Absorption - Introduction to Carbon Capture and Sequestration - page 190

form bicarbonate. Hence, the reaction of CO 2 with tertiary amines occurs
with a higher loading capacity of 1 mole of CO 2 per mole of amine, albeit
with a relatively lower reactivity with CO 2 compared with the primary
amines.
The drawback of using primary and secondary amines as weak
bases is that, as we have seen, for every 1 mole of CO 2 we want to
capture, close to 2 moles of amine are needed. Given the volumes of
CO 2 we are being asked to deal with, that could present a big problem.
The good news about CO 2 reactions with primary and secondary
amines, however, is that, even though there is a signifi cant heat of
absorption (about -80 kJ/mol CO 2 ), the reaction can be “tuned” by vary-
ing the chemical form of the R-group. Selecting amines with different
R-groups can affect the magnitude of the absorption energy of the reac-
tion, allowing us to control the solvent's reactivity with CO 2 . Some, such
as hindered amines, have been specially formulated to overcome some
of the limitations of the conventional primary and secondary amines. By
using bulkier R-groups, for example, one can induce steric hindrance.
These larger substituents lower the stability of their associated carba-
mates, allowing us to attain CO 2 loadings well in excess of 0.5 mole
equivalents.
Tertiary amines form bicarbonate directly using a 1:1 molar ratio of
amine to CO 2 . This is a plus from a cost perspective, because we will
need less solvent to process the same amount of CO 2 . Also, their
reaction with CO 2 is associated with a lower heat of absorption, which
means less energy is needed for recovery in the stripper. Finally, tertiary
amines provide the benefi t of being very tunable due to the presence of
three functional R-groups.
Question 5.3.3 Heat of absorption
Let's take a closer look at this number that is associated with the heat of
absorption of amine solutions — 40-100 kJ/mol. To desorb the CO 2 from the
solution we need to supply the heat. We do this in the stripper by boiling the
solution. Amine solutions typically consist of 30% amines and 70% water. If we
assume that the amine solution has the same properties as water, how much
heat do we need to supply if we have a heat of absorption of 40 kJ/mol? What
if we have 100 kJ/mol?