An oil refinery
The main fractions and levels
Graphic of distillation tower
This tower diagram is made from two towers: the atmospheric still and the vacuum still.

A distillation tower splits crude oil separate fractions. A fraction is a mixture of hydrocarbons that comes out of the still at a particular level.

The picture shows the familiar diagram of the names of the fractions and the levels at which they come out of the distillation tower.

In reality, a single tower could not cover the full range of temperatures needed to split up the heavier fractions.

The bottom three fractions have to be split up in a separate vacuum still.

The table below shows the names and uses of the fractions that come from the distillation process. It also shows the ranges of hydrocarbons in each fraction.

Each fraction is a mix of hydrocarbons and ach fraction has its own range of boiling points and comes off at a different level in the tower.

Notice that the ranges overlap. This is because the distillation process is not exact - some C-30 hydrocarbons will condense at the level for light gas oil and some will condense lower down.

This overlap isn't very important for the heavier fractions. However, the lighter fractions, like LPG, are usually separated more tightly in another fractionating tower.

Fraction Carbons BP °C Uses
Gases 1 to 4 < 40 • Fuel in refinery
• Bottled and sold as LPG
Napthas 5 to 10 25 – 175 • Blended into petrols
• Feedstock for making chemicals
Kerosines 10 to 16 150 – 260 • Aviation fuel
Light gas oils 14 to 50 235 – 360 • Diesel fuel production
Heavy gas oils 20 to 70 330 – 380 • Feedstock for catalytic cracker
Lubricants > 60 340 – 575 • Grease for lubrication
• Fuel additives
• Feedstock for catalytic cracker
Fuel oil > 70 > 490 • Fuel oil (power stations and ships)
Bitumen > 80 >580 • Road and roof surfaces
The fractions from distillation