Download - Scaleup Down Slides
-
8/3/2019 Scaleup Down Slides
1/46
1
Slurry Suspension
David S. Dickey
MixTech, Inc.
-
8/3/2019 Scaleup Down Slides
2/46
2
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
-
8/3/2019 Scaleup Down Slides
3/46
3
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
-
8/3/2019 Scaleup Down Slides
4/46
4
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
-
8/3/2019 Scaleup Down Slides
5/46
5
Difficulty
Primary factor settling velocitywith density adjustment
Many other factors involved
mixer geometry factors
particle property factors
fluid property factors
-
8/3/2019 Scaleup Down Slides
6/46
6
Terminal Settling Velocity
-
8/3/2019 Scaleup Down Slides
7/46
7
Terminal Settling Velocity
Settling due to density difference Slow settling
laminar
Stokes flow
Rapid settling
turbulent
Newton flow
Transition between laminar and turbulent
-
8/3/2019 Scaleup Down Slides
8/46
8
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
-
8/3/2019 Scaleup Down Slides
9/46
9
Zwietering Correlation
( ) 0 450 1 0 13 0 2 0 85.. . . .c s l js p
l
gN S X d D
=
-
8/3/2019 Scaleup Down Slides
10/46
10
Density Effect onTerminal Settling Velocity
0 28.
s l js t
l
N S u
=
-
8/3/2019 Scaleup Down Slides
11/46
11
Mixing Intensity
On-bottom motionall particles moving
not necessarily suspended
Off-bottom suspensionall particles suspended
cloud height
Uniform suspensionall particles well distributed
-
8/3/2019 Scaleup Down Slides
12/46
12
On-Bottom Motion
-
8/3/2019 Scaleup Down Slides
13/46
13
Off-Bottom Suspension
-
8/3/2019 Scaleup Down Slides
14/46
14
Uniform Suspension
-
8/3/2019 Scaleup Down Slides
15/46
15
Suspension Levels
On Bottom
Motion
Off-Bottom
Suspension
Complete
Suspension
-
8/3/2019 Scaleup Down Slides
16/46
16
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
-
8/3/2019 Scaleup Down Slides
17/46
17
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
-
8/3/2019 Scaleup Down Slides
18/46
18
Pitched-Blade Turbine
-
8/3/2019 Scaleup Down Slides
19/46
19
Hydrofoil Impeller
-
8/3/2019 Scaleup Down Slides
20/46
20
Straight-Blade Turbine
-
8/3/2019 Scaleup Down Slides
21/46
21
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
-
8/3/2019 Scaleup Down Slides
22/46
22
Straight-Blade Turbine Close Clearance
-
8/3/2019 Scaleup Down Slides
23/46
23
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
-
8/3/2019 Scaleup Down Slides
24/46
24
Pitched-Blade Turbine Close Clearance
-
8/3/2019 Scaleup Down Slides
25/46
25
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
-
8/3/2019 Scaleup Down Slides
26/46
26
Pitched-Blade Turbine Liquid Level Off-Bottom
-
8/3/2019 Scaleup Down Slides
27/46
27
Hydrofoil Impeller Liquid Level Off-Bottom
-
8/3/2019 Scaleup Down Slides
28/46
28
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
-
8/3/2019 Scaleup Down Slides
29/46
29
-
8/3/2019 Scaleup Down Slides
30/46
30
-
8/3/2019 Scaleup Down Slides
31/46
31
-
8/3/2019 Scaleup Down Slides
32/46
32
Just Suspended Speed
-
8/3/2019 Scaleup Down Slides
33/46
33
Power and Torque
-
8/3/2019 Scaleup Down Slides
34/46
34
Solids Concentration
More solids more difficult tosuspend
Increased concentration higher
speed for same suspension
-
8/3/2019 Scaleup Down Slides
35/46
35
Effect of Concentration
-
8/3/2019 Scaleup Down Slides
36/46
36
-
8/3/2019 Scaleup Down Slides
37/46
37
Cloud Height
Degree of uniformity Rapidly settling particles uniform
size distribution
Distinctive cloud appearance
More agitation higher cloud height
Suspension is never quite uniform
-
8/3/2019 Scaleup Down Slides
38/46
38
-
8/3/2019 Scaleup Down Slides
39/46
39
Similarity and Scale-up
Geometric similarity essential Kinematic and dynamic similarity
limited benefit
Scale-up by empirical relationships
-
8/3/2019 Scaleup Down Slides
40/46
40
Scale-up byGeometric Similarity
Speed adjustmentonly
Large scale speed
less than smallscale speed
Exponent
maintains oneconstantcharacteristic
12 1
2
nDN N
D
=
-
8/3/2019 Scaleup Down Slides
41/46
41
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
-
8/3/2019 Scaleup Down Slides
42/46
42
Scale-up Solids Suspension
-
8/3/2019 Scaleup Down Slides
43/46
43
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
-
8/3/2019 Scaleup Down Slides
44/46
44
Other Factors
Fluid viscosity10 to 20 cp more difficult
100 cp easier no settling
Particle shape
Size distribution
-
8/3/2019 Scaleup Down Slides
45/46
45
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
-
8/3/2019 Scaleup Down Slides
46/46
46
Slurry Suspension
David S. Dickey
MixTech, Inc.