In this work, the pseudopotential lattice Boltzmann method is employed to study the phase equilibrium in confined space. The role of capillary pressure and fluid-solid interaction are evaluated separately. In the absence of solid walls, our pore-scale simulation is consistent with the thermodynamic model coupling Peng-Robinson equation of state and capillary pressure. We prove that Young-Laplace equation and Parachor model is still valid at nanoscale. When the pore space is confined by solid walls, the heterogeneous density distributions in nanopores are discovered. The decreased critical temperature and increased critical density are observed under confinement.

In addition, the effect of fluid-solid interaction strength on phase behavior is studied. It is found that a larger fluid-solid force leads to a more heterogenous density distribution in nanopores. However, the average liquid/vapor density and critical temperature are not significantly affected by the strength of fluid-solid interaction. The critical pressure, on the other hand, is lower when the fluid-solid interaction is stronger. The coexistence curves and saturation pressures in nanopores with different fluid-solid force are presented which can be characterized by using contact angle. Finally, the phase behavior in a complex pore structure is calculated. This work illustrates that implementing capillary pressure or critical shift alone cannot fully describe the confined phase behavior.

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