(423c) A Mechanistic Modeling Framework for Describing Tertiary Recycling of Polymers

While plastic waste makes up 11.3% of municipal solid waste (MSW), only 5.3% of this waste was recycled in 2003 according to the Environmental Protection Agency. Given that plastics are the class of MSW that is recycled in the second lowest amount, improving plastic recycling technologies offers a significant opportunity for recovering value from discarded materials. Pyrolysis is a promising method for resource recovery from plastic waste that thermally converts plastics in the absence of oxygen into valuable chemical feedstocks and monomer. To further the development of pyrolysis as a resource recovery method, a greater understanding of the mechanistic and kinetic details of the underlying reaction network is needed. Because of the complexity of mechanistic modeling of macromolecular systems, we have focused on developing a mechanistic modeling framework for describing polymer pyrolysis that uses population balances and the method of moments and includes a significant level of detail that allows structure to be related to reactivity. Given that polystyrene (PS) and polypropylene (PP) comprise a significant fraction of plastic waste (8.5% and 13.5%, respectively), we have concentrated on developing detailed mechanistic models of degradation of single components and binary mixtures of PS and PP.

Recently, debate has emerged about the details of PS degradation despite decades of research. For example, a wide range of activation energies, 20 to 75 kcal/mol, has been reported, and structural anomalies have been proposed as the underlying cause. In addition, there is also a large disparity between reported quantities of the major products of PS pyrolysis. To help resolve this debate, we have implemented algorithms to increase the level of detail tracked in the PS pyrolysis model. Specifically, a methodology was included to track chain backbone triad units based on conditional probabilities in the continuum model. Individual triad concentrations are tracked using triad balance differential equations. Furthermore, the mechanistic model was coupled with a spatially dependent reactor model that was solved using a multiple timestep approach. These improvements were applied to develop a model of PS pyrolysis that allowed us to probe the effect of weak links on the activation energy and the role of small molecule diffusion on the observed low molecular weight product distribution. The methodologies developed are sufficiently general that they can be applied to study a wide range of polymer degradation systems, including multicomponent mixtures of plastics that are relevant to resource recovery.