Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/50—Catalysts, in general, characterised by their form or physical properties characterised by their shape or configuration
  • B01J35/58—Fabrics or filaments
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
  • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
  • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
  • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
  • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
  • B01J23/44—Palladium
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/19—Catalysts containing parts with different compositions
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B21/00—Nitrogen; Compounds thereof
  • C01B21/20—Nitrogen oxides; Oxyacids of nitrogen; Salts thereof
  • C01B21/24—Nitric oxide (NO)
  • C01B21/26—Preparation by catalytic or non-catalytic oxidation of ammonia
  • C01B21/267—Means for preventing deterioration or loss of catalyst or for recovering lost catalyst

Definitions

• This invention relates to ⁇ elf-gettering catalysts.
• catalysts can suffer greatly from material losses through volatilisation, especially during highly exothermic reactions such as the oxidation of ammonia. Where the catalyst is of precious metal, this loss can be extremely expensive, leading to greatly increased production costs. It is therefore common for getter systems to be employed, with the aim of recovering some of the material lost from the catalyst.
• Getter systems typically comprise one or more catchment gauzes situated downstream of one or more catalytic gauzes to recover catalytic material present in the post-reaction gases.
• UK Patent Application No. 2062486 discloses a catalyst unit comprising a pack of woven metallic gauzes in which at least some of the gauzes disposed at or towards the front of the pack are made from wire having a cross-sectional area greater than the wire of at least some of the gauzes disposed at or towards the rear of the pack.
• This 'tailoring' of the wire sizes is intended to reflect the well-known metal loss characteristic in which more metal tends to be lost from the gauzes at or towards the front of the pack than is lost fro the gauzes at or towards the rear of the pack.
• th durability of each gauze is selected to suit the rate at which it is expected to erode. In this way, no more of the expensive catalytic material need be used than is necessary; in effect, the amount of catalytic material at the lightly-loade downstream end of the pack is reduced to a minimum.
• the tailored pack disclosed in UK Patent Application No. 2062486 does not, however, propose the use of catchment or getter layers to reduce metal loss from the pack. As a result, the life of the pack is undesirably limited, and metal loss is undesirably high.
• the invention embodies a principle, namely that a quantity of a relatively expensive first material can be replaced by a relatively inexpensive second material in such a quantity as to outweigh, overcome or compensate for any intrinsic disadvantages of the second material in comparison with the first material.
• the first material may be platinum and the second material may be palladium.
• platinum is approximately four times as expensive as palladium, per unit weight. Platinum is also 1.75 times denser than palladium.
• platinum is (in some but not all cases) a more effective catalyst than palladium; it can be said that palladium is intrinsically disadvantaged in that respect with respect to platinum. Nevertheless, given the cost and density differences, approximately seven units of palladium correspond to one unit of platinum for the same overall cost. So, in a catalyst pack for example, one platinum gauze may be replaced by seven similar palladium gauzes without adding to the material cost of the pack. In many cases, such a quantity of palladium is sufficient to outweigh the intrinsic disadvantages of palladium in terms of catalytic effect, so that the catalytic efficiency of the pack is actually increased.
• a still further benefit of the invention is that it allows a useful increase in the average run length of a catalyst pack, without necessarily adding to material costs, detracting from catalytic efficiency, orworseningmetal loss characteristics.
• Increasing the run length reduces plant shutdowns and thus minimises expensive downtime.
• the run length is increased because the palladium elements 'underpin' (compensate for) the decay of the platinum elements - as platinum is evaporated from the platinum elements, it is picked up by the palladium elements which then improve as catalysts by virtue of their increasing platinum content.
• the palladium getter always provides some catalytic effect, but becomes a better catalyst as it ages. Some platinum is inevitably lost from the unit, but the remainder simply moves from one position to another within the unit and continues to do useful catalytic work.
• the various potential benefits of the invention may be expressed in terms of four performance indicators, i.e. 1. improved catalytic efficiency; 2. improved metal loss characteristics; 3. increased run time; and 4. increased resistance to organic contamination.
• a further potential benefit (not strictly a performance benefit) is, of course, reduced material costs.
• benefit may be gained by improving one or more of the four performance indicators (not necessarily all of them) and/or by reducing material costs.
• references to improved 'performance* are intended to indicate that one or more of the four performance indicators have been improved.
• the invention may thus be expressed as a method of improving the performance and/or the cost of a catalyst unit, comprising removing a quantity of a first, relatively expensive catalytic material from a region of the unit, and replacing that quantity of the first catalytic material with a larger quantity of a second, relatively inexpensive catalytic material.
• the invention may also be expressed as a catalyst unit comprising a quantity of a first catalytic material distributed non-uniformly through the unit, including a second catalytic material positioned to act as a getter for the first catalytic material and to enhance the catalytic effect of the unit.
• the distribution of the first catalytic material is suitably "tailored* such that less of the first catalytic material is situated in regions of low expected metal loss than in regions of high expected metal loss. Tailoring of the amount of the first catalytic material may be accomplished by varying the number of catalytic elements around a given position within a catalyst unit or preferably, by varying the cross-sectional area of the catalytic elements themselves.
• the distribution of the second catalytic material within the pack may also be tailored in similar fashion, so that the amount of the second catalytic material at a given position within the catalyst unit is matched to the amount of the first catalytic material expected to be present in the gas stream at that position.
• upstream getter or catchment elements generally collect more volatilised material than downstream getter or catchment elements.
• the downstream getter or catchment elements need not have the capacity of the upstream elements.
• Elements of the second catalytic material may be separated from elements of the first catalytic material by support elements.
• the support elements can, for example, be stainless steel gauzes.
• Other types of support element include ceramic elements or Pt/Pd/Rh-coated inert elements.
• the first catalytic material is platinum or a platinum-based alloy
• the second catalytic material is palladium or a palladium-based alloy
• the first catalytic material is suitably a Rh-Pt-Pd alloy, with Rh present in an amount between 1 and 15 wt%, and with Pd present in an amount between 1 and 30 wt%, preferably with additions of iridium, ruthenium, molybdenum, cobalt, manganese or zirconium.
• a US alloy, 5%Rh-90%Pt-5%Pd is a specific example.
• the second catalytic material is suitably mainly Pd with additional elements as above.
• rhodium is approximately forty times as expensive as palladium, weight for weight.
• a reduction in the amount of rhodium involves a substantially greater cost saving than simply removing some platinum. This saving can be used to pay for still more palladium, to the further benefit of the unit's performance.
• elements of the second catalytic material alternate with at least some elements of the first catalytic material in the unit.
• the elements of the first and second catalytic materials are preferably of wire woven or knitted to form a gauze screen.
• Mesh sizes for the gauze of the first catalytic material can range from, say, 100 to 1500 meshes per cm 2 , and preferably from 200 to 1200 meshes per cm 2 .
• Mesh sizes for the gauze of the second catalytic material can range from, say, 30 to 1200 meshes per cm 2 , and preferably from 50 to 1000 meshes per cm 2 .
• the diameter of wires of the first catalytic material can range, for example, from approximately 0.05 mm to 0.3 mm, and preferably from 0.05 mm to 0.15 mm.
• Wires of the second catalytic material are suitably in the range 0.05 mm to 0.25 mm.
• a further object of this invention is to develop a self-gettering catalyst arrangement which employs substantially gold-free getter materials.
• a second aspect of the invention may be expressed as a catalyst, a getter or a self-gettering catalyst of palladium or palladium alloyed with at least one element selected from the group rhenium, iron, molybdenum, zirconium, tantalum, tungsten, cobalt, yttrium, thorium, ruthenium, iridiu , osmium, rhodium, nickel, copper and chromiu .
• zirconium content does not exceed 2 wt%.
• the content of that other element does not exceed 8 wt%.
• an alloying element in general, it is desirable for an alloying element to increase the melting point of palladium (or at least not to reduce its melting point unduly) , thereby minimising self diffusion and grain growth, and resultant expansion effects, under high-temperature conditions in use, which can occur with pure palladium. Also, as gettering depends upon diffusion of platinum into the lattice of the catchment material, gettering efficiency may be expected to increase as the lattice constant of palladium is increased. It is therefore also desirable that the alloying element acts to increase the lattice constant of palladium.
• rhenium causes little or no reduction in melting point and, in small quantities, increases hardness markedly. Also, platinum readily alloys with rhenium.
• zirconium does not significantly reduce the melting point of palladium and increases its lattice constant. also, zirconium can be slagged off as the oxide on melting. It is also envisaged that a degree of internal hardening may take place in use. Zirconium may even be partially or fully oxidised before use to produce a dispersion strengthened alloy with excellent high temperature characteristics.
• cobalt can become oxidised in use to form cobalt oxide, which is itself a catalyst in the oxidation of ammonia.
• cobalt may enhance the catalytic properties of palladium when used for this reaction.
• the invention further includes a catalyst unit as defined in accordance with the first aspect of the invention, comprising catalyst, getter, or self-gettering catalyst elements as defined in accordance with the second aspect of the invention.
• Figure 1 is a schematic partial cross-sectional view through a typical known catalyst pack.
• FIGS 2, 3 and 4 are schematic partial cross-sectional views through catalyst packs constructed in accordance with this invention.
• a typical known catalyst pack 10 comprises a plurality of catalytic gauzes 12.
• twenty-eight identical 5% Rh - 90% Pt - 5% Pd gauzes 12 lie one above another.
• the gauzes 12 are connected at their edges by a heat resistant alloy foil 14 such as nickel alloy which overlaps an edge portion of the top and bottom gauzes 12 (denoted 12' and 12" respectively).
• the gauzes 12 are typically woven from wire 0.003" (0.0762 mm) in diameter, with 1024 meshes per cm 2 .
• the gauze diameter is 43y 4 inches.
• Such a configuration is suitable for a plant used to produce 450 tonnes/day of nitric acid (HN0 3 ) from ammonia (NH 3 ).
• catalyst packs 16, 18, and 20 constructed in accordance with the invention are similar in outward appearance to the typical pack 10 shown in Figure 1. That is to say, they have a top gauze 12' , a bottom gauze 12", and a nickel alloy foil 14.
• the packs 16, 18 and 20 differ from the typical pack 10 in their internal configurations, as follows:
• Pack 16 of Figure 2 contains a total of twenty-seven Rh-Pt-Pd gauzes 12 supplemented by two Pd gauzes 22.
• the Pd gauzes 22 are of 0.18 mm wire, with 140 meshes per cm 2 .
• Moving downstream through the pack 16 there are twenty-four Rh-Pt-Pd gauzes 12, one Pd gauze 22, one Rh-Pt-Pd gauze 12, one Pd gauze 22, and two Rh-Pt-Pd gauzes 12.
• the Pd gauzes 22 replace one of the Rh-Pt-Pd gauzes 12 of the typical pack and are situated adjacent the downstream end of the pack 16, it can be said that the pack 16 is tailored, albeit simply, to suit the expected 'front-loaded' metal loss profile. That is to say, the amount of Rh-Pt-Pd catalytic material in the downstream region of the pack 16 is reduced with respect to the upstream region of the pack 16.
• Pack 18 of Figure 3 contains a total of twenty-two Rh-Pt-Pd gauzes 12 plus a group of six lightweight Rh-Pt-Pd gauzes 24, supplemented by three Pd gauzes 22 adjacent the downstream end of the pack. Moving downstream through the pack, there are seventeen Rh-Pt-Pd gauzes 12, six lightweight Rh-Pt-Pd gauzes 24, one Rh-Pt-Pd gauze 12, one Pd gauze 22, one Rh-Pt-Pd gauze 12, one Pd gauze 22, one Rh-Pt-Pd gauze 12, one Pd gauze 22 and two Rh-Pd-Pt gauzes 12.
• the lightweight Rh-Pt-Pd gauzes 24 are similar in mesh size to the ordinary Rh-Pt-Pd gauzes 12 and are of similar composition, but consist of wires 0.063 mm (nominally 0.060 mm) in diameter as opposed to 0.0762 mm in diameter.
• the pack 18 of Figure 3 contains a profile of catalytic Rh-Pt-Pd elements that is more finely 'tailored' in accordance with the expected metal loss profile.
• Pack 20 of Figure 4 contains a total of twenty-two Rh-Pt-Pd gauzes 12 plus a group of seven lightweight Rh-Pt-Pd gauzes 24, supplemented by two Pd gauzes 22, one Pd gauze 26, and one Pd gauze 28.
• the Pd gauze 26 is of 0.19 mm diameter wire and has 100 meshes per cm 2 .
• the Pd gauze 28 is of 0.15 mm diameter wire and has 150 meshes per cm 2 .
• the Pd layers are also tailored in this example, in order to match them to the amount of platinum expected to be present in the gas stream at each location in the pack. It has been observed that upstream catchment gauzes tend to collect a greater quantity of platinum than downstream catchment gauzes; thus, the upstream catchment gauzes should, ideally, have a relatively greater resistance to saturation.
• Rh- Pt-Pd gauzes 12 Moving downstream through the pack 20, there are sixteen Rh- Pt-Pd gauzes 12, seven lightweight Rh-Pt-Pd gauzes 24, one Rh- Pt-Pd gauze 12, one Pd gauze 26, one Rh-Pt-Pd gauze 12, one Pd gauze 22, one Rh-Pt-Pd gauze 12, one Pd gauze 22, one Rh- Pt-Pd gauze 12, one Pd gauze 28, and two Rh-Pt-Pd gauzes 12.
• the Pd gauzes 22, 26, 28 are concealed within the pack so that the customer is presented with a unit of familiar appearance, it is also possible to arrange the pack such that one of the Pd gauzes 22, 26, 28 is exposed.
• the Pd gauzes 22, 26, 28 in effect replace one or more of the Rh-Pd- Pt gauzes 12, 24 near the downstream end of each pack.
• the number of Rh-Pd-Pt gauzes in the downstream portion of each pack is tailored to some extent to suit the relatively low metal loss expected at that end of the pack.
• the material cost saved by removing or lightening the Rh-Pd-Pt gauzes 12 allows the substitution of more, cheaper Pd gauzes 22, 26, 28. As will be shown below, this provides a highly desirable combination of lower total material cost, higher catalytic efficiency, lower platinum loss, and longer run length.
• the invention allows a marked reduction in material costs, a substantial reduction in platinum losses, and a substantial increase in the average run length.
• the increased average run length is indicative of improved catalytic efficiency. This is because the end of a run is usually determined by the catalytic efficiency falling below a given threshold level. Thus, the higher the efficiency at the start of a run, and the shallower the gradient of the efficiency decay curve, the longer the run will be before the threshold level is crossed.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Organic Chemistry (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Inorganic Chemistry (AREA)
• Catalysts (AREA)
• Exhaust Gas Treatment By Means Of Catalyst (AREA)

Priority Applications (4)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

Country Status (10)

Cited By (3)

Citations (3)

• 1990
  • 1990-07-31 GB GB909016788A patent/GB9016788D0/en active Pending
• 1991
  • 1991-07-30 JP JP3513948A patent/JPH06503743A/ja active Pending
  • 1991-07-30 AU AU83248/91A patent/AU661970B2/en not_active Ceased
  • 1991-07-30 CA CA002088149A patent/CA2088149A1/en not_active Abandoned
  • 1991-07-30 EP EP91914523A patent/EP0547071A1/en not_active Withdrawn
  • 1991-07-30 WO PCT/GB1991/001292 patent/WO1992002300A1/en not_active Application Discontinuation
  • 1991-07-31 IE IE268991A patent/IE912689A1/en unknown
  • 1991-07-31 ZA ZA916029A patent/ZA916029B/xx unknown
  • 1991-08-03 TW TW080106110A patent/TW209181B/zh active
• 1993
  • 1993-01-29 FI FI930375A patent/FI930375A0/fi not_active Application Discontinuation

Patent Citations (3)

Non-Patent Citations (1)

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