Solving Chemical Absorption Equilibria using Free Energy and Quantum Chemistry Calculations: Methodology, Limitations, and New Open-Source Software

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 μiex are very sensitive to the point charges used
in the simulations, which limits the predictive power of this method.