Page 184 - Chemical engineering design

FLOW-SHEETING
The inlet air will be at the ambient temperature, take as 15 C. With no intercooling
T 2 D 288 ð 8 0.39 D 648 K Ž 161
This is clearly too high and intercooling will be needed. Assume compressor is divided
into two sections, with approximately equal work in each section. Take the intercooler
Ž
gas outlet temperature as 60 C (which gives a reasonable approach to the normal cooling
Ž
water temperature of 30 C).
For equal work in each section the interstage pressure

p
P out
D D 8 D 2.83
P in
Taking the interstage pressure as 2.83 atm will not give exactly equal work in each section,
as the inlet temperatures are different; however, it will be near enough for the purposes
of this example.
1.64 1.64 1 /1.64
Ž
First section work, inlet 15 C D 1 ð 288 ð 8.314 ð 2.83 1
1.64 1
D 3072.9 kJ/kmol
1.64 1.64 1 /1.64
Ž
Second section work, inlet 60 C D 1 ð 333 ð 8.314 ð 2.83 1
1.64 1
D 3552.6 kJ/kmol
Total work D 3072.9 C 3552.6 D 6625.5 kJ/kmol
work/kmol ð kmol/s 6625.5 ð 0.125
Compressor power D D
efficiency 0.74
D 1119 kJ/s D 1.12 MW
Energy required per hour D 1.12 ð 3600 D 4032 MJ

Compressor outlet temperature D 333 2.83 0.39 D 500 K
Ž
say, 230 C
This temperature will be high enough for no preheating of the reactor feed to be needed
(Strelzoff).

Ammonia vaporiser
The ammonia will be stored under pressure as a liquid. The saturation temperature at
Ž
Ž
8 atm is 20 C. Assume the feed to the vaporiser is at ambient temperature, 15 C.
Specific heat at 8 bar D 4.5 kJ/kgK
Latent heat at 8 bar D 1186 kJ/kg
Flow to vaporiser D 731.0 kg/h
Ž
Heat input required to raise to 20 C and vaporise
D 731.0[4.5 20 15 C 1186] D 883,413.5 kJ/h
add 10 per cent for heat losses D 1.1 ð 883,413.5 D 971,754.9 kJ/h
say, 972 MJ