GCSE Chemistry Revision: Fractional Distillation

10th April 2017

GCSE Science – Chemistry - Fractional Distillation – Crude Oil & Fuels

KEY STAGE 4 – GCSE Science Revision Guide - Chemistry

Use this Revision Guide to remember all the Key Study Points in the GCSE Syllabus and get full marks in your GCSE Chemistry Exam.

What is distillation?

Basic distillation is a process which uses heating, evaporation, cooling and condensation to separate a mixture of liquids into the ‘pure’ liquids which make up that mixture.

For example:

When we want to separate one (pure) liquid from a mixture of two different liquids, we can do this using distillation.

 Ethanol (alcohol) and water can be separated using simple distillation

Why/how does distillation work?

Distillation relies on the following sequence of events:

HEATING —> EVAPORATION —> COOLING —> CONDENSATION

Because of the different boiling points of different liquids in a mixture, these will evaporate at different rates as they are heated.

As they evaporate at different rates, each one can be ‘distilled’ (condensed) at different stages of the process, independently of each other.

For example:

 ethanol has a lower boiling point than water both are heated in a flask to the boiling temperature of Ethanol (78.37 °C)

 the flask is kept at this temperature until all the ethanol has evaporated and has condensed in the condenser (as it cools)

 the flask is then heated to the boiling point of Water (100°C) and the water evaporates ,condensing as it cools in the condenser we end up with pure Ethanol and pure Water.

What is Fractional Distillation?

This is a more complex distillation technique which can be used to separate a number of different

parts or ‘fractions’ from a mixture of several substances.

For example:

 Instead of having only one ‘condenser’ (as in the example above), Fractional Distillation

uses a column with several condensers placed at different heights

 The distillation column is HOT at the BOTTOM and COLD at the TOP

So substances with:

 HIGH boiling point will condense at the BOTTOM of the column

 LOW boiling points will condense at the TOP of the column

Where is Fractional Distillation used?

This process is commonly used to distil and separate Crude Oil into its component parts/fractions.

Crude Oil is made up of several different ‘fractions’, so we can use Fractional Distillation to separate

it into the following substances:

 Bottled Gas (lowest boiling point / very volatile / flows easily through column)

 Petrol Fuel (slightly higher boiling point than gas, relatively volatile, flows easily)

 Naphtha (higher boiling point than petrol, quite volatile, flows quite easily)

 Kerosene/Aircraft Fuel (medium boiling point, medium volatility compared to others)

 Diesel Fuel (high boiling point, low volatility, does not flow as readily as the others)

 Fuel Oil (very high boiling point compared the other fractions, does not ignite or flow easily)

 Residue / Bitumen (Extremely high boiling point, not volatile, does not flow or ignite easily at all)

The fractions towards the top of the list (or distillation column) have small molecules and:

 lower boiling points

 more volatile

 flow more readily

 ignite more readily

 make better fuels.

The fractions towards the bottom of the list (or distillation column) have large molecules, so they are:

 higher boiling points

 less volatile

 flow less readily

 are harder to ignite

 do not make such efficient fuels.

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