A determination of the flux of gas-phase volatile organic compounds due to naturally occurring, environmentally significant driving forces

There have been few studies to date which have rigorously examined the commonly used forms of the mass and momentum transport equations for gas transport in porous media by linking equation development and numerical modeling with laboratory experiments. Initially, one-dimensional laboratory experiments were conducted to explore the transport of a dense gas (Freon-113) through air-dry Oso-Flaco sand. Gas densities and fluxes were measured during transport through a packed column. Significant differences in fluxes and density profiles were observed for the three primary flow directions (horizontal, vertically upward, vertically downward) at high source densities. Pressure gradients due to the non-equimolar diffusion of freon and air were measured throughout the column. Simulated fluxes from numerical models based on the standard Darcy-Fickian transport equation did not fit the measured fluxes. The method of volume averaging was utilized to derive microscale coupled equations for gas-phase transport in porous media. The expressions for both the advective velocity and the mass transport contained novel terms which could have significant import for flow regimes of environmental significance. New terms in the velocity expression arose from the inclusion of a slip velocity boundary condition and closure level coupling to the mass equation. A new term in the mass conservation equation, due to the coupling at the microscopic level, modifies and may act in opposition to traditional advective transport. Comparisons were made of output from numerical models based on the traditional and new volume averaged equations with experimental data. The new “slip coupling” term in the fully coupled mass conservation expression has been shown to be extremely important in accurately modeling gas transport in varied advective/diffusive flow regimes. Modeling results indicated that the “slip coupling” term will be large at low species densities and diminish as the species density approaches saturation. The exploration of the physics of gas flow in porous media conducted in this work sheds light on important transport processes that may be of interest to researchers in many fields. Further research on the exact representations of the transport parameters developed herein will increase the utility of the newly derived gas transport expressions.