Crystal Growth of Clathrate Hydrate with Methane plus Partially Water

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Crystal Growth of Clathrate Hydrate with Methane plus Partially Water Soluble Large-Molecule Guest Compound Kazuya Ozawa, and Ryo Ohmura Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01625 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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Crystal Growth & Design

Crystal Growth of Clathrate Hydrate with Methane plus Partially Water Soluble Large-Molecule Guest Compound Kazuya Ozawaa and Ryo Ohmuraa* a: Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. *: Corresponding Author Corresponding author’s name and address Please address all correspondence concerning this matter to: Dr. Ryo Ohmura Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. Tel: +81-45-566-1813 E-mail: [email protected]

This paper reports the visual observations of the formation and growth of 1.0 mm clathrate hydrate formed with 1.0 mm 1.0 mm methane plus a partially water(a) Tsub  7.1 K (b) Tsub  8.3 K (c) Tsub  14.3 soluble guest, tetrahydropyran (THP). THP is known to form a structure II hydrate with methane. The crystal growth dynamics and crystal morphology changed depending on Tsub  Teq  Tex where Teq is the equilibrium temperature and Tex is the experimental temperature. At Tsub  12 K, the hydrate crystals initially formed at the water–THP interface and then grew to form the hydrate film. After the hydrate film was formed, the hydrate formation stopped. At Tsub  12 K, the hydrate crystals formed at the water–THP interface detached from the interface. The detachment of the hydrate crystals led to the maintenance of the interface and the continual hydrate formation. This crystal growth dynamics may be ascribed to the reduced water–THP interfacial tension and the resulting enhancement of the wettability of hydrate with water in comparison with those in the system without THP.

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth of Clathrate Hydrate with Methane plus Partially Water Soluble Large-Molecule Guest Compound Kazuya Ozawaa and Ryo Ohmuraa*

a: Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan *: Corresponding Author

ABSTRACT This paper reports the visual observations of the formation and growth of clathrate hydrate formed with methane plus a partially water-soluble guest, tetrahydropyran (THP). THP is known to form a structure II hydrate with methane. The crystal growth dynamics and crystal morphology changed depending on Tsub  Teq  Tex where Teq is the equilibrium temperature and Tex is the experimental temperature. At Tsub  12 K, the hydrate crystals initially formed at the water–THP interface and then grew to form the hydrate film. After the hydrate film was formed, the hydrate formation stopped. At Tsub  12 K, the hydrate crystals formed at the water–THP interface detached from the interface. The detachment of the hydrate crystals led to the maintenance of the interface and the continual hydrate formation. This crystal growth dynamics may be ascribed to the reduced water–THP interfacial tension and the resulting enhancement of the wettability of hydrate with water in comparison with those in the system without THP. 1


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Introduction Clathrate hydrates (hydrates) are crystalline solids composed of water molecules forming cages by hydrogen bond and guest molecules enclosed in the cages. Hydrates are crystallographically composed of at least two types of small and large cages. The hydrate structures usually change into structure I (sI), structure II (sII) and structure H (sH) mainly according to the size and the shape of guest molecules. Hydrates have unique properties represented by the large gas storage capacity, the guest molecule selectivity, and the large heat of the formation and dissociation. Using these properties makes it possible for hydrates to find various industrial applications in the field of energy and environment technologies such as the gas separation [1, 2, 3, 4, 5], gas sequestration [6, 7], gas storage and transportation [8, 9, 10, 11, 12], and thermal energy storage [13, 14, 15, 16]. Generally, hydrates are formed under the low temperature and the high pressure conditions. The high pressure required the large energy, the investment in the equipment for the compression and the measure relevant to the law. Moderating the equilibrium condition is required to avoid the problems of the high pressure. One of the methods to moderate the equilibrium pressure of hydrates is adding a large molecule guest compound (LMGC) to the hydrate forming system. LMGC is a guest compound which is enclosed only inside the large cages. Hydrate formed with LMGC has the different crystal structure and may be more thermodynamically stable in comparison with that formed without LMGC. For example, the equilibrium pressure of methane hydrate of structure I is 4 MPa at 278 K [17], whereas the equilibrium pressure of methane + tetrahydropyran (THP), LMGC, hydrate of structure II is 0.3 MPa at the same temperature [18]. Furthermore, adding LMGC having the water solubility to the hydrate forming system leads to the promotion of the hydrate formation rate [19, 20]. Generally, the hydrate formation and dissociation are repeated in the hydrate-based technologies. The high hydrate formation rate enhances the efficiency of these technologies. As 2



mentioned above, adding the water soluble LMGC to the hydrate forming system is important for the technologies. Observing the crystal growth dynamics and crystal morphology needs to be performed for the realization and improvement of the hydrate-based technologies. The crystal growth dynamics and crystal morphology vary depending on the crystal growth driving force and the chemical species of the guest molecules. This change affects the fluid dynamics characteristics on the processes of the hydrate-based technologies. In the gas transportation, thermal energy storage and gas separation, the hydrate crystals are transferred as hydrate slurry. When the size of hydrate crystals is large and thus the viscosity of hydrate slurry is low, the slurry pumping process is operated with the small energy consumption [21, 22]. The understanding of the crystal growth dynamics and crystal morphology are necessary for the hydrate forming system with water soluble LMGCs. Tetrahydrofuran (THF), THP and cyclopentane (CP) are well known as LMGC with the role of the effective thermodynamic promoters [18, 23, 24]. THF is miscible with water. There are several observation studies reported the crystal growth of hydrate formed with THF [25, 26, 27]. These experimental results led to the conclusion that the hydrate formed primarily at the guest- THF solution interface and grew towards both above and below interface. However, the individual hydrate crystals were not clearly identified. THF is an air pollutant, and THF is regulated by the laws depending on the areas. It is desirable to use other LMGCs with water solubility. THP and CP are not reported to be air pollutants. The mole fraction of THP and CP in the aqueous solution are respectively 2.3  102

at 0.1 MPa at 282 K [28] and 8.7  10-5 at 0.1 MPa at 278 K [29] at most. The mole fraction

of CP and THP in the hydrate are 5.6  10-2 at most. The mole fraction of THP in the aqueous solution is comparable in the order of magnitude to that of THP in the hydrate. Based on the composition consideration, THP would be expected as better LMGC in terms of the hydrate formation kinetics. 3


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This paper reports visual observations of the crystal growth dynamics and crystal morphology of methane + THP hydrate in the range of T from 274 K to 288 K and P from 1.4 MPa to 1.8 MPa to avoid the formation of simple methane hydrate and simple THP hydrate [17, 18, 30]. The objectives of this study are to understand the crystal growth dynamics and crystal morphology of methane + THP hydrate and to reveal the effect of the partially water soluble LMGC on the hydrate crystal growth.

Experimental section

Figure 1. Schematic diagram of the experimental apparatus.

The fluid samples used in the experiments were methane gas (99.99 vol %, Takachiho Chemical Industrial Co.), liquid reagent tetrahydropyran (99 mass %, Aldrich Chemical Co.) and deionized and distilled liquid water. A schematic diagram of the experimental apparatus used in this study is shown in Figure 1. The test cell is a cylindrical vessel made of stainless steel. The inner space of the test cell is 25 mm in diameter and 20 mm in axial length. The test 4



cell has a pair of glass windows. The formation and growth of the hydrate crystals were observed through the window. The system temperature T was controlled by circulating the ethylene glycol solution as a refrigerant through a jacket covering the test cell. T was measured with an uncertainty of ±0.2 K by a platinum resistance thermometer inserted into the bulk of liquid-water phase. The system pressure P was controlled by supplying the methane gas from the high-pressure cylinder through a pressure-regulating valve. P was measured with an uncertainty of ±0.05 MPa by a strain-gauge pressure transducer. The formation and the growth of the hydrate crystals were observed and recorded using a CMOS camera (Fortissimo Co., Ltd. model CMOS130-USB2) and a microscope (Edmund Optics Co.). Firstly, 2.5 cm3 of liquid water and 2.0 cm3 of THP were charged in the test cell. Liquid water formed a pool at the bottom of the test cell, over which the other pool of the THP was formed. The air in the test cell was replaced with methane gas by repeating the pressurization with methane gas and evacuating it from the test cell. This configuration of the gas and liquids is illustrated in Figure 1. T was first decreased to about 270 K to form a hydrate (and simultaneously ice) and then raised to 1–2 K higher than Teq. After visually confirming the dissociation of all hydrate crystals, T was decreased to set the prescribed Tex to observe the formation and growth of the hydrate crystals in the test cell. The reason why we did this process is to shorten the induction time for hydrate nucleation by utilizing the memory effect of the prior hydrate formation. When the hydrate crystals were dissociated, bubbles and liquid drops appeared and the emulsifications were confirmed in the water and THP phases. This phenomenon should ensure the mutual saturations of the gas-liquid-liquid three phases. When the hydrate crystals were first confirmed, the time zero was defined and Tex and P were recorded. The subcooling Tsub is defined as the difference between the experimental temperature Tex and the equilibrium temperature Teq of methane＋THP hydrate (Tsub  Teq  Tex). Teq was determined from the phase equilibrium data of the previous study [18]. Tsub was used as the 5


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index of the driving force for crystal growth. Experiments were performed for several different subcooling Tsub in the range from 3 K to 16 K. The mole fraction of methane in THP is 2.7  10-3 at 0.1 MPa at 283 K [31]. In this system, the liquid phase was separated into water phase and THP phase. The Gibbs phase rule determined that the degree of freedom is 1 in equilibrium. Because Tsub, namely the temperature was specified, the composition of water and THP is uniquely determined. As long as two liquid phases exist in the system, there was no relation between the ratio of THP to water and the composition of water and THP.

Results and Discussion

THP

water

1.0 mm

hydrate 1.0 mm

1.0 mm (a) t = 10 s

(b) t = 70 s

(c) t = 180 s

1.0 mm 1.0 mm

1.0 mm (d) t = 284 s

(e) t = 430 s

(f) t = 3362 s

Figure 2. Sequential images of the crystal growth of methane + THP hydrate at Tex = 281.5 K, Tsub  7.1 K and P = 1.47 MPa. The time elapsed after the visual recognition of the first hydrate crystal in the experimental system is indicated below each image. 6



The sequential images of the formation and growth of the methane + THP hydrate at Tex = 281.5 K, Tsub  7.1 K and P = 1.47 MPa are shown in Figure 2. The elapsed times are shown below each image. The hydrate crystals preferentially formed at the water–THP interface as shown in Figure 2 (b). The formed hydrate crystals grew into the water phase without detaching from the water–THP interface (Figure 2 (c)). After the hydrate crystals grew to a certain size, these detached from the water–THP interface. The detached hydrate crystals sank toward the bottom of the reactor (Figure 2 (e)). The detached hydrate crystals were piled up in the water phase (Figure 2 (f)) by the repetition of the hydrate formation and detachment. The observations show that the hydrate crystal production qualitatively increased in comparison with those in methane + water system [32]. In the methane + water system, the hydrate crystals initially formed at the methane–water interface and then grew to form a thin film covering the methane–water interface.

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THP water 1.0 mm (a) t = 444 s 1.0 mm

(d) t = 669 s 1.0 mm

(g) t = 820 s

1.0 mm

1.0 mm (b) t = 492 s

(c) t = 646 s

1.0 mm

1.0 mm

(e) t = 762 s

(f) t = 793 s 1.0 mm

1.0 mm

(h) t = 837 s

(i) t = 842 s

Figure 3. Sequential images of the crystal growth of methane + THP hydrate at Tex = 284.0 K, Tsub  5.9 K and P = 1.80 MPa. The time elapsed after the visual recognition of the first hydrate crystal in the experimental system is indicated below each image. The red line indicates the specified place to trace the motion and growth of the hydrate crystal.

The sequential images of the formation and growth of the methane + THP hydrate at Tex = 284.0 K, Tsub  5.9 K and P = 1.80 MPa are shown in Figure 3. The red line indicates the specified place to trace the motion and growth of the hydrate crystal. The hydrate crystal grew at the water-THP interface (Figure 3 (a)-(i)). The hydrate crystal grew at the contact line of the water-THP-hydrate three phases (Figure 3 (e), (f)) and rotated while remaining at the interface 8



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(Figure 3 (f)-(i)). These results do not indicate the hydrate crystal growth from the hydrate film. As shown in Figure 3, the hydrate film at the interface was not visually observed.

Table 1. Mutual solubility a of organic compounds and water at T = 282.6 K, P = 0.1 MPa.

System

a

Water in organic xw

Organic in water xg

Mutual solubility xw + xg

Methane + water

1.19×10-2

7.76×10-10

1.19×10-2

THP + water

11.2×10-2

2.3×10-2

13.5×10-2

all the solubility values are expressed as mole fractions.

It would depend on the guest–water interfacial tension whether the formed hydrate crystals detach from the interface or not. The reason why the methane hydrate crystals did not detach from the methane–water interface might be the high methane–water interfacial tension [33]. The detachment of the hydrate crystals was also observed in the systems with the surfactants and Tetra-n-butylammonium Bromide (TBAB) [34, 35, 36, 37]. In these systems, the formed hydrate crystals detached from the interface and floated into the aqueous phase. The surfactants reduce the interfacial tension between the aqueous solution and guest and increase the wettability of hydrate with the aqueous solution. It is known that the surface tension of the TBAB aqueous solution is lower than that of the pure water [38]. In the water + organic compounds system, the water-organic interfacial tension decreases with increasing mutual solubility of water and the organic compounds [39]. The mutual solubility data of THP and water were reported by Stephenson [28]. The similar values for methane and water were calculated by REFPROP ver. 9.1 [40]. These data are specified in Table 1. The mutual solubility of water and THP is greater by one order of magnitude than that of water and methane. This indicates that the water–THP interfacial tension would be significantly smaller than the water– 9


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methane interfacial tension. The wettability of hydrate with aqueous liquid would be increased in the system with THP.

THP water 1.0 mm

1.0 mm

1.0 mm (a) t = 391 s

(b) t = 395 s

(c) t = 401 s

1.0 mm

1.0 mm

1.0 mm (d) t = 404 s

(e) t = 412 s

(f) t = 460 s

Figure 4. Sequential images of the crystal growth of methane + THP hydrate at Tex = 281.8 K, Tsub  8.3 K and P = 1.77 MPa. The time elapsed after the visual recognition of the first hydrate crystal in the experimental system is indicated below each image.

The sequential images of the methane + THP hydrate crystals growing at Tex = 281.8 K, Tsub  8.3 K and P = 1.77 MPa are shown in Figure 4. The process of the hydrate formation and growth was similar to that at Tsub  7.1 K (Figure 2). The hydrate crystal partially detached from the water–THP interface, but a part of the hydrate crystal kept the contact with the water– THP interface while growing at the water–THP interface (Figure 4 (b)–(e)). The repetition of the partial detachment of the hydrate crystal and the growth of the other hydrate crystal resulted in the formation of the hydrate crystal in the shape of “oligo-polygon” (Figure 4 (f)). 10



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THP water 1.0 mm

1.0 mm

(a) t = 18 s

(b) t = 23 s

(c) t = 28 s

1.0 mm

1.0 mm (d) t = 35 s

1.0 mm

(e) t = 46 s

1.0 mm (f) t = 28 s


The observational results in the methane + THP + water system at Tex = 277.7 K, Tsub  11.3 K and P = 1.56 MPa are shown in Figure 5. The nucleation of the hydrate crystals initially occurred at the water–THP interface. The formation and growth of the hydrate crystals were progressed in the lateral as well as the vertical directions (Figure 5 (a)–(e)). Figure 5 (f) was an image enlarged from Figure 5 (c). The hydrate crystals like “oligo-polygon” were confirmed in Figure 5 (f). The process of the hydrate formation and growth was similar to that at Tsub  8.3 K (Figure 4). The growth rate was high and the number of sites of the hydrate crystal formation and growth increased at high Tsub. Because the numerous hydrate crystals rapidly grew, the crystal morphology could not be identified in the images of Figure 5 (a)–(e).

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THP 1.0 mm

1.0 mm

1.0 mm

water

(a) t = 0

(b) t = 7 s

1.0 mm

(c) t = 15 s

1.0 mm

1.0 mm (d) t = 1110 s

(e) t = 12702 s

(f) t = 17502 s


Figure 6 shows the growth dynamics of the hydrate crystals in the methane + THP + water system at Tex = 275.4 K, Tsub  14.3 K and P = 1.72 MPa. The crystal growth dynamics in this system was different from that at Tsub  7.1 K (Figure 2). The water–THP interface formed a concavity by the meniscus. The site of the hydrate crystal growth was the water–THP interface. The hydrate crystals initially formed at the water–THP interface and then grew to be the hydrate film intervening between the liquid water and the liquid THP (Figure 6 (b), (c)). The hydrate crystals grew from the hydrate film into the water phase (Figure 6 (d), (e)). In three hours after the initial hydrate formation, the hydrate formation and growth were stopped (Figure 6 (e), (f)). This crystal growth dynamics is similar to that in the methane + water system [32]. At high Tsub, the interfacial tension between water and THP solution may be low, as the mutual 12



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solubility of water and THP can be high [28]. On the other hand, the driving force for the crystal growth was high at high Tsub. It is inferred that the formed hydrate crystals rapidly grew to be the hydrate film before the formed hydrate crystals detached from the water–THP interface. The observational results at Tsub  12.0 K are shown in Supporting Information (Figure S1). The crystal growth dynamics at Tsub  12.0 K was similar to that at Tsub  14.3 K (Figure 6). These observations revealed that there are two types of crystal growth dynamics, i.e. the detachment of the crystals from the water-THP interface and the film formation at the waterTHP interface. The threshold subcooling for the two different crystal growth dynamics was around Tsub  12 K. In this study, the water–THP interface formed a concavity by the meniscus as shown in Figure 1. The angle of the interface to the observation axis changed as the interface approaches the glass window. This interface was observed along the horizontal axis through the glass window. With the fluid setting and the observation axis, we observed the interface at different angles. However, the hydrate film was not observed at Tsub  12 K.

1.0 mm 1.0 mm

1.0 mm

1.0 mm

t = 270 s

t = 238 s

t = 84 s

(a) Tsub = 3.3 K

(b) Tsub = 7.1 K

(c) Tsub = 8.3 K

t = 1553 s

(d) Tsub = 15.4 K

Figure 7. Variations in the crystal morphology of the methane + THP hydrate. The images in the blue box show the hydrate crystals growing at the water-THP interface. The image of the hydrate crystals grown from the hydrate film is shown in the red box.

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The observations of the individual hydrate crystals are summarized in Figure 7 to overview the variation of the crystal morphology depending on Tsub. The elapsed time after the visual confirmation of the first hydrate crystal formation is shown below each image. At Tsub  12 K, the water–THP interface was not covered by the hydrate crystals and the crystal growth and detachment were continually repeated. The images of Figure 7 (a)–(c) show the hydrate crystals just before the detachments. At Tsub  3.3 K, the shape of the hydrate crystal was pyramidal with the longest horizontal and vertical axial lengths approximately 4 mm (Figure 7 (a)). At Tsub  7.1 K, the crystal shape was also pyramidal (Figure 7 (b)). This crystal was approximately 3 mm in the longest horizontal and vertical axial lengths. As presented in Figure 7 (c), the “oligo-polygon” crystal was observed at Tsub  8.3 K. The axial length of the main branch of the “oligo-polygon” crystal was approximately 4 mm (Figure 7 (c)). At Tsub  12 K, the water–THP interface was covered by the hydrate film and the hydrate formation was stopped by covering water–THP interface. The image of Figure 7 (d) shows the columnar crystals formed from the hydrate film at Tsub  15.4 K. Each crystal was 0.7–1.5 mm in axial length and as thick as 0.5–0.8 mm. The morphology of hydrate crystal depends on the volumetric rate of hydrate growth, which in turn depends on the rate of mass transfer of guest substance to the crystal surface [41]. The hydrate composition also affects the volumetric rate of hydrate growth. Even if the guest mass transfer rate is the same, if the hydrate composition differs, the volumetric rate of hydrate growth varies. It may be worthy to note the composition of methane + THP hydrate here. The occupancy was reportedly 93-100% in the range of T from 281 K to 288 K and P from 2.3 MPa to 4.8 MPa by means of Powder X-Ray diffraction experiments and analysis [18, 42]. Because the occupancy relevant to the experimental conditions in the present study, in the range of P from 1.4 MPa to 1.8 MPa was not reported, it is difficult to discuss the details of the effect of 14



the occupancy on the crystal morphology of methane + THP hydrate. From the observations summarized in Figure 7, an industrial implication could be derived. At Tsub  12 K, the size of hydrate crystal decreased with increasing Tsub. When the size of the hydrate crystals is large and thus the viscosity of hydrate slurry is low, the small power would be required for separating hydrate from hydrate slurry and pumping hydrate slurry through pipes. Methane + THP hydrate crystal formed at low Tsub would be suitable in terms of efficiency in the dewatering and transporting processes.

Conclusion The formation and growth of methane + THP hydrate crystals were observed. The crystal growth dynamics changed depending on Tsub. At Tsub  12 K, the water–THP interface was covered by the hydrate crystals and the crystal growth then stopped. This crystal growth dynamics was similar to that in the methane + water system. However, the crystal growth dynamics at Tsub  12 K clearly changed compared to that at Tsub  12 K. At Tsub  12 K, because the formed hydrate crystals continually detached from the water– THP interface, the water–THP interface was kept uncovered and the crystal formation continued. Based on the results, we may expect the continuous hydrate formation without the external power in the hydrate-forming system containing THP at Tsub  12 K. The crystal growth dynamics in the system with THP was significantly different from that in the methane + water system. The change in the crystal growth dynamics would occur because the partially water-soluble LMGC reduced the water–THP interfacial tension and increased the wettability of hydrate with water. This resulted in the detachment of the formed hydrate crystals. At Tsub  12 K, the morphology of methane + THP hydrate changed from pyramid to “oligopolygon” and the size of hydrate crystals decreased with increasing Tsub. Dewatering and 15


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transporting of hydrate slurry through pipes would be efficiently operated with the methane + THP hydrate crystals formed at small Tsub.

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AUTHOR INFORMATION Corresponding Addresses *(R. O.) Phone: +81-45-566-1813. E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This study was supported by a Keirin-racing-based research-promotion fund from the JKA Foundation (Grant Number 28-142) and by JSPS KAKENHI (Grant Number 17H03122). We thank Takao Shibata and Shun Oya, students in Keio University, for their contribution in the experimental works. Supporting Information description Sequential images of the crystal growth of methane + THP hydrate at Tsub  12.0 K.

References (1) Zhong, D. L.; Daraboina, N.; Englezos, P. Recovery of CH4 from coal mine model gas mixture (CH4/N2) by hydrate crystallization in the presence of cyclopentane. Fuel 2013, 106, 425-430. (2) Tomita, S.; Akatsu, S.; Ohmura, R. Experiments and thermodynamic simulations for continuous separation of CO2 from CH4 + CO2 gas mixture utilizing hydrate formation. Appl. Energy 2015, 146, 104-110. (3) Linga, P.; Kumar, R.; Englezos, P. Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures. Chem. Eng. Sci. 2007, 62, 4268-4276.

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For Table of Contents Use Only Manuscript title Crystal Growth of Clathrate Hydrate with Methane plus Partially Water Soluble LargeMolecule Guest Compound

Authors list Kazuya Ozawaa and Ryo Ohmuraa* TOC graphic

THP

water 1.0 mm

Synopsis Crystal growth of clathrate hydrate formed in the methane + THP + water system was visually observed. It was found that hydrate crystals detached from the water-THP interface, instead of covering the interface. This crystal growth dynamics may be applied to the new method of hydrate formation.

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Crystal Growth of Clathrate Hydrate with Methane plus Partially Water

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