A method for thermal cracking dechlorination and adsorption impurity removal of waste polyolefin plastics

Abstract

The present application relates to the field of plastic recycling, and particularly relates to a method for waste polyolefin plastic thermal cracking dechlorination coupled with adsorption impurity removal, comprising: waste polyolefin raw material pretreatment, melt homogenization in a partition temperature control screw reactor, directional dechlorination and carbonaceous impurity trapping particle formation, main cracking reaction, hot state pre-separation, condensation gas-liquid separation, fractionation and impurity-containing heavy phase / residue treatment. The dechlorinated carbonaceous residue contains conjugated polyenes, crosslinked structures or aromatic precursors. In the subsequent temperature rise and main cracking process, further condensation, slight carbonization and formation of carbonaceous impurity trapping particles. The carbonaceous impurity trapping particles can act as the attachment and aggregation center of impurities, can promote the enrichment and discharge of impurities to the residue or heavy phase, thereby reducing the chlorine and inorganic impurity content in light and medium quality cracking oil.

Description

Technical Field

[0001] This invention relates to the field of plastic recycling, and more specifically, to a method for removing impurities from waste polyolefin plastics through thermal pyrolysis dechlorination coupled with adsorption. Background Technology

[0002] Waste plastic pyrolysis refers to the thermal decomposition of polymers (waste plastics) at 300–600°C under an inert atmosphere, producing gases, liquids (such as naphtha, gasoline, diesel, and heavy oil), and carbon residue. As organic polymers with high hydrocarbon content, plastics can be reverse-engineered into petrochemical raw materials through pyrolysis, achieving high-value recycling of waste plastic resources while reducing environmental pollution and alleviating energy shortages.

[0003] However, in actual recycling, the sources of waste plastics are complex, and even after sorting, it is difficult to obtain completely pure polyolefin raw materials. PVC, in particular, can undergo dechlorination at relatively low temperatures, releasing HCl that not only corrodes equipment but may also react with olefins in the pyrolysis oil and gas to form organochlorides. These organochlorides, when introduced into subsequent hydrogenation or steam cracking units, can lead to catalyst poisoning, equipment corrosion, and chloride salt deposition. Furthermore, waste plastic pyrolysis systems often contain inorganic particles and metallic impurities such as Si, Ca, Fe, and Al. These impurities are small in size, have polar surfaces, and are easily encapsulated by heavy oil / colloids. Some fine inorganic particles may enter subsequent condensation, fractionation, or hydrogenation systems with the pyrolysis oil, causing risks such as filter blockage, heat exchanger deposition, increased catalyst bed pressure drop, and coking in the pyrolysis furnace.

[0004] To address the aforementioned issues, existing technologies typically employ the following treatment methods: First, strengthen sorting at the front end to minimize the content of impurities such as PVC, metals, and silt; second, install a low-temperature dechlorination unit before pyrolysis or during the melting stage to preferentially release HCl from chlorine-containing components such as PVC, and treat it through tail gas absorption or alkaline washing; third, add alkaline materials such as CaO, CaCO3, and MgO to the system to absorb or fix HCl; fourth, after the formation of pyrolyzed oil, reduce the chlorine, metal, and ash content in the oil through filtration, adsorption, water washing, and hydrotreating pretreatment; and fifth, remove some coke, heavy components, and inorganic impurities from the system through bottom slag discharge, heavy phase discharge, or asphalt phase discharge.

[0005] While the above methods can reduce the impact of chlorine and impurities on the pyrolysis process to some extent, they still have the following shortcomings: 1. If dechlorination at the front end is insufficient, residual HCl will enter light oil products, forming organochlorine pollution and increasing the difficulty of downstream treatment.

[0006] 2. When using alkaline chlorine absorbents such as CaO, new Ca-based fine particulate contaminants may be introduced while reducing chlorine content, increasing oil ash content, filtration load, and bottom sludge treatment volume.

[0007] 3. When the cracked oil is filtered, adsorbed or settled after it is formed, the inorganic fine particles are often wrapped by heavy oil, wax or colloidal components, making separation difficult and easily causing filter element blockage, increased pressure difference and oil loss. Summary of the Invention

[0008] The purpose of this invention is to provide a method for dechlorination coupled with adsorption in the thermal pyrolysis of waste polyolefin plastics. First, a small amount of chlorinated polymers such as PVC and PVDC entrained in the waste polyolefin raw material preferentially undergoes a dechlorination reaction under conditions below the main pyrolysis temperature of the polyolefin, generating HCl and forming dechlorinated carbonaceous residues containing conjugated polyenes, cross-linked structures, or aromatization precursor structures. These dechlorinated carbonaceous residues further condense and undergo slight carbonization during subsequent heating and main pyrolysis, forming carbonaceous impurity-catching particles. These carbonaceous impurity-catching particles can serve as attachment and aggregation centers for inorganic fine particles, chloride salts, metal chlorides, and heavy colloidal components, promoting the enrichment and discharge of impurities into the bottom slag or heavy phase, thereby reducing the chlorine and inorganic impurity content in the light and medium-quality pyrolysis oil.

[0009] The technical problem solved by this invention is achieved by the following technical solution.

[0010] This invention provides a method for dechlorination and impurity removal from waste polyolefin pyrolysis. Instead of using alkaline adsorbents and downstream filtration, the method utilizes a small amount of chlorinated polymer entrained in the raw material before the waste polyolefin enters the main pyrolysis reaction. This chlorination is preferentially performed at a temperature below the main pyrolysis temperature of the polyolefin, releasing HCl and forming dechlorinated carbonaceous residue. This dechlorinated carbonaceous residue continues to heat with the molten material, undergoing condensation, aromatization, or mild carbonization in the downstream section of a zone-controlled temperature screw reactor, forming carbonaceous impurity-trapping particles in situ.

[0011] Carbonaceous impurity traps primarily act as carriers for impurity attachment and aggregation during the main pyrolysis process. Their rough surface structure and defect sites facilitate the attachment and aggregation of fine inorganic particles such as Si, Ca, Fe, and Al, residual chloride salts, and metal chlorides. Simultaneously, the heavy oil, waxy components, and colloidal components formed during pyrolysis further encapsulate these impurity-containing particles, increasing their particle size and apparent density. Therefore, impurities that might otherwise enter the light and medium-quality oil products with the pyrolysis gas are more easily transferred to the heavy phase at the bottom of the reactor and removed through the heat-insulated discharge system, thereby reducing the chlorine, metal, silicon, and ash content in the pyrolysis oil.

[0012] The process route includes: pretreatment of waste polyolefin raw materials, melting and homogenization in a zoned temperature-controlled screw reactor, directional dechlorination and carbonaceous impurity-collecting particle formation, main pyrolysis reaction, hot pre-separation, condensed gas-liquid separation, fractionation, and treatment of heavy phase / bottom slag containing impurities.

[0013] This invention provides a method for removing impurities from waste polyolefin plastics through thermal pyrolysis and dechlorination coupled with adsorption, comprising the following steps: S1, pretreated waste polyolefin plastics; S2, the waste polyolefin plastic processed in step S1 is fed into a zoned temperature-controlled screw reactor to obtain molten material; the screw reactor is sequentially configured along the material conveying direction as a melting homogenization zone, a directional dechlorination zone, and a particle-forming zone; the screw reactor can be a single-screw, twin-screw, or multi-screw reactor, preferably a twin-screw reactor with zoned heating, screw mixing, exhaust port, and inert gas protection functions.

[0014] The pretreated waste polyolefin raw material first enters the melting and homogenization zone of the screw reactor. In this zone, the material gradually softens, compacts, and melts under the shearing action of the screw and external heating, forming a relatively homogeneous molten polymer phase. The temperature of the melting and homogenization zone can be controlled at 120–180℃, preferably 150–170℃; the operating pressure can be from atmospheric pressure to 0.5 MPa, preferably from atmospheric pressure to 0.2 MPa; the residence time of the material in this zone can be 0.5–15 min, preferably 1–8 min. The screw speed can be controlled at 10–300 rpm, preferably 30–150 rpm.

[0015] After being melted and homogenized, the material enters the directional dechlorination zone. By increasing the cylinder temperature, PVC, PVDC, chlorinated adhesives, chlorinated label materials, chlorinated coatings, or other chlorinated polymers entrained in the raw material preferentially undergo a dehydrochlorination reaction, releasing HCl.

[0016] The temperature in this area can be controlled between 190 and 250°C, preferably between 195 and 220°C; the material residence time can be between 2 and 60 minutes, preferably between 5 and 30 minutes; and the operating pressure can be between -20 kPa and 0.5 MPa, preferably between -5 kPa and 0.2 MPa.

[0017] The directional dechlorination zone is equipped with at least one de-volatile inlet, which is connected to the tail gas absorption, alkaline washing, or salting recovery unit. To promote the timely removal of HCl, nitrogen, pyrolysis dry gas, light hydrocarbon gas, or a small amount of water vapor can be introduced as a carrier gas, with a flow rate of 0–5 Nm³ / kg waste plastic, preferably 0.05–1 Nm³ / kg waste plastic.

[0018] Carbonaceous impurity-trapping particle formation zone After directional dechlorination, the molten material enters the carbonaceous impurity trapping particle formation zone at the rear end of the screw reactor. In this zone, the dechlorinated PVC, PVDC, or other chlorinated polymer residues continue to undergo condensation, crosslinking, aromatization, or mild carbonization, forming carbonaceous impurity trapping particles with a certain particle size and surface activity.

[0019] The temperature of the carbonaceous impurity-collecting particle formation zone can be controlled at 260–360℃, preferably 290–320℃; the residence time can be 0.5–30 min, preferably 2–15 min; and the screw speed can be 10–300 rpm, preferably 30–150 rpm.

[0020] S3, the molten material is fed into the main pyrolysis reactor and pyrolysis reaction is carried out under stirring. The first heavy phase is discharged from the bottom or bottom side of the main pyrolysis reactor, and the first pyrolysis gas is discharged from the top or top side of the main pyrolysis reactor. In the main cracking reactor, polyolefins such as polyethylene and polypropylene undergo thermal cracking under anaerobic or low-oxygen conditions to generate cracked gas, condensable cracked oil and gas, heavy cracked oil, waxy components, and a small amount of coke and bottom slag.

[0021] The main pyrolysis temperature can be controlled between 380 and 560°C, preferably between 420 and 520°C; the operating pressure can be from atmospheric pressure to 1.0 MPa, preferably from atmospheric pressure to 0.3 MPa; the average residence time can be between 1 and 60 min, preferably between 5 and 30 min, and more preferably between 8 and 20 min. The reactor is preferably kept in an oxygen-free or low-oxygen environment, which can be replaced or protected by introducing nitrogen, pyrolysis dry gas, or other inert gases.

[0022] To improve the contact efficiency between carbonaceous impurity-collecting particles and inorganic fine particles, one or more of the following can be installed in the main pyrolysis reactor: mechanical stirrer, screw propeller, scraper component, melt circulation pump, heavy liquid phase circulation pipeline, or molten material distribution component.

[0023] To reduce the risk of bottom sedimentation and blockage, the bottom of the main pyrolysis reactor is equipped with an insulated discharge port and heavy phase flow enhancement structures such as mechanical scraping, spiral discharge, heavy liquid phase circulation or circulation flushing to maintain the dischargeable state of the bottom material.

[0024] The first heavy phase discharged from the bottom or side bottom of the main pyrolysis reactor includes carbonaceous impurity-collecting particles, coke, ash, chloride salts, metal chlorides, inorganic packing particles, and some heavy pyrolysis oil. The underflow discharge rate can be 0.5–20 wt% of the waste plastic feed, preferably 1–10 wt%, and more preferably 2–8 wt%. The discharge outlet and its connecting pipelines can be heated by jacket heating, hot oil heating, electric heating, or other insulation methods to keep the impurity-containing heavy phase in a flowable state.

[0025] The first cracked gas discharged from the top or upper side stream of the main cracking reactor enters the subsequent hot pre-separation, condensation and fractionation system.

[0026] S4, the first cracked gas enters the hot pre-separator, the second heavy phase is discharged from the bottom, and the second cracked gas is discharged from the bottom; The hot pre-separation unit is located at the outlet of the main pyrolysis reactor and can be at least one of the following: hot gas-liquid separator, cyclone separator, sedimentation separator, hot filter, and pre-separation tank with demisting structure.

[0027] The hot pre-separation temperature can be controlled between 250 and 500°C, preferably between 300 and 450°C; the operating pressure can be the same as or slightly lower than that of the main pyrolysis reactor, preferably atmospheric pressure to 0.3 MPa.

[0028] The purpose of hot pre-separation is to trap heavy droplets, coke particles, carbonaceous impurity particles, and some heavy phases entrained in the cracked oil and gas, thereby reducing the content of fine particles and heavy entrainments in the upper oil and gas stream. Hot pre-separators can be equipped with baffles, swirl structures, demisters, inertial separation components, or hot filtration components to reduce the re-entry of impurity particles into the subsequent oil product system.

[0029] The impurity-containing heavy phase (second heavy phase) discharged from the bottom of the hot pre-separator can be combined with the first heavy phase discharged from the bottom of the main pyrolysis reactor for treatment, or it can be separately sent to the bottom ash treatment and hot classification unit.

[0030] S5, the second cracked gas enters the staged condensation system, and successively obtains a mixture of heavy cracked oil, medium cracked oil, light cracked oil and light hydrocarbon liquid; the non-condensable gas that is not condensed is discharged from the top of the staged condensation system; A staged condensation system may include a first-stage high-temperature condensation, a second-stage medium-temperature condensation, and a third-stage low-temperature condensation, or it may be configured as a two-stage or multi-stage condensation system depending on the product objectives.

[0031] The primary high-temperature condensation temperature can be controlled between 180 and 350°C, preferably 220 to 320°C, and is used to condense heavy cracked oil, waxy components, and high-boiling-point oil phases. This portion of the condensate can be used as a heavy cracked oil product, or partially returned to the main cracking system or used as a heavy phase carrier in the impurity trapping process. The secondary medium-temperature condensation temperature can be controlled between 80 and 220°C, preferably 100 to 180°C, and is used to condense medium-quality cracked oil. This medium-quality cracked oil can be used as an intermediate product in subsequent hydrogenation, fractionation, refinery co-processing, or as a chemical feedstock. The tertiary low-temperature condensation temperature can be controlled between 0 and 80°C, preferably 20 to 50°C, and is used to condense light cracked oil and some light hydrocarbon liquids. Uncondensed non-condensable gases are discharged from the top of the condensation system.

[0032] S6, the mixture of light cracked oil and light hydrocarbon liquid enters the gas-liquid separator for gas-liquid separation to obtain light cracked oil and non-condensable gas; The gas-liquid separation temperature can be controlled between 0 and 80°C, preferably between 20 and 50°C; the operating pressure can be from atmospheric pressure to 0.5 MPa, preferably from atmospheric pressure to 0.3 MPa. The non-condensable gas obtained after separation mainly includes C1-C4 hydrocarbons, hydrogen, carbon monoxide, carbon dioxide, and a small amount of light volatile components. The non-condensable gas can be washed, compressed, deacidified, and condensed for recovery and use as fuel gas, hydrogen production feedstock, heating fuel for cracking units, or power generation fuel.

[0033] S7, combine the heavy cracked oil and medium cracked oil from step S5 with the light cracked oil from step S6, and enter the fractionation system for fractionation to obtain light cracked oil fraction, medium cracked oil fraction, heavy cracked oil fraction, and heavy phase residue. The fractionation system can employ atmospheric distillation towers, vacuum distillation towers, flash distillation towers, thin-film evaporators, or combinations thereof.

[0034] The operating pressure of the fractionation tower can be from atmospheric pressure to negative pressure, or it can be determined according to the stability of the oil and the requirements for fraction cutting; the bottom temperature of the tower can be controlled at 150 to 400℃, preferably 180 to 350℃; the top temperature of the tower can be controlled according to the target light fraction range.

[0035] The following oil product streams can be obtained after fractionation: Light cracked oil fractions: mainly C5-C12 or similar boiling range components, which can be used as feedstocks for hydrorefining, steam cracking, blending, or further refining. Middle-grade cracked oil fractions: mainly components within the diesel distillation range or middle distillation range, which can be used as feedstock for hydrotreating or co-processing. Heavy cracked oil fraction: mainly high-boiling-point cracked oil, waxy components or heavy oil components, some of which can be discharged as products and some returned to the system as heat carrier, heavy phase medium or impurity-catching particulate circulation carrier. Heavy phase or residue at the bottom of the tower: enriched with high-boiling-point components, a small amount of solid particles, coke precursors and heavy impurities, which can be incorporated into the bottom ash treatment system or treated as a heavy by-product.

[0036] To further reduce the fine particulate content in light and medium oils, precision filtration, thermal filtration, or settling protection units can be installed before entering the fractionation tower or after the side stream products of the fractionation tower.

[0037] S8, the first heavy phase from step S3, the second heavy phase from step S4, and the heavy phase residue from step S7 are combined and sent to the bottom ash treatment unit for processing.

[0038] The bottom slag treatment unit may include one or more of the following: hot settling, cyclone classification, hot filtration, centrifugal separation, cooling solidification, solvent washing, or stabilization treatment.

[0039] After hot classification, the bottom ash is divided into two parts: one part is a coarse particle component with a larger particle size that still has the ability to capture impurities, which can be returned to the zoned temperature-controlled screw reactor, the inlet of the main pyrolysis reactor, or the lower part of the main pyrolysis reactor as a circulating impurity-capturing particle; the other part is a concentrated impurity stream enriched with chloride salts, fine ash, metal impurities and deactivated particles, which is treated as an external solid or semi-solid by-product.

[0040] The amount of recycled impurity-collecting particles returned can be 0–10 wt% of the waste plastic feed, preferably 0.1–5 wt%, and more preferably 0.2–2 wt%. The amount of impurity-containing bottom slag or heavy phase discharged can be 0.5–20 wt% of the waste plastic feed, preferably 1–10 wt%. Through partial recycling and partial discharge, the concentration of impurity-collecting particles in the system can be maintained, while preventing the continuous accumulation of chloride salts, inorganic ash, and metallic impurities in the system.

[0041] Compared with the prior art, the embodiments of the present invention have at least the following advantages or beneficial effects: This invention provides a method for dechlorination and impurity removal in waste polyolefin pyrolysis. Without using alkaline adsorbents or downstream filtration, a small amount of chlorinated polymer entrained in the waste polyolefin feedstock is preferentially dechlorinated at a temperature lower than the main pyrolysis reaction temperature, releasing HCl and forming dechlorinated carbonaceous residues. These residues continue to heat with the molten material to the main pyrolysis reaction temperature, undergoing condensation, aromatization, or mild carbonization in the downstream section of a zone-controlled temperature screw reactor, forming in-situ carbonaceous impurity-trapping particles. These carbonaceous impurity-trapping particles primarily act as carriers for impurity attachment and aggregation during the main pyrolysis process. Their rough surface structure and defect sites facilitate the attachment and aggregation of inorganic fine particles such as Si, Ca, Fe, and Al, residual chloride salts, and metal chlorides. Simultaneously, the heavy oil, waxy components, and colloidal components formed during pyrolysis further encapsulate these impurity-containing particles, increasing their particle size and apparent density. Therefore, impurities that might otherwise enter the light and medium-quality oil products with the pyrolysis gas are more easily transferred to the heavy phase at the bottom of the reactor and removed through the discharge system, thereby reducing the chlorine, metal, silicon, and ash content in the pyrolysis oil. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0043] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to specific embodiments.

[0044] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0045] Example 1:

[0046] 100 kg of sorted and crushed waste polyolefin packaging film was used as raw material, which contained 62.0 wt% polyethylene, 31.5 wt% polypropylene, 1.0 wt% PVC and chlorine-containing label material, 1.8 wt% ash, 1.5 wt% moisture, and a small amount of paper scraps, adhesives and other plastic impurities.

[0047] The above raw materials are continuously fed into a zoned temperature-controlled twin-screw reactor. The twin-screw reactor is sequentially configured with a melting and homogenization zone, a directional dechlorination zone, and a carbonaceous impurity-collecting particle formation zone along the material conveying direction.

[0048] In the melting and homogenization zone, the cylinder temperature is controlled at 165℃, the operating pressure is atmospheric pressure, the material residence time is 5 minutes, and the screw speed is 80 rpm. In this zone, the waste polyolefin raw material gradually softens, compacts, and melts under the shearing action of the screw and external heating, forming a relatively uniform molten polymer phase.

[0049] After melting and homogenization, the material enters the directional dechlorination zone. The temperature in the directional dechlorination zone is controlled at 210℃, the operating pressure is atmospheric pressure, the material residence time is 15 minutes, and the screw speed is maintained at 80 rpm. Simultaneously, nitrogen is introduced into the directional dechlorination zone as a carrier gas at a flow rate of 0.20 Nm³ / kg waste plastic. PVC and chlorine-containing label materials entrained in the raw material preferentially undergo dehydrochlorination in this zone. The generated HCl is discharged through the volatilization port with the carrier gas and then enters the alkaline washing absorption unit for further treatment.

[0050] The molten material after directional dechlorination continues into the carbonaceous impurity trapping particle formation zone. The temperature in this zone is controlled at 305℃, the residence time is 6 minutes, and the screw speed is 80 rpm. In this zone, the dechlorinated chlorinated polymer residue undergoes further condensation, crosslinking, aromatization, and mild carbonization to form carbonaceous impurity trapping particles with a rough surface structure and defect sites.

[0051] Subsequently, the molten material, along with the resulting carbonaceous impurity-catching particles, enters the main pyrolysis reactor. An oxygen-free environment is maintained within the main pyrolysis reactor, with the reaction temperature controlled at 480℃, the operating pressure at 0.10 MPa, and the residence time at 15 min. During the reaction, the main polyolefins, such as polyethylene and polypropylene, undergo thermal pyrolysis, generating pyrolysis oil and gas, heavy pyrolysis oil, waxy components, non-condensable gases, and small amounts of coke and bottom slag. Simultaneously, the carbonaceous impurity-catching particles act as an attachment and aggregation carrier for inorganic fine particles, chloride salts, metal chlorides, and heavy colloidal components, causing some impurities to accumulate in the heavy phase at the bottom of the reactor.

[0052] The main pyrolysis reactor is equipped with an insulated outlet at the bottom, and the outlet and connecting pipelines are insulated with electric heat tracing. The underflow discharge rate is controlled to be 4.0 wt% of the waste plastic feed rate. The pyrolysis oil and gas discharged from the top of the main pyrolysis reactor enters the hot pre-separation unit, where the temperature is controlled at 380℃. This unit is used to trap heavy mist droplets, coke particles, and carbonaceous impurity-trapping particle complexes entrained in the pyrolysis oil and gas.

[0053] The cracked oil and gas, after hot pre-separation, enter a three-stage condensation system. The first-stage high-temperature condensation is controlled at 280℃, yielding heavy cracked oil and waxy components; the second-stage medium-temperature condensation is controlled at 150℃, yielding medium-quality cracked oil; and the third-stage low-temperature condensation is controlled at 35℃, yielding light cracked oil and some light hydrocarbon liquid. The uncondensed non-condensable gas is discharged after gas-liquid separation and can be used as fuel gas or as feedstock for subsequent hydrogen production.

[0054] The obtained primary high-temperature condensate, secondary medium-temperature condensate, and tertiary low-temperature condensate are collected in buffer tanks and then fed into a fractionation system (fractionation column) for further fractionation. The fractionation system employs reduced-pressure fractionation, with the column top operating pressure controlled at -0.08 MPa, the column bottom temperature controlled at 285℃, and the column top temperature controlled at 75℃. After fractionation, light cracked oil fraction, medium cracked oil fraction, heavy cracked oil fraction, and a heavy phase at the bottom are obtained. Specifically, the light cracked oil fraction has an atmospheric equivalent boiling range below 180℃, the medium cracked oil fraction has an atmospheric equivalent boiling range of 180–350℃, the heavy cracked oil fraction has an atmospheric equivalent boiling range above 350℃, and the heavy phase at the bottom is a residual stream enriched with solid particles, heavy colloidal components, chloride salts, and metallic impurities.

[0055] Based on 100 kg of waste polyolefin packaging film raw material, this example yields 21.6 kg of light pyrolysis oil, 38.2 kg of medium pyrolysis oil, 17.4 kg of heavy pyrolysis oil, 13.1 kg of non-condensable gas, 5.8 kg of bottom slag and heavy phase from the bottom of the tower containing impurities, and the remainder consists of dechlorination tail gas absorbent, moisture, and a small amount of system loss.

[0056] Each of the obtained logistics samples was sampled and tested. The test results are as follows: Table 1

[0057] Example 2 The mixed waste polyolefin with high chlorine impurities was treated according to the method described in Example 1. The raw material used in this example was 300 kg of mixed waste polyolefin, of which polyethylene content was 48.0 wt%, polypropylene content was 39.0 wt%, PVC and PVDC content was 3.5 wt%, chlorine-containing adhesive and chlorine-containing coating material content was 0.8 wt%, ash content was 3.2 wt%, and moisture content was 2.0 wt%.

[0058] In this embodiment, the temperature of the melting and homogenizing zone is controlled at 170°C, the material residence time is 6 min, and the screw speed is 90 rpm; the temperature of the directional dechlorination zone is controlled at 220°C, the material residence time is 25 min, the operating pressure is 0.05 MPa, and the nitrogen carrier gas flow rate is 0.50 Nm³ / kg waste plastic; the temperature of the carbonaceous impurity-collecting particle formation zone is controlled at 320°C, the material residence time is 10 min, and the screw speed is 90 rpm.

[0059] The molten material, after being processed in a zoned temperature-controlled twin-screw reactor, enters the main pyrolysis reactor. The temperature of the main pyrolysis reactor is controlled at 500℃, the operating pressure at 0.15MPa, and the average residence time at 18min. The underflow discharge from the main pyrolysis reactor is controlled at 6.0wt% of the waste plastic feed, and the return of recycled impurity collection particles is controlled at 1.2wt% of the waste plastic feed.

[0060] The hot pre-separation temperature is controlled at 400℃. During the staged condensation process, the first-stage high-temperature condensation temperature is 300℃, the second-stage medium-temperature condensation temperature is 160℃, and the third-stage low-temperature condensation temperature is 40℃. Except for the above-mentioned raw material composition and operating conditions, the remaining steps and processing methods are the same as in Example 1.

[0061] Based on 300 kg of mixed waste polyolefin raw materials, this example yields 57.6 kg of light cracked oil, 101.4 kg of medium cracked oil, 49.8 kg of heavy cracked oil, 43.5 kg of non-condensable gas, 27.3 kg of bottom slag and heavy phase from the bottom of the tower containing impurities, and the remainder consists of dechlorination tail gas absorbent, water, and a small amount of system losses.

[0062] Each of the obtained logistics samples was sampled and tested. The test results are as follows: Table 2

[0063] The embodiments described above are some, but not all, embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for removing impurities from waste polyolefin plastics through thermal pyrolysis dechlorination coupled with adsorption, characterized in that, Includes the following steps: S1, pretreated waste polyolefin plastics; S2, the waste polyolefin plastic processed in step S1 is fed into a zoned temperature-controlled screw reactor to obtain molten material; the screw reactor is sequentially configured along the material conveying direction as a melting homogenization zone, a directional dechlorination zone, and a particle-collecting zone; S3, the molten material is fed into the main pyrolysis reactor and pyrolysis reaction is carried out under stirring. The first heavy phase is discharged from the bottom or bottom side of the main pyrolysis reactor, and the first pyrolysis gas is discharged from the top or top side of the main pyrolysis reactor. S4, the first cracked gas enters the hot pre-separator, the second heavy phase is discharged from the bottom, and the second cracked gas is discharged from the bottom; S5, the second cracked gas enters the staged condensation system, and successively obtains a mixture of heavy cracked oil, medium cracked oil, light cracked oil and light hydrocarbon liquid; the non-condensable gas that is not condensed is discharged from the top of the staged condensation system; S6, the mixture of light cracked oil and light hydrocarbon liquid enters the gas-liquid separator for gas-liquid separation to obtain light cracked oil and non-condensable gas; S7, combine the heavy cracked oil and medium cracked oil from step S5 with the light cracked oil from step S6, and enter the fractionation system for fractionation to obtain light cracked oil fraction, medium cracked oil fraction, heavy cracked oil fraction, and heavy phase residue. S8, the first heavy phase from step S3, the second heavy phase from step S4, and the heavy phase residue from step S7 are combined and sent to the bottom ash treatment unit for processing.

2. The method for removing impurities from waste polyolefin plastics by thermal pyrolysis dechlorination coupled with adsorption according to claim 1, characterized in that, In step S2, the temperature of the melting homogenization zone is 120–180°C; the operating pressure is atmospheric pressure to 0.5 MPa; the residence time of the waste polyolefin plastic in the melting homogenization zone is 0.5–15 min; and the screw speed is 10–300 rpm.

3. The method for removing impurities from waste polyolefin plastics by thermal pyrolysis dechlorination coupled with adsorption according to claim 1, characterized in that, In step S2, the temperature of the directional dechlorination zone is 190–250°C; the residence time of the waste polyolefin plastic in the directional dechlorination zone is 2–60 min; and the operating pressure is -20 kPa to 0.5 MPa.

4. The method for removing impurities from waste polyolefin plastics by thermal pyrolysis dechlorination coupled with adsorption according to claim 1, characterized in that, In step S2, the temperature of the impurity-collecting particle formation zone is 260–360°C; the residence time is 0.5–30 min; and the screw speed is 10–300 rpm.

5. The method for removing impurities from waste polyolefin plastics by thermal pyrolysis dechlorination coupled with adsorption according to claim 1, characterized in that, In step S3, the main pyrolysis temperature is 380–560°C, preferably 420–520°C; the operating pressure is atmospheric pressure to 1.0 MPa, preferably atmospheric pressure to 0.3 MPa. The average dwell time can be 1 to 60 minutes, preferably 5 to 30 minutes, and even more preferably 8 to 20 minutes.

6. The method for removing impurities from waste polyolefin plastics by thermal pyrolysis dechlorination coupled with adsorption according to claim 1, characterized in that, In step S4, the hot pre-separation temperature is 250–500°C; the operating pressure is atmospheric pressure to 1.0 MPa.

7. The method for removing impurities from waste polyolefin plastics by thermal pyrolysis dechlorination coupled with adsorption according to claim 1, characterized in that, In step S5, the staged condensation system includes a first-stage high-temperature condenser, a second-stage medium-temperature condenser, and a third-stage low-temperature condenser arranged sequentially; the temperature of the first-stage high-temperature condenser is 180-350℃, the temperature of the second-stage medium-temperature condenser is 80-220℃, and the temperature of the third-stage low-temperature condenser is 0-80℃.

8. The method for removing impurities from waste polyolefin plastics by thermal pyrolysis dechlorination coupled with adsorption according to claim 1, characterized in that, In step S6, the gas-liquid separation temperature is 0–80°C; the operating pressure is atmospheric pressure to 0.5 MPa.

9. The method for removing impurities from waste polyolefin plastics by thermal pyrolysis dechlorination coupled with adsorption according to claim 1, characterized in that, In step S7, the fractionation system is at least one of an atmospheric distillation column, a vacuum distillation column, a flash distillation column, and a thin-film evaporator.

10. The method for removing impurities from waste polyolefin plastics by thermal pyrolysis dechlorination coupled with adsorption according to claim 1, characterized in that, In step S8, the bottom slag treatment unit performs at least one of the following treatment methods: hot settling, cyclone classification, hot filtration, centrifugal separation, cooling solidification, solvent washing, and stabilization treatment.

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