Energy Fuels 2009, 23, 5596–5602 Published on Web 10/06/2009
: DOI:10.1021/ef900445n
Dielectric Properties of Crude Oil Components Helene Vra˚lstad,*,†,‡ Øyvind Spets,† Cedric Lesaint,*,† Lars Lundgaard,§ and Johan Sj€ oblom† †
Ugelstad Laboratory, Institute of Chemical Engineering, Norwegian University of Science and Technology, Trondheim, Norway, ‡ Sintef Materials and Chemistry, Sem Sælands v. 2A, No-7465 Trondheim, Norway, and §Sintef Energy Research, Sem Sælands v. 11, No-7465 Trondheim, Norway Received May 12, 2009. Revised Manuscript Received September 9, 2009
The dielectric properties of asphaltenes precipitated from four different crude oils have been studied in the frequency range of 0.01-1000 Hz by frequency domain spectroscopy (FDS). The asphaltenes were dissolved in toluene, and the dielectric response of the solutions was measured at different concentrations. To determine the precipitation point, titrations of the toluene solutions with heptane were performed and monitored by near-infrared spectroscopy. The dielectric properties of the asphaltenes were then also examined close to and well above the precipitation point. an anomalous response in the real component, ε0 , of the response at low frequencies.3 This phenomenon, called lowfrequency dispersion (LFD) is often reported for solids1b but less common for liquids. The dielectric properties of crude oil systems and individual constituents in different frequency ranges have previously been studied by a few authors. Sheu et al.4 studied asphaltene aggregates in concentrated toluene solutions. From their dielectric properties, the mechanism of charge transfer was assessed and the aggregate size was determined. At low temperatures and high concentrations, the aggregates showed a tendency to form clusters, as manifested by an additional peak in the dielectric response spectrum. Tjomsland et al.5 correlated the dielectric response at frequencies in the range of 1 kHz to 10 GHz to the IR absorption spectra of different distillation fractions of three different crude oils. The dielectric response and conductivity of different bitumen fractions in solution were measured by Chow et al.6 The experiments were performed at a single frequency, 1000 Hz, in a wide concentration range. They concluded that asphaltenes were the main constituents of the bitumens determining their conductivity, while the dielectric response was affected by all polar constituents. Therefore, the conductivity could be used as a direct method to measure the asphaltene content in bitumen samples. Sheu and Mullins7 used dielectric spectroscopy to compare the
Introduction When a dielectric material is exposed to an external electric field, dipoles in the material align to the field in a process called dielectric relaxation. The relaxation of a dipole is characterized by its relaxation time, τ. Dipoles that can readily align to the applied electric field, e.g., dipoles of a bulk liquid, have short relaxation times and are therefore found in the high-frequency range of the spectrum. Slower relaxation processes, such as dipole relaxation processes in solids, are found in the lower frequency regions. The dielectric response is commonly presented as the real and imaginary parts of the complex relative permittivity, ε*. The real component of ε* can be written as ε0 ðωÞ ¼ 1 þ χ0 ðωÞ with the imaginary part as ε00 ðωÞ ¼
σ þ χ00 ðωÞ ε0 ω
where χ0 and χ00 are the real and imaginary parts of the complex susceptibility, σ is the DC conductivity, ε0 is the permittivity of the vacuum, and ω is the angular frequency. Therefore, the DC conductivity contributes to the spectrum and to an increasing extent with decreasing frequency.1 The complex dielectric response may be expressed as a function of frequency (frequency domain spectroscopy) or time (time domain spectroscopy). At very low frequencies, DC conductivity is the dominating process of dielectric loss. The imaginary part, ε00 , therefore shows a linear frequency dependence with a slope of nearly -1 in the log-log plot, typical for DC conductivity.2 In the presence of interfacially active compounds, a high-capacitance layer may form on the electrodes, which gives rise to
(3) (a) Hill, R. M.; Cooper, J. Dielectric spectroscopy of micelle structures. J. Colloid Interface Sci. 1995, 174, 24–31. (b) Hill, R. M.; Beckford, E. S.; Rowe, R. C.; Jones, C. B.; Dissado, L. A. The characterization of oil in water emulsions by means of a dielectric technique. J. Colloid Interface Sci. 1990, 138 (2), 521–533. (4) (a) Sheu, E. Y.; Storm, D. A.; Shields, M. B. Dielectric response of asphaltenes in solvent. Energy Fuels 1994, 8, 552–556. (b) Sheu, E. Y.; Storm, D. A.; De Tar, M. M. Asphaltenes in polar solvents. J. Non-Cryst. Solids 1991, No. 131-133, 341–347. (5) Tjomsland, T.; Hilland, J.; Christy, A. A.; Sj€ oblom, J.; Riis, M.; Friisoe, T.; Folgeroe, K. Comparison of infrared and impedance spectra of petroleum fractions. Fuel 1996, 75 (3), 322–332. (6) Chow, R. S.; Tse, D. L.; Takamura, K. The conductivity and dielectric behaviour of solutions of bitumen in toluene. Can. J. Chem. Eng. 2004, 82, 840–845. (7) Sheu, E. Y.; Mullins, O. Frequency-dependent conductivity of Utah crude oil asphaltene and deposit. Energy Fuels 2004, 18, 1531– 1534.
*To whom correspondence should be addressed. E-mail: helene.
[email protected] (H.V.);
[email protected] (C.L.). (1) (a) Jonscher, A. K. Dielectric Relaxation in Solids; Chelsea Dielectric Press: London, U.K., 1983.(b) Lihnjell, D.; Lundgaard, L.; Gafvert, U. Dielectric response of mineral oil impregnated cellulose and the impact of aging. IEEE Trans. Dielectr. Electr. Insul. 2006, 14 (1), 156–169. (2) Hill, R. M.; Cooper, J. Characterization of water-in-oil emulsions by means of dielectric spectroscopy. J. Mater. Sci. 1992, 27, 4818–4827. r 2009 American Chemical Society
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Energy Fuels 2009, 23, 5596–5602
: DOI:10.1021/ef900445n
Vrålstad et al.
Figure 1. Complex relative permittivity as function of the concentration for asphaltenes A, B, C, and D diluted in toluene.
frequency-dependent dielectric properties of asphaltenes and deposit in toluene solution. The samples showed considerable differences in their dielectric properties and their temperature dependence, with the deposit being much more conducting than the corresponding asphaltene. Wattana et al.8 investigated different crude oils that differed in their tendency to precipitate asphaltenes with respect to the asphaltene polarity. The asphaltenes were fractionated with respect to polarity and compared to the corresponding deposits from different locations. The polarity of the deposits was found considerably higher than for the corresponding crude-oil-derived asphaltenes. Sheu and Acevedo9 used dielectric spectroscopy in the frequency range of 1 kHz to 10 MHz to study the changes in the crude oil upon addition of hexane to the limit of precipitation of asphaltenes and subsequent aging. The dielectric responses of a crude oil and corresponding asphaltene solutions have also been investigated at different pressures.10
In this paper, we report on the dielectric properties of asphaltenes derived from four crude oils of different origin as well as one resin fraction. The low-frequency dielectric response of the asphaltenes dissolved in toluene at different concentrations was measured to examine qualitative differences between the asphaltenes from different crude oils. The experiments were also performed on the asphaltenes in toluene-heptane mixtures to investigate the effect of the degree of aggregation on the dielectric properties. Experimental Section Chemicals. Four different crude oils labeled A, B, C, and D were kindly supplied by different producers. Toluene, pentane, and heptane, analytical grade, were purchased from SigmaAldrich and used without further purification. Precipitation of Asphaltenes. The asphaltenes were precipitated in pentane according to standard procedures.11 The asphaltenes were stored in pentane after precipitation and then filtered and washed in pentane before use. Apparatus. Heptane titrations were performed on a Titronic Universal titrator (Schott) and monitored by near-infrared
(8) Wattana, P.; Fogler, H. S.; Yen, A.; Garcia, M. D. C.; Carbognani, L. Characterization of polarity-based asphaltene subfractions. Energy Fuels 2005, 19, 101–110. (9) Sheu, E. Y.; Acevedo, S. A dielectric relaxation study of precipitation and curing of Furrial crude oil. Fuel 2006, 85, 1953–1959. (10) Syunyaev, R. Z.; Balabin, R. M. Frequency dependence of oil conductivity at high pressure. J. Dispersion Sci. Technol. 2007, 28, 419– 424.
(11) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker, Inc.: New York, 1999.
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: DOI:10.1021/ef900445n
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Figure 2. Conductivity, σ, as function of the concentration for asphaltenes A, B, C, and D diluted in toluene.
Results and Discussion
(NIR) spectroscopy using a multi-purpose analyzer (Bruker Optik, Germany). The dielectric response was measured on an IDA 200 instrument from General Electrics. The instrument was equipped with a test cell from IRlab, CL-1 142, with two cylindrical electrodes of 6 cm in length at a distance of 3.7 mm from each other immersed in the crude oil sample. The dielectric responses of the solutions were measured at ambient temperature over a frequency range from 0.01 to 1000 Hz, with 3 points per decade. The conductivity as a function of frequency was calculated from the loss curve of the dielectric response. For the sake of comparison, the conductivity was also measured under static conditions for three different crude oils by a low-voltage assembly for measurements of conduction losses in highly insulating liquids, model LDTRP-2 from IRlab, using the same test cell as above, CL-1 142. NIR Spectroscopy. The asphaltenes were dissolved in toluene to a concentration of 10 wt %. A total of 25 mL of the solutions was titrated with 200 mL of heptane under constant stirring. The heptane was added at a rate of 0.5 mL/min. The optical density (OD) was determined at 1600 nm. Dielectric Spectroscopy. The different asphaltenes were dissolved in toluene to 0.01, 0.5, and 10 wt % and analyzed by dielectric spectroscopy in the frequency range of 0.01-100 Hz. For measurements on asphaltenes in mixed solvents, the asphaltenes were first dissolved in toluene and then mixed with heptane. All measurements were performed at ambient temperature.
Conductivity Measurements. The conductivites obtained by direct measurements correlated very well with those calculated from the dielectric loss curve from dielectric spectroscopy. The conductivities calculated for toluene and heptane were in good correspondence with the values found in the literature for all temperatures and showed very little variation with frequency. Effect of the Concentration of Asphaltenes in Toluene Solutions. The complex relative permittivity and conductivity show a strong dependence of the asphaltene concentration (Figures 1 and 2). This is an expected result, because the dielectric constant of a dilute solution is expected to be a linear function of the solute concentration in the absence of intermolecular interaction.8 Because asphaltenes are the main charge-carrying species in crude oils, the conductivities of their solutions are mainly dependent upon their concentration and qualitative differences because different asphaltenes are of much less importance. The dielectric storage, ε0 , of the asphaltene solutions all show an anomalous behavior at low frequencies, LFD (Figure 1). This behavior can have a number of causes12 (12) Jonscher, A. K. Universal Relaxation Law; Chelsea Dielectrics Press: London, U.K., 1996.
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Table 1. Determination of the Water Amount Present in Asphaltene Solutions for Crude A concentration (wt %)
water amount (ppm)
0.025
176.0 175.4 170.1 173.3 174.2 177.4 179.3 171.5
0.05 0.125 0.5
Figure 4. Optical density for asphaltene solutions in toluene under titration with heptane as monitored by NIR. Compositions of the 50:50 and 20:80 (toluene/heptane) mixtures are marked.
The onset of LFD as well as its extent varies strongly with the concentration. We attribute this effect to the formation of an asphaltene film on the electrodes, which hinders the transfer of charges to the electrodes, creating a capacitive effect. Variations between different asphaltenes are small at a given concentration, although some qualitative differences exist, in both curve shape and extent of LFD (Figure 1). Because of the differences in the molecular structure, different asphaltenes may form aggregates of different size and packing on the electrodes and, therefore, give rise to qualitative differences in the LFD. The dielectric loss curve, ε00 , shows a linear dependence of frequency with a slope of -1, which is characteristic for a system where DC conductivity is the dominating charge-transfer mechanism.1b At the lowest asphaltene concentration, the concentration of conducting species in the solution is low and the DC conductivity is therefore less dominant. The effect of LFD can then also be seen in the dielectric loss curve as a deviation from linearity at low frequencies for the 0.01 wt % solutions. Because the conductivities are calculated from the dielectric loss data, this causes an apparent drop in conductivity at low frequencies. At high concentrations on the other hand, the effect of LFD is masked by the dominating DC conductivity in the dielectric loss curve. Figure 3 shows the dielectric response of crude oil D as well as its asphaltene and resin fractions, both dissolved in toluene to 0.01 wt %. At the same concentration, the resin fraction shows a significantly lower conductivity. All three samples show LFD at low frequencies, although the onset frequency varies between them. In comparison to resins, asphaltenes are more polar and have a significant content of heteroatoms, which makes them more efficient than resins as charge carriers. The dielectric storage curves vary in shape between the crude oil, the asphaltene, and the resin fraction, and the onset frequency of LFD varies between them. We believe that differences in packing of the molecules in this film because of qualitative differences between asphaltenes and resins as well as asphaltenes of different origin are the cause of the observed differences in LFD between different samples. Effect of the Solvent. To investigate the effect of the aggregation state of the asphaltenes on the dielectric properties,
Figure 3. Complex relative permittivity (1) and conductivity (2) of crude oil D and its asphaltenes and resins, respectively, dissolved in toluene to 0.01 wt %. The asphaltene and resin content in crude D is 3.3 and 7.6 wt %, respectively.
and could for instance be due to traces of water present in the solutions. To test this hypothesis, the determination of the water amount, using the Karl Fisher instrument, was achieved on several asphaltene solutions with different concentrations (Table 1). The same amount of water was found for all of the studied solutions. Therefore, if water was playing a major role at low frequency, then the effect observed should be the same for all of the systems, while an increase of the LFD effect with the asphaltene concentration is observed. Consequently, one can conclude that water alone cannot explain the behavior observed at low frequency. 5599
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Figure 6. Effect of the solvent on conductivity, σ. The dielectric response was measured for 0.5 wt % solutions of asphaltenes A, B, C, and D in toluene (1), 50:50 toluene/heptane (2), and 20:80 toluene/heptane (3), respectively. Figure 5. Effect of the solvent on complex relative permittivity, dielectric storage, ε0 , and dielectric loss, ε00 . The dielectric response was measured for 0.5 wt % solutions of asphaltenes A, B, C, and D in toluene (1), 50:50 toluene/heptane (2), and 20:80 toluene/heptane (3), respectively.
experiments were performed on asphaltenes in mixtures of heptane and toluene. The precipitation of the different asphaltenes was first monitored by NIR spectroscopy under titration of toluene solutions of the asphaltenes with heptane (Figure 4). 5600
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: DOI:10.1021/ef900445n
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Figure 8. Complex relative permittivity of 0.5 wt % asphaltene B in toluene (9 and 0) and the response obtained for the asphaltene film deposited on the electrodes when the asphaltene solution was substituted by heptane (b and O).
of magnitude smaller than that of the most dilute asphaltene solutions. Therefore, the contribution of the solvent to the conductivity of the sample and its change with composition can be assumed negligible. We believe that the decrease in conductivity reflects the decreased number of conductive species in the mixture because of the formation of larger aggregates with decreased polarity of the solvent. Some precipitation of particles may also occur during the course of the experiment for the least polar solution, removing conductive species from the mixture. To examine whether the dielectric properties changed because of aging, two solutions were left overnight in the test cell and then measured again after 17 or 24 h, respectively. The results are presented in Figure 7. From Figure 7, it is obvious that the aging had very little effect on the dielectric properties of the samples because the relative permittivities and conductivities of the solutions, aged or not, are exactly the same. In a final experiment, the 0.5 wt % asphaltene solution was removed from the test cell after the dielectric response measurement and replaced by pure heptane. The measurement was then repeated on the asphaltene film only, immersed in heptane. The results obtained are presented in Figure 8. In this nonpolar medium, a small LFD effect is still present. This supports the theory that the LFD results from the formation of a capacitive layer on the electrodes in our system.
Figure 7. Effect of aging of the films on complex relative permittivity and conductivity. Data are from dielectric spectroscopy analysis of 0.01 and 0.5 wt %, respectively, of asphaltenes A and C dissolved in 50:50 mixtures of toluene and heptane.
The NIR spectra show a rapid increase in optical density as the asphaltenes start to precipitate (Figure 4). The asphaltenes A, B, C, and D started to precipitate at 66, 61, 60, and 60 vol % heptane, respectively. Mixtures of toluene and heptane with 0.5 wt % asphaltenes, one 50:50 and one 20:80 in toluene, were then prepared for further analyses. According to NIR analysis, no precipitation has taken place for any of the asphaltenes A, B, C, or D in the 50:50 mixture. Because of the less polar nature of this solvent, the asphaltenes can be expected to form larger aggregates than in pure toluene. In the 20:80 mixture on the other hand, all asphaltenes would be expected to start to precipitate and the degree of aggregation is still larger. As the proportion of heptane increases in the solvent and the polarity decreases, the extent of LFD decreases for all asphaltenes; i.e., the amount of LFD decreases with increased aggregate size, and the ε0 curves shape changes. The change in particle size changes the packing in the layer, leading to qualitative changes in the LFD (Figure 5). The conductivity decreases more than 1 order of magnitude for all asphaltenes upon increasing the fraction of heptane in the mixture from 0 to 80% (Figure 6). The conductivities of both toluene and heptane are several orders
Conclusions The dielectric responses of dissolved asphaltenes from four different crude oils were analyzed in the frequency range of 0.01-1000 Hz, at different concentrations. All samples showed a LFD behavior unusual for liquid samples at the lowest frequencies. We attribute the LFD to the deposition of a capacitive layer of asphaltenes on the electrodes. As expected, the asphaltene concentration was the main parameter determining the complex dielectric permittivity for the different solutions in toluene. For a single concentration, experiments were performed in solvents of decreasing polarity. It appears that decreasing the polarity of the solvent 5601
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and therefore increasing the aggregation of the asphaltenes leads to a significant decrease of the LFD effect. Qualitative changes in the curve shapes were also observed, which we attribute to changes in packing behavior in the aggregates as the solvent polarity changes. All four asphaltenes in our study showed similar precipitation behavior and start to precipitate at heptane concentrations from 60 to 66 vol %. The response of one resin fraction as well as the corresponding crude oil was also analyzed. The different fractions and the crude oil all showed qualitatively and quantitatively different responses with respect to LFD. No effect of aging of the film on dielectric properties was observed. When the asphaltene film
deposited on the electrodes was immersed in heptane, LFD was still observed. Acknowledgment. This work is funded by the project ”Electrocoalescence;Criteria for an efficient process in real crude oil systems”, coordinated by SINTEF Energy Research. The contact person is L. E. Lundgaard. The project is supported by The Research Council of Norway, under the contract 169466/S30, and by the following industrial partners: Vetco Aibel AS, Aker Kvaerner Process Systems AS, Statoil ASA, Norsk Hydro ASA, BP Exploration Operating Company Ltd., Shell Technology Norway AS, and Petrobras. The companies in the JIP Flucha III program are acknowledged for providing the crude oils.
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