Transport properties of acid gasses in aqueous MDEA solvents

Abstract

Acid-gas capture systems are used to remove acid-gasses from waste gas streams from combustion or other chemical processes. A commonly used solvent is an aqueous solution with the primary amine monoethanolamine (MEA). Removing the acid gasses typically involves heating the solvent to approximately 378-383K in the stripping reactor. Using methyldiethanolamine (MDEA) instead of MEA can reduce the heating energy consumption of the stripper due to its lower reaction heat. The design of new reactors using aqueous MDEA solvents requires more data describing the properties of this solvent. Literature reporting thermophysical properties of aqueous MDEA solvents and transport properties of acid gasses in these solvents is exceedingly scarce. This work fills that information gap. Molecular dynamics were employed to compute density, viscosity and diffusivity of MDEA, CO2 and H2S in aqueous MDEA ranging in 0-50 wt% MDEA and 288-323K. The simulations were conducted using fully atomistic force fields for all species. The charges of MDEA were computed using Gaussian09 and scaled to achieve optimal agreement with experimental density and viscosity data. It has become clear that N-C-C-O dihedral in MDEA is crucial to reproduce experimental data of the viscosity and the diffusivity of MDEA. Two dihedrals were tested to achieve the best results. The resulting computed density, viscosity and diffusivity of MDEA are in good agreement with experimental data. The mixing rules between MDEA and CO2 were adjusted to increase accuracy of the prediction of diffusivity of CO2. The results are in good agreement with experimental data at 0-10wt% MDEA or 288K. The deviations become larger with higher wt% MDEA or higher temperatures.