The role of wakes in the mechanism of extraction in spray columns

Extraction in spray columns is reexamined in view of recent information on wakes of drops and the role of wakes in the mechanism of heat transfer in spray columns. In the proposed mechanism, mass is extracted from a highly mixed drop to a highly mixed wake in the wake growth zone. Elements of wakes are shed, mixed with continuous phase, and are replaced by the continuous phase in the wake shedding zone. A narrow mixing zone exists at the entrance of the continuous phase and above it the coalescence zone. A mathematical model for extraction in spray columns is presented. This model was used to calculate the top concentrations from the bottom concentrations, for 258 runs from nine papers covering very wide ranges of columns, systems, and operation parameters. Better than 20% agreement was obtained for 10 runs and the poorer agreement of most other runs can be attributed to the sensitivity of the calculations to the uncertainties in the average distribution coefficients of the solute between the two liquid phases, and, to a lesser extent, to the uncertainties in the length of the wake shedding zone and wake parameters. For very large or very small values of the distribution coefficient, the assumption of local equilibrium between drop and wake, used in the model, does not hold.