Addendum to Problem 1
As written, problem 1 is not well posed. To get the solution originally written, it is necessary to make
several assumptions which are not clear from the problem statement.
To get the original solution, you must assume the following:
•
•
•
There is no entropic contribution to the total available work. Therefore, the total available work,
ΔG = ΔH.
The oxygen feed to the system is pure and perfectly stoichiometric.
The heats of formation given are valid at 700°C.
We will accept solutions which make these assumptions.
HOWEVER,
if you’d like to treat the problem more rigorously, you can so with the data from the table below.
Species
Symbol
→
MW
ΔSf
ΔHf
C
ΔHv
y
Subscript
Molar
mass
[g mol-1]
Entropy
of
formation
[kJ mol-1]
Heat of
formation
[kJ mol-1]
Heat
Capacity
[kJ mol-1 K-1]
Heat of
vaporization
[kJ mol-1]
Mole
Fraction
in air
Ammonia
a
17.03
0.193
-46
0.047
----(NH3)
Water (g)
0.189
-242
0.035
--w
18.02
40.7
Water (l)
0.070
-286
0.075
--Hydrogen
h
2.02
0.131
0
0.029
----Oxygen
o
32.00
0.205
0
0.032
--0.2*
Nitrogen
n
28.02
0.192
0
0.030
--0.8*
*The molar content of air is truly about 70%mol nitrogen and 15%mol oxygen. These numbers have been modified to limit the number of
chemical species involved.
To pursue this approach, I suggest considering all steams entering the cracker/SOFC system do so at
700°C then exit at the same temperature before being cooled to reclaim the thermal energy in the
effluent.