A critical review of the influence of supercritical water on the pyrolysis of plastic waste: Modelling approaches and process effects

Supercritical water has emerged as a promising solvent for the pyrolysis of plastic waste, offering advantageous effects on both the process and its products. Experimental observations have shown that the presence of supercritical water reduces coke formation and enhances liquid yield during plastic waste pyrolysis. However, these effects are not significant for small non-heteroatomic hydrocarbons such as hexane or tert-butylbenzene. Consequently, the influence of supercritical water only becomes prominent for larger hydrocarbons. Despite this, modelling efforts have predominantly focused on these small, often heteroatomic, compounds. This review aims to clarify how supercritical water impacts the pyrolysis of polyolefins, highlighting significant effects and proposing approaches for modelling supercritical water pyrolysis. Essential to understand is the phase behaviour exhibited by water-hydrocarbon systems. To this end, we have further explored promising equations of state for the modelling of such systems, particularly in the supercritical domain. Additionally, the effects of supercritical water on the pyrolysis process are examined both as a medium and as a reactant. As a medium, supercritical water influences the process mainly through phase separation and dilution, but it can also stabilize molecules and affect diffusion limitations. As a reactant in non-heteroatom containing systems, water serves primarily as a radical carrier.