Crude oil taken out from the earth's crust consists of many different types of impurities. These impurities can be in very different compositions.
Impurities removal is one of the key steps before further refining. If these impurities are present in crude oil, they may cause a problem in crude distillation.
So removal of these impurities became crucial for economical refining operation.
The crude oil consists of impurities such as
- Water
- Dissolved salts
- Heavy metals
- Dissolved gases
What is the desalting operation?
Desalting operation is removal method carried out to remove dissolved salts from raw crude oil. Desalting primarily removes salts by dissolving them into the water and also removes suspended particles from raw crude oil.
Why Desalting is Required?
Dissolved salts cause great destruction. These dissolved salts cause fouling and corrosion in storage tanks where crude oil is stored and pipelines which transfer this crude oil. These salts get deposited on the furnace tubes. This furnace heats the crude oil before sending it to the atmospheric distillation unit. So salt deposits on its surface decrease the overall heat transfer coefficient and form acid while dissociating and forms an acid that causes corrosion to furnace tubes. Also, these salts cause the deactivation of catalysts in refined product conversion operations.
Desalting Operation Process
The first stage consists of three parts.
- Contact of water with crude oil
- Mixing of water and oil, Dissolving of salts to the water (Water wetting of suspended solids)
- Separation of water and oil
In the first stage of desalting operation, The crude oil is washed with water. Amount of water used for washing is dependent on the density of crude oil. Also, Crude oil is heated mildly. Heating increases the solubility of salts in water so the efficiency of operation increases.
Different quantities such as pH, gravity and viscosity of crude oil affect the efficiency of operation.
Crude oil desalting also removes very fine sand particles, clay and soil particles. Other contaminants such as iron oxide and iron sulphide are also removed.
With the help of high voltage AC or DC fields, Separation is carried out and then dewatering operation is carried out. This method is very efficient and gives nearly 90-95% removal.
Oil and water being immiscible, Simple gravity decantation can separate the emulsified mixture. In the separation tank, Crude oil is taken out from the upper part of separation and water from the bottom part of the separator.
Sometimes, the Separation of crude oil and washing water becomes difficult due to emulsion formed by water with oil. Breaking emulsion or providing high potential electric field helps in separation.
Desalting operation is often carried out in multiple stages. One stage of desalting is not enough to completely remove dissolved salts. So this operation is carried out in two or three stages.
Conclusion
Crude oil consists of different type of dissolved salts that are needed to be removed. Crude oil desalting operation is used to remove these dissolved salts by washing crude oil with water and separating this water after salts get dissolved in it. Special techniques such as separation based on the high potential electric field are also utilized to increase the efficiency of separation.