Dielectric Relaxation-Based Capacitive Heating of Oil Sands - Energy

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Dielectric Relaxation-Based Capacitive Heating of Oil Sands Tinu Abraham,* C. W. Van Neste, Artin Afacan, and Thomas Thundat* Department of Chemical and Materials Engineering, University of Alberta, Alberta, Canada T6G 2 V4 ABSTRACT: An electrothermal method of capacitive heating of oil sands was investigated. Temperature-based impedance spectroscopy experiments for a given rich grade of oil sands were conducted to identify a suitable dielectric relaxation frequency for carrying out capacitive heating, which was observed to be around 65 kHz having a full width at half-maximum of two decades. This was attributed to interfacial polarizations at water, bitumen, and silicate mineral interfaces. The relaxation frequency changed with temperature rise, indicating that frequency tuning could be suitable to optimize capacitive heating. Hence, capacitive heating of oil sands was demonstrated by exposing it to high alternating electric fields (104 V/m) at a frequency in the dispersion regime of its relaxation frequency. As temperature increased, the overall impedance of oil sands decreased as determined from impedance spectroscopy. Frequency was retuned to match the changed impedance of the system, which ensured that temperature further increased.

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INTRODUCTION Canada has the third largest oil reserves in the world, 97% of which are found in the oil sands of Alberta.1 However, the average cost of producing oil from oil sands is high, as compared to other global oil producers, at about $70−90 per bbl.2 The high cost of oil production can be attributed to currently used water-based thermal heating technologies such as hot water extraction for open pit mines and in situ steamassisted gravity drainage (SAGD) for extraction of viscous bitumen from sand, clay, and water mixtures of oil sands. Both technologies are very energy intensive and entail a host of water−oil separation issues as well as water treatment and oil upgrading problems. Furthermore, in situ SAGD extraction process is known to be inefficient in its spatial and temporal heat transfer resulting in poor bitumen recovery and slow production rates.3 Overcoming these drawbacks would mean reducing or gradually eliminating water-based thermal processes as excessive water and energy used by these technologies make them costly and inefficient in terms of the energy return on investment. Therefore, the oil sands industry needs to explore other innovative technologies which can make it reliable, economically attractive, and environmentally sustainable. Electrical heating of oil sands is one such promising innovation among many other technologies being proposed today. Unlike thermal methods, electrical heating can directly heat molecules of oil sands without the use of intermediate thermal energy carriers such as water or steam, thus eliminating the associated drawbacks.4 Electrical heating carries out volumetric heat generation as compared to surface-based thermal conduction heating.4 This is advantageous in causing uniform heating, reducing heat losses, and increasing efficiency in terms of quantity and rate of product recovery. With several foreseeable advantages, one is yet to find a full-scale commercialized plant that is heating oil sands electrically, even though the earliest studies in this field took place in the 1970s.5 Less clarity on electrical heat generation mechanisms in heterogeneous oil sands could be an important reason for this slow growth. Interestingly, oil sands are known to have © 2016 American Chemical Society

frequency-dependent electrical properties which are significantly large at low frequencies arising from trivially small electrical properties of individual components of silicate minerals, salt water, clay, and bitumen.5,6 Silicate minerals are known to be insulating dielectric materials with dielectric permittivity, ε′, in the range of 4−15 and the electrical conductivity, σ, around 10−10 S/m, and these properties are independent of frequency below gigahertz in the absence of any water.7 Distilled water devoid of any ions is also an insulating dielectric material (ε′ = 80; σ ∼ 10−4 S/m) having electrical properties that are strongly temperature-dependent but independent of frequency below several gigahertz.7 However, adding salt to water dramatically increases its conductivity (σ) while barely altering its dielectric permittivity (ε′).8 It is known that the addition of small quantities of salt water to insulating dry silicate minerals dramatically increases both conductivity and permittivity of the combined system as frequencies are reduced below gigahertz.9−12 The presence of charged clay particles in salt water further enhances the dielectric permittivity and conductivity values at frequencies below gigahertz.8,12−16 Bitumen being oil is known to have a low dielectric permittivity (ε′ ∼ 2) and low electrical conductivity (σ ∼ 10−8 S/m), which stays constant in the kilohertz− gigahertz frequency range. However, its dielectric permittivity increases significantly at frequencies lower than kilohertz attributed to the presence of polar asphaltenes in nonpolar maltenes.17 Having understood the general electrical properties of individual oil sands components, it is interesting to note that when these components are present as a mixture, they result in significantly large values of dielectric permittivity and are frequency-dependent below gigahertz.5,6 This behavior is governed mainly by physical and chemical mechanisms occurring at the solid−liquid and liquid−liquid interfaces of heterogeneous oil sands.5−18 At low frequencies between millihertz and hertz, chemical reaction mechanisms occurring Received: October 21, 2015 Revised: February 2, 2016 Published: February 16, 2016 1987

DOI: 10.1021/acs.energyfuels.5b02493 Energy Fuels 2016, 30, 1987−1996

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optimized heating. Dielectric heating is usually carried out by radiating electromagnetic waves into the reservoir through antennas.20,21 This method may have certain disadvantages such as excessive heating near the antenna and reduction in radiation intensity with distance.20,21 Carrying out dielectric heating in a capacitive manner, whereby the oil sands serve as a dielectric medium between two electrodes can be advantageous over the radiative method.4 Capacitive heating generates uniform high alternating electric fields in the entire volume of the dielectric medium between the electrodes, thus resulting in volumetric heat generation in the medium. The objective of this study is to investigate the mechanism of capacitive heating of oil sands. Impedance spectroscopy is used to characterize the dielectric relaxation frequency of the rich grade oil sands sample to examine the frequency regime of maximum heat losses as well as temperature-based changes to its impedance behavior. Frequency-tuned capacitive heating is carried out based on the temperature-based impedance spectroscopy studies to investigate the mechanism of oil sands heating. The proposed method of frequency-tuned capacitive dielectric heating could be applied to in situ reservoirs based on their electrical impedance and corresponding changes in their relaxation behavior during the heating process. The main advantages envisioned could be volumetric heat generation and hence uniform and faster heating, less dependence on ionic conduction in connected water channels, optimized frequency-tuned heating to increase electrical efficiency, feasible penetration depths for reservoir scales, as well as better control and less wasteful reservoir heating.

due to oxidation, reduction reactions between water and metallic minerals, ion-exchange reactions commonly involving clay and water, as well as clay−organic material reactions are discussed to be the reasons for high dielectric property values of heterogeneous soils which are comparable to oil sands.7−12 Physical motion mechanisms are known to be the reason for high dielectric properties between hertz and gigahertz in such heterogeneous mediums.8−11 Between hertz and megahertz, physical motion mechanisms occur because of migration of free charges or ions to grain boundaries, particle edges or phase boundaries, in other words, at interfaces and are called interfacial polarization or the Maxwell−Wagner effect.8 Physical motion mechanisms occur at gigahertz because of rotation of polarized molecules and are called orientation polarization.4 Furthermore, the electrical impedance of oil sands and soils vary with physical conditions such as temperature, pressure, and chemical composition and microstructural factors such as porosity, distribution, and connectivity of porous channels.9,10 Depending on the electrical frequency of application, different heat generation mechanisms such as joule heating and/or dielectric heating can be carried out.4,5,19−21 Joule heating is usually carried out at 60 Hz and is proportional to the resistive losses arising from the current conducted by ions in the water present in the pore spaces of oil sands.19 Connected water channels are required in this method to complete the electrical circuit and continue the heating process. The disadvantages of this process could be consumption of large amounts of current and discontinuation of heating process as water turns into vapor. Unlike joule heating, dielectric heating occurs at higher frequencies in the kilohertz−gigahertz range.4 This mechanism does not require connected water channels to form a current conduction path to initiate or continue the heating process. Dielectric heating mainly arises from interfacial and orientation polarization of molecules at different component interfaces and from individual components, respectively, in the kilohertz−gigahertz frequency range.4 Carrying out dielectric heating at microwave frequencies22−25 in the gigahertz range where orientation polarizations dominate is not feasible for large reservoirs because these frequencies operate at very small penetration depths.20,21 Penetration depths associated with radio frequencies in the kilohertz− megahertz range are most suitable for reservoir-scale heating applications. At these frequencies application of an electric field results in interfacial polarization in heterogeneous porous mediums such as oil sands.4,5 Movement of mobile charge carriers in water or bitumen toward their respective phase boundaries and toward silicate grain boundaries results in interfacial polarizations as mentioned before. As electric fields decay during the frequency cycle, the polarized mobile charge carriers tend to relax back to equilibrium through thermal motion defined as its relaxation time.11 This thermal motion causes heat dissipation and is the mechanism of dielectric heating due to interfacial polarization. When the applied frequency matches the relaxation time of the mobile charge carriers, maximum dielectric heating occurs. Due to interplay of several interfaces, the relaxation behavior due to interfacial polarizations is dispersed over a wide frequency range. Identifying the frequency range of such dispersions would be advantageous in ensuring maximum heat dissipation through dielectric heating. Additionally, tuning the frequency to keep up with temperature-based changes in relaxation behavior occurring because of increased fluid mobility and phase changes in oil sands components could result in more

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ELECTRICAL PROPERTY MEASUREMENT AND DIELECTRIC HEATING THEORY When direct current (DC) is applied to a material or circuit, the only opposition it offers to the flow of current is its resistance (R). Impedance (Z) is the total opposition a material or circuit offers to the flow of an alternating current (AC) of a given frequency (f) when an alternating voltage (V) is applied. Two additional impeding mechanisms taken into account besides the normal resistance (R) of DC circuits are inductance (L) and capacitance (C). The impedance caused by these two effects is collectively referred to as reactance (X) and forms the imaginary part of complex impedance whereas resistance (R) forms the real part. Impedance (Z) possesses both magnitude and phase, unlike resistance, which has only magnitude. Associated with impedance is a phase angle (φ) indicating that the voltage (V) and current (I) get shifted in phase depending on the conductivity (σ), permittivity (ε), and permeability (μ) of the material as shown: V = Vm sin(ωt )

(1)

I = Im sin(ωt + φ)

(2)

Z=

V I

(3)

where ω = 2πf

(4)

The complex impedance (Z) can in turn be represented as a complex quantity having a real part (Z′) and an imaginary part (Z″) as shown:

Z = Z′ + jZ″ 1988

(5) DOI: 10.1021/acs.energyfuels.5b02493 Energy Fuels 2016, 30, 1987−1996

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Energy & Fuels where j=

εT* = [ε′ − jε″] −

−1

(6)

Z′2 + Z″2

tan φ =

Z″ Z′

(7)

(8)

Impedance spectroscopy done on dielectric material results in complex impedance (Z) which is a function of its resistance (R) and capacitive reactance (XC) shown as Z = Z′ + jZ″ = R − jXC

⎛ σ⎞ ″ = ε′ − j ⎜ε″ + ⎟ εT* = ε′ − jεtotal ⎝ ω⎠

(9)

(10)

ε * = ε ′ − jε ″

(11)

εT* = ε′ =

σ″ ω

(17)

C − Z″ = 2 C0 |Z| ωC0

(18)

Z′ |Z|2 ωC0

(19)

Total AC conductivity (σT*) can also be written as σT* =

Z′*2πl |Z|2 ln(b/a)

(20)

where conductivity and free space capacitance in the above equations is calculated for coaxial capacitor as C0 =

(12)

where ε′ is the polarization term called dielectric permittivity and σ″ is a Faradaic diffusion loss term. Both of these terms describe mechanisms associated with charge polarization; however, because it is impossible to distinguish between the contributions from each in a complex impedance measurement, they remain combined in the parameter defined as εeff. Similarly, the effective conductivity (σeff) represents the ability of the material to transport charge and is given by σeff = σ ′ + ωε″

1 jωZC0

″ = εtotal

As mentioned above, during impedance spectroscopy measurements, we measure the complex impedance (Z) of the sample. The effective permittivity (εeff) and effective conductivity (σeff) can be obtained from the complex impedance measurement. The effective permittivity (εeff) represents the ability of the material to store charge through polarization and can be calculated by

εeff = ε′ +

(16)

From measurements of complex impedance (Z), phase angle (φ), resistance (R), capacitive reactance (XC), and capacitance (C) obtained for a dielectric material through impedance spectroscopy measurements, we can determine its dielectric properties mentioned in the equation above as shown:

In general, polarization processes in a material can be studied by considering its electrical response in alternating electric fields using complex dielectric permittivity (ε*) or complex electrical conductivity (σ*), which are derived from macroscopic electromagnetic properties such as resistance (R) and capacitive reactance (XC). These two electrical properties are mathematically equivalent (σ* = jωε*) and can in principle describe the same polarization processes.18 These parameters are explicitly defined as

σ * = σ ′ + jσ ″

(15)

It is commonly assumed in modeling studies that diffusion loss term (σ′) is assumed to be zero and the ohmic conduction term (σ′) is equal to the frequency-independent DC conductivity (σ) of the material. All frequency dependence in the effective parameters is attributed to the real and imaginary parts of complex permittivity, including a contribution from the relaxation of bound charges at higher frequencies through the ωε″ term. The total complex permittivity (ε*T ) can then be rewritten as

The absolute impedance, |Z|, and phase angle, φ, are then expressed by

|Z | =

⎛ j⎞ ⎜ ⎟[σ ′ + jσ ″] ⎝ω⎠

ε0 ln(b/a) 2πl

(21)

where εo is permittivity of free space, l capacitor length, b the outer coaxial electrode diameter, and a diameter of the inner electrode. The absolute permittivity , |ε|, and loss tangent or dissipation factor, tan δ, are determined by

| ε| =

″ )2 ε′2 + (εtotal

(22)

″ εtotal Z′ = Z″ ε′

(23)

tan δ =

(13)

where σ′ is the ohmic conduction and ε″ is a loss due to polarization lag. Again, it is important to note that although charge transport within the material is a result of two very different mechanisms, we measure only the combined effect in the parameter defined as σeff. In developing models to describe the dielectric behavior of multicomponent materials, it is convenient to incorporate the two effective parameters into one, the total complex permittivity (εT*), defined as σ εT* = εeff − j eff (14) ω

The electrical heat dissipated during capacitive heating in a capacitor is analogous to the electric heat dissipated in a resistance equal to V2/R and is given by P = ωε″C0V 2

(24)

From this equation, the power dissipated per unit volume, Pv [W/m3], termed power density, is given as

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Using eqs 12 and (13, the expression for the total complex permittivity (εT*) can be expanded and rearranged to yield:

Pv = ωε0ε″E2

(25)

EXPERIMENTAL METHOD

The schematic diagram of capacitive heating experiment of oil sands and theoretical explanation of the heating mechanism are given in 1989

DOI: 10.1021/acs.energyfuels.5b02493 Energy Fuels 2016, 30, 1987−1996

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Figure 1. Schematic diagram of experimental setup used for carrying out frequency-tuned capacitive heating of rich grade oil sands and theoretical interpretation of heating mechanism.

Figure 2. Illustration of coaxial cylindrical test fixture and experimental setup for conducting temperature-based impedance spectroscopy of oil sands. components that could react with electric fields and give erroneous measurements. A distributed resonator is used as the electrical energy source. This resonator source (working details given in Van Neste and co-workers26) is powered by a power amplifier (ALC Wideband, 20 Hz to 800 kHz, 0−30 Vrms) and function generator (Agilent 33500B series waveform generator) and has the ability to produce high electric fields (104 V/m) while resonating at approximately 200−230 kHz frequency range when a nominal input power of 10 V is supplied to it.

Figure 1. The capacitor test cell used for placing the oil sands is cylindrical and is made of Teflon (Enflo Co.). The capacitor cell is 20 cm in length and has 6 cm outer diameter and 5 cm inner diameter. Circular disc-shaped copper electrodes 5 cm in diameter and 0.5 cm in thickness are used as parallel plates of a capacitor inside the cell between which the oil sands are placed as a dielectric medium. Fiber optic temperature sensors (T1S Probe, Neoptix Canada) are used to measure temperature during the heating process. These sensors are preferred over thermocouples becacuse they do not have any metal 1990

DOI: 10.1021/acs.energyfuels.5b02493 Energy Fuels 2016, 30, 1987−1996

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Energy & Fuels The contents of Athabasca oil sands used for the study were determined using Dean−Stark extraction method. The sample oil sands are rich grade in nature and consisted of 12% bitumen, 1.5% water, and 86% solids of which 20% are fines less than 44 μm in size, containing mainly illite clays. Clay content and type was estimated from wet sieving and X-ray diffraction methods, respectively. For each test, three repeated experiments of tuned radio frequency-based capacitive heating of oil sands were carried out. To obtain consistent and uniform packing density for each experiment, oil sands were packed by tapping layer by layer each time and filled to a height of 10 cm in the test cell between the copper electrodes. Fiber optic temperature sensors were placed axially in the test cell along the oil sands at a distance of 2 cm at the top, middle, and bottom portions to get one-dimensional spatial variation of temperature. The oil sands capacitor cell was connected to the distributed resonator as shown in Figure 1. While heating was carried out, the temperature (T) was measured spatially (z) as a function of time (t). The electrical parameters to the distributed resonator system such as current (I), voltage (V), phase (φ), and frequency (f) were measured. As temperature increased and attained saturation, radio frequency was retuned to record further increase in temperature. Temperature-based impedance spectroscopy studies were also performed to determine the complex impedance behavior of the given rich grade oil sands to estimate their relaxation frequency peaks where optimum heating could be carried out. A Gamry Potentiostat/ Galvanostat/ZRA Reference 30000 instrument is used for impedance spectroscopy. A coaxial cylindrical test fixture as shown in Figure 2 is used to house the oil sands for this study. The test fixture was made of copper electrodes having the following dimensions: 5 cm length, 3 cm hollow outer electrode diameter, 1.5 cm solid inner electrode diameter. Both ends of the coaxial test cell are covered using Teflon caps to prevent formation of fringing electric fields. A furnace (MTI Corporation KSL-1100X) is used to house the coaxial test cell during the temperature-based impedance spectroscopy studies. For these tests, the oil sands sample was also packed layer by layer between inner and outer electrode area so that a uniform packing could be obtained just as in the heating experiments. For temperaturebased studies, a test fixture was placed inside a furnace while connected to the impedance analyzer so that impedance spectroscopy was carried out at 20, 50, 80, 120, and 150 °C. For each test, three repeated experiments of impedance spectroscopy of oil sands were carried out. Measurements were taken when a steady temperature was attained in the oil sands inside the test fixture. The impedance spectroscopy was carried out by taking a total of 10-point average measurements under frequency sweep from 1 Hz to 1 MHz at a constant voltage level of 100 mV and no DC bias. Because the oil sands sample was packed very tightly and the measurements were done using a very low current, the effect of electrode impedance was considered to be negligible.

Figure 3. (a) Total complex permittivity and total AC conductivity behavior of oil sands when swept from 1 Hz to 1 MHz. (b) Dielectric ″ ), and loss tangent (tan δ) of oil constant (ε′), dielectric losses (εtotal sands as a function of frequency at room temperature (20◦C).

frequency at room temperature. The values for εT* and σT* were obtained from impedance (Z) and phase angle (φ) data using eqs 17 and 20. It can be seen that total complex permittivity (ε*T ) of the sample oil sands showed very high values of 106 at low frequency of 1 Hz as compared to low value of 10 at high frequency of 1 MHz. This confirms data obtained by earlier studies.5,6 The high value of total complex permittivity (ε*T ) at low frequencies is indicative of electrode polarization. This could have been corrected for distinguishing the bulk behavior of oil sands at these frequencies by studying the effect of changing electrode materials and/or of the sample thickness.27 We did not carry out such corrections in our analysis because the focus of our studies was to identify dispersion behavior of oil sands in the higher-frequency range between kilohertz and megahertz, at which we intended to carry out capacitive heating. In addition, previously conducted low-frequency studies on rocks have accounted for the electrode polarization effects and have attributed the bulk behavior at these frequencies to electrochemical polarizations between salt water and clays, oil and clay, or salt water and metallic minerals.5−16,18 Figure 3a also shows that the AC conductivity (σT*) increased slightly from 10−4 to 10−3 S/m when frequency was increased from 1 Hz to 1 MHz. The rise in conductivity (σ*T ) with frequency is very small and is seen in the kilohertz frequency region. This indicates that the contribution from irreversible charge displacement causing interfacial polarization is present but is small. Identifying dispersion regimes due to interfacial polarization mechanisms in the kilohertz frequency range and their dynamic behavior with changing temperature is of importance for carrying out efficient capacitive heating. For these reasons the section below discusses further the magnitude and dispersion range of losses from interfacial polarizations. Figure 3b shows the dielectric permittivity (ε′), total dielectric losses (εtotal ″ ), and loss tangent (tan δ) as a function

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RESULTS AND DISCUSSION Temperature-Based Impedance Spectroscopy. Impedance spectroscopy of oil sands was carried out in a frequency range of 1 Hz and 1 MHz and for temperatures ranging between 20 and 150 °C. These experiments were conducted as a precursor to tuned radio frequency-based capacitive heating experiments to accomplish the following objectives: (1) determine electrical property values of oil sands such as total complex permittivity (εT*), dielectric constant (ε′), total dielectric losses (εtotal ″ ), loss tangent (tan δ), and electrical conductivity (σ*T ) as a function of frequency and temperature; (2) identify frequency dispersion range of the relaxation behavior of the given oil sands and their variations with temperature in order to select an optimum radio frequency for heating experiments as well as an optimum frequency tuning pattern to continue heating. Figure 3a depicts total complex permittivity (εT*) and total AC conductivity (σT*) behavior of oil sands as a function of 1991

DOI: 10.1021/acs.energyfuels.5b02493 Energy Fuels 2016, 30, 1987−1996

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Figure 4. Loss tangent (tan δ) spectroscopy of oil sands at different temperatures.

kHz relaxation peak would prove useful in ensuring greater heat losses in oil sands. Figure 4 shows results of temperature-based changes to loss tangent (tan δ) spectroscopy for given oil sands averaged over three repeated experiments along with their error bars. As observed in results of Figure 3b, the sample oil sands showed a mean relaxation peak averaging around 65 kHz at room temperature. As temperature is increased from 20 to 150 °C, this relaxation peak is seen to vary in magnitude and shift in position. This could be attributed to several reasons such as increased ionic mobility, increased thermal motion, and phase changes of water as well as viscosity reduction of bitumen. As temperature is increased from 20 to 50 °C, a shift in the relaxation peak, as well as a reduction in the magnitude of loss tangent (tan δ), is observed. These observations could be attributed to reduction in Maxwell−Wagner polarizations between water, bitumen, and silicate minerals as temperature

of frequency at room temperature. These values were calculated using eqs 18, 19, and 23, respectively. The dielectric ″ ) showed permittivity (ε′) and total dielectric losses (εtotal very high values at low frequency, as seen for total complex ″ ) permittivity (εT*) in Figure 3a. The total dielectric losses (εtotal curve shows dispersion in the kilohertz frequency range. For the sake of identifying specific relaxation frequencies in this regime where heat losses could be maximum, loss tangent (tan δ), which is the ratio of total losses (ε″total) to total storage (ε′), is also included in Figure 3b. Interestingly, the loss tangent curve shows a relaxation peak averaging at 65 kHz for the given sample of rich grade oil sands. This relaxation frequency at 65 kHz had an average loss tangent value of 7 and could be indicative of Maxwell−Wagner polarizations at bitumen, water, and silica interfaces.7−11 The dispersion regime of relaxation peak at 65 kHz has a full width at half-maximum of 2 decades of frequency. Carrying out heating in this dispersion regime of 65 1992

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dielectric relaxation dispersion range of the given oil sands as determined from results in Figure 3b and hence is suitable for heating. Capacitive heating results of the given oil sands are summarized in Figure 5a. In the capacitive heating experiment,

increased thermal motion of water molecules and reduced bitumen viscosity. As temperature is increased from 50 to 80 °C, the relaxation peak further shifts in position toward lower frequency and also decreases in magnitude. It is known from the literature28 that bitumen starts to phase transition at 70 °C. It can be assumed that as temperature was increased to 120 °C, the water phase in oil sands converted to vapor and the viscosity of bitumen decreased further, causing lighter components to start vaporizing. Any relaxation behavior observed could be attributed to the role of bitumen interface with silicate minerals and clays. At 120 °C, it can be seen that the relaxation peak becomes widely dispersed and also reduced in magnitude from the initial case. This relaxation peak could be reasoned to be due to Maxwell−Wagner polarizations arising from charge carriers in less viscous bitumen and silica interfaces with the disappearance of water interface. At 150 °C it is observed that the relaxation peak initially at 65 kHz completely disappears and the oil sands appears to become a low loss dielectric material. At this temperature, bitumen starts breaking down into other distillable components and also further decreases in viscosity. The interface between bitumen and silicate minerals becomes more prominent in the absence of water. As temperature was increased, a variation in the relaxation processes observed could be attributed to the vaporization of water and light ends of bitumen as well as to the reduction in viscosity of bitumen, which altered the interfacial energies governing the predicted polarization processes. With these results, we can conclude that relaxation mechanisms at 65 kHz are due to interfacial polarizations which could be due to physical charge migrations toward interfaces of bitumen, salt water, and silicate minerals. The dispersion regime of relaxation process around 65 kHz was further chosen to be suitable for carrying out radio frequency heating. Because the full width at half-maximum extended to 2 decades of frequency, operating at any frequency in this regime could prove to be useful in the heating process. As temperature was increased, a shift in relaxation frequencies and a reduction in the magnitude of loss tangent (tan δ) was observed, indicating that temperature-based changes in physical properties such as phase changes in water and bitumen and reduction in viscosity of bitumen influenced the electrical polarization properties. The relaxation frequency completely disappeared at 150 °C, indicating that once the water was vaporized the oil sands behaved more like a pure dielectric. This also ensured that retuning the applied frequency according to changed relaxation times with temperature would ensure optimized and efficient heating. These interesting observations of temperature-based variations of relaxation peaks indicated changes in polarization mechanisms as fluid mobilities, phases, and interfaces changed and gave us a strong reason to carry out frequency-tuned heating of oil sands. Dielectric Relaxation-Based Tuned Radio Frequency Capacitive Heating of Oil Sands. In this study, frequencytuned capacitive heating of oil sands was done using a distributed resonator, as shown in Figure 1. The distributed resonator has the ability to produce high electric fields (104 V/ m) when nominal input voltage (10 V) is supplied.26 With the ability to produce such high electric fields, the resonator is designed to work at resonant frequency of 220 kHz without any oil sands load. When the oil sands capacitor cell load with its electrodes are connected in circuit to the resonator, as shown in Figure 1, its total system resonance shifts to 211 kHz. This frequency lies within the full width at half-maximum of the

Figure 5. Spatial and temporal temperature profile of capacitive heating of rich grade oil sands carried out in a (a) small capacitor and (b) bigger capacitor test cell.

each test was repeated three times, and their average results with error bars are plotted. Figure 5a shows the summary of oil sands temperature rise (T) as a function of heating time (t) at the top, middle, and bottom parts of the test cell. Capacitive heating of oil sands began as the distributed resonator along with the oil sands load was tuned to the system resonance frequency of 211 kHz. The input voltage (V) supplied was kept constant at 10 V throughout the heating experiment, which translated to a high electric field of 104 V/m between the oil sands capacitor electrodes. It can be seen that the temperature at the top section of the oil sands rose to a maximum of 75 °C within 1 h 15 min, at the heating rate of 1 °C per minute. A spatial variation of (T) was observed along the z-axis with maximum temperature rise at the top and minimum temperature rise at the bottom. The main reason for this spatial variation in temperature is because we are using a distributed resonator for stepping up the voltage supplied to the capacitor. In a distributed resonator, voltage amplification is distributed along the length of the resonator with maximum amplification at the top and minimum at the bottom. Therefore, when the capacitor electrodes are tapped along two points of the resonator the electrode tapped to the top experiences more voltage as compared to the electrode tapped to the bottom. This causes a nonuniform electric field in the oil sands resulting in the spatial variation in the temperature profile. Using a higher wattage power amplifier to drive the existing distributed resonator would increase the magnitude of the nonuniform electric field in the oil sands. Higher field magnitudes, though 1993

DOI: 10.1021/acs.energyfuels.5b02493 Energy Fuels 2016, 30, 1987−1996

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Figure 6. (a) Capacitive heating results of rich grade oil sands (b) Input current change as temperature changed in oil sands while carrying out capacitive heating (c) Impedance and phase change ;(d) Real and imaginary impedance ;(e) Total dielectric losses and dielectric constant; (f) Loss tangent respectively of rich grade oil sands as temperature increased from 20 to 120 °C at 200 kHz obtained from impedance spectroscopy results.

because any changes in the oil sands impedance affected the impedance of the total resonator circuit and the new resonance frequency was a function of the impedance of the total circuit as opposed to just the oil sands impedance. When frequency retuning was carried out manually, the new resonance frequency was set at the point where we observed maximum output voltage amplitude, which was 212 kHz from 211 kHz. Setting the frequency to any other value would result in reduced voltage amplitude and thereby reduced heating or discontinuation of the process. Therefore, retuning the frequency by 1000 Hz to observe a system resonance compensated for changes in oil sands relaxation frequencies as well and caused a temperature rise in the oil sands, as shown in Figure 6a. Thus, retuning the frequency helped in catching up with the changed relaxation peaks of oil sands where maximum heat dissipation occurred and hence helped in establishing that electrical heating could be carried out by tuning the radio frequency to match the dielectric relaxation changes of oil sands with temperature. This could be advantageous in terms of energy efficiency and can prove to be an apt innovation to reduce water and energy wastes associated with current water-based thermal technologies. The reasons for the increasing input current (I) as shown in Figure 6b as temperature increased during capacitive heating of oil sands can be due to results summarized in Figure 6c−f. These mechanistic correlations of temperature rise during capacitive heating are drawn from temperature-based impedance spectroscopy results of the given oil sands at 200 kHz. The increase in current (I) during capacitive heating could be attributed to the reduction in overall impedance (Z) of oil sands, as shown in Figure 6c. This could be attributed to increased motion of water phase as well as reduction in bitumen viscosity, which tends to increase the overall thermal

nonuniform in nature, would generate higher temperatures in oil sands. The higher temperatures would cause better heat transfer due to thermal conduction and lower spatial variation of temperature. Limited by the power supply, the other way to increase the electric field in the oil sands was to increase the capacitance of the oil sands test cell by varying the capacitor geometry, which was done by us. The electrodes were increased in surface area by three times and the distance between them was reduced by half (i.e., increased area/distance, implying greater capacitance, approximately 70 pF from 10 pF in theprevious case). The heating results for this configuration are shown in Figure 5b. The results show a high degree of uniformity indicating that as the capacitance increased the electric field between the electrodes increased, which caused better uniformity of heating. A higher temperature was also attained, indicating that a higher capacitance has dual advantages due to generation of greater electric field between the capacitor plates for the same input power. As temperature rose in rich grade oil sands as shown in Figure 6a, the current (I) drawn also increased and followed the temperature rise curve, becoming steady when temperature became steady as shown in Figure 6b. Because steady-state temperature was reached for the given oil sands in 1 h 15 min for the given input power, we believed that the impedance of the oil sands had changed, thereby changing its relaxation frequency as indicated from results in Figure 4. This led to the next step of frequency retuning at 1 h and 15 min during capacitive heating of oil sands. This step helped to account for the temperature-based changes in relaxation frequency of oil sands and caused further heating of it, as shown in Figure 6a. The frequency tuning from 211 to 212 kHz as shown in Figure 6a,b was not conducted exactly according to temperature-based shifts in relaxation frequencies as shown in Figure 4. This was 1994

DOI: 10.1021/acs.energyfuels.5b02493 Energy Fuels 2016, 30, 1987−1996

Article

Energy & Fuels

dielectric losses as compared to dielectric storage with temperature rise due to increased fluid mobilities and phase transitions of water and bitumen, thereby increasing thermal motion and heat losses. Because the dielectric relaxation frequency of oil sands also changed with temperature, retuning the frequency during capacitive heating resulted in increased heating of oil sands. The spatial variation of temperature rise could be avoided if a higher-wattage power amplifier was used to drive the distributed resonator. With the limited power supply, the capacitance of the oil sands test cell was increased, which resulted in more uniform heating and maximum temperature rise of 150 °C. This also proved that the ultimate goal of volumetric heat generation and hence uniform and faster heating can be attained with some modifications to the system. This method of heating could be advantageous over other methods in ensuring volumetric heat generation, more controlled heating, less dependence on pore water, reduced heat losses, and more efficient heating. This could also be integrated with current in situ SAGD infrastructure in its preheating or heating stages to reduce the use of water and improve the efficiency and cost of heating oil sands.

motion. The phase (φ) of oil sands showed to shift toward becoming more capacitor-like (i.e., decreased to the more negative angle) though the shift was not very significant as shown in Figure 6c. This could indicate that as temperature increases, the decrease in water content due to vaporization could make the oil sands more capacitor-like. The overall reduction in impedance (Z) and phase (φ) of oil sands with increasing temperature can be further explained to be due to the reduction in the real part of its impedance (Z′), as shown in Figure 6d and thereby implying increase in the total dielectric losses (εtotal ″ ) with temperature as shown in Figure 6e. The imaginary impedance (Z″), as shown in Figure 6d, being lower in value than the real impedance (Z′) showed to stay almost constant with increasing temperature, which also reflected on the dielectric constant (ε′) as shown in Figure 6e. The combined effect of the total losses (εtotal ″ ) to the total storage (ε′) as temperature increased was reflected in the loss tangent (tan δ), shown in Figure 6f, which decreased as temperature rose, indicating that the ratio of heat generated to energy stored in the given oil sands decreased as temperature rose. This also implies that for a given input power, the ability of oil sands to heat up decreases as temperature increases. In this case, frequency tuning at the given input power could be a suitable method of increasing heating, as was done in our experiment.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 780 710 7016. *E-mail: [email protected]. Phone: 780 492 2068. Fax: 780 492 2881.

CONCLUSIONS The mechanism behind the electrothermal method of capacitive heating of a given sample of rich grade oil sands was investigated. Dielectric relaxation frequency where maximum heat losses occur for the given oil sands was identified to be approximately 65 kHz with a full width at halfmaximum of 2 decades of frequency. This was attributed to interfacial or Maxwell−Wagner polarizations occurring because of ion migrations in water and bitumen toward their respective interfaces and toward grain boundaries of silicate minerals. As temperature was increased, a shift in relaxation frequencies and a reduction in the magnitude of loss tangent (tan δ) was observed, indicating that temperature-based changes in physical properties such as phase changes in water and bitumen and reduction in viscosity of bitumen influenced the electrical polarization properties. The relaxation peak completely disappeared at 150 °C, indicating that once the water was vaporized the oil sands behaved more like a pure dielectric. Carrying out capacitive heating of such oil sands until 150 °C could be useful and feasible for the oil sands industry because it is understood that bitumen’s viscosity reduces by 3−4 orders of magnitude at these temperatures, allowing it to flow. Also, the appearance of loss tangent peaks due to interfacial polarizations from the presence of water until these temperatures makes the heating process more efficient, causing more heat losses for a given input power. Carrying out capacitive heating beyond these temperatures may result in more steady heating, but more energy would have to be supplied because the oil sand’s lossiness would have decreased significantly with the disappearance of the relaxation peak. While using the distributed resonator to carry out capacitive heating, the oil sands heated at an average rate of 1 °C/min and attained a maximum temperature of 75 °C in the smaller capacitor. As temperature rose, the impedance of oil sands decreased as indicated by the increase in current drawn by the heating system. Reduction in overall impedance was dominated by reduction in the real part of impedance as compared to imaginary impedance. This implied a dominant increase in

Funding

Canada Excellence Research Chair for Oil Sands Molecular Engineering. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support given by the Canada Excellence Research Chair for Oil Sands Molecular Engineering for this research program and Institute of Oil Sands Innovation in the Department of Chemical and Materials Engineering for giving us the oil sand samples. We also thank Priyesh Dhandharia, Faheem Khan, Ankur Goswami, Zeljka Antic, Richard Hull, and Prashanthi Kovur for their helpful discussions.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.5b02493 Energy Fuels 2016, 30, 1987−1996

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DOI: 10.1021/acs.energyfuels.5b02493 Energy Fuels 2016, 30, 1987−1996