Membranes - Introduction to Carbon Capture and Sequestration - page 308

desired purity. To achieve greater purity one would need two stages, and
hence two compression steps, which would require too large a fraction of
the produced electricity. Therefore, for a long time the logical conclusion
was that research to develop novel membranes for carbon capture does
not make much sense. The work of Baker and co-workers [7.1] completely
changed this conventional wisdom by demonstrating that one can achieve
the desired purity by invoking sound process design principles. A hint was
already given in the previous section by the demonstration that we can
increase the effi ciency of the membrane by using an N 2 -rich sweep gas.
Baker and co-workers [7.1] proposed the scheme shown in Figure 7.3.7 ,
in which two membrane units are used. The main innovation is to use air as
a sweep gas in membrane unit II and burn the coal with CO 2 -enriched air.
The net result is that the fl ue gas has a signifi cantly higher concentration of
CO 2 and, given the pressure ratio constraint of 5, this higher concentration
of CO 2 in the feed gives a higher concentration in the permeate.
Given that we now have a membrane separation process that can be
used for fl ue gas separations, it is interesting to look at how the perfor-
mance of the membrane depends on its material properties. Figure 7.3.8
shows how the capture costs depend on the selectivity and permeance
of the material. The costs are a combination of the energy requirements

Treated
flue gas

Particular
removal

Blower or
Compressor

Air sw eep

Boiler

I II

Vacuum pump

Coal feed
to boiler

CO 2 for sequestration

Figure 7.3.7 A two step counterfl ow membrane separation
In this design air is used as a sweep gas in membrane module II and hence the air fed
to the boiler is enriched with CO 2 . The energy requirement of this process is 97MW,
which is 16% of the power being produced. This fi gure is adapted from the work of
Merkel et al. [7.1] . This animation can be viewed at: http://www.worldscientifi c.com/
worldscibooks/10.1142/p911#t=suppl