Page 185 - Chemical engineering design

162
CHEMICAL ENGINEERING
Mixing tee
air 11,272.9 kg/h
230°C
t 3

NH vapour 731.0 kg/h
3
20°C
C p air D 1 kJ/kgK,
C p ammonia vapour 2.2 kJ/kgK.
Note: as the temperature of the air is only an estimate, there is no point in using other
than average values for the specific heats at the inlet temperatures.
Energy balance around mixing tee, taking as the datum temperature the inlet temperature
to the oxidiser, t 3 .
11,272.9 ð 1 230 t 3 C 731 ð 2.2 20 t 3 D 0
Ž
t 3 D 204 C
Oxidiser
The program ENERGY 1 (see Chapter 3) was used to make the balance over on the
oxidiser. Adiabatic operation was assumed (negligible heat losses) and the outlet temper-
ature found by making a series of balances with different outlet temperatures to find the
value that reduced the computed cooling required to zero (adiabatic operation). The data
used in the program are listed below:
Ž
H reaction 1 D 226,334 kJ/kmol (per kmol NH reacted)
r 3
Ž
H reaction 2 D 316,776 kJ/kmol (per kmol NH reacted)
r 3
All the reaction yield losses were taken as caused by reaction 2.
NH 3 reacted, by reaction 1
731.0 ð 0.96
Flow of NH 3 to oxidiser ð reactor yield D D 41.3 kmol/h
17
731.0 ð 0.04
balance by reaction 2 D D 1.7 kmol/h
17
Summary, flows and heat capacity data:
Ž
Feed Product C kJ/kmol K
p
Stream (3) (4)
component kmol/h kmol/h a b c d
43 27.32 23.83E-3 17.07E-6 11.85E-9
NH 3
O 2 82.1 29.2 28.11 3.68E-6 17.46E-6 10.65E-9
308.7 309.6 31.15 1.36E-2 26.80E-6 11.68E-9
N 2
NO 41.3 29.35 0.94E-3 9.75E-6 4.19E-9
H 2 O 64.5 32.24 19.24E-4 10.5E-6 3.60E-9
Temp. K 477 T 4
Ž
The outlet temperature T 4 was found to be 1180 K D 907 C.