flow_3 Flashcards
flow testing methods
- angle of repose
- orifice flow
- tap density
- avalanche flow
- powder rheometer
reason for poor powder flow
- surface forces
- inter-particle friction
- interlocking of particles
what are cohesion and adhesion due to
- non-specific van der waals forces
- moisture
- electrical charge
- particle size
- contact sites
- time of contact
- crystalline bridges
cohesion
sticking together of particles of the same substance
adhesion
adhesion
sticking together of dissimilar particles or surface
overcoming flow issues
- container (hopper)- improve design
- conveyance- paddle, auger, suction: type and design
- formulation- add glidant, lubricant
powder flow problems
- arching (bridging)
2. ratholing
what is arching
an arch-shaped obstruction forms above discharge outlet and prevents flow. It may be due to mechanical interlocking of large particles forming an arch, or a cohesive arch when particles bond together
potential problem of arching
will be significant when outlet is less than 6-10 times the maximum particle size of particles
what is ratholing
occurs when discharge takes place only in central flow channel located above the outlet, the cohesive material outside channel will not flow and may cake or agglomerate
what are gildants
additives that are added to improve flowability of powders or granules; require a certain concentration range for optimal activity, excess may be detrimental
mechanism of action of glidants
- ball bearing effect : correct surface irregularity and rolling action
- reduce interparticulate friction
- reduce surface change
how does ball bearing effect work
associated with the rolling effect of small spherical particles, decreasing friction due to rough surfaces by physical separation (eg. starch)
effect of reducing friction
fine gildant powders will coat larger host particles thereby increasing separation distance between the large particles and hence, reduce interparticulate attractive forces (van der waals forces) (eg. fumed silica)
angle of repose
angle of inclination of a heap of powder carefully built up by dropping material (usually through an appropriate sieve), through a funnel onto a horizontal plate
angle of fall
angle of repose of a heap of material obtained after the application of certain shock (eg. tap 3 times)
angle of difference
difference between the angle of repose and angle of fall
angle of spatula
angle of slope made by a powder heap formed on a spatula when inserted into a powder bed and lifting up
relationship of angle of repose to flow
lower angle, better flow
< 35 degree: free flowing
>45 degree: poorly flowing
Factors to consider during measurement
- no vibration at measurement site
- ensure elvel and horizontal plane
- conditioned surface characteristic of base
- funnel type and diameter (no arching)
- constant material moisture content
- controlled ambient humidity
- no particle fragmentation
- accurate of measurement (< +2%)
volumetric flow
using a fixed volume of powder
gravimetric flow
using a fixed weight of powder
orifice flow
flow of known quantity of material through an orifice of known opening size
how is flowability reported with orifice flow
reported as time required for known quantity (mass or volume) of material to flow through orifice
tapping method
- sieve powder into cylinder
- remove excess, level
- weigh
- put on tapping machine
- tap to constant volume
how does tapping reflect flow
the lower the hausner ratio/ carr index , the better the flow
how to count hausner ratio
tapped density/ bulk density
how to find carr index
measure compressibility index
(tapped -bulk density) / tapped density x 100
how to eradicate powder history
conditioning process involves gentle loosening and slight aeration of powder. this is to disturb and gently drop each particle in order to construct a homogenously packed powder bed, removing any precompaction or excess air and ensure the results from the following test are independent of powder hx
basic flowability energy BFE
is a measure of the powder’s flowability when forced to flow, such as through a screw feeder or in an active feed frame
- measured during the downward blade movement
- forced/ confined flow
specific energy SE
measure of the powder’s flowability when unconfined, such as during low stress filling, or low shear blending
- resistance to flow is measured as blade transverse from bottom to top
- slow stress/ unconfined flow
revolution powder tester
determine avalanche flow characteristics
what can the revolution measure
- flowability of powders in low stress situations
- study how powder behaves once it is flowing
- determine the condition of powder as it moves through process
shear cell
splits horizontally, forming shear plan between lower stationary base and upper moveable portion and could provide information regarding flowability via a number of indices
shear cell data
measure flow factor, more suitable for cohesive material
schulz rotational shear cell tester
annular type, well established test method
brookfield rotational shear cell tester
annular type
- most recent commercial shear tester
- most automated with minimum operator contact
FT4 rotational shear cell tester
- rotational type shear tester
- relatively new equipment
- equipment has other capabilities for bulk characterization
- quick operation
what are particles responsible for?
- flow
- packing
- mechanical strength
- solubility
methods of size reduction
- hammer mill
- pin mill
- cone mill
- fluid energy / jet mill <10 micrometer
particle production methods
- crystallization
- milling
- spry drying/ spray congealing
- compact and mill
- fluid bed coating
