Rheological Behavior of Concentrated Bitumen in Water Emulsions

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Rheological Behavior of Concentrated Bitumen in Water Emulsions G. A. Nun˜ez, G. Sanchez, X. Gutierrez, F. Silva, C. Dalas, and H. Rivas* PDVSA-Intevep, P.O. Box 76343, Caracas 1070-A, Venezuela Received April 9, 1999. In Final Form: April 12, 2000 The effect of bitumen concentration and mean drop diameter and distribution on the rheological behavior of bitumen in water emulsions was investigated. Results obtained showed that the emulsion viscosity is severely affected by the type of drop size distribution. In fact, emulsions with a bimodal drop diameter distribution exhibit viscosity values which were at least 1 order of magnitude lower than the viscosity of similar emulsions, having the same bitumen concentration and an equivalent mean drop diameter, but with a unimodal distribution. It was also found that concentrated bimodal emulsions behave like Newtonian fluids, while the unimodal counterparts are non-Newtonian and show some pseudoplasticity. Because of their very simple rheological behavior, bitumen in water bimodal emulsions are of great importance in the processes of transporting, handling, and commercializing extremely viscous hydrocarbons.

Introduction The development of an adequate technology for handling very viscous hydrocarbons through their emulsification in water has been studied during the past two decades by many authors.1-5 Long-distance pipeline and tanker transportation as well as in-plant storage requires that these emulsions have very good stability and appropriate rheological properties. Not very much information on these aspects has been published to date. In this paper, we shall identify the rheological behavior of concentrated emulsions of heavy Venezuelan bitumen in water, emphasizing novel features which are associated with the drop diameter distribution. Such emulsions are currently being marketed worldwide as coal-substitute fuels under the trade name of Orimulsion. Most of the studies on the rheological behavior of uniand polymodal dispersions have been carried out using solid particles with well-characterized diameter distributions. Thus, the viscosity for a system with a polymodal distribution of particle diameter can be calculated using the expression6

η ) exp[2.5φ1/(1 - k1φ1)] exp[2.5φ2/(1 - k2φ2)] ... exp[2.5φn/(1 - knφn)] (1) where the values of φ1, φ2, and φn are the volumetric fractions of particles with different diameters and k1, k2, * Corresponding author. (1) Simon, R.; Poynter, W. G. Down Hole Emulsification for Improving Viscous Crude Production. J. Pet. Technol. 1968, 20, 1349. (2) Marsden, S.; Raghavan, R. A System for Producing and transporting Crude Oil as an Oil/Water Emulsion. J. Inst. Pet. 1973, 59, 273. (3) Lamb, M. S.; Simpson, W. C. Pipeline Transportation of Wax Laden Crude Oil as Water Suspensions. Proc. Sixth World Petroleum Congress, Section VII, 1973; p 23. (4) Layrisse, I.; Rivas, H.; Quintero, L.; Rivero, M.; Martinez, A. Imulsion Technology, the Answer for the Production, Handling and Transportation of Extra-Heavy Crude Oils and Bitumen. Proc. Fourth UNITAR/UNDP Int. Conference on Heavy Crude and Tar Sands; Paper No. 179, Edmonton. Canada, 1978. (5) Nun˜ez, G.; Bricen˜o, M.; Mata, C.; Rivas, H. Flow Characteristics of Concentrated Emulsions of very Viscous Oil in Water. J. Rheol. 1996, 40, 405. (6) Parkinson, C.; Matsumoto, S.; Sherman, P. The influence of Particle Size Distribution on the Apparent Viscosity of Non-Newtonian Dispersed Systems. J. Colloid Interface Sci. 1970, 33, 150.

and kn are constants which can be calculated knowing the particle diameter in each fraction in the mixture. Highly concentrated emulsions might show anomalous rheological behavior due to the deformation of droplets. In these cases, eq 1 has no application at all. In this paper, we will discuss the rheological behavior of bimodal emulsions, obtained by mixing two unimodal emulsions, with very well differentiated mean drop diameters. The behavior of bimodal emulsions is completely different than that of unimodal counterparts used in its preparation. In fact, the viscosity of the bimodal emulsions is always much lower than the corresponding viscosity of the unimodal emulsions present in the mixture at the same bitumen loading. In the case of an ideal monodisperse emulsion, the maximum packing fraction of the dispersed phase that can be accepted prior to obtaining an “infinite” viscosity is of the order of 0.7405, which corresponds to an hexagonal close packing of spheres. This represents an upper practical limit when designing an emulsion system for transportation. Even though in reality such an emulsion would not be monodispersed (thus increasing the theoretical value of the aforementioned maximum packing value), the resulting viscosity for a, say, 0.7 bitumen fraction emulsion with 20 µm mean diameter would be very high. Such practical constraints create the need to look for alternative strategies in order to pack the maximum possible amount of dispersed phase without great increases in the resulting viscosity. In the present study, we focused our interest on unimodal emulsion with a bitumen volume fraction of 0.70 and on bimodal emulsions with a bitumen volume fraction up to 0.80. Both types of emulsions have viscosity values which allow them to be pumped and transported along pipelines. Experimental Section Emulsions of Cerro Negro bitumen (Table 1) in water stabilized with a commercial surfactant Intan-100 (nonylphenol ethoxylated having 17.5 units of ethylene oxide) were prepared with a conventional blender at 60 °C. At 30 °C this bitumen has a viscosity in excess of 100 Pa s. The surfactant concentration employed was 3000 ppm, based on the fraction of dispersed phase. The emulsions were prepared according to the HIPR method,7 which allows for the formation of highly concentrated emulsions

10.1021/la990412p CCC: $19.00 © 2000 American Chemical Society Published on Web 07/13/2000

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Table 1. Physicochemical Properties of Cerro Negro Bitumen API gravity at 60 °C saturated % (w/w) aromatics % (w/w) resins % (w/w) asphaltens % (w/w) acidity index (mg of KOH/g) carbon % (w/w) hydrogen % (w/w)

8.1 29.4 35.6 18.9 16.1 3.02 80.3 9.9

nitrogen (ppm) sulfur % (w/w) vanadium (ppm) Niquel (ppm) sodium (ppm) Conradson carbon % (w/w) water % (w/w)

6188 3.7 367.4 95.5 11.8 17.2 0.1

Figure 2. Viscosity, at 30 °C, as a function of shear rate for unimodal emulsions containing different bitumen volume fractions and a mean droplet diameter of 20 µm.

Figure 1. Typical unimodal droplet diameter distribution. with a very narrow drop diameter distribution with very low mixing energy. Model unimodal emulsions with mean drop diameter ranging between 2 and 30 µm were prepared. Bimodal emulsions were made by carefully mixing unimodal emulsions with mean drop sizes equal to or lower than 6 µm (small droplets) and with emulsion with mean diameters equal to or higher than 20 µm (large droplets) in several proportions. The total volume bitumen fractions considered in the studied emulsions were 0.70, 0.75, and 0.80. After preparation, the emulsions were stored at 25 °C and analyzed for mean drop diameter and associated distributions, using an instrument based on a light diffraction technique (Malvern, model Master Sizer). The viscosity as a function of shear rate at 30 °C was also measured using a Haake RV 20 viscometer.

Figure 3. Apparent viscosity, at 20 s-1 and 30 °C, as a function of bitumen volume fraction for unimodal emulsions having mean droplet diameters of 20 µm.

Results and Discussion Unimodal Emulsions. A typical drop diameter distribution for a unimodal emulsion is presented in Figure 1, while the rheological behavior of three unimodal emulsions containing different volume fractions of bitumen and a mean droplet diameter of approximately 20 µm is shown in Figure 2. Emulsions with bitumen fractions lower than 0.60 behave as Newtonian fluids, while those with bitumen fractions equal to or higher than 0.70 showed a non-Newtonian behavior (shear thinning). The effect of bitumen concentration on the apparent viscosity of unimodal bitumen in water emulsions measured at a shear rate of 20 s-1 and 30 °C is presented in Figure 3. For bitumen fractions higher than 0.60, the apparent viscosity increases very rapidly. This effect should be expected, since as the concentration of bitumen increases, the distance separating the droplets becomes shorter, which results in a net increase in the magnitude of the hydrodynamic interactions between droplets. On the other hand, for the case of bitumen in water emulsions stabilized by a nonionic surfactant (such as a nonylphenol ethoxylated compound, having 17 ethylene oxide units per surfactant molecule), the surfactant is adsorbed on the droplet surface through the hydrophobic portion of the molecule, while the hydrophilic chains (7) Chirinos, M. L.; Taylor, A. S.; Taylor, S. E. Preparation of HIPR Emulsions and Transportation Thereof. US Patent No. 4,934,398, 1990.

Figure 4. Interpenetration of adsorbed monolayers.

penetrate into the aqueous phase. Under these conditions, a surfactant-adsorbed monolayer with a thickness equal to δ (δ is very close to the length of the ethylene oxide chain) is formed around the bitumen droplets (Figure 4). If the bitumen droplets in the emulsion are separated by a distance (h) that is larger than 2δ+ (h > 2δ+), as in the case of emulsion containing a volume bitumen fraction lower than 0.6, there is no interaction between the ethylene oxide chains of the surfactant molecules adsorbed onto two adjacent bitumen droplets (Figure 4a). Droplets are able to move freely in the emulsion, and the viscosity is relatively low. However, when the bitumen fraction becomes equal to or larger than 0.7, the droplets approach each other to a separation distance shorter than 2δ, this is, h < 2δ. Under these conditions, there is interpenetration of the surfactant adsorbed layers (Figure 4b). This interpenetration induces the interaction between the ethylene oxide chains adsorbed on adjacent bitumen droplets, probably through hydrogen

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Figure 7. Polyhedral structure of an emulsion. Figure 5. Apparent viscosity, at 20 s-1 and 30 °C, for unimodal emulsions containing a bitumen volume fraction of 0.70 and different mean droplet diameters.

Figure 8. Disjoining pressure (π) as a function of the separation distance (h) between droplets (11).

Figure 6. Mean droplet diameter as a function of the storage time.

bonding, promoting as a consequence an increase in viscosity. On the other hand, as the separation distance between droplets decreases, the generated friction becomes stronger and stronger, which obviously promotes an increase in viscosity. As bitumen concentration increases, both phenomena, namely interpenetration of the adsorbed surfactant layers and friction, are responsible for the increase in viscosity. The more concentrated the emulsion, the higher the apparent viscosity, since the distance between droplets becomes shorter. The mean drop diameter considerably affects the apparent viscosity of bitumen in water emulsions with unimodal drop distributions. Figure 5 depicts the apparent viscosity as a function of the storage time for three different bitumen in water emulsions, containing a bitumen fraction of 0.70 and initial mean droplet diameters of 2, 20, and 30 µm. Results in Figure 5 show that, at a given bitumen concentration in the emulsion, the viscosity increases as the mean droplet diameter decreases. This effect is due to the increase in the surface areato-volume ratio as the droplet size decreases. As the interfacial area increases, the number of dispersed droplets becomes larger, and the number of interactions between ethylene oxide units is also larger. As a consequence, for a given volume fraction of bitumen, the hydrodynamic interaction between droplets is stronger and the viscosity of the emulsions increases. No changes in viscosity were observed in any of the emulsions studied during the 35 days that the experiment lasted (Figure 5). This finding suggests that there was no variation in the mean droplet diameter during this period of time. As a matter of fact, in Figure 6 the average drop diameter for each emulsion is plotted as a function of storage time. The constancy in the mean droplet diameter,

with time, indicates the absence of coalescence between droplets in all the emulsions investigated. No coalescence is an indication of good stability. When the volume fraction of dispersed phase in a monodisperse emulsion equals 0.74, the droplets can fit in a hexagonal close-packing arrangement without being deformed. When the concentration of dispersed phase exceeds this value, each droplet is deformed, and thin flat films of continuous phase are formed at each point where droplets touch (Figure 7).8 Each film is under a “compressive pressure”, which is counteracted by a “disjoining pressure” that is developed within the thin liquid film. For the emulsions discussed in this paper, droplet deformation seems to start at a bitumen fraction of 0.70, as deduced from the viscosity values shown in Figure 3. Drop deformation leads to an additional increase in interfacial area. For bitumen fractions higher than 0.70, the emulsion viscosity increases almost exponentially. This increase in viscosity can be attributed to strong attraction and friction forces, developed as the distance of separation between drops decreases. The disjoining pressure9,10 is a force normal to the droplet surfaces, which keeps them apart from each other. The magnitude of this pressure (π) is very much affected by the presence of surfactant molecules adsorbed at the bitumen/water interface and can be calculated using the expression

π ) πa + πe + πsr

(2)

where πa, πe, and πsr are the contribution to the total disjoining pressure of the van der Waals attraction forces, the double-layer repulsion forces, and the short-range attraction or repulsion forces, respectively. Figure 8 shows a typical curve obtained when the disjoining pressure is plotted as a function of the separation distance between droplets in an emulsified system.11 For (8) Princen, H. M. Highly Concentrated Emulsions. I Cylindrical Systems. J. Colloid Interface Sci. 1979, 71, 55. (9) Derjaguin, B. V.; Obukhov, E. V. Acta Physicochim. URSS 1936, 5, 1. (10) Derjaguin, B. Theory of Stability of Colloids and Thin Liquid Films; Plenum/Consultants Bureau: New York, 1989.

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a given value of the capillary pressure (Pc) in the emulsion, the equilibrium condition (Pc ) π) can be satisfied at three points, depicted in Figure 7. Point 1 corresponds to a water thin film separating the bitumen droplets, which is stabilized by the double-layer repulsion. Sometimes, this film is called common black film. Point 2 corresponds to unstable equilibrium and cannot be experimentally observed. Point 3 corresponds to a very thin liquid water film, which is stabilized by short-range repulsive forces; such a film is called the secondary or Newton black film. In our case, since the emulsions were prepared with a nonionic surfactant, double-layer repulsion forces are very weak or nonexistent; thus, the double-layer repulsion barrier tends to disappear which, in turn, means that they are stabilized by short-range steric repulsive forces. This is in point 3 in Figure 8. As the bitumen concentration in the emulsion increases with the mean drop diameter constant, the compressive pressure becomes higher and the thin liquid film separating the droplets becomes thinner, due to the drainage of the continuous phase to the Plateau borders. For an emulsion to be stable, the compressive pressure has to be balanced by the disjoining pressure (π) developed within the thin liquid film, as a result of repulsive forces between the approaching droplet surfaces. As discussed before, by increasing the concentration of bitumen in the emulsion (keeping the mean droplet diameter constant), the interfacial liquid film becomes thinner and thinner; therefore, the surfaces of the droplets are able to become closer and closer. Under these conditions, the interactions between the surfactant molecules adsorbed on adjacent bitumen droplets become stronger, and the viscosity of the emulsion increases. As the fraction of bitumen is increased further and further, a stage is reached where the disjoining pressure can no longer balance the compressive forces, and the rupture of the interfacial film occurs. Film rupture could lead to an increase in the mean droplet diameter of the emulsion (coalescence). This effect reduces the compressive pressure to a value below the maximum disjoining pressure,8 and the interfacial liquid film becomes thicker. Thus, the magnitudes of the interaction forces between surfactant molecules adsorbed on adjacent bitumen droplets are weakened, and the viscosity of the emulsion decreases. The bigger the mean droplet size, the lower the viscosity (Figure 5). If emulsified bitumen is to be transported along pipelines or in tankers, stability and rheological properties of such emulsions play a very important role. If stability and rheological properties are very much affected by the mean drop diameter, type of associated distribution, and the bitumen fraction, these parameter should be carefully controlled when unimodal emulsions are used as a vehicle for handling very viscous hydrocarbons. Stability and rheological properties can be very much improved if the emulsion systems have a bimodal drop diameter distribution. In what follows, we will discuss the behavior of this kind of emulsion. Bimodal Emulsions. Figure 9 shows a typical drop diameter distribution for a bitumen in water bimodal emulsion. This distribution is characterized by a clear differentiation between both modes. Bimodal emulsions containing bitumen fractions of 0.70, 0.75, and 0.80 as a disperse phase were prepared by mixing monomodal (11) Kralchevsky, P. A.; Danov, K. D.; Ivanov, I. V. Thin Liquid Films. In Foams: Theory and Applications; Prud’homme, R. K., Kham, S. A., Eds.; Surface Science Series Vol. 57; Marcel Dekker: New York, 1996; p 2.

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Figure 9. Typical bimodal droplet diameter distribution.

Figure 10. Viscosity, at 20 s-1 and 30 °C, as a function of the fraction of small droplets, in bimodal emulsions, containing unimodal emulsions with 2 and 20 µm, respectively.

emulsions, with mean droplet diameters of 2 and 20 µm in different proportions. Viscosities of such emulsions are presented in Figure 10. As the fraction of emulsion with a mean droplet diameter of 2 µm is increased in the mixture, the viscosity of the resulting bimodal emulsion decreases. Eventually, a minimum value in viscosity is reached. At this minimum, the fraction of emulsion with the smaller mean droplet diameter (2 µm) is around 0.3. As the fraction of the emulsion with 2 µm is further increased, the viscosity of the bimodal emulsion increases, too. The remarkable viscosity reduction shown at the minimum (around 30% fraction of small drops) in Figure 10 is probably related to a condition of minimum energy dissipation. The argument is as follows: the small drop fraction of bitumen drops in the figure is calculated with respect to the total bitumen volume. Now, both bitumen fractions can also be calculated with respect to the total emulsion volume. For the large drops, at the minimum, the “effective” bitumen fraction is 0.7 × 0.7 ) 0.49, while for the small drops the resulting value is 0.3 × 0.7 ) 0.21. With these values, the aforementioned condition of minimum energy dissipation can be explained by noting that the 0.49 figure for the large bitumen drops corresponds to simple cubic packing and represents an important reduction in dispersed phase fraction (indeed, from 0.7 to 0.49!) if it is taking into account that due to its size and fraction, the small drops virtually do not contribute to generate any friction and the area of friction (and thereby the viscosity) is almost totally controlled by the large drops. That stems from the fact that the small drops fit into the water space or “cavities” bounded by the large drops and, owing to their size, are virtually “transparent” to shear flow and hence to energy dissipation. That of

Bitumen in Water Emulsions

Figure 11. Viscosity, at 20 s-1 and 30 °C, as a function of the fraction of small droplets in bimodal emulsions, containing unimodal emulsions with 2 and 30 µm, respectively.

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Figure 13. Viscosity, at 30 °C, as a function of shear rate for emulsions with different bitumen concentrations, with unimodal and bimodal distributions.

Figure 14. Interaction between two droplets through the liquid film.

Figure 12. Viscosity, at 20 s-1 and 30 °C, for bimodal emulsions prepared under different conditions.

course has a limit, since there is a threshold in the size of the small drops beyond which they begin to generate friction. This idea is consistent with the existence of a “pseudofluid” made of a continuous phase and the small drops proposed in ref 12. Figure 12 depicts the viscosity as a function of the storage time for bimodal emulsions prepared under different conditions. In all cases, the fraction of small droplets was 0.30 and the total bitumen concentration was 70% (w/w). The viscosity of a unimodal emulsion with a mean droplet diameter of 30 µm is also included. As can be observed, the viscosity of the bimodal emulsions is very much affected by the ratio between the large mean diameter (Dl) and the small mean diameter (Ds), or Dl/Ds. The higher the ratio, the lower the viscosity of the bimodal emulsion. This is also true for bimodal emulsions containing total bitumen concentrations above 70% (w/w). It is also evident that unimodal emulsions have viscosities, which are much higher than those of the equivalent bimodal emulsions. As the bitumen fraction increases in bimodal emulsions, the viscosity also increases. However, at any bitumen concentration, the viscosity of bimodal emulsions is always several times lower than that of the equivalent unimodal emulsions (Figure 13). On the other hand, the unimodal

system behaves as a non-Newtonian fluid, while the bimodal counterpart shows Newtonian behavior. Concentrated unimodal bitumen in water emulsions are characterized for having a high bitumen fraction (>0.70). Their structure is very similar to that of the foams in that the dispersed phase exists as polyhedral cells separated by a thin film of continuous phase (Figure 7). The stability of the emulsion depends on the drainage of the continuous phase from the interfacial thin film. The capillary pressure (Pc) in the adjacent Plateau border (Figure 14) and the disjoining pressure (π) affect the drainage from films.13 The driving force for film drainage (∆P) is given by

∆P ) Pc - π

(3)

where π is positive, if the two surfaces in the film repel each other, and negative otherwise. Thus, the film is in equilibrium if ∆P ) 0, and the emulsion is stable. As bitumen concentration increases, the thin film becomes thinner, and drainage of the continuous phase to the Plateau border regions occurs. However, the emulsion remains stable as long as the disjoining pressure can be able to compensate the capillary pressure, even if the interfacial liquid film is extremely thin. The hydrophilic chains of the surfactant molecules, adsorbed on adjacent oil droplets, penetrate into the thin liquid film and interact there (Figure 14). As the liquid (12) Probstein, R. F.; Sengun, M. Z.; Tseng, T. C. J. Rheol. 1994, 38. (13) Bhakta, A.; Ruckenstein, E. Foams and Concentrated Emulsions: Dynamics and Phase Behavior. Langmuir 1995, 11, 4643.

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disappears, droplets are not longer deformed, and the distance of separation between them is large enough to avoid physical-chemical interactions. Thus, the viscosity of bimodal emulsions is significantly lower than that observed in equivalent unimodal emulsions.

Figure 15. Schematic representation of the formation of a bimodal emulsion.

film becomes thinner (by increasing the bitumen fraction), the concentration of hydrophilic chains increases in the liquid film region. In consequence, as was discussed before, the hydrophilic chains get closer and closer, and the interactions among them become stronger and stronger. These interactions directly affect the viscosity of the emulsion; thus, the stronger the interactions, the higher the viscosity. In highly concentrated bimodal emulsions (bitumen fractions above 0.7), the small droplets try to allocate themselves at the Plateau borders formed by the big droplets, where most of the continuous phase is stored. Hence, the continuous phase is pushed from the Plateau borders toward the interfacial liquid thin films, making them thicker (Figure 15). Under these conditions, the interpenetration between the adsorbed surfactant layer

Conclusions 1. The rheological behavior of bitumen in water emulsions is very much affected by the type of drop diameter distribution. 2. The viscosity of a bimodal emulsion is several times lower than the viscosity of a similar emulsion having a unimodal distribution. This effect is attributed to the fact that the viscosity is almost controlled by the large droplets, since small droplets due to its size and fraction do not contribute to generate any friction at all. 3. Viscosity in bimodal emulsion is also favored by an ideal packing, which prevents droplet deformation and inhibits strong interactions between droplets. 4. In bimodal emulsions, the viscosity depends on the relation between the mean diameters of small droplets (Ds) and big droplets (Dl). The higher the Dl/Ds, the lower the viscosity. 5. The viscosity of bimodal emulsions reaches a minimum value when the fraction of small droplets in the mixture is approximately 0.30. 6. Highly concentrated unimodal emulsions are nonNewtonian fluids, while bimodal emulsions, having bitumen concentrations up to 80%, present a Newtonian behavior. LA990412P