1 Precipitation in amino acid salt CO 2 absorption systems Magdalena Majchrowicz 1 , John P. M. Niederer 1 , Aldrik H. Velders 2 , Geert F. Versteeg 1 1 Development and Design of Industrial Processes, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands 2 Supramolecular Chemistry and Technology, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Abstract The solid products precipitating upon a reactive absorption of CO 2 in aqueous solutions of the potassium, sodium and lithium salts of a series of amino acids were identified using CHN analysis, AAS, XRD, 13 C and 1 H NMR. Among the investigated amino acids were α-aminoisobutyric acid, taurine, l-alanine and 6-aminohexanoic acid. The results from the precipitates identification together with relations between the initial solvent composition and the solid composition are reported. Keywords: precipitation, CO 2 absorption, amino acid salts, mixtures of compounds Introduction Reactive absorption is a common process in the chemical industry, among others in the treatment of CO 2 containing industrial gas streams. In these processes aqueous solutions of alkanolamines, such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA) and their mixtures, as well as sterically hindered amines such as 2-amino-2-methyl-1-propanol (AMP) are commonly used [1]. In principle, these systems could be used for the removal of the greenhouse gas CO 2 from flue gases, such as from power plants. However, these solvents suffer from a limitation in the CO 2 loading capacity, a relatively high equipment corrosion rate, a high energy consumption for regeneration and from degradation in an oxygen rich atmosphere, commonly encountered in the treatment of such flue gases [2]. Aqueous alkaline salts of amino acids could be an interesting alternative for these alkanolamines. Generally, amino acid salt solutions can be characterized by a higher stability towards oxidative degradation and a chemical reactivity with CO 2 comparable to alkanolamines due to their similar functionality [3]. A further interesting feature is their ability to precipitate when absorbing CO 2 . The solid formation was for example reported during the absorption of CO 2 in aqueous potassium taurate solutions [4] and in the potassium salts of N-methylalanine (Alkazid M), dl-alanine, α- aminoisobutyric acid and its sterically hindered derivatives. The precipitate was reported to be the amino acid [4] or the amino acid chemically bonded with carbonate species [5]. As a result of the precipitation the CO 2 equilibrium partial pressure over the resulting slurry remains almost constant for a certain solution loading [6,7]. This can then result in higher solvent loadings, a lower energy consumption for the solvent regeneration and reduction in the gas-liquid contactor size [7]. Due to the novel character of these three-phase CO 2 absorption systems, much is still unclear on for example the character of the precipitate itself, on precipitation kinetics or absorption capacities. This paper aims to shed some light on the chemical composition of the precipitating solid products, which is important for example from the slurry handling point of view and for mechanistic considerations. Different precipitate compositions will probably lead to a different absorption behaviour, affecting for example absorption capacities and regeneration conditions.