Petroleum can be extracted from tar sand and oil shale given an appropriate microwave applicator. These two materials are abundant on the North American continent and represent large reserves of low-viscosity crude petroleum. Microwave technology may provide a relatively cost-effective means of extracting the crude petroleum from these deposits of tar sand and oil shale.
The most commercially accepted method of in situ extraction of hydrocarbon from oil tar sand and oil shale is the steam flood process. It employs a combination of steam or other gaseous pressures to decrease the viscosity of the hydrocarbons and force the oil through the sand or shale to a nearby producer well. Unfortunately, this approach is costly and requires large amounts of gas as well as water. During the extraction process, a large amount of fresh water is mixed with heavy metals, sulfur, and other materials, contaminating the crude oil being collected as well as the water. The contaminated water is an environmental threat with high associated costs for proper disposal.
Microwave heating offers an attractive alternative for the oil industry compared to current oil extraction methods. Microwave heating can extract petroleum from tar sand or oil shale at a modest cost compared to present extraction methods. The approach decreases the viscosity of the tar sand or oil shale in the absence of oxygen and causes the trapped oil in the tar sands to flow much more quickly than current steam-solventbased processes. In order to implement this technique, however, an open-ended microwave oven is needed to heat the tar sand or oil shale inside the ground.
The oven should be physically open toward the heating object but electrically shielded. This can be achieved by virtue of the evanescent electromagnetic (EM) fields in a waveguide below cutoff. An evanescent waveguide connected to a resonator has been described previously by Kumar and Smith.1,2 Those researchers used the evanescent fields to measure the complex permittivity in a circular waveguide operating in a fundamental transverse electromagnetic TE111 mode. But, for heating purposes, transverse magnetic (TM) modes have been used. As shown in refs. 2-5, waveguide and cavities can be constructed for this purpose, departing from the nonseparable solution of the Helmholtz wave equation. In view of the interesting power properties of these cavities, the design and construction of a new type of TM111 mode applicator will be presented.
There are two ways to design an open-ended cavity resonator capable of producing the evanescent fields needed to heat tar sand and oil shale. The first approach involves using a metallic cavity resonator attached to an evanescent field waveguide. The second method involves a metallic resonator with partial field, using a low-loss dielectric and extension to a metallic waveguide to produce the required evanescent fields.
To explore the potential of the first approach, a new type of square waveguide resonator was constructed. The linear dimensions of the cavity resonator are represented by the line sections along OX, OY, and OZ in Fig. 1, or a, b, and d, respectively. The length of the resonator cavity is d and resonance occurs for the mnp mode, such that
d=(1/2)pλg (1)
where
λg = the waveguide wavelength of the associated TMmn wave in the waveguide whose cross section is identical with that of a resonator, and p = the elemental cells of the corresponding TMmn standing waves in a waveguide.
The waveguide wavelength, λg, is related to the free-space wavelength, λo, and cutoff wavelength, λc, by:
(1/λo)2 = (1/λg)2 + (1λc)2 (2)
It follows from Eqs. 1 and 2 that the cavity's resonant free-space wavelength is given by Eq. 3:
(1/λo)2 =
22 + (1/λc)2 (3)
The cutoff wavelength, λc, of a TMmn wave in a rectangular waveguide resonator whose cross section has linear dimensions a and b is
(1/λc)2 = 2 + 2 (4)
From eqs. 3 and 4, the resonant free-space wavelength, λmnp, of the TMmnpmode is given by Eq. 5:
(1/λmnp)2 = 2 + 2 +
2 (5)
The resonant frequency is:
fmnp = (c/2)2 + (n/b)2 + (p/d)2> (6) where
c = the velocity of the EM wave. When the cavity resonator is a cube (a = b = c = d), wavelength λ111 can be calculated using:
1/λ111 = 0.5/a (A)
For the TM111 mode, for a square cavity of 6.2 cm on a side, the cavity will resonate at a wavelength of 12.25 cm. To produce evanescent waves, a smaller waveguide is connected to the end of the cavity resonator. The size of the evanescent waveguide is 4.5 cm. A ceramic insert (Fig. 2) was placed between the main cavity and the evanescent waveguide for two reasons: to stop any liquid or vapor from entering the main cavity from the evanescent waveguide, and to improve the impedance match between the cavity and the evanescent waveguide.
For the second case, of a metallic resonator producing a partial field that is coupled via low-loss dielectric and extension to a metallic waveguide to produce evanescent fields, the main cavity resonator is partially filled with a low loss dielectric (Fig. 3) and physically open in the bottom as described by Kumar.3, 5, 6 Recently, this type of cavity was used by Sinclair et al.7 for microwave curing. The operation is based on a design that creates a resonance within the dielectric slab, with the fields being below cutoff in the open-ended, air-filled waveguide. Optimizing the design of the air-filled evanescent section allows radiation losses from the open end to be minimized. Heat is generated by the virtue of the evanescent fields in the below cutoff open-ended, air-filled waveguide. For extraction purposes, tar sand or oil shale is placed in the evanescent waveguide.
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Page Title
The power absorbed per unit volume due to the dielectric loss within the tar sand or oil shale is given by
P = fE E dV (7)
where
f = a function,
e''= the imaginary part of the complex permittivity of the tar sand, and
V = the volume of the tar sand/oil shale in the evanescent section, and
E = a conjugate.
From eq. 7, it is possible to conclude that the target samples with higher e'' value would heat more efficiently in an open microwave oven. The power absorbed by the tar sand generates heat due to the vibration of molecules at the resonant microwave frequency. The increase in temperature ( T) of the tar sand can be calculated by Eq. 8:
T/ t) = (5.563 x 10-11)fρCpE2e' 'M (8)
where
Cp = the specific heat of the tar sand in J/kg/C,
ρ = the density of the tar sands in kg/m,
E = the electric field intensity in V/ m3,
f = the frequency in Hz, and
M = the quality of the cavity, which is used to quantify the performance of the open-ended oven.
The quality of the cavity, M, is defined as the logarithmic ratio of the relative intensity of the electric field maxima within the dielectric slab in the main cavity at the maximum electric field at the target location within the cutoff volume of the cavity (evanescent region):
M = 20 log10
where
| Em| evanescent = the maximum magnitude of the electric field within the evanescent region and
| Em| dielectric = the maximum electric field within the dielectric inside the main cavity region.
Figure 4 is a plot of the change in the quality of the cavity versus variations in the thickness of the ceramic insert. The optimum dimensions of the ceramic inset were chosen from Fig. 4 for maximum cavity quality. The quality of the cavity was very poor without a ceramic insert. The impedance matching improves due to the ceramic insert between the cavity and the evanescent section.
From ref. 2, it was found that an open oven operates best in a TM resonance mode. In the present case, the longitudinal electric field is dominant for heating. The electric field is attenuated within the evanescent section of the open-ended oven, with an exponential power decay. The electric field is not propagating even the permittivity is very high in the case of wet tar sand. However, heating take place up to a few centimeters of the open-ended oven. The wet tar sand is placed 1 mm below the inserted ceramic piece inside the evanescent section of the oven. The pressure on the inserted ceramic material will be higher due to expansion of the fluid tar sand and therefore, a pressure release valve has been used in the laboratory just below the ceramic insert.
The operating frequency of the open-ended oven is 2.45 GHz with an average net power of 15 W. The table shows the measured temperature with time for both types of open-ended ovens. In this experiment, the temperature was recorded by a thermal imaging camera, which can be used without any interference to the oven. The temperature of the dielectric loaded resonator oven increases to +120C in 30 s and the temperature of the metallic resonator oven increases to +100C in 30 s, as shown in the table. The temperature increase is higher in case of a dielectric loaded resonator oven, because the impedance matching is better than for the case of the metallic resonator oven. However, an increase in temperature more than +100C is not required to heat tar sand. In operation, liquid tar sand is pumped to the ground and untreated tar sand is pushed to the oven.
An experiment was performed with oil shale and it was found that the increase in temperature was similar to that for tar sand. However, the water content of the two materials provides different heating patterns over time.
In summary, an open-ended microwave oven has been presented for heating oil tar sand and oil shale at 2.45 GHz. A ceramic piece between the cavity and evanescent oven improves the oven's impedance match and prevents particles in the cavity from entering the evanescent section.
REFERENCES
1. A. Kumar and D. G. Smith, "The measurement of the complex permittivity of sheet materials at microwave frequencies using evanescent waveguide technique," IEEE Transactions on Instrumentation and Measurements, Vol. 25, No. 2, 1976, pp. 190-193.
2. A. Kumar, "A new type of microwave applicator operating in the TM11p mode," presented at the Microwave Power Symposium, May 1974, Marquette University, Milwaukee, WI.
3. A. Kumar, "Dielectric filled open ended microwave cavity to dry paper at 2.45 GHz," Research Report, EETC Coventry, 1981.
4. A. Kumar, "Dielectric-loaded rectangular waveguide applicator," International Journal of Electronics, Vol. 57, No. 2, 1984, pp. 299-303.
5. M. Giroux, Microwave applicator research at Ecole Polytechnique, University of Montreal, Montreal, Quebec, Canada, Opening remarks at Microwave Power Symposium as Session Chairman, Session A2, Milwaukee, WI, May 1974.
6. M. Groux, "Microwave oven for microelectronics packaging," Ecole Polytechnique, University of Montreal, Montreal, Canada, 1983.
7. K. I. Sinclair, G. Goussetis, L. P. Y. Desmulliez, A. J. Sangster, T. Tilford, C. Bailey, and A.K. Parrott, "Optimization of an open-ended microwave oven for microelectronics packaging, IEEE Transactions on Instrumentation and Measurements, Vol. 56, No. 11, 2008, pp. 2635-2641.