Article pubs.acs.org/EF
Dielectric Properties of Asphaltene Solutions: Solvency Effect on Conductivity Cédric Lesaint,*,† Sébastien Simon,‡ Caterina Lesaint,‡ Wilhelm R. Glomm,‡ Gunnar Berg,† Lars E. Lundgaard,† and Johan Sjöblom‡ †
Sintef Energy Research, Sem Sælands vei 11, NO-7465 Trondheim, Norway Ugelstad Laboratory, Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), Sem Sælands vei 4, NO-7491 Trondheim, Norway
‡
ABSTRACT: Electrical demulsification is considered an elegant method to enhance the separation of water-in-crude-oil emulsions. Understanding the mechanisms involved during this process remains a challenge for the further development of this separation method. The aim of this study is to elucidate the influence of the aggregation state of the asphaltenes on their conductivity, to determine the mechanisms governing conductivity in crude oils. Dielectric properties of two different asphaltenes extracted from crude oils (labeled CrA and CrB) and their solutions diluted in toluene or heptane were measured by frequency domain spectroscopy. When the heptane volume fraction increases, conductivity decreases. It also appears that the conductivity decreases with an increasing aggregate size. Conversely, decreasing the aggregate size increases the mobility of the charge carriers in the system and, consequently, increases the conductivity. On the basis of these observations, we propose here that the conductivity of asphaltenes is mostly governed by their mobility. The possible application of this method for determining the critical nanoaggregate concentration (CNAC) of asphaltenes is also evaluated and discussed.
1. INTRODUCTION The formation of water-in-crude-oil emulsions is a major problem in oil production. Surface-active components present in crude oils, such as asphaltenes as well as inorganic particles, stabilize those emulsions, with the emulsification efficiency showing great variation between oil fields as well as production stages.1 Separation of these emulsions is crucial for the petroleum industry because they are troublesome from both a process and product-quality point of view. Presently, electrostatic demulsification2 is one of the most efficient and used methods for achieving efficient processes for solving this problem.3 The combination of high energy efficiency, because it permits a reduction of the use of heat, and also the fact that it avoids the use of chemical demulsifiers makes this technique environmentally friendly, which is of increasing importance. Dependent upon the crude oil properties, the electrocoalescer may work under different conditions regarding temperature, pressure, type of electric field (alternating current or direct current), etc. The physical and chemical properties of the crude oil and the emulsion may affect the conductivity of the crude oil and the properties of the interfacial film between oil and water. To optimize an electrocoalescer, information on how different parameters affect the coalescence process is crucial.4 Some of these parameters are the dielectric properties of the dispersed phase and the continuous phase, volume fraction of the dispersed phase, conductivity, size distribution of the dispersed droplets, intensity of the electric field, and nature of the electric field.5 Conductivity is a very important factor to be studied because it is directly linked to the field distribution and the resulting background field in the emulsion in an alternating current (AC)-operated compact coalescer. It will determine the optimum AC frequency to be used in the coalescer.1b A high time constant and, therefore, a high conductivity, will © 2012 American Chemical Society
necessitate the application of a high frequency. The dielectric properties of crude oil systems and individual constituents have previously been studied by several authors.4,6 Among the several classes of components present in crude oil, it has been demonstrated that asphaltenes play a key role in the conductivity of crude oils, while the dielectric response was affected by all polar constituents.4 Sheu et al.6a,7 studied asphaltene aggregates in concentrated toluene solutions. From their dielectric properties, the mechanism of charge transfer and, hence, mobility, 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, possibly a result of the caging effect in large clusters, which restricted their rotational dynamics. Sheu and Mullins6c used dielectric spectroscopy to compare the 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. Sheu and Acevedo6b used dielectric spectroscopy in the frequency range from 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. They observed a drastic change in the dielectric response near the critical point when flocculation occurred. Very recently, Goual8 investigated the dielectric relaxation of asphaltenes and maltenes solution at low frequency and managed to determine the concentration at which the studied asphaltenes start to Received: August 9, 2012 Revised: November 7, 2012 Published: November 20, 2012 75
dx.doi.org/10.1021/ef3013129 | Energy Fuels 2013, 27, 75−81
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2.2. Methods. 2.2.1. Onset of Precipitation in n-Heptane/ Toluene Mixtures Using Fourier Transform Near-Infrared (FT-NIR) Spectroscopy at 1600 nm. The precipitated asphaltene fractions were dissolved in toluene before n-heptane was added at different heptane volume fractions. Heptane was chosen to reduce errors as a result of vaporization when using pentane. The concentration of asphaltene solutions was 2 mg/mL. The samples were sonicated for 5 min and left on a shaker for 24 h. The onset of precipitation was detected using near-infrared (NIR) light at 1600 nm by looking at the baseline elevation as a result of scattering by particles. The instrument used for recording the NIR spectra was the multipurpose analyzer (MPA) from Bruker Optics. The procedure described herein has been described elsewhere.12 2.2.2. Dielectric Response. The dielectric response was measured on an IDA 200 instrument from General Electrics. The instrument is 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 crude oils were measured at room temperature over a frequency range from 0.01 to 1000 Hz, 3 points per decade. The applied voltage was 200 V. The conductivity as a function of frequency was calculated from the loss curve of the dielectric response. Prior to testing a new solution, the test cell was cleaned with toluene. It should also be mentioned here that the studied systems can be considered as linear, assuming an “average bulk” behavior. 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 with 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
aggregate, i.e., the critical nanoaggregate concentration (CNAC).9 Several different papers within the past few years by the same author and his co-workers show the same range of CNAC (50−200 mg L−1) using various techniques, such as high-Q ultrasonic10 or optical fluorescence.9 While a number of studies have investigated the role of asphaltenes in the conductivity of crude oils, there is a great need for an improved understanding of the mechanisms involved. Herein, we investigate the influence of the asphaltene aggregation state on their dielectric properties for two different asphaltenes. Frequency domain spectroscopy measurements were performed on asphaltenes diluted in toluene−heptane mixtures to investigate the effect of the degree of aggregation on the dielectric properties and, more specifically, the conductivity. Because asphaltenes are the main crude oil component responsible for their conductivity, a general mechanism governing conductivity in crude oil is proposed. The authors also applied this method for elucidating the CNAC of asphaltenes.
2. EXPERIMENTAL SECTION 2.1. Materials. Two asphaltene-rich crude oils were used in this study. They will be denoted herein as CrA and CrB. The asphaltenes extracted from these two crude oils will be denoted as AA and AB, respectively. 2.1.1. Precipitation of the Asphaltenes. The procedure used for extraction of the asphaltenes was as follows: Approximately 1 L of crude oil previously sampled at 60 °C was diluted in 16 L of pentane (VWR, 99.7%) and stirred overnight in darkness at room temperature. The asphaltenes were retrieved by a two-step vacuum filtration procedure using consecutively smaller filter funnels (pore sizes of 200 and 185 nm inner diameter). Following filtration, the asphaltenes were collected and stored in a Schott bottle in a N2 atmosphere to prevent oxidation. The elemental composition of both asphaltenes has been determined, and the results are given in Table 1. To investigate the
ε′(ω) = 1 + χ ′(ω) and the imaginary part can be written as ε″(ω) = σ /ε0ω + χ ″(ω)
Table 1. Elemental Composition of Asphaltenes A and B carbon hydrogen nitrogen oxygen sulfur
asphaltene A (%)
asphaltene B (%)
63.4 7.05 1.26 1.93 1.24
84.4 8.15 1.01 1.88 4.00
(1)
(2)
where χ′ and χ″ are the real and imaginary parts of the complex susceptibility, σ is the conductivity, ε0 is the permittivity of vacuum, and ω is the angular frequency. Therefore, the conductivity contributes to the spectrum and to an increasing extent with a decreasing frequency.13 In an electric field E, a particle carrying a net charge q will experience an electrostatic force Fe = qE
(3)
For ions, the resulting velocity v in an electric field is given by v = μE, where μ is the ion mobility. An estimation of the ion mobility can be found if the ions are assumed to be spherical with radius r. The relation between μ and the dynamic viscosity η of the liquid is then given by Stoke’s law
effect of conducting contaminants, two different asphaltenes were studied, one containing contaminants, such as mineral particles not soluble in pentane, asphaltene A (AA), and asphaltene B (AB), which can be considered as pure. 2.1.2. Preparation of Asphaltene Solutions. Stock solutions and their concomitant sequential dilutions were prepared by dissolving the appropriate amount of asphaltene powder in toluene or mixtures of toluene and heptane and magnetically stirred for about 24 h. Asphaltene solutions with lower concentrations were subsequently made by diluting the stock solutions in the appropriate heptane/ toluene mixtures. The resulting solutions were kept at room temperature in darkness for 24 h. Under the conditions studied here, before the flocculation onset, no sedimentation of solid asphaltene was observed for any of the systems studied here, which led to the assumption that the prepared solutions were homogeneous during the experimental time frames used. It is important to mention here that toluene is considered to be an excellent solvent for asphaltenes, while heptane is known to be a poorer choice. This means that, in the presence of heptane, the aggregation number of asphaltenes tends to increase.11
Fη = 6πηrv
(4)
valid for low Reynolds number and incompressible fluids. Balancing the two forces Fe = Fη, one obtains μ = q/6πηr
(5)
Because the viscosity is not much influenced by pressure, the mobility will also be pressure-independent. The viscosity of a liquid is generally given by
η = η0 exp(EA /RT )
(6)
thus the ion mobility will increase exponentially with the temperature. The current density because of one ion or charged particle is then given by 76
dx.doi.org/10.1021/ef3013129 | Energy Fuels 2013, 27, 75−81
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(7)
where q = e for univalent ions. Further, the conductivity σ of the liquid is given by σ = j/E, which for a single charge carrier of radius r takes the expression
σi = qμ = q2 /6πηr
(8)
Thus, the conductivity because of a charged particle is proportional to the square of the charge and inversely proportional to the viscosity and particle diameter. The link between the viscosity of crude oil solutions and conductivity has been previously studied by the authors.14
3. RESULTS AND DISCUSSION Herein, impedance measurements of asphaltene solutions extracted from two crude oils were performed to elucidate the conductivity mechanism involved and to evaluate whether this technique is suitable for CNAC determination. The organization of the Results and Discussion follows these two main objectives. 3.1. Solutions of Asphaltenes: Determination of the Mechanism of Conductivity by Impedance Measurement. 3.1.1. Onset of Precipitation. The onset of precipitation, measured by FT-NIR spectroscopy, is determined by plotting the absorbance at 1600 nm versus the heptane volume fraction for both asphaltenes AA and AB. Crude oils and asphaltenes are known to display an absorption minimum at 1600 nm.15 However, in the presence of particles, scattering at this wavelength is observed, resulting in an elevation of the baseline.15 Thus, a baseline elevation can be observed in the NIR spectra when monitoring the precipitation of asphaltenes, as reported earlier,12,16 allowing for the determination of the precipitation/flocculation onset. Figure 1 depicts the variations
Figure 2. Imaginary (ε″) part of the complex relative permittivity εr as a function of the frequency for asphaltene extracted from crude oil B in a mixture with a heptane volume fraction of 25% at 20 °C. The samples were investigated over a wide range of concentrations, from 20 to 10 000 mg/L.
range of concentrations, from 20 to 10 000 mg/L. Please note that the curves obtained for this system are similar to the curves obtained with other mixtures of solvent or with AA. Figure 2 reveals a straight line with a slope of approximately −1 for the entire concentration range, indicating pure direct conductivity as the dominating charge-transfer mechanism.13b,17 The most concentrated solution has the highest value of ε″, which is expected and in line with earlier studies.17b Asphaltenes are charge-carrying species; therefore, the conductivity of the studied solutions is mainly dependent upon their concentration.4,6a At the lowest frequency, it can be seen that the graphs of ε″ start to deflect to a small extent. The deflection appears to be quite concentration-dependent, i.e., more deflection at the lowest concentrations. This phenomenon can be attributed to low-frequency dispersion (LFD),18 as discussed below for the dielectric storage measurements. Figure 3 reveals a log−log plot of ε′, the real part of the complex relative permittivity εr, measured at 20 °C for asphaltenes extracted from crude oil B dissolved in a mixture
Figure 1. Onset of precipitation for the asphaltenes A and B measured by NIR spectroscopy at an increasing toluene volume fraction in a toluene/heptane mixture.
of the absorbance measured at 1600 nm of CrA and CrB at different heptane volume fractions. For both asphaltenes, the precipitation onset occurs for mixtures with a heptane volume fraction between 65 and 75%, as evident from the breakpoint in the absorbance curves. To investigate the behavior of solvated asphaltene molecules as well as larger flocs/aggregates, experiments were conducted with systems residing both above and below the precipitation onset. 3.1.2. Dielectric Response. Figure 2, a log−log plot of ε″, the imaginary part of the complex relative permittivity εr, measured at 20 °C, is shown for asphaltenes extracted from CrB dissolved in a mixture with a 25% heptane volume fraction. The dielectric response of the asphaltene samples was investigated over a wide
Figure 3. Real (ε′) part of the complex relative permittivity εr as a function of the frequency for asphaltene extracted from crude oil B in a mixture with a heptane volume fraction of 9% at 20 °C. The samples were investigated over a wide range of concentrations, from 20 to 10 000 mg/L. 77
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concentration. If we assume that ε′ is correlated with any parameter characteristic of an adsorption process (e.g., adsorbed amount), this result show that we have a continuous growth of the adsorbed layer, which would imply the formation of multilayers because there is no plateau of adsorption, synonymous of a monolayer formation at the surface of the electrodes. The influence of the solvent mixture can also be investigated by plotting the real ε′ part of the complex relative permittivity εr as a function of the frequency. The results obtained for solutions containing 250 mg/L of asphaltenes extracted from crude oil B are shown in Figure 5. Please note that the curves
with a 9% heptane volume fraction. Asphaltene concentrations were studied in a range, from 20 to 10 000 mg/L. Please note that the curves presented here have been selected to illustrate a tendency that all of the other systems share (other solvent mixtures or solutions with AA). At higher frequencies (>60 Hz), the curves in Figure 3 converge into a straight line with a slope of approximately 0; i.e., ε′ is independent of the frequency. On the other hand, at low frequencies (
