POWDER FLOW - skpcper.ac.in flow_0.pdf · The effect of moisture varies, depending on the degree of packing or the porosity of the powder bed. In a porous and cohesive material, flowability

POWDER FLOW

Prepared by: Dr. Geeta Patel

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Significance for including free-flowing powders

Powdres are generally considered to be composed ofsolid particles of the same or different chemicalcompositions having diameters <1000 µm.

Pharmaceutically, the largest use of powders is toproduce tablets and capsules, in addition to mixing andcompression properties, the flowability of a powder isof critical importance in production.

Success or failure in many pharmaceutical operationscan be directly linked to the flow properties of thepowder being processed.

Flowability is critically important when assessing howmaterial moves around the plant.

Industry needs the ability to develop formulations withtailored flowability, and first step is identifyingsuitable techniques for powder characterization.

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Significance for including free-flowing powders

1. Uniform feed from bulk storage container or hoppersinto feed mechanisms of tableting or capsule-fillingmachine.

2. Uniform particle packing and a constantvolume/mass ratio which maintains tablet weightuniformity.

3. Reproducible filling of tablets dies and capsule body,which improves weight uniformity and uniformphysico-mechanical properties of tablets.

4. Uneven flow leads to excess air entrapment withinpowders, which may produce capping or laminationproblems in tablets, also increase particle die-wallfriction, causing lubrication problems, and increasedust contamination risks during powder transfer.

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Capping tablet (Top) and Laminated

tablet (Right)

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Factors influencing the flow of powders

Under stress condition powder can flow like a liquids,they don’t flow if the stresses are to small.

Many manufacturing problems are attributed topowder flow, including non-uniformity (segregation)in blending, under-or0over dosage, inaccurate filling,and stoppages.

Storage, handling, production, packing, distributionand end use can all be negatively affected by commonpowder flow problems.

The factors associated with the nature of the particlesand their surface area.Particle size, particle size distribution & specific area.Particle shapeMoisture contentAdhesion and cohesion

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All matter interacts, as the dimensions of particlesincrease, the forces acting on them change.

With relatively small particles, the flow through anorifice may be restricted b’cos the cohesive forcesbetween the particles are of the same magnitude asgravitational forces.

Particle size - <100 µm – acted upon primarily by surfaceforce and >1000 µm – governed by gravitational force –Balance of interaction forces determines powderbehavior.

Fine particles (<74 µm) with very high surface area aremore cohesive than coarse particles which are influencedmore by gravitational forces.

Particles > 250 µm – free flowing, but as the size fallbelow 100 µm powders become cohesive and flowproblems are likely to occur.

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Particle size, particle size distribution & specific area

Very fine particles (<10 µm) - extremely cohesive andresist flow under gravity, except possibly as largeagglomerates.

PS has been systemically investigated – flat-bottommeter.

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Particle size, particle size distribution & specific area

Particle shape

Irregular shapes can have markedly different flowproperties due to difference contact areas.

A group of spheres has minimum interparticle contactand generally optimum flowability, whereas flakes orirregular particles have a very high surface area andpoor flow properties.

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Absorbed moisture in solids can exist either in theunbound state or as part of crystal structure.

Its effect directly change surface properties of theparticles.

It can also affect flow properties indirectly andpermanently through the granules formulation, whichare held together by solid bridges generated by hydrationand dehydration.

At higher moisture content and higher packing densitiesliquid bridges may progress.

The effect of moisture varies, depending on the degree ofpacking or the porosity of the powder bed.

In a porous and cohesive material, flowability is notaffected by moisture because the moisture can penetrateto the inside of particle without the formation of liquidbridge.

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Moisture content

A set of particles can be filled into a volume ofspace to produce a powder bed which is in staticequilibrium due to he interaction of gravitationaland adhesive/cohesive forces.

The change in bulk volume has been produced byrearrangement of the packing geometry of theparticles.

In general, such geometric rearrangements resultin a transition from loosely packed to more tightlypack.

More tightly packed powders require a higherdriving force to produce powder flow than moreloosely packed particles.

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Packing property

Cohesion occurs between like surfaces, such ascomponent particles of a bulk solid, whereasadhesion occurs between two unlike surfaces likebetween a particle and a hopper wall.

Cohesion and other particle properties affect flowproperty of powder. Vander Waals, surface tensionand electrostatic forces are important propertiesrelated with powder flow.

Adhesive/cohesive forces acting between a singlepair of particle and substrate can be accuratelydetermined using a ultracentrifuge to apply veryhigh forces strong enough to separate the twosurfaces.

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Adhesion and cohesion

Figure : Example of (a) free flowing and (b) weakly

cohesive powder blends

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Surface charges

Higher the electrostatic charges, poor the flow.

Friction generates electrostatic charges, so minimize

the friction to reduce the electrostatic charges.

Humidity

Relative humidity of the air (interstitial as well as head

space) in a storage container, such as a bin or silo, also

affects properties of bulk materials.

Many bulk materials are hygroscopic and thus the

expose to humid conditions results in increased

moisture content of the bulk. This can leads to an

increased in bulk strength and also to an increase in

angle of repose, flowability and cohesiveness of

granular powder.

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Temperature

Temperature also has a sustainable effect on bulk solid

flowability. The most drastic temperature effect is the

freezing of the moisture contained within the granular

materials and on particle surfaces. The resulting ice bonds

weaken the flow.

However, the temperature from 30 to 40 C does not

usually have a great impact on powder flowability; if there

is the component having melting point exceeds its glass

transition temperature.

Pressure

Compacting pressure is also an important factor that

affects the flow properties of bulk solids. The increased

pressure leads to a larger number of larger contact points

between particles thus causing more inter-particle

adhesion and increased compaction produces a significant

increase in critical arching dimensions. 14

It is useful to be able to quantify the type ofbehavior of powder.

Various methods are- Direct using dynamic or kinetic methods.- Indirect – measurements on static bed.

Indirect Methods – Angle of Repose (AR) It is indirect method used in many branches of

science to quantify powder flowability. AR is a characteristic related to interparticulate

friction or resistance to movement b/w particles. Many methods may produce different values for

the same powder despite its difficulties, themethod continues to be used in Pharma. industry.

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Measurement of powder flowability

Basic Methods for AR :- The AR measured by keeping the powder static

bed condition is called static angle of repose andthe AR if measured by keeping the powder inmotion is called dynamic angle of repose.

Dynamic AR – is preferred measurement as it isbetter correlated with the actual tablet or capsulemanufacturing, in which powder is usually inmotion.

Static AR can be classified – two experimentalvariables1. The height of the funnel may be fixed relative to the

base or the height may be varied as the pile forms.

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2. The base may be of fixed diameter or the diameter ofthe powder cone may be allowed to vary as the pileforms.

Variations in AR MethodsFollowing variations have been used to some extent1. Drained AR is determined by allowing an excess

quantity of material. Formation of the cone ofpowder on the fixed diameter base allowsdetermination of the drained AR.

2. Dynamic AR is determined by filling a cylinder(Flat bottom) and rotating it at a specified speed.It is the angle (relative to the horizontal plane)formed by the flowing powder.

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1. Funnel Method (Static AR) Allow the material to flow through a funnel or

orifice on to a horizontal surface below. The angleof conical heap so formed can be determined fromsimple geometry.

Figure – Fixed Funnel Method16

Various Methods for Measurement of AR

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Drawbacks :-

It’s suitable only for free flowing powders.

It doesn't give reproducible results, since thecone shape is distorted by the impact of theparticles.

2. Tilting Box Method (Dynamic AR)

A sandpaper lined rectangular box is fixed withthe powder and carefully tilted until the contentsbegin to slide.

The repose angle is the angle formed by surface ofthe box with the horizontal plane.

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Figure – Titling Box Method3. Rotating Cylinder Method (Dynamic AR) A hollow cylinder half-filled with the test powder

with one end sealed by transparent plate. Rotate it on horizontal plane, until the powder

surface cascades.19


Figure – Rotating Cylinder Method The curved wall is lined with sandpaper to

prevent slippage. The angle formed by cascading line with the

horizontal plane is the AR.20


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Figure : Dynamic angle of repose instrument

4. Drained AR

It is obtained with flow devices like rectangularvessel, cubic vessel or cubical box containingcircular disc platform.

Drained AR is usually larger than the poured ARfor the same powder.

a) Ledge Method :-

A rectangular vessel (ledge), which contains amovable or sliding shutter at the bottom side.

A vessel is filled with the sample to be tested.

Then the shutter is opened to allow the drainageof material itself.

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Figure – Ledge Method The angle formed by material left in the ledge with

horizontal plane is know as drained AR.23


b) Crater Method :-

A cubic vessel, which contains a movable or slidingshutter at the center of the bottom.

A vessel is filled with the sample to be tested.

Then the shutter is opened to allow the drainageof material itself.

Figure – Crater Method24


The angle formed by material left in the cubicvessel with horizontal plane is know as drainedAR.

c) PlatformMethod :-

A large containerwith built-in-platform is used.

The bottom of the container is provided with asliding shutter.

The container is filled with the sample to betested.

Then the shutter is opened and the material isallow to flow out at the bottom, leaving anundisturbed conical heap on the platform.

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This method eliminates wall friction from themeasurement and also avoids cone distortion dueto falling particles.

Figure – Platform Method26


AR is not an intrinsic property of powder, verymuch dependent upon type of method.

1. The cone peak of the powder can be distorted bythe impact of powder from above.

2. The nature of base upon which the powder coneis formed influences the AR. Recommended, conebe formed on a common base. This can be doneby using a base of fixed diameter with aprotruding outer edge to retain a layer ofpowder.

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Experimental consideration for AR

Pharmacopoeial specifications

Angle of Repose (AR)

The angle of repose is the constant, three

dimensional angle (relative to the horizontal

base) assumed by a cone like pile of material

formed by any of several methods discuss below.

If more material is added to the pile, it slides

down the slides until the common friction of the

particles producing a surface at an angle is in

equilibrium with the gravitational force. The

tangent of the angle of repose is equal to the co-

efficient of friction between the particles.

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tan = ; = tan-1

Or we can write as:

Where, h = height of the pile

r = radius of base of pile

= angle of repose

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Factors affecting angle of repose

Rough and irregular surface of the particles

give higher angle of repose.

Decrease in particle size leads to higher

angle of repose.

Lubricants at low concentration decreases

angle of repose whereas high concentration

increases angle of repose. So, optimum

concentration of lubricants required to

maintain angle of repose for good powder

flow.

Fines increases angle of repose. 32

Angle of Repose (AR)

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Table : Types of flow properties and corresponding AR

Flow property Angle of Repose (°)

Excellent 25 to 30

Good 31 to 35

Fair 36 to 40

Passable may hang up 41 to 45

Poor must agitate, vibrate 46 to 55

Very poor 56 to 65

Very, very poor > 66

USP recommends two methodsMethod-I Measurement in a Graduated CylinderMethod-II Measurement in a VolumeterApparatus:

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Official procedure for density measurement (USP)

Procedure: Allow an excess of powder to flow through the

apparatus into the sample receiving cup until itsoverflows (using a minimum 25 ml – powder withsquare cup, 35 ml – cylindrical cup).

Carefully scrape excess powder from the top of thecup (smoothly – spatula).

Take care to keep the spatula perpendicular toprevent packing or removal of powder.

Determine the weight, M of the powder to nearest0.1%.

Calculate the bulk density by the formula : M/VO.In which Vo is the volume in ml of the cup.

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Official procedure for density measurement (USP)

Procedure: Mechanically tapping the measuring cylinder

containing powder. After observing the initial volume, the cylinder

mechanically tapped and volume reading aretaken until little further volume change isobserved.

Tapping is achieved by raising the cylinder andallowing it to drop under its own weight aspecified distance.

Two methods.Method – I : Manual TappingMethod – II: Tapped density tester (BP)

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Tapped density measurement (Official)

Procedure: Unless otherwise specified, pass a quantity of

material through a 1.0 mm (No. 18). Into a dry 250 ml glass graduated cylinder

(readable to 2 ml) weighing 220±44 gm &mounted on a holder weighing 450±10 gm,approximately 100 gm of test sample, M, weighedwith 0.1% accuracy.

If not possible, the amount of the test sample maybe reduced and the volume of the cylinder may bemodified.

Carefully level the powder without compacting, ifnecessary and read the unsettled apparentvolume, Vo.

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Method-I- Tapped density measurement

Mechanically tap the cylinder, using a suitabletapped density tester that provides a fixed drop of14±2 mm at a nominal rate of 300 drops/min.

Unless otherwise specified, tap the cylinder 500times, measure tapped volume, Va.

Repeat the tapping an additional 750 times andmeasure tapped volume, Vb.

If the difference between two volumes is < 2%, Vb

is the final tapped volume, Vf. Repeat in increments of 1250 taps, until the

difference between succeeding measurements<2%.

Calculate the tapped density – M / Vf (gm/ml). Generally replicate determinations.

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Method-I- Tapped density measurement

It provides a fixed drop of 3 mm±10 % at a nominalrate of 250 drops/ min.

Apparatus: Consists of the following components.1. A settling apparatus, capable of producing 205 ± 15 taps

in 1 min from a height of 3 ± 0.2 mm.2. The support for the graduate cylinder, with its holder

that has a mass of 450 ± 5 gm.3. A 250 ml graduate cylinder with a mass of 220 ± 40 gm.

Method : Into a dry cylinder, pour 100 gm (M)sample. If not possible, select a test sample with anapparent volume between 50 & 250 ml.

Secure the cylinder in its holder, read the unsettledapparent volume (Vo).

Carry out 10, 500, and 1250 taps and read thecorresponding volumes V10, V500 and V1250.

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Method-II Tapped density tester (BP)

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If the difference b/w V500 and V1250 is greater than 2ml, carry out another 1250 taps.

Expression of results: Apparent volumes: Apparent volume before settling or

bulk volume is Vo in ml and apparent volume after settlingis V1250 and V2500.

Ability to settle: It is the difference b/w V10 & V500 (ml). Apparent densities: Apparent density before settling or

bulk density is M / V0 (gm/ml) and apparent density aftersettling or tapped density is M / V1250 or M / V2500 (gm/ml).

Marketed Equipments – Tapped density tester from Electrolab–ETD-1020 Tapped density tester from Quantachrome Tapped density tester from Varian

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Method-II Tapped density tester (BP)

Hausner’s Ratio (HR)

It is used to measure both bulk volume and

tapped volume of powder.

Where, V0 = unsettled apparent volume and Vf =

final tapped volume

It can also be measured in terms of density.

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Where, tapped = bulk density

bulk = tapped density

Carr’s Compressibility Index (CI)

This property of powder is also known as

“compressibility’ or “Carr’s consolidation

index.” It is simple, fast and popular method

of predicting powder flow characteristics.

It is an indirect measure of bulk density,

size, shape, surface area, moisture content

and cohesiveness of material because all of

this can influence observed CI. It is used to

measure bulk volume and tapped volume of

powder.

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Where, V0 = unsettled apparent volume

Vf = final tapped volume

It can also be measured in terms of density.

Where, tapped = bulk density

bulk = tapped density

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Compressibility Index & Hausner ratio

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Pharmacopoeial specifications

Table : Scale of flowability

Carr’s Index (%) Flow property Hausner ratio

10 Excellent 1 to 1.11

11 to 15 Good 1.12 to 1.18

16 t0 20 Fair 1.19 to 1.25

21 to 25 Passable 1.26 to 1.34

26 to 31 Poor 1.35 to 1.45

32 to 37 Very poor 1.46 to 1.59

> 38 Very, very poor > 1.60

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Monitoring the flow rate of material through anorifice has been proposed as a better measure ofpowder flowability.

Flow through an orifice – useful- for monitoring flow continuously b’cos pulsating

flow patterns have been observed for free flowingmaterial.

- Changes in flow rate can also be observed.- Empirical equations have been determined

(opening diameter, particle size and density).- The flow rate is generally measured as the

mass/flowing time from any types of containers(cylinders, funnels, hoppers).

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Flow through an orifice

Flow rate through an orifice can be classified……..1. The type of container used.2. The size and shape of the orifice used.3. The method of measuring powder flow rate. It can be

measured continuously using an electronic balance withsome sort of recording device (computer camera) or indiscrete samples, example – the time taken for 100 gm ofpowder or the amount of powder passing in 10 sec.

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Basic Methods for flow through an orifice

Apparatus: The funnel with or without stem, withdifferent angles and orifice diameters are used. Thefunnel is maintained upright. The assembly must beprotected from vibrations.

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Funnel method for flow rate measurement (BP)

Method: Introduce a test sample weighed with 0.5% accuracy in

to a dry funnel, The bottom opening of which has been blocked by

suitable means, without compacting the sample. Unblock the bottom opening of the funnel and measure

the time required for entire sample to flow out.

Expression of results: The flowability is expressed inseconds per 100 gm of sample.

Variations in methods – Either mass flow rate or volumeflow rate can be determined, but it biases the results infavor of high-density materials.

A vibrator is occasionally attached to facilitate flowfrom container; however this appears to complicateprediction of the results.

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Funnel method for flow rate measurement (BP)

Hopper flow rate: The simplest techniques, to measure the rate at which

powder discharges from a hopper. The shutter is placed over the hopper outlet and the

hopper is filled with powder. The shutter is then removed and the time taken for te

powder to discharge completely is recorded. By dividing the discharged powder mass by time, a flow

rate is obtained. Hopper or discharge tube outlets should be selected to

provide a good model for a particular flow application. Example – If a powder discharges well from a hopper

into a tablet machine feed frame, but does not flowreproducibly into the tablet die.

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Direct method for powder flow measurement

A recording flowmeter is essentially similar tothe previous method except that powder isallowed to discharge from hopper or containeron to a balance.

In case of analogue balances a chart recorder isused to produce a permanent record of theincrease in powder mass with time.

Recording flowmeters allow mass flow rates tobe determined and also provide a means ofquantifying uniformity of flow.

A. Hosokawa powder characteristics tester

B. Aero-flow42

Recording flowmeter

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A. Alteration of particle size & size distribution Coarse particles are less cohesive than fine particles

and an optimum size for free flow exists. The flowability problem can be solved by removing

a proportion of the fine particle fraction or byincreasing the proportion of coarser particles.

B. Alteration of particle Shape and texture Spherical particles – better flow properties than more

irregular particles. Spray-drying can be used (spray dried lactose). Temp cycling crystallization. Very rough surface will be more cohesive and greater

tendency to interlock than smooth-surfaced particles. Both controlled by crystallization and granulation.

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Improvement of powder flowability

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C. Alteration of surface forces Reduction of electrostatic charges can be achieved

by reducing frictional contacts or altering processconditions.

Electrostatic charges can be prevented ordichargedby efficient earth connections.

Moisture content – as absorbed surface moisturefilms tend to increase bulk density and reduceporosity.

In cases where moisture content is excessive,powder should be dried and if hygroscopic – storedproperly.

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D. Flow enhancers or flow promoters:Glidants Small amount of glidant is often used. Like talc, corn starch, silicon dioxide and colloidal

silica (Cab-O-Sil, Aerosil).Praposed mechanism for glidant action Distribution of glidant in the host particles. Dispersion of static charges from the host paticles

surface. Preferential adsorption of gases and moisture. Physical separation of particles and subsequent

reduction in Van-der walls interaction. Reduce friction between granules and surface

roughness is minimized.

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E. Alteration of process conditions

Use of vibration-assisted hoppers In cases where the powder arch strength within a bin

or hopper is greater than the stresses in it, due togravitational effects, powder flow will be broken up orprohibited.

The poor powder flow may result because of eitherrate holing or arching/bridging that may take place.

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Powder flow can be encouraged by adding stressesdue to gravitational interactions by vibrating thehopper mechanically.

Use of force feedersThe powder discharge irregularly or flood out can

be improved by fitting vibrating baffles, known aslive-bottom feeders, at the base of the conicalsection within a hopper.

Force feeders are usually made up of a single or twocounter-rotating paddles at the base of the hopperjust above the die table in place of feed frame.

The paddles presumably act by preventing powderarching over dies, improve die filling at high speeds.

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POWDER FLOW - skpcper.ac.in flow_0.pdf · The effect of moisture varies, depending on the degree of packing or the porosity of the powder bed. In a porous and cohesive material, flowability