Preparation method and preparation device of crude lactide

Abstract

The invention belongs to the technical field of heterocyclic compounds, and particularly relates to a preparation method and a preparation device of crude lactide. Adding a catalyst into the lactic acid for condensation polymerization to obtain a lactic acid oligomer; adding a terminal passivating agent and a viscosity regulating agent into the lactic acid oligomer for end capping reaction to obtain a lactic acid prepolymer; mixing the lactic acid prepolymer with a configuration locking agent, then carrying out a depolymerization reaction, and meanwhile, introducing inert gas to assist mass transfer so as to obtain lactide steam; and carrying out gas-liquid separation and condensation on the lactide steam to obtain liquid crude lactide. By introducing a ternary synergistic system of the terminal passivator, the viscosity regulator and the configuration locking agent, the yield, purity and reaction efficiency of the lactide are remarkably improved, and the problems of acid value, racemization, energy consumption and the like in the lactide preparation process are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of heterocyclic compounds, and particularly relates to a preparation method and a preparation device for 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 usually polycondensed to obtain oligolactic acid, and then depolymerized and cyclized to generate lactide. However, there are some deficiencies in the prior art: First, the traditional process requires long-term high-temperature treatment, with high energy consumption and low yield; Second, lactic acid is prone to racemization at high temperature, generating optically impure meso-lactide, which reduces the quality of lactide; Third, the oligolactic acid terminal often has a carboxyl group, and the depolymerization generates free acid, making the acid value of the crude product too high, which not only corrodes the equipment but also affects the subsequent polymerization reaction. Therefore, improving the yield and optical purity of lactide, and reducing the acid value and energy consumption of the crude product are problems that need to be solved urgently by those skilled in the art.

[0003] At present, there are already various improvement schemes for the preparation technology of crude lactide. For example, Chinese Patent CN 117886792A discloses a method and a system for efficiently preparing low-racemization lactide. After heating and melting the lactic acid oligomer, a depolymerization catalyst and a molten racemization inhibitor are added and mixed evenly, and a depolymerization reaction is carried out in a strengthened depolymerization reactor to obtain crude lactide; the crude lactide is purified and refined to obtain polymer-grade lactide. This patent uses ultrasonic waves to strengthen the depolymerization reaction system and introduces a racemization inhibitor. Although certain results have been achieved in reducing the content of meso-lactide and increasing the yield, there are still problems such as unstable meso-content between batches, high acid value of the crude product, and lack of attention to energy consumption control.

[0004] Chinese Patent CN119241492A discloses a method for catalytic synthesis of lactide with a kaolin-based catalyst, including the following steps: (1) preparing oligolactic acid: using L-lactic acid or D-lactic acid or DL-lactic acid as raw materials, carrying out lactic acid dehydration polymerization reaction to obtain oligolactic acid with a weight-average molecular weight of 1000-3000Da; (2) depolymerizing oligolactic acid: the oligolactic acid obtained in step (1) is depolymerized under the action of a kaolin-based catalyst to obtain crude lactide. This patent uses a kaolin-based catalyst to catalytically synthesize lactide. Although it obtains crude lactide with relatively high purity, its steps are cumbersome and not suitable for large-scale industrial production. Most importantly, the yield of crude lactide is relatively low.

[0005] Chinese Patent CN115677649A discloses a method for efficiently preparing and purifying lactide. The method includes polymerization, depolymerization, and purification. The polymerization includes prepolymerization, primary polymerization, and secondary polymerization. The depolymerization is carried out in a thin-film evaporator. This patent has problems such as high content of crude lactide acid, high content of meso-lactide, and low yield.

[0006] Chinese Patent CN118026990A discloses a method for preparing lactide. The depolymerization of oligolactic acid to produce lactide is carried out under the action of a zinc-based catalyst supported on mesoporous molecular sieve. In this patent, the acid content of the crude lactide is relatively high, and the synergistic optimization of acid value and optical purity has not been achieved.

[0007] In summary, the existing technologies have not achieved the synergistic optimization of low acid value, low meso content, and high yield in the preparation of crude lactide, and some processes have problems such as high energy consumption and cumbersome steps. 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 requirements of industrial production. Summary of the Invention

[0008] The object of the present invention is to provide a method for preparing crude lactide. By introducing a ternary synergistic system of a terminal passivator, a viscosity regulator, and a configuration locking agent, the yield, purity, and reaction efficiency of lactide are significantly improved, and the problems of acid value, racemization, energy consumption, etc. existing in the preparation process of lactide are solved.

[0009] The method for preparing crude lactide of the present invention includes the following steps: (1) Adding a catalyst to lactic acid for polycondensation reaction to obtain a lactic acid oligomer; adding a terminal passivator and a viscosity regulator to the lactic acid oligomer for end-capping reaction to obtain a lactic acid prepolymer; (2) Mixing the lactic acid prepolymer obtained in step (1) with a configuration locking agent and then carrying out a depolymerization reaction, while introducing an inert gas to assist mass transfer to obtain lactide vapor; (3) After gas-liquid separation and condensation of the lactide vapor, liquid crude lactide is obtained.

[0010] In step (1), the catalyst is stannous octoate, and the addition amount of the catalyst is 0.01-0.03% of the mass of lactic acid; the polycondensation reaction temperature is 140-160 °C, the polycondensation reaction vacuum degree 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.

[0011] In step (1), the terminal passivator is an acid anhydride compound, and the acid anhydride compound is one of acetic anhydride, maleic anhydride, or benzoic anhydride. The mass of the terminal passivator is 0.5-1 wt.% of the mass of the lactic acid oligomer.

[0012] 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.

[0013] In step (1), the capping reaction time is 0.5-1 hour, and the capping reaction temperature is 125-150 °C.

[0014] In step (2), the configuration locking agent is a mixture of zinc L-lactate and phenylboric acid, 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.

[0015] 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, and the flow rate of the inert gas is 0.1-0.5 m 3 / h. Introducing the inert gas can promote the escape of lactide vapor, increase the mass transfer in the system, and improve the lactide yield.

[0016] In step (3), the temperature for gas-liquid separation is 95-120 °C, and the condensation temperature is 50-60 °C.

[0017] The preparation device used in the method for preparing crude lactide according to 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 the pipelines between the prepolymerization reactor and the static mixer. The static mixer, buffer tank, depolymerization reactor, gas-liquid separator, first condenser, and liquid lactide storage tank are connected in sequence. 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, second condenser, trap, Roots water ring vacuum unit, and tail gas absorption tower are connected in sequence. The liquid lactide storage tank is connected to the second condenser. The prepolymerization reactor, third condenser, collection tank, liquid ring pump, and tail gas absorption tower are connected in sequence. The third condenser is connected to the liquid ring pump.

[0018] 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. 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.

[0019] The depolymerization reactor is a vertical scraping film reactor.

[0020] Since the configuration locking agent is a powdery substance, in order to facilitate the transportation of the configuration locking agent, in the present invention, part of the viscosity regulator is first used to ultrasonically dissolve the configuration locking agent and then transport it.

[0021] In the present invention, the role of the terminal passivator is to block the terminal carboxyl groups of lactic acid oligomers, small molecule lactic acid oligomers (such as dimers, trimers), and unreacted lactic acid monomers, and eliminate acid-catalyzed side reactions. The acid anhydride compound used in the present invention reacts with the terminal carboxyl groups of lactic acid oligomers, small molecule lactic acid oligomers (such as dimers, trimers), and unreacted lactic acid monomers to block acidic active sites, form stable ester bonds, and the generated low-boiling carboxylic acids can be efficiently removed by high temperature and vacuum; through the end-capping reaction, unstable hydrogen ions are eliminated, thereby reducing acid-catalyzed side reactions (such as meso reaction and oligomer chain breakage), and this step is crucial for reducing the acid value.

[0022] In the present invention, the viscosity regulator (PEG or PPG) destroys the original intermolecular hydrogen bond network through selective hydrogen bonds (preferentially binding to the hydroxyl groups of lactic acid oligomers), reduces chain entanglement, 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 regulator can coordinate with Zn 2+ dynamically, preventing Zn 2+ aggregation (Zn 2+ aggregation will reduce the number of effective ligand sites and thus lead to a decrease in activity).

[0023] In the present invention, Zn in zinc L-lactate 2+ preferably forms a bidentate coordination with the hydroxyl oxygen of the lactic acid prepolymer and the terminal carboxylate in the free state to fix the main chain configuration; phenylboric acid forms hydrogen bonds and π-π interactions of the aromatic ring through B-OH with the terminal hydroxyl group of the lactic acid prepolymer, restricting the rotation of the chiral center. Zinc L-lactate and phenylboric acid construct a cooperative locking system through coordination bonds and hydrogen bonds to jointly lock the lactic acid chiral center. At the same time, phenylboric acid inhibits the β-elimination chain breakage of lactic acid oligomers through the strong hydrogen bond between B-OH and the terminal hydroxyl group of the lactic acid prepolymer, reduces the generation of new carboxyl groups, helps to maintain the chain segment regularity and improve the selectivity of the depolymerization reaction. When used in an appropriate amount, it will not inhibit the main depolymerization reaction, but instead helps to improve the purity and yield of lactide.

[0024] In the present invention, the terminal passivator 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-based hydrophobic region. This hydrophobic region reduces the polarity around the Zn 2+ ligand sites, inhibits water molecules from approaching the ligand sites, and reduces hydrolysis reactions; at the same time, the hydrophobic chain of the terminal passivator forms a hydrophobic association with the benzene ring of phenylboric acid, directing 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.

[0025] When the viscosity regulator is polyethylene glycol, the long chain of polyethylene glycol makes the phenylboronic acid more evenly dispersed through the physical effect of steric hindrance. At the same time, the hydroxyl groups of polyethylene glycol can form directional hydrogen bonds with B-OH of phenylboronic acid, restricting the excessive self-association between phenylboronic acid molecules and synergistically enhancing the effect of restricting the rotation of chiral centers. The ether oxygen of polyethylene glycol and the ester group (the ester group formed by the terminal passivator and the lactic acid oligomer) form an ordered arrangement structure through dipole-dipole interaction, optimizing the diffusion path of the terminal passivator molecules in the system, enabling them to more efficiently contact and block the remaining free carboxyl groups (such as small molecule oligomers), further reducing the acid value and simultaneously inhibiting side reactions; the hydroxyl groups of polyethylene glycol form hydrogen bonds with the terminal carboxyl groups of unreacted monomers and small molecule oligomers to accelerate the removal of by-products and reduce the residual carboxyl groups.

[0026] When the viscosity regulator is polypropylene glycol, the hydrophobic association between the hydrophobic chain of polypropylene glycol and the benzene ring of phenylboronic acid can enhance the hydrogen bond stability, protect phenylboronic acid from oxidation, and indirectly enhance the configurational stability of chiral centers; the hydrophobic chain of polypropylene glycol associates with the hydrophobic region of the ester group (the ester group formed by the terminal passivator and the lactic acid oligomer), reducing phase separation, improving mass transfer efficiency, avoiding racemization caused by local overheating, and inhibiting local carbonization of oligomers; the dipole interaction between the ether oxygen of polypropylene glycol and the ester group reduces the electron cloud density of the ester bond and weakens the ester bond strength, promoting depolymerization and chain scission and improving the depolymerization efficiency. The ether oxygen of polypropylene glycol (Lewis base) forms an instantaneous coordination with the acid anhydride carbonyl (Lewis acid), reducing the diffusion energy barrier of the acid anhydride and promoting the diffusion of acid anhydride molecules to the carboxyl site.

[0027] The beneficial effects of the present invention are as follows: (1) The crude lactide has a low acid value and high optical purity: The use of the terminal passivator and the viscosity regulator has significantly reduced the acid content at the ends of the oligomers, and the acid value of the crude lactide has been reduced to less than 5 mg KOH / g, reducing the impact of acidic impurities on the product and equipment. The addition of the configuration locking agent effectively inhibits the high-temperature racemization side reaction, and the content of meso-lactide is controlled within 1.2%, which is significantly higher than the optical purity of the crude products of existing ordinary processes. This means that the crude lactide prepared by the present invention is closer to optically pure L-lactide, which is beneficial for subsequent direct polymerization to prepare high-molecular-weight PLA or reducing the refining steps.

[0028] (2) The yield is increased: The present invention has significantly shortened the residence time of lactic acid oligomers at high temperature, enabling lactide to quickly escape from the reactor, reducing thermal degradation losses, and the yield of crude lactide is increased by about 8 percentage points or more compared with the traditional process.

[0029] (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, due to the inhibition of racemization during the process, a crude product of L-lactide with high optical purity can be obtained, which is beneficial to reducing the raw material cost.

[0030] In summary, in the present invention, the terminal passivator, configuration locking agent, and viscosity regulator form a synergistic network through chemical bonds and intermolecular forces, realizing the linkage effect of acid value control, racemization control, and viscosity regulation, significantly reducing the acid value of the crude lactide, decreasing the content of meso-lactide, and increasing the yield. The acid value of the crude lactide prepared by the present invention is ≤5 mgKOH / g, the content of meso-lactide is less than 1.2%, the purity of the crude lactide is not less than 97%, and the yield of the crude lactide can reach more than 98%. The crude lactide prepared by the present invention has excellent quality indicators in all aspects, can be used for the large-scale preparation of high-purity PLA monomers, and promotes the development of the biodegradable material industry. Brief Description of the Drawings

[0031] Figure 1 is a schematic structural diagram of the preparation device used in the preparation method of the crude lactide in the present invention; 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, trap; 19, Roots water ring vacuum unit; 20, tail gas absorption tower; 21, liquid ring pump; 22, third condenser; 23, collection tank. Detailed Embodiments

[0032] The following further describes the present invention in conjunction with embodiments.

[0033] In all embodiments, unless otherwise specified, "%" refers to mass percentage.

[0034] Example 1 As Figure 1As shown in the figure, the preparation device used in the method for preparing 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 pipelines between the prepolymerization reactor 1 and the static mixer 9. 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. The pipeline between the buffer tank 10 and the depolymerization reactor 11 is connected to a third auxiliary agent storage tank 4. The bottom of the depolymerization reactor 11 is connected to an inert gas storage tank 12. The first condenser 15, a second condenser 17, a trap 18, and a Roots water ring vacuum unit 19 are connected to a tail gas absorption tower 20 in sequence. The liquid lactide storage tank 16 is connected to the second condenser 17. The prepolymerization reactor 1, a third condenser 22, a collection tank 23, a liquid ring pump 21, and the tail gas absorption tower 20 are connected in sequence. The third condenser 22 is connected to the liquid ring pump 21.

[0035] 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. 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.

[0036] The method for preparing crude lactide includes the following steps: (1) Using fermented lactic acid with a total lactic acid content of 98% (L-lactic acid content accounting for 99.4% of the total lactic acid content, D-lactic acid content accounting for 0.6% of the total lactic acid content) as the raw material, adding the fermented lactic acid and stannous octoate accounting for 0.02% of the mass of the fermented lactic acid into the prepolymerization reactor 1, and performing dehydration polycondensation at 140 °C and 5 kPa for 6 h to obtain a viscous lactic acid oligomer with a number average molecular weight of 1020. The lactic acid oligomer enters the static mixer 9 at a flow rate of 60 kg / h through the first flow regulating valve 5. At the same time, benzoic anhydride in the first auxiliary agent storage tank 2 enters the static mixer 9 at a flow rate of 10 g / min through the second flow regulating valve 6, and PEG-1000 in the second auxiliary agent storage tank 3 enters the static mixer 9 at a flow rate of 6 g / min through the third flow regulating valve 7. 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 hours to obtain a lactic acid prepolymer; (2)The lactic acid prepolymer is added to the depolymerization reactor 11 at a flow rate of 60 kg / h. Meanwhile, a mixture of PEG1000, zinc L-lactate, and phenylboric acid (the mass ratio of PEG1000, zinc L-lactate, and phenylboric acid is 2:1:0.5) in the third auxiliary agent storage tank 4 is added to the depolymerization reactor 11 at a flow rate of 7 g / min through the fourth flow regulating valve 8. The depolymerization reaction is carried out at 200 °C and a vacuum degree of 0.7 kPa for 0.5 h. The lactic acid prepolymer quickly vaporizes, generating a large amount of lactide vapor. Meanwhile, nitrogen in the inert gas storage tank 12 is introduced from the bottom of the depolymerization reactor 11. Nitrogen (flow rate 0.3 m 3 / h) assists in carrying the lactide vapor; the unreacted residual liquid in the depolymerization reactor 11 is discharged from the bottom and returned to the depolymerization reactor 11 for recycling through the circulation pump 13; among them, the mixture of PEG1000, zinc L-lactate, and phenylboric acid is a suspension prepared by ultrasonically dissolving the mixture of zinc L-lactate and phenylboric acid in PEG1000; (3)The lactide vapor and nitrogen first enter the gas-liquid separator 14 for gas-liquid separation at 100 °C, and only the high-purity lactide vapor and nitrogen enter the gas phase. Subsequently, the gas phase 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. The non-condensable gas (mainly nitrogen) generated by condensation successively passes through the second condenser 17, the trap 18, and the Roots water ring vacuum unit 19 and enters the tail gas absorption tower 20 for treatment; the water vapor obtained by polycondensation in the prepolymerization kettle 1 enters the third condenser 22 for condensation, and the condensed water enters the collection tank 23. The non-condensable gas generated by condensation passes through the liquid ring pump 21 and enters the tail gas absorption tower 20 for treatment.

[0037] The liquid crude lactide product is taken for detection. The acid value measured by the potentiometric titration method is 4.3 mgKOH / g. The content of meso-lactide measured by gas chromatography is 1.0%, the purity of the crude lactide is 97.7%, and the total yield of the crude lactide is 98.3%.

[0038] Example 2 The preparation device used for the preparation method of the crude lactide is the same as that in Example 1.

[0039] The preparation method of the crude lactide includes the following steps: (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 the raw material, add the fermented lactic acid and stannous octoate accounting for 0.01% of the mass of the fermented lactic acid into the prepolymerization reactor 1. Carry out dehydration polycondensation at 160 °C and 3 kPa for 4 h to obtain a viscous lactic acid oligomer with a number average molecular weight of 1255. The lactic acid oligomer enters the static mixer 9 at a flow rate of 60 kg / h through the first flow regulating valve 5. At the same time, maleic anhydride in the first auxiliary agent storage tank 2 enters the static mixer 9 at a flow rate of 5 g / min through the second flow regulating valve 6, and PPG-1000 in the second auxiliary agent storage tank 3 enters the static mixer 9 at a flow rate of 24.5 g / min through the third flow regulating valve 7. After the lactic acid oligomer, maleic anhydride and PPG-1000 are mixed in the static mixer 9, they enter the buffer tank 10 for capping reaction at 140 °C for 0.8 hours to obtain a lactic acid prepolymer; (2) Add the lactic acid prepolymer into the depolymerization reactor 11 at a flow rate of 60 kg / h. At the same time, a mixture of PPG-1000, zinc L-lactate and phenylboric acid (the mass ratio of PPG-1000, zinc L-lactate and phenylboric acid is 2:1:0.6) in the third auxiliary agent storage tank 4 enters the depolymerization reactor 11 at a flow rate of 9.5 g / min through the fourth flow regulating valve 8. Carry out depolymerization reaction at 180 °C and a vacuum degree of 0.3 kPa for 1 h. The lactic acid prepolymer quickly vaporizes, generating a large amount of lactide vapor. 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 0.1 m 3 / h) assists in carrying the lactide vapor; the unreacted residue liquid in the depolymerization reactor 11 is discharged from the bottom and returned to the depolymerization reactor 11 for recycling through the circulation pump 13; among them, the mixture of PPG-1000, zinc L-lactate and phenylboric acid is a suspension prepared by ultrasonic dissolution of the mixture of zinc L-lactate and phenylboric acid in PPG-1000; (3) The lactide vapor and nitrogen first enter the gas-liquid separator 14 for gas-liquid separation at 95 °C, and only the high-purity lactide vapor and nitrogen enter the gas phase. Subsequently, the gas phase 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. The non-condensable gas generated by condensation (mainly nitrogen) successively passes through the second condenser 17, the trap 18 and the roots water ring vacuum unit 19 and enters the tail gas absorption tower 20 for treatment; the water vapor obtained by polycondensation 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 generated by condensation passes through the liquid ring pump 21 and enters the tail gas absorption tower 20 for treatment.

[0040] The liquid crude lactide product was tested. The acid value measured by potentiometric titration was 4.6 mg KOH / g. The content of meso-lactide measured by gas chromatography was 1.0%, the purity of the crude lactide was 97.4%, and the total yield of the crude lactide was 98.1%.

[0041] Example 3 The preparation apparatus used in the preparation method of the crude lactide was the same as that in Example 1.

[0042] The preparation method of the crude lactide comprises the following steps: (1) Using fermented lactic acid with a total lactic acid content of 98% (the content of L-lactic acid accounts for 99.2% of the total lactic acid content, and the content of D-lactic acid accounts for 0.8% of the total lactic acid content) as the raw material, the fermented lactic acid and stannous octoate accounting for 0.03% of the mass of the fermented lactic acid were added into the prepolymerization reactor 1, and dehydration polycondensation was carried out at 150 °C and 8 kPa for 5 h to obtain a viscous lactic acid oligomer with a number average molecular weight of 1508; the lactic acid oligomer entered the static mixer 9 at a flow rate of 60 kg / h through the first flow regulating valve 5. At the same time, acetic anhydride in the first auxiliary agent storage tank 2 entered the static mixer 9 at a flow rate of 8 g / min through the second flow regulating valve 6, and PEG-400 in the second auxiliary agent storage tank 3 entered the static mixer 9 at a flow rate of 13 g / min through the third flow regulating valve 7. After the lactic acid oligomer, acetic anhydride and PEG-400 were mixed in the static mixer 9, they entered the buffer tank 10 for end-capping reaction at 125 °C for 1 h to obtain a lactic acid prepolymer; (2) The lactic acid prepolymer was added into the depolymerization reactor 11. At the same time, a mixture of PEG-400, zinc L-lactate and phenylboric acid (the mass ratio of PEG-400, zinc L-lactate and phenylboric acid is 2:1:0.8) in the third auxiliary agent storage tank 4 was added into the depolymerization reactor 11 at a flow rate of 12 g / min through the fourth flow regulating valve 8. Depolymerization reaction was carried out at 205 °C and a vacuum degree of 1 kPa for 1 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, and nitrogen (flow rate 0.5 m 3 / h) assisted in carrying the lactide vapor; the unreacted residual liquid in the depolymerization reactor 11 was discharged from the bottom and returned to the depolymerization reactor 11 for recycling through the circulation pump 13; among them, the mixture of PEG-400, zinc L-lactate and phenylboric acid was a suspension prepared by ultrasonic dissolution of the mixture of zinc L-lactate and phenylboric acid in PEG-400; (3) The lactide vapor and nitrogen first enter the gas-liquid separator 14 for gas-liquid separation at 120 °C. Only the high-purity lactide vapor and nitrogen enter the gas phase. Subsequently, the gas phase enters the first condenser 15 for condensation at 50 °C, causing the lactide vapor to condense into a liquid crude lactide product. The liquid crude lactide product enters the liquid lactide storage tank 16 for storage. The non-condensable gas (mainly nitrogen) generated by condensation sequentially passes through the second condenser 17, the trap 18, and the Roots water ring vacuum unit 19 and enters the tail gas absorption tower 20 for treatment; the water vapor obtained by polycondensation in the prepolymerization reactor 1 enters the third condenser 22 for condensation. The condensed water enters the collection tank 23, and the non-condensable gas generated by condensation passes through the liquid ring pump 21 and enters the tail gas absorption tower 20 for treatment.

[0043] The liquid crude lactide product was taken for testing. The acid value measured by potentiometric titration was 4.1 mg KOH / g. The content of meso-lactide measured by gas chromatography was 0.8%, the purity of the crude lactide was 97.6%, and the total yield of the crude lactide was 98.5%.

[0044] Comparative Example 1 PEG-1000 was not added, and other steps were the same as in Example 1.

[0045] The liquid crude lactide product was taken for testing. The acid value measured by potentiometric titration was 10 mg KOH / g. The content of meso-lactide measured by gas chromatography was 1.6%, the purity of the crude lactide was 95.2%, and the total yield of the crude lactide was 92.4%.

[0046] Comparative Example 2 Benzoic anhydride was not added, and other steps were the same as in Example 1.

[0047] The liquid crude lactide product was taken for testing. The acid value measured by potentiometric titration was 23.7 mg KOH / g. The content of meso-lactide measured by gas chromatography was 1.7%, the purity of the crude lactide was 93.7%, and the total yield of the crude lactide was 95.1%.

[0048] Comparative Example 3 The mixture of L-lactic acid zinc and phenylboric acid was not added, and other steps were the same as in Example 1.

[0049] The liquid crude lactide product was taken for testing. The acid value measured by potentiometric titration was 10.2 mg KOH / g. The content of meso-lactide measured by gas chromatography was 3.9%, the purity of the crude lactide was 93.8%, and the total yield of the crude lactide was 96.4%.

[0050] Comparative Example 4 Nitrogen was not introduced in step (2), and other steps were the same as in Example 1.

[0051] The liquid crude lactide product was taken for detection. The acid value measured by potentiometric titration was 7.5 mg KOH / g. The content of meso-lactide measured by gas chromatography was 1.9%, the purity of the crude lactide was 95.0%, and the total yield of the crude lactide was 93.5%.

[0052] Comparative Example 5 The mixture of zinc L-lactate and phenylboric acid was replaced with zinc L-lactate, and the other steps were the same as in Example 1.

[0053] The liquid crude lactide product was taken for detection. The acid value measured by potentiometric titration was 9.7 mg KOH / g. The content of meso-lactide measured by gas chromatography was 1.8%, the purity of the crude lactide was 95.9%, and the total yield of the crude lactide was 96.5%.

[0054] Comparative Example 6 The mixture of zinc L-lactate and phenylboric acid was replaced with phenylboric acid, and the other steps were the same as in Example 1.

[0055] The liquid crude lactide product was taken for detection. The acid value measured by potentiometric titration was 9.3 mg KOH / g. The content of meso-lactide measured by gas chromatography was 1.9%, the purity of the crude lactide was 96.1%, and the total yield of the crude lactide was 95.8%.

[0056] Comparative Example 7 PPG-1000 was not added, and the other steps were the same as in Example 2.

[0057] The liquid crude lactide product was taken for detection. The acid value measured by potentiometric titration was 8.4 mg KOH / g. The content of meso-lactide measured by gas chromatography was 1.5%, the purity of the crude lactide was 96.2%, and the total yield of the crude lactide was 93.6%.

[0058] The detection results of the crude lactide in Examples 1-3 and Comparative Examples 1-7 are shown in Table 1.

[0059]

Claims

1. A method for preparing crude lactide, characterized in that It includes the following steps: (1) Add a catalyst to lactic acid for polycondensation reaction to obtain lactic acid oligomers; add a terminal passivator and a viscosity regulator to the lactic acid oligomers for end-capping reaction to obtain lactic acid prepolymers; (2) Mix the lactic acid prepolymer obtained in step (1) with a configuration locking agent and then carry out depolymerization reaction, while introducing an inert gas to assist mass transfer to obtain lactide vapor; (3) After gas-liquid separation and condensation of the lactide vapor, obtain liquid crude lactide.

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

3. The preparation method of crude lactide according to claim 1, characterized in that In step (1), the terminal passivator is an acid anhydride compound, and the acid anhydride compound is one of acetic anhydride, maleic anhydride or benzoic anhydride, and the mass of the terminal passivator is 0.5-1 wt.% of the mass of the lactic acid oligomers.

4. The preparation method of crude lactide according to claim 1, characterized in that In step (1), the viscosity regulator is polyethylene glycol or polypropylene glycol, and the mass of the viscosity regulator is 1-3 wt.% of the mass of the lactic acid oligomers.

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

6. The preparation method of crude lactide according to claim 1, characterized in that In step (2), the configuration locking agent is a mixture of zinc L-lactate and phenylboric acid, and 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 preparation method of crude lactide according to claim 1, characterized in that In step (2), the depolymerization reaction temperature is 180-205 °C, the depolymerization reaction vacuum degree is 0.3-1 kPa, the depolymerization reaction time is 0.5-1 h; the inert gas is nitrogen or argon.

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

9. The preparation apparatus used in the method for preparing crude lactide according to claim 1, characterized in that It includes a prepolymerization reactor (1), the prepolymerization reactor (1) 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 pipelines between the prepolymerization reactor (1) and the static mixer (9), 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, the pipeline between the buffer tank (10) and the depolymerization reactor (11) is connected to a third auxiliary agent storage tank (4), the bottom of the depolymerization reactor (11) is connected to an inert gas storage tank (12), the first condenser (15), a second condenser (17), a trap (18), a Roots water ring vacuum unit (19) and a 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), a third condenser (22), a collection tank (23), a 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 preparation apparatus used in the method for preparing 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). 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.

Images