Hydrothermal conversion of halogen-containing plastics in supercritical water to produce oil

Abstract

A method for converting waste plastics into products by hydrothermal treatment in a supercritical unit using a supercritical water unit is provided. The waste plastics are treated in a pretreatment unit, the plastics are melted into a liquid, and the plastics for the supercritical water unit are prepared. The pretreatment unit can dehalogenate the waste plastics. The molten plastics are introduced into the supercritical water unit together with water, thereby generating products. The products from the supercritical water unit and a flushing flow of steam or water are recycled from the supercritical water unit to the pretreatment unit, preheating and pretreating the waste plastics, and acting as a catalyst in the dechlorination reaction. The purge flow removes the products of the dehalogenation reaction that occurs in the melting section.

Description

Technical Field

[0001] The present disclosure relates to a method for converting waste plastic into oil. More specifically, the present disclosure relates to a method for converting waste plastic into oil using a pretreatment unit and a supercritical water treatment unit, using supercritical water and steam. Background Art

[0002] Waste plastics are a major problem in today's society. The generation rate of plastics is so high and the recycling rate is so low that plastics and plastic-containing materials have become a significant source of waste around the world. Therefore, new methods for utilizing waste plastics are necessary. Hydrothermal treatment of plastics enables plastics to depolymerize to produce oily hydrocarbons. Conventional hydrothermal treatment uses high temperature and water or steam to degrade and depolymerize plastics, and high pressure can also be used to further process plastics. However, a large part of waste plastics contains polyvinyl chloride (PVC). Waste plastics can contain 1% to 10% by weight of PVC. PVC can contain a large amount of chlorine, including chlorine in the range of 50% to 60% by weight. When PVC is incinerated, dioxins and other toxic substances are released. PVC releases hydrochloric acid and chlorine-containing aromatic compounds when treated by anoxic pyrolysis, so these processes are not preferred. When PVC is treated by hydrothermal treatment or other conventional processes, chlorine and other corrosive substances are released, which can cause severe corrosion in the system. In addition, waste plastics also contain other pollutants that are problematic in processing, clogging the system, or causing unacceptable contamination in the final product. Therefore, new processes are needed to address these deficiencies. Summary of the invention

[0003] The present disclosure relates to a method for converting waste plastics into products by hydrothermal treatment in a supercritical unit using a supercritical water unit. The waste plastics are first treated in a pretreatment unit, which melts the plastics into liquids and prepares the plastics for use in the supercritical water unit. The pretreatment unit can remove pollutants and can also dechlorinate PVC. The pretreatment includes a melting section that heats the waste plastics to a molten state, a porous plate that filters the molten plastics, and a liquid collection section. In the melting section, a dehalogenation reaction occurs to remove chlorine from the PVC plastic. The products from the supercritical water unit and a flushing flow of steam or water are recycled from the supercritical water unit to the pretreatment unit, preheating and pretreating the waste plastics, and acting as a catalyst in the dechlorination reaction. The wastewater recovered from the supercritical water reactor is used for hydrothermal treatment of PVC plastics during the pretreatment process. The purge flow removes the products of the dehalogenation reaction that occurs in the melting section. The molten dechlorinated plastic is introduced into a supercritical water unit, which is characterized in that water and steam are under supercritical conditions of water. Reactions occurring in the supercritical water unit include cracking, pyrolysis, dehydrogenation, hydrolysis, cyclization, aromatization, alkylation, dimerization, isomerization, and other reactions known in the art. The molten plastic is further processed in the supercritical water unit to produce a product stream. The product stream may include hydrocarbons, oils, and other polymer components.

[0004] Disclosed herein is a method for treating waste plastics, comprising introducing the waste plastics into a pretreatment unit, and heating the waste plastics in the pretreatment unit to produce a molten plastic, wherein the waste plastics contain halogens. The method also includes the step of introducing the molten plastic into a supercritical water unit, wherein the supercritical water unit can be operated under conditions at, near, or above supercritical conditions of water. The method also includes the step of introducing a supercritical water flow into the supercritical water unit, and reacting the molten plastic in the supercritical water flow to produce a product, wherein the product includes hydrocarbons. The method includes removing a flushing flow from the supercritical water unit, and introducing a flushing flow from the supercritical water unit into the pretreatment unit, wherein the flushing flow includes the product and water.

[0005] In some embodiments, the method includes dehalogenating the molten plastic in a pretreatment unit, and removing a pretreatment purge from the pretreatment unit, wherein the pretreatment purge includes halogens, products, and steam. The pretreatment unit includes a porous disk that can remove solid particles from the molten plastic. The porous disk includes a metal. In some embodiments, the flushing stream is introduced into the supercritical water unit at a pressure that can force the molten plastic through the porous disk, wherein the pressure is in the range of 0.5 MPa to 22 MPa. The product includes upgraded hydrocarbons.

[0006] In some embodiments, the waste plastic includes polyvinyl chloride (PVC). In these embodiments, the pretreatment purge includes chlorine-containing compounds. In some embodiments, the flushing stream is capable of dechlorinating and partially cracking the molten plastic. The supercritical water unit operates at a pressure in the range of 22 MPa to 35 MPa and a temperature in the range of 400° C. to 550° C.

[0007] Further disclosed herein is a method for hydrothermally converting waste plastics into hydrocarbons, wherein the method includes the step of introducing the waste plastics into a pretreatment unit. The waste plastics include polyvinyl chloride (PVC). The method also includes the steps of: introducing a flushing stream into the pretreatment unit, wherein the flushing stream includes water and hydrocarbons; melting the waste plastics in a melting section of the pretreatment unit; and dechlorinating the molten plastic in the melting section to produce a dechlorinated molten plastic. The method also includes the steps of: removing a pretreatment purge from the melting section of the pretreatment unit, wherein the pretreatment purge includes chlorine, hydrocarbons, and water; filtering the dechlorinated molten plastic through a porous disk to produce a purified molten plastic; introducing the purified molten plastic into a supercritical water unit; introducing a supercritical water stream into the supercritical water unit; and reacting the supercritical water stream with the purified molten plastic to produce hydrocarbons. The method also includes the steps of: removing a product stream from the supercritical water unit, wherein the product stream includes hydrocarbons and water; and splitting the flushing stream from the product stream.

[0008] In some embodiments, the supercritical water unit is operated at a pressure in the range of 22 MPa to 35 MPa and a temperature in the range of 400° C. to 550° C. The method also includes the step of pumping the purified molten plastic so that the pressure of the purified molten plastic is at or near the pressure condition of the supercritical condition of water. The step of reacting the supercritical water and the purified molten plastic includes hydrothermal conversion. In some embodiments, the flushing stream is capable of cracking the dechlorinated molten plastic.

[0009] In some embodiments, the method further comprises the steps of: separating the product stream in a separator to produce a gas product and a liquid product; and dechlorinating the pretreatment purge in a dechlorination process to produce a dechlorinated product. In some embodiments, the pretreatment unit operates at a pressure in the range of 0.5 MPa to 22 MPa and a temperature in the range of 120° C. to 400° C. The porous disk is capable of removing solid particles. In a further embodiment, a plurality of pretreatment units are provided and the pretreatment units are arranged in series. The overhead recycle is recycled from a second pretreatment unit in the plurality of pretreatment units to a first pretreatment unit in the plurality of pretreatment units. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] These and other features, aspects and advantages of the present disclosure will be better understood through the specification, claims and drawings. However, it should be noted that the drawings only illustrate several embodiments of the present disclosure and therefore should not be considered as limiting the scope of the present disclosure, as the present disclosure may allow other equally effective embodiments.

[0011] Figure 1 is a schematic diagram of a basic supercritical water-plastic conversion process according to one embodiment.

[0012] Figure 2 is a drawing of a pre-processing unit according to one embodiment.

[0013] Figure 3 is a drawing of a pre-processing unit with multiple filters according to one embodiment.

[0014] Figure 4 is a process schematic diagram of a supercritical water-plastic conversion process according to one embodiment

[0015] picture

[0016] Figure 5 is a schematic diagram of multiple pre-processing units placed in series according to one embodiment.

[0017] In the accompanying drawings, similar components or features, or both, may have like reference labels. Figures 1 to 5 For the purpose of simplifying the schematic diagram and description, many temperature and pressure sensors, controllers, etc. that can be used and known by ordinary technicians in the field are not included. In addition, additional components in conventional industrial operations are not shown. However, components such as those described in the present disclosure that operate normally can be added to the embodiments described in the present disclosure. DETAILED DESCRIPTION

[0018] Although the present disclosure will be described through several embodiments, it should be understood that those skilled in the relevant art will appreciate that many examples, variations and modifications of the described systems and methods are within the scope and spirit of the present disclosure. Therefore, the embodiments of the present disclosure described are presented without loss of generality and are not intended to limit the claims.

[0019] The present disclosure is capable of converting waste plastics into oil by utilizing supercritical water as a reaction medium as well as a reactant. Advantageously, supercritical water acts as an efficient heat transfer medium.

[0020] Advantageously, the present disclosure removes halogens (e.g., chlorine) from plastics prior to hydrothermal conversion in a supercritical water unit, thereby reducing or preventing corrosion and preventing product hydrocarbons from being contaminated by chlorine. The flushing stream and pretreatment unit also crack some of the molten plastics. The porous disks in the pretreatment unit advantageously remove solid particles, prevent blockage of process lines, prevent product contamination, and reduce the need to pre-sort, sort, or purify waste plastics before the plastics are loaded into the pretreatment unit or a storage bin prior to the pretreatment unit.

[0021] The flushing stream of the present invention provides many advantages, including heating the waste plastic, thereby reducing electricity use; increasing the pressure of the molten plastic, reducing the need for additional pressurization; providing force to filter the molten plastic through a porous disk; and removing halogenated compounds from the molten plastic. The flushing stream includes steam and hydrocarbons. Advantageously, the hydrocarbons in the flushing stream interact with the melted plastic, causing a swelling effect that swells the plastic, making it softer, easier to melt and flow, and facilitating cracking. The swollen plastic is more easily cracked because the supercritical water in the supercritical water reactor can more easily penetrate the molten plastic. The steam in the flushing stream also introduces water molecules into the molten plastic, which helps remove chlorine from PVC. The water from the flushing stream acts as a catalyst and reactant in the dehalogenation reaction. Even at lower temperatures, adding steam from the flushing stream to the pretreatment zone will increase the dehydrochlorination rate. This is different from conventional wetting systems, which do not provide the same benefits as steam when wetting the plastic before or during the melting stage because plastics are generally hydrophobic.

[0022] Other advantages relate to the use of supercritical water reactors to process molten dechlorinated plastics. When the waste plastics include polyethylene and polypropylene, the properties of supercritical water and the reactions that occur in the supercritical water reactor beneficially produce paraffin and olefin products. The conditions of the supercritical water reactor can inhibit secondary reactions, including aromatization, dimerization, condensation, and cyclization. The use of supercritical water and supercritical water reactors can result in higher concentrations of aromatics and cycloalkanes in the product stream.

[0023] refer to Figure 1, depicts a basic supercritical water-plastic conversion process 100, which includes two main units: a pretreatment unit 110 and a supercritical water unit 140. Waste plastic 104 is introduced into the pretreatment unit 110. The waste plastic 104 may include various polymer compounds, including polyolefins, polystyrenes, polyethylene terephthalate, and combinations thereof. The waste plastic 104 may include PVC. The PVC may contain up to 50% to 60% by weight or more of chlorine. The pretreatment unit 110 allows for melting of the plastic, production of liquid plastic, and dehalogenation of the plastic. In embodiments where the plastic contains PVC, melting and heat treatment of the plastic in the presence of steam causes dechlorination of the plastic. Advantageously, chlorides in the PVC are removed by nucleophilic substitution with water to form polyols before further reactions can occur. Ionic chain reactions can also be used to form polyenes. The produced polyols are converted to polyenes by dehydration reactions. At lower temperatures below 400° C., nucleophilic substitution is dominant. At high temperatures above 400° C., ionic chain reactions are dominant. External heat may be selectively provided to the pre-treatment unit 110. The pre-treatment unit 110 also includes a treatment system to remove contaminants, such as shaped particles, from the molten liquid plastic. Multiple pre-treatment units 110 may be installed in a parallel or series configuration.

[0024] A flushing stream 180 is introduced into the pretreatment unit 110. The flushing stream 180 comprises water at supercritical or subcritical conditions and products from the supercritical water unit 140. The water of the flushing stream 180 may be in the form of steam or may be in a subcritical liquid water state. The flushing stream 180 assists in the dechlorination of the waste plastic 104. The water molecules present in the flushing stream 180 increase the dehydrochlorination reaction. The hydrocarbons present in the flushing stream 180 interact with the waste plastic 104 to swell the plastic, soften the plastic, and thereby make it easier to melt and flow through the system. A pretreatment purge 190 is removed from the pretreatment unit 110. The pretreatment purge 190 comprises halogenated components, hydrocarbons, and steam. The pretreatment purge 190 may have a chlorine content in the range of 0.005 wt % to 0.5 wt %. In an embodiment for treating PVC, the pretreatment purge 190 contains chlorine-containing compounds such as chlorine, hydrochloric acid, or other chlorine-containing hydrocarbons. Advantageously, eliminating the pretreatment purge 190 removes chlorine-containing compounds from the stream entering the subsequent reactor, which protects the subsequent reactor and downstream equipment from corrosion.

[0025] The molten plastic 118 is removed from the pretreatment unit 110 and introduced into the supercritical water unit 140. The supercritical water unit 140 is a reactor that operates near, at or above the supercritical conditions of water. The supercritical water unit 140 can be any type of reactor or container that can operate under the supercritical conditions of water. In some embodiments, the supercritical water unit 140 consists of a plurality of tubular reactors with heating elements to control the temperature of the internal fluid. The size of the tubular reactor can be determined by the Reynolds number, which is calculated by assuming that the internal fluid is 100% water. The Reynolds number can be higher than 3,000, and preferably higher than 6,000. As known in the art, the reactor can be configured in a variety of ways and at a variety of flow rates. The residence time of the component in the supercritical water unit 140 can be in the range of 1 minute to 60 minutes, and preferably in the range of 2 minutes to 30 minutes. Supercritical water 126 is introduced into the supercritical water unit 140. Supercritical water 126 contains water or steam that is at, near or above the supercritical conditions of water. The water in supercritical water unit 140 not only serves as reaction medium but also serves as reactant, and advantageously serves as effective heat transfer medium.The supercritical property of water promotes the dissociation (dissociation) of free radical-mediated polymer chains.The reaction occurring in supercritical water unit 140 comprises cracking, pyrolysis, dehydrogenation, hydrolysis, cyclization, aromatization, alkylation, dimerization, isomerization and other reactions known in the art.Dissociation produces hydrocarbons, which are removed together with water by dissolving.In the embodiment of finding PVC in waste plastics, the polyols and polyenes produced in the dechlorination process are converted into other molecules in supercritical water unit 140 by cracking, dehydrogenation, hydrolysis, cyclization, aromatization, dimerization, isomerization, alkylation or other reactions known in the art.

[0026] Remove flushing stream 180 from supercritical water unit 140. Also remove product 142 from supercritical water unit 140. Product 142 comprises steam or water and hydrocarbon products close to or in supercritical conditions of water. Product 142 may include hydrocarbons, oils, paraffins, olefins, cycloparaffins, aromatic compounds and other polymer components. In some embodiments where waste plastics 104 are mainly polyethylene and polypropylene, paraffins and olefins dominate in product 142. Product 142 may be upgraded hydrocarbons. Product 142 may contain hydrocarbons with a boiling point range from FBP (final boiling point) of 180°C (corresponding to light naphtha) to FBP of 560°C (equivalent to vacuum gas oil), and trace amounts of heavy fractions with a boiling point higher than 560°C. Flushing stream 180 may be additionally heated before being introduced into pretreatment unit 110.

[0027] refer to Figure 2, shows a pretreatment unit system 200, which includes a pretreatment unit 210. The pretreatment unit 210 can have the same characteristics as the pretreatment unit 110. The pretreatment unit is characterized by having two sections: a melting section 211 and a liquid collection section 212, which are separated by a porous disk 216. The waste plastic is added to the melting section 211 of the pretreatment unit 210. Advantageously, the waste plastic does not need to be dried or pre-washed before being added to the pretreatment unit 210. The melting section 211 heats the waste plastic, melting the plastic into a liquid state. An optional heating element can be used to melt the waste plastic in the melting section 211. The temperature of the molten plastic 118 is maintained above 120°C and below 400°C. An optional agitator or mixer can also be used for uniform heat transfer.

[0028] A flushing stream 280 is introduced into the melting section 211 of the pretreatment unit 210 and can have the same characteristics as the flushing stream 180. The flushing stream 280 advantageously heats the dechlorinated molten plastic 213 in the pretreatment unit 210 and provides additional water for efficient heat transfer. The flushing stream 280 maintains the pressure of the melting section 211 above 0.2 MPa and below 20 MPa. A pretreatment purge 290 is removed from the melting section 211 of the pretreatment unit 210. The pretreatment purge 290 can have the same characteristics as the pretreatment purge 190. The melting section 211 melts the waste plastic to produce a dechlorinated molten plastic 213. As the plastic melts, chlorine is emitted and removed by the pretreatment purge 290.

[0029] The dechlorinated molten plastic 213 passes through the porous disk 216. The flushing flow 280 can provide pressure to force the dechlorinated molten plastic 213 to pass through the porous disk 216. The porous disk 216 may include a straight-through hole with a diameter between 0.01 mm and 20 mm, and preferably between 0.1 mm and 5 mm. The porous disk 216 maintains pressure in the melting section 211. The porous disk 216 may include a metal or ceramic disk with one or more layers. Preferably, the porous disk 216 is made of a corrosion-resistant metal, such as a high nickel alloy, including Hastelloy-C and Inconel 625. In some embodiments, multiple layers have different pore sizes. The porous disk 216 removes solid particles and other contaminants (such as metals) from the dechlorinated molten plastic 213, which advantageously reduces the complexity from contamination. The porous disk 216 can be regenerated by backwashing, incineration, steaming or replacement. The purified molten plastic 217 is collected in the liquid collection section 212 of the pretreatment unit 210. The liquid collection section 212 may be heated to between 120°C and 350°C.

[0030] Molten plastic 218 is removed from liquid collection section 212 of pretreatment unit 210, and molten plastic 218 can have the same characteristics as molten plastic 118. Molten plastic 218 is introduced into supercritical water unit 240, which can have the same characteristics as supercritical water unit 140. Some hydrocarbon products in rinse stream 280 that interact with dechlorinated molten plastic 213 can be collected in molten plastic 218 and transferred to supercritical water unit 240. Supercritical water 226, which can have the same characteristics as supercritical water 126, is introduced into supercritical water unit 240. Product 242 can have the same characteristics as product 142, including steam or water at or near supercritical conditions of water, and products, and is produced by supercritical water unit 240. Product 242 is split into stream product 254 and rinse stream 280. No component separation occurs during the split process, i.e., product 242 is split into two streams, producing a flush stream 280 and a split product 254 having the same physical or chemical properties. In some embodiments, the mass flow ratio of flush stream 280 to split product 254 is in the range of 0.02 to 5.

[0031] refer to Figure 3 , a pre-treatment unit 300 with multiple filters is shown. Instead of one porous disk, the pre-treatment unit 300 with multiple filters has three porous disks: porous disk A 216A, porous disk B 216B, and porous disk C 216C. The three porous disks can be made of the same or similar materials, or they can be made of different materials. The three porous disks can have the same or similar filtering holes, or they can have different filtering holes. There can be gaps or spaces between the multiple porous disks.

[0032] refer to Figure 4, a supercritical water-plastic conversion process 400 is shown. Waste plastic 404 is stored in and removed from waste plastic storage tank 402, which can be loaded in batch or continuous mode. Waste plastic 404 can have the same or similar properties as waste plastic 104 and waste plastic 204. Waste plastic 404 is introduced into pretreatment unit 410, which can have the same or similar properties as pretreatment unit 110, pretreatment unit 210, and pretreatment unit 300 with multiple filters. Pretreatment unit 410 can be operated at a pressure in the range of 0.5 MPa to 22 MPa, preferably in the range of 1 MPa to 10 MPa, and at a temperature in the range of 120°C to 400°C, preferably in the range of 150°C to 350°C. Rinse stream 480 is introduced into pretreatment unit 410, and rinse stream 480 can have the same or similar properties as rinse stream 180 and rinse stream 280. The flushing stream 480 may have a pressure in the range of 0.5 MPa to 22 MPa, preferably in the range of 1 MPa to 10 MPa. The flushing stream 480 may have a temperature in the range of 120° C. to 400° C., preferably 150° C. to 300° C. A pretreatment purge 490 is removed from the pretreatment unit 410, and the pretreatment purge 490 may have the same or similar properties as the pretreatment purge 190 and the pretreatment purge 290. The pretreatment purge 490 may have a pressure in the range of 0.5 MPa to 22 MPa, preferably in the range of 1 MPa to 10 MPa. The pretreatment purge 490 may have a temperature in the range of 120° C. to 400° C., preferably 150° C. to 300° C. The chlorine content of pretreatment purge 490 varies depending on the flow rate of flush stream 480 and the PVC content of waste plastic 404; however, in some embodiments, the chlorine content of pretreatment purge 490 may be within the range of 0.005 wt % and 0.5 wt %.

[0033] The pretreatment purge 490 is introduced into a dechlorination process 492. The dechlorination process 492 can be any type of process, unit, or system capable of removing chlorine from a stream. The dechlorination process 492 can be an adsorption unit, an absorption unit, a neutralization tank, an alkaline solvent system, a reactor, a scrubber, or any other device known in the art. The size of the dechlorination process can depend on the mass flow ratio of the flush stream 480 to the pressurized molten plastic 422. The dechlorination product 494 is produced by the dechlorination process 492 and includes a product that is substantially free of or has a low amount of chlorine or chlorine-containing compounds.

[0034] Molten plastic 418 is removed from pretreatment unit 410, and molten plastic 418 may have the same or similar properties as molten plastic 118 and molten plastic 218. Molten plastic 418 may have a temperature in the range of 120° C. to 350° C., preferably 150° C. to 300° C. Molten plastic 418 is introduced into molten plastic pump 420. Molten plastic pump 420 may be any type of pump capable of increasing the pressure of molten plastic 418 so that the stream may be introduced into the supercritical conditions of the supercritical water unit. Pressurized molten plastic 422 is introduced into mixer 436. Pressurized molten plastic 422 may have a pressure in the range of 22 MPa to 35 MPa, preferably 23 MPa to 30 MPa.

[0035] Water 426 is introduced into a water pump 428, which increases the pressure of the water to produce a stream 430. The stream 430 may have a pressure in the range of 22 MPa to 35 MPa, preferably in the range of 23 MPa to 30 MPa. The water 426 may be demineralized water having an electrical conductivity of less than 1 microsiemens (μS) / cm (cm); preferably less than 0.5 μS / cm; and more preferably equal to or less than 0.1 μS / cm. In a preferred embodiment, the sodium content is less than 5 μg / L and the silicon content is less than 3 μg / L. The stream 430 is introduced into a preheater 432, which increases the temperature of the water to produce a supercritical water stream 434. The supercritical water stream 434 is at or above the supercritical condition of water. The supercritical water stream 434 may have a temperature in the range of 374°C to 600°C, preferably 400°C to 550°C. The supercritical water stream 434 is introduced into a mixer 436, which mixes the supercritical water stream 434 with the pressurized molten plastic 422 to produce a supercritical unit feed 438. The mixer 436 can be a static mixer, a container equipped with an agitator, a tee joint, or any other mixer known in the art. The supercritical unit feed 438 can have a pressure in the range of 22 MPa to 35 MPa, preferably in the range of 23 MPa to 30 MPa. The supercritical unit feed 438 can have a temperature in the range of 300° C. to 550° C., preferably in the range of 380° C. to 450° C. The ratio of the mass flow rate of the pressurized molten plastic 422 to the mass flow rate of the supercritical water stream 434 supplied to the mixer 436 can be in the range of 1:0.1 to 1:10; preferably in the range of 1:0.5 to 1:3.

[0036] Supercritical unit feed 438 is introduced into supercritical water unit 440. Supercritical water unit 440 may have the same or similar characteristics as supercritical water unit 140 and supercritical water unit 240. Supercritical water unit 440 produces product 442, which may have the same or similar characteristics as product 142 and product 242. Product 442 may include hydrocarbons and water. Product 442 may have a pressure in the range of 22MPa to 35MPa, preferably in the range of 23MPa to 30MPa. Product 442 may have a temperature in the range of 400°C to 550°C, preferably in the range of 420°C to 500°C. Product 442 is introduced into cooler 444, which reduces the temperature of the product to produce stream 446. Stream 446 may have a temperature in the range of 120°C to 400°C, preferably in the range of 150°C to 300°C. The material stream 446 is introduced into the decompression device 448, which reduces the pressure of the material stream 446 to produce a decompression stream 450. The decompression stream 450 can have a pressure in the range of 0.5 MPa to 22 MPa, preferably in the range of 1 MPa to 10 MPa. Preferably, the decompression device 448 is a series of devices that gradually reduce the pressure of the material stream 446.

[0037] The reduced pressure stream 450 is introduced into the splitter 452. The splitter 452 divides the reduced pressure stream 450 into a split product 454 and a flushing stream 480 having the same or similar characteristics. The split product 454 is introduced into the valve 456, which controls the amount of the flushing stream 480 delivered to the pretreatment unit 210. A flow control valve (not shown) located on the gas product 466 also controls the amount of the flushing stream 480 delivered to the pretreatment unit 210. The mass flow ratio of the flushing stream 480 to the pressurized molten plastic 422 is in the range of 0.02 to 5.0. Stream 458 is removed from the valve 456 and introduced into the product cooler 460. The product cooler 460 reduces the temperature of the stream 458 to produce a cooled product 462. The cooled product 462 may have a temperature in the range of 30°C to 90°C. The cooled product 462 may have a pressure in the range of atmospheric pressure to 0.5 MPa. The cooled product 462 is introduced into a gas-liquid separator 464, which separates the gas phase from the liquid phase, and produces a gas product 466 containing gaseous hydrocarbons. The gas-liquid separator 464 also produces a liquid product 468 containing a liquid phase. The liquid product 468 is introduced into an oil-water separator 470, which separates the oil phase from the water phase. An oil product 472 containing hydrocarbons is removed from the oil-water separator 470. A water product 474 containing water is removed from the oil-water separator 470.

[0038] refer to Figure 5, showing a plurality of pretreatment units placed in a series system 500. Three pretreatment units 510A, 510B, and 510C in series are shown. The overhead distillate from the lower pretreatment unit is transported to the upstream pretreatment unit. A waste plastic storage tank 502 having the same or similar properties as the waste plastic storage tank 402 introduces waste plastic 504 into the pretreatment unit 510A, and the waste plastic 504 has the same or similar properties as the waste plastic 104, the waste plastic 204, and the waste plastic 404. Pretreatment purge 590 is removed from the pretreatment unit 510A, and the pretreatment purge 590 has the same or similar properties as the pretreatment purge 190, the pretreatment purge 290, and the pretreatment purge 490. A second overhead recycle 584 is introduced into the pretreatment unit 510A. Molten plastic 518A from pretreatment unit 510A is introduced into pretreatment unit 510B, molten plastic 518A having the same or similar properties as molten plastic 418, and pretreatment unit 510B having the same or similar properties as pretreatment unit 510A. First overhead recycle 582 is introduced into pretreatment unit 510B. Pretreatment unit 510B produces second overhead recycle 584 and molten plastic 518B, second overhead recycle 584 having the same or similar properties as flushing stream 580, and molten plastic 518B having the same or similar properties as molten plastic 518A.

[0039] Molten plastic 518B is introduced into pretreatment unit 510C, and pretreatment unit 510C has the same or similar characteristics as 510B. Rinsing stream 580 is introduced into pretreatment unit 510C, and rinse stream 580 has the same or similar characteristics as rinse stream 180, rinse stream 280 and rinse stream 480. Pretreatment unit 510C produces first overhead recycle 582 and molten plastic 518C. Then molten plastic 518C can be introduced into supercritical water unit (not shown), and molten plastic 518C has the same or similar characteristics as molten plastic 518B. Advantageously, using multiple pretreatment units can make it possible to be exposed to the rinse stream for a longer time, thereby obtaining a higher dechlorination rate. In addition, the HCl released from the rear pretreatment unit and recycled to the front pretreatment unit can be used as a catalyst to enhance the dechlorination reaction in the front pretreatment unit, because HCl has an autocatalytic effect on PVC degradation.

[0040] The singular forms "a," "an," and "the" include plural forms unless the context clearly dictates otherwise.

[0041] As used in the description and appended claims, the words "having," "comprising," and "including" and all grammatical variations thereof are intended to have an open, non-limiting meaning that does not exclude additional elements or steps.

Claims

1. A method for treating waste plastics, the method comprising the following steps: introducing waste plastic into a pretreatment unit, wherein the waste plastic comprises a halogen; heating the waste plastic in the pretreatment unit to produce molten plastic; introducing the molten plastic into a supercritical water unit, wherein the supercritical water unit can operate under supercritical conditions of water; introducing a supercritical water flow into the supercritical water unit; reacting the molten plastic in the supercritical water stream to produce products, wherein the products include hydrocarbons; removing a flush stream from the supercritical water unit, the flush stream comprising the product and water; as well as introducing the flushing stream from the supercritical water unit into the pretreatment unit, The method further comprises the following steps: Dehalogenating the molten plastic in the pretreatment unit; as well as A pretreatment purge is removed from the pretreatment unit, the pretreatment purge comprising halogens, products, and steam.

2. The method of claim 1, wherein the pre-treatment unit comprises a porous disk capable of removing solid particles from the molten plastic.

3. The method according to claim 2, wherein: The flushing stream is introduced into the supercritical water cell at a pressure capable of forcing the molten plastic through the porous disk, wherein the pressure is in the range of 0.5 MPa to 22 MPa. The method of claim 3 , wherein the porous disk comprises metal.

5. The process of claim 1, wherein the product comprises hydrocarbons having a final boiling point range of 180°C to 560°C.

6. The method of claim 1, wherein the waste plastic comprises polyvinyl chloride (PVC).

7. The method of claim 1, wherein the pretreatment purge comprises a chlorine-containing compound.

8. The method of claim 1, wherein the flushing stream cracks a portion of the molten plastic in the pretreatment unit.

9. The method of claim 1, wherein the supercritical water unit is operated at a pressure in the range of 22 MPa to 35 MPa and a temperature in the range of 400°C to 550°C.

10. A method for hydrothermally converting waste plastics into hydrocarbons, the method comprising the following steps: introducing waste plastic into a pretreatment unit, wherein the waste plastic includes polyvinyl chloride (PVC); introducing a flush stream into the pretreatment unit, wherein the flush stream comprises water and hydrocarbons; Melting the waste plastic in a melting section of the pretreatment unit to produce molten plastic; Dechlorinating the molten plastic in the melting section to produce dechlorinated molten plastic; removing a pretreatment purge from a melting section of the pretreatment unit, wherein the pretreatment purge comprises chlorine, hydrocarbons, and water; filtering the dechlorinated molten plastic through a porous disk to produce a purified molten plastic; introducing the purified molten plastic into the supercritical water unit; introducing a supercritical water flow into the supercritical water unit; reacting the supercritical water flow with the purified molten plastic to produce hydrocarbons; removing a product stream from the supercritical water unit, wherein the product stream comprises hydrocarbons and water; as well as The flush stream is split from the product stream.

11. The method of claim 10, wherein the supercritical water unit is operated at a pressure in the range of 22 MPa to 35 MPa and a temperature in the range of 400°C to 550°C.

12. The method of claim 10, further comprising the step of pumping the purified molten plastic so that the pressure of the purified molten plastic is at or near the supercritical condition of water.

13. The method of claim 10, wherein the step of reacting supercritical water and the purified molten plastic comprises a hydrothermal conversion.

14. The method of claim 10, wherein the flushing stream partially cracks the molten plastic dechlorinated in the pretreatment unit.

15. The method according to claim 10, further comprising the steps of: separating a remaining portion of the product stream in a separator, wherein splitting the flush stream from the product stream produces the remaining portion of the product stream to produce a gas product and a liquid product; as well as The pretreatment purge is dechlorinated in a dechlorination process to produce a dechlorinated product.

16. The method of claim 10, wherein the porous disk is capable of removing solid particles.

17. The method of claim 10, further comprising a plurality of pre-processing units, the plurality of pre-processing units being arranged in series.

18. The method of claim 17, wherein the overhead recycle is recycled from a second pretreatment unit of the plurality of pretreatment units to a first pretreatment unit of the plurality of pretreatment units.

19. The method of claim 10, wherein the pretreatment unit operates at a pressure in the range of 0.5 MPa to 22 MPa and a temperature in the range of 120°C to 400°C.

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