Mechanistic Modeling of Polymer Pyrolysis: Investigation of Intrinsic Kinetics, Reaction Pathways, and Structural Heterogeneities

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Resource recovery is a promising category of polymer recycling where polymeric waste is converted via thermal or chemical means to monomer and chemical feedstocks. Specifically, pyrolysis is an attractive method because of its simplicity and ability to handle a heterogeneous feedstock. Polymer pyrolysis is characterized by a complex free radical reaction network, which often yields a diverse product spectrum. While polymer pyrolysis has been studied for over 60 years, there are still questions about the kinetics and mechanisms of these reaction systems. We have utilized detailed mechanistic modeling to gain insight into the kinetics and mechanisms of polystyrene (PS), polyethylene (PE), and poly(styrene peroxide) (PSP) pyrolysis. Mechanistic models based on the method of moments were developed to study PS and PE pyrolysis. Using the PS pyrolysis model, the possible reaction pathways for styrene dimer formation were examined. Net rate analysis demonstrated that the 7,3-hydrogen shift pathway was dominant for dimer formation, while the benzyl radical addition pathway became more competitive as temperature increased. Additionally, the PS pyrolysis model was used to determine an overall activation energy of 53.3 +/- 1.3 kcal mol-1, which was free of transport effects. The PE pyrolysis model was utilized to study the temporal evolution of the diverse product spectrum from PE decomposition. Net rate analysis was utilized to compare the general reaction pathways of product formation. Random scission was found to be dominant, while the backbiting pathway played a complementary role for product formation during PE pyrolysis. The method of moments modeling framework was extended by developing an algorithm to track backbone triad concentrations within polymer pyrolysis models. The algorithm was validated using the PS pyrolysis model. A PSP pyrolysis model was constructed using this algorithm and was of manageable size, but because of the stiffness of the model equations, it could not be solved. To address this difficulty, a kinetic Monte Carlo model for PSP pyrolysis was constructed. The model was used to test the traditional mechanism for PSP pyrolysis. A new reaction pathway relying on successive hydrogen abstraction reactions was found to be viable for formation of the minor products of PSP pyrolysis.

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  • 09/17/2018
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