A type of polylactic acid (PLA) waste recycling system and its regeneration method

Abstract

This invention relates to a polylactic acid (PLA) waste recycling system and method in the field of environmental protection and recycling technology. The system includes a screw extruder, a depolymerization vessel, an intermediate vessel, a decomposition vessel, a deoxidation tower, a purification system, a pre-polymerization vessel, a polymerization reactor, a defoliation device 1, a defoliation device 2, a granulation system, and a vacuum system. The outlet of the screw extruder is connected to the inlet of the depolymerization vessel, the depolymerization vessel is connected to the intermediate vessel, and the outlet of the intermediate vessel is connected to the feed port of the decomposition vessel via two transport pumps. The gas phase outlet of the decomposition vessel is connected to the gas phase inlet at the bottom of the deoxidation tower, a hammerhead condenser is installed at the top of the deoxidation tower, and the outlet of the deoxidation tower is connected to the purification system. The purification system is connected to the polymerization reactor via a pre-polymerization vessel, and the polymerization reactor is connected to a granulator via defoliation devices 1 and 2. This apparatus is used for recycling polylactic acid (PLA) waste, and it has high recycling efficiency and can produce high molecular weight polylactic acid (PLA). On the other hand, the depolymerization process employs a unique catalyst, lowering the depolymerization and decomposition temperatures, reducing energy consumption, and resulting in fewer racemization byproducts in the lactide product.

Description

Technical Field

[0001] The present invention relates to the field of waste environmental protection and recycling technology, and particularly relates to a polylactic acid (PLA) waste recycling and regeneration system and its regeneration method.

Background Art

[0002] Polylactic acid (PLA) is an aliphatic thermoplastic polymer raw material made by polymerization with lactic acid as the main raw material. It has excellent biodegradability and biocompatibility, and can be completely decomposed into carbon dioxide and water by compost or the action of natural microorganisms. However, from the perspective of recycling, completely decomposing polylactic acid (PLA) waste into carbon dioxide and water is a waste of certain resources. Therefore, how to efficiently recycle polylactic acid (PLA) waste has become an important research topic. Currently, the method that is often studied is to thermally decompose and depolymerize polylactic acid (PLA) waste to obtain lactide. For example, Chinese Patent No. 201910837292.7 provides an integrated polylactic acid (PLA) recycling device and a polylactic acid (PLA) recycling method. The integrated polylactic acid (PLA) recycling device includes a casing, a single-screw extruder disposed in the casing, a pre-polymerization kettle, a heater, an evaporator, a depolymerization kettle, a flash tank, a condenser, a receiving tank, a finished product tank, a finished product pump, a vacuum unit, and a temperature control device. The single-screw extruder, the pre-polymerization kettle, the heater, and the evaporator are connected in sequence. The evaporator is provided with a primary outlet and a liquid outlet. The primary outlet is connected to the flash tank by a pipe, and the liquid outlet is connected to the depolymerization kettle by a pipe. The depolymerization kettle is provided with a circulation pipeline and a secondary outlet. The secondary outlet is connected to the flash tank by a pipe. After the flash tank, a receiving tank and a receiving tank are connected in sequence. The receiving tank in the previous period is provided with a discharge port and a vacuum port. The discharge port is connected to the finished product tank, and the vacuum port is connected to the vacuum device by a pipe. This apparatus, when recycling polylactic acid (PLA), activates a heating device and a single-screw extruder. The recycled PLA material is fed into the single-screw extruder and extruded to a pre-polymerization vessel. A catalyst is added to the pre-polymerization vessel, and ring-opening and chain-breaking are performed to obtain a molten product with a set molecular weight. The molten product is then placed in a heater to further increase its temperature and transported to an evaporator. In the evaporator, the molten product undergoes a preliminary depolymerization reaction under vacuum to obtain primary lactide. The primary lactide is sent to a flash tank to complete primary evaporation. The incompletely reacted molten product is then sent to a depolymerization vessel for continued circulating depolymerization under vacuum. The secondary lactide produced by circulating depolymerization is sent to a flash tank to complete secondary evaporation. After the lactide recovered in the flash tank is cooled in a condenser, it is transported to a receiving tank and finally to a finished product tank. The preliminary depolymerization conditions for this method were 170-180°C and -0.1 MPa to -0.096 MPa, and the cyclic depolymerization conditions were 175-185°C and -0.1 MPa to -0.096 MPa, which produced lactide. The drawbacks are: (1) the need for a vacuum during preliminary depolymerization increases the cost of the equipment and makes it prone to pipeline clogging; (2) the high viscosity of the system results in low depolymerization efficiency; and (3) the insufficient productivity of the depolymerization equipment leads to low lactide yield and purity. [Overview of the Initiative]

[0003] The first objective of the present invention is to overcome the problems of the prior art and provide a polylactic acid (PLA) waste recycling system and regeneration method, thereby achieving low energy consumption in the polylactic acid (PLA) waste recycling process, and to enable the system to have a low depolymerization temperature, high recycling efficiency, and yield high molecular weight PLA.

[0004] To solve the above technical problems, the present invention provides a polylactic acid (PLA) waste recycling system comprising a screw extruder, a depolymerization vessel, a decomposition vessel, and a vacuum system. The outlet of the screw extruder is connected to the inlet of the depolymerization vessel, the bottom outlet of the depolymerization vessel is connected to the bottom inlet of one end of a horizontally installed intermediate vessel via a transport pump 1, the bottom outlet of the other end of the intermediate vessel is connected to the feed port of the decomposition vessel via a transport pump 2, a post-pump locomotive, and a preheater 1, and the bottom outlet of the decomposition vessel passes through a transport pump 3 and then runs through three circuits: one circuit is connected to the post-pump locomotive inlet, another circuit is connected to the reuse port, and the remaining circuit is connected to the vessel residue discharge port. The gas phase outlet of the intermediate part of the decomposition vessel is connected to the gas phase inlet at the bottom of the decarburization tower. A hammerhead condenser is provided at the top of the decarburization tower. The outlet of the hammerhead condenser is connected to a light component tank via a condenser 2. The condensate outlet at the bottom of the decarburization tower is connected to the purification system via a transport pump 4 and a preheater. The deoxidation tower has at least one packing layer, with an air outlet below the packing layer, which is connected to a hammerhead condenser. The outlet of the purification system is connected to a pre-polymerization vessel via a preheater 1. The outlet at the bottom of the pre-polymerization vessel is connected to the inlet at the bottom of the polymerization reactor via a transport pump 5. The outlet at the top of the polymerization reactor is connected to the inlet at the top of the devolatilization unit 1, the outlet at the bottom of the devolatilization unit 1 is connected to the top of the devolatilization unit 2 via a transport pump 6, and the outlet at the bottom of the devolatilization unit 2 is connected to the granulator via a transport pump 7. The devolatilization apparatus 1 and devolatilization apparatus 2 are each provided with gas-phase outlets on their sides, and these gas-phase outlets communicate with the vacuum system via corresponding condensers 3 and 4. The intermediate kettle is equipped with multiple agitators. Each agitator has a vertically mounted rotating shaft. A porous slow-flow partition plate is vertically installed between adjacent agitators. A heatable flowing membrane is installed inside the decomposition kettle and devolatilization apparatus, and the raw material moves from top to bottom along the flowing membrane. A partition plate is installed inside the kettle body of devolatilization apparatus 2, and multiple raw material distributors are installed on this partition plate, with a cavity below the partition plate. The feed port of devolatilization apparatus 2 is connected to the aforementioned raw material distributors. The raw material distributors include an inner support plate. A liquid distribution packing is installed above the raw material distributors, and the inner support plate has multiple drop holes. The gas-phase outlet of devolatilization apparatus 2 is located on the side of the kettle below the partition plate. In this invention, when manufacturing, polylactic acid (PLA) waste is extruded into a depolymerization vessel by a screw extruder, fatty alcohol and catalyst are added to the depolymerization vessel, and the mixture is allowed to accumulate to a certain liquid level to obtain a molten material. This molten material is then introduced into an intermediate vessel and, while being stirred, is fed from the bottom outlet through a transport pump 2, a post-pump loki and a preheater to the decomposition vessel's feed port. Decomposition allows for easier extraction of lactide from the viscous raw material. Low-boiling point impurities in the oligomer also become vapor and, together with the lactide, exit from the gas phase outlet on the side of the decomposition vessel, entering a deoxidation column, thereby shortening the thermal residence time of the raw material. The top of the deoxidation column is a hammerhead condenser, which shortens the residence time between the top of the gas phase column and the condenser, reducing pressure loss and aiding in the preparation and purification of lactide for heat-sensitive materials. The raw material at the bottom of the deoxidation column enters a purification unit, where, after purification, it is first prepolymerized, then polymerized again, secondary defoliation is performed, and finally, it is cut into granules. Compared with existing technologies, this invention has achieved the following beneficial effects. This apparatus boasts high recovery efficiency and can produce high molecular weight polylactic acid (PLA). Simultaneously, it employs a unique catalyst for depolymerization, lowering the depolymerization and decomposition temperatures, reducing energy consumption, and minimizing racemization byproducts in the lactide product. Furthermore, the boiler body of the decomposition boiler and daphne generator 1 has partition plates 1, 2 and a distribution disc installed from top to bottom. Inside the boiler body, a first chamber is formed above partition plate 1, a second chamber is formed between partition plate 1 and partition plate 2, a third chamber is formed between partition plate 2 and the distribution disc, a fourth chamber is formed below the distribution disc, and a gas phase outlet corresponding to the side of the fourth chamber is installed. The feed port at the top of the boiler body is connected to a claw distributor, and the multiple outlets of the claw distributor pass through partition plates 1 and 2 and are connected to the third chamber. An inlet for the heat transfer medium connected to the first chamber is provided at the top of the boiler body, and an outlet for the heat transfer medium is provided on the side of the second chamber. Numerous flow diaphragm tubes are installed vertically in the boiler body, and the flow diaphragm tubes include an inner tube and an outer tube installed coaxially. The inner tube is a straight tube that goes through from top to bottom, and the outer tube is a blind tube with an open top and a closed bottom. The upper port of the inner tube passes through partition plate 1 and connects to the first chamber. The lower opening of the inner tube is inserted into the lower inside of the outer tube. The distribution disk is provided with numerous flow-through membrane holes, and the outer tube passes through these flow-through membrane holes with gaps between them. The upper opening of the outer tube passes through partition plate 2 and connects to the second chamber. A further improvement of the present invention is that the corresponding descending membrane in the decomposition vessel has several weirs at different heights, with gaps between the weirs and the descending membrane tube. The oligomer raw material falls onto the weirs, and then the liquid flows downward in a membrane form. On the one hand, this solves the problem of the raw material descending too quickly, and on the other hand, it solves the problem of uneven distribution of the raw material in the descending membrane tube, such as uneven flow. A further improvement of the present invention is that the height of the corresponding flowing membrane tube for the devolving agent 1 is half the height of the fourth chamber. Further improvements to the present invention include a packing in the deoxidation tower that includes an upper packing and a lower packing, and corresponding outlet ports that include air outlet 1 and air outlet 2. Air outlet 1 is located in the tower body between the upper and lower packings, and air outlet 2 is located in the tower body below the lower packing, and air outlets 1 and 2 are connected to a hammerhead condenser. The two air outlets allow for more rapid condensation of the raw material. Furthermore, a reduction is provided at the bottom of the deoxidation tower, which reduces the amount of liquid held, shortens the residence time of the raw material, and reduces the possibility of side reactions occurring when the raw material is at high temperatures. Further improvements to the present invention include providing gas-phase ports at the top of the depolymerization vessel and intermediate vessel, connected to the gas-phase inlet of the reflux column; connecting the liquid-phase outlet at the bottom of the reflux column to the depolymerization vessel and / or intermediate vessel for reflux; connecting the top of the reflux column to the reflux tank after passing through the condenser 1; and drawing two circuits from the outlet of the reflux tank, one circuit to the reflux column and the other circuit to the receiving tank I. A further improvement of the present invention is that the condensates from condenser 3 and condenser 4 are connected to receiving tank 2 and receiving tank 3. The present invention provides a method for recycling waste polylactic acid (PLA), which is processed by the above-described waste polylactic acid (PLA) recycling system in the following procedure. (1) The catalyst is placed in the depolymerization vessel in advance. The catalyst consists of a fatty alcohol and an organic compound. The fatty alcohol is one or more of propylene glycol, ethylene glycol, and butylene glycol. The catalyst is one or more of stannous octanoate, stannous lactate, organic guanidine complex CRZnCl2, and organic guanidine complex CRFe(OAc)2. The weight ratio of the organic compound to the fatty alcohol is 0.2-2%, and the weight ratio of the catalyst to the polylactic acid (PLA) waste is 2-10%. (2) Start the screw extruder and push the polylactic acid (PLA) waste into the depolymerization vessel, where it is depolymerized at 170-180°C under normal pressure. (3) The material is fed from the bottom of the depolymerization vessel into an intermediate vessel at 150-170°C; (4) The raw materials from the intermediate kettle enter the decomposition kettle at 170-190°C through the outlet at the bottom, where a decomposition reaction takes place at an absolute pressure of 0.1-1 kPa. (5) The gaseous raw material from the decomposition vessel enters the deoxidation column, where light components are removed. The condensation temperature in the hammerhead condenser is 90-100°C, the condensation temperature in condenser 2 is 80-90°C, and the temperature at the bottom of the deoxidation column is 100-130°C. The raw material at the bottom of the deoxidation column then enters the purification system. (6) The purified lactide product is placed in a prepolymerization vessel and polymerized into a prepolymer at 150-170°C; (7) The prepolymer raw materials are placed in a polymerization reactor and polymerized at 170-180°C; (8) The polymer raw material enters devolatilizer 1 and devolatilizer 2, and secondary devolatilization is performed at 200-220°C; (9) After volatilization, the product is cut into granules using a granulator. Furthermore, the raw materials at the bottom of the decomposition vessel are either returned to the decomposition vessel for recirculation decomposition, returned to the depolymerization vessel or intermediate vessel for depolymerization, or discharged as vessel residue. This method has the advantages of low energy consumption and high production efficiency. [Brief explanation of the drawing]

[0005] [Figure 1] Figure 1 is a flowchart of the polylactic acid (PLA) waste recycling system of the present invention. [Figure 2] Figure 2 is a schematic diagram showing the structure of the daphne generator 1. [Figure 3] Figure 3 is a top view of the daphne generator 1 shown in Figure 2. [Figure 4] Figure 4 is a distribution diagram of the flowing diaphragm tubes in the devolatilization apparatus 1 shown in Figure 2. [Figure 5] Figure 5 is a schematic diagram showing the structure of the daphne generator 2. [Figure 6] Figure 6 is a top view of the daphne generator 2 shown in Figure 4. [Modes for carrying out the invention]

[0006] Example 1 As shown in Figure 1, this is a waste polylactic acid (PLA) recycling system consisting of a screw extruder 3, a depolymerization vessel 2, a decomposition vessel 29, and a vacuum device. The screw extruder 3 is a single-screw extruder, and its outlet is connected to the inlet of the depolymerization vessel 2. The outlet at the bottom of the depolymerization vessel 2 is connected to the inlet at one end of a horizontally installed intermediate vessel 32 via a transport pump 1. The outlet at the other end of the intermediate vessel 32 is connected to the inlet of the decomposition vessel 29 via a transport pump 2.30, a post-pump locator 31, and a preheater 1.8. The bottom outlet of the decomposition vessel 29 passes through a transport pump 3.33 and then runs through three circuits: one circuit is connected to the inlet of the post-pump locator 31, another circuit is connected to a reuse port and then to the depolymerization vessel 2 or intermediate vessel 32, and the remaining circuit is connected to the vessel residue discharge port. The gas phase outlet in the middle of the decomposition vessel 29 is connected to the gas phase inlet at the bottom of the deoxidation tower 9. A hammerhead condenser 10 is installed at the top of the deoxidation tower 9, and the outlet of the hammerhead condenser 10 is connected to the light component tank 12 via the condenser 2 11. The condensate outlet at the bottom of the deoxidation tower 9 is connected to the purification unit 26 via the transport pump 4 28 and the preheater 3 27. Inside the deoxidation tower 9, there is an upper packing and a lower packing, and corresponding outlet ports have air outlets 1 and 2. Air outlet 1 is located in the tower between the upper and lower packings, and air outlet 2 is located in the tower below the lower packing. Air outlets 1 and 2 are connected to the hammerhead condenser 10. A reduction is installed at the bottom of the deoxidation tower 9. The air outlet of the deoxidation tower is connected to the hammerhead condenser 10, and the outlet of the purification unit 26 is connected to the pre-polymerization vessel 14 via the preheater 2 13. The bottom outlet of the pre-polymerization vessel 14 is connected to the bottom inlet of the polymerization reactor 15 via the transport pump 5 25, the top outlet of the polymerization reactor 15 is connected to the top inlet of the devolatilization unit 1 16, and the bottom outlet of the devolatilization unit 1 16 is connected to the top inlet of the devolatilization unit 2 21 via the transport pump 6 24. The bottom outlet of the devolatilization unit 2 21 is connected to the granulator 22 via the transport pump 7 23. The devolatilization units 1 16 and 2 21 each have gas phase outlets on their sides, and these gas phase outlets are connected to the vacuum system via the corresponding condensers 3 17 and 4 19. The intermediate kettle 32 is equipped with multiple stirring devices, the rotating shafts of the stirring devices are installed vertically, and porous slow-flow partition plates are installed vertically between adjacent stirring devices. The decomposition vessel 29 and the devolatilization apparatus 116 are equipped with a heatable flow diaphragm tube, and the raw material moves from top to bottom along the descending tube. As shown in Figures 5 and 6, the kettle body 2107 of the devolatilization device 2 is provided with a partition plate 2105, and multiple material distributors 2103 are installed on the partition plate 2105, with a cavity beneath the partition plate 2105. The feed port 2102 of the devolatilization device 2 is connected to the material distributor 2103, which includes an inner support plate 2101, and a liquid distributor / filler 2104 is installed on top of the inner support plate 2101. In addition, multiple material drop holes are provided in the inner support plate 2101, and a gas phase outlet 2106 of the devolatilization device 2 is provided on the side of the kettle body 2107 of the devolatilization device 2 below the partition plate 2105. The decomposition kettle 29 and the daphne generator 1 16 have almost the same internal structure. As shown in Figures 2, 3, and 4, the structure of the daphne generator 1 is such that the kettle body 1602 of the daphne generator 1 is equipped with partition plates 1 1605, 2 1604, and a distribution disc 1603 from top to bottom. The kettle body has a first chamber above partition plate 1 1605, a second chamber between partition plate 1 1605 and partition plate 2 1604, and a third chamber between partition plate 2 1604 and the distribution disc 1603. A fourth chamber is formed below the distribution disc 1603, and the corresponding gas phase outlet 1611 of the daphne generator 1 is located on the side of the fourth chamber. The feed port 1607 of the daphne generator, located at the top of the kettle body 1602 of the daphne generator 1, is connected to a claw-type distributor 1610. Multiple outlets of the claw-type distributor 1610 are connected to the third chamber by passing through partition plates 1 1605 and 2 1604. An inlet 1608 for the heat transfer medium connected to the first chamber is provided at the top of the boiler body, and an outlet 1609 for the heat transfer medium is provided on the side of the second chamber. Several flow membrane tubes 1601 are installed vertically in the boiler body, and the flow membrane tubes 1601 include an inner tube and an outer tube installed coaxially. The inner tube is a straight tube that goes through from top to bottom, and the outer tube is a blind tube that is open at the top and closed at the bottom. The upper opening of the inner tube is connected to the first chamber by passing through partition plate 1 1605, and the lower opening of the inner tube is inserted inside and below the outer tube, and several flow membrane holes are provided in the distribution disk 1603. The outer tube passes through the flow membrane holes with a gap, and the upper opening of the outer tube is connected to the second chamber by passing through partition plate 2 1604. The differences between the decomposition vessel 29 and the devolatilization device 1 16 are as follows. The corresponding descending membrane of the decomposition vessel 29 has a gap between the surrounding weir and the descending membrane. The oligomer raw material falls onto the weir, and then the liquid flows downward in a film. On the one hand, this solves the problem of the raw material descending at too fast a speed, and on the other hand, it solves the problem of uneven distribution of the raw material in the descending membrane. Since there is no surrounding weir outside the corresponding descending membrane of the devolatilization device 1 16, the height of the descending membrane of the devolatilization device 1 16 is half the height of the fourth chamber, and space for storing the raw material inside can be secured. In Figure 1, gas-phase ports connected to the gas-phase inlets of the reflux column 4 are provided at the tops of the depolymerization vessel 2 and the intermediate vessel 32, and the liquid-phase outlet at the bottom of the reflux column 4 is connected to reflux to the depolymerization vessel 2 and / or the intermediate vessel 32. The top of the reflux column 4 is connected to the reflux tank 6 via the condenser I5, and two circuits are drawn from the outlet of the reflux tank 6, one circuit connected to the reflux column 4 and the other circuit connected to the receiving tank I7. The condensates from condenser 317 and condenser 419 are connected to receiving tanks 318 and 20. A polylactic acid (PLA) waste recycling system method. The polylactic acid (PLA) waste is recycled using the following procedure. (1) A catalyst consisting of fatty alcohols and organic compounds is placed in the depolymerization vessel 2 beforehand. The fatty alcohols are one or more of propylene glycol, ethylene glycol, and butylene glycol. The catalyst is one or more of stannous octanoate, stannous lactate, organic guanidine complex CRZnCl2, and organic guanidine complex CRFe(OAc)2. The weight ratio of the organic compound to the fatty alcohol is 0.2-2%, and the weight ratio of the catalyst to the polylactic acid (PLA) waste is 2-10%. (2) Start the screw extruder 3 and push the polylactic acid waste into the depolymerization vessel 2, and depolymerize it at atmospheric pressure and 170-180°C; (3) The raw materials are introduced from the bottom of the depolymerization vessel 2 into an intermediate vessel 32 at 150-170°C via a pump. (4) The raw materials from the intermediate kettle 32 enter the decomposition kettle 29 at 170-190°C through the outlet at the bottom, where a decomposition reaction takes place at an absolute pressure of 0.1-1 kPa. (5) The gaseous raw material from the decomposition vessel 29 enters the deoxidation column 9, where light components are removed. The condensation temperature in the hammerhead condenser 10 is 90-100°C, the condensation temperature in the condenser 2 11 is 80-90°C, the temperature at the bottom of the deoxidation column 9 is 100-130°C, and the raw material at the bottom of the deoxidation column 9 enters the purification unit 26. (6) The purified lactide product is placed in the prepolymerization vessel 14 and polymerized into a prepolymer at 150-170°C. (7) The prepolymer material is placed in the polymerization reactor 15 and polymerized at 170-180°C; (8) The polymer raw material is placed in defoliation apparatus 1 16 and defoliation apparatus 2 21 at 200-220°C and subjected to secondary defoliation. (9) The product after thawing is cut into granules by the granulator 22. Furthermore, the raw materials at the bottom of the decomposition vessel 29 are returned to the decomposition vessel 29 for cyclic decomposition, or returned to the depolymerization vessel or intermediate vessel for depolymerization, or discharged as vessel residue. This apparatus boasts high recycling efficiency and can produce high molecular weight polylactic acid (PLA). Simultaneously, it employs a unique catalyst for depolymerization, lowering the depolymerization and decomposition temperatures, reducing energy consumption, and producing fewer racemization byproducts in the lactide product.

[0007] Example 2 Using the system of Example 1 and changing the relevant parameters, different recycling processes were carried out according to the following procedures. Start the screw extruder 3, set the processing temperature to 170 - 230 °C, and start feeding the polylactic acid (PLA) waste raw material. The used polylactic acid (PLA) waste is one or several of polylactic acid (PLA) sheet products, polylactic acid (PLA) fiber and non-woven fabric products, polylactic acid (PLA) film products, polylactic acid (PLA) end materials, and polylactic acid (PLA) by-products. The extruded molten material enters the depolymerization kettle 2, and the chain flow ratio of the catalyst to the molten material is 10%. The catalyst consists of ethylene glycol, butylene glycol, organic guanidine complex CRZnCl2, and organic guanidine complex CRFe(OAc)2. The organic guanidine complex CRZnCl2 accounts for 0.5% of the total amount of diols, and the temperature of the depolymerization kettle 2 is controlled at 170 - 180 °C. After satisfying the residence time of the raw material of 2 - 3 hours by controlling the flow rate, the raw material enters the intermediate kettle 32 from the bottom of the depolymerization kettle 2, and the temperature of the intermediate kettle 32 is controlled at 150 - 160 °C. The raw material in the intermediate kettle 32 enters the decomposition kettle 29, the temperature of the decomposition kettle 29 is controlled at 170 - 180 °C, and the vacuum degree is controlled at 0.1 - 1 kPa. 0.1 - 1% of the feed amount of the decomposition kettle 29 is discharged through the transport pump of the decomposition kettle 29. The temperature at the bottom of the deacidification tower 9 is 110 - 120 °C, the condensation temperature of the hammerhead condenser 10 is 90 - 100 °C, the temperature of the condenser 2 11 is 80 - 90 °C, and the bottom temperature of the deacidification tower 9 is 100 - 130 °C. After passing through the purification system 26, the product enters the pre-polymerization kettle 14 at 150 - 170 °C, the polymerization reactor 15 at 170 - 180 °C, the devolatilization device 1 16 at 200 - 220 °C, and the devolatilization device 2 21 in sequence for devolatilization, and a polylactic acid (PLA) data weight average molecular weight of 126729 g / mol is obtained.

[0008] Example 3 Using the system from Example 1, different recycling processes were carried out by changing the relevant parameters. The procedure is as follows: The screw extruder 3 is started, the processing temperature is set to 170-230°C, and the input of polylactic acid (PLA) waste raw materials is started. The polylactic acid (PLA) waste used is one or more of the following: polylactic acid (PLA) sheet products, polylactic acid (PLA) fibers and nonwoven fabric products, polylactic acid (PLA) film products, polylactic acid (PLA) scraps, and polylactic acid (PLA) by-products. The extruded molten material enters the depolymerization vessel 2, where the chain flow ratio of catalyst to molten material is 5%, and the catalyst consists of propylene glycol and stannous octanoate, with organic stannous octanoate accounting for 1% of the total diol amount. The temperature of the depolymerization vessel 2 is controlled to 170-180°C. After satisfying a residence time of 3-4 hours for the raw materials by controlling the flow rate, the raw materials enter the intermediate vessel 32 from the bottom of the depolymerization vessel 2, and the temperature of the intermediate vessel 32 is controlled to 160-170°C. The raw materials from the intermediate vessel 32 enter the decomposition vessel 29, and the temperature of the decomposition vessel 29 is controlled to 180-190°C, and the vacuum is controlled to 0.1-1 kPa. 0.1-1% of the feed volume of the decomposition vessel 29 is discharged via the transport pump of the decomposition vessel 29. The temperature at the bottom of the deoxidation column 9 is 110-120°C, the condensation temperature of the hammerhead condenser 10 is 90-100°C, the temperature of the condenser 2 11 is 80-90°C, and the bottom temperature of the deoxidation column 9 is 100-130°C. After passing through the purification system 26, the product was sequentially defoliated in a pre-polymerization vessel 14 at 150-170°C, a polymerization reactor 15 at 170-180°C, and defoliation apparatuses 1 16 and 2 21 at 200-220°C to obtain polylactic acid (PLA) with a weight-average molecular weight of 185076 g / mol.

[0009] Example 4 Using the system of Example 1, the relevant parameters were changed to implement different recycling processes. The procedure is as follows. Start the screw extruder 3, set the processing temperature to 170 - 230 °C, and start feeding the polylactic acid (PLA) waste raw material. The polylactic acid (PLA) waste used is one or several of polylactic acid (PLA) sheet products, polylactic acid (PLA) fiber and non-woven fabric products, polylactic acid (PLA) film products, polylactic acid (PLA) trim materials, and polylactic acid (PLA) by-products. The extruded molten material enters the depolymerization kettle 2, and the chain flow ratio of the catalyst to the molten material is 2%. The catalyst consists of butylene glycol, stannous lactate, and organic guanidine complex CRFe(OAc)2, and stannous octoate and organic guanidine complex CRFe(OAc)2 account for 2% of the total amount of butylene glycol. Control the temperature of the depolymerization kettle 2 at 170 - 180 °C, and control the flow rate to satisfy the residence time of the raw material of 2 - 3 hours. Then, the raw material enters the intermediate kettle 32 from the bottom of the depolymerization kettle 2, and control the temperature of the intermediate kettle 32 at 150 - 160 °C. The raw material in the intermediate kettle 32 enters the decomposition kettle 29, control the temperature of the decomposition kettle 29 at 180 - 190 °C, control the vacuum degree at 0.1 - 1 kPa, and discharge 0.1 - 1% of the feed amount of the decomposition kettle 29 from the bottom of the decomposition kettle 29. The temperature at the bottom of the deacidification tower 9 is 100 - 110 °C, the condensation temperature of the hammerhead condenser 10 is 90 - 100 °C, the temperature of the condenser 2 11 is 80 - 90 °C, and the bottom temperature of the deacidification tower 9 is 100 - 130 °C. After passing through the purification system 26, the product enters the preliminary polymerization kettle 14 at 150 - 170 °C, the polymerization reactor 15 at 170 - 180 °C, the devolatilization device 1 16 at 200 - 220 °C, and the devolatilization device 2 21 in sequence for devolatilization, and a polylactic acid (PLA) data weight average molecular weight of 193105 g / mol is obtained.

[0010] Example 5 Using the system from Example 1, different recycling processes were carried out by changing the relevant parameters. The procedure is as follows: The screw extruder 3 is started, the processing temperature is set to 170-230°C, and the input of polylactic acid (PLA) waste raw materials is started. The polylactic acid (PLA) waste used was one or more of the following: polylactic acid (PLA) sheet products, polylactic acid (PLA) fibers and nonwoven fabric products, polylactic acid (PLA) film products, polylactic acid (PLA) scraps, and polylactic acid (PLA) by-products. The extruded molten material enters the depolymerization vessel 2, where the chain flow ratio of catalyst to molten material is 8%, and the catalyst consists of butylene glycol and the organic guanidine complex CRFe(OAc)2, with the organic guanidine complex CRFe(OAc)2 accounting for 0.2% of the total amount of butylene glycol. The temperature of the depolymerization vessel 2 is controlled to 170-180°C, and the flow rate is controlled to satisfy a residence time of 5-6 hours for the raw materials. Then, the raw materials enter the intermediate vessel 32 from the bottom of the depolymerization vessel 2, and the temperature of the intermediate vessel 32 is controlled to 150-160°C. The raw materials from the intermediate vessel 32 enter the decomposition vessel 29, and the temperature of the decomposition vessel 29 is controlled to 180-190°C, the vacuum level is controlled to 0.1-1 kPa, and 0.1-1% of the feed volume of the decomposition vessel 29 is discharged from the bottom of the decomposition vessel 29. The temperature at the bottom of the deoxidation tower 9 is 110-120°C, the condensation temperature of the hammerhead condenser 10 is 90-100°C, the temperature of the condenser 2 11 is 80-90°C, and the bottom temperature of the deoxidation tower 9 is 100-130°C. After passing through the purification system 26, the product was sequentially defoliated in a pre-polymerization vessel 14 at 150-170°C, a polymerization reactor 15 at 170-180°C, and defoliation apparatuses 1 16 and 2 21 at 200-220°C to obtain polylactic acid (PLA) with a relative molecular mass of 198054 g / mol.

[0011] The above are merely good feasible embodiments of the present invention and are not intended to limit the scope of patent protection of the present invention. In addition to the embodiments described above, the present invention may have other embodiments, and all technical solutions formed by the use of equivalent substitution or equivalent transformation fall within the scope of patent protection of the present invention. Technical features of the present invention not described can be realized by or by use of the prior art and are omitted here. [Explanation of Symbols]

[0012] 1 Transport pump 1, 2 Depolymerization kettle, 3 Screw extruder, 4 Reflux column, 5 Condenser 1, 6 Reflux tank, 7 Receiving tank 1, 8 Preheater 1, 9 Deoxidation column, 10 Hammerhead condenser, 11 Condenser 2, 12 Light component tank, 13 Preheater 2, 14 Pre-polymerization kettle, 15 Polymerization reactor, 16 Devolving unit 1, 17 Condenser 3, 18 Receiving tank 2, 19 Condenser 4, 20 Receiving tank 3, 21 Devolving unit 2, 22 Granulator, 23 Transport pump 7, 24 Transport pump 6, 25 Transport pump 5, 26 Purification system, 27 Preheater 3, 28 Transport pump 4, 29 Decomposition kettle, 30 Transport pump 2, 31 Post-pump locator, 32 Intermediate kettle, 33 Transport pump 3. 1601 Downflow membrane tube, 1602 Cauldron body of devolatilizer 1, 1603 Distribution disk, 1604 Partition plate 2, 1605 Partition plate 1, 1607 Feed port of devolatilizer 1, 1608 Inlet of heat transfer medium, 1609 Outlet of heat transfer medium, 1610 Claw-type distributor, 1611 Gas phase outlet of devolatilizer 1. 2101 Inner support plate, 2102 Feed port of devolatilization device 2, 2103 Material distributor, 2104 Liquid distribution packing, 2105 Partition plate, 2106 Gas phase outlet of devolatilization device 2, 2107 Kettle body of devolatilization device 2.

Claims

1. This is a polylactic acid (PLA) waste recycling system comprising a screw extruder, a depolymerization vessel, a decomposition vessel, and a vacuum system. The outlet at the bottom of the depolymerization vessel is connected via a transport pump to the inlet at one end of a horizontally installed intermediate vessel. The outlet at the other end of the intermediate vessel is connected to the feed port of the decomposition vessel via transport pump 2, a post-pump loki, and a preheater 1. The outlet at the bottom of the decomposition vessel passes through transport pump 3 and then runs through three circuits: one circuit is connected to the post-pump loki, another to the reuse port, and the remaining one to the vessel residue discharge port. The gas phase outlet in the middle of the decomposition vessel is connected to the gas phase inlet at the bottom of the decarburization tower. A hammerhead condenser is installed at the top of the decarburization tower. - The outlet of the hammerhead condenser is connected to the light component tank via condenser 2, the outlet of the condensed water at the bottom of the decarburization tower is connected to the purification system via transport pump 4 and preheater 3, at least one packing layer is installed inside the deoxidation tower, an air outlet is installed below the packing layer, the air outlet is connected to the hammerhead condenser, the outlet of the purification system is connected to the pre-polymerization vessel via preheater 2, the bottom outlet of the pre-polymerization vessel is connected to the bottom inlet of the polymerization reactor via transport pump 5, the top outlet of the polymerization reactor is connected to the top inlet of devolatilizer 1, the bottom outlet of devolatilizer 1 is connected to the top inlet of devolatilizer 2 via transport pump 6, and the bottom outlet of devolatilizer 2 is connected to the granulator via transport pump 7. Vapor-phase outlets are installed on the sides of devolatilizer 1 and devolatilizer 2, respectively, and these vapor-phase outlets are connected to the vacuum system via corresponding condensers 3 and 4. Numerous stirring devices are installed inside the intermediate kettle, the rotating shafts of the stirring devices are installed vertically, and porous slow-flow partitions are installed vertically between adjacent stirring devices. The decomposition vessel and the daphne apparatus 1 are equipped with a heatable flow membrane tube, and the raw material moves from top to bottom along the flow membrane tube. A polylactic acid (PLA) waste recycling system characterized in that a partition plate is installed inside the kettle body of the devolatilization device 2, several raw material distributors are installed on the partition plate, there is a cavity below the partition plate, the feed port of the devolatilization device 2 is connected to the raw material distributors, the raw material distributors include an inner support plate, a liquid distribution packing is provided on the inner support plate, the inner support plate has a plurality of drop holes, and the gas phase outlet of the devolatilization device 2 is provided on the side of the kettle body below the partition plate.

2. The decomposition kettle and davoltaic device 1 have partition plates 1, 2 and a distribution disc installed from top to bottom inside the kettle body, forming a first chamber above partition plate 1, a second chamber between partition plate 1 and partition plate 2, a third chamber between partition plate 2 and the distribution disc, and a fourth chamber below the distribution disc. The corresponding gas phase outlet is provided on the side of the fourth chamber. The feed port at the top of the kettle body is connected to a claw-type distributor, and the numerous outlets of the claw-type distributor extend downwards through partition plates 1 and 2 to connect to the third chamber. The top of the kettle body has an inlet for the heat transfer medium connected to the first chamber, and the side of the second chamber... The polylactic acid (PLA) waste recovery system according to claim 1, characterized in that a heat transfer medium outlet is provided in the section, several flow membrane tubes are vertically installed in the main body of the boiler, the flow membrane tubes include an inner tube and an outer tube installed coaxially, the inner tube is a straight tube that goes through from top to bottom, the outer tube is a blind tube with an open top and a closed bottom, the upper opening of the inner tube is connected to the first chamber by passing through a partition plate 1, the lower opening of the inner tube is inserted into the inside and bottom of the outer tube, several flow membrane holes are provided in the distribution disk, the outer tube passes through the flow membrane holes with a gap, and the upper opening of the outer tube is connected to the second chamber by passing through a partition plate 2.

3. The polylactic acid (PLA) waste recycling system according to Request 2, characterized in that the corresponding flow membrane tube of the decomposition vessel has several enclosure weirs at different heights, and gaps are provided between the enclosure weirs and the flow membrane tube.

4. The polylactic acid (PLA) waste recycling system according to claim 2, characterized in that the height of the flowing membrane corresponding to the devolatilization device 1 is half the height of the fourth chamber.

5. The polylactic acid (PLA) waste recycling system according to claim 1, characterized in that the packing in the deoxidation tower consists of an upper packing and a lower packing, the corresponding outlet ports include an air outlet 1 and an air outlet 2, the air outlet 1 is provided in the tower body between the upper packing and the lower packing, the air outlet 2 is provided in the tower body below the lower packing, and the air outlet 1 and air outlet 2 are connected to a hammerhead condenser.

6. The polylactic acid (PLA) waste recycling system according to claim 1, characterized in that a gas phase inlet connected to a reflux column is provided at the top of the depolymerization vessel and the intermediate vessel, the liquid phase outlet at the bottom of the reflux column is connected to the depolymerization vessel and / or the intermediate vessel to reflux again to the depolymerization vessel and / or the intermediate vessel, the top of the reflux column is connected to a reflux tank via a condenser 1, and two circuits are drawn from the outlet of the reflux tank, one circuit is connected to the reflux column and the other circuit is connected to a receiving tank 1.

7. The polylactic acid (PLA) waste recycling system according to claim 2, characterized in that the condensate raw materials from condenser 2 and condenser 4 are connected to receiving tank 2 and receiving tank 3.

8. When recycling polylactic acid (PLA) waste using the recycling system described in any of claims 1-7, the following procedure is followed: (1) A catalyst is placed in the depolymerization vessel in advance. The catalyst consists of a fatty alcohol and an organic compound. The fatty alcohol is one or more of propylene glycol, ethylene glycol, and butylene glycol. The catalyst is one or more of stannous octanoate, stannous lactate, organic guanidine complex CRZnCl2, and organic guanidine complex CRFe(OAc)2. The weight ratio of the organic compound to the fatty alcohol is 0.2-2%, and the weight ratio of the catalyst to waste polylactic acid (PLA) is 2-10%. (2) Start the screw extruder and push the polylactic acid (PLA) waste into the depolymerization vessel, where it is depolymerized at 170-180°C under normal pressure. (3) The raw materials are pumped from the bottom of the depolymerization vessel into an intermediate vessel at 150-170°C. (4) The raw materials from the intermediate kettle are transferred from the bottom outlet to a decomposition kettle at 170-190°C, where the decomposition reaction is carried out at an absolute pressure of 0.1-1 kPa. (5) The gaseous raw material from the decomposition vessel enters the deoxidation column, where light components are removed. The condensation temperature in the hammerhead condenser is 90-100°C, the condensation temperature in condenser 2 is 80-90°C, and the temperature at the bottom of the deoxidation column is 100-130°C. The raw material at the bottom of the deoxidation column then enters the purification system. (6) The purified lactide product is placed in a prepolymerization vessel and polymerized into a prepolymer at 150-170°C. (7) The prepolymer raw materials are placed in a polymerization reactor and polymerized at 170-180°C. (8) The polymer raw material enters devolatilization unit 1 and devolatilization unit 2 and undergoes secondary devolatilization at 200-220°C. (9) The product after evaporation is cut into granules using a granulator. The polylactic acid (PLA) waste recycling system according to claim 2, characterized in that it is the same as described in claim 2.

9. The material at the bottom of the decomposition vessel is either returned to the decomposition vessel for recirculation decomposition, returned to the depolymerization vessel or intermediate vessel for depolymerization, or discharged as vessel residue. The polylactic acid (PLA) waste recycling system according to claim 2.

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