Intrinsic Millisecond Kinetics of Polyethylene Pyrolysis via Pulse-Heated Analysis of Solid Reactions

Continued demand for polyolefins can be met by recycling plastic materials back to their constituent monomers, ethylene and propylene, via thermal cracking in a pyrolysis reactor. During pyrolysis, saturated polyolefin chains break carbon–carbon and carbon–hydrogen bonds, yielding a distribution of alkanes, alkenes, aromatic chemicals, light gases, and solid char residues at temperatures varying from 400 to 800 °C. To design a pyrolysis reactor that optimizes the chemistry for a maximum yield of light olefins, a detailed description of the chemical mechanisms and associated kinetics is required. To that end, the reaction kinetics of isothermal films of low-density polyethylene (LDPE) have been measured by the method of “pulse-heated analysis of solid reactions”, or PHASR, which allows for quantification of intrinsic kinetics via isothermal reaction-controlled experimental conditions. The evolution of LDPE films from 20 ms to 2.0 s for five temperatures (550, 575, 600, 625, and 650 °C) was characterized by measurement of the yield of chromatography-detectable compounds (<C20) in addition to the total yield of volatile products. The kinetics of volatile product evolution were interpreted via a lumped kinetic model with an activation energy of 225 ± 16 kJ mol –1, compared with existing kinetic models of polyethylene pyrolysis, and validated from first principles.