1 - powder flow

Question 1

Flowability

Answer 1

Defines the ability of a powdered material to flow from a stationary to a moving condition

A free-flowing powder moves consistently and steadily with particles moving independently of one another (in a predictive manner)

A non-free flowing powder contains cohesive (stick together) particles moving as agglomerates.

Question 2

Floodability

Answer 2

Describes the unstable, liquid-like flow of powder

Flooding can be caused by fluidising a mass of particles by air or other stimuli

This type of flow is uncontrollable, and usually undesirable.

Question 3

Poor flow problems

Answer 3

(First one) Bridging or arching
Where the powder material that is being discharged forms a bridge or arch over the discharge point in a hopper

Rat-holing
Where the powder material forms a hole or narrow channel above the feed outlet in a hopper while the remaining material is stationary against the hopper wall

Question 4

Ways to combat product flow issues:

Answer 4

Ways to combat product flow issues:
Hopper design
Conical and angled hoppers both create a distinct flow path for the material

Vibration( onto the hopper), conditioning screws, and fluidizing nozzles
On the discharge cone of a silo system or a hopper for example

Question 5

Importance of free-flowing powders

Answer 5

improve mixing
dose uniformity
reproducibility
reduce tabletting issues

Question 6

Single punch tabletting machine

Answer 6

FILL
COMPRESS
EJECT

Question 7

particle properties adhesion and cohesion

Answer 7

Adhesion
Unlike surfaces e.g. powder and feed hopper, powder and die wall.
Due to Forces of electrostatic attraction

Cohesion
Like surfaces such as component particles of a bulk solid
Short range forces: e.g. Van der Waal’s forces

Question 8

What are the different direct methods and indirect methods of powder flow

Answer 8

Direct methods: Hopper flow rate/flow meter.

Indirect measurements:
Angle of Repose
Angle of Difference
Carr’s consolidation index
Hausner ratio
Angle of Spatula
Shear strength
Tensile strength

Question 9

Flow through hopper - explain the methods that we use

Answer 9

Time taken for a known amount of powder to flow through the hopper

1. Shutter over outlet and hopper filled
2. Remove shutter
3. Time to discharge completely is monitored

AND

Recording flow meter: This method is similar to the previous method except that powder is allowed to discharge from a hopper onto a balance.

Powder onto balance
Increase in mass with time
Determine mass flow rates
Quantifies the uniformity of flow

Question 10

Angle of Repose:

Answer 10

When a powder is poured through a funnel, it forms a cone-shaped pile which makes an angle to the horizontal.

-A large angle of repose, is indicative of poor flow properties and a small angle indicates a free-flowing powder.

Question 11

Angle of Repose: Poured method

Answer 11

Flowability is performed
by flowing powder through
a funnel with a defined orifice

If the powder does not flow ?
Use drained method

Question 12

Angle of FALL

Answer 12

The results after disturbing the initial (i.e., before impact) angle of repose heap.

LOW angle of fall = GOOD flowability

Question 13

Angle of difference

Answer 13

The calculated difference between the angle of Repose and the angle of Fall.

HIGH ANGLE OF DIFFERENCE = HIGH FLOWABILITY

Question 14

Carr’s Consolidation Index (Compressibility)

Answer 14

Ability of powder to consolidate (compress)
Rate of packing down
Compares initial and final bulk volumes
Initial density: fluffed, poured, bulk
Tapped density: density after 50 taps, final, consolidated
It is a one-point determination and does not reflect the ease or speed with which consolidation can occur.

LOW CONSOLIDATION INDEX - HIGH FLOW

Question 15

Hausner ratio

Answer 15

<1.25 good flow
>1.25 poor flow
1.25 - 1.5 suggests the addition of a glidant.

its tapped density/fluff density

Question 16

Angle of Spatula

Answer 16

LOW angle of spatula = HIGH flowability

Question 17

Factors affecting flowability - PHYSICAL FACTORS

Answer 17

Physical Factors
Particle size distribution
The presence of fine particles
Shape of particles
Friction Coefficient of the particle surface
Cohesiveness of powder
Adhesiveness of powder
Electrostatic charge on particles
Compressibility of powder
Moisture content of powder
Temperature of powder

Question 18

Environmental factors influencing powder flowability

Answer 18

Humidity of the atmospheric air
External pressure
Vibrations

Question 19

Excipients used to improve powder flow

Answer 19

Can add Glidants
Some lubricant and anti-adherent
Reduce adhesion and cohesion

(inc. flow activators, glidants, lubricants, antiadherents). E.g. talc, maize starch, magnesium stearate, colloidal silica