On the use of single, dual and three process langmuir models for binary gas mixtures that exhibit unique combinations of these processes

Two scenarios were investigated for predicting binary gas adsorption equilibria when the single gas isotherms of one component is described by the three process Langmuir (TPL) model and the single gas isotherms of the other component is described by either the dual process Langmuir (DPL) or single process Langmuir (SPL) model. For the TPL–SPL and TPL–DPL models, 7 and 12 different correlations of energetic (free energy) site matching respectively exist. For these complex systems, perfect positive (PP) means the free energies of the sites of the two components align in some way from high to low, while perfect negative (PN) means they misalign in some way with high free energy sites for one component aligning with low free energy sites for the other component. Other variations of PP and PN exist where the free energies correlate in some way with the free energy of the site of one of the components distributed among two or more sites of the other component, and uncorrelated exit where the free energies do not correlate but possibly still have some site distribution. A consistent set of single and binary isotherms for CO₂ and N₂ on 13X zeolite were used to explore all 19 correlations. CO₂ fitted well only to the TPL single gas model and N₂ fitted equally well to either the DPL or SPL single gas model. Only 3 of the 12 cases for the TPL–DPL model and 2 of the 7 cases for the TPL–SPL model exhibited reasonable predictions of the experimental results; the remaining 14 cases failed. The predictions for CO₂ for both models were very good for all 19 cases; but, some of the predictions for N₂ were so overpredicted, they were not even close to reality. The simpler site matching correlations fared better than the more complex ones, such as those with the sites of N₂ distributed over two or more sites of CO₂. For this CO₂–N₂-13X binary system, PP site matching correlations provided the best predictions, with the high and low free energy sites of N₂ decidedly interacting with the high and medium free energy sites of CO₂ with the low free energy site of CO₂ unoccupied by N₂.