Published papers

Ionic semiclathrate hydrates mainly consist of water typically together with tetrabutylammonium and tetrabutylphosphonium salts. Since ionic semiclathrate hydrates have the large dissociation heat under ambient pressure and temperature conditions, various ionic semiclathrate hydrates have been studied as safety and eco-friendly phase change materials. In this study, tetrabutylammonium acetate hydrates and tetrabutylammonium hydroxide hydrates were proposed as thermal energy storage media for air conditioning and cooling lithium-ion batteries. The dissociation heat, which was a significant thermophysical property to design thermal energy storage systems, were measured at various mass fractions. The largest dissociation heats of tetrabutylammonium acetate hydrates and tetrabutylammonium hydroxide hydrates were 212.9 ± 0.9 kJ⋅kg−1 and 200.4 ± 2.2 kJ⋅kg−1. As a result of the comparison of the dissociation heats of tetrabutylammonium acetate hydrates and tetrabutylammonium hydroxide hydrates with those of other ionic semiclathrate hydrates, it was found that tetrabutylammonium acetate hydrates and tetrabutylammonium hydroxide hydrates had the promising thermophysical properties as thermal energy storage media for air conditioning and cooling lithium-ion batteries, respectively.

The interfacial behavior of tetrabutylammonium bromide (TBAB) aqueous solutions in the absence of gas and the presence of methane and carbon dioxide gases is studied by molecular dynamics simulations. The aqueous TBAB phase, at concentrations similar to the solid semiclathrate hydrate (1:38 mol ratio), has a smaller interfacial tension and an increase in the gas molecules adsorbed at the interface compared to that in pure water. Both these factors may contribute to facilitating the uptake of the gases into the solid phase during the process of semiclathrate hydrate formation. At similar gas pressures, CO2 is adsorbed preferentially compared to CH4, giving it a higher surface density, due to the stronger intermolecular interactions of CO2 molecules of the solution at the interface. The increase in relative adsorption of CH4 at the solution surface compared to that in pure water surface is due to the hydrophobic interactions between the n-alkyl chains of the TBA+ cation and methane gas.

This paper reports the phase equilibrium and crystallographic data of the hydrate formed in the CO2 + cyclohexanone + water system. We measured the phase equilibrium condition and conducted the powder X-ray diffraction measurements. The formation of the structure II hydrate in the system of CO2 + cyclohexanone + water was observed at the temperature from 270.0 K to 275.6 K, under the pressure from 0.62 MPa to 1.70 MPa. At 270.0 K – 275.6 K and 0.62 MPa – 1.70 MPa, the structure II hydrate formed, and the phase equilibrium condition alleviated in the system of CO2 + cyclohexanone + water, while the hydrate formed at 276.5 K - 280.7 K and 2.02 MPa – 3.33 MPa, displayed structure I and cyclohexanone acted as an inhibitor. Equilibrium conditions were measured using the isochoric method, revealing different p-T slopes at high and low temperatures. The high-temperature slope closely resembled that of structure I CO2 hydrate reported previously. We continuously monitored temperature and pressure during the structural phase transition, resulting in a slope change. To directly determine the hydrate structure, PXRD measurements were conducted on two samples: one from the high-temperature side and the other from the low-temperature side. The sample from the high-temperature side exhibited structure I with a lattice constant of 11.8749 (9) Å at 153 K, while the low-temperature sample displayed structure II with a lattice constant of 17.443(1) Å at 153 K.

The measurements of thermophysical and crystalline properties were conducted to propose the use of binary structure H (sH) hydrates of 1-methylpiperidine for carbon capture and storage (CCS). 1-Methylpiperidine is one of the most effective large guest compounds to relax the phase equilibrium conditions of binary sH clathrate hydrates. To reduce energy consumption in hydrate-based CCS, the hydrates should be formed as close to room temperature and ambient atmospheric pressure as possible. Phase equilibrium conditions of hydrates formed in the N2 + 1-methylpiperidine + water, CO2 + 1-methylpiperidine + water, and N2 + CO2+ 1-methylpiperidine + water systems were measured. Powder X-ray diffraction and gas composition analyses were also conducted. In the N2 + 1-methylpiperidine + water system, the binary structure H hydrate was formed under milder phase equilibrium conditions with higher temperature and lower pressure values, as compared to those of N2 + neohexane hydrate and pure N2 hydrate. In the CO2 + 1-methylpiperidine + water and N2 + CO2 + 1-methylpiperidine + water systems, structure I (sI) hydrates were formed. This is attributed to the chemical reaction between CO2, 1-methylpiperidine, and water in these systems, which prevents the sH hydrate formation. The composition of N2 + CO2 gas encapsulated into the binary sI hydrate was N2:CO2 = 0.20:0.80. CO2 capture was successful with the sI hydrate.

The four-phase equilibrium conditions were measured in carbon dioxide + cyclopentane double clathrate hydrate forming systems coexisting with either sodium chloride aqueous solution or pure water as a technological basis for developing seawater desalination technology utilizing clathrate hydrate formation and decomposition. The equilibrated phases included sodium chloride aqueous solution or pure water, carbon dioxide + cyclopentane double clathrate hydrate, liquid cyclopentane, and carbon dioxide gas. The experiments were performed using the batch isochoric procedure. Three different concentrations of sodium chloride in the aqueous solution were tested: 0.035, 0.070, or 0.105 in mass fraction. Pressures ranged from 0.385 MPa to 2.488 MPa. The results were compared with the equilibrium conditions for carbon dioxide and cyclopentane simple clathrate hydrate forming systems. The comparison revealed that the double clathrate hydrate is more thermodynamically stable than the simple clathrate hydrates, under a given sodium chloride concentration in the aqueous solution.

The equilibrium condition measurements were performed in the cyclopentane clathrate hydrate forming systems coexisting with sodium chloride as a technological basis for the development of seawater desalination technology using clathrate hydrates. The experiments were performed in the system under vacuum condition in addition to the system under atmospheric pressure. The equilibrium conditions in the system under the vacuum condition are essential to accurately construct the phase diagram in the cyclopentane clathrate hydrate forming system without the effects of the surrounding air. The equilibrated phases were sodium chloride aqueous solution + cyclopentane clathrate hydrate + liquid cyclopentane + vapor cyclopentane in the system under the vacuum condition. The concentration of sodium chloride investigated in this study, wNaCl, was 0 ≤ wNaCl ≤ 0.210 in mass fraction. The equilibrium temperature in both systems decreased with increasing wNaCl. The equilibrium temperature at a fixed concentration of sodium chloride under the vacuum condition was lower than that under the atmospheric pressure. It was attributed to the compression by the atmospheric pressure in addition to the incorporation of molecules constituting the atmospheric air.

This paper reports pressure-temperature conditions for liquid + hydrate + vapor phase equilibrium of hydrate formed with deuterium oxide (D2O), CO2 and cyclopentane. The aim of this study is to obtain the equilibrium properties of the formed hydrate for the application to hydrate-based tritium separation process. Measurements with batch isochoric method were performed. The pressure and temperature ranges of measurement were from 1.63 MPa to 2.46 MPa and from 290.6 K to 293.4 K. The obtained results indicate that D2O + CO2+ cyclopentane hydrate is formed in milder conditions than H2O + CO2+ cyclopentane hydrate by 2.1 K at the given pressure conditions.

Salt lakes are a rich source of metals used in various fields. Rubidium is found in small amounts in salt lakes, but extraction technology on an industrial scale has not been developed completely. Clathrate hydrates are crystalline compounds formed by the encapsulation of guest molecules in cage-like structures made of water molecules. One of the most important properties for engineering practices of hydrate-based technologies is the comprehension of the phase equilibrium conditions. Phase equilibrium conditions of CO2 hydrate in rubidium chloride aqueous solution with mass fractions of 0.05, 0.10, 0.15 and 0.20 were experimentally investigated in the pressure range from 1.27 MPa to 3.53 MPa, and the temperature was from 268.7 K to 280.6 K. The measured equilibrium temperature in this study decreased roughly in proportion to the concentration of the RbCl solution from the pure water system. This depression is due to the lowering of the chemical potential of water in the liquid phase by the dissolution of RbCl. Experimental results compared with other salt solution + CO2 hydrate systems showed that the equilibrium temperatures decreased to a similar degree for similar mole fractions.

This paper investigates the phase equilibrium conditions in the carbon dioxide hydrate forming system. Carbon dioxide hydrate can be utilized for carbon capture, salt manufacture, carbonated solid foods and tritium water concentration, so the phase equilibrium conditions have been substantially reported so far. However, the data from previous studies were inconsistent with each other, such as there is a difference of 1.0 K in the phase equilibrium temperature at 2 MPa. In this study, the newly three-phase (water rich liquid + hydrate + guest rich vapor) equilibrium conditions in the carbon dioxide hydrate forming system were measured at twenty different temperature conditions within the range of (271.9-282.7) K in the two different laboratories. The six pairs of the three-phase equilibrium condition data measured under equivalent pressure conditions were consistent within mutual uncertainties. The internal consistency of the data measured in this study was evaluated by the Clausius-Clapeyron equation. The data measured in this study existed within the uncertainty range of the data from several previous studies.

In this study, the effect of tetrabutylammonium halide aqueous solutions on the gas storage of CH4 and CO2 gases were studied with molecular dynamics (MD) simulations. The results show that the surface tension and the gas molecules adsorbed at the interface decreases and increases, respectively, in the presence of TBAX (X: Br, Cl, F) in the aqueous phase compared to pure water at similar gas pressures. Both of these factors may facilitate gas uptake into cages during semi-clathrate hydrate formation. CO2 showed a stronger intermolecular interaction with the water molecules since it was preferentially adsorbed at the interface, leading to a higher surface density as compared to CH4. Moreover, the relative increase in CH4 adsorption on the surface was because of the hydrophobic interactions between the CH4 molecules and the n-alkyl chains of the cation. The counter-ions of TBAXs can affect their surface activity. TBAX salts enhance the tetrahedral ordering of water molecules at the interface compared to the bulk, leading to a potential mechanism for forming semi-clathrate hydrates.

Aqueous solutions of simple ionic salts such as NaCl and NaBr have higher surface tensions compared to that of pure water and the surface tension of the solution increases with the increasing concentration of the salt. In contrast, the aqueous solutions of the ion sodium acetate (NaAc), which has an anion with both hydrophilic and hydrophobic parts, have a surface tension less than that of pure water, with the surface tension decreasing with salt concentration. The presence of large cations in the salt may change these trends. In this work, the interfacial tension of aqueous solutions of the organic ionic salt tetrabutylammonium acetate (TBAAc) is measured and compared with that of tetrabutylammonium bromide (TBAB). The surface tension of TBAAc aqueous solutions with mole fractions ranging from 0.00417 to 0.04191 were measured for temperatures ranging from 288.2 to 308.2 K and compared with the surface tension of TBAB solutions at similar concentrations. The experimental results showed that the interfacial tensions of TBAAc and TBAB aqueous solutions decreased with increasing concentration, leveling at around 0.04 mol fraction. At each concentration, the interfacial tensions of the TBAAc solutions were larger than those of the TBAB solutions. For the TBAAc and TBAB solutions, the surface excess concentrations of the salts in the solutions are calculated. The impact of these results structures and properties of these solutions are discussed.

Ionic semiclathrate hydrates, the water-based phase change materials (PCMs) have potential for thermal energy storage media. There are a number of hydrates, reported in the literature, which are suitable as PCMs for air conditioning system of the required temperature range 10–15 °C. In this study, Tetrabutylammonium picolinate hydrate (TBAPi hydrate) was newly synthesized as a PCM for the cold chain, which has a lower phase equilibrium temperature than the previously reported hydrates. Thermophysical properties of TBAPi hydrate were experimentally measured. The hydrate dissociation heat was determined using a differential scanning calorimeter (DSC). The phase equilibrium temperature was measured by visually observing system with a CMOS camera and microscope. Also, the cycling test was conducted to confirm the long-term stability of thermophysical properties. The largest dissociation heat was 157.7 ± 4.9 kJ・kg−1 at a mass fraction of TBAPi aqueous solution, wTBAPi of 0.37. The highest phase equilibrium temperature of TBAPi hydrate was 5.5 °C at 0.34 ≤ wTBAPi ≤ 0.36. These thermophysical properties did not change even after 200 cycles of hydrate formation and dissociation. Based on the experimental results, TBAPi hydrate can be applied to the transport and storage of vaccines, blood packs, and perishable foods.

We report a thermally induced phase transition of cubic structure II hydrates of tetrahydropyran (THP) and CO2 below about 140 K. The phase transition was characterized by powder X-ray diffraction measurements at variable temperatures. A dynamical ordering of the CO2 guests in small pentagonal dodecahedral 512 host water cages, not previously observed in the simple CO2 hydrate, occurs simultaneously with the symmetry lowering transition from a cubic structure II (space group Fd-3m with cell dimensions a = 17.3202(7) Å at 153 K) to a tetragonal (space group I41/amd with cell dimensions a = 17.484(4) Å and c = 12.145(1) Å at 138 K) unit cell. The effect of guest molecules on the phase transition at low temperatures is discussed, which demonstrates that the clathrate hydrate structures and thermodynamic properties can be modified by adjusting the size and chemical structure of larger and smaller guest molecules.

This paper reports characterization of clathrate hydrate formed in CO2 + cyclopentane + deuterium oxide (D2O) system with the phase equilibrium determination and the powder X-ray diffraction (PXRD) measurements towards the development of hydrate-based tritium separation technology. Four-phase (CO2 gas + liquid cyclopentane + liquid D2O + hydrate) equilibrium conditions were measured using the isochoric procedure in the pressure range from 0.279 MPa to 2.735 MPa and the temperature range from 285.35 K to 293.65 K. The equilibrium temperature of CO2 + cyclopentane + D2O system is always higher by 2.0 K to 2.4 K compared to that of CO2 + cyclopentane + H2O system. The detailed crystallographic properties of both CO2 + cyclopentane + D2O hydrate and CO2 + cyclopentane + H2O hydrate including the lattice constants, the radii and volumes of each cage and the cage occupancy were estimated by the PXRD analysis. Based on the obtained results, implications for the development and operation of D2O-hydrate-based process are discussed.

Thermal energy storage for air conditioning is increasingly becoming important because the fluctuating output of renewable energy and the growing cooling demand will need to be addressed. In this study, we proposed tetrabutylphosphonium malonate hydrate as an environment friendly thermal energy storage medium for air conditioning. Equilibrium temperature and dissociation heat, which were significant thermophysical properties for commercialization, were experimentally determined at three mass fractions of tetrabutylphosphonium malonate aqueous solution. The maximum value of equilibrium temperature was determined to be 8.9 °C. The largest dissociation heat was measured to be 148.1 kJ/kg. These results indicate that tetrabutylphosphonium malonate hydrate has appropriate thermophysical properties for a thermal energy storage medium.

Thermophysical properties relevant to clathrate hydrate-based technologies were reviewed. Clathrate hydrates are solids composed of water and guests. The clathrate hydrate-based technologies considered in this study were as follows: carbon capture, utilization, and sequestration; natural gas storage and transportation; ozone storage and transportation; carbon dioxide clathrate hydrate as food; desalination and salt production; separation of tritiated water; cold thermal energy storage; and heat pumps and heat engines. The review was based on the experimentally measured data. The reviewed thermophysical properties were phase equilibrium conditions, formation/decomposition enthalpy, heat capacity, thermal conductivity, interfacial tension, and density. The phase equilibrium conditions determine the operating conditions for the clathrate hydrate-based technologies. The formation/decomposition enthalpy, heat capacity, and thermal conductivity relate to the thermal energy exchange during hydrate formation/decomposition. The interfacial tension is a key parameter when considering the multiphase flow composed of water and guests. The density influences the behavior of clathrate hydrates within the reactor. The relevance between these properties and the clathrate hydrate-based technologies was discussed. The methods correlating the phase equilibrium conditions were also compared in terms of applicability and usefulness. It was revealed that the suitability of the model, which is based on the Clausius–Clapeyron equation or statistical thermodynamic modeling, depends on the purpose of the correlation. Future perspectives of the thermophysical properties of clathrate hydrates were also discussed.

Ionic semiclathrate hydrate is a crystalline compound mainly composed of water, which can dissociate at ordinary pressure and temperature. Ionic semiclathrate hydrates have been widely studied to develop prospective green and safety phase change materials utilized in a thermal energy storage system to cool batteries, in air conditioning, and in a cold chain system. The thermophysical properties, such as equilibrium temperature and dissociation heat, and the mass fraction dependence of such properties need to be carefully investigated to design such systems. In this study, the equilibrium temperature and the dissociation heat of tetrabutylphosphonium malonate ionic semiclathrate hydrates were experimentally determined at various mass fractions through an established method: the maximum value of the dissociation heat was 179.70 kJ·kg−1 at w = 0.357 and the equilibrium temperature was 8.9 °C at w = 0.357. By comparing tetrabutylphosphonium malonate ionic semiclathrate hydrates with other media in terms of the measured equilibrium temperature and the dissociation heat, the usability of the hydrates as thermal energy storage media for general air conditioning was discussed.

A separation method of tritium has been developed based on co-precipitation of tritiated water on porous heavy water (D2O) based clathrate hydrate. The tritium was semi-continuously removed by a circulating flow of tritium-containing light water through the D2O hydrate media crystallized in advance. The tritium concentration of 4.77 × 105 Bq/kg was reduced to 1.55 × 103 Bq/kg with HFC-134a hydrate at 0.35 MPa for 1 h, while tritiated water treatment at a concentration of 1010 Bq/kg or greater is eligible with the conventional detritiation methods. Near the D2O hydrate surfaces, solid solutions with higher concentrations of tritiated water were deposited on the hydrate substrate than those in the liquid phase. The semi-serialized operation of hydrate based detritiation is capable of high-rate processing as well as energy and resource saving. This method could be a candidate treatment for huge quantities of water with low concentration tritium discharged from nuclear fusion power generation system.

The unusual behavior of a new class of compounds that selectively function as either promoters or inhibitors of gas hydrate formation depending on the process conditions has made their exploration increasingly appealing. In this study, polyethylenimine (PEI) exhibits desirable functionality that may be a candidate for environmentally benign compounds. The present work investigated PEI's thermodynamic and kinetic influence on the formation of CH4 and CO2 hydrates for two PEI concentrations (0.5 and 5.0 mass%) and two PEI molecular weights (25,000 and 750,000 g/mol). The isochoric method was applied to measure the hydrate-liquid water–vapor-three-phase-coexisting (H-LW-V-3PC) phase boundary. PEI is a thermodynamic inhibitor regardless of CH4 or CO2 as a hydrate former, and 0.5 mass% PEI system exhibits the highest average suppression temperatures of 0.25 and 0.62 K, respectively. Kinetic findings, such as induction time, initial hydrate formation rate, and nucleation rate, on the other hand, revealed that PEI could act differently in response to specific hydrate formers. PEI kinetically inhibits CH4 hydrates formation but promotes the formation of CO2 hydrates. This promotion effect is attributed to PEI's high CO2 absorption ability to enhance the solubility of CO2. This unique behavior of PEI presents appealing advantages in the natural gas hydrate exploitation field due to their potential for greater operational flexibility and minimal environmental impact.

Seawater desalination via the crystallization using clathrate hydrate is a promising technology for displacing conventional desalination processes, yet the technology immaturity and poor cost performance are the issues to be resolved. In this study, conceptual designs of hydrate-based dual functional plant (desalination and salt production) utilizing the eutectic crystallization of hydrate and NaCl dihydrate have been presented on an industrial scale with the capacity of 24 m3/day. The conceptual design proposed in this study focuses on detailed and executive aspects, including the size of individual reactors, the selection of pumps, and the setting of operating temperature and pressure conditions. The process designs help meet these present demands on hydrate-based desalination and conform to the global standards of minimizing energy requirement and the disposal of environmental pollutants. Hydrate-based plant was designed as a novel Zero Liquid Discharge (ZLD) process to produce potable water and salt simultaneously, with the recovery ratio of pure water from seawater achieving 95%. The design-based cost estimation showed that the annual operating cost is estimated US$2.29/m3 for cyclopentane-hydrate-based plant and US$3.64/m3 for CO2-hydrate-based plant. The results suggest that the hydrate-based plants will bring gross profit comparable to the reported ZLD plants; that is, the gross profit will achieve US$20 per 1 m3 of water production. In addition to these two improvements over the existing plants, the hydrate-based process designs also exhibit favorable energy requirement compared to the eutectic system of ice. This design-based study may contribute to the promotion of additional construction of hydrate-based process unit to the existing reverse osmosis plant or even the new construction of hydrate-based desalination plant worldwide.