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Solids-stabilized oil-in-water emulsions: Scavenging of emulsion droplets by fresh oil addition
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 15 (1993) 123-132 0927-7757;93/$06.00 c 1993 - Elsevier Science Publishers B.V. All rights reserved.
123
Solids-stabilized oil-in-water emulsions: scavenging of emulsion droplets by fresh oil addition Yuhua Yan’, Jacob H. Masliyah* Department of Chemical Engineering, (Received 28 July 1992; accepted
University of Alberta, Edmonton, Alta., T6G 2G6, Canada
22 February
1993)
Abstract A novel method for breaking up solids-stabilized oil-in-water emulsions was investigated on the basis of a simple concept. Kaolinite clay particles were used as the stabilizing solids, whose wettability (contact angle) was varied by controlling the amount of asphaltene adsorption on the clay particles. A light mineral oil (Bayol-35) was used as the dispersed phase. It was found that the contact angle of the treated clay particles, measured across the water phase, increased with increasing asphaltene concentration, The prepared emulsion (oil) droplet size increased significantly with increasing clay contact angle. The demulsification process was conducted in a 1 1 beaker equipped with baffles, under stirring action. The method of scavenging emulsion oil droplets by the use of large fresh oil droplets proved to be effective. The demulsification was enhanced with increasing amount of fresh oil added. The demulsification was more effective for emulsions stabilized with clays having larger contact angles. An increase in the rotational speed of the stirrer also enhanced the demulsification process. Key words: Contact
angle; Demulsification;
Oil droplet
size; Solids-stabilized
Introduction When small (colloidal) solid particles are partially oil wettable and partially water wettable, they adsorb at the oil-water interface strong mechanical film? thus stabilizing or water-in-oil
emulsions.
and form a oil-in-water
A great deal of research
has been devoted to this area since such emulsions were identified at the beginning of this century [l]. For this type of stabilization to occur, it is accepted that the solid particles have to be much smaller than the emulsion droplets [2] and the three-phase (oil-water-solids) contact angle should neither be close to 0” nor to 180” [3]. Moreover, if the solids contact angle is less than 90” (hydrophilic), water will be the continuous phase and an oil-in-water *Corresponding author. ‘Present address: Du Pont Canada Inc., Research Box 5000, Kingston, Ont. K7L 5A5, Canada.
Centre,
P.O.
emulsions
emulsion is formed. If the contact angle is greater than 90” (hydrophobic), oil will be the continuous phase and a water-in-oil emulsion is formed [4,5]. The stability of solids-stabilized emulsions is closely related to the properties of the close-packed film of interfacial particles. Such an interfacial particle film at the oil-water interface was observed and studied by Menon et al. [6-91 and Levine et al. [lO,ll]. These authors also evaluated the “effective interfacial tension” of the solids-adsorbed film both experimentally and theoretically. They concluded that the interfacial tension was reduced owing to the adsorption of the solid particles. By analogy with other types of emulsion (surfactantstabilized emulsions), this conclusion is to be expected because the adsorption of surfactant molecules at the oil-water interface also lowers the interfacial tension and stabilizes emulsions. Levine et al. [lo] also concluded that the energy needed
124
I’. Yan and J.H. MasliyahiColloids
to remove a particle from an oil-water interface is approximately IO6 times the thermal energy (kT)
Surfaces A: Physicochem.
Eng. Aspects 75 (1993)
Water
Water
oil
oil
(a)
(b)
123-132
of the particle due to Brownian motion. All other types of potential energy (i.e. that due to the electric double layer, van der Waals attraction, capillary interaction energy among the particles) are orders of magnitude smaller. Thus solids-stabilized emulsions are very stable from a thermodynamic
point
of view. Solids-stabilized emulsions are frequently encountered in the oil industry during thermal oil recovery process or bitumen extraction from oil sands. Breaking up such emulsions has been one of the major concerns for economic and environmental reasons. For example, in the final stage of oillwatersolids separation, the continuous water phase containing a small amount of emulsion oil droplets is normally discharged into tailing ponds. In order to reduce oil losses, one has to minimize the oil content in the aqueous phase by breaking up the solids-stabilized oil droplets. Centrifugation is an efficient method used in the industry to break up such emulsions; however, capital and operating costs are fairly high. Heating has also been used in demulsification, which usually involves phase change. In the present
study, we explore
a new method
of demulsifying solids-stabilized oil-in-water emulsions. This method involves the addition of fresh oil to the emulsion under stirring action. The present study deals with two types of measurement. The first part is concerned
with the oil droplet
size
as influenced by the contact angle of the clay particles, which were used as the stabilizing solids. The second part deals with the demulsification process of such emulsions due to the addition of fresh oil. Various parameters, e.g. solids contact angle, amount of fresh oil addition, stirring speed, were investigated.
The concept It is desirable to analyze the equilibrium position of a solid particle at an oil-water interface so that
Fig. 1. Equilibrium position of a solid particle interface as influenced by the contact angle.
at an oil-water
the concept of demulsification by fresh oil addition can be readily understood. Figure 1 depicts the schematics when a biwettable solid particle is adsorbed at the interface of an oil droplet. Equilibrium partitioning (force balance) analysis indicates that if the particle is hydrophilic (contact angle 8<90”), a larger volume fraction of the particle will be in the water phase (Fig. l(a)). Otherwise (8>90”) a smaller volume fraction will be in the water phase and a larger volume fraction remains in the oil droplet (Fig. l(b)). The demulsification process is essentially a process of droplet coalescence and the ultimate separation of the two phases. When no fresh oil is added to the emulsion, the droplets is not likely
coalescence of individual to occur because of the
presence of the interfacial solid particles, which prevent the oil droplets from coming into contact with each other. This is particularly true when the oil-water interface is fully covered by the particles. When fresh oil is added and a gentle stirring action is provided to the mixture, large fresh oil droplets are formed. Thus the collection of the small emulsion droplets onto the large fresh oil droplets becomes possible. Figure 2 shows the schematics of a solids-stabilized emulsion droplet having collided with a large fresh oil droplet. Owing to the biwettable nature of the stabilizing solids, they also have a tendency to adsorb at the new surface of the fresh oil droplet. As mentioned earlier, for hydrophilic particles (Q<90’), at the equilibrium
Y. Yan and J.H. MasliyahlColloids
Surfaces
A: Physicochem. Erg. Aspects 75 (1993) 123-132
Water
Water
(a) 0 < 90” Fig. 2. Schematics
125
showing
the collision
(b) 0 > 90” between a solids-stabilized
position, only a smaller volume fraction of the particle remains in the oil phase and the emulsion droplets need not contact the large fresh oil droplets to satisfy this condition. Thus the collision does not lead to a contact between the oilLoi1 interface of the two oil droplets and consequently no coalescence would occur. However, if the solids contact angle is greater than 90”, for equilibrium partitioning both the emulsion droplet and the large fresh oil droplet have a tendency to engulf volume fraction of the particle. a larger Consequently, the two oil droplets come into contact for possible coalescence. Therefore the large fresh oil droplet acts as an scavenger which engulfs the small emulsion droplet. The larger the solids contact angle, the easier the engulfing process will be. This conceptual model would thus suggest that the efficiency of engulfing the emulsion droplets increases with increasing solids contact angle for the case of solids-stabilize! oil-in-water emulsions. Since the collision probability between the emulsion droplets and the large added fresh oil droplets increases with the number of fresh oil droplets and also with the droplet velocity, the conceptual model would also indicate that the demulsification process will be enhanced with increasing amount of fresh oil addition and with increasing rotational speed of the stirrer.
oil droplet
and a large fresh oil droplet.
Experimental Asphaltene
adsorption
Kaolinite clay particles (Hydrite UF) were used as the stabilizing solids in the present study. The individual dry clay particles have a median diameter of 0.2 pm. The procedure of varying the wettability of the clay particles was similar to that described by Menon and Wasan [6]. The asphaltene was first extracted from Alberta bitumen by adding an excess amount of asphaltene-insoluble hexane. The volume ratio of hexane to bitumen was 4 : 1. The mixture of bitumen and hexane was stirred in a 1 1 beaker for 30 min and was then left undisturbed for 2 h. Subsequent asphaltene precipitation occurred at the bottom of the beaker. A known amount of the collected asphaltene was then dissolved in approximately 1 1 of toluene-nheptane (1 : 1 by volume). Kaolinite clay particles (10 g) were added to the toluene-heptane mixture containing the asphaltene and the system was stirred for 24 h. Finally, the solvents were filtered out and the clay particles were left to dry. Contact angle measurement The three-phase (oil-water-solids) of the clay particles was measured
contact angle using the com-
Y. Yun cd
126
J.11. Masliyah,‘Col/oids Surfaces A- Pkysicochem. Enx. Aspects 75 (1993)
pressed disc method. The clay particles were first compressed into a circular disc in a 1 in die at a force of 18 000 lbf. The thickness of the compressed disc was about 3 mm. The compressed clay disc was then immersed in the oil, and a drop of water was placed on the disc. The three-phase contact angle across the water phase was directly measured using a protractor eye-piece on a goniometer. Emulsion
preparation
The treated clay particles were first added to distilled water and sheared using a homogenizer (Gifford-Wood model I-LV) for 30 min at high speed to break up possible lumps. In most cases, I.5 g of clay were added to 745 g of distilled water. A light mineral oil (Bayol-35) was used as the dispersed phase. It has a viscosity of 2.4 mPa s and a density of 780 kg mm3 at 25°C. The surface tension between the oil and the air, and the oil and the distilled water is 0.029 N m-l and 0.036 N m 1 respectively. An emulsion was prepared by slowly adding a known volume of the oil to the already prepared clay in the water dispersion and by slowly shearing the mixture using the same homogenizer for 10 min. Photomicrographs were taken of the emulsion oil droplets, and subsequent size analysis was performed using an image analyzer (Buehler, Omnimet-TM). Test proceduresfor
was used to provide
the agitation,
123-132
i.e. to break up
the fresh oil into large droplets and to provide mixing for the fresh and the emulsion oil droplets. The rotational speed of the paddle stirrer could be accurately
monitored.
The blade of the stirrer had
a diameter of 4 cm. After the emulsion and the added fresh oil were subjected to mixing for a certain time interval, the mixture was poured back into the measuring cylinder, and was allowed to separate for 10 min. The height (denoted AH) of the remaining emulsion in the cylinder was then measured. A series of such tests was performed at different time intervals for one emulsion sample at a fixed rotational speed and a fixed amount of fresh oil addition. Results and discussion Variation
of clay contuct angle with usphaltene
concentration
The variation of the clay contact angle with the asphaltene concentration is shown in Fig. 3. It is evident that the contact angle increases significantly owing to the adsorption of asphaltene. Clay particles treated with asphaltene concentrations of 0.5 g l- ’ or above have contact angles greater than 90” (hydrophobic) whereas those treated with asphaltene
concentrations
of 0.2 g 1-l
or lower
demuls$kntion
The prepared oil-in-water emulsion was left to cream for about 10 min and a certain amount of water was then withdrawn from the bottom. The final volume concentration of the oil-in-water emulsion for the demulsification tests was 35.2%. The emulsion was poured into a graduated measuring cylinder and was allowed to cream. After 10 min, the height of the upper concentrated emulsion hardly changed with time and this height was recorded as the initial height of the emulsion, AH,. The demulsification process was performed in a I 1 beaker equipped with baffles. A paddle stirrer
0.0
0.5 1.0 1.5 2.0 2.5 Asphaltene Concentration (g/L)
Fig. 3. Variation concentration.
of
clay
contact
angle
with
asphaltene
Y. Yan and J.H. MasliyahlColloids Surfaces A: Physicochem. Eng. Aspects 75 (1993)
have contact angles less than 90” (hydrophilic). The contact angle measurement is consistent with the observation that for clays treated with asphaltene concentrations below 0.2 g l- ‘, they could be easily dispersed into the aqueous phase. However, for clays treated above water
0.5 g l-‘, for quite
dispersed
with asphaltene
concentrations
they floated on the top of the some time before finally being
in the water.
It is noted that in the work of Menon and Wasan [6], the contact angle of montmorillonite clay reached 140” at an asphaltene concentration of about 0.4 g l- ‘, and further increase in the asphaltene concentration caused little change in the contact angle. However, in the present study, the variation of the contact angle with asphaltene concentration is more gradual compared with that in the work of Menon and Wasan. Variation of oil droplet size with clay contact angle Figure 4 shows a photomicrograph of an emulsion oil droplet. It is to be noted that the focusing point of the microscope is not at the droplet edge, so that the adsorbed clay particles on the droplet surface can be observed. Figure 5 shows photomicrographs where the focus point is at the droplet edge, so that a more accurate size analysis could
Fig. 4. Photomicrograph clay particles.
of an emulsion
droplet
covered
by
123-132
127
be made. Only four cases are shown here, corresponding to clay contact angles of 127”, 65 ‘, 40” and 26” respectively. It can be seen from Fig. 5 that in all cases the oil droplets have a certain size distribution. It is also evident that the oil droplet size increases with increasing clay contact angle. Figure 6 shows the variation of the Sauter mean diameter of the oil droplets with clay contact angle. The droplet size increases by a factor of 6 (from about 50 to 300 pm) as the clay contact angle changes from 30” to 150”. The dependence of the emulsion droplet size on the contact angle of the stabilizing solids has not been reported previously in the literature. Moreover, the present study indicates that it is possible to form oil-in-water emulsions with solids having contact angles greater than 90” (hydrophobic). It is worth mentioning that the untreated clay (without asphaltene adsorption) did not stabilize the oil-in-water emulsions. As soon as the homogenizer shearing action stopped, the oil droplets coalesced very quickly, and within a few minutes the oil and the water phases separated completely. The clay particles remained in the aqueous phase and settled very slowly, indicating that the untreated clay particles were very hydrophilic and had a contact angle of almost 0”. In order to test the stability of the emulsions, several emulsion samples were saved and were left undisturbed for 3 weeks. Photomicrographs of the samples were taken and a size analysis of the oil droplet was performed. The droplet size was compared with its corresponding fresh sample. Table 1 shows the ratio of the Sauter mean diameter of the droplets between the aged and the fresh samples for four cases, which correspond to clay contact angles of 143”, 102”, 40” and 26’. It can be seen that in all cases the size ratio is close to unity, indicating that all emulsions are quite stable. As a matter of fact, by the time this paper is completed, the emulsion samples will have been left for several months and at this moment they still appear quite stable. Thus the effect of clay contact angle on the emulsion stability is not observed in the present case. Although it has been conventionally believed that only hydrophilic solids stabilize oil-in-water
Y. Yan curd J.H. Mn.diyuh,lColloids
128
Clay
Contact
Angle
127’
200
Fig. 5. Photomicrographs
of emulsion
pm
Surfaces A: Physicochem.
Clay
droplets
300
30
60 Contact
Fig. 6. Variation
of the oil droplet
90
120
150
180
Angle (3 (“)
of the oil droplet
size with clay contact
angle
65’
angles.
size between
Clay contact angle (deg) Droplet size of fresh sample (pm) Droplet size of aged sample (pm) Size ratio (aged:fresh)
I
0
Angle
at different clay contact
Table I Comparison old samples 350 y
Contact
Eng. Aspects 75 (IYY3) 123-132
143 254 237 0.93
the fresh and the
102 170 168 0.99
40 87 102 I.17
26 62 69 1.11
emulsions, the present results show that both hydrophobic and hydrophilic solids give stable oilin-water emulsions. However, the present results are not startling from a theoretical (thermodynamic) point of view. The energy needed to remove a solid particle with a contact angle of 80’ (hydrophilic) at an oil-water interface is not significantly different from that needed to remove a particle with a contact angle of 100’ (hydrophobic).
Y. Yan and J.H. Masliyah/Colloids Surfaces A: Physicochem. Eng. Aspects 75 (1993)
In both cases, the dislodging energy is orders of magnitude higher than the thermal energy of the particle. As stated above, the adsorbed solids at the oil-water interface act as a strong mechanical film or barrier
which prevents
the individual
droplets
123-132
129
was performed with more clay being added, for the case of a clay contact angle of 26”. It can be seen that with an increased amount of clay addition (2.5 g), the oil droplet size is only slightly reduced. The small variation in the oil droplet size with
from coalescing, thus stabilizing the oil-in-water emulsion upon the cessation of the homogenizing action. The more solids adsorbed, the less free
increasing clay amount is consistent with the experiments of Levine et al. [lo]. These authors showed that for a 50-fold increase in the amount of added solids, the oil droplet size only decreased
oil-water interface is available for possible contact between the droplets, and thus the more effective
from 100 to 75 urn. Also worth mentioning is that although the clay amount of 1.5 g is not sufficient
is the film. For spherical solids, the maximum number of particles that can exist at the oil-water interface corresponds to an ordered hexagonal close-packed arrangement. This maximum number N p,max is related to the size ratio of the droplet to the solids by the following equation [7]:
for complete coverage in some cases in the present study, one can observe that there are quite a number of clay particles remaining in the water phase rather than being completely adsorbed on the oil droplets. This observation is also in agreement with that of Levine et al. [IO] who showed that even at the lowest solids concentration where the solids were not sufficient for full coverage, about 20% of the solid particles still remained in the aqueous phase. This may be due to the
2n R, 2 N p,max “,;? C-J R, where R, and R, are the radius of the oil droplet and the solid particle respectively. The above equation is applicable when R, >>R,, which is normally satisfied. For a given size ratio of RJR,, one can calculate N p,max. In turn, by knowing the oil concentration one can calculate the amount of clay needed for this maximum packing. Table 2 shows the comparison between the amount of clay needed and the amount of clay actually added. It can be seen that for the cases with clay contact angles above 65” which corresponds to an oil droplet size above 144 urn, the amount
of clay actually
added exceeds
the amount of clay needed for maximum packing. For the cases with clay contact angles below 40” (oil droplet sizes below 87 urn), the amount of clay actually added is not enough to cover the oil droplets completely. For this reason, an experiment Table 2 Comparison
between
the amount
Clay contact angle (deg) Droplet size (pm) Clay added (g) Clay needed (g)
of clay needed for full coverage
154 313 1.5 0.64
143 254 1.5 0.78
127 183 1.5 1.08
heterogeneity of the solid particles, especially clay particles. During the adsorption treatment, some solids may become more hydrophobic or hydrophilic than others. One should be aware that both the oil droplets and the clay particles have a certain size distribution and the use of a mean diameter in the calculation for the estimation of monolayer coverage is subject to a certain error. It is also to be noted that the above calculations are based on the assumption that the solid particles are spherical. In the present case, the shape of clay particles is known to be plate like. DemulsiJication:
visual observation
Figure 7 shows photographs zones in the graduated cylinder
and the amount 102 170 1.5 1.16
actually
65 144 1.5 1.36
added for different 40 87 1.5 2.26
26 62 1.5 3.18
of the different before and after
droplet
sizes
26 57 2.5 3.49
I’. Yan and J.H.
130
Fig. 7. Photographs
showing
the various
Masliph/Colloids
Surfaces
A: Physicochent.
Eng.
Aspects 75 11993)
zones before and after demulsiiication: (a) before demulsification; (cl after demulsification with gentle stirring.
mixing with the fresh oil. In Fig. 7(a), the fresh oil, emulsion and water remain as three distinct regions before mixing. After mixing, a fourth zone, which may also be described as a polyhedral oil-water (O/W) “foam” is also created (Fig. 7(b)). This polyhedral structure is also observed in centrifugal demulsification [12]. The polyhedral zone is very unstable. Upon gentle stirring with a stirring rod, this zone shrinks to a thin layer of flocculated clay sludge in oil (Fig. 7(c)). The flocculated clay sludge in oil appears loose and further stirring does not reduce the thickness of this layer. The demulsification of the emulsions was first studied without the addition of any fresh oil, i.e. under the same stirring action in the baffled beaker as in the case of fresh oil addition. Visual observations indicated that the polyhedral zone, as observed in the case of fresh oil addition, was not present, There only exist a few visible oil droplets on the top of the emulsion owing to the coalescence of the individual emulsion droplets when subjected to stirring. The effect of the clay contact angle on the demulsification of the emulsion without fresh oil addition was not observed.
Effect offresh
123-132
(b) after demulsification;
oil addition
Figure 8 shows the demulsification results in terms of AH/AH, vs. the mixing time when fresh oil is added, where AH is the height of the remaining emulsion in the graduated cylinder and AH, is the initial height before the demulsification process.
0.0’ 0
’
10
’ 20
’ 30
I 40
’ 50
I 60
’ 70
Mixing Time (Min.) Fig. 8. Effect of the amount of fresh oil on the demulsification process (300 rev min- I: clay contact angle 127 ~‘): 0, 20 g; V, 40 g; 0, 80 g; n , 8Og.
Y. Yan and J.H.
The stabilizing
MasliyahlColloids
clay particles
SurJaces A: Physicochem.
have a contact
Eng
angle
of 127”, and the rotational speed of the stirrer is maintained at 300 rev mini ‘. It can be seen that the amount of remaining emulsion decreases with increasing mixing time. It is also evident that the demulsification is enhanced with increasing amount
of fresh oil addition.
This is to be expected
as more fresh oil droplets become available scavenge the emulsion droplets. It is important
to to
point out that the size of the unscavenged emulsion droplets remains essentially the same during the demulsification process. This conforms to the concept of engulfing as outlined above. The coalescence between individual oil droplets is a relatively slow process. Thus the main mechanism for the significant reduction in the emulsion is due to the scavenging of small emulsion droplets by large fresh oil droplets. Effect of clay contact angle The effect of clay contact angle on the demulsification process is shown in Fig. 9. The amount of added fresh oil is 40 g and the stirrer rotational speed is 200 rev min ‘. Again, it can be seen that the remaining amount of emulsion decreases very rapidly with increasing mixing time. Moreover, the demulsification process becomes more efficient
x= 0.6
c&
with
123-132
increasing
131
clay
contact
shown in Fig. 9 conform as discussed above.
angle.
The
to the engulfing
results concept
The effect of mixing speed (rev mini ‘) on the demulsification process is shown in Fig. 10. As expected, the increase in the rotational speed of the stirrer enhances the collision rate and consequently
increases
the demulsification
efficiency.
It
should be pointed out, however, that the rotational speed of the stirrer cannot be too high. At very high stirring speed, the process becomes that of emulsification whereby the fresh oil is emulsified. Conclusions It was found in the present study that the clay contact angle was influenced substantially by the adsorption of bitumen asphaltene. All the treated clay particles, whether hydrophilic or hydrophobic, gave stable oil-in-water emulsions. The oil droplet size increased from 50 to about 300 urn as the clay contact angle increased from 30” to 150”. It proved effective to scavenge (engulf) emulsion oil droplets by the use of large fresh oil droplets. The appearances of the various zones after demulsification were similar to those observed in conventional centrifugal demulsification. The demulsification process was more effective when
0
9 S
4spects 75 (1993)
0.4
A
0.2
0.0 U
10
20
30
40
50
60
70
Mixing Time (Min.) Fig. 9. Elkct of clay contact angle on the demulsification fresh oil 40g): 0, 127-; A, 143”; process (200 rev min-‘; H, 154”.
0
10
20
30
40
50
6U
_-I 70
Mixing Time (Min.) Fig. 10. Effect of stirring speed on the demulsification process (fresh oil 80 g; clay contact angle 127”): 0, 200 rev min-‘; V, 300 rev min-‘.
Y. Yun and J.H. Musliyah/Colloids
132
Surfaces A: Physicochem.
V.B. Menon, R. Nagarajan and D.T. Wasan, Sep. Sci. Technol., 22 (1987) 2295. V.B. Menon, A.D. Nikolov and D.T. Wasan, J. Colloid Interface Sci., 124 (1988) 317. V.B. Menon and D.T. Wasan, Colloids Surfaces, 29 (1988) 7. S. Levine, B. Bowen and S.J. Partridge, Colloids Surfaces, 38 (1989) 325. S. Levine, B. Bowen and S.J. Partridge, Colloids Surfaces, 38 (1989) 345. B.J. Carroll, Surf. Colloid Sci., 9 (1976) I.
the stabilizing clay particles had a higher contact angle. Also, an increase in the amount of fresh oil addition and in the rotational speed of the stirrer enhanced
the demulsification
process.
Acknowledgments The research has been financially the Alberta Oil Sands Technology
supported by and Research
Authority and the Natural Science and Engineering Research Council of Canada.
References S.U. Pickering, J. Chem. Sot., 91 (1907) 2001. J.A. Kitchener and P.R. Musselwhite, in P. Sherman (Ed.), Emulsion Science, Academic Press, London, 1968, Chapter 2. S. Levine and E. Sanford, Can. J. Chem. Eng., 62 (1985) 258. P. Finkle, H.D. Draper and J.H. Hildebrand, J. Am. Chem. Sot., 45 (1923) 2780. J.H. Schulman and J. Leja, Trans. Faraday Sot., 50 (1954) 598. V.B. Menon and D.T. Wasan, Colloids Surfaces, I9 (1986) 89.
Eng. Aspects 75 (1993) 123-132
Appendix: Nomenclature k
Boltzmann constant (J Km ‘) N p,max maximum number of particles onto an oil droplet radius of oil droplets (pm) R0 radius of solid particles (pm) 4 T absolute temperature (K)
adsorbed
Greek letters 0
AH AH,
contact angle of clay particles height of the remaining emulsion in the graduated cylinder (cm) initial height of emulsion before demulsification (cm)
123
Solids-stabilized oil-in-water emulsions: scavenging of emulsion droplets by fresh oil addition Yuhua Yan’, Jacob H. Masliyah* Department of Chemical Engineering, (Received 28 July 1992; accepted
University of Alberta, Edmonton, Alta., T6G 2G6, Canada
22 February
1993)
Abstract A novel method for breaking up solids-stabilized oil-in-water emulsions was investigated on the basis of a simple concept. Kaolinite clay particles were used as the stabilizing solids, whose wettability (contact angle) was varied by controlling the amount of asphaltene adsorption on the clay particles. A light mineral oil (Bayol-35) was used as the dispersed phase. It was found that the contact angle of the treated clay particles, measured across the water phase, increased with increasing asphaltene concentration, The prepared emulsion (oil) droplet size increased significantly with increasing clay contact angle. The demulsification process was conducted in a 1 1 beaker equipped with baffles, under stirring action. The method of scavenging emulsion oil droplets by the use of large fresh oil droplets proved to be effective. The demulsification was enhanced with increasing amount of fresh oil added. The demulsification was more effective for emulsions stabilized with clays having larger contact angles. An increase in the rotational speed of the stirrer also enhanced the demulsification process. Key words: Contact
angle; Demulsification;
Oil droplet
size; Solids-stabilized
Introduction When small (colloidal) solid particles are partially oil wettable and partially water wettable, they adsorb at the oil-water interface strong mechanical film? thus stabilizing or water-in-oil
emulsions.
and form a oil-in-water
A great deal of research
has been devoted to this area since such emulsions were identified at the beginning of this century [l]. For this type of stabilization to occur, it is accepted that the solid particles have to be much smaller than the emulsion droplets [2] and the three-phase (oil-water-solids) contact angle should neither be close to 0” nor to 180” [3]. Moreover, if the solids contact angle is less than 90” (hydrophilic), water will be the continuous phase and an oil-in-water *Corresponding author. ‘Present address: Du Pont Canada Inc., Research Box 5000, Kingston, Ont. K7L 5A5, Canada.
Centre,
P.O.
emulsions
emulsion is formed. If the contact angle is greater than 90” (hydrophobic), oil will be the continuous phase and a water-in-oil emulsion is formed [4,5]. The stability of solids-stabilized emulsions is closely related to the properties of the close-packed film of interfacial particles. Such an interfacial particle film at the oil-water interface was observed and studied by Menon et al. [6-91 and Levine et al. [lO,ll]. These authors also evaluated the “effective interfacial tension” of the solids-adsorbed film both experimentally and theoretically. They concluded that the interfacial tension was reduced owing to the adsorption of the solid particles. By analogy with other types of emulsion (surfactantstabilized emulsions), this conclusion is to be expected because the adsorption of surfactant molecules at the oil-water interface also lowers the interfacial tension and stabilizes emulsions. Levine et al. [lo] also concluded that the energy needed
124
I’. Yan and J.H. MasliyahiColloids
to remove a particle from an oil-water interface is approximately IO6 times the thermal energy (kT)
Surfaces A: Physicochem.
Eng. Aspects 75 (1993)
Water
Water
oil
oil
(a)
(b)
123-132
of the particle due to Brownian motion. All other types of potential energy (i.e. that due to the electric double layer, van der Waals attraction, capillary interaction energy among the particles) are orders of magnitude smaller. Thus solids-stabilized emulsions are very stable from a thermodynamic
point
of view. Solids-stabilized emulsions are frequently encountered in the oil industry during thermal oil recovery process or bitumen extraction from oil sands. Breaking up such emulsions has been one of the major concerns for economic and environmental reasons. For example, in the final stage of oillwatersolids separation, the continuous water phase containing a small amount of emulsion oil droplets is normally discharged into tailing ponds. In order to reduce oil losses, one has to minimize the oil content in the aqueous phase by breaking up the solids-stabilized oil droplets. Centrifugation is an efficient method used in the industry to break up such emulsions; however, capital and operating costs are fairly high. Heating has also been used in demulsification, which usually involves phase change. In the present
study, we explore
a new method
of demulsifying solids-stabilized oil-in-water emulsions. This method involves the addition of fresh oil to the emulsion under stirring action. The present study deals with two types of measurement. The first part is concerned
with the oil droplet
size
as influenced by the contact angle of the clay particles, which were used as the stabilizing solids. The second part deals with the demulsification process of such emulsions due to the addition of fresh oil. Various parameters, e.g. solids contact angle, amount of fresh oil addition, stirring speed, were investigated.
The concept It is desirable to analyze the equilibrium position of a solid particle at an oil-water interface so that
Fig. 1. Equilibrium position of a solid particle interface as influenced by the contact angle.
at an oil-water
the concept of demulsification by fresh oil addition can be readily understood. Figure 1 depicts the schematics when a biwettable solid particle is adsorbed at the interface of an oil droplet. Equilibrium partitioning (force balance) analysis indicates that if the particle is hydrophilic (contact angle 8<90”), a larger volume fraction of the particle will be in the water phase (Fig. l(a)). Otherwise (8>90”) a smaller volume fraction will be in the water phase and a larger volume fraction remains in the oil droplet (Fig. l(b)). The demulsification process is essentially a process of droplet coalescence and the ultimate separation of the two phases. When no fresh oil is added to the emulsion, the droplets is not likely
coalescence of individual to occur because of the
presence of the interfacial solid particles, which prevent the oil droplets from coming into contact with each other. This is particularly true when the oil-water interface is fully covered by the particles. When fresh oil is added and a gentle stirring action is provided to the mixture, large fresh oil droplets are formed. Thus the collection of the small emulsion droplets onto the large fresh oil droplets becomes possible. Figure 2 shows the schematics of a solids-stabilized emulsion droplet having collided with a large fresh oil droplet. Owing to the biwettable nature of the stabilizing solids, they also have a tendency to adsorb at the new surface of the fresh oil droplet. As mentioned earlier, for hydrophilic particles (Q<90’), at the equilibrium
Y. Yan and J.H. MasliyahlColloids
Surfaces
A: Physicochem. Erg. Aspects 75 (1993) 123-132
Water
Water
(a) 0 < 90” Fig. 2. Schematics
125
showing
the collision
(b) 0 > 90” between a solids-stabilized
position, only a smaller volume fraction of the particle remains in the oil phase and the emulsion droplets need not contact the large fresh oil droplets to satisfy this condition. Thus the collision does not lead to a contact between the oilLoi1 interface of the two oil droplets and consequently no coalescence would occur. However, if the solids contact angle is greater than 90”, for equilibrium partitioning both the emulsion droplet and the large fresh oil droplet have a tendency to engulf volume fraction of the particle. a larger Consequently, the two oil droplets come into contact for possible coalescence. Therefore the large fresh oil droplet acts as an scavenger which engulfs the small emulsion droplet. The larger the solids contact angle, the easier the engulfing process will be. This conceptual model would thus suggest that the efficiency of engulfing the emulsion droplets increases with increasing solids contact angle for the case of solids-stabilize! oil-in-water emulsions. Since the collision probability between the emulsion droplets and the large added fresh oil droplets increases with the number of fresh oil droplets and also with the droplet velocity, the conceptual model would also indicate that the demulsification process will be enhanced with increasing amount of fresh oil addition and with increasing rotational speed of the stirrer.
oil droplet
and a large fresh oil droplet.
Experimental Asphaltene
adsorption
Kaolinite clay particles (Hydrite UF) were used as the stabilizing solids in the present study. The individual dry clay particles have a median diameter of 0.2 pm. The procedure of varying the wettability of the clay particles was similar to that described by Menon and Wasan [6]. The asphaltene was first extracted from Alberta bitumen by adding an excess amount of asphaltene-insoluble hexane. The volume ratio of hexane to bitumen was 4 : 1. The mixture of bitumen and hexane was stirred in a 1 1 beaker for 30 min and was then left undisturbed for 2 h. Subsequent asphaltene precipitation occurred at the bottom of the beaker. A known amount of the collected asphaltene was then dissolved in approximately 1 1 of toluene-nheptane (1 : 1 by volume). Kaolinite clay particles (10 g) were added to the toluene-heptane mixture containing the asphaltene and the system was stirred for 24 h. Finally, the solvents were filtered out and the clay particles were left to dry. Contact angle measurement The three-phase (oil-water-solids) of the clay particles was measured
contact angle using the com-
Y. Yun cd
126
J.11. Masliyah,‘Col/oids Surfaces A- Pkysicochem. Enx. Aspects 75 (1993)
pressed disc method. The clay particles were first compressed into a circular disc in a 1 in die at a force of 18 000 lbf. The thickness of the compressed disc was about 3 mm. The compressed clay disc was then immersed in the oil, and a drop of water was placed on the disc. The three-phase contact angle across the water phase was directly measured using a protractor eye-piece on a goniometer. Emulsion
preparation
The treated clay particles were first added to distilled water and sheared using a homogenizer (Gifford-Wood model I-LV) for 30 min at high speed to break up possible lumps. In most cases, I.5 g of clay were added to 745 g of distilled water. A light mineral oil (Bayol-35) was used as the dispersed phase. It has a viscosity of 2.4 mPa s and a density of 780 kg mm3 at 25°C. The surface tension between the oil and the air, and the oil and the distilled water is 0.029 N m-l and 0.036 N m 1 respectively. An emulsion was prepared by slowly adding a known volume of the oil to the already prepared clay in the water dispersion and by slowly shearing the mixture using the same homogenizer for 10 min. Photomicrographs were taken of the emulsion oil droplets, and subsequent size analysis was performed using an image analyzer (Buehler, Omnimet-TM). Test proceduresfor
was used to provide
the agitation,
123-132
i.e. to break up
the fresh oil into large droplets and to provide mixing for the fresh and the emulsion oil droplets. The rotational speed of the paddle stirrer could be accurately
monitored.
The blade of the stirrer had
a diameter of 4 cm. After the emulsion and the added fresh oil were subjected to mixing for a certain time interval, the mixture was poured back into the measuring cylinder, and was allowed to separate for 10 min. The height (denoted AH) of the remaining emulsion in the cylinder was then measured. A series of such tests was performed at different time intervals for one emulsion sample at a fixed rotational speed and a fixed amount of fresh oil addition. Results and discussion Variation
of clay contuct angle with usphaltene
concentration
The variation of the clay contact angle with the asphaltene concentration is shown in Fig. 3. It is evident that the contact angle increases significantly owing to the adsorption of asphaltene. Clay particles treated with asphaltene concentrations of 0.5 g l- ’ or above have contact angles greater than 90” (hydrophobic) whereas those treated with asphaltene
concentrations
of 0.2 g 1-l
or lower
demuls$kntion
The prepared oil-in-water emulsion was left to cream for about 10 min and a certain amount of water was then withdrawn from the bottom. The final volume concentration of the oil-in-water emulsion for the demulsification tests was 35.2%. The emulsion was poured into a graduated measuring cylinder and was allowed to cream. After 10 min, the height of the upper concentrated emulsion hardly changed with time and this height was recorded as the initial height of the emulsion, AH,. The demulsification process was performed in a I 1 beaker equipped with baffles. A paddle stirrer
0.0
0.5 1.0 1.5 2.0 2.5 Asphaltene Concentration (g/L)
Fig. 3. Variation concentration.
of
clay
contact
angle
with
asphaltene
Y. Yan and J.H. MasliyahlColloids Surfaces A: Physicochem. Eng. Aspects 75 (1993)
have contact angles less than 90” (hydrophilic). The contact angle measurement is consistent with the observation that for clays treated with asphaltene concentrations below 0.2 g l- ‘, they could be easily dispersed into the aqueous phase. However, for clays treated above water
0.5 g l-‘, for quite
dispersed
with asphaltene
concentrations
they floated on the top of the some time before finally being
in the water.
It is noted that in the work of Menon and Wasan [6], the contact angle of montmorillonite clay reached 140” at an asphaltene concentration of about 0.4 g l- ‘, and further increase in the asphaltene concentration caused little change in the contact angle. However, in the present study, the variation of the contact angle with asphaltene concentration is more gradual compared with that in the work of Menon and Wasan. Variation of oil droplet size with clay contact angle Figure 4 shows a photomicrograph of an emulsion oil droplet. It is to be noted that the focusing point of the microscope is not at the droplet edge, so that the adsorbed clay particles on the droplet surface can be observed. Figure 5 shows photomicrographs where the focus point is at the droplet edge, so that a more accurate size analysis could
Fig. 4. Photomicrograph clay particles.
of an emulsion
droplet
covered
by
123-132
127
be made. Only four cases are shown here, corresponding to clay contact angles of 127”, 65 ‘, 40” and 26” respectively. It can be seen from Fig. 5 that in all cases the oil droplets have a certain size distribution. It is also evident that the oil droplet size increases with increasing clay contact angle. Figure 6 shows the variation of the Sauter mean diameter of the oil droplets with clay contact angle. The droplet size increases by a factor of 6 (from about 50 to 300 pm) as the clay contact angle changes from 30” to 150”. The dependence of the emulsion droplet size on the contact angle of the stabilizing solids has not been reported previously in the literature. Moreover, the present study indicates that it is possible to form oil-in-water emulsions with solids having contact angles greater than 90” (hydrophobic). It is worth mentioning that the untreated clay (without asphaltene adsorption) did not stabilize the oil-in-water emulsions. As soon as the homogenizer shearing action stopped, the oil droplets coalesced very quickly, and within a few minutes the oil and the water phases separated completely. The clay particles remained in the aqueous phase and settled very slowly, indicating that the untreated clay particles were very hydrophilic and had a contact angle of almost 0”. In order to test the stability of the emulsions, several emulsion samples were saved and were left undisturbed for 3 weeks. Photomicrographs of the samples were taken and a size analysis of the oil droplet was performed. The droplet size was compared with its corresponding fresh sample. Table 1 shows the ratio of the Sauter mean diameter of the droplets between the aged and the fresh samples for four cases, which correspond to clay contact angles of 143”, 102”, 40” and 26’. It can be seen that in all cases the size ratio is close to unity, indicating that all emulsions are quite stable. As a matter of fact, by the time this paper is completed, the emulsion samples will have been left for several months and at this moment they still appear quite stable. Thus the effect of clay contact angle on the emulsion stability is not observed in the present case. Although it has been conventionally believed that only hydrophilic solids stabilize oil-in-water
Y. Yan curd J.H. Mn.diyuh,lColloids
128
Clay
Contact
Angle
127’
200
Fig. 5. Photomicrographs
of emulsion
pm
Surfaces A: Physicochem.
Clay
droplets
300
30
60 Contact
Fig. 6. Variation
of the oil droplet
90
120
150
180
Angle (3 (“)
of the oil droplet
size with clay contact
angle
65’
angles.
size between
Clay contact angle (deg) Droplet size of fresh sample (pm) Droplet size of aged sample (pm) Size ratio (aged:fresh)
I
0
Angle
at different clay contact
Table I Comparison old samples 350 y
Contact
Eng. Aspects 75 (IYY3) 123-132
143 254 237 0.93
the fresh and the
102 170 168 0.99
40 87 102 I.17
26 62 69 1.11
emulsions, the present results show that both hydrophobic and hydrophilic solids give stable oilin-water emulsions. However, the present results are not startling from a theoretical (thermodynamic) point of view. The energy needed to remove a solid particle with a contact angle of 80’ (hydrophilic) at an oil-water interface is not significantly different from that needed to remove a particle with a contact angle of 100’ (hydrophobic).
Y. Yan and J.H. Masliyah/Colloids Surfaces A: Physicochem. Eng. Aspects 75 (1993)
In both cases, the dislodging energy is orders of magnitude higher than the thermal energy of the particle. As stated above, the adsorbed solids at the oil-water interface act as a strong mechanical film or barrier
which prevents
the individual
droplets
123-132
129
was performed with more clay being added, for the case of a clay contact angle of 26”. It can be seen that with an increased amount of clay addition (2.5 g), the oil droplet size is only slightly reduced. The small variation in the oil droplet size with
from coalescing, thus stabilizing the oil-in-water emulsion upon the cessation of the homogenizing action. The more solids adsorbed, the less free
increasing clay amount is consistent with the experiments of Levine et al. [lo]. These authors showed that for a 50-fold increase in the amount of added solids, the oil droplet size only decreased
oil-water interface is available for possible contact between the droplets, and thus the more effective
from 100 to 75 urn. Also worth mentioning is that although the clay amount of 1.5 g is not sufficient
is the film. For spherical solids, the maximum number of particles that can exist at the oil-water interface corresponds to an ordered hexagonal close-packed arrangement. This maximum number N p,max is related to the size ratio of the droplet to the solids by the following equation [7]:
for complete coverage in some cases in the present study, one can observe that there are quite a number of clay particles remaining in the water phase rather than being completely adsorbed on the oil droplets. This observation is also in agreement with that of Levine et al. [IO] who showed that even at the lowest solids concentration where the solids were not sufficient for full coverage, about 20% of the solid particles still remained in the aqueous phase. This may be due to the
2n R, 2 N p,max “,;? C-J R, where R, and R, are the radius of the oil droplet and the solid particle respectively. The above equation is applicable when R, >>R,, which is normally satisfied. For a given size ratio of RJR,, one can calculate N p,max. In turn, by knowing the oil concentration one can calculate the amount of clay needed for this maximum packing. Table 2 shows the comparison between the amount of clay needed and the amount of clay actually added. It can be seen that for the cases with clay contact angles above 65” which corresponds to an oil droplet size above 144 urn, the amount
of clay actually
added exceeds
the amount of clay needed for maximum packing. For the cases with clay contact angles below 40” (oil droplet sizes below 87 urn), the amount of clay actually added is not enough to cover the oil droplets completely. For this reason, an experiment Table 2 Comparison
between
the amount
Clay contact angle (deg) Droplet size (pm) Clay added (g) Clay needed (g)
of clay needed for full coverage
154 313 1.5 0.64
143 254 1.5 0.78
127 183 1.5 1.08
heterogeneity of the solid particles, especially clay particles. During the adsorption treatment, some solids may become more hydrophobic or hydrophilic than others. One should be aware that both the oil droplets and the clay particles have a certain size distribution and the use of a mean diameter in the calculation for the estimation of monolayer coverage is subject to a certain error. It is also to be noted that the above calculations are based on the assumption that the solid particles are spherical. In the present case, the shape of clay particles is known to be plate like. DemulsiJication:
visual observation
Figure 7 shows photographs zones in the graduated cylinder
and the amount 102 170 1.5 1.16
actually
65 144 1.5 1.36
added for different 40 87 1.5 2.26
26 62 1.5 3.18
of the different before and after
droplet
sizes
26 57 2.5 3.49
I’. Yan and J.H.
130
Fig. 7. Photographs
showing
the various
Masliph/Colloids
Surfaces
A: Physicochent.
Eng.
Aspects 75 11993)
zones before and after demulsiiication: (a) before demulsification; (cl after demulsification with gentle stirring.
mixing with the fresh oil. In Fig. 7(a), the fresh oil, emulsion and water remain as three distinct regions before mixing. After mixing, a fourth zone, which may also be described as a polyhedral oil-water (O/W) “foam” is also created (Fig. 7(b)). This polyhedral structure is also observed in centrifugal demulsification [12]. The polyhedral zone is very unstable. Upon gentle stirring with a stirring rod, this zone shrinks to a thin layer of flocculated clay sludge in oil (Fig. 7(c)). The flocculated clay sludge in oil appears loose and further stirring does not reduce the thickness of this layer. The demulsification of the emulsions was first studied without the addition of any fresh oil, i.e. under the same stirring action in the baffled beaker as in the case of fresh oil addition. Visual observations indicated that the polyhedral zone, as observed in the case of fresh oil addition, was not present, There only exist a few visible oil droplets on the top of the emulsion owing to the coalescence of the individual emulsion droplets when subjected to stirring. The effect of the clay contact angle on the demulsification of the emulsion without fresh oil addition was not observed.
Effect offresh
123-132
(b) after demulsification;
oil addition
Figure 8 shows the demulsification results in terms of AH/AH, vs. the mixing time when fresh oil is added, where AH is the height of the remaining emulsion in the graduated cylinder and AH, is the initial height before the demulsification process.
0.0’ 0
’
10
’ 20
’ 30
I 40
’ 50
I 60
’ 70
Mixing Time (Min.) Fig. 8. Effect of the amount of fresh oil on the demulsification process (300 rev min- I: clay contact angle 127 ~‘): 0, 20 g; V, 40 g; 0, 80 g; n , 8Og.
Y. Yan and J.H.
The stabilizing
MasliyahlColloids
clay particles
SurJaces A: Physicochem.
have a contact
Eng
angle
of 127”, and the rotational speed of the stirrer is maintained at 300 rev mini ‘. It can be seen that the amount of remaining emulsion decreases with increasing mixing time. It is also evident that the demulsification is enhanced with increasing amount
of fresh oil addition.
This is to be expected
as more fresh oil droplets become available scavenge the emulsion droplets. It is important
to to
point out that the size of the unscavenged emulsion droplets remains essentially the same during the demulsification process. This conforms to the concept of engulfing as outlined above. The coalescence between individual oil droplets is a relatively slow process. Thus the main mechanism for the significant reduction in the emulsion is due to the scavenging of small emulsion droplets by large fresh oil droplets. Effect of clay contact angle The effect of clay contact angle on the demulsification process is shown in Fig. 9. The amount of added fresh oil is 40 g and the stirrer rotational speed is 200 rev min ‘. Again, it can be seen that the remaining amount of emulsion decreases very rapidly with increasing mixing time. Moreover, the demulsification process becomes more efficient
x= 0.6
c&
with
123-132
increasing
131
clay
contact
shown in Fig. 9 conform as discussed above.
angle.
The
to the engulfing
results concept
The effect of mixing speed (rev mini ‘) on the demulsification process is shown in Fig. 10. As expected, the increase in the rotational speed of the stirrer enhances the collision rate and consequently
increases
the demulsification
efficiency.
It
should be pointed out, however, that the rotational speed of the stirrer cannot be too high. At very high stirring speed, the process becomes that of emulsification whereby the fresh oil is emulsified. Conclusions It was found in the present study that the clay contact angle was influenced substantially by the adsorption of bitumen asphaltene. All the treated clay particles, whether hydrophilic or hydrophobic, gave stable oil-in-water emulsions. The oil droplet size increased from 50 to about 300 urn as the clay contact angle increased from 30” to 150”. It proved effective to scavenge (engulf) emulsion oil droplets by the use of large fresh oil droplets. The appearances of the various zones after demulsification were similar to those observed in conventional centrifugal demulsification. The demulsification process was more effective when
0
9 S
4spects 75 (1993)
0.4
A
0.2
0.0 U
10
20
30
40
50
60
70
Mixing Time (Min.) Fig. 9. Elkct of clay contact angle on the demulsification fresh oil 40g): 0, 127-; A, 143”; process (200 rev min-‘; H, 154”.
0
10
20
30
40
50
6U
_-I 70
Mixing Time (Min.) Fig. 10. Effect of stirring speed on the demulsification process (fresh oil 80 g; clay contact angle 127”): 0, 200 rev min-‘; V, 300 rev min-‘.
Y. Yun and J.H. Musliyah/Colloids
132
Surfaces A: Physicochem.
V.B. Menon, R. Nagarajan and D.T. Wasan, Sep. Sci. Technol., 22 (1987) 2295. V.B. Menon, A.D. Nikolov and D.T. Wasan, J. Colloid Interface Sci., 124 (1988) 317. V.B. Menon and D.T. Wasan, Colloids Surfaces, 29 (1988) 7. S. Levine, B. Bowen and S.J. Partridge, Colloids Surfaces, 38 (1989) 325. S. Levine, B. Bowen and S.J. Partridge, Colloids Surfaces, 38 (1989) 345. B.J. Carroll, Surf. Colloid Sci., 9 (1976) I.
the stabilizing clay particles had a higher contact angle. Also, an increase in the amount of fresh oil addition and in the rotational speed of the stirrer enhanced
the demulsification
process.
Acknowledgments The research has been financially the Alberta Oil Sands Technology
supported by and Research
Authority and the Natural Science and Engineering Research Council of Canada.
References S.U. Pickering, J. Chem. Sot., 91 (1907) 2001. J.A. Kitchener and P.R. Musselwhite, in P. Sherman (Ed.), Emulsion Science, Academic Press, London, 1968, Chapter 2. S. Levine and E. Sanford, Can. J. Chem. Eng., 62 (1985) 258. P. Finkle, H.D. Draper and J.H. Hildebrand, J. Am. Chem. Sot., 45 (1923) 2780. J.H. Schulman and J. Leja, Trans. Faraday Sot., 50 (1954) 598. V.B. Menon and D.T. Wasan, Colloids Surfaces, I9 (1986) 89.
Eng. Aspects 75 (1993) 123-132
Appendix: Nomenclature k
Boltzmann constant (J Km ‘) N p,max maximum number of particles onto an oil droplet radius of oil droplets (pm) R0 radius of solid particles (pm) 4 T absolute temperature (K)
adsorbed
Greek letters 0
AH AH,
contact angle of clay particles height of the remaining emulsion in the graduated cylinder (cm) initial height of emulsion before demulsification (cm)