Gas separation can refer to any of a number of techniques used to separate gases, either to give multiple products or to purify a single product.
Swing Adsorption Techniques
Pressure swing adsorption
Pressure swing adsorption (PSA) pressurizes and depressurizes gas around an adsorbent media to selectively adsorb certain components of a gas, allowing others to be selectively discarded.
Vacuum swing adsorption
Vacuum swing adsorption (VSA) uses the same principle as PSA but swings between vacuum pressures and atmospheric pressure. The two techniques may be combined and are called "vacuum pressure swing adsorption" (VPSA) in this case.
Temperature swing adsorption
Temperature swing adsorption (TSA) uses a similar technique to other swing adsorption techniques but cycles temperature instead of pressure.
Cryogenic distillation is typically only used for very high volumes because of its nonlinear cost-scale relationship, which makes the process more economical at larger scales. Because of this it is typically only used for air separation.
Membrane gas separation
Membrane technologies are not as well developed as other gas separation techniques and as a result they are less widely used. Manufacturing challenges mean the units are better suited for small to mid scale operations.
The use partially permeable membranes which allow "fast" gases to pass through and be removed, while "slow" gases remain in the airstream and emerge without the original contaminants. Membrane technology is most often used for moisture removal, hydrogen removal and nitrogen enrichment.