Agitator design and selection

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Agitator Design and Selection

Standard Engineers

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 Duty Agitator

Multi Duty Agitator

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

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

Motor Gearbox Seal Shaft Impeller

Agitator Design - Mechanical

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

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