What happens to the chemicals during the fractional distillation of crude oil?

In a fractionating column there is a temperature gradient that goes from being very hot at the bottom, to much cooler at the top. Crude oil is a mixture of hydrocarbons of different lengths, and we know that the longer the hydrocarbon chain, the more intermolecular forces there are between them and the other molecules, and the more energy is needed to break these forces. This means longer chains of hydrocarbons will have higher boiling points and this is how we separate each of the different length molecules. The crude oil is evaporated at the very high heat at the bottom and the molecules rise to higher parts of the column where it is cooler. Once it reaches a fraction where the temperature is lower than its boiling point it will condense back into a liquid and be drained away through one of the many pipes that run through the tower. This means we can separate the molecules based on their length and so can use them for their individual properties and purposes.