Accurate correlation, thermochemistry, and structural interpretation of equilibrium adsorption isotherms of water vapor in zeolite 3A by means of a generalized statistical thermodynamic adsorption model

Abstract

Due to its low energy consumption and ease of operation, the zeolite 3A molecular-sieve pressure-swing adsorption process is currently becoming the method of choice for the production of fuel ethanol worldwide. Accurate correlation of the equilibrium adsorption isotherms of water vapor in zeolite 3A is required for the reliable modeling and simulation of that process. In this paper, we firstly show that popular adsorption isotherm models such as those of Langmuir, Sips, Toth, UNILAN, and Dubinin–Astakhov are uncapable of correlating the available equilibrium adsorption isotherms of water vapor in zeolite 3A. Hill's statistical thermodynamic adsorption model, which was originally expressed in terms of absolute activities and canonical partition functions for sites with a variable number of molecules adsorbed, is rewritten here in terms of the pressure and a set of temperature-dependent adjustable parameters that turn out to correspond to the equilibrium constants of adsorption in those sites. In contrast to other recast versions of Hill's model, our version does neither parameterize nor decouple the canonical partition functions or the configurational integrals, and can be applied to the adsorption of either non-polar or polar molecules. Our generalized model allows not only an easier interpretation of the isotherms fit but also the prediction of thermochemical quantities such as the differential and integral heats of adsorption. From the application of our generalized model, we obtain a very accurate correlation of the available equilibrium adsorption isotherms of water vapor in zeolite 3A in the temperature range from 0 to 100 °C. We also provide thermochemical and structural interpretations of the isotherms fit and made predictions for the isosteric heat of adsorption and the integral heat of immersion that are in excellent agreement with the experimental data.