PVC Dechlorination for Facilitating Plastic Chemical Recycling: A Systematic Literature Review of Technical Advances, Modeling and Assessment

Abstract

Polyvinyl chloride (PVC) resins are widely used in modern society due to their acid and alkali resistance, low cost, and strong insulation properties. However, the high chlorine (Cl) content in PVC poses significant challenges for its recycling. This study reviews the treatment processes, model construction, and economic and environmental assessments to construct a methodological framework for the sustainable development of emerging dechlorination technologies. In terms of treatment processes, this study summarizes three types of processes, pretreatment, simultaneous dechlorination during chemical recycling, product purification, and emphasizes the necessity of dechlorination treatment from a systematic perspective. Additionally, the construction of models for dechlorination processes is investigated from the laboratory to the industrial production system to macro-scale material, in order to evaluate the potential inventory data and material metabolism behaviors. This review also summarized the methodology framework of Techno-Economic Analysis (TEA) and Life Cycle Assessment (LCA), which can be applied for evaluation of the economic and environmental performance of the dechlorination processes. Overall, this review provides readers with a comprehensive perspective on the state-of-the-art for PVC dechlorination technologies, meanwhile offering sustainable guidance for future research and industrial applications of chemical recycling of PVC waste.

1. Introduction

Plastic facilitates the modern daily life of human beings in various aspects. Since the mid-20th century, global plastic production has reached 9.2 billion tons, with 6.9 billion tons discarded as waste, of which 11% has been incinerated and 8% recycled [1,2]. The production and disposal of plastics result in significant environmental impacts. The production of plastic relies on non-renewable resources, such as petroleum, consuming large amounts of fossil fuels and exacerbating resource scarcity [3]. Improperly disposed of plastic waste enters the environment and is difficult to degrade under natural conditions, impacting the environment and human health [4]. To end plastic pollution and mitigate environmental impact, plastic recycling methods, such as mechanical recycling and chemical recycling, are employed [5,6]. Mechanical recycling involves processing plastic waste into secondary materials, but it is often less effective, requiring complex sorting and classification processes that increase treatment costs and difficulty [7]. Chemical recycling refers to converting polymer wastes into chemical feedstocks. Compared to mechanical recycling, chemical recycling can handle a broader range of plastics and has higher recycling efficiency [8]. In the chemical recycling process, the chlorine (Cl) element is an undesired component [4], limiting the downstream utilization of recovered feedstocks.

Polyvinyl chloride (PVC), as the third most commonly produced plastic in the world, is widely used in industries such as manufacturing, production, and transportation due to its high chemical stability, corrosion resistance, and UV resistance [9,10]. However, PVC contains a high Cl content, and the environmental issues arising from untreated PVC have garnered significant attention [11,12]. During the thermal treatment of PVC, the Cl element poses several adverse effects on facilities (e.g., corrosion and slag formation) and the environment (e.g., the formation of toxic chlorinated compounds, such as dioxins) [13,14]. To address the aforementioned issues, dechlorination processes are vital to address the potential production safety and environmental risks. The Cl control processes not only can valorize PVC waste, but the Cl issues in mixed plastics can also be resolved, facilitating the promotion and application of chemical recycling of plastic waste.

From a systems perspective, the dechlorination of PVC waste can be categorized into pretreatment, simultaneous dechlorination during chemical recycling, and product purification. The pretreatment dechlorination process refers to the removal of Cl from PVC before chemical recycling, and is of great significance for optimizing raw material quality and protecting equipment [15]. Pretreatment helps to reduce the Cl content in the polymer and bring down the environmental risks, avoiding the potential formation of undesired Cl compounds from the sources [16]. Simultaneous dechlorination during chemical recycling involves breaking down the Cl group and absorbing it into metal chlorides by using metal oxides as dechlorinating agents [17]. Although the initial two stages of dechlorination can significantly reduce chlorine content, the final dechlorinated product may still contain trace amounts of chlorine [18]. The recovered feedstocks, such as pyrolysis oil, can undergo additional purification steps for removing Cl compounds. These processes can be integrated for deep dechlorination, valorizing PVC waste and addressing Cl-related issues in mixed plastics.

Currently, most PVC dechlorination research is conducted on a laboratory scale rather than on an industrial scale. The reason why the dechlorination of PVC waste remains at the laboratory level is not only due to the energy input and consumption required but also the key limiting factor of economic cost [19]. Although dechlorination technologies hold promise for the high-value chemical recycling of waste PVC, they require significant energy and chemical inputs. This creates substantial uncertainty regarding their economic feasibility and environmental benefits at an industrial scale. Therefore, it is crucial to conduct Life Cycle Assessment (LCA), Techno-Economic Analysis (TEA), and Material Flow Analysis (MFA) to evaluate these emerging technologies comprehensively. Conducting LCA and TEA separately can lead to inconsistencies in functional units, system boundaries, and assumptions, making integrated analysis essential for accurate assessment. Therefore, it is essential to integrate TEA–LCA, which can reduce inconsistencies, enhancing the reliability of the evaluation results [20]. Additionally, conducting MFA for emerging technologies is crucial as it provides a comprehensive perspective on material metabolism, aids in policy-making, and fosters interdisciplinary collaboration in accelerating its application [21].

For technologies that have not yet been applied industrially, conducting LCA and TEA can be challenging due to the lack of available data. This data scarcity makes it necessary to estimate the input–output data of production systems using physical, chemical, and engineering principles. To this end, robust process modeling is crucial for evaluating dechlorination processes. By thoroughly analyzing literature related to multi-scale models, we can better understand the influence mechanisms of various variables in the recycling process of PVC waste. Mathematical models, as key analytical tools, allow for the prediction and optimization of reaction behaviors under different process designs and operating conditions. This approach, ultimately, reasonably quantifies the energy consumption, material input and product yield for realizing the assessment of economic viability and environmental footprints.

Overall, this paper reviews the progress in dechlorination technology for PVC waste, multi-scale modeling, and assessments for economic and environmental performance. By summarizing the development of pretreatment, simultaneous dechlorination during chemical recycling and product purification, it offers a more systematic perspective on addressing the Cl issues in plastic recycling. Additionally, we summarized a multi-scale approach for modeling the complex inventory data and potential material metabolism for Cl embodied in PVC waste. To identify the economic and environmental benefits of dechlorination technology, economic and environmental analyses are conducted to evaluate the future value and potential of dechlorination technology. This text provides references and suggestions for enterprises and policymakers.

2. Summary of Literature Review

The premise ‘summary of literature review’ requires a clear understanding of search principles and Boolean operators. The search principles are based on the topics of interest and keywords to enhance the relevance and accuracy of the search results. By using different Boolean operators, the search scope can be further expanded to cover more comprehensive literature related to the dechlorination of PVC waste. The specific search methods are as follows: to collect relevant literature on PVC waste treatment, we utilized the Web of Science search engine, employing keyword combinations with logical operators “AND”, “OR”, and “NOT”. The search focused on keywords such as “PVC” OR “polyvinyl chloride” OR “poly (vinyl chloride)” AND “dechlorination” OR “Dechlorination” to reveal the development trends in PVC dechlorination research. After the retrieval, Origin 2018 and Excel (version 2407 Build 16.0.17830.20166) software were used for data analysis and plotting.

The literature count related to PVC dechlorination from 2006 to 2023 is shown in Figure 1a. The initial search yielded 24,948 research articles published during this period, which were chosen to encapsulate the majority of relevant publications. The number of articles published in 2023 shows an 82.13% increase compared to the rapid development phase in 2010. Over the years, research has consistently involved four main fields: Engineering, Environmental Science, Ecology, Chemistry and Materials Science.

A further statistical analysis of keywords from literature published in 2013 and 2023 highlights the changes and trends over the decade, and the results are presented in Figure 1b and Figure 1c, respectively. The analysis compared the frequency and occurrence of keywords between these years. The focus of published articles has shifted from reductive dechlorination of PVC in 2013 to mechanisms for removing PVC waste pollution in 2023.

The number of keywords occurring more than four times increased from 348 in 2013 to 406 in 2023. Keywords such as ‘Degradation’, ‘Performance’, ‘Mechanical property’, and ‘Water’ remain among the top ten in frequency. However, terms like ‘Model’, ‘Mechanism’, ‘Nanoparticle’, ‘Removal’, ‘Thermal degradation’, ‘Nanocomposite’, and ‘Surface’ have seen significant increases in their frequency from 2013 to 2023. Specifically, the rise in terms related to micro and nano components, such as ‘Nanoparticle’ and ‘Nanocomposite’, indicates a growing focus on small composite pollution and its removal in PVC dechlorination research. Additionally, thermal treatment has emerged as a developing area, suggesting it may become a future hotspot in PVC dechlorination studies.

This shift reflects a broader trend from resource recycling towards addressing plastic pollution, highlighting the evolving priorities in the field.

3. Methodology

To construct a methodological framework for the sustainable development of emerging technologies, this paper reviews technical advances, modeling, and assessments related to the dechlorination of PVC waste. As illustrated in Figure 2, various dechlorination processes for PVC waste are categorized into pretreatment, simultaneous dechlorination during chemical recycling, and product purification. To estimate the inventory data for process evaluation, we summarized a comprehensive approach that spans from micro-model construction to MFA. Additionally, the paper proposes strategies for conducting TEA and LCA based on the latest advancements in industrial ecology. By evaluating the environmental and economic performance of dechlorination technologies, this study facilitates the sustainable management and recycling of PVC waste, ultimately promoting the chemical recycling of waste plastic.

4. Recent Advances of Dechlorination Technology Development

As described in the introduction, the dechlorination processes for PVC waste can be categorized into pretreatment, simultaneous dechlorination during chemical recycling, and product purification. This section provides an introduction to and analysis of the latest advancements in dechlorination technology over recent years. It covers the mechanisms, processes, and influencing factors of different dechlorination methods, highlighting their development trends and differences. The cited references are from recently published articles, reflecting the latest advancements in dechlorination technology and demonstrating strong technical representativeness and timeliness.

4.1. Pretreatment

Pretreatment refers to the removal of Cl from PVC under low-temperature conditions (200–300 °C). This dechlorination can be achieved through methods such as pyrolysis, solvothermal treatment, and the use of adsorbents.

Wang et al. [22] proposed a pretreatment method for PVC under low-temperature oxidation conditions using pyrolysis to eliminate corrosive and toxic chlorides. The experiment utilized simultaneous thermogravimetric–differential scanning calorimetry (TG–DSC) and mass spectrometry (MS) for comparative analysis under different conditions of inert gas, air, and oxygen. By constructing a symmetrical molecular model of PVC containing five repeating units and one methyl group, the study demonstrated that dechlorination occurs through the breaking of chemical bonds between atoms during pyrolysis. Due to the differences in bond dissociation energies, the C–Cl bond, which has the lowest dissociation energy, breaks first. The pyrolysis results indicated that, within the low-temperature range (200–350 °C), oxygen significantly promoted the release of HCl and caused exothermic reactions. Under optimal conditions of 275 °C with air as the oxidant, 97.26% of Cl could be removed. Overall, pyrolysis under oxygenated conditions effectively removes Cl from PVC at low temperatures (200–300 °C).

Li et al. [23] employed a solvothermal dechlorination method for PVC materials using ethylene glycol (EG) as the solvent. The study explored the effects of different alkaline additives (NaOH, KOH, K₂CO₃), temperature, and reaction time on dechlorination efficiency. The solvothermal dechlorination method primarily involves elimination and substitution reactions. In the elimination reaction, Cl is removed through a chain reaction, forming carbon–carbon double bonds in the original structure. In the substitution reaction, EG reacted with the alkaline additives to generate glycolate ions and OH⁻. The glycolate ions substituted Cl in PVC, forming C–O–C bonds, while OH⁻ participated in two substitution reactions: one where OH⁻ directly replaced Cl⁻, and another where two OH⁻ ions replaced a carbon chain segment, generating HCl and removing a molecule of H₂O. The results showed that different alkaline additives affect the solvothermal dechlorination, with the dechlorination activity ranking as follows: K₂CO₃ > KOH > NaOH. Using K₂CO₃ as the alkaline additive, a dechlorination rate of 97.51% was achieved under conditions of 180 °C, a K₂CO₃/EG solution ratio of 0.5, and a reaction time of 240 min.

Hapipi et al. [24] studied the effects of different catalysts on PVC dechlorination under low-temperature conditions using superheated steam. The results indicated that single metal oxide catalysts (such as TiO₂, MgO, and CoO) exhibited better dechlorination capabilities compared to β-zeolite and NaOH, with CoO showing the best performance. Adding more than 25 wt% ZnO to CoO improved the dechlorination efficiency, suggesting that mixed catalysts are more effective than single catalysts. Additionally, adding 25 wt%/50 wt% ZnO to CoO reduced the pyrolysis temperature from 523 K to 473 K. Therefore, incorporating catalysts in the dechlorination process not only enhances dechlorination efficiency but also lowers the pyrolysis reaction temperature, further reducing organic volatilization and energy consumption.

However, real PVC waste typically exists as large solid particles that are difficult to crush into powder due to their thermoplastic properties. In order to solve the problem, Lu et al. [25] proposed a mechanochemical approach using an up-scaled ball mill reactor in a NaOH/EG solution for dechlorination, as shown in Figure 3. The efficiency of dechlorinating real PVC waste is primarily influenced by the reaction interface, with ball milling enhancing the reaction rate and increasing the reactive surface area. This process can achieve up to 99% dechlorination for sealing strips and 92% for cable coverings. The proposed dechlorination mechanism involves initial reactions of PVC surfaces with NaOH through substitution and elimination reactions. After initial dechlorination, the surfaces become rigid and can be effectively crushed by ball milling to expose new unreacted surfaces, continuously enhancing the dechlorination reaction. The overall reaction rate is determined by a balance between chemical factors (e.g., NaOH concentration) and mechanical factors (e.g., ball size, milling speed). While higher NaOH concentrations increase the reaction rate, they also raise the solution’s viscosity, which can negatively impact the milling process.

For the pretreatment of PVC waste, this section summarizes techniques such as pyrolysis, catalytic dechlorination, solvothermal dechlorination, and mechanochemical dechlorination. These methods achieve dechlorination of waste PVC through physical and chemical reactions. Compared to other pretreatment methods, pyrolysis has a lower dechlorination efficiency and typically requires the addition of catalysts to improve efficiency. Solvothermal dechlorination, on the other hand, achieves dechlorination under low-temperature hydrothermal conditions through elimination and substitution reactions, offering high dechlorination efficiency and fewer by-products. The mechanochemical method combines physical and chemical reactions, effectively altering the particle reaction interface and utilizing a sodium hydroxide/EG solution to achieve efficient dechlorination.

4.2. Simultaneous Dechlorination during Chemical Recycling

Simultaneous dechlorination during chemical recycling refers to the removal Cl elements from PVC through chemical reactions. Methods for chemically recycling PVC include pyrolysis, electrolysis, etc.

The significant difference between simultaneous dechlorination during chemical recycling and pretreatment is that the former decomposes PVC plastic into small molecular compounds, such as hydrogen chloride and hydrocarbons, under high-temperature conditions. Cao et al. [26] studied the dechlorination characteristics of PVC during pyrolysis by adjusting the PVC content (0–12%) in mixed plastics and the heating rate (10–60 °C/min) to explore the effects of these two key experimental factors on pyrolysis. The results indicated that the proportion of PVC in the mixed plastics did not affect the shape of the HCl emission curve, but the curve shifted to the right as the heating rate increased. Additionally, the study explored the mutual influence of plastics during co-pyrolysis of mixed plastics (PVC, PP, PE), revealing that PVC inhibits the pyrolysis of PE and PP, while the presence of PP and PE has little effect on the pyrolysis of PVC.

The additives, like catalysts and absorbents, input into the pyrolysis process for halogen contained plastics significantly enhance the dechlorination process during PVC pyrolysis. Ma et al. [27] presented recent advancements in halogen–plastic pyrolysis for resource utilization and the potential pathways from “reducing to recycling to upcycling” halogens. The paper showed that, during pyrolysis, Ca(OH)₂ transitioning to CaO achieves over 96% debromination efficiency by removing bromine through charge transfer, while copper and iron oxides effectively convert organic bromides into inorganic forms. However, the strong alkalinity of Na and K additives limits their application despite their high debromination efficiency. Innovative waste-based additives, like electric arc furnace dust and red mud, improve debromination and gas yields, but challenges such as catalyst deactivation and high costs remain. Utilizing waste materials for catalyst development offers a sustainable and cost-effective solution for efficient PVC waste management.

Electrolysis involves decomposing PVC plastic into Cl gas and other chemicals through electrochemical reactions. Song et al. [28] achieved dechlorination of PVC plastic using Non-Thermal Plasma (NTP) electrolysis, resulting in gas, liquid, and solid products, with Cl elements accounting for 96.44%, 1.44%, and 2.12% of these products, respectively. During the reaction, active substances (e.g., electrons, N₂⁺) degrade PVC into chain hydrocarbons, chlorinated hydrocarbons, and a series of intermediate products, such as free radicals. Subsequently, H and Cl free radicals combine to produce HCl gas. Additionally, aromatic compounds, chlorinated hydrocarbons, and aromatic liquids are generated through aromatization, olefin chlorination, and aromatic chlorination reactions, respectively. After the dechlorination reaction, the remaining solid contains -CH₂-, C=C, and -CHCl-. The experiment investigated the effects of treatment time and discharge power on dechlorination efficiency, showing that, under conditions of 40 min and 180 W power, the maximum efficiency (98.25%) and HCl recovery rate (55.72%) were achieved. Compared to pyrolysis dechlorination, NTP dechlorination requires lower activation energy, indicating its broad potential.

For simultaneous dechlorination during chemical recycling, this section summarizes pyrolysis, catalytic dechlorination, and electrolytic dechlorination techniques. Comparative analysis shows that the principles and reaction processes of simultaneous dechlorination during chemical recycling are similar to those of pretreatment, such as pyrolysis and catalytic dechlorination. However, simultaneous dechlorination during chemical recycling achieves further dechlorination based on pretreatment, meaning that pretreatment is a prerequisite for simultaneous dechlorination during chemical recycling. In addition, as an emerging dechlorination technology, electrolysis removes chlorine from PVC waste through electrochemical reactions. Compared to traditional simultaneous dechlorination during chemical recycling (e.g., pyrolysis and catalytic dechlorination), the advantage of electrolytic dechlorination lies in its ability to remove most of the chlorine in a short time, with high dechlorination efficiency.

4.3. Product Purification

After pretreatment and simultaneous dechlorination during chemical recycling, liquid, gas, and solid products are obtained. The dechlorination of these products involves the removal of chlorine elements from all three forms.

The Cl in the liquid product (e.g., pyrolysis oil) can be removed using adsorption technology. Romero et al. [29] studied the effect of different sodium zeolites (4A, 13X, and Y) as adsorbents on the dechlorination of pyrolysis oil. Zeolites achieve Cl adsorption by interacting with Na⁺ ions and providing effective adsorption sites for organic chlorides. Among the three zeolites, 13X exhibited the best dechlorination efficiency due to its higher concentration of Lewis acid sites, larger pore size, and porosity. Experimental studies have shown that pre-treating the zeolite by dehydrating it at 150 °C to remove surface water molecules can significantly improve the dechlorination efficiency of the adsorbent. It is worth mentioning that zeolites have a certain degree of recyclability; used zeolites can be regenerated by burning at 600 °C to eliminate Cl, and the regenerated zeolites have a dechlorination percentage comparable to that of fresh zeolites within the first three hours of use.

During the combustion of PVC waste, high-Cl-content fly ash is produced. The treatment of fly ash can refer to the dechlorination of gaseous products in waste incineration processes. Zou et al. [30] used kitchen waste fermentation liquid to leach fly ash from waste incineration to achieve dechlorination. The study compared the dechlorination effects of lactic acid fermentation liquid and kitchen waste sludge. The experimental results showed that using lactic acid fermentation liquid and kitchen waste sludge in a three-step leaching process could remove 90% of the water-insoluble Cl from the fly ash, outperforming the three-step leaching effect of pure water. The Cl content in the leached fly ash residue was 0.44% and 0.39%, respectively. Additionally, the nature of the leaching solvent indicated that low pH and high concentrations of organic acids are more conducive to removing water-insoluble Cl. Overall, using lactic acid fermentation liquid and kitchen waste sludge for fly ash dechlorination is economically and technically feasible.

In addition to gaseous and liquid products, the remaining solid products also require dechlorination. For solid product dechlorination, methods for dechlorinating solid chlorinated waste can be referenced. Chen et al. [31] obtained solid-derived fuel from chlorinated food waste through dry dechlorination and steam pressurization. The study first mixed alkaline adsorbents, such as Ca(OH)₂ and NaHCO_3, with solid chlorinated waste, then treated them in a steam pressure vessel, ultimately obtaining products like calcium chloride and sodium chloride, thus achieving resource utilization of solid chlorinated waste. The study also explored the impact of the type and number of alkaline adsorbents on dechlorination efficiency, concluding that Ca(OH)₂ had a better dechlorination effect than NaHCO₃. When 15 wt% of Ca(OH)₂ was added, the highest Cl removal efficiency (77%) was achieved.

For product purification, this section summarizes the dechlorination techniques for three forms of products: liquid, gaseous, and solid. After dechlorination, the gaseous products exist in the form of HCl, which can be removed through absorption or chemical reactions. Since the chlorine in liquid and solid products is chemically bonded, dechlorination is more challenging. The chlorine in liquid and solid products can be removed through high-temperature heating, the use of catalysts, and adsorbents.

By summarizing the dechlorination techniques at different stages, we can see that each dechlorination process for PVC waste has its own advantages and limitations. Pretreatment methods can achieve high initial Cl removal but may require precise control of conditions. Simultaneous dechlorination during chemical recycling effectively breaks down PVC and removes Cl but can be limited by the properties of the additives used. Product purification is an effective way for treating residual Cl in final products but may face challenges with catalyst deactivation and regeneration. The integrated dechlorination method can be found in Figure 4 and the review of dechlorination technology literature can be found in

Table 1. Review of dechlorination technology literature.

Dechlorination Method	The Characteristic of the Method	Experimental Factors	Reference
Pyrolysis under low temperature (200–300 °C)	Through the breaking of chemical bonds between atoms during low-temperature pyrolysis to achieve dechlorination	Temperature, different gas atmosphere (inert gas, air, oxygen)	[21]
Solvothermal dechlorination method	Taking advantage of elimination and substitution reactions to achieve dechlorination	Temperature, reaction time, different alkaline additives(NaOH, KOH, K2CO3)	[22]
Metal oxide catalytic technology	Incorporating catalysts during dechlorination process can enhance dechlorination efficiency, lowering the pyrolysis reaction temperature.	Different metal oxide catalysts (TiO2, MgO, CoO)	[23]
Mechanochemical approach	Combine chemical methods (NaOH) to make the surface rigid and physical method (ball milling) for crushing in order to achieve better dechlorination	Chemical factors (e.g., NaOH concentration) and mechanical factors (e.g., ball size, milling speed)	[24]
Pyrolysis under high temperature	Pyrolysis under high temperature decomposes PVC plastic into small molecular compounds	The PVC content in mixed plastic (0–12%), heating rate (10–60 °C/min)	[26]
Catalytic dechlorination	Utilizing waste materials for catalyst offers a sustainable and cost-effective solution for PVC waste.	The interaction between metal oxides and alkalinity additives	[27]
Electrolysis method	Decomposing PVC plastic into Cl gas and chemicals through electrochemical reactions.	Treatment time, discharge power on dechlorination efficiency	[28]
Adsorption technology for liquid products (e.g., pyrolysis oil)	Zeolite adsorbents have the characteristics of recyclability, which can be regenerated at high temperature.	Temperature, different sodium zeolites as adsorbents	[30]
Adsorption technology for gas products (e.g., HCl)	Using organic acids and kitchen waste sludge for gas products dechlorination	Concentrations and pH of organic acids	[31]
Adsorption and steam pressurization technology for solid products	Achieving efficient dechlorination through the combined use of alkaline adsorption and pressurization techniques	The type and number of alkaline adsorbents on dechlorination efficiency	[32]

.

To achieve deep dechlorination while minimizing unnecessary inputs, it is essential to combine multiple Cl removal methods based on the specific conditions of industrial waste and dechlorination requirements. This means that the appropriate dechlorination process can be selected based on the Cl content in the PVC. This not only achieves the desired dechlorination effect but also ensures efficient energy utilization by matching the appropriate process. This comprehensive approach tailors the dechlorination strategy to the specific needs of the waste material, optimizing the overall process.

Additionally, the pretreatment, simultaneous dechlorination during chemical recycling, and purification are not isolated steps but interconnected and sequential processes. By integrating pretreatment, simultaneous dechlorination during chemical recycling, and purification, we can enhance efficiency, reduce environmental impact, and ensure more effective Cl removal in diverse industrial applications.

5. Multi-Scale Model Construction

To achieve the high-value recycling of PVC waste, it is essential to consider not only the dechlorination degree at the chemical reaction scale but also the material and energy metabolism of production system and macro-scale analysis to ensure sustainable industrial application.

5.1. Microscale Model Construction

At the microscale, mathematical models can be used to describe the reaction mechanisms, thermodynamics, and kinetics of chemical reactions. These models are crucial for quantifying the products of the dechlorination process of PVC waste at the macroscopic level. By simulating reactions under different temperatures, pressures, and catalyst conditions, we can quantitatively elucidate the mechanisms behind various dechlorination efficiencies and product compositions. Sensitivity analysis and system optimization of the models can aid in understanding the migration and transformation of chlorine under different processes and conditions, leading to the optimization of reaction conditions to improve dechlorination efficiency and reduce energy and material consumption.

5.1.1. Pretreatment Dechlorination Reaction Model

Since the pretreatment dechlorination process adds an additional stage to the chemical recycling process of plastic or PVC waste, it will bring extra economic and environmental burdens due to the required equipment, energy, and chemical inputs. By establishing detailed thermodynamic and kinetic models, we can provide quantitative guidance for optimizing the cost and environmental impacts of process design and operation. This allows for meeting certain dechlorination requirements while minimizing reaction time, feedstock dosage, temperature, etc., for reduction of the energy and material consumption.

Most studies indicate that the dechlorination process of PVC follows a first-order reaction rate. The dechlorination kinetics in NaOH and EG solvent substitution and elimination reactions adhere to first-order chemical reaction kinetics [32]. The accelerated rate of dechlorination at higher temperatures implies that the reaction proceeds under chemical reaction control because temperature has a much larger effect on chemical reaction control than on diffusion control. Kinetic analysis of hydrothermal dechlorination (HTD) shows that the process can be represented as a first-order reaction. According to the Arrhenius law, the apparent activation energy is 217.43 kJ/mol [33]. In neutral conditions, the HTD of PVC follows first-order kinetics, meaning the reaction rate is proportional to the concentration of unreacted PVC. Using the Kissinger method, the apparent activation energy is calculated to be 185.9 ± 6.8 kJ/mol, indicating that the dechlorination reaction is primarily controlled by chemical reactions rather than mass transfer limitations [34]. Under supercritical conditions, Cl in PVC can be completely removed, producing high-molecular-weight conjugated olefins and other products, where the dechlorination reaction also follows a first-order kinetic model [35].

However, these findings are derived based on experiments with fresh PVC resin, and the effects of actual solid waste particle size or different grain sizes were not considered [36]. Large PVC waste chunks primarily undergo surface dechlorination, necessitating mechanical methods for increasing the reactive area in the particle core. For near-industrial-scale studies on the dry ball milling co-disposal of PVC and eggshell wastes, the reaction is mainly driven by mechanical forces, following zero-order kinetics [37]. Additionally, in a NaOH/EG reaction system, the incorporation of ball milling achieves efficient dechlorination. The shrinking-core model describes this reaction system, where the PVC solid shrinks as the reaction progresses, increasing the surface area of the reactive core and considering different reactive surface changes. This model has been validated by up-scale experiments for treating various PVC wastes. The dechlorinated PVC can be crushed into a powder with a particle size of less than 100 μm [25]. However, if ball milling is stopped during the reaction, further dechlorination is limited due to the inaccessible reactive core of the PVC particles [38].

5.1.2. Simultaneous Dechlorination during Chemical Recycling Reaction Model

The simultaneous dechlorination of PVC during chemical recycling processes, mainly by pyrolysis, involves complex reaction kinetics due to the formation of multiple radical species and the potential generation of various chlorinated hydrocarbons. In the pyrolysis of PVC, dechlorination primarily occurs in two stages. Reaction models are useful for capturing the kinetics of dechlorination, treating each step as an irreversible phenomenon [39].

Firstly, during the pyrolysis of PVC at 200–300 °C, dechlorination occurs as a first-order reaction, producing HCl and small amounts of benzene [40]. After dechlorination, the polymer residue undergoes further decomposition, forming polycyclic aromatic hydrocarbons and non-condensable gases. The decomposition of cross-linked intermediates is considered a random scission reaction, described by a parallel first-order reaction model, validated through density functional theory (DFT) calculations and experimental data [41].

Other kinetic models, although not specific to PVC, provide valuable insights into dechlorination mechanisms for chlorinated organic compounds and their potential applications. Monod kinetics can frequently describe dechlorination systems but often struggle to accurately represent processes across a wide range of conditions [42]. First-order kinetics are applied in simpler chemical systems where the reaction rate is proportional to the concentration of the chlorinated compound [43]. More complex systems may require mixed-order kinetics to capture the interplay between dissolution and dechlorination rates, especially in multiphase systems. Computational models, including DFT, provide detailed insights into reaction mechanisms, aiding in the optimization of dechlorination processes [44].

The dechlorination process of PVC by pyrolysis can be thermodynamically analyzed, with the Arrhenius model commonly used to describe the kinetics of this process. Studies have shown that the dechlorination reaction of PVC follows a first-order reaction model, with an activation energy of 134 kJ/mol, reaching its maximum rate at approximately 300 °C [45]. Peng et al. [46] conducted a thermodynamic model analysis of PVC dechlorination, utilizing methods such as Ozawa–Flynn–Wall (OFW) and Kissinger–Akahira–Sunose (KAS) to estimate activation energies. The activation energies for PVC thermal degradation were initially higher without catalysts but reduced with the introduction of zeolites, indicating that catalysts help lower the energy barriers for degradation reactions. Pan et al. [47] also used multiple OFW and KAS to analyze the staged reaction mechanisms of PVC pyrolysis, revealing that two-stage reaction are endothermic and nonspontaneous. The enthalpy change for Stage I is around 70.4 kJ/mol and for Stage II is about 134 kJ/mol; Gibbs free energy for Stage I is approximately 147.6 kJ/mol and for Stage II is about 188.1 kJ/mol, highlighting the impact of reaction temperature on dechlorination efficiency. Cao et al. [26] investigated the effect of heating rate on the dechlorination process, proposing that bubble ratio and pixel area are effective morphological parameters for monitoring PVC dechlorination. DFT calculations reveals that oxygen chemisorption on alkene moieties formed dioxetane intermediates, enhancing dechlorination efficiency through exothermic chain cleavage and providing additional dechlorination pathways with similar activation energy values to conventional reactions [22]. Al-Yaari and Dubdub [48] combined the classical Arrhenius model with artificial neural networks to predict weight changes during PVC pyrolysis, demonstrating the potential of machine learning in thermodynamic modeling.

The thermodynamics and kinetics of the PVC dechlorination process during chemical recycling are more complex than those of pretreatment pathways. Current studies primarily describe overall dechlorination while lacking detailed insights into the mechanisms of high-resolution Cl-containing product formation. However, for the high-value utilization of products, as mentioned in the introduction, the petrochemical industry has stringent requirements for the Cl content in oil. For further treatment of residual Cl, it is necessary to address different Cl product components and their concentrations. On the other hand, current models are not fully equipped to address the effects of real-world variables, such as the presence of additives, particle size variations, and different PVC waste compositions. Future research should focus on developing more comprehensive dechlorination models for the chemical recycling of PVC waste, with a particular emphasis on practical applications and Cl control in the final products.

5.1.3. Dechlorination and Purification Model for Products

During the pyrolysis of waste plastics, the main products include gases, liquids, and solid residues, all of which contain various forms of chlorides. To maximize the resource utilization of these three-phase products, it is essential to purify them to remove chlorides. This not only enhances their applicability but also ensures that downstream products do not contribute to environmental pollution as they progress through the petrochemical industry chain.

The primary chloride in pyrolysis gases is HCl, along with a certain number of organic chlorides. Methods for removing these chlorides can refer to traditional waste incineration treatment processes and the latest advancements. For the dechlorination of gaseous products from the chemical recycling of PVC waste, research literature mainly focuses on dry acid removal, wet acid removal, and activated carbon adsorption modeling methods.

Dry acid removal typically uses the Modified Grain Model to describe the reaction behavior between HCl and Ca(OH)₂, approximating the reaction process through a series of Continuous Stirred-Tank Reactors (CSTRs) in a cascade model [49]. Wet acid removal employs a mass transfer-reaction model based on the two-film theory to simulate the gas-liquid mass transfer and chemical reaction processes. Studies have shown that, under actual operating conditions, the gas absorption rate is jointly controlled by gas-phase and liquid-phase diffusion resistances [50]. Additionally, numerical simulation methods have been used to model the mass transfer and reaction between flue gas and lime slurry droplets in spray towers, with results indicating that the absorption rate of HCl by the deacidification droplets is significantly higher than that of SO₂ [51].

For activated carbon adsorption, Langmuir or Freundlich isotherm models are used to describe the adsorption behavior of organic chlorides on activated carbon. Experimental data fitting is used to obtain model parameters to predict adsorption capacity under different conditions [52]. Furthermore, the mass transfer behavior of volatile organic compounds (VOCs) on wet activated carbon has been studied to evaluate the impact of activated carbon moisture on separation efficiency [53].

Modeling the dechlorination process of pyrolysis oil remains underexplored, with limited experimental studies and an absence of kinetic research. To address this gap, insights can be drawn from the dechlorination of other liquids, such as water and liquid organic compounds.

The Freundlich isotherm model is frequently employed to describe the adsorption behavior of organic chlorides on adsorbents. For instance, in the adsorption of organic chlorides onto γ-Al₂O₂ nanoparticles, the Freundlich model has demonstrated the best fit and predictive accuracy for experimental data, accounting for variations in adsorption capacity and intensity [54]. Regarding adsorption kinetics, the pseudo-second order kinetic model proposed by Ho and McKay is widely utilized. This model effectively predicts the reaction rates between the adsorbent and the adsorbate, assuming that the adsorption process is predominantly controlled by chemisorption [55]. For absorption processes, mass transfer models are crucial in describing the transfer of solutes from the gas phase to the liquid phase. The two-film theory is a commonly used mass transfer model that simulates the transport processes at the gas–liquid interface [56]. Janda et al. [57] evaluated various kinetic models for the removal of volatile chlorinated hydrocarbons by zero-valent iron based on experimental results. They found that the power law model and heterogeneous reaction models were the most suitable for describing the dechlorination reactions, with simple and interpretable parameters that are well-suited for regression analysis. The linear free energy relationship (LFER) model can predict the reductive dechlorination kinetics and congener distribution of octachlorodibenzo-p-dioxin (OCDD) by nanosized zero-valent zinc, and has been validated for effectively degrading OCDD to lower chlorinated congeners under environmental conditions [58].

The removal of chloride compounds in solid state is limited for plastic chemical recycling products. However, it is common for other solids, such as fly ash and soil. Chambon et al. [59] summarized kinetic models, including First-Order, Monod, Competitive Inhibition and Self-Inhibition, for reductive dechlorination of chlorinated ethenes in soil, highlighting the complexity for accurate field application predictions. Michaelis–Menten kinetics can be applied to quantitatively describing the sequential microbial reductive dechlorination of hexachlorobenzene, incorporating the dechlorination rate and saturation constraints [60]. The study by Liu et al. [61] developed mathematical models for dechlorination of organochloride waste using microwave irradiation, which includes the dechlorination ratio, effective ratio of HCl to volatiles, and energy yield. For the solvent extraction, a Langmuir–Hinshelwood adsorption model can be applied for the removal of chlorinated compounds from soils [62]. The finite element method can be used to simulate and optimize the electric drive desalting to separate the inorganic Cl [63].

In the complex process of waste treatment, although theoretical models provide valuable insights and guidance, their applicability faces several limitations and challenges. Firstly, most models assume that the performance of the treatment process does not vary with changes in waste composition, making it difficult for the models to accurately reflect the variations in actual operations [64]. Secondly, while complex models can offer more detailed simulation results, their high complexity and multi-parameter requirements make them challenging to apply widely in practical operations, especially when precise analysis and processing of large amounts of data are needed [65]. In waste treatment processes, although theoretical mathematical modeling is an important tool, its practical application requires consideration of model simplifications and assumptions to ensure that the model can accurately reflect the dynamic behavior of complex systems. To address the complexities of waste resource utilization and treatment, machine learning methods have shown excellent performance in predicting the dechlorination efficiency and products of various processes. In the future, combining mechanistic models with actual data can be used to guide the design and integration of different process operations.

5.2. Modeling of the Production System of PVC Dechlorination

To evaluate the potential environmental and economic benefits of PVC dechlorination for plastic chemical recycling, it is essential to obtain data on energy consumption, material inputs, and output inventories. However, most plastic chemical recycling technologies are still in the transition from lab-scale experiments to pilot-scale demonstration projects. As a result, there is no industrial-scale production system to provide detailed foreground data. Direct sampling from experiments, theoretical calculations, or process simulations can be used to construct a model of the system.

Most lab-scale evaluations focus on the unit process of dechlorination treatment, rarely including upstream and downstream processes, like feeding and product separation and purification. For the electrocatalytic dechlorination process, besides direct measurement, energy consumption can be modeled using specific electric energy consumption and current efficiency. Production analysis can be performed using High-Performance Liquid Chromatography (HPLC) and Gas Chromatography–Mass Spectrometry (GC–MS) [66,67]. The chloride ions in the solution and residual Cl in the solid can be analyzed by Ion Chromatography [68] and elemental analysis [69], respectively. The energy consumption of the mechanochemical dechlorination process can be determined by monitoring the current and voltage of the motor driving the ball milling [70].

For processes not yet industrialized, using physical, chemical, and engineering principles for energy consumption and material balance calculations is a common method for modeling production systems. These methods are widely used in LCA to determine material and energy flows within system boundaries. The literature indicates that stoichiometry can provide detailed inventory analysis through the combination of chemical reactions and process conditions [71]. Parvatker et al. [72] developed process scale-up techniques, stoichiometric calculations, and proxy data to estimate material and energy inputs. Laboratory-scale procedures were adapted to industrial-scale operations for twenty anesthetic active pharmaceutical ingredients (APIs). For complex processes that cannot be simply modeled, proxy inventories from databases of similar processes or simplified engineering calculations can be considered [73,74]. For example, authors have previously modeled the product system of a ball milling-assisted dechlorination process with NaOH/EG solutions based on stoichiometry and assumed efficiency parameters to account for energy consumption [75].

The dechlorination process is highly related to chemical engineering processes. Software like Aspen Plus (version 10.0) facilitates the acquisition of energy consumption and material input-output inventories for unit processes. By simulating individual unit operations and components, it can calculate thermodynamic and transport properties, describing the behavior of single components and mixtures, thereby achieving precise energy and material balance calculations [76]. In addition to simulating the material balance and energy consumption of reaction processes, it can also compute various separation processes, heat exchange modules, and utilities, thereby enabling accurate and comprehensive product system inventory estimation [77].

Beyond Aspen Plus simulation, computational fluid dynamics (CFD) can simulate fluid flow and heat transfer processes, predict energy consumption and material transfer in complex fluid systems, and provide precise energy and material balance data [78,79]. For processes centered on solids, the discrete element method (DEM) can be used to simulate particle collisions, friction, and flow, ultimately predicting material transfer and energy consumption in the process [80]. Additionally, machine learning methods can achieve more efficient acquisition of input–output inventory data and energy consumption estimates. Specifically, machine learning models can handle large datasets, fill data gaps, and improve prediction accuracy [81].

In summary, numerous modeling techniques are available for assessing the inventory of production systems that have not yet been industrialized. However, no single method can comprehensively acquire all the necessary data for LCA and TEA. Therefore, it is essential to clearly define system boundaries and required data, and to use methods with lower uncertainty for data estimation to ensure reliable assessment results [82]. Despite the cost of demonstration projects, pilot-scale data can provide sufficient precision for modeling the inventory data needed to assess emerging technologies.

5.3. Macro-Scale MFA

Based on the modeling foundation of the dechlorination production system for PVC waste, it is of significance to study the material flow of PVC products, the material flow related to waste treatment, and the material flow of Cl elements from a macro perspective. Through MFA, we can quantify and track the flow and transformation processes of PVC throughout its entire life cycle, identify its distribution across different sectors and applications, and gain insights into the composition and additives of various types of PVC waste [21,83]. For instance, flexible and rigid PVC contain different types and amounts of additives depending on their applications, resulting in varying rates and extents of dechlorination. This information is crucial for designing dechlorination processes [32]. Additionally, conducting a MFA from a macro perspective to quantify the potential for PVC treatment and resource recovery can help stakeholders consider how to advance its industrial application [84].

The metabolism of PVC product production, stock in human society, and waste generation has been extensively studied. In research on the material flow of PVC in Japan, 40% of PVC production is exported, with the remaining portion primarily used in rigid PVC products for public construction [85]. The study concludes that, for discarded PVC products, separation by type during collection is essential to improve recycling efficiency, particularly to avoid mixing soft and hard PVC. With the rapid economic development in China, the extensive use of PVC products will result in significant waste accumulation. Projections indicate that, by 2050, over 600 million tons of PVC waste will need to be managed [86]. Only about one-fourth of PVC waste is treated by mechanical recycling, with the remainder being improperly disposed of, landfilled, or incinerated, leading to the release of dioxins and other toxic pollutants [87]. PVC exhibits one of the highest recycling rates among common types of polymers, largely due to its predominant use in the construction sector [88]. However, recycling PVC waste from other sectors remains challenging. Even in developed regions, like the EU, a considerable portion of PVC waste is mismanaged, resulting in environmental releases as microplastics and macroplastics [89]. Due to the use of various plasticizers in PVC micro and nanoplastics production, PVC micro and nanoplastics are considered hazardous polymers [90]. Improper disposal of PVC waste micro- and nanoplastics can negatively impact organisms (including humans) [91]. Studies have shown that ingestion of PVC micro- and nanoplastics may lead to oxidative stress, endocrine and metabolic disorders, immune responses, and damage to organs such as the liver, kidneys, and intestines [92].

Compared to MFA, Element Flow Analysis (EFA) focuses more on the movement and transformation of specific elements within environmental or industrial systems. This focus allows for a more precise assessment of the impact of various waste treatment methods on the metabolism of particular elements. Pioneering work by Ayres [93] analyzed the industrial metabolism of Cl across various regions in Europe, highlighting the necessity of limiting Cl usage and controlling Cl pollution. However, his work is based on the situation in the last century and may not reflect current developments, especially in solid waste management. Ma et al. [94] calculated Cl flows based on the purity and chemical structure of different chlorinated chemicals in China. This study mapped the inputs and outputs of Cl across various industrial processes, providing targeted pollution control strategies. Furthermore, Zhang et al. [95] demonstrated that the application of new catalytic oxidation technologies for HCl in the chlor-alkali industry could improve Cl utilization efficiency from 67.6% to 90.8%, reflecting the principles of industrial symbiosis. In the context of solid waste management, Ma et al. [96] analyzed the behavior of Cl during waste incineration, including its vaporization, transformation, deposition, and corrosion processes. Their findings suggested methods to mitigate Cl-induced corrosion during incineration. Additionally, Kumagai et al. [16] examined the Cl metabolism within the chlor-alkali industry and the life cycle of PVC products in Japan; the Cl flow diagram from salt import to Cl recovery process is visualized in Figure 5. The figure illustrates the different forms of Cl element during the life cycle process, along with the changes in its mass increase and loss. In this study, novel approaches (dechlorination and electrodialysis processes) were developed to recover Cl from NaCl. The recyclable NaCl was estimated to be 293 kt-Cl, which corresponds to 7% of imported salt (4078 kt-Cl), indicating that, by recycling Cl, the input of raw materials can be reduced, thereby enhancing both environmental and economic benefits. The study innovatively proposed a chlorine resource recycling system, providing a valuable guideline for the efficient utilization and recovery of chlorine resources.

Synthesizing the findings from the studies, MFA provides crucial quantitative support for the sustainable management and resource recovery of PVC waste. By analyzing future annual generation rates, sources, and composition characteristics of PVC waste, MFA helps predict waste flow paths and identify resource recovery potentials. This quantitative analysis not only provides a scientific basis for the industrial application of relevant technologies but also lays the foundation for improving overall resource recovery rates and environmental protection levels. Given the potential significant increase in PVC waste generation in the future, it is urgent and necessary to promote the application of recycling technologies.

To sum up, the reviewed multi-scale modeling methods, ranging from chemical reactions to macro material flow, are fundamental for LCA and TEA. Chemical reaction modeling can provide detailed insights into material metabolism and process inventory changes under different process designs. Production system modeling offers data inventories that closely reflect dechlorination processes in chemical production. Macro-level modeling helps quantify the impacts of different waste plastic compositions and reveals the potential for resource recycling. In the next section, we will explore how these multi-scale modeling methods and macro-level MFA analysis support LCA, evaluating the environmental and economic benefits of different technological pathways. This approach will offer comprehensive support for developing more sustainable waste management strategies.

6. TEA and LCA of Dechlorination Technology for PVC Waste

By constructing multi-scale models, it becomes possible to generate input–output inventory data for the dechlorination and resource recovery processes of PVC waste. This enables the evaluation of their potential economic feasibility and environmental benefits. However, case studies specifically focused on PVC dechlorination are scarce. Therefore, this paper primarily draws on TEA and LCA methodologies from other similar case studies to establish a pre-assessment framework for PVC dechlorination. This framework provides a comprehensive analysis of the technical, economic, and environmental impacts of the dechlorination process for PVC waste, offering a foundation for the future industrial application of this technology.

6.1. TEA

The most common methods in TEA are Net Present Value (NPV), Internal Rate of Return (IRR), Payback Period (PBP), and Return on Investment (ROI). These methods involve a systematic evaluation of technology-related costs and revenues, considering factors such as capital investment, product or energy sales’ revenue streams, and operating expenses [97]. The dechlorination process for PVC waste is divided into pre-dechlorination technology, chemical recycling dechlorination technology, and product dechlorination technology. This study summarizes and analyzes the research of LavKumar Kaushik [98] and Leila Samiee [99] to establish a TEA–LCA model framework for the dechlorination process of PVC waste. By integrating the research objectives, scope, data, and system elements of TEA and LCA, as suggested by Roksana Mahmud, the uncertainty in model evaluation can be reduced [20]. Following a four-step framework of goal and scope definition, inventory data collection, environmental impact analysis, interpretation and explanation, the study analyzes the economic costs and environmental impacts of the PVC waste dechlorination process across six stages: equipment configuration and maintenance, raw and auxiliary material input, energy consumption, process pollutant emissions, product recovery, and final disposal of equipment and waste. Construction and storage costs can also be included [100].

$${\mathrm{C}}_{\mathrm{t}}={\mathrm{C}}_{\mathrm{s}}+{\mathrm{C}}_{\mathrm{m}}+{\mathrm{C}}_{\mathrm{r}\mathrm{a}}+{\mathrm{C}}_{\mathrm{e}\mathrm{n}}+{\mathrm{C}}_{\mathrm{e}}+{\mathrm{C}}_{\mathrm{r}\mathrm{e}}+{\mathrm{C}}_{\mathrm{d}}+{\mathrm{C}}_{\mathrm{o}}$$

(1)

${\mathrm{C}}_{\mathrm{t}}$: Total cost

${\mathrm{C}}_{\mathrm{s}}$: Capital cost

${\mathrm{C}}_{\mathrm{m}}$: Maintenance cost

${\mathrm{C}}_{\mathrm{r}\mathrm{a}}$: Raw-accessory materials cost

${\mathrm{C}}_{\mathrm{e}\mathrm{n}}$: energy cost

${\mathrm{C}}_{\mathrm{e}}$: Emission cost

${\mathrm{C}}_{\mathrm{r}\mathrm{e}}$: recycling cost

${\mathrm{C}}_{\mathrm{d}}$: Disposal cost

${\mathrm{C}}_{\mathrm{o}}$: Other cost

In Formula (1), ${\mathrm{C}}_{\mathrm{o}}$ (other costs) includes expenses, such as risk management fees and product storage costs. However, ${\mathrm{C}}_{\mathrm{o}}$ may vary depending on the chosen technology. For example, when calculating the cost of product purification process, ${\mathrm{C}}_{\mathrm{o}}$ may include the storage costs of dechlorinated products (e.g., pyrolysis oil).

NPV is used for analyzing investment decisions. It is defined as the difference between the cash inflows and outflows of an investment (typically, operating and maintenance costs subtracted from revenue) multiplied by the future value of resources recovered over a specified period at a constant rate of return. In this model, operating and maintenance costs include maintenance costs, raw material costs, energy costs, product recovery costs, disposal costs, and other costs. The equipment cost is considered the initial investment.

$$\mathrm{N}\mathrm{P}\mathrm{V}=\left(\sum _{\mathrm{i}=\mathrm{a}}^{\mathrm{n}}{\displaystyle \frac{{\mathrm{N}\mathrm{e}\mathrm{t} \mathrm{C}\mathrm{a}\mathrm{s}\mathrm{h} \mathrm{I}\mathrm{n}\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}_{\mathrm{i}}}{{\left(1+\mathrm{r}\right)}^{\mathrm{i}}}}\right)-\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{I}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}$$

(2)

i represents the iterative parameter that runs from a (starting period) to n (end period), ${\mathrm{N}\mathrm{e}\mathrm{t} \mathrm{C}\mathrm{a}\mathrm{s}\mathrm{h} \mathrm{I}\mathrm{n}\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}_{\mathrm{i}}$ represents the cash inflow at time i, r is the discount rate, and $\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{I}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}$ is the initial outlay or cost.

IRR is used to measure the profitability of potential investments and is a discount rate that makes the NPV of a specific project equal to zero (R = 0).

$$\mathrm{I}\mathrm{R}\mathrm{R}={\mathrm{r}}_{\mathrm{a}}+{\displaystyle \frac{{\mathrm{N}\mathrm{P}\mathrm{V}}_{\mathrm{a}}}{{\mathrm{N}\mathrm{P}\mathrm{V}}_{\mathrm{a}}-{\mathrm{N}\mathrm{P}\mathrm{V}}_{\mathrm{b}}}}\ast \left({\mathrm{r}}_{\mathrm{b}}-{\mathrm{r}}_{\mathrm{a}}\right)$$

(3)

${\mathrm{r}}_{\mathrm{a}}$ and ${\mathrm{r}}_{\mathrm{b}}$ represent the lower and higher discount rates, respectively. NPVa is the NPV at ${\mathrm{r}}_{\mathrm{a}}$, and NPVb is the NPV at ${\mathrm{r}}_{\mathrm{b}}$.

PBP is calculated by dividing the capital expenditure by the annual cash flow, representing the time required to recover the funds from the capital expenditure.

$${\mathrm{T}}_{\mathrm{p}\mathrm{b}\mathrm{p}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{t}}+\mathrm{d}\mathrm{e}\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}{{\mathrm{N}\mathrm{e}\mathrm{t} \mathrm{C}\mathrm{a}\mathrm{s}\mathrm{h} \mathrm{I}\mathrm{n}\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}_{\mathrm{t}}}}$$

(4)

Depreciation refers to the loss in value.

ROI evaluates the profitability performance by the ratio of net profit over the facility’s lifespan to the capital expenditure.

$$\mathrm{R}\mathrm{O}\mathrm{I}={\displaystyle \frac{{{\mathrm{C}}_{\mathrm{i}\mathrm{n}}-\mathrm{C}}_{\mathrm{t}}-\mathrm{d}\mathrm{e}\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}{{\mathrm{C}}_{\mathrm{t}}}}$$

(5)

C_in represents the income.

Comprehensive evaluation involves deriving integrated insights based on the interdependence of specific indicator results. While technology offers significant social benefits, it can also have associated negative impacts. Combining LCA and TEA allows for a systematic analysis of the relationships between technological, economic, and environmental performance, providing developers with more information for trade-off analysis.

Analyzing economic feasibility is crucial for assessing the viability of technology upgrades and commercialization outcomes. Technology assessment varies with the maturity of the technology, which can be evaluated based on TRL. The following

Table 2. Different TRL measurement standards and their indicators, respectively.

TRL	Scale	TEA	LCA
1	Concepts	Extended (Ex ante) TEA	Extended (Ex ante) LCA
2	Lab-scale
3	Bench scale
4	Engineering scale	Traditional TEA	Extended LCA
5	Full scale		Traditional LCA

provides the TRL measurement standards [101]. At low TRLs, using technology assessment tools can maximize economic benefits and minimize the environmental impact of developing technologies [102]. The dechlorination process for PVC waste includes pre-dechlorination technology, chemical dechlorination technology, and product dechlorination technology, most of which are at the experimental scale with low TRLs. This study, in conjunction with Boreum Lee’s research [103], evaluates the potential feasibility of these technologies at the early development stage by integrating TEA and process modeling. Due to the low availability and quality of data for low TRL technologies, the assessment results can differ by several orders of magnitude from industrial or commercial outcomes. Process modeling can establish a system process model through process integration and software simulation. Using ex-ante LCA methods can also reduce the uncertainty in system models. Sinke et al. [104] employed scenario and sensitivity analyses in their ex-ante LCA of commercial-scale meat production to evaluate model uncertainty. Additionally, the combined TEA–LCA framework can integrate methods such as process simulation, machine learning, multi-objective optimization, and GIS models of raw material production scenarios for multi-criteria decision-making [105].

6.2. LCA

LCA is an effective and systematic method for evaluating the environmental footprint of a product throughout its production, use, and disposal stages [106]. LCA can identify and quantify energy and material consumption as well as pollutant emissions across all life cycle stages to assess their environmental impacts [107]. According to the international standards ISO 14040/44 [108,109], the LCA framework consists of four stages: goal and scope definition, inventory analysis, impact assessment, and interpretation [110]. The environmental impact categories in LCA include resource depletion, climate change, atmospheric pollution, water and soil toxicity, etc. Yadav et al. [111] suggest that, in the process of converting waste into resources, global warming potential (GWP), terrestrial acidification, human toxicity, terrestrial eutrophication, and photochemical oxidant formation are crucial impact categories to study.

Currently, some studies use LCA to compare chemical recycling with other treatment technologies (e.g., mechanical recycling) to determine the most suitable treatment method. However, Davidson et al. [112] argue that mechanical and chemical recycling are complementary, and directly comparing the two methods has limited value. They recommend that, when using LCA to evaluate chemical recycling, it should be divided into two parts: one part should model the chemical recycling process independently to highlight environmental hotspots and potential process improvements, and the other part should model the combination of chemical recycling with other treatment technologies (e.g., mechanical recycling).

To quantify the recycling of PVC plastics using LCA, considerations can include chemical recycling and the assessment of technical feasibility. Pyrolysis is an important method for converting waste plastics into resources, but the environmental burden of the pyrolysis process must be considered. Yadav et al. [111] conducted an LCA of catalytic fast pyrolysis of mixed plastics (with 4% PVC) under four different scenarios, with the system boundary from the collection of mixed plastics to the production of pyrolysis products. The study showed that using waste plastics can reduce energy consumption and fossil fuel combustion in the supply chain. However, due to the need for pretreatment of waste plastic feedstock and the consumption of steam, methane, and electricity during processing, indicators such as carcinogens, ecotoxicity, and eutrophication worsened in the LCA impact categories.

It is worth noting that, when evaluating different types of products using different process technologies, the factors of interest in LCA may vary. Costa et al. [113] conducted a critical evaluation of the pyrolysis of waste plastics using LCA and suggested that, considering the specificity of plastic pyrolysis, LCA should focus on representative factors (temporal, geographical, and technological representativeness). When combining chemical recycling methods, the focus should be on improving carbon conversion efficiency and using renewable energy. Additionally, Liang et al. [107] quantified the carbon emissions during the conversion of PVC waste plastics into plastic pellets in the recycling stage, using 1 kg of PVC plastic as the functional unit. The study showed that the recycling process produced 0.345 kg CO₂-eq, with greenhouse gas emissions mainly coming from electricity consumption. Currently, most industrial electricity comes from coal, so increasing the proportion of clean energy in the power generation mix can reduce greenhouse gas emissions and help achieve zero or negative carbon emissions sooner, providing a reference for future PVC plastic recycling to reduce carbon emissions.

At present, the application of chemical methods in industrial production is still immature, and dechlorination technology needs to transition from laboratory scale to commercial scale. Lu et al. [114] evaluated the process of recovering Cl from NaCl/EG solution obtained from the dechlorination of PVC waste at the laboratory scale. Based on this, they modeled the scale-up from laboratory to commercial scale. The study showed that high concentrations of NaCl in the EG solution had a smaller environmental impact and, at a voltage of 2.5 V, the greatest environmental benefits were achieved in terms of Cl recovery efficiency and electricity consumption. It is worth mentioning that this experiment provided references and suggestions for emerging technologies by combining laboratory data, computational simulations, and industrial ecology. On this basis, Lu et al. [115] evaluated the environmental impact of emerging dechlorination technologies on an industrial scale using conventional LCA and ex-ante LCA, as shown in Figure 6. The experiment showed that optimizing the technology (e.g., improving the insulation system and avoiding solvent permeation during electrodialysis) would significantly reduce the environmental impact of treating PVC waste plastics.

6.3. Scenario Analysis

Scenario analysis is an important tool in LCA. By using multiple scenario assumptions, the impact of model structures on the environment can be determined [116], the uncertain parameters for assumed product system include the type of plasticizer and the content of Cl content, and the potential impact path includes the recycling mode of resources and the utilization rate of energy resources. During the dechlorination of PVC, the chemical recycling process requires a high energy input. Therefore, decarbonizing electricity in the chemical recycling process can reduce the overall carbon footprint and mitigate the environmental impact of the dechlorination process. Achieving electricity decarbonization can be pursued through the integration of renewable energy sources [117], accelerating the transition of traditional power production [118], enhancing electrification levels [119], and improving energy efficiency [120].

On the one hand, using renewable energy can reduce the carbon footprint of the chemical recycling process. Han et al. [121] evaluated the feasibility of landfill mining to terminate plastic waste leakage by setting different scenarios to assess the carbon reduction potential of plastic treatment. This study concluded that increasing the proportion of renewable energy can reduce greenhouse gas emissions from the grid, thereby offsetting the carbon emissions from plastic chemical recycling.

On the other hand, accelerating the transition from traditional power to green power can improve energy efficiency and help achieve carbon reduction goals. Additionally, the extent of electricity decarbonization varies among countries. Lu et al. [122] developed a dynamic LCA model to assess accumulated PV panels with a heterogeneous carbon footprint if manufactured and installed in the United States. In this study, the state-level carbon footprint of solar electricity from 2022 to 2050 was estimated to account for emissions stemming from electricity generated from solar PVs. The model established in the study could further estimate the carbon footprints of other electricity generation technologies under the decarbonization initiative and facilitate the supply chain planning of other renewable energy technologies. Tang et al. [123] studied the trend changes in the carbon footprint of China’s renewable energy infrastructure compared to major developed countries under different power decarbonization scenarios from a macro perspective. This study conducted a cradle-to-gate analysis of power contributions, combined with multi-source energy structure scenarios, to quantitatively assess the future carbon footprint of energy, providing decision-making and recommendations for China’s energy transition.

In addition, the decarbonization model for electricity can be used to predict potential future environmental impacts in practical processes. Xue et al. [124] employed scenario analysis to forecast the changes in the carbon footprint and carbon neutrality potential of ED desalination in four application scenarios: seawater desalination, simultaneous salt production during seawater desalination, high-salinity wastewater treatment, and organic solvent desalination, under the context of a gradually decarbonizing power grid and increasing waste recycling rates. The study indicates that, as the power grid moves towards decarbonization in the future, it is crucial to focus on reducing greenhouse gas emissions during the component production and waste treatment stages. The method of quantifying carbon footprint and carbon neutrality potential based on LCA techniques in this study can be extended to general water treatment processes to support low-carbon design and optimization of these processes.

The promotion of bio-based and biodegradable plastics is seen as a key solution to addressing plastic-related environmental issues, presenting both opportunities and challenges for PVC treatment [1]. Firstly, the adoption of bio-based PVC has the potential to reduce the carbon footprint of its life cycle [125], though it does not inherently resolve the dechlorination challenge. Additionally, bio-based additives are likely to become more prevalent in future PVC products [126], and their impact on dechlorination efficacy remains uncertain.

Moreover, the widespread use of biodegradable plastics, such as PLA, might lead to public misconceptions about disposal methods, resulting in increased environmental pollution [127]. PLA degrades slowly in natural environments, potentially leading to microplastic formation and elevated methane emissions [128]. The presence of biodegradable plastics in waste streams may necessitate different sorting and pretreatment methods to ensure efficient PVC dechlorination [129]. Therefore, integrating multiple dechlorination strategies tailored to the specific conditions of industrial waste and dechlorination requirements is essential for optimizing the process and minimizing environmental impact. Co-pyrolysis studies showed the potential for separation and selective recovery of pyrolyzes through controlled temperature processes for PLA and other biodegradable plastics [130], but this does not solve the Cl issue in PVC. HCl can act as an acid catalyst, promoting biomass pyrolysis reactions, while hydrogen atoms from PVC chains can transfer to biomass-derived radicals, accelerating the reaction [131]. Additionally, research indicates that lignin in biomass can inhibit HCl release during co-pyrolysis, thereby reducing equipment corrosion and environmental pollution [132].

7. Conclusions

This review summarized the technological advancements, modeling and evaluation of dechlorination techniques for PVC waste plastics. It discussed various dechlorination methods, multi-scale model construction and TEA–LCA analysis. Notably, this review proposed an integrated dechlorination method, which included pretreatment, simultaneous dechlorination during chemical recycling and product purification. Additionally, we summarized potential modeling methods from micro to macro perspectives. Furthermore, to better evaluate the practicality and environmental impact, the TEA and LCA frameworks for dechlorination technologies were reviewed. The results and prospects are summarized as follows:

(1)

Most current studies focused on experimental research involving single-step dechlorination processes for PVC waste. To achieve a high degree of dechlorination in the recovered hydrocarbons, we suggested an integrated dechlorination system. This method could achieve dechlorination under mild conditions—avoiding extreme conditions that increase energy consumption and reagent use—while still meeting the required chlorine content in the final product.

(2)

To identify design improvements and application potential, we summarized the mechanisms, material and energy flows of the dechlorination process from both micro and macro model levels. Currently, constructing multi-scale models faces challenges, such as parameter uncertainty and inconsistent data quality. To address these issues, future research should focus on validating and optimizing models through experiments, simulations and up-scaling applications to improve predictive capability and stability. Additionally, establishing standardized data protocols and shared platforms is recommended to enhance data quality and accessibility.

(3)

We also analyzed the TEA and LCA of the dechlorination process for PVC waste. Currently, TEA and LCA for PVC waste face challenges such as market demand, regulatory policies, product quality positioning and the complexity of environmental impact assessments. By summarizing various environmental and economic indicators, we proposed sustainable criteria for promoting these emerging technologies from laboratory to industrial applications. Stakeholders should consider both the economic feasibility and the potential for PVC waste dechlorination and plastic chemical recycling.