Note: Descriptions are shown in the official language in which they were submitted.
CA 02337123 2001-O1-11
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HY~RpGEN GENERATORS
FIELD OF THE INVENTION
This invention relates to hydragen-selective membrane walls for connecting the
high
pressure chamber to the low pressure chamber of apparatus for the preparation
of pure
hydrogen at elevated temperatures.
BACKGROUND OF THE INVENTION
to
The prior art is replete with descriptions of palladium-bearing alloy
membranes used to
prepare pure hydrogen from gaseous hydrogen-containing mixtures by permeation
through
a palladium bearing metallic membrane under pressure and at elevated
temperatures.
Reference is made to the co-pending patent application Serial No. 08/719,385
of common
~s assignee which is incorporated herein by reference, wherein an important
advantage of a
60% palladium-40% copper alloy membrane with respect to temperature-cycling
and
minimal swelling in hydrogen is disclosed. This bears also on the publication
by J. Shu, B.P.
Grandjean, A. van Neste and S. Kaliaguine, entitled "Catalytic Palladium based
Membrane
Reactors: A Review", The Canadian Jouma! of Chemical Engineering, Vol. 69,
(October,
~~0 1991) herein also incorporated by reference, wherein this particular
palladium-copper alloy is
described as showing a sharp maximum at 40 wt% Cu.
Accordingly, for the purpose of the present invention, we use such palladium
copper alloys
with copper contents sufficiently near the 40% by weight optimum, (i.e. 36-42%
Cu), in which
~~5 narrow range more than two thirds of the maximum flux is retained. Such
alloys are termed
"Pd/10% Cu" in this specification and the appended claims.
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As shown, for example, in the above reference co-pending application, Pd alloy
membranes
can be in the firm of thin flat lfoils or small diameter tubes (e.g. 2mm
O.D.). Foils of the
Pd/40% Cu membrane are readily available in thicknesses of 0.001 to 0.0025
inches,
whereas special expensive techniques are required to make such Pd/40% Cu tubes
with
wall thicknesses of no less than 0.0025 inch.
As also disclosed in the above-referenced co-pending application, a palladium
alloy foil can
be sandwiched between gaskets, and the edge area of the sandwich can be
pressed onto a
metallic frame. To so produce a leak-tight two-chamber apparatus, a uniform
weighty and
~o costly edge pressure is required to at least balance the pressure in the
high pressure
chamber. It is thus importance to replace, in a pure hydrogen generator, the
gasket seal
with a pressure-tight seal, such as a weld. Welding is preferred over brazing
or soldering to
avoid contamination of the foil by the extraneous metals of the latter.
t5 Diffusion welding employs temperatures that range from 50 to 75% of the
melting point
["Procedure Development and Practice Considerations for Diffusion Welding", by
S.B.
Dunkerton, "ASM Handbook", vol. 6, p. 883, ASM lntemational (1993)]. Diffusion
welding of
copper to a different metal or to an alloy "is conducted at a temperature
greater than one-
half of the absolute melting point" [Diffusion welding of Solid-State Welding,
by J.L. Jollison
2o and F.J. Zanner, "Metals Handbook", 9'" Ed. Vol. 6, p. 672, ASM
International (1988); see
first column on page 672 and Table 1 on page 677].
The Pd/40% Cu alloy has a melting point of approximately 1200°C (1473
K) ["ASM
Handbook", vol. 3, p. 717, ASM International (1993)]. Hence welding it to
copper is
25 expected to require' a temperature in excess of about 460°C (733 K).
However, we have
found that the hydrogen flux across the Pd/40% Cu foil deteriorates after the
foil has been
exposed to such high temperai:ures.
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Underlying the present invention is the discovery that these problems can be
overcome by
providing a hydrogen selective Pd/40% Cu membrane wall in the form of a thin
palladium-
copper alloy membrane of carefully controlled composition in an open-area
copper surfaced
metallic frame, hermetically bonded by diffusion-bonding the membrane to the
frame.
The term "diffusion-bonding", as used herein means controlling and maintaining
an elevated
bonding temperature below about 350°C, while subjecting the edge area
of the membrane in
contact with the frame to a substantially uniform high pressure, in an oxygen-
free
atmosphere.
OBJECTS OF THE INVENTION
One object of this invention is to provide a novel, much lower temperature,
diffusian-bonding
technique of Pd/40% Cu to copper which avoids flux deterioration, as well as
the need for
t5 excessive edge gasket compression.
Another object of this invention is a new and improved pure hydrogen-
permeating
:?o
:?5
membrane wall having a Pd/40°'o Cu foil hermetically bonded by such
technique to a copper
surface on a temperature and pressure-resistant, herein termed "firm", metal
frame.
Other and further objects will be; explained hereinafter and are more fully
delineated in the
appended claims.
SUMMARY OF THE INVENTIOPJ
In summary, from one of its viewpoints, this invention broadly embraces an
assembly
comprising a thin Pd/40% Cu foill having its edge area diffusion-bonded to a
copper-surfaced
metallic frame. The term "copper surface" as used in this specification and in
the appended
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claims means a copper coating as well as a copper-rich alloy coating of
sufficient copper
content for diffusion-bonding.
The novel diffusion-bonding technique of this invention comprises the step of
pressing a thin
Pd/40% Cu foil, preferably 0.001-0.0025 inches thick, onto a copper-surfaced
frame, after
exposure to or, preferably in a hydrogen-containing atmosphere, in the range
of 200° to 350°
and more particularly between 290° and 325°, for example, for a
cycle period of several
hours in a hydrogen atmosphere furnace.
no The novel wall of this invention comprises a polished firm metallic frame
made of carbon
steel or stainless steels or others, an adherent, preferably electroplated,
copper surface
thereon, and a thin Pd/40% Cu membrane foil having its edge area hermetically
sealed to
the copper surface by the above diffusion-bonding technique.
t5 Preferred and best made designs and techniques are later presented.
DESCRIPTION OF THE DRAWING
The invention will now be described in connection with the accompanying
drawing, the
2o single figure of which is a schematic top view of a metal frame having its
open area covered
by a Pd/40% Cu foil with the foil edge area diffusion-bonded to the frame, in
accordance with
the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
We propose, as above stated, to take advantage of the availability and of the
low cost
(relative to tubes) of a thin Pd/40% Cu foil membrane by diffusion-bonding it
hermetically to
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a frame to form a wall which, in a pure hydrogen generator, obviates gasketing
and the
excessive mechanical pressure required therefor.
Surprisingly, we have found that a Pd/40% Cu foil can be diffusion-bonded to
copper under
5 pressure and, after exposure to and, preferably in the presence of hydrogen,
even when the
temperature is in the low range of 200° to 350°C, and preferably
between 290° and 325°C.
While we do not wish to be held to any theory, it is believable that the bond
results, at least
in part, from copper inter-metallic diffusion and that the hydrogen exposure
eliminates any
interfering oxide layer.
to
This mode of diffusion bonding has been found not to be deleterious to the
membrane, as
evidenced by its durability, at the very same temperatures, when pure hydrogen
is
generated from a methanol-steam reformate, as described in the above-referred
to co-
pending patent application.
By way of example, referring now to the figure of the drawing, samples of
Alloy 101 (i.e.
oxygen free) copper frames 1, each 1.5 inches square and 0.03125 inch thick,
were center
punched each with a 0.625 inch diameter hole. Pd/40% Cu foils, 3, each 0.001
inch thick
and 1 inch diameter, were centered over the 0.625 inch diameter holes of the
frames 1. The
0.1875 inch overlap edge perimeters 2 of each of the coils 3 were mechanically
held under
pressure by a pair of opposing flanges (not shown). The pressure was
controlled by the
torque load on the four flange bolts (also not shown).
After evenly tightening the flange bolts, the assemblies were loaded into a
controlled
atmosphere furnace for heat-tn:ating. The furnace temperatures were varied
between 200°
and 350°C with a slow flow of pure hydrogen gas through the furnace,
which was held at
atmosphere pressure, or altem;atively under reduced pressure, for about twelve
hours.
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The hermetic seals between the foils and the frames produced by the diffusion
bonds were
tested by subjecting the finished assemblies in a separate apparatus (also not
shown) to a
pressure gradient of helium gas of up to 150 psi for several hours. No gas
leakage was
observed.
When selecting a temperature range between 190° and 325°C and
flowing hydrogen at
about atmospheric or reduced pressure, a mechanical perimeter pressure in the
order of
5000 psi or more resulted in producing hermetic seals between the frame and
the foil.
io Bonding periods in the furnace vary with operating conditions. The required
times for
hermetic sealing are readily determined experimentally by selecting and
controlling the
temperature, edge pressure and either the pre-hydrogen exposure at an elevated
temperature and/or the hydroc,~en atmosphere in the furnace, and then leak-
testing the
resultant assembly.
As the wall separating the high and low pressure chambers of a pure hydrogen
generator,
the diffusion-bonded assemblies of this invention must withstand the pressure
differential at
the elevated operating temperature. The above-described, 100% copper framed,
diffusion-
bonded assemblies deformed badly under gas pressure above about 200°C.
However,
operation of such diffusion bonded assemblies below about 200°C is
undesirable due to
poor hydrogen permeability.
Hence for the purposes of this invention, we use a structurally fimn frame
made of a metal or
an alloy subject to copper coating, which is not weakened under the operating
temperatures
and pressures of a pure hydrogen generator, such as carbon steel, stainless
steel, or others,
which, when not subject to copper coating, do not lend themselves to diffusion-
bonding.
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We have found that diffusion-bonding, as herein described, of a Pd/40% Cu foil
to a copper-
coating plated onto such a firm metallic frame, results in an assembly, in
which, surprisingly,
the bond of the copper coating to the frame metal and the bond of the Pd/40%
Cu foil to the
copper coating, are both strong.
But, in leak tests of these assemblies, helium leaks were sometimes
encountered. Though
again we do not wish to be held to any theory, we have found it plausible to
attribute the
helium leaks either to the grainy streaks of the somewhat uneven surface of
the metal frame,
(which streaks were actually showing through the foil's edge area of the
finished assembly)
m and/or to too thin a copper plate (ouch as less than 0.0005 inch thick).
Thus, to obtain consistent hermetic seals for the use of the assemblies in two-
chamber
hydrogen generators, we select thicker copper coatings and/or we polish the
frame metal to
provide a smooth even area prior to plating copper and diffusion-bonding
Pd/40% Cu
5 thereon. Those samples were leak proof in the helium leak test.
As a specific example of producing such a leak proof assembly, samples of a
commercial
grade "oil hardening" pre-ground flat stock carbon steel were prepared in the
above-
described dimensions. Prior to copper plating, they were polished to a smooth
finish, some
2o by hand and others by electropolishing, cleaned by degreasing and picked.
Some samples
were also subjected to a short electrolytic "nickel strike" to assure good
copper adhesion.
The samples were then electroplated from standard, e.g. cyanide or sulfate,
baths to copper
coating thicknesses of about 0.0005 to 0.0008 inch.
L7iffusion-bonding of 0.001 inch thick Pd/40% Cu foils to the copper plated
frames was then
pertormed as above described.
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While preferred embodiments of the invention have been described in the
foregoing, it will
be apparent to those skilled in the art that they can be varied without
departing from the
scope of the invention.
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