Flow Flashcards
Causes of poor powder flow
1) Surface forces
- Strong attractive forces (cohesion / adhesion) –> poorer flow
- Cohesion: Between same substance
- Adhesion: Between different substances
2) Inter-particle friction
- Friction due to certain amount if rubbing/cascading when flowing
- Increased friction –> poorer flow
3) Inter-locking of particles
- Interlocking of particles –> prevent movement –> poorer flow
Types of surface forces
1) Non-specific van der Waals
2) Moisture (liquid film)
- Can form stronger bond (e.g. hydrogen bonds)
3) Electrostatic forces
4) Crystalline bridges
- May occur due to temperature change
- E.g. Condensation at lower temperatures –> may cause some dissolution –> temperature increase –> condensation dries –> solute can solidify & form solid bridges
- E.g. Melting
Factors affecting strength of surface forces
1) Particle size
- Smaller particles –> able to come closer together –> stronger forces
- Smaller particles –> greater surface area available to form surface forces
2) Contact sites (smooth VS rough)
3) Time of contact
- Certain amount of adsorbed air around particles (normal phenomenon) –> helps to separate particles to certain extent
- Longer time of contact –> air might be squeezed out –> particles able to come closer together –> stronger attractive forces
Factors affecting inter-particle friction
1) Surface of particles
- Rough surface –> more friction (VS smooth surface)
2) Size of particles
- Smaller particles –> larger surface area –> increase friction
Factors affecting inter-locking of particles
1) Size of particles
- Larger particles more likely to be interlocked
2) Size distribution
- Larger size distribution –> better packing / density –> can lock particles together
3) Shape of particles
- Irregular particles –> able to interlock
- VS Round particles –> better flow
Problems arising due to poor powder flow
1) Arching
2) Ratholing
Arching
- What is arching
- How it occurs
- How to prevent
What is arching:
Arch shaped obstruction formed above funnel –> obstruct flow
Especially with coarser powders
How it occurs:
1) Mechanical inter-locking of large particles to form an arch
2) Particles bind together to form cohesive arch
How to prevent:
1) Outlet should be > 10x D90 (from size distribution)
- Since potential problems of arching will be significant when aperture size < 6-10x max particle size
Ratholing
- What is ratholing
- How to prevent
What is ratholing:
Discharge only takes place via central flow channel located above outlet
Powder along sides of outlet will stagnate / may cake/agglomerate –> no flow
How to prevent
1) Outlet should be more angular
Glidant - Function
1) Improve flowability
Glidant - Mechanisms of actions
1) Ball bearing effect
- Correct surface irregularity –> makes particle rounder
- Rolling action –> rolling effect of glidant (small round particles)
2) Decrease inter-particle friction
- Coats surface of particles –> physical separation of particles
3) Decrease surface charge / forces
- Coat particle surface –> separate particles –> decrease inter-particle attractive forces
Glidants - Concentrations used
Used at certain concentration range for optimal activity
Excess glidant is detrimental –> glidant itself may be cohesive
Glidants - Examples
1) Talc (1 - 5%)
2) Corn starch (5 - 10%)
3) Colloidal silica (0.5 - 3%)
Flow test methods
1) Angle of repose
2) Orifice flow
3) Tapping studies
Automated methods:
1) Powder rheometer
2) Revolution powder tester
3) Shear cell
Angle of repose
Angle of inclination of a heap of powder that is built by dropping powder through a funnel onto a horizontal base
Angle of fall
Angle of repose after application of a certain shock
Angle of difference
Difference between angle of repose & angle of fall
Overcomes variation in angle of repose due to poor flow
Angle of spatula
Angle of inclination made by powder heap formed on spatula
Methods to measure angle of repose
1) Fixed funnel & free standing cone
2) Fixed bed cone
- Fixed base diameter
3) Fixed height
- Funnel fixed at pre-determined height above base
- Correction of heap height needed for apex in funnel stem
Fixed funnel & free standing cone - Limitations
1) Base diameter might not be accurate / may vary
- Can overcome by doing more heaps and getting average diameter
2) A lot of measurements to make (height, base diameter)
3) May have measurement difficulties
Fixed bed cone - Advantages
1) Less measurements needed (only need to measure height)
Fixed height - Advantages
1) Less powder needed for measurements
- As little as < 10g, if height fixed is very small
Equations to measure angle of repose
tan (angle of repose) = Height / Radius
Fixed height: h / d = (H + h) / D - h = Height of apex in funnel - H = Fixed height - d = Diameter of funnel opening - D = Base diameter tan (angle of repose) = (H+h) / Radius
Limitations in measuring angle of repose
1) High variability & high angle of repose with poor flow
2) Even with good flow, difficult to measure apex of cone
- Tend to get rounded top
- Overcome by measuring angle of repose directly with angle meter (don’t measure height)
Angle of repose & flowability
Lower angle of repose –> better flowability
Note: Angle of repose is mainly for comparative studies; Whether angle of repose is considered good/poor flow is related to use of powder
Factors to consider during measurement of angle of repose
1) No vibration
2) Level/Horizontal base
3) Condition surface characteristics of base
4) Funnel type & diameter - ensure no arching
5) Constant material moisture content
6) Controlled ambient humidity
- To ensure constant moisture content
7) No particle fragmentation
8) Accuracy of measurement
- At least 5 - 6 replicated measurement
- Variation < 2%
- If there is a trend (going up/down) –> may be due to change in moisture content/fragmentation
Orifice flow - How it works
Flow a known fixed quantity of powder through orifice of known opening size
- Fixed volume (volumetric flow) or fixed weight (gravimetric flow)
Measure flow time
- May contain laser light source
- When powder is flowing, light will be blocked
- When all the powder has flown through, light can be detected by detector –> stop timer
Orifice flow - Determining flow
Flowability reported as flow rate
- Flow rate = Weight or volume of powder / Flow time
Generally used as to compare materials
- No general scale available
Orifice flow - Applications
Often used to measure flow of finished products
May be used as a quick ‘go’/’no-go’ indicator
Tapping studies - Steps
1) Fill cylinder with powder
- Recommended: Use sieve to fill
- Filling can affect bulk density (depending on height powder is poured from / if powder is disturbed)
- Avoid pouring into cylinder from beaker
2) Remove excess powder, ensure powder is level
3) Weigh
- Calculate bulk density = Weight of powder / Volume of powder
4) Put on tapping machine
5) Tap to constant volume
- Calculate tapped density = Weight of powder / Volume after tapping
Tapping studies - Determining flow
Flowability indicated by change in volume
- Smaller change –> greater flow
May be reported as Hausner ratio (HR) or Carr index/Compressibility index (CI)
HR = Tapped density / Bulk density
CI = [(Tapped density - Bulk density) / Tapped density] x 100
Lower HR/CI
- Good initial packing –> little rearrangement
- Better flow
Higher HR/CI
- Poor initial packing with void spaces –> more rearrangement
- Poorer flow
Powder rheometer - Eradication of powder history
Process:
1) Involves gentle loosening & slight aeration of powder
2) Disturb & gently drop each particles
Aim:
1) Achieve homogeneously packed powder bed
2) Removes any pre-compaction / excess air
3) Overall: Ensures the results from the following test are independent of powder history
- Not operator dependent
Powder rheometer - How it works
Rotate impeller downwards & upwards –> causes particles to interact/flow relative to one another
Resistance experienced by blade represents bulk flow properties / difficulty of relative particle movement
Harder to move blade (greater resistance) –> poorer flow
Work done = Energy = Resistance x Distance travelled
Powder rheometer - Determining flow
1) Basic flowability energy (BFE)
- Resistance measured during downward movement of impeller
- A measure of forced/confined flow (e.g. when flowing through screw feeder / active feed frame)
2) Specific energy
- Resistance measured during upward movement of impeller
- A measure of low stress/unconfined flow (e.g. low stress filling, low shear mixing)
Powder rheometer - Application
1) Used for comparative studies
2) Can get reproducible data from different batches during manufacturing
Revolution powder tester - How it works
Powder placed in test drum and rotated
Capture images of powder
Images collected used to determine powder behaviour over time
Images analyzed by software algorithms
Data generated used to calculate various parameters representing powder flowability
- E.g. Avalanche angle & frequency
Revolution powder tester - Used to measure
1) Flowability of powder in low stress situations
2) How the powder behaves once it is flowing
3) Condition of powder as it moves through process
Revolution powder tester - Determining flowability
Constant avalanche time –> better flow
Shear cell - How it works
Apply normal stress to powder
Exert shear stress
Look at amount of torque required to shear powder
Shear cell - Determining flowability
Plot unconfined failure strength (torque needed to shear powder bed) against consolidation stress (normal stress)
Determine flow factor from slope
Larger flow factor –> better flowability
Shear cell - Applications
1) Used to measure flow of very cohesive powder (i.e. very poor flow)
- Not very common in pharmaceutical industry
2) Shear cell data used in design of hopper for powder flow patterns & outlet dimensions
Types of shear cells
1) Translational shear cell
- Splits horizontally –> forms shear plane
- E.g. Jenike
2) Annular shear cell
- Rotates to shear powder
- E.g. Schulze ring shear tester
• Well established test method
• More manual
- Brookfield shear tester
• More automated, minimum operator contact
3) FT4 Powder rheometer
- Shear by rotation
- More automated –> minimize operator contact
- Quick operation
- Equipment has other capabilities for bulk characterization
