Page 355 - gas transport in porous media

Chapter 22: Environmental Remediation of Volatile Organic Compounds
sand has an effective gas permeability that is more than 1000 times higher than the
fine sand. 357
22.2.3 Diffusive Mass Transfer Limitations

Contaminant concentrations leaving SVE wells are typically well below saturation
values predicted by the chemical’s vapor pressure or Raoult’s Law. While this is
partly due to dilution effects, a number of experimental studies (Gierke et al., 1992;
Wilkins et al., 1995; Fischer et al., 1996) have evaluated the local scale interphase
mass transfer between the NAPL or aqueous phase, and the flowing gas phase. At
the small scale of these studies (cm to tens of cm) it does not appear that gas phase
transport is the limiting factor in the mass transfer. For example, Baehr et al. (1989)
observed the evaporation of gasoline in an experimental column to be an equilibrium
process. Similarly, Wilkins et al. (1995) observed relatively small deviations from
expected equilibrium values in their column NAPL evaporation tests at realistic gas
velocities, although at higher gas velocities, the effluent concentrations were only
about 60 to 80% of the equilibrium value.
For cases where the contaminant is dissolved in pore water, the local mass transfer
into the gas phase can be slower, depending on the type of soil, and the length of time
over which the experiment is operated (Gierke et al., 1992; Fischer et al., 1996). At
large times, it appears that the local interphase mass transfer is limited by aqueous
diffusion, but at early times, an equilibrium model usually provides a good fit with
the data.
As discussed previously in Section 2.6, gas flow patterns that occur in hetero-
geneous media give rise to larger scale gas diffusion limited mass transfer in SVE
systems. Referring again to Figure 22.3, for a given horizontal gas pressure gradient,
the gas flow in the coarse sand units would be hundreds of times faster than in the fine

Medium sand
Fine sand
Coarse sand
Medium sand, k =3

Coarse sand k =10
Fine sand k = 0.3 h

Coarse sand k =10

Medium sand k =3
1.0
0 0 1.0
s 1
Figure 22.3. Gravity-capillary equilibrium in a layered vadose zone (after Looney and Falta, (2000))