A three-stage synthesis process of lactide

Abstract

This invention discloses a three-stage synthesis process for lactide, a continuous and stable industrialization technology for lactide, and a three-stage synthesis process for achieving large-scale production of lactide. To solve the above-mentioned technical problems, this invention provides the following technical solution: a three-stage synthesis process for lactide, comprising a first synthesis system, a second synthesis system, and a third synthesis system connected in sequence. Each synthesis system includes a sealed synthesis reaction vessel. The middle of the first synthesis reaction vessel in the first synthesis system is fluidly connected to the lower end of the second synthesis reaction vessel in the second synthesis system. The middle of the second synthesis reaction vessel in the second synthesis system is fluidly connected to the lower end of the third synthesis reaction vessel in the third synthesis system. Each synthesis reaction vessel is fluidly connected to a condenser at its upper end.

Description

Technical Field

[0001] This invention relates to the field of lactide synthesis technology. Specifically, it relates to a three-stage process for synthesizing lactide. Background Technology

[0002] Currently, the production of commercial lactide abroad uses stannous octoate additives. During the synthesis of lactide, the by-product polymer residue undergoes severe racemization, making it unusable and only able to be discharged as solid waste. Since these solid wastes contain cytotoxic heavy metal additives, they will pollute the environment.

[0003] L-lactide, also known as levolic lactide, has the scientific name L-3,6-dimethyl-2,5-dione-1,4-dioxane and the chemical formula C6H8O4. It is a colorless, transparent needle-like crystal with a melting point of 93-95℃, a boiling point of 216℃, and a molecular weight of 144. It is readily soluble in chloroform and ethanol, but insoluble in water. It is an intermediate in the synthesis of polylactic acid.

[0004] In the preparation of lactide, lactic acid is used as a raw material. First, lactic acid oligomers are generated through condensation polymerization, followed by depolymerization and cyclization to produce lactide. The oligomers of lactic acid are a crucial factor in the synthesis of lactide. During the condensation reaction, depending on the amount of water evaporated, lactic acid tends to remain in dimers, trimers, and polymers, making it difficult to control the presence of 5-10 lactic acid oligomers. The two-step method is further divided into the reduced pressure method and the atmospheric pressure gas flow method.

[0005] The vacuum method involves heating and evacuating the lactic acid and the water produced in the reaction to remove the water, followed by depolymerization under the action of a catalyst to form lactide. This method is prone to reaction failure due to insufficient vacuum, resulting in an unstable process, and is mainly used in laboratories. The atmospheric pressure gas flow method operates on the same principle as the vacuum method, typically using an inert gas flow to reduce the partial pressure of lactide vapor and simultaneously carrying the generated lactide away from the reaction zone, avoiding reaction failure due to insufficient vacuum. This method has a high success rate, but the lactide yield is usually low.

[0006] Some researchers have proposed a one-step gas-phase method for the production of lactide, in which aqueous lactic acid is heated into steam, and the lactide product is obtained in a reactor through a catalyst. This reaction is a vapor-liquid phase reaction. Although the one-step synthesis of lactide is more economical than the two-step method, it is more sensitive to changes in process conditions and more difficult to control. Summary of the Invention

[0007] Therefore, the technical problem to be solved by the present invention is to provide a continuous and stable industrialization technology for lactide, and a three-stage synthetic lactide process for large-scale production of lactide.

[0008] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0009] A three-stage synthesis process for lactide includes a first synthesis system, a second synthesis system, and a third synthesis system connected sequentially. Each synthesis system includes a sealed synthesis reaction vessel. Fluid communication exists between the middle of the first synthesis reaction vessel in the first synthesis system and the lower end of the second synthesis reaction vessel in the second synthesis system. Fluid communication also exists between the middle of the second synthesis reaction vessel in the second synthesis system and the lower end of the third synthesis reaction vessel in the third synthesis system. A condenser is fluidly connected to the upper end of each synthesis reaction vessel. The process is characterized in that each synthesis reaction vessel in each synthesis system includes a sealed tank body. A sealing cover is detachably connected to the upper end of the sealed tank body. A vacuum pump is fixedly connected to the sealing cover. A stirring blade, coaxially arranged with the sealed tank body, is rotatably connected to the sealing cover. The stirring blade extends into the sealed tank body and is used to agitate the materials within the sealed tank body. The sealed tank has a funnel-shaped structure at the bottom with a feed inlet. A static mixer for mixing additives is located at the feed inlet. A heating coil for heating the sealed tank is located at the lower inner side of the tank, using a secondary liquid-phase heat medium. The upper inner side of the sealed tank is a gas-phase evaporation space. A condensing device, including a fractionation tower, is connected to the sealing cover at the top of the sealed tank. The gas in the tank passes through the fractionation tower and enters the first condenser connected to the fractionation tower. The first condenser is connected to a lactide collection tank and a second condenser. The condensed lactide enters the lactide collection tank, while the uncondensed gas enters the second condenser. The second condenser is connected to the lactide collection tank and a total cooler. The condensed lactide enters the lactide collection tank, while the uncondensed gas enters the total cooler. The total cooler is connected to a lactide recovery tank, and the condensate from the total cooler enters the lactide recovery tank.

[0010] Preferably, the synthesis reaction vessel in the first synthesis system is heated to a temperature of 210-240℃ and a vacuum degree of 50-100 Torr, while the fractionation tower operates at a temperature of 200℃ and a pressure of 5 Torr.

[0011] The synthesis reaction vessel in the second synthesis system is heated to 220-240℃ and has a vacuum of 50-100 Torr. The fractionation tower operates at 210℃ and at a pressure of 5 Torr. The lactide collection tank in the second synthesis system is kept at 100℃ by a secondary liquid-phase circulating heat medium.

[0012] The synthesis reaction vessel in the third synthesis system is heated to a temperature of 210-240℃ and has a vacuum degree of 50-100 Torr. The material inside the synthesis reaction vessel in the third synthesis system is a polymer. The polymer is discharged through the lower end of the synthesis reaction vessel and enters the connected dehydration oligomerization kettle and granulation equipment.

[0013] Preferably, the first synthesis reactor contains lactic acid oligomers, which enter the first synthesis reactor under the protection of 99.99% nitrogen gas. The liquid phase extract from the first synthesis reactor is discharged from the middle of the first synthesis reactor under the protection of nitrogen gas and enters the second synthesis reactor through the bottom of the second synthesis reactor. The liquid phase extract from the second synthesis reactor is discharged from the middle of the second synthesis reactor under the protection of nitrogen gas and enters the third synthesis reactor through the lower part of the third synthesis reactor. The material in the third synthesis reactor is a polymer at this time and is discharged from the lower end of the third synthesis reactor into the dehydration oligomerization reactor and granulation equipment.

[0014] Preferably, the evaporated material in the first synthesis reaction tank passes through a distillation tower and enters the first condenser for condensation. The condensed lactide enters the lactide collection tank, while the uncondensed evaporated material enters the second condenser for condensation again. The condensed lactide enters the lactide collection tank again, and the uncondensed evaporated material enters the total cooler and then enters the lactide recovery tank. The first condenser and the second condenser are used in pairs, with one in operation and the other on standby. The liquid level in the first synthesis reaction tank is controlled by the synthesis reaction tank transfer pump.

[0015] The condensation steps of the evaporated material in the second and third synthesis reaction tanks are the same as those in the first synthesis reaction tank. The liquid level in the second synthesis reaction tank is controlled by the synthesis reaction tank transfer pump, while the liquid level in the third synthesis reaction tank is controlled by the polymer transfer pump. The polymer in the third synthesis reaction tank is discharged from the bottom of the third synthesis reaction tank and then dehydrated and granulated.

[0016] Preferably, the heating coil is arranged in a spiral shape and attached to the funnel-shaped inner side of the lower end of the synthesis reaction vessel. The two ends of the heating coil extend outward from the synthesis reaction vessel and are heated by a secondary liquid phase heat medium.

[0017] Preferably, the system also includes a flow guide fixedly connected to the lower end of each synthesis reaction vessel. The flow guide is sealed to the synthesis reaction vessel. The flow guide includes a hollow flow guide cylinder. A limiting ring is fixedly connected to the outer circumference of the lower end face of the flow guide cylinder. The middle part of the flow guide cylinder extends into the interior of the synthesis reaction vessel and has multiple horizontally penetrating flow guide slots. Each flow guide slot is circumferentially distributed on the flow guide cylinder and is in fluid communication with the hollow part of the flow guide cylinder. The upper end of the flow guide cylinder is closed and circumferentially distributed flow guides extend downward from the upper end of the flow guide cylinder. The upper end of the flow guide cylinder is provided with a support slot for supporting the stirring blades.

[0018] Preferably, the stirring blade includes a stirring shaft coaxially rotatably connected to the sealing cover, and a plurality of circumferentially distributed stirring support frames are fixedly connected to the lower end of the stirring shaft. Each stirring support frame has a stirring rod fixedly connected to its extended end, and each stirring rod corresponds to the funnel-shaped structure on the lower end face of the sealed tank.

[0019] Preferably, each of the stirring rods has a hollow cavity arranged parallel to and penetrating the stirring rod. A moving rod is slidably connected inside the hollow cavity. Multiple flexible brushes are fixedly connected to the side of the moving rod facing the lower end face of the sealed tank. Multiple slots for the flexible brushes to pass through are opened on the side of the stirring rod corresponding to the flexible brushes. A limiting sliding block is fixedly connected to the side of the moving rod opposite to the direction of the flexible brushes. A limiting sliding groove is opened on the stirring rod at the position corresponding to the limiting sliding block. Small fan blades arranged obliquely are fixedly connected to the part of the limiting sliding block that extends out of the limiting sliding groove.

[0020] Preferably, the limiting sliding block is slidably connected in the limiting sliding groove, and two small fan blades are fixedly connected to the limiting sliding block and are located at both ends of the limiting sliding block and arranged perpendicularly between the limiting sliding block.

[0021] Preferably, a buffer cavity is provided between the slot on the stirring rod for the flexible brush to pass through and the side of the moving rod where the flexible brush is fixedly connected. When the moving rod slides along the stirring rod, the position of the flexible brush passing through the slot remains unchanged while the root of the flexible brush moves with the moving rod, thereby changing the position of each flexible brush protruding end. The buffer cavity provides a certain buffer space when the root of the flexible brush deforms, while preventing shearing force from being generated between the moving rod and the side of the slot on the stirring rod.

[0022] The technical solution of the present invention achieves the following beneficial technical effects:

[0023] 1. The use of organic guanidine green catalysts to replace traditional metal catalysts results in high product biosafety.

[0024] 2. The multi-stage segmented synthesis process technology makes the process continuous, controllable and stable.

[0025] 3. The three-stage continuous depolymerization and cyclization process for producing lactide is not only continuous but also results in high and controllable lactide yield. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the overall structure of the present invention;

[0027] Figure 2 This is a second perspective view of the overall structure of the present invention;

[0028] Figure 3 This is a cross-sectional view of the synthesis reaction vessel of the present invention;

[0029] Figure 4 This is a schematic diagram illustrating the stirring principle of the present invention;

[0030] Figure 5 This is a structural diagram of the condensation device of the present invention;

[0031] Figure 6 This is a schematic diagram of the flow guide tube structure of the present invention;

[0032] Figure 7 This is a diagram of the internal structure of the guide tube of the present invention;

[0033] Figure 8 This is a structural diagram of the stirring support frame of the present invention;

[0034] Figure 9 This is a cross-sectional view of the stirring rod of the present invention;

[0035] Figure 10 This is a partial enlarged view of the stirring rod of the present invention.

[0036] The reference numerals in the figure are as follows: 1. First synthesis reaction vessel; 2. Second synthesis reaction vessel; 3. Third synthesis reaction vessel; 4. Sealing cap; 5. Vacuum pump; 6. Feed inlet; 7. Static mixer; 8. Heating coil; 9. Fractionating tower; 10. First condenser; 11. Second condenser; 12. Lactide collection tank; 13. Total cooler; 14. Lactide recovery tank; 15. Flow guide tube; 16. Limiting ring; 17. Flow guide groove; 18. Flow guide vane; 19. Support groove; 20. Stirring shaft; 21. Stirring support frame; 22. Stirring rod; 23. Moving rod; 24. Flexible brush; 25. Limiting sliding block; 26. Limiting sliding groove; 27. Small fan blade; 28. Buffer cavity. Detailed Implementation

[0037] The following is in conjunction with the appendix Figure 1-10 The specific implementation method is described.

[0038] In various literature and patent reports on lactic acid polycondensation reactions, stannous salt catalysts (such as stannous chloride) are considered to be the most active catalysts. However, stannous salt catalysts have drawbacks such as low polymer yield and cytotoxicity.

[0039] Guanidine is a natural organic base found in various vegetables and seafood, and is an important raw material in the pharmaceutical industry. Guanidine derivatives also exist in the human body, such as arginine, creatine, guanidinoacetic acid, phosphoarginine, and arginine succinate, collectively known as biomass organic guanidine. The catalytic mechanism of organic guanidine mainly involves forming zwitterionic hydrogen bonds with anionic transition states, dispersing the charge of the transition state in a resonant manner to stabilize the transition state and lower the activation energy of the reaction. Furthermore, organic guanidine can also influence the reactivity of guest substances by altering their charge properties. Experiments revealed that organic guanidine was selected as a catalyst for the depolymerization and cyclization of oligolactic acid. As a liquid catalyst, it can be uniformly dispersed in the reaction system, exhibiting high catalytic efficiency and effectively reducing the formation of racemic lactide. The produced lactide has high purity and exhibits high biological and environmental safety. The dosage of the selected organic guanidine catalyst and the catalytic process conditions were optimized to determine the optimal process parameters, ensuring the production efficiency of lactide.

[0040] The depolymerization and cyclization of oligolactic acid is the key to the preparation of lactide. This process is the reverse reaction of lactic acid polycondensation. Under the action of a catalyst, the lactide generated by the depolymerization of oligolactic acid must be immediately distilled out of the reaction system. The required reaction temperature is higher than that of the polymerization process. At the same time, the pressure needs to be reduced so that the lactide can be distilled out of the reaction system in time to reduce losses.

[0041] To ensure the continuous, controllable, and stable production of lactide from oligolactic acid, a three-stage synthesis system is employed. Under the protection of nitrogen (99.99%) and the action of an organic guanidine catalyst, the lactide product is rapidly and gradually separated through appropriate reaction temperature and vacuum.

[0042] The first synthesis system involves mixing oligolactic acid with a certain amount of organic guanidine and then entering the first synthesis reaction tank 1. The first synthesis reaction tank 1 is a fully jacketed vertical reactor with heating coils 8 inside. Both the jacket and the coils are heated by secondary liquid-phase heat medium. The upper part of the first synthesis reaction tank 1 has a large gas-phase evaporation space, which allows the generated lactide product to evaporate quickly. The lactide enters the fractionation tower 9 through the gas-phase pipeline and is then cooled by the condenser. The condensed lactide enters the lactide collection tank 12, which is equipped with a jacketed insulation system. The liquid-phase product in the first synthesis reaction tank 1 is discharged from the middle of the synthesis reaction tank and enters the second synthesis reaction tank 2.

[0043] The reaction in the first synthesis reactor 1 needs to be carried out under certain temperature and vacuum conditions. The vacuum of this system is provided by vacuum pump 5. After passing through the fractionation tower 9, the lactide is cooled by the first condenser 10. Most of the lactide is cooled down and enters the lactide collection tank 12. The lactide that is still not condensed is cooled by the second condenser 11 and enters the lactide collection tank 12. Both condensers are used in pairs, one for operation and one for standby. The liquid level in the synthesis reactor is controlled by the transfer pump of the first synthesis reactor 1. The jacket and coil of the system are heated by a secondary liquid-phase circulating heat medium, and the temperature is controlled at 210-240℃. The vacuum degree of the control system is controlled by the regulating valve at 50-100 Torr. The reaction process is continuously stirred. The synthesis reactor has a large gas phase evaporation space, which allows the generated lactide product to evaporate quickly. The gas phase pipeline above the synthesis reactor is connected to the fractionation tower 9. The working temperature of the fractionation tower 9 is controlled at 200℃ and the working pressure is 5 Torr. The volatiles after fractionation enter the condenser for condensation.

[0044] Second synthesis system: The liquid phase extract from the side of the first synthesis reactor 1 enters the synthesis reactor from the bottom of the second synthesis reactor 2 under nitrogen protection. The second synthesis reactor 2 is a fully jacketed vertical reactor with internal heating coils 8. Both the jacket and the coils are heated by secondary liquid phase heat medium. The upper part of the second synthesis reactor 2 has a large gas phase evaporation space, which allows the generated lactide product to evaporate quickly. The lactide enters the fractionation tower 9 through the gas phase pipeline, and is then cooled by the condenser. The condensed lactide enters the lactide collection tank 12, which is equipped with a jacketed insulation system. The liquid phase product in the second synthesis reactor 2 is discharged from the middle of the synthesis reactor and enters the third synthesis reactor 3.

[0045] After passing through fractionation tower 9, lactide is cooled by the first condenser 10. Most of the lactide is cooled down and enters the lactide collection tank 12. The lactide that is still not condensed is cooled by the second condenser 11 and enters the lactide collection tank 12. Both condensers are used in pairs, one for operation and one for standby. The liquid level of the second synthesis reaction tank 2 is controlled by the transfer pump of the second synthesis reaction tank 2. The jacket and coil of the system are heated by secondary liquid phase heat medium, and the temperature is controlled at 220-240℃. The vacuum degree of the control system is 50-100 Torr. The reaction process is continuously stirred. The synthesis reaction tank has a large gas phase evaporation space, which allows the generated lactide product to evaporate quickly. The gas phase pipeline above the synthesis reaction tank is connected to fractionation tower 9. The working temperature of fractionation tower 9 is controlled at 210℃ and the working pressure is 5 Torr. The jacket of lactide collection tank 12 is insulated by secondary liquid phase circulating heat medium, and the temperature is controlled at about 100℃.

[0046] The third synthesis system: The liquid phase extract from the side of the second synthesis reactor 2 enters the synthesis reactor from the lower middle part of the third synthesis reactor 3 under nitrogen protection. The third synthesis reactor 3 is a fully jacketed vertical reactor with internal heating coils 8. Both the jacket and the heating coils 8 are heated by secondary liquid phase heat medium. The upper part of the third synthesis reactor 3 has a large gas phase evaporation space, which allows the generated lactide product to evaporate quickly. The lactide enters the fractionation tower 9 through the gas phase pipeline, and is then cooled by the condenser. The condensed lactide also enters the lactide collection tank 12. The polymer in the third synthesis reactor 3 is discharged from the bottom of the synthesis reactor and, after hydrolysis, is sold directly as industrial-grade lactic acid, or it can be used as a raw material for the production of lactate.

[0047] The reaction in the third synthesis reactor 3 needs to be carried out under specific temperature and vacuum conditions. The vacuum in this system is provided by a water-vacuum combined vacuum jet pump. After passing through the fractionation tower 9, the lactide is cooled by the first condenser 10. Most of the lactide is cooled and enters the lactide collection tank 12. The remaining uncooled lactide is then cooled by the second condenser 11 and enters the lactide collection tank 12. Both condensers are used in pairs, one for operation and one for standby. The liquid level in the synthesis reactor is controlled by a polymer transfer pump. The system jacket and rings are heated by a secondary liquid-phase heat medium, and the temperature is controlled at 220-240℃. The vacuum degree of the control system is maintained at approximately 50-100 Torr.

[0048] The heating coil 8 is arranged in a spiral shape on the lower end face of the synthesis reaction vessel, which is arranged in a funnel shape. Therefore, the heating coil 8 is also arranged in a spiral shape inside the synthesis reaction vessel. During the heating process of the spiral heating coil 8 by the secondary liquid phase heat medium, the synthesis reaction vessel will be heated. Since the heating part is located at the lower end of the synthesis reaction vessel, and since the space of the synthesis reaction vessel is large, it is easy to cause uneven heating inside the synthesis reaction vessel. During the heating process, it is necessary to continuously stir the synthesis reaction vessel.

[0049] During the stirring process of the synthesis reaction, a flow guide is sealed at the lower end of the synthesis reaction tank. Since the material enters from the lower end of the synthesis reaction tank, if it flows directly upwards after entering, the temperature of the upper material will decrease, thus affecting the evaporation efficiency. Therefore, multiple circumferentially distributed flow guide slots 17 are opened on the part of the flow guide that extends into the synthesis reaction tank. By sealing the upper end of the flow guide, the material passing through the flow guide will be diagonally dispersed outwards under the action of each flow guide slot 17, thereby causing the material entering the synthesis reaction tank to disperse and heat up rapidly. The outer side of the flow guide is a heating coil 8, which can further heat up the incoming material. Circumferentially distributed flow guide plates 18 extend downwards from the upper end of the flow guide, which cooperate with each flow guide slot 17. The flow guide plates 18 are polygonal and extend downwards. A cylindrical support slot 19 is provided at the upper end of the flow guide, which supports and rotates the lower end of the stirring blade to prevent the stirring blade from jumping during the stirring process.

[0050] The stirring fan blades include a stirring shaft 20, which is coaxially rotatably connected to the sealing cover 4 and driven by a drive motor fixedly connected to the sealing cover 4. The lower end of the stirring shaft 20 extends downward and rotates with the support slot 19 at the upper end of the guide. Multiple circumferentially distributed stirring support frames 21 are fixedly connected to the stirring shaft 20. Each stirring support frame 21 has a stirring rod 22 fixedly connected to its extended end. Each stirring rod 22 cooperates with the funnel-shaped structure on the lower end face of the sealed tank, so that the stirring rod 22 will drive the material at the bottom to move as the stirring shaft 20 rotates.

[0051] The stirring rod 22 is hollow, with a hollow cavity parallel to it in the middle. A moving rod 23 is slidably connected within this cavity. The moving rod 23 is square, and multiple evenly spaced flexible brushes 24 are fixedly connected to its lower end. These brushes extend towards the heating coil 8 and agitate the material around it. The stirring rod 22 has slots through which the flexible brushes 24 pass. A limiting sliding block 25 extending from the stirring rod 22 is fixedly connected to the side of the moving rod 23 opposite to the side with the flexible brushes 24. A limiting sliding groove 26, cooperating with the limiting sliding block 25, is provided on the stirring rod 22. Small, obliquely arranged fan blades 27 are fixedly connected to the portion of the limiting sliding block 25 extending beyond the limiting sliding groove 26. As the stirring rod 22 rotates with the stirring shaft 20, the small blades 27 generate a certain thrust when in contact with the material, causing the limiting sliding block 25 to move within the limiting sliding groove 26. This, in turn, causes the moving rod 23 to slide relative to the stirring rod 22. Since the flexible brushes 24 on the moving rod 23 extend from the middle of the stirring rod 22, the position of the base of the flexible brushes 24 changes when the position of the moving rod 23 changes. Therefore, the position of the extended ends of each flexible brush 24 changes synchronously, allowing the flexible brushes 24 to face the uneven parts of the heating coil 8 and preventing the material from accumulating on the heating coil 8 for a long time. The direction of the thrust of the small blades 27 is changed according to the forward and reverse rotation of the stirring shaft 20, thereby changing the orientation of each flexible brush 24, so that the flexible brushes 24 can fully contact all parts of the heating coil 8 and avoid dead zones.

[0052] A buffer cavity 28 is provided between the groove position on the moving rod 23 and the stirring rod 22. The buffer cavity 28 can avoid shearing phenomenon on the one hand, and provide a certain space for the deformation part of the flexible brush 24 on the other hand, so that the deformation angle of the flexible brush 24 can be naturally deformed instead of an acute angle, thereby avoiding the phenomenon of breakage caused by repeated deformation and thus improving the service life.

[0053] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. A three-stage lactide synthesis apparatus, comprising a first synthesis system, a second synthesis system, and a third synthesis system connected in sequence, each synthesis system comprising a sealed synthesis reaction vessel, wherein the middle of the first synthesis reaction vessel (1) in the first synthesis system is fluidly connected to the lower end of the second synthesis reaction vessel (2) in the second synthesis system, and the middle of the second synthesis reaction vessel (2) in the second synthesis system is fluidly connected to the lower end of the third synthesis reaction vessel (3) in the third synthesis system, and each synthesis reaction vessel is fluidly connected to a condenser at its upper end, characterized in that, Each synthesis system includes a sealed tank, with a detachable sealing cover (4) at the top. A vacuum pump (5) is fixedly connected to the sealing cover (4). A stirring fan blade, coaxially arranged with the sealed tank, is rotatably connected to the sealing cover (4). The stirring fan blade extends into the sealed tank and is used to stir the materials inside the sealed tank. The bottom of the sealed tank has a funnel structure and a feed inlet (6). A static mixer (7) for mixing additives is provided at the feed inlet (6). A heating coil (8) for heating the sealed tank is provided at the lower end of the inner side of the sealed tank. The heating coil (8) uses a secondary liquid phase heat medium for heating. The upper end of the inner side of the sealed tank is a gas phase evaporation space. The sealing cover at the top of the sealed tank... (4) A condensing device is connected to the tank. The condensing device includes a fractionating tower (9). The gas in the tank enters the first condenser (10) connected to the fractionating tower (9) after passing through the fractionating tower (9). The first condenser (10) is connected to the lactide collection tank (12) and the second condenser (11). The condensed lactide enters the lactide collection tank (12) and the uncondensed gas enters the second condenser (11). The second condenser (11) is connected to the lactide collection tank (12) and the total cooler (13). The condensed lactide enters the lactide collection tank (12) and the uncondensed gas enters the total cooler (13). The total cooler (13) is connected to the lactide recovery tank (14). The condensate from the total cooler (13) enters the lactide recovery tank (14). It also includes a flow guide fixedly connected to the lower end of each synthesis reaction vessel. The flow guide is sealed to the synthesis reaction vessel. The flow guide includes a hollow flow guide cylinder (15). A limit ring (16) is fixedly connected to the outer circumference of the lower end face of the flow guide cylinder (15). The middle part of the flow guide cylinder (15) extends into the interior of the synthesis reaction vessel and is provided with multiple horizontally penetrating flow guide slots (17). Each flow guide slot (17) is evenly distributed on the circumference of the flow guide cylinder (15) and is in fluid communication with the hollow part of the flow guide cylinder (15). The upper end of the flow guide cylinder (15) is closed and a circumferentially distributed flow guide plate (18) extends downward from the upper end of the flow guide cylinder (15). The upper end of the flow guide cylinder (15) is provided with a support slot (19) for supporting the stirring fan blades. The stirring fan blades include a stirring shaft (20) coaxially rotatably connected to the sealing cover (4). The lower end of the stirring shaft (20) is fixedly connected to a plurality of circumferentially distributed stirring support frames (21). Each stirring support frame (21) has a stirring rod (22) fixedly connected to its extended end. Each stirring rod (22) corresponds to the funnel-shaped structure on the lower end face of the sealed tank. Each of the stirring rods (22) has a hollow cavity arranged parallel to and penetrating the stirring rod (22). A moving rod (23) is slidably connected in the hollow cavity. Multiple flexible brushes (24) are fixedly connected to the side of the moving rod (23) facing the lower end face of the sealed tank. Multiple slots for the flexible brushes (24) to pass through are opened on the side of the stirring rod (22) corresponding to the flexible brushes (24). A limiting sliding block (25) is fixedly connected to the side of the moving rod (23) opposite to the direction of the flexible brushes (24). A limiting sliding groove (26) is opened on the stirring rod (22) at the position corresponding to the limiting sliding block (25). Small fan blades (27) arranged obliquely are fixedly connected to the part of the limiting sliding block (25) extending out of the limiting sliding groove (26).

2. The three-stage lactide synthesis apparatus according to claim 1, characterized in that, The heating coil (8) is arranged in a spiral shape and attached to the funnel-shaped inner side of the lower end of the synthesis reaction vessel. The two ends of the heating coil (8) extend outward from the synthesis reaction vessel and are heated by a secondary liquid phase heat medium.

3. The three-stage lactide synthesis apparatus according to claim 1, characterized in that, The limiting sliding block (25) is slidably connected in the limiting sliding groove (26). There are two small fan blades (27) fixedly connected to the limiting sliding block (25), which are located at both ends of the limiting sliding block (25) and arranged perpendicularly between the limiting sliding block (25).

4. The three-stage lactide synthesis apparatus according to claim 1, characterized in that, A buffer cavity (28) is provided between the slot on the stirring rod (22) for the flexible brush (24) to pass through and the side of the moving rod (23) where the flexible brush (24) is fixedly connected. When the moving rod (23) slides along the stirring rod (22), the position of the flexible brush (24) passing through the slot remains unchanged while the root of the flexible brush (24) moves with the moving rod (23), thereby changing the position of the protruding end of each flexible brush (24). The buffer cavity (28) provides a certain buffer space when the root of the flexible brush (24) deforms, and at the same time prevents shearing force from being generated between the moving rod (23) and the side of the slot on the stirring rod (22).

5. A process for synthesizing lactide using the three-stage lactide synthesis apparatus according to any one of claims 1-4, characterized in that, The synthesis reaction vessel in the first synthesis system is heated to 210-240℃ and has a vacuum of 50-100 Torr. The fractionation tower (9) operates at 200℃ and at a pressure of 5 Torr. The heating temperature of the synthesis reaction vessel in the second synthesis system is 220-240℃, the vacuum degree is 50-100 Torr, the working temperature of the fractionation tower (9) is 210℃ and the working pressure is 5 Torr, and the temperature of the lactide collection tank (12) in the second synthesis system is kept at 100℃ by the secondary liquid phase circulating heat medium. The synthesis reaction vessel in the third synthesis system is heated to a temperature of 210-240℃ and has a vacuum degree of 50-100 Torr. The material inside the synthesis reaction vessel in the third synthesis system is a polymer. The polymer is discharged through the lower end of the synthesis reaction vessel and enters the connected dehydration oligomerization kettle and granulation equipment.

6. The process for synthesizing lactide according to claim 5, characterized in that, The first synthesis reaction tank (1) contains lactic acid oligomers, and when entering the first synthesis reaction tank (1), it enters the first synthesis reaction tank (1) under the protection of nitrogen with a concentration of 99.99%. The liquid phase extract in the first synthesis reaction tank (1) is discharged from the middle of the first synthesis reaction tank (1) under the protection of nitrogen and enters the second synthesis reaction tank (2) through the bottom of the second synthesis reaction tank (2). The liquid phase extract in the second synthesis reaction tank (2) is discharged from the middle of the second synthesis reaction tank (2) under the protection of nitrogen and enters the third synthesis reaction tank (3) through the lower part of the third synthesis reaction tank (3). The material in the third synthesis reaction tank (3) is a polymer at this time and is discharged from the lower end of the third synthesis reaction tank (3) into the dehydration oligomerization kettle and granulation equipment.

7. The process for synthesizing lactide according to claim 5, characterized in that, The evaporated material in the first synthesis reaction tank (1) passes through the fractionation tower (9) and enters the first condenser (10) for condensation. The condensed lactide enters the lactide collection tank (12), while the uncondensed evaporated material enters the second condenser (11) for condensation again. The condensed lactide enters the lactide collection tank (12), and the still uncondensed evaporated material enters the total cooler (13) and then enters the lactide recovery tank (14). The first condenser (10) and the second condenser (11) are both used and standby. The liquid level in the first synthesis reaction tank (1) is controlled by the synthesis reaction tank delivery pump. The condensation steps of the evaporated material in the second synthesis reaction tank (2) and the third synthesis reaction tank (3) are the same as the condensation steps in the first synthesis reaction tank (1). The liquid level in the second synthesis reaction tank (2) is controlled by the synthesis reaction tank delivery pump, while the liquid level in the third synthesis reaction tank (3) is controlled by the polymer delivery pump. The polymer in the third synthesis reaction tank (3) is discharged from the lower end of the third synthesis reaction tank (3) and then dehydrated and granulated.

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