Desalters are one of the major components present in most oil refineries. The desalter is typically the first process unit in the chain, as its purpose is to remove the salt present in the crude before it can begin the distillation process. Rather than actually being suspended in the crude itself, various salts are typically part of a solution of brackish water. This water is usually present in the crude in an emulsified form, so a desalter may first need to demulsify and dehydrate the oil. A different type of machinery known as a desalinization plant may also be referred to as a desalter.
Crude oil is typically wet when it is extracted, as it often contains an emulsion of brackish water. The water can have a variety of salts present, including calcium, magnesium, and sodium chlorides. If these salts are not removed prior to the fractional distillation of the crude oil, many components, such as heat exchangers and catalysts, can be damaged. Another concern can be that downstream process units typically operate at very high temperatures, which could induce water hydrolysis and allow dangerous hydrochloric acid to form.
A desalter typically operates by first demulsifying crude oil and then desalting it. The first step in the process may be a wet tank, where the emulsion is allowed to settle. Any free water can drop to the bottom of the tank at this time, because oil is lighter than water. The process may then be aided by the use of electrodes or other techniques to separate water emulsified in the oil. Heat, chemicals, or additional water may also be used to help break the emulsion.
The two step desalter process may reduce salt content to below 10 pounds (approximately 4.54 kilograms) per thousand barrels (PTB). Another way that salt content may be measured is known as basic sediment and water (BS&W), and desalters often achieve 0.1% BS&W. By reducing the sediment levels, desalters may also help a refinery to comply with local environmental regulations. Any solids present in the crude may contribute to excess flue gas opacity, which is often one factor measured by regulators.
While the term desalter typically refers to the first process unit in an oil refinery, it may also be used to describe a desalinization plant. Instead of removing salty water from oil, these facilities are designed to remove salt from brackish water. The purpose behind this process can be to create potable water, acquire salt for other purposes, or to lower the saline content of agricultural runoff or certain rivers before they cross a political boundary.