BACK MIXING IN A RECIPROCATING PLATE COLUMN WITH STABLE DENSITY GRADIENTS

Axial dispersion coefficients were measured in an aqueous phase flowin g downward through a 5.08-cm-dia. Karr-type reciprocating plate column . The steady tracer injection method was used, in which the concentrat ion profile was measured upstream of tracer injection. The tracer solu tion 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 condit ions with a refined kerosene (Isopar M) dispersed in the aqueous phase as droplets. Some control experiments were done using a neutrally buo yant tracer solution containing sodium chloride and methanol. There wa s no evidence that the stable density gradient had any effect in reduc ing the values of the axial dispersion coefficient although earlier pu blished work had shown that an unstable density gradient did increase axial mixing.