Gas separation and liquefaction

Numerous methods can be used to separate gas mixtures, some without any change of state, either at ambient or at lower temperatures. However, the methods most widely used in industry involve liquefaction, i.e. low temperatures, as in the case of distillation and fractional condensation.

Some of the separation products may be required in a liquid state. In this case, separation and liquefaction are combined in the same unit.

In other cases, liquefaction is an end in itself, as in the case of natural gas for transport by sea, and hydrogen for use as rocket fuel (Ariane, etc.).

Numerous techniques based on certain physical or physico-chemical properties of the constituents of gas mixtures can be used to separate them. Among these, we will focus on the most widely used industrial techniques: permeation, adsorption, absorption, distillation and fractional condensation.

As far as the applications of these techniques are concerned, we will only describe installations of an industrial nature, as opposed to those encountered more in research activities (as is still often the case for temperatures below 4 K), and also as opposed to small-scale installations.

Topics covered include

• air gases (oxygen and nitrogen): air separation equipment, with or without liquid production of some of the components;

• natural gas: liquefaction plants and transport chains ;

• Ammonia: ammonia synthesis gas production units using liquid nitrogen scrubbing;

• mixtures of H ₂ , CO, CH ₄ ... (including purge gases from ammonia units), mainly for the production of hydrogen, carbon monoxide and methane ;

• hydrogen and helium liquefaction.