Crude oil is mainly made up of hydrocarbons – chains of carbon atoms and hydrogen atoms. Refining is a chain of physical and chemical processes to separate different hydrocarbons from each other and modify them to have better end products. Atmospheric and vacuum distiller units are to separate different hydrocarbon molecules. While the so-called conversion units are to change the carbon chains to have more valuable products.
Crude oil also contains different substances that need to be removed because they would harm refining units or damage an engine or other machinery or even are not environmental friendly. In the refineries sulphur, nitrogen, oxygen, water and other trace substances are removed from main streams.