Plastic processing using subcritical / supercritical fluids

Abstract

For example, a method is provided for recycling difficult-to-recycle plastics such as polyvinyl chloride (PVC) and / or mixed plastic streams. The plastic may be industrial water plastic or municipal waste mixed plastic in which PVC is present. In this disclosure, PVC is converted into an organic product, such as a controllable mixture of C2-C4 compounds, using a depolymerization process that utilizes a subcritical or supercritical fluid of an alcohol such as methanol.

Description

TECHNICAL FIELD

[0001] Cross-Reference to Related Applications This application claims the benefit of priority of U.S. Provisional Application No. 63 / 591,955, filed on October 20, 2023, the entire content of which is incorporated herein by reference.

[0002] The present disclosure relates to plastic recycling, and more specifically, to methods for recycling plastics that are difficult to recycle, such as polyvinyl chloride (PVC) and / or mixed plastic streams. BACKGROUND ART

[0003] In the United States, approximately 32 million tons of mixed plastic waste are sent to landfills annually. There are several reasons for this, mainly due to the lack of a robust market, the inability of recycled plastics to meet specifications suitable for product containment or storage, as well as consumer knowledge and behavior.

[0004] Supercritical fluids (SCF) have been used to recycle plastics that are difficult to recycle, such as PVC or PVC mixed with other plastics. In particular, SCF have been investigated for various applications related to polymer degradation and depolymerization. Depending on the polymer, solvent, and processing conditions, various depolymerization or degradation mechanisms are possible. Polymers with main chain linkages susceptible to solvolysis, such as polyesters, polyamides, and polycarbonates, are particularly suitable for depolymerization into well-defined products in SCF.

[0005] Depolymerization of polyesters using supercritical water (scH2O), supercritical methanol (scMeOH), and supercritical ethanol (scEtOH) has been reported. A pilot-scale process for the depolymerization of polyethylene terephthalate (PET) in bottles using scMeOH is one example.

[0006] While large-scale treatment of PET using scEtOH has not been reported, large-scale depolymerization of lignin using scEtOH has been investigated. Furthermore, SCF has been found to be effective in debrominating flame retardant additives in plastics.

[0007] For polyolefins and related polymers having an all-carbon backbone, treatment with SCF, especially with scH2O which has high oxidizing power, often results in a mixture of products. For example, polyethylene (PE) has been decomposed into a mixture of alkenes and alkenes by scH2O treatment. Poly(vinyl alcohol) (PVA) exposed to scH2O in the presence of sodium hydroxide decomposes into C2-C2. 18 It has been reported that mixtures of carboxylic acids are formed. When polystyrene (PS) is treated with scMeOH at temperatures (340-420°C) and pressures (10-30 MPa) well above the critical point (TC=240°C, PC=8.1 MPa), it has been found that a mixture of styrene, ethylbenzene, toluene, and other derivatives is obtained. It has also been found that when epoxy resins, phenolic resins, and polyether ether ketone (PEEK) resins are decomposed with scH2O and scMeOH, a mixture of products is obtained.

[0008] The situation involving PVC is more complex. While a significant portion of PVC-containing materials can be collected for mechanical recycling, the presence of PVC in mixed plastic waste streams can make processing difficult. Direct thermal decomposition of PVC forms a chlorinated oil mixture that needs to be dechlorinated before further use. PVC typically decomposes upon heating before depolymerization is observed, and therefore strategies involving catalytic dichlorination of PVC are more common.

[0009] Supercritical H2O treatment has been investigated to convert various plastics into oil mixtures intended for conversion into fuel. Currently, the use of SCF for handling PVC waste focuses on decomposition into gas / oil mixtures. However, these efforts have not focused on optimizing process selectivity to generate individual products from complex mixtures, enabling the conversion of plastics into chemical raw materials. Improving reaction selectivity by adding catalysts and / or sacrificial reagents has only been applied under superheated steam conditions, and no efforts have been made to translate this approach under SCF conditions. [Overview of the project] [Problems that the invention aims to solve]

[0010] This disclosure provides a method for recycling difficult-to-recycle plastics, such as PVC and / or mixed plastic streams. The plastics may be industrial water plastics or municipal waste mixed plastics in which PVC is present. In embodiments of this disclosure, a depolymerization process using a subcritical or supercritical fluid of alcohol is used to convert PVC into organic products, such as a controllable mixture of C2-C4 compounds. [Means for solving the problem]

[0011] The methods of this disclosure can be used to selectively depolymerize plastic materials such as PVC, which are difficult to depolymerize by thermal methods, in pure waste streams and model mixed waste streams containing other plastics. In some embodiments, depolymerization from PVC to useful products is achieved using scEtOH or a mixture of scEtOH and scH2O. A combination of synchrotron X-ray techniques (WAXS, SAXS, XPCS, and IXS) can be consistently used to explore the structure and dynamics of clusters and polymer chains in situ during depolymerization. These experimental results can be used to validate models for molecular dynamics (MD) simulations and chemical reaction rate simulations. In addition, this disclosure can be used to demonstrate correlations between cluster-forming structures, polymer structures, and reaction mechanisms of subcritical or supercritical fluids by simultaneously using synchrotron X-ray scattering and infrared (IR) experiments. By combining theoretical / computational and experimental methods, depolymerization based on subcritical or supercritical fluids can be optimized and novel pathways for scale-up can be developed.

[0012] One aspect of this disclosure provides a method for recycling plastics. The plastics may be industrial water plastics or municipal waste mixed plastics. In one embodiment, the method includes depolymerizing a waste plastic stream containing at least PVC in a reactor at a temperature below the decomposition temperature of PVC and in the presence of a subcritical or supercritical fluid, both containing alcohol. The depolymerization process converts the PVC into non-halogen-containing compounds, such as C2-C4 compounds, which can be recovered in and / or downstream of the reactor where the depolymerization process is carried out. In one embodiment, a supercritical fluid of alcohol is used. In another embodiment, a subcritical fluid of alcohol is used.

[0013] High-temperature treatment of PVC leads to dehydrochlorination and other undesirable reactions. By carrying out the depolymerization of PVC below its decomposition temperature, chlorides replace the polymer at the reactor temperature, enabling depolymerization.

[0014] In some embodiments of this disclosure, the alcohol has 1 to 8 carbon atoms. This includes methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, and / or octanol. The alcohol may be a linear alcohol or a branched alcohol. In some embodiments of this disclosure, the alcohol is ethanol. By using such an alcohol, preferably ethanol, PVC can be depolymerized into a controllable mixture of organic compounds.

[0015] In some embodiments of this disclosure, the depolymerization step is carried out in the presence of reactive clay, aluminosilicate (including zeolite), a base that neutralizes the formation of hydrochloric acid, or any combination thereof. The use of such additives can improve the selectivity of conversion to non-halogenated low molecular weight products.

[0016] In some embodiments of this disclosure, the subcritical or supercritical fluid further comprises water. Subcritical or supercritical fluids of alcohol and water can optimize the depolymerization process of waste plastic mixtures containing PVC.

[0017] In some embodiments of this disclosure, the subcritical or supercritical fluid has a ratio of water to alcohol of about 1:1 to about 10:1, more preferably about 1:1 to about 5:1, and even more preferably about 1:1 to 4:1.

[0018] Furthermore, in some embodiments of this disclosure in which water is present in a subcritical or supercritical fluid, the depolymerization step is carried out in the presence of reactive clay, aluminosilicate (including zeolite), a base that neutralizes the formation of hydrochloric acid, or any combination thereof. As described above, the use of such additives can improve the selectivity of conversion to non-halogenated low molecular weight products.

[0019] In some embodiments of this disclosure, the depolymerization step is carried out at a pressure of approximately 0.1 atmospheres to approximately 100 atmospheres.

[0020] In some embodiments of the present disclosure, the decomposition temperature is about 500°C or higher, and the depolymerization temperature is about 200°C to less than 500°C.

[0021] In some embodiments of the present disclosure, the depolymerization process of the present disclosure produces non-halogen-containing products having 2 to 12 carbon atoms, more preferably 2 to 8 carbon atoms, even more preferably 2 to 6 carbon atoms, and most preferably 2 to 4 carbon atoms. The non-halogen-containing products may include polymers.

[0022] In some embodiments of the present disclosure, the non-halogen-containing products comprise at least poly(ethyl vinyl ether).

[0023] In some embodiments of the present disclosure, the method further comprises the step of monitoring the depolymerizing step for polymer cluster formation. This allows an understanding of how the cluster structure and dynamics of subcritical or supercritical fluids affect the swelling and reactivity of PVC and other plastics, as well as mixtures thereof.

[0024] In some embodiments of the present disclosure, the monitoring step comprises at least one of synchrotron wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), X-ray photon correlation spectroscopy (XPCS), inelastic X-ray scattering (IXS), and molecular dynamics (MD) simulations.

[0025] In some embodiments of the present disclosure, the monitoring step may further comprise infrared (IR) spectroscopy.

[0026] In some embodiments of the present disclosure, the method further comprises the steps of separating and recovering the reaction products (e.g., the C2 to C4 compounds described above) formed from the step of depolymerizing the recyclable plastic.

[0027] In some embodiments of the present disclosure, the separating step comprises isothermal pressure drop after the step of depolymerizing the recyclable plastic, or comprises the step of adding a clay-based emulsion to the product formed by the depolymerizing step.

[0028] In some embodiments of the present disclosure, the recovering step comprises a fractionation process.

[0029] In another aspect of the present disclosure, a recycling plant is provided. In one embodiment, the recycling plant comprises a reactor configured to perform a depolymerization reaction at a temperature below the decomposition temperature of a recyclable plastic stream comprising at least PVC. In some embodiments, the depolymerization reaction is carried out in the presence of a subcritical or supercritical fluid comprising an alcohol.

[0030] The recycling plant can implement all the embodiments described above regarding the recycling process.

[0031] Although the depolymerization process of the present disclosure utilizes a subcritical or supercritical fluid comprising an alcohol, subcritical water can be utilized to achieve similar results.

[0032] These and other aspects of the present disclosure are described in more detail herein. [Brief Description of the Drawings]

[0033] [Figure 1] It is a graph showing size exclusion chromatography of PVC, the solid isolated after 1 hour in water, and the sample isolated after 1 hour in an ethanol (ETOH)-water mixture. [Figure 2A] Infrared spectroscopy after treatment at 250°C for 1 hour. [Figure 2B] Infrared spectroscopy after treatment at 250°C for 3 hours. [Figure 3A] 1H NMR analysis of soluble PVC after treatment at 250°C for 1 hour. [Figure 3B]This is a 1H NMR analysis of soluble PVC after treatment at 250°C for 3 hours. [Modes for carrying out the invention]

[0034] The Disclosure will be described in more detail here by reference to the following discussion and the drawings accompanying this Disclosure. The following description includes numerous specific details, such as particular structures, components, materials, dimensions, processes, and techniques, in order to understand the various embodiments of the Disclosure. However, it will be understood by those skilled in the art that the various embodiments of the Disclosure can be implemented without these specific details. Where used throughout this Disclosure, the term “about” generally means within ±10%, ±5%, ±2%, ±1%, or ±0.5% of a given number. Where a range from one number to another (e.g., 20 to 40) is given in this Disclosure, the Disclosure intends any number within that range (i.e., 22, 24, 26, 28.5, 31, 33.5, 35, 37.7, 39, or 40) or any quantity delimited by any two values ​​that may exist within that range (e.g., 28.5 to 35).

[0035] The terms used herein are for the sole purpose of describing specific embodiments and are not intended to limit the disclosure. Where used herein, the singular forms “a,” “an,” and “the” are intended to include the plural form unless otherwise clearly indicated by the context. Where used herein, the terms “include” and / or “include” identify the presence of a described feature, integer, process, operation, element, and / or component, but do not exclude the presence or addition of one or more other features, integers, processes, operations, elements, components, and / or groups thereof.

[0036] This disclosure addresses several aspects of plastics that are difficult to recycle, such as PVC, which have not been successfully recycled by conventional processes, and of mixed plastic streams. More specifically, this disclosure relates to the recycling of large volumes of general-purpose plastic streams using an expandable depolymerization process in an inexpensive supercritical solvent.

[0037] The expandable depolymerization process of this disclosure can be applied to difficult-to-treat plastics, such as pure PVC or PVC mixed with other plastics. One reason for selecting PVC for the supercritical depolymerization pathway is that approximately 80% of the 5 million tons of PVC waste generated annually ends up in landfills, and it is unsuitable for thermal decomposition (or any heat treatment) due to the generation of chlorinated byproducts. Currently, the use of supercritical fluids (SCFs) for handling PVC waste has focused on decomposition into gas / oil mixtures. However, these efforts have not focused on developing pathways that enable the conversion of plastics into chemical raw materials by generating individual products from complex mixtures.

[0038] This disclosure provides a novel method for depolymerizing plastic materials in subcritical or supercritical fluids, offering new processes, methods, and tools based on synchrotron X-ray technology and molecular dynamics (MD) simulations, and developing pathways for the separation of reaction products and the scaling up of processes.

[0039] This disclosure achieves conversion selectivity of recycled plastics by using advanced depolarization chemistry in subcritical or supercritical fluids to adjust temperature / pressure conditions and utilize reactive additives such as bases that neutralize the formation of clay and / or aluminosilicates (including zeolites) and / or hydrochloric acid. This disclosure also adds value to mixed polymers and depolymerized molecules by developing methods for fractionating polymers in and / or downstream of the reactor and recovering beneficial products by using clay-based emulsions. This disclosure can be used to control / optimize reaction rates by providing a comprehensive understanding of molecular-level structural and dynamic transformations in subcritical or supercritical fluid-based polymer recycling. This disclosure can also be used to provide a better understanding of flickering cluster processes to control subcritical or supercritical fluid-based reactions, and this disclosure can be used to develop a roadmap for the scalability of subcritical or supercritical fluid-based depolymerization.

[0040] One aspect of the present disclosure provides a method for recycling plastics. The method of the present disclosure can be used to recycle industrial water plastics or municipal waste mixed plastics. Exemplary plastics that can be recycled by the method of the present disclosure include, but are not limited to, polyethylene terephthalate (PET), high-density polyethylene (HDPE), polypropylene (PP), polyvinyl chloride (PVC), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), mixed or composite plastics, or any combination of these plastics (including all of them). Preferably, the method of the present disclosure can be used to recycle PVC, mixed or composite plastics, or a recycling stream of PVC and mixed or composite plastics.

[0041] Plastics containing PET include, for example, water bottles, plastic trays, soda bottles, and / or food containers. Plastics containing HDPE include, for example, shampoo bottles, detergent bottles, and / or milk cartons. Plastics containing PVC include, for example, pipes, toys, and / or window frames. Plastics containing LDPE include, for example, plastic bags, wraps, and / or squeeze bottles. Plastics containing PP include, for example, yogurt containers, cream containers, butter containers, and / or pre-cooked food trays. Plastics containing PS include, for example, cups, plates, egg cartons, and / or meat trays.

[0042] The methods of this disclosure are particularly preferred for recycling plastics that are difficult to recycle, such as PVC, mixed or composite plastics, or combinations thereof (i.e., plastic streams containing PVS and mixed or composite plastics). In the most preferred embodiment, the methods of this disclosure are used for recycling PVC.

[0043] In one embodiment, the method of the present disclosure includes the step of depolymerizing a waste plastic stream containing at least PVC in a reactor at a temperature below the decomposition temperature of PVC and in the presence of a subcritical or supercritical fluid containing alcohol. In another embodiment, the depolymerization may be carried out in the presence of subcritical water. In any of these embodiments, the depolymerization process of the present disclosure converts PVC into a non-halogen-containing compound, such as a C2-C4 compound (including a polymer), which can be recovered in and / or downstream of the reactor where the depolymerization process is carried out.

[0044] In this disclosure, high-temperature treatment of PVC is avoided because it leads to dehydrochlorination and other undesirable reactions. By carrying out the depolymerization of PVC below its decomposition temperature, chlorides replace the polymer at the reactor temperature, enabling depolymerization.

[0045] In preferred embodiments, the reaction treatment of PVC with ethanol under subcritical or supercritical conditions can be adjusted to allow partial / complete conversion to poly(ethyl vinyl ether) (PEVE) while minimizing the formation of polyacetylene units resulting from dehydrochlorination. Previous studies of scH2O decomposition of PVC have shown that at temperatures below 400°C, substitution to vinyl alcohol repeating units is preferred over elimination. Treatment of PVC with supercritical or subcritical EtOH, which can be performed at temperatures lower than scH2O (Tc(EtOH) = 241°C), can facilitate the nucleophilic reaction of ethanol with PVC to form ethyl vinyl ether (EVE) repeating units. Since poly(vinyl ether) can be depolymerized at readily available temperatures (below 320°C, although Ts of 130°C and 110°C have been reported in blends), this strategy allows for the depolymerization of PVC into non-halogenated molecules including EVE, acetaldehyde, acetic acid, and ethyl acetate, with the product varying depending on the degree of oxidation during treatment. Many common PVC additives (e.g., phthalates) can also be extracted and / or separated by treatment with these subcritical or supercritical fluids.

[0046] The reactors used in this disclosure include any reactor that can be used for plastic recycling and that can produce and / or maintain a subcritical or supercritical fluid environment. The reactors used include a chamber for containing the plastics to be recycled and at least one inlet port into which alcohol can be introduced. Other inlet ports may be present. At least one of the other inlet ports can be used to introduce water into the chamber. In some embodiments, a mixture of alcohol and water can be introduced through a single inlet port. The reactor may include a vacuum port for exhausting the chamber and for enabling the formation of a supercritical fluid (SCF) of alcohol or an SCF of a mixture of alcohol and water. The term “supercritical fluid or SCF” is used throughout this disclosure to refer to a substance at a given temperature and pressure that is above the critical point of the substance where different liquid and gas phases do not exist, but below the pressure required to compress the substance into a solid. For example, an SCF of methanol exists at a critical temperature of about 512°C, a critical pressure of about 79.9 atmospheres, and a critical density of 0.272. In another example, the subcritical fluid (SCF) of ethanol exists at a critical temperature of approximately 514°C, a critical pressure of approximately 60.6 atmospheres, and a critical density of 0.276. The term “subcritical fluid” is used throughout this disclosure to refer to a fluid / liquid below the critical point of a substance.

[0047] The reactor may also include means such as a conveyor belt for introducing plastics to be recycled into the reactor chamber, and means such as the same conveyor belt for removing the recycled, i.e., depolymerized material from the reactor chamber. The reactor further includes heating devices / elements that can be used to heat the chamber. Note that in some embodiments, the SFC is formed outside the reactor chamber, and the SCF is then introduced into the reactor chamber through one of the inlet ports. In such embodiments, the heating devices / elements and exhaust used for forming the SCF are carried out outside the reactor chamber. In such embodiments, the SCF may be formed within an SFC chamber connected to one of the reactor inlet ports.

[0048] In some embodiments of this disclosure, the alcohol used to provide a subcritical or supercritical fluid has 1 to 8 carbon atoms. Therefore, suitable alcohols that may be used in this disclosure include ethanol, methanol, propanol, butanol, pentanol, hexanol, heptanol, and / or octanol. The alcohol may be a straight-chain alcohol or a branched alcohol. The alcohol used in this disclosure is preferably an unsubstituted alcohol. In some embodiments, a single alcohol is used. In other embodiments, a mixture of at least two different C1-C8 alcohols may be used. In a preferred embodiment of this disclosure, the alcohol is ethanol, and therefore, a subcritical or supercritical fluid of ethanol is used in the recycling process of this disclosure.

[0049] In some embodiments of this disclosure, the alcohol-containing subcritical or supercritical fluid further contains water; that is, a mixture of alcohol and water can be used. In one preferred embodiment, the subcritical or supercritical fluid is a mixture of ethanol and water. In embodiments in which water is used, the subcritical or supercritical fluid has a water-to-alcohol ratio of about 1:1 to about 10:1, more preferably about 1:1 to about 5:1, and even more preferably about 1:1 to 4:1. Subcritical or supercritical fluids of alcohol and water can optimize the depolymerization process of waste plastic mixtures containing PVC.

[0050] Furthermore, in some embodiments of this disclosure, the depolymerization step is carried out in the presence of reactive clay, aluminosilicate (including zeolite), a base that neutralizes the formation of hydrochloric acid, or any combination thereof. Zeolite is a microporous crystalline aluminosilicate material commonly used as a commercially available adsorbent and catalyst. These “additive” materials can be added to the reactor chamber at any time during the depolymerization process. Typically, these additive materials are added to the reactor chamber before the start of the depolymerization process.

[0051] Reactive clay can be used to enhance the conversion of PVC facilitated by subcritical or supercritical fluids. Reactive clay can be used as a bulk material or on a metal support. Reactive clay is susceptible to large volume changes (swelling and shrinking) directly related to its water content. An illustrative example of a reactive clay that can be used in this disclosure is bentonite, but is not limited to this example. Reactive clay can be used in amounts effective in improving the selectivity of the conversion described above. For example, reactive clay can be used in amounts ranging from about 0.5 mass percent to about 25.0 mass percent, and other amounts less or more than the above range may be used.

[0052] When aluminosilicates (including zeolites) are used, they can be used alone or typically in combination with one of the bases listed below, such as CaO. Aluminosilicates (including zeolites) can be used in amounts ranging from about 0.5 mass percent to about 25.0 mass percent, and other amounts less or more than the above range may also be used. Aluminosilicates (including zeolites) can be used to increase the yield of low molecular weight products in subcritical or supercritical fluids while simultaneously facilitating the separation of low molecular weight products downstream. Aluminosilicates (including zeolites) can be used in bulk or on a metal support.

[0053] Bases that neutralize the formation of hydrochloric acid (HCl) may include oxides such as calcium oxide, aluminum oxide, silicon oxide, iron oxide, and / or magnesium oxide. In one example, the base may be used in amounts ranging from about 0.5 mass percent to about 25.0 mass percent, and other amounts less or more than the above range may be used. Using oxide bases can result in a high degree of conversion from chlorine to inorganic chloride while addressing any toxicity problems of chlorinated organic matter.

[0054] In some embodiments of this disclosure, the depolymerization step is carried out at a pressure of about 0.1 atmospheres to about 100 atmospheres, more preferably at a pressure of about 0.5 atmospheres to about 75 atmospheres, and more preferably at a pressure of about 1 atmosphere to about 50 atmospheres.

[0055] In some embodiments of this disclosure, the decomposition temperature of the plastic, i.e., PVC, is about 500°C or higher, more preferably about 525°C to 1200°C, and even more preferably about 550°C to 1000°C. Therefore, the depolymerization process is carried out at a temperature lower than the decomposition temperature of PVC. Typically, depolymerization is carried out at a temperature of about 200°C to less than 500°C (i.e., the depolymerization temperature), and even more preferably about 250°C to 450°C. Depolymerization is typically carried out for about 1 hour to about 5 hours, but shorter or longer depolymerization times may be used.

[0056] In some embodiments of this disclosure, a depolymerization step produces a non-halogen-containing product (compound or polymer) having 2 to 12, more preferably 2 to 8, even more preferably 2 to 6, and most preferably 2 to 4 carbon atoms. In some embodiments of this disclosure, the non-halogen-containing product includes at least poly(ethyl vinyl ether).

[0057] In some embodiments of this disclosure, the method further includes a step of monitoring the depolymerization process for polymer cluster formation. In some embodiments, the monitoring step includes at least one of synchrotron wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), X-ray photon correlation spectroscopy (XPCS), inelastic X-ray scattering (IXS), and molecular dynamics (MD) simulations. In some embodiments, a combination of these monitoring techniques may be used. These monitoring techniques can be used to determine the structure of the clusters that may be formed and the dynamics of their cluster formation. The process by which clusters gradually shrink in subcritical and supercritical fluids can significantly affect the depolymerization of PVC and other plastics. Therefore, the above monitoring techniques can be used to establish comprehensive correlations between structure, dynamics, and other reaction products, which can be crucial for maximizing the selectivity of C2-C4 products.

[0058] WAXS, SAXS, XPCS, and IXS include apparatus well known to those skilled in the art. These apparatus can be used in conjunction with the reactor in which depolymerization is performed and can be used to perform measurements at any time during the depolymerization process. The monitoring process can be performed continuously throughout the entire depolymerization process or at selected times during the depolymerization process. A baseline can be established by performing measurements before the depolymerization process is carried out.

[0059] In some embodiments of this disclosure, the monitoring step may further include infrared (IR) spectroscopy. In such embodiments, IR can be used in conjunction with at least one of WAXS, SAXS, XPCS, IXS, and MD simulations. IR monitoring can be obtained using any known IR device. IR can communicate with the reactor chamber and can be used to perform measurements at any time during the depolymerization process. IR monitoring can be performed continuously throughout the entire depolymerization process or at selected times during the depolymerization process. A baseline can be established by performing IR measurements before carrying out the depolymerization process.

[0060] In embodiments of this disclosure, a combination of IXS and IR can be used in situ to demonstrate the correlation between the cluster-forming structure of polymer chains and the potential reaction mechanism.

[0061] In embodiments of this disclosure, a fundamental understanding of the cluster-forming structures of solvent / solvent and solvent / solute can be correlated with the reaction mechanism by a combination of in-situ IR and WAXS / SAXS technologies.

[0062] In embodiments of this disclosure, fine-tuning of the reaction selectivity to low molecular weight dechlorination products can be achieved by demonstrating the correlation of time-resolved in-situ X-ray and IR results in the subcritical and supercritical fluids of this disclosure.

[0063] In some embodiments of this disclosure, the method further includes the steps of separating and recovering reaction products formed from the step of depolymerizing a recyclable plastic. In some embodiments, the separation step can be carried out within the reactor chamber. In other embodiments, the separation of reaction products is typically carried out downstream of the chamber in which depolymerization occurs. In such embodiments, the reaction products can be transferred downstream to a separation zone including one or more separation devices.

[0064] In some embodiments of the present disclosure, the separation step includes an isothermal pressure drop following the depolymerization of recyclable plastics in subcritical and supercritical fluids. In some embodiments of the present disclosure, the separation step includes adding a clay-based emulsion to the product formed by the depolymerization step. The clay emulsion can be formed by adding a previously defined reactive clay to any well known emulsifier or mixture of emulsifiers. In such embodiments, a continuous product bank can be formed by lowering the interfacial viscosity of the emulsion, and by forming this bank early in the flow process, the overall efficiency of the recovery process can be maximized.

[0065] In some embodiments of this disclosure, the recovery process includes a fractionation process. Fractionation is a separation process that divides a certain amount of a mixture (gas, solid, liquid, etc.) into several smaller amounts (fractions) whose composition changes according to a gradient during a phase transition. The fractions are collected based on differences in specific properties of the individual components. A common characteristic of the fractions is that it is necessary to find an optimal value between the amount of fraction collected and the desired purity of each fraction. Fractionation makes it possible to isolate more than two components from a mixture in a single operation. This characteristic distinguishes it from other separation techniques.

[0066] In another aspect of the present disclosure, a recycling plant is provided. In one embodiment, the recycling plant includes a reactor configured to carry out a depolymerization reaction at a temperature below the decomposition temperature of a recyclable plastic stream containing at least PVC. In some embodiments, the depolymerization reaction is carried out in the presence of a subcritical or supercritical fluid containing alcohol. That is, the depolymerization can be carried out using a subcritical or supercritical fluid containing alcohol.

[0067] A recycling plant can implement any of the above-described embodiments of the recycling process.

[0068] The reactor used for the depolymerization reaction can be used in conjunction with other types of recycling reactors present in plastic recycling plants and can be used in combination with one or more separation / product recovery devices.

[0069] All plastics destined for recycling can be treated, i.e., washed, before being introduced into the reactor. The recycling plant may include one or more recovery devices / units located downstream of the reactor where recycling takes place.

[0070] As described above, the depolymerization reaction of this disclosure can also be carried out in the presence of subcritical water. [Examples]

[0071] To initiate a low-temperature depolymerization strategy while minimizing the flammability of the solvent mixture, depolymerization experiments were performed in aqueous solution under subcritical conditions (P=3~4 MPa). Commercially available PVC was mixed with water or a 4:1 v / v water / ethanol mixture (0.2 g / mL) and heated at 250°C for 1 hour. It has been previously shown that treatment of PVC with scH2O results in the formation of aromatic decomposition products between 300 and 370°C in 1 hour, leading to complete dechlorination. Size exclusion chromatography (SEC) of the remaining solid after treatment showed a slight decrease in molecular weight (see, for example, Figure 1), but at higher temperatures and pressures, the effects of the aforementioned short-range and long-range clusters become more pronounced, and more effective depolymerization is expected.

[0072] Investigating these conditions will yield new insights into molecular dynamics and reaction pathways. In addition to a 4:1 v / v water / ethanol mixture, the ethanol decomposition of PVC was compared in pure ethanol under subcritical (240°C) and supercritical (250°C) conditions. After heating, the supernatant of each sample was analyzed.

[0073] Ion chromatography of samples heated for 3 hours showed that the chloride concentration in samples treated under supercritical conditions was six times higher than that of samples treated at subcritical temperatures.

[0074] In infrared spectroscopy (see, for example, Figures 2A-2B), a peak (612 cm) resulting from C-Cl extension was observed after 3 hours. -1 A significant decrease in (see Figure 2B), and the appearance of soluble alkene products at both 1 hour (see Figure 2A) and 3 hours (see Figure 2B) (1650 cm³). -1 , 3015cm -1 ) is shown.

[0075] ¹H NMR analysis of the supernatant revealed peaks corresponding to soluble PVC after 1 hour at 250°C (see, e.g., Figure 3A), and these peaks were significantly reduced after 3 hours (see, e.g., Figure 3B). These results provide evidence that supercritical ethanol decomposition can be used to decompose PVC.

[0076] While this disclosure has been described and illustrated in particular in relation to preferred embodiments, it will be understood by those skilled in the art that the form and details described and other modifications can be made without departing from the spirit and scope of this disclosure. Therefore, this disclosure is not limited to the form and details described and illustrated, but is intended to be included in the appended claims.

Claims

1. A method for recycling plastic, A method comprising the step of depolymerizing a waste plastic stream containing at least polyvinyl chloride (PVC) in a reactor at a temperature below the decomposition temperature of PVC and in the presence of a subcritical or supercritical fluid containing alcohol.

2. The method according to claim 1, wherein the alcohol has 1 to 8 carbon atoms.

3. The method according to claim 2, wherein the alcohol is ethanol.

4. The method according to claim 1, wherein the depolymerization step is carried out in the presence of a reactive clay, an aluminosilicate, a base that neutralizes the formation of hydrochloric acid, or any combination thereof.

5. The method according to claim 1, wherein the subcritical or supercritical fluid further comprises water.

6. The method according to claim 5, wherein the subcritical or supercritical fluid comprises ethanol and water.

7. The method according to claim 5, wherein the subcritical or supercritical fluid has a water-to-alcohol ratio of about 1:1 to about 10:

1.

8. The method according to claim 5, wherein the depolymerization step is carried out in the presence of a reactive clay, an aluminosilicate, a base that neutralizes the formation of hydrochloric acid, or any combination thereof.

9. The method according to claim 1, wherein the depolymerization step is carried out at a pressure of approximately 0.1 atmospheres to approximately 100 atmospheres.

10. The method according to claim 1, wherein the decomposition temperature is about 500°C or higher, and the depolymerization temperature is about 200°C to less than 500°C.

11. The method according to claim 1, wherein the depolymerization step produces a non-halogen-containing product having 2 to 12 carbon atoms.

12. The method according to claim 1, wherein the depolymerization step produces a non-halogen-containing product having 2 to 4 carbon atoms.

13. The method according to claim 11, wherein the non-halogen-containing product comprises at least poly(ethyl vinyl ether).

14. The method according to claim 1, further comprising the step of monitoring the depolymerization step with respect to polymer cluster formation.

15. The method according to claim 14, wherein the monitoring step includes at least one of synchrotron wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), X-ray photon correlation spectroscopy (XPCS), inelastic X-ray scattering (IXS), and molecular dynamics (MD) simulation.

16. The method according to claim 15, wherein the monitoring step further includes infrared (IR) spectroscopy.

17. The method according to claim 1, further comprising the steps of separating and recovering the reaction product formed from the depolymerization step.

18. The method according to claim 17, wherein the separation step includes an isothermal pressure drop after the depolymerization step, or a step of adding a clay-based emulsion to the product formed by the depolymerization step.

19. The method according to claim 17, wherein the recovery step includes a fractionation step.

20. A recycling plant comprising a reactor configured to carry out a depolymerization reaction at a temperature below the decomposition temperature of a recyclable plastic stream containing at least PVC, wherein the depolymerization reaction is carried out in the presence of a subcritical or supercritical fluid containing alcohol.

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