Petroleum refining by Fractional Distillation| Chemistry

In this article, we will learn how different fractions of petroleum are obtained using the fractional distillation.

What does refining mean?

Petroleum contains many types of hydrocarbons. So, a big mixture of such petroleum is less useful for us.

So the first step is to separate the compounds into groups based on their molecular size. This is called refining the petroleum. This process is carried out by fractional distillation.

Petroleum refining in the lab

The apparatus below can be used to refine petroleum in the lab.

1 As you heat the petroleum, the different compounds start to evaporate. The ones with smaller and lighter molecules go first, since less energy is required to free these from the liquid.

2 As the hot vapours rise, so does the thermometer reading. The vapours condense back in the cool test-tube.

3 When the thermometer reading is 100 °C, replace the first test-tube with an another one. The liquid in the first test-tube is your first fraction from the distillation.

4 Collect three further fractions in the same way, replacing the test-tube at 150 °C, 200 °C, and 300 °C.

Comparing the fractions

Now compare the fractions – how runny they are, how easily they burn, and so on. You can burn samples on a watch glass, like this:

It catches fire easily. The flame burns high, which shows that the liquid is volatile – it evaporates easily.

This catches fire quite easily. The flame burns less high – so this fraction is less volatile than fraction 1.

This seems less volatile than fraction 2. It does not catch fire so readily or burn so easily – it is not so flammable.

This one does not ignite easily. You need to use a wick to keep it burning. It is the least flammable of the four.

The table shows the results:

Trends which the fractions show

Those results show that, the larger the molecules in a hydrocarbon:

⦁ the higher its boiling point will be

⦁ the less volatile it will be

⦁ the less easily it will flow (or the more viscous it will be)

⦁ the less easily it will burn.

These trends help to dictate what the different fractions will be used for, as you will see below.

In the petroleum refinery

In a refinery, the fractional distillation is carried out in a tower that is kept very hot at the base, and cooler towards the top. Look at the drawing.

Petroleum is pumped in at the base. The compounds start to boil off. Those with the smallest molecules boil off first, and rise to the top of the tower. Others rise only part of the way, depending on their boiling points, and then condense.

As the molecules get larger, the fractions get less runny, or more viscous: from gas at the top of the tower to solid at the bottom. They also get less flammable. So the last two fractions in the table are not used as fuels.