Methane Storage within Dry and Wet Active Carbons: A Comparative Study

Methane storage capacities on dry and water-wetted active carbon powders are compared. Sorption isotherms of methane at temperatures of 2°C and pressures up to 8 MPa are constructed for four carbonaceous materials. Three of these materials originate from the same precursor (coconut shell), are physically activated at various burnoffs, and are mainly microporous; the fourth material is a highly mesoporous, chemically activated pinewood carbon. In the dry state, these adsorbents exhibit classical Langmuirian behavior. Wetting the materials with a constant water/carbon weight ratio of ∼1 leads to isotherms that are now characterized by a marked step. The latter occurs near the expected formation pressure of methane hydrates, thus supporting their occurrence within the porous materials. The amount of gas stored at the highest pressures investigated then ranges from 180 to 230 volumes at standard temperature and pressure (STP) per unit volume of storage vessel (V/V), depending on the material, whereas only 110−160 V/V are obtained with dry carbons (at 2°C, 8 MPa). Hence, wetting the carbonaceous adsorbents improves the methane storage capacities, thus confirming recent works. The results are discussed on the basis of the known pore texture of each adsorbent, and stoichiometries of the formed hydrates are calculated. Considerations about adsorption and desorption kinetics, and pore size distributions, are also developed.