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Energy & Fuels 1999, 13, 328-332
Predicting the Onset of Asphaltene Precipitation from Refractive Index Measurements Jill S. Buckley* New Mexico Petroleum Recovery Research Center, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801 Received September 29, 1998. Revised Manuscript Received December 29, 1998
Despite the complexity of asphaltene chemistry, a simple relationship exists between the onset of precipitation and the refractive index of the mixture in which precipitation is first observed. Separation of asphaltenes from oil is a colloidal phenomenon. Because aggregation and separation of the asphaltenes are primarily dependent on the magnitude of van der Waals forces between nonpolar species, the mixture refractive index can be used to quantify the ability of a crude oil to disperse its asphaltene fraction. In some crude oils, asphaltenes are very close to the onset of precipitation, while in other oils, the asphaltenes are relatively stable. The value of the refractive index of an oil sample and the refractive index at which precipitation is first observed are distinctive properties of each crude oil. The difference between these two values is a measure of the stability of the asphaltenes in their respective crude oils. Mixture refractive index correctly predicts the onset of asphaltene precipitation in solutions with varying concentrations of asphaltenes.
Introduction Definitions. The study of asphaltenes spans many years. A wide range of analytical techniques have been applied to their characterization.1 It is now generally agreed that asphaltenes do not constitute a chemically identifiable class of compounds.2 Still, no universally accepted definition of asphaltenes has emerged. In this work, the term asphaltene is used in its most general sense to include all the material that aggregates and ultimately separates from a crude oil in response to changes in the solvent properties of the oil. Asphaltenes are distinguished from waxes in that asphaltenes form amorphous solids that include a significant percentage of the heteroatoms present in the oil. Separation of asphaltenes from the oil phase is commonly referred to as asphaltene precipitation because the material recovered from an oil after addition of heptane is usually a powdery solid. More generally, however, the material that separates need not be solid; the term separation rather than precipitation is probably the more accurate one to describe, in general, what happens to the asphaltenes when they are no longer stable in the remainder of the oil, although both terms are used here. It is useful to distinguish between fluids that can cause precipitation of asphaltenes and those that do not. For most oils, low molecular weight paraffins such as pentane are precipitants for asphaltenes. Aromatic hydrocarbons are asphaltene solvents. * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Cimino, R.; Correra, S.; Del Bianco, A.; Lockhart, T. P. In Asphaltenes: Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995; pp 97-130. (2) Speight, J. G.; Plancher, H. Proc. Int. Symp. Chem. Bitumens 1991, 154.
Quantifying the Solvent Properties of a Crude Oil. Oil solvent properties change with variations in temperature, pressure, and oil composition. Thermodynamic models track those changes by assigning solubility parameters to oil and asphaltenes and noting the difference between them.3-7 Solubility parameters can be measured only for pure compounds. For simple mixtures they can be calculated, but for many of the components in a crude oil, solubility parameters are not known. For nonpolar molecules there is an almost linear relationship between the solubility parameter and refractive index (RI).8 Both are related to the van der Waals forces between nonpolar molecules. Unlike the solubility parameter, however, RI can be measured for mixtures. Adaptation of thermodynamic models to accommodate macromolecules using the Flory-Huggins approach has been successful in matching asphaltene phase behavior,3 but no approach has yet been shown to be predictive. Asphaltenes have also been treated as colloidal suspensions.8 Stabilization is often attributed to a peptizing effect of resins, but the relationship between asphaltenes and resins has never been conclusively demonstrated.1 If, as suggested previously,9 the nonpolar van der Waals forces are mainly responsible for separation of asphaltenes from the oil phase, then RI should (3) Hirschberg, A.; deJong, L. N. J.; Schipper, B. A.; Meijer, J. G. SPE J. 1984, 25 (3), 283-293. (4) Hildebrand, J. H.; Scott, R. L. Solubility of Non-Electrolytes, 3rd ed.; Reinhold: New York, 1950. (5) Mitchell, D. L.; Speight, J. G. Fuel 1973, 52, 149-152. (6) Burke, N. E.; Hobbs, R. D.; Kashou, S. F. J. Pet. Technol. 1990, 1440-1446. (7) Wiehe, I. A. Fuel Sci. Technol. Int. 1996, 14, 289-312. (8) Nellensteyn, F. I. In The Science of Petroleum; Dunstan, A. E., Ed.; Oxford University Press: London, 1938; Vol. 4. (9) Buckley, J. S.; Hirasaki, G. J.; Liu, Y.; Von Drasek, S.; Wang, J. X.; Gill, B. S. Pet. Sci. Technol. 1998, 16 (3/4), 251-285.
10.1021/ef980201c CCC: $18.00 © 1999 American Chemical Society Published on Web 02/09/1999
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Figure 1. Linear trend of RI with volume fraction of crude oil mixed with n-heptane.
provide a useful measure of the oil’s solvent properties with respect to the asphaltenes. RI of Mixtures. If there are no significant changes in volume when two liquid components are mixed, the mixing rules for RI are simple (see, for example, Synovec and Yeong10)
n2 - 1 n2 + 2
)
( )
∑i fvolume,i
ni2 - 1
Figure 2. RI of various components of crude oils: (a) hydrocarbon molecules and (b) resin and asphaltene fractions (in toluene solutions) from A-93 crude oil.
ni2 + 2
where n ) the mixture refractive index, ni ) the refractive index of pure i, and fvolume,i ) the volume fraction of i in the mixture. In the simple case of a binary mixture, (n2 - 1)/(n2 + 2) is linearly related to the volume fraction of either of the components. The more complicated case of a crude oil mixed with a solvent can be treated as a simple binary mixture. RI of pseudo-binary mixtures of oil from the Spraberry field (31.1° API) with n-heptane are shown in Figure 1 as a function of the volume fraction of crude oil in the oil/heptane mixture. RI of Crude Oil Components. Paraffinic compounds are the lowest RI materials in a crude oil. Cyclic compounds are higher in RI than linear ones of a similar molecular weight. Aromatics are higher still, as illustrated in Figure 2a. The resin and asphaltene fractions contain some of the highest RI materials, as shown in Figure 2b. Asphaltenes can be separated from the remaining oil components (maltenes) by addition of sufficient nheptane. After removal of the precipitated asphaltenes by filtration, the maltenes can, in principle, be recovered by evaporating off the excess precipitant. Once precipitation occurs, the crude oil can no longer be treated as a single component, as in Figure 1. Instead, the asphaltenes and maltenes should be considered separately. An example is shown in Figure 3. The RI of A-95 crude oil is 1.516. After separation of 7% asphaltenes, the remaining maltenes have an RI of about 1.495. Mixtures of maltenes with n-heptane would have RI (10) Synovec, R. E.; Yeung, E. S. Anal. Chem. 1983, 55, 1599-1603.
Figure 3. RI of A-95 crude oil, A-95 maltenes, and mixtures with n-heptane.
values on the connecting line, as shown. A second line connects maltenes and oil (fv from 0.93 to 1); the difference between RImaltenes and RIoil represents the contribution of the asphaltenes to the refractive index of the oil. RI of Mixtures during Heptane Titration. Figure 4 shows the path of a typical heptane titration experiment. Initially, heptane is added to the whole crude oil. Before the onset of precipitation, mixture RI values lie on the line between RIoil and RIheptane (Figure 4a). In this region, mixtures are stable with respect to dispersion of the asphaltenes. Precipitation is first observed when the mixture RI reaches PRI (Figure 4b). As
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Figure 4. Path of RI vs volume fractions in crude oil mixtures with heptane.
additional heptane is added, asphaltenes separate from the mixture. RI values in the unstable region now should lie below the oil-heptane line and, as additional precipitation occurs, should approach the line connecting maltenes and heptane (Figure 4c). An important element of this picture that has not been considered in previous studies of asphaltene precipitation is the contribution of the asphaltenes themselves to the RI and thus the solvent quality of the stable oil. The n-heptane titration results for two crude oils are shown in Figure 5. The best fit to all the data for mixtures in which no precipitate is present is shown as a solid line. This is also the linear interpolation between oil (as one pseudo-component) and n-heptane. Once precipitation has occurred, some deviation from this line can be expected. Little deviation from the oil-heptane line is evident in Figure 5a, where the dotted line from Figure 3 shows the RI of maltene-heptane mixtures. A more obvious deviation is shown in Figure 5b for an asphaltic oil from California. Discussion The relationship between asphaltene stability and mixture RI provides a framework for further consideration of a variety of asphaltene stability issues. Any number of paths through the RI-volume fraction diagram can be envisioned, including some that have been investigated in previous work and others that remain to be tested.
Figure 5. PRI is RI at which precipitation is first observed, as shown for n-heptane mixtures with (a) A-95 crude oil from Prudhoe Bay and (b) an asphaltic oil from California.
Figure 6. Path of RI vs volume fractions in crude oil mixtures with a solvent of higher RI and heptane.
Addition of Solvents. In previously reported experiments,11 toluene and other solvents were added to A-93 crude oil samples prior to heptane titration. The mixtures in which the onset of asphaltene instability was identified had a wide range of percentage asphaltene and the volume fractions of oil varied as well, but the value of RI at the onset of precipitation (PRI) was relatively constant for a given oil and precipitating agent. Hence, PRI is indicated as a horizontal band. Figure 6 illustrates, in general, experiments in which oil, solvent, and precipitant are mixed. For oil mixed (11) Buckley, J. S. Fuel Sci. Technol. Int. 1996, 14, 55-74.
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Figure 7. Is asphaltene precipitation “reversible”? The answer depends on how reversibility is defined. Because of the separation between asphaltenes and maltenes that occurs below PRI, the process indicated by arrow 1 (where heptane is added to oil) is not the inverse of arrow 2 (where oil is added to heptane) even though the final volume fractions are identical.
with solvent, mixture RI values fall on the line joining RIoil and RIsolvent. At some point, illustrated here for a 50:50 mixture of oil and solvent volumes, precipitant is added and RI values fall along the straight line joining the value for the mixture with RIprecipitant. Instability will occur, and asphaltenes will be observed when the RI drops below the onset value of PRI. A constant value of the ratio of solvent to precipitant volumes has been reported.1,12,13 In all these studies, the amounts of solvent and precipitant were much larger than the amount of oil. At effectively infinite dilution, it is possible to neglect the effect of the oil (maltenes + asphaltenes) on the mixture RI, although in situations of practical interest including the contribution of the oil is essential. Reversibility of Asphaltene Precipitation. Whether or not asphaltene precipitation is reversible in a true thermodynamic sense is an important, unanswered question.1 Slow kinetics of dispersion and flocculation make it difficult to determine whether the processes are reversible. Sometimes the presence of inorganic material that is separated with the asphaltenes but cannot be redispersed may be interpreted as indicating irreversibility. Apparent irreversibility may also arise because of the contribution of asphaltenes themselves to the mixture RI, as illustrated in Figure 7. A solution prepared by addition of heptane to oil (arrow 1) is different from one prepared by adding the same volume fraction of oil to heptane (arrow 2). Asphaltenes are stable in the first solution and unstable in the second. As oil is added to heptane, asphaltenes precipitate. Thus, the liquid mixture has an RI between maltenes and heptane that is lower than the RI of a stable mixture of maltenes, asphaltenes, and heptane. To judge whether asphaltene precipitation is reversible, mixtures should be compared at the same values of RI. For example, in Figure 8, RI values of mixtures of A-95 crude oil and heptane are shown. To a mixture with precipitated asphaltenes (fv,oil ) 0.25), toluene was added. Both the appearance and disappearance of asphaltenes occurred over the same narrow range of mixture RI values. Thus, asphaltene precipitation appears to be completely reversible for this crude oil. Concentration Effects. The amount of asphaltene in a mixture is not really its concentration, although the term is sometimes used. A critical micelle concen(12) Hotier, G.; Robin, M. Rev. IFP 1983, 38, 101. (13) Dubey, S. T.; Waxman, M. H. SPE Reservoir Eng. 1991, 6 (3), 389-395.
Figure 8. Addition of toluene to a mixture of A-95 crude oil and n-heptane (fv,oil ) 0.25) demonstrates that the asphaltenes disappear at the same RI at which they originally appeared.
tration is reported as a function of asphaltene concentration, by analogy with aqueous surfactant systems.14 Asphaltene adsorption isotherms are reported as a function of asphaltene concentration.15 Since the asphaltenes represent a range of materials and since they tend to increase the RI of a crude oil, some components in the asphaltene fraction may actually help to stabilize other asphaltene components, creating some confusion about the effects of asphaltene concentration. Aggregation Kinetics and Concentration. Concentration-dependent aggregation rates were reported by Yudin et al.16 They demonstrated first that asphaltenes showed aggregation kinetics similar to other colloidal systems. Dispersions that are nearly stable (less heptane) aggregate slowly with reaction-limited aggregation (RLA) kinetics. Unstable dispersions (more heptane) aggregate more rapidly with diffusion-limited aggregation (DLA) kinetics.17 More difficult to explain was a similar transition from faster DLA to slower RLA kinetics if the concentration of asphaltenes was increased from 0.1% to 1% in solutions destabilized with identical amounts of heptane. Different aggregation mechanisms were suggested, but another interpretation is suggested in Figure 9. The solution with less asphaltene has a lower RI and is therefore less stable (faster aggregation) than the more concentrated solution (higher RI, more stable, slower aggregation). The trajectory through the onset region depends on RIoil and RIprecipitant, as illustrated in Figure 10. With more paraffinic oils (lower RI), there is a greater chance of testing at near-onset conditions where the kinetics are slowest. Similarly, significantly slower kinetics might be expected with higher RI precipitants (such as (14) Sheu, E. Y.; DeTar, M. M.; Storm, D. A.; DeCanio, S. J. Fuel 1992, 71, 299-302. (15) Gonza´lez, G.; Travalloni-Louvisse, A. M. SPE Prod. Facil. 1993, 6 (3), 91-96. (16) Yudin, I. K.; Nikolaenko, G. L.; Gorodetsky, E. E.; Kosov, V. I.; Melikyan, V. R.; Markhashov, E. L.; Frot, D.; Brioland, Y. J. Pet. Sci. Eng. 1998, 20 (3/4), 297-301. (17) Weitz, D. A.; Huang, J. S.; Lin, M. Y.; Sung, J. Phys. Rev. Lett. 1985, 54 (13), 1416-1419.
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Figure 9. Difference in aggregation rates for different concentrations of asphaltenes is consistent with their probably different initial values of RI. If the 1% solution is just unstable, the 0.1% solution is more unstable and thus aggregates at a faster rate.
Figure 10. Trajectory through the onset region.
higher molecular weight paraffins) than those with lower RI (smaller molecules). Remaining Questions. For a given oil, the quantity of material precipitated, its chemical makeup, and PRI all vary with different precipitating agents. As the molecular weight of the precipitant decreases, PRI decreases,9 as indicated in Figure 11, while the amount of material precipitated and H/C ratios increase. A model that can quantitatively account for these observations remains to be developed. Conclusions Separation of asphaltenes from oil is dominated by nonpolar van der Waals forces. Thus, the refractive
Buckley
Figure 11. Onset of precipitation occurs at decreasing values of PRI for lower molecular weight precipitants.a
index can be used to quantify the solvent conditions at which asphaltenes first appear. Mixture refractive index reflects the contributions of all components in proportion to their volume fractions. Asphaltenes, resins, and condensed aromatic hydrocarbons are among the highest RI materials in a crude oil; paraffinic compounds are among the lowest. Precipitation of the asphaltenes changes the solvent properties of the remainder of the oil phase since the asphaltenes themselves represent the highest RI material in the oil. If solutions with the same RI are compared, asphaltene precipitation appears to be reversible. Colloidal aggregation kinetics also reflect the contribution of asphaltenes to the solvent quality of the oil and thus to the stability of the asphaltenes. Acknowledgment. This work has been supported by the NPTO office of the U.S. DOE, by contributions from industrial sponsors (Mobil, Norsk Hydro, Unocal), and by the State of New Mexico. The author thanks Mary Creech and Sue Von Drasek for measurements of asphaltene precipitation and RI. EF980201C
