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Determination of gas effective diffusivities in porous solids, dispersion coefficients in packed beds and molecular diffusivity of binary systems Davis, Brian Richard

Abstract

SECTION I AN EXPERIMENTAL METHOD FOR THE MEASUREMENT OF EFFECTIVE GAS DIFFUSIVITIES IN POROUS PELLETS, AND THE LONGITUDINAL DISPERSION COEFFICIENT IN PACKED BEDS Present methods of measurement of effective diffusivities are not generally adaptable to the pellets in a packed bed, for example a catalytic reactor. An unsteady state pulse method has been developed employing simple gas chromatographic rate theory. The method is generally applicable to pellet sizes down to about 2mm. With homogeneous pellets reasonable agreement was obtained on comparison of effective diffusivities measured by a steady state method. For anisotropic solids the unsteady state diffusivity can be quite different from the steady state value due to differences in diffusion path. Pulse dispersions measured in beds of non porous pellets have revealed a laminar flow regime where the dispersion coefficient is dependent on the square of the velocity. This regime was reported for flow, in straight pipes but has not previously been demonstrated in packed beds. SECTION II DEVELOPMENT OF AN UNSTEADY STATE FLOW METHOD FOR MEASURING BINARY GAS DIFFUSION COEFFICIENTS Effusion measurements of one gas from a packed bed of known geometry (porosity and tortuosity) into a second flowing gas have been evaluated as a versatile technique for the determination of binary gas diffusion coefficients. The molecular diffusivities measured (± 10%) approached the scatter encountered by other methods (±5%) and satisfactory results (± 3%) are envisaged by optimising parameters in the method.

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