Membrane Gas Separation

14.3.3
Air Separation 
A membrane process that has grown rapidly in the last few decades is the separation of 
the air into nitrogen - and oxygen - enriched streams. The main part of the membranes in 
use are oxygen selective, therefore the nitrogen - rich stream is recovered in the high pres-
sure side, whereas an O 
2
- enriched stream is obtained as permeate at a low pressure. 
Figure 14.4 Two - stage H 
2
membrane separation plant (from PRISM ™ brochure [1] ) in 
ammonia installation operating since 1979, which recycles 90% pure H 
2
to the reactor. 
Reprinted with permission from Industrial & Engineering Chemical Research, Membrane 
gas separation: a review/state of the art, by P. Bernardo et al., 48, 10, Copyright (2009) 
American Chemical Society

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Membrane Engineering: Progress and Potentialities in Gas Separations
291
Signifi cant efforts have been made for increasing O 
2
/N 
2
selectivity of the polymeric 
membranes; these two molecules have a very close kinetic diameter (nitrogen, 3.64 Å ; 
oxygen, 3.46 Å ), thus the use of a membrane whose selective properties are related only 
on the molecular size selection was very diffi cult. As reported by Baker [7] , the fi rst 
membranes used for this separation showed an O 
2
/N 
2
selectivity of ca. 4. Signifi cant 
improvement in membrane selectivity allowed values ranging from 8 to 12 to be reached, 
implying relevant reduction in compressor size and equipment costs. 
Today, nitrogen separation by membrane systems is the largest GS process in use. 
Membrane selectivity does not need to be high in order to produce a relatively pure 
nitrogen stream, thus they became the dominant technology instead of pressure swing 
adsorption (PSA) or cryogenic distillation. 
At the moment, thousands of compact on - site membrane systems generating nitrogen 
gas are installed in the offshore and petrochemical industry. Air Products Norway has 
delivered more than 670 PRISM ® systems producing N 
2
for different ship applications, 
and more than 160 PRISM ® systems for offshore installations [38] . In December 2006, 
Air Products started with another PRISM ® production plant in Missouri (USA) [39] . 
Another new air separation unit with a capacity of 550 ton/day of oxygen was installed 
by Air Liquide in Dalian (China) [40] . In Japan, Ube Industries [41] is increasing the 
production of polyimide hollow fi bres for nitrogen separation to introduce a number of 
ethanol refi ning plants, mainly in the USA and Europe, driven by the rapid increase in 
the demand for bio - ethanol as an additive for oil products. 
Concerning oxygen separation (Figure 14.4 ), great improvements in membrane per-
formances are required to produce oxygen - enriched air and, moreover, pure oxygen to be 
used for chemical industries, electronic fi elds, medical fi elds, etc. The fi rst application of 
membrane technology was carried out in 1980 using ethyl cellulose membranes, but the 
performance was not good enough to make the process competitive [7] . A compressor on 
the feed stream or a vacuum pump on the permeate side is required in order to guarantee 
the necessary driving force for the permeation of the oxygen through the membrane. Both 
these solutions are expensive, thus, for making this operation cheaper high fl ux mem-
branes are required. Depending on the quality of the oxygen to be obtained, the separation 
process can be developed in one or two stages (Figure 14.5 ). Membranes are economically 
convenient when the fi nal O 
2
concentration is in the range 25 – 50% [42] ; in this case, a 
single stage (Figure 14.5 a) is the most economic confi guration [43] . Pure oxygen can be 
produced in two stages (Figure 14.5 b), as proposed by Baker [9] . However, many studies