Direct measurements of pore fluid density by vibrating tube densimetry

Structure and dynamics of ethane confined in silica nanopores in the presence of CO2

Fundamental understanding of the subcritical/supercritical behavior of key hydrocarbon species inside nano-porous matrices at elevated pressure and temperature is less developed compared to bulk fluids, but this knowledge is of great importance for chemical and energy engineering industries. This study explores in detail the structure and dynamics of ethane (C2H6) fluid confined in silica nanopores, with a focus on the effects of pressure and different ratios of C2H6 and CO2 at non-ambient temperature. Quasi-elastic neutron scattering (QENS) experiments were carried out for the pure C2H6, C2H6:CO2 = 3:1, and 1:3 mixed fluids confined in 4-nm cylindrical silica pores at three different pressures (30 bars, 65 bars, and 100 bars) at 323 K. Two Lorentzian functions were required to fit the spectra, corresponding to fast and slow translational motions. No localized motions (rotations and vibrations) were detected. Higher pressures resulted in hindrances of the diffusivity of C2H6 molecules in all systems investigated. Pore size was found to be an important factor, i.e., the dynamics of confined C2H6 is more restricted in smaller pores compared to the larger pores used in previous studies. Molecular dynamics simulations were performed to complement the QENS experiment at 65 bars, providing supportive structure information and comparable dynamic information. The simulations indicate that CO2 molecules are more strongly attracted to the pore surface compared to C2H6. The C2H6 molecules interacting with or near the pore surface form a dense first layer (L1) close to the pore surface and a second less dense layer (L2) extending into the pore center. Both the experiments and simulations revealed the role that CO2 molecules play in enhancing C2H6 diffusion ("molecular lubrication") at high CO2:C2H6 ratios. The energy scales of the two dynamic components, fast and slow, quantified by both techniques, are in very good agreement. Herein, the simulations identified the fast component as the main contributor to the dynamics. Molecule motions in the L2 region are mostly responsible for the dynamics (fast and slow) that can be detected by the instrument.

Confinement-Induced Supercriticality and Phase Equilibria of Hydrocarbons in Nanopores

For over a century, the phase behavior of bulk fluids has been described as PVT (pressure-volume-temperature) three-dimensional properties, but it has become increasingly clear that the liquid-vapor phase behavior in confined geometries is significantly altered from the bulk. Efforts have been devoted to accessing confined phase transitions using sorption, molecular simulations, and theoretical methods. However, a comprehensive picture of PVT relationships for confined hydrocarbons remains uncertain. Herein, we introduce d (confining pore diameter) as a fourth dimension, and we present PVT-d behavior of confined fluids in nanopores. For the first time, a T-d phase diagram is presented for n-hexane, n-octane, and n-decane under multiple confinement scales (37.9, 14.8, 9.8, 6.0, 4.1, 3.3, and 2.2 nm cylindrical pore diameter) using experimental differential scanning calorimetry and PVT-d equation of state theory at atmospheric pressure. As pore diameter decreases from 37.9 to 4.1 nm, the bubble point increases by as much as 15 K above bulk, until we observe behavior consistent with a supercritical state, pointing to confinement-induced supercriticality. Remarkably, experimental and theoretical findings overlap very well, showing that this approach effectively captures the phase boundaries between the liquid, vapor, and supercritical fluid regions. The model and completed EOS are additionally extended to calculation of isothermal capillary adsorption, and its validity is discussed.

A Novel Slope Method for Measurement of Fluid Density with a Micro-cantilever under Flexural and Torsional Vibrations

A novel method, which was called a slope method, has been proposed to measure fluid density by the micro-cantilever sensing chip. The theoretical formulas of the slope method were discussed and established when the micro-cantilever sensing chip was under flexural and torsional vibrations. The slope was calculated based on the fitted curve between the excitation and output voltages of sensing chip under the nonresonant status. This measuring method need not sweep frequency to find the accurate resonant frequency. Therefore, the fluid density was measured easily based on the calculated slope. In addition, the micro-cantilver was drived by double sided excitation and free end excitation to oscillate under flexural and torsional vibrations, respectively. The corresponding experiments were carried out to measure the fluid density by the slope method. The measurement results were also analyzed when the sensing chip was under flexural and torsional nonresonant vibrations separately. The measurement accuracies under these vibrations were all better than 1.5%, and the density measuring sensitivity under torsional nonresonant vibration was about two times higher than that under flexural nonresonant vibration.