A suspended crystallization purification device and its application in lactide purification

Abstract

The present invention discloses a large-flux suspension crystallization purification device and its application in the purification of lactide, including a crystal washing tower and a crystal slurry melter, the crystal washing tower includes a separation section A, a refining section B and a washing section C, the crystal is crystallized and precipitated in the separation section and enters the refining section, a washing mother liquor outlet is provided above the separation section A, the washing mother liquor is discharged from the washing mother liquor outlet, the refining section B is provided with a stirring paddle, in the refining section B, the crystal grows and enters the washing section C, and is deposited in the washing section C to form a crystal accumulation bed, the lower end of the washing section is provided with a scraper, the bottom of the washing section is provided with a discharge port, the discharge port is connected to the crystal slurry melter, the lower end of the washing section is also provided with a liquid inlet and a distributor, the liquid inlet is connected to the crystal slurry melter. The present invention can be applied to the crystal washing process of purifying lactide by suspension crystallization, can significantly improve the lactide crystallization yield, and reduce the number of equipment and investment.

Description

Technical Field

[0001] The invention belongs to the field of separation and purification equipment, and in particular relates to a suspension crystallization purification equipment and its application in the field of purification of lactide. Background Art

[0002] Polylactic acid (PLA), also known as polylactide, is a biodegradable polymer material made from renewable plant resources through chemical synthesis. In recent years, the pollution of non-degradable plastic microspheres has become increasingly serious, attracting widespread attention around the world. Biodegradable plastics have become the most advantageous weapon to solve this stubborn problem at this stage. The global market for PLA is growing rapidly and is expected to be in short supply for a long time.

[0003] Given the current technological level, the technology for directly synthesizing PLA using lactic acid as raw material is still immature, and the product quality cannot meet the general market demand. At present, the industrial production of PLA generally adopts the lactide ring-opening polymerization method, that is, lactic acid is first synthesized into lactide, and then lactide is ring-opening polymerized to synthesize PLA in a two-step process. As a key intermediate for synthesizing PLA, the synthesis and purification process of lactide and key equipment are the key research directions in the industry.

[0004] In the preparation process of PLA, in order to obtain higher molecular weight and product performance, the impurities of raw material lactide such as isomers, moisture, acid content, etc. are very strict. Generally, the moisture content is controlled to be <100ppm, the acid content is <7meq / kg, and the optical purity is >99.5%. However, due to the relationship between physical properties, the removal of acid content and isomers is very difficult, and researchers have made different attempts in history. For example, EP0893462A2, US6277951B1, WO95 / 09879, CN100586921C, CN101821315A, CN107531663, etc. disclose distillation schemes. However, removing the above impurities by distillation consumes a lot of energy, and more side reactions will occur under the high temperature conditions of the distillation operation, sacrificing the product yield. Researchers have also developed other separation and purification technologies, such as the melt crystallization process disclosed in US5264592, US6310218, and CN208426686, and the distillation-recrystallization technology disclosed in patents such as CN105324164, CN105814110A, and CN10528994A.

[0005] Suspended melt crystallization, as a new type of high-purity purification technology, has received widespread attention, especially in the field of purification of heat-sensitive systems, where it has significant advantages. Suspended crystallization is generally divided into a crystallization unit and a washing unit. The crystallization unit achieves crystallization purification through the controlled growth of fine particles suspended in the molten liquid phase. The washing unit uses a high-purity molten product to wash the impurities adhering to the surface of the crystal, thereby further extracting a high-purity product. The washing equipment is the core equipment of suspended crystallization. In the prior art, the suspended crystallization washing equipment that has been industrially applied mainly includes gravity bed conveying washing columns, hydraulic crystal bed conveying washing columns, and mechanical bed conveying washing columns. Because the gravity conveying form generally requires a longer residence time and has lower production efficiency, the hydraulic and mechanical conveying forms can generally significantly reduce the residence time required for the washing column, and are the direction of development in the industry.

[0006] Patent CN112704901A discloses a melt crystallizer, which is divided into a crystallization zone, a crystal bed zone and a melting zone along the axial direction of the device. The liquid phase in the melting zone flows upward through the crystal bed to complete the washing effect, and the final refined product is obtained in the melting zone. This is a washing device that relies on gravity transportation, and still has the problems of low driving force, low processing capacity, and poor washing and purification effect caused by the back mixing of crystals and washing mother liquor.

[0007] Patent CN1422240A discloses a washing column with an axial filter, which has not been used in the purification of lactide systems due to its complex structure, easy clogging, and difficulty in cleaning. Currently, the commercially available ones are mainly horizontally arranged oscillating piston washing equipment with a filter end face and a residual melt discharge port.

[0008] Many scholars and process technology companies in the industry have developed piston-type suspension crystallization and washing technology. For example, Switzerland's Sulzer Company and the Netherlands' GEA Messo PT Company both use piston-type washing towers with filters. The reciprocating motion of the piston plays the role of liquid absorption, filtration and drainage, washing, etc. However, the above processes and equipment also have the following disadvantages: complex structure, high requirements for piston processing and maintenance, high equipment cost, and limited maximum processing capacity of a single set of equipment (the current maximum processing capacity of a single set of commercial piston washing towers is about 8000-10000t / a), which limits the large-scale equipment, high investment, and high production cost.

[0009] In summary, although the existing disclosed separation and purification technology of lactide has solved the problems of lactide impurity separation and quality control to a certain extent, there is still room for improvement. In particular, the core equipment washing tower in the suspension crystallization process still has problems such as complex structure, high cost, inconvenient operation, and limited processing capacity of a single set of equipment, which seriously restricts production costs and thus affects the larger-scale commercial application of PLA. How to solve the above problems existing in the existing crystallization technology, especially the problems of suspension crystallization washing equipment, is an urgent problem to be solved in the industry. Summary of the invention

[0010] The object of the present invention is to provide a method for removing acid and isomers from lactide more efficiently and at a lower cost. The method can more efficiently remove impurities such as acid and isomers that are difficult to remove in traditional separation processes, simplify production equipment, and reduce production costs.

[0011] In order to achieve the above invention object, the technical solution of the present invention is as follows:

[0012] A large-throughput suspended crystallization purification equipment, characterized in that it includes a crystallization washing tower 2 and a slurry melter 3, the crystallization washing tower 2 includes a separation section A, a refining section B and a washing section C, a feed inlet 201 is provided at the lower end of the separation section A, crystals are crystallized and precipitated in the separation section and enter the refining section, a washing mother liquor outlet 207 is provided above the separation section A, the washing mother liquor is discharged from the washing mother liquor outlet 207, the refining section B is provided with a stirring paddle 202, in the refining section B, the crystals continue to grow, the grown crystals enter the washing section C, and are deposited in the washing section C to form a crystal accumulation bed, a scraper 203 is provided at the lower end of the washing section, a discharge port 205 is provided at the bottom of the washing section, the discharge port 205 is connected to the slurry melter 3, a liquid inlet 206 and a distributor 204 are also provided at the lower end of the washing section, and the liquid inlet 206 is connected to the slurry melter 3.

[0013] The raw material slurry enters the separation section. Due to the low temperature of the separation section, crystals are precipitated. The washing mother liquor is discharged from the washing mother liquor outlet of the separation section. The crystals precipitated in the separation section enter the refining section. Under the stirring action of the refining section, the crystals are mainly suspended in the refining section. The crystals gradually grow in the refining section. After the crystals grow up, they enter the washing section and deposit to form a crystal accumulation bed of a certain height. The scraper is arranged at the lower end of the crystal accumulation bed, which plays a supporting role for the above-mentioned crystal accumulation bed. The rotating scraper cuts the crystal accumulation bed, and the crystals at the bottom of the bed are cut into fine crystal particles, which enter the lower part of the scraper and are discharged from the discharge port of the crystal washing tower. The crystal slurry delivery pump enters the crystal slurry melter to produce a molten high-purity product and a washing liquid phase that returns to the crystal purification tower.

[0014] Preferably, the liquid phase empty tower flow rate of the slurry is 0.01-1m / s, preferably 0.05-0.5m / s. In order to control the distributor outlet to cause excessive impact on the crystal bed, the liquid flow rate of the liquid inlet 206 is controlled by increasing the number of distributors, and the liquid flow rate is generally controlled to be 0.1-5m / s, preferably 0.5-1m / s. The washing liquid phase passes through the small gaps between the crystal particles of the crystal bed layer, and achieves sufficient and good contact with the crystal to achieve the washing effect, and will not cause the collapse of the crystal bed layer and the short circuit of the washing liquid phase due to excessive disturbance, so as to affect the washing effect.

[0015] In some preferred embodiments of the present invention, the tower diameters of the separation section, the refining section and the washing section are the same or different. Preferably, the tower diameters of the separation section and the washing section are both larger than the tower diameter of the refining section.

[0016] In some preferred embodiments of the present invention, the tower diameter of the separation section is 1.2 to 2 times the tower diameter of the refining section, and the tower diameter of the washing section is 1.2 to 2 times the tower diameter of the refining section.

[0017] In order to separate the crystal particles from the mother liquor, the length of the separation section is >0.5 m, preferably >1.0 m, so that almost no crystal particles are entrained in the washing mother liquor. The length of the refining section is limited by the residence time of the solid particles, and generally ensures that the residence time of the crystal particles is 1-60 min, preferably 2-40 min, and more preferably 5-30 min.

[0018] In order to prevent the back-mixing of solid particles from affecting the final washing effect, the length of the washing section is generally set to be >0.5m, preferably >1m, and more preferably >2m to control the back-mixing to the smallest possible extent and ensure sufficient washing effect.

[0019] In principle, the crude product that needs to be suspended crystallized can directly enter the crystallization washing tower from the feed port between the separation section and the refining section, and the suspension crystallization separation is achieved under the cooling effect of the heat exchange jacket or heat exchange tube of the refining section and the suspension effect of the stirring paddle. However, based on the need for equipment amplification in industrial production, it is preferred that the raw material slurry is first subjected to preliminary crystallization through one or more stages of crystallization equipment before entering the crystallization washing tower. One or two or more stages of existing crystallization equipment are set outside the crystallization washing tower to reduce the refrigeration and stirring pressure of the separation section of the crystallization washing tower, especially the need for refrigeration and heat exchange area for suspension crystallization.

[0020] Preferably, a stirring crystallization kettle (1) is provided before the crystallization washing tower.

[0021] The raw material slurry is generally a solid-liquid two-phase containing certain crystal particles. In order to facilitate the transportation of the raw material slurry and ensure that the crystal purification tower operates in the process-optimized operation range, the solid content of the raw material slurry is generally required to be 0-80%, wherein the solid content is not equal to 0, and preferably the solid content is 5-50%. For example, in some embodiments, the solid content is 20-30% for the convenience of transportation. Or in order to improve the crystallization efficiency, the solid content of the raw material slurry is controlled to be 30-50%.

[0022] For the washing process of crystals of small-sized particles, in order to ensure sufficient contact between the solid and liquid phases and the stability of the flow field, in some preferred embodiments of the present invention, the crystal stacking bed height in the washing section is greater than 0.5m, preferably greater than 1m, and more preferably greater than or equal to 2m; considering the actual operational feasibility, the crystal stacking bed height is preferably less than 10m, and the most preferred bed height is 2-5m.

[0023] In some preferred embodiments of the present invention, the washing tower may use an outer jacket and / or an internally arranged heat exchange tube and perform temperature control. The temperature settings of the separation section, the refining section, and the washing section are successively increased; the specific temperature varies according to the actual situation of the material system and the purity requirements. The new suspension crystallization purification equipment disclosed in the present invention is applied to the suspension crystallization purification process of lactide, and is generally set in the range of 70-95°C. For example, in some preferred embodiments of the present invention, the temperatures of the separation section, the refining section, and the washing section are 70-75°C, 75-85°C, and 85-95°C, respectively.

[0024] In some preferred embodiments of the present invention, the suspension crystallization purification device of the present invention adopts continuous operation.

[0025] The present application also provides an application of the suspension crystallization purification device, which is used in the field of suspension crystallization purification of lactide.

[0026] Another object of the present invention is to provide a lactide product.

[0027] A lactide product is obtained by the above-mentioned lactide separation and purification method.

[0028] The positive effects of the present invention are:

[0029] 1. Compared with gravity-type washing column equipment, the new equipment and technology disclosed in the present invention have high efficiency, no backmixing, high compatibility with crystalline particles, and higher purity of washing products;

[0030] 2. Compared with the hydraulic conveying washing column equipment, the new equipment and technology disclosed in the present invention has no filter screen, is not easy to be blocked, can operate continuously for a long time, has high efficiency and high product purity;

[0031] 3. Compared with the known oscillating piston washing column equipment with filter screen, the new equipment and technical structure disclosed by the present invention are simplified, easy to scale up, and the processing capacity of a single set is significantly increased; and there is no filter screen, which is not easy to be blocked, and can be operated continuously for a long time, with high efficiency and high product purity. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic diagram of the structure of a suspension crystallization purification device according to an embodiment of the present invention;

[0033] Among them, 1 is a stirred crystallization kettle, 2 is a crystal washing tower, 201 is a feed inlet, 202 is a stirring paddle, 203 is a scraper, 204 is a distributor, 205 is a discharge port, 206 is a liquid inlet, 207 is a washing mother liquor outlet, and 3 is a slurry melter. DETAILED DESCRIPTION

[0034] The following further describes the embodiments of the present invention in conjunction with the accompanying drawings and examples. However, the present invention is not limited to the listed examples, but also includes any other known changes within the scope of the rights claimed by the present invention.

[0035] In the specific embodiment of the present invention, the crystallization of lactide is taken as an example for description.

[0036] Raw material information: The crude lactide used in the embodiments of the present invention is homemade, using the lactic acid product of Jiangxi Musashino Company as the raw material, the chemical purity of L-lactic acid is 88%, and the optical purity is 99.7%;

[0037] Characterization equipment: GC was used for quantitative detection of substances in the examples;

[0038] Chromatographic columns and chromatographic conditions

[0039]

[0040]

[0041] Draw the standard curves of various components, and the quantitative analysis of each component is carried out according to the following formula:

[0042] Substitute the sample peak area into the standard curve formula to calculate the sample concentration. The analysis results are reported in mg / L. The lactide isomer content is calculated using the external standard curve method:

[0043]

[0044] in,

[0045] ω L ——L-lactide concentration in the sample, %

[0046] A L ——L-lactide peak area

[0047] m——weigh the sample mass, g

[0048]

[0049] Among them, ω D ——D-lactide concentration in the sample, %

[0050] A D ——D-lactide peak area

[0051] m——weigh the sample mass, g

[0052]

[0053] in,

[0054] ω m ——Concentration of m-lactide in the sample, %

[0055] A m ——m-lactide peak area

[0056] m——weighed sample mass, g.

[0057] Example 1

[0058] Reference Figure 1 As shown, this embodiment provides a suspended crystallization purification device, including a stirred crystallization kettle 1, a crystal washing tower 2 and a slurry melter 3, the crystal washing tower 2 includes a separation section A, a refining section B and a washing section C, the lower end of the separation section A is provided with a feed inlet 201, the crude lactide to be purified enters the washing device through the feed inlet 201, the crystals are crystallized and precipitated in the separation section, and enter the refining section under the action of gravity, and the remaining washing mother liquor after crystallization is discharged from the washing mother liquor outlet 207 above the separation section A, the refining section B is provided with a stirring paddle 202, the crystals are suspended in the liquid phase under the stirring action, and the crystals are in the refining section. The section continues to grow, the crystals grow up and enter the washing section C, and are deposited in the washing section C to form a crystal accumulation bed. A scraper 203 is provided at the lower end of the washing section, and a discharge port 205 is provided at the bottom of the washing section. The discharge port 205 is connected to the slurry melter 3, and the crystals are discharged from the discharge port into the slurry melter 3. A liquid inlet 206 and a distributor 204 are also provided at the lower end of the washing section. The liquid inlet 206 is connected to the slurry melter 3. The crystals are partially melted in the slurry melter 3 for further purification. The purified crystals are taken out from the slurry melter 3, and the mother liquor enters the washing section from the liquid inlet for re-crystallization.

[0059] In this embodiment, the stirred crystallization kettle 1 has a diameter of 300mm and a height of 1.5m. It is equipped with 7-stage push-up stirring paddles and a crystallization temperature of 78°C. The separation section of the crystallization washing tower has a diameter of 300mm and a height of 500mm, a diameter of 200mm and a height of 500mm in the refining section, a diameter of 300mm and a height of 1.5m in the washing section, and L-lactide is the main product. The refining section is equipped with 7-stage stirring paddles, of which the first stage and the bottom stage above use push-down propellers, and the middle 5 stages use push-up propellers. A first-stage stirred crystallizer is set in front of the crystal purification tower. The crystallizer has a diameter of 300mm and a height of 1.5m. It is equipped with 7-stage push-up stirring paddles. The stirred crystallizer is refrigerated and temperature controlled by a jacket.

[0060] Calculated based on the total flow rate of the crude lactide stream, the total residence time of the lactide stream in the stirred crystallization kettle 1, the separation section, the refining section and the washing section of the crystal washing tower is 4 hours, wherein the temperatures of the separation section, the refining section and the washing section are 74°C, 82°C and 94.5°C respectively. The outlet temperature of the slurry melter is controlled at 95.0°C, and the circulating flow rate of the washing mother liquor is adjusted to control the solid content of the slurry entering the crystal purification tower to 30%.

[0061] The relevant results are shown in the following table, where L-LD represents L-lactide, m-LD represents m-lactide, D-LD represents D-lactide, and LA represents lactic acid.

[0062] Table 1 Mass content of each component before and after reaction

[0063]

[0064]

[0065] Example 2

[0066] The main difference between this embodiment and embodiment 1 is that the feed rate is increased to reduce the material residence time in the suspension crystallization purification tower. Calculated based on the total flow rate of the crude lactide stream, the total residence time is 2.5 h. The rest is the same as embodiment 1.

[0067] The relevant results are shown in the following table.

[0068] Table 2 Mass content of each component of the product

[0069]

[0070] Example 3

[0071] Reference Figure 1As shown, the main difference between this embodiment and embodiment 1 is that the separation section of the suspension crystallization purification tower has a diameter of 200 mm and a height of 300 mm, a refining section has a diameter of 200 mm and a height of 500 mm, and a washing section has a diameter of 200 mm and a height of 0.5 m. The relevant results are shown in the following table, in which L-LD represents L-lactide, m-LD represents m-lactide, D-LD represents D-lactide, and LA represents lactic acid.

[0072] Table 3 Mass content of each component of the crystalline product

[0073]

[0074] Comparing the data of Example 1 and Example 3, it can be seen that as the diameter of the washing section is shortened, the purity and yield of the final crystal product are reduced, and the acid value (LA and oligomers) that is crucial to the quality of the lactide product is significantly increased. This represents that the treatment effect and capacity of the crystal purification equipment have been significantly reduced. At the same time, as the length of the separation section is shortened, the solid content in the washing mother liquor increases, resulting in a decrease in purification efficiency, which is also unfavorable to the purification process.

[0075] Example 4

[0076] The main difference between this embodiment and embodiment 1 is that the separation section of the suspended crystallization washing tower has a diameter of 300 mm and a height of 500 mm, a diameter of 200 mm and a height of 500 mm, a diameter of 350 mm and a height of 2.5 m, and L-lactide is the main product.

[0077] The relevant results are shown in the following table, where L-LD represents L-lactide, m-LD represents m-lactide, D-LD represents D-lactide, and LA represents lactic acid.

[0078] Table 4 Mass content of each component of the crystalline product

[0079]

[0080] By comparing the data of Example 4 with those of Examples 1 and 3, it can be seen that by increasing the diameter of the washing section and the bed height, an ultra-high purity lactide product can be obtained.

Claims

1. A large-throughput suspension crystallization purification device, characterized in that: The invention comprises a crystal washing tower (2) and a crystal slurry melter (3). The crystal washing tower (2) comprises a separation section A, a refining section B and a washing section C. The lower end of the separation section A is provided with a feed inlet (201). Crystals are crystallized and precipitated in the separation section and enter the refining section. The upper part of the separation section A is provided with a washing mother liquor outlet (207). The washing mother liquor is discharged from the washing mother liquor outlet (207). The refining section B is provided with a stirring paddle (202). In the refining section B, the crystals grow continuously. The crystals grow and enter the washing section C, where they are deposited to form a crystal accumulation bed. A scraper (203) is provided at the lower end of the washing section. The scraper (203) is provided at the lower end of the crystal accumulation bed. A discharge port (205) is provided at the bottom of the washing section. The discharge port (205) is connected to the crystal slurry melter (3). A liquid inlet (206) and a distributor (204) are also provided at the lower end of the washing section. The liquid inlet (206) is connected to the crystal slurry melter (3). The tower diameter of the washing section C is larger than the tower diameter of the refining section B; The length of the separation section A is >0.5 m.

2. The suspension crystallization purification equipment according to claim 1, characterized in that: The raw material slurry enters the separation section A. Since the temperature of the separation section A is relatively low, crystals are precipitated, and the washing mother liquor is discharged from the washing mother liquor outlet of the separation section. The crystals precipitated from the separation section A enter the refining section B. Under the stirring action of the refining section B, the crystals are mainly suspended in the refining section B. The crystals gradually grow in the refining section B. After the crystals grow, they enter the washing section C and are deposited to form a crystal accumulation bed layer of a certain height; the scraper (203) is arranged at the lower end of the crystal accumulation bed layer, which plays a supporting role for the above-mentioned crystal accumulation bed layer. The rotating scraper cuts the crystal accumulation bed layer, and the crystals at the bottom of the bed layer are cut into fine crystal particles, which enter the lower part of the scraper and are discharged from the discharge port (205). The washing tower enters the crystal slurry melter (3) through the crystal slurry delivery pump to produce a molten product and a washing liquid phase that returns to the crystal washing tower (2).

3. The suspension crystallization purification equipment according to any one of claims 1-2, characterized in that: The tower diameter of the separation section A is larger than the tower diameter of the refining section B.

4. The suspension crystallization purification equipment according to claim 1 or 2, characterized in that: The tower diameter of the separation section A is 1.2 to 2 times that of the refining section B, and the tower diameter of the washing section C is 1.2 to 2 times that of the refining section B.

5. The suspension crystallization purification equipment according to claim 1, characterized in that: The length of the separation section A is >1.0 m.

6. The suspension crystallization purification equipment according to claim 1, characterized in that: The residence time of the crystal particles in the refining section B is 1-60 minutes.

7. The suspension crystallization purification equipment according to claim 6, characterized in that: The residence time of the crystal particles in the refining section B is 2-40 minutes.

8. The suspension crystallization purification equipment according to claim 7, characterized in that: The residence time of the crystal particles in the refining section B is 5-30 minutes.

9. The suspension crystallization purification equipment according to claim 1, characterized in that: The length of the washing section is >0.5m.

10. The suspension crystallization purification equipment according to claim 9, characterized in that: The length of the washing section is >1m.

11. The suspension crystallization purification equipment according to claim 10, characterized in that: The length of the washing section is >2m.

12. The suspension crystallization purification equipment according to claim 2, characterized in that: The raw material slurry is firstly crystallized by one or more than two stages of crystallization equipment before entering the crystallization washing tower.

13. The suspension crystallization purification equipment according to claim 1, characterized in that: A stirring crystallization kettle (1) is arranged in front of the crystallization washing tower.

14. The suspension crystallization purification equipment according to claim 1, characterized in that: The solid content of the raw material slurry is 0-80%, wherein the solid content is not equal to 0.

15. The suspension crystallization purification equipment according to claim 14, characterized in that: The solid content of the raw material slurry is 5 to 50%.

16. The suspension crystallization purification equipment according to claim 1, characterized in that: The crystal accumulation bed height in the washing section is greater than 0.5m.

17. The suspension crystallization purification equipment according to claim 16, characterized in that: The crystal accumulation bed height in the washing section is greater than 1m.

18. The suspension crystallization purification equipment according to claim 17, characterized in that: The crystal accumulation bed height in the washing section is greater than or equal to 2m.

19. The suspension crystallization purification equipment according to claim 16, characterized in that: The crystal accumulation bed height is less than 10m.

20. The suspension crystallization purification equipment according to claim 19, characterized in that: The crystal stacking bed height is 2-5m.

21. The suspension crystallization purification equipment according to claim 1 or 2, characterized in that: The temperature settings of separation section A, refining section B, and washing section C are increased in sequence.

22. The suspension crystallization purification equipment according to claim 1 or 2, characterized in that: The empty tower velocity of the liquid phase of the slurry is 0.01-1m / s.

23. The suspension crystallization purification equipment according to claim 22, characterized in that: The liquid phase empty tower velocity of the slurry is 0.05-0.5m / s.

24. The suspension crystallization purification equipment according to claim 1 or 2, characterized in that: The liquid flow rate at the liquid inlet is 0.1-5m / s.

25. The suspension crystallization purification equipment according to claim 24, characterized in that: The liquid flow rate at the liquid inlet is 0.5-1m / s.

26. Use of the suspension crystallization purification device according to any one of claims 1 to 25 for crystallization purification of lactide.

27. The use according to claim 26, characterized in that In the crystallization purification of lactide, the temperature of the crystallization washing tower is set in the range of 70-90°C.

28. The use according to claim 27, characterized in that In the crystallization purification of lactide, the temperatures of the separation section A, the refining section B and the washing section C of the crystallization washing tower are 70-75°C, 75-85°C and 85-95°C respectively.

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