On the application of binary correction factors in lattice distortion calculations for methane clathrate hydrates

The lattice distortion theory of Zele and co-workers is an attractive method for amending calculated phase equilibria of clathrate hydrates, since only two molecular computations are required. The perturbation energy between the empty and loaded clathrate hydrate lattice is the quantity of interest. The effect of binary correction factors applied to the Lorentz and Berthelot combining rules for the intermolecular interaction between gas and water particles is investigated. There are clear trends for the perturbation energy and lattice constant in terms of the binary correction factors, although there is significant sensitivity to the force field parameterization of the gas species.     

The effect of lattice constant on the storage capacity of hydrogen hydrates: a Monte Carlo study

ABSTRACT

We use grand canonical Monte Carlo simulations to examine the effect of the size of hydrate cavities (as reflected through the lattice constant of the hydrate unit cell) on the efficiency of clathrate hydrates in storing hydrogen gas. With this approach, the hydrate lattice is treated as a solid substrate where gas absorption takes place. Of practical interests are cases, where the lattice-size parameters are changed in such a way that they can promote/enhance multiple cavity occupancy, namely the presence of more than one guest-gas molecule in the same hydrate cavity. This phenomenon is commonly observed in the case of hydrogen hydrates and could increase their storage capacity. A parametric analysis is also carried out to quantify the correlation between the change in the lattice constant and the storage capacity since small changes in the size of the crystal unit cell may induce significant changes in the storage capacity especially in cases where multiple cavity occupancy occurs.     

Clathrate hydrates modelled with classical density functional theory coupled with a simple lattice gas and van der Waals-Platteeuw theory

Predicting clathrate hydrate phase equilibria is of interest in the area of natural gas exploitation. This proof of concept study presents the application of a simple lattice gas model and classical density functional theory coupled with van der Waals-Platteeuw theory to predict clathrate hydrate phase equilibria for several different hydrate-forming gas species. The dissociation pressure curve is predicted using adsorption isotherms predicted for the gas species in the crystal hydrate lattice. Comparisons are made between predicted phase equilibria (and other properties) and available experimental data.     

Theoretical investigation of structures and compositions of double neon-methane clathrate hydrates,depending on gas phase composition and pressure

The region of existence of neon clathrate hydrates is an actual problem of hydrate chemistry. The current work presents theoretical study of the equilibrium formation conditions of pure neon clathrate hydrates and double clathrate hydrates of neon-methane mixture. The structures and properties of double clathrate hydrates were described within the scope of the previously developed molecular clathrate hydrate model that takes into account the influence of guest molecules on the host lattice, interaction of guest molecules between themselves, and the possibility of multiple filling of host lattice cages by guest molecules. The model makes it possible to find an equilibrium state and thermodynamic properties of clathrate hydrates at given values of p and T. In the present work, we considered the properties of double clathrate hydrates in the range of pressures from 0 to 4 kbar at 250 K. The results of modeling have shown that the mass fraction of neon in double clathrate hydrate of Ne and CH₄ mixture of cubic structure I (sI) can reach 26%, and 22.5% in double hydrate of cubic structure II (sII) even at a low methane concentration (1%) in gas phase, at high pressure. It is shown that in double clathrate hydrates of the Ne and CH₄ mixture at high pressures, phase transition sII-sI can occur.     

显微激光拉曼光谱测定甲烷水合物的水合指数

甲烷水合物是由甲烷气体分子与水分子在低温高压下形成的一种笼型结构化合物,广泛存在于海底陆架区和陆地冻土区,被认为是一种潜在的能源资源。在水合物的晶格中,水分子在氢键的作用下形成大小不同的笼子,甲烷分子可分别进入大笼(51262)和小笼(512)中。在自行研制的实验装置上,分别合成了一系列不同体系下的甲烷水合物,包括十二烷基硫酸钠(SDS)水溶液-甲烷体系、冰粉-甲烷体系以及冰粉-不同粒度砂-甲烷体系。对这些甲烷水合物样品进行了激光拉曼光谱分析,测定了其水合指数,笼占有率等结构参数。结果表明,这些甲烷水合物都为Ⅰ型结构,其水合指数和笼占有率基本不受沉积物粒径大小的影响。在3种体系中生成的水合物,大笼中甲烷分子基本占满,占有率大于97%;小笼中甲烷分子占有率为80%～86%,测得的水合指数为6.05～6.15。     

水合物研制、结构与性能及其在能源环境中的应用

天然气水合物是与能源和环境相关的物质,可以进行甲烷等能源气体的存储和提取,也可以用于对二氧化碳等废气的封存.天然气水合物主要分为三种结构:sI, sII和sH,在本文中对其稳定性、水笼类型和大小以及可俘获气体进行了论述.中子衍射技术是研究水合物的重要手段之一,有着独特的优势.如中子的穿透性可以研究在高压状态下压力腔体内的大块样品;中子对于轻元素的敏感性可以很好地确定水合物当中的碳、氢、氧元素.通过中子衍射和非弹散射可以得到水合物中H/D原子的位置、各向异性振动因子、不同温度压力下的客体分子的水笼占据率、客体分子在水笼中的无序分布、原子核密度分布(通过最大熵方法);通过时间分辨中子,可以检测水合物形成及分解过程的热力学和动力学过程.而利用非弹中子可以得到气体分子平移和旋转振动模式以及分子的量子态转变.通过二氧化碳气体注入对天然气水合物的开采可以实现能源气体甲烷的开采和废气二氧化碳的水合物封存,在减小地质灾害和开采成本上有着独特的优势.     

氮气水合物储氢的激光拉曼光谱研究

近年来,笼型水合物储氢已成为储氢研究的热点之一。采用激光拉曼光谱开展了以氮气水合物为载体的储氢实验研究。在较为温和的条件下(15 MPa, -18 ℃),使合成的氮气水合物与氢气发生反应,对反应产物的拉曼光谱分析结果显示,氢气分子进入到水合物的笼型结构中,并且呈现出多分子的笼占有状态;氮气水合物与氢气的反应时间是影响储氢效果的重要因素。研究结果表明,氮气水合物有希望成为一种有效的储氢介质。     

甲烷水合物的固体核磁共振碳谱与激光拉曼光谱研究

甲烷水合物（CH₄·nH₂O）是主要由甲烷和水分子构成的冰状笼型化合物,在自然界储量巨大．固体核磁共振（NMR）波谱和激光拉曼光谱是在分子水平分析甲烷水合物的重要手段．该文利用低温固体核磁共振碳谱（¹³C NMR）对合成的甲烷水合物结构进行了研究,分别使用¹³C交叉极化（¹³C CP）和高功率质子去偶（¹H HPDEC）2种脉冲程序采集甲烷水合物的¹³C NMR谱图,结合实验结果分析及理论推导可知,使用¹H HPDEC方法得到的¹³C NMR谱图信号更强,更利于定量分析;甲烷气体与冰粉合成的甲烷水合物为I型,其大笼和小笼占有率分别为0.988和0.824,水合数为6.07;甲烷气体与SH2站位沉积物和冰粉合成的甲烷水合物也为I型,其大笼和小笼占有率分别为0.987和0.887,水合数为5.98;SH2站位沉积物使合成的甲烷水合物的小笼占有率提高、水合数降低、水合物饱和度提高．激光拉曼光谱结果证实了上述结果的准确性．该文为甲烷水合物测试提供了重要的方法参考．     

固体核磁共振技术在气体水合物研究中的应用

气体水合物是在低温高压条件下由气体和水形成的笼型化合物,主要有I型,II型和H型3种晶体结构,而固体核磁共振（solid state NMR）是测定其水合指数、笼占有率等结构参数的重要手段. 该文综述了固体核磁共振技术的原理及其在水合物研究中的应用,着重介绍固体核磁共振在水合物结构表征、气体组分的鉴定、结构转化、以及在水合物生成/分解动力学过程监测方面的研究进展. 同时,对其实验方法及测试条件也进行了详细的探讨.     

Clathrate hydrates: recent advances on CH4 and CO2 hydrates,and possible new frontiers

Gas hydrates continue to attract enormous attention throughout the energy industry, as both a hindrance in conventional production and a substantial unconventional resource. Scientists continue to be fascinated by the hydrates’ ability of sequestering large amounts of hydrophobic gases, unusual thermal transport properties and unique molecular structures. Technologically, clathrate hydrates promise advantages in applications as diverse as carbon sequestration and water desalination. The communities interested in hydrates span traditional academic disciplines, including earth science, physical chemistry and petroleum engineering. The studies on this field are equally diverse, including field expeditions to attempt the production of natural gas from hydrate deposits accumulated naturally on the seafloor, to lab-scale studies to exchange CO₂ for CH₄ in hydrates; from theoretical studies to understand the stability of hydrates depending on the guest molecules, to molecular simulations probing nucleation mechanisms. This review highlights a few fundamental questions, with focus on knowledge gaps representing some of the barriers that must be addressed to enable growth in the practical applications of hydrate technology, including natural gas storage, water desalination, CO₂ – CH₄ exchange in hydrate deposits and prevention of hydrate plugs in conventional energy transportation.     

致冷剂简单气体水合物相平衡计算

本文从统计热力学理论出发,结合ｖａｎｄｅｒＷａａｌｓ－Ｐｌａｔｔｅｅｕｗ理想固溶休假设。给出ＨＦＣ１５２ａ和ＨＣＦＣ１４１ｂ简单气体水合物相平衡计算模型,并进行计算。计算结果与实验数据进行比较,很好吻合,正确反映了简单致冷剂水合物的相平衡规律。本研究为进一步混合气体水合物相平衡计算打下基础。     

Theoretical investigation of structures,compositions, and phase transitions of neon hydrates based on ices Ih and II

Thermodynamic conditions of existence in the p-T plane and the composition of neon hydrates based on ices Ih and II are determined. The occupancy of neon in cages (channels) of ices Ih and II at temperatures below 0°C is calculated. It is shown that the occupancy of neon in hydrate based on ices cages decreases with growing temperature. Lines of monovariant equilibria between gas phase (neon)-neon hydrate based on ice Ih-liquid water (or ice II) and neon-gas phase (neon)-hydrate based on ice II-liquidwater (or ice II) are found. The line of divariant equilibria between neon hydrate based on ice Ih-neon hydrate based on ice II has been also calculated. The possibility of ice stabilization due to inclusion of neon into ice cages (channels) is shown.     

Modeling the properties of methane + ethane (Propane) binary hydrates,depending on the composition of gas phase state in equilibrium with hydrate

The properties of methane + ethane and methane + propane hydrates of cubic structures sI and sII are theoretically investigated. It is shown that the composition of the formed binary hydrate strongly depends on the percentage of a heavier guest in gas phase. For instance, for a 1% molar ethane concentration in gas phase, even at a low pressure, ethane occupies 60% large cavities in the hydrate sII, and as the pressure grows to 100 atm, it occupies 80% large cavities at a low temperature. The tendency remains the same at a temperature of higher than the ice melting point, but the methane concentration in the hydrate decreases to 30%. In the structure sI, the influence of the component composition of the gas mixture on that of the formed hydrate is less evident. However, in this case, calculation showed also that for a 1% molar ethane concentration in gas phase, ethane molecules occupy from 8 to 10% large hydrate cavities, depending on the pressure. This work is concerned with modeling phase transitions between cubic structures sI and sII of methane + ethane binary hydrates in view of incomplete occupation of cavities in the hydrate by guest molecules. For an ethane concentration under 2% in the gas mixture, the structure sII becomes more thermodynamically stable than the structure sI. However, as the ethane concentration grows to 20% in the equilibrium ice-gas-hydrate and to 40% in the equilibrium water-gas-hydrate, the structure sI becomes more thermodynamically stable. Hence, for low ethane concentrations in a gas mixture, the structure sI can be formed only as a metastable phase. Phase equilibria of methane hydrate sI and mixed methane + propane hydrate sII are considered, depending on the gas phase composition. Similar results are obtained for this equilibrium; this can evidence simultaneous formation of hydrates sI and sII at low propane concentrations.     

Novel results on structural investigations of natural minerals of clathrate hydrates

High-resolution powder X-ray diffraction measurements were carried out on gas hydrates in the angle-dispersive mode using a synchrotron radiation source. In contrast to the structural studies of laboratory-grown gas hydrates, this study has been performed on naturally grown clathrate hydrates obtained from the sea floor at different geographic locations. While the hydrate samples of the Cascadia Margin exhibit a preponderance of structure I, those from the Gulf of Mexico consist of mixed structures, namely structure II, structure I and structure H. Ice in structure Ih is inherently present in all the clathrate hydrate samples. PACS 61.10.Nz; 61.50.Ah; 61.66.Fn; 91.50.-r     

CO2在气体水合物结构特性的固体核磁共振波谱与拉曼光谱实验研究

天然气水合物是蕴含着巨大能源潜力的非常规能源,2017年和2020年两次我国南海探索性试采的成功,加快了天然气水合物项目的进展。二氧化碳置换开采法,既能开发CH₄,又能封存CO₂。同时水合物法分离烟气中CO₂具有很好的应用前景,而CO₂在气体水合物的微观结构和特性尚不明确,实际应用存在一定的未知影响。为了考察其特性,利用13C固体核磁技术（NMR）和拉曼光谱（Raman）进行CO₂置换CH₄水合物、合成13CO₂-H₂-CP混合水合物实验表征,讨论CO₂在水合物中的定量问题,研究CO₂分子在笼型结构中的分布,探讨CO₂分子在气体水合物中的结构类型和特性。结果表明：（1）利用Raman费米低频共振1 277.5 cm-1峰积分得到CO₂在I型大笼（51262笼）的占有率为0.978 2,CH₄在Ⅰ型小笼（512笼）和大笼（51262笼）的占有率为0.059 3和0.009 5,水合数7.61,Raman费米高频共振1 381.3 m-1峰积分得到CO₂在51262笼的占有率为0.984 3,CH₄在512笼和51262笼的占有率为0.023 7和0.003 3,水合数7.70,CO₂几乎占满了大笼,CO₂气体的加入会导致水合物中,CH₄的大、小笼占有率均大幅度降低,置换后水合数略低于纯甲烷水合物,未标记的CO₂水合物在核磁中较难测出信号,CO₂气体置换后CH₄在小笼的占有率仅0.097 5,大笼占有率为0.317 2,两种方法差异主要原因为核磁的CO₂未出峰。（2）利用拉曼费米低频共振1 273.4 cm-1峰积分得到H₂、CO₂在512笼、CP在51262的占有率分别为0.124 8,0.304 2和0.997 8,水合数9.16;Raman费米高频共振1 380.6 cm-1峰积分得到H₂、CO₂在512笼、CP在51262的占有率分别为0.123 6,0.577 1和0.985 1,水合数7.12。13C标记CO₂分子在水合物中达到较好的固体核磁分辨率,首次确认CO₂在Ⅱ型小笼中的化学位移为124.8 ppm,计算得到CO₂的小笼占有率为0.783 1,CP的大笼占有率为0.971 8,水合数6.70,Raman高频频费米共振峰（1 380.6 cm-1）定量计算与13C NMR结果更接近。（3）对CO₂的13C NMR化学位移进行了归属,并结合Raman与13C NMR的对比分析,为CO₂水合物的13C NMR研究和拉曼定量提供参考。     

Characterization of Salting-Out Processes during CO2-Clathrate Formation Using Raman Spectroscopy: Planetological Application

Clathrate hydrates are particular solids that planetologists study in detail because those solids may be present in several bodies of the solar system, such as Mars, comets, and the icy satellites. The solids are formed by solid H₂O, like common water ice, but adopt open structures with cavities containing gas molecules. Clathrate hydrates are usually stable at relatively low temperature and high pressure, which are the typical conditions present inside these planetary objects. Their interest for astrobiology is that they represent potential sources of liquid water and gases when they decompose. The present work is focused on the crystallization of clathrates in Europa's (icy satellite of Jupiter) interior conditions. We postulate that clathrate hydrates may play an important role in its crust mineralogy and that it can explain some features of the satellite's surface due to their formation/destabilization. An in situ kinetic study by Raman Spectroscopy of the clathrate formation from salty solutions was performed in our laboratory. The chemical composition that we used mimics those obtained from Europa's surface during the Galileo mission. An effect of the salting-out process in the solution was monitored through the clathrate formational path. Our results demonstrate that this process may have geological consequences on Europa and confirm the suitability of Raman spectroscopy for planetary detection of clathrate hydrates and other ices.     

Pressure-induced amorphization of clathrate hydrates

Compression and decompression of clathrate hydrates have been carried out in order to investigate the onset pressures above which the crystalline forms begin to collapse at 0 K and do not revert back to the original structures upon decompression. Several proton-disordered structures of clathrate hydrate I encapsulating noble gases were subjected to compression, the steepest descent minimization of potential energy, the subsequent expansion to the original volume, and the steepest descent minimization. It was found that above the onset pressure, depending on guest species, even the fully occupied hydrates are compressed inelastically, and transformed into amorphous forms from which crystalline structures are no longer recovered by decompression at 0 K.     

Density functional theory study of structural stability for gas hydrate

Using the first-principles method based on the density functional theory(DFT),the structures and electronic properties of different gas hydrates(CO_2,CO,CH_4,and H_2) are investigated within the generalized gradient approximation.The structural stability of methane hydrate is studied in this paper.The results show that the carbon dioxide hydrate is more stable than the other three gas hydrates and its binding energy is-2.36 e V,and that the hydrogen hydrate is less stable and the binding energy is-0.36 e V.Water cages experience repulsion from inner gas molecules,which makes the hydrate structure more stable.Comparing the electronic properties of two kinds of water cages,the energy region of the hydrate with methane is low and the peak is close to the left,indicating that the existence of methane increases the stability of the hydrate structure.Comparing the methane molecule in water cages and a single methane molecule,the energy of electron distribution area of the former is low,showing that the filling of methane enhances the stability of hydrate structure.     

Methane combustion in hydrate systems: Water-methane and water-methane-isopropanol

Kinetics of dissociation of synthetic and natural methane gas hydrates, and also double isopropanol-methane hydrate is investigated. Thermal fields of the sample surfaces are measured by means of thermal imaging in combustion of released methane with clathrate dissociation. The dissociation rates of natural hydrate and double hydrate with isopropanol are many times lower than those of synthetic methane hydrate. Methane combustion is accompanied by formation of a thin water film on the powder surface, which has a strong effect on the heat and mass transfer mechanisms. The experiments demonstrated partial self-preservation for methane hydrate and the absence of self-preservation for double isopropanol-methane hydrate. The experimentally observed dissociation rate of double isopropanol-methane hydrate is considerably lower than that of methane hydrate.     

Modelling the effect of methanol,glycol inhibitors and electrolytes on the equilibrium stability of hydrates with the SAFT-VR approach

In this work we integrate the statistical associating fluid theory for fluids interacting through potentials of variable range (SAFT-VR) into a traditional van der Waals and Platteeuw framework for modelling clathrate hydrates. We incorporate a new water–guest cell potential for the hydrate phase that can be related to the potential adopted in the familiar SAFT-VR equation of state for modelling fluids. We show how the ability of this equation of state to treat a wide range of complex fluids increases the scope of hydrate modelling to incorporate, in a single framework, the presence of various inhibitors (alcohols, glycols) or brines – or, indeed, any fluid for which a model is available (for use within SAFT-VR) or can be conveniently obtained. Agreement with experimental results is good throughout and, in many cases, excellent.