Page 113 - Chemical engineering design

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CHEMICAL ENGINEERING
P mean D P 1 Ł P 2 /2 D 0.5
P r D P mean /Pc D 0.0099206 0.17
From Figure 3.2 correction to C p for pressure is negligible.
From Figures 3.8, 3.9, 3.10
Z D 1.0 X D 0 Y D 0
Essentially ideal at this pressure

m D 0.1762593 eqn 3.36a
n D 1.2139743 eqn 3.38a
W D 303.47285 kJ/kmol eqn 3.31
Actual work required D polytropic work/efficiency D 352.87541

Say 353 kJ/kmol

Second stage work
As the intermediate pressure was selected to give equal work in each stage the second
stage work could be taken as equal to the first stage work. This will be checked.
T in D 298 K
compression ratio D P 3 /P 2 D 15.822785, i.e. same as first stage
So, take gamma and efficiency as for first stage
m D 0.1811049

T 3 D 491.24593 K
T mean D 394.62296 K
C p at mean temp
Coeff. 3.806 0.15359 8.35E-05 1.755E-08
3.806 60.610141 13.00011 1.0785715
C p
sum, C p D 52.494599 kJ/kmol K
Little change from first stage, so use same gamma and T mean
Tr D 1.5
P mean D 20.4 bar
Pr D 0.4047619 (0.4)
From Figure 3.2 correction to C p for pressure is approximately 2.5 J/mol.
This is less than 5 per cent, so neglect.
From Figures 3.8, 3.9, 3.10
Z D 1.0 X D 0.1 approx. Y D 0
So, gas can be taken as ideal
W D 314.01026 slightly higher as T in is higher