Effects of Droplet Size and Droplet Size Distribution on by lgm41816


									                                                                                                                No. 1998.053

Effects of Droplet Size and Droplet Size Distribution on the Rheology
of Oil-in-Water Emulsions
Rajinder Pal, University of Waterloo, Waterloo, Ontario, Canada

Abstract                                                                 important to keep the oil concentration of the emulsion as
The effects of droplet size and droplet size distribution on the         high as possible and, at the same time, maintain the emulsion
viscosity of oil-in-water emulsions were investigated experi-            viscosity at a reasonable level. This can be achieved only by
mentally. The results indicate that the effects of droplet size on       optimizing the droplet size distribution. It is believed that the
the rheology of oil-in-water emulsions become important only             more polydispersed the droplet-size distribution, the lower the
at high values of oil volume fraction (f > 0.60). For highly             emulsion viscosity for a given dispersed-phase concentration.
concentrated emulsions, a dramatic increase in the viscosity                In this paper, experimental results dealing with the effects
occurs when the droplet size is reduced. The shear-thinning              of droplet size and droplet size distribution on the viscosity of
behavior of concentrated emulsions is also enhanced upon the             oil-in-water (o/w) emulsions are presented.
reduction in droplet size. When coarse droplets are replaced
by fine droplets (keeping the oil volume fraction constant), the
resulting emulsion exhibits a minimum in viscosity at a certain
proportion of fine droplets.                                              Experimental Work
                                                                         The emulsions were prepared using a petroleum oil (trade
Introduction                                                             name EDM) supplied by Monarch Oil Company, Kitchener,
                                                                         Canada. The viscosity of this oil is approximately 6 ± 0.5
The influence of droplet size and droplet size distribution
                                                                         mPa.s at 25°C; it varies somewhat from batch to batch. Most
upon emulsion rheology is not well understood. Surprisingly,
                                                                         of the experimental work reported in the present paper was
little work has been carried out on this problem despite its
                                                                         conducted with the oil batch having the oil viscosity of 5.52
practical significance. Most previous publications on the
                                                                         mPa.s at 25°C. The water used throughout the experiments
effects of particle size and particle size distribution deal with
                                                                         was deionized. Triton X–100 was used as a surfactant for the
suspensions of solid particles.
                                                                         preparation of oil-in-water emulsions. This non-ionic surfac-
    One practical situation where droplet-size effects on emul-          tant is manufactured by Union Carbide Co.
sion rheology could be extremely important involves the pipe-
line transportation of highly viscous crude oils (such as
bitumen and heavy oils) in the form of oil-in-water (o/w)
emulsions. The pipeline transportation of highly viscous crude           Procedure
oils in the form of oil-in-water emulsions has received special          The oil-in-water emulsions were prepared by shearing
attention in recent years.1,2 The transportation of extremely            together the known amounts of oil and aqueous surfactant
viscous crude oils, such as bitumen and heavy oils, by pipe-             solution (approximately 2% by wt. Triton X–100) in a homog-
lines is difficult, especially during cold weather. To facilitate         enizer (Gifford-Wood model 1–L). In order to determine the
the flow of very viscous crude in pipelines, it is necessary to           effects of droplet size on the viscosity of emulsions, four sets
reduce its viscosity either through the installation of heating          (Set 1, Set 2, Set 3, and Set 4) of oil-in-water emulsions with
equipment at frequent intervals along the pipeline, or through           different average droplet sizes were prepared. The droplet size
the addition of a low-viscosity hydrocarbon diluent. The                 was varied from one set to another by varying the speed and
former expedient is expensive and inconvenient, and the latter           duration of shearing in the homogenizer. For any given set, a
requires the availability of relatively large amounts of cheap           highly concentrated oil-in-water emulsion (oil volume frac-
diluent. These problems, however, can be avoided if the vis-             tion approximately 0.72) was first prepared. This emulsion
cous crudes are transported as oil-in-water (o/w) emulsions.             was then diluted with different amounts of the same continu-
This concept has already been utilized commercially in a                 ous phase (Triton X–100 solution) in order to prepare emul-
pipeline 21 km in length and 20 cm in diameter in California,            sions having lower oil volume fractions. Thus, in any given set
and in a pipeline 238 km in length and 51 cm in diameter in              of emulsions, the droplet size was kept constant but the oil
Indonesia.3 For emulsion pipelining to be economical, it is              volume fraction was varied.

   To investigate the effects of droplet size distribution on the        effect on the viscosity of emulsions. Set 4 emulsion (Sauter
viscosity of emulsions, another set (Set 5) of oil-in-water              mean diameter of 4.6 (m) is much more viscous than Set 1
emulsions was prepared. At a fixed oil volume fraction of                 emulsion (Sauter mean diameter 21.4 mm). Also, Set 4 emul-
0.745, two different droplet size emulsions were prepared —              sion is much more shear-thinning than Set 1 emulsion indicat-
fine and coarse emulsions. The coarse emulsion was prepared               ing that a decrease in droplet size enhances the shear-thinning
by keeping the shearing speed of the homogenizer low. Also,              behavior of emulsions.
the duration of shearing was kept small. The fine emulsion                   Figure 8 compares the flow curves of emulsions of Sets 2
was prepared by using a high shearing speed and increased                and 3 at an oil volume fraction of about 0.69. Note that the
duration of shearing. The fine and coarse emulsions were                  Sauter mean diameters of Set 2 and Set 3 emulsions are simi-
mixed in various proportions and the rheological behavior of             lar. Set 2 has a Sauter mean diameter of 9.12 mm whereas Set
these mixed emulsions was investigated.                                  3 has a Sauter mean diameter of 8.1 mm. The flow curves of
   The rheological measurements were carried out in a Bohlin             these emulsions nearly overlap, as expected.
controlled-stress rheometer (Bohlin CS–50). The data for Sets               The flow curves of emulsions of Sets 2 and 4 are compared
1–4 were collected using the double-gap cylindrical system;              in Figure 9 at oil volume fractions of about 0.595 and 0.495,
for Set 5, the cone-and-plate measuring system was used. The             respectively. Although the Sauter mean diameters of Set 2 and
rheological data were collected at a temperature of 26 ± 1°C.            Set 4 emulsions are quite different, the viscosities are the
The droplet sizes of emulsions were determined by taking                 same at a low oil volume fraction of 0.495. At a higher oil vol-
photomicrographs. The samples were diluted with the same                 ume fraction of 0.595, the difference in the viscosities is only
continuous phase before taking the photomicrographs. The                 marginal; the finer emulsion (Set 4) exhibits slightly higher
photomicrographs were taken with a Zeiss optical microscope              viscosities. Therefore, it can be concluded that the effect of
equipped with a camera.                                                  droplet size diminishes when the dispersed-phase concentra-
                                                                         tion is decreased.
                                                                            Figure 10 shows the viscosity versus dispersed-phase (oil)
                                                                         concentration (f) plots of emulsions at a shear-stress value of
Results and Discussion                                                   0.1137 Pa. For f £ 0.60, all four sets have the same viscosities
                                                                         indicating that the droplet size has a negligible effect on the
Effects of Droplet Size
                                                                         viscosity of oil-in-water emulsions when the oil volume frac-
Figure 1 shows the typical photomicrographs of Set 1 to Set 4
                                                                         tion is lower than 0.60. The effect of droplet size becomes
emulsions. Clearly, Set 1 emulsions are much coarser (have
                                                                         important only at higher oil volume fractions (f > 0.60).
large droplets) than Set 4 emulsions. The droplet sizes of Set 2
and Set 3 emulsions fall between those of Set 1 and Set 4
emulsions. It should be noted further that the emulsions in any
given set are not monodisperse, that is, all the droplets are not        Effects of Droplet Size Distribution
of the same size. This can be seen clearly in Figure 2 which             Figure 11 shows the photomicrographs of Set 5 emulsions.
shows the droplet size distribution of the four sets of emul-            Clearly, the droplets of the fine emulsion are much smaller
sions. It is almost impossible to generate monodisperse emul-            than those of the coarse emulsion. As mentioned earlier, these
sions using a homogenizer. As the emulsions produced were                fine and coarse emulsions were mixed together in various pro-
not monodisperse, an average droplet size was determined.                portions and the rheological behavior of these mixed emul-
About 700 droplets were counted to determine the Sauter                  sions was investigated. Figure 12 shows the viscosity versus
mean diameter. The Sauter mean diameters were as follows:                shear stress plots for various mixed emulsions. When coarse
Set 1 - 21.4 mm, Set 2 - 9.12 mm, Set 3 - 8.1 mm, and Set 4 -            droplets are replaced by fine droplets (keeping f constant), the
4.6 mm.                                                                  flow curves of the resulting emulsions initially fall below the
   The viscosity versus shear stress plots for the four sets of          flow curve of the coarse emulsion, provided that the shear
emulsions are shown in Figures 3–6. At high values of (f vol-            stress is not very high. At high shear stresses, the flow curves
ume fraction of dispersed phase), emulsions are highly non-              of the mixed emulsions fall above the coarse emulsion curve
Newtonian. The flow curve (viscosity vs. shear stress plot)               for all proportions of fine emulsion. For mixed emulsions hav-
exhibits three distinct regions: the lower Newtonian region in           ing the proportion of fine emulsion greater than 65% by vol-
the low shear stress range where the viscosity is constant, the          ume, the entire flow curve (full range of shear stress) falls
shear-thinning region in the intermediate shear stress range             above that of the coarse emulsion.
where the viscosity decreases with the increase in shear stress,            The plots of viscosity versus volume fraction of fine emul-
and the upper Newtonian region at high values of shear stress            sion in the mixed emulsions are shown in Figure 13 at two dif-
where the viscosity again becomes constant. Emulsions are                ferent values of shear stress. At a low value of shear stress (t =
generally Newtonian at f values of less than about 0.55.                 0.9 Pa), the viscosity goes through a minimum. However, no
   Figure 7 compares the flow curves of emulsions of different            minimum viscosity is observed at a high shear stress of 30 Pa.
sets (i.e., different droplet sizes) at a high oil volume fraction       This indicates that the droplet size distribution exhibits a
(f) of about 0.7223. Clearly, the droplet size has a dramatic            strong influence on the emulsion viscosity, especially at low

values of shear stress. Note that the viscosity at a low stress of       Acknowledgment
0.9 Pa decreases when coarse droplets are replaced by fine                Financial support from Norsk Hydro (Norway) and the Natu-
droplets (keeping f constant) even though there occurs a                 ral Sciences and Engineering Research Council of Canada is
reduction in the average droplet size. The observed decrease             greatly appreciated.
in viscosity with the addition of fine emulsion to a coarse
emulsion is quite consistent with the observations reported for
suspensions of solid particles. Several studies on bimodal sus-
pensions of solid particles4,5,6 indicate that a minimum in the
zero-shear viscosity occurs when the fine particles account for
about 25%–35% of the total volume fraction. In our case, the             1.   Pal, R., 1993, “Pipeline Flow of Unstable and Surfactant-
minimum occurs at a fine emulsion proportion of about 36%.                     Stabilized Emulsions.” AIChE J., V. 39, pp. 1754–1764.
                                                                         2.   Rimmer, D.P., Gregoli, A.A., Hamshar, J.A. and Yildirim,
                                                                              E., 1992, “Pipeline Emulsion Transportation for Heavy
                                                                              Oils.” Adv. Chem. Ser., V. 231, pp. 294–312.
Conclusions                                                              3.   Zakin, J.L., Pinaire, R., and Borgmeyer, M.E., 1979,
Based on the experimental results, the following conclusions                  “Transportation of Oils as Oil-in-water emulsions.” J.
can be made:                                                                  Fluids Eng., V. 101, pp. 100–104.
1. The effects of droplet size on the rheology of oil-in-water           4.   Chong, J.S., Christiansen, E.B., and Baer, A.D., 1971,
    emulsions become important only at high values of dis-                    “Rheology of concentrated suspensions.” J. Appl. Poly.
    persed phase concentration (f > 0.60).                                    Sci., V. 15, pp. 2007–2020.
2. For highly concentrated oil-in-water emulsions, the                   5.   Parkinson, C., Matsumoto, S., and Sherman, P., 1970,
    reduction in droplet size results in a dramatic increase in               “The influence of particle-size distribution on the appar-
    the viscosity. Also, the shear-thinning effect becomes                    ent viscosity of non-Newtonian dispersed systems.” J.
    even stronger when the droplet size is reduced.                           Colloid & Int. Sci., V. 33, p. 150.
3. When concentrated fine and coarse oil-in-water emul-                   6.   Rodriguez, B.E., Kaler, E.W., and Wolfe, M.S., 1992,
    sions, with different droplet sizes but the same volume                   “Binary mixtures of monodisperse latex dispersions: 2.
    fraction of oil, are mixed together in varying proportions,               Viscosity.” Langmuir, V. 8, pp. 2382–2389.
    the resulting mixed emulsion exhibits a minimum in vis-
    cosity at a certain proportion of the fine droplets. How-
    ever, the minimum in viscosity occurs only at low shear
    stresses. At high stresses, the viscosity of the mixed emul-
    sion increases with the increase in the proportion of the
    fine droplets.

Figure 1: Typical Photomicrographs of Set 1 to Set 4 Emulsions

Figure 2: Droplet Size Distributions of Set 1 to Set 4 Emulsions


Figure 3: Viscosity Versus Shear Stress Plots for Set 1 Emulsions

Figure 4: Viscosity Versus Shear Stress Plots for Set 2 Emulsions

Figure 5: Viscosity Versus Shear Stress Plots for Set 3 Emulsions

Figure 6: Viscosity Versus Shear Stress Plots for Set 4 Emulsions

Figure 7: Comparison Between Flow Curves of Emulsions of Different Sets (Different Droplet Sizes)

            Figure 8: Comparison Between Flow Curves of Emulsions of Sets 2 and 3

    Figure 9: Comparison Between Flow Curves of Emulsions of Sets 2 and 4

Figure 10: Viscosity Versus Dispersed-phase Concentration (f) Plots of Emulsions

            Figure 11: Photomicrographs of Set 5 Emulsions

Figure 12: Viscosity Versus Shear Stress Data for Various Mixed Emulsions

Figure 13: Plots of Viscosity Versus Volume Fraction of Fine Emulsion in the Mixed Emulsion


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