Polymer pyrolysis is a promising technology for resource recovery of polymeric waste. It offers the advantages of being simple since it only involves heating the waste in the absence of oxygen, applicable for addition polymers, which make up the majority of polymeric municipal solid waste and cannot undergo solvolysis, and the possibility of handling a mixed polymeric feed. Polymer pyrolysis is comprised of a large system of free radical reactions that usually lead to a broad product distribution. To utilize this technology fully, we need to increase our fundamental understanding of these systems. Mechanistic modeling provides a powerful tool for gaining insight into these complex reaction systems. In this work, the mechanistic modeling framework we have developed was applied to polystyrene (PS) pyrolysis to provide insight into outstanding areas of debate about the controlling chemistry.

The modeling framework uses the method of moments to track polymeric species, which are divided based on key structural elements. Examples of structural elements include end-chain and mid-chain radicals and both saturated and unsaturated primary and secondary carbon chain ends. Low molecular weight species and radicals are tracked explicitly. Elementary reaction types are used to create the terms in population balance equations for each species. Structure-reactivity relationships are used to link kinetic parameters to the structure and thermodynamics of the reactants and products. A hierarchical approach utilizing reliable literature values first and estimation techniques if literature values are not available is used to determine the rate parameters for the model. This framework has been successfully applied for PS pyrolysis, polypropylene (PP) pyrolysis, and PS/PP binary pyrolysis.

While PS pyrolysis has been studied for over 50 years, there are still many important questions without clear answers. A range of overall activation energies from 25-70 kcal/mol has been reported in the literature. Many different values for the ratios of the major products (styrene monomer, dimer, and trimer) produced by PS pyrolysis have been reported from different experimental studies. The reaction pathway to dimer is still not well understood. Through the inclusion of additional reaction families and structural detail to the model, a thorough analysis of the governing rate parameters, and coupling the mechanism to a reactor model accounting for physical effects, we utilized our framework to provide insight into some of these issues.