(454c) First-Principles Kinetic Modeling System for the Pyrolysis of Waste Plastics

Plastic waste accumulation in oceans and on land represents a growing concern as we produce about 300 million tonnes of plastic waste every year.¹ Waste plastic is a hydrocarbon rich material that represents a valuable feedstock to recycle into new products, increasing the circularity of plastic use. In this study, a first-principles kinetic modeling system was developed employing Aspen Plus Polymers for the pyrolysis of plastics into chemical feedstocks with the method of moments to track the evolution of inert and radical polymer species in a reactor. Each class of polymer is characterized by representative repeat units. The system automatically generates the free-radical reaction types for the polymers, including random scission, midchain β-scission, cyclic scission, depolymerization, hydrogen abstraction, hydrogen shift, debranching, termination by combination, and termination by disproportionation. A chain-length dependent probability distribution partitions the formation of low-molecular weight oligomer species tracked with one additional set of moments. The moments of the low-molecular weight oligomer species inform the Flory-Schulz distribution to discretize the formation of the specific low molecular weight products from polymer pyrolysis. Additional thermal cracking reactions modeled at the pathway level with power-law kinetics account for secondary conversion of the oligomer products to other observable product moieties. The final model captures both the evolution of the specific low-molecular weight products and the change in bulk polymer properties such as weight-average (M_w) and number-average (M_n) molecular weights, polydispersity index (PDI), and degree of polymerization (DP). Model results compare very well to experimental data reported in literature for common plastics (HDPE, LDPE, PP, PS) as well as their mixtures. This model of waste plastic pyrolysis can be utilized as a starting point for optimization and incorporation of chemical recycling of waste plastics in the circular economy paradigm.

[1] United Nations Environment Program. (n.d.). Visual feature: Beat plastic pollution. Retrieved March 20, 2022, from https://www.unep.org/interactives/beat-plastic-pollution/