516
Chapter 11
Can the dissolved oxygen concentration be maintained above 30% in the bioreactor (which for some
strains is a critical level for penicillin production) when the oxygen requirement for a
rapidly
growing
culture of
P. chrysogenum is
r0 ~
2.3 mmoles of 0 2
(g DW)"1
h'1
? Discuss how the
bioreactor can be modified to satisfy the oxygen requirement,
d.
You decide to examine the effect of increasing the stirrer diameter. Start with
d/d,
= 0.4. Can the
critical level of dissolved oxygen concentration be maintained with this diameter ratio?
Problem 11.4. Scaled-down experiment
The pH-value is often controlled using only a single pH electrode and a single point of addition of base or
acid, also in large-scale bioreactors. The pH-electrode is typically located in a well-mixed region, and
addition of base or acid is typically made at the liquid surface. Since concentrated solutions are used, pH-
gradients in large-scale reactors are likely to be present.
Amanullah et al, (2001) used of a scaled-down system to study effects of pH gradients. The scaled-down
system consisted of a 2 L standard stirred tank reactor, with a working volume of 1 L, equipped with two
Rushton turbines
{d/d, ~
0.33). To the reactor was connected a piece of tubing (L = 2.75 m, d* = 4.8 mm),
through which liquid from the reactor was pumped. The pH-value in the reactor was measured and pH
control was achieved by adding base to a small mixing bulb (volume about 1.5 ml) located just before the
tubing (see figure below).
This system was used to experimentally simulate a three compartment reactor model, with a direct feed
zone (bulb), a poorly mixed zone (tubing part), and a well-mixed zone (the reactor).
a) Discuss what residence time in the tubing that should be chosen to simulate a large-scale reactor (100
tn3). The same residence time in the tube can be achieved by different ratios of tube volume,
Vmbe
and
recirculation flow, vrec. Discuss how the values of
Vlube
and
vrec
should be chosen.
In their study, Amanullah et al. (2001) used a strain of
Bacillus subtilis.
This organism produces acetoin
and 2,3 butanediol under oxygen limited conditions. The formation of acetoin and butanediol is described
by:
2 pyruvate —» C02 + acetolactate
(acetolactate synthase)
previous page 538 Bioreaction Engineering Principles, Second Edition  read online next page 540 Bioreaction Engineering Principles, Second Edition  read online Home Toggle text on/off