Distillation Under Reduced Pressure

Distillation Under Reduced Pressure

The 'straight' distillation is suitable only for liquids which boil without decomposition at atmospheric pressure. In case of organic liquids which decompose before their boiling point is reached, the distillation is carried under reduced pressure when the liquid boils at a lower temperature. A liquid boils when its vapour pressure is equal to the atmospheric pressure. When the pressure is reduced by suction, the liquid boils at a lower temperature. The apparatus used for distillation under reduced pressure is shown in figure.

1. Claisen flask, having two necks. It is fitted with a long drawn jet, dipping in the liquid to be distilled. During the distillation, a stream of bubbles rises through the capillary of this jet and  prevents bumping, which is so pronounced here than in ordinary distillation.
2. Condenser, connected with the Claisen flask on the one hand and a filteration flask, serving as a receiver, on the other hand.
3. Manometer. The receiver flask is connected to an exhaust pump through (a) a Mercury manometer which tells the pressure under which the distillation is being carried, and (b) a trap, to eliminate any condensed liquid. 

The pressure in the apparatus is reduced with the help of a water pump. Whenever a lower pressure is desired, the water pump is replaced by a mercury pump.

An important application of this process is recovery of glycerol from spent-lye in soap industry. Glycerol decomposes  at its boiling point (298°C) but can be distilled unchanged at 12 mm pressure when it boils at 180°C. Another application of 'vacuum distillation' is the concentration of sugar juice and the reduced pressure.