Oxygen and nitrogen are common chemicals. Together they make up most of the air we breathe, yet only in the last century has there been a commercial market for those gases. In 1990, the sale of oxygen, nitrogen, and other industrial gases constituted a $16 billion business. The technology that made possible that immense market was first introduced on a commercial scale by Carl von Linde in Germany and by Georges Claude in France, around 1900. A major revolution took place in the 1930s and 1940s, when a means for generating large amounts of cheap oxygen was introduced. By 1980, another technological revolution had taken place. The low-temperature, or cryogenic, technologies pioneered by Linde and Claude were being challenged by non-cryogenic technologies utilizing the adsorbent and permeability properties of certain chemical substances.
As commercialized by Linde and Claude, air separation took place in four stages: 1) compression of the air; 2) the removal of carbon dioxide, water vapor, and other impurities; 3) liquefaction of the air; and 4) separation of the air into oxygen and nitrogen. Removal of the impurities was critical, since water vapor and carbon dioxide frozen in the liquefaction process would obstruct the equipment. The various air separation processes were called cycles and were based on the application of thermodynamic principles, that is, the manipulation of conditions of pressure and temperature. Both Linde and Claude began by liquefying air, then devised distillation columns to produce oxygen-enriched air, and finally, oxygen.