Experimental Study of Gas Transport Parameters in Unsaturated Silica Flour

Abstract

A: DGM, Dusty Gas Model.T N An experimental study was conducted to assess the rela ve contribu ons of molecular and Knudsen diff usions for gasphase transport in unsaturated silica fl our with an intrinsic permeability of 1 ? 2 × 10 −14 m 2 . Single-gas phase fl ow and binary diff usion experiments were performed on the same soils to determine the Knudsen diff usion and the eff ec ve gas-phase diff usion coeffi cients, respec vely. These results were further used to assess the diff usibility factor for Fick's law of diff usion and the obstruc on factor for the Dusty Gas Model theory. These factors were used to account for impedance to gas diff usion caused by the tortuous nature of soil pores. The Knudsen diff usion coeffi cient was found to be one to two orders of magnitude greater than the eff ec ve diff usion coeffi cient for silica fl our at water satura ons ranging from 53 to 75% (v/v), which suggested that the Knudsen eff ect has negligible impact on the eff ec ve diff usion coeffi cient for the unsaturated silica-fl our system at water satura ons lower than 75%. For unsaturated silica fl our with air-fi lled porosity <0.2, the diff usibility factor (or the obstruc on factor) value can be more than 10 mes smaller than the values es mated by other diff usibility correla ons published in the literature.

N2O emissions from a cultivated mollisol: optimal time of day for sampling and the role of soil temperature

“…Taking into account that diffusion capacity is related to impedance to gas diffusion, which depends on the shape and orientation of the soil pores (Fen et al, 2009), it can be also suggested that this pore arrangement, plus a WFPS percentage close to 90 %, could reduce the rate at which gases move from the soil to the atmosphere (Glinski & Stepniewski, 1985). This is consistent with results of Bartelt-Hunt & Smith (2002), who found that with a decrease in soil moisture content, and therefore an increase in the percentage of air-filled pores (30 to 39 %), the gas diffusion coefficient rises from 0.0133 to 0.0609 cm 2 s -1 .…”

Knudsen and Molecular Diffusion Coefficients for Gas Transport in Unconsolidated Porous Media

“…Note that the experimental results for the samples DSF 1 and DSF2, and WSFl, WSF2, and WSF3 were presented in Fen et al (2009) and are also provided here for discussion. Table 2 presents the apparent permeability values for each sample determined by Eq.…”

“…They found that the predictions obtained from this correlation wete less than the results from the difftisivity models of Millington and Qiiirk (1961), Penman (1940), Gurrie (1970), Lai et al (1976), and Millington and Shearer (1970). Fen et al (2009) obtained another correlation of the obstruction factor for silica Hour at various water saturations and with effective gas permeabilities ranging from 10"^'^ to 10"'^ m^. Their predictions of this correlation were far less than the result obtained from the diffusivity models of Millington and Qmrk (1961) and Moldrup et al (2000).…”

“…All of these studies are based on the dusty gas model (DGM) theoty, viihich provides a rigorously theoretical framework for assessing gas transport mechanisms in porous systems. Experimental work on determining gas-phase transport parameters, i.e., permeability and the Knudsen and molecular diffusion coefficients, in porous media according to the DGM theory has been conducted by Abu-El-Sha'r and Abrióla (1997), Reinecke and Sleep (2002), and Fen et al (2009. Theoretical investigation has shown that the Knudsen diffusion coefficient is related to the intrinsic permeability of a porous medium and the Klinkenberg parameter (Thorstenson and Pollock, 1989), which can also be expressed as a function of the intrinsic permeability of the medium (e.g., Heid et al, 1950;Jones and Owens, 1980).…”

“…This coefficient is also assessed by modifying the air-filled binary diffusion coefficient by a diffusivity factor (Penman, 1940). Both of these factors consider the reduced ability of gas diffusion in soils; however, they are determined with different theories and experimental methods (Fen et al, 2009). Ntimerous studies have derived correlations of the difiusivity factor in terms of the air-filled porosity and total porosity of soil (e.g.. Penman, 1940;Millington and Quirk, 1961;Ball, 1981;Shimamura, 1992;Schaefer et al, 1997;Moldrup et al, 2000Moldrup et al, , 2004.…”