Sorption and surface flow in graphitized carbon membranes - I. The steady state

Source abstract

Abstract A study has been made of the flow of inert gases and of sulphur hexafluoride through, and of the adsorption of argon and sulphur hexafluoride on, two carbons chosen for the energetic homogeneity of the sorbing surfaces and non-porosity of the individual carbon particles. A wide temperature range has been covered, ‘surface’ and ‘gas phase’ flows of the sorbed gases have been measured using helium as non-sorbed calibrating gas, and the corresponding permeability and diffusion coefficients derived. The smooth, non-porous sorbents chosen were able to produce extra flows considerably greater than those on much larger but porous and broken surfaces of some other sorbents (e. g. active carbons and alumina-silica cracking catalyst). The concentration dependence of the surface diffusion coefficients, Dss, agrees with a treatment of this dependence according to which, in the regime when gas phase diffusion coefficients are independent of pressure, Dss varies inversely as the slope of the isotherm. When fluxes at each of a series of pressures are plotted against temperature, the fluxes pass through maxima as the temperature is lowered. This behaviour can be explained in terms of the simultaneous influence of temperature on Dss and on the concentration gradients.