To separate crude oil into its fractions, we use fractional distillation. This process occurs in an oil refinery. The crude oil is heated to about 350 degrees Celsius in a column that gets cooler higher up. Each fraction will evaporate and rise up the column until it reaches a part that is cool enough to condense it. The liquid goes into its respective tube and is separated from the rest of the crude oil. The smallest alkanes, which have a lower boiling point, will rise to the top. The largest fraction, bitumen, never evaporates and falls as a liquid to the bottom.
Crude oil contains a mix of alkanes. Shorter alkanes are the most useful as fuels. We can obtain more shorter alkanes by breaking down the larger ones. This process is called cracking and requires heat and a catalyst. The results of cracking are the shorter alkane and a very short alkene. Alkenes are different because they have a double bond between the two carbons. Alkenes are used to make polymers. Each alkene is a monomer, and when added together into a chain, they form a polymer. Most polymers are made this way, and the majority of our polymers come from crude oil.