Preparation method and preparation device of crude lactide

Abstract

The present invention belongs to the technical field of heterocyclic compounds, and specifically relates to a method and apparatus for preparing crude lactide. A catalyst is added to lactic acid to conduct a condensation reaction to obtain lactic acid oligomers; a terminal deactivator and a viscosity modifier are added to the lactic acid oligomers to conduct an end-capping reaction to obtain a lactic acid prepolymer; the lactic acid prepolymer is mixed with a configuration-locking agent and subjected to a depolymerization reaction, while an inert gas is introduced to assist mass transfer to obtain lactide vapor; the lactide vapor is subjected to gas-liquid separation and condensation to obtain liquid crude lactide. The present invention significantly improves the yield, purity, and reaction efficiency of lactide by introducing a ternary synergistic system of a terminal deactivator, a viscosity modifier, and a configuration-locking agent, thereby solving problems such as acid value, racemization, and energy consumption in the lactide preparation process.

Description

Technical Field

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

[0002] Lactide is the cyclic dimer monomer of polylactic acid (PLA), and its quality directly affects the molecular weight and performance of PLA. Industrially, lactic acid is typically polycondensed to produce oligomeric lactic acid, which is then depolymerized and cyclized to produce lactide. However, existing technologies have several shortcomings: First, traditional processes require prolonged, high-temperature processing, resulting in high energy consumption and low yields. Second, lactic acid readily undergoes racemization at high temperatures, producing optically impure meso-lactide and reducing lactide quality. Third, oligomeric lactic acid often has terminal carboxyl groups. Depolymerization generates free acid, resulting in a high acid value in the crude product, which not only corrodes equipment but also affects subsequent polymerization reactions. Therefore, improving lactide yield and optical purity, while reducing crude acid value and energy consumption, are urgent challenges for those skilled in the art.

[0003] Currently, there are various improvements to crude lactide production technology. For example, Chinese patent CN 117886792A discloses a method and system for efficiently preparing low-racemized lactide. Lactic acid oligomers are heated and melted, then a depolymerization catalyst and a molten racemization inhibitor are added and mixed evenly. Depolymerization proceeds in an enhanced depolymerization reactor to produce crude lactide. The crude lactide is then purified to produce polymerization-grade lactide. While this patent utilizes an ultrasonically enhanced depolymerization reaction system and introduces a racemization inhibitor, while achieving some success in reducing meso-lactide content and improving yield, issues remain, such as batch-to-batch instability in meso-lactide content, a high crude product acid value, and a lack of attention to energy consumption control.

[0004] Chinese patent CN119241492A discloses a method for synthesizing lactide using a kaolin-based catalyst, comprising the following steps: (1) preparing oligomeric lactic acid: using L-lactic acid, D-lactic acid, or DL-lactic acid as raw material, subjecting the lactic acid to a dehydration polymerization reaction to obtain oligomeric lactic acid having an average molecular weight of 1000-3000 Da; (2) depolymerizing the oligomeric lactic acid: subjecting the oligomeric lactic acid obtained in step (1) to a depolymerization reaction under the action of a kaolin-based catalyst to obtain crude lactide. While this patent utilizes a kaolin-based catalyst to synthesize lactide, and while it produces relatively high-purity crude lactide, the process is complex and unsuitable for large-scale industrial production. Most importantly, the crude lactide yield is low.

[0005] Chinese patent CN115677649A discloses a method for efficiently preparing and purifying lactide, comprising polymerization, depolymerization, and purification. The polymerization comprises prepolymerization, primary polymerization, and secondary polymerization, and the depolymerization is carried out in a thin-film evaporator. However, this patent suffers from problems such as high crude lactide acid content, high meso-lactide content, and low yield.

[0006] Chinese patent CN118026990A discloses a method for preparing lactide, in which oligomeric lactic acid is depolymerized in the presence of a mesoporous molecular sieve-supported zinc catalyst to produce lactide. The crude lactide produced in this patent has a high acid content and fails to achieve synergistic optimization of acid value and optical purity.

[0007] In summary, existing technologies for crude lactide preparation fail to simultaneously achieve the synergistic optimization of low acid value, low meso content, and high yield, and some processes suffer from high energy consumption and cumbersome procedures. Therefore, there is an urgent need to provide a method for preparing high-quality crude lactide with low acid value, low meso content, high yield, and low energy consumption to meet the needs of industrial production. Summary of the Invention

[0008] The present invention aims to provide a method for preparing crude lactide. By introducing a ternary synergistic system of a terminal passivating agent, a viscosity modifier, and a configuration-locking agent, the yield, purity, and reaction efficiency of lactide are significantly improved, and problems such as acid value, racemization, and energy consumption that exist in the lactide preparation process are solved.

[0009] The method for preparing crude lactide of the present invention comprises the following steps:

[0010] (1) Adding a catalyst to lactic acid to carry out polycondensation reaction to obtain lactic acid oligomers; adding a terminal passivating agent and a viscosity control agent to the lactic acid oligomers to carry out end-capping reaction to obtain lactic acid prepolymers;

[0011] (2) mixing the lactic acid prepolymer obtained in step (1) with a configuration locking agent and performing a depolymerization reaction, while simultaneously introducing an inert gas to assist mass transfer, to obtain lactide vapor;

[0012] (3) After the lactide vapor is separated from the gas and liquid and condensed, liquid crude lactide is obtained.

[0013] In step (1), the catalyst is stannous octoate, and the amount of the catalyst added is 0.01-0.03% of the mass of the lactic acid; the polycondensation reaction temperature is 140-160° C., the polycondensation reaction vacuum is 3-8 kPa, the polycondensation reaction time is 4-6 h, and the number average molecular weight of the lactic acid oligomer is 1020-1508.

[0014] In step (1), the terminal passivation agent is an acid anhydride compound, which is one of acetic anhydride, maleic anhydride or benzoic anhydride, and the mass of the terminal passivation agent is 0.5-1wt.% of the mass of the lactic acid oligomer.

[0015] In step (1), the viscosity regulator is polyethylene glycol or polypropylene glycol, preferably one of PEG-400, PEG-1000, PEG-2000, PPG-400, PPG-1000 or PPG-2000; the mass of the viscosity regulator is 1-3 wt.% of the mass of the lactic acid oligomer.

[0016] The end-capping reaction time in step (1) is 0.5-1 hour, and the end-capping reaction temperature is 125-150°C.

[0017] In step (2), the configuration locking agent is a mixture of L-lactate zinc and phenylboric acid, the mass ratio of L-lactate zinc to phenylboric acid is 1:0.5-0.8, and the mass of the configuration locking agent is 0.3-0.6wt.% of the mass of the lactic acid prepolymer.

[0018] In step (2), the depolymerization reaction temperature is 180-205°C, the depolymerization reaction vacuum is 0.3-1kPa, and the depolymerization reaction time is 0.5-1h; the inert gas is nitrogen or argon, and the flow rate of the inert gas is 0.1-0.5m 3 / h. The introduction of inert gas can promote the escape of lactide vapor, increase the mass transfer effect of the system, and improve the lactide yield.

[0019] The temperature for gas-liquid separation in step (3) is 95-120°C, and the condensation temperature is 50-60°C.

[0020] The preparation device used in the method for preparing crude lactide of the present invention includes a prepolymerization reactor, which is respectively connected to a first auxiliary agent storage tank, a second auxiliary agent storage tank, and a static mixer. The first auxiliary agent storage tank and the second auxiliary agent storage tank are respectively connected to a pipeline between the prepolymerization reactor and the static mixer. The static mixer, a buffer tank, a depolymerization reactor, a gas-liquid separator, a first condenser, and a liquid lactide storage tank are sequentially connected. The pipeline between the buffer tank and the depolymerization reactor is connected to a third auxiliary agent storage tank. The bottom of the depolymerization reactor is connected to an inert gas storage tank. The first condenser, the second condenser, a collector, a Roots water ring vacuum unit, and a tail gas absorption tower are sequentially connected. The liquid lactide storage tank is connected to the second condenser. The prepolymerization reactor, the third condenser, a collecting tank, and a liquid ring pump are sequentially connected to the tail gas absorption tower. The third condenser is connected to the liquid ring pump.

[0021] A first flow regulating valve is provided on the pipeline between the prepolymerization reactor and the static mixer, a second flow regulating valve is provided on the pipeline connected to the bottom of the first auxiliary agent storage tank, a third flow regulating valve is provided on the pipeline connected to the bottom of the second auxiliary agent storage tank, and a fourth flow regulating valve is provided on the pipeline connected to the bottom of the third auxiliary agent storage tank; an external circulation pipeline is provided on the depolymerization reactor, and a circulation pump is provided on the external circulation pipeline.

[0022] The depolymerization reactor is a vertical wiped film reactor.

[0023] Since the configuration locking agent is a powdery substance, in order to facilitate the transportation of the configuration locking agent, the present invention first uses a portion of the viscosity control agent to ultrasonically dissolve the configuration locking agent before transportation.

[0024] The terminal deactivators used in the present invention block the terminal carboxyl groups of lactic acid oligomers, small-molecule lactic acid oligomers (such as dimers and trimers), and unreacted lactic acid monomers, thereby eliminating acid-catalyzed side reactions. The acid anhydride compounds employed in the present invention react with the terminal carboxyl groups of lactic acid oligomers, small-molecule lactic acid oligomers (such as dimers and trimers), and unreacted lactic acid monomers to block acidic active sites and form stable ester bonds. The resulting low-boiling carboxylic acid can be efficiently removed under high temperature and vacuum conditions. The capping reaction eliminates unstable hydrogen ions, thereby reducing acid-catalyzed side reactions (such as meso-reactions and oligomer chain scission). This step is crucial for reducing the acid value.

[0025] The viscosity control agent (PEG or PPG) in the present invention destroys the original intermolecular hydrogen bond network through selective hydrogen bonding (preferentially bonding with the hydroxyl group of lactic acid oligomers), reduces chain entanglement, and significantly reduces the intermolecular interaction of the system, thereby reducing the viscosity of the system, promoting mass transfer and uniform distribution of the system; the ether oxygen of the viscosity control agent can bind to Zn 2+ Dynamic coordination to prevent Zn 2+ Aggregation (Zn 2+ Aggregation reduces the number of available coordination sites and thus leads to decreased activity).

[0026] The Zn in the L-lactate zinc of the present invention 2+ It preferentially forms bidentate coordination with the hydroxyl oxygen of the lactic acid prepolymer and the free terminal carboxylate groups, fixing the main chain configuration. Phenylboronic acid forms hydrogen bonds with the terminal hydroxyl groups of the lactic acid prepolymer through its B-OH group, as well as π-π interactions with the aromatic ring, restricting rotation of the chiral center. Zinc L-lactate and phenylboronic acid form a synergistic locking system through coordination and hydrogen bonds, jointly locking the chiral center of lactic acid. Furthermore, phenylboronic acid, through its strong hydrogen bonding with the terminal hydroxyl groups of the lactic acid prepolymer through its B-OH group, inhibits β-elimination chain breakage in lactic acid oligomers, reducing the formation of nascent carboxyl groups. This helps maintain chain regularity and enhances the selectivity of the depolymerization reaction. When used in appropriate amounts, it does not inhibit the primary depolymerization reaction but instead helps improve the purity and yield of lactide.

[0027] The terminal passivating agent of the present invention can react with the terminal carboxyl groups of lactic acid oligomers, small molecule oligomers and unreacted lactic acid to form ester bonds, forming a stable ester hydrophobic region, which reduces the Zn 2+ The polarity around the coordination site inhibits water molecules from approaching the coordination site and reduces the hydrolysis reaction; at the same time, the hydrophobic chain of the terminal passivator hydrophobically associates with the benzene ring of phenylboronic acid, directionally guiding the B-OH group to approach the hydroxyl group of the lactic acid prepolymer to form a hydrogen bond network, synergistically enhancing the restriction on the rotation of the chiral center.

[0028] When polyethylene glycol is used as a viscosity modifier, the long polyethylene glycol chain, through the physical effect of steric hindrance, allows for a more uniform dispersion of phenylboronic acid. Furthermore, the polyethylene glycol hydroxyl group can form directional hydrogen bonds with the B-OH groups of phenylboronic acid, limiting excessive self-association between phenylboronic acid molecules and synergistically enhancing the effect of restricting rotation around the chiral center. The ether oxygen of the polyethylene glycol and the ester group (the ester group formed by the terminal passivator and lactic acid oligomers) form an ordered structure through dipole-dipole interactions, optimizing the diffusion path of the terminal passivator molecules in the system and enabling them to more efficiently contact and cap residual free carboxyl groups (such as small oligomers), further reducing the acid value while inhibiting side reactions. The polyethylene glycol hydroxyl group forms hydrogen bonds with the terminal carboxyl groups of unreacted monomers and small oligomers, accelerating the removal of byproducts and reducing residual carboxyl groups.

[0029] When polypropylene glycol is used as a viscosity modifier, the hydrophobic association between the polypropylene glycol's hydrophobic chains and the benzene rings of phenylboronic acid enhances hydrogen bond stability, protects phenylboronic acid from oxidation, and indirectly increases the configurational stability of the chiral center. The polypropylene glycol's hydrophobic chains associate with the hydrophobic regions of the ester groups (formed by the terminal passivator and the lactic acid oligomers), reducing phase separation, improving mass transfer efficiency, avoiding racemization caused by local overheating, and inhibiting localized carbonization of the oligomers. The ether oxygen of the polypropylene glycol interacts with the ester groups, reducing the electron cloud density of the ester bond and weakening the ester bond strength, promoting depolymerization and chain scission, and improving depolymerization efficiency. The ether oxygen of the polypropylene glycol (Lewis base) forms a transient coordination with the anhydride carbonyl group (Lewis acid), reducing the anhydride diffusion barrier and promoting the diffusion of anhydride molecules to the carboxyl sites.

[0030] The beneficial effects of the present invention are as follows:

[0031] (1) Low acid value and high optical purity of crude lactide: The use of terminal deactivators and viscosity modifiers significantly reduces the terminal acid content of oligomers, reducing the acid value of crude lactide to below 5 mgKOH / g, thereby alleviating the impact of acidic impurities on products and equipment. The addition of a configuration locking agent effectively inhibits high-temperature racemization side reactions, controlling the meso-lactide content to less than 1.2%, significantly higher than the optical purity of crude products from conventional processes. This means that the crude lactide produced by the present invention is closer to optically pure L-lactide, facilitating the subsequent direct polymerization to prepare high-molecular-weight PLA or reducing the number of refining steps.

[0032] (2) Improved yield: The present invention significantly shortens the residence time of lactic acid oligomers at high temperatures, allowing lactide to quickly escape from the reactor, reducing thermal degradation losses. The crude lactide yield is increased by about 8 percentage points compared to the traditional process.

[0033] (3) Strong adaptability: The present invention has relatively low requirements for the optical purity of the raw material lactic acid. Even if fermentation-grade lactic acid containing a small amount of D-lactic acid is used, crude L-lactide with high optical purity can be obtained because the process inhibits racemization, which is beneficial to reducing raw material costs.

[0034] In summary, the terminal passivator, conformation-locking agent, and viscosity modifier in the present invention form a synergistic network through chemical bonds and intermolecular forces, achieving a synergistic effect of acid value control, racemization control, and viscosity control, significantly reducing the acid value of the crude lactide, reducing the meso-lactide content, and improving the yield. The crude lactide produced by the present invention has an acid value of ≤5mgKOH / g, a meso-lactide content of less than 1.2%, a crude lactide purity of not less than 97%, and a crude lactide yield of more than 98%. The crude lactide produced by the present invention has excellent quality indicators and can be used for large-scale production of high-purity PLA monomers, promoting the development of the biodegradable materials industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Schematic diagram of the structure of the preparation device used in the method for preparing crude lactide of the present invention;

[0036] In the figure: 1. Prepolymerization reactor; 2. First auxiliary agent storage tank; 3. Second auxiliary agent storage tank; 4. Third auxiliary agent storage tank; 5. First flow regulating valve; 6. Second flow regulating valve; 7. Third flow regulating valve; 8. Fourth flow regulating valve; 9. Static mixer; 10. Buffer tank; 11. Depolymerization reactor; 12. Inert gas storage tank; 13. Circulation pump; 14. Gas-liquid separator; 15. First condenser; 16. Liquid lactide storage tank; 17. Second condenser; 18. Collector; 19. Roots water ring vacuum unit; 20. Tail gas absorption tower; 21. Liquid ring pump; 22. Third condenser; 23. Collection tank. DETAILED DESCRIPTION

[0037] The present invention is further described below with reference to the following examples.

[0038] In all examples, unless otherwise specified, “%” refers to mass percentage.

[0039] Example 1

[0040] like Figure 1As shown, the preparation device used in the preparation method of crude lactide includes a prepolymerization reactor 1, which is respectively connected to a first auxiliary agent storage tank 2, a second auxiliary agent storage tank 3, and a static mixer 9. The first auxiliary agent storage tank 2 and the second auxiliary agent storage tank 3 are respectively connected to the pipeline between the prepolymerization reactor 1 and the static mixer 9. The static mixer 9, the buffer tank 10, the depolymerization reactor 11, the gas-liquid separator 14, the first condenser 15 and the liquid lactide storage tank 16 are sequentially connected. The buffer tank 10 and the depolymerization reactor 11 are respectively connected to the first auxiliary agent storage tank 2, the second auxiliary agent storage tank 3 and the static mixer 9. The pipeline between the reactors 11 is connected to the third auxiliary agent storage tank 4, the bottom of the depolymerization reactor 11 is connected to the inert gas storage tank 12, the first condenser 15, the second condenser 17, the collector 18, the Roots water ring vacuum unit 19 and the tail gas absorption tower 20 are connected in sequence, the liquid lactide storage tank 16 is connected to the second condenser 17, the prepolymerization reactor 1, the third condenser 22, the collection tank 23, the liquid ring pump 21 and the tail gas absorption tower 20 are connected in sequence, and the third condenser 22 is connected to the liquid ring pump 21.

[0041] A first flow regulating valve 5 is provided on the pipeline between the prepolymerization reactor 1 and the static mixer 9, a second flow regulating valve 6 is provided on the pipeline connected to the bottom of the first auxiliary agent storage tank 2, a third flow regulating valve 7 is provided on the pipeline connected to the bottom of the second auxiliary agent storage tank 3, and a fourth flow regulating valve 8 is provided on the pipeline connected to the bottom of the third auxiliary agent storage tank 4; an external circulation pipeline is provided on the depolymerization reactor 11, and a circulation pump 13 is provided on the external circulation pipeline.

[0042] The preparation method of crude lactide comprises the following steps:

[0043] (1) Using fermented lactic acid with a total lactic acid content of 98% (L-lactic acid content accounts for 99.4% of the total lactic acid content, and D-lactic acid content accounts for 0.6% of the total lactic acid content) as raw materials, the fermented lactic acid and stannous octoate accounting for 0.02% of the mass of the fermented lactic acid are added into a prepolymerization reactor 1, and dehydrated and condensed at 140°C and 5kPa for 6h to obtain a viscous lactic acid oligomer with a number average molecular weight of 1020; the lactic acid oligomer enters the static chamber at a flow rate of 60kg / h through the first flow regulating valve 5. static mixer 9, while at the same time, benzoic anhydride in the first auxiliary agent storage tank 2 enters the static mixer 9 through the second flow regulating valve 6 at a flow rate of 10 g / min, and PEG-1000 in the second auxiliary agent storage tank 3 enters the static mixer 9 through the third flow regulating valve 7 at a flow rate of 6 g / min. The lactic acid oligomer, benzoic anhydride and PEG-1000 are mixed in the static mixer 9 and then enter the buffer tank 10 for end-capping reaction at 150° C. for 0.5 hour to obtain a lactic acid prepolymer;

[0044] (2) The lactic acid prepolymer was added to the depolymerization reactor 11 at a flow rate of 60 kg / h. At the same time, the mixture of PEG1000, L-lactate zinc and phenylboric acid in the third auxiliary agent storage tank 4 (the mass ratio of PEG1000, L-lactate zinc and phenylboric acid was 2:1:0.5) was added to the depolymerization reactor 11 through the fourth flow regulating valve 8 at a flow rate of 7 g / min. The depolymerization reaction was carried out at 200°C and a vacuum degree of 0.7 kPa for 0.5 h. The lactic acid prepolymer was rapidly vaporized and a large amount of lactide vapor was generated. At the same time, nitrogen in the inert gas storage tank 12 was introduced from the bottom of the depolymerization reactor 11. Nitrogen (flow rate 0.3m 3 / h) to assist in carrying lactide vapor; unreacted residual liquid in the depolymerization reactor 11 is discharged from the bottom and returned to the depolymerization reactor 11 through a circulation pump 13 for recycling; wherein the mixture of PEG1000, L-lactate zinc and phenylboric acid is a suspension prepared by ultrasonically dissolving the mixture of L-lactate zinc and phenylboric acid with PEG1000;

[0045] (3) Lactide vapor and nitrogen first enter the gas-liquid separator 14 for gas-liquid separation at 100°C, allowing only high-purity lactide vapor and nitrogen to enter the gas phase. The gas phase then enters the first condenser 15 for condensation at 55°C, condensing the lactide vapor into a liquid crude lactide product. The liquid crude lactide product enters the liquid lactide storage tank 16 for storage, and the non-condensable gas (mainly nitrogen) produced by the condensation passes through the second condenser 17, the collector 18 and the Roots water ring vacuum unit 19 in sequence and enters the tail gas absorption tower 20 for treatment; the water vapor obtained by the condensation in the prepolymerization reactor 1 enters the third condenser 22 for condensation, and the condensed water enters the collection tank 23. The non-condensable gas produced by the condensation passes through the liquid ring pump 21 and enters the tail gas absorption tower 20 for treatment.

[0046] The liquid crude lactide product was taken for testing. The acid value was determined by potentiometric titration to be 4.3 mgKOH / g. The meso-lactide content was determined by gas chromatography to be 1.0%, the crude lactide purity was 97.7%, and the total yield of crude lactide was 98.3%.

[0047] Example 2

[0048] The preparation apparatus used in the preparation method of crude lactide is the same as that in Example 1.

[0049] The preparation method of crude lactide comprises the following steps:

[0050] (1) Using fermented lactic acid with a total lactic acid content of 88% (L-lactic acid content accounts for 99.1% of the total lactic acid content, and D-lactic acid content accounts for 0.9% of the total lactic acid content) as raw materials, the fermented lactic acid and stannous octoate accounting for 0.01% of the mass of the fermented lactic acid are added to the prepolymerization reactor 1, and dehydrated and condensed at 160°C and 3kPa for 4h to obtain a viscous lactic acid oligomer with a number average molecular weight of 1255; the lactic acid oligomer enters the static reactor at a flow rate of 60kg / h through the first flow regulating valve 5. In the mixer 9, maleic anhydride in the first auxiliary agent storage tank 2 enters the static mixer 9 through the second flow regulating valve 6 at a flow rate of 5 g / min, and PPG-1000 in the second auxiliary agent storage tank 3 enters the static mixer 9 through the third flow regulating valve 7 at a flow rate of 24.5 g / min. The lactic acid oligomer, maleic anhydride, and PPG-1000 are mixed in the static mixer 9 and then enter the buffer tank 10 for end-capping reaction at 140°C for 0.8 hour to obtain a lactic acid prepolymer;

[0051] (2) The lactic acid prepolymer was added to the depolymerization reactor 11 at a flow rate of 60 kg / h. At the same time, the mixture of PPG-1000, L-lactate zinc and phenylboric acid in the third auxiliary agent storage tank 4 (the mass ratio of PPG-1000, L-lactate zinc and phenylboric acid was 2:1:0.6) was added to the depolymerization reactor 11 through the fourth flow regulating valve 8 at a flow rate of 9.5 g / min. The depolymerization reaction was carried out at 180°C and a vacuum degree of 0.3 kPa for 1 hour. The lactic acid prepolymer was rapidly vaporized and a large amount of lactide vapor was generated. At the same time, nitrogen in the inert gas storage tank 12 was introduced from the bottom of the depolymerization reactor 11. Nitrogen (flow rate 0.1m 3 / h) to assist in carrying lactide vapor; unreacted residual liquid in the depolymerization reactor 11 is discharged from the bottom and returned to the depolymerization reactor 11 through a circulation pump 13 for recycling; wherein the mixture of PPG-1000, L-lactate zinc and phenylboric acid is a suspension prepared by ultrasonically dissolving the mixture of L-lactate zinc and phenylboric acid with PPG-1000;

[0052] (3) Lactide vapor and nitrogen first enter the gas-liquid separator 14 for gas-liquid separation at 95°C, allowing only high-purity lactide vapor and nitrogen to enter the gas phase. The gas phase then enters the first condenser 15 for condensation at 60°C, condensing the lactide vapor into a liquid crude lactide product. The liquid crude lactide product enters the liquid lactide storage tank 16 for storage, and the non-condensable gas (mainly nitrogen) produced by the condensation passes through the second condenser 17, the collector 18 and the Roots water ring vacuum unit 19 in sequence and enters the tail gas absorption tower 20 for treatment; the water vapor obtained by the condensation in the prepolymerization reactor 1 enters the third condenser 22 for condensation, and the condensed water enters the collection tank 23. The non-condensable gas produced by the condensation passes through the liquid ring pump 21 and enters the tail gas absorption tower 20 for treatment.

[0053] The liquid crude lactide product was taken for testing. The acid value was determined to be 4.6 mgKOH / g by potentiometric titration. The meso-lactide content was determined to be 1.0% by gas chromatography. The crude lactide purity was 97.4%, and the total yield of crude lactide was 98.1%.

[0054] Example 3

[0055] The preparation apparatus used in the preparation method of crude lactide is the same as that in Example 1.

[0056] The preparation method of crude lactide comprises the following steps:

[0057] (1) Using fermented lactic acid with a total lactic acid content of 98% (L-lactic acid content accounts for 99.2% of the total lactic acid content, and D-lactic acid content accounts for 0.8% of the total lactic acid content) as raw materials, the fermented lactic acid and stannous octoate accounting for 0.03% of the mass of the fermented lactic acid are added to the prepolymerization reactor 1, and dehydrated and condensed at 150°C and 8kPa for 5h to obtain a viscous lactic acid oligomer with a number average molecular weight of 1508; the lactic acid oligomer is discharged through the first flow regulating valve 5 at a flow rate of 60kg / h. Entering the static mixer 9, at the same time, acetic anhydride in the first auxiliary agent storage tank 2 enters the static mixer 9 through the second flow regulating valve 6 at a flow rate of 8 g / min, and PEG-400 in the second auxiliary agent storage tank 3 enters the static mixer 9 through the third flow regulating valve 7 at a flow rate of 13 g / min. After the lactic acid oligomer, acetic anhydride and PEG-400 are mixed in the static mixer 9, they enter the buffer tank 10 and react at 125°C for 1 hour to obtain a lactic acid prepolymer;

[0058] (2) The lactic acid prepolymer is added to the depolymerization reactor 11. At the same time, the mixture of PEG-400, L-lactate zinc and phenylboric acid (the mass ratio of PEG-400, L-lactate zinc and phenylboric acid is 2:1:0.8) in the third auxiliary agent storage tank 4 is added to the depolymerization reactor 11 through the fourth flow regulating valve 8 at a flow rate of 12 g / min. The depolymerization reaction is carried out at 205°C and a vacuum degree of 1 kPa for 1 hour. The lactic acid prepolymer is rapidly vaporized and a large amount of lactide vapor is generated. At the same time, nitrogen in the inert gas storage tank 12 is introduced from the bottom of the depolymerization reactor 11. Nitrogen (flow rate 0.5m 3 / h) to assist in carrying lactide vapor; unreacted residual liquid in the depolymerization reactor 11 is discharged from the bottom through a circulation pump 13 and returned to the depolymerization reactor 11 for recycling; wherein the mixture of PEG-400, L-lactate zinc and phenylboric acid is a suspension prepared by ultrasonically dissolving the mixture of L-lactate zinc and phenylboric acid with PEG-400;

[0059] (3) Lactide vapor and nitrogen first enter the gas-liquid separator 14 for gas-liquid separation at 120°C, allowing only high-purity lactide vapor and nitrogen to enter the gas phase. The gas phase then enters the first condenser 15 for condensation at 50°C, condensing the lactide vapor into a liquid crude lactide product. The liquid crude lactide product enters the liquid lactide storage tank 16 for storage, and the non-condensable gas (mainly nitrogen) produced by the condensation passes through the second condenser 17, the collector 18 and the Roots water ring vacuum unit 19 in sequence and enters the tail gas absorption tower 20 for treatment; the water vapor obtained by the condensation in the prepolymerization reactor 1 enters the third condenser 22 for condensation, and the condensed water enters the collection tank 23. The non-condensable gas produced by the condensation passes through the liquid ring pump 21 and enters the tail gas absorption tower 20 for treatment.

[0060] The liquid crude lactide product was taken for testing. The acid value was determined to be 4.1 mgKOH / g by potentiometric titration. The meso-lactide content was determined to be 0.8% by gas chromatography. The crude lactide purity was 97.6%, and the total yield of the crude lactide was 98.5%.

[0061] Comparative Example 1

[0062] No PEG-1000 was added, and other steps were the same as in Example 1.

[0063] The liquid crude lactide product was taken for testing. The acid value was determined to be 10 mgKOH / g by potentiometric titration. The meso-lactide content was determined to be 1.6% by gas chromatography. The crude lactide purity was 95.2%, and the total yield of the crude lactide was 92.4%.

[0064] Comparative Example 2

[0065] Without adding benzoic anhydride, the other steps were the same as in Example 1.

[0066] The liquid crude lactide product was taken for testing. The acid value was determined to be 23.7 mgKOH / g by potentiometric titration. The meso-lactide content was determined to be 1.7% by gas chromatography. The crude lactide purity was 93.7%, and the total yield of the crude lactide was 95.1%.

[0067] Comparative Example 3

[0068] The other steps were the same as in Example 1 except that the mixture of L-lactate zinc and phenylboric acid was not added.

[0069] The liquid crude lactide product was taken for testing. The acid value was determined to be 10.2 mgKOH / g by potentiometric titration. The meso-lactide content was determined to be 3.9% by gas chromatography. The crude lactide purity was 93.8%, and the total yield of the crude lactide was 96.4%.

[0070] Comparative Example 4

[0071] In step (2), nitrogen is not introduced, and the other steps are the same as those in Example 1.

[0072] The liquid crude lactide product was taken for testing. The acid value was determined to be 7.5 mgKOH / g by potentiometric titration. The meso-lactide content was determined to be 1.9% by gas chromatography. The crude lactide purity was 95.0%, and the total yield of the crude lactide was 93.5%.

[0073] Comparative Example 5

[0074] The mixture of L-zinc lactate and phenylboric acid was replaced by L-zinc lactate, and the other steps were the same as in Example 1.

[0075] The liquid crude lactide product was taken for testing. The acid value was measured by potentiometric titration to be 9.7 mgKOH / g. The meso-lactide content was measured by gas chromatography to be 1.8%, the crude lactide purity was 95.9%, and the total yield of crude lactide was 96.5%.

[0076] Comparative Example 6

[0077] The mixture of L-lactate zinc and phenylboric acid was replaced by phenylboric acid, and the other steps were the same as in Example 1.

[0078] The liquid crude lactide product was taken for testing. The acid value was determined by potentiometric titration to be 9.3 mgKOH / g. The meso-lactide content was determined by gas chromatography to be 1.9%, the crude lactide purity was 96.1%, and the total yield of crude lactide was 95.8%.

[0079] Comparative Example 7

[0080] No PPG-1000 was added, and other steps were the same as in Example 2.

[0081] The liquid crude lactide product was taken for testing. The acid value was determined to be 8.4 mgKOH / g by potentiometric titration. The meso-lactide content was determined to be 1.5% by gas chromatography. The crude lactide purity was 96.2%, and the total yield of crude lactide was 93.6%.

[0082] The crude lactide detection results in Examples 1-3 and Comparative Examples 1-7 are shown in Table 1.

[0083]

Claims

1. A method for preparing crude lactide, characterized in that The steps include: (1) Adding a catalyst to lactic acid to carry out polycondensation reaction to obtain lactic acid oligomers; adding a terminal passivating agent and a viscosity control agent to the lactic acid oligomers to carry out end-capping reaction to obtain lactic acid prepolymers; (2) mixing the lactic acid prepolymer obtained in step (1) with a configuration locking agent and performing a depolymerization reaction, while simultaneously introducing an inert gas to assist mass transfer, to obtain lactide vapor; (3) The lactide vapor is separated into gas and liquid and condensed to obtain liquid crude lactide; In step (1), the terminal passivating agent is an acid anhydride compound, and the acid anhydride compound is one of acetic anhydride, maleic anhydride or benzoic anhydride; In step (1), the viscosity regulator is polyethylene glycol or polypropylene glycol; The configuration locking agent in step (2) is a mixture of L-lactate zinc and phenylboric acid.

2. The method for preparing crude lactide according to claim 1, characterized in that In step (1), the catalyst is stannous octoate, and the amount of the catalyst added is 0.01-0.03% of the mass of the lactic acid; the polycondensation reaction temperature is 140-160° C., the polycondensation reaction vacuum is 3-8 kPa, the polycondensation reaction time is 4-6 h, and the number average molecular weight of the lactic acid oligomer is 1020-1508.

3. The method for preparing crude lactide according to claim 1, characterized in that The mass of the terminal passivating agent in step (1) is 0.5-1 wt.% of the mass of the lactic acid oligomer.

4. The method for preparing crude lactide according to claim 1, characterized in that The mass of the viscosity regulator in step (1) is 1-3 wt.% of the mass of the lactic acid oligomer.

5. The method for preparing crude lactide according to claim 1, characterized in that The end-capping reaction time in step (1) is 0.5-1 hour, and the end-capping reaction temperature is 125-150°C.

6. The method for preparing crude lactide according to claim 1, characterized in that In step (2), the mass ratio of zinc L-lactate to phenylboric acid is 1:0.5-0.8, and the mass of the configuration locking agent is 0.3-0.6 wt.% of the mass of the lactic acid prepolymer.

7. The method for preparing crude lactide according to claim 1, characterized in that In step (2), the depolymerization reaction temperature is 180-205° C., the depolymerization reaction vacuum is 0.3-1 kPa, and the depolymerization reaction time is 0.5-1 h; the inert gas is nitrogen or argon.

8. The method for preparing crude lactide according to claim 1, characterized in that The temperature for gas-liquid separation in step (3) is 95-120°C, and the condensation temperature is 50-60°C.

9. A preparation device for the method for preparing crude lactide according to claim 1, characterized in that The invention comprises a prepolymerization reactor (1), wherein the prepolymerization reactor (1) is connected to a first auxiliary agent storage tank (2), a second auxiliary agent storage tank (3), and a static mixer (9), respectively; the first auxiliary agent storage tank (2) and the second auxiliary agent storage tank (3) are connected to a pipeline between the prepolymerization reactor (1) and the static mixer (9), respectively; the static mixer (9), a buffer tank (10), a depolymerization reactor (11), a gas-liquid separator (14), a first condenser (15), and a liquid lactide storage tank (16) are connected in sequence; and the pipeline between the buffer tank (10) and the depolymerization reactor (11) is connected to the first condenser (15). The depolymerization reactor (11) is connected to the third auxiliary agent storage tank (4), the bottom of the depolymerization reactor (11) is connected to the inert gas storage tank (12), the first condenser (15), the second condenser (17), the collector (18), the Roots water ring vacuum unit (19) and the tail gas absorption tower (20) are connected in sequence, the liquid lactide storage tank (16) is connected to the second condenser (17), the prepolymerization reactor (1), the third condenser (22), the collecting tank (23), the liquid ring pump (21) and the tail gas absorption tower (20) are connected in sequence, and the third condenser (22) is connected to the liquid ring pump (21).

10. The production device used in the method for producing crude lactide according to claim 9, characterized in that A first flow regulating valve (5) is provided on the pipeline between the prepolymerization reactor (1) and the static mixer (9), a second flow regulating valve (6) is provided on the pipeline connected to the bottom of the first auxiliary agent storage tank (2), a third flow regulating valve (7) is provided on the pipeline connected to the bottom of the second auxiliary agent storage tank (3), and a fourth flow regulating valve (8) is provided on the pipeline connected to the bottom of the third auxiliary agent storage tank (4); an external circulation pipeline is provided on the depolymerization reactor (11), and a circulation pump (13) is provided on the external circulation pipeline.

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