A method and device for capturing, purifying and recycling glycolide, a by-product in polyglycolic acid synthesis

Abstract

The application discloses a method and device for capturing, purifying and recycling of glycolide, a by-product in a polyglycolic acid synthesis process. The device comprises 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 with the gas phase outlet of a PGA polymerization reactor. The liquid phase outlet of the gas phase condenser is connected with the inlet of the condensate collection tank. The outlet of the condensate collection tank is connected with the glycolide elution tank. The glycolide outlet of the glycolide elution tank is connected with the glycolide solidification tank. The glycolide solidification tank is connected with the decanter. The solid glycolide obtained from the solid outlet of the decanter enters a feeding section or a pre-polycondensation section of polyglycolic acid. The glycolide treated according to the application can be returned to the PGA production process to continue participating in the reaction without affecting the product color, avoiding waste of by-products and greatly improving the conversion rate of raw materials.

Description

TECHNICAL FIELD

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

[0002] Polyglycolic acid (PGA) has excellent biocompatibility, mechanical properties, high barrier properties, high load deformation temperature and other advantages, is a rare degradable engineering polymer material, has similar biodegradation performance to natural cellulose, meets the requirements of industrial composting, household composting and seawater degradation, has obtained the safe biodegradable plastic material certification in the United States, the European Union and Japan and other countries, and is widely applied to high-end biomedical, food packaging, shale and oil exploitation and agricultural production and other fields, and has a broad market space.

[0003] At present, the synthesis methods of polyglycolic acid with industrial value include a direct polymerization method and a glycolide ring-opening polymerization method. At present, the synthesis of PGA at home and abroad is mainly based on the glycolide ring-opening polymerization method, the glycolide ring-opening polymerization method has high requirements on the purity of glycolide, the product cost is very high, and the large-scale production is mainly realized by Japan and the United States.

[0004] For the direct polymerization method, there are few reports at home and abroad. In the process of producing polyglycolic acid (PGA) by the direct polymerization method, because of the high-temperature and high-vacuum conditions, a part of oligomers will inevitably depolymerize to generate a by-product glycolide. The glycolide enters the vacuum system, which will cause the failure of the vacuum system. The glycolide itself can also be used for synthesizing PGA, and the glycolide can be captured and recycled, so that the conversion rate of raw materials can be maximized and the production cost can be reduced. However, because of the physical and chemical properties of the glycolide, the glycolide is easy to block the pipeline after being captured, and direct recycling will cause the product color to be deep and not up to standard. Therefore, the application is provided. SUMMARY

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

[0006] The application first provides a device for capturing, purifying and recycling glycolide, a by-product in a polyglycolic acid synthesis process, which comprises a gas-phase condenser, a condensate collection tank, a glycolide elution tank, a glycolide solidification tank and a decanter.

[0007] The gas-phase inlet of the gas-phase condenser is connected with the gas-phase outlet of a PGA polymerization reactor.

[0008] The liquid phase outlet of the gas phase condenser is connected with the inlet of the condensate collection tank, the outlet of the condensate collection tank is connected with the glycolide elution tank, and the glycolide outlet of the glycolide elution tank is connected with the glycolide solidification tank;

[0009] The glycolide solidification tank is connected with the decanter, the solid outlet of the decanter is connected with the feed section of polyglycolic acid or the pre-polycondensation section, solid glycolide is dried and then enters the feed section of polyglycolic acid, and under the action of the decanter, the solid glycolide and water are separated; the separated solid glycolide is dried under vacuum and then returned to the feed section of PGA to participate in the reaction again; and the separated water is sent to a subsequent treatment section.

[0010] Preferably, the gas phase condenser is a single-tube pass shell-and-tube heat exchanger, the gas phase glycolide passes through the tube, and the low-temperature heat medium passes through the shell, and the tube diameter is preferably 32-50 mm, which can alleviate the blockage.

[0011] Preferably, the trapping, purifying and recycling device comprises a plurality of groups of the condensate collection tank and the gas phase condenser connected in series;

[0012] The condensate collection tank is provided with a liquid level controller, and the tank bottom cut-off valve, the tank top cut-off valve and the 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 when the liquid level is high.

[0013] Preferably, the top of the glycolide elution tank is connected with a hydrocyclone, and the hydrocyclone and the glycolide elution tank are provided with an ethanol delivery pump;

[0014] The glycolide elution tank is provided with a plurality of sieve plates to ensure sufficient contact, and the number of sieve plates is preferably 2-3; since the density of glycolide is greater than that of ethanol, the purified glycolide is deposited at the bottom of the tank, and the tank bottom is provided with a liquid level control instrument to adjust the output flow rate by controlling the frequency of the glycolide delivery pump;

[0015] The glycolide elution tank and the glycolide solidification tank are provided with a glycolide delivery pump;

[0016] Ethanol can dissolve the colored impurities in glycolide, the ethanol dissolving the impurities is output by the ethanol delivery pump; since ethanol can entrain a small amount of glycolide, a hydrocyclone is arranged; the hydrocyclone is designed according to the physical properties of ethanol and glycolide and is used for separating ethanol and glycolide; the separated glycolide is returned to the glycolide elution tank, and the ethanol is sent to rectification and regeneration after rough adsorption of impurities in a decolorizing tank and is recycled.

[0017] On the basis of the device for capturing, purifying and recycling the by-product glycolide, the application further provides a method for capturing, purifying and recycling the by-product glycolide in the synthesis of polyglycolic acid, which comprises the following steps performed by the device:

[0018] The gas phase from the PGA polymerization reactor enters the gas phase condenser, is cooled under the action of the low-temperature heat medium, and then flows into the condensate collection tank;

[0019] The liquid glycolide condensed in the condensate collection tank enters the glycolide elution tank from the top and is in countercurrent contact with the hot ethanol entering from the bottom;

[0020] The purified glycolide is pumped into the glycolide solidification tank for solidification, and the suspension containing glycolide solid particles enters the decanter, is separated under the action of centrifugal force to obtain glycolide, and is dried to enter the PGA polymerization reactor.

[0021] Preferably, the low-temperature heat medium can be hot water, low-temperature heat-conducting oil or other conventional heat medium, the temperature of the low-temperature heat medium is 75-90°C, preferably 80-85°C, which is a key point for preventing blockage; because the freezing point of glycolide is 84°C and the gas phase glycolide from the reactor is about 210°C, if the temperature of the heat medium is too high, the gas phase glycolide cannot be completely condensed, resulting in escape, and if the temperature of the heat medium is too low, the condensed glycolide can be below the freezing point, causing solidification and blockage.

[0022] The temperature of the hot ethanol is 85-90°C to avoid glycolide solidification and ensure fluidity.

[0023] Preferably, when the high liquid level alarm in the condensate collection tank is triggered, the feed valve and the discharge valve of a group of gas phase condensers A are closed, and the feed valve and the discharge valve of another group of gas phase condensers B are opened; then the nitrogen cut-off valve is opened to charge the gas phase condensers A, after the charging is completed, the tank bottom cut-off valve of the condensate collection tank A connected with the gas phase condensers A is opened to start liquid discharge, when the liquid discharge 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, the pressure is released, and the pressure of the condensate collection tank A is extracted 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 condensers A are ready for the next cycle.

[0024] Preferably, the hot ethanol is discharged from the top of the glycolide elution tank, is transported to the hydrocyclone by an ethanol delivery pump, and is separated into ethanol and glycolide; the separated glycolide is returned to the glycolide elution tank, and the separated ethanol is regenerated by adsorption and rectification and is recycled;

[0025] The hydrocyclone is designed according to the physical properties of ethanol and glycolide, under the action of initial velocity, the mixed solution generates spiral motion, the ethanol with smaller density is concentrated in the middle of the hydrocyclone and is discharged from the upper end, and the glycolide with larger density is concentrated in the lower end of the hydrocyclone. In order to ensure the separation quality, a small amount of ethanol is controlled to flow out, and the glycolide entrains part of the ethanol to return to the washing tank.

[0026] Preferably, the water content of the glycolide separated by the decanter is 30-50%, and the vacuum drying is carried out at 35-45℃ and ≤2kPaA.

[0027] On the basis of the above-mentioned method for capturing, purifying and recycling the by-product glycolide, the application further provides a method for producing polyglycolic acid by direct polymerization, which comprises the steps of capturing, purifying and recycling the by-product glycolide in the synthesis process of the polyglycolic acid.

[0028] The method for producing polyglycolic acid by direct polymerization comprises the following steps:

[0029] (1) adding glycolic acid / glycolic acid methyl ester, a certain amount of catalyst, part of the auxiliary mixture and recycled glycolide into a reactor, and reacting at 190℃ under normal pressure or slightly negative pressure for 60 min or until no distillate is evaporated from the system;

[0030] (2) increasing the vacuum degree of the reaction system (2000-3000 PaA) and reacting for 60 min or until the theoretical water yield of the system reaches more than 98%, to obtain the oligomer of PGA;

[0031] (3) adding other auxiliaries and recycled glycolide to the system, and increasing the temperature of the system to 210-225℃, and reacting for 60 min under vacuum of 2000-3000 PaA to perform pre-polymerization and increase the polymerization degree of the oligomer;

[0032] (4) increasing the vacuum degree of the system to less than 70 PaA for polymerization until the predetermined polymerization degree is reached.

[0033] The above steps can be continuous or intermittent, wherein steps (3) and (4) produce the by-product glycolide, and step (4) is the main step for producing glycolide.

[0034] The glycolide treated according to the method of the application can be returned to the PGA production process to continue to participate in the reaction without affecting the color of the product, avoiding the waste of by-products, and greatly improving the conversion rate of raw materials. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1This is a schematic diagram of the device for collecting, purifying, and reusing glycolide, a byproduct of the synthesis of polyglycolic acid (PGA) in this invention. Detailed Implementation

[0036] like Figure 1 The diagram shows a device for collecting, purifying, and reusing glycolide, a byproduct of polyglycolic acid (PGA) synthesis, provided by this invention. The device includes a gas-phase condenser, a condensate collection tank, a glycolide washing tank, an ethanol external pump, a hydrocyclone separator, an ethanol decolorization tank, a glycolide transfer pump, a glycolide solidification tank, a decanter, a collection tank, and control instruments.

[0037] like Figure 1 As shown, the vapor-phase condenser is placed in the vacuum vapor-phase piping to capture vapor-phase glycolide. The vapor-phase condenser can be a commercially available shell-and-tube heat exchanger. The heat exchange area is calculated to be preferably a single-pass shell-and-tube heat exchanger, with the vapor-phase glycolide flowing through the tubes and the low-temperature heat medium flowing through the shell. A tube diameter of 32-50 mm is preferred to alleviate blockage. The low-temperature heat medium can be conventional heat media such as hot water or low-temperature thermal oil. The preferred temperature for the low-temperature heat medium is 80-85℃, which is crucial to prevent blockage.

[0038] like Figure 1 As shown, the condensate collection tank is used to collect condensed liquid glycolide. The condensate collection tank has a level control system. When the level is high, it is linked with the bottom shut-off valve, top shut-off valve, and nitrogen shut-off valve to isolate the collection tank, pressurize it, and discharge the glycolide into the glycolide washing tank. After drainage, the vacuum shut-off valve is adjusted to switch the condensate collection tank to a negative pressure state. The condensate collection tank is used in conjunction with a vapor phase condenser, and 2-3 sets (two sets are shown in the figure), or even more, can be set as needed.

[0039] like Figure 1 As shown, the glycolide rinsing tank is used to purify liquid glycolide. Liquid glycolide discharged from the condensate collection tank comes into countercurrent contact with recycled hot ethanol, which dissolves colored impurities in the glycolide. Multiple sieves are installed inside the tank to ensure sufficient contact. The preferred number of sieves is 2-3. The temperature of the hot ethanol is preferably 85-90℃ to prevent glycolide from solidifying and to ensure fluidity. Because glycolide is denser than ethanol, the purified glycolide will settle at the bottom of the tank. A boundary level control instrument is located at the bottom of the tank, and the output flow rate is adjusted by controlling the frequency of the glycolide delivery pump.

[0040] Ethanol containing dissolved impurities is pumped out by an ethanol delivery pump. Because ethanol may carry a small amount of glycolide, a hydrocyclone is installed. The hydrocyclone is designed based on the physical properties of ethanol and glycolide and is used to separate them. The separated glycolide is returned to the glycolide washing tank. The ethanol, after undergoing a decolorization tank to roughly remove impurities, is sent for distillation and regeneration for reuse.

[0041] like Figure 1 As shown, the purified glycolide is pumped into a glycolide curing tank by a glycolide transfer pump. The curing tank is equipped with nozzles. The tank is filled with cold water, maintained at a temperature below 30°C, preferably 20-25°C. The glycolide exiting the nozzles rapidly solidifies into fine particles due to the temperature drop and is carried by a large amount of water to the decanter.

[0042] like Figure 1 As shown, a 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. The separated solid glycolide, after vacuum drying, can be returned to the PGA feed section or prepolymerization section to participate in the reaction again. The separated water is sent to subsequent processing sections.

[0043] use Figure 1 The steps for capturing, purifying, and reusing glycolide, a byproduct of polyglycolic acid (PGA) synthesis, using the apparatus shown are as follows:

[0044] The gaseous phase from the PGA polymerization reactor enters the gas phase condenser, where it is cooled by a low-temperature heat transfer medium, and then flows into a condensate collection tank. The condensed liquid glycolate in the collection tank enters the glycolate washing tank from the top, where it comes into countercurrent contact with hot ethanol entering from the bottom. The purified glycolate is pumped to a glycolate curing tank for curing. The suspension containing glycolate solid particles enters a decanter, where glycolate is separated under centrifugal force, dried, and then returned to the PGA polymerization reactor. The low-temperature heat transfer medium can be conventional media such as hot water or low-temperature heat transfer oil, with a temperature of 75-90℃. The hot ethanol is at a temperature of 85-90℃ to prevent glycolate curing and ensure fluidity.

[0045] 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 discharge valve of one set of vapor phase condensers A are closed, and the feed valve and discharge valve of another set of vapor phase condensers B are opened; then the nitrogen shut-off valve is opened to pressurize vapor phase condensers A. After pressurization is completed, the bottom shut-off valve of the condensate collection tank A connected to vapor phase condensers A is opened to start draining liquid. When the draining is completed and the low liquid level alarm is triggered, the bottom shut-off valve of the condensate collection tank A is closed; then the vacuum shut-off valve of the condensate collection tank A is opened to release pressure and draw the pressure of the condensate collection tank A to negative pressure; after reaching the predetermined vacuum degree, the vacuum shut-off valve of the condensate collection tank A is closed, and the condensate collection tank A and the connected vapor phase condensers A are ready to enter the next cycle.

[0046] like Figure 1As shown, the hot ethanol is discharged from the top of the glycolide elution tank, and is transported to the hydrocyclone by the ethanol delivery pump for separation of ethanol and glycolide; the separated glycolide is returned to the glycolide elution tank, and the separated ethanol is recycled after being adsorbed, rectified and regenerated. The hydrocyclone used is designed according to the physical properties of ethanol and glycolide, and under the action of the initial velocity, the mixed solution undergoes spiral motion, and the ethanol with smaller density is concentrated in the middle of the hydrocyclone and is discharged from the upper end; and the glycolide with larger density is concentrated in the lower end of the hydrocyclone. In order to ensure the separation quality, a small amount of ethanol is allowed to flow out, and the glycolide with part of the ethanol is returned to the elution tank.

[0047] According to the above process, the capture, purification and reuse of the by-product glycolide in the specific polyglycolic acid (PGA) synthesis process are as follows: the gas phase material from the PGA polymerization reactor at about 210°C is first introduced into the gas phase condenser A, and is cooled to 87°C under the cooling of the low-temperature heat medium (hot water) at 85°C, and the condensable medium in the gas phase is condensed and separated out, and flows into the lower condensate collection tank A. The non-condensable gas returns to the original vacuum pipeline. The temperature of the low-temperature heat medium out of the condenser is controlled at 89°C.

[0048] As the glycolide in the condensate collection tank A increases, the high liquid level alarm is triggered. The system control closes the feed valve of the gas phase condenser A, the discharge valve of the condenser A, and opens the feed valve of the condenser B, the discharge valve of the condenser B, so that the gas phase condensation is switched to the condenser B.

[0049] Then the system control opens the nitrogen cut-off valve to pressurize the condensate collection tank A. The pressurization pressure is adjusted by the pressure control instrument. After the pressurization is completed, the system control opens the tank bottom cut-off valve of the condensate collection tank A to start liquid discharge. When the low liquid level alarm is triggered after the liquid discharge is completed, the system control closes the tank bottom cut-off valve of the condensate collection tank A. Then the vacuum cut-off valve is opened, the pressure is released, and the pressure of the condensate collection tank A is pumped to the original negative pressure, so as to avoid the impact on the vacuum degree of the main gas phase pipeline in the next use. After reaching the predetermined vacuum degree, the vacuum cut-off valve is closed, and the condensate collection tank A and the gas phase condenser A are ready for the next cycle.

[0050] The condensed liquid glycolide enters the glycolide elution tank from the upper part under the action of nitrogen pressure, and is countercurrently contacted with the hot ethanol entering from the lower part. The temperature of the hot ethanol is about 87°C. In the process of contacting the glycolide with the ethanol, the ethanol can dissolve the colored impurities in the glycolide. After the action of the three layers of sieve plates, most of the colored impurities in the glycolide are dissolved and removed by the ethanol. Due to the density difference, the purified glycolide is deposited at the bottom of the elution tank. There is a level control instrument at the bottom of the tank, which is connected to the frequency conversion motor of the glycolide delivery pump. The delivery flow of the pump is controlled according to the high and low level.

[0051] After purification, glycolide is pumped to the nozzle at the top of the glycolide solidification tank. At the pressure of the pump, glycolide at about 85°C, carrying a small amount of ethanol, is dispersed and sprayed out of the nozzle at high speed. Because the glycolide solidification tank contains a large amount of cold water (about 20°C, much lower than the freezing point of glycolide), the glycolide solidifies rapidly, and the ethanol dissolves in the water. To maintain the temperature in the tank, low-temperature cold water is continuously added.

[0052] 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, the water and glycolide particles are rapidly separated. The glycolide containing about 30-50% water is collected and sent to a vacuum dryer for drying at about 40°C for use. The water discharged from the decanter contains part of the glycolide and ethanol, which is sent to the post-treatment for recycling.

[0053] The ethanol discharged from the glycolide leaching tank contains a small amount of glycolide. It enters the hydrocyclone tangentially. Because the hydrocyclone is designed according to the physical properties of ethanol and glycolide, under the action of the initial velocity, the mixed solution undergoes spiral motion. Ethanol has a smaller density, so it is concentrated in the middle of the hydrocyclone and discharged from the upper end. Glycolide has a larger density, 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 the glycolide carries part of the ethanol back to the leaching tank.

[0054] The separated ethanol passes through the decolorizing tank to roughly remove the entrained impurities. Then it is sent to the post-stage rectification treatment for qualification and recycling.

Claims

1. A device for collecting, purifying and reusing glycolide, a byproduct of polyglycolic acid synthesis, comprising a gas phase condenser, a condensate collection tank, a glycolide rinsing 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 outlet of the vapor phase condenser is connected to the inlet of the condensate collection tank, the outlet of the condensate collection tank is connected to the glycolide washing tank, and the glycolide outlet of the glycolide washing tank is connected to the glycolide curing tank; the glycolide curing tank is equipped with nozzles and is filled with cold water. The condensed liquid glycolate in the condensate collection tank enters the glycolate washing tank from the top and comes into countercurrent contact with the hot ethanol entering from the bottom. The glycolide curing tank is connected to the decanter, and the solid glycolide obtained from the solid outlet of the decanter enters the feeding section or prepolymerization section of polyglycolic acid.

2. The collection, purification, and reuse apparatus according to claim 1, characterized in that: The gas phase condenser is a single-pass shell-and-tube heat exchanger, with gas phase glycolide flowing through the tubes and low-temperature heat medium flowing through the shell.

3. The collection, purification, and reuse apparatus according to claim 1 or 2, characterized in that: The collection, purification and reuse device includes several sets of the condensate collection tanks and the gas phase condenser connected in series; The condensate collection tank is equipped with a level controller, which enables the linkage of the bottom shut-off valve, the top shut-off valve, and the nitrogen shut-off valve of the condensate collection tank according to the liquid level.

4. The collection, purification, and reuse apparatus according to claim 1 or 2, characterized in that: The top of the glycolide rinsing tank is connected to a hydrocyclone separator, and an ethanol delivery pump is provided between the hydrocyclone separator and the glycolide rinsing tank. An glycolide transfer pump is provided between the glycolide rinsing tank and the glycolide curing tank.

5. A method for capturing, purifying, and reusing glycolide, a byproduct of polyglycolic acid synthesis, comprising the following steps using the capture, purification, and reuse apparatus described in any one of claims 1-4: The gas phase from the PGA polymerization reactor enters the gas phase condenser, is cooled by a low-temperature heat medium, and then flows into the condensate collection tank. The condensed liquid glycolate in the condensate collection tank enters the glycolate washing tank from the top and comes into countercurrent contact with the hot ethanol entering from the bottom. The purified glycolide is pumped to the glycolide curing tank for curing. The suspension containing glycolide solid particles enters the decanter, where glycolide is separated under centrifugal force and then dried before entering the PGA polymerization reactor.

6. The method for collection, purification, and reuse according to claim 5, characterized in that: The temperature of the low-temperature heat medium is 75-90℃; The temperature of the hot ethanol is 85-90℃.

7. The method for collection, purification, and reuse according to claim 5 or 6, characterized in that: When the high-level alarm in the condensate collection tank is triggered, the feed valve and discharge valve of one set of vapor phase condensers A are closed, and the feed valve and discharge valve of another set of vapor phase condensers B are opened. Then, the nitrogen shut-off valve is opened to pressurize the vapor phase condenser A. After pressurization, the bottom shut-off valve of the condensate collection tank A connected to the vapor phase condenser A is opened to start draining. When the draining is completed and the low-level alarm is triggered, the bottom shut-off valve of the condensate collection tank A is closed. Then, the vacuum shut-off valve of the condensate collection tank A is opened to release pressure and draw the pressure of the condensate collection tank A to negative pressure. After reaching the predetermined vacuum degree, the vacuum shut-off valve of the condensate collection tank A is closed, and the condensate collection tank A and the connected vapor phase condenser A are ready to enter the next cycle.

8. The method for collection, purification, and reuse according to claim 5 or 6, characterized in that: The hot ethanol is discharged from the top of the glycolide washing tank and transported by an ethanol external pump to a hydrocyclone for separation of ethanol and glycolide; the separated glycolide is returned to the glycolide washing tank, and the separated ethanol is recycled after adsorption and distillation regeneration.

9. The method for collection, purification, and reuse according to claim 5 or 6, characterized in that: The glycolide obtained by the decanter has a water content of 30-50%, and the separated glycolide is vacuum dried at 35-45°C and ≤2kPaA.

10. A method for producing polyglycolic acid by direct polymerization, comprising the steps of collecting, purifying and reusing glycolide, a byproduct of the polyglycolic acid synthesis process according to any one of claims 5-9.

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