Study of the Pyrolysis Kinetics of a Mixture of Polyethylene, Polypropylene, and Polystyrene

The principal aim of this study was the identification of possible routes for the reaction mechanism of a plastic waste mixture pyrolysis. Kinetic studies of a mixture of equal percentages of polyethylene (PE), polypropylene (PP), and polystyrene (PS) were performed in order to analyze if the direct conversion of plastic wastes into gaseous, liquid, and solid products was favored, or if parallel reactions and/or reversible elementary steps should be considered. The experiments were carried out in a six microautoclave system. On the basis of the experimental results obtained at different temperatures and reaction times, a reaction pathway was proposed. The fitting of a kinetic model to the experimental data was performed. The models reasonably fit the results and gave a satisfactory explanation of several experimental data of thermal degradation of the plastic waste mixture tested. The kinetic parameters were estimated, and a dependence of the activation energy and of the pre-exponential factor on temperature was observed, so it was verified that the rate constant of some reactions exhibited nonlinear temperature dependence on the logarithmic form. This fact probably indicates that temperature affects the reaction mechanism. The product composition was also analyzed, and on the basis of the data obtained, reaction mechanisms were proposed.