Hydrate Phase Composition Research Articles

An Accurate Model to Calculate CO2 Solubility in Pure Water and in Seawater at Hydrate–Liquid Water Two-Phase Equilibrium

Understanding of CO2 hydrate–liquid water two-phase equilibrium is very important for CO2 storage in deep sea and in submarine sediments. This study proposed an accurate thermodynamic model to calculate CO2 solubility in pure water and in seawater at hydrate–liquid water equilibrium (HLWE). The van der Waals–Platteeuw model coupling with angle-dependent ab initio intermolecular potentials was used to calculate the chemical potential of hydrate phase. Two methods were used to describe the aqueous phase. One is using the Pitzer model to calculate the activity of water and using the Poynting correction to calculate the fugacity of CO2 dissolved in water. Another is using the Lennard–Jones-referenced Statistical Associating Fluid Theory (SAFT-LJ) equation of state (EOS) to calculate the activity of water and the fugacity of dissolved CO2. There are no parameters evaluated from experimental data of HLWE in this model. Comparison with experimental data indicates that this model can calculate CO2 solubility in pure water and in seawater at HLWE with high accuracy. This model predicts that CO2 solubility at HLWE increases with the increasing temperature, which agrees well with available experimental data. In regards to the pressure and salinity dependences of CO2 solubility at HLWE, there are some discrepancies among experimental data. This model predicts that CO2 solubility at HLWE decreases with the increasing pressure and salinity, which is consistent with most of experimental data sets. Compared to previous models, this model covers a wider range of pressure (up to 1000 bar) and is generally more accurate in CO2 solubility in aqueous solutions and in composition of hydrate phase. A computer program for the calculation of CO2 solubility in pure water and in seawater at hydrate–liquid water equilibrium can be obtained from the corresponding author via email.

Improved hydration and properties of magnesium oxysulfate (MOS) cement using sodium silicate as an additive

Magnesium oxysulfate (MOS) cement has many advantages, such as high early strength, low thermal conductivity, excellent fire and corrosion resistance. However, its strength is less stable and its water resistance is relatively poor, which limits its wide use in civil engineering. In this study, the MOS cement specimens with different amount of sodium silicate (SS) as an additive were manufactured before subjected to a series of laboratory tests, including dispersion tests, strength tests, SEM, XRD and Isothermal Calorimetry analysis to evaluate their modified properties. The experimental results show that an increase of the amount of SS consistently reduces the initial setting time, dispersion and fluidity performance of MOS cement paste because of the SS accelerated endothermic reaction during hydration process of MOS. An addition of the amount of SS up to 0.5% of the mass of MgO significantly increases the long-term strength and water resistance of MOS cement. This attributes to the fact that SS enhances microstructure and refines the coarse pores of hardened MOS paste, and thus forms much denser paste, although it does not change the category of hydration phase composition in hardened MOS cement.

Theoretical study of stability zones of mixed H2 + CO2 gas hydrates CS-I and CS-II

Using the original approach that utilizes statistical thermodynamic theory that has been developed in our group the phase p-T diagram and hydrate phase composition have been calculated for “CO2 + H2 + H2O”. Depending on the CO2 content in the gas phase two hydrates types can be formed: CS-I and CS-II. It has been found that increasing CO2 content in the gas phase drastically reduces hydrate formation pressure and in the same time reduces hydrogen uptake by the hydrate. While pure H2 hydrates have CS-II structure, addition of more than 2 mol.% of CO2 into the gas phase makes CS-I hydrate structure more stable, that displays worse maximum hydrogen content. These results could be helpful for practical use of hydrates for efficient CO2 utilization and H2 transportation.

A review on hydrate composition and capability of thermodynamic modeling to predict hydrate pressure and composition

Gas hydrates are widely considered to be a crucial topic in oil and gas industries and attracting significant research due to potential applications such as gas storage, separation as well as water desalination. While the guest composition of hydrate phase is vital, due to the experimental difficulties in measuring hydrate composition, very little applicable information is available in the literature. Paradoxically, this is true, in spite of that; completing an experimental database on hydrate composition could have a significant impact on the processes design and modeling. Moreover, this should provide fundamental knowledge of kinetic effects as well as clarifying thermodynamic equilibrium. Hence, this paper was planned with the intent to fill in the gaps, classify and offer an overview of experimentally derived data on hydrate composition for literature. In addition, a thermodynamic model based on the van der Waals and Platteeuw approach and Kihara potential was utilized to simulate the hydrate composition along with equilibrium pressure.Previous experimental data shows that guest distribution in hydrate phase depends noticeably on the guest composition in vapor phase. In addition, composition of larger molecules, such as propane or butane, in the hydrate phase, is notably higher than in vapor phase. Our simulation results demonstrated that the hydrate composition data from direct measurement (microscopic tools) have been well evaluated by the thermodynamic model. Nevertheless, when structural transition can occur in a system, the thermodynamic model is no longer accurate. In the case of indirect measurements, the thermodynamic model usually predicts well the hydrate composition. Nonetheless, its capability does vary with differing hydrate composition and equilibrium pressure, to the extent that in some cases, it completely fails to predict hydrate composition. This could be due to kinetic effects on the enclathration of guest molecules during the crystallization, errors in experimental techniques to measure the hydrate composition or the model parameters like Kihara potential are not properly applied. Finally, these observations show that more reliable experimental database is needed to study the evolution of guest distribution in hydrate phase and some enhancements are required for the standard thermodynamic model.

A new approach of studying mixed gas hydrates involving propane at non-equilibrium conditions and final state: An experimental study and modeling

Metastable clathrate hydrates are a promising energy source in the shallow geosphere and present challenges in flow assurance, energy storage, and carbon capture sequestration. While they have been widely studied, little pertinent data is available for common propane hydrates concerning hydrate phase composition, nor its volume or the amount of converted water. This was the initial motivation for our work. Therefore, with a novel technique, propane hydrate composition and volume were measured dynamically at non-equilibrium conditions over time and at the final states for slow and quick rates of crystallization. Surprisingly, equilibrium pressure, hydrate volume and composition are different according to crystallization rate. The hydrate volume and water conversion in the quick crystallization process were larger. Moreover, at a slow crystallization rate, in a hydrocarbon mixture, enclathration of propane is more considerable and the hydrate crystals appear to be more homogeneous. Furthermore, the hydrate crystallization of a gas mixture is closer to the thermodynamic equilibrium at slow crystallization rates where the impact of kinetics is slight. A new compilation of propane Kihara parameters was presented. Unlike methane, ethane and carbon dioxide, for propane we strongly recommend two Kihara parameters, one for pure and the other for mixtures of propane. A thermodynamic model based on classical van der Waals and Platteuw model was also used to investigate the effects of kinetics. The simulation results have a satisfactory accordance with the experimental data from literature to predict the hydrate equilibrium pressure. The consequence of this research could have a substantial impact on design calculations in which the assumption of thermodynamic equilibrium are done. For instance, at present there would be excess hydrates volume estimations for pipe-lines, equilibrium conditions in energy storage and transportation or carbon capture sequestration and thus increase expenses or loss of productivity where propane is concerned.

A systematic kinetic study to evaluate the effect of tetrahydrofuran on the clathrate process for pre-combustion capture of carbon dioxide

THF (Tetrahydrofuran) is the most commonly employed promoter for the gas hydrate formation pertaining to applications like carbon dioxide capture and energy storage (for natural gas and hydrogen). In this work, THF was investigated as a promoter for pre-combustion capture of carbon dioxide (CO2) from fuel gas mixture (CO2/H2) in a systematic manner. 0.5, 1.0, 3.0 and 5.56 mol% THF solution were employed with different driving force conditions to study the effect of pressure, temperature and composition of gases in hydrate. The kinetic performance of the CO2/H2/THF gas hydrate system was evaluated by reviewing the IT (induction time), normalized gas uptake, normalized rate of hydrate formation and the hydrate phase composition. The optimum operating conditions and optimum THF concentration in this study was found to be 6.0 MPa, 282.2 K, and 5.56 mol% THF respectively. 5.56 mol% THF solution yielded an average gas uptake of 13.9 mmol of gas/mol of water for hydrate growth and an average normalized rate of hydrate formation of 1679.8 mmol s−1 m−3.

In situ Raman and 13C NMR spectroscopic analysis of gas hydrates formed in confined water: application to natural gas capture

This study investigates the formation characteristics of gas hydrate from bulk water as well as dispersed water in silica gel and dry water particles when they are exposed to natural gas. The inclusion process of methane, ethane, and propane molecules in hydrate cages were observed with in situ Raman spectroscopy, and the resulting cage occupancies were estimated from 13C NMR spectra. A high-pressure autoclave was used to monitor the formation process to determine hydrate onset time, initial growth rate, and conversion ratio. The obtained data from Raman spectra and gas consumption profiles suggested that hydrate formed within less than 20 min when the temperature is sufficiently lower than the hydrate equilibrium condition at a given pressure. Methane molecules started to occupy the small cages of structure II, but about 6 min later ethane and propane were also included in hydrate cages. 13C NMR spectroscopy confirms that only 23% of large cages of structure II are occupied by methane molecules when hydrate formed from dispersed water in silica gel, which was much less than 68% from dry water. These results suggest that the dispersion of water in silica gel and dry water would enhance the formation process by increasing gas-to-water ratio, although the composition of hydrate phase may vary depending on the formation condition. However the formation of hydrate in silica gel and dry water still provide an effective option to capture the natural gas without using complex rotating machineries.

Formation of methane nano-bubbles during hydrate decomposition and their effect on hydrate growth.

Molecular dynamic simulations are performed to study the conditions for methane nano-bubble formation during methane hydrate dissociation in the presence of water and a methane gas reservoir. Hydrate dissociation leads to the quick release of methane into the liquid phase which can cause methane supersaturation. If the diffusion of methane molecules out of the liquid phase is not fast enough, the methane molecules agglomerate and form bubbles. Under the conditions of our simulations, the methane-rich quasi-spherical bubbles grow to become cylindrical with a radius of ∼11 Å. The nano-bubbles remain stable for about 35 ns until they are gradually and homogeneously dispersed in the liquid phase and finally enter the gas phase reservoirs initially set up in the simulation box. We determined that the minimum mole fraction for the dissolved methane in water to form nano-bubbles is 0.044, corresponding to about 30% of hydrate phase composition (0.148). The importance of nano-bubble formation to the mechanism of methane hydrate formation, growth, and dissociation is discussed.

Hydrate-based pre-combustion capture of carbon dioxide in the presence of a thermodynamic promoter and porous silica gels

In this study, the feasibility of the hydrate-based pre-combustion capture of carbon dioxide in the presence of a thermodynamic promoter and porous silica gels was examined through stability condition measurements, gas uptake measurements, and microscopic analyses. When a fuel gas mixture of H2 (60%) and CO2 (40%) was used for gas hydrate formation, the addition of tetrahydrofuran (THF) yielded significantly enhanced hydrate stability conditions. Silica gels with a nominal diameter of 100.0nm demonstrated increased gas consumption during gas hydrate formation, indicating higher conversion of water into gas hydrate. With only one step of hydrate formation, CO2 concentrations of higher than 90% were achieved in the hydrate phase for all cases. It can be concluded that the hydrate phase composition is affected primarily by the cage occupancies of guest gases in the hydrate lattices whereas the vapor phase composition is strongly influenced by the conversion of water to gas hydrate and also partially by the cage occupancy of CO2 in the hydrate phase. From powder X-ray diffraction (PXRD) patterns and Raman spectra, it was confirmed that the H2 (60%)+CO2 (40%) gas mixture forms structure I hydrate and the inclusion of THF induces the formation of structure II hydrate.

Measurement and Modeling of Hydrate Composition during Formation of Clathrate Hydrate from Gas Mixtures

In studying the relative dissociation times of structure I (sI) and structure II (sII) gas hydrates prepared from methane + ethane mixtures, X-ray diffraction analysis indicated the simultaneous presence of both hydrate structures at conditions where only one hydrate structure was expected. The aim of this work was to develop a model describing the formation of a gas hydrate from a mixture of methane + ethane in a closed system as the hydrate forms and to validate the model by experiment. Two methods of laboratory hydrate preparation from ice particles were studied: (1) a closed-system pressure-drop formation and (2) a closed-system constant-pressure formation. Models were developed for both methods 1 and 2, and method 2 was also investigated experimentally using a mixture of mole fraction 0.897 methane to form the hydrate. Compositions of the gas were monitored periodically during the hydrate formation by gas chromatography (GC). The water-free hydrate-phase composition of methane and ethane was also mea...

Structure and kinetics of gas hydrates from methane/ethane/propane mixtures relevant to the design of natural gas hydrate storage and transport facilities

AbstractThe kinetics of gas hydrate growth from binary CH4/C2H6 and CH4/C3H8 and ternary CH4/C2H6/C3H8 gas mixtures were obtained by the gas uptake method in a semibatch stirred vessel at constant pressure and a temperature of 273.7 K. These data are of interest for the design of facilities for natural gas storage and transportation in the solid (hydrate) state. During hydrate formation, samples from the gas phase were taken and analyzed by gas chromatography. It was found that the molar composition of CH4 in the vapor phase increased as hydrate crystallization progressed. The observed fractionation effect (enrichment of the hydrate phase with propane) complicates the natural gas storage process. The fractionation effect was also confirmed with molecular‐level studies where hydrate from the CH4/C2H6/C3H8 gas mixture was characterized by powder X‐ray diffraction (PXRD), NMR, and Raman spectroscopy. The hydrate phase composition and cage occupancy of each gas were calculated with the help of information obtained by Raman spectroscopy, gas chromatography, and PXRD. The results were consistent with those obtained by NMR. The composition of the gas phase and the hydrate are found to evolve over time, suggesting that kinetic and transport factors contribute in addition to thermodynamics. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Hydrate Phase Composition for Multicomponent Gas Mixtures

Abstract: This investigation was carried out to determine the composition of guest components in the equilibrium hydrate phase. Measurements were made for six gas mixtures containing methane, ethane, propane, iso‐butane, n‐butane, and carbon dioxide. Gas hydrate was formed in the presence of water at two pressures, 2.07 and 6.89 MPa, and at temperatures 28°C below the corresponding equilibrium hydrate temperature. The results obtained showed that, with some exceptions, the van der Waals‐Platteeuw thermodynamic model can be used for hydrate phase composition calculations.

A Theory of Lattice Distortion in Gas Hydrates

Water and some nonpolar gases combine at low temperatures and high pressures to form nonstoichiometric clathrates called gas hydrates. The water molecules in gas hydrates are linked to each other through hydrogen bonds to form a host lattice with polyhedral cavities that are large enough for small gas molecules such as methane, ethane, and propane to occupy. Common gas hydrates are known to occur in three structural forms, structure 1 and structure 2 with cubic symmetry and structure H with hexagonal symmetry, although the present study is based only on structure 1 and structure 2 hydrates. Commonly cited structural details are given. The present work will show that a fixed set of structural parameters is not appropriate. Constant pressure molecular dynamic simulations of gas hydrates with guests of different size indicate that the equilibrium cell volume increases with guest size. The effect of lattice stretching due to guest size on the reference chemical potential difference between the empty lattice and water is calculated, and a new thermodynamic model with a guest dependent reference chemical potential difference, {Delta}{mu}{degree}, is used for predicting the equilibrium conditions for single component (one gas + water) gas hydrates and multicomponent gas hydrates. This model avoidsmore » potential parameter adjustment (fitting) for prediction of equilibrium conditions. A relationship for obtaining the reference chemical potential difference for multicomponent gas hydrates (mixture gas hydrates) from the hydrate phase composition is proposed.« less

Hydraulic reactions of LD steelwork slags

Five industrial LD steelwork slags were ground and reacted with water and NaOH. The formation of hydrate phases was traced by XRD, SEM and thermoanalysis. The strngth evolution and pore size distribution of LD slag blended blast furnace slag-portland cement mortars were studied. It is shown that LD slags positively react with water. NaOH accelerates the hydration, yet evokes a change in hydrate phase composition and morphology. A high free lime content of the LD slag (11.6 wt.%) does not cause unsoundness of the blended mortars investigated.