Phase field modeling of CH4 hydrate conversion into CO2 hydrate in the presence of liquid CO2

We present phase field simulations to estimate the conversion rate of CH(4) hydrate to CO(2) hydrate in the presence of liquid CO(2) under conditions typical for underwater gas hydrate reservoirs. In the computations, all model parameters are evaluated from physical properties taken from experiment or molecular dynamics simulations. It has been found that hydrate conversion is a diffusion controlled process, as after a short transient, the displacement of the conversion front scales with t(1/2). Assuming a diffusion coefficient of D(s) = 1.1 x 10(-11) m(2) s(-1) in the hydrate phase, the predicted time dependent conversion rate is in reasonable agreement with results from magnetic resonance imaging experiments. This value of the diffusion coefficient is higher than expected for the bulk hydrate phase, probably due to liquid inclusions remaining in the porous sample used in the experiment.     

Nucleation of the CO2 hydrate from three-phase contact lines

Using molecular dynamics simulations on the microsecond time scale, we investigate the nucleation and growth mechanisms of CO(2) hydrates in a water/CO(2)/silica three-phase system. Our simulation results indicate that the CO(2) hydrate nucleates near the three-phase contact line rather than at the two-phase interfaces and then grows along the contact line to form an amorphous crystal. In the nucleation stage, the hydroxylated silica surface can be understand as a stabilizer to prolong the lifetime of adsorbed hydrate cages that interact with the silica surface by hydrogen bonding, and the adsorbed cages behave as the nucleation sites for the formation of an amorphous CO(2) hydrate. After nucleation, the nucleus grows along the three-phase contact line and prefers to develop toward the CO(2) phase as a result of the hydrophilic nature of the modified solid surface and the easy availability of CO(2) molecules. During the growth process, the population of sI cages in the formed amorphous crystal is found to increase much faster than that of sII cages, being in agreement with the fact that only the sI hydrate can be formed in nature for CO(2) molecules.     

Thermodynamic properties and phase transtions in the H2O/CO2/CH4 system

The availability of free energy densities as functions of temperature, pressure and the composition of all components is required for the development of a three-component phase field theory for hydrate phase transitions. We have broadened the extended adsorption theory due to Kvamme and Tanaka (J. Phys. Chem., 1995, 99, 7114) through derivation of the free energy density surface in case of CO(2) and CH(4) hydrates. A combined free energy surface for the liquid phases has been obtained from a SRK equation of state and solubility measurements outside hydrate stability. The full thermodynamic model is shown to predict water-hydrate equilibrium properties in agreement with experiments. Molecular dynamics simulations of hydrates in contact with water at 200 bar and various temperatures allowed us to estimate hard-to-establish properties needed as input parameters for the practical applications of proposed theories. The 5-95 confidence interval for the interface thickness for the methane hydrate/liquid water is estimated to 8.54 A. With the additional information on the interface free energy, the phase field theory will contain no adjustable parameters. We provide a demonstration of how this theory can be applied to model the kinetics of hydrate phase transitions. The growth of hydrate from aqueous solution was found to be rate limited by mass transport, with the concentration of solute close to the hydrate approaching the value characterizing the equilibrium between the hydrate and the aqueous solution. The depth of the interface was estimated by means of the phase field analysis; its value is close to the interface thickness yielded by molecular simulations. The variation range of the concentration field was estimated to approximately 1/3 of the range of the phase field.     

Microsecond molecular dynamics simulations of the kinetic pathways of gas hydrate formation from solid surfaces

In this paper, we report microsecond molecular dynamics simulations of the kinetic pathway of CO(2) hydrate formation triggered by hydroxylated silica surfaces. Our simulation results show that the nucleation of the CO(2) hydrate is a three-stage process. First, an icelike layer is formed closest to the substrates on the nanosecond scale. Then, on the submicrosecond timescale, a thin layer with intermediate structure is induced to compensate for the structure mismatch between the icelike layer and the final stable CO(2) hydrate. Finally, on the microsecond timescale, the nucleation of the first CO(2) hydrate motif layer is generated from the intermediate structure that acts as nucleation seeds. We also address the effects of the distance between two surfaces.     

Molecular dynamics simulations for the growth of CH_4-CO_2 mixed hydrate

Molecular dynamics simulations are performed to study the growth mechanism of CH4-CO2 mixed hydrate in xCO2= 75%, xCO2= 50%, and xCO2= 25% systems at T = 250 K, 255 K and 260 K, respectively. Our simulation results show that the growth rate of CH4-CO2 mixed hydrate increases as the CO2 concentration in the initial solution phase increases and the temperature decreases. Via hydrate formation, the composition of CO2 in hydrate phase is higher than that in initial solution phase and the encaging capacity of CO2 in hydrates increases with the decrease in temperature. By analysis of the cage occupancy ratio of CH4 molecules and CO2 molecules in large cages to small cages, we find that CO2 molecules are preferably encaged into the large cages of the hydrate crystal as compared with CH4 molecules. Interestingly, CH4 molecules and CO2 molecules frequently replace with each other in some particular cage sites adjacent to hydrate/solution interface during the crystal growth process. These two species of guest molecules eventually act to stabilize the newly formed hydrates, with CO2 molecules occupying large cages and CH4 molecules occupying small cages in hydrate.     

Molecular dynamics simulations for the growth of CH4-CO2 mixed hydrate

Molecular dynamics simulations are performed to study the growth mechanism of CH4-CO2 mixed hydrate in xco2 = 75%, xco2 = 50%, and zco2 = 25% systems at T = 250 K, 255 K and 260 K, respectively. Our simulation results show that the growth rate of CH4-CO2 mixed hydrate increases as the CO2 concentration in the initial solution phase increases and the temperature decreases. Via hydrate formation, the composition of CO2 in hydrate phase is higher than that in initial solution phase and the encaging capacity of CO2 in hydrates increases with the decrease in temperature. By analysis of the cage occupancy ratio of CH4 molecules and CO2 molecules in large cages to small cages, we find that CO2 molecules are preferably encaged into the large cages of the hydrate crystal as compared with CH4 molecules. Interestingly, CH4 molecules and CO2 molecules frequently replace with each other in some particular cage sites adjacent to hydrate/solution interface during the crystal growth process. These two species of guest molecules eventually act to stabilize the newly formed hydrates, with CO2 molecules occupying large cages and CH4 molecules occupying small cages in hydrate.     

Accelerated nucleation of tetrahydrofuran(THF)hydrate in presence of ZIF-61

Clathrate hydrate can be used in energy gas storage and transportation,CO 2 capture and cool storage etc.However,these technologies are difficult to be used due to the low formation rate and long induction time of hydrate formation.In this paper,ZIF-61(zeolite imidazolate framework,ZIF) was first used in hydrate formation to stimulate hydrate nucleation.As an additive of clathrate hydrate,ZIF-61 promoted obviously the acceleration of tetrahydrofuran(THF) hydrate nucleation.It shortened the induction time of THF hydrate formation from 2-5 h to 0.3-1 h mainly due to the template function of ZIF-61 by which the nucleation of THF hydrate has been promoted.     

CO2 capture from binary mixture via forming hydrate with the help of tetra-n-butyl ammonium bromide

Hydrate formation rate and separation effect on the capture of CO2 from binary mixture v/a forming hydrate with 5 wt% tetra-n-butyl ammonium bromide (TBAB) solution were studied.The results showed that the induction time was 5 min,and the hydrate formation process pressure of 7.30 MPa.The CO2 recovery was about 45% in the feed pressure range from 4.30 to 7.30 MPa.Under the feed pressure of 4.30 MPa,the maximum separation factor and CO2 concentration in hydrate phase were 7.3 and 38.2 tool%,respectively.The results demonstrated that TBAB accelerated hydrate formation and enriched CO2 in hydrate phase under the gentle condition.     

Using the face-saturated incomplete cage analysis to quantify the cage compositions and cage linking structures of amorphous phase hydrates

Recent advances in the molecular dynamics simulations of spontaneous nucleation and growth of methane hydrate show that an amorphous phase of the hydrate is first reached. However, the amorphous hydrate has not been well described, due to the insufficient identification of cage structures. Here, we develop a method, called "face-saturated incomplete cage analysis", which can identify all face-saturated cages in a given system. As a result, it is found that thousands of cage types and abundant occupancy states are present in the amorphous hydrate. Moreover, the crystallinity of amorphous hydrate is evaluated according to the quantitative calculation of cage linking structures, and the critical nucleus of hydrate is also estimated on the basis of clustering analysis for all face-saturated cages.     

Two approaches for modelling hydrate growth

We consider two different approaches to model growth of CO₂ hydrate, phase field theory and a model based on cellular automata. The two approaches are applied to simulations of hydrate growth from supersaturated aqueous solution of CO₂. The thermodynamic models for the solution properties are derived from experimental solubility data while the hydrate thermodynamics is based on adsorption theory with reference properties derived from molecular simulations. We show that the cellular automata approach has the benefit of being much more computationally efficient, and are still giving results which are consistent with results from the phase field theory.     

Determining the nucleation rate from the dimer growth probability

A new method is proposed for the determination of the stationary one-component nucleation rate J with the help of data for the growth probability P2 of a dimer which is the smallest cluster of the nucleating phase. The method is based on an exact formula relating J and P2, and is readily applicable to computer simulations of nucleation. Using the method, the dependence of J on the supersaturation s is determined by kinetic Monte Carlo simulations of two-dimensional (2D) nucleation of monolayers on the (100) face of Kossel crystal. The change of J over nearly 11 orders of magnitude is followed and it is found that the classical nucleation theory overestimates the simulation J values by an s-dependent factor. The 2D nucleus size evaluated via the nucleation theorem is described satisfactorily by the classical Gibbs-Thomson equation and its corrected version accounting for the spinodal limit of 2D nucleation.     

The cages, dynamics, and structuring of incipient methane clathrate hydrates

Interest in describing clathrate hydrate formation mechanisms spans multiple fields of science and technical applications. Here, we report findings from multiple molecular dynamics simulations of spontaneous methane clathrate hydrate nucleation and growth from fully demixed and disordered two-phase fluid systems of methane and water. Across a range of thermodynamic conditions and simulation geometries and sizes, a set of seven cage types comprises approximately 95% of all cages formed in the nucleated solids. This set includes the ubiquitous 5(12) cage, the 5(12)6(n) subset (where n ranges from 2-4), and the 4(1)5(10)6(n) subset (where n also ranges from 2-4). Transformations among these cages occur via water pair insertions/removals and rotations, and may elucidate the mechanisms of solid-solid structural rearrangements observed experimentally. Some consistency is observed in the relative abundance of cages among all nucleation trajectories. 5(12) cages are always among the two most abundant cage types in the nucleated solids and are usually the most abundant cage type. In all simulations, the 5(12)6(n) cages outnumber their 4(1)5(10)6(n) counterparts with the same number of water molecules. Within these consistent features, some stochasticity is observed in certain cage ratios and in the long-range ordering of the nucleated solids. Even when comparing simulations performed at the same conditions, some trajectories yield swaths of multiple adjacent sI unit cells and long-range order over 5 nm, while others yield only isolated sI unit cells and little long-range order. The nucleated solids containing long-range order have higher 5(12)6(2)/5(12) and 5(12)6(3)/4(1)5(10)6(2) cage ratios when compared to systems that nucleate with little long-range order. The formation of multiple adjacent unit cells of sI hydrate at high driving forces suggests an alternative or addition to the prevailing hydrate nucleation hypotheses which involve formation through amorphous intermediates.     

An experimental study of crystallization and crystal growth of methane hydrates from melting ice

An experiment with well defined gas-water interfacial surface area was developed to study the crystallization and crystal growth of methane hydrates. Measurable formation rates were observed only when melting ice was involved. No hydrates nucleated from liquid water or from non-melting ice. It is concluded that melting ice, which like hydrate water is hydrogen-bonded, provides a template for hydrate nucleation as well as providing a heat sink for absorbing the heat of formation during hydrate growth. The experiment was conducted in the absence of mixing so that hydrate crystals grew under quiescent conditions.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

The conversion process of hydrocarbon hydrates into CO2 hydrates and vice versa: thermodynamic considerations

Microscopy, confocal Raman spectroscopy and powder X-ray diffraction (PXRD) were used for in situ investigations of the CO(2)-hydrocarbon exchange process in gas hydrates and its driving forces. The study comprises the exposure of simple structure I CH(4) hydrate and mixed structure II CH(4)-C(2)H(6) and CH(4)-C(3)H(8) hydrates to gaseous CO(2) as well as the reverse reaction, i.e., the conversion of CO(2)-rich structure I hydrate into structure II mixed hydrate. In the case of CH(4)-C(3)H(8) hydrates, a conversion in the presence of gaseous CO(2) from a supposedly more stable structure II hydrate to a less stable structure I CO(2)-rich hydrate was observed. PXRD data show that the reverse process requires longer initiation times, and structural changes seem to be less complete. Generally, the exchange process can be described as a decomposition and reformation process, in terms of a rearrangement of molecules, and is primarily induced by the chemical potential gradient between hydrate phase and the provided gas phase. The results show furthermore the dependency of the conversion rate on the surface area of the hydrate phase, the thermodynamic stability of the original and resulting hydrate phase, as well as the mobility of guest molecules and formation kinetics of the resulting hydrate phase.     

Hydrate-based carbon dioxide capture from simulated integrated gasification combined cycle gas

The equilibrium hydrate formation conditions for CO2/H2 gas mixtures with different CO2 concentrations in 0.29 mol% TBAB aqueous solution are firstly measured.The results illustrate that the equilibrium hydrate formation pressure increases remarkably with the decrease of CO2 concentration in the gas mixture.Based on the phase equilibrium data,a three stages hydrate CO2 separation from integrated gasification combined cycle (IGCC) synthesis gas is investigated.Because the separation efficiency is quite low for the third hydrate separation,a hybrid CO2 separation process of two hydrate stages in conjunction with one chemical absorption process (absorption with MEA) is proposed and studied.The experimental results show H2 concentration in the final residual gas released from the three stages hydrate CO2 separation process was approximately 95.0 mol% while that released from the hybrid CO2 separation process was approximately 99.4 mol%.Thus,the hybrid process is possible to be a promising technology for the industrial application in the future.     

Isothermal crystallization of poly(vinylidene fluoride) in the presence of high static electric fields. I. Primary nucleation phenomenon

Primary nucleation of poly(vinylidene fluoride) (PVF₂) γ-phase crystals from the melt is affected by the presence of an electric field. Crystallization under the electric field is studied by polarized optical microscopy, thermal analysis, and wide-angle x-ray diffraction. Preliminary results indicate that the γ-phase crystal nucleation rate and content are enhanced by the electric field. A modification of the classical theory of homogeneous nucleation of a crystalline phase is proposed to account for the experimental observations. Electrostatic interaction between the nucleus total polarization and the electric field contributes to the free energy of nucleation to a very large extent at low undercooling. Theoretical predictions indicate that a static electric field will increase the nucleation rate of a polar phase and will decrease the nucleation rate of a nonpolar phase. Confirmation of the former fact is observed experimentally.     

The effect of the water/methane interface on methane hydrate cages: the potential of mean force and cage lifetimes

Molecular dynamics simulations were used to determine the influence of a methane-water interface on the position and stability of methane hydrate cages. A potential of mean force was calculated as a function of the separation of a methane hydrate cage and a methane-water interface. The hydrate cages are found to be strongly repelled from the methane gas into the water phase. At low enough temperatures, however, the most favorable location for the hydrate cage is at the interface on the water side. Cage lifetime simulations were performed in bulk water and near a methane-water interface. The methane-water interface increases the cage lifetime by almost a factor of 2 compared to cage lifetimes of cages in bulk water. The potential of mean force and the cage lifetime results give additional explanations for the proposed nucleation of gas hydrates at gas-water interfaces.     

Phase changes of CO2 hydrate under high pressure and low temperature

High pressure and low temperature experiments with CO(2) hydrate were performed using diamond anvil cells and a helium-refrigeration cryostat in the pressure and temperature range of 0.2-3.0 GPa and 280-80 K, respectively. In situ x-ray diffractometry revealed that the phase boundary between CO(2) hydrate and water+CO(2) extended below the 280 K reported previously, toward a higher pressure and low temperature region. The results also showed the existence of a new high pressure phase above approximately 0.6 GPa and below 1.0 GPa at which the hydrate decomposed to dry ice and ice VI. In addition, in the lower temperature region of structure I, a small and abrupt lattice expansion was observed at approximately 210 K with decreasing temperature under fixed pressures. The expansion was accompanied by a release of water content from the sI structure as ice Ih, which indicates an increased cage occupancy. A similar lattice expansion was also described in another clathrate, SiO(2) clathrate, under high pressure. Such expansion with increasing cage occupancy might be a common manner to stabilize the clathrate structures under high pressure and low temperature.     

Influence of 3A molecular sieve on tetrahydrofuran (THF) hydrate formation

Visual observation of the THF hydrate formation process in the presence of a 3A molecular sieve has been made at normal atmosphere and below a temperature of zero by microscopy. The results indicate that a 3A molecular sieve can induce the nucleation of the THF hydrate and promote the THF hydrate growth. With the existence of a 3A molecular sieve, the growth rate of THF hydrate is between 0.01 and 0.05 μm/s. In comparison with the system without any 3A molecular sieve, the growth rate increases about 4 nm/s. After the THF hydrate grows into megacryst, the crystals will recombine and partially change under the same condition.     

A molecular dynamics study of ethanol-water hydrogen bonding in binary structure I clathrate hydrate with CO2

Guest-host hydrogen bonding in clathrate hydrates occurs when in addition to the hydrophilic moiety which causes the molecule to form hydrates under high pressure-low temperature conditions, the guests contain a hydrophilic, hydrogen bonding functional group. In the presence of carbon dioxide, ethanol clathrate hydrate has been synthesized with 10% of large structure I (sI) cages occupied by ethanol. In this work, we use molecular dynamics simulations to study hydrogen bonding structure and dynamics in this binary sI clathrate hydrate in the temperature range of 100-250 K. We observe that ethanol forms long-lived (>500 ps) proton-donating and accepting hydrogen bonds with cage water molecules from both hexagonal and pentagonal faces of the large cages while maintaining the general cage integrity of the sI clathrate hydrate. The presence of the nondipolar CO(2) molecules stabilizes the hydrate phase, despite the strong and prevalent alcohol-water hydrogen bonding. The distortions of the large cages from the ideal form, the radial distribution functions of the guest-host interactions, and the ethanol guest dynamics are characterized in this study. In previous work through dielectric and NMR relaxation time studies, single crystal x-ray diffraction, and molecular dynamics simulations we have observed guest-water hydrogen bonding in structure II and structure H clathrate hydrates. The present work extends the observation of hydrogen bonding to structure I hydrates.