The first process unit in an oil refinery is the desalter. Salts in crude are generally contained in residual water suspended in the oil phase. The chemical composition of these salts varies, but the major portion is nearly always sodium chloride with lesser amounts of calcium and magnesium chlorides. Crudes also contain impurities such as silt, iron oxides, sand and crystalline salt as mechanical suspensions. Removal of these contaminants is known as “desalting”.

Process Description

Since the operating range of the process is usually 100° to 150°C,the desalter is located in the pipestill preheat train. The desired temperature is obtained by heat exchange between pipe still products or recirculated reflux, in the crude charge. To dissolve the salts and/or wet the impurities, fresh water of around 3-8% by volume is added to the raw crude then mixed with the crude via a mixing device, such as a mixing valve and/or a static mixer. The resulting oil/water mixture is then resolved by electrostatic coalescence through a high voltage electrical field inside the desalting vessel. Water droplets coalesce under the influence of the electric field, and sink to the bottom of the vessel. Electrodes maintaining the electric field are spaced to produce a voltage intensity of between 2,000 to 4,000 volts per inch. Applied voltage ranges from 12,000 to 35,000 volts. In cases of very high initial salt contents, desalting may be a two stage, or even a three stage operation.

Design and Operating Features

Mackenzie Hydrocarbon desalters and dehydrators have a number of proven design features that ensure process criteria will be met. Designs typically include provision for addition of demulsifier chemicals and usually include a hydraulic wash header to enable periodic removal of solids on the bottom of the vessel (mud washing). A number of power and connection options can be offered for the electrostatic grid of the desalter to suit specific operating conditions. Proper selection of mixing devices is also crucial to the success of the operation.


High voltage transformers are essential in electrostatic treating. Usually the transformers are oil-filled, designed for hazardous location, and located close to the vessel. Power is introduced to the vessel through the entrance busing. Usually the transformers offer a number of voltage options, between 12,000 to 35,000 volts. Usually the internal voltage is AC, but DC is used in certain circumstances.