Fractionating column for crude oil: condensing points and temperature

Within the fractionating column, it’s hot at the bottom of the column and gets gradually cooler towards the top. Smaller molecules require less energy (heat) than larger molecules to change them from liquid to vapour. The smaller molecules condense (change back to liquid) further up the column. This is how the different sized molecules (fractions) are separated.

Big hydrocarbons are found at the bottom where it is hot and small hydrocarbons where it is cooler at the top

From the top to the bottom, the hydrocarbon molecules get bigger as the temperature of the column gets hotter.

For more information on hydrocarbons and to see what small C₁ and larger hydrocarbon molecules look like, follow the link to our hydrocarbons page.

But as a quick summary, see the following:

Methane CH₄: Boiling point -162°C (-259°F)

Note: Methane is the main constituent of natural gas.

Methane has a boiling point of minus 162â°C and is gathered at the top off the chamber

Ethane C₂H₆ – Boiling point -89°C (-127°F)

Ethane has a boiling point of -89â°C and is siphoned off in the chamber

Propane C₃H₈: Boiling point -42°C (-44°F)

Propane has a boiling point of -42â°C and is siphoned off in the chamber

Normal butane C₄H₁₀: Boiling point -0.51°C (31°F)

Butane has a boiling point of -0.51â°C and is siphoned off in the chamber

Isobutane C₄H₁₀: Boiling point -12°C (11°F)

Isobutane has a boiling point of -12â°C and is a mixture of propane and butane

Heneicosane C₂₁H₄₄: Boiling point 100°C (212°F)

Heneicosane has a boiling point of 100â°C