Virtually all of the crude that goes into the refinery comes out as a saleable product. The source material for bunker fuel is the residue left over from the distilling stages of the refining process. This residue is processed further to generate valuable products, including material used to blend marine fuels. Upon arrival at a refinery, crude oil is put into settling tanks to separate out the water and salt that all oil contains. Left untreated, the salt would cause severe corrosion problems in the refinery, and could poison expensive catalysts.
Salt is separated out by water washing.
The water percolates through the oil, dissolving the salt as it goes, and is drawn off at the bottom of the settling tanks. The first stage of the refining process is distillation. The oil is heated to around 350 C, and fed into a tower where the heat differential between the top and the bottom of the tower ensures that hydrocarbon components with a lower boiling point accumulate towards the top, and those with a higher boiling point collect towards the bottom. The individual hydrocarbons are drawn off at various boiling points through pipes arranged along the column. Light-end products drawn off towards the top include LPGs, naptha, and kerosene.