Engineering Evaluation Case: A Nitric-Acid Absorber Is Limited by NO Oxidation Rather Than Packing Height
Industrial nitric-acid absorption involves more than dissolving NO₂ into water.
The reaction system can also generate nitric oxide, which must be oxidized back toward more absorbable nitrogen-oxide species.
The packed tower therefore depends on both:
- gas-liquid absorption;
- gas-phase oxidation chemistry.
Project Situation
Consider a packed nitric-acid absorber receiving NOx-containing gas.
Water or weak nitric acid enters from above.
The process target is:
- acid production or recovery;
- low NOx outlet.
The plant considers adding more packing because outlet NOx remains high.
Gas analysis shows a significant fraction of nitric oxide.
NO Oxidation Requires Time and Oxygen
NO is comparatively difficult to absorb directly.
Its conversion toward NO₂ depends on:
- oxygen availability;
- concentration;
- temperature;
- residence time.
A tower with excellent liquid contact can still be limited by this gas-phase reaction.
Absorption and Reaction Change Through the Tower
Near one section, NO₂ absorption may dominate.
Elsewhere, NO generated by the chemistry must be oxidized before it can be absorbed effectively.
This means the entire bed does not perform one identical function.
Cooling Is Important
Nitrogen-oxide absorption and nitric-acid formation interact with temperature.
Heat release can reduce favorable absorption conditions.
Therefore:
- cooling;
- liquid temperature;
- acid circulation
can affect overall performance.
Acid Strength Changes Liquid Properties
As nitric acid concentration increases, liquid:
- density;
- corrosivity;
- equilibrium behavior
change.
Hydraulic and material calculations should reflect the actual acid-strength profile.
More Packing May Not Solve an Oxidation-Limited System
If NO needs additional gas-phase reaction time, simply adding wetted surface may not provide the expected improvement.
The process design may need to examine:
- oxidation volume;
- oxygen content;
- staging;
- cooling.
Material Selection Must Include Oxidizing Acid
Nitric acid is strongly oxidizing.
Packing, support, distributor, gaskets, and demister require compatibility review against the actual concentration and temperature.
Engineering Takeaway
Nitric-acid absorption is a coupled oxidation-and-absorption process.
The controlling resistance may be outside the liquid film.