Introduction:Powder flowability is the ability of a powder to flow in adesired manner in a specific piece of equipment.Flow of powders may be:Free-flowing Non-flowing or CohesiveManufacturing of tablets, capsules, filling of powder in containerinvolves several powder handling steps, including blending,transfer, storage, and feeding to a press or a dosator.The inability to achieve reliable powder flow during these stepscan have a significant adverse effect on the manufacture andrelease of a product to market.
Flow Patterns:Two flow patterns developed when powder flow from container:Funnel FlowMass FlowFunnel Flow• Side wall materialstagnant whereas flowof funnel- shapedmaterial take place• First-in-last-out• Chance of powdersegregation• Suitable only for freeflowing powderMass Flow• All of the material is inmotion• First- in-frist-out• Minimize segregationand prevent rat-holing
Factors Affecting Powder Flow Properties:The specific properties of a powder that affect its flow are known asflow properties.These flow properties depend upon:Collectiveforces acting onindividual particlesParticle variableEnvironmentalconditionPowder or Particlevariables:• Particle size, Sizedistribution, Shape, Surfacetexture, Cohesively, Surfacecoating, Particleinteraction, Wear or attritioncharacteristic, Propensity toelectro-staticcharge, Hardness, Stiffness, Strength, Fracture toughnessExternal Factorsinfluencing PowderBehaviour:• Flow rate, Compactioncondition, Humidity, Electro-static charge, Aeration,Transportation experience,Container surface effects,Storage time
Consideration of critical Factor: Particle Size Particle Nature Porosity Bulk Density Moisture Content Interparticle forceVan der WaalsElectrostaticSurface tensionInterlockingFriction, etcDuring Storage and TransportationCaking and bridgingPackaging conditionHigh to low aerationDe-aerationConsolidation by tapping
Powder Flow Problem:1. Rat-holing2. Arching3. Flooding Prevention of flow problem:Increased outlet diameterReduced filling height powderEquipment capacity can be reducedReduced cohesive strength of powderAgitation or mechanical assistance can be utilized
Measurement of Flow Properties:1. Angle of repose: The internal angle between the surface of the pile and the horizontalsurface is known as the angle of repose.Angle of repose depends upon: Density Surface area Shapes of the particles The coefficient offriction of the material This angle is in the range0 –90 .Flow Property Angle of repose (Degrees)Excellent 25–30Good 31–35Fair—aid not needed 36–40Passable—may hang up 41–45Poor—must agitate, vibrate 46–55Very poor 56-65Very, very poor ≥ 65
Angle of repose method used interparticulate friction or resistanceto movement between particles.Experimental Variable:Segregation of materialConsolidationAerationMethods for determining the static angle of repose :Fixed height methodFixed base diameter methodTilting box methodMethods for determining the dynamic angle of repose:Revolving cylinder method
Avalanching Tester:Used to determine dynamic angle ofrepose.MTA (mean time to avalanche)determine and at the time ofavalanching image was captured.Aero-Flow™ AutomatedPowder FlowabilityAnalyzer
2. Carr’s Compressibility Index And Hausner Ratio:Both are determined by measuring the bulk volume and thetapped volume of a powder.CompressibilityIndex (%)Flow Character Hausner Ratio≤10 Excellent 1.00-1.1111-15 Good 1.12-1.1816-20 Fair 1.19-1.2521-25 Passable 1.26-1.3426-31 Poor 1.35-1.4532-37 Very poor 1.46-1.59≥37 Very, very poor ≥1.60
3. Flow Through an Orifice:Useful only for free-flowing materials.Two types of flow rate of powder determined:Mass flow rateVolume flow rateExperimental variable:The type of containerThe size and shape of the orifice usedHeight of the powder bedGeneral guidelines for dimensions of the cylinder are as follows:Diameter of opening > 6 times the diameter of the particlesDiameter of the cylinder > 2 times the diameter of the opening
4. Shear Cell Method:In the shear cell method, the force necessary to shear the powderbed by moving the upper ring is determined.Types of shear cell method:Cylindrical shear cellAnnular shear cellPlate-type shear cell4.1 Brookfield PFTChoice of Test Options:Flow FunctionTime ConsolidationTest with Flow FunctionWall FrictionData Output:Flow factor IndexArching DimensionRat-hole DiameterHopper Half AngleWall FrictionBulk Density Curve
Flow factor index Flowabilityff<1 Non flowing1<ff<2 Very cohesive2<ff<4 Cohesive4<ff<10 Easy flowingff≥10 Free flowingThe standard classification of powderflowability uses the flow factor index (ff):It is ideal for manufacturers who process powders daily and want tominimize or eliminate downtime and expense that can occur whenhoppers discharge erratically or fail to discharge altogether.Consolidation stress
5. Cohesion Index:Stable Micro Systems TA.XT Plus textureanalyzer used to determine cohesioncoefficient which is determined byintegrating the negative areas underthe force displacement curve.Cohesion Index Flow behaviour≥19 Hardened, extremelycohesive16-19 Very cohesive14-16 Cohesive11-14 Easy flowing≤11 Free flowingA low cohesion index is associated withnon-cohesive free-flowing powders.
6. FT4 Powder Rheometer:This methodologies allow measurement of:Flow energyShear propertiesBulk propertiesThe forces causing the deformation and flow is measured.FT4 flowmeter Downward testing mode Upward testing modeCalibration and Data Analysis:The variables are force, torque, height, and rotational and linearvelocities, flow properties determined via extrapolation.
Downward Test Mode:It is used to measure the Basic Flow Energy (BFE)Upward Testing Mode:It is used to measure the Specific Energy (SE) which is the flow energyper gram of powder tested.
8. Penetrometry:The pressure of penetration in Pascal was usedto estimate flow rate.Particle size should be in the range of0.250-0.630 mm.Used for non-consolidated pharmaceuticalpowder excipients: sodium chloride,sodium citrate, boric acid, and sorbitol.A polynomial quadratic equation was generated using the ratio ofpenetration pressure to bulk density, from which the flow rates for thegroup of all tested powders could be estimated.
Control of powder flow1. Mucon Iris Diaphragm Valve2. Powry Valve3. Vacuum Assisted Powder Flow Nozzle4. Funken Continuous Auto Feeder5. Fluidizing Hopper Flow Control ValveMucon ValvePowry Valve
Importance of Flow Properties in Pharmacy:1. Tablet Manufacturing2. Capsule Manufacturing3. During Drug Delivery4. During Mixing and Sieving5. During Unloading of Packs6. During Transportation Through Conveyers7. During Filling of Powders, Dry Suspension and Dry Syrup
Conclusion:Powder flow is a key requirement for pharmaceutical manufacturingprocess.An understanding of the fundamentals of powder flow can steerone in the right direction to achieve reliable powder flow. Continuous and even flow of powder during tablet and capsulemanufacturing reduced weight variation, content uniformity (alsohardness in tablet).Powder flow is also crucial during mixing, packaging, and transportationReduction in process costs and maintaining the quality and consistencyof the final product
Reference:1. Jenike, A.W., Storage and Flow of Solids (Bulletin 123 of the UtahEngineering Experimental Station), 53 (26), (1964, revised 1980).2. Prescott, J.K., and Barnum R.A., On powder flowability, PharmaceuticaTechnology, October 2000, pp. 60-84 and 236.3. Prescott, J. K. and Hossfeld, R. J., Maintaining product uniformity anduninterrupted flow to direct compression tablet presses.Pharmaceutical Technology, 18 (6), 1994, pp. 99-114.4. Baxter, Thomas J., “When Powders Flow Like Water: AddressingTwo-Phase Flow Effects in Tablet Press Feed System”, Tablets &Capsules, March 2009, Volume 7, No. 2, pp. 26-32.5. Barnum, Roger, Ebb and Flow: Understanding Powder Flow Behavior,Pharmaceutical Processing, March 2009, pp. 18-21.