Pyrolysis of Polyethylene: Chemical Kinetics, Mass Transfer, and Reflux System

First, this paper investigates the interplay between reaction kinetics and mass transfer rates in the pyrolysis of polyethylene (PE) in the temperature range of 420 to 480 °C. Experimental results confirmed that the bond-breaking rate is high, shown by the reduction in average molecular weight of the reacting PE from 186 to 3.5 kDa in 2 min at 450 °C. The evaporation rate differs with reactor type and sets the mass loss rate and the molecular weight distribution of the condensed product. At 450 °C, first-order mass loss rate constants of 5 × 10^-2 and 3 × 10^-3 s^-1 were observed in a screen-heater reactor and a semibatch reactor, respectively, while the average molecular weights of the condensed product were 2.3 and 0.6 kDa (mass-average MW by gel permeation chromatography), respectively. A mathematical model, involving bond-breaking and evaporation, was able to describe the experimentally observed fast initial cracking followed by evaporation. Second, the effect of adding a reflux system to the semibatch reactor was experimentally studied, showing that tested reflux temperatures (300-350 °C) set the molecular weight distribution of the condensed product, independently of the investigated reactor temperature. Notably, at a reactor temperature of 450 °C and reflux temperature of 350 °C, the average molecular weight by gas chromatography/mass spectrometry of the condensed product was found to be 164 Da. The study concludes that while refluxing improves the quality of the condensed product by reducing its average molecular weight, it also leads to a reduction in the liquid production rate and an increase in the permanent gas yield.