Effect of convective velocity on longitudinal diffusivities in a fluidized cascade separator

Experimental results are presented on the separation of trace quantities of charcoal particles from salt crystals in a counter‐current fluidized cascade, for a range of paddle velocities extending over two decades. Interpretation of these data suggests that longitudinal diffusivity D is a function of horizontal convective velocity vH. The dependence of D on vH is attributed to mechanisms similar to those occurring when a pulse of soluble dye is suddenly injected into a solvent flowing in a pipe or a capillary, a situation first analyzed by Taylor(1,2). Experimental measurements were also made on the dispersion of a thin vertical layer of tracer material, following fluidized channel flow, using the same materials and conditions as were used in the fluidized cascade experiments. These data confirmed the general form of the dependence of D on vH found with the cascade, but yielded much lower values. The reasons for the discrepancy are not known, and further work is required to clear up this discrepancy.