Mechanism of Cohesive Forces of Cyclopentane Hydrates with and

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Mechanism of Cohesive Forces of Cyclopentane Hydrates with and without Thermodynamic Inhibitors Bo Ram Lee, Carolyn A. Koh, and Amadeu K. Sum Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie5032716 • Publication Date (Web): 03 Nov 2014 Downloaded from http://pubs.acs.org on November 8, 2014

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Industrial & Engineering Chemistry Research

Mechanism of Cohesive Forces of Cyclopentane Hydrates with and without Thermodynamic Inhibitors

Bo Ram Lee, Carolyn A. Koh, and Amadeu K. Sum*

Center for Hydrate Research, Chemical & Biological Engineering Department, Colorado School of Mines, 1500 Illinois St., Golden, CO 80401 – USA

Keywords: Hydrate, deposition, cohesive/adhesive force, thermodynamic inhibitors

ACS Paragon Plus Environment

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ABSTRACT Gas hydrates are commonly formed in oil and gas pipelines. One approach in their prevention is the injection of thermodynamic inhibitors (e.g., methanol, ethanol, monoethylene glycol) so that the hydrate stability phase equilibrium can be moved into the fluid stable region. In this study, we directly measure cohesive forces of cyclopentane hydrates with thermodynamic inhibitors (2 wt.% monoethylene glycol (MEG), methanol, ethanol, and 1 and 3.5 wt.% sodium chloride) to understand the effects of thermodynamic inhibitors (THIs) on the cohesive forces of hydrates. The cohesive forces are measured as a function of annealing time and temperature and determined from pull-off measurements based on the principle of Hooke’s law (F = Kspring × ΔD, where Kspring is spring constant of the cantilever fiber, and ΔD is the displacement of the cantilever). A mechanism for the cohesive force of partially and fully converted hydrate particles is inferred and partially demonstrated by DSC measurements. These experiments and results are essential to quantify the impact of THIs on hydrate particle interactions, with implications on the usage of these chemicals, particularly in under-inhibited conditions.


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1. INTRODUCTION Gas hydrates are stable crystalline compounds formed by water and natural gas, such as methane, ethane, and propane, at appropriate pressures and temperatures. In oil and gas transportation in multiphase flowlines, hydrates can be formed due to relatively low temperature and high pressure conditions. The formation, agglomeration, and accumulation of hydrates in these flowlines can lead to a hydrate blockage, jeopardizing production and raising safety concerns.1-2 As shown in Figure 1, a four-step formation process is generally accepted for this hydrate blockage: (1) water entrainment in the oil phase; (2) formation and growth of hydrates; (3) agglomeration between hydrate particles; and (4) jamming of agglomerates to form a hydrate plug.3-4 To prevent hydrates from forming, many studies have focused on hydrate formation and growth conditions, which are highly dependent on the kinetics and thermodynamic properties defined by the hydrate equilibrium temperature, pressure, and gas composition. In contrast, understanding the agglomeration of hydrate particles depends on physical and mechanical properties, and these are relatively scarcely studied.

Water Entrainment

Hydrate Growth

Agglomeration

Plugging

Hydrate

Water

Figure 1. Modified conceptual mechanism for hydrate plug formation in oil-dominated flowlines, from Turner3 (in collaboration with J. Abrahamson).


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The work by Yang et al.5 has shown the temperature dependence of the cohesive forces between ice or hydrate particles. Dieker et al.6 reported the adhesive force between hydrate particles in hydrocarbon oils, and Aman et al.7 presented a comprehensive overview of the interfacial mechanisms governing cyclopentane hydrate adhesion and cohesion. Recently, Lee et al.8 developed a new apparatus for measuring cohesive force of gas hydrates. For a decade, although significant work has been devoted to better understand the cohesive force of hydrates5-11, almost all of the previous measurements were performed with relatively sufficient annealing time to reach high hydrate conversion or did not consider annealing time in any detail. In real field conditions, the flowing conditions are constantly changing and it is unlikely that fully converted hydrate particles prevail.4 As such, the transient dependence for the cohesive force, that is, as a function of the annealing time, is necessary to improve the understanding of hydrate particle agglomeration in flowlines. In this study, we present measurements of the cohesive force between cyclopentane hydrate particles with and without THIs [methanol, ethanol, monoethylene glycol (MEG), and sodium chloride (NaCl)], to quantify the effect of THIs on the agglomeration properties of hydrates at varying annealing time and accompanied morphology. Cyclopentane hydrates is a model system for gas hydrates as it is convenient to form under atmospheric pressure, it is immiscible with water, and forms structure II hydrates, the same as most natural gas systems. The cohesive forces are measured as a function of annealing time and temperature. The forces are determined from pull-off measurements9 based on the principle of Hooke’s law (F = Kspring × ΔD, where Kspring is spring constant of the cantilever fiber, and ΔD is the displacement of the cantilever). A mechanism for the cohesive force of hydrate particles is inferred and partially demonstrated by DSC (Differential Scanning Calorimeter) measurements.


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2. EXPERIMENTAL SECTION 2.1 Measurements of Cohesive Force of Hydrate Particles An improved micromechanical force (MMF) apparatus was used to directly measure the cohesion force between hydrate particles. This apparatus consists of a Carl Zeiss Axiovert S100 inverted microscope with digital recording equipment on a vibration isolation table. Further information about the development of the apparatus and experimental method has been extensively discussed in the literature.7-11 THI solutions containing monoethylene glycol (2 wt%, 99% purity, Macron Chemicals), methanol (2 wt%, 99.9% purity, Macron Chemicals), ethanol (2 wt%, 99.5% purity, SigmaAldrich), and sodium chloride (1 and 3.5 wt%, 99% purity, Macron Chemicals) (each separately) is prepared, and a droplet of the solution is placed at the end of each glass fiber. The cantilevers are immersed in liquid nitrogen to freeze the sample and then placed in the precooled cyclopentane bath made by aluminum. Freezing of the solution droplets helps its conversion of hydrate upon melting of the ice7-9. As shown in Figure 2, the aluminum cell temperature is then raised above the ice point to melt any ice and induce cyclopentane (7.7ºC for pure cyclopentane hydrate dissociation) or cyclopentane + THI mixed hydrate formation. Once the experimental temperature is reached, hydrate particles are annealed for 0-3 hours at a set temperature. Cyclopentane hydrates generally form around the ice melting point, and that moment is typically taken as the starting point for the annealing time. Note that in some instances, hydrates may not form at the ice melting point (due to stochastic nature of hydrate formation) and the annealing time is taken as the point when hydrate is observed to be formed. During the experiments, dry air saturated with cyclopentane is injected to prevent evaporation of cyclopentane from the aluminum cell.


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5

Starting Point for Annealing Time

CyC5 or THI + CyC5 hydrates

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(After 1.5 hours from experimental setup)

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Temperature [oC]

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2

Annealing time: 0 to 3 hours

CyC5 bath

1 0 -1 -2 -3

Experimental materials Placing two cantilevers into the aluminum cell Stepwise heating

-4 -5 0

(1) Pure CyC5 (2) 1.0 wt.% NaCl addition (3) 3.5 wt.% NaCl addition (4) 2.0 wt.% MEG addition (5) 2.0 wt.% Methanol addition (6) 2.0 wt.% Ethanol addition

1

2

Time [Hours]

Figure 2. Experimental procedure to verify the effect of annealing time on the cohesive force of cyclopentane hydrates with and without THIs.

MMF experiments utilize “pull-off” trials, where the right-hand particle is brought in contact with the left-hand particle. The detailed procedure is provided elsewhere.7-11 For each experiment, the cohesive force is reported as the average of over 60 pull-off measurements, with 95% confidence error bounds.


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2.2 Measurements of Cohesive Force of Hydrate Particles A differential scanning calorimeter (Setaram, µDSC VII) is used for measurement of cyclopentane hydrate phase equilibrium with THIs. The stepwise scheme method12 for the measurements is as follows: 1. Inject ~30 mg of sample (cyclopentane + THI solution) to the DSC cell, initially at 20°C and atmospheric pressure. 2. Ramp down the temperature to -40°C and keep the temperature for 2 hours to freeze the entire sample. 3. Increase the temperature up to 20°C at a rate of 0.1°C/min and estimate the melting temperature. 4. Repeat step 2. 5. Ramp the temperature to near the expected melting temperature, and keep the temperature for 2 hours to allow the sample to equilibrate. 6. Increase the temperature in 0.1°C increments, holding the sample for 30 min after each step. 7. Monitor the heat flow change with temperature increase.


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3. RESULS AND DISCUSSION 3.1 Effect of Annealing Time on Cohesive Force of Hydrate Particles Figure 3 shows the cohesive force of pure cyclopentane hydrates. Without annealing of the hydrate particles, the cohesive force is about 9 mN/m, a value that gradually decreases but quickly reaches a steady value of about 4 mN/m. This value is consistent with previous measured values of about 4.2 mN/m.10 16

Cohesive Force [mN/m]

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3.5wt.% NaCl (A)

12

2wt.% Ethanol (B)

10

2wt.% Methanol (B) 2wt.% MEG (A)

8

Pure CyC5

6

1wt.% NaCl (A)

4 15 min.

2

0

1

2

3

Time [Hours] Figure 3. Cyclopentane hydrate-hydrate cohesive force with and without THIs as a function of annealing time, measured at 1°C. Error bars correspond to 95% confidence on data from at least three experiments with over 60 pull-off trials in each experiment. Groups A and B are insoluble and soluble THIs in cyclopentane, respectively.


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Of the THIs used, they can be divided into two groups: Group A for THIs insoluble in cyclopentane (NaCl and MEG), and Group B for THIs soluble in cyclopentane (Methanol and Ethanol). The cohesive forces for Group A THIs (NaCl and MEG) show similar trends with that of pure cyclopentane. The cohesive forces are significantly affected by the annealing time, especially for the higher concentration of NaCl (3.5 wt%), which shows higher cohesive force relative to pure cyclopentane hydrates. In contrast, the cohesive force shows large variation resulting from the irregular morphology of the hydrate particles in Group B. The major reason for this can be the morphology change of the hydrate particle due to the THI solubility in cyclopentane. It is visually observed that dendrites grew continuously on the surface of cyclopentane hydrate particles over annealing time. These dendrites can severely affect the cohesive force measurements in terms of contact area and particle roughness (see Supporting Information for images of the morphology of the particles). As shown in Figure 3, all cohesive forces measured in the presence of the THIs are higher than for pure cyclopentane hydrate particles. The data can be interpreted with two possible hypotheses: one is the influence of unconverted water in the hydrate particle and relatively slow conversion rate, and the other is the change in the subcooling (ΔT = TEq – TExpt.) caused by the addition of the THI.


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3.1.1 Effect of Unconverted Water in Hydrates While the cohesive force for pure cyclopentane hydrates is unchanged (after 15 mins annealing time), the cohesive force with THIs changes during the annealing time up to 3 hours. One possible reason for this difference may be due to kinetics of hydrate formation with the THIs. If the cyclopentane hydrate formation or water conversion rate is relatively slow, the interaction of the particles through the capillary liquid bridge will be impacted by the amount of unconverted water on the hydrate surface, thus affecting the cohesive force. Figure 4 shows the morphological changes to the cyclopentane hydrate particles in the presence of 1 wt.% NaCl after pull-off measurements (over 300 trials) at multi-spots. We expect some of the unconverted water in the droplet to be a source to changes in the particle surface morphology. The presence of NaCl causes the hydrate formation rate to decrease, and as such leads to a larger amount of unconverted water compared to the particles formed without THIs. The unconverted water is also a source for the capillary liquid bridge between hydrate particles upon contact, and this liquid bridge can significantly affect the cohesive force. Image B in Figure 4 shows the dendritic morphology of the hydrate on the surface, which results from the continued hydrate formation (secondary growth) from the unconverted water on the surface.


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(A)

200 µm

(C)

Fiber

(B)

200 µm

[Mechanism]

Unconverted water

Upper particle

Water bridge

Hydrates

Lower particle

Figure 4. Morphological changes on the hydrate surface for the 1 wt.% NaCl system. Hydrate particle was annealed for 1 hour. Image A and image B were recorded before and after pull-off trials (300 in total), respectively. Image C is a schematic of the possible mechanism for the changes in particle surface morphology (schematic of water bridge is exaggerated for illustration purposes).


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3.1.2 Effect of Driving Force (ΔT, Subcooling) Thermodynamic hydrate inhibitors are injected in flowlines to prevent hydrate formation by intentionally displacing the system from favorable thermodynamic conditions for hydrate formation. The addition of THIs will affect the formation of cyclopentane hydrates, thus lowering the hydrate dissociation temperature (7.7°C for pure cyclopentane hydrate1). As such, the formation of cyclopentane hydrate with THI at a given temperature will form at lower subcooling. In this section, we present the effect of subcooling on the cohesive force for almost fully converted hydrate particles. To determine the change in the subcooling, phase equilibrium DSC measurements for cyclopentane hydrate with 1wt.% NaCl were performed. A sample thermogram from the DSC measurements is shown in Figure 5. In the heat flow curve, very sharp exothermic peaks are observed from the thermal lag in the reference cell when the temperature is changed stepwise. These peaks subside, and either the heat flow returns to the baseline or endothermic peaks are observed, where the endotherms correspond to the hydrate dissociation.12 The last temperature step at which melting occurs marks the point where the last hydrate dissociates and corresponds to the equilibrium dissociation temperature. For the sample shown in Figure 5, the dissociation temperature for cyclopentane hydrate with 1 wt% NaCl is 6.9°C.


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7.4

2.0

Furnace temperature

7.2

1.5

Heat flow

Dissociation temp.

1.0

7.0 0.5 6.8 0.0 6.6

6.4

Base line

Start of dissociation (at 6.5°C)

4

6

8

10

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Heat Flow [mW]

Furnace Temperature [oC]

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-0.5 -1.0

Time [Hours] Figure 5. Heat flow-time and temperature-time plots for cyclopentane + 1wt.% NaCl hydrate sample. The last dissociation observed coincides with a furnace temperature of 6.9°C.

To consider only the effect of subcooling, cyclopentane hydrate with 1wt.% NaCl was formed and annealed for over 24 hours in a beaker immersed in the chiller bath maintained at -3°C.13 Note that for this sample, no morphological change were observed on the hydrate surface in 300 pull-off trials, suggesting there was no significant influence of unconverted water, even though there may still be some amount in unconverted water (further details are presented in the supporting information).


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10 Sample Preparation for 24 hours

Cohesive Force [mN/m]

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* TEq = 6.9°C

CyC5 + 1wt.% NaCl mixed hydrates

8

Liquid CyC5

6

4 Normalized Cohesion Force of Pure CyC5 Hydrates at 3.2°C

2

1

2

3

4

5

6

7

o

Subcooling [ C] Fig. 6 Cohesive force of cyclopentane hydrate with 1 wt.% NaCl annealed for 24 hours as a function of subcooling (hydrate equilibrium temperature = 6.9°C, measured by DSC). Normalized cohesive for CyC5 hydrates at 3.2 °C, 4.2 ± 0.4 mN/m, was presented by Aman et al. Error bounds of each experiment represents 95% confidence with over 60 pull-off trials.

Figure 6 shows the effect of subcooling on the cohesive force of the mixed hydrate particles annealed for 24 hours. The cohesive forces at the different subcooling temperatures are similar to the normalized cohesive force for pure cyclopentane hydrates, as reported by Aman et al.10 These data are also interesting in that there is no temperature dependence of hydrate particleparticle cohesive forces. While the subcooling does affect the hydrate formation rate (time dependent interactions), it has no impact on equilibrium properties (after particles have been sufficiently annealed and equilibrated).


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3.2 Possible Mechanism of Cohesive Force of Hydrate Particles Previous MMF studies5, 9-10 presented the temperature dependence of hydrate cohesion forces, supporting the presence of a quasi-liquid layer (QLL) on the hydrate surface. The height of QLL (h) increases with increasing temperature by # 1 & h ∝ ln % ( $ To − T '

(1)

where, To and T are the equilibrium and system temperatures, respectively (where To - T is the subcooling). In the data previously reported10, the hydrate-hydrate cohesion force results showed an increasing linear relationship when plotted as a function of logarithm of the inverse subcooling, and subsequently supported the liquid bridge theory10 as a mechanism of cohesive force interactions between hydrate particles. However, the data in Figure 7 suggest there is no evidence for temperature dependence on the cohesive force when the pure cyclopentane hydrate particles were annealed for 3 hours at 1°C to ensure nearly full conversion of the hydrate particles. Subsequently, pull-off trials were performed at temperatures below the ice melting point. If the liquid bridge theory is the dominant mechanism for the cohesive force, the cohesive force should be reduced at temperatures below the ice melting temperature. As shown in Figure 7(A), there are no changes to the cohesive force over a wide range of subcoolings, including for temperature below the ice melting temperature. Similar results are obtained for the system with 1 wt.% NaCl in Figure 7(B).


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10

10

(B) Cohesive Force [mN/m]

(A) Cohesive Force [mN/m]

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0 -2.6

-2.4

-2.2

-2.0

-1.8

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-1.8

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ln [1 / (T0-T)]

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

ln [1 / (T0 -T)]

Figure 7. Hydrate-hydrate cohesive force as a function of temperature/subcooling. (A): temperature dependence with pure cyclopentane hydrates at -5, -3 and -1°C. (Different symbols and colors represent different experimental runs, when checking the reproducibility.) (B): Results for cyclopentane hydrates with 1 wt.% NaCl at 1, 3 and 5°C. Error bounds of each experiment represent 95% confidence with over 60 pull-off trials for each condition.

We infer the results and mechanism for the cohesive forces in terms of the expected liquid bridge formed between hydrate particles with rough surfaces as shown in Figure 8. In the MMF experiments, the amount of hydrates formed depends on the annealing time, and the annealing time is highly related to the unconverted water in the hydrate particles. Based on the data previously reported5, 9-10 showing the cohesive force is temperature dependent, we expect those data correspond to hydrate particles with a relatively large amount of unconverted water, corresponding to the ‘Roughness regime’ shown in Figure 8 (cf. ref14).

However, as all

experiments in this study in Section 3.2.1 were performed after sufficient annealing time to maximize water conversion to hydrates, the results may be in the ‘Asperity regime’ (cf. ref15), where the dominant contribution to the cohesive forces is the roughness of particles instead of


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the liquid bridge theory. As hydrate particles are soft and deformable (elastic) spheres, the cohesive force can be highly depend on the contact time. The surface for elastic materials interpenetrate (mix) across the interface with time resulting in time-dependent (aging) effects.1415

(See the Supporting Information for further details). Therefore, a possible mechanism for the

cohesive force of hydrate particles must be attributed to the amount of surface and unconverted water, temperature, and surface roughness of the particles.

I. Asperity regime

II. Roughness regime

III. Spherical regime

Hydrate particle

Aging

Water

Liquid bridge

Soft Surface

Longer

Annealing Time

Shorter

Lower

Water Fraction

Higher

Figure 8. Schematic of the dominant interaction regimes for the liquid bridge between hydrate particles with rough surfaces14-15

4. CONCLUSION Measurements were performed of the cohesive forces of cyclopentane hydrates in the presence of thermodynamic inhibitors (such as monoethylene glycol, methanol, ethanol and sodium chloride), to understand the effects of THIs on the cohesive forces in the flowlines. All measured cohesive forces with the THIs significantly increased. This result could be a paradox, because THIs are typically injected into flowlines to prevent the hydrate formation and plugging. We


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infer from the results the possible mechanism for the cohesive forces in terms of the expected liquid bridge formed between hydrate particles with rough surfaces. From the morphological data, we expect the presence of THIs causes the hydrate formation rate to decrease, and as such leads to a larger amount of unconverted water compared to particles formed without THIs. The unconverted water is also a source for the liquid capillary bridge between hydrate particles upon contact, and this liquid bridge can significantly affect the cohesive force. Conversely, hydrate particles with sufficient annealing time show no temperature dependence on the hydrate-hydrate cohesive force, suggesting that the liquid bridge theory is not the only mechanism for the cohesive force of hydrate particles. Consequently, the mechanism for the cohesive force between hydrate particles depends on many factors, including the amount of unconverted water, temperature, and the particle roughness.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Note The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was funded by the CSM (Colorado School of Mines) Hydrate Consortium (current and past members: BP, Champion Technologies, Chevron, ConocoPhillips, ENI, ExxonMobil,


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Halliburton, Instituto Mexicano del Petróleo, Multi-Chem, Nalco, Petrobras, Schlumberger, Shell, SPT Group, Statoil, and Total).

SUPPORTING INFORMATION The supporting information document contains further details on the sample preparation, details of the hydrate particle morphology, measured hydrate dissociation temperature of hydrates in the presence of the THIs, and a longer description on the possible mechanism for hydrate cohesion forces. This information is available free of charge via the Internet at http://pubs.acs.org/.

REFERENCES (1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed; Taylor and Francis Group, LLC: Boca Raton, FL, 2007. (2) Sloan, E. D. Fundamental principles and applications of natural gas hydrates. Nature 2003, 426, 353-363. (3) Turner, D. Clathrate hydrateformation in water-in-oil dispersions. Ph.D. Thesis, Chemical Engineering, Colorado School of Mines, Golden, CO, 2006. (4) Turner, D.; Miller, K.; Sloan, E. Methane hydrate formation and an inward growing shell model in water-in-oil dispersions. Chem. Eng. Sci. 2009, 64, 3996-4004.


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(5) Yang, S.-O.; Kleehammer, D. M.; Huo, Z.; Sloan, E. D.; Miller, K. T. Temperature dependence of particle-particle adherence forces in ice and clathrate hydrates. J. Colloid Interface Sci. 2004, 277, 335-441. (6) Dieker, L. E. Cyclopentane hydrate interparticle adhesion force measurements. M.S. Thesis, Chemical Engineering, Colorado School of Mines, Golden, CO, 2009. (7) Aman, Z. M.; Brown, E. P.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Interfacial mechanisms governing cyclopentane clathrate hydrate adhesion/cohesion. Phys. Chem. Chem. Phys. 2011, 13, 19796-19806. (8) Lee, B. R.; Koh, C. A.; Sum, A. K. Development of a high pressure micromechanical force apparatus. Rev. Sci. Instrum. 2014, 85, 095120. (9) Aman, Z. M.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Lowering of clathrate hydrate cohesive forces by surface active carboxylic acids. Energy Fuels 2012, 26, 5102-5108. (10) Aman Z. M.; Joshi, S. E.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Micromechanical cohesion force measurements to determine cyclopentane hydrate interfacial properties. J. Colloid Interface Sci. 2012, 376, 283-288. (11) Taylor, C. J.; Miller, K. T.; Koh, C. A.; Sloan, E. D. Macroscopic investigation of hydrate film growth at the hydrocarbon / water interface. Chem. Eng. Sci. 2007, 62, 6524-6533. (12) Lafond, P. G.; Olcott, K. A.; Sloan, E. D.; Koh, C. A.; Sum, A. K. Measurements of methane hydrate equilibrium in systems inhibited with NaCl and methanol. J. Chem. Thermodynamics 2012, 48, 1-6.


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(13) Cha, M.; Couzis, A.; Lee, J. W. Macroscopic Investigation of Water Volume Effects on Interfacial Dynamic Behaviors between Clathrate Hydrate and Water. Langmuir 2013, 29, 5793-5800. (14) Marshall, J. S.; Li, S. Adhesive Particle Flow, Cambridge University Press, 2014. (15) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed; Academic Press, 2011.


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Mechanism of Cohesive Forces of Cyclopentane Hydrates with and

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