CN111100107B - Method for removing pigment and impurities in crude lactide - Google Patents

Method for removing pigment and impurities in crude lactide Download PDF

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CN111100107B
CN111100107B CN201811255633.1A CN201811255633A CN111100107B CN 111100107 B CN111100107 B CN 111100107B CN 201811255633 A CN201811255633 A CN 201811255633A CN 111100107 B CN111100107 B CN 111100107B
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lactide
rectifying tower
supercritical
crude
lactic acid
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CN111100107A (en
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孙启梅
彭绍忠
高大成
王鹏翔
姚新武
朱化雷
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Sinopec Dalian Petrochemical Research Institute Co ltd
China Petroleum and Chemical Corp
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Sinopec Dalian Research Institute of Petroleum and Petrochemicals
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    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07D—HETEROCYCLIC COMPOUNDS
    • C07D319/00—Heterocyclic compounds containing six-membered rings having two oxygen atoms as the only ring hetero atoms
    • C07D319/10—1,4-Dioxanes; Hydrogenated 1,4-dioxanes
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P20/00—Technologies relating to chemical industry
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P20/00—Technologies relating to chemical industry
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Abstract

The invention relates to a method for removing pigment and impurities in crude lactide, which comprises the steps of firstly adjusting the temperature and the pressure in a rectifying tower and adding supercritical CO2Feeding into a rectifying tower; then the crude lactide is sent into a rectifying tower for decolorization and impurity removal after being crushed or heated and melted; rectifying the product with CO2Separating to obtain lactide after impurity removal and decoloration. The invention converts supercritical CO2The extraction and rectification processes are combined, so that not only can pigments and impurities in the crude lactide be effectively removed, but also the operation temperature and the operation difficulty in the rectification process can be effectively reduced, the side reactions of lactide coking, oxidation, polymerization, meso-crystallization and the like caused by high temperature are reduced, the yield and the quality of the lactide product are ensured, and the purification method has the advantages of high operation feasibility, simple separation process and high efficiency in impurity removal.

Description

Method for removing pigment and impurities in crude lactide
Technical Field
The invention belongs to the technical field of high polymer materials, and particularly relates to a method for removing pigments and impurities in crude lactide.
Background
Polylactic acid (PLA) has good mechanical strength, biocompatibility, biodegradability and bioabsorbability, is a green high polymer material, and has wide application prospect and application field. With the continuous maturity of polylactic acid production technology and the continuous development of application market thereof, the production of polylactic acid by taking non-food crops → lactic acid → polylactic acid as a route has begun to enter the industrialized development period. Natureworks, usa is the largest producer of polylactic acid in the world at present, with a capacity of 14 ten thousand tons, and L-lactide capacity reaches 15 ten thousand tons. The worldwide production capacity of L-lactide and D-lactide is expected to break through 50 million tons by 2020. At present, the high molecular weight PLA produced at home and abroad is mostly obtained by lactide ring-opening polymerization. Therefore, the key to the synthesis technology of high-quality PLA lies not only in its own polymerization process, but also in the purity and quality of its raw material, L-lactide.
Pure L-lactide has the characteristics of high boiling point, high solidification point, heat sensitivity, easy water absorption, ring-opening reaction and the like, while crude L-lactide prepared by taking L-lactic acid as a raw material is generally light yellow or darker in color, and contains lactic acid, water, lactic acid oligomers (lactic acid dimers and trimers) and,m-lactide and other impurity components. The presence of these impurities can have some effect on the quality of the L-lactide monomer, such as: the existence of water, lactic acid or acidic media such as lactic acid dimer and trimer in lactide monomer seriously restricts the ring-opening polymerization of lactide, so that the molecular weight of PLA is low and the stability of PLA is influenced; whilemLactide is very easy to absorb water and hydrolyze, so that the purity of the lactide is reduced, and the mechanical property of the lactide is influenced. Therefore, the purification and refining difficulty of lactide is great, which is the main technical difficulty of the synthesis process, and is one of the main factors influencing the industrialization of the polylactide.
The commonly used purification and refining processes of L-lactide mainly comprise solvent recrystallization, rectification, solvent extraction, water extraction, melt crystallization and the like. At present, the industrialized L-lactide purification technology adopts a rectification process to pretreat crude lactide and then refine the crude lactide by melt crystallization, but because of the high boiling point, the high solidifying point and the heat sensitivity of the lactide, the rectification operation must be carried out under the high vacuum condition so as to reduce the operation temperature and reduce coking and oxidation, and a heat preservation measure is also needed so as to prevent the lactide from blocking a pipeline due to sublimation. Meanwhile, the existence of impurity water and acid components in the crude lactide can promote the polymerization of the lactide, and the yield of the lactide is not high, so that the crude lactide is pretreated by rectification with certain difficulty.
US 8053584B 2 discloses a method for purifying lactide by rectification process, which comprises passing crude lactide obtained by cracking through a first rectification column, removing heavy components such as lactic acid oligomer in the crude lactide from the bottom of the column, obtaining lactide with most of water and part of lactic acid removed from the top of the column, preferably operating at 20-50kPa, preferably at 200-250 ℃, and obtaining lactide with purity of more than 90% or preferably more than 96%; and then the lactide obtained from the tower top passes through one or more rectification columns to obtain a lactide product with the purity of 99.0 percent or more than 99.5 percent. However, the process requires high temperatures and the presence of impurity components can exacerbate the polymerization of lactide, resulting in a low lactide yield.
CN101857585A discloses a continuous high vacuum purification method of lactide, which adopts three-tower series operation, firstly adopts a first rectifying tower to remove water, lactic acid and partial meso-lactide in crude lactide, then adopts a second rectifying tower to remove all meso-lactide, and finally adopts a third rectifying tower to remove weight, so as to obtain an L-lactide product with the purity of more than 99.0% and the yield of more than 90% at the tower top. The operating pressures of the three towers are respectively 200Pa, 200Pa and 100Pa, and the required vacuum degree in the process is very high, so that the temperature in the rectification process is not too high, the decomposition and thermal polymerization of the lactide in the rectification process are reduced, and the yield and the quality of the lactide are ensured. However, an excessively high degree of vacuum is difficult to achieve in practice, and the number of theoretical plates required is also large because the boiling points of the components are close to each other.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a method for removing pigment and impurities in crude lactide. The invention converts supercritical CO2The extraction and rectification processes are combined, so that not only can pigments and impurities in the crude lactide be effectively removed, but also the operation temperature and the operation difficulty in the rectification process can be effectively reduced, the side reactions of lactide coking, oxidation, polymerization, meso-crystallization and the like caused by high temperature are reduced, the yield and the quality of the lactide product are ensured, and the purification method has the advantages of high operation feasibility, simple separation process and high efficiency in impurity removal.
The method for removing pigment and impurities in crude lactide mainly comprises the following steps:
(1) adjusting the temperature and pressure in the rectifying tower to convert supercritical CO into CO2Feeding into a rectifying tower;
(2) crushing or heating and melting crude lactide, and then feeding the crude lactide into a rectifying tower for decoloring and impurity removal;
(3) rectifying the product with CO2Separating to obtain impuritiesAnd the lactide after decolorization.
In the method, the rectifying tower in the step (1) is filled with supercritical CO2Before, the rectifying tower needs to be pressurized and preheated to 8-15MPa and 36-80 ℃. Preferably, the temperature of the rectifying tower is arranged in a sectional mode, and gradually increases from the bottom of the tower to the top of the tower, and the range is 36-80 ℃.
In the above process, CO is introduced in step (1)2Pressurizing and heating to supercritical state, wherein the pressure is 8-15MPa and the temperature is 36-60 ℃; supercritical CO introduced into a rectification column2The flow rate is 0.5-3.0 kg/h.
In the above method, supercritical CO is introduced2Adding low-boiling-point ester solvent and/or chloroform while flowing, wherein the low-boiling-point ester solvent is at least one of ethyl acetate, methyl acetate, propyl acetate, isopropyl acetate, etc., and the flow rate is CO21-10% of the flow rate. More preferably, a certain amount of petroleum ether or carbon disulfide is added at the same time, the addition amount is 1-10% of the dosage of the low-boiling-point ester solvent, so that colored substances can be further removed, and the product quality is improved.
In the method, the crude lactide in the step (2) is prepared by taking L/D-lactic acid or L/D-lactic acid ester as a raw material and carrying out polycondensation and depolymerization processes, wherein the purity of the L/D-lactide is 78-90%, the content of meso-lactide is not less than 3.7%, the content of lactic acid is not less than 3.4%, and the content of dimer and trimer is not less than 1.0%.
In the above method, the lactide in step (2) may be crushed by a mechanical crushing means such as a crusher, and the diameter after crushing is 35 to 100 mesh. The heating and melting temperature is 96-100 ℃, and the mixture is sent into a rectifying tower after the mixture is completely melted.
In the method, the flow rate of the crude lactide in the step (2) entering the rectifying tower is 0.5-5.0kg/h, and the crude lactide and the supercritical CO are mixed2And (4) carrying out countercurrent contact on the fluid to carry out decolorization and impurity removal.
In the method, the supercritical CO after the rectifying tower in the step (3)2Separating the fluid mixture by a separator to obtain the lactide after impurity removal and decoloration. Preferably using two stagesThe separator comprises a separator I and a separator II, wherein the pressure of the separator I is maintained at 5-9MPa, the temperature is maintained at 50-80 ℃, the separation pressure of the separator II is normal pressure-3 MPa, and the separation temperature is 30-50 ℃.
In the method, the chemical purity of the L/D-lactide obtained after separation in the step (3) is more than 99.0 percent, wherein the content of meso-lactide is less than 0.6 percent, the content of lactic acid is less than 0.3 percent, the content of dimer and trimer is less than 0.5 percent, and the decolorization rate is more than 95 percent; the optical purity of the product is not less than 99.0%, the melting range is 95.8-97.9 ℃, and the yield of the lactide is more than 79.3%. After two or more times of circulating treatment, the product yield can be further improved.
Compared with the prior art, the invention has the beneficial effects that:
(1) the lactide has the characteristics of high boiling point, high solidifying point, heat sensitivity and the like, and the invention uses supercritical CO2The extraction and the rectification process are combined, so that the operation temperature and the operation difficulty in the rectification process can be effectively reduced, the high-temperature and high-vacuum operation is avoided, the side reactions of lactide decomposition, thermal polymerization, meso-crystallization, coking, oxidation, polymerization and the like caused by high temperature are reduced, and the yield and the quality of lactide products are ensured; but also shortens the purification and refining process of the lactide, avoids the re-refining process after the pretreatment, namely combines the crude product pretreatment with the product refining process and adopts one-step supercritical CO2And (3) a lactide product with higher chemical purity and optical purity can be obtained through the rectification process.
(2) By using supercritical CO2The lactide is purified and refined by the rectification system, the highly polar impurity components such as lactic acid, dimer, trimer and pigment can be effectively removed at the bottom of the rectification tower, and the impurities with strong polarity can be effectively separated out in the separator at the top of the towermLactide, to obtain a lactide product with a chemical purity of above 99.0% and an optical purity of above 99.0%.
(3) By passing to supercritical CO2The low boiling point ester solvent is added into the fluid, so that the entrainment of the lactic acid, lactic acid dimer, trimer, water and other impurity components in the lactide can be effectively reduced, and the impurity components in the crude lactide can be better removed, particularly for water, lactic acid dimer, trimer, water and the like,Lactic acid, lactic acid linear polymer and colored substance have better removal effect, and the product purity is improved. After further adding petroleum ether or carbon disulfide, impurity components can be further removed, and the product quality is improved.
(4) By using supercritical CO2The rectification system purifies and refines the lactide, the product yield in the process is high, and the one-way yield is over 79.3 percent. The product yield can be improved to more than 90 percent by two or more times of circulating extraction and rectification.
Detailed Description
The method for removing impurities and pigments and the effect thereof according to the present invention will be further described by examples. The embodiments are implemented on the premise of the technical scheme of the invention, and detailed implementation modes and specific operation processes are given, but the protection scope of the invention is not limited by the following embodiments.
The experimental procedures in the following examples are, unless otherwise specified, conventional in the art. The test materials used in the following examples were purchased from biochemical reagent stores unless otherwise specified.
The yields of purified lactide in the examples were calculated as weight percent based on crude lactide.
The crude lactide adopted in the embodiment of the invention is prepared by taking L/D-lactic acid or L/D-lactic acid ester as a raw material and carrying out polycondensation and depolymerization processes, wherein the purity of the L/D-lactide is 78-90%, the content of meso-lactide is 3.7-8.3%, the content of lactic acid is 3.4-7.2%, and the content of dimer and trimer is 1.2-3.1%.
The invention adopts an HP4890D gas chromatograph to analyze the composition of each component in lactide, adopts a hydrogen ion flame monitor and an HP-INNOWAX capillary column, and adopts two sections of temperature programming, wherein the temperature rising rate is 4 ℃/min at the first section of 140 ℃ and the temperature rising rate is 8 ℃/min at the second section of 140 ℃ and 180 ℃. The melting point of the lactide sample was measured using a WRS-2A digital melting point apparatus, which uses a capillary method to measure, generally speaking, high purity with a narrow melting range and low purity with a wide melting range, thereby characterizing the relative purity of the sample, with melting point of pure L-lactide at 96 ℃ and melting point of pure D-lactide at 96 ℃. Sample analysis using a WZZ-2S automatic polarimeterThe specific optical rotation of the product is represented, so that the optical purity of the sample is represented, the specific optical rotation of the pure L-lactide is-278, the specific optical rotation of the pure D-lactide is +278,m-the specific optical rotation of lactide is 0 and the calculation formula of the optical purity of the sample is as follows;
Figure DEST_PATH_IMAGE002
wherein, XOptical purityDenotes the optical purity, alpha, of the samplePure substanceShowing the specific optical rotation of pure lactide, alphaTested sampleIndicating the specific optical rotation of the substance to be measured.
Example 1
Preparation of crude L-lactide: taking zinc oxide and stannous octoate as catalysts and 88 percent L-lactic acid as raw materials, and obtaining crude L lactide with the purity of 85.7 percent after lactic acid dehydration, polycondensation and depolymerization reaction, wherein meso-lactide (mLactide) content of 5.7%, lactic acid content of 6.3%, lactic acid dimer, trimer content of 1.8%, and also small amounts of water and other impurities.
Taking 5000g of the crude L-lactide, crushing the crude L-lactide into 35-100 meshes by a crusher, and filling the crushed crude L-lactide into a crude lactide storage tank. Before feeding, the temperature of the rectifying tower is increased by heating and boosting, the temperature of the rectifying tower is set in a three-section sectional mode, and when the pressure in the rectifying tower reaches 10MPa and the temperatures of three sections of fillers sequentially reach 36 ℃, 60 ℃ and 80 ℃ from bottom to top, supercritical CO is introduced into the rectifying tower2Feeding into a rectifying tower, and feeding into supercritical CO of the rectifying tower2The pressure of (2) is 10MPa, the temperature is 36 ℃, the flow rate is 2.0kg/h, wherein the flow rate of the entrained ethyl acetate is 0.1kg/h, and the whole supercritical CO is treated2After the rectifying system operates stably, feeding the crude lactide and the supercritical CO from the upper part of the rectifying tower at the speed of 2.0kg/h2The fluid is in countercurrent contact, lactic acid dimer, trimer and organic pigment are distilled out from the bottom of the rectifying tower, and the content of lactide (comprising L-lactide,mlactide) from the top of the rectification column by supercritical CO2The fluid is extracted and carried out, and enters a separator I and a separator II for separation, wherein the pressure of the separator I is maintained at 7.5MPa, and the temperature is 55 DEG CThe pressure of the separator II is maintained at 3MPa, the temperature is maintained at 35 ℃,mseparating the lactide from the separator II to obtain an L-lactide product with higher purity; and CO2And the entrainer is separated by the separator, enters a condenser and is cooled to liquid, is pressurized by a high-pressure pump and heated by a heater, and is sent to the rectifying tower for recycling.
By supercritical CO2The yield of L-lactide after impurity removal in the rectification system was 81.7% (single pass, the same applies below). The obtained L-lactide product has a chemical purity of 99.15%, whereinmThe content of lactide is 0.52 percent, the content of lactic acid is 0.13 percent, the content of dimer and trimer lactic acid is 0.15 percent, and the product is colorless transparent crystal with the decolorization rate of more than 95 percent. After analysis by a polarimeter and a melting point instrument, the specific optical rotation of the obtained product is-276.67, the optical purity is 99.52 percent, the melting range is 96.2-97.5 ℃, and all indexes of the product are equivalent to those of L-lactide (the chemical purity is 99.16 percent, the optical purity is 99.0 percent, the melting range is 97.6-98.2 ℃, and the specific optical rotation is-275.22) products produced by Purac company.
Example 2
Preparation of crude L-lactide: zinc oxide and stannous octoate are used as catalysts, lactate is used as a raw material (self-made, the mass fraction is 92%), the lactate is dehydrated and dealcoholized firstly, then low polymer lactate is formed through polycondensation, and finally, crude L-lactide with the purity of 83.1% is obtained through high-temperature depolymerization. Wherein the content of the first and second substances,m-lactide content of 6.2%, lactic acid content of 4.7%, dimer, trimer lactate content of 2.9%, and also small amounts of water and other impurities.
5000g of the crude L-lactide was melted by heating at 98 ℃. Before feeding, the temperature of the rectifying tower is increased by heating and boosting, the temperature of the rectifying tower is set in a three-section sectional mode, and when the pressure in the rectifying tower reaches 10MPa and the temperatures of three sections of fillers sequentially reach 36 ℃, 60 ℃ and 80 ℃ from bottom to top, supercritical CO is introduced into the rectifying tower2Feeding into a rectifying tower, and feeding into supercritical CO of the rectifying tower2The pressure of (2) is 10MPa, the temperature is 36 ℃, the flow rate is 2.0kg/h, wherein the flow rate of the entrained ethyl acetate is 0.1kg/h, and the whole supercritical CO is treated2After the rectifying system runs stably, feeding the material from the upper part of the rectifying tower at the speed of 2.0kg/h to obtain coarse materialLactide and supercritical CO2The fluid is in countercurrent contact, lactic acid dimer, trimer and organic pigment are distilled out from the bottom of the rectifying tower, and the content of lactide (comprising L-lactide,mlactide) from the top of the rectification column by supercritical CO2The fluid is extracted and carried out, and enters a separator I and a separator II for separation, the pressure of the separator I is maintained at 7.5MPa and the temperature is 55 ℃, the pressure of the separator II is maintained at 3MPa and the temperature is maintained at 35 ℃,mand (4) separating the lactide from the separator II to obtain an L-lactide product with higher purity. And CO2And the entrainer is separated by the separator, enters a condenser and is cooled to liquid, is pressurized by a high-pressure pump and heated by a heater, and is sent to the rectifying tower for recycling.
By supercritical CO2The yield of L-lactide after impurity removal in the rectification system is 80.1%. The obtained L-lactide product has a chemical purity of 99.06%, whereinmThe content of lactide is 0.50 percent, the content of lactic acid is 0.15 percent, the content of dimer and trimer lactic acid is 0.20 percent, and the product is colorless transparent crystal with the decolorization rate of more than 95 percent. After analysis by a polarimeter and a melting point instrument, the specific optical rotation of the obtained product is-276.36, the optical purity is 99.41 percent, and the melting range is 96.0-97.6 ℃.
Example 3
Preparation of crude L-lactide: taking zinc oxide and stannous octoate as catalysts and 88 percent L-lactic acid as raw materials, and obtaining crude L lactide with the purity of 85.7 percent after lactic acid dehydration, polycondensation and depolymerization reaction, wherein meso-lactide (mLactide) content of 5.7%, lactic acid content of 6.3%, lactic acid dimer, trimer content of 1.8%, and also small amounts of water and other impurities.
Taking 5000g of the crude L-lactide, crushing the crude L-lactide into 35-100 meshes by a crusher, and filling the crushed crude L-lactide into a crude lactide storage tank. Before feeding, the temperature and the pressure of the rectifying tower need to be raised, the pressure in the rectifying tower is 9MPa, the temperature of the rectifying tower is gradually raised from the bottom of the tower to the top of the tower, namely the temperature is gradually raised from 40 ℃ to 60 ℃, and the supercritical CO is subjected to2Feeding into a rectifying tower, and feeding into supercritical CO of the rectifying tower2At a pressure of 9MPa, a temperature of 40 ℃ and a flow rate of 0.5kg/h, with ethyl acetate entrained thereinThe flow rate is 0.05kg/h, and the whole supercritical CO is obtained2After the rectifying system operates stably, feeding the crude lactide and the supercritical CO from the upper part of the rectifying tower at the speed of 0.5kg/h2The fluid is in countercurrent contact, lactic acid dimer, trimer and organic pigment are distilled out from the bottom of the rectifying tower, and the content of the lactic acid dimer, the trimer and the organic pigment is sampled and analyzed; lactide (containing L-lactide,mLactide) from the top of the rectification column by supercritical CO2The fluid is extracted and carried out, and enters a separator I and a separator II for separation, the pressure of the separator I is maintained at 5MPa and the temperature is 50 ℃, the normal pressure and the temperature of the separator II are maintained at 30 ℃,mseparating the lactide from the separator II to obtain an L-lactide product with higher purity; and CO2And the entrainer is separated by the separator, enters a condenser and is cooled to liquid, is pressurized by a high-pressure pump and heated by a heater, and is sent to the rectifying tower for recycling.
By supercritical CO2The yield of the L-lactide is 79.8 percent after impurity removal and decoloration of a rectification system. The obtained L-lactide product has a chemical purity of 99.10%, whereinmThe content of lactide is 0.50 percent, the content of lactic acid is 0.21 percent, the content of dimer and trimer lactic acid is 0.16 percent, and the product is colorless transparent crystal with the decolorization rate of more than 95 percent. After analysis by a polarimeter and a melting point instrument, the specific optical rotation of the obtained product is-275.58, the optical purity is 99.13%, and the melting range is 95.8-97.5 ℃.
Example 4
Preparation of crude L-lactide: taking zinc oxide and stannous octoate as catalysts and 88 percent L-lactic acid as raw materials, and obtaining crude L lactide with the purity of 85.7 percent after lactic acid dehydration, polycondensation and depolymerization reaction, wherein meso-lactide (mLactide) content of 5.7%, lactic acid content of 6.3%, lactic acid dimer, trimer content of 1.8%, and also small amounts of water and other impurities.
Taking 5000g of the crude L-lactide, crushing the crude L-lactide into 35-100 meshes by a crusher, and filling the crushed crude L-lactide into a crude lactide storage tank. Before feeding, the temperature and the pressure of the rectifying tower need to be raised, the pressure in the rectifying tower is 15MPa, the temperature of the rectifying tower is gradually raised from the bottom of the tower to the top of the tower, namely, the temperature is gradually raised from 60 ℃ to 80 ℃, and when a rectifying tower system is stable, the temperature is beyond the critical temperatureBoundary CO2Feeding into a rectifying tower, and feeding into supercritical CO of the rectifying tower2The pressure of the fluid is 15MPa, the temperature is 60 ℃, the flow rate is 3.0kg/h, the flow rate of the entrained ethyl acetate is 0.2kg/h, and the whole supercritical CO is treated2After the normal operation of the rectification system is stable, feeding the crude lactide and the supercritical CO from the upper part of the rectification tower at the speed of 5.0kg/h2The fluid is in countercurrent contact, lactic acid dimer, trimer and organic pigment are distilled out from the bottom of the rectifying tower, and the content of lactide (comprising L-lactide,mlactide) from the top of the rectification column by supercritical CO2The fluid is extracted and carried out, and enters a separator I and a separator II for separation, the pressure of the separator I is maintained at 9MPa and the temperature is 80 ℃, the pressure of the separator II is maintained at 3MPa and the temperature is maintained at 50 ℃,mand (3) separating the lactide from the separator II to obtain the L-lactide product with higher purity. And CO2And the entrainer is separated by the separator, enters a condenser and is cooled to liquid, is pressurized by a high-pressure pump and heated by a heater, and is sent to the rectifying tower for recycling.
By supercritical CO2After purification by the rectification system, the yield of the L-lactide is 79.3 percent. The obtained L-lactide product has a chemical purity of 99.0%, whereinmThe content of lactide is 0.58 percent, the content of lactic acid is 0.20 percent, the content of dimer and trimer lactic acid is 0.20 percent, and the product is colorless transparent crystal with the decolorization rate of more than 95 percent. After analysis by a polarimeter and a melting point instrument, the specific optical rotation of the obtained product is-275.50, the optical purity is 99.10%, and the melting range is 95.8-97.6 ℃.
Example 5
Preparation of crude L-lactide: taking zinc oxide and stannous octoate as catalysts and 88 percent L-lactic acid as raw materials, and obtaining crude L-lactide with the purity of 85.7 percent after lactic acid dehydration, polycondensation and depolymerization reaction, wherein meso-lactide (I)mLactide) content of 5.7%, lactic acid content of 6.3%, lactic acid dimer, trimer content of 1.8%, and also small amounts of water and other impurities.
Taking 5000g of the crude L-lactide, crushing the crude L-lactide into 35-100 meshes by a crusher, and filling the crushed crude L-lactide into a crude lactide storage tank. Before feeding, rectification is requiredHeating and boosting the tower, setting the temperature of the rectifying tower in a three-section sectional manner, and when the pressure in the rectifying tower reaches 10MPa and the temperature of three-section fillers reaches 36 ℃, 60 ℃ and 80 ℃ from bottom to top in sequence, carrying out supercritical CO separation2Feeding into a rectifying tower, and feeding into supercritical CO of the rectifying tower2The pressure of the fluid is 10MPa, the temperature is 36 ℃, the flow rate is 2.0kg/h, the flow rate of the entrained chloroform is 0.1kg/h, and the whole supercritical CO is treated2After the rectifying system operates stably, feeding the crude lactide and the supercritical CO from the upper part of the rectifying tower at the speed of 2.0kg/h2The fluid is in countercurrent contact, lactic acid dimer, trimer and organic pigment are distilled out from the bottom of the rectifying tower, and the content of the lactide (comprising L-lactide, L-,mLactide) from the top of the rectification column by supercritical CO2The fluid is extracted and carried out, and enters a separator I and a separator II for separation, the pressure of the separator I is maintained at 7.5MPa and the temperature is 55 ℃, the pressure of the separator II is maintained at 3MPa and the temperature is 35 ℃,mand (3) separating the lactide from the separator II to obtain the L-lactide product with higher purity. And CO2And the entrainer is separated by the separator, enters a condenser and is cooled to liquid, is pressurized by a high-pressure pump and heated by a heater, and is sent to the rectifying tower for recycling.
By supercritical CO2After impurity removal and decoloration of the rectification system, the yield of the L-lactide is 82.1 percent. The obtained L-lactide product has a chemical purity of 99.27%, whereinmThe content of lactide is 0.41 percent, the content of lactic acid is 0.10 percent, the content of dimer and trimer lactic acid is 0.17 percent, and the product is colorless transparent crystal with the decolorization rate of more than 96 percent. After analysis by a polarimeter and a melting point instrument, the specific optical rotation of the obtained product is-279.39, the optical purity is 99.6%, and the melting range is 96.1-97.2 ℃.
Example 6
Preparation of crude L-lactide: taking zinc oxide and stannous octoate as catalysts and 88 percent L-lactic acid as raw materials, and obtaining crude L-lactide with the purity of 85.7 percent after lactic acid dehydration, polycondensation and depolymerization reaction, wherein meso-lactide (I)mLactide) content of 5.7%, lactic acid content of 6.3%, lactic acid dimer, trimer content of 1.8%, and small amounts of water and waterOther impurities.
Taking 5000g of the crude L-lactide, crushing and grinding into 35-100 meshes, and filling into a crude lactide storage tank. Before feeding, the temperature of the rectifying tower is increased by heating and boosting, the temperature of the rectifying tower is set in a three-section sectional mode, and when the pressure in the rectifying tower reaches 10MPa and the temperatures of three sections of fillers sequentially reach 36 ℃, 60 ℃ and 80 ℃ from bottom to top, supercritical CO is introduced into the rectifying tower2Feeding into a rectifying tower, and feeding into supercritical CO of the rectifying tower2The pressure of the fluid is 10MPa, the temperature is 36 ℃, the flow rate is 2.0kg/h, the flow rate of the entrained methyl acetate is 0.1kg/h, and the whole supercritical CO is treated2After the rectifying system operates stably, feeding the crude lactide and the supercritical CO from the upper part of the rectifying tower at the speed of 2.0kg/h2The fluid is in countercurrent contact, lactic acid dimer, trimer and organic pigment are distilled out from the bottom of the rectifying tower, and the content of the lactide (comprising L-lactide, L-,mLactide) from the top of the rectification column by supercritical CO2The fluid is extracted and carried out, and enters a separator I and a separator II for separation, the pressure of the separator I is maintained at 7.5MPa and the temperature is 55 ℃, the pressure of the separator II is maintained at 3MPa and the temperature is 35 ℃,mand (3) separating the lactide from the separator II to obtain the L-lactide product with higher purity. And CO2And the entrainer is separated by the separator, enters a condenser and is cooled to liquid, is pressurized by a high-pressure pump and heated by a heater, and is sent to the rectifying tower for recycling.
By supercritical CO2The yield of the L-lactide is 81.1 percent after impurity removal and decoloration of a rectification system. The obtained L-lactide product has a chemical purity of 99.06%, whereinmThe content of lactide is 0.55 percent, the content of lactic acid is 0.21 percent, the content of dimer and trimer lactic acid is 0.23 percent, and the product is colorless transparent crystal with the decolorization rate of more than 95 percent. After analysis by a polarimeter and a melting point instrument, the specific optical rotation of the obtained product is-276.28, the optical purity is 99.38%, and the melting range is 95.7-97.1 ℃.
Example 7
Preparation of crude L-lactide: zinc oxide and stannous octoate are used as catalysts, 88 percent L-lactic acid is used as a raw material, and the purity of the L-lactic acid is obtained after lactic acid dehydration, polycondensation and depolymerization reaction85.7% crude L-lactide, of which meso-lactide (A)mLactide) content of 5.7%, lactic acid content of 6.3%, lactic acid dimer, trimer content of 1.8%, and also small amounts of water and other impurities.
Taking 5000g of the crude L-lactide, crushing the crude L-lactide into 35-100 meshes by a crusher, and filling the crushed crude L-lactide into a crude lactide storage tank. Before feeding, the temperature of the rectifying tower is increased by heating and boosting, the temperature of the rectifying tower is set in a three-section sectional mode, and when the pressure in the rectifying tower reaches 10MPa and the temperatures of three sections of fillers sequentially reach 36 ℃, 60 ℃ and 80 ℃ from bottom to top, supercritical CO is introduced into the rectifying tower2Feeding into a rectifying tower, and feeding into supercritical CO of the rectifying tower2The pressure of the fluid is 10MPa, the temperature is 36 ℃, the flow rate is 2.0kg/h, the flow rate of the entrained isopropyl acetate is 0.1kg/h, and the whole supercritical CO is treated2After the rectifying system operates stably, feeding the crude lactide and the supercritical CO from the upper part of the rectifying tower at the speed of 2.0kg/h2The fluid is in countercurrent contact, lactic acid dimer, trimer and organic pigment are distilled out from the bottom of the rectifying tower, and the content of the lactide (comprising L-lactide, L-,mLactide) from the top of the rectification column by supercritical CO2The fluid is extracted and carried out, and enters a separator I and a separator II for separation, the pressure of the separator I is maintained at 7.5MPa and the temperature is 55 ℃, the pressure of the separator II is maintained at 3MPa and the temperature is 35 ℃,mand (3) separating the lactide from the separator II to obtain the L-lactide product with higher purity. And CO2And the entrainer is separated by the separator, enters a condenser and is cooled to liquid, is pressurized by a high-pressure pump and heated by a heater, and is sent to the rectifying tower for recycling.
By supercritical CO2The yield of the L-lactide is 80.7 percent after impurity removal and decoloration of a rectification system. The obtained L-lactide product has a chemical purity of 99.03%, whereinmThe content of lactide is 0.56 percent, the content of lactic acid is 0.21 percent, the content of dimer and trimer lactic acid is 0.24 percent, and the product is colorless transparent crystal with the decolorization rate of more than 95 percent. After analysis by a polarimeter and a melting point instrument, the specific optical rotation of the obtained product is-279.92, the optical purity is 99.31 percent, and the melting range is 96.1-97.6 ℃.
Example 8
Preparation of crude L-lactide: taking zinc oxide and stannous octoate as catalysts and 88 percent L-lactic acid as raw materials, and obtaining crude L-lactide with the purity of 85.7 percent after lactic acid dehydration, polycondensation and depolymerization reaction, wherein meso-lactide (I)mLactide) content of 5.7%, lactic acid content of 6.3%, lactic acid dimer, trimer content of 1.8%, and also small amounts of water and other impurities.
Taking 5000g of the crude L-lactide, crushing the crude L-lactide into 35-100 meshes by a crusher, and filling the crushed crude L-lactide into a crude lactide storage tank. Before feeding, the temperature of the rectifying tower is increased by heating and boosting, the temperature of the rectifying tower is set in a three-section sectional mode, and when the pressure in the rectifying tower reaches 10MPa and the temperatures of three sections of fillers sequentially reach 36 ℃, 60 ℃ and 80 ℃ from bottom to top, supercritical CO is introduced into the rectifying tower2Feeding into a rectifying tower, and feeding into supercritical CO of the rectifying tower2The pressure of the fluid is 10MPa, the temperature is 36 ℃, the flow rate is 2.0kg/h, the flow rate of the entrained propyl acetate is 0.1kg/h, and the whole supercritical CO is treated2After the rectifying system operates stably, feeding the crude lactide and the supercritical CO from the upper part of the rectifying tower at the speed of 2.0kg/h2The fluid is in countercurrent contact, lactic acid dimer, trimer and organic pigment are distilled out from the bottom of the rectifying tower, and the content of the lactide (comprising L-lactide, L-,mLactide) from the top of the rectification column by supercritical CO2The fluid is extracted and carried out, and enters a separator I and a separator II for separation, the pressure of the separator I is maintained at 7.5MPa and the temperature is 55 ℃, the pressure of the separator II is maintained at 3MPa and the temperature is 35 ℃,mand (3) separating the lactide from the separator II to obtain the L-lactide product with higher purity. And CO2And the entrainer is separated by the separator, enters a condenser and is cooled to liquid, is pressurized by a high-pressure pump and heated by a heater, and is sent to the rectifying tower for recycling.
By supercritical CO2The yield of the L-lactide is 80.1 percent after impurity removal and decoloration of a rectification system. The obtained L-lactide product has a chemical purity of 99.01%, whereinmThe content of lactide is 0.57 percent, the content of lactic acid is 0.22 percent, the content of dimer and trimer lactic acid is 0.23 percent, and the product is colorless transparent crystal with the decolorization rate of more than 99.0 percent. Warp beamThe optical rotation meter and melting point instrument analyze, the specific optical rotation of the obtained product is-280.11, the optical purity is 99.24%, and the melting range is 96.0-97.7 ℃.
Example 9
Preparation of crude L-lactide: taking zinc oxide and stannous octoate as catalysts and 88 percent L-lactic acid as raw materials, and obtaining crude L-lactide with the purity of 85.7 percent after lactic acid dehydration, polycondensation and depolymerization reaction, wherein meso-lactide (I)mLactide) content of 5.7%, lactic acid content of 6.3%, lactic acid dimer, trimer content of 1.8%, and also small amounts of water and other impurities.
Taking 5000g of the crude L-lactide, crushing the crude L-lactide into 35-100 meshes by a crusher, and filling the crushed crude L-lactide into a crude lactide storage tank. Before feeding, the temperature of the rectifying tower is increased by heating and boosting, the temperature of the rectifying tower is set in a three-section sectional mode, and when the pressure in the rectifying tower reaches 10MPa and the temperatures of three sections of fillers sequentially reach 36 ℃, 60 ℃ and 80 ℃ from bottom to top, supercritical CO is introduced into the rectifying tower2Feeding into a rectifying tower, and feeding into supercritical CO of the rectifying tower2The pressure of the fluid is 10MPa, the temperature is 36 ℃, the flow rate is 2.0kg/h, the flow rate of the entrained ethyl acetate is 0.1kg/h, the flow rate of the carbon disulfide is 0.005kg/h, and the whole supercritical CO is treated2After the rectifying system operates stably, feeding the crude lactide and the supercritical CO from the upper part of the rectifying tower at the speed of 2.0kg/h2The fluid is in countercurrent contact, lactic acid dimer, trimer and organic pigment are distilled out from the bottom of the rectifying tower, and the content of lactide (comprising L-lactide,mlactide) from the top of the rectification column by supercritical CO2The fluid is extracted and carried out, and enters a separator I and a separator II for separation, the pressure of the separator I is maintained at 7.5MPa and the temperature is 55 ℃, the pressure of the separator II is maintained at 3MPa and the temperature is 35 ℃,mand (3) separating the lactide from the separator II to obtain the L-lactide product with higher purity. And CO2And the entrainer is separated by the separator, enters a condenser and is cooled to liquid, is pressurized by a high-pressure pump and heated by a heater, and is sent to the rectifying tower for recycling.
By supercritical CO2After the purification by the rectification system, the yield of the L-lactide is 82.6 percent. The obtained L-propyl esterThe chemical purity of the lactide product was 99.30%, whereinmThe content of lactide is 0.35 percent, the content of lactic acid is 0.10 percent, the content of dimer and trimer lactic acid is 0.12 percent, and the product is colorless transparent crystal with the decolorization rate of more than 99.0 percent. After analysis by a polarimeter and a melting point instrument, the specific optical rotation of the obtained product is-276.67, the optical purity is 99.55%, and the melting range is 96.1-97.0 ℃.
Example 10
Preparation of crude L-lactide: taking zinc oxide and stannous octoate as catalysts and 88 percent L-lactic acid as raw materials, and obtaining crude L-lactide with the purity of 85.7 percent after lactic acid dehydration, polycondensation and depolymerization reaction, wherein meso-lactide (I)mLactide) content of 5.7%, lactic acid content of 6.3%, lactic acid dimer, trimer content of 1.8%, and also small amounts of water and other impurities. Taking 5000g of the crude L-lactide, crushing the crude L-lactide into 35-100 meshes by a crusher, and filling the crushed crude L-lactide into a crude lactide storage tank. Before feeding, the temperature of the rectifying tower is increased by heating and boosting, the temperature of the rectifying tower is set in a three-section sectional mode, and when the pressure in the rectifying tower reaches 10MPa and the temperatures of three sections of fillers sequentially reach 36 ℃, 60 ℃ and 80 ℃ from bottom to top, supercritical CO is introduced into the rectifying tower2Feeding into a rectifying tower, and feeding into supercritical CO of the rectifying tower2The pressure of the fluid is 10MPa, the temperature is 36 ℃, the flow rate is 2.0kg/h, the flow rate of the entrained ethyl acetate is 0.1kg/h, the flow rate of the petroleum ether is 0.005kg/h, and the whole supercritical CO is treated2After the rectifying system operates stably, feeding the crude lactide and the supercritical CO from the upper part of the rectifying tower at the speed of 2.0kg/h2The fluid is in countercurrent contact, lactic acid dimer, trimer and organic pigment are distilled out from the bottom of the rectifying tower, and the content of lactide (comprising L-lactide,mlactide) from the top of the rectification column by supercritical CO2The fluid is extracted and carried out, and enters a separator I and a separator II for separation, the pressure of the separator I is maintained at 7.5MPa and the temperature is 55 ℃, the pressure of the separator II is maintained at 3MPa and the temperature is 35 ℃,mand (3) separating the lactide from the separator II to obtain the L-lactide product with higher purity. And CO2And entrainer is separated by a separator, enters a condenser and is cooled to liquid, and then is pressurized and added by a high-pressure pumpThe heated mixture is sent to a rectifying tower for recycling.
By supercritical CO2After purification by the rectification system, the yield of the L-lactide is 82.1 percent. The obtained L-lactide product has a chemical purity of 99.27%, whereinmThe content of lactide is 0.37 percent, the content of lactic acid is 0.14 percent, the content of dimer and trimer lactic acid is 0.15 percent, and the product is colorless transparent crystal with the decolorization rate of more than 98 percent. After analysis by a polarimeter and a melting point instrument, the specific optical rotation of the obtained product is-276.67, the optical purity is 99.55%, and the melting range is 96.1-97.1 ℃.
Example 11
Preparation of crude L-lactide: taking zinc oxide and stannous octoate as catalysts and 88 percent L-lactic acid as raw materials, and obtaining crude L lactide with the purity of 85.7 percent after lactic acid dehydration, polycondensation and depolymerization reaction, wherein meso-lactide (mLactide) content of 5.7%, lactic acid content of 6.3%, lactic acid dimer, trimer content of 1.8%, and also small amounts of water and other impurities.
Taking 5000g of the crude L-lactide, crushing the crude L-lactide into 35-100 meshes by a crusher, and filling the crushed crude L-lactide into a crude lactide storage tank. Before feeding, the temperature of the rectifying tower is increased by heating and boosting, the temperature of the rectifying tower is set in a three-section sectional mode, and when the pressure in the rectifying tower reaches 10MPa and the temperatures of three sections of fillers sequentially reach 36 ℃, 60 ℃ and 80 ℃ from bottom to top, supercritical CO is introduced into the rectifying tower2Feeding into a rectifying tower, and feeding into supercritical CO of the rectifying tower2The pressure of the fluid is 10MPa, the temperature is 36 ℃, the flow rate is 2.0kg/h, the flow rate of the entrained chloroform is 0.1kg/h, the flow rate of the carbon disulfide is 0.005kg/h, and the whole supercritical CO is treated2After the rectifying system operates stably, feeding the crude lactide and the supercritical CO from the upper part of the rectifying tower at the speed of 2.0kg/h2The fluid is in countercurrent contact, lactic acid dimer, trimer and organic pigment are distilled out from the bottom of the rectifying tower, and the content of lactide (comprising L-lactide,mlactide) from the top of the rectification column by supercritical CO2The fluid is extracted and carried out, and enters a separator I and a separator II for separation, the pressure of the separator I is maintained at 7.5MPa and the temperature is 55 ℃, the pressure of the separator II is maintained at 3MPa and the temperature is 35 ℃,mand (3) separating the lactide from the separator II to obtain the L-lactide product with higher purity. And CO2And the entrainer is separated by the separator, enters a condenser and is cooled to liquid, is pressurized by a high-pressure pump and heated by a heater, and is sent to the rectifying tower for recycling.
By supercritical CO2The yield of L-lactide after purification by the rectification system is 83.7 percent. The obtained L-lactide product has a chemical purity of 99.38%, whereinmThe content of lactide is 0.31 percent, the content of lactic acid is 0.10 percent, the content of dimer and trimer lactic acid is 0.10 percent, and the product is colorless transparent crystal with the decolorization rate of more than 99.0 percent. After analysis by a polarimeter and a melting point instrument, the specific optical rotation of the obtained product is-276.97, the optical purity is 99.63 percent, and the melting range is 96.1-96.9 ℃.
Comparative example 1
Taking 500g of the crude L-lactide, crushing the crude L-lactide to 50-100 meshes, and filling the crushed crude L-lactide into an extractor for sealing. The circulation refrigerating system is opened and CO is introduced2Raising the temperature of an extraction system to 50 ℃, boosting the pressure to 10MPa, and controlling CO2The flow rate of (2) was 25g/min, and ethyl acetate was fed into the extractor at a flow rate of 1.25g/min, and extraction was carried out at constant temperature and constant pressure for 90 min. After extraction, the L-lactide is obtained through depressurization, cooling and separation. The L-lactide obtained by the process has a chemical purity of 95.7%, a lactide yield of 63.4% and a decolorization rate of more than 90%, whereinmLactide content 1.9%, lactic acid content 1.0%, lactic acid dimer, trimer content 0.7%. After three times of circulating extraction, the yield of the lactide reaches 87.3 percent.
500g of L-lactide obtained by extraction is rectified and refined, a heating jacket is adopted for heating, the vacuum degree of the system is maintained at about 600Pa, the reflux ratio is controlled at 4:5, about 392.5g of fraction with the temperature of a fraction outlet at about 106 ℃ and 110 ℃ is collected, and the yield of the product of rectification is about 81.2 percent. The total yield of crude lactide after extraction and rectification refining is 70.89%, the chemical purity of the obtained product is 99.03%, the decolorization rate is above 90%, the optical purity is 98.3%, and the melting range is 94.3-96.9 ℃.
Comparative example 2
The difference from example 1 is that no reagent was added to the supercritical CO2, which isThe operating conditions are identical. The L-lactide obtained in this process was analyzed by gas chromatography in a yield of 32.6% and the product was of a chemical purity of 93.12%, whereinmThe lactide content is 5.4%, the lactic acid content is 1.2%, the dimer and trimer lactic acid content is 1.5%, and the product decolorization rate is about 60%. After analysis by a polarimeter and a melting point instrument, the specific optical rotation of the obtained product is-256.0, the optical purity is 92.3 percent, and the melting range is 91.2-94.5 ℃.
Comparative example 3
The difference from example 1 is that supercritical CO2The reagent added in the process is acetone, and other operation conditions are completely the same. The yield of L-lactide obtained in this process was 78.5% by gas chromatography. The obtained L-lactide product has a chemical purity of 97.89%, whereinmThe lactide content is 1.2%, the lactic acid content is 0.8%, the dimer and trimer lactic acid content is 0.91%, and the decolorization rate is about 80%. After analysis by a polarimeter and a melting point instrument, the specific optical rotation of the obtained product is-274.63, the optical purity is 98.79 percent, and the melting range is 93.5-96.2 ℃.
Comparative example 4
The difference from example 1 is that supercritical CO2The reagent added in the process is ethanol, and other operation conditions are completely the same. The yield of L-lactide obtained in this process was 77.2% and the chemical purity of the L-lactide product obtained was 97.26% as analyzed by gas chromatography, wheremThe content of lactide is 1.1%, the content of lactic acid is 1.0%, the content of dimer and trimer lactic acid is 1.03%, and the decolorization rate is about 80%. After analysis by a polarimeter and a melting point instrument, the specific optical rotation of the obtained product is-270.85, the optical purity is 97.43%, and the melting range is 92.7-96.0 ℃.

Claims (7)

1. A method for removing pigment and impurities in crude lactide is characterized by comprising the following steps:
(1) adjusting the temperature and pressure in the rectifying tower to convert supercritical CO into CO2Feeding into a rectifying tower; introducing CO2Pressurizing and heating to supercritical state, wherein the pressure is 8-15MPa and the temperature is 36-60 ℃; supercritical CO introduced into a rectification column2The flow rate is 0.5-3.0 kg/h; in thatIntroduction of supercritical CO2Adding a low-boiling-point ester solvent or/and chloroform while flowing, wherein the low-boiling-point ester solvent is at least one of ethyl acetate, methyl acetate, propyl acetate and isopropyl acetate;
(2) crushing or heating and melting crude lactide, and then feeding the crude lactide into a rectifying tower for decoloring and impurity removal; the crude lactide is prepared by taking L/D-lactic acid or L/D-lactic acid ester as a raw material through polycondensation and depolymerization processes, wherein the purity of the L/D-lactide is 78-90%, the content of meso-lactide is not lower than 3.7%, the content of lactic acid is not lower than 3.4%, and the content of dimer and trimer is not lower than 1.0%;
(3) rectifying the product with CO2Separating to obtain lactide after impurity removal and decoloration; a two-stage separator, a separator I and a separator II are adopted, the pressure of the separator I is maintained at 5-9MPa, the temperature is maintained at 50-80 ℃, the separation pressure of the separator II is normal pressure-3 MPa, and the separation temperature is 30-50 ℃.
2. The method of claim 1, wherein: the rectifying tower in the step (1) is filled with supercritical CO2Before, the rectifying tower needs to be pressurized and preheated to 8-15MPa and 36-80 ℃.
3. The method according to claim 1 or 2, characterized in that: the temperature of the rectifying tower in the step (1) is set in a sectional mode, and gradually rises from the bottom of the tower to the top of the tower, wherein the range is 36-80 ℃.
4. The method of claim 1, wherein: adding a low-boiling-point ester solvent or/and chloroform into the step (1) at a flow rate of supercritical CO21-10% of the flow rate.
5. The method according to claim 1 or 4, characterized in that: in the step (1), a certain amount of petroleum ether or/and carbon disulfide is added while a low-boiling-point ester solvent or/and chloroform is added, wherein the addition amount is 1% -10% of the amount of the low-boiling-point ester solvent or/and chloroform.
6. The method of claim 1, wherein: the lactide in the step (2) is crushed by a mechanical crushing mode, and the diameter of the crushed lactide is 35-100 meshes; the heating and melting temperature is 96-100 ℃, and the mixture is sent into a rectifying tower after the mixture is completely melted.
7. The method of claim 1, wherein: the flow rate of the crude lactide entering the rectifying tower in the step (2) is 0.5-5.0kg/h, and the crude lactide and the supercritical CO are mixed2And (4) carrying out countercurrent contact on the fluid to carry out decolorization and impurity removal.
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CN103145691A (en) * 2013-04-03 2013-06-12 天津科技大学 Coupling refining method of high-purity L-lactide
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