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.