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Papers - Ohmura, Ryo

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Hydrate-based continuous hydrogen gas separation from mixing gas containing carbon dioxide with cyclopentanone

Kamiya L., Kasai R., Takeya S., Ohmura R.

International Journal of Hydrogen Energy 121 111 - 117 2025.04

ISSN 03603199

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.

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Interfacial tension between water and pentane saturated with methane

Yamashita T., Iwai T., Alavi S., Ohmura R.

Colloids and Surfaces A: Physicochemical and Engineering Aspects 710 2025.04

ISSN 09277757

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.

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Demonstration of a Practical Cooling Storage System for Lithium-Ion Batteries with Trimethylolethane (TME) Hydrate for EV

Kamiya L., Koyama R., Arai Y., Ohmura R.

Energies 18 ( 6 ) 2025.03

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.

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Phase Equilibrium of CO<inf>2</inf> Hydrate with Rubidium Chloride Aqueous Solution

Kasai R., Kamiya L., Ohmura R.

Separations 12 ( 1 ) 2025.01

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.

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Eutectic conditions of carbon dioxide clathrate hydrate and sodium Chloride: Implications for zero liquid discharge seawater desalination

Gibo A., Nakao S., Ohmura R., Yasuda K.

Journal of Industrial and Engineering Chemistry 2025

ISSN 1226086X

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.

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Effect of sucrose fatty acid ester on crystal growth of carbon dioxide hydrate in aqueous fructose solution

Nagayama S., Maruyama M., Tokunaga R., Alavi S., Ishihara K., Kawamoto M., Ohmura R.

Journal of Molecular Liquids 411 2024.10

ISSN 01677322

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.

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Phase equilibria and crystallographic structure of clathrate hydrate formed with carbon dioxide and cyclohexanone

Kamiya L., Kasai R., Takeya S., Ohmura R.

Fluid Phase Equilibria 585 2024.10

ISSN 03783812

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.

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Thermophysical Properties of Clathrate Hydrates with Various Guests for Novel Technologies: A Review

Yasuda K., Ohmura R.

International Journal of Thermophysics 45 ( 10 ) 2024.10

ISSN 0195928X

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.

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Surface tension of aqueous solutions of large organic salts: Tetrabutylammonium acetate solutions

Yamashita T., Shigehara S., Alavi S., Ohmura R.

Journal of Molecular Liquids 404 2024.06

ISSN 01677322

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.

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Crystal Growth of Clathrate Hydrate Formed with Carbon Dioxide and Deuterium Oxide: Implications for Hydrate-based Tritium Separation

Maruyama M., Dogi K., Ohmura R.

Energy and Fuels 38 ( 7 ) 6215 - 6222 2024.04

ISSN 08870624

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.

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Characterization of clathrate hydrates with CO<inf>2</inf> + 1-propanol or 2-propanol: Implications for flow assurance, refrigeration, carbon capture, and skincare applications

Maruyama M., Takeya S., Yoneyama A., Ishikawa T., Misawa T., Nagayama S., Alavi S., Ohmura R.

Journal of Industrial and Engineering Chemistry 131 305 - 314 2024.03

ISSN 1226086X

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

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Zero-point energy effects on the stability of water clusters: Implications on the uptake of hydrogen isotope substituted water on ice and clathrate hydrate phases

Ohmura R., Alavi S.

AIP Advances 14 ( 3 ) 2024.03

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

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Fiery ice: An overview of methane hydrate combustion

Dunn-Rankin D., Chien Y.C., Ueda T., Ohmura R.

Progress in Energy and Combustion Science 101 2024.03

ISSN 03601285

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.

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Phase equilibrium conditions in carbon dioxide + cyclopentane double clathrate hydrate forming system coexisting with sodium chloride aqueous solution

Mekaru K., Miyagi T., Mishima A., Uehara I., Ohmura R., Yasuda K.

Journal of Chemical Thermodynamics 189 2024.02

ISSN 00219614

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.

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Phase Equilibrium Conditions and Crystal Structure of Binary Hydrates Formed with 1-Methylpiperidine

Yamamoto K., Misawa T., Ito H., Takeya S., Alavi S., Ohmura R.

Journal of Physical Chemistry C 128 ( 3 ) 1312 - 1322 2024.01

ISSN 19327447

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.

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Growth dynamics and morphology of D<inf>2</inf>O + HFC-134a clathrate hydrate crystal toward tritiated water separation

Kamiya L., Ohmura R.

Chemical Engineering Communications 211 ( 8 ) 1191 - 1198 2024

ISSN 00986445

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.

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Unusual promotion/inhibition effect of polyethylenimine on CO<inf>2</inf>/CH<inf>4</inf> hydrate formation: A potential green chemical additive for CH<inf>4</inf>-CO<inf>2</inf> replacement process

Ho Y.H., Khoo Y.Z., Chen Y.P., Ohmura R., Chen L.J.

Chemical Engineering Journal 472 2023.09

ISSN 13858947

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.

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Renewed Measurements of Carbon Dioxide Hydrate Phase Equilibrium

Ito H., Gibo A., Shiraishi S., Yasuda K., Ohmura R.

International Journal of Thermophysics 44 ( 8 ) 2023.08

ISSN 0195928X

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.

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Study of the Effect of Tetrabutylammonium Halide Aqueous Solutions on the Gas Storage of Methane and Carbon Dioxide

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

Energies 16 ( 13 ) 2023.07

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.

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Tritiated water removal method based on hydrate formation using heavy water as co-precipitant

Nakamura S., Awata T., Kiyokawa H., Ito H., Ohmura R.

Chemical Engineering Journal 465 2023.06

ISSN 13858947

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.

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