Clathrate Hydrate Crystal Growth in Natural Gas Saturated Water Flow

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Clathrate Hydrate Crystal Growth in Natural Gas Saturated Water Flow Muhammad Aifaa, Kazuki Imasato, and Ryo Ohmura Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00281 • Publication Date (Web): 24 Apr 2015 Downloaded from http://pubs.acs.org on April 26, 2015

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Crystal Growth & Design

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Clathrate Hydrate Crystal Growth in Natural Gas Saturated Water Flow

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Muhammad Aifaa, Kazuki Imasato and Ryo Ohmura*

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Department of Mechanical Engineering, Keio University, Yokohama 223-8522, Japan

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ABSTRACT The crystal growth of clathrate hydrate in the flow of water saturated with simulated natural gas were visually observed. The simulated natural gas is a mixture of methane, ethane and propane. The compositions of simulated natural gasses were 90:7:3 and 98.5:1.4:0.1 in methane-ethane-propane molar ratio. The morphology, i.e., shape and size of hydrate crystals, which grew into water flow changed depending on the system subcooling, which denotes the difference between the hydrate equilibrium and experimental temperatures. At lower subcooling conditions, polygonal flat plate crystals were observed. When the subcooling temperature was larger than 11.5 K, polygonal crystals were completely replaced by the dendritic crystals. Crystals formed in flowing liquid water grew for a longer period of time compared to that in the quiescent system without any further guest supply. In addition, coexistence of structure I and II hydrates with 98.5:1.4:0.1 gas mixture in continuous supply system was visually confirmed. From these observations, we note that crystal morphologies also differed depending on the crystallographic structure of the hydrates. These results enable us to precisely evaluate the hydrate morphologies corresponding to the marine sediment conditions.

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Crystal growth behavior in water flow changed with guest gas composition. Coexistence of structure I and II hydrates with 98.5:1.4:0.1 gas mixture was observed. (Note: the direction of water flow is from right to left in this picture and the black part on the left side shows the porous pipe.)

Ryo Ohmura Department of Mechanical Engineering, Keio University

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3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

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+81-45-566-1813

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E-mail: [email protected]

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ACS Paragon Plus Environment


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1

Clathrate Hydrate Crystal Growth in Natural Gas

2

Saturated Water Flow

3

Muhammad Aifaa, Kazuki Imasato and Ryo Ohmura*

4

Department of Mechanical Engineering, Keio University, Yokohama 223-8522, Japan

5 6

ABSTRACT The crystal growth of clathrate hydrate in the flow of water saturated with

7

simulated natural gas were visually observed. The simulated natural gas is a mixture of

8

methane, ethane and propane. The compositions of simulated natural gasses were 90:7:3 and

9

98.5:1.4:0.1 in methane-ethane-propane molar ratio. The morphology, i.e., shape and size of

10

hydrate crystals, which grew into water flow changed depending on the system subcooling,

11

which denotes the difference between the hydrate equilibrium and experimental temperatures.

12

At lower subcooling conditions, polygonal flat plate crystals were observed. When the

13

subcooling temperature was larger than 11.5 K, polygonal crystals were completely replaced

14

by the dendritic crystals. Crystals formed in flowing liquid water grew for a longer period of

15

time compared to that in the quiescent system without any further guest supply. In addition,

16

coexistence of structure I and II hydrates with 98.5:1.4:0.1 gas mixture in continuous supply

17

system was visually confirmed. From these observations, we note that crystal morphologies

18

also differed depending on the crystallographic structure of the hydrates. These results enable

19

us to precisely evaluate the hydrate morphologies corresponding to the marine sediment

20

conditions.

2


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1

Introduction

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Clathrate hydrates are ice-like crystalline solid compounds consisting of hydrogen-bonded

3

water molecules which form cages, and other molecules called “guest” which are enclosed

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inside the cages. Three major crystal structures of clathrate hydrate have been confirmed;

5

structure I (sI), structure II (sII) and structure H (sH) which varies depending on the guest

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substances.1 Clathrate hydrates have characteristics feasible for industrial application such as

7

large amount of gas storage capacity, selectivity of guest substance and large heat of

8

formation/dissociation. Therefore, hydrate-based technologies such as transportation and

9

storage of natural gas2 and hydrogen,3,4 sequestration of carbon dioxide5,6 and development of

10

highly efficient heat pump/refrigeration systems7 utilizing the heat for hydrate

11

formation/dissociation have been proposed. Various clathrate hydrates are being investigated

12

for the development of hydrate-utilizing industrial technologies.

13

In deep marine sediments, natural gas hydrates are formed at low temperature and high

14

pressure conditions. Natural gas hydrates grow in the pore space of sedimentary particles.

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Existence of natural gas hydrate could affect the mechanical properties of marine sediments.

16

8-10

17

previously.8 The understanding on the mechanism of formation and natural gas hydrates’

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growth are important not only for practical uses but also when considering naturally formed

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hydrates in deep marine sediments from geoscience aspects. It is generally known that natural

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gas is a mixture of mainly methane, followed by ethane, and propane. Each of these

21

hydrocarbons has its own hydrate equilibrium and structural characteristics; for instance,

22

methane or ethane generally forms a sI hydrate, but large molecules such as propane form sII

23

hydrates.11 During the past few years, phase equilibrium researches on various compositions

24

of natural gas have been conducted to study the hydrate stability zone for many different gas

Submarine landslide caused by dissociation of natural gas hydrate have been reported

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fields, and the literatures have provided thermodynamic data and prediction software. 12-15

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Nonetheless, visual observations of sI and sII crystals of mixed hydrate are extremely limited.

3

Visual microscopic observations are vital in hydrate research as they would provide an

4

opportunity to analyze the mechanistic behavior of hydrate crystals. For example, observation

5

studies would improve the understanding of hydrate growth behavior, equilibrium

6

morphologies of different hydrate structures and determination of relative growth of crystal

7

planes or directions. The sI, sII and sH hydrate crystals’ growth behavior have been reported

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by Smelik and King.16 They attempted a visual observation on each hydrate crystal structure

9

but the optical images provided were dark, making it difficult to see and distinguish the

10

individual hydrates. More studies on hydrate crystal formation should be carried out in order

11

to precisely analyze clathrate hydrate formed with natural gas.

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Previous studies have revealed that the crystal morphology, i.e., the shape and size of

13

hydrate crystals, which grew into flow of water presaturated with guest substances, depended

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on the strength of driving force.17-24 With regards to natural gas (methane + propane gas)

15

hydrate, Lee et al. 19 reported the crystal morphology of natural gas hydrate. Morphology of

16

natural gas hydrate changed from needlelike to dendrite with increase in subcooling

17

temperature in batch system. Yet, since natural gases are unlimitedly supplied from natural

18

gas fields in the deep marine sediment, we have to investigate crystal growth behavior in the

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continuous guest supply system. Most of the previous studies only exhibit the hydrate crystal

20

growth behavior in batch system. Aifaa et al.25 observed crystal formation and growth of

21

methane hydrate in a methane continuous supply system. The concentration of natural gas in

22

the bulk water was kept constant by the flowing liquid water in the continuous guest supply

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system. They reported that the particle size of methane hydrate observed in the continuous

24

methane supply system is larger than that in batch system. The crystal morphology and crystal 4


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growth behavior of natural gas hydrate in the continuous guest supply system may be

2

different from those in the batch system. Therefore, we conducted experiments to reveal the

3

crystal growth behavior of natural gas hydrate in a system with continuous natural gas supply.

4

The results were analyzed to understand the effects of mass transfer of guest substances upon

5

the morphology of hydrate crystal. The continuous supply of dissolved natural gas tested in

6

this study is relevant to the hydrate growth condition in the marine sediment as the abundant

7

amount of natural gas would serve as unlimited supply for hydrate crystal growth. The effect

8

of existence of natural gas hydrate on deep marine sediment will be discussed based on the

9

results observed in this study.

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1

Experimental Apparatus and Procedure

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Three component mixed gas of methane, ethane and propane with the mole-based

3

compositions of 90:7:3 and 98.5:1.4:0.1 were used as guest substances simulating the natural

4

gas. The compositions were determined from the results of the simulation for the continuous

5

hydrate formation from the mixed gas where the composition of the feed gas was 90:7:3 and

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the steady state composition of the gas phase in the hydrate formation reactor was

7

98.5:1.4:0.1. 26,27

8

The apparatus and procedure were essentially the same as conducted by Aifaa et al.25, in

9

which CH4 was used as the guest substance. Main experimental apparatus used in this study

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were a dissolution bath with a stirrer, a piston-cylinder pump and a hydrate forming reactor.

11

The apparatus was setup by connecting these parts to form a cycle, and liquid water

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presaturated with simulated natural gas was circulated in this cycle. Hydrate crystals formed

13

in the reactor were visually observed. The reactor was a high pressure cylinder made of a

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stainless steel. The observation was made through the glass windows of the reactor. The

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temperature in the test section was set by the refrigerant flowing in the jacket surrounding the

16

reactor. The experimental pressure was set by the supply of the gas mixture from a high

17

pressure container. A porous tubing was vertically placed in the test section. The mixed gas

18

was supplied into the tubing to induce the hydrate nucleation. Porous tubing is used to form

19

the mixed-gas/water interface where preferential hydrate formation would occur. Pressure, P

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and experimental temperature, Tex were measured by strain-gauge pressure transducer and

21

thermocouple. The uncertainty of these measurement were ±0.05 MPa and ±0.2 K.

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6


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Deionized water (~220 cm3) was injected into the experimental apparatus. Then, mixed

2

gases were added from the high pressure gas containers. The air in the test sections were

3

removed by the repetition of the pressurization and depressurization of gas mixture. P was set

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at a prescribed level. The phase equilibrium temperature under a given experimental pressure

5

was calculated by using CSMGem.15 The stirrer placed in the dissolution bath was turned on

6

and the condition of experimental section was kept for several days to allow the saturation of

7

gas mixture with water. Temperature condition in the dissolution bath was kept several

8

degrees higher than Teq, the guest-hydrate-water three phase equilibrium temperature to avoid

9

plugging by hydrate formation. The temperature in the test section, Tex was first lowered to

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about 264 K to induce hydrate nucleation. When the formation of hydrates in the test section

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was observed, the temperature was raised to several degrees higher than Teq. This procedure

12

was done to exert memory to the compounds in the reactor as induction time for hydrate

13

formation may depend on the temperature at which the system was held after dissociation.28

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After, visual confirmation of the decomposition of the formed hydrate, Tex was decreased to a

15

temperature lower than Teq by about 4-12 K for the observation of hydrate formation in the

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test section. The pump was turned on when the formation of hydrates in the test section was

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observed. The water saturated with simulated natural gas was continuously supplied with 1.5

18

cm3/min flow rate. This flow rate are determined through trial and error to avoid the

19

separation of hydrate crystals from porous pipe and rheologic effects in high flow rate

20

condition. In contrast, certain flow rate are needed to modelize natural gas continuous supply

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system. Reynolds number relevant to this flow rate in the inlet tubing is approximately 110.

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A digital camera and zoom lens were used for the observation. ∆Tsub, the difference

23

between the experimental and hydrate equilibrium temperatures, was used to represent the

24

driving force for hydrate crystal growth. The hydrate equilibrium temperature Teq was 7



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calculated using CSMGem. Experimental runs were conducted for at least twice at all

2

experimental conditions and the reproducibility of the crystal-growth behavior and crystal

3

morphology was confirmed.

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1

Result and Discussion

2

The experiments were conducted at several degrees of system subcooling in the range from

3

about 4-13 K. The initial hydrate nucleation occurred at a random point at the interface

4

between mixed gas and liquid water and later grew into a hydrate film intervening the

5

interface followed by hydrate crystal growth in liquid water phase. Hydrate crystal growth

6

observed in this study generally showed the same patterns with previous observations

7

conducted by Aifaa et al.; 25 direct growth from porous pipe and growth from floating crystals

8

which attached to the porous pipe or nearby crystals. These patterns of hydrate crystal growth

9

were observed throughout the experimental runs.

10

Figure 1 displays a typical sequence of hydrate formation and growth observed in methane +

11

ethane + propane gas mixture of molar ratio 90:7:3 at the subcooling temperature ∆Tsub = 5.4

12

K and 11.1 K. Water was provided from right to the left side and the black part on the left side

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is the porous pipe. At ∆Tsub = 5.4 K, floating crystals were observed 40 minutes after

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complete coverage and polygonal flat plates crystals were seen to grow into the bulk of water

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afterward. The size of polygonal flat plates crystals were 0.4-1.0 mm. When the subcooling

16

temperature was increased to ∆Tsub = 11.1 K, the sword-like crystals grew in tens of minutes.

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In contrast to the observation at ∆Tsub = 5.4 K, hydrate crystals dominantly grew in the

18

direction into the water bulk. The axial length of the crystals was about 2 mm.

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Observations of hydrate crystal growth in the system with gas mixture of molar ratio

20

98.5:1.4:0.1 made at two different ∆Tsub are presented in Figure 2. At ∆Tsub = 6.2 K, growth of

21

granular, euhedral-shaped crystals on the porous pipe surface at one hour after hydrate film

22

coverage was observed. Observation was continued for about 8 hours and polygonal flat plate

23

crystals were seen to grow into liquid bulk together with further growth of granular crystals. 9



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Granular crystals were 0.2-0.7 mm in size while polygonal flat plate was of side 0.3-1.0 mm.

2

When ∆Tsub = 11.5 K, needlelike crystal grew towards the flow and then also other needlelike

3

crystals grew from sides of the first one. Eventually, the dendritic crystals were observed in

4

the liquid water phase. Only 16 minutes after the formation of hydrate, the longest side of

5

dendritic crystal reached approximately 2.6 mm. Granular crystal growth in the system with

6

gas mixture of molar ratio 98.5:1.4:0.1 will be discussed in detail later on.

7

Comparison of crystals observed in the present study with previous observations made by

8

Kodama and Ohmura29 and Watanabe et al.30 were shown in Figure 3. Sample gases used

9

were methane + ethane + propane mixture gas with composition of 98.5:1.4.0.1 and 90:7:3

10

and arranged using ∆Tsub. Up until 11.5 K, polygonal flat plate crystals were observed either

11

in batch or continuous supply system (98.5:1.4:0.1 mixed gas). When ∆Tsub > 11.5 K, only

12

dendritic crystals were seen to grow from the surface of the porous pipe. At ∆Tsub < 8 K,

13

polygonal crystals were observed grown into liquid water (90:7:3 mixed gas). However, as

14

the ∆Tsub increased, the polygonal crystals started to become irregular and jagged. This

15

change can be commonly observed in both batch and continuous supply system. Then when

16

∆Tsub > 11.5 K, only dendritic crystals was observed, similar to 98.5:1.4:0.1 mixed gas. At

17

this point, no new findings on crystal shape change due to the increase in ∆Tsub which

18

represents the driving force for crystal growth was obtained similar to observations made in

19

previous literatures. In addition, hydrate crystals formed in continuous supply system show no

20

noticeable difference with previous studies in term of hydrate crystals shape. However,

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hydrate crystal formation in liquid water at low subcooling was observed in continuous

22

supply system even though there was a report that observed no crystal growth at ∆Tsub below 7

23

K.30 This difference on the limit condition of crystal growth is ascribed to retention of the

24

driving force for the crystal growth by the continuous supplying of guest substances. Besides, 10


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1

growth of crystals in the batch system was stopped within 1 hour compare to present study

2

which hydrates crystals growth continued until 10 hours of experiment. As the clathrate

3

hydrate grew, the concentration of guest substances in the water phase gradually decreased in

4

the batch system. As a result, the quantity of guest substances was not enough for crystal

5

growth. In the continuous guest supply system, supply of guest saturated water prevented the

6

decrease in concentration of guest substances in the bulk water phase. Consequently, the

7

crystal growth time is longer in the continuous guest supply system.

8

Figure 4 shows hydrate crystal observed in 90:7:3 and 98.5:1.4:0.1 mixed gas and pure

9

methane at ∆Tsub = 6.3 K and 7.3 K. In pure methane system and 90:7:3 mixed gas system,

10

well-defined, granular polyhedron and polygonal flat plate crystals were observed

11

respectively. Those crystals were visually differentiated into granular and plate crystal from

12

the illumination of light on the crystals. However in 98.5:1.4:0.1 mixed gas system, both of

13

polyhedron and polygonal plate crystals were observed. From this observation we can clearly

14

state that both sI and sII crystals grow in 98.5:1.4:0.1 mixed gas system. The coexistence of sI

15

and sII hydrates in hydrocarbon + water system has been reported in the previous literature. 31,

16

32

17

by Schicks et al.,33 sI and sII crystals were presented by “dark and light” crystals yet, the

18

driving force that would cause the transformation process is still unknown. Meanwhile,

19

Kodama and Ohmura29 stated that crystal morphology change was observed when ∆Tsub >

20

13.4 K and this reflected the hydrate structure transition from sI to sII. In the present study,

21

the coexistence of sI and sII hydrate crystals was observed. It is known that during hydrate

22

formation process, sII hydrate should initially form because of the preferential intake and

23

selectivity of guest substance; in case of methane + ethane + propane mixture system, propane

24

is preferable to form hydrate. Still, sI of methane hydrate will eventually form at a certain

However, visual observations of sI and sII mixed hydrate were limited. In observation made

11



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1

point in a batch system because propane would gradually decrease and is no longer enough to

2

form sII hydrate. However, in a continuous supply system, initial formation of sI hydrate was

3

observed. Continuous supply caused excess amount of methane to be transferred to the

4

hydrate crystal growth site resulting in the initial formation of sI hydrate. Furthermore,

5

contrary to the batch system, guest gas composition in the liquid water phase remained the

6

same causing the sI and sII hydrate coexist with each other. This result was consistent with

7

the scanning electron microscopy (SEM) observations of naturally occurring gas hydrates

8

previously conducted by Klapp et al.34 They also concluded that the compositions and

9

crystallographic structures would cause the difference in the microstructure of gas hydrates.

10

However, these conclusions are derived based on the visual observations. In order to identify

11

the crystal structure difference in observational result, characterization based on diffraction

12

methods should be performed.

13

It is clear that studies of hydrate crystal growth and its structure of mixed hydrate, as

14

reported here, will offer an important understanding on the existence of natural gas hydrate

15

formed in nature. The marine sediments where natural gas hydrates exist are mainly

16

composed of sands. Since the particle size of sands is about several tens of µm to a few mm,

17

the pore spaces of marine sediments are several millimeter at most. The hydrate crystals

18

grown in this study are large enough to fill the pore spaces in the marine sediments. In

19

addition, it is known that morphology of hydrate crystals differs with its crystallographic

20

structure. From this study, we note that structural characteristics of natural gas hydrate may

21

change due to the abundant amount of natural gas in deep sea. These suggest that complex

22

characteristics of natural gas hydrates would affect the geomechanical property of marine

23

sediments.

24

Acknowledgement 12


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1 2


This study was supported by a Keirin-Racing-based research promotion fund from the JKA Foundation and by JPS KAKENHI Grant Number 25289045.

3

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1

Conclusions

2

The observation of crystal growth of clathrate hydrate in the flow of water saturated with

3

simulated natural gas was conducted. The compositions of the mixed gas was 90:7:3 and

4

98.5:1.4:0.1 in methane-ethane-propane molar ratio. The morphology of natural gas hydrate

5

crystals varied depending on subcooling temperature. At subcooling temperatures larger than

6

6.0 K, the crystals grew from the porous pipe surface into the bulk of liquid water in both of

7

the gas mixture systems. At lower subcooling conditions, polygonal flat plate crystals were

8

observed. When the subcooling temperature was greater than 11.5 K, polygonal crystals were

9

completely replaced by the dendritic crystals. The hydrate crystals observed in this study have

10

the same characteristics of crystal morphology with hydrate crystals observed in the previous

11

studies. However, hydrate crystals grew for a longer period of time in liquid water flow than

12

in the quiescent system. That is because the concentration of natural gas was kept for longer

13

time by the continuous supply of water presaturated with natural gas. Interestingly, hydrate

14

structure I and II coexist in continuous supply system as the mass transfer of guest substance

15

was greatly improved. From this observation, we also note that crystal morphologies differed

16

depending on the crystallographic structure of the hydrates. These results enable us to

17

estimate the hydrate morphologies corresponding to the marine sediment condition. The

18

natural gas hydrate would grow and fill up the pore spaces, which later would affect the

19

geomechanical stability of the marine sediments.

20

14


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(14) Subramanian, S.; Kini, R.; Dec, S. F.; Sloan, E. D. Jr. Chem. Eng. Sci. 2000, 55, 5763-5771. (15) CSMGem, a phase-equilibrium calculation program package accompanying the following book: Sloan, E.D., Jr.; Koh, C.A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2007. (16) Smelik, E. A.; King, H. E. Jr. Am. Miner. 1997, 82, 88-98. (17) Tanaka, R.; Sakemoto, R; Ohmura, R. Cryst. Growth Des. 2009, 9, 2529-2536. (18) Servio, P.; Englezos, P. AIChE J. 2003, 49, 269-276. (19) Lee, J. D.; Susilo, R.; Englezos, P. Chem. Eng. Sci. 2005, 60, 4203-4212. (20) Ohmura, R.; Shigetomi, T.; Mori, Y. H. J. Cryst. Growth 1999, 196, 164. (21) Saito, K.; Kishimoto, M.; Tanaka, R.; Ohmura, R. Cryst. Growth Des. 2011, 11, 295-301. (22) Ohmura, R.; Shimada, W.; Uchida, T.; Mori, Y. H.; Takeya, S.; Nagao, J.; Minagawa, H.; Ebinuma, T.; Narita, H. Philos. Mag. 2004, 84, 1-16. (23) Ohmura, R.; Matsuda, S.; Uchida, T.; Ebinuma, T.; Narita, H. Cryst. Growth Des. 2005, 5, 953-957. (24) Imasato, K.; Tokutomi, H.; Ohmura, R. Cryst. Growth Des. 2015, 15, 428–433. (25) Aifaa, M.; Kodama, T.; Ohmura, R. Cryst. Growth Des. 2015, 15, 559–563. (26) Tsuji, H.; Kobayashi, T.; Okano, Y.; Ohmura, R.; Yasuoka, K.; Mori, Y. H. Energy Fuels 2005, 19, 1587-1597. 16


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FIGURE CAPTIONS

Figure 1 Hydrate crystal growth process with different ∆Tsub in 90:7:3 mixed gas system.

Figure 2 Hydrate crystal growth process with different ∆Tsub in 98.5:1.4:0.1 mixed gas system.

Figure 3 Difference of hydrate crystal morphologies which depends on ∆Tsub. The optical images of batch system were re-produced from the records stored in the authors’ research group.

Figure 4 Observation of hydrate crystal morphologies of different guest gas. 1

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Figure 1 Hydrate crystal growth process with different ∆Tsub in 90:7:3 mixed gas system.

Figure 2 Hydrate crystal growth process with different ∆Tsub in 98.5:1.4:0.1 mixed gas system.

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Figure 3 Difference of hydrate crystal morphologies which depends on ∆Tsub. The optical images of batch system were re-produced from the records stored in the authors’ research group.

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Figure 4 Observation of hydrate crystal morphologies of different guest gas. 1

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1

For table of contents use only

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Title: “Clathrate Hydrate Crystal Growth in Natural Gas Saturated Water Flow”

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Author: Muhammad Aifaa, Kazuki Imasato and Ryo Ohmura

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Table of Contents Graphic and Synopsis

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Crystal growth of clathrate hydrate crystals in the flowing liquid water presaturated with

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simulated natural gas was analyzed. Crystals formed in flowing liquid water grew for a longer

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period of time compared to that in the quiescent system. Crystal morphologies differed

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depending on not only subcooling temperature but also the crystallographic structure. These

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results enable us to evaluate the hydrate morphologies corresponding to the marine sediment

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conditions.

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Clathrate Hydrate Crystal Growth in Natural Gas Saturated Water Flow

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