MODELING OF MODULATED GAS-FLOW THROUGH SOLID MEMBRANES UNDER SURFACE LIMITED CONDITIONS

Modulated permeation experiments have been proposed as a means of sepe rating surface and bulk processes in a macroscopic determination of di ffusion and solubility coefficients. In these experiments a modulated flow of hydrogen is driven by a modulated pressure difference across a thin foil (or membrane) at elevated temperature. Variations in the fr equency response can be identified with surface and bulk effects by me ans of simple model involving two rate constants (absorption and desor ption). Further details of the surface processes can also be extracted from the experiments when the model is extended to include explicitly : (i) adsorption of molecules, (ii) dissociation of adsorbed molecules and (iii) absorption of the atoms, together with the reverse processe s. The model predicts the characteristic frequency response of the rel ative modulation amplitude ratio and the phase lag of the modulated ga s flow through a membrane. In certain limiting cases a dominant surfac e process can be identified: dissociative chemisorption, accumulation of molecules, accumulation of atoms and slow surface penetration.