Phase Behavior of Methane/<i>n</i>‑Butane Binary Mixtures in Organic Nanopores under Bulk Vapor Conditions

Abstract

Nanopores can change the phase boundary of fluid mixtures. A recent study has reported that the bubble-point pressure of binary mixtures in nanopores can be merged with the dew point, showing there is no phase coexistence region but a line. On the other hand, previous molecular-scale simulations showed the existence of a phase envelope. In experiments, it is difficult to determine the composition of a mixture in nanopores. In this study, we investigated the selectivity and phase behavior of CH4/n-C4H10 binary mixtures in 10, 5, and 2 nm graphite nanopores under bulk vapor conditions by grand canonical Monte Carlo molecular simulations. The selectivity was high at low pressures, and the selectivity isotherms can be classified as classes I–III. We observed the nanopore-induced capillary condensation. When we used the bulk mole fractions to prepare the phase diagram, the dew- and bubble-point pressures were almost the same. Both were below the dew-point line of the corresponding bulk mixture. This is in good agreement with recent experiments. We observed a narrower phase envelope when we used the mole fractions of the mixture in nanopores. In general, the bubble-point pressure decreased compared with that of the bulk system, whereas the corresponding dew-point pressure increased. Furthermore, the critical pressure decreased with decreasing pore size and the supercritical region expanded accordingly. The interplay between the fluids in nanopores and bulk can yield various phase diagrams, providing us with a unified picture of the phase behavior in nanopores. The simulation results comprehensively describe the phase behavior of hydrocarbon mixtures in organic nanopores for shale gas and shale oil development