Knudsen diffusion in microporous carbon membranes with molecular sieving character

Abstract

Measurements were done on a relatively wide pore carbon molecular sieve membrane (CMSM) by means of a range of probe molecules (He, H 2, N 2, CH 4, CO 2, C 5H 12) between 25 and 500 °C. At high temperatures, where adsorption effects are attenuated, the permeance of the carbon membrane to pure gases displayed the molecular weight and temperature dependence expected from Knudsen diffusion, even though the pore size distribution lies demonstrably below 0.55 nm. Utilizing a concept of an effective diameter, the authors analyzed the temperature dependence of the probe molecules’ permeabilities and separated out the relative contribution of the different gas transport mechanisms in the pores of the carbon microporous membranes. Calculations of the total gas permeance from two different idealized membrane pore structures were made and compared to measured permeances. The comparison showed that an idealized pore structure comprising larger and smaller diameter pore regions essentially connected in series can reasonably explain the measurements. The estimate for the larger pore diameter was found by using the temperature dependence of n-pentane permeance, which showed behavior approaching that to be expected for activated diffusion typical of sieving membranes. The larger diameter region of the pore system was estimated to be 0.58 nm and the smaller diameter region to be 0.43–0.48 nm. After correction for the contribution of Knudsen diffusion, the gases tested could be ranked according to the contribution of adsorption-mediated transport as follows: CO 2>CH 4>N 2>H 2>He based on the calculated net energies for pore transport. This is consistent with the ranking to be found for adsorption isotherms of these gases on activated glassy carbon.