Separation Techniques - Obtaining Gases From Air


Mixtures are substances consisting of two or more types of matter. Mixtures are two types – homogeneous mixture and heterogeneous mixture.  In a heterogeneous mixture, the components are not uniformly distributed and there is no particle level homogeneity. Thus, we can easily separate a heterogeneous mixture into respective components of the mixture.  Sieving, filtration, handpicking, etc. are few common separation techniques used for the heterogeneous mixture. In the case of the homogeneous mixture and sometimes a heterogeneous mixture, we need to use special separation techniques. Evaporation, centrifugation, chromatography, sublimation, separating funnel, etc. are special separation techniques. Let’s see how to obtain different gases from the air.



Separation Techniques  : Obtaining Gases from Air

We know that air is a homogeneous mixture of gases. It constitutes of gases like nitrogen, oxygen, carbon dioxide, argon, etc. in different proportion. Since it is a homogeneous mixture, we need special separation techniques. Fractional distillation is the technique that is used for obtaining different components from the air.

Fractional distillation is a separation method where the difference in boiling points of components is used to separate the liquid mixture into fractions through distillation. The process begins with the liquefaction of air. Let’s try to understand the process with the help of an illustration of separation of nitrogen from the air.

In order to obtain nitrogen gas from air, we need to remove rest of the constituents of air. Before we start, air is filtered to remove the dust particles and then liquefied.

Separation Techniques - Fractional Distilation of Air

Separation Techniques-Obtaining gases from Air

Step 1:Conversion of air into liquid air

 The air which is in gaseous form is converted into liquid air. This is done under high pressure. Under high pressure, the air is compressed and then cooled by reducing the temperature. This results in liquid air.

Step 2:Fractional Distillation

The liquid air is then passed through the fractional distillation column. Here, the liquid air is allowed to warm-up. The bottom of the fractionating column is warmer than the top. Each gas starts to separate at different temperatures according to its boiling point.

Nitrogen has a boiling point of -196°C while oxygen has -183°C. The nitrogen gas will start to escape through the outlet and it is collected. The liquid oxygen will be collected in the fractionating column.

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Practise This Question

Fractional distillation is the separation of a mixture into its component parts or fractions. This is the process of separation of chemical compounds due to difference in their boiling point. The mixture is heated to a temperature at which one or more fractions (component parts) will vaporize. It is a special type of distillation.

Air is a homogeneous mixture of 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases such as helium, krypton, nitrogen dioxide etc.

Consider three gases oxygen, nitrogen and argon whose boiling points are -183, -196, -186 degree celsius, they are to be separated from air using fractional distillation.

Air is compressed by increasing the pressure and then cooled by decreasing the temperature in order to get liquid air. This liquid air is allowed to warm-up slowly in a fractional distillation column, where gases get separated at different heights depending upon their boiling points. As the height of tower increases the temperature increases.

Statement: The temperature is ____ at the base of the fractional distillation column.