Extension of Kelvin equation to CO2 adsorption in activated carbon

CO2 adsorption in activated carbon (AC) at ambient temperature was found to be mainly accomplished by filling of pores with diameter smaller than 1 nm. Kelvin equation correlates operating conditions with the critical (or maximum) diameter of pores where pore filling occurs, but it is not valid for the micropores due to deviation of the physical parameters of CO2 from their normal values. This work extends Kelvin equation to CO2 adsorption in AC by introducing a general dimensionless parameter (KCO2-AC, termed YL parameter) to account for the changes in physical parameters of CO2 in micropores. This approach is similar to the use of fugacity and activity coefficients to modify the ideal gas equation. The work includes measurements of CO2 adsorption capacity for 3 types of AC, fitting the capacity to the ACs' pores from the smallest one to the larger ones to determine the critical pore diameter at each condition, and fitting the critical pore diameter data to a modified Kelvin equation to obtain KCO2-AC. It is found that KCO2-AC varies slightly with the pore diameter but significantly with temperature and can be approximated by an empirical equation. The modified Kelvin equation excellently describes CO2 adsorption data for ACs reported in the literature and studied in this work.