Method for refining lactide

Abstract

The invention provides a method for refining lactide, which comprises the following steps: condensing and crushing crude lactide into solid lactide powder, mixing with the solid lactide powder at the temperature of not higher than 20 DEG C by using a mixed solvent of water and low-carbon alcohol as a washing solvent, separating, cleaning a filter cake, and drying to obtain refined lactide. According to the method for refining the lactide, the inherent thought in the prior art is creatively broken through, and the lactide is mixed with the washing solvent in a solid form instead of a molten form, so that on one hand, hydrolysis of m-lactide in the method in the prior art is slowed down; on the other hand, the phenomena of temperature rise caused by a molten state, non-uniform particle size distribution and easy agglomeration of a molten material after being cooled in a washing solvent and the like are avoided, and the obtained lactide is high in purity, high in yield and uniform in particle size distribution.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer material refining, and particularly relates to a method for refining lactide. Background Art

[0002] Lactide is an intermediate raw material for synthesizing high molecular weight polylactic acid, and its purity directly determines the various performance indicators of polylactic acid. Lactide has a high boiling point, a high freezing point, is heat-sensitive, and is very easy to absorb water to produce a ring-opening reaction. It is very difficult to purify and purify, so the purification and refining of lactide is a very important link in the synthesis of polylactic acid. Lactide synthesis is made from lactic acid as a raw material, and is obtained by a one-step method and a two-step method. Among them, the two-step method is a commonly used method in the industry at present, that is, lactic acid dehydration condensation forms lactic acid oligomers, and then the oligomers are catalytically cracked at high temperature to obtain lactide. The lactide obtained in this process is generally light yellow or yellow, and contains impurity components such as lactic acid, water, lactic acid polymers (mainly dimers and trimers), and meso-lactide (m-lactide). Commonly used lactide purification and refining methods mainly include solvent recrystallization, water extraction, distillation, and melt crystallization.

[0003] Solvent recrystallization is to dissolve crude lactide in a solvent and purify lactide by utilizing the different solubility of each component. Commonly used recrystallization solvents include ethanol, ethyl acetate, benzene, acetone, etc. or a mixed solvent of several kinds. The solvent recrystallization refining method has a good effect on removing lactic acid and lactic acid polymer impurities in crude lactide, but has little effect on removing impurities such as m-lactide. Although it is widely used in China due to its low cost, the effect is not ideal and the yield is low.

[0004] By utilizing the differences in dissolution rates and solubilities of lactide and other impurities in water, the water extraction method can quickly separate impurities such as m-lactide from crude lactide. However, the separation performance for lactic acid dimers, trimers and some coloring substances is poor. Moreover, when crude lactide contains lactic acid, it will promote the ring-opening hydrolysis of lactide, making it impossible to control the degree of hydrolysis and greatly reducing the yield.

[0005] The distillation method is a purification technology based on the difference in volatility between different components of crude lactide to achieve separation effect. The distillation method is easy to scale up and carry out continuous industrial production, but due to the high boiling point and heat sensitivity of lactide, the 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, too many theoretical plates are required, 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 of separating and purifying lactide by utilizing the different freezing points of the components in crude lactide. Compared with the distillation method, the operating temperature is relatively low, and high-purity or ultra-high-purity products can be prepared by controlling the number of stages while reducing energy consumption. However, the impurity content in crude lactide directly affects the yield of melt crystallization. When crude lactide with a high impurity content is processed, the yield is too low; and the operation time of a single batch of melt crystallization is too long, and large-scale production regulation requires fine control, which limits industrial scale-up.

[0007] CN101696204 A discloses a method for purifying lactide by water extraction process, wherein powdered crude L-lactide is placed in a water washing device for repeated water washing to obtain water-washed lactide, which is then dehydrated to obtain refined lactide; the lactide yield reaches more than 80%. However, repeated water washing in this method is prone to cause ring-opening hydrolysis of L-lactide, and the removal effect of dimers, trimers and colored substances in crude lactide is poor, and the purity of the obtained product is not high.

[0008] CN105646440A discloses a method for refining and purifying lactide, comprising: (1) subjecting crude lactide to water-ethanol dual solvent extraction, solid-liquid separation and drying; (2) adding lactide to a melt crystallizer, heating the material at a certain heating rate, and immediately separating the molten liquid until the temperature is 10-20°C below the melting point of lactide; (3) heating the lactide until it is completely melted, crystallizing the lactide at a certain cooling rate, and when the temperature is cooled to 85-80°C, maintaining the crystals at a constant temperature for a certain period of time, and discharging the molten liquid; (4) sweating the lactide at a certain heating rate, and when the temperature is heated to 85-92°C, maintaining the crystals at a constant temperature for a certain period of time, and discharging the molten liquid; (5) washing the lactide crystals with ethanol after cooling, and obtaining purified lactide after drying. This method optimizes the water extraction method and proposes a dual-solvent water extraction method, so that two major types of impurities with different properties in crude lactide can be removed simultaneously, and coupled with the melt crystallization method to purify the crude lactide with good results. However, the steps are numerous and the operation is complicated. In actual operation, drying is difficult. If the moisture of lactide in (1) cannot be completely removed, it will seriously affect the refining effect and yield of the subsequent process.

[0009] CN202011168019.9 discloses a method for purifying lactide, first pre-treating the crude lactide, then adding water for rapid reaction, the reaction time is ≤10min, the reaction temperature is lower than 18°C, and a water extraction liquid is obtained; a low-carbon alcohol solvent is added to the water extraction liquid, and after the reaction, the solution is filtered, washed, and dried to obtain purified lactide. This method optimizes the water extraction operation, and sequentially cleans the lactide and removes impurities contained in the crude lactide in two steps. In actual operation, if lactide is not pre-treated and directly fed in a molten state, it will inevitably agglomerate, be difficult to disperse, and fail to achieve a good cleaning effect, and easily cause blockage in the subsequent material transportation link. Summary of the invention

[0010] It is well known to those skilled in the art that the main component of crude lactide is L / D-lactide, and the impurities mainly contain m-lactide, lactic acid, lactic acid dimer, trimer and organic pigments and other impurities. It is difficult to take both into account with a single method, and purification is very difficult. Impurities such as m-lactide and lactic acid are easily soluble in water, while other impurities such as lactic acid polymers and organic pigments are easily soluble in organic solvents, but m-lactide is prone to hydrolysis after dissolution, and the hydrolysis of m-lactide will also promote the dissolution and hydrolysis of L / D-lactide. Therefore, although the traditional mixing and heat preservation stirring method can achieve the cleaning effect, the retention of the material in the cleaning solution will cause the dissolution and hydrolysis of the main components of the material, making it difficult to obtain a higher yield. In order to solve the above shortcomings, the method of the present invention creatively processes the crude lactide into a solid powder and then quickly mixes it with a washing solvent for a short time. In this process, the hydrolysis of m-lactide is slowed down, the temperature rise caused by the molten state and the uneven particle size distribution and easy agglomeration of the molten material after cooling in the washing solvent are avoided. It is found that both water-soluble and fat-soluble impurities in the crude lactide can be effectively removed in this process, the obtained lactide has high purity and high yield, and it is easy to achieve continuous refining of lactide.

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

[0012] The present invention provides a method for refining lactide, comprising:

[0013] The crude lactide is condensed and crushed into solid lactide powder, and a mixed solution of water and low-carbon alcohol in a mass ratio of 1:0.5-2 is used as a washing solvent, mixed with the solid lactide powder at a temperature not higher than 20°C, separated, the filter cake is washed, and dried to obtain refined lactide.

[0014] Furthermore, the temperature at which the washing solvent is mixed with the solid lactide powder is preferably 10-15°C.

[0015] Furthermore, the present invention utilizes a mixture of water and low-carbon alcohols to quickly contact the solid lactide powder at low temperature for a short time to remove impurities therein. The more sufficient this contact is, the more beneficial it is to remove impurities. Therefore, the solid lactide powder used in the present invention has a small and evenly distributed particle size to achieve better results. As a preferred embodiment, the average particle size of the solid lactide powder is not more than 500 μm, preferably not more than 300 μm, and most preferably 50-200 μm. Too small an average particle size results in large filtration losses.

[0016] Furthermore, the low-carbon alcohol is selected from at least one of methanol, ethanol, ethylene glycol, propanol, butanol and pentanol, preferably ethanol and / or isopropanol.

[0017] Furthermore, when the washing solvent and the solid lactide powder are mixed, the mixing means in the prior art are used to achieve sufficient mixing of the two in a short time, and the washing solvent and the solid lactide powder are mixed and contacted for 5-120 seconds, preferably 20-59 seconds, and most preferably 20-40 seconds. Too long a washing time will not enhance the washing effect on powdery materials with uniform and small particle size distribution, but will reduce the yield.

[0018] Furthermore, the mass ratio of water to low-carbon alcohol in the washing solvent is preferably 1:0.7-1.5.

[0019] Furthermore, the solvent used for cleaning the filter cake is a low-carbon alcohol, especially a low-carbon alcohol of the same type as the aforementioned washing solvent. Among them, a low-carbon alcohol with a purity of more than 95% is preferred, and the moisture in the filter cake can be removed by simple immersion. Considering the high solubility of lactide in low-carbon alcohol, the temperature for cleaning the filter cake should be ≤15°C, preferably 10-15°C. The cleaned low-carbon alcohol can be distilled, recovered and recycled.

[0020] Furthermore, the solid lactide powder is mixed with the washing solvent in a mass ratio of 1:1-3, preferably 1:1-1.5.

[0021] Furthermore, the crude lactide has L / D-lactide as a main component, contains m-lactide, lactic acid, lactic acid dimer, trimer and various impurities in organic pigments, and is a product prepared from lactic acid / lactic acid esters through dehydration, polycondensation and depolymerization processes.

[0022] Specifically, the refined crude lactide of the present invention is obtained by dehydration, polycondensation and depolymerization processes using L-lactic acid or L / D-lactic acid ester as raw materials, wherein the purity of L / D-lactide is 83%-90%, wherein the content of m-lactide is generally 3.0%-5.5%, the content of lactic acid is generally 2.4%-6.0%, and the content of dimer and trimer is generally 2.0%-3.5%.

[0023] Furthermore, the condensation and crushing process is to obtain solid powder by condensing and crushing the molten crude lactide. As one of the specific embodiments, it can be condensed and sliced ​​by a crystallization condensation scraper to obtain crude lactide flakes, especially flakes with a diameter of 5-20 mm and a thickness of 1-2 mm, and then the crude lactide flakes are ground into powder by a grinding device. The above-mentioned condensation, slicing and grinding processes are all carried out under nitrogen protection, and the grinding equipment is preferably a jet mill, especially a vortex mill.

[0024] Furthermore, the process of washing the solid lactide powder with a washing solvent is achieved by a solid-liquid mixing homogenizing pump, the solid lactide powder passes through a vacuum suction device, and the solid feed is conveyed by dry air; the solid-liquid mixture discharge is conveyed by a screw.

[0025] Furthermore, the separation is achieved by a filter, preferably a vacuum drum filter and / or a sealed crawler filter.

[0026] Furthermore, the drying is achieved by an explosion-proof rotary flash dryer and / or an explosion-proof vacuum dryer. Specifically, the material can be transported to the particle size classifier at the top of the drying chamber of the dryer by a sealed belt conveyor, and the fine powder is discharged from the top of the tower and collected by the separator behind to become the product, and the larger particles are returned to the drying chamber to continue to be crushed and dried. In the method of the present invention, after the lactide filter cake is washed with low-carbon alcohol, due to the hydrophilic effect of the low-carbon alcohol, the water content of the solid is extremely low, so it is very easy to dry, the drying temperature is 50-70°C, preferably 52-60°C, and the drying time is ≤1.5h, preferably 30-60min.

[0027] The purified lactide obtained by the method of the present invention has high purity. Under a preferred embodiment, the optical purity can reach 98.0% or more, and the yield of lactide can reach more than 95%.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] (1) The method for refining lactide of the present invention creatively breaks through the inherent ideas of the prior art and allows lactide to be mixed with the washing solvent in the form of solid powder rather than molten form. On the one hand, the hydrolysis of m-lactide in the method of the prior art is slowed down. On the other hand, the temperature rise caused by the molten state and the uneven particle size distribution and easy agglomeration of the molten material after cooling in the washing solvent are avoided. The obtained lactide has high purity, high yield and uniform particle size distribution.

[0030] (2) The present invention uses solid lactide powder and a washing solvent of water and low-carbon alcohol to wash the solid lactide powder by quickly contacting it at low temperature for a short time to remove impurities. Compared with the washing of solid or liquid lactide in the prior art, the amount of water is increased by controlling the temperature and time without significantly increasing the hydrolysis loss of lactide. Not only is the impurity removal rate high, but the product yield is also guaranteed.

[0031] (3) The method of the present invention has a simplified process, can realize continuous refining, and is easy to implement pilot test and industrial expansion production.

[0032] (4) In the refining process of the present invention, since high-purity low-carbon alcohol has good water absorption, the water content in the material obtained after washing the filter cake is extremely low, and most of the solvent in the wet material is low-carbon alcohol. In the absence of almost no water, the drying temperature can be increased, and low-carbon alcohol is easier to volatilize than water, and the residual amount is small, which greatly improves the drying efficiency, avoids the hydrolysis phenomenon of water-containing materials during the drying process under the existing process, and further improves the yield and product quality.

[0033] Other features and advantages of the present invention will be described in detail in the following detailed description. DETAILED DESCRIPTION

[0034] The method of the present invention is further described in detail below by way of examples. The examples are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes, but the protection scope of the present invention is not limited to the following examples.

[0035] The experimental methods in the following examples, unless otherwise specified, are all conventional methods in the art. The experimental materials used in the following examples, unless otherwise specified, can all be purchased from biochemical reagent stores.

[0036] The present invention adopts HP4890D gas chromatograph to analyze the composition of each component in lactide, the purity of lactide and the residual amount of ethanol, and adopts hydrogen ion flame monitor, HP-INNOWAX capillary column, two-stage program temperature rise, the first stage is 100-140°C, the heating rate is 4°C / min, the second stage is 140-180°C, the heating rate is 8°C / min. The specific rotation of the sample is analyzed by WZZ-2S automatic polarimeter to characterize the optical purity of the sample. The specific rotation of pure L-lactide is -278, the specific rotation of pure D-lactide is +278, and the specific rotation of m-lactide is 0. The calculation formula of the sample optical purity X is as follows:

[0037]

[0038] Among them, α 纯物质 Represents the specific rotation of pure lactide, α 被测样品Indicates the specific rotation of the substance being measured.

[0039] The calculation formula for the yield Y of the lactide purification process is:

[0040]

[0041] Wherein, m0 is the weight of crude lactide, y0 is the purity of L-lactide in crude lactide, m is the weight of refined lactide obtained by drying, and y is the purity of L-lactide in refined lactide.

[0042] In the following examples, the average particle size of solid lactide was measured by a particle size and shape analyzer using a matched and trained algorithm and by static image analysis, and the error was within 1% of that of a laser particle size analyzer.

[0043] The crude lactide used in the following examples and comparative examples was prepared by the following method: zinc oxide and stannous octoate were used as catalysts, 88% L-lactic acid was used as a raw material, and the lactic acid was dehydrated, polycondensed and depolymerized to obtain the crude lactide, which contained 85.6% L-lactide, 5.2% m-lactide, 4.7% lactic acid, 2.9% lactic acid dimer and trimer, and also contained a small amount of water and color impurities.

[0044] Example 1

[0045] (1) After the crude lactide is melted, it is poured into a crystallization and condensation scraper. After the bin temperature reaches 95° C. and the circulating coolant outlet temperature reaches 5° C., the crystallization and condensation scraper is turned on. After a total of 1000 g of slices are obtained, the crude lactide slices are added to a grinder and ground to obtain solid lactide powder, the average particle size of which is 112 μm. The particle size distribution is uniform overall, and no large particles are found after sieving.

[0046] (2) The solid lactide powder is put into a mixed solvent of 700 g pure water and 500 g ethanol at 15° C. at one time, and stirring is started. After the powder material is completely dispersed and soaked (the entire dispersion and soaking process takes about 30 seconds), filtering is performed.

[0047] (3) After the filtration is completed, the filter cake is washed with 500 g of 95% ethanol at 10°C and filtered again.

[0048] (4) After filtration, dry nitrogen was used to purge for 5 minutes, and then the mixture was dried at 70° C. in an explosion-proof negative pressure dryer for 30 minutes to obtain purified lactide.

[0049] After testing, the optical purity of L-lactide in the product was 98.1%, the yield was 96.7%, the water content was ≤0.02%, and the residual ethanol in the product was 82 ppm.

[0050] Example 2

[0051] Except that a mixed solvent of 600 g of pure water and 600 g of ethanol is used as the washing solvent in step (2), the rest is the same as in Example 1.

[0052] After testing, the optical purity of L-lactide in the product was 98.5%, the yield was 95.9%, the water content was ≤0.02%, and the residual ethanol in the product was 79 ppm.

[0053] Example 3

[0054] Except that a mixed solvent of 500 g of pure water and 700 g of ethanol is used as the washing solvent in step (2), the rest is the same as in Example 1.

[0055] After testing, the optical purity of L-lactide in the product was 98.7%, the yield was 95.1%, the water content was ≤0.02%, and the residual ethanol in the product was 85 ppm.

[0056] Example 4

[0057] Except that a mixed solvent of 800 g of pure water and 400 g of ethanol is used as the washing solvent in step (2), the rest is the same as in Example 1.

[0058] After testing, the optical purity of L-lactide in the product is 97.4%, the yield is 96.3%, and the water content is ≤0.02%.

[0059] Example 5

[0060] Except that a mixed solvent of 400 g of water and 800 g of ethanol is used as the washing solvent in step (2), the rest is the same as in Example 1.

[0061] After testing, the optical purity of L-lactide in the product is 97.6%, the yield is 92.9%, and the water content is ≤0.02%.

[0062] Example 6

[0063] Except that isopropanol is used instead of ethanol in step (2) and step (3), the other steps are the same as those in Example 1.

[0064] After testing, the optical purity of L-lactide in the product was 98.2%, the yield was 95.0%, the water content was ≤0.02%, and the residual amount of isopropanol in the product was 137 ppm.

[0065] Example 7

[0066] Except that the temperature of the washing solvent in step (2) is 20° C., the rest is the same as in Example 1.

[0067] After testing, the optical purity of L-lactide in the product was 97.9%, the yield was 93.2%, the water content was ≤0.02%, and the residual ethanol in the product was 141 ppm.

[0068] Comparative Example 1

[0069] Except that the temperature of the mixed solvent in step (2) is 25° C., the other steps are the same as those in Example 1.

[0070] After testing, the optical purity of L-lactide in the product is 97.2%, the yield is 91.1%, and the water content is ≤0.02%.

[0071] Comparative Example 2

[0072] Except that the temperature of the mixed solvent in step (2) is 30° C., the rest is the same as in Example 1.

[0073] After testing, the optical purity of L-lactide in the product is 96.6%, the yield is 89.3%, and the water content is ≤0.02%.

[0074] Comparative Example 3

[0075] Except that a mixed solvent of 120 g pure water and 1080 g ethanol is used as the washing solvent in step (2), the rest is the same as in Example 1.

[0076] After testing, the optical purity of L-lactide in the product is 95.9%, the yield is 88.5%, and the water content is ≤0.02%.

[0077] Comparative Example 4

[0078] Except that a mixed solvent of 1080 g of pure water and 120 g of ethanol is used as the washing solvent in step (2), the rest is the same as in Example 1.

[0079] After testing, the optical purity of L-lactide in the product is 93.1%, the yield is 94.7%, and the water content is ≤0.02%.

[0080] Comparative Example 5

[0081] Except that 25° C. and 95% concentration ethanol is used for cleaning in step (3), the rest is the same as in Example 1.

[0082] After testing, the optical purity of L-lactide in the product was 98.0%, the yield was 89.3%, and the water content was ≤0.02%. Ethanol was used to dissolve part of the product.

[0083] Comparative Example 6 (Dual Solvent Extraction Method of Prior Art)

[0084] (1) Take 700g of pure water and 500g of low-carbon alcohol, prepare the mixed solvent in advance, keep it cold at 15°C, and start high-speed stirring.

[0085] (2) Slowly pour 1000 g of molten crude lactide into the reactor in (1) over a period of about 10 minutes.

[0086] (3) Maintain the stirring rate and cold medium circulation until the temperature in the reactor is lower than 20°C, which takes about 40 minutes, and then filter.

[0087] (4) After the filtration is completed, the filter cake is washed with 500 g of 95% ethanol at 10°C and filtered again.

[0088] (4) After filtration, dry nitrogen was used to purge for 5 minutes, and then the mixture was dried at 70° C. in an explosion-proof negative pressure dryer for 30 minutes to obtain purified lactide.

[0089] After testing, the optical purity of L-lactide in the product is 95.2%, the yield is 88.2%, and the water content is ≤0.5%. The product quality of this method fluctuates greatly and the process is difficult to control.

Claims

1. A method for refining lactide, comprising: The crude lactide is condensed and crushed into solid lactide powder, and a mixed solution of water and low-carbon alcohol in a mass ratio of 1:0.5-2 is used as a washing solvent, mixed with the solid lactide powder at a temperature not higher than 20°C, separated, the filter cake is washed, and dried to obtain refined lactide.

2. The method according to claim 1, characterized in that The temperature at which the washing solvent is mixed with the solid lactide powder is 10-15°C.

3. The method according to claim 1, characterized in that The average particle size of the solid lactide powder is not greater than 500 μm.

4. The method according to claim 1, characterized in that: The washing solvent and the solid lactide powder are mixed and contacted for 5-120 seconds.

5. The method according to claim 4, characterized in that The washing solvent is mixed with the solid lactide powder for a contact time of 20-59 seconds.

6. The method according to claim 1, characterized in that The low-carbon alcohol is selected from at least one of methanol, ethanol, ethylene glycol, propanol, butanol and pentanol.

7. The method according to claim 1, characterized in that The mass ratio of water to low-carbon alcohol in the washing solvent is 1:0.7-1.

5.

8. The method according to claim 1, characterized in that The solvent used for cleaning the filter cake is low-carbon alcohol.

9. The method according to claim 8, characterized in that The temperature of the cleaned filter cake is ≤15°C.

10. The method according to claim 1, characterized in that The solid lactide powder and the washing solvent are mixed in a mass ratio of 1:1-3.

11. The method according to claim 1, characterized in that: The condensation and crushing process is to condense and crush the molten crude lactide to obtain solid powder, which is first condensed and sliced ​​by a crystallization condensation scraper to obtain crude lactide flakes, and then ground into powder by a grinding device.

12. The method according to claim 1, characterized in that The process of washing the solid lactide powder with a washing solvent is achieved by a solid-liquid mixing homogenizing pump, the solid lactide powder passes through a vacuum suction device, and the solid feed is conveyed by dry air.

13. The method according to claim 1, characterized in that The drying is as follows: the temperature is 50-70°C and the time is ≤1.5h.

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