Date of Award

2022

Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Chemical Engineering (MS)

Administrative Home Department

Department of Chemical Engineering

Advisor 1

Michael Mullins

Committee Member 1

Pradeep Agrawal

Committee Member 2

Tony Rogers

Abstract

Concerns over greenhouse gases have led to an increased interest in the Dry Reforming of Methane (DRM) which produces hydrogen and carbon monoxide from the reaction of two greenhouse gases (CO2 and CH4) over a catalyst. Currently, DRM is primarily a catalytic process which operates at temperatures between 700°C - 900°C, and 10 to 20 bar using a 1–1.5 ratio of CH4/CO2. Unfortunately, these conditions also promote the water-gas shift reaction, which produces additional CO2. Catalyst coking and sintering can also be significant problems at these harsh conditions. We have developed a non-thermal, pulsed-plasma catalytic DRM reactor which operates at ambient temperatures and pressures. When combined with an integral monolithic catalyst bed this reactor demonstrated high conversions (60 to 80%) of both methane and carbon dioxide with high yields of hydrogen and carbon monoxide (30 to 80%). To achieve this, a novel solid-state, MOSFET-based HV pulse generator was developed with controllable rise times (4-20 ns), pulse duration (0.1 to 10 ms), pulse shape, and frequency (100 -10,000 Hz). This solid-state circuit provides improved operational flexibility and higher energy efficiency. The reactor incorporates a point-to-plane electrode arrangement with an integral monolithic catalyst cell which effectively places the catalyst in direct contact with the excited state plasma. The catalysts employed are copper oxides doped with a secondary metal oxide and are tailored for low-temperature plasma DRM reactions. Bench scale reactor tests were conducted using a feed of methane and/or carbon dioxide diluted in either nitrogen or argon. To evaluate the reaction kinetics, the partial pressure of the reactants and products were measured in real time via an on-line mass spectrometer, while the excited state species were simultaneously monitored using emission spectrometry. Tests were made with the plasma alone, and the plasma plus 4 different catalyst formulations. No significant reactions were observed for the plasma without a catalyst, or for the catalyst without a plasma. The reaction kinetics were measured for a range of input power, voltages, pulse length & frequency, and electrode geometries. The feed ratio of CO2 to CH4 was found to be of great significance in the overall conversion and the yield of hydrogen and CO, with near stoichiometric reactant ratios proving to be the best. The stoichiometric ratio of carbon monoxide to hydrogen in the products depended on the combination of the metal oxides employed and to the strength of reactant adsorption on the catalyst surface. Based on the observed kinetics and emission spectroscopy results, we propose a surface moderated reaction model which explains the high reactant conversions and product yields observed. Estimates of the energy efficiency of the bench-scale process, and rate of reaction indicate the potential of this novel reactor for practical applications.

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