High performance nanoporous carbon membranes for air separation

The preparation of porous stainless steel supports was found to have a significant impact on the properties of nanoporous carbon membranes fabricated upon them. Nanofillers were incorporated into porous stainless steel supports to modify the pore structure by reducing the average pore size and porosity. Carbon membrane properties were examined as a function of support variables such as filler content, shape, size and nature of the particles. Optimum performances, in terms of the ideal selectivity ratio for oxygen to nitrogen permeances ( S O 2 / N 2 ∼ 3 – 6 ) and the oxygen permeance (10−8 mol m−2 s−1 Pa−1), were obtained when the filler completely saturated the support. This represents about a two order of magnitude improvement in oxygen permeance when compared to carbon membranes prepared on unmodified porous stainless steel supports. The origin of the improvement in the permeance is due to the formation of carbon membranes which are on average two orders of magnitude thinner than those formed on unmodified supports, i.e., the carbon membranes exists as very thin layers around and between the silica nanoparticles. A simple geometric model based on the packing of silica particles inside the porous stainless steel support is proposed to visualize and quantify this effect. The generality of the support modification concept is also demonstrated by the ability to employ different types of nanofillers and support geometries to obtain carbon membranes with high flux. Air separation experiments show that these membranes can produce both oxygen rich streams enriched to as much as 48% by volume and nitrogen rich streams enriched to over 90% by volume at reasonable operating conditions.