A method and system for preparing crude lactide by cyclic depolymerization

Abstract

The present invention relates to a method and system for preparing crude lactide by cyclic depolymerization, wherein lactic acid oligomers, a depolymerization catalyst and an inhibitor are mixed, and then transported to a reactor through a feed port provided on the wall of a depolymerization reactor for reaction, and after the reaction, the heavy component enters a circulation tank and is divided into two paths to return to the depolymerization reactor, the first path is mixed with fresh materials, i.e., lactic acid oligomers, and returned to the reactor, and the second path is returned to the reactor through a circulating material inlet provided above the feed port, wherein the area between the feed port and the circulating material inlet is set as an upper reaction zone, and the area below the feed port is a lower reaction zone, and after the heavy component reacts in the upper reaction zone, it enters the lower reaction zone to continue the reaction. The present invention adopts a cyclic depolymerization process, performs zoning treatment on the heavy component circulating material, and combines the use of a specific inhibitor, effectively solving the problem of increasing heavy components and catalyst accumulation during long-term operation, and reducing the degree of racemization of lactide and the probability of substrate coking and carbonization.

Description

Technical Field

[0001] The invention belongs to the technical field of degradable materials, and in particular relates to a method and a system for preparing crude lactide by cyclic 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 and increasing the difficulty of subsequent refining. 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 oligomers on the surface of the reactor.

[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 of 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 in 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 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%. The heavy components after depolymerization of the invention are mixed with the fresh raw materials of the reaction to form a stream of materials, which enter the depolymerization reactor for reaction. However, in long-term operation, with the increase of the molecular weight of the heavy components and the accumulation of catalysts, on the one hand, the probability of coking and carbonization of the reaction substrate on the reactor surface is easily increased, and slag discharge is required after a certain period of operation. Frequent slag discharge not only affects the stable operation of the device, but also reduces the yield of lactide; on the other hand, it is easy to cause the racemization of lactide, which increases the difficulty of subsequent refining and purification. 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 cyclic depolymerization. The present invention adopts a cyclic depolymerization process, performs partitioning treatment on the heavy component circulating material, and combines the use of a specific polymerization inhibitor, effectively solving the problem of increasing heavy components and catalyst accumulation during long-term operation, reducing the racemization degree of lactide and the probability of substrate coking and carbonization, and improving the conversion rate of oligomers.

[0008] The present invention provides a method for preparing crude lactide by cyclic depolymerization, comprising the following contents:

[0009] Lactic acid oligomers, depolymerization catalyst and polymerization inhibitor are mixed and transported to the reactor through a feed port arranged on the wall of the depolymerization reactor for reaction. After the reaction, the heavy component enters a circulation tank and is divided into two paths to return to the depolymerization reactor. The first path is mixed with fresh materials, i.e., lactic acid oligomers, and returned to the reactor. The second path returns to the reactor through a circulation material inlet arranged above the feed port, wherein the area between the feed port and the circulation material inlet is an upper reaction zone, and the area below the feed port is a lower reaction zone. After the heavy component reacts in the upper reaction zone, it enters the lower reaction zone to continue reacting, and the gas phase component, i.e., crude lactide, is extracted from the top of the reactor. After running for a period of time, it is detected that the amount of depolymerization catalyst and polymerization inhibitor in the circulation tank meets the reaction requirements, and the addition of depolymerization catalyst and polymerization inhibitor to the fresh material is stopped.

[0010] In the above method, the molecular weight of the lactic acid oligomer is 800-4000, preferably 1200-3000. 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 in 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.

[0012] In the above method, the amount of the depolymerization catalyst is 0.4%-3.0%, preferably 0.8%-2.0%, based on the amount of the lactic acid oligomers.

[0013] In the above method, the polymerization inhibitor is an iron compound, preferably a trivalent iron compound, specifically one or more of iron oxide, iron chloride, iron lactate, etc. The amount of the polymerization inhibitor added is 0.03%-0.2% of the weight of the lactic acid oligomer.

[0014] 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.

[0015] In the above method, a delivery pump is provided between the depolymerization reactor and the circulation tank for delivering materials from high vacuum to low vacuum or normal pressure, and at the same time plays the role of isolating high and low vacuum.

[0016] In the above method, the feed amount is increased, and the total feed amount of fresh materials and recycled materials is adjusted to 4-8 times the actual reaction amount, so as to reduce the residence time of lactic acid oligomers on the surface of the depolymerization reactor, inhibit the occurrence of the polymerization process, improve the substrate conversion rate, and ensure product quality.

[0017] In the above method, the feed mass ratio of fresh material to recycled material is 1:6-2:1.

[0018] In the above method, the heavy component circulating material is divided into two paths and returned to the depolymerization reactor, and the mass ratio of the first path circulating material to the second path circulating material is 1:1-1:5.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] In the above method, after the depolymerization reactor has been running for a period of time, such as 1-3 days, the amount of depolymerization catalyst and polymerization inhibitor in the circulation tank is detected. When the amount of depolymerization catalyst is 0.8%-3.0% of the mass of fresh material, the addition of catalyst is stopped; when the amount of polymerization inhibitor is 0.1%-0.2% of the mass of fresh material, the addition of polymerization inhibitor is stopped; when the detected value is less than the above required amount, additional addition is performed.

[0023] 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.

[0024] In the above method, the crude lactide product obtained has an L-lactide content of 88%-95%. m -Lactide content is 0.5%-3.0%, L-lactic acid content is 1.5%-5.0%, dimer and trimer content is 2.0%-5.0%.

[0025] The present invention also provides a system for preparing crude lactide by cyclic depolymerization, which mainly comprises a depolymerization reactor and a circulation tank, wherein the depolymerization reactor is used for carrying out a depolymerization reaction on lactic acid oligomers in the presence of a depolymerization catalyst, a feed port is arranged on the wall of the depolymerization reactor, a circulation material inlet is arranged above the feed port, an upper reaction zone is between the feed port and the circulation material inlet, and a lower reaction zone is arranged below the feed port. After the reaction, heavy components enter the circulation tank and are divided into two circulation materials to return to the depolymerization reactor. The first circulation material is mixed with fresh materials and returns to the lower reaction zone through the feed port, and the second circulation material returns to the upper reaction zone through the circulation material inlet. After the reaction, the materials enter the lower reaction zone to continue the reaction, and the gas phase components after the reaction are discharged from the top of the depolymerization reactor.

[0026] In the system of the present invention, the depolymerization reactor is a scraped film depolymerization reactor, and its main forms include any one of thin film evaporator, molecular distillation evaporator or other stirred film evaporator, etc. The upper reaction zone of the depolymerization reactor occupies 1 / 5-1 / 3 of the entire reaction zone.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] (1) The inventors of this application found in experiments that, when a cyclic depolymerization process is used, if the depolymerized heavy components are directly mixed with fresh materials and then fed into the depolymerization reactor, the heavy components and catalysts will accumulate more and more, which may easily lead to coking and carbonization of the substrate and racemization of lactide in the reactor, and long-term stable operation cannot be achieved. To this end, the inventors divided the heavy components into two routes and circulated them back to the reactor, and combined them with the use of specific inhibitors, which effectively solved the problems of increasing heavy components and catalyst accumulation during long-term operation, reduced the racemization degree of lactide and the probability of coking and carbonization of the substrate, and improved the conversion rate of lactic acid oligomers and product quality. In the obtained crude lactide, the content of m-lactide is not more than 3.0%, and the conversion rate of lactic acid oligomers can reach more than 95%. Compared with the traditional cyclic depolymerization process, the racemization degree of lactide is reduced by more than 50%, and the conversion rate of lactic acid oligomers is increased by more than 10%.

[0029] (2) The heavy component recycling material is divided into two routes and returned to two different reaction zones of the depolymerization reactor. By adjusting the appropriate reaction conditions and feed ratio, when the amount of depolymerization catalyst and inhibitor in the circulation tank reaches the reaction requirements, the addition of catalyst and inhibitor in the fresh material is stopped, avoiding the problem of excessive accumulation caused by continuous addition of catalyst and inhibitor. At the same time, the catalyst and inhibitor of the subsequent fresh material are provided in whole or in part through the heavy component of the recycling material side line, reducing the consumption of reagents.

[0030] (3) The inhibitor of the present invention can effectively prevent excessive polymerization of materials in the depolymerization reactor, avoid deterioration of the physical properties of the circulating materials, and thus adversely affect the depolymerization. It can ensure the effective depolymerization of lactic acid oligomers, improve the conversion rate of lactic acid oligomers, facilitate the continuous and stable operation of the depolymerization process, and ensure product quality.

[0031] (4) The reaction process was regulated by increasing the total feed amount of recycled materials and fresh materials to reduce the residence time of lactic acid oligomers on the reactor surface, inhibit the occurrence of the polymerization process, increase the overall yield, and reduce the degree of racemization of lactide. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A schematic flow chart of the method and system for preparing crude lactide according to the present invention;

[0033] Wherein: 1-depolymerization reactor, 2-circulation tank, 3-upper reaction zone, 4-lower reaction zone, 5-motor, 6-scraper, 7-transfer pump, 8-circulation pump, 9-mixer, 10-diversion device; 01-fresh material, 02-second circulating material, 03-heavy component, 04-gas phase crude lactide, 05-first circulating material; A-circulation feed port, B-feed port. DETAILED DESCRIPTION

[0034] 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.

[0035] 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.

[0036] 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%.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] The calculation formula for the conversion rate of lactic acid oligomers in the crude lactide synthesis process is as follows:

[0041] Lactic acid oligomer conversion rate = m / M × 100%

[0042] Wherein, m is the total mass of crude lactide, and M is the mass of initial lactic acid oligomers.

[0043] The embodiment of the present invention adopts Figure 1 The circulating depolymerization system mainly includes a depolymerization reactor 1 and a circulation tank 2. A feed port B is provided on the wall of the depolymerization reactor, and a circulating material inlet A is provided above the feed port B. The upper reaction zone 3 is between the feed port B and the circulating material inlet A, and the lower reaction zone 4 is below the feed port. First, the fresh material 01 is continuously transported from the feed port B to the depolymerization reactor 1 for depolymerization reaction. After the reaction, the heavy component is discharged into the circulation tank 2 through the delivery pump 7, a certain liquid level is established, and the circulation pump 8 is operated. Through the diversion device 10, the first circulating material 05 and the fresh material 01 are mixed in proportion in the mixer 9 and then transported to the lower reaction zone of the depolymerization reactor through the feed port B. The first circulating material 02 is transported from the circulating feed port A to the upper reaction zone of the depolymerization reactor. After the reaction, the material enters the lower reaction zone to continue the reaction. 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℃. After the reaction, the crude lactide 04 in the gas phase is discharged from the top of the depolymerization reactor and transported to the separation and purification system to finally obtain polymerization-grade monomer lactide.

[0044] 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.

[0045] Example 1

[0046] 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.

[0047] 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 and 3g of ferric lactate are 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 component is discharged into the circulation tank, and the total feed amount entering the reactor is regulated to be 4 times the actual reaction amount, wherein the feed mass ratio of fresh material to circulating material is 1:3, and the heavy component circulating material is divided into two paths to return to the depolymerization reactor, and the mass ratio of the first circulating material to the second circulating material is 1:2. The vacuum degree of the upper reaction zone is regulated at 300Pa, and the reaction temperature is 210℃. 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℃. After running for 2 days, the circulating tank material was sampled and analyzed. When the amount of depolymerization catalyst was 1.5% and the amount of inhibitor was 0.15%, the addition of depolymerization catalyst and inhibitor was stopped. After the system stabilized, the crude lactide product was sampled and analyzed. The L-lactide content was 89.3%, the m-lactide content was 2.1%, the L-lactic acid content was 2.9%, the dimer and trimer content was 3.3%, and the conversion rate of lactic acid oligomers in the crude lactide synthesis process reached 97.9%.

[0048] Example 2

[0049] 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.

[0050] Use attached Figure 1The process flow for preparing lactide is as follows: 5000g of the above-mentioned lactic acid oligomers are taken, 150g of stannous octoate catalyst is added, 10g of ferric lactate 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 200Pa, reaction temperature 220℃, and single-pass residence time 2min. After the reaction, the heavy component is discharged into the circulation tank, and the total feed amount entering the reactor is regulated to be 5 times the actual reaction amount, wherein the feed mass ratio of fresh material to circulating material is 1:4, and the heavy component circulating material is divided into two paths and returned to the depolymerization reactor, wherein the mass ratio of the first circulating material to the second circulating material is 1:3. The vacuum degree of the upper reaction zone is regulated at 200Pa, and the reaction temperature is 240℃. 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℃. After running for 2 days, the circulating tank material was sampled and analyzed. When the amount of depolymerization catalyst was 3% and the amount of inhibitor was 0.2%, the addition of depolymerization catalyst and inhibitor was stopped. After the system stabilized, the crude lactide product was sampled and analyzed. The L-lactide content was 88.2%, the m-lactide content was 2.7%, the L-lactic acid content was 3.1%, the dimer and trimer content was 3.5%, and the conversion rate of lactic acid oligomers in the crude lactide synthesis process reached 97.1%.

[0051] Example 3

[0052] 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.

[0053] Use attached Figure 1The process flow for preparing lactide is as follows: 5000g of the above-mentioned lactic acid oligomers are taken, 20g of stannous octoate catalyst is added, 1.5g of ferric lactate 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 1500Pa, reaction temperature 180℃, and single-pass residence time 2min. After the reaction, the heavy component is discharged into the circulation tank, and the total feed amount entering the reactor is regulated to be 8 times the actual reaction amount, wherein the feed mass ratio of fresh material to circulating material is 1:2, and the heavy component circulating material is divided into two paths to return to the depolymerization reactor, and the mass ratio of the first circulating material to the second circulating material is 1:1. The vacuum degree of the upper reaction zone is regulated at 1500Pa, and the reaction temperature is 210℃. 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 160℃. The circulating tank material was sampled and analyzed. When the amount of depolymerization catalyst was 1.2% and the amount of polymerization inhibitor was 0.06%, the addition of depolymerization catalyst and polymerization inhibitor was stopped. After the system was stable, the crude lactide product was sampled and analyzed. The L-lactide content was 87.7%, the m-lactide content was 2.4%, the L-lactic acid content was 3.6%, the dimer and trimer content was 3.9%, and the conversion rate of lactic acid oligomers in the crude lactide synthesis process reached 96.9%.

[0054] Example 4

[0055] The preparation process of lactic acid oligomers and lactide was the same as in 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 88.3%. m -Lactide content is 2.8%, L-lactic acid content is 3.6%, dimer and trimer content is 3.5%, and the conversion rate of lactic acid oligomers in the crude lactide synthesis process is 97.1%.

[0056] Example 5

[0057] The preparation process of lactic acid oligomers and lactide was the same as in 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 88.9%. m -The lactide content is 3.0%, the L-lactic acid content is 3.2%, the dimer and trimer content is 3.3%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process is 97.4%.

[0058] Example 6

[0059] The preparation process of lactic acid oligomers and lactide was the same as in Example 1, except that the polymerization inhibitor used was ferric chloride, and the amount added was 2 g. The crude lactide generated after the reaction was analyzed, and the L-lactide content was 88.6%. m-Lactide content is 2.6%, L-lactic acid content is 3.0%, dimer and trimer content is 3.6%, and the conversion rate of lactic acid oligomers in the crude lactide synthesis process is 97.2%.

[0060] Example 7

[0061] The preparation process of lactic acid oligomers and lactide was the same as in Example 1, except that the polymerization inhibitor used was iron oxide, and the amount added was 2 g. The crude lactide generated after the reaction was analyzed, and the L-lactide content was 88.8%, m -The lactide content is 2.9%, the L-lactic acid content is 2.4%, the dimer and trimer content is 3.5%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process is 97.5%.

[0062] Example 8

[0063] 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 89.1%. m -The lactide content is 2.0%, the L-lactic acid content is 3.5%, the dimer and trimer content is 3.3%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process is 97.3%.

[0064] Comparative Example 1

[0065] The preparation process of lactic acid oligomers and lactide is the same as that of Example 1, except that no polymerization inhibitor is added. The crude lactide product is analyzed to have an L-lactide content of 87.2%, an m-lactide content of 2.8%, an L-lactic acid content of 3.5%, a dimer and a trimer content of 3.8%, and a lactic acid oligomer conversion rate of 90.2% in the synthesis process. Without adding a polymerization inhibitor, the conversion rate of L-lactide and lactic acid oligomers is low.

[0066] Comparative Example 2

[0067] The preparation process of lactic acid oligomers and lactide was the same as in Example 1, except that a kettle reactor was used instead of a scraped film depolymerization reactor. The crude lactide obtained had an L-lactide content of 82.4%, an m-lactide content of 6.8%, an L-lactic acid content of 3.8%, a dimer and a trimer content of 5.0%, and a lactic acid oligomer conversion rate of 91.5%.

[0068] Comparative Example 3

[0069] The preparation process of lactic acid oligomers and lactide was the same as in Example 1, except that the catalyst and polymerization inhibitor were always added, and the amount of depolymerization catalyst and polymerization inhibitor in the circulation tank was not detected to stop adding catalyst and polymerization inhibitor for regulation. The L-lactide content in the obtained crude lactide was 85.4%, mThe content of -lactide is 5.2%, the content of L-lactic acid is 3.6%, the content of dimer and trimer is 4.3%, and the conversion rate of lactic acid oligomers in the synthesis process is 92.3%.

[0070] Comparative Example 4

[0071] 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%.

[0072] Comparative Example 5

[0073] The preparation process of lactic acid oligomers and lactide is the same as that of Example 1, except that the feed amount is fed at 2 times the actual reaction amount, and is not regulated according to the multiple set by 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 prone to coking and carbonization. The crude lactide obtained after condensation of the obtained gas phase component has an L-lactide content of 84.4%, an m-lactide content of 4.9%, an L-lactic acid content of 4.2%, and a dimer and trimer content of 3.4%. The conversion rate of lactic acid oligomers in the crude lactide synthesis process is only 78.5%.

[0074] Comparative Example 6

[0075] 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 cyclic depolymerization, characterized in that The invention comprises the following contents: after mixing lactic acid oligomers, depolymerization catalyst and polymerization inhibitor, the mixture is transported to the reactor through a feed port arranged on the wall of the depolymerization reactor for reaction; after the reaction, the heavy component enters a circulation tank and is divided into two paths to return to the depolymerization reactor; the first path is mixed with fresh materials, i.e. lactic acid oligomers, and returned to the reactor; the second path is returned to the reactor through a circulation material inlet arranged above the feed port; the upper reaction zone is set 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 to continue the reaction, and the gas phase component, i.e. crude lactide, is extracted from the top of the reactor; after running for a period of time, it is detected that the amount of depolymerization catalyst and polymerization inhibitor in the circulation tank meets the reaction requirements, and the addition of depolymerization catalyst and polymerization inhibitor to the fresh materials is stopped; the polymerization inhibitor is a trivalent iron compound; the depolymerization reactor is a scraped film depolymerization reactor; the upper reaction zone of the depolymerization reactor occupies the entire The reaction temperature of the upper reaction zone of the depolymerization reactor is 200-240° C., and the vacuum degree is 200-1500 Pa; the reaction temperature of the lower reaction zone of the depolymerization reactor is 180-220° C., and the vacuum degree is 200-1500 Pa; the feeding is carried out in a manner of increasing the feeding amount, and the total feeding amount of fresh materials and recycled materials is regulated to be 4-8 times of the actual reaction amount; the feeding mass ratio of fresh materials to recycled materials is 1:6-2:1; the liquid level of the circulation tank is maintained at 50%-70%; after the depolymerization reactor has been running for 1-3 days, the amounts of depolymerization catalyst and inhibitor in the circulation tank are detected, and when the amount of depolymerization catalyst is 0.8%-3.0% of the mass of fresh materials, the addition of catalyst is stopped; when the amount of inhibitor is 0.1%-0.2% of the mass of fresh materials, the addition of inhibitor is stopped; when the detected value is less than the above-mentioned required amount, additional addition is performed.

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 1, 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 amount of the depolymerization catalyst used is 0.4%-3.0% of the amount of the lactic acid oligomers.

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

9. The method according to claim 1, characterized in that: The inhibitor is one or more of ferric oxide, ferric chloride and ferric lactate.

10. The method according to claim 1 or 9, characterized in that: The amount of the polymerization inhibitor added is 0.03%-0.2% of the weight of the lactic acid oligomer.

11. 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.

12. The method according to claim 1, characterized in that: The heavy component circulating material is divided into two paths and returned to the depolymerization reactor, and the mass ratio of the first path circulating material to the second path circulating material is 1:1-1:

5.

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

14. 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.

15. The method according to claim 1, characterized in that: The pressure of the circulation tank is maintained at 10 kPa-normal pressure, and the temperature is maintained at 160-200°C.

16. The method according to claim 1, characterized in that: The crude lactide product obtained has an L-lactide content of 88%-95%. m -Lactide content is 0.5%-3.0%, L-lactic acid content is 1.5%-5.0%, dimer and trimer content is 2.0%-5.0%.

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