A method for purifying lactide

Abstract

The application provides a refining method of lactide, which comprises the following steps: mixing crude lactide with a mixed solvent of alcohol and ether at 60-65 DEG C and keeping the mixture at constant temperature until the lactide is completely dissolved; adding water at 60-65 DEG C, mixing and then standing to separate into two layers; discharging the lower water phase; cooling; filtering; and drying to obtain refined lactide product. The mixed solvent of alcohol and ether is used for refining lactide, the addition of alcohol increases the solubility of lactide in the main solvent ether, and the processing capacity is improved; and the main solvent ether is a hydrophobic solvent, so that the obtained product has very low water content after simple and rapid drying, and spontaneous hydrolysis of the product is prevented. The one-way yield of the refined product is not less than 75%, the chemical purity and optical purity of the refined lactide are not less than 99%, the water content of the product after drying is not more than 0.1%, and the obtained lactide product is colorless needle-shaped crystal and cannot spontaneously hydrolyze.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of chemical refining technology, and in particular to a method for refining lactide. BACKGROUND

[0002] Lactide is an intermediate raw material for synthesizing high molecular weight polylactic acid, and the purity of the lactide directly determines the performance indicators of the polylactic acid. Therefore, the purification of lactide is a very important link in the synthesis of polylactic acid. Common methods for purifying lactide include solvent recrystallization, water extraction, distillation, and melt crystallization.

[0003] Solvent recrystallization is a method for purifying lactide by dissolving crude lactide in a solvent and taking advantage of the different solubilities of the components. Commonly used recrystallization solvents include ethanol, ethyl acetate, benzene, acetone, or a mixture of one or more of these solvents. The current solvent recrystallization method is limited by the good solvents, although it is widely used in China due to its low cost, the effect is not ideal, and the yield is low.

[0004] Water extraction method can quickly separate m-lactide and other impurities in crude lactide by taking advantage of the difference in solubility and dissolution rate of lactide and other impurities in water. However, it is not as good as solvent recrystallization in separating lactic acid dimers, trimers, and some colored substances. Moreover, when crude lactide contains lactic acid, it will promote the ring-opening hydrolysis of lactide, making it difficult to control the degree of hydrolysis and greatly reducing the yield.

[0005] Distillation is a purification technology based on the difference in volatility between different components of crude lactide to achieve separation. Distillation is easy to scale up and continuous industrial production, but due to the high boiling point and thermal sensitivity of lactide, distillation of lactide must be carried out under high vacuum conditions to reduce the occurrence of coking, oxidation, and other phenomena. At the same time, due to the close boiling points of the key components in crude lactide, the required theoretical plate number is too high, and the presence of water and acidic components in the impurities will promote the polymerization of lactide, seriously affecting the yield.

[0006] Melt crystallization is a method for separating and purifying lactide by taking advantage of the different freezing points of the components in crude lactide. Compared with distillation, the operating temperature is relatively low, which reduces energy consumption and allows the preparation of high-purity or ultra-high-purity products through the control of the number of stages. However, the content of impurities in crude lactide directly affects the yield of melt crystallization. When dealing with crude lactide with high impurity content, the yield is too low. Moreover, the single batch operation time of melt crystallization is too long, and the scale-up production requires precise control, which limits the industrial scale-up.

[0007] CN101857585A discloses a continuous high-vacuum purification method of propiolactone, which uses three towers in series operation. Water, lactic acid and part of meso-propiolactone in crude propiolactone are removed by a first rectifying tower, then all meso-propiolactone is removed by a second rectifying tower, and finally a third rectifying tower is used for heavy removal. The L-propiolactone product with purity of more than 99.0% and yield of more than 90% is obtained at the top of the tower. The operating pressures of the three towers are 200 Pa, 200 Pa and 100 Pa respectively. The process requires a very high vacuum degree to prevent the temperature of the rectification process from being too high, thereby reducing the decomposition and thermal polymerization of propiolactone in the rectification process and ensuring the yield and quality. However, the three-tower series rectification will result in high purification cost, and due to the close boiling points of the components, a large number of theoretical plates are required.

[0008] CN101696204A discloses a method for purifying propiolactone by water extraction process. The powdered crude L-propiolactone is repeatedly washed with water in a water washing device to obtain water-washed propiolactone, which is then dehydrated to obtain refined propiolactone. The yield of propiolactone reaches more than 80%. However, the repeated water washing can easily cause ring-opening hydrolysis of L-propiolactone, and the removal effect of dimers, trimers and colored substances in the crude propiolactone is poor, and the purity of the obtained product is also not high.

[0009] CN111961028A discloses a method for refining propiolactone by alternately dissolving and recrystallizing a mixed solvent of dimethyl ketone and ethanol. The recrystallization yield and optical purity of propiolactone are low (60-80% yield, 70-92% purity).

[0010] The purification effect of the prior art on propiolactone is largely limited by the purity of the propiolactone to be purified. Especially for solvent recrystallization and water washing method, a higher purity (85-90% and above) of raw material is required to achieve the above refining effect. At present, the crude propiolactone obtained from the depolymerization section in China has a purity of only 70-75% after one rectification treatment, and the refining effect is not good using the prior art. In view of the deficiency that the prior art cannot handle propiolactone with lower purity, the present application provides a solvent recrystallization refining process for propiolactone which can quickly obtain higher yield and purity, and the process can purify crude propiolactone with a purity of 70-75%. SUMMARY

[0011] In view of the above deficiencies, the present application provides a refining method for propiolactone, which uses a mixed organic solvent to refine propiolactone, and is based on ether solvent coupling phase transfer refining technology to improve the refining yield of propiolactone.

[0012] The present application achieves the above technical purpose by the following technical scheme:

[0013] A refining method for propiolactone, comprising the following contents:

[0014] The crude lactide is mixed with the mixed solvent of alcohol and ether at 60-65℃ and incubated until it is completely dissolved. Then, water at 60-65℃ is added. After mixing, the mixture is allowed to stand and separate into layers. The lower aqueous phase is discharged. The mixture is cooled and filtered, and the product is dried to obtain refined lactide.

[0015] Further, in one embodiment of the present application, when the alcohol and the ether are mixed, the alcohol accounts for 1%-20% of the total volume of the alcohol and the ether, preferably 2%-17%.

[0016] Further, in one embodiment of the present application, the alcohol in the mixed solvent of alcohol and ether is selected from at least one of methanol, ethanol, butanol and isopropanol, preferably ethanol, especially ethanol of more than 95%. The ether is selected from at least one of propyl ether (dipropyl sulfide), butyl ether, amyl ether and hexyl ether. Therefore, the mixed solvent of alcohol and ether also contains a small amount of water and trace impurities.

[0017] Further, in one embodiment of the present application, the ratio of the crude lactide to the mixed solvent of alcohol and ether is 1:1.5-2 (w / v) by weight and volume, respectively.

[0018] Further, in one embodiment of the present application, the ratio of the amount of water added to the mixed solvent of alcohol and ether is 1:5-25 (v / v), preferably 1:5-15 by volume.

[0019] Further, in one embodiment of the present application, the incubation time of the mixture of the crude lactide and the mixed solvent of alcohol and ether is 15-40 min.

[0020] Further, in one embodiment of the present application, water at 60-65℃ is added, and the mixture is allowed to stand and separate into layers after mixing for 5-10 min.

[0021] Further, in one embodiment of the present application, the cooling is to a temperature of -10-10℃, preferably 0-5℃.

[0022] Further, in one embodiment of the present application, the crude lactide is prepared by a process of dehydration, polycondensation and depolymerization using lactic acid / lactate as raw material. The composition is: 70-75% L / D / (L+D)-lactide, 5-15% m-lactide, 1-5% L / D / (L+D)-lactic acid, 1-5% lactic acid oligomer, trace impurities and coloring substances.

[0023] Further, in one embodiment of the present application, the step of recycling the filtrate after distillation is further included.

[0024] Further, in one embodiment of the present application, the water-soluble residue after distillation is mixed with the water discharged from the purification process of crude lactide, and then decolorized and reused in the preparation of crude lactide.

[0025] Further, the decolorization uses activated carbon, and more preferably powdered carbon with a mesh size of 80-100 mesh, and the activated carbon is added in an amount of 1-5 wt% based on the weight of the liquid to be treated.

[0026] The technical solution of the present application has the following advantages:

[0027] (1) The present application uses a mixed solvent of alcohol and ether for the purification of lactide. The inventors have found that the addition of alcohol increases the solubility of lactide in the main solvent, ether, and improves the processing capacity. The main solvent, ether, is a hydrophobic solvent, which ensures that the product obtained has very low water content after simple and rapid drying, preventing spontaneous hydrolysis of the product.

[0028] (2) Because crude lactide contains m-lactide, lactic acid, lactic acid dimers, trimers and other impurities, a single method cannot be used to take into account all of them, and the purification is very difficult. The inventors have found that by mixing crude lactide with an organic solvent, lactic acid dimers, trimers and pigments are dissolved in the organic solvent, and then water washing can quickly remove m-lactide and most of the coloring substances, thereby achieving the effect of removing impurities from crude lactide.

[0029] (3) The solvents used in the present application are low-toxicity solvents, and the operating temperature is relatively low, which is relatively safe.

[0030] (4) Using the purification method of the present application, the solubility of lactide in the mixed solvent is not less than 40 g / 100 g solvent, the single-pass yield is not less than 75%, the chemical purity and optical purity of the purified lactide are not less than 99%, and the water content of the dried product is not more than 0.1%, and the lactide product obtained is colorless needle-shaped crystals, which will not spontaneously hydrolyze.

[0031] Other features and advantages of the present application will be described in detail in the following specific embodiments. DETAILED DESCRIPTION

[0032] The following non-limiting examples can enable those skilled in the art to more fully understand the present application, but in no way limit the present application.

[0033] Example 1

[0034] The mixed solvent is 200 mL of 99% n-butyl ether and 20 mL of 95% ethanol

[0035] The refining process is as follows: 110 g of L-lactide crude product is mixed with the above mixed solvent, heated to 60°C, and the solution becomes clear at 53°C. Keep stirring for 30 min. Add 20 mL of water, stir for 10 min, then separate the layers by standing for 10 min. Remove the lower aqueous phase, cool to 5°C for 2 h, filter, and dry the filter cake in a explosion-proof oven for 30 min to obtain colorless needle-shaped crystalline refined product. The product specifications are shown in Table 1.

[0036] Example 2

[0037] The mixed solvent is 200 mL of 99% n-butyl ether and 40 mL of 95% ethanol.

[0038] The refining process is as follows: 120 g of L-lactide crude product is mixed with the above mixed solvent, heated to 60°C, and the solution becomes clear at 48°C. Keep stirring for 30 min. Add 25 mL of water, stir for 10 min, then separate the layers by standing for 10 min. Remove the lower aqueous phase, cool to 5°C for 2 h, filter, and dry the filter cake in a explosion-proof oven for 60 min to obtain colorless needle-shaped crystalline refined product. The product specifications are shown in Table 1.

[0039] Example 3

[0040] The mixed solvent is 195 mL of 99% propyl ether (dipropyl sulfide) and 5 mL of 95% ethanol.

[0041] The refining process is as follows: 100 g of L-lactide crude product is mixed with the above mixed solvent, heated to 65°C, and the solution becomes clear at 63°C. Keep stirring for 30 min. Add 20 mL of water, stir for 10 min, then separate the layers by standing for 10 min. Remove the lower aqueous phase, cool to 5°C for 2 h, filter, and dry the filter cake in a explosion-proof oven for 60 min to obtain colorless needle-shaped crystalline refined product. The product specifications are shown in Table 1.

[0042] Comparative Example 1

[0043] Except that water is used instead of n-butyl ether in Example 1, the other operations are the same as in Example 1. The product specifications are shown in Table 1.

[0044] Comparative Example 2

[0045] Only 200 mL of n-butyl ether is used without adding ethanol, and the other operations are the same as in Example 1. The product specifications are shown in Table 1.

[0046] Comparative Example 3

[0047] No water is added during the refining process. After the organic solvent is dissolved, the product is directly cooled to precipitate, then filtered and dried, and the operation process is the same as in Example 1. The product specifications are shown in Table 1.

[0048] Table 1.

[0049]

Claims

1. A method for purifying a lactide, characterized by, The application comprises the following steps: The crude lactide is mixed with a mixed solvent of alcohol and ether at 60-65℃ and kept for 15-40min until it is fully dissolved, then water at 60-65℃ is added, the mixture is left to stand and separate into two layers, the lower water phase is discharged, the temperature is lowered, the product is filtered and dried to obtain the refined lactide; in the mixed solvent of alcohol and ether, the alcohol is ethanol and the ether is at least one selected from propyl ether and butyl ether, when the alcohol and ether are mixed, the alcohol accounts for 2%-17% of the total volume of the alcohol and ether.

2. The refining method according to claim 1, characterized by, The ratio of the crude lactide to the mixed solvent of alcohol and ether is 1:1.5-2 by weight and volume.

3. The refining method according to claim 1, characterized by, The ratio of the water to the mixed solvent of alcohol and ether is 1:5-25 by volume.

4. The refining method according to claim 1, characterized by, The ratio of the water to the mixed solvent of alcohol and ether is 1:5-15 by volume.

5. The method of claim 1, wherein The crude lactide is mixed with the mixed solvent of alcohol and ether and kept for 15-40min.

6. The method of claim 1, wherein The temperature is lowered to -10-10℃.

7. The method of claim 6, wherein the refining is performed by a method comprising: The temperature is lowered to 0-5℃.

8. The method of claim 1, wherein the refining is performed by a method comprising: The application further comprises the step of recycling the filtrate after distillation.

9. The method of claim 1, wherein, The application further comprises the step of mixing the water-soluble residue at the bottom of the distillation tank with the water phase discharged in the process of refining the crude lactide, decolorizing the mixture and recycling it to the process of preparing the crude lactide.

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