U.S. patent number RE35,725 [Application Number 08/636,029] was granted by the patent office on 1998-02-10 for method and apparatus for removing residual hydrogen from a purified gas.
This patent grant is currently assigned to SAES Pure Gas, Inc.. Invention is credited to Charles H. Applegarth, Jeffrey L. Briesacher, D'Arcy H. Lorimer.
United States Patent |
RE35,725 |
Briesacher , et al. |
February 10, 1998 |
**Please see images for:
( Certificate of Correction ) ** |
Method and apparatus for removing residual hydrogen from a purified
gas
Abstract
The invention relates generally to a gas purification system for
the purification of noble gasses and nitrogen. An improved method
of purification generally includes the following steps: (a) heating
an impure gas; (b) contacting the impure gas with an impurity
sorbing material to produce a purified gas; (c) cooling the
purified gas to a temperature less than about 100.degree. C.; and
(d) contacting the purified gas with a hydrogen sorbing gas to
remove residual hydrogen. The system includes an improved heat
exchange apparatus for cooling the purified gas and a low
temperature hydrogen sorption apparatus.
Inventors: |
Briesacher; Jeffrey L. (San
Luis Obispo, CA), Applegarth; Charles H. (San Luis Obispo,
CA), Lorimer; D'Arcy H. (Pismo Beach, CA) |
Assignee: |
SAES Pure Gas, Inc. (San Luis
Obispo, CA)
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Family
ID: |
25338408 |
Appl.
No.: |
08/636,029 |
Filed: |
April 22, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
862399 |
Apr 2, 1992 |
05238469 |
Aug 24, 1993 |
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Current U.S.
Class: |
95/115; 165/171;
95/116; 96/108; 96/126 |
Current CPC
Class: |
B01D
53/04 (20130101); B01D 53/0407 (20130101); B01D
2256/10 (20130101); B01D 2256/18 (20130101); B01D
2257/102 (20130101); B01D 2257/108 (20130101); B01D
2257/11 (20130101); B01D 2258/0216 (20130101); B01D
2259/402 (20130101); B01D 2259/41 (20130101) |
Current International
Class: |
B01D
53/04 (20060101); B01D 053/04 () |
Field of
Search: |
;95/115,116 ;96/108,126
;55/267-269 ;165/171 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0365490A1 |
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Apr 1990 |
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EP |
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51-28594 |
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Mar 1974 |
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JP |
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60-030919 |
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Feb 1985 |
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JP |
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2177080 |
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Jan 1987 |
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GB |
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2177079 |
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Jan 1987 |
|
GB |
|
Other References
Albrecht. H. et al., "Application of SAES and HWT Gas Purifiers for
the Removal of Impurities From Helium-Hydrogen Gas Mixtures,"
Journal of the Less-Common Metals 1991, pp. 1157-1167. .
Doni, F. et al., "Hydrogen Isotope Sorption and Recovery by a
Nonevaporable Getter Combined with a Chemical Compressor Material,"
J. Vac. Sci. Technol. A 4 (6) Nov./Dec. 1986, pp. 2447-2451. .
George M.A. et al., "Removal of Simple Hydrocarbons From a Rare Gas
by a 70%Zr-25%V-5% Fe Getter," Gas Separation & Purification
1989, vol. 3, Jun., pp. 50-55..
|
Primary Examiner: Spitzer; Robert
Attorney, Agent or Firm: Hickman Beyer & Weaver
Claims
What is claimed is:
1. A method for removing residual hydrogen from a purified gas
comprising the steps of:
(a) heating an impure gas;
(b) contacting the heated impure gas with an impurity sorbing
material to produce a purified gas having trace amounts of residual
hydrogen;
(c) cooling the purified gas to a temperature less than about
100.degree. C.; and
(d) contacting the cooled purified gas with a hydrogen sorbing
material to at least partially remove said residual hydrogen.
2. A method as recited in claim 1 wherein the impure gas is heated
to more than about 200.degree. C.
3. A method as recited in claim 2 wherein the purified gas is
cooled to less than about 60.degree. C.
4. A method as recited in claim 3 wherein the purified gas is
cooled to less than about 40.degree. C.
5. A method as recited in claim 1 wherein said impure gas includes
at least a component which is selected from the group consisting
essentially of nitrogen, helium, neon, argon, krypton, xenon, and
radon.
6. A method as recited in claim 1 wherein said impurity sorbing
material includes a non-evaporative getter material selected from
the group consisting essentially of zirconium-vanadium-iron alloys
and zirconium-iron alloys.
7. A method as recited in claim 1 wherein said hydrogen sorbing
material includes a material selected from the group consisting
essentially of zirconium, palladium, platinum, rhodium, ruthenium,
nickel, titanium, and alloys thereof.
8. A method as recited in claim 7 wherein said hydrogen sorbing
material includes a non-evaporative getter material selected from
the group consisting essentially of zirconium-vanadium-iron alloys
and zirconium-iron alloys.
9. A method as recited in claim 1 wherein said step of cooling said
purified gas comprises flowing said purified gas through at least
one conduit, wherein said conduit has a substantially constant wall
temperature which is less than about 100.degree. C.
10. A gas purification system comprising:
(a) gas inlet means for impure gas;
(b) heating means coupled to said gas inlet means for heating said
impure gas to a temperature greater than about 200.degree. C.;
(c) purifier means coupled to said heating means, said purifier
means including a getter material capable of removing impurities
from said impure gas to produce a purified gas;
(d) cooling means coupled to said purifier means for cooling said
purified gas to a temperature less than about 100.degree. C.;
(e) hydrogen sorbing means coupled to said cooling means which
includes a hydrogen sorbing material to remove residual hydrogen
from said purified gas; and
(f) a gas outlet for said purified gas.
11. A gas purification system as recited in claim 10 wherein said
impure gas is heated to about 300.degree.-450.degree. C. by said
heating means and said purified gas is cooled to less than about
60.degree. C. by said cooling means.
12. A gas purification system as recited in claim 10 wherein said
getter material comprises a non-evaporative getter alloy selected
from the group consisting essentially of zirconium-vanadium-iron
alloys and zirconium-iron alloys.
13. A gas purification system as recited in claim 10 wherein said
hydrogen sorbing material is selected from the group consisting
essentially of zirconium, palladium, platinum, rhodium, ruthenium,
nickel, titanium, and alloys thereof.
14. A gas purification system as recited in claim 10 wherein said
hydrogen sorbing material comprises a non-evaporative getter alloy
selected from the group consisting essentially of
zirconium-vanadium-iron alloys and zirconium-iron alloys.
15. A gas purification system as recited in claim 10 wherein said
cooling means comprises:
(a) a first manifold, including a first plurality of integral
connector nipples;
(b) a second manifold, including a second plurality of integral
connector nipples aligned with said connector nipples of said first
manifold;
(c) a plurality of straight tubes, each tube extending between a
first connector nipple on said first manifold and a second
connector nipple on said second manifold, each of said straight
tubes being welded in place at the first manifold and at the second
manifold.
16. A gas purification system as recited in claim 15 wherein said
cooling means further comprises heat sink means contacting said
plurality of straight tubes to maintain the temperature of said
tubes at a substantially constant temperature along their
lengths.
17. A gas purification system as recited in claim 16 wherein said
heat sink means comprises a first plate contacting said plurality
of tubes on a first side thereof and a second plate contacting said
plurality of tubes on second side thereof.
18. A gas purification system as recited in claim 16 further
comprising means for removing heat energy from said heat sink
means.
19. A heat exchange apparatus comprising:
(a) a first monoblock manifold formed from a single piece of
material and including a first plurality of integral connector
nipples such that interior surfaces of said first plurality of
connector nipples do not include a joint between a plurality of
pieces;
(b) a second monoblock manifold formed from a single piece of
material and including a second plurality of integral connector
nipples aligned with said connector nipples of said first manifold
such that interior surfaces of said second plurality of connector
nipples do not include a joint between a plurality of pieces;
(c) a plurality of straight tubes, each tube extending between a
first connector nipple on said first manifold and a second
connector nipple on said second manifold, each of said straight
tubes being welded in place at the first manifold and at the second
manifold.
20. A heat exchange apparatus as recited in claim 19 further
comprising heat sink means contacting said plurality of straight
tubes to maintain the temperature of said tubes at a substantially
constant temperature along their lengths.
21. A gas purification system as recited in claim 20 wherein said
heat sink means comprises a first plate contacting said plurality
of tubes on a first side thereof and a second plate contacting said
plurality of tubes on a second side thereof.
22. A gas purification system as recited in claim 21 further
comprising means for removing heat energy at least one of said
first plate and said second plate. .Iadd.
23. A gas purification system comprising:
a) gas inlet means for impure gas;
b) heating means having a heater inlet and a heater outlet, said
heater inlet being coupled to said gas inlet, said heating means
being for pre-heating said impure gas to a temperature greater than
about 200.degree. C.;
c) purifier means having a purifier inlet and a purifier outlet,
said purifier inlet being coupled to said heater outlet, said
purifier means including a getter material housed within a purifier
vessel, said getter material being capable of removing impurities
from said impure gas to produce a purified gas;
d) cooling means having a cooling means inlet and a cooling means
outlet, said cooling means inlet being coupled to said purifier
outlet by a first length of tubing such that said cooling means can
pre-cool said purified gas to a pre-cooled temperature less than
about 100.degree. C., said cooling means being separate from said
purifier means;
e) hydrogen sorbing means having a hydrogen sorber inlet and a
hydrogen sorber outlet, said hydrogen sorber inlet being coupled to
said cooling means outlet by a second length of tubing, said
hydrogen sorbing means including a hydrogen sorbing material housed
within a hydrogen sorber vessel which is separate from said
purifier vessel, said hydrogen sorbing means being operative to
remove residual hydrogen from said purified gas; and
f) a gas outlet coupled to said hydrogen sorber outlet for
releasing said purified gas. .Iaddend..Iadd.24. A gas purification
system as recited in claim 23 wherein said impure gas is pre-heated
to about 300.degree.-450.degree. C. by said heating means and said
purified gas is pre-cooled to less than about 60.degree. C. by said
cooling means. .Iaddend..Iadd.25. A gas purification system as
recited in claim 23 wherein said getter material comprises a
non-evaporative getter alloy selected from the group consisting
essentially of zirconium-vanadium-iron
alloys and zirconium-iron alloys. .Iaddend..Iadd.26. A gas
purification system as recited in claim 23 wherein said hydrogen
sorbing material is selected from the group consisting essentially
of zirconium, palladium, platinum, rhodium, ruthenium, nickel,
titanium, and alloys thereof. .Iaddend..Iadd.27. A gas purification
system as recited in claim 26 wherein said hydrogen sorbing
material comprises a non-evaporative getter alloy selected from the
group consisting essentially of zirconium-vanadium-iron alloys and
zirconium-iron alloys. .Iaddend..Iadd.28. A gas purification system
as recited in claim 23 wherein said cooling means comprises:
a) a first manifold, including a first plurality of integral
connector nipples;
b) a second manifold, including a second plurality of integral
connector nipples aligned with said connector nipples of said first
manifold;
c) a plurality of straight tubes, each tube extending between a
first connector nipple on said first manifold and a second
connector nipple on said second manifold, each of said straight
tubes being welded in place at the first manifold and at the second
manifold. .Iaddend..Iadd.29. A gas purification system as recited
in claim 25 wherein said cooling means further comprises heat sink
means contacting said plurality of straight tubes to maintain the
temperature of said tubes at a substantially constant temperature
along their lengths. .Iaddend..Iadd.30. A gas purification system
as recited in claim 25 wherein said heat sink means comprises a
first plate contacting said plurality of tubes on a first side
thereof and a second plate contacting said plurality of tubes on a
second side thereof. .Iaddend..Iadd.31. A method for producing a
semiconductor device under a high purity atmosphere, comprising the
steps of:
a) heating an impure gas;
b) contacting the heated impure gas with impurity absorbing
material to produce a purified gas having trace amounts of residual
hydrogen;
c) cooling the purified gas to a temperature less than about
100.degree. C.;
d) contacting the cooled purified gas with a hydrogen sorbing
material to at least partially remove said residual hydrogen from
said purified gas;
e) introducing said purified gas after said at least partial
removal of said residual hydrogen to a semiconductor wafer
processing chamber; and
f) processing a semiconductor wafer in the processing chamber to
produce at
least one semiconductor device. .Iaddend..Iadd.32. The method as
recited in claim 31, wherein said impure gas is heated to a
temperature greater than about 200.degree. C. .Iaddend..Iadd.33.
The method as recited in claim 31, wherein said purified gas is
cooled to a temperature of less than about 60.degree. C.
.Iaddend..Iadd.34. The method as recited in claim 33, wherein said
purified gas is cooled to a temperature of less than about
40.degree. C. .Iaddend..Iadd.35. The method as recited in claim 31,
wherein said step of cooling said purified gas further comprises
flowing said purified gas through at least one conduit having a
wall temperature which is less than about 100.degree. C.
.Iaddend..Iadd.36. A method as recited in claim 31 wherein said gas
includes at least a component which is selected from the group
consisting essentially of nitrogen, helium, neon, argon, krypton,
xenon, and radon. .Iaddend..Iadd.37. A method as recited in claim
33 wherein said impurity sorbing material includes a
non-evaporative getter material selected from the group consisting
essentially of zirconium-vanadium-iron alloys and zirconium-iron
alloys. .Iaddend..Iadd.38. A method as recited in claim 33 wherein
said hydrogen sorbing material includes a material selected from
the group consisting essentially of zirconium, palladium, platinum,
rhodium, ruthenium, nickel, titanium, and alloys thereof.
.Iaddend..Iadd.39. A method as recited in claim 38 wherein said
hydrogen sorbing material includes a non-evaporative getter
material selected from the group consisting essentially of
zirconium-vanadium-iron alloys and zirconium-iron alloys.
.Iaddend..Iadd. . The method of claim 31, wherein said step of
processing includes subjecting said semiconductor wafer to at least
one process selected from the group consisting of chemical vapor
deposition and plasma etching. .Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to gas purification, and more
particularly to methods and apparatus for removing impurities from
noble gasses, nitrogen, and other non-reactive gasses.
In the semiconductor manufacturing industry pure gasses are used in
a variety of manufacturing processes, such as chemical vapor
deposition (CVD), plasma etch, etc. The purity of the gas used in
the manufacturing process becomes more critical as the feature
width of integrated circuits decreases. For example, a decade ago
future widths in the range of 3 to 5 microns were standard.
Currently, integrated circuits having feature widths of 0.5 to 0.7
microns are in production. With smaller feature widths, even a very
low level of contaminants can damage an integrated circuit, thereby
destroying its functionality or degrading its performance. Typical
contemporary process specifications require process gasses to have
less than 10 parts per billion (ppb) of contaminants, and
preferably less than 1 ppb of contaminants.
One type of gas purifier utilizes hot getter materials for the
removal of impurities from noble gasses and nitrogen. The getter
materials are encased in stainless steel containers which are
typically heated to a temperature in the range of 300.degree. to
450.degree. C. Unfortunately, stainless steel outgasses a
significant mount of hydrogen at temperatures above approximately
200.degree. C. In the past when process specifications allowed 100
ppb of hydrogen in a purified process gas this was not a major
problem. However, with contemporary process specifications, the
hydrogen outgassed from hot stainless steel surfaces has become a
significant problem.
After a process gas passes through a getter-type gas purifier, it
is cooled in a heat exchanger. Absent such cooling, the hot gas
would pick up more hydrogen from the stainless steel tubing
connecting the gas purifier to the semiconductor processing
equipment and could also possibly damage valves and gas panels of
the equipment. Prior art heat exchange units used in conjunction
with semiconductor manufacturing equipment include a single heat
exchange coil made from stainless steel tubing. Typically, the coil
is immersed in a container through which a coolant liquid, such as
water, is pumped.
An advantage of coiled heat exchangers is that they are relatively
non-contaminating because the stainless steel coils have relatively
few welds (e.g. typically only one at each end) which might leak or
generate particles. Predictably, coiled heat exchangers also have
some disadvantages. One of the main disadvantages of coiled heat
exchangers is that the act of coiling the stainless steel tubing
can compromise the electropolish finish on the interior surface of
the tube, thereby increasing the amount of particulates produced by
the tube itself. Another disadvantage of these prior art heat
exchangers is that the long length of tubing can create a
substantial back-pressure to the gas purifier unit.
Another type of heat exchanger, which is not known to have been
used in the semiconductor processing industry with purified gases,
is the parallel tube heat exchanger. One type of parallel tube heat
exchanger utilizes preformed T-connectors which are welded together
to form inlet and outlet manifolds. A number of parallel, straight
tubes are coupled between the two manifolds. Another type of
parallel tube heat exchanger utilizes a pulled-T system. In such a
system, holes are punched through from the inner surface of a
manifold, and a short connecting nipple is welded to the manifold
at each punched hole. Often, a reduction fitting is welded to each
nipple to provide the appropriate diameter for the straight tubing.
With the above-described types of parallel tube heat exchanger the
number of welds are in the range of 4n+2 to 6n+2 to connect n tubes
between two manifolds.
The large number of welds required of prior art parallel tube heat
exchangers are more likely to be contaminating than the relatively
few (typically two) welds required of coiled heat exchangers.
Because of the contamination problem with parallel tube heat
exchangers of the prior art, they have been thought to be poor
choices for use with gas purification systems in the semiconductor
manufacturing field.
Nonetheless, parallel tube heat exchangers have certain advantages
over coiled tube heat exchangers. For one, parallel tube heat
exchangers exhibit much less back-pressure for a given heat removal
capacity than coiled tube heat exchangers. The parallel tube heat
exchangers also tend to exhibit less of a temperature gradient
along their relatively short lengths, thereby reducing the surface
area of the tubes exhibiting temperatures greater than about
200.degree. C. where hydrogen outgassing becomes significant.
SUMMARY OF THE INVENTION
A method for removing residual hydrogen from a purified gas in
accordance with the present invention comprises the steps of: (a)
heating an impure gas; (b) contacting the impure gas with an
impurity sorbing material to produce a purified gas; (c) cooling
the purified gas to a temperature less than about 100.degree. C.;
and (d) contacting the purified gas with a hydrogen sorbing
material to remove residual hydrogen. Since hydrogen outgassing by
hot stainless steel mostly occurs at temperatures greater than
about 200.degree. C., the removal of residual hydrogen at
temperatures less than about 100.degree. C. has been discovered to
be surprisingly effective.
The apparatus of the present invention includes a heater assembly
for heating an impure gas, a purifier assembly which purifies the
heated gas, a heat exchange assembly which cools the purified gas,
and a hydrogen sorbing assembly which removes residual hydrogen
from the cooled, purified gas. The low temperature hydrogen sorbing
unit effectively removes residual hydrogen without generating
additional hydrogen due to the stainless steel outgassing
phenomenon.
The heat exchange assembly is of a multiple, parallel,
stainless-steel cooling tube design which minimizes contamination
and maximizes heat dissipation. The heat exchange assembly is
provided with a heat sink which maintains the cooling tubes at a
relatively low and relatively constant temperature to minimize the
generation of hydrogen by the aforementioned stainless steel
outgassing phenomenon. Monoblock manifolds are used in the heat
exchange assembly to minimize particulate and gas
contamination.
These and other advantages of the present invention will become
apparent to those skilled in the art upon a reading of the
following specification of the invention and a study of the several
figures of the drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of a gas purification system in accordance
with the present invention;
FIG. 2 is a perspective view of a heat exchange assembly in
accordance with the present invention;
FIG. 2a is a perspective view of an alternate embodiment for a heat
exchange assembly in accordance with the present invention;
FIG. 3 is a cross-sectional view taken along line 3--3 of FIG.
2;
FIG. 4 is a cross-sectional view taken along line 4--4 of FIG.
2;
FIG. 5 is a cross-sectional view taken along line 5--5 of FIG. 2;
and
FIG. 6 is a cross-sectional view of a hydrogen sorbing assembly in
accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As illustrated in FIG. 1, a gas purification system 10 in
accordance with the present invention includes an inlet 12, a mass
flow meter 14, a solenoid activated valve 16, a heater assembly 18,
a purifier assembly 20, a heat exchange assembly 22, a hydrogen
sorbing assembly 24, a solenoid activated valve 26, and an outlet
28. The system 10 also includes a number of bypass valves 30, 32,
and 34, an auxiliary inlet 36, an auxiliary outlet 38, and a check
valve 40.
The interior surfaces of the gas purification system which may be
contacted by a gas should be clean and well-polished. For example,
the interior surfaces of all of the connecting tubing, the valves,
and the various gas handling assemblies should be made of polished,
non-reactive metal having a sufficiently close grain to minimize
gas adsorption. The desirable degree of smoothness of the polished
metal surface should be between 5 and 25 microinch Ra. Appropriate
metals which can provide such surfaces include stainless steel and
industry-standard alloys such as hastelloy, incoloy, and monnel,
which are available from a variety of sources. Such metals are
typically electropolished to provide the proper surface
specifications, as is well known to those skilled in the art.
A source of impure gas is coupled to gas inlet 12. By impure gas,
it is meant that the gas typically includes impurities in the range
of 0.1-10 parts per million (ppm). Therefore, these gasses actually
quite pure for many applications, but may be considered impure for
the ultra-clean processes required for semiconductor manufacturing.
Examples of such impure gasses include nitrogen or one of the noble
gasses as provided by commercial gas companies such as Air Products
Corporation of Allenstown, Pa., Airco Corporation of Murry Hill,
N.J., and Air Liquide of Chicago, III. Mass flow meter 14 is
coupled to inlet 12 by a length of tubing 42 and is used to
accurately monitor the amount of gas flowing through the system
10.
Valve 16 is coupled to mass flow meter 14 by a length of tubing 44
and is normally open during the operation of gas purification 10.
The heater assembly 18 is coupled to the valve 16 by a length of
tubing 46 and is operative to heat the gas prior to its
introduction into the purifier assembly 20. The heater assembly
includes a chamber 48, and an electric heater cartridge 50 coupled
to an electrical power source (not shown) for heating the chamber
48. The temperature to which the gas is heated is generally greater
than 200.degree. C., and is preferably greater than 300.degree. C.,
e.g. 300.degree.-450.degree. C. The operational efficiency of the
preferred purifier assembly 20 is greatly enhanced by preheating
the gas to these temperatures.
The purifier assembly 20 is coupled to the outlet of the heater
assembly 18 by a length of tubing 52. The purifier assembly 20
includes a getter vessel 54 and an electric heater cartridge 56
coupled to the aforementioned electrical power source. The heater
coil 56 is used to maintain the gas at a high operational
temperature, i.e. greater than 200.degree. C., and is preferably
greater than 300.degree. C. With some loss in efficiency, the
heater assembly 18 can be eliminated and the gas can be heated to
operational temperatures within the getter vessel 54 alone.
A getter vessel 54 preferably includes a generally cylindrical
housing provided with welded gas-tight caps at both ends. One cap
is provided with an inlet coupled to tubing 52, and the other cap
is provided with an outlet coupled to a length of tubing 58. Within
the getter vessel is a gas sorbing material (not shown) which is
preferably a high-temperature getter material. As heated gas passes
over the surface of the getter material impurities are removed from
the gas as they are chemically sorbed into its surface. The gas
released from the getter vessel 54 is therefore purified, and
typically has less than 1-10 parts per billion (ppb) of contaminant
gasses, other than residual hydrogen. Unfortunately, the hot
stainless steel surfaces of the heater assembly 18, purifier
assembly 20, and tubing 52 typically creates at least 10-25 ppb of
hydrogen.
The non-evaporable getter materials useful in the present invention
are characterized by a sorptive capacity for active gases. Examples
of suitable metals for non-evaporable getters include, among
others, zirconium, vanadium, iron. As disclosed in U.S. Pat. Nos.
3,203,901; 3,926,832; 4,071,335; 4,269,624; 4,306,887; 4,312,669;
4,405,487; and 4,907,948 (the disclosures of each of which is
incorporated herein by reference) the manufacture and use of
zirconium alloy getter materials are well known to those skilled in
the art. The specific getter material used is chosen based upon the
temperature range available, and upon the specific gas which is to
be purified.
For example, if a noble gas is to be purified, the getter material
can be an alloy of Zr--V--Fe. Such alloys generally have a weight
composition such that the percentage of weight of the three
elements when plotted on a ternary composition diagram lie with a
polygon having as its corners defined by (a) 75% Zr--20% V--5% Fe;
(b) 45% Zr--20% V--35% Fe; (c) 45% Zr--50% V--5% Fe. Such getter
materials are described in U.S. Pat. No. 4,312,669. Preferably, a
ternary alloy having a weight of Zr (70 wt. %)--V (24.6 wt. %)--Fe
(5.4 wt. %) is used. Such alloys are available commercially in
various forms as St707 Getter Alloy (SAES GETTERS S.p.A., Milan,
Italy).
A suitable getter material for both noble gasses and nitrogen is an
alloy of Zr--Fe. Such alloys are known to the art, and preferably
consists of from 15 to 30% by weight of Fe, and from 70 to 85% by
weight Zr (as described by U.S. Pat. No. 4,306,887). An especially
preferred getter material is an alloy of 84 wt. % zirconium and 16
wt. % aluminum. Such alloys are available commercially in various
forms as St101 Getter Alloy (SAES GETTERS S.p.A., Milan,
Italy).
The physical embodiment of the getter material is not critical. A
variety of getter material forms are known to the art or are
commercially available. For example, the getter material can
consist of a multiplicity of small pellets. Alternatively, the
getter material may be provided in powdered form, or as a surface
coating on a ceramic substrate.
Heat exchange assembly 22 is coupled to an outlet of getter vessel
54 by a length of tubing 58. The purpose of heat exchanger 22 is to
cool the purified gas exiting the getter vessel to a temperature
which is less than about 100.degree. C. The construction and
operation of the heat exchanger 22 will be discussed in greater
detail with reference to FIGS. 2-5.
In FIG. 2, heat exchange assembly 22 includes a number of parallel,
stainless-steel tubes 60 extending between an inlet manifold 62 and
an outlet manifold 64. A heat sink assembly 66 is clamped around
the tubes and comprises a first plate 68 and a second plate 70. The
second plate 70 is provided with a conduit 72 through which a
coolant fluid, such as water, can be circulated to remove heat from
the heat sink assembly 66.
An alternate embodiment for a heat exchange assembly 22a is
illustrated in FIG. 2a. This alternate embodiment also has a number
of parallel tubes 60 extending between an inlet manifold 62 and an
outlet manifold 64. However, the heat sink assembly 66a of the heat
exchange assembly is air cooled rather than water cooled. The heat
sink assembly 66a includes a first plate 68 similar in construction
to the first plate 68 of heat exchange assembly 22, and also
includes a second plate 70a and a number of fins 74 extending
outwardly from the second plate 70a. A blower assembly (not shown)
can be used in conjunction with the heat exchange assembly 22a to
increase the rate of heat removal from the heat sink assembly 66a
by moving air over the surface of fins 74.
FIG. 3 is a cross-sectional view of the outlet manifold 64 as taken
along line 3--3 of FIG. 2. The inlet manifold 62 is of similar
construction. The outlet manifold 64 is machined from a single
piece of semiconductor grade stainless steel bar stock and, as
such, is referred to herein as a "monoblock" manifold. The
construction of the monoblock manifold is as follows. A section of
semiconductor grade stainless steel rectangular bar stock is gun
drilled to form a central bore 76. A number of nipples 78
corresponding to the number of cooling tubes 60 (nine in the
present example) are then machined out of the bar stock. The
interior surfaces of the manifold 64 are smoothed and then
electropolished to minimize rough surfaces which might generate or
accumulate potentially damaging particulates. Ends of cooling tubes
60 are placed over the nipples 78 and are securely welded to the
manifold 62 by welds 80. The welds 80 should be gas-tight.
In FIG. 4, one of the cooling tubes 60 is shown extending between
the inlet manifold 62 and the outlet manifold 64. The heat sink
assembly 66 is located substantially centrally between the two
manifolds. As mentioned previously, the heat sink assembly includes
a first plate 68 which contacts a first side (i.e. 180.degree. or
1/2 the external surfaces) of the tubes 60 and a second plate 70
which contacts a second side (i.e. the remainder of the external
surfaces) of the tubes 60. As seen in the cross-section of FIG. 5,
first plate 68 is provided with a number of machined grooves 82
receptive to the lower halves of the tubes 60 and second plate 70
is provided with a number of machined grooves 84 receptive to the
upper halves of the tubes 60. This arrangement ensures good thermal
conductivity between the cooling tubes 60 and the heat sink
assembly 66. The heat sink assembly 66 is preferably made from a
material having good heat capacity and thermal conductivity, such
as aluminum or copper.
The heat exchange assembly 22 of the present invention has several
important benefits. The multiplicity of cooling tubes 60 reduces
the back pressure of the heat exchange assembly 22. Furthermore,
the total length of the parallel tubes 60 is less than the total
length of tubing in prior art single coiled tube systems for the
same cooling capacity. By minimizing the interior surface area of
the tubes 60 for a given cooling capacity, the potential for the
production of hydrogen and water contamination by the heat exchange
assembly 22 itself is reduced. Furthermore, the use of straight
tubing 60 minimizes the production and accumulation of particulate
contamination within the heat exchange assembly 22. The use of bent
tubing is generally undesirable in high purity applications.
The heat sink assembly 66 also plays an important roll in reducing
hydrogen contamination by the heat exchange assembly 22. The
combination of large heat capacity and high thermal conductivity
ensures that the tubing 60 which is in contact with the heat sink
assembly 66 is at a relatively constant and relatively low
temperature. For example, in the present embodiment the portions of
tubes 60 which are in contact with heat sink assembly 66 have a
substantially constant temperature of less than about 60.degree. C.
At this temperature, essentially no hydrogen is released from the
interior walls of the tubes 60. In contrast, with prior art heat
exchange units, there is a large temperature differential between
the inlet side of the heat exchange tube and the outlet side of the
heat exchange tube. Portions of the heat exchange tube which are
above 200.degree. C. will generate appreciable amounts of
hydrogen.
FIG. 6 illustrates a hydrogen sorbing assembly 24 in accordance
with the present invention. The assembly 24 includes a
substantially cylindrical sheath 86 and a pair of end caps 88 and
90. The sheath 86 and end caps 88 and 90 are preferably made from
an electropolished 316L stainless steel. End cap 88 is provided
with an inlet 92 which is coupled to the outlet manifold 64 of the
heat exchange assembly 22 by a length of tubing 94 (see FIG. 1).
End cap 90 is provided with an outlet 96 which is coupled to a
length of tubing 98 (see FIG. 1). Within the body of hydrogen
sorbing assembly 24 is a hydrogen sorbing material 100, which will
be discussed in greater detail subsequently. The hydrogen sorbing
material 100 is packed into one or more chambers within the
hydrogen sorbing assembly 24 and is held in place by a pair of
stainless steel screens 102 and 104.
The hydrogen sorbing material 100 is characterized as being a
non-evaporative getter which is effective to remove hydrogen at
temperatures below about 100.degree. C., more generally less than
about 60.degree. C. Preferably, the hydrogen sorbing material is
used at temperatures ranging from ambient temperatures to about
40.degree. C.
Hydrogen sorbing materials useful in the present invention are
characterized by a sorptive capacity for hydrogen gas. Examples of
suitable hydrogen getters include, among others, zirconium,
titanium, hafnium, uranium, thorium, vanadium, tungsten, tantalum,
niobium, and alloys of these metals. A preferred subclass of
hydrogen getter materials useful in the present invention are the
zirconium alloys.
For example, the getter material can be an alloy of Zr--V--Fe for
for both noble gasses and nitrogen. Such alloys generally have a
weight composition such that the percentage of weight of the three
elements when plotted on a ternary composition diagram lie with a
polygon having as its corners defined by (a) 75% Zr--20% V--5% Fe;
(b) 45% Zr--20% V--35% Fe; (c) 45% Zr--50% V--5% Fe. Such getter
materials are described in U.S. Pat. No. 4,312,669. Preferably, a
ternary alloy having a weight of Zr (70 wt. %)--V (24.6 wt. %)--Fe
(5.4 wt. %) is used. Such alloys are available commercially in
various forms as St707 Getter Alloy (SAES GETTERS S.p.A., Milan,
Italy).
If the gas to be purified is a noble gas or nitrogen, the getter
material can be an alloy of Zr--Fe. Such alloys are known to the
art, and preferably consists of from 15 to 30% by weight of Fe, and
from 70 to 85% by weight Zr (described by U.S. Pat. No. 4,306,887).
An especially preferred getter material is an alloy of 84 wt. %
zirconium and 16 wt. % aluminum. Such alloys are available
commercially in various forms as St101 Getter Alloy (SAES GETTERS
S.p.A., Milan, Italy).
In a preferred embodiment, the hydrogen getter is a ternary alloy
having a weight of Zr (70 wt. %)--V (24.6 wt. %)--Fe (5.4 wt. %) is
used. Such alloys are available commercially in various forms as
St707 Getter Alloy (SAES GETTERS S.p.A., Milan, Italy). Once
sorbed, oxygen, carbon and nitrogen atoms cannot be released again
by this hydrogen getter material, even at its melting point
(1400.degree. C..+-.100.degree. C.), due to the formation of strong
chemical bonds with the alloy atoms. Hydrogen atoms, however,
diffuse into the hydrogen sorbing material bulk more quickly than
other atoms and distribute almost uniformly within the bulk. Due to
well-known equilibrium phenomena, hydrogen sorption capacity of
these materials actually increases with decreasing temperatures.
However, because of the relatively weak forces which bind these
atom to the hydrogen sorbing material alloy, some of the hydrogen
sorbed at a low temperature or at room temperature can be released
at high temperatures. In other words, the hydrogen sorption is
reversible and depends upon the hydrogen sorbing material
temperature. This means that the hydrogen sorbing assembly 24 can
be regenerated by heating the assembly above about 200.degree. C.
to drive off the hydrogen trapped in the hydrogen getter material.
Hydrogen isotopes (D.sub.2, T.sub.2) are sorbed in the same way as
normal hydrogen and present the same behavior.
Since the hydrogen sorbing assembly 24 and the gas flowing through
it are at temperatures below about 100.degree. C., the stainless
steel of its construction produces almost no contaminating
hydrogen. Nonetheless, residual hydrogen in the purified gas
flowing through the hydrogen sorbing assembly 24 is effectively
scavenged from the gas, because the hydrogen sorbing material 100
is effective even at room temperatures. The hydrogen sorbing
assembly 24 of the present invention has been found to reduce
hydrogen in purified nitrogen gas from 10-25 ppb to less than 1
ppb.
Referring again to FIG. 1, during normal operation valves 30, 32,
and 34 are closed and valves 16 and 26 are open. Impure gas flows
into inlet 12 and is pre-heated in heater assembly 18 before being
purified in purifier assembly 20. The purified gas is then cooled
in heat exchange assembly 22 and passed through hydrogen sorbing
assembly 24 to remove residual hydrogen. The purified gas then
flows through valve 26 and a length of tubing 106 and out outlet 28
for use in a manufacturing process.
Valve 30 can be used to bypass the active devices within the gas
purification system. To accomplish the bypass, valves 16 and 26 are
closed, and valve 30 is opened to permit gas flowing into inlet to
flow through tubing 108, valve 30, tubing 110 and out of outlet 28.
Since valves 16, 26, and 30 are all solenoid activated, this bypass
function can be automatically initiated if the system senses a
dangerous or sub-standard operating condition.
Manual valve 32 can be used to couple auxiliary inlet 36 to the
tube 44. Manual valve 34 can be used to couple the tube 106 to
auxiliary outlet 38 through a check valve 40. The check valve 40 is
used to prevent potential contaminants from entering the system
through the auxiliary outlet 38. The valves 32 and 34 are opened to
permit the system 10 to be purged. For example, a gas source can be
connected to inlet 36, valves 32, 16, 26 and 34 can be opened, and
valve 30 can be closed to run a purging gas through the various
sub-assemblies of the gas purification system 10.
To summarize, the method for removing residual hydrogen from a
purified gas in accordance with the present invention includes the
steps of: (a) heating an impure gas; (b) contacting the impure gas
with an impurity sorbing material to produce a purified gas; (c)
cooling the purified gas to a temperature less than about
100.degree. C.; and (d) contacting the purified gas with a hydrogen
sorbing gas to remove residual hydrogen.
While this invention has been described in terms of several
preferred embodiments, it is contemplated that alterations,
modifications and permutations thereof will become apparent to
those skilled in the art upon a reading of the specification and
study of the drawings. For example, heat exchange assembly 22 can
be omitted if the pipe connecting the purifier assembly 20 to the
hydrogen sorbing assembly 24 allows the gas to cool sufficiently
due to ambient losses. In other words, the gas cooling means does
not have to be a formal heat exchange unit but can be any type of
connection or conduit which allows the gas to cool sufficiently for
the hydrogen sorbing assembly 24.
It is therefore intended that the following appended claims include
all such alterations, modifications and permutations as fall within
the true spirit and scope of the present invention.
* * * * *