HEAT AND MASS TRANSFER IN CLEAN-UP OF UNDERGROUND TOXIC WASTES
Kent S. Udell
Berkeley Environmental Restoration Center, Department of Mechanical Engineering, University of California, Berkeley, Berkeley, CA 94720
The problem of underground contamination by spills of toxic liquids is outlined in terms of the transport phenomena that determine contaminant location and rates of recovery during in situ clean-up. Residual liquid-phase contaminant trapping is quantified by balances of viscous or gravitational forces against capillary forces. As an additional mode of contaminant migration, mass transfer of the contaminant in the vapor phase due to natural convection is presented. Thermodynamic constraints of residual liquid contaminant removal by soil gas venting and groundwater pumping are identified for permeable regions. Mass transfer limitations to the recovery rates are presented for aqueous dissolution in permeable zones. For removal from low permeability zones, analytic models are summarized for receding interface evaporation of single and multiple component liquid contaminant mixtures.
The acceleration of recovery rates of second phase liquid contaminants from the subsurface during gas venting operations due to an increase in soil temperature is discussed from a thermodynamic standpoint. The use of combined steam injection and vacuum extraction to recover volatile, semi-volatile, and non-volatile contaminants from the subsurface is outlined. A review of one-dimensional studies of steam injection shows effective removal of both volatile and semi-volatile second liquid phase chemicals, as well as non-volatile aqueous phase contaminants from porous media. Two-dimensional experiments with homogeneous and layered sand packs are presented to show recovery mechanisms for volatile and semi-volatile hydrocarbon liquids. The enhancement of mass transfer from lower permeability regions during the depressurization mode of operation is discussed. Interstitial water vaporization due to depressurization or electrical heating is shown to be effective in the removal of volatile, aqueous phase contaminants to an extent that can meet clean-up objectives. Numerical simulation is discussed in reference to capabilities, limitations, and extensions. A field-scale study of steam injection with electrical heating combined with vacuum extraction is summarized to illustrate the effectiveness of thermal techniques and their applicability to contaminants found both above and below the water table. The current state of the use of thermal techniques for toxic waste clean-up is discussed, and recommendations for future work are given.