Gravity Separation

Since a separation depends upon gravity to separate the fluids, the ease with which two fluids can be separated depends upon the difference in the density or weight per unit volume of the fluids. (Density of liquid is much higher than density of gases).
In the process of separating, separation stages are as follows:
1- Separate liquid mist from the gas phase.
2- Separate gas in the form of foam from the liquid phase.
3- In case of 3 phase separation, in addition to the above two requirements, water droplets should
be separated from oil phase, and oil droplets should be separated from water phase.

Droplets of liquid mist will settle out from gas, provided:
• The gas remains in the separator long enough for mist to drop out.
• The flow of the gas through the separator is slow enough that no turbulence occurs, which will keep the gas stream stirred up so that the liquid has no chance to drop out.
The objective of ideal two-phase separation, is to separate the hydrocarbon stream into liquid-free gas and gas-free-liquid. Ideally, the gas and liquid reach a state of equilibrium at the existing conditions of Pressure and Temperature within the vessel.

Liquid droplets will settle out of a gas phase due to the difference in densities if the gravitational force acting on the droplet is greater than the drag force of the gas flowing around the droplet (see Fig. 2-3). The drag force is the force resulted from the velocity of gas and affecting the entrained droplet of liquid, forcing it to move in the gas flow direction.

Fig. 2-3 A schematic of a force balance on a droplet in a flowing gas stream.

Figures 2-4, and 2-5, illustrates the liquid droplet in gas phase and gas bubble in liquid phase in both configurations of horizontal and vertical separators. From both figures, it’s clear that, in vertical separator, the gravitational settling force is countercurrent or opposite of the drag force resulted from gas movement. While in horizontal separator, the two forces are perpendicular to each other. The same for the gas bubble entrained in liquid in vertical and horizontal separators.

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