Agitator design and selection

How to Select the Best Design of Mixer or Agitator for your Processing Needs

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Agitator Design and
Selection
Standard Engineers

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Food for Thought
You wanted to STIR THINGS UP But, your MIXING
gave only ‘AGITATION’.
There’s many a slip between the Plant and the Lab.
The Mixing arena is the Boxing ring where Chemist
and Chemical Engineer fight
In the work done for synthesising a Chemical
molecule, what % is the mass transfer, reaction and
mixing and what % is separation
Mixing can be demystified

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Lecture Flow
Single duty Agitator
Multi duty Agitator
Films showing some Interesting Mixing
systems

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Two Types
Single Duty Agitator
Multi Duty Agitator

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Agitator Selection & Design
Process
Impeller Type
Impeller Diameter
Impeller Speed
Mechanical
Power
Shaft

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Process – Imp. Type Selection
Flow (Axial)
Hydrofoil
Axial Flow Turbine
Helical
Propeller
Shear (Radial)
Cowles
Rushton Turbine
Stator – Rotor
Parabolic Disc Turbine
Suspension
Homogenisation
Heat Transfer
Dispersion
Liq-Liq & Liq-Sol Reaction
Dissolution
Blending
Liq-Liq & Liq-Gas-Sol Reaction
Emulsification
DispersionA
X
I
A
L

F
L
O
W
A
X
I
A
L

F
L
O
W
A
X
IA
L FLO
W
A
X
IA
L FLO
WR
A
D
I
A
L

F
L
O
W
R
A
D
I
A
L

F
L
O
W
R
A
D
IA
L FLO
W
R
A
D
IA
L FLO
W
Physical Duty
Process Duty

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Axial Flow Hydrofoils
High Discharge Hydrofoil
High Suction Hydrofoil
High Pitch Hydrofoil
High Solidity Hydrofoil

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High Shear Impellers
High Shear Cowles
High Shear Stator-Rotor
Gas Induction Disperser
Parabolic Disc Turbine

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Impellers with combined Flow
and Shear
UDIF or InterMig
Multi Stage
Large D/T
Flow and Shear

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D/T
Single Impeller v/s Multi Impeller
Bottom Clearance
Impeller Spacing
Baffling
Process – Imp. Diameter

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Tip Speed = ω.r = 2.π.N.r = π.N.D
High for Processes needing high Shear
Low for Processes need low Shear
Small diameter = Higher speed
Larger diameter = Lower speed
Process – Imp. Speed

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Agitator Design - Power
Power = ρ.K.N
3
.D
5
Low Power = Lower Cost ???
High Power = Higher Energy dissipated
Speed variation with constant Power
VFD and Power Issues

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Motor
Gearbox
Seal
Shaft
Impeller
Agitator Design - Mechanical

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Multi-duty mixer or Universal Mixer
Any Mixing Task requiring Flow or Shear
Any Unit Process or Any Unit Operation
From violent Dispersions to gentle
Crystallisation

All in same Reactor

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When is it Needed?
Change in Product Specifications
New R&D Results
Change of Product
New Regulations
There are many unknowns in
modern day operations

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Change in Prod. Specs
Customers (Internal as well as External) are a Demanding Lot
Change in Particle Size (Increase / Decrease)
Different PSD
Higher Purity (bigger crystals with less ML)
Different Physical characteristics (colour, shape,
solution, emulsion, dispersion)
Additional step from downstream Processing
Different output Temperature
Many others …
Need 1
Unknown

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New R&D
Competiton, Cost Reduction and New Technologies
(ionic solvents, super acids, Green Chemistries,
surfactants, nanotechnologies) throw radical
possibilities of Process Intensification for
Higher Yield / Conversion
Reduced by-Product formation
Reduced Utility consumption
Reduced batch cycle times
Need 2
Unknown

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Change of Product
Campaign based Production cycles demand
flexibility in capabilities
New Product
Different Process
Different operating conditions
New Product Developed in the Lab or Pilot Plant
needs to be Produced on plant scale
Unexpected scale-up considerations
Process modifications based on IP / quality
Need 3
Unknown

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New Regulations
Banned raw materials, stringent disposal norms
can impose variabilities
Recipe Change
Change in operating conditions
Change in Process
Need 4
UnknownUnknown

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Process Capabilities
What must it Have?
WHAT
Range of Sp. Gr.
Large solids loading
Range of Viscosities
Range of Duties from
gentle crytallisation to
violent Gas Dispersion
Th.dynmcs 1 – Temp.
Th.dynmcs 2 – Press.
Th.dynmcs 3 – pH
HOW
Surplus motor Power
Large Pumping
Large D/T, multistage
Significant Variation in
Speed with reserve
power
Good Reynolds No.
Mech. Sealing
Exotic MOC

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Mech. Capabilities
What must it Have?
Robust design of Shaft and Impellers to
deliver brute force if required
Natural Frequencies of Lateral Vibrations to
be substantially distanced from operating
speed ranges
Quick Change to be possible to handle new
conditions with minimum turn-around time

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Derivation
Mixing energy goes into Flow and Shear
Flow is Measured as Pumping (KpND
3
)
Good metric for Shear is Tip Speed (ND)
Universal Mixer therefore has to
Increase or Decrease
Flow or Shear on demand

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Derivation
Flow Proportional to D
3
Reynolds No. & so Heat Transfer prop to D
2
Bulk motion in viscous fluids needs large D
Large D/T obviously a must for Universality
Good homogenity in mixing
Multi impeller system advantageous
Axial flow required to prevent ‘air curtain’ effects

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Conventional Impellers D/T
Limit
Hydrofoils and Axial
Flow turbines have a
D/T Limit
At 0.7 D/T, The c/s area
of the impeller cylinder
equals the c/s area of
the Annulus
Any further increase is
counter productive
Flow is throttled leading
to back mixing and
inconsistencies
Annulus
Up Flow
Impeller zone
Down Flow

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Ideal Impeller System
D/T > 0.7
Achieved by up and down
flow on same impeller
Multiple staggered impeller
arrangement behaves as a
pseudo helix
Can be customised for
operation without baffles
 The up-down throw can be
configured for different
conditions of shear and flow
Proven for Crystallisation,
Gas Dispersion as well as
pretty high viscosity
blending

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UDIF - Other Advantages
Large D/T = High Wall velocities = min.build-up at
vessel walls
Good for Sticky materials
Good for Crystallisation
In Gas Dispersion, further optimisation with bottom
impeller pumping up the catalyst, while top impeller
sucks and pumps down unreacted gas
 2 bladed so easy to insert thru manhole w/o bolting
or split construction an allow location anywhere
along shaft

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Extended Universality
If the mechanical design of
the Universal mixer is made
robust, then the same shaft
can be FITTED with
different impellers with
different speeds for
performance at both
extremes of Flow and Shear
Helical can mix viscosities
upto 100,000 cP
Parabolic or gas Induction
can do gas dispersion

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Paradigm Shift
We had limited ourselves to single shaft mixers.
What about dual shaft, Co-axial systems?
Why not place the responsibility of Flow and Shear
on 2 different impellers that are ideal for their
respective duties
One Impeller provides Flow, the Other Shear
Both are on different drives so that speeds and
hence magnitudes of Flow and Shear can be
controlled at will
Impellers on the outer zone can be closed type -
Helical while inner can be hydrofoil or Cowles
operating at speeds as high as 3000RPM
Mechanical challenge, but a process marvel

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Myths to be Shattered
Lower Motor Power means lower operating
cost
Hydrofoils can do everything
D/T to be around 1/3 to 0.4
Mixer should be low capital cost.
No option to GLR
50Hz barrier
Chemist and Chemical Engineer

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Conclusion
Decide Single duty or Multi duty
If Single duty - Decide Duty driver is Flow or
Shear
Select impellers accordingly
If Multi duty, large D/T, multi impeller, variable
speed.
Ensure minimum hot spots (pH, temp.,
concentration)

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Batch Crystalliser Mixer
Flow or Shear? Obviously Flow for larger crystals,
Shear for smaller
Impeller Type. – Anchor is the worst
Temperature gradient
Conc. Gradient
Grinding at bottom
Impeller Diameter.
Impeller speed
Multi Impeller
Mechanical – VFD.

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In the end …
Some Films
Video 1: Co-rotating Co-Axial Dispertron
Thank You
Start Again