Chemistry - Distillation - Full Explanation

Distillation : 

Distillation is an important method used to separate. 

(i) Volatile liquids from non-volatile impurities
(ii) Liquids having sufficient difference in their boiling point.

Simple distillation : 

Liquids which boil without decomposition 

at atmospheric pressure are purified by the 

process of simple distillation. In this process 

the liquid is first converted into its vapour by 

boiling and then the vapour is condensed back 

into liquid by cooling, and the pure liquid 

trickles down in the reciever. 

Simple distillation

The apparatus used for simple distillation 

is shown in Figure. It consists of round bottom 

flask fitted with a cork having a thermometer. 

The flask has a side arm through which it is 

connected to a condenser. The condenser has 

a jacket with two outlets through which water 

is circulated. The liquid to be distilled is taken 

in the round bottom flask fixed by clamp. The 

flask is placed in a water bath or oil bath or 

sometimes wire gauze is kept on a stand as 

shown in Figure.

The condenser is connected to receiver to 

collect the purified liquid. Care is taken that 

the bulb of the thermometer is just below the 

side arm of the round bottom flask. The flask is heated. As the boiling point of the liquid is reached it starts boiling and the vapors rise to the neck of the flask and pass through the side arm into the cooler parts of the condenser, which is kept cool by circulating water through its jacket. The vapours condense and the liquid is collected in the receiver.

Fractional Distillation :

If in a mixture the difference in boiling points of two liquids is not appreciable, they cannot be separated from each other using the simple distillation assembly. To separate such liquids, the process called fractional distillation is employed in which a special assembly is used.

(a) : Fractional distillation

In this assembly the distillation flask is fitted with a fractionating column. Hence, the vapours first pass through the fractionating column. Vapours of more volatile liquid with lower boiling point rise up more than the vapours of liquid having higher boiling point.

Suppose we have a mixture of two liquid (A) and (B) having boiling points 363 K and 373 K respectively. A is more volatile and B is less volatile. As the mixture is heated, vapors of (A) along with a little of (B) rise up and come in contact with the large surface of the fractionating column. Vapors of (B) condense 

rapidly into the distillation flask. While passing through the fractionating column there is an exchange between the ascending vapors and descending liquid.

(b) : Fractionating Columns

The vapors of B are scrubbed off by the descending liquid, this

makes the vapors richer in (A). This process is 

repeated each time the vapors and liquid come in contact with the surface in the fractionating 

column. Rising vapors become richer in (A) 

and escape through the fractionating column 

and reach the condenser while the liquid in the distillation flask is richer in B. The separated 

components are further purified by repeating 

the process. Mixtures of acetone (b.p. 329 K) 

and methyl alcohol (b.p. 337.7 K); acetone 

and benzene (353 K) can be separated by 

fractional distillation. This process is used in petroleum industry to separate different fractions of crude oil.

Distillation under Reduced Pressure : 

Liquids having very high boiling point or 

those which decompose on heating are purified by carrying out distillation under reduced pressure. In this method the liquid is made to boil at a temperature which is below its normal boiling point by reducing the pressure on the surface of the liquid. Pressure is reduced using a water pump or vacuum pump. In soap industry glycerol is separated from soap by using this technique.

Solvent Extraction : 

When an organic substance is present in an aqueous solution, it can be extracted from that solution by shaking it with an organic solvent in which the substance is more soluble. The organic liquid should be immiscible with water and be able to form two layers. In this process the solute distributes itself between two immiscible liquids. From the aqueous phase the solute gets extracted in the organic phase. Extraction of compound takes place based on the difference in solubility of compound in two liquids. On shaking for a few times with small volumes of organic phase, most of the solute gets extracted into the organic phase. The organic solvent is, then, removed by distillation and the solute is collected. The solvent extraction process is important as it helps clean separations in a short time span. 

Solvent Extraction

If the solute is less soluble in organic phase 

then a technique called continuous extraction

is used where the same amount of organic 

solvent is used repeatedly for extraction. This 

technique involves continuous distillation of 

the solvent within the same assembly. Thus use of large quantity of organic solvent is avoided.