Direct Measurements of Contact Angles on Cyclopentane Hydrates

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Direct Measurements of Contact Angles on Cyclopentane Hydrates Erika Brown, Sijia Hu, Jonathan Wells, Xiao-Hui Wang, and Carolyn A. Koh Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00803 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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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

†Center

∗,†

for Hydrate Research, Chemical and Biological Engineering Department, Colorado School of Mines, Golden, CO 80401, USA. ‡State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, CHINA E-mail: [email protected]

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 thoroughlyexplored 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 eect on the cohesion behavior of clathrate hydrates; however, direct measurements of water on a hydrate surface are not prevalent. In order 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. Combining 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. Dierent anti-agglomerants (AAs) were tested

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for both contact angle and cohesion force, which showed that the Micromechanical Force Measurements Apparatus (MMF) 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 identied.

Introduction The environment created in subsea owlines provides favorable thermodynamic conditions for the formation of clathrate hydrates (also known as gas hydrates), which are ice-like inclusion compounds which form at low temperatures and high pressures. 1 These crystalline structures are comprised of water molecules which 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 and one for the largest cage. 1 Structure II hydrate is the most common form of hydrate found in owlines, as the gas transported there typically comprises longerchain molecules that are too large to t into the cages of Structure I. Hydrate formation in owlines is a multi-step process that can be conceptually described by splitting it into several phenomena. Figure 1 shows a conceptual model of hydrate formation in a multi-phase owline. Hydrates typically form at interfaces, so the preliminary phenomenon in hydrate formation is entrainment of the phases in one another. 3,4 Natural surfactants as well as chemical additives can alter the emulsion properties of the system, 5 as can system properties such as 2


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Figure 1: Hydrate formation in a owline can be described as the combination of multiple phenomena that can eventually result in the complete arrest of production in case of a plug. Insets show water droplets adhering to the exterior of hydrate particles, resulting in wetting of the particles. Modied from Turner et al. 2 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 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 (>100μm 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 or settle onto pipe walls. The strength and severity of the agglomeration is determined by the inter-particle 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 Equation 1: 2πγcos (θ) F = πγsin (α) sin (θ + α) + H R 1 + 2d

(1)

Where F is the cohesion/adhesion force, R is the normalized radius of the particles, γ is the interfacial tension between the bridge and the bulk phase, α is the embracing angle, θ is 3


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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 Equation 1.

Figure 2: Depiction of the physical meaning of variables represented in Equation 1 for a particle-surface interaction as well as a particle-particle interaction. 10This work aims to directly measure these parameters in order to better understand agglomeration behavior. The capillary bridge in an oil-continuous system is comprised of water that may come from several dierent 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 diuses 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 MMF 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 force using this apparatus. Aspenes and co-workers studied particle=surface interactions using cyclopentane (CyC5) as the clathrate hydrate former. The results showed that the 4


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adhesion force was strongly dependent on the surface material and the amount of water present. 12 Dieker measured the force in a system with a small amount of natural oil, nding 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) may adsorb onto the water/hydrocarbon or hydrate/hydrocarbon interface. 16 A study by Aman et al. showed that measurements of the interfacial tension were insucient 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 angle of water on dierent 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. 19Esmail et al 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 authors' knowledge. The eect of contact angle on the cohesion force may also be informative about the mechanism by which AAs reduce the inter-particle force. AAs are a class of chemicals (typically surfactants) used in owlines to encourage solid hydrate particles to ow as a transportable slurry rather than agglomerate into masses that may plug the owline. 22,23 One dominant theory for the method by which AAs reduce cohesion forces was a mechanism similar to cold ow, 24 whereby the 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 since 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 5


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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 signicantly less agglomeration due to reduced interaction with water. The interaction of water droplets on and with the hydrate surface has been shown to have a signicant eect on the agglomeration behavior. It has been shown that the hydratewater 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 dierent times. It is therefore likely that water droplets and hydrates will interact in a owline environment. The wetting behavior may also inuence 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 interfacial tension 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 eect of chemical additives on clathrate hydrate forming systems. Direct observation of morphological changes caused by hydrates can also provide more information on how these chemicals aect hydrate properties, such as agglomeration and wettability. The design and implementation of better anti-agglomerant compounds depends on an accurate hypothesis on the mechanism for chemical and hydrate interactions.

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Methods and Materials Materials

Cyclopentane (Sigma Aldrich, reagent grade, 98% purity) was chosen in this study as it forms hydrates at atmospheric pressure and temperatures below 7.7C. 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 signicant time (>24 hrs) 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. Anti-agglomerant chemicals were obtained through a third party vendor. Four AAs were utilized for this study; all four were quaternary ammonium salt-based compounds, with varied chain compositions and lengths which were proprietary to the vendor. The AAs in this work were therefore referred to by their interfacial tension values. For this work, the four anti-agglomerants are referred to as QAS-39, QAS-25, QAS-15 and QAS-2. The general structure of a quaternary ammonium salt is shown in 3.

Figure 3: General structure of a quaternary ammonium salt. The R groups may be the same or dierent alkyl or aryl groups. 1

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Interfacial Tension Apparatus

Both the interfacial tension 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 pendantdroplet method, the interfacial tension (IFT) between water and cyclopentane both with and without surfactants can be measured. A KSV Cam 200 was used to measure the interfacial tension 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 puried using a zeolite column (Beta Zeolite, ACS Material) in order to remove any residual impurities, which will be described in more detail in the results section. Experiments were repeated 3-5 times, and the reported values for the interfacial tension 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 5 separate experiments on new glass surfaces for each experiment. For measurements taken in a cyclopentane bulk phase, the cuvette was lled 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 Equation 2. F = k∆D

(2)

Where F is the cohesion force, k is the spring constant of the glass ber on which the hydrate particle is created, and ∆D is the distance between the particles when they break apart. In order to accomplish these experiments, hydrate particles are created on the end of 8


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calibrated glass cantilevers by depositing droplets onto the cantilevers and freezing them using liquid nitrogen. The ice particles are then submerged in a cyclopentane bath maintained in the range of 0-7.7C. The ice provides nucleation sites for hydrates to form, which are then allowed to anneal for 30 minutes. Pull-o measurements are then performed according to Figure 4. The particles are rst brought into contact at a constant pre-load force, and left for 10 seconds. It has been shown that hydrate growth between cyclopentane particles does not dominate the cohesion force until times greater than about 30 seconds. 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 ∆D , the particles break apart; these pull-o trials are recorded and analyzed using ImageJ software. 34 Further details may be found in Aman et. al. 10

Figure 4: Procedure for cohesion force pull-o measurements in the MMF. Error bars for cohesion force experiments represent 95% condence from 3+ individual experiments, each consisting of 40 pull-o measurements. Thus, each experimental value is the average of 120+ individual pull-os. For the QAS AA experiments, the hydrates were rst formed according to the previously described procedure and allowed to anneal for a 5 minute 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 due to the large 9


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changes in interfacial tension 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.

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 cm x 1 cm could be inserted into the glass cuvette lled 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 dierently than traditional contact angle measurements, it was rst necessary to verify that it could report accurate contact angles. In order 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 which held a glass cover slide. A water droplet (typically 600-800 µm in diameter was deposited onto a glass ber (30 µm) held by a second cantilever. Figure 5 shows the cantilever setup that was used in the MMF. This gure 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, then the glass ber was withdrawn. Measurements were performed by recording the images, 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 10


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measurements.

Figure 5: Contact angle measurement setup for the MMF apparatus as seen from above. Water droplets were deposited onto a at glass surface using a remotely operated micromanipulator. This baseline calibration was necessary because gravity is acting dierently 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 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. From the Figure 6, it is apparent that the two systems oer 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 airow in the liquid systems. Based on these measurements, the MMF appears to have the ability to accurately report contact angles. 11


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Creating a hydrate in cyclopentane requires freezing a water droplet in liquid nitrogen 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 25-50 mm in diameter using the method described above, and allowing the hydrate to anneal for 30 minutes. A large hydrate particle was necessary so that the surface was relatively at under the microscope magnication used. After the annealing, a water droplet 500-1000 μm in size was added to the cell on the tip of a glass ber. Using a micromanipulator, the droplet was brought into contact with the hydrate surface. The process was recorded and analyzed using ImageJ. Since the hydrate 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 in order to obtain an accurate average due to 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 dierent 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 eects 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 aect the measured value of the contact angle.

Figure 7: Illustration of how surface roughness can inuence the contact angle. As such, each measurement reported here is the average of a minimum of 10 measure12


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ments 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 8: Contact angle measurement of a water droplet on a cyclopentane hydrate surface in bulk cyclopentane. 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 non-intuitive, since 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 degrees in air; 35 as 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, since they have not been measured in the same bulk phase. Model Comparisons

The capillary bridge equation (Equation 1) allows the prediction of inter-particle 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 in order to predict α, the embracing angle of the liquid bridge. 10 In order to obtain α, which was determined as α = 0.1, 13


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a contact angle measurement of 29 from Asserson et al. was utilized. 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 3C was 4.2 ± 0.4 mN/m. 10 The interfacial tension of cyclopentane was assumed to be analogous to that of pentane, based on the relationship between hexane and cyclohexane, 3739 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. In order to update this estimate, it was rst necessary to obtain an accurately measured value for the interfacial tension between cyclopentane and water, rather than using a value based on pentane. Previous attempts to measure this value resulted in inaccurate measurements due to impurities present in the 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 purication system consisting of a packed cylindrical bed containing Beta Zeolite spheres (ACS Materials, SiO2/Al2O3) was employed, where the cyclopentane was poured over the spheres 4-5 times. 38 After this process, IFT measurements were performed again, and the cyclopentane was found to have a constant value with time; Figure 9 shows the IFT results after the purication process. These measurements resulted in an updated value for the IFT of cyclopentane and water of 47.8±0.5 mN/m. Obtaining an accurate value for the cyclopentane/water IFT gives every parameter in Equation 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=0.02510 and H=0.005-5 μm. The measured force using the MMF for these experiments was 14


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Figure 9: Multiple trials measuring the IFT between cyclopentane and water after purication. 4.2±0.4 mN/m, so 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: 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 10 indicates that for lower values of α, H must be (relatively) very large in order to achieve values near the measured value. Given the magnication used in MMF experiments, 15


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it is unlikely that the water layer could reach dimensions on the micron 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 the 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).

Figure 11: Invisible water layer between hydrate particles.

Figure 12: Depiction of the previous (left) and updated (right) estimations of the immersion depth, α (shown here on both sides of center for a total of 2α). The prediction of α could be improved by obtaining a better estimate of values for H. Based on measurements by Doppenschmidt and Butt on ice, the liquid layer is sensitive to the temperature dierence 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 16


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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 insights into the mechanisms of hydrate cohesion as well as the mechanism of anti-agglomerants in the future. Anti-Agglomerant 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 nal appearances of the hydrates are shown in Figure 13. With QAS-15 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 due to natural convection within the bulk cyclopentane. When QAS-2 was added to the hydrate, it was observed that small akes of hydrate began to slough o of the particle. In some experiments, enough small hydrate particles fell from the main hydrate that the unconverted water at the center was exposed. Such experiments were not used for pull-o 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 ow. 24 Based on the observations in this study, other mechanisms for decreasing the cohesion force between particles must be responsible for the anti-agglomeration behavior, as unconverted water was still present in the hydrates which was not observed to convert into hydrate. Two possible mechanisms are shown below in Figrue 14. It was found that QAS-39 and QAS-25 were poor anti-agglomerants, and did not cause a signicant 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 conrm the accuracy and wider applicability of the MMF ranking, which can be performed 17


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Figure 13: Initial and nal particle morphology for each AA used. Morphological changes observed upon the addition of AA included water expulsion and hydrate sloughing.

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Figure 14: Two proposed mechanisms describing how AAs facilitate slurry ow. (Top) AAs reduce the average droplet size in an emulsion, resulting in small hydrates that have converted the majority of the water into hydrate. Due to the lack of free water and the AA eects, the hydrates ow as a slurry. (Bottom) Hydrates form around relatively large water droplets, creating hydrates shells that surround unconverted water. Due to the morphological changes caused by AAs, the hydrates break down and release the unconverted water; however, the water and hydrates tend not to interact because of the hydrophobic hydrate shells caused by AAs. 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. Figure 15 (bottom) shows the relationship determined between the cohesion force and the contact angle measured for each system. The highly eective AAs, QAS-15 and QAS-2 both had superhydrophobic contact angles above 150◦, while the less eective AAs had contact angles which were either neutral or hydrophilic. Based on these measurements and the correlation between 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 19


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neutral contact angle for pure hydrates to a hydrophobic one for hydrate in the presence of AAs, the possible interaction with water droplets that form capillary bridges is signicantly reduced.

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.

Conclusions The micromechanical force apparatus (MMF) was veried 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 MMF. Using updated parameters, the immersion depth was estimated for a cyclopentane hydrate and 20


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water system. Based on this technique, a selection of four quaternary ammonium salt-based anti-agglomerants 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 the cohesion force were correlated for the AAs tested in this study, where eective cohesion reduction corresponded to superhydrophobic contact angles, but AAs which were shown not to be eective had neutral or even 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.

Acknowledgement The authors thank the current and past 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. CAK also acknowledges the support from the William K. Coors fund.

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Direct Measurements of Contact Angles on Cyclopentane Hydrates

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