UNIVERSITY OF PRETORIA
DEPARTEMENT OF CHEMICAL ENGINEERING
KINETICS (CKN 321)
November 2018
EXAM
TIME : 3 HOURS
TOTAL 60
Read this before you start:

THE PAPER CONSISTS OF 4 QUESTIONS AND 5 PAGES, MAKE SURE YOU
HAVE A COMPLETE QUESTION SET

All Python graphs should be redrawn on your script. No printouts allowed

Any form of communication on your computer will result in serious consequences.
There are more ways to monitor this than you are aware of.

Take care in presenting your scripts. Remember that only the scripts will be marked
and not the pynb files!

Remember that Python is only a tool to help you with calculations. This is not an exam
to test your computer/programming skills, it is an exam to test your insight in the
subject. Remember to illustrate your insight neatly on the script!

Remember to give intermediate calculated values on the script, the pynb-file will not
be marked.
1
Question 1
[15]
The following liquid phase irreversible reaction is studied in various continuous reactor
configurations with feed compositions CAo = 1 mol/L, CBo = 2 mol/L:
𝐴 + 2𝐵 → 3𝐶
The reactor configurations and the corresponding conversions of species A are shown in
the following table:
Description
Configuration
CSTR
Overall measured
conversion of A
xA = 45.25%
PFR
xA = 60.15%
Two CSTRs in series
xA = 64.39%
PFR followed by CSTR
xA = 71.99%
The volumes of all the individual reactors are identical (1L). I.e. a single PFR volume is the
same as a single CSTR. Additionally, two CSTRs in series as well as the PFR followed by
the CSTR would have double the combined volume of a single CSTR/PFR. The volumetric
flowrate through all reactors are the same: Q = 8.3 L/min
Determine the kinetics of the reaction (give the units of the kinetic constant in mol, L and
min).
2
Question 2
[15]
A complex reaction involving the limiting reagent A takes place in a liquid phase batch reactor.
1
0.9
0.8
CA (mol/L)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1
2
3
4
5
6
7
8
9
10
Time (hours)
y = 0.000175x5 - 0.0044136x4 + 0.037515x3 - 0.11643x2 - 0.02033x + 0.999108
R² = 0.9995
The curve is described nearly perfectly (r2 = 0.9995) by the fifth-order polynomial:
𝐶𝐴 = 0.000175𝑡 5 − 0.0044136𝑡 4 + 0.037517𝑡 3 − 0.11643𝑡 2 − 0.02033𝑡 + 0.999108
You may assume that the conditions used to determine the relationship between C A and t,
will hold in any continuous operation system.
a) Use the data provided for the system and draw qualitative sketches of –rA vs t and
−rA vs CA
(6)
b) Determine the minimum residence time (or combined residence time for multiple reactors) that will yield a final conversion of 95% during continuous operation.
(9)
3
Question 3
[15]
The following reversible gas phase reaction is studied in a batch reactor:
𝐴 + 𝐵 ⇋ 𝐶 + 30𝐷
The reactor is a piston reactor with a minimum volume of 5 L, a maximum volume of 70 L
and applies a constant pressure between these volume limits. Outside the volume limits
the reactor volume is constricted and consequently becomes a constant volume reactor.
The following information about the system is known:
CAo
CBo
CIo (inert)
T
Vo
EA
ko,A1
ΔH
Kc,20°C
0.1 mol/L
0.1 mol/L
0.1 mol/L
123 °C
7.5 L
60 kJ/mol
1×109 L/(mol.min)
+40 kJ/mol
3 (mol/L)29
a) What is the pressure in the reactor at the start of the experiment in bar?
b) What is the pressure in the reactor at equilibrium in bar?
4
(3)
(12)
Question 4
[15]
A system consisting of the following two gas phase reactions are studied:
𝐴 + 2𝐵 → 6𝐵
(1)
10𝐵 → 𝐶
(2)
The first reaction happens spontaneously and is known to be elementary while the second
reaction is a first order reaction and requires the presence of catalyst in the reactor.
To determine the kinetics of the system a PBR was setup and the concentration of B in the
outlet was measured for two different masses of catalyst, the catalyst is diluted by the substitution with inert packing. The operating conditions and the measurement results from the
system are summarized in the following table (only species A and B are fed to the reactor):
Parameter/Variable
Value
Units
Qo
10
L/min
CAo
1
mol/L
CBo
1
mol/L
ρB1 (catalyst packing density 450
g/L
in PBR in run 1)
ρB2 (catalyst packing density 900
g/L
in PBR in run 1)
VPBR =
1
L
CB,ρB1
1.487
mol/min
CB,ρB2
0.102
mol/min
What are the values of the kinetic constants kA1 and k’B2 in units of mol, L, min, and mol,g,
min, respectively?
5