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.     

Novel methodology for the calculation of the enthalpy of enclathration of methane hydrates using molecular dynamics simulations

Methane hydrates are encountered in a plethora of industrial and geological or environmental applications. In the current study, we present a novel methodology which is based on molecular dynamics simulations for the calculation of the enthalpy of enclathration of sI methane hydrates. Simulations are performed along the three-phase (Hydrate – Liquid water – Vapour; H–L_w–V) equilibrium line in the temperature range 274–310?K. The methodology takes into account the two different types of cages that are present in the sI methane hydrate and provides results for the enthalpy of enclathration for both types of cages, while it avoids performing calculations with the metastable, completely empty hydrate lattice. The formulation proposed is general and can be also applied to sII hydrates, while it can be modified/extended appropriately for use in the case of sH hydrates. Comparison is provided with available data from the literature and good agreement is observed.     

基于Raman光谱技术的瓦斯混合气水合过程定量分析

瓦斯水合物微观晶体结构研究对水合分离技术具有重要理论意义。利用Raman光谱技术对三种含高浓度CO₂瓦斯混合气水合反应过程进行在线观测,并对水合物相Raman光谱图进行分析,获取了瓦斯水合物不同生长阶段大、小孔穴占有率,同时利用van der Waals与Platteeuw热力学统计模型间接获得水合指数等晶体结构信息。结果表明,瓦斯水合物孔穴占有率及水合指数在水合物不同生长阶段未发生较大变化,水合物相中大孔穴几乎被客体分子填满,CO₂与CH₄分子共同占据大孔穴,但CO₂占绝大多数,达到78.58%~94.09%,CH₄分子仅为4.52%~19.12%,这主要是由于两种分子间存在竞争关系且气样中CO₂浓度明显高于CH₄,大孔穴占有率维持在97.70%~98.68%;小孔穴占有率为17.93%~82.41%,占有率普遍偏低,且仅有CH₄分子;随气样中CH₄浓度增加,CH₄在大、小孔穴中的占有率均有所增加,且CH₄分子在大孔穴中的占有率均明显低于在小孔穴中占有率;水合物生长不同阶段水合指数为6.13~7.33,随气样中CH₄浓度的增加,小孔穴占有率有所增加,致使水合指数随之降低;由于瓦斯水合物生长分布不均匀,三种气样对应的不同生长阶段水合指数均呈不规则变化。     

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.     

不同驱动力下瓦斯水合物生长过程Raman光谱特征

基于煤矿瓦斯(CH₄∶C₂H₆∶N₂=67.5∶22.5∶10)水合物相平衡曲线开展四种驱动力ΔP水合动力学实验,利用可见显微Raman光谱仪获取水合物生长过程光谱图,根据水合物相中C₂H₆ C-C键伸缩振动特征峰Raman位移确定了4组实验中水合物为sⅡ结构。基于van der Waals与Platteeuw模型获取瓦斯水合物生成过程中水合物相气体组分及水合指数变化规律。研究表明： 驱动力的大小影响水合物的稳定性,随着驱动力的增加,CH₄相比C₂H₆逐渐占据更多的孔穴结构,CH₄在水合物相内比例增加,水合物稳定性越强;瓦斯中N₂,CH₄和C₂H₆进入水合物孔穴优先级可以通过分子与水合物孔穴的直径比进行确定,分析认为在sⅡ水合物结构中小孔穴CH₄优先级最高,大孔穴C₂H₆最高;基于瓦斯水合物稳定性,对水合物生长过程客体分子的物质传递规律进行描述,为瓦斯水合物的微观生长提供理论基础。     

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.     

含乙二醇水合物形成条件理论与实验研究

利用可视化高压流体测试装置在0.78～5.17 MPa压力范围内测定了乙二醇水溶液中合成天然气（甲烷、乙烷和丙烷的混合物）水合物的形成条件。根据vanderWaals-Platteeuw的理想溶液等温吸附理论和Moshfeghian-Maddocd的数学模型,给出了含抑制剂体系气体水合物相平衡计算数学模型。计算结果表明该模型可较好地预测含抑制剂（乙二醇）体系的水合物形成条件。     

Structural changes in gas hydrates and existence of a filled ice structure of methane hydrate above 40 GPa

Recent studies have contributed a great deal to our understanding of the structural changes in gas hydrates under pressure. A feature of structural changes in terms of pressure and guest size is summarised in the former part of this paper. The gas hydrate with the guest size from argon to methane finally take a common filled ice I_h structure, although they have different initial and intermediate structures. In the latter part, retention of the filled ice I_h structure of methane hydrate up to 42 GPa is described. In situ X-ray diffractometry and optical observation revealed the existence of the filled ice structure with the volume change of 40% and large anisotropic compressibility. The distances between oxygen atoms estimated from the unit cell parameters suggested that symmetrization of hydrogen bonds occurred in the structure.     

Measurement and modeling of the sooting propensity of binary fuel mixtures

The sooting behaviour of binary fuel mixtures was evaluated both experimentally and through computer simulations. The soot volume fraction in laminar diffusion flames of mixtures of ethylene/propane, methane/ethylene, methane/propane, methane/ethane, methane/butane, ethane/propane and ethane/ethylene fuels was measured using 2-dimensional line of sight attenuation. A synergistic effect was observed for the ethylene/propane, methane/ethylene, methane/ethane and ethane/ethylene mixtures. The synergistic effect translated into a higher soot concentration for a mixture fraction than could be yielded by the added contribution of both pure fuels. Such an effect was not observed for the methane/propane, methane/butane and ethane/propane mixtures. Through experiments in which the flame temperature was kept constant, it was determined that the synergistic effect in the methane/ethylene mixture is very temperature dependent whereas, that in the ethylene/propane mixture is not. This phenomenon was further studied through the modeling of the ethylene/propane mixture. Numerical simulations were carried out using two different soot models. The simulations confirmed the presence of a synergistic effect. It was found that the effect could be directly correlated to a synergistic effect in the concentration of n-C₄H₅ and n-C₄H₃, which could be traced back to an interaction between ethylene and methyl radical species. These results yield further insight into the pathways to soot formation and highlight the importance of further analyzing binary fuel mixtures as a means of understanding soot formation in practical devices using industrial fuels.     

二元致冷剂气体水合物相平衡计算

本文在测定 ＨＦＣ１５２ａ／ＨＣＦＣ１４１ｂ和 ＨＦＣ１３４ａ／ＨＣＦＣ１４１ｂ 二元气体水合物相平衡数据的基础上［３］,建立二元混合气体水合物计算模型和方法。利用 Ｗｉｌｓｏｎ活度系数理论和Ｌａｎｇｍｕｉｒ等温吸附理论建立了水合物相各组成的逸度计算模型,并依据水合物中客体分子的逸度平衡条件进行了混合水合物的相平衡计算。计算结果正确地表征混合气体水合物相平衡特性,并与实验数据较精确吻合。     

二氧化碳-甲烷混合气体水合物四相区实验研究

以水合物的形式封存CO₂和置换海底的天然气（CH₄）水合物需要对CO₂-CH₄混合水合物的四相平衡状态及数据有清楚的了解。本文通过实验和模型计算对不同组分的CO₂-CH₄混合水合物的较高四相区（Q2）相平衡进行了测定和表述。实验温度范围为273.16～297 15 K,压力范围分为0～10 MPa。四相区的温度压力范围分别是283.51到287.04 K和4.74到8.37 MPa,甲烷的摩尔组份为0～0.225。结果揭示了相平衡温度和压力随着甲烷组分而变化情况以及四相区的范围和临界点,同时还给出了CO₂-CH₄混合气体水合物在四相状态下的融化开始和融化结束点。实验结果与热力学模型计算得出的CO₂-CH₄混合气体水合物相平衡结果进行比较,两者很好吻合,四相平衡区域的存在范围得以明确。     

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…     

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．     

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.     

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.     

粒子群优化算法在混合气体红外光谱定量分析中的应用

通过将粒子群优化技术及BP神经网络技术相结合,建立了三种烃烷混合气体的红外光谱定量分析模型。混合气体主要由甲烷、乙烷、丙烷三种组分气体组成,三种组分气体浓度范围分别为0.01%～0.1%。文章首先采用主成分分析技术从红外光谱1 866个数据中提取了5个特征变量作为神经网络的输入,将气体浓度作为网络输出。然后将粒子群优化算法与BP神经网络技术相结合,对网络的隐含层节点数进行了优化选择。再对结构优化后的网络进行训练,建立气体分析模型。分析模型的标准气体验证实验结果表明,采用此方法建立混合气体红外光谱定量分析模型所用时间(大约4 600 s)比单纯采用BP神经网络进行遍历优化建模所用时间(大约24 500 s)降低5倍以上,模型预测精度水平相当,网络结构大致相同,具有一定的实践意义和应用潜力。     

天然流体包裹体中甲烷水合物生成条件原位变温拉曼光谱研究

准确获取流体包裹体中气体水合物的生成条件一直是传统包裹体分析方法面临的一个难题。文章采用原位拉曼光谱技术分析了天然流体包裹体中甲烷水合物的生成条件。并由常温拉曼光谱分析表明,研究流体包裹体的流体组成为CH4-H2O体系。通过三种方法控制实验温度的变化,在第三种方法实验条件下获得了-170 ℃时甲烷水合物与冰的拉曼光谱,逐渐升温原位观测甲烷水合物的消失温度。原位拉曼光谱检测结果表明,研究包裹体中甲烷水合物的生成温度为7.5 ℃。结合CH4-H2O体系水合物形成条件相平衡计算,得到包裹体中甲烷水合物生成时的压力为5.587 3 MPa。研究结果表明,原位拉曼光谱技术是准确获取流体包裹体种气体水合物生成条件的一种有效方法。     

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。     

Adsorption of alkanes and their coadsorption with hydrogen on evaporated palladium thin films

Adsorption of methane, ethane and propane and their coadsorption with hydrogen on evaporated palladium thin films at 195 K and 298 K was investigated by means of surface potential change measurements, observation of gas phase composition and volumetric measurements. Adsorption at 195 K occurs without any significant decomposition of hydrocarbon admolecules, while at 298 K approximately 10% of the total deposit of ethane and propane can be decomposed. Propane admolecules strongly promote the incorporation of coadsorbed hydrogen adatoms into the bulk of palladium film at 195 K.