Page 370 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering
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              Membranes, Synthetic, Applications                                                          305

















































                     FIGURE 16  “Technology maps” showing feasibility of various air separation technologies for (a) nitrogen generation
                     and (b) oxygen enrichment, using membranes compared to competitive technologies.

                                                                leftinsidethefibersiscollectedatapressureslightlybelow
                                                                that of the feed. Increasing the residence time of the feed
                                                                air through the hollow fiber bundle reduces product out-
                                                                put, but also results in more thorough removal of oxygen.
                                                                By accepting a relatively low yield (<40% recovery), up
                                                                to 99.5%-pure nitrogen can be obtained. The membranes
                                                                available today are between three- to seven fold more per-
                                                                meable to oxygen than to nitrogen. With the most selective
                                                                of these membranes, both oxygen-enriched air and high-
                                                                purity nitrogen can be produced from the same system.
                                                                However, the purity of oxygen is limited to 45–50% with
                                                                present membranes because of the significant quantity of
                                                                nitrogen entering the permeate stream.
                                                                  Considerable effort is being devoted to developing new
                                                                polymeric membrane materials. A special type of oxygen-
                                                                enrichment membrane has also been explored, which con-
              FIGURE 17  Compact nitrogen enrichment membrane system.  sists of a solvent immobilized within a microporous solid
              (Courtesy Air Liquide.)                           support  (Fig.  7D).  Dissolved  in  the  liquid  is  a  carrier
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