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280 Chapter 6

Mist Eliminalor

Figure 6.4 A vertical gas-liquid separator.

height is calculated by allowing sufficient surge time to dampen flow-rate varia-
tions of the liquid stream, as was discussed earlier for accumulators. Presumably,
this liquid height will also be sufficient to allow vapor bubbles to rise to the top of
the liquid before being trapped in the outlet stream at the bottom of the vessel.
This can be achieved by reducing the outlet liquid velocity by increasing the di-
ameter of the outlet nozzle.
In a separator, there is not a single drop size but a distribution of drop sizes.
To prevent all drops from being carried out by the gas stream would require an
uneconomically large separator. Thus, a maximum gas velocity is specified so
that all but the very small drops are recovered. An empirical expression for the
maximum gas velocity is derived by considering the forces acting on a small drop
suspended in a gas stream. These forces are gravity acting downward and the
buoyant and drag forces acting upward. Thus,

(6.9)
F G = F B + F D

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