Clathrate Hydrate Crystal Growth in Liquid Water Saturated with a

Clathrate Hydrate Crystal Growth in Liquid Water Saturated with a Guest Substance: Observations in a Methane + Water System Ryo Ohmura,* Sadatoshi Matsuda, Tsutomu Uchida, Takao Ebinuma, and Hideo Narita

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 3 953-957

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2-17-1-2 Tsukisamu-Higashi, Toyohira-ku, Sapporo 062-8517, Japan Received September 26, 2004;

Revised Manuscript Received November 4, 2004

ABSTRACT: This paper reports on a visual study of formation and growth of clathrate hydrate crystals in liquid water saturated (prior to hydrate formation) and in contact with methane gas under the pressure of 6-10 MPa at a temperature of 273.5 K. Irrespective of the pressure set in the experimental system, in most of the experimental runs we observed that a hydrate film first formed to intervene between methane gas and liquid water, and then hydrate crystals grew in liquid water from the hydrate film. Distinct variations in the morphology of hydrate crystals grown in liquid water were observed depending on the pressure. At pressures of 6-8 MPa, hydrate crystals with skeletal, columnar morphology were observed. At the pressure of 10 MPa, the skeletal, columnar crystals were replaced by dendritic crystals. The dependency of the morphology on the degree of driving force for mass-transfercontrolled hydrate-crystal growth is discussed, comparing the present observations with those reported in the literature. Another category of hydrate formation and growth was observed in some experimental runs. The hydrate crystals first formed at the inner surface of the test cell in contact with liquid water instead of the methane-water interface. These crystals floated up to the methane-water interface, where they became a polycrystalline hydrate film, and continued to grow in liquid water. Introduction Clathrate hydrates (abbreviated to hydrates) are crystalline compounds consisting of “host” water molecules that are hydrogen-bonded to constitute cages and “guest” molecules enclosed in the cages. We encounter hydrate crystals in various energy- and environmentrelated issues such as ensuring the flow of oil or gas pipelines by avoiding plugging due to hydrate formation and the storage of natural gas in the form of hydrates. Knowledge of the crystal morphology of hydrates is necessary to deal with these issues, as they provide fundamental information on the mechanistic nature of hydrates. It is generally acknowledged that hydrate crystals grow to be a polycrystalline thin film or layer intervening between the guest fluid and liquid-water phases in the system composed of liquid water and a guest fluid, which is only sparingly soluble with water (the guestin-water solubility is typically 0.001-0.05 in mole fraction). However, recent observational studies1-10 clearly demonstrated that hydrate crystals grew into liquid water, if the guest substance had dissolved in liquid water to saturation (or, to be more exact, though not necessarily to saturation, to a certain concentration prior to the hydrate formation). Particularly, Ohmura et al.10 demonstrated in their recent observational study on the morphology of CO2 hydrate grown in liquid water presaturated with CO2 that the morphology of the hydrate crystals changed from polyhedra with facets, to skeletal polyhedra or columns, and then to dendrites, depending on the thermodynamic conditions set in the * To whom correspondence should be addressed. Phone: +81-11857-8949. Fax:+81-11-857-8471. E-mail: [email protected].

experimental system. They further analytically formulated the crystal growth process based on the assumption that hydrate-crystal growth is controlled by mass transfer of a guest substance dissolved in liquid water, and they presented a nondimensional index to predict the morphology of hydrate crystals grown in liquid water presaturated with a guest substance. The morphology variations observed in their study and previous studies were successfully related to the index. However, so far, variations in the hydrate-crystal morphology, depending on the thermodynamic conditions (or driving force for crystal growth), are reported exclusively for CO2 hydrate. Thus, observations in hydrate-forming systems with guests other than CO2 are required to acquire comprehensive knowledge on the variations in hydrate-crystal morphology. In this study, we performed a set of observations in a methane + water system. We have selected methane as a guest substance for several reasons. First, the observations of methane-hydrate crystals are relevant to various scientific or technical issues, such as the accumulation of naturally occurring methane hydrates in subsea sediments or beneath permafrost and natural gas storage in the form of hydrates. Second, the solubility of methane in liquid water, as well as the concentration of methane in liquid water in equilibrium with methane hydrate, is well documented11,12 and therefore we can calculate the above-mentioned nondimensional index relevant to the present observations to verify the utility of the nondimensional index. Subramanian and Sloan8 reported the growth of needlelike methanehydrate crystals; however, their observation was performed under a thermodynamic condition (or a single level of driving force for the hydrate-crystal growth), and

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Ohmura et al.

Figure 1. Schematic of the experimental apparatus.

the videographs are not clear enough to identify the crystal morphology. Therefore, we still need a further study on the morphology and its variation in methanehydrate crystals. The following describes the present experiments, demonstrates the representative observations, and discusses the variations in the observed crystal morphology. Experimental Section The sample fluids used to form a hydrate were methane (99.9 vol % certified purity; Sumitomo Seika Chemicals, Co., Ltd., Chiba, Japan) and liquid water, which was deionized and then distilled. The experimental apparatus is essentially the same as that used in our previous study.10 Figure 1 schematically illustrates the major portion of the experimental apparatus: a test cell made of a stainless steel cylinder with a pair of flange-type glass windows and a microscope-camera system. The inner space of the test cell was 25 mm in diameter and 20 mm in axial length. The test cell was placed horizontally to enable observations parallel to the horizontally planar methanewater interface formed in the cell. The temperature inside the test cell, T, was maintained within (0.2 K at a prescribed level by circulating a temperature-controlled ethylene glycol solution through a brass jacket integrating the test cell. T was measured with a thermocouple, inserted through a port at the bottom of the test cell into the bulk of the liquid-water phase inside the cell. The pressure inside the test cell, p, was measured with a strain-gauge pressure transducer to an accuracy of (0.06 MPa. Each experimental run was initiated by charging the test cell with liquid water and methane gas. Pouring in 4 cm3 of liquid water resulted in a pool of liquid water forming at the lower portion inside the test cell. The air in the test cell was replaced with methane gas supplied from a high-pressure bomb through a pressure-regulating valve, by repeating the pressurization of the cell with methane and evacuating it from the cell. Pressure p was then set at a prescribed level in the range from p ) 6 to 10 MPa. T was decreased to 263 K to form a hydrate (and simultaneously ice) and then raised to 1-2 K higher than Ttri, the triple methane + hydrate + liquidwater equilibrium temperature corresponding to p. After the formed hydrate crystals were dissociated, T was set at Ttri and then kept for several tens of hours, during which the test cell was manually oscillated intermittently to ensure the saturation of liquid water with methane. T was then reduced to ∼273.5 K to observe the formation and growth of hydrate crystals in the cell. The formation and growth of hydrate crystals were observed and recorded using a stereomicroscope (Nikon, model SMZ10) and a CCD video camera (Elmo Co. Ltd., model CN401), connected together on a common horizontal stage.

Results and Discussion Formation and Growth of Hydrate Crystals at the Methane-Water Interface. In most of the ex-

Figure 2. Sequential videographs of the growth of columnar methane-hydrate crystals from the hydrate film intervening between methane gas and the liquid-water phase into liquid water presaturated with methane. p ) 6.9 MPa, T ) 273.5 K. The time lapse after the hydrate nucleation at the methanewater interface is indicated below each videograph.

perimental runs, the following process of hydrate formation and growth was commonly observed, irrespective of the pressure levels set in the experimental system. Hydrate crystals initially formed at the methane-water interface and then grew into a thin film between the methane gas and liquid water. Hydrate crystals, each of which was apparently a single crystal, subsequently grew from the water-side surface of the hydrate film protruding into the liquid-water phase. Observations of this process of hydrate formation and growth obtained at each of three levels of the system pressure are presented below. Figure 2 displays a sequence of hydrate formation and growth observed at a methane-water interface of T ) 273.5 K and p ) 6.9 MPa. We observed that hydrate crystals formed at the methane-water interface grew into a thin film covering the interface within several tens of seconds (Figure 2a). The hydrate film formation was followed by the growth of columnar hydrate crystals, from the hydrate film into the bulk of liquid water (Figure 2b-d). The columnar hydrate crystals grew for several hours. Four hours after the initial hydrate formation, the columnar hydrate crystals had grown to 0.1 mm in thickness (cross-sectional dimension) and 0.3-0.4 mm in axial length. Figure 3 presents the observational results obtained at T ) 273.7 K and p ) 8.2 MPa. The process of hydrate formation and growth under this thermodynamic condition was similar to that at T ) 273.5 K and p ) 6.9 MPa. Following the formation of a hydrate film at the methane-water interface, the growth of columnar hydrate crystals from the hydrate film into liquid water was observed (Figure 3a). The growth of the columnar hydrate crystals in their axial direction nearly ceased in a few tens of minutes after the initial hydrate formation (Figure 3b), and then polygonal or polyhedral hydrate crystals grew from the sides of the first-grown columnar hydrate crystals (Figure 3c,d). A typical observational result at the highest pressure, p ) 9.7 MPa (T ) 273.3 K), set in the present study is illustrated in Figure 4. As observed at lower pressures, the hydrate film formation and subsequent growth of

Clathrate Hydrate Crystal Growth

Figure 3. Sequential videographs of the growth of methanehydrate crystals into liquid water presaturated with methane. p ) 8.2 MPa, T ) 273.7 K. The time lapse after the hydrate nucleation at the methane-water interface is indicated below each videograph.

Figure 4. Sequential videographs of the growth of dendritic methane-hydrate crystals into liquid water presaturated with methane. p ) 9.7 MPa, T ) 273.3 K. The time lapse after the hydrate nucleation at the methane-water interface is indicated below each videograph.

hydrate crystals from the hydrate film into liquid water were observed. However, the morphology of the hydrate crystals that grew into liquid water was no longer columnar, but dendritic. Dendritic hydrate crystals having axial lengths of 1-2 mm became noticeable within a few minutes after the initial hydrate formation (Figure 4a). The growth of the dendritic crystals in the axial direction of their main branch nearly ceased within some tens of minutes after the initial hydrate formation (Figure 4b). Subsequently, the change process in the morphology from dendrites to polyhedra was observed (Figure 4c,d). The crystal growth of methane hydrate presented above is believed to depend on a mechanism of mass transfer of dissolved methane to the hydrate-crystal surfaces in contact with liquid water presaturated with

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methane. The present observations of the methanehydrate crystal growth are interpreted below, based on the above-mentioned mechanism originally presented by Ohmura et al.4 for hydrate-crystal growth in an HCFC-141b + water system. Since the liquid water was equilibrated with methane at the three-phase equilibrium temperature, Ttri, corresponding to the system pressure, the methane concentration in liquid water must be cgs, the solubility of methane in liquid water in equilibrium with methane gas phase at that temperature and pressure. Concentration cgs is higher than the concentration of methane in liquid water in equilibrium with methane hydrate, cgs(h), at any temperature below Ttri, because cgs(h) has a positive temperature dependency. (This temperature dependency was previously predicted theoretically11,12 and recently confirmed experimentally.13-16) Consequently, the liquid-water phase is supersaturated with methane with respect to methane-hydrate formation when the experimental system is cooled to Tex ( 0.009. This is generally consistent with the preliminary version of Mcr-based crystal morphology classification presented by Ohmura et al.10, which predicted that dendritic crystals may be observed for Mcr > ∼0.01. This consistency proves the utility of the index for predicting the morphology of the hydrate crystals grown in liquid water presaturated with a guest substance. However, Mcr is a nondimensional index representing the mass-transfer driving force for the growth of hydrate crystals in the guestsaturated liquid water, but the mass-transfer coefficient is not included in this index. The mass-transfer coefficient should depend on various factors including convection around the hydrate crystals growing in liquid water and/or guest-in-water concentration. Thus, a more rigorous analytical and/or numerical approach to the conjugate mass-transfer/hydrate-crystal-growth process should be necessary for more detailed discussion on the variations in morphology of hydrate crystals grown in guest-saturated liquid water. Hydrate Formation on the Inner Wall of the Test Cell in Contact with Water. In some experimental runs, hydrate formed on the inner wall of the cell in contact with liquid water prior to hydrate formation at the methane-water interface. A typical observation of hydrate formation of this category, obtained at p ) 6.1 MPa and T ) 273.3 K, is demonstrated in Figure 5. Figure 5a presents the hydrate crystals formed on and then detached from the inner wall and subsequently floating in a liquid-water phase. The detached hydrate crystals were in the form of small particles, each having a size of 50 µm (the dimension of the projected area of the hydrate particles). The videograph is not clear enough to identify the shape of each of the crystals, but they were presumably polyhedra. The hydrate crystals floating in the liquid water reached the methane-water interface, where they started to grow laterally to form a polycrystalline hydrate film covering the interface (Figure 5b). Once the interface is covered with the

Figure 5. Sequential videographs of the growth of methanehydrate crystals in liquid water presaturated with methane. p ) 6.1 MPa, T ) 273.3 K. (a) Hydrate crystals formed on the inner wall of the test cell and then floated in liquid water. (b) Hydrate crystals floating in liquid water. The crystals reaching the methane-water interface grew into a hydrate film covering the methane-water interface. The crystals that floated after the formation of the hydrate film were trapped beneath the film. (c-f) Subsequently, growth of the hydrate crystals thus trapped beneath the hydrate-covered methane-water interface was observed. The time lapse after the hydrate nucleation on the inner wall of the test cell in contact with liquid water is indicated below each videograph.

hydrate film, the hydrate crystals floating from the wall were trapped at the water-side surface of the hydrate film. The hydrate formation on the wall ceased within several minutes after the initial hydrate formation, and thus the supply of the hydrate crystals from the liquidwater phase to hydrate film also ceased (Figure 5c). The hydrate crystals thus trapped at the water-side surface of the hydrate film covering the methane-water interface continued to grow into the liquid-water phase for several hours (Figure 5d-f). This observation of the hydrate formation in liquid water instead of the hydrate formation at the methanewater interface indicates that methane hydrate can be formed exclusively with methane dissolved in liquid water. In other words, the methane gas phase is not necessary for hydrate formation, which was theoretically predicted based on the phase-equilibrium considerations11,12 and was experimentally confirmed in the porous media filled with guest-saturated liquid water.7,21,22 Conclusion Formation and growth of methane-hydrate crystals in liquid water presaturated with methane at the threephase equilibrium temperature were observed at T ) 273.5 K and p ) 6-10 MPa. In most of the experimental runs, hydrate-crystal growth similar to that previously reported in the systems with CO2 or fluorocarbons was

Clathrate Hydrate Crystal Growth

observed. Hydrate crystals first formed a thin polycrystalline layer between methane and water, and then hydrate crystals grew into the liquid-water phase from the hydrate film. Columnar morphology of the hydrate crystals grown into liquid water was observed at p ) 6-8 MPa. The morphology changed to dendritic at p ) 10 MPa. The nondimensional index relevant to the driving force for the hydrate-crystal growth, which was recently proposed by Ohmura et al.10 to predict the morphology of hydrate crystals grown in liquid water presaturated with a guest substance, was evaluated for the present observations as Mcr ) 0.006-0.008 at p ) 6.9-8.2 MPa where the observed hydrate morphology was columnar and Mcr ) 0.009 at p ) 9.7 MPa where the dendritic hydrate crystals were observed. These results generally coincide with the prediction by Ohmura et al.10 based on observations of hydrate crystals in CO2 or fluorocarbon systems. From this result, we conclude that the dendritic morphology is observed under the condition that Mcr > ∼0.01. Hydrate formation at the inner wall of the test cell in contact with liquid water, instead of the methanewater interface, was also observed in some experimental runs. This observation indicates that methane-hydrate formation is possible exclusively with the methane dissolved in liquid water. Acknowledgment. This study was supported by the Industrial Technology Research Grant Program in 2003 (Grant No. 03B64003c) from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The authors thank Mr. Katsunori Matsushita (Hokkaido Branch, Suzuki Shoko Co., Sapporo, Japan) and Mr. Junji Itoh (Itoh Keiki Kogyosho, Co., Sapporo, Japan) for their help in the experimental work and in the maintenance of the apparatus.

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