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Energy & Fuels 2000, 14, 6-10
Asphaltenes: Structural Characterization, Self-Association, and Stability Behavior O. Leo´n,*,† E. Rogel,† J. Espidel,‡ and G. Torres† Production Department and Department of Analysis and Evaluation, PDVSA-Intevep, 76343 Caracas-1070A, Venezuela Received May 27, 1999
The influence of different factors on the asphaltene stability in four crude oils was evaluated. Compositional studies and structural characterization of resins and asphaltenes were carried out in order to study a possible relationship between these properties and the asphaltene deposition behavior. Low hydrogen-to-carbon ratios, high aromaticities, and high condensation of aromatic rings were the main characteristics of the asphaltenes from unstable crude oils. According to these results, the stability behavior of the crude oils studied was strongly influenced by the structural characteristics of their asphaltenes. Since the self-aggregation of asphaltenes is a first step in the formation of precipitated asphaltene particles, this study also evaluates the formation of asphaltene aggregates as a function of the structural characteristics of the asphaltenes. The self-aggregation behavior was studied on the basis of surface tension measurements. Critical micelle concentrations were determined in different solvents. Using these results, it was possible to establish a relationship between self-association and structural characteristics through the calculation of the Flory parameter. On the basis of these findings, different characteristic patterns were identified depending on the origin of the asphaltenes. A new correlation has been found to describe the critical micelle concentrations of the asphaltene solutions.
Introduction Asphaltene deposition is a very well-known problem that generates a large cost increase in the petroleum industry.1 The formation of asphaltene deposits can occur in wells, pipelines, and production equipment.2 This phenomenon seriously affects production operations and generates a large cost increase due to prevention and removal of deposits. However, although asphaltene deposition is a worldwide problem, its main causes have not been completely understood at the present moment.3 Crude oils are colloidal systems whose disperse phase is composed of asphaltenes and resins. The precipitation of asphaltenes depends on the colloidal stability of these systems.4 Among the different factors that influence the stability of crude oils, the composition plays a main role. It has been pointed out that the nature and content of asphaltenes together with the nature and content of the dispersion medium are the main factors that determine the relative stability of crude oils and related materials.5,6 In this work, two different aspects related to the asphaltene deposition problem were studied, namely, * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Production Department. ‡ Department of Analysis and Evaluation. (1) Park, S. J.; Mansoori, G. A. Energy Sources 1988, 10, 109-125. (2) Taylor, S. E. Fuel 1992, 71, 1338-1339. (3) Rassamdana, H.; Dabir, B.; Nematy, M.; Farhani, M.; Sahimi, M. AIChE J. 1996, 42, 10-22. (4) Laux, H.; Rahimian, I.; Butz, T. Fuel Process. Technol. 1997, 53, 69-79. (5) Buckley, J. S. Fuel Sci. Technol. Int. 1996, 14, 55-74.
composition of crude oils and the chemical and structural characterization of asphaltenes and resins. In particular, interest is focused on the characterization of the asphaltene fraction and its relation to the stability of the crude oil. Asphaltene and resin fractions from crude oils with and without deposition problems are examined using structural analysis. Since the selfaggregation of asphaltenes is a first step in the formation of precipitated asphaltene particles,7 this study also evaluates the effects of different solvents on the formation of asphaltene aggregates. The main objective of this work is to explore the possible relations among the structural parameters of the asphaltenes, their selfaggregation, and the unstability behavior of the crude oils. Experimental Section Crude Oil Characterization. To establish the relationship between the composition and the stability behavior of the crude oils, the main constituents of the crude oilsssaturates, aromatics, resins, and asphaltenes (SARA)8swere determined using Iatroscan thin-layer chromatography (TLC-FID) with a flame ionization detector. Preparation and Characterization of Asphaltenes. Four crude oils were studied: two (A and B) were classified as stable materials. The other two (C and D) were classified (6) Carbognani, L.; Espidel, J.; Izquierdo, A. In Asphaltenes and Asphalts: Developments in Petroleum Science, 40; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science B. V.: The Netherlands, 1999. (7) Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142, 497-502. (8) Sol, B.; Romero, E.; Carbognani, L.; Sa´nchez, V.; Sucre, L. Revista Te´ cnica de Intevep 1984, 4, 127.
10.1021/ef9901037 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/17/1999
Asphaltenes: Structure, Self-Association, and Stability
Energy & Fuels, Vol. 14, No. 1, 2000 7
Table 1. Composition of Crude Oils crude oils
saturates (wt %)
aromatics (wt %)
resins (wt %)
asphaltenes (wt %)
°API
A B C D
32.3 37.9 52.4 36.9
42.2 38.3 36.3 37.9
19.8 14.9 10.5 19.4
5.8 8.9 0.8 5.8
19.8 20.0 28.6 24.6
as unstable crude oils and presented asphaltene deposition problems. The asphaltenes were extracted from crude oils according to the method described in IP-143/90. Elemental composition was determined on a LECO CHNS 244 elemental analyzer model, and the average number molecular weight was determined on a Knauer vapor pressure osmometer in CH2Cl2 at 25 °C (MWA ) 3098 g/mol; MWB ) 3612 g/mol; MWC ) 1139 g/mol; MWD ) 2102 g/mol). Asphaltene and resin NMR spectra were obtained on a Bruker ACP-400 spectrometer, at a resonance frequency of 400 MHz for protons. A flip angle of 45° was used, with a repetition rate of 3 s, spectral width of 12 ppm, and the chemical shift was referenced relative to TMS. Samples (25 mg) were dissolved in 1 mL of dichloromethane, and 5 wt % hexamethyl cyclosiloxane was added as an internal standard. Average molecular parameters (AMP) and molecular weights (MW) were calculated according to the method developed by V. Leo´n.9 The reproducibility of the AMP is within 5%. The average molecular models for asphaltenes were constructed using a method developed by L. Carbognani et al.10 Measurement of Crude Oil Stability. The relative stability of the crude oils were estimated using flocculation onset measurements on dead oils. The flocculation points were determined by a titration method: n-heptane is added at a constant rate (1 cm3/min) to the crude under intensive stirring. The titration was monitored by means of a Guided Wave NIR spectrophotometer at the wavelength of 768 nm. The flocculation point is defined as the amount of n-heptane needed to obtain the maximum of the light intensity. Temperature, titration rate, stirring speed, and sample preparation were the same in all titrations. Surface Tension Measurements. Surface tensions of dilute solutions of asphaltenes in cyclohexane, tetrahydrofuran, and carbon tetrachloride at the interface with the air were determined using the Wilhelmy plate method, at 25 °C. The solutions of asphaltenes were pretreated by sonication for 24 h. The purpose of this pretreatment was to dissociate micelles into monomers. The instrument used to measure the surface tension was a Robal Electronics surface tension. Surface tension readings were measured at intervals over several hours until constant values were obtained.
Results and Discussion Chemical composition and stability. For this study, crude oils from different origins and stabilities were used: A and B are considered as stable crude oils while C and D are unstable crude oils that have shown asphaltene deposition problems. The stability of the four crude oils was determined by means of the flocculation points: the more stable a crude, the larger the volume of n-heptane needed to begin flocculation. According to the flocculation points, the stability of the crudes tested follows the order: A > C ≈ B > D. Table 1 shows the composition of the crude oils obtained by TLC-FID. Using these results two different stability indexes were calculated. The first one is the ratio of resins to asphaltenes which traditionally has (9) Leon, V. Fuel 1987, 66, 145-146. (10) Carbognani, L.; Espidel, J. Personal communication, 1993.
Figure 1. Flocculation onset as a function of stability indexes.
been considered as a key factor in the asphaltene dispersion. The second one is the ratio of resins plus aromatics to saturates plus asphaltenes used as a colloidal stability index for asphalts.11 It is supposed that the dispersion power of the maltenes is reflected by these ratios and, as a consequence, a relationship between the stability of the asphaltenes in the crude oil and these indexes would be expected. In Figure 1, flocculation onsets are shown as a function of two stability indexes. As can be seen, no relationship between these stability indexes and flocculation onsets was found. Therefore, the composition of the dispersion medium does not seem to play a key role in the asphaltene stability for the studied crude oils. For this reason, a deeper chemical characterization of the asphaltenes and resins was carried out in order to find other chemical factors involved in asphaltene stabilization. In fact, Schabron and Speight12 have pointed out that structural similarities in the asphaltenes and resins facilitated formation of micelles and, consequently, the stabilization of asphaltenes by resins. Earlier, Koots and Speight13 showed that the same resin has a different activity as a solubilizer for asphaltenes from different crude oils. On the other hand, recent studies14,15 have shown that the nature of the asphaltene fraction always seems to have a predominant role on the asphaltene stability. Structural Analysis of Asphaltene and Resin Fractions. Table 2 lists the most important average structural parameters for the samples. The fa parameter is the ratio between the number of aromatic carbons over the total number of carbons. CI is the number of bridging aromatic carbons, and C1 is the number of peripheric nonbridging carbons. The ratio between them gives an indication of the degree of condensation of the aromatic rings (Ar). Comparison between both types of asphaltenes (from stable and unstable crude oils) shows a significant difference in the hydrogen-to-carbon ratios (H/C), fa, and CI/C1. In particular, it was found that (11) Loeber, L.; Muller, G.; Morel, J.; Sutton, O. Fuel 1998, 77, 1443. (12) Schabron, J. F.; Speight, J. G. Pet. Sci. Technol. 1998, 16, 361. (13) Koots, J. A.; Speight, J. G. Fuel 1975, 54, 179. (14) Carbognani, L.; Orea, M.; Fonseca, M. Energy Fuels 1999, 13, 351. (15) Rogel, E. Energy Fuels 1998, 12, 875.
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Table 2. Structural Parameters of the Asphaltene and Resin Fractions Studied crude oil
MW
A B C D
800 636 615 975
A B C D
1308 1158 1212 1621
H/Ca
fab
CI/C1c
Ard
0.38 0.35 0.41 0.43
0.79 0.54 0.66 1.10
6 4 5 9
Asphaltenes 1.22 0.46 1.11 0.53 0.99 0.60 0.96 0.60
1.48 1.52 1.72 2.09
14 14 17 23
Resins 1.40 1.46 1.36 1.29
a Hydrogen-to-carbon ratio. b Aromaticity (aromatic carbons to total carbons ratio). c Aromatic condensation index (internal aromatic carbons to nonbridging aromatic carbons ratio). d Number of aromatic rings.
asphaltenes from unstable crude oils show a higher deficiency of hydrogen, higher aromaticity, higher degree of aromatic condensation, and more Ar than those from stable materials. Using average molecular formulas and the structural parameters, average structural models of the four asphaltene fractions are proposed as shown in Figure 2. With regard to the resins, the differences are not so clear as in the case of the asphaltenes, although some general tendencies can be observed. The resins from unstable crude oils (C and D) show a lower H/C ratio and a higher aromaticity (fa) than the resins of the stable crude oils. However, the condensation degree CI/ C1 and the number of aromatic rings do not show a clear tendency. These results seem to indicate some degree of compatibility or similarity between resins and asphaltenes from the same crude oil. For stable crude oils, asphaltenes and resins have more hydrogen and less aromatic carbons in comparison to asphaltenes and resins from unstable crude oils. However, further work has to be done in order to test this hypothesis. Structural Parameters and Stability. The stability of the crude oils can be related to some structural parameters of the asphaltenes as can be seen in Figure 3. In this figure, the main structural parameters of the asphaltenes (H/C, aromaticity, and aromatic condensation degree) are plotted as a function of n-heptane volume. It can be observed from this figure that the increase in stability corresponds to the increase in H/C ratio, the decrease of fa, and the significant decrease of CI/C1 ratio. Carbognani et al.14 found that asphaltenes extracted from deposits obtained from tubings show a higher hydrogen deficiency than asphaltenes from crude oils. This result indicates that the asphaltenes with low H/C ratio precipitate preferentially. This is in agreement with the results obtained in the present work indicating that the asphaltenes from unstable crude oils show low H/C ratio. According to this, it is possible to conclude that the structural characteristics of the asphaltenes are clearly related to the asphaltene deposition problems. Self-Aggregation of Asphaltenes in Different Solvents. Asphaltene critical micelle concentrations (cmc’s) were determined in cyclohexane, tetrahydrofuran, and carbon tetrachloride using surface tension (γ) measurements.
The determination of the critical micelle concentration (cmc) was based on the Gibbs excess adsorption equation:
Γ ) - (1/RT) (dγ/d ln C)
(1)
where Γ is the Gibbs surface excess, C is the concentration of asphaltenes, and γ is the surface tension of the solution. The surface tension (γ) was measured as a function of the concentration of asphaltene, and the cmc was calculated as the inflection point of the γ-ln(concentration) plot. Figure 4 shows an example of these plots, and the cmc corresponds to the break point in the curve. Table 3 shows that cmc values for the different asphaltenes range from 1 to 18.6 g/L. The cmc for asphaltenes of crude oil C in carbon tetrachloride was not determined because these asphaltenes were insoluble in this solvent. The cmc values are quite comparable to those reported by other authors16 using different measurement techniques. It is important to point out that the asphaltenes from stable crude oils show the largest cmc values in all the solvents studied. This difference indicates that the aggregation process of the asphaltenes from unstable crude oils begins earlier (at a lower concentration) than the aggregation of the asphaltenes from stable crude oils. Self-Aggregation and Structural Parameters. The self-aggregation behavior should be related to the asphaltene structure, in analogy to the observed behavior for surfactants in aqueous solution. In the case of surfactants, the cmc is related to the nature of the polar group, as well as to the polar/nonpolar groups ratio. In a similar way, it was found that the behavior of the different asphaltenes on the studied solvents could be related to the structural characteristics of the asphaltene. In general, it was observed that asphaltenes with low hydrogen content and high fa begin to aggregate at lower concentrations than the asphaltenes with high hydrogen content and low fa. However, although some general tendencies were found, it was not possible to establish a clear relationship between self-aggregation and the studied structural parameters. Self-Aggregation and Flory-Huggins Interaction Parameter. Andersen and Birdi7 have found that, in mixtures of n-alkanes and toluene, the cmc of the asphaltene is linearly related to the Hildebrand solubility parameter of the mixture. The same relationship was not found in solvents of different polarity and hydrogen bonding capacity. In the present work, it was possible to relate the solubility parameter of the asphaltenes with their cmc values in different solvents through the Flory-Huggins interaction parameter. This parameter has been used to correlate the solubilization of solutes in different solvents.17 According to the pseudophase description of the asphaltene aggregates, the cmc can be considered as the amount of asphaltene monomers solubilized in the solvent while the asphaltene micelle is considered as a new phase. For this reason, the Flory-Huggins interaction parameter can be a good correlating quantity for (16) Andersen, S. I.; Speight, J. G. Fuel 1993, 72, 1343-1344. (17) Barton, A. F. M. Handbook Of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1985; second printing.
Asphaltenes: Structure, Self-Association, and Stability
Energy & Fuels, Vol. 14, No. 1, 2000 9
Figure 2. Average structural model for the asphaltene fractions studied.
Figure 3. Relationships among flocculation points of the crude oils and structural parameters of the asphaltenes.
cmc (solubility of asphaltene monomers) in different solvents.The Flory-Huggins interaction parameter is calculated here in terms of the Hildebrand solubility parameters:
χ ) (δa - δs)2vs/kT
(2)
where δa and δs are the solubility parameters for the asphaltene and the solvent, respectively, vs is the molar volume of the solvent, and T is the temperature.
Figure 4. Surface tension as a function of ln C (concentration in g/L) for the asphaltene from Crude Oil C in cyclohexane.
The solubility parameter of the asphaltenes was calculated using equations derived by van Krevelen.18 This equation relates the solubility parameters of asphaltenes and coals with their structure and composition. The calculations of the Flory-Huggins parameters were carried out using the Hildebrand solubility parameters of the solvents. (18) van Krevelen, D. W. Fuel 1965, 44, 229-236.
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produce a theory of micellization that can a priori predict the cmc’s and the aggregation numbers based on the molecular properties of the asphaltenes. This can be the core of a predictive model for asphaltene precipitation that takes into account the particular characteristics of asphaltene fractions from different origins. The main advantage of such theory would be its versatility to describe different behaviors without the need of fitting a lot of parameters. Conclusions
Figure 5. cmc as a function of the Flory-Huggins interaction parameter. Table 3. cmc (g/L) of Asphaltenes Determined in Different Solvents asphaltene
cyclohexane
THF
CCl4
A B C D
4.3 5.9 1.0 2.1
10.8 8.2 1.7 4.8
11.1 18.6 5.2
As can be seen in Figure 4, it is possible to observe linear relationships between cmc and the Flory-Huggins parameter. The slope and intercept of the line varies depending on the solvent. On the other hand, the lower values of χ are compatible with the larger values of cmc for the same solvent, which indicates that a higher compatibility between asphaltene and solvent lead to large cmc’s in the same solvent. Although good linear correlations were obtained for CCl4 and THF, it is important to point out that for cyclohexane a quantitative description between the cmc and the χ parameter was not obtained. According to these results, the χ parameter serves as a reasonable correlating parameter for a qualitative description of the cmc of asphaltenes in different solvents. Despite the significant progress that has resulted from the precipitation models recently developed,19,20 there has not been an attempt to include the structural and chemical characteristics into the models. The results obtained in the present work indicate that there is a significant influence of the structural and chemical characteristics of the asphaltenes on their aggregation behavior. In this sense, the empirical equation developed in the present work can be introduced in these previous models in order to calculate the asphaltene monomer concentration as a function of the properties of the asphaltene. However, this is an empirical correlation and, therefore, it needs a considerable amount of experimental data to be fitted. In contrast, a theoretical understanding of the aggregation behavior can (19) Victorov, A. I.; Firoozabadi, A. AIChE J. 1996, 42, 1753. (20) Pacheco-Sanchez, J. H.; Mansoori, G. A. Pet. Sci. Technol. 1998, 16, 377.
The use of the crude oil composition as a key index to estimate stability must be revised because other factors, such as asphaltene characteristics, can be more important for some crude oils. The results obtained seem to indicate some degree of compatibility or similarity between resins and asphaltenes from the same crude oil. For stable crude oils, asphaltenes and resins have more hydrogen and less aromatic carbons in comparison to asphaltenes and resins from unstable crude oils. However, the evidence is not conclusive and further work has to be done in order to test this hypothesis. The nature of the asphaltenes is one of the most important factors involved in the stability of crude oils. In particular, the structural and compositional characteristics of the asphaltenes strongly influence their deposition problems. Asphaltenes from unstable crude oils are characterized by high aromaticity and low hydrogen content, together with a high condensation of the aromatic rings. On the contrary, asphaltenes from stable crude oils show low aromaticity and high hydrogen content, in addition to a low condensation of their aromatic rings. The self-association behavior of the asphaltenes is related to their structural characteristics. In particular, in the studied solvents, asphaltenes with low hydrogen content and high aromaticity (from unstable crude oils) begin to aggregate at lower concentrations than asphaltenes with high hydrogen content and low aromaticity (from stable crude oils). For the studied solvents, the cmc values are linearly related to the Flory-Huggins interaction parameter. This correlation suggests that higher compatibility between asphaltenes and solvent (δa - δs small) lead to significantly larger cmc in the same solvent. The Hildebrand solubility parameters of the solvents and the structural and compositional characteristics of the asphaltenes were used in the calculation of Flory-Huggins interaction parameters and directly influence the results obtained. Acknowledgment. The authors are thankful for the support provided by the CODICID Research Project “Study of the asphaltene precipitation and its effects on crude oil production”. Helpful discussions with M. Carmen Garcı´a, Simon Andersen, Lante Carbognani, and So´crates Acevedo are appreciated. EF9901037
