oxygen tansfer

8/12/2019 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

oxygen tansfer

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