Membranes - Introduction to Carbon Capture and Sequestration - page 296

The maximum separation can be obtained if we use vacuum on the
permeate side. Thus we defi ne an ideal separation factor

α * , if we

assume that the permeate pressure is zero :

′ ′

PA PA

CO N

2

p

2

p

CO , R N , R

L L

2 2

′

P HD

CO CO CO

∗

α = =

2

=

2 2

CO , N

x /x HD

22

′

P

CO , R N , R N N

N 2 2 2 2

2

This equation shows that the ideal separation factor is given by the ratio
of the permeabilities of the material.

Section 3
Separating flue gasses

Let us focus on the separation of fl ue gasses. We will discover that the low
concentration of CO 2 and the enormous fl ow rates make this design a chal-
lenging task. We will follow the analysis of Merkel et al . [7.1]. It is worth not-
ing that membranes are also used for other separations. Examples include
the separation of H 2 and CO 2 , which is important in the context of the IGCC
process, and for the separation of CH 4 and CO 2 . We refer to the literature for
details on these applications. In the remainder of this section, we focus on
the separation of CO 2 from fl ue gasses in coal-fi red power plants.

Single-stage separation

To better understand the performance of a membrane for fl ue gas appli-
cations, let us consider the design of a simple one-stage separation
shown in Figure 7.3.1 . Table 7.3.1 lists some typical values we need for
our design of a fl ue gas separation system. To illustrate a design issue,
we assume that we can change the permeability of nitrogen in our