CONTRIBUTION OF CONCENTRATION-DEPENDENT SURFACE-DIFFUSION TO RATE OF ADSORPTION

A model is presented which describes the intraparticle mass transfer considering both gas phase and adsorbed-phase diffusion. The model accounts for the concentration dependence of surface diffusion, and uses the Langmuir isotherm to describe the adsorption equilibria. A parametric study is presented showing the effects of various variables on the overall uptake curve and intraparticle concentration profiles. It is shown that surface diffusion contributes significantly to the overall intraparticle mass transfer. More importantly, inclusion of concentration dependence of surface diffusivity in the model influences significantly both the overall uptake and the intraparticle concentration profiles. The overall effect of concentration dependence of surface diffusivity is to enhance the rate of uptake during adsorption, and decrease it during desorption. Also, experimental uptake curves are presented for n-hexane on BPL activated carbon at two temperatures, and the data are correlated with the model. The surface diffusivities calculated from the model ignoring the concentration dependence of surface diffusivity are higher by a factor of 2-3 compared to the correct values obtained from the concentration-dependent model.