Method and device for cyclic depolymerization to produce crude lactide

Abstract

The present invention relates to a method and apparatus for producing crude lactide by cyclic depolymerization, comprising: (1) continuously conveying lactic acid oligomers to a depolymerization reactor for reaction, wherein the depolymerization reactor is a scraped film depolymerization reactor, the inner film of which is made of a metal oxide coating; and (2) after the reaction, the heavy components are discharged into a circulation tank, mixed with fresh lactic acid oligomers, and then conveyed to the depolymerization reactor, and the gaseous components enter a condensation system to obtain a crude lactide product. The present invention solves the problem of catalyst accumulation during the circulation process by using a specific depolymerization reactor and combining a cyclic depolymerization method, reduces the degree of racemization of lactide, ensures the stability of the preparation process, and improves product yield and production efficiency.

Description

Technical Field

[0001] The invention belongs to the technical field of degradable material preparation, and particularly relates to a method and device for cyclic depolymerization to synthesize crude lactide. Background Art

[0002] Traditional single-use plastic products have stable physical and chemical properties and take a long time to naturally degrade. Globally, 120 million tons of single-use plastic products are consumed annually, but only 10% is recycled. The rest is incinerated or discarded into the soil, air, and oceans. Both burning and disposal cause irreversible damage to ecosystems, seriously endangering the health and safety of land, water, animals, and humans. Nearly 90 countries and regions worldwide have introduced policies or regulations to control or ban the use of single-use, non-degradable plastic products.

[0003] Currently commercialized biodegradable plastics include polylactic acid (PLA), polyhydroxyalkanoates (PHA), polybutylene succinate (PBS), polybutylene succinate-adipate (PBSA), and polybutylene terephthalate-adipate (PBAT). PLA is the most widely used and offers the greatest potential for application. It not only possesses the basic properties of general polymer materials but also exhibits superior processing, physical and mechanical properties, and biodegradability. It is widely used in the packaging, textile, agricultural, and consumer goods industries, and is considered the biodegradable material most likely to replace petroleum-based polyesters.

[0004] Industrial polylactic acid synthesis is mainly produced through the ring-opening polymerization of lactide, in which lactide is the key intermediate in the entire synthesis process. The process barriers are relatively high. It is usually produced through condensation and depolymerization under a catalyst, high temperature, and high vacuum system. This process easily causes lactide racemization. When the catalyst is excessive and accumulated for a long time, the carboxylic acid anion at the end of the lactic acid oligomer attacks the chiral carbon atom on the unit adjacent to the lactic acid unit, thereby breaking the bond between the methyl carbon and the ester oxygen bond (the "backbite" process), and the configuration is reversed, resulting in meso-lactide ( m -lactide). m The presence of lactide affects the optical purity of lactide, which in turn affects the lactide ring-opening polymerization process, resulting in a lower molecular weight, reduced crystallinity, and decreased mechanical properties of the resulting PLA. Furthermore, under the long-term influence of the catalyst and high temperature, the lactic acid oligomers undergo structural changes, increasing the difficulty of recycling and reuse. This can exacerbate lactide racemization during the synthesis process over long production cycles, reducing lactide quality and yield. This is a key and challenging aspect of current lactide research both domestically and internationally.

[0005] US6326458 discloses a continuous process for preparing lactide and lactide polymers. In the depolymerization section of lactide preparation, the depolymerization reactor utilizes a falling-film tubular evaporator. Lactic acid oligomers are fed from the top of the evaporator, lactide vapor is withdrawn from the bottom of the tubular reactor, and unreacted lactic acid oligomers are discharged from the lower discharge port. The falling-film reaction in this process requires a relatively low reaction temperature, which effectively reduces the probability of lactide racemization during the depolymerization process. However, the lactide yield is low. To maintain a high lactide yield, the feed rate is generally reduced, which in turn increases the residence time of the oligomers on the falling-film reactor surface. Undepolymerized lactic acid oligomers rapidly polymerize under the high-temperature, high-vacuum system, resulting in a high molecular weight of the oligomers. This further affects the depolymerization rate and can also easily cause coking and carbonization of the oligomers on the surface of the falling-film tubular reactor.

[0006] CN111153886A discloses a method and apparatus for rapidly and efficiently synthesizing lactide. The method uses lactic acid as a single component or lactic acid plus a catalyst as a dual component, which enters an oligomer preparation system through a mixer. The oligomeric lactic acid is synthesized through a bottom circulation system to increase the residence time. The gaseous component passes through a distillation system to increase the yield of the oligomeric lactic acid. The oligomeric lactic acid passes through a purification system to remove unreacted lactic acid and water. After the light-removed oligomeric lactic acid is added with a catalyst, it passes through a mixer and enters a depolymerization reactor for depolymerization into lactide. The heavy component re-enters the depolymerization reactor through reflux, and the light component passes through a purification and recovery system to obtain a lactide product. This apparatus can efficiently synthesize lactide, achieving a yield of 94%-98% crude lactide within a short residence time of 0.5-5 minutes. After the light component passes through a simple purification system, the lactide product has a L-lactide, D-lactide, or DL-lactide content of 94%-98%. m - Lactide content 0.5%-5.5%. However, this invention directly refluxes the heavy components after depolymerization into the depolymerization reactor. Long-term operation leads to an increase in the molecular weight of the heavy components and catalyst accumulation, which can easily increase the probability of coking and carbonization of the reaction substrate on the reactor surface, and increase the degree of lactide racemization, affecting the continuous and stable operation of the reaction. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention provides a method and apparatus for producing crude lactide through cyclic depolymerization. By utilizing a specific depolymerization reactor and combining it with a cyclic depolymerization method, the present invention addresses the problem of catalyst accumulation during the cyclic process, reduces the degree of lactide racemization, ensures a stable production process, and improves product yield and production efficiency.

[0008] The present invention provides a method for cyclic depolymerization to produce crude lactide, which specifically comprises the following steps:

[0009] (1) continuously conveying lactic acid oligomers to a depolymerization reactor for reaction, wherein the depolymerization reactor is a scraped film depolymerization reactor, and the inner film is made of a metal oxide coating;

[0010] (2) After the reaction, the heavy components are discharged into the circulation tank, mixed with fresh lactic acid oligomers and then transported to the depolymerization reactor. The gaseous components enter the condensation system to obtain crude lactide product.

[0011] In the above method, the metal oxide in step (1) is mainly a metal oxide capable of depolymerizing lactic acid oligomers, such as at least one selected from stannous oxide, zinc oxide, titanium oxide, magnesium oxide, etc.

[0012] In the above method, the metal oxide coating method in step (1) includes surface coating strengthening, chemical surface heat treatment, etc., and specifically, plasma deposition, electroplating, ion plating and chemical vapor deposition can be used.

[0013] In the above method, the depolymerization reactor in step (1) is a scraped film depolymerization reactor, which mainly includes any one of a thin film evaporator, a molecular distillation evaporator or other stirred film evaporator.

[0014] In the above method, the molecular weight of the lactic acid oligomer in step (1) is 800-4000, preferably 1200-2800. Lactic acid oligomers are generally prepared from L-lactic acid or D-lactic acid by dehydration and polycondensation. The dehydration is to remove free water from the lactic acid, which can be carried out under normal pressure or reduced pressure. The reaction temperature of the polycondensation process is 140-170°C, the reaction time is 0.5-4.0 hours, and the absolute pressure is 1000-2000 Pa.

[0015] Furthermore, a certain amount of long-chain diamine solvent is added to the lactic acid oligomer, preferably at least one of dodecanediamine, tetradecanediamine, hexadecanediamine, etc. The amount of the diamine solvent is 0.1%-3.0%, preferably 0.5%-2.0% of the weight of the lactic acid oligomer.

[0016] In the above method, the depolymerization reaction temperature in step (1) is 170-220°C, and the vacuum degree is 600-1500Pa.

[0017] In the above method, the residence time of the lactic acid oligomers in the depolymerization reactor in step (1) is 2-10 minutes.

[0018] In the above method, the depolymerization reactor in step (2) and the circulation tank constitute a circulating depolymerization system, wherein the feed mass ratio of fresh material to circulating material is 1:1-4.

[0019] In the above method, in step (2), as the reaction proceeds, the liquid level in the circulation tank is controlled to be maintained at 40%-80%, the pressure is maintained at 10 kPa-normal pressure, and the temperature is maintained at 140-180°C to reduce the probability of continued intermolecular polymerization of lactic acid oligomers and reduce coking and carbonization.

[0020] In the above method, the depolymerization reactor in step (2) is provided with a gas-phase lactide outlet, and a condensation system is provided at the outlet. The condensation temperature is generally 80-95°C, and the gas-phase lactide is condensed to obtain a crude lactide product.

[0021] In the above method, the crude lactide product obtained in step (2) has an L-lactide content of 80%-88%, m -Lactide content is 2.0%-5.0%, L-lactic acid content is 2.0%-6.0%, and dimer and trimer content is 2.0%-7.0%.

[0022] The present invention also provides an apparatus for the above-mentioned method of cyclic depolymerization to produce crude lactide, which mainly includes a cyclic depolymerization system and a condensation system. The cyclic depolymerization system includes a depolymerization reactor, a circulation tank and a circulation pipeline, and is used to carry out a depolymerization reaction of lactic acid oligomers in the presence of a depolymerization catalyst. After the reaction, the heavy components are discharged into the circulation tank, mixed with fresh lactic acid oligomers, and then transported to the depolymerization reactor. After the reaction, the gaseous components are discharged from the top of the depolymerization reactor. The depolymerization reactor is a scraped film depolymerization reactor, and the inner membrane of the reactor is made of a metal oxide coating. The condensation system mainly includes a condenser and a storage tank. After the gaseous components are cooled by the condensation system, the crude lactide produced is transported to the storage tank.

[0023] In the above device, the scraped film depolymerization reactor mainly includes any one of thin film evaporator, molecular distillation evaporator or other stirred film evaporator, etc. The metal oxide is selected from at least one of stannous oxide, zinc oxide, titanium oxide, magnesium oxide, etc.

[0024] Compared with the prior art, the beneficial effects of the present invention are embodied in:

[0025] (1) The present invention adopts a specific depolymerization reactor based on a cyclic depolymerization process, that is, metal oxides are deposited on the inner wall of the depolymerization reactor through plasma deposition technology to form a thin film reaction interface, which solves the problem of catalyst accumulation in the cyclic depolymerization process, effectively reduces the probability of substrate coking and carbonization, and reduces the impact of catalyst accumulation on product quality and yield. In the long-term operation process, the regular discharge of accumulated catalyst and waste generated by excessive polymerization are avoided.

[0026] (2) The depolymerization reactor used in the present invention forms a catalyst film on the inner wall of the reactor by plasma deposition, so that the catalyst is more evenly distributed in the reaction system, the rate and stability of the depolymerization reaction are improved, and the degree of racemization of lactide is effectively reduced.

[0027] (3) After the lactic acid oligomers are treated in the depolymerization reactor of the present invention, the molecular weight of the recycled material and the fresh material is not significantly increased after mixing, which can reduce the high vacuum and high temperature reaction conditions of the depolymerization reactor and reduce the degree of product racemization.

[0028] (4) Adding an appropriate amount of diamine solvent to the reaction system can effectively reduce the racemization degree of lactide and further reduce m - Lactide content. DETAILED DESCRIPTION

[0029] The following examples further illustrate the method and apparatus for preparing lactide of the present invention. The examples are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operating processes, but the scope of protection of the present invention is not limited to the following examples.

[0030] The experimental methods in the following examples, unless otherwise specified, are all conventional methods in the art. The experimental materials used in the following examples, unless otherwise specified, can all be purchased from biochemical reagent stores.

[0031] This study used a Malvern Viscotek OMNISEC GPC / SEC gel permeation chromatography (GPC / SEC) instrument to analyze the molecular weight of lactic acid oligomers. Traditional calibration methods were used, with polystyrene (PS) as the internal standard. A T3000 column (300 mm × 8.0 mm) was used. The column temperature was 40°C, the flow rate was 1.0 mL / min, the sample concentration was 2-5 mg / mL, and the single injection volume was 500 μL.

[0032] The present invention uses an Agilent high-performance liquid chromatograph to analyze lactic acid, lactic acid dimer, and lactic acid trimer, with an ultraviolet detector, acetonitrile as the mobile phase, a column length of 150-250 mm, an inner diameter of 2.7-4.6 mm, and a filler particle size of 5 μm. The detection wavelength is 200 nm, the column temperature is 30-40°C, the flow rate is 0.4-1 mL / min, and the injection volume is 1-5 μL.

[0033] The present invention adopts Agilent gas chromatograph to analyze the content of different optical isomers of lactide, selects CYCLOSIL-B model chromatographic column, vaporization chamber temperature 250°C, detector temperature 280°C, hydrogen flame ionization detector, column temperature program heating initial temperature 100°C, hold 5min, heat to 140°C at a rate of 4°C / min, hold 7min, heat to 200°C at a rate of 8°C / min, hold 20min, carrier gas N2 flow rate 1.4mL / min, hydrogen flow rate 30mL / min, air flow rate 400mL / min, injection volume 0.5μL.

[0034] Preparation of lactic acid oligomers used in the present invention: (1) Dehydration of lactic acid: 1000 g of L-lactic acid (wherein the lactic acid content is about 88.0%) was taken and added to a four-necked flask with a stirring system. A vacuum circulating water pump was used to maintain the system pressure at about 50 kPa. Heating was started under vacuum and gradually heated to 110-120°C. Dehydration was carried out for 2 hours. At this time, the free water in the reaction system was slowly evaporated out of the reaction system. (2) Preparation of lactic acid oligomers: After the free water in the system was almost completely removed, the vacuum degree of the system was increased and the pressure of the system was slowly reduced to about 1.0 kPa. The temperature of the feed liquid was gradually raised to 160°C and the reaction was carried out for 3 hours. At this time, a polycondensation reaction occurred between the lactic acid molecules and the water generated by the reaction in the system was evaporated out of the system to obtain a lactic acid oligomer with a molecular weight of 2205.

[0035] Example 1

[0036] Stannous oxide is deposited onto the inner wall of a scraped film depolymerization reactor using plasma deposition technology. Silicon dioxide with a purity of 99.99% stannous oxide and 50 ppm of catalyst, sintered at 800-900°C under nitrogen, serves as the target material. Argon is used as the sputtering gas to deposit the catalyst coating onto the reactor wall. The substrate bias voltage is 60-80V, and the thickness is 400nm.

[0037] 3000 g of lactic acid oligomers were taken and conveyed to a scraped film depolymerization reactor with an inner membrane of stannous oxide at a flow rate of 150 g / h. The depolymerization reaction conditions were controlled as follows: vacuum degree at 800 Pa, reaction temperature at 190°C, and single-pass residence time at 2 min. After the reaction, the heavy components were discharged into a circulation tank, mixed with fresh lactic acid oligomers, and then conveyed to the depolymerization reactor. The feed mass ratio of fresh material to recycled material during the reaction process was controlled to be 1:3. After the fresh material was mixed with the recycled material, it re-entered the depolymerization reactor. As the reaction proceeded, the liquid level in the circulation tank was controlled to be maintained at 60%, the pressure was maintained at 50 kPa, and the temperature was maintained at 180°C.

[0038] After 48 hours of continuous cycle operation, the molecular weight of the lactic acid oligomers reached 2252. The gaseous components after the reaction entered the condensation system at 90°C, and the resulting liquid lactide entered the storage tank. Analysis of the crude lactide product revealed an L-lactide content of 87.2%, an m-lactide content of 3.3%, an L-lactic acid content of 4.0%, and a dimer and trimer content of 2.6%.

[0039] Example 2

[0040] 3000 g of lactic acid oligomers were taken and conveyed to a scraped film depolymerization reactor with an inner membrane of stannous oxide at a flow rate of 150 g / h. The depolymerization reaction conditions were controlled as follows: vacuum degree at 600 Pa, reaction temperature at 220°C, and single-pass residence time at 2 min. After the reaction, the heavy components were discharged into a circulation tank, mixed with fresh lactic acid oligomers, and then conveyed to the depolymerization reactor. The feed mass ratio of fresh material to recycled material during the reaction process was controlled to be 1:1. After the fresh material and recycled material were mixed, they re-entered the depolymerization reactor. As the reaction proceeded, the liquid level in the circulation tank was controlled to be maintained at 40%, the pressure was maintained at 10 kPa, and the temperature was maintained at 180°C.

[0041] After 48 hours of continuous cycle operation, the molecular weight of the lactic acid oligomers reached 2420. The gaseous components after the reaction entered the condensation system at 90°C, and the resulting liquid lactide entered the storage tank. Analysis of the crude lactide product revealed an L-lactide content of 85.3%, an m-lactide content of 3.8%, an L-lactic acid content of 4.6%, and a dimer and trimer content of 4.2%.

[0042] Example 3

[0043] 3000 g of lactic acid oligomers were taken and conveyed to a scraped film depolymerization reactor with a stannous oxide inner membrane at a flow rate of 150 g / h. The depolymerization reaction conditions were controlled as follows: vacuum degree at 1500 Pa, reaction temperature at 170°C, and single-pass residence time at 2 min. After the reaction, the heavy components were discharged into a circulation tank, mixed with fresh lactic acid oligomers, and then conveyed to the depolymerization reactor. The feed mass ratio of fresh material to recycled material during the reaction process was controlled to be 1:4. After the fresh material and recycled material were mixed, they re-entered the depolymerization reactor. As the reaction proceeded, the liquid level in the circulation tank was controlled to be maintained at 80%, the pressure was maintained at normal pressure, and the temperature was maintained at 180°C.

[0044] After 48 hours of continuous cycle operation, the molecular weight of the lactic acid oligomers reached 2308. The gaseous components after the reaction entered the condensation system at 90°C, and the resulting liquid lactide entered the storage tank. Analysis of the crude lactide product revealed an L-lactide content of 86.9%, an m-lactide content of 4.0%, an L-lactic acid content of 4.8%, and a dimer and trimer content of 4.3%.

[0045] Example 4

[0046] The lactide preparation process was the same as in Example 1, except that zinc oxide was deposited on the inner film of the scraped film depolymerization reactor. After 48 hours of continuous circulation, the molecular weight of the lactic acid oligomer was 2298. Analysis of the crude lactide product revealed an L-lactide content of 85.9%, an m-lactide content of 3.8%, an L-lactic acid content of 4.9%, and a dimer and trimer content of 5.4%.

[0047] Example 5

[0048] The lactide preparation process was the same as in Example 1, except that titanium oxide was deposited on the inner film of the scraped film depolymerization reactor. After 48 hours of continuous circulation, the molecular weight of the lactic acid oligomers was 2315. Analysis of the crude lactide product revealed an L-lactide content of 85.3%, an m-lactide content of 4.3%, an L-lactic acid content of 4.1%, and a dimer and trimer content of 6.3%.

[0049] Example 6

[0050] The lactide preparation process was the same as in Example 1, except that the molecular weight of the lactic acid oligomer was 1200. After 48 hours of continuous cycle operation of the reaction unit, the molecular weight of the lactic acid oligomer was 1462. Analysis of the crude lactide product revealed an L-lactide content of 86.4%, an m-lactide content of 4.4%, an L-lactic acid content of 4.6%, and a dimer and trimer content of 4.6%.

[0051] Example 7

[0052] The lactide preparation process was the same as in Example 1, except that a certain amount of dodecanediamine was added to the lactic acid oligomers, the amount being 1.0% by weight. After the reaction apparatus was operated continuously for 48 hours, the molecular weight of the lactic acid oligomers was 2230. Analysis of the crude lactide product revealed an L-lactide content of 87.9%, an m-lactide content of 1.9%, an L-lactic acid content of 3.9%, and a dimer and trimer content of 5.6%.

[0053] Comparative Example 1

[0054] The lactide preparation process was the same as in Example 1, except that a conventional autoclave reactor was used and the catalyst was added directly at 5% of the feed rate. After 48 hours of continuous cycle operation, the molecular weight of the lactic acid oligomers was 5231. Analysis of the crude lactide product revealed an L-lactide content of 75.3%, an m-lactide content of 9.3%, an L-lactic acid content of 9.8%, and a dimer and trimer content of 5.6%.

[0055] Comparative Example 2

[0056] The lactide preparation process was the same as in Example 1, except that a tank reactor was used instead of a scraped-film depolymerization reactor, and the catalyst was deposited on the inner wall of the tank reactor. After 48 hours of continuous cycle operation, the molecular weight of the lactic acid oligomer was 6637. Analysis of the crude lactide product revealed an L-lactide content of 70.3%, an m-lactide content of 11.3%, an L-lactic acid content of 8.8%, and a dimer and trimer content of 9.6%.

[0057] Comparative Example 3

[0058] The lactide preparation process was the same as in Example 1, except that the liquid level in the circulation tank was not controlled. After 48 hours of continuous circulation operation, the molecular weight of the lactic acid oligomer was 5806. Analysis of the crude lactide product revealed an L-lactide content of 79.3%, an m-lactide content of 10.1%, an L-lactic acid content of 5.5%, and a dimer and trimer content of 5.1%.

Claims

1. A method for cyclic depolymerization to produce crude lactide, characterized in that The steps include: (1) continuously conveying lactic acid oligomers to a depolymerization reactor for reaction, wherein the depolymerization reactor is a scraped film depolymerization reactor, and the inner film is made of a metal oxide coating; the metal oxide is at least one of stannous oxide, zinc oxide, titanium oxide, and magnesium oxide; adding a long-chain diamine solvent to the lactic acid oligomers; the amount of the long-chain diamine solvent is 0.1% to 3.0% of the mass of the lactic acid oligomers; (2) After the reaction, the heavy components are discharged into the circulation tank, mixed with fresh lactic acid oligomers and then transported to the depolymerization reactor. The liquid level of the circulation tank is controlled to maintain at 40%-80%, and the gaseous components enter the condensation system to obtain crude lactide product.

2. The method according to claim 1, wherein: The metal oxide coating method in step (1) includes surface coating strengthening or chemical surface heat treatment.

3. The method according to claim 2, wherein: The metal oxide coating method in step (1) is any one of plasma deposition, electroplating, ion plating and chemical vapor deposition.

4. The method according to claim 1, wherein: The depolymerization reactor described in step (1) is a scraped film depolymerization reactor, and its main forms include any one of a thin film evaporator, a molecular distillation evaporator or other stirred film evaporator.

5. The method according to claim 1, wherein: The molecular weight of the lactic acid oligomer in step (1) is 800-4000.

6. The method according to claim 5, characterized in that: The molecular weight of the lactic acid oligomer in step (1) is 1200-2800.

7. The method according to claim 1, 5 or 6, characterized in that: Lactic acid oligomers are prepared using L-lactic acid or D-lactic acid as raw materials through dehydration and polycondensation processes; the dehydration is to remove free water in the lactic acid, using normal pressure or reduced pressure; the reaction temperature of the polycondensation process is 140-170°C, the reaction time is 0.5-4.0h, and the absolute pressure is 1000-2000Pa.

8. The method according to claim 1, wherein: The long-chain diamine solvent is at least one of dodecanediamine, tetradecanediamine and hexadecanediamine.

9. The method according to claim 1 or 8, characterized in that: The diamine solvent is used in an amount of 0.5% to 2.0% of the weight of the lactic acid oligomer.

10. The method according to claim 1, wherein: The depolymerization reaction temperature in step (1) is 170-220° C., and the vacuum degree is 600-1500 Pa.

11. The method according to claim 1, wherein: The residence time of the lactic acid oligomers in the depolymerization reactor in step (1) is 2-10 minutes.

12. The method according to claim 1, wherein: The depolymerization reactor in step (2) and the circulation tank constitute a circulating depolymerization system, wherein the feed mass ratio of fresh material to circulating material is 1:1-4.

13. The method according to claim 1, wherein: In step (2), as the reaction proceeds, the pressure of the circulation tank is controlled to be maintained at 10 kPa-normal pressure, and the temperature is maintained at 140-180°C.

14. The method according to claim 1, wherein: The depolymerization reactor in step (2) is provided with a gas-phase lactide outlet, and a condensation system is provided at the outlet. The condensation temperature is 80-95° C. The gas-phase lactide is condensed to obtain a crude lactide product.

15. The method according to claim 1, wherein: The crude lactide product obtained in step (2) has an L-lactide content of 80%-88%. m -Lactide content is 2.0%-5.0%, L-lactic acid content is 2.0%-6.0%, and dimer and trimer content is 2.0%-7.0%.