Preparation method and preparation device of crude lactideTechnical Field
The invention belongs to the technical field of heterocyclic compounds, and particularly relates to a preparation method and a preparation device of crude lactide.
Background
Lactide (lactide) is a cyclic dimer monomer of polylactic acid (PLA), the quality of which directly affects the molecular weight and properties of PLA. Lactic acid is commonly used in industry to obtain oligomeric lactic acid by polycondensation, and then depolymerization and cyclization are carried out to generate lactide. However, the prior art has the defects that firstly, the traditional process needs long-time high-temperature treatment, the energy consumption is high, the yield is low, secondly, the lactic acid is easy to racemize at high temperature to generate optically impure meso-lactide, the quality of the lactide is reduced, and thirdly, the tail end of the oligomeric lactic acid is often provided with carboxyl, free acid is generated during depolymerization, so that the acid value of the crude product is higher, equipment is corroded, and the subsequent polymerization reaction is influenced. Therefore, the improvement of the yield and the optical purity of lactide and the reduction of the acid value and the energy consumption of crude products are the problems to be solved urgently by the person skilled in the art.
Currently, there are various improvements in the crude lactide preparation technology. For example, chinese patent CN 117886792a discloses a method and system for preparing low racemization lactide with high efficiency, after heating and melting lactic acid oligomer, adding depolymerization catalyst and melted racemization inhibitor, mixing uniformly, depolymerizing in reinforced depolymerization reactor to obtain crude lactide, purifying and refining crude lactide to obtain polymer grade lactide. The patent adopts an ultrasonic reinforced depolymerization reaction system and introduces racemization inhibitor, and has certain effects in reducing the content of meso-lactide and improving the yield, but the problems of unstable content of meso-lactide among batches, higher acid value of crude products, no attention to energy consumption control and the like still exist.
Chinese patent CN119241492A discloses a method for synthesizing lactide by using a kaolin-based catalyst, which comprises the following steps of (1) preparing oligomeric lactic acid, carrying out lactic acid dehydration polymerization reaction by using L-lactic acid or D-lactic acid or DL-lactic acid as a raw material to obtain oligomeric lactic acid with the weight average molecular weight of 1000-3000Da, and (2) depolymerizing the oligomeric lactic acid, wherein the oligomeric lactic acid obtained in the step (1) is subjected to depolymerization reaction under the action of the kaolin-based catalyst to obtain crude lactide. The patent adopts a kaolin-based catalyst to catalyze and synthesize lactide, and crude lactide with higher purity is obtained, but the steps are complicated and are not suitable for large industrial production, and the most important is that the yield of the crude lactide is lower.
Chinese patent CN115677649a discloses a method for efficiently preparing and purifying lactide, which comprises polymerization, depolymerization and purification, wherein the polymerization comprises prepolymerization, primary polymerization and secondary polymerization, and the depolymerization is performed in a thin film evaporator. The patent has the problems of high content of crude lactide acid, high content of meso-lactide, low yield and the like.
Chinese patent CN118026990a discloses a method for preparing lactide, in which the low polylactic acid is depolymerized under the action of mesoporous molecular sieve loaded zinc catalyst to produce lactide. The crude lactide in this patent has a relatively high acid content and does not allow for synergistic optimization of acid number and optical purity.
In summary, the prior art fails to realize the synergistic optimization of low acid value, low meso content and high yield in the preparation of crude lactide, and part of the process has the problems of high energy consumption, complicated steps and the like. Therefore, there is a need to provide a method for preparing high-quality crude lactide with low acid value, low meso content, high yield and low energy consumption, so as to meet the requirements of industrial production.
Disclosure of Invention
The invention aims to provide a preparation method of crude lactide, which obviously improves the yield, purity and reaction efficiency of the lactide and solves the problems of acid value, racemization, energy consumption and the like in the lactide preparation process by introducing a ternary synergistic system of a terminal passivating agent, a viscosity regulating agent and a configuration locking agent.
The preparation method of the crude lactide comprises the following steps:
(1) Adding a catalyst into lactic acid to perform polycondensation reaction to obtain a lactic acid oligomer, and adding a terminal passivator and a viscosity regulator into the lactic acid oligomer to perform end capping reaction to obtain a lactic acid prepolymer;
(2) Mixing the lactic acid prepolymer obtained in the step (1) with a configuration locking agent, performing depolymerization reaction, and simultaneously introducing inert gas to assist mass transfer to obtain lactide steam;
(3) And (3) performing gas-liquid separation and condensation on the lactide steam to obtain liquid crude lactide.
The catalyst in the step (1) is stannous octoate, the adding amount of the catalyst is 0.01-0.03% of the mass of lactic acid, the polycondensation reaction temperature is 140-160 ℃, the polycondensation reaction vacuum degree is 3-8kPa, the polycondensation reaction time is 4-6h, and the number average molecular weight of the lactic acid oligomer is 1020-1508.
The end passivating agent in the step (1) is an anhydride compound, wherein the anhydride compound is one of acetic anhydride, maleic anhydride or benzoic anhydride, and the mass of the end passivating agent is 0.5-1wt.% of the mass of the lactic acid oligomer.
The viscosity regulator in the step (1) is polyethylene glycol or polypropylene glycol, preferably one of PEG-400, PEG-1000, PEG-2000, PPG-400, PPG-1000 or PPG-2000, and the mass of the viscosity regulator is 1-3wt.% of the mass of the lactic acid oligomer.
The end capping reaction time in the step (1) is 0.5-1 hour, and the end capping reaction temperature is 125-150 ℃.
The configuration locking agent in the step (2) is a mixture of L-zinc lactate and phenylboric acid, the mass ratio of the L-zinc lactate to the 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.
The depolymerization reaction temperature in the step (2) is 180-205 ℃, the depolymerization reaction vacuum degree is 0.3-1kPa, the depolymerization reaction time is 0.5-1h, the inert gas is nitrogen or argon, the flow rate of the inert gas is 0.1-0.5m3/h, and the escape of lactide steam can be promoted by introducing the inert gas, the mass transfer effect of the system is increased, and the lactide yield is improved.
The temperature of the gas-liquid separation in the step (3) is 95-120 ℃ and the condensing temperature is 50-60 ℃.
The preparation device comprises a prepolymerization reaction kettle, wherein the prepolymerization reaction kettle is respectively connected with 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 with pipelines between the prepolymerization reaction kettle and 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 pipelines between the buffer tank and a depolymerization reactor are connected with a third auxiliary agent storage tank, the bottom of the depolymerization reactor is connected with an inert gas storage tank, the first condenser, the second condenser, a catcher and a Roots water ring vacuum unit are sequentially connected with a tail gas absorption tower, the liquid lactide storage tank is connected with the second condenser, the prepolymerization reaction kettle, the third condenser, the collection tank and a liquid ring pump are sequentially connected with the tail gas absorption tower, and the third condenser is connected with the liquid ring pump.
A first flow regulating valve is arranged on a pipeline between the prepolymerization reaction kettle and the static mixer, a second flow regulating valve is arranged on a pipeline connected with the bottom of the first auxiliary agent storage tank, a third flow regulating valve is arranged on a pipeline connected with the bottom of the second auxiliary agent storage tank, and a fourth flow regulating valve is arranged on a pipeline connected with the bottom of the third auxiliary agent storage tank; the depolymerization reactor is provided with an external circulation pipeline, and the external circulation pipeline is provided with a circulation pump.
The depolymerization reactor is a vertical wiped film reactor.
Because the configuration locking agent is a powdery substance, in order to facilitate the transportation of the configuration locking agent, the invention adopts the ultrasonic dissolution of the partial viscosity regulating agent and then the transportation is carried out.
The end passivating agent has the functions of blocking the end carboxyl of lactic acid oligomer, small molecule lactic acid oligomer (such as dimer and trimer) and unreacted lactic acid monomer, and eliminating acid catalytic side reaction. The acid anhydride compound is reacted with lactic acid oligomer, small molecule lactic acid oligomer, etc to block acid active site to form stable ester bond, and the produced low boiling point carboxylic acid may be eliminated in high temperature and vacuum, and the end capping reaction eliminates unstable hydrogen ion to reduce acid catalyzed side reaction, such as meso reaction and oligomer chain cleavage.
According to the invention, the viscosity regulator (PEG or PPG) damages the original intermolecular hydrogen bond network through selective hydrogen bonds (preferably combined with lactic acid oligomer hydroxyl groups), reduces chain entanglement, and remarkably reduces intermolecular interaction of a system, so that the viscosity of the system is reduced, mass transfer and uniform system distribution are promoted, ether oxygen of the viscosity regulator can be dynamically coordinated with Zn2+, zn2+ aggregation is prevented (Zn2+ aggregation can reduce the number of effective coordination points, thereby reducing activity).
In the invention, zn2+ in the L-zinc lactate preferentially forms bidentate coordination with hydroxyl oxygen of a lactic acid prepolymer and terminal carboxylate in a free state to fix a main chain configuration, and phenyl boric acid forms hydrogen bond with terminal hydroxyl of the lactic acid prepolymer and pi-pi action of an aromatic ring through B-OH to limit chiral center rotation. The L-zinc lactate and the phenylboric acid construct a synergistic locking system through coordination bonds and hydrogen bonds to jointly lock the chiral center of lactic acid, and meanwhile, the phenylboric acid inhibits beta-elimination chain breakage of a lactic acid oligomer through strong hydrogen bond action of B-OH and terminal hydroxyl of a lactic acid prepolymer, so that generation of new carboxyl is reduced, segment regularity is kept, depolymerization reaction selectivity is improved, main depolymerization reaction is not inhibited under proper use, and purity and yield of lactide are improved.
The terminal passivating agent can react with lactic acid oligomer, small molecular oligomer and terminal carboxyl of unreacted lactic acid to generate ester bonds to form a stable ester-based hydrophobic region, the hydrophobic region reduces the polarity around a Zn2+ coordination point, inhibits water molecules from approaching the coordination point and reduces hydrolysis reaction, and meanwhile, a hydrophobic chain of the terminal passivating agent is in hydrophobic association with a benzene ring of phenylboronic acid to directionally guide B-OH groups to be close to hydroxyl groups of lactic acid prepolymer to form a hydrogen bond network to cooperatively enhance the restriction of chiral center rotation.
When the viscosity regulator adopts polyethylene glycol, the polyethylene glycol long chain disperses phenyl boric acid more uniformly through the physical effect of steric hindrance, and simultaneously polyethylene glycol hydroxyl can form a directional hydrogen bond with B-OH of the phenyl boric acid to limit excessive self-association among phenyl boric acid molecules, so as to cooperatively enhance the function of limiting chiral center rotation. The ether oxygen and the ester group (the ester group formed by the terminal passivating agent and the lactic acid oligomer) of the polyethylene glycol form an orderly arrangement structure through dipole-dipole interaction, the diffusion path of the terminal passivating agent molecules in the system is optimized, so that the terminal passivating agent molecules are more efficiently contacted and blocked with residual free carboxyl groups (such as small molecule oligomers), the acid value is further reduced, side reactions are inhibited, and hydrogen bond between polyethylene glycol hydroxyl groups and the terminal carboxyl groups of unreacted monomers and small molecule oligomers accelerates the removal of byproducts and reduces the residual carboxyl groups.
When the viscosity regulator adopts polypropylene glycol, the hydrophobic association between the hydrophobic chain of the polypropylene glycol and the benzene ring of the phenylboronic acid can enhance the stability of hydrogen bonds, protect the phenylboronic acid from oxidation and indirectly enhance the configuration stability of chiral centers, the hydrophobic chain of the polypropylene glycol is associated with the hydrophobic region of ester groups (the ester groups formed by the terminal passivating agent and the lactic acid oligomer), the phase separation is reduced, the mass transfer efficiency is improved, racemization caused by local overheating is avoided, the local carbonization of the oligomer is inhibited, the ether oxygen of the polypropylene glycol acts with the dipole of the ester groups, the electron cloud density of ester bonds is reduced, the strength of the weakened ester bonds is reduced, the depolymerization broken chains are promoted, and the depolymerization efficiency is improved. The ether oxygen (Lewis base) of the polypropylene glycol forms transient coordination with the anhydride carbonyl (Lewis acid), so that the anhydride diffusion energy barrier is reduced, and the diffusion of anhydride molecules to carboxyl sites is promoted.
The beneficial effects of the invention are as follows:
(1) The crude lactide has low acid value and high optical purity, and the use of the terminal passivating agent and the viscosity regulating agent greatly reduces the content of the terminal acid of the oligomer, reduces the acid value of the crude lactide to below 5mgKOH/g, and reduces the influence of acid impurities on products and equipment. The configuration locking agent is added to effectively inhibit the high Wen Xiaoxuan side reaction, the meso-lactide content is controlled within 1.2 percent, and the optical purity of the crude product is obviously higher than that of the crude product of the prior common process, which means that the crude lactide prepared by the invention is more similar to the optically pure L-lactide, and is beneficial to the subsequent direct polymerization preparation of high molecular weight PLA or the reduction of refining steps.
(2) The invention greatly shortens the detention time of the lactic acid oligomer at high temperature, leads the lactide to rapidly escape from the reactor, reduces the thermal degradation loss, and improves the yield of the crude lactide by more than 8 percent compared with the traditional process.
(3) The invention has the advantages of relatively low requirement on optical purity of raw material lactic acid, and even if fermentation grade lactic acid containing a small amount of D-lactic acid is used, the L-lactide crude product with high optical purity can be obtained due to the racemization inhibition in the process, thereby being beneficial to reducing the cost of the raw material.
In conclusion, the terminal passivating agent, the configuration locking agent and the viscosity regulating agent form a synergistic network through chemical bonds and intermolecular forces, so that the linkage effect of acid value control, racemization control and viscosity regulation is realized, the acid value of crude lactide is obviously reduced, the content of meso-lactide is reduced, and the yield is improved. The acid value of the crude lactide prepared by the method is less than or equal to 5mgKOH/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 invention has excellent quality indexes, can be used for large-scale preparation of PLA high-purity monomers, and promotes the development of the biodegradable material industry.
Drawings
FIG. 1 is a schematic structural view of a preparation apparatus used in the preparation method of crude lactide in the present invention;
The device comprises a prepolymerization reactor, a first auxiliary agent storage tank, a second auxiliary agent storage tank, a third auxiliary agent storage tank, a first flow regulating valve, a second flow regulating valve, a third flow regulating valve, a fourth flow regulating valve, a static mixer, a buffer tank, a depolymerization reactor, a 12 inert gas storage tank, a 13 circulation pump, a 14 gas-liquid separator, a 15 first condenser, a 16 liquid lactide storage tank, a 17 second condenser, a 18 trap, a 19 Roots water ring vacuum unit, a 20 tail gas absorption tower, a 21 liquid ring pump, a 22 third condenser, a 23 collection tank, wherein the first auxiliary agent storage tank, the second auxiliary agent storage tank, the third auxiliary agent storage tank, the 5 first flow regulating valve, the 6 second flow regulating valve, the 7 third flow regulating valve, the 8 fourth flow regulating valve, the 9 static mixer, the 10 buffer tank, the 11 depolymerization reactor, the 12 inert gas storage tank, the 13 circulation pump, the 14, the gas-liquid lactide storage tank, the gas-liquid ring vacuum unit, the 19 and the 19.
Detailed Description
The invention is further described below with reference to examples.
In all examples, "%" refers to mass percent unless otherwise indicated.
Example 1
As shown in fig. 1, the preparation device for the preparation method of the crude lactide comprises a prepolymerization reaction kettle 1, wherein the prepolymerization reaction kettle 1 is respectively connected with 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 with pipelines between the prepolymerization reaction kettle 1 and the static mixer 9, a buffer tank 10, a depolymerization reactor 11, a gas-liquid separator 14 and a first condenser 15 are sequentially connected with a liquid lactide storage tank 16, the pipeline between the buffer tank 10 and the depolymerization reactor 11 is connected with a third auxiliary agent storage tank 4, the bottom of the depolymerization reactor 11 is connected with an inert gas storage tank 12, the first condenser 15, a second condenser 17, a catcher 18 and a Roots water ring vacuum unit 19 are sequentially connected with an exhaust gas absorption tower 20, the liquid lactide storage tank 16 is connected with the second condenser 17, the prepolymerization reaction kettle 1, a third condenser 22, a collecting tank 23 and a liquid ring pump 21 are sequentially connected with the exhaust gas absorption tower 20, and the third condenser 22 and the liquid ring pump 21 are sequentially connected.
A first flow regulating valve 5 is arranged on a pipeline between the prepolymerization reaction kettle 1 and a static mixer 9, a second flow regulating valve 6 is arranged on a pipeline connected with the bottom of the first auxiliary agent storage tank 2, a third flow regulating valve 7 is arranged on a pipeline connected with the bottom of the second auxiliary agent storage tank 3, a fourth flow regulating valve 8 is arranged on a pipeline connected with the bottom of the third auxiliary agent storage tank 4, an external circulating pipeline is arranged on the depolymerization reactor 11, and a circulating pump 13 is arranged on the external circulating pipeline.
The preparation method of the crude lactide comprises the following steps:
(1) Adding fermented lactic acid with the total lactic acid content of 98% (L-lactic acid content accounting for 99.4% of the total lactic acid content and D-lactic acid content accounting for 0.6% of the total lactic acid content) into a prepolymerization reactor 1, dehydrating and polycondensing at 140 ℃ and 5kPa for 6 hours to obtain a viscous lactic acid oligomer with the number average molecular weight of 1020, introducing the lactic acid oligomer into a static mixer 9 through a first flow regulating valve 5 at a flow rate of 60kg/h, introducing benzoic anhydride in a first auxiliary agent storage tank 2 into the static mixer 9 through a second flow regulating valve 6 at a flow rate of 10g/min, introducing PEG-1000 in a second auxiliary agent storage tank 3 into the static mixer 9 through a third flow regulating valve 7 at a flow rate of 6g/min, and introducing the lactic acid oligomer, benzoic anhydride and PEG-1000 into a buffer tank 10 for 150 ℃ end-capping reaction for 0.5 hours after being mixed in the static mixer 9 to obtain lactic acid prepolymer;
(2) Adding lactic acid prepolymer into a depolymerization reactor 11 at a flow rate of 60kg/h, adding a mixture of PEG1000, L-zinc lactate and phenylboric acid (the mass ratio of the PEG1000 to the L-zinc lactate to the phenylboric acid is 2:1:0.5) in a third auxiliary agent storage tank 4 into the depolymerization reactor 11 through a fourth flow regulating valve 8 at a flow rate of 7g/min, carrying out depolymerization reaction for 0.5h at 200 ℃ and a vacuum degree of 0.7kPa, quickly vaporizing the lactic acid prepolymer, generating a large amount of lactide steam, introducing nitrogen in an inert gas storage tank 12 from the bottom of the depolymerization reactor 11, carrying the lactide steam with the assistance of nitrogen (the flow rate of 0.3m3/h), discharging unreacted residual liquid in the depolymerization reactor 11 from the bottom, and returning the unreacted residual liquid to the depolymerization reactor 11 through a circulating pump 13 for recycling, wherein the mixture of the PEG1000, the L-zinc lactate and the phenylboric acid is a suspension prepared by ultrasonic dissolution of the mixture of the L-zinc lactate and the phenylboric acid by the PEG 1000;
(3) The lactide steam and nitrogen enter a gas-liquid separator 14 for gas-liquid separation at 100 ℃, the high-purity lactide steam and nitrogen only enter a gas phase, then the gas phase enters a first condenser 15 for condensation at 55 ℃, the lactide steam is condensed into a liquid crude lactide product, the liquid crude lactide product enters a liquid lactide storage tank 16 for storage, noncondensable gas (mainly nitrogen) generated by condensation sequentially enters a tail gas absorption tower 20 for treatment through a second condenser 17, a catcher 18 and a Roots water ring vacuum unit 19, the water steam obtained by condensation polymerization in a prepolymerization reaction kettle 1 enters a third condenser 22 for condensation, condensed water enters a collecting tank 23, and noncondensable gas generated by condensation enters the tail gas absorption tower 20 for treatment through a liquid ring pump 21.
The liquid crude lactide product is taken for detection, the acid value measured by a potentiometric titration method is 4.3mgKOH/g, the meso-lactide content measured by a gas chromatography method is 1.0%, the purity of the crude lactide is 97.7%, and the total yield of the crude lactide is 98.3%.
Example 2
The preparation apparatus used in the preparation method of crude lactide was the same as in example 1.
The preparation method of the crude lactide comprises the following steps:
(1) Adding fermented lactic acid with 88% of total lactic acid content (L-lactic acid content accounts for 99.1% of total lactic acid content and D-lactic acid content accounts for 0.9% of total lactic acid content) into a prepolymerization reactor 1, dehydrating and polycondensing for 4 hours at 160 ℃ and 3kPa to obtain a viscous lactic acid oligomer with the number average molecular weight of 1255, introducing the lactic acid oligomer into a static mixer 9 through a first flow regulating valve 5 at the flow rate of 60kg/h, introducing maleic anhydride in a first auxiliary agent storage tank 2 into the static mixer 9 through a second flow regulating valve 6 at the flow rate of 5g/min, introducing PPG-1000 in a second auxiliary agent storage tank 3 into the static mixer 9 through a third flow regulating valve 7 at the flow rate of 24.5g/min, and introducing the lactic acid oligomer, maleic anhydride and PPG-1000 into a buffer tank 10 for end-capping reaction at 140 ℃ for 0.8 hours after being mixed in the static mixer 9 to obtain lactic acid prepolymer;
(2) Adding lactic acid prepolymer into a depolymerization reactor 11 at a flow rate of 60kg/h, adding a mixture of PPG-1000, L-zinc lactate and phenylboric acid (the mass ratio of PPG-1000, L-zinc lactate and phenylboric acid is 2:1:0.6) in a third auxiliary agent storage tank 4 into the depolymerization reactor 11 through a fourth flow regulating valve 8 at a flow rate of 9.5g/min, carrying out depolymerization reaction for 1h at 180 ℃ and a vacuum degree of 0.3kPa, quickly vaporizing the lactic acid prepolymer, generating a large amount of lactide steam, introducing nitrogen in an inert gas storage tank 12 from the bottom of the depolymerization reactor 11, carrying the lactide steam with the assistance of nitrogen (the flow rate of 0.1m3/h), discharging unreacted residual liquid in the depolymerization reactor 11 from the bottom and returning the residual liquid to the depolymerization reactor 11 through a circulating pump 13 for recycling, wherein the mixture of PPG-1000, L-zinc lactate and phenylboric acid is a suspension prepared by ultrasonic dissolution of the mixture of PPG-1000 and L-zinc lactate and phenylboric acid;
(3) The lactide steam and nitrogen enter a gas-liquid separator 14 for gas-liquid separation at 95 ℃, the high-purity lactide steam and nitrogen only enter a gas phase, then the gas phase enters a first condenser 15 for condensation at 60 ℃, the lactide steam is condensed into a liquid crude lactide product, the liquid crude lactide product enters a liquid lactide storage tank 16 for storage, noncondensable gas (mainly nitrogen) generated by condensation sequentially enters a tail gas absorption tower 20 for treatment through a second condenser 17, a catcher 18 and a Roots water ring vacuum unit 19, the water steam obtained by condensation polymerization in a prepolymerization reaction kettle 1 enters a third condenser 22 for condensation, condensed water enters a collecting tank 23, and noncondensable gas generated by condensation enters the tail gas absorption tower 20 for treatment through a liquid ring pump 21.
The liquid crude lactide product is taken for detection, the acid value measured by a potentiometric titration method is 4.6mgKOH/g, the meso-lactide content measured by a gas chromatography method is 1.0%, the purity of the crude lactide is 97.4%, and the total yield of the crude lactide is 98.1%.
Example 3
The preparation apparatus used in the preparation method of crude lactide was the same as in example 1.
The preparation method of the crude lactide comprises the following steps:
(1) Adding fermented lactic acid with the total lactic acid content of 98% (L-lactic acid content accounting for 99.2% of the total lactic acid content and D-lactic acid content accounting for 0.8% of the total lactic acid content) into a prepolymerization reactor 1, dehydrating and polycondensing at 150 ℃ and 8kPa for 5 hours to obtain a viscous lactic acid oligomer with the number average molecular weight of 1508, introducing the lactic acid oligomer into a static mixer 9 through a first flow regulating valve 5 at a flow rate of 60kg/h, introducing acetic anhydride in a first auxiliary agent storage tank 2 into the static mixer 9 through a second flow regulating valve 6 at a flow rate of 8g/min, introducing PEG-400 in a second auxiliary agent storage tank 3 into the static mixer 9 through a third flow regulating valve 7 at a flow rate of 13g/min, and introducing the lactic acid oligomer, acetic anhydride and PEG-400 into a buffer tank 10 for a sealing reaction at 125 ℃ for 1 hour after being mixed in the static mixer 9 to obtain lactic acid prepolymer;
(2) Adding lactic acid prepolymer into a depolymerization reactor 11, simultaneously adding a mixture of PEG-400, L-zinc lactate and phenylboric acid (the mass ratio of the PEG-400, the L-zinc lactate and the phenylboric acid is 2:1:0.8) in a third auxiliary agent storage tank 4 into the depolymerization reactor 11 through a fourth flow regulating valve 8 at a flow rate of 12g/min, carrying out depolymerization reaction for 1h at 205 ℃ and a vacuum degree of 1kPa, quickly vaporizing the lactic acid prepolymer, generating a large amount of lactide steam, simultaneously introducing nitrogen in an inert gas storage tank 12 from the bottom of the depolymerization reactor 11, and discharging unreacted residual liquid in the depolymerization reactor 11 from the bottom and returning the residual liquid to the depolymerization reactor 11 for recycling through a circulating pump 13, wherein the mixture of the PEG-400, the L-zinc lactate and the phenylboric acid is a suspension prepared by ultrasonically dissolving the mixture of the L-zinc lactate and the phenylboric acid through the PEG-400;
(3) The lactide steam and nitrogen enter a gas-liquid separator 14 for gas-liquid separation at 120 ℃, the high-purity lactide steam and nitrogen only enter a gas phase, then the gas phase enters a first condenser 15 for condensation at 50 ℃, the lactide steam is condensed into a liquid crude lactide product, the liquid crude lactide product enters a liquid lactide storage tank 16 for storage, noncondensable gas (mainly nitrogen) generated by condensation sequentially enters a tail gas absorption tower 20 for treatment through a second condenser 17, a catcher 18 and a Roots water ring vacuum unit 19, the water steam obtained by condensation polymerization in a prepolymerization reaction kettle 1 enters a third condenser 22 for condensation, condensed water enters a collecting tank 23, and noncondensable gas generated by condensation enters the tail gas absorption tower 20 for treatment through a liquid ring pump 21.
The liquid crude lactide product is taken for detection, the acid value measured by a potentiometric titration method is 4.1mgKOH/g, the meso-lactide content measured by a gas chromatography method is 0.8%, the purity of the crude lactide is 97.6%, and the total yield of the crude lactide is 98.5%.
Comparative example 1
The other steps are the same as in example 1 without PEG-1000 added.
The liquid crude lactide product is taken for detection, the acid value measured by a potentiometric titration method is 10mgKOH/g, the meso-lactide content measured by a gas chromatography method is 1.6%, the purity of the crude lactide is 95.2%, and the total yield of the crude lactide is 92.4%.
Comparative example 2
Other steps were performed as in example 1 without adding benzoic anhydride.
The liquid crude lactide product is taken for detection, the acid value measured by a potentiometric titration method is 23.7mgKOH/g, the meso-lactide content measured by a gas chromatography method is 1.7%, the purity of the crude lactide is 93.7%, and the total yield of the crude lactide is 95.1%.
Comparative example 3
The procedure of example 1 was repeated except that a mixture of zinc L-lactate and phenylboronic acid was not added.
The liquid crude lactide product is taken for detection, the acid value measured by a potentiometric titration method is 10.2mgKOH/g, the meso-lactide content measured by a gas chromatography method is 3.9%, the purity of the crude lactide is 93.8%, and the total yield of the crude lactide is 96.4%.
Comparative example 4
In the step (2), no nitrogen was introduced, and the other steps were the same as in example 1.
The liquid crude lactide product is taken for detection, the acid value measured by a potentiometric titration method is 7.5mgKOH/g, the meso-lactide content measured by a gas chromatography method is 1.9%, the purity of the crude lactide is 95.0%, and the total yield of the crude lactide is 93.5%.
Comparative example 5
The procedure of example 1 was repeated except that the mixture of zinc L-lactate and phenylboronic acid was replaced with zinc L-lactate.
The liquid crude lactide product is taken for detection, the acid value measured by a potentiometric titration method is 9.7mgKOH/g, the meso-lactide content measured by a gas chromatography method is 1.8%, the purity of the crude lactide is 95.9%, and the total yield of the crude lactide is 96.5%.
Comparative example 6
The procedure of example 1 was repeated except that the mixture of zinc L-lactate and phenylboronic acid was replaced with phenylboronic acid.
The liquid crude lactide product is taken for detection, the acid value measured by a potentiometric titration method is 9.3mgKOH/g, the meso-lactide content measured by a gas chromatography method is 1.9%, the purity of the crude lactide is 96.1%, and the total yield of the crude lactide is 95.8%.
Comparative example 7
The procedure of example 2 was followed without adding PPG-1000.
The liquid crude lactide product is taken for detection, the acid value measured by a potentiometric titration method is 8.4mgKOH/g, the meso-lactide content measured by a gas chromatography method is 1.5%, the purity of the crude lactide is 96.2%, and the total yield of the crude lactide is 93.6%.
The results of the crude lactide measurements in examples 1-3 and comparative examples 1-7 are shown in Table 1.