Facile Method for Determination of Amine Speciation in CO₂ Capture Solutions

Abstract

A simple and quantitatively reliable method for determination of amine speciation is introduced. The method employs three experimental methods that should be readily accessible. The results for CO 2 loaded aqueous solutions of 30 wt % monoethanolamine (MEA) are used for demonstration of the method and show promising agreement with the more complicated spectroscopic methods reported in the literature. The measurements were done at ambient temperature and atmospheric pressure, since theoretical calculations and experimental data from the open literature revealed no considerable difference in speciation at different temperatures. The procedure is based on acid and base titration of the CO 2 loaded amine solution along with the determination of total CO 2 loading. The quantitative results for the different species concentration in the example MEA solution is in agreement with other available spectroscopic methods, mainly NMR, particularly for the free amine, carbamate, and protonated amine concentrations. Aspen Plus was also used for further assessment of the experimental data. ■ INTRODUCTIONRemoval of acidic gases, particularly CO 2 , is a very important operation from an industrial and environmental point of view. Primary, secondary, and tertiary amines and their blends have found widespread application in the absorption and removal of carbon dioxide from process gases. 1,2 In the designing of absorption or desorption columns for a CO 2 capture process by amine or other reactive solvents, the precise knowledge of the reaction chemistry is crucial. In this regard, having an accurate determination of liquid phase species concentration is critical in CO 2 absorption processes modeling vapor−liquid equilibria data. The contribution of the protonated amine, carbamate, and bicarbonate formed in amine solvents to, for example, the CO 2 heat of absorption makes their quantities critical for thermodynamic understanding. 3 Furthermore, providing a good estimation of the free amine concentration is important for mass transfer study of the CO 2 absorption process, where the free amine concentration explicitly appears in the liquid film mass transfer coefficient. 4 Finally, with good understanding of the above concepts it is possible to better design new solvents and blends.Various thermodynamic models, mainly based on activity coefficients, have been proposed for the CO 2 −H 2 O−alkanolamine systems. 5−7 However, many of the models employ computational models, which are not readily accessible or require parameters not necessarily available for all solvents, making experimental methods of merit. 5 Furthermore, reliable experimental values for species concentration in the liquid phase can improve the thermodynamic models. The activity coefficient models and their parameters can be better evaluated by the available amine speciation data. 3 The NMR approach for speciation study of alkanolamine solutions containing dissolved CO 2 is one of the basic methods for this purpose. 8−10 As it was discussed by Boẗtinger et al., the quantif...

Chemiresistive Sensing of Ambient CO2 by an Autogenously Hydrated Cu3(hexaiminobenzene)2 Framework

“…No characteristic modes for bicarbonate or carbamates were observed at CO 2 concentrations between 0–2500 ppm; these modes became apparent only weakly and only upon significantly raising the CO 2 level to 1–10% (i.e., 10 000–100 000 ppm; Figure S14). Analogous to the speciation of dissolved CO 2 in aqueous amine media, 45 the bicarbonate and carbamate salt fractions in Cu 3 HIB 2 are presumed to be exceedingly small, particularly at typical ambient CO 2 levels. Most revealingly, increasing CO 2 levels cause a gradual decrease of the background IR absorbance of the material, most evident in the spectral ranges that are not dominated by strong vibrational modes from the MOF itself (e.g., h v = 0.5–0.6 eV and 0.25–0.3 eV).…”

In Situ Nuclear Magnetic Resonance Mechanistic Studies of Carbon Dioxide Reactions with Liquid Amines in Non-aqueous Systems: Evidence for the Formation of Carbamic Acids and Zwitterionic Species

“…The use of titration 35,32 and FTIR for mechanistic studies has been reported 29,36-38 but quantification and real time / flow unit identification of unexpected species is cumbersome. Herein, in-situ 13 C and 1 H NMR spectroscopy using a built-in micro reactor was used to provide real time insights on reaction mechanisms and product speciation under various conditions.…”

Amine Solutions for Biogas Upgrading: Ideal versus Non‐Ideal Absorption Isotherms

“…The discrepancy in CO 2 loading was less than 5.61 %, indicating reasonable reliability of the results obtained. The reliability and the repeatability (RSD = 2.27 %) were confirmed by comparison with literature data [13,14] where, besides the gas phase analysis (chromatographic gas analysis), the liquid phase analysis (titration of the CO 2 -loaded amine solutions, nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) technique) was applied. The pH of the solution was measured with a pH meter (S20 SevenEasy™ pH; Mettler Toledo).…”

“…h a a 1 ln a 2 P CO 2 (14) where h a is the CO 2 loading (L CO 2 /L solution), and a 1 (L CO 2 /L solution) and a 2 (bar -1 ) are constants. Eq.…”