Absorption - Introduction to Carbon Capture and Sequestration - page 178

y CO 2

i ncreasing

y CO 2

flue

exh

y CO 2

x CO 2

reg

x CO 2

Figure 5.2.5 Equilibrium and operating lines
This fi gure shows Henry's law (blue) together with the mass balance equations (operat-
ing lines, black) for different ratios of gas and solvent fl uxes ( n sol / n fl ue ). Applying the mass
balance over the entire column shows that all mass balance lines have to include the
point ( x reg

CO 2 , y exh CO 2 ).

than in the case where we have an excess of gas. This is a simple con-
sequence of the conservation of mass principle, and the mass balance
equation above is a simple algebraic way of showing how much CO 2
comes into the system and how much CO 2 comes out of the system for
a given ratio of n sol / n fl ue . This conservation principle will yield the second
of the two equations we can use to graphically describe the plate tower.
From an engineering standpoint, we should note that the main
design factor we can control is n sol , the molar fl ow rate of the solvent that
enters the system. Of course, we can also tune the molar fl ow rate of the
gas ( n fl ue ). For example, we can cut n fl ue into half, but as we do need to
remove 90% of the CO 2 from our fl ue gas (see Section 4.2), this tuning
implies that we have to use two or more absorber/stripper reactor units.
In Figure 5.2.5 , we plotted the mass balance equation for different ratios
n sol / n fl ue . We are given the initial CO 2 concentration of our fl ue gas enter-
ing at the bottom of the absorber, y fl ue

CO 2 . At the top of the absorber, the
concentration of CO 2 in the fl ue gas is y exh

CO 2 and the concentration of CO 2

in the regenerated solvent is x reg

CO 2 . By defi nition, every operating line will
cross this point. For our design we need to specify the value of y exh

CO 2 , the