Fractional Distillation of Crude Oil
In the unit Introduction to Crude Oil, we saw how we can find sources of crude oil. Once the crude oil has been extracted from the ground, it is transported to
an oil refinery by pipeline or by giant oil tankers. In this unit we will see what happens to the oil in the first process
at the refinery.
Distilling crude oil in the lab
As you probably know,
Crude oil is a liquid formed from tiny sea creatures and plants that died millions of years ago. It contains a mixture of
hydrocarbons. It is the raw material for many fuels and plastics.
is a mixture of a large number of compounds (mainly hydrocarbons). The thick, gooey liquid is of little use to anyone in
At an oil refinery, the various hydrocarbons are separated into mixtures of compounds with similar boiling points. This is
done by a process called fractional distillation
. We say that the crude oil is separated into fractions
. In order to understand this process, we can look at what happens when we distil crude oil in the lab, as shown in Fig.2
As we heat the crude oil, the lighter hydrocarbons (those with the smaller molecules) are the first to boil off. They are
collected as the first (light) fraction, which is a mixture of hydrocarbons with similarly low boiling points. Then larger
molecules evaporate from the oil as heating continues. They make up the heavier fractions.
This happens because, in general, small molecules have weaker forces between their molecules than large molecules, so it takes
more heat energy to make the molecules move apart sufficiently to evaporate and form a gas.
|Figure 3. Hydrocarbons with small molecules have lower boiling points than large hydrocarbons.
Some of the molecules in crude oil have branched chains of carbon atoms. Look at the three molecules below and their boiling
For molecules of a similar mass, a branched-chain
A molecule is a group of two or more atoms bonded together.
has a lower boiling point than a straight-chain molecule. With branches sticking out, the molecules cannot pack as closely
together, so they are easier to separate from neighbouring molecules.
Notice that the molecules shown above all have the formula C5
, but they each have a different structure.
Molecules with the same formula but different arrangements of atoms within their molecules are called isomers
Properties of the different fractions
You have already seen how the larger molecules that make up the higher boiling point fractions of crude oil have compounds
that are darker in colour. Look at some other properties of the different fractions shown in Figs.5 and 6 below:
Fractional distillation in an oil refinery
In an oil refinery, the oil is separated into its fractions in giant fractionating columns, as shown in Fig.7:
|Figure 7. A fractionating column in an oil refinery.
Look at the fractionating column below and the uses we make of the different fractions:
In the fractionation process, the oil is heated and enters the column as a vapour. The column is hottest at the bottom so
only the hydrocarbons with the highest boiling points condense back to liquid there. They are piped from the bottom of the
column. The other hydrocarbons rise up the column.
As we go up the column, the temperature falls and successively lighter fractions are collected as they too condense. The lightest
fraction (which is made up of gases) comes from the top of the column.
Crude oil can be separated into its fractions (mixtures of hydrocarbons with similar boiling points) by fractional distillation.
This is sometimes called fractionation and takes place in huge fractionating columns in oil refineries.
Here are some properties of the different fractions:
||Size of molecules
|Light fraction, with low boiling point range
||thin, runny liquid
||ignites easily (burns with clean flame)
|Medium fraction, with mid-range boiling point
||more difficult to ignite (some smoke in flame)
|Heavier fraction, with higher boilng points
||viscous (thick) liquid
||difficult to ignite (burns with smoky flame)
So, as the fractions get heavier (and the chains in their molecules get longer), they get darker in colour, more viscous,
and less flammable. They also burn with smokier flames.
There are restrictions on the use of this courseware.
Please click here
for terms and conditions.