Mixed Gas Transport Studies Through Vertically Aligned Carbon Nanotube Membranes

Recent experiments demonstrated that gas flow through carbon nanotubes (CNTs) occurs at a rate 1-2 order of magnitude faster than predicted by Knudsen theory. Although single-gas permeation studies are helpful for a preliminary characterization of the CNT membrane transport properties,  they are not sufficient for a detailed understanding of their separation performance, in particular in the presence of condensable gases or when there is a specific interaction between permeating gas molecules and membrane surface. A deep understanding of the nature of gas transport through CNT membrane is still lacking. Therefore, investigation of mixed gas transport through CNT membranes is critical to advance our understanding of gas fluid flow in CNTs and for their future application is separation processes.

In this work, we will present a systematic investigation of mixed gas and single gas permeation through a CNT membrane having vertically-aligned 2.5nm-wide CNTs as only pores. Single gas permeation studies for various gases showed that hydrocarbons (e.g. CH₄, C₂H₄, and C₃H₆), CO₂ and SF₆ deviated from inverse-square-root scaling wrt. molecular weight, while weakly absorbing gases (e.g. He, N₂, and Ar) followed Knudsen behavior. For helium and nitrogen, gas diffusion occurs almost entirely in gas-phase even at low temperature.  On the other hand, carbon dioxide showed a contribution of surface diffusion at low temperature. Inert gas mixture of N₂/Ar showed a separation factor near Knudsen prediction. For the mixture of CH₄/N₂, CO₂/N₂ and C₃H₆/N₂, the separation factor increased at low temperature and high concentration of hydrocarbons or carbon dioxide. The results of this mixed gas study agree well with single-gas permeation results showing the presence of surface flow.