Ind. Eng. Chem. Res. 1998, 37, 959-965
959
Effect of Hydrophobic and Hydrophilic Clays on Bitumen Displacement by Water on a Glass Surface Suddhasatwa Basu, W. C. Kanda, K. Nandakumar, and Jacob H. Masliyah* Department of Chemical & Material Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6
Experiments were conducted to study the effect of hydrophobic and hydrophilic clays on bitumen displacement by an aqueous phase on a glass surface. A thin coating of bitumen on a glass surface displaced spontaneously in the inward radial direction upon exposure to an aqueous environment containing clay minerals. The initially circular bitumen disk took the shape of a spherical droplet. The dynamic and the static contact angles of bitumen on the glass surface were estimated by measuring the contact radius of bitumen with time. The dynamic and static contact angles in the presence of clay minerals are compared with the results when no clay is dispersed in the aqueous phase. The dynamic contact angle decreased in the presence of hydrophobic clays at higher pH when experiments were performed at 40 °C. The static contact angle also decreased in the presence of hydrophobic clays. The decrease in dynamic and static contact angles is found to be independent of clay concentrations over the range of the experimental study. The change in dynamic and static contact angles is small with increasing pH. Further, in the presence of hydrophobic clays, the dynamic and static contact angles did not decrease significantly at higher temperature, e.g., 80 °C. The effect of hydrophilic clays on the bitumen dynamic contact angle is not appreciable at different pHs and temperatures. The applicability of a previously proposed mathematical model based on the lubrication approximation for bitumen displacement is discussed. The dynamic contact angle predicted by the model is compared with experimental data for hydrophobic and hydrophilic clays at different pH conditions. The implication of the experimental results to hot water bitumen extraction is discussed. Introduction Two major commercial oil sand plants collectively provide about 18% of the Canadian oil consumption from Athabasca bitumen. Each of the two plants is fully integrated from mining the oil sand ore to upgrading the bitumen to refinery feedstock quality. At present, the technology used to produce bitumen from Athabasca oil sands utilizes the Clark hot water process (Clark and Pasternak, 1932). The process is described as follows. In the conditioning stage oil sand is digested with water, steam, and NaOH. Normally the slurry is maintained at about pH 8.2. This step involves bitumen displacement, detachment, and aeration. A rotatory drum or a hydrotransport pipeline is used in this step. The liberated and aerated bitumen present in the watersolids slurry is separated from sand and clay minerals in a quiescent gravity separator. Once the bitumen is separated from the water-solids slurry, it is diluted with a solvent and the solids and water entrained in the bitumen are subsequently removed using centrifuges or inclined plate settlers. Finally, catalytic conversion of bitumen to lighter components is achieved via coking and hydrotreatment. Oil sand consists primarily of 6-16 wt % of bitumen, 82 wt % of minerals, and the rest of water. Normally, a bitumen content of more than 10 wt % is considered as high-grade oil sand, 6-10 wt % medium-grade oil sand, and below 6 wt % a low-grade oil sand (Takamura, * Author to whom correspondence is addressed. Phone: (403) 492-4673. Fax: (403) 492-2881. E-mail: jacob.masliyah@ ualberta.ca.
1985). The minerals consist mainly of quartz and clays. The clay minerals, Kaolinite and Illite, appear within the fines fraction as fines which are defined as particles smaller than 44 µm. In a high-grade oil sand, the fines concentration is below 10 wt % of the total solids content, whereas, in medium- and low-grade oil sands, the fines concentration can be as high as 35 wt % of the solids content. It is observed that bitumen recovery in a primary separation vessel deteriorates drastically while processing low-grade oil sands (Takamura and Wallace, 1988). Such an observation necessitates a thorough study to identify the cause for decreased bitumen recovery in the presence of a high concentration clay minerals. The conditioning stage can be thought of to include (i) bitumen displacement along a sand grain, (ii) bitumen detachment, and (iii) bitumen droplet attachment to an air bubble. The present paper deals only with the effect of the presence of clay minerals on the bitumen displacement and the detachment from a solid substrate. Documentation of the clay effect on this stage of bitumen conditioning would provide in part information on whether the conditioning steps contribute to poor bitumen recovery in the presence of high concentrations of clay minerals. The tests conducted in this study deal with bitumen displacement as measured by the dynamic contact angle of bitumen/water/glass contact line and with bitumen detachment as quantified by the static contact angle of bitumen on a microscope glass surface in the presence of both hydrophobic and hydrophilic clays. Previously, Basu et al. (1996) simulated the conditioning stage of the bitumen liberation process from oil
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960 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998
sands by studying bitumen displacement by water on a glass surface. The experimental results for different water pHs, temperatures, and bitumen volumes on bitumen/water/glass contact line displacement and static contact angle of bitumen on a glass surface agree well with the features of the hot water process. Thus, the displacement velocity of the three-phase contact line and the static contact angle are a good measure of bitumen liberation from sand grains. The method employed by Basu et al. (1996) to measure the dynamic and static contact angles cannot be used due to poor visibility in the presence of clay minerals. Therefore, a different measurement technique is used in the present case which is discussed in the Experimental Section. Basu et al. (1996) provided a theoretical model for bitumen/water/glass contact line displacement based on the lubrication approximation. The mathematical model for bitumen displacement is briefly discussed here, and the predicted dynamic contact angle is compared with the experimental data in the presence of both hydrophobic and hydrophilic clays. Experimental Section Materials. Microscope glass slides were used as the substrate over which bitumen film displacement is measured. The surface of the glass slides was smooth, homogeneous, and hydrophilic in nature. The glass slides were cleaned with chromic acid and then with hot water to remove all impurities. They were rinsed with distilled water and dried before use. An adsorbed water molecular layer can be assumed to be present on the glass surface. Kaolinite clay particles (Hydrite UF) from Georgia Kaolin Company, Inc., were used to study the effect of bitumen displacement. The equivalent spherical diameter of the dry clay particles was 0.2 µm. The bitumen was a coker feed bitumen supplied by Syncrude Canada Ltd. (Edmonton, Alberta, Canada). Distilled water was used in all the experiments. Concentrated HCl or NaOH solutions were used to obtain the desired pH level. Experimental Setup. The experimental setup employed to observe bitumen displacement is shown in Figure 1. A rectangular jacketed vessel (test chamber) made of Plexiglas was fabricated. A detailed sketch of the jacketed vessel is shown in Figure 1a. The jacketed vessel consisted of an outer chamber and an inner chamber. Inlet and outlet tubes were connected to the outer chamber, through which water was circulated to keep the temperature of the inner chamber constant. The top of the inner chamber was open, whereas the bottom was closed. The top of the inner chamber was covered with a lid. A schematic sketch of the experimental setup is shown in Figure 1b. A total reflecting prism was fitted at the bottom of the inner chamber to observe the bitumen/water/glass contact line displacement in the presence of fines. This arrangement was particularly useful since contact line displacement measurements could not be made from the chamber’s side due to poor visibility. A video Hi8 camcorder (ccd V101) with a macrolens was positioned to record the experimental observations. A high-resolution TV monitor was connected to the camcorder for display purposes. Experimental Method. Modification of Clay Particles with Asphaltenes. To alter the clays’ wettability, the clay particles were modified as described by Menon and Wasan (1986) and by Yan and Masliyah (1993). Asphaltenes were first extracted from Alberta Atha-
Figure 1. Experimental setup for observation of bitumen displacement. (a) Details of the jacketed vessel. (b) Schematic view of the experimental setup.
basca bitumen by adding excess hexane to the bitumen. The volume ratio of hexane to bitumen was 4:1. The mixture of bitumen and hexane was stirred in a beaker for 30 min and left undisturbed for 2 h. The asphaltenes precipitated to the bottom of the beaker were filtered out and dried at room temperature for 24 h. A known amount of asphaltenes was dissolved in a 1:1 volume ratio of a toluene/n-heptane (Heptol) mixture. Kaolinite clay particles were then added to the Heptol mixture, and the mixture was stirred for 24 h. The concentration of Kaolinite clay particles in the Heptol mixture was 10 g/L. The treated clay particles were filtered out and were left to dry at room temperature (23 °C) for 24 h. Clays treated with different amounts of asphaltene in a Heptol solution resulted in clays with different contact angles; e.g., at a pH of 6, 1.5 g/L of asphaltene in a Heptol solution gives rise to hydrophobic clays with contact angle θf ) 143° at the air-water interface on the clay. The measured contact angle, θf, of the treated clay particles through the water phase at different asphaltene concentrations was reported by Yan and Masliyah (1996). Estimation of Dynamic and Static Contact Angles. Water from a constant-temperature circulating bath was maintained at the required temperature and circulated through the outer chamber. Bitumen was heated to the water temperature in a separate container and was used to coat a glass plate with a thin sheet of bitumen in the form of a disk (diameter D ) 9.0 mm, thickness W ) 0.762 mm). A schematic photograph of such a plate coated with bitumen is shown in Figure 2. The glass plate was placed in the inner chamber, and it completely touched the bottom of the inner chamber. The inner chamber was then gently filled with water having the same temperature, desired pH, and clay concentration. The top of the inner chamber was then covered with a lid. Because of poor visibility in the presence of clay, the bitumen contact line displacement could not be observed directly from the chamber side. Instead, the bitumen/water/glass contact line displace-
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 961 Table 1. Physical Propertiesa of Bitumen Used for Model Verification
pH
bitumen/water interfacial tension, σbw, N‚m-1
water density, Fw, kg‚m-3
bitumen density, Fb, kg‚m-3
bitumen viscosity, µb, Pa‚s
11 7
0.018 0.047
964.8 964.8
990.5
45.0
a
Temperature, 40 °C.
bitumen volume (a typical volume of bitumen used is 1.98 × 10-8 m3), was evaluated from the mass of the coated bitumen and the density of the bitumen. The static contact angle of bitumen on the glass surface in the presence of water was also measured when the static condition was reached (i.e., t f ∞, θd f θe). Model
Figure 2. (a) Schematic view of bitumen/water/glass contact line displacement. (b) Photograph of bitumen/water/glass contact line displacement viewed from the bottom (pH 11, 40 °C, and hydrophobic clays 2 g/L).
ment was observed from the bottom. The bitumen displaced spontaneously and uniformly along the inward radial direction, except for a few cases where pinning took place and some bitumen fragments were trapped at the pinning point. The data associated with pinning of the contact line were rejected. The change in contact radius of bitumen with time was recorded from the bottom using a total reflecting prism. The contact radius of the bitumen coating decreased with time as the dynamic contact angle increased from a small value to a high value. In Figure 2, a schematic illustration and photographs of the bitumen contact radius at different time intervals are shown. Finally, the contact radius did not change with time. This implies that the static contact angle was formed and the bitumen droplet remained attached to the glass slide. The variation of the contact radius of bitumen on the glass surface with time was measured by replaying the camcorder. The dynamic contact angle was then estimated from the known contact radius of the bitumen film and bitumen volume using “spherical cap” expressions. They are given as
cos θd )
1 - (h/R)2 1 + (h/R)2
π 4 h(3R2 + h2) ) πre3 6 3
(1) (2)
where h is the height of the spherical cap, R is the contact radius of the spherical cap, and re is the equivalent radius of the bitumen droplet. The validity of the spherical cap assumption for bitumen displacement on a glass surface and the use of eq 1 (Foister, 1990) for the estimation of θd are discussed in detail by Basu et al. (1996). The right-hand side of eq 2, the
The dynamics of the contact line displacement are complicated, and despite increasing attention, the problem remains only partially resolved. Different approaches used to theoretically model the liquid/liquid/ solid contact line displacement are discussed by Kistler (1993). Basu et al. (1996) theoretically modeled the bitumen/water/glass contact line displacement assuming that the lubrication theory prevails at the three-phase contact line (de Gennes, 1990). The condition for the lubrication approximation (Batchelor, 1993), (θdFbRv)/ µb , 1, is easily satisfied in this case because of the high bitumen viscosity (Table 1). It is assumed that the viscous dissipation mostly takes place within the bitumen layer (not in water) close to the glass surface, because the bitumen viscosity is much higher than that of water. In the analysis of Basu et al. (1996), viscous dissipation in the bitumen phase is equated to the driving force for contact line displacement. The driving force originates due to the difference in static and initial contact angle of bitumen on the glass surface in the presence of water. Assuming bitumen mass conservation, the final form of the expression for the bitumen dynamic contact angle is given by
() ( (
σbw dθd 1 ) - (θd sin θd) × dt 3 µbR
[
]
) )(
1 - cos θd sin θd 1 - cos θd 1+ sin θd 3+
2
2
)
1 (cos θe - cos θd) (3) ln(δ-1)
The integrated results are plotted in terms of ca. where ca. is the capillary number expressed as ca. ) µbR/(σbwt), µb is the viscosity of bitumen, t is time, σbw is the bitumen/water interfacial tension, δ is the ratio of microscopic and macroscopic scale cut-off regions. In the above expression, the inertial and gravitational effects are neglected since the Bond number (∆FgR2/σbw ∼ 10-2) and Weber number (Fb [dR/dt]2R/σbw ∼ 10-5) are small in the range of experimental parameters of this study. Thus, the dependence of dynamic contact angle on capillary number is not a function of bitumen volume. Equation 3 is solved numerically to determine the dynamic evolution of θd with ca. using an ODE integration package. The initial condition θd(t)0) ) θi is the initial dynamic contact angle of the bitumen on the glass plate in the presence of water, and it is assumed to be the same as the static contact angle of bitumen on the
962 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998
Figure 3. Comparison of side- and bottom-view techniques for dynamic contact angle measurement.
Figure 4. Effect of hydrophobic clays on bitumen displacement at pH 11.
glass plate in air. The value of θi was observed to be 18°. The measured static contact angles, θe, of the bitumen on the glass plate in the presence of water containing hydrophilic and hydrophobic clays are used in the solution of eq 3. A δ value of 5 × 10-3 is used in eq 3. This δ value is a fitted parameter which is found to match the experimental data for θd in the presence of salts and surfactants (Basu et al., 1998). Results and Discussion At first, the present experimental technique for the dynamic contact angle measurement is verified. The dynamic contact angle estimated by measuring the contact radius from the bottom of the chamber is compared with that measured from the chamber side in the presence of clays. The effect of hydrophobic clays on bitumen dynamic contact angle at different pH conditions is discussed, followed by a discussion on the effect of hydrophilic clays. Although the addition of NaOH and HCl to distilled water has the dual effect of changing the water pH and its ionic strength, the changes in bitumen dynamic and static contact angles are due to pH changes rather than to the total ionic strength of the electrolytes (Basu et al., 1998). The effect of temperature on dynamic contact angle in the presence of clays is also discussed. The static contact angles of bitumen on the glass surface in the presence of water containing clays are presented. Finally, the predicted dynamic contact angle using a model for bitumen displacement is compared with the experimental data for hydrophobic and hydrophilic clays at different pH conditions. Verification of the Estimated Dynamic Contact Angle. In Figure 3, a comparison of the dynamic contact angles estimated using the bottom- and sideview techniques is shown for low concentrations of hydrophilic clays. At a low clay concentration, e.g., 0.5 g/L, the bitumen/water/glass contact line displacement could be observed from the chamber side against a strong background light. In the side-view technique, the contact angle is estimated from eq 1 by measuring h and R as discussed by Foister (1990) and Basu et al. (1996). It is seen in Figure 3 that the estimated dynamic contact angle values are fairly similar for both
Figure 5. Effect of hydrophobic clays on bitumen displacement at pH 7.
measurement techniques. In the presence of hydrophobic clays, the estimated dynamic contact angles for the bottom- and side-view techniques were also fairly similar. Thus, the bottom-view technique can be used for the estimation of dynamic contact angle in the presence of clays. Effect of Hydrophobic Clays. Figures 4-6 show the variation of the dynamic contact angle, θd, for the bitumen/water interface with time at different pHs and hydrophobic clay concentrations. It is observed that θd increases with time until it reaches a steady value which is known as the static contact angle, θe. The rate of change of θd is a measure of the bitumen displacement rate. The value of θe quantifies the bitumen droplet detachment (Wallace et al., 1990; Dussan and Chow, 1984) from a solid substrate e.g., a bitumen droplet with θe value close to 180° is easy to detach. In Figures 4-6, results for three different pH conditions at 40 °C are shown. At pH 11, in the presence of clays, Figure 4 shows that the bitumen dynamic contact angle is lower when compared with the results for no
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 963
Figure 6. Effect of hydrophobic clays on bitumen displacement at pH 3.
Figure 7. Effect of hydrophobic clays on bitumen displacement at 80 °C for two different pH conditions.
clay case. The static contact angle of bitumen decreases from 170° to 150°. It is to be noted that the bitumen dynamic and static contact angles do not decrease further when the clay concentration is increased to a very high value, e.g., 10 g/L. For a given clay concentration, the change in dynamic and static contact angles is small with increasing pH. This is illustrated through Figures 5 and 6 for pH 7 and 3, respectively. At pH 7, Figure 5 shows that the static contact angle decreases from 138° to 122°, whereas at pH 3, no appreciable decrease in the static contact angle is observed due to the presence of clays. In Figure 7, the effect of hydrophobic clays on the bitumen dynamic and static contact angles is shown for two different pH values at a higher temperature of 80 °C. With all other conditions being the same, Figure 7 shows that, at 80 °C, the bitumen dynamic contact angle follows the same time variation. Furthermore, the bitumen static contact angle did not decrease appreciably at the higher temperature. Effect of Hydrophilic Clays. The dynamic and static contact angles are not found to decrease ap-
Figure 8. Effect of hydrophilic clays on bitumen displacement at pH 11.
preciably in the presence of hydrophilic clays at different pH conditions at both 40 and 80 °C. Figure 8 shows typical measured values of the dynamic and static contact angles at a pH of 11 and 40 °C. At this temperature and pH, hydrophilic clays have a negligible effect compared to hydrophobic clays. The hydrophilic clays are not adsorbed at the bitumen/water interface (Yan and Masliyah, 1993, 1996) and thus little change in the dynamic contact angle is observed. Further, the hydrophilic clays at pH 3 settled quickly in the bottom of the test chamber. The settling of clay particles was decreased with the increase in pH. It may be postulated that the positively charged edges of the clays are electrostatically attracted to the negatively charged faces of the clays at pH 3. This results in flocculation of the clay particles, and thus enhanced settling was observed. The enhanced settling of hydrophobic clays was not observed at a low pH. Model Verification. In Figure 9, the dynamic contact angle, θd, of the bitumen/water interface on the glass surface is plotted against capillary number, ca., for different clay conditions. The model predictions shown by the lines are generated by solving eq 3 for different static contact angles measured in the presence of hydrophobic and hydrophilic clays. The experimental data were well predicted by the model while using the same δ value of our previous study with salt and surfactant (Basu et al., 1998). The δ value (5 × 10-3) is consistent with the generally agreed upon value available in the literature (Kistler, 1993). It is seen in Figure 9 that the θd in the range of 0.01 > ca. > 1 is slightly underpredicted by the model. This may be due to the dynamic nature of the bitumen/water interfacial tension value. It should be noted that eq 3 is valid for an instantaneous change in the initial dynamic contact angle to the static contact angle. At large time scale, the dynamic interfacial tension becomes important since the bitumen/water interface composition changes with time. Due to the dynamic nature of interfacial tension, the static contact angle also changes with time. For the model verification, ca. is estimated by using an average value of the interfacial tension within the time scale of bitumen displacement. The error introduced in using the average interfacial tension value is negligible since, within the time scale of bitumen displacement, the
964 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998
Figure 9. Model prediction for hydrophobic and hydrophilic clays at different pH conditions.
interfacial tension does not change significantly (Isaacs and Smolek, 1983). The physical properties of bitumen and water used for model verification are given in Table 1 (Isaacs and Smolek, 1983; Takamura and Isaacs, 1989). The bitumen/water interfacial tension is not changed much in the presence of clays. Figure 9 shows that the dynamic contact angle depends on the capillary number through a sigmoidal curve. Initially, the contact line velocity increases rapidly and then decreases as the dynamic contact angle approaches the static contact angle value due to the decrease in the driving force. Implication of the Results to Bitumen Extraction It is well documented that the presence of large quantities of clay minerals in low-grade oil sand has a very detrimental effect on the primary and total recovery of bitumen from oil sands (Takamura and Wallace, 1988). The tests conducted here deal with the bitumen displacement as measured by the dynamic contact angle and to bitumen detachment as quantified by the static contact angle formed by the bitumen on the microscope glass slide. The microscope glass slide is a good substitute for the model sand surface to perform the laboratory test. Bitumen displacement and detachment phenomena are involved in the conditioning stage of the hot water extraction process. As would be noted from the contact angle of the three-phase contact line variation with time, by and large, addition of clays has little effect on the bitumen displacement and detachment. It is likely that the reasons for low bitumen recovery in the presence of fine clay minerals is due to poor bitumen-bitumen coalescence and/or poor bitumen droplet attachment to air bubbles. Takamura and Wallace (1988) postulated that the increase in suspension viscosity due to the agglomeration of clay particles hinders the rising of bitumen droplets in a quiescent gravity separator. Further studies are needed to confirm this suggestion. Conclusions To investigate the effect of hydrophobic and hydrophilic clays during the conditioning stage of the hot water extraction process, experiments were performed by exposing a bitumen-coated glass plate in an aqueous
environment containing clays. The thin disk-shaped bitumen coating displaced spontaneously in the inward radial direction to form a droplet. The dynamic and static contact angles of the bitumen/water interface on the glass surface were estimated from measurement of the contact radius with time. The contact radius was measured from the bottom of the test chamber using a total reflecting prism because of poor visibility in the presence of clay minerals. The measured dynamic contact angle from the bottom and that from the sides are fairly similar for a particular experimental condition. In general, the dynamic contact angles did not change significantly in the presence of hydrophobic and hydrophilic clays when compared with the results of no clay case. This implies that the low bitumen recovery in the presence of clay minerals cannot be attributed to bitumen displacement and detachment. The bitumen displacement model discussed in this study predicts the experimental data quite well in the presence of hydrophobic and hydrophilic clays. Nomenclature ca. ) capillary number, defined as ca. ) µbR/(σbwt) h ) height of the spherical cap, m re ) equivalent radius of the bitumen drop, m R ) contact radius of the spherical cap, m t ) time, s v ) velocity of the contact line ()dR/dt), m‚s-1 Greek Letters δ ) ratio of microscopic and macroscopic cut-off regions µb ) bitumen viscosity, Pa‚s θd ) dynamic contact angle of bitumen, deg θe ) static contact angle of bitumen, deg θf ) contact angle of the air-water interface through water on clay, deg Fb ) bitumen density, kg‚m-3 σbw ) bitumen/water interfacial tension, N‚m-1 ∆F ) difference in densities between bitumen and water
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Received for review July 14, 1997 Revised manuscript received December 5, 1997 Accepted December 10, 1997 IE9705012
