Absorption - Introduction to Carbon Capture and Sequestration - page 182

y CO 2 y CO 2

flue

y CO 2 y CO 2

flue

#1

#2

#3

#1

#4

exh

y CO 2 y CO 2

exh

#5 #2

reg

x CO 2

reg

x CO 2

x CO 2 x CO 2

Figure 5.2.8 Effect of changing the ratio n sol / n fl ue
In the left fi gure, we decreased the solvent fl ow n sol and in the right fi gure, this fl ow was
increased.

the n sol / n fl ue ratio affects not just the mass-transfer relationship but the
reality of what happens within each tray of the column. If the liquid fl ow
rate is too high and the vapor fl ow rate is too low, for example, we will get
poor contact between our liquid and vapor. In other words, the gas that
comes into the bottom of the absorber and enters the froth won't be there
long enough to achieve thermodynamic equilibrium. We know this is true
in the case of water, for example, because mass transfer with water is
actually very slow. Without enough time for full contact, each stage in our
diagram won't quite reach the equilibrium line, and consequently, many
more stages will be necessary to achieve the same separation.
At this point, we should also acknowledge that although we have
focused our current absorber discussion on the design potential of multi-
tray absorbers, the majority of absorbers use a packed design. In a
packed bed, one can still use the concept of a stage, but the link to a
physical plate is less direct. Nevertheless, the number of theoretical
stages in a packed bed can also be used as a measure of the height of
the column. In this topic, we will not touch on the details of how other
types of absorbers work, but the corresponding McCabe-Thiele diagrams
can still give us an intuitive sense of the effi ciency of these absorbers.
Based on what we now know about absorption column design, let's
go back to our sample coal-fi red power plant and our charge of process-
ing 400 m 3 of fl ue gas per second. What is the necessary volume of water
we would need to deal with that volume of gas? The answer, it turns out,