Ammonia oxidation: the glowing core of nitric acid! - 0 views
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1913, when direct synthesis of ammonia from nitrogen and hydrogen by the Haber-Bosch method became a practical possibility
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Alicia Stephen on 14 Feb 13More possible than ever before
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oxidation of ammonia has become the basis of commercial nitric acid manufacture displacing the old Nitre process involving the distillation of sodium nitrate (Chile saltpetre) and concentrated sulphuric acid.
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irst stage - where ammonia is burned in air to form nitric oxide (NO) - which is the platinum catalysed reaction.
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second oxidation step can take place - the further oxidation of nitric oxide in air to form nitrogen dioxide (NO.sub.2), which is then absorbed in water to form the acid.
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A stoichiometric excess of air is provided to ensure that the equilibrium reaction is biased in favour of NO formation.
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operating temperature at the catalyst gauze is between 700 and 950'C
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gases must be cooled more or less to ambient temperature for the subsequent absorption reaction.
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Usually, the heat is used to raise high pressure steam to run turbines for compressors. Even the relatively low grade heat can be used to reheat process gas to power an expansion turbine.
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cooling favours the further oxidation of nitric oxide to nitrogen dioxide. 2NO + 0.sub. 2 equalibrium 2NO.sub. 2 This is an equilibrium reaction too, and the gases must be cooled to around 200'C to achieve reasonable conversion. As the temperatures become closer to ambient, the nitrogen dioxide forms a dimer - also in an equilibrium reaction.
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the rate of reaction is higher at increased pressure, while ammonia conversion efficiency is higher at low pressure.
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The selection of operating press re in nitric acid production p been much debated over the years.
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Dual pressure systems, operating the burner section at medium pressure and the absorption section at high pressure, to take best advantage of the natural conversion efficiencies of the two stages, have also been commercially successful.
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an energy cost.
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hemical industry, there has historically been a difference in the philosophical approach in the US and in Europe. In the US, in the 1970s before the first energy crisis, relatively inefficient processes operating at high pressure throughout were in normal use, the losses in ammonia efficiency being regarded as unimportant. In Europe by contrast, where ammonia and energy were traditionally higher cost, medium pressure processes were the rule.
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Nowadays, pollution control regulations in many countries stipulate a maximum of 200 ppmv total nitrogen oxides in tail gas, and this can only be achieved directly in a high pressure absorption section. Developments in platinum catalysts
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catalysts used today is their similarity to those used when the ammonia oxidation route was conceived. In 1909, Karl Kaiser's patent indicated the use of a woven gauze made from 0.060 mm (60 micron) wire with 1204 mesh per cm 2. Today, whichever supplier is used, the majority of nitric acid plants use gauzes made from 0.076 or 0.060 mm wire at 1024 mesh to the square centimetre.
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contact times in the order of seconds are necessary to ensure that every ammonia molecule striking the catalyst is oxidized to nitric oxide, and low pressure drops across the catalyst are needed to maintain the fast gas flows. The catalyst must be capable of withstanding these high gas velocities and also significant turbulence at high temperature, for periods up to two months in high pressure plants, and longer for medium pressure facilities.
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The stated advantages of the knitted gauze are greater exposed surface area, giving increased conversion efficiency and lighter gauzes of the same size, because the cross-over points give greater accessibility to gas impingement.
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As early as the 1960s platinum losses, particularly associated with high pressure burner operation, had become a focus for technical development. The concept is simple; what does it matter what the primary loss of platinum from the catalyst gauzes is, so long as you can recover it afterwards, to get back the lost money involved. There is a catch or several catches - to this simplistic idea, of course. The most obvious, especially with the early systems proposed, is that the catchment device cannot catch all of the losses. Secondly there is a refining cost associated with the recovery operation, which must be less than the value of the platinum (and rhodium) being recovered. Thirdly, the catchment devices, especially the more effective ones also involved the use of gauzes made of other highly valuable materials.
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The first "getter gauzes" were introduced in 1968 by Degussa, the German gold-and-silver refiners. Not surprisingly, this gauze was made from an alloy containing 80% palladium and 20% gold. The catchment principle is that platinum vapour displaces the palladium on the catchment gauze, so that the palladium - which is about a quarter the value of platinum - is lost but more valuable platinum is caught for subsequent recovery. The gauze could be installed immediately after the catalyst assembly, and the presence of gold was required to enhance the mechanical strength of the palladium wire. The drawback with this very effective catchment system is that the price of gold was not only high in the late 1960s but it is even higher today, and from the start the quest was on to develop a workable system comprised of cheaper materials. Over the years, the 20% gold was reduced to 10%, and more recently has been eliminated altogether with the development of goldfree palladium alloys, containing such components as chromium, manganese, boron and carbon. Degussa has also recently announced the development of a catchment gauze made from palladium coated nickel wire (Fig. 4). Getter gauze assemblies nowadays consist of two or more gauzes separated from the catalyst only by a heat resistant mesh. They recover platinum and rhodium and, whereas in the early 1980s they were capable of recovering 7585% of primary platinum losses, recovery levels of more than 85% are claimed today, but lower recoveries are to be expected in high pressure plants. Of the specific proprietary systems, Johnson Matthey's PLUS PAC hinged, folded gold-free gauze for easy installation was introduced around 1983, while Engelhard's MTL system - originally launched in the 1970s as Palladium Plus - has very recently been revamped to what the company describes as a unique low pressure drop design. Known as LPMTL, the new system is said to reduce pressure drop effects by as much as 70%. Interestingly Degussa, the original developer of palladium catchment gauzes, has licensed gold-free "getter" gauzes from Engelhard. Dimensions of catchment gauzes have now been thoroughly investigated in regard to their gas flow characteristics, and so further improvements may be less spectacular, but the search for cheaper materials continues.
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Between the catalytic combustion section and the oxidation/absorption section of the nitric acid process, the gas stream needs t6 be efficiently cooled, and this presents an opportunity to recover heat in waste heat boilers and heat exchangers. There are two major considerations for the designer: first, in certain areas of the flow sheet, the gas stream must not be allowed to go below the dew point because of the risk of corrosion; and second the higher the grade of heat recovered, the more valuable it is. The objective is maximum heat recovery at lowest capital cost.
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