Structure and thermal expansion of natural gas clathrate hydrates

We report on the structural properties of natural gas hydrate crystals from the Sea of Okhotsk. Using powder X-ray diffraction (PXRD), it was determined that sediments from four locations contained type I gas hydrate, which encage mostly methane (96-98%) and a small amount of carbon dioxide. For all hydrates, the lattice constant was estimated to be at 113 K, which approximately equals that of pure methane hydrate. The result is in good agreement with the structure of artificially synthesized methane + carbon dioxide mixed-gas hydrates. These results suggest that the lattice constant of the natural gas hydrate does not change due to a change of CO₂ gas content. In addition, the thermal expansion of the sampled hydrate was measured for the temperature range of 83-173 K, and the resulting density of the hydrate crystal at 273 K was estimated to be . These results are essential for applying natural gas hydrates as an alternative natural fuel resources.     

Storage capacity of hydrogen in tetrahydrothiophene and furan clathrate hydrates

The storage capacity of hydrogen in the tetrahydrothiophene and furan hydrates was investigated by means of pressure-volume-temperature measurement. The hydrogen-absorption rate of tetrahydrothiophene and furan hydrates is much larger than that of tetrahydrofuran hydrate in spite of same crystal structure (structure-II). The storage amount of hydrogen at 275.1 K is about 1.2 mol (hydrogen)/mol (tetrahydrothiophene or furan hydrate) (∼0.6 mass%) at 41.5 MPa, which is coincident with that of tetrahydrofuran hydrate.     

Tuning clathrate hydrates: Application to hydrogen storage

Gas hydrates represent an attractive way of storing large quantities of gases such as methane, although to date there has been little effort to optimize the storage capacity and to understand the trade-offs between storage conditions and storage capacity. In this work, we proposed the peculiar cage occupancy dynamics observed at the mixed hydrates simultaneously containing both one gas and one liquid guest. Liquid guests participating in forming hydrates must be water-soluble and possess the sufficient interaction with surroundings in order to make strong host-guest networks. However, it seems to be almost impossible for small molecules such as hydrogen and methane to replace the relatively large liquid guest molecules and thus an appropriate condition needs to be made for small guests to be enclathrated in large cages. By lowering the concentrations of liquid guests the small gas guests are found to enter the large cages feeling like their home. We also tried to explain this natural phenomenon with solid solution theory for clathrate hydrates. Special types of water-soluble liquid materials, such as THF, 1,4-dioxane, and t-BuNH₂, form sII double hydrates upon reacting with gas guest molecules. We expect that the storage capacities of gas guests largely depend on both the chemical nature of liquid guests and their relative concentration in host water. This tuning mechanism occurring in the mixed clathrate mixtures appears to be quite general and can be applied to energy and environmental systems including gas storage and transportation fields.     

Thermodynamic model for predicting phase equilibria of simple clathrate hydrates of refrigerants

In this communication, a thermodynamic model is presented for the study of the phase equilibria of clathrate hydrates of simple refrigerants. The van der Waals–Platteeuw solid solution theory is used to model the hydrate phase while it is assumed that vapor phase is an ideal gas of refrigerant ignoring its water content and the aqueous phase is considered as pure water (activity coefficient=1) ignoring aqueous solubility. The results through this model are successfully compared with the experimental data reported in the literature for clathrate hydrates of four refrigerants namely C₂H₂F₄ (1, 1, 1, 2-tetrafluoroethane or HFC-134a or R-134a), C₂H₄F₂ (1, 1-difluoroethane or HFC-152a or R-152a), CH₂F₂ (difluoromethane or HFC-32 or R-32), and C₂H₃Cl₂F (1,1-dichloro-1-fluoroethane or HCFC-141b or R-141b).     

Kinetics of formation of carbon dioxide clathrate hydrates

The new experimental apparatus capable of observing the clathrate hydrate formation kinetics was developed in this study. Experimental data on the kinetics of carbon dioxide hydrate formation were carefully measured. The experiments were carried out in a semi-batch stirred tank reactor with stirring rate of 500 rpm at three different temperatures between 275.2 and 279.2 K and at pressures ranging from 2.0 to 3.5 MPa. The kinetic model was adopted to predict the growth of hydrates with only one adjustable parameter which represented the rate constant for the hydrate particle growth. The model was based on the crystallization theory coupled with the two-film theory for gas absorption into the liquid phase. The model predictions matched the experimental data very well with the largest deviation of 7.18%, which is within experimental error range. This study is the first for the kinetic data of carbon dioxide hydrate formation and important in developing carbon dioxide fixation process using clathrate hydrate phenomenon.     

Spectroscopic evidences of the double hydrogen hydrates stabilized with ethane and propane

Hydrogen molecules are known to occupy the small cages of structure I (sI) and II (sII) hydrates with the aid of coguests, leading to the highly stable state of their crystalline framework. For the first time, we synthesized the double hydrogen hydrates incorporated with ethane and propane that play a special role as the hydrate promoters or stabilizers. The resulting hydrate structures cage occupancy was identified by the spectroscopic methods of the PXRD and solid-state NMR. In addition, direct GC analysis confirmed that the encaged hydrogen amounts are 0.127 for sI ethane and 0.370 for sII propane at 120 bar and 270 K. The proper hydrate thermodynamics particularly focusing on the cage occupancy estimated that 0.17 and 0.33 wt% of hydrogen are observed in small cages of sI and sII hydrates. The overall spectroscopic and physicochemical analysis strongly imply that the sII cages act as much more favorable sites than sI cages in storing hydrogen.     

Structural phase transitions of methane+ethane mixed-gas hydrate induced by 1,1-dimethylcyclohexane

Methane+ethane+1,1-dimethylcyclohexane+water system was investigated by using Raman spectroscopy and isothermal phase equilibrium measurements under four-phase (gas+aqueous+large guest species+hydrate phases) equilibrium conditions at 288.15 K. The results suggest that three kinds of hydrate structures emerge at 288.15 K in the methane+ethane+1,1-dimethylcyclohexane+water system. The hydrate structure for this system changed from structure-H to structure-I via structure-II with increase in the mole ratio of ethane to methane.     

Stability boundaries of gas hydrates helped by methane—structure-H hydrates of methylcyclohexane and cis-1,2-dimethylcyclohexane

The four-phase coexistence curves for the structure-H hydrates of methylcyclohexane and cis-1,2-dimethylcyclohexane in the presence of methane are measured in the temperature range 274.09- and pressure range 1.42-. Very large pressure reductions from the pure methane hydrate are observed by forming structure-H hydrates. The present investigation on the trans-1,2-dimethylcyclohexane system reveals that the limit of the largest-cage occupancy for the structure-H hydrate is laid between the 1,2-dimethylcyclohexane stereo-isomers.     

Thermodynamic and Raman spectroscopic studies on hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrates

Thermodynamic stability and hydrogen occupancy on the hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate have been investigated by means of phase equilibrium (pressure-temperature) measurements and Raman spectroscopic analyses for two mole fractions, 0.018 and 0.034 (stoichiometric for the cubic structure) of tetra-n-butyl ammonium fluoride aqueous solutions. In the case of higher concentration (0.034), the stability boundary curve of hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate locates at about 23 K higher temperature than that of hydrogen+tetrahydrofuran mixed gas hydrate. The storage capacity of hydrogen in the cubic structure for the hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate is smaller than that of hydrogen+tetrahydrofuran mixed gas hydrate. In the case of hydrate prepared from the lower concentration (0.018) of aqueous solution, the Raman spectra and phase behavior reveal that the cubic structure of semi-clathrate hydrate is changed to a different one at about 9 MPa and 299.2 K. The new structure can entrap larger amount of hydrogen than the cubic one. The stability boundary curve of hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate obtained in the aqueous solution of lower mole fraction (0.018) is shifted to slightly low-temperature or high-pressure side from that of higher mole fraction (0.034).     

Equilibrium and crystallographic measurements of the binary tetrahydrofuran and helium clathrate hydrates

Clathrate compounds are crystalline materials formed by a physical interaction between host and relatively light guest molecules. Various types of nano-sized cages surrounded by host frameworks exist in the highly unique crystalline structures and free guest molecules are entrapped in an open host-guest network. Recently, we reported two peculiar phenomena, swapping and tuning, naturally occurring in the hydrate cages. Helium, one of the smallest light guest molecules, must be the challengeable material in the sense of physics and moreover possesses versatile applications in the field of superconductivity technology and thermonuclear industry. In this regard, we attempted for the first time to synthesize helium hydrates at moderate temperature and pressure conditions. According to inclusion phenomena, helium itself normally cannot form clathrate hydrates due to being too small molecularly without the help of hydrate former molecules (sI, sII, and sH formers). In this study, the hydrate equilibria of the binary clathrate hydrate containing tetrahydrofuran, helium, and water were determined at 2, 3, 5.56 THF mol%. Direct volumetric measurements were also carried out to confirm the exact amount of helium captured in the hydrate cages. Finally, the crystalline structure of the formed mixed hydrates was identified by powder X-ray diffraction, resulting in structure II.     

Gauche conformation of acyclic guest molecules appearing in the large cages of structure-H clathrate hydrates

In the present study, measurements and analyses were made of the High-Power Decoupling (HPDEC) solid-state ¹³C NMR spectra of structure-H (sH) methane hydrates with isopentane, one of the simplest and smallest acyclic large guest molecules, and methylcyclohexane (MCH), a commonly used cyclic guest molecule that is larger than isopentane. From the spectroscopic information, clear and definite evidence for the molecular conformation of acyclic guest molecules that are sufficiently small so as to be entrapped into the structure-H large cage (sH-L) was expected. The ¹³C NMR chemical shift change was additionally checked through the use of a hydrogen-hydrogen steric perturbation model. From the overall results, we concluded that one of the smallest acyclic guest molecules, isopentane, participating in the formation of a structure-H clathrate hydrate is encaged, confirming the gauche conformation in large cavities. The present results strongly suggest that the guest position and structure in hydrate cages are greatly influenced by both short-range interactions between guest molecules and cage frameworks and long-range interactions between small and large guests. Accordingly, cage dynamics must be carefully considered when a specific sH hydrate is designed and synthesized for the purpose of tuning material properties.     

Surfactant promoting effects on clathrate hydrate formation: Are micelles really involved?

Critical micelle concentrations (CMCs) of several surfactants in water have been determined by conductivity measurements under hydrate-forming conditions (2 °C; 40 bar methane) and under the same conditions with nitrogen. Control experiments were also performed at room temperature and pressure. Surfactants investigated were the anionics sodium dodecylsulfate (SDS), sodium laurate (SL), sodium oleate (SO), 4-dodecylbenzenesulfonic acid (DBSA), and the cationics dodecylamine hydrochloride (DAHCl) and dodecyltrimethylammonium chloride (DTACl). For SO, DBSA and DTACl, CMC values were found to vary slightly under hydrate-forming conditions with respect to normal P-T, whereas SDS, SL and DAHCl solutions underwent precipitation before reaching the CMC under hydrate-forming conditions. Interestingly, no micellar formation was observed for any surfactants in the concentration range where strong hydrate promotion was previously reported.     

Spectroscopic analysis of carbon dioxide and nitrogen mixed gas hydrates in silica gel for CO2 separation

In this study solid-state NMR spectroscopy was used to identify structure and guest distribution of the mixed N₂ + CO₂ hydrates. These results show that it is possible to recover CO₂ from flue gas by forming a mixed hydrate that removes CO₂ preferentially from CO₂/N₂ gas mixture. Hydrate phase equilibria for the ternary CO₂–N₂–water system in silica gel pores were measured, which show that the three-phase H–L_w–V equilibrium curves were shifted to higher pressures at a specific temperature when the concentration of CO₂ in the vapor phase decreased. ¹³C cross-polarization (CP) NMR spectra of the mixed hydrates at gas compositions of more than 10 mol% CO₂ with the balance N₂ identified that the crystal structure of mixed hydrates as structure I, and that the CO₂ molecules occupy mainly the abundant 5¹²6² cages. This makes it possible to achieve concentrations of more than 96 mol% CO₂ gas in the product after three cycles of hydrate formation and dissociation.     

笼型水合物为能源化工带来新机遇

笼型水合物是利用水分子通过氢键作用构建的笼型结构对甲烷等能源气体进行存储和提取,具有高安全性、高储存容量、温和储存条件、环境友好等优点。天然气水合物是传统能源和绿色能源之间的桥梁燃料,已成为世界各国科学家竞相研究开发的热点。本文综述了笼型水合物在能源与环境、流动安全、工程应用三个方面的研究成果,涵盖了固化天然气（SNG）、CO₂捕获和气体分离、蓄冷、海水淡化、汽车燃料以及制氢与储氢等能量转换、能量储存的领域。文章指出大力发展笼型水合物衍生技术,实现提取甲烷同时捕获二氧化碳,有助于实现碳中和的目标。阐述了笼型水合物生成依赖于其自身的热力学相平衡条件、反应过程的动力学性质及传递过程强化,从生成到分解的过程主要包括溶解、成核、生长、晶裂和解吸等一系列步骤,过程的微观机理复杂。展望了利用多尺度方法研究水合物生成的微观结构、界面现象、宏观应用和作用机理,有助于扩展化学工程的原理和知识,对开发能源化工领域新材料新工艺也有裨益,从而促进能源化工的发展。     

Surfactant effects on hydrate formation in an unstirred gas/liquid system: An experimental study using methane and sodium alkyl sulfates

This paper reports an experimental study on the effects of surfactant additives on the formation of a clathrate hydrate in a quiescent methane/liquid-water system, which was initially composed of a 300-cm³ aqueous phase and an ∼640-cm³ methane-gas phase, then successively provided with methane such that the system pressure was held constant. The surfactants used in the present study were three sodium alkyl sulfates appreciably different in the alkyl chain length—they were sodium dodecyl sulfate (abbreviated as SDS), sodium tetradecyl sulfate (abbreviated as STS) and sodium hexadecyl sulfate (abbreviated as SHS). For each surfactant added to water up to, at most, 1.82-3.75 times the solubility, we performed visual observations of hydrate formation simultaneously with the measurements of methane uptake due to the hydrate formation. The qualitative hydrate-formation behavior thus observed was almost the same irrespective of the species as well as the initial concentration of the surfactant used; i.e., thick, highly porous hydrate layers were formed and grew on the horizontal gas/liquid interface and also on the test-chamber wall above the level of the gas/liquid interface. In each experimental operation, hydrate formation continued for a limited time (from ∼6 to ) and then practically ceased, leaving only a small proportion (typically 15% or less) of the aqueous solution unconverted into hydrate crystals. The variations in the time-averaged rate of hydrate formation (as measured by the rate of methane uptake) and the final water-to-hydrate conversion ratio with the initial concentration of each surfactant were investigated. Moreover, we examined the promotion of hydrate formation with the aid of a water-cooled cold plate, a steel-made flat-plate-type heat sink, vertically dipped into the aqueous phase across the gas/liquid interface.     

Surfactant effects on hydrate formation in an unstirred gas/liquid system: Amendments to the previous study using HFC-32 and sodium dodecyl sulfate

This paper reports a set of experimental data of clathrate-hydrate formation from HFC-32 (difluoromethane) gas in contact with an aqueous solution of sodium dodecyl sulfate (SDS). This supersedes the corresponding data that we previously reported in this journal [Watanabe et al., 2005. Surfactant effects on hydrate formation in an unstirred gas/liquid system: an experimental study using HFC-32 and sodium dodecyl sulfate. Chemical Engineering Science 60, 4846-4857] with the new data reported herein, because of a suspicion of hydrate plugging occurring in the gas-feed line of our experimental system used to obtain the previous data. The new data show much higher levels in both the hydrate formation rate and the final water-to-hydrate conversion ratio as compared to the previous data. Neither the hydrate formation rate nor the water-to-hydrate conversion ratio exhibited a significant change with the SDS concentration in the aqueous phase over the range from 1000 to 4000 ppm.     

Experimental characterization of guest molecular occupancy in clathrate hydrate cages: A review

Study on the microscopic structure of clathrate hydrate has made significant progress in the past decades. This review aims to summarize the state of the art of the experimental characterization of guest molecular occupancy in clathrate hydrate cages, which is an important area of the microscopic structures. The characterizing method and features of different guest molecular, such as hydrocarbon, carbon dioxide, hydrogen and inhibitor/promoter, in different hydrate cages have been extensively reviewed. A comprehensive use of advanced technologies such as X-ray diffraction, Raman spectroscopy and nuclear magnetic resonance may provide better understanding on the compositions and microscopic mechanisms of clathrate hydrate.     

制氢工艺能量分析

本文用过程模拟法对以轻烃为原料的制氢工艺作了能量分析。计算表明在H_2O/C较低,转化温度较高及原料轻质化的条件下,可以获得较低的能耗。     

Ultra-stability of gas hydrates at 1 atm and 268.2 K

This paper details creation of methane sI hydrates that are much more stable at 1 atm and 268.2 K than any previously reported. Extraordinarily stable natural gas sII hydrates at 1 atm and 268.2-270.2 K are reported for the first time. Test innovations that achieved ultra-stabilities give insight into hydrate self-preservation mechanisms. Water-surfactant liquid solutions were used to nucleate hydrate crystals that adsorbed as extremely small particles on surfaces of high thermal conductivity. The small hydrate particles packed and consolidated symmetrically upon Al or Cu cylindrical surfaces, minimizing internal void spaces and fractures in the accumulated 250-400 g hydrate mass. Resulting hydrate stability window is 268.2-270.2 K at 1 atm. Methane sI, as well as natural gas sII, hydrates exhibit only minimal decomposition upon reducing confining system pressure to 1 atm in the 268.2-270.2 K stability window. Total gas that evolved after 24 h at 1 atm in the stability window typically amounted to less than 0.5% of originally stored gas, and this ultra-stability was shown to persist when the test was allowed to run 256 h before terminating. The entire methane sI or natural gas sII hydrate mass remains stable during pressure reduction to 1 atm, whereas previous reports defined hydrate anomalous stability for only about 50% of fractional hydrate remnants.