Liquid entrainment, spray height

Figure 1.8 is a realistic picture as to what we would see if our towers were made of glass. In addition to the downcomers and tray decks containing froth or foam, there is a quantity of spray, or entrained liquid, lifted above the froth level on the tray deck. The force that generates this entrainment is the flow of vapor through the tower. The spray height of this entrained liquid is a function of two factors ... 

Entrainment (Jet) Flooding Froth or spray height rises with gas velocity. As the froth or spray approaches the tray above, some of the liquid is aspirated into the tray above as entrainment. Upon a further increase in gas flow rate, massive entrainment of the froth or spray begins, causing liquid accumulation and flood on the tray above. 

The term incipient flood is that point in a trayed tower s operation when the spray height of liquid from the tray below begins to impinge on the tray above to the extent that entrainment reduces fractionation efficiency. Incipient flood in a packed column is that point in the column s operation at which liquid hold-up increases to an extent that reduces fractionation efficiency. 

The spray height of this entrained liquid is a function of two factors ... 

Clear liquid height at the froth-to-spray transition, corrected for the effect of weir height on entrainment, given by Eq. (6.29), in of liquid. 

A mote accurate (and somewhat mote complicated) correlation for predicting entrainment flooding on sieve and valve trays is proposed by Kister and Haas (1990). Their correlation is also derived from the original work of Souders and Brown, but provides a modified approach for determining Ky. They introduce, as an important parameter in the correlation, the clear liquid height at the froth-to-spray-iegime transition, and suggest that it be calculated by an equation attributed to Jeronimo and Sawistowski 0973). 

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