Page 179 - Chemical engineering design

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CHEMICAL ENGINEERING
Summary, stream compositions:
Gas (6) Acid (7)
kmol kg kmol kg
NO 15.7 471.0 Trace
48.9 2249.4 Trace
NO 2
20.0 640
O 2
720 20,160
N 2
31.4 1978.2
HNO 3
H 2 O 3.8 68.4 130.5 2349.0
Total 23,588.4 4327.2

Total, stream 6 C 7 D 23,588.4 C 4327.2 D 27,915.6 kg, checks with inlet stream (4)
total of 27,915.

Absorber
In the absorber the NO 2 in the gas stream is absorbed in water to produce acid of about
60 per cent w/w. Sufficient oxygen must be present in the inlet gases to oxidise the NO
formed to NO 2 . The rate of oxidation will be dependent on the concentration of oxygen,
so an excess is used. For satisfactory operation the tail gases from absorber should contain
about 3 per cent O 2 (Miles).

Tail
gas
10
11
Water

6 9

8
12
Secondary
air
From stream (6) composition:
NO in inlet stream to absorber D 15.7kmol andO 2 D 20.0kmol

Note: Though the NO/NO 2 ratio in this stream is not known exactly, this will not affect
the calculation of the oxygen required; the oxygen is present in the stream either as free,
uncombined oxygen or combined in the NO 2 .