(412d) Interfacial Speciation of CO2-Loaded Aqueous Solutions of Alkanolamines

Naser S. Matin¹, Janice A. Steckel², Jesse Thompson¹, Moushumi Sarma¹, and Kunlei Liu^{1, 3}

1. Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, KY 40511, United States
2. National Energy Technology Laboratory, U.S. Department of Energy, Pittsburgh, PA, 15261
3. Department of Mechanical Engineering, University of Kentucky, Lexington, KY 40506, United States

Abstract

Aqueous solutions of alkanolamines are the benchmarked solvents for reactive absorption of CO₂. Physicochemical properties like solution density, surface tension, viscosity, and CO₂ solubility are of great importance for further analysis of such solvents for use in CO₂ capture processes. Among the above-mentioned properties, surface tension at the liquid-gas interfaces exerts a significant influence on the gas absorption process kinetics. Generally for electrolyte solutions, it is well-known that the solution surface tension is increased by adding salts to the solution. According to the experimental data, for the most alkanoamines, their carbonated solutions present surface tension that increases with CO₂-loading [1-4].

In this study through a thermodynamic framework the surface behavior of the CO₂ loaded aqueous solutions of alkanolamines is related to the concentration of different species in the surface layer, which in turns can have significant impact on the overall CO₂ absorption rate. In order to apply the methodology the surface layer is assumed as a separate phase with constant composition which is in equilibrium with the adjacent bulk phase. The results will show that at a given bulk composition, what is the speciation at the surface phase and which species has more impact on the surface tension and interfacial properties. Accordingly, having a proper thermodynamic model with a detailed liquid phase activity coefficient description for solute and solvent the surface phase speciation of CO₂ loaded aqueous solution of Monoethanolamine (MEA) were estimated. Having a reliable thermodynamic model, the methodology is applicable for any CO₂ loaded alkanolamine solution.

According to the results of this study, different molecular and ionic species including molecular MEA, its protonated and carbamate forms, carbonate and bicarbonate ions present different tendency to the surface layer which can be changed by the solution CO₂ loading and system temperature. It is quantitatively shown that the total solute concentrations at the surface layer is decreased with increasing the CO₂ loading. The approach provides solute-partitioning for CO₂ loaded aqueous solution of alkanolamines, in which partition coefficients display extend of the distribution of an uncharged solute or an ion between the liquid bulk solution and the surface phase.

References

(1) Jayarathna, S. A.; Weerasooriya,A.; Dayarathna, S.; Eimer, D. A.; Melaaen, M. C. J. Chem. Eng. Data 2013, 58, 986.

(2) Jayarathna, S. A.; Jayarathna,C. K.; Kottage, D. A.; Dayarathna,S.; Eimer, D. A.; Melaaen, M. C. J. Chem. Eng. Data 2013, 58, 343.

(3) Fu, D.: Liu, F.; Li, Z. Chem. Eng. Technol. 2013, 36, 859.

(4) Fu, D.; Xu, Y.; Hua, X. Fluid Phase Equilibria 2012, 314, 121.