Nitric acid is one of the most common mineral acids in the chemical Industry. It is produced by a high temperature oxidation of ammonia with oxygen catalyzed by a platinum/rhodium contact.
This results in a nitric acid/water mixture with a strength of up to almost 68 wt% nitric acid. The majority of nitric acid is used for the production of fertilizers.
Other processes such as the production of explosives, pigments, rocket fuels or polyurethanes require higher concentrated nitric acid.
Nitrogen oxides are major pollutants in the atmosphere, being a precursor to acid rain, photochemical smog, and ozone accumulation. The oxides are mainly nitric oxide (NOx) and nitrogen dioxide (NO2) both of which are corrosive and hazardous to health. With the use of catalytic converters, the focus of controlling NOx emissions has become economically impractical. Consequently, the stationary sources of NOx emissions are now being subjected to more stringent standards in many countries. The sources include nitric acid manufacturing plants, manufacturers of nitrated materials such as fertilizer and explosives, and industrial manufacturers, metallurgical processors, glass manufacturers, cement kilns, and power generators where high processing temperatures are used. Because of the environmental concerns posed by air pollution, it is important to design and implement scrubber systems for controlling NOx emissions.
Combustion of fuels: At high temperatures, oxygen and nitrogen present in air combine to form nitrogen oxides. Typical flue gas samples contain 100-1500 ppm of nitrogen oxides.
Nitric acid manufacture: Waste gases which cannot be economically recovered in the final absorber usually contain 2-3% nitrogen oxides based on the weight of acid produced.
Metal finishing operations: Many metal surface treatment operations which use nitrates, nitrites, or nitric acid evolve nitrogen oxides.
Chemical processes: Many processes where nitric acid, nitrates, or nitrites are used as reagents evolve nitrogen oxides. Examples include the manufacture of explosives, plastics, dyes, etc.
High-temperature processes: Processes where materials are made at high temperatures such as glass manufacturing, electric furnaces, and cement kilns evolve nitrogen oxides.
"Breathing" of the tank during changes in ambient temperatures, resulting in very low rates of emissions.
Tank filling that displaces gaseous volumes equal to the volumes flowing into the storage tank.
Long-term inhalation exposures to nitric acid can cause chronic respiratory irritation, which may result in chronic bronchitis and airways hyper-reactivity. Repeated exposure to nitric acid vapours, mists, or aerosols may cause dental erosion. As nitric acid is inhaled, it may be deposited on teeth and cause decalcification resulting in erosion of tooth enamel.
OSHA: The legal airborne permissible exposure limit (PEL): 2 ppm (8-hour average)
NIOSH: The recommended airborne exposure limit (REL): 2 ppm (10-hour average)
ACGIH: The threshold limit value (TLV): 2 ppm
ACGIH: Short time exposure limit (STEL): 4 ppm
The process entails injecting H2O2 into the absorber feed water to provide a dilute solution containing 0.5 - 1 wt.% H2O2. This solution is passed through the absorber where NOx constituents are transferred from the vapor into the solution and oxidized to high-grade HNO3. The oxidation reactions are rapid at moderate temperatures (30-80 degrees C), with about 1.7 and 0.4 lbs H2O2 required per lb of NO and NO2, respectively.
The chemistry controlling the process:
3NO2 + H2O → 2HNO3 + NO
2NO + HNO3 + H2O → 3HNO2
HNO2 + H2O2 → HNO3 + H2O