The Wolfson Centre for Bulk Solids Handling Technology
THE WOLFSON CENTRE for Bulk Solids Handling Technology
Taming the Powder:
How to Control Powder Flowability
Prof MSA Bradley, Dr RJ Berry
0000
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Overview
Why does powder flowability matter?
What is “powder flowability?” - and how to measure it?
How can we modify it?
Some case studies
Why Is Powder Flow-ability Important?
Silo Intermediate Bulk Container (IBC)
Feeder
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“Hammer rash!”
Sometimes the powder flow properties and the equipment design are just not compatible!
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1827
Air Injection to Aid in
Discharge of Powder
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Losses from poor or irregular powder flow
Production downtime
Operator injury
Equipment damage
Irregular product quality
Noise from hammering or vibrators
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What is “powder flowability”?
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The Wolfson Centre for Bulk Solids Handling Technology Idealised Flow Function Test
“Flow function” of a bulk solid from a uni-axial test 0000
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The ‘Sand Castle’ (Unconfined Failure) Test
1. Consolidation Stage
σ 1 σ 1=σ c
σ 3=0
Diagonal Failure Plane
2. Failure Stage
Broken Sample
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Powder Flow Function
Consolidation stress σ 1
Unc
onfin
ed
failu
re s
tren
gth σ
c
Flow Function
σ 1 (1) σ 1 (2) σ 1 (3)
σ c (1)
σ c (2)
σ c (3)
Shear Tester
Produces:-
Powder Flow Function
Wall friction
Bulk Density Function
Internal friction
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Flow Function
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
Major Principal Consolidation Stress σ 1 [kPa]
Unc
onfin
ed fa
ilure
Str
engt
h σ
c [k
Pa]
Results- Flow Function Test
Free flowing
Easy flowing
Cohesive
Very Non flowing
Cohesive
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Bulk density function – also in Flow Function Test
Bulk Density Function 0
50 100
150 200
250
300 350
400 450
500
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
Major Principal Consolidation Stress σ 1 [kPa]
Bul
k D
ensi
ty ρ b
[kg/
m 3 ]
Critical dimensions to avoid “arching”
Cohesive arching
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Critical dimensions to avoid “rat-holing”
Tester also calculates the size and shape of hoppers that a material will flow from
(both mass flow and core flow)
Conical hopper Plane silo
Dc
Dp 6Dp>L>3Dp
θc
θp
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Modifying Powder Flow Properties:
A selection of case studies
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Effect of Particle Size
Flow Functions
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 1 2 3 4 5 6 7 8 9 10 11 Major Principal Consolidation Stress σ 1 [kPa]
Unc
onfin
ed F
ailu
re S
tren
gth
σ c
[kPa
] 170um mean dia 100um mean dia 40um mean dia 20um mean dia
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Effect of Particle Size on Arching Tendency
Critical Arching Dimension
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 20 40 60 80 100 120 140 160 180 Mean Particle diameter µ m
Crit
ical
Mas
s-flo
w A
rchi
ng
dim
ensi
on [m
]
Plane (Wedge) flow hopper
Conical hopper
Effect of Moisture Content
Liquid bridges
As moisture level increases, bond strength increases until saturation
Surface moisture
Liquid bridge
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Effect of Moisture Content
Flow function for sand increasing moisture content
Flow Functions
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0
Major Principal Consolidation Stress σ 1 [kPa]
Unc
onfin
ed F
ailu
re S
tren
gth
σ c
[kPa
]
fine sand0% fine sand 3% fine sand 6% fine sand 9% fine sand 12%
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Effect of Moisture Content
Arching dimension for sand increasing moisture content to saturation
Critical Arching dimension
0
0.05
0.1
0.15
0.2
0.25
0 2 4 6 8 10 12 14 16
Moisture content % by weight
Crit
ical
Arc
hing
dim
ensi
on [m
]
fine sand
Note on moisture: Inherent moisture (inside particle)
Has small effect - may affect particle deformation properties
Surface moisture (on the outside) Much more influential
Some particles can accommodate large quantities of inherent moisture, some not!
E.g. at 6% moisture Coal appears dry Sand is very wet and sticky
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Surface texture
Modifying Flow Properties DECREASING THE FLOW FUNCTION:
Remove fines
Reduce liquid
Roughen surface texture
Using free flow aids examples include: Mg sterate, talc, zeolite, sipernat, TCP
These can take the form of fine particles that coat the particles to reduce bond strength
Absorb free moisture, drying material to improve flow
INCREASING THE FLOW FUNCTION:
Using oils or water to make free-flowing materials more cohesive to reduce dust levels
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Dustiness Tester
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Flow Additives to Reduce Flow Function (make the
powder more free flowing)
Small particles; Hard
Unreactive
Insoluble
non-charging
Keep main particles separated
Commonly used: Clays
Talc
Zeolite
Fumed silica
Magnesium chloride
Stearic acid
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Dispersion methods for “flow aids”
Simple mixing Mechano-fusion
High shear & compression
Ordered mixing
Increasing energy input
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Modifying Flow Properties Using Additives
Effect of the flow additive on the flow function
0
0.5
1
1.5
2
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Major Principal Consolidation Stress σ 1 [kPa]
Unc
onfin
ed F
ailu
re S
tren
gth
σ c
[kPa
]
S&V 1 (Base Ing + freeflow) S&V 2 (Base Ing) S&V 3 (Base Ing + oil) chilli 2 (Base Ing) chilli 3 (base Ing + oil) chilli 4 (Base Ing + extra freeflow)
Effect of Flow Additives
Base ingredient Increasing flow additive Increasing oil
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
-4 -2 0 2 4 6 8 10 12
Additive % Weight
Crit
ical
Arc
hing
Dim
ensi
on [m
]
Easy flowing material S&V
Troublesome material chilli
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Effect of Temperature (Fatty Powder)
Flow Functions
0
1
2
3
4
5
6
7
8
0 1 2 3 4 5 6 7 8 9 10 11
Major Principal Consolidation Stress σ 1 [kPa]
Unc
onfin
ed F
ailu
re S
tren
gth
σ c
[kPa
]
15degC 20degC 30degC
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Effect of Temperature (Fatty Powder)
Critical Arching dimension
0
0.5
1
1.5
2
2.5
3
0 5 10 15 20 25 30 35
Temperature deg C
Crit
ical
arc
hing
dia
met
er [m
]
Conical Hopper Wedge hopper
Flow Function & Time Flow Functions
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10 Major Principal Consolidation Stress σ 1 [kPa]
Unc
onfin
ed F
ailu
re S
tres
s σ c
[kPa
]
30 DegC (tc=0)
30 DegC (1hour tc)
30 DegC (8hour tc)
Time Flow Function Test to indicate how time left in hopper affects powder flowability
Free Flowing
Easy Flowing
Non Flowing
Characterisation of Particle shape (Benn 93)
Shape characterised by drawing a cuboid around the particle
L - Longest length
I - Intermediate length
S - Shortest length
Platy
Equant
Elongate
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Effect of Particle Shape Free flowing materials
Increasing friction as particle become platy
Close packing structure particles cannot rotate
Cohesive (including wet) Friction less dependent on particle shape
Cohesion more influential than friction – little dependency on particle shape
Open packing structure
Can measure affect of particle shape on the internal friction with the shear cell
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Powder flowability is not the whole story
Equipment design also matters greatly!
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Equipment operational characteristics core flow
Flow from top of material
“first in last out” discharge
“dead” regions of product
errat ic discharge caused by product on product shear during emptying
central discharge channel
exaggerates segregation effects of particles
hopper half angle typically greater than 25 degrees from vertical
poor stock rotation
high storage capacity for a given headroom
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Core flow
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Core flow
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Core flow
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Core flow
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Core flow
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Core flow
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Core flow
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Core flow
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“first in, first out” discharge all storage capacity is “live”
consistent discharge encouraged by the reduced levels of shear g e n e r a t e d a s t h e p r o d u c t discharges against relatively smooth wall material - not static product
d e g r e e o f r e m i x i n g d u r i n g discharge minimises segregation effects
hopper half angle typically less than 25 degrees from vertical
relatively low storage volume for a given headroom - but all the product can be retrieved
Equipment operational characteristics mass flow
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Conclusions I “Powder flowability” = Flow Function
Remember the “sand castle” test
Bulk density and wall friction also important
Interparticle forces controlled by Size distribution – esp. fines
Presence of liquids
Chemical properties
Surface texture
This gives us various opportunities for modification!
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Conclusions II Opportunities for modification
Making flow function steeper – to reduce dusting: Liquid addition
Making Flow Function less steep – easing flow: Less fines
Reduce liquid
Modify using “flow aids”
Conclusions III Use of “flow aids”:
Small, hard, non-reactive particles work in several ways: Prevent physical contact between surfaces that
are highly attractive to one another Roughen surface texture
Sequester moisture
Choice of material is important
Means of application and final location key
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Conclusions IV
Attend to the equipment design!
Often more economic than altering the powder
Even the most favourable powder will not perform well if the equipment design is not suited to it!
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Acknowledgements Brookfield Engineering Labs
Cadburys
Glaxo-SmithKline
Kerry Ingredients
United Biscuits
Givaudan
UK Government Department for Environment, Food and Rural Affairs for funding
THE WOLFSON CENTRE for Bulk Solids Handling Technology
University of Greenwich Medway School of Engineering
Tel 020-8331-8646: Fax 020-8331-8647
www.bulksolids.com