Method and system for refining L-lactide

Abstract

The invention relates to the technical field of purification and separation of ester organic matters, and discloses a method and a system for refining L-lactide. The method comprises the following steps: (1) carrying out first falling film crystallization on crude lactide to obtain a crystallized material I and a primary mother solution; carrying out first sweating treatment on the crystallized material I to obtain L-lactide and first-grade sweat; (2) performing second falling film crystallization on the first-stage mother liquor to obtain a crystallized material II and second-stage mother liquor; carrying out second sweating treatment on the crystallized material II to obtain L-lactide and second-stage sweat; (3) carrying out third falling film crystallization on the second-stage mother liquor to obtain a crystallized material III and third-stage mother liquor; and (4) carrying out fourth falling film crystallization on the third-stage mother liquor to obtain a crystallized material IV and fourth-stage mother liquor. According to the method provided by the invention, high purity and high yield of lactide can be considered at the same time, and the industrial production requirements of high-molecular-weight polylactic acid are met.

Description

A method and system for purifying L-lactide Technical Field

[0001] This invention relates to the field of purification and separation technology of ester organic compounds, and specifically to a method and system for purifying L-lactide. Background Technology

[0002] Polylactic acid (PLA), as a fully biodegradable green polymer material, has excellent biocompatibility, mechanical processing properties and environmental friendliness. It has been widely used in food packaging, medical sutures, bone repair materials, agricultural mulch films and other fields, and is one of the core materials to replace traditional petroleum-based plastics.

[0003] Currently, the mainstream industrial production process for high molecular weight polylactic acid is the two-step process of "lactic acid-lactide-polylactic acid", which involves first preparing lactide through lactic acid condensation and depolymerization, and then obtaining polylactic acid through ring-opening polymerization of lactide.

[0004] The purity of lactide directly determines the molecular weight, mechanical properties, and degradation stability of polylactic acid (PLA). Impurities such as water, lactic acid, meso-lactide, and lactic acid oligomers in crude lactide can trigger chain termination reactions during the ring-opening polymerization of lactide, leading to a decrease in the molecular weight of PLA. Therefore, the purification of lactide is a crucial link in the PLA industry chain.

[0005] Existing lactide refining technologies mainly include solvent recrystallization, distillation (using the differences in volatility of components in crude lactide to separate impurities through vacuum distillation), and melt crystallization, but all of them have certain limitations.

[0006] CN112047920A discloses recrystallization using a mixed solvent of anhydrous ethanol and ethyl acetate, which requires multi-frequency ultrasonic treatment and subsequent melt crystallization to achieve high purity, and the yield of a single recrystallization is limited.

[0007] CN111961028A uses dimethyl ketone and ethanol as alternating solvents for recrystallization. Even after three crystallizations, the yield is only 78.1%. Furthermore, the use of a large amount of organic solvent requires supporting recycling equipment, which increases investment costs and environmental risks. Residual solvents can also affect the subsequent polymerization performance of polylactic acid.

[0008] CN1894193A discloses a method for refining crude lactide vapor product stream through distillation / condensation steps, which requires the establishment of a multi-fraction separation system under reduced pressure and subsequent distillation steps to achieve a lactide purity of 99.5 wt%.

[0009] CN101696203A employs a series distillation process of "light weight removal tower + refining tower," which can remove water, lactic acid, and some meso-lactide. However, it requires operation under high vacuum (20~500Pa) and high temperature (≤170℃) conditions. Furthermore, due to the extremely small difference in boiling points between meso-lactide and L-lactide, a large number of theoretical trays are required, resulting in complex equipment and high energy consumption.

[0010] CN118496192A discloses the use of a rotary crystallizer for continuous melt crystallization. Although it can reduce impurity inclusion through gradient temperature control, it requires pretreatment processes (such as distillation and recrystallization) and has high requirements for feed purity (≥92wt%).

[0011] CN115779481A discloses a suspension melting crystallization device. Although the core washing tower (such as a bed washing section with scrapers) can improve crystal purity, the single-unit processing capacity is limited (maximum about 10,000 t / a), which is difficult to meet the needs of large-scale industrialization.

[0012] The melt recrystallization method disclosed in CN118767468A requires strict control of the amount of seed crystals added, the cooling rate, and the filtration time, which is cumbersome to operate and has low batch processing efficiency.

[0013] In addition, although the refined lactide process equipment disclosed in CN108031141A achieves uniform film distribution and two-stage crystallization-sweating-melting through a "material distribution film device with four-headed square spiral grooves", the equipment belongs to the category of vertical heat exchange static melting and crystallization, and does not involve falling film flow characteristics. It still has problems such as uneven crystal layer thickness and difficulty in continuous material discharge.

[0014] Therefore, there is an urgent need to develop a method for refining lactide to overcome the problems of insufficient purity, low yield, high energy consumption, and environmental unfriendliness of existing lactide refining methods. Summary of the Invention

[0015] The purpose of this invention is to provide a method for refining lactide through falling film melting and crystallization, which simultaneously achieves high purity and high yield of lactide to meet the industrial production requirements of high molecular weight polylactic acid.

[0016] To achieve the above objectives, a first aspect of the present invention provides a method for refining L-lactide, the method comprising: (1) subjecting crude lactide to a first falling film crystallization to obtain crystalline material I and primary mother liquor; subjecting the crystalline material I to a first sweating treatment to obtain L-lactide and primary sweat; wherein the content of L-lactide in the crude lactide is not less than 90 wt%; (2) subjecting the primary mother liquor to a second falling film crystallization to obtain crystalline material II and secondary mother liquor; subjecting the crystalline material II to a second sweating treatment to obtain L-lactide and secondary sweat; (3) subjecting the secondary mother liquor to a third falling film crystallization to obtain crystalline material III and tertiary mother liquor; (4) The third-stage mother liquor is subjected to a fourth falling film crystallization to obtain crystallized material IV and fourth-stage mother liquor; the first-stage sweat and the crystallized material III are recycled as crude lactide raw materials to step (1) to participate in the first falling film crystallization; the control conditions are such that the content of L-lactide in the first-stage sweat and the content of L-lactide in the crystallized material III are both not less than 90 wt%; the second-stage sweat and the crystallized material IV are recycled to step (2) to participate in the second falling film crystallization; the control conditions are such that the content of L-lactide in the second-stage sweat and the content of L-lactide in the crystallized material IV are each independently 83-89 wt%.

[0017] A second aspect of the present invention provides a system for refining L-lactide, the system for implementing the method described in the first aspect above, comprising: a primary crystallization unit, a secondary crystallization unit, a tertiary crystallization unit, and a quaternary crystallization unit connected in series via pipelines; the primary crystallization unit is used at least to crystallize crude lactide and induce sweating to form L-lactide, a primary mother liquor, and a primary sweat; the secondary crystallization unit is used at least to crystallize the primary mother liquor and induce sweating to form L-lactide, a secondary mother liquor, and a secondary sweat; the tertiary crystallization unit is used at least to crystallize the secondary mother liquor to form a tertiary mother liquor and crystallized material III; the quaternary crystallization unit is used at least to crystallize the tertiary mother liquor to form a quaternary mother liquor and crystallized material IV; the system further comprises pipelines for circulating the primary sweat and crystallized material III to the first crystallization unit, and pipelines for circulating the secondary sweat and crystallized material IV to the second crystallization unit.

[0018] The method for refining L-lactide provided by this invention, through the synergy of four-stage falling film crystallization and the recycling of materials (first sweat, crystal material III, second sweat, and crystal material IV), results in an L-lactide product with a chemical purity ≥98.9%, an optical purity ≥99.1%, a water content ≤50ppm (parts per million by mass), and an ash content ≤0.06wt%. Furthermore, it can increase the total yield to over 89.6%, which is far higher than that of traditional solvent recrystallization (yield 72~78%) and single distillation (yield <85%).

[0019] The method for refining L-lactide provided by this invention does not require organic solvents such as ethyl acetate and ethanol, and has no solvent recovery costs or environmental risks, thus meeting the requirements of green chemistry; at the same time, it can reasonably arrange working hours to achieve high-efficiency production and meet the needs of large-scale production.

[0020] The system for refining L-lactide provided by this invention is simple to operate and has low setup cost. Attached Figure Description

[0021] Figure 1 is a schematic diagram of a system for refining L-lactide in a specific embodiment of the present invention.

[0022] Explanation of reference numerals in the attached diagram: 1. Primary raw material tank; 2. Primary falling film crystallizer; 3. Primary circulating pump; 4. Primary mother liquor tank; 5. Product tank; 6. Secondary falling film crystallizer; 7. Secondary circulating pump; 8. Secondary mother liquor tank; 9. Tertiary falling film crystallizer; 10. Tertiary circulating pump; 11. Tertiary mother liquor tank; 12. Quaternary falling film crystallizer; 13. Quaternary circulating pump; 14. Quaternary mother liquor tank. Detailed Implementation

[0023] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0024] It should be noted that, in this invention, the final crystallization temperature refers to the lowest temperature reached during crystallization; the final sweating temperature refers to the highest temperature reached during sweating.

[0025] In this invention, the chemical purity of L-lactide is detected by gas chromatography (Agilent 6890N); the optical purity of L-lactide is detected by polarimetry (WZZ-2S); the water content of L-lactide is determined by Karl Fischer moisture analyzer; and the ash content (referring to non-combustible mineral residues) of L-lactide is determined by high-temperature ignition and weighing method (e.g., a muffle furnace).

[0026] As previously stated, a first aspect of the present invention provides a method for refining L-lactide, the method comprising: (1) subjecting crude lactide to a first falling film crystallization to obtain crystalline material I and primary mother liquor; subjecting the crystalline material I to a first sweating treatment to obtain L-lactide and primary sweat; wherein the content of L-lactide in the crude lactide is not less than 90 wt%; (2) subjecting the primary mother liquor to a second falling film crystallization to obtain crystalline material II and secondary mother liquor; subjecting the crystalline material II to a second sweating treatment to obtain L-lactide and secondary sweat; (3) subjecting the secondary mother liquor to a third falling film crystallization to obtain crystalline material III and tertiary mother liquor; (4) The third-stage mother liquor is subjected to a fourth falling film crystallization to obtain crystallized material IV and fourth-stage mother liquor; the first-stage sweat and the crystallized material III are recycled as crude lactide raw materials to step (1) to participate in the first falling film crystallization; the control conditions are such that the content of L-lactide in the first-stage sweat and the content of L-lactide in the crystallized material III are both not less than 90 wt%; the second-stage sweat and the crystallized material IV are recycled to step (2) to participate in the second falling film crystallization; the control conditions are such that the content of L-lactide in the second-stage sweat and the content of L-lactide in the crystallized material IV are each independently 83-89 wt%.

[0027] In some embodiments, the crystal material III is heated to 95-100°C to melt into a liquid state before being recycled to step (1) to participate in the first falling film crystallization.

[0028] In some embodiments, the crystallizing material IV is heated to 95-100°C to melt into a liquid state before being recycled to step (2) to participate in the second falling film crystallization.

[0029] According to a preferred embodiment, the final crystallization temperature of the first falling film crystallization is > the final crystallization temperature of the second falling film crystallization is > the final crystallization temperature of the third falling film crystallization is > the final crystallization temperature of the fourth falling film crystallization. The inventors of this invention have discovered that, under this preferred condition, the L-lactide product has higher purity.

[0030] In some implementations, in step (1), the conditions for the first falling film crystallization include: a final crystallization temperature of 60-70°C, optionally 60-65°C.

[0031] In some embodiments, in step (2), the conditions for the second falling film crystallization include: a final crystallization temperature of 50-58°C, optionally 50-55°C.

[0032] In some embodiments, in step (3), the third falling film crystallization includes: the final crystallization temperature is 35-45°C, optionally 38-43°C.

[0033] In some embodiments, in step (4), the fourth falling film crystallization includes: the final crystallization temperature is 20-30°C, optionally 25-30°C.

[0034] In some preferred embodiments, the conditions for the first falling film crystallization include a cooling rate of 3.00-8.00℃ / h, optionally 5.00-6.67℃ / h. The inventors of this invention have found that under this preferred condition, the crystallization efficiency is high and the time is shortened.

[0035] In some implementations, the conditions for the first sweating treatment and the second sweating treatment each independently include: the final sweating temperature is 90-98°C, optionally 92-96°C.

[0036] In some embodiments, the conditions for the first sweating treatment and the second sweating treatment each independently include: a heating rate of 5-15°C / h, optionally 7-12°C / h.

[0037] In some embodiments, in step (1), the crude lactide contains 90-95 wt% L-lactide, 4-5 wt% mesolacide, and 1-5 wt% lactic acid.

[0038] In some embodiments, in step (1), the crude lactide is a crude lactide melt, and the temperature of the crude lactide melt is 95-105°C.

[0039] In some embodiments, in step (1), the falling film circulation flow rate of the crude lactide melt is 1-2 L / min, optionally 1.4-1.8 L / min.

[0040] In this invention, the falling film circulation flow rate refers to the flow rate of raw materials transported to the crystallizer.

[0041] In some embodiments, the method further includes, in step (4), extracting the fourth-stage mother liquor. In this preferred embodiment, more L-lactide can be recovered.

[0042] The present invention does not impose any special requirements on the extraction conditions. Those skilled in the art can select the conditions according to known techniques in the art. The present invention will not elaborate further here, and those skilled in the art should not understand this as a limitation of the present invention.

[0043] As previously described, a second aspect of the present invention provides a system for refining L-lactide, the system being used to implement the method described in the first aspect, comprising: a primary crystallization unit, a secondary crystallization unit, a tertiary crystallization unit, and a quaternary crystallization unit connected in series via pipelines; the primary crystallization unit being used at least to crystallize crude lactide and induce sweating to form L-lactide, a primary mother liquor, and a primary sweat; the secondary crystallization unit being used at least to crystallize the primary mother liquor and induce sweating to form L-lactide, a secondary mother liquor, and a secondary sweat; the tertiary crystallization unit being used at least to crystallize the secondary mother liquor to form a tertiary mother liquor and crystallized material III; the quaternary crystallization unit being used at least to crystallize the tertiary mother liquor to form a quaternary mother liquor and crystallized material IV; the system further comprising pipelines for circulating the primary sweat and crystallized material III to the first crystallization unit, and pipelines for circulating the secondary sweat and crystallized material IV to the second crystallization unit.

[0044] In some embodiments, the primary crystallization unit includes a primary raw material tank, a primary falling film crystallizer, a primary circulating pump, and a primary mother liquor tank.

[0045] In some embodiments, the secondary crystallization unit includes a secondary falling film crystallizer, a secondary circulation pump, and a secondary mother liquor tank.

[0046] In some embodiments, the three-stage crystallization unit includes a three-stage falling film crystallizer, a three-stage circulating pump, and a three-stage mother liquor tank.

[0047] In some embodiments, the four-stage crystallization unit includes a four-stage falling film crystallizer, a four-stage circulating pump, and a four-stage mother liquor tank.

[0048] In some embodiments, a product tank is provided between the primary mother liquor tank and the secondary falling film crystallizer, the product tank being used to store the L-lactide.

[0049] In some embodiments, an extraction vessel for extracting the fourth-stage mother liquor is also provided downstream of the fourth-stage mother liquor tank.

[0050] The present invention does not have any particular requirements for the model of the extraction vessel. Those skilled in the art can select it based on the known technical means in the art. The present invention will not elaborate further here, and those skilled in the art should not understand it as a limitation of the present invention.

[0051] In order to better achieve uniform film distribution of raw materials entering each crystallizer, in some embodiments, a material distribution device is provided in the first-stage falling film crystallizer, the second-stage falling film crystallizer, the third-stage falling film crystallizer, and the fourth-stage falling film crystallizer.

[0052] It should be noted that various control valves known to those skilled in the art can be installed in the circulation path of the system for refining L-lactide used in this invention to control the flow of fluids such as lactide.

[0053] It should be noted that all containers in the L-lactide refining system used in this invention (such as primary mother liquor tank, primary crystallizer, secondary mother liquor tank, etc.) can be equipped with independent vent valves, and can also be equipped with pressure indicators, thermocouples, etc. to display equipment operating pressure and temperature, etc.

[0054] The following describes in detail the system of the method for refining L-lactide provided by the present invention with reference to the accompanying drawings.

[0055] Figure 1 is a schematic diagram of a system for refining L-lactide applied in a specific embodiment of the present invention. As can be seen from Figure 1, the system includes a primary crystallization unit, a secondary crystallization unit, a tertiary crystallization unit, and a quaternary crystallization unit connected in series via pipelines. The method for refining L-lactide is carried out in the above system, including: First crystallization unit: Crude lactide (a crude lactide melt) is transported from the primary raw material tank 1 to the primary falling film crystallizer 2 via a primary circulating pump 3 for first falling film crystallization, yielding crystalline material I and primary mother liquor; the primary crystallization unit... The mother liquor is transported to the primary mother liquor tank 4 as raw material for the second falling film crystallization; the crystallized material I in the primary falling film crystallizer 2 undergoes a first sweating treatment to obtain L-lactide (entering the product tank 5) and primary sweat, which is then recycled back to the primary raw material tank 1 via pipeline; Second crystallization unit: the primary mother liquor in the primary mother liquor tank 4 is transported to the secondary falling film crystallizer 6 via the secondary circulation pump 7 for the second falling film crystallization to obtain crystallized material II and secondary mother liquor; the secondary mother liquor is then transported to the secondary mother liquor tank 8 via pipeline for the third falling film crystallization. Raw materials: The crystallized material II in the secondary falling film crystallizer 6 undergoes a second sweating treatment to obtain L-lactide (entering product tank 5) and secondary sweat. The secondary sweat is then recycled back to the primary mother liquor tank 4 via pipeline. Third crystallization unit: The secondary mother liquor in the secondary mother liquor tank 8 is transported to the tertiary falling film crystallizer 9 via the tertiary circulation pump 10 for third falling film crystallization to obtain crystallized material III and tertiary mother liquor. The tertiary mother liquor is then transported to the tertiary mother liquor tank 11 via pipeline as raw material for the fourth falling film crystallization. Crystallized material III is first heated to 95°C. After melting into a liquid at -100℃, the molten liquid is then returned to the primary raw material tank 1 for recycling through pipelines; Fourth crystallization unit: The tertiary mother liquor in the tertiary mother liquor tank 11 is transported to the tertiary falling film crystallizer 12 through the four-stage circulation pump 13 for fourth falling film crystallization to obtain crystallized material IV and tertiary mother liquor; The tertiary mother liquor is transported to the tertiary mother liquor tank 14 through pipelines, and then the tertiary mother liquor is extracted; The crystallized material IV is first heated to 95-100℃ to melt into a liquid, and then the molten liquid is returned to the primary mother liquor tank 4 for recycling through pipelines.

[0056] In some embodiments, the amount of sweat discharged during the first and second sweating processes is 5%-10% of the total mass of the material in the corresponding crystallizer.

[0057] The present invention will be described in detail below through examples. Unless otherwise specified, the instruments, reagents, and materials involved in the following examples are all conventional instruments, reagents, and materials, which can be obtained through legitimate commercial channels. Unless otherwise stated, all reagents used are commercially available analytical grade products.

[0058] Crude lactide: The content of L-lactide is 93.2 wt%, the content of meso lactide is 4.6 wt%, and the content of lactic acid is 2.2 wt%. It comes from the lactic acid condensation and depolymerization unit of a chemical plant to produce lactide.

[0059] Total yield of L-lactide = (L-lactide content in crude lactide - L-lactide content in mother liquor after extraction) ÷ L-lactide content in crude lactide × 100%.

[0060] Example 1 (1) First crystallization unit: Crude lactide (molten crude lactide at 98°C) is transported from the primary raw material tank 1 to the primary falling film crystallizer 2 at a falling film circulation flow rate of 1.6 L / min via the primary circulation pump 3 for the first falling film crystallization (cooled from 98°C to the final crystallization temperature of 60°C at a rate of 6°C / h, cooling time of about 3.33h), and then kept at a constant temperature for 1h), to obtain crystalline material I and primary mother liquor (the mother liquor discharged from the primary falling film crystallizer accounts for about 53wt% of the initial material (referring to the total feed of the first falling film crystallization); the primary mother liquor is transported to the primary mother liquor tank 4 via pipeline as the raw material for the second falling film crystallization; the crystals in the primary falling film crystallizer 2 are... Material I undergoes a first sweating treatment (heated at a rate of 15℃ / h to a final sweating temperature of 94℃, with a sweating duration of approximately 2.67h) to obtain L-lactide (entering product tank 5) and primary sweat (the amount of sweat discharged is 7% of the total mass of the material in the primary falling film crystallizer 2); the primary sweat (the content of L-lactide in the primary sweat is 93.2wt%) is recycled back to the primary raw material tank 1 through a pipeline; (2) Second crystallization unit: the primary mother liquor (preheated to 75℃) in the primary mother liquor tank 4 is pumped by the secondary circulation pump 7 at a rate of 1.6 A falling film flow rate of L / min is fed into the secondary falling film crystallizer 6 for a second falling film crystallization (cooling from 75°C to the final crystallization temperature of 50°C at a rate of 6.0°C / h, cooling time approximately 4.5h, followed by 1h of constant temperature), yielding crystallized material II and secondary mother liquor (approximately 25wt% of the initial material); the secondary mother liquor is transported via pipeline to the secondary mother liquor tank 8 for use as raw material for the third falling film crystallization; the crystallized material II in the secondary falling film crystallizer 6 undergoes a second sweating treatment (at a rate of 9°C / h). The temperature is increased to the final sweating temperature of 93°C at a rate of 1.6L, and the sweating time is about 5 hours. L-lactide (enters product tank 5) and secondary sweat (the amount of sweat discharged is 8% of the total mass of the material in the secondary falling film crystallizer 6) are obtained. The secondary sweat (the content of L-lactide in the secondary sweat is 88.7wt%) is returned to the primary mother liquor tank 4 for recycling through the pipeline. (3) Third crystallization unit: The secondary mother liquor (preheated to 70°C) in the secondary mother liquor tank 8 is pumped by the three-stage circulation pump 10 at a rate of 1.6L. A falling film circulation flow rate of / min is fed to the three-stage falling film crystallizer 9 for the third falling film crystallization (cooling from 70℃ to the final crystallization temperature of 40℃ at a rate of 8℃ / h, cooling time of about 3.75h, and constant temperature crystal growth for 1h), to obtain crystal material III and third-stage mother liquor (accounting for about 20wt% of the initial material); the third-stage mother liquor is transported to the third-stage mother liquor tank 11 through pipeline as raw material for the fourth falling film crystallization; firstly, crystal material III (the content of L-lactide in crystal material III is 93%) is fed to the three-stage falling film crystallizer 9 for the third falling film crystallization.(2wt%) is heated to 98°C and melted into a liquid, then returned to the primary raw material tank 1 for recycling through pipeline; (4) Fourth crystallization unit: the tertiary mother liquor (preheated to 65°C) in the tertiary mother liquor tank 11 is pumped by the four-stage circulation pump 13 at a rate of 1.4 A falling film flow rate of L / min is fed into a four-stage falling film crystallizer 12 for fourth-stage falling film crystallization (cooling from 65°C to the final crystallization temperature of 30°C at a rate of 8°C / h, cooling time approximately 4.375h, followed by 1h of constant temperature), yielding crystallized material IV and fourth-stage mother liquor (approximately 10wt% of the initial material). The fourth-stage mother liquor is then transported via pipeline to a fourth-stage mother liquor tank 14, where it undergoes extraction treatment (using an extraction vessel, refer to CN120733382A). Crystallized material IV (containing 88.7wt% L-lactide) is first heated to 97°C to melt into a liquid, then refluxed back to the first-stage mother liquor tank 4 for recycling.

[0061] The results of testing and calculation show that the chemical purity of L-lactide in product tank 5 is 99.70%, the optical purity is 99.9%, the water content is 35 ppm, the ash content is 0.03 wt%, and the total yield is 93.0%.

[0062] Example 2 This example uses a method similar to that of Example 1, except that: the final crystallization temperature of the first falling film crystallization is 65°C; all parts not listed are the same as in Example 1; in this example, the content of L-lactide in the first-stage sweat is 92.3 wt%, and the content of L-lactide in crystallized material III is 92.8 wt%; the content of L-lactide in the second-stage sweat is 87.6 wt%, and the content of L-lactide in crystallized material IV is 87.5 wt%.

[0063] The results of testing and calculation show that the chemical purity of L-lactide in product tank 5 is 99.63%, the optical purity is 99.8%, the water content is 38 ppm, the ash content is 0.05 wt%, and the total yield is 92.5%.

[0064] Example 3 This example uses a method similar to that of Example 1, except that the cooling rate of the first falling film crystallization is adjusted to 6.67℃ / h; all parts not listed are the same as in Example 1; in this example, the content of L-lactide in the first-stage sweat is 92.1wt%, and the content of L-lactide in crystallized material III is 92.2wt%; the content of L-lactide in the second-stage sweat is 87.8wt%, and the content of L-lactide in crystallized material IV is 87.6wt%.

[0065] The results of testing and calculation show that the chemical purity of L-lactide in product tank 5 is 99.21%, the optical purity is 99.65%, the water content is 42 ppm, the ash content is 0.06 wt%, and the total yield is 92.7%.

[0066] Example 4: This example uses a method similar to Example 1, except that the final temperature of the first sweating treatment is adjusted to 92°C; all parts not listed are the same as in Example 1; in this example, the L-lactide content in the first-stage sweat is 92.8 wt%, and the L-lactide content in crystal material III is 92.5 wt%; the L-lactide content in the second-stage sweat is 88.1 wt%, and the L-lactide content in crystal material IV is 88.2 wt%.

[0067] The results of testing and calculation show that the chemical purity of L-lactide in product tank 5 is 99.12%, the optical purity is 99.61%, the water content is 35 ppm, the ash content is 0.06 wt%, and the total yield is 92.1%.

[0068] Example 5: This example uses a method similar to that of Example 1, except that the final crystallization temperature of the second falling film crystallization is 60°C; all parts not listed are the same as in Example 1; in this example, the content of L-lactide in the first-stage sweat is 92.6 wt%, and the content of L-lactide in crystallized material III is 92.5 wt%; the content of L-lactide in the second-stage sweat is 87.3 wt%, and the content of L-lactide in crystallized material IV is 87.4 wt%.

[0069] The results of testing and calculation show that the chemical purity of L-lactide in product tank 5 is 98.9%, the optical purity is 99.1%, the water content is 38 ppm, the ash content is 0.06 wt%, and the total yield is 89.6%.

[0070] Comparative Example 1 was carried out using a method similar to that of Example 1, except that the third-stage mother liquor obtained from the third falling film crystallization was directly extracted (same as in Example 1), and the fourth falling film crystallization was not performed; all parts not listed were the same as in Example 1.

[0071] The results of testing and calculation show that the chemical purity of L-lactide in product tank 5 is 99.70%, the optical purity is 99.9%, the water content is 35 ppm, the ash content is 0.03 wt%, and the total yield is 75.3%.

[0072] Comparative Example 2 was carried out using a method similar to that of Example 1, except that: the primary sweat was transported with the primary mother liquor to the primary mother liquor tank 4 as raw material for the second falling film crystallization; the secondary sweat was transported with the secondary mother liquor to the secondary mother liquor tank 8 as raw material for the third falling film crystallization; and the parts not listed were the same as in Example 1.

[0073] The results of testing and calculation show that the chemical purity of L-lactide in product tank 5 is 99.69%, the optical purity is 99.8%, the water content is 36 ppm, the ash content is 0.03 wt%, and the total yield is 86.5%.

[0074] Comparative Example 3 was conducted using a method similar to that of Example 1, except that the final crystallization temperature of the four-stage crystallizer was controlled at 32°C, so that the content of L-lactide in the crystallized material IV was 80 wt%; all other parts not listed were the same as in Example 1; in this comparative example, the content of L-lactide in the first-stage sweat was 90.3 wt%, and the content of L-lactide in the crystallized material III was 89.3 wt%; the content of L-lactide in the second-stage sweat was 82.1 wt%, and the content of L-lactide in the crystallized material IV was 80 wt%.

[0075] The results of testing and calculation show that the chemical purity of L-lactide in product tank 5 is 98.12%, the optical purity is 99.1%, the water content is 56 ppm, the ash content is 0.15 wt%, and the total yield is 87.3%.

[0076] The results above show that the method for refining L-lactide provided by this invention achieves a chemical purity of ≥98.9%, an optical purity of ≥99.1%, a water content of ≤50ppm, and an ash content of ≤0.06wt%, while increasing the overall yield to over 89.6%. Furthermore, the method provided by this invention operates at a significantly lower temperature than distillation (140~170℃), and its energy consumption is only 30%~50% of that of distillation technology. It requires no organic solvents such as ethyl acetate or ethanol, eliminating solvent recovery costs and environmental risks, and thus meets the requirements of green chemistry.

[0077] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for purifying L-lactide, characterized in that, The method includes: (1) subjecting crude lactide to a first falling film crystallization to obtain crystalline material I and primary mother liquor; subjecting the crystalline material I to a first sweating treatment to obtain L-lactide and primary sweat; wherein the content of L-lactide in the crude lactide is not less than 90 wt%; (2) subjecting the primary mother liquor to a second falling film crystallization to obtain crystalline material II and secondary mother liquor; subjecting the crystalline material II to a second sweating treatment to obtain L-lactide and secondary sweat; (3) subjecting the secondary mother liquor to a third falling film crystallization to obtain crystalline material III and tertiary mother liquor; (4) subjecting the tertiary mother liquor to a fourth falling film crystallization. The resulting crystalline material IV and fourth-stage mother liquor are obtained. The first-stage sweat and the crystalline material III are recycled as crude lactide raw materials to step (1) to participate in the first falling film crystallization. The control conditions are such that the content of L-lactide in the first-stage sweat and the content of L-lactide in the crystalline material III are both not less than 90 wt%. The second-stage sweat and the crystalline material IV are recycled to step (2) to participate in the second falling film crystallization. The control conditions are such that the content of L-lactide in the second-stage sweat and the content of L-lactide in the crystalline material IV are each independently 83-89 wt%.

2. The method according to claim 1, characterized in that, The final crystallization temperature of the first falling film crystallization is greater than that of the second falling film crystallization, which is greater than that of the third falling film crystallization, which is greater than that of the fourth falling film crystallization.

3. The method according to claim 2, characterized in that, In step (1), the conditions for the first falling film crystallization include: a final crystallization temperature of 60-70℃; and / or, in step (2), the conditions for the second falling film crystallization include: a final crystallization temperature of 50-58℃; and / or, in step (3), the conditions for the third falling film crystallization include: a final crystallization temperature of 35-45℃; and / or, in step (4), the conditions for the fourth falling film crystallization include: a final crystallization temperature of 20-30℃.

4. The method according to any one of claims 1-3, characterized in that, The conditions for the first falling film crystallization include a cooling rate of 3.00-8.00℃ / h.

5. The method according to any one of claims 1-3, characterized in that, The conditions for the first sweating treatment and the second sweating treatment each independently include: the final sweating temperature is 90-98℃.

6. The method according to any one of claims 1-3, characterized in that, The conditions for the first sweating treatment and the second sweating treatment each independently include a heating rate of 5-15℃ / h.

7. The method according to any one of claims 1-3, characterized in that, In step (1), the crude lactide contains 90-95 wt% L-lactide, 4-5 wt% mesolactide, and 1-5 wt% lactic acid.

8. The method according to any one of claims 1-3, characterized in that, In step (1), the crude lactide is a crude lactide melt, and the temperature of the crude lactide melt is 95-105℃.

9. The method according to claim 8, characterized in that, In step (1), the falling film circulation flow rate of the crude lactide melt is 1-2 L / min.

10. The method according to any one of claims 1-3, characterized in that, The method further includes, in step (4), extracting the fourth-stage mother liquor.

11. A system for purifying L-lactide, characterized in that, The system is used to implement the method according to any one of claims 1-10, comprising: a primary crystallization unit, a secondary crystallization unit, a tertiary crystallization unit, and a quaternary crystallization unit connected in series via pipelines; the primary crystallization unit is used at least to crystallize crude lactide and induce sweating to form L-lactide, a primary mother liquor, and a primary sweat; the secondary crystallization unit is used at least to crystallize the primary mother liquor and induce sweating to form L-lactide, a secondary mother liquor, and a secondary sweat; the tertiary crystallization unit is used at least to crystallize the secondary mother liquor to form a tertiary mother liquor and crystallized material III; the quaternary crystallization unit is used at least to crystallize the tertiary mother liquor to form a quaternary mother liquor and crystallized material IV; the system further comprises pipelines for circulating the primary sweat and crystallized material III to the first crystallization unit, and pipelines for circulating the secondary sweat and crystallized material IV to the second crystallization unit.

12. The system according to claim 11, characterized in that, The primary crystallization unit includes a primary raw material tank, a primary falling film crystallizer, a primary circulating pump, and a primary mother liquor tank.

13. The system according to claim 12, characterized in that, The secondary crystallization unit includes a secondary falling film crystallizer, a secondary circulation pump, and a secondary mother liquor tank.

14. The system according to claim 11, characterized in that, The three-stage crystallization unit includes a three-stage falling film crystallizer, a three-stage circulating pump, and a three-stage mother liquor tank.

15. The system according to claim 11, characterized in that, The four-stage crystallization unit includes a four-stage falling film crystallizer, a four-stage circulating pump, and a four-stage mother liquor tank.

16. The system according to claim 13, characterized in that, A product tank is provided between the primary mother liquor tank and the secondary falling film crystallizer, and the product tank is used to store the L-lactide.

17. The system according to claim 15, characterized in that, Downstream of the fourth-stage mother liquor tank, there is also an extraction vessel for extracting the fourth-stage mother liquor.

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