Dispersion and diffusion in porous media under supercritical conditions

Hydrodynamic dispersion significantly impacts solute transport in a porous medium, such as a packed bed reactor or a soil column. Recent developments in the area of supercritical fluid extraction and chromatography have creat ed a need for understanding dispersion phenomena in such systems. Because t he literature in this area is sparse and limited to convection-dominated co nditions, a supercritical fluid system was constructed for measuring disper sion coefficients in porous media at low Reynolds numbers (Re < 0.1). The a pparatus allowed quantification of the axial dispersion of methane in super critical carbon dioxide by imperfect pulse chromatography. Flow rate (inter stitial velocity) was varied for a variety of solid types. A range of size fractions including Borden sand (nominal diameter, d(p) = 0.01, 0.015, 0.03 3 cm and bulk sample), Moffet aquifer sand (d(p) = 0.033 cm) and spherical glass beads (d(p) = 0.015 cm) were employed. With the temperature fixed at 45 degrees C, pressure was varied from 140 to 450 atm. Experimental Re valu es ranged from 0.01 to 3. As expected, mechanical dispersion dominated over diffusion at higher flow rates (approximately Re > 0.4, diffusion-based Pe clet number, Pe(d) > 3), and measured dispersion coefficients agree well wi th those reported elsewhere. The dispersion coefficients measured for the l ower flow rates (approximately Re < 0.1, Pe(d) < 0.4) demonstrate diffusion dominance over mechanical dispersion. The observed dispersion coefficients are demonstrated to be greater than those typical of liquid systems and le ss than those for gaseous systems for particular Re or Pe(d) values. Howeve r, the dispersion behavior is consistent with that reported for gases and l iquids when the observed dispersion coefficients are scaled by the appropri ate diffusion coefficients. Experimentally determined dispersion coefficien ts are correlated with Pe(d)(= Re Sc) for values ranging from 0.02 to 30. T he fitted expression captures the dynamics of the transition from diffusion to convection-dominated dispersion under supercritical conditions and is c onsistent with previously reported expressions for liquids and gases. (C) 1 998 Elsevier Science Ltd. All rights reserved.