Gas Hydrate Phase Equilibria Research Articles

A novel model to predict phase equilibrium state of hydrates from the relationship of gas solubility

The study of hydrate phase equilibrium is crucial for ensuring the safety of natural gas pipeline transportation and the process of hydrate recovery. While scientists typically focus on the chemical potential of hydrates, the role of gas solubility in hydrate phase equilibrium remains unclear, and this study fills this gap. This work investigated the solubility of gas at the equilibrium point of the hydrate phase through model calculations. Additionally, a new model of hydrate phase equilibrium is established based on the relationship between solubility. Firstly, a solubility model based on gas-liquid equilibrium theory showed higher prediction accuracy in comparison to the PR equation and Duan model and was then used to calculate gas solubility under hydrate phase equilibrium conditions. Afterwards, a novel model was developed to predict hydrate equilibrium state based on the relationship between gas solubility and hydrate phase equilibrium temperature, and it was further compared with the Chen–Guo model and CSMGem in terms of prediction accuracy under pure water and brine settings. The results showed: (a) The calculation deviation of the solubility model was 0.7–8.7% in pure water settings and 2.6–11.7% in brine settings; (b) A strong linear correlation between the phase equilibrium temperature of hydrates and gas solubility was also found; (c) This proposed model achieved over 10 times the accuracy of the Chen–Guo model and the CSMGem in predicting the phase equilibrium state of N2 and CO2 hydrates, and 3–10 times higher accuracy than that of the Chen–Guo model and CSMGem in brine. This work suggests that the gas solubility equilibrium theory can provide a more accurate prediction of hydrate states.

Modeling on gas hydrate phase equilibrium at high concentration of alcohols

As the major interest in flow assurance of oil and natural gas production, the concentrations of injected alcohols are commonly high exceeding mass fraction of 30 %. In this work, the non-ideality of aqueous phase is represented by the activity coefficient model, in which the water and inhibitor molecules are regarded as associating compounds, and the differences in size and shape between them are properly considered. Coupled with the Chen-Guo hydrate model, the proposed model has been extended to represent the hydrate phase equilibrium of pure gas and gas mixtures in aqueous alcohol solutions with concentrations up to 50 wt%. Moreover, the hydrate lattice parameter was reoptimized to extend the Chen-Guo hydrate model to systems below 273.15 K, when high-concentration of alcohols is present. The predictions of the proposed model have been compared with the experimental data, with the average relative deviations in pressure and temperature of 3.991 % and 0.108 %, respectively, verifying the reliability of this model. Compared with other thermodynamic models or correlations, the proposed model in this work is equivalent to or outperformed those in predicting hydrate formation conditions, especially at high alcohol concentrations.

Thermodynamic modeling of gas hydrate phase equilibrium conditions in porous media in the presence of inhibitor solution

Thermodynamic modeling of gas hydrate phase equilibrium conditions in porous media in the presence of inhibitor solution

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.

Thermodynamic modeling of gas hydrate phase equilibrium of carbon dioxide and its mixture using different equations of states

In this work, the fugacity-based approach of the Klauda and Sandler model was used to predict the equilibrium conditions of gas hydrates formation for carbon dioxide and its mixtures with nitrogen, methane, ethane, and propane. In order to calculate the Langmuir constants and empty hydrate vapor pressure, the correlations presented by Munck et al., (1988) and Dharmawardhana et al., (1980) were used, respectively. Hence, six well-known equations of state (EOS) including PR, PRSV, SRK, PT, VPT and translated and consistent versions of the Peng-Robinson (tc-PR) had been used to calculate the thermodynamic properties. The thermodynamic modeling results were compared with numerous experimental data in the literature. These results showed that although all EOSs could successfully predict the equilibrium pressure of carbon dioxide, there is a deviation, less than 3.3 %, in all of them. In the case of mixture hydrates, such as CO2/CH4 and CO2/C2H6, the equilibrium pressures from the models had a good agreement with experimental data, except for PT EOS with a relatively high average absolute deviation in CO2/CH4 mixture hydrates. The predicted equilibrium pressure from the different EOSs showed excellent results for structure I, including CO2/C3H8 mixture hydrates. In addition, it has been concluded that this modeling can’t predict the equilibrium pressure structure I of CO2/N2 mixture hydrate at high pressures.

Hydrate phase equilibrium of hydrogen-natural gas blends: Experimental study and thermodynamic modeling

Blending hydrogen (H2) into natural gas pipelines has become an effective way to transport H2 on a large scale. Due to the difference in properties between H2 and natural gas, the natural gas hydrate (NGH) phase equilibrium will be changed after blending H2, which influences the flow assurance in pipelines. In this work, hydrate phase equilibrium for CH4 + H2, CH4 + C2H6 + H2, and CH4 + C2H6 + C3H8 + H2 multicomponent systems had been measured at a temperature range from 274.24 K to 289.59 K by the isochoric pressure-search method. The results show the inhibition effect on CH4 hydrate formation of blending H2 to 20% is a little better than the 5.0 wt.%CH3OH solution. In addition, the Chen–Guo model was adopted to predict hydrogen-natural gas blends (HNGB) hydrates formation conditions, and model results have a good agreement with the experimental data. Average enthalpies of HNGB hydrates dissociation were also calculated utilizing the Clausius-Clapeyron equation, which was in the range of 68.02 kJ/mol and 90.17 kJ/mol. These studies provide basic data for flow assurance when transporting HNGB in pipelines.

Applicability research of thermodynamic models of gas hydrate phase equilibrium based on different equations of state.

Choosing an appropriate equation of state and thermodynamic model is very important for predicting the phase equilibrium of a gas hydrate. This study is based on statistical thermodynamics, considering the changes in water activity caused by gas dissolution, and deriving and summarizing four thermodynamic models. Based on the 150 collected experimental data points, the accuracy of the four thermodynamic models in predicting the phase equilibrium of methane hydrate, ethane hydrate, and carbon dioxide hydrate were compared. In addition, the influence of five equations of state on each thermodynamic model's phase equilibrium prediction accuracy is compared. The analysis results show that in the temperature range of 273.40–290.15 K, the Chen–Guo model is better than other thermodynamic models in predicting the phase equilibrium of methane hydrate by using the Patel–Teja equation of state. However, in the temperature range of 290.15–303.48 K, the John–Holder model predicts that the phase equilibrium of methane hydrate will perform better. In the temperature range of 273.44–283.09 K, the John–Holder model uses the Peng–Robinson state to predict the phase equilibrium of carbon dioxide hydrate with the highest accuracy. In the temperature range of 273.68 K to 287.6 K, the Chen–Guo model is selected to predict the phase equilibrium of ethane hydrate with the highest accuracy. However, as the temperature increases, the predicted values of the vdW–P model and the Parrish–Prausnitz model deviate further from the experimental values.

Prediction of Natural Gas Hydrate Phase Equilibrium in Presence of Amino Acid as Green Hydrate Inhibitor

Prediction of Natural Gas Hydrate Phase Equilibrium in Presence of Amino Acid as Green Hydrate Inhibitor

Effect of guest-dependent reference hydrate vapor pressure in thermodynamic modeling of gas hydrate phase equilibria, with various combinations of equations of state and activity coefficient models

Using the van der Waals solid solution theory and the fugacity-based approach of Klauda and Sandler, a thermodynamic model to predict the phase equilibria of gas hydrates using an Equation of State (EOS) and an Activity Coefficient Model (ACM) is presented. Here, we have studied five EOS models, that are, Peng-Robinson-Stryjek-Vera (PRSV), Patel-Teja (PT), Soave-Redlich-Kwong (SRK), Peng-Robinson (PR), and Redlich-Kwong (RK) to calculate the gas phase fugacity and four ACMs, that are, modified UNIFAC (mUNIFAC), UNIQUAC, Wilson and NRTL to model the liquid phase non-ideality. The Wilson and NRTL activity coefficient models have previously been used for systems containing additional chemicals in the liquid phase but not for pure water in coexistence with gas hydrates, to the best of our knowledge, and this has been explored in this work. By making the empty hydrate vapor pressure parameters guest dependent, the model improves upon the predictions from our previous model for sI and sII hydrates significantly and has drastically reduced the errors especially in modeling sII hydrates. The best combinations of EOS and ACM in L-H-V equilibrium and best EOS for I-H-V equilibrium, respectively, for each compound, to be used in the guest dependent model from the 100 combinations of EOS and ACM (for L-H-V equilibrium) and 25 possible EOS options (for I-H-V equilibrium) examined are: RK–mUNIFAC and SRK for CH4, RK–mUNIFAC and PRSV for CO2, RK–mUNIFAC and PR for C2H6, SRK–mUNIFAC and SRK for N2, and PR–NRTL and RK for C3H8. While different EOS work well for CH4, CO2, C2H6 and C3H8 hydrates in L-H-V and I-H-V equilibria, the SRK EOS works well for N2 hydrates in both L-H-V and I-H-V equilibria. We have also reported the solubilities of guest species in liquid water in equilibrium with hydrates and the hydrate cage occupancies from the models producing the least error as data on these facets is limited.

Gas hydrate stability zone in Muri coalfield, Qinghai Province, China

ABSTRACTThe gas hydrate stability zone (GHSZ) is the essential condition for gas hydrate accumulation, which is controlled by three main factors: gas component, geothermal gradient and permafrost thickness. Based on the gas component of hydrate samples from drilling in Muri coalfield, the gas hydrate phase equilibrium curve was calculated using Sloan's natural gas hydrate phase equilibrium procedure (CSMHYD) program. Through temperature data processing of coalfield boreholes, some important data such as thickness of permafrost and geothermal gradient were obtained. The GHSZ parameters of a single borehole were calculated by programming based on the above basic data. The average thickness of GHSZ of 85 boreholes in Muri coalfield amounted to approximately 1000 m, indicating very broad space for gas hydrate occurrence. The isogram of GHSZ bottom depth drawn from single borehole data in Muri coalfield demonstrated the regional distribution characteristics of GHSZ, and identified three favourable areas of gas hydrate occurrence where the bottom of GHSZ had a burial depth >1500 m – namely, the southern part of Juhugeng Mining Area, the middle part of Duosuogongma Mining Area and the eastern part of Xuehuoli Mining Area.

Prediction of equilibrium conditions for gas hydrates in the organic inhibitor aqueous solutions using a thermodynamic consistency-based model

Considering organic inhibitor plays a great role in preventing hydrate formation in oil and gas industry by shifting the hydrate phase equilibrium conditions, it is necessary to study the effects of organic inhibitors on the phase behavior of gas hydrates. In this study, a thermodynamic consistency-based model was developed to predict the phase equilibria of gas hydrates in methanol (MeOH), ethanol (EtOH), monoethylene glycol (MEG), diethylene glycol (DEG), triethylene glycol (TEG) and glycerol (GLY) solutions with concentrations of 5.44 wt%~ 65 wt%, respectively. The Chen-Guo model was applied to hydrate system. Compared with the original Hu-Lee-Sum correlation, a modified HLS type correlation that additionally considers the slight effect of temperature on calculating the activity of water was proposed. The prediction accuracy was characterized by the average absolute deviation in temperature (AADT). Results show that for CH4 hydrate the Chen-Guo model coupled with HLS correlation performs as good as the original HLS correlation and PVTSim. The corresponding AADT for each organic inhibitor can be further decreased by at least 20.95% when using the Chen-Guo model coupled with the modified HLS type correlation. Besides, this modified model also shows a better performance in predicting the phase equilibria of C2H6 and CO2 hydrates in the presence of these organic inhibitors than the Chen-Guo model coupled with HLS correlation do.

A robust and highly efficient phase boundary method for determining the thermodynamic equilibrium conditions of bulk gas hydrate systems

The phase equilibrium conditions of gas hydrate systems play a fundamental and essential role in establishing the feasibility of the various hydrate based technological applications. Although the classical isochoric and isothermal pressure search methods can provide accurate and reliable gas hydrate phase equilibrium data, these methods require researchers to devote tremendous time towards achieving each data point. Herein, we extend the phase boundary (PB) method for bulk hydrate systems and demonstrate it to be robust and highly efficient. The PB method involves a controlled pressure drop of the hydrate system to initiate the automatic trace of the pressure and temperature region upon dissociation of the hydrates. Three-phase equilibrium data for four different gas hydrate systems, namely pure methane (CH4), mixed CH4/1,3-dioxolane (DIOX), mixed hydrogen (H2)/DIOX, and mixed carbon dioxide (CO2)/DIOX, have been obtained using the PB method. Independent measurements made using the isochoric pressure search method have been obtained to validate the accuracy and reliability of the data obtained using the PB method.

Experimental study of carbon dioxide gas hydrate formation in the presence of zwitterionic compounds

In this work, the effect of a series of zwitterionic compounds (ZIs) aqueous solutions on the thermodynamic equilibrium and kinetic conditions of carbon dioxide hydrate formation is investigated. The hydrate dissociation conditions for (CO2 + ZI + water) systems are studied at different concentrations of ZI, viz. (0.10; 0.20, 0.30) mass fraction. The experiment was performed over a temperature range of T = (272–282) K and pressures range of p = (1.27–4.31 MPa) using a high pressure equilibrium cell. In this work the following ZIs: 4-(triethylammonio)butane-1-sulfonate [N222C4S], 4-(tributylammonio)-butane-1-sulfonate [N444C4S] and 4-(tributylphosphonio) butane-1-sulfonate [P444C4S] were investigated, therefore it was possible to determine the influence of alkyl chain length and the structure of the core atom on the kinetic behaviour of carbon dioxide gas hydrate formation. The results show that the gas hydrate phase equilibrium in the presence of zwitterionic compounds is shifted to higher pressures and lower temperatures so that the ZIs act slightly as a thermodynamic inhibitor of hydrate formation.

Gas Hydrates Phase Equilibria and Formation from High Concentration NaCl Brines up to 200 MPa

Gas hydrate phase equilibrium and kinetics at high NaCl concentrations (near and at saturation in solution) and very high pressures (up to ∼200 MPa) are investigated to study the interplay of hydrate formation and salt precipitation. Limited experimental data above 80 MPa exist for hydrate phase equilibrium in high salinity systems. This study reveals the unusual formation of gas hydrates under these extreme conditions of high salinity and very high pressure. In particular, the results demonstrate that hydrates can form from saturated salt solutions, and the formation of hydrates and salt precipitation are competing effects. It is determined that hydrates will remain in equilibrium with a saturated salt solution, with the amount of salt precipitation determined by the amount of hydrates formed. These data are essential fundamental data for gas hydrates applications in the oil and gas production flow assurance and seawater desalination.

Influence of tetramethylammonium hydroxide on methane and carbon dioxide gas hydrate phase equilibrium conditions

In this experimental work, the phase boundaries of TMAOH + H2O + CH4 and TMAOH + H2O + CO2 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 + H2O + CH4 and TMAOH + H2O + CO2 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 CO2 with an average suppression temperature (ΔŦ) of 2.24 K. An ample inhibition influence is observed for CH4 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.

Phase Equilibrium Data for Semiclathrate Hydrate of Synthesized Binary CO2/CH4 Gas Mixture in Tetra-n-butylammonium Bromide Aqueous Solution

Hydrate-based gas separation(HBGS) technology based on tetra-n-butyl ammonium bromide (TBAB) semiclathrare hydrate can be utilized in CO2 separation from biogas. The phase equilibria of semiclathrate hydrates are crucial for the successful industrial application of HBGS technology. In this work, the phase equilibrium data of TBAB semiclathrate hydrate with a synthesized binary CO2/CH4 mixed gas (0.5 CO2 and 0.5 CH4 in mole fraction) were reported at five different TBAB concentrations (0.001, 0.01, 0.05, 0.1, and 0.2 mass fraction). The experiments were conducted in the temperature range (278.1 to 292.6) K and pressure range (2.17 to 8.16) MPa using an isochoric method. The agreement between the experimental data and the data reported in the literature indicated the reliability of the apparatus and method. The results showed that there were no obvious thermodynamic promotion effects on the CO2/CH4 mixed gas hydrate phase equilibrium conditions when the mass fraction of TBAB was less than 0.01, while there ...

Promotion of hydrate-based CO2 capture from flue gas by additive mixtures (THF (tetrahydrofuran) + TBAB (tetra-n-butyl ammonium bromide))

HBGS (Hydrate-based gas separation) is a potential method for CO2 capture from fossil fuel power plants. High hydrate formation rates and low energy consumption are still the demands for industrial application of HBGS. The promotion effects of three additive mixtures (THF (tetrahydrofuran) + TBAB (tetra-n-butyl ammonium bromide)) on flue gas (CO2/N2) hydrate phase equilibrium and CO2 capture characteristics were experimentally investigated in this study. It was found that hydrate phase equilibrium pressure for the 5% THF + 5% TBAB mixture was almost the same as that for the 5% THF + 10% TBAB mixture. Both the gas consumption and CO2 recovery increased substantially with increases in the mass fractions of THF or/and TBAB. The experimental results also showed that the gas consumption increases with the enhancement of initial pressure. Considering the hydrate phase equilibrium conditions and gas separation efficiency, an additive mixture with a mass fraction of 5% THF + 10% TBAB was found to be a better choice for hydrate-based CO2 capture from flue gas relative to other additive mixtures investigated in this study.

Gas hydrate phase equilibrium in porous media: An assessment test for experimental data

In this communication, experimental dissociation data for carbon dioxide, methane, and ethane hydrates in the presence of pure water and different types of porous media (mesoporous silica gel, porous glass, and silica gel) were assessed, in order to detect any suspect or doubtful data. The Leverage statistical model algorithm was used in which determination of the statistical hat matrix, sketching of the Williams plot, and calculation of the residuals of selected correlation results are included. Gas hydrate dissociation conditions in porous media were predicted using of the least squares support vector machine (LSSVM) algorithm. This study provides an overview on the quality of existing experimental data (outlier detection), as well as the applicability domain of the model employed.

The role of thermal path on the accuracy of gas hydrate phase equilibrium data using isochoric method

The aim of this paper is to investigate the role of thermal path on the accuracy of gas hydrate phase equilibrium data using isochoric method. In this regard, methane hydrate equilibrium conditions have been typically determined using different heating paths – continuous and stepwise. The step-heating path, with proper size and duration of temperature increment, provided the most accurate data of methane hydrate phase equilibrium conditions compared with other experimental data available in literature. We have concluded that equilibrium step-heating path had the potential for determining reliable hydrate phase diagrams, irrespective of the present components in the system. Generally, the sufficient time given to each isothermal step is based on approaching a constant number of moles of gas phase through partial hydrate dissociation whereas the size of temperature increment is limited by the amount of gas encaged in hydrate cages.Moreover, the advantage of isochoric method for pure gas systems in utilizing a section of hydrate phase diagram instead of the distinctive condition, at which the last piece of hydrate disappears, is confirmed. Finally, the possibility of obtaining hydrate phase diagram of pure gas systems at arbitrary pressures was examined by purging the system before hydrate dissociation process.

Gas Hydrate Phase Equilibrium in Porous Media: Mathematical Modeling and Correlation

In this paper, we present two different approaches to represent/predict the gas hydrate phase equilibria for the carbon dioxide, methane, or ethane + pure water system in the presence of various types of porous media with different pore sizes. The studied porous media include silica gel, mesoporous silica, and porous silica glass. First, a correlation is presented, which estimates the hydrate suppression temperature due to the pore effects from the ice point depression (IPD). In the second place, several mathematical models are proposed using the least squares support vector machine (LSSVM) algorithm for the determination of the dissociation pressures of the corresponding systems. The results indicate that although the applied correlation based on the (IPD) leads to obtaining reliable results for the gas hydrate systems in the presence of porous silica glass media, the developed LSSVM models seem to be more general due to their predictive capability over all of the investigated systems.