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Nox Absorption with Nitric Acid Recovery Plants

NOx absorption

The absorption of nitrous gases with the formation of nitric acid can be applied to all processes where there are waste gases of relatively high nitrous content such as for example in the nitration of organic compounds, in the decomposition of nitro compounds or nitric acid in the denitration of waste acids.

Nitric acid is formed by absorption of nitrogen dioxide in water or dilute nitric acid:

3NO2+ H2O>>2HNO3+ NO

The nitrogen monoxide formed during this reaction and that already present in the exhaust gas must be oxidized:

2 NO + O2>> 2 NO2

Both reactions are strongly exothermic and are pressure and temperature dependent. The result is that at normal pressure and with the use of cooling water at approx. 25 deg C it is possible to produce nitric acid at a concentration of approx. 50% and the exhaust gas leaves the column with an NOx content of about 1000 to 2000 ppm. For relatively small quantities of exhaust gas this is mixed with air to achieve permitted waste gas contents for example, 200ppm.

As well as absorption at normal pressure the following processes have proved effective in industrial use:

  • Pressure absorber in the medium pressure range to achieve higher nitric acid concentration and to reduce the emission of NOx.
  • Cooling the absorber with cold water to achieve a lower NOx content in the exhaust gas.
  • Operation of the absorber with peroxide solution as an absorption agent to achieve very low NOx contents by a direct route.

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