Heterogeneous extended Langmuir model with multiregion surfaces for adsorption of mixtures

The multiregion or multisite adsorption theory is applied to the heterogeneous extended Langmuir (HEL) model for predicting adsorption from mixtures. A new model, multiregion HEL (MR-HEL), is derived. MR-HEL is thermodynamically consistent. It uses the same three parameters for each component of the mixture as in the HEL model. Examples, including eight binary and one ternary systems, show that both MR-HEL and HEL yield satisfactory results for relatively ideal systems with like components. For nonideal and highly nonideal mixtures, however, MR-HEL reduces the total average deviation for the predicted amount adsorbed for each component by more than 50% in comparison with the original HEL model. The improvement by MR-HEL is significant. Moreover, the new model predicts not only an azeotrope for binary system CO₂ + C₃H₈ that shows strong nonideal behavior but also the correct azeotropic composition.