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Chapter 33.4.4 Optimisation of the absorption stageDescriptionSee also Section 3.4.10 “Addition of H 2 O 2 to the last absorption stage”.Optimising the absorption step results in an efficient formation of the nitric acid and itminimises emissions of NO and NO 2 to air. In this section, the oxidation of nitrogen monoxide(NO) to nitrogen dioxide (NO 2 ), the absorption of NO 2 in water as N 2 O 4 and the reaction ofN 2 O 4 to HNO 3 are considered as one step (i.e. the “absorption step”). Various parameters effectthe absorption efficiency, mainly:1. Pressure The absorption stage, in nitric acid production, is favoured by highpressure. The benefit of high pressure is the efficient formation of nitricacid and the minimisation of NO x emissions. The absorption step iscarried out at pressures above atmospheric, and at least medium pressure(1.7 – 6.5 bar) is preferred in modern nitric acid plants, and is consideredoptimal. The effect of pressure on the absorption stage is illustrated inTable 3.10. Figure 3.6 shows applied absorption pressures in relation tothe related tail gas levels.2. Temperature The absorption stage in particular nitric acid formation takes place in thelower third of the absorption column and is exothermic, so cooling isneeded for heat removal. This is achieved by applying cooling before theabsorption column to optimise absorption.3. Optimalcontactbetween NO x ,O 2 and H 2 OOptimal contact is mainly dependent on the absorption tower design.Several parameters contribute to an optimal design, e.g. the volume, thenumber and kind of trays used, the distance between the trays and thenumber of columns. In addition, a long residence time will ensure a highrecovery of NO X to form HNO 3 , and also minimise NO X emissions. Mostnitric acid plants have a single absorption tower with sieve or bubble captrays. The tray spacing progressively increases up to the top of theabsorber.M/H L/M UnitAbsorption pressure 8 3.8 barAbsorption temperature 25 25 ºCAbsorption efficiency 99.6 98.2 %NO x concentration before SCR k 500 2000 – 3000 ppmTable 3.10: Parameters of the absorption stages in a M/H and a L/M plant[94, Austrian UBA, 2001]Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 117