Published papers

Ionic semiclathrate hydrate, which is formed under atmospheric pressure with sustainably safe properties, has a potential for solid-liquid phase change materials(PCMs). In this study, the thermophysical property of tetrabutylphosphonium glycine(TBPGly) hydrate containing no halogen guest compound, glycine (aminoacetic acid), was investigated to evaluate the availability as an eco-efficient thermal energy storage medium. The phase equilibrium temperature was obtained through visual observation method. The heat flow rate curve behavior and dissociation heat were measured via differential scanning calorimetry(DSC). In the dissociation process, a single hydrate dissociation peak emerged, indicating TBPGly hydrate has one type of crystallographic structure. The concentration dependency of the TBPGly aqueous solution on the equilibrium temperature and the dissociation heat was confirmed. The highest equilibrium temperature of 5.8 °C was confirmed at the fraction range wTBPGly =0.368 or xTBPGly =0.0306, and the largest dissociation heat was 177.7 kJ·kg−1 at the fraction range wTBPGly =0.377or xTBPGly =0.0318. The estimated hydration number in the congruent composition was found to be in 31.2 to 32.5. In addition, based on the measured physical property, the optimum operating temperature and volume fraction of TBPGly hydrate was evaluated to determine the concentration for the practical cooling system. These data revealed TBPGly hydrate is available as a solid or slurry PCM under the operating temperature range 2 °C to 5.8 °C. The experimental data as well as the illustrative calculations contribute to accelerate the expansion in the application range, and provide the prospect in selecting the suitable composition toward the actual hydrate-based cooling system.

Clathrate hydrate is considered to be a potential medium for gas storage and transportation. Slow kinetics of hydrate formation is a hindrance to the commercialized process development of such applications. The kinetics of methane hydrate formation from the reaction of ice powder and methane gas doped with/without saturated ethanol vapor at constant pressure of 16.55 ± 0.20 MPa and constant temperature ranging from −15 to −1.0 °C were investigated. The methane hydrate formation can be dramatically accelerated by simply doping ethanol into methane gas with ultralow ethanol concentration (<94 ppm by mole fraction) in the gas phase. For ethanol-doped system 80.1% of ice powder were converted into methane hydrate after a reaction time of 4 h, while only 26.6% of ice powder was converted into methane hydrate after a reaction time of 24 h when pure methane gas was used. Furthermore, this trace amount of ethanol could also substantially suppress the self-preservation effect to enhance the dissociation rate of methane hydrate (operated at 1 atm and temperatures below the ice melting point). In other words, a trace amount of ethanol doped in methane gas can act as a kinetic promoter for both the methane hydrate formation and dissociation.

This paper reports theoretical performance analysis of the hydrate-based refrigeration system. The hydrate-based refrigerator transfers thermal energy through the cycle of following processes: adiabatic compression of guest compounds and water, hydrate formation, adiabatic expansion of hydrates, hydrate dissociation. The theoretical analyses and COP calculations were performed in the each of the following hydrate-forming systems: HFC-32+cyclopentane(CP) +water, Kr+CP+water, and HFC-41+CP+water system. Practical utility of hydrate-based refrigerator is discussed based on the comparison with the Carnot cycle.

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.

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.

O3 concentration in O3-containing clathrate hydrate formed by a continuous process is predicted using the thermodynamic simulation in which the hydrate-forming process is assumed to be a series of numerous equilibrium states each slightly deviating from the preceding state. The thermodynamic simulation was originally developed for natural gas hydrate. In this study, the simulation was modified to take account of O3 self-decomposition. The O3 decomposition rate was experimentally measured in the (O3 + O2 + CO2) system to be used for the material balance calculation model. The phase-equilibrium calculation model used in this study was validated by comparison of the experimental O3 concentration in hydrate to that calculated. The thermodynamic simulation indicated that the O3 concentration in hydrate increased with an increase in the fraction of the gas discharged from the reactor. The higher rate of hydrate formation increased the O3 concentration in hydrate asymptotically. This study revealed that the O3 mass concentration in hydrate formed in a continuous process was the order of percent, which was a hundred times greater than that in the other O3 storage technologies such as ozonated ice and ozonated water.

Management for the temperature increase of lithium-ion batteries in electric vehicles (EV) is required for safety and energy efficiency. As a cooling medium for lithium-ion batteries, one suitable candidate is trimethylolethane (TME) hydrate, which may be utilized as a phase change material (PCM). This hydrate has a large dissociation heat and will form/dissociate at approximately 30 °C under atmospheric pressure conditions. A cooling system which uses TME hydrate can have a high performance with regards to the efficiency and the stability of heat transfer process. However, in previous studies, the equilibrium temperature and the dissociation heat of TME hydrate have been reported only for a TME mass fraction of 0.625. Since TME hydrate forms in an aqueous solution, it is necessary to know how these thermophysical properties change for a range of TME mass fractions. Therefore, in this study, the equilibrium temperatures and the dissociation heats were measured respectively by the visual observation method and differential scanning calorimetry measurements for TME mass fraction range from 0.20 to 0.80. The highest equilibrium temperature was 29.6 °C at the mass fraction of 0.60. For the cooling system, the largest dissociation heat was 190.1 kJ kg−1, also at the mass fraction of 0.60.

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.

To develop the “hydrate thermal energy storage technology” it is desired to reveal the mechanism of the thermophysical property manifestation on ionic semiclathrate hydrate. The influence of valence on thermophysical properties has not been investigated so far. In the present study, the phase equilibrium temperatures and the dissociation heats of tetrabutylphosphonium sulfate ((TBP)2SO4) ionic semiclathrate hydrate were experimentally studied. (TBP)2SO4 hydrate consists of the bivalent anion, SO42−. The highest phase equilibrium temperature was 6.5 °C at the mass fraction range from 0.363 to 0.384. The largest dissociation heat was (168.8 ± 2.4) kJ·kg−1 at the mass fraction 0.374. These values of the phase equilibrium temperature and the dissociation heat were respectively lower and smaller than those of the ionic semiclathrate hydrates with the monovalent anion. As a new choice for thermal energy storage medium, (TBP)2SO4 hydrate would be suitable to the automotive air conditioner and the cold chain for blood or perishable foods.

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.

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.

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.

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.

CO2 capture using clathrate hydrates is an environmentally friendly separation technology. When considering operational efficiency, it is desirable to operate the separation process continuously. In this experimental study, the continuous separation of CO2 from a model fuel gas was performed for H2 + CO2 + H2O and H2 + CO2 + tetra-n-butylammonium bromide (TBAB) + H2O systems with TBAB mass fractions of wTBAB = 0, 0.05, 0.10, and 0.32. Measurements were taken to track the time evolution of compositions of the gas phase and hydrate slurry. After between 37 and 48 h from the start of the experiment, H2 compositions in the gas phase reached steady state values of 0.87, 0.81, and 0.78 for wTBAB values of 0, 0.05, and 0.10, respectively. For the same conditions, CO2 compositions in the hydrate slurry reached steady state values of 1.00, 0.82, and 0.79, respectively. In carrying out this work we have shown that it is possible to successfully separate CO2 using structure I hydrates and ionic semiclathrate hydrates, on a continuous basis. There is the caveat, however, that for wTBAB = 0.32 a continuous separation process is not possible. The split fraction of CO2 we attained were 0.76, 0.64, and 0.62 for wTBAB values of 0, 0.05, and 0.10, respectively. The water system (wTBAB = 0) exhibited the highest H2 compositions in the gas phase, highest CO2 compositions in the hydrate slurry, and highest split fraction of CO2. Although short-term operation, specifically 18 h, is possible with wTBAB = 0, continuous hydrate formation cannot be implemented. The concentration of captured CO2 for wTBAB = 0 was comparable to that obtained from hydrate-based gas separation in multistage processes or chemical absorption.

In this study, the CO2 continuous separation experiments were performed in the system of H2 + CO2 + H2O and H2 + CO2 + tetra-n-butylammonium bromide (TBAB) + H2O for TBAB mass fraction, wTBAB of 0 (the water system), 0.05, 0.10, 0.32. The time evolutions of compositions in the gas phase and the hydrate slurry were measured. The H2 compositions in the gas phase for wTBAB of 0, 0.05, and 0.10 reached steady state at 0.87, 0.81, and 0.78 while the CO2 compositions in the hydrate slurry were 1.00, 0.82, 0.79, respectively. It is shown that the experiments succeed in continuously separating CO2 with structure I hydrate and ionic semiclathrate hydrate. On the other hand, the hydrate transportation problem in separation process occurred for wTBAB = 0.32. For wTBAB = 0, the highest values of H2 compositions in the gas phase and CO2 compositions in hydrate slurry were obtained. The concentration of captured CO2 for wTBAB = 0 was comparable to that obtained by multistage process in hydrate-based gas separation or chemical absorption used in the demonstration project of integrated gasification combined cycle.

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.

This paper reports the visual observations of the formation and growth of clathrate hydrate formed with methane plus a partially water soluble guest, tetrahydropyran (THP). THP is known to form a structure II hydrate with methane. The crystal growth dynamics and crystal morphology changed depending on T sub T eq - T ex , where T eq is the equilibrium temperature and T ex is the experimental temperature. At T sub ≥ 12 K, the hydrate crystals initially formed at the water-THP interface and then grew to form a hydrate film. After the hydrate film was formed, the hydrate formation stopped. At T sub < 12 K, the hydrate crystals that formed at the water-THP interface detached from the interface. The detachment of the hydrate crystals led to the maintenance of the interface and continual hydrate formation. These crystal growth dynamics may be ascribed to the reduced water-THP interfacial tension and the resulting enhancement of the wettability of hydrate with water in comparison with those in the system without THP.

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.

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.

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.