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TRANSCRIPT
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
BLEND TIMEBlend time: Time taken to reach C∞ ± x %
c′ ∞
′×=
Ccx 100
Exponential approach to uniformity.
)θexp('Δ kc −=
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.
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
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.
HSD
HSDs are inefficient pumps.
But efficient dispersers.
Poor flow can cause regionnear impeller to overheat.
Ideally balance circulationwith shear.
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.
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
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.