Binary Hydrate Research Articles

Structure and stability of the sH binary hydrate cavity and host‐guest versus guest‐guest interactions therein: A DFT approach

The intrinsic ability of clathrate hydrates to encage gaseous molecules is explored. Encapsulation ability depends on the cavity size and the type of guest gaseous species in addition to the physical parameters, temperature and pressure. Here we have reported the structure, stability and nature of interaction in dissimilar guest occupied sH hydrate cavity. Diatomic gas molecules and small polyatomic hydrocarbons are considered as guests. The irregular icosahedron (512 68 ) cavity of sH hydrate is the host. Different thermodynamic parameters for the guest molecules encapsulation were calculated using three different hybrid DFT functionals, B3LYP, M05-2X, M06, and moreover using dispersion correction (PBE0-D3). With the consideration of large H-bonded systems the 6-31G* and cc-pVTZ basis sets were used for two set of computations. To disclose the nature of interaction between the host-guest systems as well as the interaction between the guest molecules inside the host the non-covalent interaction (NCI) indices and energy decomposition analysis (EDA) were done. Impact of host-guest and guest-guest interactions are discussed.

Molecular hydrogen storage in binary H2-CH4 clathrate hydrates

Clathrate hydrate has emerged as a promising candidate for hydrogen storage, but the major challenge is the high pressure required for its formation. The addition of natural gas can significantly reduce the formation pressure, but also can improve the energy density. In this work, we estimated the hydrogen storage capacity of the H2-CH4 binary hydrate by performing the first-principles calculations and simulations. The thermodynamical and mechanical stability of the hydrate was studied as a function of the cage occupancy of both 512 and 51264 cages. For the most stable structure, the molar ratio of H2 and H2O is 0.353:1. The H2-CH4 binary hydrate can maintain its structure during dynamics under the moderate temperature and pressure. The self-preservation effect was observed at ∼270 K, which can be used for the hydrogen storage and transport. These findings provide a better understanding of the mixed hydrate as a viable hydrogen storage technology, which could enable us to achieve a sustainable hydrogen economy.

Thermally Induced Phase Transition of Cubic Structure II Hydrate: Crystal Structures of Tetrahydropyran–CO2 Binary Hydrate

We report a thermally induced phase transition of cubic structure II hydrates of tetrahydropyran (THP) and CO2 below about 140 K. The phase transition was characterized by powder X-ray diffraction measurements at variable temperatures. A dynamical ordering of the CO2 guests in small pentagonal dodecahedral 512 host water cages, not previously observed in the simple CO2 hydrate, occurs simultaneously with the symmetry lowering transition from a cubic structure II (space group Fd-3m with cell dimensions a = 17.3202(7) Å at 153 K) to a tetragonal (space group I41/amd with cell dimensions a = 17.484(4) Å and c = 12.145(1) Å at 138 K) unit cell. The effect of guest molecules on the phase transition at low temperatures is discussed, which demonstrates that the clathrate hydrate structures and thermodynamic properties can be modified by adjusting the size and chemical structure of larger and smaller guest molecules.

Investigation on the Structure, Formation Mechanism, and Surface Morphology of CP–CH4 Binary Hydrate

Cyclopentane (CP) is often used as a hydrate former, which shows an excellent promotion effect in hydrate-based desalination. To elucidate the guest molecule distribution and microcosmic mechanism during hydrate formation, the in situ Raman spectroscopy of CP–methane binary hydrates is performed. The Raman spectra for the intramolecular vibration mode of host and guest molecules reveal that sII CP–methane hydrate formed with CP and CH4 encaged in large and small cages, respectively. But the small cages were not completely filling in. Because of the structure of CP with a five-membered ring, hydrogen-bonded water molecules arranged around CP forming a metastable structure, which accelerates the nucleation rate of hydrate. The peak O–H stretching vibration (ROH–H = I3170/I3400) in hydrate phase is an indication of hydrogen-bond ordering. At 3.5 MPa, the CH4 occupancy in the small cages was 0.9978 and the CP occupancy in the small cages was 0.1751. According to cage occupancies and the thermodynamic model based on van der Waals–Platteeuw, the calculated hydration number was 12.6. Cryo-scanning electron microscopy was applied to observe the surface morphology of hydrate. Energy-dispersive spectroscopy was used to distinguish hydrate from ice by identifying element C.

A stable new composite phase change material based on H2C2O4⋅2H2O-NH4Al(SO4)2⋅12H2O binary eutectic into ZrO2 modified expanded graphite

As energy shortage intensifies, improving energy efficiency and reducing energy consumption have become essential topics. Herein, a form-stable composite phase change material (FCPCM) consisting of modified expanded graphite (MEG) and eutectic hydrate was designed as a reliable and efficient thermal energy storage medium. Firstly, novel binary eutectic hydrate involving oxalic acid dihydrate (H2C2O4.2H2O) and ammonium aluminum sulfate dodecahydrate (NH4Al(SO4)2.12H2O) served as an excellent thermal energy storage guest. Zirconium dioxide (ZrO2) was used as a hydrophilic modifier because of its high heat resistance, high inertness, corrosion resistance, non-toxicity and countless hydrophilic groups. Therefore, the hydrophilic modified expanded graphite (MEG) as a supporting host was constructed by dip-coating of nano ZrO2 sol on the surface of expanded graphite (EG), which improved its hydrophilicity and compatibility with a eutectic guest. Compared with the original EG (83.05°), the contact angle of MEG containing 10 % ZrO2 on the water was decreased to 37.95°. In addition, the adsorption rate and capacity of MEG to binary eutectic were much faster and larger, reaching an approximate saturation (65.58 %) within 40 min, which was three times that of the untreated EG (23.10 %) in the same adsorption time. The resultant FCPCM containing 24 wt% MEG exhibited a good phase transition temperature (54.10 °C), high phase transition enthalpy (168.5 J/g), as well as excellent anti-leakage behavior. After 100 hot and cold cycles, the thermal performances of FCPCM kept relatively stable, which guaranteed its sustained thermal benefits. It provides a potential application in the field of solar phase change heat exchangers.

Studies on Methane Gas Hydrate Formation Kinetics Enhanced by Isopentane and Sodium Dodecyl Sulfate Promoters for Seawater Desalination

Methane hydrate applications in gas storage and desalination have attracted increasing attention in recent years. In the present work, the effect of isopentane (IP), sodium dodecyl sulfate (SDS), and IP/SDS blends as promoters on methane hydrate formation kinetics, in terms of the pressure–temperature (P-T) profile, gas uptake, hydrate induction time (HIT), and water-to-hydrate conversion ratio (WHCR), were studied for distilled water and seawater samples with an IP/water sample ratio of 3:10 (by volume) and an SDS/water sample ratio of 1:1000 (by mass). Each solution was tested in a stirred tank at 600 rpm at a temperature and pressure of 2 °C and 5.2–5.3 MPa. In the case of methane hydrate formation in distilled water, the highest WHCR attained was 9.97% without additives, and 45.71% and 72.28% for SDS and isopentane additives, respectively. However, when using seawater at a salinity of 3.9%, the highest WHCR attained was 2.26% without additives and 9.89% and 18.03% for SDS and IP promoters, respectively, indicating the inhibiting effect of salinity on hydrate formation. However, the HIT was longer for seawater hydrate formation, with an average of 13.1 min compared to 9.90 min for methane hydrate formation. Isopentane enhances the HIT for methane hydrate formation in seawater by 2.23 times compared to SDS. For methane hydrate formation in seawater, the presence of IP shortened the HIT by 15.6 min compared to the seawater sample without promoters. Additionally, a synergistic effect was observed when IP and SDS were combined and used in methane hydrate formation in distilled water and seawater systems. The positive effect of IP on methane hydrate formation is possibly due to the binary hydrate formation mechanism, which improves the hydrate formation thermodynamic and kinetic parameters.

Dynamic Dissociation Behaviors of sII Hydrates in Liquid Water by Heating: A Molecular Dynamics Simulation Approach.

An understanding of the dynamic behavior of subtle hydrate dissociation in the liquid water phase is fundamental for gas production from marine hydrate reservoirs. Molecular dynamics simulations are performed in this study to investigate the dissociation kinetics of pure propane and binary propane + methane sII hydrates in a liquid water environment. The results show that faster hydrate dissociation rates are observed at higher initial temperatures. The hydrate phase dissociates from the cluster surface to the inside in a layer-by-layer manner under the simulation temperature conditions, which is similar to the behavior of sI hydrates and is independent of the hydrate crystal type. Compared to the binary sII hydrate, the pure sII hydrate dissociates more easily under the same initial temperature conditions, which can be attributed to the stabilizing effect of guest molecules in the hydrate cages. The empty cages collapse in one step, in contrast to the two-step pathway induced by the guest-host interaction. In addition, a hydrocarbon phase forms in the binary hydrate dissociation system instead of nanobubbles. These results can provide molecular-level insights into the dynamic mechanism of hydrate dissociation and theoretical guidance for gas recovery by thermal injection from marine hydrate reservoirs.

Effect of ammonium hydroxide-based ionic liquids' freezing point and hydrogen bonding on suppression temperature of different gas hydrates

The study presents the effect of freezing point depression and hydrogen bonding energy interaction on four ammonium hydroxide-based ionic liquids (AHILs) of gas hydrate systems. The AHILs investigated are tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, and tetrabutylammonium hydroxide. The considered hydrate system includes methane (CH4), carbon dioxide (CO2), and three binary mixed gas hydrates (70-30 CO2 + CH4, 50-50 CO2 + CH4, 30–70 CO2 + CH4), which are often encountered in the flow assurance pipelines. The experimental temperature range is between 274.0 and 285.0 K, corresponding to pipeline pressures for different gas systems. The thermodynamic influence, i.e., average suppression temperature (ΔŦ) of the studied system, was reported for different mass concentrations (1, 5, and 10 wt%) and correlated with the freezing point depression and hydrogen bonding energy interaction of AHILs. The study also covers the structural impact of AHILs (in the form of alkyl chain variation) on the thermodynamic hydrate inhibition (THI) behaviour via freezing point and hydrogen bonding energy interactions. Findings revealed that the increased alkyl chain length of AHILs reduced the ΔŦ due to a decrease in hydrogen bonding ability. The highest THI inhibition (ΔŦ = 2.27 K) is attained from the lower alkyl chain AHIL, i.e., TMAOH (10 wt%) for the CO2 hydrate system. The freezing point depression of AHILs is a concentration-dependent phenomenon. Increased concentration of the AHILs in the system yielded lower freezing point temperature, positively influencing hydrate mitigation. Although the study provided the initial insight between the freezing point tendency and hydrogen bonding energies of AHILs on thermodynamic inhibition (ΔŦ). Based on the freezing point depression and hydrogen bonding energy interaction, a more generalized correlation should be developed to predict any potential ionic liquids regarded as promising hydrate inhibitors.

On the importance of DIOX concentration in promoting CH4 hydrate formation: A thermodynamic and kinetic investigation

Hydrate-based CH4 storage is a promising technology for safe and economically-viable natural gas storage and transportation. Solid CH4 hydrate possesses advantages in both energy density and storage stability. The quest for an effective and economic-friendly thermodynamic promoter for CH4 hydrate formation is imminent with the aim of (a) reducing CH4 hydrate formation P-T condition and (b) increasing both the CH4 gas uptake rate and final CH4 storage capacity. In this study, a non-carcinogenic thermodynamic promoter 1,3-dioxolane (DIOX) was systematically investigated to quantify the effect of DIOX concentrations (0.15–5.56 mol%) on the process of mixed CH4 + DIOX hydrate formation. A series of thermodynamic, kinetic, Raman spectroscopy measurement, and morphology observation experiments were conducted to elucidate the mechanism of the promotion effect of DIOX at concentrations below the stoichiometric concentration (5.56 mol%). DIOX with various concentrations shifts the thermodynamic phase equilibrium of binary hydrate to the region of moderate temperature and pressure. Raman spectra provided evidence of more CH4 molecules filling in the large cages of sII hydrate with the decrease of DIOX concentration. The ratio of CH4 in large cages to CH4 in small cages of sII hydrate increases from 0.05 to 0.38 with the decrease in DIOX concentration. The gas uptake rate of CH4 declined with the decrease in DIOX concentration. We also observed that DIOX concentration is an important factor affecting the morphologies during hydrate formation. These findings provide fundamental evidence for improving the CH4 storage capacity and environmental friendliness in the application of DIOX in hydrate-based CH4 storage.

Electrochemical Pretreatment of Solid–Electrolyte Interphase Formation for Enhanced Li 4 Ti 5 O 12 Anode Performance in a Molten Li–Ca Binary Salt Hydrate Electrolyte

“Significant improvement in the cycle performance of a Li4Ti5O12 electrode was achieved using a Li–Ca binary salt hydrate (LCH) electrolyte in combination with an optimized electrochemical pretreatment process. The LCH electrolyte provided less-water-soluble Ca-based SEI components, and careful pretreatment process enabled the formation of a thicker SEI layer on the Li4Ti5O12 electrode, leading to 95.5 % capacity retention over 50 cycles…“ Learn more about the story behind the research featured on the front cover in this issue's Cover Profile. Read the corresponding Research Article at 10.1002/celc.202200061.

Front Cover: Electrochemical Pretreatment of Solid‐Electrolyte Interphase Formation for Enhanced Li 4 Ti 5 O 12 Anode Performance in a Molten Li−Ca Binary Salt Hydrate Electrolyte (ChemElectroChem 11/2022)

The Front Cover illustrates that less-water-soluble Ca-based solid electrolyte interphase formed in a Li–Ca binary salt hydrate electrolyte effectively mitigates electrochemical reduction of water, leading to improvement in the cycle performance of a Li4Ti5O12 electrode. More information can be found in the Research Article by S. Kondou et al.

Electrochemical Pretreatment of Solid‐Electrolyte Interphase Formation for Enhanced Li4Ti5O12 Anode Performance in a Molten Li−Ca Binary Salt Hydrate Electrolyte

AbstractWater‐in‐salt electrolytes have been widely explored because of their expanded electrochemical stability (>3.0 V). However, the instability of solid‐electrolyte interphase (SEI) in aqueous electrolytes leads to their reductive decomposition on the negative electrodes of low‐potential anode materials. Here, we demonstrate significant improvement in the cycle performance of a Li4Ti5O12 electrode using a Li−Ca binary salt hydrate (LCH) electrolyte in combination with an optimized electrochemical pretreatment process. Compared with a hydrate‐melt electrolyte, the LCH electrolyte provided less water‐soluble Ca‐based SEI components, and careful pretreatment process enabled the formation of a thicker SEI layer on the Li4Ti5O12 electrode. Protected with the hardly soluble, thick SEI layer, the Li4Ti5O12 electrode effectively mitigated unfavorable side reactions and achieved 95.5 % capacity retention over 50 cycles. These results offer insight into a promising route for stable SEI layer formation for the practical use of low‐potential anode materials in aqueous rechargeable lithium‐ion batteries.

Superabsorbent polymer for improved CO2 hydrate formation under a quiescent system

The hydrate formation rate and conversion yield are essential factors for hydrate-based gas storage and accordingly various thermodynamic and kinetic promoters have been extensively investigated. However, for hydrate-based CO2 capture, carbonate ions induced from dissolved CO2 in water hinder the performance of kinetic promoters (e.g., sodium dodecyl sulfate (SDS)). In this work, we introduce a superabsorbent polymer (SAP) matrix that provides an extremely high interfacial area between a sorbed tetrahydrofuran (THF) solution and gaseous CO2. The SAP enhanced the formation rate of binary THF-CO2 hydrate, despite the absence of mechanical agitation. We varied the concentration of THF, temperature, reactor volume, and the ratio of solution to the reactor for a systematic study and explored their effects on the CO2 storage capacity and the induction time. We observed that highly concentrated THF and a larger reactor with a high gas-liquid contact area enhanced the CO2 storage capacity and formation rate. Raman and PXRD spectroscopic analyses were employed to investigate the guest distribution and crystalline structure of the THF-CO2 hydrates synthesized under various conditions. All the binary hydrates were structure-II (sII) hydrates, and there was no pure structure-I (sI) CO2 hydrate. Based on the spectroscopic results, we developed a theoretical model that could calculate the amount of enclathrated CO2 and dissolved CO2 in liquid phases separately. This work reveals that SAP has excellent potential for rapid CO2 capture in hydrate media with a high water-to-hydrate conversion ratio.

Assessment of a Biocompatible Additive for Hydrate Formation Kinetics along with Morphological Observations and Model Predictions

Innovative gas hydrate-based applications and hydrate flow assurance problems of oil & gas pipelines have enabled new opportunities to develop and deploy biocompatible or green low-dosage hydrate promoters and inhibitors, respectively. In this context, we evaluate the performance of a biocompatible additive, lecithin (extracted from egg yolk), for the formation kinetics of binary cyclopentane (CP)-carbon dioxide (CO2) hydrates simulating to the structure-II hydrate of natural gas. A high-pressure visual autoclave is employed in this study to map the morphological observations with the gas hydrate kinetic data. Multiple experiments are performed to examine the inhibition or promotional effect of lecithin in saline (3.0 wt % NaCl) and non-saline water systems at low pressure of 1.0 MPa while utilizing the various CP content (1.5, 3.0, and 6.0 mol%) and 500 to 5000 ppm of lecithin. Moreover, we evaluate the effect of salinity and lecithin on the thermodynamic hydrate equilibrium conditions. The results imply that the presence of lecithin retards the hydrate formation kinetics. However, no shift in the equilibrium frontier is observed. Importantly, we propose a chemical potential based kinetic model by accounting the influence of additives, vapor-liquid interfacial area, temperature and pressure on gas hydrate formation. This model has closely interpreted the experimental data of hydrate former uptake during binary hydrate formation in the presence and absence of lecithin. Our findings highlight the potential of engaging lecithin for hydrate inhibition to mitigate the flow assurance issues and also offer comprehensive insights toward integrated hydrate-based carbon capture and seawater desalination approach.

Heat transformation performance of salt hydrate-based thermochemical energy storage sorbent during hydration

Given that the outstanding benefits of high energy storage density (ESD) and long-term energy storage ability with ignorable heat losses, thermochemical energy storage (TCES) in salt hydrates is a promising way to bridge the gap between supply and demand sides for renewables in residential heating. In this article, the hydration process (for heat discharge) of the sorbent composed of binary lithium-based salt hydrate and expanded graphite (LiOC@EG) in an individual thermal energy storage (TES) module is numerically investigated. For the airflow of 0.2 m/s and 20℃ with 80% relative humidity (RH), the bed temperature and outlet temperature dramatically rise with 30 min, and the maximum values are around 46 ℃ and 52 ℃, respectively. With hydration reaction advancement, the water vapour concentration increases in the bed due to the slowdown of adsorption kinetics. The temperature gradient and hydration reaction difference alone radial direction in the TES unit is almost negligible, and the time required to basically complete the hydration process (α > 95%) is ∼6.5 h, indicating the thermochemical sorbent coupled with the TCES unit is appropriate for home heating supply. The results of this study may provide reference and prediction on heat and mass migration behaviours of TCES system.

“Similar self-preservation” and decomposition kinetics of tetrahydrofuran-hydrogen hydrate particles

Storage hydrogen using hydrates is a promising alternative to other hydrogen storage technologies. In this work, the effects of pressure and temperature on the decomposition kinetics of binary THF-H2 hydrates was investigated systematically in the pressure range of 0.1 MPa–7 MPa and temperature from 243 K to 273 K. The results showed that the dissociation of THF-H2 hydrates could be divided into three stages, rapid decomposition, slow decomposition, and stable state. The higher the pressure, the lower the dissociation rate of THF-H2 hydrates in the final stable stage. It was worth to note that the dissociation rate of THF-H2 hydrates does not show a simple linear relationship with temperature. The anomalous phenomenon of THF-H2 hydrates dissociation was observed in the temperature region of 253 K and 268 K under 5 MPa, especially at 268 K, and it was called “similar self-preservation” phenomenon. The dissociation ratio of THF-H2 hydrates in 80 h was not more than 50 mol% at 268 K and 5 MPa. This study provides practical way for developing clathrate hydrate-based storage hydrogen (transport and application) in the future.

Exploring tuning phenomena of THF-H2 hydrates via molecular dynamics simulations

The tuning phenomenon is one of the effective strategies to enhance the gas storage density of clathrate hydrates at moderate pressure: small gaseous guest molecules can occupy some of the large cages of binary hydrates that are not occupied by thermodynamic promoters. This work explored tuning phenomena in THF-H2 hydrates at the molecular level by all-atomic molecular dynamics simulations for the first time. Before conducting molecular dynamics simulations, off-diagonal Lennard-Jones (LJ) parameters between the center of mass of H2 and oxygen of H2O were optimized from ab-initio calculations with various configurations of H2 and H2O molecules. These optimized off-diagonal parameters provided reasonable phase equilibrium of THF-H2 hydrates compared to the simple mixing rule case, which is derived from weakened H2-H2O interaction and active drifting of H2 in molecules’ 512 cages. With the ab-initio modified LJ parameters, the growth of THF-H2 hydrates was investigated by molecular dynamics simulations under various compositions and pressures of the system. Cage occupancy of H2 in 512 cages of the binary hydrates increased when the THF concentration was lower than 5.56 mol%, but double occupation of H2 in 512 cages was not observed. Cage occupancy of H2 in 51264 cages significantly increased by lowering the THF concentration, and the total storage capacity of H2 increased by multiple occupations of H2 in the 51264 cages. The growth rate of the THF-H2 hydrates depended on the THF concentration, and that dependency of the growth rate was due to the wrong adsorption of THF molecules at open small cage sites of the solution-hydrate interface. These results demonstrated that uptake of H2 could be enhanced by the tuning phenomena when mobile THF molecules in the liquid phase formed binary hydrates with H2.

Implementation of Exp-6 potential function in thermodynamic modeling of clathrate hydrates phase equilibria

Accurate modeling of clathrate hydrates phase behaviors is of significant importance to petroleum industry. It is well known that accurate consideration of the cavity force field can lead to more accurate predictions of incipient conditions of clathrate hydrates. Intermolecular repulsive force is the dominant force in cavity space that is a determinative factor in taking the guest–host interactions into account. As the result, further modifications in potential function part of van der Waals and Platteeuw (vdWP) solid solution theory could improve the model accuracy. In this study, the influence of repulsive force between guest–host molecules in hydrate cavities was investigated through replacing Kihara potential function by Exp-6 potential function. The vdWP model equations were derived based on new potential function. The pair potential parameters were regressed using equilibrium dissociation conditions data of simple hydrates of natural gas constituents. The determination performances of Kihara and Exp-6 potential functions were compared in aspects of incipient hydrate dissociation conditions and hydrate composition of binary hydrate formers and mixtures of natural gas systems. The results show a promising improvement in vdWP model in case of binary hydrate systems, while the accuracy of the model does not change dramatically in case of natural gas hydrate system. This is due to cancellation of changes in Gibbs energy of occupation for different guest species and the natural gas composition.

Preparation of Binary Mixed Inorganic Hydrate

In this paper, 54 binary mixed inorganic hydrates composed of Na2SO4·10H2O, Na2CO3·10H2O, CH3COONa·3H2O and MgSO4·7H2O were prepared, and try to research their reasonable and feasible preparation methods. The results show that the thermogravimetric ratio of binary mixtures varies with the composition and proportion of binary mixtures. There is evaporation of water in the mixtures preparation process, so appropriate deionized water should be added to supplement the evaporated water in the mixtures preparation of phase change materials.

Phase boundary of gas hydrates in single and mixed electrolyte solutions: Using a novel unified equation of state

The phase boundary of gas hydrates is key to flow assurance, energy recovery, and hydrate-based technologies. In this work, a new thermodynamic model has been proposed for calculating the phase boundary of gas hydrates based on a unified equation of state (EoS) for the non-hydrate phases and the van der Waals-Platteeuw (vdW-P) model for the hydrate phase. A modified Peng-Robinson EoS is employed to describe the interactions between water, gas, and uncharged ions, combining with a simplified explicit mean spherical approximation (MSA) term for the long-range Coulombic forces and the Born terms for the discharging-charging process of ions. A new set of reference properties have been applied and the Kihara parameters in the vdW-P model are optimized by fitting against the measured phase boundary data of hydrates formed in the pure water. The newly developed model has been validated with 734 measured phase boundary data points of pure CH4 and binary CH4-C2H6 and CH4-CO2 hydrates formed in single- and mixed-NaCl, KCl, MgCl2, and CaCl2 solutions as well as in the pure water. It is found that this model is capable of predicting the phase boundary of CH4 hydrates formed in the 1–1 electrolyte solutions (NaCl-KCl) with the average absolute relative deviation (AARD) of 3.7%. The AARD of CH4 hydrates formed in the 2–1 electrolyte solutions (MgCl2-CaCl2) is 4.2%, which is greater than that for the 1–1 electrolytes. This is acceptable given the fact that none of the ion-associated parameters have been optimized in this work. The calculated phase boundary of pure CH4 hydrate formed in mixed-electrolyte solutions deviates from the measured ones only by 2.8%. The AARD of the binary hydrates (CH4-CO2 and CH4-C2H6) formed in single- and mixed-electrolyte solutions is 3.5%. This model can also be reduced to calculate the hydrate phase boundary in pure water. The AARD of the CH4-CO2 hydrates formed in pure water is determined as 3.0%. The overall AARD of these 734 data points predicted by this newly developed model is 3.4%, which proves the capability of the newly developed model in predicting the phase boundary of hydrates in multi-component systems. It is also presented that this unified EoS is reliable to determine the gas solubility in aqueous phase with and without electrolytes.