On the Kinetics between CO2 and Alkanolamines both in Aqueous and Non-Aqueous Solutions. An Overview

Alkanolamine solutions are frequently used as solvent for the removal of acid compounds from industrial gases (Kohl and Riesenfeld, 1979). Depending on the process requirements, e.g., selective removal of H2S, CO2-bulk removal, several options for alkanolamine based treating solvents with varying compositions of the solution have been proposed. In this paper an overview is presented of the mechanisms that have been proposed in literature and the kinetic data for the various reactions are critically evaluated. Conclusions on the applicability and restrictions are discussed along with perspectives. In addition white spots in the present knowledge are indicated. The reaction between CO2 and primary/secondary amines both in aqueous and non-aqueous solutions can be described over a wide range of conditions and amine concentrations with the zwitterion-mechanism as originally proposed by Caplow (1968) and reintroduced by Danckwerts (1979). All published results, both non-aqueous and aqueous solutions, amine-promoted carbonate processes, blends of amines and sterically hindered amines can be satisfactorily explained with this mechanism. The validity of the kinetic relations that are derived is restricted to about 313 K. Above this temperature the results are severely affected by the limitations of the used experimental techniques. Both stopped flow or rapid mixing and absorption techniques show their limitations because the rates of the reactions are too fast and because of the reversibility (for absorption experiments) of the reaction. For the formation of the zwitterion, an acid-base reaction, a Br⊘nsted relation exists between the rate constant for this step of the reaction, k2, and the basic strength, pKa, of the amine. The reaction between CO2 and tertiary amines can be described with the base catalysis of the CO2 hydration as proposed by Donaldson and Nguyen (1981). The formation of monoalkylcarbonate is not responsible for the reactivity measured in aqueous tertiary amine solutions at low pH as can be concluded from the results published for TREA. In non-aqueous solvents no reaction occurs for tertiary amines. The determination of reaction mechanism and reaction rate constants from mass transfer experiments can be substantially affected by effects of reversibility of the absorption reactions. The condition of pseudo first order irreversible reaction cannot always be met, e.g., in those cases where the conversion is relatively high or the equilibrium constant is low as is the case for e.g., AMP. If this condition is not fulfilled the interpretation of the mass transfer experiments neglecting reversibility can lead to erroneous conclusions. For tertiary amines also the presence of even small amounts of fast reacting contaminants, e.g., primary or secondary amines has a pronounced effec