Membranes for Oxygen Enrichment: Higher Permeance or Higher Selectivity?

The benefits of using oxygen-enriched air (air containing more than 21% oxygen) in combustion processes have long been recognized.[1-3]  For example, if air containing 30% oxygen is used in natural gas furnaces, instead of air containing 21% oxygen, the natural gas savings can be 28% at a burner temperature of 2,500°F.[1]  The increase in savings becomes less pronounced at higher oxygen concentrations; consequently, the principal range of interest is 25% to 35% oxygen.  The use of oxygen-enriched air also decreases flue gas volume and increases the CO₂ concentration, resulting in significant reduction in the cost of CO₂ capture from flue gas.

At present, most oxygen is produced from air by cryogenic separation and vacuum swing adsorption.  Membrane technology is an attractive approach to this gas separation application because of inherent advantages such as modularity, high energy efficiency and good reliability.  However, current oxygen/nitrogen separation membranes with low or moderate oxygen permeance and high oxygen/nitrogen selectivity, optimized to produce high-purity nitrogen, are not competitive with conventional technologies.

This paper will re-examine the membrane technology for producing oxygen-enriched air containing 25-35% oxygen.  Membrane materials and process designs will be reviewed for oxygen enrichment.  Membrane materials with high permeability and high selectivity are usually preferred for gas separations.  However, polymers with higher permeability often exhibit lower selectivity, and vice versa.  This presentation will evaluate the effect of oxygen permeance and oxygen/nitrogen selectivity on the economics of the membrane processes.  It will be shown that optimal membranes for oxygen enrichment should have high oxygen permeance and moderate oxygen/nitrogen selectivity.  Several promising membrane materials will also be identified, and the challenges in making these polymers into industrial membranes will be addressed.

The presentation will also describe the net fuel savings in oxygen-enriched combustion, which is determined by calculating the gross savings from reduced fuel use in a furnace and subtracting the cost of generating oxygen-enriched air using membrane systems.  The positive savings in fuel and energy provide results encouraging enough to pursue development of membrane-based oxygen-enriched combustion.

References:

[1]        S. G. Kimura and W. R. Browall, J. Membr. Sci., 29, 69-77 (1986).

[2]        C. E. Baukal, Oxygen-Enhanced Combustion, CRC Press, 1998.

[3]        R. J. L. Hendershot, T.D. and N. C. Easterbrook, Chem. Eng. Prog., 57-61 (2010).