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Electrostatic coalescer

An electrostatic coalescer is a pressure vessel with energised electrode grids, in which a high-voltage field coalesces water droplets in crude oil so they settle. Used without wash water it is a dehydrator; with wash water it is a desalter.

Also written as: electrostatic treater, electrostatic dehydrator

Electrostatic coalescer explained

The main parts are the vessel, an inlet distributor, electrode grids hung from insulators, a high-voltage entrance bushing, an external transformer or power unit, an interface level instrument and a water outlet. Many units also have a mud-wash system for removing settled solids.

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Crude oil desalter
A crude oil desalter is an electrostatic coalescer operated with wash water mixed into the crude upstream. The water dilutes the salty droplets in the crude, and the coalescer then removes the water and most of the salt with it.
Crude oil dehydrator
A crude oil dehydrator is an electrostatic coalescer that removes produced water from crude oil to meet a water specification, usually stated as basic sediment and water. No wash water is added. It is the same equipment as a desalter.
Emulsion (water-in-oil)
A water-in-oil emulsion is a dispersion of small water droplets in crude oil, held apart by natural surface-active films of asphaltenes, resins, waxes and fine solids. The droplets will not settle until those films are broken and the droplets merge.
Electrostatic coalescence
Electrostatic coalescence is the use of a high-voltage electric field to make small water droplets dispersed in oil collide and merge into larger droplets. Larger droplets settle much faster under gravity, so water separates from crude in a smaller vessel.
Wash water
Wash water is low-salinity water injected into crude oil upstream of a desalter to dilute the salty water droplets it carries. It is typically added at 3 to 10 per cent of the crude volume, then removed in the desalter.
Mixing valve
A mixing valve is a throttling valve upstream of a desalter that shears wash water into fine droplets so they meet the salty water in the crude. Its pressure drop balances mixing against making an emulsion too tight to break.

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