Investigation on Gas Storage in Methane Hydrate

The effect of additives (anionic surfactant sodium dodecyl sulfate (SDS), nonionic surfactantalkyl polysaccharide glycoside (APG), and liquid hydrocarbon cyclopentane (CP)) on hydrate inductiontime and formation rate, and storage capacity was studied in this work. Micelle surfactant solutions werefound to reduce hydrate induction time, increase methane hydrate formation rate and improve methanestorage capacity in hydrates. In the presence of surfactant, hydrate could form quickly in a quiescentsystem and the energy costs of hydrate formation were reduced. The critical micelle concentrations of SDS and APG water solutions were found to be 300x 10-6 and 500x 10-6 for methane hydrate formation systemrespectively. The effect of anionic surfactant (SDS) on methane storage in hydrates is more pronounced compared to a nonionic surfactant (APG). CP also reduced hydrate induction time and improved hydrateformation rate, but could not improve methane storage in hydrates.     

Adsorption of surfactants on two different hydrates

The interaction between surfactants and hydrates provides insight into the role of surfactants in promoting hydrate formation. This work aims at understanding the adsorption behavior of sodium dodecyl sulfate (SDS) on cyclopentane (CP) hydrates and its derivative surfactant on tetrabutylammonium bromide (TBAB) hydrates. Cyclopentane (CP) is a hydrophobic former whereas tetrabutylammonium bromide (TBAB) is a salt that forms semiclathrate hydrates. The adsorption on these two hydrates was studied by zeta potential and pyrene fluorescence measurements. CP hydrates have a negative surface charge in the absence of SDS, and it decreases to a minimum as the SDS concentration increases from 0 to 0.17 mM. Then, it increases with further increased SDS concentration. The adsorption density of DS (-) on CP hydrates reaches a saturated value at 1.73 mM SDS. The micropolarity parameter of the TBAB hydrate/water interface starts to increase rapidly at 0.17 mM SDS and levels off at 1.73 mM SDS. The presence of Br (-) in TBAB hydrate suspensions could compete with TBADS (from association of DS (-) and TBA (+)) and DS (-) for the adsorption on the hydrate surface, but they have a much stronger affinity for the hydrates than does Br (-). From the fluorescence measurements, it was found that the micropolarity of the hydrate/water interface is mainly dependent on the polarity of hydrate formers.     

Experimental and modeling study on decomposition kinetics of methane hydrates in different media

The decomposition kinetic behaviors of methane hydrates formed in 5 cm3 porous wet activated carbon were studied experimentally in a closed system in the temperature range of 275.8-264.4 K. The decomposition rates of methane hydrates formed from 5 cm3 of pure free water and an aqueous solution of 650 g x m(-3) sodium dodecyl sulfate (SDS) were also measured for comparison. The decomposition rates of methane hydrates in seven different cases were compared. The results showed that the methane hydrates dissociate more rapidly in porous activated carbon than in free systems. A mathematical model was developed for describing the decomposition kinetic behavior of methane hydrates below ice point based on an ice-shielding mechanism in which a porous ice layer was assumed to be formed during the decomposition of hydrate, and the diffusion of methane molecules through it was assumed to be one of the control steps. The parameters of the model were determined by correlating the decomposition rate data, and the activation energies were further determined with respect to three different media. The model was found to well describe the decomposition kinetic behavior of methane hydrate in different media.     

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.     

An experimental study of crystallization and crystal growth of methane hydrates from melting ice

An experiment with well defined gas-water interfacial surface area was developed to study the crystallization and crystal growth of methane hydrates. Measurable formation rates were observed only when melting ice was involved. No hydrates nucleated from liquid water or from non-melting ice. It is concluded that melting ice, which like hydrate water is hydrogen-bonded, provides a template for hydrate nucleation as well as providing a heat sink for absorbing the heat of formation during hydrate growth. The experiment was conducted in the absence of mixing so that hydrate crystals grew under quiescent conditions.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

常见客体分子对笼型水合物晶格常数的影响

Natural gas hydrates are considered as ideal alternative energy resources for the future, and the relevant basic and applied research has become more attractive in recent years. The influence of guest molecules on the hydrate crystal lattice parameters is of great significances to the understanding of hydrate structural characteristics, hydrate formation/decomposition mechanisms, and phase stability behaviors. In this study, we test a series of artificial hydrate samples containing different guest molecules (e.g. methane, ethane, propane, iso-butane, carbon dioxide, tetrahydrofuran, methane + 2, 2-dimethylbutane, and methane + methyl cyclohexane) by a low-temperature powder X-ray diffraction (PXRD). Results show that PXRD effectively elucidates structural characteristics of the natural gas hydrate samples, including crystal lattice parameters and structure types. The relationships between guest molecule sizes and crystal lattice parameters reveal that different guest molecules have different controlling behaviors on the hydrate types and crystal lattice constants. First, a positive correlation between the lattice constants and the van der Waals diameters of homologous hydrocarbon gases was observed in the single-guest-component hydrates. Small hydrocarbon homologous gases, such as methane and ethane, tended to form sI hydrates, whereas relatively larger molecules, such as propane and iso-butane, generated sⅡ hydrates. The hydrate crystal lattice constants increased with increasing guest molecule size. The types of hydrates composed of oxygen-containing guest molecules (such as CO₂ and THF) were also controlled by the van der Waals diameters. However, no positive correlation between the lattice constants and the van der Waals diameters of guest molecules in hydrocarbon hydrates was observed for CO₂ hydrate and THF hydrate, probably due to the special interactions between the guest oxygen atoms and hydrate "cages". Furthermore, the influences of the macromolecules and auxiliary small molecules on the lengths of the different crystal axes of the sH hydrates showed inverse trends. Compared to the methane + 2, 2-dimethylbutane hydrate sample, the length of the a-axis direction of the methane + methyl cyclohexane hydrate sample was slightly smaller, whereas the length of the c-axis direction was slightly longer. The crystal a-axis length of the sH hydrate sample formed with nitrogen molecules was slightly longer, whereas the c-axis was shorter than that of the methane + 2, 2-dimethylbutane hydrate sample at the same temperature.     

P-T stability conditions of methane hydrate in sediment from South China Sea

For reasonable assessment and safe exploitation of marine gas hydrate resource, it is important to determine the stability conditions of gas hydrates in marine sediment. In this paper, the seafloor water sample and sediment sample (saturated with pore water) from Shenhu Area of South China Sea were used to synthesize methane hydrates, and the stability conditions of methane hydrates were investigated by multi-step heating dissociation method. Preliminary experimental results show that the dissociation temperature of methane hydrate both in seafloor water and marine sediment, under any given pressure, is depressed by approximately -1.4 K relative to the pure water system. This phenomenon indicates that hydrate stability in marine sediment is mainly affected by pore water ions.     

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.     

Solubility measurement of methane in aqueous solution of sodium dodecyl sulfate at ambient temperature and near hydrate conditions

Solubility data of methane in aqueous solutions of sodium dodecyl sulfate (SDS) with different concentrations were measured at ambient temperature and near hydrate conditions. The critical micelle concentration (CMC) and the number of methane molecules dissolved in each micelle of the methane + water + SDS system were calculated and compared with those of the ethylene + water + SDS system. The results demonstrated that the micelles could be formed in the SDS concentration range where an efficient promotion effect on hydrate formation was previously reported; the micelle solubilization to methane molecules was remarkable near hydrate conditions, and the ethylene molecules could be solubilized in micelles in preference to methane molecules.     

Modulated DSC for gas hydrates analysis

Modulated DSC has been applied to the study of methane, ethane and propane hydrates at different hydrate and ice concentrations. The reversing component of the TMDSC curves, makes it possible to characterize such hydrates. Methane and ethane hydrates show the melting-decomposition peak at a temperatures higher than the ice contained in the sample, while propane hydrate melts and decomposes at lower temperature than the ice present in the sample. The hydrate peaks tend to disappear if the hydrate is stored at atmospheric pressure. Guest size and cavity occupation fix the heat of dissociation and stability of the hydrates, as confirmed by parallel tests on tetrahydrofurane hydrates.     

Sorption equilibria of CO2/CH4 mixture on activated carbon in presence of water

The sorption isotherms of CO2 + CH4 mixtures on an activated carbon were collected in the presence of water at a temperature suitable for hydrate formation. The equilibrium composition of both phases was determined. The initial concentration of CO2 in mixtures was set at 33, 38 and 42%, and the total pressure was up to 10 MPa. CO2 hydrates were firstly formed following the increase of total pressure, and CO2 dominates the sorbed phase composition. CO2 concentration in the sorbed phase begins to decrease when the partial pressure of methane allows for the formation of methane hydrates. Competition for hydrate cavities was observed between CO2 and CH4 as reflected in the isotherm shape and phase composition at equilibrium. The formation pressure of hydrates is lower for mixtures than for pure gases, and the highest sorption capacity of each gas decreased in the mixture sorption either.     

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.     

Guest-Host Interaction Study in Clathrate Hydrates Using Lattice Dynamics Simulation

Lattice dynamics simulation of several gas hydrates （helium, argon, and methane） with different occupancy rates has been performed using TIP3P potential model. Results show that the coupling between the guest and host is not simple as depicted by the conventional viewpoints. For clathrate hydrate enclosing small guest, the small cages are dominantly responsible for the thermodynamic stability of clathrate hydrates. And the spectrum of methane hydrate is studied compared with argon hydrate, then as a result, shrink effect from positive hydrogen shell is proposed.     

Self-preservation of methane hydrates produced in “dry water”

The first results of studying the possibility of self-preservation of methane hydrates produced in a “dry-water” dispersion were presented. It was shown for the first time that the anomalously low rates of dissociation of gas hydrates at a temperature below 273 K and a pressure of 0.1 MPa, which were previously known for methane hydrates, are also characteristic of methane hydrates forming in dry water. Methane hydrates obtained in dry water containing no more than 5 wt % stabilizer (hydrophobized silica nanoparticles) are primarily solids at a pressure of 0.1 MPa and a temperature below 273 K. At a stabilizer content of dry water of 10 or 15 wt %, a significant part of the hydrate sample looks like a free-flowing powder. The powder fraction increases with increasing stabilizer content, which reduces the efficiency of self-preservation of methane hydrates.     

Kinetics and stability of CH4-CO2 mixed gas hydrates during formation and long-term storage.

The formation of CH4-CO2 mixed gas hydrates was observed by measuring the change of vapor-phase composition using gas chromatography and Raman spectroscopy. Preferential consumption of carbon dioxide molecules was found during hydrate formation, which agreed well with thermodynamic calculations. Both Raman spectroscopic analysis and the thermodynamic calculation indicated that the kinetics of this mixed gas hydrate system was controlled by the competition of both molecules to be enclathrated into the hydrate cages. However, the methane molecules were preferentially crystallized in the early stages of hydrate formation when the initial methane concentration was much less than that of carbon dioxide. According to the Roman spectra, pure methane hydrates first formed under this condition. This unique phenomenon suggested that methane molecules play important roles in the hydrate formation process. These mixed gas hydrates were stored at atmospheric pressure and 190 K for over two months to examine the stability of the encaged gases. During storage, CO2 was preferentially released. According to our thermodynamic analysis, this CO2 release was due to the instability of CO2 in the hydrate structure under the storage conditions.     

Experimental study and modeling of the kinetics of refrigerant hydrate formation

The purpose of this study was to identify compatible hydrate forming-refrigerants suitable for air-conditioning systems. The main challenge in designing an air conditioning system which utilises refrigerant hydrates as a media for storage of cold energy is the rate of formation and dissociation of the refrigerant hydrates. Hence, in this experimental study the kinetics of hydrate formation of three refrigerant blends, viz. R407C, R410A and R507C have been investigated. The induction time for hydrate formation, apparent rate constant of the hydrate reaction, water to hydrate conversion during hydrate growth, storage capacity, and the rate of hydrate formation of these refrigerants at various pressures and temperatures have been obtained using a kinetic model. The effect of sodium dodecyl sulfate (SDS) on the hydrate nucleation rate was also investigated.     

Estimation of ultra-stability of methane hydrate at 1 atm by thermal conductivity measurement

Thermal conductivity of methane hydrate was measured in hydrate dissociation self-preservation zone by means of the transient plane source (TPS) technique developed by Gustafsson. The sample was formed from 99.9% (volume ratio) methane gas with 280 ppm sodium dodecyl sulfate (SDS) solution under 6.6 MPa and 273.15 K. The methane hydrate sample was taken out of the cell and moved into a low temperature chamber when the conversion ratio of water was more than 90%. In order to measure the thermal conductivity, the sample was compacted into two columnar parts by compact tool at 268.15 K. The measurements are carried out in the temperature ranging from 263.15 K to 271.15 K at atmospheric pressure. Additionally, the relationship between thermal conductivity and time is also investigated at 263.15 K and 268.15 K, respectively. In 24 h, thermal conductivity increases only 5.45% at 268.15 K, but thermal conductivity increases 196.29% at 263.15 K. Methane hydrates exhibit only minimal decomposition at 1 atm and the temperature ranging from 263.15 K to 271.15 K. At 1 atm and 268.15 K, the total gas that evolved after 24 h was amounted to less than 0.71% of the originally stored gas, and this ultra-stability was maintained if the test was lasted for more than two hundreds hours before terminating.     

Crystal growth simulations of methane hydrates in the presence of silica surfaces

We present a molecular dynamics simulation study of the crystal growth of methane hydrates in the presence of model silica (SiO(2)) surfaces. The crystal growth under apparent steady-state conditions shows a clear preference for bulk solution. We observe rather disordered water arrangements very close to the silica surface within about 5 ? in both liquid and crystalline regions of the system. These disordered structures have dynamic and structural properties intermediate between those exhibited by molecules in bulk liquid and crystalline phases. The presence of methane molecules appears to help stabilize these structures. We observe that under appropriate conditions, the hydroxylated silica surfaces can serve as a source of methane molecules which can help promote hydrate growth near the surfaces.     

Gas hydrates of argon and methane synthesized at high pressures: composition, thermal expansion, and self-preservation

For the first time, the compositions of argon and methane high-pressure gas hydrates have been directly determined. The studied samples of the gas hydrates were prepared under high-pressure conditions and quenched at 77 K. The composition of the argon hydrate (structure H, stable at 460-770 MPa) was found to be Ar.(3.27 +/- 0.17)H(2)O. This result shows a good agreement with the refinement of the argon hydrate structure using neutron powder diffraction data and helps to rationalize the evolution of hydrate structures in the Ar-H(2)O system at high pressures. The quenched argon hydrate was found to dissociate in two steps. The first step (170-190 K) corresponds to a partial dissociation of the hydrate and the self-preservation of a residual part of the hydrate with an ice cover. Presumably, significant amounts of ice Ic form at this stage. The second step (210-230 K) corresponds to the dissociation of the residual part of the hydrate. The composition of the methane hydrate (cubic structure I, stable up to 620 MPa) was found to be CH(4).5.76H(2)O. Temperature dependence of the unit cell parameters for both hydrates has been also studied. Calculated from these results, the thermal expansivities for the structure H argon hydrate are alpha(a) = 76.6 K(-1) and alpha(c) = 77.4 K(-1) (in the 100-250 K temperature range) and for the cubic structure I methane hydrate are alpha(a) = 32.2 K(-1), alpha(a) = 53.0 K(-1), and alpha(a) = 73.5 K(-1) at 100, 150, and 200 K, respectively.     

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.