Aeration
&
agitation
Aparna J. Joshi
Introduction
The majority of fermentation processes are aerobic and therefore require the
provision of oxygen.
The oxidation of glucose may be represented as;
C6H12O6 + 6O2 = 6H2O + 6CO2
It is not possible to provide a microbial culture with all the oxygen it will need
for the complete oxidation of glucose in one addition.
Therefore, a microbial culture must be supplied with oxygen during growth at a
rate sufficient to satisfy the organisms demand.
The oxygen demand of an industrial fermentation process is normally satisfied
by aerating and agitating the fermentation broth.
However, the productivity of many fermentations is limited by oxygen
availability and therefore, it is important to consider the factors which affect
the fermenter’s efficiency in supplying microbial cells with oxygen.
Aeration and Agitation
Important factor in a fermenters .
Provision for adequate mixing of its contents
Mixing in fermentation
to disperse the air bubbles
to suspend the cells
to enhance heat and mass transfer in the medium
All relate to Gas-liquid mass transfer
Aeration and Agitation
– Aeration refers to the process of introducing air to
increase oxygen concentration in liquids .
– Removes unwanted volatile products of metabolism .
– Aeration may be performed by bubbling air through the
liquid, spraying the liquid into the air or agitation of the
liquid to increase surface absorption .
– Agitation – uniform suspension of microbial cells in
homogeneous nutrient medium .
Aeration and Agitation
– Agitation – uniform suspension of microbial cells in homogeneous
nutrient medium
– Importance of agitation
– 1. To increase the rate of oxygen transfer from the air bubble to the
liquid medium.
– 2. To increase the rate of oxygen and nutrients transfer from the
medium to cells.
– 3. To prevent formation of clumps of cells, aggregates of mycelium.
– 4. To increase the rate of transfer of product of metabolism from
cell to medium.
– 5. To increase the rate or efficiency of heat transfer between the
medium and the cooling surfaces of the fermenters.
Effect of agitation upon aeration
1. by dispersing the air in smaller bubble.
2.by causing the bubbles to follow a more tortuous path and
delaying their escape from the culture.
3. by preventing the coalescence of bubbles.
4. by decreasing the rate-limiting thickness of the liquid film at
the gas/liquid interface
Factors affecting Oxygen supply
1. Type of agitation:
The shape, number and arrangement of impellers and baffles.
Either 2 or 3 impellers for large fermenters at suitable level on the stirrer
shaft or 3 or 4 baffles on the wall of the vessel.
2. Speed of agitation:
1000 or more for lab. fermenters. But this is not possible for large vessels.
For penicillin fermentation requires 50rpm needs high input of energy
and uneconomical.
3. Depth of liquid in the fermenters:
Bubble remain longer in the medium of a tall, deep fermenter. Greater
hydrostatic pressure at the sparger improves solution of oxygen.
Height : diameter ratio of 3:1 or 4:1 is common.
Factors affecting Oxygen supply
4. Type of sparger:
One single opening preferred to produce large bubble.
5. Air flow:
Aeration – increased by air flow rate, expressed in vvm
(Vol. of air/vol. of medium/min.). Large fermenters cannot
be supplied with air at greater rates than 0.5 to 1.0vvm.
6. Physical properties of the medium:
Temp., viscosity, surface tension and nature of organism ,
all affects solubility of oxygen directly or by bubble size
and turbulence.
Structural components involved in
aeration and agitation
– Agitator (impeller)
– Stirrer glands and bearings
– Baffles
– Aeration system (sparger)
Agitator (impeller)
– Achieve mixing objectives –
bulk fluid and gas-phase
mixing, air dispersion, oxygen
transfer, heat transfer,
suspension of solid particles
and maintaining uniform
environment throughout
vessel contents.
–
Types of agitators
– Types: Disc turbines, vaned dics , open turbines of variable pitch and
propellers.
Disc turbine:
• It is with a series of rectangular vanes set in a vertical plane
around the circumference .
• Vane disc:
– It is a series of rectangular vanes attached vertically to the
underside.
– Air from the sparger hits the underside of the disc and is
displaced towards the vanes where the air bubbles are broken
into smaller bubbles.
Stirrer glands and bearings
– Sealing of stirrer shaft – difficult problem in the construction of
fermentation equipment .
• Entry of stirrer shaft – top, side or bottom
• Basic type of seal assembly:
• 1. the stuffing box (packed gland seal)
• 2. the simple bush seal
• 3. the mechanical seal
• 4. the magnetic drive.
Baffles
– Four baffles incorporated into agitated vessels of all sizes to
prevent vortex and to improve aeration efficiency .
– Metal strips roughly one tenth of vessel diameter and
attached radially to the wall .
– Minimizes microbial growth on baffles and fermenter walls.
– Increased agitation with wider baffles; drop in agitation with
narrower baffles.
Aeration system (sparger)
– Introduces air into liquid of fermenter
Three basic types – porous sparger
1. Orifice sparger – a perforated pipe
2. Nozzle sparger – an open or partially closed pipe
3. Combined sparger-agitator
Porous sparger and Orifice sparger
– Made of Sintered glass, ceramics or metal.
• The bubble size produced from spargers is always 10 to
100 times
larger than the pore size of the aerator block.
• Orifice sparger:
• In small stirred fermenters the perforated pipes were
arranged
below the impeller in the form of crosses or rings.
• Orifice spargers without agitation -used in yeast
manufacturing, effluent treatment and in SCP
production.
Nozzle sparger
– It’s a single open or partially closed pipe, provides
stream of air bubbles.
• The single nozzle sparger causes a lower pressure
loss than any other sparger and normally does not get
blocked.
