Published papers

This paper reports a study of the interfacial tension between natural gas (methane + ethane + propane gas mixture) and water. The composition of the (methane + ethane + propane) gas mixture was 89.95:7.05:3.00 and 95.0:3.5:1.5, in mole ratio. These measurements were performed using the pendant drop method at pressure and temperature conditions of up to 10 MPa and between 283.2 K and 298.2 K, respectively. These conditions were chosen to eliminate the possibility of clathrate hydrate formation. Interfacial tension in the (natural gas + water) system decreased with increasing pressure, but no temperature dependence was found for any of the at all conditions. The interfacial tension gradient with respect to pressure was found to depend on the composition of gas phase. The interfacial tension in a (natural gas + water) system was found to be smaller than that of a (methane + water) system.

This study reports the visual observation made of the formation and growth of clathrate hydrate on the surface boundary of a Sodium Dodecyl Sulfate (SDS) aqueous solution droplet and surrounding methane gas. The experimental temperature range was from 276 K to 282 K, under the pressure of 7.8 MPa, at pure water, wSDS = 10 ppm and wSDS = 100 ppm, where wSDS denotes the mass fraction of SDS in the aqueous solution. At pure water and wSDS = 10 ppm, the hydrate crystals were initially observed at the droplet surface, and followed by lateral growth at the droplet surface. However, at wSDS = 100 ppm, it was observed that hydrate growth behavior greatly changed compared to pure water system. At wSDS = 100 ppm and ΔTsub < 6.0 K, the structure of the droplet was not maintained and hydrate crystals grew horizontally on to the stage. We defined the subcooling, ΔTsub, the difference between the system temperature and the equilibrium temperature of methane hydrate as driving force index for crystal growth. At wSDS = 100 ppm and ΔTsub ≥ 6.0 K, the hydrate grew perpendicularly to the stage. At wSDS = 100 ppm and each ΔTsub, the amount of hydrate produced increased compared to pure water system.

This paper reports the results of an experimental study conducted to measure the thermodynamics properties of ionic semiclathrate hydrate formed with tetrabutylammonium propionate (TBAPr). The thermodynamic measurements were performed on the equilibrium temperature and on the dissociation heat of the hydrate in the range of the mass fraction of tetrabutylammonium propionate wTBAPr from 0.10 to 0.43. The highest equilibrium temperature of the TBAPr system was 17.5 °C at wTBAPr = 0.37. The largest dissociation heat of TBAPr system was 193 kJ/kg at wTBAPr = 0.37. These findings indicate that TBAPr hydrate is promising for application in cool energy storage material.

The crystal structure and phase transition of cubic structureII (sII) binary clathrate hydrates of methane (CH4) and propanol are reported from powder X-ray diffraction measurements. The deformation of host water cages at the cubic-tetragonal phase transition of 2-propanol+CH4 hydrate, but not 1-propanol+CH4 hydrate, was observed below about 110K. It is shown that the deformation of the host water cages of 2-propanol+CH4 hydrate can be explained by the restriction of the motion of 2-propanol within the 51264 host water cages. This result provides a low-temperature structure due to a temperature-induced symmetry-lowering transition of clathrate hydrate. This is the first example of a cubic structure of the common clathrate hydrate families at a fixed composition.

This paper reports three-phase (ice + vapor + clathrate hydrate) equilibrium conditions in methane + water system at temperatures between 197.3 K and 238.7 K. These equilibrium condition data could be applied for utilization of methane clathrate hydrate as a natural gas transportation medium, a working medium for heat pump/power generation cycles, and to further our understanding of the environments of planets and satellites. The equilibrium conditions were measured using the isochoric procedure. The measured equilibrium pressures are from 111.7 kPa at 197.3 K to 794.1 kPa at 238.7 K and increase with an increase in temperature. The internal consistency of the measured equilibrium data are examined by applying the Clausius-Clapeyron equation. Discussion is given on the comparison of the equilibrium data measured in this study to those reported in the literature.

This paper reports the phase equilibrium temperature and dissociation heat of ionic semiclathrate hydrate formed with tetrabutylammonium butyrate (TBABu). The temperature - composition phase diagram of the TBABu hydrate was determined in the mass fraction from 0.10 to 0.44. The highest equilibrium temperature of TBABu hydrate was 15.4 °C in the mass fraction range from 0.35 to 0.37. On the dissociation heat of DSC measurements, a single heat flow peak of TBABu hydrate excluding ice melting peak was observed at all the mass fraction systems. The largest dissociation heat was 184.9 ± 3.1 kJ/kg at the mass fraction 0.36. TBABu hydrate could potentially be used as a thermal energy storage material.

When compressed with water or ice under high pressure and low temperature conditions, some gases form solid gas hydrate inclusion compounds which have higher melting points than ice under those pressures. In this work, we study the balance of the guest-water and water-water interaction energies that lead to the formation of the clathrate hydrate phases. In particular, molecular dynamics simulations with accurate water potentials are used to study the energetics of the formation of structure I (sI) and II (sII) clathrate hydrates of methane, ethane, and propane. The dissociation enthalpy of the clathrate hydrate phases, the encapsulation enthalpy of methane, ethane, and propane guests in the corresponding phases, and the average bonding enthalpy of water molecules are calculated and compared with accurate calorimetric measurements and previous classical and quantum mechanical calculations, when available. The encapsulation energies of methane, ethane, and propane guests stabilize the small and large sI and sII hydrate cages, with the larger molecules giving larger encapsulation energies. The average water-water interactions are weakened in the sI and sII phases compared to ice. The relative magnitudes of the van der Waals potential energy in ice and the hydrate phases are similar, but in the ice phase, the electrostatic interactions are stronger. The stabilizing guest-water “hydrophobic” interactions compensate for the weaker water-water interactions and stabilize the hydrate phases. A number of common assumptions regarding the guest-cage water interactions are used in the van der Waals-Platteeuw statistical mechanical theory to predict the clathrate hydrate phase stability under different pressure-temperature conditions. The present calculations show that some of these assumptions may not accurately reflect the physical nature of the interactions between guest molecules and the lattice waters.

The quadruple point in carbon dioxide hydrate forming system, ice + water-rich liquid + hydrate + carbon dioxide-rich gas four-phase equilibrium condition, has been determined based on supercooled water-rich liquid + hydrate + carbon dioxide-rich gas and water-rich liquid + hydrate + carbon dioxide-rich gas three-phase equilibrium conditions measured in this study combined with ice + hydrate + carbon dioxide-rich gas three-phase equilibrium conditions reported in the literature (K. Yasuda, R. Ohmura, J. Chem. Eng. Data 53 (2008) 2182-2188). The intersection of the three types of the three-phase equilibrium conditions was used to determine the quadruple point. The three-phase equilibrium condition experiments were performed at temperatures between 269.45 K and 275.75 K, and at pressures between 0.825 MPa and 1.708 MPa. The determined . p-T condition at quadruple point was . T = 271.6 ± 0.2 K and . p = 1.04 ± 0.02 MPa. It was also observed that the freezing of supercooled water during some experimental runs performed below the quadruple point temperature, which causes sudden increase in both pressure and temperature.

This paper presents visual observations aiming to clarify the underlying physics of surfactant effects on clathrate hydrate crystal growth at the interface between methane gas and water with surfactant. Sodium dodecyl sulfate (SDS), which is commonly used in industrial processes, was used in this study. Various SDS mass fractions from 0 to wSDS = 100 ppm with a step of size 10 ppm were examined, where wSDS represents the mass fraction of SDS aqueous solution. The crystal growth behavior and the crystal morphology of the methane hydrate at the interface varied depending on surfactant concentration and ΔTsub. In systems with wSDS ≤ 20 ppm, the nucleation occurred on the droplet surface (gas/liquid interface), then grew laterally, and finally covered the whole droplet surface. On the contrary, at wSDS ≥ 30 ppm, the droplet shape was not maintained and enhanced hydrate crystal growth was observed compared to those systems with wSDS ≤ 20 ppm. Individual hydrate crystals at wSDS = 20 ppm were observed to be smaller than those in a pure water system at a given ΔTsub, which is ascribed to the enhanced hydrate nucleation by the addition of SDS.

This paper reports experimental values of the interfacial tension between (CO2 + N2) gas and tetrabutylammonium bromide aqueous solution (TBABaq). The experiments were performed by the pendant drop method in conditions from P = (1.00 to 4.00) MPa, from T = (288.15 to 303.15) K and from wTBAB = (0.10 to 0.40), where wTBAB denotes the mass fraction of TBAB in the aqueous solution. The interfacial tension decreased with increasing wTBAB and pressure. The depletion of the interfacial tension with increasing wTBAB in the (CO2 + N2 + TBABaq) system was consistent with that in (CO2 + TBABaq) system. The depletion ratio of the pressure gradient of the interfacial tension caused by the addition of TBAB in (CO2 + N2 + water) system was almost the same as that in (CO2 + water) system. There might be little effect on the interfacial tension caused by the interaction between gases and tetrabutylammonium ions. The temperature dependence of γ was not observed as well as in the (CO2 + TBABaq) system.

Water versus fluorine: Clathrate hydrates encaging hydrofluorocarbons as guests show both isotropic and anisotropic distributions within host water cages, depending on the number of fluorine atoms in the guest molecule; this is caused by changes in intermolecular interactions to host water molecules in the hydrates (see figure).

Methane trapped in the two distinct dodecahedral cages of the ionic clathrate hydrate of TBAB was studied by single crystal XRD and MD simulation. The relative CH4 occupancies over the cage types were opposite to those of CO2, which illustrates the interplay between the cage symmetry and guest shape and dynamics, and thus the gas selectivity.

This paper reports the measurements of the interfacial tension in a water-methane system at temperatures from 278.15 K to 298.15 K and pressures up to 10 MPa. The measurements using the pendant-drop method were performed at various pressures lower than or, at the highest, nearly equal to the hydrate/liquid/gas equilibrium pressure corresponding to each of the following prescribed temperatures: 278.15 K, 283.15 K, 288.15 K, 293.15 K and 298.15 K. The obtained data show that at each temperature the interfacial tension decreases almost linearly with the increasing pressure, indicating that the observed decrease in the interfacial tension results from the promotion of methane adsorption onto the interface with the increasing pressure.

This study reports a visual observation of the formation and growth of ionic semiclathrate hydrate on the surface of a Tetrabutylammonium bromide (TBAB) aqueous solution and CO2+N2 gas mixture. The composition of CO2+N2 gas mixture was 20: 80. The experimental temperature range was from 280 K to 290 K, under the pressures of 2.3MPa and 4.7MPa, at wTBAB=0.10 and wTBAB=0.40, where wTBAB denotes the mass fraction of TBAB in the aqueous solution. At wTBAB=0.40, the hydrate crystals were initially observed to grow within the droplet, and followed by lateral growth at the droplet surface; but at wTBAB=0.10, the hydrate crystals grew exclusively in the liquid phase and did not cover the droplet surface. Two types of different crystals with different sizes were clearly observed.