The diffusion of a CO2/CH4 mixture in carbon nanotube (CNT) bundles was studied using molecular simulations. The effect of diameter and temperature on the diffusion of the mixture was investigated. Our results show that the single-file diffusion occurs when CO2 and CH4 are confined in CNTs of diameter less than 1.0 nm. In CNTs of diameter larger than 1.0 nm, both molecules diffuse in the Fickian style. The transition from single-file to Fickian diffusion was demonstrated for both CO2 and CH4 molecules. A dual diffusion mechanism was observed in the studied (20, 0) CNT bundle, single-file diffusion of CO2 in the interstitial sites of (20, 0) CNT bundle and Fickian diffusion of CO2 and CH4 in the pores. For CO2, the interaction energies (CO2–CO2 and CO2–CNT) are larger than that of CH4 in all cases. But only a very small difference in the diffusion coefficient was observed between CO2 and CH4. Temperature has a negligible effect on the difference between diffusion coefficients of CO2 and CH4 in the studied CNT bundles. The adsorption, diffusion and permeation selectivities are discussed and compared, and the adsorption is demonstrated to be the rate limiting step for the separation of CO2/CH4 in CNT bundle membranes.
- carbon nanotube bundle
- molecular simulation