Thermodynamic calculations in the system CH4-H2O and methane hydrate phase equilibria

Using the Gibbs function of reaction, equilibrium pressure, temperature conditions for the formation of methane clathrate hydrate have been calculated from the thermodynamic properties of phases in the system CH4-H2O. The thermodynamic model accurately reproduces the published phase-equilibria data to within +/-2 K of the observed equilibrium boundaries in the range 0.08-117 MPa and 190-307 K. The model also provides an estimate of the third-law entropy of methane hydrate at 273.15 K, 0.1 MPa of 56.2 J mol(-1) K(-1) for 1/nCH4.H2O, where n is the hydrate number. Agreement between the calculated and published phase-equilibria data is optimized when the hydrate composition is fixed and independent of the pressure and temperature for the conditions modeled.     

Thermodynamic properties of methane hydrate in quartz powder

Using the experimental method of precision adiabatic calorimetry, the thermodynamic (equilibrium) properties of methane hydrate in quartz sand with a grain size of 90-100 microm have been studied in the temperature range of 260-290 K and at pressures up to 10 MPa. The equilibrium curves for the water-methane hydrate-gas and ice-methane hydrate-gas transitions, hydration number, latent heat of hydrate decomposition along the equilibrium three-phase curves, and the specific heat capacity of the hydrate have been obtained. It has been experimentally shown that the equilibrium three-phase curves of the methane hydrate in porous media are shifted to the lower temperature and high pressure with respect to the equilibrium curves of the bulk hydrate. In these experiments, we have found that the specific heat capacity of the hydrate, within the accuracy of our measurements, coincides with the heat capacity of ice. The latent heat of the hydrate dissociation for the ice-hydrate-gas transition is equal to 143 +/- 10 J/g, whereas, for the transition from hydrate to water and gas, the latent heat is 415 +/- 15 J/g. The hydration number has been evaluated in the different hydrate conditions and has been found to be equal to n = 6.16 +/- 0.06. In addition, the influence of the water saturation of the porous media and its distribution over the porous space on the measured parameters has been experimentally studied.     

Phase equilibrium measurements and crystallographic analyses on structure-H type gas hydrate formed from the CH4-CO2-neohexane-water system

Phase equilibrium conditions and the crystallographic properties of structure-H type gas hydrates containing various amounts of methane (CH4), carbon dioxide (CO2), neohexane (2,2-dimethylbutane; NH), and liquid water were investigated. When the CH4 concentration was as high as approximately 70%, the phase equilibrium pressure of the structure-H hydrate, which included NH, was about 1 MPa lower at a given temperature than that of the structure-I hydrate with the same composition (except for a lack of NH). However, as the CO2 concentration increased, the pressure difference between the structures became smaller and, at CO2 concentrations below 50%, the phase equilibrium line for the structure-H hydrate crossed that for the structure I. This cross point occurred at a lower temperature at higher CO2 concentration. Extrapolating this relation between the cross point and the CO2 concentration to 100% CO2 suggests that the cross-point temperature would be far below 273.2 K. It is then difficult to form structure-H hydrates in the CO2-NH-liquid water system. To examine the structure, guest composition, and formation process of structure-H hydrates at various CH4-CO2 compositions, we used the methods of Raman spectroscopy, X-ray diffraction, and gas chromatography. Raman spectroscopic analyses indicated that the CH4 molecules were found to occupy both 5(12) and 4(3)5(6)6(3) cages, but they preferably occupied only the 5(12) cages. On the other hand, the CO2 molecules appeared to be trapped only in the 4(3)5(6)6(3) cages. Thus, the CO2 molecules aided the formation of structure-H hydrates even though they reduced the stability of that structure. This encaged condition of guest molecules was also compared with the theoretical calculations. In the batch-type reactor, this process may cause the fractionation of the remaining vapor composition in the opposite sense as that for CH4-CO2 hydrate (structure-I), and thus may result in an alternating formation of structure-H hydrates and structure-I in the same batch-type reactor.     

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.     

Mechanisms for thermal conduction in hydrogen hydrate

Extensive equilibrium molecular dynamics simulations have been performed to investigate thermal conduction mechanisms via the Green-Kubo approach for (type II) hydrogen hydrate, at 0.05 kbar and between 30 and 250 K, for both lightly filled H(2) hydrates (1s4l) and for more densely filled H(2) systems (2s4l), in which four H(2) molecules are present in the large cavities, with respective single- and double-occupation of the small cages. The TIP4P water model was used in conjunction with a fully atomistic hydrogen potential along with long-range Ewald electrostatics. It was found that substantially less damping in guest-host energy transfer is present in hydrogen hydrate as is observed in common type I clathrates (e.g., methane hydrate), but more akin in to previous results for type II and H methane hydrate polymorphs. This gives rise to larger thermal conductivities relative to common type I hydrates, and also larger than type II and H methane hydrate polymorphs, and a more crystal-like temperature dependence of the thermal conductivity.     

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.     

Stabilization of methane hydrate by pressurization with He or N2 gas

The behavior of methane hydrate was investigated after it was pressurized with helium or nitrogen gas in a test system by monitoring the gas compositions. The results obtained indicate that even when the partial pressure of methane gas in such a system is lower than the equilibrium pressure at a certain temperature, the dissociation rate of methane hydrate is greatly depressed by pressurization with helium or nitrogen gas. This phenomenon is only observed when the total pressure of methane and helium (or nitrogen) gas in the system is greater than the equilibrium pressure required to stabilize methane hydrate with just methane gas. The following model has been proposed to explain the observed phenomenon: (1) Gas bubbles develop at the hydrate surface during hydrate dissociation, and there is a pressure balance between the methane gas inside the gas bubbles and the external pressurizing gas (methane and helium or nitrogen), as transmitted through the water film; as a result the methane gas in the gas bubbles stabilizes the hydrate surface covered with bubbles when the total gas pressure is greater than the equilibrium pressure of the methane hydrate at that temperature; this situation persists until the gas in the bubbles becomes sufficiently dilute in methane or until the surface becomes bubble-free. (2) In case of direct contact of methane hydrate with water, the water surrounding the hydrate is supersaturated with methane released upon hydrate dissociation; consequently, methane hydrate is stabilized when the hydrostatic pressure is above the equilibrium pressure of methane hydrate at a certain temperature, again until the dissolved gas at the surface becomes sufficiently dilute in methane. In essence, the phenomenon is due to the presence of a nonequilibrium state where there is a chemical potential gradient from the solid hydrate particles to the bulk solution that exists as long as solid hydrate remains.     

Measurements of methane hydrate equilibrium in systems inhibited with NaCl and methanol

Natural gas hydrates are ice-like inclusion compounds that form at high pressures and low temperatures in the presence of water and light hydrocarbons. Hydrate formation conditions are favorable in gas and oil pipelines, and their formation threatens gas and oil production. Thermodynamic hydrate inhibitors (THIs) are chemicals (e.g., methanol, monoethylene glycol) deployed in gas pipelines to depress the equilibrium temperature required for hydrate formation. This work presents a novel application of a stepwise differential scanning calorimeter (DSC) measurement to accurately determine the methane hydrate phase boundary in the presence of THIs. The scheme is first validated on a model (ice + salt water) system, and then generalized to measure hydrate equilibrium temperatures for pure systems and 0.035 mass fraction NaCl solutions diluted to 0, 0.05, 0.10, and 0.20 mass fraction methanol. The hydrate equilibrium temperatures are measured at methane pressures from (7.0 to 20.0) MPa. The measured equilibrium temperatures are compared to values computed by the predictive hydrate equilibrium tool CSMGem.     

Investigation of hydrate formation in the system H2-CH4-H2O at a pressure up to 250 MPa

Phase equilibria in the system H2-CH4-H2O are investigated by means of differential thermal analysis within hydrogen concentration range 0-70 mol % and at a pressure up to 250 MPa. All the experiments were carried out under the conditions of gas excess. With an increase in hydrogen concentration in the initial gas mixture, decomposition temperature of the formed hydrates decreased. X-ray diffraction patterns and Raman spectra of the quenched hydrate samples obtained at a pressure of 20 MPA from a gas mixture containing 40 mol % hydrogen were recorded. It turned out that the hydrate has cubic structure I under these conditions. The Raman spectra showed that hydrogen molecules are not detected in the hydrate within the sensitivity of the method, that is, almost pure methane hydrate is formed. The general view of the phase diagram of the investigated system is proposed. A thermodynamic model was proposed to explain a decrease in hydrate decomposition temperature in the system with an increase in the concentration of hydrogen in the initial mixture.     

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.     

气体分子对甲烷水合物稳定性的影响

通过B3LYP方法, 在6-31G(d,p)水平下, 分别优化了结构I型甲烷水合物十二面体和十四面体晶穴结构. 结果表明, CH4分子使晶穴的相互作用能降低, 增强了晶穴的稳定性. 计算了晶穴中甲烷分子C—H键的对称伸缩振动频率, 计算结果与实验值相符合. 研究发现CH4分子影响晶穴中氧原子的电荷分布, 从而增强了氢键的稳定性. 通过分子动力学方法研究水合物晶胞中气体分子的占有率对水合物稳定性的影响, 进一步说明气体分子对水合物晶穴稳定性的重要作用.     

Phase transformation of methane hydrate under high pressure

In situ Raman spectroscopy is employed to study the phase behavior of methane hydrate at high pressure. The structure 1 of methane hydrate can be maintained up to 950 MPa at 299 K. The transformation of structure I<-->structure H+water+CH4 occurs at 880 MPa and 323 K. The structure H of methane hydrate, however, decomposes to methane and water at 960 MPa and 348 K. The initiation mechanism of methane hydrate sI is also discussed.     

Thermodynamic stability of structure-H hydrates of methylcyclopentane and cyclooctane helped by methane

The four-phase equilibria were measured for the methylcyclopentane+methane+H₂O hydrate system (274.28–287.40 K, 1.75–9.34 MPa) and the cyclooctane+methane+H₂O hydrate system (274.08–288.57 K, 1.60–9.33 MPa). Each structure-H hydrate has the lower equilibrium pressure than the pure methane structure-I hydrate in the temperature range of the present work. The isothermal equilibrium pressures of both methylcyclopentane and cyclooctane hydrates are slightly higher than that of methylcyclohexane hydrate.     

Texture change of ice on anomalously preserved methane clathrate hydrate

We used a confocal scanning microscope to observe growth and texture change of ice due to the dissociation of methane gas clathrate hydrate (CH(4) hydrate). The experiments were done under CH(4) gas atmospheric pressure and isothermal conditions between 170 and 268 K. Above 193 K, the dissociation of CH(4) hydrate resulted in many small ice particles that covered the hydrate surface. These ice particles had roughly the same shape and density between 193 and 210 K. In contrast, above 230 K the ice particles developed into a sheet of ice that covered the hydrate surface. Moreover, the measured release of CH(4) gas decreased when the sheet of ice formed at the surface of the hydrate. These findings can explain the anomalous preservation behavior of CH(4) hydrate; that is, the known increase of storage stability of CH(4) hydrate above 240 K is likely related to the formation of the ice that we observed in the experiments.     

Nonstandard cages in the formation process of methane clathrate: stability, structure, and spectroscopic implications from first-principles

Endohedral CH(4)@(H(2)O)(n) (n = 16, 18, 20, 22, 24) clusters with standard and nonstandard cage configurations containing four-, five-, six-, seven-membered rings were generated by spiral algorithm and were systematically explored using DFT-D methods. The geometries of all isomers were optimized in vacuum and aqueous solution. In vacuum, encapsulation of methane molecules can stabilize the hollow (H(2)O)(n) cage by 2.31~5.44 kcal/mol; but the endohedral CH(4)@(H(2)O)(n) cages are still less stable than the pure (H(2)O)(n) clusters. Aqueous environment could promote the stabilities of the hollow (H(2)O)(n) cages as well as the CH(4)@(H(2)O)(n) clusters, and the CH(4)@(H(2)O)(n) clusters possess larger stabilization energies with regard to the pure (H(2)O)(n) clusters except for n = 24. The lowest energy structures of the CH(4)@(H(2)O)(20) and CH(4)@(H(2)O)(24) cages are identical to the building units in the crystalline sI clathrate hydrate. All of the low-energy cages (including both regular and irregular ones) have large structural similarity and can be connected by "dimer-insertion" operation and Stone-Wales transformation. Our calculation also showed that in the range of cluster size n = 16-24, the relative energies of cage isomers tend to decrease with increasing number of the adjacent pentagons in the oxygen skeleton structures. In addition to the regular endohedral CH(4)@(H(2)O)(20) and CH(4)@(H(2)O)(24) cage structures, some nonstandard CH(4)@(H(2)O)(n) (n = 18, 20, 22, 24) cages have lower energies and might appear during nucleation process of methane hydrate. For the methane molecules in these low-energy cage isomers, we found that the C-H symmetric stretching frequencies show a red-shift trend and the (13)C NMR chemical shifts generally move toward negative values as the cavity size increases. These theoretical results are comparable to the available experimental data and might help experimental identification of the endohedral water cages during nucleation.     

Thermal expansivity for sI and sII clathrate hydrates

Knowledge of thermal expansivity can aid in the understanding of both microscopic and macroscopic behavior of clathrate hydrates. Diffraction studies have shown that hydrate volume changes significantly (as much as 1.5% over 50 K) as a function of temperature. It has been demonstrated previously via statistical mechanics that a minor change in hydrate volume (e.g., a 1.5% change in volume or 0.5% change in lattice parameter) can lead to a major change in the predicted hydrate formation pressure (e.g., >15% at >100 MPa for methane). Because of this sensitivity, hydrate thermal expansivity measurements, for both Structures I and II with various guests, are needed help quantify volume distortions in hydrate lattices to ensure accurate hydrate phase equilibria predictions. In addition to macroscopic phase equilibria, the thermal expansion of different hydrates can give information about the interactions between the guest molecules and the host lattice. In this work, the hydrate lattice parameters for four Structure I (C2H6, CO2, 47% C2H6 + 53% CO2, and 85% CH4 + 15% CO2) and seven Structure II (C3H8, 60% CH4 + 40% C3H8, 30% C2H6 + 70% C3H8, 18% CO2 + 82% C3H8, 87.6% CH4 + 12.4% i-C4H10, 95% CH4 + 5% C5H10O, and a natural gas mixture) systems were measured as a function of temperature. The lattice parameter measurements were combined with existing literature values. Both sI and sII hydrates, with a few exceptions, had a common thermal expansivity, independent of hydrate guest. Many guest-dependent correlations for linear thermal expansivity have been proposed. However, we present two guest-independent, structure-dependent correlations for sI and sII lattices, which have been developed to express the normalized hydrate lattice parameters (and therefore volume) as a function of temperature.     

Application of the cell potential method to predict phase equilibria of multicomponent gas hydrate systems

We present the application of a mathematical method reported earlier by which the van der Waals-Platteeuw statistical mechanical model with the Lennard-Jones and Devonshire approximation can be posed as an integral equation with the unknown function being the intermolecular potential between the guest molecules and the host molecules. This method allows us to solve for the potential directly for hydrates for which the Langmuir constants are computed, either from experimental data or from ab initio data. Given the assumptions made in the van der Waals-Platteeuw model with the spherical-cell approximation, there are an infinite number of solutions; however, the only solution without cusps is a unique central-well solution in which the potential is at a finite minimum at the center to the cage. From this central-well solution, we have found the potential well depths and volumes of negative energy for 16 single-component hydrate systems: ethane (C2H6), cyclopropane (C3H6), methane (CH4), argon (Ar), and chlorodifluoromethane (R-22) in structure I; and ethane (C2H6), cyclopropane (C3H6), propane (C3H8), isobutane (C4H10), methane (CH4), argon (Ar), trichlorofluoromethane (R-11), dichlorodifluoromethane (R-12), bromotrifluoromethane (R-13B1), chloroform (CHCl3), and 1,1,1,2-tetrafluoroethane (R-134a) in structure II. This method and the calculated cell potentials were validated by predicting existing mixed hydrate phase equilibrium data without any fitting parameters and calculating mixture phase diagrams for methane, ethane, isobutane, and cyclopropane mixtures. Several structural transitions that have been determined experimentally as well as some structural transitions that have not been examined experimentally were also predicted. In the methane-cyclopropane hydrate system, a structural transition from structure I to structure II and back to structure I is predicted to occur outside of the known structure II range for the cyclopropane hydrate. Quintuple (L(w)-sI-sII-L(hc)-V) points have been predicted for the ethane-propane-water (277.3 K, 12.28 bar, and x(eth,waterfree) = 0.676) and ethane-isobutane-water (274.7 K, 7.18 bar, and x(eth,waterfree) = 0.81) systems.     

Solubility measurements for the CH4 + CO2 + H2O system under hydrate–liquid–vapor equilibrium

Phase equilibria for the CH₄ + CO₂ + H₂O system have been investigated in the past, but mole fraction of methane and carbon dioxide in the bulk liquid phase has not been measured under hydrate–liquid–vapor equilibrium. Equilibrium liquid composition is very important as it defines the driving force for hydrate growth. This study presents the solubility of methane and carbon dioxide under H–Lw–V equilibrium. Emphasis is made on the effect of pressure along the respective isotherms on the equilibrium mole fraction of the individual hydrate formers in the liquid.     

First determination of volume changes and enthalpies of the high-pressure decomposition reaction of the structure H methane hydrate to the cubic structure I methane hydrate and fluid methane

Pressure-temperature (P-T) conditions of the decomposition reaction of the structure H high-pressure methane hydrate to the cubic structure I methane hydrate and fluid methane were studied with a piston-cylinder apparatus at room temperature. For the first time, volume changes accompanying this reaction were determined. With the use of the Clausius-Clapeyron equation the enthalpies of the decomposition reaction of the structure H high-pressure methane hydrate to the cubic structure I methane hydrate and fluid methane have been calculated.     

Tuning ionic liquids for hydrate inhibition

Pyrrolidinium cation-based ionic liquids were synthesized, and their inhibition effects on methane hydrate formation were investigated. It was found that the ionic liquids shifted the hydrate equilibrium line to a lower temperature at a specific pressure, while simultaneously delaying gas hydrate formation.