Introduction to Carbon Capture - Introduction to Carbon Capture and Sequestration - page 149

We also touched on the separation of water and carbon dioxide, but
as this separation simply involves the condensation of water, there is very
little room for improving this process. The focus of this topic will be on
the separation of CO 2 from N 2 in fl ue gasses, but occasionally we will
refer to the other separations listed above.

Minimum energy for separations

Before we begin discussing the different types of technologies one can
use to separate CO 2 and N 2 , it is interesting to take a step back and ask
ourselves why it costs energy to separate gasses. Why can't we just use
a Maxwell Demon (see Box 4.2.1 ) to separate CO 2 and N 2 at zero energy
cost? The answer lies in thermodynamics, which not only teaches us that
it costs energy to separate gasses, but also allows us to estimate the
minimum energy required to carry out these separations.
Let us consider the carbon capture process shown in Figure 4.2.1.
We assume that our fl ue gasses are at a given temperature T and that the
mole fraction of CO 2 in our fl ue gasses is given by x fl ue . If we carry out the

Exhaust
CO 2 poor

Work

Flue Gas

(n exh , x exh )

(n flue , x flue )

(n cap , x cap )

Capture
CO 2 rich

Figure 4.2.1 Carbon capture process
A fl ue gas, with a fl ux of n fl ue [mol/s] and mole fraction CO 2 x fl ue is separated into a CO 2
rich capture stream with fl ux n cap and mole fraction x cap and a lean exhaust stream with
fl ux n exh and mole fraction x exh . The separation process costs energy as indicated by the
blue arrow.