Process system for preparing lactide through cooperation of microwaves and ultrasonic waves

Abstract

The invention relates to a process system for preparing lactide through cooperation of microwaves and ultrasonic waves, and belongs to the field of lactide material preparation. The microwave and ultrasonic synergistic technology is combined with the conventional lactide preparation technology, the microwave lactic acid dehydrator is used for removing water in lactic acid, the horizontal microwave and ultrasonic synergistic lactide preparation device decomposes the polycondensation and depolymerization reaction process into a plurality of reaction units for gradual polycondensation and depolymerization reaction, and the yield of lactide is improved. And respectively collecting the prepared lactide steam in different time periods according to temperature intervals. The process system has the advantages of continuous flow of process materials, continuous reaction, controllable process, rapid heating response, rapid and thorough dehydration, no use of a powder catalyst, no catalyst residue, timely volatilization of reactants, low vacuum degree requirement, and low energy consumption. The problems that volatilization of system products is hindered due to gradual increase of the viscosity of a reaction system, the reaction is inhibited from proceeding in the forward direction, and kettle residues are increased are solved, so that the purpose of preparing high-purity lactide at low cost is achieved.

Description

Technical Field

[0002] The invention belongs to the field of preparation of lactide materials, and in particular relates to a process system for preparing lactide by synergistic use of microwaves and ultrasound. Technical Background

[0004] Lactide is the most important polymer monomer for the production of polylactic acid and the most basic and core raw material for the preparation of biodegradable materials. Its chemical purity and production cost determine the quality and economy of downstream polylactic acid products. At present, domestic lactide synthesis technology generally has problems of low optical purity, low yield and high cost, and the optical purity of lactide determines the performance of ring-opening polymerization polylactic acid. In recent years, due to the rapid development of my country's polylactic acid industry, the market demand for lactide has continued to rise. If the domestic large-scale production technology of lactide cannot achieve a breakthrough, the domestic polylactic acid industry, which has just started to pick up, will continue to face problems such as shortage of raw materials or dependence on imports, which is a severe challenge. The fundamental way to promote the healthy development of my country's polylactic acid industry is to adopt new materials, new processes and new technologies to overcome the technical difficulties of lactide production and develop a process system that can achieve high purity, high yield and low cost preparation of lactide.

[0005] The synthesis of relatively high molecular weight polylactic acid requires a high purity of lactide monomer, usually requiring the optical purity of lactide to be greater than 99.5%, the chemical purity to be greater than 99.0%, and the mass fraction of water to be less than 0.15%. At present, the preparation of lactide mainly adopts a two-step synthesis process using lactic acid as raw material. In the first step, the free water in the raw lactic acid is removed to make its concentration meet the process requirements, and then the lactic acid is subjected to polycondensation reaction under a certain pressure, temperature and catalyst to obtain lactic acid oligomers; in the second step, under the action of a catalyst, the pressure and temperature are changed to make the lactic acid oligomers depolymerize and then polymerize to obtain lactide. In addition, the crude lactide obtained by the two-step method needs to be purified and refined to obtain refined lactide that meets the purity requirements of the raw material for polylactic acid synthesis.

[0006] At present, there are several difficulties in preparing lactide:

[0007] 1) Improper temperature control affects the condensation and depolymerization reactions. When the condensation temperature is low, the dehydration rate of the reaction system is slow, and the relative molecular mass of the obtained lactic acid oligomer is low, which is not suitable for the subsequent depolymerization reaction of the lactic acid oligomer; when the condensation temperature increases, the relative molecular mass of the lactic acid oligomer increases with the acceleration of the dehydration rate of the reaction system, but the lactic acid oligomer with too high a relative molecular mass is also not suitable for depolymerization into lactide. The depolymerization of lactic acid oligomers is a reversible reaction. If the temperature of the depolymerization process is too low, the lactic acid oligomers are not easy to depolymerize to form lactide; if the temperature is too high, the lactic acid oligomers will decompose and oxidize, and the generated lactide will also change color due to coking and carbonization. At present, the main device in the lactide preparation process is a vertical reactor. This traditional intermittent production method has many problems, such as the viscosity of the reactants continues to increase during the reaction, slow heat transfer of materials, uneven heating of the system, and excessively high local temperatures. The long residence time at high temperature will also trigger side reactions. The temperature range and temperature gradient in the reaction system are well known to everyone, but how to effectively control the temperature in the industrial production process to achieve uniform mass and heat transfer in the reaction system and to promptly vaporize and evaporate the water, free lactic acid, lactide, etc. in the reaction system is an urgent problem to be solved in the industrialization of lactide.

[0008] 2) High vacuum leads to high energy consumption. Currently, the preparation of lactide is often carried out under high vacuum. The reduction of the pressure of the reaction system not only helps to evaporate the water and lactide in the reaction system, but also helps to save the heat energy consumed by heating the reaction system. However, high vacuum has very high requirements on the airtightness and precision of the reaction equipment, high overall energy consumption, and requires large investment and operating costs. Therefore, it is very important to find a process technology that can quickly evaporate water and lactide under a not too high vacuum environment for the industrialization of lactide preparation.

[0009] 3) The catalyst is unevenly dispersed and many metal ions remain. At present, the mainstream of lactide production is to use metal catalysts, mainly zinc and tin compounds. Some catalysts are in powder form, which are unevenly dispersed in the reaction system, the catalytic reaction is not thorough, and many metal ions remain, resulting in carbonization of materials, reduced yield, and difficult to handle reaction residues. These are also problems that must be solved in the industrial production of lactide.

[0010] 4) The viscosity of the reaction system is too high, which inhibits the reaction from proceeding in the forward direction. During the preparation reaction of lactide, the fluidity of the lactic acid system gradually decreases and the viscosity gradually increases. This process will significantly hinder the volatilization of the system products, thereby inhibiting the reaction from proceeding in the forward direction. In addition, the system will form a kettle residue with a higher viscosity. After the temperature is lowered, the kettle residue is difficult to vaporize and evaporate by continuous discharge of the pump, which is easy to clog the pipeline and cause the entire continuous production to fail. How to develop an evaporation reaction device with special functions when the viscosity of the reaction system is too high, overcome the obstacle that the reaction products cannot be vaporized and evaporated in time due to the high viscosity of the system, and make the reaction proceed normally in the forward direction, this is the core problem in the current industrial production of lactide preparation.

[0011] It is very challenging to efficiently synthesize lactide with high chemical purity and high optical purity under conditions suitable for industrial production. People have done a lot of work in the process technology of preparing lactide by new technology, whether in one-step or two-step methods, and have achieved remarkable results. For example, a method for preparing lactide by microwave irradiation (patent number CN200410013213.4) uses a microwave oven as a test device to prepare oligomeric lactic acid by microwave irradiation polycondensation reaction of lactic acid raw materials, and then prepare lactide by microwave irradiation depolymerization reaction, which shortens the reaction time, reduces energy consumption, and improves the yield. Liu Cuijuan and others from the School of Chemistry and Pharmacy of Jiamusi University used an ultrasonic cleaning machine to ultrasonically enhance the synthesis of lactide, which greatly shortened the reaction time.

[0012] Microwaves have a unique way of intramolecular heating. Compared with traditional heating methods, chemical reactions under microwave radiation have the characteristics of strong activation, temperature conversion, fast reaction speed, high conversion rate, high selectivity, no hysteresis effect and high energy utilization efficiency. For specific chemical reactions, the role of microwaves is not only heating, but also many synergistic effects between microwaves and reactants can increase the reaction speed by dozens or even hundreds of times. Microwave synthesis has an important manifestation in depolymerization reactions, can replace catalysts, and can accelerate the reaction speed through microwave heating.

[0013] Ultrasonic waves can transfer and mix mass more effectively, thus making the catalytic reaction more uniform. The cavitation effect of ultrasonic waves can split monomers or generate reaction intermediates, which is easier to promote ring-opening reactions, with fast reaction rates, high relative molecular mass and concentrated distribution.

[0014] Microwave and ultrasonic technology has been widely used in the field of chemical synthesis. In the experimental preparation of lactide, its advantages of increasing reaction speed, reducing pollution, increasing output rate and improving product quality as well as simple subsequent processing have been highlighted. Combining microwave and ultrasonic synergistic technology with conventional lactide preparation technology can realize continuous automated production of the entire process of lactic acid dehydration, oligomerization, depolymerization, and timely vaporization and evaporation of crude lactide, and achieve the goals of short reaction time, sufficient catalyst reaction, low hybridization rate of crude lactide, high production efficiency, and low construction and production costs. However, due to the late start of microwave and ultrasonic use in the preparation of lactide, and the difficulty in matching suitable microwave and ultrasonic components and the difficulty in equipment development, microwave and ultrasonic technology has not yet been industrialized in lactide synthesis. Summary of the invention

[0016] In view of the above problems, the object of the present invention is to provide a process system for preparing lactide by synergistic use of microwaves and ultrasound, combining the application technology of microwaves and ultrasound with conventional lactide preparation technology, and providing a new lactide preparation device and process, which has the characteristics of continuous flow of process materials, continuous reaction, controllable process, rapid heating response, rapid and thorough dehydration, sufficient catalytic reaction, timely vaporization and evaporation of reactants, high lactide yield, high purity and no catalyst residue, thereby achieving the purpose of short synthesis reaction time of lactide, high production efficiency and low production cost.

[0017] The present invention is achieved through the following technical solutions:

[0018] The implementation of a process system for preparing lactide by microwave and ultrasound synergistically provided by the present invention comprises at least the following steps:

[0019] Step (1), free water is removed from lactic acid at low temperature and normal pressure using a microwave lactic acid dehydrator to obtain dehydrated lactic acid and introduce it into a dehydrated lactic acid storage tank. The microwave lactic acid dehydrator has an operating temperature below 50°C.

[0020] Step (2), heating the dehydrated lactic acid obtained in step (1) to 130°C and introducing it into a lactic acid polycondensation system.

[0021] Step (3), the lactic acid polycondensation system is subjected to the synergistic effects of microwaves, ultrasound, heating oil, mechanical stirring and energy storage catalytic pellets, and the polycondensation reaction time is about 2 hours, the reaction temperature is 130°C to 155°C, and the pressure is 15 to 20KPa. In the initial stage of the polycondensation reaction at 130°C to 135°C and within 30 minutes, free water and bound water are discharged. The water generated in the middle and late stages of the polycondensation is instantly evaporated and discharged. The reaction time, pressure and temperature are adjusted so that the molecular weight of the obtained lactic acid oligomer is controlled at 800 to 2200Da.

[0022] Step (4), heating the lactic acid oligomers obtained in step (3) to 180°C and introducing them into a depolymerization system to prepare lactide.

[0023] Step (5), further, the lactic acid oligomer obtained in step (3) is depolymerized for 7 hours under the combined action of microwave, ultrasonic wave, heating oil, mechanical stirring and energy storage catalytic pellets, at a temperature of 180°C to 250°C and a pressure of 5 to 15 KPa. Within the initial 30 minutes of depolymerization, light substances such as water and lactic acid in the depolymerization system are evaporated and discharged, at a temperature of 180°C to 185°C and a pressure of 12 to 15 KPa.

[0024] Step (6): further, the lactide vapor obtained in step (5) is vaporized and discharged in different reaction time periods according to the temperature range.

[0025] Step (7): Further, when the amount of lactide vapor evaporated in the last temperature interval of depolymerization in step (6) is significantly reduced, the residual depolymerization substrate in the last temperature reaction interval is vaporized and evaporated.

[0026] Step (8), further, purifying and diluting the depolymerization residual substrate discharged by vaporization in step (7) and returning to step (4).

[0027] Step (9), further, as the residual substrate of the depolymerization in step (7) is vaporized and discharged, step (4) introduces the lactic acid oligomers prepared in step (3) into the depolymerization system in sequence to prepare lactide.

[0028] Step (10), further, along with step (4), the lactic acid oligomers prepared in step (3) are sequentially introduced into the depolymerization system, and step (2) the dehydrated lactic acid prepared in step (1) is heated to 130° C. and introduced into the lactic acid condensation system.

[0029] Step (11), further, along with step (2) introducing the dehydrated lactic acid obtained in step (1) into the lactic acid polycondensation system, the dehydrated lactic acid obtained by the microwave lactic acid dehydration machine in step (1) is introduced into the dehydrated lactic acid storage tank.

[0030] Furthermore, steps (1) to (11) are connected in sequence and cycled in an orderly manner to form a complete lactide continuous production system.

[0031]

[018] The microwaves described generally refer to electromagnetic waves with a wavelength between 1 mm and 1 m (frequency 300-300000 MHz), the most commonly used frequencies being 915 and 2450 MHz, and the basic properties of microwaves are generally characterized by penetrability, selectivity and reflectivity. Microwaves have a special promoting effect in many chemical reactions. Compared with conventional heating chemical reactions, the chemical reaction rate under microwave radiation is fast, which can be increased to 10-10000 times of the conventional method, and stirring at the molecular level can be achieved, with a high reaction yield and few side reactions. Moreover, since different substances respond differently to microwave irradiation, microwaves can make the reaction highly selective; the role played by microwaves is not only a unique intramolecular heating method, but also a non-thermal effect in the reaction. Microwaves can act directly on substances, and in many organic reactions, solvents as heat transfer media do not need to be used, which can reduce and eliminate the use of harmful solvents; microwave heating can be precisely controlled and used, which is conducive to realizing automated operation. Microwave heating is very safe under certain technical specifications and standards. Furthermore, microwave synthesis technology has the characteristics of high heat, low energy consumption, environmental protection and easy operation, and is currently widely used in the field of organic chemistry. Microwave synthesis has an important manifestation in depolymerization reactions, can replace catalysts, and can accelerate the reaction rate through microwave heating. Further, microwaves can greatly accelerate the dehydration rate of lactic acid and polycondensation reactions. Due to the characteristics of selective heating of microwaves, different substances produce different thermal effects. Water molecules and lactic acid are both polar molecules with large dielectric loss factors. The dielectric constant of water is 78.3 and the dielectric constant of lactic acid is 22.0. They both have strong absorption capacity for microwaves, while lactide is a non-polar substance with a very small dielectric loss factor. Therefore, according to the difference in dielectric loss factors of water, lactic acid and lactide, water can be quickly evaporated and separated from the lactic acid system in the polycondensation reaction, and free lactic acid and water can be quickly and effectively separated from the depolymerization reaction system in the early stage of the depolymerization reaction, reducing the probability of side reactions of the depolymerization reaction caused by the presence of water, free lactic acid, etc.

[0032]

[018] The ultrasonic wave is a sound wave with a frequency higher than 20000Hz. When the ultrasonic wave propagates in the medium, the interaction between the ultrasonic wave and the medium causes the medium to undergo physical and chemical changes, thereby generating a series of mechanical, thermal, electromagnetic and chemical ultrasonic effects. For example, mechanical effect: the mechanical action of ultrasonic wave can promote the emulsification of liquid, liquefaction of gel and dispersion of solid; cavitation effect: when ultrasonic wave acts on liquid, a large number of small bubbles can be generated. The small bubbles formed by cavitation will continuously flip, grow or suddenly burst with the vibration of the surrounding medium. When bursting, the surrounding liquid suddenly rushes into the bubbles to generate high temperature and high pressure, and at the same time generates shock waves, which can cause monomers to split or generate reaction intermediates, making it easier to promote ring-opening reactions, with fast reaction rates, high relative molecular mass and concentrated distribution; thermal effect: due to the high frequency and large energy of ultrasonic wave, it can produce significant thermal effects when absorbed by the medium; chemical effect: the action of ultrasonic wave can promote or accelerate certain chemical reactions. Ultrasonic wave can transfer mass and mix more effectively, thereby making the catalytic reaction more uniform and thorough.

[0033] Furthermore, the energy storage catalytic pellets described in

[021] are both microwave absorbing energy storage materials and solid catalysts, and are composed of silicon carbide, four-needle zinc oxide whiskers, nano-SnO 2 It is made of materials such as tourmaline, nano-monazite, etc., with a spherical particle diameter of 3-5mm and 1-2mm.

[0034]

[033] The silicon carbide described is a high-quality microwave absorbing heat generating energy storage material.

[0035] The nano tin dioxide (SnO 2 ) material, as an important intrinsic semiconductor oxide, has excellent optical properties, electrical properties and chemical stability. It is a widely used energy storage and catalytic material, and is also the preferred catalyst for the preparation process of lactide.

[0036]

[033] The four-needle zinc oxide whisker has special tip nanoactivity and catalytic effect and strong wave absorption and heating effect; zinc oxide is also an important catalyst for the preparation of lactide.

[0037]

[033] The monazite described herein has a Chinese name of "phosphorus cerium lanthanum ore" (Ce, La, Nd, Th)PO 4 , of which the rare earth oxide content is about 65%, and it is a kind of rare earth mineral. Rare earth, as a small-volume atom with more positive charge and low polarization, belongs to hard acid; lactide is the intermediate product of synthesizing lactic acid into polylactic acid, which has the characteristics of high electronegativity, low polarizability, and difficulty in oxidation, and belongs to hard base; therefore, the two can easily form a transitional form of complex. At this time, the activation energy of the chemical reaction is low, and chemical reactions are easy to occur. Therefore, rare earth has the effect of increasing the reaction rate, and has low toxicity and low residue, and has unique catalytic advantages.

[0038]

[033] The tourmaline has the characteristics of permanent spontaneous electrode, piezoelectricity and thermoelectricity, can generate infrared rays and microcurrent, can electrolyze water and make water molecules smaller, which is beneficial to accelerate the vaporization and evaporation of water molecules from the reaction system; tourmaline has a good adsorption capacity for heavy metal ions, can adsorb the residual metal ions in the lactide preparation system, and reduce the residual metal ion content in the reaction system.

[0039]

[018] The present invention provides a process system for the coordinated preparation of lactide by microwave and ultrasonic waves, comprising: a microwave lactic acid dehydrator and a horizontal microwave and ultrasonic wave coordinated preparation device for lactide.

[0040]

[039] The microwave lactic acid dehydrator is a device for specifically implementing steps (1) and (2) of the process system for preparing lactide by microwave and ultrasonic synergistically described in

[018] , and includes a microwave source, a microwave resonant cavity, a trough conveyor belt, a lactic acid distributor, a moisture removal system and a control system, etc.

[0041]

[040] The microwave lactic acid dehydrator has a tunnel structure. Although both water and lactic acid are high-loss substances and have a very prominent microwave effect, the dielectric constant difference between water and lactic acid is relatively large. The highly selective heating characteristics of microwaves are used to separate water from lactic acid at low temperature and normal pressure. The device can produce dehydrated lactic acid with a water content of less than 0.05%.

[0042]

[040] The microwave lactic acid dehydrator adopts a 2450MHz industrial microwave source, and the output power of a single microwave magnetron is greater than 2000W. The microwave magnetrons are installed on both sides of the microwave resonant cavity, and a steam discharge port is installed on the top of the microwave resonant cavity. An infrared thermometer is also installed on the top of the microwave resonant cavity to detect the temperature of lactic acid during microwave dehydration. The temperature of lactic acid during microwave dehydration is less than 50°C. By detecting the water content of lactic acid at the discharge end of the microwave lactic acid dehydrator, the feed amount of lactic acid, conveyor belt speed, microwave power and exhaust volume and other parameters are adjusted. The conveyor belt passes through the microwave resonant cavity. There are microwave suppressors at both ends of the microwave resonant cavity to prevent microwaves from leaking from the inlet and outlet of the system. The material of the conveyor belt is a composite material of polytetrafluoroethylene and glass fiber. In order to dehydrate lactic acid evenly, a longitudinal groove is provided on the conveyor belt. The conveyor belt moves on ceramic rollers, and the ceramic rollers are supported by ceramic brackets. The feed distributor of the microwave lactic acid dehydrator is located above the microwave suppressor at the feed end. The distributor passes through the microwave suppressor to evenly distribute the lactic acid on the conveyor belt. The dehydrated lactic acid is stored in the storage tank.

[0043]

[039] The horizontal microwave and ultrasonic synergistic preparation of lactide device is a device that specifically implements steps (3) to (11) of the microwave and ultrasonic synergistic preparation of lactide process system described in

[018] , including: a lactide synthesis chamber, a mechanical stirring device, a microwave source, an ultrasonic device, a heating oil system, a moisture removal system, a lactide vapor negative pressure extraction system, a depolymerization reaction residual substrate vaporization, evaporation, discharge, purification and reuse system and a control system, etc.

[0044] According to the reaction characteristics of different temperature ranges of polycondensation and depolymerization, the lactide synthesis chamber described in

[043] is decomposed into a plurality of polycondensation reaction units and depolymerization reaction units, each reaction unit is composed of a plurality of reaction chambers. The reaction chamber is sealed and buckled together by two upper and lower U-shaped grooves, the lower U-shaped groove is made of 316L or 310S stainless steel, coated with wear-resistant nano coating, and the upper U-shaped groove is made of quartz ceramic material.

[0045]

[044] The several reaction chambers are connected to each other, and two adjacent reaction chambers are separated by a partition. The upper part or the bottom of the partition is provided with a circular hole. The partition between the polycondensation reaction unit and the depolymerization reaction unit is not provided with a hole, but there is a feed-liquid communication pipeline. A mechanical stirring device connects the reaction chambers in series. Each reaction chamber has a stirring blade and an energy storage catalytic pellet. The bottom of each reaction chamber is respectively connected to a corresponding heating oil jacket. An ultrasonic device is also installed at the bottom and lower side of each reaction chamber. The upper half of each reaction chamber is located in a microwave resonant cavity, which is located in the middle and upper part of the lactide synthesis cavity. Microwave magnetrons are installed on both sides of the microwave resonant cavity. A light material vapor outlet is installed at the top of each reaction chamber, and the light material outlet passes through the light material vapor outlet of the microwave resonant cavity reaction chamber and is connected to an external negative pressure extraction system. An infrared temperature measuring device is installed in each reaction chamber. Aluminum diamond infrared temperature measuring heat conducting elements are inlaid on both sides of the upper middle part of the lower U-shaped groove, and the aluminum diamond infrared temperature measuring heat conducting elements correspond to the far-infrared temperature measuring device. The material of the previous reaction chamber can be introduced into the next reaction chamber in sequence under certain conditions. The mechanical stirring device adopts electromagnetic clutch transmission. The lactic acid inlet of the lactide synthesis chamber is set at the front end of the lactide synthesis chamber, and the outlet of the residual substrate of the depolymerization reaction is set at the tail end of the lactide synthesis chamber.

[0046] Specifically, lactic acid with a purity of more than 85% is introduced into the distributor of the microwave lactic acid dehydrator, and the distributor evenly distributes the lactic acid on the conveyor belt, and enters the microwave resonance cavity of the microwave lactic acid dehydrator with the rotation of the conveyor belt, and the temperature is controlled to be less than 50°C, and dehydration is carried out at low temperature and normal pressure to obtain dehydrated lactic acid and introduce it into the dehydrated lactic acid storage tank for storage. The water content of the dehydrated lactic acid is less than 0.05%.

[0047] Settings: condensation reaction time 2 hours, condensation temperature 135℃~155℃, pressure 15~20KPa; depolymerization reaction time 7 hours, depolymerization temperature 180℃~250℃, pressure 5~15KPa.

[0048]

[044] Energy storage catalytic pellets are added to all the polycondensation (depolymerization) reaction chambers. 3-5 mm energy storage catalytic pellets are added to the polycondensation reaction chamber, the early depolymerization reaction chamber, and the mid-depolymerization reaction chamber. The addition ratio is 1 / 3 of the liquid volume in the reaction chamber. 1-2 mm energy storage catalytic pellets are added to the late depolymerization reaction chamber. The addition ratio is 2 / 3 of the lactic acid volume in the reaction chamber. Nitrogen replacement is done well and it is in a negative pressure working state. The oil temperature of the heating oil jacket of the polycondensation unit is 145°C.

[0049] The dehydrated lactic acid described in

[046] is heated to 130° C. and introduced into the first polycondensation chamber of the lactic acid polycondensation unit. The amount introduced is the amount of lactic acid set for one polycondensation chamber of the polycondensation unit. The lactic acid undergoes polycondensation reaction under the combined action of heating oil, microwaves, ultrasound, mechanical stirring and energy storage catalytic pellets.

[0050] Furthermore, after the lactic acid in the first polycondensation reaction chamber reaches the set polycondensation reaction time, the dehydrated lactic acid is introduced into the first polycondensation chamber. The lactic acid that has completed the polycondensation reaction in the first polycondensation chamber is introduced into the second polycondensation chamber through the circular hole channel on the upper part of the partition between the first polycondensation chamber and the second polycondensation chamber to continue the polycondensation reaction. The lactic acid is sequentially introduced into all the polycondensation chambers of the polycondensation unit, and the temperature is gradually increased to carry out the polycondensation reaction. The water vapor outlet of the polycondensation unit is vaporized and evaporated to discharge water, so as to obtain lactic acid oligomers suitable for depolymerization.

[0051]

[050] The reaction pressure of each condensation chamber in the condensation unit is 15-20 KPa, and the temperature increases gradually within the range of 135°C to 155°C. The heating oil jacket of the condensation unit provides the basic temperature, and microwave radiation adjusts the temperature of each condensation chamber.

[0052] The lactic acid oligomers described in

[050] are heated to 180°C through the feed-liquid connecting pipe between the polycondensation reaction unit and the depolymerization reaction unit, and are introduced into the depolymerization system in sequence to prepare lactide. Each reaction chamber corresponds to a different reaction temperature range and reaction time period, and the lactide vapor generated in each reaction chamber is vaporized, evaporated, discharged, and collected from the lactide vapor outlet of the corresponding reaction unit. When the amount of lactide vapor discharged by vaporization and evaporation at the lactide vapor outlet at the end of the device is significantly reduced, the residual substrate of the depolymerization reaction in the last depolymerization chamber is vaporized, evaporated, and discharged. The residual substrate of the depolymerization discharged by vaporization and evaporation is purified, diluted, and then heated to enter the first depolymerization chamber for depolymerization reaction. As the residual substrate of the depolymerization is vaporized, evaporated, and discharged, the lactic acid oligomers of the polycondensation reaction unit are introduced into the depolymerization system in sequence. Subsequently, the dehydrated lactic acid in the dehydrated lactic acid storage tank is heated to 130°C and introduced into the first polycondensation chamber of the lactic acid polycondensation system. Subsequently, the lactic acid is dehydrated by the microwave lactic acid dehydration machine and introduced into the dehydrated lactic acid storage tank to maintain the material level balance in the dehydrated lactic acid storage tank. In this way, the front and back are connected and circulated in an orderly manner, forming a complete lactic acid-lactide continuous circulation production system.

[0053]

[044] The polycondensation reaction unit is composed of a plurality of polycondensation chambers, the temperature of which increases gradually between 135°C and 155°C. Dehydrated lactic acid is successively introduced into each polycondensation chamber within two hours to carry out a polycondensation reaction.

[0054] The energy storage catalytic pellets in the reaction chamber of the polycondensation reaction unit described in

[053] are turned over in the reaction system with the stirring of the mechanical stirring device, and lactic acid continuously forms a new reaction interface on the surface of the energy storage catalytic pellets. The rare earth, tin dioxide, zinc oxide and other catalysts in the energy storage catalytic pellets are in uniform contact with the system, which has a good catalytic effect. The lactic acid polycondensation reaction generates lactic acid low molecular polymers and water. The tourmaline in the energy storage catalytic pellets electrolyzes the water and converts the water into small molecules. Under the action of mechanical stirring, the energy storage catalytic pellets are thrown above the liquid surface of the reaction system, and the surface of the energy storage catalytic pellets is covered with the feed liquid of the lactic acid polycondensation reaction system to form a liquid film. Under the appropriate microwave power radiation, microwaves heat highly polar water selectively, and the water on the liquid surface of the reaction chamber and the liquid film on the surface of the energy storage catalytic pellets instantly vaporizes and evaporates and is discharged from the steam outlet. The energy storage catalytic pellets absorb microwaves and heat up rapidly, and the energy storage catalytic pellets fall back into the liquid phase reaction system, bringing heat energy into the reaction system. The liquid film on the surface of the energy storage catalytic pellets after the water is evaporated is peeled off under the action of mechanical stirring and ultrasound, so that the reaction system is heated more evenly, the mass transfer and heat transfer effect is increased, and the rare earth, tin dioxide, zinc oxide and other catalysts in the energy storage catalytic pellets are in uniform contact with the system, which has a good catalytic effect, and a dynamic reaction interface is continuously formed on the surface of the energy storage catalytic pellets to generate lactic acid oligomers and water. The tourmaline in the energy storage catalytic pellets electrolyzes the water and makes the water molecules small. Under the action of mechanical stirring, the energy storage catalytic pellets are brought to the upper part and above the reaction system again. The reaction interface area where the vaporization products of the reaction system are vaporized and evaporated is greatly increased. The selective heating of microwaves on water molecules causes the generated water to flash evaporate instantly; and because the energy storage catalytic pellets are high-temperature sintered ceramic materials, the catalytic material will not be dispersed into the polycondensation reaction system. Therefore, there will be no catalyst residue in the polycondensation reaction system, and some side reactions caused by catalyst residues in the polycondensation reaction system will not exist. In this way, the polycondensation reaction proceeds in an orderly manner in the positive direction.

[0055] Furthermore, the several polycondensation chambers in the polycondensation reaction unit described in

[054] correspond to polycondensation reaction processes carried out in different temperature ranges and different reaction time periods, and have different functions in the polycondensation reaction. The polycondensation reaction in the previous polycondensation chamber is the basis for the polycondensation reaction in the next polycondensation chamber, and the polycondensation reaction in the next polycondensation chamber is the continuation of the polycondensation reaction in the previous polycondensation chamber. The stage functions are clear and continuous, so that the entire polycondensation reaction process can orderly move the reaction equilibrium in a positive direction. The front polycondensation chamber evaporates the free water and bound water contained in the lactic acid out of the polycondensation reaction system at a temperature of 135°C to 140°C, and starts a preliminary polycondensation reaction. The polycondensation chambers from the middle to the rear end are synchronously and orderly carried out in stages as the temperature increases in an orderly manner and the reaction time increases. The water generated by the polycondensation reaction is immediately vaporized and evaporated to leave the reaction system.

[0056] Furthermore, under the combined action of microwave, ultrasound, heating oil, mechanical stirring and energy storage catalytic pellets, the lactic acid polycondensation system has no catalyst residue, the reaction system is rapidly dehydrated, and the development of lactic acid oligomers into macromolecules can be effectively inhibited. The molecular weight of the obtained lactic acid oligomers is 800 to 2200 Da. The reaction temperature of the polycondensation system does not exceed 155°C, the pressure is 15 to 20 KPa, and the reaction time is about 2 hours.

[0057]

[044] The depolymerization initial reaction unit of the plurality of depolymerization units is composed of a plurality of depolymerization chambers, and the temperature of these plurality of depolymerization chambers increases gradually between 180°C and 200°C. The lactic acid oligomers obtained in the polycondensation stage are sequentially introduced into the depolymerization chambers of the primary depolymerization unit for the initial depolymerization reaction.

[0058] The energy storage catalytic pellets in the reaction chamber of the initial depolymerization reaction unit described in

[057] are turned over in the reaction system with the stirring of the mechanical stirring device, and the lactic acid oligomers continuously form new reaction interfaces on the surface of the energy storage catalytic pellets. The catalysts such as rare earth, tin dioxide, zinc oxide, etc. in the energy storage catalytic pellets are in uniform contact with the system, which has a good catalytic effect. The depolymerization reaction generates lactide, and the residual water in the depolymerization reaction system is electrolyzed by the tourmaline in the energy storage catalytic pellets and the water is reduced to small molecules. The surface of the energy storage catalytic pellets is covered with the feed liquid of the depolymerization reaction system to form a liquid film. Under the action of mechanical stirring, the energy storage catalytic pellets are thrown above the liquid surface of the reaction system. Under the appropriate microwave power radiation, microwaves highly selectively heat the highly polar water, lactic acid and lactic acid oligomers, and the water, lactic acid and lactic acid oligomers in the liquid film on the surface of the reaction chamber and the surface of the energy storage catalytic pellets are instantly vaporized and flash evaporated. The energy storage catalytic pellets absorb microwaves and heat up rapidly. The lactide generated in the liquid surface of the reaction chamber and the liquid film on the surface of the energy storage catalytic pellets is heated and quickly vaporized, and evaporates from the liquid surface of the reaction chamber and the liquid film on the surface of the energy storage catalytic pellets and is discharged from the vapor outlet. Then, the energy storage catalytic pellets fall into the liquid phase reaction system, bringing heat energy into the reaction system, making the reaction system more evenly heated, and increasing the mass transfer and heat transfer effect. The liquid film on the surface of the energy storage catalytic pellets after the lactide is evaporated is peeled off under the action of mechanical stirring and ultrasound, and a new reaction interface is continuously formed on the surface of the energy storage catalytic pellets to generate lactide, and a new liquid film is formed on the surface of the energy storage catalytic pellets. Under the action of mechanical stirring, the energy storage catalytic pellets are brought to the upper part and above the reaction system again, and the surface area of ​​the vaporized product of the reaction system is greatly increased. Microwaves selectively heat water molecules, free lactic acid and low molecular weight lactic acid oligomers, causing the residual water molecules, free lactic acid and low molecular weight lactic acid oligomers in the depolymerization system to evaporate instantly; and because the energy storage catalytic pellets are high-temperature sintered ceramic materials, the catalytic material will not disperse into the reaction system. Therefore, there will be no catalyst residue in the reaction system, and some side reactions caused by catalyst residue in the reaction system will not exist. In this way, the depolymerization reaction proceeds in an orderly manner in the positive direction.

[0059] Furthermore, the several depolymerization chambers in the initial depolymerization reaction unit

[057] correspond to depolymerization reaction processes carried out in different temperature ranges and different reaction time periods, and have different functions in the depolymerization reaction. The depolymerization reaction in the previous depolymerization chamber is the basis for the depolymerization reaction in the next depolymerization chamber, and the depolymerization reaction in the next depolymerization chamber is the continuation of the depolymerization reaction in the previous depolymerization chamber. The stage functions are clear and continuous, so that the entire initial depolymerization reaction process moves the reaction equilibrium in an orderly manner toward the positive direction. The main function of the front depolymerization chamber is to evaporate the water and lactic acid contained in the system at a temperature of 180°C to 185°C, and start a preliminary depolymerization reaction to generate lactide. The depolymerization chambers from the middle to the rear end are synchronously and orderly carried out in stages as the temperature increases in an orderly manner and the reaction time increases. The lactide generated in the initial depolymerization reaction unit and the generated water molecules, free lactic acid and low molecular weight lactic acid oligomers remaining in the depolymerization system are immediately evaporated and separated from the reaction system. Therefore, the obtained crude lactide contains more impurities such as water, lactic acid, and low-molecular lactic acid oligomers.

[0060]

[044] The intermediate depolymerization reaction unit of the plurality of depolymerization units is composed of a plurality of depolymerization chambers, the temperature of which increases gradually between 200°C and 230°C, and the remaining substrate of the initial depolymerization unit is introduced into each depolymerization chamber in sequence to complete the intermediate stage depolymerization reaction.

[0061] The energy storage catalytic pellets in the reaction chamber of the depolymerization intermediate reaction unit described in

[060] are turned over in the reaction system with the stirring of the mechanical stirring device, and the lactic acid oligomers continuously form new reaction interfaces on the surface of the energy storage catalytic pellets. The rare earth, tin dioxide, zinc oxide and other catalysts in the energy storage catalytic pellets are in uniform contact with the system, which has a good catalytic effect. The depolymerization reaction generates lactide. Under the action of mechanical stirring, the energy storage catalytic pellets are thrown above the liquid surface of the reaction system. The surface of the energy storage catalytic pellets is covered with the feed liquid of the depolymerization reaction system to form a liquid film. The energy storage catalytic pellets absorb microwaves and heat up rapidly. The lactide generated from the liquid surface of the reaction chamber and the surface liquid film is heated and quickly vaporized and evaporated and discharged from the steam outlet. Then, the energy storage catalytic pellets fall into the liquid phase reaction system, bringing heat energy into the reaction system, making the reaction system heated more evenly and increasing the mass transfer and heat transfer effects. The liquid film after lactide is evaporated from the surface of the energy storage catalytic pellets is peeled off under the action of mechanical stirring and ultrasound, and a new reaction interface is continuously formed on the surface of the energy storage catalytic pellets to generate lactide, and a new liquid film is formed on the surface of the energy storage catalytic pellets. Under the action of mechanical stirring, the energy storage catalytic pellets are brought to the upper part and above the reaction system again, and the surface area of ​​the vaporized product of the reaction system is greatly increased. In addition, since the energy storage catalytic pellets are high-temperature sintered ceramic materials, the catalytic material will not be dispersed into the reaction system. Therefore, there will be no catalyst residue in the reaction system, and some side reactions caused by catalyst residue in the reaction system will not exist. In this way, the depolymerization reaction proceeds in an orderly manner in the positive direction.

[0062] Furthermore, the several depolymerization chambers in the depolymerization intermediate reaction unit described in

[060] correspond to depolymerization reaction processes carried out in different temperature intervals and different reaction time periods. The depolymerization reaction in the previous depolymerization chamber is the basis for the depolymerization reaction in the next depolymerization chamber, and the depolymerization reaction in the next depolymerization chamber is the continuation of the depolymerization reaction in the previous depolymerization chamber. The stage functions are clear and continuous, so that the entire depolymerization initial reaction process orderly moves the reaction equilibrium toward the positive direction. Since the impurities such as water, lactic acid, and low molecular weight lactic acid oligomers have been removed in the initial depolymerization unit, and no powder catalyst is used, the formed lactide is also discharged in time, the depolymerization temperature is moderate, and the viscosity of the reaction system is not high. There are basically no side reaction products in the depolymerization intermediate unit. The lactide produced has high purity, and the optical purity can be above 98%. The optical purity of the lactide produced by individual depolymerization chambers is above 99.5%. It can be used for the preparation of polylactic acid and some biodegradable materials (such as PLGA) without purification.

[0063]

[044] The late depolymerization reaction unit of the plurality of depolymerization units is composed of a plurality of depolymerization chambers, the temperature of which increases gradually between 230°C and 250°C, and the remaining substrate of the mid-term depolymerization unit is introduced into each depolymerization chamber in sequence to complete the depolymerization reaction in the late stage.

[0064] The energy storage catalytic pellets in the reaction chamber of the late depolymerization reaction unit described in

[063] have a small particle size and a large proportion. As the mechanical stirring device stirs in the reaction system, lactic acid oligomers continuously form new reaction interfaces on the surface of the energy storage catalytic pellets. The rare earth, tin dioxide, zinc oxide and other catalysts in the energy storage catalytic pellets are in uniform contact with the system, which has a good catalytic effect. The depolymerization reaction generates lactide. Under the action of mechanical stirring, the energy storage catalytic pellets are thrown above the liquid surface of the reaction system. The surface of the energy storage catalytic pellets is covered with the feed liquid of the depolymerization reaction system to form a liquid film. The energy storage catalytic pellets absorb microwaves and heat up rapidly. The lactide generated from the liquid surface of the reaction chamber and the surface liquid film is heated and quickly vaporized and evaporated and discharged from the steam outlet. Then, the energy storage catalytic pellets fall into the liquid phase reaction system, bringing heat energy into the reaction system, making the reaction system more evenly heated, and increasing the mass transfer and heat transfer effects. The liquid film after lactide is evaporated from the surface of the energy storage catalytic pellets is peeled off under the action of mechanical stirring and ultrasound, and a new reaction interface is continuously formed on the surface of the energy storage catalytic pellets to generate lactide, and a new liquid film is formed on the surface of the energy storage catalytic pellets. Under the action of mechanical stirring, the energy storage catalytic pellets are brought to the upper part and above the reaction system again, and the surface area of ​​the vaporized product of the reaction system is greatly increased. In addition, since the energy storage catalytic pellets are high-temperature sintered ceramic materials, the catalytic material will not be dispersed into the reaction system. Therefore, there will be no catalyst residue in the reaction system, and some side reactions caused by catalyst residue in the reaction system will not exist. In this way, the depolymerization reaction proceeds in an orderly manner in the positive direction.

[0065] Furthermore, the several depolymerization chambers in the late depolymerization reaction unit described in

[064] correspond to depolymerization reaction processes carried out in different temperature intervals and different reaction time periods. The depolymerization reaction in the previous depolymerization chamber is the basis for the depolymerization reaction in the next depolymerization chamber, and the depolymerization reaction in the next depolymerization chamber is the continuation of the depolymerization reaction in the previous depolymerization chamber. The stage functions are clear and continuous. Since there is no influence of impurities such as water, lactic acid, low molecular weight lactic acid oligomers, catalyst residues, etc. in the late depolymerization stage, and the energy storage catalytic spheres are numerous and small in size, new reaction interfaces are continuously formed on the surface of the energy storage catalytic spheres, and the area increases, so that the reaction system is heated more evenly, the mass transfer and heat transfer effect is better, and the generated lactide is separated from the reaction system quickly, and is less affected by the increase in the viscosity of the reaction system. The optical purity of the lactide produced in the late depolymerization stage is above 95%. The comprehensive yield of lactide is above 95%.

[0066] Beneficial Effects

[0067] The present invention provides a microwave and ultrasonic collaborative process system for preparing lactide, which has the following beneficial effects:

[0068] First, the present invention has the advantages of continuous flow of process materials, continuous reaction, controllable process, rapid heating response, more uniform heating of the reaction system, better mass transfer and heat transfer effect, and solves the problem that improper temperature control affects the condensation and depolymerization reactions. Second, the process system provided by the present invention can quickly evaporate water and lactide under a not too high vacuum environment (≥5KP) using a conventional industrial water ring vacuum pump, solving the problem that high vacuum degree (0.01KPa) has high requirements on the air tightness and precision of the reaction equipment, large overall energy consumption, and requires large investment and operating costs. Third, the powdered catalyst is replaced by energy storage catalytic pellets, the catalytic reaction is uniform, and there is no catalyst residue, which solves the problem that the catalyst is unevenly dispersed, the catalytic reaction is not thorough, and there are many metal ion residues, which lead to carbonization of materials, reduced yield, and difficult to handle reaction residues. Fourth, water, lactide, free lactic acid and other small molecular polar impurities are discharged from the reaction system quickly and thoroughly, overcoming the obstacle that the reaction products cannot be vaporized and evaporated in time due to the high viscosity of the system, so that the reaction proceeds in an orderly manner in the positive direction, and solves the problem that the viscosity of the reaction system gradually increases and the viscosity is too high, which hinders the volatilization of the system products, thereby inhibiting the reaction from proceeding in the positive direction and increasing the kettle residue. Fifth, the output lactide has high purity, and more than 2 / 3 of the lactide can have a chemical purity of more than 98%, and an optical purity of more than 99.5%. It can be used for the preparation of polylactic acid and some biodegradable materials (such as PLGA) without purification. Sixth, the reaction time is short and the production cost is low. Compared with the currently known process technology, the reaction cycle can be shortened by more than 1 / 2, and the production cost can be reduced by about 1 / 3. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Figure 1 Process flow chart of microwave and ultrasonic synergistic preparation of lactide

[0071] Figure 2 Partial cross-sectional structure diagram of microwave lactic acid dehydrator

[0072] Figure 3 Partial cross-sectional structure diagram of microwave lactic acid dehydrator

[0073] Figure 4 Schematic diagram of a partial cross-sectional structure of a device for preparing lactide by microwave and ultrasonic synergy

[0074] Figure 5 Schematic diagram of a partial cross-sectional structure of a device for preparing lactide by microwave and ultrasonic synergy

[0075] Reference numerals list

[0076] 101 Microwave lactic acid dehydration machine lactic acid conveying device

[0077] 102 Microwave Suppressor for Microwave Lactic Acid Dehydration Machine

[0078] 103 Microwave lactic acid dehydrator lactic acid distributor

[0079] 104 Microwave resonant cavity of microwave lactic acid dehydrator

[0080] 105 Microwave lactic acid dehydration machine infrared thermometer

[0081] 106 Microwave lactic acid dehydration machine moisture outlet

[0082] 107 Microwave lactic acid dehydration machine conveyor belt ceramic roller

[0083] 108 Microwave lactic acid dehydrator lactic acid conveyor belt

[0084] 109 Microwave lactic acid dehydrator lactic acid conveyor belt trough

[0085] 110 Microwave lactic acid dehydration machine conveyor belt ceramic roller support

[0086] 111 Microwave magnetron for microwave lactic acid dehydration machine

[0087] 201 Microwave lactic acid dehydrator dehydrated lactic acid storage tank

[0088] 301 Condensation unit

[0089] 3011 Polycondensation Unit No. 1 Polycondensation Chamber

[0090] 3012 Polycondensation Unit Second Polycondensation Chamber

[0091] 3013 Polycondensation Unit No.3 Polycondensation Chamber

[0092] 3014 Polycondensation Unit No. 4 Polycondensation Chamber

[0093] 3015 Polycondensation unit steam outlet

[0094] 3016 Polycondensation Unit Heating Oil Jacket

[0095] 302 Depolymerization Unit

[0096] 3021 Depolymerization Unit 1 Depolymerization Chamber 1

[0097] 3022 Depolymerization Unit 1, Depolymerization Chamber 2

[0098] 3023 Depolymerization Unit 1, Depolymerization Room 3

[0099] 3024 Depolymerization Unit 1 Depolymerization Room 4

[0100] 3025 Depolymerization of one-unit lactide vapor outlet

[0101] 3026 Depolymerization Unit Heating Oil Jacket

[0102] 303 Depolymerization of Unit II

[0103] 3031 Depolymerization Unit 2, Depolymerization Room 5

[0104] 3032 Depolymerization Unit 2, Depolymerization Room 6

[0105] 3033 Depolymerization Unit 2, Depolymerization Room 7

[0106] 3034 Depolymerization Unit 2, Depolymerization Room 8

[0107] 3035 Depolymerization Unit 2 Depolymerization Room 9

[0108] 3036 Depolymerization Unit 2 Depolymerization Room 10

[0109] 3037 Depolymerization of di-lactide vapor outlet

[0110] 3038 Depolymerization three-unit heating oil jacket

[0111] 304 Depolymerization of three units

[0112] 3041 Depolymerization Unit 3, Depolymerization Room 11

[0113] 3042 Depolymerization Unit 3, Depolymerization Room 12

[0114] 3043 Depolymerization Unit 3, Depolymerization Room 13

[0115] 3044 Depolymerization Unit 3, Depolymerization Room 14

[0116] 3045 Depolymerization of di-lactide vapor outlet

[0117] 3046 Depolymerization Three-unit Heating Oil Jacket

[0118] 305 U-shaped component (microwave transparent material) of the upper part of the reaction chamber of the device for preparing lactide by microwave and ultrasonic synergy

[0119] 306 Microwave resonant cavity of a device for preparing lactide by microwave and ultrasonic synergy

[0120] 307 Microwave resonant cavity shell of a device for preparing lactide by microwave and ultrasonic synergy

[0121] 308 Insulation material for the device for preparing lactide by microwave and ultrasonic synergistic method

[0122] 309 The outlet of the residual substrate of the reaction of the device for preparing lactide by microwave and ultrasonic synergy

[0123] 310 Ultrasonic transducer

[0124] 311 Ultrasonic Horn

[0125] 312 Ultrasonic generator

[0126] 313 adjacent shrinkage chamber partition

[0127] 314 Energy storage catalytic pellets of the device for preparing lactide by microwave and ultrasonic synergy

[0120] 315 Mechanical stirring blades of a device for preparing lactide by microwave and ultrasonic synergy

[0128] 316 U-shaped structure of the lower part of the reaction chamber of the device for preparing lactide by microwave and ultrasonic synergy

[0129] 317 Mechanical stirring shaft of the device for preparing lactide by microwave and ultrasonic synergy

[0130] 318 Microwave and ultrasonic synergistic lactide preparation device dehydrated lactic acid import

[0131] 319 Mechanical stirring magnetic coupling of the device for preparing lactide by microwave and ultrasonic synergistically

[0132] 320 Far Infrared Thermometer

[0133] 321 Magnetron

[0134] Specific implementation

[0135] The following is combined with Figure 1 , Attachment Figure 2 , Attachment Figure 3 , Attachment Figure 4 and attached Figure 5 , describe the specific process and flow of the present invention in detail.

[0136] The microwave and ultrasonic collaborative lactide preparation process system is a process system for preparing lactide that combines microwave and ultrasonic application technology with conventional lactide synthesis technology. It includes: (1) a microwave lactic acid dehydrator that removes water from lactic acid under low temperature and normal pressure; (2) a microwave and ultrasonic collaborative lactide preparation device that optimizes the conventional two-step method for preparing lactide and decomposes the entire process of preparing lactide into several independent and continuous temperature intervals and reaction time periods.

[0137] like Figure 1 , Figure 2 , Figure 3 As shown, the microwave lactic acid dehydration machine described in

[136] includes: a lactic acid conveying device 101, a lactic acid distributor 103, a microwave resonant cavity 104 and a dehydrated lactic acid material storage tank 201, etc.

[0138]

[137] The lactic acid conveying device 101 includes: a conveyor belt ceramic roller 107, a lactic acid conveyor belt 108, a conveyor belt ceramic roller bracket 110, and a lactic acid conveyor belt 108 with a lactic acid conveyor belt material trough 109.

[0139]

[137] The microwave resonant cavity 104 includes: a microwave magnetron 111, a microwave suppressor 102, an infrared thermometer 105, and a moisture removal outlet 106.

[0140]

[137] The lactic acid distributor 103 is located above the microwave suppressor 102 in the feed section of the microwave resonance cavity 104.

[0141] like Figure 1 , Figure 4 , Figure 5 As shown, the device for preparing lactic acid by coordinated microwave and ultrasonic wave comprises: reaction units and related components with several independent and continuous temperature intervals and reaction time periods.

[0142] The reaction units and related components described in

[141] are labeled as: condensation unit 301, depolymerization unit 1 302, depolymerization unit 2 303, depolymerization unit 304, microwave resonant cavity 306, etc.

[0143]

[142] The polycondensation unit 301 described herein comprises: a first polycondensation chamber 3011, a second polycondensation chamber 3012, a third polycondensation chamber 3013, a fourth polycondensation chamber 3014, a polycondensation unit water vapor outlet 3015, and a polycondensation unit heating oil jacket 3026.

[0144]

[142] The depolymerization unit 302 includes: a first depolymerization chamber 3021, a second depolymerization chamber 3022, a third depolymerization chamber 3023, a fourth depolymerization chamber 3024, a depolymerization unit lactide vapor outlet 3025, and a depolymerization unit heating oil jacket 3026.

[0145]

[142] The second depolymerization unit 303 includes: a fifth depolymerization chamber 3031, a sixth depolymerization chamber 3032, a seventh depolymerization chamber 3033, an eighth depolymerization chamber 3034, a ninth depolymerization chamber 3035, a tenth depolymerization chamber 3036, a depolymerization unit two lactide vapor outlet 3037, and a depolymerization unit two heating oil jacket 3038.

[0146]

[142] The depolymerization unit 304 includes: an eleventh depolymerization chamber 3041, a twelfth depolymerization chamber 3042, a thirteenth depolymerization chamber 3043, a fourteenth depolymerization chamber 3044, a depolymerization unit 3 lactide vapor outlet 3045, and a depolymerization unit 3 heating oil jacket 3046.

[0147]

[143]

[144]

[145]

[146] The first condensation chamber 3011, the second condensation chamber 3012, the third condensation chamber 3013, the fourth condensation chamber 3014, the first depolymerization chamber 3021, the second depolymerization chamber 3022, the third depolymerization chamber 3023, the fourth depolymerization chamber 3024; the fifth depolymerization chamber 3031, the sixth depolymerization chamber 3032, the seventh depolymerization chamber 3033, the eighth depolymerization chamber 3034, the ninth depolymerization chamber 3035, the tenth depolymerization chamber 3036; the eleventh depolymerization chamber 3041, the twelfth depolymerization chamber 3042, the thirteenth depolymerization chamber 3043, the fourteenth depolymerization chamber 3044 are adjacent in sequence.

[0148]

[147] The polycondensation chambers (depolymerization chambers) are of equal size and are composed of an upper U-shaped component 305, a lower U-shaped component 316 and a partition 313. The lower U-shaped component 316 of each polycondensation chamber (depolymerization chamber) is sealed and assembled with the upper U-shaped component 305 and the partition 313. Adjacent polycondensation chambers (depolymerization chambers) are sealed and assembled together, and together with the mechanical stirring shaft 317 and the stirring blade 315, they are combined into the polycondensation unit 301, the depolymerization unit 1 302, the depolymerization unit 2 303, the depolymerization unit 304 and the depolymerization unit 4 305 to form a sealed main body component of an integrated lactide preparation device. The mechanical stirring shaft 317 is connected to the external power system using a magnetic coupling 319.

[0149]

[148] The upper U-shaped components 305 of each condensation chamber (decondensation chamber) are all made of microwave ceramic material, preferably quartz ceramic material.

[0150]

[148] The U-shaped components 316 at the lower part of each condensation chamber (depolymerization chamber) are all made of metal materials, preferably 316L stainless steel, and are coated with wear-resistant, corrosion-resistant, and non-stick nano-coating materials.

[0151]

[147] The upper U-shaped component 305 of the first condensation chamber 3011, the second condensation chamber 3012, the third condensation chamber 3013 and the fourth condensation chamber 3014 of the condensation unit 301 is connected to the water vapor outlet 3015 of the condensation unit, and the lower U-shaped component 316 is connected to the heating oil clamp 3016 of the condensation unit.

[0152]

[147] The upper U-shaped component 305 of the first depolymerization chamber 3021, the second depolymerization chamber 3022, the third depolymerization chamber 3023 and the fourth depolymerization chamber 3024 of the depolymerization unit 302 is connected to the lactide vapor outlet 3025 of the first depolymerization unit, and the lower U-shaped component 316 is connected to the heating oil clamp 3026 of the condensation unit.

[0153]

[147] The upper U-shaped component 305 of the fifth depolymerization chamber 3031, the sixth depolymerization chamber 3032, the seventh depolymerization chamber 3033, the eighth depolymerization chamber 3034, the ninth depolymerization chamber 3035, and the tenth depolymerization chamber 3036 of the second depolymerization unit 303 is connected to the second depolymerization unit lactide vapor outlet 3037, and the lower U-shaped component 316 is connected to the condensation unit heating oil clamp 3038.

[0154]

[147] The upper U-shaped assembly 305 of the eleventh depolymerization chamber 3041, the twelfth depolymerization chamber 3042, the thirteenth depolymerization chamber 3043 and the fourteenth depolymerization chamber 3044 of the third depolymerization unit 304 is connected to the third depolymerization unit lactide vapor outlet 3045, and the lower U-shaped assembly 316 is connected to the condensation unit heating oil clamp 3046.

[0155]

[147] The outer side of the lower U-shaped component 316 and the outer side of the bottom of each condensation chamber (depolymerization chamber) are detachably connected to the ultrasonic transducer 310, the ultrasonic transducer 310 is fixedly connected to the amplitude transformer 311, and the amplitude transformer 311 is fixedly connected to the ultrasonic generator 312.

[0156]

[155] The ultrasonic generator 312 is made of rare earth magnetostrictive material developed by Tsinghua University, and the single power is not less than 1KW.

[0157]

[147] The microwave resonant cavity 306 of the first polycondensation chamber 3011, the second polycondensation chamber 3012, the third polycondensation chamber 3013, the fourth polycondensation chamber 3014, the first depolymerization chamber 3021, the second depolymerization chamber 3022, the third depolymerization chamber 3023, the fourth depolymerization chamber 3024, the fifth depolymerization chamber 3031, the sixth depolymerization chamber 3032, the seventh depolymerization chamber 3033, the eighth depolymerization chamber 3034, the ninth depolymerization chamber 3035, the tenth depolymerization chamber 3036, the eleventh depolymerization chamber 3041, the twelfth depolymerization chamber 3042, the thirteenth depolymerization chamber 3043, and the fourteenth depolymerization chamber 3044 are respectively composed of a shell 307, a microwave magnetron 321, and a far-infrared thermometer 320. The microwave magnetron 321 and the far-infrared thermometer 320 are respectively installed on the left and right sides of the shell 307.

[0158] The dehydrated lactic acid inlet 318 of the device for preparing lactide by microwave and ultrasound is located at the bottom of the first polycondensation chamber 3011 , and the reaction residual substrate outlet 309 is located at the bottom of the fourteenth depolymerization chamber 3044 .

[0159]

[147] The middle partition 313 of the first condensation chamber 3011 and the second condensation chamber 3012, the third condensation chamber 3013 and the fourth condensation chamber 3014, the first depolymerization chamber 3021 and the second depolymerization chamber 3022, the third depolymerization chamber 3023 and the fourth depolymerization chamber 3024, the fifth depolymerization chamber 3031 and the sixth depolymerization chamber 3032, the seventh depolymerization chamber 3033 and the eighth depolymerization chamber 3034, the ninth depolymerization chamber 3035 and the tenth depolymerization chamber 3036, the eleventh depolymerization chamber 3041 and the twelfth depolymerization chamber 3042, the thirteenth depolymerization chamber 3043 and the fourteenth depolymerization chamber 3044 are provided with a plurality of circular holes for material flow at a position below the material liquid surface, and the diameter of the circular hole is less than the diameter of the energy storage catalytic pellet.

[0160]

[147] The bottom of the middle partition 313 of the second condensation chamber 3012 and the third condensation chamber 3013, the second depolymerization chamber 3022 and the third depolymerization chamber 3023, the fourth depolymerization chamber 3014 and the fifth depolymerization chamber 3021, the sixth depolymerization chamber 3022 and the seventh depolymerization chamber 3023, the eighth depolymerization chamber 3024 and the ninth depolymerization chamber 3025, the tenth depolymerization chamber 3026 and the eleventh depolymerization chamber 3041, the twelfth depolymerization chamber 3042 and the thirteenth depolymerization chamber 3043 are all provided with a plurality of circular holes for material flow, and the diameter of the circular hole is less than the diameter of the energy storage catalytic pellets.

[0161]

[147] The middle partition 313 between the fourth polycondensation chamber 3014 and the first depolymerization chamber 3021, the twelfth depolymerization chamber 3042 and the thirteenth depolymerization chamber 3043 does not have a circular hole for material circulation. A material circulation pipeline is installed between the fourth polycondensation chamber 3014 and the first depolymerization chamber 3011, the twelfth depolymerization chamber 3042 and the thirteenth depolymerization chamber 3043, and a solenoid valve is installed.

[0162]

[147] The polycondensation unit water vapor outlet 3015, the one-unit lactide depolymerization vapor outlet 3025, the two-unit lactide depolymerization vapor outlet 3037, and the three-unit lactide depolymerization vapor outlet 3045 are all equipped with a microwave leakage prevention and suppression structure.

[0163] Specific implementation process

[0164] Lactic acid dehydration

[0165] Combination Figure 1 , Figure 2 , Figure 3, introduce lactic acid with a purity greater than 85% into the distributor 103 of the microwave lactic acid dehydrator, the distributor 103 evenly distributes the lactic acid in the longitudinal groove 109 of the conveyor belt 108, and the lactic acid conveyor belt transmission device 101 delivers the lactic acid into the microwave resonance cavity 104. The dielectric constant of water is 78.3, and the dielectric constant of lactic acid is 22.0. Both water and lactic acid are high-loss substances, and the microwave effect is very prominent, but there is still a big difference in the dielectric constants of water and lactic acid. Due to the selective heating of microwaves, water molecules are preferentially heated and vaporized at low temperatures, and discharged through the water vapor outlet 3015 of the polycondensation unit. The far-infrared thermometer 105 detects the temperature of lactic acid in the microwave resonance cavity under microwave irradiation, and controls the dehydration temperature of lactic acid in the microwave resonance cavity to be lower than 50°C. The material of the conveyor belt 108 is a composite material of tetrafluoroethylene and glass fiber, and the conveyor belt roller 107 and the bracket 110 in the microwave resonance cavity are both made of microwave ceramic materials. Microwave suppressors 102 are provided at both ends of the microwave resonance cavity to prevent microwaves from leaking from the inlets and outlets at both ends of the microwave resonance cavity. The water content of the lactic acid after microwave dehydration is less than 0.05%. The dehydrated lactic acid is introduced into the dehydrated lactic acid storage tank 201 in a dry and clean environment.

[0166] Lactide Synthesis

[0167]

[142] Energy storage catalytic pellets are added to all the polycondensation (depolymerization) reaction chambers of the microwave and ultrasonic coordinated preparation of lactocyanate device, polycondensation unit 301, depolymerization unit 1 302, depolymerization unit 2 303, and depolymerization unit 304. Nitrogen replacement is done and the reaction chambers are in a negative pressure state. The oil temperature of the polycondensation unit heating oil jacket 3016 is 145°C.

[0168] The dehydrated lactic acid in the dehydrated lactic acid storage tank 201 described in

[165] is heated to 130°C and introduced into the lactic acid polycondensation system.

[0169] The dehydrated lactic acid heated to 130° C. as described in

[168] is pumped into the first polycondensation chamber 3011 of the polycondensation unit 301, and the amount introduced is 1 / 4 of the lactic acid amount set in the polycondensation unit 301. The lactic acid undergoes a polycondensation reaction under the combined action of the heating oil, microwaves, generated waves, and mechanical stirring.

[0170] Further, at intervals of 30 minutes, the dehydrated lactic acid in the dehydrated lactic acid storage tank 201 is pumped and introduced into the first polycondensation chamber 3011 of the polycondensation unit 301. The lactic acid introduced into the polycondensation unit is successively introduced into the second polycondensation chamber 3012 through the upper circular hole channel of the partition plate between the first polycondensation chamber 3011 and the second polycondensation chamber 3012, introduced into the third polycondensation chamber 3013 through the lower circular hole channel of the partition plate between the second polycondensation chamber 3012 and the third polycondensation chamber 3013, and introduced into the fourth polycondensation chamber 3014 through the upper circular hole channel of the partition plate between the third polycondensation chamber 3013 and the fourth polycondensation chamber 3014.

[0171] Furthermore, the polycondensation reaction time is about 2 hours, the pressure is 15-20 KPa, and the reaction temperature is 135° C.-155° C. The steam is evaporated and discharged through the steam drain hole 3015. The water is drained out to obtain a lactic acid oligomer suitable for depolymerization.

[0172] Furthermore, the reaction temperature of the first condensation chamber 3011 is 135°C~140°C, and the reaction time is 30 minutes; the reaction temperature of the second condensation chamber 3012 is 140°C~145°C, and the reaction time is 30 minutes; the reaction temperature of the third condensation chamber 3013 is 145°C~150°C, and the reaction time is 30 minutes; the reaction temperature of the fourth condensation chamber 3014 is 150°C~155°C, and the reaction time is 30 minutes.

[0173] Furthermore, the condensation unit heats the oil jacket 3016 to an oil temperature of 145°C, and microwaves are used to adjust the reaction temperatures of the first condensation chamber 3011, the second condensation chamber 3012, the third condensation chamber 3013 and the fourth condensation chamber 3014 described in

[141] .

[0174] Further, after the fourth condensation chamber 3014 was polycondensed at a pressure of 15-20 KPa and a temperature of 150°C-155°C for 30 minutes, the lactic acid in the fourth condensation chamber 3014 had been polycondensed for 120 minutes, including: polycondensation for 30 minutes each in the third condensation chamber 3013 at a temperature of 145°C-150°C, the second condensation chamber 3012 at a temperature of 140°C-145°C, and the first condensation chamber 3011 at a temperature of 135°C-140°C.

[0175] Furthermore, the lactic acid in the fourth polycondensation chamber of the polycondensation unit has undergone a polycondensation reaction at a pressure of 15 to 20 KPa, a temperature of 135°C to 155°C, and a time of 2 hours under the combined action of microwaves, ultrasound, mechanical stirring, catalysts, etc., to produce lactic acid oligomers that meet the requirements for depolymerization to prepare lactide.

[0176] Furthermore, the connecting pipe between the fourth polycondensation chamber 3014 of the polycondensation unit and the first depolymerization chamber 3021 of the first depolymerization unit is opened, and the lactic acid oligomers in the fourth polycondensation chamber 3014 of the polycondensation unit are introduced into the first depolymerization chamber 3021 of the first depolymerization unit 302 by using the pressure difference.

[0177] Furthermore, after the lactic acid oligomers in the fourth polycondensation chamber 3014 are introduced into the first depolymerization chamber 3021 of the first depolymerization unit as described in

[176] , the connecting pipe between the fourth polycondensation chamber 3014 of the polycondensation unit and the first depolymerization chamber 3021 of the first depolymerization unit is closed.

[0178] Furthermore, dehydrated lactic acid is sequentially introduced into the polycondensation reaction units at intervals of 30 minutes, and then the lactic acid oligomers obtained by the polycondensation reaction are introduced into the depolymerization reaction units to prepare lactide.

[0179] Furthermore, as the dehydrated lactic acid is continuously introduced into the first polycondensation chamber at intervals of 30 minutes, the lactic acid oligomers continuously generated by the polycondensation unit are also introduced into each depolymerization chamber alternately through the circular hole channels above and below the partitions of each adjacent depolymerization chamber at intervals of 30 minutes.

[0180] The pressure of the depolymerization reaction is 5-15 KPa, wherein the reaction pressure of the first depolymerization unit 302 is 8-10 KPa, the reaction pressure of the second depolymerization unit 303 is 6-8 KPa, and the reaction pressure of the third depolymerization unit 304 is 5-6 KPa.

[0181] The depolymerization temperature of the first depolymerization unit 302 is 180-200° C., and the corresponding oil temperature of the heating oil jacket 3026 is 180-190° C. The temperature of each chamber of the first depolymerization unit is adjusted by microwave, wherein the depolymerization temperature of the first depolymerization chamber 3021 is 180-185° C., the depolymerization temperature of the second depolymerization chamber 3022 is 185-190° C., the depolymerization temperature of the third depolymerization chamber 3013 is 190-195° C., and the depolymerization temperature of the fourth depolymerization chamber 3024 is 195-200° C., the depolymerization reaction time of each depolymerization chamber is 30 minutes, and the depolymerization time of the first depolymerization unit 302 is 2 hours.

[0182] The depolymerization temperature of the second depolymerization unit 303 is 200-230°C, and the corresponding oil temperature of the heating oil jacket 3038 is 200-210°C. The temperature of each chamber of the second depolymerization unit is adjusted by microwave, wherein the depolymerization temperature of the fifth depolymerization chamber 3031 is 200-205°C, the depolymerization temperature of the sixth depolymerization chamber 3032 is 205-210°C, the depolymerization temperature of the seventh depolymerization chamber 3033 is 210-215°C, and the depolymerization temperature of the eighth depolymerization chamber 3034 is 215-220°C. The depolymerization temperature of the ninth depolymerization chamber 3035 is 220-225°C, and the depolymerization temperature of the tenth depolymerization chamber is 225-230°C. The depolymerization time of each depolymerization chamber is 30 minutes, and the depolymerization time of the second depolymerization unit is 3 hours.

[0183] The depolymerization temperature of the third depolymerization unit 304 is 230-250°C, and the corresponding oil temperature of the heating oil jacket 3046 is 230-240°C. The temperature of each chamber of the first depolymerization unit is adjusted by microwave, wherein the depolymerization temperature of the eleventh depolymerization chamber 3041 is 230-235°C, the depolymerization temperature of the twelfth depolymerization chamber 3042 is 235-240°C, and the depolymerization temperature of the thirteenth depolymerization chamber 3043 is 240-245°C. The depolymerization temperature of the fourteenth depolymerization chamber 3044 is 245-250°C. The depolymerization time of the third depolymerization unit 304 is 2 hours.

[0184] There are fourteen depolymerization chambers from the first depolymerization chamber 3021 to the fourteenth depolymerization chamber 3044. The depolymerization temperature increases from 180°C to 250°C. The depolymerization time of each depolymerization chamber is set to 30 minutes, and the total depolymerization time is 7 hours.

[0185] Furthermore, each of the fourteen depolymerization chambers from the first depolymerization chamber 3021 to the fourteenth depolymerization chamber 3044 is provided with a lactide sampling port to detect the molecular weight, optical purity, chemical purity, impurity type and hybridization degree of the lactide produced in each depolymerization chamber during the depolymerization reaction from 180°C to 250°C for 30 minutes at every 5°C increase in temperature during the 7 hours.

[0186] Furthermore, based on the differences in molecular weight, optical purity, chemical purity, impurity type and degree of hybridization of the lactide produced by each depolymerization chamber as described in

[185] , the lactide vapor produced by one or several of the fourteen depolymerization chambers from the first depolymerization chamber 3021 to the twelfth depolymerization chamber 3044 is classified and collected.

[0187] The present invention combines the lactide vapor produced by the first depolymerization chamber 3021, the second depolymerization chamber 3022, the third depolymerization chamber 3023 and the fourth depolymerization chamber 3024, and vaporizes and evaporates the lactide vapor from the vaporization and evaporation outlet 3025 of the first depolymerization unit 302 for collection; combines the lactide vapor produced by the fifth depolymerization chamber 3031, the sixth depolymerization chamber 3032, the seventh depolymerization chamber 3033, the eighth depolymerization chamber 3034, the ninth depolymerization chamber 3035 and the tenth depolymerization chamber 3036, and vaporizes and evaporates the lactide vapor from the vaporization and evaporation outlet 3037 of the second depolymerization unit 303 for collection; combines the lactide vapor produced by the eleventh depolymerization chamber 3041, the twelfth depolymerization chamber 3042, the thirteenth depolymerization chamber 3043 and the fourteenth depolymerization chamber 3044, and vaporizes and evaporates the lactide vapor from the vaporization and evaporation outlet 3045 of the third depolymerization unit 304 for collection.

[0188] The present invention performs depolymerization for 7 hours under the combined action of microwave, ultrasonic wave, energy storage catalytic pellet, heating oil and mechanical stirring, the depolymerization temperature is 180℃~250℃, and the pressure is 5~15KPa. The prepared lactide vapor is vaporized and evaporated and discharged from the lactide vapor vaporization and evaporation outlet 3025 of the first depolymerization unit 302, the lactide vapor vaporization and evaporation outlet 3037 of the second depolymerization unit 303 and the lactide vapor vaporization and evaporation outlet 3045 of the third depolymerization unit 304 in different reaction time periods according to the temperature range, and then collected.

[0189] When the amount of lactide vapor discharged from the vaporization and evaporation outlet 3045 of the third depolymerization unit 304 described in

[188] is significantly reduced, the residual substrate of the depolymerization reaction in the last depolymerization chamber 3044 is discharged from the residual substrate reaction outlet 309 of the device for preparing lactide in coordination with microwave and ultrasound.

[0190] When the depolymerization residual substrate derived from the last depolymerization chamber 3044 described in

[189] is purified, diluted, and heated, it enters the first depolymerization chamber 3021 and then enters the depolymerization reaction system.

[0191] As described in

[189] , as the residual substrate of the depolymerization is discharged, the lactic acid oligomer sub-condensation system is supplemented and introduced into the depolymerization system in turn. Then, the dehydrated lactic acid in the dehydrated lactic acid storage tank 201 is heated to 130°C and introduced into the first condensation chamber of the lactic acid condensation system. Subsequently, the microwave lactic acid dehydrator removes free water to produce dehydrated lactic acid and introduces it into the dehydrated lactic acid storage tank to maintain the material level balance in the dehydrated lactic acid storage tank. In this way, the front and back are connected and circulated in an orderly manner, forming a complete continuous circulation production system of lactic acid-lactide under the synergistic effect of microwave and ultrasonic waves. Example

[0193] Example 1

[0194] Lactic acid with a purity of 85% is subjected to a microwave lactic acid dehydration machine to obtain dehydrated lactic acid with a water content of less than 0.05%, which is then heated to 130°C and introduced into the polycondensation unit 301 for polycondensation reaction. The pressure of the polycondensation unit 301 is 16KPa. The reaction temperature of the first polycondensation chamber 3011 is 136°C and the reaction time is 30 minutes; the reaction temperature of the second polycondensation chamber 3012 is 140°C and the reaction time is 30 minutes; the reaction temperature of the third polycondensation chamber 3013 is 145°C and the reaction time is 30 minutes; the reaction temperature of the fourth polycondensation chamber 3014 is 150°C and the reaction time is 30 minutes. The molecular weight of the obtained lactic acid oligomer is 1950Da.

[0195] The depolymerization reaction was carried out in the first depolymerization and splitting unit 302 for 2 hours at a pressure of 9 KPa, wherein the depolymerization temperature of the first depolymerization chamber 3021 was 183° C., the depolymerization temperature of the second depolymerization chamber 3022 was 187° C., the depolymerization temperature of the third depolymerization chamber 3023 was 193° C., and the depolymerization temperature of the fourth depolymerization chamber 3024 was 197° C., and the depolymerization reaction time of each depolymerization chamber was 30 minutes. The depolymerization purity of the crude lactone was 92.52%.

[0196] The depolymerization reaction was carried out in the second depolymerization splitting unit 303 for 3 hours at a pressure of 7KPa, wherein the depolymerization temperature of the fifth depolymerization chamber 3031 was 203°C, the depolymerization temperature of the sixth depolymerization chamber 3032 was 207°C, the depolymerization temperature of the seventh depolymerization chamber 3033 was 213°C, the depolymerization temperature of the eighth depolymerization chamber 3034 was 217°C, the depolymerization temperature of the ninth depolymerization chamber 3035 was 223°C, and the depolymerization temperature of the tenth depolymerization chamber was 227°C. The depolymerization time of each depolymerization chamber was 30 minutes. The depolymerization purity of lactide was 99.72, and the optical purity of lactide was 99.21%. The chemical purity of the sixth depolymerization chamber was 99.92, and the optical purity of lactide was 99.8%.

[0196] The third depolymerization unit 304 performs depolymerization reaction for 2 hours at a pressure of 5 KPa. The depolymerization temperature of the eleventh depolymerization chamber 3041 is 233°C, the depolymerization temperature of the twelfth depolymerization chamber 3042 is 237°C, and the depolymerization temperature of the thirteenth depolymerization chamber 3043 is 243°C. The depolymerization temperature of the fourteenth depolymerization chamber 3044 is 247°C. The depolymerization time of each depolymerization chamber is 30 minutes, and the depolymerization purity of lactide is 99.33, and the optical purity of lactide is 98.5%.

[0197] Example 2

[0198] Lactic acid with a purity of 95% is subjected to a microwave lactic acid dehydration machine to obtain dehydrated lactic acid with a water content of less than 0.01%, which is then heated to 130°C and introduced into the polycondensation unit 301 for polycondensation reaction. The pressure of the polycondensation unit 301 is 18KPa. The reaction temperature of the first polycondensation chamber 3011 is 136°C and the reaction time is 30 minutes; the reaction temperature of the second polycondensation chamber 3012 is 140°C and the reaction time is 30 minutes; the reaction temperature of the third polycondensation chamber 3013 is 145°C and the reaction time is 30 minutes; the reaction temperature of the fourth polycondensation chamber 3014 is 150°C and the reaction time is 30 minutes. The molecular weight of the obtained lactic acid oligomer is 2100Da.

[0199] The depolymerization reaction was carried out in the first depolymerization splitting unit 302 for 2 hours at a pressure of 10 KPa, wherein the depolymerization temperature of the first depolymerization chamber 3021 was 185°C, the depolymerization temperature of the second depolymerization chamber 3022 was 190°C, the depolymerization temperature of the third depolymerization chamber 3023 was 195°C, and the depolymerization temperature of the fourth depolymerization chamber 3024 was 200°C. The depolymerization reaction time of each depolymerization chamber was 30 min, and the depolymerization purity of the obtained crude lactone was 93.31% and the impurity content was 6.69%.

[0200] The depolymerization reaction was carried out in the second depolymerization split unit 303 for 3 hours at a pressure of 8KPa, wherein the depolymerization temperature of the fifth depolymerization chamber 3031 was 205°C, the depolymerization temperature of the sixth depolymerization chamber 3032 was 210°C, the depolymerization temperature of the seventh depolymerization chamber 3033 was 215°C, the depolymerization temperature of the eighth depolymerization chamber 3034 was 220°C, the depolymerization temperature of the ninth depolymerization chamber 3035 was 225°C, and the depolymerization temperature of the tenth depolymerization chamber was 230°C. The depolymerization time of each depolymerization chamber was 30 minutes. The depolymerization purity of lactide was 99.79, and the optical purity of lactide was 99.23%. Among them, the depolymerization purity of lactide obtained by combining the sixth split and the seventh depolymerization chambers was 99.91, and the optical purity of lactide was 99.6%.

[0201] The third depolymerization unit 304 performs depolymerization reaction for 2 hours at a pressure of 5 KPa. The depolymerization temperature of the eleventh depolymerization chamber 3041 is 233°C, the depolymerization temperature of the twelfth depolymerization chamber 3042 is 237°C, and the depolymerization temperature of the thirteenth depolymerization chamber 3043 is 243°C. The depolymerization temperature of the fourteenth depolymerization chamber 3044 is 247°C. The depolymerization time of each depolymerization chamber is 30 minutes, and the depolymerization purity of lactide is 99.46, and the optical purity of lactide is 98.7%.

[0202] Comparative Example 1

[0203] Wang Peng's patent No. 200610009860.7 uses microwave radiation to reduce pressure catalysis to prepare lactide. Specific implementation method 2: Using D, L-lactic acid as raw material, D, L-lactic acid is placed in a flask, 0.4% of stannous chloride dihydrate is added, and TSA is added as a catalyst in a ratio of 1:2 to stannous chloride dihydrate. The system pressure is 5KPa, and the polymerization is carried out under microwave radiation for 30 minutes to obtain an oligomeric lactic acid intermediate product; then, under a pressure of 1KPa and microwave radiation, the oligomeric lactic acid is depolymerized to collect crude lactide, and the collection time is 60 minutes. The total yield of the obtained lactide is 52%.

[0204] Comparative Example 2

[0205] Patent No. 202111279119.3 of Bai Yuli et al. A method and device for cyclic depolymerization to synthesize crude lactide Example 1: Take 3000g of lactic acid oligomers and transport them to a scraped film depolymerization reactor with an inner membrane of stannous oxide at a flow rate of 150g / h. The depolymerization reaction conditions are controlled as follows: vacuum degree at 800Pa, reaction temperature at 190℃, and single-pass residence time at 2min. After the reaction, the heavy components are discharged into the circulation tank, mixed with fresh lactic acid oligomers, and then transported to the depolymerization reactor. The feed mass ratio of fresh materials to circulating materials in the reaction process is controlled to be 1:3. After the fresh materials are mixed with the circulating materials, they re-enter the depolymerization reactor; as the reaction proceeds, the liquid level of the circulation tank is controlled to be maintained at 60%, the pressure is maintained at 50k Pa, and the temperature is maintained at 180℃. After the reaction device is continuously circulated for 48h, the molecular weight of the lactic acid oligomer is 2252. After the reaction, the gas phase component enters the condensation system with a condensation temperature of 90℃, and the generated liquid phase lactide enters the storage tank. The crude lactide product was analyzed and the L-lactide content was 87.2%.

[0206] Obviously, the above embodiments are only examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. For example, glycolide can be prepared based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from this are still within the scope of protection of the invention.

[0207] The structures, proportions, sizes, etc. illustrated in the drawings of the specification are only used to match the contents disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the limiting conditions for the implementation of the present invention, so they have no substantial technical significance. Any modification of the structure, change of the proportion relationship or adjustment of the size, without affecting the effects and purposes that can be achieved by the present invention, should still fall within the scope of the technical contents disclosed by the present invention. At the same time, the terms such as "upper", "lower", "front", "back", "middle" and so on quoted in this specification are only for the convenience of description, and are not used to limit the scope of the implementation of the present invention. The change or adjustment of their relative relationship should also be regarded as the scope of the implementation of the present invention without substantially changing the technical contents.

Claims

1. A process system for preparing lactide by microwave and ultrasonic synergy, characterized in that: At least the following steps are included: Step (1), free water is removed from lactic acid at low temperature and normal pressure using a microwave lactic acid dehydrator to obtain dehydrated lactic acid and introduce it into a dehydrated lactic acid storage tank. The microwave lactic acid dehydrator has an operating temperature below 50°C. Step (2), heating the dehydrated lactic acid obtained in step (1) to 130°C and introducing it into a lactic acid polycondensation system. Step (3), the lactic acid polycondensation system is subjected to the synergistic effects of microwave, ultrasonic wave, heating oil, mechanical stirring and energy storage catalytic pellets, and the polycondensation reaction time is about 2 hours, the temperature is 130°C to 155°C, and the pressure is 15 to 20KPa. Within 30 minutes at the initial stage of the polycondensation reaction at 130°C to 135°C, free water and bound water are discharged, and the water generated in the middle and late stages of the polycondensation is instantly evaporated and discharged. The reaction time, pressure and temperature are adjusted so that the molecular weight of the obtained lactic acid oligomer is controlled at 800 to 2200Da. Step (4), heating the lactic acid oligomers obtained in step (3) to 180°C and introducing them into a depolymerization system to prepare lactide. Step (5), further, the lactic acid oligomer obtained in step (3) is depolymerized for 7 hours under the combined action of microwave, ultrasonic wave, heating oil, mechanical stirring and energy storage catalytic pellets, at a temperature of 180°C to 250°C and a pressure of 5 to 15 KPa. Within the initial 30 minutes of depolymerization, light substances such as water and lactic acid in the depolymerization system are evaporated and discharged, at a temperature of 180°C to 185°C and a pressure of 12 to 15 KPa. Step (6), further, the lactide vapor obtained in step (5) is extracted separately in different reaction time periods according to the temperature range. Step (7): Further, when the amount of lactide vapor evaporated in the last temperature interval of depolymerization in step (6) is significantly reduced, the depolymerization residual substrate in the last temperature reaction interval is extracted. Step (8), further, purifying and diluting the residual depolymerization substrate extracted in step (7) and returning to step (4). Step (9), further, as the residual substrate of the depolymerization in step (7) is extracted, step (4) introduces the lactic acid oligomers prepared in step (3) into the depolymerization system in sequence to prepare lactide. Step (10), further, along with step (4), the lactic acid oligomers prepared in step (3) are sequentially introduced into the depolymerization system, and step (2) the dehydrated lactic acid prepared in step (1) is heated to 130° C. and introduced into the lactic acid condensation system. Step (11), further, along with step (2) introducing the dehydrated lactic acid obtained in step (1) into the lactic acid polycondensation system, the dehydrated lactic acid obtained by the microwave lactic acid dehydration machine in step (1) is introduced into the dehydrated lactic acid storage tank.

2. The microwave and ultrasonic synergistic process system for preparing lactide according to claim 1, characterized in that: include: Microwave lactic acid dehydrator and horizontal microwave and ultrasonic synergistic lactide continuous synthesis device.

3. The horizontal microwave and ultrasonic coordinated lactide continuous synthesis device according to claim 2, characterized in that: include: Lactide synthesis chamber, mechanical stirring device, microwave source, ultrasonic device, heating oil system, dehumidification system, lactide vapor negative pressure extraction system, depolymerization reaction residual substrate extraction, purification and reuse system and control system, etc.

4. The lactide synthesis chamber according to claim 3, characterized in that: include: Several polycondensation reaction units and depolymerization reaction units, each reaction unit is composed of several reaction chambers. The reaction chamber is sealed and buckled together by two upper and lower U-shaped grooves. The lower U-shaped groove is made of 316L or 310S stainless steel, with wear-resistant nano coating inside, and the upper U-shaped groove is made of quartz ceramic material.

5. The plurality of reaction chambers according to claim 4, characterized in that: Each reaction chamber is equipped with stirring blades and energy storage catalytic pellets.

6. The plurality of reaction chambers according to claim 4, characterized in that: The heating method for all reaction chambers is that the heating oil jacket provides the base temperature, and microwave radiation adjusts the temperature of each reaction chamber.

7. The polycondensation reaction unit according to claim 4, characterized in that The polycondensation reaction unit is composed of a plurality of polycondensation chambers, and the temperature of the polycondensation chambers increases gradually between 135°C and 155°C.

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