Mass Transfer
473
Problem 10.5. Measuring
kfl
by the hydrogen peroxide method
As described in Note 10.3, one method to measure
kta
is based on the consumption of hydrogen peroxide
in a catalase containing medium (see reaction below).
2
H 2 0 2
calai^ - -> 2 H 2 0 + 0 2
After addition of a known amount of catalase to the reactor, a continuous feed of H
2
0
2
is applied to the
reactor. Initially, hydrogen peroxide will accumulate in the reactor medium, but a steady-state will soon be
established at which the rate of decomposition of hydrogen peroxide equals the addition rate of hydrogen
peroxide. By measuring the dissolved oxygen concentration, as well as the inlet and outlet oxygen mole
fractions in the gas, it is possible to calculate the value of
kp.
In the original work by Hickman (1988), the values given in the table below, where measured dissolved
oxygen tension in liquid phase and oxygen concentrations in inlet and exit gas streams are given, can be
found.
Hydrogen peroxide
1 05
(mol s'1)
Dissolved oxygen concen-
tration
(%
of saturation)
Measured exit oxygen
concentration (mol%)
Stirrer rate
(mm)
1.6
111.5
21.14
500
3.2
123.8
21.36
500
6.4
147.2
21.78
500
12.8
195.8
22.6
500
3.2
110 .6
21.35
900
6.4
121.7
21.77
900
12.8
143.4
22.60
900
(Note that the dissolved oxygen concentration is higher than 100%, since oxygen is generated in the liquid
and transferred from the liquid phase to the gas phase.)
The following additional information is given;
Reactor volume:
V
= 0.005 m
3
Gas flow rate:
vgm=
9.2 I0
"5
m
3
s
'1
(i.e. approx. 1 wm)
Inlet oxygen concentration: 20.96%
a) Calculate the value of
kfi
for the two stirrer rates given in the Table 1. Assume that T = 20 °C.
The saturation concentration of oxygen (corresponding to 100% DOT) is 0.289 mol m
"3
under these
conditions.
b) Assume that the gas analyzer has an absolute error of 0.1 % of the measured value. How big is (in the
worst case) the error in the calculated
kta-
value? Under what conditions do you expect to get the most
accurate measurements?