Method and device for preparing lactide

Abstract

The invention relates to a method and a device for preparing lactide, a main body of a depolymerization reactor is designed into a fluidized bed form, depolymerization catalyst particles and heat accumulator particles are filled in the depolymerization reactor, and a gas distributor and a feeder are arranged at the bottom of the reactor; a lactic acid oligomer is fed into a reactor through a feeder and is subjected to a depolymerization reaction in the presence of a catalyst, a heat accumulator and airflow, heavy components generated by depolymerization overflow from the upper part of a fluidized bed and are conveyed to a circulating unit, materials with the molecular weight not greater than 4000 are detected to be circulated back to the fluidized bed to continue to participate in the reaction, and light components generated by depolymerization enter a purification unit. The method and the device provided by the invention avoid the problems of easy racemization, coking carbonization and the like of the lactide caused by cyclic accumulation of the catalyst and uneven temperature, prolong the service life of the catalyst while ensuring the yield of the lactide, and maintain long-term stable operation of the reaction.

Description

Technical Field

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

[0002] Currently, the world consumes 120 million tons of single-use plastic products annually. Only 10% is recycled, approximately 12% is incinerated, and over 70% is discarded into the soil, air, and oceans. Over 8 million tons of plastic waste enters the ocean annually, and this figure is rising. It is estimated that by 2025, the global amount of marine plastic waste will reach 250 million tons. Traditional single-use plastic products have a short lifespan, but their physical and chemical properties are stable and they are difficult to degrade naturally. The large amount of single-use plastic waste has led to a variety of environmental problems, seriously endangering the health and safety of land, water, animals, and humans. Nearly 90 countries and regions around the world have introduced policies or regulations to control or ban the use of single-use, non-degradable plastic products.

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

[0004] Industrialized polylactic acid (PLA) synthesis is primarily achieved through the ring-opening polymerization of lactide: in the first step, lactide is prepared from lactic acid; in the second step, lactide ring-opening polymerization is used to produce polylactic acid. The resulting PLA can have a molecular weight of 100,000 to 1 million. Lactide is crucial to the entire synthesis process, and the process barriers are relatively high. The current industrialized lactide synthesis method, referred to as the "two-step process," involves the polycondensation and depolymerization of lactic acid under a catalyst, high temperature, and high vacuum. This process is prone to racemization to produce m-lactide. The presence of m-lactide can affect the optical purity of lactide, which in turn affects the lactide ring-opening polymerization process, resulting in a lower molecular weight for the resulting PLA. Furthermore, it can disrupt the structural regularity of PLA, reducing its crystallinity and mechanical properties. In addition, the crude lactide obtained in the reaction process contains impurity components such as L-lactic acid, lactic acid dimer, and trimer in addition to L-lactide and m-lactide. These impurity components need to be removed through pretreatment and purification and refining processes to improve its chemical purity and optical purity.

[0005] Therefore, the crude lactide currently produced by depolymerization requires purification through processes such as solvent recrystallization, water extraction, distillation, melt crystallization, and combinations thereof to reduce impurities such as m-lactide in the product. However, due to the similar physicochemical properties of L-lactide and m-lactide, and the inherent high freezing and boiling points and heat sensitivity of lactide, separation is difficult, and the overall yield is low, only around 40%-60%, and the overall economic efficiency is also low. Therefore, racemization in the lactide synthesis process is a key factor affecting lactide quality and yield, and is currently a key difficulty in lactide technology research both domestically and internationally.

[0006] CN112876452A discloses a lactide preparation method and reaction apparatus. This method utilizes a series of reactive distillation columns to couple the reaction and purification processes. Lactic acid undergoes preliminary prepolymerization in reactive distillation column I, followed by polymerization and preliminary depolymerization in distillation column II. The product then enters reactive distillation column III for secondary depolymerization. Unreacted substrate is recycled back to reactive distillation column I to participate in the reaction. The overall depolymerization conversion rate can reach 70.0%. This method utilizes three distillation columns in series to couple the reaction and purification processes. Column III is designed with a depolymerization reaction section, a polymerization reaction section, and a distillation section, arranged from bottom to top. The depolymerization section utilizes a thin film reactor. To ensure efficient depolymerization, the thin film reactor is coupled to a catalytic distillation column, increasing the complexity of equipment design and manufacturing. Furthermore, the separation and coupling of the three reaction stages significantly increases the difficulty of equipment operation. The oligomer conversion rate in this process is only approximately 70%, which needs to be further improved.

[0007] CN116063275A discloses a method for preparing lactide by continuous coupling, comprising: (1) mixing lactic acid oligomers with a depolymerization catalyst, preheating the mixture, and continuously transferring the mixture to a depolymerization reactor for reaction, wherein the light fraction after the reaction enters the bottom of a distillation system; and (2) after treatment in the distillation system, the resulting bottom liquid phase fraction is returned to the depolymerization reactor, and the gaseous lactide is condensed to obtain a lactide product. This invention couples the depolymerization reactor with the distillation system for continuous reaction, reducing the probability of lactide racemization and substrate coking and carbonization, thereby improving the product quality and yield of the entire synthesis process. However, this method requires a falling film reactor for the depolymerization reactor, and the product yield of the single-pass depolymerization is only about 85%, which needs to be further improved. Moreover, as the reaction process proceeds cyclically, problems such as catalyst accumulation may occur, and the occurrence of racemization cannot be effectively avoided.

[0008] CN116041310A discloses a method and system for preparing crude lactide by continuous depolymerization. Lactic acid oligomers are mixed with a depolymerization catalyst and then continuously introduced into a depolymerization reactor through a feed port located in the reactor wall for reaction. After the reaction, the heavy components enter a circulation tank and are transported back to the depolymerization reactor through a recycle inlet located above the feed port. The upper reaction zone is located between the feed port and the recycle inlet, while the lower reaction zone is located below the feed port. After reacting in the upper reaction zone, the heavy components enter the lower reaction zone to continue reacting. The gaseous component, crude lactide, is extracted from the top of the reactor. This invention utilizes a cyclic depolymerization process, with recycle and fresh feed being diverted during the depolymerization process. This achieves efficient, continuous production of crude lactide, reduces the degree of lactide racemization and the probability of substrate coking and carbonization, and improves substrate conversion and product quality. However, although the partial temperature control and cyclic depolymerization in the same falling film reactor can ensure the reaction efficiency and yield, there will be a problem of catalyst accumulation as the reaction process proceeds cyclically, and the occurrence of racemization reaction cannot be effectively avoided.

[0009] The reaction mechanism of lactide synthesis reveals that temperature is a key factor influencing the depolymerization of lactic acid oligomers to produce lactide, and is also the primary factor in initiating lactide racemization. Generally, higher depolymerization temperatures increase the reaction rate, but excessively high reaction temperatures can cause lactide racemization and lead to coking and carbonization. While excessively low depolymerization temperatures reduce the probability of racemization, they slow the depolymerization reaction, accelerating the polymerization rate and increasing the molecular weight of the oligomers. This further increases the viscosity of the system, making it difficult for the generated lactide to evaporate from the system, further inhibiting the depolymerization reaction. Therefore, the depolymerization process in lactide synthesis requires strict temperature control, making it difficult to manipulate. Summary of the Invention

[0010] To address the shortcomings of existing technologies, the present invention provides a method and apparatus for preparing lactide. These methods and apparatus avoid issues such as lactide racemization and coking and carbonization caused by catalyst cycle accumulation and temperature unevenness. While ensuring lactide yield, they also extend the catalyst's service life and maintain long-term stable reaction operation.

[0011] The first aspect of the present invention provides a method for preparing lactide, comprising the following steps:

[0012] The main body of the depolymerization reactor is configured as a fluidized bed, with depolymerization catalyst particles and heat storage body particles filled inside, and a gas distributor and feeder provided at the bottom of the reactor; lactic acid oligomers are fed into the reactor by the feeder, and undergo depolymerization reaction in the presence of the catalyst, heat storage body and airflow; the heavy components produced by the depolymerization overflow from the upper portion of the fluidized bed and are transported to the circulation unit; materials with a molecular weight of no more than 4000 are circulated back to the fluidized bed to continue to participate in the reaction; the light components produced by the depolymerization enter the purification unit.

[0013] In the above method, a porous screen is set on the upper part of the fluidized bed. The porous screen can be a metal wire mesh, a glass wire mesh, etc. Its cross-sectional diameter is the same as the cross-sectional diameter of the fluidized bed. The screen aperture is smaller than the particle size of the catalyst particles and the heat storage body particles to intercept solid particles.

[0014] In the above method, the particle size of the depolymerization catalyst particles is 0.5-3.0 mm, and the amount of catalyst used is 0.05-0.5 cm 3 Catalyst / cm 3 Fluidized bed reaction zone volume. Conventional solid catalyst particles for preparing lactide using lactic acid oligomers as raw materials are used, preferably catalyst particles loaded with stannous oxide. The catalyst carrier can be any of alumina, silica, and molecular sieves, with alumina being preferred. The stannous oxide loading is 0.6% to 5.0%.

[0015] In the above method, the thermal storage particles can be selected from at least one or more of porcelain balls, cordierite, mullite, silica, zirconia, silicon carbide, etc., and the particle size of the thermal storage particles is 0.1-3.0 mm. The volume ratio of the thermal storage particles to the catalyst particles is 1-6:1.

[0016] In the above method, the gas introduced into the gas distributor is a gas capable of forming an inert atmosphere, such as any one or a mixture of nitrogen, inert gas, etc., preferably any one of N2 and Ar. The temperature of the introduced gas is 180-240°C, and the gas flow rate satisfies u / u mf =3-8, where u mf is the critical fluidization velocity.

[0017] In the above method, the molecular weight of the lactic acid oligomer is 1000-3500, preferably 1500-2500. Lactic acid oligomers are generally produced from L-lactic acid or D-lactic acid via a dehydration and polycondensation process. The dehydration process removes free water from the lactic acid and can be performed under normal or reduced pressure. The polycondensation process generally has a reaction temperature of 140-170°C, a reaction time of 0.5-4.0 hours, and an absolute pressure of 1000-2000 Pa.

[0018] In the above method, the feed flow rate of lactic acid oligomers is 0.2%-2.0% of the gas flow rate.

[0019] In the above method, the temperature of the depolymerization reaction is 180-240° C., preferably 200-220° C., and the vacuum degree is 0.5-1.0 kPa.

[0020] In this method, the heavy components produced by depolymerization flow out of the overflow port above the fluidized bed and enter a circulation unit. After testing, materials with a molecular weight of 4000 or less are recycled back to the feed port to continue the reaction, while materials with a molecular weight greater than 4000 are discharged from the system. The volume ratio of the material recycled back to the reactor to the fresh lactic acid oligomer feed is 4:1 to 1:1. This method not only ensures sufficient depolymerization of the oligomers, improving the reaction yield, but also effectively reduces the impact of increased oligomer molecular weight and system viscosity on the reaction process and product quality, thereby ensuring the system's heat and mass transfer efficiency.

[0021] In the above method, the light fraction produced by depolymerization enters a purification unit, which preferably employs a distillation process. The distillation is performed in a packed tower using structured packing such as wire mesh corrugated mesh packing or perforated plate corrugated mesh packing, with a packing height equivalent to 8-12 theoretical plates. The top of the distillation tower is fully extracted to extract light fractions, primarily lactic acid. The tower top temperature is 90-110°C, the tower top vacuum is 200-500 Pa, and the pressure drop across the distillation tower is controlled at 100-300 Pa.

[0022] In the above method, the distillation column is equipped with a lactide side-line outlet located at the 4th to 8th theoretical plates in the distillation zone. The outlet temperature is 120-140°C, and the side-line reflux ratio is 1:2-1:4. A condenser is installed after the lactide side-line outlet to quickly separate the condensate, primarily lactide, from the non-condensable vapor. The non-condensable vapor is recycled to the gas distributor, and the side-line condenser temperature is controlled at 95-110°C.

[0023] In the above method, the lactide product extracted from the lactide sideline has an L-lactide content of 96.0% or greater, an m-lactide content of 2.5% or less, and both chemical and optical purities exceeding 99%. Furthermore, the lactide yield of the entire reaction process can reach over 90%. During lactide purification and refining, it is generally required that the lactide content entering the melt crystallizer be above 95% and the m-lactide purity be 2.5% or less. The sideline product extracted from the present method fully meets these requirements and can be directly used as a raw material in the melt crystallization stage, resulting in a single-step refining step to produce a lactide product that meets polymerization-grade requirements.

[0024] In the above method, the top outlet of the distillation tower is equipped with a condenser, wherein the condenser temperature is controlled at 50-80°C to achieve rapid separation of the condensate mainly composed of lactic acid and the non-condensable steam, and the non-condensable steam is recycled to the gas distributor.

[0025] A second aspect of the present invention provides a preparation apparatus for the above-mentioned method of preparing lactide, which mainly includes a depolymerization unit, a circulation unit, and a purification unit. The depolymerization unit mainly includes a depolymerization reactor, the main body of the depolymerization reactor being a fluidized bed filled with depolymerization catalyst particles and heat storage body particles. A gas distributor and a feeder are provided at the bottom of the reactor. Lactic acid oligomers enter the reactor through the feeder and undergo a depolymerization reaction in the presence of the catalyst, heat storage body, and airflow. The circulation unit mainly includes a circulation tank and a detection device. The heavy components produced by the depolymerization enter the circulation tank. Materials with a molecular weight of no more than 4000 are circulated back to the reactor to continue the reaction. The purification unit mainly includes a distillation tower and a condenser. The light components produced by the depolymerization enter the purification unit, and lactide is extracted through a side line.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) The depolymerization reactor of the present invention is mainly composed of a fluidized bed and is simultaneously loaded with depolymerization catalyst particles and heat storage particles. While ensuring uniform reaction temperature and improving heat transfer efficiency, the reaction efficiency is improved, and the problems of lactide racemization caused by catalyst circulation accumulation and local overheating and the coking and carbonization of lactic acid oligomers on the catalyst surface are effectively avoided. The service life of the catalyst is extended while ensuring the lactide yield.

[0028] (2) Introducing thermal storage particles into the fluidized bed allows for dynamic transfer of reaction heat between the catalyst, thermal storage, exhaust gas, and feedstock, ensuring uniform reaction temperature and suppressing racemization. This also improves heat utilization. Furthermore, the fluidized abrasion of the thermal storage particles effectively removes any coking material from the catalyst surface, extending catalyst life, avoiding frequent regeneration, and ensuring long-term process stability.

[0029] (3) The gas introduced into the fluidized bed not only achieves the fluidization of the catalyst particles and the heat storage particles, but also increases the internal disturbance of the reactor, improves the heat and mass transfer effect, and helps the gas escape, shortens the residence time of lactide in the high temperature zone, and inhibits the occurrence of racemization.

[0030] (4) The heavy components produced by depolymerization are detected online and different treatment methods are adopted according to the molecular weight. On the one hand, this is conducive to the occurrence of the "back-biting" ester group catalytic transfer reaction, and on the other hand, it ensures the fluidity of the reaction system, thereby improving the product quality while ensuring the yield of lactide.

[0031] (5) The non-condensable gas produced by condensation is recycled back to the reaction zone as gas for the gas distributor, which reduces the amount of nitrogen or inert gas used and reduces production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1This is a schematic diagram of a process for preparing lactide according to the present invention;

[0033] Wherein: 100-depolymerization unit, 101-fluidized bed, 102-fresh lactic acid oligomer inlet, 103-raw material feed port, 104-gas inlet, 105-gas distributor, 106-feeder, 107-light component outlet, 108-light component reflux port, 109-lactic acid oligomer overflow port, 110-porous screen; 200-circulation unit, 201-circulation tank, 202-molecular weight detection equipment, 203-discharge port, 204- Circulating substrate discharge port; 300-purification unit, 301-distillation tower, 302-distillation tower inlet, 303-lactide side-line outlet, 304-tower top outlet, 305-distillation tower bottom heavy component outlet, 306-tower top condenser, 307-side-line condenser, 308-lactide reflux port, 309-lactide outlet, 310-tower top light component outlet, 311-tower top uncondensed component outlet, 312-side-line uncondensed component outlet. DETAILED DESCRIPTION

[0034] The following examples further illustrate the method and apparatus for preparing lactide of the present invention. The examples are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operating processes, but the scope of protection of the present invention is not limited to the following examples.

[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 uses a Malvern Viscotek OMNISEC GPC / SEC gel chromatograph to analyze the molecular weight of lactic acid oligomers. Conventional calibration methods are used, with polystyrene (PS) as the internal standard. A T3000 column, 300 mm × 8.0 mm in size, is used. The column temperature is 40°C, the flow rate is 1.0 mL / min, the sample concentration is 2-5 mg / mL, and the single injection volume is 500 μL.

[0038] The present invention uses an Agilent high-performance liquid chromatograph to analyze the chemical purity of lactide, using a UV detector, phosphoric acid and acetonitrile as the mobile phase, and a ZORBAX SB-Aq column with a length of 250 mm, an inner diameter of 4.6 mm, and a filler particle size of 5 μm. The detection wavelength is 200 nm, the column temperature is 40°C, the flow rate is 1 mL / min, and the injection volume is 5 μL.

[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 7min, heat to 200°C at a rate of 8°C / min, hold 20min, carrier gas N2 flow rate 1.4mL / min, hydrogen flow rate 30mL / min, air flow rate 400mL / min, injection volume 0.5μL.

[0040] The yield Y of the entire reaction process is the ratio of the actual amount of L-lactide produced to the theoretical amount produced, and the calculation formula is as follows:

[0041]

[0042] Where m0 is the production rate of lactide produced from the side line, y0 is the purity of L-lactide in the lactide produced from the side line, and M is the mass of lactide that can theoretically be converted from a certain amount of lactic acid oligomers, that is, the feed rate of fresh lactic acid oligomers.

[0043] The embodiment of the present invention adopts Figure 1 The device mainly includes a depolymerization unit 100, a circulation unit 200 and a purification unit 300, wherein the depolymerization unit 100 mainly includes a fluidized bed depolymerization reactor 101, a gas distributor 105, a feeder 106, and a porous screen 110; the depolymerization reactor is filled with depolymerization catalyst particles and heat storage body particles, the circulation unit 200 mainly includes a circulation tank 201 and a molecular weight detection device 202; the purification unit 300 mainly includes a distillation tower 301, a top condenser 306 and a side condenser 307.

[0044] Fresh lactic acid oligomers are mixed with the recycled material from the circulating substrate outlet 204 through the inlet 102, and then fed into the depolymerization reactor through the raw material feed port 103 and the feeder 106; the gas from the gas inlet 104 enters the reactor after passing through the gas distributor 105. The reactor is filled with catalyst particles and heat storage particles. The depolymerization reaction is carried out in the presence of the catalyst, heat storage and air flow. The light components produced by the depolymerization enter the purification unit 300, specifically through the light component outlet 107 and then through the distillation tower inlet 302 into the distillation tower 301. The lactide after distillation is taken out through the side line outlet 303 and then passed through the side line condenser 303. 07 is condensed into liquid, part of which returns to the distillation tower through the lactide reflux port 308, and part is withdrawn as a product through the lactide product withdrawal port 309. The uncondensed gas returns to the gas inlet 104 through the side line uncondensed component outlet 312, and the light component mainly composed of lactic acid enters the top condenser 306 through the top light component withdrawal port 304. The condensed liquid is discharged through the top light component withdrawal port 310, and the uncondensed components are returned to the inert gas feed port through the top uncondensed component outlet 311, and the purified heavy components return to the depolymerization unit through the heavy component outlet 305 at the bottom of the distillation tower via the light component reflux port 108. The unreacted heavy components in the fluidized bed depolymerization reactor enter the circulation tank 201 through the lactic acid oligomer overflow port 109 on the upper portion of the fluidized bed, and are tested for molecular weight by the molecular weight detection device 202. When the molecular weight is not greater than 4000, the heavy components are mixed with fresh lactic acid oligomer raw materials through the circulating substrate discharge port 204 and then circulated back to the fluidized bed depolymerization reactor through the raw material feed port 103 to continue the reaction; when the molecular weight is greater than 4000, the heavy components are directly discharged through the discharge port 203.

[0045] Example 1

[0046] This embodiment is in the attached Figure 1 In the apparatus described, the fresh lactic acid oligomer raw material has a molecular weight of 2000 and is produced from L-lactic acid via a dehydration and polycondensation process. The lactic acid oligomer feed rate is 0.75% of the gas flow rate, with the volume ratio of the material recycled back to the reactor to the fresh lactic acid oligomer feed being 3:1. Nitrogen is introduced into the gas distributor at a temperature of 200°C and a flow rate of 300 L / h into the bottom of the fluidized bed.

[0047] The depolymerization catalyst particles are alumina catalyst loaded with stannous oxide, with a loading of 1.0%, a particle size of 1 mm, and a catalyst addition amount of 0.1 cm 3 Catalyst / cm 3The volume of the fluidized bed reaction zone. The heat storage particles used were zirconium oxide with a particle size of 1.0 mm, and their dosage was three times the volume of the catalyst particles. The mesh size of the upper fluidized bed was 0.8 mm. The reaction temperature in the fluidized bed depolymerization reactor was 200°C, and the vacuum was 600 Pa. The light fractions produced by depolymerization were purified by a distillation tower with 10 theoretical stages, a top vacuum of 200 Pa, a temperature of 105°C, and an overhead condenser temperature of 60°C. This produced fractions primarily composed of L-lactic acid. The lactide sidestream outlet was located on the fourth stage, at a temperature of 126°C, a sidestream condenser temperature of 95°C, and a sidestream reflux ratio of 1:3, producing a preliminarily purified lactide product. The purified heavy fractions were returned to the fluidized bed depolymerization reactor for further reaction. The heavy components produced by depolymerization enter the circulation tank through the upper overflow port and are tested for molecular weight by an online viscometer. If the molecular weight is not greater than 4000, it will continue to return to the depolymerization reactor for reaction. If it is greater than 4000, the material will be discharged.

[0048] Analysis of the resulting side-line lactide product revealed 96.8% L-lactide, 2.0% m-lactide, 0.5% lactic acid dimer, and 0.4% trimer. The L-lactide yield during the synthesis process was 92.3%. The catalyst life was extended by 12%.

[0049] Example 2

[0050] This embodiment is in the attached Figure 1 The reaction was carried out in the apparatus described above, using the same raw materials as in Example 1. The lactic acid oligomer feed rate was 1.75% of the gas flow rate, with the volume ratio of the material recycled back to the reactor to the fresh lactic acid oligomer feed being 2:1. Nitrogen was introduced into the gas distributor at a temperature of 220°C and at a flow rate of 350 L / h into the bottom of the fluidized bed.

[0051] The depolymerization catalyst particles are alumina catalyst loaded with stannous oxide, with a loading of 3.5%, a particle size of 2.5 mm, and a catalyst addition amount of 0.25 cm 3 Catalyst / cm 3The volume of the fluidized bed reaction zone. The regenerator particles used were zirconium oxide with a particle size of 2.0 mm, and their dosage was three times the volume of the catalyst particles. The mesh size of the upper fluidized bed was 1.6 mm. The reaction temperature in the fluidized bed depolymerization reactor was 210°C, and the vacuum was 800 Pa. The light fractions produced by depolymerization were purified by a distillation tower with 8 theoretical stages, a top vacuum of 400 Pa, a temperature of 108°C, and an overhead condenser temperature of 60°C. This produced fractions primarily composed of L-lactic acid. The lactide sidestream outlet was located on the fourth stage, at a temperature of 132°C, a sidestream condenser temperature of 105°C, and a sidestream reflux ratio of 1:2, producing a preliminarily purified lactide product. The purified heavy fractions were returned to the fluidized bed depolymerization reactor for further reaction. The heavy components produced by depolymerization enter the circulation tank through the upper overflow port and are tested for molecular weight by an online viscometer. If the molecular weight is not greater than 4000, it will continue to return to the depolymerization reactor for reaction. If it is greater than 4000, the material will be discharged.

[0052] Analysis of the resulting side-line lactide product revealed 96.2% L-lactide, 2.4% m-lactide, 0.3% lactic acid dimer, and 1.0% trimer. The L-lactide yield from the synthesis process was 91.5%. The catalyst life was extended by 15%.

[0053] Example 3

[0054] This embodiment is in the attached Figure 1 The reaction was carried out in the apparatus described above, using the same raw materials as in Example 1. The feed rate of lactic acid oligomers was 0.25% of the gas flow rate, with the volume ratio of the material recycled back to the reactor to the fresh lactic acid oligomer feed being 1:1. Nitrogen was introduced into the gas distributor at a temperature of 180°C and at a flow rate of 250 L / h into the bottom of the fluidized bed.

[0055] The depolymerization catalyst particles are alumina catalyst loaded with stannous oxide, with a loading of 0.8%, a particle size of 0.5 mm, and a catalyst addition amount of 0.4 cm 3 Catalyst / cm 3The volume of the fluidized bed reaction zone. The heat storage particles used were zirconium oxide with a particle size of 0.5 mm, and their dosage was three times the volume of the catalyst particles. The mesh size of the upper fluidized bed was 0.2 mm. The reaction temperature in the fluidized bed depolymerization reactor was 190°C, and the vacuum was 500 Pa. The light fractions produced by depolymerization were purified by a distillation tower with 12 theoretical stages, a top vacuum of 200 Pa, a temperature of 105°C, and an overhead condenser temperature of 60°C. This produced fractions primarily composed of L-lactic acid. The lactide sidestream outlet was located on the seventh stage, at a temperature of 124°C, a sidestream condenser temperature of 100°C, and a sidestream reflux ratio of 1:4, producing a preliminarily purified lactide product. The purified heavy fractions were returned to the fluidized bed depolymerization reactor for further reaction. The heavy components produced by depolymerization enter the circulation tank through the upper overflow port and are tested for molecular weight by an online viscometer. If the molecular weight is not greater than 4000, it will continue to return to the depolymerization reactor for reaction. If it is greater than 4000, the material will be discharged.

[0056] Analysis of the resulting side-product lactide revealed 96.7% L-lactide, 1.7% m-lactide, 0.6% lactic acid dimer, and 0.8% trimer. The L-lactide yield during the synthesis process was 91.8%. The catalyst life was extended by 9%.

[0057] Example 4

[0058] Same as Example 1, except that the heat storage particles are cordierite with a particle size of 1.0 mm.

[0059] Analysis of the resulting lactide product revealed an L-lactide content of 97.0%, an m-lactide content of 2.0%, a lactic acid dimer content of 0.4%, and a trimer content of 0.5%. The L-lactide yield during the synthesis process was 92.0%. The catalyst service life was extended by 11%.

[0060] Example 5

[0061] Same as Example 1, except that the heat storage particles are made of silicon dioxide with a particle size of 1.0 mm.

[0062] Analysis of the resulting lactide product revealed an L-lactide content of 96.6%, an m-lactide content of 2.2%, a lactic acid dimer content of 0.6%, and a trimer content of 0.4%. The L-lactide yield during the synthesis process was 92.2%. The catalyst service life was extended by 13%.

[0063] Example 6

[0064] The same as Example 1, except that the gas introduced into the gas distributor is argon Ar.

[0065] Analysis of the resulting lactide product revealed an L-lactide content of 96.1%, an m-lactide content of 2.3%, a lactic acid dimer content of 0.7%, and a trimer content of 0.6%. The L-lactide yield during the synthesis process was 91.7%. The catalyst service life was extended by 11.5%.

[0066] Comparative Example 1

[0067] The same as Example 1, except that no thermal storage particles are loaded, only the catalyst is loaded.

[0068] Analysis of the resulting side-line lactide product revealed an L-lactide content of 92.9%, an m-lactide content of 5.6%, a lactic acid dimer content of 1.3%, and a trimer content of 1.3%. The L-lactide yield during the synthesis process was 80.9%. Compared to Example 1, the absence of thermal storage particles increased the degree of racemization and the rate of oligomer coking and carbonization on the catalyst surface, thereby impacting the catalytic efficiency and product yield.

[0069] Comparative Example 2

[0070] The same as embodiment 1, except that air is introduced into the bottom gas distributor.

[0071] Analysis revealed that the resulting lactide product contained 92.4% L-lactide, 4.7% m-lactide, 0.6% lactic acid dimer, and 0.5% trimer. The L-lactide yield during the synthesis process was 82.7%. Compared to Example 1, the presence of oxygen at high temperatures, due to the air flow, easily led to the formation of coking and carbonization of oligomers, which in turn affected racemization, shortened the catalyst life, and affected the stable operation period of the device.

[0072] Comparative Example 3

[0073] The method is the same as Example 1, except that no circulation unit is provided, that is, after the lactic acid oligomers enter the fluidized bed depolymerization reactor, the unreacted heavy components are directly withdrawn from the upper overflow port.

[0074] Analysis of the resulting lactide product revealed an L-lactide content of 96.9%, an m-lactide content of 1.8%, a lactic acid dimer content of 0.5%, and a trimer content of 0.6%. The L-lactide yield during the synthesis process was 58.9%. Compared to Example 1, the unreacted lactic acid oligomer components were not recycled, resulting in a single-pass reaction. However, during this single-pass reaction, the lactic acid oligomers were not completely depolymerized, resulting in a large amount of lactic acid oligomers being directly discharged from the system, affecting the conversion rate of lactic acid oligomers and the lactide yield.

[0075] Comparative Example 4

[0076] The same as Example 1, except that no distillation unit is provided, that is, the light components such as L-lactide generated by depolymerization are directly withdrawn from the top of the fluidized bed reactor through a condenser.

[0077] Analysis of the crude product revealed an L-lactide content of 74.6%, an m-lactide content of 6.7%, a lactic acid dimer content of 7.6%, and a trimer content of 6.4%. The L-lactide yield during the synthesis process was 72.3%. Compared to Example 1, since the light components generated by depolymerization were directly withdrawn from the tower top, the crude lactide contained a significant amount of lactic acid polymers and other components, which in turn affected the depolymerization yield and product purity.

Claims

1. A method for preparing lactide, characterized in that The invention includes the following contents: the main body of the depolymerization reactor is set in the form of a fluidized bed, and the depolymerization catalyst particles and heat storage body particles are filled inside. A gas distributor and a feeder are set at the bottom of the reactor. Lactic acid oligomers are fed into the reactor by the feeder, and depolymerization reaction is carried out in the presence of catalyst particles, heat storage body particles and airflow. The heavy components produced by depolymerization overflow from the upper part of the fluidized bed and are transported to the circulation unit. The materials with molecular weight not greater than 4000 are circulated back to the fluidized bed to continue to participate in the reaction. The light components produced by depolymerization enter the purification unit.

2. The method according to claim 1, wherein: A porous screen is provided on the upper portion of the fluidized bed, preferably a metal mesh or a glass mesh, the cross-sectional diameter of which is the same as that of the fluidized bed, and the mesh aperture is smaller than the particle size of the catalyst particles and the heat storage body particles to intercept solid particles.

3. The method according to claim 1, wherein: The particle size of the depolymerization catalyst particles is 0.5-3.0 mm, and the catalyst dosage is 0.05-0.5 cm 3 Catalyst / cm 3 Volume of fluidized bed reaction zone.

4. The method according to claim 1 or 3, characterized in that: The depolymerization catalyst uses catalyst particles loaded with stannous oxide. The catalyst carrier is any one of aluminum oxide, silicon oxide, and molecular sieve, preferably aluminum oxide. The loading amount of stannous oxide is 0.6%-5.0%.

5. The method according to claim 1, wherein: The heat storage particles are selected from at least one or more of porcelain balls, cordierite, mullite, silicon dioxide, zirconium oxide, and silicon carbide. The particle size of the heat storage particles is 0.1-3.0 mm. The volume ratio of the heat storage particles to the catalyst particles is 1-6:

1.

6. The method according to claim 1, wherein: The gas introduced into the gas distributor is a gas capable of forming an inert atmosphere, preferably any one of nitrogen and inert gases or a mixture of several thereof, more preferably any one of N2 and Ar.

7. The method according to claim 1 or 6, characterized in that: The temperature of the gas introduced into the gas distributor is 180-240°C, and the gas flow rate meets the requirements. u / u mf =3-8, where u mf is the critical fluidization velocity.

8. The method according to claim 1, wherein: The molecular weight of the lactic acid oligomer is 1000-3500, preferably 1500-2500; the feed flow rate of the lactic acid oligomer is 0.2%-2.0% of the gas flow rate.

9. The method according to claim 1, wherein: The temperature of the depolymerization reaction is 180-240° C., preferably 200-220° C., and the vacuum degree is 0.5-1.0 kPa.

10. The method according to claim 1, wherein: After testing, the material with a molecular weight of no more than 4000 is recycled back to the feed inlet to continue participating in the reaction. The volume ratio of the amount of material recycled back to the reactor to the amount of fresh lactic acid oligomer feed is 4:1-1:

1.

11. The method according to claim 1, wherein: The light components produced by depolymerization enter the purification unit. The purification unit adopts distillation method. The distillation adopts the form of a packed tower, using one of the structured packings, wire mesh corrugated mesh packing and perforated plate corrugated mesh packing. The filling height is equivalent to 8-12 theoretical tower plates.

12. The method according to claim 11, wherein: The top of the distillation tower adopts the full extraction form to extract the light components mainly composed of lactic acid. The top temperature is 90-110℃, the top vacuum is 200-500Pa, and the pressure drop of the entire distillation tower is controlled at 100-300Pa.

13. The method according to claim 11, wherein: The distillation tower is provided with a lactide side line outlet located at the 4th to 8th theoretical plates of the distillation zone, the outlet temperature is 120-140° C., and the side line reflux ratio is 1:2-1:

4.

14. The method according to claim 13, wherein: A condenser is provided behind the lactide side-line outlet, and the temperature of the side-line condenser is controlled at 95-110°C.

15. The method according to claim 13, wherein: The top outlet of the distillation tower is equipped with a condenser, wherein the condenser temperature is controlled at 50-80° C. to achieve rapid separation of the condensate mainly composed of lactic acid and the non-condensable steam, and the non-condensable steam is recycled to the gas distributor.

16. A production device for the method for preparing lactide according to any one of claims 1 to 15, characterized in that The invention comprises a depolymerization unit, a circulation unit and a purification unit, wherein the depolymerization unit mainly comprises a depolymerization reactor, the main body of the depolymerization reactor is a fluidized bed, which is filled with depolymerization catalyst particles and heat storage body particles. A gas distributor and a feeder are arranged at the bottom of the reactor. Lactic acid oligomers enter the reactor through the feeder and undergo depolymerization reaction in the presence of the catalyst, heat storage body and airflow; the circulation unit mainly comprises a circulation tank and a detection device, the heavy components produced by the depolymerization enter the circulation tank, and the materials with a molecular weight not greater than 4000 after detection are circulated back to the reactor to continue the reaction; the purification unit mainly comprises a distillation tower and a condenser, the light components produced by the depolymerization enter the purification unit, and lactide is extracted through a side line.

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