Experimental Investigation on the Interaction Forces between


contact areas and adhesion forces between the hydrate particles and water droplets ... hydrates formed on the water bridge could dramatically strength...
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Experimental Investigation on the Interaction Forces between Clathrate Hydrate Particles in the Presence of a Water Bridge Chenwei Liu, Mingzhong Li, Litao Chen, Yuxing Li, Sixu Zheng, and Guangming Han Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00364 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 16, 2017

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Energy & Fuels

1

Experimental Investigation on the Interaction Forces between Clathrate

2

Hydrate Particles in the Presence of a Water Bridge

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Chenwei Liua,b, Mingzhong Lia*, Litao Chena, Yuxing Lib, Sixu Zhengc, Guangming Hand* a

4

College of Petroleum Engineering, China University of Petroleum, Qingdao 266580, China b

5

Shandong Provincial Key Laboratory of Oil and Gas Storage and Transportation, China University of Petroleum, Qingdao 266580, China

6 7

c

Petroleum Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, Regina, S4S 0A2, Canada

8 9

d

Science and Technology Development and School-Enterprise Cooperation Section, Chengde Petroleum College, Chengde 067000, China

10 11

Corresponding Authors:

12

Mingzhong Li

E-mail: [email protected]

13

Guangming Han

E-mail: [email protected]

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ABSTRACT

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The agglomeration of hydrate particles is one of the main causes of hydrate accumulation or bedding in oil

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and gas pipelines. The unconverted water droplets in the system play a crucial role in hydrate particle

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agglomeration. In this study, a novel technique was developed to directly measure the interaction forces between

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cyclopentane hydrate particles with a water bridge between them by using a micromechanical force (MMF)

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apparatus. On the basis of the developed method, the interaction forces at different temperatures and water bridge

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volumes and the effects of mineral oil were experimentally studied. At a low subcooling level of 1.7 °C, the

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contact areas and adhesion forces between the hydrate particles and water droplets increased with the increase in

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the water droplet volume. For the case of a higher subcooling level of 6.2 °C, rapid hydrate formation between the

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bridging water droplets prevented the hydrate particles from being effectively wetted, resulting in a small contact

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area. There was no clear relationship between the contact area/adhesion force and the water droplet volume. The

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hydrates formed on the water bridge could dramatically strengthen the adhesion force, while the addition of

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mineral oil in cyclopentane could retard the water droplets from converting into hydrates, consequently decreasing

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the adhesion force. The measured adhesion forces between the hydrate particles and water droplets may provide

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further insight into the hydrate agglomeration process.

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Key words: hydrate agglomeration, micromechanical force apparatus, adhesion force, unconverted water droplet,

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hydrate particle

41 42

INTRODUCTION

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Gas clathrate hydrates are crystalline inclusion compounds comprised of hydrogen bonded water cavities that

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encapsulate suitably sized gas molecules (e.g., methane, ethane, propane, and carbon dioxide) at appropriate

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pressures and temperatures.1-3 As gas and oil exploration and production move to ultradeep water (>8000 ft of sea

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water), the oil and gas production pipelines provide favorable conditions (i.e., high pressure and low temperature) 2

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for gas hydrate formation. The aggregation and accumulation (or bedding) of hydrates in pipelines can lead to

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pipeline blockage, resulting in catastrophic operational failures and safety hazards to personnel and equipment,

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thereby presenting a major flow assurance issue in oil and gas pipelines.1 Therefore, it is of fundamental and

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practical importance to investigate and understand the agglomeration mechanism of hydrates in pipelines.

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For an oil-dominated system, a conceptual model with four successive steps has been proposed for describing

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hydrate plug formation in pipelines.4 The proposed model suggests that hydrate agglomeration is the limiting

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factor during plug formation.1 In fact, hydrate agglomeration processes can be considered as analogous to wet

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granulation processes. In the pharmaceutical industry, granulation/agglomeration technology has been widely

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employed, which refers to acts or processes in which primary powder particles are made to adhere to form larger

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multiparticle entities called granules.5 In particular, as one of the major granulation technologies applied in the

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pharmaceutical industry, wet granulation processes involve adding a liquid solution/binder to powders, forming

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bonds between the powdered particles.6 The whole process can be regarded as a combination of four sets of

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processes (see Figure 1):7-9 (i) spraying, in which liquid binder is added to the system, and the diameters of the

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binder can vary from dozens to thousands of microns;10 (ii) moistening, in which the liquid binder is brought into

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contact with a dry powder, forming granules; (iii) consolidating, in which collisions between two granules or

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granules and feed powder lead to granule compaction; and (iv) growing. In oil and gas pipelines, emulsified water

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droplets in crude oil typically have a mean diameter between 20–80 µm.11 When some water droplets convert into

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hydrate particles, as expected, the unconverted droplets can serve as the added liquid solution/binder, while the

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water–wet hydrate particles will play the role of powders during wet granulation. However, the water droplet will

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gradually convert into a solid hydrate during the agglomeration process, resulting in more complex hydrate

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agglomeration than conventional wet granulation. Although wet granulation provides evidence of the crucial role

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of unconverted droplets during the granulation/agglomeration process, multiple studies to date have merely

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focused on dry hydrate–hydrate particle interactions.12-17 It has been found that the adhesion forces between dry 3

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Wet granulation process

Powder

Spraying

Consolidating

Moistening

Growing

Hydrate 70 71

Hydrate agglomeration process Figure 1. Schematic of the wet granulation/ proposed hydrate agglomeration process

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hydrate particles are much smaller and are hard to account for regarding the remarkable hydrate agglomeration

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tendency observed in real oil-gas pipelines.18 In addition, the interactions between dry hydrate particles also

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confirm the critical role of unconverted water (i.e., quasi-liquid layer) in the adhesion force.12,16 Recently, Song et

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al.,19-20 Cha et al.21 and Liu et al.8 investigated the interaction between a hydrate particle and a water droplet,

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which can be regarded as studies on the moistening process during hydrate agglomeration. During their

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measurements, the liquid droplets had to be placed on flat plates. As suggested by previous experimental studies,

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the characteristics of the flat plates (e.g., roughness and wettability) will affect the interaction behavior/force.19, 22

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Fidel-Dufour et al.23 suggested that hydrate agglomeration occurs through three-body collisions between one

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unconverted water droplet and two already formed agglomerates/particles due to high crystal and droplet

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concentrations. However, to the best knowledge of the authors, few attempts in the literature have been made to

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investigate the important hydrate-droplet-hydrate adhesive interactions during hydrate agglomeration.

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In our previous work, we studied the interactions between hydrate particles and water droplets placed on a

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flat plate using a micromechanical force (MMF) apparatus.8 To eliminate the interference of the plate, in this study, 4

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we proposed a novel procedure to directly measure the cyclopentane hydrate particle-water droplet-hydrate

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particle interactions. On the basis of the procedure, the hydrate-droplet-hydrate interactions were experimentally

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investigated, and the effects of temperature, water droplet volume and mineral oil were discussed, which provided

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deeper insight into the hydrate agglomeration process.

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EXPERIMENTAL SECTION

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Chemicals

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Cyclopentane (CyC5) was used as a model hydrate former under atmospheric pressure because it is stable up

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to 7.7 °C at ambient pressure.1 Furthermore, CyC5 is superior to other ambient pressure hydrate formers (e.g.,

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tetrahydrofuran) because CyC5 is immiscible in the aqueous phase and forms structure II hydrates, which are

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usually encountered in petroleum fields. The CyC5 (98% purity) used in this work was purchased from Fisher

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Scientific UK Ltd. Mineral oil 70T (STE Oil Company, Inc.) with a density of 0.857 g/cm3 at 20 °C was used in

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this study, and the compositional analysis of mineral oil 70T can be found elsewhere.8

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Interaction measurement procedures

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A micromechanical force apparatus developed by the Colorado School of Mines Center for Hydrate Research

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was used for measuring the CyC5 hydrate particle-water droplet-hydrate particle interactions. The MMF apparatus

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includes a Zeiss S100 inverted light microscope that is encased in a dry box with a mean relative air humidity

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lower than ~20%. The microscope stage holds an aluminum cell (see Figure 2) that is surrounded by a cooling

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jacket connected to an external refrigeration unit (1196D, VWR International, Inc.). A circular cover glass was

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placed on the bottom of the cell for visualization. The experimental cell was filled with liquid CyC5 and housed

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two cantilevers. One cantilever was connected to a manual micromanipulator, while the other cantilever was

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connected to a remotely operated micromanipulator (Eppendorf Patchman 5173). The MMF system has been 5

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CyC5 Aluminum Rod

Microcapillary Tube

Glass Fiber

109 110

Flat Plate

Glycol Cooling Jacket

Cover Glass

Figure 2. The experimental cell setup for the hydrate particle and water droplet interaction measurements.

111 112

extensively used to measure hydrate-hydrate interaction forces.12-17 Recently, the interaction between a hydrate

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particle and a water droplet was also investigated using the MMF system.8 A more detailed description of the

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experimental system can be found elsewhere.8, 16-17 To conduct the measurements, the droplet had to be placed on

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a flat plate, and the characteristics of the plate (e.g., roughness) would affect the experimental results.19 Herein, in

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order to eliminate the interference of the plate, on the basis of previous observations, the following procedures

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were proposed for directly measuring the hydrate particle-droplet-particle interactions.6 The entire measurement

Epoxy-A Droplet Glass fiber

Ice

(a)

(g)

(h)

118 119 120 121

Hydrate

Liquid N2 (b)

(c)

(e)

(f)

(j)

(i)

(d)

(k)

Figure 3. Schematic of the proposed experimental procedure to directly measure the hydrate particle-water droplet-hydrate particle interactions. The procedures in the red dotted lines indicate the measurements that were conducted in liquid CyC5.

122 6

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process included four successive steps (see Figure 3): (i) Generate two hydrate particles (Figures 3a–c). The

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dropper technique was used to form a droplet on the end of each glass fiber cantilever (diameter: approximately

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38 µm),8 and one droplet was then transferred from one glass fiber to the Epoxy-A-coated layer in air (Figure 3a).

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The two droplets were quenched in liquid nitrogen for 20 s to completely convert them into ice particles (Figure

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3b). The ice particles were placed immediately into the CyC5 bath (maintained at -5 °C) within 2 s. The liquid

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CyC5 temperature was then slowly increased above the ice freezing temperature (0 °C) with a heating rate of

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0.1 °C/min, allowing the ice particles to convert into CyC5 hydrate particles (Figure 3c). At the experimental

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temperatures (i.e., 1.5 °C and 6 °C), the hydrate particles were immersed in the CyC5 cell for 30 min. (ii) Mount a

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droplet on the plate. The same dropper technique was used again to form a water drop on the end of the other glass

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fiber (spare) cantilever. The water droplet was then transferred from the glass fiber (spare) cantilever to the

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Epoxy-A-coated plate immersed in the CyC5 cell with a contact angle of 136.44±2.77 ° (Figure 3d). (iii) Generate

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a hydrate-droplet dimer. The position of the plate (Figure 3e) was adjusted while contacting the droplet and

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hydrate particle on the stationary glass fiber (Figure 3f). The plate was then retracted quickly. As a result, some of

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the water was left on the hydrate particle on the fiber, forming a hydrate particle-droplet dimer (Figure 3g). Quick

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retraction could reduce the amount of liquid water that was converted into the solid hydrate, especially at a lower

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temperature (i.e., higher subcooling). (iv) Measure the hydrate particle-droplet-hydrate interactions. Adjust the

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position of the plate quickly (Figure 3h), and then, begin the measurement. The measurement procedure included

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“approaching” and “retracting”. During the approaching process (Figures 3h and i), the hydrate particle on the

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plate was slowly brought into contact with the hydrate particle-droplet dimer on the glass fiber, creating a preload

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force. During the retracting process, the hydrate particle on the plate moves away from the other hydrate particle

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slowly until the hydrate particle-droplet-hydrate particle complex detaches (Figures 3j and k). The displacement of

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the glass fiber cantilever during the measurement was captured in real time at 30 frames per second. The

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displacement was multiplied by the cantilever spring constant (i.e., Hooke’s law) to determine the interaction 7

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force between the hydrate particles and droplets.

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RESULTS AND DISCUSSION

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With the aforementioned procedure, the interactions between the CyC5 hydrate particles and water droplets

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in pure CyC5 at 6 °C (i.e., subcooled by 1.7 °C) were measured. The experimental results are shown in Figures 4

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and 5. Figure 4 shows the microscope images of the preparation processes before the final measurements. In

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Figure 4a, two ice particles, which were placed on the glass fiber and flat plate, respectively, were immersed in

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CyC5. As the liquid CyC5 temperature was slowly raised above the ice-freezing temperature, the ice particle on

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the plate dissolved first and converted into a hydrate particle. The other ice particle on the glass fiber then

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dissolved and formed a hydrate particle (Figure 4c). After a period of 30 min of annealing at 6 °C, a droplet was

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introduced onto the plate with another glass fiber (spare) cantilever (Figure 4d). Figures 4e and f show the

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formation process of the hydrate-droplet dimer, and hydrate was not visibly formed from the droplet due to the

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much lower subcooling level (i.e., 1.7 °C).

Ice

Ice

Hydrate

Hydrate

a

b

c

d

e

f

Droplet

159 160 161 162

Figure 4. Microscope images of hydrate particles and hydrate particle-droplet dimer formation at 6 ºC. The black moon shape shadow on the top part of the droplet is the shadow of the flat plate on it.

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Figure 5 shows the hydrate-droplet-hydrate interaction force as a function of the plate displacement, together

164

with the respective microscope images. Before the interaction, the plate was adjusted to set the positions of the

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hydrate particle and hydrate-droplet dimer, as shown in Figure 5A. The plate then slowly approached the hydrate

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particle-droplet dimer, and no measurable net interaction force acted upon the two surfaces until point B, in which 8

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10 51.4 s

0s

200 µm

I

Interaction Force/µN

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Energy & Fuels

200 µm

4.6 s

200 µm

0 C

-10 7.5 s 51.0 s

-20

19.8 s

200 µm

E 32.6 s

168 169

-400

200 µm

200 µm

G

-30 -600

200 µm

200 µm

H 44.0 s

167

D

B

Approaching A

Retracting

F -200 0 Plate Displacement/µm

200

400

Figure 5. Typical interaction force curve of the CyC5 hydrate particle-water droplet-hydrate particle interaction in pure CyC5 at 6 °C. The images at various key locations are shown to help interpret the force curve.

170 171

the droplet rapidly spread on the top hydrate particle surface. A hydrate-CyC5-water three-phase-contact (TPC)

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line was formed and pinned on the top hydrate particle surface, resulting in a rapid drop in the interaction force

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from point B to C (initial contact force), which was ascribed to the hydrophilic properties of the hydrate particle.

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As shown in Figure 5C, the original droplet then behaved as a liquid bridge that bonded the two hydrate particles.

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As the plate continued to approach the bottom hydrate particle, the liquid bridge gradually changed from a

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stretching to a compression state. Correspondingly, the interaction force between the particles shifted from an

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attractive force to a repulsive force. At point D, the repulsive force reached the set value (preload force: ~12 µN).

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Once the preload force was reached, the plate retracted away from the bottom hydrate particle. The interaction

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force first presented an approximately straight line (D to E), and then, it curved from point E to F because of the

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large liquid bridge deformation. At point F, the interaction force reached the maximum level (i.e., adhesion force:

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~31 µN) and remained constant until point G. After the plateau, the interaction force began to decrease when the

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liquid bridge was stretched further, and necking occurred in the liquid bridge. Finally, at point H, a significantly 9

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narrow neck formed, and the liquid bridge ruptured at the neck. The liquid bridge was distributed between the two

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hydrate particles, and the shape of the droplets was approximated as a perfect sphere. During the entire

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measurement process, both TPC lines were pinned on the hydrate particle surface due to strong wetting hysteresis

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between the CyC5 hydrate and water.24 Furthermore, the outlines of the liquid bridge intersecting with the hydrate

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particles presented a smooth curve, which suggests that hydrate was not visibly converted from the liquid bridge

188

under this condition.

40 Interaction force/µN

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200 µm

200 µm

30

Cycle 1

Cycle 2

200 µm

Cycle 3

200 µm

Cycle 4

20

10

0

189 190 191 192 193 194

Rupture force

Maximum force

Figure 6. Reproducibility measurements of the hydrate particle-water droplet-hydrate particle interaction forces at 6 °C. The hydrate was not visibly converted from the liquid bridge during the measurement. The coefficient of variation of the forces was less than 3%.

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The rupture of the original liquid bridge led to the formation of two hydrate-droplet dimers. Then, the same

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approaching-retracting procedure was repeated three times for the two hydrate-droplet dimers, and the results are

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shown in Figure 6. The maximum interaction force and rupture force exhibited good repeatability, and the

198

coefficients of variation of the forces were less than 3%. The corresponding images also suggest that the amount of

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liquid distributed to the hydrate particles was approximately same for the four test cycles. The high repeatability

200

confirmed the reliability of the experimental apparatus and measurement procedure. Meanwhile, the repeatability

201

further implies that the total liquid amount remained approximately constant during the entire measurement at 10

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202

6 °C, that is, hydrate was not visibly converted from the liquid bridge. In the literature, the adhesion force between

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dry hydrate particles has exhibited considerable variability.12-17 As suggested by Yang et al.,12 the variability arises

204

because the nanoscale surface asperities have a slightly different alignment during each repeated contact, and a

205

given surface roughness distribution therefore leads to a probabilistic distribution of the measured forces. In this

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study, the presence of the water droplet on the hydrate particle surface submerged all the asperities and prevented

207

direct contact between the dry hydrate particles, thereby eliminating the variability resulting from the surface

208

roughness.

209

10 20.8 s

200 µm

A

Approaching

1.2 s

B

200 µm

D

0 C

-10

4.5 s 1.7 s

-30 -600

200 µm

200 µm

E

-20 G

210

0s

200 µm

H

Interaction Force/µN

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19.4 s

200 µm

12.6 s

F

200 µm

Retracting

-400

-200 Plate Displacement/µm

0

200

211 212 213 214

Figure 7. Typical force curve of the CyC5 hydrate particle-water droplet-hydrate particle interaction in pure CyC5 at 1.5 °C. The red lines in the images denote the TPC line. The images indicate the amount of liquid water converted into solid hydrate during the measurement.

215

The temperature/subcooling was found to play an important role in the dry hydrate-hydrate and

216

hydrate-droplet interactions.8,12,16,24 Herein, corresponding to Figure 5, the interaction behavior in pure CyC5 at a

217

lower temperature (1.5 °C, i.e., at a subcooling level of 6.2 °C) is shown in Figure 7. During the preparation

218

process of the hydrate particle-droplet dimer, although the proceeding time was reduced as much as possible, a

219

significant amount of water was still converted into hydrate due to the high level of subcooling and induction of 11

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the bottom hydrate particle (Figure 7A). At point B, the droplet contacted the top hydrate particle and

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subsequently spread on the particle surface. However, due to the high subcooling level, the water molecules

222

around the TPC line tended to rapidly form additional hydrates that prevented the effective wetting of water on the

223

top hydrate particle. As shown in Figure 7C, the contact area between the droplet and top hydrate particle was

224

quite small despite the relatively large droplet volume. In the subsequent measurements, the hydrate continued to

225

grow from both boundaries toward the middle. As indicated in Figure 7F, a significantly raised hydrate platform

226

evolved around the contact region of the liquid bridge and top hydrate particle. Finally, the liquid bridge broke,

227

and a small amount of water was distributed to the top hydrate particle due to the relatively small contact area

228

between the liquid bridge and top hydrate particle. Meanwhile, a large part of the droplet left on the bottom

229

hydrate particle was encased by the hydrate shell.

230 231

30 104.3 s

0s

200 µm

I

200 µm

Approaching

7.7 s

200 µm

C

A

0 D

Interaction Force/µN

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B

-30 0.7 s

200 µm

-60 69.0 s 90.2 s

F

-90 -120

F

200 µm

E

200 µm

84.4 s

200 µm

Retracting

-150 -800 232

-600

-400 -200 Plate Displacement/µm

0

200

233 234 235 236

Figure 8. Typical hydrate particle–water droplet-hydrate particle interaction force profile in which the hydrate shell fully wrapped the liquid droplet before the “retracting” phase. The test temperature was 1.5 °C. The purple circles are used to emphasize the deformation/ collapse of the hydrate shell.

237

To highlight the effects of the hydrate formation, the following measurements were performed (see Figure 8). 12

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238

Before retraction, the particles and droplet were allowed to rest under a constant preload force to allow the hydrate

239

shell to fully cover the liquid bridge (C to D, ~61 s). As indicated in Figure 8D, since the hydrate shell grew from

240

two opposite directions, a remarkable nucleation trace was formed near the middle of the liquid bridge. During the

241

retraction process, the convex hydrate bridge between the particles first maintained its original shape for a

242

relatively low pull-off force. With the further increase in the pull-off force, the hydrate bridge became unstable,

243

and one side of the bridge collapsed (see Figure 8E). The entire retraction force curve presents a linear region due

244

to the relatively small deformation of the solid hydrate bridge. Meanwhile, the corresponding adhesion force was

245

increased significantly (~150 µN) because the tensile strength of the hydrate shell governed the interaction force.

246

After the rupture of the hydrate bridge, the original collapsed hydrate shell retrieved its shape and became

247

spherical because of the capillary force. These phenomena suggest that although the liquid water was covered by

248

the solid hydrate shell, the strength of the hydrate shell under these conditions was not sufficiently strong to resist

249

the deformation resulting from the capillary force.

10 53.9 s

200 µm

I

Interaction Force/µN

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

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0s

200 µm

Approaching

0

A

1.5 s

200 µm

B

-10

D

C 13.3 s 1.8 s

-20

200 µm

200 µm

E

-30

H

41.8 s

200 µm

G 28.8 s

F

-40 -600 250 251 252 253 254

-400

200 µm

Retracting

-200 0 Plate Displacement/µm

200

400

Figure 9. Typical interaction force curve of the CyC5 hydrate particle-water droplet-hydrate particle interaction in a 50 wt% CyC5 and 50 wt% mineral oil 70T mixture at 1.5 °C. The images suggest that hydrate was not visibly converted from the liquid droplet during the measurement.

13

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Previous studies on dry hydrate particle-particle interactions suggested that the addition of mineral oil to

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CyC5 (1-50 wt%) did not have a significant effect on the hydrate morphology and cohesion force.15 Figure 9

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shows the force behavior of the hydrate particle-droplet-particle interaction in a mixture of CyC5 and mineral oil

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70T (mass ratio 1:1) at 1.5 °C. Compared with Figure 5, the hydrate was not visibly converted from the liquid

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bridge throughout the entire measurement despite having the same bath temperature, suggesting that the addition

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of mineral oil decreased the hydrate formation rate, which was attributed to the reduction in the subcooling level.8

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A previous study and Figure 8 indicated that hydrate formation during the measurement would increase the

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adhesion force between the hydrate and droplet.8 Thereby, the addition of mineral oil could decrease the adhesion

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force.

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For the water-in-oil emulsions, the sizes of the droplet were not uniform and presented polydispersity, e.g., a

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normal or log-normal distribution.25 Understanding the effects of the liquid volume of the droplet on the

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interaction force provided important insight into the agglomeration process. Figures 10a and b show the images

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before and after the formation of the hydrate-droplet dimer, respectively. The liquid volumes of the droplets

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(spherical cap) on the plate were calculated using the following equation:

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    6  2  

270

 



(1)



where  is the volume of the droplet on the plate and R is the apparent radius of the droplet.

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h R

272 273

200 µm

200 µm

Figure 10. Images (a) before the droplet contacted the hydrate particle and (b) after the liquid droplet ruptured.

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p u re C yC 5 , T = 6 ℃ p u re C yC 5 , T = 1 .5 ℃

400000 350000

Contact Area/µm

2

300000 250000 200000 150000 100000 50000 24 0 pu re C yC 5, T= 6 ℃ pu re C yC 5, T= 1.5 ℃

21 0 Normalized Ahesion Force/mN/m

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

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18 0 15 0 12 0 90 60 30 0 0.0

0 .3

0 .6

0.9 1.2 V d/V p

1.5

1.8

2.1

275 276 277

Figure 11. Hydrate particle-water droplet contact area and normalized adhesion force versus the volume ratio of the droplet to hydrate particle at different temperatures. The normalized force refers to the adhesion force divided

278

by the harmonic mean radius of the hydrate particle pair, ∗, where



∗







     . 



279 280

The original volume of the liquid bridge bonding the two hydrate particles was equal to the volume difference

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between the droplets on the plate in Figures 10a and 10b. Figure 11 shows the effects of the water volume on the

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contact area and the normalized adhesion force between the hydrate particles in pure CyC5 at 1.5 and 6 °C. For

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the measurements at 6 °C, at a relatively lower Vd/Vp, the increase in the contact area with the water volume was

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very slow. In addition, the contact area then increased dramatically with the further increase in the water volume.

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Correspondingly, the adhesion forces presented a similar tendency with the contact areas. A similar phenomenon

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was also reported in the experiments conducted by Cha et al..19 Compared to the measurements at 6 °C, the

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hydrate-droplet contact areas in the measurements at 1.5 °C were quite scattered. There was no clear relationship

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between the contact area and water volume. Furthermore, the contact areas from the measurements at 1.5 °C were

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generally significantly smaller than those from the measurements at 6 °C. Both phenomena were attributed to the 15

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higher hydrate formation rate at 1.5 °C. As aforementioned, once the droplet contacted the hydrate particle, a

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higher subcooling/driving force promoted the water molecules around the TPC line to rapidly form additional

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hydrates. The additionally formed hydrates prevented the effective wetting of water on the hydrate particle surface,

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resulting in a smaller contact area. In addition, due to the formation of the additional hydrates, the effects of the

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water volume on the contact area were disturbed by the high hydrate formation rate. Corresponding to the

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scattered contact areas, the adhesion forces were also scattered. Furthermore, although the contact areas from the

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measurements at 1.5 °C were significantly smaller than those from the measurements at 6 °C, the corresponding

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adhesion forces at 1.5 °C were larger than those from the measurements at 6 °C. As aforementioned, the hydrate

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formation from the liquid water could strengthen the adhesion forces between the hydrate particle and droplet.

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For oil dominated systems, the agglomerations of the hydrate particles and droplets were porous. Since the

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aggregates in the fluid acted hydrodynamically like compact spheres, screening the amount of oil (continuous

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phase) in the interior from the outside, an effective hydrate volume fraction was considered instead of the real

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volume fraction. The effective hydrate volume fraction can be written as 26

303





     

(2)

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where  is the effective hydrate volume fraction,  is the real volume fraction, RA is the radius of the

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aggregates, R is the radius of the particle/droplet, and fr is the fractal dimension of the aggregates.

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Figures 7 and 11 suggest that at high level of subcooling, a high hydrate formation rate prevented the

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effective wetting of water on the hydrate particle surface, thereby resulting in a smaller contact area. Consequently,

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it tended to form loose aggregates, resulting in a smaller fr. Furthermore, according to the fact that the aggregate

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size depended on the balance between the shear stress and adhesion force between the particles, a larger adhesion

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force tended to generate larger aggregates. Both the smaller fr and larger aggregate size led to a larger effective

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hydrate volume fraction. Therefore, the viscosity of the hydrate slurry was higher at a higher subcooling

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level/lower temperature. Such findings are in accordance with the results of Webb et al..27, 28 They investigated the 16

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313

effects of the initial pressure and temperature on the rheological properties of methane hydrate slurries formed

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from water-in-oil emulsions. They found that both the hydrate slurry viscosity and yield stress increased with

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increase in the initial pressure and decrease in temperature (i.e., increasing the hydrate formation rate). High formation rate

316 317 318

Low formation rate

Figure 12. Schematic of the microscopic structure of the hydrate particle-water droplet agglomerations at different formation rates. 240

Normalized Adhesion Force/mN/m

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

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H-D-H, 6.0℃ H-D-H, 1.5℃ 8 H-D-W,1.5℃ 8 H-D-W,7.0℃ 16 H-H, 3.2℃

200

160

120

80

40

0

0

50000 100000 150000 200000 250000 300000 350000 400000 2

Contact Area/µm

319 320 321 322 323

Figure 13. Comparisons between the cyclopentane hydrate particle-hydrate particle (H-H),16 hydrate particle-droplet-aluminum plate/wall (H-D-W) 8 and hydrate particle-droplet-hydrate particle (H-D-H) normalized adhesion forces.

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Extensive experimental and theoretical works have studied the adhesion forces between dry cyclopentane

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hydrate particle-particle (H-H) and cyclopentane hydrate particle-droplet-wall/plate (H-D-W) interactions.8, 12-17,

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19-22

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As shown in Figure 13, compared with the case of dry hydrate particles (~4.3 mN/m at 3.2 °C), the existence of

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unconverted water droplets significantly increased the adhesion forces between the hydrate particles. The

Herein, the adhesion forces of these two cases were compared with the adhesion forces in this study (H-D-H).

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adhesion forces were at least an order of magnitude larger and increased as more water converted into hydrate

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particles. Compared with the hydrate-droplet-aluminum plate interactions, at a smaller extent of subcooling (i.e.,

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1.7 or 0.7 °C), the hydrate was not visibly converted from the water droplets during both measurements, and the

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corresponding adhesion forces of these two cases were in good agreement. Notably, if the aluminum plate was

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replaced with other plate materials, the corresponding adhesion forces might have exhibited significant deviations

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in this study because the experimental work and theoretical model confirmed that the characteristics of the plate

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(e.g., wettability) could affect the adhesion force. At a high level of subcooling (i.e., 6.2 °C), the adhesion forces

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in this work were significantly larger than hydrate particle-water droplet-aluminum plate interactions. This result

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was attributed to the amount of hydrate formed from the droplet during both measurements at 1.5 °C, for the case

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of the hydrate-droplet–wall/plate interactions, the hydrate shell growth began from one side (i.e., the interface

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between the hydrate particle and water bridge) and moved to the other side, while for the measurements in this

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study, as shown in Figure 7 and 8, the hydrate shell growth began from two sides and moved along the capillary

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bridge. Hence, more hydrates were formed from the water droplets during the present measurements. According to

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the experimental and theoretical work, more hydrate formation would lead to larger adhesion forces.8, 24, 29

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Previous reports and our work suggest that the pendular liquid bridge model can account for hydrate

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particle-particle or hydrate particle-plate interactions in the presence of a water bridge.7,

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indicates that the interaction force is proportional to the water-hydrocarbon interfacial tension. Based on this

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quantitative relation, the adhesion forces of the natural gas hydrates particle in high-pressure systems can be

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preliminary estimated with the adhesion forces present in this work. The interfacial tension of a

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water-hydrocarbon system varies from approximately 72 mN/m for water/gas systems to 20 to 50 mN/m for

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water/oil systems at atmospheric conditions.31-32 With the difference in the interfacial tension from the present

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pure cyclopentane system (~48.7 mN/m),8 the adhesion forces of natural gas hydrate particles can be determined

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to be approximately 1.5 times for water/gas systems and 2/5 to 1 as large for water/oil systems compared to those 18

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24, 29-30

The model

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in this work. Notably, the above preliminary estimations were determined using the water-hydrocarbon interfacial

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tension at atmospheric conditions. In fact, the pressures and temperatures in realistic pipelines will affect the

354

interfacial tension.31-32 Furthermore, in addition to the interfacial tension, the adhesion forces were also affected

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by the contact area between the hydrate particle and water droplet, growth rate of the hydrate shell and volume of

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the droplet.24, 29 Therefore, the accuracy of the preliminary estimations needs to be verified by future natural gas

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hydrate adhesion force measurement experiments under high pressure.

358 359

CONCLUSIONS

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In this study, we proposed a novel method to directly measure the interaction between hydrate particles and

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water droplets. The effects of the temperature, liquid water volume and mineral oil on the interaction behavior

362

were investigated. Unconverted water droplets played a crucial role in the hydrate particle agglomeration process.

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At a smaller extent of subcooling, as the water droplet volumes increased, the contact areas and adhesion forces

364

between the hydrate particles and water droplets increased moderately, followed by a significant increase. For a

365

higher level of subcooling, a higher hydrate formation rate prevented effective wetting of water on the hydrate

366

particle surface, thereby resulting in smaller contact areas. In addition, no clear relationship existed between the

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contact area/adhesion force and the water droplet volume. The hydrate formed from the liquid bridge could

368

dramatically strengthen the adhesion force, and the addition of mineral oil retarded the liquid droplets from

369

converting into hydrates, consequently decreasing the adhesion force.

370 371

AUTHOR INFORMATION

372

Corresponding Authors

373

*E-mail: [email protected]

374

[email protected] 19

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375

Notes

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The authors declare no competing financial interest

377 378

ACKNOWLEDGEMENTS

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The authors acknowledge the support from the China Postdoctoral Science Foundation funded projects

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(Grant No. 2015M580619 and 2016T90659), Shandong Provincial Natural Science Foundation (Grant No.

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ZR2016EEB04), and Program for Changjiang Scholars and Innovative Research Team in the University (Grant

382

IRT1294). Chenwei Liu also thanks Professor Carolyn A. Koh (Center for Hydrate Research, Colorado School of

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Mines) for her generous support and guidance on the MMF measurements.

384 385

REFERENCES

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