Polylactic acid waste recovery system and recovery method

Abstract

The present invention relates to a polylactic acid waste recovery system and recovery method in the technical field of environment-friendly regeneration. The system comprises a screw extruder, a depolymerization kettle, an intermediate kettle, a cracking kettle, a deacidification tower, a purification system, a pre-polymerization kettle, a polymerization reactor, a first devolatilization device, a second devolatilization device, a pelletizing system, and a vacuum system; an outlet of the screw extruder is connected to an inlet of the depolymerization kettle; the depolymerization kettle is connected to the intermediate kettle; an outlet of the intermediate kettle is connected to a feeding port of the cracking kettle by means of a second conveying pump; a gas phase outlet of the cracking kettle is connected to a gas phase inlet at the lower portion of the deacidification tower; a hammer condenser is arranged at the top of the deacidification tower; an outlet of the deacidification tower is connected to the purification system; the purification system is connected to the polymerization reactor by means of the pre-polymerization kettle; and the polymerization reactor is connected to a pelletizer by means of the first devolatilization device and the second devolatilization device. Such an apparatus is used for polylactic acid waste recovery, has high recovery efficiency, and can be used for obtaining high-molecular-weight polylactic acid. Additionally, a special catalyst is used for depolymerization, such that the depolymerization and cracking temperatures are reduced, the energy consumption is reduced, and racemization by-products in a lactide product are few.

Description

A polylactic acid waste recycling system and recycling method Technical Field

[0001] The present invention relates to the technical field of environmentally friendly waste regeneration, and in particular to a polylactic acid waste recycling system and a regeneration method thereof. Background Art

[0002] Polylactic acid (PLA) is an aliphatic thermoplastic polymer made from lactic acid through polymerization. It exhibits excellent biodegradability and biocompatibility, completely degrading into carbon dioxide and water through composting or the action of natural microorganisms. However, from a recycling perspective, the complete degradation of waste PLA into carbon dioxide and water represents a degree of resource waste. Therefore, the efficient recycling of waste PLA is a key research topic.

[0003] The method that is currently being studied more is to recover lactide from polylactic acid waste through thermal degradation and depolymerization. For example, the Chinese patent application number 201910837292.7 provides an integrated polylactic acid recovery device and a polylactic acid recovery method, which includes a shell, and a single-screw extruder, a pre-depolymerization 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 arranged in the shell; the single-screw extruder, the pre-depolymerization 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 through a pipe, the liquid outlet is connected to the depolymerization kettle through a pipe, the depolymerization kettle is provided with a circulation pipe and a secondary outlet, the secondary outlet is connected to the flash tank through a pipe, the flash tank is connected in sequence with a condenser and a receiving tank, the receiving tank 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 equipment through a pipe.

[0004] When the device recovers polylactic acid, the heat preservation equipment and the single-screw extruder are started, the polylactic acid recycled material is added to the single-screw extruder, and the polylactic acid is sent to the pre-depolymerization kettle after extrusion. A catalyst is added to the pre-depolymerization kettle, and the ring is opened and the chain is broken to obtain a melt with a set molecular weight. The melt enters the heater for further heating and is then transported to the evaporator. The melt undergoes a preliminary depolymerization reaction under vacuum conditions in the evaporator to obtain first-stage lactide, which is sent to a flash tank for completion of first-stage evaporation. The unreacted melt is sent to the depolymerization kettle for continued cyclic depolymerization under vacuum conditions. The second-stage lactide produced by the cyclic depolymerization is sent to the flash tank for completion of second-stage evaporation. The lactide collected in the flash tank is cooled by a condenser and transported to a receiving tank, and finally to a finished product tank. The initial depolymerization conditions of the method are 170-180°C and -0.1Mpa--0.096Mpa, and the cyclic depolymerization conditions are 175-185°C and -0.1Mpa--0.096Mpa, to prepare lactide.

[0005] Its shortcomings are: (1) the initial depolymerization requires vacuum, which increases equipment costs and easily causes 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 purpose of the present invention is to overcome the problems existing in the prior art and provide a waste polylactic acid recovery system and recovery method to achieve low energy consumption in the waste polylactic acid recovery process. The system has a low depolymerization temperature and high recovery efficiency, and high molecular weight polylactic acid can be obtained.

[0007] In order to solve the above technical problems, the present invention provides a polylactic acid waste recycling system, which includes a screw extruder, a depolymerization kettle, a cracking kettle, and a vacuum system. The outlet of the screw extruder is connected to the inlet of the depolymerization kettle, 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 1, 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 2, a post-pump filter, and a preheater 1, the outlet of the bottom of the cracking kettle is connected to the inlet of the post-pump filter after passing through a delivery pump 3, a reuse port, and a reactor residue discharge port; the gas phase outlet in the middle of the cracking kettle is connected to the gas phase inlet at the bottom of the deacidification tower. A hammerhead condenser is provided at the top of the deacidification tower, and the outlet of the hammerhead condenser is connected to the light component tank via condenser 2. The condensate outlet at the bottom of the deacidification tower is connected to the purification system via transfer pump 4 and preheater 3. At least one layer of filler is provided in the deacidification tower, and an air outlet is provided below the filler layer, which is connected to the hammerhead condenser. The outlet of the purification system is connected to the prepolymerization kettle via preheater 2, the bottom outlet of the prepolymerization kettle is connected to the bottom inlet of the polymerization reactor via transfer 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 transfer pump 6, and the bottom outlet of devolatilizer 2 is connected to the pelletizer via transfer pump 7.

[0008] The sides of the devolatilizer 1 and the devolatilizer 2 are respectively provided with a gas phase outlet, and the gas phase outlet is connected to the vacuum system after passing through the corresponding condenser 3 and condenser 4; a plurality of stirring devices are provided in the intermediate kettle, the rotating shafts of the stirring devices are arranged vertically, and porous slow-flow baffles are vertically arranged between adjacent stirring devices; a heatable falling film tube is provided in the cracking kettle and the devolatilizer 1, and the material moves from top to bottom along the falling film tube; a baffle is provided inside the kettle body of the devolatilizer 2, and a plurality of material distributors are provided on the baffle, and a cavity is formed below the baffle; the feed port of the devolatilizer 2 is connected to the material distributor, and the material distributor includes an inner support plate, a liquid distribution filler is provided above the inner support plate, and a plurality of drop holes are provided on the inner support plate. The gas phase outlet of the devolatilizer 2 is arranged on the side of the kettle body below the baffle.

[0009] During production, the present invention extrudes polylactic acid waste into a depolymerization reactor through a screw extruder. Fatty alcohol and a catalyst are added to the depolymerization reactor and accumulated to a certain liquid level to obtain a molten material. The molten material is then pumped into an intermediate reactor and, while being stirred, is fed from the bottom outlet through a second delivery pump, a post-pump filter, and a first preheater into the cracking reactor through the feed port. The lactide produced by cracking can be more easily removed from the viscous material. Low-boiling-point impurities in oligomers also become vapor and leave the lactide through a gas phase outlet on the side of the cracking reactor, thereby reducing the thermal residence time of the material. The top of the deacidification tower is equipped with 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 the heat-sensitive lactide material. The material at the bottom of the deacidification tower enters a purification device, where it is prepolymerized after purification, followed by further polymerization reaction, two-stage devolatilization, and finally pelletization. Compared with the prior art, the present invention achieves the following beneficial effects: the device has high recovery efficiency and can produce high-molecular-weight polylactic acid. At the same time, the depolymerization adopts a special catalyst to reduce the depolymerization and cracking temperature, reduce energy consumption, and produce less racemization by-products in the lactide product.

[0010] Furthermore, the kettle bodies of the cracking kettle and the devolatilizer 1 are provided with partition 1, partition 2 and a distribution plate from top to bottom, a first chamber is formed above the partition 1 in the kettle body, a second chamber is formed between the partition 1 and the partition 2, a third chamber is formed between the partition 2 and the distribution plate, and a fourth chamber is formed below the distribution plate, and the corresponding gas phase outlet is arranged on the side of the fourth chamber; the feed port at the top of the kettle body is connected to the claw distributor, and multiple outlets of the claw distributor downwardly pass through the partition 1 and the partition 2 to connect to the third chamber, a heat medium inlet is provided at the top of the kettle body to connect to the first chamber, and a heat medium outlet is provided on the side of the second chamber; a plurality of falling film tubes are vertically arranged in the kettle body, the falling film tubes include an inner tube and an outer tube arranged coaxially, the inner tube is a straight tube with an upper end open and a bottom closed, the upper end of the inner tube passes through the partition 1 to connect to the first chamber, and the lower end of the inner tube is inserted into the inner lower part of the outer tube, the distribution plate is provided with a plurality of falling film holes, the outer tube passes through the falling film holes with a gap, and the upper end of the outer tube passes through the partition 2 to connect to the second chamber.

[0011] A further improvement of the present invention is that the falling film tube corresponding to the cracking kettle is equipped with several cofferdams at different heights, with gaps between the cofferdams and the falling film tube. The oligomer material falls onto the cofferdams and then descends as a falling film. This not only solves the problem of excessive material descent speed, but also solves the problem of uneven material distribution such as biased flow 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 first devolatilizer is half of that of the fourth chamber.

[0013] A further improvement of the present invention is that the packing in the deacidification tower includes an upper layer of packing and a lower layer of packing, and the corresponding outlets include gas outlet 1 and gas outlet 2. Gas outlet 1 is arranged on the tower body between the upper and lower layers of packing, and gas outlet 2 is arranged on the tower body below the lower layer of packing. Gas outlet 1 and gas outlet 2 are connected to a hammer condenser. Gas is discharged from the two gas outlets, further ensuring that the material can be condensed more quickly. In addition, a diameter reduction section is provided at the bottom of the deacidification tower to reduce liquid holdup, shorten the material residence time, and reduce the probability of side reactions caused by high temperature of the material.

[0014] A further improvement of the present invention is that a gas phase port is provided at the top of the depolymerization kettle and the intermediate kettle, which is 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 kettle and / or the intermediate kettle; the top of the reflux tower is connected to the reflux tank after passing through condenser 1, and the reflux tank outlet 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 condensed materials of the condenser three and the condenser four are connected to the receiving tank two and the receiving tank three.

[0016] The present invention also provides a polylactic acid waste recycling method, which utilizes the above-mentioned polylactic acid waste recycling system to recycle polylactic acid waste, and the steps are as follows:

[0017] (1) A catalyst is added to the depolymerization kettle in advance. 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, an organic guanidine complex CRZnCl2, and an 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%.

[0018] (2) Start the screw extruder and squeeze the polylactic acid waste into the depolymerization kettle for depolymerization at 170-180°C under 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 170-190℃ cracking reactor from the bottom outlet, and the cracking reaction is carried out at an absolute pressure of 0.1-1KPa;

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

[0022] (6) The purified lactide product enters the prepolymerization kettle and is polymerized into a prepolymer at 150-170°C;

[0023] (7) The prepolymer material enters the polymerization reactor and is polymerized at 170-180°C;

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

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

[0026] Furthermore, the material at the bottom of the cracking kettle is returned to the cracking kettle for cyclic cracking, or returned to the depolymerization kettle or the intermediate kettle for depolymerization, or discharged as kettle residue.

[0027] This method has the advantages of low energy consumption and high production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG1 is a flow chart of a polylactic acid waste recycling system according to the present invention.

[0029] FIG2 is a schematic structural diagram of a devolatilizer 1.

[0030] FIG3 is a top view of the devolatilizer 1 in FIG2 .

[0031] FIG4 is a distribution diagram of the falling film tubes of the devolatilizer 1 in FIG2 .

[0032] FIG5 is a schematic structural diagram of the second devolatilizer.

[0033] FIG6 is a top view of the second devolatilizer in FIG4 .

[0034] In the figure: 1 delivery pump 1; 2 depolymerization kettle; 3 screw extruder; 4 reflux tower; 5 condenser 1; 6 reflux tank; 7 receiving tank 1; 8 preheater 1; 9 deacidification tower, 10 hammer condenser; 11 condenser 2; 12 light component tank; 13 preheater 2; 14 prepolymerization kettle, 15 polymerization reactor, 16 devolatilizer 1; 17 condenser 3; 18 receiving tank 2; 19 condenser 4; 20 receiving tank 3; 21 devolatilizer 2; 22 pelletizer; 23 delivery pump 7; 24 delivery pump 6; 25 delivery pump 5; 26 purification system, 27 preheater 3; 28 delivery pump 4; 29 cracking kettle, 30 delivery pump 2; 31 post-pump filter; 32 intermediate kettle, 33 delivery pump 3.

[0035] 1601 falling film tube, 1602 kettle body of devolatilizer 1, 1603 distribution plate, 1604 partition 2, 1605 partition 1, 1607 feed port of devolatilizer 1, 1608 heat medium inlet, 1609 heat medium outlet, 1610 claw distributor, 1611 gas phase outlet of devolatilizer 1.

[0036] 2101 inner support plate, 2102 feed port of devolatilizer 2, 2103 material distributor, 2104 liquid distributor filler, 2105 partition, 2106 gas phase outlet of devolatilizer 2, 2107 kettle body of devolatilizer 2. DETAILED DESCRIPTION Example 1

[0037] As shown in Figure 1, a polylactic acid waste recycling system includes a screw extruder 3, a depolymerization kettle 2, a cracking kettle 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 kettle 2. The bottom outlet of the depolymerization kettle 2 is connected to the bottom inlet of one end of the horizontally arranged intermediate kettle 32 through a delivery pump 1, and the bottom outlet of the other end of the intermediate kettle 32 is connected to the feed port of the cracking kettle 29 through a delivery pump 2 30, a post-pump filter 31, and a preheater 8. The outlet of the bottom of the cracking kettle 29 is connected to the inlet of the post-pump filter 31 after passing through a delivery pump 33, and then to the reuse port, and is connected to the depolymerization kettle 2 or the intermediate kettle 32 through the reuse port. , and there is a way to connect the kettle residue discharge outlet; the gas phase outlet in the middle of the cracking kettle 29 is connected to the gas phase inlet at the bottom of the deacidification tower 9. A hammer condenser 10 is provided on the top of the deacidification tower 9. The outlet of the hammer condenser 10 is connected to the light component tank 12 through the condenser 2 11. The condensate outlet at the bottom of the deacidification tower 9 is connected to the purification system 26 after passing through the transfer pump 4 28 and the preheater 3 27. The deacidification tower 9 is provided with an upper layer of packing and a lower layer of packing, and the corresponding outlets include gas outlet 1 and gas outlet 2. The gas outlet 1 is arranged on the tower body between the upper layer of packing and the lower layer of packing, and the gas outlet 2 is arranged on the tower body on the lower side of the lower layer of packing. The gas outlet 1 and the gas outlet 2 are connected to the hammer condenser 10. A reduced diameter section is provided at the bottom of the deacidification tower 9.

[0038] The gas outlet of the deacidification tower is connected to the hammer 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 delivery 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 2 21 via the delivery pump 6 24, and the bottom outlet of the devolatilizer 2 21 is connected to the pelletizer 22 via the delivery pump 7 23; the sides of the devolatilizer 1 16 and the devolatilizer 2 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] A plurality of stirring devices are provided in the middle kettle 32. The rotating shafts of the stirring devices are arranged vertically. A porous slow-flow partition is arranged vertically between adjacent stirring devices.

[0040] A heatable falling film tube is provided in the cracking kettle 29 and the devolatilizer 16, and the material moves from top to bottom along the falling film tube.

[0041] As shown in Figures 5 and 6, a partition 2105 is provided inside the kettle body 2107 of the second devolatilizer, and a plurality of material distributors 2103 are provided on the partition 2105. There is a cavity below the partition 2105. The feed inlet 2102 of the second devolatilizer is connected to the material distributor 2103. The material distributor 2103 includes an inner support plate 2101. A liquid distribution filler 2104 is provided above the inner support plate 2101. The inner support plate 2101 is provided with a plurality of material drop holes. The gas phase outlet 2106 of the second devolatilizer is provided on the side of the kettle body 2107 of the second devolatilizer below the partition 2105.

[0042] The cracking kettle 29 has a substantially identical internal structure to the devolatilizer 16. As shown in Figures 2, 3, and 4, the structure of the devolatilizer 1 is shown. A partition 1605, a partition 2 1604, and a distribution plate 1603 are provided in the kettle body 1602 of the devolatilizer 1 from top to bottom. A first chamber is formed above the partition 1605 in the kettle body, a second chamber is formed between the partition 1605 and the partition 2 1604, a third chamber is formed between the partition 2 1604 and the distribution plate 1603, and a fourth chamber is formed below the distribution plate 1603. The corresponding gas phase outlet 1611 of the devolatilizer 1 is provided on the side of the fourth chamber. The feed port 1607 of the devolatilizer 1 provided on the top of the kettle body 1602 of the devolatilizer 1 is connected to the claw distributor 1610, and multiple outlets of the claw distributor 1610 pass downward through the partition 1. 1605, partition two 1604 is connected to the third chamber, a heat medium inlet 1608 is provided on the top of the kettle body and connected to the first chamber, and a heat medium outlet 1609 is provided on the side of the second chamber; a plurality of falling film tubes 1601 are vertically arranged in the kettle body, and the falling film tubes 1601 include an inner tube and an outer tube arranged coaxially, the inner tube is a straight tube up and down, and the outer tube is a blind tube with an open upper end and a closed bottom; the upper end of the inner tube passes through the partition one 1605 to connect to the first chamber, and the lower end of the inner tube is inserted into the inner lower part of the outer tube. A plurality of falling film holes are provided on the distribution plate 1603, and the outer tube passes through the falling film holes with a gap, and the upper end of the outer tube passes through the partition two 1604 to connect to the second chamber.

[0043] The cracking vessel 29 differs from the devolatilizer 16 in that the falling film tube corresponding to the cracking vessel 29 is equipped with 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 as a film, which not only solves the problem of excessive material descent speed but also alleviates uneven material distribution, such as biased flow, in the falling film tube. The falling film tube corresponding to the devolatilizer 16 lacks external weirs. Its height is half the height of the fourth chamber, leaving space for internal material collection.

[0044] In Figure 1 , the tops of depolymerization vessel 2 and intermediate vessel 32 are equipped with gas-phase ports connected to the gas-phase inlet of reflux column 4. The liquid-phase outlet at the bottom of reflux column 4 is refluxed back to depolymerization vessel 2 and / or intermediate vessel 32. The top of reflux column 4 is connected to reflux tank 6 via condenser 1 5. The outlet of reflux tank 6 is connected to reflux column 4 on one side and to receiving tank 1 7 on the other. The condensate from condenser 3 17 and condenser 4 19 is connected to receiving tank 2 18 and receiving tank 3 20.

[0045] A polylactic acid waste recycling method, which utilizes the above-mentioned polylactic acid waste recycling system to recycle polylactic acid waste, comprises the following steps:

[0046] (1) A catalyst is added to the depolymerization reactor 2 in advance. 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, an organic guanidine complex CRZnCl2, and an 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%.

[0047] (2) Start the screw extruder 3 and squeeze the polylactic acid waste into the depolymerization reactor 2 for depolymerization at 170-180°C under normal pressure;

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

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

[0050] (5) The gaseous material from the cracking kettle 29 enters the deacidification tower 9 to remove light components. The condensation temperature of the hammer condenser 10 is 90-100°C, the condensation temperature of the condenser 11 is 80-90°C, the bottom temperature of the deacidification tower 9 is 100-130°C, and the bottom material of the deacidification tower 9 enters the purification system 26;

[0051] (6) The purified lactide product enters the prepolymerization reactor 14 and is polymerized into a prepolymer at 150-170°C;

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

[0053] (8) The polymer material enters the 200~220℃ devolatilizer 16 and the 2nd devolatilizer 21 for two-stage devolatilization;

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

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

[0056] This device has high recovery efficiency and can produce high-molecular-weight polylactic acid. The depolymerization process uses a unique catalyst, which lowers depolymerization and cracking temperatures, reduces energy consumption, and produces fewer racemized lactide byproducts. Example 2

[0057] Utilize the system of embodiment 1, change relevant parameters, carry out different recycling treatment, the process is as follows: start screw extruder 3, set processing temperature 170 ~ 230 ℃, start feeding polylactic acid waste, the polylactic acid waste used is polylactic acid sheet products, polylactic acid fiber and non-woven fabric products, polylactic acid film products, polylactic acid scraps, polylactic acid secondary brand one or more. The extruded molten material enters depolymerization kettle 2, the interlocking flow ratio of catalyst and molten material is 10%, the catalyst is composed of ethylene glycol, butanediol, and organic guanidine complex CRZnCl2, and organic guanidine complex CRZnCl2 accounts for 0.5% of the total amount of diols, control the temperature of depolymerization kettle 2 at 170 ~ 180 ℃, control the flow rate so that the material residence time meets 2-3h, and then enter the intermediate kettle 32 from the bottom of depolymerization kettle 2, control the temperature of intermediate kettle 32 at 150 ~ 160 ℃; the material of intermediate kettle 32 enters cracking kettle 29, controls the temperature of cracking kettle 29 at 170 ~ 180 ℃, and the vacuum degree is 0.1 ~ 1 kpa, and 0.1-1% of the feed amount of the cracking kettle 29 is discharged from the cracking kettle 29 through the delivery pump; the bottom temperature 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 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 150-170°C prepolymerization kettle 14, the 170-180°C polymerization reactor 15, the 200-220°C devolatilizer 16 and the devolatilizer 2 21 in sequence for devolatilization, to obtain a polylactic acid data with a weight average molecular weight of 126729 kg / mol. Example 3

[0058] Utilize the system of embodiment 1, change relevant parameters, carry out different recycling treatment, process is as follows: start screw extruder 3, set processing temperature 170 ~ 230 ℃, start feeding polylactic acid waste, the polylactic acid waste used is polylactic acid sheet product, polylactic acid fiber and non-woven fabric product, polylactic acid film product, polylactic acid scrap, polylactic acid secondary brand one or more. The molten material of extrusion enters depolymerization kettle 2, the chain flow ratio of catalyst and molten material is 5%, the catalyst is propylene glycol, stannous octoate composition, stannous octoate accounts for 1% of the total amount of diols, control depolymerization kettle 2 temperature 170 ~ 180 ℃, control flow rate so that material residence time meets 3-4h, enter intermediate kettle 32 from the bottom of depolymerization kettle 2, control intermediate kettle 32 temperature 160 ~ 170 ℃; intermediate kettle 32 material enters cracking kettle 29, controls cracking kettle 29 temperature 180 ~ 190 ℃, vacuum degree 0.1 ~ 1kpa, through cracking kettle 29 delivery pump 0.1-1% of the feed amount of the cracking kettle 29 is discharged; the temperature at the bottom of the deacidification tower 9 is 120-130°C, the condensation temperature of the hammerhead condenser 10 is 90-100°C, the temperature of the condenser 11 is 80-90°C, and the temperature at the bottom of the deacidification tower 9 is 100-130°C. After passing through the purification system 26, the product sequentially enters the 150-170°C prepolymerization kettle 14, the 170-180°C polymerization reactor 15, the 200-220°C devolatilizer 16 and the devolatilizer 2 21 for devolatilization, to obtain a polylactic acid data with a weight average molecular weight of 185076 kg / mol. Example 4

[0059] The system of Example 1 is used to change relevant parameters and perform different recycling treatments. The process is as follows: start the screw extruder 3, set the processing temperature to 170-230° C., and start feeding polylactic acid waste. The polylactic acid waste used is one or more of polylactic acid sheet products, polylactic acid fibers and non-woven fabrics, polylactic acid film products, polylactic acid scraps, and polylactic acid sub-brands. The extruded molten material enters the depolymerization reactor 2, the chain flow ratio of the catalyst and the molten material is 2%, the catalyst is composed of butanediol, stannous lactate, and an organic guanidine complex CRFe(OAc)2, and the total amount of stannous octoate and the organic guanidine complex CRFe(OAc)2 accounts for 2% of the total amount of butanediol. The temperature of the depolymerization reactor 2 is controlled at 170-180°C, and the flow rate is controlled so that the material residence time meets 4-5h. Then, the material is removed from the middle reactor 32 at the bottom of the depolymerization reactor 2, and the temperature of the middle reactor 32 is controlled at 160-170°C; the material in the middle reactor 32 enters the cracking reactor 29, and the temperature of the cracking reactor 29 is controlled at 180-190°C, and the vacuum degree is 0 0.1-1 kPa, 0.1-1% of the feed to cracker 29 is discharged from the bottom of cracker 29; the bottom temperature of deacidification tower 9 is 100-110°C, the condensation temperature of hammerhead condenser 10 is 90-100°C, the temperature of condenser 2 11 is 80-90°C, and the bottom temperature of deacidification tower 9 is 100-130°C. After passing through purification system 26, the product enters prepolymerization reactor 14 at 150-170°C, polymerization reactor 15 at 170-180°C, devolatilizer 1 16 at 200-220°C, and devolatilizer 2 21 for devolatilization, resulting in polylactic acid with a weight-average molecular weight of 193105 kg / mol. Example 5

[0060] Utilize the system of embodiment 1, change relevant parameters, carry out different recycling treatment, the process is as follows: start screw extruder 3, set processing temperature 170 ~ 230 ℃, start feeding polylactic acid waste, the polylactic acid waste used is polylactic acid sheet product, polylactic acid fiber and non-woven fabric product, polylactic acid film product, polylactic acid scrap, polylactic acid secondary brand one or more. The extruded molten material enters depolymerization kettle 2, the chain flow ratio of catalyst and molten material is 8%, the catalyst is butanediol, organic guanidine complex CRFe (OAc) 2 composition, organic guanidine complex CRFe (OAc) 2 total amount accounts for 0.2% of butanediol total amount, control depolymerization kettle 2 temperature 170 ~ 180 ℃, control flow rate so that material residence time meets 5-6h, enter intermediate kettle 32 from the bottom of depolymerization kettle 2, control intermediate kettle 32 temperature 150 ~ 160 ℃; intermediate kettle 32 material enters cracking kettle 29, control cracking kettle 29 temperature 180 ~ 190 ℃, vacuum 0.1 ~1kpa, 0.1-1% of the feed amount of the cracking kettle 29 is discharged from the bottom of the cracking 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 11 is 80-90°C, the temperature at the bottom of the deacidification tower 9 is 100-130°C, and after passing through the purification system 26, the product enters the 150-170°C prepolymerization kettle 14, the 170-180°C polymerization reactor 15, the 200-220°C devolatilizer 16 and the devolatilizer 2 21 for devolatilization, and the polylactic acid data relative molecular weight is 198054kg / mol.

[0061] The foregoing description is merely a preferred embodiment of the present invention and does not limit the scope of patent protection of the present invention. In addition to the foregoing embodiment, the present invention may also have other implementations. Any technical solution formed by equivalent substitution or equivalent transformation falls within the scope of protection claimed by the present invention. Technical features not described in the present invention can be implemented by or using existing technologies and will not be described in detail here.

Claims

1. A polylactic acid waste recycling system, comprising a screw extruder, a depolymerization kettle, a cracking kettle, and a vacuum system. The outlet of the screw extruder is connected to the inlet of the depolymerization kettle. It is characterized in that: The bottom outlet of the depolymerization kettle is connected to the bottom inlet at one end of a horizontally arranged intermediate kettle through a transfer pump I. The bottom outlet at the other end of the intermediate kettle is connected to the feed inlet of the cracking kettle through a transfer pump II, a post-pump filter, and a preheater I. The outlet at the bottom of the cracking kettle is connected to the inlet of the post-pump filter through a transfer pump III, and is also connected to a reuse port and 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 a deacidification tower. A hammer condenser is provided at the top of the deacidification tower. The outlet of the hammer condenser is connected to a light component tank through a condenser II. The condensate outlet at the bottom of the deacidification tower is connected to a purification system through a transfer pump IV and a preheater III. At least one layer of packing is provided in the deacidification tower. An air outlet is provided below the packing layer and is connected to the hammer condenser. The outlet of the purification system is connected to a prepolymerization kettle through a preheater II. The bottom outlet of the prepolymerization kettle is connected to the bottom inlet of a polymerization reactor through a transfer pump V. The top outlet of the polymerization reactor is connected to the top inlet of a devolatilizer I. The bottom outlet of the devolatilizer I is connected to the top inlet of a devolatilizer II through a transfer pump VI. The bottom outlet of the devolatilizer II is connected to a pelletizer through a transfer pump VII. Gas-phase outlets are respectively provided on the sides of the devolatilizer I and the devolatilizer II, and the gas-phase outlets are connected to the vacuum system through corresponding condensers III and condensers IV. A plurality of stirring devices are provided in the intermediate kettle. The rotating shafts of the stirring devices are arranged vertically, and porous flow retardation partitions are arranged vertically between adjacent stirring devices. Heatable falling film tubes are provided in the cracking kettle and the devolatilizer I, and the material moves downward along the falling film tubes. A partition is provided inside the kettle body of the devolatilizer II, and a plurality of material distributors are arranged on the partition. A cavity is provided below the partition. The feed inlet of the devolatilizer II is connected to the material distributor. The material distributor includes an inner support plate. A liquid distribution packing is provided above the inner support plate. A plurality of blanking holes are provided on the inner support plate. The gas-phase outlet of the devolatilizer II is arranged on the side of the kettle body below the partition.

2. A polylactic acid waste recycling system according to claim 1, It is characterized in that: In the kettle body of the cracking kettle and the first devolatilizer, a first partition plate, a second partition plate and a distribution plate are arranged from top to bottom. A first chamber is formed above the first partition plate in the kettle body, a second chamber is formed between the first partition plate and the second partition plate, a third chamber is formed between the second partition plate and the distribution plate, and a fourth chamber is formed below the distribution plate. The 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 distributor. Multiple outlets of the claw distributor pass downward through the first partition plate and the second partition plate and are connected to the third chamber. A heat medium inlet is arranged at the top of the kettle body and connected to the first chamber, and a heat medium outlet is arranged on the side of the second chamber; several falling film tubes are arranged vertically in the kettle body. The falling film tube includes an inner tube and an outer tube arranged coaxially. The inner tube is a straight tube communicating up and down, and the outer tube is a blind tube with an open upper end and a closed bottom; the upper opening of the inner tube passes through the first partition plate and is connected to the first chamber, the lower opening of the inner tube is inserted into the lower inner part of the outer tube, and several falling film holes are arranged on the distribution plate. The outer tube passes through the falling film holes with a gap, and the upper opening of the outer tube passes through the second partition plate and is connected to the second chamber.

3. The poly(lactic acid) waste recycling system according to claim 2, characterized in that: Several weirs are arranged at different heights on the falling film tubes corresponding to the cracking kettle, and a gap is left between the weir and the falling film tube.

4. The poly(lactic acid) waste recycling system according to claim 2, characterized in that: The height of the falling film tube corresponding to the first devolatilizer is half of that of the fourth chamber.

5. The poly(lactic acid) waste recycling system according to claim 1, characterized in that: The packing in the deacidification tower includes upper packing and lower packing. The corresponding outlets include a first gas outlet and a second gas outlet. The first gas outlet is arranged on the tower body between the upper packing and the lower packing, and the second gas outlet is arranged on the tower body below the lower packing. The first gas outlet and the second gas outlet are connected to a hammer condenser.

6. The poly(lactic acid) waste recycling system according to claim 1, characterized in that: Gas phase ports are arranged at the tops of the depolymerization kettle and the intermediate kettle and are connected to the gas phase inlet of the reflux tower. The liquid phase outlet at the bottom of the reflux tower is reflux-connected to the depolymerization kettle and / or the intermediate kettle; the top of the reflux tower is connected to a reflux tank through a first condenser. One outlet of the reflux tank is connected to the reflux tower, and the other is connected to a first receiving tank.

7. The poly(lactic acid) waste recycling system according to claim 1, characterized in that: The condensed materials of the third condenser and the fourth condenser are connected to a second receiving tank and a third receiving tank.

8. A poly(lactic acid) waste recycling method, characterized in that, using the poly(lactic acid) waste recycling system according to any one of claims 1-7 for recycling, the steps are as follows: (1) The depolymerization kettle is pre-added with a catalyst, which is composed of fatty alcohols 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, and organic guanidine complex CRZnCl 2 , 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) Start the screw extruder, extrude the poly(lactic acid) waste into the depolymerization kettle, and depolymerize it at normal pressure of 170-180 °C; (3) Pump the material from the bottom of the depolymerization kettle into the intermediate kettle at 150-170 °C; (4) The material in the intermediate kettle enters the cracking kettle at 170-190 °C from the bottom discharge port, and undergoes a cracking reaction at an absolute pressure of 0.1-1 Kpa; (5) The gas phase material in the cracking kettle enters the deacidification tower to remove light components. The condensation temperature of the hammer condenser is 90-100 °C, the condensation temperature of the second condenser is 80-90 °C, the bottom temperature of the deacidification tower is 100-130 °C, and the material at the bottom of the deacidification tower enters the purification system; (6) The purified lactide product enters the prepolymerization kettle and is polymerized into a prepolymer at 150-170 °C; (7) The prepolymerized material enters the polymerization reactor and is polymerized at 170-180 °C; (8) The polymer material enters the devolatilizers I and II at 200-220 °C for two-stage devolatilization; (9) The product after devolatilization is cut into particles by a pelletizer.

9. A method for recycling polylactic acid waste according to claim 8, characterized in that: The material at the bottom of the cracking kettle is returned to the cracking kettle for cyclic cracking, or returned to the depolymerization kettle or the intermediate kettle for depolymerization, or discharged as kettle residue.

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