Nitrogen separation from air by pressure swing adsorption

Pressure swing adsorption is commercially used for the separation of air as an alternative to the conventional cryogenic separation process. A zeolite based process, in which the adsorbent shows preferential adsorption of nitrogen over oxygen under equilibrium conditions, is used for the production of oxygen. On the other hand, the separation of air for the production of nitrogen is carried out by pressure swing adsorption over a carbon molecular sieve. The separation is kinetically based, since the equilibrium adsorption of both oxygen and nitrogen is very similar, but the oxygen is adsorbed faster. In conventional pressure swing adsorption systems, both the productivity and yield decrease significantly as nitrogen purities are increased beyond 99.9 %. Although higher nitrogen purities were achievable, product flows were so low that it was not an economically viable process. Only recently, through both improvements in the manufacture of carbon molecular sieves and new cycle developments, higher performance is achieved in the high purity region. A theoretical dynamic model developed for the kinetic separation of nitrogen from air is presented, and the effect of the different process variables and cycle steps on the process performance is analyzed. A comparison between the experimental performance and the model predictions is discussed.