Method and system for continuous, stepwise controlled production of lactide

Abstract

The present invention belongs to the technical field of biodegradable materials and discloses a method and system for continuously producing lactide under stepwise control. The method includes the steps of (1) reacting lactic acid oligomers with a depolymerization catalyst in a first depolymerization reaction unit to obtain a first liquid phase material, (2) circulating the first liquid phase material in a second depolymerization reaction unit to react until the molecular weight of the liquid phase material is greater than 6,000 to obtain a second liquid phase material, (3) circulating the second liquid phase material in a third depolymerization reaction unit to react until the molecular weight of the liquid phase material is greater than 10,000, and (4) collecting and purifying gas-phase crude lactide from the first depolymerization reaction unit, the second depolymerization reaction unit, and the third depolymerization reaction unit. The present invention realizes efficient depolymerization of lactic acid oligomers, reduces the degree of lactide racemization and the possibility of coking and carbonization of the substrate, and ensures the conversion rate of lactic acid oligomers and the stability of the system during the entire reaction process.

Description

Detailed Description of the Invention

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of Chinese patent application No. 202111279100.9, filed on October 31, 2021, the contents of which are incorporated herein by reference.

[0002] [Technical field] The present invention relates to the technical field of biodegradable materials, and more particularly to a method and system for the stepwise controlled continuous production of lactide.

[0003] [Background technology] At present, the world's annual consumption of disposable plastic products has reached 120 million tons, of which only 10% is recycled, another 12% is incinerated, and more than 70% is dumped into the soil, atmosphere and ocean. More than 8 million tons of plastic waste are dumped into the ocean every year, and the number is still increasing. It is expected that the amount of plastic waste in the world's oceans will reach 250 million tons by 2025. Although traditional disposable plastic products have a short service life, their physical and chemical properties are stable and they are difficult to decompose naturally, so large amounts of disposable plastic product waste frequently cause various environmental problems and seriously endanger the health and safety of land, water, animals and humans. About 90 countries and regions around the world have introduced relevant policies and regulations to control or ban disposable non-degradable plastic products.

[0004] Biodegradable plastics currently available on the market include polylactic acid (PLA), polyhydroxy fatty acid esters (PHA), polybutylene succinate (PBS), polybutylene succinate adipate (PBSA), polybutylene adipate terephthalate (PBAT), etc., among which PLA is the most widely used and has the most promising future applications. PLA not only has the basic properties of general polymer materials, but also has excellent processing performance, physical and mechanical properties, and biodegradability, and can be widely used in the packaging industry, textile industry, agricultural industry, consumer goods market, etc., and is gradually developing into a basic raw material necessary for the development of the national economy and society.

[0005] Usually, polylactic acid is synthesized by a two-stage method. The specific synthesis process of polylactic acid by the two-stage method is as follows: In the first stage, lactide is produced from lactic acid, and in the second stage, lactide is ring-opening polymerized to produce polylactic acid. According to this process, PLA with a molecular weight of 100,000 to 1 million can be obtained. Lactide is the key to the entire synthesis process, and the process barrier is relatively high. It usually needs to be produced through polycondensation and depolymerization processes under catalyst, high temperature and high vacuum system, which is prone to causing lactide racemization. This is because the depolymerization process that mainly produces lactide is mainly a "backbite" ester catalysis reaction that occurs in the lactic acid oligomer molecular chain, and the specific process is as follows: Under the action of catalyst, high temperature and high vacuum, the carbonyl on the lactic acid oligomer chain is activated, and the hydroxyl at the beginning of the chain attacks the positively charged carbonyl, breaking the ester bond (the "forward bite" process), forming L / D-lactide, but if a basic oxide is present, the catalyst is in excess, or the temperature is too high, the carboxylate anion at the end of the lactic acid oligomer attacks the chiral carbon atom on the unit adjacent to this lactic acid unit, breaking the bond between the methine carbon and the ester oxygen bond (the "reverse bite" process), inverting the configuration and giving meso-lactide (m-lactide), as shown in formula (1). The presence of m-lactide affects the optical purity of lactide and further affects the ring-opening polymerization of lactide, lowering the molecular weight of the resulting PLA, while damaging the regularity of the PLA structure, reducing its crystallinity and reducing its mechanical properties.

[0006] Therefore, the crude lactide produced by the depolymerization reaction must be refined through processes such as solvent recrystallization, water extraction, rectification, and melt crystallization to reduce the amount of m-lactide in the product. However, since the physical and chemical properties of L-lactide and m-lactide are similar, and lactide itself has characteristics such as a high freezing point, boiling point, and heat sensitivity, separation is difficult, and the overall yield is low at around 40% to 60%, making the overall economic efficiency low. Therefore, racemization in the lactide synthesis process is an important factor that affects the quality and yield of lactide, and is also a focus and difficulty in current lactide technology research both at home and abroad. [ka]

[0007] Morteza Ehsani et al. in the article "Lactide synthesis optimization: investigation of the temperature, catalyst and pressure effects" conducted a detailed study on the effects of temperature, catalyst and other factors on the depolymerization process. After examining the reaction temperature, they found that when the reaction temperature was low, the production of m-lactide was low, and as the temperature increased, the content of m-lactide increased significantly, and when the reaction temperature was 230°C, the content of m-lactide reached 25.52%. After examining the effects of stannous oxide, stannous chloride, stannous octanoate, antimony trioxide and sulfuric acid on the lactide synthesis process, they found that the lactide product obtained using SnCl2 and sulfuric acid as catalysts had the highest purity and the lowest m-lactide content. However, the catalyst concentration cannot be too high. For example, taking SnCl2 as an example, the lactide production yield increases with the increase in SnCl2 concentration, but too much catalyst will accelerate the racemization reaction.

[0008] US5502215A discloses a method for purifying lactide. In this application, lactic acid oligomers are depolymerized in a three-neck flask using SnO as a catalyst to produce crude D,L-lactide. This process requires vigorous stirring and a high reaction temperature (220°C), resulting in low purity of the obtained product and serious coking and carbonization of the substrate at the bottom of the kettle. The high temperature reaction also promotes the racemization of lactide.

[0009] US6326458B discloses a continuous process for producing lactide and lactide polymers, which adopts a falling film tubular evaporator as a depolymerization reactor in the depolymerization step of producing lactide, adds lactic acid oligomers from the top of the evaporator, extracts lactide vapor from the bottom of the tubular evaporator, and discharges unreacted lactic acid oligomers from the bottom outlet. In this process, the reaction temperature required for the falling film reaction process is low, which can effectively reduce the possibility of lactide racemization in the depolymerization process, but the yield of lactide is low, and in order to maintain a high yield of lactide, the feed rate generally needs to be reduced, which also increases the residence time of oligomers on the surface of the falling film reactor, and the undepolymerized lactic acid oligomers will polymerize quickly in the high temperature and high vacuum system, which will increase the molecular weight of the oligomers, which will further affect the depolymerization rate, and is prone to coking and carbonization of oligomers on the surface of the falling film reactor.

[0010] CN111153886A discloses a method and apparatus for rapidly synthesizing lactide with high yield, which uses lactic acid alone or two components of lactic acid and catalyst to be fed into an oligomer production system through a mixer, and increases the residence time by bottom circulation to synthesize oligopolylactic acid. The gas phase component is passed through a rectification system to increase the yield of oligopolylactic acid, and the oligopolylactic acid is removed from unreacted lactic acid and water in a purification device. The light-depleted oligopolylactic acid is added with a catalyst and then passed through a mixer into a depolymerization reactor to be depolymerized into lactide. The heavy components are returned to the depolymerization reactor, and the light components are passed through a purification and recovery system to become lactide products. According to this device, lactide can be efficiently synthesized, and crude lactide can be obtained with a yield of 94%-98% within a short residence time of 0.5-5 minutes, and the content of L-lactide, D-lactide or DL-lactide in the lactide product is 94%-98%, and the content of m-lactide is 0.5%-5.5% by subjecting the light components to a simple purification system. However, in the present invention, the heavy components after depolymerization are directly refluxed to the depolymerization reactor, which not only affects the stability of the depolymerization reaction, but also increases the possibility of coking and carbonization of the reaction substrate on the reactor surface with the increase in the molecular weight of the heavy components and the accumulation of the catalyst, and the degree of racemization of lactide increases, affecting the continuous and stable operation of the reaction.

[0011] [Summary of the Invention] [Problem to be solved by the invention] In view of the shortcomings of the prior art, the present invention provides a method and system for continuously producing lactide under stepwise control. In the present invention, by controlling the multi-stage serial depolymerization stepwise, the efficient depolymerization of lactic acid oligomers is realized, the degree of lactide racemization and the possibility of coking and carbonization of the substrate are reduced, the continuous and stable operation of the depolymerization process and the stability of the composition of the crude lactide product are ensured, the overall speed of the depolymerization reaction is accelerated, and the production efficiency and lactide yield are improved.

[0012] [Means for solving the problem] The present invention relates to (1) reacting a lactic acid oligomer with a depolymerization catalyst in a first depolymerization reaction unit to obtain a first liquid phase material; (2) circulating and reacting the first liquid phase material in a second depolymerization reaction unit until the molecular weight of the liquid phase material is greater than 6,000 to obtain a second liquid phase material; (3) circulating and reacting the second liquid phase material in a third depolymerization reaction unit until the molecular weight of the liquid phase material is greater than 10,000; and (4) collecting and purifying gas-phase crude lactide from the first depolymerization reaction unit, the second depolymerization reaction unit, and the third depolymerization reaction unit.

[0013] Preferably, the first depolymerization reaction unit includes a first depolymerization reactor and a first circulation tank, the lactic acid oligomer and the depolymerization catalyst are reacted in the first depolymerization reactor, and the first liquid phase material obtained by the reaction is fed into the first circulation tank.

[0014] Preferably, said reaction in step (2) is carried out in the presence of a protonating solvent.

[0015] Preferably, the second depolymerization reaction unit includes at least one second depolymerization reactor and at least one second circulation tank, the first liquid phase material and any protonated solvent are reacted in the second depolymerization reactor, the liquid phase material after the reaction is fed into the second circulation tank, if the molecular weight of the liquid phase material is less than or equal to 6,000, the liquid phase material in the second circulation tank is circulated to the second depolymerization reactor for further reaction, if the molecular weight of the liquid phase material is greater than 6,000, the liquid phase material in the second circulation tank is sent to the third depolymerization reaction unit for reaction.

[0016] Preferably, the third depolymerization reaction unit includes at least one third depolymerization reactor and at least one third circulation tank, the second liquid phase material is reacted in the third depolymerization reactor, the liquid phase material after the reaction is fed into the third circulation tank, and when the molecular weight of the liquid phase material is less than or equal to 10,000, the liquid phase material in the third circulation tank is circulated to the third depolymerization reactor for further reaction; and when the molecular weight of the liquid phase material is greater than 10,000, the liquid phase material in the third circulation tank is discharged.

[0017] The present invention also provides a first depolymerization reaction unit; a second depolymerization reaction unit; a third depolymerization reaction unit; and and an apparatus for collecting and purifying gas-phase crude lactide from the first depolymerization reaction unit, the second depolymerization reaction unit, and the third depolymerization reaction unit; reacting a lactic acid oligomer with a depolymerization catalyst in the first depolymerization reaction unit; circulating and reacting the liquid phase material from the first depolymerization reaction unit and any protonated solvent in a second depolymerization reaction unit until the molecular weight of the liquid phase material is greater than 6,000; The system provides a stepwise controlled continuous lactide production system, in which a liquid phase material having a molecular weight of more than 6,000 from the second depolymerization reaction unit is circulated and reacted in a third depolymerization reaction unit until the molecular weight of the liquid phase material becomes higher than 10,000.

[0018] [Effects of the invention] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] (1) Multi-stage tandem depolymerization reaction is adopted, and the reaction is controlled in stages. Each stage of the depolymerization reaction unit mainly includes a depolymerization reactor and a circulation tank, which is adjusted according to the molecular weight of the lactic acid oligomer in the circulation tank to achieve efficient depolymerization, reduce the degree of lactide racemization and the possibility of coking and carbonization of the substrate, and ensure the conversion rate of lactic acid oligomer and the stability of the system during the entire reaction process. The conversion rate of lactic acid oligomer during the entire reaction process reaches more than 97.0%.

[0020] (2) By setting different operating parameters (such as reaction temperature, absolute pressure (also called vacuum degree), reaction time or single-pass reaction time) in each stage of the depolymerization reactor, the feed amount of lactic acid oligomer and the reaction operating parameters at each stage of the depolymerization reaction are stably controlled, the reaction rate of each stage of the depolymerization reactor is accelerated, thereby improving the overall reaction efficiency, ensuring the stability of the quality and yield of the discharged crude lactide, and ensuring stable operation throughout the entire depolymerization process. Compared with the process of the single circulation depolymerization system, the staged depolymerization process improves the depolymerization efficiency by 30%.

[0021] (3) In a preferred embodiment, in addition to adopting a multi-stage serial depolymerization reaction unit for stepwise control, a protonated solvent is used to further reduce the degree of racemization in the lactide synthesis process and the possibility of coking and carbonization of the substrate, thereby improving the quality of the product. The content of m-lactide in the crude lactide obtained in the three depolymerization reaction units can be controlled within 6.0%. Compared with the kettle-type circulating depolymerization reactor, the degree of racemization of lactide is reduced by more than 50%, and compared with the single-pass wiped film evaporator or multiple depolymerization reactors connected in series but not controlled in steps, the conversion rate of lactic acid oligomers is improved by more than 10%.

[0022] [Brief description of the drawings] FIG. 1 is a structural schematic diagram of a specific embodiment of a system for continuously producing lactide while controlling it in a stepwise manner according to the present invention.

[0023] [Mode for carrying out the invention] Hereinafter, a specific embodiment of the present invention will be described in detail with reference to the drawings. It should be understood that the specific embodiment described in this specification is used only to explain and interpret the present invention, and is not used to limit the present invention.

[0024] The method for continuously producing lactide in a stepwise controlled manner according to the present invention comprises the steps of: (1) reacting a lactic acid oligomer with a depolymerization catalyst in a first depolymerization reaction unit to obtain a first liquid phase material; (2) circulating and reacting the first liquid phase material in a second depolymerization reaction unit until the molecular weight of the liquid phase material is greater than 6,000 to obtain a second liquid phase material; (3) circulating and reacting the second liquid phase material in a third depolymerization reaction unit until the molecular weight of the liquid phase material is greater than 10,000; and (4) collecting and purifying the gas-phase crude lactide from the first depolymerization reaction unit, the second depolymerization reaction unit, and the third depolymerization reaction unit.

[0025] In the method according to the present invention, in step (1), a preliminary reaction is carried out, and the continuous and stable reaction of the material is realized by adjusting the conditions, and the stability of the discharge is obtained. In step (2), the material is further reacted, and the racemization of the product is controlled by controlling the time of the single-pass reaction, so that the reaction efficiency and conversion rate are further improved. In step (3), the material is further reacted, and the depolymerization reaction of the large molecular weight material is further advanced by adjusting the temperature and vacuum degree, and the racemization of the product is controlled by adjusting the time of the single-pass reaction of the material, so that the product meets the requirements of the product, and the reaction conversion rate is ensured. The multi-stage reaction (preferably three-stage reaction) of steps (1) to (3) is linked and related to each other, so that the quality and yield of the discharged crude lactide are stable, and the continuous and stable operation of the entire depolymerization process is realized.

[0026] In the method according to the present invention, in step (1), the molecular weight of the lactic acid oligomer may be from 800 to 3000, and is preferably from 1200 to 2800. In this specification, the molecular weight of the lactic acid oligomer refers to the weight average molecular weight.

[0027] In the present invention, the method may further include a step of producing a lactic acid oligomer by sequentially dehydrating L-lactic acid and / or D-lactic acid and polycondensing the L-lactic acid and / or D-lactic acid. The dehydration step mainly removes free water in lactic acid and may be in the form of normal pressure or reduced pressure. The polycondensation conditions may include a reaction temperature of 140 to 170°C, an absolute pressure of 1000 to 2000 Pa, and a reaction time of 0.5 to 4 hours.

[0028] In the method according to the present invention, the amount of the depolymerization catalyst used in step (1) is preferably 0.4% to 3%, and more preferably 0.8% to 2%, based on the mass of the lactic acid oligomer.

[0029] In the method according to the present invention, preferably in step (1), the depolymerization catalyst is a tin-based catalyst, more preferably at least one of stannous octoate, SnCl2, and SnO.

[0030] In the method according to the present invention, in step (1), the reaction conditions preferably include a reaction temperature of 180 to 200° C., an absolute pressure of 500 to 1500 Pa, and a reaction time of 3 to 8 minutes.

[0031] According to one particular embodiment of the present invention, the first depolymerization reaction unit includes a first depolymerization reactor and a first circulation tank, the lactic acid oligomer and the depolymerization catalyst are reacted in the first depolymerization reactor, and the first liquid phase material obtained by the reaction is fed into the first circulation tank.

[0032] As a preferred embodiment, in the first circulation tank, the liquid level is maintained at 50% to 70%, the pressure is maintained at 10 kPa to normal pressure, and the temperature is maintained at 160 to 200° C. In this preferred embodiment, the possibility of further intermolecular polymerization reaction of the lactic acid oligomer occurring can be reduced, and coking and carbonization can be reduced.

[0033] In the method according to the present invention, in a preferred embodiment, the conversion rate of the lactic acid oligomer in the reaction in step (1) is controlled to be between 50% and 60% by controlling the amount of the lactic acid oligomer sent to the first depolymerization reactor. In this preferred embodiment, the possibility of further intermolecular polymerization reaction of the lactic acid oligomer occurring can be reduced, and coking and carbonization can be reduced.

[0034] In the method according to the present invention, in a preferred embodiment, the reaction in step (2) is carried out in the presence of a protonated solvent. In a particular embodiment, the first liquid phase material from the first depolymerization reaction unit is mixed with a protonated solvent and sent to the second depolymerization reaction unit. In this preferred embodiment, the use of a protonated solvent can further reduce the degree of racemization during lactide synthesis and the possibility of coking and carbonization of the substrate, thereby improving the quality of the product.

[0035] In the present invention, the protonation solvent may be at least one of a diamine having 12 or more carbon atoms and a diol having 12 or more carbon atoms. Preferably, the melting temperature of the protonation solvent is 80 to 160°C, more preferably 100 to 160°C. More preferably, the protonation solvent is at least one of a diamine having C12 to C18 and a diol having C12 to C18. More preferably, the protonation solvent is at least one of dodecanediamine, tetradecanediamine, hexadecanediamine, tetradecanediol, and hexadecanediol.

[0036] In the method according to the present invention, the amount of the protonation solvent used in the reaction in step (2) may be 0.1% to 6% based on the mass of the lactic acid oligomer, but is preferably 1% to 3%.

[0037] According to one particular embodiment of the present invention, the second depolymerization reaction unit includes at least one second depolymerization reactor and at least one second circulation tank, the first liquid phase material and any protonated solvent are reacted in the second depolymerization reactor, the liquid phase material after the reaction is put into the second circulation tank, when the molecular weight of the liquid phase material is 6,000 or less, the liquid phase material in the second circulation tank is circulated to the second depolymerization reactor for further reaction, when the molecular weight of the liquid phase material is greater than 6,000, the liquid phase material in the second circulation tank is sent to the third depolymerization reaction unit for reaction. In a preferred embodiment, when the molecular weight of the liquid phase material is 3000-6,000, the liquid phase material in the second circulation tank is circulated to the second depolymerization reactor for further reaction, when the molecular weight of the liquid phase material is greater than 6,000 and less than 10,000, the liquid phase material in the second circulation tank is sent to the third depolymerization reaction unit for reaction.

[0038] In the above specific embodiment, the second depolymerization reaction unit may include one second depolymerization reactor and one second circulation tank, or may include two or more second depolymerization reactors and two or more second circulation tanks. Preferably, when the second depolymerization reaction unit includes two or more second depolymerization reactors and two or more second circulation tanks, one second circulation tank is disposed for each second depolymerization reactor, and each second depolymerization reactor and the second circulation tanks disposed corresponding to each second depolymerization reactor constitute one circulation reaction unit. In one specific example, the second depolymerization reaction unit includes two second depolymerization reactors and two second circulation tanks, where the second depolymerization reactor A and the second circulation tank a constitute one circulation reaction unit 2-1, and the second depolymerization reactor B and the second circulation tank b constitute another circulation reaction unit 2-2. The liquid phase material from the first depolymerization reaction unit (i.e., the first liquid phase material) is first fed into the second depolymerization reactor A of the circulation reaction unit 2-1 for reaction, and the liquid phase material after the reaction is fed into the second circulation tank a. If the molecular weight of the liquid phase material is 4500 or less (e.g., 3000 to 45000), the liquid phase material in the second circulation tank a is circulated to the second depolymerization reactor A for further reaction. If the molecular weight of the liquid phase material is greater than 4500 and less than 6,000, the liquid phase material in the second circulation tank a is circulated to the second depolymerization reactor A for further reaction. The liquid phase material in the second circulation tank b is sent to the second depolymerization reactor B of the circulation reaction unit 2-2 for reaction, and the liquid phase material after the reaction is put into the second circulation tank b. When the molecular weight of the liquid phase material is 6,000 or less (for example, 4500 to 6,000), the liquid phase material in the second circulation tank b is circulated to the second depolymerization reactor B for further reaction, and when the molecular weight of the liquid phase material is greater than 6,000 and less than 10,000, the liquid phase material in the second circulation tank b is sent to the third depolymerization reaction unit for reaction. In the actual operation process, the more circulation reaction units each consisting of one depolymerization reactor and one circulation tank that the second depolymerization reaction unit has, the more precisely the molecular weight of the lactic acid oligomer at each stage can be controlled and the better the reaction effect can be obtained. However, considering both the cost and the range of effect improvement, it is preferable that the second depolymerization reaction unit includes one second depolymerization reactor and one second circulation tank.

[0039] In the method according to the present invention, the reaction conditions in the second depolymerization reactor may include a reaction temperature of 200 to 220° C., an absolute pressure of 400 to 1000 Pa, and a single-pass reaction time of 2 to 5 min.

[0040] In the method according to the present invention, in a preferred embodiment, in the single-pass reaction in the second depolymerization reactor, the supply amount of the lactic acid oligomer is 3 to 5 times the actual reaction amount. In this preferred embodiment, the residence time of the lactic acid oligomer on the surface of the second depolymerization reactor can be shortened, the occurrence of polymerization reaction can be suppressed, the yield can be improved, and the quality of the product can be ensured.

[0041] In the method according to the present invention, in a preferred embodiment, the second circulation tank is maintained at a liquid level of 50% to 70%, a pressure of 10 kPa to normal pressure, and a temperature of 160 to 200°C.

[0042] In the method according to the present invention, the conversion rate of lactic acid oligomers reaches 70% or more in the depolymerization reaction in step (2).

[0043] According to one particular embodiment of the present invention, the third depolymerization reaction unit includes at least one third depolymerization reactor and at least one third circulation tank, the second liquid phase material is reacted in the third depolymerization reactor, the liquid phase material after the reaction is fed into the third circulation tank, when the molecular weight of the liquid phase material is less than or equal to 10,000, the liquid phase material in the third circulation tank is circulated to the third depolymerization reactor for further reaction, and when the molecular weight of the liquid phase material is greater than 10,000, the liquid phase material in the third circulation tank is discharged.

[0044] In the above specific embodiment, the third depolymerization reaction unit may include one third depolymerization reactor and one third circulation tank, or may include two or more third depolymerization reactors and two or more third circulation tanks. Preferably, when the third depolymerization reaction unit includes two or more third depolymerization reactors and two or more third circulation tanks, one third circulation tank is disposed for each third depolymerization reactor, and each third depolymerization reactor and the third circulation tank disposed corresponding to each third depolymerization reactor constitute one circulation reaction unit. In one specific example, the third depolymerization reaction unit includes two third depolymerization reactors and two third circulation tanks, where the third depolymerization reactor C and the third circulation tank c constitute one circulation reaction unit 3-1, and the third depolymerization reactor D and the third circulation tank d constitute another circulation reaction unit 3-2. The liquid phase material from the second depolymerization reaction unit (i.e., the second liquid phase material) is first fed into the third depolymerization reactor C of the circulation reaction unit 3-1 for reaction, and the liquid phase material after the reaction is fed into the third circulation tank c. If the molecular weight of the liquid phase material is 8000 or less (e.g., greater than 6,000 and less than 8000), the liquid phase material in the third circulation tank c is circulated to the third depolymerization reactor C for further reaction. If the molecular weight of the liquid phase material is greater than 8000 and less than 10,000, the liquid phase material in the third circulation tank c is sent to the third depolymerization reactor D of the circulation reaction unit 3-2 for reaction. The liquid phase material after the reaction is fed into the third circulation tank d. If the molecular weight of the liquid phase material is 10,000 or less, the liquid phase material in the third circulation tank d is circulated to the third depolymerization reactor D for further reaction. If the molecular weight of the liquid phase material is greater than 10,000, the liquid phase material in the third circulation tank d is discharged from the system. In the actual operation process, the more circulation reaction units each consisting of one depolymerization reactor and one circulation tank that the third depolymerization reaction unit has, the more precisely the molecular weight of lactic acid oligomer can be controlled at each stage, and the better the reaction effect can be obtained. However, considering both the cost and the degree of effect improvement, it is preferable that the third depolymerization reaction unit includes one third depolymerization reactor and one third circulation tank.

[0045] In the method according to the present invention, the reaction conditions in the third depolymerization reactor may include a reaction temperature of 220 to 240° C., an absolute pressure of 200 to 800 Pa, and a single-pass reaction time of 1 to 4 min.

[0046] In the method according to the present invention, in a preferred embodiment, in the single-pass reaction in the third depolymerization reactor, the supply amount of the lactic acid oligomer is 4 to 6 times the actual reaction amount. In this preferred embodiment, the residence time of the lactic acid oligomer on the surface of the third depolymerization reactor can be shortened, the occurrence of polymerization reaction can be suppressed, the yield can be improved, and the quality of the product can be ensured.

[0047] In the method according to the present invention, as a preferred embodiment, the third circulation tank is maintained at a liquid level of 10% to 30%, a pressure of 10 kPa to normal pressure, and a temperature of 160 to 200°C.

[0048] In the method according to the present invention, the liquid phase material having a molecular weight of more than 10,000 discharged from said third depolymerization reaction unit is a lactic acid high polymer, which may be hydrolyzed to form lactic acid for reuse.

[0049] In the method according to the present invention, the conversion rate of lactic acid oligomers reaches 70% or more in the depolymerization reaction in step (3).

[0050] According to the method of the present invention, the conversion rate of lactic acid oligomers reaches 97.0% or more throughout the process of stepwise controlling the depolymerization reaction.

[0051] In the method according to the present invention, in a preferred embodiment, the reactors of the first depolymerization reaction unit, the second depolymerization reaction unit, and the third depolymerization reaction unit (i.e., the first depolymerization reactor, the second depolymerization reactor, and the third depolymerization reactor) are wiped-film depolymerization reactors, more preferably thin-film evaporators, molecular distillation evaporators, or other agitated thin-film evaporators, respectively.

[0052] According to the method of the present invention, the gas-phase crude lactide produced in the three depolymerization reaction units is discharged from the top of each depolymerization reactor, and the composition of the obtained crude lactide is such that the L-lactide content is 82% by mass to 92% by mass, the m-lactide content is 1.0% by mass to 6% by mass, the L-lactic acid content is 0.5% by mass to 6% by mass, and the dimer and trimer content is 1.5% by mass to 6% by mass.

[0053] According to the method of the present invention, the gas-phase crude lactide produced in the three depolymerization reaction units enters into a separation and purification process, and can be directly purified by rectification or other purification and purification processes, and the quality of the obtained product meets the requirements of polymerization-grade lactide monomer.

[0054] The system for continuously producing lactide in a stepwise controlled manner according to the present invention comprises: a first depolymerization reaction unit; a second depolymerization reaction unit; a third depolymerization reaction unit; and and an apparatus for collecting and purifying gas-phase crude lactide from the first depolymerization reaction unit, the second depolymerization reaction unit, and the third depolymerization reaction unit; reacting a lactic acid oligomer with a depolymerization catalyst in the first depolymerization reaction unit; circulating and reacting the liquid phase material from the first depolymerization reaction unit and any protonated solvent in a second depolymerization reaction unit until the molecular weight of the liquid phase material is greater than 6,000; The liquid phase material having a molecular weight of more than 6,000 from the second depolymerization reaction unit is circulated and reacted in a third depolymerization reaction unit until the molecular weight of the liquid phase material is higher than 10,000.

[0055] In a preferred embodiment, the first depolymerization reaction unit includes a first depolymerization reactor and a first circulation tank, the lactic acid oligomer and the depolymerization catalyst are reacted in the first depolymerization reactor, and the liquid phase material obtained by the reaction is fed into the first circulation tank.

[0056] In a preferred embodiment, the second depolymerization reaction unit includes at least one second depolymerization reactor and at least one second circulation tank, the liquid phase material from the first depolymerization reaction unit and any protonated solvent are reacted in the second depolymerization reactor, the liquid phase material after the reaction is fed into the second circulation tank, if the molecular weight of the liquid phase material is 6,000 or less, the liquid phase material in the second circulation tank is circulated to the second depolymerization reactor for further reaction, and if the molecular weight of the liquid phase material is greater than 6,000, the liquid phase material in the second circulation tank is sent to the third depolymerization reaction unit for reaction.

[0057] In a preferred embodiment, the third depolymerization reaction unit includes at least one third depolymerization reactor and at least one third circulation tank, in which a liquid phase material having a molecular weight of more than 6,000 from the second depolymerization reaction unit is reacted in the third depolymerization reactor, and the liquid phase material after the reaction is placed in the third circulation tank; if the molecular weight of the liquid phase material is 10,000 or less, the liquid phase material in the third circulation tank is circulated to the third depolymerization reactor for further reaction; if the molecular weight of the liquid phase material is greater than 10,000, the liquid phase material in the third circulation tank is discharged from the system.

[0058] In the system described in the present invention, the reactors of the first depolymerization reaction unit, the second depolymerization reaction unit, and the third depolymerization reaction unit (i.e., the first depolymerization reactor, the second depolymerization reactor, and the third depolymerization reactor) may be wiped-film depolymerization reactors, preferably thin-film evaporators, molecular distillation evaporators, or other agitated thin-film evaporators, respectively.

[0059] In one particular embodiment of the present invention, as shown in Fig. 1, the system for continuously producing lactide under stepwise control includes a first depolymerization reaction unit, a second depolymerization reaction unit, and a third depolymerization reaction unit, in which the first depolymerization reaction unit includes a first depolymerization reactor I and a first circulation tank IV, the second depolymerization reaction unit includes a second depolymerization reactor II and a second circulation tank V, the second circulation tank V is disposed with a first molecular weight detection control assembly VII, the third depolymerization reaction unit includes a third depolymerization reactor III and a third circulation tank VI, the third circulation tank VI is disposed with a second molecular weight detection control assembly VIII. The specific operation process of the system is as follows: First, the lactic acid oligomer 01 is sent to the first depolymerization reactor I for reaction, the unreacted lactic acid oligomer is put into the first circulation tank IV, the pressure is controlled to 10 kPa to normal pressure, and the temperature is controlled to 160 to 200°C. When the liquid level in the tank reaches 50% to 70%, the first circulation tank discharge material 05 is sent alone or mixed with a protonated solvent 10 to the second depolymerization reactor II for reaction, the unreacted lactic acid oligomer is put into the second circulation tank V, the pressure is controlled to 10 kPa to normal pressure, and the temperature is controlled to 160 to 200°C. When the liquid level in the tank reaches 50% to 70%, the molecular weight of the lactic acid oligomer in the second circulation tank V is detected. If the molecular weight is 6,000 or less (preferably 3,000 to 6,000), the second depolymerization reactor 1 is discharged as the circulation material 06 in the second circulation tank. The unreacted lactic acid oligomer is put into the third circulation tank VI, and the pressure is adjusted to 10 kPa to normal pressure and the temperature to 160 to 200°C. When the liquid level in the third circulation tank VI reaches 10% to 30%, the molecular weight of the oligomer in the third circulation tank VI is detected. When the molecular weight is less than 10,000, the material is returned to the third depolymerization reactor III as the third circulation tank circulating material 08. When the molecular weight is greater than 10,000, the material is discharged from the reaction system as the lactic acid high polymer 09. The discharged material is the lactic acid high polymer, which may be hydrolyzed to form lactic acid for reuse. The gas-phase crude lactide produced in the three depolymerization reactors is discharged from the top of each depolymerization reactor, and the resulting crude lactide product 04 enters the separation and purification process.

[0060] The method and system for producing lactide of the present invention will be further described below with reference to examples. The examples are carried out on the basis of the technical solution of the present invention, and provide detailed embodiments and specific operation processes, but the scope of protection of the present invention is not limited to the following examples.

[0061] Unless otherwise specified, all experimental methods in the following examples are conventional methods in the art. Unless otherwise specified, the experimental materials used in the following examples can be purchased from biochemical reagent stores.

[0062] The lactic acid used in the examples of the present invention is heat-resistant grade L-lactic acid having a lactic acid content of 88% or more and an optical purity of 99.0% or more.

[0063] The present invention uses Malvern Viscotek OMNISEC GPC / SEC gel chromatograph to analyze the molecular weight of lactic acid oligomers. Conventional calibration method is adopted, polystyrene (PS) is used as the internal standard, the column is model number T3000, size is 300 mmL x 8.0 mm, column temperature is 40°C, flow rate is 1.0 mL / min, sample concentration is 2-5 mg / mL, and sample injection volume is 500 μL.

[0064] The present invention uses Agilent high performance liquid chromatography to analyze the chemical purity of lactide, the content of L-lactic acid, dimer and trimer. An ultraviolet detector is used, phosphoric acid and acetonitrile are used as the mobile phase, the column is model number ZORBAX SB-Aq, the column length is 250 mm, the column inner diameter is 4.6 mm, and the particle size of the inner packing material is 5 μm. Detection wavelength: 200 nm, column temperature: 40° C., flow rate: 1 mL / min, sample injection volume: 5 μL.

[0065] The present invention uses an Agilent gas chromatograph to analyze the lactide content of various optical isomers. The column is CYCLOSIL-B model, the vaporizer temperature is 250°C, the detector temperature is 280°C, and a hydrogen flame ionization detector is used. The column temperature is programmed to start at 100°C, hold for 5 min, raise to 140°C at a rate of 4°C / min, hold for 7 min, raise to 200°C at a rate of 8°C / min, and hold for 20 min. The carrier gas N2 flow rate is 1.4 mL / min, hydrogen flow rate is 30 mL / min, air flow rate is 400 mL / min, and sample injection volume is 0.5 μL.

[0066] Yield of lactide purification process Y, and product yield of the entire purification process Y 総 The calculation formula is as follows:

number

[0067] Use WZZ-2S automatic polarimeter to analyze the specific rotation of the sample and express the optical purity of the sample. The specific rotation of pure L-lactide is -278, the specific rotation of pure D-lactide is +278, and the specific rotation of m-lactide is 0, and the calculation formula for the optical purity X of the sample is as follows:

number

[0068] The embodiment of the present invention is carried out according to the apparatus and procedure of Fig. 1. First, lactic acid oligomer 01 is sent to the first wiped film depolymerization reactor I for reaction, and unreacted lactic acid oligomer is put into the circulation tank IV, and the pressure is controlled to 10 kPa to normal pressure and the temperature is controlled to 160 to 200°C. When the liquid level in the tank reaches 50% to 70%, the first circulation tank discharge material 05 is sent alone or mixed with a protonated solvent 10 to the second wiped film depolymerization reactor II for reaction, and unreacted lactic acid oligomer is put into the circulation tank V, and the pressure is controlled to 10 kPa to normal pressure and the temperature is controlled to 160 to 200°C. When the liquid level in the tank reaches 50% to 70%, the molecular weight of the oligomer in the circulation tank V is detected. If the molecular weight is lower than 6,000, preferably 3,000 to 6,000, the second circulation tank circulating material 06 is sent to the second circulation tank circulating material 06. The unreacted lactic acid oligomer is put into the circulation tank VI, and the pressure is controlled to 10 kPa to normal pressure and the temperature to 160 to 200°C. When the liquid level in the circulation tank VI reaches 10% to 30%, the molecular weight of the oligomer in the circulation tank VI is detected. When the molecular weight is less than 10,000, the material is returned to the third depolymerization reactor III as the circulation material 08 in the third circulation tank. When the molecular weight is greater than 10,000, the material is discharged from the reaction system as the lactic acid polymer 09. The discharged material is the lactic acid polymer 10, which may be hydrolyzed to form lactic acid for reuse. The gas-phase crude lactide produced in the three depolymerization reactors is discharged from the top of each depolymerization reactor, and the resulting crude lactide product 04 enters the separation and purification stage. Production of lactic acid oligomers

[0069] (1) Removal of free water from lactic acid: Add 4000g of L-lactic acid (lactic acid content is about 88.0%, optical purity is 99.2%) into a reaction vessel equipped with a stirring system, maintain the system pressure at about 50kPa with a vacuum circulating water pump, start to heat under vacuum, gradually heat to 110-120℃, and dehydrate for 2h, thus gradually evaporating the free water in the reaction system from the reaction system. (2) Preparation of lactic acid oligomer: After removing most of the free water in the system, increase the vacuum of the system, gradually reduce the system pressure to about 1.2 kPa, gradually increase the temperature of the liquid material to 160°C, and react for 2.5 hours, during which a polycondensation reaction occurs between the lactic acid molecules, and the water generated by the reaction in the system is evaporated from the system to obtain a lactic acid oligomer with a molecular weight of 1901. Example 1

[0070] 3000g of lactic acid oligomer was mixed with 30g of stannous octanoate catalyst and sent to the first wiped film depolymerization reactor, and the depolymerization reaction conditions were controlled as follows: vacuum degree 600Pa, reaction temperature 190℃, single-pass reaction time 4min. Unreacted lactic acid oligomer was put into the first circulation tank, and the temperature was controlled at 180℃ and the pressure at 20kPa. When the liquid level rose to 60%, the molecular weight of the lactic acid oligomer was detected as 3216. In this process, the conversion rate of the lactic acid oligomer was 54.6%. The lactic acid oligomer in the first circulation tank was sent to the second wiped film depolymerization reactor, and the reaction temperature was controlled at 210°C, the vacuum was controlled at 400Pa, and the single-pass reaction time was controlled at 3min. The amount of lactic acid oligomer fed was 4 times the actual reaction amount. The unreacted lactic acid oligomer was put into the second circulation tank, and the liquid level was maintained at 50%, the pressure at 10kPa, and the temperature at 180°C. The detection result showed that the molecular weight of the lactic acid oligomer in the second circulation tank was 4915. The lactic acid oligomer was circulated to the second depolymerization reactor for further reaction. If the molecular weight of the oligomer at the outlet of the second circulation tank was greater than 6,000, the oligomer was sent to the third depolymerization reactor. In this process, the conversion rate of the lactic acid oligomer was 72.4%. The lactic acid oligomer in the second circulation tank is sent to the third depolymerization reactor, the reaction temperature is controlled at 230℃, the vacuum degree is controlled at 300Pa, the single-pass reaction time is controlled at 2min, the amount of lactic acid oligomer fed is 5 times the actual reaction amount, the unreacted lactic acid oligomer is put into the third circulation tank, the temperature is controlled at 180℃, the pressure is controlled at 20kPa, when the liquid level rises to 20%, the detection result shows that the molecular weight of the lactic acid oligomer is 7864, this lactic acid oligomer is circulated to the third depolymerization reactor for further reaction, when the molecular weight of the lactic acid oligomer rises to 10,000, this lactic acid oligomer is discharged from the system. In this process, the conversion rate of the lactic acid oligomer is 71.1%. The composition of crude lactide obtained throughout the process is 89.7% L-lactide, 3.2% m-lactide, 2.5% L-lactic acid, and 3.4% lactic acid dimer and trimer. The conversion rate of lactic acid oligomers reaches 98.3% throughout the stepwise cyclic depolymerization process. The crude lactide product is purified and refined using a two-stage rectification system, and the chemical purity and optical purity of the product meet the requirements for polymerization grade lactide monomer. Example 2

[0071] 3000g of lactic acid oligomer was mixed with 30g of stannous octanoate catalyst and sent to the first wiped film depolymerization reactor, and the depolymerization reaction conditions were controlled as follows: vacuum 1500Pa, reaction temperature 230℃, single-pass reaction time 8min. Unreacted lactic acid oligomer was put into the first circulation tank, and the temperature was controlled at 200℃ and the pressure at 20kPa. When the liquid level rose to 60%, the molecular weight of the lactic acid oligomer was detected as 3713. In this process, the conversion rate of the lactic acid oligomer was 50.8%. The lactic acid oligomer in the first circulation tank was sent to the second wiped film depolymerization reactor, and the reaction temperature was controlled at 220°C, the vacuum degree was controlled at 1000Pa, and the single-pass reaction time was controlled at 5min. The amount of lactic acid oligomer fed was three times the actual reaction amount. The unreacted lactic acid oligomer was put into the second circulation tank, and the liquid level was maintained at 50%, the pressure at 20kPa, and the temperature at 180°C. The detection result showed that the molecular weight of the lactic acid oligomer in the second circulation tank was 6032, and the oligomer was sent to the third depolymerization reactor. In this process, the conversion rate of the lactic acid oligomer was 71.2%. The lactic acid oligomer in the second circulation tank is sent to the third depolymerization reactor, the reaction temperature is controlled at 240°C, the vacuum degree is controlled at 800Pa, the single-pass reaction time is controlled at 4min, the amount of lactic acid oligomer fed is 4 times the actual reaction amount, the unreacted lactic acid oligomer is put into the third circulation tank, the temperature is controlled at 180°C, the pressure is controlled at 20kPa, when the liquid level rises to 20%, the detection result shows that the molecular weight of the lactic acid oligomer is 7357, this lactic acid oligomer is circulated to the third depolymerization reactor for further reaction, when the molecular weight of the lactic acid oligomer rises to 10,000, this lactic acid oligomer is discharged from the system. In this process, the conversion rate of the lactic acid oligomer is 70.3%. The composition of crude lactide obtained throughout the process was 84.6% L-lactide, 5.9% m-lactide, 1.5% L-lactic acid, and 4.0% lactic acid dimer and trimer. The conversion of lactic acid oligomers throughout the stepwise cyclic depolymerization process was 97.1%. The crude lactide product is purified and refined using a two-stage rectification system, and the chemical purity and optical purity of the product meet the requirements for polymerization grade lactide monomer. Example 3

[0072] 3000g of lactic acid oligomer was mixed with 30g of stannous octanoate catalyst and sent to the first wiped film depolymerization reactor, and the depolymerization reaction conditions were controlled as follows: vacuum degree 500Pa, reaction temperature 180℃, single-pass reaction time 3min, the amount of lactic acid oligomer fed was 4 times the actual reaction amount, and the unreacted lactic acid oligomer was put into the first circulation tank, the temperature was controlled at 180℃, and the pressure was controlled at 20kPa, and when the liquid level rose to 60%, the molecular weight of the lactic acid oligomer was detected as 3461. In this process, the conversion rate of the lactic acid oligomer was 51.8%. The lactic acid oligomer in the first circulation tank is sent to the second wiped film depolymerization reactor, the reaction temperature is controlled at 200°C, the vacuum degree is controlled at 400Pa, the single-pass reaction time is controlled at 2min, the amount of lactic acid oligomer fed is 5 times the actual reaction amount, the unreacted lactic acid oligomer is put into the second circulation tank, the liquid level is maintained at 50%, the pressure is controlled at 20kPa, and the temperature is controlled at 180°C, the molecular weight of the lactic acid oligomer in the second circulation tank is 4456, the lactic acid oligomer is circulated to the second depolymerization reactor and further reacted, the molecular weight of the oligomer at the outlet of the second circulation tank is detected to be greater than 6,000, the oligomer is sent to the third depolymerization reactor. In this process, the conversion rate of the lactic acid oligomer is 71.9%. The lactic acid oligomer in the second circulation tank is sent to the third depolymerization reactor, the reaction temperature is controlled at 220°C, the vacuum degree is controlled at 200Pa, the single-pass reaction time is controlled at 1min, the amount of lactic acid oligomer fed is 6 times the actual reaction amount, the unreacted lactic acid oligomer is put into the third circulation tank, the temperature is controlled at 180°C, the pressure is controlled at 20kPa, when the liquid level rises to 20%, the detection result shows that the molecular weight of the lactic acid oligomer is 7071, this lactic acid oligomer is circulated to the third depolymerization reactor for further reaction, when the molecular weight of the lactic acid oligomer rises to 10,000, this lactic acid oligomer is discharged from the system. In this process, the conversion rate of the lactic acid oligomer is 72.4%. The composition of crude lactide obtained throughout the process was 89.2% L-lactide, 3.2% m-lactide, 2.9% L-lactic acid, and 4.7% lactic acid dimer and trimer. The conversion of lactic acid oligomers throughout the stepwise cyclic depolymerization process was 97.6%. The crude lactide product is purified and refined using a two-stage rectification system, and the chemical purity and optical purity of the product meet the requirements for polymerization grade lactide monomer. Example 4

[0073] Except for using the same mass of stannous chloride catalyst, lactide was produced according to the method of Example 1. After reacting in the first depolymerization reactor, the molecular weight of the lactic acid oligomer at the outlet of the first circulation tank was 3527, and the conversion rate of the lactic acid oligomer was 53.8%. After the lactic acid oligomer was put into the second depolymerization reactor and reacted, the molecular weight of the lactic acid oligomer in the second circulation tank was 5319. The lactic acid oligomer was directly circulated to the second depolymerization reactor and further reacted. The detection result showed that the molecular weight of the lactic acid oligomer at the outlet of the second circulation tank was greater than 6,000. In this process, the conversion rate of the lactic acid oligomer was 71.7%. After reacting in the third depolymerization reactor, the detection results showed that the molecular weight of the lactic acid oligomer at the outlet of the third circulation tank was 8213, and the lactic acid oligomer was circulated to the third depolymerization reactor for further reaction. When the molecular weight of the lactic acid oligomer increased to 10,000, the lactic acid oligomer was discharged from the system. In this process, the conversion rate of the lactic acid oligomer was 70.8%. The composition of crude lactide obtained throughout the process was 88.2% L-lactide, 3.9% m-lactide, 2.7% L-lactic acid, and 4.4% lactic acid dimer and trimer. The conversion of lactic acid oligomers throughout the stepwise cyclic depolymerization process was 97.8%. The crude lactide product is purified and refined using a two-stage rectification system, and the chemical purity and optical purity of the product meet the requirements for polymerization grade lactide monomer. Example 5

[0074] Except for using a molecular distillation type depolymerization evaporator, lactide was prepared according to the method of Example 1. After reacting in the first depolymerization reactor, the molecular weight of the lactic acid oligomer at the outlet of the first circulation tank was 3009, and the conversion rate of the lactic acid oligomer was 55.3%. After the lactic acid oligomer was fed into the second depolymerization reactor and reacted, the conversion rate of the lactic acid oligomer in this process was 73.5%. After reacting in the third depolymerization reactor, the conversion rate of the lactic acid oligomer in this process was 72.7%. The composition of crude lactide obtained throughout the process was 91.3% L-lactide, 2.2% m-lactide, 2.9% L-lactic acid, and 3.0% lactic acid dimer and trimer. The conversion of lactic acid oligomers throughout the stepwise cyclic depolymerization process was 98.7%. The crude lactide product is purified and refined using a two-stage rectification system, and the chemical purity and optical purity of the product meet the requirements for polymerization grade lactide monomer. Example 6

[0075] Lactide was produced according to the method of Example 1, except that 1.0% tetradecanediol was added to the unreacted lactic acid oligomer in the first depolymerization reactor before it was sent to the second depolymerization reactor. The conversion rate of the lactic acid oligomer in the second depolymerization reactor was 74.4%. The conversion rate of the lactic acid oligomer in the third depolymerization reactor was 72.4%. The composition of the crude lactide obtained throughout the process was 91.3% L-lactide, 1.8% m-lactide, 2.1% L-lactic acid, and 3.3% lactic acid dimer and trimer content. The conversion of lactic acid oligomers was 98.6% throughout the stepwise cyclic depolymerization process. The crude lactide product obtained at each stage had a low degree of racemization, and the material throughput was high throughout the process. The crude lactide product is purified and refined using a two-stage rectification system, and the chemical purity and optical purity of the product meet the requirements for polymerization grade lactide monomer. Example 7

[0076] Lactide was produced according to the method of Example 1, except that 1.0% dodecanediamine was added to the unreacted lactic acid oligomer in the first depolymerization reactor before it was sent to the second depolymerization reactor. The conversion rate of the lactic acid oligomer in the second depolymerization reactor was 72.9%. The conversion rate of the lactic acid oligomer in the third depolymerization reactor was 71.9%. The composition of crude lactide obtained throughout the process was 91.5% L-lactide, 1.6% m-lactide, 2.2% L-lactic acid, and 3.1% lactic acid dimer and trimer. The conversion of lactic acid oligomers throughout the stepwise cyclic depolymerization process was 98.7%. The crude lactide product is purified and refined using a two-stage rectification system, and the chemical purity and optical purity of the product meet the requirements for polymerization grade lactide monomer. Comparative Example 1

[0077] Lactide was produced according to the method of Example 1, except that the molecular weight of the circulation tank was not detected and adjusted according to the detection results. Throughout the process, the composition of the crude lactide obtained was 85.4% L-lactide, 6.5% m-lactide, 4.7% L-lactic acid, and 5.8% dimer and trimer. Throughout the stepwise circulation depolymerization process, the conversion rate of lactic acid oligomers was 95.3%. The obtained crude lactide product corresponds to the case where the quality is controlled in stages, but the conversion rate of lactic acid oligomer is low, and the reaction throughput throughout the entire process is only about 200 g / h (in Example 1, the throughput is more than 300 g / h). In addition, with the increase in the molecular weight of the circulating oligomer, the composition at the inlet of the depolymerization reactor changes constantly, the depolymerization reaction rate changes constantly, the degree of racemization changes constantly, and the composition of the product at the outlet changes dynamically. The reaction stability in the continuous process is poor, and it is also necessary to periodically stop the device and remove the slag. Comparative Example 2

[0078] The depolymerization scheme of lactic acid oligomers described in the CN111153886A application by Nanjing University, i.e., a one-stage circulating depolymerization process, was used to produce lactide. In this comparative example, the configuration of the depolymerization reactor was the same as that of Example 1. The depolymerization reaction conditions were controlled at a vacuum of 300 Pa, a reaction temperature of 210°C, and a single-pass residence time of about 2 min. After the reaction, the heavy components were put into a circulation tank and mixed with new lactic acid oligomers and sent to the depolymerization reactor, and the mass ratio of the new material to the circulating material during the reaction was controlled to be 1:3. As the reaction proceeded, the liquid level of the circulation tank was controlled to be 60%, the pressure to be 50 kPa, and the temperature to be 180°C. The slag was periodically removed based on the molecular weight of the lactic acid oligomers at the outlet of the circulation tank or the cumulative amount of catalyst circulation, and slag removal was performed when the molecular weight of the oligomers was greater than 10,000. Analysis showed that the crude lactide product contained 85.2% L-lactide, 8.6% m-lactide, 2.4% L-lactic acid, and 3.2% dimer and trimer. The conversion rate of lactic acid oligomers in the process of synthesizing crude lactide was 95.2%. The product yield reached 95%, but the product had serious racemization. Moreover, as the reaction proceeded, the material composition at the inlet of the depolymerization reactor changed significantly, and as a result, the depolymerization reaction rate also changed significantly, which prolonged the depolymerization reaction time of the system, and the material composition of the product at the outlet became unstable. Comparative Example 3

[0079] A test was carried out to produce lactide by depolymerizing lactic acid oligomers according to a scheme similar to that of Japanese Patent Application JPH08333359A, that is, depolymerization was carried out in a form in which three reactors were connected in series, the material of the third reactor was circulated to the first reactor for reaction, and slag was periodically removed from the bottom of the third reactor, and the form of the depolymerization reactor used was the same as that of Example 1. The reaction temperatures in the three reactors connected in series were all 200°C, the vacuum degree was increased sequentially to 600 Pa, 400 Pa, and 300 Pa, respectively (similar to Example 1), the feed rate of the first depolymerization reactor was the same as Example 1, in the first depolymerization reactor, the conversion rate of lactic acid oligomer was 54.9%, the unreacted material in the first depolymerization reactor was continuously sent to the second depolymerization reactor, and the conversion rate was 49.3%, while the unreacted material in the second depolymerization reactor was sent to the third depolymerization reactor to react, and the conversion rate was 40.1%. At this time, if the unreacted components were removed as slag, the conversion rate of lactic acid oligomer in the entire reaction was 86%, and the content of m-lactide in the crude lactide product was 5.8%, but the composition of the crude lactide was relatively stable. On the other hand, if the unreacted components were circulated back to the first depolymerization reactor, the composition at the inlet of each reactor would change significantly, improving the product yield but compromising the reaction efficiency and stability of the reaction system.

[0080] From the above examples and comparative examples, it can be seen that the present invention adopts a multi-stage serial depolymerization reaction, and controls the reaction step by step, adjusting according to the molecular weight of the lactic acid oligomer in the circulation tank, thereby realizing efficient depolymerization of the lactic acid oligomer, reducing the degree of lactide racemization and the possibility of the substrate being coked and carbonized, and ensuring the conversion rate of the lactic acid oligomer and the stability of the system in the entire reaction process. The conversion rate of the lactic acid oligomer in the entire process reaches more than 97.0%.

[0081] Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, the technical solution of the present invention may be modified in a number of simple ways, including combining each technical feature in another suitable manner, and these simple modifications and combinations should also be regarded as the disclosed content of the present invention, and all of them belong to the protection scope of the present invention.

[0082] [Explanation of symbols] I First depolymerization reactor II. Second depolymerization reactor III. Third depolymerization reactor IV First Circulation Tank V Second Circulation Tank VI 3rd Circulation Tank VII First molecular weight detection control assembly VIII Second molecular weight detection control assembly 01 Lactic acid oligomer 02 Second Circulation Tank Supply Material 03 3rd Circulation Tank Supply Material 04 Crude lactide 05 First circulation tank discharge material 06 Second Circulation Tank Circulation Material 07 Second circulation tank discharge material 08 3rd Circulation Tank Circulation Material 09 Lactic acid high polymer 10 Protonated Solvents [Brief description of the drawings]

[0083] [Figure 1] FIG. 1 is a structural schematic diagram of a specific embodiment of a system for continuously producing lactide in a stepwise controlled manner according to the present invention.

Claims

1. A process for continuously and stepwise controlled production of lactide, comprising the steps of: (1) reacting lactic acid oligomers with a depolymerization catalyst in a first depolymerization reaction unit to obtain a first liquid phase material; (2) circulating and reacting the first liquid phase material in a second depolymerization reaction unit until the molecular weight of the liquid phase material is greater than 6,000 to obtain a second liquid phase material; (3) circulating and reacting the second liquid phase material in a third depolymerization reaction unit until the molecular weight of the liquid phase material is greater than 10,000; and (4) collecting and purifying the gas-phase crude lactide from the first depolymerization reaction unit, the second depolymerization reaction unit, and the third depolymerization reaction unit.

2. The method according to claim 1, characterized in that in step (1), the molecular weight of the lactic acid oligomer is 800 to 3,000.

3. The method described in claim 2, characterized in that in step (1), the molecular weight of the lactic acid oligomer is 1,200 to 2,800.

4. The method further comprises the step of producing a lactic acid oligomer by sequentially dehydrating and polycondensing L-lactic acid and / or D-lactic acid, 2. The method according to claim 1, wherein the polycondensation conditions include a reaction temperature of 140 to 170° C., an absolute pressure of 1000 to 2000 Pa, and a reaction time of 0.5 to 4 h.

5. The method according to claim 1, characterized in that in step (1), the amount of the depolymerization catalyst used is 0.4% to 3% based on the mass of the lactic acid oligomer.

6. The method described in claim 5, characterized in that in step (1), the amount of the depolymerization catalyst used is 0.8% to 2% relative to the mass of the lactic acid oligomer.

7. 2. The method of claim 1, wherein in step (1), the depolymerization catalyst is a tin-based catalyst.

8. The method of claim 7, wherein in step (1), the depolymerization catalyst is at least one of stannous octoate, SnCl 2 , and SnO.

9. 2. The method of claim 1, wherein in step (1), the reaction conditions include a reaction temperature of 180-200° C., an absolute pressure of 500-1500 Pa, and a reaction time of 3-8 min.

10. The first depolymerization reaction unit includes a first depolymerization reactor and a first circulation tank, 2. The method according to claim 1, wherein the lactic acid oligomers and the depolymerization catalyst are reacted in the first depolymerization reactor, and the first liquid phase material obtained by the reaction is placed in the first circulation tank.

11. The method according to claim 10, wherein the first circulation tank is maintained at a liquid level of 50% to 70%, a pressure of 10 kPa to atmospheric pressure, and a temperature of 160 to 200°C.

12. 2. The method according to claim 1, wherein in the reaction of step (1), the conversion of lactic acid oligomers is controlled to be between 50% and 60%.

13. 2. The method of claim 1, wherein the reacting in step (2) is carried out in the presence of a protonated solvent.

14. 14. The method of claim 13, wherein the protonated solvent is at least one of a diamine having 12 or more carbon atoms and a diol having 12 or more carbon atoms.

15. 15. The method of claim 14, wherein the melting temperature of the protonated solvent is between 80 and 160°C.

16. 15. The method of claim 14, wherein the protonated solvent is at least one of a C12-C18 diamine and a C12-C18 diol.

17. The method of claim 16, wherein the protonated solvent is at least one of dodecanediamine, tetradecanediamine, hexadecanediamine, tetradecanediol, and hexadecanediol.

18. The method according to claim 13, characterized in that in the reaction of step (2), the amount of the protonated solvent used is 0.1% to 6% based on the mass of the lactic acid oligomer.

19. The method according to claim 18, characterized in that in the reaction of step (2), the amount of the protonated solvent used is 1% to 3% relative to the mass of the lactic acid oligomer.

20. The second depolymerization reaction unit includes at least one second depolymerization reactor and at least one second circulation tank; 2. The method according to claim 1, further comprising: reacting the first liquid phase material and any protonated solvent in the second depolymerization reactor, and feeding the liquid phase material after the reaction into the second circulation tank; if the molecular weight of the liquid phase material is less than or equal to 6,000, circulating the liquid phase material in the second circulation tank to the second depolymerization reactor for further reaction; if the molecular weight of the liquid phase material is greater than 6,000, sending the liquid phase material in the second circulation tank to the third depolymerization reaction unit for reaction.

21. The method according to claim 20, wherein when the molecular weight of the liquid phase material is 3000-6,000, the liquid phase material in the second circulation tank is circulated to the second depolymerization reactor for further reaction; when the molecular weight of the liquid phase material is more than 6,000 and less than 10,000, the liquid phase material in the second circulation tank is sent to the third depolymerization reaction unit for further reaction.

22. 21. The method of claim 20, wherein the reaction conditions in the second depolymerization reactor include a reaction temperature of 200 to 220° C., an absolute pressure of 400 to 1000 Pa, and a single-pass reaction time of 2 to 5 min.

23. The method according to claim 20, wherein in the single-pass reaction of the second depolymerization reactor, the amount of lactic acid oligomers fed is 3 to 5 times the actual reaction amount.

24. 21. The method according to claim 20, wherein the second circulation tank is maintained at a liquid level of 50% to 70%, a pressure of 10 kPa to atmospheric pressure, and a temperature of 160 to 200°C.

25. The third depolymerization reaction unit includes at least one third depolymerization reactor and at least one third circulation tank; 2. The method according to claim 1, further comprising: reacting the second liquid phase material in the third depolymerization reactor; introducing the reacted liquid phase material into the third circulation tank; circulating the liquid phase material in the third circulation tank to the third depolymerization reactor for further reaction when the molecular weight of the liquid phase material is 10,000 or less; and discharging the liquid phase material in the third circulation tank when the molecular weight of the liquid phase material is greater than 10,000.

26. 26. The method of claim 25, wherein the reaction conditions in the third depolymerization reactor include a reaction temperature of 220 to 240° C., an absolute pressure of 200 to 800 Pa, and a single-pass reaction time of 1 to 4 min.

27. The method according to claim 25, wherein in the single-pass reaction of the third depolymerization reactor, the amount of lactic acid oligomers fed is 4 to 6 times the actual reaction amount.

28. 26. The method according to claim 25, wherein the third circulation tank is maintained at a liquid level of 10% to 30%, a pressure of 10 kPa to atmospheric pressure, and a temperature of 160 to 200°C.

29. 2. The method of claim 1, wherein the reactors of the first depolymerization reaction unit, the second depolymerization reaction unit, and the third depolymerization reaction unit are each a wiped film depolymerization reactor.

30. The method described in claim 1, characterized in that the reactors of the first depolymerization reaction unit, the second depolymerization reaction unit, and the third depolymerization reaction unit are each a thin film evaporator, a molecular distillation evaporator, or other agitated thin film evaporator.

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