Polyethylene decomposition via pyrolysis and partial oxidation in supercritical water

Experimental studies on pyrolysis and partial oxidation of polyethylene (PE) in supercritical water were conducted by use of batch reactors at a reaction temperature of 420°C and reaction time of 30 min. In the case of pyrolysis, PE decomposition was enhanced with increasing water density. Since the effect of supercritical water on pyrolysis itself was found to be not significant through the basic studies using n-hexadecane, the enhancement of PE decomposition by supercritical water was considered to be due to dissolution of high molecular weight hydrocarbons into supercritical water and diffusion of water into the molten PE phase. For partial oxidation, the yield of partial oxidation products, such as CO, CO₂, alcohols, aldehydes, and ketones, increased with increasing water density of supercritical water. In addition, the yield of H₂ and n-alkane also increased with increasing water density. The partial oxidation of PE was influenced by the area of the interface between the molten PE phase and water-O₂ phase. The reaction rate of partial oxidation of PE increased with an increase of the interfacial area. Through the studies, PE decomposition in supercritical water was found to be controlled by the changes of phase behavior and mass transfer with temperature and pressure.