Introduction to Desalting of Crude Oil

Now, in this article we will discuss about a very important topic in crude oil distillation which is 'Desalting of Crude Oil'.  

DESALTING OF CRUDE OIL

If the content in the crude oil of salt is greater than 10 lb/1000 bbl (expressed as NaCl), the crude requires desalting to lower fouling and corrosion happen due to deposition of salt on heat transfer surfaces and acids formed by decomposition of the chloride salts. Afterwards, some metals in inorganic components dissolved in water emulsified with the crude oil, which can cause deactivation of catalyst in catalytic processing units, are partially rejected in the desalting process. 

Until lately, the standard for desalting crude oils was 10 lb salt/1000 bbl (expressed as NaCl) or more, but now many companies desalt all crude oils. Shorten equipment fouling and corrosion and longer catalyst life provide justification for this additional treatment. Two-stage de salting is used if the crude oil salt content is more than 20 lb/1000 bbl and, in the situations where residua are catalytically developed, there are some crudes for which three-stage desalting is used. The salt in the crude is in the form of dissolved or suspended salt crystals in water emulsified with the crude oil. 

The main objective or principle is washing the salt from the crude oil with water. Problems occur in obtaining efficient and economical water/oil mixing, water-wetting of suspended solids, and separation of the wash water from the oil. The pH, gravity, and viscosity of the crude oil, as well as the volume of wash water used per volume of crude, affect the separation comfort and competence.

A secondary but important function of the desalting process is the removal of suspended solids from the crude oil. These are usually very fine sand, clay, and soil particles; iron oxide and iron sulphide particles from pipelines, tanks, or tankers; and other contaminants picked up in transit or production. 

Total suspended solids removal should be 60% or better with 80% removal of particles greater than 0.8 micron in size. Desalting is carried out by mixing the crude oil with from 3 to 10 volume% water at temperatures from 200 to 300°F (90 to 150°C). Both the ratio of the water to oil and the temperature of operation are functions of the density of the oil. The salts are dissolved in the wash water and the oil and water phases separated in a settling vessel either by adding chemicals to see through in breaking the emulsion or by developing a high-potential electrical field across the settling vessel to mobilize the droplets of salty water more rapidly. 

Either AC or DC fields may be used or potentials from 12,000 - 35,000 volts are used to promote mobilization. For single-stage desalting units 90 to 95% efficiencies are obtained and two-stage processes achieve 99% or better efficiency.