The determination of the Henry’s Coefficient of reactive gases – An example of CO2 in aqueous solutions of monoethanolamine (MEA)

Abstract

The Henry’s Coefficient of CO2 is a fundamental property, it quantifies the equilibrium between the partial pressure of CO2 in the gas phase and the concentration of dissolved free CO2 in the liquid phase. This value is critical for simulation and may lead to improvements in the process of post combustion capture of CO2. However, because of its reactive nature, CO2 reacts in amine solutions and forms multiple carbon-containing species, so that the free CO2 concentration is very small and difficult to determine. The “N2O analogy” has been used to estimate the physical solubility of CO2 based on the assumption that the two gases behave similarly in water and amine solutions. In the current study, a direct way is proposed for the determination of the Henry’s Coefficient of CO2 in MEA solutions. The method only requires vapor-liquid equilibrium (VLE) measurements of the MEA-CO2-H2O system; and all equilibrium/protonation constants for all solution reactions are sourced from the open literature while activity coefficients for all ionic species were estimated using the Davis equation. The free CO2 concentration in solution can be computed using the total MEA concentration, total CO2 concentration and temperature, which allows the determination of the Henry’s Coefficient from the CO2 partial pressure. A 10-parameter polynomial is used to approximate the Henry’s Coefficient as a function of the total MEA concentration, total CO2 concentration and temperature. VLE calculations were performed using the liquid-phase equilibrium calculations and the polynomial function calculating the Henry’s Coefficient, and the results show the VLE calculations are adequate to describe the VLE data.