Clathrate hydrates of hexagonal structure III at high pressures: Structures and phase diagrams

It was shown in this work that the clathrate hydrates of Hexagonal Structure III, formed in the ternary systems 1-methylpiperazine-help gas-water and iso-amyl alcohol-help gas-water are stable in a wide range of pressures. The decomposition curves of these hydrates were studied for the first time up to the pressures 1 GPa. Ar, Kr, Xe and CH₄ were utilized as the help gases. In a number of the systems studied, high pressure phases were revealed that presumably form due to the distortion of the corresponding low pressure hydrate structures.     

Double clathrate hydrates of cross-linked tetrabutylammonium polyacrylate and noble gases at high pressures

Temperatures of hydrate decomposition were measured by means of the differential thermal analysis at a pressures up to 800–900 Mpa in the systems: cross-linked tetrabutylammonium polyacrylate–water and cross-linked tetrabutylammonium polyacrylate–water–noble gas (He, Ne, Ar, Kr, Xe). The effect of the deformation of D- cavities of the hydrates on the temperature of their decomposition is discussed on the basis of the experimental data.     

The thermal analysis of polymers at high pressures

The extension of two techniques of thermal analysis into the region high pressures (50–100 MPa) are discussed. One is the extension of dilatometry (thus becoming pressure-volume-temperature measurements, PVT). This technique has been well established over the past few years. Some results obtained on typical polymer systems are presented and discussed. The second is the extension of the differential thermal analysis (DTA) principle to high pressures, trying to maintain some of the advantages of the DTA technique when compared to the PVT method, such as small sample size and productivity. DTA determinations of the pressure dependence of the melting points of pure metals and polymers are presented and compared with results from the PVT technique. Satisfactory agreement is obtained. The advantages and limitations of our current high-pressure DTA method are discussed.We gratefully acknowledge the partial financial support of the high-pressure DTA development by Metler Instrumente AG, Switzerland.     

Monte Carlo and molecular dynamics simulations of methane in potassium montmorillonite clay hydrates at elevated pressures and temperatures

The structure and dynamics of methane in hydrated potassium montmorillonite clay have been studied under conditions encountered in sedimentary basin and compared to those of hydrated sodium montmorillonite clay using computer simulation techniques. The simulated systems contain two molecular layers of water and followed gradients of 150 bar km(-1) and 30 K km(-1) up to a maximum burial depth of 6 km. Methane particle is coordinated to about 19 oxygen atoms, with 6 of these coming from the clay surface oxygen. Potassium ions tend to move away from the center towards the clay surface, in contrast to the behavior observed with the hydrated sodium form. The clay surface affinity for methane was found to be higher in the hydrated K-form. Methane diffusion in the two-layer hydrated K-montmorillonite increases from 0.39 x 10(-9) m2 s(-1) at 280 K to 3.27 x 10(-9) m2 s(-1) at 460 K compared to 0.36 x 10(-9) m2 s(-1) at 280 K to 4.26 x 10(-9) m2 s(-1) at 460 K in Na-montmorillonite hydrate. The distributions of the potassium ions were found to vary in the hydrates when compared to those of sodium form. Water molecules were also found to be very mobile in the potassium clay hydrates compared to sodium clay hydrates.     

Entropy and mobility of nitrogen and argon adsorbed on active carbon at high pressures

Summary Entropy changes have been calculated for the adsorption of nitrogen and argon on active carbon at high pressures, measured at –78, –25, 0 and 20 C byvon Antropoff. The results indicate that both the adsorbed gases behave as mobile two-dimensional layers with restricted freedom for translational movements. At surface coverage values above 0.13 the adsorbed nitrogen molecules undergo greater entropy loss than the argon molecules at same surface coverage, suggesting new restrictions on the rotational freedom of the former. The onset of lateral interactions and the consequent restricted mobility at such low surface coverages are explained as due to the over-crowding of adsorbed molecules on the relatively more active parts of the surface rather than being distributed uniformly on the entire surface.

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Zusammenfassung Für die Adsorption von N₂ und Argon wurden EntropieÄnderungen bei Adsorption an aktivem Kohlenstoff und hohen Drücken berechnet aus Messungen bei –78, –25, 0 und 20 C vonvon Antropoff. Die Ergebnisse zeigen, da\ sich beide Gase als bewegliche zweidimensionale Schichten mit behinderter Freiheit für Translationsbewegungen benehmen.Bei Bedeckungswerten über 0,13 der adsorbierten N₂-Molekule besitzen die adsorbierten N₂-Moleküle grö\eren Entropieverlust als Argonmoleküle. Das lÄ\t neue Behinderungen hinsichtlich der Rotation bei ersteren vermuten. Der Einsatz lateraler Wechselwirkung und die daraus folgende behinderte Beweglichkeit wird bei diesen niedrigen OberflÄchenbedeckungen aus der Ansammlung adsorbierter Moleküle in relativ stÄrker aktiven OberflÄchenteilen erklÄrt. Das ist besser, als eine gleichmÄ\ige Verteilung auf der ganzen OberflÄche anzunehmen.

     

A new and reliable model for predicting methane viscosity at high pressures and high temperatures

In recent years, there has been an increase of interest in the flow of gases at relatively high pressures and high temperatures. Hydrodynamic calculation of the energy losses in the flow of gases in conduits, as well as through the porous media constituting natural petroleum reservoirs, requires knowledge of the viscosity of the fluid at the pressure and temperature involved. Although there are numerous publications concerning the viscosity of methane at atmospheric pressure, there appears to be little information available relating to the effect of pressure and temperature upon the viscosity. A survey of the literature reveals that the disagreements between published data on the viscosity of methane are common and that most investigations have been conducted over restricted temperature and pressure ranges. Experimental viscosity data for methane are presented for temperatures from 320 to 400 K and pressures from 3000 to 140000 kPa by using falling body viscometer. A summary is given to evaluate the available data for methane, and a comparison is presented for that data common to the experimental range reported in this paper. A new and reliable correlation for methane gas viscosity is presented. Predicted values are given for temperatures up to 400 K and pressures up to 140000 kPa with Average Absolute Percent Relative Error (EABS) of 0.794.     

Gas sorption and diffusion processes in polymer matrices at high pressures

A theoretical approach has been developed to describe the processes of gases diffusion and sorption in rubbery and glassy polymers. Various models (Flory-Huggins, dual-mode sorption, gas-polymer-matrix) used for interpreting the sorption-diffusion experiments are discussed within this approach framework. Experimental data on carbon dioxide sorption in glassy and rubbery polymers have been considered using the proposed approach. The comparison of the experimental and theoretical data has permitted to make the conclusion on the developed concepts adequacy. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1339–1348, 1997     

Molecular dynamics simulations of the mechanisms of thermal conduction in methane hydrates

The thermal conductivity of methane hydrate is an important physical parameter affecting the processes of methane hydrate exploration,mining,gas hydrate storage and transportation as well as other applications.Equilibrium molecular dynamics simulations and the Green-Kubo method have been employed for systems from fully occupied to vacant occupied sI methane hydrate in order to estimate their thermal conductivity.The estimations were carried out at temperatures from 203.15 to 263.15 K and at pressures from 3 to 100 MPa.Potential models selected for water were TIP4P,TIP4P-Ew,TIP4P/2005,TIP4P-FQ and TIP4P/Ice.The effects of varying the ratio of the host and guest molecules and the external thermobaric conditions on the thermal conductivity of methane hydrate were studied.The results indicated that the thermal conductivity of methane hydrate is essentially determined by the cage framework which constitutes the hydrate lattice and the cage framework has only slightly higher thermal conductivity in the presence of the guest molecules.Inclusion of more guest molecules in the cage improves the thermal conductivity of methane hydrate.It is also revealed that the thermal conductivity of the sI hydrate shows a similar variation with temperature.Pressure also has an effect on the thermal conductivity,particularly at higher pressures.As the pressure increases,slightly higher thermal conductivities result.Changes in density have little impact on the thermal conductivity of methane hydrate.     

甲烷水合物导热机理的分子动力学模拟

甲烷水合物导热系数是甲烷水合物勘探、开采、储运以及其他应用过程中一个十分重要的物理参数.我们采用平衡分子动力学（EMD）方法Green-Kubo理论计算温度203.15～263.15K、压力范围3～100MPa、晶穴占有率为0～1的sI甲烷水合物的导热系数,采用的水分子模型包括TIP4P、TIP4P-Ew、TIP4P-FQ、TIP4P/2005、TIP4P/Ice.研究了主客体分子、外界温压条件等对甲烷水合物导热性能的影响.研究结果显示甲烷水合物的低导热性能由主体分子构建的sI笼型结构决定,而客体分子进入笼型结构后,使得笼型结构导热性能增强,同时进入笼型结构的客体分子越多,甲烷水合物导热性能越强.研究结果还显示在高温区域（T〉TDebye/3）内不同温度作用下,所有sI水合物具有相似的导热规律.压力对导热系数有一定影响,尤其是在较高压力条件下,压力越高,导热系数越大.而在不同温度和不同压力作用过程中,密度的改变对导热系数的增大或减小几乎没有影响.     

Determination of molecular surface structure, composition, and dynamics under reaction conditions at high pressures and at the solid-liquid interface

In the last two decades, surface-science experiments and techniques have been developed to focus on obtaining molecular information under reaction conditions at high pressures (near or above 1 bar) and liquid interfaces. This Minireview describes the results of these studies obtained by surface-sensitive laser spectroscopies, scanning tunneling microscopy, and X-ray spectroscopies usually practiced at a synchrotron light source. The use of model surfaces, single crystals, and monodisperse nanoparticles with variable size (1-10 nm) and shape facilitates meaningful interpretation of the experimental data. These methods allow evaluation of the molecular structures of intermediates, oxidation states of metals, and mobility of adsorbants. New techniques that are likely to make major contributions to the investigation of surfaces under reaction conditions are also discussed.     

Specific features of the growth,composition, and content of natural gas hydrates synthesized in inverted oil emulsions

A study of specific features of the growth, composition, and content of natural gas hydrates formed in a water-in-oil emulsion demonstrated that the process in which hydrates are formed in a water-oil emulsion occurs in stages and depends on the saturation of hydrate growth zones with the hydrate-forming gas via diffusion of natural-gas components across the oil phase. Hydrates enriched in methane are formed in water-oil emulsions, compared with the hydrates grown from distilled water, which is accounted for by the difference in solubility between natural-gas components in oil and water, and also by the presence of a surfactant layer on the surface of emulsified water drops. With increasing fraction of water in an emulsion, the content of hydrates decreases, and the mass of a hydrate being formed is independent of the composition of the water-oil emulsion.     

CS-II binary clathrate hydrates at pressures of up to 15 kbar

The pressure dependence of the decomposition temperatures of binary clathrate hydrates of tetra-hydrofuran with xenon and methane as well as of chloroform and carbon tetrachloride clathrates with xenon has been studied. The absence of phase transitions at pressures from 1 to 15,000 bar indicates that the structure of all the hydrates remains constant (CS-II). The decomposition temperatures of the binary hydrates of tetrahydrofuran and carbon tetrachloride with xenon at 15 kbar (above 124ℴC) are exceedingly high for polyhedral clathrate hydrates because the guest molecules are highly complementary to the cavities of the clathrate lattice. The paper also considers the packing density effect in the crystal structure of hydrates on the behavior of the latter at elevated pressure. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 3, pp. 582-589, May–June, 2000.     

The deaquation reactions of some metal salt hydrates at elevated pressures

The deaquation reactions of BaCl₂·2H₂O, BaBr₂·2H₂O and CoCl₂·6H₂0 were studied by the thermal analysis techniques of thermogravimetry, differential thermal analysis (DTA), and electrical conductivity in the pressure range from one to 170 atm. In general, the effect of pressure on the TG curves increased the T_i and T_f values and also the reaction interval, (T_t—T_i). The DTA curves exhibited splittings into multiple peaks as a result of the increased pressure. These splittings were interpreted as due to the evolution of a liquid water phase followed b     

Thermal expansion and impurity effects on lattice thermal conductivity of solid argon

Thermal expansion and impurity effects on the lattice thermal conductivity of solid argon have been investigated with equilibrium molecular dynamics simulation. Thermal conductivity is simulated over the temperature range of 20-80 K. Thermal expansion effects, which strongly reduce thermal conductivity, are incorporated into the simulations using experimentally measured lattice constants of solid argon at different temperatures. It is found that the experimentally measured deviations from a T(-1) high-temperature dependence in thermal conductivity can be quantitatively attributed to thermal expansion effects. Phonon scattering on defects also contributes to the deviations. Comparison of simulation results on argon lattices with vacancy and impurity defects to those predicted from the theoretical models of Klemens and Ashegi et al. demonstrates that phonon scattering on impurities due to lattice strain is stronger than that due to differences in mass between the defect and the surrounding matrix. In addition, the results indicate the utility of molecular dynamics simulation for determining parameters in theoretical impurity scattering models under a wide range of conditions. It is also confirmed from the simulation results that thermal conductivity is not sensitive to the impurity concentration at high temperatures.     

Comparison of the thermal stability of β- and β″-alumina at high pressures

The high-pressure thermal stability of Na⁺ β-alumina and magnesium-stabilized Na⁺ β″-alumina was studied from 5 to 50 Kb and from room temperature to 1400°C. Above 5 Kb and 560°C, Na⁺ β-alumina decomposes into α-Al₂O₃ and NaAlO₂. Magnesium-stabilized Na⁺ β″-alumina decomposes at slightly higher temperatures to α-Al₂O₃, MgAl₂O₄, and probably NaAlO₂. Estimates of activation volume and enthalpy support a model for decomposition that depends on cation vacancies and proceeds by sodium diffusion in quasi-liquid conduction planes.