Kinetics of Carbon Dioxide Absorption into Aqueous Potassium Carbonate and Piperazine

Abstract

The absorption rate of CO 2 was measured in a wetted-wall column in 0.45−3.6 m piperazine (PZ) and 0.0−3.1 m potassium carbonate (K 2 CO 3 ) at 25−110 °C. A rigorous kinetic model was used to model the data and interpret diffusivities and rate constants. The rate approaches second-order behavior with PZ and is highly dependent on other strong bases. In 1 M PZ, the overall rate constant is 102 000 s -1, 20 times higher than in monoethanolamine. The activation energy is 35 kJ/mol, similar to other amine−CO 2 reactions. Rate constants for contributions of carbonate, PZ carbamate, and water to the rate were determined according to base catalysis theory. The addition of neutral salts to aqueous PZ increases the apparent rate constant. In 2.7 M NaCl/0.6 M PZ, the overall rate constant is increased by a factor of 7. Ionic strength effects were accounted for within the rigorous model of K + /PZ mixtures. The absorption rate in concentrated K + /PZ mixtures is up to 3 times faster than in 30 wt % monoethanolamine. At low temperatures and low CO 2 loadings, a pseudo-first-order approximation adequately represents the absorption rate. At high loadings, the reaction approaches instantaneous behavior but is still influenced by reaction kinetics. Under industrial conditions, gas film resistance may account for >80% of the total mass transfer resistance at low loadings.