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Surfactant Effects on Crystal Growth Dynamics and Crystal Morphology of Methane Hydrate Formed at Gas/Liquid Interface Hiroaki Hayama, Makoto Mitarai, Hiroyuki Mori, Jonathan Verrett, Phillip Servio, and Ryo Ohmura Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01124 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 4, 2016

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Crystal Growth & Design

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Surfactant Effects on Crystal Growth Dynamics and Crystal

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Morphology of Methane Hydrate Formed at Gas/Liquid Interface

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Hiroaki Hayama†, Makoto Mitarai†, Hiroyuki Mori†, Jonathan Verrett‡, Phillip Servio‡ and Ryo Ohmura*, †

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†

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

‡

Department of Chemical Engineering, McGill University, 3610 University Street, Montréal, Québec, H3A 0C5, Canada. ABSTRACT This paper presents the visual observations aims to clarify the underlying physics of surfactant effects on clathrate hydrate crystal growth at the interface between methane gas and water with surfactant. Sodium dodecyl sulfate (SDS), which is commonly used in industrial process, was used in this study. Various SDS mass fractions from 0 to wSDS =100 ppm with a step of size 10 ppm were examined, where wSDS represents the mass fraction of SDS aqueous solution. The crystal growth behavior and the crystal morphology of the methane hydrate at the interface varied depending on surfactant concentration and ∆Tsub. In systems with wSDS ≤ 20 ppm, the nucleation occurred on the droplet surface (gas/liquid interface), then grew laterally and finally covered the whole droplet surface. Contrary, at wSDS ≥ 30 ppm, the shape of droplet was not maintained and the enhanced hydrate crystal growth was observed compared to those systems with wSDS ≤ 20 ppm. Individual hydrate crystals at wSDS = 20 ppm were observed to be smaller than those in a pure water system at a given ∆Tsub , which is ascribed to the enhanced hydrate nucleation by the addition of SDS.

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Crystal growth behavior depending on SDS concentration and subcooling temperature. Ryo Ohmura Department of Mechanical Engineering, Keio University

Yokohama 223-8522, Japan Phone: +81-45-566-1813 E-mail: [email protected]

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ACS Paragon Plus Environment


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Surfactant Effects on Crystal Growth Dynamics and Crystal

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Morphology of Methane Hydrate Formed at Gas/Liquid Interface

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Hiroaki Hayama†, Makoto Mitarai†, Hiroyuki Mori†, Jonathan Verrett‡, Phillip Servio‡ and

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Ryo Ohmura*, †

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†

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

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‡

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Québec, H3A 0C5, Canada.

Department of Chemical Engineering, McGill University, 3610 University Street, Montréal,

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ABSTRACT

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This paper presents the visual observations aims to clarify the underlying physics of

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surfactant effects on clathrate hydrate crystal growth at the interface between methane gas and

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water with surfactant. Sodium dodecyl sulfate (SDS), which is commonly used in industrial

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process, was used in this study.

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step of size 10 ppm were examined, where wSDS represents the mass fraction of SDS aqueous

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solution.

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the interface varied depending on surfactant concentration and ∆Tsub.

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20 ppm, the nucleation occurred on the droplet surface (gas/liquid interface), then grew

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laterally and finally covered the whole droplet surface. Contrary, at wSDS ≥ 30 ppm, the

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shape of droplet was not maintained and the enhanced hydrate crystal growth was observed

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compared to those systems with wSDS ≤ 20 ppm.

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ppm were observed to be smaller than those in a pure water system at a given ∆Tsub , which is

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ascribed to the enhanced hydrate nucleation by the addition of SDS.

Various SDS mass fractions from 0 to wSDS =100 ppm with a

The crystal growth behavior and the crystal morphology of the methane hydrate at In systems with wSDS ≤

Individual hydrate crystals at wSDS = 20

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INTRODUCTION Clathrate hydrates (hereafter simply hydrates) are crystalline compounds formed at

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low temperatures and high pressures.

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bonded water molecules forming cages and guest molecules occupying the inside of the cages.

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There has been much research interest in hydrates due to their unique properties.

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have a large gas-storage capacity and can contain 160 times more gas in the same volume

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compared to gas at standard temperature and pressure, a large heat of formation and

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decomposition which is comparable to that of ice, and guest selectivity which comes from the

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interaction of the water-molecule cages and the occupying guest molecules.

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properties may lead to the development of the various technologies, for instance, storage and

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transportation of natural gas1 and hydrogen2, thermal energy storage3, high efficient heat

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pumps4 and, carbon dioxide separation5, 6, 7. With so many applications and given their slow

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formation rate, the enhancement of hydrate formation is one of the key challenges toward

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developing hydrate technologies.

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compound in oil and gas industries as the formation and growth of hydrates followed by their

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agglomeration results in plugging oil and gas pipelines8.

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gone into finding inhibitors which prevent hydrate formation or growth which block pipelines.

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In this way, both hydrate formation and prevention methods are necessary for their respective

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engineering practices.

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Their structures are constructed of the hydrogen

Hydrates

These

Meanwhile, hydrates have been viewed as a problematic

Because of this, much research has

Recently, several studies regarding the surfactant effect on the hydrate formation Aman et al.9 reported the reduction of agglomeration in surfactant

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have been conducted.

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system compared to pure system by measuring the adhesion force between hydrate particles.

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On the other hand, Okutani et al.10 reported the increase in the rate of hydrate formation and

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water-to-hydrate conversion ratio by adding surfactant.

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surfactants have both effects for prevention of hydrate agglomeration and promotion of

These reports indicate that

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hydrate formation, which are apparently different effects.

Why surfactants provide two

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different effects for hydrate formation is still not well understood.

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of hydrate formation in the system including surfactant can be better understood using crystal

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growth and morphology date.

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the mechanistic aspects of hydrate crystal growth by providing crystal size and shape data11,

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The physical mechanism

Crystal morphology studies provide valuable knowledge of

. It is generally known that hydrates preferentially grow at the guest/water interface13,

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chemical species of the guests and the addition of surfactant.

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critically reviewed the previous studies related the surfactant effect on the hydrate formation

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and they mentioned that SDS would be suitable for the application in the engineering

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technologies15.

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methane gas, tetrahydrofuran and sodium dodecyl sulfate (SDS) at near ambient temperatures

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for large scale natural gas storage.

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uptake by the addition of SDS compared to the pure water system.

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the effect of surfactant such as SDS on the hydrate growth from methane-propane gas mixture.

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They observed the leaf-like and fiber-like hydrate crystals in the bulk water.

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relationship between hydrate growth dynamics and the hydrate morphology was not

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elucidated.

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between water and cyclopentane with surfactant such as PPG, naphthenic acid, and Span 80.

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They revealed that the hydrate crystal production increased and the size of individual hydrate

, but the hydrate growth behavior and hydrate crystal morphology changes depending on the Recently, Kumar et al.

Veluswamy et al.16 observed hydrate formation kinetics in the presence of

They reported the substantial increase in methane gas Yoslim et al.17 reported

However, the

Mitarai et al.18 observed hydrate crystal growth at the liquid/liquid interface

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crystals was larger compared to the system without surfactants.

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formation and the formation of smaller sized crystals were caused by the detachment of the

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hydrate crystals from the interface in the presence of surfactant.

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The enhanced hydrate

SDS is commonly used in industrial process because it is one of the most economical 4


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surfactants.

Though the morphology of hydrate systems with SDS have been studied, there

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has been little work, explaining the mechanism of hydrate growth.

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the mechanism of the SDS effect on the hydrate growth at the gas/liquid interface with the

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visual observations of hydrate crystal growth and morphology in the methane/water system

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with SDS.

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were examined to understand how SDS affects hydrate crystal growth and crystal

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morphology.

This study aims to reveal

Various SDS concentrations from 0 to wSDS =100 ppm with a step of size 10 ppm

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EXPERIMENTAL SECTION

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The fluid samples used in the experiments were deionized and distilled water and

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methane (99.99 vol %, Takachiho Chemical, Co.). The surfactant used in the study was SDS

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(99.0 %, Aldrich Chemical Co.). SDS aqueous solution samples were prepared at mass

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fractions from 0 to wSDS =100 ppm with increasing step of 10 ppm, where wSDS represents the

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mass fraction of SDS aqueous solution. The system subcooling ∆Tsub was defined as the

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difference between the experimental temperature and the equilibrium temperature of methane

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hydrate (∆Tsub ≡ Teq - Tex) under the given experimental pressure as an indicator of the

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driving force for the crystal growth.

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A schematic diagram of the main portion of the experimental apparatus was shown in

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Fig. 1. The experimental section holding hydrate crystals and methane gas is a cylinder made

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from stainless steel and a pair of glass windows for the observation. The internal space of test

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section is 20 mm in axial length by 25 mm in diameter. To control the temperature of test

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section, ethylene glycol aqueous solution was circulated in the jacket which covered the

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experimental vessel. The experimental temperature, Tex was measured by Pt-wire

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thermometer inserted directly under the Teflon stage which is 6 mm or 13 mm in diameter.

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The SDS aqueous solution was deposited and hold on the Teflon stage. Methane gas 5



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is supplied and depressured repeatedly to replace the air in test section with methane gas. The

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experimental pressure, P was set to a prescribed level by supplying methane gas. P was

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measured by a stain gauge pressure transducer. The uncertainty of temperature and pressure

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measurements were ±0.2 K and ±0.05 MPa, respectively.

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It was confirmed that equilibrium temperature of the hydrate formed between

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methane and SDS aqueous solution was equal to the methane hydrate equilibrium temperature

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in any conditions19.

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explained in the previous study by Mitarai et al.18

The detailed procedure of equilibrium temperature measurements was

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After the droplet of SDS solution and methane were placed in the test cell, the

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experimental pressure was set to P = 7.8 MPa and the experimental temperature T was first

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decreased to approximately 270 K to form a hydrate.

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was increased to 1 K higher than the equilibrium temperature of methane hydrate, Teq = 283.9

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K to dissociate the hydrate under the experimental pressure.

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dissociation of all hydrate crystals, Tex was set at a prescribed experimental temperature, such

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that ∆Tsub was in the range from 1.3 K to 9.0 K, to observe the formation and growth of

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hydrate crystals in each experimental condition. In this manner, the memory effect was used

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to shorten the induction time for hydrate formation.

Then, the temperature of the test cell

After visually confirming the

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Concurrent monitoring and recording of the formation and growth of the hydrate

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crystals were conducted using a CCD camera (Olympus, DP72) and a microscope

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(VZMis450i, Edmond optics).

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RESULTS AND DISCUSSION

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Hydrate crystal growth behavior

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The sequential images of the methane hydrate crystal growth at the interface of methane and water with/without SDS are shown in Fig. 2.

The figure shows images of the 6


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system with SDS concentration at wSDS = 0 and wSDS = 20 ppm.

The time when the first

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hydrate crystal was visually confirmed is defined as t = 0 and the elapsed time is shown under

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each image in Fig. 2.

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(gas/liquid interface) (Fig. 2 (1a, 2a)), and then laterally grew (Fig. 2 (1b, 2b)) and finally

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covered the whole droplet surface (Fig. 2, (1c, 2c)).

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covered the droplet, no remarkable subsequent hydrate growth was observed. The crystal

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growth behavior was almost similar in wSDS ≤ 20 ppm system but crystal morphology was

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distinct between pure water and wSDS = 20 ppm as described in detail below crystal

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morphology section.

Nucleation occurred at random points on the droplet surface

After the hydrate film completely

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The hydrate growth behavior in the systems with SDS in the range from wSDS = 30

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ppm to wSDS = 100 ppm showed a distinct difference compared to those system where wSDS ≤

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20 ppm.

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shown in Fig. 3.

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formed hydrate at the gas/liquid interface detached and migrated to the bulk (Fig. 3, 1b).

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The accumulation of the detached hydrate crystals resulted in the conversion of the droplet of

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SDS aqueous solution to a chunk of crystals.

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out on the stage (Fig. 3, 1c).

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the droplet surface after the nucleation was observed (Fig. 3, 2a), then the fiber crystals grew

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vertically from the point of contact between the droplet of SDS aqueous solution and the stage

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(Fig. 3, 2c).

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compared to those in the wSDS ≤ 20 ppm systems based on visual observation.

The representative hydrate growth behavior in this range of the concentration was At ∆Tsub < 6.0 K, the hydrate crystals did not cover the droplet surface but

The droplet with the hydrate crystals spread

At ∆Tsub ≥ 6.0 K, the hydrate crystals instantaneously covered

The observation showed that the amount of formed hydrate crystals increased

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Crystal morphology It was difficult to observe the crystal morphology because the droplet shape was not maintained and thus the individual crystals could not be identified at wSDS ≥ 30 ppm.

In the 7



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previous study, morphology change was observed when the surfactant such as SDS is present

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at the condition with wSDS ≥ 25 ppm20.

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morphology that was clearly influenced by SDS, the concentration wSDS = 20 ppm may be

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favorable because the effect of SDS on the methane hydrate crystal growth was observed at

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this concentration while the droplet shape was maintained.

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morphology of the individual hydrate crystals between pure water system21 and wSDS = 20

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ppm system.

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The size of individual hydrate crystals was smaller and their shape varied from polygonal to

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sword-like with increasing ∆Tsub in both concentration.

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individual hydrate crystals in wSDS = 20 ppm system was smaller than those in the pure water

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system at a given ∆Tsub.

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nucleation by SDS reported in the previous studies22, 23. Crystal morphology should be fixed

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by the rate of nucleation in this system because the hydrate growth was prevented from

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contacting with other formed hydrates.

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morphology was determined from the balance between the rates of nucleation and crystal

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growth.

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restricted and the size of hydrate crystals was smaller in comparison to pure water system.

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These results were different from those found in Mitarai et al.18

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explained as follows: they observed the morphology in the condition that individual crystals

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could grow without ever contacting any other crystals as the hydrate crystals detached from

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the interface and fell into water.

With regard to the observation of hydrate crystal

Fig. 4 shows the comparison of

The typical individual hydrate crystals were enclosed by red lines in Fig. 4.

It should be noted that the size of

This observation may be ascribed to the enhancement of hydrate

As discussed in the previous study24, hydrate

When the rate of nucleation increased, hydrate crystal growth was strongly

This difference may be

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Crystal Growth Mechanism Fig. 5 shows the visual summary of the representative behavior for methane hydrate formation at various SDS concentrations.

As mentioned above, there were two types of 8


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characteristic behaviors occurring for the methane and SDS aqueous solution at the gas/liquid

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interface.

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growth mechanisms.

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Covering the interface

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The following figures from Fig. 6 through Fig. 8 further explained these hydrate

The first mechanism was observed when hydrate crystals initially forming on the

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interface and spread to cover the whole droplet surface.

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were in the red box in Fig. 5 at the conditions with wSDS ≤ 20 ppm, 1.3 K ≤ ∆Tsub ≤ 6.0 K, P =

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7.8 MPa.

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how the hydrate crystals covered the interface. These figures show how the hydrate crystal

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covers the interface and hydrate growth stops.

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covering the droplet surface, and this crystal barrier prevents the necessary interaction

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between water and methane to continue methane hydrate formation.

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Detached from the interface

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The conditions, at which this occurs,

Fig. 6 shows the representative sequential images of hydrate growth and explains

Hydrate crystals grow on the interface with

Another mechanism observed was the detaching of hydrate crystals from the

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interface after the crystal formed on the droplet surface.

This behavior is highlighted in the

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blue box in Fig. 5 at the conditions with wSDS ≥ 30 ppm, 1.3 K ≤ ∆Tsub ≤ 9.0 K, P = 7.8 MPa.

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Depending on the intensity of subcooling, the crystal growth differed.

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crystal growth direction was tangent to the substrate, whereas at ∆Tsub ≥ 6.0 K, the crystal

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growth direction was vertical.

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the hydrate crystal grow, then detached from the interface and then grow tangential to the

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substrate.

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substrate with the SDS aqueous solution, and therefore SDS aqueous solution inside the

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droplet permeated beyond the initial droplet edge and continued to outward crystallize newly

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formed interfaces.

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the increased wettability of the hydrate crystals with SDS aqueous solution.

At ∆Tsub < 6.0 K, the

Fig. 7 refers to wSDS = 100 ppm and ∆Tsub < 6.0 K, and shows

This spreading growth behavior would be caused by increased wettability of the

Why the crystals detached from the interface could also be found from When the 9



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hydrate crystals detached from the gas/liquid interface and formed new hydrate crystals on the

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gas/liquid interface, the concentration of SDS within the droplet increased because SDS was

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not enclosed into the hydrate.

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wettability of the stage surface would increase in response to the decreasing interfacial tension

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thereby resulting in the spreading of the droplet in the tangent direction.

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interfacial tension on the SDS concentration was previously reported by Watanabe et al.25.

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The permeated SDS aqueous solution continued to contact with methane gas. Therefore, it

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was observed that hydrate crystal growth keeps this spreading behavior.

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From the above, as the hydrate crystals formed in SDS system,

The dependency of

Fig. 8 is the explanatory diagram of vertical hydrate growth, which occurred for wSDS

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= 100 ppm and ∆Tsub ≥ 6.0 K systems.

The droplet of SDS aqueous solution rapidly

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crystalized because of the high subcooling, which promoted crystal nucleation and growth.

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Thereafter, the droplet was elevated from the substrate by pillars of fiber hydrate crystals.

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Similar to the aforementioned spreading mechanism, once the gas/liquid interface was

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covered, the remaining SDS increased the wettability of the substrate.

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permeated out of the initial droplet border.

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hydrate crystals in the system with SDS were smaller than those in the system without SDS as

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shown in the crystal morphology observation (Fig. 4).

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the pores between hydrate crystals were smaller, the SDS aqueous solution tended to permeate

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outside, contacted the methane gas and then crystallized very quickly.

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solution inside the lower part of the droplet continued to permeate outside and contacted the

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already formed crystal column and continue to grow by contact with the methane gas.

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Finally, columns of fiber-like hydrate crystals were generated.

Aqueous solution

The capillary force increased because formed

Since ∆Tsub was extremely high and

The SDS aqueous

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Macroscopic hydrate growth dynamics is known to be quite similar irrespective to

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the chemical species of the surfactants as clearly reported by Okutani et al10. We consider that

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the surfactant-effect mechanism found in this study would be common for the other surfactant 10


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systems but further experimental confirmation is necessary.

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CONCLUSION

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Visual observations of methane hydrate were used to understand the underlying

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physics of the two different surfactant effects, the antiagglomeration of hydrate crystals and

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the hydrate formation enhancement.

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phenomena that explain the surfactant effects.

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These observations successfully clarified the distinct

The amount of formed hydrate crystals increased, which was caused by detachment

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of hydrate crystals from the interface because of increased wettability.

Detachment of

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hydrate crystals is the phenomenon common both for the antiagglomeration and the enhanced

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hydrate formation.

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smaller compared to pure water system because of increased hydrate nucleation.

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subcooling temperature was extremely high, the hydrates formed upright crystal fibers which

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was distinct from the existing observed hydrate morphology in SDS system because capillary

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force was larger due to smaller pores between hydrate crystals.

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it is concluded that increased wettability and strong capillary action, both induced by SDS,

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control the crystal growth and morphology in the system with surfactant.

The size of individual hydrate crystals in systems with SDS present was When the

Based on these observations,

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FIGURE CAPTIONS Figure 1 Schematic diagram of the experimental apparatus. Figure 2 Sequential images of typical methane hydrate growth between methane and water with/without SDS at wSDS ≤ 20 ppm, P = 7.8 MPa. Figure 3 Sequential images of typical methane hydrate growth between methane and SDS aqueous solution at wSDS ≥ 30 ppm, P = 7.8 MPa. Figure 4 Summary of crystal morphology comparison between pure water system and wSDS = 20 ppm system depending on ∆Tsub. The typical individual hydrate crystals were enclosed by red lined. Figure 5 Visual summary of crystal growth behavior on gas/liquid interface for different ∆Tsub and mass fractions. The upper images were methane hydrate crystals without SDS, P = 8.15 MPa, the middle images and lower images were methane hydrate crystals with SDS at wSDS = 20 ppm, wSDS = 100 ppm, respectively, P = 7.8 MPa. Figure 6 Sequential images and schematic illustration explaining how the hydrate crystals covered the interface at wSDS = 10 ppm, ∆Tsub = 1.3 K, P = 7.8 MPa. Figure 7 Sequential images and schematic illustration explaining how crystals form and slide to the edges of the interface at wSDS ≥ 30 ppm, ∆Tsub < 6.0 K, P = 7.8 MPa. Figure 8. Sequential photographs and schematic illustration explaining how columnar growth occurs on the edges of the interface for wSDS ≥ 30 ppm, ∆Tsub ≥ 6.0 K, P = 7.8 MPa. 272 273

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Figure 1. Schematic diagram of experimental apparatus.

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Figure 2. Sequential images of typical methane hydrate growth between methane and water

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with/without SDS at wSDS ≤ 20 ppm, P = 7.8 MPa. 13



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Figure 3. Sequential images of typical methane hydrate growth between methane and SDS

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aqueous solution at wSDS ≥ 30 ppm, P = 7.8 MPa.

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Figure 4. Summary of crystal morphology comparison between pure water system and wSDS =

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20 ppm system depending on ∆Tsub. The typical individual hydrate crystals were enclosed by

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red lined.

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Figure 5. Visual summary of crystal growth behavior on gas/liquid interface for different

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∆Tsub and mass fractions. The upper images were methane hydrate crystals without SDS, P =

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8.15 MPa, the middle images and lower images were methane hydrate crystals with SDS at

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wSDS = 20 ppm, wSDS = 100 ppm, respectively, P = 7.8 MPa.

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Figure 6. Sequential images and schematic illustration explaining how the hydrate crystals

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covered the interface at wSDS = 10 ppm, ∆Tsub = 1.3 K, P = 7.8 MPa. 15



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Figure 7. Sequential images and schematic illustration explaining how crystals form and slide

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to the edges of the interface at wSDS ≥ 30 ppm, ∆Tsub < 6.0 K, P = 7.8 MPa.

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Figure 8. Sequential photographs and schematic illustration explaining how columnar growth

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occurs on the edges of the interface for wSDS ≥ 30 ppm, ∆Tsub ≥ 6.0 K, P = 7.8 MPa. 16


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AUTHOR INFORMATION

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Corresponding Author

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*E-mail: [email protected]

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENT

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This study was supported by a Keirin-racing-based research-promotion fund from the JKA

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Foundation and by JSPS KAKENHI Grant Number 25289045.

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For Table of Contents Use Only

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Title: “Surfactant Effects on Crystal Growth Dynamics and Crystal Morphology of Methane

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Hydrate Formed at Gas/Liquid Interface”

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Author: Hiroaki Hayama, Makoto Mitarai, Hiroyuki Mori, Jonathan Verrett, Phillip Servio

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and Ryo Ohmura

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Table of Contents Graphic and Synopsis

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Crystal growth of clathrate hydrate formed in the methane + SDS aqueous solution system

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was visually analyzed. Based on the visual observations, the controlling mechanism of the

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surfactant effect was discussed. It was found that the increased wettability and strong

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capillary action, both induced by SDS, control the crystal growth and morphology in the

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system with surfactant.

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Surfactant Effects on Crystal Growth Dynamics and ... - ACS Publications

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