Fractional distillation is the process of separating out different
hydrocarbons from crude oil. It works because different
hydrocarbons all
boil at different
temperatures but most of them
boil at lower than 500°C.
In a fractional distillation tower crude oil is heated at the bottom. The different hydrocarbons turn to vapor and rise up through the tower, as they do this, they cool. There temperature lowers back down to around their boiling point and the liquid is drained off and because the tower is separated into stages, hydrocarbons with have the same boiling points can be collected. Obviously the hydrocarbons that don't evaporate a certain stage rise up the tower until they do. All that is left at the bottom is a sticky tar called bitumen. Bellow is listed the different stages of the tower.
The reason that the hydrocarbons boil at different temperatures is due to the different amount of molecular bonds in each one. The more bonds a hydrocarbon has the higher its boiling point. So bitumen has more bonds than, for example, naphtha. This is also actually what makes bitumen so sticky, and if you were wondering they use it to surface roads.