R.J. Littel

Kinetics of CO2 with primary and secondary amines in aqueous solutions - II. Influence of temperature on zwitterion formation and deprotonation rates

The kinetics of the reaction of CO2 with various alkanolamines (MEA, DGA, DIPA, DEA, MMEA) in aqueous solutions has been studied as a function of temperature. Also kinetic data at 303 K were obtained for the reaction between CO2 and the cyclic amine morpholine in aqueous solutions. All observed phenomena can be explained very satisfactorily with the zwitterion mechanism proposed by Caplow. With respect to the temperature influence on the overall reaction rate for primary and secondary amines, two classes can be distinguished: when the zwitterion formation is rate determining a significant temperature influence is observed whereas only a slight temperature dependence is observed when the zwitterion deprotonation is rate determining. All kinetic experiments were interpreted with the aid of a numerically solved absorption model which describes gas absorption accompanied by reversible chemical reactions. For last reversible reactions like those in the present study, only in this way reliable reaction-rate data can be deduced from absorption experiments. The Bronsted relationship between the zwitterion-formation rate constant and the acid dissociation constant of the alkanolamine, as proposed by Versteeg and van Swaaij (1988a), seems to be valid over a wide range of temperatures and for a great variety of alkanolamines. This relationship is not valid for cyclic amines like MOR.

Kinetics of CO2 with primary and secondary amines in aqueous solutions—I. Zwitterion deprotonation kinetics for DEA and DIPA in aqueous blends of alkanolamines

The deprotonation kinetics of the DEA—CO2 and the DIPA—CO2 zwitterions have been studied in aqueous blends of amines at 298 K. Amine mixtures investigated were: DEA—TEA, DEA—MDEA, DEA—DMMEA, DEA—DEMEA, DIPA—TEA. DIPA—MDEA, DIPA—DMMEA, DIPA—DEMEA. For each blend the zwitterion deprotonation constant of the additional base present in solution (i.e. the tertiary amine) was determined. The observed deprotonation rate constants for the DEA-zwitterion and for the DIPA-zwitterion could be summarized in two Bronsted-type relationships. These relationships can be used to estimate the overall reaction rate of CO2 with DEA or DIPA in aqueous blends of amines. The present work on the zwitterion deprotonation kinetics of the reaction of CO2 with DEA and DIPA in aqueous amine blends provides additional verification for the validity of the zwitterion mechanism proposed by Caplow(1968) for the description of the reaction between CO2 and primary and secondary alkanolamines.

Modelling of simultaneous absorption of H2S and CO2 in alkanolamine solutions : The influence of parallel and consecutive reversible reactions and the coupled diffusion of ionic species

Numerical models, based on Higbies penetration theory, were developed to study the effect of the coupled diffusion of ions and the effect of parallel and consecutive chemical reactions on the mass transfer rate for the simultaneous absorption of H2S and CO2 in aqueous solutions of (mixtures of) alkanolamines. Prior to this complicated system, gas absorption accompanied by a single reversible reaction and in the presence of an inert salt has been studied in order to determine clearly the effect of coupled ion diffusion on the mass transfer rate. From the latter model simulations it was concluded that ion diffusion and consequently ion decoupling can have significant effect on the mass transfer rate, although a rather special set of conditions is required. Model simulations for the simultaneous absorption of CO2 and H2S showed that incorporation in the flux model of all relevant reactions, instead of only the direct reactions between CO2 and H2S and alkanolamines, only leads to more realistic concentration profiles and not to changes in absorption rate predictions for practical conditions. Correct modelling of ion diffusion does give significant though minor changes in absorption rate predictions: CO2 absorption is reduced and H2S absorption increased.