Hydrothermal processing of polyethylene in superheated steam and supercritical water into fuels and chemicals

Abstract

This study aims to elucidate the mass transfer behavior and reaction mechanisms governing the depolymerization of polyethylene (PE) during hydrothermal processing (HTP). It specifically focuses on the comparative effects of superheated steam (SHS, defined as water heated beyond its boiling point at a given pressure without undergoing condensation) and supercritical water (SCW, formed above the critical point of water) conditions on PE depolymerization, providing an unprecedented analysis of these two environments. The effects of pressure ranging from 5.5 to 23 MPa, polymer concentrations between 30–83 wt%, and temperatures of 425 and 450 °C. The oil, gas, and solid products obtained were analyzed by quantifying their total yields, and the chemical compositions of the oil, aqueous phase products and gas were analyzed using gas chromatography mass spectrometry (GC–MS) and gas chromatography coupled to a thermal conductivity detector (GC-TCD) respectively. The results show comparable oil yields (∼83 %) between reactions conducted at lower pressures with SHS and those operated at 23 MPa with SCW. Notably, the chemical composition of the oils − primarily olefins and paraffins − remained identical regardless of changes in pressure; the same was observed when increasing polymer concentration under conditions with SHS. At higher temperatures (450 °C), complete conversion of PE into valuable chemicals, including benzene, toluene, ethylbenzene, and xylene (BTEX) in the oil was observed. The chemical composition of the gas revealed the absence of CO2 from reactions conducted with SHS, representing a notable advantage for this technology considering CO2 and its role as a major greenhouse gas. By understanding the differences between SHS and SCW, this study provides insights into optimizing HTP for efficient PE conversion and the development of sustainable technologies for plastic waste management and resource recovery.