Fundamentals of methane adsorption in microporous carbons

Abstract

This paper analyzes from a fundamental point of view the adsorption of methane on microporous carbons. CH 4 adsorption on a series of microporous activated carbons (AC), activated carbon fibers (ACF) and superactivated carbons (SAC), at 298 K and 4 MPa (supercritical conditions) is analyzed. These results are compared with those obtained from the adsorption of other gases (N 2 at 77 K up to 0.1 MPa and CO 2 at 273–298 K up to 4 MPa). A detailed analysis is made to obtain realistic values of the saturation pressure, the affinity coefficient and density of adsorbed CH 4. The results show that CH 4 adsorbs in narrow micropores (<0.7 nm), which are well characterized by CO 2 adsorption. For samples with uniform micropore size distribution (MPSD), CH 4 adsorption data fit very well the characteristic curve. Contrarily, on samples with heterogeneous MPSD, CH 4 adsorption data do not fit the characteristic curve. These results point out the dependence of CH 4 density with the pore size at supercritical conditions. Although CH 4 adsorbs in the microporosity and fills it as a compressed gas, the adsorption data has been also related to the specific BET surface area. A linear relationship between the estimated BET areas obtained from both adsorptive (N 2 and CH 4) is observed. In this paper, a new molecular surface area for CH 4 at 298 K has been estimated (0.38 nm 2).