阅读说明：本技术 一种循环制备丙交酯的设备 (Equipment for circularly preparing lactide ) 是由 张颂培 翁云宣 付烨 张学武 刁晓倩 于 2021-10-28 设计创作，主要内容包括：本发明提供了一种循环制备丙交酯的设备,包括混料罐、反应罐、真空冷凝器、纯化罐和真空泵,混料罐上安装第一搅拌器,混料罐罐体顶部开设回料口和进料口,进料口与反应罐顶部连通；反应罐顶部还开设导气口,导气口与真空冷凝管连通；反应罐外壁包覆加热夹套,反应罐内部中轴具有至少两层转盘筛板桨,反应罐内壁上设置至少两层固定筛板组,转盘筛板桨与固定筛板交错间隔设置,其上均开设若干筛孔,反应罐底部通过循环管道与纯化罐顶部连接。本发明利用转盘筛板薄膜反应罐结合降温、析晶、过滤一体化技术,使得丙交酯脱离反应体系,乳酸脱水平衡向着生成丙交酯方向移动,既提高了传质效率及反应速率,又同步脱水环化提高了丙交酯收率。(The invention provides equipment for circularly preparing lactide, which comprises a mixing tank, a reaction tank, a vacuum condenser, a purification tank and a vacuum pump, wherein a first stirrer is arranged on the mixing tank; the top of the reaction tank is also provided with an air guide port which is communicated with the vacuum condensation pipe; the heating jacket is wrapped on the outer wall of the reaction tank, at least two layers of rotary disc sieve plate paddles are arranged on a middle shaft in the reaction tank, at least two layers of fixed sieve plate groups are arranged on the inner wall of the reaction tank, the rotary disc sieve plate paddles and the fixed sieve plates are arranged in a staggered and spaced mode, a plurality of sieve holes are formed in the rotary disc sieve plate paddles, and the bottom of the reaction tank is connected with the top of the purification tank through a circulating pipeline. The invention utilizes the integration technology of cooling, crystallization and filtration of the turntable sieve plate film reaction tank, so that the lactide is separated from the reaction system, the dehydration balance of the lactic acid moves towards the direction of generating the lactide, the mass transfer efficiency and the reaction rate are improved, and the yield of the lactide is improved by synchronous dehydration and cyclization.)

1. The equipment for circularly preparing lactide is characterized by comprising a mixing tank (1), a reaction tank (2), a vacuum condenser (3), a purification tank (4) and a vacuum pump (5), wherein a first stirrer (12) is arranged on the mixing tank, the top of the mixing tank body is provided with a feed back port (14) and a feed inlet (11), and the feed inlet is communicated with the top of the reaction tank;

the top of the reaction tank is also provided with an air guide port (24) which is communicated with the vacuum condensation pipe; the outer wall of the reaction tank is coated with a heating jacket (21), the middle shaft in the reaction tank is provided with at least two layers of rotary disc sieve plate paddles (22), the inner wall of the reaction tank is provided with at least two layers of fixed sieve plate groups (23), the rotary disc sieve plate paddles and the fixed sieve plates are arranged in a staggered and spaced mode, and the bottom of the reaction tank is connected with the top of the purification tank through a circulating pipeline (8);

the vacuum condenser comprises a condensation sleeve (31) and a liquid collecting tank (34) arranged at the bottom of the condensation sleeve, the lower part of the condensation sleeve is provided with an air inlet (32) which is communicated with an air guide port, the condensation sleeve is provided with a vacuum suction port (33), the vacuum suction port is hermetically communicated with a vacuum pump (5), one side of the top of the liquid collecting tank is provided with an entrainer overflow port (35), and the entrainer overflow port is communicated with the bottom of the reaction tank through a circulating pipeline;

the bottom of the reaction tank is communicated with the purification tank, a second stirrer (41) is arranged at the top of the purification tank, a product discharge hole (42) is formed in one side of the bottom of the reaction tank, a filter screen (44) is arranged at the bottom of the purification tank, and the bottom of the purification tank is connected with a feed back hole of the mixing tank through a circulating pipeline.

2. The apparatus for cyclic production of lactide according to claim 1, wherein a turntable motor (25) is installed on top of the reaction tank, and the turntable screen paddle is driven by the turntable motor.

3. The apparatus for cyclically preparing lactide according to claim 1, wherein the rotating disc sieve plate paddle is composed of a plurality of blades (221) fixed along the rotating shaft, the blades are uniformly provided with rotating disc sieve holes (222), and the diameter of each rotating disc sieve hole is 0.1-2 cm.

4. The apparatus for cyclically producing lactide according to claim 3, wherein the paddles of the rotating disk screen plate paddles of adjacent layers are arranged in a staggered manner.

5. The apparatus for cyclically producing lactide according to claim 3, wherein the diameter of the mesh of the rotary table increases from the upper layer to the lower layer.

6. The apparatus for cyclically preparing lactide according to claim 1, wherein the fixed sieve plate is of a paddle-shaped or arc-shaped protruding structure protruding inwards from the inner wall of the reaction tank, and the fixed sieve plate protrudes inwards to partially overlap with the outer edges of the paddles of the rotating disc sieve plate paddles in an up-and-down staggered manner.

7. The apparatus for cyclically preparing lactide according to claim 1, wherein the fixed sieve plates are uniformly provided with fixed sieve holes (231), and the diameter of each fixed sieve hole is 0.1-2 cm.

8. The apparatus for the cyclic production of lactide according to claim 1, wherein the outer wall of the purification tank is covered with a heat exchange jacket (42).

9. The equipment for the cyclic production of lactide according to claim 1, wherein the bottom of the liquid collecting tank is provided with a water discharge port (36).

10. The apparatus for cyclically preparing lactide according to claim 1, wherein the outer wall of the circulation pipeline is coated with an insulating layer.

Technical Field

The invention relates to the field of chemical raw material synthesis, in particular to equipment for circularly preparing lactide.

Background

With the continuous advance of industrialization, the problem of environmental pollution is widely concerned and regarded by society. White pollution control is also a hot topic of research in industry and academia. Polylactic acid has biodegradability, good mechanical property and processability, is an environment-friendly material with wide application, and is mainly prepared by a lactide ring-opening polymerization method.

In the prior art, the preparation of lactide mainly adopts a two-step method: heating lactic acid at 140-170 ℃ for dehydration to form low molecular weight polylactic acid; then under the high vacuum condition of 210 ℃ and 250 ℃, the oligomer is cracked and cyclized to lactide under the action of a catalyst.

The reaction formula is as follows:

the lactide formed in the process generally contains impurities such as water, lactic acid oligomer and the like, so further dehydration and purification are technical problems for preparing lactide monomers. The operation process often has the problems of low water removal efficiency, more reaction byproducts, low utilization rate of lactic acid and the like. In the polycondensation stage which plays a key role in the process for preparing lactide by direct polycondensation of lactic acid, although water-carrying agents and the like are added in the process, the generated lactide is not separated from a reaction system, so that a large amount of oligolactic acid can be cracked and converted into lactide only at a higher temperature of 230 ℃, and the high-temperature high-vacuum energy consumption is high, the conversion rate is low, and the molecular weight of a polymer in the next step is easily influenced by racemization.

Disclosure of Invention

In order to solve the technical problems, the invention provides equipment for circularly preparing lactide by catalyzing lactic acid solution with hydrogen acid, which can realize that the lactide is quickly crystallized, separated out and separated from a reaction system, and reaction mother liquor can be circularly prepared. The reaction is simple to control, and the product has high purity, low reaction temperature and controllable optical activity.

The invention provides equipment for circularly preparing lactide, which comprises a mixing tank, a reaction tank, a vacuum condenser, a purification tank and a vacuum pump, wherein a first stirrer is arranged on the mixing tank;

the top of the reaction tank is also provided with an air guide port which is communicated with the vacuum condensation pipe; the outer wall of the reaction tank is coated with a heating jacket, the central shaft in the reaction tank is provided with at least two layers of rotary disc sieve plate paddles, the inner wall of the reaction tank is provided with at least two layers of fixed sieve plate groups, the rotary disc sieve plate paddles and the fixed sieve plates are arranged in a staggered and spaced mode, a plurality of sieve holes are formed in each rotary disc sieve plate paddle, and the bottom of the reaction tank is connected with the top of the purification tank through a circulating pipeline;

the vacuum condenser comprises a condensation sleeve and a liquid collecting tank arranged at the bottom of the condensation sleeve, the lower part of the condensation sleeve is provided with an air inlet which is communicated with an air guide port, the condensation sleeve is provided with a vacuum suction port, the vacuum suction port is hermetically communicated with a vacuum pump, one side of the top of the liquid collecting tank is provided with an entrainer overflow port, and the entrainer overflow port is communicated with the bottom of the reaction tank through a circulating pipeline;

the reaction tank bottom communicates with the purification jar, and purification tank top installation second agitator, and the product discharge gate is seted up to bottom one side, and bottom installation filter screen in the purification jar, purification jar bottom are connected through the feed back mouth of circulating line with the compounding jar.

Furthermore, a turntable motor is arranged at the top of the reaction tank, and a turntable sieve plate paddle is driven by the turntable motor.

Furthermore, the rotary disc sieve plate paddle is composed of a plurality of blades fixed along the rotating shaft, rotary disc sieve holes are uniformly distributed on the blades, and the diameter of each sieve hole is 0.1-2 cm.

Further, the paddles of the rotary disc sieve plate paddles on two adjacent layers are arranged in a staggered mode.

Further, the diameter of the sieve holes of the rotary table increases from the upper layer to the lower layer.

Furthermore, the fixed sieve plate is of a paddle-shaped or arc-shaped protruding structure protruding inwards from the inner wall of the reaction tank, and protrudes inwards to be partially overlapped with the outer edge of a paddle of the rotary disc sieve plate paddle in a vertically staggered manner.

Furthermore, the fixed sieve plates are uniformly provided with fixed sieve pores, and the diameter of each fixed sieve pore is 0.1-2 cm.

Further, the outer wall of the purification tank is covered with a heat exchange jacket.

Furthermore, a water outlet is formed in the bottom of the liquid collecting tank.

Furthermore, the outer wall of the circulating pipeline is coated with an insulating layer.

The invention utilizes the integration technology of cooling, crystallization and filtration of the turntable sieve plate film reaction tank, so that the lactide is separated from the reaction system, the dehydration balance of the lactic acid moves towards the direction of generating the lactide, the mass transfer efficiency and the reaction rate are improved, and the yield and the purity of the lactide are improved by synchronous dehydration and cyclization. The reaction is carried out homogeneously, and the dehydrated hydrogen-containing acid quickly catalyzes the reaction for polycondensation; the used hydrogen-containing acid is cheap and easy to obtain, and the dosage of the catalyst is small; the entrainer is used for removing water and the lactide mother liquor is used for double-circulation reaction, the operation temperature is low, the energy consumption is low, and the industrial implementation is easy.

Mixing hydrogen-containing acid, an entrainer and a lactic acid solution, removing water by using the entrainer in a heating and reduced pressure distillation mode, and then carrying out catalytic polycondensation reaction to obtain lactide; cooling and crystallizing the obtained product to separate crude lactide; and further washing, filtering and drying the crude lactide crystal obtained in the purification tank by using a solvent, wherein the operation temperature below 180 ℃ is far lower than the vacuum rectification temperature of the lactide at 230 ℃, and the obtained lactide has controllable optical rotation and high purity. The mother liquor circularly supplements lactic acid to continue reaction, the crude lactide is washed by solvent, filtered and dried to obtain refined lactide, the purity of the obtained product can reach more than 98.3 percent, and the yield reaches more than 86 percent.

Drawings

Fig. 1 is a schematic diagram of a lactide synthesis apparatus.

In the figure: 1-mixing tank; 11-a feed inlet; 12-a first stirrer; 13-an exhaust port; 14-a feed back port; 2-a reaction tank; 21-heating jacket; 22-rotating disc sieve plate paddle; 221-a paddle; 222-rotating disc sieve holes; 23-fixing the sieve plate; 231-fixing sieve pores; 24-a gas guide port; 25-a turntable motor; 26-a first bleeder valve; 3-vacuum condenser; 31-a condenser sleeve; 32-an air inlet; 33-vacuum suction; 34-a liquid collecting tank; 35-entrainer overflow; 36-a water discharge port; 4-a purification tank; 41-a second stirrer; 42-product discharge port; 43-heat exchange jacket; 44-a filter screen; 45-a second bleeder valve; 5-a vacuum pump; 6-a first delivery pump; 7-a second delivery pump; 8-a circulation pipeline.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and examples.

The invention provides equipment for circularly preparing lactide, which comprises a mixing tank 1, a reaction tank 2, a vacuum condenser 3, a purification tank 4 and a vacuum pump 5, wherein a first stirrer 12 is arranged on the mixing tank 1, the top of the tank body of the mixing tank 1 is provided with a feed back port 14 and a feed inlet 11, and the feed inlet 11 is communicated with the top of the reaction tank 2. The reaction materials are firstly added into the mixing tank 1 for mixing and then are introduced into the reaction tank 2 through the circulating pipeline 8. The top of the mixing tank 1 is provided with an exhaust port 13 for discharging gas in the feeding and stirring processes, so that the safety of the mixing tank 1 is ensured.

The top of the reaction tank 2 is also provided with an air guide port 24, and the air guide port 24 is communicated with the vacuum condensation pipe 3; the heating jacket 21 is coated on the outer wall of the reaction tank 2, and the heating jacket 21 can be heated by water vapor or heat-conducting oil electrically, so as to improve the reaction rate of lactide and accelerate the evaporation and discharge of water generated by the reaction.

The inside axis of retort 2 has two-layer at least carousel sieve oar 22, sets up two-layer at least fixed sieve group 23 on the 2 inner walls of retort, and carousel sieve oar 22 sets up with fixed sieve 23 crisscross interval, all sets up a plurality of sieve meshes on it, and 2 bottoms of retort are passed through circulating line 8 and are connected with purification jar 4 tops. Further, a turntable motor 25 is installed on the top of the reaction tank 2, and the turntable screen paddle 222 is driven by the turntable motor 25. The rotary disc sieve plate paddle 22 is composed of a plurality of blades 221 fixed along the rotating shaft, rotary disc sieve holes 222 are uniformly distributed on the blades, and the diameter of each sieve hole is 0.1-2 cm. In order to improve reactant mixing and stirring, the blades 221 of the rotary disc sieve plate paddles of the two adjacent layers are arranged in a staggered mode. The diameter of the disk openings 222 increases from the upper tier to the lower tier. The fixed sieve plate 23 is a paddle-shaped or arc-shaped protruding structure protruding inwards from the inner wall of the reaction tank 2, and protrudes inwards to be overlapped with the outer edge of the paddle of the rotary disc sieve plate paddle 22 in a staggered manner. The fixed sieve plates 23 are uniformly provided with fixed sieve holes 231. Carousel sieve board oar 22 sets up with fixed sieve 23 is crisscross, flows into the reactant from the top, is driven by pivoted carousel sieve board oar 22, and high-efficient stirring is strikeed with fixed sieve 23, improves reaction efficiency. The fluid reactant is stirred, and under the action of gravity, a part of the fluid reactant flows downwards from the rotary disc sieve plate paddle 22 and the fixed sieve plate 23, and a part of the fluid reactant is sieved from the rotary disc sieve holes 222 and the fixed sieve holes 231, so that the surface area of the fluid reactant is greatly increased. Under the action of high temperature, on one hand, the contact reaction between reactants is improved, and on the other hand, water generated by the reaction is quickly evaporated and carried by the entrainer and is sucked and separated by a gas guide port 24 under negative pressure.

The mixed product after the reaction of the reactants in the reaction tank 2 is discharged into the next treatment process, namely into the purification tank 4, under the control of the first discharge valve 26 and the first delivery pump 6 on the circulating pipeline 8. The outer wall of the purification tank 4 is coated with a heat exchange jacket 43, the heat exchange jacket 43 is used for cooling the purification tank 4, the lactide crude crystals are promoted to be separated out from the mixed reactant, the purification tank 4 is utilized for filtering and collecting, and the lactide crude crystals are washed, filtered and dried after the reaction is finished and then discharged from a product discharge hole 42.

The vacuum condenser 3 comprises a condensation sleeve 31 and a liquid collecting tank 34 arranged at the bottom of the condensation sleeve 31, wherein the lower part of the condensation sleeve 31 is provided with an air inlet 32, the air inlet 32 is communicated with an air guide port 24 at the top of the reaction tank 2, the condensation sleeve 31 is provided with a vacuum air suction port 33, the vacuum air suction port 33 is hermetically communicated with the vacuum pump 5 through an air suction pipeline, and a liquid blocking plug is arranged at the opening of the air suction pipeline. An entrainer overflow port 35 is formed in one side of the top of the liquid collection tank 34, and the entrainer overflow port 35 is communicated with the bottom of the reaction tank 2 through a circulating pipeline 8. Preferably, the bottom of the liquid collection tank 34 is provided with a water outlet 36. The liquid condensed by the vacuum condenser 3 flows to a bottom liquid collecting tank 34, an entrainer overflow port 35 is arranged at the top of the liquid collecting tank 34, light liquid components, namely the entrainer, in the liquid collecting tank 34 are led out from the entrainer overflow port 35 and conveyed to the bottom of the reaction tank 2 to be circularly heated and lifted, and a water outlet 36 is arranged at the bottom of the liquid collecting tank 34 to discharge the taken water.

2 bottoms of retort are passed through circulating line 8 and are communicated with purification jar 4, and 4 top installation second agitators 41 of purification jar, product discharge gate 42 are seted up to bottom one side, and bottom installation filter screen 44 in the purification jar 4, have in the purification jar 4 by the rotatory scraper of second agitator 41 driven, and the lactide crystal that will mix among the reaction product is cooled down on 4 inner walls of purification jar and is appeared, and rotatory scraper stirring is scraped off, and the lactide crystal of being convenient for is discharged from product discharge gate 42. And (3) transforming the purification tank 4, arranging a washing liquid discharge port isolated from the original reaction filtrate at the bottom of the purification tank 4, further washing the crude lactide crystal by using ethyl acetate in the purification tank 4, and drying and filtering to obtain the refined lactide crystal.

The bottom of the purification tank 4 is connected with a feed back port 14 of the mixing tank 1 through a circulating pipeline 8, filtered reactant filtrate flows back to the mixing tank 1 through a second discharge valve 45 and a second delivery pump 6 on the circulating pipeline 8, and the mixture is continuously mixed in the mixing tank 1 and then enters the reaction tank 2 for circular reaction until the reactant reaction is complete as much as possible. In the invention, the process of the lactide reaction process is separated, the discharge of water in the reaction product is accelerated, the forward reaction is realized, the yield of the lactide is improved under the action of the catalyst, the separation is carried out by utilizing the purification tank 4 of the independent tank body, and the original reactant can be recycled until the lactide is generated, and then the lactide is separated out and collected. The lactide solution obtained by the reaction is conveyed to a purification tank 4 through a pipeline and a first conveying pump 6 from a reaction tank 2, after cooling, crystallization and filtration, the filtered mother liquor is recovered and is re-injected into a material mixing tank 1 for utilization, and the mother liquor is conveyed into the material mixing tank 1 through a second conveying pump 7 to be mixed with unreacted reactants and then is introduced into the reaction tank 2 for circulating and participating in the reaction.

Furthermore, the outer wall of the circulating pipeline 8 is coated with a heat-insulating layer, the heat-insulating layer can ensure the temperature stability of the whole circulating pipeline 8, the treatment temperatures of the two processes are well connected, and no adverse effect is caused on the whole reaction.

The method for preparing lactide by using the equipment comprises the following steps:

(a) adding a lactic acid solution, a hydrogen-containing acid catalyst and an entrainer into a mixing tank 1, stirring and mixing, and introducing the mixture serving as a reactant into a reaction tank 2;

(b) the reactants are heated and catalytically condensed in a reaction tank 2 to obtain a lactide solution, the lactide solution is subjected to reduced pressure distillation in the reaction tank 2, an entrainer is evaporated to carry water vapor into a vacuum condenser 3, the entrainer flows back into the reaction tank 2 after condensation and water separation, the evaporation is continued, and the water vapor is carried into the vacuum condenser 3;

(c) the lactide solution which flows to the bottom of the reaction tank 2 after stirring reaction is conveyed into a purification tank 4 through a first conveying pump 6, crude lactide crystals are obtained after cooling, crystallization and filtration, and lactide mother liquor obtained after filtration is conveyed into a mixing tank 1 through a second conveying pump 7 to be mixed with reactants and then is introduced into the reaction tank 2 to participate in reaction circularly;

(d) and when the crystal reaches 30-50% of the volume of the purification tank, further washing, filtering and drying the crude lactide crystal in the purification tank to obtain refined lactide, and discharging the refined lactide from a product discharge port 42 of the purification tank 4.

The lactic acid solution is a 60 wt% -90 wt% lactic acid aqueous solution, and in a preferred technical scheme, the concentration of the lactic acid aqueous solution is 75 wt% -85 wt%.

The hydrogen-containing acid catalyst is one or more of sulfuric acid, benzenesulfonic acid and p-toluenesulfonic acid; the volume ratio of the addition volume to the lactic acid solution is 0.2-1.5%.

The entrainer in the step (a) is one or a mixture of a plurality of toluene, xylene, diphenyl ether, dipropylene glycol methyl ether, diethylene glycol monomethyl ether, ethylene glycol butyl ether, ethylene glycol monoethyl ether, n-octane, butyl acetate and propylene glycol monoethyl ether. In a preferred technical scheme, the ratio of the addition volume of the entrainer to the volume of the lactic acid solution is 20-60%.

In step (b), retort 2 is multilayer carousel sieve film retort, the axis has the multilayer by carousel motor 25 coaxial drive's carousel sieve oar 22 in retort 2, set up multilayer fixed sieve 23 on retort 2 inner wall, carousel sieve oar 22 sets up with fixed sieve 23 crisscross interval, a plurality of sieve meshes have all been seted up on it, the sieve mesh diameter is 0.5-2cm, the sieve mesh diameter from top to bottom scales up, upper portion solution viscosity is lower, go back the continuous evaporation of steam and take out along with going on of reaction, 2 bottom solution viscosity of retort increase gradually, in order to guarantee the stirring resistance equilibrium, the continuous stirring, the sieve mesh suitable increase with retort 2 lower part, can guarantee the unobstructed passageway that bottom volatile gas rises simultaneously.

And heating the reaction tank 2 to 90-180 ℃ for catalytic condensation reaction, wherein the catalytic condensation reaction time is 2-5 hours. The reaction tank 2 is heated to carry out the polycondensation reaction, simultaneously the entrainer, the water in the solution and the water generated by the polycondensation reaction are evaporated, the entrainer in the gas phase entrains the water vapor to be continuously sucked away from the gas guide port 24 at the top of the reaction tank 2 by the vacuum pump 5, the water content in the reaction tank 2 is reduced, and the reaction is carried out towards the oligomerization and cyclization directions. The lactic acid after dehydration is rapidly subjected to polycondensation reaction under the catalysis of hydrogen-containing acid to generate low molecular weight polylactic acid such as dimerization and trimerization, lactide and the like, and the generated low molecular weight polylactic acid such as dimerization and trimerization is continuously subjected to catalytic cracking and cyclization to form the lactide.

In the step (b), the pressure of the reduced pressure distillation is 80 to 95 kPa. The vacuum distillation is performed to accelerate the evaporation rate of the entrainer and water and improve the reaction efficiency; when the system pressure is lower than 80kPa, the energy consumption is increased and the reaction rate is decreased.

In the step (b), the vacuum distillation further comprises the following steps: an air guide port 24 is arranged above the reaction tank 2, the air guide port 24 is connected with an air inlet 32 of the vacuum condenser 3 through a pipeline, an air outlet 33 of the vacuum condenser 3 is connected with the vacuum pump 5, liquid condensed by the vacuum condenser 3 flows into a liquid collecting tank 34 at the bottom of the vacuum condenser 3, an entrainer overflow port 35 is arranged at the upper part of the liquid collecting tank 34, and light liquid components in the liquid collecting tank 34, namely the entrainer, are guided out from the entrainer overflow port 35 and are conveyed to the lower part of the reaction tank 2 to circularly participate in the reaction.

Wherein, the temperature reduction crystallization in the step (c) is to reduce the temperature to 40-60 ℃ in the purification tank 4. And (c) further washing the crude lactide crystal obtained in the step (c) by using ethyl acetate, drying and filtering to obtain a refined lactide crystal.

The invention adopts the hydrogen-containing acid as the catalyst and utilizes the reaction tank with the multilayer rotary disc sieve plate to fully stir and react, thereby reducing the reaction temperature and simultaneously improving the reaction efficiency, and through the cyclic batch reaction process, the yield of the final product is greatly and obviously improved, and simultaneously the productivity consumption is obviously reduced. The preparation reaction of the lactide is carried out in a homogeneous phase state, lactic acid solution is directly condensed under the catalysis of hydrogen-containing acid, the dehydrated lactic acid is rapidly subjected to polycondensation and cyclization under the catalysis of the hydrogen-containing acid, and oligomers in the circulating lactide mother liquor are continuously cracked and cyclized to form the lactide; the used hydrogen-containing acid is cheap and easy to obtain, and the dosage of the catalyst is small; the entrainer is used for removing water and the lactide mother liquor is used for double-circulation reaction, the operation temperature is low, the energy consumption is low, and the industrial implementation is easy.

The embodiments of the present invention are merely illustrative and not restrictive, and those skilled in the art can modify the embodiments without inventive contribution as required after reading the present specification, but the present invention is protected by patent law within the scope of the appended claims.