Distillation

This method is used to purify liquids that boil without decomposition and have non-volatile impurities. Impure liquid is boiled and the resulting vapors are condensed to get the pure liquid.

Aim

To separate a solution of a solid in a liquid and for separating a solution of two liquids whose boiling points are different.

Principle

Distillation involves the conversion of a liquid into its vapors upon heating and then cooling the vapors back into the liquid. Depending on the difference in boiling points of liquids, distillation is of two types: simple distillation and fractional distillation.

Types of distillation

Simple Distillation

It is used for separating liquids having boiling points differing by 10-20 degrees. The liquid having the lower boiling point distills over first, and the other liquid component is left behind. In this process, vaporization and condensation occur side by side.

Process

The impure liquid or a mixture of two liquids is taken in a distillation flask fitted with a thermometer and a condenser. The flask is heated on a sand bath, on a wire gauze or in a water bath. The more volatile liquid, i.e. the one having a lower boiling point, boils first and the vapors distill over from the outlet near the top. These vapors pass through the condenser and get condensed into the liquid. This condensed liquid collected in a receiver is called the distillate. The less volatile liquid, i.e. the one having a higher boiling point, gets left behind in the distillation flask. To avoid bumping of liquid, a few glass beads or porcelain pieces are placed in the distillation flask.

Fig: 16.4 Simple distillation

Fractional Distillation

It is used for separating two liquids in any mixture, which have boiling points within a narrow range of temperatures. In such cases, simple distillation does not give complete separation and a modified version called fractional distillation is employed.

Process

Due to the small difference in the boiling points, both liquids form vapors simultaneously. Vapors of the boiling liquid mixture are made to pass through a glass fractionating column. The fractionating column is a simple packed column, packed with glass beads, or it is a specially designed column called the bubble plate column. The vapors of the low volatile liquid (the liquid having a higher boiling point) get condensed in this column and return to the distillation flask. The condensate consisting of the pure, more-volatile component (the compound having lower boiling point) leaves the column from the exit near the top and enters the condenser and gets collected in the receiver.

A typical example of this technique on a commercial scale is the refining of crude petroleum by fractional distillation to get different fractions such as petrol, kerosene, oil, grease etc.

 

Two typical fractionating columns Experimental setup for fractional distillation

Fig: 16.5 Fractional distillation

 

In this technique, the design and the total length of the column are very important factors to obtain a high degree of purity of the two separated components. If the mixture contains more than two components, the fraction of the liquid mixture left behind in the flask is subjected to fractional distillation repeatedly.

On a commercial scale, fractional distillation is done in carefully designed fractionating columns (these designs are made in terms of the composition of the feed-in mixture and the desired number and nature of the fractions).

One typical example of the applications of this technique on a commercial scale is the fractional distillation (refining) of crude petroleum to get different fractions such as gasoline, kerosene, lubricating oil etc.

Distillation Under Reduced Pressure or Vacuum Distillation

This technique is used for purifying or separating thermally unstable liquid compounds that decompose at their normal boiling points.

Principle

The lowering of pressure on the surface of a liquid lowers its boiling point. As a result of this, a liquid can be boiled and distilled, without any decomposition, at temperature much below its normal boiling point.

Process

Distillation under reduced pressure or vacuum is carried out in a specially designed glass apparatus. A two necked 'Claisen's flask' is used, the main neck of which is fitted with a long capillary tube and the side neck being fitted with a thermometer. The side tube is connected to a condenser carrying a receiver at the other end. The receiver is attached to a vacuum pump to reduce the pressure. The pressure is measured with the help of a manometer.

Fig: 16.6 Setup for distillation under reduced pressure

Advantages of Distillation Under Reduced Pressure

The advantages of distillation under reduced pressure are:

• The compounds that decompose on heating to their boiling points can be purified by distillation under reduced pressure. This is because at the reduced pressure, a liquid would boil at a temperature much below its normal boiling point.

• Distillation under reduced pressure is more fuel-economical as it makes the liquid boil at temperatures well below the normal boiling point.

Steam Distillation

This technique is used for separating/purifying liquids, which are immiscible with water, volatile in steam, and have high vapor pressure at the boiling temperature of water.

Principle

This method is based on the fact that:

• Total vapor pressure above a mixture of two immiscible liquids is equal to the sum of the vapor pressures of the individual liquids i.e.,

P_total = P₁ + P₂

Hence, P₂ = P_total - P₁

• A liquid boils at the temperature when its vapor pressure becomes equal to the atmospheric pressure.

Steam is continuously passed over the impure organic liquid. The steam heats the liquid and gets condensed into water itself. The resulting mixture of liquid and water begins to boil when the vapor pressure above the mixture becomes equal to the atmospheric pressure. Hence the mixture of the two immiscible liquids will boil at a temperature lower than the normal boiling points of both the liquids. The mixture will continue to boil at the same temperature until one of the liquids is completely distilled out. The distillate, which contains water and the liquid, separates out into two layers as both are immiscible with each other. The two layers can be separated using a separating funnel.

Process

The impure compound and water are placed in a distillation flask kept at a slight slant position and heated on a sand bath. Steam is then bubbled through this mixture. The vapors of the compound, along with steam, leave the flask from the outlet and get condensed in the water condenser. The condensate collected in the receiver is transferred to a separating funnel. The liquid compound being immiscible with water forms a separate layer and can be separated.

Fig: 16.7 Apparatus for steam distillation

The proportion of the organic substance that distills over with steam is related to the vapor pressures as well as molecular masses obtained by the following equation:

where, w₁ and w₂ are the masses of water and the compound (which is steam distilled) respectively.

p₁ and p₂ are the vapor pressures of water and the compound respectively at the temperature of distillation.

M₁ and M₂ are the molar masses of water and the compound respectively.