- Email: [email protected]
water contact line displacement due to instability
Fuel 79 (2000) 837–841 www.elsevier.com/locate/fuel
A study on daughter droplets formation in bitumen/glass/water contact line displacement due to instability S. Basu a,*, K. Nandakumar b, J.H. Masliyah b a
Department of Chemical Engineering, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India b Department of Chemical and Material Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6
Abstract Experiments were performed to study the displacement of a rectangular strip bitumen, coated on a glass plate, in the presence of water at different pH and temperature. During the displacement, modulated structure formed at the bitumen/water/glass contact line and ridge-like structure formed at the bitumen/water interface. The modulated and ridge structures formed due to the contact line and the free surface instabilities. These instabilities grew further, which led to the formation of daughter droplets. On the contrary, circular bitumen disk displaced uniformly in the inward radial direction to form single droplet in the presence of water. The number of daughter droplets formed increases with the decrease in the width of the bitumen strip for a constant strip length. It was observed that the number of daughter droplets increases with the increase in pH and temperature of water. The observed growth of instability and the experimental results were compared with the instability analyses on dewetting process available in the literature. It is recommended that a non-symmetrical shape of bitumen film on sand grain may be detrimental to bitumen separation in an oil sand extraction unit. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: Oil sand extraction; Bitumen displacement; Contact line instability; Free surface instability; Dynamic contact angle; Equilibrium contact angle
1. Introduction Hot Water Process [1] of oil sand extraction and enhanced-oil-recovery [2] of crude oil involve displacement and detachment of oil from sand grain. The mechanism of oil sand disintegration in Hot Water Process extensively studied by Takamura and Chow [3], Takamura [4], Takamura and Wallace [5] and Basu et al. [6]. Takamura and Chow [3] dipped oil sand grain in water and observed it under the microscope. They elucidated the mechanism of oil sand extraction in Hot Water Process based on microscopic observations and experimental results. The different steps involved in Hot Water Process are bitumen film displacement and droplet formation and bitumen droplet detachment from sand grain in digestion stage, collection of bitumen from bitumen–solid mixture in floatation column and conversion of bitumen to lighter components by catalytic coking and hydrotreatment. Our series of investigations mainly focused on the detail study of the bitumen displacement and detachment from the solid surface. Basu et al. [6–8] used bitumen coated glass plate to study the bitumen displacement in the presence of water containing salt, surfactants and clays at different temperature and * Corresponding author. E-mail address: [email protected] (S. Basu).
pH. Further, oil droplet detachment from solid surface in a shear field was also studied by Basu et al. [9]. In the bitumen displacement study of Basu et al. [6], bitumen was coated as the shape of a thin circular disk on the microscope glass slide and it was found to displace in the inward radial direction to form single droplet in the presence of water. The bitumen will be present on the sand grain surface in an arbitrary fashion instead of a symmetric fashion like a thin circular disk. In the present investigation, a rectangular-strip bitumen is coated on the glass surface and the bitumen displacement is observed in the presence of water at different pH and temperature. Further, the width of the bitumen strip is varied for constant strip length to study the effect of strip dimension. Brochard-Wyart and Redon [10] studied the drying of hexadecane, dodecane, poly(dimethylsiloxane)silicone oil (PDMS) on florinated wafers and silicon wafers, respectively. The drying of these oils occurs through the formation and extension of holes. They observed that during the extension of a hole, the initial circular rim with uniform width changed to modulated rim and finally it broke down to droplets. They observed varicose instability, i.e. the amplitude of the contact line instability at the two sides of the rim was out of phase. They analyzed the dynamics of liquid rim instability following the work of Plateau [11] and Rayleigh [12,13] on the break-up of liquid jets, Sekimoto et al. [14] on
0016-2361/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(99)00203-3
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Fig. 2. Schematic view of growth of bitumen/water/glass contact line and bitumen/water interface instabilities and daughter droplets formation. Fig. 1. Experimental setup for observation of instability during bitumen displacement. (a) Details of the jacketed vessel. (A) Outer chamber; (B) inner chamber; (C) inlet tube; (D) outlet tube; (E) protrusion; (F) rectangular bitumen strip; (G) lid. (b) Schematic view of the experimental setup.
instability of a liquid ribbons deposited on flat solid surface under static condition. Brochard-Wyart and Redon [10] extended the static theory to apply in dynamic condition of the drying process. These studies mainly consider growth of instability at the oil/solid contact line. In the present study, it is observed that the instability grows both at bitumen/water interface, as well as, bitumen/water/glass contact line simultaneously during the displacement of bitumen. Spaid and Homsy [15] theoretically studied the stability of contact line by considering equation for free surface evolution and applying contact line displacement models, e.g. precursor film and slip, as the boundary condition. The theoretical results of Spaid and Homsy [15] in the given form cannot be used to verify our experimental observations and results. The present experimental observations and results are compared with the theoretical model developed by Brochard-Wyart and Redon [10].
in all the experiments. Concentrated HCl or NaOH solutions were used to obtain the desired pH level. 2.2. Experimental setup The experimental setup used to estimate bitumen displacement rate is shown in Fig. 1a and a schematic of the video recording setup is shown in Fig. 1b. A rectangular jacketed vessel (test chamber) made of Plexiglas was fabricated. A detailed sketch of the jacketed vessel is shown in Fig. 1a. The jacketed vessel consisted of an outer chamber (A) and an inner chamber (B). Tubes (C) and (D) were connected to the outer chamber through which water was circulated to keep the temperature of the inner chamber constant. Small protrusions (E) on the wall were made at the middle of the inner chamber to hold the test plate (F). The top of the inner chamber was covered with a lid (G). 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.
2. Experimental
2.3. Experimental method
2.1. Materials
Water from a constant temperature circulating bath was maintained at the required temperature and circulated through the outer chamber. The inner chamber was half filled with water having the desired pH and temperature. 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 rectangular strip. The coating was made with the help of rectangular template so that in each case the initial geometry of the bitumen strip was the same. The length to width ratio, L=D were varied from 5 to 64. The width of the bitumen strip was varied from 0.001 to
Microscope glass slides were used as the substrate over which displacement of rectangular shaped bitumen strip was observed. 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 may be present on the glass surface. The bitumen is a coker feed bitumen supplied by Syncrude Canada Ltd. Distilled water was used
S. Basu et al. / Fuel 79 (2000) 837–841
Fig. 3. Comparison of rectangular strip and disk shaped bitumen displacement. Disk shape bitumen displaced uniformly to form single droplet (L=D 1; pH 11; 40⬚C). Growth of bitumen/water interface instability during the displacement of rectangular bitumen strip and the formation of two daughter droplets (L=D 10; pH 8:0; 40⬚C).
0.012 m and the L was kept constant (0.064 m). The strip thickness was kept constant which is 7:62 × 10⫺4 m: The glass plate was then placed in the inner chamber and held by the protrusion on the walls. The remaining part of the inner chamber was filled gently with water having the appropriate temperature, pH and its top was then covered with the lid. The displacement of bitumen by water was recorded by the video Hi8 camcorder using a macrolens. The bitumen displaced spontaneously in the span wise and length-wise direction of the rectangular strip. Finally, the thin rectangular shaped strip of bitumen fragmented into daughter droplets. A schematic view of the bitumen/water interface and bitumen/water/glass contact line evolution and the formation of daughter droplets are shown in Fig. 2. The number of droplets formed for different L=D ratio, pH and temperature were counted by replaying the camcorder. In few cases, pinning took place because of the presence of dust on the glass surface and some bitumen fragments were trapped at the pinning point.
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of daughter droplets 1. The bitumen/water/glass contact line velocity was not constant throughout the strip length. The contact line velocity at the contracted region was faster than that at the thicker region. The bitumen/water/glass contact line velocity was also observed to decrease with time as the dynamic contact angle approached the static contact angle. The growth of instability, both at the bitumen/water interface and bitumen/water/glass contact line, led to the formation of modulated contact line and ridge structure at the free interface (Fig. 2b). Finally, modulated contact line and ridge structure grew further with time resulting in the formation of daughter droplets (Fig. 2c). In the previous investigation of Basu et al. [6], the disk shaped bitumen coating on a glass slide moved in the inward radial direction to form single droplet in the presence of water. The bitumen/water/glass contact line displacement was uniform throughout the circular disk. The modulated contact line and ridge like structure at the free interface were not observed. In Fig. 3, bitumen displacement for a rectangular strip and circular disk are compared. It is seen in Fig. 3b that the ridge like structure is formed for rectangular strip bitumen of L=D 10; whereas in the case of disk shaped bitumen L=D 1; the contact line displacement was uniform and no ridge like structure is formed. Fig. 3d shows the formation of two daughter droplets for rectangular strip bitumen whereas disk shaped bitumen displaced to form single droplet. In Fig. 3d, it is seen that small daughter droplets were formed between two large daughter droplets for rectangular strip bitumen. They are formed because of pinning in the presence of dust on glass surface. No particular pattern was observed regarding the number of these droplets formed and their sizes. The static contact angle of bitumen droplet for disk shaped bitumen is larger than that for rectangular bitumen strip because of the difference in pH values. The effect of pH on the static contact angle of bitumen droplet were studied by Basu et al. [6]. The visual observation conducted by Basu et al. [16] showed the formation of irregular shape of contact line during bitumen displacement. The bitumen is present in the oil sand grain in an arbitrary fashion and not as a circular disk. Thus, during the displacement of bitumen by water on sand grain, the instability may grow at the free interface as well as at the contact line leading to the formation of a large number of daughter droplets. This may be detrimental to the bitumen separation in an oil sand extraction unit. 3.2. Comparison with other studies
3. Results and discussion 3.1. Daughter droplets formation The rectangular shaped bitumen strip with L=D q 1; moved in the spanwise and lengthwise direction when it was exposed to water and the daughter droplets formed. Fig. 2 shows the sequence of steps involved in the formation
The experimental conditions, observations and results are compared with that of Brochard-Wyart and Redon [10]. The Bitumen viscosity is high mb 45 Pa: s at 40⬚C) and thus bitumen displacement is mainly governed by viscous 1 This is true for an ideal situation. In oil sand grain, the bitumen displacement may not be uniform as shown in Fig. 2 because of surface roughness and impurities.
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glass contact line displacement the contact angle changes from equilibrium contact angle in the presence of air to that in the presence of water at a given pH and temperature. Finally, the velocity of the contact line is not constant during bitumen displacement whereas propagation of dry patches take place at a constant velocity. 3.3. L=D ratio
Fig. 4. Number of daughter droplets formed for different L=D ratio (pH 8, 40⬚C).
regime and not by viscoinertial regime. This is validated on the basis of theoretical model developed by Brochard-Wyart and Redon [10] on the dynamics of liquid rim instability. According to them the viscous regime prevails when wave vector, q ⴱ, multiplied by width of the strip, D, is less than 1 qⴱ D ⬍ 1 and viscoinertial regime prevails when qⴱ D ⬎ 1: The wave vector is defined as, qⴱ sbw rb u5e =m2b ; where s bw is the bitumen/water interfacial tension, r b is the bitumen density, u e is the equilibrium contact of bitumen and m b is the bitumen viscosity. Brochard-Wyart and Redon [10] developed the instability theory for the expansion of dry patches on a solid surface for a constant contact angle during the contact line displacement. They observed that the contact angle of the rim remains constant during the expansion of dry patches. It should be noted that the contact angle varied during the bitumen displacement. Thus an average value of contact angle is used to estimate q ⴱ. The value q ⴱ for the bitumen–glass–water system 2 is 0.043 m ⫺1. q ⴱD value varies from 4:3 × 10⫺5 to 5:18 × 10⫺4 for the range of D value employed in the experiment. Here qⴱ D p 1 and thus bitumen displacement is governed by viscous regime only. Further, Brochard-Wyart and Redon [10] pointed out that the growth of contact line instability is faster for capillary length, k⫺1 ⬎ D than that for k⫺1 ⬍ D: The capillary length is defined as, k⫺1 sbw =rg1=2 : The k ⫺1 for bitumen–water sytem is 0.0015 m. It was indeed observed that the growth of instability is faster for D 0:001 m than that for D 0.004 m. According to Brochard-Wyart and Redon [10], the growth of instability is very slow for k⫺1 p D and thus the instability can not be observed during the time scale of the experiment. However, the growth of instability during the bitumen displacement was observed for D 0:012 m: This may be due to the fact that the instability grows both at the bitumen/water/glass contact line, as well as at bitumen/water interface for bitumen/water/glass system. Further, during the bitumen/water/ 2 Physical properties (pH 8.0, 40⬚C): 990:5 kg=m3 ; ue avg 1:37; mb 45 Pa s:
sbw 22 mN=m;
rb
In Fig. 4, number of daughter droplets formed, N, is plotted against length to width ratio, L=D; of the bitumen strip for pH 8 and at 40⬚C. The disjointed straight lines indicate that the number of daughter droplets formed is constant for a particular range of L=D ratio. The disjointed portion of the straight line corresponds to a transition region, where the number of daughter droplets formation changes with L=D ratio. The number of daughter droplets formed decreases with the decrease in L=D ratio in a stepwise manner. The L=D ratio was changed by varying the width, D, of the bitumen strip. At a low value of L=D ratio, D becomes greater than k ⫺1 which leads to slow growth of instability during bitumen/water/glass contact line displacement. The slow bitumen/water/glass contact line velocity and high relaxation time give rise to decrease in number of daughter droplets. In Fig. 3b (rectangular strip), four peaks are visible on bitumen/water interface indicating the possibility of four daughter droplets formation. However, due to slow bitumen/water/glass contact line velocity and high relaxation time, two daughter droplets are formed (Fig. 3d). The distance between daughter droplets was not constant for a particular L=D ratio. This is due to the end effect since the bitumen strip length is finite. 3.4. pH and temperature The preliminary study shows that the number of daughter droplets formation increases with the increase in pH and temperature. However, the effect of pH on the number of daughter droplets formation is less than the effect of temperature on the same. The bitumen/water interfacial tension decreases with the increase in pH. The effect of temperature on bitumen/water interfacial tension is minimal whereas the bitumen viscosity drastically decreases with the increase in temperature. Further, the equilibrium contact angle of bitumen on the glass slide in the presence of water increases with increase in pH [6]. The detail physical properties of bitumen/water system is available in Takamura and Isaacs [17] and Basu et al. [6]. The combining effect of all these parameters influences the growth of instability at bitumen/water interface and bitumen/water/ glass contact line. It will be interesting to know that whether the large number of daughter droplets are formed at the operating pH and temperature, in the digestion stage of an oil sand extraction unit.
S. Basu et al. / Fuel 79 (2000) 837–841
4. Conclusion The rectangular shaped bitumen strip coated on glass plate shows the growth of ridge and modulated structures at bitumen/water interface and bitumen/water/glass contact line, respectively, upon exposure to water. The modulated contact line and ridge like structure at the interface grows further and daughter droplets are formed. Whereas the circular bitumen disk displaced uniformly in the inward radial direction to form single bitumen droplet in the presence of water. The number of daughter droplets formed decreases with the increase in width of the bitumen strip for a constant strip length. Further, the number of bitumen droplets formed increases with the increase in pH and temperature. The present experimental results follow the contact line instability analyses developed by BrochardWyart and Redon [10].
References [1] Clark KA, Pasternak SD. Ind Engng Chem 1932;24:1410–16.
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[2] Moore TF, Slobod RL. Producers Mon Penn Oil Prod Assoc 1956;20:20–30. [3] Takamura K, Chow RS. J Can Petrol Technol 1983;22(6):22–30. [4] Takamura K. AOSTRA J Res 1985;2(1):1. [5] Takamura K, Wallace D. J Can Petrol Technol 1988;27(6):98–106. [6] Basu S, Nandakumar K, Masliyah JH. J Colloid Interface Sci 1996;182:82–94. [7] Basu S, Nandakumar K, Masliyah JH. Colloid Surf A: Physicochem Engng Aspects 1998;136:71–80. [8] Basu S, Nandakumar K, Masliyah JH. Ind Engng Chem Res 1998;37:959–65. [9] Basu S, Nandakumar K, Masliyah JH. J Colloid Interf Sci 1997;190:253–7. [10] Brochard-Wyart F, Redon C. Langmuir 1992;8:2324–9. [11] Plateau J. Statique expe´rimentale et the´orique des liquides soumis aux seules forces mole´culaires, Paris: Gauthier-Villars, 1873. [12] Rayleigh JWS. Lord Rayleigh Scientific Papers, Cambridge: Cambridge University Press, 1899. [13] Rayleigh JWS. Phil Mag 1892;34:145. [14] Sekimoto K, Oguma R, Kawasaki K. Ann Phys 1987;176:359–92. [15] Spaid MA, Homsy GM. Phys Fluids 1996;8(2):460–78. [16] Basu S, Nandakumar K, Masliyah JH. J Colloid Interf Sci 1998;205:201–3. [17] Takamura K, Isaacs EE. Interfacial proposeties. In: Hepler LG, His C, editors. AOSTRA technical handbook on oil sands, bitumen and heavy oils, 6. Edmonton, Alberta, Canada: AOSTRA, 1989. p. 101.
A study on daughter droplets formation in bitumen/glass/water contact line displacement due to instability S. Basu a,*, K. Nandakumar b, J.H. Masliyah b a
Department of Chemical Engineering, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India b Department of Chemical and Material Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6
Abstract Experiments were performed to study the displacement of a rectangular strip bitumen, coated on a glass plate, in the presence of water at different pH and temperature. During the displacement, modulated structure formed at the bitumen/water/glass contact line and ridge-like structure formed at the bitumen/water interface. The modulated and ridge structures formed due to the contact line and the free surface instabilities. These instabilities grew further, which led to the formation of daughter droplets. On the contrary, circular bitumen disk displaced uniformly in the inward radial direction to form single droplet in the presence of water. The number of daughter droplets formed increases with the decrease in the width of the bitumen strip for a constant strip length. It was observed that the number of daughter droplets increases with the increase in pH and temperature of water. The observed growth of instability and the experimental results were compared with the instability analyses on dewetting process available in the literature. It is recommended that a non-symmetrical shape of bitumen film on sand grain may be detrimental to bitumen separation in an oil sand extraction unit. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: Oil sand extraction; Bitumen displacement; Contact line instability; Free surface instability; Dynamic contact angle; Equilibrium contact angle
1. Introduction Hot Water Process [1] of oil sand extraction and enhanced-oil-recovery [2] of crude oil involve displacement and detachment of oil from sand grain. The mechanism of oil sand disintegration in Hot Water Process extensively studied by Takamura and Chow [3], Takamura [4], Takamura and Wallace [5] and Basu et al. [6]. Takamura and Chow [3] dipped oil sand grain in water and observed it under the microscope. They elucidated the mechanism of oil sand extraction in Hot Water Process based on microscopic observations and experimental results. The different steps involved in Hot Water Process are bitumen film displacement and droplet formation and bitumen droplet detachment from sand grain in digestion stage, collection of bitumen from bitumen–solid mixture in floatation column and conversion of bitumen to lighter components by catalytic coking and hydrotreatment. Our series of investigations mainly focused on the detail study of the bitumen displacement and detachment from the solid surface. Basu et al. [6–8] used bitumen coated glass plate to study the bitumen displacement in the presence of water containing salt, surfactants and clays at different temperature and * Corresponding author. E-mail address: [email protected] (S. Basu).
pH. Further, oil droplet detachment from solid surface in a shear field was also studied by Basu et al. [9]. In the bitumen displacement study of Basu et al. [6], bitumen was coated as the shape of a thin circular disk on the microscope glass slide and it was found to displace in the inward radial direction to form single droplet in the presence of water. The bitumen will be present on the sand grain surface in an arbitrary fashion instead of a symmetric fashion like a thin circular disk. In the present investigation, a rectangular-strip bitumen is coated on the glass surface and the bitumen displacement is observed in the presence of water at different pH and temperature. Further, the width of the bitumen strip is varied for constant strip length to study the effect of strip dimension. Brochard-Wyart and Redon [10] studied the drying of hexadecane, dodecane, poly(dimethylsiloxane)silicone oil (PDMS) on florinated wafers and silicon wafers, respectively. The drying of these oils occurs through the formation and extension of holes. They observed that during the extension of a hole, the initial circular rim with uniform width changed to modulated rim and finally it broke down to droplets. They observed varicose instability, i.e. the amplitude of the contact line instability at the two sides of the rim was out of phase. They analyzed the dynamics of liquid rim instability following the work of Plateau [11] and Rayleigh [12,13] on the break-up of liquid jets, Sekimoto et al. [14] on
0016-2361/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(99)00203-3
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S. Basu et al. / Fuel 79 (2000) 837–841
Fig. 2. Schematic view of growth of bitumen/water/glass contact line and bitumen/water interface instabilities and daughter droplets formation. Fig. 1. Experimental setup for observation of instability during bitumen displacement. (a) Details of the jacketed vessel. (A) Outer chamber; (B) inner chamber; (C) inlet tube; (D) outlet tube; (E) protrusion; (F) rectangular bitumen strip; (G) lid. (b) Schematic view of the experimental setup.
instability of a liquid ribbons deposited on flat solid surface under static condition. Brochard-Wyart and Redon [10] extended the static theory to apply in dynamic condition of the drying process. These studies mainly consider growth of instability at the oil/solid contact line. In the present study, it is observed that the instability grows both at bitumen/water interface, as well as, bitumen/water/glass contact line simultaneously during the displacement of bitumen. Spaid and Homsy [15] theoretically studied the stability of contact line by considering equation for free surface evolution and applying contact line displacement models, e.g. precursor film and slip, as the boundary condition. The theoretical results of Spaid and Homsy [15] in the given form cannot be used to verify our experimental observations and results. The present experimental observations and results are compared with the theoretical model developed by Brochard-Wyart and Redon [10].
in all the experiments. Concentrated HCl or NaOH solutions were used to obtain the desired pH level. 2.2. Experimental setup The experimental setup used to estimate bitumen displacement rate is shown in Fig. 1a and a schematic of the video recording setup is shown in Fig. 1b. A rectangular jacketed vessel (test chamber) made of Plexiglas was fabricated. A detailed sketch of the jacketed vessel is shown in Fig. 1a. The jacketed vessel consisted of an outer chamber (A) and an inner chamber (B). Tubes (C) and (D) were connected to the outer chamber through which water was circulated to keep the temperature of the inner chamber constant. Small protrusions (E) on the wall were made at the middle of the inner chamber to hold the test plate (F). The top of the inner chamber was covered with a lid (G). 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.
2. Experimental
2.3. Experimental method
2.1. Materials
Water from a constant temperature circulating bath was maintained at the required temperature and circulated through the outer chamber. The inner chamber was half filled with water having the desired pH and temperature. 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 rectangular strip. The coating was made with the help of rectangular template so that in each case the initial geometry of the bitumen strip was the same. The length to width ratio, L=D were varied from 5 to 64. The width of the bitumen strip was varied from 0.001 to
Microscope glass slides were used as the substrate over which displacement of rectangular shaped bitumen strip was observed. 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 may be present on the glass surface. The bitumen is a coker feed bitumen supplied by Syncrude Canada Ltd. Distilled water was used
S. Basu et al. / Fuel 79 (2000) 837–841
Fig. 3. Comparison of rectangular strip and disk shaped bitumen displacement. Disk shape bitumen displaced uniformly to form single droplet (L=D 1; pH 11; 40⬚C). Growth of bitumen/water interface instability during the displacement of rectangular bitumen strip and the formation of two daughter droplets (L=D 10; pH 8:0; 40⬚C).
0.012 m and the L was kept constant (0.064 m). The strip thickness was kept constant which is 7:62 × 10⫺4 m: The glass plate was then placed in the inner chamber and held by the protrusion on the walls. The remaining part of the inner chamber was filled gently with water having the appropriate temperature, pH and its top was then covered with the lid. The displacement of bitumen by water was recorded by the video Hi8 camcorder using a macrolens. The bitumen displaced spontaneously in the span wise and length-wise direction of the rectangular strip. Finally, the thin rectangular shaped strip of bitumen fragmented into daughter droplets. A schematic view of the bitumen/water interface and bitumen/water/glass contact line evolution and the formation of daughter droplets are shown in Fig. 2. The number of droplets formed for different L=D ratio, pH and temperature were counted by replaying the camcorder. In few cases, pinning took place because of the presence of dust on the glass surface and some bitumen fragments were trapped at the pinning point.
839
of daughter droplets 1. The bitumen/water/glass contact line velocity was not constant throughout the strip length. The contact line velocity at the contracted region was faster than that at the thicker region. The bitumen/water/glass contact line velocity was also observed to decrease with time as the dynamic contact angle approached the static contact angle. The growth of instability, both at the bitumen/water interface and bitumen/water/glass contact line, led to the formation of modulated contact line and ridge structure at the free interface (Fig. 2b). Finally, modulated contact line and ridge structure grew further with time resulting in the formation of daughter droplets (Fig. 2c). In the previous investigation of Basu et al. [6], the disk shaped bitumen coating on a glass slide moved in the inward radial direction to form single droplet in the presence of water. The bitumen/water/glass contact line displacement was uniform throughout the circular disk. The modulated contact line and ridge like structure at the free interface were not observed. In Fig. 3, bitumen displacement for a rectangular strip and circular disk are compared. It is seen in Fig. 3b that the ridge like structure is formed for rectangular strip bitumen of L=D 10; whereas in the case of disk shaped bitumen L=D 1; the contact line displacement was uniform and no ridge like structure is formed. Fig. 3d shows the formation of two daughter droplets for rectangular strip bitumen whereas disk shaped bitumen displaced to form single droplet. In Fig. 3d, it is seen that small daughter droplets were formed between two large daughter droplets for rectangular strip bitumen. They are formed because of pinning in the presence of dust on glass surface. No particular pattern was observed regarding the number of these droplets formed and their sizes. The static contact angle of bitumen droplet for disk shaped bitumen is larger than that for rectangular bitumen strip because of the difference in pH values. The effect of pH on the static contact angle of bitumen droplet were studied by Basu et al. [6]. The visual observation conducted by Basu et al. [16] showed the formation of irregular shape of contact line during bitumen displacement. The bitumen is present in the oil sand grain in an arbitrary fashion and not as a circular disk. Thus, during the displacement of bitumen by water on sand grain, the instability may grow at the free interface as well as at the contact line leading to the formation of a large number of daughter droplets. This may be detrimental to the bitumen separation in an oil sand extraction unit. 3.2. Comparison with other studies
3. Results and discussion 3.1. Daughter droplets formation The rectangular shaped bitumen strip with L=D q 1; moved in the spanwise and lengthwise direction when it was exposed to water and the daughter droplets formed. Fig. 2 shows the sequence of steps involved in the formation
The experimental conditions, observations and results are compared with that of Brochard-Wyart and Redon [10]. The Bitumen viscosity is high mb 45 Pa: s at 40⬚C) and thus bitumen displacement is mainly governed by viscous 1 This is true for an ideal situation. In oil sand grain, the bitumen displacement may not be uniform as shown in Fig. 2 because of surface roughness and impurities.
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S. Basu et al. / Fuel 79 (2000) 837–841
glass contact line displacement the contact angle changes from equilibrium contact angle in the presence of air to that in the presence of water at a given pH and temperature. Finally, the velocity of the contact line is not constant during bitumen displacement whereas propagation of dry patches take place at a constant velocity. 3.3. L=D ratio
Fig. 4. Number of daughter droplets formed for different L=D ratio (pH 8, 40⬚C).
regime and not by viscoinertial regime. This is validated on the basis of theoretical model developed by Brochard-Wyart and Redon [10] on the dynamics of liquid rim instability. According to them the viscous regime prevails when wave vector, q ⴱ, multiplied by width of the strip, D, is less than 1 qⴱ D ⬍ 1 and viscoinertial regime prevails when qⴱ D ⬎ 1: The wave vector is defined as, qⴱ sbw rb u5e =m2b ; where s bw is the bitumen/water interfacial tension, r b is the bitumen density, u e is the equilibrium contact of bitumen and m b is the bitumen viscosity. Brochard-Wyart and Redon [10] developed the instability theory for the expansion of dry patches on a solid surface for a constant contact angle during the contact line displacement. They observed that the contact angle of the rim remains constant during the expansion of dry patches. It should be noted that the contact angle varied during the bitumen displacement. Thus an average value of contact angle is used to estimate q ⴱ. The value q ⴱ for the bitumen–glass–water system 2 is 0.043 m ⫺1. q ⴱD value varies from 4:3 × 10⫺5 to 5:18 × 10⫺4 for the range of D value employed in the experiment. Here qⴱ D p 1 and thus bitumen displacement is governed by viscous regime only. Further, Brochard-Wyart and Redon [10] pointed out that the growth of contact line instability is faster for capillary length, k⫺1 ⬎ D than that for k⫺1 ⬍ D: The capillary length is defined as, k⫺1 sbw =rg1=2 : The k ⫺1 for bitumen–water sytem is 0.0015 m. It was indeed observed that the growth of instability is faster for D 0:001 m than that for D 0.004 m. According to Brochard-Wyart and Redon [10], the growth of instability is very slow for k⫺1 p D and thus the instability can not be observed during the time scale of the experiment. However, the growth of instability during the bitumen displacement was observed for D 0:012 m: This may be due to the fact that the instability grows both at the bitumen/water/glass contact line, as well as at bitumen/water interface for bitumen/water/glass system. Further, during the bitumen/water/ 2 Physical properties (pH 8.0, 40⬚C): 990:5 kg=m3 ; ue avg 1:37; mb 45 Pa s:
sbw 22 mN=m;
rb
In Fig. 4, number of daughter droplets formed, N, is plotted against length to width ratio, L=D; of the bitumen strip for pH 8 and at 40⬚C. The disjointed straight lines indicate that the number of daughter droplets formed is constant for a particular range of L=D ratio. The disjointed portion of the straight line corresponds to a transition region, where the number of daughter droplets formation changes with L=D ratio. The number of daughter droplets formed decreases with the decrease in L=D ratio in a stepwise manner. The L=D ratio was changed by varying the width, D, of the bitumen strip. At a low value of L=D ratio, D becomes greater than k ⫺1 which leads to slow growth of instability during bitumen/water/glass contact line displacement. The slow bitumen/water/glass contact line velocity and high relaxation time give rise to decrease in number of daughter droplets. In Fig. 3b (rectangular strip), four peaks are visible on bitumen/water interface indicating the possibility of four daughter droplets formation. However, due to slow bitumen/water/glass contact line velocity and high relaxation time, two daughter droplets are formed (Fig. 3d). The distance between daughter droplets was not constant for a particular L=D ratio. This is due to the end effect since the bitumen strip length is finite. 3.4. pH and temperature The preliminary study shows that the number of daughter droplets formation increases with the increase in pH and temperature. However, the effect of pH on the number of daughter droplets formation is less than the effect of temperature on the same. The bitumen/water interfacial tension decreases with the increase in pH. The effect of temperature on bitumen/water interfacial tension is minimal whereas the bitumen viscosity drastically decreases with the increase in temperature. Further, the equilibrium contact angle of bitumen on the glass slide in the presence of water increases with increase in pH [6]. The detail physical properties of bitumen/water system is available in Takamura and Isaacs [17] and Basu et al. [6]. The combining effect of all these parameters influences the growth of instability at bitumen/water interface and bitumen/water/ glass contact line. It will be interesting to know that whether the large number of daughter droplets are formed at the operating pH and temperature, in the digestion stage of an oil sand extraction unit.
S. Basu et al. / Fuel 79 (2000) 837–841
4. Conclusion The rectangular shaped bitumen strip coated on glass plate shows the growth of ridge and modulated structures at bitumen/water interface and bitumen/water/glass contact line, respectively, upon exposure to water. The modulated contact line and ridge like structure at the interface grows further and daughter droplets are formed. Whereas the circular bitumen disk displaced uniformly in the inward radial direction to form single bitumen droplet in the presence of water. The number of daughter droplets formed decreases with the increase in width of the bitumen strip for a constant strip length. Further, the number of bitumen droplets formed increases with the increase in pH and temperature. The present experimental results follow the contact line instability analyses developed by BrochardWyart and Redon [10].
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