Energy Fuels 2010, 24, 2275–2280 Published on Web 12/03/2009
: DOI:10.1021/ef9009235
Role of Resins on Asphaltene Stability† Mohammad Sedghi and Lamia Goual* Department of Chemical and Petroleum Engineering, University of Wyoming, 1000 East University Avenue, Laramie, Wyoming 82071 Received August 25, 2009. Revised Manuscript Received November 10, 2009
The objective of this study is to clarify through impedance analysis some of the controversial issues related to the role of resins on asphaltene stability. From the variations of the direct-current (DC) conductivity of charge carriers in petroleum fluids with their concentration in toluene, we find that resins are unlikely to coat asphaltene nanoaggregates within the concentration range investigated (10-1000 ppm). Thus, the long-time-standing Nellensteyn hypothetical model, where resins adsorb on asphaltenes to provide a steric stabilizing layer, is not valid. In heptane/toluene mixtures, the least soluble resins tend to aggregate with asphaltenes to enhance their stability in the mixture, in agreement with a recently proposed molecular thermodynamic approach to the formation of mixed asphaltene-resin aggregates (Rogel, E. Molecular thermodynamic approach to the formation of mixed asphaltene-resin aggregates.Energy Fuels 2008, 22 (6), 3922-3929).
permanent or induced dipoles.3 To better understand the nature of the charge carriers in petroleum fluids, electrodeposition studies were undertaken on asphaltenes and maltenes extracted from solid-free Athabasca bitumen.4 The method consisted of applying a constant DC voltage of 250 V between two gold crystals of a quartz crystal microbalance. The application of the voltage resulted in a reversible electrocollection of large positively charged species that could not electrodeposit at the cathode possibly because of steric constraints. Interestingly, both asphaltenes and resins in the maltenes exhibited the same behavior. Resins affect asphaltene aggregation and precipitation. They can increase the stability of asphaltenes,5-11 affect the amount of precipitation from gravimetric analysis,11,12 and decrease the size of asphaltene aggregates from small-angle X-ray scattering (SAXS) and small-angle neutron scattering
1. Introduction The role of resins on asphaltene stability has long been controversial, and it is only recently that a more unified view is gradually emerging. Resins are defined as the fraction of maltenes that is insoluble in propane or the fraction of maltenes recovered by adsorption chromatography after elution of oils by nonpolar solvents.2 The result of this separation is a very sticky dark brown resin fraction, whose properties are intermediate between those of asphaltenes and oils. Figure 1 presents a schematic of the molecular-weight distribution of asphaltene and resin fractions. The border region between asphaltenes and resins depends upon the n-alkane used to separate asphaltenes. For instance, if n-heptane (nC7) is used to separate asphaltenes, then the C5-C7 fraction of C5 asphaltenes is part of C7 resins. Similarly, if n-decane (nC10) is used to separate asphaltenes, then the C7-C10 fraction of C7 asphaltenes is part of C10 resins, as shown in Figure 2. In a previous study, the dielectric constant of asphaltenes, resins, and oils extracted from eight different crude oils was measured at 800 Hz.2 The large values recorded in asphaltenes and to a lower extent resins are attributed to the presence of aromatic structures as well as dipoles and charge carriers. Even though present in trace amounts in low dielectric solvents, such as toluene, the charge carriers in asphaltenes and resins usually form because of the ionization of labile protons in protic molecules under the action of an external electric field. These ions may remain stabilized by other protic molecules or by
(4) Goual, L.; Horvath-Szabo, G.; Masliyah, J. H.; Xu, Z. Characterization of the charge carriers in bitumen. Energy Fuels 2006, 20, 2099. (5) Lian, H.; Lin, J.-R.; Yen, T. F. Peptization studies of asphaltene and solubility parameter spectra. Fuel 1994, 73 (3), 423. (6) Hammami, A.; Ferworn, K. A.; Nighswander, J. A.; Overa, S.; Stange, E. Asphaltenic petroleum fluid oil characterization: An experimental investigation of the effect of resins on the stability of asphaltenes. Pet. Sci. Technol. 1998, 16 (3-4), 227. (7) Carnahan, N. F.; Salager, J. L.; Anton, R.; Davila, A. Properties of resins extracted from boscan crude oil and their effect on the stability of asphaltenes in Boscan and Hamaca crude oils. Energy Fuels 1999, 13 (2), 309. (8) Al-Sahhaf, T. A.; Mohammed, A. F.; Elkilani, A. S. Retardation of asphaltene precipitation by addition of toluene, resins, deasphalted oil and surfactants. Fluid Phase Equilib. 2002, 194, 1045. (9) Spiecker, P. M.; Gawrys, K. L.; Trail, C. B.; Kilpatrick, P. K. Effects of petroleum resins on asphaltene aggregation and water-in-oil emulsion formation. Colloids Surf., A 2003, 220, 9. (10) Rogel, E.; Leon, O.; Contreras, E.; Carbognani, L.; Torres, G.; Espidel, J.; Zambrano, A. Assessment of asphaltene stability in crude oils using conventional techniques. Energy Fuels 2003, 17 (6), 1583. (11) Goual, L.; Firoozabadi, A. Effect of resins and DBSA on asphaltene precipitation from petroleum fluids. AIChE J. 2004, 50 (2), 470. (12) Murzakov, R. M.; Sabanenkov, S. A.; Syunyaev, Z. I. Influence of petroleum resins on colloidal stability of asphaltene-containing disperse systems. Z. I. Khim. Tekhnol. Topl. Masel 1981, 10, 40.
† Presented at the 10th International Conference on Petroleum Phase Behavior and Fouling. *To whom correspondence should be addressed. Telephone: 307-7663278. E-mail:
[email protected]. (1) Rogel, E. Molecular thermodynamic approach to the formation of mixed asphaltene-resin aggregates. Energy Fuels 2008, 22 (6), 3922. (2) Goual, L.; Firoozabadi, A. Measurement of asphaltenes and resins, and their dipole moment in petroleum fluids. AIChE J. 2002, 48 (11), 2646. (3) Fotland, P.; Anfindsen, H. Conductivity of asphaltenes. In Structure and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum: New York, 1998.
r 2009 American Chemical Society
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Energy Fuels 2010, 24, 2275–2280
: DOI:10.1021/ef9009235
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could be modeled solely by using Flory-Huggins theory of regular solutions.18 Electrophoretic studies on asphaltenes in heptane showed that resins do not change the electrophoretic mobility of asphaltene precipitates.19 Moreover, microcalorimetric studies recorded very small enthalpies of interactions between resins and asphaltene clusters in toluene, much smaller than those found in dipole-dipole or hydrogen-bonding interactions.20 More recently, downhole analysis of reservoir fluids reported a large concentration gradient with depth for C7 asphaltene precipitates but no resin gradient.21 A model was then suggested where asphaltene nanoaggregates are composed of stacks of aromatic sheets sterically stabilized by their own aliphatic chains. Ultrafiltration studies on C5 asphaltenes22 as well as centrifugation studies of live crude oils23 indicate that asphaltene nanoaggregates may contain some heavy resins. This picture is in agreement with a recent molecular thermodynamic approach to the formation of mixed asphaltene-resin aggregates, where the effectiveness of different resins to split asphaltene aggregates depends upon the solubility of their polyaromatic rings.1 The objective of this work is to clarify through impedance analysis some of the controversial issues related to the role of resins on asphaltene stability. In particular, we investigate the validity of the Nellensteyn model. Impedance analysis has recently proven to be a powerful technique to study aggregation phenomena in petroleum fluids at very low frequency where the DC conductivity of the ionizable fractions can be determined.24,25
Figure 1. Schematic of the molecular-weight distribution of asphaltenes and resins.
Figure 2. Effect of n-alkanes on the nature of asphaltenes and resins.
(SANS) measurements.9,13,14 The extent by which resins affect asphaltenes depends upon their dielectric properties.11 The very first attempt to explain the role of resins on asphaltene stability was made in the late 1930s by Nellensteyn, who proposed a hypothetical model where resins adsorb on asphaltenes and provide a steric stabilizing layer.15 Since then, this model has been extensively used by other researchers to interpret their data. For example, electrical conductivity studies showed that resins can decrease the DC conductivity of asphaltene clusters in nitrotoluene.16 It was then concluded that resins are able to neutralize part of asphaltene charges by adsorbing on their surface. Soxhlet extraction of C7 asphaltene deposits also revealed a decrease in molecular weight and increase in polarity with extraction time. The authors interpreted the data in terms of the Nellensteyn model. However, the observed gradient in molecular weight and polarity of the deposit could very well be explained by a more recent model where C7 asphaltene precipitates are composed of aromatic regions surrounded by aliphatic zones that can swell in n-alkanes.17 In fact, an increasing number of papers that do not support the Nellensteyn model are emerging in the literature. Among them, refractive index studies on crude oils in n-alkanes suggest that precipitation is dominated by London dispersion interactions. In this case, asphaltene precipitation
2. Experimental Section 2.1. Materials. Materials include toluene, methylene chloride, and n-heptane, all 99.9% HPLC grade from Fisher Scientific, acetone 99.4% ultra resi-analyzed (J.T. Baker), dry benzene (Mallinckrodt, Inc.), chromium(III) acetylacetone 99.99% (Aldrich), deuterated chloroform 99.8% (Cambridge Isotope Laboratories), n-nonane 99% (Aldrich), tridecane 99% (Acros Organics), and attapulgus clay 30:60 (Forcoven Products). The crude oils selected are from two different origins, and their physical properties are presented in Table 1. WP crude from Alaska is medium, whereas Boscan crude from Venezuela is heavy. The medium crude is filtered with a 20-25 μm poresize Whatman filter paper (Fisher Scientific) prior to each measurement. 2.2. Properties of Crude Oils. Densities, viscosities, and refractive indices are determined using a DMA 45 density meter (Anton-Paar, Ashland, VA), Cannon-Fenske viscometer (18) Buckley, J. S.; Hirasaki, G. J.; Liu, Y.; Von Drasek, S.; Wang, J.-X.; Gill, B. S. Asphaltene precipitation and solvent properties on crude oils. Pet. Sci. Technol. Int. 1998, 16 (3-4), 251. (19) Gonzalez, G.; Neves, G. B. M.; Saraiva, S.; Lucas, E. F.; dos Anjos de Sousa, M. Electrokinetic characterization of asphaltenes and the asphaltenes-resins interaction. Energy Fuels 2003, 17, 879. (20) Merino-Garcia, D.; Andersen, S. I. Thermodynamic characterization of asphaltene-resin interaction by microcalorimetry. Langmuir 2004, 20 (11), 4559–4565. (21) Mullins, O. C.; Betancourt, S. S.; Cribbs, M. E.; Dubost, F. X.; Creek, J. L.; Andrews, A. B.; Venkataramanan, L. The colloidal structure of crude oil and the structure of oil reservoirs. Energy Fuels 2007, 21 (5), 2785. (22) Zhao, B.; Shaw, J. M. Composition and size distribution of coherent nanostructures in Athabasca bitumen and Maya crude oil. Energy Fuels 2007, 21, 2795. (23) Indo, K.; Ratulowski, J.; Dindoruk, B.; Gao, J.; Zuo, J.; Mullins, O. C. Asphaltene nanoaggregates measured in a live crude oil by centrifugation. Energy Fuels 2009, 23 (9), 4460. (24) Zeng, H.; Song, Y.-Q.; Johnson, D. L.; Mullins, O. C. Critical nanoaggregate concentration of asphaltenes by direct-current (DC) electrical conductivity. Energy Fuels 2009, 23 (3), 1201. (25) Goual, L. Impedance spectroscopy of petroleum fluids at low frequency. Energy Fuels 2009, 23 (4), 2090.
(13) Espinat, D.; Ravey, J. C. Colloidal structure of asphaltene solutions and heavy-oil fractions studied by small-angle neutron and X-ray scattering. Presented at the SPE International Symposium on Oilfield Chemistry, New Orleans, LA, March 2-5, 1993; SPE Paper 25187. (14) Barre, L.; Espinat, D.; Rosenberg, E.; Scarsella, M. Colloidal structure of heavy crudes and asphaltene solutions. Rev. Inst. Fr. Pet. 1997, 52 (2), 161. (15) Nellensteyn, F. I. In The Science of Petroleum; Dunstan, A. E., Ed.; Oxford University Press: London, U.K., 1938; Vol. 4. (16) Hasnaoui, N.; Achard, C.; Rogalski, M.; Behar, E. Study of asphaltene solutions by electrical conductivity measurements. Rev. Inst. Fr. Pet. 1998, 53 (1), 41. (17) Carbognani, L.; Rogel, E. Solvent swelling of petroleum asphaltenes. Energy Fuels 2002, 16 (6), 1348.
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where t is the gap between electrodes (t = 10 m) and A is the area of the electrodes (A = 0.001 134 m2). The DC conductivity is related to the effective diffusion coefficient of asphaltene monomers via the Nernst-Einstein equation kB TMσdc D ¼ ð2Þ CNA xe2
Table 1. Selected Properties of Crude Oils density at 23 °C (g/cm ) gravity (°API) refractive index at 23 °C viscosity at 23 °C (cP) molecular weights (g/mol) acid number (mg of KOH/g of oil) base number (mg of KOH/g of oil) C7 resins (wt %) C7 asphaltenes (wt %) resin/asphaltene ratio (R/A) elemental analysis of C7 asphaltenes (wt %) C H N O S H/C ratio 3
Boscan
WP
1.0290 6 175965 342 2.9 4.2 35.7 18.8 1.9
0.9214 22 1.5222 112 282 0.2 2.3 25.0 7.9 3.2
81.05 8.06 1.85 1.56 7.87 1.19
85.07 7.75 1.09 1.61 4.63 1.09
where kB is the Boltzmann constant (1.380 65 10-23 m2 kg s-2 K-1), T is the absolute temperature, e is the ionic charge (1.602 176 5 10-19 C), NA is Avogadro’s number (6.022 141 8 1023), M is the molecular weight of asphaltenes (∼1 kg/mol), C is the concentration of asphaltenes in kg/L, and x is the ionic fraction of asphaltenes (∼10-4).24,26 Because the translational diffusion coefficient is linear in radius, the cube of the diffusion coefficients before and after aggregation gives an estimate of the aggregation number according to27 !3 Dmonomer AN ¼ ð3Þ Daggregate
(Fisher Scientific), and GPR12-70E high-temperature refractometer (Index Instruments Ltd., Cambridgeshire, U.K.), respectively. Molecular weights are measured with a 5009 Cryette wide range cryoscope (Precision Systems, Natick, MA). The total acid and base numbers are measured with a Metrohm 808 Titrando autotitrator (CH-9101 Herisau, Switzerland). 2.3. Separation of Asphaltenes, Maltenes, and Resins. Crude oils are separated into asphaltenes and maltenes by mixing a known amount of crude oil with n-heptane at a ratio of 1:40 g/mL. The mixture is allowed to equilibrate after stirring and left overnight at room temperature. It is then filtered under vacuum using a 0.2 μm pore size Whatman filter paper. The filter cake, mainly asphaltenes, is repeatedly washed with n-heptane until the effluent from the filter becomes colorless. The asphaltenes are recovered from the filter cake by dissolution in toluene and then dried after toluene evaporation. Maltenes are obtained from the liquid filtrate after n-heptane evaporation. Resins are extracted from maltenes by adsorption on attapulgus clay according to a procedure described elsewhere.2 2.4. 13C NMR Analysis. 13C NMR studies are performed in 5 mm tubes at 300 K on a Bruker Avance DRX-400 spectrometer at 100 MHz using deuterated chloroform (CDCl3) as the solvent. Because of the low sensitivity of NMR for 13C, highly concentrated solutions of asphaltenes in CDCL3 are used (100 mg of asphaltenes in 0.5 mL of CDCL3). Chromium(III) acetylacetone (0.02 M in asphaltene-CDCL3 solution) is also used as a relaxation reagent to reduce the delay time between each scan to 2 s. The pulse program is set to inverse gated decoupling with a flip angle of 30°. At least 8000 scans are collected and averaged for each asphaltene to increase the accuracy. Shape analysis is performed using XWin NMR version 3.6 software. 2.5. Impedance Analysis. Low-frequency impedance measurements are conducted using a 4294A precision impedance analyzer (Agilent Technologies, Santa Clara, CA), with a wide frequency range from 40 Hz to 110 MHz. The impedance analyzer is connected to a 16452A liquid test fixture (Agilent, Santa Clara, CA) through a 16048G 1 m port extension cable with four terminals (Agilent, Santa Clara, CA). The operating frequency of the test fixture is from 40 Hz to 30 MHz. The cell has a volume capacity of 4.8 mL and comports two nickelcoated cobalt electrodes with 1 mm spacing between them. The area of the electrodes is 0.001 134 m2. A total of 40 point average measurements are performed under frequency sweep from 40 Hz to 1 Hz at a constant voltage level of 0.5 V and no DC bias. More details on the equations and calibration procedure are presented elsewhere.25 The parallel resistance of the system, Rp, is determined from a fit with Z-view software and is used to calculate the DC conductivity according to t σdc ¼ ð1Þ ARp
Assuming particles with a spherical shape, their diameters d are calculated from the Stokes-Einstein equation, where ηs is the viscosity of toluene (0.000 545 Pa s) kB T d ¼ ð4Þ 3πηs D
3. Results and Discussion Figure 3 depicts the variations of the DC conductivity of C7 asphaltenes from Boscan and WP crude oils versus the concentration in toluene. The conductivity increases with the concentration. The data can be fit to two straight lines with different slopes. The break between the two lines represents the critical nanoaggregate concentration (CNAC).24,25 The CNAC occurs at around 150 mg/L, in line with the literature.28 The decrease in the slope upon aggregation is due to either charge annihilation3 or an increase in the drag of asphaltenes. The former is unlikely because the ionic fraction of asphaltenes in toluene is very small.24 Thus, the charge carriers behave as tracers, upon which neutral asphaltenes would stack to form nanoaggregates. Table 2 provides the Stokes translational diffusion coefficient of Boscan and WP asphaltenes calculated at different concentrations according to eq 2. The data are in the same range as those found in the literature.29-32 The average diameters and aggregation numbers of asphaltenes are determined from the diffusion coefficients and presented in Table 2. Examination of the aggregation data reveals a higher (26) Goual, L.; Abudu, A. Predicting the adsorption of asphaltenes from their electrical conductivity. Energy Fuels 2009, manuscript submitted. (27) Mostowfi, F.; Indo, K.; Mullins, O. C. Asphaltene nanoaggregates studied by centrifugation. Energy Fuels 2009, 23, 1194. (28) Andreatta, G.; Bostrom, N.; Mullins, O. C. High-Q ultrasonic determination of the critical nanoaggregate concentration of asphaltenes and the critical micelle concentration of standard surfactants. Langmuir 2005, 21 (7), 2728. (29) Groenzin, H.; Mullins, O. C. Asphaltene molecular size and structure. J. Phys. Chem. A 1999, 103, 11257. (30) Andrews, A. B.; Guerra, R. E.; Mullins, O. C.; Sen, P. N. Diffusivity of asphaltene molecules by fluorescence correlation spectroscopy. J. Phys. Chem. 2006, 110, 8093. (31) Badre, S.; Goncalves, C. C.; Norinaga, K.; Gustavson, G.; Mullins, O. C. Molecular size and weight of asphaltene and asphaltene solubility fractions from coals, crude oils, and bitumen. Fuel 2006, 85 (1), 1. (32) Guerra, R.; Andrews, A. B.; Mullins, O. C.; Sen, P. N. Comparison of asphaltene molecular diffusivity of various asphaltenes by fluorescence correlation spectroscopy. Fuel 2007, 86, 2016.
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Table 2. Aggregation Numbers and Average Diameters of Boscan and WP Asphaltenes in Toluene Boscan asphaltenes C (mg/L) 0 10 20 30 40 50 60 70 80 90 100 150 200 250 350 400 500 1000
C (kg/m3) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.15 0.20 0.25 0.35 0.40 0.50 1.00
σ (S/m)
D (m2/s)
WP asphaltenes d (nm)
σ (S/m)
AN
-1
2.84 10 0 1.90 10-9 3.54 10-9 5.24 10-9 6.73 10-9 8.23 10-9 9.37 10-9 1.11 10-8 1.20 10-8 1.39 10-8 1.50 10-8 2.16 10-8 2.78 10-8 3.45 10-8 4.68 10-8 5.29 10-8 6.42 10-8 1.20 10-7
D (m2/s)
d (nm)
AN
4.324 10-10 3.506 10-10 3.336 10-10 3.294 10-10 3.124 10-10 3.106 10-10 2.957 10-10 2.954 10-10 2.919 10-10 2.881 10-10 2.737 10-10 2.693 10-10 2.585 10-10 2.543 10-10 2.516 10-10 2.409 10-10 2.271 10-10
1.84 2.27 2.39 2.42 2.55 2.56 2.69 2.70 2.73 2.76 2.91 2.96 3.08 3.14 3.16 3.31 3.51
1 2 2 2 3 3 3 3 3 3 4 4 5 5 5 6 7
-1
5.140 10-10 4.793 10-10 4.735 10-10 4.559 10-10 4.465 10-10 4.235 10-10 4.285 10-10 4.081 10-10 4.193 10-10 4.065 10-10 3.905 10-10 3.765 10-10 3.743 10-10 3.622 10-10 3.585 10-10 3.483 10-10 3.244 10-10
1.55 1.66 1.68 1.75 1.78 1.88 1.86 1.95 1.90 1.96 2.04 2.12 2.13 2.20 2.22 2.29 2.46
2.84 10 0 1.65 10-9 2.67 10-9 3.81 10-9 5.02 10-9 5.95 10-9 7.09 10-9 7.88 10-9 9.00 10-9 1.00 10-8 1.10 10-8 1.56 10-8 2.05 10-8 2.46 10-8 3.38 10-8 3.83 10-8 4.59 10-8 8.65 10-8
1 1 1 1 2 2 2 2 2 2 2 3 3 3 3 3 4
Figure 4. Variation of the average diameter of Boscan and WP asphaltenes versus the concentration in toluene.
Figure 3. Variation of DC conductivity of Boscan and WP asphaltenes versus the concentration in toluene.
propensity of WP asphaltenes to aggregate in toluene as compared to Boscan asphaltenes, which translates into higher diameters for WP asphaltenes (see Figure 4). The diameters increase from 16-18 A˚ for monomers to 20-40 A˚ for nanoaggregates, in line with previous work.27 Furthermore, Figure 4 shows a steep increase in diameters up to CNAC and then a much smaller increase in nanoaggregate size, in agreement with past studies.33
Figure 5. CDCl3.
(33) Freed, D. E.; Lisitza, N. V.; Sen, P. N.; Song, Y.-Q. Molecular composition and dynamics of oils from diffusion measurements. In Asphaltenes, Heavy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007.
On the basis of the 13C NMR spectra of asphaltenes in CDCl3 (see Figure 5), the aromaticity of Boscan and WP asphaltenes is 42 and 56%, respectively. This is obtained by dividing the area of the aromatic carbon peak (100-150 ppm) 2278
13
C NMR spectra of Boscan and WP asphaltenes in
Energy Fuels 2010, 24, 2275–2280
: DOI:10.1021/ef9009235
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Figure 6. Variation of the amount of asphaltene precipitation with the heptane/toluene ratio.
Figure 7. Schematic of asphaltene nanoaggregates and resins according to the Nellensteyn model. Figure 8. Variation of DC conductivity of Boscan and WP crudes and their C7 asphaltenes and maltenes versus the concentration in toluene.
by the total area of the spectrum (excluding the solvent peak). Thus, Boscan asphaltenes are less aromatic than WP asphaltenes, in accordance with the H/C ratios in Table 1. Despite their high conductivity in toluene (see Table 2), Boscan asphaltenes aggregate less than WP asphaltenes (see Figure 4) indicating that aromaticity is the driving force for aggregation and not polarity. This aggregation tendency is confirmed in Figure 6, where the slope of the line representing the variations of precipitate amount relative to the total asphaltene amount versus the heptane/toluene ratio is smaller for Boscan than WP asphaltenes. On the other hand, the onset of asphaltene precipitation is 50:50 and 57:43 for Boscan and WP asphaltenes, respectively. This is because Boscan asphaltenes are heavier than WP asphaltenes and thus have a lower solubility in heptane/toluene mixtures. Indeed, the molar mass of 8784 ppm asphaltenes in benzene is 1700 g/mol for Boscan asphaltenes and 1000 g/mol for WP asphaltenes. The relatively small AN values of asphaltenes from both crudes are in good agreement with recent work21 and suggest that the total surface area of asphaltene nanoaggregates is rather large compared to aggregates with a high AN (see Figure 7). If resins were to coat asphaltenes, then large amounts of resins would be required, much larger than the ones provided in Table 1. This indicates that the Nellensteyn model is not valid. To confirm this result, the variation of the DC conductivity of crude oils (σcrude) versus the concentration of crude oils (Ccrude) in toluene is determined for both crude oils. At each Ccrude, the corresponding concentrations of asphaltenes (Casph) and maltenes (Cmalt) are considered, so that Ccrude is equal to the sum of Casph and Cmalt. The conductivities of asphaltenes (σasph) and maltenes (σmalt) are then measured separately in toluene, and their sum is compared to σcrude. Figure 8 illustrates the variations of σcrude versus Ccrude as well as the variations of (σasph þ σmalt) versus
Figure 9. Variation of DC conductivity of Boscan and WP crudes and their C7 asphaltenes and maltenes versus the concentration in the heptane/toluene mixture (30:70, vol/vol).
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(Casph þ Cmalt) for Boscan and WP crude oils. For both crudes, the two curves coincide, indicating that asphaltene nanoaggregates may not contain any resins. Indeed, if there were enough resins in the nanoaggregates to increase their drag, then σcrude would be smaller than (σasph þ σmalt). On the other hand, when the same experiments are performed in heptane/toluene mixtures (30:70, vol/vol), the behavior of Boscan crude is different than in toluene; σcrude becomes smaller than (σasph þ σmalt) after 300 mg/L, as indicated in Figure 9. This is because the solubility of Boscan resins is smaller in the heptane/toluene mixture than in toluene.11 This is not the case for WP crude oil, where resins are completely soluble in the mixture. Indeed, the kinetics of dissolution of 4 wt % Boscan resins in heptane is slower than that of WP resins, and only 92% of Boscan resins are soluble in heptane compared to 98% of WP resins. This difference in solubility could be attributed to the difference in their average molecular weight (649 g/mol for Boscan resins and 433 g/mol for WP resins). Thus, the least soluble resins would aggregate with asphaltenes to enhance their stability, in agreement with a recently proposed molecular thermodynamic approach to the formation of mixed asphaltene-resin aggregates.1 As a result, the number of resins in asphaltene nanoaggregates may be slightly higher in crude oils than in toluene.
Nomenclature A = area of the electrodes (0.001 134 m2) AN = aggregation number or number of molecules per aggregate C = concentration (kg/L) C = carbon CDCl3 = deuterated chloroform CNAC = critical nanoaggregate concentration 13 C NMR = carbon 13 nuclear magnetic resonance d = diameter of asphaltene spherical particles (m) D = transnational diffusion coefficient (m2/s) DC = direct current e = ionic charge (1.602 176 5 10-19 C) H = hydrogen H/C = hydrogen/carbon ratio ((wt % of H/molecular weight of H)/(wt % of C/molecular weight of C)) 1 H NMR = proton nuclear magnetic resonance kB = Boltzmann constant (1.380 65 10-23 m2 kg s-2 K-1) M = molecular weight of asphaltenes (∼1 kg/mol) MW = molecular weight N = nitrogen NA = Avogadro’s number (6.022 141 8 1023 molecules/ mol) O = oxygen Rp = parallel resistance (Ω) S = sulfur SANS = small-angle neutron scattering SAXS = small-angle X-ray scattering t = gap between electrodes (10-3 m) T = absolute temperature (K) TMS = tetramethylsilane x = ionic fraction of asphaltenes (∼10-4)
4. Conclusions The charge carriers in asphaltenes are used as tracers to probe the aggregation behavior of asphaltenes in toluene and heptane/toluene mixtures. The measurement of their DC conductivity in toluene indicates that asphaltene nanoaggregates start to form at low concentrations and that their aggregation number is rather small (less than 10). The amount of resins in asphaltene nanoaggregates is not high enough to increase their drag; thus, resins are unlikely to coat asphaltenes. This study shows that the Nellensteyn hypothetical model, in which asphaltenes are sterically stabilized by resins, is not valid within the concentration range investigated. In heptane/toluene mixtures (30:70, vol/vol), the least soluble resins tend to aggregate with asphaltenes to enhance their stability, in agreement with a recently proposed molecular thermodynamic approach to the formation of mixed asphaltene-resin aggregates.1
Greek Symbols ηs = viscosity of toluene (0.000 545 Pa s) σ = DC conductivity (S/m) Subscripts aggregate = asphaltene nanoaggregate asph = asphaltenes crude = crude oil DC = direct current malt = maltenes monomer = asphaltene monomer 5 = in C5 for n-pentane 7 = in C7 for n-heptane 10 = in C10 for n-decane
Acknowledgment. The authors acknowledge discussions with Oliver Mullins (Schlumberger) and Daniel Merino-Garcia (Repsol-YFP).
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