Diffusion of methane in supercritical carbon dioxide across the Widom line

Abstract

Diffusion of methane diluted in supercritical carbon dioxide is studied by experiment and molecular simulation in the temperature range from 292.55 to 332.85 K along the isobars 9.0, 12.5 and 14.7 MPa. Measurements of the Fick diffusion coefficient are carried out with the Taylor dispersion technique. Molecular dynamics simulation and the Green-Kubo formalism are employed to obtain Fick, Maxwell-Stefan and intradiffusion coefficients as well as shear viscosity. The obtained diffusion coefficients are on the order of 10 −8 m 2 /s. The composition, temperature and density dependence of diffusion is analyzed. The Fick diffusion coefficient of methane in carbon dioxide shows an anomaly in the near-critical region. This behavior can be attributed to the crossing of the so-called Widom line, where the supercritical fluid goes through a transition between liquid-like and gas-like states. Further, several classical equations are tested on their ability to predict this behavior and it is found that equations that explicitly include the density are better suited to predict the sharp variation of the diffusion coefficient near the critical region predicted by molecular simulation.

Diffusion of Quinine with Ethanol as a Co-Solvent in Supercritical CO2

“…One of the first observations of a change in slope or even the appearance of a V-shaped region on a diffusion curve was reported by Nishiumi& Kubota [ 18 ], who attributed this to a decrease in thermodynamic factor. Recent molecular dynamic simulations also showed that the thermodynamic factor in mixtures manifests a deep well in the vicinity of the Widom line [ 8 ]. At the critical point, the thermodynamic factor is zero per definition.…”

“…It was recently reported that the Fick diffusion coefficient in scCO 2 may vary strongly at low solute concentrations, for example, in the case of methane [ 8 ]. It can be also noted that the content of quinine used in medical and industrial products is very low, for example, tonic water contains slightly less than 0.01% quinine by mass fraction.…”

“…The shape of this distribution at the end of the capillary, known as Taylor peak, is monitored by a detector. Figure 2a presents a schematic of the Taylor dispersion instrument used in this and previous studies [6][7][8]. It consists of four modules: a carrier fluid conditioning module, a CO 2 delivery system with a solute injection valve, an air bath thermostat housing the diffusion capillary and a FT-IR detector.…”

The Anomalous Behavior of Thermodynamic Parameters in the Three Widom Deltas of Carbon Dioxide-Ethanol Mixture

“…One of the most common methods at an early stage of studying supercritical fluids’ properties is molecular dynamics (MD), which is a bridge between experimentally observed macroscopic properties of a substance with microscopic ones. This approach has been previously used to study the Widom delta separating the region of liquid-like from gas-like SCF in the p-T diagram [ 53 , 54 , 55 ]. Molecular dynamics (MD) is a method used for numerical simulation of the system evolution in time by integrating the equations of motion of atoms or particles.…”

Dynamics of hydrocarbons diluted in CO2 at the Widom region: A molecular simulation study

“…The results of a molecular simulation study of methane and small aromatics infinitely diluted in supercritical CO 2 at different temperatures, pressures, and compositions are discussed. In the Widom region, an anomalous behavior of the Fick diffusion coefficient and other transport properties was found even for methane diluted in CO 2 [1].…”