Experiment 7
Experiment 7
Experiment 7
Objectives :
Introduction:
Most of the biochemical processes require oxygen as the source to yield the output. One
of the examples is fermentation. Fermentation is defined as a process of producing chemicals
from substrates by using organisms. The presence of organisms necessitates the oxygen
supply to initiate the reaction and yield the desired product. Hence, dissolved oxygen
concentration becomes one of the important control variables in any aerobic fermentation. It is
very important to thoroughly understand the oxygen transfer to cells in a reactor. During an
aerobic fermentation, oxygen has to be supplied continuously to the reaction liquid in order to
maintain the design concentration while oxygen is consumed by the organism. The oxygen is
transferred from air bubbles and then sparged into the reaction solution and broken up and
mixed by mechanical stirring. There are two factors that affect the capability of a reactors
oxygen mass transfer namely air flow rate and the level of agitation. These two parameters give
a significant effect on mass transfer coefficient, k La in aerobic bioreactors. Mass transfer
coefficient, kLa is a function of the mechanics of a particular reactor namely its constant
dimension and its operating parameters.
Among numerous methods of determining the mass transfer coefficient, k LA are:
1. The static or gassing out method
2. The dynamic method with growing culture
Both of these methods utilize graphical techniques to calculate experimental values for kLa and
share a common underlying principle of biological aerobic reactions:
The oxygen transfer rate is the rate at which oxygen is transferred into solution and can be
expressed in term of kLA:
Where
kL = oxygen transfer coefficient (cm/h)
a = gas-liquid interfacial area (cm2/cm3)
kLa = Volumetric Oxygen transfer coefficient (h-1)
C* = saturated dissolved oxygen concentration (mg/L)
CL = actual dissolved oxygen concentration in the broth (mg/L)
OTR = oxygen transfer rate (mg O2/L.h)
The oxygen uptake rate (OUR) is the rate at which bacteria or other microorganisms
consume oxygen (typical units of mmol O2/L.h)
OUR = qO2. X
Where;
qO2 = specific rate of oxygen consumption (mmol O2/gdw.h)
X = bacteria concentration (gdw/L)
gdw = gram dry weight of cells
The saturated oxygen concentration in water (C*) varies with temperature. Values for C* are
given in the table below:
Expectations:
1. Prepare a suitable method to determine the Mass Transfer Coefficient (kLa) in a
bioreactor using distilled water only as the simulating media.
2. Choose three manipulating variables that influencing the kLa values in bioreactor
3. Discuss in the report how the chosen manipulating variables affecting the kLa.