In large chemical plants multicomponent fluid mixtures are separated in fractions or in pure components. Dealing with gaseous mixtures, separations can be carried out by cryogenic distillation, adsorption followed by temperature swing adsorption (TSA) or pressure swing adsorption (PSA) or membrane processes, as gas permeation or pervaporation, When separation is carried out with membrane processes, the purification efficiency is often not sufficient if ultrapure products are the objective of the process. In the case of cryogenic distillation and adsorption product purity is not a problem. The investment cost of membrane as well as cryogenic separation processes are high in comparison to adsorption separation plants but the increase of investment cost versus capacity is smaller for cryogenic plants compared to membrane and adsorption units [1]. Therefore, the installation of a cryogenic process may be advantageous in very large plants designed for ultrapure products whereas adsorption is often the domain for plants with medium capacity. The future of membrane separation depends on the development of more efficient membranes.