Hydrodynamic Flow of Gases in Porous Solids

When the gas is a mixture, the hydrodynamic flux given by equation (1 -112) or (1 -113) is the flux of the mixture that is, the mixture moves as a whole under the total pressure gradient. All species move at the same speed (i. e., no separation is achieved), and the individual flux of each component is the product of its mole fraction times the total flux. In that case, the viscosity in equations (1-112) and (1-113) is the viscosity of the mixture. If the gas is a mixture with different concentrations and different total pressure on either side of the porous solid, the flow may be a combination of hydro-dynamic, Knudsen, and diffusive. 

A porous carbon diaphragm 25.4 mm thick of average pore diameter 0.01 cm allowed the flow of nitrogen at the rate of 0.05 m3 (measured at 300 K and 1 atm)/m2-s with a pressure difference across the diaphragm of 500 Pa. The temperature was 300 K and the downstream pressure was 0.1 atm. The viscosity of nitrogen at 300 K is 180 iP. 

---