Oxygen membrane separation process

A simplified flow scheme of a membrane-separation process to produce oxygen-enriched air is shown in Fig. 7.10(a). Feed air containing 21% oxygen is passed across the surface of a membrane that preferentially permeates oxygen. In the scheme shown, the pressure differential across the membrane required to drive the process is maintained by dravting a vacuum on the permeate gas. The alternative is to compress the feed gas. 

Fig. 7.10 Oxygen/air separation process designs (a) one-stage membrane-separation process, (b) two-stage separation process.

The oxygen-hemoglobin system is but one of many examples of adsorption processes that occur in the human body. These are often irreversible in nature (i.e., involve chemisorption) and do not properly fall into the category of phase equilibria. All such events, however, do involve the transport of mass in one form or another, with both membranes and fluid barriers defining the mass transfer resistance. We will return to this topic in Section 8.2 dealing with membrane separation processes. 

It is known that fluorinated compounds dissolve considerable amounts of oxygen and that membranes containing ultrahydrophobic moieties show a high selectivity in gas-separation processes, e.g., in O2/N2 separation. 

Figure 8.29 Cost of oxygen-enriched air produced by membrane separation on an EP02 basis as a function of the oxygen permeability and oxygen/nitrogen selectivity of the membrane. The performance of today s best membranes is represented by the upper bound performance line from Robeson s plot (Figure 8.24) [35,47]. Reprinted from J. Membr. Sci. 62, B.O. Bhide and S.A. Stem, A New Evaluation of Membrane Processes for the Oxygen-enrichment of Air, p. 87. Copyright 1991, with permission from Elsevier...

Another gas separation process uses special hollow-fiber membranes designed to allow the oxygen and water vapor in a stream of air to leak through the walls of the fibers as the air flows through, producing a stream of nitrogen at the end of the tube. 

These gas separation processes have also been adapted for use in other industries. For example, the Fetzer Winery in Redwood Valley, California, uses a membrane separator to produce a nitrogen-rich atmosphere for its fermentation tanks to prevent oxygen-driven decomposition of the wine. 

Zakrzewska-Trznadel, G., Chmielewski, A.G., and Miljevic, N., Separation of protium/deuterium and oxygen-16/oxygenl8 by membrane distillation process, J. Membr. Sci, 113, 337, 1996. 

Air separation by membranes to produce N2 or O2 is interesting especially for small to medium capacities and medium (up to 99%) purity. The highest purities of oxygen (>99%) are to be obtained with dense (non-porous) membranes. The economics of gas separation processes is discussed in detail by Spilman [7] together with a number of potentially interesting applications. 

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