Method and system for liquefying solid waste plastics

Abstract

The invention relates to a method and system for liquefying solid waste plastics, and the method comprises the following steps: S1, enabling the solid waste plastics to enter a deoxidation device for deoxidation treatment to obtain deoxidized plastics; s2, the deoxidized plastic enters a softening device to be subjected to softening treatment and 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 for continuous treatment. And liquid feed can be continuously provided for waste plastic pyrolysis.

Description

Technical Field

[0001] The present disclosure relates to the field of waste plastic utilization, and in particular, to a method and system for liquefying solid waste plastic. Background Art

[0002] Waste plastics are mainly disposed of by landfill and incineration. Landfilled waste plastics are difficult to degrade naturally, resulting in the long-term occupation of a large amount of land resources; during the landfill cycle, harmful substances in waste plastics may also penetrate and cause secondary land pollution. Incineration has negative effects such as poor economic benefits and the production of highly toxic substances such as dioxins during the incineration process.

[0003] Plastics have a polymer structure, and the carbon-carbon bonds of waste plastics can be broken by thermal cracking to produce petrochemical products such as cracked oil. The thermal cracking technology of waste plastics can realize the reuse of waste resources, and its economic value is good. It is an important measure for my country to develop a circular economy, but the current development of technology is not perfect. The pyrolysis of waste plastics needs to be carried out under high temperature and certain pressure conditions, and the raw materials of waste plastics are mostly plastic films or plastic particles. The porosity of solid plastics is large, and it is difficult to achieve self-sealing of logistics with continuous solid feeding, which is very likely to cause subsequent pyrolysis products to leak upstream. Therefore, the existing pyrolysis technology mostly uses intermittent pyrolysis production, and the production efficiency is low. In addition, the porosity of plastic films or particles is large, and there is more air inside. The pyrolysis process will produce a certain amount of light hydrocarbons due to the high temperature. The direct feeding of plastic films or particles increases the possibility of mixed explosion of oxygen and light hydrocarbons. At the same time, oxygen participates in the pyrolysis reaction to produce oxidation products, which affects the product quality and liquid phase yield. Furthermore, solid waste plastics contain a certain amount of PVC, which decomposes into various types of chlorides when heated. Improper treatment methods will cause serious corrosion of equipment and affect product quality.

[0004] CN114262621A discloses a waste plastic liquefaction pyrolysis system and method. The invention proposes that: by introducing hot nitrogen into the bottom of the liquid phase tank to heat the waste plastic to liquefy it, two sealing plates are arranged on the top of the liquid phase tank, and the alternate opening and closing realizes continuous sealing and feeding. There is cracking gas between the two sealing plates of this method. When the first sealing plate is opened for feeding, cracking gas will be discharged. In addition, this method cannot effectively avoid the contact between air and high-temperature pyrolysis gas. Summary of the invention

[0005] The purpose of the present disclosure is to provide a method and system for liquefying solid waste plastics, which can continuously provide liquid feed for pyrolysis of waste plastics.

[0006] In order to achieve the above-mentioned object, the present disclosure provides a method for liquefying solid waste plastics in a first aspect, comprising the following steps:

[0007] S1, allowing solid waste plastics to enter a deoxidation device for deoxidation treatment to obtain deoxidized plastics;

[0008] S2, allowing the deoxidized plastic to enter a softening device for softening and dechlorination treatment to obtain chlorine-containing gas and softened dechlorinated plastic;

[0009] S3, allowing the softened dechlorinated plastic to enter a liquefaction device for liquefaction treatment to obtain pyrolysis gas and liquid plastic;

[0010] S4, allowing the chlorine-containing gas in step S2 and the pyrolysis gas in step S3 to enter the alkali washing device for alkali washing treatment; allowing at least part of the liquid plastic in step S3 to be heated as a circulating material and then returned to the liquefaction device for further treatment.

[0011] Optionally, the deoxidation device comprises a solid waste plastic inlet and a deoxidized plastic outlet; a sealing unit is respectively provided on the inlet pipeline of the solid waste plastic inlet and the outlet pipeline of the deoxidized plastic outlet;

[0012] Optionally, the sealing unit comprises a valve upstream pipeline, a valve and a valve downstream pipeline which are sequentially connected and arranged; the inner diameter of the valve upstream pipeline is smaller than the inner diameter of the valve, and the inner diameter of the valve downstream pipeline is the same as the inner diameter of the valve; the outlet of the valve upstream pipeline is connected to the valve, and a protective gas inlet pipeline is arranged on the side wall of the valve upstream pipeline near the outlet;

[0013] Optionally, the valve upstream pipeline includes a constant diameter section and a variable diameter section arranged in sequence from top to bottom, the inner diameter of the constant diameter section is smaller than the inner diameter of the valve, the inner diameter of the top of the variable diameter section is the same as the inner diameter of the constant diameter section, and the inner diameter of the bottom of the variable diameter section is the same as the inner diameter of the valve; the protective gas inlet pipeline is arranged on the side wall of the variable diameter section;

[0014] Preferably, the inclination angle between the protective gas inlet pipe and the central axis of the valve upstream pipe is an acute angle, preferably 10 to 80°; optionally, the ratio of the inner diameter of the equal diameter section of the valve upstream pipe to the inner diameter of the valve is 0.5 to 0.9; the height of the variable diameter section of the valve upstream pipe is 10 to 500 mm;

[0015] Alternatively, the sealing unit includes a valve upstream pipeline, a valve and a valve downstream pipeline which are connected in sequence; the inner diameter of the valve upstream pipeline and the inner diameter of the valve downstream pipeline are respectively the same as the inner diameter of the valve; the outlet of the valve upstream pipeline is connected to the valve, and a protective gas inlet pipeline is arranged on the side wall of the valve upstream pipeline near the outlet; and a protective baffle is circumferentially arranged on the inner wall near the outlet of the valve upstream pipeline; preferably, the inclination angle between the protective gas inlet pipeline and the central axis of the valve upstream pipeline is an acute angle, preferably 10 to 80°; optionally, the width of the protective baffle is 2 to 150 mm.

[0016] Optionally, step S1 includes:

[0017] The solid waste plastic is allowed to enter the deoxidation device through the solid waste plastic inlet at the top of the deoxidation device via the sealing unit; an inert gas is continuously introduced into the deoxidation device through the inert gas inlet on the side wall of the deoxidation device for continuous purge to displace the air in the deoxidation device out of the deoxidation device; optionally, the rate of the inert gas is 10 to 2000 L / min, preferably 50 to 500 L / min;

[0018] Alternatively, the solid waste plastics are allowed to enter the deoxidation device through the solid waste plastics inlet at the top of the deoxidation device via a sealing unit; inert gas is filled into the deoxidation device through an inert inlet on the side wall of the deoxidation device to 0.1-1.0 MPag, preferably 0.3-0.5 MPag, and then the deoxidation device is depressurized to below 0.1 MPag; this is repeated 1-10 times, preferably 2-5 times; preferably, in the process of the solid waste plastics flowing through the sealing unit into the deoxidation device, protective gas is introduced into the sealing unit through the protective gas inlet pipe to purge the solid waste plastics in the sealing unit; preferably, the protective gas is selected from one or more of air, inert gas and liquid hydrocarbons, preferably nitrogen; optionally, the flow rate of the protective gas is 10-2000 L / min, preferably 50-500 L / min.

[0019] Optionally, in step S2, the softening device is a first screw machine; the first screw machine includes a screw inlet, a screw outlet, a first exhaust port and a first heating element;

[0020] Preferably, the inlet temperature of the first screw machine is 0-100°C, the material residence time is 30-1200s, and the outlet temperature of the first screw machine is 80-300°C; preferably, the inlet temperature of the first screw machine is 20-60°C, the material residence time is 120-300s, and the outlet temperature of the first screw machine is 120-200°C;

[0021] Preferably, the chlorine content of the softened dechlorinated plastic is less than 0.1 wt%.

[0022] 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;

[0023] Optionally, the conditions of the liquefaction treatment include: temperature of 150-500°C, pressure of -0.09-1.0Mpa, and time of 0.1-1.0h; preferably, temperature of 250-400°C, pressure of 0.05-0.5Mpa, and time of 0.1-0.2h; optionally, stirring rate of 10-500r / min; preferably 50-200r / min.

[0024] Optionally, in step S3, the liquefaction device is a second screw machine, and the second screw machine includes a third heating element for heating the material in the second screw machine;

[0025] The conditions for the liquefaction treatment include: the inlet temperature of the second screw machine is 80-300°C, the material residence time is 30-1200s, and the outlet temperature of the second screw machine is 250-500°C; preferably, the inlet temperature of the second screw machine is 120-200°C, the material residence time is 100-300s, and the outlet temperature of the second screw machine is 250-400°C.

[0026] Optionally, in step S4, the circulating material accounts for 10 to 90% by weight of all liquid materials from the liquefaction device, preferably 30 to 60% by weight; preferably, the temperature of the circulating material after heating is 300 to 500° C., preferably 350 to 450° C.;

[0027] Optionally, the weight ratio of the recycled material to the softened dechlorinated plastic from the softening device is 0.1 to 9:1, preferably 0.5 to 3:1.

[0028] Optionally, the method further comprises:

[0029] Allowing the pyrolysis gas from the liquefaction device to enter a gas-liquid separation device for gas-liquid separation treatment to obtain a gas phase material and a liquid phase material;

[0030] The gas phase material enters the alkali washing device for alkali washing treatment; the liquid phase material enters the subsequent liquefied plastic processing device;

[0031] Optionally, the alkali washing agent in the alkali washing device is selected from one or more of sodium hydroxide, potassium hydroxide, sodium carbonate and potassium carbonate.

[0032] A second aspect of the present disclosure provides a system for liquefying solid waste plastics, the system comprising: a deoxidation device, a softening device, a liquefaction device and an alkali washing device;

[0033] The deoxidation device comprises a solid waste plastic inlet, an inert gas inlet and a deoxidized plastic outlet;

[0034] The softening device comprises a deoxidized plastic inlet, a chlorine-containing gas outlet and a softened dechlorinated plastic outlet, and the deoxidized plastic inlet is connected to the deoxidized plastic outlet of the deoxidation device;

[0035] The liquefaction device comprises a softened dechlorinated plastic inlet, a pyrolysis gas outlet and a liquid plastic outlet, wherein the softened dechlorinated plastic inlet is connected to the softened dechlorinated plastic outlet of the softening device, and the liquid plastic outlet is connected to the softened dechlorinated plastic inlet of the liquefaction device via a liquid plastic circulation pipeline;

[0036] The alkali washing device comprises an inlet for materials to be alkali washed, an inlet for alkali washing agent, an outlet for waste liquid and an exhaust port; the inlet for materials to be alkali washed is respectively connected with the chlorine-containing gas outlet of the softening device and the pyrolysis gas outlet of the liquefaction device.

[0037] Optionally, the system further comprises a sealing unit, wherein the sealing unit comprises a valve upstream pipeline, a valve, and a valve downstream pipeline which are sequentially connected;

[0038] Optionally, the system comprises a plurality of the deoxidation devices; preferably, the system comprises two deoxidation devices arranged in parallel; a sealing unit is respectively arranged on the introduction pipeline of the waste plastic inlet and the outlet pipeline of the deoxidation plastic outlet of each deoxidation device; wherein the outlet of the pipeline downstream of the valve of the sealing unit located on the introduction pipeline of the waste plastic inlet is communicated with the solid waste plastic inlet of the deoxidation device; the inlet of the pipeline downstream of the valve of the sealing unit located on the outlet pipeline of the deoxidation plastic outlet is communicated with the deoxidation plastic outlet of the deoxidation device, and the outlet of the pipeline downstream of the valve is communicated with the deoxidation plastic inlet of the softening device;

[0039] Optionally, the deoxygenation device is a vertical tank; the softening device is a first screw machine; the liquefaction device is a stirring tank or a second screw machine; and the alkali washing device is a washing tower.

[0040] Through the above technical solution, the present disclosure provides a method and system for liquefying solid waste plastics, which at least include the following beneficial effects:

[0041] (1) Deoxidation of solid waste plastics in a deoxidation device with good sealing performance, low failure rate of the device and long continuous operation cycle; the oxygen content of the plastic after deoxidation is low, which can effectively reduce the possibility of mixed explosion of oxygen and subsequent high-temperature pyrolysis gas, and at the same time reduce the participation of oxygen in the pyrolysis reaction, reduce the gas phase yield and increase the liquid yield, and reduce the content of oxygen-containing compounds in the liquid phase product;

[0042] (2) In the softening device, the plastic is softened at a relatively low temperature, wherein the chlorine-containing plastic decomposes to release chlorine-containing gas, which is then extracted through a plurality of exhaust ports provided in the softening device. The chlorine content of the softened plastic is low, which can effectively reduce the corrosion to subsequent equipment and reduce the chlorine content in the liquid product;

[0043] (3) The residence time in the plastic liquefaction device is short, and the asphaltene content of the product is low; the plastic liquefaction device itself has a low pressure drop, and the internal gas and liquid flow is smooth, which can effectively prevent the liquid plastic from coking and clogging the equipment and causing the internal material to flow back upstream, and the device operation is highly safe;

[0044] (4) Convert solid plastic into liquid plastic and continuously provide liquid plastic to the downstream pyrolysis device to achieve continuous and safe production of the device.

[0045] Other features and advantages of the present disclosure will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The accompanying drawings are used to provide a further understanding of the present disclosure and constitute a part of the specification. Together with the following specific embodiments, they are used to explain the present disclosure but do not constitute a limitation of the present disclosure. In the accompanying drawings:

[0047] Figure 1 A schematic diagram of a process for liquefying solid waste plastics provided in the present disclosure;

[0048] Figure 2 A schematic structural diagram of a sealing unit provided in one embodiment of the present disclosure;

[0049] Figure 3 A schematic structural diagram of a sealing unit provided in another embodiment of the present disclosure.

[0050] Reference numerals

[0051] 1-solid waste plastics; 2-sealing unit; 3-deoxidation device; 4-deoxidized plastics; 5-softening device; 6-liquefaction device; 7-liquid plastics; 8-gas-liquid separation device; 9-liquefied gas; 10-alkali washing device; 11-alkali solution; 12-exhaust gas; 13-waste alkali solution; 14-softened plastics; 15-circulating materials; 16-heating furnace; 17-nitrogen; 18-acid gas; 19-valve; 20-protective gas inlet pipeline; 21-protective baffle; 23-valve upstream pipeline; 24-valve downstream pipeline. DETAILED DESCRIPTION

[0052] The specific embodiments of the present disclosure are described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present disclosure, and are not used to limit the present disclosure.

[0053] The first aspect of the present disclosure provides a method for liquefying solid waste plastics, such as Figure 1 As shown, the following steps are included:

[0054] S1, allowing the solid waste plastic to enter the deoxidation device 3 for deoxidation treatment to obtain deoxidized plastic;

[0055] S2, allowing the deoxidized plastic to enter the softening device 5 for softening and dechlorination treatment to obtain chlorine-containing gas and softened dechlorinated plastic;

[0056] S3, allowing the softened dechlorinated plastic to enter the liquefaction device 6 for liquefaction treatment to obtain pyrolysis gas and liquid plastic;

[0057] S4, allowing the chlorine-containing gas in step S2 and the pyrolysis gas in step S3 to enter the alkali washing device 10 for alkali washing treatment; allowing at least part of the liquid plastic in step S3 to be heated as a circulating material and then returned to the liquefaction device 6 for further treatment.

[0058] The present disclosure provides a method for liquefying solid waste plastics, which comprises deoxidizing the solid waste plastics in a deoxidizing device with good sealing performance. After deoxidation, the oxygen content of the plastics is low, which can effectively reduce the possibility of mixed explosion of oxygen and subsequent high-temperature pyrolysis gas. In the softening device, the plastics are softened at a relatively low temperature, wherein the chlorine-containing plastics are decomposed to release chlorine-containing gas, and the chlorine-containing gas is extracted through a plurality of exhaust ports arranged in the softening device. The chlorine content of the softened plastics is low, which can effectively reduce the corrosion to subsequent equipment. The plastic liquefaction device itself has a low pressure drop, and the internal gas-liquid flow is smooth, which can effectively prevent the coking of liquid plastics from clogging the device and causing the internal materials to flow back upstream, and the device operation safety is high. The solid plastics are converted into liquid plastics, and the liquid plastics are continuously provided to the downstream pyrolysis device, so as to realize continuous and safe production of the device.

[0059] In a 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 chlorine content in the solid waste plastic is 0.01 to 3% by weight; the water content is less than 30% by weight, and the organic impurity content is less than 20% by weight. The solid waste plastic can be a film or granules.

[0060] In one embodiment, the deoxidation device 3 includes a solid waste plastic inlet and a deoxidation plastic outlet; the inlet pipeline of the solid waste plastic inlet and the outlet pipeline of the deoxidation plastic outlet are respectively provided with a sealing unit 2. The sealing unit 2 used in the present disclosure is composed of a valve, an anti-blocking structure, and a protective fluid to achieve effective sealing of the deoxidation device 3. The sealing unit 2 provided in the present disclosure includes two forms:

[0061] In a first specific embodiment, Figure 2 As shown, the sealing unit 2 includes a valve upstream pipeline 23, a valve 19 and a valve downstream pipeline 24 which are connected in sequence; the inner diameter of the valve upstream pipeline 23 is smaller than the inner diameter of the valve 19, and the inner diameter of the valve downstream pipeline 24 is the same as the inner diameter of the valve 19; the outlet of the valve upstream pipeline 23 is connected to the valve 19, and a protective gas inlet pipeline 20 is provided on the side wall of the valve upstream pipeline 23 near the outlet; optionally, the valve upstream pipeline 23 includes a constant diameter section and a variable diameter section which are arranged in sequence from top to bottom, the inner diameter of the constant diameter section is smaller than the inner diameter of the valve 19, the inner diameter of the top of the variable diameter section is the same as the inner diameter of the constant diameter section, and the inner diameter of the bottom of the variable diameter section is the same as the inner diameter of the valve 19; the protective gas inlet pipeline 20 is arranged on the side wall of the variable diameter section. In this embodiment, the inclination angle between the protective gas inlet pipe 20 and the central axis of the valve upstream pipe 23 is an acute angle, preferably 10 to 80°; the ratio of the inner diameter of the equal-diameter section of the valve upstream pipe 23 to the inner diameter of the valve 19 is 0.5 to 0.9; the height of the variable-diameter section of the valve upstream pipe 23 is 10 to 500 mm.

[0062] In a second embodiment, if Figure 3 As shown, the sealing unit 2 includes a valve upstream pipeline 23, a valve 19 and a valve downstream pipeline 24 which are connected in sequence; the inner diameter of the valve upstream pipeline 23 and the inner diameter of the valve downstream pipeline 24 are respectively the same as the inner diameter of the valve 19; the outlet of the valve upstream pipeline 23 is connected to the valve 19, and a protective gas inlet pipeline 20 is arranged on the side wall of the valve upstream pipeline 23 near the outlet; and a protective baffle 21 is circumferentially arranged on the inner wall near the outlet of the valve upstream pipeline 23; in this embodiment, the inclination angle between the protective gas inlet pipeline 20 and the central axis of the valve upstream pipeline 23 is an acute angle, preferably 10 to 80°; optionally, the width of the protective baffle 21 is 2 to 150 mm.

[0063] In the present disclosure, during the deoxidation treatment process of solid waste plastics, the inlet and outlet of the deoxidation device 3 are respectively provided with Figure 2 or Figure 3 In the sealing unit shown, the inner diameter of the pipe upstream of the valve in the sealing unit is smaller than the inner diameter of the valve ( Figure 2 ) or install a protective baffle on the inner wall of the upstream pipeline near the valve position ( Figure 3), both of which can limit the circulation area of ​​solid plastics, and to a certain extent prevent solid plastics from falling into the valve slideway and hindering the normal opening and closing of the valve. To further reduce the probability of solid plastics falling into the valve slideway, when solid plastics pass through the valve, nitrogen is used to purge the valve slideway at a suitable angle. Nitrogen purges the solid plastics to prevent solid plastics from falling into the valve slideway. When there is no solid plastic flowing through the valve, nitrogen purge is stopped. The sealing unit disclosed in the present invention can avoid the consequences of poor valve sealing caused by solid plastics falling into the valve slideway during the valve opening and closing process, thereby achieving long-term sealing on solid conveying pipelines that need to be opened and closed frequently.

[0064] In a specific embodiment, during the process of the solid waste plastic flowing through the sealing unit 2 and entering the deoxidation device 3, the protective gas is introduced into the sealing unit 2 through the protective gas inlet pipe 20 to purge the solid waste plastic in the sealing unit 2; preferably, the protective gas is selected from one or more of air, inert gas and liquid hydrocarbons, preferably nitrogen; optionally, the flow rate of the protective gas is 10 to 2000 L / min, preferably 50 to 500 L / min.

[0065] In one embodiment, step S1 includes the following two methods for deoxidation treatment:

[0066] The solid waste plastic is allowed to enter the deoxidation device 3 from the solid waste plastic inlet at the top of the deoxidation device 3 through the sealing unit 2; an inert gas is continuously introduced into the deoxidation device 3 from the inert gas inlet on the side wall of the deoxidation device 3 for continuous purge to replace the air in the deoxidation device 3 out of the deoxidation device 3; optionally, the rate of the inert gas is 10 to 2000 L / min, preferably 50 to 500 L / min; preferably, in this method, a tank can be used as a deoxidation device;

[0067] Alternatively, the solid waste plastic is allowed to enter the deoxidation device 3 through the solid waste plastic inlet at the top of the deoxidation device 3 via the sealing unit 2; the deoxidation device 3 is filled with inert gas to 0.1-1.0MPag, preferably 0.3-0.5MPag, through the inert inlet on the side wall of the deoxidation device 3, and then the deoxidation device 3 is depressurized to below 0.1MPag; repeat 1-10 times, preferably 2-5 times; preferably, two tanks can be used in parallel as deoxidation devices in this manner. In the present disclosure, an inert gas can be continuously purged, or an inert gas can be filled into the deoxidation device to a certain pressure, and then the gas in the deoxidation device is discharged, and the above process is repeated to replace oxygen, thereby effectively reducing the oxygen content of the solid plastic after deoxidation.

[0068] In a specific embodiment, when two tanks arranged in parallel are used as deoxygenation devices, one of the deoxygenation devices is controlled to be in the deoxygenation treatment process, and the other deoxygenation device is in the process of completing the deoxygenation treatment and conveying the deoxygenated plastic to the softening device 5. The treatment process of the two deoxygenation devices is switched as the reaction progresses to ensure that the deoxygenation device can continuously provide deoxygenated plastic to the softening device, thereby continuously providing liquid feed for the pyrolysis of waste plastics.

[0069] In one embodiment, in step S2, the softening device 5 is a first screw machine; the first screw machine includes a screw inlet, a screw outlet, a first exhaust port and a first heating element;

[0070] Preferably, the inlet temperature of the first screw machine is 0-100°C, the material residence time is 30-1200s, and the outlet temperature is 80-300°C; preferably, the inlet temperature of the first screw machine is 20-60°C, the material residence time is 120-300s, and the outlet temperature is 120-200°C;

[0071] Preferably, the chlorine content of the softened dechlorinated plastic is less than 0.1% by weight. The softening device used in the present disclosure is a screw machine with a heating function (first heating element) and an air extraction port (first exhaust port). In the plastic softening part, the solid plastic moves under the push of the screw and softens at a relatively low temperature. Most of the chlorine in the plastic is decomposed and extracted from the air extraction port of the plastic softener. The softened plastic has a low chlorine content, which can greatly reduce the corrosion of chlorine impurities to subsequent equipment. In the present disclosure, the softening device is provided with a plurality of first exhaust ports at different temperatures, preferably two first exhaust ports.

[0072] The liquefaction device used in the present disclosure may adopt a stirring tank with a heating function; or may adopt a screw machine with a heating function and a structure that facilitates gas-liquid outflow.

[0073] In a specific 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;

[0074] Optionally, the conditions of the liquefaction treatment include: temperature of 150-500°C, pressure of -0.09-1.0Mpa, and time of 0.1-1.0h; preferably, temperature of 250-400°C, pressure of 0.05-0.5Mpa, and time of 0.1-0.2h;

[0075] Optionally, the stirring rate is 10 to 500 r / min; preferably 50 to 200 r / min.

[0076] In another specific embodiment, in step S3, the liquefaction device 6 is a second screw machine, and the second screw includes a third heating element for heating the material in the second screw machine; optionally, when the liquefaction device 6 is a second screw machine, the cross-sectional area of ​​the liquefaction device outlet pipe is 50% to 300% of the cross-sectional area of ​​the second screw machine body; preferably, the cross-sectional area of ​​the liquefaction device outlet pipe is equal to the cross-sectional area of ​​the second screw machine body;

[0077] The conditions for the liquefaction treatment include: the inlet temperature of the second screw machine is 80-300°C, the residence time is 30-1200s, and the outlet temperature of the second screw machine is 250-500°C; preferably, the inlet temperature of the second screw machine is 120-200°C, the residence time is 100-300s, and the outlet temperature of the second screw machine is 250-400°C.

[0078] In the present disclosure, in the plastic liquefaction part, the plastic liquefaction operation temperature is relatively high, and a small amount of plastic is cracked to produce a certain amount of gas. The screw machine with a large stirring tank or outlet cross-sectional area has the characteristics of low pressure drop, and the liquefied flow with high viscosity and gas-liquid two-phase characteristics can pass through the liquefaction equipment smoothly, thereby avoiding the problem of local overheating of high-temperature liquid plastic due to poor circulation, and then coking and clogging the equipment.

[0079] In a specific embodiment, the stirring device or the second screw machine can be set to one stage or multiple stages in series to improve the processing efficiency.

[0080] In a preferred embodiment, in step S4, the circulating material accounts for 10 to 90% by weight of the total liquid material from the liquefaction device 6, preferably 30 to 60% by weight; preferably, the temperature of the circulating material after heating is 300 to 500° C., preferably 350 to 450° C.;

[0081] Optionally, the weight ratio of the recycled material to the softened dechlorinated plastic from the softening device 5 is 0.1 to 9:1, preferably 0.5 to 3:1.

[0082] In a specific embodiment, the liquefaction device 6 is a stirring device, and liquid plastic is obtained at the bottom of the stirring tank. The top gas phase outlet is connected to the gas-liquid separation device 8, and pyrolysis gas is obtained at the top of the gas-liquid separation device 8. Another portion of liquid plastic is obtained at the bottom of the gas-liquid separation device 8, which can be used for subsequent liquefied plastic processing devices;

[0083] The liquefaction device 6 is the second screw machine, and the outlet of the last screw machine is connected to the gas-liquid separation device 8. The pyrolysis gas is obtained at the top of the gas-liquid separation device for entering the subsequent alkali washing device 10, and the liquid plastic is obtained at the bottom. The liquid plastic at the bottom of the gas-liquid separation device 8 can be used in the subsequent liquefied plastic processing device.

[0084] In one embodiment, if Figure 1 As shown, the method also includes:

[0085] The pyrolysis gas from the liquefaction device 6 enters the gas-liquid separation device 8 for gas-liquid separation treatment to obtain gas phase material and liquid phase material;

[0086] The gas phase material is allowed to enter the alkali washing device 10 for alkali washing treatment; and the liquid phase material is allowed to enter the subsequent liquefied plastic processing device.

[0087] In a specific embodiment, the alkali cleaning agent in the alkali cleaning device is selected from one or more of sodium hydroxide, potassium hydroxide, sodium carbonate and potassium carbonate.

[0088] A second aspect of the present disclosure provides a system for liquefying solid waste plastics, such as Figure 1 As shown, the system includes: a deoxidation device 3, a softening device 5, a liquefaction device 6 and an alkali washing device 10;

[0089] The deoxidation device 3 includes a solid waste plastic inlet, an inert gas inlet and a deoxidized plastic outlet;

[0090] The softening device 5 comprises a deoxidized plastic inlet, a chlorine-containing gas outlet and a softened dechlorinated plastic outlet, and the deoxidized plastic inlet is connected to the deoxidized plastic outlet of the deoxidizing device 3;

[0091] The liquefaction device 6 includes a softened dechlorinated plastic inlet, a pyrolysis gas outlet and a liquid plastic outlet, the softened dechlorinated plastic inlet is connected to the softened dechlorinated plastic outlet of the softening device 5, and the liquid plastic outlet is connected to the softened dechlorinated plastic inlet of the liquefaction device 6 via a liquid plastic circulation pipeline;

[0092] The alkali washing device 10 includes an inlet for materials to be washed, an inlet for alkali washing agent, an outlet for waste liquid and an exhaust port; the inlet for materials to be washed is connected to the chlorine-containing gas outlet of the softening device 5 and the pyrolysis gas outlet of the liquefaction device 6 respectively.

[0093] In one embodiment, if Figure 1 As shown, the system further includes a sealing unit 2, which includes a valve upstream pipeline 23, a valve 19 and a valve downstream pipeline 24 which are sequentially connected. Figure 2 or Figure 3 The specific structure of the sealing unit 2 has been described in detail in the previous content, and will not be repeated here.

[0094] In a preferred embodiment, Figure 1As shown, the system includes multiple deoxygenation devices 3; preferably, the system includes two deoxygenation devices 3 arranged in parallel; a sealing unit 2 is respectively arranged on the inlet pipeline of the waste plastic inlet and the outlet pipeline of the deoxygenated plastic outlet of each deoxygenation device 3; wherein the outlet of the valve downstream pipeline 24 of the sealing unit 2 located on the inlet pipeline of the waste plastic inlet is connected to the solid waste plastic inlet of the deoxygenation device 3; the inlet of the valve downstream pipeline 24 of the sealing unit 2 located on the outlet pipeline of the deoxygenated plastic outlet is connected to the deoxygenated plastic outlet of the deoxygenation device 3, and the outlet of the valve downstream pipeline 24 is connected to the deoxygenated plastic inlet of the softening device 5.

[0095] In a specific implementation, the deoxygenation device 3 is a vertical tank structure; the softening device 5 is a first screw machine; the chemical device 6 is a stirring tank or a second screw machine; and the alkali washing device 10 is a washing tower.

[0096] In a specific implementation manner, Figure 1 As shown, the system also includes a heating furnace 16, the inlet of the heating furnace 16 is connected to the liquid plastic outlet of the liquefaction device 6, and the outlet of the heating furnace 16 is connected to the softened dechlorinated plastic inlet of the liquefaction device 6, so as to heat the liquid plastic returned to the liquefaction device 6 for further processing.

[0097] In a specific implementation manner, Figure 1 As shown, the system further includes a gas-liquid separation device 8 for further separating the pyrolysis gas from the liquefaction device 6; the inlet of the gas-liquid separation device 8 is connected to the pyrolysis gas outlet of the liquefaction device 6, the gas phase outlet of the gas-liquid separation device 8 is connected to the inlet of the material to be alkali-washed of the alkali-washing device 10, and the liquid phase outlet of the gas-liquid separation device 8 can be combined with the material flow (liquid plastic) 7 to flow out. The gas-liquid separation device 8 can be a gas-liquid separation tank.

[0098] use Figure 1 The process of liquefying solid waste plastics in the system shown includes:

[0099] The system includes two deoxidation devices 3 arranged in parallel, and the crushed solid waste plastics 1 are introduced into the two deoxidation devices 3 through the sealing units 2 on the solid waste plastic inlet introduction pipeline of the deoxidation device 3. The two deoxidation devices 3 are operated in parallel: the sealing unit 2 at the top of one deoxidation device 3 is in a closed state and cut off from the upstream, and no nitrogen protective gas is passed, the sealing unit 2 at the bottom of the deoxidation device 3 is in an open state and passes nitrogen protective fluid, and the deoxidized plastic 4 is sent to the softening device 5; the sealing unit 2 at the bottom of the other deoxidation device 3 is in a closed state and no nitrogen protective gas is passed, and the connection with the downstream softening device 5 is cut off, the sealing unit 2 at the top of the deoxidation device 3 is opened and passes nitrogen protective fluid, receives the solid waste plastics 1, and after receiving a sufficient amount of raw materials, closes the top sealing unit 2 and does not pass nitrogen protective gas, passes nitrogen 17 to fill the deoxidation device 3 until the pressure reaches 0.3MPag, and then relieves the pressure to 0.05MPag, repeats 3 times to complete the plastic deoxidation. The deoxidized plastic 4 enters the softening device 5, where the plastic is heated to a softened state, and the chlorine-containing plastic decomposes to release acidic gas 18, which enters the alkali washing device 10. The softened plastic 14 enters the liquefaction device 6. The gas at the top of the liquefaction device 6 is introduced into the gas-liquid separation device 8, and the liquefied gas at the top of the gas-liquid separation device 8 enters the alkali washing device 10; the liquid plastic at the bottom of the liquefaction device 6 is divided into two parts, one part is the circulating material 15, which is heated by the heating furnace 16 and sent to the entrance of the liquefaction device 6 to provide part of the heat for the plastic liquefaction, and the other part is the liquid plastic 7, which can be used for subsequent treatment, such as entering the delayed coking device.

[0100] The present disclosure is further described in detail by way of examples. The raw materials used in the examples can all be obtained through commercial sources.

[0101] Example 1

[0102] use Figure 1 The system for liquefying solid waste plastics shown in the figure, wherein the structural parameters of each device include:

[0103] The structure of the sealing unit 2 is as follows: Figure 2 As shown, the inclination angle between the protective gas inlet pipe 20 and the central axis of the valve upstream pipe 23 is an acute angle of 45°; the ratio of the inner diameter of the equal-diameter section of the valve upstream pipe 23 to the inner diameter of the valve 19 is 0.8; the height of the variable-diameter section of the valve upstream pipe 23 is 100 mm; the size of the deoxidation device 3 is 600 mm (inner diameter) × 1500 mm (height); the softening device 5 is a first screw machine, the major diameter of the screw of the first screw machine is 50 mm, and the screw length is 3000 mm; the liquefaction device 6 is a stirring device, and the size of the stirring device is 300*mm (inner diameter) × 500 mm (height).

[0104] The process of liquefying solid waste plastics using the above system includes:

[0105] Solid waste plastics include polypropylene, polyethylene and polyvinyl chloride, with a chlorine content of 0.5% by weight;

[0106] The crushed solid waste plastics 1 are introduced into the two deoxidation devices 3 respectively through the sealing units 2 on the solid waste plastic inlet introduction pipeline of the deoxidation device 3 . Two deoxidation devices 3 are operated in parallel: the sealing unit 2 at the top of one deoxidation device 3 is in a closed state and cut off from the upstream, and no nitrogen protective gas is passed. The deoxidation treatment has been completed by the deoxidation device. The sealing unit 2 at the bottom of the deoxidation device 3 is in an open state and nitrogen protective gas is passed. The flow rate of the protective gas is 150L / min, and the deoxidized plastic 4 is sent to the softening device 5; the sealing unit 2 at the bottom of the other deoxidation device 3 is in a closed state and no nitrogen protective gas is passed. The connection with the downstream softening device 5 is cut off. The sealing unit 2 at the top of the deoxidation device 3 is opened and nitrogen protective fluid is passed to receive the solid waste plastic 1. After receiving a sufficient amount of raw materials, the top sealing unit 2 is closed and no nitrogen protective gas is passed. Nitrogen 17 is passed to fill the deoxidation device 3 until the pressure reaches 0.3MPag, and then the pressure is relieved to 0.05MPag. This is repeated 3 times to complete the deoxidation of the plastic. During the continuous treatment process, the two deoxidation devices are switched and used, and the conditions for the deoxidation treatment are the same.

[0107] The deoxygenated plastic 4 enters the softening device 5, in which the plastic is heated to a softened state, and the chlorine-containing plastic decomposes to release acid gas 18, and the acid gas 18 enters the alkali washing device 10. During the softening treatment, the inlet temperature of the first screw machine is 25°C, the feed rate of the deoxygenated plastic is 41.7g / s, the screw linear speed is 0.1m / s, the material residence time is 300s, and the outlet temperature is 150°C.

[0108] The softened plastic 14 enters the liquefaction device 6; the conditions of the liquefaction treatment include: temperature of 300° C., pressure of 0.05 MPa, time of 0.2 h; stirring rate of 100 r / min.

[0109] The gas at the top of the liquefaction device 6 is introduced into the gas-liquid separation device 8, and the liquefied gas at the top of the gas-liquid separation device 8 enters the alkali washing device 10; the liquid plastic at the bottom of the liquefaction device 6 is divided into two parts, one part is the circulating material 15 (the circulating material accounts for 33 weight % of all the liquid materials from the liquefaction device 6; the weight ratio of the circulating material to the softened dechlorinated plastic from the softening device 5 is 0.5:1) which is heated to 350°C by the heating furnace 16 and then sent to the entrance of the liquefaction device 6 to provide part of the heat for plastic liquefaction, and the other part is the liquid plastic 7, which can be used for subsequent treatment.

[0110] Example 2

[0111] This embodiment refers to the system and method of Embodiment 1, but differs from Embodiment 1 in that no sealing unit 2 is provided, and the remaining devices and processes are the same as those of Embodiment 1.

[0112] Example 3

[0113] This embodiment adopts the same method and system as Embodiment 1, and the difference from Embodiment 1 is that:

[0114] The softened plastic 14 enters the liquefaction device 6; the conditions of the liquefaction treatment include: temperature of 300° C., pressure of 0.05 MPa, time of 0.3 h; stirring rate of 100 r / min.

[0115] The gas at the top of the liquefaction device 6 is introduced into the gas-liquid separation device 8, and the liquefied gas at the top of the gas-liquid separation device 8 enters the alkali washing device 10; the liquid plastic at the bottom of the liquefaction device 6 is divided into two parts, one part is the circulating material 15 (the circulating material accounts for 20 weight% of the total liquid material from the liquefaction device 6; the weight ratio of the circulating material to the softened dechlorinated plastic from the softening device 5 is 0.25:1) which is heated to 350°C by the heating furnace 16 and then sent to the entrance of the liquefaction device 6 to provide part of the heat for plastic liquefaction, and the other part is the liquid plastic 7, which can be used for subsequent treatment.

[0116] Example 4

[0117] This embodiment adopts the same method and system as Embodiment 3, and the difference from Embodiment 3 is that:

[0118] The softened plastic 14 enters the liquefaction device 6; the conditions of the liquefaction treatment include: temperature of 300° C., pressure of 0.05 MPa, time of 0.5 h; stirring rate of 100 r / min.

[0119] The gas at the top of the liquefaction device 6 is introduced into the gas-liquid separation device 8, and the liquefied gas at the top of the gas-liquid separation device 8 enters the alkali washing device 10; the liquid plastic at the bottom of the liquefaction device 6 is divided into two parts, one part is the circulating material 15 (the circulating material accounts for 5 weight% of the total liquid material from the liquefaction device 6; the weight ratio of the circulating material to the softened dechlorinated plastic from the softening device 5 is 0.05:1) which is heated to 350°C by the heating furnace 16 and then sent to the entrance of the liquefaction device 6 to provide part of the heat for plastic liquefaction, and the other part is the liquid plastic 7, which can be used for subsequent treatment.

[0120] Example 5

[0121] This embodiment adopts the same method and system as Embodiment 1, and the difference from Embodiment 1 is that:

[0122] A second screw machine is used as a liquefaction device, the major diameter of the screw of the second screw machine is 50 mm, and the screw length is 4000 mm;

[0123] The conditions for the liquefaction treatment include: the inlet temperature of the second screw machine is 150° C., the material residence time is 260 s, the outlet temperature of the second screw machine is 300° C., and the rest of the process is the same as in Example 1.

[0124] Example 6

[0125] This embodiment adopts the same method and system as Embodiment 1, and the difference from Embodiment 1 is that:

[0126] The solid waste plastic is allowed to enter the deoxidation device 3 from the solid waste plastic inlet at the top of the deoxidation device 3 through the sealing unit 2; an inert gas is continuously introduced into the deoxidation device 3 from the inert gas inlet on the side wall of the deoxidation device 3 for continuous purge to replace the air in the deoxidation device 3 out of the deoxidation device 3; the rate of the inert gas is 200L / min; and the rest of the process is the same as in Example 1.

[0127] Example 7

[0128] This embodiment refers to the system and method in Embodiment 1, which is different from Embodiment 1 in that: Figure 3 The sealing unit shown in the figure comprises a valve upstream pipeline 23, a valve 19 and a valve downstream pipeline 24 which are connected in sequence; the inner diameter of the valve upstream pipeline 23 and the inner diameter of the valve downstream pipeline 24 are respectively the same as the inner diameter of the valve 19; the outlet of the valve upstream pipeline 23 is connected to the valve 19, and a protective gas inlet pipeline 20 is arranged on the side wall of the valve upstream pipeline 23 near the outlet; and a protective baffle 21 is circumferentially arranged on the inner wall near the outlet of the valve upstream pipeline 23; the inclination angle between the protective gas inlet pipeline 20 and the central axis of the valve upstream pipeline 23 is 45°; the width of the protective baffle 21 is 20 mm, and the rest of the process is the same as that of Example 1.

[0129] Comparative Example 1

[0130] This embodiment refers to the system and method of Embodiment 1, but differs from Embodiment 1 in that no deoxidation device is provided, and the remaining devices and processes are the same as those of Embodiment 1.

[0131] Comparative Example 2

[0132] This embodiment refers to the system and method of Embodiment 1, but differs from Embodiment 1 in that the liquid plastic from the liquefaction device 6 is not refluxed, and the remaining devices and processes are the same as those of Embodiment 1.

[0133] The processing results of the above embodiments and comparative examples are listed in the following Table 1.

[0134] Table 1

[0135]

[0136] In Table 1, the symbol “>” represents “more than”, and the symbol “~” represents “approximately”.

[0137] Comparing Example 1 with Comparative Examples 1 to 2, it can be seen that in Comparative Example 1, no deoxidation device is provided, and in Comparative Example 2, the liquid plastic is not refluxed. In Example 1, the method for liquefying solid waste plastic provided by the present disclosure can obtain a lower CO+CO2 yield, and the asphaltene content in the liquid plastic is also lower;

[0138] Comparing Example 1 with Example 2, it can be seen that in Example 2, no sealing unit is provided, and the continuous operation period is only about 20 hours, indicating that the use of the sealing unit provided by the present disclosure can avoid the consequences of poor valve sealing caused by solid plastic falling into the valve slideway during the valve opening and closing process, thereby achieving long-term sealing on the solid conveying pipeline that needs to be opened and closed frequently;

[0139] By comparing Example 1 with Examples 3 to 4, it can be seen that the process conditions in the liquefaction device in Example 1 are within the optimized condition range provided by the present disclosure, and the asphaltene content in the liquid plastic 7 prepared in Example 1 is lower than that in Examples 3 to 4.

[0140] The preferred embodiments of the present disclosure are described in detail above; however, the present disclosure is not limited to the specific details in the above embodiments. Within the technical concept of the present disclosure, a variety of simple modifications can be made to the technical solution of the present disclosure, and these simple modifications all fall within the protection scope of the present disclosure.

[0141] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present disclosure will not further describe various possible combinations.

[0142] In addition, various embodiments of the present disclosure may be arbitrarily combined, and as long as they do not violate the concept of the present disclosure, they should also be regarded as the contents disclosed by the present disclosure.

Claims

1. A method for liquefying solid waste plastics, characterized in that: The following steps are involved: S1, allowing the solid waste plastic to enter the deoxidation device (3) for deoxidation treatment to obtain deoxidized plastic; S2, allowing the deoxidized plastic to enter a softening device (5) for softening and dechlorination treatment to obtain chlorine-containing gas and softened dechlorinated plastic; S3, allowing the softened dechlorinated plastic to enter a liquefaction device (6) for liquefaction treatment to obtain pyrolysis gas and liquid plastic; S4, allowing the chlorine-containing gas in step S2 and the pyrolysis gas in step S3 to enter the alkali washing device (10) for alkali washing treatment; allowing at least part of the liquid plastic in step S3 to be heated as a circulating material and then returned to the liquefaction device (6) for further treatment.

2. The method according to claim 1, characterized in that The deoxidation device (3) comprises a solid waste plastic inlet and a deoxidized plastic outlet; a sealing unit (2) is respectively provided on the inlet pipeline of the solid waste plastic inlet and the outlet pipeline of the deoxidized plastic outlet; Optionally, the sealing unit (2) comprises a valve upstream pipeline (23), a valve (19) and a valve downstream pipeline (24) which are connected in sequence; the inner diameter of the valve upstream pipeline (23) is smaller than the inner diameter of the valve (19), and the inner diameter of the valve downstream pipeline (24) is the same as the inner diameter of the valve (19); the outlet of the valve upstream pipeline (23) is connected to the valve (19), and a protective gas inlet pipeline (20) is provided on the side wall of the valve upstream pipeline (23) close to the outlet; Optionally, the valve upstream pipeline (23) comprises a constant diameter section and a variable diameter section arranged in sequence from top to bottom, the inner diameter of the constant diameter section is smaller than the inner diameter of the valve (19), the inner diameter of the top of the variable diameter section is the same as the inner diameter of the constant diameter section, and the inner diameter of the bottom of the variable diameter section is the same as the inner diameter of the valve (19); the protective gas inlet pipeline (20) is arranged on the side wall of the variable diameter section; Preferably, the inclination angle between the central axis of the protective gas inlet pipe (20) and the valve upstream pipe (23) is an acute angle, preferably 10 to 80°; optionally, the ratio of the inner diameter of the equal diameter section of the valve upstream pipe (23) to the inner diameter of the valve (19) is 0.5 to 0.9; the height of the variable diameter section of the valve upstream pipe (23) is 10 to 500 mm; Alternatively, the sealing unit (2) comprises a valve upstream pipeline (23), a valve (19) and a valve downstream pipeline (24) which are connected in sequence; the inner diameter of the valve upstream pipeline (23) and the inner diameter of the valve downstream pipeline (24) are respectively the same as the inner diameter of the valve (19); the outlet of the valve upstream pipeline (23) is connected to the valve (19), and a protective gas inlet pipeline (20) is arranged on the side wall of the valve upstream pipeline (23) near the outlet; and a protective baffle (21) is circumferentially arranged on the inner wall near the outlet of the valve upstream pipeline (23); preferably, the inclination angle between the protective gas inlet pipeline (20) and the central axis of the valve upstream pipeline (23) is an acute angle, preferably 10 to 80 degrees; optionally, the width of the protective baffle (21) is 2 to 150 mm.

3. The method according to claim 2, characterized in that Step S1 includes: The solid waste plastic is allowed to enter the deoxidation device (3) through the solid waste plastic inlet at the top of the deoxidation device (3) via the sealing unit (2); an inert gas is continuously introduced into the deoxidation device (3) through the inert gas inlet on the side wall of the deoxidation device (3) to continuously purge the air in the deoxidation device (3) out of the deoxidation device (3); optionally, the rate of the inert gas is 10 to 2000 L / min, preferably 50 to 500 L / min; Alternatively, the solid waste plastics are allowed to enter the deoxidation device (3) through the solid waste plastics inlet at the top of the deoxidation device (3) via the sealing unit (2); the deoxidation device (3) is filled with inert gas to 0.1-1.0 MPag, preferably 0.3-0.5 MPag, through the inert inlet on the side wall of the deoxidation device (3); and then the deoxidation device (3) is depressurized to below 0.1 MPag; this is repeated 1-10 times, preferably 2-5 times; preferably, in the process of the solid waste plastics flowing through the sealing unit (2) into the deoxidation device (3), protective gas is introduced into the sealing unit (2) through the protective gas inlet pipe (20) to purge the solid waste plastics in the sealing unit (2); preferably, the protective gas is selected from one or more of air, inert gas and liquid hydrocarbon, preferably nitrogen; optionally, the flow rate of the protective gas is 10-2000 L / min, preferably 50-500 L / min.

4. The method according to claim 1, characterized in that: In step S2, the softening device (5) is a first screw machine; the first screw machine comprises a screw inlet, a screw outlet, a first exhaust port and a first heating element; Preferably, the inlet temperature of the first screw machine is 0-100°C, the material residence time is 30-1200s, and the outlet temperature of the first screw machine is 80-300°C; preferably, the inlet temperature of the first screw machine is 20-60°C, the material residence time is 120-300s, and the outlet temperature of the first screw machine is 120-200°C; Preferably, the chlorine content of the softened dechlorinated plastic is less than 0.1 wt%.

5. 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 conditions of the liquefaction treatment include: temperature of 150-500°C, pressure of -0.09-1.0Mpa, and time of 0.1-1.0h; preferably, temperature of 250-400°C, pressure of 0.05-0.5Mpa, and time of 0.1-0.2h; optionally, stirring rate of 10-500r / min; preferably 50-200r / min.

6. The method according to claim 1, characterized in that In step S3, the liquefaction device (6) is a second screw machine, and the second screw machine includes a third heating element for heating the material in the second screw machine; The conditions for the liquefaction treatment include: the inlet temperature of the second screw machine is 80-300°C, the material residence time is 30-1200s, and the outlet temperature of the second screw machine is 250-500°C; preferably, the inlet temperature of the second screw machine is 120-200°C, the material residence time is 100-300s, and the outlet temperature of the second screw machine is 250-400°C.

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

1.

8. The method according to claim 1, characterized in that The method further includes: The pyrolysis gas from the liquefaction device (6) enters the gas-liquid separation device (8) for gas-liquid separation treatment to obtain gas phase material and liquid phase material; The gas phase material is allowed to enter the alkali washing device (10) for alkali washing treatment; the liquid phase material is allowed to enter a subsequent liquefied plastic processing device; Optionally, the alkali washing agent in the alkali washing device is selected from one or more of sodium hydroxide, potassium hydroxide, sodium carbonate and potassium carbonate.

9. A system for liquefying solid waste plastics, characterized in that: The system comprises: a deoxidation device (3), a softening device (5), a liquefaction device (6) and an alkali washing device (10); The deoxidation device (3) comprises an inlet for solid waste plastics, an inert gas inlet and an outlet for deoxidized plastics; The softening device (5) comprises a deoxygenated plastic inlet, a chlorine-containing gas outlet and a softened dechlorinated plastic outlet, and the deoxygenated plastic inlet is connected to the deoxygenated plastic outlet of the deoxygenating device (3); The liquefaction device (6) comprises a softened dechlorinated plastic inlet, a pyrolysis gas outlet and a liquid plastic outlet, the softened dechlorinated plastic inlet is connected to the softened dechlorinated plastic outlet of the softening device (5), and the liquid plastic outlet is connected to the softened dechlorinated plastic inlet of the liquefaction device (6) via a liquid plastic circulation pipeline; The alkali washing device (10) comprises an inlet for the material to be alkali washed, an inlet for the alkali washing agent, an outlet for the waste liquid and an exhaust port; the inlet for the material to be alkali washed is respectively connected to the chlorine-containing gas outlet of the softening device (5) and the pyrolysis gas outlet of the liquefaction device (6).

10. The system according to claim 9, characterized in that The system further comprises a sealing unit (2), wherein the sealing unit (2) comprises a valve upstream pipeline (23), a valve (19) and a valve downstream pipeline (24) which are sequentially connected. Optionally, the system comprises a plurality of the deoxidation devices (3); preferably, the system comprises two deoxidation devices (3) arranged in parallel; one sealing unit (2) is respectively arranged on the inlet pipeline of the waste plastic inlet and the outlet pipeline of the deoxidation plastic outlet of each deoxidation device (3); wherein the outlet of the valve downstream pipeline (24) of the sealing unit (2) located on the inlet pipeline of the waste plastic inlet is communicated with the solid waste plastic inlet of the deoxidation device (3); the inlet of the valve downstream pipeline (24) of the sealing unit (2) located on the outlet pipeline of the deoxidation plastic outlet is communicated with the deoxidation plastic outlet of the deoxidation device (3), and the outlet of the valve downstream pipeline (24) is communicated with the deoxidation plastic inlet of the softening device (5); Optionally, the deoxidation device (3) is a vertical tank; the softening device (5) is a first screw machine; the liquefaction device (6) is a stirring tank or a second screw machine; and the alkali washing device (10) is a washing tower.

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