Influence of Model Oil with Surfactants and Amphiphilic Polymers on

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Energy Fuels 2010, 24, 5441–5445 Published on Web 09/13/2010

: DOI:10.1021/ef100762r

Influence of Model Oil with Surfactants and Amphiphilic Polymers on Cyclopentane Hydrate Adhesion Forces Zachary M. Aman,† Laura E. Dieker,† Guro Aspenes,‡ Amadeu K. Sum,† E. Dendy Sloan,† and Carolyn A. Koh*,† †

Center for Hydrate Research, Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401, and ‡ Department of Chemistry, University of Bergen, All egaten 41, 5007 Bergen, Norway Received June 17, 2010. Revised Manuscript Received August 25, 2010

Adhesion forces between cyclopentane hydrate particles were measured at atmospheric pressure and 3.2 °C using an improved micromechanical force apparatus. Because of the complexity of crude oil systems, a series of model oils was prepared by adding surface-active components to 200 cP mineral oil as analogues to crude oil systems. The addition of 1 wt % sorbitan monooleate (Span80, a commercial anti-agglomerant), 1 wt % polypropylene glycol (an amphiphilic polymer), and 0.6 wt % commercial naphthenic acid mixture, separately, to a mineral oil and cyclopentane continuous phase, reduced the average interparticle hydrate adhesion force by 37, 65, and 80%, respectively, compared to pure mineral oil and cyclopentane. The 95% confidence bounds of the Span80 and mineral oil data points overlap; therefore, we cannot conclude that Span80 was effective at reducing the adhesion force between hydrate particles. These results indicate that model amphiphilic polymers and commercial naphthenic acid mixtures may be surface-active on the hydrate particle, drastically reducing the agglomeration tendency between hydrate particles; naphthenic acids are found to be the most effective at lowering the adhesive force between particles. The structure of the additive plays a role in determining the extent of surface activity and effectiveness. Compounds with small hydrophilic groups can more efficiently adsorb to the hydrate surface, while additives that induce morphological changes to the hydrate surface may cause non-uniform growth and are more effective in preventing hydrate agglomeration.

adhesion forces.4-7 In this model, a liquid bridge acts to hold two smooth, spherical particles together; the acting force is a product of the Laplace pressure acting between the particles. The adhesion force between hydrate particles has been measured using an improved micromechanical device.4 In the development of this apparatus, several hydrate formers were tested, including tetrahydrofuran (THF), ethylene oxide (EtO), and cyclopentane (CyC5). Dieker et al.9 concluded that CyC5 hydrates are preferable to the cyclic ether compounds for study in the MMF apparatus. The dissociation point for CyC5 hydrate (7.7 °C) is well above the ice point, ensuring that no ice is present during the course of the experiment,9 when the experiment is performed between 0 and 7.7 °C. Furthermore, CyC5 is immiscible in water and forms structure II (sII) hydrate, similar to natural gas hydrate formers.10 Dieker et al.9 examined the effect of crude oils on the interparticle adhesion force of CyC5 hydrate. In all test cases, adhesion forces decreased with an increasing oil concentration in the continuous phase. Those results suggested that the naturally occurring additives in crude oil decrease interparticle adhesion force and reduce hydrate agglomeration tendency, which is a key step toward forming hydrate blockages.9 While studies on crude oil have shown qualitative trends, the

Introduction Natural gas hydrates are ice-like compounds where guest molecules, such as methane or ethane, are enclathrated in a hydrogen-bonded network of water cages.1 These compounds typically form under high-pressure and low-temperature conditions.2 Turner et al.3 in collaboration with Abrahamson proposed a mechanism for hydrate formation in oil-dominated systems, where water droplets are entrained in the oil phase. Changes in pipeline pressure and temperature, as well as readily available guest molecules in the continuous phase, provide a driving force for hydrate formation.1 Hydrate particles then begin to agglomerate, which may eventually lead to a hydrate plug in the pipeline. Understanding the key factors that may control this agglomeration process is paramount to the prediction and prevention of hydrate plugs in oil-dominated systems. Multiple studies in our laboratory have concluded that the capillary bridge theory is the most appropriate model to describe observed hydrate interparticle *To whom correspondence should be addressed. Telephone: (303) 273-3237. Fax: (303) 273-3730. E-mail: [email protected]. (1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press (Taylor and Francis Group): Boca Raton, FL, 2008. (2) Koh, C. A.; Westacott, R. E.; Zhang, W.; Hirachand, K.; Creek, J. L.; Soper, A. K. Fluid Phase Equilib. 2002, 194, 143–151. (3) Turner, D. J.; Miller, K. T.; Sloan, E. D. Chem. Eng. Sci. 2009, 64, 3996–4004. (4) Yang, S. O.; Kleehammer, D. M.; Huo, Z.; Sloan, E. D., Jr.; Miller, K. T. J. Colloid Interface Sci. 2004, 277, 335–341. (5) Dieker, L. E. M.S. Thesis, Colorado School of Mines, Golden, CO, 2009. (6) Taylor, C. J.; Dieker, L. E.; Miller, K. T.; Koh, C. A.; Sloan, E. D. J. Colloid Interface Sci. 2007, 306, 255–261. r 2010 American Chemical Society

(7) Taylor, C. J.; Dieker, L. E.; Miller, K. T.; Koh, C. A.; Sloan, E. D. Proceedings of the 6th International Conference on Gas Hydrates; Vancouver, British Columbia, Canada, 2008. (8) Huo, Z.; Freer, E.; Lamar, M; Sannigrahi, B.; Knauss, D. M.; Sloan, E. D. Chem. Eng. Sci. 2001, 56 (17), 4979–4991. (9) Dieker, L. E.; Aman, Z. M.; George, N. C.; Sum, A. K.; Sloan, E. D.; Koh, C. A. Energy Fuels 2009, 23, 5966–5971. (10) Sloan, E. D., Jr. Energy Fuels 1998, 12, 191–196.

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Figure 1. Chemical structures for the monomer unit of PPG (left), Span80 (middle), and a generic carboxylic acid (with a variable R group) that is commonly found in naphthenic acids (right).19,20

natural complexity of crude oil systems makes it difficult to fully characterize the effect of oil modifications, such as pH 14 extraction and deasphalting.9 To address this question, and to examine the product of isolated components on hydrate interparticle adhesion forces, model oil systems were investigated. Simple model oil systems by Greaves11 used CyC5 as both a hydrate former and a continuous phase. Aspenes et al.12 used petroleum ether as a model oil for studying the isolated effect of naphthenic acids on the wettability of model pipeline surfaces (petroleum ether is comprised of C-5-C-7 variants). A complex model oil system was developed by Vignati et al.,13 composed of normal and iso-paraffin carbon chains between C-21 and C-55; Vignati concluded that the addition of isoparaffins to the system was essential to model the behavior of a crude oil. Previous studies have also successfully used mineral oil, which typically has a paraffin and cyclic paraffin distribution of approximately C-20-C-40, for emulsion stability experiments.11,14 Additionally, white mineral oil has been successfully used for modeling phase inversion in a two-phase flowline.15 Several model surfactants have previously been investigated with regard to hydrate plugging. Taylor et al.6,7 hypothesized that model anti-agglomerant Span20, a sorbitan acid ester, reduced hydrate interparticle adhesion force by adsorbing onto the hydrate surface, ultimately creating shortrange steric repulsion between the particles. Additionally, Span20 induced finger-like morphological changes on the hydrate surface. That observation suggested that hydrophilic, molecular head groups will adhere to the hydrate surface, possibly extending the hydrophobic tails into the organic phase. Accordingly, species with sufficiently large hydrophobic tails may be more effective at reducing adhesion force between hydrate particles. Sorbitan acid esters have been shown to act as anti-agglomerants,6-8 suggesting surfaceactive molecular behavior. Aspenes et al.12 concluded that natural acids effectively alter the wettability of model pipeline surfaces, ultimately making the pipeline wall more oil-wet and reducing the tendency for hydrate deposition. Further, Erstad et al.16 have suggested that natural acids may affect the wettability of the hydrate surface, instead of solely decreasing the water-oil interfacial tension. Polypropylene glycol (PPG) has been shown to increase the stability of water-in-oil emulsions.17,18 In this laboratory,

Figure 2. (A) Hydrate particles are brought into contact at a known preload force. (B) Micromanipulator then moves the right-hand particle down at a constant velocity. (C) Final displacement is used to calculate the adhesion force between the two particles.5

emulsion stability experiments have demonstrated that PPG enhances the stability of simple water-in-oil emulsions, with mineral oil as the continuous phase. As an amphiphilic polymer, the chemical structure for PPG suggests that it will adsorb to the hydrate-oil interface (Figure 1). In the work presented in this paper, mineral oil was selected as the model fluid to minimize the complexity of the model oil system, while maintaining a broad carbon distribution. Three initial additives were incorporated into the model oil: a commercially available naphthenic acid, sorbitan monooleate acid ester (Span80), and PPG. The purpose of selecting these three additives was to investigate whether hydrate interparticle adhesion forces may be controlled by the chemistry of the surrounding medium. Furthermore, these three additives were selected to investigate the structure-dependent nature of chemical additives that may be active on the hydrate surface. Experimental Section Experimental Apparatus. An improved micromechanical force (MMF) apparatus was used in this study.4 The experimental setup consisted of an inverted-light Carl Zeiss Axiovert S100 microscope and a digital recording system. Glass-fiber cantilevers were placed within a temperature-controlled aluminum cell.4 The left-hand cantilever remained stationary during the experiment, while the right-hand cantilever was maneuvered by a highprecision, remote-operated Eppendorf Patchman 5173 micromanipulator. The left-hand cantilever is calibrated indirectly with a tungsten wire of known spring constant; this tungsten wire is calibrated directly through a Denver Instruments TB-215-D analytical laboratory weighing scale. Additional information, including calibration methodology, on the improved MMF apparatus may be found elsewhere.5

(11) Greaves, D. P. M.S. Thesis, Colorado School of Mines, Golden, CO, 2007. (12) Aspenes, G.; Hoiland, S.; Barth, T.; Askvik, K. M. J. Colloid Interface Sci. 2009, 333, 533–539. (13) Vignati, E.; Piazza, R.; Visintin, R. F. G.; Lapasin, R.; D’Antona, P.; Lockhart, T. P. J. Phys.: Condens. Matter 2005, 17, S3651–S3660. (14) Boxall, J. A. Ph.D. Thesis, Colorado School of Mines, Golden, CO, 2009. (15) Gong, J.; Li, Q.; Yao, H.; Yu, D. J. Hydrodyn. 2006, 18, 310–314. (16) Erstad, K.; Hoiland, S.; Fotland, P.; Barth, T. Energy Fuels 2009, 23, 2213–2219. (17) Shim, J. Stability improvers for water-in-oil emulsions. U.S. Patent 4,483,777, Nov 20, 1984. (18) Oelscher, H.; Geke, J. Emulsifier system, anti-corrosive and lowtemperature lubricant emulsion. U.S. Patent 6,780,824, Aug 24, 2004.

(19) Rogers, V. V.; Liber, K.; MacKinnon, M. D. Chemosphere 2002, 48, 519–527. (20) Clemente, J.; Prasad, N. G. N.; MacKinnon, M. D.; Fedorak, P. M. Chemosphere 2003, 50, 1265–1274.

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A schematic of the experimental procedure is provided in Figure 2, showing the manipulation of the right-hand cantilever to obtain the adhesion force as a product of particle displacement and known spring constant. The interparticle adhesion force (F) is calculated using Hooke’s law F ¼ kx

ð1Þ

where k is the calibrated spring constant of the left-hand cantilever and x is the final particle displacement. A preload force of approximately 0.7 N was maintained for 10 s between the particles in each pull-off. Both the preload force and displacement are captured through a digital recording system and are analyzed with ImageJ.21 In each complete experiment, 40 pull-off measurements were performed. The final value reported for each experiment represents the average of these 40 measurements, and the corresponding error bounds represent the standard deviation among the 40 measurements. Each adhesion force measurement is normalized by the harmonic mean radius R* based on the radius (R1 and R2) for each hydrate particle.   1 1 1 1 ¼ ð2Þ þ R 2 R1 R2

Figure 3. Normalized CyC5 interparticle adhesion forces with the addition of pure mineral oil, 1 wt % Span80 in mineral oil, 1 wt % PPG in mineral oil, and 0.6 wt % naphthenic acid in mineral oil. The dashed horizontal line corresponds to the adhesion force for the system with pure CyC5. Vertical error bars represent ( one standard deviation in each set of 40 pull-off measurements.

The procedure for hydrate particle formation was identical for the left- and right-hand cantilevers. Using a dropper technique, a droplet of deionized water was placed on the end of each cantilever, immediately quenched in liquid nitrogen, and then placed in a CyC5 bath (99%, Sigma-Aldrich Co.). Once both cantilevers were secured in the bath, the temperature in the bath was slowly raised to 3.2 °C. After initial hydrate formation, the particles were left for 30 min to allow the hydrate to form at the surface of the droplet, usually forming a shell. Adhesion force experiments were performed with a continuous CyC5 phase containing variable amounts of model oil and additives. For the model oil, technical-grade white mineral oil (200 cP, 32° API (D287), specific gravity of 0.856 at 25 °C, CAS number 8042-47-5) was acquired from STE Oil Company (San Marcos, TX). To prevent the cell composition from changing over the course of the experiment, nitrogen gas was bubbled through CyC5 into the experimental chamber. Dieker et al.22 concluded that the normalized interparticle adhesion force for CyC5 hydrate at 3.2 °C is 3.8 ( 2.3 mN/m, where the bounds represent one standard deviation. Model oil adhesion force measurements, with and without additive components, may be compared to this pure CyC5 value, because the hydrate former is the same. It is important to note that these normalized, interparticle adhesion forces may not be directly compared to the THF hydrate results reported by Taylor et al.,6,7 because a change in the continuous phase and contact force will naturally result in different adhesion forces. Model Additives. Three additives were added into mineral oil: a commercially available naphthenic acid, Span80, and PPG. The technical-grade naphthenic acid mixture with an acid value of 230 was obtained from Sigma-Aldrich. The interparticle adhesion force was tested with a mixture of 0.6 wt % naphthenic acid in mineral oil. Span80 is a commercial surfactant (SigmaAldrich). The interparticle adhesion force was tested with a mixture of 1.0 wt % Span80 in mineral oil. PPG is an amphiphilic polymer; that is, it contains a hydrophilic group within a long polymer chain. For this study, hydrate interparticle adhesion forces were tested with a mixture of 1.0 wt % PPG (SigmaAldrich, average Mn of approximately 2000) in mineral oil. It is important to note that the bulk liquid in the cell must contain CyC5 at all times to preserve the thermodynamic stability of the

hydrates. Aspenes et al.12 examined the effect of 0-5000 ppm commercial naphthenic acids on water-surface interfacial tension, contact angle, and adhesion energy; the most significant changes in each measurement occurred between approximately 0 and 500 ppm. In this work, the concentration of mineral oil was varied up to 54 wt % for PPG, 42 wt % for Span80, and 30 wt % for naphthenic acids; the additive concentration in each model oil was maintained at around 1 wt % to reinforce an upper limit of approximately 500 ppm additive in the system. The amount of each model oil added to the system was varied to investigate the effect of the additive concentration on the interparticle adhesion force.

Results and Discussion Performance of Model Oil. Because the model oil (200 cP mineral oil) used for this study was visibly clearer than crude oil, hydrate interparticle adhesion forces could be visualized with the current setup of the MMF apparatus beyond the 8 wt % limitation with crude oils.9 Figure 3 shows the normalized interparticle adhesion force for the mineral oil with and without additives with increasing mineral oil concentrations in CyC5. As shown in Figure 3, the interparticle adhesion force of CyC5 hydrate is relatively constant in the presence of mineral oil between 1 and 50 wt % in the CyC5 bulk. From the data points displayed in Figure 3 (where each point is an average of 40 individual pull-off trials), the average normalized adhesion force with mineral oil is 6.1 ( 1.3 mN/m (95% confidence). Effect of Naphthenic Acids on the Interparticle Adhesion Force. Also shown in Figure 3 are the results for the mixture of 0.6 wt % naphthenic acid in mineral oil tested in the MMF apparatus for concentrations between 2 and 30 wt % in CyC5 bulk. Although the standard deviation of each data set (comprised of 40 pull-offs) increases with the addition of the naphthenic acid mixture to the cell, the average interparticle adhesion force remains relatively consistent over the course of the trials; this average adhesion force at 95% confidence is 1.2 ( 0.4 mN/m, which is significantly lower than the adhesion force without naphthenic acid. Erstad et al.23 suggested that ester compounds, which are found in non-plugging oils, do not typically appear in the

(21) Rasband, W. S. ImageJ; U.S. National Institutes of Health (NIH): Bethesda, MD, 2008. (22) Dieker, L. E.; Taylor, C. J.; Koh, C. A.; Sloan, E. D., Jr. Proceedings of the 6th International Conference on Gas Hydrates; Vancouver, British Columbia, Canada, 2008.

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Figure 4. Typical CyC5 hydrate particles without additives (left). Morphological changes to the surface of CyC5 hydrate caused by the addition of Span80 (right). Bulk composition includes liquid cyclopentane and between 11 and 42 wt % model oil (mineral oil with 1 wt % Span80).

spectra of plugging oils. Carboxylic acids are the primary component of naphthenic acids, where the composition varies by a range of hydrogen deficiency (approximately 0-12) and total carbon number (approximately 5-30).19,20 Clemente et al.20 showed that most naphthenic acid constituents are smaller in total carbon number (approximately 11-17) and range in hydrogen deficiency from 2 to 6. These common acids, which are primarily paraffinic and contain one or two aliphatic rings, exhibit the same hydrophilic and hydrophobic molecular ends found in Span80.20 Unlike Span80, however, the carboxylic acids discussed above are generally smaller molecules; a generic carboxylic acid structure is shown in Figure 1. The results presented here are consistent with the observations by Aspenes et al.,12 in that the addition of naphthenic acids to the system results in oil-wet surfaces. This condition reduces the water bridging between hydrate particles, which can potentially reduce the agglomeration of hydrate particles. In comparison to the established baseline of pure mineral oil, the addition of 0.6 wt % commercial naphthenic acid to the mineral oil system reduced the hydrate interparticle adhesion force by 80%. The results presented here indicate that naphthenic acids in crude oils may play a significant role in the agglomeration tendency of hydrate particles; crude oils that have a more significant naphthenic acid fraction may result in decreased particle agglomeration. However, the scope of this study does not explore how the specific chemical identity of naphthenic acids (e.g., various carbon chain lengths of carboxylic acids) affects adhesion between hydrate particles. It is important to note that, in the data presented here, the hydrate interparticle adhesion forces seem to be independent of the weight percentage of mineral oil in CyC5 bulk liquid. As discussed by Aspenes et al.,12 surface-active components will saturate model pipeline surfaces in sufficient concentration. On the basis of this observation, it is reasonable to suggest that the surface of the hydrate particles becomes saturated at low additive concentrations. Effect of the Span80 Model Surfactant on the Interparticle Adhesion Force. Figure 3 shows the results for the addition of 1 wt % Span80 to mineral oil, which produced a similar effect

to the naphthenic acids. The adhesive force was measured for concentrations between 11 and 42 wt % of the mixture in the CyC5 bulk. The average adhesion force for 1 wt % Span80 in mineral oil is 3.8 ( 1.5 mN/m. This Span80 average is 37% lower than that of mineral oil but lies within the 95% confidence bounds of mineral oil. Span20 was shown by Taylor et al.6,7 to reduce the adhesion force between THF hydrates. Taylor et al.6,7 suggested that Span20 stabilized a water-in-oil emulsion, where hydrophilic ends remained bound to the hydrate surface and hydrophobic tails extended into the continuous bulk phase. Taylor et al.6,7 also showed that the addition of 3 wt % Span20 reduced the THF hydrate interparticle adhesion force by approximately 80%, which is qualitatively consistent with the data presented here. Taylor et al.6,7 also observed that the addition of Span20 induced morphological changes on the hydrate surface, where hair-like extrusions extended from the surface into the bulk phase. The experimental study reported here with Span80 and mineral oil in CyC5 bulk fluid with CyC5 hydrate produced a similar phenomenon, which is seen from the sample images shown in Figure 4. Further studies are needed to fully understand the mechanism behind these morphological changes. However, from these two independent studies, we hypothesize that the observed surface morphology changes are independent of the hydrate former and are primarily related to the addition of Span-class additives to the bulk fluid phase. We did not observe morphological changes through the addition of naphthenic acid or an amphiphilic polymer (PPG). Although Span80 has distinct hydrophilic and hydrophobic ends, it is less effective at reducing hydrate interparticle adhesion forces compared to naphthenic acids; the molecular structure of Span80 is provided in Figure 1. One possible explanation for the lower effectiveness of Span80 can be related to the morphological changes observed in the present work. Instead of decreasing the interfacial energy between the bulk fluid and the hydrate surface or quasi-liquid surface layer, the observed increase in surface roughness may alter the capillary bridge forming between the two particles. Evidence for this observation is shown in the experimental images of Figure 4. As such, the functional behavior of Span80 is independent of its affect on interfacial energy. Effect of Amphiphilic Polymers on the Interparticle Adhesion Force. Figure 3 shows the results for the addition of a 1.0 wt % mixture of PPG in mineral oil to CyC5 bulk, which

(23) Erstad, K.; Hoiland, S.; Barth, T.; Fotland, P. Proceedings of the 6th International Conference on Gas Hydrates; Vancouver, British Columbia, Canada, 2008.

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and, in turn, modify the adhesion between the hydrate particles that results from capillary bridge forces. Naphthenic acids were notably more effective than Span80, reducing the average adhesion force by 80%, while Span80 lies within the error bounds of mineral oil. Both Span80 and naphthenic acids have distinct hydrophilic and hydrophobic active groups, but the hydrophilic active group in Span80 is structurally much larger than the typical acid group found in naphthenic acids.19,20 One possible explanation for this observation is that molecules with smaller hydrophilic groups are more effective at adsorbing and saturating the hydrate surface, thereby reducing the tendency of hydrates to agglomerate. PPG contains equidistant hydrophilic groups throughout the length of the polymer that may encourage adsorption to the hydrate surface, as compared to the increased size and complexity of the hydrophilic group in Span80; this difference may explain the increased effectiveness of PPG in decreasing hydrate interparticle adhesion force. The hydrophobic tails within PPG contain only one methyl group, whereas the hydrophobic ends of naphthenic acids can extend up to 30 carbons in length.19,20 Naphthenic acids were more effective than PPG in reducing the adhesive forces between hydrate particles, lowering the interparticle adhesion force by 80% compared to 65%, respectively. One possible explanation for this observation is that molecules with larger hydrophobic groups will extend farther into the bulk fluid phase.

Figure 5. Summary of the average CyC5 hydrate interparticle adhesion force with the addition of pure mineral oil, 1.0 wt % Span80 in mineral oil, 1.0 wt % PPG in mineral oil, 0.6 wt % commercial naphthenic acid in mineral oil, and 4.6 wt % unmodified Caratinga crude oil.9 Error bounds represent 95% confidence of multiple experimental trials (shown in Figure 2).

resulted in a decrease in hydrate interparticle adhesion force. Concentrations between 26 and 54 wt % of the mixture were added to the cell. From the data presented in Figure 3, the average adhesion force is 2.1 ( 0.8 mN/m (at 95% confidence). This corresponds to a 65% reduction in the adhesion force from pure model oil. The PPG used in this study had a molecular weight of approximately 2000, which corresponds to an average of 34 monomer units (see Figure 1). One possible explanation for the observed behavior of PPG is that the polymer backbone adsorbs to the hydrate particle surface and modifies the interfacial properties, leading to a lower adhesion force between hydrate particles. Comparison of Adhesive Forces to Crude Oil. Interparticle adhesion forces for 4.6 wt % unmodified Caratinga crude oil24 in CyC5 bulk are at least 60% lower than all model systems presented here. This suggests that Caratinga oil has effective natural compounds, which may be similar in nature to naphthenic acids that are surface-active and cause the adhesive forces between hydrate particles to decrease. Because of the complexity of crude oils, however, the present study does not investigate the chemical characterization of the natural compounds within Caratinga oil. Dieker et al.9 showed that Caratinga crude oil was more effective at reducing hydrate interparticle adhesion force than other crude oils (Dalia3 or Troika). Impact of the Chemical Structure on the Interparticle Adhesion Force. A comparison of CyC5 hydrate interparticle adhesion forces immersed in pure mineral oil, mineral oil with each of the three selected additives (commercial naphthenic acids, Span80, and PPG), and Caratinga crude oil (from Dieker et al.9) is shown in Figure 5. Because the adhesion force for each model oil remained relatively unchanged over a range of concentrations, the average values for the adhesive forces are plotted in Figure 5, with error bars corresponding to the 95% confidence intervals. The experimental results presented, with commercial naphthenic acids, PPG, and Span80, show that hydrate interparticle adhesion forces are affected by changes in the chemical composition of the surrounding medium. Even though additional work is required to quantitatively understand the structure-function relationship of the additives, the results presented here suggest that chemical additives in the surrounding medium interact with the hydrate surface

Conclusions Mineral oil was selected as a model oil for examining CyC5 interparticle adhesion force in an improved MMF apparatus. The adhesive force for hydrate particles with mineral oil added in bulk CyC5 shows a relatively constant adhesion force of 6.1 ( 1.3 mN/m from 1 to 50 wt % in the CyC5 bulk fluid. The addition of a mixture of 1.0 wt % Span80 in mineral oil resulted in a relatively constant adhesion force of 3.8 ( 1.5 mN/m. This corresponds to a 37% reduction in average adhesion force relative to that with pure mineral oil, but the averages overlap at 95% confidence. The addition of a mixture of 1.0 wt % PPG in mineral oil decreased adhesion forces by 65% from pure mineral oil, with an average adhesion force of 2.1 ( 0.8 mN/m. The addition of a mixture of 0.6 wt % commercial naphthenic acids in mineral oil showed an average adhesion force of 1.2 ( 0.4 mN/m, which corresponds to an 80% reduction in adhesion force from pure mineral oil. These results show that naphthenic acids, Span-class surfactants (sorbitan acid esters), and PPG are all surface-active components, which are shown to significantly reduce hydrate interparticle adhesion forces in the order: naphthenic acids > amphiphillic polymer > model anti-agglomerant. On the basis of the data presented here, we hypothesize that hydrate agglomeration tendency may be reduced by the presence of compounds with small hydrophilic groups that will more effectively adsorb and saturate the hydrate surface or through induced morphological changes that preclude the capillary bridge between particles. Acknowledgment. The authors acknowledge the funding and support from the Colorado School of Mines Hydrate Consortium (BP, Champion, Chevron, ConocoPhillips, ExxonMobil, Halliburton, Multi-Chem, Nalco, Schlumberger, Shell, SPT Group, StatoilHydro, and Total). A.K.S. acknowledges the support of DuPont for a DuPont Young Professor Award.

(24) Davies, S. R.; Boxall, J. A.; Dieker, L. E.; Sum, A. K.; Koh, C. A.; Sloan, E. D. J. Dispersion Sci. Technol., 2010, in press.

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