Fractional distillation

Crude oil has hundreds of parts, many with boiling points close together, too many for simple distillation. In a refinery it is heated to about 400 degrees so that most of it turns to vapour, which rises up a tall fractionating tower that is hot at the bottom and cool at the top. Each part condenses on the level whose temperature is just below its boiling point and is piped off. What comes off each level is still a mixture of hydrocarbons with similar boiling points: a fraction. From top to bottom: gas, petrol, kerosene, diesel, lubricating oil and bitumen; lower fractions have bigger molecules, higher boiling points, darker colour and thicker flow. Heavy fractions are often cracked into smaller, more useful molecules.