Bioreactor Design and Operation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 954

) 1/3 g/dm 2 . Thus, the productivity per unit surface area, X S ¼ 1.25 O (5 p

For the large reactor, the productivity is then

(1) from suspended cells, 5 60,000 ½g ¼ 150 kg

) 1/3 ] ) 1/3 60,000 2/3

(2) from surface attached cells, [1.25 O (5 p 2(5 p

60,000 2/3 g g ¼ 2.5 ¼ 3.8315 kg

Thus, the total productivity from the 60,000-L reactor is 153.8315 kg.
If no wall growth had been present (but with the same level of productivity in the lab
reactor), the 60,000-L reactor would have yielded 300-kg target product. Thus, wall growth,
if present, can seriously alter the productivity of a large-scale reactor. Scale-up option should
also be adjusted.

18.10. SUMMARY

For a well-mixed reactor, there are three common operational modes: batch, CSTR, and
fed-batch. The selection of the operational modes depends on the catalyst stability, substrate
supply, product demand, and regulatory requirements. A summary of the effect of opera-
tional mode on bioreactor operation is listed in Table 18.3 . Batch operation has the best flex-
ibility (at each batch of operation), while CSTR has the highest productivity. CSTR has the
effect of screening for fast growth cells and in general not suited for fermentation with genet-
ically modified cells.
There are three basic reactor types for aerobic cultivation of suspended cells: (1) systems
with internal mixing mechanisms, (2) bubble columns, and (3) loop reactors. While bubble
and loop reactors offer advantages of higher efficiency and lower shear, the traditional
stirred-tank reactor is more flexible and can handle broths that become highly viscous.
Design of a bioreactor is a balance of productivity and sterility. The desired product is to
produce a pure culture where only the desired organism is detectably present.
For industrial-scale fermentors, oxygen supply andheat removal are key design limitations.
K L a is dependent on themedia, salts, surfactants, pressure, and temperaturemaking it difficult
to predict. K L a is however measurable. Once K L a is established, it can be used to estimate the
rate of metabolic heat generation and the total amount of cooling surface required.
Scale-up is difficult because conditions in a large vessel are more heterogeneous than in
a small vessel. Scale-up problems are all related to transport processes. If geometrically
similar vessels are used, it is impossible to maintain shear, mixing times, and K L a identically
in both large and small vessels. Scale-down techniques are useful in identifying the control-
ling regime at the smaller scale, where many parameters can be tested more quickly and less
expensively than at the production scale.
Bioreactor instrumentation and control are less advanced than in the petrochemical
industry. Improvements in sensor technology and the dynamical models of bioreactors are
critical to improvements in control technology.
Sterilization is the removal of biocontaminants or foreign organisms for a fermentation
system and the surrounding equipment. Scientists and engineers who are familiar with bio-
logical testing, as in a fermentation laboratory, will be familiar with autoclaves. These
systems apply steam to an internal vessel at 121 C, usually for 15 min. Instruments,