We report here the results of our recent studies describing the preparation of carbon molecular sieve membranes, their separation and transport characteristics with gas mixtures, and their molecular simulation. Poly(furfuryl alcohol) (PFFA) was used as the polymeric precursor for the preparation of the carbon films. The membranes were tested using single gases H2, CO2, CO, CH4, and Ar, as well as binary mixtures of CO2/CH4 and a four-gas mixture consisting of CO2/CO/H2/CH4. Separation factors for CO2/CH4 in the range of 34−37 were obtained for the binary and the four-gas mixtures. The membrane permeance decreased slowly during continuous testing with the four-gas mixture. This decline in performance was found, however, to be reversible. The initial membrane permeance was recovered by heating the membrane in an inert atmosphere of Ar. Modeling of the membrane's transport characteristics was carried out using a nonequilibrium molecular dynamics simulation. The modeling results agree qualitatively with the experimental data.

A new class of carbon molecular sieve membranes (CMSMs) has been prepared by carbonization of polyetherimide-coated mesoporous tubular supports. The membranes show higher permeance and better separation factors than other supported CMSMs reported in the literature for the CO2/CH4 and H2/CH4 binary mixtures as well as for the CO2/H2/CH4 ternary mixture. CO2/CH4 separation factors as high as 145 for the equimolar binary and 155 for the ternary mixture were obtained with a CO2 permeance about 0.15 (cm3/cm2·psi·min). The corresponding H2/CH4 separation factors for the equimolar binary and ternary mixtures were 68 and 50, respectively, with a H2 permeance of 0.13 (cm3/cm2·psi·min). The membrane also shows good stability when tested with CO2 and Ar single gases, as well as with an equimolar mixture of CO2/CH4. To study the mechanism of permeation and separation in CMSMs, tests with single gases as well as with binary and ternary mixtures were performed at different temperatures, transmembrane pressure differences, and feed compositions. Elemental analysis, scanning electron microscopy, and gas adsorption were also employed to study the morphology of the resulting membranes. Elemental analysis shows that although the structure consists mostly of carbon, it also still contains oxygen, nitrogen and hydrogen. Scanning electron microscopy of the cross section of the carbonized membrane shows that the carbonized layer lies essentially within the mesoporous γ-alumina layer, a result also verified by N2 adsorption analysis at 77 K. The experimental data were compared with simulation results with the same mixtures using a nonequilibrium molecular dynamics method.

No abstract

We present the results of extensive nonequilibrium molecular dynamics simulations of transport and separation characteristics of binary and ternary gas mixtures consisting of CO2, CH4, and H2 through a carbon nanopore, in the presence of an external chemical potential gradient. The gas molecules are represented as Lennard-Jones (LJ) hard spheres. The effect of the various factors, such as the temperature, feed composition, and the pore size, on the transport, adsorption, and separation characteristics is investigated in detail. The simulations’ predictions are compared with experimental data obtained with a carbon molecular-sieve membrane. In some cases, there is good agreement between the predictions and the experimental data, while in other cases the simulations’ results and the data do not agree. Possible causes for the (dis)agreement are discussed, including the crucial interplay between two main factors in gas separation in a pore space, namely, adsorption on the pores’ walls versus the morphology (the pores’ interconnectivity and size distribution) of the porous material. Improved models are thus suggested.

No abstract