Rheology of Hydrate-Forming Emulsions Stabilized by Surfactant and

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Rheology of hydrate-forming emulsions stabilized by surfactant and hydrophobic silica nanoparticles Amit Ahuja, Anam Iqbal, Mohsin Iqbal, Jae W Lee, and Jeffrey F. Morris Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00795 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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Rheology of hydrate-forming emulsions stabilized by surfactant and hydrophobic silica nanoparticles Amit Ahuja∗1,2 , Anam Iqbal2 , Mohsin Iqbal2 , Jae W. Lee2,3 , Jeffrey F. Morris1,2 1 Benjamin

Levich Institute and 2 Department of Chemical Engineering, City College of City University of New York, New York, NY, 10031, USA

3 Department

of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea

Abstract Observed effects of hydrophobic fumed silica nanoparticles (of average primary particle size 7 nm) on the rheological behavior of hydrate-forming emulsions are presented. Liquid cyclopentane (CP) is the hydrate former. The hydrate slurry is prepared in a Couette geometry at atmospheric pressure from a water-in-oil emulsion with the phases density matched to avoid segregation. Hydrates are formed upon quenching to a low temperature at a fixed shear rate. Dispersed water droplets convert to hydrate particles, leading to an effective viscosity increase by orders of magnitude. The hydrate inhibition by silica nanoparticles at the water-oil interface, forming a Pickering type of emulsion, is characterized using the onset time of steep viscosity rise after seeding with small hydrate particles; this is termed the critical time. Seeding eliminates stochasticity associated with nucleation of the hydrate. The critical time is increased when the interface is covered with silica nanoparticles. For a particle concentration range of 0.05−0.5% (by weight based on total oil mass) at the interface, the hydrate crystallization process is delayed by five hours in comparison to the particle-free case for a 20 vol% water-in-oil emulsion at T = −2◦ C and shear rate of γ˙ = 100 s−1 . The final hydrate slurry viscosity was the same as observed ∗ Corresponding

author presently at: Cresilon Inc., 122 18th Street, Brooklyn, NY 11215, Tel.: +1 848-202-5761; Email: [email protected].

Preprint submitted to Journal of LATEX Templates

ACS Paragon Plus Environment

April 20, 2018

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in the slurry with no particles. At particle concentrations greater than 1 wt%, the viscosity increased abruptly and ultimately jammed the rheometer during hydrate formation. A hypothesis is presented to explain this latter behavior and indicates some of the limitations of this method of inhibition by nanoparticles. A discussion of factors which may complicate application of the method in the field is provided. Keywords: flow assurance, clathrate hydrate, cyclopentane, rheology, Pickering emulsion

1. Introduction Maintaining flow in pipelines, or flow assurance, is a critical topic for the petroleum industry. One of the foremost concerns is flow stoppage due to clathrate hydrates. The formation and deposition of these materials in pipelines can lead to excessive pressure drop, or complete blockage. The materials of concern, i.e. clathrate hydrates, are also called gas hydrates or simply hydrates. These are non-stoichiometric crystals composed of “host” water molecules and “guest” molecules, which may be gases or small-molecule liquids. The cage around a guest molecule is formed by water molecules through hydrogen bonds [1–3]. In oil and gas pipelines containing hydrocarbon gas and/or liquid oil together with an aqueous phase, hydrates form at the water-hydrocarbon interface under the conditions of low temperature and high pressure. Hydrate management has shifted from a focus on use of thermodynamic hydrate inhibitors (e.g., methanol and monoethylene glycol) toward hydrate risk management by use of kinetic hydrate inhibitors (KHIs) and anti-agglomerants (AAs). Common practice in field operation for hydrate prevention involves the use of KHIs and AAs. The KHIs have been used successfully for retarding the hydrate formation in pipelines. The hydrate inhibition performance of KHIs can be assessed by measuring the hydrate onset time. The inhibition mechanism of KHIs involves their adsorption to the surface of a growing hydrate crystal, which modifies the energy of the surface and thus changes its growth kinetics giving

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time to the fluids reach storage facilities without causing blockage. In general, AAs are surface-active species, which tend to make hydrate particles remain well-dispersed without making large agglomerates in the continuous oil phase. In this work, we study the possibility of using hydrophobic silica nanoparticles at a surfactant-stabilized water-oil interface as a method of hydrate inhibition and its limitations. The underlying mechanism for this approach is different from the established methods of using KHIs and AAs; here nanoparticles are employed to physically block the water-cyclopentane interface (thus limiting contact between the hydrate-forming components coming from the two phases): this delays or slows crystallization. Motivation for this work is found in the study by Cha et al. [4], who considered the effect of adding hydrophobic silica nanoparticles into a planar water-CP interface and carried out visualization and micro-differential scanning calorimetry studies to investigate the hydrate crystal growth inhibition performance of these particles. Varying the hydrophobic silica nanoparticle concentration in the oil (CP) phase resulted in different crystal growth at the water-oil interface. Nanoparticles adsorbing at the interface led to a significant retardation in hydrate growth. As the hydrate formation occurs preferentially at the water-oil interface, it is useful to monitor the hydrate crystal growth at the interface [4–7]. These observations provide important insights to hydrate crystal growth inhibition at the water-oil interface. Here, we seek to advance understanding of the consequences at the bulk effect of this interfacial modification, by studying the mechanical properties of particle-laden interfaces in hydrate-forming emulsions under shearing. These complex multiphase mixtures arise in petroleum pipelines, where water-in-crude oil emulsions, stabilized by naturally present surface-active materials – asphaltenes, resins and inorganic and organic solid particles (calcium oxide, iron oxide, sand, minerals and waxes) – flow under hydrate forming conditions. To this end, we have chosen to work with liquid cyclopentane (CP) as the hydrate former; CP forms structure II hydrate at atmospheric conditions [8, 9]. Rheological properties of cyclopentane hydrate slurry have been studied in non-pressurized rheometers [10–17]. Peixinho et al. [10] studied cyclopentane 3


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hydrate-forming water-in-oil emulsions and reported that these hydrate-forming emulsions have characteristic times, defined as critical time and growth time. The critical time is described as the onset of steep viscosity rise after the seeding, while the growth time is the time measured from the critical time to a final steady state. The formation of hydrate particles in emulsions leads to abrupt and large (orders of magnitude) changes in the mechanical properties as it has been observed in many previous studies using commercial rheometers and custom-built flow loop setups [14, 18–29]. A detailed survey of the typical trends of critical and growth times observed in these works is presented in Ahuja et al. [12]. Some of the factors influencing the critical time of hydrate formation are salt concentration in aqueous phase, degree of subcooling, shear rate and mixing. Zylyftari et al. [14] studied the effect of salt concentrations on the thermodynamic and rheological properties of CP hydrate-forming emulsions and found that the critical time decreases with higher subcooling and higher shear rate. The critical time of hydrate formation involves two primary steps: nucleation and growth. During hydrate nucleation, which is experimentally challenging to observe, hydrate nuclei grow and attain a critical size at the water-oil interface. Despite its significance, the hydrate nucleation mechanism is still not well understood and thus is a difficult step to control. On the other hand, hydrate crystal growth after the formation of critical size nuclei at the interface, can be manipulated [4–7]. Karanjkar et al. [5] studied the effects of an oil-soluble surfactant (Span 80) on CP hydrate crystallization at the water-oil interface through visualization and they observed the formation, in the early part of the transition of water to hydrate, of unique hollow-conical crystals followed by a radial growth of needle-like crystals which leads to a porous structure ultimately. Cha et al. [4], and Baek et al. [6] found that the size and shape of the conical crystals can be altered by manipulating the water-CP interface. With the addition of 2 wt% (based on CP mass) silica nanoparticles at the water-oil interface, Cha et al. found that the hydrate growth was retarded and very localized in the vicinity of the added seed under quiescent conditions. Recently, Baek et al. [6] observed a 4


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similar behavior with activated carbon particles of size O(100) µm at water-oil interface using cyclopentane and propane as hydrate guest molecules. Raman et al. [30] investigated the ramifications of CP hydrate formation and dissociation on solid stabilized and surfactant stabilized water-in-oil emulsions. Span 80 and two grades of silica nanoparticles with varying hydrophobicity were chosen as emulsifiers. An important conclusion from their study was that emulsions prepared with solid particles of moderate hydrophobicity prevented destabilization after hydrate formation and dissociation. By contrast, emulsions prepared either by using particles with relatively higher hydrophobicity or by using Span 80 led to destabilization of emulsions after hydrate formation and dissociation. Min et al. [7] studied the roles of hydrophobic silica nanoparticles and Span 20 at the interface in preventing adhesion between the CP hydrate and the aqueous phase by using microbalance measurements along with visualization under a controlled temperature environment. Using different concentrations of silica particles or Span 20 leading to partial or full interfacial coverage, different behaviors including instant adhesion, delayed adhesion, and non-adhesion between CP hydrate and aqueous phase were observed. The underlying conceptual mechanisms for different adhesion behaviors for both silica particles and Span 20 were discussed. Furthermore, the impact of adding water-soluble sodium dodecyl sulfate (SDS) surfactant on the anti-adhesive property of the silica particle system and Span 20 system were studied. Interestingly, the presence of SDS did not influence the anti-adhesive behavior of silica particle system, while the dual surfactant system of SDS and Span 20 led to the formation of emulsion drops which immediately adsorbed on the CP hydrate surface and caused more hydrate growth. The main objective of the present study is to determine the impact on hydrate crystallization rate and crystal morphology of silica nanoparticles at the water-oil interface. This is done primarily through measurement of the bulk rheological behavior of emulsions undergoing hydrate formation in shearing conditions. The work carried out here deals with water-in-oil emulsions stabilized by a non-ionic surfactant, Span 80, solely and in combination with hydrophobic silica nanoparticles. To understand the role of silica nanoparticles at the 5


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interface in the hydrate crystallization process we investigated the evolving rheological behavior of water-in-oil emulsions under hydrate-forming conditions. To this end, we studied the effect on the overall rheology of increasing particle concentration in the emulsions for varying water volume fraction. In particular, we evaluated the hydrate inhibition performance of the silica nanoparticles by extracting the critical time from the measured rheological data. We have also evaluated the performance of these particles at different levels of subcoolings. Dynamic oscillatory measurements have been performed to further examine the influence of particles at the interface in the hydrate formation process.

2. Materials and Methods 2.1. Materials Deionized water is used as the dispersed phase of emulsions in this work and it is purified by a Millipore QTM water system. The continuous oil phase is prepared by mixing 50 vol% cyclopentane, 31.2 vol% light mineral oil (Fisher Chemicals) and 18.7 vol% Halocarbon 27 (polychlorotrifluoroethylene polymer, Halocarbon Products Corporation). Halocarbon 27 is used here to adjust the density of continuous oil mixture and aqueous dispersed phase in order to minimize the impact of sedimentation. The density and viscosity of Halocarbon 27 oil are 1.9 gm/cm3 and about 100 cP, respectively, while the same physical properties for the mineral oil are 0.86 gm/cm3 and 46 cP, respectively, at 25◦ C. An oil soluble and nonionic surfactant, sorbitan monooleate, commonly known as Span 80 (Sigma Aldrich) with density 0.986 gm/cm3 and viscosity 1600 ± 400 cP at 20◦ C is used to stabilize the water-in-oil emulsions, at a loading of 0.1 vol% of the total organic phase. The solid particles utilized are hydrophobic fumed silica nanoparticles with average particle size of 7 nm (Aerosil R812, Evonik Industries). To measure contact angle, the traditional methods such as sessile drop on a compressed pellet made of particles present difficulties as wetting properties are affected by the local surface roughness due to the agglomerated and irregular particle structures. To overcome this difficulty, Forney et

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al. [31] developed a new method to measure contact angles. The measured contact angle of pure water with silica particles using this indirect method was 118 degrees. The contact angles measured by the sessile drop method on compressed fumed silica pellet can be much higher (close to 180 degrees). Petroleum fluids are generally complex mixtures of many hydrocarbon compounds among which surface-active agents are often present. The formation of emulsions in the presence of surface-active materials in pipelines becomes likely when sufficient surface-active agents are present. Here, we focus on an emulsion instead of segregated phases, as the segregated system is not appropriate for rheological measurements. This complex model mixture is carefully designed not only to achieve a density-matched mixture, but also to mimic a realistic oil-field petroleum emulsion. The temperature dependence of the viscosity of each liquid can be found in Peixinho et al. and Zylyftari et al. [10, 14]. The composition of continuous oil phase with 0.1 vol% (based on total oil volume) of Span 80 and known amount of particles is formulated first. The concentration of Span 80 is fixed here for all the formulations. The emulsions of varying compositions are formulated by mixing aqueous phase into the continuous oil phase, using a high shear homogenizer (IKA T25 digital Ultra-Turrax) operating at a speed of 7000 rpm for a duration of 5 minutes; a 30 ml batch-size is prepared in a standard 100 ml glass beaker. Emulsions are observed to be stable against phase separation or coalescence for 24 hours. To estimate the droplet size, 400 droplets were measured from photomicrographs. Table 1 shows mean drop size for different conditions. Similar drop size distribution has been observed in other studies using similar systems of water, oil, silica particles, and Span 80 [32, 35]. 2.2. Rheometry and Methods The rheological measurements were performed on a stress-controlled rheometer (TA Instruments AR 2000ex) using a concentric cylindrical cup and a cylindrical rotor. The exterior stationary cup has a 15 mm radius, the inner rotor has a 14 mm radius, and thus this Couette geometry has a 1 mm gap. The 7


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Table 1: Emulsion droplet size.

Water vol.

Surfactant

Particle

Drop size (µm)

Drop size (µm)

fraction (vol%)

conc. (vol%)

conc. (wt%)

t=0

t = 24 hours

20

0.1

0.0

8.5 ± 3.7

9.1 ± 4.6

20

0.1

0.2

9.6 ± 4.2

11.7 ± 4.8

outer cup is equipped into a Peltier jacket for temperature control, which is attached to the rheometer. The emulsion formulated at ambient temperature is steadily transferred to the cylindrical cup which is previously equilibrated to a temperature of −2◦ C. To avoid long and erratic induction times associated with hydrate nucleation, the emulsion is seeded with separately prepared cyclopentane hydrate crystals (5 seeds of mm scale gently placed on the upper surface) as soon as the shearing test is started at t = 0. The test volume used in all rheology experiments is 19.6 ml. To measure the viscosity of continuous oil phase with particles, a cone−plate set up with cone diameter of 40 mm and a cone angle of 2◦ is exploited. The lower plate of this set up is also a Peltier system for temperature control. The oil mixture with varying concentrations of silica particle are examined at a fixed temperature of −2◦ C.

3. Results 3.1. Effect of particle concentration In Figure 1, measured rheological properties of the continuous oil phase (mixture of light mineral oil, Halocarbon 27, cyclopentane and Span 80) with varying particle concentrations are presented. For particle concentrations ranging from 0−0.75 wt% (based on total oil mass), the oil dispersions show a Newtonian behavior with a constant viscosity for the shear stress range of 0.05−5 Pa. For 1 wt% and higher particle concentrations, the oil dispersions are found to develop a small yield stress and a shear-thinning behavior. A similar behavior was observed in Drelich et al. [33] where water-in-paraffin oil emulsions were studied using Span 80 and hydrophobic silica particles (Aerosil R711, particle 8


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100

0.0% 0.05% 0.10% 0.75% 1.0% 1.5% 2.5%

10

viscosity (Pa.s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

0.1

0.01 5 6 7

0.1

2

3

4

5 6 7

1 shear stress (Pa)

2

3

4

Figure 1: Viscosity-shear stress curves for continuous oil mixture for particle concentration ranging from 0−2.5 wt% (shown in legend) obtained at −2◦ C.

size = 12 nm). The continuous paraffin oil dispersions with particles exhibited a small yield stress. This was attributed to the formation of a network of silica particles. Figure 2 shows the viscosity as a function of time for 20 vol% waterin-oil emulsions with 0.1 vol% Span 80 and different particle concentrations at T = 15◦ C (above the hydrate-dissociation temperature). For no particles and 0.1 wt% particle concentration, the viscosities increase over 100 minutes which is presumably due to rising/settling of droplets in the Couette geometry. On the other hand, with 2.5 wt% particles, a more dramatic increase in the viscosity is observed. The viscosity increases from an initial value of 0.02 Pa.s to 0.07 Pa.s. We postulate that this increase is due to adsorption of surfactant from the bulk oil phase and water-oil interface to the solid particles, which ultimately leads to destabilization of the emulsion. The drops undergo coalescence and can be seen as large globules in figure 3(b). Figure 3 shows images of the emulsions taken i) immediately after preparation at t = 0 and ii) after 100 minutes of shearing at γ˙ = 100 s−1 and T = 15◦ C. The latter case shows large drops due to some destabilization and coalescence of drops. A detrimental consequence of this behavior during hydrate formation is discussed below in association with figures 4 and 5.

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0.0% 0.1% 2.5%

4 2

0.1

viscosity (Pa.s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8 6 4 2

0.01

8 6 4 2

0.001 0

20

40 60 time (min)

80

100

Figure 2: Viscosity versus time for 20 vol% water-in-oil emulsions with 0.1 vol% Span 80 and with different particle concentrations in wt% (shown in legend) at γ˙ = 100 s−1 and T = 15◦ C under no hydrate-forming conditions.

(a)

(b)

Figure 3: Images of vials containing 20 vol% water-in-oil emulsions with 0.1 vol% Span 80 and with 2.5 wt% particles (a) after preparation at t = 0 and (b) emulsion collected after 100 minutes of shearing at T = 15◦ C and γ˙ = 100 s−1 .

10


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Figure 4 shows the typical viscosity plots obtained for 20 vol% water-inoil emulsions with varying particle concentrations in the continuous oil phase undergoing hydrate formation. This is done at a temperature of −2◦ C and a shear rate of 100 s−1 . The critical time for the particle-free system is approximately 40 minutes. With 0.01 wt% particle concentration, the critical time is approximately 70 minutes, while for 0.1 and 0.5 wt% particle concentrations, the critical times are 370 and 350 minutes respectively. For these particle-laden interfaces, there is a significant delay in the mechanical transition, presumably due to a delay in the hydrate crystallization process. This is in agreement with the observations of Cha et al. [4], where with 1 wt% (based on CP mass) silica nanoparticles at the interface, the water to hydrate conversion time was 120 minutes, while with 2 wt% particles, only a small hydrate growth takes place in the vicinity of the added seed in the first 180 minutes (reaction stops thereafter and no further hydrate growth was observed in next 180 minutes), in comparison to a conversion time of 40 minutes with a particle-free interface. These observations are further supported by the recent work of Baek et al. [6], where the hydrate inhibition performance of the activated carbon particles at the interface was quantified by measuring the dissociation enthalpies of cyclopentane + propane gas mixed hydrates using high-pressure micro differential scanning calorimetry. With 1 wt% particles (based on oil weight) at the interface, approximately two orders of magnitude decrease in the dissociation enthalpies was measured in comparison to no particle case. This delay appears to be due to the particle coverage of the water-oil interface which prevents contact between the hydrate forming components (water and cyclopentane). With small particle concentrations (0.01 wt%), the interface may not be fully covered with the particles and thus there is not a significant delay. At higher particle concentrations (0.1 wt% and 0.5 wt%) at the interface, the hydrate crystallization process is delayed by almost 5 hours in comparison to the particle-free case. On increasing the particle concentration further, to 2 wt%, we observe a different behavior, in which the rheometer Couette gap jams as stresses increase above the instrument limit of 3000 Pa for this fixture. A 11


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plausible explanation for this behavior is that a significant amount of surfactant available in bulk oil phase and at water-oil interface adsorbs on the silica nanoparticles due to their high specific area and the surfactant concentration in the bulk phase and water-oil interface may decrease. This leads to water droplet coalescence and ultimately destabilization of the emulsion as we observed in figures 2 and 3. This can also lead to rising/settling of water droplets due to slight density mismatch and thus local increase of the internal phase volume fraction. Under this scenario, the hydrate growth would be very abrupt as we observed in figures 4 and 5. Karanjkar [34] showed that Span 80 loading alters the rheological behavior of our hydrate-forming emulsion. A higher surfactant loading leads to a lower slurry viscosity and improves the flowability, thus acting as an anti-agglomerant. A similar line of reasoning was presented in the recent work of Nesterenko et al. [35], where water-in-oil emulsions, consisting of water and paraffin oil and stabilized by using a dual emulsifier system including hydrophobic silica particles and non-ionic surfactant, were studied. It was found that in the presence of particles, due to the adsorption of Span 80 on the particles, more surfactant was needed to reduce interfacial tension to the level obtained in the particle-free emulsion. As a result of this adsorption, the apparent CMC value in the presence of particles was found to be considerably higher than the particle-free case. Similar behavior of plugging at higher particle loadings is observed for other water volume fractions as well (even for as low as 10 vol% water-in-oil emulsion) as shown in figure 5.

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plugged

0.0% 0.01% 0.1% 0.5% 2.0%

viscosity (Pa.s)

10

1

growth time

0.1

final viscosity

critical time

0.01 0

100

200 300 time (min)

400

500

Figure 4: Evolving viscosity profiles for 20 vol% water-in-oil emulsion with particle concentration in the oil phase ranging from 0−2.0 wt% (shown in legend) undergoing hydrate formation at T = −2◦ C and γ˙ = 100 s−1 . Dashed black lines are used for schematic depiction of critical time, growth time and final viscosity.

100

10% 20% 25%

10

viscosity (Pa.s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

0.1

0.01 0

20

40 60 time (min)

80

100

Figure 5: Evolving viscosity profiles during hydrate formation for varying water volume fractions (shown in legend) with a fixed particle concentration of 2.5 wt% in the oil phase at T = −2◦ C and γ˙ = 100 s−1 .

13


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3.2. Effect of water volume fraction Figure 6 shows a comparison of critical times obtained for varying water volume fractions for particle-free and 0.1 wt% (based on total oil mass) particle concentration. The Span 80 concentration is 0.1 vol% (based on oil volume) in both the cases. Only one particle concentration is chosen to more clearly show the comparison with particle-free system and the trend of delays in critical times. Evidently, for both the systems, the critical time increases for decreasing water volume fraction. This observation agrees with previous work [12, 34]. Higher water fraction presumably leads to faster hydrate growth because of enhanced droplet interactions and thus a shorter critical time; a droplet with hydrate crystals at the interface would act as a nucleating agent for neighboring droplets. Particle loadings at the interface cause a consistent delay in critical times for all the water fractions studied here. Table 2 shows the data on critical time obtained for water volume fractions ranging from 10−30 vol% and varying particle loadings. An important point is that the minimum amount of silica particles required to cover all the droplets (or interfacial area) is different for different water volume fractions. For example, for 20 vol% water-in-oil emulsion, 0.05 wt% particles effectively delay the hydrate crystallization presumably due to complete interfacial coverage by these silica particles, while for 30 vol% waterin-oil emulsion, where the droplet number density is higher, 0.05 wt% may not cover the interfacial area completely. This may explain the shorter critical time at 30 vol% water (see table 2). 3.3. Effect of subcooling In figure 7, we show the effect of subcooling on the critical time of hydrate formation for 20 vol% water fraction with no particles and in the presence of 0.1 wt% (based on total oil mass) particles in the oil phase. All data are taken at a fixed shear rate of 100 s−1 and a fixed concentration of 0.1 vol% (based on total oil volume) Span 80, with or without particles present. Subcooling is defined as the difference between the equilibrium temperature for hydrate dissociation and the set temperature for the experiment. The equilibrium temperature measured 14


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Table 2: Effect of particle concentrations on the critical time for emulsions with varying water volume fractions.

Water volume

Particle

Critical

fraction

concentration

time (tc )

(vol%)

(wt%)

(minutes)

10

15

20

25

30

∗

Comments

0.0

240

Base case (for comparison)

0.1

490

Effective - delays tc

1.0

100

Not effective - beginning of gelation∗

2.5

50

Not effective - leads to jamming

0.0

140


0.2

290


1.0

40


0.0

40


0.01

70

Not effective - partial interface coverage

0.05

250


0.1

370


0.2

250


0.5

350


2.0

30


0.0

20


0.1

132


0.2

142


0.75

70

Not effective - beginning of gelation∗

2.0

48


2.5

20


0.0

35


0.05

45

Not effective - partial interface coverage

0.2

125


For this case, the viscosity is higher than the no particle case but does not lead to

plugging. This is observed for the particle concentration range between 0.75−1.0 wt%, which seems to be borderline particle concentrations above which jamming occurs.

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600

0.0% 0.1%

500

critical time (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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400 300 200 100 0 0.10

0.15 0.20 0.25 water volume fraction

0.30

Figure 6: Comparison of critical times for varying water volume fractions for particle-free and 0.1 wt% particle-laden systems (shown in legend).

using calorimetry was found to be 5.4◦ C for this system with 50 vol% cyclopentane in the organic phase [14]. The critical time for both the systems decreases exponentially with increasing degree of subcooling. This trend agrees with Zylyftari et al. [14]. At a subcooling of 4.4◦ C, the hydrate crystallization process is delayed by approximately 7 hours in comparison to the particle-free case, while at a subcooling of 10.4◦ C, the critical time is delayed by approximately 90 minutes. 3.4. Oscillatory rheology Figure 8 shows evolution of storage modulus (G0 ) as hydrate formation occurs for 20 vol% water-in-oil emulsions with 0.1 vol% Span 80 and with varying particle concentrations. The applied strain is 10% and the frequency is fixed at 1 Hz. The storage modulus increases by orders of magnitude as a result of hydrate formation similar to the rapid viscosity increase observed in the shear rheology experiments. As the water-to-hydrate conversion approaches completion, G0 attains a statistically constant value. The trends for critical times for particle concentrations of 0 and 0.1 wt% are also similar to those observed in shear rheology experiments. The critical time for the no-particle case is about 50 minutes, while with 0.1 wt% particles in the sample, the critical time is ap16


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700 600

critical time (minute)

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500 -B.x

y = A.e A =1420 B = 0.21

400 300

-B.x

y = A.e A = 639 B = 0.38

200 100 0 4

5

6

7 8 9 subcooling (°C)

10

11

Figure 7: Critical time of viscosity evolution plotted as a function of the subcooling for 20 vol% water-in-oil emulsion with no particles (blue circles) and 0.1 wt% particles (black markers) at γ˙ = 100 s−1 . The lines show exponential fits to the data with the values of coefficients A and B in the box.

proximately 300 minutes and final average G0 is O(10) Pa for both the cases. The critical time for 1 wt% particle concentration is about 200 minutes with the final G0 of O(103 ) Pa indicative of gelation. The evolution of G0 with 2.5 wt% particles exhibits a similar trend as the viscosity in shear experiments. The initial G0 is O(1) Pa and the final average G0 is O(104 ) Pa. Both the values are approximately an order of magnitude higher than that at the other particle concentrations. The critical time is approximately 80 minutes and again this behavior is attributed to the destabilization of emulsion due to migration of surfactant from the bulk and the interface to the solid silica particles.

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10 10 10

G' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10 10 10 10 10

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5

0.0% 0.1% 1.0% 2.5%

4

3

2

1

0

-1

-2

0

100

200 time (min)

300

400

Figure 8: Evolution of G0 for 20 vol% water-in-oil emulsions undergoing hydrate formation with varying particle concentrations in wt% (shown in legend) at T = −2◦ C, strain = 10%, and frequency = 1 Hz.

4. Discussion We have explored hydrate inhibition by addition of nanoparticles, establishing the concept as one having some merit for possible application. However, we have only examined limited idealized conditions, and ultimate utility will require a number of considerations. We discuss here the work, considering both positive features and limitations. The use of cyclopentane as hydrate former yields sII, and for liquid-dominated flows an emulsion of organic phase containing cyclopentane (CP) thus provides a good density-matching model (equal density organic and aqueous phases) of the oil-field emulsion. In particular, mass transfer to the interface between the aqueous and organic phases is required. As a model system, CP offers advantages over THF as it is immiscible with water as opposed to THF which is fully miscible with water. The immiscibility introduces the mass transfer limitations usually present in gas hydrate emulsion systems, making CP a suitable model material for study of properties of oil-field emulsions. While features of the overall process of pipeline transport are not captured by study of this model system, it is a quality material model. The stabilization of this interface in a crude oil emulsion would involve as-

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phaltenic or other interfacially active components. An observed influence of surfactant is to lead to a marked change in the morphology of the hydrate formed: a very porous needle-like structure results with surfactant present, with the key point being that this generates a much larger effective internal phase fraction, φeff , [5]. The porosity and its effect on φeff have a pronounced influence on bulk rheological properties. The structure observed when hydrate-forming gas, probed using propane as hydrate former through propane-saturation of the oil phase, is similarly porous [13, 16]. Outstanding questions are: What can be done to interrupt the small-scale structure development, and thus cut off the large viscosity and yield stress development? Cyclopentane hydrate is a difficult material to nucleate. This leads to difficulties in reliable dynamics in experimental analysis of emulsions which undergo hydrate formation in rheological tests. To alleviate the uncertainty, we generally seed the emulsion, using externally grown hydrate crystals of mm scale; this sets a reliable zero time, and reproducibility of dynamics of the hydrate growth leading to rheological transition. The seeding is seen to yield reproducible growth and rheological response, even at modestly different internal phase fractions (see Peixinho et al. [10] and Ahuja et al. [12] for the impact of seeding on the critical time). In general, there is a lack of understanding in the literature on the role of solid particles (sand, asphaltenes, minerals and waxes) on hydrate formation. Use of silica nanoparticles at the interface to prevent hydrate formation was tested under a certain set of experimental conditions, and several practical limitations of this approach must be noted and considered. Examples (by no means an exhaustive list) are that: the hydrate formers are usually smaller molecules like methane and ethane and could potentially penetrate an interfacial layer more readily than cyclopentane; often only some fraction of the water is emulsified in the field, and the fluids are often under turbulent multiphase conditions which could disturb the interface and this may impact the inhibition by nanoparticles; and finally there is understandable reluctance to add large amounts of nanoparticles which would have to be separated in the end of the 19


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production. All these practical concerns deserve detailed investigation, but are outside the scope of this paper. It is likely that the water droplets are covered with multiple layers of aggregated, 3D networked nanoparticles. The approach described here only works for a certain concentration of nanoparticles (presumably maintaining a balance of interface coverage and surfactant-nanoparticle ratio) and this particle concentration may not work effectively for smaller hydrate-forming molecules like methane and ethane. However, the general concept of using nanoparticles as a hydrate inhibitor has been generating interest in the community. For example, a recent study by Wang et al. [36] answers some questions raised here; the authors employed hydrophilic silica nanoparticles (20 nm and 50 nm grain sizes) as hydrate inhibitors for methane gas hydrates in an autoclave with agitation of the material. The key result of this study was that a concentration of 4 wt% silica nanoparticles with a size of 50 nm has a strong inhibition effect of hydrate formation under the dynamic mixing conditions. The induction time for hydrate formation was increased by almost 200% in the presence of silica nanoparticles in comparison to pure water. However, the working mechanism of hydrophilic silica nanoparticles in this non-emulsified, water-gas system may be quite different from ours. A key observation of our work was the use of a small quantity (0.05 – 0.5 wt%) of nanoparticles for it to be effective unlike relatively higher amount of 4 wt% of nanoparticles in the work of Wang et al. [36]. The primary mechanism by which particles stabilize emulsions involves particle attachment to the drop surfaces and significant energy is required to displace attached particles from the interface. However, turbulent flows commonly encountered in the petroleum production may disrupt nanoparticle-decorated interfaces, making them vulnerable to hydrate formation. The turbulence shear stresses, and in particular their relation to the capillary pressure in the drops, are key issues to consider in assessing this point. The addition of various materials – surfactants, solid nanoparticles, salts and polymers – is known to influence hydrate formation. The degree of impact of 20


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these additives on the hydrate formation can vary significantly. In fact, a particular additive can reverse its role from an inhibitor to a promoter with changing experimental conditions or formulations. The benefits of utilizing nanoparticles for flow assurance purposes come with its own additional risks. The addition of solid nanoparticles can aid rapid hydrate formation with shorter induction times, as these solid particles could act as seeds for hydrate nucleation. A deeper analysis of the phenomena controlling hydrate effects on rheology and its impact on flow assurance demands consideration of the detailed interfacial structure, including molecular scale analysis both by experiment and simulation, as well as interfacial rheology. As such, the interfacial environment plays a controlling role and should be a focus of further examination for the purposes of understanding the risk factors associated with a given emulsion (interfacially active agent and its effect on hydrate growth and morphology) and possible interventions.

5. Conclusion The effects of hydrophobic silica nanoparticles on the rheological behavior of cyclopentane hydrate-forming emulsions has been examined. It is observed that in the particle concentration range of roughly 0.05−0.5 wt%, the hydrate crystallization event is delayed to a large extent for water fractions ranging from 10 to 25 vol%. At lower particle concentrations (below 0.05 wt%), the interface coverage is insufficient and thus the presence of particles do not affect the hydrate crystallization adequately, while at high particle concentrations (above 1 wt%), the surfactant molecules present in the system migrate to the silica particles due to the availability of high specific area and the surfactant concentration falls below the CMC value. This scenario essentially leads to destabilization of the emulsion and eventually progresses to jamming of the rheometer during hydrate formation. A comparison of critical times for particle-free and particleladen systems shows that at lower subcoolings (temperatures closer to the hydrate dissociation temperature) the critical times can be delayed significantly when particles are at the interface.

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Acknowledgements We acknowledge support from Chevron. This research was also made possible in part by a grant from BP/GoMRI.

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[9] Nakajima, M.; Ohmura, R.; Mori, Y. H. Clathrate hydrate formation from cyclopentane-in-water emulsions. Ind. Eng. Chem. Res. 2008, 47, 8933– 8939. [10] Peixinho, J.; Karanjkar, P. U.; Lee, J. W.; Morris, J. F. Rheology of hydrate forming emulsions. Langmuir 2010, 26, 11699–11704. [11] Ahuja, A.; Zylyftari, G.; Morris, J. F. Calorimetric and rheological studies on cyclopentane hydrate-forming water-in-kerosene emulsions. J. Chem. Eng. Data 2015, 60 (2), 362–368. [12] Ahuja, A.; Zylyftari, G.; Morris, J. F. Yield stress measurements of cyclopentane hydrate slurry. Journal of non-Newtonian Fluid Mechanics 2015, 220, 116–125. [13] Ahuja, A. Hydrate forming emulsion: rheology and morphology analysis for flow assurance. PhD Thesis, The City University of New York 2015. [14] Zylyftari, G.; Lee, J. W.; Morris, J. F. Salt effects on thermodynamic and rheological properties of hydrate forming emulsions. Chem. Eng. Sci. 2013, 95, 148–160. [15] Zylyftari, G.; Ahuja, A.; Morris, J. F. Nucleation of cyclopentane hydrate by ice studied by morphology and rheology. Chem. Eng. Sci. 2014, 116, 497–507. [16] Zylyftari, G.; Ahuja, A.; Morris, J. F. Modeling oilfield emulsions: comparison of cyclopentane hydrate and ice. Energy Fuels, 2015, 29 (10), 6286– 6295. [17] Karanjkar, P. U.; Ahuja, A.; Zylyftari, G.; Lee, J. W.; Morris, J. F. Rheology of cyclopentane hydrate slurry in a model oil-continuous emulsion. Rheol. Acta, 2016, 55, 235–243. [18] Rensing, P. J.; Liberatore, M. W.; Sum, A. K.; Koh, C. A.; Sloan, E. D. Viscosity and yield stress of ice slurries formed in water-in-oil emulsions. Journal of non-Newtonian Fluid Mechanics 2011, 166, 859–866. 23


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[19] Webb, E. B.; Rensing, P. J.; Koh, C. A.; Sloan, E. D.; Sum, A. K.; Liberatore, M. W. High-pressure rheology of hydrate slurries formed from waterin-oil emulsions. Energy and Fuels 2012, 26, 3504–3509. [20] Sch¨ uller, R. B.; Tande, M.; Kvandal, H. K. Rheological hydrate detection and characterization. Annual Transactions of the Nordic Rheology Society 2005, 13, 83–90. [21] Fidel-Dufour, A.; Gruy, F.; Herri, J. M. Rheology of methane hydrate slurries during their crystallization in a water in dodecane emulsion under flow. Chem. Eng. Sci. 2006, 61, 505–515. [22] Delahaye, A.; Marinhas, S.; Martinez, M. C. Rheological study of CO2 hydrate slurry in a dynamic loop applied to secondary refrigeration. Chem. Eng. Sci. 2008, 63, 3551–3559. [23] Joshi, S. V.; Grasso, G. A.; Lafond, P. G.; Rao, I.; Webb, E.; Zerpa, L. E.; Sloan, E. D.; Koh, C. A.; Sum, A. K. Experimental flowloop investigations of gas hydrate formation in high water cut systems. Chem. Eng. Sci. 2013, 97, 198–209. [24] Peng, B.; Chen, J.; Sun, C.; Dandekar, A.; Guo, S.; Liu, B.; Mu, L.; Yang, L; Li, W.; Chen, G. Flow characteristics and morphology of hydrate slurry formed from (natural gas + diesel oil/condensate oil + water) system containing anti-agglomerant. Chem. Engg. Sci. 2012, 84, 333–344. [25] Andersson, V.; Gudmundsson, J. S. Flow properties of hydrate-in-water slurries. Annals of New York Academy of Sciences 2000, 912, 322–329. [26] Camargo, R.; Palermo, T. Rheological properties of hydrate suspensions in an asphaltenic crude oil. Proceedings of the 4th International Conference on Gas Hydrates (ICGH) 2002. [27] Webb, E. B.; Koh, C. A.; Liberatore, M. W. Rheological properties of methane hydrate slurries formed from AOT + Water + Oil Microemulsions. Langmuir 2013, 29, 10997–11004. 24


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! ! Oil!phase!+!surfactant!

!!Water!phase!

Oil!phase!+!surfactant!+!nanoparticles!

!!Water!phase!

!

Figure 9: Table of Contents Graphic.

! !

!

! !

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Rheology of Hydrate-Forming Emulsions Stabilized by Surfactant and

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