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Liquid–liquid interface controllers will function effectively as long as
there is an appreciable difference between the densities of the two liquids.
In most three-phase separator applications, water–oil emulsion forms and
a water–emulsion interface will be present in the separator instead of a
water–oil interface. The density of the emulsion is higher than that of the
oil and may be too close to that of the water. Therefore, the smaller
density difference at the water–emulsion interface will adversely affect the
operation of the interface controller. The presence of emulsion in the
separator takes up space that otherwise would be available for the oil
and/or the water. This reduces the retention time of the oil and/or water
and, thus results in a less efficient oil–water separation. In most operations
where the presence of emulsion is problematic, chemicals known as
deemulsifying agents are injected into the fluid stream to mix with the
liquid phase. These chemicals help in breaking the emulsion, as will be
described in Chapter 5. Another method that is also used for the same
purpose is the addition of heat to the liquid within the separator. In both
cases, however, the economics of the operations have to be weighted
against the technical constraints.

4.4 SEPARATION THEORY
The basic separation concepts and settling equations developed for two-
phase separators in Chapter 3 are, in general, valid for three-phase
separators. In particular, the equations developed for separation of liquid
droplets from the gas phase, which determined the gas capacity constraint,
are exactly the same for three-phase separators.
Treatment of the liquid phase for three-phase separators is, however,
different from that used for two-phase separators. The liquid retention
time constraint was the only criterion used for determining the liquid
capacity of two-phase separators. For three-phase separators, however, the
settling and separation of the oil droplets from water and of the water
droplets from oil must be considered in addition to the retention time
constraint. Further, the retention time for both water and oil, which might
be different, must also be considered.
In separating oil droplets from water, or water droplets from oil, a
relative motion exists between the droplet and the surrounding continuous
phase. An oil droplet, being smaller in density than the water, tends to
move vertically upward under the gravitational or buoyant force, F g . The
continuous phase (water), on the other hand, exerts a drag force, F d ,on
the oil droplet in the opposite direction. The oil droplet will accelerate
until the fractional resistance of the fluid drag force, F d , approaches and

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