Back mixing in a reciprocating plate column with stable density gradients

Abstract Axial dispersion coeficients were measured in an aqueous phase flowing downward through a 5.08‐cm‐dia. Karr‐type reciprocating plate column. The steady tracer injection method was used, in which the concentration profile was measured upstream of tracer injection. The tracer solution was a strong (150–170 g/L) solution of sodium chloride, whereby a significant stable density gradient (density decreasing with height) was created in the upstream section. The operating variables included frequency (0 to 7.1 Hz), amplitude (0.7 to 1.4 cm), and plate spacing (2.65 to 7.65 cm). Some data were also obtained under two‐phase conditions with a refined kerosene (Isopar M) dispersed in the aqueous phase as droplets. Some control experiments were done using a neutrally buoyant tracer solution containing sodium chloride and methanol. There was no evidence that the stable density gradient had any effect in reducing the values of the axial dispersion coejjicient, although earlier published work had shown that an unstable density gradient did increase axial miring.