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==Description of petroleum refinery electrostatic desalters==

The [[Petroleum refining processes#The crude oil distillation unit|crude oil distillation unit]] (CDU) is the first processing unit in virtually all petroleum refineries. The CDU distills the incoming crude oil into various fractions of different [[Boiling point|boiling]] ranges, each of which are then processed further in the other refinery processing units. Figure 1 below is a schematic flow diagram of a typical CDU and, as can be seen, the desalter (colored red for clarity) is typically installed in the [[Heat exchanger|heat exchange]] train that heats the incoming crude oil before it flows through a [[fired heater]] and into the distillation tower. The desalter is usually located at the point where the incoming crude oil has been heated to about 100 to 150 °[[Celsius|C]]. The optimum desalter temperature varies somewhat with the crude oil source.<ref name=Wauquier/>

At that point, wash water is injected and mixed into the continuous flow of crude oil and the resulting oil-water emulsion then continuously enters the electrostatic desalter. The rate of wash water required is about 4 to 10 % by volume of the crude oil rate. The optimum wash water rate varies with the [[API gravity]]<ref>'''Note:''' API gravity = °API = (141.5 / SG) – 131.5 &nbsp; &nbsp; where SG = specific gravity 60°F/60°F = (density of oil at 60 °F) / (density of water at 60 °F) &nbsp; and 60 °F = 15.56 °C</ref> of the crude oil and with the desalter temperature.<ref name=Wauquier/>

Externally viewed, the typical electrostatic desalter is a horizontal, cylindrical vessel as depicted in Figure 1. A cross-sectional end-view of the of the desalter's interior is shown in Figure 2 below. The oil-water emulsion that enters from the bottom of the desalter through the feed line is a thorough mixture of two non-miscible liquids consisting of a continuous phase (the crude oil) and a dispersed phase (water in the form of very small droplets with dimensions ranging from 1 to 10 micrometres). Asphaltenes and finely divided sediment solids are [[adsorption|adsorbed]] on the oil-water interface and stabilize the emulsion. Thus the degree of difficulty involved in [[coalescence|coalescing]] the droplets into large globules which can be settled and removed is related to the presence of asphaltenes, sediments and other water-insoluble contaminants.

An electrical system connected to the [[electrode]]s within the desalter (see Figure 2) generates an [[electrostatic field]] at potentials ranging from about 6,000 [[volt]]s to about 20,000 volts that induce [[dipole]] attractive forces between neighboring droplets of water. In other words, the electrostatic field results in each droplet having a positive charge on one side and a negative charge on the other which cause the droplets to [[coalescence|coalesce]] because of the attractive force generated by the opposite charges on neighboring droplets. The resulting larger water droplets (globules), along with water-insoluble solids, then settle to the bottom of the desalter. The  settled water is continuously withdrawn from the desalter from a point somewhat above the desalter bottom (see Figure 2) and is referred to as a brine because it contains the inorganic salts that originally entered the desalter with the water in the crude oil. The settled sediment at the bottom of desalter is withdrawn as a sludge at intermittent intervals as needed to prevent solids from entering the settled water withdrawal outlet.

{| border="0" align="center"
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|{{Image|Crude oil distillation 2.png|center|420px|Figure 1: Schematic flow diagram of a typical crude oil distillation unit.}}
|{{Image|Desalter Diagram.png|right|350px|Figure 2: Cross-sectional end-view diagram of a horizontal, cylindrical crude oil desalter}} 
|}

The desalter depicted in Figure 2 and described above is referred to as a ''single-stage'' desalter and represents but one of many available configurations, including configurations such as:

* Flowing the crude oil through two stages in series and recycling part of the brine from the second stage for use as wash water to the first stage.

* Flowing the crude oil through two stages in series with no recycle of brine from the second stage. 

* Using multiple electrostatic fields in a single vessel so as to create, in effect, two or three stages of desalting within that single vessel.