Laboratory study evaluating electrical resistance heating of pooled trichloroethylene
Martin, Eric John
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A laboratory scale study was conducted to evaluate the thermal remediation of trichloroethylene (TCE) in a saturated groundwater system using electrical resistance heating (ERH). Two experiments were conducted using a two-dimensional polycarbonate test cell, the first consisting of a single pool of TCE perched above a capillary barrier, the second consisting of two pools of TCE each perched on separate capillary barriers. Temperature data was collected during the heating process from an array of 32 thermocouples located throughout the test cell. Visualization of the vaporization of liquid phase TCE, as well as the upward migration of the produced vapour was recorded using a digital camera. Chemical testing was performed 48 hours after experiment termination to measure post heating soil concentrations. A co-boiling plateau in temperature was found to be a clear and evident earmark of an ongoing phase change in the pooled TCE. Temperature was found to increase more rapidly in the second experiment that included a fully spanning barrier. As temperatures increased above the co-boiling plateau, vapour rise originating from the source zone was observed, and was found to create a high saturation gas zone beneath the upper capillary barrier when no clear pathway was available for it to escape upwards. When the source zones had reached the target temperature of 100°C and the ERH process stopped, this high saturation gas zone condensed, leading to elevated TCE concentrations below as well as within the capillary barrier itself. The water table within the experimental cell was also noted to drop measurably when the gas zone collapsed. Post-testing chemical analysis showed reductions in TCE concentrations of over 99.04% compared to the source zone, although due to condensation of entrapped gas and convective mixing, there was a net redistribution of TCE within the experimental domain, especially within confined areas below the capillary barriers. A secondary set of experiments were conducted using a homogenous silica sand pack with no chemical contaminants to determine the effect, if any, of the wave shape of electrical input on the ERH process. It was found that in early time heating, square wave inputs consistently produced a more localized heating pattern when compared to the standard sine wave electrical input. This effect equalized between the two experiments as the ERH process went on, perhaps due to the increased dominance of conduction and convection as the mode of heat transfer in the test cell at higher temperatures. It is believed that the localization of heating in square wave experiments is due to a consistent power supply due to the lack of a sinusoidal ramping in power delivery.