Gas product distribution from polyethylene pyrolysis

The disposal of plastic waste is extensively studied nowadays. Of the treatments available, thermal degradation is attracting great interest because it offers the possibility of recovering energy and useful chemicals. PE pyrolysis has already been discussed in a previous paper [1] where the rate of decomposition of a sample was modeled using a detailed kinetic scheme. Its predictions were validated by thermogravimetric experimental results across wide operative pressure and heating rate ranges. The gas product distribution was derived from the random scission hypothesis. Thus, the ratio of alkane, alkene and dialkene species was 1:2:1 and, within the same class of products, the distribution was equimolecular, except for those species which are more favored by allyl resonance. This paper aims to continue the previous work in attempting to model gas products correctly. The principal improvements made here are the introduction of backbiting reactions into the kinetic scheme to predict the larger amounts of selected products and the consideration of the physical aspects of the degradation process. These phenomena are responsible for the production rate and the distribution of volatiles. A simplified model of the in-depth mass transfer through bubble formation is presented which describes the degradation progress for a small-sized polyethylene sample. The model predictions are compared with both the experimental results of polyethylene pyrolysis performed under various temperature conditions and with thermogravimetric analysis. (C) 1999 Elsevier Science B.V. All rights reserved.