Dissociation of Methane Hydrate in Aqueous NaCl Solutions

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Dissociation of Methane Hydrate in Aqueous NaCl Solutions Takuma Yagasaki, Masakazu Matsumoto, Yoshimichi Andoh, Susumu Okazaki, and Hideki Tanaka J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp507978u • Publication Date (Web): 19 Sep 2014 Downloaded from http://pubs.acs.org on September 26, 2014

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The Journal of Physical Chemistry

Dissociation of Methane Hydrate in Aqueous NaCl Solutions

Takuma Yagasaki,† Masakazu Matsumoto,† Yoshimichi Andoh,§ Susumu Okazaki,§ and Hideki Tanaka†, ‡,* †

Department of Chemistry, Faculty of Science, Okayama University, Okayama,

700-8530, Japan ‡

Research Center of New Functional Materials for Energy Production, Storage and

Transport, Okayama, 700-8530, Japan §

*

Department of Applied Chemistry, Nagoya University, Nagoya 464-8603, Japan

Email: [email protected].

Phone and Fax: +81-86-251-7769

1

ACS Paragon Plus Environment


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Abstract Molecular dynamics simulations of the dissociation of methane hydrate in aqueous NaCl solutions are performed.

It is shown that the dissociation of the hydrate

is accelerated by the formation of methane bubbles both in NaCl solutions and in pure water.

We find two significant effects on the kinetics of the hydrate dissociation by

NaCl.

One is slowing down in an early stage before bubble formation and another is

swift bubble formation that enhances the dissociation.

These effects arise from the low

solubility of methane in NaCl solution which gives rise to a non-uniform spatial distribution of solvated methane in the aqueous phase.

We also demonstrate that

bubbles form near the hydrate interface in dense NaCl solutions, and that the hydrate dissociation proceeds inhomogeneously due to the bubbles.

Keywords Molecular dynamics, salt effects, bubble formation.

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Introduction Clathrate hydrate is a crystalline inclusion compound in which small molecules, such as methane and hydrogen, are held within polyhedral water cages.1

It is known

that a huge amount of natural gas is stored as clathrate hydrates in ocean floor and permafrost regions, and these naturally occurring gas hydrates are expected as a future energy resource.2-4

It has also been proposed that gas hydrates can be used for energy

storage, transportation, and gas separation purposes.3,5-11 Molecular dynamics (MD) is a powerful tool to examine microscopic features of condensed phases, and has provided much insight into static and dynamic properties of gas hydrates,

such as

thermal stability,12-23 structure

of the

interface,24-26

dissociation,27-42 formation,43-67 and molecular diffusion in the cage structure.68-74

In a

previous paper, we performed large-scale MD simulations of the dissociation of a methane hydrate cluster in pure water.41

It was shown that the concentration of

methane in the liquid phase increases with time, and this results in a decrease in the dissociation rate due to reconstruction of hydrate cages. further, bubbles of methane form. phase.

As the dissociation proceeds

Once formed, they rapidly grow in the aqueous

The dissociation rate turns to increase after the bubble formation because the

bubbles absorb surrounding methane molecules supersaturated in the solution.

We

also demonstrated that methane hydrate can be preserved as a metastable superheated solid if there are no pre-existent bubbles near the hydrate/liquid interface. It is experimentally known that properties of gas hydrates are affected by the presence of solutes in the surrounding aqueous phase.

For example, the

hydrate-liquid-gas three-phase equilibrium temperature is lowered by adding alcohols

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into the aqueous phase.1,3

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This is due to the lower chemical potential of water in the

aqueous solution of alcohol than in pure water.

To the best of our knowledge, however,

there are only a limited number of MD simulations to examine roles of solutes other than the guest species,22,64 and thus the effects of solutes on the dissociation kinetics are still unclear at the molecular scale. In this paper, we perform MD simulations of the dissociation of methane hydrate in

aqueous

NaCl solutions.

Experimental

studies have

shown

that the

hydrate-liquid-gas three-phase equilibrium temperature, Teq, in aqueous NaCl solutions becomes lower with increasing NaCl concentrations.75,76

This fact suggests that the

dissociation of methane hydrate is also faster in solutions of a higher NaCl concentration at a given temperature.

This study demonstrates, however, that the

effects of NaCl are not so simple as what we anticipated.

We find that NaCl has both

deceleration and acceleration effects on the kinetics of hydrate dissociation.

These

effects are explained by the spatial distribution of methane molecules released from the hydrate.

We also discuss the temperature dependence of the NaCl effects.

Methods The initial structure consists of a hydrate cluster and the surrounding aqueous phase.

The hydrate cluster is a 9 × 9 × 9 unit cell replica of fully occupied structure I

methane hydrate. 0.6 mol kg-1.

Simulations are performed for two NaCl concentrations, m = 4.8 and

The concentration of m = 4.8 mol kg-1 is somewhat lower than the

experimental saturation condition at ambient temperature, m ~ 6 mol kg-1, and m = 0.6 mol kg-1 is close to the NaCl concentration of seawater. 4


The analyses are also

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performed for the m = 0.0 mol kg-1 solution, i.e., pure water.

The trajectories of the

pure water system are the same as those examined in our previous paper.41

The

number of each chemical species in the initial configuration is summarized in Table I. The sum of molecules and ions in the system is 124510 for all three concentrations. This large system size is required to treat methane bubbles that grow quite rapidly.

Table I.

The number of each chemical species in the initial configuration. Hydrate

Liquid phase

m / mol kg-1

CH4

H2O

Na+

Cl-

H2 O

0.0

5832

33534

0

0

85144

0.6

5832

33534

900

900

83344

4.8

5832

33534

6273

6273

72598

In accordance with the previous study,41 we employ the TIP4P/2005 model for water,77 and the OPLS united model for methane molecules.78 parameters for Na+ and Cl- are taken from ref. 79.

The MD simulations are carried out

using the MODYLAS package with a time step of 2 fs.80 MPa in all simulations. directions.

The potential

The pressure is kept at 0.1

Periodic boundary conditions are applied in all three

Long-range Coulomb interactions are treated with the fast multipole

method.80 The equilibration procedure is the same as that of our previous paper.41

First,

the aqueous phase is equilibrated at 300 K with fixing all the degrees of freedom of the hydrate cluster.

The simulation is continued for 2 ns at 220 K without any constrains

to relax the structure of the hydrate cluster.

Then, the temperature is suddenly changed 5



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to a higher temperature.

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We set this instant as the time origin, t = 0, and analyze the

following dissociation process.

Methane molecules are classified into three types,

methane in the hydrate cluster, solvated in the aqueous phase, and in the gas phase at each time step based on a procedure given in the previous paper.41

The F3 parameter is

used for the classification.27

Results and Discussion In Figure 1, we present the number of methane molecules in the hydrate cluster, Ng, at T = 292 K.

The hydrate-liquid-gas three-phase equilibrium temperature, Teq, in

aqueous NaCl solutions is lower for higher NaCl concentrations.75,76

If the

dissociation rate is simply proportional to the degree of superheating, T – Teq, Ng would be smaller for solutions of higher concentrations at any moment. demonstrates that this simple expectation is not true.

Figure 1 clearly

It is seen that Ng is larger for 10

ns < t < 90 ns for the 0.6 mol kg-1 solution than for the 0.0 mol kg-1 solution. indicates that the presence of NaCl slows down the hydrate dissociation.

This

The slowing

down is also seen in the 4.8 mol kg-1 solution.

Figure 1. T = 292 K.

Time evolution of the number of methane molecules in the hydrate cluster at Black, green, and red curves are the results of the 0.0, 0.6, and 4.8 mol kg-1 6


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solutions, respectively.

It is well known that the formation of methane hydrate is enhanced by pressure. This is because the pressurization increases the possibility of finding methane molecules near the hydrate surface in water.

In our simulations, the dissociation of the hydrate

cluster gradually increases the number of methane molecules at the surface.

This

results in a similar effect to the pressurization, i.e., the enhancement of the construction of hydrate cages.

Since the temperature is higher than Teq, this is seen as the decrease

in the dissociation rate.

The slowing down of the dissociation in the 0.0 mol kg-1

solution for t < 80 ns shown in Figure 1 arises from this mechanism.

The dissociation

rate turns to increase at t ~ 80 ns because two bubbles form at t = 73 and 84 ns. similar behavior is observed in the NaCl solutions.

A

An increase in the dissociation rate

is seen at ~70 ns in the 0.6 mol kg-1 solution and at ~20 ns in the 4.8 mol kg-1 solution. This result suggests that bubble formation occurs earlier in denser NaCl solutions. NaCl has two effects on the kinetics of the hydrate dissociation: slowing down at an early stage before bubble formation and a rapid bubble formation that enhances the dissociation.

Hereafter, we focus on the difference between the dissociation processes

in the 4.8 and 0.0 mol kg-1 solutions because both effects are more significant in the denser NaCl solution.

Figure 2 presents snapshots of the dissociation process in the

0.0 mol kg-1 solution at T = 292 K. initial condition.

The hydrate cluster is cubic at t = 0 owing to the

The hydrate cluster changes its shape from a cube to a sphere (Figure

2b) because acute parts of the hydrate cluster dissociate faster than planar parts due to the Gibbs-Thomson effect81 (note that the contribution of the Gibbs-Thomson effect to 7



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the dissociation rate of methane hydrate was examined in detail in our previous paper41). The spherical shape of the cluster is maintained during the dissociation for t > ~15 ns. The concentration of methane in the aqueous phase increases as the dissociation proceeds until the formation of bubbles at t = 73 ns and 84 ns.

The number of solvated

methane molecules in the aqueous phase begins to decrease after the formation of bubbles because the bubbles absorb the surrounding methane molecules.

Figure 2.

Several snapshots along the hydrate dissociation process in the 0.0 mol kg-1

solution at T = 292 K.

Gray, blue, and red particles represent methane molecules in the

hydrate cluster, solvated in the aqueous phase, and in bubbles, respectively.

Water

molecules are not shown.

Figure 3 shows snapshots of the hydrate dissociation in the 4.8 mol kg-1 solution at T = 292 K.

There are several significant differences in dissociation behavior between

the 0.0 and 4.8 mol kg-1 solutions.

One is the location of bubbles: All bubbles form in 8


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the vicinity of the hydrate-liquid interface in the dense NaCl solution. elapsed time before bubble formation.

Another is the

As expected from Figure 1, the bubbles form

much earlier in the 4.8 mol kg-1 solution than in the 0.0 mol kg-1 solution. the shape of the hydrate cluster.

The other is

As shown in Figures 3d and 3e, the hydrate cluster is

nonspherical in the dense NaCl solution.

Figure 3.

Snapshots of the hydrate dissociation in 4.8 mol kg-1 solution at T = 292 K.

Gray, blue, and red particles represent methane molecules in the hydrate cluster, solvated in the aqueous phase, and in bubbles, respectively. Water molecules and ions are not shown.

To analyze the spatial distribution of each chemical species, we consider a cylindrical coordinate system whose origin is set to the position of G0, where G0 is the guest molecule that is the closest to the center of mass (COM) of the hydrate cluster at t = 0.

The number density profile along the cylinder axis is calculated with a grid size 9



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of 0.5 Å.

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The number of molecules located within 15 Å from the axis in each grid is

counted to construct the density profile (i.e., we consider a cylinder of a radius of 15 Å). Figures 4a-4d present the number density profiles in a cylinder in the 0.0 mol kg-1 solution.

We refer to this cylinder as Cylinder-0.

As shown in Figure 5a, the

direction of Cylinder-0 is so chosen as to include a given bubble. Before the formation of the bubble, the cylinder axis passes through a point where the bubble emerges at t = 73 ns.

The cylinder axis passes through the COM of the bubble when t > 73 ns.

Figure 4a shows that the hydrate-liquid interface resides in c = 80 Å, i.e., 80 Å away from G0, at t = 1 ns.

Since the initial structure is prepared without caring for open

cages at the interface, a certain amount of guest molecules is quickly released from the hydrate.

This gives rise to a peak in the distribution of the solvated methane near the

interface.

As the dissociation proceeds, the interface shifts inward and the number of

solvated methane molecules increases.

Figure 4c shows that the density profile of

solvated methane is almost flat everywhere in the liquid phase.

This indicates that the

timescale of the diffusion of methane in the aqueous phase is faster than that of the release of methane from the hydrate.

The concentration of methane in the aqueous

phase increases with time, and a bubble forms at t = 73 ns.

Because the distribution of

the solvated methane is uniform, there is no particular place where bubbles would form more easily.

In this case, the bubble forms at c ~ 75 Å, about 40 Å away from the

hydrate/liquid interface, due to the concentration fluctuation. supersaturation is roughly 0.002 Å-3.

The limit of

Note that similar results are obtained for another

bubble in the 0.0 mol kg-1 solution because the dissociation proceeds isotropically.

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Figure 4.

The number density profiles along the cylinder axis.

Black, red, and green

curves are the number density profiles of methane molecules in the hydrate cluster, aqueous phase, and bubbles, respectively.

Blue curves show the number density of the

Na+ ion. Curves for Cl- are not shown because they are quite similar to the profiles of Na+.

A moving average is taken with a window of 1 ns to reduce noisy fluctuations.

Figure 5.

(a) Cylinder-0 at t = 80 ns.

The blue arrow is the cylinder axis that passes 11



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through the COM of a bubble.

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(b) Cylinder-4.8a (green) and Cylinder-4.8b (blue) at t

= 20 ns.

The number density profiles in a cylinder in the 4.8 mol kg-1 solution are shown in Figures 4e-4g.

This cylinder, referred to as Cylinder-4.8a, does not include bubbles

(Figure 5b), and can be straightforwardly compared with Cylinder-0 in which no bubble exists at t = 1, 7, and 20 ns.

Note that the results do not depend on the angle of the

cylinder when it does not include any bubble.

At t = 1 ns, the density profiles of

methane molecules in the 4.8 mol kg-1 solution (Figures 4e) are similar to those in the 0.0 mol kg-1 system (Figure 4a).

Water and methane molecules are released into the

aqueous phase as the dissociation proceeds. hydrate interface (Figures 4f and 4g).

This results in an ion-poor region near the

In pure water, as mentioned above, the timescale

of the diffusion of methane in the aqueous phase is faster than that of the release of methane from the hydrate.

The distribution of methane in solution therefore becomes

flat at t = 20 ns (Figure 4c).

In contrast, the solvated methane molecules in the NaCl

solution prefer to stay near the interface, and do not enter into the ion-rich region as shown in Figure 4g.

This non-uniform distribution is caused by the low solubility of

methane in aqueous NaCl solutions. The lower solubility cause by increasing NaCl concentration is examined for the present model interactions by a simple particle insertion method.82

MD simulations

are performed for aqueous solutions containing 1,000 molecules (ions) using the GROMACS package.83,84 The excess chemical potentials are calculated by trial insertion of methane for the configurations generated by MD simulations. 12


As shown

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in Figure 6, the calculated excess chemical potential is higher for solutions of higher NaCl concentrations.

This result is consistent with experimental observations.85,86

Salt changes the structure of water in a similar way as pressurization.87

The increase in

NaCl concentration reduces the number of large voids in the solution which can accommodate methane molecules.

As a result, the excess chemical potential increases

as the NaCl concentration increases.

Figure 6.

Excess chemical potential of methane in aqueous NaCl solutions at 300 K.

The presence of solvated methane near the hydrate/liquid interface facilitates the reconstruction of hydrate cages.41

This fact, together with the comparison between

Figures 4c and 4g, suggests that the hydrate dissociation rate is slower in Cylinder-4.8a than in Cylinder-0.

In Figure 7, we show the position of the hydrate/liquid interface

against time for these cylinders.

The position of the interface is defined as the most

distant grid from G0 in which the number density of the methane molecules in the hydrate is larger than 0.005 Å-3.

The dissociation rate is indeed slower in

Cylinder-4.8a than in Cylinder-0 for t > 10 ns.

The dissociation rates are the almost

same for t < 10 ns because both cylinders include an edge of the cubic hydrate cluster and the dissociation kinetics is dominated by the Gibbs-Thomson effect in this period. 13



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The present result demonstrates that the slowing down effect caused by NaCl arises from the non-uniform distribution of the solvated methane.

Figure 7.

Position of the hydrate interface against time.

Data for the 4.8 mol kg-1

solution is truncated at t = 41 ns because G0 dissolves into the aqueous phase at this time.

A bubble forms when the local concentration of solvated methane exceeds the limit of supersaturation.

Because the solvated methane molecules can stay only in a

small area near the interface, the bubble forms at an early stage of the dissociation in the dense NaCl solution.

In Figures 4h-4j, we present the number density profile of a

cylinder in the 4.8 mol kg-1 solution that includes a bubble. Cylinder-4.8b and shown in Figure 5b.

This cylinder is named

Similar results are obtained for other bubbles.

The local concentration exceeds the limit of supersaturation, 0.002 Å-3 as early as t = 7 ns and a bubble forms near the hydrate interface. bubbles form at t = 7, 8, 13, and 21 ns.

In the 4.8 mol kg-1 solution, four

Acceleration of the dissociation found at t ~ 20

ns in Figure 1 is caused by these bubbles.

Note that bubble formation is a stochastic

phenomenon, and thus it is not surprising that no bubble occurs in Cylinder-4.8a even though the local concentration exceeds the limit of supersaturation. 14


It should also be

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noted that the probability of bubble formation would be the same anywhere on the hydrate/liquid interface if the initial hydrate cluster is spherical.

In this study, however,

the hydrate cluster is initially cubic, and each bubble forms near a vertex of the cube which dissociates faster than planar parts.

In macroscopic systems, there must be

asperity on the hydrate interface where the dissociation proceeds faster than other regions.

The present simulation suggests that methane bubbles tend to form near such

places in dense NaCl solutions. The acceleration of the hydrate dissociation due to bubbles is explained from the chemical potential of methane in the hydrate (µH), liquid (µL), and gas phases (µG). Before bubble formation, the driving force for the dissociation can be given as F0 = C(µH – µL) + Fw, where C is a constant and Fw is the driving force arising from the difference between the chemical potentials of water in the hydrate and liquid phases. After bubble formation, the driving force can reach F’ = C(µH – µG) + Fw.

Because of

the nature of the metastable state, the chemical potential of methane in water is higher than that in the gas phase, µL > µG, and hence F’ > F0. is accelerated after the bubble formation.

Thus, the hydrate dissociation

The driving force is maximized when a

bubble is in contact with the hydrate interface.

This is evidently seen in Figure 8.

The configurations presented in Figure 8 are the same as those shown in Figure 3d and 3e but are drawn in a different way to show the shape of hydrate cluster more clearly. Figure 8 demonstrates that the cluster interface near each bubble is concave inward in the 4.8 mol kg-1 solution.

Figure 7 also shows that the dissociation is much faster in

Cylinder-4.8b than in Cylinder-4.8a.

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Figure 8.

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Shape of the hydrate cluster in the 4.8 mol kg-1 solution at (a) t = 30 ns and

(b) t = 40 ns.

Large particles are the methane molecules in the hydrate cluster that are

colored based on the distance from the COM of the cluster.

Green methane molecules

indicate the sphere surface of the equivalent volume with the cluster, and red and blue ones indicate the 20% elevated and concaved surface regions, respectively.

Small

white particles are methane molecules in bubbles.

The condition of bubble formation depends on the system size and the ratio of the number of methane molecules to that of water, R.88,89

As shown above, bubbles form

in the bulk region of the aqueous phase in the 0.0 mol kg-1 solution.

If the simulation

is performed with much smaller R, i.e., larger aqueous region, the dissociation kinetics of this system would change drastically because the system cannot reach the limit of supersaturation and bubbles never form.

In contrast, the dissociation behavior of the

4.8 mol kg-1 solution is expected to be insensitive to the ratio R because the bubbles form in the vicinity of the hydrate interface. still a size effect.

Even when R is kept constant, there is

It is known that the free energy barrier of bubble formation

decreases with increasing system size.88

In the 0.0 mol kg-1 solution, the first bubble

forms at t = 73 ns, at which about 70 % of methane molecules dissolved into the

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aqueous phase.

If both the aqueous part and the initial hydrate crystal of this system

are equally enlarged, the bubble formation would occur on the earlier stage in the dissociation process at which less than 70 % of methane molecules dissolved into water (note that the elapsed time before bubble formation might be longer than 73 ns because it takes longer time for methane molecule in larger systems to diffuse throughout the system).

This size effect is also expected to be small in the 4.8 mol kg-1 system for the

same reason. This study assumes that the hydrate phase is well distant from the gas phase, which is also a stable phase at the present thermodynamic condition.

This assumption

would be relevant to real bulk systems, in which bubble formation dominates the dissociation kinetics as shown above.

However, there is another possible condition

where the gas phase initially exists near the hydrate interface.37

This could be

observed, for example, in dissociation of gas hydrates in porous media, in which the dissociation rate is determined by the timescale of transfer of methane molecules from the vicinity of the hydrate to the gas phase.

In between these two extreme cases, both

the bubble formation and the mass transfer would affect the dissociation kinetics. We finally discuss the temperature dependence of the dissociation rate.

Figure 9

presents the number of methane molecules in the hydrate at T = 296, 308, and 324 K. It is shown that the effects of NaCl decrease with increasing temperature.

The rate of

cage decomposition grows exponentially with temperature because it follows the Arrhenius law with an activation energy that is related to the mechanical stability of the hydrate.41

Other effects on the dissociation rate become negligible at higher

temperatures.

Thus, the dissociation kinetics does not depend on the NaCl

concentration at the highest temperature, T = 324 K.

At T = 296K, only the slowing

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down effect is observed in the 0.6 mol kg-1 solution.

In contrast, the deceleration

effect is small while the acceleration effect is evident in the 4.8 mol kg-1 solution at T = 308 K.

There is no simple explanation why only one of the two effects of NaCl is

dominant under these conditions.

The kinetics of hydrate dissociation involves many

factors, including the activation energy of cage breaking, the rate of cage reconstruction, free energy barrier for bubble formation, and diffusivity of methane in the aqueous phase.

A model accounting for these factors, as well as the dependence on temperature

and the NaCl concentration, and knowledge as to how much they contribute to the non-equilibrium dissociation process is required for a quantitative understanding of the effects of NaCl.

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Figure 9.

Time evolution of the number of methane molecules in the hydrate cluster at

T = 296, 308, and 324 K.

Black, green, and red curves are the results of the 0.0, 0.6,

and 4.8 mol kg-1 solutions, respectively.

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Conclusions We have investigated the dissociation of methane hydrate in aqueous NaCl solutions using MD simulations.

The dissociation rate decreases with time, and then it

turns to increase by the formation of the methane bubbles both in pure water and in NaCl solutions.

It is found that two effects of NaCl on the dissociation kinetics.

One

is the slowing down which is significant before bubble formation, and another is the acceleration due to rapid bubble formation.

These effects arise from the low solubility

of methane in NaCl solutions which results in a locally high concentration of solvated methane near the hydrate interface. We also find that the dissociation occurs rather inhomogeneously in dense NaCl solutions because the methane bubbles, which enhance the dissociation, form in proximity to the interface. Other salts, such as CaCl2 and K2SO4, also decrease the solubility of methane.85 These salts would have similar effects on the kinetics of hydrate dissociation. interesting to consider the effects of other types of solute.

It is

Amphiphilic solutes may

exhibit a different mechanism because they can stabilize bubbles in aqueous solutions. The presence of solutes should affect the formation mechanism as well as the dissociation rate.

Controlling formation/dissociation kinetics using solutes might

become a useful technique in the industrial use of gas hydrates in the future.

Acknowledgments The present work was supported by a Grant-in-Aid by JSPS and by HPCI Strategic Programs for Innovative Research (SPIRE) and Computational Materials Science 20


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Initiative (CMSI), MEXT, Japan.

Calculations were performed on the K computer at

the RIKEN Advanced Institute for Computational Science (Project ID: hp140216).

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Dissociation of Methane Hydrate in Aqueous NaCl Solutions

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