A process system and a process method for continuously depolymerizing to prepare lactide

Abstract

Provided are a process system and method for continuously depolymerizing to prepare lactide, comprising a polycondensation reactor, a static mixer and a depolymerization reactor connected in sequence, the depolymerization reactor is provided with a light phase material outlet and a heavy phase material outlet, the heavy phase material outlet is connected to a circulating tank, a depolymerization catalyst material tank is connected to a front end pipeline of the static mixer, and an outlet of the circulating tank is connected to an inlet pipeline of the static mixer. In the process of preparing lactide, the reaction temperature in the depolymerization reactor is maintained at 180-220 DEG C, the temperature of the gaseous crude lactide is maintained above 170 DEG C, as the reaction proceeds, the temperature of the gaseous crude lactide decreases, when the temperature decreases to 160-165 DEG C, the valve of the depolymerization catalyst material tank is opened to supplement the catalyst, when the temperature of the gaseous crude lactide reaches 175 DEG C-185 DEG C, the valve of the depolymerization catalyst material tank is closed, and the supplement of the catalyst is stopped. The application creatively uses the temperature of the gaseous crude lactide at the outlet of the depolymerization reactor as a basis for judging the activity of the catalyst, and provides a data reference for when the catalyst is supplemented, the amount of the catalyst is reduced by more than 50%, and the cost is greatly reduced.

Description

Technical Field

[0001] This invention relates to the field of lactide preparation technology, and in particular to a method for controlling the amount of catalyst used to achieve continuous reaction, and the corresponding process system. Background Technology

[0002] Controlling the use of single-use non-degradable plastic products has become a consensus, presenting a significant development opportunity for biodegradable materials. Currently, commercially available biodegradable plastics include polylactic acid (PLA), polyhydroxyalkanoates (PHA), polybutylene succinate (PBS), polybutylene succinate-adipate (PBSA), and polybutylene terephthalate-adipate (PBAT), among others. PLA is currently the most widely used and has broad future prospects. It not only possesses the basic properties of general polymer materials but also excels in processing performance, physical and mechanical properties, and biodegradability. It can be widely applied in the packaging, textile, agricultural, and consumer goods markets, and is considered the most likely biodegradable material to replace petroleum-based polyesters.

[0003] Industrialized polylactic acid (PLA) synthesis primarily involves the ring-opening polymerization of lactide. Lactic acid is the key component of the entire synthesis process, which has relatively high production barriers. It typically requires a catalyst, high temperature, and high vacuum system, where lactic acid feedstock undergoes polycondensation and depolymerization processes. Continuous lactide production is crucial for its industrial-scale production. To improve feedstock utilization, a cyclic depolymerization process is commonly employed in continuous lactide production. This involves mixing the heavy components generated during depolymerization with fresh feedstock and then re-feeding them to the depolymerization reactor. A significant problem with this process is the accumulation of high-molecular-weight heavy components and catalyst during synthesis, which can easily lead to coking and carbonization of the substrate on the reactor surface.

[0004] CN111153886A discloses a rapid and high-yield synthesis method and apparatus for lactide. After removing the light components, oligolactic acid is added to a catalyst and passed through a mixer before entering a depolymerization reactor to depolymerize into lactide. The heavy components are refluxed and reintroduced into the depolymerization reactor, while the light components are purified and recovered to obtain the lactide product. This apparatus can efficiently synthesize lactide, obtaining crude lactide with a yield of 94%-98% within a short residence time of 0.5-5 minutes. After a simple purification system using the light components, the lactide product contains 94%-98% L-lactide, D-lactide, or DL-lactide, and 0.5%-5.5% m-lactide. However, the heavy components generated by the depolymerization reaction of this invention directly enter the depolymerization feed mixer and mix with the reaction raw materials to form a stream, which is then re-entered into the depolymerization reactor for reaction. In long-term operation, this not only affects the stable operation of the depolymerization reactor, but also, with the increase of the molecular weight of the heavy components and the accumulation of catalyst, it easily increases the probability of coking and carbonization of the reaction substrate on the reactor surface, and increases the degree of racemization of lactide. At the same time, the catalyst deactivates in the reaction system. How to judge the activity of the catalyst and replenish it in time to adjust the amount of effective catalyst in the depolymerization reaction system will directly affect the quality of crude lactide and the long-term stable operation of the equipment. Summary of the Invention

[0005] To address the above shortcomings, this invention provides a process system and method for the continuous depolymerization of lactide. By adding a catalyst addition control system, the amount of catalyst added can be controlled. In particular, the amount of fresh catalyst entering the depolymerization reactor can be adjusted in a timely manner according to the gas phase outlet temperature of the depolymerization reactor. Through interlocking control, automated control can be achieved, flexibly realizing the continuous production of lactide and significantly reducing the amount of catalyst used.

[0006] To achieve the above-mentioned technical objectives, the technical solution of the present invention is as follows:

[0007] The technical objective of the first aspect of this invention is to provide a process system for the continuous depolymerization of lactide, comprising a polycondensation reactor, a static mixer, and a depolymerization reactor connected in sequence. The depolymerization reactor is provided with a light phase material outlet and a heavy phase material outlet. A temperature measuring device is provided at the light phase material outlet, and the heavy phase material outlet is connected to a circulation tank. A depolymerization catalyst material tank is connected to the front end pipeline of the static mixer, and the outlet of the circulation tank is connected to the inlet pipeline of the static mixer.

[0008] Furthermore, a pump is provided between the depolymerization reactor and the circulation tank for transporting materials from high vacuum to low vacuum or atmospheric pressure, while also serving to isolate the high and low vacuum.

[0009] Furthermore, the light phase material outlet of the depolymerization reactor is connected to a crude lactide purification system, which includes a process for purifying and refining crude lactide.

[0010] The process system of the present invention has a temperature measuring device installed on the outlet pipeline of the light phase material of the depolymerization reactor to monitor the gas phase temperature in real time. At the same time, a depolymerization catalyst material tank is set on the inlet pipeline of the depolymerization reactor for adding fresh depolymerization catalyst. The material addition point is located at the front end of the lactic acid oligomer entering the depolymerization reactor, and a static mixer is set on the pipeline at the rear end of the depolymerization catalyst addition point.

[0011] Furthermore, the depolymerization catalyst material tank is equipped with a stirring device and a temperature control device, which can fully stir the material and control the temperature. The depolymerization catalyst material tank is also equipped with a valve to control whether the catalyst is fed and the amount of feed.

[0012] Furthermore, the above adjustment process can be performed manually or automatically through interlocking.

[0013] The second aspect of this invention aims to provide a process for the continuous depolymerization preparation of lactide using the above-mentioned process system, comprising: fresh lactic acid oligomer raw material from a polycondensation reactor is mixed with material including a depolymerization catalyst via a static mixer and then enters a depolymerization reactor; the gaseous crude lactide produced by the depolymerization reaction leaves the depolymerization reactor from the light phase material outlet; unreacted material and heavy components are discharged from the heavy phase material outlet and enter a circulation tank; the material in the circulation tank is mixed with fresh lactic acid oligomer and depolymerization catalyst and then transported back to the depolymerization reactor; throughout the above process, the reaction temperature in the depolymerization reactor is maintained. The temperature range is 180-220℃. A temperature measuring device at the outlet of the light phase material is used to monitor the temperature of the gaseous crude lactide in real time to determine the deactivation of the catalyst in the depolymerization reactor. Under normal reaction conditions, the temperature of the gaseous crude lactide is maintained above 170℃. As the catalyst in the circulating depolymerization system deactivates, the temperature of the gaseous crude lactide shows a downward trend. When the temperature drops to 160-165℃, the valve of the depolymerization catalyst material tank is opened to replenish the catalyst. When the temperature of the gaseous crude lactide reaches the range of 175℃-185℃, the valve of the depolymerization catalyst material tank is closed and the catalyst replenishment is stopped.

[0014] Furthermore, when the temperature of the gas-phase crude lactide drops to 160-165°C, the amount of catalyst added is 0.2%-5% of the real-time feed amount of fresh lactic acid oligomers, preferably 0.3%-3%.

[0015] Furthermore, the molecular weight of the lactic acid oligomer is 800-4000, preferably 1200-3000, and it is generally obtained from L-lactic acid or D-lactic acid through dehydration and polycondensation processes. The dehydration removes free water from the lactic acid and can be carried out under normal or reduced pressure. The reaction temperature in the polycondensation reactor is 140-170℃, the reaction time is 0.5-4.0 h, and the absolute pressure is 1000-2000 Pa.

[0016] Furthermore, the material in the depolymerization catalyst material tank is a depolymerization catalyst, or a mixture of a depolymerization catalyst and lactic acid oligomers, preferably a mixture of a depolymerization catalyst and lactic acid oligomers. The amount of depolymerization catalyst is smaller than that of oligolactic acid. A portion of the lactic acid oligomer raw material is used to first disperse the depolymerization catalyst, and then stirring is used to achieve uniform mixing of the catalyst and lactic acid oligomers. The mass concentration of the depolymerization catalyst in the mixture is controlled at 5%-45%, preferably 5-35%. The depolymerization catalyst material tank is equipped with a stirring paddle and a temperature monitoring device to ensure thorough stirring and temperature control of the material. The temperature is controlled at 160℃-170℃.

[0017] Furthermore, the depolymerization catalyst in the depolymerization reactor is entirely supplied by the depolymerization catalyst material tank, and the discharge valve of the depolymerization catalyst material tank can be precisely controlled.

[0018] Furthermore, in the above process, the depolymerization catalyst is a tin-based catalyst, selected from at least one of stannous octoate, SnCl2, and SnO. The initial amount of depolymerization catalyst is 0.4%-3.0% of the weight of the lactic acid oligomer, preferably 0.8%-2.0%.

[0019] Furthermore, in the above process method, the depolymerization reactor is a scraped film depolymerization reactor, mainly including any one of thin film evaporators, molecular distillation evaporators, or other stirred film evaporators.

[0020] Furthermore, in the above process, a pump is provided between the depolymerization reactor and the circulation tank for transporting materials from high vacuum to low vacuum or atmospheric pressure, while also serving to isolate the high and low vacuum.

[0021] Furthermore, in the above process, the reaction temperature of the depolymerization reactor is preferably 190℃-220℃, and the vacuum degree is 200Pa-1500Pa.

[0022] Furthermore, in the above process, the feed weight ratio of the fresh material to the recycled material from the circulation tank is 1:6-2:1.

[0023] Furthermore, in the above process, the feed rate is increased, and the total feed rate of fresh material and recycled material is controlled to be 4-8 times the actual reaction amount. This reduces the residence time of lactic acid oligomers in the scraped membrane depolymerization reactor, inhibits the polymerization process, improves the overall yield, and ensures product quality.

[0024] Furthermore, in the above process, the liquid level in the circulating tank is maintained at 30%-70%, the pressure is maintained at 10kPa-atmospheric pressure, and the temperature is maintained at 160-200℃, in order to reduce the probability of lactic acid oligomers continuing to undergo intermolecular polymerization reactions and reduce coking and carbonization.

[0025] Furthermore, in the above process, slag is periodically discharged based on the cumulative amount of catalyst circulating in the circulating tank. To achieve optimized continuous operation of the unit, slag is discharged when the cumulative amount of catalyst circulating in the circulating tank is detected to be greater than 15% of the circulating material; preferably, slag is discharged when it is greater than 15% but not greater than 20%, and most preferably, slag is discharged when it is greater than 15% but not greater than 18%. The lactic acid polymer discharged from the slag can be hydrolyzed and reused as lactic acid under certain conditions. The cumulative amount of catalyst circulating in the circulating tank is monitored in real time by sampling and analyzing the catalyst content from the rear outlet of the circulating tank every 2-4 hours.

[0026] Furthermore, in the above process, the gaseous crude lactide is directly fed into the crude lactide purification system. The crude lactide purification system is any combination of single-stage or multi-stage distillation, solvent extraction, and melt crystallization.

[0027] Furthermore, in the above process, the crude lactide product obtained contains: L-lactide content of 85%-90%, m-lactide content of 0.5%-3.5%, L-lactic acid content of 2.0%-5.6%, and dimer and trimer content of 2.0%-4.7%.

[0028] Compared with the prior art, the beneficial effects of the present invention are reflected in:

[0029] (1) This invention creatively proposes to use the temperature of the gaseous crude lactide at the outlet of the depolymerization reactor as the basis for determining the catalyst activity, providing data reference for when to add catalyst.

[0030] (2) Compared with adding depolymerization catalyst in the same proportion, the catalyst dosage is reduced by more than 50% by judging the appropriate addition of depolymerization catalyst based on the temperature of crude lactide in the gas phase, which greatly reduces the cost.

[0031] (3) This invention effectively controls the circulation rate of the depolymerization catalyst in the depolymerization reaction system, reducing the risk of catalyst accumulation leading to coking and carbonization of the reaction substrate on the reactor surface. It can effectively extend the slag discharge interval and improve oligomer conversion rate and product quality. The obtained crude lactide contains no more than 3.5% m-lactide, and the lactic acid oligomer conversion rate can reach over 95%. Compared to the traditional circulating depolymerization process, the racemization degree of lactide is reduced by more than 40%, and the lactic acid oligomer conversion rate is increased by more than 10%.

[0032] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the process system for preparing crude lactide according to the present invention. Detailed Implementation

[0034] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the invention, but do not limit the invention in any way.

[0035] The lactic acid used in the embodiments of the present invention is heat-resistant L-lactic acid with a lactic acid content of 88% or higher and an optical purity of not less than 99.0%.

[0036] This invention utilizes a Malvern Viscotek OMNISEC GPC / SEC gel chromatography system to analyze the molecular weight of lactic acid oligomers. A conventional calibration method was employed, with polystyrene (PS) as the internal standard. The chromatographic column was a T3000, 300 mm × 8.0 mm, with a column temperature of 40℃, a flow rate of 1.0 mL / min, a sample concentration of 2-5 mg / mL, and a single injection volume of 500 μL.

[0037] This invention employs an Agilent high-performance liquid chromatograph (HPLC) to analyze the chemical purity of lactide, the content of L-lactic acid and its dimer and trimer, and uses a UV detector. The mobile phase consists of phosphoric acid and acetonitrile. The chromatographic column is a ZORBAX SB-Aq, 250 mm long, with an inner diameter of 4.6 mm and packed with 5 μm particle size. 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.

[0038] This invention uses an Agilent gas chromatograph to analyze the content of different optical isomers of lactide. A CYCLOSIL-B column was selected, with a vaporization chamber temperature of 250°C, a detector temperature of 280°C, and a flame ionization detector. The column temperature program was as follows: initial temperature 100°C, held for 5 min, increased to 140°C at a rate of 4°C / min, held for 7 min, increased to 200°C at a rate of 8°C / min, held for 20 min, N2 flow rate 1.4 mL / min, hydrogen flow rate 30 mL / min, air flow rate 400 mL / min, and injection volume 0.5 μL.

[0039] The formula for calculating the conversion rate of lactic acid oligomers during the synthesis of crude lactide is as follows:

[0040] Lactic acid oligomer conversion rate = m / M × 100%

[0041] Where m is the total mass of crude lactide and M is the mass of the initial lactic acid oligomer.

[0042] Example 1

[0043] A process system for the continuous depolymerization to prepare lactide, such as Figure 1As shown, the reactor includes a polycondensation reactor 1, a static mixer 3, and a depolymerization reactor 4 connected in sequence. The depolymerization reactor 4 is equipped with a light phase material outlet and a heavy phase material outlet. A temperature measuring device 6 is installed at the light phase material outlet, and the temperature measuring device 6 is installed as close as possible to the light phase material outlet. It is then connected to a lactide purification system 7. The heavy phase material outlet is connected to a circulation tank 5, and the outlet of the circulation tank 5 is connected to the inlet pipeline of the static mixer. A depolymerization catalyst material tank 2 is connected to the front end pipeline of the static mixer 3. A branch pipeline is provided in the polycondensation reactor 1 to transport part of the oligomeric lactic acid raw material 13 into the depolymerization catalyst material tank 2.

[0044] The method for producing lactide using the above process system is as follows: Lactic acid oligomer raw material 8 from polycondensation reactor 1, recycled material 11 from the circulation tank, and a mixture 9 of partially mixed oligomerized lactic acid raw material 13 and depolymerization catalyst from depolymerization catalyst material tank 2, are passed through a static mixer and then enter depolymerization reactor 4. The gaseous lactide 12 produced by depolymerization exits depolymerization reactor 4 from the top. Unreacted materials and heavy components 10 are discharged from the bottom of depolymerization reactor 1, passing through circulation tank 5 and returning to depolymerization reactor 4. Slag is discharged when the amount of catalyst in the recycled material at the outlet of the circulation tank reaches the slag discharge condition.

[0045] The specific production process and results of lactide using the above system and methods are as follows:

[0046] Example 2

[0047] Preparation of lactic acid oligomers: (1) Dehydration of lactic acid: Take 10 kg of L-lactic acid (with a lactic acid content of about 88.0%) and add it to a four-necked flask with a stirring system. Use a vacuum circulating water pump to maintain the system pressure at about 50 kPa. Start heating under vacuum and gradually heat to 110-120℃ for 2 hours to dehydrate. At this time, the free water in the reaction system is slowly evaporated out of the reaction system. (2) Preparation of lactic acid oligomers: After the free water in the system is almost completely removed, increase the vacuum degree of the system and slowly reduce the system pressure to about 1.0 kPa. Gradually raise the temperature of the liquid to 160℃ and react for 3 hours. At this time, the lactic acid molecules undergo a condensation reaction, and the water generated in the system is evaporated out of the system to obtain a lactic acid oligomer with a molecular weight of 2200.

[0048] Add 950g of the above-mentioned lactic acid oligomer and 50g of stannous octoate catalyst to the depolymerization catalyst storage tank, and start stirring to mix the materials evenly. Fresh material is fed into the scraped membrane depolymerization reactor at a flow rate of 150g / h, and catalyst material at a flow rate of 37.5g / h. The depolymerization reaction conditions in the lower reaction zone are controlled as follows: vacuum degree 300Pa, reaction temperature 200℃, and single-pass residence time 2min. Unreacted material and heavy components after reaction are discharged into the circulation tank. The material in the circulation tank is discharged from the bottom and returned to the depolymerization reactor for recirculation and depolymerization. After establishing the circulation, the catalyst feeding is stopped. The temperature of the gas-phase crude lactide is approximately 185℃. After a period of depolymerization reaction, the temperature of the gas-phase crude lactide gradually decreases. When it drops to 165℃, catalyst addition is started at a rate of 45g / h. When the temperature of the gas-phase crude lactide rises back to 180℃, catalyst addition is stopped. During the subsequent reaction, catalyst addition continues according to the temperature changes of the gas-phase crude lactide.

[0049] The catalyst circulation accumulation was monitored in real time by sampling and analyzing samples taken from the rear end of the circulation tank. Cleaning and slag removal were performed when the accumulated catalyst circulation reached 15% of the circulating material. During the reaction, the mass ratio of fresh material to circulating material was controlled at 1:2, and the total feed rate was controlled at 4 times the actual reaction volume. As the reaction proceeded, the liquid level in the circulation tank was maintained at 50%, the pressure at 50 kPa, and the temperature at 180°C.

[0050] After the reaction system stabilized and the temperature of the crude lactide in the gas phase was between 175-180℃, samples were taken for analysis. During the reaction, samples were taken every 1 hour. If the reaction was in the catalyst replenishment stage, samples were taken after the system stabilized following the replenishment. Three samples were taken to analyze the composition of the crude lactide product, and the average value was used. The content of L-lactide was 86.7%, m-lactide was 2.2%, L-lactic acid was 3.1%, and the content of dimers and trimers was 3.7%. The conversion rate of lactic acid oligomers during the synthesis of crude lactide reached 98.0%.

[0051] The reaction was carried out continuously for 72 hours, during which the catalyst was added three times. The reactor operated smoothly and there was no obvious coking inside.

[0052] When the reaction was carried out continuously for 50 hours, the total feed amount of catalyst (excluding oligolactic acid) was 21.6 g, and the crude lactide yield was 8370 g.

[0053] Example 3

[0054] The preparation process of the lactic acid oligomer is the same as in Example 2, 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.

[0055] Add 55g of the above-mentioned lactic acid oligomer and 45g of stannous octoate catalyst to the depolymerization catalyst storage tank, and start stirring to mix the materials evenly. Fresh material is fed into the scraped membrane depolymerization reactor at a flow rate of 150g / h, and catalyst material at a flow rate of 10.7g / h. The depolymerization reaction conditions in the lower reaction zone are controlled as follows: vacuum degree 200Pa, reaction temperature 220℃, and single-pass residence time 2min. Unreacted material and heavy components after reaction are discharged into the circulation tank. The material in the circulation tank is discharged from the bottom and returned to the depolymerization reactor for recirculation and depolymerization. After establishing the circulation, the catalyst feeding is stopped. The temperature of the gas-phase crude lactide is 180℃. After a period of depolymerization reaction, the temperature of the gas-phase crude lactide gradually decreases. When it drops to 165℃, catalyst addition is started at a rate of 10g / h. When the temperature of the gas-phase crude lactide rises back to 175℃, catalyst addition is stopped. Afterward, catalyst addition continues according to the temperature change of the gas-phase crude lactide.

[0056] The catalyst circulation accumulation was monitored in real time by sampling and analyzing samples taken from the rear end of the circulation tank. Cleaning and slag removal were performed when the accumulated catalyst circulation reached 15% of the circulating material. During the reaction, the mass ratio of fresh material to circulating material was controlled at 2:1, and the total feed rate was adjusted to four times the actual reaction volume. As the reaction proceeded, the liquid level in the circulation tank was maintained at 60%, the pressure at atmospheric pressure, and the temperature at 200℃.

[0057] After the reaction system stabilized and the temperature of the crude lactide in the gas phase was between 175-180℃, samples were taken for analysis. During the reaction, samples were taken every 1 hour. If the reaction was in the catalyst replenishment stage, samples were taken after the system stabilized following the replenishment. Three samples were taken to analyze the composition of the crude lactide product, and the average value was used. The content of L-lactide was 86.3%, m-lactide was 3.2%, L-lactic acid was 2.7%, and the content of dimers and trimers was 3.6%. The conversion rate of lactic acid oligomers during the synthesis of crude lactide reached 96.3%.

[0058] Example 4

[0059] The preparation process of the lactic acid oligomer is the same as in Example 2, except that the reaction time is 2.0 h and the molecular weight of the obtained lactic acid oligomer is 800.

[0060] Add 70g of the above-mentioned lactic acid oligomer and 30g of stannous octoate catalyst to the depolymerization catalyst storage tank, and start stirring to mix the materials evenly. Fresh material is fed into the scraped membrane depolymerization reactor at a flow rate of 150g / h, and catalyst material at a flow rate of 2.0g / h. The depolymerization reaction conditions in the lower reaction zone are controlled as follows: vacuum degree 1500Pa, reaction temperature 180℃, and single-pass residence time 2min. Unreacted material and heavy components after reaction are discharged into the circulation tank. The material in the circulation tank is discharged from the bottom and returned to the depolymerization reactor for recirculation and depolymerization. After establishing the circulation, the catalyst feeding is stopped. The temperature of the gas-phase crude lactide is 178℃. After a period of depolymerization reaction, the temperature of the gas-phase crude lactide gradually decreases. When it drops to 165℃, catalyst addition is started at a rate of 23g / h. When the temperature of the gas-phase crude lactide rises back to 175℃, catalyst addition is stopped. Afterward, catalyst addition continues according to the temperature change of the gas-phase crude lactide. The catalyst circulation accumulation was monitored in real time by sampling and analyzing samples taken from the rear end of the circulation tank. Cleaning and slag removal were performed when the accumulated catalyst circulation reached 15% of the circulating material. During the reaction, the mass ratio of fresh material to circulating material was controlled at 1:6, and the total feed rate was controlled to be 8 times the actual reaction volume. As the reaction proceeded, the liquid level in the circulation tank was maintained at 70%, the pressure at 10 kPa, and the temperature at 160℃.

[0061] After the reaction system stabilized and the temperature of the crude lactide in the gas phase was between 175-180℃, samples were taken for analysis. During the reaction, samples were taken every 1 hour. If the reaction was in the catalyst replenishment stage, samples were taken after the system stabilized following the replenishment. Three samples were taken to analyze the composition of the crude lactide product, and the average value was taken. The content of L-lactide was 85.6%, the content of m-lactide was 2.2%, the content of L-lactic acid was 5.1%, and the content of dimers and trimers was 4.5%. The conversion rate of lactic acid oligomers in the crude lactide synthesis process reached 95.7%.

[0062] Example 5

[0063] The preparation process of lactic acid oligomers and lactide was the same as in Example 2, except that SnCl2 was used as the tin catalyst. Analysis of the crude lactide product revealed that the L-lactide content was 86.7%, the m-lactide content was 2.8%, the L-lactic acid content was 3.5%, and the dimer and trimer contents were 3.8%. The conversion rate of lactic acid oligomers in the crude lactide synthesis process reached 96.2%.

[0064] Example 6

[0065] The preparation process of lactic acid oligomers and lactide was the same as in Example 3, except that SnO was used as the tin catalyst. Analysis of the crude lactide product revealed that the L-lactide content was 86.1%, the m-lactide content was 2.4%, the L-lactic acid content was 3.6%, and the dimer and trimer contents were 4.1%. The conversion rate of lactic acid oligomers in the crude lactide synthesis process reached 95.8%.

[0066] Example 7

[0067] The preparation process of lactic acid oligomers and lactide was the same as in Example 3, except that a molecular distillation evaporator was used as the depolymerization reactor. Analysis of the crude lactide product revealed that the L-lactide content was 87.2%, the m-lactide content was 2.4%, the L-lactic acid content was 3.5%, and the dimer and trimer contents were 4.5%. The conversion rate of lactic acid oligomers in the crude lactide synthesis process reached 96.8%.

[0068] Comparative Example 1

[0069] The preparation process of lactic acid oligomers and lactide is the same as in Example 2, except that the catalyst is continuously added at a rate of 0.6 wt% of the weight of the lactic acid oligomers. It has already been added in the polycondensation reactor. During the long-term reaction, the catalyst accumulates quickly, and the heavy components accumulate more and more, which easily causes the reaction substrate to coke and carbonize on the surface of the reactor. After more than 50 hours, the lactide content in the product is less than 80%.

[0070] When the reaction was carried out continuously for 50 hours, the total feed amount of the catalyst was 45g, and the yield of crude lactide was 6145g.

[0071] Comparative Example 2

[0072] The preparation process of lactic acid oligomers and lactide was the same as in Example 3, except that slag was discharged when the cumulative amount of catalyst circulating in the circulation tank reached 25%. It was found that slag was discharged when the cumulative amount of catalyst circulating was large, and the slag contained black lumpy slag, presumably caused by excessive catalyst accumulation leading to coking on the reactor surface. Furthermore, the lactide product showed a high content of m-lactide. Analysis of the crude lactide product revealed that the L-lactide content was 85.5%, the m-lactide content was 6.4%, the L-lactic acid content was 3.4%, and the dimer and trimer content was 4.5%. The conversion rate of lactic acid oligomers in the crude lactide synthesis process was 74.7%.

Claims

1. A process system for the continuous depolymerization of lactide, comprising a polycondensation reactor, a static mixer and a depolymerization reactor connected in sequence, wherein the depolymerization reactor is provided with a light phase material outlet and a heavy phase material outlet, the light phase material outlet is provided with a temperature measuring device, the heavy phase material outlet is connected to a circulation tank, a depolymerization catalyst material tank is connected to the front end pipeline of the static mixer, and the outlet of the circulation tank is connected to the inlet pipeline of the static mixer.

2. The process system according to claim 1, characterized in that, A pump is also installed between the depolymerization reactor and the circulation tank for transporting materials from high vacuum to low vacuum or atmospheric pressure, while also serving to isolate the high and low vacuum.

3. The process system according to claim 1, characterized in that, The light phase material outlet of the depolymerization reactor is connected to a crude lactide purification system, which includes a process for purifying and refining crude lactide.

4. The process system according to claim 1, characterized in that, The depolymerization catalyst material tank is equipped with a stirring device and a temperature control device.

5. A process for continuously depolymerizing and preparing lactide using the process system of claim 1, comprising: Fresh lactic acid oligomer feedstock from the polycondensation reactor is mixed with materials including the depolymerization catalyst in a static mixer before entering the depolymerization reactor. The gaseous crude lactide produced by the depolymerization reaction leaves the depolymerization reactor from the light phase material outlet, while unreacted materials and heavy components exit from the heavy phase material outlet and enter a circulation tank. The materials in the circulation tank are mixed with fresh lactic acid oligomer and depolymerization catalyst and then transported back to the depolymerization reactor. Throughout the process, the reaction temperature in the depolymerization reactor is maintained at 180-220℃. A temperature measuring device at the light phase material outlet is used to monitor the temperature of the gaseous crude lactide in real time to determine the deactivation of the catalyst in the depolymerization reactor. Under normal reaction conditions, the temperature of the gaseous crude lactide is maintained above 170℃. As the catalyst in the circulating depolymerization system deactivates, the temperature of the gaseous crude lactide shows a decreasing trend. When the temperature drops to 160-165℃, the valve of the depolymerization catalyst material tank is opened to replenish the catalyst. When the temperature of the gaseous crude lactide reaches the range of 175℃-185℃, the valve of the depolymerization catalyst material tank is closed, and catalyst replenishment is stopped.

6. The process method according to claim 5, characterized in that, When the temperature of the gas-phase crude lactide drops to 160-165℃, the amount of catalyst added is 0.2%-0.5% of the real-time feed amount of fresh lactic acid oligomers.

7. The process method according to claim 5, characterized in that, The molecular weight of the lactic acid oligomer is 800-4000.

8. The process method according to claim 5, characterized in that, The reaction temperature in the polycondensation reactor is 140-170℃, the reaction time is 0.5-4.0h, and the absolute pressure is 1000-2000Pa.

9. The process method according to claim 5, characterized in that, The material in the depolymerization catalyst material tank is a depolymerization catalyst, or a mixture of a depolymerization catalyst and lactic acid oligomers, wherein the mass concentration of the depolymerization catalyst in the mixture is controlled at 5%-45%.

10. The process method according to claim 5, characterized in that, The depolymerization catalyst material tank is equipped with a stirring paddle and a temperature monitoring device, which are used to fully stir the material and control the temperature at 160℃-170℃.

11. The process method according to claim 5, characterized in that, The depolymerization catalyst is a tin-based catalyst, selected from at least one of stannous octoate, SnCl2, and SnO; the initial amount of depolymerization catalyst is 0.4%-3.0% of the weight of the lactic acid oligomer.

12. The process method according to claim 5, characterized in that, The depolymerization reactor is a scraped film depolymerization reactor, and its main forms include any one of thin film evaporators, molecular distillation evaporators, or other stirred film evaporators.

13. The process method according to claim 5, characterized in that, The reaction temperature of the depolymerization reactor is 190℃-220℃, and the vacuum degree is 200Pa-1500Pa.

14. The process method according to claim 5, characterized in that, The feed weight ratio of fresh material to recycled material from the circulation tank is 1:6-2:

1.

15. The process method according to claim 5, characterized in that, The feeding method is to increase the feed rate, and the total feed rate of fresh material and recycled material is controlled to be 4-8 times the actual reaction amount.

16. The process method according to claim 5, characterized in that, The liquid level in the circulation tank is maintained at 30%-70%, the pressure is maintained at 10kPa-atmospheric pressure, and the temperature is maintained at 160-200℃.

Images