Method and system for refining and post-processing crude cyclic ester and application

Abstract

The invention discloses a cyclic ester refining and post-processing method, a cyclic ester refining and post-processing system and application. The method comprises the following steps: (1) sequentially carrying out melt crystallization and recrystallization on a melt of crude cyclic ester to obtain a cyclic ester product; (2) rectifying the recrystallized filtrate in a first rectifying tower to obtain a recycled solvent at the top of the first rectifying tower, circulating the recycled solvent back to the recrystallization tower, and mixing the material at the bottom of the first rectifying tower with the mother liquor of melt crystallization and the alcohol solvent to perform alcoholysis reaction to obtain alcoholysis liquid; and (3) carrying out rectification treatment on the alcoholysis solution in a second rectifying tower, externally collecting a tower top material, and circulating a tower kettle material back to the first rectifying tower in the step (2). According to the method and the system disclosed by the invention, not only can the cyclic ester product with relatively high purity be obtained, but also the recycling and cyclic utilization of the recrystallization solvent and the cyclic ester precursor can be realized by retreating the filtrate.

Description

Technical Field

[0001] The present invention belongs to the field of cyclic ester synthesis, and particularly relates to a method, a system and an application for refining and post-treating crude cyclic esters. Background Art

[0002] Cyclic esters such as glycolide, lactide, etc. can obtain corresponding polymers through ring-opening polymerization reactions, such as polyglycolic acid and polylactic acid. Such polymers are a new type of degradable plastic with good biocompatibility and are widely used in industries such as medicine, agriculture, and food.

[0003] The synthesis methods of cyclic esters are divided into one-step method and two-step method. The one-step method refers to the direct reaction of raw materials (lactic acid / glycolic acid) to generate the corresponding cyclic esters. However, the yield of cyclic esters in this method is low, so the two-step method is currently used in industry to synthesize cyclic esters. The two-step method means that the raw materials (lactic acid / glycolic acid) first undergo a polycondensation reaction to obtain oligomers, and then the oligomers undergo a depolymerization reaction to obtain cyclic esters. The purity of the cyclic esters obtained through the above reactions cannot meet the requirements of their polymerization reactions and need to be refined.

[0004] The refining methods of cyclic esters include distillation, crystallization, recrystallization, and melt crystallization. Since cyclic esters are thermosensitive substances, there are risks in using the distillation method. Therefore, the refining process of cyclic esters generally uses crystallization, which can be one or a combination of crystallization, recrystallization, and melt crystallization.

[0005] This patent purifies cyclic esters by combining melt crystallization and recrystallization. First, a large amount of impurities in the crude cyclic esters are removed through melt crystallization, reducing the risk of side reactions between the impurities and the recrystallization solvent during the recrystallization process. Then, the cyclic esters are refined through recrystallization to obtain cyclic esters that meet the requirements of the polymerization reaction. This patent sets up an alcoholysis unit to alcoholyze the oligomers in the crude cyclic esters through an alcoholysis reaction to obtain raw materials that can be used in the polycondensation reaction. At the same time, a solvent recovery unit is also set up to recover the recrystallization solution, realizing the recovery and recycling of materials in the system. Summary of the Invention

[0006] In order to overcome the problems existing in the prior art, the present invention provides a method, a system and an application for refining and post-treating crude cyclic esters. The melt of the crude cyclic esters is subjected to melt crystallization and recrystallization to obtain cyclic ester products, and the filtrates of the melt crystallization and the recrystallization are respectively processed. The method and system of the present invention can not only obtain cyclic ester products with higher purity, but also realize the recovery and recycling of the recrystallization solvent and the cyclic ester precursor through the reprocessing of the filtrates.

[0007] One of the purposes of the present invention is to provide a method for refining and post-treating crude cyclic esters, including:

[0008] (1) The melt of the crude cyclic ester is subjected to melt crystallization and recrystallization in sequence to obtain a cyclic ester product;

[0009] (2) The filtrate from the recrystallization is subjected to rectification treatment in a first rectification column. A recycled solvent is obtained at the top of the first rectification column and recycled back to the recrystallization. The material at the bottom of the first rectification column is mixed with the mother liquor from the melt crystallization and an alcohol solvent to carry out an alcoholysis reaction to obtain an alcoholysis solution;

[0010] (3) The alcoholysis solution is subjected to rectification treatment in a second rectification column. The material at the top is taken out, and the material at the bottom is recycled back to the first rectification column in step (2).

[0011] Among them, the melt crystallization includes cooling crystallization and sweating treatment.

[0012] In a preferred embodiment, step (1') is carried out before step (1): The cyclic ester polycondensation precursor is subjected to polycondensation to obtain an alcohol solvent and a polycondensation product, and the polycondensation product is subjected to a depolymerization reaction to obtain the crude cyclic ester crystal and / or its melt.

[0013] Among them, the polycondensation and depolymerization of the cyclic ester polycondensation precursor can be carried out by the methods and conditions disclosed in the prior art, that is, the cyclic oligomer precursor is subjected to a polycondensation reaction in the presence of a catalyst to obtain an alcohol solvent and a polycondensation product respectively, and the polycondensation product undergoes a depolymerization reaction under the action of a catalyst to obtain a crude cyclic ester.

[0014] In a further preferred embodiment, the cyclic ester polycondensation precursor is selected from at least one of the compounds shown in formula (I); the alcohol solvent is selected from at least one of the alcohols shown as R 1 OH, where R 1 is selected from C1-C10 alkyl or substituted alkyl, preferably C1-C5 alkyl or substituted alkyl, such as C1, C2, C3, C4, C5, C6, C7, C8, C9 or C10 alkyl or substituted alkyl

[0015]

[0016] In formula (I), R 1 is selected from C1-C10 alkyl or C1-C10 substituted alkyl, preferably C1-C5 alkyl or C1-C5 substituted alkyl, such as C1, C2, C3, C4, C5, C6, C7, C8, C9 or C10 alkyl or substituted alkyl; R 2 is selected from hydrogen, C1-C10 alkyl or C1-C10 substituted alkyl, preferably hydrogen, C1-C5 alkyl or C1-C5 substituted alkyl.

[0017] In a still further preferred embodiment, the cyclic ester is selected from at least one of the compounds shown in formula (II):

[0018]

[0019] In formula (I), R 2 is selected from hydrogen, C1-C10 alkyl or C1-C10 substituted alkyl, preferably from hydrogen, C1-C5 alkyl or C1-C10 substituted alkyl, such as hydrogen, C1, C2, C3, C4, C5, C6, C7, C8, C9 or C10 alkyl or substituted alkyl.

[0020] In a preferred embodiment, the melt crystallization comprises: cooling and crystallizing the melt of the crude cyclic ester to obtain cyclic ester crystal I and mother liquor I, and then sweating the cyclic ester crystal I to obtain cyclic ester crystal II and mother liquor II.

[0021] Wherein, the melt crystallization is carried out in a melt crystallizer, and the melt crystallizer includes but is not limited to a falling film crystallizer, a static crystallizer, etc.

[0022] In a further preferred embodiment, the conditions for the cooling crystallization include: cooling and crystallizing at 0.05-7 °C / h, and the final crystallization temperature is 65-82 °C; preferably, cooling and crystallizing at 0.05-5 °C / h, and the final crystallization temperature is 70-80 °C.

[0023] For example, the conditions for the cooling crystallization include: cooling and crystallizing at 0.05 °C / h, 0.08 °C / h, 0.1 °C / h, 0.2 °C / h, 0.5 °C / h, 1 °C / h, 2 °C / h, 3 °C / h, 4 °C / h, 5 °C / h, 6 °C / h or 7 °C / h, and the final crystallization temperature is 65 °C, 66 °C, 68 °C, 70 °C, 72 °C, 74 °C, 76 °C, 78 °C, 80 °C or 82 °C.

[0024] In a still further preferred embodiment, the conditions for the sweating treatment include: heating and sweating at 0.5-8 °C / h, and the final sweating temperature is 74-88 °C; preferably, heating and sweating at 0.5-5 °C / h, and the final sweating temperature is 76-86 °C.

[0025] For example, the conditions for the sweating treatment include: heating and sweating at 0.5 °C / h, 0.5 °C / h, 0.5 °C / h, 0.5 °C / h, 0.5 °C / h, 0.5 °C / h, 0.5 °C / h, 0.5 °C / h, 8 °C / h, and the final sweating temperature is 74 °C, 76 °C, 78 °C, 80 °C, 82 °C, 84 °C, 86 °C or 88 °C.

[0026] In a preferred embodiment, the recrystallization comprises: mixing fresh solvent and / or recycled solvent from the top of the first distillation column with the cyclic ester crystal II obtained by the melt crystallization, heating and dissolving, and then cooling.

[0027] In a further preferred embodiment, the solvent in the fresh solvent and the recycled solvent is selected from at least one of alcohols, esters, and ether solvents, preferably at least one of ethanol, n-propanol, isopropanol, n-butanol, tert-butanol, isobutanol, ethyl acetate, butyl acetate, and polyol ethers.

[0028] In a still further preferred embodiment, the weight ratio of the total weight of the fresh solvent and / or the recycled solvent from the top of the first distillation column to the weight of the cyclic ester crystal II is (0.5 - 8):1, preferably (0.8 - 4):1, such as 0.5:1, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, or 8:1.

[0029] In a preferred embodiment, during the recrystallization: when heating for dissolution, the temperature is raised to 50 - 80°C, preferably 60 - 70°C, such as 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, or 80°C; and / or, the temperature is lowered to 5 - 30°C, preferably 5 - 25°C (such as 5°C, 10°C, 15°C, 20°C, 25°C, or 30°C) for the recrystallization.

[0030] In a preferred embodiment, step (1) includes: (1.1) cooling and crystallizing the melt of the crude cyclic ester from the synthesis unit to obtain cyclic ester crystal I and mother liquor I, and sweating the cyclic ester crystal I to obtain cyclic ester crystal II and mother liquor II; (1.2) mixing the cyclic ester crystal II with the fresh solvent and / or the recycled solvent in step (2), heating for dissolution, and then cooling for recrystallization; (1.4) subjecting the material after recrystallization to solid-liquid separation to obtain a cyclic ester product and a filtrate.

[0031] In a preferred embodiment, a heavy organic substance is introduced into the first distillation column. Preferably, the heavy organic substance enters the first distillation column together with or independently of the filtrate from the recrystallization (preferably enters the middle of the distillation column).

[0032] In a further preferred embodiment, the heavy organic substance is selected from organic solvents with a boiling point higher than that of the recycled solvent and the cyclic ester prepolymer, preferably at least one of alcohol solvents, ether solvents, and alkane solvents with a boiling point higher than that of the recycled solvent and the cyclic ester prepolymer.

[0033] Preferably, the heavy organic substance is selected from C2 - C10 diols, C5 - C20 alkane solvents, diaryl ethers, and C5 - C20 dialkyl ethers, preferably at least one of ethylene glycol, n-decane, and diphenyl ether.

[0034] In a further preferred embodiment, the weight ratio of the heavy organic matter entering the first distillation column to the filtrate of the recrystallization is (0.05 - 1):1, preferably (0.1 - 0.5):1, such as 0.05:1, 0.08:1, 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1 or 1:1.

[0035] Among them, the weight of the heavy organic matter includes the weight of the freshly fed heavy organic matter 23 and the weight of the bottoms liquid 19 or 22 from the bottoms of the second distillation column.

[0036] In a preferred embodiment, the number of theoretical plates of the first distillation column is 5 - 30, preferably 10 - 25, such as 5, 10, 15, 20, 25 or 30.

[0037] In a further preferred embodiment, based on the plates from the top to the bottom of the first distillation column being 0 - 100% of the plates, a feed inlet is preferably provided at the 25 - 80% plates, preferably at the 25 - 80% plates.

[0038] Among them, the filtrate and / or the heavy organic matter are fed from the 25 - 80% plates, preferably from the 35 - 70% plates, such as 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75% or 80%.

[0039] In a preferred embodiment, the conditions of the first distillation column include: the bottom temperature is 120 - 285°C, and the top operating pressure is -0.09 - 0.5 MPaG.

[0040] In the present invention, the distillation can be controlled to be carried out at a relatively high temperature, so that the recovery rate of the solvent in the filtrate can be maximized.

[0041] In a further preferred embodiment, the conditions of the first distillation column include: the bottom temperature is 150 - 200°C, and the operating pressure is -0.03 - 0.3 MPaG.

[0042] For example, control the bottom temperature of the first rectification column at 150°C, 160°C, 180°C, 200°C, 220°C, 240°C, 260°C, 280°C or 285°C, and the operating pressure at -0.09 MPaG, -0.08 MPaG, -0.07 MPaG, -0.06 MPaG, -0.05 MPaG, -0.04 MPaG, -0.03 MPaG, -0.02 MPaG, -0.01 MPaG, 0 MPaG, 0.1 MPaG, 0.15 MPaG, 0.2 MPaG, 0.25 MPaG, 0.3 MPaG, 0.35 MPaG, 0.4 MPaG, 0.45 MPaG or 0.5 MPaG.

[0043] In a preferred embodiment, the alcohol solvent in step (2) is selected from at least one of the alcohols represented by R 1 OH, where R 1 is selected from C1-C10 alkyl or C1-C10 substituted alkyl, preferably C1-C5 alkyl or C1-C5 substituted alkyl, such as C1, C2, C3, C4, C5, C6, C7, C8, C9 or C10 alkyl or substituted alkyl.

[0044] In a further preferred embodiment, the alcohol solvent in step (2) is from the alcohol solvent in step (1'), that is, from the alcohol solvent in the polycondensation product.

[0045] In an even more preferred embodiment, in step (2), control the weight ratio of the alcohol solvent to the oligomers in the bottom material of the first rectification column and the mother liquor (i.e., the material in the alcoholysis reactor) to be 0.2-5, preferably 0.5-2 (such as 0.2, 0.5, 1, 2, 3, 4 or 5), where the oligomers refer to dimers to eicosamers of cyclic esters, such as dimers, tetramers, hexamers, octamers, decamers, dodecamers, tetradecamers, hexadecamers, octadecamers or eicosamers of cyclic esters.

[0046] In a preferred embodiment, the temperature of the alcoholysis reaction is 40-150°C, and / or the pressure is 0.1-1.5 MPaG, and / or the time is 0.2-6 h.

[0047] For example, the temperature of the alcoholysis reaction is 40°C, 60°C, 80°C, 100°C, 120°C, 140°C or 150°C, the pressure is 0.1 MPaG, 0.2 MPaG, 0.4 MPaG, 0.6 MPaG, 0.8 MPaG, 1 MPaG, 1.2 MPaG, 1.4 MPaG or 1.5 MPaG, and the time is 0.2 h, 0.5 h, 0.8 h, 1 h, 2 h, 3 h, 4 h, 5 h or 6 h.

[0048] In a further preferred embodiment, the temperature of the alcoholysis reaction is 60 to 130 °C, and / or the pressure is 0.1 to 0.8 MPaG, and / or the time is 0.5 to 4 h.

[0049] In a still further preferred embodiment, the alcoholysis reaction is carried out under a protective atmosphere. Preferably, the protective atmosphere is selected from at least one of nitrogen and inert gases, more preferably nitrogen and / or argon.

[0050] In a preferred embodiment, the number of theoretical plates of the second distillation column is 15 to 50, preferably 25 to 40, such as 15, 20, 25, 30, 35, 40, 45 or 50.

[0051] In a further preferred embodiment, based on the plates from the top to the bottom of the second distillation column being 0 to 100% of the total plates, the alcoholysis solution is fed at the 20 to 70% plate, preferably at the 35 to 55% plate, such as 20%, 30%, 40%, 50%, 60% or 70%.

[0052] In a preferred embodiment, the conditions of the second distillation column include: the bottom temperature is 150 to 285 °C, and the top operating pressure is -0.09 to 0.5 MPaG.

[0053] For example, control the bottom temperature of the second distillation column to be 150 °C, 160 °C, 180 °C, 200 °C, 220 °C, 240 °C, 260 °C, 280 °C or 285 °C, and the operating pressure to be -0.09 MPaG, -0.08 MPaG, -0.07 MPaG, -0.06 MPaG, -0.05 MPaG, -0.04 MPaG, -0.03 MPaG, -0.02 MPaG, -0.01 MPaG, 0 MPaG, 0.1 MPaG, 0.15 MPaG, 0.2 MPaG, 0.25 MPaG, 0.3 MPaG, 0.35 MPaG, 0.4 MPaG, 0.45 MPaG or 0.5 MPaG.

[0054] In a further preferred embodiment, the conditions of the second distillation column include: the bottom temperature is 185 to 250 °C, and the operating pressure is -0.03 to 0.3 MPaG.

[0055] In the present invention, the distillation can be controlled to be carried out at a relatively high temperature. In this way, the cyclic ester polycondensation precursor and the alcohol solvent can be separated out to the greatest extent, and the recovery rate is relatively high. The overhead material of the second distillation column includes the cyclic ester polycondensation precursor described in step (1) and the alcohol solvent described in step (1).

[0056] In a preferred embodiment, the bottom material of the second distillation column includes the heavy organic matter.

[0057] In a further preferred embodiment, when the content of the heavy organic matter in the bottom material of the second rectification column is 90% (preferably above 95%), part or all of the bottom material is recycled back into the first rectification column.

[0058] In another further preferred embodiment, when the content of the heavy organic matter in the bottom material of the second rectification column is less than 90% (preferably 95%), the bottom material is first subjected to rectification treatment or membrane separation treatment until the content of the heavy organic matter reaches 90% (preferably above 95%), and then part or all of it is recycled back into the first rectification column.

[0059] In the present invention, the method comprises:

[0060] Step A: Placing the melt of the crude cyclic ester in a melt crystallizer, performing melt crystallization according to a temperature reduction procedure (including temperature reduction crystallization and sweating treatment), sending the obtained cyclic ester crystals II to a downstream recrystallizer, and sending the mother liquor to an alcoholysis reactor;

[0061] Step B: Placing a certain amount of solvent in the recrystallizer and mixing it with the cyclic ester crystals II, heating to dissolve the cyclic ester under stirring, and after temperature reduction recrystallization, solid-liquid separation and drying, obtaining a high-purity cyclic ester product, and sending the filtrate to the first rectification column;

[0062] Step C: Mixing the filtrate of Step B with the heavy organic matter and entering the first rectification column, recovering the high-purity recycled solvent at the top of the column and recycling it to the recrystallizer, and sending the bottom liquid to an alcoholysis reactor;

[0063] Step D: The mother liquor in Step A, the bottom liquid of the first rectification column and the alcohol solvent undergo an alcoholysis reaction in the alcoholysis reactor, sending the obtained alcoholysis liquid to the second rectification column, sending the mixture of the cyclic ester prepolymer (such as methyl glycolate) and the alcohol solvent (such as methanol) obtained at the top of the second rectification column to the upstream cyclic ester synthesis unit, and directly and / or after purification, recycling the bottom material of the second rectification column to the inlet of the first rectification column.

[0064] A second object of the present invention is to provide a system for refining and post-treating cyclic esters, preferably used for performing the method described in the first object of the present invention. The system comprises a cyclic ester synthesis unit, a melt crystallization unit, a recrystallization unit, a first rectification column, an alcoholysis unit and a second rectification column.

[0065] In a preferred embodiment, the cyclic ester synthesis unit includes a polycondensation reactor and a depolymerization reactor. Further, a cyclic ester polycondensation precursor inlet, a polycondensation product outlet, and an alcohol solvent outlet are provided on the polycondensation reactor, and a polycondensation product inlet and a crude cyclic ester outlet are provided on the depolymerization reactor; and / or, the melt crystallization unit includes a melt crystallizer, and further includes a crude cyclic ester inlet, a mother liquor outlet, and a cyclic ester crystal II outlet.

[0066] In a further preferred embodiment, the cyclic ester polycondensation precursor inlet of the cyclic ester synthesis unit (in the polycondensation reactor) is connected to a cyclic ester polycondensation precursor feed pipeline; and / or, the crude cyclic ester outlet of the cyclic ester synthesis unit (in the depolymerization reactor) is connected to the crude cyclic ester inlet of the melt crystallization unit through a pipeline; and / or, an alcohol solvent discharge pipeline and an alcohol solvent transfer pipeline are arranged in parallel on the alcohol solvent outlet of the cyclic ester synthesis unit (in the polycondensation reactor), and the alcohol solvent transfer pipeline connects the cyclic ester synthesis unit (in the polycondensation reactor) to the alcoholysis unit; and / or, a mother liquor transfer pipeline is arranged between the mother liquor outlet of the melt crystallization unit and the alcoholysis unit; and / or, a crystal transfer pipeline is arranged between the cyclic ester crystal II outlet of the melt crystallization unit and the recrystallization unit.

[0067] In a preferred embodiment, the recrystallization unit includes a recrystallizer, and a cyclic ester crystal II inlet, a solvent inlet, a filtrate outlet, and a cyclic ester product outlet are provided on the recrystallization unit (or the recrystallizer).

[0068] In a further preferred embodiment, the filtrate outlet of the recrystallization unit is connected to the feed inlet of the first distillation column through a filtrate transfer pipeline, the solvent inlet of the recrystallization unit is connected to the top of the first distillation column through a recycled solvent circulation pipeline, and a fresh solvent feed pipeline is further provided on the solvent inlet of the recrystallization unit.

[0069] In an even more preferred embodiment, a fresh heavy organic matter feed pipeline is further provided on the feed inlet of the first distillation column.

[0070] In a preferred embodiment, the bottom of the first distillation column is connected to the alcoholysis unit through a pipeline.

[0071] In a further preferred embodiment, the number of theoretical plates of the first distillation column is 5 to 30, preferably 10 to 25, such as 5, 10, 15, 20, 25 or 30.

[0072] In a further preferred embodiment, a feed inlet is provided on the first rectification column. Calculated based on the trays from top to bottom of the first rectification column being 0 to 100% of the total trays, the feed inlet is provided at the 25 to 80% tray position, such as 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75% or 80%.

[0073] In a preferred embodiment, the feed inlet of the second rectification column is connected to the alcoholysis unit through an alcoholysis liquid transfer pipeline.

[0074] In a further preferred embodiment, the top of the second rectification column is connected to the cyclic ester synthesis unit through a pipeline, for recycling the mixture including the cyclic ester polycondensation precursor and the alcohol solvent at the top of the column back to the cyclic ester synthesis unit.

[0075] In a still further preferred embodiment, the bottom of the second rectification column is connected to the feed inlet of the first rectification column through a parallel-connected heavy organic matter circulation pipeline I and heavy organic matter circulation pipeline II.

[0076] Further preferably, a refining unit and / or a membrane treatment unit are provided on the heavy organic matter circulation pipeline II. Preferably, the refining unit includes a rectification column, and the membrane separation unit includes a membrane separator.

[0077] Wherein, when the content of the heavy organic matter in the bottom material of the second rectification column is less than 90% (preferably 95%), it is first treated in the refining unit through the heavy organic matter circulation pipeline II until the content of the heavy organic matter reaches 90% (preferably more than 95%) and then partially or fully recycled back into the first rectification column. When the content of the heavy organic matter in the bottom material of the second rectification column is more than 95%, it enters the first rectification column through the heavy organic matter circulation pipeline I.

[0078] In a preferred embodiment, the number of theoretical trays of the second rectification column is 15 to 50, preferably 25 to 40, such as 15, 20, 25, 30, 35, 40, 45 or 50.

[0079] In a further preferred embodiment, a feed inlet is provided on the second rectification column. Calculated based on the trays from top to bottom of the second rectification column being 0 to 100% of the total trays, the feed inlet is provided at the 20 to 70% tray position, such as 20%, 30%, 40%, 50%, 60% or 70%.

[0080] The third object of the present invention is to provide the application of the method described in the first object of the present invention or the system described in the second object of the present invention in the process of synthesizing cyclic esters.

[0081] In the ranges disclosed in the present invention, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. In the following text, in principle, the various technical solutions can be combined with each other to obtain new technical solutions, which should also be regarded as specifically disclosed herein.

[0082] Compared with the prior art, the present invention has the following beneficial effects:

[0083] (1) By means of the refining method combining melt crystallization and recrystallization, the method of the present invention obtains a high-purity cyclic substance and reduces the generation of impurities during the refining process;

[0084] (2) The method of the present invention realizes the separate recovery of the solvent and the oligomer, and the mixture of the ester substance and the alcoholysis solvent obtained can be recycled to the cyclic ester synthesis unit;

[0085] (3) After the method or system of the present invention is stable, there is basically no external discharge of waste liquid. The recrystallization solvent, the alcoholysis solvent and the heavy components can circulate in the system. Among them, a small amount of heavy components may be externally discharged according to the separation situation of the second rectification column, and the material recycling rate in the system is high;

[0086] (4) The method of the present invention is simple to operate, easy to implement, and can be applied in large-scale industrial production. Description of the Drawings

[0087] Figure 1 Shows a schematic structural diagram of the system of the present invention.

[0088] In Figure 1 1 - Feed pipeline for cyclic ester polycondensation precursor, 2 - Cyclic ester synthesis unit, 3 - Alcohol solvent to alcoholysis unit, 4 - External discharge of alcohol solvent, 5 - Crude cyclic ester crystals and / or its melt, 6 - Melt crystallization unit, 7 - Cyclic ester crystals II, 8 - Mother liquor, 9 - Recrystallization unit, 10 - Cyclic ester product, 11 - Filtrate, 12 - First rectification column, 13 - Recycled solvent, 14 - Bottom liquid of the first rectification column, 15 - Alcoholysis unit, 16 - Alcoholysis liquid, 17 - Second rectification column, 18 - Mixture of recycled cyclic ester polycondensation precursor and alcohol solvent, 19 - Bottom liquid containing more than 95 wt% heavy organic matter, 20 - Bottom liquid containing less than 95 wt% heavy organic matter, 21 - Refining unit, 22 - Bottom liquid after refining, 23 - Fresh heavy organic matter, 24 - Fresh recrystallization solvent, 25 - Fresh alcohol solvent.

[0089] Using Figure 1The system shown: The cyclic ester polycondensation precursor 1 undergoes a polycondensation reaction in the cyclic ester synthesis unit 2. The polycondensation product is subjected to solid-liquid separation to obtain crude cyclic ester 5 and an alcohol solvent 3. The alcohol solvent 3 goes to the alcoholysis unit 15, and a part of the alcohol solvent is discharged externally 4. The crude cyclic ester is subjected to melt crystallization and sweating treatment in the melt crystallization unit 6 to obtain cyclic ester crystals II 7 and mother liquor 8. The mother liquor 8 enters the alcoholysis unit 15. The cyclic ester crystals II 7 are mixed with the recycled solvent 13 and an optional fresh recrystallization solvent 24 and enter the recrystallization unit 9. After recrystallization and solid-liquid separation, a cyclic ester product 10 and a filtrate 11 are obtained. The filtrate 11 is subjected to rectification treatment in the first rectification tower 12. At the same time, fresh heavy organic matter 23 and / or the bottom liquid of the second rectification tower 17 are introduced into the first rectification tower. The recycled solvent 13 is obtained at the top of the tower and recycled back to the recrystallization unit 9. The bottom liquid 14 of the first rectification tower enters the alcoholysis unit 15. The alcohol solvent 3, the mother liquor 8 from melt crystallization, and the bottom liquid 14 of the first rectification tower react in the alcoholysis unit 15 to obtain an alcoholysis solution 16. The alcoholysis solution 16 is subjected to rectification treatment in the second rectification tower 17. A mixture 18 of the recycled cyclic ester polycondensation precursor and the alcohol solvent obtained at the top of the tower is recycled back to the cyclic ester synthesis unit 2. When the content of heavy organic matter in the bottom material of the second rectification tower is above 90 wt% (preferably 95 wt%), it is directly recycled back to the first rectification tower. When the content of heavy organic matter in the bottom material is below 90 wt% (preferably 95%), it needs to pass through the refining unit 21 first and then be recycled back to the first rectification tower. Detailed Embodiments

[0090] The present invention will be specifically described below in conjunction with specific embodiments. It is necessary to point out here that the following embodiments are only for further illustration of the present invention and should not be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention still fall within the protection scope of the present invention.

[0091] In addition, it should be noted that the various specific technical features described in the following detailed embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

[0092] Furthermore, any combination can be made between different embodiments of the present invention as long as it does not violate the idea of the present invention. The technical solutions formed thereby belong to a part of the original disclosure of this specification and also fall within the protection scope of the present invention.

[0093] If there is no special limitation on the raw materials used in the examples and comparative examples, they are all disclosed in the prior art. For example, they can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0094] The 85.21% crude glycolide in the examples and comparative examples can be directly purchased or obtained through the following glycolide synthesis unit: Methyl glycolate is polycondensed to obtain methanol and a polycondensation product, and the polycondensation product is depolymerized to obtain a 85.21% crude glycolide melt.

[0095] Example 1

[0096] Adopt Figure 1 the system shown.

[0097] 100 kg of the crude glycolide melt with a purity of 85.21% is fed into a melt crystallizer. The surface temperature of the crystallizer is slowly cooled from 84 °C at a cooling rate of 2 °C / h for 2 hours to 80 °C. After the cooling is completed, the mother liquor is sent to an alcoholysis reactor; the glycolide crystals are heated to 82 °C at a rate of 3 °C / h for sweating operation. After the sweating is completed, the mother liquor is sent to the alcoholysis reactor. The glycolide crystals are melted by heating and then sent to a recrystallizer, and the purity of the glycolide is 98.15%.

[0098] 155 kg of ethanol and 78 kg of glycolide from melt crystallization are mixed in the recrystallizer. The mixture is heated to 80 °C to make the two phases mutually soluble, and then cooled for recrystallization. The final recrystallization temperature is controlled at 20 °C. After the cooling is completed, filtration and drying are carried out, and the purity of the obtained glycolide product is 99.91%. The filtrate is collected and sent to the first distillation column.

[0099] 50 kg / h of the filtrate is mixed with 10 kg / h of n-decane and then enters the first distillation column (at the 9th tray from top to bottom) for solvent recovery. The number of theoretical trays of the first distillation column is 20, the operating pressure of the column is 0 MPaG, the temperature of the column bottom is 123 °C, and the ethanol concentration obtained at the top of the column is 99.97 wt%, which can be directly recycled to the recrystallization unit as the raw material ethanol. The ethanol recovery rate in the filtrate is 99.99%.

[0100] The bottom liquid of the first distillation column is sent to an alcoholysis reactor. The mass ratio of methanol added to the oligomers in the reactor (i.e., the oligomers in the bottom material of the first distillation column and the mother liquor) is 0.6. The alcoholysis reaction temperature is 80 °C, the reaction pressure is 0.3 MPaG, and the reaction is carried out for 4 h. The alcoholysis rate of the oligomers reaches 98%.

[0101] The alcoholysis liquid obtained from the alcoholysis reaction enters the second distillation column (at the 18th tray from top to bottom). The number of theoretical trays of the second distillation column is 35, the operating pressure of the column is 0.1 MPaG, the temperature of the column bottom is 203 °C, the n-decane content at the top of the column is 30 ppm, and it can be directly recycled to the glycolide synthesis unit. The concentration of n-decane at the bottom of the column is 99.99 wt%, and it can be directly recycled to the inlet of the first distillation column.

[0102] Example 2

[0103] Adopt Figure 1 the system shown

[0104] 100 kg of crude glycolide melt with a purity of 85.21% is fed into the melt crystallizer. The surface temperature of the crystallizer is slowly decreased from 83 °C at a cooling rate of 1.5 °C / h for 3 hours to 78.5 °C. After the cooling is completed, the mother liquor is sent to the alcoholysis reactor; the glycolide crystals are heated at a rate of 2 °C / h to 84 °C for sweating operation. After the sweating is completed, the mother liquor is sent to the alcoholysis reactor. The glycolide crystals are melted by heating and then sent to the recrystallizer, and the purity of the glycolide is 98.85%.

[0105] 80 kg of ethyl acetate and 76 g of glycolide from melt crystallization are mixed in the recrystallizer. The mixture is heated to 75 °C to make the two phases mutually soluble, and then cooled for recrystallization. The final temperature of recrystallization is controlled at 20 °C. After the cooling is completed, filtration and drying are carried out, and the purity of the obtained glycolide product is 99.96%. The filtrate is collected and sent to the first distillation column.

[0106] 50 kg / h of the filtrate is mixed with 10 kg / h of ethylene glycol and then enters the first distillation column (at the 11th tray from top to bottom) for solvent recovery. The number of theoretical trays of the first distillation column is 15. The operating pressure of the column is -0.05 MPaG, the temperature of the column bottom is 174 °C, the concentration of ethyl acetate obtained at the top of the column is 99.99 wt%, which can be directly recycled to the recrystallization unit, and the recovery rate of ethyl acetate in the filtrate is 99.98%.

[0107] The bottom liquid of the first distillation column is sent to the alcoholysis reactor. The mass ratio of methanol added to the oligomer in the reactor is 1.0. The alcoholysis reaction temperature is 100 °C, the reaction pressure is 0.5 MPaG, and the reaction lasts for 2.5 h. The alcoholysis rate of the oligomer reaches 95%.

[0108] The alcoholysis liquid obtained from the alcoholysis reaction enters the second distillation column (at the 7th tray from top to bottom). The number of theoretical trays of the second distillation column is 15. The operating pressure of the column is 0.35 MPaG, the temperature of the column bottom is 221 °C, the content of ethylene glycol at the top of the column is 50 ppm, which can be directly recycled to the glycolide synthesis unit. The concentration of ethylene glycol in the column bottom is 99.95 wt%, which can be directly recycled to the inlet of the first distillation column.

[0109] Example 3

[0110] Adopt Figure 1 the system shown

[0111] 100 kg of crude glycolide melt with a purity of 85.21% is fed into a melt crystallizer. The surface temperature of the crystallizer is slowly decreased from 82 °C at a rate of 1 °C / h for 2.5 hours. After the temperature reduction, the mother liquor is sent to an alcoholysis reactor; the glycolide crystals are heated to 86 °C at a rate of 4 °C / h for sweating operation. After the sweating operation, the mother liquor is sent to the alcoholysis reactor. The glycolide crystals are melted by heating and then sent to a recrystallizer, and the purity of the glycolide is 98.75%.

[0112] 220 kg of n-propanol and 66 g of glycolide from melt crystallization are mixed in a recrystallizer. The mixture is heated to 78 °C to make the two phases miscible, and then cooled for recrystallization. The final temperature of recrystallization is controlled at 20 °C. After the temperature reduction, filtration and drying are carried out, and the purity of the obtained glycolide product is 99.94%. The filtrate is collected and sent to the first distillation column.

[0113] 50 kg / h of filtrate and 10 kg / h of diphenyl ether are mixed and then enter the first distillation column (at the 10th tray from top to bottom) for solvent recovery. The number of theoretical trays of the first distillation column is 25, the operating pressure of the column is 0.35 MPaG, the temperature of the column bottom is 174 °C, and the concentration of n-propanol obtained at the top of the column is 99.99 wt%, which can be directly recycled to the recrystallization unit as the raw material n-propanol. The recovery rate of n-propanol in the filtrate is 99.98%.

[0114] The bottom liquid of the first distillation column is sent to an alcoholysis reactor. The mass ratio of methanol added to the oligomer in the reactor is 1.6. The alcoholysis reaction temperature is 120 °C, the reaction pressure is 0.8 MPaG, and the reaction time is 1 h. The alcoholysis rate of the oligomer reaches 100%.

[0115] The alcoholysis liquid obtained from the alcoholysis reaction enters the second distillation column (at the 29th tray from top to bottom). The number of theoretical trays of the second distillation column is 40, the operating pressure of the column is -0.03 MPaG, the temperature of the column bottom is 285 °C, the content of diphenyl ether at the top of the column is 50 ppm, which can be directly recycled to the glycolide synthesis unit. The concentration of diphenyl ether in the column bottom is 99.95 wt%, which can be directly recycled to the inlet of the first distillation column.

[0116] Comparative Example 1

[0117] 100 kg of crude glycolide melt with a purity of 85.21% is fed into a melt crystallizer. The surface temperature of the crystallizer is slowly decreased from 84 °C at a rate of 3 °C / h for 1.5 hours to 79.5 °C. After the temperature reduction, the mother liquor is sent to an alcoholysis reactor; the glycolide crystals are heated to 85 °C at a rate of 2.5 °C / h for sweating operation. After the sweating operation, the mother liquor is sent to the alcoholysis reactor. The glycolide crystals are melted by heating and then sent out of the device, and the purity of the glycolide is 98.75%.

[0118] Comparative Example 2

[0119] 220 kg of n-propanol and 100 kg of crude glycolide with a purity of 85.21% are mixed in a recrystallizer. The mixture is heated to 78 °C to make the two phases mutually soluble, and then cooled for recrystallization. The final temperature of recrystallization is controlled at 20 °C. After cooling, filtration and drying are carried out, and the purity of the obtained glycolide product is 98.64%. The filtrate is collected and sent to the first distillation column.

[0120] 50 kg / h of filtrate is mixed with 40 kg / h of water and then enters the first distillation column for solvent recovery. The number of theoretical plates of the first distillation column is 30, the column operating pressure is 0.1 MPaG, the column bottom temperature is 121 °C, and the concentration of ethyl acetate obtained at the top of the column is 71.2 wt%. It needs to be dehydrated and then reused in the recrystallization unit.

[0121] The bottom liquid of the first distillation column is sent to a hydrolysis reactor. The hydrolysis reaction temperature is 100 °C, the reaction pressure is 0.3 MPaG, and the reaction lasts for 3 h. The alcoholysis rate of the oligomer reaches 95%. The alcoholysis liquid can be reused in the glycolide synthesis unit after membrane separation.

[0122] Comparative Example 3

[0123] 100 kg of crude glycolide melt with a purity of 85.21% enters a melting crystallizer. The surface temperature of the crystallizer slowly decreases from 84 °C at a rate of 2 °C / h for 2 hours to 80 °C. After cooling, the mother liquor is sent to an alcoholysis reactor; the glycolide crystals are heated at a rate of 3 °C / h to 82 °C for sweating operation. After sweating, the mother liquor is sent to the alcoholysis reactor. The glycolide crystals are melted by heating and then sent to a recrystallizer, and the purity of the glycolide is 98.15%.

[0124] 155 kg of ethanol and 78 g of glycolide from melting crystallization are mixed in a recrystallizer. The mixture is heated to 80 °C to make the two phases mutually soluble, and then cooled for recrystallization. The final temperature of recrystallization is controlled at 20 °C. After cooling, filtration and drying are carried out, and the purity of the obtained glycolide product is 99.91%. The filtrate is collected and sent to the first distillation column.

[0125] A mixture of 50 kg / h of filtrate and the mother liquor from melting crystallization enters the first distillation column for solvent recovery. The number of theoretical plates of the first distillation column is 20, the column operating pressure is 0 MPaG, the column bottom temperature is 123 °C, and the concentration of ethanol obtained at the top of the column is 99.97 wt%. It can be directly reused in the recrystallization unit, and the ethanol recovery rate in the filtrate is 60%.

[0126] The bottoms liquid of the first rectification column is fed into the alcoholysis reactor. The mass ratio of methanol added to the oligomer in the reactor is 0.6. The alcoholysis reaction temperature is 80 °C, the reaction pressure is 0.3 MPaG, and the reaction lasts for 4 h. The alcoholysis rate of the oligomer reaches 99%. However, methyl glycolate, ethyl glycolate, methanol, and ethanol are present in the reaction products. They cannot be recovered by rectification subsequently. If directly reused, it will not only introduce the recrystallization solvent but also introduce the oligomer that has not been completely alcoholyzed.

[0127] The present invention has been described in detail above in conjunction with specific embodiments and exemplary examples. However, these descriptions should not be construed as limiting the present invention. Those skilled in the art understand that without departing from the spirit and scope of the present invention, various equivalent substitutions, modifications, or improvements can be made to the technical solutions and their implementation manners of the present invention, and these all fall within the scope of the present invention. The protection scope of the present invention shall be subject to the appended claims.

Claims

1. A method for refining and post - treating crude cyclic esters, comprising: (1) successively performing melt crystallization and recrystallization on the melt of the crude cyclic ester to obtain a cyclic ester product; (2) subjecting the filtrate of the recrystallization to distillation treatment in a first distillation column, obtaining a recycled solvent at the top of the first distillation column, recycling it back to the recrystallization, and mixing the bottom material of the first distillation column with the mother liquor of the melt crystallization and an alcohol solvent to carry out an alcoholysis reaction to obtain an alcoholysis solution; (3) subjecting the alcoholysis solution to distillation treatment in a second distillation column, taking the overhead material out, and recycling the bottom material back to the first distillation column in step (2).

2. The method according to claim 1, characterized in that, before step (1), step (1’) is carried out: polycondensing a cyclic ester polycondensation precursor to obtain an alcohol solvent and a polycondensation product, and carrying out a depolymerization reaction on the polycondensation product to obtain the crude cyclic ester crystal and / or its melt; preferably, the cyclic ester polycondensation precursor is selected from at least one of the compounds shown in formula (I); In formula (I), R 1 is selected from alkyl groups having 1 to 10 carbon atoms or substituted alkyl groups having 1 to 10 carbon atoms, preferably alkyl groups having 1 to 5 carbon atoms or substituted alkyl groups having 1 to 5 carbon atoms; R 2 is selected from hydrogen, alkyl groups having 1 to 10 carbon atoms or substituted alkyl groups having 1 to 10 carbon atoms, preferably hydrogen, alkyl groups having 1 to 5 carbon atoms or substituted alkyl groups having 1 to 5 carbon atoms.

3. The method according to claim 1, characterized in that, the melt crystallization includes: cooling the melt of the crude cyclic ester to crystallize to obtain cyclic ester crystal I and mother liquor I, and then subjecting the cyclic ester crystal I to sweating treatment to obtain cyclic ester crystal II and mother liquor II; preferably, the conditions for the cooling crystallization include: cooling and crystallizing at a rate of 0.05 - 7 °C / h, and the final crystallization temperature is 65 - 82 °C; preferably, cooling and crystallizing at a rate of 0.05 - 5 °C / h, and the final crystallization temperature is 70 - 80 °C; preferably, the conditions for the sweating treatment include: heating and sweating at a rate of 0.5 - 8 °C / h, and the final sweating temperature is 74 - 88 °C; preferably, heating and sweating at a rate of 0.5 - 5 °C / h, and the final sweating temperature is 76 - 86 °C.

4. The method according to claim 3, characterized in that, the recrystallization includes: mixing fresh solvent and / or the recycled solvent from the top of the first distillation column with the cyclic ester crystal II obtained by the melt crystallization, heating to dissolve, and then cooling to crystallize; preferably, the solvent in the fresh solvent and the recycled solvent is selected from at least one of alcohol solvents, ester solvents, and ether solvents, preferably selected from at least one of ethanol, n - propanol, isopropanol, n - butanol, tert - butanol, isobutanol, ethyl acetate, butyl acetate, and polyol ethers; preferably, when carrying out the recrystallization: during the heating and dissolution, heating to 50 - 80 °C, preferably 60 - 70 °C; and / or, cooling to 5 - 30 °C, preferably 5 - 25 °C for the recrystallization.

5. The method according to claim 1, characterized in that, introducing a heavy organic substance into the first distillation column, preferably, the heavy organic substance enters the first distillation column together with or independently of the filtrate of the recrystallization; more preferably, the heavy organic substance is selected from organic solvents with a boiling point higher than the recycled solvent and the cyclic ester polycondensation precursor, preferably selected from at least one of alcohol solvents, ether solvents, and alkane solvents with a boiling point higher than the recycled solvent and the cyclic ester polycondensation precursor; Further preferably, the weight ratio of the heavy organic matter entering the first distillation column to the filtrate of the recrystallization is (0.05 - 1):1, preferably (0.1 - 0.5):1, wherein the weight of the heavy organic matter includes the weight of the fresh heavy organic matter feed and the weight of the bottoms liquid from the bottom of the second distillation column.

6. The method according to claim 1, wherein, the number of theoretical plates of the first distillation column is 5 - 30, preferably 10 - 25; and / or, counting from the top to the bottom of the first distillation column as 0 - 100% of the plates, a feed inlet is preferably provided at 25 - 80% of the plates, preferably at 25 - 80% of the plates; and / or, the conditions of the first distillation column include: the bottom temperature is 120 - 285 °C, and the top operating pressure is -0.09 - 0.5 MPaG.

7. The method according to claim 1, wherein, The alcohol solvent described in step (2) is selected from at least one of the alcohols represented by R 1 OH, wherein R 1 is selected from an alkyl group having 1 to 10 carbon atoms or a substituted alkyl group having 1 to 10 carbon atoms, preferably an alkyl group having 1 to 5 carbon atoms or a substituted alkyl group having 1 to 5 carbon atoms; and / or, the temperature of the alcoholysis reaction is 40 - 150 °C, and / or the pressure is 0.1 - 1.5 MPaG, and / or the time is 0.2 - 6 h; and / or, the alcoholysis reaction is carried out under a protective atmosphere. Preferably, the protective atmosphere is selected from at least one of nitrogen and inert gases, preferably nitrogen and / or argon.

8. The method according to any one of claims 1 - 7, wherein, the number of theoretical plates of the second distillation column is 15 - 50, preferably 25 - 40; and / or, counting from the top to the bottom of the second distillation column as 0 - 100% of the plates, the alcoholysis solution is fed from 20 - 70% of the plates, preferably from 35 - 55% of the plates; and / or, the conditions of the second distillation column include: the bottom temperature is 150 - 285 °C, and the operating pressure is -0.09 - 0.5 MPaG; and / or, the bottom material of the second distillation column includes the heavy organic matter; preferably, when the content of the heavy organic matter in the bottom material of the second distillation column is 90%, part or all of the bottom material is recycled back to the first distillation column; or, preferably, when the content of the heavy organic matter in the bottom material of the second distillation column is less than 90%, the bottom material is first subjected to distillation treatment or membrane separation treatment until the content of the heavy organic matter reaches more than 90%, and then part or all of it is recycled back to the first distillation column.

9. A system for refining and post - treating cyclic esters, preferably for carrying out the method according to any one of claims 1 - 8, the system includes a cyclic ester synthesis unit, a melt crystallization unit, a recrystallization unit, a first distillation column, an alcoholysis unit, and a second distillation column.

10. The system according to claim 9, wherein, the cyclic ester synthesis unit includes a polycondensation reactor and a depolymerization reactor. Among them, a cyclic ester polycondensation precursor inlet, a polycondensation product outlet, and an alcohol solvent outlet are provided on the polycondensation reactor, and a polycondensation product inlet and a crude cyclic ester outlet are provided on the depolymerization reactor; and / or, the melt crystallization unit includes a melt crystallizer, and further includes a crude cyclic ester inlet, a mother liquor outlet, and a cyclic ester crystal outlet; Preferably, the inlet of the cyclic ester polycondensation precursor is connected to a cyclic ester polycondensation precursor feed pipeline; and / or, the outlet of the crude cyclic ester is connected to the inlet of the crude cyclic ester of the melt crystallization unit through a pipeline; and / or, an alcohol solvent discharge pipeline and an alcohol solvent transfer pipeline are arranged in parallel on the outlet of the alcohol solvent. The alcohol solvent transfer pipeline connects the cyclic ester synthesis unit and the alcoholysis unit; and / or, a mother liquor transfer pipeline is arranged between the mother liquor outlet of the melt crystallization unit and the alcoholysis unit; and / or, a crystal transfer pipeline is arranged between the crystal outlet of the melt crystallization unit and the recrystallization unit.

11. The system according to claim 9, wherein, the recrystallization unit includes a recrystallizer, and a crystal inlet, a solvent inlet, a filtrate outlet and a cyclic ester product outlet are arranged on the recrystallization unit; preferably, the filtrate outlet of the recrystallization unit is connected to the feed inlet of the first distillation column through a filtrate transfer pipeline, and the solvent inlet of the recrystallization unit is connected to the top of the first distillation column through a recycled solvent circulation pipeline; more preferably, a fresh heavy organic matter feed pipeline is further arranged at the feed inlet of the first distillation column.

12. The system according to claim 9, wherein, the bottom of the first distillation column is connected to the alcoholysis unit through a pipeline; preferably, the number of theoretical plates of the first distillation column is 5 to 30, preferably 10 to 25; more preferably, a feed inlet is arranged on the first distillation column. Calculated based on the 0 to 100% plates from top to bottom of the first distillation column, the feed inlet is arranged at the 25 to 80% plates; and / or, the feed inlet of the second distillation column is connected to the alcoholysis unit through an alcoholysis liquid transfer pipeline; preferably, the number of theoretical plates of the second distillation column is 15 to 50, preferably 25 to 40; more preferably, a feed inlet is arranged on the second distillation column. Calculated based on the 0 to 100% plates from top to bottom of the second distillation column, the feed inlet is arranged at the 20 to 70% plates; and / or, the bottom of the second distillation column is connected to the feed inlet of the first distillation column through a parallel-connected heavy organic matter circulation pipeline I and heavy organic matter circulation pipeline II; preferably, a refining unit and / or a membrane treatment unit are arranged on the heavy organic matter circulation pipeline II.

13. Application of the method according to any one of claims 1 to 8 or the system according to any one of claims 9 to 12 in the process of synthesizing cyclic ester.

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