Polystyrene Upcycling using a Two-Ring Non-Thermal Plasma-Catalysis Reactor

Abstract

Current plastic recycling technologies strain to effectively mitigate the rapid increase and disposal of plastic waste. Polystyrene (PS) is one plastic of particular concern due to its chemical stability and resistance to degradation without special treatment. Cost-intensive separation of PS from other waste contaminants constrains mechanical recycling methods whereas chemical recycling techniques, such as pyrolysis or gasification, require high temperatures (~780 K) to achieve PS depolymerization. Non-thermal plasma (NTP)-facilitated plastic upcycling has drawn increasing attention due to the presence of energized electrons, ions, and other species within the NTP able to initiate chemical reactions at ambient conditions. However, more research is needed to understand how NTP-driven reactor design (reactor configuration, packing conditions, and other parameters) can be tailored for desired PS upcycling outcomes. This thesis seeks to characterize the overall conversion and product selectivity of polystyrene degradation under varying reactor conditions with a two-ring dielectric barrier discharge (DBD) reactor operating under a hydrogen (H2) and argon (Ar) co-fed gas mixture. Key parameters studied included the influence of packing material, reaction time, reactor pressure, and catalyst loading. Our results starting at ambient temperature, 300 Torr, 15.3 W discharge power, and a flow of 100 mL min-1 (sccm) 40% H2/Ar showed that ~ 30 wt% of the initial PS mass can be converted to gaseous products within the first 15 minutes and up to 81 wt% within the first hour. Gaseous yields were considerably higher than liquid yields, and methane was the primary gas component. The highest yield of ethane and ethylene, at 11 and 1.5 mol% respectively, were achieved within the first five minutes under non-catalytic conditions. Within the reactor volume, the configuration of the quartz wool (QW) support clearly influenced plasma discharge characteristics. Under the studied parameters, the partial packing of QW created higher average power in the plasma discharge, and as high as a 76% increase, in contrast to when fully packed with QW. While varying reactor conditions and catalyst selection showed only a marginal change of product selectivity, this NTP reactor configuration demonstrates a promising approach for rapid PS depolymerization under ambient conditions with moderate discharge power.