Published papers

To study the effect of hydrogen isotope substitution on the uptake of water during formation of clathrate hydrates, the harmonic intermolecular librational modes of selected water clusters (X2O)n with n = 2-6 and hydrogen isotopes X = H, D, and T are studied. The effects of the quantum mechanical zero-point energy (ZPE) in each cluster on the binding energies of the H2O, D2O, and T2O clusters are determined, with ZPE leading to the smallest binding energies in the H2O clusters and the largest binding energies in the T2O clusters. Corrections for anharmonicity of the librational modes are considered, and these bring the frequency ranges of the calculated intermolecular librational modes in the clusters to the experimental ranges of the librational modes in the infrared spectra of H2O and D2O solid ice and clathrate hydrate phases, and liquid H2O water. These calculations show the expected ranges of the binding energy of tritiated water onto a solid ice and clathrate hydrate surface and can help quantify the isotopic enrichment on a growing clathrate hydrate phase from the solution.

There has been growing interest in the use of hydrate-based technology for energy and environmental applications; however, structural characterization of some hydrate phases relevant to these applications has not been dealt with sufficiently in previous studies. This paper reports crystallographic characterization of hydrates formed in [CO2 + 1-propanol (propan-1-ol) + water] and [CO2 + 2-propanol (propan-2-ol) + water] systems with powder X-ray diffraction (PXRD) and X-ray computed tomography (CT) measurements. It was revealed that structure I CO2 hydrate and a non-crystalline phase involving 1-propanol are formed in CO2 + 1-propanol system. As for CO2 + 2-propanol system, crystalline structure of the formed hydrate differs depending on thermodynamic conditions; at 3.3 MPa, 274 K and at 2.2 MPa, 263 K, sI CO2 hydrate was formed, whereas at lower pressure and temperature, i.e., 0.5 MPa and 255 K, a new tetragonal structure outside the canonical hydrates was identified. Implications of using 1-propanol as a thermodynamic inhibitor and kinetic promoter of hydrate formation are discussed, which opens some possibilities into petroleum exploration/production applications and the development of hydrate-based refrigeration systems. The structural identification of tetragonal (sII′) CO2 + 2-propanol hydrate sheds new light on developing hydrate-based CCS process and novel skin care products.

Clathrate-hydrate-based CO2 capture from a post-combustion N2 + CO2 gas mixture has been experimentally investigated for decarbonization. In this study, we performed hydrate-based continuous CO2 separation experiments for N2 + CO2 + tetrahydropyran (THP) + water system. The experimental conditions were set at 280.1 K under 3.0 MPa with THP mass fractions wTHP of 0.22 and 0.11. For each experimental run, the time-dependent changes in compositions in the gas phase and the hydrate slurry were recorded hourly until the system reached a steady state. The cage occupancies of N2 and CO2 in the structure II 512 small cages were determined to be 43 and 24%, respectively, with the powder X-ray diffraction analysis. The obtained results showed that the continuous carbon capture is a practicable scheme, yet the CO2 concentration rate and CO2 recovery ratio are comparable to or lower than those of the pre-combustion systems reported in previous studies. Still, our investigation revealed that the development of an efficient hydrate formation method, such as the improvement of the geometry of mixing impellers, could enhance the CO2 recovery ratio for the continuous operation.

In a previous study, we have experimentally demonstrated the continuous formation of O3+O2+CO2 hydrate. Here, improved results for the O3+O2+CO2 hydrate formation are reported. By forming O3+O2+CO2 hydrate at a low temperature and higher O3 gas concentration in the gas phase, the O3 concentration in the hydrates was increased to be 0.41 mass %, which was 1.6 times higher than our past record. One-quarter reduction of the induction time for O3+O2+CO2 hydrate formation was also achieved by raising the initial volume ratio of CO2 in the feed gas in the range of 2–2.9 times that of our previous study. Also, the overall heat transfer coefficient during O3+O2+CO2 hydrate continuous formation was experimentally determined.

Continuous hydrate-based CO2 separation is a compelling technology both from an environmental and economical perspective. We developed a method to continuously capture and separate CO2 from a gas mixture with H2 and determined the time-dependent changes of compositions in the gas and hydrate containing aqueous phases. Tetrahydropyran (THP), a representative water-soluble large molecular guest compound for structure II (sII) clathrate hydrate, was added in the reactor. THP forms hydrates under moderate conditions. The effects of THP mass fraction in the solution, wTHP, on the gas separation was simultaneously investigated by using wTHP = 0.11 and the stoichiometric value for sII hydrate, wTHP = 0.22. The results showed the gas phase hydrogen concentration reached steady state at 0.92 mol fraction from 0.60 in the feed gas. Also, the results demonstrated that sII hydrate has the highest CO2 loading capacity compared to ionic semi-clathrate hydrate phases. A continuous hydrate-based gas separation with sII hydrate has yet to be investigated. This study provides new experimental evidence which proves that the sII hydrate is the best structure for hydrate-based gas separation.

The development of the eco-friendly technology generating energy will be a gradual requirement all over the world to replace thermal power releasing CO2 and nuclear power for which the wastes are disposed of with great difficultly. The hydrate-heat engine is a new cycle obtaining mechanical work from temperature differences that normally cannot generate energy, for example, heat differences between the top and bottom seawater. Although the hydrate cycle is a more sustainable power generation system than typically used Rankine cycle from the aspect of using environmental harmless working media, this new cycle has an issue as the low thermal efficiency. To increase the thermal efficiency, the performance of the hydrate-based heat engine utilizing structure II and H hydrates is theoretically evaluated for various models of low-grade heat sources. The results showed that the structure II and H hydrates formed by adding large guest molecule compounds to a second small guest gas system were effective in improving thermal efficiency. Especially, the double hydrate with Kr and cyclopentane enable the hydrate cycle to perform as efficiently as Rankine cycle, having the thermal efficiency around 3.0%, for the ocean thermal energy conversion in the temperature range from 282 K to 293 K.

The presence of small hydrocarbons is known to reduce the interfacial tension of the gas-water interface, and this phenomenon can affect the formation of the clathrate hydrates of these gases. In this work, the interfacial behavior of the pure methane-, ethane-, and propane-water, and the ternary 90:7:3 mol. % gas mixture of (methane + ethane + propane)-water were studied with molecular dynamics simulations. The interfacial tension, γ, and z-density profiles for the gases and water from simulations of the gas-water systems were determined at the temperatures of 275.15 and 298.15 K, and pressures up to 10 MPa for methane and up to near the experimental saturation pressures of ethane and propane. The goal is to accurately calculate the interfacial tension for the hydrocarbon/water systems and to analyze the molecular behaviors at the interfaces which lead to the observed trends. At the same hydrostatic gas phase pressure, propane, ethane, and methane reduce the gas-water interfacial tension in that order. The local density of the gas molecules at the interface is enhanced relative to the bulk gas, and it was determined that about 13%-20%, 33%-40%, and 54%-59% of the gas molecules in the simulation congregated at the interfaces for the CH 4 -, C 2 H 6 -, and C 3 H 8 -water systems, respectively, at the different simulated hydrostatic pressure ranges. For all gases in the pressure range studied, a complete monolayer of gas had not formed at the water interface. Furthermore, a dynamic equilibrium with fast exchange between molecules at the interface and in the gas phase was observed. For the gas mixture, deviations were observed between total calculated interfacial tension, γ mix , and the "ideal mixture" value, ςx i γ i,pure , calculated from the interfacial tensions of the pure gases, where x i is the mole fraction of each substance in the simulation. Some possible implications of the results on the mechanism of clathrate hydrate formation are discussed.

Interfacial tension is one of the most important physical properties for high-precision simulations to develop the methods of preventing plugging of pipelines in the oil and natural gas industry. This paper reports experimental data with the pendant drop method for the interfacial tension of a decane + methane + water system at temperatures between 278.2 K to 298.2 K and pressures up to 10 MPa. The data show that in this temperature range the interfacial tension in the decane + methane + water system decreases almost linearly with increasing temperature. The results also show that by increasing the pressure of methane, the interfacial tension decreases from 53.98 mN m−1 to 50.23 mN m−1 at 283.2 K and 52.23 mN m−1 to 49.74 mN m−1 at 288.2 K. The nature of the methane pressure dependence of the interfacial tension changes for pressures above around 2.00 MPa. The interfacial tension decreases with the pressure up to 2.00 MPa, but has no pressure dependence above 2.00 MPa. It may be inferred that the decane/water interface is saturated with methane at pressures around 2.00 MPa and at higher pressure the interfacial tension is no longer affected by the presence of methane.

CO2 hydrate is applicable to solid carbonated foods. The hydrate crystal morphology, which represents the crystal size and shape, is an important characteristic that changes the texture of foods. We report an observational study of the crystal growth of CO2 hydrate in aqueous fructose solution. The difference between the phase equilibrium temperature and the experimental temperature, ΔTsub, is applied as an index of the driving force. Experiments were performed at ΔTsub range of 0.9 K to 5.4 K. At all ΔTsub, initial crystal formed at the gas-solution interface and grew along the interface. After covering the interface, the crystals grew in the liquid phase The individual crystals were identified as polyhedral with facets (ΔTsub = 0.9 K), skeletal crystals (ΔTsub = 2.0 K) and dendrites (ΔTsub = 3.0 K and 5.4 K). Based on these results, the potential effect of gas hydrate morphology on texture of foods has been discussed.

Blockages in natural gas and oil pipelines can be caused by the aggregation of clathrate hydrate crystals which form at the oil-water-gas interfaces. Knowledge of interfacial tensions between components are crucial to understanding the interface physical chemistry and numerically simulating pipeline multiphase flow. This study measures the effect of the presence of natural gas components on the oil-water interfacial tension and its potential implications on flow assurance in oil-gas pipelines. Decane is used to mimic oil and is saturated with the gaseous mixture of methane, ethane, and propane as is consistent with simulations of multiphase dynamics. Water droplets were generated in decane and photographed through a window using pendant drop method in order to determine the oil-water interfacial tension. This study examined both mechanical and thermodynamic phenomena by considering interfacial tension under different pressure/temperature conditions covering the natural gas hydrate formation region. The effect of the gas mixture composition and pressure on the interfacial tension was determined at different temperatures in the range of pipeline operating conditions. The behavior was explained from both experimental and theoretical points of view. At lower pressures, the presence of the C2 and C3 natural gas components lowers the decane-water interfacial tension compared to the case of the decane-water-methane system. In particular, the pressure dependence of the interfacial tension was found to be almost five times higher than that in the absence of C2 and C3. By decreasing the interfacial tension, the presence of ethane and propane may facilitate the mixing of the water and oil phases in pipelines, therefore affecting the rate of hydrate formation.

Molecular dynamics simulations were performed to study the interfacial behavior of the pure carbon dioxide-water system and a binary 40:60 mol. % gas mixture of (carbon dioxide + methane)-water at the temperatures of 275.15 K and 298.15 K and pressures near 4 MPa for CO2 and up to 10 MPa for methane. The simulations are used to study the dynamic equilibrium of the gases at the water-gas interface, to determine the z-density profiles for the gases and water, and calculate the interfacial tension γ under the different temperature/pressure conditions close to those of the formation of clathrate hydrates of these gases. At the same hydrostatic gas phase pressure, the CO2-water interface has a lower interfacial tension than the CH4-water interface. A greater number of CO2 molecules, as much as three times more than methane at the same pressure, were adsorbed at the interfacial layer, which reflects the stronger electrostatic quadrupolar and van der Waals interactions between CO2 and water molecules at the interface. The water surfaces are covered by less than a monolayer of gas even when the pressure of the system goes near the saturation pressure of CO2. The surface adsorbed molecules are in dynamic equilibrium with the bulk gas and with exchange between the gas and interface regions occurring repeatedly within the timescale of the simulations. The effects of the changes in the CO2-water interfacial tension with external temperature and pressure conditions on the formation of the clathrate hydrates and other CO2 capture and sequestration processes are discussed.

Three phase equilibrium conditions of synthetic standard (seawater + carbon dioxide hydrate + carbon dioxide gas) were experimentally determined based on the batch, isochoric procedure. The total mass fraction of the electrolytes in the synthetic standard seawater, wss, was 0.0351, 0.0700, or 0.1036 that was composed of pure water and seven salts. The equilibrium pressure–temperature conditions were as follows: 271.95 K < T < 279.05 K and 1.298 MPa < p < 3.144 MPa in the system having wss = 0.0351, 272.25 K < T < 278.15 K and 1.530 MPa < p < 3.361 MPa in the wss = 0.0700 system and 271.15 K < T < 275.65 K and 1.643 MPa < p < 2.909 MPa in the wss = 0.1036 system. The quantitative evaluation of the changes in the equilibrium temperature at a fixed pressure was performed by applying the equilibrium conditions to the Clausius–Clapeyron equation. The results are essential to design the thermodynamic conditions for the clathrate hydrate-based seawater desalination technology.

Three phase equilibrium conditions of (sodium chloride aqueous solution + carbon dioxide hydrate + vapour carbon dioxide) were measured to establish seawater desalination technology using clathrate hydrates. The three phase equilibrium condition measurements were performed at temperatures between 271.85 K and 279.05 K, and at pressures between 1.283 MPa and 3.617 MPa. The concentrations of sodium chloride aqueous solution were 0.0350, 0.0700, 0.1000, and 0.1050 in mass fraction. It was found that the equilibrium temperature difference between pure water system and each sodium chloride concentration system at the same pressure was hardly influenced by the pressure level. In addition, it was found that the equilibrium temperature depression of carbon dioxide hydrate by the addition of sodium chloride is smaller than that of ice and cyclopentane hydrate especially at the high concentration of sodium chloride.

The phase equilibrium conditions of clathrate hydrate of argon were measured by the isochoric procedure at the temperatures from 197.6 K to 274.1 K. Argon hydrate would have industrial applications including a working medium of the hydrate-based heat pump systems as well as influence the analysis of the past climate change on the Earth. Nevertheless, the equilibrium conditions of argon hydrate were limited in the literatures and not reported below the temperature 264 K. The lowest and the highest measured equilibrium pressure is the pressure 0.825 MPa at the temperature 197.6 K and the pressure 9.881 MPa at the temperature 274.1 K. The equilibrium pressure increases with an increase in temperature. The reliability of the measured equilibrium conditions is supported by the internal consistency of those and the coincidence of the experimentally determined quadruple point (vapor + hydrate + ice + aqueous phases in equilibrium) temperature with the water freezing temperature theoretically predicted. The discussion is given on the comparison of the equilibrium data measured in this study and those reported in the literature.

This paper reports the pressure-temperature conditions for the three-phase aqueous liquid + hydrate + vapor equilibrium in the hydrate formed with water, difluoromethane and methylpiperdine as large molecule guest compound (LMGC). The aim of the study is to obtain a hydrate formed in mild pressure-temperature conditions which could be applied to various technologies. To measure the pressure-temperature phase equilibrium, isochoric method[1] was used. The pressure and temperature ranges of the present measurements are from 61.9 to 104.2 kPa and from 259.3 to 272.2 K. Obtained results were comparatively mild conditions than the phase equilibrium p-T condition of the structure I hydrates formed with water + difluoromethane. To alleviate the phase equilibrium condition, the LMGCs showed its significance through this study. In addition, the cage structure of formed hydrates was measured by powder X-ray diffraction measurements. The hydrate samples were made using the difluoromethane and N-methylpiperidine. The measurement results indicated the cage structure of hydrate as structure H. Thus, the molecular size of difluoromethane was confirmed as the suitable size for the small cages of structure H.

This paper reports the thermodynamic and crystallographic properties of the binary clathrate hydrate with methane + 1,1,1,3,3-pentafluoropropane (HFC-245fa) by phase equilibrium measurements, powder X-ray diffraction (PXRD) measurements, and first-principles and thermodynamic calculations. The four-phase (methane gas + water liquid + HFC-245fa liquid + hydrate) equilibrium conditions are lower in pressure by approximately 2−3 MPa than those of the methane + trans-1,3,3,3tetrafluoropropene [HFO-1234ze(E)] hydrates at each temperature from 274.5 to 282.17 K, in spite of the similar molecular structure of HFC-245fa and HFO-1234ze(E). The crystallographic structure of the hydrate with methane + HFC-245fa was identified to be structure II by the PXRD measurement; although because of its molecular length, HFC-245fa would be expected to be encapsulated into a 51268 cage of structure H hydrate. First-principles calculations revealed that these thermodynamic and crystallographic phenomena are the result of two physical factors. First, the total HFC-245fa−water interactions in the cages are stronger than the HFO-1234ze(E)−water interactions due to the presence of more fluorine atoms at the two edge groups in the HFC-245fa molecule and the resulting higher dipole moment of HFC-245fa, and second, the HFC-245fa−HFC-245fa interaction is more greatly enhanced by the presence of methane compared to the HFO-1234ze(E)−HFO-1234ze(E) interactions.

Movement towards the gradual replacement of gasoline combustion vehicles to electrical vehicles is occurring on a global scale. Heat management of lithium-ion batteries (LIB) is necessary for long-term operation and safety of the power sources of electrical vehicles. This research suggests pinacol hydrate as a phase change material (PCM) for use as an alternative cooling media for LIB. The pinacol hydrate crystallizes under atmospheric pressure and temperatures around 45 °C, which are compatible with LIB operating conditions. Its dissociation enthalpy can control the excessive heating of the battery. The equilibrium phase transition temperature of pinacol hydrate was reported about a decade ago, but its thermal storage capacity has not been reported yet and updated quantitative measurements are required. Hence, in this study the equilibrium temperatures and dissociation enthalpies are experimentally measured for pinacol aqueous solutions prepared with mass fractions between 0.40 and 0.85. The maximum thermal storage capacity is measured by differential scanning calorimetry to be 301.9 kJ/kg for mass fraction of 0.50 which is compatible with operating conditions of up to 45.6 °C. A minimization of reservoir tank mass by 35% of that needed for conventional technologies may be possible by using pinacol hydrate.

Simultaneous crystallization of cyclopentane hydrate and sodium chloride dihydrate was implemented to demonstrate a dual-functional system for hydrate-based seawater desalination and salt manufacture that could be performed with zero emission. A sodium chloride aqueous solution having an initial mass fraction of sodium chloride of 0.235 or 0.264 was used for the experiments as an alternative to seawater. When using the aqueous solution having a sodium chloride concentration of 0.264 in mass fraction, the precipitation of polyhedral sodium chloride dihydrate crystals was first observed, followed by the formation of particulate cyclopentane hydrate crystals. In contrast, when using the aqueous solution having a sodium chloride concentration of 0.235 in mass fraction, the formation of particulate cyclopentane hydrate was first observed and then the particulate sodium chloride dihydrate precipitated. The formed cyclopentane hydrate and the precipitated sodium chloride dihydrate were separated by the remaining sodium chloride aqueous solution due to the density difference even after the agitation of the system. The eutectic temperature and concentration were determined to be 259.75 K and 0.238 in mass fraction of sodium chloride, respectively.

We report the phase diagram and structure of the binary clathrate hydrate (abbreviated as hydrate) of methane + trans-1,3,3,3-tetrafluoropropene (HFO-1234ze(E)) guest molecules by phase equilibrium measurements, powder X-ray diffraction measurements, and molecular dynamics simulations. The four-phase (methane-rich gas + water + HFO-1234ze(E) liquid + solid clathrate hydrate) equilibrium data in the system were measured in the temperature range of 274.3 K to 280.2 K. At temperatures from 274.3 K to 276.6 K, the phase equilibrium pressures of the binary hydrate were lower, by a maximum of 0.27 MPa, than those in a pure methane structure I clathrate hydrate system three phase equilibrium line. This indicates that the hydrate formed from the binary mixture was different from a simple methane hydrate. Powder X-ray diffraction measurements were performed on the binary hydrate sample prepared at 274.3 K and pressures from 2.65 to 2.94 MPa. The X-ray diffraction measurements revealed that a structure II hydrate was formed in the system. This is somewhat surprising as the longest molecular dimension of HFO-1234ze(E) is similar to that of 2-methylbutane that forms a structure H hydrate. To clarify some of the aspects of the behavior of the binary hydrate, molecular dynamics simulations were performed. These calculations show that the HFO-1234ze(E) molecules do not form hydrogen bonds with the cage water molecules but are closely interacting with the cage water molecules via electrostatic and van der Waals interactions.

A tetrabutylammonium bromide (TBAB) hydrate is the only ionic semiclathrate hydrate that has been commercialized as a medium for efficient thermal energy storage targeting general air conditioning. However, TBAB is an environmental pollutant, while liquid water and ice that have been widely used as conventional media, also have difficulty in increasing the thermal energy storage density as well as the coefficient of performance (COP) of a refrigerator to form solid. In this study, a tetrabutylphosphonium phosphate ((TBP)3PO4) hydrate was proposed as an alternative green medium. The dissociation heat and the phase equilibrium temperature, which were both crucial factors to evaluate the thermal energy storage density, and COP was experimentally determined. It was found that the maximum dissociation heat was 154.1 ± 2.0 kJ・kg−1 at the mass fraction of 0.309. The highest phase equilibrium temperature was 10.0 °C at the mass fractions of 0.290, 0.299, and 0.309. The thermal energy storage density of (TBP)3PO4 hydrates was 3.7 times larger than that of water. (TBP)3PO4 hydrates increased the COP of a refrigerator to form solid by 42% as compared with ice, indicating the potential size-reduction of conventional devices with the decrease in the power consumption of a refrigerator. The effective balance between environmental protection and the system performance through (TBP)3PO4 hydrates was also discussed.