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 TM_mn wave in the waveguide whose cross section is identical with that of a resonator, and p = the elemental cells of the corresponding TM_mn 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 = [p/(2d)²2 + (1/λ_c)² (3)

The cutoff wavelength, λ_c, of a TMmn wave in a rectangular waveguide resonator whose cross section has linear dimensions a and b is

(1/λc)² = [m/(2a)]² + [n/(2b)]² (4)

From eqs. 3 and 4, the resonant free-space wavelength, λ_mnp, of the TM_mnpmode is given by Eq. 5:

(1/λ_mnp)² = [m/(2a)]² + [n/(2b)]² + [p/( 2d)]² (5)

The resonant frequency is:

f_mnp = (c/2)[(m/a)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[(3)]/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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