Knitted Gauze for Ammonia Oxidation


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KnittedGauze forAmmonia Oxidation
NEW INDUSTRIAL RHODIUM-PLATINUM FABRIC CATALYST
By B. T. Horner
Johnson Matthey, Materials Technology Division, Royston

Nitric acid is used in the manufacture offer-

tilisers, explosives, plastics and other chemicals,

and its production has been the subject of

substantialtechnical development since its incep-

tion on an industrial scale at the turn of the cen-

tury. Thus, a modem manufacturing plant is as

different from its early predecessorsas the com-

puter is from the abacus. The exception to this

is the catalyst, the design of which differs little

from that used in the early years of this century.

An early patent filed by Karl Kaiser in 1909sug-

gested the use of a platinum gauze (l), and he

settled on “a thickness of thread of 0.06

millimetres, and netting of about 1050 meshes
per square centimetre.. .”. Today the vast ma-

jority of nitric acid plants use 1024 mesh gauze

of 0.076 mm and/or 0.060 mm diameter wire.

Nitric acid production plants can be broadly

classified into two types according to the reac-

tion process:

(i) A single pressure process carried out at either

medium pressure (4-6 atmospheres) or high

pressure (7 - 14 atmospheres)

(ii) A dual pressure process where ammonia

oxidation is carried out at medium pressure,

and the absorption stage is completed at high

pressure.

The overall chemical reaction sequence in both

processes is described by the following equations:

4NH, + 5 0 , = 4 N 0 + 6H,O ammonia

2NO + 0 , = 2 N 0 ,

oxidation

3N0, + H,O = 2HN0, + NO absorption
The ammonia oxidation stage utilises a catalyst pack of up to 36 platinum group alloy gauzes, the number being dependent on the pressure in the plant, and the make-up of the pack is designed to maximise the efficiency of the reaction.
The process is fast, but during the reaction contact between the gas and the catalyst is

required for times of the order of seconds in order to maintain process efficiency. The catalysts employed have a typical service life of 50 - 300 days, depending upon the pressure of the system. Conventional woven rhodiumplatinum catalyst gauzes fulfil the requirements of the plant quite successfully.
Initial work on catalyst development was performed using flat gauzes (2), and has progressed by the almost exclusive use of woven gauzes. Woven gauzes, however, restrict catalyst development, because only alloys with mechanical properties that satisfy the requirements of the weaving loom can be considered for use. Gauze and catalyst pack designs, therefore, have evolved together from the initial concept, rather than the concept driving the development.
Why Develop New Gauzes?
Since very little has changed in the design of catalysts from almost the start of the industrial ammonia oxidation process, it appears that this could be an area for potential improvement. There are numerous pressures on the fertiliser manufacturing industry, not least of which is concern for the environment which ha; increased in recent years. A reduction in the rate of application of nitrogen-based fertiliser seems inevitable, and this in turn could result in a decline in industrial output. There are many indications that the industry is already streamlining itself for this eventuality.
One of the major costs of the nitric acid production process is the catalyst. In a typical plant, the catalyst could consist of 25 kg of 10 per cent rhodium-platinum, which at present metal price levels (platinum f;6860/kg and rhodium f;90,000/kg) would cost approximately E350,000. Fluctuations in metal prices and especially the

Platinum Metals Rev., 1991, 35, (2), 58-64

58

recent high cost of rhodium further increasesthe need for the nitric acid producers to make optimum use of this asset. With the widespread adoption of the “just in time” philosophy, pressure is being put upon the catalyst gauze manufacturers to improve hrther their response to the needs of the catalyst users. The process of weaving gauze is, however, inherently inflexible, since it ties up large amounts of metal for long periods of time, to the possible financial detriment of both the supplier and the user.
As previously stated the catalyst used for ammonia oxidation must fulfil several needs. Lowering the amount of rhodium in a catalyst will substantially reduce its cost, but then it will not be able to meet its full requirements. Changing the alloy composition and seeking to develop new alloys would be difficult also, under such con’straints. A compromise is needed between cost and requirements, and an appreciation that any benefits from alternative alloys, such as higher conversion efficiencies and longer life, will be more difficult to instigate.
An alternative flexible catalyst manufacturing system was therefore believed to offer solutions, providing that the resultant catalyst was able to meet specification. Knitting was identified as a possible method of manufacture. No one, however, had been able to successfully knit the relatively low tensile strength platinum group alloys used for ammonia oxidation catalysts, beyond laboratory scale.
Gauze Manufacturing Trials
Initial attempts to produce knitted gauzes on a commercial knitting machine proved difficult, the wire breaking frequently. Sufficient material was produced, however, to enable basic catalyst proving trials to take place. This knitted material was found to be significantly lighter than the same area of conventionallywoven catalyst gauze.
Trials were carried out in a high pressure ammonia oxidation development reactor built by Johnson Matthey (3). The initial trials were restricted due to the limited availability of material, the catalyst pack consisting of 16 knitted gauzes backed up with 4 woven catalyst gauzes. The results, however, were encouraging.

Fig. 1 Initially, gauze produced by knitting was

significantly lighter than the same area of woven

gauze, and this was allowed for by using four knitted

gauzes as a substitute for one woven gauze. Dur-

ing use knitted gauzes developed a faceted

appearance, comparable to that of conventional

woven material

x 80 approx.

As the knitted catalyst, shown in Figure 1, was 75 per cent lighter than the conventional gauze, four knitted catalysts were used as a substitute for one woven gauze, thus maintainingthe weight of the catalyst charge. The resulting ammonia conversion efficiency was recorded as being 3 per cent higher than normal, and the light-off characteristics were also improved. Subsequent examination of the surface of the knitted gauzes by scanning electron microscopy revealed a well-developed, faceted structure, comparable to that observed on used conventional gauzes. It was found that no reaction occurred on the surface of the woven gauze, indicating that any improvements could be attributed to the knitted material.
Additional fabrication work was undertaken, but this did not produce a knitted gauze having a 1:l weight ratio with the woven gauze. Encouraged by previous success, however, further pilot plant catalyst trials were carried out at both medium and high pressures using various industrial pack simulations. Every permutation gave improved plant performance, Table I. The make up of the packs and the test conditions are given in Table 11.
In view of the novelty of the concept and the lighter weight catalyst fabric, a continuous 4 day trial at medium pressure was undertaken to assess

Platinum Meialr Rev., 1991, 35, (2)

59

Platinum Metals Rev., 1991, 35, (2)

Table I
Results of Pilot Plant Trials

.Conditions

L
W
cn
25
C

Pack construction

Run number

Campaign hours

.m c
EI$
a

Ammonia range, per cent

Inlet temperature,
OC

~ - m o r -N

Medium pressure

Standard Standard 50% knitted 100% knitted (a 100% knitted Reduced weight* (b)

85/34 85/35 8711 6 8819 8811 10
8811 1 A

10.5 12.5 13.3
9.8 26.5
9.8

9.9- 10.1 9.9- 10.2 9.8- 10.1 10.0- 10.2 9.5- 10.3
10.2-1 0.4

267-272 269-271 287-298 273-280 259-267
2 59-272

N

N

N

N

"

I-

r-r-mcoa

r-

t

rt -

r-

0

r-

I\

m

(0

m

0

Ln

t t

t
PI

xr-

Reactor temperature,
OC
741-748 737-743 733-740 765- 795 730-769
747-760

r-

r-

(D

(D

m

m

Ln

m

Average conversion efficiency,
per cent -

Pressure
drop, iwg

90.4

1.o

89.3

1 .o

94.5

2.7

94.5

0.2

94.7

1.2

87.7

0.8

2 rm Dw Ih
60

N

x N

High

Standard

8715

10.8

zL?

t

10.5- 10.9

268-270

co

Ln

825-845

91.6

-

pressure

cow

mm

cob

N

tt

zF ! " z!

Standard

88/22

6.5

10.2-10.5

2 7 5-283

850-863

90.4

-

40% knitted

88/17

9.5

10.2- 10.9

2 72- 273

824- 874

93.3

3.5

N

. 100% knitted pack but with 28 17 weave equivalentl rather than 32 18 weave equivalentl knitted gauzes

Table II
Test Conditions and Pack Construction for the Evaluation of Knitted 10 Per Cent Gauze Catalysts

Medium pressure trials

Pack construction
Standard pack 8 x 1024iY10.076 mm
50% knitted pack 16 x 2 6 0 # / 0 . 0 7 6 mm knit 4 x 1024#/0.76 mm knit

I

Targeted operating conditions

Loading 12 tonneslday NH,

Pressure 56 psi

Percentage of ammonia 10.0

Inlet temperature 27OOC

Reactor size 1.25" dia

100% knitted pack 3 2 x 2 6 0 # / 0 . 0 7 6 mm knit (a) 28 x 2 6 0 # / 0 . 0 7 6 mm knit (b)

High pressure trials

Standard pack 21 x 1 0 2 4 # / 0 . 0 7 6 m m
4 0 % knitted pack 3 2 x 2 6 0 # / 0 . 0 7 6 mm knit 1 3 x 1 0 2 4 # / 0 . 0 7 6 mm knit

Loading 78.9 tonneslday NH, Pressure 135 psi Percentage of ammonia 10.5 Inlet temperature 27OOC Reactor size 2.0" dia

the durability of the knitted catalyst. The results confirmed those of the short term trials.
Some possible explanations for the improved efficiency of the knitted catalyst could be its open weave, or a feature associated with it, coupled with the increasednumber of gauzes required to provide the equivalent weight. Scanning electron microscopy showed that the reactants had penetrated to greater depths than normal, presumably because the gas needed to travel further before finding a reaction site, thus increasing contact time. However, an important feature of the knitted material is that, unlike woven gauze, the crossover points of the wires are more accessible to gas impingement. The loss of catalytic area on a woven gauze due to masking at crossover points is 10 per cent of the total area theoretically available. Therefore, it was postulated, if equivalent weight knitted catalysts could be manufactured, then fewer gauzes would

be required to fulfil the same function. Alternatively, lighter weight packs could be used to achieve the same result.
To ensure initial acceptance by the chemical industry, the production of a knitted gauze of equivalent weight to the conventional woven gauze was considered desirable, although not essential to ultimate performance. Using the experience gained, a knitting machine believed to be suitablefor producing such a fabricwas identified; this being a commercially available 6 inch diameter circular knitting machine with a single feed. The single feed system reduced the need to carry large stock levels of wire during the development stages, although multiple feed units were available for the knitting machine, and could be used to scale up the operation and increase the flow of knitted material from the machine. With such a feature it is possible to commence knitting as soon as wire of the

Platinum Metah Rev., 1991, 35, (2)

61

r;CLEARING YARN FEEDING

LATCH CL0SING

LATCH ON HOOK

CAST - OFF

LATCH OPENING

Fig. 2 The knittingprocess imposes severe distortions on the metal feed wire, but co-knitting a polyester yam gives support to the wire and reduces friction at the needles

correct diameter and with the necessary mechanical properties, becomes available. It soon became apparent that, due to the relatively low tensile strength of the alloys being used, and to the s u r i c e friction generated, the wire could not withstand the severe deformationit was subjected to. Therefore changes to the basic design of the knitting machine were made and special features were developed to assist in the production of knitted noble metal gauzes.
The knitting process is shown in Figure 2. Here the degree of deformation, as well as the mechanical operations that produce it, are shown. To overcome the problems resulting from such severe distortions, a polyester yarn was added to the metal wire feed. This had the effect of wrapping the wire, givingi t mpport, and also reducing friction in the needles. After fabrication the co-knitted polyester is removed. Furthermore, the mechanical properties of the rhodium-platinum wire being knitted were optimised by varying the annealing conditions, so as to produce properties which were more amenable to the high degree of deformation experienced by the wire during the knitting process. To reduce further the friction forces, several

lubricantswere investigated and the optimum one identified. Using a knitting head with 370 needles (19 per inch), a fabric was produced with
an effective weight of 590 g/m*,the same as for
typical woven gauze. During trials it was found that by careful adjustment the machine could be made to produce i b r i c with a weight tolerance o f f 25 per cent. Thus, a flexible manufacturing technique was successfully developed enabling fabrics of different cloth densities to be produced, using noble metals and alloy wires of various diameters, including rhodium-platinum and rhodium-palladium-platinum.
Commercial Trials
An industrialplant operated under atmospheric conditions with a production capacity of 160 tomes per day of nitric acid was used for the initial trials, the campaign lasting for 75 days. An examination of the catalyst pack by scanning electron microscopy again revealed excellent surface development, and also showed that activationhad occurred in the areas of potential masking, see Figure 3. This may be compared with the woven gauze shown as Figure 4. The results of an EDXA (energy dispersive

Platinum Metah Rev., 1991, 35, (2)

62

Fig. 3 After commercial trials under atmospheric conditiona. excellent aurface development wm evidenton a hutted gauze, and this extended to areas of potential maaking
x 100 approx.

X-ray analysis) of the gauze surface were most informative, and are given in Table 111. Only low levels of rhodium were detected on the surface of the knitted samples. These may be explained by a better gas distribution throughout the catalyst pack, which promotes a more consistent temperature profile and thus reduces the redistribution of rhodium, which usually occurs at low operating temperatures. This unexpected finding is still under investigation, and is supported by the results of further trials. Generally a pack becomes uneconomic to use when rhodium levels on the surface reach 50 per cent. Any lowering of this rate of rhodium redistribution would be expected to extend the life of the catalyst. A reduction in the redistribution of rhodium should also reduce rhodium losses

which, at the present time, could provide a significant financial saving. Total noble metal loss when using a knitted catalyst was, however, the same as when a woven catalyst gauze was employed.
Several industrial trials are at present being conducted at low, medium and high pressures, and initial results show that knitted catalysts give a performance superior to that of woven gauzes. These trials will be reported in a later issue of Platinum Metals Review.
Summary
The development of knitted gauzes has many potential benefits both for the catalyst supplier and the nitric acid producer, and thus the present project has met most of its initial aims. The

Fig. 4 Characteristic faceted development on the surface of aa conventionally woven rhodium-platinum catalyst gauze, after use for the oxidation of ammonia x 100 approx. x 100 approx.

Platinum Metals Rev., 1991, 35, ( 2 )

63

Tabla 111

Surface Rhodium Concentrations as Determined by EDXA
per relit

Knitted pack

I

Conventional woven pack

Gauze 1 knitted 8.9

2.5

Gauze 2 knitted 9.0

2.7

Gauze 3 woven 1 1 . 5

1.4

Gauze 4 woven 11.9

1.5

Gauze 5 woven

9.7

1.2

Starting rhodium content of knitted gauze 10.05 per cent

potential benefits can be summarised as follows: (a) Increased conversion efficiency (b) Reduced rhodium oxide formation (c) Higher surface area available for catalysis (d) Fabric flexibility providing greater resistance to damage by thermal shock (e) Stronger material, the ability to resist tearing being superior to that of woven gauze (f) Choice of alloy: any changes from the standard 10 per cent rhodium-platinum catalyst can be considered; indeed any combination of rhodium, platinum and palladium can be knitted into gauzes, and many have been. If a chosen alloy can be processed to wire of a suitable diameter, then the possibility of knitting it is extremely high

(g) Only minimal stock levels are required; this enables the supplier to respond more quickly to the needs of the consumer.
Johnson Matthey believe that this range of products and the associated technology, will have a major impact on the chemical industry and will encourage platinum metals catalyst development to take place in ways that, previously, have only been considered in theory.
References 1 K. Kaiser, German Paten? 271,517, 1909 2 S. L. Handforth and J. N. Tilley, Id.Eng. Ind.,
1934, 26, (12), 1287
3 K. G. Gough and B. L. Wibberley, Platinum Metals Rev., 1986, 30, (4), 168

Platinum and Iridium Silicide Infrared Imagers

INCREASING OPPORTUNITIES FOR DIVERSE APPLICATIONS

In a recent communication from the David
Sarnoff Research Center, Princeton, New Jersey, the background to the continuing
development of platinum silicide infrared imaging devices is discussed (J. R. Tower, Influred Technol., 1991, 25, (2), 103-106).
When the concept of Schottky-barrier infrared focal plane arrays was first put forward
by researchers at the Rome Air Development Center, in 1973, it was proposed that detectors formed by the reaction of platinum, or
palladium, with p-type silicon would be sensitive in the 1 to 5 pm band. Before the end
of the decade the Sarnoff Laboratory had demonstrated a 25 x 50 platinum silicide imager

operating in the 3 to 5 pm band, and five years later they had developed a 160 x 244 array with a noise equivalent temperature difference of 0.1 K. Now they are developing cameras around three focal plane arrays, namely 64x 128 and 320 x 244 infrared charge-coupled devices and a 640x 480 complementary metal-oxide silicon infhred imager. Applications include thermography, radiometry, industrial process control and scientific imaging.
In the future it is planned to produce iridium silicide complementary metal-oxide silicon infmred imagers with wavelength capability from 1 to 10 pm or 0.2 to 10 with back- or front-side illumination, respectively.

Platinum Metals Rev.,1991, 35, (2)

64

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Knitted Gauze for Ammonia Oxidation