Unraveling Mixed Hydrate Formation: Microscopic Insights into Early

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Unraveling Mixed Hydrate Formation: Microscopic Insights into Early Stage Behavior Kyle Wm. Hall, Zhengcai Zhang, and Peter G. Kusalik J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b11961 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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The Journal of Physical Chemistry

Unraveling Mixed Hydrate Formation: Microscopic Insights into Early Stage Behavior Kyle Wm. Hall,† Zhengcai Zhang,‡ and Peter G. Kusalik*,† †

Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, T2N

1N4, Alberta, Canada ‡

Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese

Academy of Sciences, Beijing 100029, China. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Tel: +1 403-220-6244

ACS Paragon Plus Environment

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ABSTRACT

The molecular-level details of mixed hydrate nucleation remain unclear despite the broad implications of this process for a variety of scientific domains. Through analysis of mixed hydrate nucleation in a prototypical CH4/H2S/H2O system, we demonstrate that high-level kinetic similarities between mixed hydrate systems and corresponding pure hydrate systems are not a reliable basis for estimating the composition of early-stage mixed hydrate nuclei. Moreover, we show that solution compositions prior to and during nucleation are not necessarily effective proxies for the composition of early-stage mixed hydrate nuclei. Rather, microscopic details, (e.g., guest-host interactions and previously neglected cage types) apparently play key roles in determining early stage behavior of mixed hydrates. This work thus provides key foundational concepts and insights for understanding mixed hydrate nucleation.

INTRODUCTION

Clathrate hydrates are multicomponent crystals consisting of water and guest species (e.g., methane) where the water molecules form polyhedral hydrogen-bond cages, and the guest species occupy these cages. Gas hydrates have gained considerable interest both scientifically and industrially. As highlighted by a recent review,1 there is significant interest in global gas hydrate deposits as a potential source of natural gas, and there has been much work on developing natural gas extraction strategies for hydrates. Previous work has also explored optimizing gas hydrates as an energy storage strategy.2,3 However, hydrates can also be undesirable. Hydrates are a major challenge to overcome when ensuring continuous flow in oil


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and gas pipelines, and insights into the molecular-level details of hydrate formation could enable the development of improved hydrate inhibitors.4

Current experimental techniques cannot probe the time and length scales needed to study gas hydrate nucleation at a molecular level. Consequently, simulations have become the major tool for studying the molecular details of hydrate nucleation.5-11 Molecular simulations have been used to probe the hydrate nucleation behavior of a variety of guest species.5-11 This previous work has focused on hydrate nucleation in binary systems (e.g., CH4/H2O mixtures) with the exception of one study on high-level nucleation behavior in a ternary CH4/THF/H2O mixture.12 In contrast, mixed hydrates containing multiple guest species are very relevant (e.g., to energy storage2,3 and hydrate deposits13). It remains an open challenge to understand the molecularlevel details of mixed hydrate nucleation, particularly key factors impacting the rates at which guest species are incorporated into a mixed hydrate phase. The aim of this study is to address this challenge by studying mixed CH4/H2S hydrate nucleation.

Mixed CH4/H2S hydrate nucleation is an industrially relevant process given the vast energy reserves in H2S-containing (i.e., sour) oil and natural gas fields. The United States has historically produced hundreds of millions of barrels of high-sulfur content crude oil14 while sour gas reservoirs in Alberta (the center of the Canadian oil and gas industry) and Russia can contain over 20% H2S.15,16 Given that hydrate formation can pose significant challenges to oil and gas extraction, understanding mixed CH4/H2S hydrate nucleation is relevant to oil and gas production in sour fields.


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CH4 and H2S individually favor formation of the same hydrate phase (i.e., sI hydrate17,18) while their pure hydrates exhibit distinct thermodynamic properties. For a given temperature, the H2O(l)/hydrate/vapor three phase coexistence line for CH4/H2O systems is located at an order-ofmagnitude higher pressure than for H2S/H2O systems (e.g., 5.48 MPa vs. 0.21 MPa at ~280.6 K).19 Therefore, H2S hydrates exhibit enhanced stability with respect to CH4 hydrates. The H2O(l)/hydrate/vapor three phase coexistence line of ternary CH4/H2S/H2O systems is shifted to significantly higher temperatures and lower pressures relative to the same coexistence line for CH4/H2O systems even when H2S constitutes only a small percentage of the vapor phase.20 This implies an unexpected stabilization of the mixed hydrate phase by H2S. Based on the properties of the pure CH4 and H2S hydrates alone, it is unclear how the enclathration of CH4 and H2S compare during mixed CH4/H2S hydrate nucleation. This is in contrast to mixed CH4/THF hydrate nucleation, for example, where THF and CH4 have different pure hydrate structures, and thus unsurprisingly have been reported to occupy distinct cages during mixed hydrate nucleation.12

Given that both CH4 and H2S individually form the same hydrate structure and have similar diffusion coefficients in water,21,22 their relative aqueous solubilities, which are substantially different, might be expected to control their enclathration and incorporation into a hydrate nucleus. For example, the aqueous solubility of CH4 is two orders of magnitude lower than that of H2S for partial pressures of ~2.5 MPa at 40 °C.23,24 When differences in guest species’ aqueous mole fractions are large, the nucleation behavior of mixed hydrates can be expected to depend strongly on the behavior of the predominant species. Under conditions where the aqueous


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mole fractions of CH4 and H2S are comparable, their organization within aqueous phases (e.g., distributions of solvent-separated pairs) is also comparable.9 In this case, it is unclear what factors would determine the relative enclathration of CH4 and H2S. This study elucidates molecular-level mechanistic details of mixed CH4/H2S hydrate nucleation in the regime of comparable CH4 and H2S aqueous mole fractions, thereby revealing key factors impacting guest enclathration rates for mixed hydrates. For example, previously neglected types of water cages can apparently have an unexpectedly large impact on guest species uptake.

METHODS

Mixed CH4/H2S hydrate nucleation was probed using extensive Molecular Dynamics (MD) simulations of a ternary CH4/H2S/H2O system exhibiting comparable CH4 and H2S aqueous mole fractions. The methodological details of these simulations are the same as our previous work on the potential energy landscapes associated with hydrate nucleation.25 Briefly, ten independent NPT MD simulations were performed at 50 MPa and 250 K for a system initially composed of a CH4/H2S nanobubble immersed in water. Under these conditions and for the specific models employed, the system was metastable with respect to the CH4 and H2S hydrate phases, but not hexagonal ice (see ref. 25). Each simulation was run for 200 ns. The system consisted of 3375 molecules with a H2O/CH4/H2S molar ratio of 90/5/5.


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RESULTS & DISCUSSION

F4 order parameter26 values as averaged over the water molecules within a simulation have been previously used to monitor hydrate nucleation in simulations.7,9,27,28 In gas hydrate nomenclature, a 4b5c6d water cage consists of b quadrilateral, c pentagonal, and d hexagonal water-based hydrogen bond rings. Previous visual29 and algorithmic30 analysis has revealed that the appearance of 5126n – 415106m cages (where n=0, 2, 3, or 4 and m=2, 3 or 4) is associated with hydrate formation. In this study, water cages and their occupancies were analyzed using a modified version of the Face-Saturated Incomplete Cage Analysis (FSICA) method.30 For a given trajectory, increases in average F4 value (which align well with decreases in potential energy) essentially coincide with the appearance of a cluster of 5126n – 415106m cages. For example, compare the black curve in Figure 1a for Run 2 with the snapshots from Run 2 in Figure 1b. On average less than 0.5% of 5126n – 415106m cages contain multiple guest species with multi-occupancy only being observed for large cages (specifically, 51263, 51264, and 4151064 cages). Consequently, the enclathration of CH4 and H2S was analyzed based on singly-occupied cages.

Based on Figure 1a, nucleation had occurred and the hydrate phase was in a growth regime by the time six trajectories (Runs 1-4, 6 and 10) reached 200 ns. For these nucleating trajectories, their induction times are on the order of one hundred nanoseconds and the hydrate phase becomes the dominant phase within approximately fifty nanoseconds following nucleation. This behaviour is comparable to analogous pure H2S nanobubble systems at 250 K and 50 MPa (e.g., see Figure 1 and Figure S4 in ref. 9). In contrast, CH4 nanobubble systems have induction times


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of several hundreds of nanoseconds to well over a microsecond depending on the exact pressuretemperature conditions, and hydrate growth following nucleation takes on the order of several hundreds of nanoseconds (e.g., see Figures 3 and 6 in ref. 27 as well as Figures 1 and 3 in ref. 7). Therefore, the high-level nucleation behaviour of the CH4/H2S/H2O system more closely mirrors H2S/H2O systems.

Figure 1. Detecting hydrate nucleation. A) The time evolution of the average F4 value for each simulation. The dashed lines are the times corresponding to the snapshots in panel B. B) Snapshots during the nucleation process 12 n

1 10 m

in Run 2. The oxygen atoms of the water molecules comprising the largest cluster of 5 6 – 4 5 6 cages are connected by maroon tubes. The oxygen atoms of additional water molecules are represented as dots. The enclathrated H2S and CH4 are yellow and blue spheres.

Previous work has suggested that the homogeneous nucleation of CH4 hydrates is unlikely.31 The results of this study highlight how the presence of other guest species (e.g., H2S) can considerably enhance hydrate formation. By extension, it may be possible for small in situ amounts of hydrate promoter (e.g., H2S) to enhance hydrate nucleation and enable formation of an initial hydrate phase that subsequently grows in a CH4-rich environment, thereby yielding hydrates with a high fraction of CH4 without requiring nucleation of a pure CH4 hydrate. Current


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work is underway to establish the fraction of H2S required to realize significantly enhanced nucleation.

The hydrate structures formed in this study’s six nucleating trajectories exhibit relatively low levels of crystallinity. The unit cell for the sI hydrate structure (i.e., the favoured structure for pure CH4 and H2S hydrates) is composed of six 51262 cages and two 512 cages,32 so the ratio of 51262 cages to 512 cages should be 3.0 for a defect-free sI hydrate phase. In contrast, across this study’s six nucleating trajectories, the 51262:512 ratio on average lies between 0.25 and 0.6 during hydrate nucleation and growth, thus indicating that this study’s CH4/H2S/H2O system has a preference for nucleating via a comparatively amorphous phase at the current temperaturepressure conditions. Ratio values between 0.25 and 0.6 are comparable to 51262:512 ratios extracted from simulations of CH4 hydrates nucleating via amorphous-like structures (e.g., see runs A, B, and D-F in Figure 6 of ref. 28), and 51262:512 ratios during H2S hydrate nucleation when using the NPT ensemble (based on data from ref. 33). Therefore, the nucleating trajectories for this study’s CH4/H2S/H2O system yielded hydrate structures with low crystallinity that is comparable to appropriate single-guest reference systems.

More detailed analysis of hydrate nucleation requires temporal alignment of the nucleating trajectories since hydrate nucleation is stochastic (see Figure 1a). As previously detailed,25 the nucleating trajectories can be temporally aligned by setting the inflection points of the F4 curves in Figure 1a to be trelative = 0 ns. Nucleation occurs when trelative < 0 ns, and the aligned F4 curves exhibit excellent overlap for trelative < 0 ns as is highlighted by the max-mean-min distribution in


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Figure 2a. As is apparent in Figure 2b, trelative = 0 ns coincides with start of a precipitous decline in the fraction of guest molecules free in solution.

The aqueous phase in the nucleating trajectories is generally enriched in H2S prior to and during nucleation (see Figure 2c for trelative < 0 ns). Therefore, it might be inferred that this study’s CH4/H2S/H2O system is nucleating via H2S-rich nuclei. H2S-rich nuclei could have similar behavior to pure H2S nuclei, which might then explain the high-level similarities between the nucleation kinetics of this study’s mixed CH4/H2S hydrates and previous work on pure H2S hydrates. However, this is not the case. Despite high-level similarities, this study’s CH4/H2S/H2O system does not on average nucleate through H2S-rich nuclei.


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Figure 2. Temporal alignment and aqueous phase composition. A) The max-mean-min distribution for the average F4 values among the nucleating trajectories following temporal alignment. A max-mean-min distribution shows the evolution of the maximum, mean and minimum values for a specified property (e.g., average F4 values) as a function of relative time for the set of temporally aligned nucleating trajectories. B) The max-mean-min distribution for the fraction of guest molecules free in solution (i.e., CH4 and H2S that are neither in the CH4/H2S nanobubble nor enclathrated). C) The max-mean-min distribution for the ratio of H2S to CH4 free in solution.


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The max-mean-min distribution for the number of H2S-filled cages essentially overlaps with that of the CH4-filled cages for 512 (see Figure 3a) and 51262 cages (see Figure 3b). There is thus, on average, no marked preferential uptake of CH4 or H2S by either 512 or 51262 cages during hydrate nucleation. Similar analysis for the other 5126n – 415106m cages is subject to a high degree of noise given the low populations of these cages; however, none of the other 5126n – 415106m cages appear to have a preference for H2S. Therefore, nucleation in the CH4/H2S/H2O system proceeds on average with the initial formation of a mixed CH4/H2S hydrate phase rather than with the anticipated H2S-rich nuclei. Given that the aqueous phase is enriched in H2S prior to and during nucleation, the results in Figure 3a and Figure 3b indicate that other factors must play important roles in determining the enclathration rates of CH4 and H2S.

The hydrate nuclei formed in this study’s CH4/H2S/H2O system contain additional cages beside the 5126n – 415106m cages. Given that most previous simulation-based work has tended to focus on 5126n – 415106m cages, these additional cages will be referred to as nonstandard cages. The set of nonstandard cages includes: 1) complete cages (i.e. perfect polyhedra)30 excluding the set of 5126n – 415106m cages, and 2) face-saturated incomplete cages (i.e., defective polyhedra).30,34 These nonstandard cages represent a significant percentage of the singly-occupied cages present during hydrate nucleation and early-stage growth for this study’s set of nucleating trajectories (see Figure 3c). As formation of the hydrate phase proceeds, the prevalence of these nonstandard cages decreases somewhat; however, they continue to account for 30-50% of all singly-occupied cages even at trelative = 0 ns, where the hydrate phase is definitively in a growth regime. Therefore, nonstandard cages make an important contribution to the formation of the hydrate phase. As can be seen from Figure 3d, nonstandard cages tend to form an interfacial region


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between the growing hydrate phase and the aqueous phase, which is consistent with previous work.12 The max-mean-min distributions for the number of CH4-filled and H2S-filled nonstandard cages reveal that these cages preferentially enclathrate CH4 during the hydrate nucleation process (see Figure 4a, particularly between -20 ns and 0 ns). The volume distributions for the CH4-filled and H2S-filled nonstandard cages have the same overall shape, though the latter has an overall lower intensity than the former (see Figure 4b). Therefore, the preferential filling of nonstandard cages with CH4 is not a volume effect. It is apparent from Figure 4c that the nonstandard cages containing odd numbers of water molecules are more abundant than those with even numbers of water molecules.

All of the 5126n – 415106m cages contain even numbers of water molecules and are known to be more stable, in terms of lifetimes, than cages containing odd numbers of water molecules (i.e., odd-number cages).29 Odd-number cages are proposed to exhibit reduced stability because not all of the water molecules comprising such a cage can form three-fold hydrogen bond connectivity with the other vertex water molecules while still maintaining four-fold connectivity in the extended hydrogen bond network.29 That is to say, odd-number cages correspond to higher energy hydrogen bond topologies, so the majority of the nonstandard cages are higher energy structures.


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Figure 3. Composition and structure of hydrate nuclei. The max-mean-min distributions for the H2S-filled and CH412

12 2

filled 5 cages (A) and 5 6 cages (B). The color scheme is the same for both panels. C) The max-mean-min distribution for the percentage of singly-occupied cages that correspond to nonstandard cages. The appearance of singly-occupied standard cages is coupled to the presence of singly-occupied nonstandard cages such that the max-mean-min distribution is undefined at relative times earlier than -15 ns due to the absence of singly-occupied 12 n

1 10 m

cages in some trajectories. D) Snapshots taken from Run 2 showing the largest cluster of 5 6 – 4 5 6 and 12 n

1 10 m

nonstandard cages. The oxygen atoms of the water molecules comprising the largest cluster of 5 6 – 4 5 6

cages are connected by maroon tubes while oxygen atoms of additional cages are connected by purple tubes. The oxygen atoms of water molecules not participating in the largest cluster of cages are represented as dots. Guest molecules have been omitted for visual clarity.


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Figure 4. Properties of singly-occupied nonstandard cages. A) The max-mean-min distributions for the number of H2S-filled and CH4-filled nonstandard cages. B) Histograms of the volumes of the nonstandard H2S-filled and CH43

filled cages. The histogram bin is 5 Å . C) Histograms of the number of water molecules comprising nonstandard H2S-filled and CH4-filled cages. Major peaks have been labeled for clarity. The same color scheme is used in panels B and C. Both histograms are based on the cages extracted from the configurations between trelative = -20 ns and 0 ns for the set of nucleating trajectories.


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When a nonstandard cage contains H2S, the local hydrogen bond network will have a greater opportunity to relax through guest-host interactions (e.g., through a weak H2S-H2O hydrogen bond). Previous studies have demonstrated that the presence of H2S in ether/H2S mixed hydrates stabilizes crystal defects in the hydrate crystal lattice.18 In the nucleating trajectories for this study’s CH4/H2S/H2O system, H2S-H2O hydrogen bonds occur frequently prior to nucleation and are decreasingly prevalent as nucleation proceeds (see Figure 5a). Individual H2S-H2O hydrogen bonds are transient (generally not lasting longer than 10 ps) with H2S generally acting as the hydrogen bond donor. H2S-H2O hydrogen bonds occur in a variety of local structures, including in the aqueous phase (see Figure 5b) and for nonstandard cages (see Figure 5c). Consistent with previous work,35 H2S-H2O hydrogen bonds are also occasionally observed in standard cages (see Figure 5d). Consequently, CH4 and H2S demonstrate markedly different guest-water interactions during hydrate nucleation in the present study. In the case of THF/H2O systems, guest-water hydrogen bonding interactions can lead to cage destruction.12 H2S-H2O hydrogen bonding could enhance destruction rates for H2S-filled nonstandard cages compared to their CH4-filled counterparts, thereby explaining the lower population of H2S-filled nonstandard cages in spite of H2S enrichment in the aqueous phase. Similar phenomenology is expected in other mixed hydrate systems when one guest species can interact more strongly and anisotropically with water molecules than other guest species, though other factors such as differences in guest species size may be more important in some systems.


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Figure 5. Hydrogen bonding interactions between H2S and H2O molecules during hydrate formation. A) The maxmean-min distribution for the fraction of H2S molecules that are hydrogen bonded to water molecules. In accord 35

with previous work on H2S hydrate phases, a H2S-H2O pair is classified as hydrogen bonded when the S-O distance is less than 3.5 Å and the D–H···A angle is greater than 150° (where D=S and A=O, or vice versa). B-D) Representative local structures involving H2S-H2O hydrogen bonds as extracted from the 100 ns configuration from Run 2. Red and blue tubes correspond to H2O-H2O and H2S-H2O hydrogen bonds, respectively. H2S molecules are represented by scaled Van der Waals spheres. B) The local structure surrounding a H2S molecule that lies within the aqueous solution phase of the right-most snapshot in Figure 3d. Only hydrogen bonds within 6 Å of the central H2S are shown. Assignment of H2O-H2O hydrogen bonds was done using the same criteria as for detecting H2S-H2O hydrogen bonds. C) A H2S-filled face-saturated incomplete cage (i.e., a nonstandard cage) located at the surface of 12 2

the cluster of cages shown in the right-most snapshot in Figure 3d. D) A H2S-filled 5 6 cage extracted from the cluster of cages shown in the right-most snapshot in Figure 3d.

Given that nonstandard cages appear to dominate the interface between the hydrate nucleus and the aqueous phase (see Figure 3d), the nonstandard cages apparently serve as key intermediate structures during the formation of the 5126n – 415106m cluster that constitutes the core of the nucleus. Previous work has made similar inferences.12 Hence, 512 and 51262 cages do not exhibit a preference for either H2S or CH4 in the current system because the CH4 enrichment in the nonstandard cages, which stems from differences in guest-host interactions, is effectively cancelling out the effects of the overall H2S enrichment in the aqueous phase. Hydrate nucleation


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thus involves a delicate interplay between kinetics (e.g., local structures and interactions) and thermodynamics (e.g., the partitioning of guest species between different phases).

CONCLUSIONS

The aim of this study was to explore mixed hydrate nucleation and to elucidate key factors influencing guest enclathration rates. This study revealed that the high-level nucleation kinetics of mixed systems can closely mirror the overall nucleation kinetics of one guest species, but that this does not necessarily imply that nuclei are enriched in that guest species. Similarly, solution composition was shown to not be the sole determinant of guest enclathration rates. Nonstandard cages were revealed to represent mechanistically important interfacial structures between the hydrate nucleus core of 5126n – 415106m cages and the aqueous phase. Guest-host interactions, particularly for nonstandard cages, are proposed to constitute a key factor affecting guest enclathration rates, thereby providing an explanation for how this study’s CH4/H2S/H2O system yielded nuclei not enriched in H2S despite displaying H2S enrichment in the aqueous phase and H2S-like nucleation kinetics. Previous simulation-based studies5-9,11 of hydrate nucleation support a two-step mechanism for hydrate nucleation.5,6 This work highlights that hydrate nucleation is even more nuanced, suggesting a delicate interplay between cage-type interconversions and guest-host interactions. Work is currently underway to further elucidate guest-host interactions and cage dynamics.


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AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (Grant No. 41372059) and the Natural Sciences and Engineering Research Council of Canada (Grant No. RGPIN-2016-03845 ). We acknowledge further support from Alberta Innovation and Science; Alberta Innovates - Technology Futures (AITF); the Canada Foundation for Innovation (CFI); the NSERC Vanier CGS Program; and the University of Calgary. The authors thank Dr. Shuai Liang for providing the F4 analysis code, and Dr. Guang-Jun Guo for his comments during manuscript preparation.


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Unraveling Mixed Hydrate Formation: Microscopic Insights into Early

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