A method and system for purifying cyclic ester feed streams

Abstract

This invention provides a method and system for purifying a cyclic ester stream. The method includes the step of continuously evaporating a solution stream containing cyclic esters, thereby converting all or part of the cyclic esters into a solid state. Specifically, it includes the following steps: (a) introducing the solution stream containing cyclic esters into a continuous evaporation concentration unit, thereby converting all or part of the cyclic esters into a solid state, to obtain a gaseous solvent and a stream A containing purified solid cyclic esters; (b) introducing the stream A containing purified solid cyclic esters obtained in step (a) into a particle size control unit, to obtain a stream B containing purified solid cyclic esters; optionally, (c) introducing the stream B containing purified solid cyclic esters obtained in step (b) into a solid-liquid separation unit, to obtain a filtrate and a stream C containing purified solid cyclic esters; (d) drying the stream B containing purified solid cyclic esters obtained in step (b) or the stream C containing purified solid cyclic esters obtained in step (c), to obtain dried glycolide particles. This invention solves the technical problems of high difficulty in particle size control, insufficient chemical purity of the product, and inability to operate continuously in existing glycolide purification technologies.

Description

Technical Field

[0001] This invention relates to the purification of cyclic esters, and more specifically, to a method and system for purifying cyclic ester streams. Background Technology

[0002] Ring-opening polymerization is an important chemical reaction process with wide applications in various fields. With advancements in industrial catalysis and polymerization technologies, the application scope of ring-opening polymerization continues to expand, and an increasing number of types of cyclic monomers are being used in the synthesis of polymer materials. Ring-opening polymerization is one of the key steps in the manufacture of polymer materials; for example, the preparation of biodegradable polymers such as polyglycolic acid (PGA), polylactic acid (PLA), and polycaprolactone (PCL) all rely on it. These polymer materials have wide applications in the medical, packaging, and biomedical fields and have a positive impact on environmental protection.

[0003] Ring-opening polymerization typically uses cyclic monomers as raw materials. These monomers usually possess reactive groups that readily open the ring, such as esters, ethers, and ketones, and under specific conditions and with the aid of a catalyst, they form linear polymers. However, these cyclic monomers may contain trace impurities originating from the synthesis, separation, and purification processes of the raw materials, or from improper storage conditions. Common trace impurities include unreacted raw materials, byproducts, catalyst residues, solvent residues, oxidation products, and accidental ring-opening products.

[0004] Removing these impurities is crucial because they can affect the efficiency of the ring-opening polymerization reaction and product quality. Necessary purification and separation steps can reduce these impurities, ensuring product quality, improving reaction efficiency, meeting environmental requirements, and ensuring application safety. In actual production, the purification of ring-opening monomers is essential for the successful application of materials.

[0005] Crystallization is a widely used technique for refining ring-opening polymerizable monomers such as glycolide. The main purpose of crystallization is to separate the target compound from solution, melt, or gas phase by forming solid crystals. Specific purposes include purification, controlling crystal form, and controlling particle size distribution.

[0006] Crystallization is commonly used to improve the purity of compounds. By selecting appropriate solvents and controlling crystallization conditions, impurities can be effectively removed, allowing the target compound to reach the desired purity level. Crystallization is also used to control the crystal form of products. Different crystal forms (i.e., different crystal structures) significantly affect the physical and chemical properties of compounds, such as solubility and stability. By controlling crystallization conditions, specific crystal forms can be obtained, thereby optimizing product performance. Crystallization can also be used to control the particle size distribution of products. The nucleation and crystal growth mechanisms during crystallization directly affect the final crystal size and morphology. By adjusting parameters such as supersaturation, stirring rate, and temperature, the crystal particle size distribution can be controlled to meet the needs of different applications. The crystallization process is not merely a physical separation process, but also a finely controlled technical means.

[0007] Evaporation crystallization is an important crystallization method in which organic solvents are separated from a solution through evaporation, bringing the solution to a supersaturated state and promoting the precipitation of solute crystals. This method is easier to recover organic solvents than cooling crystallization. The solvent is evaporated through a physical process, turning it into a gaseous state, which is then condensed and collected, thus recovering the solvent. This process does not involve complex chemical reactions or changes, resulting in high solvent purity and a high recovery rate. Furthermore, the evaporation process is usually carried out under controlled temperature and pressure, allowing for adjustable solvent evaporation rates. By controlling the evaporation and condensation processes, evaporation crystallization can efficiently and purely recover organic solvents, giving it significant advantages in industrial applications.

[0008] Invention patent CN107868074A discloses a technical solution for purifying glycolide using a combination of evaporation crystallization and extraction. In this solution, crude glycolide is collected in a cold state, dissolved in a solvent, and filtered to remove insoluble substances. The resulting transparent filtrate is then subjected to rotary evaporation to remove most of the solvent, thus removing large molecular impurities from the glycolide. However, this invention does not disclose equipment and systems suitable for industrial-scale production and fails to recognize that large molecular impurities can continue to be generated during the purification process. There is still room for improvement and innovation in terms of process sequence and continuous operation.

[0009] Therefore, the purpose of this invention is to provide a method for continuous purification of cyclic esters, which can realize continuous crystallization of solution streams, particle size control, solid-liquid separation, solvent recovery, and obtain refined glycolide products with high chemical purity. Summary of the Invention

[0010] To address the technical problems existing in the prior art, the present invention provides a method and system for purifying cyclic ester streams.

[0011] This invention solves the technical problems of high difficulty in particle size control, insufficient chemical purity of the product, and inability to operate continuously in existing glycolide purification technologies.

[0012] The inventors discovered that oligomer impurities easily remain in cyclic ester monomers such as glycolide. When the acid value of glycolide is already below 10 mol / t, the purity of glycolide can be further improved simply by removing the residual oligomer impurities. A solution is prepared by dissolving glycolide and an organic solvent. Due to the presence of oligomer impurities, the solution is turbid. After clarification, a clear solution (turbidity less than 10 NTU) is obtained. Then, the solvent is completely evaporated, which can further purify some of the glycolide that has reached the polymerization grade (if it still contains a small amount of oligomer impurities). Partial evaporation allows the impurities to dissolve in the remaining unevaporated solvent, resulting in a better purification effect.

[0013] Vacuum belt dryers are commonly used as drying equipment in pharmaceutical, chemical, and materials industries. However, through experimental research, the inventors discovered that this equipment can also be used as a continuous evaporation crystallizer, suitable for the continuous evaporation crystallization process of glycolide and similar materials, offering advantages such as continuity and good stability. In cases of total evaporation, liquid vacuum belt dryers are ideal, simultaneously completing the evaporation and drying processes with significant advantages. In cases of partial evaporation, selecting appropriate equipment is also necessary. Using a liquid vacuum belt dryer, the residence time of the material in the belt dryer is consistent compared to conventional continuous kettle evaporation crystallizers. Vibrating screen solid-liquid separators are a specific type of solid-liquid separation equipment used for the solid-liquid separation of cyclic ester materials. Through the special design and arrangement of the vibrators, continuous solid-liquid separation can be achieved.

[0014] One of the objectives of this invention is to provide a method for purifying cyclic ester streams, comprising the step of continuously evaporating a solution stream containing cyclic esters, thereby converting all or part of the cyclic esters into a solid state.

[0015] In a preferred embodiment of the present invention,

[0016] The method includes the following steps:

[0017] (a) Introduce a solution stream containing cyclic esters into a continuous evaporation and concentration unit, so that all or part of the cyclic esters are converted into solids, to obtain a gaseous solvent and a stream A containing purified solid cyclic esters.

[0018] (b) The feed stream A containing purified solid cyclic esters obtained in step (a) is introduced into the particle size control unit to obtain feed stream B containing purified solid cyclic esters.

[0019] Optionally, (c) the feed stream B containing purified solid cyclic esters obtained in step (b) is introduced into a solid-liquid separation unit to obtain filtrate and feed stream C containing purified solid cyclic esters.

[0020] (d) Dry the feed stream B containing purified solid cyclic esters obtained in step (b) or the feed stream C containing purified solid cyclic esters obtained in step (c) to obtain dried glycolide particles.

[0021] Steps (a-d) include two cases: complete evaporation and partial evaporation. In the case of complete evaporation, step (c) is not required, and steps (a, b, d) are performed instead.

[0022] The higher the evaporation ratio, the more solid phase is precipitated. The lower limit of the evaporation ratio is essentially a limit on the solid phase recovery rate (yield), which should not be too low.

[0023] In a preferred embodiment of the present invention,

[0024] Step (a),

[0025] The cyclic ester is glycolide, a cyclic dimer of glycolic acid; and / or,

[0026] The turbidity of the solution stream is less than 10 NTU; preferably, the solution stream is obtained by dissolving crude glycolide in an organic solvent to form a solution, and then clarifying the solution until the turbidity is below 10 NTU; more preferably, the crude glycolide is obtained as a pyrolysis product of glycolic acid oligomers without purification or after purification by any method; and / or, the clarification treatment is at least one of filtration and sedimentation, filtration including cake filtration, and centrifugation can be used before filtration; sedimentation includes gravity sedimentation (also known as natural sedimentation) and centrifugal sedimentation; the key point of the present invention is to make the turbidity of the solution stream less than 10 NTU through various methods. 0 NTU, which can be achieved using all methods of the prior art; and / or, the boiling point of the organic solvent at an absolute pressure of 0.1 MPa does not exceed 120°C; more preferably, the glycolic acid oligomer is prepared by polycondensation or transesterification of glycolic acid crystals, aqueous glycolic acid solution, or methyl glycolate; and / or, the weight-average molecular weight of the glycolic acid oligomer is less than 50,000 Da; and / or, the organic solvent is at least one of saturated monohydric alcohol, ethyl acetate, acetone, and ethylene glycol dimethyl ether; the saturated monohydric alcohol is most preferably at least one of methanol, ethanol, n-propanol, isopropanol, n-butanol, tert-butanol, and isobutanol; and / or,

[0027] The continuous evaporation and concentration unit includes a liquid vacuum belt dryer; preferably, the liquid vacuum belt dryer has at least three conveyor belts; more preferably, the temperature of the three conveyor belts is 20–60°C, more preferably 30–40°C; and / or, the internal pressure of the liquid vacuum belt dryer is absolute pressure 5–20 kPa; and / or, the residence time of the glycolide solution in the three conveyor belts is independently selected from 15–90 min; and / or,

[0028] The solution stream containing the cyclic ester is concentrated in a continuous evaporation and concentration unit; preferably, the evaporation ratio is 25-100 wt%, which is the ratio of the mass of solvent evaporated into the gas phase in the feed solution to the mass of solvent in the feed solution. In the case of total evaporation, the evaporation ratio is as close to 100% as possible, and step (b) yields glycolide crystal particles; in the case of partial evaporation, a solid-liquid mixture with a solid phase mass fraction greater than or equal to 10% is obtained; and / or,

[0029] The feed stream A containing solid cyclic esters contains particulate or flake-like solids with a median diameter greater than 1000 micrometers.

[0030] Regarding the turbidity of the solution stream entering the continuous evaporation and concentration unit, the inventors have the following understanding: Typically, crude lactide obtained through pyrolysis contains high molecular weight impurities exceeding 200 ppm. Since these high molecular weight impurities are insoluble in most organic solvents, the crude lactide, after dissolving in the solvent, yields a solution with a certain degree of turbidity. Furthermore, conventional extraction, washing, and crystallization methods not only fail to sufficiently reduce the content of high molecular weight impurities but also lead to the generation of even more high molecular weight impurities during the purification process. This makes it common for refined lactide to contain high molecular weight impurities exceeding 200 ppm, severely affecting the absolute chemical purity of the refined lactide product. Therefore, utilizing a specially designed system to perform necessary clarification treatment on the lactide solution can effectively remove high molecular weight impurities, ensuring that the crystallization process is unaffected by these impurities and guaranteeing the chemical purity of the product.

[0031] Regarding the continuous generation of high molecular weight impurities during the purification process, the inventors hypothesize that the generation of high molecular weight impurities during purification originates from the unique physical properties of crude glycolide. Crude glycolide is essentially a composition comprising glycolide, an initiator, and a catalyst, thus exhibiting strong self-polymerization characteristics. The initiator is a class of hydroxyl compounds, specifically including water, glycolic acid, and linear oligomers of glycolic acid commonly found in crude glycolide products; the catalyst is a class of carboxyl compounds, specifically including glycolic acid and linear oligomers of glycolic acid that can provide protic acids.

[0032] Vacuum belt dryers are a type of high-efficiency and energy-saving drying equipment. They operate in a continuous belt environment under vacuum and are suitable for drying solid and liquid materials. When the feed end is designed to be suitable for liquid materials, the equipment is called a liquid vacuum belt dryer.

[0033] For vacuum drying processes, it is required that most of the solvent evaporates in the dryer to obtain a dry solid with the lowest possible residual solvent content. Therefore, liquid vacuum belt dryers can essentially be used for the total evaporation crystallization of cyclic ester solutions. This is suitable for cases where the acid content in the cyclic ester solution is low, for example, below 5 μmol / g. In this case, the evaporation crystallization is not aimed at further reducing the acid content. The cyclic ester solution undergoes total evaporation and drying in the dryer, directly obtaining the dried product, which is then processed by a particle size control unit to obtain the final product. A schematic diagram of the system under total evaporation conditions is shown below. Figure 2 .

[0034] Because the vacuum level, residence time (evaporation duration), and material temperature of a vacuum belt dryer are precisely controllable, it can also be used for the partial evaporation of cyclic ester solutions. The clarified cyclic ester solution is transformed into a solid-liquid mixture in this equipment before entering subsequent units. Under partial evaporation conditions, since the acid in the cyclic ester solution can still dissolve in the unevaporated solvent and does not precipitate with the product crystals, partial evaporation has a further purification effect by reducing the acid content, making it suitable for cyclic ester solutions with an acid content higher than 5 μmol / g. A system schematic diagram under partial evaporation conditions is shown below. Figure 1 .

[0035] Based on the above inferences, the inventors further realized that removing the catalyst (carboxyl compounds, i.e., acids) from the crude lactide first would inhibit the continuous formation of high molecular weight impurities in subsequent refining processes. Then, crystallization could yield a product with extremely high chemical purity. Considering the need to obtain a product with extremely high chemical purity, total evaporation (…) Figure 2 While partial evaporation is preferred, it also has significant industrial application value. After obtaining crystals, the acid content in the crystals can still be further reduced by means of spray washing, so that the purity and impurity content of the product still meet the application requirements.

[0036] In the above technical solution, step (a) is specifically carried out in a liquid vacuum belt dryer. Since the solution is relatively static on the conveyor belt, the evaporation process is close to the static crystallization process, which makes it easy to obtain cyclic ester crystals with large particle size. The resulting material stream contains granular or flaky solids with a median diameter greater than 1000 micrometers, which is not conducive to subsequent extraction and deacidification and has a longer melting time when used for polymerization. Therefore, it is necessary to introduce a continuous particle size control unit in step (b).

[0037] In a preferred embodiment of the present invention,

[0038] Step (b),

[0039] The particle size control unit is a continuous particle size control unit, and the particle size control unit is connected in series with the continuous evaporation and concentration unit; and / or

[0040] The particle size control unit controls the solid particle size by squeezing or shearing the solid particles; and / or,

[0041] The median diameter of the particles in the feed stream B containing solid cyclic esters is less than or equal to 700 micrometers.

[0042] The particle size control unit is connected in series with a liquid vacuum belt dryer. Under full evaporation conditions, large-diameter dried products are crushed into uniformly sized particles, and the material is conveyed from the dryer hopper to a crusher (hammer or roller) via a screw conveyor to achieve particle size control. Under partial evaporation conditions, solid-liquid mixtures with high solid content are crushed by an agitator or high-speed disperser and then conveyed to a solid-liquid separation unit.

[0043] In a preferred embodiment of the present invention,

[0044] Step (c),

[0045] The solid-liquid separation unit includes a solid-liquid separation device; the solid-liquid separation device is preferably a vibrating screen type solid-liquid separator; more preferably, the screen aperture size of the vibrating screen type solid-liquid separator is 25 to 250 mesh.

[0046] The vibrating screen solid-liquid separator is a highly efficient separation device. Its working principle utilizes vibrating screening technology to effectively separate solid particles from the liquid in a solid-liquid mixture, achieving the collection of solid matter and the recycling of the liquid. It features high efficiency, reliability, and strong adaptability. Through the special design and arrangement of the vibrator, continuous solid-liquid separation can be achieved. The solid-liquid mixture is fed from one side of the screen. Under the action of the vibrator, the solid phase moves to the other side, gradually removing the solvent and being collected and transported to the drying unit. The liquid phase passes through the screen and is collected on the opposite side from the solid phase.

[0047] In a preferred embodiment of the present invention,

[0048] Step (d),

[0049] Drying is performed using at least one of a vacuum spiral dryer and a fluidized bed dryer; and / or,

[0050] The median diameter of the dried glycolide particles obtained is less than 700 micrometers.

[0051] In a preferred embodiment of the present invention,

[0052] Following step (d), the method further includes the following steps:

[0053] (e) Introduce the gaseous solvent obtained from evaporation in step (a) into the condensation recovery unit;

[0054] Optionally, (f) the filtrate obtained from the solid-liquid separation in step (c) is introduced into the evaporation recovery unit to completely evaporate the solvent in the filtrate, and the evaporated gas phase is introduced into the condensation recovery unit;

[0055] Optionally, (g) the solid phase obtained by solid-liquid separation in step (c) is subjected to at least one organic solvent spray washing; preferably, the spray washing is achieved by a solvent spray head disposed above the screen of the vibrating screen type solid-liquid separator.

[0056] Step (e) applies to all and part of the evaporation. The implementation of step (e) is precisely where the advantage of evaporation crystallization lies. The vacuum phase pipe of the liquid vacuum dryer is directly connected to the solvent condensation and recovery unit. After the organic vapor is condensed into liquid and collected, it can be directly used for the preparation of more cyclic ester solutions, or optionally, it can be treated by adsorption or other means and then reused.

[0057] Steps (f-g) apply to partial evaporation. In step (c), the filtrate obtained from solid-liquid separation contains solvent, cyclic esters, and impurities. It needs to be introduced into the evaporation recovery unit to completely evaporate the solvent in the filtrate and introduce the evaporated gas phase into the condensation recovery unit. The evaporation recovery unit is a distillation column or an evaporator. The composition of the solid residue obtained after complete evaporation is similar to that of the crude glycolide obtained from the pyrolysis reaction. It can be treated as crude glycolide and purified to obtain refined glycolide. The solid filter cake obtained from solid-liquid separation may still need further reduction of acid content. The solid filter cake can be further subjected to at least one organic solvent spray washing. The spray washing is achieved by a solvent spray head set above the screen of the vibrating screen solid-liquid separator.

[0058] Regarding the number of evaporation steps in step (f), a single evaporation can fully recover the solvent, but it can also be divided into multiple evaporations. The purpose is to optimize the material state in the evaporator and maintain good fluidity. For example, a single evaporation may cause a large amount of non-fluid solids to precipitate in the evaporator, making it impossible for the evaporator to operate for a long period of time. If step (f) is optimized to recover most of the solvent (A) in the first evaporation, the material in the evaporator can still remain in the liquid phase. Another solvent (B) is added, and the remaining solvent (A) is recovered in the second evaporation. The evaporator then obtains a fluid solution with solvent B as the main component.

[0059] The second objective of this invention is to provide a method for purifying cyclic ester feed streams, comprising an evaporation and concentration unit, a particle size control unit, a solid-liquid separation unit, and a drying unit;

[0060] The evaporation and concentration unit, particle size control unit, solid-liquid separation unit, and drying unit are connected in sequence via pipelines;

[0061] Preferably,

[0062] The system also includes a condensation recovery unit and an evaporation recovery unit;

[0063] The evaporation recovery unit is connected to the solid-liquid separation unit via pipelines, and the condensation recovery unit is connected to the evaporation concentration unit and the evaporation recovery unit via pipelines.

[0064] More preferably, the system for purifying cyclic ester streams is used in the above-described method for purifying cyclic ester streams.

[0065] In a preferred embodiment of the present invention,

[0066] The continuous evaporation and concentration unit includes a liquid vacuum belt dryer; and / or,

[0067] The particle size control unit is a continuous particle size control unit, and the particle size control unit is connected in series with the continuous evaporation and concentration unit; and / or

[0068] The particle size control unit controls the solid particle size by squeezing or shearing the solid particles; and / or,

[0069] The solid-liquid separation unit includes a solid-liquid separation device; the solid-liquid separation device is preferably a vibrating screen type solid-liquid separator; more preferably, the screen aperture size of the vibrating screen type solid-liquid separator is 25-250 mesh; and / or,

[0070] The drying unit includes drying equipment, preferably at least one of a vacuum spiral dryer and a fluidized bed dryer; and / or,

[0071] The evaporation recovery unit includes at least one of a distillation column and an evaporator; and / or,

[0072] The condensation recovery unit includes a condenser and a matching collection tank; the condenser is preferably at least one of the following: tubular, plate, spiral, and annular condensers.

[0073] A third objective of this invention is to provide a refined glycolide obtained by the above-described method or system.

[0074] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0075] This invention discloses a method for further purifying polymerization-grade cyclic ester monomers by total evaporation, wherein the cyclic ester monomers contain a small amount of oligomer impurities. This invention can not only fully remove oligomer impurities, but also recover cyclic esters with a yield close to 100%, and the evaporation operation temperature is much lower than that of existing technologies such as vacuum distillation and rectification, making it more suitable for cyclic ester monomers with heat-sensitive characteristics.

[0076] The technical solution of the present invention uses a type of equipment that can simultaneously perform evaporation crystallization and crystal drying. Compared with the existing technical solutions that usually set crystallizers and dryers separately, the equipment investment is smaller. This type of equipment can also be used in series with a dryer, which is equivalent to implementing multi-stage drying, and can obtain better drying effect and higher purity products.

[0077] The technical solution of the present invention also discloses a special type of continuous evaporation crystallization equipment. Compared with conventional continuous evaporation crystallizers (partial evaporation, kettle type), the residence time of materials in the equipment is completely consistent, which is beneficial to product stability. Attached Figure Description

[0078] Figure 1 In a system for purifying a feed stream containing glycolide, the solution is partially evaporated in an evaporation and concentration unit (2);

[0079] Among them, 1-(clarified solution state) cyclic ester feed line; 2-evaporation and concentration unit; 3-cyclic ester feed line A; 4-particle size control unit; 5-cyclic ester feed line B; 6-solid-liquid separation unit; 7-cyclic ester feed line C; 8-drying unit; 9-cyclic ester feed line D; 10-cyclic ester storage unit; 11a-gas phase solvent line; 11b-gas phase solvent line; 12-condensation and recovery unit; 13-recovery solvent line; 14-evaporation and recovery unit; 15-cyclic ester feed line E.

[0080] Figure 2 In a system for purifying a feed stream containing glycolide, the solution is completely evaporated in an evaporation and concentration unit (2);

[0081] Among them, 16-(clarified solution state) cyclic ester feed line; 17-evaporation and concentration unit; 18-cyclic ester feed line A; 19-cyclic ester storage unit; 20-gas phase solvent line; 21-condensation and recovery unit; 22-recovery solvent line. Detailed Implementation

[0082] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection 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 are still within the scope of protection of the present invention.

[0083] All raw materials used in the examples are commercially available.

[0084] Test method:

[0085] The method for testing the average particle size of glycolide particles in this invention is as follows:

[0086] The average particle size (median diameter, i.e., the particle size at which the cumulative distribution of particles is 50%) of glycolide was determined using a Malvern Mastersizer 3000 laser particle size analyzer, with isopropanol as the dispersion medium.

[0087] The method for determining the free acid concentration (acid content) of crude glycolide and glycolide in this invention is as follows:

[0088] The concentration of free acid in crude glycolide was determined using an acid-base titration method. The specific procedure is as follows: The crude glycolide sample was dissolved in approximately 30 mL of dry dimethyl sulfoxide. After dissolution, a few drops of bromophenol blue indicator solution were added, resulting in a yellow solution. Titration was then performed using a dilute solution of sodium hydroxide in benzyl alcohol of known concentration. The endpoint was reached when the solution color changed from yellow to green. The terminal carboxyl group content (in μmol) in the glycolide was calculated by calculating the volume of sodium hydroxide solution used to reach the titration endpoint. This value was then divided by the mass of the crude glycolide sample to obtain the free acid concentration (acid content) (in μmol / g).

[0089] The method for determining the purity of crude glycolide in this invention is as follows:

[0090] The purity of crude lactide was determined by gas chromatography (GC). 200 mg of the lactide sample and 40 mg of p-chlorobenzophenone (PCP) as an internal standard were dissolved in 10 mL of acetone. 2 μL of the solution was injected into the gas chromatograph to determine the amount of lactide. The purity of the lactide was determined using a standard calibration curve prepared beforehand with at least 5 sites of lactide standard (160–200 mg) and the internal standard (40 mg PCP). The apparatus used was an Agilent 7890B, with an HP-5 capillary column (30 m × 0.32 mm, 0.25 μm), a column temperature of 280 °C, an injection port temperature of 150 °C, and an FID detector.

[0091] Figure 1 For purifying a flow containing glycolide, the solution is partially evaporated in an evaporation and concentration unit (2); wherein, 1-(clarified solution state) cyclic ester flow line; 2-evaporation and concentration unit; 3-cyclic ester flow line A; 4-particle size control unit; 5-cyclic ester flow line B; 6-solid-liquid separation unit; 7-cyclic ester flow line C; 8-drying unit; 9-cyclic ester flow line D; 10-cyclic ester storage unit; 11a-gas phase solvent line; 11b-gas phase solvent line; 12-condensation and recovery unit; 13-recovery solvent line; 14-evaporation and recovery unit; 15-cyclic ester flow line E.

[0092] Figure 2For purifying the glycolide-containing feed stream, the solution is completely evaporated in the evaporation and concentration unit (2); wherein, 16-(clarified solution state) cyclic ester feed stream line; 17-evaporation and concentration unit; 18-cyclic ester feed stream A line; 19-cyclic ester storage unit; 20-gas phase solvent line; 21-condensation and recovery unit; 22-recovery solvent line.

[0093] Example 1

[0094] Preparation of a clear solution of glycolide:

[0095] Using a 150L stainless steel stirred reactor as the glycolide dissolver, 10.0 kg of glycolide (commercially available, white powder) and 100.0 kg of dehydrated ethyl acetate (dehydrated to 10 ppm water content using 4A molecular sieve) were added. The mixture was stirred and heated by a jacket to prepare a turbid solution at 25°C. The turbidity of the glycolide solution was 112 NTU.

[0096] A centrifugal pump is used to transport the turbid glycolide solution from the bottom of the glycolide dissolver to the inlet of a jacketed filter (jacket water temperature 25°C; polypropylene pleated filter element, filtration accuracy 1 micron). A clear stream containing glycolide is obtained at the clear liquid outlet of the filter. The clear solution is collected in a storage tank (jacket water temperature 25°C). The average turbidity of the solution in the storage tank is 5.4 NTU.

[0097] Example 2

[0098] Preparation of a clear solution of glycolide:

[0099] The difference from Example 1 is that 110.0 kg of turbid glycolide solution was transferred to a three-legged sedimentation centrifuge (1500 rpm, centrifugation operation for 30 min) at one time. The clear liquid stream containing glycolide obtained in the centrifuge was collected into a storage tank (jacket water temperature 25°C) by a liquid suction device. The average turbidity of the solution in the storage tank was 2.2 NTU.

[0100] Example 3 (Total Evaporation)

[0101] The purification system used in the method of the present invention consists of the following components:

[0102] Step (a) involves complete evaporation, crystallization, and drying in a 12 m² liquid vacuum belt dryer (three-layer conveyor) at a feed rate of 11 kg / h. The average turbidity of the glycolide solution obtained in Example 1 is 5.4 NTU. The temperature of all three conveyor layers is set to 40°C, the internal pressure of the dryer is 5 kPa(A), and the residence time of all three conveyor layers is set to 75 min. Large particles obtained at the end of the third conveyor layer are collected in a hopper and conveyed to step (b) by a screw conveyor. Step (b) is carried out in a ring hammer mill, and the resulting fine particles are further conveyed at a rate of 1.0 kg / h to a 50 L vacuum screw dryer (operating conditions 60°C, 3 kPa(A)) for further drying to obtain refined glycolide product.

[0103] Table 1 shows the results of testing the solid particle size, acid content, and purity after sampling the material flow at each step.

[0104] Table 1

[0105]

[0106] Example 4 (Partial Evaporation)

[0107] The purification system used in the method of the present invention consists of the following components:

[0108] Step (a) Partial evaporation and crystallization were carried out in a 12 square meter liquid vacuum belt dryer (three-layer conveyor). The feed rate was 11 kg / h. The average turbidity of the glycolide solution obtained in Example 1 was 5.4 NTU. The temperature of the three conveyors was set to 30°C. The internal pressure of the dryer was 15 kPa(A). The residence times of the three conveyors were set to 55 min, 45 min, and 35 min, respectively. The evaporation ratio was 75 wt%. The solid-liquid mixture (containing large particles) obtained at the end of the third conveyor belt is collected in a hopper; step (b) is carried out in a hopper equipped with a stirring paddle (3000 rpm). Under the shearing action of the stirring paddle, the resulting solid-liquid mixture (containing fine particles) is further conveyed to step (c) by a screw pump; step (c) is carried out in a linear vibrating screen solid-liquid separator (single layer, screen area 4 square meters, screen aperture 150 mesh). The solid phase after solvent removal is conveyed to the dryer by a screw conveyor at a rate of 0.65 kg / h; step (d) is further dried in a 50L vacuum screw dryer (operating conditions 60℃, 3 kPa(A)) to obtain the refined glycolide product. Samples of the solid-liquid mixtures from steps (a) and (b) are taken, and the crystal particles are collected by vacuum filtration through a sand core funnel. The particles are then dried in a 60℃ vacuum oven for 4 hours for analysis.

[0109] Table 2 shows the results of testing the solid particle size, acid content, and purity after sampling the material flow at each step.

[0110] Table 2

[0111]

[0112] Example 5 (Partial Evaporation Case)

[0113] The difference from Example 4 is that the residence time of the three layers of tracks is adjusted to 40 min, 30 min, and 20 min respectively, and the evaporation ratio is 50 wt%.

[0114] Table 3 shows the results of testing the solid particle size, acid content, and purity after sampling the material flow at each step.

[0115] Table 3

[0116]

[0117] Example 6 (Partial Evaporation)

[0118] The difference from Example 4 is that the feed to the vacuum belt dryer was a glycolide solution with an average turbidity of 2.2 NTU obtained in Example 2. The temperature of all three conveyor belts was set to 35°C, the internal pressure of the dryer was 20 kPa(A), the residence times of the three conveyor belts were set to 50 min, 40 min, and 30 min, respectively, and the evaporation ratio was 25 wt%. Table 4 shows the results of testing the solid particle size, acid content, and purity after sampling the material flow at each step.

[0119] Table 4

[0120]

[0121] Example 7 (Partial Evaporation)

[0122] Step (e): In step (a) of Example 4, two tubular heat exchangers (operating temperature -35°C) and a matching storage tank (jacket temperature -35°C) are installed on the vacuum phase pipeline of the liquid belt dryer. Solvent is recovered at a mass recovery rate of 95% during the evaporation process, and liquid ethyl acetate (batch A) is collected.

[0123] Step (f): In step (c) of Example 4, the filtrate that has passed through the screen of the linear vibrating screen solid-liquid separator is sent to a multi-effect evaporator. Two series-connected shell-and-tube heat exchangers (operating temperature -35°C) and a matching storage tank (jacket temperature -35°C) are connected to the vapor phase pipeline of the evaporator. During the total evaporation of the filtrate, the solvent is recovered at a mass recovery rate of 98%, and the liquid phase ethyl acetate (batch ethyl) is collected.

[0124] Preparation of clear glycolide solution: The only difference from Example 1 is that 100.0 kg of recycled ethyl acetate (the ratio of batch A to batch B is 1:1) and 10 kg of commercially available glycolide with a high acid value due to improper storage and partial hydrolysis were used to prepare the turbid glycolide solution. All other conditions were the same as in Example 1, and the average turbidity of the solution was 5.6 NTU.

[0125] Step (g): In step (c) of Example 4, an ethyl acetate (30°C) spray head is installed above the first half of the screen of the vibrating screen solid-liquid separator (the screen area covered by the first half of the solid phase's movement path on the screen), with a spray rate of 0.52 kg / h. Table 5 shows the results of testing the solid phase particle size, acid content, and purity after sampling the material flow in each step.

[0126] Table 5

[0127]

[0128] As shown in Table 1, the purified glycolide obtained during total evaporation in Example 3 had a median diameter of 471 micrometers, an acid content of 1.9 μmol / g, and a purity of 99.65%.

[0129] As can be seen from Tables 2-5, during the partial evaporation in Examples 4-7, the median diameter of the refined glycolide obtained was 288-383 micrometers, the acid content was 0.25-0.5 μmol / g, and the purity was 99.79-99.91%.

[0130] Compared to the commercially available glycolide added to the feed solution in Examples 1-2 (median diameter 556 μm, acid content 3.4 μmol / g, purity 99.10%), Examples 3-6 further improved purity, reduced acid content and median diameter, allowing for further purification of refined glycolide with good purification effect. Examples 3-6 demonstrated excellent performance in controlling particle size, improving purity, and enabling continuous operation. In Example 7, the solvent was recycled, and glycolide with a high acid value was treated, reducing the acid value from 15.4 μmol / g to 0.8 μmol / g. The treated glycolide had a median diameter of 288 μm and a purity of 99.82%, demonstrating good purification effect, convenient continuous operation, and suitability for widespread application.

Claims

1. A method for purifying a cyclic ester stream, comprising the step of continuously evaporating a solution stream containing cyclic esters to convert all or part of the cyclic esters into a solid state.

2. The method for purifying cyclic ester streams as described in claim 1, characterized in that: The method includes the following steps: (a) Introduce a solution stream containing cyclic esters into a continuous evaporation and concentration unit, so that all or part of the cyclic esters are converted into solids, to obtain a gaseous solvent and a stream A containing purified solid cyclic esters. (b) The feed stream A containing purified solid cyclic esters obtained in step (a) is introduced into the particle size control unit to obtain feed stream B containing purified solid cyclic esters. Optionally, (c) the feed stream B containing solid cyclic esters obtained in step (b) is introduced into a solid-liquid separation unit to obtain filtrate and feed stream C containing purified solid cyclic esters. (d) Dry the feed stream B containing purified solid cyclic esters obtained in step (b) or the feed stream C containing purified solid cyclic esters obtained in step (c) to obtain dried glycolide particles.

3. The method for purifying cyclic ester streams as described in claim 2, characterized in that: Step (a), The cyclic ester is glycolide; and / or, The turbidity of the solution stream is less than 10 NTU; preferably, the solution stream is obtained by dissolving crude glycolide in an organic solvent to form a solution, and then clarifying the solution until the turbidity is below 10 NTU; more preferably, the crude glycolide is obtained by the pyrolysis reaction product of glycolic acid oligomers without purification or by any method; and / or, the clarification treatment is at least one of filtration and sedimentation; and / or, the boiling point of the organic solvent at absolute pressure of 0.1 MPa does not exceed 120°C; even more preferably, the glycolic acid oligomers are prepared by glycolic acid crystals, glycolic acid aqueous solution, or methyl glycolate through polycondensation or transesterification; and / or, the weight-average molecular weight of the glycolic acid oligomers is less than 50,000 Da; and / or, the organic solvent is at least one of saturated monohydric alcohol, ethyl acetate, acetone, and ethylene glycol dimethyl ether; the saturated monohydric alcohol is most preferably at least one of methanol, ethanol, n-propanol, isopropanol, n-butanol, tert-butanol, and isobutanol; and / or, The continuous evaporation and concentration unit includes a liquid vacuum belt dryer; and / or, The solution stream containing the cyclic ester is concentrated in a continuous evaporation and concentration unit; preferably, the evaporation ratio is 25-100 wt%, which is the ratio of the mass of solvent evaporated into the gas phase in the feed solution to the mass of solvent in the feed solution; and / or, The feed stream A containing solid cyclic esters contains particulate or flake-like solids with a median diameter greater than 1000 micrometers.

4. The method for purifying cyclic ester streams as described in claim 2, characterized in that: Step (b), The particle size control unit is a continuous particle size control unit, and the particle size control unit is connected in series with the continuous evaporation and concentration unit; and / or The particle size control unit controls the solid particle size by squeezing or shearing the solid particles; and / or, The median diameter of the particles in the feed stream B containing solid cyclic esters is less than or equal to 700 micrometers.

5. The method for purifying cyclic ester streams as described in claim 2, characterized in that: Step (c), The solid-liquid separation unit includes a solid-liquid separation device; the solid-liquid separation device is preferably a vibrating screen type solid-liquid separator; more preferably, the screen aperture size of the vibrating screen type solid-liquid separator is 25 to 250 mesh.

6. The method for purifying cyclic ester streams as described in claim 2, characterized in that: Step (d), Drying is performed using at least one of a vacuum spiral dryer and a fluidized bed dryer; and / or, The median diameter of the dried glycolide particles obtained is less than 700 micrometers.

7. The method for purifying cyclic ester streams as described in claim 2, characterized in that: The method further includes the following steps: (e) Introduce the gaseous solvent obtained from evaporation in step (a) into the condensation recovery unit; Optionally, (f) the filtrate obtained from the solid-liquid separation in step (c) is introduced into the evaporation recovery unit to completely evaporate the solvent in the filtrate, and the evaporated gas phase is introduced into the condensation recovery unit; Optionally, (g) the solid phase obtained by solid-liquid separation in step (c) is subjected to at least one organic solvent spray washing; preferably, the spray washing is achieved by a solvent spray head disposed above the screen of the vibrating screen type solid-liquid separator.

8. A system for purifying cyclic ester streams, comprising an evaporation and concentration unit, a particle size control unit, a solid-liquid separation unit, and a drying unit; The evaporation and concentration unit, particle size control unit, solid-liquid separation unit, and drying unit are connected in sequence via pipelines; Preferably, The system also includes a condensation recovery unit and an evaporation recovery unit; The evaporation recovery unit is connected to the solid-liquid separation unit via pipelines, and the condensation recovery unit is connected to the evaporation concentration unit and the evaporation recovery unit via pipelines. More preferably, the system for purifying cyclic ester streams is used in the method for purifying cyclic ester streams according to any one of claims 1 to 6.

9. The system for purifying cyclic ester streams as described in claim 8, characterized in that: The continuous evaporation and concentration unit includes a liquid vacuum belt dryer; and / or, The particle size control unit is a continuous particle size control unit, and the particle size control unit is connected in series with the continuous evaporation and concentration unit; and / or The particle size control unit controls the solid particle size by squeezing or shearing the solid particles; and / or, The solid-liquid separation unit includes a solid-liquid separation device; the solid-liquid separation device is preferably a vibrating screen type solid-liquid separator; more preferably, the screen aperture size of the vibrating screen type solid-liquid separator is 25-250 mesh; and / or, The drying unit includes drying equipment, preferably at least one of a vacuum spiral dryer and a fluidized bed dryer; and / or, The evaporation recovery unit includes at least one of a distillation column and an evaporator; and / or, The condensation recovery unit includes a condenser and a matching collection tank; the condenser is preferably at least one of the following: tubular, plate, spiral, and annular condensers.

10. A method for purifying a cyclic ester stream as described in any one of claims 1 to 7, or refined glycolide obtained by purification using the system for purifying a cyclic ester stream as described in claim 8 or 9.

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