Effect of unfavorable continuous phase density gradient on axial mixing

Abstract Field experience with several industrial reciprocating plate liquid‐liquid extraction columns indicated that large density gradients in the continuous phase resulted in excessive backmixing. Since there are no published data on the effect of higher density in the upper section compared to lower density in the bottom section of the column, experiments were performed with the calcium chloride brine‐water system in a 76.2 mm diameter reciprocating plate column. Axial dispersion coefficients, E, were found to be 10–20 times higher than the values in systems with uniform densities. E decreased with increasing agitation to 30% of the value with no agitation. The effect of increasing flow rate of water up the column was to increase both E and the Peclet number. E was correlated with the density difference between any point in the column and tap water. Another correlation was obtained showing the effects of buoyant and mechanical energy input on axial dispersion.