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Over the last two decades, gas permeation has seen a sharp increase in industrial applications for gas separation and purification. It is now widely used to produce nitrogen from air, to recover hydrogen from dilute sources, to treat natural gas, and to dry or treat volatile organic compounds (VOCs).

If gas permeation has found its technical and economic place in fields traditionally reserved for well-established processes (distillation, adsorption, washing), it's because of the progress made in all the constituent elements of a "membrane solution":

• in the field of polymer materials – improving selectivity;

• in module technology, in size, in quality, in fluid flow arrangements;

• in the implementation of these modules, in systems integrating the functions required to protect the films (or fibers) against the risk of premature aging or accidental pollution, as well as the regulations and controls required for proper operation of the membranes, in production mode as well as in shutdown or startup phases;

• and finally, in the technical-economic analysis to better position membranes in the range of solutions. Gas permeation is primarily used to concentrate or deplete a mixture in one of its components; it is less suited to complete separations aimed at producing high-purity gases.

The main advantage of gas permeation is the simplicity of a "dry" process at moderate temperature, enabling continuous treatment after a compression stage or taking advantage of available pressure. A number of interesting variants have been described, including recirculation, multiple compression and sweeping. However, the membrane solution will be all the more likely to be the most suitable if we try to use it for what it can do, and by favoring the simplest arrangements.

For engineers wishing to learn more about the fundamentals, we offer information on materials and the theoretical basis of membrane separation.

For readers faced with a particular problem and wishing to evaluate a "membrane solution", we have provided a guide to help them think through the choice of materials, the influence of intrinsic membrane characteristics and operating variables, etc.

Finally, we wanted to present the state of the art, giving examples of the main industrial applications and highlighting current developments.