Carbon dioxide absorption into hydroxide and carbonate systems

Abstract

In the global warming scenario, a literature review on carbon dioxide capture technologies shows that CO2 capture by chemical absorption seems to be the immediately viable route for sustainable energy supply in near future. The technology for chemical absorption is available and has been practiced in the industry. Development and identification of optimal absorbents for absorption is required to employ it on the scale required for CO2 capture and to minimize the energy penalty of capture.

Hydroxide and carbonate systems have been used for absorption of CO2 from the beginning of the 20th century in different industrial processes. Though the use of hydroxides and carbonates remained persistent in special applications but the use of these systems in industrial gas cleaning units decreased after the introduction of alkanolamines as absorbents. During the last two decades, hydroxide and carbonate systems have regained interest in post combustion CO2 capture by absorption. Potentially low energy requirements for the capture process based on hydroxide and carbonate systems and being environment friendly are advantages over the energy intensive amine based CO2 capture solvents and environmental issues arising from degradation of amines.

This thesis contributes to the kinetics and equilibrium of CO2 absorption into hydroxide and carbonate systems. The measured experimental data on CO2 absorption and physical solubility of CO2 (from N2O solubility using N2O analogy) into these systems, in addition to vapor liquid equilibrium (VLE) data were used to evaluate the activity based kinetics of the reaction of CO2 with hydroxyl ion (OH-) containing Li+, Na+ and K+ counter ions. To study the kinetics of the CO2 reaction with hydroxyl ion is important not only for hydroxide and carbonate systems but it is significant as this reaction occurs in all alkaline systems including alkanolamines. The density and N2O solubility data into aqueous hydroxides and blends of hydroxides with carbonates containing Li+, Na+ and K+ counter ions were experimentally determined. The measured density data were compared with an empirical density model available in the literature.

The measured N2O solubility data were used for the refitting of parameters in an extensively used solubility model available in the literature for up gradation to wider ranges of temperature and concentration. The measured N2O solubility data and VLE data collected from the literature were simultaneously regressed using an in-house equilibrium model to determine the interaction parameters in the Electrolyte-NRTL model. The determined Electrolyte- NRTL parameters were subsequently used for the estimation of liquid phase activities of CO2 and OH- in the systems containing Li+, Na+ and K+ counter ions. The kinetics of aqueous hydroxides and blends of hydroxides with carbonates containing Li+, Na+ and K+ counter ions were experimentally measured using a string of discs contactor (SDC). The measured data were used for the parameter optimization in a widely used kinetics model available in the literature to a broader range of temperature.

Finally, the activity based kinetics of the CO2 reaction with OH- were determined using the measured kinetics data and the calculated liquid phase activities of CO2 and OH- in the aqueous solutions containing Li+, Na+ and K+ counter ions.