Method for refining lactide by coupling secondary extraction with rectification

Abstract

The invention relates to a method for refining lactide by coupling secondary extraction with rectification, which comprises the following steps: carrying out first extraction on crude lactide, carrying out second extraction on a first raffinate phase by using a first extraction agent which is water or a mixed solvent of water and low-carbon alcohol, the second extraction agent is a solvent which is immiscible with water and lactide or is low in solubility and has the density between that of water and lactide, obtaining a wet solid with the main component of L-lactide, carrying out reduced pressure drying and melting, then carrying out rectification, and collecting a light component which is a lactide product. According to the method, the lactide is purified and refined by adopting a secondary extraction coupling rectification process, two solvents are adopted for secondary extraction, optical purification is performed firstly, and then chemical purification is performed through rectification, so that the design and operation difficulty of rectifying and purifying the lactide is effectively reduced, and the product yield and quality are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer materials, and specifically relates to the purification and refinement of lactide, specifically a method for purifying lactide by using a secondary extraction process and coupling it with a rectification process. Background Art

[0002] Currently, commercially available biodegradable plastics include polylactic acid (PLA), polyhydroxyalkanoates (PHA), polybutylene succinate (PBS), polybutylene succinate adipate (PBSA), polybutylene adipate terephthalate (PBAT), etc. Among them, PLA is currently the most widely used and has the most prominent application prospects. It not only has the basic properties of general polymer materials, but also is more excellent in terms of processing performance, physical and mechanical properties, and biodegradability. It can be widely applied to the packaging industry, textile industry, agricultural industry, consumer goods market, etc., and is considered the most likely biodegradable material to replace petroleum-based polyesters. Currently, the global PLA production capacity is about 500,000 tons, and the output is more than 200,000 tons. The main production enterprises include NatureWorks in the United States, Corbin Purac in the Netherlands, Mitsubishi in Japan, Zhejiang Hisun, etc. Among them, NatureWorks and Corbin Purac in the Netherlands account for about 70% of the global production capacity. There are not many production lines that have been built and put into production in China. Among them, Zhejiang Hisun is currently the largest in scale. At present, the expansion of China's PLA production capacity has accelerated. As of the end of November 2020, according to incomplete statistics, there are at least 1.53 million tons of PLA projects under construction or proposed in China.

[0003] Industrial polylactic acid synthesis is mainly achieved through ring-opening polymerization of lactide: first, lactide is prepared from lactic acid; then, polylactic acid is obtained by ring-opening polymerization of lactide. The molecular weight of PLA obtained in this process can reach from one hundred thousand to one million. Among them, lactide is the key in the whole synthesis process, and the process barrier is relatively high. Currently, lactide is synthesized from lactic acid by one-step and two-step methods. The two-step method is the commonly used method in industry at present, that is, lactic acid is dehydrated and condensed to form 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), meso-lactide, etc. For example, the purity of L-lactide in current industrial production is generally between 80% and 85%, among which the content of m-lactide is between 6.0% and 10.0%, the content of lactic acid polymers is between 4.0% and 10.0%, and the content of lactic acid is between 2.0% and 3.0%. The presence of these impurities will have a certain impact on the quality of PLA. For example, the presence of acidic media such as water and lactic acid will affect the stability and polymerization molecular weight of PLA. Meso-lactide is easy to hydrolyze, and its presence will affect the ring-opening polymerization process of lactide, resulting in a lower molecular weight of PLA, and then affecting the mechanical properties, stability, etc. of polylactic acid. Moreover, lactide has a relatively high boiling point and freezing point, is heat-sensitive, and is extremely easy to absorb water and open the ring, making its purification and refinement very difficult.

[0004] Common L-lactide purification and refining processes include solvent recrystallization, distillation, water extraction, and melt crystallization. Solvent recrystallization suffers from poor recrystallization results, making it difficult to achieve the high-purity requirements required for industrial production. While increasing the number of crystallizations can improve product purity, this can lead to increased product losses and reduced yields. This is particularly true when the crude lactide contains a high concentration of m-lactide, further reducing the effectiveness of this method in purifying L-lactide. Melt crystallization, which utilizes differences in freezing points between components for crystallization and purification, is typically the final step in the refining process. This process places high demands on feed purity. To achieve a high-yield, high-quality lactide product, the raw material generally requires a lactic acid content of no more than 2.0%, a polymer content of no more than 1.5%, and an m-lactide content ≤ 2.5%. Excessive levels of any one impurity can affect the purification process due to co-crystallization and oversaturation. The water extraction process mainly uses the difference in hydrolysis rates between L-lactide and m-lactide and the solubility of impurity components in the water polarity system to purify crude lactide. It is particularly suitable for situations where the m-lactide content is high. It also has a good removal effect on lactic acid, but it is not effective in removing lactic acid dimers, trimers and colored substances. The distillation process mainly uses the boiling point difference between different components of crude lactide to purify lactide. It is the current mainstream industrial purification process. The key impurity components in crude lactide, such as lactic acid, have a boiling point of 215°C at atmospheric pressure, lactic acid dimer has a boiling point of 350°C at atmospheric pressure, and trimers and tetramers have even higher boiling points. Compared with the boiling point of lactide at atmospheric pressure of 260°C, the difference is large and it is easier to separate. However, the boiling point of its optical isomer m-lactide is smaller than that of L-lactide. At the same time, because lactide is a heat-sensitive substance, To avoid operating at excessively high temperatures, high vacuum distillation is generally required. However, under high vacuum conditions, the boiling point difference between m-lactide and L-lactide is even smaller. For example, when the system pressure is below 1.0 kPa, the boiling point difference is only 10°C. Therefore, the number of theoretical plates required for separation is typically 40-60, which can easily lead to thermal polymerization at the bottom of the reactor. Consequently, the lactide yield in a single-pass distillation purification process is relatively low. Furthermore, obtaining high-quality lactide generally requires three or four distillation columns in series, resulting in low overall product yield. Therefore, to isolate lactide of high optical purity, it is usually necessary to couple the above processes.

[0005] CN110498787B discloses a purification system and method for crude lactide. The process of purifying lactide is carried out by first distillation and then melt crystallization. Specifically, the crude lactide is first purified by the distillation process, and medium-purity lactide is taken out from the side line. Then it is refined by melt crystallization. The mother liquor after refining is purified by recrystallization and then returned to the distillation or melt crystallization system. The whole system is coupled to purify lactide. The crude lactide directly enters the distillation system. Since the boiling points of m-lactide and L-lactide are similar, and the presence of a large amount of acidic components will cause the ring-opening thermal polymerization of lactide at the bottom of the kettle, which not only affects the yield of the distillation system but also results in high energy consumption and great operation difficulty in the whole process.

[0006] CN114478471A discloses a lactide purification system and purification process, which couples melt crystallization and distillation processes. The process of purifying lactide is carried out by first melt crystallization, then distillation, and then coupling with static melt crystallization. Specifically, the crude lactide first enters the continuous crystallization unit for melt crystallization purification, and then mother liquor, sweating liquid and product are obtained. The sweating liquid is returned to the continuous crystallizer. The mother liquor contains a relatively large amount of m-lactide and is further purified by the distillation process. The components mainly composed of m-lactide are taken out from the top of the column and crystallized by the static melt crystallizer to obtain m-lactide products. The components mainly composed of L-lactide are taken out from the side line of the distillation column and returned to the continuous crystallization section. Although this invention realizes the co-production of L-lactide and m-lactide through the coupling of several processes, the crude lactide directly enters the melt crystallizer, and the content of impurity components is about 10%. The operation difficulty is relatively large, the product yield and quality cannot be guaranteed, and the mother liquor containing a large amount of m-lactide enters the distillation system, resulting in high energy consumption of the distillation column, increased degree of bottom thermal polymerization, and great separation difficulty.

[0007] CN114507209A discloses a method for purifying lactide with high yield and high purity. In this process, water is first used to purify the crude lactide, and then lower-carbon alcohol is added for further reaction. After filtration, the filtrate is re-extracted with a water-insoluble organic solvent that can dissolve lactide to obtain lactide therein. After cooling crystallization and filtration, the obtained filter cake is washed and dried together with the filter cake in the previous step to obtain lactide products. This process can preferably remove racemized lactide and other impurity components in the crude lactide. However, the water content in the filter cake is generally 5.0%-10%, which is extremely easy to cause secondary hydrolysis of lactide during the drying process, affecting the product quality, and causing an excessive burden on the subsequent drying. In addition, chloroform has a relatively high solubility in lactide, and the crystallization curve changes insignificantly in the medium and low temperature regions, resulting in a relatively low yield in the crystallization process, and the whole purification process is an intermittent operation, so the production efficiency is relatively low. Summary of the Invention

[0008] Aiming at the deficiencies of the prior art, the present invention provides a method for refining lactide by secondary extraction coupled with rectification. By adopting the secondary extraction process and selecting appropriate extractants, impurities mainly composed of m-lactide, as well as water, lactic acid, etc. are removed first, mainly for optical purification to reduce the acidity of the rectification feedstock. Then, impurities mainly composed of lactic acid polymers and organic pigments are removed through the rectification process for chemical purification, obtaining a lactide product meeting the requirements of polymerization grade. The whole process couples optical purification with chemical purification, effectively reducing the load and design difficulty of the rectification column, with low process energy consumption, high product yield, and good product quality.

[0009] To achieve the above technical objectives, the technical solution adopted by the present invention is as follows:

[0010] The technical objective of the present invention is to provide a method for refining lactide by secondary extraction coupled with rectification, including:

[0011] Mix the crude lactide with a first extractant for the first extraction. The first extractant is water or a mixed solvent of water and lower alcohols. Stir to make m-lactide enter the extraction phase and L-lactide remain in the raffinate phase. After standing, discharge the first extraction phase. Add a second extractant to the first raffinate for the second extraction. The second extractant is a solvent that is immiscible with water, insoluble or has low solubility in lactide, and has a density between that of water and lactide. After adding the second extractant, stir and mix, and then stand. The mixture is divided into three phases. The main components of the first phase are water and lactic acid. The middle phase is the second extractant and part of the impurities dissolved therein, which is the second extraction phase. The third phase mainly consists of L-lactide. Discharge the first phase, separate the solid-liquid of the middle phase and the third phase, perform vacuum drying - melting treatment on the separated wet solid phase to separate the remaining second extractant, and rectify the molten material after vacuum drying to collect the light components, which are the lactide products.

[0012] Those skilled in the art should understand that a solvent with insoluble or low solubility in lactide is a poorly soluble solvent of lactide recognized in the prior art. Specifically, low solubility in lactide means that the solubility of lactide is no higher than 2.0 g / 100 g of solvent (25 °C).

[0013] Furthermore, the function of the first extractant is to optically purify the product, that is, to remove impurity components mainly composed of m-lactide. Therefore, one of the optimal embodiments of the first extractant is water. By mainly utilizing the difference in hydrolysis rates of L-lactide and m-lactide in water, and by controlling the weight ratio of the materials, the extraction temperature, and the extraction time, it is possible to ensure that all or nearly all of the m-lactide is hydrolyzed, while reducing the hydrolysis rate of L-lactide to the lowest level, thereby achieving the removal of this key component of m-lactide and ensuring the yield of the L-lactide product. At the same time, through the first extraction process, most of the lactic acid, a small amount of polymers, and pigment components in the crude lactide are also removed simultaneously. In addition, the inventor believes that mixing water with a solvent that is miscible with water may also achieve the same effect. After repeated attempts by the inventor, it is determined that this mixed solvent can be a lower alcohol. Therefore, another optimal embodiment of the first extractant is a mixed solvent of water and a lower alcohol. The lower alcohol is miscible with water and can increase the dissolution rate of lactide. By selecting appropriate conditions, it is also possible to make all of the m-lactide enter the extraction phase while reducing the loss of L-lactide. Among them, in the mixed solvent of water and a lower alcohol, the weight ratio of water to the lower alcohol is 1:1 - 1:0.25, and the lower alcohol is selected from at least one of methanol, ethanol, n-propanol, and isopropanol.

[0014] Furthermore, to determine the first extraction conditions, the inventors of this case conducted an in-depth exploration of the solubility and hydrolysis rates of the two lactide optical isomers, m-lactide and L-lactide, in water, and found that the solubility and hydrolysis rates of L-lactide and m-lactide in water both increase significantly with the increase in temperature. By comprehensively considering the variation laws of the solubility and hydrolysis rates of the two isomers in water at different temperatures, the difference in their hydrolysis rates, the material ratio, and the content characteristics of m-lactide and L-lactide in the crude lactide, in order to achieve good removal of m-lactide by the first extractant and at the same time ensure the yield of L-lactide, the first extraction conditions are as follows: the mixed weight ratio of the crude lactide and the first extractant is 1:2 - 1:0.3, preferably 1:1 - 1:0.5; the temperature of the first extraction is 3 - 25°C, preferably 5 - 20°C; the time of the first extraction is 10 - 40 min, preferably 10 - 25 min.

[0015] Furthermore, the first extraction phase is a homogeneous liquid, mainly a solution of the first extractant containing lactic acid, and also contains a very small amount of L-lactide. Among them, lactic acid can be recovered through processes such as concentration. The optical purity of L-lactic acid is between 55% and 70%, and it can be used to synthesize lactate esters for industrial application fields; the first raffinate phase is a wet solid component, which mainly consists of L-lactide, and contains a small amount of components such as lactic acid, lactic acid polymers, and water.

[0016] Furthermore, the above method also includes a process of treating the first extraction phase to recover lactic acid.

[0017] Further, the second extractant is chlorobenzene. Chlorobenzene is a solvent that is immiscible with water, insoluble or has low solubility in lactide, and has a density between that of water and lactide. The main function of the second extractant is to separate water, lactic acid from L-lactide, reducing the re-hydrolysis caused by the subsequent re-treatment of water-containing L-lactide.

[0018] More specifically, chlorobenzene mainly extracts some organic pigments into the extract phase, while water and lactic acid are in the raffinate phase and located in the upper layer. The solid-phase component mainly composed of L-lactide is heavier and is at the bottom of the extract phase. By separation, an L-lactide component mainly composed of solid phase with a small amount of extract phase is obtained, achieving the purpose of removing water, lactic acid and some organic pigments.

[0019] Further, the addition amount of the second extractant is 0.5 - 1.5 times that of the initial crude lactide feedstock, the temperature of the second extraction is 5 - 25 °C, and the time of the second extraction is 10 - 40 min.

[0020] Further, the above method also includes the process of recycling and reusing the second extractant after solid-liquid separation in the secondary extraction.

[0021] Further, the first extraction and the second extraction are respectively completed by an extraction device, and the extraction devices are all solid-liquid extraction and separation devices, which can be in the form of an extraction tower or a centrifugal extractor, etc. Preferably, the stirring and mixing process and the extraction process of the first extraction are respectively completed by a stirring and mixing device and an extraction and separation device, so that the materials are first mixed in the stirring and mixing device and then extracted and separated by the extraction and separation device, which will have better effects.

[0022] Further, the crude lactide is prepared from lactic acid by one-step direct cyclization or two-step polycondensation - depolymerization, and can be melted into a liquid first or mixed with the first extractant after being pulverized. The composition of the crude lactide is generally: L-lactide ≥ 78%, m-lactide 6.0% - 12.0%, L-lactic acid 1.0% - 4.0%, lactic acid polymers (dimers, trimers, etc.) 3.0% - 10.0%.

[0023] Further, the purpose of the reduced-pressure drying-melting treatment is to remove the residual second extractant in the wet solid phase and melt the solid lactide simultaneously. The material is preheated before reduced-pressure drying, and the pressure control for reduced-pressure drying needs to be different, which can be determined by those skilled in the art according to the physical properties of the second extractant. Specifically, when the second extractant is chlorobenzene, the normal boiling point of chlorobenzene is 132.2 °C, the boiling point is about 95 °C under 34 kPa, and the boiling point is about 84 °C under 20 kPa. Therefore, the temperature of reduced-pressure drying needs to be controlled at 100-130 °C, and the pressure is controlled at 30-45 kPa. At this temperature and pressure, on the one hand, the rapid removal of the second extractant can be ensured, and at the same time, the transformation of lactide from solid state to liquid state can be realized.

[0024] Further, the above method also includes the process of recycling the solvent obtained by reduced-pressure drying as the second extractant for reuse.

[0025] Further, the lactide after reduced-pressure drying-melting is heated to 120-150 °C by a heater and then sent to the rectification process for purification. The rectification process is carried out using a rectification column. The rectification column is filled with low-pressure-drop integral packing, and the number of theoretical plates is 8-14. The bottom of the column is heated by a falling-film reboiler, and the top of the column is equipped with an in-built condenser. The heating temperature at the bottom of the rectification column is 140-170 °C, the top condenser further reduces the column pressure drop, the top pressure is 200-600 Pa, the condensation temperature is 97-105 °C, the reflux ratio is 1:2-1:5, and the lactide product is obtained at the top of the column, and the heavy components are discharged at the bottom of the column.

[0026] Further, the heavy components at the bottom of the rectification column are mainly lactic acid polymers, among which the content of L-lactide is 50.0%-60.0%, the content of lactic acid dimer is 2.3%-4.9%, the content of lactic acid trimer is 2.6%-7.5%, the content of lactic acid tetramer is 3.2%-5.8%, the content of lactic acid pentamer is 3.5%-6.9%, and the rest are polymers of higher lactic acid units such as lactic acid hexamer.

[0027] Under the process method of the present invention, the chemical purity of the L-lactide product obtained after rectification is ≮99.0%, and the optical purity is ≮99.5%, meeting the requirements of polymerization-grade lactide.

[0028] Under the process method of the present invention, the yield of lactide in the whole purification and refinement process is not less than 92.0%.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] (1) The present invention purifies and refines lactide by a secondary extraction coupled with rectification process. Two solvents are used for secondary extraction. First, optical purification is carried out, and then chemical purification is carried out through rectification, effectively reducing the difficulty of the design and operation of rectifying and purifying lactide, improving the product yield and quality. The lactide yield is not less than 92.0%, and the chemical purity of the product is ≮99.0%, and the optical purity is ≮99.5%.

[0031] (2) The present invention first removes the impurity components mainly composed of m-lactide through a first extractant, and then uses a second extractant that is immiscible with water and insoluble in lactide to extract and remove pigments, and removes impurities such as lactic acid and water by using the density difference, achieving the purpose of optical purification and partial chemical purification, and also removing the water component therein to prevent the hydrolysis of L-lactide during subsequent treatment. The order of these two steps is crucial; then, further chemical purification is carried out again through vacuum drying and rectification processes. The entire design process ensures the product yield and quality.

[0032] Other features and advantages of the present invention will be described in detail in the subsequent specific implementation section. Specific implementation mode

[0033] The method for purifying lactide and its effects of the present invention will be further described below through examples. The examples are implemented on the premise of the technical solution of the present invention, and detailed implementation methods and specific operation processes are given, but the protection scope of the present invention is not limited to the following examples.

[0034] In the following examples, the experimental methods are all conventional methods in the art unless otherwise specified. The experimental materials used in the following examples can be obtained from biochemical reagent stores unless otherwise specified.

[0035] The present invention uses an Agilent high-performance liquid chromatograph to analyze the chemical purity of lactide, with a UV detector, using phosphoric acid and acetonitrile as the mobile phase. The chromatographic column model is ZORBAX SB-Aq, with a column length of 250 mm, a column inner diameter of 4.6 mm, and the inner packing particle size of 5 μm. Detection wavelength: 200 nm, column temperature: 40 °C, flow rate: 1 mL / min, injection volume: 5 μL.

[0036] The present invention uses an Agilent gas chromatograph to analyze the content of different optical isomers of lactide. A CYCLOSIL-B type chromatographic column is selected, the vaporization chamber temperature is 250 °C, the detector temperature is 280 °C, a hydrogen flame ionization detector is used, and the column temperature is programmed with an initial temperature of 100 °C, held for 5 min, heated at a rate of 4 °C / min to 140 °C, held for 7 min, and then heated at a rate of 8 °C / min to 200 °C, held for 20 min. The carrier gas N2 flow rate is 1.4 mL / min, the hydrogen flow rate is 30 mL / min, the air flow rate is 400 mL / min, and the injection volume is 0.5 μL.

[0037] The calculation formula for the yield Y of the lactide purification process is as follows:

[0038]

[0039] Among them, m0 is the weight of the crude lactide, y0 is the purity of L-lactide in the crude lactide, m is the weight of the lactide in the gas-phase product obtained by rectification, and y is the purity of L-lactide in the refined lactide.

[0040] The specific rotation of the sample is analyzed using a 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 for the optical purity X of the sample is as follows;

[0041]

[0042] Among them, α 纯物质 represents the specific rotation of pure lactide, and α 被测样品 represents the specific rotation of the substance to be measured.

[0043] Example 1

[0044] This example provides a method for refining lactide by secondary extraction coupled with rectification:

[0045] Composition of crude lactide (the following are weight percentages): L-lactide 80.4%, m-lactide 8.2%, L-lactic acid 2.1%, lactic acid polymers (dimers, trimers, etc.) 9.1%.

[0046] (1) The crude lactide is mixed with the first extractant at a weight ratio of 1:1. The first extractant is water, and the extraction temperature of the first extraction is 10°C. After stirring for 20 min, it is left to stand, and solid-liquid separation is carried out to obtain a wet first raffinate phase;

[0047] (2) The second extractant is added to the first raffinate phase. The second extractant is chlorobenzene, and the addition amount is 0.5 times the mass of the crude lactide in step (1). The extraction temperature of the second extraction is 10°C, and the extraction time is 20 min; it is stirred and left to stand. The mixture is divided into three phases. The upper part is a mixed liquid mainly composed of water and lactic acid, the middle part is the second extraction phase that has extracted components such as pigments in the first raffinate phase, and the lower part is mainly solid L-lactide. Separation is carried out on it to obtain a wet L-lactide solid, and the second extraction phase is recycled directly or after simple treatment.

[0048] (3) The wet L-lactide solid is heated to 105°C for vacuum drying, and the pressure is 35 kPa. During this process, as the vacuum drying process proceeds, the lactide gradually melts into a liquid state.

[0049] (4) The liquid molten lactide is sent to a rectification device for purification. The rectification process is carried out using a rectification column. The rectification column is filled with low-pressure-drop integral packing, and the number of theoretical plates is 8. The bottom of the rectification column is heated by a falling-film reboiler, and the top of the column is equipped with an internal condenser. The heating temperature at the bottom of the rectification column is 165 °C. The condenser at the top further reduces the column pressure drop. The top pressure is 250 Pa, the condensation temperature is 98 °C, the reflux ratio is 1:3, and the lactide product is obtained at the top of the column, while the heavy components are discharged at the bottom of the column.

[0050] After analysis, in the lactide product, the chemical purity of lactide is 99.2%, and the optical purity is 99.6%, meeting the requirements of the polymerization-grade lactide product. The yield of the entire lactide purification process is 92.5%.

[0051] Example 2

[0052] Composition of crude lactide (the following are weight percentages): L-lactide 78.5%, m-lactide 11.2%, L-lactic acid 1.9%, lactic acid polymers (dimers, trimers, etc.) 8.5%.

[0053] (1) The crude lactide is mixed with the first extractant at a weight percentage of 1:0.8. The first extractant is a mixed solution of water and ethanol, and the weight ratio of water to ethanol is 1:0.5. The extraction temperature of the first extraction is 20 °C. After stirring for 15 min, it is left to stand, and solid-liquid separation is carried out to obtain a wet first raffinate phase.

[0054] (2) The second extractant, which is chlorobenzene, is added to the first raffinate phase. The addition amount is 1.0 times the mass of the crude lactide in step (1). The extraction temperature of the second extraction is 15 °C, and the extraction time is 15 min. After stirring and standing, the mixture is divided into three phases. The upper part is a mixed solution mainly composed of water and lactic acid, the middle part is the second extraction phase that has extracted components such as pigments in the first raffinate phase, and the lower part is mainly solid L-lactide. It is separated to obtain a wet L-lactide solid, and the second extraction phase is reused directly or after simple treatment.

[0055] (3) The wet L-lactide solid is heated to 110 °C for vacuum drying, and the pressure is 40 kPa. During this process, as the vacuum drying progresses, the lactide gradually melts into a liquid state.

[0056] (4) The liquid molten lactide is sent to a rectification device for purification. The rectification process is carried out using a rectification column. The rectification column is filled with low-pressure-drop integral packing, and the number of theoretical plates is 10. The bottom of the rectification column is heated by a falling-film reboiler, and the top of the column is equipped with an internal condenser. The heating temperature at the bottom of the rectification column is 165 °C. The condenser at the top further reduces the column pressure drop. The top pressure is 200 Pa, the condensation temperature is 98 °C, the reflux ratio is 1:2, and the lactide product is obtained at the top of the column, while the heavy components are discharged at the bottom of the column.

[0057] After analysis, in the lactide product, the chemical purity of lactide is 99.1%, and the optical purity is 99.5%, meeting the requirements of polymer-grade lactide products. The yield of the entire lactide purification process is 92.1%.

[0058] Example 3

[0059] Composition of crude lactide (the following are weight percentages): L-lactide 84.4%, meso-lactide 6.7%, L-lactic acid 3.2%, lactic acid polymers (dimers, trimers, etc.) 5.6%.

[0060] (1) Mix the crude lactide with the first extractant at a weight ratio of 1:1.2. The first extractant is a mixture of water and isopropanol, and the weight ratio of water to isopropanol is 1:1. The extraction temperature of the first extraction is 5°C. After stirring for 30 min, let it stand, and perform solid-liquid separation to obtain the wet first raffinate phase;

[0061] (2) Add the second extractant to the first raffinate phase. The second extractant is chlorobenzene, and the addition amount is 0.75 times the mass of the crude lactide in step (1). The extraction temperature of the second extraction is 10°C, and the extraction time is 20 min; stir and let it stand. The mixture is divided into three phases. The upper part is a mixed liquid mainly composed of water and lactic acid, the middle part is the second extraction phase that extracts components such as pigments in the first raffinate phase, and the lower part is mainly L-lactide solid. Separate it to obtain the wet L-lactide solid, and the second extraction phase can be recycled directly or after simple treatment.

[0062] (3) Heat the wet L-lactide solid to 120°C for vacuum drying, and the pressure is 45 kPa. During this process, as the vacuum drying process progresses, lactide gradually melts into a liquid state.

[0063] (4) Send the liquid molten lactide to a rectification device for purification. The rectification process is carried out using a rectification column. The rectification column is equipped with low-pressure-drop integral packing, and the number of theoretical plates is 8. The bottom of the column is heated by a falling-film reboiler, and the top of the column is equipped with an internal condenser. The heating temperature at the bottom of the rectification column is 170°C, the top condenser further reduces the column pressure drop, the top pressure is 300 Pa, the condensation temperature is 100°C, the reflux ratio is 1:4, and the lactide product is obtained at the top of the column, and the heavy components are discharged at the bottom of the column.

[0064] After analysis, in the lactide product, the chemical purity of lactide is 99.3%, and the optical purity is 99.7%, meeting the requirements of polymer-grade lactide products. The yield of the entire lactide purification process is 92.8%.

[0065] Comparative Example 1

[0066] The crude lactide is directly fed into the rectification system after chemical purification, indicating that the technical solution of the present invention can reduce the operation difficulty of the rectification column:

[0067] Using the crude lactide of Example 1 as the raw material, the chemical purification of the crude lactide was carried out by ethanol recrystallization process. The specific process is as follows:

[0068] (1) The crude lactide and ethanol were mixed at a mass ratio of 1:1. After heating to 60 °C until the lactide was completely dissolved, the program cooling was started, and the temperature was decreased at a cooling rate of 0.5 °C / min to about 15 °C. Then, it was filtered and dried to obtain the purified lactide product. After analysis, the content of L-lactide was 93.2%, the content of meso-lactide was 4.3%, and the yield was 81.4%.

[0069] (2) After the above purified lactide was melted at 120 °C, it was fed into the distillation system for further purification and refinement. The form of two towers in series was adopted. The first distillation tower was mainly for optical purification, removing a small amount of light components and meso-lactide in the lactide. Since meso-lactide and L-lactide are optical isomers and the physical system data are similar, the number of theoretical plates required for general separation is 40 - 60. Here, 45 theoretical plates were selected, and the same low-pressure-drop packing as in Example 3 was selected. To ensure the rise of gas-phase components, the bottom heating temperature needed to be increased to 180 - 190 °C. On the one hand, the equipment energy consumption was relatively high. On the other hand, since lactide is a thermosensitive physical system, the degree of thermal polymerization at the bottom of the kettle increased, reducing the product yield. And the second distillation tower was mainly for chemical purification. The same distillation tower and process conditions as in Example 3 were selected. Finally, the chemical purity of the obtained lactide was 99.4%, and the optical purity was 99.6%. Although the purity could be guaranteed, the yield was relatively low, only 70.3%.

[0070] Compared with the present invention, when adopting the method of first recrystallization and then distillation, generally two distillation towers or the form of side-stream extraction of one distillation tower are required to obtain the lactide product that meets the requirements. However, since the effect of removing meso-lactide in the recrystallization process is not obvious, and the distillation process requires optical purification, the number of theoretical plates required is large and the energy consumption increases by about 30% relatively.

[0071] If the direct purification of the crude lactide adopts the form of multiple distillation towers in series, there is also a process of optical purification using the distillation tower, and the energy consumption is also very large.

[0072] Comparative Example 2

[0073] After being dried by the first extraction device, it was directly fed into the distillation system:

[0074] The crude lactide used and step (1) are the same as those in Example 1. After obtaining the wet first raffinate phase, a wet filter cake is obtained through filtration. It is measured that the water content in the filter cake is 7.5%. It is placed in a vacuum drying system at 60 °C and 1 kPa for drying for 6.0 h to obtain purified lactide. After analysis, the content of L-lactide is 94.5%, and the content of meso-lactide is 0.2%. There is a rehydrolysis phenomenon during the drying process of the product.

[0075] After melting the above lactide at 120 °C, it is sent to the rectification system described in Example 1 for refining, and a lactide product with a chemical purity of 99.0% and an optical purity of 99.5% can be obtained. The yield of the entire purification process is about 88.5%.

[0076] Compared with Example 1, the main reason for the low yield in this process is that the first extraction phase obtained by the first extraction is filtered and dried and then sent to the rectification system. The moisture content in the filter cake after filtration is relatively high, causing the rehydrolysis of lactide during the drying process, affecting the quality of the obtained purified lactide, increasing the acidity of the product, and thus affecting the yield and purity of the product during the rectification process.

[0077] Comparative Example 3

[0078] As a comparison, the crude lactide used in Example 3 of the present invention is used as the raw material, and the process conditions in Example 1 of CN 114507209A are used for product purification. Specifically as follows: 1) Take 200 g of the above crude lactide, crush it to a diameter of 0.5 - 1.0 mm with a crusher, add it to the reaction kettle at one time, and then add 200 g of deionized water. Keep the system temperature at 5 °C and react at 200 rpm for 5 min to obtain a water extraction feed liquid; 2) Add 100 g of absolute ethanol to the water extraction feed liquid, maintain the reaction temperature at 5 °C, continue stirring and reacting for 1 min, and then filter to obtain a filter cake and a filtrate; 3) Take the above filtrate, add chloroform according to the mass ratio of the filtrate to chloroform of 7:1, react at room temperature for 10 min, then let it stand for 20 min to obtain a feed liquid mainly composed of chloroform phase. Cool it at a rate of 0.5 °C / min. After the temperature drops to 0 °C, keep it at a constant temperature for 2.0 h, and lactide crystals precipitate out. Filter to obtain a filter cake; Wash it together with the filter cake in step 2) with water and dry it at 70 °C for 8.0 h to obtain purified lactide.

[0079] After detection and calculation, its chemical purity is 99.0%, its optical purity is 99.2%, and the yield of L-lactide is 90.12%.

[0080] Compared with the method of the present invention, on the one hand, the water content in the filter cake obtained by this method is about 6.5%. Since lactide is easily hydrolyzed in water, it is difficult to dry, and hydrolysis is likely to occur in the product. On the other hand, this method recovers the residual L-lactide in the filtrate after extraction with water and lower alcohols. Due to the relatively high solubility of chloroform in lactide, the amount of lactide recovered in a single pass is small, and multiple crystallizations are required, which increases the operation process and complexity. Considering the overall efficiency, the present invention does not recover this part of L-lactide. On the other hand, this method is an intermittent batch operation process and it is difficult to achieve continuous operation, while the present invention can achieve continuous and stable operation through the series operation of an extraction tower and a distillation tower, greatly improving the purification efficiency.

Claims

1. A method for refining lactide by secondary extraction coupling with rectification, comprising: Mixing crude lactide with a first extractant for first extraction, wherein the first extractant is water or a mixed solvent of water and a lower alcohol, stirring to make m-lactide enter the extraction phase and L-lactide remain in the raffinate phase. After standing, the first extraction phase is discharged. A second extractant is added to the first raffinate for second extraction. The second extractant is a solvent that is immiscible with water, insoluble or has a low solubility in lactide, and has a density between that of water and lactide. After adding the second extractant, stir and mix, and then stand. The mixture is divided into three phases. The main component of the first phase is water and lactic acid, the middle phase is the second extractant and part of the impurities dissolved therein, which is the second extraction phase, and the third phase mainly consists of L-lactide. The first phase is discharged, the middle phase and the third phase are subjected to solid-liquid separation, the separated wet solid phase is subjected to vacuum drying-melting treatment to separate the remaining second extractant, and the melted material after vacuum drying is rectified to collect the light components, which are lactide products.

2. The method according to claim 1, characterized in that, The mixing weight ratio of the crude lactide to the first extractant is 1:2 - 1:0.

3.

3. The method according to claim 1, characterized in that In the mixed solvent of water and a lower alcohol, the weight ratio of water to the lower alcohol is 1:1 - 1:0.

25.

4. The method according to claim 1, wherein The lower alcohol is selected from at least one of methanol, ethanol, n-propanol, and isopropanol.

5. The method according to claim 1, characterized in that, The temperature of the first extraction is 3 - 25 °C, and the time is 10 - 40 min.

6. The method according to claim 1, characterized in that It also includes a process of treating the first extraction phase to recover lactic acid.

7. The method according to claim 1, wherein The second extractant is chlorobenzene.

8. The method according to claim 1, wherein The addition amount of the second extractant is 0.5 - 2.5 times that of the crude lactide based on the initial feeding amount of the crude lactide.

9. The method according to claim 1, characterized in that, The temperature of the second extraction is 5 - 25 °C, and the time of the second extraction is 10 - 40 min.

10. The method according to claim 1, wherein The first extraction and the second extraction are respectively completed by extraction devices, and the extraction devices are all solid-liquid extraction and separation devices.

11. The method according to claim 10, wherein The stirring and mixing process and the extraction process of the first extraction are respectively completed by a stirring and mixing device and an extraction and separation device, so that the material is first mixed in the stirring and mixing device and then extracted and separated by the extraction and separation device.

12. The method according to claim 1, wherein When the second extractant is chlorobenzene, the feeding temperature of the vacuum drying is 100 - 130 °C, and the pressure is 30 - 40 kPa.

13. The method according to claim 1, wherein It also includes a process of recycling the solvent obtained by vacuum drying as the second extractant for reuse.

14. The method according to claim 1, characterized in that, The solid after vacuum drying is heated to 120 - 150 °C by a heater and then depressurized, and then rectified.

15. The method according to claim 1, characterized in that The rectification process is carried out in a rectification column. The rectification column is filled with low-pressure-drop integral packing, and the number of theoretical plates is 8 - 14. The bottom of the column is heated by a falling-film reboiler, and the top of the column is equipped with an internal condenser.

16. The method according to claim 15, wherein, The heating temperature at the bottom of the rectification column is 140 - 170 °C. The condenser at the top of the column further reduces the column pressure drop. The pressure at the top of the column is 200 - 600 Pa, the condensation temperature is 97 - 105 °C, the reflux ratio is 1:2 - 1:5, lactide products are obtained at the top of the column, and heavy components are discharged at the bottom of the column.

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