CN115010696B

CN115010696B - Method for continuously preparing and purifying lactide

Info

Publication number: CN115010696B
Application number: CN202210883125.8A
Authority: CN
Inventors: 韩梦超; 陈志明; 王良波; 马高琪; 梁伟; 裴承度; 徐宁; 林秀锦; 金洁
Original assignee: Zhejiang Hisun Biomaterials Co ltd
Current assignee: Zhejiang Hisun Biomaterials Co ltd
Priority date: 2022-07-26
Filing date: 2022-07-26
Publication date: 2023-10-27
Anticipated expiration: 2042-07-26
Also published as: CN115010696A

Abstract

The invention relates to a method for continuously preparing and purifying lactide, which comprises the following steps: feeding the lactic acid oligomer and polyether polyol into a hypergravity device for step reaction to obtain crude lactide vapor; conveying the obtained crude lactide vapor to a rectifying tower for a rectifying process to obtain distillate; liquefying the obtained distillate to obtain liquefied product, sending a part of liquefied product into a crystallizer, performing cooling crystallization-crystal growth process in the crystallizer to obtain lactide crystals, and sending the rest liquefied product back to a rectifying tower to participate in the rectifying process; heating-insulating the obtained lactide crystal in a crystallizer, and melting and discharging the remaining lactide crystal after the heating-insulating process to obtain purified lactide; wherein the stepwise reaction comprises: the lactic acid oligomer and polyether polyol are polymerized under the first temperature gradient, and then the temperature is raised to the second temperature gradient to carry out the cracking reaction.

Description

Method for continuously preparing and purifying lactide

Technical Field

The invention relates to the technical field of organic chemical purification, and particularly provides a method for continuously preparing and purifying lactide.

Background

Biodegradable plastics have been pursued by the academia and industry from the beginning of their advent, and are typically represented by polylactic acid (PLA). Polylactic acid has excellent biocompatibility and good mechanical processability, and can be completely degraded into CO under natural conditions ₂ And H ₂ O has great application prospect in the fields of 3D printing, spinning, medical use and the like. In addition, the global plastic exclusion regulations bring huge market demands for degradable materials such as polylactic acid.

Polylactic acid (PLA) is generally prepared by two methods: direct polycondensation of lactic acid monomers (i.e., one-step process) and ring-opening polymerization of lactide (i.e., two-step process). The polylactic acid synthesized by the one-step method has wider molecular weight distribution and lower average molecular weight, and the processability of the product is limited; the two-step method is to obtain a lactide intermediate by cracking a lactic acid oligomer, and then obtain polylactic acid by lactide ring opening.

In actual production, the purity and reaction conditions of lactide are controlled to prepare PLA with ultrahigh molecular weight, controllable chemical structure and good mechanical property, which is a synthesis method commonly adopted in industry nowadays. The core technology of the process is the synthesis and purification of lactide, as the purity of lactide shows a clear positive correlation with PLA quality. Therefore, the purification and production cost of lactide is a main limiting factor for limiting the PLA productivity.

Lactide is mainly obtained by cleavage of lactic acid oligomers. Cleavage of lactic acid oligomers into rings and self-polymerization between oligomers are competing reactions. The traditional cracking device has short plates with uneven dispersion or high system viscosity, the yield of continuous cracking for a long time is lower, and the carbon formation risk is greatly improved. In the prior art, the oligomer cleavage process is typically carried out in a conventional tank reactor. The shearing force of the traditional kettle type reactor can not realize uniform dispersion of the catalyst in the high-viscosity materials, and gravity sedimentation in the stirring process can not be eliminated by simply increasing the rotating speed, and the prior art lacks an effective description on how to control the stability of gas-phase feeding so as to achieve the purpose of continuous production.

Currently, the mainstream lactide purification method is: vacuum rectifying, recrystallizing and hydrolyzing. CN112934139A, CN112876452A, CN112480064a discloses that the lactide is purified by multistage rectification, and the purity of the obtained lactide is higher. Purification of lactide by recrystallization is disclosed in CN112047920A, CN111961028A, CN102875522 a. Purification by suitable hydrolysis of lactide is disclosed in CN101696204 a.

Disclosure of Invention

The invention relates to a method for continuously preparing and purifying lactide, which comprises the following steps:

feeding the lactic acid oligomer and polyether polyol into a hypergravity device for step reaction to obtain crude lactide vapor;

conveying the obtained crude lactide vapor to a rectifying tower for a rectifying process to obtain distillate;

liquefying the obtained distillate to obtain liquefied product, sending a part of liquefied product into a crystallizer, performing cooling crystallization-crystal growth process in the crystallizer to obtain lactide crystals, and sending the rest liquefied product back to a rectifying tower to participate in the rectifying process; and

heating-insulating the obtained lactide crystal in a crystallizer, and melting and discharging the remaining lactide crystal after the heating-insulating process to obtain purified lactide;

wherein the stepwise reaction comprises: the lactic acid oligomer and polyether polyol are polymerized under the first temperature gradient, and then the temperature is raised to the second temperature gradient to carry out the cracking reaction.

In one embodiment, the polyether polyol has a functionality of 2 to 3 and is selected from one or more of the following: polyoxypropylene polyol, polyoxyethylene polyol, polytetrahydrofuran diol.

In another embodiment, the polyether polyol has a functionality of 2 to 3 and is selected from one or more of the following: a polyoxypropylene polyol having a number average molecular weight of 400-20000, a polyoxyethylene polyol having a number average molecular weight of 400-20000, a polytetrahydrofuran diol having a number average molecular weight of 1000-10000.

Drawings

Fig. 1: the invention relates to a process flow chart for preparing and purifying lactide.

Fig. 2: (a) The invention relates to a cross-section view of a supergravity device and (b) a material distribution effect diagram

Reference numerals: 1: a lactic acid oligomer feed inlet; 2: a catalyst feed port; 3: a mixer; 4: a supergravity device; 5. 8: connecting with a vacuum device interface; 6. 17, 18, 19, 20, 21: a transfer line; 7: a rectifying tower; 9: a heat exchange condenser; 10. 13: a liquid storage tank; 11. 14, 16: a circulation metering pump; 12: a crystallizer; 15: a reboiler; 22. 23: a waste discharge port; 24: purified lactide discharge port.

Detailed Description

General definitions and terms

All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety, if not indicated otherwise.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In the event of a conflict, the definitions provided herein will control.

All percentages, parts, ratios, etc. are by weight unless otherwise specified.

When an amount, concentration, or other value or parameter is given as either a range, preferred range or upper and lower limit or a particular value, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. When numerical ranges are recited herein, unless otherwise stated, the stated ranges are meant to include the endpoints thereof, and all integers and fractions within the range. The scope of the invention is not limited to the specific values recited when defining the scope. For example, "1-8" encompasses 1, 2, 3, 4, 5, 6, 7, 8 and any subrange comprised of any two values therein, e.g., 2-6, 3-5.

The terms "about", "about" when used in conjunction with a numerical variable generally refer to the value of the variable and all values of the variable being within experimental error (e.g., within a confidence interval of 95% for the average) or within + -10% of the specified value, or more broadly.

The terms "comprising," "including," "having," "containing," or "involving," and other variations thereof herein, are inclusive or open-ended and do not exclude additional unrecited elements or method steps. Those skilled in the art will appreciate that such terms as "comprising" encompass the meaning of "consisting of …". The expression "consisting of …" excludes any element, step or ingredient not specified. The expression "consisting essentially of …" means that the scope is limited to the specified elements, steps, or components, plus any elements, steps, or components that are optionally present that do not materially affect the basic and novel characteristics of the claimed subject matter. It should be understood that the expression "comprising" encompasses the expressions "consisting essentially of …" and "consisting of …".

The term "selected from …" means that one or more elements in the group listed below are independently selected and may include a combination of two or more elements.

When numerical values or range endpoints are described herein, it is to be understood that the disclosure includes the specific value or endpoint cited.

The terms "one or more" or "at least one" as used herein mean one, two, three, four, five, six, seven, eight, nine or more.

Unless otherwise indicated, the terms "combination thereof" and "mixtures thereof" refer to multicomponent mixtures of the elements, e.g., two, three, four, and up to the maximum possible multicomponent mixtures.

Furthermore, the number of components or groups of components of the present invention not previously indicated is not limiting with respect to the number of occurrences (or existence) of components or groups of components. Thus, the singular forms of a component or a constituent should be interpreted to include one or at least one, and the plural unless the numerical value clearly indicates the singular.

The term "optional" or "optionally" as used herein means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.

The method of the invention

Lactide is a heat-sensitive material, and is extremely easy to thermally degrade at high temperature in the presence of moisture. And the existing single purification mechanism can not give consideration to purity, yield and continuous production.

To this end, the inventors devised a process for the continuous preparation, purification of lactide, which has at least the following characteristics:

and the supergravity reaction equipment is introduced to replace the original kettle type device, so that the reaction rate and the cracking yield are improved. Under the same operation condition, the dispersion speed and uniformity of the materials are greatly improved.

Secondly, creatively adopts a step-type lactide preparation scheme (end group modification-high-temperature cracking), inhibits the viscosity surge and high-temperature coking loss of the system, and improves the yield.

The cracking process parameters are designed, so that the feeding mode of the rectifying tower is gas-phase feeding, and the real-time conveying is realized by utilizing the pressure difference between the cracking device and the rectifying tower, thereby avoiding the additional energy supply of the traditional liquid-phase feeding. The parallel connection of the equipment is adopted to realize the stable gas phase feeding in the whole day and the continuous production.

The purification mechanism of coupling rectification and melt crystallization phase is adopted to realize the complementary advantages of the rectification and melt crystallization phase in terms of purity, yield and continuous production. On the basis of not introducing solvent and not increasing energy consumption, the capacity and the quality are greatly improved.

The following description is made in connection with the specific steps:

liquefying the obtained distillate to obtain liquefied product, sending a part of liquefied product into a crystallizer, performing cooling crystallization-crystal growth process through the crystallizer to obtain lactide crystals, and sending the rest liquefied product back to a rectifying tower to participate in the rectifying process; and

the lactide crystal obtained in the process is subjected to a heating-heat preservation process in a crystallizer, the remaining lactide crystal after the heating-heat preservation process is melted and discharged to obtain purified lactide,

Step (a): feeding the lactic acid oligomer and polyether polyol into a supergravity device for step reaction to obtain crude lactide vapor. Wherein the stepwise reaction comprises: the lactic acid oligomer and polyether polyol are polymerized under the first temperature gradient, and then the temperature is raised to the second temperature gradient to carry out the cracking reaction.

The supergravity technology is to strengthen the relative speed and mutual contact between phases by utilizing the unique flow behavior of a multiphase flow system under the supergravity condition, thereby realizing the efficient mass and heat transfer process and chemical reaction process. The mode of obtaining the supergravity mainly forms a centrifugal force field by rotating the whole or parts of the equipment, and the related multiphase flow system mainly comprises a gas system, a solid system and a gas-liquid system.

In this context, the supergravity technique is implemented using a supergravity device. The supergravity device is basically constructed as a device for generating centrifugal force in a high-speed rotation mechanism. Fig. 2 (a) shows a cross-sectional view of a supergravity device of the present invention.

The supergravity device is an emerging high-efficiency reaction device, and micro-nano mixing among materials is carried out by centrifugal force which is several times of the conventional gravity field. The rotor of the supergravity device rotates at a certain rotation speed so as to generate a supergravity condition. Under the action of strong centrifugal force generated in the hypergravity device, huge shearing stress overcomes surface tension, so that a reaction substrate stretches out a huge inter-phase contact interface, and the mass transfer process is greatly strengthened.

The material is fully contacted by utilizing the supergravity device to form the micro-nano reaction unit, so that the step reaction process is accelerated, the heating time of lactide is reduced, the degradation process is inhibited, and the reaction rate and the cracking yield are improved. Under the same operation condition, the supergravity reactor can greatly improve the dispersion speed and uniformity of materials, form a micro-nano reaction unit to accelerate the cracking process, reduce the heating time of lactide, inhibit the degradation process of lactide and improve the yield of lactide.

When the reaction materials enter the supergravity device, the reaction materials are thrown outwards under the action of centrifugal force generated by a rotor, and after the reaction materials pass through a porous structure in the device, the reaction materials (such as polylactic acid oligomer liquid and polyether polyol liquid) are dispersed and crushed to form a large and continuously updated surface area, and extremely fine liquid particles are formed, so that an extremely good mass transfer condition is formed. As shown in FIG. 2 (b), the reaction mass is uniformly dispersed in the hypergravity apparatus by centrifugal force. The stepwise reaction comprises in particular two stages as follows:

the first stage of the stepwise reaction is the polymerization stage of the lactic acid oligomer and the polyether polyol. In one embodiment, the lactic acid oligomer is transported from top to bottom or from the middle to a supergravity cracker, followed by the addition of polyether polyol and heating to a first temperature gradient to effect polymerization. In the polymerization reaction process, the lactic acid oligomer and the polyether polyol are polymerized, and the polyether polyol is introduced into the chain end of the oligomer, so that the chain growth reaction is inhibited, and the viscosity surge caused by the self-polymerization of the lactic acid oligomer is avoided. In the polymerization process, the lactic acid oligomer and the polyether polyol are polymerized by an esterification reaction, and thus, the polymerization process may be also referred to herein as an esterification process, an esterification process. The polymerization time of the polymerization reaction may also be referred to as the esterification time. Meanwhile, polyether polyol which does not participate in the reaction and polyether polyol which is generated by bond breaking in the subsequent cracking reaction process can participate in the polymerization reaction with the lactic acid oligomer again, and the reaction system can be diluted to avoid excessive coking in the cracking process. The specific polymerization process can be illustrated in the following figures:

The second stage of the stepwise reaction is a cleavage reaction. In one embodiment, after a period of polymerization, the temperature of the supergravity device is raised to a second temperature gradient to perform a cleavage reaction to obtain crude lactide.

The lactic acid oligomer used in the present invention may be D-lactic acid oligomer or L-lactic acid oligomer. In one embodiment, the D-lactic acid oligomer is cleaved by the method of the invention, and purified, to obtain purified D-lactide. In another embodiment, the L-lactic acid oligomer is cleaved by the method of the present invention, and purified to obtain purified L-lactide.

In one embodiment, the lactic acid oligomer of the present invention has a weight average molecular weight of 600-3000Da. When the weight average molecular weight is too high and the polymerization degree of the oligomers is too high, the viscosity of the system is increased, so that the heat transfer and mass transfer effects in the reaction process are poor, the depolymerization reaction rate is reduced, further polymerization is easy to occur between the lactic acid oligomers, and the lactide needs to be distilled out of the reaction system at a higher reaction temperature, thereby further increasing racemization and coking carbonization of a substrate; when the weight average molecular weight is too low and the oligomer content is low, the system viscosity is low, the content of free acid in the oligomer is high, and under the conditions of high temperature and high vacuum, the free lactic acid is easier to evaporate out of the reaction system, so that the content of free acid in the obtained crude lactide is increased, and the product quality is reduced.

The polyol with proper molecular weight is selected to ensure that the polyol is not easy to escape in the earlier polymerization and the subsequent cracking process, so that the overload of the pressure of the reaction kettle can be avoided, and the purity of a cracking product is not influenced. The molecular weight of the polyether polyol can influence the viscosity and the reactivity of the system, the molecular weight is too small and is easy to escape, the molecular weight is too large, and the reactivity of the polyether polyol and the lactic acid oligomer is inhibited. The polyether polyol in the invention can be aliphatic long-chain type and does not contain rigid multi-ring such as benzene ring. The aliphatic polyether polyol has a long linear structure, good flexibility and low glass transition temperature, and is suitable for reducing the viscosity of a system. The glass transition temperature of aromatic rigid polyether polyol such as benzene ring is higher, and the compatibility with the lactic acid oligomer system is poor. The polyether polyols of the present invention may have a functionality of 2 to 3 and a suitable range of functionality may provide certain reaction sites while avoiding cross-linking gelation between prepolymers. The functionality is too high, the oligomeric lactic acid is possibly converted from a linear polymer into a star polymer due to the excessive hydroxyl sites, the molecular weight is in nonlinear growth, the glass transition temperature is suddenly increased, and the polymer in the reaction vessel is possibly crosslinked and gelled, so that the cracking efficiency is low, the kettle wall base materials are more, the production efficiency is reduced, and the kettle cleaning cost is increased.

In one embodiment, the polyether polyol used has a functionality of 2-3 and is selected from one or more of the following: polyoxypropylene polyol, polyoxyethylene polyol, polytetrahydrofuran diol. In a preferred embodiment, polyether polyols having a functionality of from 2 to 3 are used and are selected from one or more of the following: a polyoxypropylene polyol having a number average molecular weight of 400-20000, a polyoxyethylene polyol having a number average molecular weight of 400-20000, a polytetrahydrofuran diol having a number average molecular weight of 1000-10000.

In one embodiment, the weight ratio of polyether polyol to lactic acid oligomer is from 0.05 to 0.2. The proper weight ratio is helpful to regulate the viscosity of the system and the purity of the cracked product. The molecular weight distribution of the lactic acid oligomer in the polymerization system is not unique, and the lactic acid oligomer with smaller molecular weight is easy to escape along with lactide during the cracking process of the lactic acid oligomer, so that the quality of a product is influenced; but the lactic acid oligomer with smaller molecular weight has higher reaction priority, can react with polyether polyol preferentially, and has increased stability after reaction, thereby ensuring the high-efficiency and high-quality operation of the system. The weight ratio of polyether polyol to lactic acid oligomer is too low to effectively prevent the escape of the lactic acid oligomer with small molecular weight, which affects the purity of the product; the weight ratio is too high, the cost is increased, and no additional gain effect is achieved.

The invention improves the yield and purity of the obtained crude lactide by adjusting various reaction parameters in the polymerization reaction and the cracking reaction.

In one embodiment, the first temperature gradient (i.e., polymerization temperature) of the supergravity device of the present invention is 140-180 ℃ during the polymerization reaction. Suitable temperatures ensure efficient polymerization. When the polymerization temperature is too high, the esterification speed is increased to a certain extent, but the self-polymerization rate and the cleavage cyclization rate of the lactic acid oligomer are also greatly increased, and the reaction probability of the polyether polyol and the lactic acid oligomer is reduced. At temperatures above 180 ℃, the cleavage rate of the lactic acid oligomer may be higher than the esterification rate of the polyether polyol with the lactic acid oligomer, failing to proceed as expected. When the polymerization temperature is too low, the reaction rate of polyether polyol and lactic acid oligomer is slow, the reaction time is long, the operation duration of the cracking process section is prolonged, and the co-production in equipment maintenance and production line is unfavorable. When the first temperature gradient (i.e., polymerization temperature) of the supergravity device is lower than 140 ℃, the polymerization time is prolonged by about 1 hour every 10 ℃ and when the first temperature gradient (i.e., polymerization temperature) is lower than about 120 ℃, the esterification reaction tends to stop, and the reaction cannot be completed.

The polymerization reaction may be carried out under reduced pressure. The reduced pressure conditions may be performed by a vacuum apparatus. In one embodiment, the internal pressure of the hypergravity device is 500 to 1000Pa in the polymerization reaction. The esterification polymerization is a reversible reaction, and in order to make the reaction proceed forward, the water of the system needs to be removed in time, and the high-viscosity reaction system is not added with solvent and entrainer, so that the water removal is needed under the condition of high vacuum degree, and the continuous reaction is ensured; however, the vacuum cannot be too low due to the life of the equipment and the safety of production, which would result in increased material suck-back and equipment maintenance costs.

In one embodiment, the polymerization time for the polymerization reaction is from 90 to 180 minutes. Suitable polymerization times facilitate adequate completion of the polymerization reaction and reduce byproducts. The polymerization system has the advantages that the lactic acid oligomer is self-polymerized, the lactic acid oligomer is cracked, the lactic acid oligomer and the polyether polyol are esterified, the polyether polyol and the lactic acid oligomer are uniformly dispersed in the early stage of polymerization, and the esterification reaction of the two is a main proceeding mode; and as the time is prolonged, the esterification rate of the target reaction is reduced, the side reaction yield is accelerated, the components in the system are complex, the quality of the subsequent products is reduced, and the energy consumption and the production cost are increased due to the overlong reaction time.

In one embodiment, the present invention is 180-260 ℃ during the cleavage reaction. The proper temperature can ensure the efficient implementation of the cracking reaction. Because of the competition between polymerization and depolymerization in the cracking reaction, when the cracking temperature is too low, the racemization probability is reduced, but the depolymerization reaction rate is slowed down, the self-polymerization reaction rate of the polylactic acid is increased, the molecular weight of the polylactic acid oligomer is increased continuously, the viscosity of the system is increased, and the generated lactide is not easy to be distilled out of the system, so that the depolymerization reaction is inhibited. When the cracking temperature is too high, the depolymerization reaction rate is accelerated, the generated crude lactide can be quickly removed from the reaction system, but the racemization of the lactide can be caused by the too high reaction temperature, and coking carbonization is initiated. When the second temperature gradient (namely cracking temperature) of the hypergravity device is lower than 180 ℃, the cracking rate is very low or tends to zero; above 260 ℃, the heavy component in the cracked product increases by about 8% per 10 ℃ rise, and the purity of the product decreases.

The cleavage reaction may be carried out under reduced pressure. The reduced pressure conditions may be performed by a vacuum apparatus. In one embodiment, the internal pressure of the supergravity device is in the range of 1000 to 3500Pa during the cleavage reaction. The proper pressure is beneficial to obtaining a gaseous lactide product, separating the lactide product from raw materials and entering a subsequent purification process, and reducing the impurity content entering a rectifying tower; the depolymerization process must be operated under high vacuum due to the high congealing point, high boiling point and heat sensitivity characteristics of lactide and the limitation of the reaction conditions of the depolymerization system of lactic acid oligomer. The higher the vacuum, the faster the reaction rate, the higher the yield and purity of the product obtained, and the lower the meso-lactide content. In the invention, higher vacuum degree is ensured on the premise of ensuring the safe production of the device.

In one embodiment, the cleavage time for the cleavage reaction is 45 to 100 minutes. The proper cracking time is favorable for the complete completion of the cracking reaction and the reduction of the generation of byproducts so as to obtain higher cracking yield (in actual production, the reaction yield and the reaction time are positively correlated, i.e. the longer the cracking time is, the higher the yield is, but the longer the reaction time is, the serious coking is caused at the bottom of the reactor, so that the excessive carbonized substrate enters a circulating system, and the quality of the cracked product is reduced.

In one embodiment, the step reaction of the polylactic acid oligomer in the supergravity device is performed under the catalysis of a catalyst. When the ultra-fine polylactic acid oligomer liquid particles are contacted with the catalyst, the cracking reaction rapidly occurs to generate lactide as a target product, and under proper conditions, the lactide is carried out of the supergravity device to enter the rectifying tower.

The proper catalyst is favorable for improving the reaction rate of cracking the polylactic acid oligomer into lactide and shortening the time of the cracking reaction. Catalysts useful in the present invention include, but are not limited to: tin-based catalysts, zinc-based catalysts, and combinations thereof, for example: zinc oxide, stannous chloride, zinc lactate, and the like.

In one embodiment, the catalyst of the present invention is used in an amount of 0.015 to 0.25wt% based on the weight of the lactic acid oligomer. The catalyst with proper dosage is helpful for reducing the cost while ensuring the cracking rate and the yield. The particle size of the very small liquid particles formed by the polylactic acid oligomer is related to the operating conditions of the supergravity device, for example: rotor speed, porous structure, etc. The suitable particle size of the liquid particles helps to increase the yield of the cleavage reaction.

The proper rotor rotating speed of the supergravity device is favorable for obtaining liquid particles with proper particle size, so that the material presents the shapes of films, filaments, drops or very small bubbles on the micro-nano scale; the rapid updating of the high-dispersion and high-turbulence and strong-mixing interface can greatly improve the mass and heat transfer efficiency. In actual production, the setting of the rotating speed is adapted to physical properties and productivity, the rotating speed of the rotor is too high, the liquid phase of the outside filling area flows along the radial direction, the shearing action is weakened, and the mixing and transferring processes of the catalyst between the liquid phases are attenuated from inside to outside; the rotating speed is too low, the viscosity of the materials is high, the machine is easy to be blocked, the catalyst cannot reach the expected uniformity in the reaction medium, and the reaction is insufficient. In one embodiment, the rotor speed of the supergravity device is 500-1100r/min.

The porous structure in the hypergravity device can be a filler, and the proper filler is favorable for uniformly dispersing the polylactic acid oligomer and the catalyst, so that the speed of the cracking reaction is increased, and the time of the cracking reaction is shortened. In one embodiment, the packing of the supergravity device of the present invention is selected from the group consisting of: silk screen packing, baffles, corrugated packing, and combinations thereof.

In one embodiment, the hypergravity conditions of the hypergravity device of the present invention are 5-10G. The radial polylactic acid oligomer liquid particles with the hypergravity are not easy to be carried out of the hypergravity device, and the subsequent purification process can be simplified. The proper hypergravity condition is helpful to improve the efficiency and the income of the cracking process end. The super-gravity condition is provided by the rotating speed of the rotor, and the rotating speed is high or low, namely, the rotating speed corresponds to the super-gravity condition.

In one embodiment, the step reaction process uses multiple supergravity devices (e.g., 2-3) in parallel. Realizes the stable feeding of gas phase in the whole day, and further realizes the continuous production.

By selecting parameters (such as raw materials, a supergravity device, reaction conditions and the like) in the step-type reaction, the cracking yield is improved, crude lactide vapor is obtained, the feeding mode of the rectifying tower is gas-phase feeding, real-time conveying is realized by utilizing the pressure difference between the two devices, the additional energy supply of the traditional liquid-phase feeding is avoided, and the energy consumption is reduced. The parallel connection of the equipment is adopted to realize the stable gas phase feeding in the whole day and the continuous production.

Step (a): and conveying the obtained crude lactide vapor to a rectifying tower for a rectifying process to obtain distillate.

The rectification process is a unit operation process for separating substances by utilizing different volatilities of different substances through multiple vaporization and multiple condensation, and the energy required by multiple vaporization is provided through a reboiler. Proper conditions in the rectification process are favorable for being combined with the cracking process in a high-efficiency manner, purification is performed in a high-efficiency manner, the yield is improved, and the real-time feeding by utilizing the air pressure difference between the rectification tower and the supergravity device is realized, so that the energy consumption is reduced.

The distillate can be obtained by the rectification process in the rectification tower and sent out from the top of the rectification tower to enter the next purification process. The distillate is a gaseous component, which generally comprises: lactide, lactic acid and small amounts of polylactic acid oligomers.

In one embodiment, the internal temperature of the rectification column is 115-150 ℃. The temperature is too high, the energy consumption is increased, the steam rising speed is increased, bad phenomena such as flooding and the like are easy to form, and the quality of the product is reduced due to the increase of the top heavy component content. The temperature is too low, the rising speed of steam is reduced, liquid leakage is easy to occur, the mass transfer efficiency is obviously reduced, the rectification effect is seriously affected, or the device cannot operate.

In one embodiment, the internal pressure of the rectification column is from 500 to 3000Pa. The pressure is too high, the boiling point of lactide is increased, the distilled lactide needs higher temperature, and the lactide is degraded in the rectification process while the energy consumption is greatly increased; the pressure is too low, the boiling points of the components are similar, the separation effect is negatively optimized, the requirement on the precision and the tightness of the equipment is higher due to the too low internal pressure, and the equipment investment and the later maintenance cost are increased.

In one embodiment, the reflux ratio of the rectification process is from 0.5 to 3. Reflux ratio (r=l/D), i.e. essenceDistillation columnThe ratio of reflux flow L to overhead product flow D in the overhead return column. In the rectification operation related to the invention, the reflux ratio needs to be controlled within a certain range, and the increase of the reflux ratio can improve the quality of the target product at the top of the tower, but can reduce the production capacity of the tower, so that the consumption of water, electricity and gas is caused. The excessive reflux ratio can cause excessive circulation of materials in the tower and even form flooding, thereby damaging the normal operation of the tower; too low a reflux ratio, reduced overhead purity, increased trays required, increased equipment manufacturing costs and reduced product quality, and inability to commercial production.

In the rectification process, the bottom liquid of the rectification tower is liquid, and the liquid contains lactide and other impurities and can be purified again through the rectification tower. In one embodiment, the rectification process further comprises: and after the liquid phase at the bottom of the rectifying tower is gasified by a reboiler, conveying the obtained gas phase back to the rectifying tower to participate in the rectifying process, and conveying the unvaporized liquid phase in the reboiler back to the supergravity device to participate in the cracking reaction.

The reboiler heats the liquid phase to vaporize it, thereby ensuring the gas-liquid phase balance between the rising gas phase and the refluxing liquid phase, and maintaining the heat balance in the tower. The proper parameter range in the reboiler is favorable for obtaining proper pressure in the tower and reflux ratio, so that the purity and yield of the product are improved.

In one embodiment, the reboiler temperature is 125-155 ℃. The proper temperature helps to increase the rate of liquid phase vaporization at the bottom while reducing degradation of lactide, thereby increasing yield. Meanwhile, the proper temperature is favorable for reducing impurities contained in the gas phase sent back to the rectifying tower, so that the high-boiling-point heavy component is used as a liquid phase and is sent back to the supergravity device to participate in the cracking reaction. In one embodiment, the unvaporized liquid phase is fed back to the supergravity device by means of a circulating metering pump

Step (a): liquefying the obtained distillate to obtain liquefied product, sending a part of liquefied product into a crystallizer, performing cooling crystallization-crystal growth process in the crystallizer to obtain lactide crystals, and sending the rest liquefied product back to a rectifying tower to participate in the rectifying process.

The distillate fraction is liquefied to obtain a liquefied product, which is used for the subsequent purification process. The liquefaction may be carried out in a suitable heat exchange device, which may be a heat exchange condenser, to effect liquefaction by heat exchange.

The liquefied product extracted by the heat exchange equipment can be directly sent to subsequent purification, or can be stored in a storage device after liquid phase extraction.

A portion of the liquefied product (which is also referred to herein as "medium purity lactide") is fed to a crystallizer for a reduced temperature crystallization-seeding process to obtain lactide crystals. And conveying the rest liquefied products back to the rectifying tower to participate in the rectifying process.

In one embodiment, the ratio of the flow of liquefied product fed into the crystallizer to the flow of liquefied product fed back to the rectification column is the same as the reflux ratio (as defined herein).

In one embodiment, the crystallizer of the present invention is selected from: falling film crystallizer, static crystallizer, and combinations thereof.

The proper cooling crystallization-crystal growing process is helpful to promote the crystallization rate and improve the crystallization purity, thereby improving the purification effect and the yield.

In one embodiment, the temperature-reducing crystallization-crystal growing process adopts multistage stepwise temperature control. The temperature is gradually decreased, and the enrichment purity of the target product is superposed, so that a better purification effect is realized on the basis of reducing energy consumption. Furthermore, the gradual temperature gradient facilitates continuous operation.

In one embodiment, the temperature of the reduced crystallization is 95-75 ℃. In one embodiment, the seeding time is 45-75 minutes.

In one embodiment, after the medium-purity lactide is subjected to cooling crystallization-crystal growth in the crystallizer, mother liquor is discharged out of the liquid storage tank b, and the mother liquor in the liquid storage tank b can be conveyed back to the reboiler by a circulating pump to continue to participate in circulation.

And liquefying the distillate to obtain a liquefied product, wherein a part of liquefied product is sent into a crystallizer for cooling crystallization-crystal growth process, and the other part of liquefied product is sent back to a rectifying tower to participate in the rectifying process. In one embodiment, the further portion of the liquefied product is fed back to the top of the column via a circulation pump to participate in the reflux.

Step (a): and (3) carrying out a heating-heat preservation process on the obtained lactide crystals in a crystallizer, and melting and discharging the remaining lactide crystals after the heating-heat preservation process to obtain purified lactide.

The lactide crystal is subjected to a heating-heat preservation process through a crystallizer, so that low-melting-point impurities on the surface of the crystal can be melted, and the impurities are melted and then become liquid so as to be separated from the lactide crystal. The melted liquid impurities can be discharged to a liquid storage tank b, and are conveyed back to a reboiler at the bottom of the rectifying tower by a circulating pump together with mother liquor obtained by cooling, crystallizing and crystallizing the medium-purity lactide in a crystallizer, so as to continue to participate in circulation.

The proper temperature and heat preservation time of the heating are helpful to fully melt the impurities attached to the surface of the crystal, thereby reducing the melting and degradation of lactide, improving the yield and reducing the energy consumption. In one embodiment, the temperature of the present invention is raised to a temperature of 70-90 ℃. In another embodiment, the incubation time of the present invention is 10 to 30 minutes.

After the heating-heat preservation process, the rest lactide crystals are melted and discharged to obtain purified lactide.

According to the invention, the continuous preparation and purification of lactide are realized by adopting a supergravity device and combining a coupling purification process, and the method has the advantages of low energy consumption, high yield, good purification effect and the like.

In the invention, the cracking yield of crude lactide vapor obtained by lactic acid oligomer is more than or equal to 90 percent. Cleavage yield y=w ₁ /w ₂ X 100%, where w ₁ Is the mass of crude lactide, w ₂ Is the mass of the lactic acid oligomer.

The purity of the purified lactide obtained by the process of the present invention is not less than 99%, preferably not less than 99.6%, for example about 99.60%, 99.62%, 99.65%, 99.70%.

The yield of lactide in the method of the present invention is not less than 75%, for example, about 75.31%, 76.21%, 77.2% and 77.15%. The yield of lactide was calculated by: y=i ₂ /I ₁ X 100%, wherein Y denotes the lactide yield, I ₁ Refers to the feed amount, i.e. the weight (mass) of the lactic acid oligomer fed into the supergravity device for the stepwise reaction, I ₂ Refers to the weight (mass) of the purified lactide obtained.

The energy consumption of the process is less than or equal to 1.2N, preferably less than or equal to 1.1N, for example about 1.01N, 1.03N, 0.98N, 1.01N.

An embodiment of the present invention will be described with reference to fig. 1.1

The lactic acid oligomer 1, the catalyst 2 and the polyether polyol are fed to the supergravity device 4 from top to bottom or from the middle after preliminary blending in the mixer 3. The supergravity device 4 is connected with a vacuum device through an external vacuum device interface 5, and the system is in negative pressure through the vacuum device. Under the given temperature and pressure conditions, the stepwise reaction is carried out, and the obtained crude lactide vapor is conveyed to a rectifying tower 7 through a conveying pipeline 6 for further separation and purification.

The crude lactide steam conveyed in the conveying pipeline 6 and the top condensed reflux material carry out countercurrent heat and mass transfer; the gas phase distillate 19 at the top of the tower enters a heat exchange condenser 9 to be condensed and liquefied, the obtained liquid phase is extracted to a liquid storage tank 10, a part of the liquid phase in the liquid storage tank 10 is conveyed back to the top of the tower to participate in reflux through a circulating pump 11, the liquid phase 17 at the bottom of the rectifying tower is heated and gasified through a reboiler 15 to form a gas phase 18, the gas phase 18 is conveyed back to the rectifying tower, and the unvaporized liquid (high-boiling-point heavy component) in the reboiler is conveyed back to a supergravity device through a circulating pump 16 to participate in the cracking reaction again. Another portion of the liquid phase (i.e., medium purity lactide) in the holding tank 10 is withdrawn via transfer line 21 to the crystallizer 12 for subsequent purification.

The medium-purity lactide conveyed by the conveying pipeline 21 is subjected to cooling crystallization-crystal growth in the crystallizer 12, and mother liquor is discharged out of the liquid storage tank 13. The crystals in the crystallizer 12 are then subjected to a temperature raising-maintaining process to melt the low-melting impurities on the surfaces of the crystals and discharge the melt to the liquid tank 13. The remaining crystals are rapidly melted and discharged to obtain purified lactide, and discharged through 24. Wherein, the liquid in the liquid storage tank 13 can be conveyed back to the reboiler 15 at the bottom of the rectifying tower by the circulating pump 14 to continue to participate in circulation; the liquid storage tank 13 is recycled to be rich in the meso-lactide mother solution, and can also be used for raw materials for the purification of the meso-lactide.

Advantageous effects

The invention provides a method for continuously preparing and purifying lactide, which has the following remarkable advantages:

1) The lactide preparation link adopts a two-step ladder-type scheme, so that the viscosity surge and the high-temperature coking loss of the system are inhibited.

2) In the lactide stepped preparation process, an embodiment of parallel equipment is adopted to cooperatively perform 'prepolymerization-pyrolysis', so that gas phase stable feeding during the whole day is realized, and continuous production is realized.

3) The coupling purification mechanism of the hypergravity rectification and the melt crystallization is utilized to improve the cracking yield and the purity of the lactide.

4) The introduction of the supergravity device solves the common problems of uneven dispersion and high system viscosity inherent in the traditional cracking device, improves the long-time continuous cracking yield and greatly reduces the coking risk; the coupling purification mechanism is adopted in the purification stage, so that the problems of excessive number of tower plates of a traditional single rectifying tower, excessive high equipment and lactide degradation caused by a multi-stage rectifying tower in the rectifying process are solved, and the defects of incoherence in single melting crystallization production and lower yield and purity are also overcome.

5) On the premise of not sacrificing productivity, the production system provided by the invention has less equipment holding capacity and low maintenance cost, and the cracking device and the rectifying device are fed in real time by utilizing the air pressure difference, so that extra energy consumption is not needed, and the energy consumption is obviously reduced; and the whole coupling purification system has no solvent intervention, accords with the green chemical industry instruction specification, and is suitable for large-scale industrial production.

Examples

The following describes the aspects of the invention in further detail with reference to specific examples.

It should be noted that the following examples are only examples for clearly illustrating the technical solution of the present invention, and are not limiting. Other variations or modifications of the above description will be apparent to those of ordinary skill in the art, and it is not necessary or exhaustive of all embodiments, and obvious variations or modifications of the invention are intended to be within the scope of the invention. The instrumentation and reagent materials used herein are commercially available unless otherwise indicated.

Material and apparatus

Supergravity device: BZ1K1-3P, hangzhou Keli chemical plant Co., ltd.

Purity measurement: the purity of lactide was determined by gas chromatography.

Examples and comparative examples were obtained according to the specific embodiment shown in fig. 1 (see above) and according to the following parameters.

Example 1

Feeding:

the weight average molecular weight of the lactic acid oligomer used in example 1 was 1000Da; the polyether polyol is polyoxyethylene polyol (functionality is 2) with number average molecular weight of 400, wherein the polyoxyethylene polyol is used in an amount of 5% by weight of the lactic acid oligomer; the catalyst was zinc oxide in an amount of 0.05% by weight of the lactic acid oligomer.

The cracking process comprises the following steps:

the hypergravity condition (acceleration) of the hypergravity device is 5G, the rotating speed of the rotor is 700 revolutions per minute, and the filler in the hypergravity device is silk screen filler. For the step reaction, in the process of the polymerization reaction, the first temperature gradient of the hypergravity device is 160 ℃, the polymerization time is 90 minutes, and the internal pressure is 800Pa; during the cleavage reaction, the second temperature gradient was 190℃and the cleavage time was 60 minutes with an internal pressure of 2000Pa.

The purification process comprises the following steps:

the pressure of a rectifying section in the rectifying tower is 1000Pa, and the corresponding rectifying temperature is 125 ℃; the internal pressure of the stripping section is 1800Pa, and the corresponding temperature is 135 ℃; the reboiler temperature was 140℃and the reflux ratio was 0.75.

The crystallizer is a series combination of a falling film crystallizer and a static crystallizer. The temperature is reduced to 80 ℃ and the crystal growing time is 50min; in the heating-heat preservation process, the impurity removal temperature of heating is 85 ℃, and the heat preservation is carried out for 20min.

The fluid medium in the heat exchange condenser 9 is hot water at 80 ℃, and the fluid medium in the bottom reboiler 15 is heat conduction oil.

Example 2

Feeding: the weight average molecular weight of the lactic acid oligomer used in example 2 was 1200Da; the polyether polyol is polyoxypropylene polyol with the number average molecular weight of 1000, wherein the polyoxypropylene polyol is used for 7% of the lactic acid oligomer by weight; the catalyst was stannous chloride in an amount of 0.05% by weight of the lactic acid oligomer.

The cracking process comprises the following steps:

for the step reaction, in the polymerization process, the first temperature gradient of the hypergravity device is 180 ℃, the polymerization time is 120 minutes, and the corresponding pressure is 1000Pa; in the course of the cleavage reaction, the second temperature gradient was 200℃and the cleavage time was 75 minutes, corresponding to a pressure of 2500Pa. The other conditions and parameters were the same as in example 1.

The purification process comprises the following steps:

the pressure of a rectifying section in the rectifying tower is 1200Pa, and the corresponding rectifying temperature is 130 ℃; the internal pressure of the stripping section is 2000Pa, and the corresponding temperature is 140 ℃; the reboiler temperature was 145℃and the reflux ratio was 1.

The crystallizer is a series combination of falling film crystallizers of the same type. The temperature is reduced to 85 ℃ and the crystal growing time is 60min; in the heating-heat preservation process, the impurity removal temperature of the heating is 90 ℃, and the heat preservation is carried out for 30min. The other conditions and parameters were the same as in example 1.

Example 3

Feeding: the catalyst in example 3 was a mixture of stannous chloride and zinc oxide in an amount of 0.03% by weight of lactic acid oligomer, and the other conditions and parameters were the same as in example 1.

Conditions and parameters of the cleavage process and the purification process were the same as those of example 1.

Example 4

Feeding: the catalyst in example 4 was a mixture of stannous chloride, zinc oxide and zinc lactate in an amount of 0.03% by weight of the lactic acid oligomer, and the other conditions and parameters were the same as in example 2.

Comparative example 1

Feeding: the charge of comparative example 1 was the same as in example 1.

The cracking process comprises the following steps: the cleavage apparatus used in comparative example 1 was a conventional tank reactor, and the other conditions and parameters were the same as those in example 1.

The conditions and parameters of the purification process were the same as in example 1.

Comparative example 2

Feeding: the charge of comparative example 2 was the same as in example 1.

The cracking process comprises the following steps: the cleavage apparatus used in comparative example 2 was a conventional tank reactor, and the other conditions and parameters were the same as those in example 1.

The purification process comprises the following steps: the purification process is that a multistage rectifying tower is used for purification, the pressure of a rectifying section in the rectifying tower is 1200Pa, and the corresponding rectifying temperature is 130 ℃; the internal pressure of the stripping section is 2000Pa, and the corresponding temperature is 140 ℃; the reboiler temperature was 145℃and the reflux ratio was 1.

Comparative example 3

Feeding: the charge of comparative example 3 was the same as in example 1.

The cracking process comprises the following steps: the cleavage apparatus used in comparative example 3 was a conventional tank reactor, and the other conditions and parameters were the same as those in example 1.

The purification process comprises the following steps: the purification process is multistage melt crystallization, and the crystallizer is a series combination of a falling film crystallizer and a static crystallizer. The temperature is reduced to 80 ℃ and the crystal growing time is 50min; in the heating-heat preservation process, the impurity removal temperature of heating is 85 ℃, and the heat preservation is carried out for 20min.

Comparative example 4

Feeding: in comparative example 4, ethylene glycol was used instead of polyether polyol, and the other conditions and parameters were the same as in example 1.

Comparative example 5

Feeding: in comparative example 5, propylene glycol was used instead of polyether polyol, and the other conditions and parameters were the same as in example 1.

Comparative example 6

Feeding: in comparative example 6, glycerol was used instead of polyether polyol, and the other conditions and parameters were the same as in example 1.

Comparative example 7

Feeding: in comparative example 7, pentaerythritol was used instead of polyether polyol, and the other conditions and parameters were the same as in example 1.

Comparative example 8

Feeding: in comparative example 8, xylitol was used instead of polyether polyol, and the other conditions and parameters were the same as in example 1.

Comparative example 9

Feeding: in comparative example 9, the ratio of the polyether polyol to the lactic acid oligomer was 0.03 by mass, and the other conditions and parameters were the same as in example 1.

Comparative example 10

Feeding: in comparative example 10, the ratio of polyether polyol to lactic acid oligomer was 0.3 by mass, and the other conditions and parameters were the same as in example 1.

Comparative example 11

Feeding: in comparative example 11, the ratio of the polyether polyol to the lactic acid oligomer was 0.4 by mass, and the other conditions and parameters were the same as in example 1.

Comparative example 12

Feeding: the charge of comparative example 12 was the same as in example 1.

The cracking process comprises the following steps: the first temperature gradient of the hypergravity device was 110 ℃, and the other conditions and parameters were the same as in example 1.

Comparative example 13

Feeding: the charge of comparative example 13 was the same as in example 1.

The cracking process comprises the following steps: the first temperature gradient of the hypergravity device was 130 ℃, and the other conditions and parameters were the same as in example 1.

Comparative example 14

Feeding: the charge of comparative example 14 was the same as in example 1.

The cracking process comprises the following steps: the first temperature gradient of the hypergravity device was 190 ℃, and the other conditions and parameters were the same as in example 1.

Comparative example 15

Feeding: the charge of comparative example 15 was the same as in example 1.

The cracking process comprises the following steps: the second temperature gradient of the hypergravity device was 170 ℃, and the other conditions and parameters were the same as in example 1.

Comparative example 16

Feeding: the charge of comparative example 15 was the same as in example 1.

The cracking process comprises the following steps: the second temperature gradient of the hypergravity device was 270 ℃, and the other conditions and parameters were the same as in example 1.

Comparative example 17

Feeding: in comparative example 17, polyether polyol was not added, and the other conditions and parameters were the same as in example 1.

Comparative example 18

Feeding: in comparative example 18, a polyether polyol was used as a polyoxyethylene polyol having a number average molecular weight of 200, and the other conditions and parameters were the same as in example 1.

Comparative example 19

Feeding: in comparative example 18, a polyether polyol was used as a polyoxyethylene polyol having a number average molecular weight of 30000, and the other conditions and parameters were the same as in example 1.

The data relating to the examples and comparative examples are shown in the following table (the data are all obtained in the steady state of the system).

TABLE 1

Yield: y=i ₂ /I ₁ *100, wherein Y denotes yield, I ₁ Refer to the feed amount, I ₂ Refers to the amount of the target product.

For cleavage yield: i ₁ The mass of the lactic acid oligomer; i ₂ Is the mass of crude lactide.

Yield for purified lactide: i ₁ Feeding amount of lactic acid oligomer to the feed inlet 1; i ₂ Is the output of purified lactide from outlet 24.

The theoretical energy consumption of 500kg/h of the production line is taken as a benchmark N.

TABLE 2

As can be seen from the data in tables 1 and 2, the method can realize that the cracking yield of the lactic acid oligomer is higher than 91%, the purity of the purified lactide is higher than 99.6%, the continuous yield is higher than 75%, and the energy consumption is lower than 1.1N.

Claims

1. A method for continuously preparing and purifying lactide, which comprises the following steps:

wherein the stepwise reaction comprises: the lactic acid oligomer and polyether polyol are subjected to polymerization reaction under a first temperature gradient, and then the temperature is raised to a second temperature gradient to carry out cracking reaction;

the polyether polyol has a functionality of 2-3 and is selected from one or more of the following: a polyoxypropylene polyol having a number average molecular weight of 400-20000, a polyoxyethylene polyol having a number average molecular weight of 400-20000, a polytetrahydrofuran diol having a number average molecular weight of 1000-10000;

the stepwise reaction uses the following parameters:

the weight ratio of the polyether polyol to the lactic acid oligomer is 0.05-0.2;

the first temperature gradient of the polymerization reaction is 140-180 ℃;

the polymerization time of the polymerization reaction is 90-180 minutes;

In the polymerization reaction, the internal pressure of the hypergravity device is 500-1000Pa;

the second temperature gradient of the cracking reaction is 180-260 ℃;

the cracking time of the cracking reaction is 45-100 minutes;

in the cracking reaction, the internal pressure of the hypergravity device is 1000-3500Pa;

the weight average molecular weight of the lactic acid oligomer is 600-3000Da;

the hypergravity condition of the hypergravity device is 5-10G;

the rotating speed of the rotor of the hypergravity device is 500-1100r/min;

the packing of the hypergravity device is selected from the group consisting of: silk screen packing, baffles, corrugated packing, and combinations thereof.

2. The method of claim 1, wherein the rectification process uses at least one of the following parameters:

the internal temperature of the rectifying tower is 115-150 ℃;

the internal pressure of the rectifying tower is 500Pa to 3000Pa,

the reflux ratio in the rectification process is 0.5-3.

3. The method of claim 1, wherein the step of,

the rectification process further comprises: the liquid phase at the bottom of the rectifying tower is gasified by a reboiler, the obtained gas phase is conveyed back to the rectifying tower to participate in the rectifying process, and the unvaporized liquid phase in the reboiler is conveyed back to the hypergravity device to participate in the cracking reaction.

4. The method of claim 3, wherein the step of,

The temperature of the reboiler is 125-155 ℃.

5. The method of claim 1, wherein the step of,

in the cooling crystallization-crystal growing process and/or the heating-heat preserving process, the crystallizer is selected from the following components: falling film crystallizer, static crystallizer, and combinations thereof.

6. The method of claim 1, wherein the step of,

in the cooling crystallization-crystal growing process, the temperature of the cooling crystallization is 95-75 ℃; and/or

The crystal growing time is 45-75min.

7. The method of claim 1, wherein the step of,

in the heating-heat preservation process,

the temperature of the heating is 70-90 ℃;

the heat preservation time is 10-30min.

8. The method of claim 1, wherein the step of,

obtaining the cracking yield of the crude lactide vapor by the lactic acid oligomer to be more than or equal to 90 percent; and/or

The purity of the purified lactide is more than or equal to 99 percent; and/or

The yield of lactide of the method is more than or equal to 75 percent; and/or

The energy consumption of the method is less than or equal to 1.2N.