Page 176 - Chemical engineering design

FLOW-SHEETING
Summary, stream compositions:
Feed (3) Outlet (4) 153
kmol kg kmol kg

100 1700 nil
NH 3
NO nil 96 2880
H 2 O trace 150 2700
191 6112 68 2176
O 2
718 20,104 720 20,016
N 2
Total 27,916 27,916

Notes
(1) The small amount of water in the inlet air is neglected.
(2) Some NO 2 will be present in the outlet gases, but at the oxidiser temperature used,
1100 to 1200 K, the amount will be small, typically <1 per cent.
(3) It is good practice always to check the balance across a unit by calculating the
totals; total flow in must equal total flow out.

Waste-heat boiler (WHB) and cooler-condenser
The temperature of the gases leaving the oxidiser is reduced in a waste-heat boiler and
cooler-condenser. There will be no separation of material in the WHB but the composition
will change, as NO is oxidised to NO 2 as the temperature falls. The amount oxidised
will depend on the residence time and temperature (see Stephenson). The oxidation is
essentially complete at the cooler-condenser outlet. The water in the gas condenses in the
cooler-condenser to form dilute nitric acid, 40 to 50 per cent w/w.

Balance on cooler-condenser

5 6

7

The inlet stream (5) will be taken as having the same composition as the reactor outlet
stream (4).
Ž
Let the cooler-condenser outlet temperature be 40 C. The maximum temperature of the
Ž
Ž
cooling water will be about 30 C, so this gives a 10 C approach temperature.
If the composition of the acid leaving the unit is taken as 45 per cent w/w (a typical
value) the composition of the gas phase can be estimated by assuming that the gas and
condensed liquid are in equilibrium at the outlet temperature.