2.0 Flowsheet for the Production of Nitric Acid
2.1 Simplified Schematic Flowsheet for Nitric Acid Production
Air
Filter
Compressor/
Heater
Ammonia
Ammonia
Converter
(Oxidation of
Ammonia)
Evaporator
Cold water
NO
Air
Absorption
Tower
Tailgas HNO3
Air
NO2
Condenser
NO2
Tail-gas
HNO3
Bleacher
Tail-gas
Vent
Waste Heat
Boiler
Heater
Exchanger
Tail-gas
Catalytic
Combustor
Compressor
Figure # - the simplified flowsheet for the production of nitric acid
2.2 Flowsheet Description for Nitric Acid Production
Section 1: (-----)
 Air is filtered, compressed to approximately 860 kP and preheated to 250°C
 Ammonia evaporate (NH3) is mixed with the air in the ammonia converter –
mixture is 10% ammonia (vol)
 The mixture flows through a pack of flat gauzes (platinum catalyst), producing
nitric oxide (NO) – 95% efficiency, at approximately 900°C
Section 2: (-----)
 The nitric oxide gas is cooled in a tail-gas heat exchanger and reacts with residual
oxygen in the system producing nitrogen dioxide (NO2)
Section 3: (-----)
 The cooled nitrogen dioxide gas flows through a condenser, where part of the gas
is condensed to a weak acid
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The uncondensed nitrogen dioxide gas flows to the bottom of an absorption tower
The weak acid is pumped to an intermediate bubble-cap tray in the absorption
tower
Section 4: (-----)
 Cold water is introduced at the top of the absorption tower and flow down through
the tower
 The uncondensed gas flows countercurrent to the water through the tower
 The nitrogen dioxide absorbs to the water on the bubble cap trays, producing
nitric acid and nitric oxide
 The resulting nitric oxide is reoxidized with the supplied air stream in the tower,
producing nitrogen dioxide which is then absorbed, producing nitric acid
 The nitric acid flows from the bottom of the absorption tower to a bleacher, where
dissolved nitrogen oxides are removed
 The removed nitrogen oxides are returned to the absorption tower where they are
reoxidized
 The finished product, nitric acid, is collected after the bleaching process
Section 5: (-----)
 The inert tail-gas leaves the top of the absorption tower and is reheated in the heat
exchanger by the hot nitric acid in section 2
 The gas then flows through a catalytic converter where the nitrogen oxides are
turned into elemental nitrogen
 The remaining gas then flows through a compressor, a waste heat boiler (where
the temperature of the gas is reduced significantly), and vented to the atmosphere
2.3 Delineation of the Major Components of the Production of Nitric Acid
Oxidation of Ammonia
NH3 +1.25O2 → NO + 1.5H2O
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∆Hrxn = -226 kJ/mol
mixture of 1:9 of ammonia:air
oxidation occurs over a platinum catalyst at a temperature of 900°C
the rate of reaction is extremely rapid
the catalyst consists of 90% platinum and 10% rhodium for increased strength
the higher the catalytic temperature, the higher the tendency for the products to go
to nitric oxide
the lower the catalytic temperature, the higher the tendency for the products to go
to the unwanted by-products of nitrogen (N2) and nitrous dioxide (N2O), a
greenhouse gas
the negative enthalpy of reaction indicates that heat is released from the reaction
Air @ T=250°C, P= 860kPa
Ammonia
Converter
(Oxidation of
Ammonia)
Ammonia evaporate
Q
NO @ T = 900°C, η = 93 to 98%
Figure # - the inflows and outflows for the ammonia converter
Oxidation of Nitric Oxide
2NO + O2 → 2NO2
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∆Hrxn = -114 kJ/mol
the nitric oxide is cooled from 900°C to approximately 40°C in a heat exchanger
the nitric oxide combines with residual oxygen within the system
the reaction rate is slow
the rate constant increases with lowering temperatures
the lower the temperature, the higher the tendency the reaction has to go to
nitrogen dioxide
the lower the temperature, the lower the required reaction time
NO2
Heater
Exchanger
Tail-gas
NO @ 900°C,
η = 93 to 98%
Tail-gas
Figure # - the oxidation of nitric acid to nitrogen dioxide
Absorption of Nitric Oxides to Water to Produce Nitric Acid
3NO2(g) + H2O(l) ↔ 2HNO3(aq) + NO(g)
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∆Hrxn = -135.6 kJ/mol
nitrogen dioxide gas flows to the bottom of the absorption tower and continues to
flow up through the tower
water condensate is introduced at the top of the tower and flows countercurrent to
the gas stream
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the nitrogen dioxide is absorbed by the water on bubble cap absorption trays
which are located are various heights within the tower, producing nitric acid and
nitric oxide
the resulting nitric oxide gas is oxidized by the supplied air stream within the
tower, producing nitrogen dioxide which is then absorbed by the water, producing
nitric acid
the resulting nitric acid flows downward through the tower and is collected at the
bottom, where it is then passed on to a bleacher where the impurities are further
removed
the nitric acid recovered at the end of the process can range in concentration form
30 to 70%
the tail-gas, consisting of NO, NO2, and N2O, produced in the absorption process
exits the tower at the top
Tail-gas
Condensate
Bubble cap absorption tray
Air
NO2
Nitric Acid, 30-70%
Treatment of Tail-gases
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The tail-gas from the absorption tower contains nitric oxide (NO) , nitrogen
dioxide (NO2) and nitrous oxide (N2O)
the gas flow from the absorption tower to a catalytic converter where the nitrogen
based gases are converted to elemental nitrogen
once the tail-gases have been treated they are sent to a heat waste boiler and then
out into the atmosphere
Untreated Tail-gas (NO, NO2, N2O)
Catalytic
Combustor
Treated Tail-gas (N2)