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

* Author for correspondence: Dr. Zhiwu Liang  E-mail: zwliang@hnu.edu.cn

 Tel: +86-13618481627 and +86-73188573033

ABSTRACT

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.

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