A method for purifying propiolactone

Abstract

This invention provides a method for refining lactide, comprising: condensing and crushing crude lactide into solid lactide powder; using a mixed solvent of water and low-carbon alcohol as a washing solvent; mixing the solid lactide powder with the powder at a temperature not exceeding 20°C; separating the powder; washing the filter cake; and drying to obtain refined lactide. This invention creatively breaks through the inherent thinking of existing technologies by mixing lactide in solid form rather than in molten form with the washing solvent. This slows down the hydrolysis of m-lactide in existing methods and avoids 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. The resulting lactide has high purity, high yield, and uniform particle size distribution.

Description

Technical Field

[0001] This invention belongs to the field of polymer material refining technology, specifically relating to a method for refining lactide. Background Technology

[0002] Lactide is an intermediate raw material for the synthesis of high molecular weight polylactic acid (PLA), and its purity directly determines the various performance indicators of PLA. Lactide has a high boiling point, high freezing point, is heat-sensitive, and readily absorbs water, undergoing a ring-opening reaction, making purification very difficult. Therefore, the purification of lactide is a crucial step in PLA synthesis. Lactide synthesis uses lactic acid as a raw material and can be achieved through one-step and two-step methods. The two-step method is currently the most commonly used in industry: lactic acid undergoes dehydration and condensation to form lactic acid oligomers, which are then subjected to high-temperature catalytic cracking to obtain lactide. The lactide obtained from this process is generally pale yellow or yellow and contains impurities such as lactic acid, water, lactic acid polymers (mainly dimers and trimers), and meso lactide (m-lactide). Commonly used methods for purifying lactide include solvent recrystallization, water extraction, distillation, and melt crystallization.

[0003] Solvent recrystallization involves dissolving crude lactide in a solvent and purifying the lactide by utilizing the different solubilities of its components. Commonly used recrystallization solvents include one or more mixtures of ethanol, ethyl acetate, benzene, and acetone. Solvent recrystallization is effective at removing lactic acid and lactic acid polymers from crude lactide, but it has virtually no effect on impurities such as m-lactide. Although it is widely used in China due to its low cost, its effectiveness is not ideal, and the yield is low.

[0004] Taking advantage of the differences in the dissolution rate and solubility of lactide and other impurities in water, water extraction can quickly separate impurities such as m-lactide from crude lactide. However, it has poor separation performance for lactic acid dimers, trimers, and some coloring substances. Moreover, when crude lactide contains lactic acid, it will promote the ring-opening hydrolysis of lactide. The degree of hydrolysis cannot be controlled, which greatly reduces the yield.

[0005] Distillation is a purification technique that separates different components of crude lactide based on their volatility differences. While distillation is suitable for large-scale and continuous industrial production, the high boiling point and heat sensitivity of lactide necessitate high-vacuum distillation to minimize coking and oxidation. Furthermore, the close boiling points of key components in crude lactide necessitate an excessive number of theoretical plates, and the presence of water and acidic components in impurities can promote lactide polymerization, severely impacting yield.

[0006] Melt crystallization is a method for separating and purifying lactide by utilizing the different freezing points of its components. Compared to distillation, it operates at relatively lower temperatures, reducing energy consumption, and allows for the production of high-purity or ultra-high-purity products through stage control. However, the impurity content in crude lactide directly affects the melt crystallization yield; when processing crude lactide with high impurity content, the yield is too low. Furthermore, the single-batch melt crystallization operation time is too long, requiring precise control for large-scale production, thus limiting industrial scale-up.

[0007] CN101696204 A discloses a method for purifying lactide using a water extraction process. Powdered crude L-lactide is repeatedly washed in a water washing device to obtain washed lactide, which is then dehydrated to obtain refined lactide; the lactide yield reaches over 80%. However, this method's repeated washing easily causes ring-opening hydrolysis of L-lactide, and it is ineffective in removing dimers, trimers, and colored substances from the crude lactide, resulting in a low purity of the obtained product.

[0008] CN105646440A discloses a method for refining and purifying lactide, comprising: (1) extracting crude lactide with a water-ethanol dual solvent, separating the solid and liquid, and drying it; (2) adding lactide to a melt crystallizer, heating the material at a certain heating rate, and immediately separating the molten liquid until it is 10-20°C below the melting point of lactide; (3) heating lactide until it is completely melted, crystallizing lactide at a certain cooling rate, cooling it to 85-80°C, maintaining the temperature for a certain time, and discharging the molten liquid; (4) evaporating lactide at a certain heating rate, heating it to 85-92°C, maintaining the temperature for a certain time, and discharging the molten liquid; (5) rinsing the lactide crystals with ethanol after cooling, and drying them to obtain purified lactide. This method optimizes the water extraction method and proposes dual-solvent water extraction, which allows two major categories of impurities with different properties in crude lactide to be removed simultaneously. It is coupled with the melt crystallization method to purify crude lactide, which has a good effect. However, the steps are numerous and the operation is complicated. In actual operation, the drying is difficult. If the moisture in lactide in (1) cannot be completely removed, it will seriously affect the refining effect and yield of subsequent processes.

[0009] CN202011168019.9 discloses a method for purifying lactide. First, crude lactide is pretreated, then water is added for a rapid reaction (reaction time ≤ 10 min, reaction temperature below 18℃) to obtain an aqueous extract. A low-carbon alcohol solvent is added to the aqueous extract, and after reaction, the mixture is filtered, washed, and dried to obtain purified lactide. This method optimizes the aqueous extraction process by sequentially washing the lactide and removing impurities from the crude lactide in two steps. In practice, if lactide is fed directly in a molten state without pretreatment, agglomeration is inevitable, making dispersion difficult and hindering effective cleaning. Furthermore, it can easily cause blockages in subsequent material conveying processes. Summary of the Invention

[0010] As is known to those skilled in the art, the main component of crude lactide is L / D-lactide, while impurities include m-lactide, lactic acid, lactic acid dimers, trimers, and organic pigments, among others. A single method is insufficient to address all these impurities, making purification very difficult. Impurities such as m-lactide and lactic acid are readily soluble in water, while other impurities like lactic acid polymers and organic pigments are easily soluble in organic solvents. However, m-lactide is prone to hydrolysis after dissolution, and this hydrolysis simultaneously promotes the dissolution and hydrolysis of L / D-lactide. Therefore, while traditional mixing and heat-maintaining stirring methods can achieve a cleaning effect, the retention of the material in the cleaning solution leads to the dissolution and hydrolysis of the main components, making it difficult to achieve a high yield. To address the above shortcomings, the method of this invention creatively processes crude lactide into a solid powder before quickly mixing it with a washing solvent. This process slows down the hydrolysis of m-lactide, avoids the temperature rise caused by the molten state, and prevents uneven particle size distribution and easy agglomeration of the molten material after cooling in the washing solvent. It was also found that both water-soluble and fat-soluble impurities in crude lactide can be effectively removed in this process, resulting in lactide with high purity and high yield, and facilitating continuous purification of lactide.

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

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

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

[0014] Furthermore, the preferred temperature for mixing the washing solvent with the solid lactide powder is 10-15°C.

[0015] Furthermore, this invention utilizes a mixture of water and low-carbon alcohols to rapidly and briefly contact solid lactide powder at low temperatures to remove impurities. The more thorough this contact, the better the impurity removal. Therefore, solid lactide powder with a small and uniformly distributed particle size is more effective. In a preferred embodiment, the average particle size of the solid lactide powder is no greater than 500 μm, preferably no greater than 300 μm, and most preferably 50-200 μm. A smaller average particle size results in greater filtration loss.

[0016] Furthermore, the lower 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 solid lactide powder are mixed, existing mixing methods are used to ensure thorough mixing within a short time. The contact time between the washing solvent and the solid lactide powder is 5-120 seconds, preferably 20-59 seconds, and most preferably 20-40 seconds. Excessive washing time does not enhance the washing effect on powdery materials with uniform and small particle size distribution; instead, it reduces 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. Preferably, it is a low-carbon alcohol with a purity of 95% or higher, which can remove moisture from the filter cake with only simple rinsing. Considering the high solubility of lactide in low-carbon alcohols, the cleaning temperature should be ≤15℃, preferably 10-15℃. The cleaned low-carbon alcohol can be recovered and recycled by distillation.

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

[0021] Furthermore, the crude lactide is mainly composed of L / D-lactide and contains m-lactide, lactic acid, lactic acid dimer, trimer and various impurities in organic pigments. It is a product prepared from lactic acid / lactate as raw material through dehydration, condensation and depolymerization processes.

[0022] Specifically, the crude lactide refined in this invention is obtained by dehydration, polycondensation, and depolymerization of L-lactic acid or L / D-lactic acid ester as raw materials. The purity of L / D-lactide is 83%-90%, of which 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 involves condensing and crushing the molten crude lactide to obtain a solid powder. As one specific embodiment, the crude lactide can first be condensed and sliced ​​using a crystallization condenser to obtain crude lactide flakes, particularly flakes with a diameter of 5-20 mm and a thickness of 1-2 mm. These flakes are then ground into powder using a grinding device. All the above condensation, slicing, and grinding processes are carried out under nitrogen protection, and the grinding device is preferably an air jet mill, particularly a vortex mill.

[0024] Furthermore, the process of washing solid lactide powder with washing solvent is achieved by a solid-liquid mixing homogenizing pump. The solid lactide powder is fed through a vacuum suction device, and the solid feed is delivered by dry air. The solid-liquid mixture is discharged through a screw conveyor.

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

[0026] Furthermore, the drying is achieved using an explosion-proof rotary flash dryer and / or an explosion-proof vacuum dryer. Specifically, the material can be conveyed to a particle size classifier at the top of the drying chamber of the dryer via a sealed belt conveyor. Fine powder is discharged from the top of the tower and collected as the product by a separator at the rear, while larger particles are returned to the drying chamber for further crushing and drying. In the method of this invention, after the lactide filter cake is washed with a low-carbon alcohol, the solid has an extremely low water content due to the hydrophilic effect of the low-carbon alcohol, making it very easy to dry. The drying temperature is 50-70℃, preferably 52-60℃, 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. In the preferred embodiment, the optical purity can reach 98.0% or higher, and the yield of lactide can reach 95% or higher.

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

[0029] (1) In the method for refining lactide of the present invention, the inherent ideas of the prior art are creatively broken through, so that lactide is mixed with washing solvent in the form of solid powder instead of molten form. On the one hand, the hydrolysis of m-lactide in the prior art method is slowed down, and 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 lactide obtained has high purity, high yield and uniform particle size distribution.

[0030] (2) The present invention uses solid lactide powder to wash solid lactide powder with water and low-carbon alcohol washing solvent through short-term and rapid contact at low temperature, thereby removing impurities. Compared with the washing of solid or liquid lactide in the prior art, by controlling the temperature and time, the amount of water is increased 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 is simplified, can realize continuous refining, and is easy to realize pilot-scale and industrial-scale production.

[0032] (4) In the refining process of the present invention, since high-purity low-carbon alcohol has good water absorption, the material obtained after washing the filter cake has extremely low water content. Most of the solvent in the wet material is low-carbon alcohol. With almost no water involved, the drying temperature can be increased. Moreover, low-carbon alcohol is more volatile than water and has less residue, which greatly improves the drying efficiency and avoids the hydrolysis phenomenon that occurs in the drying process of water-containing materials under the existing process, further improving the yield and product quality.

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

[0034] The method of the present invention will be further described in detail below through embodiments. These embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

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

[0036] This invention uses an HP4890D gas chromatograph to analyze the composition of each component in lactide, the purity of lactide, and the residual amount of ethanol. A flame ionization detector and an HP-INNOWAX capillary column were used, with a two-stage temperature program: the first stage was 100-140℃ at a heating rate of 4℃ / min, and the second stage was 140-180℃ at a heating rate of 8℃ / min. The specific rotation of the samples was analyzed using a WZZ-2S automatic polarimeter to characterize the optical purity of the samples. The specific rotation of pure L-lactide was -278, that of pure D-lactide was +278, and that of m-lactide was 0. The formula for calculating the optical purity X of the sample is as follows:

[0037]

[0038] Where, α 纯物质 The specific rotation of pure lactide, α 被测样品This indicates the specific optical rotation of the substance being measured.

[0039] The formula for calculating the yield Y in the lactide purification process is:

[0040]

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

[0042] In the following examples, the average particle size of solid lactide was determined by a particle size analyzer using a matched and trained algorithm and static image analysis, with an error of less than 1% compared to 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: using zinc oxide and stannous octoate as catalysts, and 88% L-lactic acid as raw material, it was obtained by lactic acid dehydration, condensation and depolymerization reactions. By mass content, L-lactide was 85.6%, m-lactide was 5.2%, lactic acid was 4.7%, lactic acid dimer and trimer was 2.9%, and it also contained a small amount of water and coloring impurities.

[0044] Example 1

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

[0046] (2) Add solid lactide powder into a mixed solvent of 700g pure water and 500g ethanol at 15°C, start stirring, and filter after the powder is completely dispersed and soaked (the whole dispersion and soaking process takes about 30s).

[0047] (3) After filtration, use 500g of 95% ethanol at 10℃ to wash the filter cake and then filter again.

[0048] (4) After filtration, purge with dry nitrogen for 5 minutes and dry with an explosion-proof negative pressure dryer at 70°C for 30 minutes to obtain refined 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 content in the product was 82ppm.

[0050] Example 2

[0051] Except for step (2), in which a mixed solvent of 600g pure water and 600g ethanol is used as the washing solvent, 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 content in the product was 79ppm.

[0053] Example 3

[0054] Except for step (2), in which a mixed solvent of 500g pure water and 700g ethanol is used as the washing solvent, 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 content in the product was 85ppm.

[0056] Example 4

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

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

[0059] Example 5

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

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

[0062] Example 6

[0063] Except for replacing ethanol with isopropanol in steps (2) and (3), the rest is the same as 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 for the temperature of the washing solvent in step (2) being 20°C, everything else 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 content in the product was 141 ppm.

[0068] Comparative Example 1

[0069] Except for the temperature of the mixed solvent being 25°C in step (2), everything else is the same as in Example 1.

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

[0071] Comparative Example 2

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

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

[0074] Comparative Example 3

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

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

[0077] Comparative Example 4

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

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

[0080] Comparative Example 5

[0081] Except for step (3), in which 25°C and 95% ethanol are used for cleaning, 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%. A portion of the product was dissolved in ethanol.

[0083] Comparative Example 6 (Prioritized Dual-Solvent Extraction Method)

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

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

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

[0087] (4) After filtration, use 500g of 95% ethanol at 10℃ to wash the filter cake and then filter again.

[0088] (4) After filtration, purge with dry nitrogen for 5 minutes and dry with an explosion-proof negative pressure dryer at 70°C for 30 minutes to obtain refined lactide.

[0089] Testing revealed that the optical purity of L-lactide in the product was 95.2%, the yield was 88.2%, and the water content was ≤0.5%. This method results in significant fluctuations in product quality and makes the process difficult to control.

Claims

1. A method of purifying propiolactone, comprising: The crude lactide is treated by condensation and crushing to obtain solid lactide powder, the average particle size of the solid lactide powder is not more than 500 μm, the solid lactide powder is mixed with a mixed solution of water and a low-carbon alcohol at a mass ratio of 1:0.7-1.5 as a washing solvent at a temperature of 10-15 ℃, the mixing and contacting time is 20-59 s, the solid lactide powder is separated, the filter cake is washed, the solvent used for washing the filter cake is a low-carbon alcohol, the temperature for washing the filter cake is ≤15 ℃, and the refined lactide is obtained by drying, wherein the low-carbon alcohol is at least one selected from ethanol and propanol.

2. The method of claim 1, wherein, The solid lactide powder is mixed with the washing solvent at a mass ratio of 1:1-3.

3. The method of claim 1, wherein, The condensation and crushing treatment is that the molten crude lactide is treated by condensation and crushing to obtain a solid powder, the crude lactide is first condensed and sliced by a crystallization condensation slicer to obtain crude lactide slices, and then the crude lactide slices are ground into powder by a grinding device.

4. The method of claim 1, wherein, The process of washing the solid lactide powder with the washing solvent is realized by a solid-liquid mixing homogenizer pump, the solid lactide powder is sucked by a vacuum suction device, and the solid feed is dry air.

5. The method of claim 1, wherein, The drying is performed at a temperature of 50-70 ℃ for ≤1.5 h.

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