Difference Between Fractional Distillation and Simple Distillation

Main Difference – Fractional Distillation vs Simple Distillation

Distillation is a separation technique that is used to purify a liquid or a mixture of liquids by heating followed by cooling. When a mixture of liquids is heated, the components which have different boiling points go into gaseous phase at different instances. Condensation of the vapour allows it to be liquefied and thus, different components can be separated. There are several different distillation techniques adapted according to the composition of the liquid mixture. These are simple distillation, fractional distillation and steam distillation. The main difference between fractional distillation and simple distillation is that simple distillation separate liquids with boiling point gaps of at least 50 degrees whereas fractional distillation separates liquids with closer boiling points.

This article looks at,

1. What is Fractional Distillation
      – Process, Mixture Characteristics, Apparatus, Uses
2. What is Simple Distillation
      – Process, Mixture Characteristics, Apparatus, Uses
3. What is the difference between Fractional Distillation and Simple Distillation

What is Simple Distillation

Simple distillation is employed when the component of the liquid mixture have boiling point gaps close to fifty degrees. This involves boiling the mixture at first, and the compound with the lowest boiling point will start vaporizing. Let’s consider a mixture of two liquids. The vapour formed will be rich in the compound which has the lowest boiling point. At this point, the temperature remains steady until the fraction of the compound with the lowest boiling point in the liquid mixture reaches zero. Thereafter, the temperature again starts to rise until it reaches the boiling point of the compound with the highest boiling point.

Figure 1: Simple Distillation

As shown in this image, the mixture of liquids is kept in the flask and heated until the compound with lowest boiling point starts to evaporate. Then the vapour is sent through a condenser where it is cooled and liquefied to collect as a separate compound. This process is repeated until the two compounds are separated.

This is employed to purify a solvent which has a solute dissolved in. If we take salt water as an example, water will be purified leaving salt in the flask.

What is Fractional Distillation

Fractional distillation is a technique employed for many complex liquid mixtures, i.e., mixtures with components that have closer boiling points. For such systems, simple distillation will be meaningless. Therefore, a modified distillation is required. For fractional distillation to be effective, the components should be miscible.

The apparatus of this method is somewhat different to simple distillation apparatus. There is a fractionating column fixed to the flask which contains the liquid mixture. The fractionating column contains glass beads to provide a larger surface area for vaporization and condensation to happen. For example, let’s take a liquid mixture of two components. On heating the mixture, the vapour formed will consist of both components. At some point, the top of the fractionating column will reach the temperature of the lower boiling point, the component which with the lower boiling point will be distilled. The whole amount of the component with the lower boiling point will be gradually collected into the flask, and the one with higher boiling point remains in the flask.

Fractional distillation is the technique used to purify crude oil into numerous components.

Figure 2: Fractional Distillation

Difference Between Fractional Distillation and Simple Distillation

Boiling Point

Fractional Distillation: Fractional distillation is used to separate liquids with closer boiling points.

Simple Distillation: Simple distillation is used to separate liquids with boiling point gaps of at least 50 degrees. 

Apparatus 

Fractional Distillation: Fractional distillation uses a complex apparatus with a fractionating column.

Simple Distillation: A simple apparatus with a flask to contain the mixture, a condenser and a flask to collect purified components is used.

Repetition

Fractional Distillation: The process has to be repeated several times to get the pure components.

Simple Distillation: This is a one run process. 

Separation of a Solvent from a Solute

Fractional Distillation: This cannot be used to separate a solvent from a solute.

Simple Distillation: This can be used to separate a solvent from a solute.

Uses

Fractional Distillation: This is used in crude oil refining. 

Simple Distillation: This is used to purify sea water.

Conclusion

As explained above, the main difference between fractional and simple distillation is that fractional distillation is used for complex liquid mixtures with closer boiling point. The apparatus and process used for fractional distillation is also complex, unlike simple distillation. 

Reference:
1. “Fractional Distillation.” Chem 211 – Techniques. Wellesley College, n.d. Web. 23 Feb. 2017.
2. Helmenstine, Anne Marie. “Understand How Distillation Works.” About.com Education. N.p., 10 Aug. 2016. Web. 23 Feb. 2017.

Image Courtesy:
1. “Distillation using Liebig condenser_set up” by Siyavula Education (CC BY 2.0) via Flickr
2. “Fractional distillation lab apparatus” By derivative work: John Kershaw Fractional_distillation_lab_apparatus.png: User:Theresa knott – Fractional_distillation_lab_apparatus.svg, (CC BY-SA 3.0) via Commons Wikimedia