Method and device for trapping, purifying and recycling by-product glycolide in polyglycolic acid synthesis process

Abstract

The invention discloses a method and a device for capturing, purifying and recycling by-product glycolide in a polyglycolic acid synthesis process. The device comprises a gas phase condenser, a condensate collecting tank, a glycolide leaching tank, a glycolide curing tank and a decanter, a gas phase inlet of the gas phase condenser is connected with a gas phase outlet of the PGA polymerization reaction kettle; a liquid phase outlet of the gas phase condenser is connected with an inlet of a condensate collecting tank, an outlet of the condensate collecting tank is connected with a glycolide leaching tank, and a glycolide outlet of the glycolide leaching tank is connected with a glycolide curing tank; the glycolide curing tank is connected with the decanter, and solid glycolide obtained at a solid outlet of the decanter enters a feeding section or a pre-polycondensation section of polyglycolic acid. The glycolide treated according to the method can be returned to the PGA production process to continuously participate in the reaction without influencing the color of the product, so that the waste of byproducts is avoided, and the conversion rate of the raw materials is greatly improved.

Description

Technical Field

[0001] The invention discloses a method and a device for capturing, purifying and recycling glycolide, a by-product in the synthesis process of polyglycolic acid (PGA), and belongs to the technical field of purification of chemical products. Background Art

[0002] Polyglycolic acid (PGA) has the advantages of excellent biocompatibility, mechanical properties, high barrier properties, high load deformation temperature, etc. It is a rare degradable engineering polymer material. Its biodegradability is similar to that of natural cellulose. At the same time, it meets the requirements of industrial composting, household composting and seawater degradation. It has been certified as a safe biodegradable plastic material in the United States, the European Union, Japan and other countries. It is widely used in high-end biomedicine, food packaging, shale and oil extraction, agricultural production and other fields, and has broad market space.

[0003] At present, the synthesis methods of polyglycolic acid with industrial value include direct polymerization and glycolide ring-opening polymerization. At present, the synthesis of PGA at home and abroad is mainly based on glycolide ring-opening polymerization. This route has high requirements for the purity of glycolide and high product cost. It is mainly mass-produced in Japan and the United States.

[0004] There are few reports on direct polymerization at home and abroad. In the process of producing polyglycolic acid (PGA) by direct polymerization, due to high temperature and high vacuum conditions, some oligomers will inevitably depolymerize to form by-product glycolide. Glycolide entering the vacuum system will cause vacuum system failure. Glycolide itself can also be used to synthesize PGA. Capturing and reusing it can maximize the conversion rate of raw materials and reduce production costs. However, due to the physical and chemical properties of glycolide, it is easy to clog the pipeline after capture; and direct reuse will cause the product to have a dark color and fail to meet the standards. Therefore, the present invention is proposed. Summary of the invention

[0005] The purpose of the present invention is to provide a method and device for capturing, purifying and recycling glycolide, a by-product in the synthesis process of polyglycolic acid (PGA), which can be used to capture glycolide in the process of producing PGA by direct polymerization, and through purification treatment, the recycled glycolide does not affect the product color.

[0006] The present invention first provides a device for capturing, purifying and recycling glycolide, a byproduct in the synthesis process of polyglycolic acid, comprising a gas phase condenser, a condensate collection tank, a glycolide elution tank, a glycolide solidification tank and a decanter; The gas phase inlet of the gas phase condenser is connected to the gas phase outlet of the PGA polymerization reactor; The liquid phase outlet of the gas phase condenser is connected to the inlet of the condensate collection tank, the outlet of the condensate collection tank is connected to the glycolide elution tank, and the glycolide outlet of the glycolide elution tank is connected to the glycolide solidification tank; The glycolide solidification tank is connected to the decanter. The solid glycolide obtained from the solid outlet of the decanter enters the feeding section or pre-condensation section of polyglycolic acid after drying. Under the action of the decanter, the solid glycolide and water are separated. The separated solid glycolide can be returned to the feeding section of PGA after vacuum drying to participate in the reaction again. The separated water is sent to the subsequent treatment section.

[0007] Preferably, the gas phase condenser is a single-pass shell-and-tube heat exchanger, the gas phase glycolide flows through the tube side, and the low-temperature heat medium flows through the shell side. The tube diameter is preferably 32-50 mm, which can alleviate blockage.

[0008] Preferably, the capture, purification and recycling device comprises a plurality of groups of the condensate collection tanks and the gas phase condensers connected in series; A liquid level controller is provided in the condensate collection tank, and the bottom cut-off valve, top cut-off valve and nitrogen cut-off valve of the condensate collection tank are linked according to the liquid level. When the liquid level is high, the bottom cut-off valve, top cut-off valve and nitrogen cut-off valve are linked to isolate the condensate collection tank, pressurize the condensate collection tank, and discharge glycolide into the glycolide elution tank. After the discharge is completed, the condensate collection tank is converted into a negative pressure state by adjusting the vacuum cut-off valve.

[0009] Preferably, the top of the glycolide elution tank is connected to a hydrocyclone separator, and an ethanol delivery pump is provided between the hydrocyclone separator and the glycolide elution tank; The glycolide elution tank is provided with a plurality of sieve plates to ensure sufficient contact. Preferably, the number of sieve plates is 2 to 3. Since the density of glycolide is greater than that of ethanol, the purified glycolide will be deposited at the bottom of the tank. There is a position control instrument at the bottom of the tank to adjust the output flow rate by controlling the frequency of the glycolide delivery pump. A glycolide delivery pump is provided between the glycolide elution tank and the glycolide solidification tank; Ethanol will dissolve the colored impurities in glycolide, and the ethanol with dissolved impurities will be output by the ethanol delivery pump; since ethanol will carry a small amount of glycolide, a hydrocyclone separator is provided; the hydrocyclone separator is designed according to the physical properties of ethanol and glycolide and is used to separate ethanol and glycolide; the separated glycolide is returned to the glycolide washing tank, while the ethanol is sent to distillation regeneration for recycling after roughly adsorbing impurities in the decolorization tank.

[0010] Based on the device for capturing, purifying and recycling the by-product glycolide, the present invention further provides a method for capturing, purifying and recycling the by-product glycolide in the process of synthesizing polyglycolic acid, comprising the following steps performed using the device for capturing, purifying and recycling: The gas phase from the PGA polymerization reactor enters the gas phase condenser, is cooled by the low-temperature heat medium, and then flows into the condensate collection tank; The liquid glycolide condensed in the condensate collection tank enters the glycolide elution tank from the top and contacts with the hot ethanol entering from the bottom in countercurrent; The purified glycolide is pumped to the glycolide solidification tank for solidification, and the suspension containing glycolide solid particles enters the decanter, where glycolide is separated under centrifugal force and enters the PGA polymerization reactor after drying.

[0011] Preferably, the low-temperature heat medium can be a conventional heat medium such as hot water, low-temperature heat transfer oil, etc. The temperature of the low-temperature heat medium is 75-90°C, preferably 80-85°C, which is the key point to prevent blockage; because the freezing point of glycolide is 84°C, and the gaseous glycolide coming out of the reactor is about 210°C. If the temperature of the heat medium is too high, the gaseous glycolide cannot be completely condensed and escapes, and if the temperature of the heat medium is too low, the condensed glycolide may be lower than the freezing point, which will solidify and block.

[0012] The temperature of the hot ethanol is 85-90° C. to avoid solidification of glycolide and ensure fluidity.

[0013] Preferably, when the high liquid level alarm in the condensate collection tank is triggered, the feed valve and the discharge valve of one group of gas phase condenser A are closed, and the feed valve and the discharge valve of the other group of gas phase condenser B are opened; then the nitrogen cut-off valve is opened to pressurize the gas phase condenser A, and after the pressurization is completed, the bottom cut-off valve of the condensate collection tank A connected to the gas phase condenser A is opened to start draining, and when the drainage is completed and the low liquid level alarm is triggered, the bottom cut-off valve of the condensate collection tank A is closed; then the vacuum cut-off valve of the condensate collection tank A is opened to release the pressure, and the pressure of the condensate collection tank A is pumped to negative pressure; after reaching a predetermined vacuum degree, the vacuum cut-off valve of the condensate collection tank A is closed, and the condensate collection tank A and the connected gas phase condenser A are ready to enter the next cycle.

[0014] Preferably, the hot ethanol is discharged from the top of the glycolide elution tank and transported to a hydrocyclone by an ethanol external delivery pump to separate ethanol and glycolide; the separated glycolide is returned to the glycolide elution tank, and the separated ethanol is recycled after adsorption and distillation regeneration; The hydrocyclone is designed based on the physical properties of ethanol and glycolide. Under the action of the initial velocity, the mixed solution undergoes spiral motion. The density of ethanol is smaller, so it is concentrated in the middle of the hydrocyclone and discharged from the upper end; while the density of glycolide is larger, so it is concentrated at the lower end of the hydrocyclone. In order to ensure the separation quality, a small amount of ethanol is controlled to flow out, and glycolide is allowed to return to the elution tank with part of the ethanol.

[0015] Preferably, the glycolide separated by the decanter has a water content of 30-50%, and is vacuum dried at 35-45° C. and ≤2 kPaA.

[0016] Based on the above-mentioned method for capturing, purifying and recycling the by-product glycolide, the present invention also provides a method for producing polyglycolic acid by direct polymerization, comprising the steps of capturing, purifying and recycling the by-product glycolide in the polyglycolic acid synthesis process; The method for producing polyglycolic acid by direct polymerization comprises the following steps: (1) Add glycolic acid / methyl glycolate, a certain amount of catalyst, and a portion of the auxiliary agent mixture into a reactor. Recycled glycolide may also be added. React at 190°C, normal pressure or slightly negative pressure for 60 minutes or until no distillate is evaporated from the system. (2) increasing the vacuum degree of the reaction system (2000-3000 PaA), reacting for 60 min or until the theoretical water yield of the system reaches more than 98%, to obtain PGA oligomers; (3) Add recycled glycolide to other additives in the system, raise the system temperature to 210-225°C, and react for 60 minutes under vacuum at 2000-3000 PaA to perform pre-polycondensation to increase the degree of polymerization of the oligomers; (4) Raise the system vacuum to below 70 PaA and continue polymerization until the desired degree of polymerization is reached.

[0017] The above steps can be continuous or intermittent, wherein steps (3) and (4) will produce glycolide as a by-product, and (4) is the main step for producing glycolide.

[0018] The glycolide treated according to the method of the present invention can be returned to the PGA production process to continue to participate in the reaction without affecting the product color, avoiding the waste of by-products and greatly improving the conversion rate of raw materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the device for capturing, purifying and recycling glycolide, a by-product in the synthesis process of polyglycolic acid (PGA) of the present invention. DETAILED DESCRIPTION

[0020] like Figure 1 As shown, it is a schematic diagram of the capture, purification and recycling device of glycolide by-product in the synthesis process of polyglycolic acid (PGA) provided by the present invention, including a gas phase condenser, a condensate collection tank, a glycolide elution tank, an ethanol delivery pump, a cyclone separator, an ethanol decolorization tank, a glycolide delivery pump, a glycolide solidification tank, a decanter, a collection tank and control instruments.

[0021] like Figure 1As shown, the gas phase condenser is placed in front of the vacuum gas phase pipeline to capture gas phase glycolide. The gas phase condenser can be a commercially available shell and tube heat exchanger. The heat exchange area is calculated by the process, and it is preferably a single tube pass shell and tube heat exchanger. The gas phase glycolide goes through the tube pass and the low temperature heat medium goes through the shell pass. The preferred tube diameter is 32-50mm, which can alleviate blockage. The low temperature heat medium can be a conventional heat medium such as hot water, low temperature heat transfer oil, etc. The preferred temperature of the low temperature heat medium is 80-85°C, which is the key point to prevent blockage.

[0022] like Figure 1 As shown in the figure, the condensate collection tank is used to collect the condensed liquid glycolide. There is a liquid level control in the condensate collection tank. When the liquid level is high, it is linked with the tank bottom cut-off valve, tank top cut-off valve and nitrogen cut-off valve to isolate the collection tank, pressurize the collection tank, and discharge glycolide into the glycolide elution tank. After the discharge is completed, the condensate collection tank is converted to a negative pressure state by adjusting the vacuum cut-off valve. The condensate collection tank is used in conjunction with the gas phase condenser, and 2 to 3 groups (two groups are shown in the figure) or even more can be set as needed.

[0023] like Figure 1 As shown, the glycolide elution tank is used to purify liquid glycolide. The liquid glycolide discharged from the condensate collection tank is in countercurrent contact with the recycled hot ethanol, and the ethanol will dissolve the colored impurities in the glycolide. Multiple sieve plates are arranged in the tank to ensure sufficient contact. The preferred number of sieve plates is 2 to 3. The temperature of the hot ethanol is preferably 85-90°C to avoid solidification of glycolide and ensure fluidity. Because the density of glycolide is greater than that of ethanol, the purified glycolide will be deposited at the bottom of the tank. There is a position control instrument at the bottom of the tank to adjust the output flow by controlling the frequency of the glycolide delivery pump.

[0024] The ethanol with dissolved impurities is discharged by the ethanol delivery pump. Since ethanol will carry a small amount of glycolide, a hydrocyclone is set up. The hydrocyclone is designed based on the physical properties of ethanol and glycolide and is used to separate ethanol and glycolide. The separated glycolide is returned to the glycolide washing tank. After the ethanol passes through the decolorization tank to roughly absorb the impurities, it is sent to distillation regeneration and recycled.

[0025] like Figure 1 As shown, the glycolide after elution and purification is sent to the glycolide solidification tank by the glycolide delivery pump. A nozzle is set in the solidification tank. The solidification tank is filled with cold water, and the water temperature is kept below 30°C, preferably 20-25°C. The glycolide coming out of the nozzle is quickly solidified into fine particles due to the decrease in temperature, and goes to the decanter with a large amount of water.

[0026] like Figure 1As shown in the figure, the decanter is a device that uses centrifugal force to separate solids and liquids. Under the action of the decanter, solid glycolide and water are separated. After vacuum drying, the separated solid glycolide can be returned to the PGA feeding section or pre-condensation section to participate in the reaction again. The separated water is sent to the subsequent treatment section.

[0027] use Figure 1 The steps of capturing, purifying and recycling the by-product glycolide in the synthesis process of polyglycolic acid (PGA) are as follows: The gas phase from the PGA polymerization reactor enters the gas phase condenser, is cooled by the low-temperature heat medium, and then flows into the condensate collection tank; the liquid glycolide condensed in the condensate collection tank enters the glycolide elution tank from the top, and countercurrently contacts with the hot ethanol entering from the bottom; the purified glycolide is pumped to the glycolide solidification tank for solidification, and the suspension containing glycolide solid particles enters the decanter, where glycolide is separated under the action of centrifugal force and enters the PGA polymerization reactor after drying. Among them, the low-temperature heat medium can be conventional heat medium such as hot water and low-temperature heat transfer oil, and the temperature of the low-temperature heat medium is 75-90°C; the temperature of the hot ethanol is 85-90°C to avoid glycolide solidification and ensure fluidity.

[0028] Among them, the liquid level control process in the condensate collection tank is as follows: when the high liquid level alarm in the condensate collection tank is triggered, the feed valve and the discharge valve of one group of gas phase condenser A are closed, and the feed valve and the discharge valve of the other group of gas phase condenser B are opened; then the nitrogen cut-off valve is opened to pressurize the gas phase condenser A, and after the pressurization is completed, the tank bottom cut-off valve of the condensate collection tank A connected to the gas phase condenser A is opened to start draining, and when the drainage is completed and the low liquid level alarm is triggered, the tank bottom cut-off valve of the condensate collection tank A is closed; then the vacuum cut-off valve of the condensate collection tank A is opened to release the pressure, and the pressure of the condensate collection tank A is pumped to negative pressure; after reaching the predetermined vacuum degree, the vacuum cut-off valve of the condensate collection tank A is closed, and the condensate collection tank A and the connected gas phase condenser A are ready to enter the next cycle.

[0029] like Figure 1 As shown, after the hot ethanol is discharged from the top of the glycolide elution tank, it is transported to the hydrocyclone by the ethanol external pump to separate ethanol and glycolide; the separated glycolide is returned to the glycolide elution tank, and the separated ethanol is recycled after adsorption and distillation regeneration. Among them, the hydrocyclone used is designed according to the physical properties of ethanol and glycolide. Under the action of the initial speed, the mixed solution undergoes spiral motion, and the density of ethanol is smaller, so it is concentrated in the middle of the hydrocyclone and discharged from the upper end; while the density of glycolide is larger, so it is concentrated at the lower end of the hydrocyclone. In order to ensure the separation quality, the ethanol outflow is controlled to be small, and the glycolide is allowed to return to the elution tank with part of the ethanol.

[0030] According to the above process, the capture, purification and reuse of glycolide, a byproduct of the synthesis of polyglycolic acid (PGA), are carried out: the gas phase material from the PGA polymerization reactor is about 210°C, and first enters the gas phase condenser A. Under the cooling of 85°C low-temperature heat medium (hot water), the gas is cooled to 87°C, and the condensable medium in the gas phase is condensed and precipitated, and flows into the condensate collection tank A below. The non-condensable gas returns to the original vacuum pipeline. The temperature of the low-temperature heat medium out of the condenser is controlled to be 89°C.

[0031] The amount of glycolide in the condensate collection tank A is increasing, triggering a high liquid level alarm. The system controls the closing of the feed valve and discharge valve of gas phase condenser A, and opens the feed valve and discharge valve of condenser B, switching the gas phase condensation to condenser B.

[0032] After that, the system controls to open the nitrogen shut-off valve and pressurize the condensate collection tank A. The charging pressure is adjusted by the pressure control instrument. After the charging is completed, the system controls to open the tank bottom shut-off valve of the condensate collection tank A to start draining. When the drainage is completed and the low liquid level alarm is triggered, the system controls to close the tank bottom shut-off valve of the condensate collection tank A. Then open the vacuum shut-off valve to release the pressure, and pump the pressure of the condensate collection tank A to the original negative pressure to avoid the impact on the vacuum degree of the main gas phase pipeline during the next use. After reaching the predetermined vacuum degree, close the vacuum shut-off valve, and the condensate collection tank A and the gas phase condenser A are ready to enter the next cycle.

[0033] The condensed liquid glycolide enters the glycolide elution tank from the top under the pressure of nitrogen and contacts with the hot ethanol entering from the bottom in countercurrent. The temperature of hot ethanol is about 87℃. During the contact between glycolide and ethanol, ethanol will dissolve the colored impurities in glycolide. After the three-layer sieve plate, most of the colored impurities in glycolide are dissolved and taken away by ethanol. Due to the density difference, the purified glycolide is deposited at the bottom of the elution tank. There is a boundary control instrument at the bottom of the tank, which is connected to the variable frequency motor that controls the glycolide delivery pump. The delivery flow of the pump is controlled according to the boundary height.

[0034] After purification, glycolide is pumped to the nozzle on the top of the glycolide solidification tank. Under the pressure of the pump, glycolide at about 85°C is dispersed and sprayed out from the nozzle at high speed with a small amount of ethanol. Because there is a large amount of cold water (about 20°C, which is much lower than the solidification point of glycolide) in the glycolide solidification tank, glycolide solidifies quickly and ethanol dissolves in the water. In order to maintain the temperature in the tank, low-temperature cold water is continuously added.

[0035] The suspension carrying a large amount of glycolide solid particles is sent to the decanter under self-pressure. Under the centrifugal force of the decanter, water and glycolide particles are quickly separated. The glycolide out of the decanter contains about 30-50% water. After being collected, it is sent to a vacuum dryer and dried at about 40°C for use. The water out of the decanter contains part of glycolide and ethanol, so it is sent for post-processing and recycling.

[0036] The ethanol coming out of the glycolide elution tank contains a small amount of glycolide. It enters the hydrocyclone from the tangential direction. Because the hydrocyclone is designed based on the physical properties of ethanol and glycolide, the mixed solution undergoes spiral motion under the action of the initial velocity. The density of ethanol is smaller, so it is concentrated in the middle of the hydrocyclone and discharged from the upper end. The density of glycolide is larger, so it is concentrated at the lower end of the hydrocyclone. In order to ensure the separation quality, the ethanol outflow is controlled to be small, and the glycolide is allowed to carry part of the ethanol back to the elution tank.

[0037] The separated ethanol passes through a decolorization tank to roughly remove the impurities carried by it. It is then sent to the rear distillation stage for recycling after passing the treatment.

Claims

1. A device for capturing, purifying and recycling glycolide, a by-product in the synthesis process of polyglycolic acid, comprising a gas phase condenser, a condensate collection tank, a glycolide elution tank, a glycolide solidification tank and a decanter; The gas phase inlet of the gas phase condenser is connected to the gas phase outlet of the PGA polymerization reactor; The liquid phase outlet of the gas phase condenser is connected to the inlet of the condensate collection tank, the outlet of the condensate collection tank is connected to the glycolide elution tank, and the glycolide outlet of the glycolide elution tank is connected to the glycolide solidification tank; The glycolide solidification tank is connected to the decanter, and the solid glycolide obtained at the solid outlet of the decanter enters the feeding section or pre-condensation section of polyglycolic acid.

2. The capture, purification and recycling device according to claim 1, characterized in that: The gas phase condenser is a single-tube shell-and-tube heat exchanger, gas phase glycolide flows through the tube side, and low-temperature heat medium flows through the shell side.

3. The capture, purification and recycling device according to claim 1 or 2, characterized in that: The capture, purification and recycling device includes a plurality of groups of the condensate collection tanks and the gas phase condensers connected in series; A liquid level controller is provided in the condensate collection tank, and linkage is achieved between the tank bottom cut-off valve, tank top cut-off valve and nitrogen cut-off valve of the condensate collection tank according to the liquid level.

4. The capture, purification and recycling device according to claim 1 or 2, characterized in that: The top of the glycolide elution tank is connected to a hydrocyclone separator, and an ethanol delivery pump is provided between the hydrocyclone separator and the glycolide elution tank; A glycolide delivery pump is provided between the glycolide elution tank and the glycolide solidification tank.

5. A method for capturing, purifying and recycling glycolide, a byproduct in the synthesis process of polyglycolic acid, comprising the following steps using the capturing, purifying and recycling device according to any one of claims 1 to 4: The gas phase from the PGA polymerization reactor enters the gas phase condenser, is cooled by the low-temperature heat medium, and then flows into the condensate collection tank; The liquid glycolide condensed in the condensate collection tank enters the glycolide elution tank from the top and contacts with the hot ethanol entering from the bottom in countercurrent; The purified glycolide is pumped to the glycolide solidification tank for solidification, and the suspension containing glycolide solid particles enters the decanter, where glycolide is separated under centrifugal force and enters the PGA polymerization reactor after drying.

6. The capture, purification and recycling method according to claim 5, characterized in that: The temperature of the low temperature heat medium is 75-90°C; The temperature of the hot ethanol is 85-90°C.

7. The capture, purification and recycling method according to claim 5 or 6, characterized in that: When the high liquid level alarm in the condensate collection tank is triggered, the feed valve and the discharge valve of one group of gas phase condenser A are closed, and the feed valve and the discharge valve of the other group of gas phase condenser B are opened; then the nitrogen cut-off valve is opened to pressurize the gas phase condenser A. After the pressurization is completed, the tank bottom cut-off valve of the condensate collection tank A connected to the gas phase condenser A is opened to start draining. When the drainage is completed and the low liquid level alarm is triggered, the tank bottom cut-off valve of the condensate collection tank A is closed; then the vacuum cut-off valve of the condensate collection tank A is opened to release the pressure, and the pressure of the condensate collection tank A is pumped to negative pressure; after reaching the predetermined vacuum degree, the vacuum cut-off valve of the condensate collection tank A is closed, and the condensate collection tank A and the connected gas phase condenser A are ready to enter the next cycle.

8. The capture, purification and recycling method according to any one of claims 5 to 7, characterized in that: The hot ethanol is discharged from the top of the glycolide elution tank and transported to the hydrocyclone separator by the ethanol export pump to separate ethanol and glycolide; the separated glycolide is returned to the glycolide elution tank, and the separated ethanol is recycled after adsorption and distillation regeneration.

9. The capture, purification and recycling method according to any one of claims 5 to 8, characterized in that: The glycolide separated by the decanter has a water content of 30-50%, and is vacuum dried at 35-45° C. and ≤2 kPaA.

10. A method for producing polyglycolic acid by direct polymerization, comprising the steps of capturing, purifying and recycling the by-product glycolide produced during the polyglycolic acid synthesis process as claimed in any one of claims 5 to 9.

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