Energy & Fuels 2009, 23, 3139–3149
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Study of Solvent-Bitumen-Water Rag Layers Sumit K. Kiran,† Edgar J. Acosta,*,† and Kevin Moran‡,§ Department of Chemical Engineering and Applied Chemistry, UniVersity of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada, and Edmonton Research Centre, Syncrude Canada Limited, Edmonton, Alberta T9H 3H5, Canada ReceiVed October 7, 2008. ReVised Manuscript ReceiVed February 2, 2009
A major operational issue in the crude oil industry is the formation of intermediate rag layers, (primarily water-in-oil emulsions) in oil-water separation processes that limit the amount and quality of recoverable oil. In this study, the formation of rag layers is evaluated as a function of solvent-bitumen-water ratios, solvent aromaticity, and temperature, with various imaging techniques. Using these techniques, it is possible to obtain an estimate of the amount of oil, water, and asphalthenes in the rag layer and excess phases. On the basis of these material balances, it was observed that when bitumen is diluted with a more paraffinic (poor) solvent, such as Heptol 80/20 (80% heptane and 20% toluene), the asphaltenes in solution tend to adsorb/segregate at exposed oil-water interfaces, impacting the extent of rag layer formation. Diluting similar systems with a more aromatic solvent (Heptol 50/50) reduces the surface activity of the asphaltenes, and the stability of rag layers, as evidenced by lower asphaltene and oil losses to the rag layer. Furthermore, it was observed that increasing the temperature of the system minimizes rag layer formation and the fraction of oil lost to the rag layer. The better separation at high temperature could be explained by the lower viscosity of the oil, which results in improved oil drainage from the rag layer.
Introduction The stability of water-in-oil emulsions involving heavy oils and bitumen is detrimental to the operations of extraction, separation, transportation, and upgrading of these oils.1-4 The presence of asphaltenes (pentane-insoluble fraction of oil) in heavy oils and bitumen has been linked to the formation of such emulsions by adsorbing at exposed oil-water interfaces.5-7 Complete dewatering of the bituminous oil product using mechanical separation techniques is limited because adsorbed asphaltenes as well as fine solids produce an interfacial “skin” barrier that adjacent water droplets must overcome to coalesce.8,9 Further complications may arise because of the presence of lamellar liquid crystals that may cover the oil-water interface.10 The term “rag layer” is typically reserved for stable oil-water * To whom correspondence should be addressed. Telephone: 416-9460742. Fax: 416-978-8605. E-mail:
[email protected]. † University of Toronto. ‡ Syncrude Canada Ltd. § Present address: Titanium Corporation, 10025 10th Street, Suite 1400, Edmonton, Alberta, Canada. (1) Horva´th-Szabo´, G.; Masliyah, J. H.; Czarnecki, J. J. Colloid Interface Sci. 2003, 257, 299–309. (2) Havre, T. E.; Sjo¨blom, J.; Vindstad, J. E. J. Dispersion Sci. Technol. 2003, 24, 789–801. (3) Gutierrez, X.; Silva, F.; Morles, A.; Pazos, D.; Rivas, H. Pet. Sci. Technol. 2003, 21, 1219–1240. (4) Nour, A. H.; Abu Hassan, M. A.; Yunus, R. M. J. Appl. Sci. 2007, 7, 1437–1441. (5) Rondo´n, M.; Bouriat, P.; Lachaise, J. Energy Fuels 2006, 20, 1600– 1604. (6) Hamadou, R.; Khodja, M.; Kartout, M.; Jada, A. Fuel 2008, 87, 2178–2185. (7) Mofidi, A. M.; Edalat, M. Fuel 2006, 85, 2616–2621. (8) Sztukowski, D. M.; Jafari, M.; Alboudwarej, H.; Yarranton, H. W. J. Colloid Interface Sci. 2003, 265, 179–186. (9) Jiang, T.; Hirasaki, G.; Miller, C.; Moran, K.; Fleury, M. Energy Fuels 2006, 21, 1325–1336. (10) Ha¨ger, M.; Ese, M.-H.; Sjo¨blom, J. J. Dispersion Sci. Technol. 2005, 26, 673–682.
emulsions (water-in-oil, oil-in-water, or multiple emulsions) that form in separation vessels.11,12 Yan et al. studied the role of various bitumen components on the stability of water in diluted bitumen emulsions.13 In that study, it was shown that both asphaltenes and fine solids synergistically promote the stability of bitumen-water emulsions because the emulsifying capacity of deasphalted bitumen was significantly reduced. Yarranton et al. later showed that, at a low concentration of asphaltenes (e2 kg/m3), these molecules act as surfactant monomers capable of stabilizing emulsions.14 Rondo´n et al. demonstrated that a critical aggregation concentration of asphaltenes exists (≈1000 ppm in their system), where the oil-water interface is saturated.15 It was hypothesized that the observed reduction in the rate of demulsification in the presence of excess asphaltenes (beyond the critical aggregation concentration) was due to the formation of micellar aggregates as well as a type of gel phase adjacent to the interfacial layer. Rondo´n et al. also showed that, in polar diluents, such as toluene, asphaltenes are less surface-active, leading to less stable emulsions. Additional results obtained by Yang et al. support those of Rondo´n et al. for formulations containing an excess concentration of asphaltenes.16 In such systems, emulsion stability was most prominent in regimes where asphaltenes were only partially soluble in the oil phase. By deflating water in diluted bitumen emulsions using micropipette techniques, Yeung (11) Dabros, T. Advanced Separation Technologies. http://www.nrcan.gc. ca/se/etb/cwrc/English/AST/Areas/Bitumen/bitumen_e.html (accessed in Nov 2002). (12) Varadaraj, R.; Brons, C. Energy Fuels 2007, 21, 1617–1621. (13) Yan, Z.; Elliott, J. A. W.; Masliyah, J. J. Colloid Interface Sci. 1999, 220, 329–337. (14) Yarranton, H. W.; Hussein, H.; Masliyah, J. J. Colloid Interface Sci. 2000, 228, 52–63. (15) Rondo´n, M.; Pereira, J. C.; Bouriat, P.; Graciaa, A.; Lachaise, J.; Salager, J.-L. Energy Fuels 2008, 22, 702–707. (16) Yang, X.; Hamza, H.; Czarnecki, J. Energy Fuels 2004, 18, 770– 777.
10.1021/ef8008597 CCC: $40.75 2009 American Chemical Society Published on Web 05/07/2009
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et al. and Tsamantakis et al. showed that asphaltenes adsorb at oil-water interfaces as rigid asphaltene skins that contribute to stabilize oil emulsions.17,18 Asekomhe et al. confirmed that these asphaltene skins represent a barrier against droplet coalescence.19 A factor to consider when studying the stability of emulsions produced with heavy oils in the presence of solvents (diluents) is that the solubility of asphaltenes is greater in polar solvents and at elevated temperatures, which, according to the discussion above, may influence the surface activity of asphaltenes and their tendency to produce rigid interfacial skins.16,20 Furthermore, Wu demonstrated that the formation of such asphaltene skins is enhanced at an increased oil-water interfacial area for a fixed quantity of surface-active material in the sample.21 This was later confirmed by Gelin et al., who analyzed the effect of the water concentration on the behavior of asphaltenes within reservoir hydrocarbon fluids.22 Results obtained indicate that the concentration of emulsified water significantly affects the amount and physical properties of asphaltenes lost to the rag layer. In systems containing high concentrations of naphthenic acid, the formation of liquid crystals out of these species is another contributing factor to rag layer formation. In the absence of naphthenic acids, such as the case evaluated in this work, it is less likely to find liquid crystals. However, there is some evidence of their existence.10,23,24 Despite the numerous studies with model systems that point to the interfacial activity of asphaltenes in rag layer formation, there is lack of literature characterizing the effectiveness of various intensive crude oil processing variables in producing asphaltene skins capable of stabilizing emulsions. One of the goals of this work is to introduce a batch emulsificationseparation protocol and method of analysis, to study rag layer formation in solvent-bitumen-water systems. The second goal of this work is to study the effect that variables such as solventbitumen-water ratios, solvent aromaticity, and temperature have on the interfacial activity of asphaltenes and rag layer formation. These variables were evaluated in batch emulsification-separation experiments by measuring the composition of the rag layer, the fraction of asphaltene losses, and the oil-water interfacial tension of all of the systems considered. Experimental Section Chemicals. All chemicals were used as supplied: anhydrous toluene was purchased from Sigma-Aldrich Canada (99.8%); anhydrous heptane was purchased from Caledon Laboratory Chemicals (g99%); coker feed bitumen was donated by Syncrude Canada Ltd. (16.3 wt % saturates, 39.8 wt % aromatics, 28.5 wt % resins, and 14.7 wt % asphaltenes according to SARA analysis);25 and Kam Generator Solution Anode (premixed with reagent-grade xylene in a 60/40 volume ratio) and cathode solution were purchased (17) Tsamantakis, C.; Masliyah, J.; Yeung, A.; Gentzis, T. J. Colloid Interface Sci. 2005, 284, 176–183. (18) Yeung, A.; Dabros, T.; Masliyah, J.; Czarnecki, J. Colloids Surf. 2000, 174, 169–181. (19) Asekomhe, S. O.; Chiang, R.; Masliyah, J. H.; Elliott, J. A. W. Ind. Eng. Chem. 2005, 44, 1241–1249. (20) Fahim, M. A.; Al-Sahhaf, T. A.; Elkilani, A. S. Ind. Eng. Chem. 2005, 40, 2748–2756. (21) Wu, X. Energy Fuels 2003, 17, 179–190. (22) Gelin, F.; Grutters, M.; Cornelisse, P.; Taylor, S. Asphaltene precipitation from live oil containing emulsified water. Proceedings of the 5th International Conference on Petroleum Phase Behaviour and Fouling, Banff, Alberta, Canada, 2004. (23) Horva´th-Szabo´, G.; Czarnecki, J.; Masliyah, J. H. J. Colloid Interface Sci. 2002, 253, 427–434. (24) Yang, X.; Czarnecki, J. Colloids Surf., A 2002, 211, 213–222. (25) Akbarzadeh, K.; Alboudwarej, H.; Svrcek, W. Y.; Yarranton, W. Fluid Phase Equilib. 2005, 232, 159170].
Kiran et al. Table 1. Composition (wt %) of Heptol 80/20 (H)-Bitumen (B)-Saline Water (SW) Systems Heptol/bitumen ratio f
1:1.5
1:1
3:1
4:1
10:1
SW
H
B
H
B
H
B
H
B
H
B
75 50 25 9.1 0
10.0 20.0 30.0 36.4 40.0
15.0 30.0 45.0 54.5 60.0
12.5 25.0 37.5 45.5 50.0
12.5 25.0 37.5 45.5 50.0
18.8 37.5 56.3 68.2 75.0
6.3 12.5 18.8 22.7 25.0
20.0 40.0 60.0 72.7 80.0
5.0 10.0 15.0 18.2 20.0
22.7 45.5 68.2 82.6 90.9
2.3 4.5 6.8 8.3 9.1
from Kam Controls, Inc.26 A saline water (SW) solution was prepared by dissolving 25 mmol/L NaCl, 15 mmol/L NaHCO3, 2 mmol/L Na2SO4, 0.3 mmol/L CaCl2, and 0.3 mmol/L MgCl2 in deionized water to simulate typical water compositions in oil field operations.27 This solution had a pH of 7.5-8. Batch Emulsification-Separation Protocol. The oil phase was prepared by mixing Heptol 80/20 (80 vol % heptane and 20 vol % toluene) and bitumen together at various ratios in individual glass jars overnight using a wrist-action shaker, with a stroke length and frequency of approximately 1.5 in. and 180 strokes/min, respectively. Such mixing parameters were used to ensure that a homogeneous oil phase was obtained. Upon completion, the diluted bitumen solutions were added to prescribed amounts of SW in 15 mL glass centrifuge tubes. These formulations were mixed (VWR Vortex Mixer) at 3200 rpm for 2 min and then centrifuged (IEC Clinical Centrifuge) at approximately 500g for 1 min at room temperature (25 °C). The development of these emulsification and separation parameters will be discussed later. The compositions of solvent-bitumen-water systems considered in this work are presented in Table 1. The same systems described in Table 1 were also studied at 80 °C by placing the test tubes in a hot water bath prior to mixing and centrifuging. Another set of phase behavior studies was carried out using Heptol 50/50 (50 vol % heptane and 50 vol % toluene) instead of Heptol 80/20 as the solvent at 25 °C. Microscopy. The oil phase, aqueous phase, and rag layer produced in all formulations after mixing and centrifuging were sampled and analyzed according to slight variations in the procedure developed by Varadaraj et al.12 To sample the excess oil phase (top phase), a small volume of oil was slowly extracted using a 5 mL glass pasteur pipet at three distinct regions of interest. When sampling the rag layer (middle phase) and aqueous phase (bottom phase), a small positive pressure was applied during the insertion and extraction of the pasteur pipet to ensure that no additional phases were sampled along with the phase of interest. To prevent sample contamination, small air bubbles were blown out from the pipet upon entry into the phase of interest and the first few drops of the extracted sample were discarded prior to deposition onto a microscope slide. Three distinct sampling points were used for each of the phases to guarantee that the sample was representative. All extracted samples were analyzed using a BX-51 Olympus microscope at a 50× magnification, and pictures of these images were taken using an Olympus C-7070 wide zoom digital camera set to its maximum magnification (4×). Three different microscope configurations were used: (1) optical (transmitted light), which is used to differentiate between solid and liquid phases, (2) transmitted light using cross-polarized lenses, which is used to detect the presence of liquid crystals, and (3) fluorescence microscopy, which differentiates between oil (green) and aqueous (black) phases.12,23 Material Balances. Because of the variable composition and density of the rag layer in all systems analyzed, material balances were performed by tracking the volume of each formulation component in all phases produced after mixing and centrifuging. The volume of each phase produced was calculated from measurements of the relative heights of the separated “free” oil phase, rag layer, and “free” aqueous phase. The separated oil phase was considered a recoverable and useful product, in which only minor (26) Kam Controls, Inc. Karl Fischer Moisture Analyzer. http://kam.ewhiteboard.com/new/html/product.php?id)17 (accessed in March 2004). (27) Allen, E. W. J. EnViron. Eng. Sci. 2008, 7, 123–138.
Study of SolVent-Bitumen-Water Rag Layers traces of water were dispersed throughout. This was confirmed by measuring its water content using Karl Fischer titration (
