A spiral reactor for preparing lactide by one-step lactic acid and its application method

Abstract

This invention discloses a spiral reactor for one-step preparation of lactic acid to lactide and its application method, belonging to the technical field of lactic acid to lactide preparation. It includes a vessel body with a reaction chamber, a heating unit disposed within the reaction chamber, and several spiral pipes placed within the reaction chamber and surrounding the heating unit. Several through holes are formed in the pipe walls facing away from gravity. An inlet at the top of the spiral pipe extends to the outside of the vessel body, and an outlet at the bottom of the spiral pipe connects to a collection chamber at the bottom of the reaction chamber. The vessel body has an exhaust port connected to the reaction chamber and a drain port connected to the collection chamber. This spiral reactor and its application method for one-step preparation of lactic acid to lactide promote uniform flow of lactic acid and organic solvent through the catalyst bed to achieve sufficient contact and reduce the differences in reaction progress in different regions.

Description

Technical Field

[0001] This invention belongs to the field of lactic acid to lactide technology, specifically relating to a spiral reactor for one-step lactic acid to lactide and its application method. Background Technology

[0002] Polylactic acid (PLA), a high-performance biodegradable polymer, can be completely degraded into carbon dioxide and water, making it a core material for replacing traditional petroleum-based plastics and alleviating marine plastic pollution and "white pollution." In the industrial production of PLA, lactide, as a key cyclic dimer precursor, directly determines the quality and industrial scale of PLA products through its synthesis efficiency, purity, and cost.

[0003] Chinese invention patent application No. 2025105112567 discloses a reaction apparatus and its application method for one-step preparation of lactic acid and lactide. This apparatus employs a layered structure and a push-flow design, allowing water vapor generated during the reaction to be directionally removed sequentially through a water separator and a condenser layer, thereby effectively reducing the hydrolysis side reaction of the lactide product. However, while this apparatus achieves high lactic acid conversion and good lactide selectivity under experimental conditions with small-volume feed, significant technical shortcomings remain for industrial-scale applications, specifically: (1) The toluene solvent used in the reaction has poor miscibility with lactic acid material. Under small feed conditions, the two phases can maintain a relatively stable feed flow matching relationship. Once the feed scale is increased, the two phases are very easy to separate into layers, breaking the material flow matching relationship set by the original process, thus causing the molar ratio of each component in the reaction system to deviate from the design optimal value, causing the catalytic reaction to deviate from the optimal process range. (2) Due to the high viscosity of lactic acid material itself, it is easy to form wall-adhering flow and deflection phenomenon along the bottom surface of the reaction tank in this device. This causes some high viscosity lactic acid material to bypass the catalyst bed and flow directly out of the reactor without fully contacting the catalytic active sites, thereby greatly reducing the proportion of material participating in the effective catalytic reaction, and ultimately causing a significant decline in the overall catalytic reaction efficiency and product selectivity. (3) The raw material lactic acid and solvent flowing on the surface of the reaction tank in the device are mainly heated by the heat transfer oil circulating in the oil guide layer. At the same time, the raw material lactic acid and solvent will continuously consume heat when they react with the catalyst. This will cause the heat transfer oil flowing from the oil inlet to the oil outlet in the oil guide layer to have a significant temperature deviation. Consequently, the reaction temperature in different areas on the surface of the reaction tank will be uneven, and the reaction process in each area will also be different. Ultimately, this will reduce the evaporation rate of the reaction-generated water, thereby affecting the efficiency of the reaction-generated water leaving the reaction system. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a spiral reactor for one-step preparation of lactic acid and its application method, which can promote the uniform flow of lactic acid and organic solvent through the catalyst bed to achieve full contact and reduce the differences in reaction process in different regions.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a spiral reactor for one-step preparation of lactic acid lactide, comprising a body having a reaction chamber, a heating unit disposed in the reaction chamber, and a plurality of spiral pipes disposed in the reaction chamber and surrounding the outer periphery of the heating unit; The spiral pipe has several through holes on its wall facing away from gravity. The feed inlet at the top of the spiral pipe extends to the outside of the device body. The liquid outlet at the bottom of the spiral pipe is connected to the collection chamber at the bottom of the reaction chamber. The device body is provided with an exhaust port connected to the reaction chamber and a drain port connected to the collection chamber.

[0006] Optionally, the position of the liquid outlet communicating with the collection chamber is higher than the bottom of the reaction chamber, and the device body is provided with a second liquid outlet communicating with the bottom of the reaction chamber.

[0007] Optionally, a plurality of the spiral pipes are arranged in a circumferential array along the heating unit, and a gap is reserved between the outer wall of the spiral pipes and the heating unit.

[0008] Optionally, the feed inlet is used to introduce fully mixed and preheated lactic acid and organic solvent.

[0009] Optionally, the heating unit includes a hollow cylinder and a heater disposed within the hollow cylinder; The hollow column is placed upright in the reaction chamber and is isolated from the internal cavity of the reaction chamber.

[0010] Optionally, the through holes are evenly spaced.

[0011] A method for using a spiral reactor in a one-step process for preparing lactide from lactic acid includes: Step 1: Mix and preheat the lactic acid raw material and organic solvent, and fill the spiral pipe with catalyst. Then start the heating unit to raise the temperature to the reaction temperature and preheat the spiral pipe and the catalyst filled in the spiral pipe. Step 2: The preheated mixture is fed into the spiral pipe through the feed inlet. The lactic acid raw material reacts under the catalysis of the catalyst filled in the spiral pipe to produce lactide and water. The water produced by the reaction forms an azeotropic vapor with the organic solvent. The azeotropic vapor enters the reaction chamber through several through holes and is discharged through the exhaust port. The lactide solution produced by the reaction flows along the spiral pipe to the collection chamber for centralized storage.

[0012] Optionally, the reaction temperature is 120℃~250℃, and the reaction chamber is maintained at atmospheric pressure or reduced pressure.

[0013] Optionally, the organic solvent is selected from one or more of toluene, xylene, and cyclohexane.

[0014] Optionally, after the azeotropic vapor in step two is condensed and separated from the oil and water, the organic solvent obtained can be reused for mixing and preheating with the lactic acid raw material.

[0015] Compared with existing technologies, the beneficial effects achieved by this invention are as follows: By setting up a spiral pipe surrounding the heating unit, the spiral pipe and the catalyst filled inside it can be heated uniformly, thereby reducing the differences in catalytic reactions in different regions. Simultaneously, by filling the spiral pipe with catalyst to construct a catalyst bed, large quantities of lactic acid and organic solvents, after being fed into the spiral pipe through the inlet, can be dispersed and flow under the influence of gravity and the catalyst's guiding effect to fully contact the catalyst filled inside the spiral pipe, thereby effectively increasing the proportion of materials participating in the effective catalytic reaction and meeting the requirements for large-volume lactide preparation. Attached Figure Description

[0016] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0017] Figure 1 This is a schematic diagram of the spiral reactor in a preferred embodiment of the present invention; Figure 2 This is a cross-sectional view of the spiral reactor in a preferred embodiment of the present invention; Figure 3 This is a schematic diagram of the structure of the spiral pipe surrounding the heating unit and connected to the reaction chamber in a preferred embodiment of the present invention; Among them, 1. Body; 101. Reaction chamber; 102. Collection chamber; 103. Exhaust port; 104. Drain port one; 105. Drain port two; 106. Filter hole; 2. Heating unit; 201. Hollow cylinder; 202. Heater; 3. Spiral pipe; 301. Through hole; 302. Feed inlet; 303. Liquid outlet. Detailed Implementation

[0018] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. These drawings are simplified schematic diagrams, which are only used to illustrate the basic structure of the present invention and therefore only show the components relevant to the present invention.

[0019] It should be noted that if directional indicators (such as up, down, bottom, top, etc.) are involved in this embodiment, these directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly. The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, features defined with "first" and "second" may explicitly or implicitly include one or more of that feature. Unless otherwise explicitly specified and limited, the terms "set," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal connection of two components. For those skilled in the art, the specific meaning of the above terms in this invention can be understood according to the specific circumstances. Example 1

[0020] like Figures 1-3As shown, a spiral reactor for one-step preparation of lactic acid to lactide includes a vessel body 1 with a reaction chamber 101, a heating unit 2 disposed within the reaction chamber 101, and several spiral pipes 3 disposed within the reaction chamber 101 and surrounding the heating unit 2. The spiral pipes 3 are continuous pipes with a spiral central axis. Their inner cavities are filled with a catalyst for the preparation of lactic acid to lactide. In this technical solution, the catalyst used is a solid catalyst or a catalyst-supported packing material to construct a catalyst bed within the spiral pipes 3. It should be noted that several through holes 301 are formed on the pipe wall facing away from the direction of gravity on the spiral pipes 3, while the pipe wall facing the direction of gravity is continuous and smooth. Meanwhile, the feed inlet 302 located at the top of the spiral pipe 3 extends to the outside of the vessel body 1. Lactic acid and organic solvents involved in the preparation of lactide can be transported into the spiral pipe 3 through the feed inlet 302. The spiral pipe 3 can be heated to a suitable environment for lactic acid catalytic polycondensation and cracking reaction to generate lactide under the heating action of the heating unit 2. Since the liquid outlet 303 located at the bottom of the spiral pipe 3 is connected to the collection chamber 102 located at the bottom of the reaction chamber 101, and the collection chamber 102 and the reaction chamber 101 are isolated from each other and do not communicate with each other, the lactide generated by the reaction system can flow into the collection chamber 102 along the pipe wall of the spiral pipe 3 and can be discharged through the liquid outlet 104 connected to the collection chamber 102. A valve is provided at the liquid outlet 104. The water vapor generated in the reaction system can form an azeotrope with the organic solvent. The azeotrope can enter the reaction chamber 101 continuously through the through hole 301 and be discharged from the exhaust port 103 on the body 1. Since the pipe wall of the spiral pipe 3 facing the direction of gravity is sealed, it can effectively prevent the azeotrope from re-entering the reaction system in the spiral pipe 3, avoiding the side reactions of lactide hydrolysis, ring-opening reverse reaction and lactic acid polymerization, and effectively improving the production efficiency of lactide.

[0021] As described above, the diameter, number of spiral rings, and pitch of the spiral pipe 3 can be matched according to the flow rate of lactic acid and organic solvent and the reaction residence time. That is, the lactic acid and organic solvent flowing into the spiral pipe 3 have sufficient reaction time with the catalyst packed in the spiral pipe 3 during the process of flowing along the spiral pipe 3. At the same time, by setting the spiral pipe 3 around the outer periphery of the heating unit 2, the spiral pipe 3 and the catalyst bed formed therein can be uniformly heated under the action of the heating unit 2, thereby reducing the difference in catalytic reaction in different areas of the catalyst bed. Furthermore, in this embodiment, by filling the spiral pipe 3 with solid catalyst or catalyst-supported packing, the lactic acid and organic solvent can be dispersed and flow through the contact surface between the catalysts during the downward flow along the direction of gravity, thereby ensuring sufficient contact between the lactic acid and the catalyst. At the same time, the large amount of catalyst filled in the spiral pipe 3 can greatly increase the resistance to the downward flow of lactic acid and organic solvent along the direction of gravity, thereby effectively slowing down the flow rate of lactic acid and organic solvent and reducing the occurrence of spontaneous two-phase stratification of lactic acid and organic solvent.

[0022] As described above, several through holes 301 are evenly spaced to enable the azeotrope to quickly exit the reaction system through the through holes 301.

[0023] As described above, the exhaust port 103 can be connected to the exhaust port of a conventional extraction device in this field via a pipe, thereby increasing the efficiency of azeotrope discharge and preventing excessive liquefaction and residue of azeotrope due to failure to be discharged from the exhaust port 103 in time, which would accumulate in the reaction chamber 101. However, to prevent a small amount of residue from accumulating in the reaction chamber 101 and entering the spiral pipe 3 through the through hole 301, in this embodiment, the position where the liquid outlet 303 communicates with the collection chamber 102 is higher than the bottom of the reaction chamber 101, and the device body 1 is provided with a second liquid outlet 105 communicating with the bottom of the reaction chamber 101, so as to facilitate the timely discharge of liquid residue in the reaction chamber 101.

[0024] The above, such as Figure 3 As shown, multiple spiral pipes 3 can be arranged around the heating unit 2, and the multiple spiral pipes 3 can be arranged in a circumferential array along the heating unit 2. While ensuring that the temperature of each spiral pipe 3 is relatively uniform after being heated by the heating unit 2, the preparation efficiency of lactide can also be further increased.

[0025] It should be noted that a gap is reserved between the outer wall of the spiral pipe 3 and the heating unit 2 to facilitate the flow of azeotropic material to the exhaust port 103.

[0026] In this embodiment, the feed inlet 302 is used to introduce fully mixed and preheated lactic acid and organic solvent. Specifically, the feed inlet 302 on the spiral pipe 3 can be connected via a pipe to a conveying pump used in conventional technology for conveying materials and a premixing tank equipped with a stirrer and a jacketed heater. The premixing tank can uniformly mix the lactic acid and organic solvent and preheat the resulting mixture, reducing the time required for the lactic acid and organic solvent to reach the reaction temperature. Furthermore, while the feed inlet 302 is connected to the premixing tank via a pipe, it can also be connected via other pipes to a pump body and a container holding pure toluene for pumping pure toluene into the spiral pipe 3, thereby impregnating the catalyst bed within the spiral pipe 3.

[0027] Furthermore, the heating unit 2 includes a hollow cylinder 201 and a heater 202 disposed within the hollow cylinder 201. The heater 202 may employ, but is not limited to, electric heating furnaces, thermal oil jackets, microwave heaters, etc., as conventional technologies in this field. In this technical solution, the heater 202 is an electric heating furnace. It should be noted that the hollow cylinder 201 is placed upright within the reaction chamber 101 and is isolated from the internal cavity of the reaction chamber 101. Specifically, as... Figure 1 , Figure 3 As shown, in this embodiment, the bottom opening of the hollow column 201 can be sealed to the bottom of the reaction chamber 101 by seamless welding, flange sealing connection or other means, and the top opening of the hollow column 201 extends to the outside of the body 1. The outer periphery of the neck near its top opening is sealed to the top of the reaction chamber 101 by seamless welding, flange sealing connection or other means, thereby isolating the cavity of the hollow column 201 from the internal cavity of the reaction chamber 101, thus facilitating the installation of the heater 202 in the cavity of the hollow column 201.

[0028] It should be noted that in this embodiment, the catalyst can be filled into the spiral pipe 3 in a state including but not limited to solid spheres, solid columns, solid strips, etc.; the through hole 301 opened on the spiral pipe 3 can be one or more of the following shapes: circular, elliptical, or slit. Regardless of the shape of the catalyst and the shape of the through hole 301, the catalyst will not easily pass through the through hole 301 before the reaction.

[0029] Furthermore, in this embodiment, a plurality of filter holes 106 are provided on the side wall of the reaction chamber 101 corresponding to the bottom end of the spiral pipe 3, for connecting the spiral pipe 3 and the collection chamber 102. It should be noted that, in order to prevent the catalyst from directly entering the collection chamber 102 through the filter holes 106, preferably, in this technical solution, a filter screen, filter element, etc., can be filled in the bottom of the spiral pipe 3 connected to the collection chamber 102. The filter screen, filter element, etc., can be made of materials including but not limited to PTFE, PP, high-temperature nylon, ceramics, etc., which meet the usage requirements using conventional technologies in this field. Example 2

[0030] Based on Example 1, a method for applying a spiral reactor for one-step preparation of lactic acid lactide includes: Step 1: Mix and preheat the lactic acid raw material and organic solvent, and fill the catalyst into the spiral pipe 3. Then start the heating unit 2 to raise the temperature to the reaction temperature, preheat the spiral pipe 3 and the catalyst filled in the spiral pipe 3. The reaction temperature is 120℃~250℃, and the reaction chamber 101 is kept under normal pressure or reduced pressure. Step 2: The preheated mixture is fed into the spiral pipe 3 through the feed inlet 302. The lactic acid raw material reacts under the catalysis of the catalyst filled in the spiral pipe 3 to produce lactide and water. The water produced by the reaction forms an azeotropic vapor with the organic solvent. The azeotropic vapor enters the reaction chamber 101 through several through holes 301 and is discharged through the exhaust port 103. The lactide solution produced by the reaction flows along the spiral pipe 3 to the collection chamber 102 for centralized storage.

[0031] The organic solvent mentioned above is selected from one or more of toluene, xylene, and cyclohexane.

[0032] As described above, after the azeotropic vapor in step two is condensed and separated into oil and water, the separated organic solvent can be reused for mixing and preheating with lactic acid raw materials. Specifically, the azeotropic vapor discharged through exhaust port 103 can enter an external condenser through a vacuum device and pipeline. The condenser can condense the azeotropic vapor into a liquid and transport it to an oil-water separator, thereby performing oil-water separation on the condensed liquid of the azeotropic vapor. This allows the separated organic solvent to be pumped to a premixing tank or directly into the spiral pipeline 3 using a reflux pump, which is a conventional technique in this field. Example 3

[0033] Based on Examples 1 and 2, a method for applying a spiral reactor for one-step preparation of lactic acid lactide includes: Before feeding, lactic acid and toluene are mixed in a premix tank at a mass ratio of 1:2, and the mixture is preheated to 80°C and stirred continuously for 30 minutes to ensure thorough mixing. The electric heating furnace is then started to gradually increase the temperature of the reaction chamber 101 and the spiral pipe 3 and stabilize it at 140°C. Meanwhile, pure toluene is slowly pumped in from the feed inlet 302 at the top of the spiral pipe 3, allowing it to flow through the spiral pipe 3, thereby impregnating the catalyst bed inside the spiral pipe 3 and expelling the air inside the device 1; the pure toluene eventually flows into the collection chamber 102 and can be discharged and collected through the drain outlet 104. After the system stabilizes, the lactic acid-toluene mixture preheated to 80°C in the premix tank is continuously pumped into the top inlet of the spiral pipe 3, and the mixture flows downward along the spiral pipe 3 under the action of gravity. At a reaction temperature of 140℃ and under the action of a catalyst, lactic acid first undergoes a condensation reaction to generate lactic acid oligomers, which then further undergo cleavage and cyclization to generate lactide. The water molecules generated in the reaction, along with a large amount of toluene solvent in the system, rapidly form azeotropic vapors (the main azeotropic composition being water-toluene) at the reaction temperature. Because the spiral pipe 3 has several through-holes 301 on its wall facing away from gravity, driven by the azeotropic vapor pressure difference, the azeotropic vapor preferentially escapes from the catalyst bed through these through-holes 301 and enters the outer reaction chamber 101. The azeotropic vapor rises in the reaction chamber 101, is discharged through the exhaust port 103, and can be condensed into liquid in a condenser at a condensation temperature of 10℃. The condensed liquid can then be transported to an oil-water separator. In the oil-water separator, toluene and water naturally separate into layers. The lower aqueous phase is continuously discharged, while the upper toluene phase is continuously returned to the premixing tank by a reflux pump or directly pumped into the spiral pipe 3, achieving the recycling of the organic solvent.

[0034] As the reaction proceeds, the generated lactide dissolves in toluene to form a lactide solution. This solution continues to flow downwards along the spiral pipe 3, and because the lower half of the pipe is a sealed structure, the solution does not leak along the way, but flows to the bottom of the spiral pipe 3 until it flows into the collection chamber 102.

[0035] The above-mentioned reaction conditions for lactic acid to lactide catalysis are as follows: the raw material is 80wt% lactic acid aqueous solution, the solvent is toluene, the catalyst is H-β molecular sieve (Si / Al=12.5), the reaction temperature is 140℃, and the continuous catalytic operation lasts for five hours.

[0036] The resulting product was tested and showed a lactic acid conversion rate of 100% and a lactide selectivity of 99%.

[0037] Working principle: By opening several through holes 301 on the wall of the spiral pipe 3 facing away from the direction of gravity, the mixture of lactic acid and organic solvent flows along the spiral pipe 3 under the action of gravity, allowing sufficient reaction time between it and the catalyst filled in the spiral pipe 3. This allows the lactide produced in the reaction to flow along the spiral pipe 3 into the collection chamber 102 for centralized storage. The water vapor generated in the reaction can enter the reaction chamber 101 in real time and continuously through the through holes 301 after forming an azeotrope with the organic solvent. The azeotrope can rise in the reaction chamber 101 and be discharged from the exhaust port 103. Since the wall of the spiral pipe 3 facing the direction of gravity is sealed, it can effectively prevent the azeotrope from re-entering the reaction system in the spiral pipe 3, avoiding lactide hydrolysis, ring-opening reverse reaction and lactic acid polymerization side reaction, thus effectively improving the production efficiency of lactide.

[0038] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A helical reactor for the one-step preparation of lactide from lactic acid, characterized in that: It includes a vessel body (1) with a reaction chamber (101), a heating unit (2) disposed in the reaction chamber (101), and a plurality of spiral pipes (3) disposed in the reaction chamber (101) and surrounding the heating unit (2). The spiral pipe (3) has several through holes (301) on its wall facing away from the direction of gravity. The feed inlet (302) at the top of the spiral pipe (3) extends to the outside of the device body (1). The liquid outlet (303) at the bottom of the spiral pipe (3) is connected to the collection chamber (102) at the bottom of the reaction chamber (101). The device body (1) is provided with an exhaust port (103) connected to the reaction chamber (101) and a drain port (104) connected to the collection chamber (102).

2. The spiral reactor for the one-step preparation of lactide from lactic acid according to claim 1, characterized in that: The position of the outlet (303) communicating with the collection chamber (102) is higher than the bottom of the reaction chamber (101), and the body (1) is provided with a second outlet (105) communicating with the bottom of the reaction chamber (101).

3. The spiral reactor for one-step preparation of lactide from lactic acid according to claim 1, characterized in that: Several spiral pipes (3) are arranged in a circumferential array along the heating unit (2), and a gap is reserved between the outer wall of the spiral pipes (3) and the heating unit (2).

4. The spiral reactor for one-step lactic acid preparation of lactide according to claim 1, characterized in that: The feed inlet (302) is used to introduce fully mixed and preheated lactic acid and organic solvent.

5. The spiral reactor for one-step lactic acid preparation of lactide according to claim 1, characterized in that: The heating unit (2) includes a hollow cylinder (201) and a heater (202) disposed inside the hollow cylinder (201). The hollow column (201) is placed upright in the reaction chamber (101) and is isolated from the internal cavity of the reaction chamber (101).

6. The spiral reactor for one-step lactic acid preparation of lactide according to claim 1, characterized in that: The through holes (301) are evenly spaced.

7. The application method of the spiral reactor for the one-step preparation of lactide from lactic acid according to any one of claims 1 to 6, characterized in that, include: Step 1: Mix and preheat the lactic acid raw material and organic solvent, and fill the catalyst in the spiral pipe (3). Then start the heating unit (2) to raise the temperature to the reaction temperature and preheat the spiral pipe (3) and the catalyst filled in the spiral pipe (3). Step 2: The preheated mixture is fed into the spiral pipe (3) through the feed port (302). The lactic acid raw material reacts under the catalysis of the catalyst filled in the spiral pipe (3) to generate lactide and water. The water generated by the reaction forms an azeotropic vapor with the organic solvent. The azeotropic vapor enters the reaction chamber (101) through several through holes (301) and is discharged through the exhaust port (103). The lactide solution generated by the reaction flows along the spiral pipe (3) to the collection chamber (102) for centralized storage.

8. The application method of the spiral reactor for one-step preparation of lactide from lactic acid according to claim 7, characterized in that: The reaction temperature is 120℃~250℃, and the reaction chamber (101) is kept under normal or reduced pressure.

9. The application method of the spiral reactor for one-step preparation of lactide from lactic acid according to claim 7, characterized in that: The organic solvent is selected from one or more of toluene, xylene, and cyclohexane.

10. The application method of the spiral reactor for one-step preparation of lactide from lactic acid according to claim 7, characterized in that: The azeotropic vapor obtained in step two is condensed and separated from oil and water. The separated organic solvent can be reused for mixing and preheating with lactic acid raw materials.

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