Absorption - Introduction to Carbon Capture and Sequestration - page 214

We now see that staying close to equilibrium and minimizing ( T H - T L )
can be achieved by increasing the area ( A ) and thus the size of the heat
exchanger. As heat exchangers are not cheap, we have to make the
tradeoff between energy savings and capital costs. We see that because
of this temperature difference, the fl uid entering the stripper does not
have the same temperature as the fl uid leaving the stripper. This differ-
ence needs to be supplied by the steam from our power plant, which
gives a lost heat per unit CO 2 that we remove:

( )

in out

CT − T

pH H

W = ,

exc,loss

∆σ

CO

2

where C p is the heat capacity of the solvent and ∆σ CO 2 is the working

capacity of the fl uid. The working capacity is defi ned as the amount of
CO 2 removed in a cycle. We see that increasing the working capacity will
decrease our losses in the exchanger. The numbers reported in
Figure 5.7.3 are converted to parasitic energy, the corresponding
amount of electricity that we are unable to generate due to this lost
work.
One puzzle that may have occurred to you by now is why we do not
simply run our process with a higher percentage of MEA, say a 20 molar
solution. It sounds like a good idea, but it turns out that such a high
concentration of MEA is not soluble in water. Twelve molar solutions are
also problematic, as they turn out to be too viscous. The main trouble
with viscosity, in addition to the pumping costs, is the effect on the size
of the heat exchanger. Because of the increase in the viscosity in going
from 8 to 12 molar solution, the effective heat transfer coeffi cient
decreases by a factor of between 3 and 10. We would need to increase
the size of the heat exchanger by the same factor in order to compen-
sate. As you're now beginning to see, optimizing a heat exchanger is a
diffi cult task, akin to slaying a seven-headed dragon. Just when you
think you've found a way to approach it, another problem will sneak up
on you.
Dragon-slaying aside, we will now look at the absorber, where we
seek to stay as close to equilibrium as possible. In the absorber, equi-
librium is achieved by mass-transfer; CO 2 needs to go from the gas to
the liquid phase. As we have seen in Section 5.4 this is a combination of