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Dust Emission by Powders

Renaud ANSART, John DODDS, Alain De RYCK

Centre RAPSODEEEcole des Mines d’Albi , France

Dust Emission by Powders

Renaud ANSART, John DODDS, Alain De RYCK

Centre RAPSODEE, Ecole des Mines d’Albi

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Une poudre de ciment ordinaire

Poudre de ciment Lafarge traité anti poussiere

Handling powders Handling powders = Emission of dust= Emission of dust

Handling powders = Emission of dustHandling powders = Emission of dust

Particles in Suspension =

• Product losses• Risk of product contamination• Health risks for operators • Risk of dust explosions

Handling powders = Emission of dustHandling powders = Emission of dust

Measures :

• Product design (such as granulation)

• Surface treatment of particles

• Ventilation and dust capture systems

To be effective we must :

• Be able to measure dustiness

• Understand the mechanisms of dust emission

Many Tests for Measuring “Dustiness”

Tests for Measuring “Dustiness”

• A great variety of “dustiness tests”, all empirical.

• Many different ways of stressing a powder to produce dust.

• No real relation to processing conditions.

• Many different ways of characterising the dust produced (total wt.of dust, wt. of specific size fractions)

• Useful for comparing formulations, but not for designing ventilation and dust capture systems.

A study case of dust emission

• Forces in action

• Experimental set-up

• Results

• Perspectives

Powder falling from a silo into a container

A study case of dust emission

• Forces in action

• Experimental set-up

• Results

• Perspectives

Powder falling from a silo into a container

Experimental SetExperimental Set--upup

Silo

Pneumaticconveyor

Protectionsleeve

Measuring chamber

SpraytecOpen path

Laser diffractionPSA instrument

Particle Image Velocimetry

(PIV)equipment

Load cells

DantecDantec PIV System PIV System

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Malvern Malvern SpraytecSpraytec InstrumentInstrument

Non intrusive, in-situ, real time PSA by Mie diffraction

Particle size range: 0.5 μm - 2mm

Beam length 50 CmBeam width was reduced

10mm ⇒ 6mm

A study case of dust emission

• Forces in action

• Experimental set-up

• Results• Particle motion• Particle flow rate• Particle size distribution

• Perspectives

Powder falling from a silo into a container

•• The stream first retracts beforeThe stream first retracts beforeexpandingexpanding

•• Non expansion zone near the outletNon expansion zone near the outlet(velocity close to that of free fall).(velocity close to that of free fall).

A stream of particles falling from the silo

Velocity vector field (PIV)Velocity vector field (PIV)

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Gel de Silice-Qm=1,5 g/s-h=50cm

Higher velocity at centre.Higher velocity at centre.Particle stream grows wider with height of fall.Particle stream grows wider with height of fall.

||V||

Vertical velocity of the particlesVertical velocity of the particles

Gaussian velocity profile.Gaussian velocity profile.The stream grows wider as it falls .The stream grows wider as it falls .

Gel de Silice-Qm=1,5 g/s

D=10mm

Silica gel 1,5 g/s D0 =10mm PIV results

Free fall velocity

Particle velocity at stream centre Particle velocity at stream centre ((VmaxVmax))

Velocity stabilises after a certain height of fallVelocity stabilises after a certain height of fall

Horizontal velocity of the particlesHorizontal velocity of the particles

Particles move towards the edge of the stream.Particles move towards the edge of the stream.The speed of migration lessens with height of fall.The speed of migration lessens with height of fall.

Silica gel 0,6 g/sNormalized w.r.t. max velocity

Changes in PSD with height of fall Changes in PSD with height of fall

The fines migrate to the edge of the stream.The fines migrate to the edge of the stream.

Mastersizer 2000 0,5bar MonomodalD10= 34 μmD50= 59 μmD90= 97 μm

Silica gel 1,5 g/s

34,3%

14,5%

A study case of dust emission

• Forces in action

• Experimental set-up

• Results

• Perspectives•• ModellingModelling

Powder falling from a silo into a container

Numerical modellingNumerical modelling

• Allows calculating : :• Particle velocity (solid phase),• Air velocity (gas phase),• Flow of air induced into the stream of particles.

• Input parameters:• Mass flow rate, particle size, density, outlet diameter• Arbitrary choice of the rate of expansion of the stream

•Liu’s two phase model (negative buoyancy plume)

α =Vinduced air

Vz(stream )

=Ve

Vmax

Vmax

Ve Ve

Velocity profile of the stream of particles and the entrainment factor α

• Here slope 6% for Liu’s model• Origin slightly below silo outlet

Width at Vmax/2

Model results for velocity at centre (Model results for velocity at centre (VmaxVmax))

Good agreement between measurements and predictions for Good agreement between measurements and predictions for αα=6,2%.=6,2%.

Gel de Silice-Qm=1,5 g/s

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Velocity profile of the stream of particles and the entrainment factor α

• α seems to depend on powder characteristics• Could give a way to characterise “dustiness”

Silica gel 40-63 µm

Silica gel 40-200 µm

PerspectivesPerspectives

• Development of more rigorous characterisation methods for the dustiness of powders

• With the Fluid Mechanics Institute of Toulouse (IMFT), development of a new two phase model of particle stream to eliminate the need for parameter α.

• Experiments underway with different powders. (particle size, density, cohesion, fines content…..)

• Tests on anti-dust treatments for powders

• Tests on inserts and air injection/extraction tominimise dust emission

Thank you for listening

This research programme is financed by the CNRS, EMAC and the Institut National de Recherche et Securité (INRS)

Tests for Measuring “Dustiness”

• Single drop column tests

• Single drop chamber tests

• Fluidisation tests

• Rotating drum tests

An example of a palliative An example of a palliative mesuremesure

A study case of dust emission

Powder falling from a silo into a container Forces acting on a particle:Forces acting on a particle:

•• gravity.gravity.•• bouyancybouyancy ((negligeablenegligeable in air).in air).•• drag.drag.

Parameters of the streamParameters of the stream::•• height of fall.height of fall.•• mass flow rate.mass flow rate.•• particle size and distribution.particle size and distribution.•• air induced in the flow.air induced in the flow.•• cohesion.cohesion.•• humidityhumidity

EquipmentEquipment•• Pneumatic conveyor (Pneumatic conveyor (GerikeGerike))•• Silo 20 litreSilo 20 litre•• Measuring chamber 250 lMeasuring chamber 250 l

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Experimental SetExperimental Set--upup

Instrumentation• Powder flow rate by load cells• Air flow into measuring chamber• PIV for particle trajectories• Laser diffraction for PSA

Tests for Measuring “Dustiness”

• Single drop column tests: example PALAS apparatus

• Drop powder sample down tube• Measure changes in light transmission

Tests for Measuring “Dustiness”

• Single drop chamber tests : example Material Research Institute (MRI) apparatus

• Rotate and vibrate beaker to pour powder in the chamber

• Inlet air flow of 10 l/min

• Collect dust in stages of an impactor with additional air flow

Tests for Measuring “Dustiness”

• Fluidisation testsexample : HSE fluidisation test using inert particles

• Fluidisation of cohesive particles in a bed of inert sand particles

• High stress dispersion

• Measurement of particle concentration above bed by collection and micro-balance

Tests for Measuring “Dustiness”

• Rotating drum testsexample : Heubach rotating drum test

• 20 g powder sample put in a rotating drum with lifters

• Air flow carries dust to impactor for particle size and concentration measurement

Handling powders = Emission of dustHandling powders = Emission of dust

Product lossesHealth risks for operators Risk of dust explosions

Particles in suspension

Visualisations du jet

moy 500 images

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h = 50cm, 10x10 cm

Image instantanée

h = 50cm, 10x10 cm

Moy. de 500 instantanées

Visualisation at silo outletVisualisation at silo outlet

The stream first retracts before expandingThe stream first retracts before expandingNon expansion zone near the outlet (velocity close to that of fNon expansion zone near the outlet (velocity close to that of free fall).ree fall).

Experimental results for stream expansion

Slope=6%Slope=6%Validates LiuValidates Liu’’s model Gives a value of s model Gives a value of ααThe origin of the linear section is below the silo outlet (initThe origin of the linear section is below the silo outlet (initial retraction)ial retraction)

Silica gelQm=1,5 g/sDorifice = 10 mm

width=Vmax/2

PlanPlan

Forces in action

• Experimental set-up

• Results• Particle motion• Particle flow rate• Particle size distribution

• Perspectives

PlanPlan

Forces in action

• Experimental set-up

• Results• Particle motion• Particle flow rate• Particle size distribution

• Perspectives

Handling powders = Emission of dustHandling powders = Emission of dust

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Test with two different powders• powder A > 100 µm• powder B < 100 µm

Velocity profile of the stream of particles and the entrainment factor α

Vmax

α = Ve/Vmax

Numerical modellingNumerical modelling

• Allows calculating : :• Particle velocity (solid phase),• Air velocity (gas phase),• Flow of air induced into the stream of particles.

• Input parameters:• Mass flow rate, particle size, density, outlet diameter• Arbitrary choice of the rate of expansion of the stream

i.e. α = V(induced air)/Vz(stream)

ddz

π a2r av( )= 2π ar av α

mQd pvdz

=1

pv*B − 1k pv − av( )4 / 3

sC⎡ ⎣ ⎢

⎤ ⎦ ⎥

ddz aρ π a

2r a2v( )= 1k pv − av( )4 / 3

sC 1

pv Momentum balance (air)

Momentum balance (particles)

Mass balance.

•Liu’s two phase model (negative buoyancy plume)

Handling powders = Emission of dustHandling powders = Emission of dust

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Test with two different powders (BASF)• powder A > 100 µm• powder B < 100 µm

Visualisation of the particle stream

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2 zones : core + powder boundary layer .Mixing zone : Kelvin-Helmholtz instability.

Silo outlet30x30 cm

h = 50cm, 10x10 cm h = 50cm, 10x10 cm

Instantaneous image Mean of 500 instants

Concentration profile by masking and weighingConcentration profile by masking and weighingGel de Silice-Qm=15 g/s-Y/Do=53.5

Particle size distribution of the Silica GelParticle size distribution of the Silica GelMastersizer 2000: dispersion 0,5barMonomodal distributionD10= 34 μm D50= 59 μm D90= 97 μm

ρapparent = 500 kg/m3ρsolid = 1000 kg/m3

EquipmentEquipment•• Pneumatic conveyor (Pneumatic conveyor (GerikeGerike))•• Silo 20 litreSilo 20 litre•• Measuring chamber 250 lMeasuring chamber 250 l

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Experimental SetExperimental Set--upup

Different size entry disks to receiving vessel

Concentration profile by masking and weighingConcentration profile by masking and weighing

High concentration at the centreHigh concentration at the centre

Gel de Silice-Qm=15 g/s-Y/Do=53.5

Concentration of particles in the stream Concentration of particles in the stream

The porosity of the stream increases with height of fall.The porosity of the stream increases with height of fall.The particle concentration becomes more homogeneous with heightThe particle concentration becomes more homogeneous with height of fall.of fall.

Gel de Silice-Qm=15 g/sD0=20mm

Concentration profile by Concentration profile by SpraytecSpraytec

Gaussian concentration ,profile.Gaussian concentration ,profile.Not absolute values, Not absolute values, pbpb: assume uniform concentration over beam length: assume uniform concentration over beam length

Silica gel-Qm=1,5 g/sLaser beam

To conclude on the experimental resultsTo conclude on the experimental results

• Characterisation of a stream of falling particles :

• Particle velocity ↓ and →,

• Width of the stream,

• Concentration profile of the stream,

• Particle size profile in the stream.