Binary Gas Hydrates Research Articles

Evaluation of a Simplified Model for Three-Phase Equilibrium Calculations of Mixed Gas Hydrates

In this study, we perform an extensive evaluation of a simple model for hydrate equilibrium calculations of binary, ternary, and limited quaternary gas hydrate systems that are of practical interest for separation of gas mixtures. We adopt the model developed by Lipenkov and Istomin and analyze its performance at temperature conditions higher than the lower quadruple point. The model of interest calculates the dissociation pressure of mixed gas hydrate systems using a simple combination rule that involves the hydrate dissociation pressures of the pure gases and the gas mixture composition, which is at equilibrium with the aqueous and hydrate phases. Such an approach has been used extensively and successfully in polar science, as well as research related to space science where the temperatures are very low. However, the particular method has not been examined for cases of higher temperatures (i.e., above the melting point of the pure water). Such temperatures are of interest to practical industrial applications. Gases of interest for this study include eleven chemical components that are related to industrial gas-mixture separations. Calculations using the examined methodology, along with the commercial simulator CSMGem, are compared against experimental measurements, and the range of applicability of the method is delineated. Reasonable agreement (particularly at lower hydrate equilibrium pressures) between experiments and calculations is obtained considering the simplicity of the methodology. Depending on the hydrate-forming mixture considered, the percentage of absolute average deviation in predicting the hydrate equilibrium pressure is found to be in the range 3–91%, with the majority of systems having deviations that are less than 30%.

Experimental study of the formation and decomposition of mixed gas hydrates in water-hydrocarbon system

The phase transition of gas hydrate with two or more guests is associated with gas recovery, storage and separation. In this work, the formation and decomposition of binary and ternary gas hydrates containing CH4, C2H6 and C3H8 were investigated experimentally in high-pressure reactors equipped with visual windows. The gas compositions during mixed hydrates formation and decomposition were analyzed to track the occupation of guests in hydrate structures and the release of guests from hydrate cages. It was found that of CH4 was adsorbed in the hydrate cages constructed by C2H6 and C3H8 in the initial stage. In the depressurized decomposition of CH4-C3H8 hydrates and CH4-C2H6-C3H8 hydrates slurry, CH4 and C2H6 molecules were preferentially released from hydrate structures while more C3H8 molecules were able to stay in hydrate phase, which was in accordance with the order of the diameter ratio of guest molecule to hydrate cage. The preservation of CH4-C3H8 hydrates and CH4-C2H6-C3H8 hydrates above 0 ℃ was observed under a low pressure. It was proposed that the reconstitution or reformation of hydrate layer rich of C3H8 may encrust the remaining hydrate particles, preventing hydrate from further quick decomposition. In-situ Raman spectra confirmed the enrichment of C3H8 molecules at the surface of CH4-C3H8 hydrates, which revealed that the metastable hydrate layer rich of C3H8 molecules may act as mass transfer resistance of gas release from hydrate decomposition. Differently, the decompositions of CH4-C2H6 hydrates and C2H6-C3H8 hydrates were uniform as different guests were produced concurrently without preservation. The new insights into mixed hydrates decomposition in this work may provide a new basis for mixed gas production from gas hydrate, and gas storage and separation with hydrate.

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.

Ammonium hydroxide ILs as dual-functional gas hydrate inhibitors for binary mixed gas (carbon dioxide and methane) hydrates

In this study the dual-functionality of four ammonium based Ionic Liquids (AILs) having hydroxide anion namely: tetramethyl ammonium hydroxide (TMAOH), tetraethyl ammonium hydroxide (TEAOH), tetrapropyl ammonium hydroxide (TPrAOH) and, tetrabutyl ammonium hydroxide (TBAOH) is investigated on binary mixed gas hydrates (50 mol%-50 mol% CO2-CH4). For thermodynamic behaviour, the hydrate liquid vapour equilibrium (HLVE) data are generated in the presence of 10 wt% AILs solutions by the T-Cycle method at temperature and pressure conditions of 275.0–284.0 K and 2.0–6.50 MPa, respectively. THI results revealed that apart from TBAOH, all the considered AILs can shift HLVE curve towards higher pressure and lower temperature regions. Obtained results are further compared with commercial THI inhibitors. Average suppression temperature (Ŧ), Enthalpy of hydrate dissociation (∆Hdiss) and Freezing point temperature (Tf) are also calculated for considered systems. The electrolyte based model is applied for validation of experimental HLVE data. Also, formation kinetics of studied AILs is reported for 1 wt% aqueous AILs solutions at 6.50 MPa with different experimental temperatures (274.0 K and 277.0 K). Induction time, initial formation rate, Relative Inhibition Power (RIP) and total gas uptake are the reported parameters for kinetic study. Furthermore, kinetic results are compared with commercial kinetic inhibitor namely: polyvinyl pyrrolidinium (PVP) at 274.0 K condition. Kinetic data suggested that all the studied AILs can perform as KHI inhibitors. Therefore, the considered AILs (except TBAOH) are able to perform as dual-functional inhibitors which highlight the use of AILs as a potential dual-functional gas hydrate inhibitors for sustainable flow in gas transmission pipelines.

Study of Selected Factors Influencing Carbon Dioxide Separation from Simulated Biogas by Hydrate Formation

CH4 concentration and CO2 separation from biogas (CH4/CO2) by an one-stage hydrate formation was supposed to be a prospective and economical practicable technology compared with traditional gas separation methods. We investigated the influence of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM] PF6) on simulated biogas (64.0 mol %CH4/CO2) hydrate separation in this work. The thermodynamic promoter TBAB was combined with [BMIM] PF6 to investigate binary gas hydrate formation and separation efficiency. Different parameters which affected hydrate formation were investigated. The concentration of [BMIM] PF6 was 1000/2000/3000 ppm, and the optimized concentration of TBAB was 4 wt %. With respect to the hydrate gas separation results, the addition of TBAB can decrease CH4/CO2 hydrate formation pressure greatly and the hydrate formation induction time. CH4 in the remaining gas can be increased from 64.0 to 79.90 mol %, and CH4 recovery can arrive at 95.83%. Meanwhile, the addition of [BMIM] PF6 could increase the CO2 mole friction in hydrate from 36.0 to 58.63 mol %. So it was demonstrated that the combination of TBAB + [BMIM] PF6 would have a great application prospect on biogas purification technology.

Selective Encaging of N2O in N2O-N2 Binary Gas Hydrates via Hydrate-Based Gas Separation.

The crystal structure and guest inclusion behaviors of nitrous oxide-nitrogen (N2O-N2) binary gas hydrates formed from N2O/N2 gas mixtures are determined through spectroscopic analysis. Powder X-ray diffraction results indicate that the crystal structure of all the N2O-N2 binary gas hydrates is identified as the structure I (sI) hydrate. Raman spectra for the N2O-N2 binary gas hydrate formed from N2O/N2 (80/20, 60/40, 40/60 mol %) gas mixtures reveal that N2O molecules occupy both large and small cages of the sI hydrate. In contrast, there is a single Raman band of N2O molecules for the N2O-N2 binary gas hydrate formed from the N2O/N2 (20/80 mol %) gas mixture, indicating that N2O molecules are trapped in only large cages of the sI hydrate. From temperature-dependent Raman spectra and the Predictive Soave-Redlich-Kwong (PSRK) model calculation, we confirm the self-preservation of N2O-N2 binary gas hydrates in the temperature range of 210-270 K. Both the experimental measurements and the PSRK model calculations demonstrate the preferential occupation of N2O molecules rather than N2 molecules in the hydrate cages, leading to a possible process for separating N2O from gas mixtures via hydrate formation. The phase equilibrium conditions, pseudo-pressure-composition (P-x) diagram, and gas storage capacity of N2O-N2 binary gas hydrates are discussed in detail.

Theoretical modeling of the thermodynamic properties and the phase diagram of binary gas hydrates of argon and hydrogen

Theoretical calculation of the thermodynamic properties and determining of equilibrium hydrate composition of binary argon + hydrogen hydrates has been performed with taking into account the concurrent cage occupation by different guest types. For this purpose the original previously developed approach has been used that allows to take into account multiple cage occupancy and the possibility of mixed clusters inside the cages as well as the influence of guest molecules on the host lattice. Separately, argon and hydrogen form cubic structure II (sII) hydrates. Therefore, in this work we considered only sII gas hydrates formation. It is shown that thermodynamic stability of binary argon + hydrogen hydrates strongly depends on the presence of argon in the gas phase as the heavier component. Thus, with increasing argon content in the system the hydrate stability field extends to low pressures with increase of argon fraction in the small cavities.

Modeling nitrogen and methane with ethane and propane gas hydrates at low temperatures (173–290 K) with applications to Titan

The FREZCHEM model was primarily designed for cold temperatures (173–298K) and high pressures (1–1000bars). Nitrogen gas (95.0%) and methane gas (5.0%) are major gases on the surface of Titan. Recently, we added nitrogen and methane gas hydrates to FREZCHEM model on Titan; and nitrogen–methane gas hydrates formed on Titan at 178K. The other common but less abundant gases on Titan are ethane (C2H6) and propane (C3H8) that can also form gas hydrates with nitrogen and methane. The specific objectives of this study were to (1) add ethane and propane to gas hydrates, including mixtures with nitrogen, methane, and carbon dioxide, and (2) explore the potential roles of gas hydrates on Titan. At 273K, the Ln(gas hydrates) were 5.095 for N2, 3.217 for CH4, 2.327 for CO2, 1.288 for C2H6, and 0.281 for C3H8. At 173K, the Ln(gas hydrates) were −4.968 for N2, −6.102 for CH4, −7.803 for CO2, −5.125 for C2H6, and −5.512 for C3H8. Apparently C2H6 and C3H8 gas hydrates change less at lower temperatures than N2, CH4, and CO2 gas hydrates. In previous papers, we added three mixed CH4–CO2, N2–CH4, and N2–CO2 binary gas hydrates. In this paper, we added ethane and propane to include new binary gas hydrate mixtures of N2–C2H6, N2–C3H8, C2H6–C3H8, CH4–C2H6, CH4–C3H8, CO2–C2H6, and CO2–C3H8. Today, there are ten binary gas hydrates in the FREZCHEM model. In the text, how to cope with more than two species is described. Simulations from 273K to 173K used a surface Titan pressure of 1.467 bars with a major gas of nitrogen (94.24%), a minor gas of methane (5.65%), and extremely minor gases of ethane (0.0038%), and propane (0.000343%). Eventually at 178K, N2·6H2O formed with 0.17694mol, CH4·6H2O formed with 0.04101mol, C2H6·6H2O formed with 6.48e−6mol, and C3H8·6H2O formed with 9.36e−7mol. Based on the atmospheric conditions of Titan, the trace gases of ethane and propane led to low gas hydrate precipitations of ethane and propane with nitrogen and methane. However, the gas concentrations could be very different in an ocean of Titan that could be 50 to 175km beneath the surface where the temperatures may increase as high as 260K. Methane gas hydrates are likely dominant beneath the surface of Titan with some minor contributions from nitrogen, ethane, and propane. Below the surface of Titan in the ocean and icy shell, gas hydrates clearly depend on the gas concentrations of methane, nitrogen, ethane, and propane as well as temperature and pressure.

Thermodynamic model for predicting equilibrium conditions of clathrate hydrates of noble gases + light hydrocarbons: Combination of Van der Waals–Platteeuw model and sPC-SAFT EoS

In this communication, equilibrium conditions of clathrate hydrates containing mixtures of noble gases (Argon, Krypton and Xenon) and light hydrocarbons (C1–C3), which form structure I and II, are modeled. The thermodynamic model is based on the solid solution theory of Van der Waals–Platteeuw combined with the simplified Perturbed-Chain Statistical Association Fluid Theory equation of state (sPC-SAFT EoS). In dispersion term of sPC-SAFT EoS, the temperature dependent binary interaction parameters (kij) are adjusted; taking advantage of the well described (vapor+liquid) phase equilibria. Furthermore, the Kihara potential parameters are optimized based on the P–T data of pure hydrate former. Subsequently, these obtained parameters are used to predict the binary gas hydrate dissociation conditions. The equilibrium conditions of the binary gas hydrates predicted by this model agree well with experimental data (overall AAD P∼2.17).

Investigating the Thermodynamic Stabilities of Hydrogen and Methane Binary Gas Hydrates

When hydrogen (H2) is mixed with small amounts of methane (CH4), the conditions required for clathrate hydrate formation can be significantly reduced when compared to that of simple H2 hydrate. With growing demand for CH4 as a commercially viable source of energy, H2 + CH4 binary hydrates may be more appealing than extensively studied H2 + tetrahydrofuran (THF) hydrates from an energy density standpoint. Using Raman spectroscopic and powder X-ray diffraction measurements, we show that hydrate structure and storage capacities of H2 + CH4 mixed hydrates are largely dependent on the composition of the initial gas mixture, total system pressure, and formation period. In some cases, H2 + CH4 hydrate kinetically forms structure I first, even though the thermodynamically stable phase is structure II.

Prediction of Equilibrium Conditions for Hydrate Formation in Binary Gaseous Systems Using Artificial Neural Networks

AbstractThe present work attempts to indicate the potential of artificial neural networks (ANN) for the fast and reliable estimation of the equilibrium conditions of single, binary, and multiple hydrocarbon gas hydrates. The ANN used in this study was a network with the tangent‐sigmoid (tansig) propagation transfer function in the hidden layer and a final layer with the linear (purelin) transfer function. The number of hidden neurons has been determined by minimizing the error of the calculation. The obtained results and the ANN model reliability were compared with other predictive methods. Results showed that the ANN method is able to reliably predict the hydrate equilibrium conditions of hydrocarbons, particularly for binary gas systems, using simple input parameters such as the weight fractions and normal boiling points of the components.

Guest molecule dynamics and guest-specific degassing phenomenon of binary gas hydrate investigated by terahertz time-domain spectroscopy

The behaviour of clathrate hydrates in the THz region could provide useful information about inclusion compounds. In this study, terahertz time-domain spectroscopy (THz-TDS) revealed the dynamics of gaseous guest molecules inside the water framework of a clathrate hydrate. Distinct footprints of each gaseous molecule (CH4, H2 and O2) inside the gas hydrate lattice were analysed. Thermally stimulated gas hydrates give out gas molecules (secondary guests) to remain secure, and each gas has a specific degassing temperature. In this way, they form the stable sII THF hydrate with empty small cages (512) which can survive at temperatures up to 278 K.

CO2 Removal from a CO2–CH4 Gas Mixture by Clathrate Hydrate Formation Using THF and SDS as Water-Soluble Hydrate Promoters

This study investigates the use of hydrate formation to separate the CO2 from a CO2–CH4 gas mixture in the presence of water-soluble additives—tetrahydrofuran (THF) and/or sodium dodecyl sulfate (SDS)—at low concentrations. The influence of additive concentration and process operating conditions on the gas enclathration kinetics, the quantity of gas removed, and the selectivity of the separation are studied under quiescent hydrate-forming conditions in a batch reactor. Gas consumption and enclathration occur at high rates only when the two additives are used in combination. Similarly to what has been observed with pure CO2, the proposed mechanism is that a mixed hydrate of structure sII containing THF forms first, which triggers the formation of CO2–CH4 binary gas hydrate of structure sI. However, the gas separation was found not to be selective enough to the CO2 for envisaging any practical application.

Spectroscopic Identification on Cage Occupancies of Binary Gas Hydrates in the Presence of Ethanol

Ethanol has been widely used for inhibiting gas hydrate formation due to its cost and efficiency. However, recent research showed that ethanol can act as a hydrate former when coguested with CH(4) molecules at various ethanol concentrations. Herein, we report tuning phenomenon of the gas hydrate in the presence of ethanol by means of spectroscopic measurements. On the basis of the experimental results, it is verified that ethanol molecules cannot inhibit hydrate formation effectively, but enhance the gas storage in the hydrate phase when a much less amount of the inhibitor than the stoichiometric concentration is used. The cage occupancies of binary hydrate systems in the presence of a thermodynamic inhibitor, showing similar guest behaviors in the presence of a promoter such as tetrahydrofuran (THF), can provide useful information on the molecular behaviors of guest species.

Experimental Determination of the Equilibrium Conditions of Binary Gas Hydrates of Cyclopentane + Oxygen, Cyclopentane + Nitrogen, and Cyclopentane + Hydrogen

In this work, four-phase hydrate equilibrium data for each of the systems cyclopentane (CP) + water + hydrogen, CP + water + nitrogen, and CP + water + oxygen were measured and are reported in the temperature range of 281.3−303.1 K and the pressure range of 2.27−30.40 MPa. Measurements were made using an isochoric method. Experimental data on the equilibrium conditions of the two systems nitrogen + water and hydrogen + water with CP are reported over a previously uninvestigated extended temperature span. The hydrate dissociation data for the CP + water + hydrogen system are compared with some selected experimental data from the literature, and the acceptable agreement demonstrates the reliability of the experimental method used in this work. Finally, the first quadruple phase equilibrium data in the ternary system CP + water + oxygen have been achieved.

Study on thermal properties of TBAB–THF hydrate mixture for cold storage by DSC

In order to study the thermal properties of new type environment-friendly binary hydrate for cold storage in air-conditioning system, tests have been carried out by DSC comprehensively on the phase-change temperature and fusion heat of TBAB hydrate, THF hydrate, and TBAB–THF hydrate mixture. The results show a good trend that TBAB–THF hydrate has the superiority for more proper phase-change temperature and increased fusion heat. A broader and more developed view is that adding appropriate amount of hydrate with lower phase-change temperature to hydrate with higher one can make the hydrate mixture more suitable for cold storage (especially for 278–281 K); some hydrates with lower phase-change temperature can even make the fusion heat of mixture hydrate increased greatly. Several new environmental working pairs for binary gas hydrates have been listed to help to promote the application.

Modeling of phase transition sI−sII in binary gas hydrates of methane and ethane in dependence on composition of gas phase

Calculations of phase transformations ‘structure sI–structure sII’ for binary methane–ethane hydrates were performed in the framework of molecular level model. The conditions of hydrate formation in equilibrium with gas phase and ice have been determined at different gas-phase compositions and pressures. It was shown that even at very low ethane concentration in the gas phase (about 0.5%) hydrate structure sII becomes more stable than sI. Regions of stability in T, P plane of sI and sII structures for mixed hydrates have been predicted. For mixed hydrates with ethane content about 5% in gas phase pressure-induced phase transformation at low temperatures is predicted.

Dissociation studies of CH 4–C 2H 6 and CH 4–CO 2 binary gas hydrates

The objective of this work was to enhance understanding of gas hydrate dissociation, and provide information to help evaluate gas hydrate processes, such as ocean CO 2 sequestration and CH 4 recovery from natural gas hydrates in deep sea sediments. The composition and dissociation behavior of CH 4 + C 2H 6 and CH 4 + CO 2 structure I binary gas hydrates were determined using NMR spectral analysis. Single-pulse excitation was used in combination with magic-angle spinning (MAS). Time-resolved decomposition experiments were carried out on sealed, pressurized samples of both binary gas hydrate systems. Decomposition profiles have been obtained for each case. At the experimental conditions of this work, the gas hydrate composition did not change after the dissociation experiments. These results suggest that the unit cell decomposes as a single entity.

Excess Gibbs Potential Model for Multicomponent Hydrogen Clathrates

A new thermodynamic calculation procedure is introduced to predict the equilibrium conditions of multicomponent gas hydrates containing hydrogen. This new approach utilizes an excess Gibbs potential term to account for second- or higher-order water-cavity distortions due to the presence of multiple guest species. The excess Gibbs potential describes changes in reference chemical potentials according to different compositions of guest mixtures in the hydrate phase. To determine the equilibrium conditions of multicomponent gas hydrates, the excess Gibbs potential term is incorporated to the Lee-Holder model along with the Zele-Lee-Holder cell distortion model. For binary gas hydrates between hydrogen and the other gas molecule, the predicted equilibrium pressure deviates within 10-20% from the experimental value. For the ternary and quaternary mixture hydrates, the model prediction is reasonably good but its error increases with increasing pressure and temperature under the presence of THF.