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"Nitric Acid, Nitrous Acid, and Nitrogen Oxides," In: Ullmann's Encyclopedia of Industrial Chemistry

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Article No : a17_293 Nitric Acid, Nitrous Acid, and Nitrogen Oxides MICHAEL THIEMANN, Uhde GmbH, Dortmund, Federal Republic of Germany ERICH SCHEIBLER, Uhde GmbH, Dortmund, Federal Republic of Germany KARL WILHELM WIEGAND, Uhde GmbH, Dortmund, Federal Republic of Germany 1. Nitric Acid ....................... 177 1.4.1. Wastewater . ...................... 211 1.1. Introduction...................... 177 1.4.2. Stack Gas . ...................... 211 1.2. Properties ....................... 178 1.4.2.1. Emission Limits . .................. 211 1.3. Industrial Production............... 179 1.4.2.2. Analysis . ...................... 212 1.3.1. Oxidation of Ammonia . ............. 180 1.4.2.3. Control of NOx Emissions . ......... 212 1.3.2. Oxidation and Absorption of Nitrogen 1.5. Storage and Transportation.......... 216 Oxides .......................... 184 1.6. Uses and Economic Aspects .......... 216 1.3.3. Equipment. ...................... 191 2. Nitrous Acid ..................... 217 1.3.3.1. Filters and Mixers .................. 191 3. Nitrogen Oxides................... 218 1.3.3.2. Burners and Waste-Heat Boilers........ 192 3.1. Dinitrogen Monoxide ............... 218 1.3.3.3. Compressors and Turbines . ......... 193 3.2. Nitrogen Monoxide ................ 219 1.3.3.4. Heat Exchangers and Columns......... 195 3.3. Nitrogen Dioxide and Dinitrogen 1.3.3.5. Construction Materials. ............. 198 Tetroxide ........................ 220 1.3.4. Processes . ...................... 200 3.4. Dinitrogen Trioxide ................ 221 1.3.4.1. Weak Acid Processes. ............. 200 3.5. Dinitrogen Pentoxide ............... 222 1.3.4.2. Concentrated Acid Processes . ......... 206 4. Toxicology and Occupational Health ... 222 1.4. Environmental Protection ........... 211 References ....................... 223 1. Nitric Acid The direct combustion of air in an electric arc, developed by BIRKELAND and EYDE, was aban- 1.1. Introduction doned because of its poor energy efficiency. Later direct processes, such as thermal nitrogen Nitric acid is a strong acid that occurs in nature monoxide synthesis with fossil fuels or nitrogen only in the form of nitrate salts. When large-scale monoxide synthesis in nuclear reactors, did not production of nitric acid began, sodium nitrate gain widespread acceptance. Ammonia produc- (soda saltpeter, Chile saltpeter) was used as the tion from calcium cyanamide was of only transi- feedstock. At the beginning of the 20th century tory value. Ammonia produced from nitrogen the reserves of Chile saltpeter were thought to be and hydrogen by the Haber – Bosch process nearing exhaustion, so processes were developed (! Ammonia, 2. Production Processes) is, for replacing nitrogen from natural nitrates however, still used as feedstock for nitric acid with atmospheric nitrogen. Three techniques production. were used industrially: The crucial step in nitric acid production, the catalytic combustion of ammonia, was 1. Production of nitrogen monoxide by reacting developed by OSTWALD around the turn of the atmospheric nitrogen and oxygen at > 2000 century. The most important design parameters C (direct processes) for a nitric acid plant were determined first in 2. Production of ammonia by hydrolysis of laboratory tests and later in a pilot plant. The calcium cyanamide under pressure first production facility employing the Ostwald 3. Production of ammonia from nitrogen and process came on stream in 1906 at Gerthe in hydrogen Germany [1–3]. Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/14356007.a17_293 178 Nitric Acid, Nitrous Acid, and Nitrogen Oxides Vol. 24 Most of the nitric acid produced is used to Heat of vaporization (20 C) 626.3 J/g Specific heat form inorganic fertilizers (! Phosphate Fertili- at 0 C 1.7601 J gÀ1 KÀ1 zers); it is mostly neutralized with ammonia to at 20 C 1.7481 J gÀ1 KÀ1 form ammonium nitrate (! Ammonium Com- pounds, Section 1.2.1.). The decomposition of nitric acid makes its physical properties difficult to determine at high- er temperature. Up to ca. 50 C, conventional 1.2. Properties methods of measurement are possible; beyond this, indirect thermodynamic calculations or Nitric acid was known to the ancient Egyptians special short-time measuring methods must be because of its special ability to separate gold employed. Figure 1 illustrates the vapor pressure and silver. Many well-known alchemists in the curve of pure nitric acid. Table 1 lists important Middle Ages experimented with the acid. In the physical properties of aqueous nitric acid. A middle of the 17th century, GLAUBER reported its comprehensive review of the physical and chem- preparation from saltpeter and sulfuric acid. ical properties of nitric acid is given in [6]. Physical Properties. Nitric acid [7697-37- Chemical Properties. Concentrated nitric 2], Mr 63.013, is miscible with water in all acid, with nitrogen in the þ 5 oxidation state, proportions. At a concentration of 69.2 wt %, it acts as a strong oxidizing agent. The reaction forms a maximum-boiling azeotrope with water. À þ The azeotropic mixture boils at 121.8 C. Pure NO3 þ4H NOþ2H2O anhydrous nitric acid boils at 83 – 87 C; the goes to the right for all substances with oxidation reason a range of boiling points are cited in potentials more negative than þ 0.93 V [7]. For the literature is that the acid decomposes on example, copper (þ 0.337 V) and silver heating [4]: (þ 0.799 V) are dissolved by nitric acid, where- 4 HNO !2H Oþ4NO þO as gold (þ 1.498 V) and platinum (þ 1.2 V) are 3 2 2 2 resistant. In practice, 50 % nitric acid (aqua The nitrogen dioxide formed on decomposi- tion colors the acid yellow or even red at higher concentrations. Because the vapors can also absorb moisture, the term ‘‘red fuming nitric acid’’ is used. In the pure anhydrous state, nitric acid is a colorless liquid. The most important physical properties of pure nitric acid follow: fp À41.59 C bp 82.6 Æ 0.2 C Density, liquid at 0 C 1549.2 kg/m3 at 20 C 1512.8 kg/m3 at 40 C 1476.4 kg/m3 24 Refractive index nD 1.3970 Dynamic viscosity at 0 C 1.092 mPa Á s at 25 C 0.746 mPa Á s at 40 C 0.617 mPa Á s Surface tension at 0 C 0.04356 N/m at 20 C 0.04115 N/m at 40 C 0.03776 N/m Thermal conductivity (20 C) 0.343 W mÀ1 KÀ1 Standard enthalpy of formation Liquid 2.7474 J/g Gas 2.1258 J/g Figure 1. Vapor pressure curve for pure nitric acid Vol. 24 Nitric Acid, Nitrous Acid, and Nitrogen Oxides 179 Table 1. Physical properties of aqueous nitric acid as a function of composition [5] HNO3 Density mp, C bp, C Partial concentration, (20 C), pressure wt % g/cm3 (20 C), kPa HNO3 H2O 0 0.99823 0 100.0 2.23 10 1.0543 À7 101.2 2.26 20 1.1150 À17 103.4 2.02 30 1.1800 À36 107.0 1.76 40 1.2463 À30 112.0 1.44 50 1.3100 À20 116.4 0.03 1.05 60 1.3667 À22 120.4 0.12 0.65 70 1.4134 À41 121.6 0.39 0.35 80 1.4521 À39 116.6 1.4 0.12 90 1.4826 À60 102.0 3.6 0.03 100 1.5129 À42 86.0 6.0 0 fortis) is used for separating gold from silver. Some base metals, such as aluminum and chro- mium, are attacked by nitric acid only at their surfaces because a thin oxide layer is formed, Figure 2. Ostwald process which protects (i.e., passivates) the core against further oxidation. As a result of this passivation, alloyed steel equipment can be used in nitric acid technology. Highly diluted nitric acid is almost completely 2NOþO2!2NO2 N2O4 ð2Þ dissociated 2. Absorption of the nitrogen oxides to yield HNO þH O H OþþNOÀ 3 2 3 3 nitric acid: and does not attack copper and more noble 3NO þH O!2 HNO þNO ð3Þ metals. Due to its acid nature, however, it reacts 2 2 3 with base metals, liberating hydrogen and form- ing nitrates. A mixture (volume ratio 3 : 1) of concentrated The way in which these three steps are nitric acid and concentrated hydrochloric acid implemented characterizes the various nitric acid (aqua regia) also dissolves gold. processes. In monopressure (single-pressure) processes, ammonia combustion and NOx ab- sorption take place at the same working pressure. 1.3. Industrial Production These include medium-pressure (230 – 600 kPa) and high-pressure (700 – 1100 kPa) The industrial production of nitric acid by the processes. Very few plants currently employ low Ostwald process is described in this section. The pressure (100 – 220 kPa) for both combustion process involves three chemical steps (Fig. 2): and absorption. In dual-pressure (split-pressure) processes, 1. Catalytic oxidation of ammonia with atmo- the absorption pressure is higher than the com- spheric oxygen to yield nitrogen monoxide: bustion pressure. Modern dual-pressure plants feature combustion at 400 – 600 kPa and ab- 4NH3þ5O2!4NOþ6H2O ð1Þ sorption at 900 – 1400 kPa. Some older plants Oxidation of the nitrogen monoxide product still employ atmospheric combustion and medi- to nitrogen dioxide or dinitrogen tetroxide: um-pressure absorption. 180 Nitric Acid, Nitrous Acid, and Nitrogen Oxides Vol. 24 Intensive work on ammonia combustion as O2 À 2Oðadsorb:Þ well as the oxidation and absorption of nitrogen oxides has been well documented since the early O ðadsorb:ÞþNH3!NH2OH 1920s. Up to now the combustion of ammonia has been considered as a heterogeneous catalytic NH2OHþO2!HNO2þH2O surface reaction with undesirable side reactions. À The oxidation of nitrogen monoxide is a rare HNO2!NOþOH example of a homogeneous third-order gas-phase À reaction. The absorption of NOx is a heteroge- 2OH !H2OþO neous reaction between fluid (i.e., gas and liquid) phases. In oxidation experiments under high vacuum, BODENSTEIN and coworkers detected both hydrox- ylamine and nitrous acid as intermediate pro- 1.3.1.
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