Hydrocarbons have different boiling points and therefore different condensation points. The linger the hydrocarbon, the more intermolecular forces exerting between each molecule. This means that it requires more energy to separate the molecules and boil it. A fractionating column boils the mixture of hydrocarbons. The temperature decreases as you go up the levels of the column. The hydrocarbon with the lowest boiling point condenses lowest down and runs off. The hydrocarbon with the highest boiling point condenses closest to the top and runs out.