Various inputs that can be controlled to impact mixing

8/11/2019 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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Various inputs that can be controlled to impact mixing

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