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Richard K. Grenville Director of Mixing Technology ASC Convention Atlanta Marriott Marquis, Atlanta GA 5 th April 2017 DEVELOPMENTS IN IMPELLER TECHNOLOGY FOR MIXING VISCOUS, NON-NEWTONIAN FLUIDS

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Richard K. GrenvilleDirector of Mixing Technology

ASC ConventionAtlanta Marriott Marquis, Atlanta GA

5th April 2017

DEVELOPMENTS IN IMPELLER TECHNOLOGY FOR MIXING VISCOUS, NON-NEWTONIAN FLUIDS

TOPICS• Introduction.• Why mixing?• Impeller characteristics:

– Flow versus Shear– Efficiency

• Flow.• Shear.• New developments:

– Flow and Shear

• Examples.

BUSINESS DRIVERS• External pressures:

– Market for expected to grow by 4.5 % world wide.– Reduction in solvent use:

• Government and Consumer• Different regulations across globe

– New uses / markets.

• Internal pressures:– Product development to meet new markets– Fit new formulations into existing plant facilities– Increase production in existing facilities:

• Mix faster?!– Batch to batch uniformity:

• Raw material costs• Cost of rejected batches

CRITICAL SUCCESS FACTORS• Compare with other industries:

– Paint and coatings:• Reduce VOCs• Higher solids / pigments• Batch uniformity

– Food “feel” is attribute:• Tomato paste• Chocolate

– Polymers:• Molecular weight and distribution

– Paper stock:• Reduce water increase concentration

• All addressed by understanding the impact of fluid properties (viscosity) on mixing.

IMPORTANCE OF MIXING• Smith (Trans IChemE., 1990):

– US chemical industry loses $ 1010 ($ 10 billion) each year due to poor mixing:

• 1 % increase in yield ~ $ 1 × 106

• One day of down time ~ $ 1 × 106

• Examples:– Low yields in chemical reactions:

• Change in selectivity on scale-up• By-products, MWD, etc

– Longer than expected batch / cycle times.– Poor heat transfer / hot spots.– Others………………..?

WHAT IS MIXING? (Etchells)Mixing is the application of

mechanical motion

in order to create

fluid dynamic effects

which achieve a

desired process result.

Mean Flow Shear

IMPELLERS FOR FLOW

BLEND TIMEBlend time: Time taken to reach C∞ ± x %

c′ ∞

′×=

Ccx 100

Exponential approach to uniformity.

)θexp('Δ kc −=

IMPELLERS FOR SHEAR

www.indiamart.com

PUMPING EFFICIENCYBrown (2010) and PMSL

BLEND TIME EXPERIMENTS

• Objective method:– Conductivity– Measure in regions of differing “mixing intensity”

• Try and correlate results:– Universal blend time correlation?

• Dimensionless blend time is product of impeller rotational speed and measured blend time:

θN

TEST METHOD• Experimental set-up:

• Three probes in regions of different mixing intensity:– 1 - Under Impeller– 2 - Between Shaft and Wall– 3 - Behind Baffle

• Tracer added to liquid surface.

• Geometrically identical at all three scales.

RMS RESULTS - 4PBT (T/3 & T/2)

D = T/3

D = T/2

CORRELATION

D = T/3

D = T/2

TURBINE IMPELLERS - OPERATING REGIMESTurbulent Regime:Po = ConstantNθ = Constant

Transitional Regime:Po ≈ ConstantNθ ∝ Re-1

Laminar Regime:Po ∝ Re-1

Nθ ∝ Re-10

Hoogendoorn, Private Communication, 1988

BLEND TIME• Grenville et al. (MIXING XVIII, 2001) using Rieger’s

data:

• Empirical correlation – no physical basis.• Concluded that high KP impeller is most efficient for

blending:– Lowest power input for desired blend time running costs– Need bigger impeller (more steel) capital costs

69.1-P

3K10×896=θN

Constant =θN

OTHER DATA SOURCES

SHEAR

• Impellers that generate shear are required for size reduction:– De-agglomeration and wetting of pigment added to

liquid– Reduction of droplet size in dispersion of immiscible

liquids

• Rarely break down primary particles:– Use Media or Sand Mills– High energy

• High-pressure homogenizers for emulsions.

DROP SIZE vs. POWER PER MASS

HSD

HSDs are inefficient pumps.

But efficient dispersers.

Poor flow can cause regionnear impeller to overheat.

Ideally balance circulationwith shear.

X-RAY STREAK PHOTOGRAPHY

Impeller Blade

Elson, Chem. Eng. Comm., 1990

NEW DEVELOPMENT• Need impeller that combines flow and shear

generation capabilities.

• Alternative is double shaft system with HSD and Anchor / Ribbon combination:– Still generate “hot spots”– Two drives / motors– Two sealing systems

• Use Bi-Directional impeller – CounterFlow.

COUNTERFLOW

PUMPING EFFICIENCYBrown (2010) and PMSL

ATTRIBUTES• Generally supplied with large impeller to tank

diameter ratio:– > 0.6

• High shear zone where down and up-pumping blades meet:– Large velocity gradient high shear rate

• Currently tested:– Viscosity of 2 million cP– Reynolds number < 0.1– Yield stress > 75 Pa

COMPARISON: TURBINE vs. CF

High solids slurry

Impeller diameter: 320 inchesMotor power: 30 HP

EXAMPLE – COAL SLURRY

CONCLUSIONS• Impellers in stirred tanks are pumps:

– Machines that move fluid

• Need to decide if process requires:– Efficient pumping Flow– Inefficient pumping Dispersion– Or combination of flow and shear

• Commonly used impellers provide one – not both.• CounterFlow™ impeller can provide both.• PMSL lab can:

– Measure fluid rheology– Perform mixing tests

• Scale-up fluid mechanics / physics meet chemistry.