Polyethylene high-pressure pyrolysis: Better product distribution and process mechanism analysis

Pyrolysis is currently an effective way to recycle plastics. High-pressure conditions can change the pyrolysis product component distribution, but the microscopic mechanism has not been well elucidated. To explore the relationship of product distribution versus pressure and explain the microscopic mechanism of polyethylene high-pressure pyrolysis, experiments under a large initial pressure range from 1 bar to 51 bar at initial temperatures of 330–380 °C were carried out in an autoclave. In the process of polyethylene high-pressure pyrolysis, the temperature within the reactor exceeded the set temperature by 100 °C at a rate of 150 °C/min. The thermal runaway phenomenon was caused by the polymerization of concentrated olefins in liquid form, which was initiated by hydrocarbon radicals. As the pressure increased, the reaction peak temperature was risen and more small molecules were produced. Under an initial temperature of 340 °C and high-pressure conditions, polyethylene was completely converted into liquid and gas products. The experimental results also revealed that high-pressure conditions led to the production of aromatic compounds and isoparaffins, as well as more cycloalkanes and fewer olefins in the liquid product, making the product characteristics closer to the fuel standard. Finally, this paper proposes the radical microscopic mechanisms of polyethylene thermal degradation under atmospheric-pressure and high-pressure conditions.