Chemistry Reference
In-Depth Information
815
∘
C to 930
∘
C with most plants operating at about 10 atmospheres [26].
The temperature is chosen by balancing nitric oxide yield, which increases
with increasing temperature, and catalyst loss, which also increases with
increasing temperature. Commonly, the oxidation takes place at about 900
∘
C.
Catalyst
4 NH
3(g)
+ 5 O
2(g)
4 NO
(g)
+ 6 H
2
O
(g)
2 NO + O
2(g)
2 NO
2(g)
N
2
O
4
3 NO
2
+ H
2
O
2 HNO
3
+ NO
The nitric oxide which is formed reacts with oxygen to form nitrogen
dioxide. Nitrogen dioxide exists in equilibrium with its dimer, dinitrogen
tetroxide. The nitrogen dioxide/dimer mixture is sent to a column, sometimes
called an absorption tower. Water is added at the top of the column. The
nitrogen dioxide is converted to nitric acid. Byproduct nitric oxide is oxidized
to nitrogen dioxide by means of a stream of air passed into the absorption
column. The aqueous nitric acid is removed continuously from the base of
the column. Overall, the reaction can be written as:
NH
3
+ 2 O
2
HNO
3
+ H
2
O
Industrial processes are single-pressure or two-pressure. In the
single-pressure process, the absorption takes place at the same pres-
sure as the catalytic oxidation. In the two-pressure process, the absorption
pressure is higher than the catalytic oxidation. The catalytic oxidation (first
reaction) is favored by low pressures and the two subsequent reactions (nitric
oxide oxidation and reaction with water) by high pressure. Therefore, newer
plants tend to operate with the two-pressure process. The main advantage is
less platinum consumption.
Recognize that the reaction to convert ammonia to nitric acid is more
complicated than summarized here. One set of authors proclaimed, “Never
has a reaction system been studied so long with ultimately so little consensus
on mechanistic explanation [27].” For example, ammonia can be oxidized to
form nitrogen or nitrous oxide (N
2
O).
4 NH
3
+ 3 O
2
2 N
2
+ 6 H
2
O
2 NH
3
+ 2 O
2
N
2
O + 3 H
2
O
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