Hydrothermal dehalogenation of chemical substances

Abstract

A method for treating plastic waste and used caustic substances, the method comprising the steps of mixing a feed plastic and a used caustic substance stream in a feed mixer to produce a mixed feed, wherein the feed plastic comprises plastic waste in the form of plastic waste chips; introducing the mixed feed into a hydrothermal reactor; reacting the mixed feed in the hydrothermal reactor to produce an effluent, wherein chlorine is removed from the plastic waste in the presence of sodium hydroxide and this chlorine reacts with sodium hydroxide to produce sodium chloride and water; introducing the effluent into a washing and dewatering unit, wherein the effluent contains a liquid phase substance and a solid phase substance, and the solid phase substance contains dechlorinated plastic; and separating the liquid phase substance and the solid phase substance in the washing and dewatering unit to produce dechlorinated plastic waste and neutralized wastewater.

Description

Technical Field

[0001] A method for treating chemical substances is disclosed. Specifically, a method and system for removing halogen from waste plastics using spent caustic substances are disclosed.

Background Art

[0002] Polyvinyl chloride (PVC) is one of the most abundant plastics used in consumer goods. It has relatively high abrasion resistance, mechanical strength, hardness, and durability. It is also stable in various chemical environments. By adding plasticizers, it can be very flexible.

[0003] However, recycling of used PVC is very difficult because chlorine is present in its structure. Incineration of PVC waste generates highly harmful substances such as dioxins. Pyrolysis is not a suitable recycling method because it produces hydrogen chloride (HCl) and chloroaromatic compounds that can be harmful to the environment.

[0004] Spent caustic solution is an alkaline aqueous solution generated from a sweetening process in which hydrogen sulfide (H2S) and mercaptans are removed. The MEROX® process is one common specific example of a sweetening process. Spent caustic solution is a by-product of the sweetening process and needs to be treated before recycling or discharging to the environment.

[0005] There are many wastewater treatment methods for destroying organic compounds in water before recycling or discharging to the environment, such as biological treatment, adsorption, and membrane treatment. However, the presence of sulfur compounds and high pH in spent caustic solution are problems in wastewater treatment methods.

[0006] Several methods have been used to treat spent caustic solutions. Chemical oxidation and wet air oxidation are effective methods for converting organic compounds into carbon dioxide and other small organic compounds, such as formates, acetates, and carboxylates, and sulfur compounds into sulfates. Such oxidation methods require oxidizing agents such as air, hydrogen peroxide, or other peroxides, and in the Fenton process, a catalyst, Fe 2+ is required, resulting in high operating costs. Biological treatment, which is one of the most common wastewater treatment methods in refineries and petrochemical plants, requires pretreatment of the spent caustic solution, i.e., neutralization and removal of sulfur compounds. Neutralizing the spent caustic solution with an acid and then steam stripping (to remove H2S and mercaptans) is a cost-effective method for pretreating such wastewater prior to the biological treatment step. By lowering the pH of the spent caustic solution, sulfides are converted to H2S. Neutralization of the spent caustic solution is accompanied by the spontaneous generation of H2S, which must be considered for safety reasons. Therefore, neutralization of the spent caustic solution must be carried out in a well-controlled atmosphere, preferably using a sulfur recovery unit in which H2S is converted to solid sulfur.

Summary of the Invention

[0007] A method for treating a chemical substance is disclosed. Specifically, a method and system for removing halogen from waste plastic using spent caustic substance are disclosed.

[0008] In a first aspect, a method for treating plastic waste and used caustic substances is provided. The method includes the step of mixing a feed plastic and a used caustic stream in a feed mixer to produce a mixed feed, wherein the feed plastic includes plastic waste in the form of plastic waste chips, the plastic waste is selected from polyvinyl chloride (PVC), halogenated plastics, and combinations thereof, and the used caustic stream includes sodium hydroxide. The method includes the step of introducing the mixed feed into a hydrothermal reactor, and the step of reacting the mixed feed in the hydrothermal reactor to produce an effluent, wherein the residence time in the hydrothermal reactor ranges from 0.2 hours to 5 hours, the pressure in the hydrothermal reactor is higher than the saturation pressure of water at the temperature in the hydrothermal reactor such that the water in the hydrothermal reactor is liquid-phase water, the temperature in the hydrothermal reactor ranges from 150 °C to 350 °C, chlorine is removed from the plastic waste in the presence of sodium hydroxide, and the chlorine reacts with sodium hydroxide to produce sodium chloride and water, and the step of introducing the effluent into a washing and dewatering unit, wherein the effluent includes a liquid-phase substance and a solid substance, and the solid substance includes dechlorinated plastic, and further includes the step of separating the liquid-phase substance and the solid substance in the washing and dewatering unit to produce dechlorinated plastic waste and neutralized wastewater.

[0009] In one aspect, the plastic waste chips are less than 1 mm. In one aspect, the halogenated plastic is selected from the group consisting of chlorinated polyvinyl chloride (CPVC), polyvinylidene dichloride (PVDC), and combinations thereof. In one aspect, the plastic waste further comprises a thermoplastic polymer selected from the group consisting of polyethylene, polystyrene, polypropylene, and combinations thereof. In one aspect, the flow rates of the feed plastic and the spent caustic substance are adjusted to achieve a target pH of 5-6 in the hydrothermal reactor. In one aspect, the method further comprises treating the neutralized wastewater in a desalination unit to produce desalinated water. In one aspect, the method further comprises introducing desalinated water into the hydrothermal reactor, and the desalinated water can adjust the target pH in the hydrothermal reactor. In one aspect, the chlorine content of the feed plastic is at least 5 wt%. In one aspect, the spent caustic substance in the spent caustic substance stream has a chemical oxygen demand of 1,000 mg / L to 100,000 mg / L, a total organic carbon of 500 mg / L to 10,000 mg / L, a sulfide content of 1,000 mg / L to 40,000 mg / L, an alkali metal content of 1.5 wt% to 8.5 wt%, and a pH of 11.5 to 13.9.

[0010] In a second aspect, a system for treating plastic waste and spent caustic substances is provided. The system includes a plastic waste storage container configured to store plastic waste in the form of plastic waste chips, where the plastic waste is selected from polyvinyl chloride (PVC), halogenated plastics, and combinations thereof; a spent caustic storage tank configured to store the spent caustic substance, where the spent caustic substance contains sodium hydroxide; a feed mixer fluidly connected to the plastic waste storage container and the spent caustic storage tank, where the feed mixer is configured to mix a feed plastic and a spent caustic stream to produce a mixed feed, the feed plastic includes the plastic waste and the spent caustic stream includes the spent caustic substance; a hydrothermal reactor fluidly connected to the feed mixer, the hydrothermal reactor being configured to react the mixed feed to produce an effluent, where the residence time in the hydrothermal reactor is in the range of 0.2 hours to 5 hours, the pressure in the hydrothermal reactor is higher than the saturation pressure of water at the temperature in the hydrothermal reactor such that the water in the hydrothermal reactor is in the liquid phase, the temperature in the hydrothermal reactor is in the range of 150 °C to 350 °C, chlorine is removed from the plastic waste in the presence of sodium hydroxide, and the chlorine reacts with the sodium hydroxide to produce sodium chloride and water; and a washing and dewatering unit fluidly connected to the hydrothermal reactor, the washing and dewatering unit being configured to separate solid and liquid phase substances in the effluent to produce dechlorinated plastic waste and neutralized wastewater, the solid substance including dechlorinated plastic.

[0011] In one aspect, the feed mixer is selected from an in-line mixer, a T-fitting, a Y-fitting, and combinations thereof. In one aspect, the hydrothermal reactor is selected from a batch reactor having an internal mixing device and a CSTR.

[0012] These and other features, aspects, and advantages of the present invention will be better understood with reference to the following description, claims, and accompanying drawings. However, it should be noted that the drawings illustrate only some embodiments and, therefore, should not be considered as limiting the scope, as other equally effective embodiments may be recognized.

Brief Description of the Drawings

[0013]

Figure 1

[0014] In the accompanying drawings, like components or features, or both, may have the same reference labels.

Mode for Carrying Out the Invention

[0015] The scope of the apparatus and method is described with reference to several embodiments, but those skilled in the art will understand that many examples, variations, and modifications of the apparatus and method described herein are within the scope and spirit of this embodiment.

[0016] Accordingly, the described embodiments are presented without loss of generality and without imposing limitations, and are described in this embodiment. Those skilled in the art will understand that its scope includes all possible combinations and uses of the specific features described herein.

[0017] The described system and method simultaneously process plastic waste streams and used caustic substances. More specifically, the system and method combine a plastic waste stream and a used caustic substance stream in a hydrothermal reaction, and this combination simultaneously processes both while enhancing each other's treatment in a mutually beneficial way. This system and method describe using a used caustic solution for the highly effective dechlorination of waste PVC by the hydrothermal method.

[0018] Advantageously, the described systems and methods can process two waste streams simultaneously, resulting in a product stream that can be recycled or released to the environment. Advantageously, each waste stream provides the materials necessary for the treatment by the other stream. The plastic waste stream provides a neutralizing agent, an acid, necessary to neutralize the spent caustic, and the spent caustic provides an alkaline compound that acts as an alkali catalyst to facilitate a nucleophilic substitution reaction. Advantageously, the systems and methods result in neutralization and dechlorination in a single process. Advantageously, the hydrothermal systems and methods enable the recycling of plastics and minimization of waste. Advantageously, the hydrothermal systems and methods enable the dechlorination of plastic waste and the neutralization of spent caustic in one reactor. Advantageously, this system and method can utilize spent caustic to enhance the hydrothermal dechlorination of PVC. Advantageously, this system and method can utilize PVC to neutralize the spent caustic.

[0019] As used throughout, "degrade" refers to the breakdown, by the breaking of carbon-carbon bonds, into smaller hydrocarbons containing fewer carbon atoms.

[0020] Referring to FIG. 1, one embodiment of a method and apparatus for removing halogen from a chemical is shown.

[0021] A feed plastic 10 containing plastic waste chips can be removed from a plastic waste storage container 2. The plastic waste storage container 2 can collect plastic waste chips and store them until needed in the process. The plastic waste chips are produced from plastic waste that has been discarded or sent for recycling. The plastic waste can be produced by grinding, crushing, shredding, cutting, or combinations thereof to mechanically produce the plastic waste chips. The plastic waste chips can be less than 5 millimeters (mm), or less than 1 mm, or 0.5 mm to 1.5 mm, or further 0.1 mm to 1 mm.

[0022] Plastic waste can include plastics such as polyvinyl chloride (PVC), halogenated plastics, thermoplastic polymers, and combinations thereof. Halogenated plastics can include chlorinated polyvinyl chloride (CPVC), polyvinylidene dichloride (PVDC), and combinations thereof. Thermoplastic polymers can include polyethylene, polystyrene, polypropylene, and combinations thereof. Advantageously and unexpectedly, the chlorine content of the feed plastic 10 must be at least 5 weight percent (wt%) in order to achieve the desired result. The chlorine content of PVC is 56 - 57 wt%. The plastic waste storage container 2 can contain at least 9 wt% PVC so as to have a chlorine content of 5 wt%. Alternatively, the plastic waste storage container 2 can contain at least 9 wt% PVC so as to have a chlorine content of 5 wt%.

[0023] The spent caustic stream 11 can be withdrawn from the spent caustic storage tank 3. The spent caustic stream 11 can include spent caustic generated from a sweetening process. The spent caustic stream 11 is an aqueous caustic solution. The spent caustic contains sodium hydroxide and water. Any sweetening process that can remove hydrogen sulfide and mercaptans from an alkaline aqueous solution and generate spent caustic as a by-product is suitable. In at least one embodiment, the sweetening process is a MEROX unit from LPG treatment, naphtha treatment, or kerosene treatment.

[0024] The used caustic solution in the used caustic substance storage tank 3 has a chemical oxygen demand of 1,000 mg / L to 100,000 mg / L, a total organic carbon of 500 mg / L to 10,000 mg / L, a sulfide content of 1,000 mg / L to 40,000 mg / L, an alkali metal content of 1.5 wt% to 8.5 wt%, and a pH of 11.5 to 13.9. The alkali metal in the used caustic substance can exist as dissolved alkali hydroxide. When the used caustic solution in the used caustic substance storage tank 3 has a pH less than 11.5, an alkali compound such as sodium hydroxide can be added to raise the pH. Maintaining the pH in the range of 11.5 to 13.9 plays a role in promoting the nucleophilic substitution by the hydroxide of chloride. The used caustic substance storage tank 3 can include a measuring instrument for measuring the parameters and conditions in the used caustic substance storage tank 3.

[0025] Supply plastic 10 and spent caustic stream 11 are mixed in supply mixer 4 to produce a mixed feed 12. Supply mixer 4 can be any type of mixing unit capable of mixing the two streams. Examples of supply mixer 4 include in-line mixers, T-fittings, Y-fittings, and combinations thereof introduced into hydrothermal reactor 1. The flow rates of supply plastic 10 and spent caustic stream 11 are adjusted to achieve the target pH of effluent 14 from hydrothermal reactor 1. The target pH of effluent 14 from hydrothermal reactor 1 is less than 6, or less than 5, or between 0 and 5, or further between 0 and 6. One skilled in the art will understand that the pH of the internal fluid changes dynamically throughout the neutralization reaction. Hydrothermal reactor 1 can include a pH meter internally to monitor the pH of the internal fluid. In at least one embodiment, a steady and consistent pH measurement can be one of the indicators for determining the end of the neutralization and dechlorination reactions. To control the pH in hydrothermal reactor 1, water supply 18 can be used. Water supply 18 can be a slip stream from demineralized water 13. Demineralized water 13 can be from any source of demineralized water. The demineralized water can have a conductivity of less than 5 microsiemens (μS) / centimeter (cm), or less than 2 μS / cm, or less than 1 μS / cm, or further between 1 μS / cm and 5 μS / cm. The flow rate of water supply 18 can be based on the target pH.

[0026] The hydrothermal reactor 1 can be selected from a batch reactor having an internal mixing device and a CSTR. The internal mixing device in the batch reactor in the hydrothermal reactor 1 can be an agitator. The residence time of the internal fluid in the hydrothermal reactor 1 can be determined by the pH of the internal fluid. The higher the pH of the spent caustic stream 11, the shorter the residence time, and vice versa. Dechlorination is not completely controlled by the reaction rate (kinetic characteristics) and also depends on the mass transfer of chlorides embedded in the bulk plastic. Since dechlorination depends on mass transfer in addition to the reaction rate, it is important to monitor the pH change for the residence time. The residence time of the internal fluid in the hydrothermal reactor 1 ranges from 0.2 hours to 5 hours, or alternatively from 0.5 hours to 2 hours. The pressure of the hydrothermal reactor 1 is controlled by the purge gas 16 and a pressure controller disposed in the process line carrying the gas product 20. The pressure in the hydrothermal reactor 1 is controlled to be higher than the saturation pressure of water at the temperature in the hydrothermal reactor 1 so that any water in the hydrothermal reactor 1 is in the liquid phase. Maintaining water in the liquid phase is necessary for dechlorination with the assistance of alkali because water vapor causes precipitation of alkali. Further, since the dielectric constant of water vapor is lower than that of liquid water, in the presence of water vapor, the alkali compound exists as a solid and does not dissolve. The purge gas 16 can be any inert gas for purging hydrogen sulfide (H2S), chlorine, methane, ethane, ethylene, and any other gas generated in the neutralization of the spent caustic with HCl. The inert gas can include nitrogen, helium, argon, and combinations thereof.

[0027] The temperature of the internal fluid in the hydrothermal reactor 1 can be in the range of 150°C to 350°C, or alternatively in the range of 200°C to 250°C. The temperature can be controlled by a heater associated with the reactor. Examples of the heater include an internal heater, a heater jacket, and combinations thereof. The neutralization reaction between sodium hydroxide and hydrogen chloride is an exothermic reaction, and a part of the thermal energy can be supplied to the hydrothermal reactor 1.

[0028] In the hydrothermal reactor 1, chlorine in PVC or halogenated plastics can be removed as hydrogen chloride (HCl) under hydrothermal conditions. Chloride is removed via a nucleophilic substitution reaction with water to form a polyol, or via an ionic chain reaction to form a polyene. The polyol can be converted to a polyene by an intramolecular dehydration reaction. At temperatures below 400 °C, the nucleophilic substitution reaction is dominant. Advantageously, the used caustic acts as an alkali source to enhance the dechlorination of PVC. Advantageously, the presence of sodium hydroxide from the used caustic acts as a catalyst to promote the nucleophilic substitution reaction of PVC and halogenated plastics. At temperatures above 400 °C, the ionic chain reaction is dominant. Therefore, under the reaction conditions in the hydrothermal reactor 1, the nucleophilic substitution reaction is dominant and the polyol is more abundant than the polyene. Then, together with the thermoplastic polymer, the polyol and polyene are converted to other compounds by decomposition reactions, condensation reactions, cross-linking reactions, and other reactions occurring in the hydrothermal reactor, or by subsequent treatments such as anaerobic pyrolysis at temperatures above 450 °C. The degree of decomposition reaction in the hydrothermal reactor 1 is limited due to the lower temperature. Advantageously, when chlorine is removed from PVC and halogenated plastics, the ability to convert the dechlorinated material to other compounds is accelerated. Chlorine released in the nucleophilic substitution reaction of PVC and halogenated plastics forms HCl. Then, HCl neutralizes the sodium hydroxide in the used caustic. Therefore, both the plastic waste chip and the used caustic contribute to the treatment of others in the hydrothermal reactor 1. The reaction between HCl and NaOH produces NaCl and water.

[0029] H2S and other gases are removed from the hydrothermal reactor 1 via the gas product 20. The gas product 20 is fed to a sulfur recovery unit or other sulfur capture unit to remove sulfur compounds from the exhaust gas in the gas product 20. The effluent 14 is transferred to the washing and dehydration unit 5. Demineralized water 13 is introduced into the washing and dehydration unit 5 together with the effluent 14. The effluent 14 contains a liquid phase material and a solid material. The liquid phase material contains water and hydrocarbons. The solid material contains dechlorinated plastics.

[0030] The washing and dewatering unit 5 can be any type of separation unit capable of separating liquid and solid substances. In the washing and dewatering unit 5, the liquid phase substance is separated from the solid substance. The water in the demineralized water 13 washes away residual chloride ions, sodium ions, and other water-soluble compounds. In the washing and dewatering unit 5, dewatering can be performed at a temperature below 100 °C and atmospheric pressure or a pressure below 100 psig. The dewatering unit can have an internal mixing device such as a stirrer.

[0031] The solid substance is removed from the washing and dewatering unit 5 as dechlorinated plastic waste 21. The dechlorinated plastic waste 21 can be subjected to a further treatment process including a recycling process. One specific recycling process is anaerobic pyrolysis.

[0032] The liquid phase substance is removed from the washing and dewatering unit 5 as neutralized wastewater 22. The neutralized wastewater 22 can be further treated. In at least one embodiment, the neutralized wastewater 22 is treated in a demineralization unit 6 and returned to the process as demineralized water 13. The demineralization unit 6 can be selected from reverse osmosis membranes and ion exchange. In any embodiment, the neutralized wastewater 22 can be discharged to a wastewater treatment unit via a discharge stream 23. In the wastewater treatment unit, the discharge stream 23 is treated and can be released to the environment or used in other processes.

[0033] The method for treating the spent caustic substance is carried out in the absence of Fe as a catalyst 2+ and in the absence of hydrogen peroxide. The method for treating plastic waste and spent caustic substance is carried out in the absence of biomass.

[0034] (Example)

[0035] The examples compared the treatment of used caustic substances with waste plastics in water. In Experiment 1, the feed plastics were treated in used caustic substances. The feed plastics were composed of 45 wt% PVC, 24 wt% polyethylene, 19 wt% polypropylene, and 12 wt% polystyrene. The plastic waste chips in the feed plastics 10 were cut to a size of 0.8 mm to 1.5 mm. The used caustic substances were produced from an LPG sweetening process with the characteristics shown in Table 1.

[0036]

Table 1

[0037] The hydrothermal reactor was an autoclave-type reactor with an internal volume of 1,000 mL equipped with an internal stirrer. An amount of 500 mL of used caustic substances and 71 g of the feed plastics by weight were added to the hydrothermal reactor. The hydrothermal reactor was purged with nitrogen, pressurized with nitrogen at 0.3 barg under stirring at 200 rpm, and then air was removed by discharging nitrogen to 0.05 barg under stirring at 200 rpm. This process was repeated three times. Then, the hydrothermal reactor was filled with nitrogen at 3 barg. The hydrothermal reactor was heated to 240 °C at a rate of 50 °C / 10 min and maintained at 240 °C for 0.5 h.

[0038] After the reaction, the hydrothermal reactor was cooled to 60 °C. The reaction effluent in the hydrothermal reactor was collected, filtered, and the weight of the solid product was measured. The weight of the solid product was 52.8 grams. The solid product was analyzed to determine the chlorine content, which was then used for the evaluation of dechlorination. The dechlorination was about 96%. The pH of the neutralized wastewater was measured to be about 5.7.

[0039] In Experiment 2, the feed plastics were treated in deionized water. The feed plastics, reactor fillings, operating conditions, and operating processes were the same as in Experiment 1. In Experiment 2, the dechlorination efficiency was 57%.

[0040] This example shows the beneficial effect of the dechlorination reaction in a hydrothermal reactor in the presence of caustic substances.

[0041] Although the present invention has been described in detail, it should be understood that various changes, substitutions, and modifications can be made without departing from the principles and scope of the present invention. Therefore, the scope of the present invention should be determined by the following claims and their appropriate legal equivalents.

[0042] Unless otherwise indicated, the various elements described can be used in combination with all other elements described herein.

[0043] The singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise.

[0044] "Optional" or "optionally" means that the subsequently described event or circumstance may or may not occur. The description includes both the case where the event or circumstance occurs and the case where it does not occur.

[0045] Ranges can be expressed herein as from approximately one particular value to approximately another particular value and, unless otherwise indicated, are inclusive. When such a range is expressed, it should be understood that another embodiment includes from one particular value to another particular value, along with all combinations within the range.

[0046] Throughout this application, when patents or publications are referenced, the disclosures of these references are hereby incorporated by reference in their entirety into this application to more fully describe the state of the art to which this invention pertains, unless these references are inconsistent with the description made herein.

[0047] As used in this specification and the appended claims, the terms "comprise", "has", and "include", and all grammatical variations thereof, are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps.

Claims

**Claim 1** A method for treating plastic waste and used caustic substances, the method comprising: mixing a feed plastic and a used caustic substance stream in a feed mixer to produce a mixed feed, wherein the feed plastic comprises plastic waste in the form of plastic waste chips, the plastic waste being selected from polyvinyl chloride (PVC), halogenated plastics, and combinations thereof, and the used caustic substance stream comprises sodium hydroxide; introducing the mixed feed into a hydrothermal reactor; reacting the mixed feed in the hydrothermal reactor to produce an effluent, wherein the residence time in the hydrothermal reactor ranges from 0.2 hours to 5 hours, the pressure in the hydrothermal reactor is higher than the saturation pressure of water at the temperature in the hydrothermal reactor such that the water in the hydrothermal reactor is in the liquid phase, the temperature in the hydrothermal reactor ranges from 150°C to 350°C, chlorine is removed from the plastic waste in the presence of the sodium hydroxide, and the chlorine reacts with the sodium hydroxide to produce sodium chloride and water; introducing the effluent into a washing and dewatering unit, wherein the effluent comprises a liquid phase substance and a solid phase substance, and the solid phase substance comprises dechlorinated plastic; and separating the liquid phase substance and the solid phase substance in the washing and dewatering unit to produce dechlorinated plastic waste and neutralized wastewater. **Claim 2** The method according to claim 1, wherein the plastic waste chips are less than 1 mm. **Claim 3** The method according to claim 1, wherein the halogenated plastic is selected from the group consisting of chlorinated polyvinyl chloride (CPVC), polyvinylidene dichloride (PVDC), and combinations thereof. **Claim 4** The method according to claim 1, wherein the plastic waste further comprises a thermoplastic polymer selected from the group consisting of polyethylene, polystyrene, polypropylene, and combinations thereof. **Claim 5** The method according to claim 1, wherein the flow rates of the feed plastic and the used caustic substance are adjusted in the hydrothermal reactor to achieve a target pH of 5-6. **Claim 6** The method according to claim 1, further comprising treating the neutralized wastewater in a desalination unit to produce desalinated water. **Claim 7** The method according to claim 1, further comprising a step of introducing demineralized water into the hydrothermal reactor, wherein the demineralized water can adjust the target pH in the hydrothermal reactor.

8. The method according to claim 1, wherein the chlorine content of the supplied plastic is at least 5% by weight.

9. The used caustic substance in the used caustic substance stream has a chemical oxygen demand of 1,000 mg / L to 100,000 mg / L, a total organic carbon of 500 mg / L to 10,000 mg / L, a sulfide content of 1,000 mg / L to 40,000 mg / L, an alkali metal content of 1.5 wt% to 8.5 wt%, and a pH of 11.5 to 13.

9. The method according to claim 1.

10. A system for treating plastic waste and used caustic substances, the system comprising: A plastic waste storage container configured to store the plastic waste in the form of plastic waste chips, wherein the plastic waste is selected from polyvinyl chloride (PVC), halogenated plastics, and combinations thereof. Plastic waste storage container; A used caustic substance storage tank configured to store the used caustic substance, wherein the used caustic substance contains sodium hydroxide. Used caustic substance storage tank; A supply mixer fluidly connected to the plastic waste storage container and the used caustic substance storage tank, the supply mixer being configured to mix a supply plastic and a used caustic substance stream to produce a mixed supply, wherein the supply plastic includes the plastic waste and the used caustic substance stream includes the used caustic substance. Supply mixer; A hydrothermal reactor, the hydrothermal reactor being fluidly connected to the supply mixer, the hydrothermal reactor being configured to react the mixed supply to produce an effluent, the residence time in the hydrothermal reactor being in the range of 0.2 hours to 5 hours, and the pressure in the hydrothermal reactor being greater than the saturation pressure of water at the temperature in the hydrothermal reactor such that the water in the hydrothermal reactor is in the liquid phase. The temperature in the hydrothermal reactor is in the range of 150°C to 350°C, and chlorine is removed from the plastic waste in the presence of sodium hydroxide. The chlorine reacts with sodium hydroxide to produce sodium chloride and water. Hydrothermal reactor; and A cleaning and dewatering unit fluidly connected to the hydrothermal reactor, wherein the cleaning and dewatering unit is configured to separate solid and liquid phase materials in the effluent to produce dechlorinated plastic waste and neutralized wastewater, and the solid material includes dechlorinated plastic; cleaning and dewatering unit; A system comprising. **Claim 11** The system according to claim 10, wherein the plastic waste chips are less than 1 mm. **Claim 12** The system according to claim 10, wherein the halogenated plastic is selected from the group consisting of chlorinated polyvinyl chloride (CPVC), polyvinylidene dichloride (PVDC), and combinations thereof. **Claim 13** The system according to claim 10, wherein the plastic waste further comprises a thermoplastic polymer selected from the group consisting of polyethylene, polystyrene, polypropylene, and combinations thereof. **Claim 14** The system according to claim 10, wherein the flow rates of the feed plastic and the spent caustic are adjusted to achieve a target pH of 5-6 in the hydrothermal reactor. **Claim 15** The system according to claim 10, further comprising a desalting unit fluidly connected to the cleaning and dewatering unit, wherein the desalting unit is configured to treat the neutralized wastewater to produce desalted water. **Claim 16** The system according to claim 10, wherein the chlorine content of the feed plastic is at least 5 wt%. **Claim 17** The spent caustic in the spent caustic stream has a chemical oxygen demand of 1,000 mg / L to 100,000 mg / L, a total organic carbon of 500 mg / L to 10,000 mg / L, a sulfide content of 1,000 mg / L to 40,000 mg / L, an alkali metal content of 1.5 wt% to 8.5 wt%, and a pH of 11.5 to 13.

9. The system according to claim 10. **Claim 18** The system according to claim 10, wherein the feed mixer is selected from an in-line mixer, a T-fitting, a Y-fitting, and combinations thereof. **Claim 19** The system according to claim 10, wherein the hydrothermal reactor is selected from a batch reactor having an internal mixing device and a CSTR.

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