Classical and reactive simulations of plastic co-pyrolysis: Effects of polystyrene and its fragments on product yields, aggregation, and reaction mechanisms

Abstract

Mixtures of polyethylene (PE), polypropylene (PP), polystyrene (PS), and their fragments such as ethylbenzene (EB) and linear alpha olefin (LAO) are simulated at different ratios using classical and reactive force fields. Using all-atom models validated by calculating conformations and diffusivities of differently sized PE melts in agreement with experiments, simulations are performed at 450 or 513 K that mimics a low temperature under the thermal co-pyrolysis condition, showing that the decomposition of PE into LAO influences densities and diffusivities of PE, to an extent dependent on PE lengths. In PE/PP/PS simulations, PS polymers aggregate at PS concentrations of 20 wt% or higher, leading to the separation between PS and other polymers. With the inclusion of EB that are assumed to be decomposed from PS, polymers do not aggregate. To understand reaction mechanisms, reactive simulations of PE and PS are performed, showing more heavy oil and less gas products at higher PS concentrations, in agreement with experiments, because PS fragments such as benzene, EB, and methylbenzene bind to PE and end up in heavy or light oils. In particular, the transfer of H radicals between PE and PS not only prevents further decomposition of polymers but also yields free radicals of polymers that can expedite co-pyrolysis. These findings help explain experimental observations regarding the effect of PS on liquid and gas product yields and suggest the optimal PS concentration without aggregation under the thermal co-pyrolysis condition, interpreted by reaction mechanisms of the transfer of H radicals and benzene fragments.