Page 94 - Petroleum and Gas Field Processing

4. For vertical separators, liquid droplets (oil) separate by settling
downward against an up-flowing gas stream; for horizontal
ones, liquid droplets assume a trajectory like path while it flows
through the vessel (the trajectory of a bullet fired from a gun).
5. For vertical separators, the gas capacity is proportional
to the cross-sectional area of the separator, whereas for
horizontal separators, gas capacity is proportional to area of
disengagement (LD) (i.e., length diameter).

3.6.3 Settling of Oil Droplets
In separating oil droplets from the gas in the gravity settling section of a
separator, a relative motion exists between the particle, which is the oil
droplet, and the surrounding fluid, which is the gas.
An oil droplet, being much greater in density than the gas, tends to
move vertically downward under the gravitational or buoyant force, F g .
The fluid (gas), 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
frictional resistance of the fluid drag force, F d , approaches and balances
F g ; and, thereafter, the oil droplet continues to fall at a constant velocity
known as the settling or terminal velocity.
The drag force, F d , is proportional to the droplet surface area
perpendicular to the direction of gas flow, and its kinetic energy per unit
volume. Hence,
2 g u 2
F d ¼ C d d ð1Þ
4 2
whereas F g is given by
3
F g ¼ d ð o g Þg ð2Þ
6
where C d is the drag coefficient, d is the diameter of the oil droplet (ft), u is
the settling velocity of the oil droplet (ft/s), o and g are the oil and gas
2
3
densities, (lb/ft ), respectively, and, g is gravitational acceleration (ft/s ).
The settling terminal velocity, u, is reached when F d ¼ F g . Therefore,
equating Eqs. (1) and (2) and solving for u, the droplet settling velocity, we
obtain

2 8 ð o g Þ d
u ¼ g
6 g C d
The droplet diameter d is normally expressed in microns, where 1 mmis
equal to 3.2808 10 6 ft. Let d m be the droplet diameter (in mm) and

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