Hydrate Equilibrium Research Articles

Thermodynamics, Kinetics, Morphology, and Raman studies for sH Hydrate of Methane and Cyclooctane

Natural gas is expected to be the major energy source in the near future, and storing it in the form of gas hydrate is a safe, clean, and economical approach. However, required thermodynamic conditions and slow kinetics are the key challenges that need to address for process viability. This study involves an experimental investigation of methane and cyclooctane sH hydrate formation for possible applications in gas storage using thermodynamics, kinetics, morphology, and Raman analysis. The hydrate formation is carried out at such thermodynamic conditions where only sH hydrate would form. The four-phase (Lw-LHC-H-V) sH hydrate equilibrium is studied for the methane and cyclooctane system via dissociation along the phase boundary method which is a robust method as it delivers a greater number of equilibrium data points in a single experimental run compared to other available methods. The sH hydrate formation helps in lowering the equilibrium conditions compared with sI hydrate formation. The slow sH hydrate formation kinetics can be improved by using low tryptophan concentrations. In this work, 0.1 wt % is the optimum tryptophan concentration as the gas uptake, and the hydrate formation rate is found to be the highest compared to 0.01, 0.05, and 1 wt % tryptophan concentrations. Here, we also visually investigate the sH hydrate formation and observed that the hydrate formation occurs below the interface for the system with no tryptophan; however, hydrate formation occurrence above the interface increases with an increase in the tryptophan concentration. The increase in the hydrate formation could be dedicated to the increased gas uptake due to the increasingly porous nature of hydrate formation. The Raman analysis confirmed the presence of methane and cyclooctane in sH hydrate cages. The higher intensity of the peaks using tryptophan additionally confirms the higher hydrate formation compared to the system with no tryptophan.

Experimental measurement and model prediction on methane hydrate equilibrium conditions in the presence of organic carboxylic sodium salts

Gas hydrate inhibitors are employed to mitigate gas hydrate formation in pipelines. The effects of thermodynamics inhibition were evaluated by measuring the vapor - liquid aqueous solution - solid hydrate (VLH) equilibrium conditions on methane (CH4) hydrate in the presence of three organic carboxylic sodium salts (sodium formate, sodium acetate, sodium propionate) in this study. The experiments were carried out in the temperature range of 273.65 K to 281.65 K. Compared with deionized water, in the electrolyte solutions of sodium formate, sodium acetate and sodium propionate with mole fraction of 0.010 and 0.020, the VLH equilibrium pressure change (ΔP) increased by 12.93, 27.68 %; 18.78, 38.15 %; 23.51, 49.41 %, respectively. Both of the three organic carboxylic sodium salts have obvious thermodynamic inhibition effect on CH4 hydrate. The strength of hydrate inhibition is related to the carbon chain length of organic carboxylate. The dissociation enthalpy (ΔHdiss) of CH4 hydrate in three organic carboxylic sodium salts solutions remains quite uniform with deionized water, which indicates that three organic carboxylic sodium salts have not been involved in the formation of hydrate nucleus. The Chen-Guo hydrate model and N-NRTL-NRF activity model were employed to predict the VLH equilibrium conditions in electrolyte solutions. In the three organic carboxylic sodium salts solutions, the average absolute relative deviations (AARD) of the VLH equilibrium condition data between the predicted and the experimental were 0.81, 2.17, and 3.02 %, respectively. In the sodium chloride (NaCl) and potassium chloride (KCl) solutions, the AARD between the predicted and literature were 1.44, 1.69 %, respectively. The model has good universality and accuracy. Both sodium acetate and sodium propionate are food grade preservatives. Consequently, these are beneficial for the development of environmental-friendly hydrate inhibitors.

Thermodynamic effects of the interaction of multiple solutes and dodecahedral-cage deformation on the semi-clathrate hydrate formation with CH4-CO2

The accurate prediction of thermodynamic equilibrium hydrate formation pressure (Peq) is crucial to the industrial application of hydrate-based gas separation. This study evaluated Peq of CH4-CO2 gas mixtures with different TBAB solutions at different temperatures. Unlike previously reported models, the proposed model accounted for the effects of interaction of multiple solutes and the deformation of dodecahedral (D)-cages on Peq, thereby minimizing deviations in predictions. The effects of the interaction of multiple solutes on Peq can be attributed to gas dissolution and the presence of electrolytes. For accurate prediction, the proposed model incorporated the aforementioned effects based on the electrolyte-cubic plus association equation of state and the parameters for interaction among guest molecules in cages Furthermore, the proposed model can quantitatively describe the effects of gas dissolution, the presence of electrolytes and deformation of D-cages on the Peq values for corresponding systems.

A cage-specific hydrate equilibrium model for robust predictions of industrially-relevant mixtures.

Understanding the thermophysical properties and phase behaviour of gas hydrates is essential for industrial applications ranging from energy transport and storage, CO2 capture and sequestration, to gas production from hydrates found on the seabed. Current tools for predicting hydrate equilibrium boundaries typically use van der Waals-Platteeuw-type models which are over-parameterised containing terms with limited physical basis. Here we present a new model for hydrate equilibrium calculations with 40% fewer parameters than existing tools but with equivalent accuracy, including for multicomponent gas mixtures and/or systems with thermodynamic inhibitors. By eliminating multi-layered shells from the model's conceptual basis and focusing on Kihara potential parameters for guest-water interactions specific to each hydrate cavity type, this new model provides insight into the physical chemistry governing hydrate thermodynamics. The model retains the improved description of the empty lattice developed recently by Hielscher et al. but couples the hydrate model with a Cubic-Plus-Association Equation of State (CPA-EOS) to describe fluid mixtures with many more components including inhibitors such as methanol and mono-ethylene glycol used by industry. An extensive database of over 4000 data points was used to train and evaluate the new model and compare its performance against existing tools. The absolute average deviation in temperature (AADT) achieved with the new model is 0.92 K for multicomponent gas mixtures, compared with 1.00 K for the widely-known model of Ballard and Sloan, and 0.86 K for the CPA-hydrates model implemented in the MultiFlash 7.0 software package. With fewer, more physically justified parameters, this new cage-specific model provides a robust basis for improved hydrate equilibrium predictions particularly for industrially-important, multi-component mixtures containing thermodynamic inhibitors.

Effect of hydration equilibria on the relaxometric properties of Gd(III) complexes: new insights into old systems.

We present a detailed relaxometric and computational investigation of three Gd(III) complexes that exist in solution as an equilibrium of two species with a different number of coordinated water molecules: [Gd(H2O)q]3+ (q = 8, 9), [Gd(EDTA)(H2O)q]- and [Gd(CDTA)(H2O)q]- (q = 2, 3). 1H nuclear magnetic relaxation dispersion (NMRD) data were recorded from aqueous solutions of these complexes using a wide Larmor frequency range (0.01-500 MHz). These data were complemented with 17O transverse relaxation rates and chemical shifts recorded at different temperatures. The simultaneous fit of the NMRD and 17O NMR data was guided by computational studies performed at the DFT and CASSCF/NEVPT2 levels, which provided information on Gd⋯H distances, 17O hyperfine coupling constants and the zero-field splitting (ZFS) energy, which affects electronic relaxation. The hydration equilibrium did not have a very important effect in the fits of the experimental data for [Gd(H2O)q]3+ and [Gd(CDTA)(H2O)q]-, as the hydration equilibrium is largely shifted to the species with the lowest hydration number (q = 8 and 2, respectively). The quality of the analysis improves however considerably for [Gd(EDTA)(H2O)q]- upon considering the effect of the hydration equilibrium. As a result, this study provides for the first time an analysis of the relaxation properties of this important model system, as well as accurate parameters for [Gd(H2O)q]3+ and [Gd(CDTA)(H2O)q]-.

CO2 hydrate formation in NaCl systems and undersaturated aqueous solutions

A high-pressure experimental setup was used to obtain experimental data on the Hydrate Equilibrium (melting) Temperature (HET) of aqueous phases undersaturated with CO2, at different CO2 saturation levels and with different aqueous phase compositions (sodium chloride and sodium chloride/calcium chloride). This paper details the experimental equipment, methods, test fluids, and results. To study the phase equilibria of CO2-brine systems and further validate the results, hydrate dissociation conditions of CO2 in different concentrations of NaCl aqueous solutions were measured in the low-temperature region and over a wide range of pressure. Experimental results were compared against predictions of the simplified Cubic Plus Association Equation of State (sCPA-EoS) coupled with the van der Waals and Platteeuw solid solution theory.

New insights into methane hydrate inhibition with blends of vinyl lactam polymer and methanol, monoethylene glycol, or diethylene glycol as hybrid inhibitors

Gas hydrate inhibition is a relevant topic of flow assurance. Thermodynamic hydrate inhibitors (THIs) and kinetic hydrate inhibitors (KHIs) are applied to prevent gas hydrates formation in oil and gas production facilities. We have carried out extensive experimental studies to further develop the hybrid inhibition strategy involving KHI/THI formulations. We determined methane hydrate equilibrium conditions for an aqueous solution of methanol, MEG, and DEG (≤50mass%). Subsequently, we studied the kinetics of methane hydrate nucleation and growth for an aqueous solution of commercial vinyl lactam KHI and its blends with alcohols. KHI loses its ability to inhibit hydrate nucleation at ≥ 20 mass%methanol. Adding MeOH to KHI solution also accelerated the growth of methane hydrate. The addition of glycols to KHI increases the hydrate onset subcooling. KHI/MEG blends are optimal for hybrid inhibition as they demonstrate the enhanced delay of gas uptake and reduced rate of methane hydrate growth.

CIRCULAR ECONOMY APPLIED TO METHANE PRODUCTION FROM NATURAL GAS HYDRATE RESERVOIRS: POTENTIALITIES OF RESIDUAL DUST COMING FROM STEEL PLANTS

Natural gas hydrate represents one of the most promising solutions to answer to the constantly increasing energy demand; in addition, the possibility of recover methane via carbon dioxide injection, with a theoretical exchange ratio equal to 1, makes it a potential carbon neutral energy source. Among them, energetical costs associated to practical operations in marine deposits. The use of chemical inhibitors and or promoters to improve the exchange process is gaining increasing interest and researchers are mainly focused on finding less environmental unfriendly additives and on reducing their costs. In that direction, the present work deals with the possible use of waste dust, produced during steel mill processes, as promoter of the CO2/CH4 replacement process. That sand commonly contains a great variety of compounds, such as metal oxides, alumina, salts, and so on. Some of them have a chemical composition close to well-known hydrate inhibitors/promoters. Moreover, that application could be a further energetic cycle for a waste product. In this work, both methane and carbon dioxide hydrate formation were tested in absence and in presence of cupper oxides, with different concentrations. Hydrate formation and dissociation results where then compared among each other and with hydrate equilibrium values for those compounds.

Impact of Atmospheric CO2 on Thermochemical Heat Storage Capabilities of K2CO3.

This work investigates the reactions occurring in K2CO3-H2O-CO2 under ambient CO2 pressures in temperature and vapor pressure ranges applicable for domestic thermochemical heat storage. The investigation shows that depending on reaction conditions, the primary product of a reaction is K2CO3·1.5H2O, K2CO3·2KHCO3·1.5H2O, or a mixture of both. The formation of K2CO3·1.5H2O is preferred far above the equilibrium conditions for the hydration reaction. On the other hand, the formation of double salt is preferred at conditions where hydration reaction is inhibited or impossible, as the thermogravimetric measurements identified a new phase transition line below the hydration equilibrium line. The combined X-ray diffraction, thermogravimetric analysis, and Fourier-transform infrared spectroscopy study indicates that this transition line corresponds to the formation of K2CO3·2KHCO3, which was not observed in any earlier study. In view of thermochemical heat storage, the formation of K2CO3·2KHCO3·(1.5H2O) increases the minimum charging temperature by approximately 40 °C. Nevertheless, the energy density and cyclability of the storage material can be preserved if the double salt is decomposed after each cycle.

Thermal transport of charging/discharging for hydrogen storage in a metal hydride reactor coupled with thermochemical heat storage materials

Metal hydride is a promising alternative for hydrogen storage. A novel metal hydride reactor coupled with thermochemical heat storage material is proposed to reduce the energy loss. The hydrogen charging and discharging processes are numerically and analytically investigated in this paper. Hydrogen is stored in a compacted metal hydride disk, so the reaction characteristics under low permeability are studied. The dehydration kinetic equation has been re-established to determine the equilibrium temperature at a corresponding pressure. The reactor takes 139 min for the hydrogen charging process and takes 512.8 min for the hydrogen discharging process. The reaction characteristics are found to be affected by heat transfer and gas pressure at the permeability of 5.9×10-16m2. The analytical model is found to be only applicable to the prediction of reaction time with high permeability of hydrogen in metal hydride. The thermal management of thermochemical heat storage materials has been proven to be feasible. By increasing the hydration equilibrium temperature from 321.8 °C to 350 °C, the reaction time can be decreased to 43.1 %. By decreasing the dehydration equilibrium temperature from 300 °C to 270 °C, the reaction time can be decreased to 84.5 %. To enhance the heat transfer, meal foam is added to the thermochemical heat storage material. Adding metal foam can effectively shorten the reaction time, but will reduce the hydrogen production. A metal foam with a porosity of 0.96 will reduce the production of hydrogen by 3 %.

Prediction of organic and inorganic gas hydrates equilibrium conditions in the presence and absence of thermodynamic inhibitors

The formation of gas hydrates in surface production equipment is common and poses a significant loss to the oil and gas industry sectors. It arises primarily in the pipeline located beneath sea level, and this causes equipment blockages, increasing the operational cost. Most drilling processes incorporate thermodynamic inhibitors such as methanol, ethylene glycol, and triethylene glycol to address these issues. One method to study this relationship is by developing an empirical correlation to predict the hydrate equilibrium condition. In the current work, new empirical correlations for organic and inorganic gas hydrates in pure water and thermodynamic inhibitors are developed. It was observed that the overall mean deviation between the experimental data and the predicted correlations for organic and inorganic gas hydrates in pure water was 5.61% and 3.27%, respectively. Similarly, the overall mean deviation for CO2 in MeOH, EG, and TEG was 5.77%, 8.90%, and 2.09%, respectively. In the case of CH4 and C2H6 in TEG, the overall mean deviation was 2.03% and 2.16%, respectively. The developed correlations are in good agreement with the experimental data in the literature. The present work may be helpful in predicting the various gas hydrate equilibrium conditions, especially in natural gas production.

Empirical correlation for calculating the pure liquid water–methane hydrate–methane equilibrium pressure

A new empirical correlation for calculating the pure liquid water–methane hydrate–methane equilibrium pressure in the temperature range of 273.15–301.70 K is obtained. This correlation was obtained on the basis of 398 experimental points taken from 44 sources. A statistical error analysis was carried out, as a result of which it was found that the obtained empirical correlation is the best of the currently existing empirical correlations for calculating the pure liquid water–methane hydrate–methane equilibrium pressure. The obtained empirical correlation is satisfied with a mean absolute percentage error of 1.55% and a coefficient of determination of 0.997. A theoretical analysis was carried out, as a result of which the general form of any empirical correlation for predicting gas hydrate equilibrium conditions in the water–gas system was established. It is shown that the form of such empirical correlation follows from the Clausius–Clapeyron equation.

(Invited) Atomistic Insights on Prussian Blue Analogues for Aqueous Electrochemical Separations

Prussian blue analogues (PBAs) containing metal centers bridged by cyanide ligands exhibit one of the simplest structures among metal organic frameworks, which lends them to facile and reversible insertion of monovalent cations. Such a property has motivated their use in aqueous electrochemical separations, including our most recent demonstration of brackish water desalination with only 25% more energy consumption than thermodynamic minimum.1 Despite their selectivity bias toward cations of small hydrated radius being known for 36 years,2 the microscopic mechanisms responsible for such bias that could be used to inform the rational design of future materials are not well understood.Accordingly, we report on our recent efforts3,4 to uncover the interactions between inserted cations, interstitial water, and PBA lattices that produce selectivity bias. To understand the interplay between structural distortion, bonding, and water uptake from contacting electrolyte we use a first-principles approach that combines machine learning with density functional theory and grand potential analysis to explore the potential energy landscapes of disordered arrangements of interstitial species in nickel hexacyanoferrate PBA. Here, a competition for dative bonds of acidic interstitial cations between basic oxygen in interstitial water and basic cyanide ligands is shown to result in complexation between cations and water that deviate fundamentally from bulk hydration in pure water, despite the common attribution of PBA selectivity to purely steric effects of cation hydration. Van der Waals interactions are further shown to enhance the formation of water clusters and extended hydrogen-bonded networks within the PBA host, depending on the bare ionic radius of inserted cations. Grand potential analysis is performed to ultimately produce previously observed selectivity bias (Cs+ > K+ > Na+) and to determine the equilibrium hydration degree of the PBA in both reduced and oxidized forms. This analysis shows that small (Na+) and moderately sized (K+) cations require two to four water molecules per formula unit in both reduced and oxidized forms of the PBA, while large cations (Cs+) exhibit much smaller hydration levels in oxidized form and exhibit no hydration in reduced form. References Reale, E. R., Regenwetter, L., Agrawal, A., Dardón, B., Dicola, N., Sanagala, S. & Smith, K. C. Low porosity, high areal-capacity Prussian blue analogue electrodes enhance salt removal and thermodynamic efficiency in symmetric Faradaic deionization with automated fluid control. Water Res. X 13, 100116 (2021).Ikeshoji, T. Separation of Alkali Metal Ions by Intercalation into a Prussian Blue Electrode. J. Electrochem. Soc. 133, 2108 (1986).Liu, S. & Smith, K. C. Linking the polyatomic structure of interstitial H2O and cations to bonding within Prussian blue analogues ab initio using gradient-boosted machine learning. Phys. Rev. Mater. 5, 035003 (2021).Liu, S. & Smith, K. C. Effects of interstitial water and alkali cations on the expansion, intercalation potential, and orbital coupling of nickel hexacyanoferrate from first principles. J. Appl. Phys. 131, 105101 (2022).

Simulation of the CO2 hydrate-water interfacial energy: The mold integration-guest methodology.

The growth pattern and nucleation rate of carbon dioxide hydrate critically depend on the precise value of the hydrate-water interfacial free energy. There exist in the literature only two independent experimental measurements of this thermodynamic magnitude: one obtained by Uchida et al. [J. Phys. Chem. B 106, 8202 (2002)], 28(6)mJ/m2, and the other by Anderson and co-workers [J. Phys. Chem. B 107, 3507 (2003)], 30(3)mJ/m2. Recently, Algaba et al. [J. Colloid Interface Sci. 623, 354 (2022)] have extended the mold integration method proposed by Espinosa and co-workers [J. Chem. Phys. 141, 134709 (2014)] to deal with the CO2 hydrate-water interfacial free energy (mold integration-guest or MI-H). Computer simulations predict a value of 29(2)mJ/m2, in excellent agreement with experimental data. The method is based on the use of a mold of attractive wells located at the crystallographic positions of the oxygen atoms of water molecules in equilibrium hydrate structures to induce the formation of a thin hydrate slab in the liquid phase at coexistence conditions. We propose here a new implementation of the mold integration technique using a mold of attractive wells located now at the crystallographic positions of the carbon atoms of the CO2 molecules in the equilibrium hydrate structure. We find that the new mold integration-guest methodology, which does not introduce positional or orientational information of the water molecules in the hydrate phase, is able to induce the formation of CO2 hydrates in an efficient way. More importantly, this new version of the method predicts a CO2 hydrate-water interfacial energy value of 30(2)mJ/m2, in excellent agreement with experimental data, which is also fully consistent with the results obtained using the previous methodology.

Mathematical Modeling of Gas Hydrates Dissociation in Porous Media with Water-Ice Phase Transformations Using Differential Constrains

2D numerical modeling algorithms of multi-component, multi-phase filtration processes of mass transfer in frost-susceptible rocks using nonlinear partial differential equations are a valuable tool for problems of subsurface hydrodynamics considering the presence of free gas, free water, gas hydrates, ice formation and phase transitions. In this work, a previously developed one-dimensional numerical modeling approach is modified and 2D algorithms are formulated through means of the support-operators method (SOM) and presented for the entire area of the process extension. The SOM is used to generalize the method of finite difference for spatially irregular grids case. The approach is useful for objects where a lithological heterogeneity of rocks has a big influence on formation and accumulation of gas hydrates and therefore it allows to achieve a sufficiently good spatial approximation for numerical modeling of objects related to gas hydrates dissociation in porous media. The modeling approach presented here consistently applies the method of physical process splitting which allows to split the system into dissipative equation and hyperbolic unit. The governing variables were determined in flow areas of the hydrate equilibrium zone by applying the Gibbs phase rule. The problem of interaction of a vertical fault and horizontal formation containing gas hydrates was investigated and test calculations were done for understanding of influence of thermal effect of the fault on the formation fluid dynamic.

The coexistence of multiple hydrates triggered by varied H2 molecule occupancy during CO2/H2 hydrate dissociation

The participation of small H2 molecules in hydrate formation generally includes single and multiple occupancy, and the state could vary frequently due to their high fluidity. In this study, distinctly split DSC peaks for hydrate dissociation in constant CO2/H2 gas phase systems were found, which generally means the coexistence of several hydrates with different phase equilibrium temperatures. Based on the results of hydrate equilibrium decomposition conditions and decomposition enthalpies, we attributed the occurrence of this peculiar phenomenon mainly to different occupation patterns of H2 molecules in hydrate cages. However, several occupancies in the mild conditions of this study may be metastable, which was significantly related to H2 escape. Raman results demonstrated the existence of single occupancy of H2 molecule in both of the 512 and 51262 cages. In addition, co-existing of CO2 and H2 in 51262 cage may also be detected. The effects of hydrate formation time and morphology on the split DSC peaks further indicated the occupation-related H2 molecule aggregation and probabilistic escape. The discovery in this work may provide some new insights into the deeper understanding of the physical/chemical properties behind clathrate hydrate in the future.

Determination of clathrate hydrates dissociation conditions in the presence of gas dehydration, sweetening, and other nitrogenated additives using a predictive thermodynamic approach

–Despite numerous experimental data on gas hydrate equilibrium conditions in the presence of glycols, alkanolamines, and nitrogenated additives that are frequently utilized in the gas refinery, the apparent lack of a precise predictive thermodynamic model is still perceived. This study presents an unprecedented thermodynamic framework benefitting from the modified van der Waals-Platteeuw (vdW-P) model for the hydrate phase, the Peng-Robinson equation of state (PR EoS) for the vapor/gas phase, and combinations of free-volume Flory Huggins (FVFH) and Pitzer-Debye-Hückel (PDH) equations for the water activity in the aqueous phase, in which the FVFH activity model is utilized for the additives with molecular interactions solely, while the PDH model is employed when the ionic interactions also exist. When the model assessed a databank of 1075 data points, 0.29% (0.80 K) and 9.67% (0.49 MPa) deviations were observed in the temperature and pressure calculations, respectively. In particular, for 877 data points (glycols, urea, acetamide, and formamide), employing FVFH solely resulted in 0.32% (0.88 K) and 10.54% (0.50 MPa) temperature and pressure deviations, respectively, whereas the combination of FVFH + PDH yielded 0.17% (0.48 K) and 5.81% (0.47 MPa) errors in temperature and pressure estimations, respectively in 198 data points of the systems comprised of amines, hydrazine, and piperazine. The maximum deviation of temperature prediction did not exceed 6.80 K (2.39%). The results reveal the effective performance of the proposed calculation approach.

Phase equilibria of methane/TBAC mixed hydrates in the presence of produced water

AbstractProduced water (PW) is a by‐product and one of the major pollutants from the oil and gas industry. PW contains various pollutants and must be treated as per environmental regulations before its disposal or reuse. Thus, it presents a substantial economic and ecological problem in the oil and gas industry. Currently, there is ongoing research on various methods to treat PW efficiently. Hydrate‐based water desalination is one of the promising technologies. There is a continuing effort to identify a suitable hydrate former that can enhance the commercial feasibility of the process. This study investigates tetra‐n‐butyl ammonium chloride (TBAC) + methane as a potential clathrate former. The hydrate equilibrium temperatures of the mixed hydrates (TBAC + methane) are significantly higher at a given pressure than methane hydrates, suggesting the suitability of mixed hydrates for process development. There was a significant increase in the phase equilibrium temperature at 5 wt.% TBAC in the presence of PW compared to deionized water (DI), but the effect faded away as TBAC concentration increased in the solution. This can be attributed to a higher amount of chloride ions as the TBAC concentration is increased. These results elucidate the necessity to optimize the TBAC concentrations to develop an efficient hydrate desalination process for higher saline concentrations.

Gas Hydrate Phase Equilibrium Data for the CO2 + TBPB + THF + Water System

In this study, phase equilibria of gas hydrates of CO2 in the presence of tetra n-butyl phosphonium bromide (TBPB) and tetrahydrofuran (THF) have been investigated. The experiments were performed using the isochoric pressure search method at varying compositions of promoters. The phase equilibrium data have been reported at 0.01, 0.05, and 0.1 mass fractions of TBPB in aqueous solutions at the temperature and pressure ranges of 278–295 K and 1.1–4.5 MPa, respectively. The results showed that adding the aqueous mixtures of TBPB and THF to the system changes the hydrate phase equilibrium conditions as expected. In comparison with the system including only TBPB, the promotion effect of the applied mixture is low at all studied concentrations of TBPB in the presence of 0.01 mass fraction of THF. However, at 0.05 mass fraction of THF, the hydrate equilibrium pressures at constant temperatures decrease dramatically.

Equilibrium conditions of CO2+C3H8 hydrates in pure and saline water solutions of NaCl

Gas hydrate formation and dissociation have been extensively studied in past decades after introducing hydrate-based technology in the energy and environmental fields. Among all, hydrate-based water desalination (HBWD) has been receiving significant attention because of the shortage in water resources. The present study investigates the equilibrium conditions for gas hydrate formation from propane and carbon dioxide gas mixtures in the presence of pure water and low saline aqueous solutions for the first time. According to the carried out sensitivity analysis on thermodynamic equilibrium conditions of the CO2+C3H8 mixture with different mole fractions of propane (5–50% C3H8 + balanced CO2) it was concluded that the addition of propane above 20% did not significantly affect the hydrate equilibrium condition. Experiments have been carried out with an initial gas molar composition of 20% C3H8 and 80% CO2 in a temperature range of 275.15–280.15 K, at isochoric conditions for the first time. The inhibition effect on the equilibrium conditions is reported in the case of NaCl addition (1, 1.5, 2, and 3 wt%). A reasonable agreement between the experimental data and those calculated from the thermodynamic model applied in this study has been obtained. According to the tests results, the coefficient of determination (R squared) of the final results of the conducted tests and the model predictions are 0.9987 and 0.9950 for pure and saline water, respectively.