-Separation of CO 2 from CO 2 -CH 4 mixtures is an important issue in natural gas and biogas purification. The design of such separation processes depends on the knowledge of the behavior of multicomponent adsorption, particularly that of CO 2 -CH 4 mixtures. In this study, we present a series of experimental binary equilibrium isotherms for CO 2 -CH 4 mixtures on an activated carbon at 293 K and compare them with predicted values using the Ideal Adsorption Solution Theory (IAST) and the Extended Langmuir (EL) model. Even at concentrations of ca. 20% for all binary isotherms, CO 2 already presents higher adsorbed amounts with respect to CH 4 . A maximum selectivity of around 8.7 was observed for a nearly equimolar mixture at 0.1 MPa. The IAST in conjunction with the Toth equation showed slightly better results than IAST using the Langmuir equation and both showed better results than the EL model.
ABSTRACT:The problem of predicting the adsorptive properties of activated carbon (AC) towards a mixture of gases from the simple knowledge of the adsorption properties of the pure components is addressed, with special reference to the CO 2 /CH 4 mixture. The adsorption process for the pure gases and their mixtures was simulated using the Grand Canonical Monte Carlo (GCMC) method and the calculations were then used to analyze experimental isotherms for the pure gases and for mixtures with different molar fractions in the gaseous phase. It was shown that the pore-size distributions (PSDs) "sensed" by each of the pure probe gases was different one from the other and also from the PSDs "seen" by the mixture. A mixing rule for combining the PSDs corresponding to the pure gases is proposed for obtaining predictions regarding the adsorption of the corresponding mixtures, which are then compared with those arising from the classical IAST approximation. For this purpose, selectivity curves for CO 2 relative to CH 4 have been calculated and compared with experimental values. It was concluded that, for the adsorbate/adsorbent system under study, the proposed GCMC mixed model was capable of predicting the binary adsorption equilibrium, and especially the selectivity, more accurately than the IAST.
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