In this video you will learn how fractional
distillation separates crude oil into useful fractions.
Crude oil is the term used to describe unprocessed oil. That is oil that has been taken directly
out of the ground either on land or under the sea. It is an exceptionally valuable resource.
It provides us with a great number of organic compounds some of which are used as fuels
and others are used in the manufacture of many different chemicals and even plastics.
However, in the raw form as crude oil, it can be of a viscous dark coloured tar like
consistency and the different fractions of hydrocarbons must be separated by fractional
distillation for them to be useful. Before we understand how fractional distillation
works, we should be clear that crude oil is a mixture of hydrocarbons with different chain
lengths. That means that there are different numbers of carbon atoms making up the molecule
— some molecules are short with only a few carbons in the chain and some are very long.
Intermolecular forces act between molecules and the longer the molecule, the greater the
intermolecular force. As you can see here, the small molecules have weaker intermolecular
forces and so will require less energy to break them apart and turn them into a gas
— they have a low boiling point. The longer molecules have greater intermolecular forces
and more energy, therefore higher temperature will be needed to evaporate these molecules
— they have a higher boiling point. Now we understand how chain length is related
to the boiling point of a molecule, let us look at how this method works.
As you can see, crude oil is heated up to a high temperature outside of the fractionating
column. The hot crude oil, now mostly in vapour form is pumped into the column. The column
has a heat gradient and is very hot at the bottom going cooler as we move up to the top.
Even at the very bottom of the column where the temperature is still high, some long chain
molecules with high boiling points begin to condense back into a liquid and are collected
at the bottom of the column. The rest of the molecules start to rise up the column, making
their way through bubble caps in each tray. The bubble caps slow down the rate of the
rising vapour and eventually the vapours gets too cool, condense and are collected trays.
Small molecules have low boiling points and so condense much higher in the column where
the temperature is cooler still. As you can see, hydrocarbons with similar boiling points
are collected in the same tray and this is why they are known as fractions — they are
mixtures of hydrocarbons with similar boiling points.
Each fraction has important uses. Some examples of fractions are ,Petrol useful as a fuel
for cars, Naptha used in the manufacture of chemicals and as a fuel, Kerosine as aircraft
fuel, Diesel oil used as a fuel for cars,vans and lorrys and Buitmen, a mixture of large
chain hydrocarbons used to lay roads and on roofs.
Now, at the end of this video you should understand that crude oil is a mixture of important hydrocarbons
and that fractional distillation is the method used to separate crude oil into useful fractions
with similar boiling points. You should understand that small chain molecules are collected at
the top of the column since they have lower boiling points and large chain molecules further
down the column as these have higher boiling points.