oxygen transfer to bioreactors

8/10/2019 oxygen transfer to bioreactors

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Oxygen transfer in bioreactors

The majority of fermentation processesare aerobic

Therefore oxygen is an important

requirement in aerobic processes like

Therefore, 192 grams of oxygen are

required for the complete oxidation of 180grams of glucose.


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The oxygen demand of an industrial fermentation

process is normally satisfied by aerating and agitating the

fermentation broth.

The productivity of many fermentations is limited by oxygen

availability.

Therefore, it is an important factors which affect a efficiencyof fermenter in aerobic fermentation processes.


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The effect of dissolved oxygen concentration

on the specific oxygen uptake rate is follow

the Michaelis-Menten type curve given below

(Qo2)

is mmoles of oxygen consumed per gram dry

weight of cells per hour.


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The above figure shows the specific oxygen uptake rate increases with increase in

the dissolved oxygen concentration up to a certain point (referred to as Ccrit ) above

which no further increase in oxygen uptake rate occurs.


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Some examples of the critical oxygen levels for a range of micro-organisms are given

in Table


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Therefore, maximum biomass production may beachieved by satisfying the organism's maximum specificoxygen demand by maintaining the dissolved oxygenconcentration greater than the critical level.

If the dissolved oxygen concentration were to fall belowthe critical level then the cells may be metabolicallydisturbed.


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Critical dissolved oxygen concentrations for a

range of micro-organisms (Riviere, 1977)


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Oxygen is normally supplied to microbial cultures in

the form of air, because air is the cheapest available

source of the gas.

The method for supply of air in a culture is varies

with the scale of the process:

Oxygen transfer in large vessels


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(i) At laboratory-scale cultures may be aerated

by means of the shake-flask technique

where the culture is grown in a conical flaskshaken on a platform contained in a

controlled environment of chamber.

(ii) Pilot- and industrial-scale fermentations

broth or culture is aerated by

-stirred or agitation method,


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(iii) Some bioreactor or fermenter are so

designed that adequate supply of oxygen is

obtained without agitation and such

bioreactor or fermenter are called bubble

columns and air-lift fermenter


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Bartholomew et at. (1950) represented the transfer

of oxygen from air to the cell, during a fermentation,as occurring in a number of steps:

(i) The transfer of oxygen from an air bubble into solution.

(ii) The transfer of the dissolved oxygen through thefermentation medium to the microbial cell.

(iii) The uptake of the dissolved oxygen by the cell.


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The individual steps involved in oxygen transfer from a gas

bubble to the reaction site inside the individual cell


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The rate of oxygen transfer from air bubble to the liquid phase may be described by theequation:

dCL/ dt = KLa(C*-CL) (1)

where ,

CL is the concentration of dissolved oxygen in the fermentation broth (inmmoles dm-3)

t is time (in hours),

dCL/ dt is the change in oxygen concentration over a time period, i.e. theoxygen transfer rate (mmoles O2 dm-3 h- 1),

KL is the mass transfer coefficient (cm h- 1),

a is the gas/liquid interface area per liquid volume (cm2 cm- 3),

C* is the saturated dissolved oxygen concentration (mmoles dm-3 ).


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KLmay be considered as the sum of the

reciprocals of the resistances to the transfer of

oxygen from gas to liquid

(C* - CL ) may be considered as the 'driving

force' across the resistances.

oxygen transfer to bioreactors

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