Utilization of Computational Fluid Dynamics and Aqueous Organic Oxidation Experiments to Aid the Development of a Tubular High-Density Plasma Reactor

Derek C. Johnson and David S. Dandy. Department of Chemistry, Colorado State University, Fort Collins, CO 80523

Plasma treatment of contaminated water is a promising alternative for the oxidation of aqueous organic pollutants and biological disinfection.  Experiments have yielded a number of important insights into the sparging and oxidation of methyl tert-butyl ether (MTBE), benzene, ethylbenzene, toluene, m- and p-xylene, and o-xylene (BTEX) in a plasma reactor utilizing a submersed point-to-plane electrode configuration.  Rate constants associated with plasma initiated oxidation, interphase mass transfer and photolysis were determined using a combination of nonlinear least squares analysis and Matlab® optimization techniques for each species.  Computational fluid dynamics was then applied to the study of three-dimensional fluid flow in the reactor under different operating conditions.  The reaction mechanisms and rates previously developed for the removal of MTBE were used to determine the plasma discharge volume, the rate of interphase mass transfer, and the photolysis rate of MTBE via UV emission from the plasma.  The simulations show that the volume of fluid directly interacting with the plasma in the reactor only constitutes a maximum of approximately 10% of the fluid intended to be cycled through the plasma arcs.  The simulations also predict appreciable pressure gradients on the surfaces of the pin electrodes, resulting in a small discharge area located away from the region in which the electric field strength is a maximum.  This result has been confirmed indirectly through observation in that the pin electrodes sputter metal from an area of similar size and location to the low-pressure region predicted by the simulations.  The pressure gradients are shown to be a function of operating conditions as well as pin location, indicating that the plasma discharge conditions are not consistent throughout the reactor.  Given the experimental and computation fluid dynamics results, a prototype tubular high-density plasma reactor has been designed.


The rate constants developed for the original plasma reactor, in conjunction with a species mass balance on the prototype tubular high-density plasma reactor, have been applied to determine the removal rates of MTBE and BTEX when operating in batch and continuous flow configurations.  The dependence of contaminant concentration on parameters such as treatment time, number of pin electrodes, electrode gap, and volumetric flow rate has been determined. It was found that, under different design specifications and operating conditions, the tubular high-density plasma reactor might be an effective tool for the removal of organic compounds and biological agents from aqueous solutions.  Based on these results, a prototype tubular high-density plasma reactor has been fabricated.  Characterization of the aqueous plasma discharge has been performed as an initial step in determining the feasibility of the new reactor to oxidize aqueous organic compounds.  Current and voltage measurements will be presented for varying operating conditions such as electrode gap, solution conductivity, number of pin electrodes and feed gas.  The sputtering rate of the pin electrodes has also been examined to determine the time for which the plasma discharge can be sustained without electrode maintenance.