Low pressure drop, high surface area oxidation catalyst and catalyst for production of hydrocyanic acid

Abstract: This invention is a catalytic element for use in the catalytic oxidation of ammonia or ammonia and methane. The element comprises a foraminous structure fabricated from a material consisting essentially of a metal selected from the group consisting of platinum, rhodium, palladium and alloys of mixtures thereof characterized by (a) a novel configuration whereby the initial product of the formula: curve to flat ratio (C/F) multiplied by mesh count (N) and wire diameter (d w ), for said element is greater than at least about 0.08 and (b) where, for a given throughput, the conversion efficiency is a function of the curve to flat ratio (C/F), wire diameter (d w ) and mesh count (N) combination and conversion efficiency is improved by increasing the mesh count (N) for a given wire diameter, increasing the wire diameter (d w ) for a given mesh count, and increasing the curve to flat ratio (C/F) to a ratio of above 1.0.

Description: FIELD OF THE INVENTION This invention relates to a low pressure drop, high surface area ammonia oxidation catalyst system and method. This invention also relates to a low pressure drop, high surface area catalyst system and method for production of hydrocyanic acid. BACKGROUND OF THE INVENTION Nitric acid is produced commercially by passing ammonia and air across a flat gauze woven from platinum-rhodium-palladium alloy wire. The ammonia, initially, is converted to nitric oxide over the precious metal gauze, and the nitric oxide is, subsequently, oxidised and absorbed to form a nitric acid. The conversion efficiency of ammonia to nitric oxide is a function of temperature, pressure, velocity of gas stream, volume of catalyst, and purity of the ammonia and air streams. The ammonia oxidation to nitric oxide generates a large exotherm and raises the temperature of the catalyst in the range of 810°C to 960°C. During the ammonia oxidation process some of the precious metal is volatilised from the surface of the gauze wire. The rate of loss is dependent on the temperature, pressure, and flow rate of gases across the catalyst surface. The cost of the precious metal lost from the ammonia oxidation catalyst is a significant part of the cost of operating a nitric acid plant. In the acid production, the catalyst pack consists of 3 to 50 sheets of flat woven gauze. The conventional flat woven gauze is typically made with 3.15 mesh per mm (80 mesh per inch) and 0.076mm (.003") wire. However, UK patent GB 2062486 B discloses the use of a system where the diameter of the wire is reduced from the front to the back of the flat woven gauze pack resulting in maximum conversion efficiency with minimum precious metal content and therefore also minimum metal loss from the catalyst. Other recent disclosures include US 4,863,893 which claims improved catalyst "light-off" by the use of a high surface area per unit area of catalyst by the deposition of fine platinum particles onto the surface of the flat woven gauze. US 4,863,893 is a variation of the technology of US 3,470,019 which had a different method of deposition. Patent application EP 0 364 153 A1 claims the use of a flat knitted fabric of precious metal for the oxidation of ammonia to nitric oxide. Recently, a flat woven gauze of 2.756 mesh per mm (70 mesh per inch), 0.076mm (.003") wire diameter was introduced to the market place. A platinum-rhodium catalyst is utilized in the Andrussow process, for the production of hydrocyanic acid or hydrogen cyanide (HCN). In the Andrussow process, methane, ammonia, and oxygen are reacted by passing that gas mixture through a platinum alloy gauze pad. The reaction takes place at about 1000°C, and the heat liberated enables it to be self-sustaining. HCN formation proceeds according to one or other of the following reactions: CH₄ + NH₃ + 3/2 O₂ -----> HCN + 3H₂ O NH₃ + CH₄ -----> HCN + 3H₂ The mixed gases are preheated to about 500°C, and the methane:ammonia mixture ratio is in the range 0.8 to 1.2:1. Sufficient air is added to provide 25 to 40% of the theoretical requirement to oxidise the methane and ammonia. Reactors are similar to ammonia oxidation converters, although usually smaller in diameter. Since the gauze operates at about 1000°C, its radiant heat causes the converter walls to be hotter than in ammonia oxidation, and some ammonia is lost from the reaction by pre-decomposition. Some carbon monoxide and nitrogen are also formed by undesirable side reactions. In hydrocyanic acid production, the catalyst pack consists of flat woven gauze. The conventional flat woven gauze is also typically made with 3.15 mesh per mm (80 mesh per inch) and 0.076mm (.003") wire. The catalyst pack of elements or sheets of gauze of precious metals and its use to catalyse ammonia is disclosed in US 4,412,859, US 4,497,657 and US 4,526,614, all of which are hereby incorporated by reference, in toto , By curve to flat ratio is meant the ratio of that portion of a wire in an element of a catalyst (sheet or gauze etc) that is not in the base plane of the element to the portion that is in that plane. When curved, it is the ratio of curved section to the flat section. By mesh count is meant the number of openings in a given area. The wire diameter and area for mesh count must be in the same units, such as millimetres or, alternatively inches. SUMMARY OF THE INVENTION This invention is a catalytic element for use in the catalytic oxidation of ammonia and for catalytic production of hydrocyanic acid. The element comprises a foraminous structure fabricated from a material consisting essentially of a metal selected from the group consisting of platinum, rhodium, palladium and alloys of mixtures thereof characterised by (a) a novel configuration whereby the initial product of the formula: curved the flat ratio (C/F) multiplied by mesh count (N) and wire diameter (d w ), for said element is greater than at least about 0.08 and (b) wherefore a given nitrogen throughput, the conversion efficiency is a function of the curved flat ratio (C/F), wire diameter (d w ) and mesh count (N) combination and conversion efficiency is improved by increasing the mesh count (N) for a given wire diameter, increasing the wire diameter (d w ) for a given mesh count, and increasing the curve to flat ratio (C/F) to a ratio of above 1.0. Preferably the initial product of the formula is greater than 0.2 and the curve to flat ratio (C/F) is above 1.00 (most preferably π/2). The preferred initial product of the formula is in the range of from 0.08 to about 10 and more preferred from about 0.2 to about 10. The preferred element is woven gauze, knitted fabric, fibres and combinations thereof. Preferably the element is in a series of said elements as a plurality of screens. Preferably the element consists of platinum or essentially of platinum alloyed with one or more metals selected from the group consisting of nickel, cobalt, palladium, ruthenium, iridium, gold, silver and copper. The preferred element contains platinum present in an amount of at least of about 70% by weight. The preferred embodiment of this invention is an element wherein (C/F) is in the range of from above 1.0 to about 4, N is the range of from 1.575 to 4.724 mesh per mm (40 to 120 per inch), d w is in the range of from about 0.025 to 0.46mm (0.001 to 0.018 inch) and their respective values are such that the initial product of the formula is greater than at least 0.08, more preferably C/F is from above 1.00 (most preferably π/2) to about 4 and the product of the formula is greater than 0.2. In another embodiment, the initial product is in the range of from 0.08 to about 10, more preferably from about 0.9 to about 8. In a preferred embodiment of this invention, the C/F ratio is achieved by means of forming the element into a pleat-like configuration. The pleats are preferably parallel. Alternatively, the ratio can be achieved by means of pleats in concentric patterns. Such patterns can be circles, parallel lines or polygons. The ratio can also be achieved by means of intersecting patterns resulting in the waffle-like pattern. The waffle-like pattern can be regular comprising straight lines or curved lines, or both. The waffle-like pattern could also be random with either straight or curved lines, or both. In another embodiment the C/F ratio can be achieved by means of shaped depressions on the surface of the element. One method of this invention is a method for catalytic oxidation of ammonia at temperatures above 850°C which comprises using as catalyst a foraminate element fabricated from metal consisting essentially of a metal from the group consisting of platinum, rhodium, palladium and alloys of mixtures thereof characterised by (a) a novel configuration whereby the initial product of the formula: curve to flat ratio (C/F) multiplied by mesh count (N) and wire diameter (d w ) for the element, is greater than at least about 0.08 and (b) where, for a given nitrogen throughput, the conversion efficiency is a function of the curve to flat ratio (C/F), wire diameter (d w ) and mesh count (N) combination and conversion efficiency is improved by increasing the mesh count (N) at a given wire diameter, increasing the wire diameter (d w ) at a given mesh count, and increasing the curve to flat ratio (C/F) to a ratio of above 1.0. Preferably the formula product is greater than about 0.9 and the C/F ratio is above about π/2. Preferably the product of the formula is in the range of from about 0.08 to about 10, more preferably to about 8, most preferably about 0.20 to about 8. The method preferably uses elements of woven gauze, knitted fabric, fibres or combinations thereof. Preferably the elements are in a plurality of screens. The preferred method of this invention uses elements consisting essentially of platinum or platinum alloys of one or more metals selected from the group consisting of nickel, cobalt, palladium, rhodium, ruthenium, iridium, gold, silver and copper. Preferably platinum is present in the amount of at least about 70% by weight. Another embodiment of this invention when using multiple elements of this invention is modulating the initial product of the formula across the elements. The product can be modulated to change from higher at those elements initially contacted by ammonia to lower where the ammonia last contacts the elements. Or vice versa, from lower as the elements initially contact the ammonia to higher where the ammonia last contacts the elements. The product of the formula can change by a recognisable pattern across the system, preferably a pattern described by a variety of mathematical functions, most preferably the mathematical function selected from the groups consisting of linear, parabolic, hyperbolic, step, sinusoidal and combinations thereof. This modulation described both in the method of this invention and in the catalyst system using elements described by this invention. The modulation may be varying any one of the product multipliers C/F, d w or N independently and/or in coordination, C/F, N and d w may be constant or variable both screen-to-screen and/or across each screen and independently varied across the warp and/or across the weft of the woven or knitted screen. Thus, when variable within the screen, the definition of C/F, N and d w in this invention shall all be average numbers for each individual screen. For example, varying both N and d w , the formula would be calculated as follows: Warp: 1.969 wires per mm at 0.147mm wire diameter (50 wires per inch at .0058 inch wire diameter). Weft: 1.18 wires per mm at 0.25mm wire diameter (30 wires per inch at .0097 inch wire diameter). Developed Curve to Flat: 2.61 (using inch units). The other method of this invention is a method for catalytic production of HCN which comprises using as catalyst the same catalyst as described above. This invention provides the following benefits. 1. By using a C/F ratio over 1, formerly flat screens are made three-dimensional, thereby increasing the surface area for greatly improved catalytic activity of the catalyst element. For example, using typical 3.15 mesh/mm, 0.076mm wire (80 mesh, 3mil (0.003")) screen, surface area per cross-sectional area of the reactor can be increased three-fold from 1.5 units at C/F equals 1, ie flat, to 4.5 units at C/F equals 3. 2. The higher surface area, per screen, made possible by increasing C/F ratio permits fewer elements or screens in the system and lower pressure difference across the total system used. This in turn permits higher throughput rates of ammonia through the system, ie, more tons per day conversion of ammonia from the same equipment. 3. The conversion of ammonia or methane and ammonia over the catalyst elements (screens) is also improved by the higher surface area of each element, more ammonia or methane and ammonia, per ton of throughput, is converted to nitric acid or hydrocyanic acid through the same equipment. 4. By another embodiment of this invention, namely modulation of the factors determining surface area, either individually or by the formula, C/F (N) d w , the catalyst surface area can be tailored to be maximum at the point in the system (screen pack) it is most effective and efficient. The formation of a non-planar element according to the present invention may be accomplished in different ways. A particularly suitable and simple method is to pass a planar screen having the desired N and d w values through two horizontal axis cooperating toothed corrugating rollers, causing the plastic deformation of the screen. The rollers are spaced according to the normal compressed thickness of the screen or screens, and the tooth profile is, of course, designed to give the desired curve to flat ratio. Normally, two to seven catalyst screens would be used to form a pad for corrugating. It may be desirable, for example if flexible knitted catalyst screen is to be used, to provide additional support on the bottom of the assembly fed to the corrugating rollers. Such support may be provided by a woven catalyst screen, or even by a non-catalytic support screen, for example a stainless steel screen. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a computer model generated set of curves projecting how the low pressure drop catalyst system would lower pressure drop and increase residence time in 100 T/m²/day plant using 30 sheets of 0.076mm wire 3.15 mesh per mm (3mil (0.003") 80 mesh) sheets (elements) at varying C/F ratios. Figure 2 is a prior art curve of actual values of conversion efficiency at various residence time (sec)/number of gauzes (elements). Figure 3 is a set of curves showing improved residence time at higher C/F (curve to flat) ratios. Figure 4 is a curve showing conversion efficiency at a 100 ton nitric acid plant using 30 sheets (elements) of catalyst at various C/F rations. Figure 5 is a second curve showing conversion efficiency but with 20 sheets (elements). Figure 6 is a curve showing the relation of flow to the curve to flat ratio of elements at a specific residence time for HCN production. Discussion Pressure drop across the system would be significantly lower, especially above C/F of π/2 and residence times can be significantly increased at higher C/F ratios than 1. For example, residence time is doubled between C/F values of about 1.1 and about 1.6. Conversion efficiency increases with residence time to an optimum at a residence time of about 7 x 10⁻⁴ seconds with about 50 gauzes (elements or sheets). Thus in a preferred embodiment of this invention, the optimum residence time would be found for the commercial plant in which the system will be used. Then the curves would be generated using the gas flow loading, number of sheets in the catalyst system and the desired mesh and wire diameter of each sheet desired. A series of such curves could be generated for various desired wire diameters and meshes. Then the computer generated residence time and pressure drop curves would make possible picking a C/F ratio for the actual commercial application. Figure 1 shows that pressure drop across the system would be significantly lower, especially above C/F of π/2 and residence times can be significantly increased at higher C/F ratios than 1. For example, residence time is doubled between C/F values of about 1.1 and about 1.6. Figure 2 shows the importance of residence time in a prior art system at the conditions shown. The curve shows that conversion efficiency increases with residence time to an optimum at a residence time of about 7 x 10⁻⁴ seconds with about 50 gauzes (elements or sheets). Thus in a preferred embodiment of this invention, the optimum residence time would be found for the commercial plant in which the system will be used. Then the curves similar to Figure 1 would be generated using the nitrogen loading, number of sheets in the catalyst system and the desired mesh and wire diameter of each sheet desired. A series of such curves could be generated for various desired wire diameters and meshes. Then the computer generated residence time and pressure drop curves would make possible picking a C/F ratio for the actual commercial application. For a crude example, for the 100 T/m²/day ammonia conversion plant using a 30 sheet (elements) catalyst system of 3mil wire diameter, 80 mesh catalyst (90% Pt:10% Rh) would pick the highest C/F ratio available (or practicable) using Figure 1. DESCRIPTION OF PREFERRED EMBODIMENT A computer model of the system of this invention is used to predict performance of various catalyst systems is various commercial plant situations. Example 1 Nitric Acid Production: 100 tons per day Nitrogen Loading: 116 tons N/meter²/day Inlet Temperature: 250°C Number of Woven Sheets: 30 The current residence time for a flat system is 4 x 10⁻⁴ seconds with a conversion efficiency of 96% (see Figure 4 for 30 sheets). It has been found through experimentation that the optimum efficiency is obtained at a residence time of 6.0 x 10⁻⁴ seconds. To obtain this optimum residence time, the developed curve to flat ratio must be increased from 1 to 1.5. This will increase the conversion of ammonia from 96% to 99% at the same operating conditions. Example 2 The operating conditions are the same as the preceding example. The nitric acid producer is not interested in increasing conversion efficiency, but would like to lower the precious metal loading in his reactor. This can be achieved by arbitrarily reducing the number of sheets being used. For the number of sheets equal to 20, the curve to flat ratio must be increased from 1 to 1.5 to obtain a conversion efficiency of 96% (see Figure 5 for 20 sheets). The following are examples for use in low pressure drop, high production capacity gauze for the production of Hydrogen Cyanide. Example 3 (Prior Art HCN) A reactor for the production of hydrogen cyanide has a reactor diameter of 35 inches. The volumetric flow rate of ammonia, air and methane is 2.266m³/sec (80ft 3/sec). It has been found that the maximum selectivity of the reactants to product is reached at a residence time of 6.8 x 10⁻⁴ seconds. With a volumetric flow rate of 2.266m³/sec and a reactor diameter 88.9cm (35 inches), 15 sheets of gauze are required to obtain a residence time of 6.8 x 10⁻⁴ seconds. Example 4 The HCN producer wants to increase the volumetric flow rate of ammonia, air, and methane to 5.66m³/sec (200ft 3/sec) by increasing the curve to flat ratio of the gauze. Figure 1 is a plot of volumetric flow rate vs developed curve to flat ratio at a residence time of 6.8 x 10⁻⁴ seconds and 15 sheets of gauze. To increase the production flow rate to 5.66m³/sec, the curve to flat ratio must be increased to 2.5. The increased developed curve to flat ratios in excess of 1:1, independent of how they are formed, accomplish the following: 1. If it is assumed that the same precious metal alloy mesh and wire diameter system (3.15 mesh/mm, 0.076mm wire diameter) is used, the amount of available open area in the corrugated woven gauze system per unit area of reactor cross section increases; the amount of precious metal alloy available for catalysis increases per cross-sectional area of reactor; the initial pressure drop and the rate of increase of pressure drop across corrugated woven gauze system decrease; the amount of hydrogen cyanide production per cross-sectional area of reactor is increased; the increase of decrease or the above are functions of the developed curve to flat ratio. 2. If it is assumed that a precious metal alloy gauze system of equal weight per cross-sectional area of the reactor is used either the meshes per inch in the warp or the weft or both, wire diameter in the warp or the weft or both, of both must be changed. Example 5 Designated C/F ratio = 2.6:1 Open Area = 2.61 x equivalent flat pack open area. Precious Metal Alloy Weight = 2.61 x equivalent of flat pack precious metal alloy weight (Assumption that flat pack has pressure drop of 1 unit.) While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, the invention is not limited to the disclosed embodiment but, on the contrary, is intended to cover various modifications and equivalents included within the spirit and scope of the following claims.

Claims: A catalytic element for use in the catalytic oxidation of ammonia or ammonia in conjunction with methane, said element comprising a foraminous structure fabricated from a material consisting essentially of a metal selected from the group consisting of platinum, rhodium, palladium and alloys of mixtures thereof characterised by (a) a novel non-planar configuration whereby the initial product of the formula: curve to flat ratio (C/F) multiplied by mesh count (N) and wire diameter (d w ), for said element is greater than at least about 0.08 and (b) where, for a given ammonia throughput, the conversion efficiency is a function of the curve to flat ratio (C/F), wire diameter (d w ) and mesh count (N) combination and conversion efficiency is improved by increasing the mesh content (N) for a given wire diameter, increasing the wire diameter (d w ) for a given mesh count, and increasing the curve to flat ratio (C/F) to a ratio of above 1.0. An element according to claim 1, wherein the initial product of said formula is greater than 0.9 and the curve to flat ratio (C/F) is above about π/2. An element according to claim 1 or 2, wherein the initial product of said formula is in the range of from about 0.08 to about 10. An element according to claim 3, wherein said product is from about 0.2 to about 10. An element according to any one of the preceding claims, wherein said element is a woven gauze, knitted fabric, fibres or combinations thereof. An element according to any one of the preceding claims, in combination with one or more other catalytic elements to form a catalyst pack. An element according to any one of the preceding claims, wherein said element consists essentially of platinum alloyed with one or more metals selected from the group consisting of nickel, cobalt, palladium, rhodium, ruthenium, iridium, gold, silver and copper. An element according to claim 7, wherein the platinum is present in an amount of at least about 70% by weight. An element according to any one of the preceding claims, wherein C/F is in the range of from about 1.00 to about 4 and the product of said formula is greater than about 0.08. An element according to any one of the preceding claims, wherein C/F is in the range of from above 1.0 to about 4, N is in the range of from about 1.57 to 4.724 mesh per mm (40 or 120 per inch), d w is in the range of from about 0.025 to 0.46mm (0.001 to 0.018 inches) and their respective values are such that the initial product of said formula is greater than at least about 0.08. An element according to any one of the preceding claims, wherein said C/F ratio is achieved by means of forming said element into a pleat-like configuration. An element according to claim 11, wherein the pleats are parallel. An element according to claim 11, wherein said C/F ratio is achieved by means of pleats in concentric patterns. An element according to any one of the preceding claims, wherein said C/F ratio is achieved by means of shaped depressions on the surface of said element. A method for the catalytic oxidation of ammonia or ammonia in conjunction with methane at temperatures above 850°C, wherein the catalyst comprises a catalytic element according to any one of claims 1 to 14. A method according to claim 15, wherein said product changes from higher at the elements initially contacted by ammonia to lower where said ammonia last contacts the elements. A method according to claim 15, wherein said product changes from lower at the elements initially contacted by ammonia to higher where said ammonia last contacts the elements. The use of a catalytic element according to any one of claims 1 to 14, as a catalyst in the catalytic oxidation of ammonia or of ammonia in conjunction with methane.

Inventors: HEFFERNEN STEVEN A, HOCHELLA WILLIAM A, HEFFERNEN, STEVEN A., HOCHELLA, WILLIAM A.
Applicants: JOHNSON MATTHEY PLC, JOHNSON MATTHEY PUBLIC LIMITED COMPANY
Application year: 1992
Main classification: B01J35
IPC codes: C01B21, B01J23, C01C3, B01J35