Influence of gravitational potential on the thermodynamic stability of pure and mixed clathrate hydrates

Clathrate hydrates are an ice-like material consisting of gas molecules confined within cavities in a crystalline water lattice. Phase equilibria of clathrate hydrates systems was described using the statistical mechanical theory of van der Waals and Platteeuw. This theory makes use of the fractional occupancy of cavities within the clathrate hydrate lattice in the determination of chemical equilibria. Classical density functional theory with intermolecular interactions restricted to the first hydration shell was employed to determine the fractional occupancy. In addition to the external field describing the gas-water interactions, the effect of a gravitational field was introduced. The results of the calculations show that although the gravitational potential term may be orders of magnitude smaller than the thermal kinetic energy of the gas species or the hydrogen-bond energy holding the clathrate lattice together, it can nevertheless influence the phase equilibrium of the clathrate hydrate system to some degree. The effect of the magnitudes of both the gravitational potential and the local gravitational field are considered too.     

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.     

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.     

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。     

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.     

CO2水合物在电解质溶液中的相平衡预测

关于二氧化碳水合物基础物性的研究,对二氧化碳置换开采天然气水合物和工业废气二氧化碳的深海埋藏具有重要意义。本文在Van der Waals-Platteeuw理论和Pizter的电解质模型的基础上,考虑了电解质和温度对二氧化碳在水中溶解度的影响,从理论上对水合物相平衡模型进行了修正,提高了预测的精度。本文预测了二氧化碳水合物在NaCl、KCl、CaCl₂、MgCl₂的一元及二元溶液体系中的相平衡条件,预测的最大压力误差7.92%,平均压力误差在1%～3%之间。结果表明,本文的理论预测结果和实验数据吻合较好。     

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.     

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     

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.     

Theoretical investigation of the possibility of using multicomponent (N2-O2-CH4-Y2O) clathrate hydrates for methane recovery from mine gas

Phase equilibria in a multicomponent ice-gas mixture-hydrate system are investigated for a mixture of nitrogen, oxygen, and methane, depending on the gas phase composition, pressure, and temperature. Equilibrium compositions of the formed hydrates are found, depending on the gas mixture composition. Calculations show that with increasing concentration ofmethane in gas phase the pressure of hydrate formation gradually decreases. It is shown that this pressure at considerable methane concentrations approximately corresponds to partial pressure of methane in the gas phase. Conditions of hydrate formation are calculated in the range of temperatures from 258 to 273 K, at pressures from 1 to 350 atm. The obtained results are in agreement with the known experimental data for hydrates of pure gases-nitrogen, oxygen, and methane.     

用分子动力学模拟甲烷水合物热激法结合化学试剂法分解

用分子动力学模拟方法研究甲烷水合物的热激法，化学试剂法，以及热激法结合化学试剂法分解，系统研究温度为277K和340K时添加液态水(WTR)和30wt％乙二醇(EG)溶液对水合物分解的影响.模拟显示WTR与水合物表面水分子形成氢键，破坏水合物原有的氢键平衡，造成笼状结构坍塌，水合物分解.EG分子中的羟基与水合物表面水分子形成氢键，从而破坏原有的稳定结构，造成水合物笼状结构被破坏，达到促进水合物分解，释放甲烷气体的效果.比较温度为277K和340K时添加WTR和30wt％EG溶液对水合物分解效果得出EG(340K)> WTR(340K)>EG(277K)>WTR(277K)，热激法结合化学试剂法能更好促进水合物分解.     

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.     

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.     

Molecular dynamics simulations of PVP kinetic inhibitor in liquid water and hydrate/liquid water systems

The possible effects of PVP (poly(N-vinylpyrrolidone)) on the properties of liquid and water in clathrate hydrate has been investigated using NVT molecular dynamics simulations. A model for a monomer of the PVP polymer is immersed in three systems, liquid water, a unit cell of a hydrate in liquid water with a hydrate former and a third system where some of the liquid water molecules of this last system are replaced by a PVP monomer. Both molecular dynamics simulation and integral equation theory predict hydrogen bonding between the double bonded oxygen in the PVP ring and hydrogens in water. For the composite system, the PVP monomer has a preference for hydrogen bonding to hydrogens from the water molecules at the surface of the hydrate lattice. The simulations indicate that the PVP monomer tends to orient perpendicular to the hydrate surface. For the model systems in this study PVP may form hydrogen bonds with liquid water through the double bonded oxygen in the ring. When a hydrate crystal is immersed in the liquid water phase this hydrogen bonding is shifted towards the hydrate due to a more favourable Coulomb interaction involving hydrogens from more than one water molecule at the hydrate surface. The PVP monomer has a preference for perpendicular orientation with respect to the hydrate surface. A scheme is suggested for the characterization of kinetic hydrate inhibitors based on molecular dynamics simulations and on three basic properties. In addition to the energy between the active groups of the inhibitor and hydrate water another point of focus is the free energy changes in the interactions between the inhibitor and water as the charges are changed from zero to the original model charges. In particular the difference between this integral for the (hydrate water)–(PVP monomer) interaction and the (liquid water)–(PVP inhibitor) interaction should reflect the driving forces in freezing the inhibitor out from the liquid water phase and onto the hydrate surface. The third property in the characterization scheme is the diffusivities of groups connecting to the hydrate crystal, relative to the diffusivities of the hydrate crystal. Results are presented from simulations where a small cavity with a methane model as a guest is immersed in liquid water with free methane molecules at a temperature of 150K. Changes in structure, diffusivities and energy indicate a tendency towards a more solid–like structurde around the cavity.     

Island formation and condensation of a chemisorbed overlayer

The condensation of a chemisorbed overlayer from a lattice gas into a particular ordered structure in discussed in terms of the lattice-gas theory of Lee and Yang. The formation of islands of ordered structure is identified with the condensation phenomenon predicted by the theory. The phase diagram (transition temperature versus coverage) based on the theory of a two-dimensional Ising ferromagnet in zero magnetic field is constructed for the condensation of a lattice gas system with net attractive interactions between the particles. It is demonstrated that critical points at coverages other than θ = 0.5 are achieved for overlayer systems with unit meshes larger than (1 × 1). Low-energy electron diffraction results of the thermal disordering (island dissolution) for oxygen chemisorbed on W(110) are compared with the theory, and the effect of substrate surface heterogeneity on the phase diagram is discussed.     

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。     

HFC152a/HCFC141b混合气体水合物相平衡特性研究

1引言气体水合可使水在8—12”C络合结晶形成水合物，同时释放反应热（蓄冷）。其反应热与传热性能均较理想，被认为是理想的空调蓄冷介质山。早期研究的主要是Rll和R12的水合物。由于Rll和R12严重破坏大气臭氧层而受控，因此寻找替代CFC的新型致冷剂气体水合物成为当前的研究重点［‘－‘］。近年来我们根据混合物性能互补原理，提出了利用混合气体水合物构造性能优良的蓄冷材料的设想［‘1。HFC152a和HCFC141b分别是R12和Rll的首选替代物之一。HFC152a是较活跃的致水合物质，但相变温度和压力偏高。而HCFC141b水合物的相变温度…     

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.     

Progress and bottlenecks in simulating lattice gauge theory with fermions

We present a progress report in lattice gauge theory computer simulations which includes the effects of light, dynamical fermions. Microcanonical and hybrid microcanonical-Langevin alogrithms are presented and discussed. A method for “accelerating” stochastic differential equations and defeating critical slowing down is reviewed. Physics applications such as the thermodynamics of quantum chromodynamics, hierarchal energy scales in unified gauge theories, and the phase diagram of theories with many fermion species are discussed. Prospects for future research are assessed.