One-step reaction device for preparing lactide from lactic acid and its application method

Abstract

The present invention belongs to the technical field of lactic acid preparation of lactide, and relates to a reaction device for preparing lactide by a one-step method of lactic acid and an application method thereof. The device comprises a reaction layer, a water separation layer and a condensation layer which are arranged in sequence from bottom to top. A hollow oil guide layer is arranged inside the reaction layer; a reaction tank is formed above the concave part of the oil guide layer. 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 surface 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 at the top of the condensation layer. The bottom surface of the cold water plate and the top surface of the water separation layer form a closed space. In the reaction device of the present invention, water in the reaction system is discharged upward in the form of gas and leaves the reaction layer, effectively preventing the hydrolysis of the product, solving the problem of dehydration in the reaction process that cannot be solved by the existing reaction device, and improving the yield of lactide.

Description

Technical Field

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

[0002] As a current mainstream synthetic bio-based plastic, polylactic acid (PLA) has attracted much attention due to its wide application prospects and 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 lactide synthesis process 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 first, and then cracked under high temperature and high vacuum, but there are problems such as low product purity, difficult separation and high energy consumption; the one-step method directly converts lactic acid molecules into cyclic dimers under the action of a catalyst, which has significant advantages such as high reaction selectivity and simplified process. In recent years, it has been found that molecular sieve catalysts can efficiently catalyze the one-step synthesis of lactide from lactic acid, and its characteristics such as low cost and easy recovery of by-products provide the possibility for industrial application. However, the continuously generated water in this reaction process will trigger the reverse reaction of lactide, resulting in a decrease in the yield.

[0003] Due to the limitations of the vertical mass transfer mode of the existing reaction devices, it is difficult to achieve rapid removal of water, resulting in a low conversion rate of lactic acid, which has become the key bottleneck restricting the industrial development of the one-step method. Therefore, developing efficient water separation technology is the core challenge in promoting the industrialization process of preparing lactide by the one-step method of lactic acid. Summary of the Invention

[0004] To solve the problems mentioned in the above background art, the present invention provides a reaction device for preparing lactide by a one-step method of lactic acid and an application method thereof. This 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 by a one-step method from lactic acid, comprising a reaction layer, a water separation layer and a condensation layer arranged in sequence from bottom to top. The reaction layer is used for carrying out a lactic acid catalytic reaction to prepare lactide. A hollow-structured oil guide layer is arranged inside the reaction layer. A reaction tank is formed above the concave part of the oil guide layer, and a catalyst is arranged in the reaction tank. Heat-conducting oil is arranged inside the oil guide layer and is used 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 separators are arranged on the water separation plate. Water separation holes are arranged on the side surfaces of the water separators. The 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 separators. A cold water plate is arranged at the top of the condensation layer. The cold water plate is of a hollow structure, and condensed water passes through it. The bottom surface of the cold water plate and the top surface of the water separation layer form a closed space.

[0007] Further, the reaction tank, the oil guide layer, the water separation plate and the cold water plate are parallel to each other, and the angles of the reaction tank, the oil guide layer, the water separation plate and the cold water plate with the ground plane are 2 to 20°.

[0008] Further, 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 clamping plate, a baffle plate and the side. The water separation layer and the condensation layer are fixedly connected through a clamping plate.

[0009] Further, a feed inlet and a discharge outlet are arranged at both ends of the reaction tank. The feed inlet is used for introducing raw lactic acid and a solvent, and the product and the solvent are discharged from the discharge outlet. The outer walls of the feed inlet and the discharge outlet are cylindrical, and the lower half part inside is solid. The bottom surface where the material flows through the feed inlet and the discharge outlet is a plane.

[0010] Further, the feed inlet is connected with a slope. Raw lactic acid and a solvent are introduced through the feed inlet, flow through the slope and enter the reaction tank.

[0011] Further, the oil guide 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 arranged at both ends of the oil guide layer. The oil inlet and the oil outlet are externally connected to a circulation pump. The heat-conducting oil enters the oil guide layer through the oil inlet and is discharged from the oil outlet into the circulation pump. The temperature of the heat-conducting oil is 100 to 300 °C.

[0012] Further, the reaction tank is rectangular, circular or square, and the aspect ratio of its length to width or the ratio of its major axis to minor axis is 100:1 to 1:1; the ratio of its length to depth is 100:1 to 1:1; the material of the reaction tank is resin, stainless steel, cast iron, glass, ceramic or titanium alloy.

[0013] Further, 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 can also be other nanoporous molecular sieve materials with certain acidity; the solvent used is selected from at least one of benzene, toluene, xylene, mesitylene, straight-chain alkanes, and cycloalkanes.

[0014] Further, a water inlet and a water outlet are connected to the cold water plate, and a cold water circulator is externally connected to the water inlet and the water outlet; a side plate is arranged on the side of the condensation layer, a collection port is arranged on the side plate, and the inner side of the side plate is in contact with the water distribution plate.

[0015] Further, the water divider is conical, and the number is 4 to 20; there are 1 to 10 water distribution holes on each water divider.

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

[0017] Beneficial effects

[0018] For the reaction device for preparing lactide by one-step method from lactic acid of the present invention, the raw materials and solvents are fed into the reaction system by a flat-pushing method. During the flowing process, the water generated in the reaction process is continuously evaporated and removed through the water separation layer, and the designed condensation layer makes the removed water not return to the reaction system, effectively solving the problem that lactic acid depolymerizes due to the influence of water after catalysis, and avoiding the reaction of the water and lactide generated in the process of catalyzing lactic acid to prepare lactide to cause a depolymerization reaction, resulting in the conversion of lactide into lactic acid monomers or linear polymers, leading to a decrease in the selectivity of lactide. The reaction device of the present invention sets the water divider as a conical shape, sets water distribution holes on the side, and designs an inclined surface for the condensation layer, so that the evaporated water will not enter the reaction tank again but be directly discharged.

[0019] For the reaction device for preparing lactide by one-step method from lactic acid of the present invention, the optimal catalytic result shows a 100% conversion rate and a 99% lactide selectivity, which is much higher than that of the laboratory three-neck flask device, achieving good technical effects. Description of the drawings

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

[0021] Figure 2 Split view of the present invention

[0022] Figure 3 Right view of the present invention

[0023] Figure 4 Sectional view of the present invention

[0024] Figure 5 Front sectional view of the present invention

[0025] Figure 6 Structure diagram of the reaction layer of the present invention

[0026] Figure 7 Sectional view of the water separation layer of the present invention

[0027] Figure 8 Sectional view of the water separator of the present invention

[0028] Reference numerals: 1, condensation layer; 2, water separation layer; 3, reaction layer; 4, base; 5, cold water plate; 6, water inlet; 7, water outlet; 8, side plate; 9, collection port; 10, water separation plate; 11, water separator; 12, water separation hole; 13, clamping plate; 14, baffle; 15, oil guide layer; 1, slope; 17, discharge port; 18, feed port; 19, oil inlet; 20, side; 21, card slot; 22, oil outlet; 23, reaction tank Detailed implementation mode

[0029] Example 1

[0030] As Figures 1 to 8 shown, this example provides a reaction device in the process of catalytic preparation of lactide from lactic acid, including a base 4, a reaction layer 3, a water separation layer 2 and a condensation layer 1 arranged in sequence from bottom to top. The base 4, the reaction layer 3, the water separation layer 2 and the condensation layer 1 can be assembled and disassembled. The base 4 is connected to the reaction layer 3 through a card slot 21, and the reaction layer 3 is connected to the water separation layer 2 through a clamping plate 13, a baffle 14 and a side 20. The condensation layer 1 covers the water separation layer 2, and the water separation layer 2 and the condensation layer 1 are fixedly connected through a clamping plate 13. The four parts of the reaction device maintain a long side of L and a short side of D, and L / D = 10

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

[0032] A water separating plate 10 is arranged on the upper side of the water separating layer 2, a plurality of water separators 11 are arranged on the water separating plate 10, the water separators 11 are conical, and the number is 12; a water separating hole 12 is arranged on the side surface of the water separator 11, and each water separator has 1 water separating hole; the water vapor generated by the lactic acid catalytic reaction system reaches the water separating layer 2 and enters the condensation layer 1 through the water separating hole 12 on the water separator 11.

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

[0034] The reaction tank 23, the oil guide layer 15, the water separator plate 10, and the cold water plate 5 are parallel to each other and are all at an angle of 15° to the ground plane. This angle allows the reaction raw materials to slowly flow towards the outlet under the action of gravity, while enabling the water generated during the reaction to continuously evaporate upwards to reach the water separation layer and be further removed.

[0035] The spherical catalyst H-β zeolite (Si / Al = 15) is placed in the reaction tank 23 and fills the entire reaction tank 23. The raw material lactic acid and the solvent toluene are introduced through the feed inlet 18. The oil inlet 19 and the oil outlet 22 are externally connected to a circulating pump. The heat transfer oil enters the oil guide layer 15 through the oil inlet 19 and is discharged from the oil outlet 22 into the circulating pump. Due to the inclined plane design of the reaction tank 23, the raw material lactic acid and the solvent toluene continuously flow towards the discharge port 17 under the action of gravity after entering the reaction tank 23. Lactic acid reacts in the reaction tank 23 under the catalysis of the catalyst. The 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 holes 12 on the water separator 11. The water vapor continues to rise to reach the cold water plate 5, liquefies into water droplets, drips onto the water separator plate 10, converges into a water flow, and flows to the side plate 8 and is discharged from the collection 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 circulating machine. The product and the solvent are discharged from the discharge port 17 and sent to high performance liquid chromatography for product analysis.

[0036] The reaction conditions for the catalytic conversion of lactic acid to lactide are as follows: the raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the catalysis time is 5 h.

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

[0038] Example 2

[0039] This example provides a reaction device for the catalytic conversion of lactic acid to lactide. The difference from Example 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 Example 1.

[0040] The reaction conditions for the catalytic conversion of lactic acid to lactide are as follows: the raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the catalysis time is 5 h.

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

[0042] Example 3

[0043] This example provides a reaction device for the catalytic conversion of lactic acid to lactide. The difference from Example 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 Example 1.

[0044] The reaction conditions for the catalytic production of lactide from lactic acid are as follows: the raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the catalysis time is 5 h.

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

[0046] Example 4

[0047] This example provides a reaction device for the catalytic production of lactide from lactic acid. The difference from Example 1 is that the catalyst used is H-ZSM-5 molecular sieve, and the rest is the same as Example 1.

[0048] The reaction conditions for the catalytic production of lactide from lactic acid are as follows: the raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the catalysis time is 5 h.

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

[0050] Example 5

[0051] This example provides a reaction device for the catalytic production of lactide from lactic acid. The difference from Example 1 is that the catalyst used is H-Y molecular sieve, and the rest is the same as Example 1.

[0052] The reaction conditions for the catalytic production of lactide from lactic acid are as follows: the raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the catalysis time is 5 h.

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

[0054] Example 6

[0055] This example provides a reaction device for the catalytic production of lactide from lactic acid. The difference from Example 1 is that ramp 16 is not used, and the rest is the same as Example 1.

[0056] The reaction conditions for the catalytic production of lactide from lactic acid are as follows: the raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the catalysis time is 5 h.

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

[0058] Comparative Example 1

[0059] First, mix L-lysine, tetraethylammonium hydroxide, and deionized water at room temperature and stir until the solution is clear and transparent. Then, add potassium chloride and sodium metaaluminate and stir until the solution is clear and transparent. After that, slowly add silica white and stir for 12 h. Finally, transfer the gel precursor to a stainless-steel autoclave lined with polytetrafluoroethylene and crystallize at 140 °C for 4 days. Wash, centrifuge, and dry the crystallized mixed solution overnight, and then calcine it in a muffle furnace at 550 °C for 6 h to obtain β zeolite.

[0060] Place the prepared zeolite sample in 1 M NH4NO3 solution and perform ion exchange at 80 °C in an oil bath for 6 h. After repeating this process twice, wash, centrifuge, and dry it overnight. Finally, calcine it at 550 °C for 6 h to obtain H-β zeolite (Si / Al = 15).

[0061] The reaction conditions for the catalytic conversion of lactic acid to lactide are as follows: the raw material is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the reaction is catalyzed in a three-necked flask for 5 h.

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

[0063] Comparative Example 2

[0064] This comparative example provides a reaction device for the catalytic conversion of lactic acid to lactide. The difference from Example 1 is that there is no water separator on the water separation plate, and only the water holes at the water separator are retained, and the rest is the same as in Example 1.

[0065] The reaction conditions for the catalytic conversion of lactic acid to lactide are as follows: the raw material is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the reaction is catalyzed for 5 h.

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

[0067] Comparative Example 3

[0068] This comparative example provides a reaction device for the catalytic conversion of lactic acid to lactide. The difference from Example 1 is that there is no water separation layer, and the condensation layer and the reaction layer are connected by a baffle and a clamping plate, and the rest is the same as in Example 1.

[0069] The reaction conditions for the catalytic conversion of lactic acid to lactide are as follows: the raw material is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the reaction is catalyzed for 5 h.

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

[0071] Comparative Example 4

[0072] This comparative example provides a reaction device in the process of preparing lactide by catalytic reaction of lactic acid. The difference from Example 1 is that the water separator on the water separation plate is provided with one water separation hole, which is located at the top of the water separator, and the rest is the same as in Example 1.

[0073] The reaction conditions for preparing lactide by catalytic reaction of lactic acid are as follows: the raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the reaction is catalyzed for 5 h.

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

[0075] Comparative Example 5

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

[0077] The reaction conditions for preparing lactide by catalytic reaction of lactic acid are as follows: the raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the reaction is catalyzed for 5 h.

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

[0079] Comparative Example 6

[0080] This comparative example provides a reaction device in the process of preparing lactide by catalytic reaction of lactic acid. The difference from Example 1 is that the condensation plate is changed to a solid one, no condensed water is passed, and no condensation system is provided, and the rest is the same as in Example 1.

[0081] The reaction conditions for preparing lactide by catalytic reaction of lactic acid are as follows: the raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the reaction is catalyzed for 5 h.

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

Claims

1. A reaction device for preparing lactide by a one-step method of lactic acid, characterized in that, It includes a reaction layer, a water separation layer and a condensation layer arranged in sequence from bottom to top. The reaction layer is used for carrying out a lactic acid catalytic reaction to prepare lactide. A hollow-structured oil guide layer is arranged inside the reaction layer. A reaction tank is formed above the depression of the oil guide layer, and a catalyst is arranged in the reaction tank. Heat-conducting oil is arranged inside the oil guide layer and is used 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 separators are arranged on the water separation plate. Water separation holes are arranged on the side surface of the water separator. The 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 at the top of the condensation layer. The cold water plate is of a hollow structure, and condensed water passes through it. The bottom surface of the cold water plate and the top surface of the water separation layer form a closed space. The reaction tank, the oil guide layer, the water separation plate and the cold water plate are parallel to each other, and the angles of the reaction tank, the oil guide layer, the water separation plate and the cold water plate with the ground plane are 2-20°.

2. The reaction device for preparing lactide by a one-step method of lactic acid 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 clamping plate, a baffle plate and the side. The water separation layer and the condensation layer are fixedly connected through a clamping plate.

3. The reaction device for preparing lactide by one-step method of lactic acid according to claim 1, characterized in that, An inlet and an outlet are arranged at both ends of the reaction tank. The inlet is used for introducing raw lactic acid and a solvent, and the product and the solvent are discharged from the outlet. The outer walls of the inlet and the outlet are cylindrical, and the lower half of the inside is solid. The bottom surface of the material flowing through the inlet and the outlet is flat.

4. The reaction device for preparing lactide by one-step method of lactic acid according to claim 3, characterized in that, The inlet is connected with a slope. Raw lactic acid and a solvent are introduced through the inlet, flow through the slope and enter the reaction tank.

5. The reaction device for preparing lactide by one-step method of lactic acid according to claim 1, characterized in that, The oil guide 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 arranged at both ends of the oil guide layer. The oil inlet and the oil outlet are externally connected to a circulation pump. The heat-conducting oil enters the oil guide layer through the oil inlet and is discharged from the oil outlet into the circulation pump. The temperature of the heat-conducting oil is 100-300°C.

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

7. The reaction device for preparing lactide by a one-step method of lactic acid according to claim 1, characterized in that, The catalyst is at least one of BEA, FAU, MOR; the solvent used is selected from at least one of toluene, xylene and mesitylene.

8. The reaction device for preparing lactide by a one-step method of lactic acid according to claim 1, characterized in that, An inlet and an outlet are connected to the cold water plate. The inlet and the outlet are externally connected to a cold water circulator; a side plate is arranged on the side of the condensation layer. A collection port is arranged on the side plate, and the inner side of the side plate is in contact with the water separation plate.

9. Method for applying the reaction device for preparing lactide by one-step method of lactic acid according to any one of claims 1 to 8, characterized in that, It includes: The catalyst is placed in the reaction tank. Raw lactic acid and a solvent are introduced into the reaction layer. Heat-conducting 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. The water in the lactic acid catalytic reaction system rises in the form of water vapor to the water separation layer, and rises through the water separation holes on the water separator to 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 and is discharged; the product and the solvent generated by the reaction tank are discharged from the outlet of the reaction tank.

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