Hazardous or toxic waste destruction or containment – Containment – Solidification – vitrification – or cementation
Reexamination Certificate
2001-12-12
2003-07-29
Wong, Don (Department: 2821)
Hazardous or toxic waste destruction or containment
Containment
Solidification, vitrification, or cementation
C588S253000, C315S111210
Reexamination Certificate
active
06600084
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a microwave plasma generator useful to decompose particularly organic halides, such as freon and trichloromethane, in which the structure of a discharge tube for microwave plasma generation is improved, and a method and system for decomposing organic halides such as freon and trichloromethane.
Large amounts of organic halides such as freon, trichloromethane, and halon, containing fluorine, chlorine, bromine, and the like in molecules, are used in a variety of applications such as refrigerants, solvents, and fire extinguishers. However, these compounds are highly volatile, so many of them are emitted, without being processed, to environments such as the air, soil, and water. Such emitted gases are found to have large influence on environments, e.g., produce carcinogens, destroy the ozonosphere, and cause global warming. Hence, it is necessary to make these organic halides harmless from the viewpoint of environmental protection.
Conventionally reported organic halide processing methods primarily use decomposition reaction at high temperatures. These processing methods are roughly classified into the incineration method and the plasma method.
In the incineration method, volatile organic halides are incinerated together with common wastes such as resins. To burn them in a waste incinerator, corrosion resistance must be improved as a measure against hydrogen chloride which is a strong acid and corrosive. Also, since a combustion temperature different from those for combustion of common resins is set, these organic halides must be separately burned. Furthermore, the exhaust amounts of, e.g., hydrogen chloride and dioxine are strictly restricted. As a consequence, the processing amount cannot be thoughtlessly increased to avoid a primary cause of, e.g., an unstable combustion temperature.
As the plasma method, a method of decomposing a volatile organic halide into carbon dioxide, hydrogen chloride, and hydrogen fluoride by reacting the halide with water vapor in a plasma is known as a dedicated decomposing apparatus for, e.g., freon.
Jpn. Pat. Appln. KOKAI Publication No. 3-222298 has disclosed a microwave plasma trace element analyzer including a double-structure discharge tube shown in
FIG. 17. A
double-structure discharge tube
201
shown in
FIG. 17
is made of quartz and comprises a cylindrical outer tube
202
and a cylindrical inner tube
203
. A reaction gas supply pipe
204
is connected to the outer tube
202
in the direction of tangent of this outer tube
202
. A tapered large-diameter cylindrical portion
205
of thick section is formed at the end portion of the inner tube
203
, thereby narrowing the space between the cylindrical portion
205
of this inner tube
203
and the outer tube
202
. The discharge tube
201
is inserted through a microwave cavity
207
having a metal conductor
206
and a rectangular waveguide
208
.
A reaction gas
209
is supplied through the supply pipe
204
to the space between the outer tube
202
and the inner tube
203
in the discharge tube
201
, where the gap between an antenna and a cavity end plate, or between inner and outer conductors, in the cavity
207
is positioned. This reaction gas
209
is injected from the exit end of this space. At the same time, a carrier gas
210
is supplied into the inner tube
203
and blown off from an injection opening
211
open at the end portion of the inner tube
103
, thereby generating a plasma
212
by discharge. In elemental analysis, for example, nitrogen gas or the like is used as the reaction gas
209
, and Ar or He is used as the carrier gas
210
.
The plasma is ignited by using a Tesla coil placed outside and near the discharge tube
201
at the microwave cavity exit or an ignition coil
214
connected to an ignition power supply
213
such as a neon transformer.
When Ar gas is used as the carrier gas, however, the running cost increases.
If the flow rate of the carrier gas is increased, the generation of a plasma becomes unstable.
Also, since the injection opening
211
of the inner tube
203
for injecting the carrier gas is very small, this portion is readily damaged by, e.g., melting by a plasma.
Additionally, the gas flow path between the inner tube
203
and the outer tube
202
is narrowed by the tapered large-diameter cylindrical portion
205
of thick section formed at the end portion of the inner tube
203
. Although this raises the reaction gas injection rate, the plasma
212
flows backward in the space around the cylindrical portion
205
. Consequently, the cylindrical portion
205
of the inner tube
203
melts and breaks.
On the other hand, to prevent the contact of a plasma with the wall surface of the discharge tube
201
, the gas flow in the discharge tube
201
is usually given a swirling flow effect in the gap between the outer tube
202
and the inner tube
203
by connecting the gas supply pipe
204
to the outer tube
202
in the direction of tangent of this outer tube
202
. However, this effect is lost because the thick cylindrical portion
205
of the inner tube
203
narrows the flow path formed by the gap. Consequently, even a slight change in the plasma state causes nonuniform discharge or melts the discharge tube
201
.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a microwave plasma generator capable of stably and efficiently generating a plasma.
It is another object of the present invention to provide a method of decomposing an organic halide capable of efficiently decomposing volatile organic halides such as freon and trichloromethane.
It is still another object of the present invention to provide an organic halide decomposing system capable of efficiently decomposing volatile organic halides such as freon and trichloromethane.
A microwave plasma generator according to the present invention comprises:
a rectangular waveguide having a hole to transmit a microwave;
a coaxial microwave cavity connected to the rectangular waveguide by communicating with the hole; and
a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends through the cavity so as to be coaxial with a central axis of the cavity,
wherein the discharge tube has a double-tube structure including outer and inner tubes,
a sectional area of an annular gap formed between the outer and inner tubes is constant over an entire length of the inner tube, and
an end portion of the inner tube has a gas injection opening having the same diameter as an internal cavity of the inner tube.
Another microwave plasma generator according to the present invention comprises:
a rectangular waveguide having a hole to transmit a microwave;
a cylindrical microwave resonance cavity which is connected to the rectangular waveguide by communicating with the hole and is placed such that a central axis of the resonance cavity aligns with the direction of electric field in the rectangular waveguide; and
a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends through the resonance cavity so as to be coaxial with the central axis of the cavity,
wherein the discharge tube has a double-tube structure including outer and inner tubes,
a sectional area of an annular gap formed between the outer and inner tubes is constant over an entire length of the inner tube, and
an end portion of the inner tube has a gas injection opening having the same diameter as an internal cavity of the inner tube.
In a method of decomposing an organic halide according to the present invention, which decomposes an organic halide in a plasma by using a microwave plasma generator comprising a rectangular waveguide having a hole to transmit a microwave, a coaxial microwave cavity connected to the rectangular waveguide by communicating with the hole, and a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends through the cavity so as to be coaxial with a central
Danno Minoru
Ikeda Tetsuya
Cooper & Dunham LLP
Lee Wilson
Mitsubishi Heay Industries, Ltd.
White John P.
Wong Don
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