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Fractional Distillation

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Fractional Distillation is a technique used in the separation of miscible liquids. The process requires repeated distillations and condensations of the mixture. The separation of the mixture happens when the mixture is heated at a certain temperature where fractions of the mixture start to vaporize and are collected separately. Fraction Distillation works on the liquid that has liquid mixed together with different heat of vaporization. Petroleum Oil is distilled into Petrol, Diesel, Petroleum Jelly, etc. using the Fractional Distillation method.

In this article, we will learn about fractional distillation in chemistry, the fractional distillation definition, the fractional distillation process, and others in detail.

Fractional Distillation Definition

Fractional Distillation is used to separate miscible liquids that are volatile in nature. The boiling points of these liquids are close enough. The fractionating column apparatus is used to simulate the separation. Also known as liquification, since the vapour is condensed partially and returned as a liquid. It is basically a process where the vaporization of a liquid mixture gives rise to a mixture of constituents, followed by the extraction of the required component in its pure form.

However, this technique can be only used to separate miscible liquids, which form pure azeotropic mixtures.

Fractionating Column

The apparatus used in this process is similar to that for simple distillation except for a fractionating column which is fitted in between the distillation flask and the condenser. A simple fractionating column is a tube packed with glass beads. The beads provide a surface for the vapours to cool and condense repeatedly.

Principle of Fractional Distillation

On distillation of the liquid mixture, the partial condensation of the vapour occurs in a fractionating column.

In the column, advancing vapour from the still comes in touch with the condensing vapour that returns back to the still. This causes the enrichment of vapour with the more volatile component.

By constant condensation and heating of the vapour, an equilibrium is achieved between liquid and vapour. This results in the extraction of a more volatile component from the mixture. The image added below shows the principle of fractional distillation.


Fractional Distillation Procedure

Apparatuses required for the process of Fractional Distillation includes:

  • Distilling Flask
  • Condenser
  • Receiver
  • Fractionating Column
  • Thermometer
  • Heat Source

After setting up the apparatus, a mixture of two miscible liquids A and B is taken where A has more volatility than substance B. The solution is added to the distilling flask while the fractionating column is connected at the tip of the flask. Heat is applied which increases the temperature slowly. The mixture then starts to boil and vapours start rising in the flask. The vapours are from the volatile component A. The vapours then start moving through the fractionating column into the condenser where it is cooled down to form a liquid which is collected in the receiver.

Throughout the process, vaporization and condensation take place repeatedly until the two mixtures are separated completely.

Applications of Fractional Distillation

Fractional distillation can be used to separate the mixtures like:

  • Acetone and water
  • Chloroform and benzene
  • Separation of gases in the air

Separating Funnel

The technique used for the separation of two immiscible liquids is done using an apparatus called a separating funnel. It is a special type of funnel which has a stop-cock in its stem which permits or stops the flow of a liquid from it. The separation of two immiscible liquids by a separating funnel depends on the differences in the densities of the composing liquids. The heavier liquid settles at the bottom and the lighter liquid remains as an upper layer.

Applications of Separating Funnel

  • Separating a mixture of water and kerosene
  • Separating a mixture of petrol and water
  • Groundnut oil or mustard oil from water
  • Mercury, carbon disulphide, chloroform, and benzene from water

Principal of Separating Funnel

A separating funnel is used for the separation of components of a mixture between two immiscible liquid phases. One phase is the aqueous phase and the other phase is an organic solvent. This separation is based on the differences in the densities of the liquids. The liquid having more density forms the lower layer and the liquid having less density forms the upper layer.

How Separating Funnel Works?

When we pour the mixture into a separating funnel Then liquid with the lower density floats on top. When the tap is opened, the liquid with a higher density starts to flow through the separating funnel into the container. The tap is then closed just before the liquid with the lower density starts to flow through. The liquid with the lower density remaining in the separating funnel can then be drained into a different container to separate the two immiscible liquids.

The image added below shows the separating funnel,


Industrial Distillation

Fractional distillation has a lot of industrial applications. The principle behind the process of Industrial Distillation remains the same, this distillation is carried out on a very large scale. Huge vertical cylindrical columns called “distillation columns”, “distillation towers” or “fractionation towers” are used. Reflux is used in these towers to ensure complete separation of the mixtures.

Fractional Distillation of Crude Oil

The most widespread use of  Fractional Distillation is in the distillation of Crude Oil. Fractional distillation separates various components of crude oil. Crude oil contains substances such as

  • Paraffin Wax
  • Gasoline
  • Diesel
  • Naphtha
  • Lubricating Oil
  • Kerosene

Fractional Distillation separates these components efficiently. For this process, crude oil is added to the chambers and then heated with high-pressure steam. When the mixture starts to boil and vapours are formed the vapour then rises up in the fractional distillation column. These columns have various plates that have holes which allow the vapour to pass through them. The temperature at the top of the fractionating column is kept low. Now, the components with the highest boiling point will condense in the lower part of the column while components with low boiling points will condense at the top. 

The condensed vapours are then removed from the column and then collected to get the required components.

Learn More, Fractional Distillation of Petroleum

Difference between Simple Distillation and Fractional Distillation

The difference between Simple Distillation and Fractional Distillation is shown in the table added below:

Simple Distillation

Fractional Distillation

Simple distillation is used to separate a mixture of miscible liquids with sufficiently large differences in their boiling points.Fractional distillation is used when the difference in boiling point is quite less.
It consists of simple apparatus with two flasks and a condenser.It consists of more complex apparatus with a fractionating column.
Example: To purify seawaterExample: Crude oil refining

Read, More

FAQs on Fractional Distillation

1. What is Fractional Distillation?

Fractional Distillation is process of separating two or more miscible liquids at different boiling temperature.

2. How can you Separate two liquids that have less than 25 K difference in Boiling Points?

We can separate a mixture of two or more miscible liquids for which the difference in boiling points is less than 25 K by fractional distillation. 

3. How will you separate a mixture of Oil and Water?

To separate a mixture of oil and water, we need a separating funnel as both are immiscible liquids. Pour the mixture in separating funnel and let the funnel stand undisturbed for sometime. So that separate layer of oil and water are formed. Open the stopcock of the separating funnel and pour out the lower layer of water carefully.

4. Can we separate Alcohol dissolved in Water by using a Separating Funnel?

No, being miscible they cannot be separated by using a separating funnel.

5. What is the difference between Separating Funnel and Fractional Distillation?

Separating Funnel

Fractional Distillation

Separating funnel is used to separate two immiscible liquids.Fractional distillation is used to separate two miscible liquids.
Fractional distillation separates two miscible liquids based on their boiling point. Separating funnel separates two liquids of different density.
Example: Water and Alcohol.Example: Oil and Water.

6. Which liquids are separated using the Fractional Distillation method?

Liquids with identical boiling points are separated using the fractional distillation method.

7. What steps are involved in the Fractional distillation method?

Steps involved in the Fractional distillation method are:

  • Evaporation
  • Condensation
  • Collection

8. What are the essential conditions for Fractional Distillation?

Eritical conditions for fractional distillation are:

  • Liquids must be miscible with each other.
  • Boiling points of the two liquids must have a difference of fewer than 25 degrees Celsius.

Last Updated : 21 Sep, 2023
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