Obtaining Nitrogen

Obtaining nitrogen: Fractional distillation of liquefied air

Fractional distillation is a separation method. It uses the difference in boiling temperatures of the components of a mixture to separate those components.

• The mixture is heated to convert the components into the vapour (gas) phase.

• The vapour mixture is then pumped into a tall separation column (called a fractional distillation column), usually at the bottom of the column.

• As the vapour mixture moves up the column and cools, the different components (called fractions) condense as the temperature drops below the various boiling point temperatures.

• These fractions are collected using collection trays.

• The fractions can be removed from the mixture in this way and thus the components are separated.

This process is used to separate the components of air or crude oil. Air is a mixture of gases, mainly nitrogen (\(\text{N}_{2}\)) and oxygen (\(\text{O}_{2}\)). Liquefied air (compressed and cooled to \(-\text{200}\)\(\text{°C}\)) is pumped into the fractional distillation column. Nitrogen gas has the lowest boiling point temperature and is collected at the top of the column (see figure below).

A fractional distillation column for the separation of the components of air.