Gas Production and Mass Transfer During Electrical Resistance Heating of Clay Lenses
The objective of this study was to develop a mechanistic understanding of remediation in clay lenses in sand by electrical resistance heating (ERH). Clay lenses are areas of accumulation for dense non-aqueous phase liquid (DNAPL) and are difficult to remediate. Experiments were performed in a two-dimensional, saturated porous medium comprising of an electrically conductive, low permeability clay lens embedded within a less electrically conductive, higher permeability silica sand. This study is based on an experimental program and mathematical modeling of experimentally measured data. To study the lens exterior, experiments compared the differences in heating and gas production during ERH between pure sand geologies and one containing a clay lens. Preferential heating occurred in the lens interiors with higher rates of heating. Gas production occurred preferentially around the sand-clay interface. Gas production in the interior of clay lenses after ERH was quantified for lenses composed of kaolin and #20-30 silica sand (40%, 70% and 100% kaolin by mass). A measurement technique was developed to interpret electrical conductivity as gas saturation in the lens. Gas saturations after ERH increased with decreasing clay fraction, indicating that a connected gas phase capable of rapid mass removal can be produced in lenses containing moderate amounts of clay. While gas was produced in the pure clay lens, it is possible that a connected gas phase was not achieved at the applied power and treatment time considered. To assess mass transfer mechanisms in lenses, pure clay lenses containing dissolved-phase trichloroethene (TCE) were subjected to ERH to produce gas saturations that were either negligible, disconnected, or connected. Electrical and chemical concentration data were analyzed using diffusion models and indicated that TCE concentration reductions where no gas was produced were consistent with liquid phase diffusion. High gas saturations were likely connected and resulted in non-detect levels of TCE after heating. Concentration reductions at moderate gas saturations were consistent with multi-phase (gas and liquid) diffusive transport when the connected gas phase at the lens exterior is considered. The rate of diffusion increases nonlinearly with gas saturations that are disconnected and immobile, and enhanced by connected gas at the lens interface.
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