Nitric Acid High Efficiency Absorption

All nitric acid plants are based on the same basic chemical operations 1) Oxidation of ammonia with air to give nitric oxide, 2) Oxidation of the nitric oxide to nitrogen dioxide and 3) Absorption in water to give a solution of nitric acid. The efficiency of the first step is favored by low pressure whereas that of the second step is favored by high pressure. These considerations, combined with economic reasons give rise to two types of nitric acid plants - single pressure and dual pressure97. 

The principle of the high-efficiency absorption (HEA) column is liquid-phase oxidation and absorption. The product is a dilute nitric acid, which is recycled to tiie main absorption system. The nonabsorbed gases are heated and directed to the catalytic reactor (Rhone-... 

As demand for nitric acid increased, efforts were made to improve the process by operation at higher pressures, which would allow the use of smaller equipment with lower capital costs. High pressure could also provide higher acid concentrations with more efficient absorption and an increased rate of reaction in converting nitrogen oxides to nitric acid. High-pressure operation was made possible when chromium and chromium/nickel alloy steels replaced ceramic materials. 

Modern pressure-absorption s retems permit absorption efficiencies of more than 99% and acid concentrations in the range of 55% to 65% to be obtained. In some processes, concentrations of up to 70% HNO3 are achieved by using high pressures and cooled water in conjunction with specially designed absorption columns. In some plants, excess air may be injected into either the cooler or absorption column to speed the oxidation reaction. Process water for absorption systems must be very pure to reduce corrosion effects in the nitric acid plant and also in applications involving subsequent use of the acid. In particular, the chloride content must be very low. Hence, water from condensate sources or ion-exchange purification units is often used. 

There is little opportunity to improve further the Oswald nitric acid process. The overall efficiency of ammonia conversion into acid is in the range of 949fr 96%, The process is self-sufficient in energy supply and can even export steam. Up to 80% of platinum catalyst losses can now be recovered. Capital investments were significantly reduced by the development of high-pressure absorption and with the design of efficient and reliable compressors and expanders. Economies of scale improve little for single-train units with capacities above 1,000 tpd. 

The NH3 into NO conversion efficiency increases with decreasing pressure, whereas the conversion of NO into NO2 and the subsequent absorption is favored by high pressures. Thus, modern nitric acid piants are duai pressure processes, that is, the product gas of ammonia oxidation (at 6 bar) is compressed to 12 bar and then fed to the absorption tower for NO oxidation and for NO2 absorption. 

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