ON THE KINETICS BETWEEN CO2 AND ALKANOLAMINES BOTH IN AQUEOUS AND NONAQUEOUS SOLUTIONS - AN OVERVIEW

Alkanolamine solutions are frequently used as solvent for the removal of acid compounds from industrial gases (Kohl and Riesenfeld, 1979). D epending on the process requirements, e.g., selective removal of H2S, CO2-bulk removal, several options for alkanolamine based treating solv ents with varying compositions of the solution have been proposed. In this paper an overview is presented of the mechanisms that have been p roposed in literature and the kinetic data for the various reactions a re critically evaluated. Conclusions on the applicability and restrict ions are discussed along with perspectives. In addition white spots in the present knowledge are indicated. The reaction between CO2 and pri mary/secondary amines both in aqueous and non-aqueous Solutions can be described over a wide range of conditions and amine; concentrations w ith the zwitterion-mechanism as originally proposed by Caplow (1968) a nd reintroduced by Danckwerts (1979). All published results, both non- aqueous and aqueous solutions, amine-promoted carbonate processes, ble nds of amines and sterically hindered amines can be satisfactorily exp lained 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 experimen tal techniques. Both stopped flow or rapid mixing and absorption techn iques show their limitations because the rates of the reactions are to o fast and because of the reversibility (for absorption experiments) o f the reaction. For the formation of the zwitterion, an acid-base reac tion, a Bronsted relation exists between the rate constant for this st ep of the reaction, k(2), and the basic strength, pKa, of the amine. T he reaction between CO2 and tertiary amines can be described with the base catalysis of the CO2 hydration as proposed by Donaldson and Nguye n (1981). The formation of monoalkylcarbonate is not responsible for t he reactivity measured in aqueous tertiary amine solutions at low pH a s 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 exp eriments can be substantially affected by effects of reversibility of the absorption reactions. The condition of pseudo first order irrevers ible reaction cannot always be met, e.g., in those cases where the con version is relatively high or the equilibrium constant is low as is th e case for e.g., AMP. If this condition is not fulfilled the interpret ation of the mass transfer experiments neglecting reversibility can le ad to erroneous conclusions. For tertiary amines also the presence of even small amounts of fast reacting contaminants, e.g., primary or sec ondary amines has a pronounced effect.