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Energy & Fuels 2003, 17, 508-509
Examination of Asphaltenes Precipitation and Self-Aggregation Kyeongseok Oh, Scott C. Oblad, Francis V. Hanson, and Milind D. Deo* Department of Chemical and Fuels Engineering, University of Utah, Salt Lake City, Utah 84112 Received June 24, 2002 It is believed that asphaltenes self-aggregation is initiated above a threshold concentration, which is called the critical micellization concentration (CMC). Self-aggregation of asphaltenes has been understood to be the first step in the formation of small particles.1 Terms such as self-association and self-aggregation are often used interchangeably. It was hypothesized that asphaltene molecular moieties associated to form micelles through hydrogen bonds.2 Later studies have shown that charge transfer between molecules may be the significant mechanism leading to self-aggregation.3 Selfaggregation of asphaltenes in organic solvents has been observed by calorimetric,1 surface tension3-5 and interfacial tension measurements,5,6 vapor pressure osmometry,6,7 and small-angle neutron scattering.3,8 Andersen and Birdi1 used calorimetric titration to investigate the behavior of asphaltenes in different solvents. They identified CMC by finding the existence of distinct break points in the heats evolved by the dilution of higher concentration asphaltene solutions. They attributed the heats evolved to the dilution, demicellization, and injection processes. Sheu and co-workers9 measured the surface tensions of C5S (pentane-soluble) and C7I (heptane-insoluble) asphaltenes extracted from Ratawi vacuum residue in pyridine and nitrobenzene. The surface tensions were plotted as a function of asphaltenes concentrations. They defined the discontinuous point as a CMC. A typical plot consisted of an initial sharp decrease at concentrations below the CMC (discontinuous point) and a constant or a mildly decreasing zone above the CMC. After Taylor’s interpretation,10 the Gibbs adsorption equation has been widely accepted in analyzing the surface tension data.3,5,11 Meanwhile, Andersen and Speight11 reported that resins and aromatics do not show self-aggregation behavior. Thus, consistent trends with respect to discontinuities or break points were observed in the results of surface tension and calorimetric measurements of asphaltenes in organic solvents. These breaks have been interpreted to be CMC * Author to whom correspondence should be addressed. Fax: (801) 585-9291. E-mail:
[email protected]. (1) Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142, 497502. (2) Ho, B.; Briggs, D. E. Colloids Surf. 1982, 4, 285-303. (3) Sheu, E. Y.; Storm, D. A. Asphaltenes: Fundamentals and Applications; Sheu, E. Y.; Mullins, O. C., Eds.; Plenum Press: New York, 1995; Chapter 1, pp 1-52. (4) Rogacheva, O. V.; Rimaev, R. N.; Gubaidullin, V. Z.; Khakimov, D. K. Collod. J. USSR 1980, 490-493. (5) Mohamed, R. S.; Ramos, A. C. S.; Loh, W. Energy Fuels 1999, 13, 323-327. (6) Yarranton, H. W.; Alboudwarej, H.; Jakher, R. Ind. Eng. Chem. Res. 2000, 39, 2916-2924. (7) Moschopedis, S. E.; Fryer, J. F.; Speight, J. G. Fuel 1976, 55, 227232. (8) Sheu, E. Y.; Storm, D. A.; De Tar, M. M. J. Non-Cryst. Solids 1991, 131-133, 341-347. (9) Sheu, E. Y.; De Tar, M. M.; Strom, D. A.; DeCanio, S. J. Fuel 1992, 71, 299-302. (10) Taylor, S. E. Fuel 1992, 71, 1338-1339. (11) Andersen, S. I.; Speight, J. G. Fuel 1993, 72, 1343-1344.
Figure 1. Precipitation onset values for two asphaltene samples, A1 and A2, with respect to their concentrations in toluene.
values. The physical properties of the mixtures changed critically between concentrations below and above the defined CMC values. It is shown in this paper that asphaltene precipitation onset points measured using titration and near-infrared (NIR) spectroscopy can be used effectively to establish CMC values. Onset is defined as the minimum amount of precipitant necessary to induce precipitation of asphaltenes. The basic ideas were to expect differences in onset features just below and just above CMC by a titration method. NIR spectroscopy has been used in the past to determine the onset of asphaltene precipitation.12-15 The onsets measured in this manner are consistently reproducible. Two different field-asphaltenes (A1 and A2 from Rangely oil field in northwestern Colorado) were used in this study. The asphaltenic solids were formed in the course of a carbon dioxide flood in the field. Each of the asphaltenes was mixed separately with solvent (toluene or pyridine) at different concentrations (1-15 g/L) and was stirred for over 24 h. Mixtures were also prepared by the dilution of a primary 15 g/L concentration mixture. Titration with n-heptane was carried out with 40 mL of asphaltenes-solvent mixtures. A peristaltic pump controlled the heptane flow rates at 1 mL/min. Onset determination was described in a previous paper.15 The onsets of precipitation with respect to the asphaltenes concentrations are plotted in Figure 1. The ratios of precipitant (heptane) volume to solvent (toluene) volume are plotted on the y-axis while the asphaltenes concentrations are plotted on the x-axis. Two distinct zones are evident on the plot. Initially, the onset values decrease rapidly, followed by a zone where the onsets decrease more gradually. The two zones are separated by an easily identifiable break point or discontinu(12) Mullins, O. C. Anal. Chem. 1990, 62, 508-514. (13) Leontaritis, K. J. World Oil 1997, Nov., 101-104. (14) Fuhr, B. J.; Cathrea, C.; Coates, L.; Kalra, H.; Majeed, A. I. Fuel 1991, 70, 1293-1297. (15) Oh, K.; Deo, M. D. Energy Fuels 2002, 16, 694-699.
10.1021/ef020138y CCC: $25.00 © 2003 American Chemical Society Published on Web 02/28/2003
Communications
Energy & Fuels, Vol. 17, No. 2, 2003 509
Table 1. Elemental Analysis and Molecular Weights of Asphaltenes A1 A2 A2-F11 A2-F12 A2-F21 a
C, %
H, %
N, %
S, %
H/C
MWa
83.34 87.13 87.33 82.90 87.40
7.11 7.33 6.39 7.92 6.33
0.83 0.79 0.91 0.62 0.94
3.53 2.81 3.50 2.27 3.64
1.02 1.01 0.88 1.15 0.87
1149 930 3601 523 3756
MW- number average molecular weight.
ous point. It should be noted that the onset values, overall, are higher for A2 in comparison to A1. This signifies that A2 is relatively more stable in toluene than A1. The onset break points were observed at 3.0 g/L for A1 and 4.8 g/L for A2. The average molecular weights of the two asphaltenes were measured using a vapor pressure osmometer (Knauer, K-7000). The average molecular weights were 1149 for A1 and 930 for A2. Some important questions arose from the CMC and solubility characteristics of A1 and A2: Is CMC the sole determinant of the solubility behavior of asphaltenes? What would be the characteristics of the different fractions from asphaltenes? Asphaltene fractions were prepared to investigate these questions. A fixed amount of A2 (10 g) was dissolved in dichloromethane (DCM, 200 mL). The solution was mixed (with a magnetic stirring device) for about 12 h, following which 500 mL of pentane was added to the mixture. The resultant system was mixed for six more hours. Then, the mixture was filtered under vacuum with a 11-µm filter paper. The filtered asphaltenes were termed A2-F11. The remaining mixture was dried under vacuum conditions evaporating the solvent. The fraction left behind was named A2-F12. The weight of A2 was more or less equally partitioned between A2-F11 and A2-F12. When a different volumetric ratio (1:2) of DCM to pentane was used for the A2 fractionation, about 42 wt % of the asphaltenes were precipitated in the first fraction (A2-F21). Elemental analysis (Leco Corp., CHNS-932) and molecular weights of asphaltenes and fractioned asphaltenes are shown in Table 1. Molecular weights were measured at 60 °C in pyridine as a solvent. The error range (reproducibility) in molecular weight measurements was
