Page 181 - Chemical engineering design

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CHEMICAL ENGINEERING
Ž
The tail gases will be saturated with water at the inlet water temperature, say 25 C.
Ž
Partial pressure of water at 25 C D 0.032 atm. The absorber pressure will be approx-
imately 8 atm, so mol fraction water D 0.032/8 D 4 ð 10 3 and H 2 O in tail gas D
857.5 ð 4 ð 10 3 D 3.4kmol.
Water required, stream (11).
The nitrogen oxides absorbed, allowing for the NO in the tail gases, will equal the
HNO 3 formed
D 48.9 C 15.7 1.7 D 62.9kmol D 3962.7kg
Stoichiometric H 2 O required, from reaction 6
62.9
D ð 2 D 31.5kmol
4
The acid strength leaving the absorber will be taken as 60 per cent w/w. Then, water
required for dilution
3962.7
D ð 0.4 D 2641.8kg D 146.8kmol
0.6
So, total water required, allowing for the water vapour in the inlet stream (6), but
neglecting the small amount in the secondary air
D 31.5 C 146.8 C 3.4 3.8 D 177.9kmol
Summary, stream compositions:

Secondary air (8) Inlet (9) Acid (12) Tail gas (10) Water feed (11)
Stream
kmol kg kmol kg kmol kg kmol kg kmol kg
NO 15.7 471.0 1.7 51.0
NO 2 48.9 2249.4 trace
O 2 29.7 950.4 49.7 1590.4 27.0 864
N 2 111.8 3130.4 831.8 23,290.0 831.8 23,290.4
HNO 3 62.9 3962.7
H 2 O trace 3.8 68.4 146.8 2641.8 3.4 61.2 177.9 3202.2
Total 4080.8 27,669.2 6604.5 24,266.6 3202.6
Check on totals: Stream 6 C 8 D 9 ? 4080.8 C 23,588.4 D 27,669.2
27,669.2 D 27,669.2 checks
Stream 9 C 11 D 10 C 12 ? 27,669.2 C 3203.2 D 24,266.6 C 6604.5
30,871.4 D 30,871.1 near enough.

Acid produced

12
7
Mixer

13
Product