Transport and sorption of water vapor in activated carbon

Control of organic vapors by adsorption on activated carbon can be impacted by the presence of water vapor in the gas stream. As a foundation to better understand this issue, the transport and sorption of water vapor within activated carbon grains was studied. Adsorption/desorption kinetics and equilibrium partitioning of water between gas and sorbed phases were determined at relative humidities (RH) in the range of 0-86% and at 20°C using an electrobalance. A model that accounts for transport within grains by pore and surface diffusion was used to interpret the sorption kinetic data. The model assumes instantaneously attained local equilibrium between sorbed and gas phases within the pores, based on a piecewise-linear isotherm measured for a small ensemble of activated carbon grains. Despite the complicated observed shape of the isotherms, including hysteresis, the model conforms well to all of the experimental data, and the asymmetry between adsorption and desorption is well resolved. The model results indicate that gas-phase diffusion dominates transport through the pores at RH ≤ 60%. Surface diffusion is found to be important at higher RH values.