Thermodynamic and NMR spectroscopic characteristics of sterically hindered amine used for $CO_2$ absorption

Abstract

The removal of acid gases from refinery and natural gases has been of great importance. Aqueous solutions of alkanolamines have been widely used to the gas treatment. Recently, the growing interest in aqueous solutions of sterically hindered amines for the use in gas treatment processes is due to the high cyclic capacity, and relatively high absorption rates at high $CO_2$ loadings. Because of the hindrance of the bulky group attached to the tertiary carbon atom, unstable carbamate ions are formed in the reaction of the aqueous solutions of hindered amine with carbon dioxide resulting in the theoretical loading capacity up to 1.0 mol of $CO_2$/mol of amine. Prior to the practical use of aqueous solutions of hindered amines, it is important to understand the thermodynamic and NMR spectroscopic characteristics of them as $CO_2$ absorbents. In this work, aqueous solutions of 2-amino-2-ethyl-1,3-propanediol (AEPD)/2-amino-2-hydroxymethyl-1,3-propanediol (AHPD), sterically hindered amines, were proposed as potential $CO_2$ absorption agents. Density, viscosity, and solubility of carbon dioxide in aqueous AEPD/AHPD solutions were measured to estimate thermodynamic characteristics of the solutions. It is required to use physical properties such as density and viscosity of aqueous amine solutions for the determination of reaction kinetics and the design of appropriate acid gas treatment equipments. Equilibrium solubility data must be also essential to the design of gas treating process and the modeling of absorption phenomena. $13^C$ NMR spectroscopy was used to identify the produced ions from the reactions of aqueous AHPD solutions with carbon dioxide and investigate the reaction mechanism.