Figure 3.8 is a realistic picture of 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:
• The foam height on the tray
• The vapor velocity through the tray
High vapor velocities, combined with high foam levels, will cause the spray height to hit the underside of the tray above. This causes mixing of the liquid from a lower tray with the liquid on the upper tray. This backmixing of liquid reduces the separation, or tray efficiency, of a distillation tower.
When the vapor flow through a tray increases, the height of froth in the downcomer draining the tray will also increase. This does not affect the foam height on the tray deck until the downcomer fills with foam. Then a further increase in vapor flow causes a noticeable increase in the foam height of the tray deck, which then increases the spray height.
When the spray height from the tray below hits the tray above, this is called the incipient flood point, or the initiation of jet flooding. Note, though, that jet flood may be caused by excessive downcomer backup. It is simple to see in a glass column separating colored water from clear methanol how tray separation efficiency is reduced as soon as the spray height equals the tray spacing. And while this observation of the onset of incipient flood is straightforward in a transparent tower, how do we observe the incipient flooding point in a commercial distillation tower?