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Hysteresis in Asphaltene Precipitation and Redissolution J. Beck,† W. Y. Svrcek, and H. W. Yarranton* Department of Chemical & Petroleum Engineering, University of Calgary, Calgary, Alberta, Canada T2N 1N4 Received November 17, 2004. Revised Manuscript Received March 4, 2005
The precipitation and redissolution of asphaltenes from mixtures of Athabasca bitumen and n-heptane was measured over time in both air and nitrogen atmospheres at 23 °C. In air, it appears that oxidation of the bitumen increased the asphaltene yield for as long as the experiments were conducted; that is, for several months. When oxidation effects are excluded, asphaltene precipitation and redissolution both appear to reach steady state within 24 h. A hysteresis between asphaltene precipitation and redissolution was also observed in both atmospheres. The hysteresis was exaggerated in the air atmosphere due to oxidation effects. In both air and nitrogen atmospheres, complete reversibility was attained when the heptane-to-bitumen ratio was reduced to 1.5 cm3/g. The hysteresis is attributed to an energy barrier to asphaltene dissociation. As more heptane is added, the asphaltenes likely self-associate to higher apparent molar masses. When the heptane is removed, the asphaltenes may remain in the higher association state and hence precipitate to a greater extent than before at a given heptane-to-bitumen ratio. If the system is heated, the asphaltene yields return to their original values, indicating that the original association state can be restored.
Introduction Asphaltenes are the crude oil constituents with the highest molar mass, aromaticity, and heteroatom content. They are defined as the crude oil fraction that precipitates upon addition of an n-alkane, usually n-pentane or n-heptane.1 Asphaltenes can precipitate upon a change in pressure, temperature, or composition and can be a major problem for oil producers. For example, asphaltene precipitation in the reservoir triggered by a change in pressure can significantly reduce production. Asphaltene deposition in surface facilities and pipelines can occur upon depressurization or the addition of condensate diluent. Treatment to remove the deposits increases operating costs. One issue in the mitigation of asphaltene deposition is the reversibility of asphaltene precipitation. In addition, a better understanding of the reversibility of asphaltene precipitation may contribute to the development of predictive models of asphaltene precipitation. For example, thermodynamic theories assume that asphaltene precipitation is fully reversible, while the colloidal model predicts irreversible precipitation.3,4 Several investigators have demonstrated that there is a hysteresis between the precipitation and redissolution of asphaltenes. This hysteresis has been observed in asphaltene and solvent mixtures with the addition or removal of a solvent or precipitant such as * Corresponding author. † Present address: Suncor Energy Ltd., Fort MacMurray, Canada. (1) Wiehe, I. A.; Liang, K. S. Asphaltenes, Resins, and Other Petroleum Macromolecules. Fluid Phase Equilibria 1996, 117, 201210.
toluene or heptane2,5 and with heating or cooling.2,9 A similar hysteresis has also been observed in crude oil and diluent mixtures with the addition or removal of a precipitant,2,6-8,10,11 with heating or cooling2,6-8 and with an increase or decrease in pressure.12 A more complete review of these studies is provided elsewhere.2 Most authors hypothesize that the hysteresis arises from slow (2) Peramanu, S.; Singh, C.; Agrawala, M.; Yarranton, H. W. Investigation on the Reversibility of Asphaltene Precipitation. Energy Fuels 2001, 15, 910-917. (3) Hirschberg, A.; deJong, L. N. J.; Schipper, B. A.; Meijer, J. G. Influence of Temperature and Pressure on Asphaltene Flocculation. SPE J. 1984, June, 283-289. (4) Leontaritis, K. J.; Mansoori, G. A. Asphaltene Flocculation During Oil Production and Processing: A Thermodynamic Colloidal Model. SPE International Symposium on Oilfield Chemistry, San Antonio, TX, February 4-6, 1987; SPE Paper 16258. (5) Andersen, S. I. Hysteresis in Precipitation and Dissolution of Petroleum Asphaltenes. Fuel Sci. Technol. Int. 1992, 10, 1743-1749. (6) Clarke, P. F.; Pruden, B. B. Heat Transfer Analysis for Detection of Asphaltene Precipitation and Resuspension. 47th Annual Technical Meeting of the Petroleum Society, Calgary, Alberta, Canada, June 1012, 1996; Paper 96-112. (7) Clarke, P. F.; Pruden, B. B. Asphaltene Precipitation: Detection using Heat Transfer Analysis, and Inhibition using Chemical Additives. Fuel 1997, 76 (7), 607-614. (8) Clarke, P. F.; Pruden, B. B. Asphaltene Precipitation from Cold Lake and Athabasca Bitumens. Pet. Sci. Technol. 1998, 16 (3/4), 287305. (9) Andersen, S. I.; Stenby, E. H. Thermodynamics of Asphaltene Precipitation and Dissolution Investigation of Temperature and Solvent Effects. Fuel Sci. Technol. Int. 1996, 14 (1/2), 261-287. (10) Mohamed, R. S.; Loh, W.; Ramos, A. C. S.; Delgado, C. C.; Almeida, V. R. Reversibility and Inhibition of Asphaltene Precipitation in Brazilian Crude Oils. Pet. Sci. Technol. 1999, 17, 877-896. (11) Rassamdana, H.; Dabir, B.; Nematy, M.; Farhani, M.; Sahimi, M. Asphalt Flocculation and Deposition: 1. The Onset of Precipitation. AIChE J. 1996, 42, 10-22. (12) Hammami, A.; Phelps, C. H.; Monger-McClure, T.; Little, T. M. Asphaltene Precipitaiton from Live Oils: An Experimental Investigation of Onset Conditions and Reversibility. Energy Fuels 2000, 14, 14-18.
10.1021/ef049707n CCC: $30.25 © 2005 American Chemical Society Published on Web 04/05/2005
Asphaltene Precipitation/Redissolution Hysteresis
kinetics of asphaltene precipitation and/or redissolution. However, this hypothesis has not yet been directly tested. Another factor that may give rise to hysteresis is oxidation or oxygen-catalyzed polymerization of resins to asphaltenes.13 Many benchtop experiments are conducted in an air atmosphere. Redissolution experiments typically involve more time than precipitation experiments and hence may exhibit an increase in asphaltene precipitation if any oxidation occurs. In other words, the observed hysteresis may be an artifact. Yet another possibility is that the level of asphaltene association is different for precipitation and redissolution. Asphaltene molecules are known to selfassociate into aggregates consisting of 2-10 molecules/ aggregate on average, and the extent of self-association depends on temperature and composition.14 It is possible that the extent of asphaltene association plays a role in the hysteresis between precipitation and redissolution. In this work, the time dependency of asphaltene precipitation and redissolution is investigated under air and nitrogen atmospheres. The role of asphaltene selfassociation is also considered, and a possible cause of the hysteresis is identified. Experimental Procedures Materials. Athabasca coker feed bitumen was obtained from Syncrude Canada Ltd. This oil sands bitumen had been processed to remove water and solids, such as sand and clays, and was ready for upgrading. The asphaltenes precipitated from the bitumen contain 0.56 wt % non-asphaltene solids, such as sand, clays, and associated organics.15 The asphaltene content, including the solids, of the Athabasca bitumen sample used in this work is 9.3 wt % from ASTM D4124. Note that non-asphaltene solids have been shown to collect in the first material to precipitate from bitumens and heavy oils.15 All the asphaltene yields reported in this work include the non-asphaltene solids. Toluene and n-heptane were obtained from Aldrich Chemical Co. and were 99%+ pure. Precipitation and Redissolution of Asphaltenes. For precipitation tests, n-heptane was added to bitumen to a desired heptane-to-bitumen (H/B) ratio, and the mixture was shaken in a mechanical shaker for 30 min and left to equilibrate for the desired time. The mixture was then shaken for 5 min to disperse the sediment and centrifuged for 6 min at 3600 rpm. The supernatant was decanted and 20 cm3 of fresh n-heptane was added to wash the precipitate. The new mixture was then sonicated for 15 min and centrifuged for 6 min at 3600 rpm. The supernatant was decanted and the heptane-washing step was repeated twice. The remaining precipitate was dried at room temperature until no change in mass was detected. The asphaltene yield is the mass of dried (13) Wilson, D. I.; Watkinson, A. P. A Study of Autoxidation Reaction Fouling in Heat Exchangers. Can. J. Chem. Eng. 1996, 74, 236-246. (14) Yarranton, H. W.; Alboudwarej, H.; Jakher, R. Investigation of Asphaltene Association with Vapor Pressure Osmometry and Interfacial Tension Measurements. Ind. Eng. Chem. Res. 2000, 39 (8), 2916-2924. (15) Alboudwarej, H.; Beck, J.; Akbarzadeh, K.; Svrcek, W. Y.; Yarranton, H. W. Sensitivity of Asphaltene Properties to Extraction Techniques. Energy Fuels 2002, 16 (2), 462-469.
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precipitate divided by the mass of the bitumen sample. The bitumen samples in these tests ranged from 1 to 10 g in mass and the volume of solvent added ranged from 23 to 38 cm3, so that the total volume was approximately 40 cm3. The data presented later were not repeated at any given data point but the repeatability of the measurements was determined from the scatter about the observed trendlines and is approximately (1.5% yield. For redissolution tests, asphaltenes were first precipitated by adding n-heptane to bitumen at a desired H/B ratio, the initial H/B ratio. The mixture was shaken vigorously for 30 min and left to settle for 24 h. To redissolve asphaltenes, heptane was evaporated under vacuum at 5-20 °C until a targeted H/B ratio was achieved (as determined from the mass of the mixture). The evaporation of heptane required 10-30 min. After the evaporation step, the remaining mixture was thoroughly mixed by shaking and allowed to settle for 24 h. Samples were recovered, washed, and dried as for the precipitation experiments. It was found that the yields after redissolution were insensitive to the initial H/B ratio and depended only on the H/B ratio after redissolution; that is, the final H/B ratio. Therefore, for experimental convenience, the initial H/B ratio was set to be 1 cm3/g greater than the desired final H/B ratio. For instance, for a final H/B ratio of 2.5 cm3/g, the asphaltenes were first precipitated at an H/B ratio of 3.5 cm3/g. The experimental procedures for the precipitation and redissolution experiments are summarized schematically in Figure 1. Precipitation and redissolution experiments were also conducted in a nitrogen atmosphere. These experiments were performed in a glove chamber filled with 99.999% pure nitrogen. The desired mass of bitumen was measured into test tubes and placed in the glove chamber. To remove trace air from the glove chamber, the chamber and apparatus within were emptied under vacuum and then filled with nitrogen. This cycle was repeated at least 10 times. To prepare oxygen-free solvent, 400 cm3 of n-heptane was purged with nitrogen for 3 h. After the deoxygenation, the chamber was voided and nitrogen-purged another 5-10 times. In all other respects, the same procedures were followed as for an air atmosphere. Results and Discussion A typical hysteresis between precipitation and redissolution in an air atmosphere is shown in Figure 2. While asphaltenes appear to redissolve at low heptaneto-bitumen (H/B) ratios, there is a significant gap between the precipitation and redissolution curves. One possible source of the hysteresis is slow kinetics of precipitation or redissolution; that is, at least one of the curves does not represent equilibrium data. To test this idea, asphaltene precipitation and redissolution were measured over time for several H/B ratios as shown in Figures 3 and 4. Figure 3 demonstrates that in the first 24 h there is a rapid increase in yield with precipitation and a rapid decrease in yield with redissolution. The change in yield in the first 24 h is shown in more detail in Figure 4. After 24 h, there is a gradual increase in yield in both cases. The yield continues to increase for as long as
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Figure 1. Schematic of experimental procedure for (a) precipitation, (b) redissolution, and (c) cycling experiments.
Figure 2. Hysteresis between asphaltene precipitation and redissolution for Athabasca bitumen and n-heptane system at 23 °C in air and nitrogen atmospheres.
these samples were observed; that is, several months. Oxidation is the likely mechanism for the gradual increase in yield.16 When the same experiment is repeated in a nitrogen atmosphere, no increase in yield is observed after the first 24 h, as shown in Figure 5. Hence, oxidation effects are significant in diluted bitumens even at 23 °C. Figure 5 also shows that although there is less yield at any given time in a nitrogen atmosphere, a hysteresis persists even after the yield reaches what appears to be steady-state values. The hysteresis after 24 h is smaller than that observed in air but still significant, as shown in Figure 2. Hence, neither oxidation effects nor slow kinetics of precipitation and/or redissolution can fully explain the hysteresis. An alternative explanation for the hysteresis is a change in the molecular association of asphaltenes. (16) Asomansing, S.; Watkinson, A. P. Petroleum Stability and Heteroatom Species Effects in Fouling of Heat Exchangers by Asphaltenes. Heat Transfer Eng. 2000, 21 (3), 10-16.
Figure 3. Time dependence of asphaltene precipitation and redissolution from Athabasca bitumen and n-heptane system at 23 °C in an air atmosphere.
Figure 4. Asphaltene precipitation over 24 h from Athabasca bitumen diluted with n-heptane at 40 cm3/g in air at 23 °C.
Asphaltenes are known to self-associate into molecular aggregates or macromolecules of approximately 210 monomers/aggregate.14 The extent of the self-as-
Asphaltene Precipitation/Redissolution Hysteresis
Figure 5. Time dependence of asphaltene precipitation and redissolution from Athabasca bitumen and n-heptane system at 23 °C in a nitrogen atmosphere.
sociation depends on composition, temperature, and possibly pressure. When heptane is added to bitumen, the asphaltenes likely self-associate to a greater extent. There may be an energy barrier to dissociation so that when the solvent is removed, the asphaltenes remain in a higher state of association. Since the effective molar mass is higher, the amount of precipitation is higher and a hysteresis arises. If this explanation is correct, then the asphaltene yields are not expected to return to the original precipitation curve unless the original state of association is restored. The original association state may possibly be restored by heating the mixture of bitumen and heptane. To test this hypothesis, a bitumen sample that had been contacted with heptane was required. To create this sample, bitumen was diluted to an H/B ratio of 4 cm3/g as described previously. At this ratio, a substantial fraction of the asphaltenes precipitated. Then, heptane was evaporated until the H/B ratio was reduced to less than 0.04 cm3/g and the mixture was left to equilibrate for 24 h. At this ratio, all of the precipitate redissolved and a single-phase “cycled” bitumen was obtained. Then, heptane was added to the heated or unheated cycled bitumen and the asphaltene yields were compared. In one set of experiments, the cycled bitumen was left at room temperature and the yields were determined as described previously (pathway I in Figure 1). In a second set of experiments, the cycled bitumen was heated to 75 °C for 3 h. After it was cooled to 23 °C, heptane was added and the yield was determined after 21 h of equilibration (pathway II in Figure 1). Both sets of experiments were conducted in a nitrogen atmosphere except for the heating step, which was conducted under a vacuum at 23 °C. The results are given in Figure 6. Figure 6 shows that precipitation from the unheated cycled bitumen falls between the original precipitation and redissolution curves. It is likely that when the heptane is removed, some but not all of the asphaltenes dissociate. Hence, the effective molar mass is greater than in the original bitumen but less than the diluted bitumen. Therefore, the yield is greater than the original precipitation curve but less than the redissolution curve. Now consider the heated cycled bitumen. Figure 6 shows
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Figure 6. Effect of heating on yield from “cycled” bitumen diluted with n-heptane at 23 °C in a nitrogen atmosphere.
that the yield of the heated bitumen does follow the original precipitation curve. As a final confirmation, the previous redissolution experiments were repeated except that after solvent evaporation the mixtures were heated at 75 °C for 3 h in sealed test tubes (pathway III in Figure 1). As shown in Figure 6, the original precipitation curve was again restored. Hence, it appears that heating restores the original association state and removes the hysteresis. Conclusion The precipitation and redissolution of asphaltenes from mixtures of Athabasca bitumen and n-heptane was measured over time at 23 °C. Both precipitation and redissolution appeared to reach equilibrium values within 24 h. A hysteresis between asphaltene precipitation and redissolution was also observed in both air and nitrogen atmospheres. The hysteresis was exaggerated in the air atmosphere due to the oxidation effects. However, the hysteresis could not be explained in terms of the kinetics of oxidation. It is postulated that the hysteresis arises from an energy barrier to asphaltene dissociation. As more heptane is added, the asphaltenes self-associate to higher molar masses. When the heptane is removed, the asphaltenes may remain in the higher association state and hence precipitate to a greater extent than before at a given heptane-to-bitumen ratio. If the system is heated, the asphaltene yields return to their original values, indicating that the original association state can be restored. Hence, it appears that asphaltene precipitation is a reversible process and that the hysteresis between precipitation and redissolution can be explained in terms of an energy barrier to asphaltene dissociation. Acknowledgment. We acknowledge Albian Sands Energy Ltd. and the Province of Alberta COURSE program for financial support and Syncrude Canada Ltd. for providing bitumen samples. We also thank Dr. W. Power and Dr. R. Tipman of Albian Sands Energy Ltd. for their invaluable input. EF049707N
