Dielectric heating using spread-spectrum energy

Details Dielectric heating using spread-spectrum energy Dielectric heating using spread-spectrum energy

1999-03-19

2001-01-02

Walberg, Teresa (Department: 3742)

Electric heating

Capacitive dielectric heating

With power supply system

C219S770000

Utility Patent

active

06169278

ABSTRACT:

BACKGROUND
This invention relates to soil decontamination using dielectric heating. Specifically, the invention involves spread-spectrum radio energy to achieve dielectric heating.
It has been known to use radio-frequency energy (“RF energy”) to heat dielectric substances. For example, RF energy has been used to heat thermosetting resins during molding processes. See, for example, U.S. patent application Ser. Nos. 08/906,414 and 08/906,061 which are incorporated by reference. Also, RF energy may be used to heat undisturbed soil for purposes of decontaminating the soil. Using RF energy to heat soil may be referred to as “dielectric heating” of the soil. Dielectric heating of soil is known to remove many types of organic and hydrocarbon compounds from the soil. Steam “purging” of the soil's water content and pyrolization (heat induced molecular breakdown) of organic compounds are two mechanisms attributable to the heating which effect the decontamination.
Steam purging occurs when the water content of the soil is boiled off by the heating, and the contaminants are carried out of the soil by the steam. Steam purging requires only that the soil be heated to the boiling point of water. Pyrolization is the breakdown of molecular bonds in complex hydrocarbon contaminants, and, depending on the contaminant in question, requires the soil to be raised to a temperature from 100 to 250 degrees Celsius.
In many cases, in situ (“in place”) heating of soil is preferred over conventional methods which disturb the soil. Methods which require the soil to be disturbed, such as moving the soil to an incinerator and then replacing it, are expensive, time consuming, and may encounter additional regulatory hurdles, when compared to in situ heating. However, known methods of in situ soil heating require expensive, high-powered RF signal generators and impedance matching networks.
For example, one known attempt to dielectrically heat soil in situ applies RF energy to the soil. Three parallel, linear rows of electrodes are driven into the soil. The center row is connected through an impedance matching network to a High Frequency (“HF”) transmitter. The two outer rows are a ground counterpoise to the center row. It is believed that HF transmitters of more than 100 kW are used. The high-power transmitter, and associated impedance matching network, are expensive, and reduce the economic advantages of in situ decontamination. Similarly, radio frequency molding often requires high power transmitters and impedance matching networks.
In addition to the expense of high-powered transmitters, regulations relating to emissions of radio frequency signals limit the use of high-powered transmitters to a limited number of predetermined frequencies, eg., 6.78 MHz, 13.56 MHz, or 27.15 MHz. The use of other frequencies is not feasible because the high power levels required often lead to leakage of RF energy in excess of levels permitted by the FCC. The limitation to predetermined frequencies is disadvantageous because the predetermined frequencies may not be optimal, or even suitable, for heating certain materials or a particular thickness of a material. For example, the dielectric constant of materials such as soil or a thermosetting resin varies with the frequency that is applied to the subject material. Also, for any given frequency, various subject materials are likely to have various different dielectric constants. For example, the impedance of an electrode-and-soil network, as known in the prior art, has been observed to vary depending on the frequency to which the RF source is tuned and with the actual soil itself. Thus, the impedance matching network, coupling the RF source to the electrode-and-soil network, is essential to efficient power transmission to the soil. Moreover, the limited number of frequencies available for dielectric heating of soil may make it difficult to adequately match the RF source to the electrode-and-soil network.
SUMMARY
The present invention provides for dielectric heating of subject material by connecting an RF source, modulated to produce a spread-spectrum signal to a radio frequency applicator for the subject material, without requiring an impedance matching network. The carrier frequency of the RF source may be selected to match the impedance of the subject material and applicator network.


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Application Ser. No. 08/906,061, Filed Aug. 5, 1997, Entitled “Efficient Dielectrically Heatable Compound And Method” R. H. Sternowski.
Application Ser. No. 08/906,414, Filed Aug. 5, 1997, Entitled “Efficient Dielectric Heater” R. H. Sternowski.

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