Reaction device for preparing lactide from lactic acid through one-step method and application method of reaction device

Abstract

The invention belongs to the technical field of preparation of lactide from lactic acid, and relates to a reaction device for preparing lactide from lactic acid through a one-step method and an application method thereof, the device comprises a reaction layer, a water separation layer and a condensation layer which are sequentially arranged from bottom to top, and an oil guide layer of a hollow structure is arranged in the reaction layer; a reaction tank is formed above the sunken part of the oil guide layer, a water distribution plate is arranged on the upper side of the water distribution layer, a plurality of water distributors are arranged on the water distribution plate, water distribution holes are formed in the side surfaces of the water distributors, and water vapor generated by a lactic acid catalytic reaction system reaches the water distribution layer and enters the condensation layer through the water distribution holes in the water distributors; a cold water plate is arranged at the top of the condensation layer, and a closed space is formed by the bottom surface of the cold water plate and the top surface of the water separation layer. According to the reaction device disclosed by the invention, water in a reaction system is upwards discharged in a gas form to leave a reaction layer, so that hydrolysis of a product is effectively prevented, the problem of dehydration in a reaction process, which cannot be solved by an existing reaction device, is solved, and the yield of lactide is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of preparing lactide from lactic acid, and in particular relates to a reaction device for preparing lactide from lactic acid in a one-step process and an application method thereof. Background Art

[0002] As the current mainstream synthetic bio-based plastic, polylactic acid (PLA) has attracted much attention due to its wide application prospects and its positive role in alleviating marine plastic pollution. In the industrial production of PLA, lactide, as a key precursor, is mainly prepared by the dimerization reaction of lactic acid. At present, the synthesis process of lactide is mainly divided into two-step method and one-step method: the two-step method requires lactic acid to be polymerized into oligomers under low temperature and reduced pressure conditions, and then cracked at high temperature and high vacuum, but there are problems such as low product purity, difficult separation and high energy consumption; the one-step method is to directly convert lactic acid molecules into cyclic dimers under the action of a catalyst, which has significant advantages such as high reaction selectivity and simplified process. Recent studies have found that molecular sieve catalysts can efficiently catalyze the one-step synthesis of lactide from lactic acid, and its low cost and easy recovery of by-products make it possible for industrial application. However, the water continuously generated during the reaction will trigger the reverse reaction of lactide, resulting in a decrease in yield.

[0003] Due to the limitation of vertical mass transfer mode, it is difficult to remove water quickly in existing reaction devices, resulting in low lactic acid conversion rate, which has become a key bottleneck restricting the industrial development of one-step process. Therefore, the development of efficient water separation technology is the core challenge to promote the industrialization of lactide preparation from lactic acid in one step. Summary of the invention

[0004] In order to solve the problems mentioned in the above background technology, the present invention provides a reaction device for preparing lactide from lactic acid in one step and an application method thereof, wherein the device can effectively remove water during the reaction and improve the lactide selectivity of the reaction.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A reaction device for preparing lactide from lactic acid by a one-step method comprises a reaction layer, a water separation layer and a condensation layer which are arranged in sequence from bottom to top, wherein the reaction layer is used for carrying out a lactic acid catalytic reaction to prepare lactide; an oil conducting layer with a hollow structure is arranged inside the reaction layer, a reaction tank is formed above a depression of the oil conducting layer, and a catalyst is arranged in the reaction tank; heat transfer oil is arranged inside the oil conducting layer for heating a lactic acid catalytic reaction system in the reaction tank; a water separation plate is arranged on the upper side of the water separation layer, a plurality of water separators are arranged on the water separation plate, water separation holes are arranged on the side of the water separator, water vapor generated by the lactic acid catalytic reaction system reaches the water separation layer and enters the condensation layer through the water separation holes on the water separator; a cold water plate is arranged on the top of the condensation layer, the cold water plate is a hollow structure, condensed water passes through it, and the bottom surface of the cold water plate and the top surface of the water separation layer form a closed space.

[0007] Furthermore, the reaction tank, the oil conducting layer, the water distribution plate and the cold water plate are parallel to each other, and the angle between the reaction tank, the oil conducting layer, the water distribution plate and the cold water plate and the ground plane is 2-20°.

[0008] Furthermore, a base is provided below the reaction layer, the base is connected to the reaction layer via a card slot, the reaction layer is connected to the water separation layer via a card plate, a baffle and a side, and the water separation layer is fixedly connected to the condensation layer via a card plate.

[0009] Furthermore, a feed port and a discharge port are provided at both ends of the reaction tank, the feed port is used to introduce raw material lactic acid and solvent, and the product and solvent are discharged from the discharge port; the outer walls of the feed port and the discharge port are cylindrical, the lower half of the interior is solid, and the bottom surface through which the material flows through the feed port and the discharge port is flat.

[0010] Furthermore, the feed port is connected with a slope, and the raw materials lactic acid and solvent are introduced from the feed port, flow through the slope, and enter the reaction tank.

[0011] Furthermore, the oil conducting layer is fixed on both sides of the inner wall of the reaction layer and is connected through the bottom. An oil inlet and an oil outlet are provided at both ends of the oil conducting layer. The oil inlet and the oil outlet are externally connected to a circulating pump. The heat transfer oil enters the oil conducting layer through the oil inlet and is discharged from the oil outlet into the circulating pump. The temperature of the heat transfer oil is 100~300℃.

[0012] Furthermore, the reaction tank is rectangular, circular or square, and its aspect ratio or length-to-diameter ratio is 100:1 to 1:1; its length-to-depth ratio is 100:1 to 1:1; the reaction tank is made of resin, stainless steel, cast iron, glass, ceramic or titanium alloy.

[0013] Furthermore, the catalyst is a silica-alumina molecular sieve material with 12-membered ring channels, selected from at least one of BEA, FAU, and MOR, or other nanoporous molecular sieve materials with certain acidity; the solvent used is selected from at least one of benzene, toluene, xylene, trimethylolbenzene, linear alkanes, and cycloalkanes.

[0014] Furthermore, the cold water plate is connected to a water inlet and a water outlet, and the water inlet and the water outlet are externally connected to a cold water circulation machine; a side panel is provided on the side of the condensation layer, a collecting port is provided on the side panel, and the inner side of the side panel is in contact with the water distribution plate.

[0015] Furthermore, the water distributors are conical, and the number is 4 to 20; each water distributor has 1 to 10 water distribution holes.

[0016] The present invention also provides an application method of the reaction device for preparing lactide from lactic acid in one step, comprising: placing a catalyst in a reaction tank, introducing raw lactic acid and a solvent into a reaction layer, introducing heat transfer oil with a temperature of 100-300°C into an oil-conducting layer, lactic acid reacts in the reaction tank under the catalysis of the catalyst, water in the lactic acid catalytic reaction system rises in the form of water vapor to reach the water separation layer, rises through the water separation holes on the water separator to reach the cold water plate, liquefies into water droplets and drips onto the water separation plate, and converges into a water flow along the water separation plate to be discharged; and the product and solvent produced in the reaction tank are discharged from the discharge port of the reaction tank.

[0017] Beneficial Effects

[0018] The reaction device for preparing lactide from lactic acid in one step of the present invention delivers raw materials and solvents into the reaction system by a flat push method, and the water generated in the reaction process is continuously evaporated and removed through the water separation layer during the flow process, and the removed water is designed so that it will not return to the reaction system, which effectively solves the problem of lactic acid depolymerization due to the influence of water after catalysis, and avoids the reaction of water generated by the reaction with lactide during the lactic acid catalysis process to depolymerize, so that lactide is converted into lactic acid monomers or linear polymers, resulting in a decrease in the selectivity of lactide. The reaction device of the present invention sets the water separator to a cone, sets a water separation hole on the side, and designs the condensation layer with an inclined surface, so that the evaporated water will not enter the reaction tank again but will be directly discharged.

[0019] The reaction device for preparing lactide from lactic acid in one step of the present invention has the best catalytic results of 100% conversion rate and 99% lactide selectivity, which are much higher than the laboratory three-necked flask device and have achieved good technical effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is the overall structure diagram of the present invention;

[0021] Figure 2 It is a split diagram of the present invention;

[0022] Figure 3 It is a right side view of the present invention;

[0023] Figure 4 is a cross-sectional view of the present invention;

[0024] Figure 5 It is a front cross-sectional view of the present invention;

[0025] Figure 6 It is a structural diagram of the reaction layer of the present invention;

[0026] Figure 7 It is a cross-sectional view of the water-dividing layer of the present invention;

[0027] Figure 8 It is a cross-sectional view of the water divider of the present invention.

[0028] Figure numerals: 1. condensation layer; 2. water-dividing layer; 3. reaction layer; 4. base; 5. cold water plate; 6. water inlet; 7. water outlet; 8. side plate; 9. collecting port; 10. water-dividing plate; 11. water distributor; 12. water-dividing hole; 13. card plate; 14. baffle; 15. oil-guiding layer; 16. slope; 17. discharge port; 18. feed port; 19. oil inlet; 20. side; 21. card slot; 22. oil outlet; 23. reaction tank. DETAILED DESCRIPTION

[0029] Example 1

[0030] like Figures 1 to 8 As shown, this embodiment provides a reaction device in the process of preparing lactide by lactic acid catalysis, comprising a base 4, a reaction layer 3, a water-dividing layer 2 and a condensing layer 1 arranged in sequence from bottom to top, the base 4, the reaction layer 3, the water-dividing layer 2 and the condensing layer 1 can be assembled and disassembled, the base 4 is connected to the reaction layer 3 through a card slot 21, the reaction layer 3 is connected to the water-dividing layer 2 through a card plate 13, a baffle 14 and a side 20, the condensing layer 1 is covered above the water-dividing layer 2, and the water-dividing layer 2 and the condensing layer 1 are fixedly connected through a card plate 13. The four parts of the reaction device keep the long side L, the short side D, and L / D=10.

[0031] The reaction layer 3 is used for carrying out lactic acid catalytic reaction to prepare lactide; a hollow oil-conducting layer 15 is arranged inside the reaction layer 3, and heat-conducting oil is filled inside the oil-conducting layer 15. The oil-conducting layer 15 is fixed to both sides of the inner wall of the reaction layer 3 and is connected through the bottom to form a "concave" shape. A reaction tank 23 is formed above the concave part of the oil-conducting layer 15, so that the high-temperature heat-conducting oil can more fully heat the lactic acid catalytic reaction system in the reaction tank 23; a feed port 18 and a discharge port 17 are arranged at both ends of the reaction tank 23, and the feed port 18 is used to introduce raw lactic acid and solvent, and the product and solvent are discharged from the discharge port 17; a spherical catalyst is arranged in the reaction tank 23. The outer wall of the feed port 18 and the discharge port 17 is cylindrical, and the lower half of the inner part is solid. The bottom surface of the material flowing through the feed port 18 and the discharge port 17 is flat; the feed port 18 is connected to the slope 16, and the raw material lactic acid and solvent are introduced from the feed port 18, flow through the slope 16, and enter the reaction tank 23, so that the raw material lactic acid and solvent can slowly pass through the catalyst to make them fully react; the two ends of the oil-conducting layer 15 are provided with an oil inlet 19 and an oil outlet 22, and the oil inlet 19 and the oil outlet 22 are externally connected to a circulating pump. The oil-conducting layer 15 is not connected to the reaction tank 23, and the heat-conducting oil enters the oil-conducting layer 15 through the oil inlet 19 and is discharged from the oil outlet 22 to enter the circulating pump. The component of the heat-conducting oil is a compound of a benzene ring with an alkane branched type, and the temperature is 100~300℃, which is used to heat the lactic acid catalytic reaction system in the reaction tank 23 to vaporize the water in the lactic acid catalytic reaction system into water vapor.

[0032] A water dividing plate 10 is arranged on the upper side of the water dividing layer 2, and a plurality of water dividing devices 11 are arranged on the water dividing plate 10. The water dividing devices 11 are conical and 12 in number; water dividing holes 12 are arranged on the side of the water dividing devices 11, and there is one water dividing hole on each water dividing device; the water vapor generated by the lactic acid catalytic reaction system reaches the water dividing layer 2 and enters the condensation layer 1 through the water dividing holes 12 on the water dividing devices 11.

[0033] A cold water plate 5 is arranged on the top of the condensation layer 1. The cold water plate 5 is a hollow structure, through which condensed water passes. The bottom surface of the cold water plate 5 and the top surface of the water-dividing layer 2 form a closed space. The cold water plate 5 is connected with a water inlet 6 and a water outlet 7, and the water inlet 6 and the water outlet 7 are externally connected to a cold water circulation machine. A side plate 8 is arranged on the shorter side of the condensation layer 1, and a collecting port 9 is arranged on the side plate 8. The inner side of the side plate 8 is in contact with the water-dividing plate 10; the temperature of the cold water plate 5 is 5-20°C. The lower temperature of the cold water plate 5 causes the water vapor passing through the water-dividing layer 2 to liquefy into water droplets, which drip onto the water-dividing plate 10, converge into a water stream, slide down along the water-dividing plate 10, flow to the side plate 8 and are discharged from the collecting port 9.

[0034] The reaction tank 23, the oil conducting layer 15, the water dividing plate 10 and the cold water plate 5 are parallel to each other and form an angle of 15° with the ground plane. This angle allows the reaction raw materials to flow slowly toward the outlet under the action of gravity, while allowing the water generated during the reaction to continuously evaporate upward to reach the water dividing layer and be further removed.

[0035] A ball-shaped catalyst H-β molecular sieve (Si / Al=15) is placed in a reaction tank 23 and fills the entire reaction tank 23. Raw lactic acid and solvent toluene are introduced from the feed port 18. The oil inlet 19 and the oil outlet 22 are externally connected to a circulating pump. The heat transfer oil enters the oil transfer layer 15 through the oil inlet 19 and is discharged from the oil outlet 22 to enter the circulating pump. Due to the inclined design of the reaction tank 23, the raw lactic acid and solvent toluene flow continuously toward the discharge port 17 due to gravity after entering the reaction tank 23. Lactic acid reacts in the reaction tank 23 under the catalysis of the catalyst. Water in the reaction system rises in the form of water vapor to reach the water separation layer 2, and enters the condensation layer 1 through the water separation hole 12 on the water separator 11. The water vapor continues to rise to the cold water plate 5, liquefies into water droplets and drips onto the water separation plate 10, converges into a water flow, flows to the side plate 8 and is discharged from the collecting port 9. The cold water plate 5 is connected with a water inlet 6 and a water outlet 7, and the water inlet 6 and the water outlet 7 are externally connected to a cold water circulation machine. The product and the solvent are discharged from the discharge port 17 and sent to the high performance liquid chromatography for product analysis.

[0036] The reaction conditions for preparing lactide catalyzed by lactic acid are as follows: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140°C, and catalysis for 5 h.

[0037] The obtained product was tested and the lactic acid conversion rate was 99% and the lactide selectivity was 99%.

[0038] Example 2

[0039] This embodiment provides a reaction device for preparing lactide by catalysis of lactic acid. The difference from Embodiment 1 is the ratio of the long side L to the short side D of the reaction device, L / D=5, and the rest of the structure is the same as that of Embodiment 1.

[0040] The reaction conditions for preparing lactide catalyzed by lactic acid are as follows: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140°C, and catalysis for 5 h.

[0041] The obtained product was tested and the lactic acid conversion rate was 99% and the lactide selectivity was 95%.

[0042] Example 3

[0043] This embodiment provides a reaction device for preparing lactide by catalysis of lactic acid. The difference from Embodiment 1 is the ratio of the long side L to the short side D of the reaction device, L / D=1, and the rest of the structure is the same as that of Embodiment 1.

[0044] The reaction conditions for preparing lactide catalyzed by lactic acid are as follows: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140°C, and catalysis for 5 h.

[0045] The obtained product was tested and the lactic acid conversion rate was 100% and the lactide selectivity was 99%.

[0046] Example 4

[0047] This embodiment provides a reaction device for preparing lactide by catalysis of lactic acid. The difference from Embodiment 1 is that the catalyst used is H-ZSM-5 molecular sieve, and the rest is the same as Embodiment 1.

[0048] The reaction conditions for preparing lactide catalyzed by lactic acid are as follows: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140°C, and catalysis for 5 h.

[0049] The obtained product was tested and the lactic acid conversion rate was 92% and the lactide selectivity was 83%.

[0050] Example 5

[0051] This embodiment provides a reaction device for preparing lactide by catalysis of lactic acid. The difference from Embodiment 1 is that the catalyst used is HY molecular sieve, and the rest is the same as Embodiment 1.

[0052] The reaction conditions for preparing lactide by lactic acid catalysis are: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140 °C, and catalysis for 5 h.

[0053] The obtained product was tested and the lactic acid conversion rate was 87% and the lactide selectivity was 68%.

[0054] Example 6

[0055] This embodiment provides a reaction device for preparing lactide by catalysis of lactic acid. The difference from the embodiment 1 is that the slope 16 is not used, and the rest is the same as the embodiment 1.

[0056] The reaction conditions for preparing lactide by lactic acid catalysis are: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140 °C, and catalysis for 5 h.

[0057] The obtained product was tested and the lactic acid conversion rate was 98% and the lactide selectivity was 95%.

[0058] Comparative Example 1

[0059] First, l-lysine, tetraethylammonium hydroxide and deionized water were mixed and stirred at room temperature until the solution was clear and transparent, then potassium chloride and sodium aluminate were added and stirred until the solution was clear and transparent, then white carbon black was slowly added and stirred for 12 h, and finally the gel precursor was transferred to a stainless steel reactor lined with polytetrafluoroethylene and crystallized at 140 ° C for 4 days. After washing, centrifuging and drying overnight, the crystallized mixed solution was calcined in a muffle furnace at 550 ° C for 6 h to obtain β molecular sieve.

[0060] The prepared molecular sieve sample was placed in 1 M NH 4 NO 3 The solution was ion exchanged in an 80°C oil bath for 6 h, which was repeated twice, followed by washing, centrifugation and overnight drying, and finally calcined at 550°C for 6 h to obtain H-β molecular sieve (Si / Al = 15).

[0061] The reaction conditions for preparing lactide catalyzed by lactic acid are as follows: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140°C, and catalysis in a three-necked flask for 5 h.

[0062] The obtained product was tested and the lactic acid conversion rate was 98% and the lactide selectivity was 75%.

[0063] Comparative Example 2

[0064] This comparative example provides a reaction device for preparing lactide by catalysis of lactic acid, which is different from Example 1 in that no water distributor is provided on the water distributor plate, and only water distribution holes at the water distributor are retained, and the rest is the same as Example 1.

[0065] The reaction conditions for preparing lactide by lactic acid catalysis are: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140 °C, and catalysis for 5 h.

[0066] The obtained product was tested and the lactic acid conversion rate was 88% and the lactide selectivity was 65%.

[0067] Comparative Example 3

[0068] This comparative example provides a reaction device for the process of preparing lactide catalyzed by lactic acid. The difference from Example 1 is that no water separation layer is provided, and the condensation layer and the reaction layer are connected by a baffle and a card plate, and the rest is the same as Example 1.

[0069] The reaction conditions for preparing lactide by lactic acid catalysis are: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140 °C, and catalysis for 5 h.

[0070] The obtained product was tested and the lactic acid conversion rate was 50% and the lactide selectivity was 0%.

[0071] Comparative Example 4

[0072] This comparative example provides a reaction device for preparing lactide by catalysis of lactic acid, which is different from Example 1 in that a water dividing hole is provided on the water dividing plate, and the hole is located at the top of the water dividing hole, and the rest is the same as Example 1.

[0073] The reaction conditions for preparing lactide by lactic acid catalysis are: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140 °C, and catalysis for 5 h.

[0074] The obtained product was tested and the lactic acid conversion rate was 92% and the lactide selectivity was 71%.

[0075] Comparative Example 5

[0076] This comparative example provides a reaction device for preparing lactide by catalysis of lactic acid, which is different from Example 1 in that no condensation layer is provided, and the rest is the same as Example 1.

[0077] The reaction conditions for preparing lactide by lactic acid catalysis are: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140 °C, and catalysis for 5 h.

[0078] The obtained product was tested and the lactic acid conversion rate was 53% and the lactide selectivity was 40%.

[0079] Comparative Example 6

[0080] This comparative example provides a reaction device for preparing lactide by catalysis of lactic acid, which is different from Example 1 in that the condensation plate is changed to solid, condensation water is not passed, and a condensation system is not provided, and the rest is the same as Example 1.

[0081] The reaction conditions for preparing lactide by lactic acid catalysis are: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140 °C, and catalysis for 5 h.

[0082] The obtained product was tested and the lactic acid conversion rate was 89% and the lactide selectivity was 74%.

Claims

1. A reaction device for preparing lactide from lactic acid in one step, characterized in that: It comprises a reaction layer, a water separation layer and a condensation layer which are arranged in sequence from bottom to top, wherein the reaction layer is used for carrying out a lactic acid catalytic reaction to prepare lactide; an oil conducting layer with a hollow structure is arranged inside the reaction layer, a reaction tank is formed above the depression of the oil conducting layer, and a catalyst is arranged in the reaction tank; heat transfer oil is arranged inside the oil conducting layer for heating the lactic acid catalytic reaction system in the reaction tank; a water separation plate is arranged on the upper side of the water separation layer, a plurality of water distributors are arranged on the water separation plate, and water separation holes are arranged on the sides of the water distributors, and water vapor generated by the lactic acid catalytic reaction system reaches the water separation layer and enters the condensation layer through the water separation holes on the water distributors; a cold water plate is arranged on the top of the condensation layer, and the cold water plate is a hollow structure, through which condensed water passes, and the bottom surface of the cold water plate and the top surface of the water separation layer form a closed space.

2. The reaction device for preparing lactide from lactic acid by one-step method according to claim 1, characterized in that: The reaction tank, the oil conducting layer, the water distribution plate and the cold water plate are parallel to each other, and the angle between the reaction tank, the oil conducting layer, the water distribution plate and the cold water plate and the ground plane is 2-20 degrees.

3. The reaction device for preparing lactide from lactic acid by one-step method according to claim 1, characterized in that: A base is arranged below the reaction layer, the base is connected to the reaction layer through a card slot, the reaction layer is connected to the water separation layer through a card plate, a baffle and a side, and the water separation layer is fixedly connected to the condensation layer through a card plate.

4. The reaction device for preparing lactide from lactic acid by one-step method according to claim 1, characterized in that: The reaction tank is provided with a feed port and a discharge port at both ends, the feed port is used to introduce raw material lactic acid and solvent, and the product and solvent are discharged from the discharge port; the outer walls of the feed port and the discharge port are cylindrical, the lower half of the interior is solid, and the bottom surface through which the material flows through the feed port and the discharge port is flat.

5. The reaction device for preparing lactide from lactic acid by one-step method according to claim 4, characterized in that: The feed port is connected with a slope, and raw materials lactic acid and solvent are introduced from the feed port, flow through the slope, and enter the reaction tank.

6. The reaction device for preparing lactide from lactic acid by one-step method according to claim 1, characterized in that: The oil conducting layer is fixed on both sides of the inner wall of the reaction layer and is connected through the bottom. An oil inlet and an oil outlet are provided at both ends of the oil conducting layer. The oil inlet and the oil outlet are externally connected to a circulating pump. The heat conducting oil enters the oil conducting layer through the oil inlet and is discharged from the oil outlet into the circulating pump. The temperature of the heat conducting oil is 100-300°C.

7. The reaction device for preparing lactide from lactic acid by one-step method according to claim 1, characterized in that: The reaction tank is rectangular, circular or square, and its aspect ratio or length-to-diameter ratio is 100:1 to 1:1; its length-to-depth ratio is 100:1 to 1:1; the material of the reaction tank is resin, stainless steel, cast iron, glass, ceramic or titanium alloy.

8. The reaction device for preparing lactide from lactic acid by one-step method according to claim 1, characterized in that: The catalyst is at least one of BEA, FAU and MOR; the solvent is at least one of benzene, toluene, xylene, trimethylbenzene, straight-chain alkane and cycloalkane.

9. The reaction device for preparing lactide from lactic acid by one-step method according to claim 1, characterized in that: The cold water plate is connected with a water inlet and a water outlet, and the water inlet and the water outlet are externally connected with a cold water circulation machine; a side plate is arranged on the side of the condensation layer, a collecting port is arranged on the side plate, and the inner side of the side plate is in contact with the water distribution plate.

10. The application method of the reaction device for preparing lactide from lactic acid by one-step method according to any one of claims 1 to 9, characterized in that: include: The catalyst is placed in a reaction tank, raw lactic acid and solvent are introduced into the reaction layer, and heat transfer oil with a temperature of 100-300°C is introduced into the oil guide layer. Lactic acid reacts in the reaction tank under the catalysis of the catalyst, and water in the lactic acid catalytic reaction system rises to the water separation layer in the form of water vapor, and rises to the cold water plate through the water separation holes on the water separator, liquefies into water droplets and falls onto the water separation plate, and converges into a water flow along the water separation plate and is discharged; the products and solvents produced in the reaction tank are discharged from the discharge port of the reaction tank.

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