Absorption - Introduction to Carbon Capture and Sequestration - page 215

CO 2 dissolving in the solvent and subsequent reactions. The effective
fl ux is given by the square root of the reaction kinetics and Henry
coeffi cient:

[ ]

2

DkAm

( )

CO

0 ∗

2

j = p − p ,

CO CO

CO

H

2 2

2

CO

2

where p CO 2 , i is the actual partial CO 2 pressure in the gas phase of the
absorber and p CO 2 the equilibrium partial pressure corresponding to the
actual solvent loading. For a particular solvent we can defi ne an effective
mass transfer coeffi cient k g :

( )

0 ∗

j = k g p − p

CO CO

CO

2 2

2

Similarly, in our heat exchanger we would like to keep the driving
force, p CO 2 - p ∗ CO 2 , as small as possible, and selecting a solvent
with the highest value of k g will do this. In order to select the optimal
solvent, we need to know the diffusion coeffi cient, solubility, and
reactivity.
In the compression step we lose work for two reasons. One is related
to the pressure of the stripper: if we were to operate at a higher pressure,
our compressor wouldn't have to do as much work. The second reason
is that there is a signifi cant amount of water vapor coming off the CO 2
stream from the stripper, and we lose work in the condensers.
Interestingly, these losses can also be reduced by changing the solvent.
The key factors here are the temperature and corresponding equilibrium
partial pressure of CO 2 . At higher temperature the vapor pressures of
both CO 2 and water increase. The rate of increase depends on the heat
of absorption. So by selecting a solvent with a high heat of absorption
the relative amount of water in the vapor phase will decrease, and we will
therefore have less water to condense per kg CO 2 . Another important
factor is the stability of the amine solution. If we can operate at a higher
temperature at the bottom of our stripper, the pressure at the top will be
larger and we save on compression costs. The lost work for compression
can be computed very accurately, as this involves compressors that are
well characterized.