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CN116041310A - Method and system for preparing crude lactide through continuous depolymerization - Google Patents

Method and system for preparing crude lactide through continuous depolymerization Download PDF

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Publication number
CN116041310A
CN116041310A CN202111262402.5A CN202111262402A CN116041310A CN 116041310 A CN116041310 A CN 116041310A CN 202111262402 A CN202111262402 A CN 202111262402A CN 116041310 A CN116041310 A CN 116041310A
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depolymerization
reaction
lactic acid
lactide
reaction zone
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白富栋
孙启梅
刘来伍
周峰
白毓黎
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Sinopec Dalian Petrochemical Research Institute Co ltd
China Petroleum and Chemical Corp
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China Petroleum and Chemical Corp
Sinopec Dalian Research Institute of Petroleum and Petrochemicals
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Abstract

The invention relates to a method and a system for preparing crude lactide by continuous depolymerization, which are characterized in that after lactic acid oligomer and a depolymerization catalyst are mixed, the mixture continuously enters a depolymerization reactor through a feed inlet arranged on the wall of the depolymerization reactor for reaction, heavy components enter a circulating tank after the reaction and are conveyed back to the depolymerization reactor through a circulating material inlet arranged above the feed inlet, an upper reaction zone is arranged between the feed inlet and the circulating material inlet, a lower reaction zone is arranged below the feed inlet, the heavy components enter the lower reaction zone for continuous participation in the reaction after the reaction in the upper reaction zone, and gas phase components, namely crude lactide, are extracted from the top of the reactor. The invention adopts the cyclic depolymerization process, and the cyclic material and the fresh material are split in the depolymerization process, so that the efficient and continuous preparation of the crude lactide is realized, the racemization degree of the lactide and the coking carbonization probability of the substrate are reduced, and the substrate conversion rate and the product quality are improved.

Description

Method and system for preparing crude lactide through continuous depolymerization
Technical Field
The invention belongs to the technical field of biodegradable materials, and particularly relates to a method and a system for preparing crude lactide through continuous depolymerization.
Background
The disposable plastic products bring convenience to people and simultaneously the harm of the disposable plastic products is also increasingly remarkable. At present, once-a-year plastic products consume 1.2 hundred million tons worldwide, only 10% of which are recycled, and the other about 12% of which are incinerated, and more than 70% of which are discarded into soil, air and the ocean. The amount of plastic waste put into the ocean exceeds 800 ten thousand tons per year, and this figure is rising, and by 2025, the global ocean plastic waste amount will be up to 2.5 hundred million tons. The traditional disposable plastic products have short service life, but have stable physical and chemical properties, are difficult to degrade naturally, have frequent various environmental problems caused by a large amount of disposable plastic product wastes, and have seriously endangered the health and safety of land, water bodies, animals and human beings.
There are nearly 90 countries and regions worldwide that have come to bear relevant policies or regulations that govern or prohibit disposable non-degradable plastic products, and degradable materials are coming to great development opportunities. The currently commercialized biodegradable plastics include polylactic acid (PLA), polyhydroxyalkanoate (PHA), polybutylene succinate (PBS), polybutylene succinate-adipate (PBSA), polybutylene terephthalate-adipate (PBAT) and the like, wherein the PLA has the most wide application at present and has wide future development prospect. The material has the basic performance of common high polymer materials, is better in processing performance, physical and mechanical properties, biodegradability and the like, can be widely applied to packaging industry, textile industry, agricultural industry, consumer product market and the like, and is considered to be the degradable material most likely to replace petroleum-based polyester.
The industrialized polylactic acid synthesis is mainly obtained by lactide ring-opening polymerization: the first step of preparing lactide from lactic acid; and the polylactic acid is prepared by lactide ring-opening polymerization in the second step, and the molecular weight of PLA obtained by the process can reach hundreds of thousands to millions. The lactide is the key of the whole synthesis process, the production process has relatively high barrier, and the lactide is usually prepared by a polycondensation and depolymerization process under a catalyst, a high temperature and a high vacuum system, and the racemization of the lactide is easily caused in the process, so that the purity of the product is influenced. In addition, in order to improve the utilization rate of raw materials, a cyclic depolymerization process is generally adopted, and heavy components generated by depolymerization are mixed with fresh materials and then are conveyed into a depolymerization reactor, so that high molecular weight heavy components and catalysts accumulate in the synthesis process, and coking and carbonization of substrates on the surface of the reactor are easily caused.
US6326458 discloses a continuous process for the preparation of lactide and lactide polymers, wherein a falling film type tube array evaporator is used as the depolymerization reactor in the lactide preparation depolymerization section, lactic acid oligomer is added from the top end of the evaporator, lactide vapor is extracted from the bottom of the tube array reactor, and unreacted lactic acid oligomer is discharged from a lower discharge port. The reaction temperature required in the falling film reaction process is relatively low, but the lactide yield is low, and in order to keep high lactide yield, the feeding speed is generally required to be reduced, so that the retention time of the oligomer on the surface of the falling film reactor is increased, the undeployed lactic acid oligomer is rapidly polymerized under a high-temperature high-vacuum system, the molecular weight of the oligomer is increased, the depolymerization rate is further influenced on one hand, and the coking and carbonization of the oligomer on the surface of the falling film tube reactor are also easily caused on the other hand.
CN111153886a discloses a method and a device for synthesizing lactide rapidly and high yield, the de-light oligolactic acid is added with a catalyst and then enters a depolymerization reactor to be depolymerized into lactide through a mixer, heavy components enter the depolymerization reactor again through reflux, and the light components are purified and recycled to obtain lactide products. The device can be used for efficiently synthesizing lactide, the crude lactide with the yield of 94-98% can be obtained in the short residence time of 0.5-5 minutes, the L-lactide, D-lactide or DL-lactide content in the lactide product is 94-98% after the light component passes through a simple purification system,m-lactide content 0.5% -5.5%. However, the hairThe heavy components generated by the depolymerization reaction directly enter a depolymerization feeding mixer to be mixed with reaction raw materials to form a strand of materials, and the strand of materials enter the depolymerization reactor again to react, so that in long-term operation, the stable operation of the depolymerization reactor is influenced, and along with the increase of the molecular weight of the heavy components and the accumulation of a catalyst, the coking and carbonization probability of reaction substrates on the surface of the reactor are easily increased, the racemization degree of lactide is increased, and the continuous stable operation of the reaction and the product quality are influenced.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a method and a system for preparing crude lactide by continuous depolymerization. The invention adopts the cyclic depolymerization process, and the cyclic material and the fresh material are split in the depolymerization process, so that the efficient and continuous preparation of the crude lactide is realized, the racemization degree of the lactide and the coking carbonization probability of the substrate are reduced, and the substrate conversion rate and the product quality are improved.
In one aspect, the invention provides a method for preparing crude lactide by continuous depolymerization, comprising the following steps:
after the lactic acid oligomer and the depolymerization catalyst are mixed, the mixture continuously enters the depolymerization reactor through a feed inlet arranged on the wall of the depolymerization reactor to react, the heavy component enters a circulating tank after the reaction, the heavy component is conveyed back to the depolymerization reactor through a circulating material inlet arranged above the feed inlet, an upper reaction zone is arranged between the feed inlet and the circulating material inlet, a lower reaction zone is arranged below the feed inlet, the heavy component enters the lower reaction zone to continuously participate in subsequent reaction after reacting in the upper reaction zone, and the gas phase component, namely crude lactide, is continuously extracted from the top of the reactor.
In the above method, the molecular weight of the lactic acid oligomer is 800-4000, preferably 1200-3000, and is generally prepared by using L-lactic acid or D-lactic acid as raw materials through dehydration and polycondensation processes. The dehydration is to remove free water in lactic acid, and can be in a normal pressure or reduced pressure form; the reaction temperature in the polycondensation process is 140-170 ℃, the reaction time is 0.5-4.0h, and the absolute pressure is 1000-2000Pa.
In the above method, the depolymerization catalyst is a tin catalyst, such as stannous octoate, snCl 2 At least one of SnO, etc.The depolymerization catalyst is used in an amount of 0.4% to 3.0%, preferably 0.8% to 2.0% of the mass of the lactic acid oligomer.
In the above method, the depolymerization reactor is a wiped film depolymerization reactor, and the main form includes any one of a thin film evaporator, a molecular distillation evaporator, or other stirred film evaporator.
In the method, a pump is arranged between the depolymerization reactor and the circulating tank and used for conveying materials from high vacuum to low vacuum or normal pressure, and meanwhile, the high vacuum and the low vacuum are isolated.
In the method, the upper reaction zone of the depolymerization reactor occupies 1/5-1/3 of the whole reaction zone. The upper reaction zone has a reaction temperature of 200 to 240 ℃, preferably 210 to 240 ℃ and a vacuum of 200 to 1500Pa. Further, with the progress of the reaction, the reaction process is regulated and controlled by adopting a mode of gradually increasing the reaction temperature by monitoring the molecular weight of the heavy component in the circulating tank in real time, when the molecular weight is higher than 3000, the temperature is increased by 2-10 ℃, when the molecular weight is higher than 4000, the temperature is increased by 2-10 ℃, when the molecular weight is higher than 5000, the temperature is increased by 2-10 ℃ and finally the temperature is not higher than 240 ℃, so that the effective depolymerization of the heavy component is realized.
In the above method, the reaction temperature in the lower reaction zone of the depolymerization reactor is 180-220 ℃, preferably 190-220 ℃, and the vacuum degree is 200-1500Pa.
In the method, the feeding mass ratio of the fresh material to the recycled material is 1:6-2:1.
In the method, the mode of increasing the feeding amount is adopted for feeding, and the total feeding amount of fresh materials and recycled materials is regulated and controlled to be 4-8 times of the actual reaction amount, so that the residence time of the lactic acid oligomer in the wiped film depolymerization reactor is reduced, the occurrence of the polymerization process is inhibited, the overall yield is improved, and the product quality is ensured.
In the method, the liquid level of the circulating tank is maintained at 50% -70%, the pressure is maintained at 10 kPa-normal pressure, and the temperature is maintained at 160-200 ℃, so that the probability of continuous intermolecular polymerization of the lactic acid oligomer is reduced, and coking carbonization is reduced.
In the above method, the periodic deslagging is performed with reference to the catalyst circulation accumulation amount, and when the catalyst circulation accumulation amount is greater than 15%, the deslagging is performed. The lactic acid polymer discharged from the slag discharge can be hydrolyzed and recycled into lactic acid under certain conditions.
In the method, the depolymerization reactor is provided with a gas-phase lactide outlet, and the gas-phase lactide is directly conveyed into a separation and purification working section. The separation and purification section can be a single-stage or multi-stage rectification, solvent extraction and melt crystallization in various combinations.
In the above process, the crude lactide product obtained: the L-lactide content is 85% -90%,m-lactide content 0.5% -3.5%, L-lactic acid content 2.0% -6.0%, dimer content 2.0% -5.0%.
The invention also provides a system for preparing crude lactide by continuous depolymerization, which mainly comprises a depolymerization reactor and a circulating tank, wherein the depolymerization reactor is used for carrying out depolymerization reaction on the lactic acid oligomer in the presence of a depolymerization catalyst, heavy components are discharged into the circulating tank after the reaction, a feeding hole and a circulating material inlet on the feeding hole are arranged on the wall of the depolymerization reactor, an upper reaction zone is arranged between the feeding hole and the circulating material inlet, a lower reaction zone is arranged below the feeding hole, the heavy components enter the lower reaction zone for continuous subsequent reaction after the reaction in the upper reaction zone, and gas phase components are discharged from the top of the depolymerization reactor after the reaction.
In the system, the depolymerization reactor is a wiped film depolymerization reactor, and the main forms comprise any one of a thin film evaporator, a molecular distillation evaporator or other stirring film evaporators and the like.
In the system of the invention, the upper reaction zone of the depolymerization reactor occupies 1/5-1/3 of the whole reaction zone.
Compared with the prior art, the invention has the beneficial effects that:
(1) In experiments, the inventor finds that if the depolymerized heavy components are mixed with fresh materials and then fed into a depolymerization reactor, the heavy components are accumulated more and more in the long-term cyclic reaction process, so that the reaction substrates are easy to coke and carbonize on the surface of the reactor, and long-term stable operation cannot be ensured. Therefore, the inventor circulates the heavy component generated by depolymerization back to the upper part of the fresh material reaction zone, and the reacted material enters the fresh material reaction zone to continue to react, so that the adverse effect of mixing the heavy component and the fresh material on the reaction is reduced, the racemization degree of lactide and the coking carbonization probability of a substrate are reduced, the deslagging interval is effectively prolonged, and the oligomer conversion rate and the product quality are improved. In the obtained crude lactide, the m-lactide content is not more than 3.5%, and the lactic acid oligomer conversion rate can reach more than 95%. Compared with the traditional cyclic depolymerization process, the racemization degree of lactide is reduced by more than 50%, and the conversion rate of lactic acid oligomer is improved by more than 10%.
(2) On the basis of adopting a cyclic depolymerization process, the continuous and stable operation of the depolymerization process can be effectively ensured by monitoring the accumulation condition of the catalyst in the circulating tank in real time to carry out periodic deslagging, and the substrate conversion rate and the product quality are ensured.
(3) The method of increasing the total feeding amount of fresh materials and recycle materials is adopted to reduce the residence time of the lactic acid oligomer on the surface of the reactor, inhibit the polymerization process, improve the overall conversion rate and reduce the racemization degree of lactide.
Drawings
FIG. 1 is a schematic flow diagram of a process and apparatus for preparing crude lactide according to the present invention;
wherein: 1-depolymerization reactor, 2-circulating tank, 3-upper reaction zone, 4-lower reaction zone, 5-motor, 6-scraper, 7-transmission pump, 8-circulating pump; 01-fresh material, 02-recycled material, 03-heavy component, 04-gas phase crude lactide; a-circulating material inlet and B-feeding port.
Detailed Description
The method and apparatus for preparing lactide according to the present invention are further illustrated by the following examples. The embodiment is implemented on the premise of the technical scheme of the invention, and detailed implementation modes and specific operation processes are given, but the protection scope of the invention is not limited to the following embodiment.
The experimental methods in the following examples, unless otherwise specified, are all conventional in the art. The experimental materials used in the examples described below were purchased from biochemical reagent stores unless otherwise specified.
The lactic acid adopted in the embodiment of the invention is heat-resistant L-lactic acid with the lactic acid content of 88% or more, and the optical purity of the lactic acid is not lower than 99.0%.
The invention adopts a Markov Viscotek OMNISEC GPC/SEC gel chromatograph to analyze the molecular weight of the lactic acid oligomer. The traditional correction method is adopted, polystyrene (PS) is used as an internal standard, the type of a chromatographic column is T3000, the size is 300mmL multiplied by 8.0mm, the column temperature is 40 ℃, the flow rate is 1.0mL/min, the sample concentration is 2-5mg/mL, and the single sample injection amount is 500 mu L.
The invention adopts Agilent high performance liquid chromatograph to analyze the chemical purity of lactide, the content of L-lactic acid, dimer and trimer, the ultraviolet detector adopts phosphoric acid and acetonitrile as mobile phase, the model of chromatographic column is ZORBAX SB-Aq, the column length is 250mm, the inner diameter of column is 4.6mm, and the particle diameter of the built-in filler is 5 μm. Detection wavelength: 200nm, column temperature: 40 ℃, flow rate: 1mL/min, sample injection amount: 5. Mu.L.
The invention adopts Agilent gas chromatograph to analyze the lactide content of different optical isomers, and selects a CYCLOSIL-B type chromatographic column, a gasification chamber temperature of 250 ℃, a detector temperature of 280 ℃, a hydrogen flame ion detector, a column temperature programming initial temperature of 100 ℃, a temperature of 5min, a temperature of 140 ℃ at a speed of 4 ℃/min, a temperature of 7 min, a temperature of 200 ℃ at a speed of 8 ℃/min, a temperature of 20min, a carrier gas N2 flow of 1.4mL/min, a hydrogen flow of 30mL/min, an air flow of 400mL/min and a sample injection amount of 0.5 mu L.
The calculation formula of the conversion rate of lactic acid oligomer in the process of synthesizing crude lactide is as follows:
lactic acid oligomer conversion = M/mx100%
Where M is the total mass of crude lactide and M is the mass of the initial lactic acid oligomer.
According to the embodiment of the invention, a circulating depolymerization system shown in fig. 1 is adopted, firstly, fresh lactic acid oligomer 01 is continuously pumped into a depolymerization reactor 1 from an inlet B for reaction, undeployed heavy components are discharged into a circulating tank 2 through a pump 7 after the reaction, a certain liquid level is established, a circulating pump 8 is operated, heavy component circulating material is conveyed into the depolymerization reactor from an inlet A through a pipeline 02, the circulating depolymerization system starts to carry out, the regulating circulating material 02 and the fresh lactic acid oligomer are mixed in proportion and then conveyed into the depolymerization reactor, the liquid level of the circulating tank is maintained at 30% -70% along with the progress of the reaction, the pressure is maintained at 10 kPa-normal pressure, the temperature is maintained at 160-200 ℃, and gas-phase lactide after the reaction is conveyed into a separation and purification system through the pipeline 04, so that the polymerization-grade monomer lactide is finally obtained.
The depolymerization reactor is a wiped film depolymerization reactor and adopts a thin film evaporator mode, wherein the upper reaction zone accounts for 1/4 of the whole reaction zone.
Example 1
Preparation of lactic acid oligomer: (1) lactic acid-releasing water: taking 10kg of L-lactic acid (the lactic acid content is about 88.0 percent), adding the L-lactic acid into a four-neck flask with a stirring system, adopting a vacuum circulating water pump, maintaining the pressure of the system at about 50kPa, starting heating under vacuum, gradually heating to 110-120 ℃, dehydrating for 2 hours, and slowly steaming free water in a reaction system out of the reaction system. (2) preparation of lactic acid oligomer: after the free water in the system is almost completely removed, the vacuum degree of the system is increased, the pressure of the system is slowly reduced to about 1.0kPa, the temperature of the feed liquid is gradually increased to 160 ℃, the reaction is carried out for 3 hours, at the moment, the polycondensation reaction between lactic acid molecules occurs, and the water generated by the reaction in the system is distilled out of the system, so that the lactic acid oligomer with the molecular weight of 2200 is obtained.
Lactide was prepared using the process flow of fig. 1: taking 5000g of the lactic acid oligomer, adding 50g of stannous octoate catalyst, uniformly mixing, and then conveying the mixture to a wiped film depolymerization reactor at a flow rate of 150g/h, wherein depolymerization reaction conditions in a lower reaction zone are controlled as follows: the vacuum degree is 300Pa, the reaction temperature is 200 ℃, the one-way residence time is 2min, heavy components are discharged into a circulating tank after the reaction, the heavy components are pumped back to the upper reaction zone of the depolymerization reactor through a circulating pump, the vacuum degree is controlled at 300Pa, and the reaction temperature is 210 ℃. In the reaction process, the mass ratio of the fresh material to the circulating material is regulated to be 1:2, and the total feeding quantity is regulated to be 4 times of the actual reaction quantity. As the reaction proceeded, the recycle tank level was maintained at 50%, the pressure was maintained at 50kPa, and the temperature was maintained at 180 ℃. And deslagging by taking the catalyst circulating accumulation amount as a reference, and deslagging when the catalyst circulating accumulation amount is more than 15%. The deslagging of the invention is judged by 15% of accumulated catalyst, and compared with the method which uses the molecular weight of the oligomer in the circulating tank as a criterion, the method has the advantages that the molecular weight reaches 10000 and is generally much faster than 15% of accumulated catalyst, so that the deslagging interval is effectively prolonged.
In the analyzed crude lactide product, the content of L-lactide is 87.1%, the content of m-lactide is 2.9%, the content of L-lactic acid is 3.5%, the content of dimer and trimer is 3.8%, and the conversion rate of lactic acid oligomer in the synthesis process of the crude lactide is 96.6%.
Example 2
The procedure of example 1 was followed except that the final reaction temperature was 170℃and the reaction time was 4.0 hours, whereby the molecular weight of the lactic acid oligomer was 4000.
Lactide was prepared using the same process flow as in fig. 1: taking 5000g of the lactic acid oligomer, adding 150g of stannous octoate catalyst, uniformly mixing, and then conveying the mixture to a wiped film depolymerization reactor at a flow rate of 150g/h, wherein depolymerization reaction conditions in a lower reaction zone are controlled as follows: the vacuum degree is 200Pa, the reaction temperature is 220 ℃, the one-way residence time is 2min, heavy components are discharged into a circulating tank after the reaction, the heavy components are pumped back to the upper reaction zone of the depolymerization reactor through a circulating pump, the vacuum degree is controlled at 200Pa, and the reaction temperature is 240 ℃. In the reaction process, the mass ratio of fresh materials to circulating materials is controlled to be 2:1, and the total feeding amount is regulated and controlled to be 4 times of the actual reaction amount. With the progress of the reaction, the liquid level of the circulating tank was controlled to be 60%, the pressure was maintained at normal pressure, and the temperature was maintained at 200 ℃. And (3) taking the circulating accumulation amount of the catalyst at the outlet of the circulating tank as a reference to carry out periodic deslagging, and carrying out deslagging when the circulating accumulation amount of the catalyst is more than 15%.
In the analyzed crude lactide product, the content of L-lactide is 86.5%, the content of m-lactide is 3.4%, the content of L-lactic acid is 2.5%, the content of dimer and trimer is 4.1%, and the conversion rate of lactic acid oligomer in the synthesis process of crude lactide reaches 95.1%.
Example 3
The procedure for the preparation of lactic acid oligomer was as in example 1, except that the reaction time was 2.0 hours, and the molecular weight of the obtained lactic acid oligomer was 800.
Lactide was prepared using the same process flow as in fig. 1: taking 5000g of the lactic acid oligomer, adding 20g of stannous octoate catalyst, uniformly mixing, and then conveying the mixture to a film depolymerization reactor at a flow rate of 150g/h, wherein depolymerization reaction conditions in a lower reaction zone are controlled as follows: the vacuum degree is 1500Pa, the reaction temperature is 180 ℃, the one-way residence time is 2min, heavy components are discharged into a circulating tank after the reaction, the heavy components are pumped back to the upper reaction zone of the depolymerization reactor by a pump, the vacuum degree is controlled to be 1500Pa, and the reaction temperature is 210 ℃. In the reaction process, the mass ratio of the fresh material to the circulating material is regulated to be 1:6, and the total feeding amount is regulated to be 8 times of the actual reaction amount. As the reaction proceeded, the recycle tank level was controlled to be 70%, the pressure to be 10kPa, and the temperature to be 160 ℃. And (3) taking the circulating accumulation amount of the catalyst at the outlet of the circulating tank as a reference to carry out periodic deslagging, and carrying out deslagging when the circulating accumulation amount of the catalyst is more than 15%.
In the analyzed crude lactide product, the content of L-lactide is 85.7%, the content of m-lactide is 2.0%, the content of L-lactic acid is 6.0%, the content of dimer and trimer is 5.0%, and the conversion rate of lactic acid oligomer in the synthesis process of crude lactide reaches 95.7%.
Example 4
The procedure for the preparation of lactic acid oligomers and lactide was as in example 1, except that the tin catalyst used was SnCl 2 The addition amount was 25g. The crude lactide produced after the reaction was analyzed to have an L-lactide content of 85.2%,mthe lactide content is 3.1%, the L-lactic acid content is 4.0%, the dimer and trimer content is 4.3%, and the lactic acid oligomer conversion rate in the course of synthesizing crude lactide is 96.0%.
Example 5
The procedure for the preparation of lactic acid oligomer and lactide was as in example 1, except that the tin catalyst used was SnO and the amount added was 17g. The crude lactide produced after the reaction was analyzed to have an L-lactide content of 85.6%,mthe lactide content is 3.2%, the L-lactic acid content is 3.7%, the dimer and trimer content is 4.3%, and the lactic acid oligomer conversion rate in the course of synthesizing crude lactide is up to 95.4%.
Example 6
The procedure for the preparation of lactic acid oligomers and lactide was as in example 1, except that a molecular distillation depolymerization evaporator was used. By analysis, the L-lactide content in the lactide product was 88.1%,m-lactide content of 2.0%, L-lactic acid content of 4.5%, dimer, trimer content of3.6 percent, and the conversion rate of the lactic acid oligomer in the synthesis process of the crude lactide reaches 97.5 percent.
Example 7
The procedure for the preparation of lactic acid oligomers and lactide was as in example 1, except that the reaction was controlled by stepwise increasing the reaction temperature by monitoring the molecular weight of the heavy components in the circulation tank in real time as the reaction proceeded, and when the molecular weight was higher than 3000, the temperature was increased by 5 ℃, when the molecular weight was higher than 4000, the temperature was increased by 8 ℃, and when the molecular weight was higher than 5000, the temperature was increased by 10 ℃ and the final temperature was 233 ℃. By analysis, the L-lactide content in the lactide product was 88.1%,mthe lactide content is 2.0%, the L-lactic acid content is 3.2%, the dimer and trimer content is 3.3%, and the lactic acid oligomer conversion rate in the course of synthesizing crude lactide is up to 97.8%.
Comparative example 1
The lactic acid oligomer and lactide preparation process was the same as in example 1, except that the depolymerization reactor was a kettle reactor. In the obtained crude lactide, the content of L-lactide is 82.7%, the content of m-lactide is 6.7%, the content of L-lactic acid is 3.8%, the content of dimer and trimer is 4.9%, and the conversion rate of lactic acid oligomer in the synthesis process of the crude lactide is 92.6%.
Comparative example 2
The lactic acid oligomer and lactide preparation process was the same as in example 1 except that the recycle tank level was not controlled by feedback adjustment, the recycle tank pressure was the same as the wiped film depolymerization reactor pressure, and the temperature was 180 ℃. The test result shows that the reaction rate is slow due to no feedback regulation and the circulation flow rate is unchanged, so that the liquid level in the storage tank is continuously increased, the stable balance operation of feeding and discharging is influenced, and the lactide content in the reaction product is less than 50%.
Comparative example 3
The procedure for the preparation of lactic acid oligomers and lactide was as in example 1, except that the feed was fed in accordance with the actual reaction amount, and the feed was not controlled in the manner of the present invention. The test result shows that the lower half part of the surface of the film scraping depolymerization reactor is in a viscous state, the bottom is easy to coke and carbonize, the content of L-lactide is 86.2%, the content of m-lactide is 6.2%, the content of L-lactic acid is 4.2%, the content of dimer and trimer is 3.6%, and the conversion rate of lactic acid oligomer in the synthesis process of the crude lactide is only 69.4%.
Comparative example 4
The lactic acid oligomer and lactide preparation process is the same as in example 1, except that after depolymerization, the heavy component is mixed with fresh material and then sent into a depolymerization reactor, and in the long-term reaction process, the more the heavy component is accumulated, the more the reaction substrate is coked and carbonized on the surface of the reactor, frequent discharging is needed, and the lactide content in the product is lower than 80%.

Claims (16)

1. A method for preparing crude lactide by continuous depolymerization, which is characterized by comprising the following steps: after the lactic acid oligomer and the depolymerization catalyst are mixed, the mixture continuously enters the depolymerization reactor through a feed inlet arranged on the wall of the depolymerization reactor to react, the heavy component enters a circulating tank after the reaction, and is conveyed back to the depolymerization reactor through a circulating material inlet arranged above the feed inlet, wherein an upper reaction zone is arranged between the feed inlet and the circulating material inlet, a lower reaction zone is arranged below the feed inlet, the heavy component enters the lower reaction zone to continuously react after reacting in the upper reaction zone, and the gas phase component, namely crude lactide, is extracted from the top of the reactor.
2. The method according to claim 1, characterized in that: the molecular weight of the lactic acid oligomer is 800-4000, preferably 1200-3000.
3. The method according to claim 1 or 2, characterized in that: the lactic acid oligomer is prepared by taking L-lactic acid or D-lactic acid as a raw material through a dehydration and polycondensation process; the dehydration is to remove free water in lactic acid, and adopts normal pressure or decompression mode; the reaction temperature in the polycondensation process is 140-170 ℃, the reaction time is 0.5-4.0h, and the absolute pressure is 1000-2000Pa.
4. The method according to claim 1, characterized in that: the depolymerization catalyst is tin catalyst, preferably stannous octoate and SnCl 2 At least one of SnO.
5. The method according to claim 1 or 4, characterized in that: the depolymerization catalyst is used in an amount of 0.4% to 3.0%, preferably 0.8% to 2.0% of the mass of the lactic acid oligomer.
6. The method according to claim 1, characterized in that: the depolymerization reactor is a wiped film depolymerization reactor, and the main forms comprise any one of a thin film evaporator, a molecular distillation evaporator or other stirring film evaporators.
7. The method according to claim 1, characterized in that: the upper reaction zone of the depolymerization reactor accounts for 1/5-1/3 of the whole reaction zone.
8. The method according to claim 1 or 7, characterized in that: the upper reaction zone has a reaction temperature of 200 to 240 ℃, preferably 210 to 240 ℃ and a vacuum of 200 to 1500Pa.
9. The method according to claim 1, characterized in that: along with the progress of the reaction, the reaction process is regulated and controlled by adopting a mode of gradually increasing the reaction temperature by the real-time monitoring of the molecular weight of the heavy components in the circulating tank, when the molecular weight is higher than 3000, the temperature is increased by 2-10 ℃, when the molecular weight is higher than 4000, the temperature is increased by 2-10 ℃, and when the molecular weight is higher than 5000, the temperature is increased by 2-10 ℃ and the final temperature is not higher than 240 ℃, so that the effective depolymerization of the heavy components is realized.
10. The method according to claim 1 or 7, characterized in that: the reaction temperature of the lower reaction zone of the depolymerization reactor is 180-220 ℃, preferably 190-220 ℃ and the vacuum degree is 200-1500Pa.
11. The method according to claim 1, characterized in that: the feeding mass ratio of the fresh material to the recycled material is 1:6-2:1.
12. The method according to claim 1, characterized in that: the method of increasing the feeding amount is adopted for feeding, and the total feeding amount of the fresh material and the recycle material is regulated and controlled to be 4-8 times of the actual reaction amount.
13. The method according to claim 1, characterized in that: the liquid level of the circulating tank is maintained at 50% -70%, the pressure is maintained at 10 kPa-normal pressure, and the temperature is maintained at 160-200 ℃.
14. The method according to claim 1, characterized in that: and (3) taking the circulating accumulation amount of the catalyst as a reference to carry out periodic deslagging, and carrying out deslagging when the circulating accumulation amount of the catalyst is more than 15%.
15. The method according to claim 1, characterized in that: the crude lactide product obtained: the L-lactide content is 85% -90%,m-lactide content 0.5% -3.5%, L-lactic acid content 2.0% -6.0%, dimer content 2.0% -5.0%.
16. A system for the continuous depolymerization process for preparing crude lactide according to any one of claims 1-15, characterized in that it mainly comprises a depolymerization reactor and a circulation tank, wherein the depolymerization reactor is used for depolymerizing lactic acid oligomer in the presence of a depolymerization catalyst, heavy components after the reaction are discharged into the circulation tank, the wall of the depolymerization reactor is provided with a feed inlet and a circulation inlet on the feed inlet, an upper reaction zone is arranged between the feed inlet and the circulation inlet, a lower reaction zone is arranged below the feed inlet, the heavy components after the reaction in the upper reaction zone enter the lower reaction zone to continue the subsequent reaction, and gas phase components after the reaction are discharged from the top of the depolymerization reactor.
CN202111262402.5A 2021-10-28 2021-10-28 Method and system for preparing crude lactide through continuous depolymerization Pending CN116041310A (en)

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Citations (6)

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US6326458B1 (en) * 1992-01-24 2001-12-04 Cargill, Inc. Continuous process for the manufacture of lactide and lactide polymers
CN1429814A (en) * 2002-12-26 2003-07-16 浙江大学 Method of preparing lactide using recovered lactic acid polymer as raw material
CN1986540A (en) * 2005-12-19 2007-06-27 奇美实业股份有限公司 Method for producing α-hydroxy organic acid cyclic ester compound
CN101054371A (en) * 2007-05-24 2007-10-17 复旦大学 Preparation method for glycolide
CN111153886A (en) * 2020-01-16 2020-05-15 南京大学 A kind of synthetic method and device of fast high-yield lactide
CN112876452A (en) * 2021-01-19 2021-06-01 万华化学(四川)有限公司 Preparation method and reaction device of lactide

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6326458B1 (en) * 1992-01-24 2001-12-04 Cargill, Inc. Continuous process for the manufacture of lactide and lactide polymers
CN1429814A (en) * 2002-12-26 2003-07-16 浙江大学 Method of preparing lactide using recovered lactic acid polymer as raw material
CN1986540A (en) * 2005-12-19 2007-06-27 奇美实业股份有限公司 Method for producing α-hydroxy organic acid cyclic ester compound
CN101054371A (en) * 2007-05-24 2007-10-17 复旦大学 Preparation method for glycolide
CN111153886A (en) * 2020-01-16 2020-05-15 南京大学 A kind of synthetic method and device of fast high-yield lactide
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