Reaction Kinetics and Dissociation constants of Tertiary Alkanolamines for Carbon Capture

Abstract

This research work studies the absorption kinetics of CO2 into aqueous solutions of four amines; {1-[Bis [3-(dimethylamino) propyl] amino]-2-propanol, N, n, n′, n′′, n′′-pentamethyldiethylenetriamine, N, n, n′, n′-tetrakis (2-hydroxyethyl) ethylenediamine, and N, n, n′, n′-tetrakis (2-hydroxypropyl) ethylenediamine}. The experimental results were reported in the form of secondorder reaction rate constants (k2). The kinetics data were measured using the Stopped-flow technique within an amine concentration range of 200 to 1000 mol/m3 and a temperature range of 298 to 318K. The equipment can determine the solution-based kinetics on a millisecond time scale by measuring the reaction rate of the reactants. The different tertiary alkanolamine solvents studied in this research work have no kinetics data published in the literature, and the solvents were selected the solvents for research studies because of their unique characteristics and properties. The experimental procedure was validated using kinetic experiments using a Methydiethanolamine (MDEA) solution prepared at concentrations ranging from 200 to 1000 mol/m3. Experimental pseudo-first order reaction rate constants were found to increase steadily with an increase in amine concentration and temperature. In terms of pseudo first-order constant, the reaction rates of CO2 absorption in 1-[Bis [3- (dimethylamino) propyl] amino]-2-propanol, N, n, n′′, n′′-pentamethyldiethylenetriamine and N, n, n′, n′-tetrakis (2-hydroxyethyl) ethylenediamine and N, n, n′, n′-tetrakis (2-hydroxypropyl) ethylenediamine were found to be faster than that of Methydiethanolamine at low concentrations and temperature. The values of pseudo-first order reaction rate constant increased steadily at higher concentrations and temperature. An aqueous solution of 1-[Bis [3-(dimethylamino) propyl] amino]-2-propanol has the best overall reaction rate at the experimental conditions when compared with the other amines studied. The values of the pseudo first-order rate constants (k0) were interpreted using the base-catalyzed hydration mechanism. All experimental values of the pseudo first-order rate constants (k0) obtained from the stopped-flow apparatus were compared with calculated values determined using the power-law equation. All calculated values of (k0) were found to be in good agreement with the experimental values with an acceptable AAD of 4.3%. In addition, the dissociation constants (pKa) of 1-[Bis [3-(dimethylamino) propyl] amino]-2- propanol, N, n, n′′, n′′-pentamethyldiethylenetriamine, N, n, n′, n′-tetrakis (2-hydroxypropyl) ethylenediamine and N, n, n′, n′-tetrakis (2-hydroxyethyl) ethylenediamine were determined using the potentiometric titration method at (298, 303, 313 and 323) K. The pKa values obtained for each solvent reflect the basicity of that solution and are considered a critical parameter in the selection of alkanolamine solutions for CO2 removal. The pKa values were predicted using the Perrin-Dempsey-Serjeant empirical (PDS) method. The actual PDS method is somewhat better than the updated PDS with both approaches producing average absolute errors of 0.04 and 0.14, respectively.