Thermodynamic excess functions for mixture adsorption on zeolites

Thermodynamic excess functions have been widely used to describe liquid properties because they quantify deviations from ideal behavior. In this work, thermodynamic excess functions are used as a tool to understand and predict the behavior of mixtures in microporous materials such as zeolites. The use of excess functions for describing deviations from ideal mixing in the adsorbed phase differs from liquid solutions in several subtle but important ways. Prediction of mixture adsorption is a key factor in the design of adsorption separation processes. Measuring single-component adsorption properties is easy compared to multicomponent properties. Therefore it is important to have a reliable method of calculating mixture behavior from pure-component properties. The main obstacle to progress is a scarcity of accurate and consistent experimental data over a wide range of temperature and loading for testing theories. Almost no data are available on the enthalpy of adsorbed mixtures, even though such information is necessary for the modeling of fixed bed adsorbers. A custom-made calorimeter was used to measure mixture properties. Thermodynamic excess functions such as excess enthalpy (heat of mixing) and excess free energy (activity coefficients) provide a complete thermodynamic description of the effect of temperature, pressure and composition variables. The mixtures studied are described within experimental error by a 3-constant equation, which is thermodynamically consistent and has the correct asymptotic properties at high and low coverage for gases adsorbed in zeolites. More importantly, it is shown that pure component properties such as heats of adsorption and saturation capacity can be used to predict the magnitude of the non-idealities in mixture adsorption. Predictions of mixture properties for SF6-CH4 mixtures on silicalite using molecular simulation agree with experimental measurements. Molecular simulation results show segregation of SF6 and CH 4 molecules in different sections of the silicalite pore network. Deviations from ideal solution are consequence of a non-uniform composition of the adsorbed phase