Towards sustainable polyethylene terephthalate (PET) recycling: Kinetic modelling, parametric analysis, and process optimisation

Abstract

Polyethylene terephthalate (PET) poses persistent environmental challenges due to its accumulation in waste streams. This study investigates the kinetics and mechanistic pathways of PET hydrolysis in subcritical water (225 – 300) °C as a sustainable chemical recycling route. A mechanistic model incorporating non-catalytic hydrolysis, autocatalysis by in-situ terephthalic acid (TPA), and TPA degradation was developed and fitted to experimental data, showing excellent agreement (R² > 0.99). Distinct activation energies were determined for non-catalytic (112.7 kJ·mol⁻¹), autocatalytic (48.7 kJ·mol⁻¹), and degradation (38.4 kJ·mol⁻¹) pathways. Experiments revealed complete PET conversion within (20 – 40) min at 275 °C, with recovery of > 97 % pure TPA directly without additional purification. Regime classification using dimensionless parameters (autocatalytic efficiency, K; degradation severity, D) identified 275 °C as the optimal condition balancing reaction rate and product stability. Parametric studies on residence time, PET-to-water ratio, agitation, heating profile, and morphology confirmed the robustness of the process. Catalyst effects were also examined, with both TPA and zinc acetate enhancing depolymerisation efficiency. Environmental performance, evaluated via energy economy coefficients and E-factors, highlighted favourable energy input and low waste generation. This work uniquely integrates experimental validation, mechanistic kinetic modelling, and dimensionless regime analysis to advance the mechanistic understanding and practical optimisation of PET hydrolysis. The findings provide a quantitative foundation for reactor design and process intensification, supporting the industrial development of closed-loop PET recycling aligned with green chemistry and circular economy principles.