N2 Hydrates Research Articles

Dissociation line and driving force for nucleation of the nitrogen hydrate from computer simulation. II. Effect of multiple occupancy.

In this work, we determine the dissociation line of the nitrogen (N2) hydrate by computer simulation using the TIP4P/Ice model for water and the TraPPE force field for N2. This work is the natural extension of Paper I, in which the dissociation temperature of the N2 hydrate has been obtained at 500, 1000, and 1500bar [Algaba et al., J. Chem. Phys. 159, 224707 (2023)] using the solubility method and assuming single occupancy. We extend our previous study and determine the dissociation temperature of the N2 hydrate at different pressures, from 500 to 4500bar, taking into account the single and double occupancy of the N2 molecules in the hydrate structure. We calculate the solubility of N2 in the aqueous solution as a function of temperature when it is in contact with a N2-rich liquid phase and when in contact with the hydrate phase with single and double occupancy via planar interfaces. Both curves intersect at a certain temperature that determines the dissociation temperature at a given pressure. We observe a negligible effect of occupancy on the dissociation temperature. Our findings are in very good agreement with the experimental data taken from the literature. We have also obtained the driving force for the nucleation of the hydrate as a function of temperature and occupancy at several pressures. As in the case of the dissociation line, the effect of occupancy on the driving force for nucleation is negligible. To the best of our knowledge, this is the first time that the effect of the occupancy on the driving force for nucleation of a hydrate that exhibits sII crystallographic structure is studied from computer simulation.

The dual action of N2 on morphology regulation and mass-transfer acceleration of CO2 hydrate film

The morphology characteristics of CH4, CO2 and CO2+N2 hydrate film forming on the suspending gas bubbles are studied using microscopic visual method at supercooling conditions from 1.0 to 3.0 K. The hydrate film vertical growth rate and thickness along the planar gas-water interface are measured to study the hydrate formation kinetics and mass transfer process. Adding N2 in the gas mixture plays the same role as lowering the supercooling conditions, both retarding the crystal nucleation and growth rates, which results in larger single crystal size and rough hydrate morphology. N2 in the gas mixture helps to delay the secondary nucleation on the hydrate film, which is beneficial to maintain the pore-throat structure and enhance the mass transfer. The vertical growth rate of hydrate film mainly depends on the supercooling conditions and gas compositions but has weak dependence with the experimental temperature and pressure. Under the same gas composition condition, the final film thickness shows a linear relationship with the supercooling conditions. The mass transfer coefficient of CH4 molecules in hydrates ranges from 4.54 to 7.54×10–8 mol·cm–2·s–1·MPa–1. The maximum mass transfer coefficient for CO2+N2 hydrate occurs at the composition of 60% CO2+40% N2, which is 3.98×10–8 mol·cm–2·s–1·MPa–1.

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.

NaCl-induced enhancement of thermodynamic and kinetic CO2 selectivity in CO2 + N2 hydrate formation and its significance for CO2 sequestration

The importance of carbon capture and storage (CCS) has recently been emphasized owing to the ever-increasing global warming. Direct CO2 sequestration in marine sediments is an attractive option for CO2 storage, and some amount of injected CO2 can be stored in the form of solid gas hydrates. In this study, the effects of NaCl on the hydrate phase equilibria, thermodynamic and kinetic CO2 selectivity, and time-dependent growth behaviors of CO2 + N2 hydrates were experimentally investigated to elucidate their implications for hydrate-based CO2 sequestration. The presence of NaCl shifted the equilibrium conditions of CO2 + N2 hydrates to the higher-pressure or lower-temperature region, whereas it increased thermodynamic CO2 selectivity at a specified temperature and pressure, which was also confirmed by the pressure-composition diagram. As revealed by powder X-ray diffraction analysis, the conversion into CO2 + N2 hydrate was lower in the saline water system because of the lower initial driving force and gradual salt enrichment in the residual solution during hydrate growth. In situ Raman spectroscopic measurements demonstrated that CO2 was kinetically selective at the early stage of CO2 + N2 hydrate formation and that kinetic CO2 selectivity was more noticeable in the saline water system. The overall results provide an in-depth understanding of the role of salts in CO2 + N2 hydrate formation and thus offer valuable insights into hydrate-based CO2 storage and geological CO2 sequestration.

Salt-Induced Enhancement of Thermodynamic and Kinetic Co2 Selectivity in Co2 + N2 Hydrate Formation and its Significance for Co2 Sequestration

Salt-Induced Enhancement of Thermodynamic and Kinetic Co2 Selectivity in Co2 + N2 Hydrate Formation and its Significance for Co2 Sequestration

The dual-functional roles of N2 gas for the exploitation of natural gas hydrates: An inhibitor for dissociation and an external guest for replacement

In this study, the dissociation behavior and the time-dependent guest distributions of methane (CH4) hydrate after the injection of gaseous N2 were closely investigated using gas chromatography, in situ Raman spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy for two different N2 gas injecting pressures (4.0 and 8.0 MPa) at three different temperatures (268.8, 274.2, and 278.2 K). The dissociation kinetics of the CH4 hydrate were accelerated at a higher temperature and a lower N2 injecting pressure. Time-dependent Raman spectra confirmed that the N2 molecules began to be captured in the hydrate cages immediately after the N2 gas injection at 268.8 K, and the extent of N2 incorporation in the hydrate phase increased at a higher N2 gas injecting pressure. The 13C NMR spectra revealed that an increase in the N2 composition in the hydrate phase induced a structural transition of the CH4 + N2 hydrates from sI to sII and that N2 molecules were preferentially captured in the small (512) cages of the CH4 + N2 hydrates. The overall results provide deep insight into the exact role of N2 molecules in the inhibitor injection method and CH4 – CO2 + N2 replacement methods for the exploitation of natural gas hydrates.

Phase Equilibria of the Synthesized Ternary CH4/CO2/N2 Mixed-Gas Hydrates in Tetrabutylammonium Bromide Aqueous Solutions at Different Concentrations

Hydrate-based gas separation (HBGS) technology can effectively separate CH4 and CO2 in multicomponent gases. To provide the necessary basic data for the HBGS technology, the hydrate phase equilibrium conditions of a ternary gas mixture (CH4/CO2/N2 = 0.5/0.4/0.1) were determined and compared in tetrabutylammonium bromide (TBAB) aqueous solutions with different mass fractions (0.05, 0.1, 0.2, and 0.4). The results indicated that all four concentrations of TBAB aqueous solution could shift the hydrate phase equilibrium conditions of the ternary gas mixture to higher temperatures and lower pressures; this influence was enhanced with higher TBAB concentrations. When the TBAB mass fraction was 0.4, under the same phase equilibrium pressure conditions, the phase equilibrium temperature increased by a range of approximately 9–14 K compared with that of a pure water system. Meanwhile, TBAB had a greater impact on the phase equilibrium conditions of hydrates containing larger proportions of CO2 and CH4 gas and a greater impact on the phase equilibrium conditions of CO2 and CH4 gas hydrates than on N2 hydrate.

Separation efficiency and equilibrium recovery ratio of SF6 in hydrate-based greenhouse gas separation

The feasibility of hydrate-based sulfur hexafluoride (SF6) separation was investigated by primarily focusing on the thermodynamic, kinetic, and structural characteristics of SF6 + N2 hydrates, the separation efficiency, and the equilibrium recovery ratio. Three-phase (hydrate (H)–water (LW)–vapor (V)) equilibria of SF6 + N2 hydrates were measured to examine the effect of guest occupation on their thermodynamic stability. A pressure–composition diagram, which was obtained at 275.15 K, was constructed to elucidate the separation efficiency. The final SF6 compositions in the vapor phase during hydrate formation in isochoric and isobaric conditions showed agreement with the corresponding equilibrium compositions. SF6 + N2 hydrates were identified as sII via powder X-ray diffraction (PXRD). The Rietveld refinement of the PXRD patterns offered quantitative cage occupancy of SF6 and N2 in the SF6 + N2 hydrates. The dissociation enthalpy (ΔHd) of SF6 + N2 hydrates was measured using a high-pressure micro-differential scanning calorimeter (HP μ-DSC). The overall experimental results clearly demonstrated that SF6 was selectively captured in the hydrate phase. The hydrate-based method required a lower initial SF6 concentration and pressure to attain a specified recovery ratio of SF6 compared with the liquefaction method; however, it offered lower SF6 purity. Therefore, the hydrate-liquefaction combined method is suggested to supplement the drawbacks of each method and conserve power consumption for pressurization.

Molecular Selectivity of CO–N2 Mixed Hydrates: Raman Spectroscopy and GCMC Studies

This paper reports a novel quantitative investigation concerning the CO selectivity properties for mixed CO–N2 hydrates. The study was developed by combining Raman scattering experiments and grand ...

Experimental study and thermodynamic modeling of CCl4 + O2 and CCl4 + N2 hydrate equilibria

Nitrogen and oxygen are the main components of air. Liquid nitrogen is an alternative to fossil fuels. However, little attention has been paid to this energy resource. Liquid oxygen is not a fuel in itself, however, it can be used as an oxidizer to run vehicles. In this respect, gas hydrate technology can be of interest, as applicable media for nitrogen/oxygen transport or nitrogen/oxygen separation. Both nitrogen and oxygen have the tendency to form hydrates, especially at high pressures, with double occupancy of the large cavities of structure sII. Thermodynamic hydrate promoters (THPs) can be used to facilitate the formation of nitrogen or oxygen hydrates at more moderate equilibrium conditions. For this purpose, CCl4 can be a potential candidate. In this contribution, hydrate equilibrium conditions for binary mixtures of nitrogen + CCl4 (50 data points) and oxygen + CCl4 (41 data points) were experimentally measured. This was done using three different experimental equipment, namely: a tensimeter, the Cailletet apparatus, and a high-pressure autoclave. A thermodynamic model, according to the van der Waals-Platteeuw (vdW-P) solid solution theory, was then employed to predict the hydrate equilibrium temperatures of the aforementioned mixtures. The fugacities of the components in the mixtures were calculated through the Peng-Robinson (PR) EoS and the classical van der Waals mixing rules. The Kihara potential model was applied to represent the guest-host interactions. The results reveal that the model has the ability to predict the hydrate equilibrium temperatures with the average absolute deviation (AAD) of 0.32 K. The fractional occupancy calculations indicate that the CCl4 molecules mostly occupy the large cages and do not enter the small cages of sII hydrates because of their size, while nitrogen and oxygen have small contributions in the filling of the large cages of sII.

Elucidating the guest disorder in structure II argon hydrate – A neutron diffraction isotopic substitution study

Clathrate hydrates with the cubic structure II (CS-II) form typically with large guest molecules, such as tetrahydrofuran, trimethylamine oxide, or propane. However, CS-II is also realized for argon hydrate despite the comparatively small van der Waals diameter of the guest (around 3.8 ​Å). Here, the structure of deuterated argon hydrate was studied at ambient pressure in the temperature range 20–95 ​K using neutron diffraction and comparing natural Ar with 36Ar, which scatters neutrons more than 13 times more efficiently. The procedure allowed to unambiguously establish the positional disorder within the large cages of CS-II, while simultaneously refining host and guest structures. These cages are singly occupied and off-centered argon atoms distribute on two tetrahedron-shaped split positions with a ratio 3:1. Molecular dynamics (MD) simulations revealed that the crystallographic positional disorder structure is due to mobile argon atoms even at 20 ​K. The MD potential energy distribution confirmed the diffraction model. It is noted that the unit cell volumes of argon hydrate in the investigated temperature range are virtually identical to N2 hydrate, which has a similar composition at ambient pressure, indicating a very similar (slightly attractive) host-guest interaction.

Molecular Dynamics Simulation Study on the Growth of Structure II Nitrogen Hydrate.

Crystal growth of N2 hydrate in a three-phase system consisting of N2 hydrate, liquid water, and gaseous N2 was performed by molecular dynamics simulation at 260 K. Pressure influence on hydrate growth was evaluated. The kinetic properties including the growth rates and cage occupancies of the newly formed hydrate and the diffusion coefficient and concentration of N2 molecules in liquid phase were measured. The results showed that the growth of N2 hydrate could be divided into two stages where N2 molecules in gas phase had to dissolve in liquid phase and then form hydrate cages at the liquid-hydrate interface. The diffusion coefficient and concentration of N2 in liquid phase increased linearly with increasing pressure. As the pressure rose from 50 to 100 MPa, the hydrate growth rate kept increasing from 0.11 to 0.62 cages·ns-1·Å-2 and then dropped down to around 0.40 cages·ns-1·Å-2 once the pressure surpassed 100 MPa. During the hydrate formation, the initial sII N2 hydrate phase set in the system served as a template for the subsequent growth of N2 hydrate so that no new crystal structure was found. Analysis on the cage occupancies revealed that the amount of cages occupied by two N2 molecules increased evidently when the pressure was above 100 MPa, which slowed down the growth rate of hydrate cages. Additionally, a small fraction of defective cages including two N2 molecules trapped in 51265 cages and three N2 molecules trapped 51268 cages was observed during the hydrate growth.

Greenhouse Gas (CHF3) Separation by Gas Hydrate Formation

In this study, the feasibility of gas hydrate-based greenhouse gas (CHF3) separation was investigated with a primary focus on thermodynamic, structural, and cage-filling characteristics of CHF3 + N2 hydrates. The three-phase (hydrate (H)–liquid water (LW)–vapor (V)) equilibria of CHF3 (10%, 20%, 40%, 60%, and 80%) + N2 + water systems provided the thermodynamic stability conditions of CHF3 + N2 hydrates. Powder X-ray diffraction revealed that the structure of the CHF3 + N2 hydrates was identified as sI (Pm3n) for all the CHF3 concentration ranges considered in this study. A pressure–composition diagram obtained at two different temperature conditions (279.15 and 283.15 K) demonstrated that 40% CHF3 could be enriched to 88% CHF3 by only one step of hydrate formation and that separation efficiency was higher at the lower temperature. Furthermore, Raman spectroscopy revealed that CHF3 molecules preferentially occupy large (51262) cages of the structure I (sI) hydrate during CHF3 + N2 hydrate formation. The o...

Thermodynamic modeling of equilibrium conditions of CH4/CO2/N2 clathrate hydrate in presence of aqueous solution of sodium chloride inhibitor

ABSTRACTIn this work, the effect of sodium chloride (NaCl) on thermodynamic properties of CH4+CO2+N2 hydrate formation and equilibrium condition has been studied. The three-phase (hydrate–liquid–gas) equilibrium calculation has been carried out using the Peng–Robinson equation of state (PR EoS) and Universal Quasi Chemical (UNIQUAC) activity coefficient models. The PR EoS coupled with classic mixing rule is applied for the vapor phase. The calculations of the gas hydrate formation pressures are performed in the absence and presence of sodium chloride inhibitor for the gas hydrate systems. The Chen–Guo model has been used for the hydrate phase and the UNIQUAC activity coefficient is applied for non-ideality of the liquid phase. To obtain higher accuracy, the solubility of the gases in the aqueous phase is also taken into account using pressure corrected Henry's law. Finally, the stepwise procedure has been followed to obtain the results and compared with the experimental results. The addition of 2% (by volume) sodium chloride to water results in large shifts in phase equilibrium boundary to increase pressure for the same temperature condition.

Guest enclathration and structural transition in CO2 + N2 + methylcyclopentane hydrates and their significance for CO2 capture and sequestration

The phase equilibria, structural transition, gas uptake, and composition of CO2+N2+methylcyclopentane (MCP) hydrates were investigated for their potential applications in CO2 capture and sequestration. Thermodynamic stability conditions indicated that flue gas could form sH hydrates in the presence of MCP at a reduced pressure and an elevated temperature. Powder X-ray diffraction (PXRD) and Raman analyses revealed that the structural transition of CO2+N2+MCP hydrates occurs from sH to sI with increasing CO2 concentration and that both sH and sI coexist at a boundary composition of CO2 (20%)+N2 (80%). In situ Raman spectroscopy provided direct evidence of the enclathration of CO2 and MCP in cages of sH hydrates under highly N2-rich conditions. The sH CO2+N2+MCP hydrates demonstrated slightly lower gas uptakes and CO2 selectivity in the hydrate phase than corresponding sI CO2+N2 hydrates, despite their thermodynamic advantages. The overall experimental results obtained in this study can help create a better understanding of guest enclathration and structural transition in sH hydrates and thus provide new insights into sH hydrate-based CO2 capture and sequestration using flue gas.

Thermodynamic stability conditions for semi-clathrate hydrates of CO2, CH4, or N2 with tetrabutyl ammonium nitrate (TBANO3) aqueous solution

In this communication, the effect of tetrabutyl ammonium nitrate (TBANO3) on CO2, CH4, or N2 hydrate was studied. Hydrate dissociation conditions for CO2+TBANO3+water, CH4+TBANO3+water and N2+TBANO3+water were investigated at different concentrations of TBANO3, viz. (0.05, 0.10, and 0.15) mass fraction, and over a temperature and pressure range of T=(275.1 to 285.1)K and (1.21 to 7.27)MPa, respectively. The results show that the presence of TBANO3 causes the phase equilibrium curve to be shifted to moderate conditions (lower pressure and higher temperature), especially at higher concentrations of TBANO3.

Enclathration of tert-butyl alcohol in sII hydrates and its implications in gas storage and CO2 sequestration

The inclusion of tert-butyl alcohol (tBA) as a co-guest of clathrate hydrates in the presence of CH4, CO2, and N2 was investigated for its potential role in gas storage and CO2 sequestration. The 13C NMR, Raman spectroscopy, and powder X-ray diffraction revealed that the guest gas (CH4, CO2, and N2)+tBA+water systems form sII hydrates. The enclathration of tBA molecules in the sII large (51264) cages resulted in significant thermodynamic stabilization of the CH4+tBA and N2+tBA hydrates. However, the hydrate phase equilibrium curves of the CO2+tBA hydrates were shifted to inhibited regions despite the participation of tBA molecules as a co-guest in the sII hydrate lattices. tBA was found to function as a thermodynamic promoter for both CH4 and N2 hydrates, whereas it functioned as a thermodynamic inhibitor for CO2 hydrate. The overall experimental results provide a better understanding of the thermodynamic behaviors, structural transitions, and guest distributions of the guest gas (CH4, CO2, and N2)+tBA hydrates for the potential use of tBA in gas storage and CO2 sequestration.

Phase Equilibria of Double Semiclathrate Hydrates Formed with Tetraamylammonium Bromide Plus CH4, CO2, or N2

Dissociation data for the semiclathrate hydrates formed with tetraamylammonium bromide (TAAB: C20H44NBr) + CH4, + CO2, or + N2 were measured in the pressure range of (1.33 to 20.37) MPa and temperature range of (279.8 to 291.9) K at (0.050 and 0.100) mass fraction of TAAB. The experimental data were obtained by employing an isochoric pressure-search method. The results showed that at a given pressure, the temperature required to form double TAAB + CH4, + CO2, or + N2 hydrates was higher than that required for the corresponding pure CH4, CO2, or N2 hydrates, and as the TAAB concentration increased, its effect of enlarging the hydrate stability zone was strengthened. However, the experimental dissociation temperatures for double semiclathrate hydrates were lower than those for TAAB hydrates formed without guest gases. In addition, the data for double semiclathrate hydrates formed from TAAB generated in this work were compared with those formed from some other semiclathrate hydrate formers (such as tetrabutylammonium bromide, tetrabutylammonium chloride, and tetrabutylammonium fluoride) reported in literature.

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.

Structural transition and phase behavior of N2 gas hydrates with pinacolyl alcohol and tert-amyl alcohol

The influences of large molecular alcohols (LMAs) (pinacolyl alcohol (PCA) and tert-amyl alcohol (tAA)) on N2 hydrate were examined with a primary focus on the hydrate phase equilibria and structural transition. The four-phase (H–Lw–LLMA–V) equilibria of the N2+PCA+water and N2+tAA+water systems were experimentally measured in order to determine the thermodynamic stability conditions of the N2 hydrates with LMAs. The H–Lw–LLMA–V curves of both N2+PCA and N2+tAA hydrates were significantly shifted to more thermodynamically stable regions. Powder X-ray diffraction (PXRD) and Raman analyses verified that the structure of N2 hydrates with LMAs was transformed from the original structure of sII to sH (P6/mmm) as a result of the inclusion of LMAs. In addition, the formation of sH hydrate from the N2+PCA+water system was further verified using an endothermic dissociation thermogram from a high pressure micro-differential scanning calorimeter (HP μ-DSC) by confirming the appearance of three distinct peaks that correspond to ice, sII hydrate, and sH hydrate. Therefore, the experimental results that were obtained in this study are expected to be informative for understanding the roles of LMAs in affecting the macroscopic hydrate phase behavior and microscopic hydrate structure.