Molecular thermodynamics of adsorption from gas mixtures: composition of adsorbed phase from gravimetric data

Abstract

A gravimetric procedure is proposed for measuring equilibrium adsorption from gas mixtures. The total mass adsorbed is measured with a microbalance as a function of temperature, pressure, and composition of the gas phase. The composition of the adsorbed phase is then calculated from the gravimetric data using the principles of adsorption thermodynamics. The increased computational work is more than compensated for by a reduction in experimental effort. The model proposed here, a Bragg—Williams approximation for the local isotherm and a beta distribution of adsorptive energies, provides a consistent framework for the rigorous thermodynamic equations that govern adsorption equilibrium for gas mixtures. The beta distribution is distinguished by its flexibility and its finite range of energies, and generates adsorption isotherms with continuous derivatives at the limit of zero pressure. The BWB equation for adsorption of mixtures is thermodynamically consistent and therefore suitable for calculating the composition of the adsorbed phase from measurements of the total amount adsorbed. The reliability of the procedure is tested by comparing it with both experimental and Monte Carlo data for which the composition of the adsorbed phase is already known. It is found that deviations between experimental and calculated mole fractions lie within the estimated error of the experimental data.