Published papers

Application of CO2 clathrate hydrate as a constituent of solid carbonated foods is an emerging new idea to utilize the large gas-storage capacity of this material. The CO2 gas concentration of CO2 hydrate is 50 times higher than the CO2 concentration in carbonated water. To design an industrial process for making CO2 clathrate hydrates and to determine the texture of hydrate foodstuffs, understanding the crystal growth dynamics and crystal morphology is essential. This study reports the visual observations of crystal growth of CO2 hydrate in the presence of trehalose under conditions relevant to potential food production. To classify the observation results, the subcooling temperature ΔTsub defined as the difference between equilibrium hydrate formation temperature, Teq and the experimental temperature, Tex was used as an index of the driving force for the crystal growth. The experiments were performed at the ∆Tsub range of 0.8 K to 4.9 K. At any ΔTsub, the formation and growth of the crystals were observed at the interface between the CO2 gas phase and the trehalose solution. The hydrate crystals covered the interface and grew into the liquid phase. The crystal morphology significantly depends on ΔTsub. The shape of crystals was polygonal at ΔTsub < 2.5 K, columnar at ΔTsub ≈ 3.5 K, and dendritic at ΔTsub > 3.8 K. The effects of CO2 hydrate crystal morphology in the system with trehalose, on the dynamics of hydrate slurry and the texture of hydrate food are discussed.

Clathrate-hydrate-based desalination would surpass reverse osmosis membrane technology in terms of the water recovery ratio and energy consumption on an industrial scale. For engineering practice, comprehension of hydrate crystal growth and morphology is essential because these kinetics affect the efficiency of the hydrate formation and dewatering process. We measured the three-phase equilibrium conditions of CO2+ NaCl aqueous solution with a mass fraction of 0.20. The equilibrium temperature at 2.1 MPa was determined to be 265.9 K. We observed crystal growth and the morphology of the CO2hydrate in the same system at various subcooling temperatures under 2.1 MPa. Dendritic and polyhedral crystals were observed at large and small subcooling temperatures. The crystal growth rate was higher at large subcooling temperatures in the same system. At a given subcooling temperature, the growth rate in NaCl aqueous solution was obviously lower than in pure water under a given driving force of crystal growth. We discussed this difference in the aspects of viscosity, CO2solubility, and thermal conductivity. These observations indicate that both hydrate formation rate and dewatering efficiency depend on subcooling temperature and are in a trade-off relation. The results would facilitate the optimization of the operation conditions of hydrate-based desalination plants.

The eutectic crystallization of cyclopentane clathrate hydrate and solid calcium sulfate was performed to reduce the amount of calcium sulfate in concentrated seawater for the development of seawater desalination and salt production technology. The experiments were performed in terms of eutectic conditions and dynamics of eutectic crystallization. From the eutectic condition measurements, it was found that the eutectic temperature is 280.1 K, and the eutectic concentration corresponds to a mass fraction of 0.002 of calcium sulfate. In the eutectic crystallization experiments, the remaining unreacted solution was analyzed. An increase in the water-to-hydrate conversion ratio led to an increase in the reduction ratio of calcium sulfate; it was found that an average water-to-hydrate conversion ratio of 14 % yielded an average reduction ratio of calcium sulfate of 18 %. Based on these results, a conceptual design for a methodology was presented combining reverse osmosis, multi-stage clathrate hydrate-based seawater desalination, multi-stage clathrate hydrate-based seawater desalination with calcium sulfate reduction, and clathrate hydrate-based seawater desalination and salt production.

Natural hydrogen is generated via serpentinization, radiolysis and organic metagenesis in geological settings. After expulsion from the source, and along its upward migration path, the free gas may encounter hydrate-bearing sediments. To simulate this natural scenario, CH4 hydrate and CH4 + C3H8 hydrate were synthesized at 5.0 MPa and exposed to a hydrogen-containing gas mixture. In-situ Raman spectroscopic measurements demonstrated the incorporation of H2 molecules into the hydrate phase even at a partial pressure of 0.5 MPa. Ex-situ Raman spectroscopic characterization of hydrates formed from a CH4 + H2 gas mixture at 5.0 MPa confirmed the H2 inclusion within the large cavities of structure I. The results show that the interactions between H2 and the natural gas hydrate phase range from the incorporation of H2 molecules into the hydrate phase to the rapid dissociation of the gas hydrate, depending on thermodynamic conditions and H2 concentration in the coexisting gas phase.

Toward the widespread adoption of biomass energy for carbon neutral age, demand for biogas separation technologies that enables easier operation and maintenance is increasing. CH4 enrichment in biogas in solid structure II clathrate hydrate has recently attracted attention, yet the knowledge is limited to the batch-type operation, which is far from engineering practice. In the present study, we have conducted hydrate-based CH4 enrichment experiments with semi-batch and continuous separation methods in CH4 + CO2 + tetrahydropyran (THP) + water system, which are closer to applied-side operations. Semi-batch CH4/CO2 separation experiment with powder X-ray diffraction (PXRD) measurement revealed that CH4 is enriched in the hydrate phase with its mole fraction from 0.60 to 0.81 via the formation of structure II hydrate with THP. The continuous CH4/CO2 separation experiment showed that CH4 is concentrated in the hydrate slurry during the continuous formation process. The concentration of CH4 in the hydrate slurry reached 0.65 in mole fraction at the steady state at t = 24 h from 0.45 at t = 1h. The experimental results of time-dependent changes in the compositions in the gas phase as well as in the hydrate slurry exhibited that the long-term operation of hydrate-based CH4 enrichment was achieved and therefore could be implemented on a continuous industrial scale. The overall results demonstrated that the continuous CH4 enrichment in biogas is an applicable scheme utilizing structure II hydrate.

Clathrate-hydrate-based tritium separation from isotope water is a promising process for removing tritium that is not effectively separated by conventional methods. Clathrate hydrates (hereafter hydrates) are crystalline compounds composed of water and guest molecules. Hydrate-based tritium separation utilizes the property that heavy water (D2O) forms hydrates under milder temperatures than light water (H2O). Efficient industrial operation requires a guest compound that forms hydrates at high temperatures and low pressures and has a large difference in phase equilibrium temperature between H2O and D2O hydrates (ΔTDH). In this study, we measured the phase equilibrium conditions of D2O hydrates formed with HFC-134a, HFC-32, and HFC-23. The formation of D2O hydrates with these guests can be a route to tritium separation through co-precipitation of T2O. HFC-134a formed hydrates under the mildest conditions, with ΔTDH values of 2.8 K, 1.8 K, and 2.4 K for HFC-134a, HFC-32, and HFC-23. In addition to the three investigated guests, the potentials of propane, cyclopentane, and cyclopentane + CO₂ hydrate systems for hydrogen isotope separations were also compared, suggesting that HFC-134a and cyclopentane may be suitable guests for tritium separation. Present and previous studies have also shown a strong positive correlation between the hydration number and ΔTDH (correlation coefficient = 0.76). This trend may be ascribed to the fact that a higher proportion of water molecules in the hydrate amplifies the effect of replacing H2O with D2O. These results indicate that the equilibrium conditions of D₂O hydrates may be approximately predicted to identify suitable guests for tritium separation.

The thermal properties of clathrate hydrates are determined by the combination of water molecules and encapsulated types of guest molecules through the resulting crystal structure. Takaya et al. have reported that the 2-propanol + CH₄ hydrate possesses a tetragonal structure II’ (sII′) below 110 K. This finding shows that thermodynamic conditions (T, P) could change the crystal structural transition in the same chemical system. This study investigated crystallographic characteristics of the HFC-41 + 2-propanol hydrate and demonstrated that a single-atom fluorination stabilizes the sII’, replaced with methane in small cages. We have employed variable-temperature powder X-ray diffraction (PXRD) measurement, and the results show that the binary HFC-41 + 2-propanol hydrate exhibits canonical cubic sII hydrate with a lattice constant of a = 17.2259(5) Å at 168 K. Cooling down to 93 K, HFC-41 + 2-propanol hydrate transits to a tetragonal structure (sII’) from sII with lattice constants at=12.304(1) Å and ct=16.925(2) Å. The transition temperature from sII′ to sII is near 153 K, 40 K higher than that of the methane + 2-propanol hydrate. This result demonstrated that a single-atom fluorination stabilizes the sII’, which was replaced with methane in the small cage.

Electric vehicles (EVs) play key roles in realizing a sustainable society. Lithium-ion batteries (LIBs) implemented in EVs deteriorate due to significant temperature changes. To maintain LIBs in an appropriate temperature range, a competitive thermal energy storage (TES) medium is needed. Phase change materials (PCMs) that can utilize their latent heat have great potential. In this study, we focused on piperazine hexahydrate as a promising candidate for a PCM and investigated its thermodynamic properties that are essential for practical design. The equilibrium temperature and dissociation enthalpy were determined in the mass fraction range of 0.250 to 0.550. The results showed that piperazine hexahydrate had a maximum equilibrium temperature of 42.9 °C and a maximum dissociation enthalpy of 250.0 ± 3.7 kJ kg−1 at a mass fraction of 0.443, which is the congruent point of piperazine hexahydrate. Piperazine hexahydrate can be utilized as a PCM in the temperature range of 34.4 °C to 42.9 °C. By utilizing piperazine hexahydrate instead of a conventional water-cooling system, the volume of the TES tank may be reduced by 31% approximately. The crystal system of the piperazine hexahydrate prepared in the present study was determined to be pseudomonoclinic by powder X-ray diffraction (PXRD) measurements, which is the same as that reported in the literature.

CO2 separation from H2 + CO2 gas mixture is a necessary technology for a stable supply of blue hydrogen. Among CO2 separation technology, hydrate-based CO2 separation is an environmentally and economically superior technology. Cyclopentane (CP) was selected as guest compound to enable hydrate-based CO2 separation under thermodynamic conditions closer to normal pressure and temperature. Hydrate-based continuous CO2 separation experiments are conducted in the H2 + CO2 + CP + H2O system at 284 K. The results of these experiments showed that mole fraction of H2 in the gas phase reached 0.9 at the steady state in the H2 + CO2 + CP + H2O system after the CO2 was extracted via hydrate formation. At the same time, more H2 was incorporated into the mixed hydrate during this process as the concentration of H2 in the gas phase increases post hydrate formation. Therefore, it was demonstrated that hydrate-based CO2 separation was successful in H2 + CO2 + CP + H2O system at 284 K but there is also increase of energy costs caused by H2 loss. The PXRD results identified H2 + CO2 + CP hydrate as structure II hydrate. Although H2 + CO2 + Tetrahydropyran (THP) hydrate also forms structure II hydrate, more H2 is encapsulated in the H2 + CO2 + CP mixed hydrate than in the H2 + CO2 + THP mixed hydrate. This difference may be caused by the difference in the lattice constants of CP and THP hydrates, which arises from the difference in molecular size of CP and THP, from the differences between the large guest–host interactions, and from differences arising from hydrogen bonding of THP with the host water framework which can decrease the H2 holding capacity. In this study, it was revealed that the molecular diameter of the guest molecules need also to be considered in addition to the phase equilibrium conditions of the hydrates when selecting guest compound of hydrates.

Tritium separation technology is a key method for treating contaminated water from nuclear power plants. Recently, a novel tritium separation method using deuterium oxide (D2O)-based clathrate hydrate has attracted attention because it can reduce the tritium concentration in contaminated water from 4.77 × 105 Bq/kg to 1.55 × 103 Bq/kg. In this hydrate-based tritium separation method, tritium is concentrated on the surface of a D2O-based hydrate substrate. The mechanistic characteristics and formation kinetics of D2O-based hydrates need to be clarified to develop an efficient process for substrate formation. This study contributes to the process design by revealing the crystal growth of the CO2 + D2O hydrate. The crystal growth behavior of the hydrate formed in the CO2 + D2O system was experimentally observed under different subcooling temperatures (ΔTsub) at 3.0 MPa. At all ΔTsub, hydrate crystals were formed at the gas-liquid interface and extended along the interface. After the interface was covered, hydrate crystals grew in the liquid phase. The variation in the morphology of CO2 + D2O hydrate depended on ΔTsub. At ΔTsub = 1.6 K, polygonal crystals with side lengths of 0.2-0.6 mm were formed. At ΔTsub = 2.7 K, columnar crystals with a length of 1-2 mm were formed. At ΔTsub ≥ 3.3 K, dendritic crystals were formed. With an increase in ΔTsub, the size of the dendritic branches increased. The crystal growth dynamics and morphological tendency of the CO2 + D2O hydrate were comparable to those of the CO2 + H2O hydrate at fixed ΔTsub. Detailed theoretical discussions on the obtained results of surface kinetics, mass transfer, and heat transfer during hydrate formation from H2O and D2O were provided, considering the chemical potential, viscosity, and heat transfer of the liquids.

Lithium-ion batteries are required to equip the cooling system for high power performance, quick charge, and safety. Trimethylolethane (TME) hydrate is known as phase change material (PCM), a cooling medium that offers large dissociation heat and 30 °C form/dissociation temperature under atmospheric pressure. This study demonstrates the design of a practical cooling system with TME hydrate. This cooling method involves cooling outdoor air through water contact, exchanging heat between the cooled outdoor air and the aqueous solution of TME hydrate, exchanging heat from the resulting TME hydrate with a refrigerant, and finally supplying the processed refrigerant to the cooling LIB. Koyama et al. conducted a study on the physical properties of TME hydrate. They measured equilibrium temperatures and dissociation heats of TME mass fractions from 0.20 to 0.80. Their findings revealed that at a mass fraction of 0.60, the system reached its highest equilibrium temperature of 29.6 °C, and the dissociation heat peaked at 190.1 kJ/kg. This cooling system employed TME hydrate. These results underscore the potential of TME hydrates in energy-efficient cooling systems and demonstrate that they are suitable for lithium-ion battery cooling systems.

Clathrate-hydrate-based seawater desalination is a novel technology that is performed by reaction between seawater and guest compounds to obtain fresh water. This study proposed a dual functional methodology for seawater desalination and salt manufacture by using carbon dioxide clathrate hydrate. In this methodology, the clathrate hydrate formation continues until the concentration of electrolytes in unreacted brine reaches the saturated solubility for the salt precipitation. The visual observation of the clathrate hydrate formation and the salt precipitation coupled with the analysis utilizing the powder X-ray diffraction (PXRD) technique was implemented to demonstrate the methodology. The carbon dioxide clathrate hydrate formation was first performed with sodium chloride aqueous solution. The carbon dioxide clathrate hydrate crystals floated on high salinity water. This experiment was followed by the occurrence of the sodium chloride dihydrate precipitation upon the carbon dioxide clathrate hydrate formation. The formed compounds, carbon dioxide clathrate hydrate and sodium chloride dihydrate, were identified by PXRD analysis. The mass fraction of the compounds was also determined by PXRD.

CO2 hydrate is a remarkable candidate for novel solid carbonated foods because of its large gas capacity. This paper reports the effect of surfactants, essential additives in food production, on hydrate crystal growth by visually observing the CO2 hydrate crystal growth in aqueous fructose + sucrose fatty acid ester solution + gaseous CO2 system. Under all thermodynamic conditions, the surfactants affected the crystal growth of CO2 hydrates at liquid/gas interface, while not influencing growth in the liquid bulk phase. Porous hydrate crystals were formed at the liquid/CO2 interface and grew continuously along the reactor wall. The hydrate crystal morphology in liquid bulk phase was not affected by surfactants, changing from polyhedral and polygonal to dendritic shapes as temperature decreased. Discussion is provided on how the observation results can be specifically applied to intensification of hydrate formation process design and evaluation of hydrated food texture.

This study reports thermodynamic properties and crystal growth observations of D2O + cyclopentane hydrate toward the development of a hydrate-based tritium separation process. We employed D2O as a host of hydrate with respect to the investigation into the formation of D2O hydrate substrate, which is a core component of the tritium separation process. The hydrate phase equilibrium temperature in the liquid D2O + liquid cyclopentane system was 3.2°C higher than the corresponding H2O hydrate. Experiments on crystal growth were conducted at temperatures ranging from 5.1 to 8.5°C under atmospheric pressure. Under each thermodynamic condition, polygonal hydrate crystals appeared, growing along the D2O/cyclopentane interface. The geometric shape and size of the crystals varied depending on the temperature. The variation in crystal morphology was comparable to that of the H2O + cyclopentane hydrate. Implications based on the obtained results for the hydrate-based isotopic water separation process design are discussed.

The eutectic temperature, pressure, and concentration conditions of carbon dioxide clathrate hydrate and sodium chloride dihydrate were measured to advance clathrate hydrate-based seawater desalination and salt production technology that enables seawater desalination through zero liquid discharge. The measured eutectic conditions were within the temperature range of 256.2 K ≤ Teu ≤ 263.5 K, the pressure range of 0.91 MPa ≤ peu ≤ 2.70 MPa, and the concentration range, expressed as the mass fraction of sodium chloride, of 0.235 ≤ wm,eu ≤ 0.242. These eutectic conditions correspond to the four-phase equilibrium of a saturated sodium chloride aqueous solution, carbon dioxide clathrate hydrate, carbon dioxide gas, and sodium chloride dihydrate, as well as the five-phase equilibrium of a saturated sodium chloride aqueous solution, carbon dioxide clathrate hydrate, carbon dioxide gas, liquid carbon dioxide, and sodium chloride dihydrate. Based on the measured eutectic conditions, implications for implementing clathrate hydrate-based seawater desalination and salt production were presented, including a conceptual design for a methodology involving reverse osmosis, multi-stage clathrate hydrate-based seawater desalination, and clathrate hydrate-based seawater desalination and salt production.

This article describes the combustion behavior of combustible gases as they are released from the clathrate cages of a hydrate. Gas hydrates (clathrates) are ice-like crystalline solids that encapsulate guest gas molecules. It has become known that a significant methane storehouse is in the form of methane hydrates on the sea floor and in the arctic permafrost. There is great interest in this large fuel storehouse, particularly how to extract the methane from the clathrates. One of the unique features of methane clathrate is that it is flammable, despite being 85% water – fiery ice. While methane clathrates are the most prevalent in nature, other combustible gas hydrates (notably, propane and hydrogen) also have potential energy technology implications. In addition, carbon-dioxide hydrates have been proposed as a potential post-combustion greenhouse gas sequestration strategy, and there is a wide range of separation technologies and thermal management that take advantage of the unique thermodynamic and kinetic features of hydrate formation. To better understand the important implications of direct utilization of fuel clathrates and the related potential environmental consequences of CO2 hydrates, we describe the state-of-the-art knowledge regarding the formation and structure of gas hydrates, and the combustion behavior of flammable gas hydrates. The combustion studies involve determining the rate of ice melt and water evaporation during the hydrate burn, as well as the interesting phenomenon of self-healing, where the hydrates stop burning by forming an ice sheet on their surface. Experimental results are used to estimate the heat transfer from the flame into the hydrate and to calculate the amount of energy released to sustain the flame. This article provides the reader with a comprehensive understanding of the basics and the subtleties of hydrates and their combustion, thereby explaining the true meaning of fiery ice.

This work represents the crystal growth and morphology of Deuterium water (D2O) and HFC-134a clathrate hydrate toward enhancing the hydrate-based tritiated water separation method. The utilization of the method involves the use of D2O + HFC-134a hydrate as a substrate to capture tritiated water. It is imperative to have a comprehensive understanding of the morphology of each crystal and the precise temperature and pressure conditions required for their production, given that the substrate consists of individual crystals. Experiments were conducted within Δ (Formula presented.) range from 1.0 K to 7.8 K. The pressure was set at 0.305 to 0.398 MPa to avoid the liquefaction of HFC-134a. The results revealed that at Δ (Formula presented.) less than 5.2 K, the hydrate crystals were plates and polyhedrons with a size of 1.0–3.0 mm. When Δ (Formula presented.) exceeded 5.2 K, the crystals of columnar or dendritic morphologies were observed. They formed elongated shapes of 3.0 mm in length and 0.1 mm in width. Columnar or dendritic crystals are suitable for increasing surface area. In contrast plate and polyhedral crystals decrease the surface area of the substrate and reduce flow resistance. Selecting the favorable crystalline form to intensify the equipment and processes is essential.

Clathrate-hydrate-based H2 separation is superior to conventional separation methods in terms of energy consumption, but the criteria for selecting promoters that facilitate stable hydrate formation and gas separation are unclear. We conducted continuous and batch separation experiments in a CO2 + H2 + cyclopentanone (CP-one) system and powder X-ray diffraction (PXRD) measurements of the hydrate formed in this system. While the mole fraction of H2 in the gas phase increased from 0.60 to 0.90 in continuous separation, CO2 in the hydrate phase remained at 0.90. PXRD measurements revealed that CO2 + H2 + cyclopentanone hydrate possesses the structure II hydrate, suggesting CO2 and CP-one encapsulation in small and large cages of hydrate. Compared to previous studies using tetrahydropyran and cyclopentane as guest compounds, CP-one with high water solubility facilitated kinetics of the hydrate, encapsulation of CO2 in the hydrate and more stable separation. This study would provide a comprehensive understanding of selecting guest compounds for hydrate-based separation.

The plugging of natural gas pipelines caused by hydrate formation has been recognized as a problem in the oil and gas industry for approximately 90 years. Interfacial tension is an important physical property to understand the dynamics of multiphase flow, namely, natural gas, condensate, and water, inside the pipelines. This paper describes our measurements of the interfacial tension between water and pentane saturated with methane as a model for the fluids in the pipelines transporting gas and condensate. The measurements were conducted by the pendant drop method in the temperature range between 278.2 K and 293.2 K, and the methane pressure range between 1.1 MPa and 13.7 MPa. The uncertainty of the measurements is typically below 1.5 mN∙m−1. The interfacial tension decreased with increasing temperature at 2 MPa. As for the pressure dependence at 288.2 K, with increasing pressure, the interfacial tension decreased from 49.12 mN∙m−1 at 1.1 MPa to 47.64 mN∙m−1 at 4.0 MPa. The interfacial tension remained roughly constant until 8.7 MPa with the value being 48.02 mN∙m−1 and then increased to 59.81 mN∙m−1 at 12.9 MPa. The decrease of the interfacial tension between 1.1 MPa and 4.0 MPa could be attributed to the increase of the methane molecule adsorption at the interface. In the pressure range between 4.0 MPa and 8.7 MPa, it is likely that the methane adsorption at the water – pentane interface became saturated. The increase of the interfacial tension above 8.7 MPa is a novel phenomenon which requires further investigation.

Thermophysical properties such as phase equilibrium temperature and dissociation heat are important to determine the performance of phase change materials targeting thermal energy storage. The phase equilibrium temperature and dissociation heat of tetrabutylphosphoniumglutarate ionic semiclathrate hydrate were measured in this study. The highest phase equilibrium temperature of tetrabutylphosphoniumglutarate hydrate was experimentally determined 287.4 K at the mass fraction 0.347 and 0.357. The largest dissociation heat was 193.3 ± 1.6 kJ/kg at the mass fraction 0.357. The measurement results in this work show that the dissociation heat can be utilized in the suitable temperature range targeting general air conditioning and data center.