CN-121991021-A - Lactide separation and purification method and system

Abstract

The invention belongs to the technical field of chemical engineering, and relates to a method and a system for separating and purifying lactide. The system comprises a raw material preprocessor, a solution crystallizer, a first dryer, a melting crystallizer and a second dryer which are sequentially connected in series. Compared with the prior art, the method has the advantages of short flow, low energy consumption, good separation effect, high product purity and the like, and can meet the production requirement of high-purity lactide.

Inventors

• YU YANG
• HOU JINGWEI
• QIU BO
• XU PENG
• WANG HAO
• SONG ZIHANG
• YE HAIBO

Assignees

• 中石油(上海)新材料研究院有限公司
• 中国石油天然气股份有限公司

Dates

Publication Date

20260508

Application Date

20241101

Claims (10)

1. 1. A method for separating and purifying lactide is characterized by comprising the steps of dissolving crude lactide in a solvent, adding an anti-solvent for elution crystallization, and then carrying out melt crystallization to obtain separated and purified lactide.
2. 2. The method according to claim 1, wherein the solvent is one or more selected from the group consisting of methanol, ethanol, n-propanol, isopropanol, acetone, butanone, chloroform, methyl acetate and ethyl acetate.
3. 3. The method according to claim 2, wherein the mass ratio of the crude lactide to the solvent is (1-20): 1, and/or the dissolution temperature of the crude lactide in the solvent is 30-35 ℃.
4. 4. The method for separating and purifying lactide according to claim 1, wherein the antisolvent is selected from benzene, toluene, xylene, methyl acetate, ethyl acetate and butyl acetate.
5. 5. The method according to claim 4, wherein the mass ratio of the antisolvent to the solvent is (1-5): 1, and/or the crystallization temperature in the elution crystallization is 28-32 ℃.
6. 6. The method according to claim 1, wherein in the solvent-out crystallization, the solvent-out crystallization mother liquor is recovered from the solvent and the anti-solvent by rectification.
7. 7. The method according to claim 6, wherein in the rectification, the theoretical plate number is 10, the operation pressure is 100-110kPa, the column top temperature is 56-77 ℃, and the column bottom temperature is 66-118 ℃.
8. 8. The method according to claim 1, wherein the crystallization temperature in the melt crystallization is 74 to 110 ℃.
9. 9. The lactide separation and purification system is characterized by comprising a raw material preprocessor (3), a elution crystallizer (6), a first dryer (8), a melting crystallizer (10) and a second dryer (13) which are sequentially connected in series; the raw material preprocessor (3) is used for mixing the crude lactide (1) with the solvent (2) to obtain a crude lactide solution (4); The solution crystallization device (6) is used for mixing the crude lactide solution with the anti-solvent (5) and performing solution crystallization to obtain lactide (7) subjected to solution crystallization and solution crystallization mother liquor (15); the first dryer (8) is used for carrying out primary drying on the lactide (7) subjected to the solution crystallization to obtain a primary purified lactide product (9); the melting crystallizer (10) is used for performing melting crystallization on the primary purified lactide product (9) to obtain lactide (11) after melting crystallization and a melting crystallization mother solution (12); The second dryer (13) is used for carrying out secondary drying on the lactide (11) after melting crystallization to obtain a high-purity lactide product (14).
10. 10. The lactide separation and purification system according to claim 1, wherein a solvent recoverer (16) is further arranged between the solvent feed port of the raw material preprocessor (3) and the mother liquor discharge port of the solvency crystallizer (6), and the solvent recoverer (16) obtains the solvent (2) and the anti-solvent (5) through rectification separation and returns to the raw material preprocessor (3) and the solvency crystallizer (6) respectively.

Description

Lactide separation and purification method and system Technical Field The invention belongs to the technical field of chemical engineering, and relates to a method and a system for separating and purifying high-purity lactide. Background Polylactic acid is a biodegradable polymer material and is widely applied to the fields of environment-friendly packaging, medical materials, textile fibers and the like. The method for producing polylactic acid at home and abroad is a lactide ring-opening polymerization method, firstly, lactic acid is dehydrated and polymerized into an oligomer with lower molecular weight, and then depolymerized at high temperature under the action of a catalyst to generate cyclic lactide. Finally, lactide monomer is used as raw material to carry out ring-opening polymerization to obtain polylactic acid with high molecular weight. The crude lactide contains L-lactide, M-lactide, lactic acid dimer, lactic acid trimer and water. In the lactide ring-opening polymerization process, in order to obtain polylactic acid products with higher molecular weight, the lactide as a raw material has strict requirements on indexes such as optical purity, acidity, moisture and the like. For a long time, the purification cost of lactide has been one of the reasons for restricting the mass production and popularization of polylactic acid. The purification method of lactide comprises recrystallization, rectification, melt crystallization and the like. The recrystallization method has longer operation period, and can meet the requirement of polymerization reaction only by multiple purifications, thus leading to lower overall yield of lactide, and simultaneously leading to high operation energy consumption by multiple cooling. The rectification method has harsh conditions, and is required to be operated under high vacuum conditions in consideration of the characteristics of high boiling point, sensitivity to temperature and the like of lactide, and has high equipment requirements. The melt crystallization method is suitable for further refining of high-purity lactide, has high requirements on the purity of raw materials, and is not suitable for purifying crude lactide. The existing patent cases with relatively large relevance are as follows: Patent CN113292531B discloses a method for purifying lactide by recrystallization using isopropanol as solvent. Dissolving lactide at 15-45 ℃, standing for more than 5 hours at-20-4 ℃ to enable the lactide to be recrystallized and separated out, and repeating the operation for 2-5 times. Although the lactide yield can be improved from 37.2% to 68.6% compared with the conventional mixed solvent of ethyl acetate and ethanol, the method has the advantages of complicated process, long time consumption and high energy consumption requirement for repeated cooling crystallization. Chinese patent CN110498787a discloses a method for purifying crude lactide. Specifically, rectification is adopted to purify the crude lactide. Firstly preheating lactide to 110-130 ℃, adding the lactide into a rectifying tower, wherein the temperature of a reboiler at the bottom of the rectifying tower is 135-155 ℃, the temperature in the rectifying tower is 115-145 ℃, and the top pressure is 300-1000 Pa. The lactide gas obtained from the top of the tower is condensed to obtain crude purified lactide. Although the purity of the L-lactide obtained from the top of the rectification column can be improved from 86-92% to about 95%. However, the rectification operation conditions are severe, the lactide is easy to react in a high-temperature area, the requirements on equipment and operation are high, and the energy consumption is high. Chinese patent CN109776482B discloses an evaporation-coupling melt crystallization process for purification of lactide. In particular, a two-stage tandem evaporation apparatus is used for removing most of the oligomers, lactic acid and pigments. And then the lactide crude product collected by the condenser is pumped into a melting crystallizer, and further purified by sweating, so as to obtain a high-purity lactide product. Although high purity lactide can be obtained by a melt crystallization process, the process requires a raw material crude lactide with high purity requirements (about 92.9%) and is not suitable for direct purification of crude lactide. Chinese patent CN105646440A discloses a refining and purifying method of lactide, which comprises (1) extracting crude lactide with water-ethanol double solvent, drying after solid-liquid separation, (2) adding lactide into a melting crystallizer, heating materials at a certain heating speed, immediately separating out melted liquid until the temperature is 10-20 ℃ below the melting point of the lactide, (3) heating the lactide to be completely melted, crystallizing the lactide at a certain cooling speed, cooling to 85-80 ℃, maintaining the temperature for a certain time, discharging the melted liquid, (4) sweating the lactide