Thermal Heating/Thermal Enhancements
The physical and chemical properties of organic contaminants, together with the properties of porous media, limit the removal of contaminants from the subsurface through traditional technologies, such as groundwater extraction or soil-vapor extraction (SVE). Most of the properties of organic contaminants are temperature-dependent. Furthermore, various types of liquid and vapor movement can be induced in porous media as a result of temperature gradients.
Understanding these temperature effects and their relative magnitudes is important in understanding how in-situ thermal remediation processes might possibly be used to enhance the recovery of organic contaminants. The primary mechanism for enhanced recovery of volatile organic compounds (VOCs), which include fuels such as gasoline and diesel, and chlorinated solvents, is the increase in vapor pressure that accompanies increases in temperature, which allows these compounds to be recovered in the vapor phase. Enhanced solubility and decreased adsorption at higher temperatures will also contribute to the recovery of these chemicals.
The recovery of semivolatile and (essentially) non-volatile oils also can be enhanced to some degree at higher temperatures through viscosity reduction, increased relative permeability, and decreased capillary forces. These changes in physical properties allow oils to be displaced in the liquid phase when the degree of saturation is greater than residual saturation, and also tend to decrease the amount of residual saturation. Field experience demonstrates that even semivolatile organic compounds (SVOCs) can be recovered to some degree when high temperatures and low vapor-phase pressures conditions are created in the subsurface.
Thermally enhanced extraction is a combination of groundwater extraction, SVE, steam or hot-water injection, hot-air injection, radio-frequency (RF) heating, or in-situ resistive heating, coupled with control technologies, which is intended to accelerate and more completely remove contaminants from the subsurface. Heat is introduced to the subsurface by any of several methods (hot water, steam or hot-air injection, RF or resistive heating) to vaporize volatile and semivolatile contaminants. Vaporized components rise through the vadose zone where they are removed by SVE and then treated. In-situ biological treatment may follow the application of Thermally enhanced extraction, and is continued until the concentrations of contaminants in groundwater satisfy established remediation objectives.
The process can be used to remove oily waste accumulations, and to retard the continued downward or lateral migration of mobile organic contaminants. The process is applicable to contaminant source zones at shallow or greater depths in the subsurface, and can be used in soils of low, moderate, or high permeability. The target contaminant groups for Thermally enhanced extraction are SVOCs and fuels, including heavy oils. VOCs also can be treated using this technology, but there are more cost-effective processes for sites contaminated with VOCs. This technology can be applied at manufactured gas plants, wood-treating sites, petroleum-refining facilities, and other sites with soils containing light to dense organic liquids, such as coal tars, pentachlorophenol solutions, creosote, and petroleum by-products.
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