Axial mixing of binary gas mixtures flowing in a random bed of spheres

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Abstract
Measurements are reported on axial mixing of binary gas mixtures at room temperature and atmospheric pressure in a random bed of spherical particles. By means of the Fick's Law equation for diffusion an axial Peclet number, dp U/Ez (where dp is particle diameter, U is interstitial velocity, and Ez is axial eddy diffusivity), was computed in terms of the ratio of the amplitudes of a sinusoidal concentration wave at the inlet and outlet of the bed. An experimental method was devised to eliminate end effects in the system. For the gas systems H2N2 and C2H4N2 and for Reynolds numbers between 100 and 400 the mean of twenty‐one determinations of axial Peclet number was 1.88 ± 0.15. This value is in excellent agreement with a value of 2.0 predicted theoretically on the assumption that the bed acts as a series of n perfect mixers, where n is the number of particles traversed between inlet and outlet.Axial diffusivity for turbulent flow of gases among particles is about sixfold larger than radial diffusivity, previously determined. It is suggested that axial diffusivity may not, perhaps, be neglected in contacting devices, such as adsorbers and catalytic reactors.

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