Desalter is the first process unit in an oil refinery. Salts in crude are generally contained in residual water suspended in the oil phase. The chemical composition of these salts varies depending on source of the crude oil, 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”.
Usually the operating range of the process is about 90° to 160°C, the desalter is located in the pipe still pre-heat train. The desired temperature is obtained by heat exchange between pipe still products or re-circulated reflux, in the crude charge. To dissolve the salts and/or wet the impurities, fresh water of around 3-7% 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 to 35 Kilo volts. The technology is Dual polarity (at the same time AC and DC filed is used). In cases of very high initial salt contents, desalting may be a two stage, or even a three stage operation. The produced water is discharged to the water treatment system (effluent water). It can also be used as wash water for mud washing process during operation.
Crude oil generally contains different type of contaminants, such as suspended solids, inorganic salts, corrosion byproducts, sand, drilling mud, polymer, dust, rust and more. and all of them make difficulties in distillation or its downstream equipment. Desalting process is the first step of purification foreseen at upstream portion of each refiner feed stream, with beneath benefits:
- Generally, it increases crude throughput
- There will be less plugging, coking and scaling of the downstream exchangers and furnace tubes
- Less erosion problem by solids in the pumps exchanger, control valves and tubes will be observed
Corrosion in the pipe lines, Heat Exchanger, Columns and Trays will