A device and method for preparing high-purity L-lactide

Abstract

The invention discloses a device and method for preparing high-purity L-lactide. The device comprises a first crystallization separation unit and a second crystallization separation unit for crystallization and solid-liquid separation, a rectification unit for removing light and heavy components, and a falling film crystallization unit. The material is subjected to multi-stage crystallization to achieve a gradient increase in purity, and the rectification can achieve the treatment of materials whose content is too low to be separated by crystallization. The yield of high-purity products is improved by applying the mother liquor, and a yield of more than 90% and a purity of more than 99.5wt% can be achieved within a range of more than 70wt% of the raw material content.

Description

Technical Field

[0001] The present invention relates to the technical field of L-lactide separation and purification, and in particular to a device and method for preparing high-purity L-lactide. Background Art

[0002] Polylactic acid (PLA), also known as polylactide, is a degradable polymer material with good biocompatibility. It has a wide range of uses. Driven by the global ban on plastics, the demand for polylactic acid in the field of disposable degradable plastic products is expanding rapidly, and the potential market is huge.

[0003] The synthesis of high molecular weight polylactic acid is mainly prepared by the ring-opening polymerization of lactide. Among them, the impurities in the raw material lactide will directly affect the ring-opening polymerization reaction, and then affect the molecular weight and product performance of the polylactic acid product. At present, most of the industrial preparation of lactide adopts lactic acid dehydration condensation, recyclization depolymerization to obtain crude lactide, and finally purifies to obtain refined lactide. Therefore, the purification capacity and purification cost of lactide are one of the important reasons affecting the development of the domestic polylactic acid industry.

[0004] The existing methods for purifying lactide by suspension crystallization have limited operational flexibility. When the raw material content is reduced to a certain level, the separation effect will be greatly reduced. For example, a method for separating and purifying lactide disclosed in Chinese invention patent application CN112500389A uses distillation and melt crystallization, but the single-stage crystallization yield is not high, and distillation as the main purification process consumes a lot of energy. Another example is a method and system for continuous purification of L-lactide by suspension crystallization disclosed in Chinese invention patent application CN112321560A. Although the yield has been improved, the raw material content is also required to be 85-95%.

[0005] Obviously, the above existing separation and purification technology has great limitations for crude lactide raw materials and cannot play a significant purification role for crude lactide with a lower content, which limits its application scope in industrial separation and purification.

[0006] In summary, if the selectivity of the purification method for raw materials can be reduced and the operational flexibility can be increased, crude lactide raw materials of various sources and different contents can be effectively utilized, the utilization rate of the crude lactide product from the front-end cyclization and depolymerization can be improved, thereby improving the overall yield and reducing costs. This has significant value for the industrial separation and purification of crude lactide of different contents. Summary of the invention

[0007] The present invention provides a device and method for preparing high-purity L-lactide, so as to at least solve one of the many defects existing in the above-mentioned prior art.

[0008] To achieve the above object, the scheme of the present invention is as follows:

[0009] A device for preparing high-purity L-lactide, comprising a first crystallization separation unit for crystallization and solid-liquid separation, a second crystallization separation unit, a distillation unit for removing light and heavy fractions and recovering L-lactide, and a falling film crystallization unit;

[0010] The feed end and the recovery discharge end of the distillation unit are correspondingly connected to the liquid phase discharge end and the crystallization feed end of the first crystallization separation unit; the feed end of the second crystallization separation unit is connected to the solid phase discharge end of the first crystallization separation unit;

[0011] The feeding end of the falling film crystallization unit is connected to the solid phase discharging end of the second crystallization separation unit, and a high-purity product is produced from the solid phase discharging end of the falling film crystallization unit.

[0012] In the present invention, the liquid phase discharge end of the falling film crystallization unit is connected to the crystallization feed end of the second crystallization separation unit.

[0013] In the present invention, the liquid phase output end of the second crystallization separation unit is connected to the crystallization feed end of the first crystallization separation unit.

[0014] In the present invention, the distillation unit is provided with a light component extraction pipe at the top, a recovery pipe in the middle, and a heavy component extraction pipe at the bottom; the recovery pipe is connected to the crystallization feed end of the first crystallization separation unit.

[0015] Preferably, the device further comprises a first buffer storage tank, and the recovery pipe is connected to the first buffer storage tank; the crude L-lactide and the top reflux material enter the first crystallization separation unit from the first buffer storage tank.

[0016] In the present invention, the first crystallization separation unit and the second crystallization separation unit adopt membrane separation.

[0017] The present invention provides a method for preparing high-purity L-lactide, comprising the following steps:

[0018] S1. crystallizing the crude L-lactide and separating it into a crystal suspension and a mother liquor;

[0019] S2. The mother liquor 1 is subjected to distillation to remove light and heavy substances, and the L-lactide recovery component at the top of the tower is collected and incorporated into the crude product of step S1; the crystal suspension 1 is recrystallized and separated to obtain a crystal suspension 2;

[0020] S3. Perform falling film crystallization on the crystal suspension to obtain a high-purity product.

[0021] In the above preparation method of the present invention, the mother liquor formed in the re-crystallization separation process in step S2 is incorporated into the crude product of step S1; the mother liquor formed by the falling film crystallization in step S3 is incorporated into the crystal suspension 1 of step S2.

[0022] In the above preparation method of the present invention, the crude product also contains meso-lactide, lactic acid, water, and oligomers; in the step S2, the heavy part is removed from the bottom of the tower to obtain the oligomers, and the light part is removed from the top of the tower to obtain meso-lactide, lactic acid, and water, which are then incorporated into the crude product of step S1.

[0023] In the above preparation method of the present invention, the L-lactide content in the lactide crude product is 70-95wt%.

[0024] Compared with the prior art, the beneficial effects of the present invention are at least:

[0025] 1. The high-purity lactide preparation device and method provided by the present invention purifies the material in multiple stages by means of two-stage crystallization and one-stage falling film crystallization, thereby achieving a gradient increase in purity and making maximum use of the separation effect of crystallization; in addition, the yield of high-purity products is improved by applying the mother liquor, and a yield of more than 90% and a purity of more than 99.5wt% can be achieved within a range of more than 70% of the raw material content.

[0026] 2. The high-purity lactide preparation device and method provided by the present invention utilize distillation to process materials whose content is too low to be separated by crystallization, thereby breaking the restriction of crystallization on the minimum content of raw materials. At the same time, distillation is only used as a means of impurity enrichment, thereby avoiding the adverse effects of long-term high temperature on lactide products.

[0027] 3. The high-purity lactide preparation device provided by the present invention can realize the continuous operation of partial slurry solid-liquid separation by adopting a membrane separation unit, thereby improving reliability and production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is an overall schematic diagram of the device for preparing high-purity L-lactide according to the present invention.

[0029] Marked in the figure are: 10, first crystallizer; 20, first separator; 30, second crystallizer; 40, second separator; 50, falling film crystallizer; 60, distillation tower; 70, first buffer tank; 80, second buffer tank; 90, third buffer tank; 100, fourth buffer tank; 201, first solid phase output pipe; 202, first liquid phase output pipe; 401, second solid phase output pipe; 402, second liquid phase output pipe; 501, falling film crystal output pipe; 502, falling film mother liquor output pipe; 601, recovery pipe; 602, light component extraction pipe; 603, heavy component extraction pipe; 701, crude product feed pipe. DETAILED DESCRIPTION

[0030] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] It should be understood that the orientations or positional relationships indicated by terms such as “upper”, “lower”, “top”, “bottom”, “inside” and “outside” are based on the orientations or positional relationships shown in the accompanying drawings and are only for the convenience of describing the present invention and simplifying the description. They do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0032] like Figure 1 As shown, the present invention provides an apparatus for preparing high-purity L-lactide, wherein a crude product feed pipe 701 is connected to a first buffer tank 70, and the first buffer tank 70, a first crystal separator 10, and a first solid-liquid separator 20 are connected in sequence; the first solid-liquid separator 20 is provided with a first solid phase output pipe 201 and a first liquid phase output pipe 202 for outputting the separated solid phase and liquid phase respectively.

[0033] The first solid phase output pipe 201 is connected to the second buffer tank 80, and the second buffer tank 80, the second crystallizer 30 and the second separator 40 are connected in sequence. The second separator 40 is provided with a second solid phase output pipe 402 and a second liquid phase output pipe 401 to respectively output the separated solid phase and liquid phase; the second liquid phase output pipe 401 is connected to the crude product feed pipe 701, the second solid phase output pipe 402 is connected to the third buffer tank 90, and further connected to the feed end of the falling film crystallizer 50, the falling film crystallizer 50 is provided with a falling film crystal output pipe 501 and a falling film mother liquor output pipe 502, the falling film mother liquor output pipe 502 is further connected to the second buffer tank 80, and the purified L-lactide is extracted through the falling film crystal output pipe 501.

[0034] The first liquid phase output pipe 202 is connected to the fourth buffer tank 100, and further connected to the feed end of the distillation tower 60. The distillation tower 60 is provided with a light component extraction pipe 602 at the top, a recovery pipe 601 in the middle, and a heavy component extraction pipe 603 at the bottom. The recovery pipe 601 is connected to the first buffer tank 70. The recovery pipe 601 is used to recover L-lactide, the light component extraction pipe 602 is used to extract low-boiling impurities such as water, lactic acid, meso-lactide, etc.; the heavy component extraction pipe 603 is used to extract high-boiling impurities such as oligomers.

[0035] The first crystallizer 10 and the second crystallizer 30 are vertical or horizontal jacketed crystallizers with wall scraping and stirring, which are used for primary and secondary suspension crystallization separation respectively; the falling film crystallizer 50 is a common falling film crystallizer. After the fluid enters, it falls in the form of a film on the inner wall of the tube, forming a liquid film in each heat exchange tube. The liquid film flows from the heat exchange tube from top to bottom while crystallizing due to the gradual decrease in temperature, and the generated crystals are recovered. The first solid-liquid separator 20 and the second solid-liquid separator 40 are common solid-liquid separators, preferably membrane separators, and the membrane material is preferably an inorganic inert material. The pore size can be selected according to the size of the crystal particles. In the separation process, the solid phase that cannot be passed through will be output as a solid phase under the entrainment of part of the liquid phase, and the permeated mother liquor will be output as a liquid phase.

[0036] As described above, the first buffer tank 70 is provided with heating, and is used to melt and mix the crude lactide raw material with one or a combination of the mother liquor from the second solid-liquid separator 40 and the recovered lactide from the distillation tower 60; the second buffer tank 80 is used to melt and mix the crystals separated by the first solid-liquid separator 20 with the mother liquor from the falling film crystallizer 50; the third buffer tank 90 and the fourth buffer tank 100 are respectively used to store the solid phase of the second solid-liquid separator 40 and the mother liquor of the first separator 20, and provide raw materials for the falling film crystallizer 50 and the distillation tower 60 under the premise of maintaining a certain liquid level.

[0037] In a certain stable working condition, the raw material and the mother liquor from the second solid-liquid separator 40 and one or both of the recovered lactide from the distillation tower 60 enter the first buffer tank 70 for melt mixing, the raw material after melt mixing enters the first crystallizer 10 for suspension crystallization, the slurry after crystallization enters the first solid-liquid separator 20 for solid-liquid separation, the separated mother liquor enters the fourth buffer tank 100 and then enters the distillation tower 60 for vacuum distillation separation, the light component impurities rich in water, lactic acid, and meso-lactide are discharged from the light component extraction pipe 602 at low temperature, and the lactide in the middle is returned to the first buffer tank 70 through the recovery pipe 601, and the heavy component impurities rich in lactic acid oligomers are discharged from the system through the heavy component extraction pipe 603. The crystals entering the first solid-liquid separator for separation are mixed with the mother liquor from the falling film crystallizer 50 in the second buffer tank 80 and then enter the second crystallizer 30 for suspension crystallization. The crystal slurry after crystallization enters the second solid-liquid separator 40 for solid-liquid separation. The separated mother liquor returns to the first buffer tank 70. The crystals entering the second solid-liquid separator 40 for separation enter the third buffer tank 90 and then enter the falling film crystallizer 50 for falling film crystallization. The mother liquor after falling film crystallization returns to the second buffer tank 80. The crystals after falling film crystallization are extracted as products, thereby realizing the purification of L-lactide.

[0038] In the above process, the first crystallizer 10 and the second crystallizer 30 respectively perform suspension crystallization on the material, and the falling film crystallizer 50 performs falling film crystallization. In addition, the distillation tower 60 performs distillation separation on the components that cannot be separated by crystallization to achieve the purification of L-lactide, thereby obtaining a high-purity product. The distillation tower 60 controls the top temperature of the tower under a certain vacuum to achieve the extraction of components with different boiling ranges. The boiling points of each component increase in sequence from water, lactic acid, meso-lactide, L-lactide, and oligomers. By controlling the temperature of the top and bottom of the tower, the light components of water, lactic acid, meso-lactide, and the oligomers of the heavy components are respectively extracted, and the L-lactide with an intermediate boiling point is separated; the above vacuum distillation conditions can avoid the polymerization of lactide due to excessively high temperature, thereby reducing the yield or purity of the product.

[0039] In particular, the utilization of the mother liquors from the first separator 20 and the second separator 40 further improves the yield of L-lactide.

[0040] In this embodiment, the crude lactide raw material contains the following components calculated by weight percentage: L-lactide 70-95%, meso-lactide 2%-15%, lactic acid 1%-5%, water 0.5%-2%, oligomers 1.5%-8%. The purity of L-lactide extracted after purification and separation is above 99.5wt%, and the yield of L-lactide can reach above 90%.

[0041] The following are specific experimental examples of the method for preparing high-purity L-lactide of the present invention, which are only preferred and do not represent limitations of the present invention.

[0042] Example 1

[0043] The crude lactide raw material at a temperature of 100° C. is mixed with other recycled materials at a flow rate of 300 kg / h into the first buffer tank 70, and then enters the first crystallizer 10 for suspension crystallization. The crude lactide raw material is composed of 94.7 wt% L-lactide, 2.1 wt% meso-lactide, 0.9 wt% lactic acid, 0.6 wt% water, and 1.7 wt% oligomers. The crystal slurry A after crystallization enters the first solid-liquid separator 20 for solid-liquid separation. After separation, the mother liquor A with a weight of 10% enters the fourth buffer tank 100 and then enters the distillation tower 60. The crystal A with a weight of 90% enters the second buffer tank 80; enters the distillation tower 60 for vacuum purification. After separation by distillation, 65% by weight of the recovered lactide returns to the first buffer tank 70, and the remaining 35% of impurities are discharged out of the system; the crystals A separated by the first solid-liquid separator 20 and the mother liquor C from the falling film crystallizer 50 enter the second buffer tank 80 for mixing and then enter the second crystallizer 30 for suspension crystallization, and the crystal slurry B after crystallization enters the second solid-liquid separator 40 for solid-liquid separation, and the mother liquor B with a weight of 5% after separation returns to the first buffer tank 70, and the crystals B with a weight of 95% enter the third buffer tank 90 and then enter the falling film crystallizer 50 for falling film crystallization, and the mother liquor C with a weight of 4% after crystallization returns to the second buffer tank 80 for suspension crystallization, and the crystals C with a weight of 96% are discharged out of the system as products.

[0044] The purity of L-lactide obtained in this example is 99.8%, and the yield is 93%.

[0045] Example 2

[0046] like Figure 1As shown, a crude lactide raw material at a temperature of 100° C. is mixed with other recycled materials at a flow rate of 300 kg / h into a first buffer tank 70, and then enters a first crystallizer 10 for suspension crystallization, wherein the crude lactide raw material is composed of 90.2 wt % L-lactide, 4.5 wt % meso-lactide, 2 wt % lactic acid, 0.9 wt % water, and 2.4 wt % oligomers. The crystal slurry A after crystallization enters a first solid-liquid separator 20 for solid-liquid separation, and the mother liquor A with a weight of 16% after separation enters a fourth buffer tank 100 and then enters a distillation tower 60, and the crystal A with a weight of 84% enters a second buffer tank 80; enters a distillation tower After vacuum distillation separation at 60, 60% of the recovered lactide by weight returns to the first buffer tank 70, and the remaining 40% of impurities are discharged out of the system; the crystals A and the mother liquor C from the falling film crystallizer enter the second buffer tank 80 for mixing and then enter the second crystallizer 30 for suspension crystallization, and the crystal slurry B after crystallization enters the second solid-liquid separator 40 for solid-liquid separation, and after separation, 10% of the mother liquor B by weight returns to the first buffer tank 70, and 90% of the crystals B enter the third buffer tank 90 and then enter the falling film crystallizer 50 for falling film crystallization, and 5% of the mother liquor C after crystallization returns to the second buffer tank 80 for suspension crystallization, and 95% of the crystals C by weight are discharged out of the system as products.

[0047] The purity of L-lactide obtained in this example is 99.6%, and the yield is 92.2%.

[0048] Example 3

[0049] like Figure 1As shown, a crude lactide raw material at a temperature of 100° C. is mixed with other recycled materials at a flow rate of 300 kg / h into a first buffer tank 70, and then enters a first crystallizer 10 for suspension crystallization, wherein the crude lactide raw material is composed of 80.5 wt % L-lactide, 8.8 wt % meso-lactide, 4 wt % lactic acid, 1.3 wt % water, and 5.4 wt % oligomers. The crystal slurry A after crystallization enters a first solid-liquid separator 20 for solid-liquid separation, and the mother liquor A with a weight of 35% after separation enters a fourth buffer tank 100 and then enters a distillation tower 60, and the crystal A with a weight of 65% enters a second buffer tank 80; enters a distillation tower After vacuum distillation separation at 60, 59% by weight of the recovered lactide returns to the first buffer tank 70, and the remaining 41% of impurities are discharged out of the system; the crystals A and the mother liquor C from the falling film crystallizer enter the second buffer tank 80 for mixing and then enter the second crystallizer 30 for suspension crystallization, and the crystal slurry B after crystallization enters the second solid-liquid separator 40 for solid-liquid separation. After separation, the mother liquor B with a weight of 17% returns to the first buffer tank 70, and the crystals B with a weight of 83% enter the third buffer tank 90 and then enter the falling film crystallizer 50 for falling film crystallization. After crystallization, the mother liquor C with a weight of 9% returns to the second buffer tank 80 for suspension crystallization, and the crystals C with a weight of 91% are discharged out of the system as products.

[0050] The purity of L-lactide obtained in this example is 99.5%, and the yield is 91.5%.

[0051] Example 4

[0052] like Figure 1As shown, a crude lactide raw material at a temperature of 100° C. is mixed with other recycled materials at a flow rate of 300 kg / h into a first buffer tank 70, and then enters a first crystallizer 10 for suspension crystallization, wherein the crude lactide raw material comprises 72.1 wt % L-lactide, 14.2 wt % meso-lactide, 4.8 wt % lactic acid, 1.8 wt % water, and 7.1 wt % oligomers. The crystal slurry A after crystallization enters a first solid-liquid separator 20 for solid-liquid separation, and the mother liquor A with a weight of 49% after separation enters a fourth buffer tank 100 and then enters a distillation tower 60, and the crystal A with a weight of 51% enters a second buffer tank 80; enters a distillation tower 60, and then enters a second buffer tank 80; enters a distillation tower 60, and then enters a second crystallizer ... first crystallizer 80; enters a distillation tower 60, and then enters a first After vacuum distillation and separation in tower 60, 57% by weight of the recovered lactide returns to the first buffer tank 70, and the remaining 43% of impurities are discharged out of the system; crystal A comes from the mother liquor C of the falling film crystallizer and enters the second buffer tank 80 for mixing and then enters the second crystallizer 30 for suspension crystallization, and the crystal slurry B after crystallization enters the second solid-liquid separator 40 for solid-liquid separation, and the mother liquor B with a weight of 33% after separation returns to the first buffer tank 70, and the crystal B with a weight of 67% enters the third buffer tank 90 and then enters the falling film crystallizer 50 for falling film crystallization, and the mother liquor C with a weight of 14% after crystallization returns to the second buffer tank 80 for suspension crystallization, and the crystal C with a weight of 86% is discharged out of the system as a product.

[0053] The purity of L-lactide obtained in this example is 99.5 wt % and the yield is 91.7%.

[0054] Comparative Example 1

[0055] In this comparative example, the first crystallizer 10 is used for a suspended crystallization, and the falling film crystallizer 50 is used for a falling film crystallization (the second buffer tank 80 is directly connected to the falling film crystallizer 50). The crude product composition and other operating conditions are the same as those in Example 4. After the above purification, the purity of the crystals extracted from the falling film crystallizer 50 is 97.7wt%, and the yield is 69.3%.

[0056] Comparative Example 2

[0057] This comparative example does not use the distillation tower 50 for distillation and purification. The first liquid phase output pipe 202 is connected to the crude product feed pipe 701. The crude product composition and other operating conditions are the same as those in Example 4. After the above purification, the purity of the crystals extracted from the falling film crystallizer 50 is 99.5wt%, and the yield is 40.5%.

[0058] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A device for preparing high-purity L-lactide, characterized in that: It comprises a first crystallization separation unit for crystallization and solid-liquid separation, a second crystallization separation unit, a distillation unit (60) for removing light and heavy fractions and recovering L-lactide, and a falling film crystallization unit (50); The first crystallization separation unit and the second crystallization separation unit are respectively used for suspension crystallization separation; The feed end and the recovery discharge end of the distillation unit (60) are connected to the liquid phase discharge end and the crystallization feed end of the first crystallization separation unit correspondingly; The feed end of the second crystallization separation unit is connected to the solid phase discharge end of the first crystallization separation unit; The feeding end of the falling film crystallization unit (50) is connected to the solid phase discharging end of the second crystallization separation unit, and a high-purity product is produced from the solid phase discharging end of the falling film crystallization unit (50); The liquid phase discharge end of the falling film crystallization unit (50) is connected to the crystallization feed end of the second crystallization separation unit; The liquid phase output end of the second crystallization separation unit is connected to the crystallization feed end of the first crystallization separation unit; The distillation unit (60) is provided with a light component extraction pipe (602) at the top, a recovery pipe (601) in the middle, and a heavy component extraction pipe (603) at the bottom; the recovery pipe (601) is connected to the crystallization feed end of the first crystallization separation unit; The device further comprises a first buffer storage tank (70), and the recovery pipe (601) is connected to the first buffer storage tank (70); the crude L-lactide and the top reflux material enter the first crystallization separation unit from the first buffer storage tank (70).

2. The device according to claim 1, characterized in that The first crystallization separation unit and the second crystallization separation unit use membrane separation.

3. A method for preparing high-purity L-lactide, characterized in that: The steps include: S1. performing suspension crystallization on the crude L-lactide and separating the crystal suspension A and the mother liquor A; S2. The mother liquor A is subjected to distillation to remove light and heavy substances, and the L-lactide recovery component at the top of the tower is collected and incorporated into the crude L-lactide product described in step S1; the crystal suspension A is resuspended and crystallized and separated to obtain a crystal suspension B; S3. The crystalline suspension B is subjected to falling film crystallization to obtain a high purity product; The mother liquor formed in the re-suspension crystallization separation process in step S2 is incorporated into the crude L-lactide product in step S1; the mother liquor formed in the falling film crystallization in step S3 is incorporated into the crystal suspension A in step S2; The crude L-lactide also contains meso-lactide, lactic acid, water and oligomers; in step S2, the heavy components are removed from the bottom of the tower to obtain oligomers, and the light components are removed from the top of the tower to obtain meso-lactide, lactic acid and water, which are then incorporated into the crude L-lactide in step S1.

4. The method according to claim 3, characterized in that The L-lactide content in the lactide crude product is 70-95wt%.

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