研究者詳細 - 大村　亮

論文 - 大村　亮

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Crystal growth of clathrate hydrate formed with H<inf>2</inf> + CO<inf>2</inf> mixed gas and tetrahydropyran

Maruyama M., Matsuura R., Ohmura R.

Scientific Reports （Scientific Reports） 11 （ 1 ） 2021年12月

Hydrate-based gas separation technology is applicable to the CO2 capture and storage from synthesis gas mixture generated through gasification of fuel sources including biomass. This paper reports visual observations of crystal growth dynamics and crystal morphology of hydrate formed in the H2 + CO2 + tetrahydropyran (THP) + water system with a target for developing the hydrate-based CO2 separation process design. Experiments were conducted at a temperature range of 279.5–284.9 K under the pressure of 4.9–5.3 MPa. To simulate the synthesis gas, gas composition in the gas phase was maintained around H2:CO2 = 0.6:0.4 in mole fraction. Hydrate crystals were formed and extended along the THP/water interface. After the complete coverage of the interface to shape a polycrystalline shell, hydrate crystals continued to grow further into the bulk of liquid water. The individual crystals were identified as hexagonal, tetragonal and other polygonal-shaped formations. The crystal growth rate and the crystal size varied depending on thermodynamic conditions. Implications from the obtained results for the arrangement of operating conditions at the hydrate formation-, transportation-, and dissociation processes are discussed.

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Measurement and analysis of interfacial tension of decane/water system pressurized with methane + ethane + propane gas mixture

Kiyokawa H., Kodera M., Lassiège M., Alavi S., Ohmura R.

Journal of Natural Gas Science and Engineering （Journal of Natural Gas Science and Engineering） 96 2021年12月

ISSN 18755100

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.

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Improved Operation of Continuous Ozone Hydrate Production

Hatsugai T., Kiyokawa H., Takeya S., Ohmura R.

Chemical Engineering and Technology （Chemical Engineering and Technology） 44 （ 9 ） 1677 - 1685 2021年09月

ISSN 09307516

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.

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Simultaneous Crystallization of Cyclopentane Hydrate and Sodium Chloride for Desalination and Salt Manufacture

Tanaka M., Tsugane K., Suga D., Tomura S., Ohmura R., Yasuda K.

ACS Sustainable Chemistry and Engineering （ACS Sustainable Chemistry and Engineering） 9 （ 27 ） 9078 - 9084 2021年07月

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.

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Characterization of clathrate hydrate formed in H<inf>2</inf> + CO<inf>2</inf> + tetrahydropyran + water system as carbon capture materials

Kodera M., Takeya S., Lassiège M., Alavi S., Ohmura R.

Fuel （Fuel） 295 2021年07月

ISSN 00162361

This paper reports the thermodynamic and crystallographic characterization of the clathrate hydrate formed in H2 + CO2 + tetrahydropyran (THP) + water system by phase equilibrium measurements and powder X-ray diffraction measurements with a goal of developing a hydrate-based carbon capture technology in an integrated coal gasification combined cycle. The four-phase [(H2 + CO2) gas + water liquid + THP liquid + hydrate] equilibrium conditions were measured with two series of gas compositions (around H2:CO2 = 0.7:0.3 and 0.9:0.1 in mole fractions) in the range of 2.64 MPa to 7.51 MPa. At around H2:CO2 = 0.7:0.3 in gas phase, the equilibrium temperatures in (H2 + CO2) gas + THP + water system are always higher by approximately 9 to 10 K compared to the equilibrium temperatures of H2 + CO2 + water system. This means that THP acts as the hydrate thermodynamic promoter. The equilibrium conditions in (H2 + CO2) gas + THP + water system, which is only considered with CO2 partial pressure, greatly agree with those in CO2 + THP + water system. Therefore, the equilibrium conditions in (H2 + CO2) gas + THP + water system would only depend on the partial pressure of CO2. Moreover, the crystallographic structure of the hydrate formed in H2 + CO2 + THP + water system was identified to be structure II, which is the same structure as the hydrate formed in H2 + CO2 + water system. These results can contribute to the study of a hydrate-based gas separation and carbon capture.

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Thermal energy storage performance of tetrabutylammonium acrylate hydrate as phase change materials

Kiyokawa H., Tokutomi H., Ishida S., Nishi H., Ohmura R.

Applied Sciences (Switzerland) （Applied Sciences (Switzerland)） 11 （ 11 ） 2021年06月

Kinetic characteristics of thermal energy storage (TES) using tetrabutylammonium acrylate (TBAAc) hydrate were experimentally evaluated for practical use as PCMs. Mechanical agitation or ultrasonic vibration was added to detach the hydrate adhesion on the heat exchanger, which could be a thermal resistance. The effect of the external forces also was evaluated by changing their rotation rate and frequency. When the agitation rate was 600 rpm, the system achieved TES density of 140 MJ/m3 in 2.9 hours. This value is comparable to the ideal performance of ice TES when its solid phase fraction is 45%. UA/V (U: thermal transfer coefficient, A: surface area of the heat ex-change coil, V: volume of the TES medium) is known as an index of the ease of heat transfer in a heat exchanger. UA/V obtained in this study was comparable to that of other common heat exchang-ers, which means the equivalent performance would be available by setting the similar UA/V. In this study, we succeeded in obtaining practical data for heat storage by TBAAc hydrate. The data obtained in this study will be a great help for the practical application of hydrate heat storage in the future.

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Thermodynamic analysis of hydrate-based refrigeration cycle

Matsuura R., Watanabe K., Yamauchi Y., Sato H., Chen L.J., Ohmura R.

Energy （Energy） 220 2021年04月

ISSN 03605442

Clathrate hydrates (hydrates) have a larger dissociation heat than an evaporation heat of the working fluid of conventional refrigeration systems. This property can be applied to a novel refrigeration system. In this study, theoretical performance analysis of the refrigeration system utilizing hydrates as its working medium was conducted. We modeled the thermodynamic cycle of the hydrate-based refrigeration system composed of following processes: adiabatic compression, hydrate formation at high temperature, adiabatic expansion, and hydrate dissociation at low temperature. Based on the thermodynamic cycle, the coefficient of performance (COP) of the hydrate cycle was theoretically formulated with thermodynamic state functions of the working medium. Using the formula, COP was calculated on the three hydrate forming systems including HFC-32 + cyclopentane (CP) + water, Kr + CP + water, and HFC-41 + CP + water. The analysis based on the calculated results revealed that the dissociation heat of hydrates and the enthalpy change of guest gas were dominant factors to COP and polyatomic molecules would be appropriate for guest gas of hydrates. The maximum COP values on the hydrate cycle were comparable to those of the reversed Rankine cycle. The hydrate-based refrigeration system outperformed conventional refrigeration systems in terms of safety and environmental-friendliness.

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Crystal Growth of Structure-H Hydrate with Water-Soluble Large Molecule Guest Compound: 1-Methylpiperidine as a Case Study

Matsuura R., Alavi S., Ohmura R.

Crystal Growth and Design （Crystal Growth and Design） 21 （ 2 ） 1351 - 1357 2021年02月

ISSN 15287483

This paper presents visual observations of formation and growth of structure-H hydrate formed with methane and 1-methylpiperidine (MPD). MPD is one of the water-miscible guest compounds which form structure-H hydrates with a help gas. This guest promotes hydrate formation of the help gas and has an advantage of omitting the dissolution process in water during potential industrial utilization. The observed crystal growth dynamics were categorized into two types depending on the subcooling temperature ΔTsub, the difference between the phase equilibrium and experimental temperatures. At ΔTsub = 2.8 K, the gas/liquid interface was covered by an agglomeration of hydrate grains. No remarkable hydrate growth was recognized after coverage of the gas/liquid interface. At ΔTsub ≥ 4.9 K, hydrate crystals continued forming and growing even after the gas/liquid interface was covered by hydrates, which increased the amount of formed hydrate crystals compared to lower ΔTsub. The crystal shape changed from polygonal or platelike into pyramidal, and the size of individual crystals increased with the rise of ΔTsub, opposite the common trend where crystals become smaller at higher supercooling. Implications of the results of crystal morphology on suitable conditions for industrial utilization of this hydrate for gas capture applications are discussed.

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Characterization of the Clathrate Hydrate Formed with Fluoromethane and Pinacolone: The Thermodynamic Stability and Volumetric Behavior of the Structure H Binary Hydrate

Kondo Y., Alavi S., Takeya S., Ohmura R.

Journal of Physical Chemistry B （Journal of Physical Chemistry B） 125 （ 1 ） 328 - 337 2021年01月

ISSN 15206106

To reveal the relation of guest dynamics within the structure H clathrate hydrate and its macroscopic physical properties, experimental and computational works have been conducted on the system of fluoromethane (HFC-41) and pinacolone coexisting with water. The phase boundaries of the hydrate formed from HFC-41 and pinacolone within the pressure range of (0.25-2.48) MPa and the temperature range of (277-293) K were measured. The equilibrium hydrate formation pressure incorporating HFC-41 was lowered by adding the pinacolone as a large guest molecule compound to form a sH phase compared to the HFC-41 single hydrate. Powder X-ray diffraction measurements confirmed the formation of the structure H hydrate with the HFC-41 and pinacolone binary hydrate. The lattice constants of the sH hydrate were also measured to see the effect of the help guest molecular size, which showed a different trend from that of the previous studies of sH pinacolone hydrates. Molecular dynamics simulations of the binary sH phase indicate weak hydrogen bonding of the pinacolone molecules with the water in the cages in the phase with HFC-41. The oblate HFC-41 molecules showed strong orientational preference to the equatorial planes of the D′ cages, which may explain some of the trends in the behavior of this phase.

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Phase equilibrium condition measurements in carbon dioxide hydrate forming system coexisting with seawater

Nakane R., Shimosato Y., Gima E., Ohmura R., Senaha I., Yasuda K.

Journal of Chemical Thermodynamics （Journal of Chemical Thermodynamics） 152 2021年01月

ISSN 00219614

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.

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X-Ray attenuation and image contrast in the X-ray computed tomography of clathrate hydrates depending on guest species

Takeya S., Muromachi S., Hachikubo A., Ohmura R., Hyodo K., Yoneyama A.

Physical Chemistry Chemical Physics （Physical Chemistry Chemical Physics） 22 （ 47 ） 27658 - 27665 2020年12月

ISSN 14639076

In this study, X-ray imaging of inclusion compounds encapsulating various guest species was investigated based on the calculation of X-ray attenuation coefficients. The optimal photon energies of clathrate hydrates were simulated for high-contrast X-ray imaging based on the type of guest species. The proof of concept was provided by observations of Kr hydrate and tetra-n-butylammonium bromide (TBAB) semi-clathrate hydrate using absorption-contrast X-ray computed tomography (CT) and radiography with monochromated synchrotron X-rays. The radiographic image of the Kr hydrate also revealed a sudden change in its attenuation coefficient owing to the K-Absorption edge of Kr as the guest element. With a photon energy of 35 keV, X-ray CT provided sufficient segmentation for the TBAB semi-clathrate hydrate coexisting with ice. In contrast, the simulation did not achieve the sufficient segmentation of the CH4 and CO2 hydrates coexisting with water or ice, but it revealed the capability of absorption-contrast X-ray CT to model the physical properties of clathrate hydrates, such as Ar and Cl2 hydrates. These results demonstrate that the proposed method can be used to investigate the spatial distribution of specific elements within inclusion compounds or porous materials.

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Development and Continuous Operation of a Bench-Scale System for the Production of O<inf>3</inf> + O<inf>2</inf> + CO<inf>2</inf> Hydrates

Hatsugai T., Nakayama R., Tomura S., Akiyoshi R., Nishitsuka S., Nakamura R., Takeya S., Ohmura R.

Chemical Engineering and Technology （Chemical Engineering and Technology） 43 （ 11 ） 2307 - 2314 2020年11月

ISSN 09307516

Ozone (O3) has many industrial applications such as in sterilization. One of the long-term O3 preservation methods is molecular storage in clathrate hydrate. In this study, an experimental system was developed for continuously forming O3 + O2 + CO2 hydrates. The parameters that affect the continuous operation of the system and that lead to increases in the concentration of O3 in the hydrates were also experimentally evaluated, implementing the method of quality engineering. After optimizing these operating parameters, the O3 storage capacity in the hydrates was measured to be 0.26 wt % at 2 h of total operation time. By X-ray diffraction, it was found that the produced sample contained hydrates, and long-term preservation for 6 months was possible at the temperature of general freezing warehouses.

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Crystal Growth of Clathrate Hydrate with Ozone: Implication for Ozone Preservation

Matsuura R., Ozawa K., Alavi S., Ohmura R.

ACS Sustainable Chemistry and Engineering （ACS Sustainable Chemistry and Engineering） 8 （ 41 ） 15678 - 15684 2020年10月

Preservation using hydrates as a storage medium is the only technology that enables long-term and high-concentration O3 preservation. This study reports the visual observations of the crystal growth of O3-containing hydrate for the process design of the hydrate production plant. We conducted experiments with the feed gas involving O3, O2, and CO2 in the molar ratio of O3:O2:CO2 = 3.4:42.0:52.6. During observations, O3 in the mixed gas remained at 1.31-2.43 mol %, which is comparable to those of the previous experiments. The observed results of hydrate crystal growth dynamics were classified into two types, which are lateral growth along the gas/water interface and growth into the bulk of the liquid water phase. Crystal morphology varied from polygonal to columnar, dendritic, or haze like with increasing subcooling temperature relative to equilibrium. The diversity of the crystal morphology of O3-containing hydrate was comparable to those of O2 + CO2 hydrate and CO2 hydrate. The implications of the obtained results for the operating conditions of an O3-containing hydrate production plant are discussed.

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Improvement of continuous hydrate-based CO<inf>2</inf> separation by forming structure II hydrate in the system of H<inf>2</inf> + CO<inf>2</inf> + H<inf>2</inf>O + Tetrahydropyran (THP)

Kiyokawa H., Horii S., Alavi S., Ohmura R.

Fuel （Fuel） 278 2020年10月

ISSN 00162361

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.

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Physical properties and characterization of the binary clathrate hydrate with methane + 1,1,1,3,3-Pentafluoropropane (HFC-245fa) + Water

Kodera M., Matsueda T., Belosludov R.V., Zhdanov R.K., Belosludov V.R., Takeya S., Alavi S., Ohmura R.

Journal of Physical Chemistry C （Journal of Physical Chemistry C） 124 （ 38 ） 20736 - 20745 2020年09月

ISSN 19327447

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.

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Clathrate hydrates at temperatures below the freezing point of water: A review

Nagashima H.D., Miyagi T., Yasuda K., Ohmura R.

Fluid Phase Equilibria （Fluid Phase Equilibria） 517 2020年08月

ISSN 03783812

This paper provides the review focusing on the clathrate hydrates at the temperatures below the freezing point of water. The three main topics of the review are the chemical thermodynamics, naturally-occurring hydrates and novel industrial applications utilizing hydrates. Fundamental phase equilibria in the hydrate forming systems at the temperatures below the freezing point of water are first reviewed. The effects of water-soluble compound on the hydrate phase equilibria are extensively discussed based on the experimental data reported in the literature and the newly obtained experimental data on the solid ice + sodium chloride aqueous solution + carbon dioxide hydrate + carbon dioxide gas four-phase equilibria that were measured in the present study. Natural occurrence of hydrates is discussed by comparing the thermodynamic conditions in the planets and satellites with the relevant hydrate phase equilibrium data. The naturally-occurring hydrates would exist in the following planets and satellites; Mars and Titan. The storage of natural gas in the form of the hydrate has been well investigated in the past two decades. CO2 hydrate can be used as the fire-fighting agent and the solid carbonated foods. The refrigeration and engine utilizing hydrates as the working media may be the challenging technologies having the prospects in terms of the efficiency, environment friendliness and the safety.

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Molecular dynamics simulations of interfacial properties of the CO<inf>2</inf>-water and CO<inf>2</inf>-CH<inf>4</inf>-water systems

Naeiji P., Woo T.K., Alavi S., Ohmura R.

Journal of Chemical Physics （Journal of Chemical Physics） 153 （ 4 ） 2020年07月

ISSN 00219606

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.

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Thermophysical Property Measurements of Tetrabutylphosphonium Oxalate (TBPOx) Ionic Semiclathrate Hydrate as a Media for the Thermal Energy Storage System

Miyamoto T., Koyama R., Kurokawa N., Hotta A., Alavi S., Ohmura R.

Frontiers in Chemistry （Frontiers in Chemistry） 8 2020年07月

With increasing global power demand, thermal energy storage technology could play a role ensuring a sustainable energy supply in power generation from renewable energy sources and power demand concentration. Hydrates have high potential as phase change materials (PCMs) for the use as a thermal energy storage medium. To develop thermal energy storage technology using a hydrate-based material, further investigation of thermophysical properties and the selection of a suitable hydrate are required. Tetrabutylphosphonium oxalate (TBPOx) ionic semiclathrate hydrate contains oxalic acid in salt form, as a guest compound, which is classified as carboxylic acid group with low environmental impact. In the present study, the phase equilibrium temperature and the dissociation heat of TBPOx hydrate were measured. The highest equilibrium temperature of the solid hydrate formed was 9.4°C at the mass fraction 0.35 of TBPOx in aqueous solution. The largest dissociation heat was 186.0 ± 0.5 kJ·kg−1 at the mass fraction of 0.35. Comparing with other PCMs with close phase equilibrium temperatures, TBPOx hydrate is superior in safety and sustainability. These results indicate that TBPOx hydrate would be suitable as the thermal storage medium for the general air conditioning systems.

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Improving thermal efficiency of hydrate-based heat engine generating renewable energy from low-grade heat sources using a crystal engineering approach

Koyama R., Chen L.J., Alavi S., Ohmura R.

Energy （Energy） 198 2020年05月

ISSN 03605442

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.

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Equilibrium temperature and dissociation heat of tetrabutylphosphonium acrylate (TBPAc) ionic semi-clathrate hydrate as a medium for the hydrate-based thermal energy storage system

Koyama R., Hotta A., Ohmura R.

Journal of Chemical Thermodynamics （Journal of Chemical Thermodynamics） 144 2020年05月

ISSN 00219614

The hydrate-based thermal energy storage system is one of the most effective supplemental technologies to stabilize the energy supply for renewable energies. The system is expected to solve the global energy-management issues related to energy-demand increase and energy supply chain. In fact, this eco-friendly technology requires prospective hydrate candidates for the thermal energy storage media, followed by the extensive investigation of the thermophysical properties of the selected hydrates. In this study, to satisfy these requirements, the equilibrium temperature and the dissociation heat of tetrabutylphosphonium acrylate (TBPAc) ionic semi-clathrate hydrate were studied. It was found that the highest equilibrium temperature was 14.4 °C at the mass fractions from 0.37 to 0.40. It was also found that the largest dissociation heat was (210.4 ± 5.3) kJ kg−1 at the mass fraction 0.39. These values indicate that TBPAc hydrate could be a new promising thermal-storage medium for the general or data-centre air-conditioning systems. The experimental results of this study also revealed an underlying tendency between the thermophysical properties of the tetrabutylphosphonium (TBP) + carboxylate salt hydrates and the molar mass of the guest anion in salt.

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