Introduction to Chemical Engineering
Institute of Process Engineering
Exercise 2: Energy Balances for Ideal Reactors
Date of distribution: March 06, 2017
Due Date: March 13, 2017
(If you choose to hand in the exercise, please write down name and legi number!)
2.1 Chemical Reaction Analysis
The standard heat of reaction for the production of NO (important intermediate) via
4NH3 + 5O2 −→ 4NO + 6H2 O
is ∆Hr0 = −904.7 kJ mol−1
NH3 .
(a) Is the reaction exothermic or endothermic ?
(b) 1.7 kg of NH3 and 3.68 kg of O2 are allowed to react according to the reaction above. Assume the
reactions is irreversible and proceeds until one of the reactants runs out (this is called the limiting
reactant). How many moles of each of the four species are present when the reaction stops? Assume
that at the beginning there are no products present.
(c) How much energy do you have to remove/add if the reactor has to be kept at a constant temperature
of 25 ◦ C?
Introduction to Chemical Engineering
Institute of Process Engineering
2.2 CSTR Energy Balance
Consider an irreversible, exothermic reaction taking place in a CSTR in which species A is consumed. The
heat generation line G(T ) is plotted in Figure 1. The heat removal line R(T ) for this CSTR is given by the
following expression:
R(T ) = cp,0 [(1 + β) (T − Tc )]
where
cp,0 =
X
cin
i cp,i = 0.15
i
Tc =
kJ
m3 K
T in + Ta β
1+β
The variable Tc is an intermediate temperature between the feed temperature T in and the temperature of
the heat transfer fluid Ta , and β is a dimensionless parameter relating the effect of the heat exchanger to
that of the entering feed. Assuming a heat transfer fluid temperature of Ta = 300 K, an inlet temperature
T in = 400 K, and β = 1
a) What is the value of the temperature Tc ?
b) Plot the resulting heat removal line in Figure 1, and label it with I.
c) Mark all possible steady states in the figure as A, B, ...
d) For each steady state in c), indicate whether it is stable or unstable.
Consider now the case where the heat exchanger cooling the reactor is excluded, so that the reactor becomes
an adiabatic CSTR. This means that now β = 0, and the only cooling effect comes from the entering feed.
With the same inlet temperature T in = 400 K,
e) What is the new value of Tc ?
f) Plot the new heat removal line in Figure 1, and label it with II.
g) What is the new steady state temperature?
h) If the heat exchanger is turned on again and β = 1 is restored, so as the heat removal function R(T )
is given again by the curve I, which of the steady-states found in c) will the reactor attain?
Introduction to Chemical Engineering
Institute of Process Engineering
60
R(T ), G(T ) [kJ/m3 ]
G(T )
50
40
30
20
10
0
300
400
500
600
Temperature [K]
Figure 1: Heat generation curve
700
800
Introduction to Chemical Engineering
Institute of Process Engineering