The produced fluid from oil formation consists of a mixture of oil, gas, free water and water in oil emulsion. For several reasons each of three components i.e. oil, water and gas must be freed from contamination by other two.
Oil processing involves separating the oil from other fluids (gas and water) for the following reasons:
1. Crude oil must be free of gas so that storage tanks will not be a hazard due to escaping gas.
2. Liquid must be removed from a gas stream to prevent it from accumulating in low sections of a pipeline and restricting the flow of gas. If the gas is to be compressed liquids must be removed to avoid damaging the compressor.
3. The water content in the oil must be low in order to maximize its commercial value. The separated water tends to have oil associated with it and the oil must be removed before the water is injected into the reservoir.
The vessel used to separate the liquids from the gases is called a separator. Separators are usually made as cylindrical vessels and placed either vertically or horizontally. Vertical vessels are more suitable in gas service and horizontal vessels are usually employed in oil (liquid) service and where long residence times are required.
Figure – 1 Typical separator
The function of a separator is to produce clean separation of free gas from oil at any pressure and temperature and to prevent or eliminate liquid entrainment in the gas stream leaving the separator.
The principle ofseparation is essentially by means of gravity based on the difference in density between the fluids involved. Fluids which are not soluble in each other and have different densities are usually separated by gravity force. The drop of liquid settling out of a flowing stream depends among different things:
(1) Relative densities of the liquid and the gas.
(2) Size of the drop.
(3) Velocity of the gas in the stream.
The gravity separation perform well when particle sizes are relatively large and where flow velocities are low. Therefore various methods are used to make the flow and make liquid drops large. Turbulence and high velocities must be prevented in order to avoid re-mixing (re-entrainment). If the fluid enter the vessel at high velocity ,it may create more turbulence and result in liquid getting entrained in the gas stream.
When oil enters a separator it flows from the restricted volume of the flow line where a high velocity is maintained, into the larger volume of the separator. In the separator the speed of the oil and gas is greatly reduced. Due to the separator volume, expansion takes place and consequently the gas comes out of solution.
The normal performance requirement of a separator is to produce gas free of liquid and liquid free of gas provided that:
(1) The oil is not foaming.
(2) Flow to the separator is steady and is not surging.
(3) The gas temperature is above the cloud point of the oil.