A method and system for pyrolysis treatment of waste plastics

Abstract

This invention relates to a pyrolysis treatment method and system for waste plastics. The waste plastics undergo segmented dehydration and dechlorination in a waste plastic extrusion dehydration and dechlorination unit to obtain dechlorinated waste plastics. The obtained dechlorinated waste plastics then enter a waste plastic impurity removal treatment unit, where they are contacted with pretreatment agents and solvent oil for impurity removal treatment. The resulting effluent undergoes solid-liquid separation to obtain insoluble matter and a depurified plastic-containing solution. This depurified plastic-containing solution enters a pyrolysis unit for pyrolysis to obtain pyrolysis oil. The pyrolysis oil has very low metal, chlorine, and silicon content, effectively reducing the impact of impurities in the waste plastics on subsequent treatment units. This invention has low pollution discharge during production, good environmental performance, reduces carbon emissions, and has good environmental, social, and economic benefits.

Description

Technical Field

[0001] This invention relates to the field of solid waste plastic treatment technology, and specifically to a pyrolysis treatment method and system for waste plastics. Background Technology

[0002] The plastics industry has made a significant contribution to social development. Currently, my country's annual plastic production has reached approximately 120 million tons, the vast majority of which is discarded into the environment after a single use. Statistics show that my country's waste plastic production is approximately 2.4 to 4.8 million tons per year; by 2035, approximately 8 billion tons of waste plastic will exist in the natural environment globally. Since plastic takes 200 to 500 years to fully degrade, the continuous accumulation of waste plastic not only causes serious environmental pollution but also affects the ecological balance of the natural environment.

[0003] Existing waste plastic treatment technologies mainly include landfill and incineration. Landfill, besides occupying a large amount of land, is prone to causing secondary pollution and cannot fundamentally solve the problem. Currently, there are only about 400 waste-to-energy plants in China capable of processing waste plastics. If all waste plastics were to be disposed of through waste-to-energy incineration, it is estimated that more than 1,000 plants would still need to be built, which is clearly impractical. In addition, the high cost and low economic value of physical recycling processes for waste plastics result in insufficient profits for enterprises and are prone to causing secondary pollution during the recycling process, all of which contribute to the slow development of waste plastic resource recycling.

[0004] Chemical conversion of waste plastics is an effective means to achieve rapid recycling and conversion of waste plastics. In particular, thermoplastic waste plastics can be converted into oils and gases through pyrolysis. Therefore, pyrolysis is one of the main process routes for the chemical conversion of waste plastics.

[0005] CN106635115B discloses a method and hydrothermal reaction system for efficient and clean oil production from mixed waste plastics. The method includes hydrolyzing the mixed waste plastics using an alkaline aqueous solution at a temperature of 160–300°C and a pressure of 20–220 bar. The mixed waste plastics contain one or more of polyethylene (PE), polypropylene (PP), and polystyrene (PS), and PET; optionally, the mixed waste plastics may also include polyvinyl chloride (PVC). The aqueous phase and solid phase of the hydrothermally treated material are separated, and the separated solid phase is used to produce oil. This application not only avoids the adverse effects of PET and PVC in the mixed waste plastics on oil quality, achieving efficient and clean oil production, but also produces high-value-added products, such as TA powder or granules, and recycled PS plastics.

[0006] CN108456328A discloses a method for treating waste plastics, comprising: adding waste plastics, a modified catalyst, and a reaction solvent into a catalytic cracking reactor and mixing them evenly, and then carrying out a catalytic cracking reaction. The modified catalyst is a composite molecular sieve catalyst of HZSM-5 and HY modified with oxide modifiers, the modifier being selected from one or more of Sn, Fe, Ti, and Zn; the reaction solvent is a mixture of tetrahydronaphthalene and n-hexadecane; and the catalytic cracking reaction conditions are as follows: reaction temperature of 150-300℃, reaction time of 120-240 minutes, the reaction being carried out under stirring at a stirring rate of 600-1000 rpm, and hydrogen gas being introduced during the reaction at a hydrogen partial pressure of 4-7 MPa.

[0007] Due to the complex composition of waste plastics, containing a large number of heteroatoms, the addition of various organic and inorganic additives during plastic production to improve plastic performance, and the tendency for plastics to adhere to a large number of impurities during disposal, the main problem with existing pyrolysis processes is the substandard quality of pyrolysis oil, particularly high levels of Cl and Si impurities. Cl in pyrolysis oil mainly comes from the decomposition of PVC in waste plastics, and is mostly small-molecule organic chlorine, which easily decomposes into HCl during subsequent oil processing, causing severe corrosion. Current technologies for dechlorinating pyrolysis oil primarily use hydrogenation to convert organic chlorine into HCl, which is then neutralized with alkaline substances. However, this generates a large amount of solid waste, consumes a significant amount of hydrogen resources, and increases catalyst costs. Si in pyrolysis oil mainly comes from the decomposition of polymeric additives such as silicone oil, silicone resin, and silicone rubber powder, as well as inorganic additives such as SiO2, primarily alkylepoxysilanes. These are poisons for catalysts in subsequent processing, leading to permanent catalyst deactivation.

[0008] Therefore, there is still an urgent need for a new waste plastic treatment technology to solve the problem that the high content of harmful elements Cl and Si impurities in the pyrolysis oil of waste plastics in the existing technology leads to easy corrosion of pipelines and equipment in subsequent processes and easy deactivation of catalysts. Summary of the Invention

[0009] This invention addresses the problem in existing technologies where high levels of harmful elements Cl and Si impurities in waste plastic pyrolysis oil lead to easy corrosion of pipelines and equipment in subsequent processes, as well as catalyst deactivation. Therefore, it provides a method and system for the pyrolysis treatment of waste plastics.

[0010] The first aspect of the present invention provides a method for pyrolysis treatment of waste plastics, comprising the following steps:

[0011] (1) Waste plastic extrusion dehydration and dechlorination unit: Waste plastic enters the waste plastic extrusion dehydration and dechlorination unit, is first dehydrated at temperature T1, and then dechlorinated at temperature T2. The temperature T2 is 50-360℃ higher than the temperature T1, resulting in dechlorinated waste plastic.

[0012] (2) The dechlorinated waste plastics obtained in step (1) enter the waste plastic impurity removal treatment unit, and are treated by contacting the pretreatment agent and solvent oil. The resulting effluent is subjected to solid-liquid separation to obtain insoluble matter and a depurified plastic-containing solution. The depurified plastic-containing solution contains less than 5 μg / g of metal, less than 20 μg / g of chlorine, and less than 3 μg / g of silicon. The pretreatment agent is selected from one or more of humus, red mud, waste catalyst from oil refining units, kaolin, semi-coke, activated carbon, and gasification ash, as well as optional alkaline oxides.

[0013] (3) The plasticized solution obtained in step (2) enters the pyrolysis reactor of the pyrolysis unit, comes into contact with the pyrolysis catalyst, and undergoes pyrolysis reaction at a temperature of 350-550℃ to obtain semi-coke and pyrolysis oil and gas. After separation, the pyrolysis oil and gas yield coke powder, pyrolysis gas and pyrolysis oil.

[0014] In this invention, the waste plastic is one or more of the following: waste plastic from fresh household waste, waste plastic from industrial and agricultural production, and waste plastic from aged waste. It can be one or more of the following: waste plastic rods, waste plastic granules, waste plastic sheets, etc., formed by the preliminary processing of the above-mentioned waste plastics. Preferably, it is a low-quality waste plastic that cannot be physically recycled. The type of waste plastic is one or more of PE, PP, PS, and PVC.

[0015] In one embodiment of the present invention, the waste plastic is coarsely crushed before entering the waste plastic extrusion dehydration and dechlorination unit, and the particle size of the crushed waste plastic is 1-200 mm, preferably 1-50 mm. In one embodiment of the present invention, the waste plastic is washed and dried before crushing.

[0016] In one embodiment of the present invention, in the waste plastic extrusion dehydration and dechlorination unit, the waste plastic is subjected to pressure and the water is first squeezed out at temperature T1, and then further heated to temperature T2, so that the PVC decomposes and discharges chlorine-containing compounds, such as HCl, to obtain dechlorinated waste plastic.

[0017] In one embodiment of the present invention, in the waste plastic extrusion dehydration and dechlorination unit, the temperature range of T1 is 0 to 50°C, the temperature range of T2 is 100 to 350°C, and the extrusion pressure range is 5 to 30 MPa.

[0018] In a preferred embodiment, the temperature range of T1 is 0–20°C, the temperature range of T2 is 200–300°C, and the extrusion pressure range is 10–20 MPa.

[0019] In one embodiment of the present invention, a spiral extruder with an external heat exchange device can be used in the waste plastic extrusion dehydration and dechlorination unit. This external heat exchange device is a two-stage heat exchange system: one stage is a refrigeration and insulation device to control the temperature of the dehydration process T1, and the other stage is a heating and insulation device to control the temperature of the dechlorination process T2. The removed water is mainly discharged in liquid form, while the removed chlorinated compounds enter an HCl adsorption device for contact with the adsorbent for recovery. The adsorbent is selected from one or more of water, alkaline solution, brine, and solid alkaline substances.

[0020] In one embodiment of the present invention, in step (2), the processing conditions of the waste plastic impurity removal unit are: temperature of 250–410°C, pressure of 0.1–5 MPa, and residence time of the dechlorinated waste plastic of 10–60 min. Preferably, the processing conditions of the waste plastic impurity removal unit are: temperature of 320–390°C, pressure of 0.5–4.0 MPa, and residence time of the dechlorinated waste plastic of 15–45 min.

[0021] In this invention, the term "humus" refers to the mixture of plant matter and various organic wastes that are excavated from landfills and carried by waste plastics.

[0022] In this invention, the term "semi-coke" refers to the solid product generated by the pyrolysis of carbonaceous materials such as coal, biomass, and waste plastics at 350–600°C.

[0023] In this invention, the term "gasification ash" refers to the solid product remaining after carbonaceous materials such as coal, semi-coke, coke, biomass, petroleum coke, and waste plastics react with a gasifying agent at temperatures above 600°C and under normal or pressurized conditions.

[0024] In this invention, the term "red mud" also refers to the industrial solid waste discharged after alumina is extracted from bauxite.

[0025] In this invention, the term "solvent oil" has a meaning known in the art, and is generally a complex mixture of hydrocarbons. Preferably, the "solvent oil" is an aromatic-rich distillate oil, which may be one or a mixture of several liquid distillate oils obtained from petroleum processing, coal pyrolysis, direct coal liquefaction, biomass and / or waste plastic pyrolysis.

[0026] In one embodiment of the present invention, the solvent oil has a distillation range of 80–550°C, and the total aromatic hydrocarbon content in the solvent oil is higher than 50% by mass, and the monocyclic aromatic hydrocarbon content is higher than 20% by mass; preferably, the monocyclic aromatic hydrocarbon content in the solvent oil is higher than 40% by mass. The weight ratio of solvent oil to waste plastic is 1:10 to 10:1, and preferably, the weight ratio of solvent oil to waste plastic is 1:1 to 7:1.

[0027] In one embodiment of the present invention, the particle size range of the pretreatment agent is 75-150 μm; the weight ratio of the pretreatment agent to waste plastic is 1:10-2:1, preferably 1:7-1:5.

[0028] In a preferred embodiment of the present invention, the pretreatment agent is one or more of the following: waste catalytic cracking catalyst, humus, semi-coke, activated carbon, and optional alkaline oxide.

[0029] In a preferred embodiment, the semi-coke is the semi-coke obtained from the pyrolysis unit in step (3) of the present invention.

[0030] In this invention, "optional" means optional components. In one embodiment of this invention, the pretreatment agent contains, in addition to other components, an alkaline oxide.

[0031] Within the waste plastic impurity removal unit, most of the plastic dissolves in solvent oil and undergoes desiliconization and demetallization reactions under the action of a pretreatment agent. The effluent after impurity removal undergoes solid-liquid separation, separating it into insoluble matter and a plastic-containing solution containing impurities. In the method of this application, by employing the carefully selected pretreatment agent and solvent oil type, as well as the processing conditions, the impurity content in the resulting plastic-containing solution containing impurities can be controlled within a desired range, which is beneficial for subsequent further processing and treatment.

[0032] This invention does not limit the specific method of solid-liquid separation; it can be one or more of various types of filters and centrifuges. In one embodiment of this invention, the solid-liquid separation device is equipped with a heating and heat preservation device. The heating device is one or more types of external heating or internal heating, and the operating temperature of the solid-liquid separation is 250–410°C. When the solid-liquid separation device is a filter, the filter screen pore size is 0.1–75 μm, preferably 0.1–10 μm.

[0033] This invention reduces the chlorine content in waste plastics through a dehydration and dechlorination process via extrusion, effectively preventing chlorine impurities from affecting subsequent processing units. Simultaneously, in the waste plastic impurity removal unit described in this invention, metallic and silicon impurities are effectively removed from the dechlorinated waste plastics. In one embodiment of this invention, the metal content, chlorine content, and silicon content in the de-impuration plastic solution are less than 3 μg / g, less than 1 μg / g, and less than 1 μg / g. The silicon impurities are removed in their original form as organosilicon polymers added during plastic processing, effectively preventing the silicon impurities from affecting the catalysts in subsequent processing units.

[0034] In one embodiment of the present invention, in step (3), the reaction temperature of the pyrolysis reactor is 450-500°C, the reaction pressure is 0.1-5 MPa, and the residence time is 10-30 min.

[0035] In one embodiment of the present invention, the pyrolysis reactor of the present invention is a rotary kiln pyrolysis reactor, and the pyrolysis reaction is carried out in a pyrolysis gas atmosphere. The pyrolysis gas can be the full-component pyrolysis gas generated by pyrolysis, or it can be the treated pyrolysis gas, and is more preferably hydrogen-rich pyrolysis gas.

[0036] In one embodiment of the present invention, the pyrolysis catalyst is a catalyst containing one or more of macroporous molecular sieves, mesoporous molecular sieves, alumina, and silica-alumina.

[0037] The preferred pyrolysis catalyst contains a metal component, which is selected from one or more of K, Na, Ca, Fe, Co, Ni, and Mo.

[0038] In one embodiment of the present invention, in step (3) of the present invention, part of the pyrolysis gas obtained by separation is sent to the boiler of the heating unit for combustion and heating, part is sent to the pyrolysis gas separation and purification unit, and after separation and purification, it is discharged as pyrolysis gas product, and part is sent to the pyrolysis reactor as pyrolysis reaction atmosphere.

[0039] In one embodiment of the present invention, the pyrolysis oil is fractionated to obtain light and heavy fractions with a cut-off point of 180–230°C. The resulting light fraction is a high-quality light oil product. This light oil product is essentially free of Cl and Si, as well as other metallic impurities, and can be directly processed and upgraded in subsequent petroleum refining units to obtain fuel products or chemical products.

[0040] In one preferred embodiment of the present invention, the obtained heavy fraction is used as solvent oil and recycled to the waste plastic impurity removal treatment unit.

[0041] In one embodiment of the present invention, the following steps are also included:

[0042] (4) The insoluble matter obtained in step (2) is introduced into the extraction unit and solvent extracted by contacting with an organic solvent. The product is separated into solid phase material and liquid phase material by solid-liquid separation. The organic solvent is selected from one or more of benzene, toluene, chloroform, cyclohexanone, ethyl acetate, butyl acetate, carbon disulfide, tetrahydrofuran, and gasoline.

[0043] (5) The solid material obtained in step (4) is processed in the pretreatment agent recovery unit to obtain the recovered pretreatment agent, and part of the recovered pretreatment agent is returned to the waste plastic impurity removal treatment unit in step (2).

[0044] (6) The obtained liquid phase material is subjected to precipitation treatment and separation in the solvent recovery unit to obtain solid plastic particles and recovered organic solvent.

[0045] In one embodiment of the present invention, the extraction unit is provided with one or more of the following: a solvent extraction tower or a static mixing extractor. The specific operating conditions of the extraction unit are matched with the selected organic solvent. The purpose of extraction is to remove solvent oil adhering to the discharged solid material, reduce the total amount of discharged solid material, and ultimately achieve the purpose of emission reduction.

[0046] In a preferred embodiment, the organic solvent is selected from one or more of benzene, toluene, chloroform, cyclohexanone, ethyl acetate, butyl acetate, carbon disulfide, tetrahydrofuran, and gasoline.

[0047] In one embodiment of the present invention, the solid material obtained by the extraction unit is processed in the pretreatment agent recovery unit, wherein the processing is selected from one or more of the screening and regeneration methods.

[0048] In one embodiment of the present invention, the liquid material obtained from the extraction unit undergoes precipitation treatment and separation in the solvent recovery unit to obtain solid plastic particles and recovered organic solvent. The precipitation treatment is selected from one or more of the following methods: back-extraction, simple distillation, flash evaporation, and rectification. The specific operating temperature is flexibly adjusted according to the type and ratio of the selected solvent.

[0049] In another aspect, the present invention provides a system for any of the above methods, comprising a waste plastic extrusion dehydration and dechlorination unit, a waste plastic impurity removal treatment unit, a solid-liquid separation unit, and a pyrolysis unit;

[0050] The waste plastic extrusion dehydration and dechlorination unit is equipped with a waste plastic inlet, a moisture outlet, a chlorine-containing compound outlet, and a dechlorinated waste plastic outlet.

[0051] The waste plastic impurity removal treatment unit is equipped with a dechlorinated waste plastic inlet, a pretreatment agent inlet, a solvent oil inlet, and a pretreated material outlet. The pretreated material outlet is connected to the inlet of the solid-liquid separation unit, which is equipped with an insoluble matter outlet and a depurified plastic-containing solution outlet.

[0052] The pyrolysis unit includes a pyrolysis reactor and a pyrolysis oil-gas separation system. The pyrolysis reactor is provided with a de-impure plastic-containing solution inlet, a semi-coke outlet, and a pyrolysis oil-gas outlet. The de-impure plastic-containing solution outlet of the solid-liquid separation unit is connected to the de-impure plastic-containing solution inlet of the pyrolysis reactor. The pyrolysis oil-gas outlet of the pyrolysis reactor is connected to the inlet of the pyrolysis oil-gas separation system. The pyrolysis oil-gas separation system is provided with at least a pyrolysis gas outlet and a pyrolysis oil outlet.

[0053] In one embodiment of the present invention, the moisture outlet of the waste plastic extrusion dehydration and dechlorination unit is connected to a condensation device, and further connected to a wastewater treatment device.

[0054] In one embodiment of the present invention, the outlet of the chlorine-containing compound is connected to the inlet of the HCl adsorption device, and the chlorine-containing compound is recovered by contacting the adsorbent, wherein the adsorbent is selected from one or more of water, alkaline solution, salt water, and solid alkaline substances.

[0055] In one embodiment of the present invention, a screw extruder with an external heat exchange device can be used in the waste plastic extrusion dehydration and dechlorination unit. The external heat exchange device is a two-stage heat exchange device, one stage being a refrigeration and cold preservation device to control the temperature T1 of the dehydration process, and the other stage being a heating and heat preservation device to control the temperature T2 of the dechlorination process. The screw extruder can be continuous or intermittent, horizontal or vertical, and can be one or more of the following: screwless, single-screw, twin-screw, and multi-screw.

[0056] In one embodiment of the present invention, the waste plastic impurity removal treatment unit can be a single-stage or multi-stage series or parallel kettle-type dissolving device, or a non-standard dissolving device with a dissolving function, or a device with the same function such as a continuous spiral extractor. More preferably, the waste plastic impurity removal treatment unit is equipped with a stirring device and a heating device. The stirring device can be electrically driven or magnetically driven, and can be one or more of paddle, anchor, frame, and screw types. The stirring speed of the stirring device can be 1-300 r / min, preferably 60-100 r / min. The heating device can be one or more of internal heating and / or external heating devices. For example, the external heating device can be one or more of a jacketed or semi-circular coil, and the internal heating device can be various types of built-in coils. The heat source can be one or more of electric, heat transfer oil, steam, and open flame heating.

[0057] This invention does not impose any particular requirement on the specific form of the solid-liquid separation unit; it can be various types of filters, centrifuges, or combinations thereof. In a preferred embodiment, the solid-liquid separation unit can be a single-stage or multi-stage series-connected continuous automatic solid-liquid separation device with heat insulation function, resistant to oil and organic solvents; or one or more of the following: filters with automatically replaceable screens, scraper filter presses, cross-flow filters, horizontal screw centrifuges, and screw extrusion solvent removal devices. More preferably, the solid-liquid separation unit can be equipped with heating and heat preservation devices. The heating device can be one or more of external or internal heating devices. External heating devices can be one or more of jacketed or semi-circular coils, while internal heating devices can be various types of built-in coils. The heat source can be one or more of electricity, thermal oil, steam, or open flame heating. Preferably, the operating temperature of the solid-liquid separation unit can be 250-410℃. When the solid-liquid separation unit is a filter, the filter screen pore size can be 0.1-75μm, preferably 0.1-10μm.

[0058] In one embodiment of the present invention, the semi-coke outlet of the pyrolysis reactor is connected to the pretreatment agent inlet of the waste plastic impurity removal treatment unit.

[0059] In one embodiment of the present invention, a fractionation system is provided in the pyrolysis unit, the pyrolysis oil outlet is connected to the inlet of the fractionation system, and the fractionation system is provided with a light fraction outlet and a heavy fraction outlet.

[0060] The heavy fraction outlet of the fractionation system is connected to the solvent oil inlet of the waste plastic impurity removal treatment unit.

[0061] In one embodiment of the present invention, the pyrolysis reactor is a rotary kiln pyrolysis reactor.

[0062] In one embodiment of the present invention, the rotary kiln pyrolysis reactor is provided with fins and liquid guiding channels inside, the fins and liquid guiding channels being parallel or perpendicular to the central axis of the rotary kiln cylinder, or being spiral-shaped; a metal connecting rod is provided at the center inside the rotary kiln pyrolysis reactor, and an elastic scraper is provided on the connecting rod.

[0063] In one embodiment of the present invention, the rotary kiln pyrolysis reactor is an externally jacketed rotary kiln pyrolysis reactor, with fins and liquid guide channels inside the rotary kiln to increase the heat transfer area; a metal connecting rod is provided in the center inside the rotary kiln, and an elastic scraper is provided on the metal connecting rod to scrape off the semi-coke adhering to the inner wall of the rotary kiln after the reaction is completed.

[0064] In one embodiment of the present invention, the rotary kiln pyrolysis reactor is provided with a feed inlet, and the pyrolysis gas, the de-impure plasticizing solution and the pyrolysis catalyst are mixed and fed together, or the rotary kiln pyrolysis reactor is provided with separate inlets for pyrolysis gas, de-impure plasticizing solution and pyrolysis catalyst, and the three are fed separately.

[0065] In one embodiment of the present invention, the pyrolysis oil-gas separation system is provided with a cyclone separator and a three-phase separator; the cyclone separator is provided with a pyrolysis oil-gas inlet, a pyrolysis oil-gas outlet, and a semi-coke outlet; the cyclone separator is one or more of a single-stage or multi-stage cyclone separator connected in series or in parallel.

[0066] In one embodiment of the present invention, the three-phase separator is provided with a pyrolysis oil and gas inlet, a pyrolysis gas outlet, a pyrolysis water outlet, and a pyrolysis oil outlet. The pyrolysis gas outlet is located above the three-phase separator, the pyrolysis water outlet is located below the three-phase separator, the pyrolysis oil outlet is located in the middle of the three-phase separator, and the pyrolysis oil outlet is connected to the inlet of the fractionation system.

[0067] In one embodiment of the present invention, the system further includes an extraction unit, a pretreatment agent recovery unit, and a solvent recovery unit;

[0068] The extraction unit is provided with an insoluble matter inlet, an organic solvent inlet, a solid phase material outlet, and a liquid phase material outlet. The insoluble matter outlet of the solid-liquid separation unit is connected to the insoluble matter inlet of the extraction unit.

[0069] The pretreatment agent recovery unit is equipped with a solid material inlet and a pretreatment agent recovery outlet. The solid material outlet of the extraction unit is connected to the solid material inlet of the pretreatment agent recovery unit, and the pretreatment agent recovery outlet of the pretreatment agent recovery unit is connected to the pretreatment agent inlet of the waste plastic impurity removal treatment unit.

[0070] The solvent recovery unit is equipped with a liquid material inlet, a solid plastic particle outlet, and a recovered organic solvent outlet. The liquid material outlet of the extraction unit is connected to the liquid material inlet of the solvent recovery unit.

[0071] In one embodiment of the present invention, the extraction unit may be equipped with one or more of a solvent extraction tower or a static mixing extractor. The specific operating conditions of the extraction unit are matched with the selected second organic solvent. The purpose of extraction is to remove solvent oil adhering to the discharged solid phase material, reduce the total amount of discharged solid material, and ultimately achieve the purpose of emission reduction.

[0072] In a preferred embodiment, the solid material obtained from the extraction unit is processed in a pretreatment agent recovery unit, and part or all of it is returned to the waste plastic pretreatment unit as a recycled pretreatment agent. The processing method of the pretreatment agent recovery unit can be selected from screening, regeneration, or a combination thereof. The pretreatment agent recovery unit can be in the form of a vibrating screen, a drum screen, a single-stage or multi-stage fluidized bed regeneration furnace, a rotary kiln regeneration furnace, an infrared heating furnace, etc.

[0073] Features of this invention:

[0074] This invention provides a pyrolysis treatment method and system for waste plastics. First, chlorine impurities are effectively removed from the waste plastics to prevent their impact on subsequent processing units. Then, metallic and silicon impurities are effectively removed, with silicon impurities removed in their original form as organosilicon polymers added during plastic processing, effectively avoiding catalyst deactivation issues in subsequent processing. The purified waste plastics undergo pyrolysis in a pyrolysis reactor. The light fraction of the resulting pyrolysis oil, as a light oil product, can be further processed in subsequent units to obtain fuel products or chemical products. The heavy fraction of the resulting pyrolysis oil can be recycled back to the waste plastic purification unit for reuse.

[0075] In addition, the present invention preferably recycles the semi-coke obtained after pyrolysis to the waste plastic impurity removal treatment unit as an adsorbent for recycling, to adsorb and remove silicon impurities in the waste plastic.

[0076] The method and system provided by this invention have low pollution discharge and good environmental performance during the production process, reducing carbon emissions. This invention helps to solve "white pollution". Moreover, the light oil products prepared have low content of harmful elements and high added value, and have good environmental, social and economic benefits. Attached Figure Description

[0077] Figure 1 This is a schematic diagram of one embodiment of the pyrolysis treatment method for waste plastics provided by the present invention. Detailed Implementation

[0078] The present invention will be further described below with reference to the accompanying drawings, but this description does not limit the scope of the invention.

[0079] Figure 1 This is a schematic diagram of one embodiment of the pyrolysis treatment method for waste plastics provided by the present invention, as shown below. Figure 1 As shown, the crushed waste plastic 8 enters the waste plastic extrusion dehydration and dechlorination unit 1, where water is removed at temperature T1. The resulting water 10 enters the wastewater treatment device, where it is dechlorinated at temperature T2. The resulting chlorinated compound 11 enters the HCl adsorption device 2 and contacts the adsorbent 12 to obtain recovered chlorinated compound 13. The waste plastic is then dehydrated and dechlorinated to obtain dechlorinated waste plastic 9.

[0080] The resulting dechlorinated waste plastic 9 enters the waste plastic impurity removal treatment unit 3, where it is contacted with solvent oil 14 and pretreatment agent 15 for impurity removal treatment. The resulting effluent 16 is separated into insoluble matter 17 and a depurified plastic-containing solution 24 by the solid-liquid separation unit 4. The resulting insoluble matter 17 enters the extraction unit 5, where it is contacted with organic solvent 18 for solvent extraction. After solid-liquid separation, the resulting product yields solid material 21 and liquid material 19. The resulting solid material 21 is processed in the pretreatment agent recovery unit 7, and at least a portion is returned to the waste plastic impurity removal treatment unit 3 as recovered pretreatment agent 22. The remaining solid material 23 is discharged from the device. The resulting liquid material 19 undergoes precipitation treatment and separation in the solvent recovery unit 6 to obtain solid plastic particles 25 and recovered organic solvent 20, which is recycled.

[0081] The resulting purified plastic solution 24 enters the pyrolysis reactor 26 of the pyrolysis unit, where it contacts the pyrolysis catalyst to carry out a pyrolysis reaction, yielding semi-coke 28 and pyrolysis oil gas 27. The pyrolysis oil gas 27 enters the pyrolysis oil gas separation system 29, where it is separated to obtain pyrolysis gas 30 and pyrolysis oil 31.

[0082] The present invention will be further described below with reference to embodiments, but this does not limit the present invention in any way.

[0083] In this embodiment, the chlorine content in the solid mixture was determined by coulometric method, specifically according to the standard method of RIPP 64-90 (see "Analytical Methods in Petrochemical Industry" (RIPP Test Methods), edited by Yang Cuiding et al., Science Press, 1990, pp. 164-167, "Determination of Total Chlorine Content in Crude Oil by Coulometric Method"). The instrument used was a Thermo Fisher ECS300 microcoulometric analyzer, and the sample was a cryogenically pulverized waste plastic powder sample. The obtained chlorine content was the total chlorine content of the sample, including both organic and inorganic chlorine.

[0084] In the examples, the chlorine content in the liquid mixture was also determined using the RIPP 64-90 method, except that the sample was a liquid mixture.

[0085] In the embodiments, the silicon content in the liquid mixture was determined using the method of "Determination of Additive Elements, Wear Metals and Contaminants in Used Lubricating Oils and Certain Elements in Base Oils (Inductively Coupled Plasma Atomic Emission Spectrometry)" (GB17476-1998).

[0086] In the following embodiments, unless otherwise specified, the waste plastic is crushed using a shear crusher with a filter screen aperture of 50 mm.

[0087] The waste plastic raw materials used in the examples and comparative examples are as follows:

[0088] Waste plastic A consists of greenhouse film and mulch film from a certain region in Shandong Province. It contains some soil and biomass components. After simple washing and drying, it is crushed. The resulting waste plastic A has an ash content of 4.3% and a moisture content of 10.5%. The main plastic components are PE and a small amount of EVA. It also contains trace amounts of other materials that cannot be separated, such as PP and PS. The average total chlorine content on a dry basis is approximately 850 μg / g.

[0089] Waste plastic B is waste plastic separated from the waste of a paper mill in Jiangsu Province. After preliminary drying, it is crushed. The resulting waste plastic B has an ash content of 9.5% and a moisture content of 15%. The main components of the waste plastic are PE and PP, and it also contains a small amount of PVC, PET, PS, etc. The average total chlorine content on a dry basis is 2.2%.

[0090] Waste plastic C is waste plastic from aged waste excavated from a landfill in Guangdong. After multi-stage air separation, preliminary drying, and crushing, the resulting waste plastic C has an ash content of 11.9% and a moisture content of 12%. This waste plastic is a mixed waste plastic including PE, PP, PS, PET, and PVC, with an average total chlorine content of 1.9% on a dry basis.

[0091] Example 1

[0092] The waste plastic extrusion dehydration and dechlorination unit includes a single-screw extruder with an external heat exchange device. This external heat exchange device is a two-stage system: one stage is a refrigeration and insulation device to control the temperature T1 of the dehydration process, and the other stage is a heating and insulation device to control the temperature T2 of the dechlorination process. Crushed waste plastic A is fed into the single-screw extruder, where it is first dehydrated in a liquid state at 25°C, and then dechlorinated and the remaining moisture is removed at 200°C. The removed chlorinated compounds enter an HCl adsorption device, where they are recovered by contacting with CaO adsorbent. Dechlorinated waste plastic A is obtained from the dechlorinated waste plastic outlet of the single-screw extruder and enters the waste plastic impurity removal treatment unit. The chlorine content in the obtained dechlorinated waste plastic is 79 μg / g.

[0093] The waste plastic impurity removal treatment unit uses an externally heated dissolving kettle. Dechlorinated waste plastic A is mixed with pretreatment agent and solvent oil, and then subjected to impurity removal treatment at 350℃ and 1.5MPa. The residence time of dechlorinated waste plastic A is 30 minutes. After treatment, the material is separated into insoluble matter and a depurified plastic-containing solution by a solid-liquid separation unit. The plastic components in the obtained depurified plastic-containing solution are mainly PE and PP, with a concentration of 16%. The silicon content is <1μg / g, the chlorine content is 13μg / g, and the total metal content is 3.5μg / g. The insoluble matter is sent to the extraction unit for solvent extraction with tetrahydrofuran. After separation, solid and liquid phases are obtained. The solid phase is processed in the pretreatment agent recovery unit, and part of the recovered pretreatment agent is returned to the waste plastic impurity removal treatment unit. In the solvent recovery unit, the liquid material is back-extracted by contacting the back-extractant water. The resulting solid-liquid mixture is filtered to obtain solid plastic particles and a mixed liquid. The resulting mixed liquid is then distilled to recover tetrahydrofuran and the back-extractant for recycling.

[0094] The solvent oil used is catalytic cracking light cycle oil with a distillation range of 180–350°C, a total aromatic hydrocarbon content of 69% by mass, and a monocyclic aromatic hydrocarbon content of 48% by mass; the weight ratio of solvent oil to waste plastic is 5:1.

[0095] The pretreatment agent used was a waste FCC balancing agent provided by the refinery, with a particle size of 75-150 μm; the weight ratio of the pretreatment agent to waste plastic was 1:9.

[0096] The back-extraction agent used is water, and the amount used is three times that of the liquid phase material. The back-extraction temperature is room temperature.

[0097] Example 2

[0098] The waste plastic extrusion dehydration and dechlorination unit includes a single-screw extruder with an external heat exchange device. This external heat exchange device is a two-stage system: one stage is a refrigeration and insulation device to control the temperature T1 of the dehydration process, and the other stage is a heating and insulation device to control the temperature T2 of the dechlorination process. Crushed waste plastic B is fed into the single-screw extruder, where it is first dehydrated at 40°C and then dechlorinated at 400°C. The removed chlorine-containing compounds enter an HCl adsorption device, where they are recovered by contacting with CaO adsorbent. Dechlorinated waste plastic B is obtained from the dechlorinated waste plastic outlet of the single-screw extruder and enters the waste plastic impurity removal treatment unit. The total chlorine content in the obtained dechlorinated waste plastic is 1.18%, mainly inorganic chlorine.

[0099] The waste plastic pretreatment unit uses an externally heated dissolving kettle. After mixing dechlorinated waste plastic B with pretreatment agent and solvent oil, it undergoes impurity removal treatment at 300℃ and 1.2MPa. The residence time of dechlorinated waste plastic B is 40 minutes. The treated material is then separated into insoluble matter and a depurified plastic-containing solution by a solid-liquid separation unit. The plastic components in the obtained depurified plastic-containing solution are mainly PE and PP, with a concentration of approximately 14.8%. The silicon content is <1μg / g, the total chlorine content is 12μg / g, and the total metal content is 2.1μg / g. The insoluble matter is sent to the extraction unit for solvent extraction with tetrahydrofuran. After separation, solid and liquid phases are obtained. The solid phase is processed in the pretreatment agent recovery unit, and a portion of the recovered pretreatment agent is returned to the waste plastic impurity removal treatment unit. In the solvent recovery unit, the liquid material is back-extracted by contacting the back-extractant water. The resulting solid-liquid mixture is filtered to obtain solid plastic particles and a mixed liquid. The resulting mixed liquid is then distilled to recover tetrahydrofuran and the back-extractant for recycling.

[0100] The solvent oil used is catalytic cracking light cycle oil with a distillation range of 180–350°C, a total aromatic hydrocarbon content of 69% by mass, and a monocyclic aromatic hydrocarbon content of 48% by mass; the weight ratio of solvent oil to waste plastic is 5:1.

[0101] The pretreatment agent used was humus and CaO in a mass ratio of 8:2 with a particle size of 75–150 μm; the weight ratio of the pretreatment agent to waste plastic was 1:7.

[0102] Example 3

[0103] The waste plastic extrusion dehydration and dechlorination unit uses the same processing equipment and conditions as in Example 2, and obtains dechlorinated waste plastic B.

[0104] The waste plastic impurity removal treatment unit adopts an externally heated dissolving kettle. After being mixed with pretreatment agent and solvent oil, the dechlorinated waste plastic B undergoes impurity removal treatment at 350℃ and 1.8MPa. The residence time of the dechlorinated waste plastic B is 45 minutes. After the reaction, the material is separated into insoluble matter and a depurified plastic-containing solution by a solid-liquid separation unit. The plastic components in the obtained depurified plastic-containing solution are mainly PE and PP, with a concentration of 14.8%. The silicon content is less than 3 μg / g, the chlorine content is 18 μg / g, and the total metal content is 3.8 μg / g.

[0105] The solvent oil used is refinery VGO with a distillation range of 275–581°C, a total aromatic hydrocarbon content of 45.1% by mass, and a monocyclic aromatic hydrocarbon content of 21% by mass; the weight ratio of solvent oil to waste plastic is 5:1.

[0106] The pretreatment agents used were activated carbon and Fe2O3 in a mass ratio of 9:1, with a particle size of 75–150 μm. The weight ratio of the pretreatment agent to waste plastic was 1:8.

[0107] Example 4

[0108] The waste plastic extrusion dehydration and dechlorination unit uses the same processing equipment and conditions as in Example 2, and obtains dechlorinated waste plastic B.

[0109] The waste plastic impurity removal treatment unit adopts an externally heated dissolving kettle. After being mixed with pretreatment agent and solvent oil, the dechlorinated waste plastic B undergoes impurity removal treatment at 350℃ and 1.8MPa. The residence time of the dechlorinated waste plastic B is 30 minutes. After the reaction, the material is separated into insoluble matter and a depurified plastic-containing solution by a solid-liquid separation unit. The plastic components in the obtained depurified plastic-containing solution are mainly PE and PP, with a concentration of 14.8%. The silicon content is less than 1 μg / g, the chlorine content is 11 μg / g, and the total metal content is 4.1 μg / g.

[0110] The solvent oil used is the heavy fraction of the pyrolysis oil obtained from the pyrolysis unit, with a distillation range of 208–509℃, a total aromatic hydrocarbon content of 68.9% by mass, and a monocyclic aromatic hydrocarbon content of 50.7% by mass; the weight ratio of solvent oil to waste plastic is 5:1.

[0111] The pretreatment agents used were semi-coke and Fe2O3 obtained from the pyrolysis unit, with a mass ratio of 9:1 and a particle size of 75–150 μm. The weight ratio of the pretreatment agent to the waste plastic was 1:8.

[0112] The resulting purified plastic-containing solution was sent to the rotary kiln pyrolysis reactor of the pyrolysis unit, where it contacted the pyrolysis catalyst. The catalyst used was a Ni-based Al2O3 catalyst, with a Ni content of 8% and an Al2O3 content of 92% by mass. The pyrolysis reaction was carried out at 450℃ and 2MPa for a residence time of 30 min. After separation, the reaction products yielded semi-coke and pyrolysis oil and gas. The pyrolysis oil and gas were further separated to obtain fine coke powder, pyrolysis gas, and pyrolysis oil. The pyrolysis oil was fractionated to obtain light and heavy fractions. The basic properties of the light and heavy fractions are listed in Table 1.

[0113] Comparative Example 1

[0114] This comparative example uses the same waste plastic raw materials, solvent oil, and impurity removal reaction conditions as Example 4. The difference is that no pretreatment agent is used in this comparative example. After the reaction, the materials are separated into insoluble matter and a purified plastic-containing solution by a solid-liquid separation unit. The obtained purified plastic-containing solution contains 42 μg / g silicon, 770 μg / g chlorine, and 13 μg / g total metals.

[0115] The impurity content in the plasticized solution obtained in this comparative example is too high, especially the chlorine content, which is much higher than that in crude oil. This can easily cause corrosion to subsequent processing equipment and pose a safety hazard.

[0116] Example 5

[0117] The waste plastic extrusion dehydration and dechlorination unit includes a single-screw extruder with an external heat exchange device. This external heat exchange device is a two-stage system: one stage controls the temperature (T1) of the dehydration process (refrigeration / heating), and the other stage controls the temperature (T2) of the dechlorination process (heating). Crushed waste plastic C is fed into the single-screw extruder, where it is first dehydrated at 50°C and then dechlorinated at 300°C. The removed chlorinated compounds enter an HCl adsorption device, where they are recovered by contacting a CaO adsorbent. Dechlorinated waste plastic C is obtained from the dechlorinated waste plastic outlet of the single-screw extruder and then enters the waste plastic impurity removal treatment unit.

[0118] The waste plastic pretreatment unit uses an externally heated dissolving kettle. After the dechlorinated waste plastic C is mixed with the pretreatment agent and solvent oil, it undergoes impurity removal treatment at 350℃ and 1.2MPa. The residence time of the dechlorinated waste plastic C is 35 minutes. The treated material is then separated into insoluble matter and a depurified plastic-containing solution by a solid-liquid separation unit. The plastic components in the obtained depurified plastic-containing solution are mainly PE and PP, with a concentration of 14.4%. The silicon content is 2.3 μg / g, the chlorine content is 8.8 μg / g, and the total metal content is 1.8 μg / g.

[0119] The heavy fraction of the pyrolysis oil obtained from the solvent oil pyrolysis unit has a distillation range of 210–537℃, a total aromatic hydrocarbon content of 75.8% by mass, and a monocyclic aromatic hydrocarbon content of 63.9% by mass; the weight ratio of solvent oil to dechlorinated waste plastic is 5:1.

[0120] The pretreatment agent used was semi-coke obtained from the pyrolysis unit and Fe2O3, with a mass ratio of 8:2 and a particle size of 75–150 μm. The weight ratio of the pretreatment agent to the waste plastic was 1:8.

[0121] The resulting purified plastic-containing solution was sent to the rotary kiln pyrolysis reactor in the pyrolysis unit, where it contacted with a pyrolysis catalyst, specifically a Y-type molecular sieve. The purified plastic-containing solution underwent pyrolysis at 470℃ and 2.5 MPa for 30 min. The reaction products were separated to obtain semi-coke and pyrolysis oil / gas. Further separation of the pyrolysis oil / gas yielded fine coke powder, pyrolysis gas, and pyrolysis oil. The pyrolysis gas was returned to the rotary kiln pyrolysis reactor, and the pyrolysis oil was fractionated to obtain light and heavy fractions. The basic properties of the light and heavy fractions are listed in Table 1.

[0122] The resulting oil-containing insoluble matter is sent to the extraction unit for solvent extraction with toluene. The resulting product is separated into solid and liquid phases. The solid phase is processed in the pretreatment agent recovery unit, with a portion returned to the waste plastic pretreatment unit as a recycled pretreatment agent. The liquid phase undergoes distillation in the solvent recovery unit to obtain a small amount of solid plastic particles (PS), recovered toluene, and a small amount of adhering solvent oil.

[0123] Table 1

[0124]

[0125]

[0126] It should be noted that the above descriptions are merely arbitrary embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for pyrolysis treatment of waste plastics, comprising the following steps: (1) Waste plastic extrusion dehydration and dechlorination unit: Waste plastic enters the waste plastic extrusion dehydration and dechlorination unit, is first dehydrated at temperature T1, and then dechlorinated at temperature T2. The temperature T2 is 50~360℃ higher than the temperature T1, resulting in dechlorinated waste plastic. In the waste plastic extrusion dehydration and dechlorination unit, the temperature range of T1 is 0~50℃, the temperature range of T2 is 100~350℃, and the working pressure range of extrusion is 5~30MPa. The waste plastics are one or more of PE, PP, PS, and PVC. (2) The dechlorinated waste plastics obtained in step (1) enter the waste plastic impurity removal treatment unit and are subjected to impurity removal treatment by contacting the pretreatment agent and solvent oil. The resulting effluent is subjected to solid-liquid separation to obtain insoluble matter and a depurified plastic-containing solution. The depurified plastic-containing solution contains less than 5 μg / g of metal, less than 20 μg / g of chlorine, and less than 3 μg / g of silicon. The solvent oil has a distillation range of 80~550℃ and a total aromatic hydrocarbon content of more than 50% by mass and a monocyclic aromatic hydrocarbon content of more than 20% by mass. The pretreatment agent is selected from one or more of humus, red mud, waste catalyst from oil refining unit, kaolin, semi-coke, activated carbon, and gasification ash, as well as optional alkaline oxides. The treatment conditions of the waste plastic impurity removal treatment unit are: temperature of 250~410℃, pressure of 0.1~5MPa, and residence time of dechlorinated waste plastics of 10~60min. (3) The plasticized solution obtained in step (2) enters the pyrolysis reactor of the pyrolysis unit, comes into contact with the pyrolysis catalyst, and undergoes pyrolysis reaction at a temperature of 350~550℃ to obtain semi-coke and pyrolysis oil and gas. After separation, the pyrolysis oil and gas yield coke powder, pyrolysis gas and pyrolysis oil.

2. The method according to claim 1, characterized in that, The waste plastics mentioned are one or more of the following: waste plastics from fresh household waste, waste plastics from industrial and agricultural production, and waste plastics from aged waste.

3. The method according to claim 1, characterized in that, Waste plastics are coarsely crushed before entering the waste plastic extrusion, dehydration, and dechlorination unit, and the particle size of the crushed waste plastics is 1~200mm.

4. The method according to claim 1, characterized in that, In the waste plastic extrusion dehydration and dechlorination unit, the temperature range of T1 is 0~20℃, the temperature range of T2 is 200~300℃, and the working pressure range of extrusion is 10~20MPa.

5. The method according to claim 1, characterized in that, The chlorine-containing compounds removed by the waste plastic extrusion dehydration and dechlorination unit enter the HCl adsorption device and are recovered by contacting the adsorbent. The adsorbent is selected from one or more of water, alkaline solution, brine, and solid alkaline substances.

6. The method according to claim 1, characterized in that, The processing conditions for the waste plastic impurity removal unit are: temperature 320~390℃, pressure 0.5~4.0MPa, and residence time of dechlorinated waste plastic 15~45min.

7. The method according to claim 1, characterized in that, The weight ratio of solvent oil to waste plastic is 1:10 to 10:

1.

8. The method according to claim 1, characterized in that, The content of monocyclic aromatic hydrocarbons in the solvent oil is higher than 40% by mass. The weight ratio of solvent oil to waste plastic is 1:1 to 7:

1.

9. The method according to claim 1, characterized in that, The particle size range of the pretreatment agent is 75~150μm; the weight ratio of the pretreatment agent to waste plastic is 1:10~2:

1.

10. The method according to claim 1, characterized in that, The weight ratio of pretreatment agent to waste plastic is 1:7 to 1:

5.

11. The method according to claim 1, characterized in that, The pretreatment agent is one or more of the following: waste catalytic cracking catalyst, humus, semi-coke, activated carbon, and optional alkaline oxide.

12. The method according to claim 11, characterized in that, The semi-coke is the semi-coke obtained from the pyrolysis unit in step (3).

13. The method according to claim 1, characterized in that, The operating temperature for solid-liquid separation in step (2) is 250~410℃; The impurity-removing plastic solution contains less than 3 μg / g of metal, less than 1 μg / g of chlorine, and less than 1 μg / g of silicon.

14. The method according to claim 1, characterized in that, In step (3), the reaction temperature of the pyrolysis reactor is 450~500℃, the reaction pressure is 0.1~5MPa, and the residence time is 10~30min.

15. The method according to claim 1, characterized in that, In step (3), the pyrolysis catalyst is a catalyst containing one or more of the following: macroporous molecular sieve, mesoporous molecular sieve, alumina, and silica-alumina.

16. The method according to claim 15, characterized in that, The pyrolysis catalyst contains a metal component, which is selected from one or more of K, Na, Ca, Fe, Co, Ni, and Mo.

17. The method according to claim 1, characterized in that, In step (3), the pyrolysis oil is fractionated to obtain light and heavy fractions, with a cut-off point of 180~230℃; The heavy fraction obtained from the pyrolysis oil fractionation is used as solvent oil and recycled to the waste plastic impurity removal treatment unit in step (2).

18. The method according to claim 1, characterized in that, The pyrolysis gas obtained in step (3) is returned to the pyrolysis reactor.

19. The method according to claim 1, characterized in that, It also includes the following steps: (4) The insoluble matter obtained in step (2) is introduced into the extraction unit and solvent extracted by contact with organic solvent. The product is separated into solid phase material and liquid phase material by solid-liquid separation. The organic solvent is selected from one or more of benzene, toluene, chloroform, cyclohexanone, ethyl acetate, butyl acetate, carbon disulfide, tetrahydrofuran, and gasoline. (5) The solid material obtained in step (4) is processed in the pretreatment agent recovery unit to obtain the recovered pretreatment agent, and part of the recovered pretreatment agent is returned to the waste plastic impurity removal treatment unit in step (2). (6) The obtained liquid phase material is subjected to precipitation treatment and separation in the solvent recovery unit to obtain solid plastic particles and recovered organic solvent.

20. The method according to claim 19, characterized in that, In step (5), the pretreatment agent recovery unit processes one or more of the following methods: screening and regeneration.

21. The method according to claim 19, characterized in that, In step (6) solvent recovery unit, the precipitation treatment is selected from one or more of back extraction, simple distillation, and rectification.

22. The method according to claim 19, characterized in that, In step (6) solvent recovery unit, the precipitation treatment is performed by flash evaporation.

23. A system for use in any of the methods of claims 1-22, It includes a waste plastic extrusion dehydration and dechlorination unit, a waste plastic impurity removal unit, a solid-liquid separation unit, and a pyrolysis unit; The waste plastic extrusion dehydration and dechlorination unit is equipped with a waste plastic inlet, a moisture outlet, a chlorinated compound outlet, and a dechlorinated waste plastic outlet. The waste plastic extrusion dehydration and dechlorination unit is a spiral extruder with an external heat exchange device. The external heat exchange device is a two-stage heat exchange device, one stage is a refrigeration and cold preservation device that controls the temperature of the dehydration process T1, and the other stage is a heating and heat preservation device that controls the temperature of the dechlorination process T2. The waste plastic impurity removal treatment unit is equipped with a dechlorinated waste plastic inlet, a pretreatment agent inlet, a solvent oil inlet, and a pretreated material outlet. The pretreated material outlet is connected to the inlet of the solid-liquid separation unit, which is equipped with an insoluble matter outlet and a depurified plastic-containing solution outlet. The pyrolysis unit includes a pyrolysis reactor and a pyrolysis oil-gas separation system; The pyrolysis reactor is provided with a plastic-containing solution inlet, a semi-coke outlet, and a pyrolysis oil and gas outlet. The plastic-containing solution outlet of the solid-liquid separation unit is connected to the plastic-containing solution inlet of the pyrolysis reactor. The pyrolysis oil and gas outlet of the pyrolysis reactor is connected to the inlet of the pyrolysis oil and gas separation system. The pyrolysis oil and gas separation system is provided with at least a pyrolysis gas outlet and a pyrolysis oil outlet.

24. The system according to claim 23, characterized in that, A fractionation system is set up in the pyrolysis unit, and the pyrolysis oil outlet is connected to the inlet of the fractionation system. The fractionation system is set up with light distillate outlet and heavy distillate outlet. The heavy fraction outlet of the fractionation system is connected to the solvent oil inlet of the waste plastic impurity removal treatment unit.

25. The system according to claim 23, characterized in that, The semi-coke outlet of the pyrolysis reactor is connected to the pretreatment agent inlet of the waste plastic impurity removal unit.

26. The system according to claim 23, characterized in that, The pyrolysis reactor is a rotary kiln pyrolysis reactor.

27. The system according to claim 23, characterized in that, The system also includes an extraction unit, a pretreatment agent recovery unit, and a solvent recovery unit; The extraction unit is provided with an insoluble matter inlet, an organic solvent inlet, a solid phase material outlet, and a liquid phase material outlet. The insoluble matter outlet of the solid-liquid separation unit is connected to the insoluble matter inlet of the extraction unit. The pretreatment agent recovery unit is equipped with a solid material inlet and a pretreatment agent recovery outlet. The solid material outlet of the extraction unit is connected to the solid material inlet of the pretreatment agent recovery unit, and the pretreatment agent recovery outlet of the pretreatment agent recovery unit is connected to the pretreatment agent inlet of the waste plastic impurity removal treatment unit. The solvent recovery unit is equipped with a liquid material inlet, a solid plastic particle outlet, and a recovered organic solvent outlet. The liquid material outlet of the extraction unit is connected to the liquid material inlet of the solvent recovery unit.

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