Crystal Growth of Clathrate Hydrate in Liquid Water Saturated with a

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Crystal Growth of Clathrate Hydrate in Liquid Water Saturated with a Simulated Natural Gas Sho Watanabe, Kota Saito, and Ryo Ohmura* Department of Mechanical Engineering, Keio University, Yokohama 223-8522, Japan ABSTRACT:

This paper reports the visual observation of the formation and growth of clathrate hydrate crystals in liquid water presaturated with a simulated natural gas (methane þ ethane þ propane mixture). The compositions of the methane þ ethane þ propane gas mixtures are (i) 90:7:3, (ii) 94.1:5.8:0.1, and (iii) 99.47:0.51:0.02 in molar ratio. A hydrate film first formed to intervene between the mixed gas and liquid water, and then hydrate crystals grew in the liquid water phase. The morphology of hydrate crystals grown in liquid water distinctly varied depending on the system subcooling ΔTsub. When ΔTsub is smaller than ∼7 K, hydrate crystal growth in liquid water was not observed. At ∼7 K < ΔTsub < ∼12 K, hydrate crystals with polygonal morphology were observed. At ΔTsub > ∼12 K, polygonal crystals were replaced by dendritic crystals. These changes in morphology were observed with all three gas mixtures. It was found that the morphology in the system with mixed gas of the molar ratio 99.47:0.51:0.02 was different from those of the other two systems. We also observed the hydrate crystals floating to the hydrate film from liquid water phase. Floating crystals formed in the bulk of liquid water, attached the hydrate film, and then continued to grow in liquid water. The morphology of floating crystals varied with ΔTsub and the gas composition.

’ INTRODUCTION Clathrate hydrates are crystalline solid compounds consisting of hydrogen-bonded water molecules called “host” that form cages and other different molecules called “guest” enclosed in the cages. Hydrates are known to form various crystal structures. The most common hydrate crystal structures are structures I, II, and H. Recently, novel hydrate-related technologies have attracted attention in the energy and environmental fields. Some of these technologies are the transportation and storage of natural gas1 and hydrogen,2 cool energy storage for air conditioning,3 highly efficient heat pump/refrigeration systems,4,5 etc. Various hydrates are studied for the development of these hydrate-based technologies. Natural gas hydrate is one of the most important hydrates because it is relevant to the natural gas transportation/ r 2011 American Chemical Society

storage and the plugging of the flow of oil or gas pipelines.6 Furthermore, natural gas hydrate naturally occurring in marine sediments is expected to be an energy resource in the future.7 This study focuses on the crystal-growth characteristics of the natural gas hydrate. Knowledge of the morphology of hydrate crystals is necessary for development of the above-mentioned technologies utilizing hydrates since the morphology provide fundamental information on the mechanistic nature of hydrates. “Crystal morphology” means the geometric configuration of crystals such as their size or Received: April 20, 2011 Revised: May 19, 2011 Published: May 20, 2011 3235

dx.doi.org/10.1021/cg2005024 | Cryst. Growth Des. 2011, 11, 3235–3242

Crystal Growth & Design

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shape. The technology influenced by crystal morphology follows below. The transportation and storage of natural gas in the form of hydrates have been developed by Japanese industrial groups.8,9 During the hydrate production process, the crystal morphology of natural gas hydrate is an important characteristic to impact the slurry conveyance and dewatering process of hydrate crystals. In addition, it is considered that the morphology of natural gas hydrates formed in geological layers under the sea bed affects the mechanical and thermal characteristics of the geological layer including the hydrate. Morphology studies10,11 on the recovery of naturally occurring hydrate samples and scanning electron microscopy (SEM) observations of the samples have been recently reported. It is generally acknowledged that hydrate crystals preferentially form to intervene between the guest and water as a polycrystalline thin film. After the hydrate film covers the interface, hydrate crystals grow into liquid water when the guest substances sufficiently dissolve in liquid water. Several studies1215 on hydrate crystal morphology at the guest gasliquid water interface have been reported. Some single gas hydrates12,13 and mixed gas hydrates14,15 were reported in these studies. Morphology studies that are relevant to engineering practice, such as simulated continuous hydrate production from natural gas,16 the system including kinetic inhibitors,17 and the system with seawater,18 also have been reported. It appears that comprehensive understanding of the morphology at the guest gasliquid water interface has been obtained. On the other hand, we find a limited number of previous studies on the morphology of hydrate crystals grown into liquid water. Ohmura et al.19 demonstrated the morphology of CO2 hydrates grown into liquid water presaturated with CO2. The morphology of hydrate crystals changed from polyhedra with facets to skeletal polyhedra or columns and then to dendrites, depending on the thermodynamic conditions. They further analyzed the crystal growth process by assuming that hydrate crystal growth was controlled by mass transfer of a guest substance dissolved in liquid water, and they presented a nondimensional index to predict the crystal morphology of hydrate grown in liquid water phase. Ohmura et al.20 also observed the crystal morphology of CH4

hydrates grown into liquid water. The observational crystal morphology was similar to that previously reported in the system with CO2. The morphology changed from columnar to dendrite with increasing driving force of crystal growth. Lee et al.21 presented crystal growth of CH4 þ C3H8 clathrate hydrate grown into liquid water with or without n-heptane. This study reported not only the crystal growth in liquid water but also the hydrate crystals floating to the hydrate film from liquid water. The shapes of floating crystals were octahedral and triangular or hexagonal flat plate. The shape of floating crystals became dendritic as the driving force increased. As reviewed above, studies on hydrate crystal morphology grown into liquid water are scarce and hence there is still room for future studies. Specifically, we find no previous study on the morphology of hydrate crystals grown in liquid water in a situation of producing hydrates from natural gas. During continuous hydrate formation in a fixed volume reactor from a gas mixture such as natural gas, the vapor-phase and hydrate-phase compositions change in series, because some species are preferentially taken in the hydrate from the vapor phase.22,23 For example, if the guest substance is a methane þ ethane þ propane gas mixture, propane will preferentially occupy the hydrate cages. The composition of the gas phase in equilibrium with the hydrates is different from that of the feed gas, and the methane ratio becomes rich because of the preferential uptake of ethane and propane in the hydrates over methane. Thus, the multiphase system inside the reactor is generally in an unsteady state at each instant during the transient process asymptotically approaching a steady state, at which the composition of the feed gas and that in the hydrate should coincide with each other.22,23 Therefore, it is important to understand the morphology of the hydrates corresponding to the various vapor-phase compositions for process design of continuous production of natural gas hydrates. In this study, we visually observed the formation and growth of hydrate crystals in liquid water saturated with simulated natural gas (methane þ ethane þ propane gas mixture). This paper follows on the study of formation and growth of hydrate crystals at the interface between simulated natural gas and liquid water reported by Saito et al.16 The observed variations in the morphology of hydrate crystals were systematically classified with the magnitudes of the driving force for hydrate crystal growth.

’ EXPERIMENTAL APPARATUS AND PROCEDURE The sample fluids used in the experiments were deionized and distilled liquid water and three ternary gas mixtures of methane, ethane, and propane of the following compositions in molar ratio: (i) 90% methane, 7% ethane, and 3% propane; (ii) 94.1% methane, 5.8% ethane, and 0.1% propane; and (iii) 99.47% methane, 0.51% ethane, and 0.02% propane. These compositions were selected from the results of simulation of the isobaric hydrate-forming operation from multiple guest substances.22,23 Mixed gas (90:7:3) was the initial feed gas composition

Figure 1. Schematic diagram of the major experimental apparatus.

Table 1. Temperature and Pressure Conditions of the Experiments with Mixed Gas Systems (Methane þ Ethane þ Propane)a gas-phase compositions in molar ratio (methane/ethane/propane) 90:7:3

a

94.1:5.8:0.1

99.47:0.51:0.02

P/MPa

Teq/K

ΔTsub/K

P/MPa

Teq/K

ΔTsub/K

P/MPa

Teq/K

ΔTsub/K

6.5

290.7

5.016.5

9.5

289.2

5.015.5

10.5

286.6

5.013.5

Equilibrium temperatures at the given experimental pressures. 3236

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Figure 2. General process of hydrate crystal growth behavior [mixed gas (90:7:3) system, P = 6.5 MPa, ΔTsub = 12.8 K].

Figure 3. Difference in hydrate crystal growth depending on ΔTsub [mixed gas (90:7:3) system, P = 6.5 MPa]. simulated natural gas on the hydrate-forming operation; mixed gas (94.1:5.8:0.1) was the gas-phase composition in a transient state of the hydrate-forming operation; and mixed gas (99.47:0.51:0.02) was the gas-phase composition in the steady-state condition of the hydrateforming operation.22,23 Figure 1 schematically illustrates the main portion of the experimental apparatus: a test cell made of a stainless steel cylinder, a pair of flangetype glass windows, and a microscope-camera system. The inner space of the test cell to form the hydrate crystals was 25 mm in diameter and 20 mm in axial length. The temperature inside the test cell, Tex, was controlled by circulating a refrigerant through a brass jacket around the test cell. Tex was measured within the uncertainty of (0.2 K by a platinum resistance thermometer inserted into the bulk of liquid-water phase inside the cell from the bottom of the test cell. The pressure inside the test cell, P, was controlled by supplying the mixed gas from a gas cylinder through a pressure-regulating valve. P was measured by a

strain-gauge pressure transducer with the uncertainty of 0.2% over the span 015 MPa, that is, (0.03 MPa. Five cubic centimeters of liquid water was poured in the lower half of the test cell to form a pool. The air in the test cell was replaced with a mixed gas supplied from the highpressure cylinder through the pressure-regulating valve, by repeating the pressurization of the cell with gas mixture and evacuating it from the cell. P was then set at a prescribed level in the range P = 6.510.5 MPa as specified in Table 1. Tex was decreased to about 268 K to form a hydrate (and simultaneously ice) and then raised 12 K higher than Teq, the guest gashydrateliquid water three- to phase equilibrium temperature corresponding P. After dissociation of the formed hydrate crystals, the test cell was manually oscillated to ensure the saturation of liquid water with the gas mixture. Tex was then set at the prescribed level that is lower by about 5.016.5 K than Teq to observe the formation and growth of the hydrate crystals in the test cell. As each of the experiments was performed with a batch procedure, the composition of the gas phase 3237



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Figure 4. Difference in hydrate crystal growth depending on ΔTsub [mixed gas (94.1:5.8:0.1) system, P = 9.5 MPa].

Figure 5. Difference in hydrate crystal growth depending on ΔTsub [mixed gas (99.47:0.51:0.02) system, P = 10.5 MPa].

should change before and after gas dissolution. The gas-phase compositions after dissolution of the gas into liquid water were estimated with a

phase-equilibrium calculation program, CSMGem.24 In this estimation, we calculated the guest gasliquid water two-phase equilibrium. The 3238



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Figure 6. Arrangement of hydrate crystal observations depending on ΔTsub. change of composition was predicted to be less than 16% for each of the gas compositions based on the mass balance calculations with a fixed volume. The formation and growth of hydrate crystals were monitored and recorded by use of a charge-coupled device (CCD) camera (Pixelink, model PL-A662) and a microscope (Elmo, model TVZ610M). We defined the system subcooling ΔTsub as the index of the driving force for the crystal growth. ΔTsub is the difference between the system temperature Tex and the equilibrium temperature Teq corresponding to the system pressure (ΔTsub Teq Tex). The equilibrium temperature was calculated with CSMGem.24

’ RESULTS AND DISCUSSION Formation and Growth of Hydrate Crystals in Liquid Water Saturated with Methane þ Ethane þ Propane Gas Mixture. Figure 2 shows the general growth behavior of hydrate

crystals [mixed gas (90:7:3) system, P = 6.5 MPa, ΔTsub = 12.8 K]. The nucleation of hydrate crystals initially occurred at the mixed gasliquid water interface, and then thin hydrate film grew to intervene between the mixed gas and liquid water.

Hydrate crystals subsequently grew in liquid water phase. Two modes were observed for the hydrate crystal growth in liquid water. One mode was crystal growth from the hydrate film. Another mode was that the hydrate formed in the bulk of liquid water and floated to the hydrate film, thereby growing in liquid water after attachment to the hydrate film. Hydrate crystals grew in liquid water by occurrence of both modes. This process of hydrate formation and growth was commonly observed in all experimental systems. Figure 3 displays sequential images of formation and growth of hydrate crystals in the system with a methane þ ethane þ propane gas mixture of molar ratio 90:7:3 at the subcooling temperatures ΔTsub = 9.0, 12.3, and 14.0 K, under the same pressure P = 6.5 MPa. At ΔTsub = 9.0 K, we observed that hydrate crystals at the mixed gaswater interface grew into a thin hydrate film covering the interface within several tens of seconds. After the complete coverage, polygonal flat plate hydrate crystals grew in the bulk of liquid water in an hour or two. At ΔTsub = 12.3 K, the growth of dendritelike crystals in the liquid water was 3239



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Figure 7. Difference in morphology of floating hydrate crystals depending on ΔTsub [mixed gas (90:7:3) system, P = 6.5 MPa].

observed after the formation of a hydrate film at the gas mixturewater interface. The dendritic hydrate crystals initially grew in a few tens of minutes and then the shape of the dendritic crystals changed to polygonal flat plates connected in a longitudinal direction (as shown in the closeup view in the middle of Figure 3). The observational results obtained at ΔTsub = 14.0 K were generally similar to those at ΔTsub = 12.3 K. However, the morphology of hydrate crystals grown in the liquid water became clearly dendrite crystals. The dendritic hydrate crystals initially grew in their axial directions within a few minutes, and then side branches grew from the main branch. Growth of the dendritic crystals ceased in some tens of minutes after the initial hydrate formation. Figure 4 presents observational results in the system with mixed gas (94.1:5.8:0.1) obtained at the three different levels of ΔTsub and at P = 9.5 MPa. The growth behavior in this system was similar to that in the system with mixed gas (90:7:3). When ΔTsub = 9.5 K, the polygonal or triangular flat plate hydrate crystals grew into the bulk of liquid water in an hour or two after the hydrate film covered the interface. At ΔTsub = 11.8 K, the needlelike crystals initially grew in a few tens of minutes, and then the needlelike shape changed to the polygonal flat plates connected in a longitudinal direction (as shown in the closeup view in the middle of Figure 4). The dendrite crystals grown into liquid water were observed at ΔTsub = 14.3 K. The dendrite crystals initially grew in their axial directions within a few minutes. After that, the dendrite crystals branched from the main branch of the dendrite crystals. Observational results in the most methane-rich mixed gas (99.47:0.51:0.02) system at various degrees of ΔTsub and at P = 10.5 MPa are illustrated in Figure 5. When ΔTsub = 9.2 K, after the hydrate film formation, the polygonal crystals growing downward into the bulk of water were observed within a few hours. At ΔTsub = 11.0 K, the polygonal crystals first grew into the liquid water, and then the polygonal shape changed to columnar in an hour or two. The dendrite crystals grew in a few tens of minutes at ΔTsub = 13.5 K. The dendrite crystals initially grew in their axial directions within a few minutes. After the main branch formed, side branches of dendrite crystals grew in a lateral direction.

Figure 8. Difference in morphology of floating hydrate crystals depending on ΔTsub [mixed gas (94.1:5.8:0.1) system, P = 9.5 MPa].

It was found from these results that the hydrate crystal morphology varied depending on the system subcooling temperature ΔTsub. The crystal shape changed from polygonal or triangular flat plate to dendritic with increasing ΔTsub. Hydrate crystal morphology in the systems with three types of mixed gases (90:7:3, 94.1:5.8:0.1, and 99.47:0.51:0.02) observed in the present study and with methane reported by Ohmura et al.20 are arranged along the horizontal axis of ΔTsub in Figure 6. In all experimental systems, when ΔTsub was smaller than 7.0 K, hydrate crystal growth into liquid water was not observed. In the case of the mixed gas (90:7:3) system, when the subcooling is 7.7 K < ΔTsub < 10.1 K, the shape of hydrate crystals was polygonal flat plates with a length on a side of 0.51.0 mm. When 10.1 K < ΔTsub < 12.3 K, the hydrate crystal shape was triangular flat plates or polygonal flat plates connected in a longitudinal direction. At ΔTsub > 12.8 K, the triangular or polygonal crystals were replaced by dendritic crystals. Main 3240



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Figure 9. Difference in morphology of floating hydrate crystals depending on ΔTsub [mixed gas (99.47:0.51:0.02) system, P = 10.5 MPa].

branch of the dendritic hydrate crystals grew to 23 mm in axial length. In the case of the mixed gas (94.1:5.8:0.1) system, at 7.1 < ΔTsub < 10.7 K, the shape of hydrate crystals was polygonal or triangular flat plates with a length of about 0.51.5 mm. The crystal shape changed to polygonal flat plates connected in a longitudinal direction at ΔTsub = 10.711.8 K. At ΔTsub > 12.7 K, the polygonal flat plates were replaced by dendritic crystals. The axial length of the main branch of the dendritic crystals was 23 mm. In the case of the mixed gas (99.47:0.51:0.02) system, when 7.5 < ΔTsub < 11.0 K, the crystal shape was polygonal with a length of about 0.10.2 mm. At ΔTsub = 11.012.0 K, columnar hydrate crystals having a length of 0.20.4 mm were observed. The columnar crystals were replaced by dendritic crystals at ΔTsub > 12.0 K. The main branch of the dendritic crystal was 0.51.0 mm in axial length. It is seen in the comparison of observational results summarized in Figure 6 that the hydrate crystal morphology in the system with mixed gas (90:7:3) is similar to that of the system with mixed gas (94.1:5.8:0.1) and also to that was previously observed in the system with methane and propane.21 The morphology in the system with gas mixture (99.47:0.51:0.02) was similar to those of simple methane hydrate20 and simple carbon dioxide hydrate.19 We also found that the crystal morphology in mixed gas (94.1:5.8:0.1 and 90:7:3) systems was different from those of methane20 and mixed gas (99.47:0.51:0.02) systems. These results suggest that the hydrate crystal morphology varied depending on the gas composition. It is known that the morphology of hydrate crystals varies with differences in crystallographic structures. Single crystals of structure I grow to be rhombic dodecahedra along exhibited {110} planes, and those of structure II grow to be octahedral along exhibited {111} planes.2528 Klapp et al.10 and Saito et al.16 suggested that structure II hydrate crystals were smaller than the structure I hydrate. The crystal morphology difference between the systems with gas mixtures (90:7:3 and 94.1:5.8:0.1) and the system with gas mixture (99.47:0.51:0.02) may be ascribed to the difference in crystallographic structures of the hydrates. While the mixed gas (90:7:3) and (94.1:5.8:0.1) hydrates are calculated to be structure II iwith CSMGem under the present experimental

conditions, the hydrates with gas mixture (99.47:0.51:0.02) are calculated to be structure I. This estimation is consistent with the similarity between the morphology of mixed gas (99.47:0.51:0.02) hydrate and that of simple methane hydrate by Ohmura et al.20 The estimation of the crystallographic structures is simply dependent on thermodynamic predictions, and thus additional diffraction measurements are necessary for experimental confirmation. Floating Hydrate Crystals. The formation of hydrate crystals in the bulk of liquid water and their floating to the hydrate film were observed in all experimental systems, as mentioned in the previous section. Density of the structure II hydrates formed with the gas mixtures used in the present study is deduced to be 920960 kg/m3 on the basis of the hydrate compositions predicted by CSMGem24 and the unit-cell lattice constant29 of 1.73 nm. The morphology of floating hydrate crystals varied depending on the system ΔTsub and the gas composition. Figure 7 presents the floating crystal morphology in the system with gas mixture (90:7:3) at ΔTsub = 8.3, 10.6, and 16.3 K, under the same pressure P = 6.5 MPa. When ΔTsub = 8.3 K, the shape of floating hydrate crystals was hexagonal flat plates with length on a side of 0.10.5 mm, and the crystal shape changed to triangular flat plates with a size less than 0.3 mm at ΔTsub = 10.6 K. At ΔTsub = 16.3 K, needlelike crystals growing from the apexes of the triangular crystals were observed. The crystal shape changed from a hexagonal flat plate, triangular flat plate, and then needlelike in planar three directions with increasing ΔTsub. Figure 8 shows the floating crystal morphology in the mixed gas (94.1:5.8:0.1) at the three different levels of ΔTsub and at P = 9.5 MPa. The floating crystal morphology in this system was similar to those of the system with the mixed gas (90:7:3). However, needlelike crystals growing in planar three directions were not observed. When ΔTsub = 7.1 K, the shape of floating crystals was hexagonal flat plates with a length of 0.10.5 mm. The hexagonal crystals floating to the hydrate film were replaced by the triangular flat plate crystals having a length on a side of 0.10.2 mm at ΔTsub = 13.3 K. The morphology of floating crystals observed in the gas mixture (99.47:0.51:0.02) at various degree of ΔTsub and at P = 10.5 MPa is displayed in Figure 9. When ΔTsub = 8.3 K, 3241


Crystal Growth & Design floating hexagonal crystals were observed. The size of the hexagonal crystals was less than 0.1 mm. At ΔTsub = 11.8 K, needlelike crystals floating up to the film three-dimensionally grew in six different directions, and we observed the needlelike crystals remarkably grown in six directions at the larger ΔTsub = 13.5 K. We recognized from these results that the floating crystals in the systems with gas mixtures (90:7:3 and 94.1:5.8:0.1) grew two-dimensionally, and those of the system with mixed gas (99.47:0.51:0.02) grew three-dimensionally. It is inferred that these differences in the growth behavior are related to the difference in crystallographic structures as previously mentioned.

’ CONCLUSIONS Formation and growth of clathrate hydrate crystals in liquid water presaturated with methane þ ethane þ propane gas mixtures were observed. The compositions of gas mixtures were (i) 90:7:3, (ii) 94.1:5.8:0.1, and (iii) 99.47:0.51:0.02 in molar ratio. A hydrate film initially formed to intervene between mixed gas and liquid water, and then hydrate crystals grew into liquid water from the hydrate film. The morphology of hydrate crystals grown in liquid water changed from polygonal or triangular flat plates to dendritic with increasing ΔTsub. The crystal morphology in the system with mixed gas (90:7:3) was similar to that of mixed gas (94.1:5.80.1). On the other hand, the morphology of the hydrate crystals in the system with gas mixture (99.47:0.51:0.02) was different from those of the other two systems. We also observed the hydrate crystals floating to the hydrate film from the bulk of liquid water. The floating crystals in the systems with gas mixtures (90:7:3 and 94.1:5.8:0.1) grew in two dimensions, and those in the system with mixed gas (99.47:0.51:0.02) grew in three dimensions. These observational results indicate that the hydrate crystals formed with natural gas varied depending on the gas composition. ’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected]; phone þ81-45-566-1813.

’ ACKNOWLEDGMENT This study was supported by a Grant-in-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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Crystal Growth of Clathrate Hydrate in Liquid Water Saturated with a

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