How does fractional distillation separate different hydrocarbons in crude oil?

Crude oil is a mixture of different hydrocarbons, which all have different lengths and must be separated by length into different useful 'fractions'. Fractional distillation achieves this by using the fact that longer hydrocarbons have higher boiling points. The fractionating column has a temperature gradient, which means that it is hottest at the bottom and gets progressively cooler closer to the top. Heated crude oil is added to the bottom of the column and, in gaseous form, rises. As it becomes cooler, longer hydrocarbons (with higher boiling points) begin to condense first, and leave the column near the bottom. Shorter hydrocarbons travel further up the column until eventually it is cool enough for them to condense, and they leave closer to the top.