Kazunari Ohgaki

Isothermal vapor-liquid equilibrium data for the ethane—carbon dioxide system at high pressures

Abstract Isothermal vapor-liquid equilibrium data for the ethane—carbon dioxide system at high pressures were obtained at 10, 15, 18, 20, and 25°C. The gas-liquid critical data (pressure—temperature—composition) for the system were also measured at several temperatures, and the azeotropic points were obtained at 10, 15, and 18°C. This system exhibits two critical points and one azeotropic point in the temperature range from 17.5°C to 18.5°C.

Solubilities of naphthalene and indole in supercritical fluids

Abstract Solubilities of naphthalene and indole in carbon dioxide, ethylene, and ethane were measured at 308.15 K over a pressure range of 5 to 20 MPa. The solubility data were obtained, using a newly developed static method that utilizes supercritical fluid chromatography. The data obtained are well correlated by applying the Soave-Redlich-Kwong equation of state to the fugacity coefficient of the fluid phase. In addition, the relation between the solubility and the fluid phase density is briefly discussed for each binary system by use of the virial equation of state.

Stability boundaries and small hydrate-cage occupancy of ethylene hydrate system

Abstract Thermodynamical stability boundaries of ethylene hydrate system were investigated in a temperature range from 279 to 328 K and pressure range up to 465 MPa . The three-phase coexisting curve of the ethylene hydrate + saturated water + saturated fluid ethylene shows the characteristic “S-shape” behavior in the pressure–temperature projection. From the fugacity–temperature projection, it can be concluded that such curious behavior is caused by the anomalous critical phenomenon. The laser Raman microprobe spectroscopic analysis reveals that the stretching and bending vibration energies of ethylene entrapped in the hydrate cage, fulfill two different conditions. It has been argued that ethylene occupies only the large cage of structure-I. However, based on the double peaks from the Raman scattering on ethylene hydrate crystals, we give direct evidence that, in spite of ethylenes large van der Waals diameter, it also occupies the small cage.

Heterogeneity in aqueous solutions: electron microscopy of citric acid solutions

Abstract The structures of aqueous citric acid solutions of various concentrations, including supersaturated solutions, were examined directly by scanning electron microscopy (SEM). Electron microscopic samples were prepared by solidifying a droplet of solution in a refrigerant at very high quenching speed, then replicating it with electron-beam-gunned platinum and carbon. Solute clusters and their aggregates were observed in all the samples irrespective of the solute concentration. The solutions showed a double-zone structure in scanning electron microphotographs, consisting of a smooth zone and a compartment of many solute clusters. With the increase in solute concentration, the number density of compartments increased, while their volume decreased. Electron microscopy thus strongly suggested that the solutions examined were heterogeneous, namely, the so-called colloidal mixtures. X-ray diffraction analysis revealed the very important fact that the crystal of citric acid monohydrate contains fine particles which seem to be the remnants of solute clusters in solution. This is supported by the observation in the boundary region between the crystal and solution of fine particles which stretch like elastic to form a network structure.

Solute clusters in aqueous citric acid solutions

Abstract Densities of aqueous citric acid monohydrate solutions are examined at three different concentrations. Two supersaturated solutions exhibit density gradients in a vertical column, but a saturated solution does not. The density gradients can be expressed satisfactorily by use of a theoretical model based on the Brownian diffusion equation for solute clusters under gravity. Electron microscopy also strongly suggests the existence of solute clusters in supersaturated solutions. The cluster size is estimated to be about 16 nm in diameter. Ultraviolet-ray scattering suggests a possible secondary structure of cluster aggregations.

Whisker formation from jet of supercritical fluid solution

Abstract The growth mechanism of a whisker of stigmasterol particles deposited by rapid expansion from supercritical CO 2 solutions has been investigated by scanning electron microscope. Amorphous fine particles were found to be under relatively low pre-expansion pressures while whisker-like crystals were observed under relatively high pre-expansion pressures. The turning point was about 13 MPa at 373 K. It was found that the secondary products grew on a screen in the expansion jet after deposition of primary particles of about 10-nm diameter. The morphology of secondary products was found to strongly depend on the surface condition of the primary particle. Because of the temperature decrease caused by the rapid expansion, the pre-expansion pressure plays an important role in determining the thermal roughening degree on the surface of primary particles, and consequently the morphology of the secondary products.

Pressure-density-temperature (p-p-T) relations of CHF3, N2O, and C3H6 in the critical region

Abstract The pressure-density-temperature (p-ϱ-T) relations in the critical region (0.996

Hydrogen transfer from guest molecule to radical in adjacent hydrate-cages

Electron spin resonance measurement of gamma-ray-irradiated propane hydrates shows that the normal propyl radical withdraws hydrogen from the adjacent propane molecule through the hexagonal planes of the hydrate cage without water molecule bridging.

Electron spin resonance study on γ-ray-induced methyl radicals in methane hydrates

Electron spin resonance (ESR) studies have been performed to investigate radicals induced in ethane hydrate irradiated by γ-rays at 77 K. Two ESR spectra are observed and identified as the induced ethyl radical (g=2.0031±0.0005, Aα⊥=2.2±0.1 mT, Aα|| =2.5±0.1 mT, Aβ=2.7±0.1 mT) and induced atomic hydrogen (g=2.0026±0.0005, A=50.5±0.1 mT). From the results of ESR analysis and gas mass spectroscopy, it is concluded that the ethyl radical decays into butane by dimerization in the first-order reaction in the temperature region of 250–265 K. The activation energy of the decay reaction is 73.1±6.3 kJ/mol, which is near the dissociation enthalpy change of ethane hydrate to liquid water and gaseous ethane. This finding implies that ethane hydrate does not dissociate into ice but supercooled water in the present temperature region, similar to the dissociation of methane hydrate in our previous study.

Isothermal vapor-liquid equilibria for the binary systems propylene-carbon dioxide, propylene-ethylene and propylene-ethane at high pressure

Abstract isothermal vapor-liquid equilibria for the binary systems propylene-carbon dioxide, propylene-ethylene and propylene-ethane have been measured at 283.15 and 298.15 K using a static method. The equilibrium data obtained have been correlated with the total-pressure method using both the Redlich-Kwong equation of state modified by Soave and the perturbation equation proposed by Gubbins and Twu. From normal probability plots of the residuals in the correlations versus the theoretical residuals in the normal distribution, the applicability of each of these two equations has been tested statistically. Furthermore, the fugacity coefficients calculated from the virial equation have been compared with those obtained from the correlations for the low-pressure regions.