Solving Chemical Absorption Equilibria using Free Energy and Quantum Chemistry Calculations

Abstract

We developed an open-source chemical reaction equilibrium solver in Python (CASpy, https://github.com/omoultosEthTuDelft/CASpy) to compute the concentration of species in any reactive liquid-phase absorption system. We derived an expression for a mole fraction-based equilibrium constant as a function of excess chemical potential, standard ideal gas chemical potential, temperature, and volume. As a case study, we computed the CO₂ absorption isotherm and speciation in a 23 wt % N-methyldiethanolamine (MDEA)/water solution at 313.15 K, and compared the results with available data from the literature. The results show that the computed CO₂ isotherms and speciations are in excellent agreement with experimental data, demonstrating the accuracy and the precision of our solver. The binary absorptions of CO₂ and H₂S in 50 wt % MDEA/water solutions at 323.15 K were computed and compared with available data from the literature. The computed CO₂ isotherms showed good agreement with other modeling studies from the literature while the computed H₂S isotherms did not agree well with experimental data. The experimental equilibrium constants used as an input were not adjusted for H₂S/CO₂/MDEA/water systems and need to be adjusted for this system. Using free energy calculations with two different force fields (GAFF and OPLS-AA) and quantum chemistry calculations, we computed the equilibrium constant (K) of the protonated MDEA dissociation reaction. Despite the good agreement of the OPLS-AA force field (ln[K] = −24.91) with the experiments (ln[K] = −23.04), the computed CO₂ pressures were significantly underestimated. We systematically investigated the limitations of computing CO₂ absorption isotherms using free energy and quantum chemistry calculations and showed that the computed values of μ_i^ex are very sensitive to the point charges used in the simulations, which limits the predictive power of this method.