Gas hydrate formation in the system C2H6–H2–H2O at pressures up to 250 MPa

Decomposition curves of gas hydrates formed in the ethane–hydrogen–water system were studied in the pressure interval 2–250 MPa. Gas hydrates synthesized at low (up to 5 MPa) pressures were also studied with use of X-ray powder diffraction and Raman spectroscopy. It was shown that ethane–hydrogen mixtures with hydrogen contents 0–30 mol.% form cubic structure I gas hydrates. Higher hydrogen concentration most probably results in appearance of another hydrate phase. We speculate that the gas mixtures with the hydrogen content above 60 mol.% form cubic structure II double hydrate of hydrogen and ethane at temperatures below ≈280 K and pressures above 25 MPa.     

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.     

Theoretical investigation of ozone hydrate formation conditions

Structural, dynamic, and thermodynamic properties of ozone, oxygen, and mixed ozone-oxygen hydrates are investigated. The thermodynamic stability regions of these hydrates are found. Ozone can form hydrates at ambient pressure and temperatures below 230 K. Strong dependence of the binary hydrate formation pressure on the ozone concentration in the gas phase is shown. In the formation of the hydrate, ozone concentrates in the hydrate phase. At an ozone concentration of 5 mol.% in the gas phase, the ozone content in the hydrate reaches 40%.     

Dissociation behavior of “dry water” C_3H_8 hydrate below ice point:Effect of phase state of unreacted residual water on a mechanism of gas hydrates dissociation

The results on a dissociation behavior of propane hydrates prepared from "dry water" and contained unreacted residual water in the form of ice inclusions or supercooled liquid water(water solution of gas) were presented for temperatures below 273 K.The temperature ramping or pressure release method was used for the dissociation of propane hydrate samples.It was found that the mechanism of gas hydrate dissociation at temperatures below 273 K depended on the phase state of unreacted water in the hydrate sample.Gas hydrates dissociated into ice and gas if the ice inclusions were in the hydrate sample.The samples of propane hydrates with inclusions of unreacted supercooled water only(without ice inclusions) dissociated into supercooled water and gas below the pressure of the supercooled water-hydrate-gas metastable equilibrium.     

Double clathrate hydrates of tetrabutylammonium fluoride + helium, neon, hydrogen and argon at high pressures

Decomposition curves of double ionic clathrate hydrates of tetrabutylammonium fluoride with helium, neon, hydrogen and argon were studied at pressures up to 800 MPa. Formation of double hydrates with helium, neon and hydrogen does not lead to any significant increase of the temperatures of decomposition of these hydrates; at high temperatures the hydrates may decompose even at lower temperatures than the hydrate of pure tetraalkylammonium salt does. Decomposition temperatures of double hydrates with argon in all cases were 4–8 °C higher in comparison with the decomposition temperature of ionic clathrate hydrates of tetrabutylammonium fluoride. We suppose that this behavior is caused by simultaneous effect of three factors on hydrate decomposition temperature: (1) partial filling of the small cavities in the framework of the hydrate with water molecules, (2) weakness of the van der Waals interactions between the gas molecules and the host water molecules, and (3) dissolution of helium, hydrogen and neon in the solution of tetrabutylammonium salt causing a decrease of melting temperatures of the hydrates formed from these solutions.     

Dissociation behavior of C2H6 hydrate at temperatures below the ice point: melting to liquid water followed by ice nucleation

The dissociation of C(2)H(6) hydrate particles by slow depressurization at temperatures slightly below the ice melting point was studied using optical microscopy and Raman spectroscopy. Visual observations and Raman measurements revealed that ethane hydrates can be present as a metastable state at pressures lower than the dissociation pressures of the three components: ice, hydrate, and free gas. However, they decompose into liquid water and gas phases once the system pressure drops to the equilibrium boundary for supercooled water, hydrate, and free gas. Structural analyses of obtained Raman spectra indicate that structures of the metastable hydrates and liquid water from the hydrate decay are fundamentally identical to those of the stable hydrates and supercooled water without experience of the hydration. These results imply a considerably high energy barrier for the direct hydrate-to-ice transition. Water solidification, probably induced by dynamic nucleation, was also observed during melting.     

Gas hydrates in low water content gases: Experimental measurements and modelling using the CPA equation of state

In this communication, new experimental data are reported for the water content of methane and two synthetic gas mixtures in equilibrium with hydrates at pressures range from 5 to 40 MPa and temperature down to 251.65 K. The measurements have been made on equilibrated samples taken from a high-pressure variable volume hydrate cell using a new analyser based upon tuneable diode laser absorption spectroscopy (TDLAS) technology. A statistical thermodynamic approach, with the Cubic-Plus-Association equation of state, is employed to model the phase equilibria. The hydrate-forming conditions are modelled by the solid solution theory of van der Waals and Platteeuw. The thermodynamic model was used to predict the water content of methane and synthetic gases in equilibrium with gas hydrates.     

Communication: quantitative Fourier-transform infrared data for competitive loading of small cages during all-vapor instantaneous formation of gas-hydrate aerosols

A simple method has been developed for the measurement of high quality FTIR spectra of aerosols of gas-hydrate nanoparticles. The application of this method enables quantitative observation of gas hydrates that form on subsecond timescales using our all-vapor approach that includes an ether catalyst rather than high pressures to promote hydrate formation. The sampling method is versatile allowing routine studies at temperatures ranging from 120 to 210 K of either a single gas or the competitive uptake of different gas molecules in small cages of the hydrates. The present study emphasizes hydrate aerosols formed by pulsing vapor mixtures into a cold chamber held at 160 or 180 K. We emphasize aerosol spectra from 6 scans recorded an average of 8 s after "instantaneous" hydrate formation as well as of the gas hydrates as they evolve with time. Quantitative aerosol data are reported and analyzed for single small-cage guests and for mixed hydrates of CO(2), CH(4), C(2)H(2), N(2)O, N(2), and air. The approach, combined with the instant formation of gas hydrates from vapors only, offers promise with respect to optimization of methods for the formation and control of gas hydrates.     

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.     

Calculating the separation coefficients in argon, krypton and xenon gas mixture separation by gas hydrate crystallization

The separation of gas mixtures based on noble gases by means of gas hydrate crystallization is considered for various schemes of the process. The separation coefficient and the separation factor of the basic mixture components at 273 K and at different pressures and compositions of the gas mixture are calculated. It is shown that xenon can be almost completely separated from a mixture in two stages of gas hydrate crystallization even without the use of booster gases. Directional crystallization allows us to separate xenon more completely and crystallization at constant pressure enables us to obtain a more highly purified product.     

Isothermal phase equilibria for the (HFC-32 + HFC-134a) mixed-gas hydrate system

Isothermal phase equilibria (pressure-composition relations in hydrate, gas, and aqueous phases) in the {difluoromethane (HFC-32) + 1,1,1,2-tetrafluoroethane (HFC-134a)} mixed-gas hydrate system were measured at the temperatures 274.15 K, 279.15 K, and 283.15 K. The heterogeneous azeotropic-like behaviour derived from the structural phase transition of (HFC-32 + HFC-134a) mixed-gas hydrates appears over the whole temperature range of the present study. In addition to the heterogeneous azeotropic-like behaviour, the isothermal phase equilibrium curves of the (HFC-32 + HFC-134a) mixed-gas hydrate system exhibit the negative homogeneous azeotropic-like behaviour at temperatures 279.15 K and 283.15 K. The negative azeotropic-like behaviour, which becomes more remarkable at higher temperatures, results in the lower equilibrium pressure of (HFC-32 + HFC-134a) mixed-gas hydrates than those of both simple HFC-32 and HFC-134a hydrates. Although the HFC-134a molecule forms the simple structure-II hydrate at the temperatures, the present findings reveal that HFC-134a molecules occupy a part of the large cages of the structure-I mixed-gas hydrate.     

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.     

Compressibility of Gas Hydrates

Experimental data on the pressure dependence of unit cell parameters for the gas hydrates of ethane (cubic structure I, pressure range 0–2 GPa), xenon (cubic structure I, pressure range 0–1.5 GPa) and the double hydrate of tetrahydrofuran+xenon (cubic structure II, pressure range 0–3 GPa) are presented. Approximation of the data using the cubic Birch–Murnaghan equation, P=1.5B₀[(V₀/V)^7/3?(V₀/V)^5/3], gave the following results: for ethane hydrate V₀=1781 ų, B₀=11.2 GPa; for xenon hydrate V₀=1726 ų, B₀=9.3 GPa; for the double hydrate of tetrahydrofuran+xenon V₀=5323 ų, B₀=8.8 GPa. In the last case, the approximation was performed within the pressure range 0–1.5 GPa; it is impossible to describe the results within a broader pressure range using the cubic Birch–Murnaghan equation. At the maximum pressure of the existence of the double hydrate of tetrahydrofuran+xenon (3.1 GPa), the unit cell volume was 86 % of the unit cell volume at zero pressure. Analysis of the experimental data obtained by us and data available from the literature showed that 1) the bulk modulus of gas hydrates with classical polyhedral structures, in most cases, are close to each other and 2) the bulk modulus is mainly determined by the elasticity of the hydrogen‐bonded water framework. Variable filling of the cavities with guest molecules also has a substantial effect on the bulk modulus. On the basis of the obtained results, we concluded that the bulk modulus of gas hydrates with classical polyhedral structures and existing at pressures up to 1.5 GPa was equal to (9±2) GPa. In cases when data on the equations of state for the hydrates were unavailable, the indicated values may be recommended as the most probable ones.     

Formation and decomposition of ethane, propane, and carbon dioxide hydrates in silica gel mesopores under high pressure

The experimental data on decomposition temperatures for the gas hydrates of ethane, propane, and carbon dioxide dispersed in silica gel mesopores are reported. The studies were performed at pressures up to 1 GPa. It is shown that the experimental dependence of hydrate decomposition temperature on the size of pores that limit the size of hydrate particles can be described on the basis of the Gibbs-Thomson equation only if one takes into account changes in the shape coefficient that is present in the equation; in turn, the value of this coefficient depends on a method of mesopore size determination. A mechanism of hydrate formation in mesoporous medium is proposed. Experimental data providing evidence of the possibility of the formation of hydrate compounds in hydrophobic matrixes under high pressure are reported. Decomposition temperature of those hydrate compounds is higher than that for the bulk hydrates of the corresponding gases.     

Highly selective encaging of carbon dioxide molecules in the mixed carbon dioxide and nitrogen hydrate at low temperatures

The structural identification and guest compositions of the mixed CO(2) and N(2) hydrates at low temperature conditions were investigated by both theoretical predictions and experimental measurements. From the model calculations, at very low temperatures, the highly CO(2)-concentrated hydrates over 95 mol % CO(2) on the basis of water-free concentration could coexist with the gas mixtures of low CO(2) concentrations in equilibrium. X-ray diffraction measurements of the hydrates formed with the gas mixture of 3.16 mol % CO(2) and balanced N(2) indicate that the formed hydrates at all conditions considered in this study were identified as structure I, whereas the model predicts a structural transition to structure II around 220 K. However, it was also found that the formed hydrate samples contain a considerable amount of hexagonal ice resulting from incomplete conversion of ice to the hydrates. The compositional analysis suggests that a favorable encaging of CO(2) in the mixed hydrate can be obtained by the hydrate formation at low temperatures and relative amount of CO(2) molecules in the mixed hydrates increases with a decrease of temperature.     

Clathrate formation in water-noble gas (Hydrogen) systems at high pressures

Phase equilibria in helium-water, neon-water, and hxdrogen-water svstems were studied at pressures up to 15 kbar. The results are compared with the data for the previously investigated water systems with argon, crypton, and xenon. It is concluded that classical polyhedral clathrate hydrates are formed in all the systems, the stability of the hydrates diminishing from xenon to neon. In all the systems, except the xenon system, the hydrates are based on the crystalline framework of ice II. Their formation demands high pressures; the larger the guest molecule, the higher the pressure required. The xenon molecule seems to be too large to fit the cage of the ice II framework; therefore, the xenon hydrate CS-I remains stable up to at least 15 kbar. Translated fromZhurnal Strukturnoi Khimii, Vol. 40, No. 5, pp. 974–980, September–October, 1999.     

Molecular Dynamics Simulation of Structure II Clathrate Hydrates of Xenon and Large Hydrocarbon Guest Molecules

Molecular dynamics (MD) simulations of structure II clathrate hydrates are performed under canonical (NVT) and isobaric–isothermal (NPT) ensembles. The guest molecule as a small help gas is xenon and gases such as cyclopropane, isobutane and propane are used as large hydrocarbon guest molecule (LHGM). The dynamics of structure II clathrate hydrate is considered in two cases: empty small cages and small cages containing xenon. Therefore, the MD results for structure II clathrate hydrates of LHGM and LHGM + Xe are obtained to clarify the effects of guest molecules on host lattice structure. To understand the characteristic configurations of structure II clathrate hydrate the radial distribution functions (RDFs) are calculated for the studied hydrate system. The obtained results indicate the significance of interactions of the guest molecules on stabilizing the hydrate host lattice and these results is consistent with most previous experimental and theoretical investigations.     

Spectroscopic methods in gas hydrate research

Gas hydrates are crystalline structures comprising a guest molecule surrounded by a water cage, and are particularly relevant due to their natural occurrence in the deep sea and in permafrost areas. Low molecular weight molecules such as methane and carbon dioxide can be sequestered into that cage at suitable temperatures and pressures, facilitating the transition to the solid phase. While the composition and structure of gas hydrates appear to be well understood, their formation and dissociation mechanisms, along with the dynamics and kinetics associated with those processes, remain ambiguous. In order to take advantage of gas hydrates as an energy resource (e.g., methane hydrate), as a sequestration matrix in (for example) CO₂ storage, or for chemical energy conservation/storage, a more detailed molecular level understanding of their formation and dissociation processes, as well as the chemical, physical, and biological parameters that affect these processes, is required. Spectroscopic techniques appear to be most suitable for analyzing the structures of gas hydrates (sometimes in situ), thus providing access to such information across the electromagnetic spectrum. A variety of spectroscopic methods are currently used in gas hydrate research to determine the composition, structure, cage occupancy, guest molecule position, and binding/formation/dissociation mechanisms of the hydrate. To date, the most commonly applied techniques are Raman spectroscopy and solid-state nuclear magnetic resonance (NMR) spectroscopy. Diffraction methods such as neutron and X-ray diffraction are used to determine gas hydrate structures, and to study lattice expansions. Furthermore, UV-vis spectroscopic techniques and scanning electron microscopy (SEM) have assisted in structural studies of gas hydrates. Most recently, waveguide-coupled mid-infrared spectroscopy in the 3–20 μm spectral range has demonstrated its value for in situ studies on the formation and dissociation of gas hydrates. This comprehensive review summarizes the importance of spectroscopic analytical techniques to our understanding of the structure and dynamics of gas hydrate systems, and highlights selected examples that illustrate the utility of these individual methods.     

Equilibrium conditions of clathrate hydrates formed from carbon dioxide and aqueous acetone solutions

Equilibrium conditions of clathrate hydrates formed from carbon dioxide and aqueous acetone solutions were experimentally measured at temperatures between 269.2 and 281.4 K and pressures up to 3.98 MPa. The acetone concentrations in solutions were investigated from 0.04 to 0.40 mass fractions. The experimental results suggested a transition in hydrate structure from structure I to another structure for acetone solutions between 0.04 and 0.12 mass fractions of acetone. The hydrate structure was suggested to be structure II which was the most stable with a 0.16 mass fraction acetone solution. For more than 0.16 mass fraction of acetone, the equilibrium conditions of the hydrate were shifted to lower temperatures as acetone concentrations increased.