Mixing Consoles Designations Inputs Outputs Signal processing.
Various inputs that can be controlled to impact mixing
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Transcript of Various inputs that can be controlled to impact mixing
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-ABHAY TANKSALI2012BB50003
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Aphysical process which aims at reducing
non-uniformitiesin fluids by eliminating
gradients of concentration , temperature , and
other properties.
Dominant factor in deciding the performance
or efficiency of a bioreactor.
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FACTORS
AFFECTING
Impeller
speed
Power
requirements
Rheological
properties
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Electrical power is used to drive impellers in
stirred tanks.
Depends on the resistance offered by the fluid
to rotation of the impeller.
[Electrical power consumed]>[Mixing power]
{depending on the efficiency}
Energy costs for operation of stirrers in
bioreactors is an important consideration.
On an avg.
10kW m3 for small vessels to
1~2 kW m3 for large vessels
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MIXING TIME
Useful parameter for assessing mixing efficiency
The mixing time tm is the time required toachieve a given degree of homogeneity starting
from the completely segregated state. It can be measured by injecting a tracer into the
vessel and following its concentration at a fixedpoint in the tank
Tracers in common use include acids, bases andconcentrated salt solutions ; correspondingdetectors are pH probes and conductivity cells
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MEASURING MIXING EFFICIENCY
small pulse of tracer is added to fluid in a
stirred tank already containing tracer material
at concentration Ci.
If circular flow, tracer concentration measured
at some fixed point in the tank will follow a
pattern similar to that shown in Figure
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Concentration response after tracer is
injected into a stirred tank
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MEASURING MIXING EFFICIENCY
Definition of the mixing time tm depends on
the degree of homogeneity required
Usually, mixing time is defined as the time
after which the concentration of tracer differs
from the final concentration Cf by less than
10% of the total concentration difference
(Cf
Ci)
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Effective mixing
Intensity of turbulence is represented by the
impellers Reynolds number.
once Re falls below criteria, turbulence is
damped and mixing time increases
significantly.
Re decreases in direct proportion to increase
in viscosity.
Turbulent conditions
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Variation of mixing time with Reynolds number
for a six-blade Rushton turbine in a baffled tank
Here, Ni is the number of rotations per second and the
product Ni*ti is called Dimensionless mixing time.
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With increase in viscosity non-turbulent conditions
and poor mixing are likely to occur..
This can be overcomed by increasing the impeller speed
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Increase in impeller speed
Increases eynol s
number
Increases
shear stress to
cells
Increases power
consumption.
Decreases
mixing time
Increases cost
of production
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May not be
feasible!!
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