The adsorption of methane on an activated carbon of high surface was measured in the range of 233−333 K and 0−10 MPa. The isotherms showed a considerably different feature than that measured on low surface carbon. A maximum appeared on isotherms at relatively low temperatures. All the experimental isotherms were well modeled by the Langmuir−Freundlich equation. The difference between the measured and the so-called absolute adsorption was properly accounted for in modeling. An assumption of monolayer adsorbate adsorbed was included in the model. The layer volume evaluated was consistent with the pore volume of adsorbent reported by CO2 adsorption. The intermolecular distance in the adsorbed phase was evaluated from a model parameter. This intermolecular distance was compared with that in the free sate, which revealed a basic picture of the physical state of the adsorbed phase at above-critical temperatures.

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The adsorption of CO2 on superactivated carbon was measured for the near-critical region, and a comparative study between the excess and the absolute adsorption was presented. The quantity of absolute adsorption was determined based on the principle that it is equal to the excess one if the product of the gas-phase density and the volume of the adsorbed phase can be neglected. However, all isotherms in the ranges of 273−360 K and 0−18 MPa can be satisfactorily modeled by properly accounting for such product. The number of molecular layers in the adsorbed phase was estimated based on the density and volume of the adsorbed phase as evaluated. It was shown that multilayer adsorption is possible for near-critical temperatures but seems impossible if the temperature is fairly far away from the critical. Finally, it was shown that the difference between the absolute and the excess isotherms of CO2 at 273 K could yield about 20% difference in adsorbent characterization.

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