Dual diffusion and finite mass exchange model for adsorption kinetics in activated carbon

A model allowing for the finite mass exchange between the two phases is proposed for the description of adsorption kinetics in activated carbon. This model based on Do's earlier structural model for activated carbon involves three mass-transfer processes: pore diffusion, adsorbed phase diffusion, and finite mass interchange between the fluid and adsorbed phases. The solid phase is heterogeneous, which is characterized by the micropore size distribution. The interaction between the adsorbate molecule and the micropore is calculated from the Lennard-Jones potential theory. The model developed for nonpolar adsorbates is tested with the experimental data of seven adsorbates (paraffin gases, aromatics, carbon dioxide, and sulfur dioxide) on pellets of different shapes and sites and at various operating conditions. The finite kinetics play an important role in the overall kinetics. Failure to account for this finite kinetics makes the model unable to describe correctly the desorption behavior since under such conditions, the ability of the particle to release adsorbed molecules is dictated mostly by the resistance at the pore mouth of the micropore.