
101Modes of Operation
(or “theoretical unit”) within the column (i.e., the bubbly liquid and foam
layer combined constitutes one equilibrium stage). However, if some of the
enriched product is added to the top, in a manner reminiscent of distillation
columns in which condensed overhead vapor is added as a reux, then
the surface of the bubbles contacts an interstitial liquid of higher concen-
tration and this increases the surface excess. Thus, a higher enrichment is
manifest. In fact, the addition of reux potentially enables the establishment
of multiple theoretical units. Mass balances upon such a column were rst
given by Stevenson and J ...