A thermodynamically consistent Langmuir model for mixed gas adsorption

A multiregion, extended Langmuir model (MREL) is proposed for predicting multicomponent adsorption equilibria. For a binary mixture, it is assumed that two regions or sites exist on the surfaces. One region or site adsorbs both species from the mixture whereas the other region or site is excluded from one species due to size exclusion or energetic exclusion (or lack of competition). On the region where both species are adsorbed, the saturated adsorption capacities are the same, and consequently the extended Langmuir model (EL) applied to this region attains thermodynamic consistency. The model is also extended to multicomponent mixtures. The total adsorbed amount of a component is the sum of that adsorbed on all regions/sites. Only the two parameters of the Langmuir isotherm for each pure component are used. The MREL model is computationally as simple as the EL model. Eight binary and one ternary systems, including some highly nonideal ones, have been tested, and significant improvements are seen with the MREL over the EL. In particular, the MREL is capable of predicting the azeotropic behavior of highly nonideal systems.