Molecular simulation of CO2/N2 separation using vertically-aligned carbon nanotube membranes

The CO2/N2 permeation through carbon nanotube (CNT) membrane was studied using molecular simulations to explore its potential for flue gas separation. The theory of gas permeation was derived on the basis of the Maxwell–Stefan formulas. Adsorption isotherms calculated using grand-canonical Monte Carlo methods show a higher loading of pure CO2 than that of N2, while an approximately identical loading was found for CO2/N2 mixture with a feed pressure ratio of 1:9. The molecular snapshot indicates a concentrically layered structure inside the cylindrical channel of CNT. Results of molecular dynamics simulations show that the diffusion of N2 is a few times faster than that of CO2. For binary mixtures, the diffusion coefficients of CO2/N2 mixtures become identical at high concentrations due to the correlation effect that causes a slowing-down of the mobile species, and a speed-up of the other tardy component. The calculated N2 permeance agrees with the experimental measurement. The computed CO2 flux through CNT membranes is higher than N2 for single components, while they become nearly equal under flue gas conditions.

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► Theory of gas permeation through microporous membranes was developed and validated.
► Transport mechanism of CO2/N2 in CNT membranes was analyzed at a molecular level.
► Permeation selectivity of CO2 under flue gas condition is low in pure CNT membrane.
► Increasing CO2 solubility via CNT functionalization is key for separation efficiency