Question

When is it necessary to use a vacuum distillation? Give some explanation and provide examples

Answer 1

Vacuum distillation is used when the materials to be distilled have very high boiling points at atmospheric pressures.

Or when a mixture is a close boiling mixture at atmospheric pressure, we need to reduce the pressure to increase relative volatility of the key components. Lower pressures give better separation due to increase in relative volatility.

It is also used in case of heat sensitive materials because in vacuum distillation, lower tower bottom temperature is maintained. Vacuum distillation is also called 'low temperature distillation'.

Examples: Vacuum distillation is widely used in petroleum industry where temperature sensitive materials are handled. In crude oil distillation, we cannot go above the temperatures of 380 deg celcius due to constraint of thermal cracking of some high molecular weight components. But at atmoshperic pressure, separation of crude oil does not take palce below 380 deg celcius. So in this case, high vacuum is maintained (around 40 torr) to reduce separation temperature.