Fractionating Column - QS Study
top logo

Fractionating Column

In practice, however, instead of so many individual steps of vaporization and condensation, a device is utilized in which only one distillation is carried out with the effect of many distillations. Such a device is known as fractionating column which is a very important modem appliance in many industries.

Consider a solution of A and B in a flask heated by an immersed heating coil (Figure a). Compound A has a lower boiling point than B. On boiling, the liquid gives off vapour which rises. The vapour will be richer in A, since the hotline point of A is lower. The ascending vapour reaches the plate 1 which has a small hole. The vapour partially condenses on the plate. Some liquid falls downward the remaining vapour becomes still richer in A and the solutions in the flask gets richer in B, the higher boiling liquid. The vapour travels upward and is again partially condensed on the plate 2, the liquid drains down and the vapour still more enriched in A travels upwards. This process shall continue till the vapour phase becomes pure A, if sufficient number of plates are available, pure A will escape by the side-tube and can be condensed to give the liquid. The liquid in the flask becomes richer and richer in B as more A escapes until pure B is left in the flask.

(a) Fractionating Column

Fig:  (b) bubble cap fractionating tower

In practice, however, the column does not contain plates as shown. The wide tube is generally filled with glass beads, narrow small glass tubes, glass rings and other materials of various shapes and designs to meet individual needs. Industrial columns are very big indeed. One type used in refining crude oil or petroleum oil is known as bubble-cap fractionating tower (figure b. For general laboratory use a glass tube of 3.5 to 4 cm diameter and about a metre long filled with broken glass-pieces is quite satisfactory.