141Column and Process Design
enhance enrichment. In order to take advantage of the internal
reux, a total height of at least 60 cm is recommended for the reason
given above. Any further height will enhance enrichment but reduce
recovery in a manner that cannot be predicted from rst principles.
If the foam stability is too low, such as in most protein skimmers, a
stable foam height of 60 cm may not be achievable. In those circum-
stances, the diameter of the foam column may need to be reduced to
increase the supercial gas velocity, and thus nal foam height.
3. A system undergoing external reux but with no signicant change
in bubble size distribution requires enough foam height to manifest,
say, four theoretical units. By adopting a conser ...