Adsorption, Desorption and Single-File Diffusion in Carbon Membranes:
What is the Rate-limiting step?
Department of Chemical Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel
In our recent study [G.A. Sznejer, I. Efremenko and M. Sheintuch, AIChE Journal, 50, 596 (2004)], we found that counter diffusion of nitrogen and C2 to C4 alkanes through a carbon membrane module in the temperature range of 25-400°C significantly inhibits the fluxes of both, while hydrogen flux was only slightly diminished. This was a very promising result in the search for a highly selective, yet relatively inexpensive, membrane that may be employed in membrane reactors for separation of hydrogen, simultaneously with equilibrium-limited reactions. To account for this result, in the present study, we use molecular mechanics simulations to find the energetics of adsorption, diffusion and desorption of individual gases in cylindrical nanopores modeled by carbon nanotubes. Several specific features that are characteristic of van der Waals interaction of a molecule with narrow carbon nanopore are found. The hydrogen-hydrocarbon separation selectivities calculated using computed thermodynamic values and pore size distribution compares favorably with the measured selectivities.