Diffusion and flow of gases and vapours through micropores IV. Flow of capillary condensate

Abstract

If capillary condensation occurs in a porous plug and a steady pressure difference of an adsorbed gas is maintained across the plug, capillary condensate behaves as if it were bulk liquid in viscous flow through the pore-space. The driving force producing flow is not the pressure difference of the gas, but a very much larger difference of capillary pressure resulting from this pressure difference, presumably due to action of surface tension at curved menisci in the pores. The mechanism contrasts sharply with flow in adsorbed films, which has earlier been shown to conform to a process of surface diffusion. In a saturated plug, observed permeabilities agree very well with calculated values, assuming capillary condensate has the viscosity and density of bulk liquid. Apart from confirming the mechanism of flow, this suggests that liquid properties are not much altered in capillaries of radii down to at least 20 Å. At a given mean pressure of gaseous adsorbate, the permeability has a very different value if the pressure is approached by desorption from a saturated plug from that when the pressure is approached by adsorption from a degassed plug. This is because, due to adsorption hysteresis, the saturation of the plug is different in the two experiments. For a given degree of saturation, the permeability is the same in both adsorption and desorption runs.