Gas separation: processes – With control responsive to sensed condition – Pressure sensed
Reexamination Certificate
2001-12-26
2002-11-19
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
With control responsive to sensed condition
Pressure sensed
C095S023000, C095S045000, C095S047000, C096S004000, C096S009000
Reexamination Certificate
active
06482251
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a perfluoro compound separation recovery method to separate and recover a perfluoro compound with a membrane from diluted mixed gas in which the perfluoro compound is diluted, and a perfluoro compound separation recovery system therefor.
BACKGROUND ART
Perfluoro compound (simply mentioned as “PFC” hereinafter) gas such as CF
4
, C
2
F
6
, C
3
F
8
, C
4
F
8
, C
4
F
10
, CHF
3
, SF
6
and NF
3
has been conventionally diluted with a large amount of N
2
gas or the like for use such as in etching, cleaning a chamber and the like in the manufacturing process of semiconductors. Also, in the field of electrical machines, SF
6
is similarly used as insulating gas.
However, exhaust gas (diluted mixed gas) contains unreacted (undecomposed) PFC gas after use, causing such environmental problems as ozone layer depletion when the gas is released into the atmosphere.
Thus, PFC gas in exhaust gas has been conventionally recovered. A PFC separation recovery system shown in
FIG. 8
is used to recover the gas. In the order of exhaust gas treatment, a pre-treatnent unit
1
, a blower
2
, a first tank
3
, a compressor
4
, a second tank
5
, a heat exchanger
6
, a PFC separation unit
51
and a PFC recovery unit
28
are connected in series in the PFC separation recovery system.
More specifically, a first membrane
7
, a second membrane
8
and a third membrane
9
ate arranged in series in the PFC separation unit
51
. Each membrane
7
,
8
,
9
has a tendency to permeate more diluent gas such as N
2
gas than PFC gas. The materials of each membrane
7
,
8
,
9
include polysulfone, polyetherimide, polypropylene, cellulose acetate, polymethylpentane, amorphous copolymer having 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxysol as a radical, polyvinyltrimethylsilane, polyimide, polyamide, polyaramid or ethyl cellulose polymer, and the like. The materials mentioned above are formed in a hollow fiber, spiral, roll or flat sheet shape.
Moreover, spaces in the front and rear of each membrane
7
,
8
,
9
are a permeation side and a non-perineation side, respectively. The permeation side of the first membrane
7
is linked to the atmosphere through a needle valve
52
, releasing permeated gas into the atmosphere. The non-permeation side of the first membrane
7
is connected to the second membrane
8
. Also, the permeation side of the second membrane
8
is connected to return the gas to the first tank
3
, and the non-permeation side of the second membrane
8
is connected to the third membrane
9
. Furthermore, the permeation side of the third membrane
9
is connected to return the gas to the first tank
3
, and the non-permeation side of the third membrane
9
is connected to the PFC recovery unit
28
through a first control valve
10
.
Furthermore, a concentration analyzer
53
is provided to sample a portion of a third non-permeable gas flow between the first control valve
10
and the PFC recovery unit
28
so as to measure the concentration of PFC gas. A control unit
53
a
is also provided to control the opening of the first control valve
10
in response to the output of the concentration analyzer
53
. In other words, when the concentration analyzer
53
takes in a portion of the third non-permeable gas flow and it is determined that the concentration of PFC gas is lower than a predetermined level, the control unit
53
a
reduces the opening of the first control valve
10
, thus increasing a third permeable gas flow. Accordingly, diluent gas that was flowing to the non-permeation side flows to the permeation side, and recovery concentration (concentration of recovered PFC gas) improves. Thus, PFC gas of high concentration may be recovered by controlling the flow amount of the third non-permeable gas flow. The portion of the third non-permeable gas flow that was taken into the concentration analyzer
53
returns to the first tank
3
after the measurement of concentration.
A third tank
11
, a booster pump
12
and a recovery container
13
are arranged in series in the PFC recovery unit
28
.
The separation recovery of PFC gas is carried out as follows by the PFC separation recovery system. In other words, exhaust gas is first passed through the pre-treatment unit
1
, removing an acid-based portion, a particle portion and the like which provide negative effects on the compressor
4
and each membrane
7
,
8
,
9
, with a wet or dry scrubber, a filter and the like. Subsequently, the treated gas flow is stored in the first tank
3
by the blower
2
. Then, the compressor
4
raises the pressure of a gas flow from the first tank
3
to store it in the second tank
5
. The heat exchanger
6
raises the temperature of exhaust gas stored in the second tank
5
. Then, the gas contacts the first membrane
7
, thus separating the gas into a first permeable gas flow rich in diluent gas and a first non-permeable gas flow rich in PFC gas. Additionally, the first permeable gas flow is released into the atmosphere by opening and closing the needle valve
52
. At the same time, the first non-permeable gas flow contacts the second membrane
8
, thereby further separating the gas into a second permeable gas flow rich in diluent gas and a second non-permeable gas flow rich in PFC gas. Then, the second permeable gas flow is returned to the first tank
3
. At the same time, the second non-permeable gas flow contacts the third membrane
9
, thus separating the gas into a third permeable gas flow having diluent gas at high concentration and a third non-permeable gas flow having PFC gas at high concentration. Then, the third permeable gas flow is returned to the first tank
3
. At the same time, the third non-permeable gas flow is stored into the third tank
11
through the first control valve
10
. Subsequently, a gas flow from the third tank
11
is stored in the recovery container
13
by a booster pump
12
. PFC gas can be separated and recovered as noted above.
In the above-noted PFC separation recovery method, pressure (internal pressure) is controlled at a constant level at the upstream side of the blower
2
and the upstream side (first tank
3
) and the downstream side (second tank
5
) of the compressor
4
for its stable operation.
In other words, a first pressure sensor
14
is provided before a section where exhaust gas flows into the PFC separation recovery system (the upstream side of the pre-treatment unit
1
) in order to indicate the pressure of exhaust gas. Also, an inverter
2
a
is provided to the blower
2
to control its operation. At the same time, the inverter
2
a
is provided with a control unit
14
a
to send signals to itself in response to the output of the first pressure sensor
14
. The inverter
2
a
starts the blower
2
to provide the output of the first pressure sensor
14
at a predetermined fixed level. Pressure at the upstream side of the blower
2
is controlled at a constant level thereby. The first pressure sensor
14
indicates strength values for sucking exhaust gas. The flow of exhaust gas is made stable and smooth by controlling the values so as to be constant.
As described above, gas flows into the first tank
3
after the blower
2
, the permeation side of the second membrane
8
, the permeation side of the third Membrane
9
and the concentration analyzer
53
, and then flows out to the compressor
4
. Additionally, there provided are a first line
15
to take in outside air, and a second line
16
returning from the second tank
5
. At the same time, a second pressure sensor
17
is provided to indicate the internal pressure of the first tank
3
. Furthermore, a second control valve
18
is Provided at the first line
15
, and a control unit
17
a
is provided to the second control valve
18
to control its opening in response to the output of the second pressure sensor
17
. The second tank
5
is also provided with a third pressure sensor
19
to show its internal pressure. Furthermore, the second line
16
is provided with a third control valve
20
, and a control unit
19
a
is provided to the third co
Kawasaki Fumio
Matsubayashi Rosuke
Daido Air Products Electronics, Inc.
Spitzer Robert H.
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