Powder flow Flashcards
why is good flow important
helps evenly distribute drugs and excipients
list 6 things that affect powder flow
-particle size
-particle shape
-moisture
-surface properties
-history of handling
-static changes
how does the particle size affect powder flow
smaller particles have poorer flows as they have a reduced mass so their surface area increases so any forces applied will have an increased effect because there will be more interactions, as size decreases the surface area increases and the cohesive forces increase, small particles cohere to each other causing agglomeration
how does the particle size distribution affect powder flow
monodispersed- one size, minimal contact so good flow
different sizes- more contact, poor flow, sieve to decrease size range, more packing, more interactions
how does the shape of the particle affect powder flow
most ideal shape is spherical however the most common is irregular and flake/needle shaped, these particles interlock with each other making the powder harder to flow, causes bridging which causes rougher particles
how does the surface propeties affect powder flow
material could have suffered from electrostatic charge or adsorption of water, hygroscopic
how does the history of handling affect powder flow
powder could have collapsed during transport
what does hygroscopic mean
attracts moisture
name two ways to predict flow properties
angle of repose and carr’s index
what is the angle of repose
angle of free surface of a pile of powder to a horizontal plane, the lower the angle the better flow, 40< is poor flow
how to measure the angle of repose
pour powder from funnel, trace around edge of powder cone, measure from surface to tip of the triangular powder shape, use tan to calculate angle
what is bulk density
effected density of a powder, volume of powder with air idk
equation of bulk density
Pb=m/v
units of bulk density
Kg.m^-3
what angle of repose is considered good flow
angles below 40 degrees
why does powder segregate when pouring
difference in particle sizes, smaller powder clumps in the middle, larger powder has more kinetic energy so rolls off of the smaller ones and travels to the edge of the cone, based on size
what is Carr’s index
relates to the change in volume of a powder in its fluffed to tapped state
what is Carr’s index formula
tapped-fluffed/tapped x100
what does fluffed state and tapped state mean
fluffed- just poured, lots of air
tapped- particle collapsed, air removed, controlled agitation
what percentage is considered good and bad in Carr’s index
good = around 10%
bad = 25%<
what does a bigger % calculated in the Carr’s index mean
bigger %=bigger cjange in density
what is mass flow and tunnel flow in hoppers
mass flow- all material in hopper in motion
tunnel flow- centrally moving core, slow
why is tunnel flow an issue
uneven mixture, stuff at side slowly degrades, centre moves faster, demixes
how can you improve the powder flow in hoppers
reduce hopper angle
