottom residue liquid rich in high-boiling-point polyol chain transfer agent. The product is separated and purified by condensing the gas phase component through a condensation system to obtain crude lactide. The crude lactide is then melt-crystallized to remove impurities, yielding the lactide product. The active carrier is recycled by using a booster pump to return the residue discharged from the vacuum reactive distillation column to the raw material pretreatment and melt mixing step or the step of preparing polylactic acid oligomer melt for reuse.

2. The method for preparing lactide by chemically depolymerizing and recovering polylactic acid according to claim 1, characterized in that, The high-boiling-point polyol chain transfer agent has a normal-pressure boiling point greater than 280°C; The high-boiling-point polyol chain transfer agent is selected from glycerol, pentaerythritol, diglycerol, and other compounds with a number average molecular weight greater than [missing information]. At least one of polyethylene glycol; The catalyst is selected from at least one of stannous octoate, zinc lactate, and aluminum lactate.

3. The method for preparing lactide by chemically depolymerizing and recovering polylactic acid according to claim 1, characterized in that, In the raw material pretreatment and melt mixing, the amount of catalyst added is 0.01%-0.5% of the mass of polylactic acid waste; The amount of the high-boiling-point polyol chain transfer agent added is equal to the mass of the polylactic acid waste. To ensure the number average molecular weight of the reaction products Controlled .

4. The method for preparing lactide by chemically depolymerizing and recovering polylactic acid according to claim 1, characterized in that, The melt mixing unit employs a twin-screw extruder or a melt pump with a static mixer. The chain transfer reactor is a tubular reactor, and a static mixing element is installed inside the reactor.

5. The method for preparing lactide by chemically depolymerizing and recovering polylactic acid according to claim 1, characterized in that, In the vacuum reactive distillation, the vacuum reactive distillation column adopts a scraped film evaporator or a falling film evaporator; The absolute pressure is controlled between 0.1 and 1 kPa; The temperature of the cyclization depolymerization reaction is controlled at 200-220℃.

6. The method for preparing lactide by chemically depolymerizing and recovering polylactic acid according to claim 1, characterized in that, In the product separation and purification process, the condensation temperature of the condensation system is controlled at 70-90℃. During the melting and crystallization process, moisture, free acid, and racemic impurities in crude lactide are removed through programmed cooling crystallization and sweating operations.

7. The method for preparing lactide by chemically depolymerizing and recovering polylactic acid according to claim 1, characterized in that, In the circulation of the active carrier, the mass recycling ratio of the residual liquid in the reactor is equal to the total mass of the feed. ; The residual liquid in the reactor is pressurized by a gear pump before reflux and directly injected into the inlet of the chain transfer reactor to merge with the fresh mixture.

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