Pyrolysis\nof Polyethylene: Chemical Kinetics, Mass\nTransfer, and Reflux System

First, this paper investigates the interplay between\nreaction kinetics\nand mass transfer rates in the pyrolysis of polyethylene (PE) in the\ntemperature range of 420 to 480 °C. Experimental results confirmed\nthat the bond-breaking rate is high, shown by the reduction in average\nmolecular weight of the reacting PE from 186 to 3.5 kDa in 2 min at\n450 °C. The evaporation rate differs with reactor type and sets\nthe mass loss rate and the molecular weight distribution of the condensed\nproduct. At 450 °C, first-order mass loss rate constants of 5\n× 10<sup>–2</sup> and 3 × 10<sup>–3</sup> s<sup>–1</sup> were observed in a screen-heater reactor and a semibatch\nreactor, respectively, while the average molecular weights of the\ncondensed product were 2.3 and 0.6 kDa (mass-average MW by gel permeation\nchromatography), respectively. A mathematical model, involving bond-breaking\nand evaporation, was able to describe the experimentally observed\nfast initial cracking followed by evaporation. Second, the effect\nof adding a reflux system to the semibatch reactor was experimentally\nstudied, showing that tested reflux temperatures (300–350 °C)\nset the molecular weight distribution of the condensed product, independently\nof the investigated reactor temperature. Notably, at a reactor temperature\nof 450 °C and reflux temperature of 350 °C, the average\nmolecular weight by gas chromatography/mass spectrometry of the condensed\nproduct was found to be 164 Da. The study concludes that while refluxing\nimproves the quality of the condensed product by reducing its average\nmolecular weight, it also leads to a reduction in the liquid production\nrate and an increase in the permanent gas yield.