UNIFIED FORMULATION OF ISOTHERMAL ADSORBATE FLOW IN MESOPOROUS MEDIA OVER THE FULL VAPOR-PRESSURE RANGE

Surface flow plays an important role in phenomena of gas or vapor tran sport in mesoporous pellets or membranes, which are of considerable pr actical interest; but a satisfactory unified formulation of surface fl ow over the whole applicable pressure range is as yet lacking. In anal ogy with gas phase flow (which varies from diffusive at low, to viscou s at high, pressures), surface transport varies from diffusive, in the dilute monolayer region, to hydrodynamic, when the adsorbate is in th e form of multimolecular layers or liquid condensate. Transport in the se cases has been formulated in terms of a Fick surface diffusion coef ficient or a hydrodynamic surface permeability respectively. We demons trate here that attempts which have been made to extend each one of th ese formulations into the region of the other do not lead to physicall y acceptable results. We further show that an irreversible thermodynam ic approach leads to the desired unified formulation of a ''thermodyna mic'' diffusion coefficient, which appears to vary smoothly between th e Fick value at infinite dilution and the value derived from the liqui d permeability of the porous medium.