Determination of absolute gas adsorption isotherms: simple method based on the potential theory for buoyancy effect correction of pure gas and gas mixtures adsorption

The absolute adsorption isotherms are necessary to correctly evaluate the selectivity of the adsorbent material or to design adsorption processes at high pressure (e.g., H₂ purification from syngas processes, removal of acid gas from natural gas,…). The aim of this work is thus to propose an easy method to correct the buoyancy effect of the bulk phase on the adsorbed phase volume during both pure gas and gas mixtures adsorption for pressures up to 10 MPa. The potential theory of adsorption and the Dubinin–Radushkevich relation are adapted by introducing mixing parameters based on simple Berthelot rules. The concept of internal pressure used to characterize the adsorbed phase is also adapted for mixtures. The method is then improved on a commercial activated carbon (AC), when adsorbing pure H₂S and CH₄, and their mixtures up to 5 MPa. The study points out the importance to carefully consider the buoyancy effect of the bulk phase on the adsorbed phase volume. Its impact on the adsorbent material selectivity at high pressures could affect the design and the performances of PSA or TSA processes. For example, only considering the excess adsorption data leads to an apparent selectivity 13 % greater than the absolute one for a concentration of 6 ppm of H₂S in a CH₄ matrix at 5 MPa (298 K) on the AC.