Densities, viscosities, and diffusivities of loaded and unloaded aqueous CO2/H2S/MDEA mixtures: A molecular dynamics simulation study

Experimentally measuring the diffusivities of CO2 and H2S in aqueous alkanolamine solutions presents an extremely challenging task. To overcome this challenge, we performed Molecular Dynamics (MD) simulations to study the effects of temperature and N-methyldiethanolamine (NDEA) concentration on self-diffusivities of CO2 (DCO2) and H2S (DH2S) in aqueous MDEA solutions. We compute the densities and viscosities of aqueous MDEA solutions for an MDEA concentration range of 10 – 50 wt% and a temperature range of 288 – 333 K showing an excellent agreement with experimental data from literature. We compute the self-diffusivity of MDEA (DMDEA) in aqueous MDEA solutions and our findings show that the computed values of DMDEA are in excellent agreement with experimental and simulation results from literature. The self-diffusivities DCO2 and DH2S in aqueous MDEA solutions are computed for a wide range of temperatures and MDEA concentrations and our results show that both DCO2 and DH2S depend significantly on temperature and MDEA concentration. We also show that both CO2 and H2S diffuse slower in aqueous MDEA solutions than in aqueous MEA solutions. By comparing the radial distribution functions of CO2, H2S, water, and MDEA, we show that H2S has stronger interactions with the surrounding molecules than CO2, which makes H2S diffuse slower in aqueous MDEA solutions. We also investigate the densities and viscosities of acid gas loaded aqueous MDEA solutions and self-diffusivities of the reaction products of CO2 and H2S with aqueous MDEA solutions. We show that the self-diffusivities of CO2-loaded solutions significantly decrease with increasing CO2 loading while the self-diffusivities of H2S-loaded solutions do not change with changing H2S loading. Our results will be helpful in the design and optimization of acid gas removal units.