principles and measurement of dynamic powder behaviourpowders under consolidation measure the onset...
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Powder Flow 2009
Tim FreemanFreeman Technology
Principles and Measurementof Dynamic Powder
Behaviour
Powder Flow 2009
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Powder Flow 2009
Behaviour is a function of the properties of each component and the way the components interact
= Solids Gases++ LiquidsPowders
Powders are complex materials
They consist of….
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Powder Flow 2009
SEM Images of Particles
Ref:
Microparticles
SciELO
Trinity College Dublin
U.S. Geological Survey
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Powder Flow 2009
Surface Texture
Ref:
Microparticles
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Powder Flow 2009
Solid components ‐
variable and complex
Each particle defined by a set of physical and chemical properties
Elasticity
Plasticity
Porosity
Potential for Charge
Cohesivity
Adhesivity
Amorphous content
Particle Size & Distribution
Shape
Surface Texture
Surface Area
Density
Hardness / Friability
Hygroscopicity
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Powder Flow 2009
Examples of Powder Behaviour (observational behaviour)
Loose packing state
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Powder Flow 2009
Consolidated packing state
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Powder Flow 2009
Aerated / fluidised packing state
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Powder Flow 2009
Fluidised Consolidated
Low HighModerateStress:
Variation in behaviour with level of aeration and consolidation
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Powder Flow 2009
External Variables
Consolidation
Vibration / tapping
Direct pressure ‐
container, keg, hopper
Aeration
Gravity discharge, filling, blending, aerosolisation, pneumatic transfer
Flow Rate
Powder/powder, powder/container wall
Moisture
Increases particle adhesion, reduces particle stiffness
Electrostatic Charge
Discharge, high shear mixing, high flow rates
Storage Time
Consolidation, caking
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Powder Flow 2009
Mechanisms of Powder Flow
Friction between particles
Smooth / low friction particles
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Powder Flow 2009
Rough / high friction particles
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Powder Flow 2009
Friction between particles and wall
Smooth / low friction particles
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Powder Flow 2009
Friction between particles and wall
Rough / high friction particles
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Powder Flow 2009
Mechanical Interlocking of particles
Strong interlockingHigh force required for separation of particles
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Powder Flow 2009
Same particles, but different orientation….
Weak interlockingRelatively low force required for separation of particles
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Powder Flow 2009
Liquid bridges between particles
Increased particle –
particle adhesion
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Powder Flow 2009
Increased particle – wall adhesion
Liquid bridges between particle and wall
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Powder Flow 2009
Cohesive, inter‐particulate forces
Increased particle –
particle attraction
Van der Waals
Electrostatics
Covalent
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Powder Flow 2009
Gravitational forces
F
F = mg
g = acceleration due to gravity
m
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Powder Flow 2009
Friction
Particle – Particle, Particle ‐
Wall
Mechanical Interlocking
Adhesion / Liquid Bridges
Particle – Particle, Particle ‐
Wall
Cohesion
Van der Waals, Electrostatics, Covalent
Gravity
Mechanisms of particle interaction
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Powder Flow 2009
m1
g m2
g
m3
gfa
fb
fc
Interparticulate forces (fn
) are a function of: ‐
Frictional forces
Mechanical interlocking
Adhesion
Cohesion
The relationship between fn
and mg depends on
particle physical properties
and process
environment.
When powder is in low stress state
(die filling / mixing), cohesive forces
most influential.
When powder is consolidated
(hopper / extrusion / compression), mechanical
friction
and particle interlocking
are most important.
Cohesion still contributes to flow, but it’s effect is less significant.
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Powder Flow 2009
Examples of processes and applications
Flow in the hopper
High stress, low flow rates, gravitationally induced flow
Filling
Low stress, dynamic, gravitationally induced flow
Transport / storage
Moderate / high stress, de‐aeration, consolidation, caking
Aerosolisation
Very low stress, powder needs to fluidise
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Powder Flow 2009
Athletic ability!
Consider an athlete competing at the Olympic Games
Properties: ‐
Tall, long stride length, slim, fast runner, stamina
Suitable event: ‐
Long jump, runner, pole vault
Properties: ‐
Short, strong, explosive power
Suitable event: ‐
Shot put, javelin
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Powder Flow 2009
Are they “good”
athletes?
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Powder Flow 2009
So what can be measured?
Several traditional techniquesCarr’s Index, Hausner Ratio, Angle of Repose, Flow through a funnel
Pragmatic solution for their day
Basic & insensitive, developed before modern technologies
existed
Don’t represent conditions that powders experience in
processing and application
Each method typically produces one parameter
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Powder Flow 2009
More recently, shear cells ‐
Measure powders under consolidation
Measure the onset of flow, transition from static to dynamic
Good for understanding behaviour in hoppers
Most recently, dynamic characterisation methods
Measure response of powder to various environments
Simulating range of process conditions
Measure directly response to aeration, consolidation, moisture, flow rate
Measure bulk properties of density, compressibility and permeability
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Powder Flow 2009
Principles of Operation
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Powder Flow 2009
H2H1
Force
H1 H2
Torque
H1
H2
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Powder Flow 2009
H2
Energy Gradien
t, m
J/mm
H1
Total Flow Energy = Area under Curve
Work Done = Energy = “Force”
x Distance = (Force + Torque) x Distance
Energy Gradient = Work Done per mm
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Powder Flow 2009
Flow Energy vs. Moisture Content
Increasing Water Content
Flow
Ene
rgy
A
B
CA
B
C
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Powder Flow 2009
Flow Energy vs. Additive & Morphology
A
B
C
D
E F G
Flow Additive, %
BFE, m
J
A
B
C
D
E F
G
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Powder Flow 2009
Aeration
Air supply
Non – CohesivePowder
Cohesive Powder
Air supply
or
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Powder Flow 2009
Generally, if: ‐
mg > fcoh
powder is non‐cohesive
mg < fcoh
– powder is cohesive
m1
g m2
g
m3
gfcoh‐a
fcoh‐b
fcoh‐c
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Powder Flow 2009
Increasing Air Velocity
Flow
Ene
rgy
Cohesive
Non-cohesive
Used for measuring: ‐
Cohesion
Low stress, gravitationally induced flow
Dosing / Mass Uniformity
Aerosolisation / DPI
Fluidisation behaviour
Blending / mixing
Segregation potential
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Powder Flow 2009
Consolidation
Direct Pressure
Tapping / vibration
Before Tapping
After Tapping
Understanding theeffects of: ‐
Transport
Storage
Processing
Caking
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Powder Flow 2009
Increased Tapping
Flow
Ene
rgy Flowability
change > 1000%
Den
sity
Density change ~ 40% (max)
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Powder Flow 2009
Filling and Compression case study
Initial Compression
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Powder Flow 2009
GoodUniform filling, low porosity
PoorNon‐uniform filling, high porosity
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Powder Flow 2009
High cohesion Cohesive forces >> mg
High mechanical friction & Interlocking
Low permeability
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Powder Flow 2009
CohesionAeration Test – low value of Aerated Energy equals low cohesion
Increasing Air Velocity
Flow
Ene
rgy
Aerated Energy
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Powder Flow 2009
Mechanical interlocking & frictionSpecific Energy – low value equals low interlocking and friction
Blade movement
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Powder Flow 2009
PermeabilityHigh Permeability – entrained air can easily escape
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Powder Flow 2009
Material Material/Powder description D50 (m) Shape
(a) GL Glass beads 174 Spherical
(b) GS Glass beads 68 Spherical
(c) Granular Aluminium powder 134 Irregular
(d) Tungsten powder 4 Angular
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Powder Flow 2009
Filling RatioRatio of actual mass in die compared to mass calculated
from bulk density and volume of die
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Powder Flow 2009
Filling Ratio & Specific Energy Aeration
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Powder Flow 2009
Measurements: Glass GL Glass GS Aluminium Tungsten
Fill Ratio 0.82 0.40 0.29 0.14
Aeration Energy, AE (mJ) < 10 < 10 < 10 ~300
Specific Energy, SE (mJ/g) 3.4 2.4 4.4 6.7
Pressure Drop across the powder bed at 2mm/s air velocity, PD15 (mbar) 0.8 5.2 1.4 15.3
Consolidation Index, CI20Taps - factor by which flow energy increases after tapping, relative to BFE
1.31 1.11 1.43 2.32
Measurements: Glass GL Glass GS Aluminium Tungsten
Fill Ratio 0.82 0.40 0.29 0.14
Measurements: Glass GL Glass GS Aluminium Tungsten
Fill Ratio 0.82 0.40 0.29 0.14
Aeration Energy, AE (mJ) < 10 < 10 < 10 ~300
Measurements: Glass GL Glass GS Aluminium Tungsten
Fill Ratio 0.82 0.40 0.29 0.14
Aeration Energy, AE (mJ) < 10 < 10 < 10 ~300
Specific Energy, SE (mJ/g) 3.4 2.4 4.4 6.7
Measurements: Glass GL Glass GS Aluminium Tungsten
Fill Ratio 0.82 0.40 0.29 0.14
Aeration Energy, AE (mJ) < 10 < 10 < 10 ~300
Specific Energy, SE (mJ/g) 3.4 2.4 4.4 6.7
Pressure Drop across the powder bed at 2mm/s air velocity, PD15 (mbar) 0.8 5.2 1.4 15.3
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Powder Flow 2009
Other applications
Mixing / Blending – Cohesion, Aeration, Specific Energy
Compression – Basic Flowability Energy, Permeability,
Compressibility
Dry Powder Inhalers – Aeration, Permeability
Storage –
Consolidation, Caking
Size Reduction –
Particle attrition, Basic Flowability Energy
Hopper Flow – Shear Cell, Wall Friction, Permeability
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Powder Flow 2009
Conclusions
1.
Powder behaviour is complex
2.
Flow properties will change depending on the conditions imposed
3.
Each process / application will subject the powder to a specific
set of conditions
4.
Essential that powder characteristics match the demands of the process or application
5.
Now possible to simulate range of conditions and measure powder response
6.
Important to measure powder in representative state – e.g. aeration tests for aerosolisation & cohesion, shear cell tests for behaviour in
hoppers, etc
7.
Database of flow properties can be established, leading to enhanced process understanding, efficient formulation and development of
new materials and higher quality of final product
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Powder Flow 2009
Thank you for your attention
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