Coupled Population Balance Models for Evolution of Alkane, α-Olefin, and α,ω-Olefin Molecular Weight Distributions During Polypropylene Thermal Pyrolysis.

Pyrolysis is a relatively mature process for recycling plastic waste, yet predictive kinetic models remain elusive due to the enormous number of radical reaction pathways, intermediates, and products. Starting from elementary hydrogen abstraction, random and chain-end β-scission reactions, we construct continuum population balance equations (PBEs) and discrete species balance equations (SBEs) to model polypropylene (PP) pyrolysis for realistic initial molecular weight distributions (MWDs) and reactor time and length scales. The framework accounts for the separate but coupled MWDs of alkanes, α-olefins, and α,ω-olefins polymers, along with the amounts of volatile alkanes and olefins generated during the reaction. We parameterized the model using rate constants from prior ab initio calculations. The model predicts the evolution of the polymer MWD, the degree of double-bond functionality, the number of scission events, and the amounts of volatile alkane and alkene products as a function of time. The predictions agree with experimental MWD data and resolve prior questions about the double-bond functionality (f) of volatile and nonvolatile products. We discuss how the model can help design processes to obtain products of the desired molecular weight and functionality.