A process study on the pyrolysis of waste polyethylene

Abstract

The thermo-catalytic cracking of waste polyethylene bottles was thoroughly examined to determine the possibility of operating the process in continuous mode. Two types of zeolite catalysts renowned for their strong catalytic properties and precise selectivity, HUSY and HBeta, were used to study the slates and yields of the products generated under various operating conditions. The liquid and gas samples were analyzed via GC–MS to determine the hydrocarbons chain distribution in terms of paraffins, olefins and aromatics. Under thermal heating, the gas yield reached 17.8% in the absence of catalysts and it increased to 93–96% when HUSY and HBeta were used in vapor contact mode. This yield dropped to 57% when energy-efficient induction heating was applied. Under hydrogen cracking, the gas yield decreased further to 51.6% while the P/O ratio increased due to a rise in the C13–C23+ paraffin contents. The catalyst deactivation stage showed that both HUSY and HBeta started to lose their activity as waste HDPE charges were continuously fed to the reactor. After the third HDPE charge, the amount of solid and liquid wax products dramatically increased and the process started to resemble to thermal pyrolysis. High gas yields of greater than 90% were achieved when the catalysts were regenerated with industrial air at 550°C, indicating full catalyst recovery.