Chemical recycling refers to a set of technologies that convert plastic waste into raw materials by breaking down long polymer chains into shorter molecules (monomers), enabling the creation new chemicals, fuels or virgin-quality plastics. Key chemical recycling methods for plastics include pyrolysis (breaking down polymers with heat, in the absence of oxygen), dissolution (selectively dissolving polymers, then precipitating and filtering the dissolved polymer) and solvolysis (use of solvents, such as glycols and alcohols, to chemically break polymers into monomers).
Feedstocks for these recycling processes are inherently complex, because plastics are rarely singular substances. Rather, they are often intricate mixtures of thousands of chemicals, including monomers, additives, processing aids and a vast array of non-intentionally added substances. Real-world chemical-recycling feedstocks are not only chemically diverse and often unknown, but also burdened with potentially hazardous and difficult-to-track compounds. Addressing this complexity is essential for advancing safe and effective recycling technologies, as well as for fostering a truly circular and non-toxic plastics economy.
For plastics recycling, feedstock complexity necessitates robust separation and purification steps to minimize chemical contamination of recycled products and environmental releases (Figure 1). The following are descriptions of some of the purification approaches used for each of the types of chemical recycling processes.
FIGURE 1. Purification, using the unit operations shown above, is required for effective chemical recycling of plastics
Purification of pyrolysis oils
Purification of pyrolysis oil derived from plastic waste is essential to rendering it suitable for downstream industrial use, such as in steam crackers or as feedstock for new chemical production. Overall, pyrolysis oil purification requires an integrated approach combining adsorption, hydrotreatment and careful fractionation to manage contaminants and meet industrial quality specifications. Advanced and selective catalysts and adsorbents are critical to improve purification efficiency, and ongoing research and pilot projects are addressing purification challenges to better enable chemical recycling. Common purification steps in pyrolysis include the following:
Adsorption. Adsorbents are used to selectively remove contaminants such as halogens (chlorine, bromine), heavy metals and other impurities (such as silicon) that interfere with the performance of the hydrotreating catalysts, as well as with end-product quality. These adsorption technologies are being specially developed and tailored for pyrolysis oils from mixed plastic waste.
Hydrotreating. Treating pyrolysis oil with H2 in the presence of catalysts removes heteroatoms (such as sulfur, nitrogen, chlorine and oxygen). It also saturates olefins, improving oil stability and compatibility with refinery operations.
Distillation. Fractionation separates the pyrolysis oil into different boiling range cuts (light, middle, heavy fractions). The light fractions can replace naphtha for steam-cracker applocations, but currently, pyrolysis oil is often blended with conventionally produced naphtha, rather than being used in its pure form, due to residual impurities and low volumes.
Despite purification efforts, naphtha from pyrolysis oil is not yet fully achieved as a pure drop-in replacement product, mainly because of the variable contaminants from mixed plastic feedstocks. Therefore, pyrolysis oil is frequently blended with fossil naphtha. Compared to conventional crude oil hydroprocessing, pyrolysis oil from plastic waste introduces contaminants that have not been extensively studied in this context before, such as silicon, phosphorus and arsenic.
Dissolution recycling
Dissolution recycling relies heavily on effective purification to ensure the quality and safety of the recovered polymers. A critical part of this process is the solvent-recovery system, which is designed to separate and reclaim solvents used to selectively dissolve target polymers from the plastic waste stream. Efficient solvent recovery not only reduces environmental impact and operational costs, but also minimizes solvent loss.
Equally important is the rigorous removal of solvent residues from the purified polymer (must be parts-per-million levels or lower). Residual solvents in the recycled plastic can adversely affect the recycled polymer’s physical properties, safety and suitability for high-value applications, especially in food-contact or sensitive uses. Therefore, advanced drying techniques, vacuum stripping, devolatilization and purification steps are employed to ensure near-complete solvent removal.
Solvolysis purification
Monomer purification is a crucial step in solvolysis, where the goal is to recover pure monomers suitable for repolymerization. Solvolysis has been most effective with polyesters, such as polyethylene terephthalate (PET). In the case of PET solvolysis, crystallization or recrystallization steps may be employed to further refine the monomers, removing isomeric or similar impurities, and ensuring a consistent grade for polymer production. Crystallization of monomers may be necessary to obtain high-purity monomers free of colorants or interfering substances, which is important for ensuring the quality and performance of the recycled PET polymer. Thus, integrated purification processes combining distillation and crystallization are key to successful monomer recovery in PET chemical recycling.
Editor’s note: Information for this column was supplied by Irina Yarulina, head of recycling technology and innovation at Sulzer Chemtech Ltd. (Winterthur, Switzerland; www.sulzer.com).