Agitator design and selection

Agitator Design and
Selection
Standard Engineers

vineet.shroff@gmail.com

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

Lecture Flow
 Single

duty Agitator
 Multi duty Agitator
 Films showing some Interesting Mixing
systems


Two Types
 Single

 Multi

Duty Agitator

Duty Agitator


Agitator Selection & Design
 Process




Impeller Type
Impeller Diameter
Impeller Speed

 Mechanical



Power
Shaft


Process – Imp. Type Selection
OW
LLOW
F
LLF
A
IA
X
AX I
A

OW
LLOW
F
LLF
IA
A
D
AD I
RA
R



Flow (Axial)
 Hydrofoil
 Axial Flow Turbine
 Helical
 Propeller



Shear (Radial)
 Cowles
 Rushton Turbine
 Stator – Rotor
 Parabolic Disc Turbine



Suspension
Homogenisation
Heat Transfer



Dispersion

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

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

Physical Duty

Liq-Liq & Liq-Sol Reaction
Dissolution
Process Duty
Blending





Liq-Liq & Liq-Gas-Sol Reaction
Emulsification
Dispersion


Axial Flow Hydrofoils
High Discharge Hydrofoil

High Suction Hydrofoil

High Pitch Hydrofoil

High Solidity Hydrofoil

High Shear Impellers
High Shear Cowles

High Shear Stator-Rotor

Gas Induction Disperser

Parabolic Disc Turbine


Impellers with combined Flow
and Shear





UDIF or InterMig
Multi Stage
Large D/T
Flow and Shear


Process – Imp. Diameter
 D/T
 Single

Impeller v/s Multi Impeller
 Bottom Clearance
 Impeller Spacing
 Baffling


Process – Imp. Speed
 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


Agitator Design - Power
 Power

= ρ.K.N3.D5
 Low Power = Lower Cost ???
 High Power = Higher Energy dissipated
 Speed variation with constant Power
 VFD and Power Issues


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


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

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

Need 1

Change in Prod. Specs
Customers (Internal as well as External) are a Demanding Lot


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

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

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
n
now
Many others …
Unk

Need 2

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

U

w
nkno

n

Need 3

Change of Product
Campaign based Production cycles demand
flexibility in capabilities
 New

Product
 Different Process
 Different operating conditions

U

w
kno
n

n

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 4

New Regulations
Banned raw materials, stringent disposal norms
can impose variabilities
 Recipe

Change
 Change in operating conditions
 Change in Process

wn
n own
nk

U nkno
U

Process Capabilities

What must it Have?
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



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

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

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HOW
Surplus motor Power
Large Pumping
Large D/T, multistage
Significant Variation in
Speed with reserve
power
Good Reynolds No.
Mech. Sealing
Exotic MOC


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


Derivation
 Mixing



energy goes into Flow and Shear

Flow is Measured as Pumping (KpND3)
Good metric for Shear is Tip Speed (ND)

 Universal

Mixer therefore has to
 Increase or Decrease
Flow or Shear on demand


Derivation
 Flow

Proportional to D3
 Reynolds No. & so Heat Transfer prop to D2
 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


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
Impeller zone


Annulus
Up Flow

Down Flow

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


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

Extended Universality




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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

Paradigm Shift
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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

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

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)

Batch Crystalliser Mixer



Flow or Shear? Obviously Flow for larger crystals,
Shear for smaller
Impeller Type. – Anchor is the worst
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Temperature gradient
Conc. Gradient
Grinding at bottom

Impeller Diameter.
Impeller speed
Multi Impeller
Mechanical – VFD.

In the end …
 Some

Films
 Video 1: Co-rotating Co-Axial Dispertron
 Thank

You
 Start Again


Agitator design and selection

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