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Neutralization
This section comprises a first and second neutralizer, an ammonium nitrate storage tank,
a UAN storage tank and off-gas purification equipment. Essentially all ammonia which is not
converted into urea in the synthesis section is converted here into ammonium nitrate.
The first neutralizer consists of a U-type combination of a circulation tube and a mixing tube
with a separator. It operates just above the atmospheric pressure. The NH3-containing gases
from the urea plant are fed into the bottom of the mixing tube; the nitric acid is introduced
somewhat lower in the circulation tube. The two components react exothermically to form ammonium
nitrate according to the following reaction equation,
The presence of liquid in the circulation tube and of the gas-liquid mixture in the mixing
tube causes the fluid to circulate. The heat of reaction is used for the evaporation of water
and to increase the temperature to about 135°C. The pH of the solution is controlled
by addition of nitric acid.
The gas and liquid phases are separated in the separator, the liquid being sent to
the AN storage tank.
In the second neutralizer the following substances are mixed:
- urea solution, still containing some NH3, temperature
110 °C;
- ammonium nitrate solution, containing some 0,4% HNO3,
temperature 135 °C;
- overhead vapors from the urea storage tank;
- nitric acid;
- liquid effluent from the dust purification scrubber;
Here again, the heat of reaction is utilized to increase the temperature and for evaporation
of water. The ammonium nitrate to urea ratio is controlled at about 1.35. The pH is controlled
at 5 to 6 by means of the HNO3 feed.
The resultant UAN solution is either stored or pumped to battery limits.
The off-gases from the first and second neutralizer are treated in the off-gas purification Section.
Nitric acid is supplied to this section to neutralize any NH3 which is contained
in the off-gases from both neutralizers. The liquid effluent from the purifier flows down
to the second neutralizer.
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UREA 
