Agitator Design for Solids Suspension under Gassed Conditions
Agitator design and selection
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How to Select the Best Design of Mixer or Agitator for your Processing Needs
Transcript of Agitator design and selection
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
Agitator Selection & Design
Process Impeller Type Impeller Diameter Impeller Speed
Mechanical Power Shaft
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 Dispersion
AXIAL FLOW
AXIAL FLOW
AXIAL FLOW
AXIAL FLOW
RADIAL FLOW
RADIAL FLOW
RADIAL FLOW
RADIAL FLOW
Physical Duty
Process Duty
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
D/T Single Impeller v/s Multi Impeller Bottom Clearance Impeller Spacing Baffling
Process – Imp. Diameter
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
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
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
Change in Prod. SpecsCustomers (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
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
Change of Product
Campaign based Production cycles demand flexibility in capabilities
New Product Different Process Different operating conditionsNew 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
New Regulations
Banned raw materials, stringent disposal norms can impose variabilities
Recipe Change Change in operating conditions Change in Process
Need 4
UnknownUnknown
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
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 DLarge 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
AnnulusUp Flow
Impeller zoneDown 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 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 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 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
