Surfactant Effect on Hydrate Crystallization at the OilâWater Interface

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Surfactant effect on hydrate crystallization at oil-water interface Kevin Dann, and Liat Rosenfeld Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00333 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Surfactant effect on hydrate crystallization at oil-water interface Kevin Dann and Liat Rosenfeld∗ Department of Chemical Engineering, San Jos´ e State University E-mail: [email protected]

Keywords crystallization; cyclopentane; hydrate; morphology; rheology; surfactant Abstract Gas hydrates pose economic and environmental risks to the oil and gas industry when plug formation occurs in pipelines. A novel approach was applied to understand cyclopentane clathrate hydrate formation in the presence of nonionic surfactant to achieve hydrate inhibition at low percent weight compared to thermodynamic inhibitors. The hydrate-inhibiting performance of low (CMC) concentrations of Span 20, Span 80, Pluronic L31, and Tween 65 at 2 ◦ C on a manually nucleated 2 µL droplet showed a morphological shift in crystallization from planar shell growth to conical growth. Monitoring the internal pressure of the water droplet undergoing hydrate crystallization provides information on the change of interfacial tension during crystallization process. The results of this study will provide information on surfactant effect on hydrate crystallization and inhibition. At low surfactant concentrations (below CMC), planar hydrate crystal was formed. A decreasing interfacial tension was observed, which can be related to the shrinking area of the

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water-cyclopentane interface. At high surfactant concentration crystal morphology was shifted to conical. Interfacial tension measurements reveal oscillations of interfacial tension during the crystallization process. The oscillations of the interfacial tension result from the fact that once the crystal has reached a critical size, a portion of the cone breaks free from the droplet surface which results in a sudden increase in the available surface for the surfactant molecules. Hence, a temporary increase in the interfacial tension can be observed. The oscillatory behaviour of the interfacial tension is a result of the growth and release of the hydrate cones from the surface of the droplet. We have found that the most efficient surfactant in hydrate inhibition would be the one with HLB closest to 10 (equal hydrophilic-hydrophobic parts). This way the surfactant molecules will stay at the interface as they observe equal affinities to both the oil and water phases. Surfactant molecules that have the strongest affinity to the interface will be able to inhibit the growth of the crystal as they will force the cones to break and will not allow them to grow.

Introduction Clathrate hydrates are simultaneously nonstoichiometric and crystalline structures composed of guest molecules trapped inside cavities of the surrounding hydrogen-bonded water molecule cages. 1 They are readily found in nature along the sea floor, permafrost, and in glaciers. 2 Hydrates have important applications in many areas, including flow assurance of oil and gas lines, as a potential source of natural gas (primarily methane) from permafrost and deep-sea hydrate deposits, water desalination, 3–5 carbon dioxide capture to regulate global warming, 6–12 and as a medium for energy storage and transportation. 13,14 Despite widespread use, there remains a demand to understand the crystallization mechanism of clathrate hydrates as these fields of research continue to explore methods of manipulating hydrate formation and dissociation. Understanding hydrate formation plays a primary role in subverting future environmental disasters as seen in the 2010 Gulf of Mexico oil spill. 15

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With knowledge of the formation mechanism at hand, effective additives can be developed and utilized to control pipeline blockages, increase petroleum capture, and improve the health prospects of the environment. Gas hydrate formation can severely disrupt production in the context of offshore oil pipelines. The high pressures often used for throughput combined with low temperatures on the seabed floor near 4 ◦ C leave many pipelines susceptible to conditions favorable to hydrate formation. 16 Even when proper precautions are taken, favorable formation conditions relentlessly plague the oil and gas industry; it is the leading cause of technical difficulties for offshore oil pipeline flow assurance by large margins. 17 Oil spills involving hydrates represent both a loss of profit to the oil entity and ecological disaster. The most well known and example of hydrate blockage is the greatly publicized 2010 BP Deepwater Horizon oil spill that took place in the Gulf of Mexico. 18 It was found that containment issues were caused when gas contacted the sea water, formed hydrates, and plugged the cofferdam and relief pipe before it could be maneuvered over the leak. 15 In the past century much of the driving force for hydrate research is owed to the oil industry’s effort to circumvent hydrate plug agglomeration and the subsequent blocking of flow. In 1934 Hammerschmidt was first to determine that hydrates were responsible for plugged flowlines when above the ice point of water. 19 Still today, flow assurance is a continued motivator behind current hydrate research. It is expensive (on the order of $1 million/km additional cost) to attempt sufficient insulation of deep water pipelines for hydrate prevention. 17 Other means of prevention have been utilized, including injecting thermodynamic inhibitors into wellheads. Methanol would be an effective choice; however, large volumetric ratios as great as 1:1 of water to alcohol are often needed. 20 This amount of alcohol usage is not sustainable, not to mention costly; there is a $220 million/year global cost attributed to the methanol used for hydrate prevention. 16 In addition, methanol is environmentally hazardous, rendering it an impractical solution for large-scale transport. 16 There is an alternative method for hydrate prevention, namely surface modification through the use of surfactants. Nonionic surfactants have shown the ability to hinder hydrate growth through kinetic inhibition and

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anti-agglomeration at undercoolings of up to about 20 ◦ C. 1 The altered surface properties have been suggested to delay hydrate nucleation. 20 Hydrate inhibition is thought to result from either direct surface modification or the prevention of formed individual hydrate cells from further agglomeration down the line. 21 The effect of sodium dodecyl sulfate (SDS) on the hydrate formation morphology of carbon dioxide-hydrogen-cyclopentane hydrates was investigated by Lim et al.

22

They have

found that the addition of surfactant (SDS) had led to a change in the hydrate crystal morphology, forming fiber-like crystals from the hydrate layer. Mitarai et al.

23

conducted visual

observation of the effect of surfactants on hydrate crystal growth at the interface of water and cyclopentane. They have found that the crystalline morphology and crystal growth behaviour of the hydrate at the interface were substantially affected by the addition of the surfactants. However, despite these efforts, the mechanism through which surfactants inhibit hydrate formation has not yet been fully defined. Due to the absence of a complete model to explain the effect of surfactants on hydrate crystallization and inhibition mechanisms, contributions from researchers have been inconclusive and occasionally contradictory. While it has been demonstrated that surfactants can prolong hydrate induction time at certain subcoolings, others have found exceptions in low surfactant concentrations, where the surfactants actually collapse the droplets and accelerate the process of hydrate formation. 24 It has been proposed that planar hydrate growth is interrupted by the presence of surfactant molecules, and the hollow-conical crystal formation is due to surfactant crowding and increased surface pressure, thus creating a mechanical barrier for growth. 25 In this study we have applied a novel approach using visualization techniques and internal pressure measurements to understand cyclopentane clathrate hydrate formation in the presence of nonionic surfactants. The hydrate-inhibiting performance of low (CMC) concentrations of Span 20, Span 80, Pluronic L31, and Tween 65 at 2 ◦ C on a 2 µL water droplet showed a morphological shift in crystallization from planar shell growth to conical growth. Monitoring the internal pressure of the water

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droplet undergoing hydrate crystallization provides information on the change of interfacial tension during crystallization process. The results of this study will provide information on surfactant effect on hydrate crystallization. At low surfactant concentrations (below CMC), a planar hydrate crystal was formed. A decreasing interfacial tension was observed which can be related to the shrinking area of the water-cyclopentane interface. At high surfactant concentration crystal morphology was shifted to conical. Interfacial tension measurements reveal oscillations of interfacial tension during the crystallization process. The oscillations of the interfacial tension result from the fact that once the crystal reaches a critical size, a portion of the cone breaks free from the droplet surface which results in a sudden increase in the available surface for the surfactant molecules. Hence, a temporary increase in the interfacial tension can be observed. The oscillatory behaviour of the interfacial tension is a result of the growth and release of the hydrate cones from the surface of the droplet. We have found that the most efficient surfactant in hydrate inhibition would be the one with HLB closest to 10 (equal hydrophilic-hydrophobic parts). This way the surfactant molecules will stay at the interface as they observe equal affinities to both the oil and water phases. Surfactant molecules that have the strongest affinity to the interface will be able to inhibit the growth of the crystal as they will force the cones to break before reaching full size.

Experimental Hydrate Cell Apparatus A purpose-built experimental setup, referred to as the hydrate-visualization cell, was utilized to characterize hydrate formation. This multi-component system is capable of processing visual measurements at the interface of a droplet along with internal pressure measurements as hydrate formation occurs. Temperature control is achieved by the implementation of a programmable temperature regulator via solid-state Peltier component. A full description of the programmable temper5


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ature regulator is described in. 26 The hydrate visualization cell, shown schematically in Figure 1, is comprised of several key elements. The Peltier plate maintains constant cell temperatures in regions of hydrate stability (−10 ◦ C to 10 ◦ C). A camera is fixed at the window and is used to visualize a side view of the water droplet. A pressure transducer monitors the internal pressure of the water droplet. By syncing these systems, the growth rate and interfacial tension can be linked. The existence of a seed hydrate is required for consistent nucleation and tracking of growth rate. Since the presence of melting ice is a necessity for timely initial hydrate conversion, a small volume (50 − 100 µL) of pure water is deposited on the floor of the hydrate cell as indicated by the seed hydrate in Figure 1. Approximately 30 mL of cyclopentane is added to the brass cell. The temperature is lowered to −5 ◦ C until the small volume of water forms ice; nucleation may be expedited by disturbing the supercooled water with a clean suitable probe. Once ice has been formed, the temperature is raised to 2 ◦ C and held there for the remainder of the experiment. This temperature ensures the solid ice is converted to hydrate as the system is above the melting point of ice, yet below that of cyclopentane hydrates. 27 At this time the plumbing is primed and the brass hook is lowered into the cyclopentane to equilibrate for 5 min to reduce expansion and contraction oscillations seen in the droplet volume from the temperature change.

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Transducer

P

Syringe

Cyclopentane Water Droplet

Window

Seed Hydrate Brass Cell Peltier Plate Figure 1: Camera-eyed view of hydrate-visualization cell schematic of necessary components. The brass cell sits atop the Peltier plate, the water droplet is delivered by tubing connecting the syringe and pressure transducer.

Plumbing was achieved with a combination of flexible Polytetrafluoroethylene (PTFE) tubing and rigid brass tubing. A full description of the hydrate-visualization cell is given in SI. To supply metered water and surfactant solutions, a Chemyx Fusion 100 Infusion Pump was used with a 1 mL Hamilton syringe and a 19 gauge needle (model: 1001 LTSN SYR). The 19 gauge needle was sized to allow a press fit of the PTFE tubing. Pressure changes in the line were monitored with an OMEGA PX409-10WGUSBH pressure transducer. This transducer is particularly sensitive, with a maximum pressure of 2500 P a, or the pressure equal to submersion in water at 10 in. The transducer has an accuracy of 0.08% BSL, which denotes the furthest deviation for the measured data from the best straight line fit. The custom-built brass cell shown in Figure 2 was used to contain the bath of cyclopentane and facilitate the observation of hydrate formation.

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Figure 2: Brass visualization-cell with cover (left). Visualization-cell window showing silica desiccant (right).

An acrylic cover (with cutouts for the temperature probe and brass hook) is fixed to the top of the cell in order to limit evaporation of the volatile cyclopentane, therefore stabilizing the submersion depth and preserving the external pressure acting on the outside of the droplet. The viewing windows are double-paned with additional silica desiccant inserted in between the glass slides to combat frosting and fogging which otherwise occurred at the R temperature differentials seen in experimentation. Application of Rain-X to the outer

window further alleviates fogging. A Basler acA640-750um camera was used to capture monochrome images with VGA resolution (640 × 480). A Kipon EOS to C-mount adapter coupled with 35 mm of Fotodiox macro extension tubes were used to connect the camera body to a Canon 28-90 mm lens. This allowed for closeup macro images to be observed at the cost of decreased depth of field. Illumination was provided by an AmScope 150 W fiber optic goose-neck lamp.

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Materials An attractive guest phase for hydrate study is one stable at ambient pressure and moderate temperatures. This guest phase would offer benefits in visualization, ease of access, cost, and safety. Since tetrahydrofuran and cyclopentane are commonly used hydrate formers which meet the aforementioned criteria, 28 cyclopentane was the guest phase of choice for this study. Nonionic surfactants have demonstrated the ability to suppress hydrate growth through kinetic inhibition and anti-agglomeration at undercoolings of up to 20o C . 1 The onset of hydrate nucleation can also be delayed by surfactants, which alter surface properties such as surface tension. 20 There are at least two hypotheses that describe a surfactant’s effectiveness in hydrate inhibition: 1) Surfactants are useful because the hydrates are inhibited through the surface modification, and 2) surfactants aid the initial formation of hydrate cells but prevent further agglomeration down the line. 21 Surfactant presence in the bulk phase that will inhibite the growth of the hydrate, would drastically reduce, if not eliminate, the large amount of glycols or alcohols needed for hydrate prevention. For example, Karanjkar et al. determined that a volumetric concentration of 0.03% (v/v) Span 80 (a nonionic surfactant) was enough to saturate the water/bulk phase interface. 25 Surfactants are proposed to work on two fronts: 1) inhibit crystal growth mechanically and 2) reduce cohesion through a weakening of the capillary bridge forces. 29 When surfactant molecules migrate to an interface, the surface tension will lower according to the Gibbs adsorption isotherm given in Equation 1, where γ is the surface tension, C is the surfactant concentration, Γ is the amount of adsorbed surfactant (mol/m2 ), R is the gas constant (8.314 J/K mol), and T is the temperature (K). 30 dγ = −Γ RT d Ln C

(1)

The period of time that surfactants are able to delay hydrate formation is called the induction time, and more effective surfactants have longer associated induction times, which

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are determined experimentally at the onset of clouding in the liquid. Surfactant performance is commonly categorized by the induction time for a specific magnitude of undercooling. High subcoolings result in higher driving forces for hydrate formation, and the efficacy of the surfactant inhibition subsequently decreases. 31 The proposed work that follows aims to explore alternative solutions for hydrate suppression through surfactant utilization. Analysis was conducted with the assorted nonionic surfactants listed in Table 1. The surfactants include various hydrophilic-lipophilic balance (HLB) values (defined in Equation 2) as well as various molecular weights. This variety of surfactants will provide information concerning the influence of the different components in the surfactant molecule on hydrate crystallization and destabilization. Table 1: Surfactants and their properties. Span 20 Span 80 Pluronic L31 Tween 65

HLB Molecular Weight (g/mol) Formula 8.6 346.5 C18 H34 O6 4.6 428.6 C24 H44 O6 3.2 1105 C56 H112 O20 10.5 1845 C100 H194 O28

HLB = 20 ·

Mh M

(2)

where Mh is the molecular mass of the hydrophilic portion of the molecule and M is the molecular mass of the whole molecule. The HLB value can be used to predict the surfactant properties of a molecule. HLB < 10: lipid-soluble (water insoluble), HLB > 10: water-soluble (lipid insoluble).

Results and Discussion Image Processing: Surface Area Two distinct types of hydrate growth were observed: planar and conical. 10


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Tracking the conical and planar hydrate growth was performed with visual analysis methods. ImageJ was used to set the scale for the images based on the known diameter of the 1/16 in (1.588 mm) brass tube at the base of every frame. Seven equally spaced snapshots were selected from each experiment for analysis between the point of nucleation and full droplet conversion. As can be seen in Figure 3, the hydrate coverage in each image was painted black, while the droplet edge was outlined in red. Since the camera only captured the 2D projection of the spherical droplet, a 3D reconstruction was created in Mathematica as a correction to surface area. The code rastered the images from top to bottom, set the outermost black or red pixel as the radius, applied radial mapping, and applied the correct pixel-to-mm ratio. The surface area was taken as the summation of the arc lengths over all rows. An example of the reconstructed 3D image can be seen in Figure 3 (bottom).

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Figure 3: Example hydrate region for surface area analysis of Span 20low , 0.01 gr/100mL. Observed hydrate regions (left) are painted black (right) to map a 3D surface (bottom).

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Apparent Average Interfacial Stress Measurements Since a dilating or shrinking droplet undergoes a change in surface area, surface tension is a dynamic quantity following initial droplet formation and subsequent hydrate surface coverage. 32 However, once the hydrates start to form, the surface of the droplet become heterogeneous and is not characterized by a single interfacial tension. The pressure being measured gives an apparent average interfacial stress that can be calculated using Equation 3. The Young-Laplace relation 33 given by Equation 3 is used to determine the interfacial tension (or in this case, the apparent average interfacial stress), γ, between cyclopentane and the surfactant solution droplet, at time t ≥ 0.

∆P =

1 R1

γ +

1 R2

≈

2γ R

(3)

where R1 and R2 are the droplet radii of curvature and ∆P is the change in pressure within the droplet relative to t=0. In the initial period following droplet formation, the two radii are approximately equal, and Equation 3 can be approximated further, with the radius of the predetermined 2 µL droplet equal to R = 782 µm. Solving for γ, the apparent average interfacial stress can now be linearly approximated from changes in pressure. It should be noted that this method of apparent average interfacial stress approximation is only valid while the droplet maintains a spherical shape with some area of liquid interface remaining. The initial apparent average interfacial stress value (t = 0, when the drop was generated) of water/cyclopentene was measured to be γ = 28.6mN/m. Although this value is not very close to prior reports of water/cyclopentane interfacial tension measured with pendant drop method (γ = 48mN/m

25,34,35

). It is close to prior reports of the interfacial stress between

water and cyclopentene (γ = 28mN/m 36 at 20o C). The initial apparent average interfacial stress was set to 28 mN/m to show the relative change with time. The apparent average interfacial stress values are calculated as a secondary value, whereas the pressure in the

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droplet was the primary measurement. ∆ρgR2 The Bond number, Bo = = 0.052, where ∆ρ is the difference in densities, R is γ the radius of the drop and γ is the surface tension. Such a small Bo number means that surface tension changes dominate the process.

Planar Shell Growth A time lapse of planar hydrate growth for pure water (with no surfactant) is depicted in Figure 4.

1.5 min

5.7 min

10 min

Figure 4: Time lapse of the planar hydrate growth for pure water (with no surfactant) at 2 ◦ C in cyclopentane. In the planar shell growth, the crystal starts to form at the two poles and grows towards the equator. The undercooling acts as a driving force to propagate the hydrate front at the interface. The outcome is that the hydrate shell is constantly increasing in size. In order to prevent the brass tube from being nucleation site we removed the tube from the solution between experiments to melt any residual hydrates. The hydrates are buoyant in general, so when we nucleated the drop they tended to float to the top of the droplet away from the brass tube, but it is not impossible for some find their way to the bottom of the drop and stick to the edge at the brass wall, spurring growth from there. In all the experiments, the brass tube would not nucleate any hydrate by itself, we had to add the "seed hydrate" before any growth occurred anywhere due to the requirement for ice as a precursor to hydrate growth. The seed hydrate is largely maintained and untouched in the corner of the hydrate cell (as described in Figure 1) throughout the experiment. Only a 14


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small portion of the seed hydrate is transferred to make brief contact with the water droplet interface by manual probing with a needle. The growth rate of the cyclopentane hydrate on a pure water droplet is plotted in Figure 5. The hydrate shell completely covered the droplet after 10 min, as can also be seen from Figure 4. From this plot we can calculate the hydrate growth rate when uninhibited for a 2 µL droplet at 2 ◦ C to be 0.590 mm2 /min. This growth rate will be the baseline upon which the surfactant performance can be judged. 5 Water Regression 4 Hydrate Area (mm2 )

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A = 0.590t − 0.276 R = 0.998

3

2

1

0

0

1

2

3

4

5 6 Time (min)

7

8

9

10

Figure 5: Hydrate area as a function of time at 2 ◦ C for pure water (no surfactant) in cyclopentane. The area was found using the method described above in the image processing section. In addition to pure water, the planar shell morphology was observed in surfactant concentration low to medium (0.1×CMC to CMC). An example of the planar shell growth in Span 80med , 0.03 gr/100mL, is shown in Figure 6. 15


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2.1 min

4.2 min

6.3 min

8.5 min

10.6 min

12.7 min

Figure 6: Time lapse of the planar hydrate growth for Span 80med , 0.03 gr/100mL, at 2 ◦ C in cyclopentane.

Comparison of the planar growth of Span 80med in Figure 6 with that of pure water in Figure 4 shows a greater sphericity is maintained for pure water, presumably due to the higher apparent average interfacial stress maintained throughout hydrate propagation. The presumed Span 80 concentration buildup and apparent average interfacial stress decrease contributed to the deformity and eventual droplet collapse seen from the 8.5 min mark onward. Since a collapsed or deformed droplet greatly increases the surface area to volume ratio, crystallization proceeded rapidly along the newly available interface until complete hydrate conversion was achieved. As the presence of surfactant is linked to lower apparent average interfacial stress, a droplet is more easily distorted by the advancing hydrate front when surfactant is present. The surfactant initially inhibits the formation of the hydrate but once the hydrate is being formed and the droplet is starting to collapse a rapid formation of the crystal will occur. The change in pressure and inferred apparent average interfacial stress over time for 16


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Span 80med is presented in Figure 7. It can be seen that with planar hydrate formation there

0

28

−10

24

−20

20

−30

16

−40

12

−50

8 0

2

4

6

8 Time (min)

10

12

14

Apparent average interfacial stress (mN/m)

is a gradual decrease in apparent average interfacial stress as the hydrate growth progresses.

Droplet Internal ∆P (Pa)

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Figure 7: Pressure difference inside and outside the drop as being read by the pressure transducer of Span 80med , 0.03 gr/100mL. The apparent average interfacial stress values were evaluated using the Young-Laplace equation as described in the apparent average interfacial stress section. A clear reduction in the internal droplet pressure can be seen. That reduction in pressure can be related to decreasing apparent average interfacial stress according to Equation 3. As the hydrate grows, the available interface for surfactant molecules decreases. The decreased interfacial area results in an increased surfactant concentration on the droplet areas that are not yet covered by the hydrate. According to the Gibbs adsorption isotherm, 37 increasing surfactant concentration relates to decreasing surface tension or interfacial stress. Apparent average interfacial stress as a function of % of the remaining liquid interfacial area during hydrate growth with Span 80med is presented in Figure 8. It can be seen that as the available surface area for surfactant decreases, the concentration of surfactant molecules will increase which will result in a decrease of the apparent average interfacial stress. In the planar hydrate formation this decrease in the apparent average interfacial stress is linear. 17


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We can conclude here that the decreased internal droplet pressure is indicative of decreased apparent average interfacial stress from increased surfactant concentration as hydrate growth displaced and crowded the surfactant molecules. Apparent average interfacial stress (mN/m)

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22

Span 80med Regression

20 18

R = −0.989

16 14 12 10 8 100

80

60 40 20 Remaining Liquid Interfacial Area (%)

0

Figure 8: Apparent average interfacial stress as a function of available liquid area (%) in the planar hydrate growth of Span 80med , 0.03 gr/100mL. Points correspond to snapshots in Figure 6, starting at 2.1 min

Conical Morphologies Conical crystal formations were observed when the surfactant concentration was increased past the CMC. This was true for all surfactants except Pluronic L31. Here the hydrate starts to grow as a conical crystal. When the conical crystal becomes large enough, a portion of the circumference of the cone breaks free from the droplet surface. Then the remaining solution from inside the droplet redistributes to fill and engulf the cone. This phenomenon was observed by Karanjkar et al.

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for Span80. Higher surfactant concentrations yielded

larger cone growths before separation, sometimes outgrowing the size of the droplet. Figure 9 exhibits such a case for Tween 65high , 0.15 gr/100mL.

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7.6 min

15.2 min

22.7 min

30.3 min

37.9 min

53 min

Figure 9: Time lapse of conical hydrate growth for Tween 65high , 0.15 gr/100mL, at 2 ◦ C in cyclopentane.

The slow conical growth proceeds at the circumference of the opening until the physical size of the crystal is too large for the droplet. After the droplet collapsed, rapid hydrate crystallization was observed until complete droplet conversion. Apparent average interfacial stress measurements of the conical hydrate crystallization are presented in Figure 10. It can be seen that there is an initial decrease in apparent average interfacial stress. The decrease in apparent average interfacial stress results from the formation of the conical crystal and the reduction of available surface area for the surfactant molecules. Once the crystal has reaches a critical size a portion of the cone breaks free from the droplet surface which results in a sudden increase in the available surface for the surfactant molecules. Hence, a temporary increase in the apparent average interfacial stress can be observed. The oscillatory behaviour of the Apparent average interfacial stress is a result of the growth and release of the hydrate cones from the surface of the droplet. Karanjkar et al.

25

hypothesized that there is a relation between crowding of surfactant

molecules at the interface and the hollow-conical crystal shape. Our measurements show that, indeed, when there is excess of surfactant molecules, apparent average interfacial stress 19


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decreases, and the hollow-conical crystal forms. When the crystal reaches a critical size it gets released into the bulk, resulting in increase in apparent average interfacial stress due to

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increase of the available area for surfactant molecules.

Droplet Internal ∆P (Pa)

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Figure 10: The change in pressure within the droplet relative to t=0 and the corresponding apparent average interfacial stress values as a function of time during the hydrate growth process of Tween 65high , 0.15 gr/100mL. The apparent average interfacial stress values were evaluated using the Young-Laplace equation as described in the apparent average interfacial stress measurements section.

Evaluation of Growth Rates The growth rate of the hydrate crystal on a fresh water droplet as well as on the surfactant solutions droplets is presented in Figure 11. It can be seen that all but one of the surfactant solutions performed better than pure water at retarding hydrate growth. The most effective surfactant studied was Tween 65high , 0.15 gr/100mL, with a growth rate (0.068 mm2 /min) nearly three times slower than the next best surfactant (Span 20high , 1 gr/100mL, at 0.178 mm2 /min). Tween 65 showed the largest range in performance, between low, medium and high concentrations, and it was not until the concentration was increased

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beyond the CMC that growth rate plummeted. It can also be noted that the most efficient crystal formation in terms of hydrate inhibition is the conical crystal. Due to the nature of the growth and the fact that the crystal cannot grow past a certain size, the time that it takes for the hydrate to form increase dramatically. Hence, surfactants that will force the hydrate to form a conical crystal will be the most efficient. 0.7 water

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0.1

0 Span 20

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Water

Figure 11: Hydrate growth rate across all solutions at 2 ◦ C in cyclopentane.

By scanning the surfactant growth rates shown in Figure 11 from low, medium, to high concentrations, it becomes clear that there is a downward trend for growth rates with increasing concentration regardless of the specific brand of surfactant. The high-concentration group exhibited the least amount of variance (0.004 mm2 /min) across the surfactant types. Hydrate crystallization proceeded linearly for all droplets in the initial stages after nucleation. The low-concentration surfactants performed similar to pure water. The low concentra21


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tions were not sufficient to alter the growth morphology. The growth behavior of the differing low-concentration surfactants and pure water are compared in Figure 12. Tween 65low was the only solution to display an accelerated growth rate when compared to pure water. Image analysis of Tween 65low indicated that shortly after nucleation there were multiple growth sites which spread from both the top and at the bottom at the droplet-brass tube interface, and growth was therefore propagating on several fronts.

7 6 Hydrate Area (mm2 )

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5 4 3 Span 20low Span 80low Plur L31low Tween 65low Pure Water

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Figure 12: Hydrate growth evolution at 2 ◦ C for low-concentration surfactants in cyclopentane. Span 20low (0.01gr/100mL), Span 80low (0.003gr/100mL), Plur L31low (0.01gr/100mL), Tween 65low (0.0026gr/100mL) and Pure Water. The medium concentrations were chosen to be close to the CMC for each respective surfactant. All morphologies at this concentration resulted in a shell of planar growth. Because of the similar growth mechanism between these and the low concentrations, the growth curves follow fairly the same pattern as the low concentration and pure water plots. From the high concentration plot in Figure 13, it can be seen that all growth rates began slow, and ramped up towards the end of droplet conversion. The conical crystals observed 22


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with the high-concentration solutions would frequently become so large that the droplet would flatten and rupture, causing many fresh nucleation sites and exponential hydrate conversion. There appears to a clear benefit to using surfactant concentrations past the CMC to inhibit hydrate growth, although there is a downside when high concentrations result in low apparent average interfacial stress, and reduced droplet stability causes droplet annihilation. Since unnecessarily high surfactant concentrations would impact the cost for usage on a large scale, it would be most economical to use a minimum concentration greater than the CMC. Span 20high Span 80high Plur L31high Tween 65high Pure Water

10

Hydrate Area (mm2 )

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8

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Figure 13: Hydrate growth evolution at 2 ◦ C for high-concentration surfactants in cyclopentane. Span 20high (1gr/100mL), Span 80high (0.3gr/100mL), Plur L31high (1gr/100mL), Tween 65high (0.15gr/100mL) and Pure Water. From the various structures of the different surfactants it can be noted that the most efficient surfactant in hydrate inhibition would be the one with HLB closest to 10. This way the surfactant molecules will stay at the interface as they observe equal affinities to both the oil and water phases. Surfactant molecules that have the strongest affinity to the interface 23


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will be able to inhibit the growth rate and size of the crystal as they will force the cones to break and will not allow the more rapid planar morphology to propagate. The standard deviation between three independent measurements of every surfactant concentration were smaller than 5%. Hence, error bars are not presented.

Conclusions In this study we have applied a novel approach using visualization techniques and internal pressure measurements to understand cyclopentane clathrate hydrate formation in the presence of nonionic surfactants. • Two types of crystals were found: planar shell and conical crystal. – For plain water and surfactant concentration below CMC, the hydrate formed as a planar shell. – In the planar shell growth, the crystal starts to form at the nucleation point and grows towards the equator. – The undercooling acts as a driving force to propagate the hydrate front at the interface. The outcome is that the hydrate shell is constantly increasing in size. – Apparent average interfacial stress measurements reveal a constant decrease in apparent average interfacial stress values as a result of a constant decrease in available surface area for the surfactant molecules. – For surfactant concentrations above the CMC, the hydrate formed as a conical crystal. – When the conical crystal growth becomes large enough, a portion of the circumference of the cone breaks free from the droplet surface. Then the remaining solution from inside the droplet redistributes to fill and engulf the cone.

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• We note here that as for the location of the surfactant molecules in the system, we believe that the majority of surfactant gets expelled from the interface and pushed into either the water or the cyclopentane. We also believe that some surfactant does get decorated or adsorbed to the exterior hydrate surface based on results of others who specifically tested adhesion among hydrate particles formed with (and in the presence of) surfactant . 38,39 We do not believe that the surfactants are within the bulk hydrate structure at very significant concentrations due to the relatively large size of the surfactants compared to the guest molecules (surfactants would not make a stable hydrate). • The conical shape is induced by planar pressure from the crowding on the interface as the crystal wants to grow faster than the surfactants can get out of the way of the progressing hydrate front. • Apparent average interfacial stress measurements showed an oscillatory behavior during cone generation and separation. There was an initial decrease in apparent average interfacial stress which results from the formation of the conical crystal and the reduction of available surface area for the surfactant molecules. Once the crystal has reached a critical size, a portion of the cone breaks free from the droplet surface. That results in a sudden increase in the available surface for the surfactant molecules. Hence, a temporary increase in the apparent average interfacial stress was observed. The oscillatory behaviour of the apparent average interfacial stress is a result of the growth and release of the hydrate cones from the surface of the droplet. • The hydrate morphology was not a function of the droplet curvature but only of the type and concentration of the surfactants. • The lowest hydrate growth rate observed was 0.068 mm2 /min with Tween 65high , proving it to be the most effective inhibitor tested when used at concentrations above the CMC. The most influential properties associated with Tween 65 are believed to 25


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be its comparatively large molecular weight (1845 g/mol) and central HLB (10.5) close to 10, which work to preserve adsorption at the interface. The remainder of the surfactants all had lower weight and were of varying degrees more hydrophobic than Tween 65. In this paper we have established a set of general conditions for the formation of the different types of hydrate crystals in the presence of nonionic surfactants. We have confirmed previous observations regarding formation of hollow conical crystals at high surfactant concentration and provided proof for previous hypothesis regarding oscillatory behaviour due to crowding of surfactant molecules at the interface. This work will help develop a better understanding of hydrate formation in the presence of surfactant molecules and can lead to the design of more effective, eco-friendly surfactants which will have broad applications in offshore natural gas production and seabed oil capture.

Supporting Information Description of the plumbing of the hydrate-visualization cell; method of surfactant CMC determination; description of the data acquisition.

Acknowledgement The authors thank American Chemical Society - Petroleum Research Fund (ACS - PFR), grant number: PRF # 57216-UNI9, for financial support.

References (1) Sloan, E. D.; Koh, C. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC press: Boca Raton, FL, 2007; pp. 78–619.

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(29) Aman, Z. M.; Koh, C. A. Interfacial phenomena in gas hydrate systems. Chemical Society Reviews 2016, 45, 1678–1690. (30) Tadros, T. Encyclopedia of Colloid and Interface Science; Springer: New York, NY, 2013; pp. 626. (31) Kelland, M. A. History of the development of low dosage hydrate inhibitors. Energy & Fuels 2006, 20, 825–847. (32) Soós, J.; Koczó, K. Measurement of dynamic surface tension of surfactant solutions with the drop volume method using an automatic drop detector. Periodica Polytechnica. Chemical Engineering 1989, 33, 269. (33) Ibach, H. Physics of surfaces and interfaces; Springer Science & Business Media, 2006; pp. 8. (34) Song, J. H. K.; Couzis, A.; Lee, J. W. Direct Measurements of Contact Force between Clathrate Hydrates and Water. Langmuir 2010, 26, 9187–9190, PMID: 20481459. (35) Aman, Z. M.; Olcott, K.; Pfeiffer, K.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Surfactant Adsorption and Interfacial Tension Investigations on Cyclopentane Hydrate. Langmuir 2013, 29, 2676–2682. (36) Montgomery, J. H. Groundwater chemicals desk reference, 4th ed.; CRC Press, 2007; pp. 295. (37) Tadros, T. Encyclopedia of Colloid and Interface Science; Springer Berlin Heidelberg: Berlin, Heidelberg, 2013; p 626. (38) Taylor, C. J.; Dieker, L. E.; Miller, K. T.; Koh, C. A.; Sloan, E. D. Micromechanical adhesion force measurements between tetrahydrofuran hydrate particles. Journal of Colloid and Interface Science 2007, 306, 255 – 261.

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(39) Dieker, L. E.; Aman, Z. M.; George, N. C.; Sum, A. K.; Sloan, E. D.; Koh, C. A. Micromechanical adhesion force measurements between hydrate particles in hydrocarbon oils and their modifications. Energy & Fuels 2009, 23, 5966–5971.

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