Adsorption equilibria and kinetics of propane and propylene in silica gel

Adsorption equilibrium isotherms for propane and propylene on narrow pore silica gel (NSG) and wide pore silica gel (WSG) were measured by gravimetry in the temperature range 303-343 K and at pressures up to 101 kPa. The isotherms were correlated by the generalized Dubinin isotherm and Polanyi's theory. Both adsorbents have a higher affinity for propylene. The loading at 100 kPa of propylene is 2 and 0.7 mmol/g on NSG and WSG, respectively. These loadings are 1.5 higher than those for propane with both adsorbents. The heat of adsorption estimated by the Clausius-Clapeyron equation decreases with the degree of coverage. Adsorption kinetics in the same temperature range was studied by the zero-length column (ZLC) method using the long time response. The diffusion of propane and propylene in both silicas is controlled by Knudsen diffusion. The pore diffusivities determined in the temperature range 303-343 K are 0.0024-0.0028 and 0.0017-0.0024 cm2/s for propane and propylene on NSG, respectively. For WSG these values are 0.0064-0.0099 cm2/s for propane and 0.0066-0.0121 cm2/s for propylene. Average tortuosity factors of 2.5 for NSG and 2.0 for WSG were obtained.