Molecular Mechanisms of the Effect of Water on CO2/CH4 Mixture Adsorption in Slitlike Carbon Pores

The grand canonical ensemble Monte Carlo method has been used to study adsorption of carbon dioxide, methane, and their mixtures with different compositions in slitlike carbon pores at a temperature of 318 K and pressures below 60 atm. The data obtained have been used to show the effect of fixed amounts of pre-adsorbed water (19, 37, and 70 vol %) on the adsorption capacity and selectivity of carbon micro- and mesopores. The presence of water reduces the adsorption capacity throughout the studied pressure range upon adsorption of gaseous mixtures containing less than 50% CO₂, as well as in narrow micropores (with widths of 8?12 Å). Upon adsorption of mixtures with CO₂ contents higher than 50%, the adsorption capacity of pores with low water contents appears, in some region of the isotherm, to be higher than that in dry pores. In the case of wide pores (16 and 20 Å), this region is located at low and moderate pressures, while for mesopores it is located at high pressures. The analysis of the calculated data has shown that the molecular mechanism of the influence of preadsorbed water on the adsorption capacity is based on the competition between the volume accessible for adsorption (decreases the capacity) and the strength of the interaction between carbon dioxide molecules and water molecules (increases the capacity). Therewith, the larger the surface area of the water–gas contact, the stronger the H₂O–CO₂ interactions.