What refining process separates crude oil into fractions by boiling point?

Fractional distillation separates crude oil into fractions by boiling point.

In a refinery, heated crude oil enters a tall fractionating column. Hydrocarbons vaporize and rise through the tower, then condense at different levels because lighter molecules have lower boiling points while heavier molecules require more heat. The resulting streams are called fractions rather than pure individual compounds, because each contains hydrocarbons with overlapping boiling ranges.

Light fractions collected near the top include refinery gases, naphtha and kerosene-range material. Heavier fractions settle lower in the column and include gas oils and residual fuel components. Refineries then process these streams further through methods such as catalytic cracking, reforming, hydrotreating and alkylation.

A frequent misconception is that fractional distillation alone turns crude oil directly into finished gasoline. It mainly separates the mixture; additional conversion and cleaning processes improve octane, remove sulfur and produce fuels that meet specifications. Catalytic cracking is therefore a different process: it breaks large hydrocarbon molecules into smaller ones rather than separating them by boiling point.