Extraction - Principles of Chemical Separations with Environmental Applications - page 123

Inlet
heavyphase

Outlet
light phase

Distributor

Inlet
lightphase

Outlet
heavyphase

Figure 5.2 Spray-tower extractor.

more dense, or heavy, liquid, which flows continuously downward. The drops of lighter
liquid are collected at the top and the heavy liquid exits the bottom of the column. This
scheme can also be reversed so that the heavy liquid is dispersed into droplets at the top
of the column that fall through a continuously upward-flowing lighter fluid. The choice of
which phase is dispersed depends upon mass transfer rates and which phase is the diluent
or the solvent. Whichever scenario results in the highest rate of mass transfer is the best
for a specific separation.
Regardless of which phase is dispersed there is a continuous transfer of solute from
the diluent to the solvent. Equilibrium is never completely reached at each point in the
column. Rather, the difference between equilibrium and operating conditions governs the
driving force for mass transfer.