阅读说明：本技术 一种便于催化剂回用的乳酸合成丙交酯的工艺方法 (Process method for synthesizing lactide from lactic acid convenient for catalyst recycling ) 是由 孙启梅 彭绍忠 张通 白富栋 姚新武 于 2019-10-31 设计创作，主要内容包括：一种便于催化剂回用的乳酸合成丙交酯的工艺方法：以乳酸为原料,脱除自由水分,加入负载过渡金属氧化物的催化剂,缩合脱水,形成乳酸低聚物,再经解聚,得到粗L/D-丙交酯；反应完成后,采用高温旋风分离装置将解聚后的带催化剂的未完全反应的乳酸低聚物进行分离,得到脱除催化剂后的乳酸低聚物流；将其返回解聚单元,继续进行反应,得到粗L/D-丙交酯产品,脱除的催化剂处理后回用；所述催化剂为以过渡金属为活性组分,以Al-2O-3或ZSM-5为载体的粒径为2-8mm的球形或圆柱状颗粒。采用特定大小的Al-2O-3载体和ZSM-5分子筛载体负载过渡金属作为催化剂,能在保证反应活性的前提下,便于后续利用高温旋风分离装置将催化剂从反应底物中脱除,提高整个过程乳酸的转化率,降低整个丙交酯制备过程的成本。(A process method for synthesizing lactide by lactic acid convenient for catalyst recycling comprises the following steps: removing free water from lactic acid serving as a raw material, adding a catalyst loaded with transition metal oxide, condensing and dehydrating to form lactic acid oligomer, and depolymerizing to obtain crude L/D-lactide; after the reaction is finished, separating the depolymerized lactic acid oligomer with the catalyst and not completely reacted by adopting a high-temperature cyclone separation device to obtain a lactic acid oligomer material flow after the catalyst is removed; returning the crude L/D-lactide product to a depolymerization unit for continuous reaction to obtain a crude L/D-lactide product, and recycling the removed catalyst after treatment; the catalyst takes transition metal as an active component and Al 2 O 3 Or ZSM-5 as carrierSpherical or cylindrical particles with a diameter of 2-8 mm. Using Al of a specific size 2 O 3 The carrier and the ZSM-5 molecular sieve carrier load transition metal as the catalyst, so that the catalyst can be conveniently removed from a reaction substrate by utilizing a high-temperature cyclone separation device on the premise of ensuring the reaction activity, the conversion rate of lactic acid in the whole process is improved, and the cost in the whole lactide preparation process is reduced.)

1. A process for synthesizing lactide from lactic acid convenient for catalyst reuse comprises removing free water from lactic acid as raw material, adding catalyst loaded with transition metal oxide, condensing and dehydrating to form lactic acid oligomer, depolymerizing the lactic acid oligomer to obtain crude L/D-lactide; after the reaction is finished, separating the depolymerized lactic acid oligomer with the catalyst and not completely reacted by adopting a high-temperature cyclone separation device to obtain a lactic acid oligomer material flow after the catalyst is removed; returning the crude L/D-lactide product to a depolymerization unit for continuous reaction to obtain a crude L/D-lactide product, and recycling the removed catalyst after treatment; the catalyst takes transition metal as an active component and Al₂O₃Or ZSM-5 as carrier and spherical or cylindrical particle with 2-8mm diameter.

2. The process of claim 1, wherein the catalyst is prepared by the following method: adding Al to soluble salt solution of transition metal₂O₃Stirring the carrier or ZSM-5 molecular sieve carrier, fully soaking, filtering, drying and roasting to obtain the catalyst; the transition metal is at least one of zinc, antimony, titanium, zirconium, cobalt, copper and nickel.

3. The process of claim 2, wherein the transition metal loading is: based on the amount of transition metal substance, it is mixed with Al₂O₃The molar ratio of (1: 10) - (1: 1), or the loading capacity of the catalyst on the ZSM-5 molecular sieve is 0.1% -10% by mass of the transition metal.

4. The process of claim 2, wherein said Al is₂O₃The specific surface area of the carrier is 150-220m²The pore volume is not less than 1.0 mL/g; the silicon and aluminum of the ZSM-5 molecular sieve carrierThe ratio is less than or equal to 50.

5. The process method of claim 2, wherein in the preparation process of the catalyst, the roasting adopts a sectional roasting mode: heating from normal temperature to 180 deg.C at a heating rate of 1-2 deg.C/min, calcining for 0.5-1h, further heating to 400 deg.C at a heating rate of 1-5 deg.C/min, calcining for 0.5-2h, and finally heating to 600 deg.C at a heating rate of 5-10 deg.C/min, and calcining for 2-6 h.

6. The process method according to claim 1, wherein the lactic acid is L-lactic acid or D-lactic acid, wherein the removal of free water is carried out under normal pressure or reduced pressure, the temperature is controlled to be lower than 120 ℃, and the content of lactic acid in the feed liquid is not lower than 96% after dehydration.

7. The process method according to claim 1, wherein the amount of the catalyst added is 2-10% by mass of the lactic acid raw material.

8. The process as claimed in claim 1, wherein the condensation and dehydration temperature is 120-160 ℃, the vacuum degree is 1000-2000Pa, and the reaction time is 1.5-3 h.

9. The process as claimed in claim 1, wherein the depolymerization temperature is 170-220 ℃, the vacuum degree is 500-1000P, and the reaction time is 1-2 h.

10. The process of claim 1, further comprising a step of purifying the obtained crude L/D-lactide product by pre-treating the crude L/D-lactide product by solvent water extraction and then refining the crude L/D-lactide product by fractional melt crystallization.

11. The process as claimed in claim 1, wherein the high temperature cyclone separation device is used for treating at a temperature of 120 ℃ to 200 ℃, an inlet pressure of 0.1 to 1.5MPa and a feed flow of 0.2 to 1.0m³/h。

Technical Field

The invention belongs to the field of preparation of high polymer materials, relates to a preparation method of a bio-based material polylactic acid intermediate, and particularly relates to a process method for recycling a special catalyst in the process of synthesizing lactide by catalyzing lactic acid with a metal oxide.

Background

The polylactic acid has good mechanical strength, biocompatibility, biodegradability and bioabsorbability, is a green high polymer material, and has wide application prospect and application field. With the continuous maturity of polylactic acid production technology and the continuous development of application market thereof, the production of polylactic acid by taking non-food crops → lactic acid → polylactic acid as a route has begun to enter the industrialized development period. Natureworks, usa is the largest producer of polylactic acid in the world at present, with a capacity of 14 ten thousand tons, and L-lactide capacity reaches 15 ten thousand tons. The worldwide production capacity of L-lactide and D-lactide is expected to break through 50 million tons by 2020. At present, the high molecular weight PLA produced at home and abroad is mostly obtained by lactide ring-opening polymerization. Therefore, the key to the synthesis technology of high-quality PLA lies not only in its own polymerization process but also in the quality of lactide as a raw material.

The synthesis of lactide usually adopts a two-step catalytic method, i.e. lactic acid/lactate ester dehydration/alcohol polycondensation is firstly carried out to form lactic acid oligomer under the action of a catalyst, and then the oligomer is depolymerized to obtain the lactide product. In the preparation process, the yield and the quality of the catalyst corresponding to the lactide have important influence, so that the selection of a proper catalyst is the key for improving the yield and the purity of the lactide product. At present, inorganic metal oxide catalysts, inorganic metal salt catalysts (mainly metal chlorides) and organic metal salt catalysts are commonly used for synthesizing lactide, wherein an inorganic metal oxide catalytic system is a catalyst type which is researched more and widely applied and mainly comprises aluminum oxide, zinc oxide and other transition metal oxides, such as antimony trioxide, titanium dioxide, zirconium oxide and the like. The addition of a proper amount of these metal oxides increases the activity of the whole reaction and improves the yield and quality of lactide.

The conversion per pass of the whole lactide preparation process is only about 60%, and the rest 40% is discharged as a reaction substrate. If the conversion rate of the whole process is to be improved, the cost of lactide synthesis is to be reduced, and the industrial development of polylactic acid is to be realized, the reaction substrate needs to be recycled and returned to the depolymerization unit or the polycondensation reaction unit, the reaction substrate contains the catalyst required in the polycondensation or depolymerization process, and a large amount of catalyst is returned to the polycondensation or depolymerization, which can cause the circulation accumulation of the catalyst and the adverse effect on the reaction. Therefore, the catalyst in the reaction substrate needs to be removed in time, the recycling of the reaction substrate and the activity of the catalyst in the whole reaction process are ensured, and meanwhile, the recycled catalyst can be recycled, so that the reaction cost is reduced.

CN101585827A discloses a preparation method of high-yield lactide, which takes D, L-lactic acid as a raw material and zinc oxide or zinc lactate as a catalyst to promote the condensation reaction of lactic acid to obtain low-molecular-weight lactic acid, and then crude lactide is obtained through high-temperature cracking, wherein the dosage of the catalyst is 1-2%, the yield of the crude lactide product can reach 85.02%, but the recycling of a reaction substrate in the depolymerization process and the separation and recovery technology of the catalyst are not mentioned.

CN 102863420A discloses a method for preparing medical lactide, which comprises adding catalyst such as antimony trioxide, phosphorus pentoxide or zinc oxide and high boiling point solvent into lactic acid to obtain oligomeric lactic acid, and then depolymerizing under the catalysis of tin salt such as stannous octoate to obtain crude lactide, wherein the product purity can reach 93%, and the yield can reach 98%.

Disclosure of Invention

Aiming at the problems that in the prior art, a catalyst in a reaction substrate is difficult to recover in the preparation process of lactide, so that a large amount of the catalyst is accumulated in the recycling process of the reaction substrate, and the activity and the product yield of the whole reaction process are influenced, the invention provides a process method for synthesizing lactide by lactic acid, which is convenient for catalyst recycling, for the prior art, the process method loads an active component on alumina or low-silicon aluminum molecular sieve particles with specific sizes, performs catalytic reaction, and then adopts a cyclone separation device to recover the catalyst in the reaction substrate. The invention ensures the reaction activity of the whole process, improves the recycling efficiency of reaction substrates, reduces the discharge of wastes, improves the conversion rate of lactic acid and the use efficiency of the catalyst, ensures the stability of the catalyst and reduces the reaction cost in the preparation process of lactide on the one hand.

A process for synthesizing lactide from lactic acid convenient for catalyst reuse comprises removing free water from lactic acid as raw material, adding catalyst loaded with transition metal oxide, condensing and dehydrating to form lactic acid oligomer, depolymerizing the lactic acid oligomer to obtain crude L/D-lactide; after the reaction is finished, separating the depolymerized lactic acid oligomer with the catalyst and not completely reacted by adopting a high-temperature cyclone separation device to obtain a lactic acid oligomer material flow after the catalyst is removed; returning the crude L/D-lactide product to a depolymerization unit for continuous reaction to obtain a crude L/D-lactide product, and recycling the removed catalyst after treatment; the catalyst takes transition metal as an active component and Al₂O₃Or ZSM-5 as carrier and spherical or cylindrical particle with 2-8mm diameter.

Wherein the particle size of the cylindrical particles means that both the length and the diameter are within the above-mentioned range of particle size requirements.

Further, the specific preparation method of the catalyst is as follows: adding Al to soluble salt solution of transition metal₂O₃And stirring the carrier or the ZSM-5 molecular sieve carrier, fully soaking, filtering, drying and roasting to obtain the catalyst. Wherein the transition metal is at least one selected from zinc, antimony, titanium, zirconium, cobalt, copper and nickel; the loading of the transition metal is as follows: based on the amount of transition metal substance, it is mixed with Al₂O₃The molar ratio of (1: 10) - (1: 1), or the loading capacity of the catalyst on the ZSM-5 molecular sieve is 0.1% -10% by mass of the transition metal.

Further, said Al₂O₃The specific surface area of the carrier is 150-220m²The pore volume is not less than 1.0 mL/g.

Furthermore, the silica-alumina ratio of the ZSM-5 molecular sieve carrier is less than or equal to 50, and preferably 10-40.

Furthermore, in the preparation process of the catalyst, the dipping time is 8-12h, the drying temperature is 100-120 ℃, and the drying time is 6-12 h.

Further, in the preparation process of the catalyst, the roasting adopts a sectional roasting mode: heating from normal temperature to 180 deg.C at a heating rate of 1-2 deg.C/min, calcining for 0.5-1h, further heating to 400 deg.C at a heating rate of 1-5 deg.C/min, calcining for 0.5-2h, and finally heating to 600 deg.C at a heating rate of 5-10 deg.C/min, and calcining for 2-6 h.

Furthermore, the lactic acid is L-lactic acid or D-lactic acid raw material, wherein the free water is removed in a normal pressure or reduced pressure mode, the temperature is controlled to be lower than 120 ℃, and the content of the lactic acid in the feed liquid is not lower than 96% after dehydration is finished.

Furthermore, the addition amount of the catalyst is 2-10% of the mass of the lactic acid raw material.

Further, the temperature of the condensation dehydration is 120-160 ℃, the vacuum degree is 1000-2000Pa, the reaction time is 1.5-3h, and the molecular weight of the obtained lactic acid oligomer is 500-2500.

Further, the temperature of the depolymerization is 170-220 ℃, the vacuum degree is 500-1000P, and the reaction time is 1-2 h.

Further, the method also comprises the step of purifying the obtained crude L/D-lactide product, wherein the purification method comprises the steps of carrying out pretreatment by solvent water extraction, and then carrying out fractional melting crystallization and refining.

Furthermore, the purity of the crude lactide product is not less than 86.0%, in order to ensure the fluidity of a reaction substrate and the continuity of the reaction, the one-way molecular yield of the lactide obtained in the preparation process is not less than 83.5%, and the one-way molecular yield of the refined and purified lactide can reach 62.0%.

Furthermore, the chemical purity of the product obtained after the crude lactide is refined and purified is not lower than 99.2 percent, and the optical purity is not lower than 99.0 percent.

Further, the high-temperature cyclone separation device processesThe temperature of the reaction system is 120 ℃ plus 200 ℃, the inlet pressure is 0.1-1.5MPa, and the feeding flow is 0.2-1.0m³/h。

Furthermore, the recovery rate of the catalyst after separation by adopting the high-temperature cyclone separation device can reach more than 85 percent.

Further, the lactic acid oligomer stream separated by the high-temperature cyclone separator and the fresh material from the polycondensation reactor are mixed and then return to the depolymerization section for reaction, the conversion rate of lactic acid in the whole circulation process can reach 98.0 percent, and the yield of lactide can reach 95.0 percent

Compared with the prior art, the process method has the beneficial effects that:

(1) using Al of a specific size₂O₃The carrier and the ZSM-5 molecular sieve carrier load transition metal as the catalyst, so that the catalyst can be conveniently removed from a reaction substrate by using a high-temperature cyclone separation device in the follow-up process on the premise of ensuring the reaction activity, the reaction substrate returns to the next reaction unit, the activity of the whole reaction is not influenced, the conversion rate of the lactic acid in the whole process is improved, and meanwhile, the separated catalyst can be used as a fresh catalyst to return to the reaction unit after simple treatment, so that the cost of the whole lactide preparation process can be reduced.

(2) The carrier has larger specific surface area, pore diameter and acid-base reaction site, on one hand, more active centers can be provided for the reaction, and the yield and product quality of the lactide in the whole reaction process are ensured; on the other hand, the active components of the loaded metal oxide can be effectively dispersed, agglomeration is avoided, the reaction activity is improved, and the strength of the catalyst and the stability of the catalyst in the reaction process are ensured. Meanwhile, due to the electronic effect of the transition metal, the reactivity of the carboxyl carbon and the terminal hydroxyl carbon is increased, and the depolymerization of the oligomer into the lactide is facilitated.

(3) The catalyst has a good catalytic effect on the whole reaction, the chemical purity of the obtained crude lactide is more than 86%, the chemical purity of the refined and purified product is not less than 99.2%, the optical purity is not less than 99.0%, and the once-through yield of the lactide product in the whole process can reach 62.0%.

Detailed Description

The following describes in detail specific embodiments of the present invention. It should be understood that the detailed description and specific examples, while indicating the present invention, are given by way of illustration and explanation only, not limitation.

The molecular yield of lactide in the reaction process was calculated by the following formula.

Wherein m is_{Crude lactide}Denotes the mass of the crude lactide product obtained, m_{Lactic acid starting material}Denotes the mass of the starting lactic acid, x_{Purity of lactide}Indicates the purity of lactide in the crude lactide product, x_{Purity of lactic acid}Indicates the purity of the starting lactic acid, M_{Lactic acid}Denotes the relative molecular weight of lactic acid, M_LactideIndicates the relative molecular weight of the lactide.

The carriers used in the following examples were prepared by the following method:

self-made mesoporous Al₂O₃Powder: weighing 3.0g of Pluronic P123, adding 60.0mL of absolute ethyl alcohol, stirring for dissolving, adding 6.3g of aluminum isopropoxide under strong stirring, continuously stirring until the aluminum isopropoxide is completely dissolved, then adding 4.8mL of concentrated hydrochloric acid, adding 1.4g of citric acid, reacting for 12 hours under the closed condition of 40 ℃, transferring to a culture dish, drying at 60 ℃ to obtain a white solid, grinding, filtering, and roasting for 6 hours at 500 ℃ in a muffle furnace to obtain the mesoporous alumina powder.

Al₂O₃Preparing a carrier: using self-made mesoporous alumina or commercial gamma-Al₂O₃Adding appropriate amount of pore-forming agent, adjuvant and water into the powder as raw materials, stirring, pressing to support columnar carrier with diameter and length of 2-8mm, air drying, oven aging at 120 deg.C, and roasting at 500 deg.C to obtain Al₂O₃A carrier particle.

Self-made ZSM-5 molecular sieve with low silica-alumina ratio: weighing a certain amount of deionized water, aluminum sulfate, potassium hydroxide and n-butylamine, and stirring for 20min to form a solution A; mixing a certain amount of deionized water and silica solForming a solution B; adding ZSM-5 seed crystals and the B solution into the A solution, continuously stirring for 30min to obtain white gel, transferring the white gel into a high-pressure kettle, performing temperature-changing crystallization for 1-3 days, cooling to normal temperature after crystallization is finished, performing suction filtration, washing until the pH value is 8, and drying at 120 ℃ for 2h to obtain ZSM-5 with different silicon-aluminum ratios. By controlling SiO₂And Al₂O₃The mol ratio of the molecular sieve is 10-50, and the ZSM-5 molecular sieves with different low silica-alumina ratios are obtained by controlling the crystallization temperature and the crystallization time.

Preparation of a ZSM-5 molecular sieve carrier: taking a self-made ZSM-5 molecular sieve with low silica-alumina ratio or a commercial ZSM-5 molecular sieve as a raw material, adding a proper amount of pore-forming agent, auxiliary agent and water, stirring uniformly to form a strip-shaped carrier with a spherical or circular cross section, airing, and aging in a 120 ℃ oven to obtain the ZSM-5 molecular sieve.

Example 1

(1) Preparation of the catalyst: weighing Zn (NO)₃)₂•6H₂O29.7 g, dissolved in 100mL of deionized water, and 51g of Al was added thereto₂O₃Carrier particles are stirred and soaked for 8 hours, then dehydrated in a water bath kettle at 90 ℃ to remove excessive moisture, then dried for 8 hours in a drying oven at 120 ℃, and finally roasted in a muffle furnace, wherein the roasting process adopts a three-section program heating mode, namely, the temperature is increased from normal temperature to 180 ℃ at the heating rate of 1 ℃/min, the roasting is carried out for 0.5 hour, then the temperature is continuously increased to 400 ℃ at the heating rate of 2 ℃/min, the roasting is carried out for 1 hour, then the temperature is continuously increased to 600 ℃ at the heating rate of 5 ℃/min, and the roasting is carried out for 4 hours, so that ZnO/Al is obtained₂O₃A catalyst.

The lactide synthesis process comprises the following steps: taking 88% L-lactic acid of Anhui Feng original as raw material, adding 5kg into a dehydration kettle, removing free water at 120 ℃ under normal pressure, sending the lactic acid to a polycondensation reaction kettle when the content of lactic acid in an outlet of the dehydration kettle is not less than 96%, and simultaneously adding 0.20kg of ZnO/Al₂O₃Adding catalyst into polycondensation kettle, controlling temperature of reaction kettle at 160 deg.C and vacuum degree at 1.0kPa, reacting for 2.5h, measuring average molecular weight of oligomer by online viscometer to be 1980, feeding into depolymerization reaction kettle, controlling vacuum degree of depolymerization reactor at 500pa and temperature at 210 deg.C, continuously depolymerizing for 1.5h to obtain crude product of 3.32kg at kettle topLactide product and reaction substrate with flowability and containing undeployed lactic acid oligomer, catalyst and other components are obtained at the bottom of the kettle. Through gas chromatographic analysis, the purity of L-lactide in the crude lactide is 89.0 percent, the yield of L-lactide molecules is 83.9 percent, and the mass of a reaction substrate remained at the bottom of the kettle is 0.69 kg.

Removing and recycling the catalyst: removing the ZnO/Al in the reaction substrate remained at the bottom of the kettle by a high-temperature cyclone separation device₂O₃A catalyst. In the high-temperature cyclone, the feed rate of the reaction substrate was 0.5m³The inlet pressure is controlled to be 0.8MPa, the overflow pressure is 0.17MPa, the temperature of the separator is controlled to be 180 ℃, the recovery efficiency of the catalyst can reach 86.9 percent, the partial catalyst is roasted at 500 ℃ and can be regenerated for use, the reaction substrate obtained by the cyclone separation device can return to the depolymerization section to be mixed with the lactic acid oligomer flow from the polycondensation reactor, the addition amount of the catalyst is controlled by a flowmeter, the mixture enters a depolymerization tower to carry out lactide synthesis reaction, the conversion rate of lactic acid in the whole circulation process is 98.2 percent, and the yield of the lactide is 95.3 percent.

Example 2

Preparation of a supported transition metal oxide ZSM-5 catalyst: weighing Cu (NO)₃)₂•3H₂Dissolving 7.6g of O in 100ml of deionized water, adding 100g of ZSM-5 carrier with the silica-alumina ratio of 20, stirring, dipping for 12h, dehydrating in a water bath kettle at 90 ℃, drying for 10h in an oven at 120 ℃, and finally roasting in a muffle furnace, wherein the roasting process adopts a three-section temperature programming mode, namely, firstly heating from normal temperature to 150 ℃ at the heating rate of 1 ℃/min, roasting for 0.5h, then continuously heating to 350 ℃ at the heating rate of 2 ℃/min, roasting for 1h, then continuously heating to 500 ℃ at the heating rate of 5 ℃/min, and roasting for 4h to obtain the CuO/ZSM-5 catalyst (metal loading is 2%).

The lactide synthesis process comprises the following steps: taking 88% L-lactic acid of Anhui Feng original as a raw material, adding 5kg of L-lactic acid into a dehydration kettle, removing free water at 120 ℃ under normal pressure, sending the L-lactic acid into a polycondensation reaction kettle when the content of lactic acid in an outlet of the dehydration kettle is not less than 96%, simultaneously adding 0.35kg of CuO/ZSM-5 catalyst into the polycondensation kettle, controlling the temperature of the reaction kettle at 150 ℃ and the vacuum degree at 1.2kPa, after reacting for 2.5h, measuring the average molecular weight of the oligomer by an online viscometer to be 1250, then sending the oligomer into a depolymerization reaction kettle, controlling the vacuum degree of the depolymerization reactor at 600pa and the temperature at 220 ℃, continuously depolymerizing for 1.8h, obtaining 3.37kg of crude lactide product at the top of the tower, and obtaining a reaction substrate which has flowability and contains components such as undeploymerized lactic acid oligomer and catalyst at the bottom of the kettle. Through gas chromatographic analysis, the purity of L-lactide in the crude lactide is 87.3 percent, the yield of L-lactide molecules is 83.6 percent, and the mass of a reaction substrate remained at the bottom of the kettle is 0.71 kg.

And removing the CuO/ZSM-5 catalyst from the reaction substrate remained at the bottom of the kettle by a high-temperature cyclone separation device. In the high-temperature cyclone separator, the feeding flow of a reaction substrate is 0.8m3/h, the inlet pressure is controlled at 1.0MPa, the overflow pressure is 0.18MPa, the temperature of the separator is controlled at 160 ℃, the recovery efficiency of the catalyst can reach 85.7%, the catalyst is calcined at 550 ℃ and can be recycled, the reaction substrate obtained by the cyclone separator can be returned to a depolymerization section to be mixed with lactic acid oligomer flow from a polycondensation reactor, the addition amount of the catalyst is controlled by a flowmeter, and the mixture enters a depolymerization tower to carry out lactide synthesis reaction, the conversion rate of lactic acid in the whole circulation process is 98.0%, and the yield of lactide is 95.1%.

Comparative example 1

Taking 88% L-lactic acid of Anhui Feng original as a raw material, adding 5kg of L-lactic acid into a dehydration kettle, maintaining the pressure of the system at about 50kPa, gradually heating to 110-120 ℃, dehydrating until the content of lactic acid in an outlet at the bottom of the kettle is not lower than 96%, sending the L-lactic acid to a polycondensation reaction kettle, simultaneously adding 150g of ZnO and 200g of stannous octoate into the polycondensation kettle, controlling the temperature of the reaction kettle to 140 and 150 ℃, the vacuum degree to 1.1kPa, reacting for 4.0h, measuring the average molecular weight of an oligomer by an online viscometer to 1350, sending the oligomer to a depolymerization reaction kettle, controlling the vacuum degree of the depolymerization reactor to 300pa, the temperature to 220 ℃, continuously depolymerizing for 3.0h, obtaining 3.45kg of crude lactide product (the purity of L-lactide is 87.1%) at the top of the depolymerization kettle, obtaining 0.57kg of a reaction substrate at the bottom of the kettle, and adding a high single-pass depolymerization degree because a catalyst in the reaction substrate cannot be effectively removed, the polymerization degree of the substrate is too large, the substrate cannot be returned to be mixed with the lactic acid oligomer after polycondensation for depolymerization reaction, and the yield of the lactide in the whole preparation process can only reach 85.3 percent.