Article pubs.acs.org/JPCC
Anomalously Preserved Clathrate Hydrate of Natural Gas in Pellet Form at 253 K Satoshi Takeya,*,† Akio Yoneyama,‡ Kazuhiro Ueda,‡ Hiroko Mimachi,§ Masahiro Takahashi,§ Kenichi Sano,§ Kazuyuki Hyodo,∥ Tohoru Takeda,⊥ and Yoshito Gotoh† †
National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Hitachi Ltd., 2520 Akanuma, Hatoyama, Saitama 350-0395, Japan § Mitsui Engineering and Shipbuilding Co., Ltd., 1 Yawatakaigandori, Ichihara, Chiba 290-8531, Japan ∥ High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan ⊥ Kitasato University, 1-15-1 Kitasato, Sagamihara, Kanagawa 228-8555, Japan ABSTRACT: Storage of natural gas (NG) is required for a range of applications. In this study, we succeeded to maintain NG within water cages of anomalously preserved NG hydrate crystal for more than three weeks under thermodynamically unstable conditions at 253 K and atmospheric pressure. This anomalous preservation phenomenon of NG hydrate pellet was measured quantitatively by powder X-ray diffraction and gas chromatography as well as measuring the change in weight caused by hydrate dissociation. In addition, two different types of phase contrast X-ray imaging techniques, X-ray interferometric imaging and diffraction enhanced imaging, also revealed that the NG hydrate was maintained inside the pellet. Also, scanning electron microscopy showed that the outer ice layer formed contained many pores while the internal microstructure of NG hydrate pellet was dense without pores. These experimental results suggest that formation of the outer ice layer may not be related to anomalous preservation of NG hydrate, but formation of pore space may cause hydrate dissociation. thickness of less than about 100 μm using phase-contrast X-ray imaging techniques.28 However, it has been reported that C2H6 and C3H8 hydrates do not show the preservation phenomenon, and a mixture of C2H6 or C3H8 with CH4 diminish the preservation of CH4 hydrate12,22,29 except the CH4 + C2H6 + C3H8 hydrate system formed from water including surfactant.30 This implies that the preservation phenomenon of NG hydrate should not be expected without any additives. In this respect, it is important to study the dissociation process of NG hydrate. Study of this phenomenon from a physicochemical point of view will also increase understanding of the chemistry of water and hydraterelated climate change on Earth.5 Herein, NG hydrate pellets stored at 253 K, which is outside their thermodynamically stable zone, were examined. Gas chromatography and powder X-ray diffraction (PXRD) were used for quantitative analysis of an ice/hydrate mixture. Nondestructive observation of preserved NG hydrate was performed using two different phase-contrast X-ray imaging techniques, diffraction enhanced imaging (DEI), and X-ray interferometric imaging (XII). These techniques enabled ice coexisting with CH4 hydrate to be visualized because of their high-density resolution.28 Cryo-field emission scanning electron
1. INTRODUCTION Clathrate hydrates, also known as gas hydrates, are host−guest inclusion compounds that are crystalline materials consisting of water molecules that incorporate guest molecules inside hydrogen-bonded water cages.1 Natural gas (NG) hydrate, which mainly contains methane (CH4) with some ethane (C2H6), propane (C3H8), butane (C4H10), and pentane (C5H12) as guest molecules, is a possible source of NG.2 NG hydrate is attracting attention for use in storage and transportation applications3−6 because it is composed of only NG and water, making it one of the most favorable and environmentally friendly candidates for NG storage. It is known that some gas hydrates can be preserved at atmospheric pressure just below the melting point of ice (273.2 K), even though this is well outside their zone of thermodynamic stability.7 This phenomenon has been termed self-preservation8 and anomalous preservation,9 and control of this effect is important for practical use of gas hydrate as an unconventional medium for gas storage and transportation. The mechanisms of preservation of CH4 hydrate and CO2 hydrate have been extensively studied.9−27 It has been concluded that the formation of an ice layer with different morphology after hydrate dissociation and gas diffusion through the ice layer control the preservation of gas hydrate below about 240 K.10,11,13,15,18 Recently, we found that anomalously preserved CH4 hydrate pellets can be kept for a month by being enveloped by a thin ice layer with an average © 2012 American Chemical Society
Received: March 8, 2012 Revised: May 28, 2012 Published: May 31, 2012 13842
dx.doi.org/10.1021/jp302269v | J. Phys. Chem. C 2012, 116, 13842−13848
The Journal of Physical Chemistry C
Article
atmosphere of N2 gas at less than 100 K. The powder was loaded into a specimen holder made of Cu. The sample was then placed in a cryo-chamber attached to an X-ray diffractometer. PXRD measurements were performed in the θ/2θ step scan mode with a step width of 0.02° over a 2θ range of 6° to 60° using Cu Kα radiation and parallel beam optics (40 kV, 40 mA; Ultima III, Rigaku, Japan). To analyze the unit cell parameters of the hydrate crystal and mass fraction of ice/ hydrate in the pellet sample, Rietveld analysis taking into account the crystal structures of NG hydrate and hexagonal ice (P63/mmc) was performed using the RIETAN-FP program.31 Temperature-dependent PXRD measurements from 123 to 243 K were performed every 10 K under isothermal temperature conditions with a total scan time of 2.5 min. Measurements from 123 to 163 K were performed under vacuum and those from 163 to 243 K were performed under an atmosphere of dry N2 gas to prevent vapor condensation on the sample surface. For gas analysis, a pellet sample was allowed to dissociate at room temperature, and the gas coming from the pellet was measured by a gas chromatograph (Micro GC CP4900, Varian, Walnut Creek, CA). For nondestructive internal imaging of NG hydrate pellets, phase contrast X-ray images were collected using a monochromatic synchrotron X-ray at a vertical wiggler beamline (BL14C) at the Photon Factory in Tsukuba, Japan. DEI allows a wide dynamic range of densities to be probed, which enables observation of samples containing regions with large density gradients, and XII is suitable for detecting gradual phase shifts and the absolute density of materials with high resolution.32 For DEI, an incident monochromatic X-ray beam enlarged horizontally by an asymmetric Si(220) crystal was used to irradiate the sample. The X-ray beam that passed through the sample was diffracted by a Si(220) analyzer crystal and then entered a charge-coupled device (CCD)-based X-ray imager (detector dimensions 12.5 × 12.5 μm2 per pixel).33 A phase map was produced using two images obtained by scanning the analyzer crystal at each position for 1 s, giving a total measurement time of about 30 min. The field of view of the DEI system was 25 × 35 mm with a spatial resolution of 40 μm. To obtain three-dimensional (3-D) phase contrast X-ray computed tomography (CT) images, the pellet was rotated 180° in 0.72° steps. For XII, an X-ray interferometer was used to detect phase shifts by converting phase shifts into interferograms. One of the two generated interference patterns was detected using a CCD-based X-ray imager, whereas the other was detected using a feedback positioning system.34 Three fringe scans were used to obtain a phase map, which is a spatial distribution of the phase shift. The field of view of the XII system was 25 × 35 mm, with a spatial resolution of 40 μm. To obtain a phase contrast X-ray CT image, the NG hydrate sample was rotated 180° in 0.45° steps. Three fringe scans with an exposure time of 1 s were used to obtain each interference pattern, and the total measurement time was about 30 min. During each DEI and XII measurement, the samples were immersed in liquid methyl acetate (99.5%, Kishida Chemical Co., Japan) maintained at 193 ± 1 K. The presence of liquid methyl acetate around the sample prevented undesirable outline contrasts of the X-ray from the outer surface of the sample, allowing internal observation. More experimental details are reported elsewhere.28 Observations of microstructure within the NG hydrate pellet were performed using an FE-SEM (JSM-6301F, JEOL, Japan) equipped with a cryostat (CRU-40). Before observation, each
microscopy (cryo-FE-SEM) was also used for high spatialresolution imaging. Experimental results obtained in this study revealed anomalous preservation in NG hydrate pellets, but not in NG hydrate powder, and the mechanism of NG hydrate preservation is discussed.
2. EXPERIMENTAL SECTION NG hydrate pellets were formed as follows. NG hydrate slurry (containing about 3−6 wt % NG hydrate) was formed in a high-pressure reactor by mixing liquid water and NG (89.83% CH4, 5.59% C2H6, 3.10% C3H8, 0.64% iso-C4H10, 0.81% nC4H10, and 0.03% iso-C5H12). The hydrate slurry was mechanically compressed by a piston with a diameter of 33 mm to form cylindrical NG hydrate pellets (Figure 1). This
Figure 1. (a) Schematic diagram of the experimental apparatus used to synthesize NG hydrate pellets. (b) Cylindrical NG hydrate pellet with a diameter of 3.3 cm.
process was performed at 281 ± 0.5 K under 5.4 MPa of natural gas to avoid hydrate dissociation. After a rapid drop in pressure from 5.4 MPa to atmospheric pressure at 253 K, the cylindrical pellets were stored at 253 K at atmospheric pressure under a mixture of air and natural gas formed by hydrate dissociation for up to three weeks. Each sample was transferred under an atmosphere of N2 gas at a temperature of less than 100 K before measurements were performed. Characterization of the NG hydrate sample was performed by PXRD and gas chromatography. For PXRD measurements, a hydrate pellet was finely ground into a powder under an 13843
dx.doi.org/10.1021/jp302269v | J. Phys. Chem. C 2012, 116, 13842−13848
The Journal of Physical Chemistry C
Article
The lattice constants of the NG hydrate as a function of temperature are shown in Figure 3. Here, the density of NG
sample was immersed in liquid nitrogen and attached to a brass sample holder. The sample was quickly transferred into a sample stage in an evacuated and precooled cryo-preparation section. The sample was fractured using a cold blade to produce fresh sample surfaces for observation without contamination caused by surface condensation. Observations were performed in the cryo-stage at about 140 K under a pressure of ∼10−4 Pa at an acceleration voltage of 5 keV. Measured spots were reobserved to confirm that the electron beam did not cause surface etching.
3. RESULTS AND DISCUSSION 3.1. Characterization of NG Hydrate. The crystal structure of NG hydrate formed was identified by PXRD as cubic structure II hydrate (Fd3m) with a lattice constant of 17.1088(5) Å at 100 K (Figure 2). Even for a freshly prepared
Figure 3. Temperature dependence of the lattice constant of NG hydrate measured by PXRD and corresponding densities calculated using a cage occupancy for a large cage of 50.7% CH4, 33.3% C2H6, 13.5% C3H8, 1.4% iso-C4H10, 1.1% n-C4H10, and ∼0% iso-C5H12 and for a small cage of 90% CH4. Here, the densities of CH4 hydrate (dotted line) and hexagonal ice (dashed line) are also shown for comparison.
hydrate as a function of temperature was also estimated using the same cage occupancy as that estimated above. The densities of NG hydrate are higher than those of hexagonal ice and CH4 hydrate (Figure 3), and the difference in density decreases as the temperature increases. The difference in density between the hydrate and ice is an important factor to distinguish clathrate hydrate and ice using X-ray imaging techniques. For example, in an earlier study, a single crystal of structure II air hydrate with a density of 0.937 g/cm3 was distinguished from hexagonal ice (0.923 g/cm3) at 233 K39 by XII, even though the difference in density between the hydrate and hexagonal ice is just 0.014 g/cm3.40 In this work, the difference in density between NG hydrate (0.944 g/cm3 at 193 K) and hexagonal ice (0.927 g/cm3 at 193 K) is 0.017 g/cm3. Thus, it is possible to distinguish NG hydrate coexisting with ice by XII with a spatial resolution of 40 μm. 3.2. Time-Dependent Change of NG Hydrate. The change in the mass fraction of NG hydrate pellets being stored at 253 K under an air atmosphere was measured. Figure 4 shows that more than 95% of the NG hydrate remained even after three weeks of being kept outside its zone of thermodynamic stability. As estimated above, the NG hydrate
Figure 2. PXRD pattern of NG hydrate, which was stored under atmospheric pressure at 253 K for 40 min, obtained at 100 K. The plus signs (+) denote the observed intensities; the black solid line was calculated from the crystal structure model. The bottom curve shows the deviation between the observed and calculated intensities, where blank regions are excluded from the analysis because of the presence of Bragg reflections from the Cu sample holder. The upper dashes represent the calculated peak positions for a structure II hydrate and the lower dashes represent those for hexagonal ice.
NG hydrate pellet, diffraction peaks from hexagonal ice (Ih), which result from untransformed water in the hydrate or hydrate dissociation, were also detected. Gas chromatography measurements showed that the NG hydrate contained 82.4% CH4, 11.9% C2H6, 4.8% C3H8, 0.5% iso-C4H10, 0.4% n-C4H10, and
