FTIR as a Powerful Tool for Measurements of Diffusion in Supercritical Carbon Dioxide Using Taylor Dispersion Method

A new experimental high-pressure setup for measuring diffusion coefficients in supercritical fluids, based on Taylor dispersion method, and using an FTIR detector to operate up to 25.0 MPa was designed and optimized. Tracer diffusivities, D12, of toluene and benzene in supercritical carbon dioxide were measured in the temperature range of 306.15–320.15 K, and pressure range of 7.5–17 MPa to evaluate the setup and experimental protocol. The effects of flow velocity, volume of the cell, absorbance at different wavenumbers on the diffusion coefficient as well as all parameters respecting the Taylor dispersion method have been analyzed. The obtained diffusion coefficients are in excellent agreement with the available literature data. The dependence of D12 on temperature, pressure, and solvent density were examined. Some correlation models based on the hydrodynamic theory were used to estimate the diffusion coefficients in supercritical carbon dioxide, which is the best agreement obtained for an improved version of the Wilke–Chang model

Pattern selection for convective flow in a liquid bridge subjected to remote thermal action

The stability of thermocapillary/buoyant flows is affected by a remote thermal source. We present a nonlinear two-phase computational study of convection in a liquid bridge that develops under the action of Marangoni and buoyancy forces as well as under the influence of distant thermal disturbances. The gas phase (air) occupies a typical annular container holding a liquid bridge (n-decane, Pr = 14), and the disturbances are locally imposed in the form of hot/cold spots on the outer wall of the container. The hydrothermal wave instability and pattern selection have been explored for two temperature differences Δ���� by varying the intensity of thermal source Hf over a wide range. Not far from the critical point, in all the cases, the instability emerges in the form of a standing wave, but the azimuthal wavenumber depends on whether the external perturbation is caused by cooling (m = 2) or by heating (m = 1). Further into supercritical area, 45% above the threshold, in the region with thermal perturbations ⎯200<��������<50, the flow pattern comprises, but is not limited to, a hydrothermal traveling wave with the azimuthal wavenumber m = 2. For hotter perturbations, the instability develops either in the form of traveling or standing waves, depending on Hf, with the prevailing mode m = 1, but with a strong presence of other modes

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

This study aims at contributing to quinine extraction using supercritical CO2 and ethanol as a co-solvent. The diffusion coefficients of quinine in supercritical CO2 are measured using the Taylor dispersion technique when quinine is pre-dissolved in ethanol. First, the diffusion coefficients of pure ethanol in the supercritical state of CO2 were investigated in order to get a basis for seeing a relative change in the diffusion coefficient with the addition of quinine. We report measurements of the diffusion coefficients of ethanol in scCO2 in the temperature range from 304.3 to 343 K and pressures of 9.5, 10 and 12 MPa. Next, the diffusion coefficients of different amounts of quinine dissolved in ethanol and injected into supercritical CO2 were measured in the same range of temperatures at p = 12 Mpa. At the pressure p = 9.5 MPa, which is close to the critical pressure, the diffusion coefficients were measured at the temperature, T = 343 K, far from the critical value. It was found that the diffusion coefficients are significantly dependent on the amount of quinine in a small range of its content, less than 0.1%. It is quite likely that this behavior is associated with a change in the spatial structure, that is, the formation of clusters or compounds, and a subsequent increase in the molecular weight of the diffusive substance

The Soret coefficients of the ternary system water/ethanol/triethylene glycol and its corresponding binary mixtures

Thermodiffusion in ternary mixtures is considered prototypic for the Soret effect of truly multicomponent systems. We discuss ground-based measurements of the Soret coefficient along the binary borders of the Gibbs triangle of the highly polar and hydrogen bonding ternary DCMIX3-system water/ethanol/triethylene glycol. All three Soret coefficients decay with increasing concentration, irrespective of the choice of the independent component, and show a characteristic sign change as a function of temperature and/or composition. With the exception of triethylene glycol/ethanol at high temperatures, the minority component always migrates toward the cold side. All three binaries exhibit temperature-independent fixed points of the Soret coefficient. The decay of the Soret coefficient with concentration can be related to negative excess volumes of mixing. The sign changes of the Soret coefficients of the binaries allow to draw far-reaching conclusions about the signs of the Soret coefficients of the corresponding ternary mixtures. In particular, we show that at least one ternary composition must exist, where all three Soret coefficients vanish simultaneously and no steady-state separation is observable

Diffusion of methane in supercritical carbon dioxide across the Widom line

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 m2/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.DFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische Universität Berli

Density, refractive index, and derived properties of binary mixtures of water + ionic liquid 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate

The density and refractive index were experimentally determined for binary mixtures of water + ionic liquid 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate throughout the ionic liquid mass frac- tion range, at atmospheric pressure and various temperatures between 293.15 K and 323.15 K. The refractive index was measured at five wavelengths between 589.2 nm and 935 nm. From the experimen- tal data on density, volumetric properties such as the excess molar volume and thermal expansion coef- ficient were calculated. The excess molar volume was negative throughout the ionic liquid mass fraction range and its magnitude decreased with temperature. From the experimental data on the refractive index, the deviation in the refractive index and its coefficients of concentration, temperature and chro- matic dispersion were obtained. The values of the deviation in the refractive index were positive and decreased with temperature. In order to simultaneously investigate the dependence of the refractive index on concentration, temperature and wavelength, we correlated the experimental data with a two-term Cauchy equation. Furthermore, a comparative study of 11 refractive index mixing rules was performed to assess their prediction ability. More advanced mixing rules do not lead to any improvement in comparison with the simple linear mixing rule (Arago-Biot) for estimating refractive index and the concentration contrast factor of the mixture studied. The results are expected to be useful for tuning the properties of an ionic liquid by adding water or selecting the temperature or optical region.Peer ReviewedPostprint (author's final draft

G-Jitter, Vibrations, Diffusion: The IVIDIL Experiment

Experiments aboard the International Space Station (ISS) provide a large but still limited amount of data. A complete set of data is usually returned to Earth on a flash disk a few months later, by which time the experimental facility has already been put into storage or trashed. Thus, scientists have no possibility to repeat the experiment, even if some ambiguities are later found. Therefore, onboard experiments require careful preparation on the ground with multiple tests in the laboratory and in parabolic flights, if possible. Furthermore, during postflight analysis, it is important to clarify all unknown sources of errors. One of the most obvious sources of perturbations on the ISS is g-jitter. Here, we present the preparation and implementation of the Influence of Vibrations on Diffusion in Liquids (IVIDIL) experiment on the ISS, which studied the effect of random g-jitter and given vibrations on diffusion-controlled experiments in liquid mixtures. Since diffusion in liquids is a slow process, only vibrational effects were examined in parabolic flights. A methodology for the analysis of diffusion and thermodiffusion processes was developed in the laboratory