Effect of unstable density gradients on back-mixing in a reciprocating plate column

Axial dispersion coefficients were measured in the continuous phase in a 5-cm-dia. Karr column, operated cocurrently with the system kerosene(d)-water(c). The effect of unstable density gradient in the continuous phase was studied by injecting a steady stream of sodium chloride solution as a tracer near the top of the column. The effects of energy dispersed phase were investigated as the three principle variables, and two plate spacings (2.55 and 5.10 cm) were also employed. The axial dispersion results were correlated in terms of a weighted power-law model based on Kolmogoroff's mixing length concept and incorporating contributions due to each of the three modes of energy dissipation. Axial mixing was significantly increased in the presence of unstable density gradients even though these contributed very little (typically less than 1 mW/kg) to the total specific energy dissipation rate.