Flow Flashcards
What are the causes of poor powder flow?
- surface forces (smaller particles have larger surfaces, creates more attractive forces)
- inter-particle friction (may require lubrication)
- interlocking of particles (may be present in fibrous materials)
What factors results in the cohesion and adhesion of surface forces?
- non-specific VDW forces
- moisture
- electrical charge
- particle size
- contact sites
- time of contact
- crystalline bridges
What is the difference btw cohesion and adhesion?
Cohesion: particles of same substance stick together
Adhesion: particles of dissimilar particles or surfaces stick together
How to overcome flow issues?
- Container (hopper) - improve design
- Conveyance - paddle, auger, suction: type, design
- Formulation - add glidant, lubricant
What are some powder flow problems (esp in orifices)?
a) Arching (bridging): arch-shaped obstruction forms above discharge outlet and prevents flow; may be due to mechanical interlocking of large particles/ cohesive arch when particles bond together; potential problem significant when outlet <6-10 times the maximum particle size of particles
b) Ratholing: discharge only in central flow channel located above outlet; stuck at sides; cohesive material outside channel will not flow and may cake or agglomerate. (solution = need a steeper design)
When does arching occurs?
Opening too small
funnel diameter needs to be >6-10x the largest particle of the powder
What is the purpose of glidants?
- additive; improves flowability of powders or granules
- require certain conc range for optimal activity, excess may be detrimental
What is the mechanism of action of glidants?
- ball bearing effect: correct surface irregularity; rolling action
- reduce interparticulate friction
- reduce surface charge
glidants can also go in btw the particles and fill the irregular spaces –> reduce surface irregularity –> thus, reduce friction and charge
What are some examples of glidants?
talc (1-5%) corn starch (5-10%) colloidal silica (0.5-3%)
How does the ball bearing effect works?
Rolling effect of small spherical particles, decreasing friction due to rough surfaces by physical separation
e.g. starch
How does reducing friction in powders work?
Fine glidant powders will coat larger host particles, thus increasing the separation distance btw the large particles, and hence, reduce interparticle attractive forces (VDW forces)
e.g. fumed silica
What are the types of angles of repose (flow test methods)?
a) Angle of repose: angle of inclination of a heap of powder carefully built up by dropping material (usually through appropriate sieve) through a funnel onto a horizontal plate.
b) Angle of fall: angle of repose of a heap of material obtained after application of certain shock (movement)
c) Angle of difference: diff btw angle of repose and angle of fall
What is an angle of spatula?
angle of slope made by powder heap formed on spatula; insert spatula in powder bed and lift spatula OR form powder bed over spatula and lower the base or lift the spatula
the angle of repose is a qualitative assessment of the internal cohesive and frictional effects under low levels of external loading in die filling or mixing
How to measure angle of repose?
a) fixed funnel and free-standing cone: tan (tether) = Height / Radius (1/2 of base diameter, 2R)
b) Fixed bed cone: tan (tether) = Height / Radius (1/2 of base diameter, 2R)
c) fixed height: correction of heap height needed for apex in funnel stem if fixed funnel method is employed;
H = H’ + h = the heap height + heap contact with the funnel)
tan (tether) = H / R
the lower value of angle of repose –> better flowability
How to compare the angles of repose?
Angle of repose: max angle of inclination
<35o = free flowing >45o = poorly flowing
Use: for comparative flow studies, e.g. btw granules of different formulations or process conditions
What are some precautions for angle of repose measurements?
- no vibration at measurement site
- ensure level and horizontal plane
- conditioned surface characteristics of base
- funnel type and diameter (there should not be arching)
- constant material moisture content
- controlled ambient humidity
- no particle fragmentation
- accuracy of measurement (< +2%)
What are the 2 types of orifice flow (flow test methods)?
A) Volumetric flow (use fixed volume of powder)
B) Gravimetric flow (use fixed weight of powder) - preferred
How to measure orifice flow?
Flow rate = Weight or Vol / Flow time
Laser will be blocked by powder once orifice is open
Detector stops once there is no longer change in laser blocking
- flow a known qty of material through an orifice of known opening size
- flowability reported as time required for known qty (mass/vol) of material to flow through the orifice
- Orifice flow can be adopted as a quick ‘go’; ‘no go’ indicator, particularly for free flowing particulates
- no general scale is available, comparison of materials under the same conditions
What is the method for tapping studies?
- Sieve powder into cylinder
- Remove excess, level
- Weigh
- Put on tapping machine
- Tap to constant volume
How do we read the compressibility index and the hausner ratio?
Tapped density: final volume after tapping (constant volume)
Bulk density: quotient of weight of powder and volume of cylinder
Hausner ratio (HR): Tapped density/ Bulk density (<1.26-1.34)
Carr index (Compressibility Index) (CI): (Tapped - Bulk density) / Tapped density x 100 (<21-25% to pass)
(V0-Vf)/V0 = when value is low, little rearrangement of the particles, indicates good initial packing due to good flow
(V0-Vf)/V0 = when value is high, much rearrangement of the particles, indicates initial packing is loose with void spaces, material is sticky/ cohesive with poor flow
What is the function of powder rheometer in tapping studies/ flow measurements?
Eradication of powder history - conditioning process
gentle loosening and slight aeration of powder; disturb and gently drop each particle in order to construct a homogenously packed powder bed, removing any precompaction or excess air and ensuring the results from the following test are independent of powder history
How does the powder rheometer works?
Rotate impeller down and upwards to measure the resistance of powder –> cause particles to interact, or flow relative to one another
resistance experienced by blade –> difficulty of this relative particle movement/ bulk flow properties
The more particles resist motion and the harder it is to get the powder to flow, the more difficult it is to move the blade.
Work done = energy = (Resistance x Dist travelled)
‘Resistance’ = combined torque and force
Energy gradient = energy per mm of blade travel (calculated from measurements of torque and force)
What is basic flowability energy (BFE) for forced (or confined) flow in powder rheometer?
BFE measured during downward blade movement. Powder confined by closed bottom end of test vessel
Push powder at the side
Shows resistance
Measure the powder flowability; hard to remove from sides or not
e.g. through a screw feeder / active feed frame
What is Specific Energy (SE) for low stress (or unconfined) flow in powder rheometer?
SE: resistance to flow is measured as blade traverses from bottom of vessel to top.
e.g. during low stress filling / low shear blending
As there is no solid surface at the top of the vessel preventing the powder from dilating and moving upwards, the powder is unconfined during the test
