OCR Text |
Show the membrane effective permeability, II? ' defined as .... cm3 (STP) -~~2--;~~-~~-_Hg (3) As will be discussed, an oxygen to nitrogen separation factor greater than 3 is necessary to achieve over 35% OEA from air under realistic operating conditions. To perform these separations most economica!1r, eqective PEtmeability values on the order of 10 cm (STP)/cm sec cm-Hg are preferred to reduce the membrane area requirement which is directly related to the system capital cost. OPERATING MODES - Membrane air separation systems can be operated in either of two modes: pressurized (above atmospheric) or vacuum (subatmospheric), as depicted in the Figure 2 flow schematics. In the pressurized mode, feed air is compressed, filtered, cooled and passed through a condensate trap before entering the membrane modules. The OEA permeant stream exits the modules at atmospheric pressure and the NEA retentate stream is available at essentially the same pressure as the compressed feed air. In the vacuum mode, feed air is pressurized to slightly above atmospheric (about 1 psig). This prefiltered air is introduced into the membrane module and a vacuum pump on the permeant-side provides the driving force for gas permeation. The retentate is vented at atmospheric pressure. AIR INTAKE AIR INTAKE Figure 2. -----OEA MEMBRANE CARTRIDGES PRESSURIZED MODE OF OPERATION VACUUM MODE OF OPERATION VACUUM PUMP Alternate Modes of Operation for OEA Mem brane Systems. The pressurized mode allows higher driving forces (differential pressure) than the vacuum mode resulting in reduced membrane area requirements 181 for a given capacity system. This mode is, however, more energy intensive for OEA generation since larger volumes of air have to be compressed to higher pressures. As a general rule, OEA systems are operated in the vacuum mode to take advantage of the lower energy requirements. A useful term in comparing these two modes of operation is OEA system recovery, defined as the ratio of OEA flow to the feed air flow. INTRINSIC MEMBRANE PROPERTIES COMPARATIVE MEMBRANE PROPERTIES - Membrane systems for OEA generation are being commercialized by 3 companies: Fluid Systems Division of VOP, Inc. Dow Chemical Company A/G Technology Corporation. Table 1 compares the best estimates of the intrinsic properties of membranes offered by these companies. Table 1 - Best Estimates of Cqmparative Membrane Properties OEA A/G TECHNOLOGY DOW CONCEm-RA TION 35'11> to 40'11> 27'11> to 30'11> to 35'11> (typical lingle-it age) OXYGEN/NITROGEN SEPARATION FACTOR 3 to 4 OXYGEN EFFECIIVE PERMEABILITY 1 to 4 1.8 to 2 3.5 to 4.5 5 to 8.5 0.03 to 0.04 I. All performance data at ambient temperature (25 C). 2. References 4 and 10. 3. Reference 11. 3 4. 4. Unltl of Effective Permeablllty are cm (STP)/cm tee cm-Hg x 10 Low separation factor «2) membranes are limited in their single-stage system's OEA concentration to about 30%, even at recoveries as low as 5% to 10% (5). Thus, these membranes are not suitable for OEA applications requiring over 30% oxygen. Membranes with low oxygen effective permeability values are not well-suited for OEA generation under the preferrable OEA vacuum operating conditions. The resultant low productivity for OEA adversely affects system economics by requiring excessive membrane areas for these membrane systems versus other membranes which have one to two orders-of-magnitude higher permeability. To compensate for this low productivity, the pressurized mode of operation may be employed. However, even' when operated at high recoveries under |