Distillation & Fractional Distillation

Distillation

The operation of distillation is employed for the purification of liquids from non-volatile impurities. The impure liquid is boiled in a flask and the vapours so formed are collected and condensed to give back the pure liquid in another vessel. The non-volatile impurities are left behind in the flask.

The apparatus used for distillation is shown in figure 2.5. It consists of a distillation flask fitted with a thermometer in its neck and condenser at the side-tube. The liquid to be purified is placed in the distillation flask and the thermometer so adjust that its bulb stands just below the opening of the side-tube. This ensures the correct recording of the temperature of the vapour passing over the condenser.A suitable vessel is attached to the lower end of the condenser to receive the condensed liquid. On heating the distillation flask, the thermometer first records a rise in temperature which soon becomes constant. At this point, which is the boiling temperature of the pure liquid, most of the liquid passes over. Towards the end of the operation the temperature rises once again on account of the super heating of the vapour. The distillation is stopped at this stage and the receiver disconnected.

In case of liquids having boiling point lower than 110°C, the water-condenser is replaced by air condenser. To prevent bumping, it is customary to put a few pieces of unglazed porcelain in the distillation flask. While distilling a very volatile and inflammable liquid such as ether, the distillation flask is heated on a water-bath and not on a wire guage. In case of very high-boiling liquids, the flask is heated directly with a naked flame.

 Fractional Distillation

A mixture of two or more volatile liquids can be separated by fractional distillation. When their boiling points differ by more than 40°C, the operation can be carried with the help of simple distillation apparatus described in fig. 2.5. The more volatile liquid passes over first and is collected in a receiver. When the temperature begins to rise for the second time, the first receiver is disconnected. A new receiver is attached as soon as the temperature becomes constant once again. Thus, the distillate it is collected in fractions and the process termed 'fractional distillation'. 

When the liquids present in the mixture have their boiling points close to each other, the separation is best effected by fitting the distillation flask with a fractionating column which in turn is connected to the condenser (fig. 2.6.). On heating, the vapours of the more volatile liquid A, along with the little of the vapours of less volatile liquid B, rise up and come in contact with a large cooling surface of the fractionating column. The vapours of B condense first and that of A pass on. The condensed liquid flowing down the column meets the fresh hot ascending vapour. It snatches more of B from the vapour mixture and gives up any dissolved vapour of A. This process is repeated at every bulb of the fractionating column, so that the vapour escaping at its top consists almost exclusively of A and the condensed liquid flowing back into the distillation flask is rich in B. If necessary, the process can be repeated with the distillate and the liquid left in the distillation flask.

The use of fractionating column has found a remarkable application in modern industry, especially, in the distillation of petroleum, Coal-tar and crude alcohol.