Nitrogen and Oxygen Separation from Air

The largest gas-separation process in current use is the production of nitrogen from air. The first membranes used for this process were based on poly(4-methyl-l-pentene) (TPX) and ethyl cellulose. These polymer materials have oxy-gen/nitrogen selectivities of 4 the economics of the membrane processes were marginal. The second-generation membrane materials now used have selectivities of 6-8, providing very favorable economics, especially for small plants pro- 

Improvements in membrane selectivity have now reached a point of diminishing returns. A major improvement in selectivity, from 8 to 12, would reduce compressor size by 20%. Unfortunately, based on Robesoifs trade-off curve, such an improvement in selectivity is likely to be accompanied by a tenfold reduction in membrane permeability, and a tenfold reduction in permeabihty means a tenfold increase in membrane area. These effects are shown graphically in Fig. 7.9. 

For the reasons outlined above, it is unlikely that significantly improved membranes, or the separation of nitrogen from air, will be introduced in the near future. One place where improved oxygen/nitrogen-selective membranes could make a difference is the production of oxygen from air. 

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. 

A few trial calculations show that a process using a feed-gas compressor, even if coupled with an energy-recovery turbine on the residue side, cannot produce low-cost oxygen because of the quantity of electricity consumed. All the feed air must be compressed but only a small portion permeates the membrane. The power consumption of a vacuum pump is less because the only gas evacuated by the vacuum pump is the oxygen-enriched product that permeates the membrane. 

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