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Abstract

Knudsen diffusion, surface diffusion and Poiseuille flux of capillary condensate can contribute to the flux of a condensable gas in the small pores of Vycor membrane (pore radius 4 nm). It is very difficult to distinguish between individual the contributions, especially because the pores have no uniform pore diameter. Two basic types of experiments were made to study the transport mechanisms of a condensable gas (butane) through a Vycor membrane: steady state permeation and measurements of liquid flux. Steady state experiments enable to study butane transport for different concentration profiles in the pores. Measurements of liquid flux are indispensable for the verification of the presence of condensate in pores during gas permeation. Differences between the liquid butane flux (on high pressure side of membrane is liquid, on opposite side is gas) and the gas flux (at both sides butane is in the gas phase) for the same pressure gradients across the membrane were observed. The difference between these fluxes decreases with decreasing of pressure gradients. For a pressure gradient higher than 2 bars (liquid butane P1=2.25 bar; P2= 0 bar, gas butane P1=2.24 bar; P2= 0 bar) the flux of the liquid butane is approximately 2 times higher. The ratio of liquid and gas fluxes decreases to 1.7 for pressure conditions P1=2.24-2.25 and P2=0.98 bar. No difference between liquid and gas fluxes was registered for the low pressure gradient investigated (P1=2.22 bar; P2= 1.91 bar). It can be concluded that the condensation of butane in pores occurs at high pressures on both sides of pores if simultaneously the pressure difference is small.