Page 180 - Chemical engineering design

So, O 2 required to oxidise the NO in the inlet to stream to NO 2 , from reaction 4, D
15.7 ð 1 D 7.85 kmol. FLOW-SHEETING 157
2
Hence, the “free” oxygen in the inlet stream D 20.0 7.85 D 12.15 kmol.
Combining reactions (4) and (5) gives the overall reaction for the absorption of NO 2
to produce HNO 3 .
Reaction 6. 4NO 2 C 2H 2 O C O 2 ! 4HNO 3
Using this reaction, the oxygen required to oxidise the NO formed in the absorber can be
calculated:
1
O 2 required to oxidise NO formed Df NO C NO 2 in stream 6 gð
4
D 48.9 C 15.7 ð 1 D 16.15 kmol
4
So O 2 required for complete oxidation, in addition to that in inlet gas
D 16.15 12.15 D 4kmol
Let the secondary air flow be y kmol. Then the O 2 in the secondary air will be D
0.21 y kmol. Of this, 4 kmol react with NO in the absorber, so the free O 2 in the tail
gases will be D 0.21 y 4kmol.

N 2 passes through the absorber unchanged, so the N 2 in the tail gases D the N 2 entering
the absorber from the cooler-condenser and the secondary air. Hence:

N 2 in tail gas D 720 C 0.79 y kmol.
The tail gases are essentially all N 2 and O 2 (the quantity of other constituents is
negligible) so the percentage O 2 in the tail gas will be given by:
0.21 y 4 100
O 2 per cent D 3 D
720 C 0.79 y C 0.21 y 4
from which
y D 141.6kmol
and the O 2 in the tail gases D 141.6 ð 0.21 4 D 25.7kmol
and the N 2 in the tail gases D 720 C 111.8 D 831.8kmol.

Tail gas composition, the tail gases will contain from 0.2 to 0.3 per cent NO, say 0.2
per cent, then:
0.2
NO in tail gas D total flow ð D N 2 C O 2 flow ð 0.002
100
D 831.8 C 25.7 0.002 D 1.7kmol
The quantity of the secondary air was based on the assumption that all the nitric oxides
were absorbed. This figure will not be changed as it was calculated from an assumed
(approximate) value for the concentration of the O 2 in the tail gases. The figure for O 2
in the tail gases must, however, be adjusted to maintain the balance.
The unreacted O 2 can be calculated from Reactions (4) and (6). 1.7 kmol of NO are not
1
1
oxidised or absorbed, so the adjusted O 2 in tail gases D 25.7 C 1.7 C D 27.0 kmol.
4 2