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water emulsion
Colloids and Surfaces A: Physicochem. Eng. Aspects 215 (2003) 141 /153 www.elsevier.com/locate/colsurfa
Role of fine kaolinite clay in toluene-diluted bitumen/water emulsion Guoxing Gu, Zhiang Zhou, Zhenghe Xu, Jacob H. Masliyah * Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alta, Canada T6G 2G6 Received 7 December 2000; accepted 12 September 2002
Abstract The role of suspension pH and added fine kaolinite clays in the emulsification of a toluene-diluted bitumen/water system was investigated. Within five distinct pH ranges of 2.9 /3.7, 4.1 /5.3, 7.4 /7.9, 8.8 /9.4 and 10.5 /10.8, mixing toluene-diluted bitumen with de-ionized water at a water:diluted bitumen volume ratio of 4:1 resulted in a stable waterin-oil (W/O) emulsion. Over these pH ranges, the mixture was completely emulsified to a homogeneous, gel-like creamed phase. However, a creamed layer of varying volume was separated on the top of an aqueous phase when mixed outside these pH ranges. The creamed layer was found to be mainly an oil-in-water (O/W) emulsion. This zigzag type of change in emulsion-type inversion over these narrow pH ranges is a unique feature of the current system. The removal of the original fine solids in bitumen prior to emulsification resulted in a constant volume of O/W emulsion separated on the top of the aqueous phase over the pH range 1.5 /11 studied. The addition of kaolinite clays to the emulsifying water was found to suppress the original zigzag type emulsification characteristics. Consequently, the volume fraction of the emulsion reduced significantly with a corresponding reduction of water content in the emulsion. For a suspension of pH B/3 and clay concentration at 4 g l 1, /98% of the added clay was retained in the emulsion. At pH /3, on the other hand, the majority of the added clay remained in the separated aqueous phase. Measurement of water contact angle (u ) on the collected clay showed that the added clay, originally being hydrophilic (u :/48), was rendered hydrophobic (u /1208) at pH B/3, while remained hydrophilic (u :/208) at pH /3. Release of natural surfactants from bitumen into the aqueous phase was confirmed by the conductivity, surface tension and contact angle measurements. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Toluene-diluted bitumen; Kaolinite clay; Emulsion; Volume fraction; Clay partitioning
1. Introduction * Corresponding author. Tel.: /1-780-492-4673; fax: /1780-492-2881 E-mail address: [email protected] (J.H. Masliyah).
The oil sands deposits in Northern Alberta, Canada, are complex mixtures of sand grains, fine clays, bitumen and water. To extract bitumen from
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Athabasca oil sands, both Syncrude Canada Ltd. and Suncor Energy Corporation use a variation of the Clark Hot Water Extraction process. In this process, mined oil sands are digested in a tumbler or in a hydrotransport pipeline with the addition of hot water (50 /80 8C) and small quantities of sodium hydroxide (caustic) to facilitate bitumen liberation from oil sand grains. The detached bitumen droplets are recovered by attaching to or engulfing air bubbles followed by flotation in a gravity separation vessel. The recovered bitumen froth normally consists of 60% bitumen, 30% water and 10% solids [1]. Prior to bitumen upgrading, the solids and water are removed by centrifuging the diluted bitumen froth, known as the froth treatment. In the upcoming low energy extraction (LEE) process, bitumen is to be extracted at an operating temperature as low as 25 8C, resulting in substantially lower energy and capital costs [2]. However, in the LEE process, an increased amount of solids and water in bitumen froth is anticipated, posing an additional challenge to satisfactory removal of micro-emulsified water droplets during the froth treatment. However, the removal of micro-emulsified water in oil droplets to below 0.5% is essential as the chlorines entrained in water would cause unpredicted, yet catastrophic blow-out to up-grading units. To achieve a satisfactory removal of emulsified water from the diluted bitumen, it is essential to understand the stabilization mechanisms in diluted bitumen systems. In addition to the natural surfactants in bitumen, fine clays, such as kaolinite, illite, chlorite and montmorillonite, are known to be present in oil sands with kaolinite being the most abundant [3]. Although the amount of clay minerals represents only a small portion of the total solids, their presence has been recognized to be detrimental to the oil sand ore processibility by either the hot-water process or in-situ production [4]. Extensive studies have been conducted to investigate fine solids-stabilized emulsions [5 /9]. A general observation is that solids with contact angle B/908 tend to stabilize oil-in-water (O/W) emulsions, while those with contact angle /908 favor stabilizing water-in-oil (W/O) emulsions. This observation is consistent with thermodynamic
analysis [10,11]. The shape of solids was also found to play a role in emulsion stabilization. Van Boekel and Walstra [5] observed that globule- or needle-type crystals could stabilize O/W emulsions, whereas the mixture of the globule- and needle-type crystals could decrease the stability of the emulsions or even induce droplet coalescence. In heavy oil systems, such as bitumen, the presence of asphaltenes and water-soluble natural surfactants in the organic phase tends to alter the surface properties of the minerals and clays, which in turn changes the emulsion stability drastically. Gelot et al. [6] studied the role of fine solids and surface active species in stabilization of toluene-inwater emulsion and found that in the presence of surfactants, clay particles caused the formation of a stable toluene-in-water emulsion, while naturally hydrophobic carbon black stabilized water-intoluene emulsions. However, in the absence of surfactants, the added clay did not produce any stable emulsion. These observations suggest that surfactants modified the clay surface from waterwet to oil-wet or bi-wettable solids, thereby stabilizing the toluene-in-water emulsion. Clearly, solids accumulated at an oil/water interface provide a steric barrier to coalescence [7] and modify the rheological [12] properties of the interfacial region, thereby enhancing the emulsion stability. Yan and Masliyah [13,14] examined the effects of asphaltene-modified clay on the stability of a mineral oil-in-water emulsion. They found that the amount of asphaltene coated on the clay has significant impact on the contact angle of the clay, thereby affecting the stability of the emulsion, the size of the resultant oil droplets and the distribution of the clays in the continuous aqueous phase and at water/oil droplet interfaces. The pH of the aqueous phase was also found to be an important parameter in a solid-stabilized emulsion system [15]. To gain a better understanding of the role of solids in bitumen/water emulsions encountered in oil sands processing, this communication focuses mainly on the role of fine kaolinite clay and aqueous pH in a toluene-diluted bitumen/water emulsion system.
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2. Experimental 2.1. Materials and apparatus Coker feed bitumen, a stock feed ready for upgrading, was obtained from Syncrude Canada Ltd. and used without further purification. Fine kaolinite clay with an equivalent spherical diameter of 0.2 mm was purchased from Georgia Kaolin Co. Inc. and used as received. Toluene (/ 99.93%) from Fisher Scientific was used as the bitumen diluent. De-ionized water of conductivity 0.8 mS cm 1 was used in all the experiments. The pH of the aqueous solution was adjusted by analytical grade hydrochloric acid and sodium hydroxide solutions purchased from Fisher Scientific. A horizontal shaker (Fisher) was used to prepare the emulsions. A pH/conductivity meter from Fisher Scientific (Accumet Research 50) was used to measure the pH and conductivity of the aqueous phase. The surface tension of the aqueous phase was measured by a K-12 processor tensiometer (Kruss, USA). The contact angle of water on a layer of clay was measured using a video-assisted goniometer (Rame-Hart, USA). The viscosity of the emulsion phase was measured using a VT 550 viscometer (Haake, USA). An optical microscope equipped with a CCD camera, a monitor and a VCR was used to observe the type of emulsions and record the images which were grabbed into electronic image files by the Snappy software. Sigma Scan Pro software was used to analyze the droplet size. 2.2. Methods 2.2.1. Volume fraction of emulsion in diluted bitumen/water system A stock solution of bitumen in toluene (450 g l 1), referred to as diluted bitumen, was prepared by dissolving 450-g bitumen in toluene to a total volume of 1-l solution. The viscosity of the diluted bitumen was measured to be 5.15 mPa s 1 and was found independent of the shear rate in the
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range of 0 /500 s 1, indicating a Newtonian fluid behavior. A mixture of 80-ml deionized water and 20-ml diluted bitumen in a 250-ml glass bottle was shaken at 280 cycles/min with an amplitude of 3.5 cm for 20 min. The emulsified mixture was allowed to cream until no visible ascending of the interface between the separated aqueous phase and the formed emulsion, referred to as the emulsion phase in this communication. The volume of the emulsion phase was recorded and used as a measure of emulsification and emulsion stability. It is evident that a larger volume of the emulsion phase represents a stronger emulsification and a more stable emulsion. In a set of comparison tests, the solids originally being present in the diluted bitumen were removed by centrifugation at 35,000 /g force for 2 h before the dilute bitumen was emulsified using the experimental procedures described above. Whenever kaolinite clay was involved, a given amount of clay was added to 80-ml water prior to emulsion preparation. The amount of the clay added is expressed in terms of gram of clay per liter of water. After thoroughly mixing the kaolinite clay with the water for 20 min on the shaker, 20-ml diluted bitumen was added to the resultant suspension and emulsified by shaking for an additional 20 min. The emulsification and emulsion stability were collectively evaluated by measuring the volume of emulsion phase after the interface of emulsion/clear aqueous phases stopped ascending. The viscosity of the collected emulsion phase was measured at different shear rates. The emulsion preparation procedures used for the systems with and without kaolinite clay addition are shown in Fig. 1. Measurements of the conductivity and pH were performed for the water (k0 and pH0), the initial clay suspension (k1 and pH1) prior to emulsification and the separated aqueous phase from the emulsion before solids removal (k2 and pH2) and after solids removal (k3 and pH3). To determine the partition of fine clays, the mass of the clay remaining in the aqueous phase (ma) was determined after evaporating the water and drying the solids in a vacuum oven at 220 8C for :/5 h. Knowing the mass of the added clay (m ), the mass
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Fig. 1. Procedures used for preparing dilute bitumen in water emulsions without and with kaolinite clay addition.
of the clay in the emulsion phase (mo) was estimated from the mass balance, mo /m /ma.
were determined soon after the interface between emulsion and aqueous phases stopped rising.
2.2.2. Visual observations Visual observations were made under an optical microscope to identify the type of emulsions formed. In the case of O/W emulsion, bitumen droplets in water could be easily observed under the microscope. However, water droplets in the black diluted bitumen could not be observed directly under the microscope. It was therefore difficult to identify a water-in-diluted bitumen emulsion directly. In order to observe water droplets in the diluted bitumen, a small drop of the viscous emulsion sample was taken and placed gently on a glass slide. A small volume of toluene was placed on the glass slide, in contact with the emulsion drop only at one corner. The emulsion was diffused into the toluene through the contact region, which made the water droplets visible under the optical microscope. Both the type of the emulsion and the size of the dispersed droplets
2.2.3. Washing tests To determine whether the added clay particles in the emulsion phase were located mainly inside or outside the diluted bitumen droplets, the following tests were performed. The separated emulsion phase was diluted using the make-up water at the volume of the released aqueous phase. The pH of the make-up water was kept the same as the initial emulsification water. The diluted emulsion was shaken gently by hand for 1 min and then left still for phase separation. After the interface between the emulsion and aqueous phases stopped ascending, the aqueous phase was released into an Erlenmeyer flask. This washing procedure was repeated until the separated aqueous phase became clear. The mass of the clay in the collected aqueous phase was determined after evaporating the water and drying the clay in a vacuum oven at 220 8C for :/5 h.
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2.2.4. Surface tension and contact angle measurements Surface tension of the aqueous phase was measured using a processor tensiometer (K-12). For contact angle measurement, the clay particles in the aqueous and emulsion phases were collected separately by centrifuging corresponding phases. To minimize the effect of any attached bitumen on the collected clays on contact angle measurements, the recovered clay from the emulsion phase was thoroughly washed using toluene until the toluene became clear. The clay samples were dried in a vacuum oven at 220 8C for 5 h. A layer of the collected clay sample was spread onto a piece of double sides Scotch tape fixed on a piece of glass slide. A water drop was then placed on the clay surface and the contact angle was measured through the aqueous phase directly from the goniometer. The procedure described here is similar to that used by Ryan and Hemmingsen [16] and the reproducibility of the measurement is about 9/58.
3. Results and discussion 3.1. Emulsion characteristics It is well documented that the emulsification and emulsion stability are affected by the pH of the aqueous phase and the presence of surfactants. Depending on the wettability of the solids, the presence of fine solids in an emulsion system can either enhance or reduce the stability of the emulsion. To examine the relative impact of the kaolinite clay on the emulsification and emulsion stability in the toluene-diluted bitumen/water/clay system, a baseline test was conducted with the toluene-diluted bitumen/water system without kaolinite clay addition. The mixture of the diluted bitumen and water formed a stable gel-like W/O emulsion over five distinct pH ranges of the water (pH0): 2.9 /3.7, 4.1 /5.3, 7.4 /7.9, 8.8 /9.4 and 10.5 /10.8. Over these pH ranges, the mixture was completely emulsified to a homogeneous phase as evidenced by the absence of any excess oil (on the top) and aqueous (on the bottom) phases after the prepared emulsion was left for
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several weeks at room temperature. In this case, the measured volume fraction of the emulsion phase is 1.0, as shown in Fig. 2. However, the mixture formed a creamed layer on the top of the clear aqueous phase when emulsified outside these pH ranges. An interface between the emulsion and aqueous phases was formed at locations shown in Fig. 2. It is important to note that the remnant emulsion phase in this case was found under the optical microscope to be primarily an O/W type, which is an inversion of that formed within the five distinct pH regions. This zigzag type of change in emulsion type and emulsion phase volume fraction over these narrow pH ranges was reproducible. To the best of our knowledge, such a pH-sensitive zigzag type of variation has not been reported in the literature, but bears significant practical implications. In practice, a slight fluctuation in operating pH is not uncommon. This fluctuation would cause an inversion in emulsion type, significantly changing the processing and handling properties of the system. During this set of experiments, the presence of solids was noticed. It is therefore possible that the solids in the original bitumen may have played a role in emulsification and emulsion stabilization of diluted bitumen in water. This hypothesis was tested by conducting emulsification tests after removal of the solids in the original bitumen. The results presented also in Fig. 2 (triangles) showed that the zigzag type of variation in the volume fraction of emulsion phase with aqueous pH disappeared in this case. The volume fraction of the emulsion phase remained constant at 0.35 and the emulsion phase formed under this condition was found to be O/W type. Since, in the absence of surfactants, toluene or its mixture with heptane (hep-tol) in water does not form a stable emulsion [7,17], the observed emulsification in the diluted bitumen/water system appears to be related to the natural surfactants present in the bitumen. The suppression of zigzag type of change in emulsion phase volume by removing fine solids from bitumen clearly demonstrates that the fine solids in the bitumen are indeed a critical component in determining emulsification and emulsion stabilization. The original fine solids in bitumen promoted the formation of W/O emulsions over
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Fig. 2. Effect of emulsifying water pH on emulsion volume fraction for the toluene-diluted bitumen/water system with (triangle) and without (circle) removal of the original fine solids from bitumen.
the five distinctive pH regions. In a separate emulsion test, however, it was found that the isolated solids without further treatment showed similar behavior in stabilizing water in hep-tol emulsion as in stabilizing water in dilute bitumen emulsion. However, the solids after a thorough cleaning with toluene /water mixture did not cause the formation of a stable water in hep-tol emulsion. These findings suggest that the zigzag type of variation was caused by a combination of the natural surfactants and solids present originally in the bitumen, showing a pH-sensitive synergistic effect of natural surfactant and fine solids. Although the exact cause for this zigzag type of change in the volume fraction of the emulsion phase remains to be identified, the irregular pattern observed does suggest the complex nature of the emulsification phenomenon. We believe that the zigzag phenomenon is largely related to the complex nature of the fine solids in bitumen, which range from heavy minerals to various types of fine clays (kaolinite, illite and smectite) and unmatured hydrocarbons. It appears that the wettability of fine solids present in bitumen is
highly pH dependent in a complex, surfactantcontaining oil/water system. To further test the role of solids in emulsification and emulsion stability, fine kaolinite clay was added to the original emulsification process. The variation of the volume fraction of the emulsion phase with the amount of the added solids is shown in Fig. 3. Two selected pH0 values of 4.7 and 9.0, which were within the pH0 ranges of 4.1 / 5.3 and 8.8 /9.4, respectively, were examined. The results show that, for these two cases, the volume fraction of the emulsion phase (solid symbols) decreased drastically with increasing clay concentration up to 1 g l 1. The ultimate volume fraction of the released aqueous phase was found to be 0.72 and 0.65 for pH0 of 4.7 and 9.0, respectively, decreasing with increasing the pH of the aqueous phase. The volume fraction of the diluted bitumen in the emulsion phase (open symbols), on the other hand, showed an opposite trend, increasing with clay concentration and the effect is more pronounced at higher pH0. Examination of the emulsions formed with added clay revealed the emulsion inversion from a W/O type to an O/W
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Fig. 3. The effect of fine kaolinite clay addition on emulsion volume fraction of toluene-diluted bitumen/water system at two emulsifying water pH 4.7 and 9.0.
type at a clay concentration around 1 g l 1. Clearly, the current study demonstrates that the added clay in the current system acts as a demulsifier and its effect depends on the pH of aqueous phase. More importantly, the added clay caused the inversion of emulsion type and the minimum amount (0.1%) needed for the inversion is pH-independent. This observation is consistent with the reported emulsion stabilization mechanism that preferential wetting of solids by water leads to an O/W emulsion [18]. In the present case, the crowd effect by the added water-wettable clay at 0.1% appears to override the role of the oilwettable solids present in bitumen, causing the inversion of emulsion type. A typical photograph of a W/O emulsion formed in the absence of added clay and that of an O/W emulsion formed at a clay concentration of 4 g l 1 are shown in Fig. 4. It can be observed that the size of the water droplets in Fig. 4(a) is much smaller than that of the oil droplets in Fig. 4(b). Without clay addition, the size of the water droplets (white dots on black background) is :/10 mm, while with 4 g l 1 kaolinite addition, the size of oil droplets (black circles on white background) is :/200 mm. A plot of the mean droplet diameter
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versus clay concentration for emulsion system at pH0 of 4.7 is shown in Fig. 5. A steep change in droplet size occurred at a clay concentration :/1 g l 1. Interestingly, this solids level corresponded to the change of emulsion type from W/O dominant to O/W dominant. The change in the volume fraction of the emulsion phase with the added clay concentration (Fig. 2) is not as drastic as the droplet size (Fig. 5) at the emulsion type inversion point. It is important to note that for a given volume ratio of the dispersed phase to the continuous phase, the droplet size of the dispersed phase can vary by orders of magnitude depending on the physicochemical condition of the system. A typical example is the change of an emulsion to a microemulsion system by changing the characteristics of the emulsifiers for a given volume ratio of two emulsifying phases. Therefore, the observed disproportional changes in emulsion phase volume and droplet size is not unexpected. The volume fraction of emulsion decreased dramatically then leveled off with the emulsion type inversion from a water-in-oil to an oil-in-water, as shown in Fig. 3. The dramatic decrease in the volume fraction corresponded to the abrupt droplet size increase shown in Fig. 5. Since pH is seen to affect the emulsification and emulsion stability, the effect of the resulting suspension pH (pH1) on the volume fraction of the O/W emulsion phase was studied at a clay concentration of 4 g l 1. The results in Fig. 6 show that the volume fraction of the O/W emulsion phase decreased monotonically with increasing pH1. A clear transition at pH1 around 3 is evident, but the emulsion remained O/W type. Visual observation of the sample taken at an intermediate depth of the emulsion phase formed at suspension pH1 of 5.4 revealed a wide distribution of diluted bitumen droplet size from 130 to 570 mm with an area-average droplet size of :/366 mm. The average drop size increased slightly from the bottom to the top of the emulsion phase. 3.2. Rheology of the formed emulsions In the handling, mixing, storage and pipeline transportation of emulsions, knowledge of the rheological properties is required for the design,
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Fig. 4. Photographs of sampled emulsions: (a) water-in-oil emulsion without kaolinite clay addition; and (b) oil-in-water emulsion at a kaolinite clay concentration of 4 g l 1.
Fig. 5. Variation of mean drop diameter with added kaolinite clay concentration at an emulsifying water pH 4.7.
selection and operation of the equipment involved. The rheology of emulsions has been studied extensively [19 /22]. A general observation is that the viscosity of emulsions is shear rate dependent.
Fig. 6. Effect of kaolinite clay suspension pH on emulsion phase volume fraction at a kaolinite clay concentration of 4 g l 1.
In the present study, a shear-thinning characteristic of the formed emulsion was observed. The dependency of the emulsion viscosity on clay
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concentration at a shear rate of 200 s 1 is shown in Fig. 7. It can be seen that the viscosity of the W/ O emulsion decreased with increasing clay concentration up to 1 g l 1.The drastic decrease in the emulsion viscosity corresponded well to the observed inversion of emulsion to O/W type. For the oil-in-water emulsion, the effect of clay on the emulsion viscosity is less significant. This finding suggests that the inversion of W/O to O/W emulsion by the addition of water-wettable clays would assist the transport of the formed emulsion systems. 3.3. Clay partition It is known that oil-wettable fine solids can destabilize and eventually invert an O/W to a W/O emulsion [23]. To understand W/O to O/W typeemulsion inversion by kaolinite addition in the present system, the distribution of kaolinite clay between the aqueous phase and the emulsion phase was determined. In this case, the initial kaolinite clay concentration was fixed at 4 g l 1. The results in Fig. 8(a) show that /98% of the added clay was retained in the upper emulsion phase when the suspension pH was B/3. However,
Fig. 7. Effect of kaolinite clay concentration on viscosity of emulsion measured at a shear rate of 200 s 1 for two emulsifying water pH 4.7 and 9.0.
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for a suspension pH /3, B/15% of the added clay was retained in the emulsion phase. Yan and Masliyah reported a similar observation for a mineral oil system [15]. In the current system, the transition of added clay partition at pH :/3 correlated well with the observed transition in volume fraction of the emulsion phase (Fig. 6). Fig. 8(b) shows the variation of the clay concentration in the aqueous and the emulsion phases with the suspension pH (pH1). In order to appreciate clay partitioning after phase separation, the clay concentration is evaluated based on the weight of the clay to the volume of water present within the phase. For pH1 B/3, the concentration of the clay in the emulsion phase reached a value as high as 13 g l 1 which is much higher than the initial value of 4 g l 1. This would suggest that the excess clay particles would be located either within the diluted bitumen or on the droplet surface. Such a shift in the clay partitioning would also suggest a modification clay surface to become more hydrophobic. For pH1 /3, the clay concentration in both the aqueous and emulsion phases is fairly close to that of the initial suspension concentration. This distribution of clay in aqueous phase would suggest that the clay remained hydrophilic and uniformly distributed among the total aqueous volume. To confirm that the clay particles collected from the upper emulsion phase for pH1 B/3 were hydrophobic, while the clay particles collected from the lower aqueous phase for pH1 /3 remained hydrophilic, the water contact angle on a layer of the clay sample was measured as a function of pH. The results presented in Fig. 9 show that the original clay was hydrophilic with a contact angle value :/48. A contact angle value of B/208 was determined with the clays isolated from the aqueous phase for pH1 /3. The clay collected from the upper emulsion phase at pH1 B/3 gave a contact angle value /1208. Such a high value was obtained even though the solids were washed thoroughly with toluene. The significant decrease in contact angle from 120 to 208 with increasing pH1 correlated well with the significant decrease in the amount of clay remained in the emulsion phase.
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Fig. 8. Effect of kaolinite clay suspension pH on clay partition: (a) mass fraction of clay in emulsion (solid) and aqueous (open) phases; and (b) clay concentration in the emulsion (solid) and in the aqueous (open) phases at an initial clay concentration of 4 g l 1.
Fig. 9. Contact angle of water on clay particles obtained from the upper emulsion phase for suspensions at pH (pH1) B/3 and from the lower aqueous phase for pH1 /3. The initial clay concentration in emulsifying water is 4 g l 1.
To determine whether the clay particles in an O/ W emulsion phase were located mainly inside the diluted bitumen droplets or accumulated at diluted bitumen droplet/water interface, two washing tests were conducted. One test was at pH1 /5.4, another was at pH1 /2. When the O/W emulsions were diluted with water, as described in Section 2,
solids would be removed if they were present outside the diluted bitumen droplets and would be retained if they were present inside the diluted bitumen droplets or if they were strongly attached to the droplet surface. The washing test showed that :/91% of the added clay was removed at pH1 of 5.4, indicating that marginally hydrophobic clay particles (contact angle B/208) were mainly attached weakly to diluted bitumen droplets. However, only 12% of the added clay was washed away at pH1 /2, indicating that most of the clay particles were extracted into the diluted bitumen droplets instead of accumulating at diluted bitumen/water interface to stabilize otherwise expected W/O emulsions. This finding is consistent with a strong hydrophobic characteristics of clays with a contact angle value of /1208. Such a strong hydrophobicity of the clay particles favors the extraction of the particles in oil phase than the accumulation at oil/water interface that would favor the formation of W/O emulsions. Our finding suggests that to stabilize a W/O emulsion by fine solids, the wettability of the solids needs to be controlled within a desired contact angle range. For too strong hydrophobic solids, as in the present case, hydrophobic extraction of solids into organic phase would dominate. As a result, the ability of stabilizing W/O emulsion by solids diminishes due to the lack of the solids at the interface.
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3.4. Conductivity and surface tension In the absence of surfactant, paraffin oil or toluene does not form a stable emulsion in water. The addition of hydrophilic kaolinite clay alone has little effect on the emulsification of pure immiscible organic solvents in water. However, a stable diluted bitumen-in-water emulsion was obtained in the present study, suggesting that some surface-active components were released from bitumen and played a role in bitumen emulsification. It has been confirmed experimentally [24,25] that both cationic and anionic watersoluble surfactants are released from bitumen into the aqueous phase during bitumen extraction. The changes in the conductivity (Fig. 10) and surface tension (Fig. 11) of the aqueous phase before and after contact with the diluted bitumen provided a direct evidence of surfactant release. Due to ion exchange, the addition of the clay to the deionized water altered the conductivity of the water, as shown in Fig. 10 by the difference between the k0 and k1 values. Upon the addition of the diluted bitumen to the suspension, however, the conductivity of the suspension changed drastically, as seen by comparing k1 and k2. Removal of the suspending solids had little effect on the
Fig. 10. Comparison of aqueous phase conductivity measured during emulsification.
Fig. 11. Surface tension of the aqueous phase collected after emulsion creaming.
aqueous phase conductivity (compare k2 and k3). These results suggest the release of ionic species from the diluted bitumen. Whether the release ionic species is surface active species or inorganic salt was confirmed by the results of surface tension measurements, as shown in Fig. 11. It is interesting to note the similarity in the shape of the conductivity and surface tension profiles. This similarity suggests that the release of ionic surface active species is a contributor to the observed increase in aqueous conductivity. Over the alkaline pH range, the decrease of surface tension with increasing pH1 is associated with the increased solubility of anionic surfactants, e.g. carboxylate, sulphate and sulphonates. In this case, anionic surfactant is the predominant contributor to the observed increase in aqueous conductivity and over the acidic pH range, the decrease of surface tension with decreasing pH is associated with the increased solubility of cationic surfactants, e.g. primary and/or secondary amines. Here, cationic surfactants became predominant. The variable extraction of both cationic and anionic surfactants into aqueous phase would account for the measured conductivity and surface tension profiles shown in Figs. 10 and 11, respectively. Accepting the release of surfactants from bitumen, the clay
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surface becomes inevitably modified from initially hydrophilic to hydrophobic, resulting in a pHdependent partitioning behavior, as shown in Fig. 8. The observed transition of contact angle and clay partition (Fig. 9) at pH :/3 is the result of a combined effect from the released surfactants and the added fine clays.
4. Industrial implications In order to accommodate oil sands production levels accompanied by an increased distance between a mine face and the base plant, the mined oil sand lumps are transported via slurry pipeline to the base plant. During the transportation in the slurry pipeline, bitumen is liberated from the oil sand grains and attaches to the available air bubbles within the slurring mixture. At the base plant, the liberated bitumen is floated in a gravity separation vessel for further processing. In addition, in a recent commercial development, bitumen froth recovery is accomplished at the mined phase with a de-aerated bituminous froth containing :/ 60% bitumen, 30% water and 10% coarse solids and fines, which is pipeline-hydrotransported to the base plant. In the two approaches of bituminous transportation, it is essential to avoid waterin-bitumen emulsion, as it would significantly hinder the capacity of transportation due to an excessively high viscosity, as illustrated in this study. This study strongly suggests that the presence of fines would prevent the formation of water-in-bitumen emulsions. It appears that the presence of the fines is beneficial to bituminous transportation, as a reduced viscosity is anticipated. The results of this study confirm those of Joseph’s studies, where it was found beneficial to add fines to froth pipeline transportation [26]. We also identified the existence of a minimal amount of fines needed to avoid W/O emulsification. Determining this critical level should provide a guideline for practical operations. More important, the current study demonstrated tuning the type of emulsion by solids addition, which suppressed the complex zigzag change in emulsion type in the current system, significantly simplifying the control of industrial operations.
5. Conclusions A stable, gel-like water-in-oil emulsion was obtained when toluene-diluted bitumen was mixed with water over five distinct pH regions. A zigzag type of variation of emulsion type with pH was observed. The removal of the original fine solids from bitumen or the addition of kaolinite clay /1 g l 1 suppressed the zigzag type of variation and the gellike W/O emulsion became an oil-in-water emulsion. At a given aqueous pH where W/O emulsion forms, the volume fraction of emulsion phase reduced significantly with the addition of kaolinite clay up to 1 g l 1. Solids addition was able to tune the type of emulsion and hence to control the viscosity of emulsion system. Without the clay addition, the viscosity of the water-in-oil emulsion was excessively high, which can be reduced to a minimum by adding the clay /1 g l 1, at which the inversion of emulsion type was observed. For a suspension pH B/3, the clay obtained from the upper emulsion phase had a water contact angle /1208 and /98% of the added clay was present in the emulsion phase. However, for a suspension pH /3, the contact angle of the clay increased to B/208 and the majority of the added clay remained in the aqueous phase.
Acknowledgements The financial support from NSERC-Industry Research Chair in Oil Sands (held by JHM) and from Alberta Department of Energy is greatly appreciated. One of the authors (ZZ) wishes to thank the Quebec Government for providing an FCAR Postdoctoral Research Fellowship.
References [1] L.G. Hepler, R.G. Smith, The Alberta Oil Sands: Industrial Procedures for Extraction and Some Recent Fundamental Research, AOSTRA Technical Publication Series No. 14, Alberta Oil Sands Technology and Research Authority, Edmonton, Canada, 1994.
G. Gu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 215 (2003) 141 /153 [2] P. Mankowski, S. Ng, R. Siy, J. Spence, P. Stapleton, in: R. May (Eds.), Proceedings of the Thirty-first Annual Meeting of the Canadian Mineral Processors, Ottawa, Ontario, 19 /21 January 1999, The Canadian Institute of Mining, Metallurgy and Petroleum, Ottawa, 1999, p. 154. [3] W.D. Gunter, Z. Zhou, E.H. Perkins, SPE 23786. [4] T.M. Ignasiak, L. Kotlyar, F.J. Longstaffe, O.P. Strause, D.S. Montgomery, Fuel 62 (1983) 353. [5] M.A.J.S. van Boekel, P. Walatra, Colloids Surf. 3 (1981) 109. [6] A. Gelot, W. Friesen, H.A. Hamza, Colloids Surf. 12 (1984) 271. [7] D.E. Tambe, M.M. Sharma, J. Colloid Interface Sci. 157 (1993) 244. [8] V.B. Menon, A.D. Nikolov, D.T. Wasan, J. Colloid Interface Sci. 124 (1) (1988) 317. [9] V.B. Menon, A.D. Nikolov, D.T. Wasan, J. Dispers. Sci. Tech. 9 (5/6) (1989) 575. [10] M.T. Jacques, AIChE J. 25 (1979) 160. [11] V.B. Menon, R. Nagarajan, D.T. Wasan, Separ. Sci. Technol. 22 (12) (1987) 2295. [12] D.E. Tambe, M.M. Sharma, J. Colloid Interface Sci. 162 (1994) 1.
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[13] Y. Yan, J.H. Masliyah, Colloids Surf. A 75 (1993) 123. [14] N. Yan, J.H. Masliyah, Colloid Surf. A 96 (1995) 229. [15] N. Yan, J.H. Masliyah, J. Colloid Interface Sci. 181 (1996) 20. [16] W.L. Ryan, E.A. Hemmingsen, J. Colloid Interface Sci. 157 (1993) 312. [17] Z.A. Zhou, Z. Xu, J.H. Masliyah, J. Czarnecki, Colloids Surf. A 148 (1998) 199. [18] D. Shaw, Introduction to Colloid and Surface Chemistry, fourth ed, Butterworth-Heinemann, Oxford, 1992, p. 267. [19] P. Manoj, A.D. Watson, D.J. Hibberd, A.J. Fillery-Travis, M.M. Robins, J. Colloid Interface Sci. 207 (1998) 294. [20] R. Pal, AIChE J. 42 (11) (1996) 3181. [21] Y. Yan, R. Pal, J. Masliyah, Chem. Eng. Sci. 46 (4) (1991) 985. [22] A.M. Howe, A. Clarke, Langmuir 13 (1997) 2617. [23] J.H. Schulman, J. Leja, Trans. Faraday Soc. 50 (1954) 598. [24] L.L. Schramm, R.G. Smith, J.A. Stone, Colloids Surf. 11 (1984) 247. [25] L.L. Schramm, R.G. Smith, Colloids Surf. 14 (1985) 67. [26] D. Joseph, R. Bai, C. Mata, K. Sury, C. Grant, J. Fluid Mech. 386 (1999) 127.
Role of fine kaolinite clay in toluene-diluted bitumen/water emulsion Guoxing Gu, Zhiang Zhou, Zhenghe Xu, Jacob H. Masliyah * Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alta, Canada T6G 2G6 Received 7 December 2000; accepted 12 September 2002
Abstract The role of suspension pH and added fine kaolinite clays in the emulsification of a toluene-diluted bitumen/water system was investigated. Within five distinct pH ranges of 2.9 /3.7, 4.1 /5.3, 7.4 /7.9, 8.8 /9.4 and 10.5 /10.8, mixing toluene-diluted bitumen with de-ionized water at a water:diluted bitumen volume ratio of 4:1 resulted in a stable waterin-oil (W/O) emulsion. Over these pH ranges, the mixture was completely emulsified to a homogeneous, gel-like creamed phase. However, a creamed layer of varying volume was separated on the top of an aqueous phase when mixed outside these pH ranges. The creamed layer was found to be mainly an oil-in-water (O/W) emulsion. This zigzag type of change in emulsion-type inversion over these narrow pH ranges is a unique feature of the current system. The removal of the original fine solids in bitumen prior to emulsification resulted in a constant volume of O/W emulsion separated on the top of the aqueous phase over the pH range 1.5 /11 studied. The addition of kaolinite clays to the emulsifying water was found to suppress the original zigzag type emulsification characteristics. Consequently, the volume fraction of the emulsion reduced significantly with a corresponding reduction of water content in the emulsion. For a suspension of pH B/3 and clay concentration at 4 g l 1, /98% of the added clay was retained in the emulsion. At pH /3, on the other hand, the majority of the added clay remained in the separated aqueous phase. Measurement of water contact angle (u ) on the collected clay showed that the added clay, originally being hydrophilic (u :/48), was rendered hydrophobic (u /1208) at pH B/3, while remained hydrophilic (u :/208) at pH /3. Release of natural surfactants from bitumen into the aqueous phase was confirmed by the conductivity, surface tension and contact angle measurements. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Toluene-diluted bitumen; Kaolinite clay; Emulsion; Volume fraction; Clay partitioning
1. Introduction * Corresponding author. Tel.: /1-780-492-4673; fax: /1780-492-2881 E-mail address: [email protected] (J.H. Masliyah).
The oil sands deposits in Northern Alberta, Canada, are complex mixtures of sand grains, fine clays, bitumen and water. To extract bitumen from
0927-7757/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 2 ) 0 0 4 2 2 - 3
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Athabasca oil sands, both Syncrude Canada Ltd. and Suncor Energy Corporation use a variation of the Clark Hot Water Extraction process. In this process, mined oil sands are digested in a tumbler or in a hydrotransport pipeline with the addition of hot water (50 /80 8C) and small quantities of sodium hydroxide (caustic) to facilitate bitumen liberation from oil sand grains. The detached bitumen droplets are recovered by attaching to or engulfing air bubbles followed by flotation in a gravity separation vessel. The recovered bitumen froth normally consists of 60% bitumen, 30% water and 10% solids [1]. Prior to bitumen upgrading, the solids and water are removed by centrifuging the diluted bitumen froth, known as the froth treatment. In the upcoming low energy extraction (LEE) process, bitumen is to be extracted at an operating temperature as low as 25 8C, resulting in substantially lower energy and capital costs [2]. However, in the LEE process, an increased amount of solids and water in bitumen froth is anticipated, posing an additional challenge to satisfactory removal of micro-emulsified water droplets during the froth treatment. However, the removal of micro-emulsified water in oil droplets to below 0.5% is essential as the chlorines entrained in water would cause unpredicted, yet catastrophic blow-out to up-grading units. To achieve a satisfactory removal of emulsified water from the diluted bitumen, it is essential to understand the stabilization mechanisms in diluted bitumen systems. In addition to the natural surfactants in bitumen, fine clays, such as kaolinite, illite, chlorite and montmorillonite, are known to be present in oil sands with kaolinite being the most abundant [3]. Although the amount of clay minerals represents only a small portion of the total solids, their presence has been recognized to be detrimental to the oil sand ore processibility by either the hot-water process or in-situ production [4]. Extensive studies have been conducted to investigate fine solids-stabilized emulsions [5 /9]. A general observation is that solids with contact angle B/908 tend to stabilize oil-in-water (O/W) emulsions, while those with contact angle /908 favor stabilizing water-in-oil (W/O) emulsions. This observation is consistent with thermodynamic
analysis [10,11]. The shape of solids was also found to play a role in emulsion stabilization. Van Boekel and Walstra [5] observed that globule- or needle-type crystals could stabilize O/W emulsions, whereas the mixture of the globule- and needle-type crystals could decrease the stability of the emulsions or even induce droplet coalescence. In heavy oil systems, such as bitumen, the presence of asphaltenes and water-soluble natural surfactants in the organic phase tends to alter the surface properties of the minerals and clays, which in turn changes the emulsion stability drastically. Gelot et al. [6] studied the role of fine solids and surface active species in stabilization of toluene-inwater emulsion and found that in the presence of surfactants, clay particles caused the formation of a stable toluene-in-water emulsion, while naturally hydrophobic carbon black stabilized water-intoluene emulsions. However, in the absence of surfactants, the added clay did not produce any stable emulsion. These observations suggest that surfactants modified the clay surface from waterwet to oil-wet or bi-wettable solids, thereby stabilizing the toluene-in-water emulsion. Clearly, solids accumulated at an oil/water interface provide a steric barrier to coalescence [7] and modify the rheological [12] properties of the interfacial region, thereby enhancing the emulsion stability. Yan and Masliyah [13,14] examined the effects of asphaltene-modified clay on the stability of a mineral oil-in-water emulsion. They found that the amount of asphaltene coated on the clay has significant impact on the contact angle of the clay, thereby affecting the stability of the emulsion, the size of the resultant oil droplets and the distribution of the clays in the continuous aqueous phase and at water/oil droplet interfaces. The pH of the aqueous phase was also found to be an important parameter in a solid-stabilized emulsion system [15]. To gain a better understanding of the role of solids in bitumen/water emulsions encountered in oil sands processing, this communication focuses mainly on the role of fine kaolinite clay and aqueous pH in a toluene-diluted bitumen/water emulsion system.
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2. Experimental 2.1. Materials and apparatus Coker feed bitumen, a stock feed ready for upgrading, was obtained from Syncrude Canada Ltd. and used without further purification. Fine kaolinite clay with an equivalent spherical diameter of 0.2 mm was purchased from Georgia Kaolin Co. Inc. and used as received. Toluene (/ 99.93%) from Fisher Scientific was used as the bitumen diluent. De-ionized water of conductivity 0.8 mS cm 1 was used in all the experiments. The pH of the aqueous solution was adjusted by analytical grade hydrochloric acid and sodium hydroxide solutions purchased from Fisher Scientific. A horizontal shaker (Fisher) was used to prepare the emulsions. A pH/conductivity meter from Fisher Scientific (Accumet Research 50) was used to measure the pH and conductivity of the aqueous phase. The surface tension of the aqueous phase was measured by a K-12 processor tensiometer (Kruss, USA). The contact angle of water on a layer of clay was measured using a video-assisted goniometer (Rame-Hart, USA). The viscosity of the emulsion phase was measured using a VT 550 viscometer (Haake, USA). An optical microscope equipped with a CCD camera, a monitor and a VCR was used to observe the type of emulsions and record the images which were grabbed into electronic image files by the Snappy software. Sigma Scan Pro software was used to analyze the droplet size. 2.2. Methods 2.2.1. Volume fraction of emulsion in diluted bitumen/water system A stock solution of bitumen in toluene (450 g l 1), referred to as diluted bitumen, was prepared by dissolving 450-g bitumen in toluene to a total volume of 1-l solution. The viscosity of the diluted bitumen was measured to be 5.15 mPa s 1 and was found independent of the shear rate in the
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range of 0 /500 s 1, indicating a Newtonian fluid behavior. A mixture of 80-ml deionized water and 20-ml diluted bitumen in a 250-ml glass bottle was shaken at 280 cycles/min with an amplitude of 3.5 cm for 20 min. The emulsified mixture was allowed to cream until no visible ascending of the interface between the separated aqueous phase and the formed emulsion, referred to as the emulsion phase in this communication. The volume of the emulsion phase was recorded and used as a measure of emulsification and emulsion stability. It is evident that a larger volume of the emulsion phase represents a stronger emulsification and a more stable emulsion. In a set of comparison tests, the solids originally being present in the diluted bitumen were removed by centrifugation at 35,000 /g force for 2 h before the dilute bitumen was emulsified using the experimental procedures described above. Whenever kaolinite clay was involved, a given amount of clay was added to 80-ml water prior to emulsion preparation. The amount of the clay added is expressed in terms of gram of clay per liter of water. After thoroughly mixing the kaolinite clay with the water for 20 min on the shaker, 20-ml diluted bitumen was added to the resultant suspension and emulsified by shaking for an additional 20 min. The emulsification and emulsion stability were collectively evaluated by measuring the volume of emulsion phase after the interface of emulsion/clear aqueous phases stopped ascending. The viscosity of the collected emulsion phase was measured at different shear rates. The emulsion preparation procedures used for the systems with and without kaolinite clay addition are shown in Fig. 1. Measurements of the conductivity and pH were performed for the water (k0 and pH0), the initial clay suspension (k1 and pH1) prior to emulsification and the separated aqueous phase from the emulsion before solids removal (k2 and pH2) and after solids removal (k3 and pH3). To determine the partition of fine clays, the mass of the clay remaining in the aqueous phase (ma) was determined after evaporating the water and drying the solids in a vacuum oven at 220 8C for :/5 h. Knowing the mass of the added clay (m ), the mass
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Fig. 1. Procedures used for preparing dilute bitumen in water emulsions without and with kaolinite clay addition.
of the clay in the emulsion phase (mo) was estimated from the mass balance, mo /m /ma.
were determined soon after the interface between emulsion and aqueous phases stopped rising.
2.2.2. Visual observations Visual observations were made under an optical microscope to identify the type of emulsions formed. In the case of O/W emulsion, bitumen droplets in water could be easily observed under the microscope. However, water droplets in the black diluted bitumen could not be observed directly under the microscope. It was therefore difficult to identify a water-in-diluted bitumen emulsion directly. In order to observe water droplets in the diluted bitumen, a small drop of the viscous emulsion sample was taken and placed gently on a glass slide. A small volume of toluene was placed on the glass slide, in contact with the emulsion drop only at one corner. The emulsion was diffused into the toluene through the contact region, which made the water droplets visible under the optical microscope. Both the type of the emulsion and the size of the dispersed droplets
2.2.3. Washing tests To determine whether the added clay particles in the emulsion phase were located mainly inside or outside the diluted bitumen droplets, the following tests were performed. The separated emulsion phase was diluted using the make-up water at the volume of the released aqueous phase. The pH of the make-up water was kept the same as the initial emulsification water. The diluted emulsion was shaken gently by hand for 1 min and then left still for phase separation. After the interface between the emulsion and aqueous phases stopped ascending, the aqueous phase was released into an Erlenmeyer flask. This washing procedure was repeated until the separated aqueous phase became clear. The mass of the clay in the collected aqueous phase was determined after evaporating the water and drying the clay in a vacuum oven at 220 8C for :/5 h.
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2.2.4. Surface tension and contact angle measurements Surface tension of the aqueous phase was measured using a processor tensiometer (K-12). For contact angle measurement, the clay particles in the aqueous and emulsion phases were collected separately by centrifuging corresponding phases. To minimize the effect of any attached bitumen on the collected clays on contact angle measurements, the recovered clay from the emulsion phase was thoroughly washed using toluene until the toluene became clear. The clay samples were dried in a vacuum oven at 220 8C for 5 h. A layer of the collected clay sample was spread onto a piece of double sides Scotch tape fixed on a piece of glass slide. A water drop was then placed on the clay surface and the contact angle was measured through the aqueous phase directly from the goniometer. The procedure described here is similar to that used by Ryan and Hemmingsen [16] and the reproducibility of the measurement is about 9/58.
3. Results and discussion 3.1. Emulsion characteristics It is well documented that the emulsification and emulsion stability are affected by the pH of the aqueous phase and the presence of surfactants. Depending on the wettability of the solids, the presence of fine solids in an emulsion system can either enhance or reduce the stability of the emulsion. To examine the relative impact of the kaolinite clay on the emulsification and emulsion stability in the toluene-diluted bitumen/water/clay system, a baseline test was conducted with the toluene-diluted bitumen/water system without kaolinite clay addition. The mixture of the diluted bitumen and water formed a stable gel-like W/O emulsion over five distinct pH ranges of the water (pH0): 2.9 /3.7, 4.1 /5.3, 7.4 /7.9, 8.8 /9.4 and 10.5 /10.8. Over these pH ranges, the mixture was completely emulsified to a homogeneous phase as evidenced by the absence of any excess oil (on the top) and aqueous (on the bottom) phases after the prepared emulsion was left for
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several weeks at room temperature. In this case, the measured volume fraction of the emulsion phase is 1.0, as shown in Fig. 2. However, the mixture formed a creamed layer on the top of the clear aqueous phase when emulsified outside these pH ranges. An interface between the emulsion and aqueous phases was formed at locations shown in Fig. 2. It is important to note that the remnant emulsion phase in this case was found under the optical microscope to be primarily an O/W type, which is an inversion of that formed within the five distinct pH regions. This zigzag type of change in emulsion type and emulsion phase volume fraction over these narrow pH ranges was reproducible. To the best of our knowledge, such a pH-sensitive zigzag type of variation has not been reported in the literature, but bears significant practical implications. In practice, a slight fluctuation in operating pH is not uncommon. This fluctuation would cause an inversion in emulsion type, significantly changing the processing and handling properties of the system. During this set of experiments, the presence of solids was noticed. It is therefore possible that the solids in the original bitumen may have played a role in emulsification and emulsion stabilization of diluted bitumen in water. This hypothesis was tested by conducting emulsification tests after removal of the solids in the original bitumen. The results presented also in Fig. 2 (triangles) showed that the zigzag type of variation in the volume fraction of emulsion phase with aqueous pH disappeared in this case. The volume fraction of the emulsion phase remained constant at 0.35 and the emulsion phase formed under this condition was found to be O/W type. Since, in the absence of surfactants, toluene or its mixture with heptane (hep-tol) in water does not form a stable emulsion [7,17], the observed emulsification in the diluted bitumen/water system appears to be related to the natural surfactants present in the bitumen. The suppression of zigzag type of change in emulsion phase volume by removing fine solids from bitumen clearly demonstrates that the fine solids in the bitumen are indeed a critical component in determining emulsification and emulsion stabilization. The original fine solids in bitumen promoted the formation of W/O emulsions over
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Fig. 2. Effect of emulsifying water pH on emulsion volume fraction for the toluene-diluted bitumen/water system with (triangle) and without (circle) removal of the original fine solids from bitumen.
the five distinctive pH regions. In a separate emulsion test, however, it was found that the isolated solids without further treatment showed similar behavior in stabilizing water in hep-tol emulsion as in stabilizing water in dilute bitumen emulsion. However, the solids after a thorough cleaning with toluene /water mixture did not cause the formation of a stable water in hep-tol emulsion. These findings suggest that the zigzag type of variation was caused by a combination of the natural surfactants and solids present originally in the bitumen, showing a pH-sensitive synergistic effect of natural surfactant and fine solids. Although the exact cause for this zigzag type of change in the volume fraction of the emulsion phase remains to be identified, the irregular pattern observed does suggest the complex nature of the emulsification phenomenon. We believe that the zigzag phenomenon is largely related to the complex nature of the fine solids in bitumen, which range from heavy minerals to various types of fine clays (kaolinite, illite and smectite) and unmatured hydrocarbons. It appears that the wettability of fine solids present in bitumen is
highly pH dependent in a complex, surfactantcontaining oil/water system. To further test the role of solids in emulsification and emulsion stability, fine kaolinite clay was added to the original emulsification process. The variation of the volume fraction of the emulsion phase with the amount of the added solids is shown in Fig. 3. Two selected pH0 values of 4.7 and 9.0, which were within the pH0 ranges of 4.1 / 5.3 and 8.8 /9.4, respectively, were examined. The results show that, for these two cases, the volume fraction of the emulsion phase (solid symbols) decreased drastically with increasing clay concentration up to 1 g l 1. The ultimate volume fraction of the released aqueous phase was found to be 0.72 and 0.65 for pH0 of 4.7 and 9.0, respectively, decreasing with increasing the pH of the aqueous phase. The volume fraction of the diluted bitumen in the emulsion phase (open symbols), on the other hand, showed an opposite trend, increasing with clay concentration and the effect is more pronounced at higher pH0. Examination of the emulsions formed with added clay revealed the emulsion inversion from a W/O type to an O/W
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Fig. 3. The effect of fine kaolinite clay addition on emulsion volume fraction of toluene-diluted bitumen/water system at two emulsifying water pH 4.7 and 9.0.
type at a clay concentration around 1 g l 1. Clearly, the current study demonstrates that the added clay in the current system acts as a demulsifier and its effect depends on the pH of aqueous phase. More importantly, the added clay caused the inversion of emulsion type and the minimum amount (0.1%) needed for the inversion is pH-independent. This observation is consistent with the reported emulsion stabilization mechanism that preferential wetting of solids by water leads to an O/W emulsion [18]. In the present case, the crowd effect by the added water-wettable clay at 0.1% appears to override the role of the oilwettable solids present in bitumen, causing the inversion of emulsion type. A typical photograph of a W/O emulsion formed in the absence of added clay and that of an O/W emulsion formed at a clay concentration of 4 g l 1 are shown in Fig. 4. It can be observed that the size of the water droplets in Fig. 4(a) is much smaller than that of the oil droplets in Fig. 4(b). Without clay addition, the size of the water droplets (white dots on black background) is :/10 mm, while with 4 g l 1 kaolinite addition, the size of oil droplets (black circles on white background) is :/200 mm. A plot of the mean droplet diameter
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versus clay concentration for emulsion system at pH0 of 4.7 is shown in Fig. 5. A steep change in droplet size occurred at a clay concentration :/1 g l 1. Interestingly, this solids level corresponded to the change of emulsion type from W/O dominant to O/W dominant. The change in the volume fraction of the emulsion phase with the added clay concentration (Fig. 2) is not as drastic as the droplet size (Fig. 5) at the emulsion type inversion point. It is important to note that for a given volume ratio of the dispersed phase to the continuous phase, the droplet size of the dispersed phase can vary by orders of magnitude depending on the physicochemical condition of the system. A typical example is the change of an emulsion to a microemulsion system by changing the characteristics of the emulsifiers for a given volume ratio of two emulsifying phases. Therefore, the observed disproportional changes in emulsion phase volume and droplet size is not unexpected. The volume fraction of emulsion decreased dramatically then leveled off with the emulsion type inversion from a water-in-oil to an oil-in-water, as shown in Fig. 3. The dramatic decrease in the volume fraction corresponded to the abrupt droplet size increase shown in Fig. 5. Since pH is seen to affect the emulsification and emulsion stability, the effect of the resulting suspension pH (pH1) on the volume fraction of the O/W emulsion phase was studied at a clay concentration of 4 g l 1. The results in Fig. 6 show that the volume fraction of the O/W emulsion phase decreased monotonically with increasing pH1. A clear transition at pH1 around 3 is evident, but the emulsion remained O/W type. Visual observation of the sample taken at an intermediate depth of the emulsion phase formed at suspension pH1 of 5.4 revealed a wide distribution of diluted bitumen droplet size from 130 to 570 mm with an area-average droplet size of :/366 mm. The average drop size increased slightly from the bottom to the top of the emulsion phase. 3.2. Rheology of the formed emulsions In the handling, mixing, storage and pipeline transportation of emulsions, knowledge of the rheological properties is required for the design,
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Fig. 4. Photographs of sampled emulsions: (a) water-in-oil emulsion without kaolinite clay addition; and (b) oil-in-water emulsion at a kaolinite clay concentration of 4 g l 1.
Fig. 5. Variation of mean drop diameter with added kaolinite clay concentration at an emulsifying water pH 4.7.
selection and operation of the equipment involved. The rheology of emulsions has been studied extensively [19 /22]. A general observation is that the viscosity of emulsions is shear rate dependent.
Fig. 6. Effect of kaolinite clay suspension pH on emulsion phase volume fraction at a kaolinite clay concentration of 4 g l 1.
In the present study, a shear-thinning characteristic of the formed emulsion was observed. The dependency of the emulsion viscosity on clay
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concentration at a shear rate of 200 s 1 is shown in Fig. 7. It can be seen that the viscosity of the W/ O emulsion decreased with increasing clay concentration up to 1 g l 1.The drastic decrease in the emulsion viscosity corresponded well to the observed inversion of emulsion to O/W type. For the oil-in-water emulsion, the effect of clay on the emulsion viscosity is less significant. This finding suggests that the inversion of W/O to O/W emulsion by the addition of water-wettable clays would assist the transport of the formed emulsion systems. 3.3. Clay partition It is known that oil-wettable fine solids can destabilize and eventually invert an O/W to a W/O emulsion [23]. To understand W/O to O/W typeemulsion inversion by kaolinite addition in the present system, the distribution of kaolinite clay between the aqueous phase and the emulsion phase was determined. In this case, the initial kaolinite clay concentration was fixed at 4 g l 1. The results in Fig. 8(a) show that /98% of the added clay was retained in the upper emulsion phase when the suspension pH was B/3. However,
Fig. 7. Effect of kaolinite clay concentration on viscosity of emulsion measured at a shear rate of 200 s 1 for two emulsifying water pH 4.7 and 9.0.
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for a suspension pH /3, B/15% of the added clay was retained in the emulsion phase. Yan and Masliyah reported a similar observation for a mineral oil system [15]. In the current system, the transition of added clay partition at pH :/3 correlated well with the observed transition in volume fraction of the emulsion phase (Fig. 6). Fig. 8(b) shows the variation of the clay concentration in the aqueous and the emulsion phases with the suspension pH (pH1). In order to appreciate clay partitioning after phase separation, the clay concentration is evaluated based on the weight of the clay to the volume of water present within the phase. For pH1 B/3, the concentration of the clay in the emulsion phase reached a value as high as 13 g l 1 which is much higher than the initial value of 4 g l 1. This would suggest that the excess clay particles would be located either within the diluted bitumen or on the droplet surface. Such a shift in the clay partitioning would also suggest a modification clay surface to become more hydrophobic. For pH1 /3, the clay concentration in both the aqueous and emulsion phases is fairly close to that of the initial suspension concentration. This distribution of clay in aqueous phase would suggest that the clay remained hydrophilic and uniformly distributed among the total aqueous volume. To confirm that the clay particles collected from the upper emulsion phase for pH1 B/3 were hydrophobic, while the clay particles collected from the lower aqueous phase for pH1 /3 remained hydrophilic, the water contact angle on a layer of the clay sample was measured as a function of pH. The results presented in Fig. 9 show that the original clay was hydrophilic with a contact angle value :/48. A contact angle value of B/208 was determined with the clays isolated from the aqueous phase for pH1 /3. The clay collected from the upper emulsion phase at pH1 B/3 gave a contact angle value /1208. Such a high value was obtained even though the solids were washed thoroughly with toluene. The significant decrease in contact angle from 120 to 208 with increasing pH1 correlated well with the significant decrease in the amount of clay remained in the emulsion phase.
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Fig. 8. Effect of kaolinite clay suspension pH on clay partition: (a) mass fraction of clay in emulsion (solid) and aqueous (open) phases; and (b) clay concentration in the emulsion (solid) and in the aqueous (open) phases at an initial clay concentration of 4 g l 1.
Fig. 9. Contact angle of water on clay particles obtained from the upper emulsion phase for suspensions at pH (pH1) B/3 and from the lower aqueous phase for pH1 /3. The initial clay concentration in emulsifying water is 4 g l 1.
To determine whether the clay particles in an O/ W emulsion phase were located mainly inside the diluted bitumen droplets or accumulated at diluted bitumen droplet/water interface, two washing tests were conducted. One test was at pH1 /5.4, another was at pH1 /2. When the O/W emulsions were diluted with water, as described in Section 2,
solids would be removed if they were present outside the diluted bitumen droplets and would be retained if they were present inside the diluted bitumen droplets or if they were strongly attached to the droplet surface. The washing test showed that :/91% of the added clay was removed at pH1 of 5.4, indicating that marginally hydrophobic clay particles (contact angle B/208) were mainly attached weakly to diluted bitumen droplets. However, only 12% of the added clay was washed away at pH1 /2, indicating that most of the clay particles were extracted into the diluted bitumen droplets instead of accumulating at diluted bitumen/water interface to stabilize otherwise expected W/O emulsions. This finding is consistent with a strong hydrophobic characteristics of clays with a contact angle value of /1208. Such a strong hydrophobicity of the clay particles favors the extraction of the particles in oil phase than the accumulation at oil/water interface that would favor the formation of W/O emulsions. Our finding suggests that to stabilize a W/O emulsion by fine solids, the wettability of the solids needs to be controlled within a desired contact angle range. For too strong hydrophobic solids, as in the present case, hydrophobic extraction of solids into organic phase would dominate. As a result, the ability of stabilizing W/O emulsion by solids diminishes due to the lack of the solids at the interface.
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3.4. Conductivity and surface tension In the absence of surfactant, paraffin oil or toluene does not form a stable emulsion in water. The addition of hydrophilic kaolinite clay alone has little effect on the emulsification of pure immiscible organic solvents in water. However, a stable diluted bitumen-in-water emulsion was obtained in the present study, suggesting that some surface-active components were released from bitumen and played a role in bitumen emulsification. It has been confirmed experimentally [24,25] that both cationic and anionic watersoluble surfactants are released from bitumen into the aqueous phase during bitumen extraction. The changes in the conductivity (Fig. 10) and surface tension (Fig. 11) of the aqueous phase before and after contact with the diluted bitumen provided a direct evidence of surfactant release. Due to ion exchange, the addition of the clay to the deionized water altered the conductivity of the water, as shown in Fig. 10 by the difference between the k0 and k1 values. Upon the addition of the diluted bitumen to the suspension, however, the conductivity of the suspension changed drastically, as seen by comparing k1 and k2. Removal of the suspending solids had little effect on the
Fig. 10. Comparison of aqueous phase conductivity measured during emulsification.
Fig. 11. Surface tension of the aqueous phase collected after emulsion creaming.
aqueous phase conductivity (compare k2 and k3). These results suggest the release of ionic species from the diluted bitumen. Whether the release ionic species is surface active species or inorganic salt was confirmed by the results of surface tension measurements, as shown in Fig. 11. It is interesting to note the similarity in the shape of the conductivity and surface tension profiles. This similarity suggests that the release of ionic surface active species is a contributor to the observed increase in aqueous conductivity. Over the alkaline pH range, the decrease of surface tension with increasing pH1 is associated with the increased solubility of anionic surfactants, e.g. carboxylate, sulphate and sulphonates. In this case, anionic surfactant is the predominant contributor to the observed increase in aqueous conductivity and over the acidic pH range, the decrease of surface tension with decreasing pH is associated with the increased solubility of cationic surfactants, e.g. primary and/or secondary amines. Here, cationic surfactants became predominant. The variable extraction of both cationic and anionic surfactants into aqueous phase would account for the measured conductivity and surface tension profiles shown in Figs. 10 and 11, respectively. Accepting the release of surfactants from bitumen, the clay
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surface becomes inevitably modified from initially hydrophilic to hydrophobic, resulting in a pHdependent partitioning behavior, as shown in Fig. 8. The observed transition of contact angle and clay partition (Fig. 9) at pH :/3 is the result of a combined effect from the released surfactants and the added fine clays.
4. Industrial implications In order to accommodate oil sands production levels accompanied by an increased distance between a mine face and the base plant, the mined oil sand lumps are transported via slurry pipeline to the base plant. During the transportation in the slurry pipeline, bitumen is liberated from the oil sand grains and attaches to the available air bubbles within the slurring mixture. At the base plant, the liberated bitumen is floated in a gravity separation vessel for further processing. In addition, in a recent commercial development, bitumen froth recovery is accomplished at the mined phase with a de-aerated bituminous froth containing :/ 60% bitumen, 30% water and 10% coarse solids and fines, which is pipeline-hydrotransported to the base plant. In the two approaches of bituminous transportation, it is essential to avoid waterin-bitumen emulsion, as it would significantly hinder the capacity of transportation due to an excessively high viscosity, as illustrated in this study. This study strongly suggests that the presence of fines would prevent the formation of water-in-bitumen emulsions. It appears that the presence of the fines is beneficial to bituminous transportation, as a reduced viscosity is anticipated. The results of this study confirm those of Joseph’s studies, where it was found beneficial to add fines to froth pipeline transportation [26]. We also identified the existence of a minimal amount of fines needed to avoid W/O emulsification. Determining this critical level should provide a guideline for practical operations. More important, the current study demonstrated tuning the type of emulsion by solids addition, which suppressed the complex zigzag change in emulsion type in the current system, significantly simplifying the control of industrial operations.
5. Conclusions A stable, gel-like water-in-oil emulsion was obtained when toluene-diluted bitumen was mixed with water over five distinct pH regions. A zigzag type of variation of emulsion type with pH was observed. The removal of the original fine solids from bitumen or the addition of kaolinite clay /1 g l 1 suppressed the zigzag type of variation and the gellike W/O emulsion became an oil-in-water emulsion. At a given aqueous pH where W/O emulsion forms, the volume fraction of emulsion phase reduced significantly with the addition of kaolinite clay up to 1 g l 1. Solids addition was able to tune the type of emulsion and hence to control the viscosity of emulsion system. Without the clay addition, the viscosity of the water-in-oil emulsion was excessively high, which can be reduced to a minimum by adding the clay /1 g l 1, at which the inversion of emulsion type was observed. For a suspension pH B/3, the clay obtained from the upper emulsion phase had a water contact angle /1208 and /98% of the added clay was present in the emulsion phase. However, for a suspension pH /3, the contact angle of the clay increased to B/208 and the majority of the added clay remained in the aqueous phase.
Acknowledgements The financial support from NSERC-Industry Research Chair in Oil Sands (held by JHM) and from Alberta Department of Energy is greatly appreciated. One of the authors (ZZ) wishes to thank the Quebec Government for providing an FCAR Postdoctoral Research Fellowship.
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