Diffusion of ethane and ethylene in carbon molecular sieve membranes by pulsed field gradient NMR

Carbon molecular sieve (CMS) membranes are promising new materials which have potential applications in light paraffin/olefin gas separations. Despite the importance of mass transport in these materials there have been no reported diffusion studies of C-2 (or larger) hydrocarbons in CMS membranes using microscopic techniques. In this work, a pulsed field gradient (PFG) NMR technique employing high magnetic field and large magnetic field gradients enabled the measurement of microscopic self-diffusion of ethane and ethylene in selected CMS membranes. For the studied Matrimid CMS membranes, the results of PFG NMR diffusion studies are compared with corresponding data obtained from macroscopic permeation measurements reported in the literature. It was found that although the ratios of ethane and ethylene diffusivities measured by these two techniques were the same within the experimental uncertainty, the diffusivities obtained by PFG NMR were several times larger than those obtained from permeation. The reasons for such difference are discussed. (c) 2013 Elsevier Inc. All rights reserved.