Oil Production from Thermal Liquefaction of Polyethylene in Low-Pressure Superheated Methanol
Improper disposal of plastic waste represents both the loss of valuable resources and significant environmental threat. This study investigates the thermal liquefaction of high-density polyethylene (HDPE) using low-pressure superheated methanol. It systematically evaluates the effects of reaction temperature and the ratio of reactant to methanol on liquefaction efficiency and product characteristics. Results indicate that complete conversion of HDPE can be achieved in low-pressure superheated methanol (<0.5 MPa) at a minimum external heating temperature of 260 °C. Under this condition, oil yield reached 77.1% with alkanes comprising 62.3% of the product alongside minor oxygenated compounds. As temperature increased, the average carbon number of hydrocarbons gradually decreased. Below 260 °C, HDPE conversion decreased significantly, and products were primarily waxy. At 290 °C, the proportion of gasoline-like fractions (C6–C12) increased markedly from 16.6% to 80.9%. Furthermore, reactant ratio plays a critical regulatory role; extremes in ratio—either too high or too low—diminish heat transfer efficiency and reduce conversion. Mechanistically, liquefaction primarily involved cleavage of secondary C−C bonds, where resulting oligomers further cracked into free radicals to form diverse hydrocarbons through secondary reactions. This work demonstrates that low-pressure superheated methanol liquefaction is a mild, efficient, and pretreatment-free method to upcycle polyethylene into valuable fuels. Optimizing these process parameters can pave the way for industrial application, aiding in both plastic pollution management and sustainable resource recovery.