school energy&environment

Treatment and storage

On extraction, crude oil  contains a mixture of hydrocarbons along with water, dissolved natural gas, salts, sulphur and inert substances such as sand and heavy metals. Prior to being introduced into the pipelines, crude oil must undergo a number of processes such as degasification, dehydration, desalting and desulphurisation.
During the degasification stage, crude oil is separated from associated gas. In order to do this, crude oil is made to pass through a series of separators (3 or 4); separators are particular pressure vessels. This separation process in different stages allows the maximum recovery of liquid hydrocarbons.
During the dehydration stage, water is removed from crude oil. Water in oil can be free or emulsified. In the former case, water can be separated easily due to differences in specific gravity, using a separator; in the latter, separation is more complex and can be carried out with the help of chemical emulsifiers (tensio-active agents) or by heating the mixture.
Often crude oil must undergo another important refining process, desulphurisation. Very often, in fact, crude oil contains hydrogen sulphide, a very corrosive and toxic gas, which must be removed. The most commonly used process is “stripping”, which consists in bringing crude oil into contact with a sweet natural gas within special vertical, cylindrical vessels (stripping towers) in counterflow with the gas. In this way the stripping gas removes hydrogen sulphide from  crude oil.
Lastly, a desalting process is carried out to remove sodium chloride and possible sediments present in crude oil. This process also allows the removal of other contaminants which are soluble in water, such as carbonates or sulphates  along with heavy metal chlorides.
After having undergone the different processes, crude oil is generally stored in cylindrical steel tanks, which are fire resistant and equipped with a cooling system and containment basins in case of rupture of the tank, until it is transported to refineries by oil tankers and transmission pipelines.