Search

• Dissociation enthalpies of CH 4 hydrate in the absence/presence of ionic liquids using the Clausius-Clapeyron equation calculated. • The Clausius-Clapeyron equation was analyzed in the temperature and pressure range of 273 K – 290 K and up to 10 MPa, respectively. • The Peng-Robinson equation is preferred for the calculation of the compressibility factor. • The obtained methane hydrate enthalpies show overestimated values and temperature dependence compared to the experimental ones. • This approach can be used as a rule of thumb, but care should be taken to get molecular-level insights. The Clausius-Clapeyron (CC) equation is generally preferred to obtain dissociation enthalpies (ΔH) of hydrate-forming systems due to its ease of use. The application of other direct and indirect methods becomes more problematic if complex additives such as ionic liquids (ILs) are also present in the system. In this work, around 400 equilibrium data points for methane hydrates in the presence of over 80 ILs were collected from the literature in the temperature and pressure ranges of (272.10 – 306.07) K and (2.48 – 100.34) MPa, respectively. The ΔH of methane hydrates in the absence and presence of ionic liquids (ILs) have been calculated using the CC equation. The compressibility factor (z), required to calculate ΔH at each phase equilibrium condition has been obtained from three different approaches viz., Peng-Robinson (PR) equation of state, Soave-Redlich-Kwong (SRK) equation of state and Pitzer (Pz) correlation. The results were compared to the experimentally reported dissociation enthalpy (54.5 ± 1.5 kJ.mol−1) of methane hydrates. The role of the compressibility factor along with the slope of the equilibrium data set and the temperature/pressure range in determining the outcome of the CC equation has been discussed. The effect of molar mass, molar volume, and hydrate suppression temperature of the ILs on the ΔH of methane hydrates has been explored. The tested ILs do not show a systematic and significant influence on enthalpies, rather they show a large scattering in the ΔH values, which might mask any existing subtle effect of the ILs on the dissociation enthalpies. Therefore, this approach should be dealt with care to obtain molecular-level insights. [ABSTRACT FROM AUTHOR]

Understanding gas hydrate behaviour under confinement is crucial to the development of strategies to efficiently extract methane from hydrate reservoirs. Thus, we have performed molecular dynamics simulations of methane hydrate dissociation inside a hydrophilic silica slit nanopore (representing the pores present in naturally occurring hydrate reservoirs) in the canonical ensemble at 290, 300, 305, and 310 K. Methane hydrate dissociates at lower temperatures under confinement than in bulk. Hydrate dissociation under confinement proceeds in a shrinking core manner showing an increased dissociation rate in the confined system compared to the bulk system where the dissociation is layer-by-layer only. Under confinement, the observed Arrhenius-type behaviour of the methane hydrate dissociation rate (in the initial 5 ns) with temperature leads to a value of the activation energy of dissociation (i.e., 46.885 kJ/mol) to be twice the hydrogen bond energy. In contrast to the confined system, the activation energy of dissociation in the bulk system is higher (i.e., 56.928 kJ/mol). The hydrophobic methane nanobubble formed after the dissociation tends to adhere to the hydrophilic silica substrate and there is an ordered bound water layer on the hydrophilic silica surface underneath the methane nanobubble, with the water molecules in this bound water layer region ordered in a square lattice arrangement unlike the random orientation of water molecules in the bound water layer at other regions on the hydroxylated silica surface. This ordered arrangement of the bound water molecules underneath the nanobubble maximizes the hydrogen bonding between bound water molecules and the surface hydroxyl groups (i.e., one water molecule is associated with a pair of hydroxyl groups). Our study, thus brings this detailed molecular-level structural insight into the complex interactions that exist among methane, water, and the hydrophilic silica surface under confinement for the first-time, to the best of our knowledge. [Display omitted] [ABSTRACT FROM AUTHOR]

Present work details the determination of three phase hydrate equilibrium data (H- L -V) using temperature search method for the binary CO 2 /CH 4 gas mixture (50:50 molar ratio) in presence of 1,3 dioxolane (DIOX). DIOX concentration used for phase equilibrium determination were 1,3 and 5.56 mol% respectively. In the presence of DIOX, it was observed that phase equilibrium curve was shifted right with respect to the curve using same gas mixture with no additive (pure water). This indicates that the presence of DIOX moderates the hydrate formation equilibrium conditions. Also, as DIOX concentration increases from 1 mol% to 5.56 mol%, a decrement in phase equilibrium pressure at same temperatures was observed, confirming the potential of DIOX as an effective thermodynamic promoter. Enthalpy of hydrate dissociation was calculated for CO 2 /CH 4 gas mixture in presence of DIOX using Clausius- Clapeyron plot and it was found to be 90.81±6.55 KJ/mol which confirms the sII structure of CO 2 -CH 4 -DIOX mixed hydrate. Besides providing the phase equilibrium data of formed hydrates with CO 2 /CH 4 gas mixture using different concentration of DIOX, the present study will aid in determining suitable experimental conditions for examining kinetics and performing separation studies of CO 2 /CH 4 gas mixture through hydrate formation. [Display omitted] [ABSTRACT FROM AUTHOR]

Semiclathrate hydrates are water-based materials formed from aqueous solutions of ionic substances. To enhance the gas capture performance of these materials, it is necessary to direct focus on the cation, which is a key part of construction of the hydrogen-bonding framework. In this paper, we focus on the partly-asymmetric cations, i.e., the n -propyl, tri- n -butylammonium bromide (N3444Br) and tri- n ‑butyl, n -pentylammonium bromide (N4445Br), which are yet to be subjected to gas hydrate formation. The three phase equilibrium (gas–hydrate–liquid) conditions for the systems of (N3444Br or N4445Br) + H 2 O + (CH 4 , CO 2 or N 2) in the range of the pressure and mass fractions of the aqueous solutions between 1 and 11 MPa and 0.10–0.40, respectively, are reported. In all the systems, the semiclathrate hydrates were successfully formed under gas pressure. It was demonstrated that both the N3444Br and N4445Br salts promoted hydrate formation under these gases, while in the case with lean aqueous solutions N3444Br acted as an inhibitor of methane hydrate formation in pure water system. The present data are compared with the literature data, and the effect of the side chain length on the phase behavior of the semiclathrate hydrate phase is discussed. [ABSTRACT FROM AUTHOR]

Natural gas hydrate is widely distributed in silty-clayey sediments, that occur largely in a highly dispersed state. However, the phase equilibria of methane hydrate in the presence of clay and how surface adsorption associated with clay particles affect natural gas hydrate thermodynamics and stability are not fully elucidated and warrant investigation. In this study, the dissociation conditions of methane hydrate in representative silty-clayey sediments (sand, montmorillonite and illite) with a water content of ∼20 % at different clay contents (10, 30, 50, 70 and 100 wt%) were measured using the multi-step heating method. The results indicated that montmorillonite has a greater dissociation temperature depression compared to illite. In the pure montmorillonite system, the temperature depression of methane hydrate is up to -2.24 K at a pressure of 11.69 MPa compared to pure water, however, that for illite is very small, less than -0.5 K. When the montmorillonite content decreases from 100 wt% to 70 wt%, the dissociation temperature of methane hydrate decreases significantly. As the montmorillonite content decreases below 50 wt%, the dissociation temperature of methane hydrate in silty-clayey sediments is almost the same as bulk hydrates, which due to the clay sediments is almost saturated with water. Furthermore, we introduce the Van Genuchten (VG) model to quantify the effect of clay content on water potential (matric suction) in unsaturated clays. Based on the classical van der Waals model, the VG model has successfully been applied to calculate methane hydrate phase equilibria in clay-bearing sediments, with a maximum absolute deviation (AADP) <5 %. It reveals that montmorillonite, characterized by its strong capacity for interlayer water absorption, shows an approximately exponential increase in matric suction as clay content rises. In contrast, the matrix suction of illite is weak, which can be approximately considered as a linear increase with the increase of clay content. [ABSTRACT FROM AUTHOR]

The synthesis process of methane hydrate exhibits strong non-equilibrium phenomena at all stages, with the nucleation stage being the most prominent. In this study, we used molecular dynamics simulations to investigate the spontaneous synthesis of methane hydrate on solid surfaces, focusing on the mass transfer non-equilibrium characteristics during the nucleation stage. From the overall perspective of nucleation events, we applied the Onsager hypothesis and introduced normalized autocorrelation functions to analyze the fluctuation-dissipation characteristics of hydrate cage structures on different substrate surfaces. We also examined the relationship between the orderliness of water molecules forming the cages and the solubility and diffusion of guest molecules. By comparing the effects of temperature and the presence of SDS and THF additives, we regulated the non-equilibrium characteristics during nucleation. The results indicate that nucleation on aluminum surfaces exhibits stronger non-equilibrium characteristics; there is a synergy between the components forming the hydrate and the overall fluctuation characteristics; lowering the temperature inhibits the non-equilibrium characteristics of the nucleation process but also reduces the number of nucleation events; the addition of additives effectively enhances the non-equilibrium characteristics, with the effect being more pronounced when SDS is in the gas phase, shortening the fluctuation period and relaxation time by 90.2 % and 66.3 %, respectively. This study demonstrates the mass transfer-driven non-equilibrium characteristics of the spontaneous nucleation of methane hydrates on solid surfaces and provides regulatory methods, contributing to a deeper understanding of the synthesis mechanism of hydrates. [ABSTRACT FROM AUTHOR]

• The SAFT-VR Mie EOS was solved analytically by deriving all contribution terms with respect to the packing fraction. • Molecular parameters for methane and water are extracted using SAFT-VR Mie EOS to fit vapor pressure and saturated liquid density data. • A model is developed using the vdW-P for the hydrate phase and the SAFT-VR Mie EOS for the vapor and liquid phases. • MD simulation is performed using Mie potential parameters extracted from SAFT-VR Mie EOS for non-bonded interactions. • The MD simulation was performed with three force fields for methane, OPLS-UA, GAFF, and UFF, to compare with our approach. This paper explores the three-phase equilibrium of structure-I methane hydrate by utilizing the extracted Mie potential parameters for non-bonded molecular interactions in the LAMMPS molecular dynamics simulator. An in-house code was developed to numerically solve the SAFT-VR Mie equation of state (EOS). To reduce numerical errors, unlike common practices, we incorporated all partial derivatives analytically in the code. The microscopic simulation parameters for methane and water molecules were selected so that saturation pressure and liquid density data from EOS were fitted on experimental data. Using this approach, the three-phase (V − L w − H) equilibrium temperatures for pressure values of 30, 70, 100, and 300 bar were calculated. The solubility of methane in the aqueous phase at three-phase equilibrium was also determined. The results were validated against available laboratory data. To evaluate the performance of Mie potential parameters, the MD simulations were repeated using three well-known force fields for methane molecules, namely, Optimized Potentials for Liquid Simulations-United Atom (OPLS-UA), Universal force field (UFF), and General Amber force field (GAFF). The TIP4P/Ice force field was used for water molecules in all cases. The comparison with experimental data reveals that while the accuracy of the UFF force field is the highest, our simulation approach outperforms OPLS-UA and GAFF force fields in predicting methane solubility in the aqueous phase. Besides molecular dynamics simulation, the van der Waals-Platteeuw model was combined with the SAFT VR Mie EOS to account for the hydrate phase. In our case study, the SAFT-VR Mie + vdW-P model proved to be more accurate in predicting the three-phase equilibrium data compared to the MD approach. [ABSTRACT FROM AUTHOR]

The present study investigates hydrate dissociation conditions in methane + cyclopentane (CP) and tetra-n‑butyl ammonium chloride (TBAC) aqueous solution. The experimental work was done using an isochoric vessel with varying compositions of the promoters in the pressure and temperature ranges of (1.42 to 4.90) MPa and (291.2 to 298.9) K, respectively. The methane hydrate dissociation pressure was predicted by the Gibbs minimization method. The findings denote that adding CP to the TBAC-containing system shifts the hydrate dissociation temperature of methane to higher values. The results also show an average increase of 12, 9, 5, and 3 K in hydrate dissociation temperature for 5, 10, 20, and 30 wt% TBAC in aqueous solution within the pressures. These results can be caused by the existence of more dodecahedral cages that are inhabited by methane. Thus, it makes hydrates more stable and increases the temperature of hydrate dissociation. On the other hand, adding TBAC to CP containing system has an inhibitor effect. The methane hydrate dissociation temperature was not changed sensibly when a 5 wt% TBAC was added to CP containing system. When the addition of the TBAC weight percentage to CP containing system is increased to 10, the average hydrate dissociation temperature decreases by 1 K within the pressures. When adding 20 and 30 wt% TBAC to CP containing system an average decrease of 2 K and 3 K in the hydrate dissociation temperature is resulted, respectively, within the pressures. The reduction in the hydrate dissociation temperatures can be attributed to salting-out and ion clustering. The experimental data and modeling results for the hydrate dissociation pressures exhibit an AARD of 2.4 %. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, we performed experiments to obtain phase equilibrium data for methane hydrates in the presence of ammonium chloride (NH 4 Cl) and methanol/ethylene glycol (MeOH/MEG), for which there is currently no data. The measured three-phase (liquid–hydrate–vapor, L–H V) equilibrium data for methane hydrates containing MeOH/MEG and NH 4 Cl solutions in a temperature range 264–281 K and a pressure range 4–12 MPa may be useful in engineering applications. In addition, the hydrate inhibition performance of our aqueous inhibitor systems can be summarized as follows: (10 wt% NH 4 Cl + 20 wt% MeOH/MEG) > (5 wt% NH 4 Cl + 20 wt% MeOH/MEG) > (10 wt% NH 4 Cl + 10 wt% MeOH/MEG) > (5 wt% NH 4 Cl + 10 wt% MeOH/MEG). The dissociation enthalpies of methane hydrates containing the mixed inhibitor solutions were also calculated using the Clausius–Clapeyron equation, and the predicted values matched the experimental values well. [ABSTRACT FROM AUTHOR]

One of the challenges associated with vinasse, the by-product of bioethanol production, is the high concentration of dissolved solids, largely in the form of K+, SO 4 2−, Mg2+, Ca2+, and Cl−. While traditional desalination technologies such as evaporation and distillation are employed in the concentration of salts to recover purified or potable water, these methods are energy intensive. The application of a novel technology known as gas hydrate technology can make the process of desalination less energy intensive, since the latent heat of fusion is much less compared to the heat of vaporisation for water. Reliable gas hydrate equilibrium experimental data plays a crucial role in the design of a hydrate-based process to treat vinasse. The systems measured in this study were based on the results of an extensive literature review conducted on the characterisation of vinasse. The isochoric pressure search method was used to measure the dissociation points for carbon dioxide hydrate in the presence of mixed salt solutions and synthesized vinasse mixtures containing (5 wt % KCl + 1 wt % Na 2 SO 4 + 0.5 wt % MgCl 2 + 0.5 wt % CaCl 2 + 2.2 wt % ethanol + 0.5 wt % propionic acid + 0.3 wt % acetic acid). A thermodynamic model based on the solid solution theory of van der Waals and Platteeuw, UNIQUAC model along with Debye-Hückel term were used to calculate the hydrate dissociation conditions in the presence of mixed salts and synthetic vinasse mixtures. New UNIQUAC interaction parameters were optimized. Good agreement between the experimental data and model results were achieved. [ABSTRACT FROM AUTHOR]

Abstract By revealing that a key parameter D in Chen-Guo model is related to the temperature T , a new temperature-dependent parameter D (T) trained by the reported experimental data is proposed with Chen-Guo model to improve the prediction accuracy of phase equilibrium conditions of methane hydrate in the range of 145.75 K- 272.2 K. Four equations of state (EOS) such as Redlich-Kwong (RK), Soave- Redlich-Kwong (SRK), Peng-Robinson (PR) and Patel-Teja (PT) are chosen to calculate the gas fugacity respectively and the corresponding prediction accuracy is evaluated by the average absolute deviation percentage (AADP). The process of training D (T) based on the Chen-Guo model employed with SRK EOS is described in detail first, and the prediction results show that the corresponding prediction accuracy is greatly improved, whose AADP decreases from 10.83% to 3.31%. In order to verify that training D (T) is an effective way to improve the prediction accuracy, the same procedure is also used to the Chen-Guo models employed with RK, PR and PT EOS respectively, and the corresponding AADP decreases from ∼11% to ∼3%. In addition, the prediction accuracy according to the proposed D (T) is also evaluated by other temperatures different from the data used for training D (T) to verify its feasibility and effectiveness. [ABSTRACT FROM AUTHOR]

Abstract The objective of this work was to investigate diffusion phenomena in methane clathrate hydrates during depressurization that lead to their dissociation. A thermodynamic mass transfer model is proposed that considers cage occupancy in structure I clathrate hydrates and methane diffusion from both S-cages (dodecahedron: 12 pentagons) and M-cages (tetradecahedron: 12 pentagons, 2 hexagons). Methane occupancies during dissociation were measured in situ with Raman spectroscopy with a newly-constructed optical system and data were estimated with Langmuir constants calculated by assuming a spherically symmetric cell potential. Model results were in agreement with experimentally-measured changes in average occupancy with time, so that dissociation kinetics at the interface were concluded to be strongly related to both methane diffusion and cage occupancy. Dissociation kinetics of methane hydrate can be reliably estimated with the proposed model. The proposed model is applicable to the study of methane hydrate dissociation-limited mechanisms or to larger-scale geological systems being used to estimate gas production rates. [ABSTRACT FROM AUTHOR]

Abstract We present a detailed overview of the calculation of the enthalpy of dissociation of methane hydrates, focusing primarily on methods that are based on either the Clapeyron equation or the direct calculation of the enthalpies of all the components that are involved in the dissociation reaction. Molecular dynamics simulations are used extensively in order to calculate the enthalpies and molar volumes of water, methane, and sI methane hydrate (with variant degree of occupancy) at pressure and temperature conditions along the three-phase (Hydrate – Liquid water – Vapor) equilibrium line. While for temperatures lower than 304 K there is a consensus that the enthalpy of dissociation is independent of temperature, the case for temperatures higher than 304 K is not that clear, particularly for the case when the Clapeyron equation is used. Therefore, new experimental measurements are required for the higher temperatures to shed light to the problem. In addition, the aqueous solubility of methane is calculated using the phase coexistence approach, resulting in accurate predictions when using the TIP4P/Ice water model and the OPLS-UA model for methane. [ABSTRACT FROM AUTHOR]

Abstract Thermodynamic stabilities of tetra- n -butylphosphonium acetate (TBP-Ace) semiclathrate hydrates in the presence of methane (CH 4), carbon dioxide (CO 2), nitrogen (N 2), or ethane (C 2 H 6) were measured in a pressure range up to approximately 5 MPa. The dissociation temperature of TBP-Ace + CH 4 , TBP-Ace + CO 2 , and TBP-Ace + N 2 semiclathrate hydrates increased drastically with an increase in pressure, which means that CH 4 , CO 2 , and N 2 molecules occupy the vacant cages of the TBP-Ace semiclathrate hydrate. On the other hand, the C 2 H 6 molecules hardly occupied the cages, resulting in small pressure dependence of the dissociation temperature. Raman spectra and powder X-ray diffraction patterns of TBP-Ace + CO 2 semiclathrate hydrate reveal that the phase transition occurs at 1.04 ± 0.04 MPa and 285.88 ± 0.05 K. One of the possible reasons why the phase transition occurs is that the carbonate and/or hydrogen carbonate anions derived from the CO 2 molecules are replaced with some of acetate anions in the TBP-Ace + CO 2 semiclathrate hydrate. Graphical abstract Image 1 Highlights • Thermodynamic stabilities of TBP-Acetate (TBP-Ace) + gas hydrates were measured. • The gases used in the present study are CH4, CO2, N2, and C2H6. • TBP-Ace + CH4 hydrate showed the highest equilibrium temperature. • Raman spectra and PXRD patterns reveal a phase transition of TBP-Ace + CO2 hydrate. • A possible reason of phase transition is the existence of anions derived from CO2. [ABSTRACT FROM AUTHOR]

Abstract Electrolyte solution can effectively inhibit the formation of hydrate. In this work, molecular dynamics simulations of hydrate nucleation and growth in aqueous NaCl solutions were performed to study the effect of ions on hydrate formation. We found that the electrolyte ions can inhibit hydrate formation on both the nucleation and the growth stage. Hydrates prefer nucleation in a region with low ionic concentration because the availability of water molecules in the ionized hydration layer is reduced. The slight change in orientation distribution of water around methane molecules in a NaCl solution system may be another factor to inhibit nucleation. The induction time is approximately proportional to the ionic strength, indicating that the charge has a more obvious inhibitory effect on hydrate formation. In hydrate growth stage, ions are excluded from the hydrate phase and a certain amount of free water molecules exist around the hydrate cluster. The ions near the hydrate can reduce its growth rate, causing a higher crystallinity of hydrate formed in salt solution. Finally, a dual influence of the ions on methane mass transfer was found to be that the ions increase the apparent concentration of methane to facilitate nucleation, but inhibit their diffusion at the same time. [ABSTRACT FROM AUTHOR]

Abstract Formation of gas hydrate cause a lot of problems for gas industry in the generation, transmission and processing which caused blockage in the equipment. One of the most common treatments is injection of electrolytes as chemical inhibitors. In this paper, hydrate formation conditions of methane gas in both water and NaCl, CaCl 2 , MgCl 2 electrolyte solutions were measured and a new thermodynamic calculation method based on the original PR model and Chen-Guo model was proposed by introducing a water activity calculation equation. The comparison demonstrates that the inhibiting effect of MgCl 2 , NaCl, CaCl 2 on the hydrate formation decrease respectively. The experimental results showed that the hydrate formation conditions are related to the electrolyte molecular mass and electrolyte concentration to a large extent, the deviation will increase as the temperature or the electrolyte concentration increases, so three parameters t 1 , t 2 , t 3 (0.034, 0.15, 0.01) which are related to temperature, concentration and molecular weight of electrolyte are introduced in the new calculation method. The absolute average deviation of 41 sets of data is 7.98 and 1.96 respectively before and after the correction parameters were introduced, the accuracy has more than quadrupled. [ABSTRACT FROM AUTHOR]

Abstract Currently there are few amino acids hydrate based thermodynamic studies compared to kinetic studies, thus, the thermodynamic effect of four amino acids on methane hydrate formation are presented herein. The amino acids (valine, threonine, asparagine, and phenylalanine) were tested as gas hydrate inhibitors using the isochoric pressure search method in a high pressure stirring reactor. They were tested at 1 wt% and 5 wt% in the temperature and pressure ranges of 275.71–286.10 K and 3.52–10.25 MPa, respectively. All studied amino acids thermodynamically inhibited methane gas hydrate formation, with valine exhibiting the best inhibition impact with an average depression temperature of 0.529 K at 5 wt%. Amino acids side chain properties were found to cause the variations in their inhibition impact. This study is useful to fully understand the thermodynamic inhibition effect of amino acids on gas hydrate formation. Inferring from the results in this study and literature, amino acids are thermodynamic gas hydrate inhibitors, while most of them are kinetic promoters. Graphical abstract Image 1 Highlights • New CH 4 hydrate phase boundary data in the presence of amino acids is reported. • The thermodynamic effect of valine, threonine, asparagine, and phenylalanine on CH 4 hydrate is reported. • All studied amino acids show thermodynamic hydrate inhibition in CH 4 hydrates. [ABSTRACT FROM AUTHOR]

The effects of mung starch on methane hydrate formation equilibria/rate and gas storage capacity were investigated in this work. Mung starch at three concentrations of 100, 500, and 800 ppm were tested comparing with the pure water. The results show that mung starch has slight thermodynamic promotion effect on methane hydrate formation, because it decreases the gas-liquid-hydrate three-phase equilibria pressure at the same temperature. The formation rate and gas storage capacity of methane hydrate in the presence of mung starch were studied at 8.0 MPa and 275.15 K–281.15 K. The results demonstrate that mung starch significantly accelerates methane hydrate formation rate. It could shorten the induction time and reaction time of methane hydration process. In addition, the gas storage capacity of methane hydrate is also increased greatly. The solubility data of methane indicate that mung starch indeed plays a role of solubility enhancement. The green and environmental friendly characteristics of mung starch could be helpful to promote the application of hydrate-based technology. [ABSTRACT FROM AUTHOR]

In this study, thermodynamic modeling is performed to predict the P-T equilibrium conditions of methane gas hydrate formation in presence of ionic liquids. The imidazolium-based ionic liquids including [EMIM][HSO 4 ], [EMIM][EtSO 4 ], [BMIM][BF 4 ], [OH-EMIM][BF 4 ], [BMIM][Cl] and [BMIM][Br] are modeled through two-step hydrate formation theory of Chen and Guo coupled with CPA EoS for calculating the fugacities of species in liquid and gas phase. The CPA pure component parameters for ionic liquids and also the majority of binary interaction parameters are adjusted using the liquid density and binary VLE data, respectively. The thermodynamic modeling is carried out in the presence of 0.1 weight fractions of all the studied imidazolium based ionic liquids. Moreover, the effect of three various concentrations of [BMIM][BF 4 ] on the P-T equilibrium diagram of methane hydrate are predicted at 0.1, 0.15 and 0.20 weight fractions of this ionic liquid. The results indicate that the CPA EoS can satisfactorily model the phase behavior of aqueous solutions of imidazolium based ionic liquids considering 2B scheme for the association term. In addition, the results indicate good predictions of the proposed model for methane hydrate equilibrium conditions in the presence of imidazolium-based studied ionic liquids including concentration changes such that the absolute average deviations of methane hydrate formation pressures is obtained below 4.2%. [ABSTRACT FROM AUTHOR]

Reliable hydrate phase behavior predictions are critical to petroleum and natural gas processing and design, and operation of process equipment. Inaccurate predictions of phase equilibria can also lead to erroneous design of process facilities and subsequently may cause safety hazards and flow assurance issues. This work reviews the experimental data on hydrate phase equilibria in the presence of inhibitors (Salts + Organic Inhibitors). The statistical thermodynamic model of van der Waals and Platteeuw for hydrate phase equilibria prediction is also critically reviewed. Recent studies have shown that the basic assumptions of the van der Waals and Platteeuw model, including the spherical symmetry of molecules, no guest–guest interactions, and no lattice distortions (due to guest molecules) can introduce errors in the predicted results. In addition, the limitations of the fluid phase models, which do not account for the effect of hydrogen bonding and electrolyte contributions, can lead to severe prediction errors in hydrate forming systems containing polar hydrate formers, inhibitors, and salts, especially at high concentrations. Thermodynamic predictions of gas hydrate phase equilibria for polar hydrate formers and inhibited systems (e.g., NaCl , KCl , CaCl 2 , and also methanol, ethanol, Ethane-1,2-diol) are of major concern because of the large errors in fluid phase equilibrium predictions. The unavailability of phase equilibria data, an appropriate electrolyte model, and an associative equation of state as well leads to various problems in predicting the properties of aqueous associating fluids and inhibited systems. [ABSTRACT FROM AUTHOR]

Comprehensive information about the hydrate formation and dissociation conditions is necessary for design and operation of different processes in the petroleum industry. This study investigated the natural gas hydrate equilibrium conditions in the presence of mono ethylene glycol aqueous solution as a widely used thermodynamic hydrate inhibitor in the oil and gas industries. Stepwise heating method was used for the hydrate formation and dissociation tests in a 250 cm 3 stainless steel equilibrium cell. To validate and verify the performance of the apparatus and the method used, methane hydrate equilibrium data were measured and compared to a number of selected experimental data in the literature. This verification showed that the experimental procedure was accurate. The equilibrium temperatures of synthetic natural gas (SNG) hydrates in the absence of MEG aqueous solutions at various pressures of 3.95, 4.49, 5.36, 6.50, 6.85, and 8.20 MPa were 289.9, 290.9, 291.7, 293.1, 293.5 and 294.5 K, respectively. The equilibrium data for natural gas hydrate in the presence of 0.10 and 0.20 mass fractions of MEG in the water for pressure range of 3.90–8.20 MPa were also comparatively measured. The results showed that the natural gas hydrate equilibrium temperature decreased about 2.5 and 5 K in the presence of 0.10 and 0.20 mass fractions of MEG, respectively. The modified statistical thermodynamic model based on van der Waals and Platteeuw solid solution theory and the PVTsim software were applied to predict and evaluate hydrate equilibrium conditions. Thermodynamic model predictions matched reasonably well with the experimental results. [ABSTRACT FROM AUTHOR]

• Methane hydrate equilibrium in the presence of amino acids was computed using the cubic plus association (CPA) and trebble-bishnoi (TB) equations of state (EOSs). • The EOSs were used to describe both the liquid and vapour phases. • The use of both EOSs led to an excellent correlation of methane hydrate pressures, when interaction parameters were regressed from hydrate data. An initial attempt at using an equation of state (EOS) to compute the liquid solution properties of aqueous amino acids at elevated pressures is presented. Specifically, this paper evaluates the performance of two different EOSs for use in correlating methane hydrate equilibrium in amino acid solutions. The two EOSs are the Cubic Plus Association (CPA) and Trebble-Bishnoi (TB) EOSs; the latter is a four-parameter cubic EOSs. CPA EOS parameters for the amino acids (glycine, alanine, l -proline, and l -valine), as well as the water-amino acid binary interaction parameters, were available in the literature; however, parameters had to be regressed for the TB EOS. The parameters were regressed from amino acid activity coefficient data at 298 K, and it was found that the TB EOS was able to accurately correlate that data; the average relative deviation of an amino acid activity coefficient, as correlated by the TB EOS, ranged from 0.08% to 0.11%. Subsequently, methane hydrate equilibrium conditions were computed with the aid of these two EOSs. When the EOS parameters regressed from amino acid activity coefficients at 298 K were used, the predictions of methane hydrate equilibrium were fair, with average relative deviations ranging from 5.01% to 11.64% for the TB EOS and from 6.47% to 14.58% for the CPA EOS. It was subsequently shown that correlation of methane hydrate equilibrium pressures could be improved by regressing water/amino-acid binary interaction parameters from hydrate equilibrium data. With the tuned binary interaction parameters, the average relative deviations ranged between 1.81% to 4.44% for the TB EOS and between 2.92% to 4.94% for the CPA EOS. In addition to evaluating the EOS' capacity to correlate methane hydrate equilibrium, this work also discusses the limitations associated with the regressed equation of state parameters. [ABSTRACT FROM AUTHOR]

In present work, phase equilibrium of cyclopentane-methane hydrate formed in different salt solution systems was studied using an orthogonal test method. The target ions including four cations (K + , Na + , Mg 2+ , Ca 2+ ) and two anions (Cl − , SO 4 2− ) were employed. The experimental results showed that the equilibrium temperature of cyclopentane - methane hydrate decreased when four cations (K + , Na + , Mg 2+ , Ca 2+ ) and two anions (Cl − , SO 4 2− ) were added. The equilibrium temperature decreased with the increase of ion concentrations. Analysis of variance suggested that cations presented a sequential inhibition effect on hydrate formation as follows: Mg 2+ > Ca 2+ > Na + > K + , while Cl − ion had a much stronger hydrate inhibition effect than SO 4 2− ion. The hydrate inhibition strength of an ion depended on the charge and radii of ion. The inhibitory effects of ions became intensified with the charge increased and radius decreased. And the radius of ions played a more significant role than charge of ions in altering hydrate phase equilibrium. [ABSTRACT FROM AUTHOR]

The dissociation enthalpy of methane hydrate is an important thermal parameter for hydrate-related work. In this paper, the dissociation enthalpies of methane hydrate in various salt solutions are calculated using the previously measured H-L W -V equilibrium data by the Clapeyron equation and the Clausius-Clapeyron equation, respectively. The calculated results by the Clapeyron equation do not show temperature dependence and the order of the dissociation enthalpy of methane hydrate in different salt solutions is 0.5NaCl>0.5MgCl 2 >0.5CaCl 2 > 1.0NaCl >1.0CaCl 2 >2.0NaCl>1.0MgCl 2 , the corresponding value of which is 52.6 ± 0.1 kJ/mol, 51.3 ± 0.3 kJ/mol, 50.4 ± 0.5 kJ/mol, 49.4 ± 0.3 kJ/mol, 46.0 ± 0.2 kJ/mol, 45.1 ± 0.2 kJ/mol and 44.0 ± 0.3 kJ/mol. Both the values and the errors calculated by the Clausius-Clapeyron equation are bigger than those by the Clapeyron equation. Moreover, the calculated values obtained by the Clausius-Clapeyron equation decrease with the temperature increase. Cations have less effect on the dissociation enthalpy than anions due to the different ability in affecting the ambient water networks. However, how the two effects (the colligative effect and the salting out effect) combine to influence the dissociation enthalpy of hydrate needs to be investigated further. [ABSTRACT FROM AUTHOR]

The methane hydrate dissociation conditions are measured at high pressures (up to 200 MPa) to fundamentally understand the hydrate properties. The results show an increased enthalpy of dissociation, from about 53 to 58 kJ/mol, as the hydrate equilibrium pressures transition from low (<80 MPa) to higher pressures (up to 200 MPa), indicating greater stability for the hydrates. Molecular simulations are used to understand and explain the increased stability with increasing pressure, by calculating the enthalpy and molecular structure of the phases. [ABSTRACT FROM AUTHOR]

In this experimental work, the phase boundaries of TMAOH + H 2 O + CH 4 and TMAOH + H 2 O + CO 2 hydrates are measured at different concentrations of aqueous TMAOH solution. The temperature-cycle (T-cycle) method is applied to measure the hydrate equilibrium temperature of TMAOH + H 2 O + CH 4 and TMAOH + H 2 O + CO 2 systems within the ranges of 3.5–8.0 MPa and 1.8–4.2 MPa, respectively. Results reveals that, TMAOH acts as a thermodynamic inhibitor for both gases. In the presence of 10 wt% of TMAOH, the inhibition effect appears to be very substantial for CO 2 with an average suppression temperature (ΔŦ) of 2.24 K. An ample inhibition influence is observed for CH 4 hydrate at 10 wt% with ΔŦ of 1.52 K. The inhibition effect of TMAOH is observed to increase with increasing TMAOH concentration. Confirmed via COSMO-RS analysis, the TMAOH inhibition effect is due to its hydrogen bonding affinity for water molecules. Furthermore, the calculated hydrate dissociation enthalpies in both systems revealed that TMAOH does not participate in the hydrate crystalline structure. [ABSTRACT FROM AUTHOR]

The phase equilibrium conditions of methane hydrate in single ionic liquid aqueous solutions containing low dosage 1-ethyl-3-methylimidazolium nitrate ([Emim][NO 3 ]) at mass fractions of 0.01, 0.025, 0.05 are measured in a pressure range of 3.50–15.0 MPa via an isochoric step-heating pressure search method. The thermodynamic inhibition effect of mixed ionic liquids containing [Emim][NO 3 ] in conjunction with 1-ethyl-3-methylimidazolium chloride ([Emim][Cl]) at low and high concentrations on methane hydrate formation is also investigated for the possible presence of synergistic effects. It is observed that for all the inhibitor systems containing [Emim][NO 3 ] or [Emim][Cl], the inhibition effect increases progressively with the increased concentration and pressure. In addition, the mixture of [Emim][NO 3 ] and [Emim][Cl] does not always exhibit synergetic effect on the thermodynamic inhibition of methane hydrate at higher concentrations up to 0.2 mass fraction. The higher concentrations are, the positive interactions between [Emim][NO 3 ] and [Emim][Cl] possibly happen at higher pressures. [ABSTRACT FROM AUTHOR]

• Comparison between SRK and CPA use on hydrates equilibrium calculations in the presence of ionic liquids. • CPA parameter optimization of pure ILs from density data. • Binary interaction parameter optimization from methane hydrate equilibrium data. • Study on anion basicity and cation chain lenght influence on EoS accuracy in hydrate equilibrium prediction. Gas hydrates are solid solutions which cause flow assurance problems and risks to line integrity in the petroleum industry. Thermodynamic inhibitors are usually injected during production to avoid hydrate formation. In recent years, many authors have reported Ionic Liquids (ILs) as inhibitors. However, there are not many studies on thermodynamic modeling in the literature involving ILs as hydrate inhibitors, especially when it comes to the applicability of the Cubic Plus Association (CPA) and Soave-Redlich Kwong (SRK) Equation of State. In light of this, the aim of this work was to evaluate methane hydrate equilibrium in the presence of ILs as inhibitors using van der Waals-Platteeuw solid solution theory along with these Equations of State to describe liquid and gas phases. The commercial software Multiflash and its interface with excel were employed to calculate hydrate equilibrium pressures. Data from the literature on 12 imidazolium-based ILs were taken into account by varying carbon chain length and anion basicity. Both Equations of State resulted in accurate equilibrium predictions with similar deviations between calculated and experimental data (overall ARD 4.64% for CPA, and 4.49% for SRK) considering p and T conditions in the range of 2.5–35 Mpa and 272–295 K respectively. 2B association was considered for ILs, while 4C association was used for water. In general, longer cation carbon chain lengths and higher anion basicity resulted in lower accuracy. [ABSTRACT FROM AUTHOR]

Subsea methane hydrates are expected as a potential natural gas resource. To develop gas production methods for the subsea methane hydrates, flow assurance technologies are necessary. Urea is one of the environmentally-friendly thermodynamic hydrate inhibitors (THIs) which is expected to be applicable for subsea methane hydrate systems. To evaluate its performance as a THI, a set of phase equilibrium data are necessary. In this study, we report phase equilibrium data for systems of urea + methane + water and urea + carbon dioxide + water in the range of aqueous compositions of urea up to its solubility limits. The present data for urea + methane + water ranged in pressures from 5 to 13 MPa and in urea mass fractions from 0.05 to 0.5. The comparison with the literature found that the inhibition effect of urea is a few kelvin weaker than that of methanol in the present measurement range. With the dense urea solutions, the system reached four phase equilibrium, i.e., solid urea–aqueous solution–gas–hydrate. At 13 MPa, approximately 13 K of inhibition temperature was obtained with 0.500 of feed mass fraction where solid urea was precipitated from the aqueous phase. In the urea + carbon dioxide + water systems, urea also worked as a THI. The maximum inhibition temperature was approximately 10 K with 0.400 of urea mass feed fraction at the four phase equilibrium. Based on the present equilibrium data and seafloor conditions of the subsea methane hydrates, it was found that urea can be used for the both systems of the methane gas production and the hydrate based carbon capture and storage. [ABSTRACT FROM AUTHOR]

• A correlation for the activity of water in the scenario of gas hydrate formation is proposed. • Relationship between the hydrate dissociation enthalpy, hydrate number, and pressure is presented. • The proposed gas hydrate correlation is suitable for inhibitor concentrations up to 70 mol%. The calculation result of activity of water (a w) is an important factor affecting the description of phase equilibria for gas hydrate in the inhibited system. In this work, the Chen-Guo model was used to calculate the phase equilibria of simple gas hydrates. In addition, a new a w correlation was proposed to realize the description of gas hydrates in the organic inhibitor solutions containing methanol (MeOH), ethanol (EtOH), monoethylene glycol (MEG), diethylene glycol (DEG), triethylene glycol (TEG) and glycerol (GLY). Mole fraction of these inhibitors ranges from 0.003 to 0.761. Moreover, the calculated results were compared with available experimental data in the literature. Results showed that the existing Margules equation has limited accuracy in the description of methane hydrate within a wide concentration range. Meanwhile, the proposed a w correlation performed much better than the Hu-Lee-Sum (HLS) correlation when incorporated with the Chen-Guo model, which increased the calculation accuracy for methane hydrate by ∼54%. Furthermore, this work also presented an accurate description for ethane and carbon dioxide hydrates in the organic inhibitor solutions, which was evidenced by a series of experimental data. [ABSTRACT FROM AUTHOR]

• The different sets of the KPFP reported in the literature are used to predict the hydrate dissociation conditions. • Using the KPFP reported in the literature, the hydrate structure cannot be predicted correctly. • Using genetic algorithm, new set of KPFP were optimized considering hydrate structure. • Good agreement with experimental data, both in determination of hydrate dissociation conditions and hydrate structure. Gas hydrates, or clathrate hydrates, are solid crystalline compounds, which are formed by combination of water and gas and/or some volatile liquid molecules. Prediction of hydrate stability/dissociation/equilibrium conditions of natural gases is important in separation processes, gas storage, and in preventing blockage of gas transmission pipelines. In this study, initially, the different sets of the Kihara Potential Function Parameters, KPFP, reported in the literature were used to predict the experimental hydrate dissociation conditions of methane, ethane, propane and i-butane and mixtures of these four compounds. In most cases, however, based on these sets of KPFP, the hydrate structure cannot be predicted correctly. Consequently due to incorrect estimation of the hydrate structure, especially for natural gas mixture, the predicted hydrate dissociation conditions are found inaccurate. For overcoming this fault and by using a genetic algorithm, a new set of KPFP were optimized based on the new definition of the objective function considering hydrate structure. The results show good agreement with experimental data, both in the prediction of hydrate dissociation conditions and hydrate structure. [ABSTRACT FROM AUTHOR]

We have recently developed an algorithm for multiphase equilibrium calculations and stability analysis involving hydrates. This algorithm allows phase equilibrium calculations in scenarios presenting different number of phases that may also be in distinct physical states. Here, we present P × T phase diagrams calculated using our algorithm that involve mixed hydrates of natural gas beyond incipient condition and show complex behavior. Our phase diagrams show a global overview of the behavior of phase equilibrium involving hydrates in a mixture of water, methane and propane and in a mixture of water, carbon dioxide and iso-butane. We accomplish that using a global phase diagram in a wide range of pressure and temperature to show diverse phase equilibrium regions. We present an approach to tune the Peng–Robinson equation of state to improve the calculations of the chemical potential of water in liquid state at high pressure and therefore, improve the prediction of the phase equilibrium between this phase and ice or hydrates, up to pressure conditions as high as 100 MPa. Our calculations predict 3-phase equilibrium regions and 4-phase equilibrium univariant lines in binary hydrates of methane and propane and in binary hydrates of carbon dioxide and iso-butane. We use local phase diagrams highlighting details in regions with complex behavior of phase equilibrium to show how the phase boundary isopleths and univariant lines relate in a phase diagram for water, methane and propane. Our calculations for a mixture of water, carbon dioxide and iso-butane predicts structural transitions occurring below the dew point line of the guest components, in a mixture with excess of guest components, and 5- phase equilibrium invariant point. We show equilibrium lines that present retrograde behavior in the phase diagram for this mixture. [ABSTRACT FROM AUTHOR]

In this study, experimental data on methane hydrate phase equilibria containing electrolytes, sodium chloride (NaCl), potassium chloride (KCl), and ammonium chloride (NH 4 Cl) were measured for concentrations up to about 10 wt%. The concentration of the aqueous salt solution in the system containing hydrate and salt solution is continuously changed during the hydrate formation and dissociation processes, so applying the isochoric method with the continuously temperature ramping to measure hydrate phase equilibria in the presence of salts may be unsuitable for precise and accurate measurements. An isochoric method, using a step-wise increase of temperature with sufficient equilibration time at every step, was introduced for measuring the hydrate equilibrium conditions for these systems containing electrolytes. To compare the results from the isochoric method, measurements using a high-pressure differential scanning calorimetry (DSC) were also performed using the same step-wise increase of temperature method. The results from both isochoric and DSC methods showed good agreement. The effects of the cation in the electrolyte on the hydrate inhibition were identified through the measurements, showing that hydrate inhibition strength by the sodium cation was slightly stronger than that of potassium and ammonium cations. [ABSTRACT FROM AUTHOR]

Hydrate formation and remediation in oil flowlines facilities represent a major concern for oil industry in respect of capital and operational costs. It is necessary to have a better understanding on the hydrate formation process to be more efficient in hydrate prevention, especially in respect to additive dosage. This work is a contribution to enhance the knowledge of hydrate formation at high water cuts, by introducing new techniques of analysis in the Archimede flow loop: a Focus Beam Reflectance Measurement (FBRM) probe and a Particle Video Microscope (PVM) probe. These results will be supported by Pressure Drop, Flow Rate, Density and Temperature probes. From experimental observations, a method to determine the continuous phase (water or oil) of the system under flowing is proposed. It is based on the time evolution of the most representative chord class measured by the FBRM. In order to predict morphology and size of hydrates, a topological model was developed. It represents hydrate crystallization from different emulsion systems with and without low dosage of hydrate inhibitor additive (anti-agglomerant type). The key parameters are the gas transfer rate at the gas/liquid interface and at the hydrocarbon/water interface, and the role of the hydrocarbon as gas transfer phase. Gas/liquid transfer is low as water phase remains the continuous phase, but is enhanced as hydrocarbon content is increased. Hydrocarbon gas transfer property is depleted as continuous and rigid crust is formed around droplets, especially in well dispersed emulsions. This behavior is highlighted for experiments without anti-agglomerant additive (AA-LDHI) and at high water cut, as a small fraction of hydrocarbon is well dispersed in the water continuous phase. The maximum hydrate plugging risk is in between 70% and 30% water cut (intermediate and low water cut). In this work, experiments at high water cuts (more than 80%) never plug. In order to prevent agglomeration, the AA-LDHI works better if it shows a secondary surfactant benefit, well-dispersing the droplets (and later the formed hydrates) in the continuous phase, which was identified as being preferentially the hydrocarbon phase. [ABSTRACT FROM AUTHOR]

The experimental study on phase equilibrium conditions for semi-clathrate hydrates formed in the systems of CH 4 /N 2 gas mixture and tetra-butyl-ammonium fluoride (TBAF) with different ratios (0.210, 0.293 and 0.500 mol%) to water was conducted in this paper. The experiments were carried out under the conditions of 281.15–291.15 K and 0.30–3.70 MPa. The structures of the hydrates were characterized using in-situ Raman spectroscopy at 276.15 K and 2.50 MPa. At a certain given pressure, the equilibrium temperature of the semi-clathrate hydrates formed in the systems with 0.500 mol% TBAF is significantly higher than that of semi-clathrate hydrates formed the systems with 0.210 or 0.293 mol% TBAF, and even higher than that of structure II hydrates formed in the systems with THF-SDS. Moreover, TBAF has more positive influence on enhancing dissociation enthalpies of the hydrates than THF. Due to the quite weak signal of N–N triple bond vibration or/and seldom N 2 molecules encaged into the cavities, only CH 4 molecules could be determined clearly in the mixed hydrates. In addition, the crystal morphology of the semi-clathrate hydrates is affected by the ratio of TBAF to water. [ABSTRACT FROM AUTHOR]

Isobaric-isothermal Gibbs ensemble Monte Carlo simulations of sI methane hydrates in equilibrium with bulk methane are performed to calculate large and small cage occupancies. The OPLS united-atom Lennard-Jones potential, and a variation, are used to model methane, while the TIP4P/Ice model represents water. This model system produces systematically higher total cage occupancies than those of simulations by Henley and Lucia [1] using TIP4P-Ew water and TraPPE-UA methane. We also see higher total occupancies than those observed in prior Raman spectrometry studies by Uchida et al. [2], but find good agreement for large cage occupancies. The simulations provide comprehensive predictions of large and small cage occupancies for temperatures and pressures within the ranges 270–290 K and 20–400 bar, respectively. These predictions demonstrate the advantages of the Gibbs ensemble for the simulation of clathrate hydrates; the ability to control the temperature and pressure, while not constraining the chemical composition of the hydrate phase is the key advantage of this ensemble. [ABSTRACT FROM AUTHOR]

The properties of gas hydrates with additives such as electrolytes and surfactants have been studied because their properties are greatly affected by additives. Although sodium chloride is a well known thermodynamic inhibitor of gas hydrate formation and a main component of seawater, little is known about its influence on gas hydrate dissociation. In this study, we formed methane hydrate containing sodium chloride up to 2.7 wt% and conducted a storage test at 253 K under ambient pressure. We used phase contrast X-ray computed tomography, cryo-SEM, and powder X-ray diffraction to investigate the stability of methane hydrate with up to a few percent of sodium chloride. It was revealed that the dissociation rate of methane hydrate containing sodium chloride was faster than that of pure methane hydrate. In addition, smaller particles of methane hydrate exhibited more influence of sodium chloride on dissociation. These experimental results obtained in this study may be caused by occurrences of NaCl solutions even at 253 K. [ABSTRACT FROM AUTHOR]

A high pressure differential scanning calorimeter was used to determine the dissociation temperature of methane hydrate in the presence of ionic liquid 1-alkyl-3-methylimidazolium chloride under a constant pressure ranging from 5 to 35 MPa. A homologous series of 1-alkyl-3-methylimidazolium chloride with different alkyl chain lengths: 1-ethyl-3-methylimidazolium chloride, 1-hexyl-3-methylimidazolium chloride and 1-decyl-3-methylimidazolium chloride, were chosen to examine the chain length effect on the dissociation temperature for methane hydrates. All these ionic liquids have inhibition effect on methane hydrate formation. Moreover, the shorter the alkyl chain length is, the stronger the inhibition effect is. That is, the inhibition effect of these ionic liquids on the methane hydrate formation is in the order of 1-ethyl-3-methylimidazolium chloride > 1-hexyl-3-methylimidazolium chloride > 1-decyl-3-methylimidazolium chloride. The three-phase vapor–liquid–hydrate equilibrium condition of methane hydrate in the presence of 1-alkyl-3-methylimidazolium chloride was successfully described by a predictive thermodynamic model. The Peng–Robinson–Stryjek–Vera equation of state incorporated with COSMO-SAC activity coefficient model and the first order modified Huron–Vidal mixing rule were applied to evaluate the fugacity of vapor and liquid phase. An explicit pressure dependence of the Langmuir adsorption constant in the modified van der Waals and Platteeuw model was applied to determine the fugacity of hydrate phase. The average absolute relative deviation in predicted dissociation temperature from the predictive thermodynamic model was 0.54%. In addition, these experimental data of dissociation temperatures were further correlated to a general correlation of Østergaard et al. (J. Petroleum Sci. Eng. 48 (2005) 70–80) and the average absolute relative deviation in the correlated dissociation temperature was 0.05%. [ABSTRACT FROM AUTHOR]

Recently, several alkylamine hydrates have been studied in an effort to reveal the structural transitions from semi-to ‘canonical’ clathrate hydrate in the presence of secondary guest molecules. Trimethylamine (TMA) is known to form the semi-clathrate hydrate, and it has been reported that the structural transition of the TMA semi-clathrate hydrate may not occur in the presence of hydrogen gas as a secondary guest molecule. In this study, we investigate the structural transition of TMA hydrate by CH 4 gas. Powder X-ray diffraction was used to analyze the crystal structure of a binary (TMA + CH 4 ) clathrate hydrate, and the results showed that the structural transition from semi-to cubic structure II hydrate can occur in the presence of methane gas. 13 C solid-state NMR and Raman spectroscopy were used to confirm the distribution of guest molecules. Finally, we measured the phase equilibrium conditions of a binary (TMA + CH 4 ) clathrate hydrate. The guest-induced structural transformation in amine semi-clathrate hydrates can provide useful insight into the complex nature of the host–guest inclusion system, and these results can be applied to research on potential gas storage and transportation areas. [ABSTRACT FROM AUTHOR]

The bromomethane clathrate hydrate crystallizes into the type I structure with the six large cages and the two small cages both filled with bromomethane guest molecules. Dynamic properties of the guest molecules have been investigated by means of incoherent quasi-elastic neutron scattering (QENS) in temperatures between 50 K and 260 K at atmospheric pressure. These investigations provide direct experimental evidence about the fundamental aspect of rotational motions of the bromomethane molecules. A comprehensive study has been achieved through the analysis of the structure factors (i.e., the spatial extend of the motion) as well as of the QENS broadening (i.e., the characteristic time of the motion). Such analysis reveals the existence of two types of motions: methyl group rotations about the C–Br bond with activation energy of 0.19 ± 0.08 kJ/mol and whole molecule reorientations thermally activated above 100 K with activation energy of 4.3 ± 1.2 kJ/mol. At the lowest studied temperature (50 K), the methyl group rotates faster by almost one order of magnitude in the large cage (characteristic time of 1.25 ps) than in the small cage (characteristic time of 10 ps). Such results provide new experimental signatures of the cage occupancy by evidencing different Brownian dynamics for the guest molecules in small cages and those in large cages. [ABSTRACT FROM AUTHOR]

Decomposition conditions of methane hydrate in marine sediments from South China Sea were measured using multi-step decomposition method. Four different samples of the marine sediments were used in the experiments. The pore distribution, the surface area, particle size and the surface texture were measured and observed. The experimental results indicated that the final decomposition temperatures are shifted lower than those for bulk hydrates at the same pressure for different marine sediments and different water saturations. Temperature shifts are more negative for smaller initial water saturation. The surface textures and pore of the sediments both affect the equilibrium condition of methane hydrate. Using the Clausius-Clapeyron equation, the enthalpy of hydrate dissociation in marine sediments was calculated. It was found that the enthalpy of hydrate dissociation in marine sediments with lower water saturation is higher than that at the bulk state. [ABSTRACT FROM AUTHOR]

In an attempt to study the accuracy and utility of ‘coarse grained’ models for methane-clathrate systems, molecular-dynamics simulations were run for three different potential models. One was fully atomistic of TIP4P water and fully atomistic methane, the next model was atomistic SPC water and coarse-grained UA methane, whilst the final model was the fully coursed-grained mW model. All models were run at two different sizes (8 and 64 fully-occupied sI clathrate unit cells) at 250 K and 60 bar. It was found that the coarse-grained models had a high level of accuracy in recreating structural properties, such as density or radial distribution functions (RDFs), with the obvious exception of not being able to create RDFs for atoms which are neglected by the model. More coarse grained models were shown to have lower accuracy for time-dependent phenomena, such as identifying the density or velocity fluctuations' frequencies. [ABSTRACT FROM AUTHOR]

Hydrate or semiclathrate hydrate systems formed using tetrahydrofuran (THF) and tetra- n -butyl ammonium bromide (TBAB) show potential use for natural gas storage and transportation. Information on the effect of various inhibitors on their phase stability is necessary to effectively dissociate these hydrate systems for the end use application. In this work, the experimental data for the phase equilibrium of methane (CH 4 ) hydrate system have been obtained in the presence of THF (0.005 and 0.01 mass fraction) and TBAB (0.1 and 0.2 mass fraction) with various inhibitors. Various inhibitors used in the study include sodium chloride (NaCl), methanol (MeOH) and ethylene glycol (EG) with mass fraction (0.03 and 0.1) of each inhibitors in aqueous solutions of THF and TBAB. The reported data lie in the temperature and pressure range of (277.56–290.43) K and (2.20–6.16) MPa, respectively. Comparative effects of NaCl, MeOH and EG on hydrate/semiclathrate hydrates of CH 4 + H 2 O + THF/TBAB have been studied. The inhibition effect of NaCl on hydrate of CH 4 + H 2 O + THF is observed to be higher as compared to MeOH and EG, whereas for semiclathrate hydrate of CH 4 + H 2 O + TBAB, MeOH is observed to be an effective inhibitor than NaCl and EG. In addition, as the concentration of promoters (THF and TBAB) increases, the effect of inhibitors is found to decrease. This indicates that lower concentrations of promoters along with inhibitors may be suitable for efficient formation and dissociation of natural gas storage and transportation. The heat of dissociation ( H diss ) determined using Clausius–Clapeyron equation is reported for various hydrate/semiclathrate hydrate system. The values of heat of dissociation are observed to be in good agreement with literature trend. H diss is observed to decrease with increase in concentration of NaCl, MeOH and EG for hydrate/semiclathrate hydrate of CH 4 + H 2 O + THF/TBAB. [ABSTRACT FROM AUTHOR]

The equilibrium conditions of methane hydrate in the presence of additives of cyclopentanone (C 5 H 8 O) or 4-hydroxy-4-methyl-2-pentanone (C 6 H 12 O 2 , also known as diacetone alcohol) in aqueous solutions were experimentally measured in this study. The hydrate-liquid water-vapor (H-L w -V) three-phase equilibrium conditions were determined by employing the isochoric method within a high pressure cell at low temperature conditions. New experimental results are reported for pressures ranging from 7 to 13 MPa with various concentrations of these additives. It is demonstrated that the cyclopentanone additive has a promotion effect on the formation of methane hydrate, while the other additive of 4-hydroxy-4-methyl-2-pentanone shows an inhibition effect. The structures of methane hydrates with each of the two additives were determined using the Clausius–Clapeyron equation. To simulate the salinity of the seawater environment, this study also measured the hydrate-liquid water-vapor (H-L w -V) three-phase equilibrium temperatures and pressures by adding both additives to brine solution consisted of NaCl at 0.035 mass fraction. In both cases, the additional component of NaCl in the aqueous phase reduces the equilibrium temperature by about 2 K. [ABSTRACT FROM AUTHOR]

In this work, a thermodynamic model is developed and used to predict the phase stability conditions for methane hydrate–ionic liquid (IL)–water system. The hydrate phase is computed from modified van der Waals–Platteeuw model. The Peng–Robinson equation of state (PR-EoS) and developed activity model as a combination of Pitzer–Mayorga–Zavitsas-hydration model is used to evaluate the fugacities of gas and liquid phases, respectively. The hydrate phase stability prediction is also computed using the liquid phase activity predicted by NRTL and Pitzer–Mayogra models, separately, and is compared with the results predicted from the developed model. The model predictions are compared with experimental results on the phase stability of methane hydrate reported in open literatures for 21 ILs. The 21 ILs chosen from various ionic groups such as tetraalkylammonium, pyrrolidinium, imidazolium cationic family with various anion group such as halides (Cl, Br, I), sulphate (HSO 4 , ethylsulphate), tetrafluoroborate (BF 4 ) and dicyanamide (DCA). The absolute average relative deviation in predicted pressure (AARD-P) with developed Pitzer–Mayorga–Zavitsas-hydration-model is improved to 1.60% and non-random two liquid (NRTL), Pitzer–Mayorga model showed 2.02% and 1.77% with 120 data points in the temperature range of 272.1–291.59 K and pressure range of 2.48–20.67 MPa. For 120 data points of phase stability conditions of 21 ILs, 39.2% of the predicted equilibrium pressures (47 data points) were within relative absolute deviation of 0.0–1.0%, 29.2% of the equilibrium pressures (35 data points) were within absolute deviation of 1.01–2.5%, 25.8% of data (31 data points) were within 2.51–7.5% which are mainly for data with low concentrations of ILs and only 5.8% of data (7 data points) showed relative absolute deviations above 7.5% which are observed mainly for data with high concentrations of ILs. Further, the model is used to calculate the inhibition effect of selected 21 ILs on methane hydrate formation. [ABSTRACT FROM AUTHOR]

Studies of the thermodynamic stability of clathrate hydrates of natural gas (mostly methane) is important in fields such as offshore gas exploitation and energy storage. Two approaches were used to study the effect of unlike dispersion interactions on methane clathrate hydrates: grand canonical Monte Carlo simulations (which yield adsorption data directly and can be used to infer phase equilibria), and estimation of the heat of dissociation coupled with the Clausius–Clapeyron equation (to calculate the phase equilibria, at the expense of providing no information about the adsorption behavior). It was found that the adsorption isotherm parameters change monotonically with respect to unlike dispersion interactions, although a perfect fit to experimentally-derived values may not be possible, at least using the force fields considered in this study. The heat of dissociation changes monotonically due to changes in the unlike dispersion interaction, and a best fit value of the Berthelot correction factor is achieved. [ABSTRACT FROM AUTHOR]

Highlights: [•] Occupancy of cavities in the hydrate lattice determined using Grand Canonical Monte Carlo simulation. [•] Simulations performed using several force fields. [•] Langmuir-type adsorption isotherms fitted to simulation results. [•] Clathrate hydrate phase equilibria computed from adsorption isotherms. [•] Predicted phase equilibria compare favourably with literature. [Copyright &y& Elsevier]

Highlights: [•] Phase equilibria of semiclathrate hydrate of methane in TBAB are investigated. [•] This study clarifies the discrepancy of published data in the literature and their reliability. [•] Two distinct equilibrium points for semiclathrate hydrates within pure hydrate stability zone are emphasized. [•] One closely corresponds to the pure methane hydrate phase stability curve. [•] Phase equilibria of semiclathrate hydrate of methane in TBAB+NaCl are investigated. [Copyright &y& Elsevier]

Abstract: We simulated decomposition of structure I methane hydrate (H) with all cages filled in contact with two reservoirs (pools) of liquid water (W) which in turn are in contact with either two methane gas reservoirs (G), or with vacuum (V), under constant volume–constant energy conditions. By adding gas or empty spaces to the simulation box we allow the released methane to diffuse out of the liquid phase and into the gas phase similar to what happens during methane hydrate dissociation. The evolution of the released methane molecules during the hydrate dissociation process was carefully monitored. We found that some of the released methane gas reaches the gas phase and contributes to the increase of gas pressure on the hydrate phase. As the hydrate dissociates, liquid water phase becomes supersaturated with methane, methane molecules aggregate, and spherical regions of high concentration of methane form which we identify as “nano-bubbles”. These nano-bubbles grew to a specific size range which depends on simulation conditions and remained stable in the liquid phase for the duration of the simulations (5ns). [Copyright &y& Elsevier]

Highlights: [•] Dissociation conditions of methane hydrate in the presence of pure water and three ionic liquids are reported. [•] A thermodynamic model is developed for prediction of the obtained data. [•] Acceptable agreement is found between the obtained data and the predicted results. [ABSTRACT FROM AUTHOR]