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Different Modes of Gas Hydrate Dissociation to Ice Observed by Microfocus X-ray Computed Tomography Hiroshi Ohno,*,†,‡ Hideo Narita,‡ and Jiro Nagao*,†,‡ †
Production Technology Team and ‡Methane Hydrate Research Center, National Institute of Advanced Industrial Science and Technology, 2-17-2-1 Tsukisamu-Higashi, Toyohiraku, Sapporo 062-8517, Japan
ABSTRACT To investigate the kinetics of gas hydrate dissociation to ice, we conducted in situ observations of argon and krypton hydrates decomposed at atmospheric pressure and 245 K using microfocus X-ray computed tomography (MFXCT). The MFXCT observations for Ar samples revealed encapsulation of shrinking pure hydrate cores by ice products, confirming the previously proposed model of hydrate dissociation to ice. In stark contrast, CT images taken for Kr samples revealed entirely different distributions of the two phases: preserved hydrates homogeneously trapped in the ice matrix. Results obtained in this work indicate that the mode of hydrate dissociation to ice depends on the hydrate systems, probably because of differences in interaction between gas and water molecules. SECTION Energy Conversion and Storage
as hydrates (also called clathrate hydrates) are nonstoichiometric solid compounds that form when small guest molecules of suitable size and shape are enclathrated by host cage structures comprising hydrogenbonded water molecules under appropriate conditions (pressure, temperature, and concentration).1-3 Over the past few decades, numerous studies have examined gas hydrates because of their involvement in widely various natural phenomena and human activities. Those studies have greatly enriched our knowledge of time-independent hydrate properties such as the structures and equilibrium thermodynamics of hydrates. However, many investigative challenges remain for time-dependent hydrate processes, including hydrate formation and decomposition.4 A remaining puzzle is the kinetics of gas hydrate dissociation to ice. Decomposition rates of gas hydrates are known to be suppressed considerably when systems are subjected to temperatures lower than the ice melting point (the so-called self-preservation effect).5-9 Previous reports have described that preservation behavior varies nonlinearly with temperature7 and pressure10 and that it depends strongly on the guestgas composition.11,12 Although the detailed mechanism remains unclear, a consensus holds that the ice produced by hydrate dissociation might play an important role in selfpreservation. A shrinking hydrate core mantled with an ice rind has been commonly assumed to be a model of hydrate dissociation to ice.8,13,14 The ice coating on the remaining hydrate prevents further dissociation either by maintaining internal gas pressure at or near the equilibrium pressure or by limiting gas diffusion through the reaction boundary. Such an ice barrier model is supported by confocal scanning microscopic (CSM) observations of methane hydrates destabilized by temperature ramping. Shimada et al.13 showed
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that CH4 hydrate was covered by small ice particles in the initial stage of decomposition. These particles subsequently grew into a dense sheet because of sintering, particularly at temperatures higher than 230 K. Additionally, they observed that the amount of CH4 gas released by hydrate dissociation decreased when the ice sheet formed. More recently, Falenty and Kuhs10 reported the evolution of ice films on decaying CO2 hydrates, as observed using scanning electron microscopy (SEM). In contrast, Stern et al.11 reported from their SEM observations of self-preserved (partly dissociated) methane hydrate particles that no evidence was found for ice-rind development around individual hydrate grains. Nevertheless, detailed distributions of hydrate and ice phases during decomposition remain unclear because previous observations were limited to specimen surfaces. Additionally, it is extremely difficult to differentiate the two components using microscopy. In this Letter, we repot the first experimental attempt to elucidate the internal structure of self-preserved hydrates using microfocus X-ray computed tomography (MFXCT). Figure 1 depicts powder X-ray diffraction (PXRD) profiles of the as-grown and dissociated hydrates. For as-grown samples, observed diffraction peaks were all from clathrate structure II (Figure 1), which shows that the synthetic methods produced pure hydrates. With elapsed time at atmospheric pressure and 245 K, diffraction peaks assigned to hexagonal ice appeared and became stronger, although hydrate PXRD intensities decreased because of hydrate dissociation to ice (Figure 1). Using the relation between the Received Date: November 25, 2010 Accepted Date: January 7, 2011 Published on Web Date: January 11, 2011
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Figure 2. Hydrate-to-ice conversion ratio versus time for hydrate samples dissociated at atmospheric pressure and 245 K. The open circles and crosses, respectively, show estimations for the argon and krypton systems. The vertical axis between 0.95 and 1 is expanded in the inset.
Figure 1. PXRD profiles of (a) Ar and (b) Kr hydrates dissociated for a desired period of time at atmospheric pressure and 245 K. The asterisks denote signals from hexagonal ice.
The MFXCT observations of argon samples are consistent with the hydrate-core/ice-rind model. Results show that CT images of Ar hydrate particles darkened over time, as a rule, from their peripheries (Figure 3a-e), which suggests that hydrate dissociation started from the surfaces. Subsequently, the ice formation proceeded to the particle interior. The CT images show that hydrate cores shrank and finally disappeared (Figure 3a-e). As predicted from PXRD measurements (Figure 2), no hydrate was observable at the end of measurements (∼10 h). At the interior of hydrate samples (e.g., the area shown in Figure 3), the relative CT values (the average CT values for a dissociating hydrate, bdh, divided by that for the reference ice, bi) remained unchanged before the arrival of the reaction boundary between a hydrate core and an ice envelope (before 200 min in Figure 4), which indicates no internal breakdown of hydrate cores. Relative CT values also show that products of the hydrate dissociation had slightly lower bdh/bi values than the reference ice (after 400 min in Figure 4). This fact might be attributed to the probable inclusion of pores and bubbles inside ice products because the density of water molecules in the hydrate phase is lower than that in the ice phase (water density of hydrate structure II is ∼0.79 g/cm3, whereas that of hexagonal ice is 0.92 g/cm3).1-3 In fact, previous SEM-based reports have described that dissociated gas hydrates have porous and frothy textures.10,11 Another type of hydrate and ice distribution was observed for the krypton system. The PXRD analyses indicate the presence of strongly preserved hydrates in the second regime of hydrate dissociation (after 30 min in the Figure 2 inset). Indeed, the CT values of krypton samples in the second decomposition step were higher than those of the reference ice (Figures 3f-h and 4). The bdh/bi values decreased slowly with time, but they remained higher than 1 after approximately 1 day (Figure 4), which again suggests the survival of hydrates at the end of observations. It is particularly interesting that no region with a considerably higher CT value, corresponding to a concentrated hydrate phase (a hydrate
hydrate and ice phase fractions and the relative PXRD intensity (Supporting Information), the hydrate-to-ice conversion ratio was estimated from the PXRD data as a function of time (Figure 2). For argon samples, the estimations show that the dissociation rate decreased gradually with time, but hydrates decomposed completely in 12 h (Figure 2). In contrast, results showed that the dissociation of krypton samples occurred in two steps: >95% of Kr hydrates decomposed rapidly within 30 min, whereas the remaining hydrates were preserved strongly thereafter (Figure 2 inset). The Kr hydrates were detectable even after several days (not shown). The dissociation rate of argon samples was considerably lower than that of krypton samples during the main dissociation period (Figure 2), although the Ar system is thermodynamically less stable under the pressure and temperature conditions for dissociation (0.1 MPa and 245 K) than the Kr system is. (The respective dissociation temperatures of Ar and Kr hydrates at atmospheric pressure are approximately 149 and 204 K.)15,16 Similar observations were reported by Takeya and Ripmeester.12 Although 3D data were obtained using MFXCT, typical cross-sectional CT images are presented in this report to highlight the main findings (Figure 3). The CT images show a 2D distribution of the X-ray linear attenuation coefficient, which is a function of density of a subject, its chemical composition, and the photon energy of the X-ray beam. The Ar and Kr hydrate phases have higher CT values (brighter pixels in a CT image) than those of the ice phase because of their higher density and larger X-ray mass absorption coefficient. Images taken immediately after sample heating are blurred considerably and show artifacts (ghosts and flares), probably because rapid hydrate dissociation caused marked changes in distributions of samples during CT scans. Therefore, results of samples exposed at 245 K for longer than 30 min are shown.
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Figure 3. Cross-sectional MFXCT images of (a-e) Ar and (f-h) Kr hydrate samples during decomposition to ice at atmospheric pressure and 245 K: (a) 30, (b) 120, (c) 180, (d) 270, (e) 450, (f) 30, (g) 480, and (h) 1260 min. The asterisks denote reference hexagonal-ice particles. The open squares enclose areas analyzed for relative CT values. (See Figure 4.) The scale bars are 300 μm.
core), was observed in CT images of krypton samples (Figure 3f-h), unlike the case for argon samples (Figure 3a-e). Independent of the size and location (interior or periphery) of the dissociating-hydrate particles, bdh/bi values were similar (Figure 3f-h). Therefore, we infer that some Kr hydrates were trapped in ice products during the course of the first dissociation regime (before 30 min in Figure 2) and that the preserved hydrates were distributed uniformly in sample particles, as observed with a spatial resolution of several micrometers. This finding is at variance with the previously reported concept that a self-preserved sample particle is divisible into two distinct layers: an outer ice layer and a central hydrate core.8,13,14 Previous SEM observations of mostly decomposed CO2 hydrates suggest that remaining hydrates were present as microinclusions in the ice matrix.10 They appear to be consistent with our results. However, further work with a technique providing explicit information about compositions will be necessary to confirm the microstructural distributions of hydrates left in the ice. Observed differences in the rate of gas hydrate dissociation have often been attributed to differences in the texture of ice rinds: the thicker and denser the ice sheet, the stronger the shielding effect.8,10,13,14 This idea, based on the simple hydrate-core/ice-rind geometry, might explain the observation that the dissociation rate of argon samples was significantly lower than that of krypton samples in the first reaction regime (Figure 2), although spatial distributions of hydrate and ice phases in the Kr system during rapid decomposition are unknown because of the lack of MFXCT data. Our preliminary SEM observations indicate that ice from Ar hydrate dissociation has a less granular (more cohesive) appearance than that of Kr hydrate decomposition, whereas
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Figure 4. Average CT values of dissociating hydrate samples (area presented in Figure 3), bdh, relative to those of reference hexagonal ice, bi.
both as-synthesized samples exhibit similar dense features (not shown). However, the ice envelope model is not applicable for the second dissociation regime of krypton samples. The homogeneous mixture of preserved hydrates and ice products was observed only in the krypton system, as described above. To explain these facts, another physicochemical mechanism might be necessary. If the distribution of ice during hydrate dissociation is related to the degree of self-preservation, then a new question arises: why does the geometry of ice products depend on the hydrate system?11,12 It is considered that water molecules from hydrate dissociation transform into hexagonal ice via vapor- or liquid-like phases because a direct solid-solid transition might require the hurdling of a high energy barrier to nucleate a new solid phase. Increasing evidence indicates
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the formation of supercooled liquid water during gas hydrate dissociation below the ice melting point; examples are an electron spin resonance investigation by Takeya et al.17 and visual observations by Melnikov et al.18 Assuming the presence of liquid-like water phase, gas molecules will be released into the supercooled liquid water by hydrate dissociation; subsequently, they diffuse to the atmosphere.19,20 This process might affect the ice product morphology. For example, some gas molecules dissolved in metastable liquid water might nucleate bubbles and be trapped in ice when crystallizing. Probably a larger amount of krypton gas than argon gas is retained in the water phase because of lower diffusivity in liquid water.21 Therefore, more bubble inclusions exist in ice products for the Kr system, engendering a weaker ice-shielding effect. It was also suggested, based on observations of frost-like ice formation on dissociating CH4 hydrates, that the ice phase is produced by water evaporation-condensation processes during hydrate dissociation.13 A crystal habit of hexagonal ice grown in a gas phase can be modified according to the type of atmospheric gas.22-24 Takeya and Ripmeester25 proposed that possible modification of ice morphology caused by this phenomenon might change the dissociation rates of gas hydrates. In the case of an argon atmosphere, its influence on growing ice shapes was reported,23 although the effects of krypton gas have not been tested. Several possible causes of the atmospheric dependence of ice morphology have been proposed (e.g., change in diffusivity of water vapor through foreign gas22 and a selective poisoning of ice growth sites by adsorption of gas species24), but the primary mechanism remains unclear. Herein we demonstrated that MFXCT measurement is a powerful tool to investigate the internal structure (spatial distributions of hydrate and ice phases) of self-preserved gas hydrates. The dissociation rate of Ar hydrates was observed to decrease monotonically with time, although twostep dissociation was observed for Kr hydrates: rapid decomposition was followed by a strong self-preservation regime. The MFXCT observations for the argon samples show the development of ice envelopes around pure hydrate cores during dissociation processes. Furthermore, CT images taken for krypton samples suggest that strongly preserved hydrates were distributed uniformly in the matrix of ice products, although no evidence exists for such a phenomenon in the argon system. These observations indicate that the two systems' modes of hydrate dissociation differ markedly, probably because of differences in the mutual interaction of gas and water molecules. Taken together, these findings present important implications for understanding of the mechanism of gas hydrate dissociation to ice. These results are therefore important to develop methods to control hydrate decomposition in industrial processes.
Electron) set at 253 K and pressurized with feed gas to a desired pressure (10 MPa for the Ar system and 3 MPa for the Kr system). After hydrate nucleation was observed as a pressure drop, the system temperature was maintained for 7 h and then increased to the ice melting point of 273 K to promote hydrate formation. The freeze-thaw cycle was repeated until the hydrate gas consumption ceased. Microfocus X-ray Computed Tomography Imaging. In situ observations of hydrate dissociation processes were performed using a MFXCT system equipped with a white X-ray source26 (SMX-225CTX-SV; Shimadzu). The synthetic hydrates were mixed with reference ice particles of diameter >500 μm. They were then loaded into a sample cell made of a fluorocarbon polymer (PFA) tube (4 mm inner diameter, 6 mm outer diameter, and 10 mm length) at a temperature of
