2. 2
Fermenter
The heart of the fermentation process is the fermenter.
In general:
• Stirred vessel, H/D 3
• Volume 1-1000 m3 (80 % filled)
• Biomass up to 100 kg dry weight/m3
• Product 10 mg/l –200 g/l
8. Basic Functions of a Fermenter
1. It should provide a controlled environment for
optimum biomass/product yields.
2. It should permit aseptic fermentation for a number of
days reliably and dependably, and meet the requirements
of containment regulations. Containment involves
prevention of escape of viable cells from a fermenter or
downstream processing equipment into the environment.
3. It should provide adequate mixing and aeration for
optimum growth and production, without damaging the
microorganisms/cells. The above two points (items 2 and
3) are perhaps the most important of all.
4. The power consumption should be minimum.
5. It should provide easy and dependable temperature
control.
9. 6. Facility for sampling should be provided.
7. It should have a system for monitoring and regulating
pH of the fermentation broth.
8. Evaporation losses should be as low as possible.
9. It should require a minimum of labour in
maintenance, cleaning, operating and harvesting
operations.
10. It should be suitable for a range of fermentation
processes. But this range may often be restricted by the
containment regulations.
10. Fermenter Design
• A bioreactor is a device in which a substrate of low value is
utilized by living cells or enzymes to generate a product of
higher value. Bioreactors are extensively used for food
processing, fermentation, waste treatment, etc.
• On the basis of the agent used, bioreactors are grouped into
the following two broad classes:
(i) Those based on living cells
(ii) Those employing enzymes.
But in terms of process requirements, they are of the following
types:
(i) Aerobic
(ii) Anaerobic
(iii) solid state
(iv) immobilized cell bioreactors.
11. Theoretical explanation usually lags behind technical
realization. A bioreactor should provide for the following:
Agitation
Aeration
Agitation
The medium must be suitably stirred to keep the cells in
suspension and to make the culture homogeneous; it
becomes increasingly difficult with the scaling up.
Various types of stirrers range from simple magnetic
stirrers, flat blade turbine impellers, to marine
impellers, to those using pneumatic energy, e.g., airlift
fermenter, and those using hydraulic energy, e.g.,
medium perfusion.
12. Aeration
Aeration may be achieved by medium perfusion, in which
medium is continuously taken from culture vessel, passed
through an oxygenation chamber and returned to the culture.
The cells are removed from the medium taken for perfusion so
that the medium can be suitably altered, e.g., for pH control.
Perfusion is used with glass bead and, more particularly, with
micro-carrier systems.
The following components of the fermenter are required for
aeration and agitation:
(i) agitator (impeller)
(ii) stirrer glands and bearings
(iii) Baffles
(iv) sparger (the aeration system).
13. 1. Agitator (Impeller):
• Agitators achieve the following objectives;
(a) Bulk fluid and gas-phase mixing
(b) Air dispersion
(c) Oxygen transfer
(d) Heat transfer
(e) Suspension of solid particles
(f) Maintenance of a uniform environment
throughout the vessel.
14. These objectives are achieved by a suitable combination
of the most appropriate agitator, air sparger and baffles,
and the best positions for nutrient feeds, acid or alkali for
pH control and antifoam addition.
Agitators are of several different types:
(i) Disc turbines
(ii) Vaned discs
(iii)Open turbines of variable pitch
(iv) Propellers.
15. 2. Stirrer Glands and Bearings:
• The satisfactory sealing of the stirrer shaft
assembly has been one of the most difficult
problems; this is very important for maintaining
aseptic conditions over long periods. Four basic
types of seal assembly have been used in
fermenters:
(1)The stuffing box (packed- gland seal)
(2)The simple bush seal
(3)The mechanical seal
(4)The magnetic drive.
16.
17. 3. Baffles:
Baffles are metal strips roughly one-tenth of the vessel
diameter and attached radially to the fermenter wall.They
are normally used in fermenters having agitators to
prevent vortex formation and to improve aeration
efficiency
Usually, four baffles are used, but larger fermenters may
have 6 or 8 baffles. Extra cooling coils may be attached
to baffles to improve cooling. Further, the baffles may be
installed in such a way that a gap exists between the
baffles and the fermenter wall. This would lead to a
scouring action around and behind the baffles, which
would minimise microbial growth on the baffles and the
fermenter wall.
18. 4. Aeration System (Sparger):
• The device used to introduce air into the fermenter
broth is called sparger.
Spargers are of the following three basic types:
(1) Porous spargers
(2) Orifice spargers
(3) Nozzle spargers.
Porous spargers may be made of sintered glass,
ceramics or a metal.
19.
20. Temperature Regulation:
• The fermenter must have an adequate provision for temperature
control. Both microbial activity and agitation will generate heat.
If this heat generates a temperature that is optimum for the
fermentation process, then heat removal or addition may not be
required.
• But in most cases, this may not be the case; in all such cases,
either additional heating or removal of the excess heat would be
required. Temperature control may be considered at laboratory
scale, and pilot and production scales.
The heating/cooling requirements for a specific fermentation
process can be accurately estimated by taking into account the
overall energy balance of the process, which is described by the
following formula.
Qmet + Qag + Qgas = Qacc + Qexch + Qevap
+ Qsen
21. where, Qmet – the rate of heat generated by microbial
metabolism;
Qag = the rate of heat produced by mechanical agitation;
Qgas = the rate of heat generated by aeration power input; Qacc =
the rate of heat accumulation in the system;
Qexch = the rate of heat transfer to the surroundings and/or heat
exchanger, i.e., heating/cooling device;
Qevap = the rate of heat loss due to evaporation; and
Qsen = the rate of sensible enthalpy gain by the flow streams
(exit-inlet). This equation may be arranged as follows.
Qexch = Qmet + Qag + Qgas – Qacc – Qsen – Qevap
In this equation, Qexch provides the estimate of heat that has to be
removed by the cooling system. The values for Qmet are
experimentally determined for different substrates, while those of
Qag, Qgas, Qevap,and Qsen are computed using appropriate
methods/formulae.
22. Foam Control:
Foam is produced during most microbial
fermentations. Foaming may occur either due to a
medium component, e.g., protein present in the
medium, or due to some compound produced by the
microorganism. Proteins are present in corn-steep
liquor, pharma media, peanut meal, soybean meal,
etc.
These proteins may denature at the air-broth interface
and form a protein film that does not rupture readily.
Foaming can cause removal of cells from the
medium; such cell wills undergo autolysis and release
more proteins into the medium. This, in turn, will
further stabilize the foam.
23. Five different patterns of foaming are recognized; these
are listed below.
1. Foaming remains at a constant level throughout the
fermentation. Initial foaming is due to the medium, but
later microbial activity contributes to it.
2. Foaming declines steadily in the initial stages, but
remains constant thereafter. This type of foaming is due to
the medium.
3. The foaming increases after a slight initial fall’, in this
case, microbial activity is the major cause of foaming.
4. The foaming level increases with fermentation
duration; such foaming pattern is solely due to microbial
activity.
5. A complex foaming pattern that combines features of
two or more of the above patterns.
24. Ideal antifoam should have the following properties.
1. It should disperse rapidly and act fast on existing foam.
2. It should be used at a low concentration.
3. It should prevent new foam formation for a long time.
4. It should not be used up or degraded by the
microorganism.
5. It should be nontoxic (to the microorganism as well as
animals, including humans).
6. It should not interfere with downstream processing.
7. It should not cause problems in effluent treatment.
8. It should be safe to handle.
9. It should be cheap.
10. It should not affect oxygen transfer.
25. Types of Fermenters:
Stirred Tank Fermenter:
These are glass (smaller vessels) or
stainless steel (larger volumes) vessels
of 1-1,000 1 or even 8,000 1 (Namalva
cells grown for interferon; but in
practice their maximum size is 20 1
since larger vessels are difficult to
handle, autoclave and to agitate the
culture effectively).
These are closed systems with fixed
volumes and are usually agitated with
motor-driven stirrers with considerable
variation in design details, e.g., water
jacket in place of heater type
temperature control, curved bottom for
better mixing at low speeds, mirror
internal finishes to reduce cell damage,
etc. Many heteroploid cell lines can be
grown in such vessels.
26. Airlift Fermenter:
• An airlift fermenter consists of a gas light baffled riser tube
or draught tube (broth rises through this tube) connected to a
down-comer tube (broth flows down through this tube). The
riser tube may be placed within the down-comer tube as
shown in Fig. 14.4, or it may be externally located and
connected to the latter (Fig. 14.5). Air/gas mixture is
introduced into the base of the riser tube by a sparger.
• The aerated medium/broth of the riser tube has a lower
density, while that in the down-flow tube it is relatively
much less aerated and, as a consequence, has a higher
density. This density difference drives the circulation of
broth.
• The lighter medium in the rise tube flows upward till it
reaches the gas disengagement space of the fermenter. The
27.
28.
29. This air-lift fermenter of 43 m3 volume is used in a continuous
mode for the production of mycoprotein Quorn from Fusarium
gaminareum grown on wheat starch-based medium. It allows
production of long hyphae due to low shear, which is the
preferred form of the product.
However, it gives lower biomass yields (only 20 g l-1) due to
lower oxygen transfer rates in the high viscosity broth
resulting from fungal hyphae. This fermenter is a modification
of that designed by ICI pic, U.K. for SCP production using
methanol as substrate.
The fermenter was developed to reduce production costs by
minimising cooling costs since agitated vessels would
generate additional heat. ICI pic used it in a continuous
process to produce SCP for animal feed, but the process had to
be discontinued because of high methanol cost and
competition from animal feeds based on protein-rich crop
produce. The mycoprotein production is, however, primarily
for animal food.
30. Animal cell cultures are also grown in such vessels that are both aerated
and agitated by air bubbles introduced at the bottom of vessels (Fig.
14.7). The vessel has an inner draft lube through which the air bubbles
and the aerated medium rise since aerated medium is lighter than non-
aerated one; this results in mixing of the culture as well as aeration. The
air bubbles lift to the top of the medium and the air passes out through an
outlet.
The cells and the medium that lift out of the draft tube move down
outside the tube and are re-circulated. O2 supply is quite efficient but
31. Tower Fermenter:
• A tower fermenter has been defined by Green-
shields and co-workers as an elongated non-
mechanically stirred fermenter that has an aspect
ratio (height to diameter ratio) of at least 6 : 1 for
the tubular section and 10 ; 1 overall, and there is a
unidirectional How of gases through the fermenter.
There are several different types of tower fermenters,
which are grouped as follows on the basis of their
design:
(1) Bubble columns
(2) Vertical-tower beer fermenter
(3) Multistage fermenter systems.
32. 1. Bubble Column Tower Fermenters:
These are the simplest type of tower fermenters; they
consist of glass or metal tubes into which air is introduced
at the base. Fermenter volumes from 3 / to up to 950 / have
been used, and the aspect ratio may be up to 16 : 1. These
tower fermenters have been used for citric acid and
tetracycline production, and for a range of other
fermentations based on mycelial fungi.
2. Vertical-Tower Beer Fermenters:
These fermenters were designed for beer production and to
maximise yeast biomass yields. A series of perforated
plates are placed at intervals to maximise yeast yields. It
has a settling zone free of gas; in this zone, yeast cells
settle down to the bottom and return to the main body of
the tower fermenter, and clear beer could be removed from
the fermenter. Tower of up to 20,000 / capacity and capable
of producing up to 90,000 I beer per day have been
installed.
33. 3. Multistage Tower Fermenters:
In these fermenters, a column forms the body of
vessel, which is divided into compartments by
placing perforated plates across the fermenter. About
10% of the horizontal area of plates is perforated. In
a variant of this type of fermenter (down-flow tower
fermenter), the substrate is fed in at the top and
overflowed through down spouts to the next section,
and the air is supplied from the base. These
fermenters have been used for continuous culture of
E. coli, S. cerevisiae (baker’s yeast), and activated
sludge.
34.
35. • Bubble-up Fermenter:
It is a bubble column fermenter that is fitted with an
internal cooling coil (Fig. 14.8). Air is introduced
from the bottom of the column. In this vessel, the
cooling coil effectively separates the column into an
inner riser/draught tube and the outer down-flow
tube. The cooling coil assembly functions as a leaky
draught tube.
The culture broth rises in the compartment enclosed
by the cooling coils and it moves down in the
compartment outside the coil, although back- mixing
also occurs through the coils. The region above the
cooling coil shows good mixing, and there were no
poorly oxygenated zones in the vessel.
36. These are the simplest type of tower
fermenters; they consist of glass or metal tubes
into which air is introduced at the base. These
tower fermenters have been used for citric acid
and tetracycline production, and for a range of
other fermentations based on mycelial fungi.
