Method for preparing lactide by solvent-assisted lactic acid two-step method and production device

Abstract

The invention discloses a method for synthesizing lactide by a solvent-assisted two-step method and a production device. A lactic acid raw material is subjected to dehydration polycondensation and depolymerization to obtain a lactide product, a catalyst and a low-boiling-point solvent are added in the dehydration polycondensation step to obtain a lactic acid oligomer, and then the low-boiling-point solvent in the system is evaporated out and stored for subsequent recycling; a high-boiling-point solvent is added in the depolymerization step, low-boiling-point solvent residues are evaporated out at a low temperature, then the temperature is increased to the depolymerization temperature, and a lactide crude product is obtained through reduced pressure distillation. The method has the advantage that the reaction requirements of different stages in the two-step lactide preparation process are met in a targeted manner by using different solvents. The low-boiling-point solvent continuously and efficiently removes water generated in the reaction process through azeotropy, and the polycondensation rate is increased; the high-boiling-point solvent relieves the problem that the product is hindered from being evaporated due to the fact that the viscosity of a reaction system is increased in the high-temperature depolymerization process, forward proceeding of the reaction is facilitated, and the lactide production efficiency and selectivity are improved.

Description

Technical Field

[0001] The invention belongs to the field of biodegradable material synthesis, and particularly relates to a method and a production device for synthesizing lactide by a solvent-assisted two-step lactic acid process. Background Art

[0002] At present, polylactic acid is considered to be one of the preferred bio-based polymers for the preparation of alternatives to petroleum-based plastics. Compared with other bio-based plastics, polylactic acid is favored for its superior properties in production process, output, biocompatibility and environmentally friendly raw material sources, and has been applied in many fields. At present, the production of polylactic acid mainly relies on lactic acid and lactide as precursors, requiring a large supply of lactic acid and lactide. The synthesis method of polylactic acid involves two main pathways: direct polymerization and ring-opening polymerization: the former is synthesized using lactic acid as raw material; the latter is based on lactide monomer. However, in the process of direct polymerization of lactic acid, the presence of water will inhibit the reaction process, and it is difficult to obtain high molecular weight polylactic acid products. Therefore, using lactide as a precursor to prepare high molecular weight and high quality polylactic acid has become a more preferred method. The supply of high-quality lactide is crucial to promoting the development of the current polylactic acid industry.

[0003] The commercial production of lactide usually adopts a two-step process: the first step is to generate prepolymer from lactic acid, which is a reversible equilibrium process with the production of water. Under specific temperature conditions, in order to increase the degree of polymerization, the content of water molecules in the system must be reduced, because even very small amounts of water will significantly reduce the degree of polymerization of the prepolymer. At present, the industry usually uses reduced pressure operation to effectively remove water in this process, thereby promoting the formation of higher molecular weight prepolymers. In the second step, the lactic acid oligomers are depolymerized to form cyclic lactic acid dimers, namely lactide. Whether it is the oligomeric lactic acid system in the polycondensation reaction or the oligomeric lactic acid system in the depolymerization reaction, the reaction material flow rate gradually decreases and the viscosity gradually increases during the reaction. This change significantly hinders the volatilization of the product, thereby inhibiting the reaction from proceeding in the positive direction. The entire process needs to be operated under high temperature and vacuum environment to promote the continuous and effective separation of water and lactide. This highly energy-intensive production process requires strict control of thermal and vacuum conditions as well as complex recovery and subsequent purification steps. In addition, as the viscosity of the material increases during the reaction, the reaction system will form high-viscosity bottom residues, which are difficult to discharge continuously through conventional pumping after the temperature drops, and are prone to cause pipeline blockage, leading to interruption of the continuous production process. In order to overcome the problem of poor diffusion and mass transfer of reaction products caused by excessive system viscosity, ensure that the reaction can proceed smoothly, and maintain a continuous production process, it is also necessary to use specially designed evaporation reactors and pump devices, which significantly increases production costs.

[0004] Therefore, finding effective methods to increase the dehydration rate in the polycondensation stage and the evaporation rate of lactide in the depolymerization stage is crucial to improving production efficiency and reducing production costs. At the same time, reducing the problem of excessive system viscosity and carbonization causing product yellowing during the depolymerization process is also a key factor in improving product purity and further reducing costs.

[0005] CN101434594A discloses a method for preparing lactide with a mixed solvent, wherein a low boiling point solvent is added after the lactic acid reaction system is dehydrated and polycondensed to a viscous state, and a lactic acid oligomer is produced by vacuum distillation; the depolymerization reaction is carried out under high temperature and high vacuum conditions, and the reaction material changes from dark yellow to brown or black oligomers and begins to crack, at which time the low boiling point solvent takes out the generated lactide; and then a high boiling point solvent is added after reacting at a high temperature of 250-280°C for 2-3 hours to prevent carbonization of lactide, and vacuum distillation is continued for 4 hours. The method adds solvents separately during the dehydration polycondensation and depolymerization processes, but both are used to play a role in the depolymerization stage, the temperature and vacuum control procedures of the whole process are complicated, the water removal efficiency in the polycondensation stage is not high, the depolymerization takes a long time, and the lactide product is yellow and has a low yield.

[0006] CN102863420A discloses a method for preparing lactide, which includes adding a catalyst and a high-boiling-point solvent to lactic acid for polycondensation reaction, and further adding a catalyst for depolymerization reaction, so that the reaction system will not become viscous, difficult to evaporate, carbonized, or yellowed due to reduced distillation of lactide during depolymerization. However, the dehydration polycondensation step in this method still requires relatively high temperature and vacuum conditions, is time-consuming, and has a low purity of the lactide product.

[0007] In addition to the exploration of the above two-step synthesis technology, CN114853719B discloses a reaction enhancement method and device for producing lactide by a one-step method of lactic acid liquid phase. This method uses molecular sieves as catalysts and directly uses boiling organic solvents to carry out lactic acid dehydration reaction. The evaporated organic solvent is continuously refluxed to the reactor after phase separation and dehydration, and finally an organic solvent containing a lactide product is obtained. A combination of one or more salt chemicals and a physical water absorbent is used in the phase separation process. This method simplifies the steps of lactide synthesis and has mild reaction conditions, but the reaction rate is much lower than the two-step method, and the purity of the lactide product is low, and separation and purification are difficult. Summary of the invention

[0008] In view of the shortcomings of the prior art, the present invention proposes a method and a production device for synthesizing lactide by a two-step process using different solvents to assist lactic acid in stages. In the dehydration prepolymerization stage, under positive pressure conditions without high vacuum, a low-boiling-point solvent is used to azeotropically remove water, which reduces the energy consumption and operation difficulty of the prepolymerization reaction. The prepolymer formed has a narrow molecular weight distribution and a low lactic acid content, which is conducive to the depolymerization reaction; a high-boiling-point solvent is added in the depolymerization stage to prevent the system viscosity from increasing and coking and carbonization, and the lactide product is obtained by reducing the pressure.

[0009] In order to achieve the above object, the technical solution adopted by the present invention is as follows: A method for preparing lactide by a solvent-assisted two-step lactic acid process comprises the following steps: (1) adding a catalyst and a low boiling point solvent to a polymerization-grade lactic acid monomer aqueous solution to carry out a dehydration polycondensation reaction to obtain a lactic acid oligomer, and then distilling off the low boiling point solvent and storing it for subsequent recycling; (2) Add a high boiling point solvent to the lactic acid oligomer obtained in step (1), first stir and heat at a relatively low temperature to evaporate the residual low boiling point solvent, then continue to heat to the depolymerization temperature and connect a vacuum pump to generate a crude lactide product under reduced pressure conditions.

[0010] Furthermore, the polymerization-grade monomer aqueous solution in step (1) is a polymerization-grade L-lactic acid or D-lactic acid aqueous solution with a concentration of 30%-105%.

[0011] Furthermore, the low boiling point solvent in step (1) is at least one of toluene, ethylbenzene, o-xylene, m-xylene, p-xylene, cyclohexane, n-heptane, n-octane or n-nonane; and the mass ratio of the low boiling point solvent to the polymerization grade lactic acid monomer aqueous solution is 0.1-10:1.

[0012] Furthermore, the catalyst in step (1) is at least one of stannous oxide, tin oxide, stannous chloride, stannous octoate, tin chloride, zinc oxide, zinc lactate, and zinc sulfate; and the molar ratio of the catalyst to the polymerization-grade lactic acid monomer aqueous solution is 0.001-0.05:1.

[0013] Furthermore, the temperature of the dehydration polycondensation reaction in step (1) is 120-160°C and the time is 0.5-2.5 hours. The free water in the raw material is quickly taken out by the organic solvent vapor at the initial stage of heating, the water phase is separated and settled in the liquid separation pipeline of the water separator, and the organic phase is refluxed to the reaction system at the upper end of the liquid separation pipeline.

[0014] Furthermore, the molecular weight of the lactic acid oligomer in step (1) is 800-6000, preferably 2000-4500.

[0015] Furthermore, the high boiling point solvent in step (2) is an organic alcohol with a boiling point greater than 200°C, including at least one of hexadecanol, octadecanol, eicosanol or tetraethylene glycol; and the mass ratio of the high boiling point solvent to the lactic acid oligomer is 0.03-1:1.

[0016] Furthermore, in step (2), the temperature for distilling off the residual low boiling point solvent is 120-160°C; the depolymerization temperature is 190-250°C, preferably 200-240°C; the reduced pressure condition is less than 50 kPa, preferably 1-5 kPa; and the reaction time is 5-45 min.

[0017] A production device for preparing lactide by a solvent-assisted two-step lactic acid process comprises a dehydration prepolymerization reactor, a depolymerization reactor, a water separator, a condenser, a low-boiling-point solvent storage tank, a lactide product collection tank and a vacuum pump; the upper part of the dehydration prepolymerization reactor is connected to the water separator, the condenser and the low-boiling-point solvent storage tank; the inlet of the depolymerization reactor is connected to the outlet of the dehydration prepolymerization reactor, and the outlet of the depolymerization reactor is connected to the lactide product collection tank and the vacuum pump.

[0018] The depolymerization reactor is provided with a high boiling point solvent feed port and a discharge port connected to a low boiling point solvent storage tank to evaporate and recover the low boiling point solvent residue, and both ports are in a closed state during the depolymerization reaction.

[0019] Furthermore, a combined agitator is arranged inside the depolymerization reactor, and the combined agitator is composed of an anchor agitator at the bottom and a high-speed turbine or propeller agitator in the middle.

[0020] Bottom anchor agitator: Anchor agitators have blades with a basic outer edge shape that is consistent with the inner wall of the bottom of the reactor. The blades are close to the inner wall of the bottom of the reactor to ensure that the agitator can effectively reduce the adhesion and accumulation of materials on the wall when rotating, preventing agglomeration or the formation of dead zones. Anchor agitators can cause the reaction system to move up along the reactor wall and then sink to the center, forming a circular flow pattern. This flow pattern helps to fully contact and mix the materials, thereby effectively avoiding local overheating or uneven reaction problems. The choice of anchor agitators is not limited to adding beams or additional blades on the basis of the standard anchor type.

[0021] Middle high-speed stirring paddle: Use turbine or propeller stirring paddle. The high-speed rotating stirring paddle can effectively disperse the polymer in the organic solvent to prevent agglomeration. It has a turbulent effect, and the strong turbulence helps to improve the mass transfer efficiency and promote the volatilization of the product.

[0022] The combined agitator keeps the high-boiling-point solvent in the depolymerization reactor in a high-speed shaking state, ensuring that when the prepolymer reaction system of the dehydration prepolymerization reactor is introduced into the depolymerization reactor, it can immediately enter a dispersed state. The combined stirring effectively prevents the viscous polymer from adhering to the reactor wall. High-speed stirring is conducive to the effective evaporation of low-boiling-point solvent residues at a lower temperature, reducing energy consumption. After heating to the depolymerization temperature, the lactide generated by the depolymerization reaction can escape quickly, improving the yield and purity of the product. Through this design, the depolymerization reactor achieves efficient mixing, dispersion and product volatilization in the high-viscosity polymer system, avoiding material adhesion and local overheating, thereby significantly improving the reaction efficiency and product quality.

[0023] The beneficial effects of the present invention are: (1) Different solvents are used to specifically address the reaction requirements of different stages of the two-step method for preparing lactide. During the reaction, low-boiling-point solvents can continuously and efficiently remove the water generated during the reaction through azeotropy, accelerating the rate of polycondensation to obtain lactic acid oligomers with the target molecular weight; the use of high-boiling-point solvents alleviates the problem of increased viscosity of the reaction system during high-temperature depolymerization, which hinders the evaporation of the product, is conducive to the forward progress of the reaction, and improves the efficiency and selectivity of lactide production.

[0024] (2) The dehydration condensation reaction conditions are milder and more efficient, which is conducive to reducing production costs.

[0025] (3) The use of a combined agitator in the depolymerization reactor improves depolymerization efficiency and product quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the reaction process of the present invention.

[0027] Figure 2 This is the GPC analysis test spectrum of the lactic acid oligomer prepared in Example 1 of the present invention.

[0028] Figure 3 This is the GPC analysis test spectrum of the lactic acid oligomer prepared in Comparative Example 1 of the present invention.

[0029] Figure 4 It is a liquid chromatogram of lactide prepared in Example 4 of the present invention.

[0030] Figure 5 It is a gas chromatogram of lactide prepared in Example 4 of the present invention. DETAILED DESCRIPTION

[0031] The technical solution of the present invention will be described in detail and comprehensively through specific embodiments below. It should be pointed out that these descriptions only cover some embodiments of the present invention, not all. Based on the embodiments provided herein, all other variants and implementations that can be implemented by ordinary technicians in the field without creative work should be deemed to fall within the scope of protection of the present invention.

[0032] In order to further illustrate the present invention, the following examples are provided for detailed description. The raw materials used in the following examples of the present invention are all commercially available commodities.

[0033] The device of the present invention is as follows Figure 1 As shown: A production device for synthesizing lactide by a two-step method of lactic acid assisted by a solvent, comprising a dehydration prepolymerization reactor, a depolymerization reactor, a water separator, a condenser, a low-boiling-point solvent storage tank, a lactide product collection tank and a vacuum pump; the upper part of the dehydration prepolymerization reactor is connected to the water separator, the condenser and the low-boiling-point solvent storage tank; the inlet of the depolymerization reactor is connected to the outlet of the dehydration prepolymerization reactor, and the outlet of the depolymerization reactor is connected to the lactide product collection tank and the vacuum pump. The depolymerization reactor is provided with a high-boiling-point solvent feed port and a discharge port connected to the low-boiling-point solvent storage tank to evaporate and recover the low-boiling-point solvent residue. A combined agitator is arranged inside the depolymerization reactor, and the combined agitator is composed of an anchor agitator at the bottom and a high-speed turbine or propeller agitator in the middle.

[0034] Add polymerization-grade lactic acid raw materials and catalysts from the feed port of the dehydration prepolymerization reactor, fill a certain amount of low-boiling-point solvent in the low-boiling-point solvent storage tank, open the valve before the reaction to add the low-boiling-point solvent to the dehydration prepolymerization reactor, then start stirring and mix evenly and heat up. Lactic acid undergoes a dehydration reaction in a boiling low-boiling-point organic solvent, and the generated steam undergoes phase separation and removes moisture after passing through a water separator and a condenser. Continuously reflux the low-boiling-point organic solvent to remove moisture, and finally obtain lactic acid oligomers mixed with organic solvents. Subsequently, open the valve at the lower end of the liquid separator on the right side of the water separator to discharge the low-boiling-point solvent evaporated and condensed from the above-mentioned material system. After dehydration, the solvent is stored in a low-boiling-point container tank for subsequent recycling.

[0035] Furthermore, the obtained lactic acid oligomers are transferred to a depolymerization reactor, a high boiling point solvent is added to the reaction system from a feed port, and the low boiling point solvent residue is first evaporated by stirring and heating at a relatively low temperature, and then recovered to a low boiling point solvent storage tank through a pipeline, and then the temperature is continued to be raised to a depolymerization temperature and a vacuum pump is turned on to depolymerize under reduced pressure to generate a crude lactide product, which is evaporated and stored in a lactide product collection tank.

[0036] Example 1 300 parts of 50% L-lactic acid aqueous solution, 200 parts of toluene and 3 parts of stannous chloride catalyst were added to the dehydration prepolymerization reactor, stirred and heated to 120°C. After 5 minutes, almost all the free water in the lactic acid solution entered the separator tube in the water separator along with the toluene vapor and settled in phases. Continue heating and stirring at 120°C for 1 hour to obtain lactic acid oligomers. Then open the valve at the lower end of the separator tube, continue heating at 120°C to evaporate and dehydrate the toluene, and recover it in a low-boiling point solvent storage tank. The molecular weight of the obtained lactic acid oligomers was determined by gel permeation chromatography, and the number average molecular weight of the lactic acid oligomers was measured to be 1011 ( Figure 2 ).

[0037] The obtained 120 parts of lactic acid oligomers were transported to a depolymerization reactor equipped with 10 parts of octadecyl alcohol and a combined agitator. The temperature in the depolymerization reactor was 140°C. The valve connected to the low-boiling point solvent storage tank was opened, and the residual low-boiling point solvent was evaporated and then the valve was closed. Then the valve connecting the depolymerization reactor and the lactide product collection tank was opened, the temperature was raised to 210°C and the vacuum pump was turned on, the pressure was reduced to 5kPa, and the reaction was carried out for 25 minutes until no product was evaporated, and a white lactide product was obtained with a yield of 87.5%. The lactide product was tested by liquid chromatography, and the chemical purity of the product was detected to be 92.2%; the optical purity of the product was detected by gas chromatography, and it was detected to be 98.5%.

[0038] Example 2 200 parts of 80% D-lactic acid aqueous solution, 150 parts of mixed solvent of ethylbenzene and o-xylene (mass ratio 1:1) and 7 parts of stannous octoate catalyst were added to the dehydration prepolymerization reactor, stirred and heated to 145°C, and the free water in the lactic acid solution quickly entered the separator pipe of the water separator along with the mixed solvent vapor of ethylbenzene and o-xylene and settled in phases, and continued to heat and stir at 145°C for 1.5h to obtain lactic acid oligomers with a number average molecular weight of 1930. Then, the valve at the lower end of the separator pipe was opened, and the ethylbenzene and o-xylene were continued to be heated at 145°C to evaporate and dehydrate, and then recovered in the low boiling point solvent storage tank.

[0039] The obtained 125 parts of lactic acid oligomers were transported to a depolymerization reactor containing 50 parts of eicosanol and a combined agitator turned on. The temperature in the depolymerization reactor was 150°C. The low-boiling point solvent residue was first evaporated, and then the temperature was raised to 230°C and the vacuum pump was turned on. The pressure was reduced to 3kPa, and the reaction was continued for 20 minutes until no product was evaporated. A white lactide product was obtained with a yield of 97.3%. The lactide product was tested by liquid chromatography, and the chemical purity of the product was detected to be 92.1%; the optical purity of the product was detected by gas chromatography, and the optical purity of the product was detected to be 91.6%.

[0040] Example 3 150 parts of 105% L-lactic acid aqueous solution, 100 parts of n-nonane and (3 parts of stannous oxide + 4 parts of zinc lactate) catalyst were added to the dehydration prepolymerization reactor. The other processes were the same as those in Example 2. The mixture was heated and stirred to 160° C. to obtain a lactic acid oligomer with a number average molecular weight of 3300.

[0041] The obtained 125 parts of lactic acid oligomers were transported to a depolymerization reactor containing 50 parts of a mixed solvent of tetraethylene glycol and eicosanol (mass ratio 1:1) and a combined agitator. The temperature in the depolymerization reactor was 170°C. The low-boiling point solvent residue was first evaporated, and then the temperature was raised to 220°C and the vacuum pump was turned on. The pressure was reduced to 1kPa, and the reaction was continued for 20 minutes until no product was evaporated. A white lactide product was obtained with a yield of 98.7%. The lactide product was tested by liquid chromatography, and the chemical purity of the product was detected to be 98.2%; the optical purity of the product was detected by gas chromatography, and the optical purity of the product was detected to be 98.3%.

[0042] Example 4 150 parts of 105% concentration of L-lactic acid aqueous solution, 100 parts of o-xylene and (5 parts of stannous octoate + 2 parts of zinc oxide) catalyst were added to the dehydration prepolymerization reactor, stirred and heated to 150°C, and reacted for 2 hours to obtain lactic acid oligomers with a number average molecular weight of 4150.

[0043] The obtained 125 parts of lactic acid oligomers were transported to a depolymerization reactor containing 80 parts of a mixed solvent of octadecyl alcohol and eicosanol (mass ratio 1:1) and a combined agitator. The other processes were the same as those in Example 3, and a white lactide product was obtained with a yield of 98.5%. The lactide product was subjected to liquid chromatography testing, and the chemical purity of the product was detected to be 98.5% ( Figure 4 ); Gas chromatography test showed that the optical purity of the product was 99.0% ( Figure 5 ).

[0044] Example 5 The other conditions were the same as those in Example 4, except that the low boiling point solvent was replaced with a mixed solvent of 100 parts of p-xylene and n-nonane (mass ratio 1:1), and the mixture was heated to 160° C. with stirring to obtain a lactic acid oligomer with a number average molecular weight of 5970. The white lactide product was obtained by depolymerization with a yield of 96.5%, a chemical purity of 95.4%, and an optical purity of 97.1%.

[0045] Example 6 150 parts of 105% concentration L-lactic acid aqueous solution, 20 parts of o-xylene and (10 parts of stannous octoate + 5 parts of zinc oxide) catalyst were added to the dehydration prepolymerization reactor, stirred and heated to 150°C, and reacted for 2.5 hours to obtain lactic acid oligomers with a number average molecular weight of 2550.

[0046] The obtained 125 parts of lactic acid oligomers were conveyed to a depolymerization reactor containing 50 parts of octadecyl alcohol and a combined agitator turned on. After the low-boiling point solvent was evaporated, the temperature was raised to 230°C and the vacuum pump was turned on. The pressure was reduced to 1 kPa and the reaction was carried out for 20 minutes to obtain a white lactide product with a yield of 95.9%, a chemical purity of 91.0%, and an optical purity of 92.1%.

[0047] Comparative Example 1 The other conditions were the same as those in Example 4. No solvent was added during the dehydration and depolymerization stage. An external vacuum pump was connected. The temperature was first raised to 120°C, and the vacuum was slowly evacuated to 50kPa. The water was stirred and dehydrated (the water was pumped to the condenser and condensed and then flowed out through the water separator) for 1 hour. The temperature was then raised to 150°C, and the dehydration and polycondensation were stirred at 20kPa and 5kPa pressures for 4 hours to obtain lactic acid oligomers. The total reaction time in the dehydration prepolymerization reactor was 9 hours. The number average molecular weight of the obtained lactic acid oligomer was 806. The GPC analysis test spectrum is shown in Figure 3 , which is similar to the prepolymer molecular weight in Example 1 ( Figure 2 ), the polydispersity index (PDI) of the prepolymer is significantly larger, indicating that the molecular weight distribution of the lactic acid oligomers obtained by vacuum heating is wider. Depolymerization gave a white lactide product with a yield of 90.9%, a chemical purity of 82.1%, and an optical purity of 94.3%. Comparing the experimental results, it can be seen that the use of solvents in the dehydration polycondensation process can significantly shorten the reaction time for obtaining lactic acid oligomers of the target molecular weight, while improving the uniformity of the molecular weight distribution and the yield and purity of the lactide product. In contrast, although the method relying solely on vacuum heating can also obtain lactic acid oligomers, its molecular weight is lower, the molecular weight distribution is wider, and the yield and purity of the product are relatively low. Therefore, it is more effective and feasible to use solvent-assisted methods in actual production.

[0048] Comparative Example 2 The dehydration polycondensation procedure was the same as in Example 4, and lactic acid oligomers with a number average molecular weight of 4370 were obtained. After 125 parts of lactic acid oligomers were transferred to the depolymerization reactor and the low boiling point solvent residue was evaporated, the temperature was raised to 220°C without adding a high boiling point solvent, and the lactide product was evaporated to 3 kPa with a yield of 86.9%, a chemical purity of 95.3%, and an optical purity of 96.4%. After the depolymerization reaction was completed, serious carbon deposition was observed on the bottom and inner wall of the reactor. In comparison, the use of a high boiling point solvent in the depolymerization stage can avoid carbon deposition, improve the quality of the product, and reduce the maintenance cost of the equipment and the energy consumption in the production process.

[0049] Comparative Example 3 Other conditions were the same as those in Example 4. In the depolymerization stage, the bottom anchor stirrer was turned off and only the middle high-speed stirring blade was turned on. The number average molecular weight of the obtained oligomeric lactic acid was 4080, and the depolymerization gave a white lactide product with a yield of 91.3%, a chemical purity of 97.2%, and an optical purity of 97.8%.

[0050] Comparative Example 4 Other conditions were the same as those in Example 4. In the depolymerization stage, the middle high-speed stirring blade was turned off and only the bottom anchor stirrer was turned on. The number average molecular weight of the obtained oligomeric lactic acid was 4210, and the depolymerization gave a white lactide product with a yield of 84.0%, a chemical purity of 95.8%, and an optical purity of 98.0%.

[0051] From the comparison of the experimental results of Example 4 with Comparative Examples 3 and 4, it can be seen that the use of a combined agitator is one of the key factors for improving the efficiency of the depolymerization reaction and the quality of the product. The use of a combined agitator in the depolymerization reaction ensures uniform mixing of the materials and efficient mass transfer, thereby improving the yield and purity of the product.

[0052] The disclosed technical solutions enable those skilled in the art to implement the invention by referring to the contents of this document and adjusting the conditions and steps appropriately. The general principles described herein can be applied to other embodiments without departing from the spirit and scope of the invention. Therefore, the invention is not limited to the specific embodiments described herein, but covers the broadest range consistent with all the principles and novel features disclosed herein.

Claims

1. A method for preparing lactide by a solvent-assisted two-step lactic acid process, characterized in that: The following steps are involved: (1) adding a catalyst and a low boiling point solvent to a polymerization-grade lactic acid monomer aqueous solution to carry out a dehydration polycondensation reaction to obtain a lactic acid oligomer, and then distilling off the low boiling point solvent and storing it for subsequent recycling; (2) Add a high boiling point solvent to the lactic acid oligomer obtained in step (1), first stir and heat at a relatively low temperature to evaporate the residual low boiling point solvent, then continue to heat to the depolymerization temperature and connect a vacuum pump to generate a crude lactide product under reduced pressure conditions.

2. The method according to claim 1, characterized in that: The polymerization-grade lactic acid monomer aqueous solution in step (1) is an L-lactic acid or D-lactic acid aqueous solution with a concentration of 30%-105%.

3. The method according to claim 1, characterized in that: The low boiling point solvent in step (1) is at least one of toluene, ethylbenzene, o-xylene, m-xylene, p-xylene, cyclohexane, n-heptane, n-octane or n-nonane; the mass ratio of the low boiling point solvent to the polymerization grade lactic acid monomer aqueous solution is 0.1-10:

1.

4. The method according to claim 1, characterized in that: The catalyst in step (1) is at least one of stannous oxide, tin oxide, stannous chloride, stannous octoate, tin chloride, zinc oxide, zinc lactate, and zinc sulfate; and the molar ratio of the catalyst to the polymerization-grade lactic acid monomer aqueous solution is 0.001-0.05:

1.

5. The method according to claim 1, characterized in that: The temperature of the dehydration polycondensation reaction in step (1) is 120-160° C. and the time is 0.5-2.5 hours.

6. The method according to claim 1, characterized in that: The molecular weight of the lactic acid oligomer in step (1) is 800-6000.

7. The method according to claim 1, characterized in that: The high boiling point solvent in step (2) is an organic alcohol with a boiling point greater than 200° C., including at least one of hexadecanol, octadecanol, eicosanol or tetraethylene glycol; the mass ratio of the high boiling point solvent to the lactic acid oligomer is 0.03-1:

1.

8. The method according to claim 1, characterized in that: In step (2), the temperature for distilling off the residual low boiling point solvent is 120-160° C.; the depolymerization temperature is 190-250° C., the reduced pressure condition is less than 50 kPa, and the reaction time is 5-45 min.

9. A production device for preparing lactide by a solvent-assisted two-step lactic acid process, characterized in that: It includes a dehydration prepolymerization reactor, a depolymerization reactor, a water separator, a condenser, a low boiling point solvent storage tank, a lactide product collection tank, and a vacuum pump; the upper part of the dehydration prepolymerization reactor is connected with the water separator, the condenser and the low boiling point solvent storage tank; the inlet of the depolymerization reactor is connected with the outlet of the dehydration prepolymerization reactor, and the outlet of the depolymerization reactor is connected with the lactide product collection tank and the vacuum pump.

10. The production device according to claim 9, characterized in that: A combined agitator is arranged inside the depolymerization reactor, and the combined agitator is composed of an anchor agitator at the bottom and a high-speed turbine or propeller agitator in the middle.

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