HUNGARIAN JOURNAL

OF INDUSTRIAL CHEMISTRY
VESZPRM
Vol. 34. pp. 35-39 (2006)

OPTIMIZATION PROBLEMS OF FERMENTOR AERATION-AGITATION SYSTEM

L. KOZMA1, L. NYESTE2 and A. SZENTIRMAI3
1

Chartered engineer, former leader of the Technical Division of BIOGAL Pharmaceuticals,

Postal address: rsg u. 17., H-1112 Budapest, HUNGARY, E-mail: multiplan@axelero.hu
2
Professor of Department of Biochemical Engineering, Budapest University of Technology, Szt. Gellrt tr 4.,
H-1521 Budapest, HUNGARY
3
Professor of Microbiology and Biotechnology, University of Debrecen, Haraszt u. 21., H-4010 Debrecen, HUNGARY

After four decades of almost exclusive application of Rushton turbines a number of types of impellers were developed for
the fermentation industry in the last 20 years which were proven to be more efficient despite their lower power
requirements.
The efficiency of an impeller is affected strongly by the degree of their hydromechanical properties corresponding
with the specific characteristics and requirements of a certain fermentation process. Evaluation in case of non-Newtonian
broths cannot be carried out with proper accuracy, and their optimization becomes difficult.
An increase in yield even by a few percent can be of great importance in large-scale fermentation vessels.
Consequently, the optimization of agitation system is a very important factor but it is only partially provided by scale-up
using pilot plant data and similarity criteria. This is the reason why we need newer methods for optimization of aerationagitation systems for large-scale fermentation vessels by agitators equipped with changeable flow modifying parts.
Keywords: power number, non-Newtonian broth, Rushton turbine, shear power, scale-up, flooding

Advantages and Disadvantages of Newer Agitation

Systems
The disadvantages of flat-blade Rushton turbines less
axial circulation capability and large power requirement
- applied in penicillin fermentation for many years due
to their excellent dispersion capability became more and
more obvious with the increase in size of fermentation
vessels. Due to these disadvantages newer types of
impellers and complex agitation systems were developed.
Turbines
The efficiency of Rushton turbines (Fig. 1) was
increased by the application of impellers with parabolic
profile instead of flat-blades in Scaba 6SRGT system
(Fig. 2). Their power number has decreased from 5,56,0 to 3,2. The number of higher energy peaks around
the impeller endangering more sensitive microorganisms
decreased and by increasing impellers diameter larger
volumes could be blended with the same power
requirements. The impellers sensitivity to flooding
phenomenon and increased viscosity had decreased, but
still they had lower circulation capability. In the event
of applying more impellers compart-mentalization
(inadequately blended areas) may occur. That is why in
newer agitation systems these are used only at the
lowest, dispersion level. (A. Baker et al. 1)
Closed turbines have better circulation capability but
these are rarely applied due to their little dispersion
capability.

Propeller agitators
Propeller agitators have excellent axial circulation
properties but weak dispersion capability. Due to their
little power number of 0,5-1,1 their diameter could be
larger with the same power requirement and this
facilitates full blending of viscous broths and filamentous
microorganisms.
First EKATO has developed propeller type impellers
with pitched blades called MIG and INTERMIG (Fig.
3). Recently streamlined propeller agitators with twisted
surface have been applied at the upper levels of
complex agitation systems. Impellers with larger
diameter ratios of 1:0,5-1:0,6 narrower blades such as
Lightnin 310 are applied for blending of lower viscosity
broths. Impellers with less diameter and diameter ratio
of 1:0,45 and broader blades which have power number
of 1,0-1,1 such as Lightnin A 315 and Prochem Maxflo
(Fig. 4 and Fig. 5) are applied for blending of higher
viscosity broths.
According to A.W. Nienow propeller agitators
provide better blending efficiency for both the lower
and higher viscosity broths and better mass and heat
transfer than Rushton turbines. Other advantages of
these agitators are their power number and indulgence
with sensitive microorganisms (A.E. Nienow 2.)
Vacuum agitators
Vacuum agitators has low power requirement, good
dispersion capability but lesser circulation properties.
These types can be used for blending less air volumes.

36
They are used only in certain technological processes
such as in flotation devices and in yeast production.
Newer Complex Agitation Sysems
Merely the last few decades the researchers and
manufacturers have realized that the efficiency if
agitation systems can be increased by development of
complex systems including more agitators of different
types and properties which satisfy better the
requirements of the particular levels. Despite many
published paper literature dealt not so much with the
problems of agitation of large-scale fermentation
vessels. Perhaps on the Symposium in Firenze in 1993
data on the mass transfer problems due to differences
between the levels of large-scale fermentation vessels
were published for the first time. These differences are
stemming partially from the position of levels and
partially from their different functions i.e. could be local
or functional differences.
Local differences are mainly caused by the pressure
differences due to 8-12 m height of fermentors and this
may affect bubble size and the density of foaming
broths, etc.
Functional differences are because the function of
the lowest agitator is efficiently disperse air input, the
function of the middle agitator(s) is the best intensity
circulation of the broth-air mixture and the function of
the upper agitator is recirculation of the foaming broths
on the surface with less further foam formation possible.
In the nineties the increase in differences due to
larger and larger-scale fermentation vessels had led to
the development of complex agitation systems
considering the differences between levels. 6SRGT
modified turbine agitator with good dispersion
capability is generally used on the lowest level, and high
efficiency propeller agitators e.g. Lightnin, Prochem are
applied on the highest level. (K. Myers, 3.) These
complex systems have better energy dissipation,
dispersion and circulation capabilities, they are more
efficient and more sensitive to flooding than the older
systems built from components of the same type and
size.
Optimization Problems of Agitation Systems
Sizing and development of large-scale agitation systems
is still based mainly on data from and experiences with
pilot plant fermentors, and relations developed through
the theory of similarity and dimension analysis. Lately
industrial measurements have been used more and more
often.
The application of the results of experimental
measurements during scale-up is limited very much by
the significant differences in the hydrodynamic fields
and flow patterns of large-scale fermentors mainly due
to the following reasons:
a) because of the nearness of the baffles and impellers
the large velocity gradient between the flowing

b)

c)

d)

e)

layers in the experimental device results in large

shear velocity and shear power, while in large-scale
devices the much less velocity gradient due to larger
sizes results less values.
unlike in the large-scale vessels no free turbulence
facilitating mass transfer is evolved because of the
less Reynolds number value due to the less size of
the experimental device.
flow of high viscosity broths can slow down so
much in the large-scale vessels that inadequately
mixed areas are formed even when newer agitation
systems are applied. No secondary dispersion can
occur along the baffles which may mask the
deficiencies of the impeller type itself in pilot plant
fermentors.
Due to the high pressure of large-scale fermentation
vessels bubble size affecting oxygen transfer is
decreased, solubility of gases, the density of liquidgas mixture and coalescence of bubbles are
increased.
In large-scale vessels the agitation time and crosssectional air flow velocity are increased with the
same specific air volume and v/v input.

Kipkes example can be cited for demonstration of

the increase in agitation time, namely if a given
agitation time can be produced in a laboratory fermentor
of 5 liters with P/V = 1 kW/m3 power/unit volume, in a
large-scale fermentation vessel of 50 m3 the same
agitation time can only be achieved with 5000 kW
power! The differences in magnitude show the problems
of scale-up and the limitations of the application of
experimental results.
The scales are changed considerably during scale-up
even during entirely proportioned geometric scale-up.
For example, if the size of a model is increased only by
tenfold, its surface increases hundred-fold but its
volume increases thousand-fold. That is why even the
name of similarity criteria is false since their application
provides merely partial similarity. Due to the unequal
change in size and value ratios, physical, geometric,
kinetic and dynamic similarity criteria cannot be
selected simultaneously.
Due to the lack of a generally valid procedure many
scale-up processes had been developed.
The most often is to rely on power requirement per
volume (P/V), volumetric oxygen transfer coefficient
(kLa), gas-holdup and shear stress (viscosity/velocity
gradient).
The variation of chosen considerations may lead to
great differences. That is why many researchers opinion
is that results do not comply with the technical and
economical requirements of biotechnology and can only
be informative data for developers of sizing procedures.
According to M. Charles: in practice, scale-up strategies
tend to be mixed bags engendering art enpricism,
conventional wisdom and (frequently) wishful thinking
(4).
For the optimization of fermentation process i.e. for
the achievement of largest possible yields even
distribution of the dissolved oxygen (DO), medium and
ingredients added during fermentation and optimum

37
mass transfer conditions should be provided besides
application of high productivity microorganisms and
adequate mediums.
Adequate oxygen level can be achieved by both
proper air volume input and its best possible dispersion
i.e. the least oxygen bubbles and their most even
distribution. To achieve this, adequate agitation power,
air volume and an agitation system is necessary which is
suitable for effective dispersion of air, for creation of
intensive circulation and for even distribution of bubbles.
The level of dissolved oxygen (DO) can be measured
during fermentation and can be adjusted by the regulation
of power input and/or air volume if there are adequate
quantities.
Considering the sometimes high values e.g. in case
of penicillin fermentation the efficiency of the process is
a significant factor and it is affected by the structure of
the agitation system besides the power input and
adequate air volume. The problem is that however, we
can calculate at least approximately - the diameter and
power requirement of the agitators and the air volume
by the available procedures and relations, these data
provide very little information on optimum design
Similarly to the added nutrients oxygen transfer
occurs on the interfaces of air bubbles and medium
particles and through the walls of microorganisms
cells. According to the double layer theory thinning of
the laminar layers on the interfaces by creating turbulent
liquid flow and shear stress due to this turbulence is
necessary for the acceleration of mass transfer.
It is well known that vortexes are arisen during real
liquid flow due to their viscosity and because of the
collision of these vortexes turbulence proportional to the
velocity of flow occurs. Shear stresses proportional to
the velocity of flow occur between turbulent liquid
layers which have important role in oxygen (DO) and
mass transfer: these stresses thin the laminar layers of
transferring interfaces, micromix the components of
broths, disperse oil particles and air bubbles facilitating
and accelerating mass transfer processes, disintegrate
clots and in some cases cause morphological changes in
the structure of microorganisms as in the case of
penicillin fermentation.
The magnitude of hydrodynamic forces created by
agitation can be seen from the fact that according to the
calculations of Vant Riet and Smith the centrifugal
acceleration behind the vortexes created by impellers
can be seven-hundredfold of the gravity (5).
Shear powers may, however, damage microorganisms
which are especially sensitive, contribute to the creation
of stable liquefied foams which generally decrease
oxygen transfer, and aeration of carbon-dioxide and
other gases partially on direct way and partially due to
antifoaming oils.
Microorganisms on the interface of vortexes can be
disrupted while those in the centre of the vortex may
abrade each other.
Consequently the intensity of agitation should remain
within a narrow range for keeping damaging effect at a
minimum level while maintaining maximum advantages
and this is the purpose of optimization.

The characteristics of fermentation processes may

vary due to the differences in viscosity, foaming
properties, density, etc. A typical feature is that while
foaming generally decreases the oxygen transfer, in
certain cases the increased persistence of bubble may
raise the rate of oxygen transfer in liquefied foams, and
antifoaming agents may decrease it.
Some microorganisms such as oxytetracycline
producers do not need agitation, and their fermentation
can be made in slim vessels without agitator which are
much cheaper.
Air inflated into the broths dispersed, distributed and
circulated in the medium of the fermentor by the
agitation system proportionally to power input. Due to
this procedure the volume of the medium is increasing
and oxygen will be dissolved in the medium depending
on the intensity of agitation, characteristics of medium
and surface gas velocity vs. The degree of oxygen
transfer is depending on the viscosity of the medium the
characteristics of air, medium and microorganism system,
and coalescing properties of air bubbles. The entrapment
of air and this way oxygen fusion can decrease greatly
due to increased viscosity and bubble coalescence
(Vant Riet, Smith 5., and Buchholz et al. 6.)
Besides the mentioned air entrapment broths volume
can also be increased by the often very intensive foam
formation depending on broths characteristics. Stable so
called liquefied foams are formed on more viscous
mediums such as in penicillin fermentation. Contrary to
the air entrapment mentioned above this foam formation
is detrimental since it limits oxygen transfer partially
directly and partially through antifoaming agents,
however rarely the opposite situation may occur.
Consequently maintaining the air input and power
within a narrow range based on continuous instrumental
measurement of fermentation parameters is an important
requirement for dissolving oxygen and nutrients and
also their transfer to microorganisms with adequate rate.
Considering economical importance of mass transfer
problems arising from increasing size of fermentor
vessels more efficient complex agitation systems were
developed with lower power requirement, better
dispersion and including far better circulation levels. In
their paper published in 1987 B.C. Buckland et al. had
revealed that application of Lightnin and Prochem
propeller agitators providing better top to bottom
blending of viscous broths is cost effective due to
saving power input and/or by the application of these
agitators the production can be increased because of the
higher cell concentration due to better agitation
(Buckland et al. 7). Papers on complex agitation systems
have been published more often since the beginning of
1990s (Chemineer, 8).
During their developmental activities manufacturers
besides the relations for calculation of main sizes and
powers could mainly rely on experimental results
which, however, provided merely informative results
due to the above reasons. It should also be noticed that
uniformization, development of systems which can be
distributed widely and production of their own types
and licensed products are the main interests of
manufacturers. All these factors eventually lead to

38
negligence of specific requirements of fermentations.
Exerting themselves to protect their trade secrets,
factories generally provide very little possibilities for
carrying out profound studies fermentation process by
the professionals of manufacturers.
According to the above characteristics and
requirements of fermentation processes may differ very
much. It follows from the above written that besides
applied technologies and materials the success of
fermentation processes also depends upon whether the
agitation system used during fermentation is adequate
for the specific requirements of fermentation.
Consequently the characteristics, dispersion and
circulation capabilities of agitation levels should be
adjusted to the features and requirements of the
fermentation which may, conversely, vary because of
the differences between the experimental and industrial
levels. In case of viscous liquids experimental levels do
not provide data and indications of adequate accuracy
for the adjustment. Although the analysis of experimental
data has been improved very much since V. Charles
through the application of computers, lesser changes
may also be of significance due to the large volume of
industrial fermentors and these changes cannot be
designed with adequate accuracy.
It follows from the above that there is no
adequate procedure available for actual optimization
of industrial agitation systems and for establishment
how much an agitation system can be considered
optimal for a certain fermentation procedure.
The efficiency of an agitation system is depending on its
structure and considering fermentation it is depending
on how the agitation systems levels use power input for
dispersion and circulation and how adequate this is for
the requirements of a given fermentation process.
A solution for this problem may be if manufacturers
provide special separated parts for the particular levels
of the agitation system which could be fixed on the
system by screw this was changing the characteristics of
agitation. It would not be especially difficult to solve
since power input is proportionally changed with the
fifth degree of the diameter of the agitator and the
characteristic of flows can be modified within a wide
range merely with changing the shape and angle of
blades of the impeller.
The application of this idea requires some change in
viewpoint according to the following:
1. It cannot be expected that a manufacturer will
provide an optimal agitation system, but it is
expected to provide an agitation system of which
perfusion properties can be modified within a wide
range with auxiliary parts. It would be, of course, the
obligation of the manufacturer to provide detailed
user manuals and information sheet for the
expectable effects of these auxiliary parts and
provide professional assistance for testing on
demand.
2. The obligation of the user would be the actual
optimization of the agitation system according to
provided directives and thorough analysis of the
effects of the auxiliary parts. Inclusion of factory
professionals in the selection of the most efficient

system may solve the problems of scale-up sometime

mentioned as dream by M. Charles (4) and may assist
the establishment really optimal agitation systems.
Biogal
Pharmaceuticals established for the production of
antibiotics together with research centers has endeavored
to develop its devices since the beginning. According to
the knowledge learned in the international symposium
in Prague in 1964 where both European and US
professionals attended, BIOGAL Pharmaceutical was
the first pharmaceutical company applying two-turns
driver engine which increased power utilization by 3040%. At the beginning of 1970s the company changed
the systems with Rushton agitators which had
asymmetric structure, and 20% better power on the
lowest level. This was due to the cognition of the fact
that in the applied asymmetric systems the efficiency of
the lower agitators compared to the upper ones was
considerably decreased by the function of dispersion.
Since the beginning of seventies the company had started
to apply a complex system including propeller agitators
and Rushton turbines and with this method narrow OTC
fermentors without agitators could successfully be
adapted for penicillin fermentation.
Based on these experiences also considering the
construction of BIOGALs newer complex agitation
systems it can be concluded that there are more
possibilities for the increase of efficiency and
optimization of agitation systems through the application
of modifiable impellers recommended above.
Conclusions
Conditions of optimization of the aeration-agitation
systems of large-scale fermentors in case of viscous
broths:
1. Providing flow modifying parts for the agitations
system for variation of dispersion and circulation
capabilities and adjustment for the requirements of a
specific fermentation process.
2. Evaluation of the results of variation by fermentation
professionals and choosing optimum variation.
Considering these on a long term basis may lead to
gain profound knowledge about specific requirements of
fermentation processes and industrial optimization may
become unnecessary in the future.
NONATION
DO
P/V
k La
vs

dissolves oxygen
power/volume
volumetric oxygen transfer coefficient
gas velocity

39
REFERENCES
1. BAKKER A., SMITH J. M., MEYERS K. J., Chemineer,
PO Box 1123, Daytona, OH45401, Reprinted from
Chemical Engineering
2. NIENOW A. W.: 9th Biotech Symposium, Crystal
City, USA, 1992 pp. 196-196
3. MEYERS K., REEDER M., BAKKER A., RIGDEN M.:
Agitating for Success, The Chemical Engineering
4. CHARLES M.: Trends in Biotechnology, Vol 3., No.6
180

5. VANT RIET K., SMITH J. M., Chemical Eng. Sci.

1975, 30. 1083
6. BUCHHOLZ H., BUCHHOLZ R., NIEBENSCHUTZ H.,
SCHGERL K.: Eur. J. Appl. Microb. And Biotechn.
6. 1978, 115.
7. BUCKLAND et al. Bioengineering Vol. 31. 70. 737742. I. 1988
8. Chemineer, Inc. Reprinted for Chemical Engineer
Crammer Road, West Meadows, Daby, DE 21 6XT,
England