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