Investigating the Stability Mechanism of Water-in-Diluted Bitumen

Dec 13, 2002 - The presence of sodium naphthenates might hinder the conformational change of asphaltene at the interface, necessary for rigid film for...
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Energy & Fuels 2003, 17, 179-190

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Investigating the Stability Mechanism of Water-in-Diluted Bitumen Emulsions through Isolation and Characterization of the Stabilizing Materials at the Interface X. Wu Syncrude Canada Ltd., Edmonton Research Center, Edmonton, Alberta, Canada, T6N 1H4 Received April 30, 2002

In oil sand industry, formation of a stable salty water-in-diluted bitumen emulsion poses a serious corrosion problem at the bitumen upgrading plant. Previous studies indicated the presence of a “critical concentration” of bitumen in the emulsion, above which the oil-water interface is flexible and the water-in-diluted bitumen emulsion is stable, but below which the interface becomes rigid and the emulsion is unstable. The cause of this critical concentration was investigated through isolation and characterization of the interfacial material from the emulsions both below and above the critical concentration. The analytical data showed that, in heptane/ toluene(1:1)-diluted bitumen emulsions, the flexible interfacial film is composed of a mixture of asphaltene and carboxylic salts with a combined H/C ratio of 1.32 while the rigid interfacial film is composed of asphaltene alone with a H/C ratio of 1.13. The carboxylic salts are water insoluble and are likely sodium naphthenates containing >20 carbons. The presence of sodium naphthenates might hinder the conformational change of asphaltene at the interface, necessary for rigid film formation.

Introduction Bitumen is commercially produced from Athabasca oil sands using the Clark hot water process,1 an extraction method of mixing oil sands and hot water with some caustic added. The mixture, which is a slurry containing suspended bitumen drops (∼200 µm), clays, sand grains, and air bubbles, is fed into a separation vessel where aerated bitumen drops are collected as froth. Sand and water form tailings and are discarded. Froth is subsequently diluted with naphtha for further separation of hydrocarbons from water and solids using both gravity settling and centrifugation methods. Commercial demulsifiers are used in the separation. This separation process has only achieved limited success. There are still significant numbers of water droplets, typically less than 10 µm, in the naphtha-diluted bitumen after the treatment. These small water droplets carry chlorides and solids to the downstream upgrading units and pose serious corrosion and fouling problems. Effective demulsification requires understanding of the stability mechanism of the water-in-diluted bitumen emulsion. This stability mechanism was investigated using a wide variety of techniques including colloidal force measurement,2 interfacial tension and rheology studies,3,4 thin liquid film thickness/disjoining pressure (1) Clark, K. A.; Pasternack, D. S. Research Council of Alberta Report 1949 (53). (2) Wu, X.; van de Ven, T. G. M., Czarnecki, J. Colloids Surf., A 1999, 149, 577-583. (3) Yeung, A.; Dabros, T.; Czarnecki, J.; Masliyah, J. Proc. R. Soc. London A 1999, 455, 3709-3723. (4) Dabros, T.; Yeung, A.; Masliyah, J.; Czarnecki, J. J. Colloid Interface Sci. 1999, 210, 222-224.

measurements,5 and emulsion stability studies6-8 in the past. Perhaps the most important discovery is the socalled “critical concentration” of bitumen in emulsion,3 above which the oil-water interface is flexible and a stable water-in-diluted bitumen emulsion is formed almost spontaneously, but below which rigid film covers the oil-water interface and the diluted bitumen mixed with water under low shear is surprisingly free of suspended water droplets (see Figure 1). This finding has a significant impact on a newly developed commercial dewatering process. However, the cause of the critical concentration is poorly understood and the underlying mechanisms for stable and unstable emulsions are by no means clear. Petroleum dewatering has also been a challenge in conventional crude oil industry for several decades.9 Most of the studies on stability mechanisms of waterin-crude oil emulsions can be divided into three categories: interfacial tension and interfacial rheology studies,10-15 comparative studies on emulsion stability using (5) Khristov, K.; Taylor, S. D.; Czarnecki, J.; Masliyah, J. Colloids Surf. A 2000, 174, 183-196. (6) Xu, Y.; Dabros, T.; Hamza, H.; Shefantook, W. Pet. Sci. Technol. 1999, 17, 1051-1070. (7) Yan, Z.; Elliott, J. A. W.; Masliyah, J. H. J. Colloid Interface Sci. 1999, 220, 329-337. (8) Gafonova, O. V.; Yarranton, H. W. J. Colloid Interface Sci. 2001, 241, 469-478. (9) McLean, J. D.; Spiecher, P. M.; Sullivan, A. P.; Kilpatrick, P. K. The Role of Petroleum Asphaltenes in the Stabilization of Water-inOil Emulsions. Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998. (10) Reisberg, J.; Doscher, T. M. Producers Monthly 1956, 21, 4350. (11) Dodd, C. G. J. Phys. Chem. 1960, 64(5), 544-550. (12) Strassner, J. E. J. Pet. Technol. 1968, 20, 303-312.

10.1021/ef020098y CCC: $25.00 © 2003 American Chemical Society Published on Web 12/13/2002

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Figure 1. Contraction of a water drop in heptol-diluted bitumen by withdrawing water back into a micropipette shown on the left of the photo. (a) The continuous phase in the background is 10 vol % bitumen in heptol. During contraction, the surface of the water drop remains spherical and small water droplets are “budding” from the surface. Water droplets in the background were created in the previous contractions. (b) The continuous phase in the background is 0.1 vol % bitumen in heptol. During contraction, the interface crumples such as a paper bag. No small water droplets are created in the contraction. (Adapted from ref 4 with permission.)

model components and solvents,16,17 and characterization on interfacial material responsible for stable emulsions.17-23 The majority of the studies showed that water-in-crude oil emulsions are stabilized by a rigid film or “skin” at the oil-water interface.10-12,14,24,25 This finding, however, seems to be contradictory to the result obtained from diluted bitumen. As mentioned above, stable emulsions are associated with flexible rather than rigid interfacial films in diluted bitumen. Further comparison of the studies on crude oil and bitumen emulsions indicates that what lacks in the bitumen emulsion studies is the characterization work on the interfacial material (IM). Knowing the chemical composition of the IM in diluted bitumen could be the key (13) Mohammed, R. A.; Bailey, A. I.; Luckham, P. F.; Taylor, S. E. Colloids Surf., A 1993, 80, 223-235. (14) Acevedo, S.; Escobar, G.; Gutierrez, L. B.; Rivas, H.; Gutierrez, X. Colloids Surf., A 1993, 71, 65-71. (15) Ese, M.-H.; Yang, X.; Sjoblom, J. Colloid Polymer Sci. 1998, 276, 800-809. (16) McLean, J. D.; Kilpatrick, P. K. J. Colloid Interface Sci. 1997, 189, 242-253. (17) McLean, J. D.; Kilpatrick, P. K. J. Colloid Interface Sci. 1997, 196, 23-34. (18) Bartell, F. E.; Niederhauser, D. O. Fundamental Research on Occurrence and Recovery of Petroleum 1946-1947; API: 1949; pp 5780. (19) Denekas, M. O.; Carlson, F. T.; Moore, J. W.; Dodd, C. G. Ind. Eng. Chem. 1951, 43, 1165-1169. (20) Dodd, C. G.; Moore, J. W.; Denekas, M. O. Ind. Eng. Chem. 1952, 44, 2585-2590. (21) Layrisse, I.; Rivas, H.; Acevedo, S. J. Dispersion Sci. Technol. 1984, 5, 1-18. (22) Acevedo, S.; Escobar, G.; Gutierrez, L.; Rivas, H. Fuel 1992, 71, 619-623. (23) Hasiba, H. H.; Jessen, F. W. J. Can. Pet. Technol. 1968, JanMarch, 1-12. (24) Blair, C. M. Chem. Ind. 1960, May 14, 538-544. (25) Taylor, S. Chem. Ind. 1992, 20, 770-773.

to elucidating the mechanisms behind emulsion stability and interfacial rigidity. The most common method of isolating the IM from the oil-water interface is to artificially emulsify water in oil and collect water droplets together with the IM adsorbed on the droplet surfaces by sedimentation or centrifugation.18-22,26 The advantage of this method is the relatively high yield of IM due to the large interfacial area generated by emulsification. Another method of isolating IM is the modified Langmuir-Blodgett technique.23 The resulting IM typically contains smaller amount of contaminants from the bulk oil phase than those isolated with the first method. However, the interfacial area for IM adsorption is usually small in a Langmuir trough. A small interfacial area not only affects the yield of IM but, more importantly, makes the resulting IM poorly represent the IM in a real emulsion, which typically contains a large interfacial area. Hence, the first method was chosen for the present work. In principle, all analytical techniques used in petroleum analyses can be applied to studying the IM. The restrictive factor, however, is the limited quantity of IM that can be isolated within a reasonable time frame. This, for example, eliminates 13C NMR as a useful technique. Two techniques, which have been frequently used to analyze IM samples in the literature, are CH elemental analysis17,19-22 and FTIR.6,23,26 The latter can be replaced with a more recent photoacoustic FTIR (PAFTIR) technique.27,28 The PA-FTIR has the advantage (26) Guo, S.; Qian, J. Petroleum Sci. Technol. 1998, 16, 433-447. (27) Michaelian, K. H.; Friesen, W. I. Fuel 1990, 69, 1271-1275. (28) Michaelian, K. H.; Zhang, S. L.; Hall, R. H.; Bulmer, J. T. Can. J. Anal. Sci. Spectrosc. 2001, 46(1), 10-22.

Water-in-Diluted Bitumen Emulsion

of being able to directly analyze dry IM samples without sample preparation. This is especially valuable for some IM samples that are not fully soluble in common organic solvents and are difficult to handle in the KBr-plate making. In addition to these two techniques, some IM samples were analyzed using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS)29,30 in this study. On the bases of the analytical results, a hypothetical mechanism is proposed to explain the changes of the interfacial and emulsion properties in diluted bitumen around the critical concentration. Experimental Section Materials. Diluted bitumen solutions used in this study were prepared from Syncrude coke-feed bitumen, which had been extracted from Athabasca oil sands using the abovementioned Clark hot water method and had been atmospherically topped to remove naphtha diluent and bitumen light ends at the Syncrude plant. The solvent used for bitumen dilution is heptane/toluene 1:1 mixture by volume, which will be called “heptol”. Both solvents are HPLC grade reagents, supplied by Fisher Scientific. Two types of water were used in the experiment. They are deionized water from a MilliQ deionization system equipped with a 0.22 µm filter and deuterium oxide (heavy water) of 99.9% D, supplied by Acros Organics (density ∼1100 kg/m3). The reason of using heavy water will be given later. Asphaltene and maltene fractions were extracted from the coker-feed bitumen by n-pentane (Fisher HPLC grade) precipitation at a dilution ratio of 40:1 by mass. Small quantity of SARA fractions of the same bitumen were prepared by Norwest Labs following Syncrude Method 5.1 (pentane precipitation at 40:1).31 The contents of asphaltene, resin 1, and resin 2 in bitumen are 16.5%, 31.6%, and 3.9%, respectively. Two commercial sodium naphthenates samples, supplied by Acros Organics and Eastman Kodak, were used as reference compounds. According to manufacturer’s information, the former is mainly composed of molecules with 8-12 carbons and one cyclopentane ring. The latter was analyzed with electrospray ionization mass spectrometry (A. Morales, Syncrude Contract Report), which shows that the main components are molecules with 15-17 carbons and 1 or 2 cyclic rings. Calcium naphthenates were prepared by reacting the Acros sodium naphthenates with CaCl2 in an aqueous solution. A higher molecular weight compound, 5β-cholanic acid (C24H40O2), supplied by Sigma Chemicals, was also used as a reference. It was converted to its sodium salt by reacting with NaOH in ethanol. Procedure for IM Isolation and Analysis. A series of bitumen solutions in heptol were prepared for IM isolation. The bitumen concentrations were 0.1, 0.5, 1, 2, 3, 4, 5, 7.5, and 10 wt %. IM isolated from a 5 wt % bitumen solution will be called “IM5%”. Similarly, “IM0.5%” represents the IM from a 0.5% solution. A ∼1.2 wt % asphaltene solution and a ∼6.3 wt % maltene solution in heptol were prepared to imitate a 7.5 wt % bitumen solution with its maltene or asphaltene fraction removed, respectively. The IM samples isolated from these asphaltene and the maltene solutions will be called “IMA” and “IMM”, respectively. The IM isolation procedure is as follows. The bitumen solution was centrifuged at approximately 27000g for 30 min, (29) Bensebaa, F.; Kotlyar, L.; Pleizier, G.; Sparks, B.; Deslandes, Y.; Chung, K. Surf. Interface Anal. 2000, 30, 207-211. (30) Bensebaa, F.; Kotlyar, L. S.; Sparks, B. D.; Chung, K. H. Can. J. Chem. Eng. 2000, 78, 610-616. (31) Bulmer, J. T., Starr, J., Eds. Syncrude Analytical Methods for Oil Sand and Bitumen Processing; Alberta Oil Sands Technology and Research Authority: Edmonton, 1979; p 121.

Energy & Fuels, Vol. 17, No. 1, 2003 181 and the bottom 1/10 of the solution was discarded to remove solids. A sample of 3 wt % deuterium oxide (D2O) was added to the centrifuged bitumen solution. The mixture (∼55 g) was then placed in a 125 mL glass bottle and was shaken in an ultrasonic bath (Fisher FS-10) for 30 min. Temperature in the ultrasonic bath usually rose to 60-80°C. The glass bottle was closed during sonication to prevent solvent evaporation. The resulting emulsion contains water droplets mostly of 3-5 µm in diameter (see Figure 2). The emulsion was then poured into two 125 mL glass bottles with the bottom one-third of the bottles filled with deionized water (H2O). These two bottles containing the emulsion were centrifuged at 1300g for 15 min. In the centrifugal field, D2O droplets in the emulsion broke through the interface between the diluted bitumen layer (or the emulsion layer) and the H2O layer, and settled on the bottom of the glass bottle. These D2O droplets, in the form of an agglomerated oily mass, will be called a “wet cake”. After the centrifugation, the top diluted bitumen and H2O layers were pumped out to retrieve the wet cake, which was subsequently placed in a Plexiglas chamber continuously flushed with argon at room temperature for several days to evaporate water and organic solvents. It should be noted that the term “IM” in this paper only refers to this dried material. The yield of IM is usually around 10 mg. IM typically has a porous, honeycomblike structure (see Figure 3). The voids were filled with emulsified water prior to drying the wet cake. It is important to verify that the emulsified water droplets did not coalesce and lose their interfacial area and IM during centrifugation. IM samples were further dried in a 60°C helium-purged vacuum oven for 20 h immediately prior to PA-FTIR measurement. The PA-FTIR spectroscopy is based on the principle that when infrared radiation is absorbed by the sample and is converted to heat, gas adjacent to the sample expands. Because the intensity of the incident infrared beam is modulated at an acoustic frequency, it is possible to detect the periodic expansion of the gas with a microphone.32 The intensity of the acoustic signal is directly related to the absorbance of the sample. The PA-FTIR instrument (Thermo-Nicolet Magna 560 spectrometer equipped with an MTEC Model 300 photoacoutic cell) was operated using the following parameters: aperture of 150, interferometer mirror velocity of 0.1581 cm/s, 256 scans per run, and resolution of 4 cm-1 with a data spacing of 0.964 cm-1. Photoacoustic samples underwent two-staged purging with helium to remove CO2 and moisture in the instrument. The total purging time was more than 1 h. Four IM samples, IM0.5%, IM5%, IMA, and IMM, were submitted to ICPET, NRC in Ottawa for XPS and ToF-SIMS analyses. XPS data were obtained with a Kratos Axis HS X-ray photoelectron spectrometer (Kratos, UK). Monochromated Al KR radiation was used for excitation. The X-ray gun was operated at 15 kV and 20 mA. ToF-SIMS spectra were recorded with a Kratos Prism instrument (Kratos, UK). The samples were bombarded with 25 keV Ga+ ions and the current was less than 1 × 10-9 Å. Charge neutralization was accomplished by means of a beam of electrons. High-resolution spectra were obtained using resolution time of 1.25 ns. Only positive-ion spectra were recorded. A Perkin-Elmer CHNS/O 2400 Series II Analyzer was used for the elemental analysis. This instrument requires a sample of ∼1.5 mg in an aluminum foil cup for CH analysis, which suites the IM sample of only limited quantity quite well. The samples were heated in a 60°C helium-purged vacuum oven for at least 5 h before being submitted to NCUT, CANMET in Devon, Alberta, for the actual analysis. It has been noticed that the absolute values of C and H contents often do not (32) McClelland, J. F.; Jones, R. W.; Luo, S.; Seaverson, L. M. A Practical Guide to FTIR Photoacoustic Spectroscopy. Practical Sampling Techniques for Infrared Analysis; Coleman, P. B., Ed.; CRC Press: Boca Raton, 1992.

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Figure 2. Water-in-heptol diluted bitumen emulsions observed under a microscope. (a) Stable emulsion containing well dispersed water droplets. The bitumen concentration is 5 wt % in the continuous phase. (b) Unstable emulsion containing clustering water droplets. The bitumen concentration is 0.5 wt % in the continuous phase. indicate any trend. However, the H/C atomic ratios are far more informative. Therefore, only H/C ratios are reported in this paper. Minimization of Contamination in IM Samples. The main concern of the above-mentioned isolation procedure is the contamination in IM samples. The main contaminant is bitumen, which originates from the diluted bitumen trapped in the space between water droplets inside the “wet cake”. A common way of minimizing the IM contamination is to wash the wet cake with an organic solvent until the solvent becomes colorless.21,22,26 However, since one of the main objectives of this work is to investigate whether the bitumen concentration affects the IM composition, solvent washing could alter the bitumen concentration around the IM and therefore was not adopted. Bitumen contamination was minimized by reducing the bitumen concentration in the original solution and by using D2O as the water source for emulsification. Isolating IM from a low bitumen-content solution is the most effective way of minimizing the contamination for an apparent

reason. However, the selected bitumen concentrations must cover both sides of the critical concentration as required by the objective of this study. It is known that the value of the critical concentration varies greatly with the type of solvent used for bitumen dilution. For example, the critical concentrations are around 40 wt % bitumen for heptane-diluted bitumen, 20 wt % for Syncrude naphtha-diluted bitumen and approximately 1 vol % (1.3 wt %) for heptol-diluted bitumen.3 Among these solvents, Heptol is obviously the best choice. IM contamination can also be reduced by passing emulsified D2O droplets through a clean layer of H2O (refer to Procedures and Instruments). Ideally, if individual D2O droplets (3-5 µm) broke through the interface between the top diluted bitumen layer and the H2O layer, the contamination in IM would be zero. This type of breakthrough, however, does not occur due to the tension at that interface (∼20 mN/m).3 For the observed breakthrough of D2O droplets to occur, the droplets must initially agglomerate at the interface until the gravity force exerted on the agglomerate is sufficiently large to overcome

Water-in-Diluted Bitumen Emulsion

Energy & Fuels, Vol. 17, No. 1, 2003 183 Three heptol-diluted bitumen solutions containing 0.1, 0.5, and 3 wt % bitumen were centrifuged at 27000g for 30 min to remove mineral solids of larger than 50 nm, which accounts for almost all mineral solids by weight in bitumen. The supernatants were filtered through Millipore VSWP04700 membrane filters with a mean pore size of 25 nm and a thickness of 105 µm in a standard Millipore glass vacuum filtration device. The pressure difference across the filter was kept constant at 97 kPa. The amount of solution filtered by each filter disk was kept at 37, 7.4, and 1.2 g for 0.1%, 0.5%, and 3% solutions, respectively, to ensure same amount of bitumen passing through each filter. Filtering bitumen solutions more concentrated than 3 wt % encountered technical difficulties. Sixteen filter disks were used for each solution. Filtration was performed at room temperature. The filtrate was subsequently heated in an 80 °C helium-purged vacuum oven for 20 h to remove Heptol solvent. A reference solution of the same bitumen concentration, which had been centrifuged at 27000g but not filtered, was placed in the same oven for solvent evaporation. Asphaltene contents in both bitumen samples were determined by pentane precipitation with a dilution ratio of 40:1. The difference thus represents the amount of asphaltene removed by the filters, which can serve as an indicator for size increase of asphaltene aggregates close to 25 nm.

Results and Discussion

Figure 3. SEM micrograph of an interfacial material sample (IM3%). The voids were filled with emulsified water before drying. the interfacial tension. The resulting wet cake inevitably contains some contaminants, but the contamination is less severe compared with a wet cake collected without the H2O layer. This is because the main contaminant in the agglomerate is heptol-diluted bitumen, which has a density of 780 kg/ m3, significantly lower than that of H2O. Lighter-than-H2O agglomerates, containing more contaminants, are effectively separated from the heavy ones that sank to the bottom of the H2O layer. These light agglomerates were frequently observed in the experiment. Additional benefit of using the D2O/H2O combination is that the exterior of the wet cake is completely free of bitumen contaminant. The degree of bitumen contamination in IM will be further discussed in the Results and Discussion section. Gravity Settling Test. Two water-in-heptol-diluted bitumen emulsions were tested for water settling performance. The bitumen concentrations are 0.5 and 5 wt %. The water content is 2.4 wt % in both emulsions. Bitumen solutions were centrifuged at 27000g for 30 min before water addition. The added deionized water was then emulsified using an ultrasonic bath. Samples were taken from the middle positions of the emulsions left for gravity settling after 30 min of sonication. The sampling time was 0, 10, and 30 min after turning off the ultrasonic bath. The water contents in emulsion samples were determined with a Karl-Fisher moisture analyzer. Detection of Possible Onset of Asphaltene Precipitation. Heptol-diluted bitumen solutions of various bitumen concentrations were examined under an optical microscope for signs of asphaltene precipitation. Refractive indexes of the solutions were also determined using a GPR12-70 refractometer supplied by Index Instruments US. As will be discussed later, no asphaltene precipitation has been observed. A filtration test was subsequently performed to detect possible growth of asphaltene aggregates, which might be undetectable using optical methods. The procedure is as follows.

Physical Properties of Bitumen Emulsions and Oil-Water Interfaces. Critical Concentration and Emulsion Stability. As mentioned above, the critical concentration essentially sets the boundary of rigid and flexible oil-water interfaces in diluted bitumen. This critical concentration was experimentally determined by blowing a water drop of 10-20 µm into diluted bitumen with a micropipet and contracting the drop after several minutes of aging.3,4 Longer aging time has no effect on the appearance of interfacial films. Above the critical concentration, the oil-water interface (drop surface) was flexible and remained spherical in the contraction. This appearance indicates that the interfacial film is a 2-D liquid with no resistance to in-plane shearing. An example of the 2-D liquid is a soap film. In addition to the flexible appearance of the interface, small water droplets of 1.32 as showed above. The molecular weights of these compounds are in the range of 300-400. The sodium naphthenates found in IM are unique to bitumen extracted with the caustic/water-based method (refer to the Introduction section). It is known that water-soluble “sodium naphthenates” in tailings water are generated through neutralization of naphthenic acids of C15-17 in bitumen by aqueous NaOH on bitumen drop surfaces at the slurry stage of the bitumen extraction process.49 It is very likely that sodium naphthenates of poor solubility in water are generated in the same way and are retained in bitumen when bitumen drops (49) Schramm, L. L.; Stasiuk, E. N.; MacKinnon, M. Surfactants in Athabasca Oil Sands Slurry Conditioning, Flotation Recovery, and Tailings Processes. In Surfactants: Fundamentals and Applications in the Petroleum Industry; Schramm, L. L., Ed; Cambridge University Press: New York, 2000.

start to coalesce in froth and lose their surface area. Since generation of the sodium naphthenates is limited by the relatively small surface area of bitumen drops in slurry, the sodium naphthenate concentration in bitumen is probably very low. This might be the reason that no sodium carboxylates were detected in previous characterization works on Athabasca bitumen and its fractions.41-44 Asphaltene in IM. As a solubility class, asphaltene contains many types of molecules different in both chemical properties and surface activity. It is possible that the asphaltene detected in IM is only one small fraction, likely the most surface active one. However, due to the limited sample quantity and the limitation of the analytical tools presently available, the exact molecular structure of the asphaltene in IM is unknown. In the literature, it has been reported that the acidic asphaltene fraction is a more effective emulsion stabilizer than its neutral counterpart.9 According to the relatively weak strength of the 1700 cm-1 band in IM spectra, it is unlikely that the asphaltene in IM is solely the acidic one. There was another concern that the sodium carboxylates detected in IM2%-IM7.5% actually originate from acidic asphaltene through NaOH neutralization. This is also unlikely since the H/C ratios of acidic, neutral, and basic Athabasca asphaltene fractions eluted from ion exchangers are mostly similar to that of the whole asphaltene. In particular, the H/C ratio of one fraction that is rich in carboxylic acids, was determined to be 1.19,44 which is significantly lower than the value of carboxylate-containing IM (1.32). Hypothetical Mechanisms Behind Interfacial Rigidity and Emulsion Stability. Two hypothetical mechanisms are proposed here to explain the dramatic changes of interfacial rigidity and emulsion stability at the critical bitumen concentration. One mechanism assumes that the compositional change in IM causes the changes of interfacial and emulsion properties. The other mechanism assumes that the onset of asphaltene precipitation occurs at the critical concentration. According to literature studies,9,17 the solubility state of asphaltene plays a pivotal role in emulsion stability and rheological properties of an oil-water interface regardless of the interfacial composition. The latter is consistent with what was observed in heptane-diluted bitumen since the critical bitumen concentration of 40 wt % in heptane coincides with the onset of asphaltene precipitation.50 However, no asphaltene precipitation has been observed under an optical microscope in heptoldiluted bitumen solutions of any concentrations. Refrac(50) Taylor, S. D.; Czarnecki, J.; Masliyah, J. Fuel 2001, 80, 20132018.

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Figure 8. (n2 - 1)/(n2 + 2) plotted as a function of bitumen volume fraction in Heptol, n being the solution refractive index. The filled circles are experimental data, which are fitted by the solid line with a linear correlation coefficient of 0.9999. Table 4. Asphaltene Concentrations in Unfiltered Bitumen and Filtered Bitumen bitumen concentration in heptol before filtration (wt %)

asphaltene concentration in unfiltered bitumen (wt %)

asphaltee concentration in 25 nm filtered bitumen (wt %)

percentage of asphaltene removed by filters

3 0.5 0.1

16.6 16.4 16.4

10.3 9.7 9.2

38.0 40.9 43.9

tive index measurement supports this finding. A plot of (n2 - 1)/(n2 + 2), where n is the refractive index of a heptol-diluted bitumen solution, vs volume fraction of bitumen is shown in Figure 8. Good linearity of the data points indicates no phase separation. By contrast, a similar plot for heptane-diluted bitumen or crude oil showed very pronounced discontinuity at the onset of asphaltene precipitation.50,51 Although no asphaltene precipitation has been detected in heptol, there is still an possibility that asphaltene aggregates become larger in heptol as bitumen concentration decreased, yet still too small for optical observation. A filtration test was therefore performed to explore this possibility (refer to Experimental section). Essentially, the amounts of asphaltene removed by 25 nm filters in three centrifuged bitumen solutions were determined and compared. Among these three solutions, one containing 3% bitumen is above the critical concentration and the others containing 0.5% and 0.1% bitumen are below. The results are shown in Table 4. The asphaltene concentrations in all three centrifuged but unfiltered bitumen samples are in good agreement with the value (16.5 wt %) determined by Norwest Labs using the standard Syncrude method 5.1 (see Experimental section). This indicates that no asphaltene has been removed by centrifugation prior to the filtration. For 3% heptol-diluted bitumen, 38% of asphaltene was removed by the filters. Literature study on size distribution of Athabasca asphaltene entities (molecules or aggregates) in a 5% asphaltene-in-toluene solution using small-angle X-ray scattering (SAXS) technique52 showed that 38% of asphaltene entities, (51) Buckley, J. S. Energy Fuels 1999, 13, 328-332. (52) Xu, Y.; Koga, Y.; Strausz, O. P. Fuel 1995, 74, 960-964.

which are assumed spherical, have a diameter larger than ∼9 nm. The discrepancy between 9 and 25 nm (mean filter pore size) can be explained by the following facts. First, a filter regularly captures particles smaller than its mean pore size. This is especially true when captured particles partially block the pores.53 Second, asphaltene entities may not be spherical as assumed in the SAXS data interpretation. The effective sphere diameter of an asphaltene “particle” is smaller than its largest dimension, which might be the determining factor in filtration. Third, the asphaltene-in-toluene solution used in the SAXS experiment is quite different from the bitumen-in-heptol solution used in the present study. Hence, the “effective pore size” of the 25 nm filter must be between 9 and 25 nm and it should not vary significantly if filtration conditions, e.g., pressure difference across the filter, amount of materials blocking the filter pores, are kept constant. The latter depends strongly on the total amount of bitumen in the feed, which was kept constant in all three solutions. Despite the uncertainty of the effective pore size, filtration method is a useful tool to detect changes in the size of asphaltene aggregates at different bitumen concentrations. The small difference among the percentages of filtered asphaltene in all three solutions indicates no significant size increase of asphaltene aggregates close to 25 nm when bitumen concentration decreases. Although asphaltene aggregates smaller than the effective pore size cannot be studied using the filtration method, the result in Table 4 certainly indicates no onset of asphaltene precipitation around the critical concentration in heptol-diluted bitumen. Furthermore, the critical bitumen concentration in toluene-diluted bitumen has been determined to be around 1% using the micropipet technique (C. Tsamantakis, private communication), which is very close to the value of heptol-diluted bitumen. The apparent similarity between toluene and heptol suggests that no dramatic change of the solubility state of asphaltene should be expected in heptol at the critical concentration. In heptol-diluted bitumen, the compositional change of IM more likely causes the changes of interfacial rigidity and emulsion stability. Literature studies showed that asphaltene tends to change its conformation and cross-links with other asphaltene entities at the oilwater interface via hydrogen bonds or electron donoracceptor interactions9,43 to form a rigid film. The conformational change and/or the cross-linking of asphaltene might be hindered in the presence of sodium naphthenates. The resulting interfacial film thus becomes flexible. Although this hypothesis has not been finally proven yet, what has been observed in this study seems to be consistent with some literature results showing that an interfacial film composed of asphaltene and resin has higher compressibility or flexibility than a pure asphaltene film.54 It has been found that the acid form of the sodium naphthenates is a resinous species.43 In addition, commercial sodium naphthenates doped in bitumen can be found in the maltene fraction, not in the asphaltene fraction (X. Yang, private communication). Hence, in terms of solubility and functionality, the (53) Ives, K. J., Ed. The Scientific Basis of Filtration; Noordhoff: Leyden, 1975. (54) Ese, M.-H.; Galet, L.; Clausse, D.; Sjoblom, J. J. Colloid Interface Sci. 1999, 220, 293-301.

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sodium naphthenates in IM could be regarded as a special resinous species as well. The above-mentioned film structure can explain the solubility difference between IM0.5% and IM5% in toluene well. The asphaltene found in IM0.5% would be fully soluble in toluene unless it is cross-linked or physically transformed. Lacking similar cross-linking or other changes, the asphaltene in IM5% is fully soluble in toluene as the definition of asphaltene suggests. The same hypothesis can also explain the clustering/ nonclustering behavior of water droplets in heptoldiluted bitumen. Below the critical concentration, crosslinking may occur among asphaltene entities on different water droplet surfaces due to random collisions among water droplets and thus causes droplet clustering. The adsorbed asphaltene can nevertheless provide sufficient steric repulsion to prevent droplet coalescence. Above the critical concentration, because of the presence of the naphthenates, asphaltene seems to lose its tendency of association and meanwhile, provides steric repulsion for water droplets. Hence, the emulsion is stable without droplet clustering. An important question yet to be answered is whether sodium naphthenates and asphaltene are individually adsorbed at the oil-water interface or they associate in the bulk and adsorb at the interface as aggregates. It is also not completely understood what triggers the change of IM composition at the critical concentration. This could be caused by the limited amount of sodium naphthenates in bitumen, which are completely “consumed” at the interface when bitumen concentration decreases below certain value. In the literature, rigid interfacial film is often regarded as an emulsion stabilizer in conventional crude oil systems. If water droplet coalescence is used as the emulsion stability criterion, both rigid and flexible films in bitumen emulsions actually result in “stable” emulsions. Furthermore, it has been observed that emulsified water in many diluted bitumen systems can rarely be resolved under any conditions. Therefore, there are no fundamental differences between the roles of rigid interfacial films in preventing water droplet coalescence in bitumen and crude oil emulsions. However, some differences may exist in these two types of emulsions when flexible interfacial films dominate the systems. Industrial Implication. As mentioned above, the heptane-diluted bitumen, which shows noticeable as-

Wu

phaltene precipitation, probably has a different mechanism behind its critical concentration as compared with the heptol-diluted bitumen, which shows no asphaltene precipitation at all. Syncrude naphtha with an aromaticity of 0.1 by 13C NMR resembles a paraffinic solvent like heptane rather than an aromatic-dominant solvent like heptol. However, because of its high cycloalkane content, Syncrude naphtha has some properties similar to an aromatic solvent as well, e.g., the majority of asphaltene remains soluble in naphtha-diluted bitumen at 40:1 dilution ratio. In terms of IM composition, preliminary studies show that IM isolated from naphthadiluted bitumen above its critical concentration is almost identical to its heptol counterpart (e.g., IM5%) in both H/C ratio and PA-FTIR features. Since the bitumen concentration in the naphtha-diluted bitumen encountered in the commercial operation is almost exclusively in this regime, understanding its IM composition being a mixture of asphaltene and sodium naphthenates is of great value in designing a more effective demulsifier. Conclusions Critical bitumen concentrations exist in many diluted bitumen systems, which set the boundaries between rigid and flexible oil-water interfaces and between stable and unstable water-in-oil emulsions. In heptoldiluted bitumen, above the critical concentration, the interfacial film has been found to comprise a mixture of asphaltene and sodium naphthenates containing >20 carbons. Below the critical concentration, the interfacial film is composed of asphaltene alone. It is believed that this compositional difference results in different interfacial and emulsion properties above and below the critical concentration. Acknowledgment. Special thanks to Mr. D. Rioux from NCUT, CANMET for CH analysis, Dr. F. Bensebaa from ICPET, NRC for XPS and ToF-SIMS analyses, and Dr. R. Hall from Syncrude for PA-FTIR instruction. The author is also grateful to Dr. J. Czarnecki from Syncrude and Dr. A. Yeung from University of Alberta for support and helpful discussion. EF020098Y