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How does fractional distillation separate crude oil into different petroleum fractions?

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Aerial view of an oil refinery complex near New Orleans, Louisiana, with storage tanks, industrial structures, and smoke rising from the facility, surrounded by open land. — petroleum refining Aerial view of an oil refinery near New Orleans.

Fractional distillation separates crude oil into distinct fractions by exploiting differences in boiling points through a continuous, multi-stage process.1

After being heated to 315–400 °C, crude oil enters a tall fractionating column as a mixture of vapor and liquid. As vapors rise through the tower, they pass through a series of trays that facilitate contact between rising vapors and descending liquid, known as reflux.1

This interaction creates a temperature gradient:
* Vapor-Liquid Equilibrium: On each tray, condensation and re-evaporation occur. The liquid absorbs heavier constituents from the vapor while releasing lighter ones.1
* Gradation: This process results in a column where the lightest components concentrate at the top and the heaviest at the bottom.1
* Extraction: Intermediate products, or "sidestreams," are withdrawn at various heights based on their specific boiling ranges, such as naphtha, kerosene, and gas oils.1

Steam is injected at the base to assist in stripping light constituents from the heavy residue, ensuring efficient separation of the various hydrocarbon components.1