Baffles:

 To augment mixing and gas dispersion, baffles are employed. They are normally incorporated into
agitated vessels of all sizes to prevent vortex and to improve aeration efficiency.
 Baffles are metal strips roughly one-tenth of vessel diameter and attached radially to the wall of
bioreactor.
 The agitation effect is only slightly increased with increase in width of baffles, but drops sharply with
narrower baffles. Generally four to eight baffles are incorporated.
 Baffles should be installed in such a way that a gap exist between them and vessel wall, so that there is
scouring acting around and behind the baffles thus minimizing microbial growth on the baffles and
fermenter walls.
 Extra cooling coils may be attached to baffles to improve the cooling capacity of the fermenter without
affecting the geometry. With animal cell culture baffles causes shear damage, instead of baffles bottom
drive axial impellers slightly off sight of centre is used.
 The aeration system (sparger): - Gas under pressure is supplied to the sparger (usually either a ring with
holes or a tube with singl
The aeration system (sparger): -
Gas under pressure is supplied to the sparger (usually either a ring with holes or a tube with
single orifice). It is defined as a device for introducing air into the liquid fermenter.
Three basic types of sparger have been used and they are: the porous sparger, the orifice sparger
(a perforated pipe), and he nozzle sparger (an open or partially closed pipe).
A combined sparger-agitator may be used in laboratory fermenter.

Followings are adequate for good performance:

a) The sparge holes in the ring should be in line with the inner edges of the impeller blades.
b) The sparger holes should face downward to minimize medium retention in the sparger.
c) Hole diameter should be chosen such that each hole is a critical orifice at maximum glass
flow.
d) The sparger inlet pipe should be placed so as to allow free draining back into the vessel.
 The porous sparger is made up of sintered glass, ceramics or metal and has been used primarily on a
laboratory scale in non-agitated vessels. Here, throughput of air is low because of the pressure drop
across the sparger and there is also problem of the fine holes becoming blocked by the growth of
the microbial culture.

 In small stirred fermenter the perforated piped (orifice sparger) are arranged below the impeller in
the form of crosses or rings, approximately three quarters of the impeller diameter.
 Sparger holes should be at least 6 mm diameter because of the tendency of smaller holes to block
and to minimize the pressure drop.

 In low viscosity fermentations sparged at 1 vvm with a power input of 1 W kg-1 , the power often
falls below 50 % of its unaerated value when using a single Rushton disc turbine which is one third
of the diameter of the vessel and a ring sparger smaller than the diameter of the agitator.

 If the ring sparger is placed close to disc turbine and its diameter is 1.2 times of that of disc turbine,
a number of benefits can be obtained. A 50% higher aeration rate can be obtained before flooding
occurs, the power drawn is 75% of the unaerated value, and a higher KLa can be obtained at same
agitator speed and aeration rate.
 Orifice spargers without agitation have been used in limited extent in yeast manufacture, effluent
treatment and in production of single cell protein in air lift fermenter.

 In most modern mechanically stirred fermenter from laboratory to industrial scale have a single open
or partially closed pipe (Nozzle sparger) as a sparger to provide the stream of air bubbles.

 Ideally the pipe should be positioned centrally below the impeller and as away as possible from it to
ensure that impeller is not flooded by air stream.

 The single nozzle sparger causes a lower pressure loss than any other sparger and normally does not
get block.
 Bubbles from a single orifice, bubble volume is directly proportional to orifice diameter and
surface tension, and it is inversely proportional to the density of the liquid.

 Neither gas pressure, temperature, nor liquid viscosity is reported to have much effect on bubble
size, but unfortunately no work has been done with systems exhibiting non-Newtonian viscosity
(plastic flow) such as occurs in mold cultures.

 Bubble size is always larger than the pore size by a factor of 10 to 100 for porous spargers. Surface-
active agents which are present in most complex media reduce the bubble size, and one might
suppose that the resulting increase in interfacial area would cause higher values for KLa.

 On the other hand, tiny bubbles which are unable to create much turbulence in their passage
through the broth would be surrounded by rather thick liquid films, and these in turn would
adversely affect KLa.
Foam control: -
 The problem often encounters in fermentation is foaming. It is very important to control foaming.

 When foaming becomes excessive, there is a danger that filters become wet resulting in contamination,
increasing pressure drop and decreasing gas flow.

 Foam can be controlled with mechanical foam breaker or the addition of surface active chemical agents,
called anti foaming agents.

 Foam breaking chemicals usually lowers KLa values, reducing reactors capacity to supply to supply oxygen
or other gases, and some cases they inhibit the cell growth.
 Mechanical foam breaker available is “turbosep”, in which foam is directed over stationary
turbine blades in a separator and the liquid is returned to fermenter. Foam is also controlled
by addition of oils. Control of foams by oil additions is of large economic importance to the
fermentation industry.

 Excessive foaming causes loss of material and contamination, while excessive oil additions may
decrease the product formation.

 Antifoam oils may be synthetic, such as silicones or polyglycols, or natural, such as lard oil or
soybean oil.

 Either will substantially change the physical structure of foam, principally by reducing surface
elasticity. Industrial antifoam systems usually operate automatically from level-sensing
devices.

 Methods for metering of oil under aseptic conditions are: timed delivery through a solenoid,
two solenoids with an expansion chamber between, a motor-driven hypodermic syringe, and
certain industrial pumps.
Temperature control: -
• Normally in the design and construction of a fermenter there must be adequate provision for
temperature control which will affect the design of the vessel body.

• Heat will be produced by microbial activity and mechanical agitation and if generated by these
two processes is not ideal for the particular manufacturing process then heat may have to be
added to or removed from, the system.

• On laboratory scale little heat is normally generated and extra heat has to be provided by
placing fermenter in a thermostatically controlled bath, or by use of internal heating coils or by
a heating jacket through which water is circulated or a silicone heating jacket.

• When certain size has been exceeded, the surface area covered by the jacket becomes too
small to remove the heat produced by the fermentation.
1. Four other types of auxiliary equipment are flanking the main fermenter in a plant or
biorefinery, i.e. the seed train, the utilities, equipment for cell separation and buffer/feed tanks).
The seed train is usually a sequence of vessels that go up in size in steps of a factor 10, to propagate
the initial amount of cells from a working cell bank into an inoculum for the main fermenter.

2. Utilities involve equipment for sterilization (e.g. in line heat shocks, or steam), a compressor for air
(or other gas) supply, a motor for driving the agitator and a system for pumping cooling water
through coils or a jacket in contact with the fermentation broth.

3. The separation of the cells from the rest of the fermentation broth can be accomplished with
membranes or centrifuges.

4. Finally, supply of feed solutions, titrants, anti-foam agent and nitrogen sources, as well as storage
of broth between fermentation and the purification line, requires a series of separate vessels,
which all need to be cleanable, sterilizable and require controlled filling, mixing and discharge into
the bioreactor or downstream processing unit.