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The oxygen transfer process
• The oxygen transfer process
• The oxygen transfer model
• Oxygen transfer coefficient (kL)
and interfacial area (a)
• Oxygen concentration at the gasliquid interface(Co*)
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The oxygen transfer process
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The oxygen transfer model
dCo
*
OTR 
 k L a(Co  Co )
dt
CO is the concentration of oxygen
dissolved in the bulk liquid
kL is the mass transfer coefficient for the
bubble boundary layer
a is the interfacial area per unit volume
CO* is the concentration of oxygen in the
bubble boundary layer.
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Oxygen transfer coefficient (kL)
and interfacial area (a)
• Because it not possible to accurately
measure the total interfacial area of
the gas bubbles (a) and kL.
• The kL a represents the oxygen
transfer rate per unit volume.
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Oxygen concentration at the
gas-liquid interface(Co*)
• There is no probe small
enough to directly
measure.
• the value of Co* can be
approximated by
indirect methods.
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transfer coefficient (kL)
• The mass transfer coefficient (kL)
represents the rate at which oxygen
molecules move through the boundary layer
to the bulk liquid.
• The value of kL can be increased by
– reducing the size of the boundary layer
– increasing the rate at which molecules
travel through the boundary layer
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previous chapter
• the size of the boundary layer is
determined by the level of mixing.
• the diffusivity of the molecule
through the boundary layer is
determined by
–Medium viscosity
–Temperature
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Oxygen uptake
• Oxygen as a growth limiting nutrient
• Steady state analysis
• Critical Oxygen Concentration
• Integrating the oxygen uptake and
oxygen transfer equations
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Oxygen as a growth
limiting nutrient
Monod model

m S
Ks  S
The growth limiting substrate may be the carbon and
energy source or it may be a nutrient such as a nitrogen
compound.
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In many fermentation
systems, oxygen is
often the growth
limiting substrate.
The following figure
shows a typical Monod
relationship.
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