(572d) The Influence of Dissociation Constants on the Kinetics of Carbon Dioxide Absorption in Aqueous Tertiary Amines Solutions Containing Carbonic Anhydrase

American Institute of Chemical Engineers (AIChE)
Annual Meeting

The influence of dissociation constants
on the kinetics of carbon dioxide absorption in aqueous tertiary amines
solutions containing carbonic anhydrase

Bin Liu^a, Xiao Luo^a*, Zhiwu
Liang^a,b*, Paitoon
Tontiwachwuthikul^a,b*

^aJoint International Center
for CO₂ Capture and Storage (iCCS), Hunan Provincial Key Laboratory
for Cost-effective Utilization of Fossil Fuel Aimed at Reducing CO₂
Emissions, College of Chemistry and Chemical Engineering, Hunan University,
Changsha 410082, PR China

^bClean Energy Technologies Research
Institute (CETRI), Faculty of Engineering and Applied Science, University of Regina,
Regina, Saskatchewan, S4S 0A2, Canada

Keywords: Tertiary amines; CO₂ absorption; carbonic
anhydrase

The chemical
absorption of CO₂ into primary amine and secondary amine, such as
monoethanolamine (MEA), diethanolamine (DEA), is currently the most widely
accepted commercial approach to carbon capture.^[1] This solvent is
extremely effective in forming stable carbamates when reacted with CO₂,
leading to a very efficient removal of CO₂ with a 2:1 reaction
stoichiometry.^[2,3] However, regeneration of the carbamate species requires
a large parasitic supply of energy to release the CO₂ during
stripping.^[1] This high energy requirement accounts for up to 80% of
the operating costs for CO₂ capture and recovery.^[4] It
is also a highly corrosive substance that readily undergoes degradation in the
presence of oxygen, emitting harmful volatile organic compounds.^[5]

Considering the
high absorption heat and high energy consumption during CO₂ capture
process, there are a number of tertiary amines such as methyldiethanolamine
(MDEA), dimethylaminoethanol (DMEA), in place of primary and secondary amine
for carbon capture. While many of tertiary amines solvents are able to reduce
the stripping energy requirement, the reaction rate for CO₂ sorption
in the absorber is often slow. This means that it is not possible to recover
high purity carbon dioxide within a reasonable column height. Carbonic
anhydrase (CA), a naturally occurring enzyme and very efficient catalyst that
enhances the reversible reaction of CO₂ to HCO₃^-
, was first identified in 1933 in red blood cells and known to catalyze the
conversion of CO₂ into bicarbonate (HCO₃^-) at
extremely high turnover rates, ^[6−7] which may be useful for
promoting the absorption rates of CO₂ from gas streams when these
alternative solvents are used. There is great potential for using CA to hydrate
CO₂, especially when it is known that the hydration step is the rate
determining step in the CO₂ absorption process. Carbonic anhydrase
is not just a single enzyme form, but a broad group of zinc metallo-proteins
(enzymes) that exists in three genetically unrelated families of isoforms (¦Á, ¦Â
and ¦Ã). Its catalyzed mechanism of CO₂ hydration has been introduced
by Lindskog et al. ^[6-7] and simplified as shown in Figure 1. Penders et al. ^[8]
has been working on CO₂ absorption into amine solutions
catalyzed by human carbonic anhydrase in string cell reactor since 2012. This
work focuses on the study of effect of dissociation constants (pKa) on the
kinetics of carbon dioxide absorption in aqueous tertiary amines solutions
containing carbonic anhydrase at various temperature using stopped-flow
techniques. The examined alkanolamines were (Figure 2): Triethanolamine (TEA), Methyldiethanolamine (MDEA), Dimethylethanolamine
(DMEA), 2-(Diethylamino)ethanol (DEEA), 1-Diethylamino-2-propanol (1DEA-2P), 3-Diethylamino-1-propanol
(3DEA-1P) and 2-(Dimethylamino)isobutanol (2DEA-2M-1P).
The values of pKa of different amine was determined using conventional pH
method as shown in Figure 3
and its final results was presented a function of temperature in Figure 4.

It can be seen from
the results (Figure 5)
that DMEA with a higher pKa value than MDEA and TEA hence showed a higher
reactivity with carbon dioxide in the absence of enzyme. It can also been found
that the reaction rate constant (k₀) in amine-CO₂-H₂O
system was significantly enhanced in the present of BCA, in which the enzyme
activity is higher in TEA solutions. This is because Enzymes often display a
pH-dependent changes in activity, in characterizing the pH dependence of the enzyme
activity, the experimenter often observes a bell-shaped curve in plot of
activity versus pH or s-shaped curves in plots of activity versus pH (either falling
from optimal activity or rising to optimal activity) with an inflection point
at some pH value. This result would provide a
useful information to understand the reaction activity of CO₂
absorption into various amines system.

Figure 1. The catalytic
Mechanism of enzyme in aqueous solutions

 Figure 2.
The chemical structure of amines studied in this work

Figure 3. String cell for
determination of amine pKa

Figure 4. The dissociation
constants show as a function of temperature

Figure 5. Comparison of enzymes
activity in different aqueous amine solutions

Acknowledgment: The financial supports from
the National Natural Science Foundation of China (Nos. 21476064 and 21406057), National
Key Technology R&D Program (Nos. 2012BAC26B01 and2014BAC18B04), Innovative
Research Team Development Plan-Ministry of Education of China (No. IRT1238),
and China¡¯s State ¡°Project 985¡± in Hunan University-Novel Technology Research
and Development for CO₂ Capture as well as Hunan University to the
Joint International Center for CO₂ Capture and Storage (ICCS) is
gratefully acknowledged.

References

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