Molecular simulation of CO2 capture from flue gas and natural gas using carbon nanotubes

Abstract

Nanoporous carbons including the well-shaped carbon nanotubes (CNTs) and the amorphous carbons, activated carbon fiber-15 (ACF-15) and silicon carbide derived carbon (SiC-DC) have been investigated regarding CO2 separation in this study, in order to reveal the performance of CNTs in separating CO2 from flue/natural gas relative to conventional amorphous carbons. A full understanding of the performance of CNTs in separation of CO2 requests fundamental knowledge of the adsorption and diffusion of sorbates in the CNTs. Since both flue and natural gases are saturated with water, and the carbons will be wetted in the humid separation environment, a thorough investigation of the effect of water vapour and pre-adsorbed water on the adsorption of CO2 and CO2/CH4 and CO2/N2 mixtures in the CNTs and amorphous carbons is vital to determine the potential of CNTs on CO2 separation. On the other hand, the diffusion of sorbates through the CNT has to overcome the interfacial barriers located at the entrance and exit of the CNT, which drag down the transport diffusion of sorbates dramatically, 2-3 orders of magnitude observed in this study, compared to the diffusion in the infinitely long CNT. As there lacks a feasible and effective method to quantitatively measure the interfacial barriers and its relative importance to the overall diffusion resistance, a novel equilibrium molecular dynamics (EMD) simulation method is proposed in this study and employed to measure the interfacial barriers for methane at the entrance and exit of the CNTs.Initially, the adsorption of CO2 in different sizes of CNTs with and without pre-adsorbed water is studied using grand canonical Monte Carlo simulations (GCMC). It is found while the adsorption capacity shows strong dependency on the diameter of the CNT, the chirality of the CNT has negligible impact on the adsorption of CO2. Meanwhile, due to the strong hydrogen-bonding, preadsorbed water molecules form clusters during the adsorption of CO2. It is intriguing to observe that the water clusters are split into smaller ones by the enhanced adsorption of CO2 at high pressures, and this splitting effect in turn enhances the adsorption of CO2 in the CNT, which is because those split smaller water clusters distribute in the CNT, further facilitating the water-CO2 interactions.Further, the adsorption of CO2/CH4 and CO2/N2 mixtures in the CNTs, CNT bundles and the amorphous carbons, in the presence of water vapour and pre-adsorbed water clusters is studied. At first, negligible adsorption of water vapour was observed in the carbons considered, both for the CO2/CH4/H2O and CO2/N2/H2O ternary mixtures saturated water. For the CO2/CH4/H2O ternary mixture, pre-adsorbed water clusters in the carbons attract the water vapour molecules to adsorb on them, leading to noticeable adsorption of water in the CNTs and amorphous carbons, which reduces the adsorption of CO2 and CH4. Nevertheless, the enhanced additional water-adsorbate (CO2+CH4) interactions prompt the adsorption of CO2 over CH4 and hence facili...