oxygen tansfer
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Transfer of mass from one location to another
Occurs in areas of concentration variation
Occurs till equilibrium is established
Occurs in many processes such as evaporation, adsorption,drying, precipitation, filtration and distillation.
In bioprocessConcentration of compounds are not uniform
Mass transfer principle is made use of to achieve this
E.g., Oxygen transfer
Solvent extraction
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AerationOxygen is normally supplied to microbial cultures in the form of
air, i.e cheapest source
Sterile air / oxygen which must be dispersed throughout
the fermenter
Air introduced into the fermentor is filter sterilized and
introduced via sparger which is located below the agitator
Sparger structure affects oxygen transfer in the medium as it
influences the size of the gas bubble produced
Smaller the bubble, larger the surface area to volume ratio
which provides greater oxygen transfer.
Spargers with small pore size are effective in producing smallbubbles but are prone to clogging and require high energy
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Oxygen transferOxygen transfer is complex and it involves a phase change
from its gaseous phase to the liquid phase which is influenced
by following factors
1. Temperature, pressure and surface area of oxygen bubbles
2. Chemical composition of the medium3. Volume of gas introduced per unit reactor volume per
unit time
4. Type of sparger system used to introduce air into the
fermenter
5. Speed of agitation
In aerobic fermentation- oxygen should be maintained
at optimal concentration for maximal yeild
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Oxygen mass balance
-The rate at which O2can be delivered to the biological
sysytem (OTR-Oxygen transfer rate) and the rate at which
it is utilised by microorganism (COD- Critical oxygen
demand)
Anaerobic conditions develop when the rate of Oxygen
utilization is > than OTR ( This limit growth and production)
OTR can be increased by elevating the pressure,
enriching the inlet air with O2 and increasing agitation and
aeration
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Solubility of oxygen depends on temperature
and pressure
Temperature
in Cp(O2)=100 kPa
Solubility in mg/l
p(O2)=20.9 kPa
Solubility in mg/l
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Determination of OTR
OTR = Oxygen gradient
Resistance to oxygen transfer
= Oxygen gradient ( C*-CL)
eqn 1KLa
C* =is the saturated dissolved oxygen conc.(mmol/dm3)
CL=is the concentration of dissolved O2 at time (t) (mmol/dm3)
KL=is the mass transfer coefficient at the gas to liquid phase(phase boundary)(cm/h)
a = is the gas/liquid interface area per liquid volume (cm2cm-3)
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Stages of resistance of oxygen transfer from gaseous phase
to an individual cell
1. Resistance within the gas film to the phase boundary
2. Penetration of the phase boundary between gas bubble and
liquid
3. Transfer from the phase boundary to the liquid
4. Movement within the nutrient solution
5. Transfer to the surface of the cell
6. Entry into the cell
7. Transport to the site of reaction within the cell
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Gas bubbleGas
filmFluid
film
Fluid
Fluid
film
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Rate of O2transfer from air bubble to the liquid phase is described as
dCL = kLa(C*- CL) eqn. 2
dt
Integration of eqn 1 gives
C*-CL = e-KLat eqn 3
Interms of natural log, for kLa determination
ln (C*- CL) = -KLa (t) eqn 4
KLa for the specific conditions is determined by plotting a semilog
graph of ln(C*- CL) against time where slope is mass transfercoefficient (KLa)
Therefore KLa is a measure of the aeration capacity of a fermentor and
must be maintained above a minimum critical level to satisfy oxygen
requirements
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KLa
KL= Is the mass transfer coefficient
a = Is the gas/liquid interface area per liquid volume (cm2cm-3)
These are difficult to measure individually and are generallylinked to give as
KLa = volumetric mass transfer coefficient per hr
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How to improve the oxygen transport?
Increase of the O2-solubility Pressure increase from 100 to 200 kPa
Increase the O2-content in the air
enrichment of the aeration with O2
Use of pure O2
Change in the phase boundary (gas/liquid)
size and distribution of the gas bubbles
contact time between the gaseous phase and the liquid
phase
Viscosity of the nutrient solution
viscosity reductionincreases the relative velocity of the
gas bubbles thinner liquid film higher kLa-value
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Scale-up
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What is Scale-Up? Increasing the scale of a fermentation. (i.e.
From lab scale to pilot scale and from pilotscale to industrial scale
3 Stages
Bench Scale ( 220 L)
Pilot Scale (100 500 L)
Plant Scale (500 20,000 L)
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What is a scale-up problem?
A scale-up problem is something that we do not see in
the small-scale experiment(lab scale) and are
surprised and disappointed to find in the large scale
process..
Problems associated with scale up are due to the
different ways in which process parameters are
affected by increase in size of the unit
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1) Inoculum development when a scale is increased -extra stages have to be
incorporated in the inoculum development programme
2)Sterilization It is a scale dependent factor
When there is increase in scale of a fermentation process,
the sterilization regime should be adjusted according to the
scale This may result in change in quality of the medium after
sterilization
Major factors involved in
Scale-Up
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3. Environmental parametersIncrease in scalemay results in a change in the environment
for the microorganism.
Environmental parameters may changed due to increase in scale:i) Nutrient availability
ii) pH
iii) Temperature
iv) Shear conditionsv) Dissolved oxygen concentration
vi) Dissolved CO2 concentration
vii) Foam production
Agitation
Aeration
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agitation
aeration
shearcost
foam
mixing
oxygen
CO2
Scale up window based on AGITATION /
AERATION
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This illustrates the"scale-up" window defining the
operating boundariesfor aeration and agitation in the scale-
up of a typical fermentation.
Agitation and aeration rate must fall between a minimum
and maximum value
Problemsthat may arise, when these values are not falling
within the limits.
ACTION RESULTMinimise aeration below the limit Decrease in CO2 and
O2 levels
Maximise aeration above the limit Increased Foam formation takesplace
Minimise agitation below the limit Bulk mixing poor
Maximise agitation above the limit Shear and cost increased
S l ti f l bl
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Solution for scale up problemIdentify the environmental parameter affected by aeration and
agittaion. eg: Oxygen concentration, Shear, bulk mixing
Identify the process variable or variables which affects the
identified environmental parameter
Calculate the value of the process variable to be used on large
scale which results in the same environmental conditions on bothscalesProcess variables which affect the mixing and mass transfer
Process Variable Mass transfer or Mixing
property affected
Power consumption perUnit volume
Oxygen transfer rate
Volumetric air flow rate Oxygen transfer rate
Impeller tip speed Shear rate
Pumping rate Mixing time
Reynolds number Heat transfer
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Scale-Up Parameters
First scale-up criterion is the preservation of
Geometrical similarity when building a new bioreactor vessel, the geometry is
usually scaled linearly.
height to diameter ratio (H/D) or aspect ratio is kept
constant to ensure the tanks will operate similarly.Vessel 1
Vessel 2
The aspect ratio of the vessels is 1.5.
The height and diameter between the two
vessels is scaled linearly.
By multiplying the dimensions of Vessel 1 by6.4, the dimensions of Vessel 2 are determined.
Note: the volume of the vessels does not scale
linearly (volume of vessel 1 is 147 ft3and vessel
2 is 38,603 ft3, 262 times larger.)
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To scale-up a manufacturing process from onebioreactor to another, the process parameters arescaled based on the following :
Agitation-based scaling parameters Gassing-based scaling parameters
NOTE: Cannot keep all parameters constant during
scale up because they scale by different values
Scale-Up Parameters
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Common Scale-Up Parameters Agitation-based scaling parameters:
Mixing Time Power Input per Volume (P/V)
Tip Speed
Notes: These three parameters are all dependent on agitationrate, so all three cannot be held constant when scaling-up. For
example:Keeping mixing time constant might cause a high
P/V that the cells cannot handle.
Scaling based on constant tip speed might cause alow agitation rate that will not deliver oxygenadequately.
- Thus all three scaling parameters must beevaluated and the final scale-up agitation rate mustproduce acceptable values for all three parameters.
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Gassing-based scaling parameters Vessel Volumes per Minute, VVM
Superficial Gas Velocity, Vs Note: The two gas flow rate scaling parameters are both
dependent on the dimensions of the vessel. Scaling based on
one will greatly affect the other.
Scale-Up Parameters
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Agitation Parameters
Parameter Definition Scale-Up Factor Why is this
Important?
Mixing Time
Amount of time it takes thebioreactor to create a
homogeneous environment
N2=N1(D1/D2)1/4
N2agitation speed in scale-upN1agitation speed in scale-downD1impeller diameter of scale -downD2impeller diameter of scale-
up
Want to ensure that thematerials are well-mixed in a
timely manner
Power Input perVolume (P/V)
Amount of powertransferred to a volume of
cell culture through theagitator shaft and impellers
P/V N3/D2
P- power suppliedV- Volume of BioreactorN- Agitation SpeedD- Impeller Diameter
Mammalian cells cannothandle a lot of powerintroduced into the culturemedia as it can cause smalleddies that will shear thefragile cell membranes
Tip SpeedRelated to the shear rate
produced from the impellersmoving through the cell
culture media
N2=N1(D1/D2)
N2agitation speed in scale-upN1agitation speed in scale-downD1impeller diameter of scale -downD2impeller diameter of scale-up
High shear rates can causethe cell membrane to tearand the cells to die.If scale-up based onconstant tip speed isattempted, P/V and mixingtime will decrease
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Gassing Parameters
Parameter Definition Scale-Up Factor Why is thisImportant?
Vessel Volumesper Minute
(VVM)
means the volume ofgas flow per bioreactor
volume per minute. Volume of Gas Flow/time
necessary to ensure thatenough oxygen will be
supplied to the cells
Superficial GasVelocity (Vs)
volume of gas percross-sectional area of
the vessel.
Vs= Qgas/Av
Vs- superficial gas velocityQgas- gas volumetric flow rateAv- inside cross-sectional area
of vessel
increasing Vscauses
An increase in foamgeneration
A decrease in P/VAn increase in oxygentransfer
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Scale-Up Review Overall, scaling up process parameters is tricky
Each scale-up parameter is dependent on another.Scaling-up based on constant P/V will affect the mixingtime and the tip speed in the bioreactor. As well, for gasflow rates, scaling-up on constant Vswill affect the VVM.
Cannot keep all constant during scale-up
Not one scale-up process is correct
Technicians determine which parameter is critical to theprocess and try to find a happy medium between each ofthe remaining parameter