How can one refinery column separate crude oil into several streams?

Crude oil is a mixture of many hydrocarbons. A refinery heats it and sends the resulting liquid and vapor into a distillation column. Conditions are generally hotter near the bottom and cooler toward the top.

As vapor rises, some components condense while liquids moving downward can partly evaporate again. Repeated contact between liquid and vapor helps separate components according to their volatility, which describes how readily they enter the vapor phase.

Different heights provide streams called fractions, each covering a range of boiling behavior. Lighter, more volatile components tend to be collected higher in the column, while heavier fractions leave lower down or from the bottom.

Distillation sorts molecules; it does not by itself break heavy molecules into smaller ones. Refineries use additional processes such as cracking and treatment to change composition and meet product requirements. A fraction labeled for a familiar fuel is therefore usually an intermediate stream rather than a finished product.