On the intrinsic reaction rate of polyethylene pyrolysis and its interplay with mass transfer

An attempt to determine the the intrinsic kinetic of polyolefins pyrolysis is presented. For this, pyrolysis experiments of polyethylene were performed in a screen heater reactor, where the effects of mass and heat transfer on the pyrolysis process are minimized. First-order mass loss rate constants obtained at 500 °C were circa 0.5 and 1.3 s⁻¹ for HDPE and LDPE, respectively, which are significantly higher than the majority of the values reported in the literature. At 450 °C, the mass loss rate of HDPE was lower, circa 10⁻² s⁻¹. Additionally, we have observed that the interplay between mass transfer and the depolymerization reactions can be used to steer the product distribution. For instance, in a 50 g scale batch reactor, the product obtained at 420 °C (only oil) is much lighter than at 500 °C (mixture of oil and wax), which can be attributed to the much lower evaporation rate of larger cracking products at 420 °C as a result of which these fragments crack further in the reacting liquid phase.