Powder flow testing and control

Introduction:
Powder flowability is the ability of a powder to flow in a
desired manner in a specific piece of equipment.
Flow of powders may be:
Free-flowing
 Non-flowing or Cohesive
Manufacturing of tablets, capsules, filling of powder in container
involves several powder handling steps, including blending,
transfer, storage, and feeding to a press or a dosator.
The inability to achieve reliable powder flow during these steps
can have a significant adverse effect on the manufacture and
release of a product to market.

Flow Patterns:
Two flow patterns developed when powder flow from container:
Funnel Flow
Mass Flow
Funnel Flow
• Side wall material
stagnant whereas flow
of funnel- shaped
material take place
• First-in-last-out
• Chance of powder
segregation
• Suitable only for free
flowing powder
Mass Flow
• All of the material is in
motion
• First- in-frist-out
• Minimize segregation
and prevent rat-holing

Factors Affecting Powder Flow Properties:
The specific properties of a powder that affect its flow are known as
flow properties.
These flow properties depend upon:
Collective
forces acting on
individual particles
Particle variable
Environmental
condition
Powder or Particle
variables:
• Particle size, Size
distribution, Shape, Surface
texture, Cohesively, Surface
coating, Particle
interaction, Wear or attrition
characteristic, Propensity to
electro-static
charge, Hardness, Stiffness, Str
ength, Fracture toughness
External Factors
influencing Powder
Behaviour:
• Flow rate, Compaction
condition, Humidity, Electro-
static charge, Aeration,
Transportation experience,
Container surface effects,
Storage time

Consideration of critical Factor:
 Particle Size
 Particle Nature
 Porosity
 Bulk Density
 Moisture Content
 Interparticle force
Van der Waals
Electrostatic
Surface tension
Interlocking
Friction, etc
During Storage and Transportation
Caking and bridging
Packaging condition
High to low aeration
De-aeration
Consolidation by tapping

Powder Flow Problem:
1. Rat-holing
2. Arching
3. Flooding
 Prevention of flow problem:
Increased outlet diameter
Reduced filling height powder
Equipment capacity can be reduced
Reduced cohesive strength of powder
Agitation or mechanical assistance can be utilized

Measurement of Flow Properties:
1. Angle of repose:
 The internal angle between the surface of the pile and the horizontal
surface is known as the angle of repose.
Angle of repose depends upon:
 Density
 Surface area
 Shapes of the particles
 The coefficient of
friction of the material
 This angle is in the range
0 –90 .
Flow Property Angle of repose (Degrees)
Excellent 25–30
Good 31–35
Fair—aid not needed 36–40
Passable—may hang up 41–45
Poor—must agitate, vibrate 46–55
Very poor 56-65
Very, very poor ≥ 65

Angle of repose method used interparticulate friction or resistance
to movement between particles.
Experimental Variable:
Segregation of material
Consolidation
Aeration
Methods for determining the static angle of repose :
Fixed height method
Fixed base diameter method
Tilting box method
Methods for determining the dynamic angle of repose:
Revolving cylinder method

Avalanching Tester:
Used to determine dynamic angle of
repose.
MTA (mean time to avalanche)
determine and at the time of
avalanching image was captured.
Aero-Flow™ Automated
Powder Flowability
Analyzer

2. Carr’s Compressibility Index And Hausner Ratio:
Both are determined by measuring the bulk volume and the
tapped volume of a powder.
Compressibility
Index (%)
Flow Character Hausner Ratio
≤10 Excellent 1.00-1.11
11-15 Good 1.12-1.18
16-20 Fair 1.19-1.25
21-25 Passable 1.26-1.34
26-31 Poor 1.35-1.45
32-37 Very poor 1.46-1.59
≥37 Very, very poor ≥1.60

3. Flow Through an Orifice:
Useful only for free-flowing materials.
Two types of flow rate of powder determined:
Mass flow rate
Volume flow rate
Experimental variable:
The type of container
The size and shape of the orifice used
Height of the powder bed
General guidelines for dimensions of the cylinder are as follows:
Diameter of opening > 6 times the diameter of the particles
Diameter of the cylinder > 2 times the diameter of the opening

4. Shear Cell Method:
In the shear cell method, the force necessary to shear the powder
bed by moving the upper ring is determined.
Types of shear cell method:
Cylindrical shear cell
Annular shear cell
Plate-type shear cell
4.1 Brookfield PFT
Choice of Test Options:
Flow Function
Time Consolidation
Test with Flow Function
Wall Friction
Data Output:
Flow factor Index
Arching Dimension
Rat-hole Diameter
Hopper Half Angle
Wall Friction
Bulk Density Curve

Flow factor index Flowability
ff<1 Non flowing
1<ff<2 Very cohesive
2<ff<4 Cohesive
4<ff<10 Easy flowing
ff≥10 Free flowing
The standard classification of powder
flowability uses the flow factor index (ff):
It is ideal for manufacturers who process powders daily and want to
minimize or eliminate downtime and expense that can occur when
hoppers discharge erratically or fail to discharge altogether.
Consolidation stress

5. Cohesion Index:
Stable Micro Systems TA.XT Plus texture
analyzer used to determine cohesion
coefficient which is determined by
integrating the negative areas under
the force displacement curve.
Cohesion Index Flow behaviour
≥19 Hardened, extremely
cohesive
16-19 Very cohesive
14-16 Cohesive
11-14 Easy flowing
≤11 Free flowing
A low cohesion index is associated with
non-cohesive free-flowing powders.

6. FT4 Powder Rheometer:
This methodologies allow measurement of:
Flow energy
Shear properties
Bulk properties
The forces causing the deformation and flow is measured.
FT4 flowmeter Downward testing mode Upward testing mode
Calibration and Data Analysis:
The variables are force, torque, height, and rotational and linear
velocities, flow properties determined via extrapolation.

Downward Test Mode:
It is used to measure the Basic Flow Energy (BFE)
Upward Testing Mode:
It is used to measure the Specific Energy (SE) which is the flow energy
per gram of powder tested.

7. PTG-S4 Powder Characterization Instrument:
Features:
Fully USP <1174>, EP <2.9.36> and
ISO 4324 (12/83) compliant
Measure flow-time, cone angle,
flowability
PTG-ER stirrer included
Integrated analytical balance cell
Integrated dust protection bonnet
Built-in printer

8. Penetrometry:
The pressure of penetration in Pascal was used
to estimate flow rate.
Particle size should be in the range of
0.250-0.630 mm.
Used for non-consolidated pharmaceutical
powder excipients: sodium chloride,
sodium citrate, boric acid, and sorbitol.
A polynomial quadratic equation was generated using the ratio of
penetration pressure to bulk density, from which the flow rates for the
group of all tested powders could be estimated.

Control of powder flow
1. Mucon Iris Diaphragm Valve
2. Powry Valve
3. Vacuum Assisted Powder Flow Nozzle
4. Funken Continuous Auto Feeder
5. Fluidizing Hopper Flow Control Valve
Mucon Valve
Powry Valve

Importance of Flow Properties in Pharmacy:
1. Tablet Manufacturing
2. Capsule Manufacturing
3. During Drug Delivery
4. During Mixing and Sieving
5. During Unloading of Packs
6. During Transportation Through Conveyers
7. During Filling of Powders, Dry Suspension and Dry Syrup

Conclusion:
Powder flow is a key requirement for pharmaceutical manufacturing
process.
An understanding of the fundamentals of powder flow can steer
one in the right direction to achieve reliable powder flow.
 Continuous and even flow of powder during tablet and capsule
manufacturing reduced weight variation, content uniformity (also
hardness in tablet).
Powder flow is also crucial during mixing, packaging, and transportation
Reduction in process costs and maintaining the quality and consistency
of the final product

Reference:
1. Jenike, A.W., Storage and Flow of Solids (Bulletin 123 of the Utah
Engineering Experimental Station), 53 (26), (1964, revised 1980).
2. Prescott, J.K., and Barnum R.A., On powder flowability, Pharmaceutica
Technology, October 2000, pp. 60-84 and 236.
3. Prescott, J. K. and Hossfeld, R. J., Maintaining product uniformity and
uninterrupted flow to direct compression tablet presses.
Pharmaceutical Technology, 18 (6), 1994, pp. 99-114.
4. Baxter, Thomas J., “When Powders Flow Like Water: Addressing
Two-Phase Flow Effects in Tablet Press Feed System”, Tablets &
Capsules, March 2009, Volume 7, No. 2, pp. 26-32.
5. Barnum, Roger, Ebb and Flow: Understanding Powder Flow Behavior,
Pharmaceutical Processing, March 2009, pp. 18-21.

Powder flow testing and control

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