Membranes - Introduction to Carbon Capture and Sequestration - page 345

suggests, however, that the selectivity is equally important as the perme-
ability. In practice, however, permeability is far more important than
selectivity.
The only difference between the materials in Figure 7.6.8 is the pore
structure. If we, somewhat naively, assume that all these structures can
be converted into membranes at equal cost, the optimal material is the
one that gives a membrane with the smallest area. The smaller the area,
the smaller the capital costs for the membrane separation unit. If we now
look at the simple separation process; we see that we have the following
variables:

• The area of the membrane;
• The fl ow and concentration of the N 2 -rich retentate;
• The fl ow and concentration of the CO 2 -rich permeate.

The process requires us to remove a percentage of the CO 2 . Also,
the purity of the CO 2 -rich permeate is usually set by the process
parameters.
To illustrate how one can fi nd the optimal material, we assume that
we remove a given fraction of the CO 2 with a given purity. Let us consider
a simple membrane separation of CO 2 and N 2 as was shown in
Figure 7.2.1 . Assume that our material has a given CO 2 permeation and
permeation selectivity α CO 2 ,N 2 . In addition, the concentration of CO 2 in the

feed as well as the tota l fl ue gas per second that needs to be separated
is known. If we assume that we use the design given in Figure 7.3.7 , in
which air is used as a sweep gas and this CO 2 -enriched sweep gas is
subsequently used in the boiler, we have a fl ue gas with a higher CO 2
concentration (~25%) than is typical for a coal-fi red power plant. Recall
that in the optimized process the use of a second membrane ensures
that 90% of the CO 2 will be removed from the fl ue gas and that the purity
required for sequestration is reached.
In the design, we are looking for the zeolite structure that will give us
the smallest membrane area. In Section 7.3, we saw that if we assume
that all membranes have the same thickness, for a given permeation
selectivity the area follows from the mass balances for CO 2 and N 2 . The
best material in this analysis is the one with the highest permeation for
which the permeation selectivity is suffi ciently high that it meets the
required purity.