Crude oil contains water, salts and sulfur that need to be removed before the crude oil can be refined to its useful components. To remove the salt and the water, crude oil undergoes a step known as desalting. In this step the crude oil undergoes an initial dilution with the addition of clean water. The new diluted mixture is then emulsified so that the salty water initially present in the crude oil comes into contact with the clean water. The emulsion then enters the desalter which separates the emulsion into two separate layers, a layer of water and a layer of crude oil. Salt easily dissolves in water and because of this it is also separated along with the water.
The water is then sent to the water treatment system and the crude oil progresses to the next stage. The next stage is to desulfur the crude oil, this is important as this should reduce the emission of sulfur dioxide (SO2) when any fuel created in the fractional distillation is burnt.
Desulfurisation requires heating and pressurising the crude oil to specific conditions and then the addition of hydrogen gas. The addition of the hydrogen gas should cause a reaction with any sulfur present and form hydrogen sulfide (H2S). The hydrogen sulfide is then converted into elemental sulfur and hydrogen, the sulfur can be sold on as a by-product whereas the hydrogen can be recycled and used again in the previous step.
Water treatment is very important in an oil refinery as water is used or removed in most parts of the plant. Initially oil skimmers are used, which are pieces of equipment that remove any oil that would be floating on the surface of the water. The next stage incorporates several large tanks that force air through the water. The water is mixed with biological agents that will consume any unwanted waste in the water and the air that is forced into the water helps promote the growth of these agents. To find out more about one method for water treatment click here.