A method and system for preparing crude lactide by continuous depolymerization

Abstract

The present invention relates to a method and system for preparing crude lactide by continuous depolymerization, wherein lactic acid oligomers are mixed with a depolymerization catalyst, and then continuously enter a depolymerization reactor through a feed port provided on the wall of the depolymerization reactor for reaction, and after the reaction, a heavy component enters a circulation tank, and is transported back to the depolymerization reactor through a circulating material inlet provided above the feed port, and an upper reaction zone is between the feed port and the circulating material inlet, and a lower reaction zone is provided below the feed port, and after the heavy component reacts in the upper reaction zone, it enters the lower reaction zone to continue to participate in the reaction, and a gas phase component, i.e., crude lactide, is extracted from the top of the reactor. The present invention adopts a cyclic depolymerization process, and the circulating material and the fresh material are diverted and processed during the depolymerization process, so as to realize efficient and continuous preparation of crude lactide, reduce the racemization degree of lactide and the probability of substrate coking and carbonization, and improve the substrate conversion rate and product quality.

Description

Technical Field

[0001] The invention belongs to the technical field of biodegradable materials, and in particular relates to a method and a system for preparing crude lactide by continuous depolymerization. Background Art

[0002] While disposable plastic products bring convenience to people, their hazards are becoming increasingly apparent. At present, the global annual consumption of disposable plastic products reaches 120 million tons, of which only 10% are recycled, about 12% are incinerated, and more than 70% are discarded into the soil, air and ocean. More than 8 million tons of plastic waste are dumped into the ocean every year, and this number is still rising. It is estimated that by 2025, the amount of global marine plastic waste will reach 250 million tons. Traditional disposable plastic products have a short service life, but their physical and chemical properties are stable and difficult to degrade naturally. Various environmental problems caused by a large amount of disposable plastic waste have occurred frequently, which has seriously endangered the health and safety of land, water, animals and humans.

[0003] Nearly 90 countries and regions around the world have introduced policies or regulations to control or ban disposable non-degradable plastic products, and degradable materials have ushered in major development opportunities. Currently, commercial biodegradable plastics include polylactic acid (PLA), polyhydroxyalkanoates (PHA), polybutylene succinate (PBS), polybutylene succinate-adipate (PBSA), polybutylene terephthalate-adipate (PBAT), etc. Among them, PLA is currently the most widely used and has broad development prospects in the future. It not only has the basic properties of general polymer materials, but also has better processing properties, physical and mechanical properties and biodegradability. It can be widely used in the packaging industry, textile industry, agricultural industry, consumer goods market, etc., and is considered to be the most likely biodegradable material to replace petroleum-based polyester.

[0004] The industrialized synthesis of polylactic acid is mainly obtained through the ring-opening polymerization of lactide: the first step is to prepare lactide from lactic acid; the second step is to prepare polylactic acid through the ring-opening polymerization of lactide. The molecular weight of the PLA obtained in this process can reach 100,000 to 1 million. Among them, lactide is the key to the entire synthesis process, and the production process barriers are relatively high. It is usually prepared through polycondensation and depolymerization processes under a catalyst, high temperature, and high vacuum system. This process is prone to cause the racemization of lactide, affecting the purity of the product. In addition, in order to improve the utilization rate of raw materials, a cyclic depolymerization process is usually adopted. The heavy components produced by depolymerization are mixed with fresh materials and transported to the depolymerization reactor, resulting in the accumulation of high molecular weight heavy components and catalysts during the synthesis process, which is easy to cause coking and carbonization of the substrate on the reactor surface.

[0005] US6326458 discloses a continuous process for preparing lactide and lactide polymers. In the depolymerization reactor of the lactide preparation depolymerization section, a falling film tube evaporator is used. Lactic acid oligomers are added from the top of the evaporator, lactide vapor is extracted from the bottom of the tube reactor, and unreacted lactic acid oligomers are discharged from the lower discharge port. The reaction temperature required for the falling film reaction process in this process is relatively low, but the lactide yield is low. In order to maintain a high lactide yield, the feed rate is generally required to be reduced, which will increase the residence time of the oligomers on the surface of the falling film reactor. The undepolymerized lactic acid oligomers are rapidly polymerized under a high temperature and high vacuum system, resulting in a large molecular weight of the oligomers, which further affects the depolymerization rate on the one hand, and is also prone to coking and carbonization of the oligomers on the surface of the falling film tube reactor on the other hand.

[0006] CN111153886A discloses a method and device for synthesizing lactide quickly and in high yield. After the light components are removed, the oligomeric lactic acid is added with a catalyst and passed through a mixer, and then enters a depolymerization reactor for depolymerization into lactide. The heavy components are refluxed and enter the depolymerization reactor again. The light components are purified and recovered to obtain a lactide product. This device can be used to efficiently synthesize lactide, and crude lactide with a yield of 94%-98% can be obtained within a short residence time of 0.5-5 minutes. After the light components are passed through a simple purification system, the content of L-lactide, D-lactide or DL-lactide in the lactide product is 94%-98%. m - Lactide content 0.5%-5.5%. However, the heavy components produced by the depolymerization reaction of the invention directly enter the depolymerization feed mixer and mix with the reaction raw materials to form a stream of materials, which then re-enter the depolymerization reactor for reaction. In the long-term operation, this not only affects the stable operation of the depolymerization reactor, but also increases the probability of coking and carbonization of the reaction substrate on the reactor surface as the molecular weight of the heavy components increases and the catalyst accumulates, which increases the degree of racemization of lactide and affects the continuous and stable operation of the reaction and the product quality. Summary of the invention

[0007] In view of the shortcomings of the prior art, the present invention provides a method and system for preparing crude lactide by continuous depolymerization. The present invention adopts a cyclic depolymerization process, and the circulating material and the fresh material are diverted during the depolymerization process, thereby achieving efficient and continuous preparation of crude lactide, reducing the racemization degree of lactide and the probability of substrate coking and carbonization, and improving the substrate conversion rate and product quality.

[0008] In one aspect, the present invention provides a method for preparing crude lactide by continuous depolymerization, comprising the following contents:

[0009] After the lactic acid oligomers are mixed with the depolymerization catalyst, they continuously enter the depolymerization reactor through the feed port arranged on the wall of the depolymerization reactor for reaction. After the reaction, the heavy components enter the circulation tank and are transported back to the depolymerization reactor through the circulation material inlet arranged above the feed port. The area between the feed port and the circulation material inlet is the upper reaction zone, and the area below the feed port is the lower reaction zone. After the heavy components react in the upper reaction zone, they enter the lower reaction zone to continue to participate in subsequent reactions, and the gas phase component, i.e., crude lactide, is continuously extracted from the top of the reactor.

[0010] In the above method, the molecular weight of the lactic acid oligomer is 800-4000, preferably 1200-3000, and is generally prepared from L-lactic acid or D-lactic acid by dehydration and polycondensation. The dehydration is to remove free water in the lactic acid, which can be done under 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.

[0011] In the above method, the depolymerization catalyst is a tin catalyst, such as at least one of stannous octoate, SnCl2, SnO, etc. The amount of the depolymerization catalyst is 0.4%-3.0%, preferably 0.8%-2.0% of the amount of the lactic acid oligomer.

[0012] In the above method, the depolymerization reactor is a scraped film depolymerization reactor, and the main forms include any one of a thin film evaporator, a molecular distillation evaporator or other stirred film evaporator.

[0013] In the above method, a pump is provided between the depolymerization reactor and the circulation tank for conveying materials from high vacuum to low vacuum or normal pressure, and at the same time plays a role in isolating high and low vacuum.

[0014] In the above method, the upper reaction zone of the depolymerization reactor occupies 1 / 5-1 / 3 of the entire reaction zone. The reaction temperature of the upper reaction zone is 200-240°C, preferably 210-240°C, and the vacuum degree is 200-1500Pa. Furthermore, as the reaction proceeds, the reaction progress is regulated by real-time monitoring of the molecular weight of the heavy component in the circulation tank and gradually increasing the reaction temperature. When the molecular weight is higher than 3000, the temperature is increased by 2-10°C. When the molecular weight is higher than 4000, the temperature is further increased by 2-10°C. When the molecular weight is higher than 5000, the temperature is further increased by 2-10°C. The final temperature does not exceed 240°C, thereby achieving effective depolymerization of the heavy component.

[0015] In the above method, the reaction temperature of the lower reaction zone of the depolymerization reactor is 180-220°C, preferably 190-220°C, and the vacuum degree is 200-1500Pa.

[0016] In the above method, the feed mass ratio of the fresh material to the recycled material is 1:6-2:1.

[0017] In the above method, the feed amount is increased, and the total feed amount of fresh material and recycled material is adjusted to 4-8 times the actual reaction amount, so as to reduce the residence time of lactic acid oligomers in the scraped film depolymerization reactor, inhibit the occurrence of the polymerization process, improve the overall yield, and ensure product quality.

[0018] In the above method, the liquid level of the circulation tank is maintained at 50%-70%, the pressure is maintained at 10kPa-normal pressure, and the temperature is maintained at 160-200°C to reduce the probability of continued intermolecular polymerization of lactic acid oligomers and reduce coking and carbonization.

[0019] In the above method, the slag is discharged regularly with reference to the cumulative amount of catalyst circulation, and when the cumulative amount of catalyst circulation is greater than 15%, the slag is discharged. The lactic acid polymer discharged by the slag discharge can be hydrolyzed under certain conditions to recycle lactic acid.

[0020] In the above method, the depolymerization reactor is provided with a gas phase lactide outlet, which is directly transported to the separation and purification section. The separation and purification section can be a combination of single-stage or multi-stage distillation, solvent extraction and melt crystallization.

[0021] In the above method, the crude lactide product obtained has an L-lactide content of 85%-90%. m -Lactide content is 0.5%-3.5%, L-lactic acid content is 2.0%-6.0%, dimer and trimer content is 2.0%-5.0%.

[0022] On the other hand, the present invention also provides a system for preparing crude lactide by continuous depolymerization, which mainly includes a depolymerization reactor and a circulation tank, wherein the depolymerization reactor is used to carry out a depolymerization reaction on lactic acid oligomers in the presence of a depolymerization catalyst, and the heavy components are discharged into the circulation tank after the reaction, and the wall of the depolymerization reactor is provided with a feed port and a circulation material inlet on the feed port, an upper reaction zone is between the feed port and the circulation material inlet, and a lower reaction zone is below the feed port, and after the heavy components react in the upper reaction zone, they enter the lower reaction zone to continue the subsequent reaction, and the gas phase components after the reaction are discharged from the top of the depolymerization reactor.

[0023] In the system of the present invention, the depolymerization reactor 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.

[0024] In the system of the present invention, the upper reaction zone of the depolymerization reactor occupies 1 / 5-1 / 3 of the entire reaction zone.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] (1) The inventors of the present application found in experiments that if the heavy components after depolymerization are mixed with fresh materials and then fed into the depolymerization reactor, the heavy components will accumulate more and more during the long-term cyclic reaction process, which may easily lead to coking and carbonization of the reaction substrate on the reactor surface, and long-term stable operation cannot be guaranteed. To this end, the inventors circulate the heavy components produced by depolymerization back to the upper part of the fresh material reaction zone, and the materials after reaction enter the fresh material reaction zone to continue the reaction, thereby reducing the adverse effects of the mixing of heavy components and fresh materials on the reaction, reducing the degree of racemization of lactide and the probability of coking and carbonization of the substrate, effectively extending the slag discharge interval, and improving the oligomer conversion rate and product quality. The content of m-lactide in the obtained crude lactide is not more than 3.5%, and the conversion rate of lactic acid oligomers can reach more than 95%. Compared with the traditional cyclic depolymerization process, the degree of racemization of lactide is reduced by more than 50%, and the conversion rate of lactic acid oligomers is increased by more than 10%.

[0027] (2) Based on the cyclic depolymerization process, the real-time monitoring of the catalyst accumulation in the circulation tank and regular slag discharge can effectively ensure the continuous and stable operation of the depolymerization process and ensure the substrate conversion rate and product quality.

[0028] (3) Increasing the total feed rate of fresh and recycled materials can reduce the residence time of lactic acid oligomers on the reactor surface, inhibit the occurrence of polymerization, increase the overall conversion rate, and reduce the degree of racemization of lactide. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A schematic diagram of a process of preparing crude lactide by a method and apparatus according to the present invention;

[0030] Wherein: 1-depolymerization reactor, 2-circulation tank, 3-upper reaction zone, 4-lower reaction zone, 5-motor, 6-scraper, 7-transmission pump, 8-circulation pump; 01-fresh material, 02-circulation material, 03-heavy component, 04-gas phase crude lactide; A-circulation material inlet, B-feed port. DETAILED DESCRIPTION

[0031] The following examples further illustrate the method and device 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 operation processes, but the protection scope of the present invention is not limited to the following examples.

[0032] 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.

[0033] The lactic acid used in the embodiment of the present invention is heat-resistant grade L-lactic acid with a lactic acid content of 88% or more, and an optical purity of not less than 99.0%.

[0034] The present invention adopts Malvern Viscotek OMNISEC GPC / SEC gel chromatograph to analyze the molecular weight of lactic acid oligomers. The conventional calibration method is adopted, polystyrene (PS) is used as the internal standard, the chromatographic column model is T3000, the size is 300mmL×8.0mm, the column temperature is 40℃, the flow rate is 1.0mL / min, the sample concentration is 2-5mg / mL, and the single injection volume is 500μL.

[0035] The present invention adopts Agilent high performance liquid chromatograph to analyze the chemical purity of lactide, L-lactic acid and dimer and trimer content, ultraviolet detector, phosphoric acid and acetonitrile as mobile phase, chromatographic column model is ZORBAX SB-Aq, column length is 250mm, column inner diameter is 4.6mm, internal filler particle size is 5μm, detection wavelength is 200nm, column temperature is 40℃, flow rate is 1mL / min, injection volume is 5μL.

[0036] 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 7 min, 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.

[0037] The calculation formula for the conversion rate of lactic acid oligomers in the crude lactide synthesis process is as follows:

[0038] Lactic acid oligomer conversion rate = m / M × 100%

[0039] Wherein, m is the total mass of crude lactide, and M is the mass of initial lactic acid oligomers.

[0040] The embodiment of the present invention adopts Figure 1 The circulating depolymerization system firstly continuously pumps fresh material 01 lactic acid oligomers into the depolymerization reactor 1 from the inlet B for reaction. After the reaction, the undepolymerized heavy components are discharged into the circulation tank 2 through the pump 7 to establish a certain liquid level. The circulation pump 8 is operated to transport the heavy component circulating material from the inlet A into the depolymerization reactor through the pipeline 02. The circulating depolymerization system starts, and the circulating material 02 and the fresh material 01 are mixed in proportion and then transported to the depolymerization reactor. As the reaction proceeds, the liquid level of the circulation tank is maintained at 30%-70%, the pressure is maintained at 10kPa-normal pressure, and the temperature is maintained at 160-200°C. After the reaction, the gas phase lactide is transported to the separation and purification system through the pipeline 04 to finally obtain polymerization grade monomer lactide.

[0041] The depolymerization reactor is a scraped film depolymerization reactor in the form of a thin film evaporator, wherein the upper reaction zone occupies 1 / 4 of the entire reaction zone.

[0042] Example 1

[0043] Preparation of lactic acid oligomers: (1) Dehydration of lactic acid: Take 10 kg of L-lactic acid (lactic acid content of about 88.0%) and add it to a four-necked flask with a stirring system. Use a vacuum circulating water pump to maintain the system pressure at about 50 kPa. Start heating under vacuum and gradually heat to 110-120 °C. Dehydrate for 2 hours. At this time, the free water in the reaction system is slowly evaporated out of the reaction system. (2) Preparation of lactic acid oligomers: After the free water in the system is almost completely removed, increase the vacuum degree of the system and slowly reduce the pressure of the system to about 1.0 kPa. The temperature of the feed liquid is gradually raised to 160 °C and reacted for 3 hours. At this time, a polycondensation reaction occurs between lactic acid molecules. The water generated by the reaction in the system is evaporated out of the system to obtain a lactic acid oligomer with a molecular weight of 2200.

[0044] Use attached Figure 1 The process flow for preparing lactide is as follows: 5000g of the above-mentioned lactic acid oligomers are taken, 50g of stannous octoate catalyst is added, and after mixing, they are transported to the scraped film depolymerization reactor at a flow rate of 150g / h. The depolymerization reaction conditions in the lower reaction zone are controlled as follows: vacuum degree 300Pa, reaction temperature 200℃, and single-pass residence time 2min. After the reaction, the heavy components are discharged into the circulation tank, and the heavy components are pumped back to the upper reaction zone of the depolymerization reactor through the circulation pump. The vacuum degree is controlled at 300Pa and the reaction temperature is 210℃. During the reaction, the mass ratio of fresh material to recycled material is regulated to 1:2, and the total feed amount is regulated to 4 times the actual reaction amount. As the reaction proceeds, the liquid level of the circulation tank is controlled to be maintained at 50%, the pressure is maintained at 50kPa, and the temperature is maintained at 180℃. Slag discharge is performed with reference to the cumulative amount of catalyst circulation. When the cumulative amount of catalyst circulation is greater than 15%, slag discharge is performed. The slag discharge of the present invention is judged by the accumulation of catalyst reaching 15%. Compared with the judgment based on the molecular weight of the oligomer in the circulation tank, the time for the molecular weight to reach 10,000 is generally much faster than the time for the accumulation of catalyst to reach 15%, thereby effectively extending the slag discharge interval.

[0045] Analysis of the crude lactide product showed that the L-lactide content was 87.1%, the m-lactide content was 2.9%, the L-lactic acid content was 3.5%, the dimer and trimer content was 3.8%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process reached 96.6%.

[0046] Example 2

[0047] The preparation process of lactic acid oligomer is the same as that of Example 1, except that the final reaction temperature is 170° C., the reaction time is 4.0 h, and the molecular weight of the obtained lactic acid oligomer is 4000.

[0048] Use attached Figure 1 The same process flow is used to prepare lactide: 5000g of the above-mentioned lactic acid oligomers are taken, 150g of stannous octoate catalyst is added, and after mixing, it is transported to the scraped film depolymerization reactor at a flow rate of 150g / h. The depolymerization reaction conditions in the lower reaction zone are controlled as follows: vacuum degree 200Pa, reaction temperature 220℃, and one-way residence time of 2min. After the reaction, the heavy component is discharged into the circulation tank, and the heavy component is pumped back to the upper reaction zone of the depolymerization reactor through the circulation pump. The vacuum degree is controlled at 200Pa and the reaction temperature is 240℃. During the reaction, the mass ratio of fresh material to recycled material is controlled to be 2:1, and the total feed amount is regulated to be 4 times the actual reaction amount. As the reaction proceeds, the liquid level of the circulation tank is controlled to be maintained at 60%, the pressure is maintained at normal pressure, and the temperature is maintained at 200℃. Regular slag discharge is carried out with reference to the cumulative amount of catalyst circulation at the outlet of the circulation tank. When the cumulative amount of catalyst circulation is greater than 15%, slag discharge is carried out.

[0049] Analysis of the crude lactide product showed that the L-lactide content was 86.5%, the m-lactide content was 3.4%, the L-lactic acid content was 2.5%, the dimer and trimer content was 4.1%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process reached 95.1%.

[0050] Example 3

[0051] The preparation process of lactic acid oligomer is the same as that of Example 1, except that the reaction time is 2.0 h, and the molecular weight of the prepared lactic acid oligomer is 800.

[0052] Use attached Figure 1 The same process flow is used to prepare lactide: 5000g of the above-mentioned lactic acid oligomer is taken, 20g of stannous octoate catalyst is added, and after mixing, it is transported to the thin film depolymerization reactor at a flow rate of 150g / h. The depolymerization reaction conditions in the lower reaction zone are controlled as follows: vacuum degree 1500Pa, reaction temperature 180℃, and one-way residence time of 2min. After the reaction, the heavy component is discharged into the circulation tank, and the heavy component is pumped back to the upper reaction zone of the depolymerization reactor by a pump, and the vacuum degree is controlled to be 1500Pa and the reaction temperature is 210℃. During the reaction, the mass ratio of fresh material to circulating material is regulated to be 1:6, and the total feed amount is regulated to be 8 times the actual reaction amount. As the reaction proceeds, the liquid level of the circulation tank is controlled to be maintained at 70%, the pressure is maintained at 10kPa, and the temperature is maintained at 160℃. Regular slag discharge is carried out with reference to the cumulative amount of catalyst circulation at the outlet of the circulation tank. When the cumulative amount of catalyst circulation is greater than 15%, slag discharge is carried out.

[0053] Analysis of the crude lactide product showed that the L-lactide content was 85.7%, the m-lactide content was 2.0%, the L-lactic acid content was 6.0%, the dimer and trimer content was 5.0%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process reached 95.7%.

[0054] Example 4

[0055] The preparation process of lactic acid oligomers and lactide was the same as that of Example 1, except that the tin catalyst used was SnCl2, and the addition amount was 25 g. The crude lactide generated after the reaction was analyzed, and the L-lactide content was 85.2%. m -The lactide content is 3.1%, the L-lactic acid content is 4.0%, the dimer and trimer content is 4.3%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process is 96.0%.

[0056] Example 5

[0057] The preparation process of lactic acid oligomers and lactide was the same as that of Example 1, except that the tin catalyst used was SnO, and the addition amount was 17 g. The crude lactide generated after the reaction was analyzed, and the L-lactide content was 85.6%. m -Lactide content is 3.2%, L-lactic acid content is 3.7%, dimer and trimer content is 4.3%, and the conversion rate of lactic acid oligomers in the crude lactide synthesis process is 95.4%.

[0058] Example 6

[0059] The preparation process of lactic acid oligomers and lactide was the same as in Example 1, except that a molecular distillation depolymerization evaporator was used. After analysis, the L-lactide content in the lactide product was 88.1%. m -The lactide content is 2.0%, the L-lactic acid content is 4.5%, the dimer and trimer content is 3.6%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process is 97.5%.

[0060] Example 7

[0061] The preparation process of lactic acid oligomers and lactide is the same as that of Example 1, except that as the reaction proceeds, the molecular weight of the heavy component in the circulation tank is monitored in real time, and the reaction temperature is gradually increased to control the reaction process. When the molecular weight is higher than 3000, the temperature is increased by 5°C, when the molecular weight is higher than 4000, the temperature is further increased by 8°C, and when the molecular weight is higher than 5000, the temperature is further increased by 10°C, and the final temperature is 233°C. After analysis, the L-lactide content in the lactide product is 88.1%, m -Lactide content is 2.0%, L-lactic acid content is 3.2%, dimer and trimer content is 3.3%, and the conversion rate of lactic acid oligomers in the crude lactide synthesis process is 97.8%.

[0062] Comparative Example 1

[0063] The preparation process of lactic acid oligomers and lactide was the same as that of Example 1, except that the depolymerization reactor was a kettle reactor. The crude lactide obtained had an L-lactide content of 82.7%, an m-lactide content of 6.7%, an L-lactic acid content of 3.8%, a dimer and a trimer content of 4.9%, and a lactic acid oligomer conversion rate of 92.6% in the crude lactide synthesis process.

[0064] Comparative Example 2

[0065] The preparation process of lactic acid oligomers and lactide is the same as that of Example 1, except that the liquid level of the circulation tank is not controlled by feedback regulation, the pressure of the circulation tank is the same as the pressure of the scraped film depolymerization reactor, and the temperature is 180° C. The test results show that because there is no feedback regulation, the circulation flow rate remains unchanged, which slows down the reaction rate, and the liquid level in the storage tank continues to rise, affecting the stable balance operation of the inlet and outlet materials, and the lactide content in the reaction product is less than 50%.

[0066] Comparative Example 3

[0067] The preparation process of lactic acid oligomers and lactide is the same as that of Example 1, except that the feed amount is fed according to the actual reaction amount, and the feed amount is not regulated according to the method of the present invention. The test results show that the lower half of the membrane surface of the scraped film depolymerization reactor is in a viscous state, and the bottom is easy to coke and carbonize. The crude lactide obtained after condensation of the obtained gas phase components has an L-lactide content of 86.2%, an m-lactide content of 6.2%, an L-lactic acid content of 4.2%, and a dimer and trimer content of 3.6%. The conversion rate of lactic acid oligomers in the crude lactide synthesis process is only 69.4%.

[0068] Comparative Example 4

[0069] The preparation process of lactic acid oligomers and lactide is the same as that of Example 1, except that the heavy components after depolymerization are mixed with fresh materials and then fed into the depolymerization reactor. During the long-term reaction process, the heavy components will accumulate more and more, which may easily cause the reaction substrate to coke and carbonize on the reactor surface, requiring frequent discharge, and the lactide content in the product is less than 80%.

Claims

1. A method for preparing crude lactide by continuous depolymerization, characterized in that The invention comprises the following contents: after the lactic acid oligomers are mixed with the depolymerization catalyst, the mixture is continuously fed into the depolymerization reactor through the feed port arranged on the wall of the depolymerization reactor for reaction; after the reaction, the heavy component enters the circulation tank and is transported back to the depolymerization reactor through the circulation material inlet arranged above the feed port; the upper reaction zone is between the feed port and the circulation material inlet, and the lower part of the feed port is the lower reaction zone; after the heavy component reacts in the upper reaction zone, the heavy component enters the lower reaction zone for further reaction, and the gas phase component, i.e., the crude lactide, is extracted from the top of the reactor; the depolymerization reactor is a scraped film depolymerization reactor; the upper reaction zone of the depolymerization reactor occupies 1 / 5-1 / 3 of the entire reaction zone; the upper reaction zone The reaction temperature is 200-240°C, and the vacuum degree is 200-1500Pa; the reaction temperature of the lower reaction zone is 180-220°C, and the vacuum degree is 200-1500Pa; the feed mass ratio of fresh material to recycled material is 1:6-2:1; the feed is fed in an increased feed amount manner, and the total feed amount of fresh material and recycled material is regulated to be 4-8 times the actual reaction amount; the liquid level of the circulation tank is maintained at 50%-70%, the pressure is maintained at 10kPa-normal pressure, and the temperature is maintained at 160-200°C; regular slagging is performed with reference to the cumulative amount of catalyst circulation, and slagging is performed when the cumulative amount of catalyst circulation is greater than 15%.

2. The method according to claim 1, characterized in that: The molecular weight of the lactic acid oligomer is 800-4000.

3. The method according to claim 2, characterized in that: The molecular weight of the lactic acid oligomer is 1200-3000.

4. The method according to claim 1, 2 or 3, characterized in that: The lactic acid oligomer is prepared by dehydration and polycondensation process with L-lactic acid or D-lactic acid as raw material; the dehydration is to remove free water in the lactic acid under 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.

5. The method according to claim 1, characterized in that: The depolymerization catalyst is a tin catalyst.

6. The method according to claim 5, characterized in that: The depolymerization catalyst is at least one of stannous octoate, SnCl2, and SnO.

7. The method according to claim 1, 5 or 6, characterized in that: The depolymerization catalyst is used in an amount of 0.4%-3.0% of the amount of the lactic acid oligomers.

8. The method according to claim 7, characterized in that: The depolymerization catalyst is used in an amount of 0.8%-2.0% of the amount of the lactic acid oligomers.

9. The method according to claim 1, characterized in that: The main forms of the scraped film depolymerization reactor include any one of a thin film evaporator, a molecular distillation evaporator or other stirred film evaporators.

10. The method according to claim 1, characterized in that: The reaction temperature in the upper reaction zone is 210-240°C.

11. The method according to claim 1, characterized in that: As the reaction proceeds, the molecular weight of the heavy component in the circulation tank is monitored in real time and the reaction progress is regulated by gradually increasing the reaction temperature. When the molecular weight is higher than 3000, the temperature is increased by 2-10°C. When the molecular weight is higher than 4000, the temperature is further increased by 2-10°C. When the molecular weight is higher than 5000, the temperature is further increased by 2-10°C. The final temperature does not exceed 240°C, thereby achieving effective depolymerization of the heavy component.

12. The method according to claim 1, characterized in that: The reaction temperature of the lower reaction zone of the depolymerization reactor is 190-220°C.

13. The method according to claim 1, characterized in that: The crude lactide product obtained has an L-lactide content of 85%-90%. m -Lactide content is 0.5%-3.5%, L-lactic acid content is 2.0%-6.0%, dimer and trimer content is 2.0%-5.0%.

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