taming the powder: how to control powder flowability · powder flow function consolidation stress...

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

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

“Hammer rash!”

Sometimes the powder flow properties and the equipment design are just not compatible!

Air Injection to Aid in

Discharge of Powder

Losses from poor or irregular powder flow

Production downtime

Operator injury

Equipment damage

Irregular product quality

Noise from hammering or vibrators

What is “powder flowability”?

The Wolfson Centre for Bulk Solids Handling Technology Idealised Flow Function Test

“Flow function” of a bulk solid from a uni-axial test 0000

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

Powder Flow Function

Consolidation stress σ 1

gth σ

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

Flow Function

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]

Results- Flow Function Test

Free flowing

Easy flowing

Cohesive

Very Non flowing

Cohesive

Bulk density function – also in Flow Function Test

Bulk Density Function 0

50 100

150 200

300 350

400 450

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

ty ρ b

Critical dimensions to avoid “arching”

Cohesive arching

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

Dp 6Dp>L>3Dp

Modifying Powder Flow Properties:

A selection of case studies

Effect of Particle Size

Flow Functions

0 1 2 3 4 5 6 7 8 9 10 11 Major Principal Consolidation Stress σ 1 [kPa]

] 170um mean dia 100um mean dia 40um mean dia 20um mean dia

Effect of Particle Size on Arching Tendency

Critical Arching Dimension

0 20 40 60 80 100 120 140 160 180 Mean Particle diameter µ 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

Flow function for sand increasing moisture content

Flow Functions

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0

fine sand0% fine sand 3% fine sand 6% fine sand 9% fine sand 12%

Arching dimension for sand increasing moisture content to saturation

Critical Arching dimension

0 2 4 6 8 10 12 14 16

Moisture content % by weight

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

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

Dustiness Tester

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

Zeolite

Fumed silica

Magnesium chloride

Stearic acid

Dispersion methods for “flow aids”

Simple mixing Mechano-fusion

High shear & compression

Ordered mixing

Increasing energy input

Modifying Flow Properties Using Additives

Effect of the flow additive on the flow function

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Major Principal Consolidation Stress σ 1 [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

-4 -2 0 2 4 6 8 10 12

Additive % Weight

Easy flowing material S&V

Troublesome material chilli

Effect of Temperature (Fatty Powder)

Flow Functions

0 1 2 3 4 5 6 7 8 9 10 11

15degC 20degC 30degC

Effect of Temperature (Fatty Powder)

Critical Arching dimension

0 5 10 15 20 25 30 35

Temperature deg C

Conical Hopper Wedge hopper

Flow Function & Time Flow Functions

0 1 2 3 4 5 6 7 8 9 10 Major Principal Consolidation Stress σ 1 [kPa]

s σ c

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

Equant

Elongate

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

Powder flowability is not the whole story

Equipment design also matters greatly!

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

Core flow

“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

The Wolfson Centre for Bulk Solids Handling Technology Discharge number

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!

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

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!

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

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