Supercritical water liquefaction of mixed plastic waste ‘contaminated’ with PET (polyethylene terephthalate): Influence on yields and product composition

Supercritical water liquefaction of mixed plastic waste produces a hydrocarbon oil for the production of fuels or chemicals. However, a major processing challenge is the presence of polyethylene terephthalate (PET), which behaves as a process contaminant by generating solid terephthalic acid (TPA) and oxygenated intermediates that disrupt product quality and cause operational fouling. This study systematically evaluates the influence of PET content (10–50 wt%) and process operating severity on product yields and composition during supercritical water liquefaction of representative mixed plastics (polyethylene, polypropylene, and polystyrene). Increasing PET concentration consistently reduced oil yields and enhanced gas production, particularly CO₂ and CH₄. Oil-phase composition remained dominated by monoaromatic hydrocarbons derived from polystyrene, with additional aromatic species formed via decarboxylation of PET-derived intermediates. Higher PET levels also resulted in substantial accumulation of isophthalic and benzoic acids in the aqueous phase. Increased severity of the process conditions revealed that PET contents above 30 wt% progressively divert carbon from the oil to the aqueous phase, increasing oxygenate formation and imposing greater downstream treatment requirements. These results demonstrate that PET levels above 30 wt% compromise both process operability and product quality, establishing a practical upper limit for PET in supercritical water liquefaction of mixed plastics.