Method and system for recycling waste plastics

Abstract

The invention relates to a method and system for recycling waste plastics, and the method comprises the steps that S1, solid waste plastics and replacement gas enter a deoxidation device, so that the replacement gas conducts purging deoxidation treatment on the solid waste plastics, deoxidized plastics are obtained, and the deoxidation device comprises a gas stripping functional area and a diffusion functional area which communicate with each other; s2, the deoxidized plastic enters a softening-dechlorination device to be subjected to softening-dechlorination treatment, and chlorine-containing gas and softened dechlorinated plastic are obtained; s3, the softened dechlorinated plastic enters a liquefying device to be subjected to liquefying treatment, and pyrolysis gas and liquid plastic are obtained; s4, enabling the chlorine-containing gas in the step S2 and the pyrolysis gas in the step S3 to enter an alkali washing device for alkali washing treatment; and at least part of the liquid plastic in the step S3 serves as a circulating material to be heated and then returns to the liquefying device to be continuously treated. The present disclosure can significantly reduce the yield of by-products and continuously and stably provide a liquid feed for waste plastic pyrolysis.

Description

Technical Field

[0001] This disclosure relates to the field of waste plastic utilization, and more specifically, to a method and system for recycling waste plastics. Background Technology

[0002] Plastics, with their high molecular structure, can have their carbon-carbon bonds broken through thermal cracking to produce petrochemical products such as cracked oil. Waste plastic pyrolysis technology enables the reuse of waste resources and has significant economic value, making it an important measure for developing a circular economy in my country. However, the technology is not yet fully developed. Existing pyrolysis technologies mostly employ intermittent methods (rotary kilns, etc.), resulting in low production efficiency and unstable equipment operation. Furthermore, plastic films or granules have high porosity and contain a large amount of air. The high temperatures during pyrolysis generate a certain amount of light hydrocarbons, and direct feeding of plastic films or granules increases the possibility of an explosion due to the mixing of oxygen and light hydrocarbons. Moreover, the softening and liquefaction process of plastics typically produces a significant amount of byproduct gaseous phases, gums, and asphaltenes, affecting product quality and liquid phase yield. In addition, solid waste plastics contain a certain amount of PVC, which decomposes upon heating to produce various chlorides. Improper handling can cause severe equipment corrosion and affect product quality.

[0003] CN114262621A discloses a waste plastic liquefaction pyrolysis system and method. This invention proposes liquefying waste plastics by introducing hot nitrogen gas into the bottom of a liquid phase tank, and installing two sealing plates at the top of the tank, which are alternately opened and closed to achieve continuous sealed feeding. However, this method allows air carried by the particles into the pyrolysis process, failing to effectively prevent contact between air and high-temperature pyrolysis gas; furthermore, pyrolysis gas exists between the two sealing plates, and when the first sealing plate is opened for feeding, pyrolysis gas mixed with air is released. Summary of the Invention

[0004] The purpose of this disclosure is to provide a method and system for recycling waste plastics, which can significantly reduce the yield of by-products and provide a continuous and stable liquid feed for the pyrolysis of waste plastics.

[0005] To achieve the above objectives, the first aspect of this disclosure provides a method for recycling waste plastics, comprising the following steps: S1. Solid waste plastic and displacement gas are introduced into a deoxygenation device, so that the displacement gas purges and deoxygenates the solid waste plastic to obtain deoxygenated plastic. The deoxygenation device includes a gas stripping functional area and a diffusion functional area. S2. The deoxidized plastic is fed into a softening-dechlorination device for softening-dechlorination treatment to obtain chlorine-containing gas and softened dechlorinated plastic; S3. The softened dechlorinated plastic is fed into a liquefaction device for liquefaction treatment to obtain pyrolysis gas and liquid plastic. S4. The chlorine-containing gas mentioned in step S2 and the pyrolysis gas mentioned in step S3 are introduced into the alkaline washing device for alkaline washing treatment; at least a portion of the liquid plastic mentioned in step S3 is heated as a circulating material and then returned to the liquefaction device for further processing.

[0006] Optionally, the deoxygenation device includes a first cylinder, the first cylinder including an air-lift section and a diffusion section, the air-lift section forming the air-lift functional area, and the diffusion section forming the diffusion functional area; The inner diameter of the air-lift section is smaller than the inner diameter of the diffusion section; The air-lift section and the diffusion section are connected by a variable-diameter section; the variable-diameter section includes a first port and a second port, the inner diameter of the first port is smaller than the inner diameter of the second port, the first port of the variable-diameter section is connected to the air-lift section, and the second port of the variable-diameter section is connected to the diffusion section. Preferably, the ratio of the cross-sectional area of ​​the gas lift section cylinder to the cross-sectional area of ​​the diffuser section cylinder is 0.02~0.95:1, more preferably 0.05~0.5:1; Preferably, the first cylinder includes a plurality of the air-lift sections, the variable-diameter section, and the diffusion section; Preferably, the top of the first cylinder is a gas lift section, and the top of the first cylinder is provided with a solid waste plastic inlet; the bottom of the first cylinder is a diffusion section, and the side wall of the diffusion section at the bottom of the first cylinder is provided with a displacement gas inlet.

[0007] Optionally, the deoxygenation device includes a second cylinder; the second cylinder includes an air-lifting component and a diffusion component; the air-lifting component forms the air-lifting functional area, and the diffusion component forms the diffusion functional area; preferably, the air-lifting component and the diffusion component are coaxially arranged with the second cylinder; The air-lift component includes a funnel section and a constant-diameter section. The funnel section includes an open end and a closed end. The inner diameter of the constant-diameter section is the same as the inner diameter of the closed end of the funnel section, and one section of the constant-diameter section is connected to the closed end of the funnel section. The open end of the funnel section is fixedly connected along the side wall of the second cylinder, and the open end of the funnel section is positioned facing the solid waste plastic inlet of the second cylinder. Preferably, the outer diameter of the open end is the same as the inner diameter of the second cylinder. Preferably, the funnel section has a cone structure with uniformly varying diameter; the angle between the sidewall of the funnel section and the central axis of the funnel section is an acute angle, preferably 10~80°; preferably, the ratio of the cross-sectional area of ​​the equal-diameter section of the air-lifting component to the cross-sectional area of ​​the second cylinder is 0.02~0.95:1, preferably 0.05~0.5:1; The diffusion component is a closed cone; the cone apex faces the equal-diameter section of the air-lifting component, with its port spaced apart from the funnel section; optionally, the angle between the sidewall of the cone and the central axis of the cone is an acute angle, preferably 10~80°. Optionally, the bottom surface of the cone is smaller than the inner diameter of the second cylinder, so that an annular space is formed between the side wall of the cone and the inner wall of the second cylinder; the width of the annular space gradually decreases along the flow direction of solid waste plastic in the second cylinder; Preferably, the ratio of the area of ​​the annular space in the plane containing the bottom surface of the cone to the cross-sectional area of ​​the second cylinder is 0.02~0.95:1, more preferably 0.05~0.5:1; More preferably, the second cylinder is provided with a plurality of air-lifting components and a plurality of diffusion components, with a diffusion component disposed between every two air-lifting components, and the funnel section of the air-lifting component located downstream of the diffusion component is spaced apart along the flow direction of solid waste plastic in the second cylinder, with its opening end facing the bottom surface of the diffusion component; preferably, the top of the second cylinder is provided with a solid waste plastic inlet, and the top of the second cylinder is provided with an air-lifting component, so that the solid waste plastic entering the second cylinder through the solid waste plastic inlet first enters the air-lifting component.

[0008] Optionally, step S1 includes: allowing the solid waste plastic to enter the deoxidation device through the solid waste plastic inlet at the top of the deoxidation device, and continuously introducing the displacement gas into the deoxidation device through the displacement gas inlet on the bottom side wall of the deoxidation device to continuously purge the solid waste plastic, thereby displacing the air in the deoxidation device and obtaining deoxidized plastic; Optionally, the ratio of the amount of the displacement gas to the feed volume of the solid waste plastic is 0.01~50:1, preferably 0.5~10:1; Optionally, the replacement gas is selected from one or more of carbon dioxide, inert gases and liquid hydrocarbons, preferably nitrogen or carbon dioxide.

[0009] Optionally, in step S2, the softening-dechlorination device is a first screw compressor; the first screw compressor includes a screw inlet, a screw outlet, a first exhaust port, and a first heating element; Preferably, the inlet temperature of the first screw compressor is 0~120℃, the material residence time is 10~1500s, and the outlet temperature of the first screw compressor is 80~290℃; more preferably, the inlet temperature of the first screw compressor is 20~70℃, the material residence time is 6~400s, and the outlet temperature of the first screw compressor is 120~240℃. Preferably, the chlorine content of the softened dechlorinated plastic is less than 0.1% by weight.

[0010] Optionally, in step S3, the liquefaction device is a stirring device, and a second heating element is provided on the outer side of the bottom of the stirring device to heat the material in the stirring device; Optionally, the liquefaction conditions include: a temperature of 150~490℃, a pressure of -0.09~1.0 MPa, and a time of 0.1~0.9 h; preferably, a temperature of 250~410℃, a pressure of 0.05~0.5 MPa, and a time of 0.2~0.8 h; optionally, a stirring rate of 10~500 r / min, preferably 50~200 r / min.

[0011] Optionally, in step S3, the liquefaction device is a second screw compressor, which includes a third heating element for heating the material inside the second screw compressor; The liquefaction conditions include: the inlet temperature of the second screw compressor is 80~290℃, the material residence time is 10~1200s, and the outlet temperature of the second screw compressor is 250~490℃; preferably, the inlet temperature of the second screw compressor is 120~240℃, the material residence time is 60~300s, and the outlet temperature of the second screw compressor is 250~410℃.

[0012] Optionally, in step S4, the circulating material accounts for 10-90% by weight of all liquid material from the liquefaction unit, preferably 30-60% by weight; preferably, the temperature of the circulating material after heating is 300-500°C, preferably 350-450°C. Optionally, the weight ratio of the recycled material to the softened and dechlorinated plastic from the softening-dechlorination unit is 0.1 to 9:1, preferably 0.5 to 3:1.

[0013] Optionally, the method further includes: The pyrolysis gas from the liquefaction unit is fed into a gas-liquid separation unit for gas-liquid separation to obtain gaseous and liquid materials. The gaseous material is introduced into the alkaline washing device for alkaline washing treatment; the liquid material is then introduced into the subsequent liquefied plastics processing device. Optionally, the alkaline washing agent in the alkaline washing device is selected from one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate.

[0014] Optionally, the geometric equivalent diameter of the solid waste plastic particles ranges from 0.1 to 100 mm; preferably, the bulk density of the solid waste plastic particles in the deoxidation device is 50 to 800 kg / m³. 3 ; Optionally, the solid waste plastic particles are lifted into a deoxygenation device by a lifting device; the lifting device is selected from mechanical lifting equipment or gas conveying equipment.

[0015] A second aspect of this disclosure provides a system for recycling waste plastics, including a deoxidation unit, a softening-dechlorination unit, a liquefaction unit, and an alkaline washing unit; The deoxygenation device includes a solid waste plastic inlet, a displacement gas inlet, and a deoxygenated plastic outlet; the deoxygenation device includes a gas stripping functional zone and a diffusion functional zone; The softening-dechlorination device includes a deoxygenated plastic inlet, a chlorine-containing gas outlet, and a softened dechlorinated plastic outlet, wherein the deoxygenated plastic inlet is connected to the deoxygenated plastic outlet of the deoxygenation device. The liquefaction device includes a softening and dechlorinating plastic inlet, a pyrolysis gas outlet, and a liquid plastic outlet. The softening and dechlorinating plastic inlet is connected to the softening and dechlorinating plastic outlet of the softening-dechlorinating device, and the liquid plastic outlet is connected to the softening and dechlorinating plastic inlet of the liquefaction device via a liquid plastic circulation pipeline. The alkaline washing device includes an inlet for the material to be washed, an inlet for the alkaline washing agent, an outlet for the waste liquid, and an exhaust port; the inlet for the material to be washed is connected to the chlorine-containing gas outlet of the softening-dechlorination device and the pyrolysis gas outlet of the liquefaction device, respectively.

[0016] Through the above technical solution, this disclosure provides a method and system for recycling waste plastics, which includes at least the following beneficial effects: (1) Solid waste plastics are deoxygenated in a deoxygenation device with good deoxygenation performance. The device operates continuously and the amount of replacement medium used is low. After deoxygenation, the plastic has a low oxygen content, which can effectively reduce the possibility of explosion caused by mixing oxygen and subsequent high-temperature pyrolysis gas, and greatly improve the intrinsic safety of the device. It reduces the participation of oxygen in the pyrolysis reaction, greatly reduces the gas phase yield and increases the liquid yield, and at the same time reduces the content of oxygen and oxygen-containing compounds generated by plastic pyrolysis intermediates, and improves the properties of liquid phase products. (2) In the softening-dechlorination device, the plastic softens at a relatively low temperature, where the chlorine-containing plastic decomposes and releases chlorine-containing gas, which is extracted through multiple exhaust ports set in the softening-dechlorination device. The chlorine content of the softened and dechlorinated plastic is low, which can effectively reduce the corrosion of subsequent equipment and reduce the chlorine content in the liquid product. (3) The plastic liquefaction device itself has a low pressure drop and smooth internal gas-liquid flow, which can effectively prevent liquid plastic from coking and clogging the equipment and causing the internal material to flow upstream. The device has high operational safety. (4) Solid plastics are continuously converted into liquid plastics to continuously supply liquid plastics to downstream pyrolysis units, thereby enabling continuous and safe production of the units.

[0017] Other features and advantages of this disclosure will be described in detail in the following detailed description section. Attached Figure Description

[0018] The accompanying drawings are provided to further illustrate the present disclosure and form part of the specification. They are used together with the following detailed description to explain the present disclosure, but do not constitute a limitation thereof. In the drawings: Figure 1 This is a flowchart illustrating the method for recycling waste plastics provided in this disclosure; Figure 2 A schematic diagram of the structure of a deoxygenation device provided in one embodiment of this disclosure; Figure 3 This is a schematic diagram of the deoxygenation device provided in another embodiment of this disclosure.

[0019] Figure Labels 1-Solid waste plastic; 2-Displacement gas outlet; 3-Deoxidation device; 4-Deoxidized plastic; 5-Softening-dechlorination device; 6-Liquefaction device; 7-Liquid plastic; 8-Gas-liquid separation device; 9-Liquefied gas; 10-Alkali washing device; 11-Alkali solution; 12-Emission gas; 13-Waste alkali solution; 14-Softened and dechlorinated plastic; 15-Circulating material; 16-Heating furnace; 17-Displacement gas inlet; 18-Acidic gas; 19-Gas stripping section; 20-Diffusion section; 21-Gas stripping component; 22-Diffusion component; 23-Including angle; 24-Including angle. Detailed Implementation

[0020] The following provides a detailed description of specific embodiments of this disclosure. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit this disclosure.

[0021] The first aspect of this disclosure provides a method for recycling waste plastics, comprising the following steps: S1. Solid waste plastic and displacement gas are introduced into deoxygenation device 3 so that the displacement gas can purge and deoxygenate the solid waste plastic to obtain deoxygenated plastic. The deoxygenation device 3 includes a gas stripping functional area and a diffusion functional area. S2. The deoxidized plastic is fed into the softening-dechlorination device 5 for softening-dechlorination treatment to obtain chlorine-containing gas and softened dechlorinated plastic; S3. The softened dechlorinated plastic is fed into the liquefaction device 6 for liquefaction treatment to obtain pyrolysis gas and liquid plastic. S4. The chlorine-containing gas mentioned in step S2 and the pyrolysis gas mentioned in step S3 are introduced into the alkaline washing device 10 for alkaline washing treatment; at least a portion of the liquid plastic mentioned in step S3 is heated as a circulating material and then returned to the liquefaction device 6 for further processing.

[0022] This disclosure provides a method for recycling waste plastics. The method involves deoxygenating solid waste plastics in a deoxygenation device with excellent deoxygenation performance. The device operates continuously with low displacement medium usage. The deoxygenated plastics have a low oxygen content, effectively reducing the possibility of an explosion caused by the mixing of oxygen and subsequent high-temperature pyrolysis gases, greatly improving the intrinsic safety of the device. Reducing oxygen participation in the pyrolysis reaction significantly reduces the gas phase yield and increases liquid recovery, while simultaneously reducing the content of oxygen-containing compounds generated from oxygen and plastic pyrolysis intermediates, improving the properties of the liquid phase product. Furthermore, the deoxygenation device incorporates a gas stripping functional zone and a diffusion functional zone, further enhancing the deoxygenation effect on solid waste plastics. In the softening- In the dechlorination unit, plastics soften at relatively low temperatures, during which chlorinated plastics decompose and release chlorine-containing gases, which are extracted through multiple exhaust ports within the softening-dechlorination unit. The softened and dechlorinated plastics have low chlorine content, effectively reducing corrosion to downstream equipment and lowering the chlorine content in the liquid product. The short residence time within the plastic liquefaction unit results in low asphaltene content in the product. The unit itself has a low pressure drop and smooth gas-liquid flow, effectively preventing coking and blockage of the equipment by liquid plastics and avoiding upstream backflow of internal materials, ensuring high operational safety. By converting solid plastics into liquid plastics, the unit continuously supplies liquid plastics to the downstream pyrolysis unit, enabling continuous and safe production.

[0023] In one specific embodiment, the solid waste plastic includes one or more of polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET); the solid waste plastic has a chlorine content of 0.01-3% by weight, a water content of less than 30% by weight, and an organic impurity content of less than 20% by weight; the solid waste plastic can be film sheets or granules. The method provided in this disclosure is applicable to a wide range of waste plastics.

[0024] In one specific embodiment, the deoxygenation device 3 includes a solid waste plastic inlet and a deoxygenated plastic outlet, a displacement gas inlet and a displacement gas outlet; wherein the solid waste plastic inlet and the displacement gas outlet are disposed at the top of the deoxygenation device, the displacement gas inlet is disposed on the bottom side wall of the deoxygenation device, and the deoxygenated plastic outlet is disposed at the bottom of the deoxygenation device, so that the displacement gas introduced through the displacement gas inlet moves upward from the bottom of the deoxygenation device, and the deoxygenation device includes a gas lift functional area and a diffusion functional area connected in sequence from top to bottom, so as to achieve deep deoxygenation with less displacement gas.

[0025] The deoxygenation device provided in this disclosure can be designed by shaping and structured the cylinder of the deoxygenation device itself to form a gas-lifting functional zone and a diffusion functional zone (such as...). Figure 2 (as shown); or, by setting up airlift and diffusion components within the deoxygenation device to form airlift and diffusion functional zones (e.g. Figure 3 (As shown).

[0026] In the first implementation, such as Figure 2 As shown, the deoxygenation device 3 includes a first cylinder, which includes an air-lift section and a diffusion section. The air-lift section forms the air-lift functional area, and the diffusion section forms the diffusion functional area. The air-lift section and the diffusion section are cylindrical bodies of equal diameter, and the inner diameter of the air-lift section is smaller than the inner diameter of the diffusion section. The air-lift section and the diffusion section are connected by a variable-diameter section. The variable-diameter section includes a first port and a second port, where the inner diameter of the first port is smaller than that of the second port. The first port of the variable-diameter section is connected to the air-lift section, and the second port of the variable-diameter section is connected to the diffusion section. In this embodiment, the deoxygenation device, by setting a smaller-diameter air-lift section and a larger-diameter diffusion section, allows waste plastics to flow sequentially from top to bottom through the air-lift section, the variable-diameter section, and the diffusion section, while the replacement gas moves from bottom to top to remove oxygen from the waste plastics. The smaller-diameter air-lift section restricts the flow of solid waste plastics and also restricts the replacement gas from entering the smaller-diameter air-lift section from the larger-diameter diffusion section. This creates an inert gas barrier in the deoxygenation device, preventing oxygen from flowing downstream and avoiding the downward flow of oxygen carried by the solid plastics, thus improving the deoxygenation effect. After the solid waste plastics enter the diffusion section from the air-lift section, the flow space increases, which is more conducive to the deep deoxygenation of the solid waste plastics by the replacement gas.

[0027] In a preferred embodiment, the ratio of the cross-sectional area of ​​the gas-lift section to the cross-sectional area of ​​the diffusion section is 0.02 to 0.95:1, preferably 0.05 to 0.5:1. When the ratio of the cross-sectional areas of the gas-lift section and the diffusion section is within the range of this embodiment, especially within the preferred range, the deoxygenation device has a better deoxygenation effect.

[0028] In one specific implementation, such as Figure 2 As shown, the first cylinder includes multiple air-lift sections, variable-diameter sections, and diffusion sections, allowing solid waste plastics to flow through the air-lift and diffusion sections multiple times from top to bottom. In each air-lift section, an inert gas barrier is formed to prevent the waste plastics from carrying oxygen. After passing through multiple air-lift sections, the amount of oxygen carried by the waste plastics can be further reduced, improving the deoxygenation effect. Similarly, after passing through multiple diffusion sections, the deep deoxygenation effect can be further improved.

[0029] In one specific implementation, such as Figure 2 As shown, the top of the first cylinder is a stripping section, and a solid waste plastic inlet is provided at the top of the first cylinder; the bottom of the first cylinder is a diffusion section, and a displacement gas inlet is provided on the side wall of the diffusion section at the bottom of the first cylinder. Setting the stripping section at the very top and the diffusion section at the very bottom of the deoxygenation device is more suitable for the deoxygenation process.

[0030] In the second implementation method, such as Figure 3 As shown, the deoxygenation device 3 includes a second cylinder; the second cylinder includes an air-lifting component and a diffusion component; the air-lifting component forms the air-lifting functional area, and the diffusion component forms the diffusion functional area; preferably, the air-lifting component and the diffusion component are coaxially arranged with the second cylinder; The air-lift component includes a funnel section and a constant-diameter section. The funnel section includes an open end and a closed end. The inner diameter of the constant-diameter section is the same as the inner diameter of the closed end of the funnel section, and one end of the constant-diameter section is connected to the closed end of the funnel section. The open end of the funnel section is fixedly connected along the side wall of the second cylinder, and the open end of the funnel section faces the solid waste plastic inlet of the second cylinder. Preferably, the outer diameter of the open end is the same as the inner diameter of the second cylinder. Using the deoxygenation device provided in this embodiment, when solid waste plastic particles flow through the air-lift component, they first enter the funnel section and then enter the diffusion section from the closed end. During this process, the flow area of ​​the solid waste plastic is restricted, and the flow of the replacement gas is also restricted. Therefore, an inert gas resistance is formed in the deoxygenation equipment, preventing oxygen from flowing downstream, reducing the oxygen entrained in the solid waste plastic, and improving the deoxygenation effect. After the solid waste plastic contacts the cone apex of the diffusion component below at the outlet of the air-lift component, it diffuses and flows into the surrounding annular gap, increasing the flow area and further improving the deep deoxygenation effect.

[0031] In a preferred embodiment, the funnel section has a cone structure with uniformly varying diameter; the included angle 23 between the sidewall of the funnel section and the central axis of the funnel section is an acute angle, preferably 10~80°; preferably, the ratio of the cross-sectional area of ​​the equal-diameter section of the air-lifting component to the cross-sectional area of ​​the second cylinder is 0.02~0.95:1, preferably 0.05~0.5:1; The diffusion component is a closed cone; the cone apex faces the equal diameter section of the air-lift component and is spaced apart on the side away from the funnel section; optionally, the angle 24 between the side wall of the cone and the central axis of the cone is an acute angle, preferably 10~80°. The deoxygenation device with the structure in this embodiment can have a better deoxygenation effect.

[0032] In one embodiment, the bottom surface of the cone is smaller than the inner diameter of the second cylinder, so that an annular space is formed between the side wall of the cone and the inner wall of the second cylinder; the width of the annular space gradually decreases along the flow direction of solid waste plastic in the second cylinder. Preferably, the ratio of the area of ​​the annular space in the plane containing the bottom surface of the cone to the cross-sectional area of ​​the second cylinder is 0.02~0.95:1, more preferably 0.05~0.5:1. Having the minimum ratio of the annular space area to the cross-sectional area of ​​the second cylinder provided by this embodiment can improve the deoxidation effect.

[0033] In a preferred embodiment, such as Figure 3 As shown, the second cylinder is provided with multiple air-lifting components and multiple diffusion components. A diffusion component is positioned between every two air-lifting components. Along the flow direction of solid waste plastic in the second cylinder, the funnel-shaped openings of the air-lifting components downstream of the diffusion components are spaced apart, facing the bottom surface of the diffusion components. Preferably, the top of the second cylinder has a solid waste plastic inlet, and the top of the second cylinder contains air-lifting components, so that solid waste plastic entering the second cylinder through the solid waste plastic inlet first enters the air-lifting components. Providing multiple air-lifting components and diffusion components in the deoxygenation device can further improve the effect of reducing oxygen entrainment in solid waste plastic and improve the effect of deep deoxygenation.

[0034] In one implementation, such as Figure 1 As shown, step S1 includes: allowing the solid waste plastic to enter the deoxygenation device 3 through the solid waste plastic inlet at the top of the deoxygenation device 3, and continuously introducing the displacement gas into the deoxygenation device 3 through the displacement gas inlet on the bottom side wall of the deoxygenation device 3 to continuously purge the solid waste plastic, thereby displacing the air in the deoxygenation device 3 and obtaining deoxygenated plastic.

[0035] In one specific embodiment, the ratio of the amount of the displacement gas to the feed volume of the solid waste plastic is 0.01~50:1, preferably 0.5~10:1; Optionally, the replacement gas is selected from one or more of carbon dioxide, inert gases, and liquid hydrocarbons, preferably nitrogen or carbon dioxide. According to the optimized volume ratio in this embodiment, especially the preferred volume ratio, the deoxygenation effect of waste plastics can be further improved.

[0036] In one implementation, such as Figure 1 As shown, in step S2, the softening-dechlorination device 5 is a first screw compressor; the first screw compressor includes a screw inlet, a screw outlet, a first exhaust port and a first heating element.

[0037] In a preferred embodiment, the inlet temperature of the first screw compressor is 0~120℃, the material residence time is 10~1500s, and the outlet temperature of the first screw compressor is 80~290℃; more preferably, the inlet temperature of the first screw compressor is 20~70℃, the material residence time is 6~400s, and the outlet temperature of the first screw compressor is 120~240℃. Preferably, the chlorine content of the softened dechlorinated plastic is less than 0.1% by weight.

[0038] The softening-dechlorination device used in this disclosure is a screw compressor with a heating function (first heating element) and an exhaust port (first exhaust port). In the plastic softening section, solid plastic moves under the push of the screw and softens at a relatively low temperature. Most of the chlorine in the plastic decomposes and is extracted from the exhaust port of the plastic softener. The softened and dechlorinated plastic has a low chlorine content, which can significantly reduce the corrosion of subsequent equipment by chlorine impurities. In this disclosure, the softening-dechlorination device is provided with multiple first exhaust ports at different temperatures, preferably two first exhaust ports.

[0039] The liquefaction device used in this disclosure can be a stirred tank with heating function; or it can be a screw compressor with heating function and a structure that facilitates gas-liquid flow.

[0040] In the first embodiment, in step S3, the liquefaction device 6 is a stirring device, and a second heating element is provided on the outer side of the bottom of the stirring device to heat the material in the stirring device. Optionally, the liquefaction conditions include: a temperature of 150~490℃, a pressure of -0.09~1.0 MPa, and a time of 0.1~0.9 h; preferably, a temperature of 250~410℃, a pressure of 0.05~0.5 MPa, and a time of 0.2~0.8 h; optionally, a stirring rate of 10~500 r / min, preferably 50~200 r / min. According to the process conditions in this embodiment, especially the preferred process conditions, the liquefaction effect of solid plastics can be improved.

[0041] In the second embodiment, in step S3, the liquefaction device 6 is a second screw compressor, which includes a third heating element for heating the material inside the second screw compressor; The liquefaction conditions include: the inlet temperature of the second screw compressor is 80~290℃, the material residence time is 10~1200s, and the outlet temperature of the second screw compressor is 250~490℃; preferably, the inlet temperature of the second screw compressor is 120~240℃, the material residence time is 60~300s, and the outlet temperature of the second screw compressor is 250~410℃. In this disclosure, in the plastic liquefaction section, the plastic liquefaction operation temperature is relatively high, and a small amount of plastic decomposes to produce a certain amount of gas. Screw compressors with large mixing tanks or large outlet cross-sectional areas have the characteristic of low pressure drop, allowing liquefied materials with high viscosity and gas-liquid two-phase characteristics to pass smoothly through the liquefaction equipment, thereby avoiding the problem of local overheating of high-temperature liquid plastics due to poor flow, leading to coking and equipment blockage. According to the process conditions in this embodiment, especially the preferred process conditions, the liquefaction effect of solid waste plastics can be further improved.

[0042] In one specific embodiment, the stirring device or the second screw compressor can be configured as a single stage or a multi-stage system connected in series to improve processing efficiency.

[0043] In one embodiment, in step S4, the circulating material accounts for 10-90% by weight of all liquid material from the liquefaction device 6, preferably 30-60% by weight; preferably, the temperature of the circulating material after heating is 300-500°C, more preferably 350-450°C. Optionally, the weight ratio of the recycled material to the softened and dechlorinated plastic from the softening-dechlorination unit 5 is 0.1 to 9:1, preferably 0.5 to 3:1. According to the process conditions provided in this embodiment, especially the preferred process conditions, better waste plastic recycling results can be obtained.

[0044] In one specific embodiment, the liquefaction device 6 is a stirring device. Liquid plastic is obtained at the bottom of the stirring tank, and the gas phase outlet at the top is connected to the gas-liquid separation device 8. The top of the gas-liquid separation device 8 obtains pyrolysis gas, and the bottom of the gas-liquid separation device 8 obtains another part of liquid plastic, which can be used in subsequent liquefaction plastic processing devices. The liquefaction unit 6 is the second screw compressor, and the outlet of the last screw compressor is connected to the gas-liquid separator 8. The top of the gas-liquid separator receives pyrolysis gas, which is used to enter the subsequent alkaline washing unit 10, and the bottom receives liquid plastic. The liquid plastic at the bottom of the gas-liquid separator 8 can be used in the subsequent liquefaction plastic processing unit.

[0045] In one implementation, such as Figure 1 As shown, the method also includes: The pyrolysis gas from the liquefaction device 6 is introduced into the gas-liquid separation device 8 for gas-liquid separation to obtain gaseous and liquid materials. The gaseous material is introduced into the alkaline washing device 10 for alkaline washing treatment; the liquid material is introduced into the subsequent liquefied plastic processing device.

[0046] In one specific embodiment, the alkaline washing agent in the alkaline washing device is selected from one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate.

[0047] A second aspect of this disclosure provides a system for recycling waste plastics, such as... Figures 1-3 As shown, it includes a deoxidation unit 3, a softening-dechlorination unit 5, a liquefaction unit 6, and an alkaline washing unit 10; The deoxygenation device 3 includes a solid waste plastic inlet, a displacement gas inlet, and a deoxygenated plastic outlet; the deoxygenation device 3 includes a gas stripping functional zone and a diffusion functional zone; The softening-dechlorination device 5 includes a deoxygenated plastic inlet, a chlorine-containing gas outlet, and a softened and dechlorinated plastic outlet. The deoxygenated plastic inlet is connected to the deoxygenated plastic outlet of the deoxygenation device 3. The liquefaction device 6 includes a softening and dechlorinating plastic inlet, a pyrolysis gas outlet, and a liquid plastic outlet. The softening and dechlorinating plastic inlet is connected to the softening and dechlorinating plastic outlet of the softening-dechlorinating device 5, and the liquid plastic outlet is connected to the softening and dechlorinating plastic inlet of the liquefaction device 6 via a liquid plastic circulation pipeline. The alkaline washing device 10 includes an inlet for the material to be washed, an inlet for the alkaline washing agent, an outlet for waste liquid, and an exhaust port. The inlet for the material to be washed is connected to the chlorine-containing gas outlet of the softening-dechlorination device 5 and the pyrolysis gas outlet of the liquefaction device 6, respectively. The device structure in the system provided in this disclosure has been described in detail above and will not be repeated here.

[0048] In one specific embodiment, the deoxygenation device 3 is a vertical tank structure; the softening-dechlorination device 5 is a first screw compressor; the chemical treatment device 6 is a stirred tank or a second screw compressor; and the alkaline washing device 10 is a washing tower.

[0049] In one specific implementation, such as Figure 1 As shown, the system also includes a heating furnace 16, the inlet of which is connected to the liquid plastic outlet of the liquefaction device 6, and the outlet of which is connected to the softened and dechlorinated plastic inlet of the liquefaction device 6, for heating the liquid plastic returned to the liquefaction device 6 for further processing.

[0050] In one specific implementation, such as Figure 1As shown, the system also includes a gas-liquid separator 8 for further separating the pyrolysis gas from the liquefaction unit 6. The inlet of the gas-liquid separator 8 is connected to the pyrolysis gas outlet of the liquefaction unit 6, the gas phase outlet of the gas-liquid separator 8 is connected to the inlet of the material to be alkali-washed in the alkali washing unit 10, and the liquid phase outlet of the gas-liquid separator 8 can collect and flow out with the material stream (liquid plastic) 7. The gas-liquid separator 8 can be a gas-liquid separation tank.

[0051] use Figure 1 The system shown includes the following processes for liquefying solid waste plastics: Solid waste plastic 1, after crushing or granulation, is introduced into the deoxidation unit 3 through the solid waste plastic inlet. Simultaneously, displacement gas is introduced into the deoxidation unit 3. This process is a continuous feeding operation. Deoxidized plastic 4 enters the softening-dechlorination unit 5, where the plastic is heated to a softened state. The chlorinated plastic decomposes, releasing acidic gas 18, which enters the alkaline washing unit 10. Softened and dechlorinated plastic 14 enters the liquefaction unit 6. The gas at the top of the liquefaction unit 6 is introduced into the gas-liquid separation unit 8, and the liquefied gas at the top of the gas-liquid separation unit 8 enters the alkaline washing unit 10. The liquid plastic at the bottom of the liquefaction unit 6 is divided into two parts: one part is recycled material 15, which is heated by the heating furnace 16 and sent to the inlet of the liquefaction unit 6 to provide some heat for plastic liquefaction; the other part is liquid plastic 7, which can be used for subsequent processing, such as entering the delayed coking unit.

[0052] The present disclosure is further described in detail below through examples. All raw materials used in the examples are commercially available.

[0053] In the following examples and comparative examples, the CO / CO2 yield was obtained by measuring material flow rate and analyzing material composition (gas chromatography).

[0054] The content of asphaltene in liquid plastics was determined by the four-component determination method of petroleum asphalt (NB / SH / T0509-2010).

[0055] The dechlorination rate was obtained by measuring the quality of the raw materials and the quality of the liquefied plastic, combined with XRF quantitative analysis.

[0056] In the following examples and comparative examples, the geometric equivalent diameter of the solid waste plastic particles ranges from 2 to 4 mm.

[0057] Example 1 use Figure 1 The solid waste plastic liquefaction system shown includes the following structural parameters for each device: The structure of the deoxygenation device 3 is as follows: Figure 2As shown, the dimensions of the gas stripping section are 50mm (inner diameter) × 200mm (height), and the dimensions of the diffusion section are 150mm (inner diameter) × 200mm (height). A total of 3 sets of gas stripping and diffusion section combinations are set up. The ratio of the cross-sectional area of ​​the gas stripping section cylinder to the cross-sectional area of ​​the diffusion section cylinder is 0.11:1. The softening-dechlorination device 5 is a first screw compressor with a screw major diameter of 50mm and a screw length of 3000mm. The liquefaction device 6 is a stirring device with dimensions of 300mm (inner diameter) × 500mm (height).

[0058] The process of liquefying solid waste plastics using the above system includes: Solid waste plastics include polypropylene, polyethylene, and polyvinyl chloride, with a chlorine content of 0.7% by weight. The crushed solid waste plastic 1 is introduced into the deoxidation unit 3 through the solid waste plastic inlet. Nitrogen gas is injected into the bottom of the deoxidation unit 3, and the amount of nitrogen is 5 times the volume of the fed plastic. The bulk density of the solid waste plastic particles in the deoxidation unit 3 is approximately 390 kg / m³. 3 .

[0059] Deoxidized plastic 4 enters softening-dechlorination unit 5. In softening-dechlorination unit 5, the plastic is heated to a softened state. The chlorinated plastic decomposes and releases acidic gas 18. Acidic gas 18 enters alkaline washing unit 10. In the softening process, the inlet temperature of the first screw is 25℃, the feed rate of the deoxidized plastic is 41.7g / s, the screw linear speed is 0.1m / s, the material residence time is 300s, and the outlet temperature is 140℃.

[0060] The softened and dechlorinated plastic 14 enters the liquefaction unit 6. The liquefaction conditions include: temperature of 310℃, pressure of 0.05Mpa, time of 0.2h, and stirring rate of 100r / min.

[0061] The gas at the top of the liquefaction unit 6 is introduced into the gas-liquid separation unit 8, and the liquefied gas at the top of the gas-liquid separation unit 8 enters the alkaline washing unit 10. The liquid plastic at the bottom of the liquefaction unit 6 is divided into two parts. One part is the circulating material 15 (the circulating material accounts for 33% of the total weight of the liquid material from the liquefaction unit 6; the weight ratio of the circulating material to the softened and dechlorinated plastic from the softening-dechlorination unit 5 is 0.5:1). After being heated to 360°C by the heating furnace 16, it is sent to the inlet of the liquefaction unit 6 to provide some heat for the liquefaction of the plastic. The other part is the liquid plastic 7, which can be used for subsequent processing.

[0062] Example 2 This embodiment refers to the system and method of Embodiment 1, but differs from Embodiment 1 in that: the structure of the deoxygenation device 3 is as follows... Figure 3As shown, the deoxygenation device has a diameter (inner diameter) of 150 mm, the top diameter of the air-lifting component (i.e., the inner diameter of the opening end of the funnel section) is 150 mm, the bottom notch diameter (i.e., the inner diameter of the closing end) is 50 mm, and the height of the air-lifting component is 150 mm (of which the height of the funnel section is 50 mm and the height of the equal-diameter section is 100 mm; the ratio of the cross-sectional area of ​​the equal-diameter section of the air-lifting component to the cross-sectional area of ​​the second cylinder is 0.11:1). The included angle between the sidewall of the funnel section and the central axis of the funnel section is 45°; the bottom diameter of the diffuser section is 130 mm. The cone is 65 mm high and has a sidewall angle of 45° with its central axis. It consists of three sets of air-lift and diffusion components. The distance between the cone apex and the equal-diameter section of the air-lift component away from the funnel section is 50 mm. The distance between the opening of the funnel section and the bottom surface of the upstream diffusion component is also 50 mm. The ratio of the area of ​​the annular space on the plane containing the bottom surface of the cone to the cross-sectional area of ​​the second cylinder is 0.25:1. The bulk density of the solid waste plastic particles in deoxidation unit 3 is approximately 390 kg / m³. 3 The remaining apparatus and processes are the same as in Example 1.

[0063] Example 3 This embodiment uses the same method and system as Embodiment 1, but differs from Embodiment 1 in that: the softened dechlorinated plastic 14 enters the liquefaction device 6 (stirring device); the liquefaction conditions include: temperature of 310℃, pressure of 0.05Mpa, time of 0.3h; and stirring rate of 100r / min.

[0064] The gas at the top of the liquefaction unit 6 is introduced into the gas-liquid separation unit 8, and the liquefied gas at the top of the gas-liquid separation unit 8 enters the alkaline washing unit 10. The liquid plastic at the bottom of the liquefaction unit 6 is divided into two parts. One part is the circulating material 15 (the circulating material accounts for 20% of the total weight of the liquid material from the liquefaction unit 6; the weight ratio of the circulating material to the softened and dechlorinated plastic from the softening-dechlorination unit 5 is 0.25:1). After being heated to 360°C by the heating furnace 16, it is sent to the inlet of the liquefaction unit 6 to provide some heat for the liquefaction of the plastic. The other part is the liquid plastic 7, which can be used for subsequent processing.

[0065] Example 4 This embodiment uses the same method and system as Embodiment 3, but differs from Embodiment 3 in that: The softened and dechlorinated plastic 14 enters the liquefaction unit 6; the liquefaction conditions include: temperature of 310℃, pressure of 0.05Mpa, time of 0.5h, and stirring rate of 100r / min.

[0066] The gas at the top of the liquefaction unit 6 is introduced into the gas-liquid separation unit 8, and the liquefied gas at the top of the gas-liquid separation unit 8 enters the alkaline washing unit 10. The liquid plastic at the bottom of the liquefaction unit 6 is divided into two parts. One part is the circulating material 15 (the circulating material accounts for 5% of the total weight of the liquid material from the liquefaction unit 6; the weight ratio of the circulating material to the softened and dechlorinated plastic from the softening-dechlorination unit 5 is 0.05:1). After being heated to 360°C by the heating furnace 16, it is sent to the inlet of the liquefaction unit 6 to provide some heat for the liquefaction of the plastic. The other part is the liquid plastic 7, which can be used for subsequent processing.

[0067] Example 5 This embodiment uses the same method and system as Embodiment 1, but differs from Embodiment 1 in that: The second screw compressor is used as the liquefaction device. The screw of the second screw compressor has a major diameter of 50 mm and a screw length of 4000 mm. The liquefaction conditions include: the inlet temperature of the second screw compressor is 140°C, the material residence time is 260s, the outlet temperature of the second screw compressor is 290°C, and the rest of the process is the same as in Example 1.

[0068] Example 6 This embodiment refers to the system and method in Embodiment 1, but differs from Embodiment 1 in that: The softened and dechlorinated plastic 14 enters the liquefaction unit 6 (stirring device); the liquefaction conditions include: temperature of 450℃, pressure of 0.05Mpa, time of 0.8h; stirring rate of 100r / min.

[0069] The gas at the top of the liquefaction unit 6 is introduced into the gas-liquid separation unit 8, and the liquefied gas at the top of the gas-liquid separation unit 8 enters the alkaline washing unit 10. The liquid plastic at the bottom of the liquefaction unit 6 is divided into two parts. One part is the circulating material 15 (the circulating material accounts for 9% of the total weight of the liquid material from the liquefaction unit 6; the weight ratio of the circulating material to the softened and dechlorinated plastic from the softening-dechlorination unit 5 is 0.1:1). After being heated to 470°C by the heating furnace 16, it is sent to the inlet of the liquefaction unit 6 to provide some heat for the liquefaction of the plastic. The other part is the liquid plastic 7, which can be used for subsequent processing.

[0070] Example 7 This embodiment refers to the system and method in Embodiment 1, but differs from Embodiment 1 in that: Deoxidized plastic 4 enters the softening-dechlorination unit 5. Inside the softening-dechlorination unit 5, the plastic is heated to a softened state. The chlorinated plastic decomposes and releases acidic gas 18. Acidic gas 18 enters the alkaline washing unit 10. In the dechlorination-softening treatment, the inlet temperature of the first screw is 25℃, the feed rate of the deoxidized plastic is 41.7g / s, the screw linear speed is 0.1m / s, the material residence time is 300s, and the outlet temperature is 100℃.

[0071] Example 8 This embodiment refers to the system and method in Embodiment 1, but differs from Embodiment 1 in that: Deoxidized plastic 4 enters softening-dechlorination unit 5. Inside softening-dechlorination unit 5, the plastic is heated to a softened state. The chlorinated plastic decomposes and releases acidic gas 18. Acidic gas 18 enters alkaline washing unit 10. In the dechlorination-softening treatment, the inlet temperature of the first screw is 25℃, the feed rate of the deoxidized plastic is 41.7g / s, the screw linear speed is 0.1m / s, the material residence time is 300s, and the outlet temperature is 70℃.

[0072] Comparative Example 1 This comparative example refers to the system and method of Example 1, but differs from Example 1 in that: no deoxygenation device is provided, while the rest of the equipment and processes are the same as in Example 1.

[0073] Comparative Example 2 This comparative example refers to the system and method of Example 1, except that the liquid plastic from the liquefaction device 6 is not recirculated, while the rest of the device and process are the same as in Example 1.

[0074] Comparative Example 3 This comparative example refers to the system and method in Example 1, but differs from Example 1 in that: The deoxygenation device does not have a gas lift and diffusion functional area; the replacement gas is used to purge solid waste plastics only in the cylinder. The rest of the device and process are the same as in Example 1.

[0075] The waste plastic treatment results of the above embodiments and comparative examples are listed in Table 1 below.

[0076] Table 1

[0077] In Table 1, the symbol "~" means "approximately".

[0078] According to the data in Table 1: Comparing Comparative Example 1 and Comparative Example 3, it can be seen that Comparative Example 1 does not have a deoxygenation device, while Comparative Example 3 has a deoxygenation device but does not have an air-lift functional zone and a diffusion functional zone. The CO / CO2 yield of the waste plastic recycling system in Comparative Example 3 is lower. However, comparing Examples 1-2 with Comparative Example 3, Examples 1-2 use a deoxygenation device with an air-lift functional zone and a diffusion functional zone. Examples 1-2 can effectively reduce the CO / CO2 yield, that is, the deoxygenation effect is better, and the liquefied plastic resin asphalt content obtained in Examples 1-2 is lower. Comparing Example 1 with Comparative Example 2, it can be seen that Example 1 uses liquid plastic recycling, and the liquefied plastic resin asphalt content obtained in Example 1 is lower. Comparing Examples 3 and 4, it can be seen that the weight ratio of recycled material to softened dechlorinated plastic in Example 3 is within the optimized range (0.1~9:1) provided in this disclosure, while the weight ratio in Example 4 is not within this optimized range. The liquefied plastic obtained by liquefaction treatment in Example 3 has a lower content of asphaltenes. Comparing Examples 1 and 3, the weight ratio of recycled material to softened dechlorinated plastic in Example 1 is within the further preferred range (0.5~3:1) provided in this disclosure. The liquefied plastic obtained by liquefaction treatment in Example 1 has a lower content of asphaltenes. Comparing Example 1 with Example 6, it can be seen that the waste plastics treated according to the preferred process conditions provided in this disclosure in Example 1 have a lower content of asphaltene in the liquefied plastics obtained by liquefaction treatment. Comparing Example 7 with Example 8, it can be seen that the dechlorination-softening process conditions in Example 7 are within the optimized range provided in this disclosure (the outlet temperature of the first screw compressor is 80~290℃), and the dechlorination rate of waste plastics in Example 7 is higher; comparing Example 1 with Example 7, it can be seen that the dechlorination rate of waste plastics in Example 1 is higher.

[0079] The preferred embodiments of this disclosure have been described in detail above. However, this disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, and these simple modifications all fall within the protection scope of this disclosure.

[0080] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0081] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A method for recycling waste plastics, characterized in that, Includes the following steps: S1. Solid waste plastic and displacement gas are introduced into the deoxygenation device (3) so that the displacement gas can purge and deoxygenate the solid waste plastic to obtain deoxygenated plastic. The deoxygenation device (3) includes a gas lift functional area and a diffusion functional area connected to each other. S2. The deoxidized plastic is fed into the softening-dechlorination device (5) for softening-dechlorination treatment to obtain chlorine-containing gas and softened dechlorinated plastic; S3. The softened dechlorinated plastic is fed into the liquefaction device (6) for liquefaction treatment to obtain pyrolysis gas and liquid plastic; S4. The chlorine-containing gas in step S2 and the pyrolysis gas in step S3 are introduced into the alkaline washing device (10) for alkaline washing treatment; at least a portion of the liquid plastic in step S3 is heated as a circulating material and then returned to the liquefaction device (6) for further processing.

2. The method according to claim 1, characterized in that, The deoxygenation device (3) includes a first cylinder, which includes an air-lift section and a diffusion section. The air-lift section forms the air-lift functional area, and the diffusion section forms the diffusion functional area. The inner diameter of the air-lift section is smaller than the inner diameter of the diffusion section; The air-lift section and the diffusion section are connected by a variable-diameter section; the variable-diameter section includes a first port and a second port, the inner diameter of the first port is smaller than the inner diameter of the second port, the first port of the variable-diameter section is connected to the air-lift section, and the second port of the variable-diameter section is connected to the diffusion section. Preferably, the ratio of the cross-sectional area of ​​the gas lift section cylinder to the cross-sectional area of ​​the diffuser section cylinder is 0.02~0.95:1, more preferably 0.05~0.5:1; Preferably, the first cylinder includes a plurality of the air-lift sections, the variable-diameter section, and the diffusion section; Preferably, the top of the first cylinder is a gas lift section, and the top of the first cylinder is provided with a solid waste plastic inlet; the bottom of the first cylinder is a diffusion section, and the side wall of the diffusion section at the bottom of the first cylinder is provided with a displacement gas inlet.

3. The method according to claim 1, characterized in that, The deoxygenation device (3) includes a second cylinder; the second cylinder includes an air-lifting component and a diffusion component; the air-lifting component forms the air-lifting functional area, and the diffusion component forms the diffusion functional area; preferably, the air-lifting component and the diffusion component are coaxially arranged with the second cylinder; The air-lift component includes a funnel section and a constant-diameter section. The funnel section includes an open end and a closed end. The inner diameter of the constant-diameter section is the same as the inner diameter of the closed end of the funnel section, and one section of the constant-diameter section is connected to the closed end of the funnel section. The open end of the funnel section is fixedly connected along the side wall of the second cylinder, and the open end of the funnel section is positioned facing the solid waste plastic inlet of the second cylinder. Preferably, the outer diameter of the open end is the same as the inner diameter of the second cylinder. Preferably, the funnel section has a cone structure with uniformly varying diameter; the angle between the sidewall of the funnel section and the central axis of the funnel section is an acute angle, preferably 10~80°; preferably, the ratio of the cross-sectional area of ​​the equal-diameter section of the air-lifting component to the cross-sectional area of ​​the second cylinder is 0.02~0.95:1, preferably 0.05~0.5:1; The diffusion component is a closed cone; the cone apex faces the equal-diameter section of the air-lifting component, with its port spaced apart from the funnel section; optionally, the angle between the sidewall of the cone and the central axis of the cone is an acute angle, preferably 10~80°. Optionally, the bottom surface of the cone is smaller than the inner diameter of the second cylinder, so that an annular space is formed between the side wall of the cone and the inner wall of the second cylinder; the width of the annular space gradually decreases along the flow direction of solid waste plastic in the second cylinder; Preferably, the ratio of the area of ​​the annular space in the plane containing the bottom surface of the cone to the cross-sectional area of ​​the second cylinder is 0.02~0.95:1, more preferably 0.05~0.5:1; More preferably, the second cylinder is provided with a plurality of air-lifting components and a plurality of diffusion components, with a diffusion component disposed between every two air-lifting components, and the funnel section of the air-lifting component located downstream of the diffusion component is spaced apart along the flow direction of solid waste plastic in the second cylinder, with its opening end facing the bottom surface of the diffusion component; preferably, the top of the second cylinder is provided with a solid waste plastic inlet, and the top of the second cylinder is provided with an air-lifting component, so that the solid waste plastic entering the second cylinder through the solid waste plastic inlet first enters the air-lifting component.

4. The method according to claim 1, characterized in that, Step S1 includes: allowing the solid waste plastic to enter the deoxygenation device (3) through the solid waste plastic inlet at the top of the deoxygenation device (3), and continuously introducing the displacement gas into the deoxygenation device (3) through the displacement gas inlet on the bottom side wall of the deoxygenation device (3) to continuously purge the solid waste plastic, thereby displacing the air in the deoxygenation device (3) from the deoxygenation device (3) to obtain deoxygenated plastic; Optionally, the ratio of the amount of the displacement gas to the feed volume of the solid waste plastic is 0.01~50:1, preferably 0.5~10:1; Optionally, the replacement gas is selected from one or more of carbon dioxide, inert gases and liquid hydrocarbons, preferably nitrogen or carbon dioxide.

5. The method according to claim 1, characterized in that, In step S2, the softening-dechlorination device (5) is a first screw compressor; the first screw compressor includes a screw inlet, a screw outlet, a first exhaust port and a first heating element; Preferably, the inlet temperature of the first screw compressor is 0~120℃, the material residence time is 10~1500s, and the outlet temperature of the first screw compressor is 80~290℃; more preferably, the inlet temperature of the first screw compressor is 20~70℃, the material residence time is 6~400s, and the outlet temperature of the first screw compressor is 120~240℃. Preferably, the chlorine content of the softened dechlorinated plastic is less than 0.1% by weight.

6. The method according to claim 1, characterized in that, In step S3, the liquefaction device (6) is a stirring device, and a second heating element is provided on the outer side of the bottom of the stirring device to heat the material in the stirring device; Optionally, the liquefaction conditions include: a temperature of 150~490℃, a pressure of -0.09~1.0 MPa, and a time of 0.1~0.9 h; preferably, a temperature of 250~410℃, a pressure of 0.05~0.5 MPa, and a time of 0.2~0.8 h; optionally, a stirring rate of 10~500 r / min, preferably 50~200 r / min.

7. The method according to claim 1, characterized in that, In step S3, the liquefaction device (6) is a second screw compressor, which includes a third heating element for heating the material inside the second screw compressor; The liquefaction conditions include: the inlet temperature of the second screw compressor is 80~290℃, the material residence time is 10~1200s, and the outlet temperature of the second screw compressor is 250~490℃; preferably, the inlet temperature of the second screw compressor is 120~240℃, the material residence time is 60~300s, and the outlet temperature of the second screw compressor is 250~410℃.

8. The method according to claim 1, characterized in that, In step S4, the circulating material accounts for 10-90% by weight of all liquid material from the liquefaction device (6), preferably 30-60% by weight; preferably, the temperature of the circulating material after heating is 300-500°C, preferably 350-450°C; Optionally, the weight ratio of the recycled material to the softened and dechlorinated plastic from the softening-dechlorination unit (5) is 0.1 to 9:1, preferably 0.5 to 3:

1.

9. The method according to claim 1, characterized in that, The method also includes: The pyrolysis gas from the liquefaction device (6) is introduced into the gas-liquid separation device (8) for gas-liquid separation to obtain gaseous material and liquid material; The gaseous material is introduced into the alkaline washing device (10) for alkaline washing treatment; the liquid material is introduced into the subsequent liquefied plastic processing device. Optionally, the alkaline washing agent in the alkaline washing device is selected from one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate.

10. The method according to claim 1, characterized in that, The geometric equivalent diameter of the solid waste plastic particles ranges from 0.1 to 100 mm; preferably, the bulk density of the solid waste plastic particles in the deoxidation device (3) is 50 to 800 kg / m³. 3 ; Optionally, the solid waste plastic particles are lifted into a deoxygenation device by a lifting device; the lifting device is selected from mechanical lifting equipment or gas conveying equipment.

11. A system for recycling waste plastics, characterized in that, It includes a deoxygenation unit (3), a softening-dechlorination unit (5), a liquefaction unit (6), and an alkaline washing unit (10). The deoxygenation device (3) includes a solid waste plastic inlet, a displacement gas inlet, and a deoxygenated plastic outlet; the deoxygenation device (3) includes a gas stripping functional area and a diffusion functional area; The softening-dechlorination device (5) includes a deoxygenated plastic inlet, a chlorine-containing gas outlet, and a softened dechlorinated plastic outlet. The deoxygenated plastic inlet is connected to the deoxygenated plastic outlet of the deoxygenation device (3). The liquefaction device (6) includes a softening and dechlorinating plastic inlet, a pyrolysis gas outlet and a liquid plastic outlet. The softening and dechlorinating plastic inlet is connected to the softening and dechlorinating plastic outlet of the softening-dechlorinating device (5), and the liquid plastic outlet is connected to the softening and dechlorinating plastic inlet of the liquefaction device (6) via a liquid plastic circulation pipeline. The alkaline washing device (10) includes an inlet for the material to be washed, an inlet for the alkaline washing agent, an outlet for the waste liquid, and an exhaust port; the inlet for the material to be washed is connected to the chlorine-containing gas outlet of the softening-dechlorination device (5) and the pyrolysis gas outlet of the liquefaction device (6), respectively.

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