Three phase separators are commonly used in the oil and gas industry for the separation of oil, water and hydrocarbon gases from raw product streams. All crude oils contain dissolved gases that will come out of solution once oil is brought to the surface. The amount of gas in the oil is referred to as the gas/oil ratio (or GOR).
Vessel Orientation
Depending on the GOR of the product stream, three phase separators may be aligned horizontally or vertically. Vertical separators are often used when the GOR is low and there is a small volume of gas to be separated from the liquid constituents.
Horizontal vessels, on the other hand, are used to separate mixtures that have a high GOR. Much of this is due to the fact that they have increased surface area, which facilitates separation of gases from the oil. In many raw crude oil streams, certain hydrocarbon gases cannot be separated from solution in a primary separator. In these instances, the gases are stripped in other unit processes downstream.
Basic Design Principles
Both horizontal and vertical three phase separators have three main sections:
• A primary separation section – The primary separation section is situated at the inlet to the vessel and is designed to separate the fluids from any entrained gas.
• Secondary separation – The secondary separation section is designed to facilitate the separation of the liquid constituents into light and heavy phases according to their specific gravity. Typically oil comprises the light phase and water the heavy phase.
• Coalescing section – The coalescing section includes a vapor coalescer or mist extractor to remove liquid droplets from the gas. A wire mesh eliminator is often used for this purpose.
In a vertical tank, the heavy liquids are pumped out of the bottom of the vessel and the light phase is drawn off the side of the tank. Horizontal vessels use a sump arrangement to separate light and heavy liquids. They are then both drawn off via separate outlets from the bottom of the tank.