(224a) Predicting the Vapor-Liquid Equilibrium Curves of CO2 and H2s in Aqueous Blends of Mdea and Piperazine Using Molecular Simulations and Quantum Chemistry Calculations

Vapor-Liquid Equilibrium (VLE) curves of acid-gases (CO2 and H2S) in aqueous alkanolamine solvents are used to model the acid gas removal from natural gas streams using process modeling. Accurate modeling of the VLE curves especially at very low loading of acid gases (<0.05 molAG / molamine) is significant for the modeling of acid gas removal process since the acid gas concentration in the natural gas stream required to be quite low by the natural gas specifications (<50 ppm for CO2 and <4 ppm for H2S). Measuring acid gas VLEs experimentally at exceptionally low concentrations is challenging due to very low partial pressures of acid gases in the gas phase. Recently, we developed a Python library called CASpy to compute the speciation in reactive liquid-phase absorption systems (https://pypi.org/project/CASpy-ReactionEquilibria/). Using CASpy, free energy and quantum chemical calculations, we showed that accurate point charges for the molecules and ions are needed to compute accurate absorption isotherms of CO2 and H2S in aqueous alkanolamine solutions.

In this study, we investigate the absorption of CO2 and H2S in aqueous blends of MDEA and piperazine. We investigate aqueous blends of MDEA and piperazine because MDEA is a common alkanolamine that is used in selective absorption of H2S from natural gas streams and piperazine is a common activator in the acid gas removal process. We investigate the absorption of CO2 and H2S using two different force fields for MDEA and piperazine (and the reaction products such as MDEAH+), General Amber Force Field (GAFF) and OPLS-AA force field. Our results show that the absorption isotherms computed using OPLS-AA agree much better with experimental isotherms from literature compared to the results computed using GAFF. Results also show that even though the reaction equilibrium constants computed using OPLS-AA force field are slightly different from experimental equilibrium constants (ca. 1-2 units of ln[K]), the computed absorption isotherms are not accurate because the absorption isotherms are sensitive to the value of equilibrium constants. We show that the sensitivity is mostly due to inaccurate point charges, and accurate point charges are needed to compute accurate acid gas absorption isotherms.

[1] Polat, H. M., Salehi, H. S., Hens, R., Wasik, D. O., Rahbari, A., de Meyer, F., Houriez, C., Coquelet, C., Calero, S., Dubbeldam, D., Moultos, O. A., & Vlugt, T. J. H. (2021). New Features of the Open Source Monte Carlo Software Brick-CFCMC: Thermodynamic Integration and Hybrid Trial Moves. Journal of Chemical Information and Modeling, 61(8), 3752–3757. https://doi.org/10.1021/acs.jcim.1c00652

[2] Polat, H. M., de Meyer, F., Houriez, C., Coquelet, C., Moultos, O. A., & Vlugt, T. J. H. (2023). Transport properties of mixtures of acid gases with aqueous monoethanolamine solutions: A molecular dynamics study. Fluid Phase Equilibria, 564, 113587. https://doi.org/10.1016/j.fluid.2022.113587