DOI: 10.1021/cg101310z
Crystal Growth of Clathrate Hydrate at the Interface between Hydrocarbon Gas Mixture and Liquid Water
2011, Vol. 11 295–301
Kota Saito, Masatoshi Kishimoto, Ryo Tanaka, and Ryo Ohmura* Department of Mechanical Engineering, Keio University, Yokohama 223-8522, Japan Received October 6, 2010; Revised Manuscript Received November 5, 2010
ABSTRACT: This paper reports the visual observations of the formation and growth of clathrate hydrate crystals on the surface of a water droplet exposed to a methane þ ethane þ propane gas mixture. The compositions of the methane þ ethane þ propane gas mixtures are (i) 99.47:0.51:0.02, (ii) 94.1:5.8:0.1, and (iii) 90:7:3 molar ratio. The nucleation of the hydrate first occurred at a random point on the water droplet and then the hydrate grew to form a polycrystalline layer covering the droplet. We visually observed the individual crystals that constitute the polycrystalline hydrate layer and classified the morphology of the hydrate crystals depending on the system subcooling ΔTsub and system pressure. It was found that the size of the individual mixed-gas hydrate crystals decreased with the increasing ΔTsub as observed for the simple hydrates each formed with methane, ethane, or propane. The size of the individual crystals decreased with the increasing molar ratio of methane in the gas mixtures. The mixed-gas hydrate crystals exhibited a morphology different from that of the simple methane hydrate. We also measured the lateral growth rates of the hydrate-film propagation. In any of the systems, the rates increased with increasing ΔTsub as observed in other hydrate-forming systems.
*To whom correspondence should be addressed. Address: Department of Mechanical Engineering, Keio University 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. Phone: þ81-45-566-1813. E-mail: rohmura@ mech.keio.ac.jp.
smaller hydrate crystals. Furthermore, the morphology of the hydrate crystals should have a significant influence on the storage stability of the natural gas hydrate. Consequently, it is important to understand the systematic behavior of the morphology of hydrate crystals formed at the guest-water interface. Generally, hydrate crystals preferentially form at the guestwater interface as a polycrystalline thin film. After the hydrate film covers the interface, the hydrate crystals may grow into the liquid water in various forms such as columns, dendrites,10,11 when the water is saturated with guest substances prior to the hydrate formation. Several studies12-15 on the morphology of the hydrate crystals at the guest gas-water interface have previously been reported. Servio and Englezos12 observed the morphology of the methane hydrate and carbon dioxide hydrate crystals formed on water droplets exposed to gaseous methane and carbon dioxide by changing the pressure. They reported that the size of a water droplet had no pronounced effect on the induction time or crystal morphology of the hydrates, while the driving force significantly affected the crystal morphology. Freer et al.13 also observed methane hydrate crystals formed at the interface between gaseous methane and liquid water. They reported that the crystal morphology was different under different driving force conditions and the growth rate of the hydrate-film was proportional to the driving force. Peng et al.14 observed hydrate-film growth on the gas bubble surfaces exposed to a liquid water at different temperatures. They concluded that the hydrate-film growth rate of the mixed-gas CH4 þ C2H4 hydrate films was slower than that of the pure gases (CH4 or C2H4) for the same driving force, and the rate of the CH4 þ C3H8 mixed-gas hydrate-film growth was the slowest. In addition, their results showed that the thickness of the hydrate-film was inversely proportional to the driving force and the thicknesses of mixed-gas hydrate films were thicker than those of the pure gases. Tanaka et al.15 reported detailed observations of the morphology of individual hydrate crystals on the surface of a water droplet exposed to guest gases, that is, methane, ethane, or propane. Their results
r 2010 American Chemical Society
Published on Web 11/30/2010
Introduction Clathrate hydrates are crystalline solid compounds consisting of water molecules called the “host” that are hydrogen-bonded to form cages and enclose different molecules called “guests” within the cages. Hydrocarbons and noble gases are the typical guest substances that form clathrate hydrates. Clathrate hydrates have several unique properties, such as a high gas storage capacity, guest-substance selectivity, and a large heat of formation/decomposition. Thus, industrial technologies utilizing hydrates are proposed, such as the transportation and storage of natural gases1 and hydrogen,2 development of heat pump/refrigeration systems,3 ground/ocean sequestration of carbon dioxide,4-6 etc. With reference to these developments of the novel hydrate-based technologies, we need to understand the formation/decomposition characteristics of the clathrate hydrates. Among the various aspects of the formation characteristics, this study focuses on the morphology of the hydrate crystals and determines which factors control the morphology. “Crystal morphology” means the geometric configuration of crystals such as their sizes or shapes. The crystal morphology of hydrates is one of the important factors in the hydrate-based technologies as exemplified by the following example. A Japanese industrial group7,8 has been developing the technology for the transportation and storage of natural gas by forming a hydrate. They made the natural gas hydrate in the form of pellets, with a size of 20 mm, which had advantages over the powdery or slurry form in filling efficiency, storage stability depending on the “self-preservation effect9”, and quality assurance of the cargo in a ship. During the hydrate production process, it is required to dewater the natural gas hydrate formed in the powdery or slurry form. In this process, a greater dewatering power is required for the
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Figure 1. Schematic diagram of the apparatus.
indicated that the hydrate crystal morphology has a significant dependence on the system subcooling ΔTsub, and the size of the individual hydrate crystals decreased with the increasing ΔTsub irrespective of the guest substances. The pressure difference had no significant effect on the hydrate crystal morphology, and thus they concluded that the crystal morphology of the hydrate formed at the interface between the methane, ethane, or propane gas and liquid water are classified with ΔTsub as the common criterion. They also reported that the hydrate-film growth rate decreased with a decrease in ΔTsub. As reviewed above, the crystal morphology of the single gas hydrates12,13,15 and some mixed gas hydrates14 are reported. However, we find no previous study that systematically classified the morphology of the hydrate crystals formed in a situation of producing hydrates from natural gas. During hydrate formation from a gas mixture such as natural gas, the vapor-phase and hydrate-phase compositions continuously change, because of the preferential uptake of some species from the vapor-phase into the hydrate.16,17 For example, if the guest gas substance is a methane þ ethane þ propane gas mixture, propane will preferentially occupy the hydrate cavities.18 Thus, the composition of the vapor-phase in equilibrium with the hydrate becomes different from the feed gas (natural gas), and the methane ratio becomes rich because ethane and propane is preferentially taken into the hydrates over methane. The guest composition in the hydrate phase gradually changes during the continuous hydrate formation and is finally equal to the feed gas composition when the hydrate formation process achieves a steady state. Therefore, it is important to determine the morphology of the hydrates corresponding to the various vapor-phase compositions for developing the process design for the continuous production of natural gas hydrates. Variations in the hydrate crystal morphology may also have a significant relevance to the characteristics of the naturally occurring hydrates. Recently, Klapp et al.19,20 reported the scanning electron microscopy (SEM) observations of the recovered naturally occurring hydrate samples. Klapp et al.19 analyzed the natural hydrate samples recovered from various locations in the Gulf of Mexico. Their compositional analyses indicated that the structure II hydrate samples were composed of 70-80% methane and C2-C5 hydrocarbons of up to 30%, and the structure I hydrate samples of 98% methane or more. Their SEM observations revealed the difference in the microstructures of the hydrates depending on the compositions and crystallographic structures; that is, the structure II hydrates were less porous and had more dense structures. Klapp et al.20
reported that the grain size of the natural hydrate samples recovered from the Black Sea, Gulf of Mexico, and West Coast of North America (Hydrate Ridge, Offshore Oregon) are in the range of from 200 to 600 μm. This paper reports a visual study of the formation and growth of hydrate crystals formed at the interface between simulated natural gas (methane þ ethane þ propane mixture) and liquid water. The observed variations in the morphology of the hydrate crystals were classified based on the magnitudes of the driving force for hydrate crystal growth. Apparatus and Procedures The fluid samples used in the experiments were deionized and distilled liquid water and three ternary gas mixtures of methane, ethane, and propane of the following molar ratios, that is, 99.47:0.51:0.02, 94.1:5.8:0.1, and 90:7:3. These compositions were selected from the simulation results of the isobaric hydrate-forming operations using multiple guest substances.16 The mixed gas (i) 99.47% methane, 0.51% ethane, and 0.02% propane was the gas-phase composition in the steady-state condition of the hydrate-forming operation from a simulated natural gas, the mixed gas (ii) 94.1% methane, 5.8% ethane, and 0.1% propane was the gas-phase composition in a midstream process of the hydrate-forming operation, and the mixed gas (iii) 90% methane, 7% ethane, and 3% propane was simply the initial feed gas composition for the hydrateforming operation.16 Figure 1 is a schematic diagram of the apparatus. Figure 2 schematically illustrates the major portion of the experimental apparatus, that is, a test cell made of a stainless-steel cylinder and a pair of flange-type glass windows. The inner space of the test cell that holds the test fluids and hydrate crystals is 25 mm in diameter and 20 mm in axial length. The temperature inside the cylinder is controlled by a refrigerant circulating inside the brass jacket around the cylinder and the pressure is controlled by supplying the guest gas from a cylinder through a pressureregulating valve. A Teflon stage with a 6 mm diameter was placed inside the cylinder to keep the water droplet. Figure 3 depicts the configuration of the liquid water with methane þ ethane þ propane mixed gas placed in the test section. A water droplet of approximately 6 mm diameter was held on the Teflon stage placed in the test section. The temperature Tex inside the container was measured within the uncertainty of (0.2 K by a thermometer (platinum resistance thermometer) inserted from the bottom of cylinder directly under the stage. The pressure P inside the cylinder was measured
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within the uncertainty of (0.02 MPa by a strain gauge pressure transducer. The water droplet was placed as indicated in Figure 3. The air in the test cell was then replaced with the guest gas supplied from a high-pressure cylinder through a pressure-regulating valve by repeating the pressurization of the cell with the guest gas and evacuating it from the cell. P was then set to a prescribed level (specified in Table 1). T was first decreased to about 268 K to form a hydrate (and possibly simultaneously ice) and
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then raised to a level higher by 1 K than Teq, the guest gashydrate-liquid water three phase equilibrium temperature corresponding to the system pressure P, thereby dissociating the formed hydrate crystals. After visual confirmation of the dissociation of all the hydrate crystals, T was set at the prescribed level Tex that is lower by about 1.2-4.7 K than Teq (specified in Table 1) to observe the formation and growth of the hydrate crystals in the cell. We used this procedure to shorten the induction time for hydrate reformation by providing the system with a “memory” of the prior hydrate formation. As the experiments were performed with a batch procedure, the composition of the gas phase should change before/after the hydrate formation in the test cell. We estimated the change of the composition to be typically less than 7% for each of the components based on the mass balance calculations. In this calculation, the hydrate phase composition was estimated with a phase-equilibrium calculation program CSMGem,21 and the thickness of the formed hydrate film was assumed to be 100 μm referring to the previous work on the film thickness measurements for the hydrates formed at guest/ water interfaces.22,23 We monitored and recorded the formation and growth of the hydrate crystals using a CCD camera (Pixelink, model PL-A662) and a microscope (Elmo, model TVZ610M). The guest substance is a methane þ ethane þ propane (99.47:0.51:0.02, 94.1:5.8:0.1, or 90:7:3) mixed gas. We defined the system subcooling ΔTsub, the deficiency of the system temperature from the triple guest-hydrate-water equilibrium temperature corresponding to the system pressure (ΔTsub Teq - Tex) as the index of the driving force for the crystal growth. The lateral growth rates of the hydrate crystals formed at the guest-gas/water interface were determined from the optical observational records. The equilibrium temperature at a given experimental pressure was estimated with CSMGem.21 Results and Discussion Figure 4a,b shows the sequential images of the hydrate crystals formed with the methane þ ethane þ propane gas mixture of the molar ratio 99.47:0.51:0.02 that are growing on the surface of the water droplet. The experimental system temperatures are (a) Tex = 277.9 K, ΔTsub = 1.3 K and (b) Tex = 277.0 K, ΔTsub =2.2 K at the same pressure, P=4.5 MPa. As observed for the simple hydrates15 each formed with methane, ethane, or propane, the nucleation of the hydrate first occurred at a random point on the water droplet and then floated up to the apex of the water droplet along the surface. It should be noted that the densities of the hydrates formed with the gas mixture used in the present experiments are estimated to be 910940 kg/m3. In this estimation, the cage occupancies were predicted with CSMGem21 and the lattice constant of the structure II hydrate unit cell was assumed to be 1.73 nm referring to the powder X-ray diffraction data on various structure II hydrates by Hester et al.24 The hydrate then grew down to
Figure 2. Schematic diagram of the main section of the apparatus.
Figure 3. Schematic illustration of the configuration of the fluid placed in the test section.
Table 1. Temperature and Pressure Conditions of the Experiments with Mixed Gas Systems (Methane þ Ethane þ Propane)a composition in molar ratio (methane/ethane/propane) (99.47:0.51:0.02)
(94.1:5.8:0.1)
(90:7:3)
P/MPa
Teq/K
ΔTsub/K
P/MPa
Teq/K
ΔTsub/K
P/MPa
Teq/K
ΔTsub/K
5.1 4.5 3.9
280.2 278.8 277.3
1.3-4.0 1.3-2.6 1.3-2.5
5.1 4.5 3.9
284.4 283.3 282.0
1.3-3.7 1.3-3.6 1.3-3.5
4.2 3.2 2.0
287.3 285.1 281.2
1.2-4.7 1.2-3.9 1.4-6.5
a
Equilibrium temperatures at the given experimental pressures.
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Figure 4. Difference in hydrate crystal growth depending on ΔTsub (mixed gas (99.47:0.51:0.02) system, P = 4.5 MPa).
Figure 5. Classification of hydrate crystal morphology based on ΔTsub (mixed gas (99.47:0.51:0.02) system).
form a polycrystalline layer covering the surface of the water droplet. A comparison of the observations at the two different degrees of the subcooling in Figure 4 indicated that the water droplet completely covered by the hydrate had more roughness at a smaller ΔTsub. The unevenness of the hydrate layer formed at ΔTsub =1.3 K is quite noticeable, while the hydrate layer formed at ΔTsub = 2.2 K was smooth. This may be ascribed to the morphology of the individual hydrate crystal changes significantly depending on the degree of ΔTsub, and the size of the individual mixed-gas hydrate crystals decrease with the increasing ΔTsub as observed for the simple hydrates15 each formed with methane, ethane, or propane. The time required
for the complete coverage of the water-droplet surface by the hydrate layer also significantly depended on ΔTsub. The time was shorter at the larger ΔTsub. This indicated that the lateral growth rate of the hydrate-film propagation (the rate of twodimensional hydrate crystal growth) increased with the increasing ΔTsub. At ΔTsub = 1.3 K (Figure 4a), it took 1874 s until the droplet surface was completely covered, while at ΔTsub = 2.2 K (Figure 4b), the time was 418 s. These trends were observed for all cases involving the other mixed gases (94.1:5.8:0.1 and 90:7:3). Figures 5-7 show the magnified views of the hydrate crystals formed with mixed gases (99.47:0.51:0.02, 94.1:5.8:0.1, and
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Figure 6. Classification of hydrate crystal morphology based on ΔTsub (mixed gas (94.1:5.8:0.1) system).
Figure 7. Classification of hydrate crystal morphology based on ΔTsub (mixed gas (90:7:3) system).
90:7:3), respectively, at different levels of ΔTsub and at different pressures. The red lines in the images highlight the typical individual crystals. Figure 5 shows a comparison of the crystal morphology of the mixed gas (99.47:0.51:0.02) hydrate observed at the three different levels of ΔTsub in the pressure range from 3.9 to 5.1 MPa. Figures 6 and 7 show the crystal morphology in a similar way for the other two mixed gases (94.1:5.8:0.1 and 90:7:3). All of the images in these figures are pictured just after the complete coverage of the water-droplet surface by the hydrate-crystal layer. On the basis of these results, no noticeable effect by the system pressure on the crystal morphology was observed in the same level of ΔTsub as observed for the simple hydrates.15 In addition, we clearly recognized that the size of the individual mixed-gas hydrate crystals decreased
with increasing ΔTsub, as shown in Figure 4, and the crystal shape changed from polygonal to swordlike. The faces of these crystals are estimated to be {111} planes of the cubic structure II hydrate as previously discussed by Smelik and King25 and Knight and Rider.26 A comparison of the crystal morphology observed in the systems with three different mixed gases (99.47:0.51:0.02, 94.1:5.8:0.1, and 90:7:3) and with methane by Tanaka et al.15 are arranged along the horizontal axis of ΔTsub in Figure 8. For the most methane-rich mixed gas (99.47:0.51:0.02) system, when the subcooling is ΔTsub 2.5 K, we could not
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Figure 8. Arrangement and comparison of hydrate crystal observations depending on ΔTsub.
Figure 9. Crystal hydrate growth rates at the interface of the liquid water and gaseous guest.
discriminate the individual hydrate crystals because of the resolution limit of the present observations. For the mixed gas (94.1: 5.8: 0.1) system, at ΔTsub 2.7 K, the crystal shape was sword-like with a size of less than 0.3 mm. For the mixed gas (90:7:3) system, at
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ΔTsub = 1.3-3.0 K, the crystal shape was polygonal or triangular with a length of about 0.5-1.5 mm, and at ΔTsub > 2.7 K, the crystal shape was sword-like with a size less than 0.3 mm. It is interesting to note that the size of the hydrate crystals observed in the present study is typically less than 0.3 mm at ΔTsub > 2.7 K, which is consistent with the SEM observation results for the natural hydrate samples by Klapp et al.20 A comparison of the observational results for the gases with different compositions is summarized in Figure 8; the size of the individual mixed-gas hydrates crystals decreased with the increasing molar ratio of methane in the mixed gases from 90% to 99.47%. These results indicate that the concentration of methane in the mixed gas has a significant impact on the hydrate crystal morphology. A difference in the size of the hydrate crystals was seen between the methane hydrate and mixed gas (99.47:0.51:0.02); that is, the methane hydrate crystal was larger at a given ΔTsub. This difference in the crystal size may be ascribed to the difference in the crystallographic structures of the hydrates. Simple methane hydrate is of structure I, whereas the mixed gas hydrate crystal structure was of structure II.16 This tendency that the structure II hydrate crystals are smaller than the structure I hydrate is consistent with the SEM observations of the naturally occurring hydrates by Klapp et al.19 Figure 9 shows the relation between the ΔTsub and the lateral hydrate growth rates that were deduced from the optical observational records. In determing these growth rate data, the lateral growth rate is defined as the length of the periphery of the water droplet from its apex to its bottom divided by the time required for the complete surface coverage from the onset of the hydrate layer propagation on the top of the droplet. This lateral growth rate represents the rate of two-dimensional hydrate crystal growth, sometimes referred to as the “rate of hydrate-film/layer propagation”. Consistent with previous studies,13,14,22,27 the hydrate film growth rate decreased with the decreasing ΔTsub in any of the systems. In addition, the mixed gas hydrate data fit between those of the methane and propane hydrates. Conclusions Visual observations were carried out on the formation and growth of clathrate hydrate crystals on the surface of a water droplet exposed to methane þ ethane þ propane gas mixtures of the following compositions: (i) 99.47:0.51:0.02, (ii) 94.1:5.8:0.1, and (iii) 90:7:3 molar ratios. The size of the individual mixedgas hydrate crystals decreased with the increasing ΔTsub as previously observed for the simple hydrates each formed with methane, ethane, or propane. At a given ΔTsub, the size of the hydrate crystals decreased with the increasing methane concentration in the gas mixture. However, the difference in the size of the hydrate crystals was observed between the methane hydrate and mixed-gas hydrates; that is, the methane hydrate crystal was larger at a given ΔTsub. These observations indicated that the hydrate crystals formed with natural gas containing ethane and propane would be smaller than the simple methane hydrate crystals at a given thermodynamic condition, which should be taken into account during the process design of the hydrate production for natural gas
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storage/transportation and the analyses of the recovered natural gas hydrate samples. Acknowledgment. This study was supported by a Grantin-Aid for Science Research from the Japan Society for the Promotion of Science (Grant 22760157) and by a Grant-in-Aid for the Global Center of Excellent Program for “Center for Education and Research of Symbiotic, Safe and Secure System Design” from the Ministry of Education, Culture, Sport, and Technology in Japan.
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