Crude oil is seperated at refineries by fractional distillation. The crude oil is vaporised and fed into a fractionating column. This is a tall tower that is hot at the bottom and cooler at the top.
Oil is vapourised before it enters the column and enters the column at 350 degrees, any residue (a thick sticky mixture of long chain hydrocarbons used to make roads and flat roofs) then drops out the the column. Diesel and gas oils used as fuels for diesel engines and as bolier are the next to come out the column. Next Keronsene, used for aircraft fuel, condenses and leaves the column. Petrol, used for car engines, condenses next. Lastly at around 50 degrees refinery/petroleum gases (short chain hydrocarbons and low boiling point alkanes, used as fuel) condenses.
Inside the column there are many trays with holes to allow gases through, as the vapours move up the column and condense on the tray when they reach their boiling points. There are outlets are different levels to collect the liquid fractions. Hydrocarbons with the lowest boiling point and smallest molecules are collected at the top of the tower, the fractions collected at the bottom have the highest boiling points. Frations with low boiling points burn better and so are more useful.
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