You can learn about how the Power Requirement for Mixing in bioreactor .they were Gased fluid, ungased fluid , Rheological properties.

1. 10/21/20191
POWER REQUIREMENTS FOR
MIXING -EFFECT OF
RHEOLOGICAL PROPERTIES ON
MIXING IN A BIOREACTOR
G.VIJAY CHITHRA
(117011101311)
H.VISHALACHI
(117011101313)
R.YOKESH
(117012101314)

2. POWER REQUIREMENTS FOR
MIXING
10/21/20192
 Electrical power is used to drive impellers in
stirred vessels.
 Average power consumption per unit volume for
industrial bioreactors
10kWm^-3 for small vessels (ca. 0.1 m^3)
1-2kWm^-3 for large vessels (ca. 100

3. 10/21/20193
 Stirrer motor gearbox and seals reduces the energy
transmitted to the fluid.
 Electrical power consumed by stirrer motors is
always greater than the mixing power by an amount
depending on the efficiency of the drive.
 Energy cost for operation of stirrer are an important
consideration in process economics.

4. UNGASSED NEWTONIAN
FLUIDS
10/21/20194
 Non-aerated fluids.
 It depends on the stirrer speed, the impeller
diameter and geometry, and properties of the fluid
such as density and viscosity.
------- (1)

5. The results for 5 impeller designs
10/21/20195

6. 10/21/20196

7. Once the value of Np is known, the
power required is calculated from
equation (1) as:
10/21/20197
------- (2)

8. 10/21/20198
 For a given impeller, the general relationship
between power number and reynolds number
depends on the flow regime in the tank.
 Three flow regimes:
1. laminar regime
2. turbulent regime
3. transition regime

9. LAMINAR REGIME
10/21/20199
 Reynolds number< 10 for many impeller.
 Stirrer with very small clearance such as anchor and
helical-ribbon mixer, laminar flow persists until
reynolds number = 100 or greater.
(3)
 K1 is the proportionality constant.
 Power required is Independent of the density, but
directly proportional to the fluid viscosity.

10. Values of k1 for the impellers
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11. TURBULENT REGIME
10/21/201911
 Power number is independent of Reynolds
number.
 Np’ – constant value of the power number.
 The Np’ values for the impeller are listed in the
above table.
-------(4)

12. 10/21/201912
 For most small impellers in baffles, reynolds
number > 10^3 or 10^4.
 Without baffles turbulence is not fully developed
until reynolds number > 10^5.

13. TRANSITION REGIME
10/21/201913
 Transition regime lies between the laminar and
turbulent flow.
 Both density and viscosity affect the power
requirements in this regime.
 The flow pattern and reynolds - number range for
transition depend on system geometry.

14. 10/21/201914
 Small changes in impeller size have a large effect
on power requirements, as would be expected
from dependency on impeller diameter raised to
the third or fifth power.
 In turbulent regime, 10% increase in impeller
diameter increase the power required more than
60%

15. 10/21/201915
 Therefore the power required for stirring, depend on
the geometry of the impeller and configuration of the
tank.
 If the number or size of baffles, the number, length,
width, pitch or angle of blades on the impeller, the
height of impeller from the bottom of the tank, etc are
changed, the particular geometries will change.

16. UNGASSED NON-NEWTONIAN
FLUIDS
10/21/201916
 Estimation of power requirements for non-
newtonian fluids is more difficult.
 Impossible with highly viscous fluids.
 The power number is always dependent on the
reynolds number.
 – apparent viscosity.

17. 10/21/201917
 For power – law fluids:
n is the flow behaviour index and K is the
consistency index.
where k is a constant with magnitude dependent on

18. 10/21/201918
 Appropriate reynolds number for pseudoplastic fluids
is:
 The laminar region extends to higher reynolds
numbers in pseudoplastic fluids than in newtonian
system.
 At, reynolds number below 10 and above 200 the
results for newtonian and non-newtonian are same.

19. 10/21/201919
 In the intermediate range, pseudoplastic liquids
consume less power than newtonian fluids.
 The flow patterns in pseudoplastic and newtonian
fluids differ significantly.
 Even when there is high turbulence near the impeller
in pseudoplastic systems, the bulk liquid may be
moving very slowly and consuming relatively little
power.
 Another problem is that, the non-newtonian
parameters K and n, and therefore can vary

20. GASSED FLUIDS
10/21/201920
 Liquids into which gas is spared have reduced
power requirement.
 Gas bubbles decrease density of fluid. Hence,
the power requirement equation for turbulent
regime cannot be used.
 It does not explain the power characteristics of
gas-liquid systems.

21. 10/21/201921
 Certainly, the hydrodynamic behavior of fluid
around the impeller is affected.
 Large gas filled cavities are formed behind the
stirrer blades.
 These cavities reduce resistant to fluid flow liquid
and decrease drag coefficient of the impeller.

22. 10/21/201922
 The hydrodynamic behavior of gassed fluids are
not yet understood.
 Power consumption is majorly controlled by gas
cavity formation leading to the process being
discontinuous and random
 Certainly, the reduction in power consumption is
non uniform.
 Hence it is difficult to obtain accurate prediction of
power requirements.

23. 10/21/201923
 The expression for ratio of gassed to ungassed
power as function of operating conditions has
been obtained:
Power consumption with sparging
Power consumption without sparging
Volumetric gas flow rate
Stirrer speed
Impeller diameter
Impeller blade width

24. 10/21/201924
 The average deviation of experimental values
from the equation is about 12%.
 With sparging, the power consumed could be
reduced to as little as half the power requirement
of ungassed value, depending on the gas flow
rate.

25. 10/21/201925
EFFECT OF
RHEOLOGICAL
PROPERTIES ON

26. 10/21/201926
•For effective mixing there must be turbulent condition in the mixing
vessel.
•Intensity of the turbulence is represented by the Impeller Reynolds
number Rei.
•Baffled tank with turbine impeller, once Rei falls below about 5X10^3
turbulence is damped and mixing time increases significantly
•Rei when it’s increasing decrease in direct proportion and
increase in velocity.
•According to the rheological property on mixing , non-turbulent
conditions and poor mixing are likely to acquire during agitation of
OVERVIEW

27. 10/21/201927
 Because the apparent viscosity of
this fluids depend on the shear rate ,
The rheological behaviour of many
culture broths depends on shear in
the fermenter. Metzner and Otto have
proposed that the average shear rate
in a stirred vessel in a linear function
of stirrer speed.

28. 10/21/201928
Ꝩav =kNi
Where as,
Ꝩav → Average stirrer rate,
K → Constant,
Ni → Stirrer speed.
 However shear rate in stirred vessels is far from
uniform, Being strongly dependent on distance from
the impeller.
 The maximum shear rate close to the impeller is
much higher than the average calculated from the
equation.
 A small circulating pool of highly stirred fluids and
rounds the impeller, while the bulk liquid scarcely
moves at all.

29. 10/21/201929
 Stirrers of large diameter are recommended for
turbine impellers, instead of the conventional
tank-to-the impeller diameter ratio of 3:1 used
with low velocity fluids, This ratio is reduced
between 1.6 and 2.
 The most common types used for viscous
mixing are helical impellers and gate and
paddle-anchors mounted with small clearance
between the impeller and tank wall.
 However their application in fermenters is only
possible when oxygen demand in the culture is
relatively low.
 In design of fermenter for viscous culture ,
compromise is usually required between mixing

30. REFERENCE
10/21/201930
 Bioprocess engineering principles by Pauline
M.Doran.

31. 10/21/201931