Adsorption performance indicators for the CO2/CH4 separation: Application to biomass-based activated carbons

A deep understanding of the interaction and competition between different gases on adsorption processes is essential for the design and optimization of industrial units. Two biomass-derived activated carbons (CS-CO2 and CS-H2O), synthesized in our laboratory, were evaluated as selective adsorbents for the separation of CO2 from CO2/CH4 mixtures. Adsorption isotherms of the pure gases (i.e., CO2 and CH4) were performed in a high-pressure magnetic suspension balance at three different temperatures (303, 323 and 343 K) up to 1000 kPa and the data were correlated using the Sips and Toth models. The Ideal Adsorbed Solution Theory (IAST) was applied to predict the binary adsorption equilibrium. The results were validated by means of experimental breakthrough tests in a fixed-bed set-up. The isosteric heats of adsorption were estimated from the pure component adsorption data by means of the Clausius-Clapeyron equation and the Sips and Toth equations. All of the equilibrium data were integrated in a performance indicator defined so as to be able to evaluate the adsorbent in terms of selectivity, working capacity and adsorption enthalpy under conditions relevant to the specific application. Although both CS activated carbons had similar textural features, CS-CO2 showed a better overall performance than CS-H2O for the separation of CO2 from CO2/CH4 mixtures. Our results highlight the importance of carrying out a deep analysis of the adsorption equilibrium under conditions relevant to the foreseen application in order to identify the most suitable adsorbent.