Co-production process of lactide and preparation method of polylactic acid

Abstract

The invention relates to a co-production process of lactide and a preparation method of polylactic acid. The co-production process comprises the following steps: dehydrating lactic acid, and carrying out polycondensation to obtain a lactic acid oligomer; a lactic acid oligomer is subjected to primary depolymerization under the action of a first catalyst, a first coarse material and a leftover material are obtained, and the conversion rate of the first coarse material formed by depolymerization of the lactic acid oligomer is 50%-90%; refining and purifying the first coarse material to obtain lactide; the leftovers are subjected to secondary depolymerization under the action of a second catalyst, a racemic second coarse material is obtained, the second coarse material is refined and purified, racemic lactide is obtained, and the second catalyst is selected from alkaline ionic liquid. According to the co-production process, the utilization rate of the lactic acid raw material and the yield and the optical purity of the lactide product are effectively improved, meanwhile, the proportion of the lactide and the racemic lactide prepared in the co-production process can be regulated and controlled, so that polylactic acid products with different melting points are obtained, and the method has a wide application prospect.

Description

Technical Field

[0001] This invention relates to the field of lactide production technology, and in particular to the co-production process of lactide and the preparation method of polylactic acid. Background Technology

[0002] Lactide is a key intermediate in the industrial two-step production of polylactic acid (PLA), and its chemical and optical purity determines the quality of the PLA product. The two-step process involves first polycondensing lactic acid into oligomers, then depolymerizing them to obtain crude lactide. The purified lactide is then subjected to ring-opening polymerization to obtain PLA. However, because the depolymerization process involves high temperature and high vacuum conditions, the lactic acid oligomers tend to further condense, increasing their molecular weight. Simultaneously, side reactions such as carbonization occur, leading to increased system viscosity and a decreased depolymerization rate. To ensure the depolymerization rate and product quality, the depolymerization process requires continuous discharge of waste material. Furthermore, during the purification of crude lactide, if distillation is used, oligomer waste material is easily formed in the distillation column.

[0003] The rational utilization of by-products directly determines the utilization rate of raw material lactic acid and affects the purity of lactide. Existing methods typically hydrolyze by-products into lactic acid for recycling; however, the resulting lactide product has low optical purity and is more difficult to separate. On the other hand, alcoholystothening by-products into lactate esters with lower optical purity requirements requires the use of alcohol raw materials, greatly increasing the process complexity, and the lactate ester products cannot be directly used in the production of polylactic acid.

[0004] Therefore, there is an urgent need for a method that can make reasonable use of the by-products generated during the synthesis of lactide, improve the utilization rate of raw material lactic acid, and increase the yield and optical purity of lactide products. Summary of the Invention

[0005] Therefore, it is necessary to provide a co-production process for lactide and a method for preparing polylactic acid to address the above-mentioned problems. The co-production process of the present invention effectively improves the utilization rate of lactic acid raw materials and the yield and optical purity of lactide products. At the same time, it can control the ratio of lactide and racemic lactide obtained in the co-production process, so as to directly obtain polylactic acid products with different melting points using the lactide produced by the co-production process of the present invention, which has broad application prospects.

[0006] A process for the co-production of lactide includes the following steps:

[0007] The lactic acid is dehydrated, wherein the lactic acid is L-lactic acid or D-lactic acid;

[0008] Dehydrated lactic acid is polycondensed to obtain lactic acid oligomers;

[0009] The lactic acid oligomer is depolymerized once under the action of a first catalyst to obtain a first coarse material and waste material, wherein the conversion rate of the lactic acid oligomer to form the first coarse material is 50% to 90%.

[0010] The first crude material was refined and purified to obtain lactide;

[0011] The waste material is subjected to secondary depolymerization under the action of a second catalyst to obtain a racemic second crude material. The second crude material is then refined and purified to obtain racemic lactide. The second catalyst is selected from alkaline ionic liquids.

[0012] In one embodiment, the mass ratio of the second catalyst to the waste material is 0.5:100 to 5:100;

[0013] And / or, the second catalyst is selected from at least one of guanidine-based ionic liquids or choline-based ionic liquids.

[0014] In one embodiment, the structural formula of the cation donor in the guanidine-based ionic liquid is as follows: R1 to R6 are each independently selected from C1 to C6 alkyl, C1 to C6 alkoxy, C1 to C6 alkyl ester, C6 to C6 alkyl groups. 12 The aromatic group, and any two adjacent R1 to R6 can be linked to form a ring;

[0015] And / or, the anion donor in the guanidine-based ionic liquid is selected from... CH3CH(OH)COO - CF3COO - Or CH3(CH2) n COO - , where n is an integer from 0 to 4.

[0016] In one embodiment, the cation donor contains at least one cycloalkyl, heterocycloalkyl, or aromatic group.

[0017] In one embodiment, the cation donor in the choline-based ionic liquid is selected from choline and its derivatives;

[0018] And / or, the anion donor in the choline-based ionic liquid is selected from... or

[0019] In one embodiment, the alkaline ionic liquid in the secondary depolymerization process is recycled and purified for reuse.

[0020] In one embodiment, the first light component produced in the refining and purification process of the first crude material is recycled to the dehydration process;

[0021] And / or, the second light component generated in the refining and purification process of the second crude material is recycled to the dehydration process.

[0022] And / or, the first heavy component formed in the refining and purification process of the first crude material is recycled to the secondary depolymerization process;

[0023] And / or, the second heavy component formed in the refining and purification process of the second crude material is recycled to the secondary depolymerization process.

[0024] In one embodiment, the polycondensation reaction is carried out at a temperature of 120°C to 180°C and a pressure of 2 kPa to 10 kPa.

[0025] And / or, the temperature of the first depolymerization is 180℃~240℃, and the pressure is 0.1kPa~2kPa;

[0026] And / or, the temperature of the secondary depolymerization is 200℃~260℃, and the pressure is 0.1kPa~2kPa.

[0027] In one embodiment, the molecular weight of the lactic acid oligomer is 500 Da to 5000 Da.

[0028] In one embodiment, the mass ratio of the first catalyst to the lactic acid oligomer is 0.2:100 to 5:100;

[0029] And / or, the first catalyst is selected from tin-based catalysts.

[0030] A method for preparing polylactic acid includes the following steps:

[0031] Lactide and racemic lactide were prepared using the co-production process of lactide described above;

[0032] By adjusting the mass ratio of the lactide and the racemic lactide, polylactic acid with a target melting point is obtained through ring-opening polymerization.

[0033] The aforementioned co-production process for lactide effectively yields high-purity lactide and racemic lactide by controlling the conversion rate of lactic acid oligomers during the primary depolymerization and subjecting the recovered waste to secondary depolymerization under a second catalyst with racemic properties. Simultaneously, the ratio of lactide to racemic lactide in the product can be controlled, thereby effectively improving the utilization rate of raw material lactic acid and the yield and optical purity of the lactide product. Furthermore, it effectively reduces waste emissions and lowers processing costs. Moreover, directly utilizing the lactide and racemic lactide co-produced by this invention to prepare polylactic acid (PLA) products can yield PLA products with different melting points, demonstrating broad application prospects. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 This is a process flow diagram of the co-production of lactide in one embodiment of the present invention. Detailed Implementation

[0036] To facilitate understanding of the present invention, it will be described in more detail below. However, it should be understood that the present invention can be implemented in many different forms and is not limited to the embodiments or examples described herein. Rather, these embodiments or examples are provided to make the disclosure of the present invention more thorough and complete.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments or examples only and is not intended to limit the invention. The optional scope of the term "and / or" as used herein includes any one of two or more of the related listed items, as well as any and all combinations of the related listed items, including any two related listed items, any more related listed items, or a combination of all related listed items.

[0038] Combination Figure 1 As shown, the co-production process of lactide provided by the present invention includes the following steps:

[0039] S1. Dehydrate the lactic acid, wherein the lactic acid is L-lactic acid or D-lactic acid;

[0040] S2. The dehydrated lactic acid is subjected to polycondensation to obtain lactic acid oligomers;

[0041] S3. The lactic acid oligomer is depolymerized once under the action of a first catalyst to obtain a first coarse material and waste material, wherein the conversion rate of the lactic acid oligomer to form the first coarse material is 50% to 90%.

[0042] S4. The first crude material is refined and purified to obtain lactide;

[0043] S5. The waste material is subjected to secondary depolymerization under the action of the second catalyst to obtain a racemic second crude material. The second crude material is refined and purified to obtain racemic lactide. The second catalyst is selected from alkaline ionic liquid.

[0044] The co-production process described in this invention effectively produces high-purity lactide and racemic lactide by controlling the conversion rate of lactic acid oligomers during the first depolymerization and subjecting the recovered waste to a second depolymerization under the condition of a second catalyst with racemic effect. Simultaneously, it can regulate the ratio of lactide and racemic lactide in the product, thereby effectively improving the utilization rate of raw material lactic acid and the yield and optical purity of the lactide product. Furthermore, it effectively reduces waste emissions and lowers treatment costs.

[0045] It should be noted that when the lactic acid is L-lactic acid, the lactide obtained from the purification of the first crude material is L-lactide; when the lactic acid is D-lactic acid, the lactide obtained from the purification of the first crude material is D-lactide. The conversion rate of the above-mentioned lactic acid oligomers to form the first crude material is expressed as the ratio of the mass of the first crude material to the mass of the lactic acid oligomers. The by-product mainly consists of unconverted lactic acid oligomers and the first catalyst. For example, when the conversion rate of the lactic acid oligomers is 60%, 60% of the first crude material and 40% of unconverted lactic acid oligomers are obtained.

[0046] In step S1, the temperature for dehydration is preferably 90℃~120℃, and the pressure is preferably 5kPa~20kPa.

[0047] In step S2, the preferred temperature for the polycondensation reaction is 120℃~180℃, and the preferred pressure is 2kPa~10kPa.

[0048] In one embodiment, the molecular weight of the lactic acid oligomer is 500 Da to 5000 Da.

[0049] In step S3, the conversion rate of the lactic acid oligomer to form the first crude material is preferably 60% to 80%, which is beneficial for further adjusting the mass ratio of lactide to racemic lactide and improving the yield and optical purity of the lactide product.

[0050] In one embodiment, the mass ratio of the first catalyst to the lactic acid oligomer is preferably 0.2:100 to 5:100. By adjusting the ratio of the first catalyst to the lactic acid oligomer, it is beneficial to improve the efficiency of the primary depolymerization reaction.

[0051] More preferably, the mass ratio of the first catalyst to the lactic acid oligomer is 0.5:100 to 3:100.

[0052] Preferably, the first catalyst is selected from tin-based catalysts.

[0053] Specifically, the first catalyst is selected from at least one of stannous oxide, stannous octoate, stannous acetate, stannous chloride, stannous bromide, and stannous benzoate.

[0054] In one embodiment, the temperature of the first depolymerization is preferably 180°C to 240°C, and the pressure is preferably 0.1 kPa to 2 kPa.

[0055] In step S4, the refining and purification process of the first crude material preferably adopts a coupled process of distillation and melt crystallization. The first light component at the top of the distillation column is mainly lactic acid, and also includes a small amount of water, Meso-lactide and lactide. The first heavy component at the bottom of the distillation column is mainly lactic acid oligomer, and also includes a small amount of lactide. A large amount of lactide is taken out through the side stream of the distillation column and further purified by melt crystallization to obtain lactide.

[0056] In one embodiment, the mother liquor and sweating liquid separated during melt crystallization are returned to the distillation column for recycling, thereby increasing the yield of lactide.

[0057] To further improve the utilization rate of raw material lactic acid and the yield of lactide, and reduce waste emissions and production costs, it is preferable to return the first light component obtained by distillation to the dehydration process in the refining and purification process of the first crude material for recycling.

[0058] In one embodiment, in the refining and purification process of the first crude material, the first heavy component obtained by distillation is returned to the secondary depolymerization process, which is beneficial to further improve the utilization rate of raw material lactic acid and the yield of lactide, and reduce waste discharge and production costs.

[0059] In step S5, the mass ratio of the second catalyst to the by-product is preferably 0.5:100 to 5:100, which is beneficial to promoting the production of racemic lactide, while inhibiting the ring-opening polymerization reaction of lactide, improving the yield and purity of racemic lactide, and facilitating the separation of racemic lactide products.

[0060] More preferably, the mass ratio of the second catalyst to the by-product is 1:100 to 2:100.

[0061] In one embodiment, the second catalyst is selected from at least one of guanidine-based ionic liquids or choline-based ionic liquids.

[0062] Specifically, the structural formula of the cation donor in the guanidine-based ionic liquid is as follows: R1 to R6 are each independently selected from C1 to C6 alkyl, C1 to C6 alkoxy, C1 to C6 alkyl ester, C6 to C6 alkyl groups. 12 The aromatic group, and any two adjacent R1 to R6 can be linked to form a ring.

[0063] Preferably, the cation donor contains at least one cycloalkyl, heterocycloalkyl, or aromatic group to further improve the steric hindrance of the guanidine-based ionic liquid, promote the production of racemic lactide, and inhibit the ring-opening polymerization reaction of lactide.

[0064] It should be noted that the heterocyclic alkyl group refers to a cycloalkyl group containing heteroatoms, which include, but are not limited to, N atoms or O atoms.

[0065] More preferably, R4 and R6 are connected to form a ring.

[0066] Understandably, it is feasible to connect R3 and R6 to form a loop, R5 and R3 to form a loop, or R5 and R4 to form a loop.

[0067] Specifically, the structural formulas of the aforementioned cation donors include, but are not limited to, those mentioned above.

[0068] In one embodiment, the anion donor in the guanidine-based ionic liquid is selected from... CH3CH(OH)COO - CF3COO - Or CH3(CH2) n COO - , where n is an integer from 0 to 4.

[0069] In one embodiment, the cation donor in the choline-based ionic liquid is selected from choline and its derivatives, wherein the derivatives of choline include, but are not limited to, acetylcholine, butyrylcholine, or benzoylcholine.

[0070] In one embodiment, the anion donor in the choline-based ionic liquid is selected from... or

[0071] Preferably, in the secondary depolymerization process, reactor residue is periodically discharged, and the reactor residue includes at least an alkaline ionic liquid.

[0072] In one embodiment, the alkaline ionic liquid in the secondary depolymerization process is recycled and purified for reuse, which effectively improves the utilization rate of the alkaline ionic liquid, reduces resource waste, and lowers production costs.

[0073] In one embodiment, the temperature of the secondary depolymerization is preferably 200°C to 260°C, and the pressure is preferably 0.1 kPa to 2 kPa.

[0074] In one embodiment, the refining and purification process of the second crude material preferably adopts a coupled process of distillation and melt crystallization. The second light component at the top of the distillation column is mainly lactic acid, and also includes a small amount of water and Meso-lactide. The second heavy component at the bottom of the distillation column is mainly lactic acid oligomer, and also includes a small amount of racemic lactide. A large amount of racemic lactide is collected from the side stream of the distillation column and further purified by melt crystallization to obtain racemic lactide.

[0075] Preferably, in the refining and purification process of the second crude material, the second light component obtained by distillation is returned to the dehydration process for recycling.

[0076] Preferably, in the refining and purification process of the second crude material, the second heavy component obtained by distillation is returned to the secondary depolymerization process.

[0077] It is understood that the mass ratio of the second catalyst to the by-product is the same as the mass ratio of the second catalyst to the mass of the raw material undergoing secondary depolymerization. For example, when the first heavy component, the second heavy component, and the by-product undergo secondary depolymerization together, the mass ratio of the second catalyst to the by-product is the same as the mass ratio of the second catalyst to the sum of the masses of the first heavy component, the second heavy component, and the by-product.

[0078] It should be noted that the present invention does not limit the order of steps S4 and S5. They can be performed simultaneously, or step S4 can be performed first and then step S5, or step S5 can be performed first and then step S4. The refining and purification of the first crude material and the refining and purification of the second crude material in the present invention both adopt existing processes, including but not limited to distillation and melt crystallization coupled processes, which will not be described in detail in the present invention.

[0079] Using the co-production process provided by this invention, the total yield of lactide and racemic lactide obtained reaches 95% to 99.5%, wherein the total yield is expressed as the percentage of the sum of the masses of the lactide and racemic lactide to the mass of the lactic acid oligomer.

[0080] This invention also provides a method for preparing polylactic acid, comprising the following steps:

[0081] Lactide and racemic lactide were prepared using the co-production process of lactide described above;

[0082] By adjusting the mass ratio of the lactide and the racemic lactide, polylactic acid with a target melting point is obtained through ring-opening polymerization.

[0083] The above-mentioned method for preparing polylactic acid utilizes lactide and racemic lactide obtained by the co-production process provided by this invention to prepare polylactic acid products with different melting points, which has broad application prospects.

[0084] Specifically, polylactic acid (PLA) prepared using L-lactide or D-lactide has a melting point greater than 170°C and can be used in fields requiring high heat resistance, such as spinning and injection molding. PLA prepared by mixing L-lactide with racemic lactide in a certain proportion, or by mixing D-lactide with racemic lactide in a certain proportion, has a melting point of 125°C to 170°C and can be used in fields such as films, extrusion, thermoforming, or nonwoven fabrics. PLA prepared using racemic lactide is an amorphous polymer and does not have crystallinity, and can be used in medical materials such as surgical sutures and bone screws.

[0085] It should be noted that by adjusting the mass ratio of L-lactide to racemic lactide or the mass ratio of D-lactide to racemic lactide, polylactic acid with various melting points can be achieved, such as melting points of 135℃~145℃, 150℃~160℃, etc., which can be adjusted according to actual needs.

[0086] The following specific examples will further illustrate the co-production process of lactide and the preparation method of polylactic acid.

[0087] Example 1

[0088] S1 and 80% L-lactic acid (optical purity 99.7%) were added to the dehydration unit at a flow rate of 50 kg / h to remove free water under conditions of 100℃ and 10 kPa.

[0089] S2. L-lactic acid, after being dehydrated, is introduced into the polycondensation unit at a flow rate of 40 kg / h. Under conditions of 140 °C and 2 kPa, it undergoes dehydration and polycondensation to obtain lactic acid oligomers with a molecular weight of 2600 Da.

[0090] S3. Lactic acid oligomers are mixed with 0.2 kg / h of stannous octoate and fed into the first depolymerization unit at a flow rate of 32.2 kg / h. The mixture is depolymerized once at 200°C and 0.5 kPa to obtain the first coarse material. The discharge rate of the first coarse material from the first depolymerization unit is controlled at 25.6 kg / h (the conversion rate of the first coarse material is 80%), and the discharge rate of the waste material is 6.6 kg / h (including 6.4 kg / h of unconverted lactic acid oligomers and 0.2 kg / h of stannous octoate).

[0091] S4. The first crude feedstock is separated and purified using a coupled process of distillation and melt crystallization. The first light component at the top of the distillation column mainly consists of water, lactic acid, Meso-lactide, and a small amount of L-lactide, which is recycled back to the dehydration unit. The first heavy component at the bottom of the column consists of lactic acid oligomers and a small amount of L-lactide, which is recycled back to the second depolymerization unit. L-lactide is collected from the side stream of the distillation column and further purified by melt crystallization, with a yield of 24.8 kg / h. Gas chromatography analysis shows that the L-lactide content is 99.4%.

[0092] S5. After mixing the waste from the first depolymerization unit, the first heavy component from the L-lactide distillation column, and the second heavy component from the racemic lactide distillation column, add the second catalyst. The second catalyst was fed into the second depolymerization unit at a rate of 0.07 kg / h, and depolymerization was carried out at 220℃ and 0.5 kPa to obtain a racemic second crude feed. The racemic second crude feed was separated and purified using a coupled process of distillation and melt crystallization. The second light component at the top of the distillation column mainly consisted of small amounts of water, lactic acid, and Meso-lactide, while the second heavy component at the bottom of the column consisted of lactic acid oligomers and a small amount of racemic lactide, which was returned to the second depolymerization unit. The racemic lactide was collected from the side stream of the distillation column and further purified by melt crystallization, with a yield of 6.1 kg / h. Gas chromatography analysis showed that the L-lactide content was 49.7% and the D-lactide content was 49.6%.

[0093] The total yield of lactide (L-lactide and racemic lactide) prepared in Example 1 was 96.6%.

[0094] Example 2

[0095] S1 and 80% L-lactic acid (optical purity 99.7%) were added to the dehydration unit at a flow rate of 50 kg / h to remove free water under conditions of 120℃ and 10 kPa.

[0096] S2. L-lactic acid, after being dehydrated, is fed into the polycondensation unit at a flow rate of 40 kg / h. Under conditions of 120 °C and 4 kPa, it undergoes dehydration and polycondensation to obtain lactic acid oligomers with a molecular weight of 3000 Da.

[0097] S3. Lactic acid oligomers are mixed with 0.2 kg / h of stannous octoate and then fed into the first depolymerization unit at a flow rate of 32.2 kg / h. The mixture is depolymerized once at 200°C and 0.5 kPa to obtain the first coarse material. The discharge rate of the first coarse material from the first depolymerization unit is controlled at 22.4 kg / h (the conversion rate of the first coarse material is 70%), and the discharge rate of the waste material is 9.8 kg / h (including 9.6 kg / h of unconverted lactic acid oligomers and 0.2 kg / h of stannous octoate).

[0098] S4. The first crude feedstock is separated and purified using a coupled process of distillation and melt crystallization. The first light component at the top of the distillation column mainly consists of water, lactic acid, Meso-lactide, and a small amount of L-lactide, which is recycled back to the dehydration unit. The first heavy component at the bottom of the column consists of lactic acid oligomers and a small amount of L-lactide, which is recycled back to the second depolymerization unit. L-lactide is collected from the side stream of the distillation column and further purified by melt crystallization, with a yield of 22.1 kg / h. Gas chromatography analysis shows that the L-lactide content is 99.6%.

[0099] S5. After mixing the waste from the first depolymerization unit, the first heavy component from the L-lactide distillation column, and the second heavy component from the racemic lactide distillation column, add the second catalyst. The second catalyst was fed into the second depolymerization unit at a rate of 0.10 kg / h, and depolymerization was carried out at 220℃ and 0.5 kPa to obtain a racemic second crude feed. The racemic second crude feed was separated and purified using a coupled process of distillation and melt crystallization. The second light component at the top of the distillation column mainly consisted of small amounts of water, lactic acid, and Meso-lactide, while the second heavy component at the bottom of the column consisted of lactic acid oligomers and a small amount of racemic lactide, which was returned to the second depolymerization unit. The racemic lactide was collected from the side stream of the distillation column and further purified by melt crystallization, with a yield of 9.4 kg / h. Gas chromatography analysis showed that the L-lactide content was 49.6% and the D-lactide content was 49.6%.

[0100] The total yield of lactide (L-lactide and racemic lactide) prepared in Example 2 was 98.4%.

[0101] Example 3

[0102] S1 and 80% L-lactic acid (optical purity 99.7%) were added to the dehydration unit at a flow rate of 50 kg / h to remove free water under conditions of 100℃ and 10 kPa.

[0103] S2. L-lactic acid, after being dehydrated, is fed into the polycondensation unit at a flow rate of 40 kg / h. Under conditions of 130 °C and 2 kPa, it undergoes dehydration and polycondensation to obtain lactic acid oligomers with a molecular weight of 1800 Da.

[0104] S3. Lactic acid oligomers are mixed with 0.4 kg / h of stannous chloride and fed into the first depolymerization unit at a flow rate of 32.4 kg / h. The mixture undergoes one depolymerization at 190°C and 0.2 kPa to obtain the first coarse material. The discharge rate of the first coarse material from the first depolymerization unit is controlled at 19.2 kg / h (the conversion rate of the first coarse material is 60%), and the discharge rate of the waste material is 13.2 kg / h (containing 12.8 kg / h of unconverted lactic acid oligomers and 0.4 kg / h of stannous chloride).

[0105] S4. The first crude feedstock is separated and purified using a coupled process of distillation and melt crystallization. The first light component at the top of the distillation column mainly consists of water, lactic acid, Meso-lactide, and a small amount of L-lactide, which is recycled back to the dehydration unit. The first heavy component at the bottom of the column consists of lactic acid oligomers and a small amount of L-lactide, which is recycled back to the second depolymerization unit. L-lactide is collected from the side stream of the distillation column and further purified by melt crystallization, with a yield of 19.1 kg / h. Gas chromatography analysis shows that the L-lactide content is 99.7%.

[0106] S5. After mixing the waste from the first depolymerization unit, the first heavy component from the L-lactide distillation column, and the second heavy component from the racemic lactide distillation column, add the second catalyst. The second catalyst was fed into the second depolymerization unit at a rate of 0.26 kg / h, and depolymerization was carried out at 240℃ and 0.8 kPa to obtain a racemic second crude feed. The racemic second crude feed was separated and purified using a coupled process of distillation and melt crystallization. The second light component at the top of the distillation column mainly consisted of small amounts of water, lactic acid, and Meso-lactide, while the second heavy component at the bottom of the column consisted of lactic acid oligomers and a small amount of racemic lactide, which was returned to the second depolymerization unit. The racemic lactide was collected from the side stream of the distillation column and further purified by melt crystallization, with a yield of 12.6 kg / h. Gas chromatography analysis showed that the L-lactide content was 49.8% and the D-lactide content was 49.7%.

[0107] The total yield of lactide (L-lactide and racemic lactide) obtained in Example 3 was 99.1%.

[0108] Example 4

[0109] The difference between Example 4 and Example 1 is that in step S3, the discharge rate of the first crude material from the first depolymerization unit is controlled at 16 kg / h (the conversion rate of the first crude material is 50%), and the discharge rate of the by-product is 16.2 kg / h; in step S4, the yield of L-lactide is 15.6 kg / h, and the L-lactide content is 99.5% as determined by gas chromatography; in step S5, a second catalyst is added. The second catalyst was controlled to enter the second depolymerization unit at a rate of 0.16 kg / h, and the yield of racemic lactide was 14.9 kg / h. Gas chromatography analysis showed that the L-lactide content was 49.7% and the D-lactide content was 49.6%.

[0110] The total yield of lactide (L-lactide and racemic lactide) prepared in Example 5 was 95.3%.

[0111] Example 5

[0112] The difference between Example 5 and Example 1 is that 80% D-lactic acid (optical purity 99.7%) is used in step S1; in step S3, the discharge rate of the first crude material from the first depolymerization unit is controlled at 22.4 kg / h (the conversion rate of the first crude material is 70%), and the discharge rate of the by-product is 9.8 kg / h; in step S4, the yield of D-lactide is 22.1 kg / h, and the D-lactide content is 99.5% as detected by gas chromatography; in step S5, the second catalyst is controlled to enter the second depolymerization unit at 0.10 kg / h, the yield of racemic lactide is 9.5 kg / h, and the L-lactide content is 49.6% and the D-lactide content is 49.7% as detected by gas chromatography.

[0113] The total yield of lactide (D-lactide and racemic lactide) prepared in Example 5 was 98.8%.

[0114] Comparative Example 1

[0115] The difference between Comparative Example 1 and Example 1 is that in step S3, the lactic acid oligomer is mixed with 0.2 kg / h of stannous octoate and then fed into the first depolymerization unit at a flow rate of 38.6 kg / h. Depolymerization is performed at 200°C and 0.5 kPa to obtain the first coarse material. The discharge rate of the first coarse material in the first depolymerization unit is controlled at 32.0 kg / h (the conversion rate of the first coarse material is 83.3%), and the discharge rate of the by-product is 6.6 kg / h. In step S4, the by-product... The feed and the first heavy component in the distillation column are mixed and fed into the hydrolysis unit. After being re-hydrolyzed into lactic acid, the feed is returned to the dehydration unit without secondary depolymerization or refining of the second crude feed. The feed, after hydrolysis and purification, is fed into the dehydration unit at a flow rate of 10.0 kg / h, calculated based on 80% L-lactic acid mass. L-lactide is collected from the side stream of the distillation column and further purified by melt crystallization, with a yield of 29.2 kg / h. Gas chromatography analysis shows that the L-lactide content is 98.9%.

[0116] The total yield of lactide (L-lactide) prepared in Comparative Example 1 was 91.3%.

[0117] Comparative Example 2

[0118] The difference between Comparative Example 2 and Example 1 is that, in step S3, the discharge rate of the first crude material from the first depolymerization unit is controlled at 30.4 kg / h (the conversion rate of the first crude material is 95%), and the discharge rate of the by-product is 1.8 kg / h; in step S4, the yield of L-lactide is 28.2 kg / h, and the L-lactide content is 99.1% as detected by gas chromatography; in step S5, the second catalyst is controlled to enter the second depolymerization unit at a rate of 0.02 kg / h, the yield of racemic lactide is 1.4 kg / h, and the L-lactide content and D-lactide content are 49.7% as detected by gas chromatography.

[0119] The total yield of lactide (L-lactide and racemic lactide) obtained in Comparative Example 2 was 92.5%.

[0120] Comparative Example 3

[0121] The difference between Comparative Example 3 and Example 1 is that, in step S5, stannous octoate is added in the second depolymerization unit to obtain the second crude material; lactide is collected from the side stream of the distillation column and further purified by melt crystallization, with a yield of 4.2 kg / h, and the L-lactide content is 98.6% as detected by gas chromatography.

[0122] In Comparative Example 3, the use of stannous octoate as a second catalyst failed to achieve the purpose of depolymerizing lactic acid oligomers to prepare racemic lactide. The total yield of lactide (L-lactide) obtained in Comparative Example 3 was 90.6%.

[0123] Comparative Example 4

[0124] The difference between Comparative Example 4 and Example 1 is that, in step S5, an additive is added to the second depolymerization unit. Depolymerization yields a second crude feedstock; lactide is collected from the side stream of the distillation column and further purified by melt crystallization, with a yield of 3.9 kg / h. Gas chromatography analysis shows that the L-lactide content is 98.3%.

[0125] Comparative Example 4, which used a neutral ionic liquid as a second catalyst, could not achieve the purpose of depolymerizing lactic acid oligomers to prepare racemic lactide. The total yield of lactide (L-lactide) obtained in Comparative Example 4 was 89.7%.

[0126] Comparative Example 5

[0127] The difference between Comparative Example 5 and Example 1 is that, in step S5, an additive is added to the second depolymerization unit. The yield of racemic lactide was 2.6 kg / h, and the L-lactide content was 49.7% and the D-lactide content was 49.6% as determined by gas chromatography.

[0128] Comparative Example 5 used a strongly basic ionic liquid as a second catalyst to promote the ring-opening polymerization of lactide, which led to a decrease in the yield of lactide. The total yield of lactide (L-lactide and racemic lactide) obtained in Comparative Example 5 was 85.6%.

[0129] Application examples

[0130] According to the formulation in Table 1, lactide was added to a 10L titanium alloy reactor, preheated to 140℃, and stirred until the lactide was completely melted. 1.0g of stannous octoate and 6.0g of benzyl alcohol were added, the temperature was raised to 180℃, and the reaction was stirred for 2 hours. After the reaction, the polymer melt was conveyed to a vented twin-screw extruder for devolatilization to remove unreacted lactide. The extruded melt was cooled and pelletized to obtain polylactic acid particles, wherein lactide was added according to Table 1.

[0131] Melting point tests were performed on polylactic acid particles from all application examples using a differential scanning calorimeter (DSC) model TA DSC250. The test temperature range was 25℃~200℃, and the heating / cooling rate was 10℃ / min. The test results are shown in Table 1.

[0132] Table 1

[0133]

[0134] In summary, the lactide obtained in all examples exhibits higher optical purity and overall yield than that in all comparative examples. Furthermore, all examples simultaneously yield a certain proportion of lactide and racemic lactide. Polymerizing different proportions of lactide and racemic lactide can produce polylactic acid products with specific melting points. Therefore, the co-production process of this invention effectively improves the utilization rate of raw material lactic acid, as well as the yield and optical purity of lactide products, effectively reducing waste discharge and treatment costs. Moreover, the lactide directly produced using the co-production process of this invention can yield polylactic acid products with different melting points, demonstrating broad application prospects.

[0135] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0136] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A co-production process for lactide, characterized in that, The co-production process includes the following steps: The lactic acid is dehydrated, wherein the lactic acid is L-lactic acid or D-lactic acid; Dehydrated lactic acid is polycondensed to obtain lactic acid oligomers; The lactic acid oligomer is depolymerized once under the action of a first catalyst to obtain a first coarse material and waste material, wherein the conversion rate of the lactic acid oligomer to form the first coarse material is 50% to 90%. The first crude material was refined and purified to obtain lactide; The waste material is subjected to secondary depolymerization under the action of a second catalyst to obtain a racemic second crude material. The second crude material is then refined and purified to obtain racemic lactide. The second catalyst is selected from alkaline ionic liquids.

2. The co-production process of lactide according to claim 1, characterized in that, The mass ratio of the second catalyst to the by-product is 0.5:100 to 5:100; And / or, the second catalyst is selected from at least one of guanidine-based ionic liquids or choline-based ionic liquids.

3. The co-production process of lactide according to claim 2, characterized in that, The structural formula of the cation donor in the guanidine-based ionic liquid is as follows: R1 to R6 are each independently selected from C1 to C6 alkyl, C1 to C6 alkoxy, C1 to C6 alkyl ester, C6 to C6 alkyl groups. 12 The aromatic group, and any two adjacent R1 to R6 can be linked to form a ring; And / or, the anion donor in the guanidine-based ionic liquid is selected from... CH3CH(OH)COO - CF3COO - Or CH3(CH2) n COO - , where n is an integer from 0 to 4.

4. The co-production process of lactide according to claim 3, characterized in that, The cation donor contains at least one cycloalkyl, heterocycloalkyl, or aromatic group.

5. The co-production process of lactide according to claim 2, characterized in that, The cation donor in the choline-based ionic liquid is selected from choline and its derivatives. And / or, the anion donor in the choline-based ionic liquid is selected from... or 6. The co-production process of lactide according to claim 1, characterized in that, The alkaline ionic liquid in the secondary depolymerization process is recycled and purified for reuse.

7. The co-production process of lactide according to claim 1, characterized in that, The first light component produced in the refining and purification process of the first crude material is recycled to the dehydration process; And / or, the second light component generated in the refining and purification process of the second crude material is recycled to the dehydration process; And / or, the first heavy component formed in the refining and purification process of the first crude material is recycled to the secondary depolymerization process; And / or, the second heavy component formed in the refining and purification process of the second crude material is recycled to the secondary depolymerization process.

8. The co-production process of lactide according to claim 1, characterized in that, The polycondensation reaction is carried out at a temperature of 120℃ to 180℃ and a pressure of 2kPa to 10kPa. And / or, the temperature of the first depolymerization is 180℃~240℃, and the pressure is 0.1kPa~2kPa; And / or, the temperature of the secondary depolymerization is 200℃~260℃, and the pressure is 0.1kPa~2kPa.

9. The co-production process of lactide according to claim 1, characterized in that, The molecular weight of the lactic acid oligomer is 500 Da to 5000 Da.

10. The co-production process of lactide according to claim 1, characterized in that, The mass ratio of the first catalyst to the lactic acid oligomer is 0.2:100 to 5:100; And / or, the first catalyst is selected from tin-based catalysts.

11. A method for preparing polylactic acid, characterized in that, Includes the following steps: Lactose and racemic lactide are prepared by the co-production process of lactide as described in any one of claims 1 to 10; By adjusting the mass ratio of the lactide and the racemic lactide, polylactic acid with a target melting point is obtained through ring-opening polymerization.

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