Impact of H2O on CO2 Separation from Natural Gas: Comparison of Carbon Nanotubes and Disordered Carbon

The adsorption of pure H2O, CO2/CH4, and H2O/CO2/CH4 mixtures in carbon nanotubes (CNTs) ranging from (6,6) to (15,15), and in a reconstructed model of a silicon carbide derived carbon (SiC-DC), was studied using Monte Carlo simulations, at 300 K. We also investigated the effect of preadsorbed water and the choice of water model including SPC, SPC/E, TIP3P, TIP4P, and TIP5P, on the adsorption of CO2/CH4 and H2O/CO2/CH4 mixtures in the CNTs and SiC-DC. Using the SPC model, our simulations reveal that, below saturation pressure, the adsorption of water is negligible in CNTs with diameters ranging from 0.81 to 2.03 nm, and in the SiC-DC. Water fills the CNTs and the SiC-DC model suddenly when the pressure reaches a critical value that is above the saturation pressure, and exhibits nonmonotonic variation with CNT diameter. It was also found that the small amount of water adsorbed from the saturated H2O/CO2/CH4 mixture has little impact on the adsorption of the components CO2 and CH4. When water is preadsorbed, it is found to be present in the form of hydrogen-bonded clusters, and the average water cluster size is reduced when the amount of adsorbed CO2 and CH4 is increased. For the adsorption of a CO2/CH4 mixture, the CO2 selectivities of the (7,7) CNT and SiC-DC increase almost linearly with the density of preadsorbed water, while the capacities of CH4 and CO2 in the narrow (7,7) CNT are reduced, and the capacity of CO2 in the SiC-DC is slightly enhanced by the preadsorbed water. It was found that the choice of water model has little impact on the adsorption of the CO2/CH4 mixtures in either carbon. Noticeable adsorption of H2O was observed for the H2O/CO2/CH4 mixture in the (7,7) CNT and SiC-DC having 0.05 g/cm3 preadsorbed water, as a result of strong hydrogen bonding between the preadsorbed water and water molecules from the gas phase. For the H2O/CO2/CH4 mixture, the CO2 selectivities of the (7,7) CNT and SiC-DC are subsequently enhanced by the additional cooperative wateradsorbate interactions. We conclude that, among the CNTs investigated, the (7,7) CNT shows the best performance for separating CO2 from natural gas at atmospheric pressure and is superior to the amorphous carbon, SiC-DC.