Empirical Theory of Vacuum-Assisted Steam Stripping of Organic Pollutants from Contaminated Soils

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Courtesy of ASTM International

A theoretical model was developed for the removal of organic chemicals from soil via vacuum-assisted steam stripping. The model considers a homogeneous, isotropic soil described by a single steam permeability coefficient. (The author realizes that this is an idealization of most field situations.) It is assumed that the entire soil mass has been steam heated to 100°C (or slightly above), so the temperature can be considered a constant for the entire decontamination process. The steady-state steam pressure and velocities are determined by solving Laplace´s equation for two geometries: a circular symmetry model and a point source model (results for the latter are available upon request). One-dimensional laboratory experiments were conducted to determine the needed steam permeability and decontamination rate of a particular pollutant during steady-state steam flow. The rate of decontamination was determined over a range of steam velocities. The soils used in this particular study were 100% sand, 75% sand/25% Delaware River silt, and 50% sand/50% Delaware River silt. The “pollutant” was commercially available kerosene at a level of 5% by weight in the soil. The soil was initially moisture-free.

Results are presented as the time needed to produce either 80 or 100% decontamination. These times depend on the position in the soil—the higher the steam-flow velocity, the shorter the times to decontaminate. Drawing on previous work, the times to reach steady state are also presented. For contaminated regions as large as 10 m in diameter for these three soil types, the times to decontaminate to at least 98% appear reasonable.

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