che_april-2024 - 38

FIGURE 6. This " combined gassing " concept can
be applied for " pure gas " fermenters
ating point to the flooding curve.
If there is a significant difference
between the inlet and outlet gassing
rates due to the metabolic conversion,
it must also be checked
whether there are significant gradients
in the superficial gas velocities
in the fermenter. This would also result
in significant kLa and GTR gradients,
which must be considered in
the process design and geometric
optimization of the fermenter.
Apart from the challenges from the
process engineering point of view,
the ongoing increase in reactor sizes
also bears huge challenges from the
mechanical point of view.
Agitator components
The following three key factors have
to be considered when designing
agitator components for fermentation
applications:
cleanability,
cost
and flexibility.
Agitators must be designed to
allow for thorough cleaning to prevent
contamination of subsequent
batches. Cleanability can be improved
by using smooth surfaces,
avoiding tight spaces, and using materials
that are resistant to corrosion
and chemical attack.
Agitator components must be
cost-effective to manufacture and to
install. This may require using less expensive
materials or simpler designs.
Agitators must be able to be disassembled
and reassembled easily for
maintenance or repair. This may require
using components that are not
welded together.
As a consequence, there are three
main approaches to design agitator
38
components for fermentation applications,
as follows:
* Gaskets are used to seal connections
between components. This
approach is relatively inexpensive
and flexible, but cleanability might
be more complicated compared to
alternative approaches
* Welding components together
creates a continuous, sealed surface.
This is the best cleanable solution,
but the least flexible
* Special solutions can be used to
combine the advantages of gaskets
and weldings. For example, components
can be welded together with a
smooth, flush surface
In addition to the design of the agitator
components themselves, there
are several guidelines to consider
when designing the interior of a fermenter,
including the following:
* Horizontal surfaces should be
avoided as they might trap product
residue, leading to contamination
* Surface quality is important. A
polished surface is easier to clean
than a rough surface
Mechanical design principles
Aerated fermenters tend to have a
very low natural-vibration frequency.
Main reasons for this are the usually
quite large height-to-diameter ratio
of the apparatus in combination with
very thin vessel walls, as operating
and design pressures are usually
quite low.
When operating an aerated fermenter,
the frequencies created by
agitation run the risk of matching
the natural frequency of the vessel,
the agitator, or the vessel-agitator
system. This can lead to strong vibration
or even serious damage of
the system.
To identify the natural frequency
of the vessel-agitator system, it is
necessary
to
gather
information
from both the agitator manufacturer
and the vessel manufacturer. This
can be a complex and time-intensive
process.
To visualize the interaction of frequencies
and agitator speed, a socalled
Campbell diagram (Figure 7)
can be used. This diagram shows
all the excitation frequencies and
natural frequencies. It can be used
to identify potential resonance risks.
To take different filling levels into
account, it is recommended to
perform a fluid-structure interaction
(FSI) calculation. This calculation
uses finite element analysis to
model the interaction of the fluid
and the structure.
With this information, it is possible
to check the agitation frequencies
against the natural frequencies of the
vessel-agitator system. If there is no
match, the operation of the system
is non-critical. If there is a match, design
changes are required.
Final remarks
Industrial
fermentation
is becoming
more and more important in the
chemical process industries. The
Shaft Rotation n2, min-1
FIGURE 7. The Campbell diagram can help visualize the interaction of vibration frequencies and agitator
speed, which enables the identification of potential resonance risks
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
APRIL 2024
Frequency: f(n2), Hz
Amplitude (zero to peak): A(f)(0-p) , mm/s

http://WWW.CHEMENGONLINE.COM

che_april-2024

Table of Contents for the Digital Edition of che_april-2024