Modeling of CO2 Absorption Kinetics in Aqueous 2-Methylpiperazine

Abstract

CO2 absorption into 8 molal (m) 2-methylpiperazine (2MPZ) is modeled in Aspen Plus using a previously developed rigorous thermodynamic model. With the regression of three reaction rate constants for carbamate and bicarbonate and one parameter for diffusion activation energy, the kinetic model represents CO2 flux measured in a wetted-wall column at 40–100 °C with 0.1–0.4 mol CO2/mol alkalinity with a relative deviation from −20% to 20%. The kinetic reaction rate is interpreted with activity-based termolecular kinetics and the rate constants are correlated with the Bro̷nsted theory. The liquid film mass transfer coefficient (kg′) is well represented by the model at medium to rich CO2 loading. The power to which kg′ is dependent on the rate constant of 2MPZ carbamate formation (k2MPZ–2MPZ) decreases from ∼0.5 at very lean loading to nearly 0 at rich loading, whereas it is increasingly dependent upon the diffusion coefficient of the reactants and products and the physical liquid film mass transfer coefficient (kl0) as loading and temperature increases. In the liquid film, 2MPZ and 2MPZCOO– are the major reactants and protonated 2MPZ carbamate and bicarbonate are the major products. The pseudo-first-order region for 2MPZ narrows and shifts to higher value of kl0 as temperature increases. Under typical industrial conditions, the gas film resistance contributes less than 25% of the total resistance.