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8/3/2019 Scaleup Down Slides

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Slurry Suspension

David S. Dickey

MixTech, Inc.


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What We Know aboutMechanical Agitation

Thoroughly studied for 40 years Characteristics

off-bottom suspension

on-bottom motion

uniform suspension

cloud height

Visual observation

Reliable means of measurement


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What We Dont Know aboutPulse Jet Mixers

We dont know how a pulse willeffect solids suspension

Pulsed operation will not be betterthan continuous agitation

We need to know about the effects of

geometry on solids suspension We need to know how to scale-up


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Everything We Know aboutSolids Suspension is Scale-up

All of solids suspension is empirical Observation of suspension

characteristics

Testing in different scales

Scale-up of small-scale tests


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Difficulty

Primary factor settling velocitywith density adjustment

Many other factors involved

mixer geometry factors

particle property factors

fluid property factors


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Terminal Settling Velocity


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Terminal Settling Velocity

Settling due to density difference Slow settling

laminar

Stokes flow

Rapid settling

turbulent

Newton flow

Transition between laminar and turbulent


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Agitated Suspension

Not terminal settling velocity Agitated flow not vertical at all points

Initial suspension by horizontal flow

across bottom Suspension characteristics a stronger

function of density than predicted by

terminal settling velocity Density more important than diameter


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Zwietering Correlation

( ) 0 450 1 0 13 0 2 0 85.. . . .c s l js p

l

gN S X d D

=


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Density Effect onTerminal Settling Velocity

0 28.

s l js t

l

N S u

=


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Mixing Intensity

On-bottom motionall particles moving

not necessarily suspended

Off-bottom suspensionall particles suspended

cloud height

Uniform suspensionall particles well distributed


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On-Bottom Motion


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Off-Bottom Suspension


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Uniform Suspension


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Suspension Levels

On Bottom

Motion

Off-Bottom

Suspension

Complete

Suspension


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Mixer Design Factor

Just suspended speed NJS Classical correlation by Zwietering

adapted for practical use

Other more complicated and moreaccurate correlations

Testing and scale-up best method


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Mixer Geometry Factors

Impeller type axial flow is best Impeller to tank diameter

Off-bottom clearance

Number of impellers

Liquid level

Baffle length off-bottom clearance


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Pitched-Blade Turbine


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Hydrofoil Impeller


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Straight-Blade Turbine


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Suspension with Radial Flow

The straight-blade radial flowimpeller did not work with typical off-bottom clearance

Close clearance does work for solidssuspension

Impeller location can be a majorfactor in solids suspension


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Straight-Blade Turbine Close Clearance


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Suspension with Axial Flow

Axial flow impellers work at closeclearance

pitched-blade turbine

hydrofoil impeller

Pitched-blade turbine requires less

power than straight-blade turbine forthe same results


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Pitched-Blade Turbine Close Clearance


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High Clearance CausesProblems

Pitched-blade turbine fails tosuspend solids when too far off-bottom

Hydrofoil impeller works better evenat high clearance position

Different degrees of axial flow areimportant for solids suspension


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Pitched-Blade Turbine Liquid Level Off-Bottom


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Hydrofoil Impeller Liquid Level Off-Bottom


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Impeller-to-Tank DiameterRatio

Small impellers require more powerthan larger ones

Definite optimum impeller-to-tank

diameter ratio about 0.4

Large impellers do not sweep the

bottom


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Just Suspended Speed


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Power and Torque


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Solids Concentration

More solids more difficult tosuspend

Increased concentration higher

speed for same suspension


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Effect of Concentration


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Cloud Height

Degree of uniformity Rapidly settling particles uniform

size distribution

Distinctive cloud appearance

More agitation higher cloud height

Suspension is never quite uniform


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Similarity and Scale-up

Geometric similarity essential Kinematic and dynamic similarity

limited benefit

Scale-up by empirical relationships


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Scale-up byGeometric Similarity

Speed adjustmentonly

Large scale speed

less than smallscale speed

Exponent

maintains oneconstantcharacteristic

12 1

2

nDN N

D

=


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Scale-up Exponent

Equal tip speed (fluid velocity)n=1

Equal power per volume

n=2/3 Equal blend time

n=0

Zwietering suspensionn=0.85


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Scale-up Solids Suspension


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Scale-up

Depends on settling velocityslow settling follows flow

rapid settling difficult

Scale-up may also depend on size oftest

Larger tests less conservative scale-up


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Other Factors

Fluid viscosity10 to 20 cp more difficult

100 cp easier no settling

Particle shape

Size distribution


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Conclusions

Good understanding of solidssuspension with mechanical mixers

Understanding of steady jet mixers

Little understanding of the effects ofpulse mixingcycle duration

refill period Uncertain scale-up criteria


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Slurry Suspension

David S. Dickey

MixTech, Inc.

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