How does fractional distillation work?

Different length hydrocarbon chains are separated by their different boiling points as longer chains will have stronger intermolecular attractions. The crude oil is heated and injected into the base of the column which is therefore hottest at the bottom and coolest at the top, and compounds with the lowest boiling points will rise further and be collected as they condense towards the top of the column. Hydrocarbons with a very high boiling point will remain in liquid or solid phase and are collected in the base of the column as bitumen.