(121b) Phase Equilibria of CO2 and N-Alkanes in Bulk and Confined Space Using Parallelized Wang-Landau Transition-Matrix Monte Carlo Simulations

Accurate and efficient simulations of CO₂-n-alkanes vapor-liquid equilibria (VLE) are essential for various engineering applications such as enhanced oil recovery (EOR) through CO₂ miscible flooding and CO₂ storage in subsurface porous media under in-situ conditions. These conditions, characterized by high-pressure and high-temperature, pose significant challenges for experimental measurements, making molecular simulations a powerful alternative. For bulk systems, the Gibbs Ensemble Monte Carlo (GEMC) method is widely used to simulate VLE, while confined systems typically employ techniques like grand canonical Monte Carlo (GCMC), pore-pore GEMC, and gauge-cell methods. However, these approaches face challenges such as difficulties in accurately determining equilibrium phase transition points, limitations imposed by pore geometry, and the need for multiple simulations to obtain a pair of VLE points.

Transition-Matrix Monte Carlo (TMMC), combined with histogram reweighting, addresses these challenges by enabling the accurate determination of a pair of VLE points from a single simulation without constraints on pre-specified chemical potentials or pore geometry. In this work, we apply the Wang−Landau TMMC (WL-TMMC) method using the Free Energy and Advanced Sampling Simulation Toolkit (FEASST) to simulate the VLE of CO₂-methane and CO₂-hexane systems in both bulk and confined spaces. We validate our results against NVT-GEMC (bulk), pore-pore GEMC (confined), and gauge-GCMC (confined) simulations conducted with the MCCCS Towhee software, achieving relative errors of less than 6% and 8% for bulk and confined systems, respectively. This method also provides free energy information that can distinguish between stable, metastable, and unstable fluid states, enabling the construction of a complete van der Waals loop from a single simulation. Furthermore, the liquid-vapor free energy barrier obtained from these simulations offers a novel approach for determining critical points, consistent with the vanishing interfacial tension method. This capability opens avenues for studying nucleation phenomena and adsorption hysteresis, which are vital for understanding subsurface fluid behaviors.

Our results demonstrate that the WL-TMMC method implemented in FEASST is a robust and reliable tool for investigating the VLE of CO₂-n-alkane systems. This work lays the foundation for our ongoing research into the phase equilibria of CO₂-hydrocarbon-formation water systems within various mineral and kerogen nanopores, with direct implications for subsurface EOR and geological carbon storage.