A method and device for continuously preparing lactide

Abstract

The present invention relates to a method and device for continuously preparing lactide, wherein lactic acid oligomers are mixed with a depolymerization catalyst, and then continuously transported to a depolymerization reactor for reaction, and after the reaction, the heavy components are discharged into a circulation tank, mixed with fresh lactic acid oligomers, and then transported to a depolymerization reactor, and after the reaction, the gas phase components enter a condensation system, and the generated crude lactide enters a storage tank; the crude lactide is transported to a distillation system for purification to obtain polymerization-grade monomer lactide. The present invention adopts a cyclic depolymerization-condensation-distillation combined process to achieve continuous preparation of lactide, especially reducing the racemization degree of lactide and the probability of substrate coking and carbonization, and improving product yield and production efficiency.

Description

Technical Field

[0001] The invention belongs to the technical field of degradable materials, and particularly relates to a method and a device for continuously preparing lactide. Background Art

[0002] At present, the world consumes 120 million tons of disposable plastic products every year, only 10% of which 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 plastic waste in the world's oceans 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. Nearly 90 countries and regions around the world have introduced relevant policies or regulations to control or prohibit disposable non-degradable plastic products.

[0003] 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 the most outstanding application prospects. 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. At present, the global PLA production capacity is about 500,000 tons, and the output is more than 200,000 tons. The main manufacturers include NatureWorks in the United States, Corbin Purac in the Netherlands, Mitsubishi in Japan, Zhejiang Hisun, etc., among which NatureWorks and Corbin Purac in the Netherlands account for about 70% of the global production capacity. There are not many production lines that have been built and put into production in China. Zhejiang Haizheng is currently the largest one. With the gradual implementation of the domestic plastic ban, my country's PLA production capacity expansion has accelerated. As of the end of November 2020, according to incomplete statistics, there are at least 1.53 million tons of PLA projects under construction or planned in China.

[0004] Industrialized polylactic acid synthesis 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 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 process barriers are relatively high. It is usually prepared through condensation and depolymerization processes under catalysts, high temperatures, and high vacuum systems, and this process is prone to cause lactide racemization. This is mainly because the depolymerization process mainly occurs in the "back-biting" ester catalytic reaction on the lactic acid oligomer molecular chain. The specific process is: under the action of catalyst, high temperature and high vacuum, the carbonyl group on the lactic acid oligomer chain is activated, and the hydroxyl group at the beginning of the chain attacks the positively charged carbonyl group, so that the ester bond breaks ("positive bite" process), forming L / D-lactide; but in the presence of alkaline oxides, excess catalyst or too high temperature, 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 ("back bite" process), and the configuration is reversed to obtain meso-lactide ( m -lactide), as shown in formula (1). m -The presence of lactide, on the one hand, will affect the optical purity of lactide, and then affect the ring-opening polymerization process of lactide, making the molecular weight of the obtained PLA low; on the other hand, it will destroy the regularity of the PLA structure, reduce its crystallinity and mechanical properties.

[0005] Therefore, the crude lactide obtained by depolymerization usually needs to be refined through processes such as solvent recrystallization, water extraction, distillation, and melt crystallization to reduce the content of lactide in the product. m -lactide; and due to L-lactide and m -Lactide has similar physical and chemical properties, and lactide itself has high freezing point, boiling point and thermal sensitivity, which makes separation difficult, and the overall yield is low, only about 40%-60%, and the overall economic efficiency is also low. Therefore, the racemization of lactide synthesis is a key factor affecting the quality and yield of lactide, and is also the focus and difficulty of current lactide technology research at home and abroad.

[0006]

[0007] Formula (1)

[0008] Morteza Ehsani et al. studied the effects of temperature, catalyst and pressure on the depolymerization process in detail in the article "Lactide synthesis optimization: investigation of the temperature, catalyst and pressure effects". When investigating the reaction temperature, they found that when the reaction temperature was low, m -Lactide is produced less, as the temperature rises,m -Lactide content increased significantly when the reaction temperature was 230℃. m -The lactide content reached 25.52%; when investigating the effects of stannous oxide, stannous chloride, stannous octoate, antimony trioxide and sulfuric acid on the lactide synthesis process, it was found that the lactide product obtained by using SnCl2 and sulfuric acid as catalysts had the highest purity, and m -The lactide content is the lowest; but the catalyst concentration cannot be too high. Taking SnCl2 as an example, as the SnCl2 concentration increases, the lactide production yield increases, but too much catalyst will increase the racemization reaction rate.

[0009] 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, which can effectively reduce the probability of lactide racemization in the depolymerization process, 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 cause coking and carbonization of the oligomers on the surface of the falling film tube reactor on the other hand.

[0010] CN111153886A discloses a method and device for synthesizing lactide in a fast and high yield, wherein a single component of lactic acid or a two-component of lactic acid and a catalyst is used to enter an oligomer preparation system through a mixer, and the residence time is increased through bottom circulation to synthesize oligomeric lactic acid. The gas phase component is passed through a distillation system to increase the yield of oligomeric lactic acid; the oligomeric lactic acid is passed through a purification device to remove unreacted lactic acid and water; the oligomeric lactic acid after light removal is added with a catalyst and passed through a mixer to enter a depolymerization reactor for depolymerization into lactide, and the heavy component is refluxed and enters the depolymerization reactor again, and the light component is passed through a purification and recovery system to obtain a lactide product. This device can be used to efficiently synthesize lactide, and a crude lactide with a yield of 94%-98% can be obtained within a short residence time of 0.5-5 minutes. After the light component is 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 invention directly refluxes the heavy components into the depolymerization reactor after depolymerization, which not only affects the stability of the depolymerization reaction, 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. Summary of the invention

[0011] In view of the shortcomings of the prior art, the present invention provides a method and device for continuously preparing lactide. The present invention realizes the continuous preparation of lactide by adopting a cyclic depolymerization-condensation-distillation combined process, especially reduces the degree of lactide racemization and the probability of substrate coking and carbonization, and improves the product yield and production efficiency.

[0012] The present invention provides a method for continuously preparing lactide, comprising the following steps:

[0013] (1) After the lactic acid oligomers are mixed with the depolymerization catalyst, they are continuously transported to the depolymerization reactor for reaction. 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 gas phase components enter the condensation system;

[0014] (2) After condensation, the crude lactide produced enters the storage tank; then the crude lactide is transported to the distillation system for purification to obtain polymerization-grade monomer lactide.

[0015] In the above method, the molecular weight of the lactic acid oligomer in step (1) is 800-4000, preferably 1200-2800. The lactic acid oligomer is prepared by dehydration and polycondensation process using L-lactic acid or / and D-lactic acid as raw materials. 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 vacuum degree is 1000-2000Pa, and the reaction time is 0.5-4.0h.

[0016] In the above method, the depolymerization catalyst in step (1) 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 mass of the lactic acid oligomers.

[0017] In the above method, the depolymerization reactor 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.

[0018] In the above method, the depolymerization reaction temperature in step (1) is 180-240°C, preferably 190-210°C, and the vacuum degree is 200-1500Pa.

[0019] In the above method, the depolymerization reactor in step (1) and the circulation tank constitute a circulating depolymerization system, wherein the feed mass ratio of fresh material to circulating material is 1:6-2:1.

[0020] In the above method, in step (1), as the reaction proceeds, the liquid level in the circulation tank is controlled to be maintained at 50%-70%, the pressure is maintained at 10 kPa-normal pressure, and the temperature is maintained at 160-200°C, so as to reduce the probability of continued intermolecular polymerization of lactic acid oligomers and reduce coking and carbonization.

[0021] In the above method, step (1) adopts a method of increasing the feed amount. After the fresh material is mixed with the recycled material, the feed amount 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 scraped film depolymerization reactor, inhibit the occurrence of the polymerization process, reduce the degree of racemization of lactide, improve the overall yield, and ensure product quality.

[0022] In the above method, step (1) performs regular slag discharge based on the molecular weight of the lactic acid oligomer at the outlet of the circulation tank or the cumulative amount of catalyst circulation, and performs slag discharge when the molecular weight of the oligomer is greater than 6000 or the cumulative amount of catalyst circulation is greater than 10%. The lactic acid polymer discharged by the slag discharge can be hydrolyzed under certain conditions to recover lactic acid.

[0023] In the above method, the conversion rate of lactic acid oligomers in the depolymerization process in step (1) can reach more than 95.0%.

[0024] In the above method, the depolymerization reactor in step (1) 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 enters the crude lactide storage tank after condensation.

[0025] In the above method, the crude lactide product obtained in step (2) has an L-lactide content of 82%-88%, m -Lactide content is 2.0%-3.5%, L-lactic acid content is 3.0%-6.0%, dimer and trimer content is 2.0%-5.0%.

[0026] In the above method, the crude lactide distillation system in step (2) is used to remove impurity components in crude lactide, and mainly includes two or more distillation towers connected in series, at least one of which is a crude fractionation tower, which is mainly used to quickly remove water and light components such as part of lactic acid in crude lactide; at least one of which is a refining tower, which is mainly used to remove residual lactic acid, lactic acid and other light components in crude lactide. m -Lactide and lactic acid polymers, etc.

[0027] In the above method, the distillation tower used as the crude fraction tower in step (2) is in the form of packing or plates, with a bottom temperature of 150-170°C, 2-6 plates, a vacuum degree of 1.0-2.0 kPa, a tower body insulation of 60-110°C, and a top temperature of 80-120°C.

[0028] In the above method, the distillation tower used as the refining tower in step (2) adopts a plate or filler with small resistance and small pressure drop that can improve gas-liquid distribution, such as a floating jet tower, etc. The bottom reboiler temperature is 170-200°C, the number of plates is 10-20, the top vacuum degree is 200-600Pa, the top temperature is gradually increased from 110-120°C to 120-130°C, the top reflux ratio is 1:1-1:3, and the L-lactide product is withdrawn through a side line.

[0029] In the above method, the product yield of the distillation and purification process in step (2) is not less than 80%, the chemical purity and optical purity of the obtained L-lactide are not less than 99%, and the m-lactide content is not more than 2.0%. The total product yield of the entire lactide synthesis process can reach more than 78%.

[0030] The present invention also provides a device for continuously preparing lactide, which mainly includes a circulating depolymerization system, a condensation system and a distillation system, wherein the circulating depolymerization system includes a scraped film depolymerization reactor, a circulation tank and a circulation pipeline, which is used to carry out a depolymerization reaction of lactic acid oligomers in the presence of a depolymerization catalyst, and the heavy components after the reaction are discharged into the circulation tank, mixed with fresh lactic acid oligomers and then transported to the depolymerization reactor, and the gaseous components after the reaction are discharged from the top of the depolymerization reactor; the condensation system mainly includes a condenser and a storage tank, and the gaseous components are cooled by the condensation system to produce crude lactide, which is transported to the storage tank; the distillation system mainly includes a plurality of series-connected distillation towers, which are used to purify the crude lactide to obtain a polymerization-grade monomer lactide product.

[0031] In the above device, the scraped film depolymerization reactor mainly includes any one of a thin film evaporator, a molecular distillation evaporator or other stirred film evaporator.

[0032] In the above-mentioned device, the distillation system is used to remove impurity components in crude lactide, and mainly includes two or more distillation towers connected in series, wherein at least one distillation tower is a coarse fractionation tower, which adopts fillers or tower plates, and the number of tower plates is 2-6; at least one distillation tower is a refining tower, which adopts tower plates or fillers with small resistance and small pressure drop that can improve gas-liquid distribution, such as a floating jet tower, and the number of tower plates is 10-20.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] (1) The scraped film depolymerization reactor and the circulation tank are combined into a circulating depolymerization system, and combined with liquid level control, doubling the material feed amount and other operating methods, the degree of lactide racemization and the probability of substrate coking and carbonization are effectively reduced, and the product quality and lactic acid oligomer conversion rate are improved. The m-lactide content in the obtained crude lactide is not more than 3.5%, and the lactic acid oligomer conversion rate can reach more than 95%. Compared with the kettle type circulating depolymerization reactor, the degree of lactide racemization is reduced by more than 50%, and compared with the single-pass scraped film evaporator, the lactic acid oligomer conversion rate is increased by more than 10%.

[0035] (2) Based on the use of a circulating depolymerization system, the molecular weight of oligomers or the accumulation of catalysts in the circulation tank is monitored in real time and slag is discharged regularly to effectively ensure the continuous and stable operation of the depolymerization process and to ensure product quality and yield.

[0036] (3) When the recycled material is mixed with the fresh material and fed, the feed rate is increased to inhibit the polymerization process and reduce the degree of racemization of lactide. The content of m-lactide in the obtained crude lactide is not more than 3.5%.

[0037] (4) The crude lactide produced by the circulating depolymerization-condensation system is distilled and purified without the need for separation methods such as melt crystallization. Since the previous process reduces the degree of racemization, the burden of subsequent distillation and purification is reduced. In particular, the probability of subsequent distillation thermal polymerization reaction can be reduced, thereby improving the product yield while ensuring product quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 A process flow chart of the method for continuously preparing lactide according to the present invention;

[0039] Wherein: 1-depolymerization reactor, 2-circulation tank, 3-crude lactide storage tank, 4-first distillation tower, 5-second distillation tower, 6-condenser, 7-first external cooler, 8-second external cooler; 01-fresh material, 02-gas phase crude lactide, 03-circulation material, 04-liquid phase crude lactide, 05-light components at the top of the tower, 06-pre-purified lactide, 07-distillation tower reflux, 08-refined lactide, 09-heavy components at the bottom of the refining tower. DETAILED DESCRIPTION

[0040] 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.

[0041] 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.

[0042] 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%.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] The yield Y of the lactide purification process and the yield Y of the product in the entire preparation and purification process 总 The calculation formula is as follows:

[0047]

[0048] in, m 0 is the mass of crude lactide, y 0 is the purity of L-lactide in crude lactide, m is the mass of lactide product, M is the mass of lactide that a certain amount of lactic acid oligomers can theoretically be converted into, that is, the mass of lactic acid oligomers.

[0049] The specific rotation of the sample was analyzed by WZZ-2S automatic polarimeter to characterize the optical purity of the sample. The specific rotation of pure L-lactide was -278, and the specific rotation of pure D-lactide was +278. m-The specific rotation of lactide is 0, and the calculation formula of the sample optical purity X is as follows;

[0050]

[0051] Among them, α 纯物质 Represents the specific rotation of pure lactide, α 被测样品 Indicates the specific optical rotation of the substance being measured.

[0052] The embodiment of the present invention is as follows Figure 1 The process is carried out, firstly, the fresh material 01 lactic acid oligomer is continuously pumped into the scraped film depolymerization reactor 1 for reaction through a pump, and the heavy components after the reaction are discharged into the circulation tank 2. After a certain liquid level is established, the circulating depolymerization system starts, and the circulating material 03 is regulated to be mixed with the fresh material 01 in proportion and then transported to the depolymerization reactor. As the reaction proceeds, 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. After the reaction, the gas phase enters the condenser 06 for condensation, and the generated liquid phase crude lactide 04 enters the storage tank 3; the liquid phase crude lactide is transported to the distillation system for purification, first transported to the first distillation tower 4 for separation, and the gas phase 05 such as water and most of the lactic acid is discharged after condensation by the first external cooler 7. The lactide after pretreatment and purification is purified by the second distillation tower 5, and the residual lactic acid, m -Lactide 07 is returned to the distillation tower 5 for circulation, and L-lactide product 08 is taken out from the side line, and lactic acid polymers and undistilled lactide 09 are discharged from the bottom of the tower.

[0053] Example 1

[0054] Preparation of lactic acid oligomers: (1) Dehydration of lactic acid: Take 600g of L-lactic acid (lactic acid content of about 88.0%), add it to a four-necked flask with a stirring system, use a vacuum circulating water pump to maintain the system pressure at about 50kPa, start heating under vacuum, gradually heat to 110-120℃, dehydrate for 2h, at which 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, slowly reduce the pressure of the system to about 1.0kPa, gradually increase the temperature of the feed liquid to 160℃, react for 3h, at which time a polycondensation reaction occurs between lactic acid molecules, and 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.

[0055] Use attached Figure 1The process flow for preparing lactide is as follows: 3000g of the above-mentioned lactic acid oligomers are taken, 30g 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 are controlled as follows: vacuum degree is 300Pa, reaction temperature is 200℃, and single-pass residence time is about 2min. After the reaction, the heavy components are discharged into the circulation tank and mixed with fresh lactic acid oligomers and then transported to the depolymerization reactor. The mass ratio of fresh materials to circulating materials in the reaction process is controlled to be 1:2. After the fresh materials are mixed with the circulating materials, the feed amount is adjusted 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 50kPa, and the temperature is maintained at 180℃. Regular slag discharge is carried out based on the molecular weight of lactic acid oligomers at the outlet of the circulation tank or the cumulative amount of catalyst circulation. When the molecular weight of the oligomer is greater than 6000 or the cumulative amount of catalyst circulation is greater than 10%, slag discharge is carried out.

[0056] After the reaction, the gaseous components enter the condensation system at a condensation temperature of 90°C, and the resulting crude liquid lactide enters the storage tank. Analysis of the crude lactide product showed that the L-lactide content was 86.9%, the m-lactide content was 2.6%, the L-lactic acid content was 4.1%, the dimer and trimer content was 3.2%, and the conversion rate of lactic acid oligomers in the crude lactide synthesis process reached 96.7%.

[0057] The crude lactide is transported to the distillation system for purification. A two-stage distillation system is used. The first distillation tower adopts a plate tower, with a bottom temperature of 160°C, 5 plates, a vacuum degree of 1.2kPa, a tower body insulation at 105°C, and a tower top temperature of 105°C. The second distillation tower adopts a floating jet tower, with a bottom reboiler temperature of 190°C, 15 plates, a tower top vacuum degree of 200Pa, a tower top temperature gradually increased from 112°C to 123°C, a tower top reflux ratio of 1:2, and L-lactide products are produced through the side line.

[0058] After analysis, the yield of the lactide synthesis process was 81.9%, the chemical purity of the product was 99.3%, the optical purity was 99.1%, and the m-lactide content was 1.0%, which met the requirements of polymerization-grade lactide monomer.

[0059] Example 2

[0060] 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.

[0061] Use attached Figure 1The same process flow is used to prepare lactide: 3000g of the above-mentioned lactic acid oligomers are taken, 90g 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 are controlled as follows: vacuum degree (absolute pressure) is 200Pa, reaction temperature is 240℃, and single-pass residence time is about 2min. After the reaction, the heavy components are discharged into the circulation tank and mixed with fresh lactic acid oligomers and then transported to the depolymerization reactor. The mass ratio of fresh materials to circulating materials in the reaction process is controlled to be 2:1. After the fresh materials are mixed with the circulating materials, the feed amount is adjusted to be 5 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 normal pressure, and the temperature is maintained at 200℃. Regular slag discharge is carried out based on the molecular weight of lactic acid oligomers at the outlet of the circulation tank and the cumulative amount of catalyst circulation. When the molecular weight of the oligomer is greater than 6000 or the cumulative amount of catalyst circulation is greater than 10%, slag discharge is carried out.

[0062] After the reaction, the gaseous components enter the condensation system at a condensation temperature of 90°C, and the resulting crude liquid lactide enters the storage tank. Analysis of the crude lactide product showed that the L-lactide content was 83.6%, the m-lactide content was 3.4%, the L-lactic acid content was 3.0%, the dimer and trimer content was 5.0%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process reached 95.2%.

[0063] The crude lactide is transported to the distillation system for purification. A two-stage distillation system is used. The first distillation tower adopts a plate tower, with a bottom temperature of 150°C, 2 plates, a vacuum degree of 2.0 kPa, a tower body insulation at 105°C, and a tower top temperature of 105°C. The second distillation tower adopts a floating jet tower, with a bottom reboiler temperature of 200°C, 20 plates, a tower top vacuum degree of 500 Pa, a tower top temperature gradually increased from 115°C to 130°C, a tower top reflux ratio of 1:3, and L-lactide products are extracted through the side line.

[0064] After analysis, the yield of the lactide synthesis process was 80.3%, the chemical purity of the product was 99.4%, and the optical purity was 99.0%. The m-lactide content was 1.4%, which met the requirements of polymerization-grade lactide monomer.

[0065] Example 3

[0066] The preparation process of lactic acid oligomer was the same as that of Example 1, except that the reaction time was 2.0 h. GPC analysis showed that the molecular weight of the obtained lactic acid oligomer was 800.

[0067] Use attached Figure 1The same process flow is used to prepare lactide: 3000g of the above-mentioned lactic acid oligomers are taken, 12g of stannous octoate catalyst is added, and after mixing, they are transported to the thin film depolymerization reactor at a flow rate of 150g / h. The depolymerization reaction conditions are controlled as follows: vacuum degree is 1500Pa, reaction temperature is 180℃, and single-pass residence time is about 2min. After the reaction, the heavy components are discharged into the circulation tank and mixed with fresh lactic acid oligomers and then transported to the depolymerization reactor. The mass ratio of fresh materials to circulating materials in the reaction process is controlled to be 1:6. After the fresh materials are mixed with the circulating materials, the feed amount is adjusted to 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 based on the molecular weight of the lactic acid oligomer at the outlet of the circulation tank and the cumulative amount of catalyst circulation. When the molecular weight of the oligomer is greater than 6000, or the cumulative amount of catalyst circulation is greater than 10%, slag discharge is carried out.

[0068] After the reaction, the gaseous components enter the condensation system at a condensation temperature of 90°C, and the resulting crude liquid lactide enters the storage tank. Analysis of the crude lactide product showed that the L-lactide content was 82.2%, the m-lactide content was 2.1%, 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 96.1%.

[0069] The crude lactide is transported to the distillation system for purification. A two-stage distillation system is used. The first distillation tower adopts a plate tower, with a bottom temperature of 170°C, 6 plates, a vacuum degree of 1.0 kPa, a tower body insulation at 105°C, and a tower top temperature of 105°C. The second distillation tower adopts a floating jet tower, with a bottom reboiler temperature of 170°C, 10 plates, a tower top vacuum degree of 200 Pa, a tower top temperature gradually increased from 110°C to 118°C, a tower top reflux ratio of 1:1, and L-lactide products are produced through the side line.

[0070] According to analysis, the yield of the lactide synthesis process is 78.9%, the chemical purity of the product is 99.1%, the optical purity is 99.4%, and the m-lactide content is 0.6%, which meets the requirements of polymerization-grade lactide monomer.

[0071] Example 4

[0072] 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 15 g. The crude lactide generated after the reaction was analyzed, and the L-lactide content was 84.3%, m -The lactide content is 2.9%, the L-lactic acid content is 4.4%, the dimer and trimer content is 4.3%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process is 95.9%.

[0073] The distillation and purification process was the same as in Example 1. After analysis, the yield of the lactide synthesis process was 80.5%, the chemical purity of the product was 99.2%, the optical purity was 99.0%, and the m-lactide content was 1.2%, meeting the requirements of polymerization-grade lactide monomer.

[0074] Example 5

[0075] 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 10 g. The crude lactide generated after the reaction was analyzed, and the L-lactide content was 85.5%, m -Lactide content is 3.2%, L-lactic acid content is 3.7%, dimer and trimer content is 4.5%, and the conversion rate of lactic acid oligomers in the crude lactide synthesis process is 95.4%.

[0076] The distillation and purification process was the same as in Example 1. After analysis, the yield of the lactide synthesis process was 81.0%, the chemical purity of the product was 99.3%, the optical purity was 99.0%, and the m-lactide content was 1.5%, which met the requirements of polymerization-grade lactide monomer.

[0077] Example 6

[0078] The preparation process of lactic acid oligomers and lactide is the same as that of Example 1, except that a molecular distillation depolymerization evaporator is used. The gaseous lactide generated during the reaction is directly condensed in the molecular distillation built-in condenser, and the liquid phase enters the crude lactide storage tank. After analysis, the L-lactide content in the lactide product is 87.9%. m -The lactide content is 2.3%, the L-lactic acid content is 4.5%, the dimer and trimer content is 4.8%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process is 97.5%.

[0079] The distillation and purification process was the same as in Example 1. Analysis showed that the yield of the lactide synthesis process was 82.4%, the chemical purity of the product was 99.2%, the optical purity was 99.3%, and the m-lactide content was 0.7%, which met the requirements for polymerization-grade lactide monomer.

[0080] Comparative Example 1

[0081] The preparation process of lactic acid oligomers and lactide is the same as that of Example 1, except that the lactide preparation adopts a tank reactor, and the depolymerization reaction conditions are: vacuum degree at 300 Pa, reaction temperature at 200° C., and reaction time at 1.5 h. The crude lactide obtained has 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.

[0082] The distillation and purification process was the same as in Example 1. The lactide product yield was 72.3% after analysis, wherein the chemical purity of the lactide product was 97.6% and the m-lactide content was 4.6%. The product could not meet the requirements of polymerization-grade monomers and still needed to be refined by melt crystallization process.

[0083] Comparative Example 2

[0084] 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 test results show that under the high temperature and high vacuum system, the molecular weight of lactic acid oligomers in the circulation tank continues to increase. Because there is no liquid level 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 reaction yield.

[0085] The crude lactide obtained after condensation has an L-lactide content of 85.2%, an m-lactide content of 5.3%, an L-lactic acid content of 3.9%, and a dimer and trimer content of 4.4%. The conversion rate of lactic acid oligomers in the crude lactide synthesis process is only 80.6%.

[0086] Comparative Example 3

[0087] 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 doubled. The test results show that the lower half of the membrane surface of the scraped film depolymerization reactor is in a viscous state, the material stays on the reactor surface for a long time, about 10 minutes, 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 84.2%, an m-lactide content of 6.2%, an L-lactic acid content of 4.3%, and a dimer and trimer content of 3.4%. The conversion rate of lactic acid oligomers in the crude lactide synthesis process is only 69.4%.

[0088] Comparative Example 4

[0089] The preparation process of lactic acid oligomers and lactide is the same as that of Example 1, except that the feed amount is twice the actual reaction amount. The test results show that at the bottom of the membrane surface of the scraped film depolymerization reactor, it is in a viscous state, the material stays on the reactor surface for about 6 minutes, the molecular weight of the unreacted oligomer rises to 3500, and it is recycled for reaction. The crude lactide obtained after condensation of the obtained gas phase component has an L-lactide content of 85.6%, an m-lactide content of 5.7%, an L-lactic acid content of 4.0%, a dimer and a trimer content of 3.3%, and a lactic acid oligomer conversion rate of only 84.3% in the crude lactide synthesis process.

[0090] Comparative Example 5

[0091] The preparation process of lactic acid oligomers and lactide is the same as that of Example 1, except that the molecular weight and catalyst accumulation at the bottom of the circulation tank are not monitored in real time, and the circulation depolymerization system has been reacting continuously without slag discharge. The test found that after 48 hours of continuous circulation, the discharge rate of crude lactide slowed down significantly, and the content of m-lactide in the product increased significantly by more than 15%. The reaction continued for 72 hours, coking appeared on the wall of the circulation tank, and the agitator resistance increased significantly. At this time, the reaction was stopped and the heavy component was discharged. According to GPC analysis, the molecular weight of the heavy component was 7779.

[0092] The crude lactide obtained after condensing the obtained gas phase through a condensation system has an L-lactide content of 84.3%, an m-lactide content of 5.9%, an L-lactic acid content of 3.9%, a dimer and a trimer content of 4.0%, and a lactic acid oligomer conversion rate of 88.7% in the crude lactide synthesis process.

Claims

1. A method for continuously preparing lactide, characterized in that The steps include: (1) After the lactic acid oligomers are mixed with the depolymerization catalyst, they are continuously transported to the depolymerization reactor for reaction. After the reaction, the heavy components are discharged into the circulation tank, mixed with the fresh lactic acid oligomers and transported to the depolymerization reactor. After the reaction, the gas phase components enter the condensation system. The molecular weight of the lactic acid oligomers is 800-4000. The depolymerization reactor is a scraped film depolymerization reactor. The depolymerization reactor and the circulation tank constitute a circulating depolymerization system, wherein the feed mass ratio of fresh material to circulating material is 1:6-2:

1. As the reaction proceeds, the liquid level of the circulation tank is controlled to be maintained at 50%-70%, the pressure is maintained at 10kPa-normal pressure, and the temperature is maintained at 160-200°C. The feed is fed in a manner of increasing the feed amount. After the fresh material is mixed with the circulating material, the feed amount is adjusted to be 4-8 times the actual reaction amount. Regular slag discharge is carried out based on the molecular weight of lactic acid oligomers at the outlet of the circulation tank or the cumulative amount of catalyst circulation. When the molecular weight of the oligomer is greater than 6000 or the cumulative amount of catalyst circulation is greater than 10%, slag discharge is carried out; (2) After condensation, the crude lactide produced enters the storage tank; then the crude lactide is transported to the distillation system for purification to obtain polymerization-grade monomer lactide.

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

3. The method according to claim 1 or 2, characterized in that: Step (1) The lactic acid oligomer is prepared from L-lactic acid and / or D-lactic acid as raw materials through a dehydration and polycondensation process.

4. The method according to claim 3, characterized in that: The dehydration is to remove free water in lactic acid, using normal pressure or reduced pressure; the reaction temperature of the polycondensation is 140-170°C, the vacuum degree is 1000-2000Pa, and the reaction time is 0.5-4.0h.

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

6. The method according to claim 5, characterized in that: The depolymerization catalyst in step (1) 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 in step (1) is 0.4%-3.0% of the mass of the lactic acid oligomers.

8. The method according to claim 7, characterized in that: The amount of the depolymerization catalyst used in step (1) is 0.8%-2.0% of the mass of the lactic acid oligomers.

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

10. The method according to claim 1 or 9, characterized in that: The depolymerization reaction temperature in step (1) is 180-240° C. and the vacuum degree is 200-1500 Pa.

11. The method according to claim 10, characterized in that: The depolymerization reaction temperature in step (1) is 190-210°C.

12. The method according to claim 1, characterized in that: The crude lactide product obtained in step (2) has an L-lactide content of 82%-88%. m -Lactide content is 2.0%-3.5%, L-lactic acid content is 3.0%-6.0%, dimer and trimer content is 2.0%-5.0%.

13. The method according to claim 1, characterized in that: The crude lactide distillation system in step (2) is used to remove impurity components in crude lactide, and mainly includes two or more distillation towers connected in series, wherein at least one distillation tower is a crude fractionation tower, which is used to quickly remove water and part of lactic acid in crude lactide; and at least one distillation tower is a refining tower, which is used to remove residual lactic acid, lactic acid and lactic acid in crude lactide. m -Lactide and lactic acid polymers.

14. The method according to claim 13, characterized in that: The distillation tower used as the crude fraction tower in step (2) is in the form of packing or plates, with a bottom temperature of 150-170°C, 2-6 plates, a vacuum degree of 1.0-2.0 kPa, a tower body insulation of 60-110°C, and a top temperature of 80-120°C.

15. The method according to claim 13, characterized in that: The distillation tower used as the refining tower in step (2) is a floating jet tower, the reboiler temperature at the bottom of the tower is 170-200°C, the number of tower plates is 10-20, the vacuum degree at the top of the tower is 200-600Pa, the top temperature of the tower gradually increases from 110-120°C to 120-130°C, and the top reflux ratio is 1:1-1:

3.

16. The method according to claim 1 or 13, characterized in that: The product yield of the distillation purification process in step (2) is not less than 80%, the chemical purity and optical purity of the obtained L-lactide are not less than 99%, and the m-lactide content is not more than 2.0%.

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