Nitric acid is manufactured through the Ostwald process which involves oxidizing ammonia with air over a platinum-rhodium catalyst at 850°C to produce nitric oxide, which is then oxidized to nitrogen dioxide and absorbed in water to form nitric acid. The manufacturing process can operate at single or dual pressures and involves filtering air, oxidizing and absorbing NO to form weak nitric acid, and concentrating the weak acid through extractive distillation with sulfuric acid to produce strong nitric acid at 99% concentration. Emissions are controlled by further absorbing tail gases to produce sodium nitrate and nitrite or with wet scrubbers.

1. Description of Nitric acid manufacturing process
Sameer Pandey1
National Fertilizers Limited ,Naya Nanagal, Ropar, Punjab,140126
Abstract: Nitric acid is a very valuable chemical and finds intensive use in polymer industry, as an oxidizing agent and
in producing other chemicals. The basis of Nitric acid manufacturing process has remained the same over the years, but
changes have been made to the operations involved, to make the process more environmently friendly and less energy
intensive. This paper presents a Technical description of the manufacturing process of Nitric acid.
INTRODUCTION:
Nitric acid is manufactured by Ostwald's process, main reaction is as follows:
4NH3 + 5O2 → 4NO + 6H2O (1) ∆H= -217Kcal.
Other possible side reactions are:
4NH3 + 3O2 → 2N2 + 6H2O (2)
2NH3 + 2O2 → N2O + 3H2O (3)
4NH3 + 7O2 → 4NO2 + 6H2O (4)
2NH3↔N2 + 3H2 (5)
4NH3 + 6NO → 5N2 + 6H2O (6)
2NO → N2 + O2 (7)
Conditions in the reactor are maintained so as to favour the reaction 1, if the flow rate of the
reactants is too high, ammonia may escape and reaction conversion drops. Escaped ammonia may
then reac with NO according to reaction 6. If the flowrate is too low, NO formed may decompose
according to reaction 7, due to prolonged exposure to catalyst.
NO formed is oxidized in the oxidation towers with the following reaction:
2NO + O2 → 2NO2 ∆H= -26.9 Kcal (8)
the formed NO2 is absorbed in water to form nitric acid:
3NO2 +H2O → 2HNO3 + NO ∆H=-32.5 Kcal (9)
Both reactions 8 and 9 are exothermic hence heats of absorption are removed to maintain acid
production.
INDUSTRIAL MANUFACTURING PROCESS:
Ammonia is oxidized by air in reactors (reaction 1) , over Pt-Rh catalyst at a temperature of about
850 o
C. The catalyst are in the form of closely knitted gauges, composition is around 95% Pt. Some
Platinum and Rhodium may vaporize during the process, so a catchment system is provided below
the catalyst, which is a palladium alloy, this saves 80% to 90% of the catalyst loses.
Impurities from Air may harm the catalyst activity so filteration of process air mixture is a must.
The NO formed is further oxidized to NO2 according to reaction 8, and absorbed in De-mineralised
Water to form Nitric acid.
The various process differ on operating pressures of the plant:
a) Single medium pressure process
b) Single high pressure process
c) Dual pressure process
as the name suggests the dual pressure process involves combination of medium and high pressures.
Medium pressure process gives higher efficiency in reactors, high pressure process gives better
economics in absorption, dual pressure process combines both the advantages.

2. Figure 1: flow diagram showing the process for weak Nitric acid manufacturing.
The dual pressure process has a compressor after the cooling step before the absorption step.
The technology has changed involving the air compressors used in large capacity plants. Bull Gear
compressors are now being used for dual pressure and high pressure systems.
Burners with better design and larger diameter are being used for large capacity plants, diameter
ranging from 5.7 m to 6 m.
weak Nitric acid is converted to strong nitric acid by simple steps.

3. Figure 2: Strong nitric acid production.
In the first step, the dehydration of weak nitric acid is done by extractive distillation. H2 SO4 upto
60% is added in the Dehydrating tower, concentrated nitric acid upto 99% is obtained as vapors
from the top. Bleaching removes oxygen, cooling the vapors gives strong acid as product, the
remaining gases are sent to absorption coloumn and mixed with air to give weak nitric acid.
The Nox emmissions are controlled by further absorbing the tail gas in caustic solution to produce
sodium nitrate and sodium nitrite. Wet scrubbers can also be used.