Graduate Thesis Or Dissertation
 

Applications of Nonthermal Microplasmas in Chemical Reaction Engineering

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/6969z527z

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  • Nonthermal plasmas generate high concentrations of excited species that can simultaneously exist at high energy and far from thermodynamic equilibrium, making them useful tools in chemistry and engineering. Microplasmas, roughly defined as plasmas that are generated within sub-millimeter dimensions, provide enhanced stability, improved excited species density, increased nonequilibrium properties, higher electron temperature, and better energy efficiency along with reduced onset voltages compared to traditional nonthermal plasmas, making them promising candidates for novel chemical processing pathways. This work summarizes current knowledge regarding the advantages gained by generating nonthermal microplasmas in constricted spaces, on reduced timescales, and with engineered electrodes. Those insights are then used in the experimental evaluation of DC microplasma reaction systems in methane processing and the oxidation of model refractory sulfur compounds in fuel-like media.The reaction environment generated by nonthermal plasmas is well suited for the activation of non-spontaneous gas phase reactions. Here, a microreactor capable of generating low power atmospheric pressure glow discharges is used in methane processing. The reactor effectively performs oxidative methane coupling to C2 and C3 hydrocarbons with methane conversions up to 50% and selectivity to C2 C3 products greater than 90%, achieving one pass yields that surpass state-of-the-art catalysis.The generation of DC nonthermal plasmas in fuel-like media for the oxidative desulfurization of dibenzothiphene has also been investigated. At discharge gaps around 250 microns, plasmas can be initiated with DC potentials above 6 kV for short periods of time before carbon bridges are formed that short the reactor. These simple DC discharges show little promise for continuous flow desulfurization processes. However, in flat plate reaction systems with silver epoxy electrode surfaces with discharge gaps less than 50 microns, electrically driven reactions can occur at much less than 1,000 volts. These discharges warrant further investigation and characterization in future works, and could be promising systems for the oxidative desulfurization of diesel fuel.Complementary to experimental investigations, COMSOL multiphysics models have been developed to provide insight into the kinetics of gas phase plasmachemical reactions, as well as the electric field of point-to-plane microplasma reactor designs. The kinetic models of the oxidative coupling of methane are preliminary, however, the current simulations produce the same compounds as the experimental system with realistic kinetic parameters. These models provide an excellent platform for more complicated kinetic modeling. Increasing the number of modeled plasmachemical reaction pathways will likely allow the model to converge on experimental data and be used in predictive analysis of the constructed microplasma reactor in the oxidative coupling of methane.
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