Volume-temperature flash calculation for multiphase equilibrium of hydrocarbon mixtures confined in nanopores

Accurate prediction of the thermodynamic properties and phase behavior of fluid confined in nanopores is essential for modeling transport processes in subsurface formations, membranes, and nanofluidic systems. Conventional cubic equations of state (EOS) often fail to account for fluid-wall interactions, leading to substantial deviations from experimental observations. Although molecular simulations provide detailed insights, their high computational cost limits their use in large-scale simulations. This study develops a robust Volume-Temperature flash calculation algorithm for determining the multiphase equilibrium of confined fluids using a modified Peng-Robinson EOS that explicitly incorporates confinement and fluid-wall interactions. Phase stability and equilibrium are determined through direct minimization of Helmholtz free energy using a trust-region modified Newton iteration method, ensuring robust convergence for any initial guess. The model is applied to pure and multicomponent hydrocarbon system within nanopores. Results reveal that both confinement effects and fluid-wall interactions significantly alter phase behavior, leading to the shrinkage of two-phase envelopes with decreasing pore size. Confinement effects increase the effective molecular covolume, while fluid-wall interactions weaken intermolecular attractions, collectively lowering the critical temperature. Critical density exhibits a nonmonotonic trend: rising in moderately sized pores due to dense sorbed layers but declining in smaller pores dominated by confinement. The proposed algorithm provides an efficient and physically consistent tool for predicting confined-fluid phase behavior in subsurface or engineered nanoporous media.