Hazardous or toxic waste destruction or containment – Containment – Solidification – vitrification – or cementation
Reexamination Certificate
2000-12-08
2003-08-12
Bos, Steven (Department: 1754)
Hazardous or toxic waste destruction or containment
Containment
Solidification, vitrification, or cementation
C204S169000, C204S157150
Reexamination Certificate
active
06605750
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a technique of decomposing organohalogen compounds by using plasma and, more particularly, to a method of and an apparatus for generating plasma by means of microwaves.
BACKGROUND OF THE INVENTION
Organohalogen compounds such as CFCs (flon), trichloromethane and halon that include such elements as fluorine, chlorine and bromine in the molecule thereof are used in large quantities in broad applications including refrigerants, solvents and fire extinguishers, and have very high practical values for industrial applications.
However, these compounds have high volatility and may have adverse effects on the environment such as generation of carcinogens and destruction of the ozone layer, when released to the atmosphere, soil, or waters without proper treatment. Thus, the organohalogen compounds must be processed for neutralization in order to protect the environment.
Known methods of the prior art for processing the organohalogen compounds mostly utilize the reaction of decomposition at high temperatures. These treatment processes are roughly divided into burning process and plasma process. In the burning process, organohalogen compounds are burned together with ordinary wastes such as resin.
In the plasma process, the organohalogen compounds are caused to react with steam in plasma thereby to decompose into carbon dioxide, hydrogen chloride and hydrogen fluoride. A method of generating high-frequency plasma by using a high-frequency power source to decompose the organohalogen compounds is proposed in Japanese Patent Application No. Sho 63-284098 (Japanese Patent No. 2134675). A method of generating arc plasma by using a DC power source and applying the plasma for decomposition is proposed in Japanese Patent Application No. Hei 7-3089452 and Japanese Patent Application No. Hei 7-332580.
As to an apparatus that utilizes plasma to decompose organohalogen compounds, an apparatus which utilizes microwave to generate plasma has recently been developed. This decomposition apparatus includes a waste gas processing tank that contains an alkali solution, a reactor tube disposed to open at the lower end thereof in the alkali solution, a circular waveguide that extends vertically above the reactor tube, a discharge tube disposed in the circular waveguide while the lower end thereof communicates with the reactor tube, a rectangular waveguide connected at a position near one end thereof to the circular waveguide and a microwave oscillator mounted on the other end of the rectangular waveguide.
In this decomposition apparatus, CFC gas and steam are supplied to the discharge tube and microwave generated by the microwave oscillator is transmitted through the rectangular waveguide to the circular waveguide. Electric field generated by the microwave inside the circular waveguide causes arc discharge, so that the CFC gas is decomposed by the thermal plasma in the reactor tube. Product gas generated in the decomposition reaction is neutralized while passing through the alkali solution. Other gas containing carbon dioxide is released through an exhaust duct.
Neutralization of the decomposition product gas generates neutralization products such as calcium chloride and calcium fluoride which precipitate in the form of slurry in the alkali solution. The alkali solution is returned to the waste gas processing tank to be reused. In this decomposition apparatus, a solid-liquid separator is installed in the waste gas processing tank where the neutralization product and the alkali solution are separated. The neutralization product separated from the alkaline solution is disposed of as waste.
The decomposition apparatus described above consumes a large quantity of water for cooling the reactor tube and other purposes, as well as the alkali solution. Thus it is desired to reduce the water consumption through improvement of the solid-liquid separation efficiency of the neutralization product and the alkali solution and by decreasing the amount of residual alkali solution included in the neutralization product to be discarded as waste.
While the decomposition apparatus described above has a mixer installed in the waste gas processing tank to mix the alkali solution, the mixer is of the ordinary propeller type that has twisted blades. This mixer can effectively mix the alkali solution, but hardly contributes to the breakdown of bubbles (dividing into fine bubbles) of the decomposition product gas released into the alkali solution, and is therefore not capable of increasing the contact area between the bubbles and the alkali solution. This leads to longer time required for the neutralization reaction and/or larger capacity required of the neutralization tank (waste gas processing tank). It may also result in the generation of acidic gas due to insufficient neutralization.
Moreover, in the decomposition apparatus described above, in case the product gas generated through decomposition of CFC gas is neutralized with the alkali solution over an extended period of time, capability of the alkali solution to neutralize decreases resulting in insufficient neutralization. When the rotation of a motor of the bubble breakdown means decreases the speed thereof or stops altogether for some reason, bubbles in the alkali solution cannot be broken sufficiently, thus resulting in insufficient neutralization. In this case, continuation of the decomposition may cause an acidic gas exhausted out.
The reactor of the apparatus described above includes the reactor tube located above and a blow tube connected to the bottom end of the reactor tube, so that the decomposition product gas (strongly acidic gas) of high temperature (around 1000° C., for instance) is blown into the alkali solution through this blow tube. For this reason, a metallic pipe having high heat resistance is normally used for the blow tube. However, when the blow tube is made of a metal, since the metallic pipe is not resistant to acid and alkali despite the high heat resistance, it must be frequently replaced thus leading to higher costs in the equipment and maintenance. When consideration is given only to the resistance to acid and alkali, use of a resin tube may be worthwhile but weak heat resistance thereof has been inhibiting the practical application.
DISCLOSURE OF THE INVENTION
The present invention has been made to solve the problems described above, and a first object thereof is to provide a method of reducing the water consumption in the organohalogen compound decomposition process and effectively and reliably feed CFC gas and other gases.
In order to achieve the object described above, first aspect of the present invention provides a method of decomposing organohalogen compounds by irradiating a gas containing organohalogen compound with microwave to generate thermal plasma, and reacting the organohalogen compounds with the steam in the thermal plasma, said method includes: neutralizing a product gas generated through the decomposition reaction of the organohalogen compounds and steam with an alkali solution, leaving to stand for a predetermined period of time to precipitate a neutralization product generated in this neutralization reaction in the alkali solution, and removing the neutralization product.
With this decomposition process, since leaving the alkali solution, wherein the decomposition product gas has been neutralized, to stand for a predetermined period of time causes the neutralization product to precipitate in the alkali solution, the neutralization product can be easily taken out and the solid-liquid separation efficiency of the neutralization product and the alkali solution can be improved. This makes it possible to improve the yield of recovering the alkali solution from the neutralization product for the purpose of effective reuse, while the quantity of the neutralization product to be disposed of is decreased.
In the decomposition process described above, after stopping the decomposing reaction between the organohalogen compounds and steam, the decomposing reaction may be restarted after remov
Bessho Masahiro
Hamada Takayoshi
Hattori Toshio
Tsubaki Yasuhiro
Bos Steven
Kuhar Anthony
Mitsubishi Heavy Industries Ltd.
Scully Scott Murphy & Presser
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