The document discusses oxygen transfer in aerobic fermentation processes. It states that the majority of fermentation processes require oxygen, which has low solubility in water. For efficient oxygen transfer, dissolved oxygen must be continuously supplied to microorganisms at a rate equal to their demand. Key factors that influence oxygen transfer rate include bubble size, agitation intensity, viscosity, foaming, and vessel geometry. Equations are provided to characterize oxygen transfer rates and model maximum cell densities supported by reactors based on process conditions. Scale-up of fermentation processes requires matching critical environmental parameters like dissolved oxygen levels between small and large scales.

1. Oxygen Transfer
 Majority of fermentation processes are
aerobic and therefore require the provision of
oxygen.
 If the stoichiometry of respiration is
considered then the oxidation of glucose is
represented by
C6H12O6 + 6 O2 6H2O+6CO2

2.  Thus 192 gms of O2 is required for 180 gm glucose.
 Also oxygen is required for product formation ex: for
pencillin production 2.2 gms of oxygen is required
for 1g pencillin formation
 Microorganisms take up the substrate/ nutrient from
liquid
 Hence both O2 and glucose have to be available in
liquid
 Solubility of oxygen in water is 1.26 mmol/l at 25o
C
OR approx 8 mg/l
 This will reduce further in the presence of salt or
acid
 This solubility is 6000 times lesser than the solubility
of glucose

3.  Thus it is not possible to supply entire oxygen
required for batch fermentation in one
addition.
 Oxygen will be supplied continuously at the
rate of demand by the microorganism.
 Oxygen is consumed quickly in aerobic
cultures. For actively respirating yeast with
109
cells /ml oxygen in liquid have to be
replaced 12 times in minute to keep up the
cellular demand.
 O2 in the liquid is measured as Dissolved
oxygen concentration

4. OUR = qo X
OUR = oxygen uptake rate
qo= specific oxygen uptake rate
X = biomass concentration
Cell concentration increases oxygen
requirement increases
Upto certain DO concentration in the
liquid the specific oxygen uptake rate
increases with DO and remain
constant after that.
E.coli = 0.008 mmol/l
S.cereviseae = 0.004 mmol/l
Pencillium sp = 0.022 mmol/l
Time
qo
X
qo
X
qo
DO
1

5. Flux = resistance X driving force
NA = kLa X (C*-C)
NA = Oxygen transfer rate
kLa = Volumetric Mass transfer coefficient
C*-C = Concentration gradient.
The concentration gradient is small because of
poor solubility of oxygen. Hence it is not easy
task.
2

6. 1
2
3
4
5
6
7 8
1.1. Transfer from the interior of the bubble to the gas liquid interfaceTransfer from the interior of the bubble to the gas liquid interface
2.2. Movement across the gas liquid interfaceMovement across the gas liquid interface
3.3. Diffusion through relatively stagnant film surrounding the bubbleDiffusion through relatively stagnant film surrounding the bubble
4.4. Transport through the bulk liquidTransport through the bulk liquid
5.5. Diffusion through the relatively stagnant film surrounding the cellsDiffusion through the relatively stagnant film surrounding the cells
6.6. Movement across the liquid cell interfaceMovement across the liquid cell interface
7.7. If the cells are in a floc, clump diffusion through the solid to theIf the cells are in a floc, clump diffusion through the solid to the
individual cellindividual cell
8.8. Transport through the cytoplasm to the site of the reactionTransport through the cytoplasm to the site of the reaction
Note: Resistance due to the gas boundary layer on the inside of the bubble has
been neglected. If the cells are individually suspended step 7 disappears.

7.  Magnitude of various mass transfer
resistance depend on the composition and
rheological properties of the liquid, mixing
intensity, bubble size, cell clump size, etc.,
For most of the bioreactors the following
analysis is valid
 Step 1 – relatively fast
 Step 2 – negligible resistance
 Step 3- major resistance

8.  Step 4 – In well mixed fermenter
concentration gradients in the bulk liquid
are minimised and resistance is small.
However rapid mixing is affected in the
viscous broths. In this case the bulk liquid
resistance is important example: Xanthan
gum production, high viscous mycelial
fermentation etc.,

9.  Step 5: Because single cells are much
smaller than the gas bubbles, the liquid
film surrounding each cell is much thinner
than that around the bubbles and its effect
on mass transfer can generally be
neglected. On the other hand if the cells
form large clumps, liquid film resistance
can be significant example Citric acid
production. A.niger form pellets.

10.  Step 6 – Very small
 Step 7 – When the cells are in clumps,
intraparticle resistance is significant as
oxygen has to diffuse through the solid pellet.
Magnitude of this depend on the size of the
clumps.
 Step 8 – Intracellular transfer resistance is
small since distance is very small.

11. At steady state the rate of oxygen transfer from the bubbles must be
equal to the rate of oxygen consumption by the cells. Equations
1 and 2 will be equal.
kLa (C*-C)= qo X
kLa is used to characterize the oxygen mass transfer
capability
If it is small then the ability of the reactor to deliver oxygen
is small.
At steady state if stirrer speed is increased i.e kLa is
increased (Raising stirrer speed will reduce the thickness
of the boundary layer surrounding the bubble) the
dissolved oxygen concentration increases
At the same time if the cell concentration is increased at
constant kLa the DO will decrease
3

12. Equation 3 can be used for deriving
relationships for fermenters.
For a given set of operating conditions the
maximum rate of oxygen transfer occurs
when the driving force is maximum.
i.e. C*-C is highest. In otherwords when C = 0
Sub in eqn 3 for obtaining maximum cell
concentration supported by the reactor
Xmax = kLa C*/qo
If Xmax is lower than the required cell concentration
in fermentation then kLa should be improved.
(kLa)crit =qoX/(C*-Ccrit)

13. FACTORS AFFECTING OXYGEN
TRANSFER
OTR ∝ kL
a
(C*-C)

14. Bubbles
 Efficiency of OTR depends to large extent on
the characteristic of bubble
 Bubble behaviour mostly affect kLa
 In stirred fermenter air is sparged under the
impeller
 In lab fermenters very good mixing will be there
hence bubbles in the system are frequently subject
to distortions (10-20 KW/m3
)
 In contrast in industrial fermenters most of the time
bubbles are freely floating after initial dispersion (0.5
– 5 KW/m3
)
Reason P/V value are low in large fermenters

15.  The most important property of air bubbles in
fermenter is its size
 Smaller the size
 Greater is the interfacial area a
 Slow bubble rise velocity and stays for
longer time in fermenter giving more
time for oxygen transfer
 Create high gas holdup (ε)
 ε = VG/(VL+VG)
VG – Vol of gas bubbles in reactor
VL – Vol of liquid

16.  Interfacial area largely depend on gas holdup.
It varies between 0.01 to 0.2
 One side it is desirable to have smaller
bubbles.
 But bubbles less than 1 mm dia can become
nuisance in bioreactors
 O2 concentration in the bubbles immediately
transferred to medium and attains equilibrium.
These bubbles staying in the reactor is of no
use.

17.  Bubble size also affects kL
 Bubbles less than 2-3 mm dia acts as rigid
spheres. This lowers kL values
 On the other hand bubbles greater in size have
relatively mobile surfaces.
 These bubbles are able to wobble and move in
spirals during free rise
 Due to this they have beneficial effect
 2-3 mm bubbles 3-4 X 10-4
m/s kL values
 Bubble size reduced kL reduces to 1 X 10-4
 Above 3 mm kL values are constant

18. Air bubbles are formed at the sparger
Sparger design varies from open pipe, porous
diffusers, perforated pipes and complex
injectors
Normally air flow rates used
are 0.5 to 1.5 vvm
The effect of air flow rate
on kLa values is given in
the diagram
Aeration and agitation
0.50 1.0

19.  This air flow rate is maintained during
scaleup
 If the impeller is unable to disperse the
incoming air then OTR decreases extremely
due to impeller flooding.
 Flooding is the phenomena where the air
flow dominates the flow pattern
Air velocityStirrer speed

20.  The degree of agitation played major role in
oxygen transfer
 Agitation increases the air available by dispersing
the air in the culture in the form of bubbles
 It delays escape of air bubbles from the system
 It prevents coalescence of bubbles to bigger ones.
 It decreases the thickness of liquid film at the gas
liquid interface by creating turbulence in the culture

21.  To avoid flooding minimum impeller tip speed
for dispersion of air bubbles 1.5 – 2.5 m/s
 Flooding could be avoided if
F/ND3
< 0.3 N2
D/g
F – Volumetric air flow rate
N – stirrer speed
D – stirrer diameter
g – accelaration due to gravity

22. Viscosity changes flow properties such as
surface tension etc will affect kLa
Increase in viscosity decreases kLa
Increase in viscosity may occur due to biomass
in case of fungal mycelia formation or some
products such as polysachharides production
Broth Viscosity

23. High degree of aeration and agitation will result in
foam formation.
Foaming reduces oxygen transfer. Air bubbles
entrapped in the foam and again and again
they recirculate in the medium. This will result
in oxygen depleted bubbles residing in the
system
To control foam antifoam agents are added.
Antifoam agents

24.  Most of the antifoams are surface tension
lowering substances
 This will result in rigid bubble formation and
resistance to oxygen transfer.
 Also antifoams in the liquid may favour
coalescence of bubbles in freely moving areas
which again will decrease oxygen transfer.
 OTR can be reduced dramatically even by
factor of 10.

25. Normally salts suppress the coalescence of bubbles
hence it favors OTR
Increase in suspended solids will decrease OTR in
High cell density cultivation
Temperature increase beyond 40 o
C will decrease
oxygen solubility hence OTR
Vessel geometry will influence OTR. If H/D ratio is
more bubble residence time is more and hence
OTR may increase.
Other factors

26. kLa and Power consumption
 A large number of empirical relationships
have been developed between kLa, power
consumption and superficial gas velocity
kLa = k(P/V)x
Vs
y
P –Power absorption
V- Volume of the reactor
Vs – Superficial airvelocity
k,x,y – empirical constants

27.  Value of x is dependant on the size of the
vessel
 Laboratory it is 0.95
 Pilot plant – 0.67
 Production fermenter – 0.5
k – 0.026
x- 0.4
y- 0.5

28. Major Factors in ScaleupMajor Factors in Scaleup
 Inoculum developmentInoculum development
 SterilizationSterilization
 Environmental parametersEnvironmental parameters
 Nutrient availabilityNutrient availability
 pHpH
 TemperatureTemperature
 Dissolved oxygen concentrationDissolved oxygen concentration
 Shear conditionsShear conditions
 Dissolved CODissolved CO22 concentrationconcentration
 Foam productionFoam production

29. AerationAeration
AgitationAgitation
COCO22
Bulk mixingBulk mixing
FoamFoam
CostCost
ShearShear
OO22

30. Steps in scaleupSteps in scaleup
 Identification of the principalIdentification of the principal
environmental domain affected by theenvironmental domain affected by the
aeration and agitationaeration and agitation
 Identification of the process variable whichIdentification of the process variable which
affects the identified environmentalaffects the identified environmental
domaindomain
 Calculation of the value of the processCalculation of the value of the process
variable to be used on the large scalevariable to be used on the large scale

31. Process VariableProcess Variable CharacteristicsCharacteristics
affectedaffected
Power consumptionPower consumption
per unit volumeper unit volume
Oxygen Transfer RateOxygen Transfer Rate
Impeller tip speedImpeller tip speed Shear RateShear Rate
Volumetric air flow rateVolumetric air flow rate Oxygen Transfer RateOxygen Transfer Rate
Pumping ratePumping rate Mixing timeMixing time
Reynolds numberReynolds number Heat transferHeat transfer

32. Criterion usedCriterion used
in scale upin scale up
from 80 tofrom 80 to
10000 l10000 l
Effect on the operating conditionsEffect on the operating conditions
on the large scaleon the large scale
PP P/VP/V FlowFlow
min-1 vol-1min-1 vol-1
NDiNDi
P/VP/V 125125 1.01.0 0.340.34 1.71.7
FlowFlow min-1 vol-1min-1 vol-1 31253125 25.025.0 1.01.0 5.05.0
NDiNDi 2525 0.20.2 0.20.2 1.01.0
ReynoldsReynolds
numbernumber
0.20.2 0.00160.0016 0.040.04 0.20.2

33. Scale down methodScale down method
 Medium designMedium design
 Medium sterilizationMedium sterilization
 Inoculation proceduresInoculation procedures
 Number of generationsNumber of generations
 MixingMixing
 Oxygen transfer rateOxygen transfer rate