Flow of Adsorbed Molecules

Abstract

JUST as Knudsen and others¹ have shown for single capillaries, flow of a gas through a porous medium also undergoes a transition from viscous flow to molecular flow when pore diameters become comparable in size with the mean free path of the gaseous molecules. The position has been reviewed recently², but more data in the molecular flow region are desirable. I have therefore been studying the flow of various gases through plugs of 'Linde silica', a finely divided, amorphous silica stated by the makers to have a maximum particle size of 0.05 µ. The experiments cited here were carried out on a single plug compressed to a porosity (ϵ) of 0.678. The specific surface, S₀, calculated from the permeability equation of Carman and Arnell², was 2.35 × 10⁶ cm.²/cm.³. This gives an equivalent pore diameter which was usually well below the mean free path of the various gases studied. Application of the Brunauer—Emmett—Teller calculation³ to gas adsorption isotherms gave S₀ = 2.8 × 10⁶ to 4.1 × 10⁶ cm.²/cm.³, indicating that particles possibly possess an internal surface which is not accessible to flow. To cover a wide range of molecular weights, flow-rates were measured at 22° C. for hydrogen, air, carbon dioxide and 'Freon-12', CF₂Cl₂, all of which gave consistent values for S₀.