CN115010696A

CN115010696A - Method for continuously preparing and purifying lactide

Info

Publication number: CN115010696A
Application number: CN202210883125.8A
Authority: CN
Inventors: 韩孟超; 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: 2022-09-06
Anticipated expiration: 2042-07-26
Also published as: CN115010696B

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 supergravity device to carry out step-type reaction so as to obtain crude lactide vapor; conveying the obtained crude lactide vapor to a rectifying tower for a rectifying process to obtain distillate fractions; liquefying the obtained distillate fraction to obtain a liquefied product, sending a part of the liquefied product into a crystallizer, performing a cooling crystallization-crystal growing process in the crystallizer to obtain lactide crystals, and sending the rest of the liquefied product back to the rectifying tower to participate in the rectifying process; heating and insulating the obtained lactide crystal in a crystallizer, melting the remaining lactide crystal after the heating and insulating process, and discharging to obtain purified lactide; wherein the step reaction comprises: and carrying out polymerization reaction on the lactic acid oligomer and polyether polyol under a first temperature gradient, and then heating to a second temperature gradient to carry out 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 sought by the academic and industrial fields from the beginning of their appearance, and polylactic acid (PLA) is a typical example. The polylactic acid has excellent biocompatibility and good machinability, and can be completely degraded into CO under natural conditions ₂ And H ₂ And O, the method has great application prospect in the fields of 3D printing, spinning, medical use and the like. In addition, the plastic prohibition regulations in the global range also bring huge market demands for degradable materials such as polylactic acid and the like.

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 processing performance of the product is limited; the two-step method is to obtain lactide intermediate by cracking lactic acid oligomer and obtain polylactic acid by ring opening of lactide.

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, and the method is a synthesis method generally adopted in the industry nowadays. The core technology of this process is the synthesis and purification of lactide, since the purity of lactide appears to be clearly positively correlated to PLA quality. Therefore, the purification and production cost of lactide is a major limiting factor that limits the productivity of PLA.

Lactide is mainly obtained by the cleavage of lactic acid oligomers. The lactic acid oligomer is cleaved to form a ring and self-polymerized between the oligomers, which are competing reactions. And the traditional cracking device has short plates with uneven dispersion or high system viscosity, the yield of long-time continuous cracking is low, and the risk of carbon formation is greatly improved. In the prior art, the oligomer cracking 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 high-viscosity materials, the gravity settling in the stirring process can not be eliminated by simply increasing the rotating speed, and the prior art lacks effective explanation on how to control the stability of gas-phase feeding to achieve the purpose of continuous production.

Currently, the mainstream lactide purification method is as follows: vacuum rectification, recrystallization and hydrolysis. The purification of lactide by adopting a multi-stage rectification mode is disclosed in CN112934139A, CN112876452A and CN112480064A, and the purity of the obtained lactide is higher. Purification of lactide by recrystallization is disclosed in CN112047920A, CN111961028A, CN 102875522A. CN101696204A discloses that purification is achieved by appropriate hydrolysis of lactide.

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 supergravity device to carry out step-type reaction so as to obtain crude lactide vapor;

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

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

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

wherein the step reaction comprises: and carrying out polymerization reaction on the lactic acid oligomer and polyether polyol under a first temperature gradient, and then heating to a second temperature gradient to carry out cracking reaction.

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

In another embodiment, in the process of the present invention, the polyether polyol has a functionality of 2 to 3 and is selected from one or more of the following: polyoxypropylene polyol having a number average molecular weight of 400-20000, polyoxyethylene polyol having a number average molecular weight of 400-20000, and 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 section view of the supergravity device of the invention and (b) the material distribution effect view

Reference numerals: 1: a lactic acid oligomer feed port; 2: a catalyst feed port; 3: a mixer; 4: a supergravity device; 5. 8: an external 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 circulating metering pump; 12: a crystallizer; 15: a reboiler; 22. 23: a waste discharge port; 24: and discharging the purified lactide.

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 otherwise indicated.

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 case of conflict, the definitions provided herein will control.

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

When an amount, concentration, or other value or parameter is given as either a range, preferred range, or a pair of upper and lower preferable values or specific values, 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 ranges. The scope of the invention is not limited to the specific values recited when defining a range. For example, "1-8" encompasses 1, 2, 3, 4, 5, 6, 7, 8, as well as any subrange consisting of any two values therein, e.g., 2-6, 3-5.

The terms "about" and "approximately," when used in conjunction with a numerical variable, generally mean that the value of the variable and all values of the variable are within experimental error (e.g., within 95% confidence interval for the mean) or within ± 10% of the specified value, or more.

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. It will be understood by those skilled in the art that terms such as "including" and "comprising" encompass the meaning of "consisting of …. The expression "consisting of …" excludes any element, step or ingredient not specified. The phrase "consisting essentially of …" means that the scope is limited to the specified elements, steps or components, plus optional elements, steps or components that do not materially affect the basic and novel characteristics of the claimed subject matter. It is to be understood that the expression "comprising" covers the expressions "consisting essentially of …" and "consisting of …".

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

When values or range ends are described herein, it is understood that the disclosure includes the particular values or ends recited.

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

Unless otherwise indicated, the terms "combination thereof" and "mixture thereof" refer to a multi-component mixture of the elements described, such as two, three, four, and up to the maximum possible multi-component mixture.

Furthermore, no number of elements or components of the invention has been previously indicated and no limitation on the number of occurrences (or presence) of an element or component is intended. Thus, it should be understood that the singular includes one or at least one and that the singular of an element or component also includes the plural unless the numerical value explicitly indicates the singular.

The terms "optionally" or "optionally" as used herein mean 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.

Method of the invention

Lactide is a thermo-sensitive material and is very easily degraded thermally at high temperature and in the presence of moisture. And the existing single purification mechanism cannot give consideration to purity, yield and continuous production.

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

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 the uniformity of the materials are greatly improved.

And secondly, a step-type lactide preparation scheme (end group modification-pyrolysis) is innovatively adopted, the system viscosity surge and the high-temperature coking loss are inhibited, and the yield is improved.

The design cracking process parameter for the feeding mode of rectifying column is gaseous phase feeding, utilizes the pressure differential between cracker and the rectifying column to realize real-time transport, has avoided the extra energy supply of traditional liquid phase feeding. The implementation scheme of parallel connection of equipment is adopted, so that stable gas-phase feeding in all the day is realized, and continuous production is realized.

And a purification mechanism of coupling rectification and a melting crystallization phase is adopted, so that the advantages of the rectification and the melting crystallization phases on purity, yield and continuous production are complemented. The productivity and the quality are greatly improved on the basis of not introducing a solvent and not increasing the energy consumption.

The following is described with reference to specific steps:

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

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

Step (ii) of: feeding the lactic acid oligomer and the polyether polyol into a supergravity device for stepwise reaction to obtain crude lactide vapor. Wherein the step reaction comprises: and carrying out polymerization reaction on the lactic acid oligomer and polyether polyol under a first temperature gradient, and then heating to a second temperature gradient to carry out cracking reaction.

The supergravity technology is to utilize the unique flow behavior of multiphase flow system under supergravity condition to strengthen the relative speed and mutual contact between phase and phase, so as to realize efficient mass and heat transfer process and chemical reaction process. The mode of acquiring the supergravity is mainly to form a centrifugal force field by rotating the whole or parts of equipment, and the related multiphase flow system mainly comprises a gas system, a solid system and a gas-liquid system.

Herein, supergravity techniques are implemented using supergravity devices. The supergravity device is basically constructed as a device that generates centrifugal force in a high-speed rotation mechanism manner. Fig. 2(a) shows a cross-sectional view of a hypergravity apparatus of the present invention.

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

Utilize the hypergravity device to make the material fully contact and form micro-nano reaction unit to step-like reaction process accelerates, reduces the heated time of lactide, makes the degradation process inhibited, promotes reaction rate and schizolysis yield. Under the same operation condition, the hypergravity reactor can greatly improve the dispersion speed and the 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 the lactide and improve the yield of the lactide.

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

the first stage of the stepwise reaction is the polymerization of the lactic acid oligomer and the polyether polyol. In one embodiment, the lactic acid oligomer is transferred from top to bottom or from the middle to a supergravity cracking unit, followed by addition of polyether polyol and warming to a first temperature gradient for polymerization. In the polymerization reaction process, the lactic acid oligomer and polyether polyol are polymerized, and polyether polyol is introduced at 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 esterification, and therefore, the polymerization process may also be referred to as an esterification process and an esterification procedure herein. The polymerization time of the polymerization reaction may also be referred to as the esterification time. Meanwhile, the polyether polyol which does not participate in the reaction and the polyether polyol 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 shown in the following figure:

the second stage of the stepwise reaction is the 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 cracking reaction to obtain crude lactide.

The lactic acid oligomer used in the present invention may be a D-lactic acid oligomer or an L-lactic acid oligomer. In one embodiment, after the D-lactic acid oligomer is cracked and purified by the method of the present invention, purified D-lactide is obtained. In another embodiment, the L-lactic acid oligomer is obtained as a purified L-lactide after cleavage and purification by the process of the present invention.

In one embodiment, the weight average molecular weight of the lactic acid oligomers of the present invention is 600-3000 Da. When the weight average molecular weight is too high and the polymerization degree of the oligomer 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, and the lactic acid oligomer is easy to further polymerize, so that the higher reaction temperature is needed for distilling lactide out of the reaction system, and racemization and coking and carbonization of a substrate are further aggravated; when the weight average molecular weight is too low and the oligomer component is too low, the viscosity of the system is low, the content of free acid in the oligomer is too high, and the free lactic acid is easier to evaporate out of the reaction system under the conditions of high temperature and high vacuum, so that the content of the free acid in the obtained crude lactide is increased, and the product quality is reduced.

The polyol with proper molecular weight can ensure that the polyol is not easy to escape in the early polymerization and subsequent cracking processes, can avoid the pressure overload of the reaction kettle, and does not influence the purity of the cracked product. The molecular weight of the polyether polyol can influence the viscosity and the reactivity of a system, when the molecular weight is too low, the polyether polyol is easy to dissipate, and when the molecular weight is too high, the reactivity of the polyether polyol and the lactic acid oligomer is inhibited. The polyether polyol can be aliphatic long-chain type and does not contain rigid polycyclic rings such as benzene rings and the like. The aliphatic polyether polyol has a long straight chain structure, good flexibility and low glass transition temperature, and is suitable for reducing the viscosity of a system. Aromatic rigid polyether polyols such as benzene rings have high glass transition temperatures and poor compatibility with the lactic acid oligomer system of the present invention. The polyether polyols of the present invention may have a functionality of from 2 to 3, with a suitable range of functionalities providing certain reactive sites while avoiding cross-linking gelation between prepolymers. Due to the fact that the functionality is too high, due to the fact that the functionality has too many hydroxyl sites, the low-polymer lactic acid oligomer can be converted into a star-shaped polymer from a straight-chain polymer, the molecular weight is increased in a nonlinear mode, the glass transition temperature rises suddenly, the cross-linking gelation of polymers in a reaction container can be caused, the cracking efficiency is low, the amount of kettle wall base materials is large, the production efficiency is reduced, and the kettle cleaning cost is increased.

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

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 for regulating and controlling the viscosity of the system and the purity of the cracking 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 cracking in the reaction process, so that the product quality is influenced; but the lactic acid oligomer with smaller molecular weight has higher reaction priority and can preferentially react with polyether polyol, the stability is increased after the reaction, and the high-efficiency and high-quality operation of a system is ensured. The weight ratio of the polyether polyol to the lactic acid oligomer is too low, so that the dissipation of the low-molecular-weight lactic acid oligomer cannot be effectively prevented, and the product purity is influenced; the weight ratio is too high, the cost is increased and no additional gain effect is produced.

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

In one embodiment, the first temperature gradient (i.e., the polymerization temperature) of the hypergravity apparatus of the present invention is 140-. The proper temperature can ensure the polymerization reaction to be carried out efficiently. When the polymerization temperature is too high, although the esterification speed is accelerated to a certain extent, the self-polymerization rate and the cracking and cyclization rate of the lactic acid oligomer are also greatly increased, and the reaction probability of polyether polyol and the lactic acid oligomer is reduced. When the temperature is higher than 180 ℃, the cleavage rate of the lactic acid oligomer is higher than the esterification reaction rate of the polyether polyol and the lactic acid oligomer, and the desired reaction cannot be performed. When the polymerization temperature is too low, the reaction rate of the polyether polyol and the lactic acid oligomer is slow, and the reaction time is long, so that the operation time of the cracking process section is increased, and the equipment maintenance and the production line collaborative production are not favorable. When the first temperature gradient (i.e., polymerization temperature) of the hypergravity apparatus is lower than 140 ℃, the polymerization time needs to be prolonged by about 1 hour for every 10 ℃ reduction, and when the first temperature gradient (i.e., polymerization temperature) is lower than about 120 ℃, the esterification reaction tends to stop and cannot be completed.

The polymerization reaction may be carried out under reduced pressure. The reduced pressure condition may be performed by an evacuation device. In one embodiment, the internal pressure of the hypergravity apparatus during the polymerization reaction is 500-1000 Pa. The esterification polymerization is a reversible reaction, in order to remove the water in the system in time when the reaction is carried out in the forward direction, a solvent and an entrainer are not added in the high-viscosity reaction system, so that the water removal is carried out under the condition of high vacuum degree, and the continuous reaction is ensured; however, limited to the service life of the equipment and production safety, the vacuum degree cannot be too low, which would result in material suck-back and increased equipment maintenance cost.

In one embodiment, the polymerization reaction has a polymerization time of from 90 to 180 minutes. The proper polymerization time helps to complete the polymerization reaction and reduce by-products. The polymerization system simultaneously has the functions of lactic acid oligomer self polymerization, lactic acid oligomer cracking and esterification of the lactic acid oligomer and polyether polyol, 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 continuously prolonged, the method is limited by the increase of the viscosity of the system and the blocking of chain segment slippage, the esterification rate of the target reaction is reduced, the yield of side reaction is increased, the components in the system are complex, the quality of subsequent products is reduced, and the energy consumption and the production cost are increased due to overlong reaction time.

In one embodiment, the present invention is 180-260 ℃ during the cleavage reaction. The proper temperature can ensure that the cracking reaction is carried out efficiently. Because of the competitive relationship of polymerization-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 accelerated, so that the molecular weight of the polylactic acid oligomer is continuously increased, the viscosity of the system is increased, the generated lactide is not easy to evaporate out of the system, and the depolymerization reaction is inhibited from proceeding. When the cracking temperature is too high, the depolymerization reaction rate is accelerated, the generated crude lactide can be quickly moved out of a reaction system, but the too high reaction temperature can cause the racemization of the lactide and cause coking and carbonization. At a second temperature gradient (i.e., cracking temperature) of the hypergravity apparatus below 180 ℃, the cracking rate is very low or tends to zero; when the temperature is higher than 260 ℃, the temperature is higher than 10 ℃ per liter, the heavy components in the cracked product are increased by about 8 percent, and the product purity is reduced.

The cleavage reaction may be carried out under reduced pressure. The reduced pressure condition may be performed by an evacuation device. In one embodiment, the internal pressure of the hypergravity apparatus is 1000-3500Pa in the cleavage reaction. The appropriate pressure helps to obtain gaseous lactide product, helps to separate the gaseous lactide product from the raw material and enter a subsequent purification process, and reduces the content of impurities entering the rectifying tower; due to the characteristics of high condensation point, high boiling point and heat sensitivity of lactide and the limitation of reaction conditions of the depolymerization system of the lactic acid oligomer, the depolymerization process must be operated under high vacuum. 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, the higher vacuum degree is ensured on the premise of ensuring the safe production of the device.

In one embodiment, the cleavage time of the cleavage reaction is 45 to 100 minutes. The proper cracking time helps to complete the cracking reaction fully and reduce the generation of byproducts, so as to obtain higher cracking yield (in actual production, the reaction yield is in positive correlation with the reaction time, namely the yield is higher when the cracking time is longer, but the excessive reaction time can cause serious coking at the bottom of the reactor, so that the substrate which is carbonized excessively enters a circulating system, and the quality of the cracked product is reduced.

In one embodiment, the step-wise reaction of the polylactic acid oligomer in the hypergravity apparatus is carried out under catalysis of a catalyst. When the superfine polylactic acid oligomer liquid particles are contacted with a catalyst, the cracking reaction is quickly carried out to generate the target product lactide, and the lactide is taken out of the supergravity device and enters the rectifying tower under a proper condition.

Suitable catalysts help to increase the rate of the reaction for the cleavage of the polylactic acid oligomer into lactide and shorten the time for the cleavage 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 0.25 wt%, based on the weight of the lactic acid oligomer. The proper amount of catalyst helps to ensure cracking rate and yield and reduce cost. The particle size of the very fine liquid particles formed by the polylactic acid oligomer is dependent on the operating conditions of the hypergravity apparatus, for example: rotor speed, porous structure, etc. Suitable particle sizes of the liquid particles help to increase the yield of the cleavage reaction.

The appropriate rotor speed of the supergravity device is beneficial to obtaining liquid particles with appropriate particle size, so that the material presents the shapes of films, filaments, drops or extremely tiny bubbles on the micro-nano scale; the rapid updating of the high dispersion, high turbulence and strong mixing interface can greatly improve the mass and heat transfer efficiency. The setting of the rotating speed in the actual production is matched with the physical property and the productivity, the rotating speed of the rotor is too high, the liquid phase in the outer packing area flows along the radial direction, the shearing action is weakened, and the mixing and transferring process of the catalyst between the liquid phases is attenuated from inside to outside; the rotating speed is too low, the viscosity of materials is high, the machine is easy to block, the catalyst cannot reach the expected uniformity in a reaction medium, and the reaction is insufficient. In one embodiment, the rotor speed of the supergravity device is 500-.

The porous structure inside the supergravity device can be a filler, and a suitable filler is helpful for uniformly dispersing the polylactic acid oligomer and the catalyst, so that the rate of the cracking reaction is improved, and the time of the cracking reaction is shortened. In one embodiment, the filler of the hypergravity device of the invention is selected from the group consisting of: wire mesh packing, baffles, corrugated packing, and combinations thereof.

In one embodiment, the hypergravity conditions of the hypergravity apparatus of the present invention are from 5 to 10G. The radial polylactic acid oligomer liquid particles with supergravity are not easy to be taken out of the supergravity device, so that the subsequent purification process can be simplified. Suitable hypergravity conditions help to improve the efficiency and yield of the cracking process end. The supergravity condition is provided by the rotating speed of the rotor, and the rotating speed is corresponding to the supergravity condition.

In one embodiment, the step reaction process uses multiple parallel hypergravity devices (e.g., 2-3). Realize the gas phase stable feeding all the day, and further realize the continuous production.

Through the selection of various parameters (such as raw materials, a hypergravity device, reaction conditions and the like) in the step reaction, the cracking yield is improved, crude lactide vapor is obtained, the feeding mode of the rectifying tower is gas-phase feeding, and real-time conveying is realized by utilizing the pressure difference between the two devices, so that the additional energy supply of the traditional liquid-phase feeding is avoided, and the energy consumption is reduced. The implementation scheme of parallel connection of equipment is adopted, so that stable gas-phase feeding in all the day is realized, and continuous production is realized.

Step (ii) of: the obtained crude lactide vapor is transferred to a rectifying column for a rectifying process to obtain a distillate fraction.

The rectification process is a unit operation process for separating substances by utilizing different volatility of different substances through multiple vaporization and multiple condensation, and energy required by multiple vaporization is provided by a reboiler. The proper conditions in the rectification process are favorable for forming efficient combination with the cracking process, the purification is efficiently carried out, the yield is improved, and the real-time feeding is realized by utilizing the air pressure difference between the rectification tower and the hypergravity device, thereby reducing the energy consumption.

The distillate fraction can be obtained by the rectification process in the rectification tower and is sent out from the top of the rectification tower to enter the next purification process. The distillate fractions are gaseous components which generally comprise: lactide, lactic acid and a small amount of polylactic acid oligomer.

In one embodiment, the internal temperature of the rectification column is 115-150 ℃. The temperature is too high, the energy consumption is aggravated, the rising speed of steam is accelerated, and adverse phenomena such as flooding are easily formed, so that the product quality is reduced due to the increase of the content of heavy components at the top. The temperature is too low, the rising speed of steam is reduced, liquid leakage is easily caused, the mass transfer efficiency is obviously reduced, and the rectification effect is seriously influenced or the device cannot operate.

In one embodiment, the internal pressure of the rectification column is 500-3000 Pa. The pressure is too high, the boiling point of the lactide is increased, higher temperature is needed for distilling the lactide, and the degradation of the lactide is caused in the rectification process while the energy consumption is greatly improved; the pressure is too low, the boiling points of the components are close, the separation effect is negative and optimized, the too low internal pressure has higher requirements on the precision and the sealing property of the equipment, and the investment and later maintenance cost of the equipment is increased.

In one embodiment, the reflux ratio during rectification is from 0.5 to 3. Reflux ratio (R ═ L/D), i.e. essenceDistillation columnReflux liquid returned to the tower from the tower topThe ratio of the flow L to the flow D of the overhead product. In the rectification operation of the invention, the reflux ratio needs to be controlled within a certain interval, and the quality of the target product at the top of the tower can be improved by increasing the reflux ratio, but the production capacity of the tower is reduced, so that the consumption of water, electricity and gas is caused. The reflux ratio is too high, which causes the circulation of the materials in the tower to be too large, even flooding is formed, and the normal operation of the tower is damaged; the reflux ratio is too low, the purity of the distillate at the tower top is reduced, the required tower plates are increased, the manufacturing cost of equipment is increased, the product quality is reduced, and the commercial production cannot be carried out.

In the rectification process, liquid bottom liquid containing lactide and other impurities is at the bottom of the rectification tower, 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, 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 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 ascending gas phase and the refluxing liquid phase, and simultaneously maintaining the heat balance in the tower. The proper parameter range in the reboiler is helpful for obtaining proper pressure and reflux ratio in the tower, thereby improving the purity and yield of the product.

In one embodiment, the temperature of the reboiler is 125-155 ℃. Suitable temperatures help to increase the rate of vaporization of the bottom liquid phase while reducing degradation of the 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 taken as a liquid phase and is conveyed back to the supergravity device to participate in the cracking reaction. In one embodiment, the unvaporized liquid phase is transported back to the hypergravity apparatus by means of a circulating metering pump

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

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

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

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

In one embodiment, the ratio of the flow of liquefied product fed to 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 crystallizers, static crystallizers and combinations thereof.

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

In one embodiment, multistage staged temperature control is adopted in the temperature reduction crystallization-crystal growing process. The temperature is gradually decreased, and the enrichment purity of the target product is overlapped, so that a better purification effect is realized on the basis of reducing energy consumption. In addition, the gradual temperature gradient is beneficial to continuous operation.

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

In one embodiment, after the medium-purity lactide is crystallized and crystallized in the crystallizer through temperature reduction, the 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 through the circulating pump to continue to participate in circulation.

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

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

The lactide crystal is heated and insulated by the crystallizer, so that low-melting-point impurities on the surface of the crystal can be melted, and the impurities are melted and 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 rectification tower together with mother liquor obtained by cooling, crystallizing and growing crystal of the medium-pure lactide in a crystallizer by a circulating pump to continuously participate in circulation.

The proper temperature rise and the proper heat preservation time are beneficial to fully melting impurities attached to the surface of the crystal, so that the melting and degradation of the lactide are reduced, the yield is improved, and the energy consumption is reduced. In one embodiment, the temperature for the present invention for increasing the temperature is 70 to 90 ℃. In another embodiment, the incubation time of the present invention is 10-30 min.

After the heating-heat preservation process, the remaining lactide crystals are discharged after being melted, and the purified lactide is obtained.

In the invention, the continuous preparation and purification of the lactide are realized by adopting the supergravity device and combining the 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. Cracking yield Y ═ w ₁ /w ₂ X 100%, wherein 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 invention is > 99%, preferably > 99.6%, for example about 99.60%, 99.62%, 99.65%, 99.70%.

The yield of lactide in the process of the present invention is 75% or more, for example, about 75.31%, 76.21%, 77.2%, 77.15%. The yield of lactide was calculated by: y ═ I ₂ /I ₁ X 100%, wherein Y denotes lactide yield, I ₁ Means the feed amount, i.e., the weight (mass) of the lactic acid oligomer charged into the supergravity apparatus for stepwise reaction, I ₂ Refers to the weight (mass) of the purified lactide obtained.

The energy consumption of the process is ≦ 1.2N, preferably ≦ 1.1N, for example about 1.01N, 1.03N, 0.98N, 1.01N.

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

Lactic acid oligomer 1, catalyst 2 and polyether polyol are initially blended in a mixer 3 and then conveyed from top to bottom or from the middle to a supergravity device 4. Wherein, 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 conditions of given temperature and pressure, 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 condensed reflux material at the top part carry out countercurrent heat and mass transfer; the gas phase distillate 19 at the top of the tower enters a heat exchange condenser 9 for condensation and liquefaction, the obtained liquid phase is extracted to a liquid storage tank 10, a part of liquid phase in the liquid storage tank 10 is conveyed back to the top of the tower by a circulating pump 11 to participate in reflux, the liquid phase 17 at the bottom of the rectifying tower is heated and gasified by a reboiler 15 to form a gas phase 18 which is conveyed back to the rectifying tower, and the liquid (high boiling point heavy component) which is not gasified in the reboiler is conveyed back to the supergravity device by the circulating pump 16 to participate in the cracking reaction again. Another portion of the liquid phase in reservoir 10 (i.e., the medium-purity lactide) is withdrawn via transfer line 21 to crystallizer 12 for subsequent purification.

The medium-purity lactide conveyed by the conveying pipeline 21 is cooled, crystallized and crystallized in the crystallizer 12, and the mother liquor is discharged from the liquid storage tank 13. Then, the temperature rise and heat preservation process is carried out on the crystals in the crystallizer 12, low-melting-point impurities on the surfaces of the crystals are melted, and the molten liquid is discharged to the liquid storage tank 13. The remaining crystals are rapidly melted and discharged to obtain purified lactide, and the purified lactide is discharged through 24. Wherein, the liquid in the liquid storage tank 13 can be conveyed back to a reboiler 15 at the bottom of the rectification tower by a circulating pump 14 to continuously participate in the circulation; the liquid storage tank 13 is recycled to be a mother liquor rich in meso-lactide, and can also be used as a raw material for meso-lactide purification.

Advantageous effects

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

1) a two-step scheme is adopted in the lactide preparation link, so that the system viscosity surge and high-temperature coking loss are inhibited.

2) In the step-type preparation process of lactide, the implementation scheme of parallel equipment is adopted to carry out prepolymerization-cracking in a synergistic manner, so that the gas-phase stable feeding in all days is realized, and the continuous production is realized.

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

4) The supergravity device is introduced to solve the common problems of uneven dispersion and high system viscosity inherent in the traditional cracking device, the long-time continuous cracking yield is improved, and the carbon formation risk is greatly lower; and a coupling purification mechanism is adopted in a purification stage, so that the problems of excessive tower plates, overhigh equipment and lactide degradation caused by a multi-stage rectification tower in the rectification process of the traditional single rectification tower are solved, and the defects of incoherent single melt crystallization production and low yield and purity are overcome.

5) On the premise of not sacrificing productivity, the production system provided by the invention has less equipment holding amount and low maintenance cost, and the cracking device and the rectifying device can feed materials in real time by utilizing air pressure difference without additional energy consumption, so that the energy consumption performance is remarkably reduced; and the whole coupling purification system has no solvent intervention, accords with green chemical engineering guide specifications, and is suitable for large-scale industrial production.

Examples

The present invention will be described in further detail with reference to specific examples.

It should be noted that the following examples are only for clearly illustrating the technical solutions of the present invention, and are not intended to limit the present invention. It will be apparent to those skilled in the art that other variations and modifications may be made in the foregoing disclosure without departing from the spirit or essential characteristics of the invention, and it is not desired to exhaustively enumerate all embodiments, but rather those obvious variations and modifications are within the scope of the invention. Unless otherwise indicated, both the instrumentation and reagent materials used herein are commercially available.

Materials and apparatus

The supergravity device: BZ1K1-3P, Hangzhou Ke Li chemical engineering Equipment Co.

And (3) purity measurement: the purity of the 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 parameters described below.

Example 1

Feeding:

the weight average molecular weight of the lactic acid oligomer used in example 1 was 1000 Da; the polyether polyol is polyoxyethylene polyol (functionality is 2) with the 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 is zinc oxide, and the dosage of the zinc oxide is 0.05 percent of the weight of the lactic acid oligomer.

And (3) cracking:

the supergravity condition (acceleration) of the supergravity device is 5G, the rotating speed of the rotor is 700 r/min, and the filler in the supergravity device is a wire mesh filler. For the stepwise reaction, in the polymerization process, the first temperature gradient of the hypergravity device is 160 ℃, the polymerization time is 90 minutes, and the internal pressure is 800 Pa; during the cracking reaction, the second temperature gradient was 190 ℃, the cracking time was 60 minutes, and the internal pressure was 2000 Pa.

And (3) purification process:

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. Cooling to crystallize at 80 deg.C for 50 min; in the heating-heat preservation process, the impurity removal temperature of heating is 85 ℃, and the heat preservation is carried out for 20 min.

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

Example 2

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

And (3) cracking:

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

And (3) purification process:

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. Cooling and crystallizing at 85 deg.C for 60 min; in the heating-heat preservation process, the impurity removal temperature of the heating is 90 ℃, and the heat preservation is carried out for 30 min. The other conditions and parameters were the same as in example 1.

Example 3

Feeding: the catalyst in example 3 is a mixture of stannous chloride and zinc oxide, and the amount is 0.03% of the weight of the lactic acid oligomer, and the rest conditions and parameters are the same as those in example 1.

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

Example 4

Feeding: in example 4, the catalyst is a mixture of stannous chloride, zinc oxide and zinc lactate, the amount of the catalyst is 0.03 percent of the weight of the lactic acid oligomer, and the rest conditions and parameters are the same as those in example 2.

Comparative example 1

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

And (3) cracking: the cracking apparatus used in comparative example 1 was a conventional tank reactor, and the remaining conditions and parameters were the same as those in example 1.

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

Comparative example 2

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

And (3) cracking: the cracking apparatus used in comparative example 2 was a conventional tank reactor, and the remaining conditions and parameters were the same as those in example 1.

And (3) purification process: the purification process is the purification of a multi-stage rectifying tower, 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 batch charge for comparative example 3 was the same as in example 1.

And (3) cracking: the cracking apparatus used in comparative example 3 was a conventional tank reactor, and the remaining conditions and parameters were the same as those in example 1.

And (3) purification process: the purification process is multistage melt crystallization, and the crystallizer is a serial combination of a falling film crystallizer and a static crystallizer. Cooling to crystallize at 80 deg.C for 50 min; in the heating-heat preservation process, the impurity removal temperature of heating is 85 ℃, and the heat preservation is carried out for 20 min.

Comparative example 4

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

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

Comparative example 5

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

Comparative example 6

Feeding: in comparative example 6, glycerin was used instead of the 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 remaining conditions and parameters were the same as in example 1.

Comparative example 8

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

Comparative example 9

Feeding: in comparative example 9, the charge ratio of polyether polyol to 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 charge 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 charging ratio of polyether polyol to 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 batch charge for comparative example 12 was the same as in example 1.

And (3) cracking: the first temperature gradient of the hypergravity apparatus was 110 ℃ and the remaining conditions and parameters were the same as in example 1.

Comparative example 13

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

And (3) cracking: the first temperature gradient of the hypergravity apparatus was 130 ℃ and the remaining conditions and parameters were the same as in example 1.

Comparative example 14

Feeding: the batch charge for comparative example 14 was the same as example 1.

And (3) cracking: the first temperature gradient of the hypergravity apparatus was 190 ℃ and the remaining conditions and parameters were the same as in example 1.

Comparative example 15

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

And (3) cracking: the second temperature gradient of the hypergravity apparatus was 170 ℃ and the remaining conditions and parameters were the same as in example 1.

Comparative example 16

And (3) cracking: the second temperature gradient of the hypergravity apparatus was 270 ℃ and the remaining conditions and parameters were the same as in example 1.

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

Comparative example 17

Feeding: in comparative example 17, no polyether polyol was added, and the remaining conditions and parameters were the same as in example 1.

Comparative example 18

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

Comparative example 19

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

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

TABLE 1

Yield: y ═ I ₂ /I ₁ 100% of the total, wherein Y denotes the yield, I ₁ Means amount of feed, I ₂ Indicating the target product amount.

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

For the purified lactide yield: i is ₁ The feeding amount of the lactic acid oligomer at the feed port 1; i is ₂ Is the discharge amount of purified lactide at the discharge port 24.

The theoretical energy consumption of the production line with the handling capacity of 500kg/h is taken as a reference N.

TABLE 2

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

Claims

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

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

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

2. The method of claim 1, wherein,

the polyether polyol has a functionality of 2 to 3 and is selected from one or more of the following: polyoxypropylene polyols, polyoxyethylene polyols, polytetrahydrofuran diols;

preferably, the polyether polyol has a functionality of 2 to 3 and is selected from one or more of the following: polyoxypropylene polyol having a number average molecular weight of 400-.

3. The method of claim 1 or 2, wherein the stepwise reaction uses at least one of 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 to 180 minutes;

in the polymerization reaction, the internal pressure of the supergravity device is 500-1000 Pa;

a second temperature gradient of 180 ℃ and 260 ℃ for the cracking reaction;

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

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

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

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

the rotating speed of the rotor of the supergravity device is 500-;

the filler of the supergravity device is selected from: wire mesh packing, baffles, corrugated packing, and combinations thereof.

4. The method of any of claims 1-3, 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 500-3000Pa,

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

5. The method of any one of claims 1-4, wherein,

the rectification process further comprises: gasifying a liquid phase at the bottom of the rectifying tower through a reboiler, conveying an obtained gas phase back to the rectifying tower to participate in the rectifying process, and conveying an unvaporized liquid phase in the reboiler back to the supergravity device to participate in the cracking reaction;

preferably, the temperature of the reboiler is 125-155 ℃.

6. The method of any one of claims 1-5, wherein,

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

7. The method of any one of claims 1-6, wherein,

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

The crystal growth time is 45-75 min.

8. The method of any one of claims 1-7, wherein,

in the process of temperature rise and heat preservation,

the temperature of the temperature rise is 70-90 ℃;

the heat preservation time is 10-30 min.

9. The method of any one of claims 1-8, wherein,

the cracking yield of the crude lactide vapor obtained by the lactic acid oligomer is 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 the lactide obtained by 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.