Dispersed-phase residence times and axial drop velocities in a multistage column contactor

The residence time of fluid elements in a control volume of any chemical process has a direct effect on the process efficiency. This effect is more important in liquid-liquid extraction where the residence time of the dispersed-phase droplets determines not only the extraction/reaction time but also the interfacial area of mass transfer. Dispersed-phase residence times and drop velocities were studied with operating conditions in a 127 mm diameter multistage contactor of pilot plant scale for the toluene-water physically equilibrated system. The two-point dynamic tracer technique was employed in combination with a laser photometric concentration technique. The tracer concentration distributions, measured at the entrance and exit of a single stage, and the simultaneously measured drop size distribution in the monitored stage were discretized appropriately to provide drop residence times and axial velocities in agitated dispersions. Results showed that the variance of the residence time distribution is proportional to the square of the mean residence time, suggesting that the dispersion model is not appropriate for the analysis of such flows. The obtained drop residence times or axial velocities can be directly applied in a comprehensive approach which considers the convective property of the droplets to predict the extraction performance of the contactor.