Direct Measurements of Contact Angles on Cyclopentane Hydrates

(10) An alternate mechanism, therefore, could include changes in the contact angle of water on the hydrate particles, creating particles that are hydr...
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Article Cite This: Energy Fuels 2018, 32, 6619−6626

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Direct Measurements of Contact Angles on Cyclopentane Hydrates Erika P. Brown,† Sijia Hu,† Jon Wells,† Xiaohui Wang,†,‡ and Carolyn A. Koh*,† †

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Center for Hydrate Research, Chemical and Biological Engineering Department, Colorado School of Mines, Golden, Colorado 80401, United States ‡ State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China ABSTRACT: When clathrate hydrate particles come into contact, they tend to agglomerate together, resulting from capillary liquid bridges that form between the particles. The strength of these capillary bridges is a function of several physical variables, many of which have been directly obtained for clathrate hydrate systems. A less thoroughly explored variable is the contact angle of water on the clathrate hydrate surface. Analyses have shown that the contact angle of a surface can have a strong effect on the cohesion behavior of clathrate hydrates; however, direct measurements of water on a hydrate surface are not prevalent. To better understand this important parameter, a method was developed to directly measure the contact angle of a water droplet deposited onto the surface of a cyclopentane hydrate particle. Using a novel method, the contact angles of water droplets on cyclopentane hydrate surfaces were directly measured. In combination of these new measurements with an updated interfacial tension value for a cyclopentane and water system, the immersion depth of the capillary bridge on a clathrate hydrate particle was estimated. Different anti-agglomerants (AAs) were tested for both contact angle and cohesion force, which showed that the micromechanical force measurement apparatus was capable of ranking the anti-agglomeration tendency of clathrate hydrate particles in the presence of AAs. These tests revealed a correlation between low cohesion force and higher contact angle, corresponding to a hydrophobic surface. Morphological changes were also observed in the hydrate particles upon the addition of AAs, and two main types of morphological changes, water extrusion and hydrate sloughing, were identified.

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INTRODUCTION The environment created in subsea flowlines provides favorable thermodynamic conditions for the formation of clathrate hydrates (also known as gas hydrates), which are ice-like inclusion compounds that form at low temperatures and high pressures.1 These crystalline structures are comprised of water molecules that form hydrogen-bonded cages around guest molecules, usually light hydrocarbons. Three structures of hydrates are common, which are characterized by their crystal structure created by the guest molecules that stabilize the cages. Small molecules, such as methane, stabilize structure I, while larger molecules, such as propane or cyclopentane, stabilize structure II. Structure H requires at least two guest species, one for the smallest cage and one for the largest cage.1 The structure II hydrate is the most common form of hydrate found in flowlines, because the gas transported there typically comprises longer chain molecules that are too large to fit into the cages of structure I. Hydrate formation in flowlines is a multistep process that can be conceptually described by splitting it into several phenomena. Figure 1 shows a conceptual model of hydrate formation in a multiphase flowline. Hydrates typically form at interfaces; therefore, the preliminary phenomenon in hydrate formation is entrainment of the phases in one another.3,4 Natural surfactants and chemical additives can alter the emulsion properties of the system5 as well as system properties, such as shear rate and water cut. Once thermodynamically favorable hydrate formation conditions have been reached, nucleation occurs stochastically, followed by hydrate growth along the water− oil or water−gas interface.6 After a thin shell of hydrate forms over the entire interface, annealing and further growth are © 2018 American Chemical Society

Figure 1. Hydrate formation in a flowline can be described as the combination of multiple phenomena that can eventually result in the complete arrest of production in the case of a plug. Insets show water droplets adhering to the exterior of hydrate particles, resulting in wetting of the particles. This figure was modified with permission from ref 2. Copyright 2009 Elsevier.

limited by mass transfer of hydrate formers across the hydrate shell.7 As such, large water droplets, such as those formed from coalescence of smaller droplets (>100 μm in diameter) can retain an unconverted liquid core. After hydrates have formed, they can agglomerate through collisions with other particles or water droplets, or deposit and settle onto pipe walls. The strength and severity of the agglomeration is determined by the interparticle forces that occur when the particles are in contact.8,9 The dominant mechanism of cohesion has been determined to be capillary bridging.10 Capillary bridging can be described using eq 1 Received: March 9, 2018 Revised: May 16, 2018 Published: May 23, 2018 6619

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may adsorb onto the water/hydrocarbon or hydrate/hydrocarbon interface.16 A study by Aman et al. showed that measurements of the IFT were insufficient to predict changes in the cohesion force when adding a surfactant.17,18 This indicated that other physical variables in the system were also changing when surfactants were added to the system. Contact angles of water on different surfaces were measured by Aspenes et al. and compared to the surface energy of the material as well as the adhesion force between the surfaces and hydrate particles.19 Esmail and Beltran also performed similar tests and measured propagation velocities.20 Asserson et al. measured contact angles for hydrate systems where water was the bulk phase and a droplet of a third phase was deposited onto a Freon hydrate surface.21 However, direct measurements of the contact angle of a water droplet on a hydrate surface have been lacking to the knowledge of the authors. The effect of the contact angle on the cohesion force may also be informative about the mechanism by which antiagglomerants (AAs) reduce the interparticle force. AAs are a class of chemicals (typically surfactants) used in flowlines to encourage solid hydrate particles to flow as a transportable slurry rather than agglomerate into masses that may plug the flowline.22,23 One dominant theory for the method by which AAs reduce cohesion forces was a mechanism similar to cold flow,24 whereby AA promotes the conversion of water to hydrate, eliminating any free water in the system that could lead to increased agglomeration.25 However, in high-water-content or gas-limited systems, this mechanism is not plausible because the free water phase could not be completely eliminated. Additionally, this mechanism does not take into account the possibility of a thermodynamic water layer that exists even when particles have fully converted.10 An alternate mechanism, therefore, could include changes in the contact angle of water on the hydrate particles, creating particles that are hydrophobic.26,27 These systems would display significantly less agglomeration as a result of the reduced interaction with water. The interaction of water droplets on and with the hydrate surface has been shown to have a significant effect on the agglomeration behavior. It has been shown that the hydrate− water interaction is much stronger than the hydrate−hydrate interaction for a dry system.28,29 Water-wetted particles may be more likely to agglomerate into large masses or to deposit on pipe walls to form hydrate deposits.30,31 In addition, hydrate nucleation is a stochastic process, which means that, even when favorable thermodynamic conditions are met, droplets may convert to hydrate at different times. It is therefore likely that water droplets and hydrates will interact in a flowline environment. The wetting behavior may also influence the behavior of hydrates that form naturally in the pore space in sediments.32 The purpose of this work is to develop a better understanding of the interaction between water and hydrates by directly measuring the contact angle between a water droplet and a hydrate surface. These measurements can be compared to IFT measurements for the cyclopentane system and used to evaluate the accuracy of assumptions that have been used in the prediction of cohesion forces previously. Understanding these relationships will give further insight into the agglomeration and plugging behavior of clathrate hydrates, which can then be used to investigate the effect of chemical additives on clathratehydrate-forming systems. Direct observation of morphological changes caused by AAs can also provide more information on how these chemicals affect hydrate properties, such as

(1)

where F is the cohesion/adhesion force, R is the normalized radius of the particles, γ is the interfacial tension (IFT) between the bridge and the bulk phase, α is the embracing angle, θ is the contact angle between the liquid bridge and the particle surface, H is the height of the liquid bridge, and d is the immersion depth. Figure 2 shows a physical representation of the variables from eq 1.

Figure 2. Depiction of the physical meaning of variables represented in eq 1 for a particle−surface interaction as well as a particle−particle interaction.10 This work aims to directly measure these parameters to better understand agglomeration behavior.

The capillary bridge in an oil continuous system is comprised of water that may come from several different sources. A water layer may be supplied through excess water in the system that has yet to convert to hydrates, such as in a gas-limited system, or through water that diffuses outward from the liquid core of a hydrate particle that did not convert fully.7 A thermodynamic water layer that acts to minimize the free energy of the system may also supply the water used in the capillary bridge.10,11 A micromechanical force (MMF) apparatus has been used in the past to directly measure the cohesion force between hydrate particles. Previous studies have been performed to investigate the cohesion and adhesion forces using this apparatus. Aspenes studied particle−surface interactions using cyclopentane (CyC5) as the clathrate hydrate former. The results showed that the adhesion force was strongly dependent upon the surface material and the amount of water present.12 Dieker et al. measured the force in a system with a small amount of natural oil, finding that natural oils with a higher content of asphaltenes and acids may exhibit non-plugging tendencies.13 Therefore, additions of surfactants or crude oils to model systems have repeatedly been shown to alter the cohesion behavior of hydrate particles.14,15 It is suspected that these surfactant molecules (or surfactants present in the natural oils) 6620

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them using liquid nitrogen. The ice particles are then submerged in a cyclopentane bath maintained in the range of 0−7.7 °C. The ice provides nucleation sites for hydrates to form, which are then allowed to anneal for 30 min. Pull-off measurements are then performed according to Figure 4. The particles are first brought into contact at a

agglomeration and wettability. The design and implementation of better AA compounds depends upon an accurate hypothesis on the mechanism for chemical and hydrate interactions.

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MATERIALS AND METHODS

Materials. Cyclopentane (98% purity, reagent grade, SigmaAldrich) was chosen in this study because it forms hydrates at atmospheric pressure and temperatures below 7.7 °C.1 Cyclopentane hydrates are also ideal as a model system because they form structure II hydrates (the same structure that is commonly found to form in oil and gas production systems) and cyclopentane is immiscible with water. Without an aid to induce nucleation, it has been observed that water droplets in cyclopentane take significant time (>24 h) to stochastically begin conversion to hydrate.33 This property allows water droplets to be added to the system without rapid conversion to hydrate, which is necessary for testing the contact angle of a water droplet on the hydrate surface. AA chemicals were obtained through a third-party vendor. Four AAs were used for this study; all four were quaternary-ammonium-saltbased compounds, with varied chain compositions and lengths, which were proprietary to the vendor. The AAs in this work were therefore referred to by their IFT values. For this work, the four AAs are referred to as QAS-39, QAS-25, QAS-15, and QAS-2. The general structure of a quaternary ammonium salt (QAS) is shown in Figure 3.

Figure 4. Procedure for cohesion force pull-off measurements in the MMF. constant preload force and left for 10 s. It has been shown that hydrate growth between cyclopentane particles does not dominate the cohesion force until times greater than about 30 s.10 The top particle, which is controlled using a remote micromanipulator (Eppindorf Patchman), is then pulled away from the bottom at a constant velocity. At distance ΔD, the particles break apart; these pull-off trials are recorded and analyzed using ImageJ software.34 Further details may be found in the study by Aman et al.10 Error bars for cohesion force experiments represent 95% confidence from 3+ individual experiments, each consisting of 40 pull-off measurements. Thus, each experimental value is the average of 120+ individual pull-offs. For the QAS AA experiments, the hydrates were first formed according to the previously described procedure and allowed to anneal for a 5 min period. After the initial shell growth was completed, the AA was added at a concentration of 0.1 wt % with respect to the liquid hydrocarbon phase. The hydrate was then allowed to complete the rest of the annealing time in the presence of the AA. This procedure was selected as a result of the large changes in IFT that the AAs can cause; as the ice particle melted in the cyclopentane solution, a droplet containing AA was likely to fall from the cantilever. By allowing a thin shell to form using pure hydrate before the AA was added, the cohesion force and contact angles could be measured consistently for each AA solution.

Figure 3. General structure of a quaternary ammonium salt. The R groups may be the same or different alkyl or aryl groups.1 IFT Apparatus. Both the IFT between water and cyclopentane and the contact angle of water on a solid surface are able to be measured using the IFT apparatus. Using the pendant-droplet method, the IFT between water and cyclopentane both with and without surfactants can be measured. A KSV Cam 200 was used to measure the IFT as a function of time. The heavy phase for these experiments was deionized water, while for the light phase, cyclopentane was used. The cyclopentane phase was purified using a zeolite column (beta zeolite, ACS Material) to remove any residual impurities, which will be described in more detail in the Results and Discussion. Experiments were repeated 3−5 times, and the reported values for the IFT represent the averages of the steady-state values for each chemical tested. Error reported indicates the standard deviation of these measurements. The contact angle of water on a glass surface was also measured by depositing a water droplet through the needle onto a glass cover slide. The software automatically registers the edges and reports a contact angle. These measurements were also repeated with a minimum of five separate experiments on new glass surfaces for each experiment. For measurements taken in a cyclopentane bulk phase, the cuvette was filled with cyclopentane and a glass cover slide was placed at the bottom of the cuvette. Cohesion Force. A MMF was used to measure the cohesion force between hydrate particles. This is accomplished using Hooke’s law, shown in eq 2

F = k ΔD

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RESULTS AND DISCUSSION Method Development for Contact Angle Measurements. The contact angle of water/surfactant systems on glass was measured using a KSV Cam 200 apparatus. A glass cover slide was cut so that a square, approximately 1 × 1 cm, could be inserted into the glass cuvette filled with air or cyclopentane. A needle was used to deposit a drop of water onto the surface where the stationary contact angle was measured. Because the MMF apparatus is oriented differently than traditional contact angle measurements, it was first necessary to verify that it could report accurate contact angles. To do this, a system was needed that could be recreated on both the IFT and MMF apparatus. Measurements began on an air/water/glass system and progressed to a cyclopentane/water/glass system. In the IFT, the apparatus was used to automatically measure the contact angle. For the MMF, a cantilever was created that held a glass cover slide. A water droplet (typically 600−800 μm in diameter was deposited onto a glass fiber (30 μm) held by a second cantilever. Figure 5 shows the cantilever setup that was

(2)

where F is the cohesion force, k is the spring constant of the glass fiber on which the hydrate particle is created, and ΔD is the distance between the particles when they break apart. To accomplish these experiments, hydrate particles are created on the end of calibrated glass cantilevers by depositing droplets onto the cantilevers and freezing 6621

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will quickly begin to grow along the surface of the water droplet, the image just after the droplet was deposited was used to measure the contact angle.8 A large number of measurements were necessary to obtain an accurate average as a result of the variation caused by the surface roughness of the hydrates. The hydrate shell does not grow smoothly and results in local areas that may have very different topography than the larger scale particle. Surface roughness can also vary based on the formation conditions, such as the magnitude of the driving force present, while the hydrate grows. This parameter is, therefore, highly variable, and the effects of surface roughness on hydrate cohesion and adhesion are not well understood. Figure 7 shows an illustration of how the roughness of the surface can affect the measured value of the contact angle.

Figure 5. Contact angle measurement setup for the MMF apparatus, as seen from above. Water droplets were deposited onto a flat glass surface using a remotely operated micromanipulator.

used in the MMF. This figure represents a top-down view of the particle, representing a 90° shift from the IFT measurements. Using a micromanipulator (Eppindorf Patchman), the water droplet was brought into contact with the cover slide and then the glass fiber was withdrawn. Measurements were performed by recording the images and then processing the angles using ImageJ. Error in the MMF measurements may arise from alignment variations. While the IFT has the camera mounted to look straight onto the droplet/surface interface, the surface in the MMF is free to rotate and, therefore, is not as accurate in capturing images for contact angle measurements. This baseline calibration was necessary because gravity is acting differently on the water droplet in each case. For the IFT, gravity is a force perpendicular to the glass surface, while in the MMF, gravity acts in a parallel direction. Figure 6 shows the results from the comparison of the two apparatuses.

Figure 7. Illustration of how surface roughness can influence the contact angle.

As such, each measurement reported here is the average of a minimum of 10 measurements on 5+ droplets. Error reported for contact angles are the standard deviation of all measurements at a particular condition. Figure 8 shows an image of a water droplet on the surface of a cyclopentane hydrate particle.

Figure 6. Comparison of contact angle measurements in the IFT and MMF apparatuses; these experiments were used to verify that the MMF was suitable to make contact angle measurements.

Figure 8. Contact angle measurement of a water droplet on a cyclopentane hydrate surface in bulk cyclopentane.

From Figure 6, it is apparent that the two systems offer very comparable results (±6%). The experiments performed in a liquid bulk appear to be in closer agreement than those in a system where the bulk phase was air; this may be due to reduced evaporation and airflow in the liquid systems. On the basis of these measurements, the MMF appears to have the ability to accurately report contact angles. Creating a hydrate in cyclopentane requires freezing a water droplet in liquid nitrogen and then submerging it into a cyclopentane bath maintained at the desired experimental temperature. The ice melts because the bulk phase is maintained above the ice point, but the ice serves as a template to aid in the nucleation of hydrates. Hydrate contact angle experiments were carried out by creating a large hydrate particle 10−15 mm in diameter using the method described above and allowing the hydrate to anneal for 30 min. A large hydrate particle was necessary so that the surface was relatively flat under the microscope magnification used. After the annealing, a water droplet 500−1000 μm in size was added to the cell on the tip of a glass fiber. Using a micromanipulator, the droplet was brought into contact with the hydrate surface. The process was recorded and analyzed using ImageJ. Because the hydrate

Measurements of the contact angle of a water droplet on a cyclopentane hydrate surface yielded a value of 94.2° ± 8.5°. This value represents 70 measurements on over 35 individual droplets on hydrate surfaces. This result is somewhat nonintuitive; because the hydrate shell is predominantly composed of water, it was expected that the hydrate shell would have a lower contact angle, more similar to that of ice. For comparison, the contact angle of water on ice has been experimentally measured to be 12° in air;35 because both surfaces are crystalline structures of water, initial estimates for hydrate hydrophilicity tend to be in similar ranges. However, more experimentation is necessary to compare ice and hydrate surfaces, because they have not been measured in the same bulk phase. Model Comparisons. The capillary bridge equation (eq 1) allows for the prediction of interparticle force if several interfacial properties are known about a given system. Aman et al. used the cohesion force measured in the MMF along with IFT measurements to predict α, the embracing angle of the liquid bridge.10 To obtain α, which was determined as α = 0.1°, a contact angle measurement of 29° from Asserson et al. was 6622

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micrometer range without being visually detected (as shown in Figure 11). It is therefore more likely that the embracing angle is larger than previous estimates. Using the value of 50 nm for the height of the liquid layer, the embracing angle for a cyclopentane hydrate system was estimated as α = 4.7°. Figure 12 shows a visualization of both the previous and updated estimates for α. The images represent the total width of the liquid bridge on the hydrate particle, with the embracing angle extending out to either side from the midline rather than just a single side, as indicated in Figure 2 (i.e., the distance shown in Figure 12 is 2α). The prediction of α could be improved by obtaining a better estimate of values for H. On the basis of measurements by Doppenschmidt and Butt on ice, the liquid layer is sensitive to the temperature difference between the system and the thermodynamic equilibrium of the solid phase (e.g., the ice point in frozen systems).36 Therefore, it would be ideal to perform atomic force microscopy directly on a hydrate particle to investigate the water layer under a variety of conditions. Despite this, the measurements described in this work show that the contact angle is directly obtainable for hydrate systems, which may allow for insights into the mechanisms of hydrate cohesion as well as the mechanism of AAs in the future. AA Tests. After the AAs were added to the cyclopentane bulk phase, many of the AAs tested caused morphological changes to occur in the hydrate particles. The initial and final appearances of the hydrates are shown in Figure 13. With QAS15 and QAS-25, unconverted water from the core of the hydrate was observed extruding through the hydrate shell. The water drops did not appear to stick to the hydrate shell and were swept away as a result of natural convection within bulk cyclopentane. When QAS-2 was added to the hydrate, it was observed that small flakes of hydrate began to slough off of the particle. In some experiments, enough small hydrate particles fell from the main hydrate that unconverted water at the center was exposed. Such experiments were not used for pull-off testing in case the exposed water altered the results. Previously, it was theorized that AAs function by rapidly converting the water in the system into hydrates, reducing the amount of water available to form capillary bridges and, thereby, enabling cold flow.24 On the basis of the observations in this study, other mechanisms for decreasing the cohesion force between particles must be responsible for the anti-agglomeration behavior, because unconverted water was still present in the hydrates, which was not observed to convert into hydrate. Two possible mechanisms are shown below in Figure 14. It was found that QAS-39 and QAS-25 were poor AAs and did not cause a significant reduction in the cohesion force; however, QAS-15 and QAS-2 both performed well, with the force reduced by over 90% from the baseline force measured for pure hydrate. The results from these experiments are shown in Figure 15. More AA testing will confirm the accuracy and wider applicability of the MMF ranking, which can be performed more quickly/conveniently than other ranking tests, such as rocking cell experiments. In addition, MMF tests require chemical volumes an order of magnitude smaller than other testing methods. IFT values are also shown in the top of Figure 15, and while they correlate with decreasing cohesion force in this case, it has been shown previously that IFT is not always a good predictor of cohesion force.14 Contact angle measurements were performed and compared to the cohesion force measurements for each AA system tested. The bottom of Figure 15 shows the relationship determined

used. This measurement was obtained from the analysis of a chloroform droplet deposited on a Freon hydrate surface in a bulk water phase. It was also assumed that the height of the liquid layer, H, was 50 nm based on atomic force microscopy measurements performed on ice particles.36 Through a large number of baseline experiments, it was determined that the cohesion force between two cyclopentane hydrate particles at 3 °C was 4.2 ± 0.4 mN/m.10 The IFT of cyclopentane was assumed to be analogous to that of pentane based on the relationship between hexane and cyclohexane,37−39 which gave an IFT value of 50.9 mN/m. The calculated value of α = 0.1° corresponds to a minimal amount of spreading from the liquid layer on the particle. To update this estimate, it was first necessary to obtain an accurately measured value for the IFT between cyclopentane and water rather than using a value based on pentane. Previous attempts to measure this value resulted in inaccurate measurements as a result of impurities present in cyclopentane. The impurities caused the IFT values obtained to decrease over time, as is typically observed for systems containing surfactants. For pure systems, the IFT value should be constant with time. A purification system consisting of a packed cylindrical bed containing beta zeolite spheres (SiO2/Al2O3, ACS Materials) was employed, where cyclopentane was poured over the spheres 4−5 times.38 After this process, IFT measurements were performed again and cyclopentane was found to have a constant value with time; Figure 9 shows the IFT results after the purification process. These measurements resulted in an updated value for the IFT of cyclopentane and water of 47.8 ± 0.5 mN/m.

Figure 9. Multiple trials measuring the IFT between cyclopentane and water after purification.

Obtaining an accurate value for the cyclopentane/water IFT gives every parameter in eq 1 measured directly for a cyclopentane/hydrate system, except for H, the height of the liquid bridge, and α, the immersion depth. The force was predicted for a range of values for both α and H. Figure 10 illustrates a plot showing the predicted force for α = 0.025−10° and H = 0.005−5 μm. The measured force using the MMF for these experiments was 4.2 ± 0.4 mN/m; therefore, using the surface plot in Figure 10 to match the measured force, one can draw insight on possible combinations of H and α, which give results that match what was physically observed. Figure 10 indicates that, for lower values of α, H must be (relatively) very large to achieve values near the measured value. Given the magnification used in MMF experiments, it is unlikely that the water layer could reach dimensions on the 6623

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Figure 10. Surface plot showing the cohesion force for various combinations of H and α. The measured force for this system was 4.2 ± 0.4 mN/m, meaning that the possible combinations of H and α must intersect the plane at F = 4.2 mN/m.

Figure 11. Invisible water layer between hydrate particles.

Figure 12. Depiction of the (left) previous and (right) updated estimations of the immersion depth, α (shown here on both sides of center for a total of 2α). Figure 13. Initial and final particle morphology for each AA used. Morphological changes observed upon the addition of AA included water expulsion and hydrate sloughing.

between the cohesion force and the contact angle measured for each system. The highly effective AAs, QAS-15 and QAS-2, both had superhydrophobic contact angles above 150°, while the less effective AAs had contact angles that were either neutral or hydrophilic. On the basis of these measurements and the correlation between the contact angle and cohesion force for the AAs tested in this study, it was proposed that one mechanism by which AAs reduce the cohesion between hydrate particles is by altering the hydrophobicity of the hydrate surface. By causing the hydrate surface to change from a neutral contact angle for pure hydrates to a hydrophobic contact angle for hydrate in the presence of AAs, the possible interaction with

water droplets that form capillary bridges is significantly reduced.

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CONCLUSION The MMF apparatus was verified to produce contact angle measurements comparable to traditional pendant-droplet measurement techniques, and the contact angle of water droplets on the hydrate surface was directly measured using 6624

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hydrophilic contact angles. In addition, the MMF allowed for the visual observation of morphological changes caused by the addition of AAs into the system. The two major morphological changes observed were water extrusion out from the unconverted center of the hydrate and hydrate sloughing, where small pieces of hydrate detached from the exterior of the hydrate particle.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Figure 14. Two proposed mechanisms describing how AAs facilitate slurry flow. (Top) AAs reduce the average droplet size in an emulsion, resulting in small hydrates that have converted the majority of the water into hydrate. As a result of the lack of free water and the AA effects, the hydrates flow as a slurry. (Bottom) Hydrates form around relatively large water droplets, creating hydrate shells that surround unconverted water. As a result of the morphological changes caused by AAs, the hydrates break down and release unconverted water; however, water and hydrates tend not to interact because of the hydrophobic hydrate shells caused by AAs.

Carolyn A. Koh: 0000-0003-3452-4032 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the current and past Colorado School of Mines (CSM) Hydrate Consortium members for their support: BP, Chevron, ConocoPhillips, ENI, ExxonMobil, Halliburton, IMP, MultiChem, Nalco Champion, One Subsea, Petrobras, Schlumberger, Shell, Statoil, and Total. Carolyn A. Koh also acknowledges the support from the William K. Coors fund.

MMF. Using updated parameters, the immersion depth was estimated for a cyclopentane hydrate and water system. On the basis of this technique, a selection of four quaternaryammonium-salt-based AAs were tested using the MMF for both cohesion force and contact angle measurements. It was found that the MMF was capable of rapidly ranking the AAs. In addition, the contact angle and cohesion force were correlated for the AAs tested in this study, where effective cohesion reduction corresponded to superhydrophobic contact angles, but AAs that were shown not to be effective had neutral or even

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REFERENCES

(1) Sloan, E. D.; Koh, C. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2007. (2) Turner, D. J.; Miller, K. T.; Sloan, E. D. Methane hydrate formation and an inward growing shell model in water-in-oil dispersions. Chem. Eng. Sci. 2009, 64, 3996−4004. (3) Aman, Z. M.; Koh, C. A. Interfacial phenomena in gas hydrate systems. Chem. Soc. Rev. 2016, 45, 1678−1690.

Figure 15. (Top) Cohesion force and IFT values for each of the AAs tested. (Bottom) Cohesion force appears to decrease as contact angles become more hydrophobic. 6625

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Energy & Fuels

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DOI: 10.1021/acs.energyfuels.8b00803 Energy Fuels 2018, 32, 6619−6626