(704b) Analysis of the Non-Isothermal Model for Volumetric Diffusion Measurements

Experiments carried out in a volumetric apparatus can be performed with small pressure steps to ensure linearity; nonetheless, for fast diffusing species it is difficult to achieve isothermal conditions. For this reason, a model that combines heat and mass transfer in a porous solid is developed and solved analytically to include the main effect of non-isothermality, which is the temperature dependence of the equilibrium amount. The model leads to a set of dimensionless groups and eigenvalues that can be used to understand under which limiting conditions simplified versions are applicable: isothermal; full thermal control; valve control; and regions where distinct mass or heat transfer parameters dominate the kinetics. The analysis will also show that there are ranges of parameters, where the isothermal and non-isothermal model will have the same shape. Finally, a set of recommendations will be given on how to detect the influence of the combined mass and heat transfer mechanisms.