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368 12 Carbon Capture and Storage

amines is usually described by the zwitterion mechanism, whereas the reaction with
tertiary amines is described by the base-catalyzed hydration of CO 2 [54].
According to the zwitterion mechanism [16] the reaction between CO 2 and the
amine (AmH) proceeds through the formation of a zwitterion as an intermediate
(R1, Eq. (12.27)).

k f1
þ
CO 2 + AmH $ AmH COO ðR1Þ ð12:27Þ
k b1
The zwitterion reacts with a base or bases (Bs) to form carbamate as the final
product (R2, Eq. (12.28)).

k B
AmH COO +Bs ! AmCOO + BsH þ ðR2Þ ð12:28Þ
þ

When the base is MEA its self, Eqs. (12.27) and (12.28) lead to an overall
reaction. It shows that the stoichiometric CO 2 to MEA mole ratio is 0.5 mol CO 2
per mole of MEA.
With water in the solution, the following reactions may also take place:.
þ
H 2 O $ H þ OH ð12:29Þ
k OH
CO 2 þ OH $ HCO ðR3Þ ð12:30Þ

3
k H 2 O

CO 2 þ H 2 O $ HCO þ H þ ð R4Þ ð12:31Þ
3
Assuming zwitterion concentration at quasi-steady state, the overall rate of
reaction for R1 and R2 is

þ þ
½
½
½
½
r 1;2 ¼ k f1 CO 2 Š AmH½ Š k b1 AmH COO Š ¼ k B AmH COO Š BsŠ ð12:32Þ
Reorganizing Eq. (12.32) leads to
r 1;2 k f1
¼ ð12:33Þ
½ CO 2 Š AmHŠ 1 þ k b1 = k B BsŠÞ
½
½
ð
When R2 is instantaneous, k b1 k B , as it is for some amines. Almost every
zwitterion is deprotonated before it can revert to CO 2 and amine through the
backward reaction in R1. Mathematically, the denominator in Eq. (12.33) can be
simplified by k b1 = k B Bs½ð ŠÞ ! 0 and Eq. (12.33) becomes
½
r 1;2 ¼ k f1 CO 2 Š AmH½ Š ð12:34Þ