Diffusivities and viscosities of some gases at elevated temperatures: Gas diffusivities in porous solids
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I. I N T R O D U C T I O N
II. T H E O R Y
of gas-solid reactions have long been of interest in process metallurgy as well as in other fields of engineering and science. In most cases of practical interest, the rates of such reactions are determined by transport of gaseous reactants (and products) rather than by the intrinsic chemistry of the reaction. In some instances, mass transport is a diffusion of gases within the pores of a solid, while viscous flow of gases under pressure gradients can also be important. A quantitative description of the progress of reaction must then depend on a knowledge of the transport properties (diffusivity and viscosity) of the gas and of how these properties are affected by the pore structure of the solid. Sometimes the transport properties can be predicted, for example, by using the Chapman-Enskog correlation, but there are occasions when such predictions are unreliable, as will be seen below. The second task of determining the effect of solid structure on transport properties is a much more formidable one. These impediments to prediction of the rates of mass transport controlled gas-solid reactions have provided the motivation for the research described below, as has the role played by mass transport in other engineering systems (nuclear fuel enrichment, drying of solids, etc.). The research has had both a theoretical and an experimental component. The former entails a Monte Carlo simulation of gaseous diffusion in porous solids that is fully described in a separate publication; m only a brief summary is presented here. The experimental program involved measurement of diffusion coefficients and viscosities over a broad range of temperature by means of a diffusion bridge apparatus. These measurements were compared with those of others and various predictions.
This research has focused on the transport in the "ordinary" diffusion regime of pore diffusion where the mean free path of the gas molecules is small compared to the mean pore size. The alternative Knudsen diffusion regime (in which these length scales are reversed) has been treated in previous publicationsJ 2,31 In discussing ordinary diffusion, it is common to write a Fickian equation
THE rates
B.Q. LI, Graduate Student, and J.W. EVANS, Professor and Chairman, are with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. This paper is based on a presentation made in the T.B. King Memorial Symposium on "Physical Chemistry in Metals Processing" presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS. METALLURGICAL TRANSACTIONS B
NA = --DABeff~TCA + XA(NA + NB)
[1]
to describe the diffusion of component A of binary A-B within the porous solid. Here, NA is the flux of A in moles per unit area of total (pores plus solid) solid, CA is the concentration of A within the solid (moles per unit volume of pore), and DABcff is an "effective diffusion coefficient" which is
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