Modeling the Fluid Phase Behavior of Carbon Dioxide in Aqueous Solutions of Monoethanolamine Using Transferable Parameters with the SAFT-VR Approach

Abstract

The current method of choice for large-scale carbon dioxide (CO2) capture is amine-based chemisorption, typically in packed columns, with the benchmark solvent being aqueous solutions of a primary alkanolamine: monoethanolamine (MEA). In this contribution, we use the statistical associating fluid theory for potentials of variable range (SAFT-VR) to describe the fluid phase behavior of MEA + H2O + CO2 mixtures. The physical chemistry of CO2 in aqueous solutions of amines is highly complex owing to the chemical equilibria between the various species that are formed in solution at ambient conditions. We explicitly consider the multifunctional nature of MEA and, in so doing, are able to represent accurately the thermodynamic properties and phase equilibria of this highly nonideal mixture over a wide range of temperatures, pressures, and compositions. MEA is modeled as an associating chain molecule formed from homonuclear spherical segments with six distinct association sites incorporated to mediate the asymmetric hydrogen bonding interactions exhibited by this molecule. The models for H2O and CO2 are taken from previous work. In order to describe the chemisorption, which is the key to the CO2 capture process, two additional effective sites are incorporated on the otherwise nonassociating CO2 molecule to describe the chemical interaction between the MEA and CO2, so that the correct maximal stoichiometry of two amine molecules per CO2 molecule is retained. The vapor−liquid phase equilibria of the various binary mixtures and of the MEA + H2O + CO2 ternary mixture are accurately described with our approach, including the degree of absorption of CO2 in the solvent for wide ranges of temperature and pressure. This suggests that the underlying complexity of the chemical equilibria associated with this system are correctly captured by the model and provides great promise for the modeling of the overall process of CO2 capture.