Powder Flow & Flow Measurements Flashcards
Causes of poor powder flow
1) Surface forces (Cohesion/Adhesion)
Result from: Non-specific vDW forces
Moisture (stick particles tgt)
Electrical charge (static charges)
Particle size
Contact sites
Times of contact
Crystalline bridges
2) Inter-particle friction
3) Interlocking of particles
How to overcome flow issues
1) Improving design of container (hopper)
2) Conveyance (How to move powder from 1 spot to another)
- Paddle, auger, suction: Type, design
3) Formulation - Adding glidant/lubricant
Types of powder flow problems
1) Arching -> Alter diameter of orrifice opening
- Arch-shaped obstruction formed above discharge outlet & preventing flow
- Significant when outlet < 6-10x max particle size
- Tends to happen with coarse particles
2) Ratholing -> Alter slope design
- When discharge takes place only in central flow channel located above outlet (cohesive material outside channel will not flow & cake/agglomerate)
Glidants. What are they & how do they help to improve flow?
- Additives to improve flowability
- Requires certain [ ] range for optimal activity (excess may be detrimental)
MOA:
1) Ball bearing effect (eg. Starch)
- Rolling effect of small spherical particles decreases friction due to rough surfaces by physical separation
- Decreases surface irregularity
2) Reducing interparticulate friction (eg. fumed silica)
- Fine glidant particles will coat larger host particles & increase the separation dist between large particles and hence, reduce interparticulare attractive forces
3) Reduce surface charge
Flow test methods
- Angle of repose (+ Angle of fall, difference)
- Orrifice flow
- Tapping density
- Avalanche flow
- Powder rheometer
What is the diff between Angle of Repose vs Fall vs Difference
Repose: Angle of inclination of a heap of powder
Fall: Angle of repose after application of a certain shock
Difference: Repose - Fall
(More reproducible measurement)
What’s the use of angle of spatula?
- Simple technique to measure resistance to particle movement
- Qualitative assessment of the internal cohesive & frictional effects under low levels of external loading
How to measure angle of repose?
1) Fixed funnel & free-standing cone: tan(0) = H/R
2) Fixed bed cone: H
(since base is fixed, only measure height)
3) Correction of heap height needed for apex in funnel stem if fixed funnel method is employed:
tan(0) = H/R
H = H’ + h
Lower value indicates better flowability
Use of measuring Angle of Repose
Mainly for comparative flow of good flowing powders
Factors to consider when measuring Angle of Repose
- No vibration at measurement site
- Ensure level & horizontal plane
- Conditioned surface characteristics of base
- Funnel type & diameter (no arching)
- Constant material moisture content
- Controlled ambient humidity
- No particle fragmentation
- Accuracy of measurement (<2%)
How to measure Orrifice Flow?
Flow rate = Weight or Volume/ Flow time
Gravimetric flow: Fixed WEIGHT of powder
Volumtetric flow: Fixed VOLUME of powder
Use of measuring Orrifice Flow
Popular for QC
How to measure Tapping density?
Compressibility index/ Carr index:
(Tapped - Bulk Density)/Tapped Density x 100
–> Good: <= 10%
Hausner Ratio: Tapped/Bulk Density
–> Good: 1.00 - 1.11
How to evaluate Tapping studies?
(Vo-Vf)/Vo low: Little rearrangement indicates good initial packing due to good flow
How does powder rheometer work?
It measures resistance of the powder to flow by rotating an impeller downwards & upwards, causing particles to interact (or flow relative to one another) & the resistance experienced by the blade represents the difficulty of this relative particle movement or the bulk flow properties
