Pressed graphite crystals as gas separation membrane for steam reforming of ethanol

Pressed graphite was evaluated as a potential membrane for steam reforming of ethanol in membrane reactors. In ethanol steam reforming, hydrogen has to be in situ removed selectively from a mixture with ethanol, CO2, and H2O. Commercial graphite flakes (single crystals) have been pressed into disc membranes of different thicknesses. Both single gas permeation and H-2/CO2/H2O mixed gas permeation were studied. From single gas permeation, a relatively high ideal separation factor of 35,...,60 for the H-2/CO2 mixture could be predicted. However, the study of the real separation factor of this mixture by gas-chromatographic analysis gave real mixture separation factors around 5. This experimental finding is explained by a Knudsentype mechanism with permeation paths along the grain boundaries of the pressed graphite flakes. At temperatures between 100 and 250 degrees C, the pressed graphite membrane is indeed hydrogen-selective. Hydrogen is separated from a H-2/CO2/H2O mixture with a separation factor of 5 relative to CO2 and 12 relative to H2O, but only 2.4 relative to ethanol. Pressing of the graphite crystals results in a self-orientation (brick layer structure) of the individual graphite crystals. Hydrogen permeation parallel to the aligned flake-shaped crystals is by the factor 25 faster than perpendicular to them. The hydrogen permeabilities through the pressed graphite membranes are about one to two orders of magnitude higher than those through molecular sieve membranes such as supported zeolite or MOF membranes. (C) 2014 Elsevier B.V. All rights reserved.