CA1051412A - Catalyst supporting screen - Google Patents

Abstract

CATALYST SUPPORTING SCREEN

ABSTRACT OF THE DISCLOSURE
A catalyst supporting structure comprising a layer of heat-resistant wire elements composed of knitted wire, placed in contiguous relationship with each other to provide a pad of sub-stantially uniform thickness and having flat top and bottom sur-faces, with a woven sheet of heat-resistant material bonded to each surface. The heat resistant wire elements and optionally the woven sheet consist essentially of an alloy of iron, chromium and aluminum.

Description

``` l(~S1~12 The present invention relates to certain improvements in the catalyst supporting structure described and claimed in Canadian application No. 153,235, filed October 4, 1972.
In numerous types of chemical processes involving a catalyzed reaction of gaseous reactants, it is desirable to sup-port the catalyst by means which will permit gaseous reactants to pass through the catalyst and its support at elevated tempera-tures and pressures. For example, in the oxidation of alrunonia in the preparation of nitric acid, ammonia and oxygen are passed 10 through a platinum gauze catalyst supported on nichrome bars and heavy mesh screen (see, for example, U.S. Patent 3,195,988). It has also been proposed to position getter netting or gauze between the catalyst and the support to recover platinum lost by volatil-ization from the platinum catalyst (see U.S. Patent 3,434,826 which describes the use of one or more nettings or screens of palladium or palladium/gold alloy between platinum wire screens constituting the catalyst and a coarse metal netting as support for the catalyst and getter).
The above-mentioned Canadlan application No. 153,235 20 describes and claims a catalyst supporting structure comprising a layer of heat-resistant rope elements composed of knitted wire, each of which is rolled up on itself and shaped into elements having essentially flat outer surfaces, said elements being placed in contiguous relationship with each other to provide a pad of substantially uniform thickness and having flat top and bottom surfaces and a woven foraminous sheet of heat-resistant material bonded to each surface. This structure has been found to be emin-

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ently suitable for use in, for example, ammonia oxidation proces-ses The present invention has as its principal object an im-provement in the structure of Serial No. 153,235 which makes it possible for the support to be used for a longer period of time.
Other objects will also be hereinafter apparent.
Broadly described, the supporting structure of this in-vention comprises a layer of heat-resistant wire elements composed of knitted wire and positioned so that adjacent elements are in contiguous relationship with each other to provide a pad of sub-stantially uniform thickness throughout and having flat top andbottom surfaces, and a woven sheet of heat-resistant material bonded to each flat surface of the pad. According to the inven-tion, the wire elements consist essentially of an alloy of iron, ; chromium and aluminum optionally with minor amounts of cobalt, silicon and/or manganese but no nickel (or at most not more than 5% nickel). The wire utilized usually will have a diameter in the range of .005 to 0.009 inch, preferably about 0.006 to 0.008 inch, although diameter sizes outside this range may also be used.
In Serial No. 153,235, the rope elements are disclosed as preferably being made of nichrome wire. It has been found that the nichrome-wire of the diameter indicated tends to lose .
its integrity in use in ammonia oxidation and this may reduce the i~ useful life of the structure. Because the structure of Serial No.
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153,235 includes a woven sheet of larger diameter wire on both sides of the rope elements which is not significantly affected by ammonia oxidation conditions, the structure of Serial No. 153,235 maintains its integrity and can be used for long and effective periods of time even though the nichrome rope elements themselves, .~

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because of t~ieir finer wire size, may break down or lose their integrity in useO According to the present invention, however, it has been found that if the wire in the rope elements is com-po~ed of an iron/chromium/aluminum alloy, optionally containing cobalt and from 0 to 5% nickel, the life of the rope elements is very markedly improved thus greatly extending the period for which the structure can be effectively used.
Two particularly useful alloy compositions for the rope ~- elements used herein are available as "Kanthal" and "Megapyr"~
These alloys have the following compositions, percentages being by weight:
"Kanthal"
aluminum 4O5%
chromium 22.0%
cobalt ~5%
iron balance "~"
aluminum 5~
chromium 22-25%
silicon less than 1.0%
: manganese less than 1.~
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iron balance ;~ . It will be recognized that the two alloy compositions described above are given only as examples. Broadly described, the alloy used hereln for the rope elements, and preferably for the woven sheets as well, consist essentially of 20-30% chromium, 4-6%
aluminum, balance essentially iron with the optional presence of not more than 1% cobalt, silicon and/or manganese. Preferably ~ d e h ~ ~ es t ~d ~ ~ ~a r~ 4~

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the alloy used is essentially nickel~free although, as stated above, up to 5% nickel can be tolerated.
It is essential, according to the invention, that the rope elements be composed of a nickel-free alloy, or one of low nickel content as describqd above. Preferably the woven screens are also composed of such alloy although, because of the larger di-ameter wire used therein, conventionally available woven nichrome screens may also be used. T~pically the sheets, whether of ni-chrome or alloy as described above, are screens of 50 mesh or coarser (U.S. Sieve). Particularly useful results have been ob tained with 10-20 mesh woven screens (e.g. 18 mesh, v016'' diameter wire).
The wire rope used herein represents a particularly impor-tant feature of this invention. ~s indicated above, the rope com-prises knitted wire tubing flattenedO rolled and shaped to form asquare or rectangular cross section, preferably squareO It is essential for best results that the outer surfaces of the rope be substantially flat so that when segments of the rope are arranged in contiguous relationship a pad having a substantially uniform thickness is formed. ~ypically the rope is such that it has cross-sectional dimensions in the range of l/8" to l/2" with a square ; ~
cross section of l/4" x l/4" preferred although it will be appre-ciated that the cross-sectional dimensions may be varied.
The wire rope referred to above may be formed into a pad ~` 25 for use herein in several ways, eOg. by cutting the rope into rope elements of desired length and bringing these into parallel contig-uous relationship or by simply winding the rope into a flat spiral so that adjacent elements thereof are in contiguous relationship.

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Both of these methods start with a knitted tube of the indicated alloy wire. Tubes of this sort are known in the art (see, for example, U.S. Patent 3,245,206). According to the present inven-tion, the knitted tube is flattened, rolled into a cylindrical form and then shaped into a porous flexible flat-sided rope,.pref~
erably of relatively square cross section, In one embodiment of the invention, the rope is cut into rope elements of appropriate length and the~e are placed side by side.in parallel contacting relationship in a common plane to form the pad. The pad is com-pleted by wrapping another flat sided rope element around thecircumference or outer extremity defined by the parallel rope ele-ments. The supporting structure is completed by placing one or more woven Rheets on the top and bottom surfaces of the resulting pad and bonding, e.g. spot welding, these woven sheets to the rope elements at spaced intervals over the entire surface of the padO
In the alternative fabricating method, the rope is not cut into rop.e elements but instead is simply wound in a spiral to give a circular pad of desired diameter followed by positioning the woven outer screens on the top and bottom of the pad and spot welding or otherwise bonding t~e sheets to the pad.
-The invention is hereinafter described in more detail by reference to the accompanying drawings wherein:
Figure 1 is a vertical sectional view of a catalytic re-.actor showin~ the manner in which a catalyst supporting structure 25 ~acco~ding to this invention may be used;
Fiyures ?-5 diagrammatically show various phases in one method of preparing the rope elements of the present invention;
Flgures 6A and 6B are plan views showing how the rope . . .

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elements may be assembled to make a p~d;
- Figure 7 is a fragmentary top plan view showing the com-pleted supporting structure using the pad of Figure 6A;
Figure 8 is a fragmentary sectional view of the support-ing structure along the lines 8-8 of Figure 7;
Figure 9 diagrammatically illustrates another embodiment of this invention; and Figure 10 diagra~matically shows an alternative method for making the rope elements used herein.
Referring more specifically to the drawlngs, a typical way of u~ing~ a catalyst supporting structure according to this in-vention is shown in Figure 1 wherein (2) represents a reactor which, for the purposes of illustration, may be a convqntional type ammonia oxidation burner, including a catalyst package (4), through which a gaséous mixtu~e of NH3, 2 and N2 is fed, at ele-vated temp~rature and pressure. Thé catalyst package (4) com-.. . .
prises a woven gauze or pack of such gauzes (6) of platinum metal,preferably platinum/rhodium alloy. Typically the gauze or gauze pack comprises woven 80 mesh screen with 9~%~platinum/1~ rhodium alloy wire (.003") although different mesh sizes, wire composi-, . . ~, , ; tions and the like may be used. ~
~ Preferably, but not necessarily, the catalyst package -~ also comprises one or more screens (8) or the equivalent of a "getter" material, as described in U.S. Patent 3,434,826~ As ~25 shown, and as understood~in the art, the getter is positioned just below the catalyst (6) and serves to collect platinum volatilized from the catalyst.
The supporting structure (10), constituting the essen-1~51~
tial aspect of the present invention, serves to support the cata-lyst (6) and getter (8) although it will be appreciated that in certain circumstances, the getter may be omitted. Additionally, in a further modification of the invention as described below, the getter may be made a part of the supporting structure (10).
Figures 2-5 illustrate the steps in~olved in preparing the rope used for the supporting structure of this invention. The rope is formed from a tube ~11) knitted of alloy wire as shown in Figure 2. Typically, but not necessarlly, the tube iB knitted by feeding 0.005 inch wire of the indicated alloy composition through a knitting machine to produce a knitted tube or cylinder, it being appreciated that the size of the wire used and the diameter of the tubing can be varied as desired. The tube (11) is flattened as shown in Figure 3 and rolled up as tightly as convenient around its longitudinal axis as in Figure 40 to produce a cylinder (12), the latter being thereafter compacted and shaped in any convenient fashion, eOg. by drawing through a die, and calenderlng to give j the desired flat-sided rope (13), preferably of square cross sec-tion, as shown in Figure 5. The compacting and shaping of the 20 knitted tubing results in a rope wherein the various layers of ~; wire are intermeshed and integrated with each other to give a ~; structure w~ich does not tend to unwind or unravel.
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The dçnsity of the knitted wire rope can be varied as desired to fit the lntended use. However, the rope advantageous-ly has a volume density of about 10-3~%, preferably 10-2~% (i.e.
the degr~e of compacting is such that the rope comprises 10-3~%
or 10-2~/o metal, balance void). As noted earlier herein, the rope may be from 1/8 - 1/2" in cross-sectional dimension, preferably 1/4" square.

l(~S1412 The pad (14)~ see Figs. 6A and 6B particularly, is made by cutting the rope (13) into rope elements or pieces (16) of ap-propriate length to form the desired shape when the pieces are brought together in parallel contiguous relationship~ preferably circular or hexagonal as shown,in the embodiments illustrated in Figures 6A and Bo One or more pieces (18) of the rope (13) are then placed around the abutting pleces (16) as ~hown in Figures 6A and 6B. Woven sheets (20) and (22) of wire, preferably of an alloy composition as de~scribed e~rlier herein, are posit~3~e~ on the top and bottom surfaces of the resulting pad (14), as shown in Figures 7 and 8, and spot welded on a grid pattern as shown at (24) (FigO 7) to the encompassing rope piece or pieces (18) and to the elements (161 to complete the supporting structure. The sheets (20) and (22) extehd outwardly over piece (18) as shown ~' ' 1. .
by the numeral (23) in Figure 8 and serve to firmly h~ld the piece or pieces (18) and encircled elements (16) in place to give a com-pact unitary structure which is adapted for easy insta~lation in conventional type ammonia oxidation reactors such as shown in Fig-ure 1~ or the like, to provide a firm, controlled density base or support for the catalytic gauze.
In a further modificatiqn of the invention, the tube (11) used to make t~e wire rope (I3j is knitted with a combina-tion of the alloy wire and palladium or palladium/gold wire, e.g~
the wires are plied together or otherwise combined so as to pro-vide a catalyst support which includes the getter material as anintegral part of the support itself rather than as a separate layer. This makes it possible to eliminate the use of the sepa-rate getter layer (8) shown in Figure 1. Figure 9 illustrates _9_ 1(~514~2 this embodiment of the invention~ the numeral (25) representing the getter wire intermixed with the alloy wire in the pad (14)o It will be recognized that the amount of getter in the pad may be varied buto as an illustration, a ratio of palladium/gold alloy 5 to alloy wire of from 1:1 to 1:3 may be mentionçd with best re~ -sults obtainable at a 1:2 ratio. In lieu of the method described above for fabricating the pad (14), an alternative method is shown in Figure 10. This method replaces the steps of cutting -the rope into lengths~ (16) and encircling the same with rope (18) as in Figures 6A and 6B and involves using a continuous length (26) of the rope (13) and simply winding the rope on itself to provide the pad (14), the support being completed as in the Fig~
ure 6A by placing the woven sheets (20) and (22) on both sides of the pad and welding as in Figure 7.
While the invention has been described above with par-; ticular reference to use of a structure to support catalysts com-prising platinumO eOg. platinum/rhodium alloys, for oxidation of ammonia in the production of nitric acid, it will be recognized that the structure may be used in other types of reactions, such as the-Andrussow hydrocyanic acid synthesis, using the same or different catalysts. The structure may also be used as a demister and for other purposes obvious to one in the art~
,~ Various other modificat1ons will also be apparent, the scope of the invention being defined in the following claims wherein:

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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A catalyst supporting structure comprising a pad of heat-resistant wire elements wherein the wire elements are composed of knitted wire, said elements being placed in lateral contacting relationship with each other to provide a pad of sub-stantially uniform thickness and having flat top and bottom sur-faces and a woven sheet of heat-resistant material, in addition to said heat-resistant wire elements, positioned directly on the top and bottom surfaces of said pad, said wire being composed of an alloy consisting essentially of aluminum, chromium and iron.

2. The structure of claim 1 wherein the woven sheets are also composed of an alloy consisting essentially of aluminum, chromium and iron.

3. A catalyst supporting structure according to claim 1 wherein the wire elements are rolled and shaped into rope ele-ments having flat sides.

4. A catalyst supporting structure according to claim 1 wherein the wire elements are essentially square in cross sec-tion.

5. A catalyst supporting structure according to claim 1 and comprising said wire elements arranged in parallel in a common plane and of varying lengths so as to form said pad, an-other such element or elements peripherally encompassing said pad to enclose the elements arranged in parallel, and woven sheets of heat-resistant material on the top and bottom of said pad, said sheets being bonded to the pad at spaced intervals.

6. A catalyst supporting structure according to claim 1 wherein the wire elements are composed of an alloy consisting essentially of 20-30% chromium, 4-6% aluminum, the balance being iron.