Principle:
A liquid boils at lower temperature when the external pressure on its surface is reduced.
Distillation under reduced pressure / Vacuum Distillation is the process in which distillation is carried out under a reduced temperature than its boiling point by applying vacuum in the system.
Construction:
The general assembly of laboratory scale vacuum distillation unit is as shown in above Fig.
It consists of a double neck distillation flask known as Claisen flask.
Thick walled glass apparatus with interchangeable standard glass joints are used for vacuum distillation.
A thermometer is fitted in one of the necks of the Claisen flask.
The splashing of the violently agitated liquid is prevented by the second neck.
Bumping occurs continuously during vacuum distillation.
A fine capillary tube in the second neck of the flask can prevent bumping.
The capillary tube is dipped in the boiling liquid, so that a stream of air bubbles is drawn out.
Water bath or oil bath is used for heating.
The Claisen flask is connected to a condenser which empties its content in a receiver.
Vacuum pump is attached to the receiver through an adapter.
A manometer should be inserted between the pump and the receiver.
Working:
The distillation flask is filled one-half to two-third of its volume with the liquid to be distilled.
Add small pieces of porcelain to the liquid to facilitate distillation and prevent bumping.
The capillary tube and a thermometer are positioned in place in the flask.
Cold water is circulated through the jacket of the condenser.
The required vacuum is applied and then heat is supplied to the flask.
Heating should not be commenced until the required vacuum is attained.
The contents are heated gradually.
The temperature rises and liquid begins to boil rapidly due to vacuum.
The vapour passes through the condenser and the condensate is collected in the receiver.
The temperature is noted down, which would be less than the boiling point of the liquid.
Uses:
To separate a volatile mixture of two miscible liquids.
e.g. Acetone water mixture.
Separation of air components.
Advantages:
Useful for thermolabile substances.
Useful for temperature sensitive substances: There is a shorter time of thermal exposure of the distillant.
Higher production rates.
Fractional distillation under vacuum leads to easier separation of components of a mixture.
Change in position of the azeotropic point at reduced pressure. This enables separation under reduced pressure/vacuum of hard to separate materials.
Energy consumption is reduced by lowering the boiling point under vacuum.
Losses due to oxidation of the feedstock are reduced under good vacuum condition
Disadvantages:
Persistent foaming occurs (Can be controlled by inserting capillary tube in second neck and addition of capryl alcohol.)
Not suitable to separate solid and semisolid mixtures.
Commonly Asked Questions.
Define distillation and write in detail about Distillation under reduced pressure.
With help of a well labelled diagram describe the principle and working of the apparatus used for Distillation under reduced pressure.