The flow of butane and isobutane vapors near saturation through porous Vycor glass membranes

The pressure-driven flow of vapors through Vycor glass membranes is investigated theoretically and experimentally. The mass flow is measured for a range of pore diameters between 20 nm and 200 nm and for a range of upstream conditions and pressure differences. The pressures of the vapors upstream of the membrane lie between the saturation pressure and approximately half the saturation pressure. The process is described assuming an adiabatic flow of the fluid. As an important result, the adiabatic description yields a characteristic pore diameter. For upstream pressures close to the saturation pressure and pore diameters smaller than the characteristic value, complete condensation of the fluid and flow of the liquid phase through a part of the membrane is predicted. Under these circumstances, due to the action of capillary pressure, the mass flux is greatly increased with respect to the mass flux of a vapor that does not undergo phase changes. Isothermal descriptions do not yield a characteristic pore diameter. There is qualitative agreement between our description and experimental data with respect to the mass transport. Quantitatively, the agreement is poor to moderate.