The Ideal Adsorbed Solution Theory (IAST) of Myers and Prausnitz is used in adsorption engineering to predict multicomponent mixtures from pure component adsorption isotherms. It is the equivalent of Rault's law in vapour liquid equilibrium, but the reference state is calculated from the Gibbs isotherm and cannot be measured directly and very accurately as is the case for pure component vapour pressures. Because the reference state is calculated, any error in this calculation leads to the incorrect evaluation of the adsorbed phase excess Gibbs energy and thus makes it not possible to develop robust correlation also for the more general case where non-ideality in the adsorbed phase is introduced.
In theory many gas-solid systems should be represented well by the IAST because in microporous solids the main interaction is between the solid and the adsorbed molecule. The only main exception would be due to entropic effects, where a molecule can access regions of the micropore volume that a second molecule cannot. A recent review by Wu and Sircar (2016) shows that data reported in the literature can deviate substantially and what is more worrying is that even for the same binary system different authors report different deviations from the IAST, amongst these, oxygen and nitrogen on LiLSX is an example of roughly equally sized molecules which should follow closely the IAST but in some cases literature results point to poor predictions.
Clearly one possible explanation for the observed discrepancy may be that the binary experimental data are inconsistent, but this is not the only reason for such differences. This contribution is aimed at addressing two main points:
1) The correct representation of pure component isotherms when the gas phase is not ideal;
2) The discussion of the assumptions that are implied by the actual procedure with which the ideal adsorbed phase is calculated;
Examples from the literature will be used to highlight several inconsistencies which point to the incorrect use of the IAST. As a more in depth case study, the binary mixtures of nitrogen and carbon dioxide on NaX zeolites at pressures up to 10 bar will be considered. This system is chosen for its industrial relevance, especially in carbon capture applications. It provides also a stringent test for the IAST since the ideal selectivity is > 100 and therefore allows to highlight issues that may not be as evident when selectivities are relatively low.
