RESUMEN
We investigated the effect of confinement on the phase behavior of hexane in nanopores of mesoporous silica at varying pore diameters and temperatures using a patented gravimetric apparatus. The adsorption and desorption isotherms were experimentally measured, and the capillary condensation and evaporation pressures were calculated from the isotherms. The results show that, for all pore sizes and temperatures utilized here, the confinement of fluids significantly lowers the vapor-liquid phase transition pressures. However, its evaporation, i.e., liquid-vapor phase transition, occurs at a lower pressure than its capillary condensation counterpart. The experimental findings demonstrate that the confinement effect becomes weaker in wider nanopores due to the reduced solid-fluid interactions in larger spaces. Furthermore, it is evident from isotherms that hexane rapidly approaches a supercritical-like state at high temperatures when confined in smaller pores, resulting in an ambiguous vapor-liquid phase transition. In contrast, this behavior disappears in larger pores at similar temperatures. Moreover, the present study compares the fully gravimetric adsorption method against the thermogravimetric approach. The results show that the fully gravimetric method, which directly measures the mass of the adsorbed or condensed fluids, provides significant advantages over the thermogravimetric counterpart. The findings of this study are expected to be of fundamental interest to a wide range of science and engineering communities concerned about the behavior of heavier hydrocarbons in various industrial applications, and modeling the confined phase behavior of fluids and developing robust equations of state (EOS).
RESUMEN
Fundamental understanding of the phase behavior and properties of fluids under confinement is of great significance for multiple fields of engineering and science, as well as for many practical industrial applications. In particular, unconventional geological systems, such as shale reservoirs, possess nanometer-scale pores, which impose nanoconfinement on the fluid molecules. In large pores, the bulk phase behavior of fluids can be modeled by the well-established methods, such as equation of state (EOS) approaches. However, under confinement the thermodynamic properties of fluids deviate significantly from those in the bulk, thus rendering the traditional EOS methods ineffective in predicting the phase behavior of confined fluids. Recently, the PC-SAFT/Laplace EOS has been developed to better represent the fluid phase equilibria in nanopores, which incorporates a new parameter that needs to be determined from experimental data. In this study, a new dataset is presented to reflect the phase properties of propane confined within the MCM-41 pores, with the aim to improve both the general understanding of the phase behavior of hydrocarbons under confinement and to parameterize the PC-SAFT/Laplace EOS for the nanoconfined propane. For this purpose, propane adsorption and desorption isotherms are determined experimentally for a wide range of temperatures (-27 to 20 °C) in MCM-41 of three different pore sizes (nominal pore diameters of 60, 80, and 100 Å). The effects of temperature and pore diameter on the capillary condensation and evaporation pressures are discussed in detail. Furthermore, the adsorption-desorption hysteresis behavior and its progression for different pore sizes were discussed. The experimental data are modeled using the parameterized PC-SAFT/Laplace EOS, which accurately captured the effects of confinement on the capillary condensation of propane in MCM-41. In addition, this study enriches the field of nanoconfinement research by providing a new dataset exemplifying the thermodynamic characteristics of hydrocarbons in nanopores.
