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.     

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.     

Three-phase equation of state by the lattice gas model

A rigid lattice gas model of solid-liquid, liquid-gas, and solid-gas transformations is proposed. The total free energy is separated into a lattice part and a vibrational correction part. It is reported that the vibrational part plays an important role in obtaining the gas, liquid and solid phases and the correct phase transitions. A method of combining the two lowest order approximations (single site, two-site) in the cluster variation method is suggested in describing the lattice free energy. The critical point and the triple point of argon are reproduced fairly well. Results are compared with the expandable lattice theory demonstrating the appropriateness of the present model in the vicinity of the triple point.     

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.     

Spin-lattice relaxation of CH4 gas in a deuterated clathrate lattice

The proton T₁ temperature dependence of CH₄ gas in a deuterated clathrate lattice deviates from the semiclassifical predictions. The discrepancy is partially accounted for by including the effects of symmetry and the quantization of rotational states.     

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.     

Interacting hadron resonance gas meets lattice QCD

We present, in the framework of the interacting hadron resonance gas, an evaluation of thermodynamical quantities. The interaction is modelled via a correction for the finite size of the hadrons. We investigate the sensitivity of the model calculations on the radius of the hadrons, which is a parameter of the model. Our calculations for thermodynamical quantities as energy and entropy densities and pressure are confronted with predictions using the lattice Quantum Chromodynamics (QCD) formalism.     

Correlation correction to the free energy for the ising model of a binary alloy

We consider correlation corrections to the free energy of the Ising model of a binary solid solution. The correlation free energy is represented in the form of a high-temperature series, in which only the leading terms in the parameter 1/z are retained, where z is the number of lattice sites inside the effective interaction sphere of the interatomic potential. In some cases the series can be assumed and an explicit formula can be obtained for the correlation corrections. As one of the consequences of our results, we obtain a correction to the mean field theory expression for the order-disorder transition point.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 86–91, March, 1987.     

Critical point field mixing in an asymmetric lattice gas model

The field mixing that manifests broken particlehole symmetry, is studied for a 2-D asymmetric lattice gas model having tunable field mixing properties. Monte Carlo simulations within the grand canonical ensemble are used to obtain the critical density distribution for different degrees of particle-hole asymmetry. Except in the special case when this asymmetry vanishes, the density distributions exhibit an antisymmetric correction to the limiting scale-invariant form. The presence of this correction reflects the mixing of the critical energy density into the ordering operator. Its functional form is found to be in excellent agreement with that predicted by the mixed-fied finite-size-scaling theory of Bruce and Wilding. A computational procedure for measuring the signiffcant field mixing parameter is also described, and its accuracy gauged by comparing the results with exact values obtained analytically.     

Gaussian Mean Field Lattice Gas

We study rigorously a lattice gas version of the Sherrington–Kirckpatrick spin glass model. In discrete optimization literature this problem is known as unconstrained binary quadratic programming and it belongs to the class NP-hard. We prove that the fluctuations of the ground state energy tend to vanish in the thermodynamic limit, and we give a lower bound of such ground state energy. Then we present a heuristic algorithm, based on a probabilistic cellular automaton, which seems to be able to find configurations with energy very close to the minimum, even for quite large instances.     

Simulation of ultrasonic-driven gas separations

The separation of components in a gas mixture is important for a wide range of applications. One method for achieving this separation is by passing a traveling acoustic wave through the gas mixture, which creates a flux of the lighter components away from the transducer. A series of simulations was performed to assess the effectiveness of this method for separating a binary mixture of argon and helium using the lattice kinetics method. The energy transport equation was modified to account for adiabatic expansion and compression. The species transport equation was modified to include a barodiffusion term. Simulations were performed on two different scales; detailed acoustic wave simulations to determine the net component flux as a function of local concentration, pressure, etc. and device scale simulations to predict the gas composition as a function of time inside a gas separation cylinder. The method is first validated using data from literature and then applied to mixtures of argon and helium. Results are presented and discussed.     

A non-Hamiltonian formulation of the Ising chain

The Gibbs states of binary lattice systems can be characterized by their stability with respect to certain microscopic transitions which have a simple physical interpretation. A detailed analysis is provided for the case of a one-dimensional lattice gas with nearest-neighbor interactions.     

空间辐射效应的蒙特-卡罗模拟

 概述了天然宇宙空间的辐射环境,简要地分析了辐射效应机制,在辐射效应的蒙特-卡罗模拟中对靶材料作了无定形假设,入射粒子在靶材料中的弹性能量损失采用经典二体散射公式,非弹性能量损失高能时采用Beth-Bloch公式,低能时采用Lindhard-Scharff公式,中能时采用插值公式,撞出晶格原子引起的次级损伤用Kinchin-Pease模型计算,最后对100KeV硼离子入射于硅材料引起的辐射效应进行了模拟计算,并给出了计算结果和分析。     

用分子动力学模拟甲烷水合物热激法分解

用分子动力学模拟方法研究甲烷水合物热激法分解，系统地研究注入340 K液态水的结构Ⅰ型甲烷水合物的分解机理.模拟显示水合物表层水分子与高温液态水分子接触获得热能，分子运动激烈，摆脱水分子间的氢键束缚，笼状结构被破坏.甲烷分子获得热能从笼中挣脱，向外体系扩散.热能通过分子碰撞从外层传递给内层水分子，水合物逐层分解.对比注入277K液态水体系模拟结果，得出热激法促进水合物分解. 关键词： 甲烷水合物 分子动力学模拟 热激法     

Computer simulations of particle flux and energy peaking

A computer simulation of the channeling of 300 keV deuterons in a niobium lattice has been performed. The model calculations included: binary Thomas-Fermi interaction between projectiles and lattice atoms, thermal displacements of the lattice atoms in the transverse plane of particle motion, electron scattering of the deuterons dependent on the local electron density, and electronic energy loss dependent on the local electron density. Flux distributions and mean particle energy distributions of the deuterons were calculated in their development with penetration depth. Flux peaking and energy peaking was found to depend sensitively on the local variation of the electron density in the crystal.     

Quantum fluctuations and collective oscillations of a Bose-Einstein condensate in a 2D optical lattice

We use Bogoliubov theory to calculate the beyond mean field correction to the equation of state of a weakly interacting Bose gas in the presence of a tight 2D optical lattice. We show that the lattice induces a characteristic 3D to 1D crossover in the behavior of quantum fluctuations. Using the hydrodynamic theory of superfluids, we calculate the corresponding shift of the collective frequencies of a harmonically trapped gas. We find that this correction can be of the order of a few percent and hence easily measurable in current experiments. The behavior of the quantum depletion of the condensate is also discussed.     

The two-dimensional Peierls-Nabarro model for interfacial misfit dislocation networks of cubic lattice

A model is developed to investigate the two-dimensional interfacial misfit dislocation networks that follows the original Peierls-Nabarro idea. Structure and energies of heterophase interfaces are considered for the cubic lattice. To examine the energy contribution of misfit dislocations, where interactions between two dislocation arrays are concerned, a generalized stacking fault energy is proposed. Combined with first-principles calculations, we apply this model to a practical metal-ceramic example: the Ag/MgO(100) interface. An important correction to the adhesive energy is proposed in addition to its dislocation structure being confirmed.     

Variational Cumulant Expansion on SU(2)⊗ SU(2) Chiral Model

The phase transition of the two-dimensional SU(2)⊗ SU(2) lattice chiral model has been studied by a variational cumulant expansion method. The internal energy and the specific heat are calculated to the 4th order correction and they are in good agreement with the Monte Carlo (MC) resdits. Except the behavior of the crossover region, similarities between this model and the four-dimensional SU(2) lattice gauge model are also discussed.     

Thermal distribution functions and finite-size effects for lattice fermions

We perform, analytically, the sum over the finite set of Matsubara frequencies that arises in the lattice thermodynamics of a free Fermi gas. The resultant expressions provide an easy demonstration that the correction continuum expressions are obtained in the limit of small lattice spacing.