Absorption - Introduction to Carbon Capture and Sequestration - page 216

We can quantify the lost work associated with the condensation by
the amount of heat that is taken up by the cooling water used to run the
condenser:

con

∆ h ∆σ

T − T

HO

HO

cond env

2

2

W = ,

exc,loss

∆σ T

CO cond

2

where h H 2 O is

the heat of vaporization. The term in brackets is the Carnot factor which
takes into account the effi ciency at which this heat can be converted into
electricity, and depends on the temperature of the condenser T cond and
the temperature of the environment T env .
Finally, we come to the stripper process confi guration. This is a
typical example in which spending more on capital costs makes the
process signifi cantly more effi cient: The improvements in effi ciency
come from increasing the stripper area and an advanced two-stage
process confi guration to recover the CO 2 at an elevated pressure. In the
fi rst stage, we evaporate the solution and fl ash off CO 2 using steam at
150ºC and a system pressure of 17 bar. In that stage, half of the CO 2
evaporates at a very favorable high pressure. We then reduce the pres-
sure of the solution to 11 bar, heat it again to 150ºC with steam, and
release the rest of the CO 2 at a somewhat lower pressure. This two-
stage process also reduces the amount of water evaporating from the
system, which, as we have seen, will save energy in the compression
stage. For more details, we again refer to the work of Rochelle and his
co-workers.

∆σ H 2 O is the amount of water that is condensed and the ∆

Solvent selection

In the previous section we have seen that in various steps of the process,
the choice of the solvent has a signifi cant effect on the effi ciency of the
absorption process. Let us summarize the most important properties:

1. Working capacity: the amount of CO 2 a solvent can capture per

cycle. A high working capacity reduces the amount of solvent we
have to cycle.

2. Heat of absorption: Solvents with higher heats of absorption tend to

have larger vapor pressures at high pressure, and therefore the gas