(299i) Utilization of Computational Fluid Dynamics and Aqueous Organic Oxidation Experiments to Aid the Development of a Tubular High-Density Plasma Reactor

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.