Problems Associated with Thermodynamic Analysis of Gas-Solid Adsorption Isotherms Measured at High Pressures

Abstract

Adsorption equilibria of Ar, N₂, CH₄ and C₂H₆ on the 13X molecular sieve and the AS activated carbon were measured at five temperatures over a wide pressure range from 0.1 to 20 MPa using a microbalance. Experimental adsorption isotherms, which are excess data, were used to calculate the absolute isotherms according to the following equation:

$$ {\Gamma^S} = {\Gamma^0} + {\Gamma^8} $$

((1))

where Γ⁰ and Γ^g are respectively the experimental excess isotherm and the gas isotherm. Grand canonical Monte Carlo simulations and density functional theory calculations were carried out in order to explain specific features of the resulting isotherms. Thermodynamic functions calculated from the excess and absolute adsorption isotherms were analyzed over a wide pressure region at 288.15 K, which is the middle temperature in the range studied.