Adsorption and desorption kinetics in activated carbon

Modelling of fixed bed adsorption and desorption kinetics for gas separation and purification is based on the correct representation of mass transport mechanisms on the single-pellet scale. In the case of adsorption it is commonly assumed that pore mass transport within the adsorbent particle is rate limiting. It has been questioned whether this is also true for desorption. Here the exchange step between the adsorbed state on the solid surface and the mobile state within the pore fluid was considered as being the dominant kinetic resistance. Therefore, experimental and theoretical investigations of single component adsorption and desorption of water vapor, n-hexane, cyclohexane and tetrachloroethylene on single pellets of activated carbon were performed. From the results it can be concluded that fore most cases of physical gas-phase adsorption as well as desorption the overall kinetics is controlled by mass transport within the pore system of the adsorbent. The exchange step between the adsorbed phase and the fluid phase happens infinitely fast justifying the assumption of adsorption equilibrium at every position along the pellet radius.