(57d) Micropyrolysis of Polyethylene and Polypropylene: The Effect of Reactor Temperature and Vapor Residence Time on Product Distribution Prior to Bioconversion

The thermal degradation of plastics is a promising recycling technology that both addresses the waste management issues of landfill disposal and produces useful products. Primary thermal degradation of polymers usually yields large quantity of high molecular weight compounds with limited applicability. This makes secondary degradation necessary to improve the product’s quality. We have found that the liquid product can be used as a feedstock for microbial biodegradation in enrichment cultures derived from environmental inocula. Oil-degrading microorganisms can colonize and break down the liquid alkenes, using the alkenes as carbon and energy source to produce microbial biomass or other value-added compounds.

In this study, pyrolysis vapors from polyethylene and polypropylene were subjected to secondary degradation using a new two-stage micropyrolysis reactor (TSMR) accessory to a commercial micropyrolysis unit (CDS Analytics). The reactor temperature (550-600 °C) and vapor residence time (VRT) (1.4-5.6 seconds) were varied. Pyrolysis products were analyzed using gas chromatography/mass spectrometry (GC/MS). The temperature and VRT variations showed a strong effect on the product distribution, comprising mostly alkene hydrocarbons with minor production of alkanes and alkadienes. At a low temperature (550 °C) and short VRT (1.4 s), a wide range of liquid (C5-C20 hydrocarbons) and wax products (C21-C30 hydrocarbons) are produced. Increasing temperature and VRT caused higher proportions of gas (C2-C4 hydrocarbons) and aromatics products are generated. Polypropylene was found to break down faster than polyethylene (HDPE and LDPE), producing lower molecular weight compounds under the same reactor conditions. A lumped kinetic model comprised of 10 reactions and 6 “species” was created to describe the plastic pyrolysis and to understand how the temperature and VRT effect product distribution into different product classes of compounds (plastic, wax, heavy oil, light oil, gas, and aromatics). Kinetic parameters, such as activation energy and frequency factor, are also reported.