A lactide microcrystal purification system and purification method

Abstract

The application discloses a lactide microcrystal purification system and a purification method and a purification process. The lactide microcrystal purification system and the purification method comprise a continuous microcrystal unit, the continuous microcrystal unit comprises a continuous microcrystalizer and a microcrystal temperature control pry block, lactide can be rapidly nucleated in the continuous microcrystalizer, and the lactide is slowly grown into lactide microcrystals; the continuous microcrystalizer is connected with a lactide crystal slurry conveying pump through a pipeline; a filtration and drying unit comprises a filter, a washing tower and a drying tower; high-purity lactide crystals are obtained after lactide crystal slurry is filtered and dried; and filtrate is introduced into a static crystallization unit for recovery. The crude lactide purification system provided by the application has low energy consumption, high product purity and high yield, is suitable for lactide industrial purification, and is suitable for high-quality polylactic acid product production processes.

Description

Technical Field

[0001] This invention relates to the field of lactide purification technology, specifically to a lactide microcrystallization purification system, purification method, and purification process. Background Technology

[0002] With the implementation of my country's carbon peaking and carbon neutrality strategy and the continuous advancement of plastic restriction and ban policies, the development and application of green chemical new materials are of great significance to the implementation of these policies. Among them, polylactic acid (PLA), as a completely bio-based biodegradable polymer material, has gained popularity due to its high strength and excellent comprehensive performance. Currently, global PLA production capacity is rapidly increasing, with leading foreign PLA companies occupying the majority of the global market share. NatureWorks and Total Corbion, the two major PLA producers, have capacities of 150,000 tons / year and 75,000 tons / year, respectively. The main reason for these two companies' dominant market share lies in the technological barriers of the PLA industry, possessing a high-purity, low-cost preparation process for PLA intermediate—lactide. Industrially, PLA is often prepared using a two-step method: lactic acid molecules are first dehydrated to generate oligolactic acid, which is then cleaved and cyclized to obtain lactide. The lactide monomer is then ring-opening polymerized to form high-molecular-weight PLA products. Research results indicate that the purity of lactide, the intermediate in the two-step process, has a significant impact on the quality of the final polylactic acid (PLA). Therefore, obtaining high-purity lactide monomers is crucial for this process. However, my country's PLA industry started relatively late and currently faces numerous challenges in both technology and industrialization, such as low-level PLA production processes, high costs, and poor stability. The lactide production process suffers from severe racemization and purification difficulties, significantly affecting the quality and stability of PLA products. Therefore, the preparation of high-purity, low-cost lactide has become a critical technological bottleneck in my country's PLA production process. Because lactide contains two chiral carbon atoms, it exists in three isomers: L-lactide, D-lactide, and Meso-lactide. Limited by current lactide synthesis equipment and processes, the crude lactide obtained through pyrolysis is prone to racemization at high temperatures, generating meso-lactide. Furthermore, it contains byproducts such as lactic acid and lactic acid oligomers, making the separation and purification of L-lactide difficult. L-lactide has a boiling point of 304.5℃, Meso-lactide has a boiling point of 296.8℃, and lactic acid has a boiling point of 230.8℃. Therefore, it is difficult to achieve good separation results using the commonly used industrial distillation unit, and it is necessary to use it under reduced pressure and high temperature conditions, which consumes a lot of energy.

[0003] Melt crystallization, as a green chemical separation method, is a novel chemical unit operation that separates mixtures through solid-liquid phase equilibrium and is increasingly widely used in the chemical industry. Studies have shown that L-lactide, Meso-lactide, and lactic acid are all eutectic and completely immiscible systems, suitable for separation and purification via melt crystallization. Although many processes employ a distillation-crystallization coupling method to purify crude lactide—for example, patents CN110498787A and CN113387920A report methods for separating and purifying crude lactide by first distilling and then crystallizing—advanced distillation systems for crude lactide suffer from complex operating conditions and certain side reactions at high temperatures. Particularly under reduced pressure, the boiling points of L-lactide and Meso-lactide are close, requiring precise control of process parameters for separation via distillation, and also resulting in high energy consumption. In addition, patent CN112500389A reported a crude lactide refining process of crystallization followed by distillation. Although the operation conditions of crystallization are relatively mild and can reduce the occurrence of side reactions, the process has low operational flexibility and requires high purity of raw materials to obtain high-purity L-lactide. Moreover, no recovery process for the by-product Meso-lactide has been designed.

[0004] In summary, while existing patents have solved the problem of lactide purification in industry to some extent, issues such as obtaining high-purity lactide with low energy consumption and recovering its by-products still need to be addressed. Summary of the Invention

[0005] To address the shortcomings mentioned in the background art, the present invention aims to provide a lactide microcrystallization purification system and method. The crude lactide purification system provided by the present invention has low energy consumption, high product purity, and high yield. The entire purification process is carried out at a low temperature, avoiding the occurrence of side reactions. It is suitable for polylactic acid industrial production. The L-lactide monomer obtained by purifying lactide using the system of the present invention can reach a purity of 99.9% and a yield as high as 85-92%.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A lactide microcrystallization purification system includes: a continuous microcrystallization unit, which includes a lactide microcrystallizer, a temperature control skid, and a filter drying skid;

[0008] The static crystallization recovery unit includes a static crystallizer, which is connected to a recovery tank, a mother liquor tank, and a sweat tank via pipelines.

[0009] A method for purifying lactide using microcrystals includes the following steps:

[0010] S1. The crude lactide raw material obtained from the upstream is pumped into the lactide microcrystal crystallizer C-101. The temperature of the crystallizer is maintained at 50~70℃ by the temperature control skid, and the crystal size is controlled to not exceed 1000μm.

[0011] S2. The lactide slurry obtained in step S1 is transported to the filtration and drying unit via the bottom rotor pump P-102 of the microcrystallizer. In this unit, solid-liquid separation is first completed through a filter. The solid phase is thrown into a washing tower for washing. The washing liquid is a high-purity lactide solution. The washed solid phase enters the drying tower for drying. The drying temperature is controlled at 70~90℃ to obtain a high-purity lactide crystal product with a purity greater than 99.9%. Alternatively, the washed solid phase is passed through a melting tank to obtain a high-purity lactide liquid product with a melting temperature set at 100~120℃.

[0012] S3. The filtrate obtained from the filter in step S2 is pumped into the static crystallization recovery unit via a centrifugal pump: First, the temperature is lowered using a temperature-controlled skid at a rate of 5-10℃ / h, and the crystallization temperature is controlled at 55-65℃; the uncrystallized mother liquor enters the mother liquor tank V-202 and is pumped into the upstream reaction unit via pump P-202; then, sweating is performed, with the heating rate controlled at 5-20℃ / h using a temperature-controlled skid, and the final temperature controlled at 88-95℃; the resulting sweat enters the sweat tank V-203 and is pumped back to the static crystallizer C-201 via pump P-203; finally, the crystals are rapidly heated and melted at a temperature controlled at 100-120℃, and the lactide product is obtained and enters the recovery tank, which is then pumped into the crude lactide raw material tank V-101 via pump P-201.

[0013] The beneficial effects of this invention are:

[0014] The crude lactide purification system provided by this invention has low energy consumption, high product purity and yield, and is simple to operate. The entire process is carried out at low temperature under normal pressure, which is safe and reliable. At the same time, the low temperature can avoid the occurrence of side reactions such as oligomerization and racemization. It is suitable for polylactic acid industrial production. The purity of L-lactide monomer obtained by purifying lactide using the system of this invention can reach more than 99.9%, and the yield is as high as 85%~92%. Attached Figure Description

[0015] The invention will now be further described with reference to the accompanying drawings.

[0016] Figure 1 This is a diagram of the microcrystallization purification system of the lactide microcrystallization purification system and purification method of the present invention.

[0017] Figure 2 This is a schematic diagram of the microcrystallizer unit of the lactide microcrystallization purification system and purification method of the present invention.

[0018] Figure 3This is a schematic diagram of the static crystallization unit of the lactide microcrystallization purification system and method of the present invention.

[0019] In the picture:

[0020] V-101 crude lactide raw material tank, P-101 raw material transfer pump, C-101 lactide microcrystal crystallizer, P-102 crystal slurry transfer pump, W-101 filter drying skid, C-201 static crystallizer, P-201 recovery transfer pump. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] like Figure 1 As shown, a lactide microcrystallization purification system and purification method includes: a continuous microcrystallization unit, which includes a lactide microcrystallizer. The lactide microcrystallizer is crystallized and the temperature is controlled by a temperature control skid. The obtained crystal slurry enters a filter drying skid for filtration, washing and drying to obtain a solid or liquid product.

[0023] The static crystallization recovery unit includes a static crystallizer, which is connected to a recovery tank, a mother liquor tank, and a sweat tank via pipelines. The sweat from the static crystallizer is recycled back to the plate crystallizer, the mother liquor is returned to the upstream reaction unit, and the product is recycled back to the microcrystallizer.

[0024] A method for purifying lactide using microcrystals includes the following steps:

[0025] S1. The crude lactide raw material obtained from the upstream is pumped into the lactide microcrystal crystallizer C-101. The temperature of the crystallizer is maintained at 50~70℃ by the temperature control skid, and the crystal size is controlled to not exceed 1000μm.

[0026] S2. The lactide slurry obtained in step S1 is transported to the filtration and drying unit via the bottom rotor pump P-102 of the microcrystallizer. In this unit, solid-liquid separation is first completed through a filter. The solid phase is thrown into a washing tower for washing. The washing liquid is a high-purity lactide solution. The washed solid phase enters the drying tower for drying. The drying temperature is controlled at 70~90℃ to obtain a high-purity lactide crystal product with a purity greater than 99.9%. Alternatively, the washed solid phase is passed through a melting tank to obtain a high-purity lactide liquid product with a melting temperature set at 100~120℃.

[0027] S3. The filtrate obtained from the filter in step S2 is pumped into the static crystallization recovery unit via a centrifugal pump: First, the temperature is lowered using a temperature-controlled skid at a rate of 5-10℃ / h, and the crystallization temperature is controlled at 55-65℃; the uncrystallized mother liquor enters the mother liquor tank V-202 and is pumped into the upstream reaction unit via pump P-202; then, sweating is performed, with the heating rate controlled at 5-20℃ / h using a temperature-controlled skid, and the final temperature controlled at 88-95℃; the resulting sweat enters the sweat tank V-203 and is pumped back to the static crystallizer C-201 via pump P-203; finally, the crystals are rapidly heated and melted at a temperature controlled at 100-120℃, and the lactide product is obtained and enters the recovery tank, which is then pumped into the crude lactide raw material tank V-101 via pump P-201.

[0028] Example 1

[0029] The crude lactide purified in this case consisted of: 93% L-lactide, 5% M-lactide, 0.5% lactic acid, and 1.5% oligomers.

[0030] The crude lactide in crude lactide raw material tank V-101 is pumped into lactide microcrystal continuous crystallizer by raw material transfer pump P-101. The crystallizer wall temperature is controlled at 55°C by temperature control skid. At this temperature, lactide nucleates in large quantities under the flow field inside the microcrystal and "slowly" grows into microcrystal state.

[0031] The obtained lactide microcrystalline slurry is pumped to a filter via pump P-102 to filter the filtrate. The resulting lactide crystals are then thrown into a washing tower and washed with 99% pure L-lactide liquid phase to obtain high-purity lactide. Subsequently, the product is dried in a drying tower at 80℃ to obtain a lactide product with a purity of 99.93%.

[0032] The filtrate enters the static crystallization recovery unit, where it is rapidly cooled from 100°C to 85°C via a temperature-controlled skid, then further cooled to 50°C at a rate of 10°C / h, and then held at 50°C for 1 hour to allow complete crystallization. The mother liquor is then discharged into mother liquor tank V-202. After the mother liquor is discharged, a heating and sweating process is initiated, with the temperature increased to 90°C at a rate of 8°C / h, and then held at 90°C for 1 hour to allow complete sweating. The sweat is then discharged into sweat tank V-203. After the sweat is discharged, the temperature is rapidly increased to 100°C to melt the crystals, and the resulting product is discharged into recovery tank V-201. The product purity is 92%.

[0033] In this example, the yield of L-lactide can reach 88.5%.

[0034] Example 2 and Example 3 were carried out with reference to the process flow of Implementation Case 1. The specific conditions and result parameters are shown in Table 1:

[0035] Table 1 Parameters of Examples 2 and 3

[0036]

[0037] Through the above three embodiments, the purification process for crude lactide provided by the present invention adopts a method that couples continuous microcrystallization and static crystallization. First, a crystallization process is used to refine and purify L-lactide by utilizing the difference in melting points of substances, resulting in refined lactide with a purity of over 99.9%. This process has low temperature, few side reactions, and low energy consumption. Through precise temperature control and flow field control, lactide nucleates in large quantities in the continuous microcrystallizer and "slowly" grows into high-purity lactide microcrystals. The final product is obtained through subsequent filtration, washing, and drying.

[0038] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0039] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for purifying lactide through microcrystallization, characterized in that, The method employs a lactide microcrystallization purification system, the system comprising: A microcrystallization continuous crystallization unit, comprising a lactide microcrystallizer (C-101), wherein the lactide microcrystallizer (C-101) is subjected to crystallization temperature control by a temperature control skid, the temperature control skid comprising a heater and a cooler; A filtration and drying unit is used to purify the solid phase of lactide slurry, including a filter, a washer, and a dryer, which are connected by pipelines. The static crystallization unit includes a static crystallizer (C-201), a recovery tank (V-201), a mother liquor tank (V-202), and a sweat tank (V-203). The static crystallizer (C-201) is connected to the recovery tank (V-201), the mother liquor tank (V-202), and the sweat tank (V-203) through pipes. The method includes the following steps: S1. The crude lactide raw material obtained from the upstream is pumped into the lactide microcrystal crystallizer (C-101). The temperature of the lactide microcrystal crystallizer is maintained at 50-70℃ and the crystal size is controlled not to exceed 1000μm by the temperature control skid. S2. The lactide slurry obtained in step S1 is transported to the filtration and drying unit: solid-liquid separation is completed in this unit, the solid phase is washed, the washing liquid is a high-purity lactide solution, the washed solid phase is dried, and the drying temperature is controlled at 70-90℃, thereby obtaining a high-purity lactide crystalline product, or the washed solid phase is melted to obtain a high-purity lactide liquid product. S3. The filtrate obtained from the filter in step S2 is fed into the static crystallization recovery unit: First, it is cooled down at a rate of 5-10℃ / h and the crystallization temperature is controlled at 55-65℃; the uncrystallized mother liquor enters the upstream reaction unit; then, sweating is performed and the resulting sweat is returned to the static crystallizer (C-201); finally, the crystals are rapidly heated and melted to obtain lactide product which enters the crude lactide raw material tank (V-101).

2. The method for purifying lactide by microcrystallization according to claim 1, characterized in that, It also includes a crude lactide raw material tank (V-101), which is connected to the lactide microcrystal crystallizer (C-101) via a raw material transfer pump (P-101).

3. The method for purifying lactide by microcrystallization according to claim 1, characterized in that, The temperature control medium of the temperature control skid is an aqueous solution of ethylene glycol.

4. The method for purifying lactide by microcrystallization according to claim 1, characterized in that, It also includes a filter drying skid (W-101), which includes a drying tower for obtaining L-lactide crystalline product and a melter for obtaining L-lactide liquid product.

5. The method for purifying lactide by microcrystallization according to claim 1, characterized in that, The static crystallizer (C-201) is connected to the recovery tank (V-201), the mother liquor tank (V-202), and the sweat tank (V-203) via feed pipelines.

6. The method for purifying lactide by microcrystallization according to claim 1, characterized in that, The static crystallized product in the recovery tank (V-201) is recycled to the microcrystalline raw material tank through a pipeline for reuse.

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