Literature data for kinetics in water-lean solvents

“Alvarez-Fuster et al. [141] have reported reduced absorption and kinetic rates in mixtures of ethylene glycol and MEA when compared to aqueous MEA. Indeed, most of the literature on amine-MEG mixtures reports a reduction in kinetic rates when operating with MEG [133], [134], [135], [142]. Another reduction in mass transfer rates is exemplified in the data of Jiru and Eimer [143] when shifting from a water-MEA to a MEG-MEA solvent. On the other hand, Kang et al. [88] report an enormous increase both in kinetic and mass transfer rates by shifting from aqueous 30 %wt. MEA to water-free MEG-MEA. Garcia et al. [144] also describe moderate enhancements in both rates when adding MEG to MEA and DEEA/MAPA (3/2 in mol∙l⁻¹) blends, and Zheng et al. [145] report an increment in kinetic rates (but not in mass-transfer rates) in AMP-MEG mixtures. We shall discuss more about the distinction between kinetic rates and mass transfer rates in the next section, but it is remarkable that so many discrepancies transpire from data regarding amine-glycol mixing.

More notably, the data from Garcia et al. [144] shows that the kinetics of water-lean solvents containing MEA surpass those of aqueous MEA at higher temperatures (>50 °C). Since most of the kinetic data obtained by other researchers consists only in measurements taken at a single temperature, usually 25 °C, it is quite difficult to properly discuss these results.

Through analysis of these exceptions, one can begin to understand the troubles of dealing with separate sets of kinetic data. This mostly stems from the fact that this data is not directly measurable. Reaction rates have to be extracted from mass transfer rates under a predetermined set of assumptions, such as pseudo-first order conditions, and mass transfer rates themselves have to be calculated under the assumption of an estimated gas phase resistance. Jiru and Eimer [143] have demonstrated how small deviations from these conditions might propagate into enormous errors in the measured kinetic rates.

Additionally, trouble also comes from the fact that we are trying to treat an enormous array of chemicals, both amines and organic diluents, through the perspective of a single reaction mechanism. Surely one is bound to find exceptions. And finally, though both the zwitterion mechanism and the termolecular mechanism offer practical frameworks for treating and analyzing data, none of them is entirely satisfying. It might be that another framework would lead to a better understanding of the discrepancies observed regarding amine-glycol mixtures. Though we attempt to offer an overview on what is known about kinetics in water-lean solvents, there is no consensus among the investigators in this field.”