Preparation method of glycolide

Abstract

The present invention relates to a method for continuously producing glycolide, comprising: a prepolymerization step in which glycolic acid or methyl glycolate is prepolymerized into respective oligomers; a depolymerization step in which these oligomers are depolymerized into crude glycolide using a reactive distillation system, in which a small amount of a concentrated glycollic acid solution, a concentrated methyl glycolate solution, or a combination thereof is introduced into the depolymerization reactor as part of the reaction feed; and a purification step, wherein the obtained crude glycolide is processed into ultra-pure glycolide through a melt crystallization system.

Description

Technical Field

[0001] This invention relates to a method for the continuous production of high-purity glycolide from glycolic acid oligomers or methyl glycolate oligomers. The process involves depolymerizing these oligomers to generate crude glycolide, followed by refining the crude product through melt crystallization to obtain ultrapure glycolide. Background Technology

[0002] Polyglycolic acid (PGA) is a biodegradable and biocompatible polymer known for its excellent mechanical properties, gas barrier properties, and hydrolysis. Originally developed for medical applications such as sutures and tissue engineering, PGA's uses have expanded to packaging, industrial products, and environmental applications due to its environmentally friendly characteristics. Its natural biodegradability makes it an attractive option for reducing environmental impact.

[0003] Glycolide is a cyclic dimer of glycolic acid and an intermediate in the production of high molecular weight PGA. Its production process involves a technique called ring-opening polymerization. In this process, glycolide molecules are subjected to heat and catalysis, causing the cyclic dimer ring structure to break. This action initiates a chain reaction, where glycolide monomers link together to form long polymer chains that constitute high molecular weight PGA.

[0004] One established method for synthesizing glycolide includes a prepolymerization step and a subsequent depolymerization step. In this process, glycolic acid reacts, producing water as a byproduct, forming glycolic acid oligomers. These oligomers are then depolymerized to produce crude glycolide. Alternatively, alkyl glycolates such as methyl glycolate can be used as starting materials. In this method, prepolymerization is carried out by removing the alcohol (de-alcoholization), resulting in the formation of alkyl glycolate oligomers, producing alcohol as a byproduct. These oligomers are then depolymerized in a similar manner to produce crude glycolide.

[0005] Several processes for preparing glycolide from generated glycolic acid oligomers have been proposed. For example, U.S. Patent No. 9,512,100 describes a method in which, during depolymerization, the glycolic acid oligomer is heated together with a large amount of tetraethylene glycol dibutyl ether as a polar organic solvent and octyltriethylene glycol as a solubilizer. In this method, the resulting crude glycolide is co-distilled with the tetraethylene glycol dibutyl ether. The resulting co-distillate is then subjected to phase separation to separate a glycolide-enriched phase and a polar organic solvent-enriched phase. Crude glycolide can be obtained by separating and recovering glycolide from the liquid glycolide-enriched phase.

[0006] While the method described in U.S. Patent No. 9,512,100 allows the use of polar organic solvents and solubilizers to obtain crude glycolide from glycolic acid oligomers, it has several drawbacks. First, the process requires co-distillation of a large amount of polar organic solvent with the crude glycolide, resulting in significant energy consumption due to the need for boiling the solvent. Second, separating the co-distillate phase into a glycolide-enriched phase and a polar organic solvent-enriched phase, and then recovering the crude glycolide from the glycolide-enriched phase, requires additional steps, increasing the complexity of the process. Furthermore, some of the organic solvents or solubilizers cited in U.S. Patent No. 9,512,100 are known to be toxic, and some are easily decomposed, making them unsuitable for reuse. Summary of the Invention

[0007] Therefore, one object of the present invention is to develop a method for producing crude glycolide by depolymerizing glycolic acid oligomers or alkyl glycolate oligomers. This method involves introducing a small amount of liquid material as part of the reaction feed into the depolymerization reactor during the depolymerization process. This liquid material is readily soluble in the oligomers, forming a homogeneous solution with reduced viscosity, and acts as a stripping agent to promote the efficient formation of crude glycolide from the oligomers.

[0008] Another object of the present invention is to provide a method for producing high-purity glycolide from crude glycolide by means of suspension-based melt crystallization, while returning the mother liquor from the crystallization process to the prepolymerization system. Attached Figure Description

[0009] Figure 1 This is a schematic diagram of a preferred glycolide production system according to the present invention, using glycolic acid as a starting material.

[0010] Figure 2 This is a schematic diagram of a preferred glycolide production system according to the present invention, using methyl glycolate as a starting material.

[0011] Figure 3 This is another schematic diagram of a preferred glycolide production system according to the present invention, using glycolic acid as a starting material. Detailed Implementation

[0012] Two common methods for synthesizing oligomers include prepolymerizing glycolic acid or using alkyl glycolates as starting materials for prepolymerization via dealcoholization. In the first method, glycolic acid undergoes a condensation reaction to generate glycolic acid oligomers, with water as a byproduct which is subsequently removed. In the second method, dealcoholization converts alkyl glycolates into alkyl glycolate oligomers by removing alcohol groups. For example, methyl glycolate is dealcoholized to form methyl glycolate oligomers and methanol, while ethyl glycolate is dealcoholized to produce ethyl glycolate oligomers and ethanol.

[0013] Glycolic acid and alkyl glycolates, especially methyl glycolate, are commercially available and commonly used in the production of glycolide. Glycolic acid can also be synthesized from methyl glycolate through a hydrolysis process, in which the ester bond in methyl glycolate is broken down in water (usually with the aid of a catalyst) to form glycolic acid and methanol.

[0014] In glycolide production, the choice between glycolic acid and methyl glycolate requires a trade-off between factors such as ease of operation, reactivity, and supply considerations. Glycolic acid is more reactive in the prepolymerization step and is an effective starting material for producing glycolic acid oligomers. However, its high melting point of 80°C requires careful handling to prevent freezing in equipment such as vacuum systems, pipes, and fittings. Conversely, methyl glycolate has a significantly lower melting point, making it easier to handle and process, but its reactivity is lower than that of glycolic acid. Therefore, the choice between glycolic acid and methyl glycolate in glycolide production depends on a trade-off between reactivity and ease of operation.

[0015] Glycolic acid has a simpler molecular structure and lacks a chiral center, thus exhibiting no optical isomers. This simplicity endows glycolic acid and its derivative, methyl glycolate, with inherent stability, preventing racemization even at prolonged high temperatures. This stability allows for the production of high molecular weight oligomers without the risk of racemization. In contrast, lactic acid, due to its chiral center, has two optical isomers and is prone to racemization at high temperatures. This racemization can lead to the formation of unwanted meso-lactide during the depolymerization of lactic acid oligomers, even if pure enantiomeric lactic acid is used as the starting material.

[0016] During the depolymerization of glycolic acid or methyl glycolate oligomers, an intramolecular reaction occurs, where the ends of the polymer chains "bite back" to form cyclic dimers, thereby producing glycolide. The efficiency of this biting-back process is significantly affected by the oligomer chain length (corresponding to the number of carbon atoms).

[0017] Longer oligomer chains can enhance the intramolecular reactions required for backbiting, thus promoting the formation of more glycolide. However, when the chains become too long, the viscosity of the oligomer increases. This presents a challenge because glycolide has a much higher boiling point than lactide, making it more difficult to distill from viscous oligomers.

[0018] Furthermore, glycolide is far more reactive than lactide, and if not removed promptly, it will rapidly repolymerize due to the reversible nature of oligomer depolymerization. This repolymerization leads to the formation of more viscous oligomers, thus prolonging their residence time in the depolymerization reactor. Prolonged exposure to high temperatures increases the risk of degradation and coking, ultimately reducing the overall yield of glycolide production. As described in U.S. Patent No. 9,512,100, one approach to addressing these challenges is to use large amounts of polar organic solvents and solubilizers during the depolymerization process. While effective, this method requires significant energy to evaporate the co-distilled solvent, increasing process complexity and cost.

[0019] On the other hand, if the chains are too short, their biting efficiency decreases, leading to reduced glycolide formation. As a result, these short-chain oligomers accumulate in the depolymerization reactor, causing the liquid level in the prepolymerization reactor to rise and the residence time to increase due to their low conversion efficiency to glycolide. This, in turn, increases the risk of degradation and coking.

[0020] The inventors discovered that oligomers of glycolic acid or methyl glycolate with a number average molecular weight in the range of 800 to 2,000 exhibit satisfactory re-biting efficiency. These oligomers are mainly composed of chains with a degree of polymerization between 10 and 40, making them ideal feedstocks for subsequent depolymerization reactions. However, these oligomers still have relatively high viscosity, making it difficult to effectively remove the generated glycolide. To address this issue, a small amount of liquid substance can be introduced into the depolymerization reactor as part of the reaction feed. This substance mixes well with the oligomers, helps reduce their viscosity, and acts as a stripping agent, thereby improving the formation and extraction of glycolide.

[0021] The production of glycolide typically begins with an aqueous solution consisting of 70 wt% glycolic acid and 30 wt% water. The first step in the production process is prepolymerization, which involves a first reactive distillation process. In this step, the glycolic acid concentration increases, leading to a condensation reaction that produces glycolic acid oligomers and water as a byproduct. This prepolymerization can be carried out in a single stage or in multiple stages (e.g., two stages in series), especially when processing large quantities of feedstock.

[0022] After forming glycolic acid oligomers, they undergo a second reactive distillation process, during which the oligomers are depolymerized to produce crude glycolide. The inventors discovered that introducing a small amount of concentrated glycolic acid (preferably glycolic acid with a concentration exceeding 95 wt%) into the depolymerization reactor can significantly increase glycolide yield.

[0023] Concentrated glycolic acid is preferred over an initial glycolic acid solution containing 30 wt% water. Water in the initial solution increases water vapor formation when introduced into the hot depolymerization feed. Excessive water vapor in the depolymerization reactor reduces the heat transfer coefficient on the process fluid side, leading to a decrease in the overall heat transfer efficiency of the reactor. Furthermore, water vapor is difficult to condense and may introduce other substances, such as glycolic acid, into the vacuum system, further complicating the process.

[0024] Adding concentrated glycolic acid solution offers several key advantages. First, it dissolves effectively in the oligomers, forming a homogeneous solution and significantly reducing the viscosity of the mixture. This reduced viscosity makes it easier for the generated glycolide to escape from the oligomers. Second, due to the residence time in the depolymerization reactor being less than three minutes, only a portion of the added glycolic acid has the opportunity to form glycolic acid dimers or react with the oligomers, both of which produce water as a byproduct. These dimers, along with water and unreacted glycolic acid, have boiling points much lower than glycolide, allowing them to act as stripping agents to assist in the removal of glycolide from the oligomers. Finally, because the added glycolic acid constitutes only a small fraction of the depolymerization feed, subsequent glycolide purification processes are simplified and require significantly less energy.

[0025] In the first reactive distillation, glycolic acid is concentrated and condensed to produce glycolic acid oligomers. The number average molecular weight of the glycolic acid oligomers obtained by the glycolic acid condensation reaction is typically in the range of 400 to 5,000, preferably in the range of 600 to 3,000, and more preferably in the range of 800 to 2,000.

[0026] According to the invention, the first reactive distillation system preferably includes at least one vessel, a distillation column, a condenser, and an evaporator. In this system, the distillation column (with the condenser horizontally mounted at the top) and the evaporator are preferably mounted on top of the vessel, respectively. This arrangement creates a single, enclosed space where the glycolic acid concentration, condensation, and related distillation processes take place.

[0027] The reboiler can be horizontal or vertical, depending on the process conditions. The distillation column can be a conventional column or a partitioned column with partition walls that separate the internal space. This invention does not impose any particular limitation on the type of mass transfer element installed in the distillation column. Good results can be obtained using suitable mass transfer elements selected from trays, random packing, structured packing, and any combination thereof. However, structured packing is particularly suitable as a mass transfer element, offering advantages such as reduced pressure drop and liquid holdup within the column. Preferably, the specific surface area of ​​the structured packing is between 50 and 750 m². 2 / m 3 Within the range, more preferably within 125 to 500m 2 / m 3Within the specified range. The evaporator can be any type commonly used in the chemical industry, including but not limited to falling film evaporators, forced circulation evaporators, and thermosiphon evaporators. However, due to its particularly low liquid holdup, a falling film evaporator is preferred to reduce the residence time of glycolic acid and oligomers, thereby reducing any undesirable side reactions such as degradation and coking. The condenser can be any type commonly used in the chemical industry, including co-flow and counter-flow condensers. However, the improved horizontal shell-and-tube heat exchanger described in WO2022 / 177500, which is installed at the top of the distillation column, is preferred. Due to its specific geometry, this condenser produces only a very small pressure drop. This small pressure drop translates into lower operating pressures in the evaporator and kettle, which in turn leads to lower condensation reaction temperatures in the evaporator, thereby minimizing any undesirable side reactions such as degradation and coking.

[0028] The initial glycolic acid solution is continuously fed into the distillation column through an inlet located between the top and bottom of the column. The reaction feed enters the top of the falling film evaporator and flows downward through long vertical tubes that serve as the heat transfer and reaction zone. These tubes are surrounded by a heat transfer medium (such as heat transfer oil or steam) circulating in the shell side. As the reaction solution flows through the tubes, it exits at the bottom as a vapor-liquid two-phase mixture. This two-phase flow flows directly into a connected vessel, where the vapor and liquid separate. The separated vapor rises to the bottom of the top-mounted distillation column, while the liquid is collected in the vessel. To prevent the liquid film inside the falling film evaporator tubes from breaking, the evaporation rate of the reaction solution is typically kept below 30 wt%. Most of the liquid collected at the bottom of the vessel is circulated back to the top of the falling film evaporator via a transfer pump for continuous glycolic acid condensation. A small portion of the liquid is fed into a subsequent depolymerization reactor. The reaction solution is typically heated at 120 to 220 °C, preferably at 160 to 200 °C.

[0029] In the rectification column of the first reactive distillation system, a concentration gradient is established. Water is concentrated in the rectification section, while higher-boiling components, such as glycolic acid and its oligomers, are concentrated in the stripping section. Water present in the glycolic acid aqueous stream, as well as water generated during the glycolic acid condensation process, is distilled off as the overhead vapor stream. This vapor is condensed by a modified horizontal condenser mounted at the top, producing a condensate stream primarily composed of water. A portion of this condensate is preferably refluxed back into the column, while the remainder can be discarded. Any uncondensed vapor is removed via a vacuum system. The high-boiling fraction, primarily composed of glycolic acid and its oligomers, is liquefied within the column and flows back into the reactor.

[0030] According to the invention, the depolymerization of glycolic acid oligomers takes place within a second reactive distillation system, which preferably comprises at least one reboiler, a distillation column, a condenser, and a falling film evaporator. The distillation column and the falling film evaporator are mounted directly on top of the reboiler, forming a single closed space where depolymerization and distillation occur simultaneously. Mass transfer elements installed in the distillation column include trays, random packing, structured packing, and any combination thereof. Structured packing is particularly advantageous because it reduces pressure drop and minimizes liquid holdup within the column. Preferably, the specific surface area of ​​the structured packing is between 50 and 750 m². 2 / m 3 Within the range, a more preferred range is 125 to 500 m 2 / m 3 A falling film evaporator was chosen for this process because it significantly reduces liquid holdup, thereby decreasing the residence time of glycolic acid oligomers. This reduction in residence time is crucial for minimizing undesirable side reactions such as degradation and coking. The system also incorporates a modified horizontal shell-and-tube heat exchanger, as detailed in WO2022 / 177500, which serves as a condenser and is mounted at the top of the distillation column. The unique geometry of this condenser results in a very small pressure drop, which in turn reduces the operating pressure of the evaporator and kettle. This pressure reduction leads to a lower depolymerization temperature within the evaporator, further minimizing the risk of undesirable side reactions such as degradation and coking.

[0031] A small portion of the initial glycolic acid solution (comprising 70 wt% glycolic acid and 30 wt% water) is first introduced into a separate distillation column, where the glycolic acid concentration is preferably increased to over 95 wt%. The concentrated glycolic acid stream is then introduced and mixed with the glycolic acid oligomer from the first reactive distillation system. The concentration of glycolic acid solution in the glycolic acid oligomer stream from the first reactive distillation system is preferably between 1% and 10 wt%, more preferably between 3% and 6 wt%.

[0032] In the second reactive distillation system, a catalyst such as stannous octoate is mixed with glycolic acid oligomers from the first reactive distillation system and a small amount of concentrated glycolic acid solution introduced. This mixture is then fed into the circulating stream of a falling film evaporator as the reaction feed. In this setup, the falling film evaporator serves both as a depolymerization reactor (converting glycolic acid oligomers into crude glycolide) and as an energy source for the evaporated glycolide and other low-boiling volatiles (such as glycolic acid dimers, unreacted glycolic acid, and water).

[0033] The reaction feed enters the top of the falling film evaporator and flows downward through long vertical tubes heated by a shell-side circulating medium (such as heat transfer oil or steam). As the reaction proceeds, the solution exits at the bottom of the tubes as a vapor-liquid two-phase mixture.

[0034] The vapor-liquid two-phase flow exits from the bottom of the falling film evaporator tubes and flows directly into the connected vessel, where the vapor and liquid separate. The separated vapor then rises to the bottom of a distillation column mounted above the vessel, while the separated liquid is collected in the vessel. To prevent the liquid film inside the falling film evaporator tubes from rupturing, the evaporation rate of the reaction solution is typically kept below 30 wt%.

[0035] Most of the liquid collected at the bottom of the reactor is circulated back to the top of the falling film evaporator via a transfer pump for continuous depolymerization. A small portion of this liquid is removed as a purge stream (discharge stream), which contains residues of the tin catalyst and any metals leached from the system.

[0036] The operating pressure of the horizontal condenser is set in the range of 3 to 30 mbar, more preferably in the range of 5 to 10 mbar. The temperature at which the reaction solution is heated in the falling film evaporator is preferably in the range of 200 to 280 °C, more preferably in the range of 220 to 240 °C. A low-boiling-point distillate stream is generated at the top of the column, called crude glycolide. This stream mainly consists of 75 to 97 wt% glycolide, and small amounts of glycolic acid dimer (0 to 10 wt%), unreacted glycolic acid (0 to 8 wt%), water (0 to 6 wt%), and glycolic acid oligomers (0 to 1 wt%). The high-boiling-point fraction, mainly composed of unconverted glycolic acid oligomers, is returned to the reactor for further processing.

[0037] With the increasing demand in the medical industry for ultrapure glycolide with a purity of 99.9% or higher, further purification of crude glycolide to achieve the required purity is crucial. However, due to its high reactivity, purifying crude glycolide in a distillation system carries the risk of repolymerization into oligomers at the bottom of the distillation column (where temperatures are high). In contrast, melt crystallization has become an optimal solution. This method operates at lower temperatures and provides a highly selective separation process that is both energy-efficient and environmentally friendly, requiring no additional solvents. Based on the technique used to obtain the crystalline product, melt crystallization falls into two categories: suspension-based melt crystallization and layer crystallization. While both techniques are suitable for purification, suspension-based melt crystallization is generally used to achieve the highest purity levels, although layer crystallization remains a viable option.

[0038] To achieve high-purity glycolide, the crystallization system typically includes at least one stage of suspension-based melt crystallization to purify the crude glycolide obtained from the depolymerization step. This suspension-based melt crystallization system employs a single-wall scraped rotary drum crystallizer and at least one washing column, which together ensure effective separation of glycolide crystals from the impurity-rich mother liquor, thereby producing an ultrapure liquid glycolide product.

[0039] Crude glycolide enters a scraped-wall rotary drum crystallizer, where it is cooled by a heat transfer fluid circulating through the outer jacket. This cooling process forms a supersaturated film of glycolide on the inner wall of the crystallizer. To prevent scaling and maintain efficient continuous operation, the inner wall is periodically scraped to ensure the cooled surfaces remain clean. The supercooled liquid is then rapidly mixed with the volume of the crystallizer, dispersing the supersaturation throughout the slurry and establishing a stable environment for controlled crystal growth. The resulting slurry is then directed to a washing column to completely separate the ultrapure glycolide crystals from the impurity-rich mother liquor.

[0040] In the washing column, most of the impurity-rich mother liquor is removed from the glycolide crystals by filtration or other means. A large portion of this mother liquor is recycled to combine with the initial glycolic acid solution, while a small portion is removed as a purge stream (discharge stream) to prevent impurities from accumulating in the system. The now essentially mother liquor-free crystals form a compacted crystal bed. Although the crystals themselves are ultrapure glycolide, a thin layer of residual mother liquor may remain on their surface. To address this, a portion of the ultrapure glycolide crystals is cut from the crystal bed and melted using steam or another suitable heat source. A small amount of molten glycolide is then pressed through the crystal bed to wash away the adhering mother liquor, while the majority of the molten glycolide is discharged as the final product through an automatically controlled valve, achieving a purity of 99.9% or higher.

[0041] To improve the yield of glycolide, a secondary stage of suspension-based melt crystallization was implemented downstream of the primary stage. In this secondary stage, glycolide was recovered from the impurity-rich mother liquor separated from the primary stage. The recovered glycolide was then combined with crude glycolide from the depolymerization step and reintroduced into the scraped-wall drum crystallizer of the primary stage. Simultaneously, the impurity-rich mother liquor from the secondary stage was split into a purge stream (discharge stream) and a recirculation stream, the latter of which was combined with the initial glycolic acid solution. The combined yield of glycolide from the two stages of suspension-based melt crystallization exceeded 80%.

[0042] The uncondensed vapor from the horizontal condenser installed at the top of the second reactive distillation column consists primarily of water, along with small amounts of leaked air, unreacted glycolic acid, and glycolic acid dimers. This vapor is introduced into a scrubbing column, where a stream of cold, concentrated methyl glycolate solution is introduced at the top. The purpose of this stream is to absorb the unreacted glycolic acid and dimers. Methyl glycolate has a very low freezing point, allowing for sufficient cooling to effectively condense and absorb these components, thus preventing blockage of the vacuum system and related piping. Vapor not recovered at the top of the scrubbing column (consisting mainly of methyl glycolate, leaked air, and water) is then introduced into the vacuum system. The liquid mixture collected at the bottom of the scrubbing column is recycled by combining it with the initial glycolic acid solution, which is then fed into the prepolymerization reaction in the first reactive distillation system. This recycling is feasible because methyl glycolate can also be used as a starting material for glycolide production. Furthermore, the uncondensed vapor from the first reactive distillation column (containing a significant amount of methanol) is introduced into the vacuum system for recovery.

[0043] Similarly, the production of ultrapure glycolide from a concentrated methyl glycolate solution involves two reactive distillation systems followed by at least one stage of suspension-based melt crystallization. In the first reactive distillation system, methyl glycolate is prepolymerized into methyl glycolate oligomers. These oligomers are then fed into a second reactive distillation system, where they are depolymerized to produce crude glycolide.

[0044] It has been found that adding a small amount of concentrated methyl glycolate solution (preferably at a concentration exceeding 95 wt%) to the methyl glycolate oligomer from the first reactive distillation stream significantly increases the yield of glycolide in the depolymerization step. In the second reactive distillation system, a catalyst such as stannous octoate is mixed with the methyl glycolate oligomer from the first reactive distillation system and the introduced small amount of concentrated methyl glycolate solution. This mixture is then introduced into the circulating stream of a falling film evaporator as a reaction feed. In this setup, the falling film evaporator serves both as a depolymerization reactor (converting the methyl glycolate oligomer to crude glycolide) and as an energy source for the evaporated glycolide and other low-boiling volatiles (such as methyl glycolate dimers, unreacted methyl glycolate, and methanol).

[0045] The concentration of methyl glycolate solution in the oligomerized methyl glycolate stream from the first reactive distillation system is preferably between 1% and 10 wt%, more preferably between 3% and 6 wt%. The operating pressure of the horizontal condenser in the second reactive distillation system is set in the range of 3 to 30 mbar, more preferably between 8 and 15 mbar.

[0046] Adding methyl glycolate offers several key benefits. First, methyl glycolate dissolves efficiently in the oligomers, forming a homogeneous solution and significantly reducing the viscosity of the mixture. This reduced viscosity facilitates the escape of crude glycolide from the oligomers. Second, due to the residence time in the depolymerization reactor being less than three minutes, and given that methyl glycolate is less reactive than glycolic acid, only a small fraction of the added methyl glycolate forms dimers or reacts with the oligomers, producing methanol as a byproduct. These dimers, along with methanol and unreacted methyl glycolate, have boiling points far lower than glycolide, allowing them to act as stripping agents to aid in the removal of glycolide from the oligomers. Finally, because the added methyl glycolate constitutes only a small portion of the depolymerization feed, subsequent glycolide purification processes become much simpler and require significantly less energy.

[0047] Because methyl glycolate has a much lower freezing point than glycolic acid and lower reactivity, the process using methyl glycolate as a starting material differs from that using glycolic acid in several ways. First, methyl glycolate polymerizes into oligomers in the presence of a catalyst, which may include tin salts, zinc salts, titanium salts, or combinations thereof. Second, unlike the process using glycolic acid as a starting material, a washing tower is not required when using methyl glycolate because of its very low freezing point. This low freezing point prevents the risk of blockage in the vacuum system and connecting pipes. Therefore, uncondensed vapors from the condenser installed at the top horizontally of the second reactive distillation column can be further condensed in the cold trap. The condensate from the bottom of the cold trap, containing a significant amount of methyl glycolate, is then recycled back to the prepolymerization step. The remaining uncondensed vapors (mainly composed of methanol, methyl glycolate, leaked air, and water) are introduced into the vacuum system.

[0048] It has been observed that adding a small amount of concentrated glycolic acid or methyl glycolate solution to the oligomers from the first reactive distillation serves a similar dual purpose: both reducing the viscosity of the mixture and acting as a stripping agent in the depolymerization step. Therefore, when glycolic acid solution is the starting material, concentrated methyl glycolate solution can be used alone, or in combination with concentrated methyl glycolate solution, as a substitute for concentrated glycolic acid solution in the depolymerization step feed.

[0049] Conversely, when methyl glycolate solution is the starting material, concentrated glycolic acid solution can be used alone, or a mixture of concentrated methyl glycolate and glycolic acid solutions, as a substitute for the concentrated methyl glycolate solution in the depolymerization step feed. However, when using glycolic acid solution (alone or in combination with other substances), a washing tower is typically required to replace the cold trap. This adjustment is necessary because glycolic acid has a significantly higher freezing point than methyl glycolate, and uncondensed glycolic acid vapor from the condenser of the second reactive distillation column needs to be absorbed to prevent freezing in the vacuum system.

[0050] Figure 1This invention describes a preferred system for producing glycolide from a glycolic acid solution as a starting material. The system includes a first reactive distillation column 6, a condenser 7, a kettle 12, a falling film evaporator 13, a pump 15, a second reactive distillation column 27, a condenser 28, a kettle 20, a falling film evaporator 26, a pump 22, a washing column 32, a concentrating distillation column 41, and a suspension melt crystallization system 46.

[0051] An aqueous glycolic acid solution is continuously introduced into a static mixer 2 via stream 1, where it is mixed with stream 35 from the bottom of a washing column 32 and stream 49 from a suspension melt crystallization system 46. The resulting mixture is heated to a temperature above the melting point of glycolic acid in a preheater 4 via stream 3, and then enters a first reactive distillation column 6. The rising vapor, primarily composed of water, in the distillation column 6 is condensed in a condenser 7. The condensate is separated into a top liquid product stream 9 and a reflux stream 8, with the reflux stream 8 returning to the top of column 6. Uncondensed vapor is removed via stream 10 through a vacuum system 11. Glycolic acid and glycolic acid oligomers accumulate at the bottom of column 6 and flow back to a reactor 12. The bottom stream 14 from reactor 12 is pumped by a pump 15 and separated into two streams: a bottom product stream 17 and a recycle stream 16, with the recycle stream 16 being sent to the top of a falling film evaporator 13. In evaporator 13, the reaction solution is partially vaporized and then returned to vessel 12. The vapor generated in vessel 12 is separated from the liquid. The vapor rises through tower 6, while the liquid remains in vessel 12.

[0052] A small portion of stream 1, as diverter stream 40, is directed to a concentrating distillation column 41, where it is distilled into a concentrated glycolic acid solution, collected via stream 42, while water is removed via overhead stream 43. Any uncondensed vapor is removed via vacuum system 45 through stream 44. The concentrated glycolic acid in stream 42 is then mixed in static mixer 18 with the bottom product from stream 17 and the depolymerization catalyst from stream 39 to form stream 19. Bottom stream 21 from reactor 20 is pumped by pump 22 and divided into an exhaust stream 23 and a recycle stream 24. This recycle stream is combined with stream 19 to form the reaction feed, which is then sent to the top of falling film evaporator 26. After partial vaporization, the reaction solution enters reactor 20, where vapor and liquid are separated; the vapor rises through column 27, while the liquid remains in reactor 20. The overhead vapor, containing a high concentration of glycolide and some glycolic acid dimers, unreacted glycolic acid, and water, is condensed in condenser 28. The condensate is separated into a top liquid product stream 30 and a reflux stream 29, with the reflux stream 29 returning to the top of column 27. Uncondensed vapors are removed via stream 31. Unconverted glycolic acid oligomers are concentrated at the bottom of column 27 and flow back to reactor 20.

[0053] A concentrated methyl glycolate solution is continuously introduced into cooler 37 via stream 36, and then sent to the top of scrubbing tower 32 to help remove glycolic acid and glycolic acid dimer from stream 31 entering the bottom of the scrubbing tower. The resulting liquid stream 35 is recycled back to static mixer 2 to mix with stream 1. Uncondensed vapor from the top of scrubbing tower 32 is removed via stream 33 through vacuum system 34.

[0054] The overhead product stream 30 from the top of distillation column 27 is sent to the suspension melt crystallization system 46. The mother liquor stream 47, rich in impurities, is divided into a discharge stream 48 and a recirculation stream 49, with the recirculation stream 49 being recycled back to the static mixer 2. The ultrapure glycolide product is collected via stream 50 for further processing.

[0055] Figure 2 This invention describes a preferred system for producing glycolide from a methyl glycolate solution as a starting material. The system includes a first reactive distillation column 56, a condenser 57, a kettle 62, a falling film evaporator 63, a pump 65, a second reactive distillation column 77, a condenser 78, a kettle 70, a falling film evaporator 76, a pump 72, a cold trap 82, and a suspension melt crystallization system 91.

[0056] A concentrated methyl glycolate solution is continuously introduced into a static mixer 52 via stream 51, where it is mixed with catalyst stream 89, stream 85 from the bottom of cold trap 82, and stream 94 from suspension melt crystallization system 91. The resulting mixture is preheated in preheater 54 via stream 53 and then enters the first reactive distillation column 56. The rising vapor, primarily composed of methanol, in distillation column 56 is condensed in condenser 57. The condensate is separated into a top liquid product stream 59 and a reflux stream 58, with the reflux stream 58 returning to the top of column 56. Uncondensed vapor is removed via stream 60 through vacuum system 61. Methyl glycolate and methyl glycolate oligomers accumulate at the bottom of column 56 and flow back to reactor 62. The bottom stream 64 from reactor 62 is pumped by pump 65 and separated into two streams: a bottom product stream 67 and a recycle stream 66, with the recycle stream 66 being sent to the top of falling film evaporator 63. In evaporator 63, the reaction solution is partially vaporized and then returned to reactor 62. The vapor generated in reactor 62 is separated from the liquid. The vapor rises through tower 56, while the liquid remains in reactor 62.

[0057] A small portion of stream 51 is diverted as stream 86 and preheated in preheater 87. The heated concentrated methyl glycolate is mixed in stream 88 with the bottom product from stream 67 and the depolymerization catalyst from stream 90 in static mixer 68 to form stream 69. The bottom stream 71 from reactor 70 is pumped by pump 72 and divided into an exhaust stream 73 and a recycle stream 74. This recycle stream is combined with stream 69 to form the reaction feed, which is then sent to the top of falling film evaporator 76. After partial vaporization, the reaction solution enters reactor 70, where the vapor generated is separated from the liquid. The vapor rises through column 77, while the liquid remains in reactor 70. The overhead vapor contains a high concentration of glycolide, as well as some methyl glycolate dimers, unreacted methyl glycolate, and methanol, which is condensed in condenser 78. The condensate is divided into overhead liquid product stream 80 and reflux stream 79, with reflux stream 79 returning to the top of column 77. Unconverted methyl glycolate oligomers are concentrated at the bottom of column 77 and returned to reactor 70. Uncondensed vapors are removed via stream 81 and fed into cold trap 82, where they are further condensed. The resulting liquid stream 85 is recycled back to static mixer 52 to mix with stream 51. Uncondensed vapors from cold trap 82 are removed via stream 83 through vacuum system 84.

[0058] The overhead product stream 80 from the top of distillation column 77 is sent to the suspension melt crystallization system 91. The mother liquor stream 92, rich in impurities, is divided into a discharge stream 93 and a recirculation stream 94, with the recirculation stream 94 being recycled back to the static mixer 52. The ultrapure glycolide product is collected via stream 95 for further processing.

[0059] Figure 3 This invention describes an alternative system for producing glycolide using glycolic acid solution as a starting material. Figure 1 The process differs from the one shown, and this system introduces a key modification: the previously diverted stream 40 from stream 1 and processed by the concentration tower 41 is now replaced by a separate stream 101 and a preheater 102. Stream 101 is preheated in the preheater 102 to obtain a heated stream 103, which is then combined with streams 17 and 39 to form stream 19. The bottom stream 21 from the reactor 20 is pumped by the pump 22 and divided into a discharge stream 23 and a recirculation stream 24. The recirculation stream 24, after being combined with stream 19, is introduced into the top of the falling film evaporator 26. Stream 101 may consist only of a methyl glycolate solution or a mixture of glycolic acid and methyl glycolate solutions. All other streams remain consistent with the original process.

[0060] The present invention will now be described in more detail with reference to the accompanying drawings and embodiments.

[0061]

Example

[0062] Example 1

[0063] Perform according to the present invention as follows Figure 1 The first reactive distillation system shown is a reactive distillation process. Distillation column 6 has a total of 10 theoretical stages. An aqueous glycolic acid solution from stream 1, containing 70 wt% glycolic acid, at a mass flow rate of 500 kg / h, is combined with streams 35 and 49 in static mixer 2. The resulting mixture, stream 3, is preheated to 100°C by preheater 4 before entering distillation column 6. In the prepolymerization step, glycolic acid oligomers are formed and by-product water is distilled off. The heat required for this process is provided by falling film evaporator 13, which raises the temperature of the reaction solution to 210°C. The top product stream 9, mainly composed of water, at a mass flow rate of 238 kg / h, is removed for further water treatment. Bottom product stream 17 is obtained, containing mostly glycolic acid oligomers. The operating pressure of distillation column 6 (pressure of condenser 7) is 40 mbar, and the bottom pressure is 43 mbar. Titration of the acid content in the bottom product indicates that the number-average molecular weight of the glycolic acid oligomers is 800 g / mol.

[0064] Stream 40, with a mass flow rate of 17 kg / h, is fed into a concentrating distillation column 41, where it is purified to a solution containing 96 wt% glycolic acid and extracted via stream 42. Distilled water is removed via overhead stream 43. Any uncondensed vapors are removed via stream 44 by a vacuum system 45. Bottom product stream 17 from the first reactive distillation system is combined with the catalyst (stannous octoate) from stream 39 and the concentrated glycolic acid solution from stream 42 in a static mixer 18 to form stream 19. This resulting stream 19 is then combined with the recycle stream 24. The combined stream is then fed to the top of a falling film evaporator 26. Glycol, along with a small amount of glycolic acid dimer formed, water, and unreacted glycolic acid, is distilled off via stream 30 while the evaporator heats the reaction solution to 230°C. Overhead product stream 30, with a mass flow rate of 372 kg / h, containing 93.2 wt% glycolic acid, is collected. Stream 23 is discharged as a purge stream (discharge stream). Distillation column 27 has a total of 7 theoretical stages, with an operating pressure (pressure of condenser 28) of 5 mbar and a bottom pressure of 8 mbar.

[0065] Scrubber 32 has a total of 6 theoretical stages. A concentrated methyl glycolate stream containing 97 wt% methyl glycolate is first cooled in cooler 37 and then fed to the top of scrubber 32, where an internal pump circulation configuration is used in its stripping section. Unreacted glycolic acid and glycolic acid dimers from stream 31 are absorbed by the cooled methyl glycolate upon entering the bottom of the scrubber. The resulting liquid, at a mass flow rate of 45 kg / h, is then recycled back to static mixer 2 to mix with stream 1. Any uncondensed vapors from the top of the scrubber are removed by vacuum system 34 via stream 33.

[0066] The glycolide-rich overhead product stream 30 undergoes additional processing in a two-stage suspension-based melt crystallization system 46. This process produces an ultrapure glycolide product with a purity of 99.92 wt% and a mass flow rate of 280 kg / h, which is collected via stream 50.

[0067] Example 2

[0068] Perform according to the present invention as follows Figure 1 The first reactive distillation system shown is a reactive distillation process. Distillation column 6 has a total of 10 theoretical stages. An aqueous glycolic acid solution from stream 1, containing 70 wt% glycolic acid, at a mass flow rate of 500 kg / h, is combined with streams 35 and 49 in static mixer 2. The resulting mixture, stream 3, is preheated to 100°C by preheater 4 before entering distillation column 6. In the prepolymerization step, glycolic acid oligomers are formed and by-product water is distilled off. The heat required for this process is provided by falling film evaporator 13, which raises the temperature of the reaction solution to 215°C. The top product stream 9, mainly composed of water, at a mass flow rate of 242 kg / h, is removed for further water treatment. Bottom product stream 17 is obtained, containing the majority of glycolic acid oligomers. The operating pressure of distillation column 6 (pressure of condenser 7) is 40 mbar, and the bottom pressure is 43 mbar. Titration of the acid content in the bottom product indicates that the number-average molecular weight of the glycolic acid oligomers is 2,000 g / mol.

[0069] Stream 40, with a mass flow rate of 34 kg / h, is fed into a concentrating distillation column 41, where it is purified into a solution containing 96 wt% glycolic acid and extracted via stream 42. Distilled water is removed via overhead stream 43. Any uncondensed vapors are removed via stream 44 by a vacuum system 45. Bottom product stream 17 from the first reactive distillation system is combined with the catalyst (stannous octoate) from stream 39 and the concentrated glycolic acid solution from stream 42 in a static mixer 18 to form stream 19. This resulting stream 19 is then combined with the recycle stream 24. The combined stream is then fed to the top of a falling film evaporator 26. Glycol, along with a small amount of glycolic acid dimer formed, water, and unreacted glycolic acid, is distilled off via stream 30 while the evaporator heats the reaction solution to 240°C. Overhead product stream 30, with a mass flow rate of 386 kg / h, containing 90.9 wt% glycolic acid, is collected. Stream 23 is discharged as a purge stream (discharge stream). Distillation column 27 has a total of 7 theoretical stages, with an operating pressure (pressure of condenser 28) of 10 mbar and a bottom pressure of 13 mbar.

[0070] Scrubber 32 has a total of 6 theoretical stages. A concentrated methyl glycolate stream containing 97 wt% methyl glycolate is first cooled in cooler 37 and then fed to the top of scrubber 32, where an internal pump circulation configuration is used in its stripping section. Unreacted glycolic acid and glycolic acid dimers from stream 31 are absorbed by the cooled methyl glycolate upon entering the bottom of the scrubber. The resulting liquid, at a mass flow rate of 55 kg / h, is then recycled back to static mixer 2 to mix with stream 1. Any uncondensed vapors from the top of the scrubber are removed by vacuum system 34 via stream 33.

[0071] The glycolide-rich overhead product stream 30 undergoes additional processing in a two-stage suspension-based melt crystallization system 46. This process produces an ultrapure glycolide product with a purity of 99.90 wt% and a mass flow rate of 284 kg / h, which is collected via stream 50.

[0072] Example 3

[0073] Perform according to the present invention as follows Figure 2 The first reactive distillation system shown is a reactive distillation process. Distillation column 56 has a total of 10 theoretical stages. A methyl glycolate solution from stream 51, with a concentration of 97 wt% and a mass flow rate of 500 kg / h, is combined with streams 85, 94, and catalyst stream 89 in static mixer 52. The resulting mixture, stream 53, is preheated to 70°C by preheater 54 before entering distillation column 56. In the prepolymerization step, methyl glycolate oligomers are formed and methanol by-product is distilled off. The heat required for this process is provided by falling film evaporator 63, which raises the temperature of the reaction solution to 210°C. The top product stream 59, mainly composed of methanol, with a mass flow rate of 190 kg / h, is removed for further processing. The bottom product stream 67 is obtained, containing mostly methyl glycolate oligomers. The operating pressure of distillation column 56 (pressure of condenser 57) is 200 mbar, and the bottom pressure is 203 mbar. Titration of the acid group content in the bottom product showed that the number average molecular weight of the methyl glycolate oligomer was 800 g / mol.

[0074] Stream 86, with a mass flow rate of 12 kg / h and a concentration of 97 wt% methyl glycolate, is preheated in preheater 87. The heated concentrated methyl glycolate in stream 88 is mixed with the bottom product from stream 67 and the depolymerization catalyst from stream 90 in static mixer 68 to form stream 69. Bottom stream 71 from reactor 70 is split into purge stream 73 and recirculation stream 74. This recirculation stream is combined with stream 69 to form the reaction feed, which is then fed to the top of falling film evaporator 76. Glycol, along with a small amount of formed methyl glycolate dimer, methanol, and unreacted methyl glycolate, is distilled through stream 80, while the evaporator heats the reaction solution to 230°C. Top product stream 80, with a mass flow rate of 388 kg / h and containing 94.9 wt% glycolate, is collected. Distillation column 77 has a total of 7 theoretical stages, operating at a pressure (condenser pressure 78) of 8 mbar and a bottom pressure of 11 mbar.

[0075] Uncondensed vapor is removed via stream 81 and fed into cold trap 82, where it is further condensed. The resulting liquid stream 85, with a mass flow rate of 8 kg / h, is recycled back to static mixer 52 to mix with stream 51. Uncondensed vapor from cold trap 82 is removed via stream 83 by vacuum system 84.

[0076] The glycolide-rich overhead product stream 80 undergoes additional processing in a two-stage suspension-based melt crystallization system 91. This process produces an ultrapure glycolide product with a purity of 99.93 wt% and a mass flow rate of 298 kg / h, which is collected via stream 95.

[0077] Example 4

[0078] Perform according to the present invention as follows Figure 2 The first reactive distillation system shown is a reactive distillation process. Distillation column 56 has a total of 10 theoretical stages. A methyl glycolate solution from stream 51, with a concentration of 97 wt% and a mass flow rate of 500 kg / h, is combined with streams 85, 94, and catalyst stream 89 in static mixer 52. The resulting mixture, stream 53, is preheated to 70°C by preheater 54 before entering distillation column 56. In the prepolymerization step, methyl glycolate oligomers are formed and methanol by-product is distilled off. The heat required for this process is provided by falling film evaporator 63, which raises the temperature of the reaction solution to 215°C. The overhead product stream 59, mainly composed of methanol, with a mass flow rate of 194 kg / h, is removed for further processing. The bottom product stream 67 is obtained, which contains mostly methyl glycolate oligomers. The operating pressure of distillation column 56 (pressure of condenser 57) is 200 mbar, and the bottom pressure is 203 mbar. Titration of the acid group content in the bottom product showed that the number average molecular weight of the methyl glycolate oligomer was 2,000 g / mol.

[0079] Stream 86, with a mass flow rate of 24 kg / h and a concentration of 97 wt% methyl glycolate, is preheated in preheater 87. The heated concentrated methyl glycolate in stream 88 is mixed with the bottom product from stream 67 and the depolymerization catalyst from stream 90 in static mixer 68 to form stream 69. Bottom stream 71 from reactor 70 is split into purge stream 73 and recirculation stream 74. This recirculation stream is combined with stream 69 to form the reaction feed, which is then fed to the top of falling film evaporator 76. Glycol, along with a small amount of formed methyl glycolate dimer, methanol, and unreacted methyl glycolate, is distilled through stream 80 while the evaporator heats the reaction solution to 240°C. Top product stream 80, with a mass flow rate of 396 kg / h, contains 92.4 wt% glycolate. The operating pressure of distillation column 77 (pressure of condenser 78) is 15 mbar, and the bottom pressure is 18 mbar.

[0080] Uncondensed vapor is removed via stream 81 and fed into cold trap 82, where it is further condensed. The resulting liquid stream 85, with a mass flow rate of 10 kg / h, is recycled back to static mixer 52 to mix with stream 51. Uncondensed vapor from cold trap 82 is removed via stream 83 by vacuum system 84.

[0081] The glycolide-rich overhead product stream 80 undergoes additional processing in a two-stage suspension-based melt crystallization system 91. This process produces an ultrapure glycolide product with a purity of 99.91 wt% and a mass flow rate of 296 kg / h, which is collected via stream 95.

[0082] Example 5

[0083] Perform according to the present invention as follows Figure 3 The first reactive distillation system is shown. Distillation column 6 has a total of 10 theoretical stages. An aqueous glycolic acid solution from stream 1, containing 70 wt% glycolic acid, at a mass flow rate of 500 kg / h, is combined with streams 35 and 49 in static mixer 2. The resulting mixture, stream 3, is preheated to 100°C by preheater 4 before entering distillation column 6. In the prepolymerization step, glycolic acid oligomers are formed and by-product water is distilled off. The heat required for this process is provided by falling film evaporator 13, which raises the temperature of the reaction solution to 212°C. The top product stream 9, mainly composed of water, at a mass flow rate of 240 kg / h, is removed for further water treatment. The bottom product stream 17 is obtained, which contains the majority of glycolic acid oligomers. The operating pressure of distillation column 6 (pressure of condenser 7) is 40 mbar, and the bottom pressure is 43 mbar. Titration of the acid content in the bottom product indicates that the number average molecular weight of the glycolic acid oligomers is 1,000 g / mol.

[0084] Stream 101, with a mass flow rate of 20 kg / h and a concentration of 97 wt% methyl glycolate, is preheated in preheater 102. The heated concentrated methyl glycolate in stream 103 is mixed with the bottom product from stream 17 and the depolymerization catalyst from stream 39 in static mixer 18 to form stream 19. The bottom stream 21 from kettle 20 is split into purge stream 23 and recirculation stream 24. This recirculation stream is merged with stream 19 to form the reaction feed, which is then fed to the top of falling film evaporator 26. Glycol, along with a small amount of formed methyl glycolate dimer, water, and unreacted glycolic acid, is distilled out through stream 30, while the evaporator heats the reaction solution to 230°C. The top product stream 30, with a mass flow rate of 392 kg / h, contains 91.3 wt% glycolate. Distillation column 27 has a total of 7 theoretical stages, operating at a pressure (condenser pressure 28) of 7 mbar and a bottom pressure of 10 mbar.

[0085] Scrubber 32 has a total of 6 theoretical stages. A concentrated methyl glycolate stream containing 97 wt% methyl glycolate is first cooled in cooler 37 and then fed to the top of scrubber 32, where an internal pump circulation configuration is used in its stripping section. Unreacted glycolic acid, dimers, and methyl glycolate from stream 31 are absorbed by the cooled methyl glycolate upon entering the bottom of the scrubber. The resulting liquid, at a mass flow rate of 60 kg / h, is then recycled back to static mixer 2 to mix with stream 1. Any uncondensed vapors from the top of the scrubber are removed by vacuum system 34 via stream 33.

[0086] The glycolide-rich overhead product stream 30 undergoes additional processing in a two-stage suspension-based melt crystallization system 46. This process produces an ultrapure glycolide product with a purity of 99.91 wt% and a mass flow rate of 288 kg / h, which is collected via stream 50.

[0087] Example 6

[0088] Use such as Figure 3 The first reactive distillation system shown performs a reactive distillation process according to an embodiment of the invention. Following the same procedure detailed in Example 5, glycolic acid oligomers with a number-average molecular weight of 1,000 g / mol were produced. The depolymerization and purification steps were carried out under the same conditions as described in Example 5, except that stream 101 consisted of a mixture of concentrated glycolic acid (96 wt%) and a methyl glycolate solution (97 wt%), with a total mass flow rate of 30 kg / h. In this mixture, the concentrated glycolic acid solution accounted for 20% of the total mass, while the methyl glycolate solution accounted for the remaining 80%.

[0089] The glycolide-rich overhead product stream 30 undergoes additional processing in a two-stage suspension-based melt crystallization system 46. This process produces an ultrapure glycolide product with a purity of 99.90 wt% and a mass flow rate of 294 kg / h, which is collected via stream 50.

Claims

1. A method for continuous production of glycolide, comprising: A prepolymerization step in which glycolic acid or methyl glycolate is prepolymerized into their respective oligomers; The depolymerization step includes depolymerizing the oligomer into crude glycolide using a reactive distillation system, wherein a small amount of liquid material is introduced into the depolymerization reactor as part of the reaction feed; and The purification step involves processing the crude glycolide into ultrapure glycolide using a melt crystallization system.

2. The method according to claim 1, wherein, The number-average molecular weight of the oligomer is in the range of 400 to 5000, preferably in the range of 600 to 3000, and more preferably in the range of 800 to 2000.

3. The method according to claim 1, wherein, The reactive distillation system includes at least one vessel, a distillation column, a horizontal condenser, and a falling film evaporator.

4. The method according to claim 3, wherein, The distillation column and falling film evaporator are respectively installed on the top of the kettle, and the horizontal condenser is installed on the top of the distillation column.

5. The method according to claim 1, wherein, The liquid substance is selected from concentrated glycolic acid solution, concentrated methyl glycolate solution, or a combination thereof.

6. The method according to claim 1, wherein, In the oligomer produced in the prepolymerization step, the content of the liquid substance is between 1 wt% and 10 wt%, more preferably between 3 wt% and 6 wt%.

7. The method according to any one of claims 1 and 3, wherein, The depolymerization reactor is a falling film evaporator.

8. The method according to claim 3, wherein, The bottom flow from the vessel is divided into a discharge flow and a recirculation flow.

9. The method according to any one of claims 1 and 8, wherein, The reaction feed comprises the liquid substance, the depolymerization catalyst, the oligomer from the prepolymerization step, and a mixture of circulating streams.

10. The method according to claim 1, wherein, The reaction feed is introduced at the top of the depolymerization reactor and flows downward through a long vertical tube that serves as a heat transfer and reaction zone, the long vertical tube being surrounded by a heat transfer medium.

11. The method according to claim 1, wherein, The crude glycolide mainly consists of glycolide produced in a depolymerization reactor, and a small portion of low-boiling-point volatiles, including the liquid substance, its dimer, and related byproducts including water and methanol.

12. The method according to claim 1, wherein, The melt crystallization system is a suspension-based melt crystallization system.

13. The method according to claim 12, wherein, The suspension-based melt crystallization system includes at least one washing column for producing ultrapure glycolide.

14. The method according to claim 12, wherein, The mother liquor from the suspension-based melt crystallization system is recycled back to the prepolymerization step.

15. The method according to any one of claims 1 and 3, wherein, The operating pressure of the horizontal condenser in the reactive distillation system is set in the range of 3 to 30 mbar.

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