Experimental assessment of gas transport mechanisms in natural porous media: Parameter evaluation

An understanding of vapor transport in natural porous media is critical to the assessment of a wide range of environmental problems. In this work a comprehensive experimental program was undertaken to evaluate the relative importance of different gaseous transport mechanisms in natural porous media. The dusty gas model was used as a framework for this evaluation. The experimental program was divided into two parts: the first emphasizes the measurement of porous media transport parameters, and the second explores flux mechanisms in organic vapor transport. Results of the first part are presented in this paper. Single and binary gas experiments were conducted to obtain governing transport parameters (coefficients of permeability, Knudsen diffusion, and diffusibility) in dry porous media. To conduct these experiments, a special experimental apparatus was built that incorporates a diffusion cell that represents an open system where the pressure gradient and absolute pressures can be regulated by controlling the flow rates of the component gases at both ends of the soil sample. Soils tested included three uniform materials, a sea sand, an Ottawa sand, and kaolinite clay, and five graded mixtures of these uniform soils, Results of the single-gas experiments illustrate the importance of Knudsen diffusion in permeability measurements. Two new transport parameter correlations are developed from the presented data and compared with available literature correlations. The correlation for the Knudsen diffusion radius has a functional dependence upon the square root of intrinsic permeability, selected as a characteristic length of the porous medium. Binary diffusion experiments, using an equimolar pair of gases, are employed to develop a correlation for a composition independent diffusibility.