Experimental studies of the formation and dissociation of methane hydrate in loess

In order to study the nature of gas hydrate in porous media, the formation and dissociation processes of methane hydrate in loess were investigated. Five cooling rates were applied to form methane hydrate. The nucleation times of methane hydrate formation at each cooling rate were measured for comparison. The experimental results show that cooling rate is a significant factor affecting the nucleation of methane hydrate and gas conversion. Under the same initial conditions, the faster the cooling rate, the shorter the nucleation time, and the lower the methane gas conversion. Five dissociating temperatures were applied to conduct the dissociation experiment of methane hydrate formed in loess. The experimental results indicated that the temperature evidently controlled the dissociation of methane hydrate in loess and the higher the dissociating temperature, the faster the dissociating rates of methane hydrate.     

Methane hydrate dissociation above 0 °C and below 0 °C

The kinetic data of methane hydrate dissociation at various temperatures and pressures were measured in a sapphire cell apparatus by depressurizing method. When the temperature was higher than 0 °C, the experimental results showed that the hydrate dissociation rate was controlled by intrinsic dissociation reaction. When the temperature was lower than 0 °C, water generated from the hydrate dissociation would transform into ice rapidly at the surface of hydrate crystal. The released gas diffused from the hydrate and ice mixture to the bulk of gas phase. With the hydrate continuous dissociation, the boundary of ice–hydrate moved toward water/ice phase. The hydrate dissociation was controlled by gas diffusion, and the hydrate dissociation process was treated as a moving boundary problem. Corresponding kinetic models for hydrate dissociation were established and good agreements with experimental data were achieved.     

Water transfer characteristics during methane hydrate formation and dissociation processes inside saturated sand

Gas hydrates formation and dissociation processes inside porous media are always accompanied by water transfer behavior, which is similar to the water behavior of ice freezing and thawing processes. These processes have been studied by many researchers, but all the studies are so far on the water transfer characteristics outside porous media and the water transfer characteristics inside porous media have been little known. In this study, in order to study the water transfer characteristics inside porous med...     

Methane hydrate formation and decomposition: structural studies via neutron diffraction and empirical potential structure refinement

Neutron diffraction studies with hydrogen/deuterium isotope substitution measurements are performed to investigate the water structure at the early, medium, and late periods of methane clathrate hydrate formation and decomposition. These measurements are coupled with simultaneous gas consumption measurements to track the formation of methane hydrate from a gas/water mixture, and then the complete decomposition of hydrate. Empirical potential structure refinement computer simulations are used to analyze the neutron diffraction data and extract from the data the water structure in the bulk methane hydrate solution. The results highlight the significant changes in the water structure of the remaining liquid at various stages of hydrate formation and decomposition, and give further insight into the way in which hydrates form. The results also have important implications on the memory effect, suggesting that the water structure in the presence of hydrate crystallites is significantly different at equivalent stages of forming compared to decomposing. These results are in sharp contrast to the previously reported cases when all remaining hydrate crystallites are absent from the solution. For these systems there is no detectable change in the water structure or the methane hydration shell before hydrate formation and after decomposition. Based on the new results presented in this paper, it is clear that the local water structure is affected by the presence of hydrate crystallites, which may in turn be responsible for the "history" or "memory" effect where the production of hydrate from a solution of formed and then subsequently melted hydrate is reportedly much quicker than producing hydrate from a fresh water/gas mixture.     

Molecular-dynamics simulations of methane hydrate dissociation

Nonequilibrium molecular-dynamics simulations have been carried out at 276.65 K and 68 bar for the dissolution of spherical methane hydrate crystallites surrounded by a liquid phase. The liquid was composed of pure water or a water-methane mixture ranging in methane composition from 50% to 100% of the corresponding theoretical maximum for the hydrate and ranged in size from about 1600 to 2200 water molecules. Four different crystallites ranging in size from 115 to 230 water molecules were used in the two-phase systems; the nanocrystals were either empty or had a methane occupation from 80% to 100% of the theoretical maximum. The crystal-liquid systems were prepared in two distinct ways, involving constrained melting of a bulk hydrate system or implantation of the crystallite into a separate liquid phase. The breakup rates were very similar for the four different crystal sizes investigated. The method of system preparation was not found to affect the eventual dissociation rates, despite a lag time of approximately 70 ps associated with relaxation of the liquid interfacial layer in the constrained melting approach. The dissolution rates were not affected substantially by methane occupation of the hydrate phase in the 80%-100% range. In contrast, empty hydrate clusters were found to break up significantly more quickly. Our simulations indicate that the diffusion of methane molecules to the surrounding liquid layer from the crystal surface appears to be the rate-controlling step in hydrate breakup. Increasing the size of the liquid phase was found to reduce the initial delay in breakup. We have compared breakup rates computed using different long-range electrostatic methods. Use of the Ewald, minimum image, and spherical cut-off techniques led to more rapid dissociation relative to the Lekner method.     

Thermodynamics of the dissociation of 2-aminopyridinium ion in synthetic seawater and a standard for pH in marine systems

Buffer solutions composed of 2-aminopyridinium chloride and 2-aminopyridine in synthetic seawater are useful as a supplement to buffers of Tris (pH 8.2) and Bis (pH 8.8) in standardizing measurements of hydrogen ion concentration (pm _H or pH(SWS)) in oceanography. The dissociation constant of 2-amino-pyridinium ion over the range of salinities (S) from 30 to 40 has now been determined from the emf of cells without liquid junction at eight temperatures (T) from 278.15 to 313.15 K. The results fit the equation pK=2498.31/T–15.3274+2.4050 lnT+S(0.012928–2.9417×10^–5T) with a standard deviation of 0.0023. Thermodynamic constants for the dissociation process and standard reference values of pm _H and pH(SWS) were derived from the data. The pm _H of the buffer consisting of 2-aminopyridinium chloride and 2-aminopyridine (each 0.04 molal) in synthetic seawater of salinity 35 varies from 7.356 at 278.15 K to 6.601 at 313.15 K.     

Water transfer characteristics in the vertical direction during methane hydrate formation and dissociation processes inside non-saturated media

In order to study water transfer characteristics inside non-saturated media during methane hydrate formation and dissociation processes, water changes on the top, middle and bottom locations of experimental media during the reaction processes were continuously followed with a novel apparatus with three pF-meter sensors. Coarse sand, fine sand and loess were chosen as experimental media. It was experimentally observed that methane hydrate was easier formed inside coarse sand and fine sand than inside loess. Methane hydrate formation configuration and water transfer characteristics during methane hydrate formation processes were very different among the different non-saturated media, which were important for understanding methane hydrate formation and dissociation mechanism inside sediments in nature.     

Molecular dynamics study of carbon dioxide hydrate dissociation

We present results from a molecular dynamics study of the dissociation behavior of carbon dioxide (CO(2)) hydrates. We explore the effects of hydrate occupancy and temperature on the rate of hydrate dissociation. We quantify the rate of dissociation by tracking CO(2) release into the liquid water phase as well as the velocity of the hydrate-liquid water interface. Our results show that the rate of dissociation is dependent on the fractional occupancy of each cage type and cannot be described simply in terms of overall hydrate occupancy. Specifically, we find that hydrates with similar overall occupancy differ in their dissociation behavior depending on whether the small or large cages are empty. In addition, individual cages behave differently depending on their surrounding environment. For the same overall occupancy, filled small and large cages dissociate faster in the presence of empty large cages than when empty small cages are present. Therefore, hydrate dissociation is a collective phenomenon that cannot be described by focusing solely on individual cage behavior.     

Modeling heating curve for gas hydrate dissociation in porous media

A method for modeling the heating curve for gas hydrate dissociation in porous media at isochoric conditions (constant cell volume) is presented. This method consists of using an equation of state of the gas, the cumulative volume distribution (CVD) of the porous medium, and a van der Waals-Platteeuw-type thermodynamic model that includes a capillary term. The proposed method was tested to predict the heating curves for methane hydrate dissociation in a mesoporous silica glass for saturated conditions (liquid volume = pore volume) and for a fractional conversion of water to hydrate of 1 (100% of the available water was converted to hydrate). The shape factor (F) of the hydrate-water interface was found equal to 1, supporting a cylindrical shape for the hydrate particles during hydrate dissociation. Using F = 1, it has been possible to predict the heating curve for different ranges of pressure and temperature. The excellent agreement between the calculated and experimental heating curves supports the validity of our approach.     

Methane hydrate stability in the presence of water-soluble hydroxyalkyl cellulose

The effect of low-dosage water-soluble hydroxyethyl cellulose (approximate MW～90,000 and 250,000) as a member of hydroxyalkyl cellulosic polymer group on methane hydrate stability was investigated by monitoring hydrate dissociation at pressures greater than atmospheric pressure in a closed vessel. In particular, the influence of molecular weight and mass concentration of hydroxyethyl cellulose (HEC) was studied with respect to hydrate formation and dissociation. Methane hydrate formation was performed at 2℃ and at a pressure greater than 100 bar. Afterwards, hydrate dissociation was initiated by step heating from -10℃ at a mild pressure of 13 bar to 3℃, 0℃ and 2℃. With respect to the results obtained for methane hydrate formation/dissociation and the amount of gas uptake, we concluded that HEC 90,000 at 5000 ppm is suitable for long-term gas storage and transportation under a mild pressure of 13 bar and at temperatures below the freezing point.     

The electrochemical behaviour of bronze in synthetic seawater

This paper reports a voltammetric study of bronze in synthetic seawater (SSW). The effects of buffering and deoxygenating were particularly visible in the transpassive region. The breakdown of the anodic passive film on bronze leads to a well-defined activation peak in the transpassive region typical of a nucleation and growth of pits. The breakdown potential of the passivity was shown to vary with the experimental conditions, namely, with buffering and deoxygenating. Buffering has shown to lead to more stable passive films and deoxygenating to higher oxidation currents. Scanning electron microscopy with energy dispersive spectrometer (SEM/EDS) studies of bronze samples with 1-month exposure in non-deoxygenated buffered and non-buffered SSW under open circuit potential have shown significant differences in their morphology: a uniformly cracked surface and a surface showing large and spherical precipitates of about 50 μm uniformly distributed along the surface, respectively, for bronze coupons in buffered (pH 9) and in non-buffered SSW. The EDS technique has identified Cu, O, Cl and Na on the corrosion products of bronze in non-buffered SSW, whilst in buffered media, Sn was also identified. In non-buffered media, open circuit potentials have shown to be all the time less negative than in the buffered media. After 1-month exposure the E _OCP of bronze samples in both media seem to converge to −0.131 and −0.155 V vs Ag|AgCl, respectively. This potential can be assigned to the formation of cuprite, Cu₂O and nantokite, CuCl. The analysis of the SEM images after the removal of the corrosion products has shown descuprification with higher intensity on the surface from coupons in non-buffered SSW.     

Search for memory effects in methane hydrate: structure of water before hydrate formation and after hydrate decomposition

Neutron diffraction with HD isotope substitution has been used to study the formation and decomposition of the methane clathrate hydrate. Using this atomistic technique coupled with simultaneous gas consumption measurements, we have successfully tracked the formation of the sI methane hydrate from a water/gas mixture and then the subsequent decomposition of the hydrate from initiation to completion. These studies demonstrate that the application of neutron diffraction with simultaneous gas consumption measurements provides a powerful method for studying the clathrate hydrate crystal growth and decomposition. We have also used neutron diffraction to examine the water structure before the hydrate growth and after the hydrate decomposition. From the neutron-scattering curves and the empirical potential structure refinement analysis of the data, we find that there is no significant difference between the structure of water before the hydrate formation and the structure of water after the hydrate decomposition. Nor is there any significant change to the methane hydration shell. These results are discussed in the context of widely held views on the existence of memory effects after the hydrate decomposition.     

Kinetic studies of gas hydrate formation with low-dosage hydrate inhibitors

Pipeline blockage by gas hydrates is a serious problem in the petroleum industry. Low-dosage inhibitors have been developed for its cost-effective and environmentally acceptable characteristics. In a 1.072-L reactor with methane, ethane and propane gas mixture under the pressure of about 8.5 MPa at 4 °C, hydrate formation was investigated with low-dosage hydrate inhibitors PVP and GHI1, the change of the compressibility factor and gas composition in the gas phase was analyzed, the gas contents in hydrates were compared with PVP and GHI1 added, and the inhibition mechanism of GHI1 was discussed. The results show that PVP and GHI1 could effectively inhibit the growth of gas hydrates but not nucleation. Under the experimental condition with PVP added, methane and ethane occupied the small cavities of the hydrate crystal unit and the ability of ethane entering into hydrate cavities was weaker than that of methane. GHI1 could effectively inhibit molecules which could more readily form hydrates. The ether and hydroxy group of diethylene glycol monobutyl ether have the responsibility for stronger inhibition ability of GHI1 than PVP.     

Comparison of the water change characteristics between the formation and dissociation of methane hydrate and the freezing and thawing of ice in sand

Hydrate formation and dissociation processes are always accompanied by water migration in porous media, which is similar to the ice. In our study, a novel pF-meter sensor which could detect the changes of water content inside sand was first applied to hydrate formation and dissociation processes. It also can study the water change characteristics in the core scale of a partially saturated silica sand sample and compare the differences of water changes between the processes of formation and dissociation of methane hydrate and freezing and thawing of ice. The experimental results showed that the water changes in the processes of formation and dissociation of methane hydrate were basically similar to that of the freezing and thawing of ice in sand. When methane hydrate or ice was formed, water changes showed the decrease in water content on the whole and the pF values rose following the formation processes. However, there were very obvious differences between the ice thawing and hydrate dissociation.     

Effect of rapidly depressurizing and rising temperature on methane hydrate dissociation

Two methods, rapidly depressurizing to 0.1 MPa at a constant temperature and rising temperature under equilibrium P, T conditions, were used to study the dissociation of pure CH4 hydrate formed below the ice point. At a constant temperature with rapidly depressurizing to 0.1 MPa, CH4 hydrate dissociated rapidly at initial dissociation and then the dissociation rate gradually decreased. However, the dissociation of CH4 hydrate at temperatures of 261 to 266 K was much faster than that at temperatures of 269 to 272 K, indicating its anomalous preservation. Under an equilibrium P, T conditions, rising temperature had extensively controlling impact on dissociation of CH4 hydrate at equilibrium pressures of 2.31, 2.16 and 1.96 MPa. In this study, we report the effect of pressure on CH4 hydrate dissociation, especially the effect of equilibrium pressure on dissociation at various melting temperatures. And we find that the ice particles size of CH4 hydrate formed may dominant the CH4 hydrate dissociation. Dissociation of CH4 hydrate formed from ice particles of smaller than 250 μm may not have an anomalous preservation below the ice point, while particles larger than 250 μm may have more extensive anomalous preservation.     

Some aspects of hydrate formation and wetting

Experimental observations of gas hydrate formation have shown that, in the initial nucleation and crystallization process, water-oil emulsions may be generated, destabilized or even inverted. These phenomena are consistent with the effects of particles on emulsions. In this work we relate experimental observations of hydrate formation to the phenomenon of wettability. It is shown that details of hydrate wetting are important for both the morphology and the kinetics of the formed hydrates. For the cases of hydrate lenses and spheres, it is shown that the various wetting states can be illustrated and analyzed by using wetting diagrams. Metastability is a function of the surface energies of the hydrate formation, i.e., the wetting state, and it is shown that in some cases metastability vanishes, and thus hydrates nucleates instantly at all positive driving forces. The magnitude of buoyancy and turbulence forces acting on a hydrate sphere are compared to the capillary force and it is concluded that capillary energy dominates when the hydrate spheres is less than 1 mm.     

Study of CCl3F hydrate formation and dissociation in W/O emulsion by differential scanning calorimetry and X-ray diffraction

Differential scanning calorimetry (DSC) and time-resolved synchrotron X-ray diffraction as a function of temperature (XRDT) were combined in a novel way in order to study conditions of formation and the amount of gas clathrate formed in dispersed systems. The formation and dissociation of trichlorofluoromethane hydrate CCl₃F·(H₂O)₁₇ in a water-in-oil emulsion were followed by using these combined techniques. An emulsion containing 3 wt.% NaCl was submitted to a cooling and heating cycle between 20 and −50 °C. During cooling, a single exothermic peak at −43 °C, found in DCS thermograms was assigned to the freezing of under-cooled water droplets; however, no noticeable signal related to hydrate crystallisation was detected. Conversely, during subsequent heating, the progressive melting of ice was followed by an endothermic signal indicative of hydrate decomposition. From X-ray diffraction performed on an emulsion sample, it was possible to identify the exact condition of CCl₃F·(H₂O)₁₇ formation. XRDT diffraction patterns clearly demonstrated that only ice crystallised in the aqueous droplets during cooling and that the hydrate only formed during heating simultaneously with melting of ice. From the solid–liquid phase diagrams of systems H₂ONaCl and CCl₃FH₂ONaCl and from the DSC and XRDT experiments, the composition of the droplets was deduced. The upper limit of the amount of hydrate that could form in the system was calculated.     

含低剂量抑制剂体系气体水合物生成动力学

水合物管道堵塞是油气工业安全生产的重要问题之一, 目前低剂量抑制剂以其经济性、环境友好性等优点, 逐步取代传统抑制剂. 文中在8.5 MPa、4 ℃条件下, 1.072 L反应釜内, 采用甲烷、乙烷和丙烷混和气, 研究了含低剂量抑制剂聚乙烯吡咯烷酮(PVP)和GHI1的水合物生成体系反应过程, 计算分析了压缩因子和自由气量随反应时间的变化, 对比了在相同反应程度下添加PVP和GHI1后水合物含气量的区别, 探讨了GHI1组合抑制剂的抑制机理. 实验结果表明PVP和GHI1能抑制水合物生长, 不能有效抑制水合物成核; 添加PVP的体系, 在实验气体组成下, 甲烷乙烷进入水合物小晶穴, 并且甲烷优先进入小晶穴; GHI1对丙烷乙烷的抑制能力强于甲烷; 对比GHI1和PVP的反应过程, 认为协同剂二乙二醇丁醚的羟基和醚类结构加强反应体系中的氢键, 和PVP结合使用, 通过氢键和空阻达到抑制效果.