Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 3Pressure in bar Temperature (°C) NO yield (%)6.5 900 – 940 95Table 3.2: NO dependence on pressure and temperature[102, EFMA, 2000]The reaction is carried out in the presence of a catalyst. The catalyst typically consists of severalwoven or knitted gauzes formed from wire containing approximately 90 % platinum alloyedwith rhodium for greater strength and sometimes containing palladium.The enthalpy of the hot reaction gas is used to produce steam and/or preheat the tail gas. Afterthis, the reaction gas has a temperature of 100 to 200 °C, depending on the process and it is thenfurther cooled with water. The water formed in the oxidation reactions is condensed in a coolercondenserand transferred to the absorption column.3.2.4 Oxidation of NO and absorption in H 2 ONitric oxide is oxidised to nitrogen dioxide as the combustion gases are cooled, according to theequation:2 NO + O 2 d 2 NO 2For this purpose, secondary air is added to the gas mixture obtained from the ammoniaoxidation. Demineralised water, steam condensate or process condensate is added at the top ofthe absorption column. The weak acid solution (approximately 43 %) produced in the coolercondenser is also added to the absorption column. The NO 2 in the absorption column iscontacted countercurrently with flowing H 2 O, reacting to give HNO 3 and NO:3 NO 2 + H 2 O d 2 HNO 3 + NOThe oxidation, absorption of the nitrogen dioxide and its reaction to nitric acid and nitric oxidetake place simultaneously in the gaseous and liquid phases. Both reactions (oxidation and HNO 3formation) depend on pressure and temperature and are favoured by higher pressure and lowertemperature.The formation of HNO 3 is exothermic and continuous cooling is needed within the absorber. Asthe conversion of NO to NO 2 is favoured by low temperature, this will be the significantreaction taking place until the gases leave the absorption column. The nitric acid produced in theabsorber contains dissolved nitrogen oxides and is then ‘bleached’ by the secondary air.An aqueous solution of nitric acid is withdrawn from the bottom of the absorption tower. Theacid concentration can vary from 50 – 65 wt-% nitric acid, depending on the temperature,pressure, the number of absorption stages and the concentration of nitrogen oxides entering theabsorber. The gases that are not absorbed in the nitric acid solution leave the absorption columnat the top, at a temperature of approximately 20 – 30 ºC. This gas mixture is commonly referredto as tail gas and is heated by heat exchange. The hot tail gas is led through a NO X abatementsystem and through a tail gas expander for energy recovery. The resulting expanded tail gas (inmost cases with a temperature above 100 °C to prevent the deposition of ammonium nitrate andammonium nitrite) is vented through a stack.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 97