Page 186 - Chemical engineering design

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FLOW-SHEETING
Waste-heat boiler (WHB)
As the amount of NO oxidised to NO 2 in this unit has not been estimated, it is not possible
to make an exact energy balance over the unit. However, the maximum possible quantity
of steam generated can be estimated by assuming that all the NO is oxidised; and the
minimum quantity by assuming that none is. The plant steam pressure would be typically
Ž
150 to 200 psig ³ 11 bar, saturation temperature 184 C. Taking the approach temperature
Ž
of the outlet gases (difference between gas and steam temperature) to be 50 C, the gas
Ž
outlet temperature will be D 184 C 50 D 234 C (507 K).
1238.4
From the flow-sheet, NO entering WHB D D 41.3kmol
30
935.7
O 2 entering D D 29.2 kmol/h
32
If all the NO is oxidised, reaction 4, the oxygen leaving the WHB will be reduced to
41.3
29.2 D 8.6 kmol/h
2
Ž
H D 57,120 kJ/kmol, NO oxidised
r
If no NO is oxidised the composition of the outlet gas will be the same as the inlet. The
inlet gas has the same composition as the reactor outlet, which is summarised above.
Summarised below are the flow changes if the NO is oxidised:
Ž
C (kJ/kmol K)
p
(kmol/h) a b c d
O 2 7.46 as above
NO 2 41.3 24.23 4.84 E-2 20.81 E-2 0.29 E-9
Temp. 507K

Using the program ENERGY 1, the following values were calculated for the heat trans-
ferred to the steam:
no NO oxidised 9.88 GJ/h
all NO oxidised 12.29 GJ/h
Ž
Steam generated; take feed water temperature as 20 C,
enthalpy of saturated steam at 11 bar D 2781 kJ/kg
Ž
enthalpy of water at 20 C D 84 kJ/kg
heat to form 1 kg steam D 2781 84 D 2697 kJ
heat transferred
steam generated D
enthalpy change per kg
9,880,000
so, minimum quantity generated D D 3662 kg/h
2697
12,290,000
maximum D D 4555 kg/h
2697