Phase Equilibria of CO2 and n-Alkanes in Bulk and Confined Space Using Parallelized Wang–Landau Transition-Matrix Monte Carlo Simulations

The accurate and fast simulation of CO₂ and n-alkane phase equilibria is crucial for guiding their industrial applications. We used Wang–Landau Transition-Matrix Monte Carlo (WL-TMMC) with the Free Energy and Advanced Sampling Simulation Toolkit (FEASST) software to compute the vapor–liquid equilibrium (VLE) of CO₂-methane and CO₂-hexane systems in both bulk and confined spaces. The bulk-phase simulation results were compared with literature data and constant volume Gibbs Ensemble (NVT-GEMC) results, with relative errors less than 6%. For confined systems, the results were compared with gauge cell grand-canonical Monte Carlo (gauge-GCMC) and pore–pore GEMC, with relative errors less than 8%. Notably, the WL-TMMC exhibits significant advantages in computing VLE for confined spaces. It requires only a single simulation to determine a pair of VLE points without being constrained by prespecified chemical potentials or pore geometry. Furthermore, the method provides free energy information for different fluid states, enabling the construction of a complete van der Waals loop from a single simulation. In conclusion, we demonstrate that WL-TMMC in FEASST is a robust and reliable tool for studying CO₂-n-alkane VLE.