Nov 21, 2019
Oil and Gas Separation
Principles of separation theory
The fundamentals of two-phase gas-liquid separation looking at principal theories and practical points which influence the choice and sizing of horizontal and vertical separators are covered in this chapter.
Mainly, separation mechanisms can be divided into the following categories:
- Density – A relatively simple operation requiring no energy input. Virtually, all bulk separations are based on gravity in the oil and gas field.
- Filtration – Based on difference in molecular size on some inert media. Filtration is a batch operation as eventually material captured on the filter must be taken away.
- Absorption/Adsorption – To separate particular components from a mixture, the affinity of one component to be attracted to another can be used. Absorption is the transfer of a solute into the bulk of the absorbent, while adsorption is the capture of solute molecules to a surface. In oilfield operations, both techniques can be found in gas handling systems.
- Volatility – Due to the difference in vapor pressures of material.
The basic mechanism of separating a mixture of gas and liquid is density difference. The principle of operation is to give enough time to gas to be liberated from the bulk liquid phase and time for liquid droplets to fall out of the bulk gas phase. A functional way is to use concept of retention time (the holdup time of gas and liquid inside a volume) which is usually based on physical measurement.
Figure 1.1 illustrates the principle; there is a cylinder holding gas and liquid which is shaken and then left standing for a set length of time. Gravity is the reason for falling of the droplets with heavier density, while the droplets of lighter density are rising through the respective bulk layers; in this case liquid droplets in the gas phase and gas bubbles in the liquid phase. Coalescence between individual drops is possible but unlikely without the aid of some mechanical device. However, coalescence of individual droplets with the bulk layer will not usually be rapid, but is the last stage which finalizes the separation.
Figure 1.1. Principle of gas-liquid separation
In gravity separators, settling and sedimentation lead to separation process which depends on gravitational force. Liquid droplets or solid particles will settle out of a gas phase if the gravitational force acting on the droplet or particle is greater than the drag force of the gas flowing around the droplet or particle. Solid particles in liquid phase and immiscible sphere of a liquid immersed in another liquid encounter the same phenomenon.
The principle forces acting on a particle:
Drag is the force of friction between an object and the fluid through which it is moving. Drag always acts in the opposite direction to motion. This can exist between two fluid layers (or surfaces) or a fluid and a solid surface. Unlike other resistive forces, such as dry friction, which are nearly independent of velocity, drag forces depend on velocity.