Mechanisms of Stabilization of Water-in-Crude Oil Emulsions

K. Kumar, A. D. Nikolov, and D. T. Wasan*. Department of Chemical and Environmental Engineering, Illinois Institute of Technology,. Chicago, Illinois ...
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Ind. Eng. Chem. Res. 2001, 40, 3009-3014

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Mechanisms of Stabilization of Water-in-Crude Oil Emulsions K. Kumar, A. D. Nikolov, and D. T. Wasan* Department of Chemical and Environmental Engineering, Illinois Institute of Technology, Chicago, Illinois 60616

The mechanisms of stabilization of water-in-crude oil emulsions have been investigated by changing the solvent-solute interactions in crude oil. Diluting the original crude oil with varying amounts of heptane, which is a poor solvent for asphaltenes, changes the solvent-solute interactions, leading to flocculation of asphaltenes and thus changing the emulsion stability. The interactions between the water droplets in an emulsion system have been quantified by measuring the radial distribution function and thereby the pair potential using the digitized optical imaging technique. It has been observed that the force of interaction between water droplets is oscillatory. This shows that non-DLVO forces, such as attractive depletion and repulsive structural forces, exist between the droplets. The interaction between the water droplets has been modeled by studying the properties of a thin liquid film sandwiched between the water droplets. Because of the film confinement effect, asphaltene-resin particles form a layered structure inside the thin liquid film. Also, the role of hydrodynamic interactions has been studied by using the film rheometer to measure the dynamic film tension and film elasticity. It has been found that, because of the adsorption of asphaltene at the film interfaces, the film elasticity plays a significant role in stabilizing these emulsions. Introduction Crude oil is a complex fluid comprising colloidal particles such as asphaltenes and resins dispersed in a mixture of aliphatic and aromatic solvents. Water-inoil (W/O) emulsions are extremely stable because of the presence of these particles. To reduce the water content of the produced crude oil, the water/crude oil emulsions have to be broken (i.e., demulsified). Thus, it is important to understand the mechanisms responsible for stabilization of these emulsions.1-7 The effect of asphaltene solvency on the stability of water-in-crude oil emulsions has been investigated.8 The role of asphaltenes in the stability of water-in-bitumen emulsions has recently been studied.9 Depending on the crude oil aliphatic/aromatic ratio which governs the solubility of asphaltene-resins and their interfacial activity, one can expect three major stabilization mechanisms for a crude oil film between water deposits:10,11 (1) steric stabilization due to an adsorbed layer of asphaltene at a low concentration of asphaltene-resin submicron particles; (2) depletion destabilization due to an excluded-volume effect, leading to attraction between water droplets; and (3) structural stabilization due to long-range colloidal structure formation inside the film in the presence of a sufficient effective volume fraction of colloidal particles. Generally, an emulsion system is expected to have a combination of these three mechanisms. The goal of the present research is to study the solvent-solute interactions in relation with dispersion stability and understand the various mechanisms in stabilizing the emulsion. This paper explains the role of long-range oscillatory structural and attractive depletion forces in the stability of water-in-crude oil emulsions. Also, the effect of increasing the aliphatic/ * To whom correspondence should be addressed. Phone: 312 567-3001. Fax: 312 567-3003. E-mail: [email protected].

aromatic ratio on the surface activity of asphalteneresin has been elucidated by measuring the film tension. Experiment Materials. Crude oil used for this research was a Venezuelan crude oil (API ) 24.5°) with 54.5% aliphatic concentration and a resin/asphaltene ratio of 4.3. The crude oil was also found to have no water and solid content. Also, the W/O emulsions formed with the original crude oil were found to be extremely stable, and there was no separation observed (under gravity) even after a few weeks. Water-in-Crude Oil Emulsion Stability. The original crude oil (aliphatic concentration ) 54.5 wt %) was diluted with heptane to change the crude oil solvent property. This modified crude oil was then mixed with preequilibrated water. The emulsion samples were prepared by vigorously stirring the water (30 vol %) and crude oil mixture. An emulsion sample of 25 mL was taken and tested for stability under gravity. The amount of water resolved is a measure of the emulsion stability. The measurements were done in a water bath at 40 °C, and after 4 days, the amount of water resolved was recorded. Droplet-Droplet Interaction in a Water-inCrude Oil Emulsion. A digitized optical imaging technique was used to study the microstructure of the water-in-crude oil emulsion. The emulsion sample was taken under a microscope to record a microstructural image. This image was recorded using a video attached to the microscope. Using an imaging software (Image Pro), the microstructural image was magnified and then the analysis was done to measure the interdroplet distance. These acquired data were processed in MATLAB to calculate the radial distribution function (RDF) and the pair potential of the interaction between droplets. The RDF g(r) measures the probability to find a water droplet center at a distance r from a reference

10.1021/ie000663p CCC: $20.00 © 2001 American Chemical Society Published on Web 04/17/2001

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water droplet. It is oscillatory in nature and tends to 1 as the distance from the reference water droplet tends to infinity, implying that the probability of finding a water droplet at infinity is the same as that for bulk. It typically has a maximum at a distance of about one droplet diameter for a monodisperse system. The pair potential can also characterize the bulk packing structure of dispersion systems such as a waterin-crude oil emulsion. It is related to the radial distribution function according to the following relationship:

U(r) ) -kT ln[g(r)]

(1)

The curve is observed to be oscillatory with the period of the droplet diameter, indicating the formation of a layering structure of colloidal particles.12 At the contact position of particles, a deep minimum exists. This minimum, generally named as the attractive depletion well, is one of the central issues of colloidal stability. The basic principle behind this is when two large spheres (like water droplets in an emulsion) approach each other, the small spheres (like micelles/particles/ crude oil colloids) are expelled from the closing gap, leading to an imbalance in the osmotic pressure in the gap and outside. This induces an effective attraction between the large spheres, which is called the depletion interaction.13-15 Emulsion Film Rheology and Dynamic Film Tension Measurement. Another way to probe the interaction of solvent and solute and their effect on the short-term stability of the water-in-crude oil emulsion is by studying the film rheology. The emulsion film rheology was measured by a film rheometer developed at the Illinois Institute of Technology.16-18 The measurements were carried out at a temperature of 25 °C. A sensitive pressure transducer was used to measure the capillary pressure versus time. From the capillary pressure data, the film tension was calculated using the Young-Laplace equation. The transient film tension is related by the Young-Laplace equation to the film radius (R) and the capillary pressure (Pc)

Pc ) 2f/R

(2)

where f is the film tension. The Gibbs film elasticity is defined as the change of the initial film tension versus the logarithm of the relative film area expansion (A/A0)

E)-

dfi d ln(A/A0)

(3)

where A is the final area of the film, A0 is the initial area of the film, fi is the film tension, and E is the Gibbs film elasticity. Results and Discussion The emulsion stability of water-in-crude oil emulsion samples with varying aliphatic/aromatic ratios was tested under gravity. These samples were made using a crude oil of varying aliphatic concentration (54.5-78.6 wt %). The aliphatic concentration of the crude oil was changed by adding heptane. The change in the aliphatic concentration leads to a change in the interaction between crude oil colloids,8 and this leads to the flocculation of asphaltenes. The mixture was left overnight

Figure 1. Emulsion stability: effect of the aliphatic component and correlation with film elasticity.

Figure 2. Droplet size distribution for a water-in-crude oil emulsion.

for crude oil colloids (e.g., asphaltenes and resins) to flocculate. The results of these studies are shown in Figure 1. It has been observed that, as the solvent is changed (i.e., the aliphatic concentration is increased by adding heptane to the original crude oil), the stability of the emulsion decreases; i.e., the amount of waterresolved increases and goes through a maximum. It is worth noticing that at an aliphatic concentration of 54.5 wt % we did not observe any separation even after a month, showing that the sample is extremely stable. However, a small change in the aliphatic concentration from 54.5% to 54.7% caused the emulsion stability to change drastically. Upon a further increase of the aliphatic concentration (greater than 55.2 wt %), the stability of the emulsion increases, the amount of water resolved goes through a minimum at 56.7 wt %, and the amount of water resolved was less than 4% of the original amount of water. Also, it is important to note that the amount of water resolved is more than the amount of water resolved for the 54.5 wt % sample (initial point). As the aliphatic concentration is further increased, the stability starts to decrease again, and at an aliphatic concentration of 78.6 wt %, the amount of water resolved after 4 days is about 39 wt %. Although the separation after 4 days is 39 wt %, after several days (15 days) the water separates out completely (90%). However, the sample with an aliphatic concentration of 54.7 wt % reaches a steady state and does not have much separation after 4 days. To understand the stability of the emulsion, we have investigated the interactions between the water droplets by studying the pair potential of interactions between

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Figure 3. Radial distribution function.

Figure 4. Effective pair potential between emulsion droplets.

the emulsion droplets. This study was conducted using the digitized optical imaging technique. The droplet size distribution for the emulsion sample was measured using this technique and is shown in Figure 2. The microstructural image was used to measure the interdroplet distance for the emulsion sample. The interdroplet distance was used to calculate the radial distribution function. The radial distribution function of the emulsion system is shown in Figure 3. The radial distribution is found to be oscillatory, confirming that the asphaltene submicron particles form a layered structure in the film. The effective pair potential of the mean forces between water droplets as calculated using eq 1 is shown in Figure 4. The first peak corresponds to the repulsive structure barrier, which the water droplets have to overcome in order to flocculate. Once again, the oscillatory nature of the curve confirms that the layered structure of asphaltene particles in the thin liquid film between water droplets gives rise to a repulsive structural barrier. The water droplets are separated by a crude oil medium containing colloidal particles (e.g., asphaltenes and resins) dispersed in the continuous oil phase. When the droplets approach each other because of the manybody interactions between the droplets, a thin oil film is formed between these water droplets. Because of the confinement effect of the film surfaces, the asphaltene submicron particles (crude oil colloids) form a layered structure inside the thin liquid film. In a model system

study, Wasan and Nikolov10 have shown the layering phenomena of asphaltenes in the oil film between water droplets. They used a model oil consisting of heptane and toluene with 7 wt % asphaltenes. They have reported that at sufficiently high concentration asphaltenes form a layered structure inside the thin oil film between water droplets. This long-range structure induces a repulsive structural barrier, which contributes to the prevention of water droplets flocculating and coalescing. This layered structure causes the asphaltene particles to come out of the film in a layerwise manner as described by Kralchevsky et al.19 In the “diffusiveosmotic mechanism”, vacancies inside the film diffuse to form larger vacancies until they cover the whole film. When the film size is small, the layer of asphaltene stays inside the film and stabilizes the film, but if the film size is large, the layers leave the film at a faster rate and destabilize the film.20 Because of the layered structure inside the film, the structural disjoining pressure becomes oscillatory. Nikolov and Wasan21 have experimentally shown that a number of thickness transitions are strong functions of the micellar/particle concentration for a thin liquid film. It was found that the emulsion film was very stable if the draining emulsion film contained layers of surfactant micelles.22 Chu et al.23 have shown by Monte Carlo simulation that the structural disjoining pressure increases with the micellar/particle concentration. This suggests that an

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Figure 5. Stress relaxation experiment: crude oil samples with and without heptane.

increase in the micellar/particle concentration leads to an increased stability because the thin liquid film containing particles drains slowly. Upon addition of the nonsolvent (heptane), the asphaltene particles precipitate and the emulsion stability changes because of the change in the solvent-solute interactions.8 At an aliphatic concentration of 54.5 wt %, the asphaltenes are in a well-dispersed state and stay peptized in the crude medium by resins. At a high asphaltene concentration (2.4 wt %), asphaltene submicron particles form a layered structure in the thin crude oil film separating the water droplets. However, when the aliphatic concentration is increased further, the emulsion becomes unstable, and at 54.7 wt % aliphatic concentration, 33 vol % of water was resolved after 4 days. This maximum in the curve is attributed to the attractive depletion forces. When two interacting water droplets are so close to each other that the distance between them is less than the diameter of the asphaltene particles, because of the excluded-volume effect, there is an attractive depletion force, which forces the droplets closer to each other. When the aliphatic concentration is increased, the concentration of the asphaltene submicron particle decreases because of precipitation. The concentration reaches a point where the depletion forces become strong enough to destabilize the emulsion system. Upon a further increase in the aliphatic concentration (>55.2 wt %), asphaltene particles precipitate, leading to lesser concentration of asphaltene particles inside the film. Because of lesser concentration of asphaltene particles, attractive depletion forces become weak and the system is stabilized because of adsorbed layer steric stabilization. It is known that the adsorbed layer of asphaltene plays an important role in the stability of water-in-crude oil emulsions. The film rheometer was used to understand the role of the adsorbed asphaltene layer on the stability of the emulsion. It was used to investigate the effect of asphaltene particles on the curved emulsion film. The stress relaxation experimental results for two samples; i.e., crude oil with aliphatic concentrations of 54.5 and 54.7 wt % are shown in Figure 5. It was found that the change in the film tension for a crude oil with 54.7 wt % heptane was higher compared to that of the original crude oil (54.5 wt %). This shows that the elasticity of the crude oil sample with a higher aliphatic concentration is more than that of the original crude oil as shown in Figure 6. The crude oil with a higher aliphatic concentration has an elasticity of 24.2 dyn/cm, and the original crude

Figure 6. Emulsion film elasticity for crude oil samples with and without heptane.

Figure 7. Emulsion film elasticity for crude oil samples with varying amounts of heptane.

oil has an elasticity of 20.3 dyn/cm. The film elasticity values as given by the slope of each of these curves for other crude oil samples along with their aliphatic concentrations are shown in Figure 7. The correlation between the film elasticity and the emulsion stability is shown in Figure 1. It can be seen clearly that, at higher aliphatic concentration, the film elasticity correlates with the emulsion stability. Following the classical understanding, it would be expected that the sample with higher elasticity would be more stable because of the Gibbs-Marangoni effect.24 Consider the emulsion samples with an aliphatic concentration larger than 54.7 wt %. It can be seen clearly that the film elasticity correlates with the emulsion stability; i.e., the higher the film elasticity is, the higher is the stability of the emulsion. However, the classical understanding cannot explain the behavior of the sample with a 54.5 wt % aliphatic concentration. At this concentration the emulsion stability trend is reversed; i.e., the sample with a lower elasticity is more stable than the sample with a higher elasticity. To better understand this behavior, let us compare the samples with 54.5, 54.7, 55.2, and 56.7 wt % aliphatic concentration. It is clearly seen from the emulsion stability curve that the emulsion sample with a 54.5 wt % aliphatic concentration is the more stable sample. However, the film elasticity for this sample is the least of all four samples. The effect is even more pronounced if we compare the emulsion stability and elasticity of only samples with 54.5 and 54.7 wt % aliphatic concentration. Although, the sample with a 54.7 wt % aliphatic concentration has a higher elasticity (24.2 dyn/cm) compared to the sample with a 54.5 wt % aliphatic concentration, it is extremely unstable.

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Figure 8. Comparison between the experimental data of Kilpatrick and McLean and our experimental data.

aromatic ratio. The emulsion stability depends on the asphaltene-resin colloidal particle interaction and their adsorption at the water-oil interface. To quantify these forces in an emulsion system, a digitized optical imaging technique was used. The oscillatory nature of both the radial distribution function and the effective pair potential of interaction between droplets also reveals that the crude oil colloids (like asphaltenes and resins) form a layered structure in the emulsion film, which leads to an increased stability of the emulsion. However, when the concentration of heptane (aliphatic concentration) increases, the asphaltenes dissolved in the crude oil precipitate. This leads to higher size and polydispersity of submicron asphaltene particles, and the stability of the emulsion decreases because of attractive depletion forces. The surface activity of asphaltene particles increases upon an increase of the concentration of nonsolvent (heptane). Thus, adsorbed layer stabilization was found to play an important role at higher aliphatic concentration. The film elasticity correlates well with the emulsion stability in this concentration region where structural forces do not play a significant role. Acknowledgment This research was supported by the National Science Foundation (Grant CTS9612322). Literature Cited

Figure 9. Comparison between experimental data of Kessel et al. and our experimental data.

The adsorbed layer of asphaltene prevents the water droplets from coming close together. However, when the aliphatic concentration is further changed by adding heptane, the asphaltene particles precipitate, the concentration of the particles is not enough to cover the water droplets, and the emulsion becomes unstable. A photomicrograph of precipitated asphaltene particles for crude oil with a heptane concentration of 65.9 wt % (333 000 ppm) shows that the asphaltene particle size is very large (6 µm approximately) and is inefficient in stabilizing the emulsions.25 Similar results for the water-in-crude oil emulsion stability have been reported by Kilpatrick and McLean.8 They studied a model water-in-crude oil system where crude oil is modeled by a heptane-toluene mixture (Heptol). They show that the emulsion stability goes through a maximum as the aliphatic concentration of Heptol is increased (Figure 8). In the region where structure and depletion forces are not significant, Kessel et al.26 have reported similar results, where they show that the emulsion stability decreases as the precipitation of asphaltene increases (Figure 9). Conclusions The mechanisms of stabilization of a water-in-crude oil emulsion system were investigated by changing the aliphatic/aromatic ratio of the continuous crude oil phase. To understand the asphaltene-resin colloidal particle interactions and their role in the stability of water-in-crude oil emulsion, the solvent-solute interactions were varied by changing the crude oil aliphatic/

(1) Berger, P. D.; Hsu, C.; Arendell, J. P. Designing and Selecting Demulsifier for Optimum Field Performance Based on Production Fluid Characteristics. Soc. Pet. Eng. 1987, SPE-16285, 457. (2) Mohammed, R. A.; Baily, A. I.; Luckham, P. F.; Taylor, S. E. Dewatering of Crude Oil Emulsions. 1. Rheological Behavior of the Crude Oil-Water Interface. Colloids Surf. 1993, 80, 222. (3) Mohammed, R. A.; Baily, A. I.; Luckham, P. F.; Taylor, S. E. Dewatering of Crude Oil Emulsions. 2. Interfacial Properties of the Asphaltene Constituents of Crude Oil. Colloids Surf. 1993, 80, 237. (4) Mohammed, R. A.; Baily, A. I.; Luckham, P. F.; Taylor, S. E. Dewatering of Crude Oil Emulsions. 3. Emulsion Resolution by Chemical Means. Colloids Surf. 1993, 83, 261. (5) Sjoblom, J.; Hoiland, H.; Urdahl, O.; Christy, A. A.; Johansen, E. J. Water-in-Crude Oil Emulsions. Formation, Characterization and Destabilization. Prog. Colloid Polym. Sci. 1990, 82, 131. (6) Sjoblom, J.; Mingyan, L.; Christy, A. A.; Gu, T. Water-inCrude Oil Emulsions from the Norwegian Continental Shelf. Part 7. Interfacial Pressure and Emulsion Stability. Colloids Surf. 1992, 66, 55. (7) Sjoblom, J.; Mingyan, L.; Christy, A. A.; Ronningsen, H. P. Water-in-crude Oil Emulsions from the Norwegian Continental Shelf. Part 10. Aging of the Interfacially Active Components and the Influence on the Emulsion Stability. Colloids Surf. 1995, 96, 261. (8) Kilpatrick, P. K.; McLean, J. D. Effects of Asphaltene Solvency on Stability of Water-in-Crude Oil Emulsions. J. Colloid Interface Sci. 1997, 189, 242. (9) Yan, Z.; Elliott, J. A. W.; Masliyah, J. H. Role of Various Bitumen Components in the Stability of Water-in-Diluted-Bitumen Emulsions. J. Colloid Interface Sci. 1999, 220, 329. (10) Wasan, D. T.; Nikolov, A. Emulsion Stability Mechanisms. Proceedings of the First World Congress on Emulsions, Paris, France, Oct. 19-22, 1993; Vol. 4, p 93. (11) Krawczyk, M. A.; Wasan, D. T.; Shetty, C. S. Chemical Demulsification of Petroleum Emulsions using Oil-Soluble Demulsifiers. Ind. Eng. Chem. Res. 1991, 30, 367.

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(12) Chu, X. L.; Nikolov, A. D.; Wasan, D. T. Monte Carlo Simulation of In-layer Structure Formation in Thin Liquid Films. Langmuir 1994, 10, 4403. (13) Aronson, M., P. The Role of Free Surfactant in Destabilizing Oil-in-Water Emulsions. Langmuir 1989, 5, 494. (14) Aronson, M. P. Flocculation of Emulsions by Free Surfactant in Purified Systems. Colloids Surf. 1991, 58, 195 (15) Bibette, J. Depletion Interactions and Fractionated Crystallization for Polydisperse Emulsion Purification. J. Colloid Interface Sci. 1991, 147, 474. (16) Soos, J. M.; Koczo, K.; Erdos, E.; Wasan, D. T. An Automatic Apparatus for Measuring Interfacial and Film Tension Under Static and Dynamic Conditions. Rev. Sci. Instrum. 1994, 65, 3555. (17) Kim, Y. H.; Wasan, D. T.; Breen, P. J. A Study of Dynamic Interfacial Mechanisms for Demulsification of Water-in-Oil Emulsions. Colloids Surf. 1995, 95, 235. (18) Nagarajan, R.; Koczo, K.; Erdos, E.; Wasan, D. T. Controlled Drop Tensiometer for Measuring Dynamic Interfacial Tension and Film Tension. AIChE J. 1995, 41 (4), 915. (19) Kralchevsky, P. A.; Nikolov, A. D.; Wasan, D. T.; Ivanov, I. B. Formation and Expansion of Dark Spots in Stratifying Foam Films. Langmuir 1990, 6 (6), 1180. (20) Nikolov, A. D.; Wasan, D. T. Effects of Film Size and Micellar Polydispersity on Film Stratification. Colloids Surf. 1997, 128, 243.

(21) Nikolov, A. D.; Wasan, D. T. Dispersion Stability Due to Structural Contributions to the Particle Interaction as Probed by Thin Liquid Film Dynamics. Langmuir 1992, 8 (12), 2986. (22) Manev, E. D.; Sazdanova, S. V.; Wasan, D. T. Stratification in Emulsion Films. J. Dispers. Sci. Technol. 1984, 5, 111. (23) Chu, X. L.; Nikolov, A. D.; Wasan, D. T. Thin Liquid Film Structure and Stability: The Role of Depletion and Surfaceinduced Structural Forces. J. Chem. Phys. 1995, 103 (15), 6653. (24) Kim, Y. H.; Nikolov, A. D.; Wasan, D. T.; Diaz-Arauzo, H.; Shetty, C. S. Demulsification of Water-in-Crude Oil Emulsions: Effects of Film Tension, Elasticity, Diffusive, and Interfacial Activity of Demulsified Individual Components and Their Blends. J. Dispers. Sci. Technol. 1996, 17 (1), 33. (25) Kumar, K. Mechanisms of Stabilization and Destabilization of Emulsions and Foams. M.S. Thesis, Illinois Institute of Technology, Chicago, IL, May 1999. (26) Kessel, D.; Neumann, H. J.; Rahimian, I. Asphaltics in Crude Oil-Water Emulsions. Proceedings of the First World Congress on Emulsions, Paris, France, Oct. 19-22, 1993; Vol. 4, p 22.

Received for review July 11, 2000 Revised manuscript received October 23, 2000 Accepted October 25, 2000 IE000663P