Micropyrolysis of Polyethylene and Polypropylene Prior to Bioconversion: The Effect of Reactor Temperature and Vapor Residence Time on Product Distribution

The rapid thermal degradation of olefin plastics is a promising chemical recycling technology to create useful products from waste plastics. In this study, pyrolysis vapors from polyethylene (HDPE and LDPE) and polypropylene were subjected to secondary degradation using a new two-stage micropyrolysis reactor (TSMR) accessory to a commercial micropyrolysis unit. Variations in reactor temperature (550-600 °C) and vapor residence time (VRT) (1.4-5.6 s) showed a strong effect on the product distribution, which was comprised of mostly alkene hydrocarbons over a broad carbon number range, with minor production of alkanes and alkadienes. On the basis of the generated micropyrolysis data, a very practical lumped kinetic model comprised of 10 reactions and 6 lumped "species"was created to describe the plastic pyrolysis and to understand how temperature and VRT turn the product distribution into different product classes of compounds (plastic, wax, heavy oil, light oil, gas, and aromatics). The kinetic parameters, such as the activation energy and frequency factor, were solved for using the method of least squares. The presented kinetic model shows good agreement with the data and with known degradation mechanisms.