A polylactic acid waste recycling system and a recycling method

Abstract

This invention relates to a polylactic acid (PLA) waste recycling system and method in the field of environmental recycling technology. The system includes a screw extruder, a depolymerization reactor, an intermediate reactor, a pyrolysis reactor, a deacidification tower, a purification system, a prepolymerization reactor, a polymerization reactor, a first devolatilizer, a second devolatilizer, a pelletizing system, and a vacuum system. The screw extruder outlet is connected to the depolymerization reactor inlet, which is connected to the intermediate reactor. The intermediate reactor outlet is connected to the pyrolysis reactor inlet via a second pump. The pyrolysis reactor's gas phase outlet is connected to the gas phase inlet at the bottom of the deacidification tower. A hammerhead condenser is installed at the top of the deacidification tower, and its outlet is connected to the purification system. The purification system is connected to the polymerization reactor after passing through the prepolymerization reactor. The polymerization reactor is connected to the pelletizer after passing through devolatilizers one and two. This device is used for PLA waste recycling, achieving high recycling efficiency and yielding high molecular weight PLA. Simultaneously, a unique catalyst is used for depolymerization, reducing depolymerization and pyrolysis temperatures, lowering energy consumption, and reducing racemic byproducts in the lactide product.

Description

Technical Field

[0001] This invention relates to the field of environmentally friendly waste recycling technology, and in particular to a polylactic acid waste recycling system and recycling method. Background Technology

[0002] Polylactic acid (PLA) is an aliphatic thermoplastic polymer material prepared by polymerization of lactic acid as the main raw material. It possesses excellent biodegradability and biocompatibility, and can be completely degraded into carbon dioxide and water through composting or the action of microorganisms in nature. However, from a recycling perspective, the complete degradation of waste PLA into carbon dioxide and water represents a certain degree of resource waste. Therefore, how to efficiently recycle and utilize waste PLA is an important research topic.

[0003] Currently, the most researched method is to recover lactide from polylactic acid waste through thermal degradation and depolymerization. For example, Chinese Patent Application No. 201910837292.7 provides an integrated polylactic acid (PLA) recovery device and a PLA recovery method. The integrated PLA recovery device includes a housing, and within the housing are a single-screw extruder, a pre-depolymerization kettle, a heater, an evaporator, a depolymerization kettle, a flash evaporation 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-depolymerization kettle, the heater, and the evaporator are connected sequentially. The evaporator has a primary outlet and a liquid outlet. The primary outlet is connected to the flash evaporation tank via a pipe, and the liquid outlet is connected to the depolymerization kettle via a pipe. The depolymerization kettle has a circulation pipe and a secondary outlet. The secondary outlet is connected to the flash evaporation tank via a pipe. A condenser and a receiving tank are connected sequentially after the flash evaporation tank. The receiving tank has a discharge port and a vacuum port. The discharge port is connected to the finished product tank, and the vacuum port is connected to a vacuum device via a pipe.

[0004] When recovering polylactic acid (PLA), the device starts the insulation equipment and a single-screw extruder. PLA recycled material is added to the single-screw extruder and extruded into a pre-depolymerization reactor. A catalyst is added to the pre-depolymerization reactor to break the ring-chain and obtain a melt of a set molecular weight. The melt is then further heated in a heater and transported to an evaporator. In the evaporator, the melt undergoes a preliminary depolymerization reaction under vacuum conditions to obtain first-stage lactide. The first-stage lactide is sent to a flash tank for first-stage evaporation. Unreacted melt is sent to a depolymerization reactor for continued depolymerization under vacuum conditions. The second-stage lactide produced by this depolymerization process is sent to a flash tank for second-stage evaporation. The lactide collected in the flash tank is cooled by a condenser and then transported to a receiving tank, and finally to a finished product tank. The initial depolymerization conditions of this method are 170~180℃ and -0.1Mpa~-0.096Mpa, and the cyclic depolymerization conditions are 175℃~185℃ and -0.1Mpa~-0.096Mpa, to prepare lactide.

[0005] Its shortcomings are: (1) the initial depolymerization requires a vacuum degree, which increases the equipment cost and is prone to pipeline blockage; (2) the system viscosity is high, resulting in low depolymerization efficiency; (3) the depolymerization equipment has insufficient production efficiency, resulting in low lactide yield and purity. Summary of the Invention

[0006] The primary objective of this invention is to overcome the problems existing in the prior art and provide a waste polylactic acid (PLA) recycling system and method that achieves low energy consumption in the waste PLA recycling process. This system has a low depolymerization temperature, high recycling efficiency, and can obtain high molecular weight PLA.

[0007] To address the above technical problems, this invention provides a polylactic acid (PLA) waste recycling system, comprising a screw extruder, a depolymerization reactor, a pyrolysis reactor, and a vacuum system. The screw extruder outlet is connected to the depolymerization reactor inlet. The bottom outlet of the depolymerization reactor is connected via a first conveying pump to the bottom inlet of one end of a horizontally arranged intermediate reactor. The bottom outlet of the other end of the intermediate reactor is connected via a second conveying pump, a post-pump filter, and a first preheater to the feed inlet of the pyrolysis reactor. The bottom outlet of the pyrolysis reactor is connected via a third conveying pump to the inlet of the post-pump filter, a reuse port, and a reactor residue discharge port. The gas phase outlet in the middle of the pyrolysis reactor is connected to the gas phase inlet at the bottom of the deacidification tower. The top of the deacidification tower is equipped with a hammerhead condenser. The outlet of the hammerhead condenser is connected to the light component tank via condenser two. The condensate outlet at the bottom of the deacidification tower is connected to the purification system via transfer pump four and preheater three. The deacidification tower is equipped with at least one layer of packing. An outlet is provided below the packing layer and is connected to the hammerhead condenser. The outlet of the purification system is connected to the prepolymerization kettle via preheater two. The bottom outlet of the prepolymerization kettle is connected to the bottom inlet of the polymerization reactor via transfer pump five. The top outlet of the polymerization reactor is connected to the top inlet of devolatilizer one. The bottom outlet of devolatilizer one is connected to the top inlet of devolatilizer two via transfer pump six. The bottom outlet of devolatilizer two is connected to the pelletizer via transfer pump seven.

[0008] The devolatilizer 1 and devolatilizer 2 are respectively provided with gas phase outlets on their sides. The gas phase outlets are connected to the vacuum system after passing through the corresponding condenser 3 and condenser 4. The intermediate vessel is provided with multiple stirring devices. The rotating shaft of the stirring device is vertically arranged. A porous slow-flow baffle is vertically arranged between adjacent stirring devices. The pyrolysis vessel and devolatilizer 1 are provided with heated falling film tubes. The material moves from top to bottom along the falling film tubes. The vessel body of devolatilizer 2 is provided with a baffle. Several material distributors are arranged on the baffle. There is a cavity below the baffle. The feed port of devolatilizer 2 is connected to the material distributor. The material distributor includes an inner support plate. Liquid distribution packing is provided above the inner support plate. Multiple material discharge holes are provided on the inner support plate. The gas phase outlet of devolatilizer 2 is located on the side of the vessel body below the baffle.

[0009] In this invention, polylactic acid waste is extruded into a depolymerization reactor via a screw extruder. Fatty alcohol and a catalyst are added to the depolymerization reactor, and the mixture accumulates to a certain level, yielding molten material. This molten material is then pumped into an intermediate reactor, and while being stirred, it enters the pyrolysis reactor through a bottom outlet via a second transfer pump, a post-pump filter, and a preheater. The resulting lactide is more easily removed from the viscous material, and low-boiling-point impurities in the oligomers are vaporized and exit with the lactide through the gas phase outlet on the side of the pyrolysis reactor, entering a deacidification tower. This reduces the material's thermal residence time. The top of the deacidification tower is a hammerhead condenser, which shortens the residence time of the vapor phase between the tower top and the condenser and reduces the pressure drop, facilitating the preparation and purification of heat-sensitive lactide. The material at the bottom of the deacidification tower enters a purification unit, where it undergoes prepolymerization, further polymerization, two-stage devolatilization, and pelletizing. Compared to existing technologies, this invention achieves the following advantages: the device has high recovery efficiency and can obtain high molecular weight polylactic acid. Meanwhile, a unique catalyst is used for depolymerization, which reduces the depolymerization and cracking temperature, reduces energy consumption, and results in fewer racemic byproducts in lactide products.

[0010] Furthermore, the pyrolysis vessel and devolatilizer 1 are provided with baffle 1, baffle 2 and distribution plate from top to bottom inside the vessel. A first chamber is formed above baffle 1, a second chamber is formed between baffle 1 and baffle 2, a third chamber is formed between baffle 2 and distribution plate, and a fourth chamber is formed below distribution plate. The corresponding gas phase outlet is located on the side of the fourth chamber. The feed inlet at the top of the vessel is connected to a claw distributor. Multiple outlets of the claw distributor pass downward through baffle 1 and baffle 2 to connect to the third chamber. A heat medium inlet is provided at the top of the vessel to connect to the first chamber, and a heat medium outlet is provided on the side of the second chamber. Several falling film tubes are vertically arranged inside the vessel. The falling film tube includes an inner tube and an outer tube arranged coaxially. The inner tube is a straight tube that runs vertically, 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 passes through baffle 1 to connect to the first chamber, and the lower opening of the inner tube is inserted into the lower part of the outer tube. Several falling film holes are provided on the distribution plate. The outer tube passes through the falling film holes with gaps, and the upper opening of the outer tube passes through baffle 2 to connect to the second chamber.

[0011] A further improvement of this invention is that the falling film tube corresponding to the pyrolysis reactor is provided with several weirs at different heights, with gaps left between the weirs and the falling film tube. The oligomer material falls onto the weirs and then descends through the falling film tube. This solves the problem of excessively fast material descent and also addresses the problem of uneven material distribution, such as flow deviation, in the falling film tube.

[0012] A further improvement of the present invention is that the height of the falling film tube corresponding to the devolatilizer is half that of the fourth chamber.

[0013] A further improvement of this invention is that the packing material inside the deacidification tower includes an upper packing layer and a lower packing layer, and the corresponding outlets include an outlet one and an outlet two. Outlet one is located on the tower body between the upper and lower packing layers, and outlet two is located on the tower body below the lower packing layer. Outlet one and outlet two are connected to a hammerhead condenser. The two outlets allow for faster condensation of the material. Furthermore, a narrowing section is provided at the bottom of the deacidification tower, which reduces liquid holdup, material residence time, and the probability of high-temperature side reactions.

[0014] A further improvement of the present invention is that the top of the depolymerization vessel and the intermediate vessel are provided with a gas phase port connected to the gas phase inlet of the reflux tower, and the liquid phase outlet at the bottom of the reflux tower is refluxed and connected to the depolymerization vessel and / or the intermediate vessel; the top of the reflux tower is connected to the reflux tank after passing through a condenser, and the outlet of the reflux tank is connected to the reflux tower in one way and to the receiving tank in the other way.

[0015] A further improvement of the present invention is that the condensate from condenser three and condenser four is connected to receiving tank two and receiving tank three.

[0016] This invention also provides a method for recycling polylactic acid (PLA) waste, which utilizes the aforementioned PLA waste recycling system to recycle PLA waste, comprising the following steps:

[0017] (1) A catalyst is pre-added to the depolymerization reactor. The catalyst is composed of fatty alcohol and organic compounds. The fatty alcohol is one or more of propylene glycol, ethylene glycol, and butanediol. The catalyst is one or more of stannous octoate, stannous lactate, organic guanidine complex CRZnCl2, and organic guanidine complex CRFe(OAc)2. The organic compounds account for 0.2-2% of the weight of the fatty alcohol, and the catalyst accounts for 2-10% of the weight of the polylactic acid waste.

[0018] (2) Start the screw extruder and extrude the polylactic acid waste into the depolymerization kettle, and depolymerize it at 170~180℃ and normal pressure;

[0019] (3) The material is pumped from the bottom of the depolymerization reactor into the intermediate reactor at 150~170℃;

[0020] (4) The material in the intermediate reactor enters the pyrolysis reactor at 170~190℃ from the bottom outlet and undergoes pyrolysis reaction at an absolute pressure of 0.1-1Kpa;

[0021] (5) The gaseous material from the pyrolysis reactor enters the deacidification tower to remove light components. The condensation temperature of the hammerhead condenser is 90~100℃, the condensation temperature of the second condenser is 80~90℃, the bottom temperature of the deacidification tower is 100~130℃, and the bottom material of the deacidification tower enters the purification system.

[0022] (6) The purified lactide product enters the prepolymer reactor and polymerizes into a prepolymer at 150-170℃;

[0023] (7) The prepolymer material enters the polymerization reactor and polymerizes at 170~180℃;

[0024] (8) The polymer material enters the devolatilizer 1 and devolatilizer 2 at 200~220℃ for two-stage devolatilization;

[0025] (9) The devolatilized product is cut into granules by a pelletizer.

[0026] Furthermore, the material at the bottom of the pyrolysis reactor is returned to the pyrolysis reactor for recycling pyrolysis, or returned to the depolymerization reactor or intermediate reactor for depolymerization, or discharged as reactor residue.

[0027] This method has the advantages of low energy consumption and high production efficiency. Attached Figure Description

[0028] Figure 1 This is a flowchart of the polylactic acid waste recycling system of the present invention.

[0029] Figure 2 This is a schematic diagram of the structure of devolatilizer one.

[0030] Figure 3 for Figure 2 Top view of the central devourer.

[0031] Figure 4 for Figure 2 Distribution diagram of falling film tubes in devolatilizer 1.

[0032] Figure 5 This is a schematic diagram of the second devourer.

[0033] Figure 6 for Figure 4 Top view of the second de-idler.

[0034] In the diagram: 1. Transfer pump 1; 2. Depolymerization reactor; 3. Screw extruder; 4. Reflux tower; 5. Condenser 1; 6. Reflux tank; 7. Receiving tank 1; 8. Preheater 1; 9. Deacidification tower; 10. Hammerhead condenser; 11. Condenser 2; 12. Light component tank; 13. Preheater 2; 14. Prepolymerization reactor; 15. Polymerization reactor; 16. Deviation reactor 1; 17. Condenser 3; 18. Receiving tank 2; 19. Condenser 4; 20. Receiving tank 3; 21. Deviation reactor 2; 22. Pelletizer; 23. Transfer pump 7; 24. Transfer pump 6; 25. Transfer pump 5; 26. Purification system; 27. Preheater 3; 28. Transfer pump 4; 29. ​​Cracking reactor; 30. Transfer pump 2; 31. Post-pump filter; 32. Intermediate reactor; 33. Transfer pump 3.

[0035] 1601 Falling film tube, 1602 Boiler body of devolatilizer one, 1603 Distribution plate, 1604 Second baffle, 1605 First baffle, 1607 Feed inlet of devolatilizer one, 1608 Heat medium inlet, 1609 Heat medium outlet, 1610 Claw distributor, 1611 Gas phase outlet of devolatilizer one.

[0036] 2101 Inner support plate, 2102 Feed inlet of devolatilizer 2, 2103 Material distributor, 2104 Liquid distribution packing, 2105 Baffle plate, 2106 Gas phase outlet of devolatilizer 2, 2107 Kettle body of devolatilizer 2. Detailed Implementation Example 1

[0037] like Figure 1 The diagram shows a polylactic acid (PLA) waste recycling system, comprising a screw extruder 3, a depolymerization reactor 2, a pyrolysis reactor 29, and a vacuum system. The screw extruder 3 is a single-screw extruder, and its outlet is connected to the inlet of the depolymerization reactor 2. The bottom outlet of the depolymerization reactor 2 is connected via a transfer pump 1 to the bottom inlet of one end of a horizontally positioned intermediate reactor 32. The bottom outlet of the other end of the intermediate reactor 32 is connected via a transfer pump 20, a post-pump filter 31, and a preheater 8 to the inlet of the pyrolysis reactor 29. The bottom outlet of the pyrolysis reactor 29 is connected via a transfer pump 33, with one path leading to the inlet of the post-pump filter 31 and the other path leading to a reuse port. The reuse port then connects to either the depolymerization reactor 2 or the intermediate reactor 32. There is also a connection to the reactor residue discharge port; the gas phase outlet in the middle of the pyrolysis reactor 29 is connected to the gas phase inlet at the bottom of the deacidification tower 9. The top of the deacidification tower 9 is equipped with a hammerhead condenser 10. The outlet of the hammerhead condenser 10 is connected to the light component tank 12 via condenser two 11. The condensate outlet at the bottom of the deacidification tower 9 is connected to the purification system 26 via transfer pump four 28 and preheater three 27. The deacidification tower 9 is equipped with upper packing and lower packing. The corresponding outlets include gas outlet one and gas outlet two. Gas outlet one is located on the tower body between the upper and lower packing, and gas outlet two is located on the tower body below the lower packing. Gas outlet one and gas outlet two are connected to the hammerhead condenser 10. The bottom of the deacidification tower 9 is equipped with a narrowing section.

[0038] The outlet of the deacidification tower is connected to the hammerhead condenser 10; the outlet of the purification system 26 is connected to the prepolymerization kettle 14 via the preheater 2 13, the bottom outlet of the prepolymerization kettle 14 is connected to the bottom inlet of the polymerization reactor 15 via the transfer pump 5 25, the top outlet of the polymerization reactor 15 is connected to the top inlet of the devolatilizer 1 16, the bottom outlet of the devolatilizer 1 16 is connected to the top inlet of the devolatilizer 21 via the transfer pump 6 24, and the bottom outlet of the devolatilizer 21 is connected to the pelletizer 22 via the transfer pump 7 23; the sides of the devolatilizer 1 16 and the devolatilizer 21 are respectively provided with gas phase outlets, and the gas phase outlets are connected to the vacuum system after passing through the corresponding condenser 3 17 and condenser 4 19.

[0039] The intermediate vessel 32 is equipped with multiple stirring devices. The rotating shaft of the stirring device is vertically arranged, and a porous slow-flow baffle is vertically arranged between adjacent stirring devices.

[0040] The pyrolysis vessel 29 and the devolatilizer 16 are equipped with heated falling film tubes, and the material moves from top to bottom along the falling film tubes.

[0041] like Figure 5 , 6 As shown, the vessel body 2107 of the second devolatilizer is provided with a partition 2105 inside, and a number of material distributors 2103 are provided on the partition 2105. The space below the partition 2105 is a cavity. The feed inlet 2102 of the second devolatilizer is connected to the material distributors 2103. The material distributors 2103 include an inner support plate 2101. A liquid distribution packing 2104 is provided above the inner support plate 2101. The inner support plate 2101 is provided with a number of material discharge holes. The gas phase outlet 2106 of the second devolatilizer is located on the side of the vessel body 2107 of the second devolatilizer below the partition 2105.

[0042] The pyrolysis vessel 29 and the devolatilizer 16 have essentially the same internal structure, such as Figure 2 , 3 Figure 4 shows the structure of devolatilizer one. Inside the vessel body 1602 of devolatilizer one, from top to bottom, are a first partition 1605, a second partition 1604, and a distribution plate 1603. A first chamber is formed above the first partition 1605 inside the vessel body; a second chamber is formed between the first partition 1605 and the second partition 1604; a third chamber is formed between the second partition 1604 and the distribution plate 1603; and a fourth chamber is formed below the distribution plate 1603. The corresponding gas phase outlet 1611 of devolatilizer one is located on the side of the fourth chamber. The feed inlet 1607 of devolatilizer one, located at the top of the vessel body 1602, is connected to a claw distributor 1610. Multiple outlets of the claw distributor 1610 pass downwards through the first partition. 1605. Partition 2 1604 connects to the third chamber. A heat medium inlet 1608 is provided at the top of the vessel body, connecting to the first chamber. A heat medium outlet 1609 is provided on the side of the second chamber. Several falling film tubes 1601 are vertically arranged in the vessel body. Each falling film tube 1601 includes an inner tube and an outer tube arranged coaxially. The inner tube is a straight tube running vertically, and the outer tube is a blind tube with an open top and a closed bottom. The upper opening of the inner tube passes through partition 1 1605 and connects to the first chamber. The lower opening of the inner tube is inserted into the lower part of the outer tube. Several falling film holes are provided on the distribution plate 1603. The outer tube passes through the falling film holes with gaps. The upper opening of the outer tube passes through partition 2 1604 and connects to the second chamber.

[0043] The difference between the pyrolysis reactor 29 and the devolatilizer 16 is that the falling film tube corresponding to the pyrolysis reactor 29 has several weirs at different heights, with gaps between the weirs and the falling film tube. The oligomer material falls onto the weirs and then descends through the falling film tube. This solves the problem of excessively fast material descent and uneven material distribution, such as flow deviation, in the falling film tube. The falling film tube corresponding to the devolatilizer 16 has no weirs, and its height is half the height of the fourth chamber, allowing space for internal material collection.

[0044] Figure 1 In this system, the top of the depolymerization vessel 2 and the intermediate vessel 32 are equipped with gas phase inlets connected to the gas phase inlet of the reflux tower 4. The liquid phase outlet at the bottom of the reflux tower 4 is connected back to the depolymerization vessel 2 and / or the intermediate vessel 32. The top of the reflux tower 4 is connected to the reflux tank 6 after passing through condenser 5. One outlet of the reflux tank 6 is connected to the reflux tower 4, and the other outlet is connected to the receiving tank 7. The condensed materials from condensers 17 and 19 are connected to receiving tanks 18 and 20.

[0045] A method for recycling polylactic acid (PLA) waste, which utilizes the aforementioned PLA waste recycling system to recycle PLA waste, comprises the following steps:

[0046] (1) A catalyst is pre-added to the depolymerization reactor 2. The catalyst is composed of fatty alcohol and organic compounds. The fatty alcohol is one or more of propylene glycol, ethylene glycol, and butanediol. The catalyst is one or more of stannous octoate, stannous lactate, organic guanidine complex CRZnCl2, and organic guanidine complex CRFe(OAc)2. The organic compounds account for 0.2-2% of the weight of the fatty alcohol, and the catalyst accounts for 2-10% of the weight of the polylactic acid waste.

[0047] (2) Start the screw extruder 3 and extrude the polylactic acid waste into the depolymerization kettle 2, and depolymerize it at 170~180℃ and normal pressure;

[0048] (3) The material is pumped from the bottom of the depolymerization reactor 2 into the intermediate reactor 32 at 150~170℃;

[0049] (4) The material in the intermediate reactor 32 enters the pyrolysis reactor 29 at 170~190℃ from the bottom outlet and undergoes pyrolysis reaction at an absolute pressure of 0.1-1Kpa;

[0050] (5) The gaseous material from the cracking vessel 29 enters the deacidification tower 9 to remove light components. The condensing temperature of the hammerhead condenser 10 is 90~100℃, the condensing temperature of the second condenser 11 is 80~90℃, the bottom temperature of the deacidification tower 9 is 100~130℃, and the bottom material of the deacidification tower 9 enters the purification system 26.

[0051] (6) The purified lactide product enters the prepolymer reactor 14 and polymerizes into a prepolymer at 150-170℃;

[0052] (7) The prepolymer material enters the polymerization reactor 15 and is polymerized at 170~180℃;

[0053] (8) The polymer material enters the devolatilizer 16 and devolatilizer 21 at 200~220℃ for two-stage devolatilization;

[0054] (9) The devolatilized product is cut into granules by pelletizer 22.

[0055] Furthermore, the material at the bottom of the pyrolysis reactor 29 is returned to the pyrolysis reactor 29 for cyclic pyrolysis, or returned to the depolymerization reactor 2 or intermediate reactor 32 for depolymerization, or discharged as reactor residue.

[0056] This device boasts high recovery efficiency, yielding high molecular weight polylactic acid. Simultaneously, a unique catalyst is employed for depolymerization, lowering the depolymerization and pyrolysis temperatures, reducing energy consumption, and minimizing racemic byproducts in the lactide product. Example 2

[0057] Using the system of Example 1, different recycling processes were performed by changing relevant parameters. The process is as follows: The screw extruder 3 was started, and the processing temperature was set to 170~230℃. Polylactic acid (PLA) waste was then fed in. The PLA waste used included one or more of PLA sheet products, PLA fiber and nonwoven fabric products, PLA film products, PLA scraps, and PLA by-products. The extruded molten material entered the depolymerization reactor 2. The catalyst-to-molten material flow ratio was 10%. The catalyst consisted of ethylene glycol, butanediol, and the organic guanidine complex CRZnCl2, with the organic guanidine complex CRZnCl2 accounting for 0.5% of the total diols. The temperature of the depolymerization reactor 2 was controlled at 170~180℃, and the flow rate was controlled to ensure a material residence time of 2-3 hours. The material then entered the intermediate reactor 32 from the bottom of the depolymerization reactor 2, and the temperature of the intermediate reactor 32 was controlled at 150~160℃. The material from the intermediate reactor 32 then entered the pyrolysis reactor 29, and the temperature of the pyrolysis reactor 29 was controlled at 170~180℃, with a vacuum degree of 0.1~1. kPa, 0.1-1% of the feed amount of pyrolysis reactor 29 is discharged from the outside of pyrolysis reactor 29 via the transfer pump; the bottom temperature of deacidification tower 9 is 110~120℃, the condensation temperature of hammerhead condenser 10 is 90~100℃, the temperature of condenser 11 is 80~90℃, and the bottom temperature of deacidification tower 9 is 100~130℃. After purification system 26, the product enters the prepolymerization reactor 14 at 150-170℃, the polymerization reactor 15 at 170~180℃, and the devolatilizer 16 and devolatilizer 21 at 200~220℃ for devolatilization, resulting in polylactic acid with a weight average molecular weight of 126729 kg / mol. Example 3

[0058] Using the system of Example 1, different recycling processes were performed by changing relevant parameters. The process is as follows: The screw extruder 3 was started, and the processing temperature was set to 170~230℃. Polylactic acid (PLA) waste was then fed in. The PLA waste used included one or more of the following: PLA sheet products, PLA fiber and nonwoven fabric products, PLA film products, PLA scraps, and PLA by-products. The extruded molten material entered the depolymerization reactor 2. The catalyst-to-molten material flow ratio was 5%. The catalyst consisted of propylene glycol and stannous octoate, with stannous octoate accounting for 1% of the total diol. The temperature of the depolymerization reactor 2 was controlled at 170~180℃, and the flow rate was controlled to ensure a material residence time of 3-4 hours. The material then entered the intermediate reactor 32 from the bottom of the depolymerization reactor 2. The temperature of the intermediate reactor 32 was controlled at 160~170℃. The material from the intermediate reactor 32 entered the pyrolysis reactor 29. The temperature of the pyrolysis reactor 29 was controlled at 180~190℃, and the vacuum degree was 0.1~1 kPa. The material was then transferred to the pyrolysis reactor 29 via a pump. 0.1-1% of the feed amount to the pyrolysis reactor 29 is discharged externally; the bottom temperature of the deacidification tower 9 is 120-130℃, the condensation temperature of the hammerhead condenser 10 is 90-100℃, the temperature of the second condenser 11 is 80-90℃, and the bottom temperature of the deacidification tower 9 is 100-130℃. After passing through the purification system 26, the product sequentially enters the prepolymerization reactor 14 at 150-170℃, the polymerization reactor 15 at 170-180℃, and the devolatilizers 16 and 21 at 200-220℃ for devolatilization, resulting in polylactic acid with a weight-average molecular weight of 185076 kg / mol. Example 4

[0059] Using the system of Example 1, different recycling processes were carried out by changing relevant parameters. The process is as follows: Start the screw extruder 3, set the processing temperature to 170~230℃, and start feeding polylactic acid waste. The polylactic acid waste used is one or more of polylactic acid sheet products, polylactic acid fiber and non-woven fabric products, polylactic acid film products, polylactic acid scraps, and polylactic acid by-products. The extruded molten material enters depolymerization reactor 2. The catalyst-to-molten material flow ratio is 2%. The catalyst consists of butanediol, stannous lactate, and the organic guanidine complex CRFe(OAc)2. The total amount of stannous lactate and the organic guanidine complex CRFe(OAc)2 accounts for 2% of the total butanediol. The temperature of depolymerization reactor 2 is controlled at 170-180℃, and the flow rate is controlled to ensure a material residence time of 4-5 hours. The material then enters intermediate reactor 32 from the bottom of depolymerization reactor 2, with the temperature of intermediate reactor 32 controlled at 160-170℃. The material from intermediate reactor 32 then enters pyrolysis reactor 29, with the temperature of pyrolysis reactor 29 controlled at 180-190℃ and a vacuum degree of 0. The feed rate of the cracking reactor 29 is 0.1-1 kPa, and 0.1-1% of the feed rate of the cracking reactor 29 is discharged from the bottom of the cracking reactor 29. The bottom temperature of the deacidification tower 9 is 100-110℃, the condensation temperature of the hammerhead condenser 10 is 90-100℃, the temperature of the second condenser 11 is 80-90℃, and the bottom temperature of the deacidification tower 9 is 100-130℃. After passing through the purification system 26, the product enters the prepolymerization reactor 14 at 150-170℃, the polymerization reactor 15 at 170-180℃, and the devolatilizers 16 and 21 at 200-220℃ for devolatilization, resulting in polylactic acid with a weight-average molecular weight of 193105 kg / mol. Example 5

[0060] Using the system of Example 1, different recycling processes were performed by changing relevant parameters, as follows: The screw extruder 3 was started, and the processing temperature was set to 170~230℃. Polylactic acid (PLA) waste was then fed in. The PLA waste used included one or more of PLA sheet products, PLA fiber and nonwoven fabric products, PLA film products, PLA scraps, and PLA by-products. The extruded molten material entered the depolymerization reactor 2. The catalyst-to-molten material flow ratio was 8%. The catalyst consisted of butanediol and an organic guanidine complex CRFe(OAc)2, with the total amount of CRFe(OAc)2 accounting for 0.2% of the total butanediol. The temperature of the depolymerization reactor 2 was controlled at 170~180℃, and the flow rate was controlled to ensure a material residence time of 5-6 hours. The material then entered the intermediate reactor 32 from the bottom of the depolymerization reactor 2, with the temperature of the intermediate reactor 32 controlled at 150~160℃. The material from the intermediate reactor 32 then entered the pyrolysis reactor 29, with the temperature of the pyrolysis reactor 29 controlled at 180~190℃ and a vacuum degree of 0.1. ~1kPa, 0.1-1% of the feed amount of pyrolysis reactor 29 is discharged from the bottom of pyrolysis reactor 29; the bottom temperature of deacidification tower 9 is 110~120℃, the condensation temperature of hammerhead condenser 10 is 90~100℃, the temperature of condenser 21 is 80~90℃, the bottom temperature of deacidification tower 9 is 100~130℃, after purification system 26, the product enters the prepolymerization reactor 14 at 150-170℃, the polymerization reactor 15 at 170~180℃, and the devolatilizer 16 and devolatilizer 21 at 200~220℃ for devolatilization, and the relative molecular weight of polylactic acid is 198054kg / mol.

[0061] The above description is merely a preferred embodiment of the present invention and does not limit the scope of patent protection of the present invention. Besides the above embodiments, the present invention may have other implementations, and all technical solutions formed by equivalent substitution or equivalent transformation fall within the scope of protection claimed by the present invention. Technical features of the present invention not described can be implemented by or using existing technology, and will not be elaborated here.

Claims

1. A polylactic acid waste recycling system comprising a screw extruder, a depolymerization kettle, a cracking kettle, a vacuum system, the outlet of the screw extruder being connected to the inlet of the depolymerization kettle, characterized in that: The bottom outlet of the depolymerization kettle is connected to the bottom inlet of one end of the horizontally arranged intermediate kettle through a delivery pump, the bottom outlet of the other end of the intermediate kettle is connected to the feed inlet of the cracking kettle through a delivery pump two, a post-pump filter and a preheater, the outlet at the bottom of the cracking kettle is connected to the inlet of the post-pump filter through a delivery pump three, and one way is connected to a recycling port, and the other way is connected to a kettle residue discharge port; the gas phase outlet in the middle of the cracking kettle is connected to the gas phase inlet at the lower part of the deacidification tower, a hammer head condenser is arranged at the top of the deacidification tower, the outlet of the hammer head condenser is connected to a light component tank through a condenser two, the condensed liquid outlet at the bottom of the deacidification tower is connected to a purification system through a delivery pump four and a preheater three; at least one layer of filler is arranged in the deacidification tower, an air outlet is arranged below the filler layer, and the air outlet is connected to the hammer head condenser; the outlet of the purification system is connected to a prepolymerization kettle through a preheater two, the outlet at the bottom of the prepolymerization kettle is connected to the bottom inlet of a polymerization reactor through a delivery pump five, the top outlet of the polymerization reactor is connected to the top inlet of a devolatilizer one, the bottom outlet of the devolatilizer one is connected to the top inlet of a devolatilizer two through a delivery pump six, and the bottom outlet of the devolatilizer two is connected to a pelletizer through a delivery pump seven. The side of the devolatilizer one and the devolatilizer two is respectively provided with a gas phase outlet, and the gas phase outlet is connected to the vacuum system through a corresponding condenser three and a condenser four. A plurality of stirring devices are arranged in the intermediate kettle, the rotating shafts of the stirring devices are vertically arranged, and a plurality of porous slow-flow partition plates are vertically arranged between adjacent stirring devices. The cracking kettle and the devolatilizer one are provided with heatable falling film pipes, and the material moves downward along the falling film pipes. The kettle body of the devolatilizer two is provided with a partition plate, a plurality of material distributors are arranged on the partition plate, and a cavity is arranged below the partition plate; the feed inlet of the devolatilizer two is connected to the material distributor, the material distributor comprises an inner support plate, a liquid distribution filler is arranged above the inner support plate, a plurality of material falling holes are arranged on the inner support plate, and the gas phase outlet of the devolatilizer two is arranged on the side of the kettle body below the partition plate.

2. The polylactic acid waste recycling system of claim 1, wherein: The kettle body of the cracking kettle and the devolatilizer one is provided with a partition plate one, a partition plate two and a distribution disc from top to bottom, a first chamber is formed above the partition plate one, a second chamber is formed between the partition plate one and the partition plate two, a third chamber is formed between the partition plate two and the distribution disc, a fourth chamber is formed below the distribution disc, and a corresponding gas phase outlet is arranged on the side of the fourth chamber; the feed inlet at the top of the kettle body is connected to a claw type distributor, a plurality of outlets of the claw type distributor pass through the partition plate one and the partition plate two downward and are connected to the third chamber, a heat medium inlet is arranged at the top of the kettle body and is connected to the first chamber, a heat medium outlet is arranged on the side of the second chamber; a plurality of falling film pipes are vertically arranged in the kettle body, the falling film pipe comprises coaxially arranged inner and outer pipes, the inner pipe is a straight pipe, the outer pipe is a blind pipe with an open upper end and a closed bottom end, the upper end of the inner pipe penetrates through the partition plate one and is connected to the first chamber, the lower end of the inner pipe is inserted into the inner lower part of the outer pipe, a plurality of falling film holes are arranged on the distribution disc, the outer pipe passes through the falling film holes with a gap, and the upper end of the outer pipe penetrates through the partition plate two and is connected to the second chamber.

3. The polylactic acid waste recycling system of claim 2, wherein: The corresponding falling film pipes of the cracking kettle are provided with a plurality of cofferdams at different heights, and a gap is arranged between the cofferdams and the falling film pipes.

4. The polylactic acid waste recycling system of claim 2, wherein: The height of the corresponding falling film pipe of the devolatilizer one is half of the height of the fourth chamber. The height of the corresponding falling film pipe of the devolatilizer one is half of the height of the fourth chamber.

5. The polylactic acid waste recycling system of claim 1, wherein: The packing in the deacidification tower comprises upper packing and lower packing, and the corresponding gas outlets comprise gas outlet one and gas outlet two, the gas outlet one is arranged on the tower body between the upper packing and the lower packing, and the gas outlet two is arranged on the tower body at the lower side of the lower packing, and the gas outlet one and the gas outlet two are connected with the hammer head condenser.

6. The polylactic acid waste recycling system of claim 1, wherein: The top of the depolymerization kettle and the intermediate kettle is provided with a gas phase port connected to a gas phase inlet of a reflux tower, and a liquid phase outlet at the bottom of the reflux tower is connected to the depolymerization kettle and / or the intermediate kettle in reflux connection; the top of the reflux tower is connected to a reflux tank through a condenser one, and the outlet of the reflux tank is connected to the reflux tower in one way and to a receiving tank one in another way.

7. The polylactic acid waste recycling system of claim 1, wherein: The condensate of the condenser three and the condenser four is connected to the receiving tank two and the receiving tank three.

8. A polylactic acid waste recycling method characterized by, The polylactic acid waste recycling system according to any one of claims 1-7 is used for recycling, and the steps are as follows: (1) a catalyst is added in advance to the depolymerization kettle, the catalyst is composed of a fatty alcohol and an organic compound, the fatty alcohol is one or more of propylene glycol, ethylene glycol and butanediol, the catalyst is one or more of stannous octoate, 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 waste is 2-10%; (2) the screw extruder is started, and the polylactic acid waste is extruded into the depolymerization kettle for depolymerization at 170-180 DEG C under normal pressure; (3) the material is pumped from the bottom of the depolymerization kettle into the intermediate kettle at 150-170 DEG C; (4) the material in the intermediate kettle is discharged from the bottom discharge port into the cracking kettle at 170-190 DEG C for cracking reaction under 0.1-1 KPa absolute pressure; (5) the gas phase material in the cracking kettle is introduced into the deacidification tower to remove light components, the condensing temperature of the hammer head condenser is 90-100 DEG C, the condensing temperature of the condenser two is 80-90 DEG C, the bottom temperature of the deacidification tower is 100-130 DEG C, and the bottom material of the deacidification tower is introduced into a purification system; (6) the propylene lactone product after purification is introduced into a prepolymerization kettle to polymerize into a prepolymer at 150-170 DEG C; (7) the prepolymer material is introduced into a polymerization reactor for polymerization at 170-180 DEG C; (8) the polymer material is introduced into two-stage devolatilizers one and two at 200-220 DEG C for devolatilization; (9) the product after devolatilization is cut into particles by a cutting machine.

9. The method of claim 8, wherein: The material at the bottom of the cracking kettle is recycled into the cracking kettle, or the depolymerization kettle or the intermediate kettle for depolymerization, or discharged as kettle residue.

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