Capillary condensation and evaporation of n-butane and iso-butane in nanoporous media: Experimental study of the effects of pore size and temperature using an automated gravimetric apparatus

Abstract

Understanding the confinement-induced phase behavior of fluids stored in nanometer-sized pores of solid materials is paramount in various science and engineering applications. Experimental findings describing fluid phase behavior in confinement are often used to develop the efficient and accurate equation of state models that can then provide a cost-effective approach to predict such information for different applications. However, there is a scarcity of experimental data in the literature that can offer insights into the confinement effects under varying conditions. The present work endeavors to provide new experimental data on the effects of pore size and temperature on the confined phase behavior of two chemical species (n-Butane and iso-Butane) and thereby improve the current understanding of the impact of these parameters on capillary condensation, evaporation, and hysteresis. We use a patented gravimetric apparatus that was upgraded in this study by the development and incorporation of advanced automated data acquisition modules. The automation module improved the data collection quality by fine-tuning the fluid injection and withdrawal processes which significantly enhanced the accuracy and precision of the measured isotherms. The capillary condensation and evaporation processes were then studied using a nanoporous material known as Mobil Composition of Matter No. 41 (MCM-41), a mesoporous medium with well-defined pore shape and sizes. The isotherms were obtained for a wide range of temperatures (−7, −3, 0, 7, 8.7, 17, and 27 °C) and using MCM-41 with various pore sizes (60, 80, 100, and 120 Å). The accuracy of the measurements was validated against the phase behavior data of bulk n-Butane and iso-Butane reported by the National Institute of Standards and Technology (NIST). The capillary condensation pressures obtained from the isotherms for different pore sizes agreed with the trends reported in the literature. The results show that the capillary condensation pressures of the fluids increase with the pore size and temperature. The measured isotherms and capillary condensation pressures provide new data sets on the condensation characteristics of n-Butane and iso-Butane. They are expected to contribute to the fundamental understanding of the confined phase behavior of hydrocarbons and thereby advance the development of improved equations of state models that can accurately describe the phase behavior of fluids in nanoporous media. PC-SAFT/Laplace EOS was used to model the phase behavior of the fluids under confinement. The predictions from the PC-SAFT model agree with previous data therefore the results from this study can be integrated into building more robust models.