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Form & Tech CA3 Rev > Particle Flow & Flow measurements > Flashcards

Particle Flow & Flow measurements Flashcards

1
Q

Three causes of poor powder flow

A
  1. Surface forces
  2. Inter-particle friction
  3. Interlocking of particles, which can be affected by shape of particle
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2
Q

Name some examples of surface forces that causes cohesion and adhesion of particles

A
  1. Non-specific vdw forces
  2. Hydrogen bond from moisture
  3. Electrical charge in dry particles
  4. Small particle size providing large surface area for bonding
  5. Contact sites
  6. Time of contact which affect air-cushion around particles
  7. Crystalline bridges from drying and condensation
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3
Q

Describe 2 powder flow problems that may arise in hoppers. What is one modification that can be made to the hopper to overcome each problem?

A
  1. Arching (bridging): arch-shaped obstruction formed due to interlocking or bonding of particles to form an arch. Significant when outlet is less than 6-10x maximum particle size
    - Solution: Increase size of hopper outlet
  2. Ratholing: discharge only happens in the central flow channel above outlet, while cohesive material outside channel does not flow, and may agglomerate
    - Solution: Use steeper angled hopper
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4
Q

Define Glidants

A
  • Additives added to improve flowability of powders or granules
  • Requires a certain concentration range for optimal activity, as excess may be detrimental
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5
Q

Describe the different Mechanisms of Action of glidants that help to improve flowability of powders or granules. Name an example of glidant that uses each mechanism of action

A
  1. Ball bearing effect: Help to separate particles and correct surface irregularity which make the particles rounder to enable better rolling.
    E.g. Fumed silica
  2. Reduce friction: fine glidant particles coat larger host particle, increasing separation distance between large particles and reducing interparticle attractive forces (vdw)
    E.g. fumed silica
  3. Reduce surface charge: disrupts ionic charges at the surface of particles, hence decreasing electrostatic attraction between them
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6
Q

Name some test methods to determine powder flowability

A
  1. Measurement base on angles
    - Repose, fall, difference, spatula
  2. Flow rate via orifice flow
  3. Resistance of powder to flow via Powder rheometer
  4. Avalanche angle via Revolution powder tester
  5. Flow factor via Shear cell
  6. Hausner ratio and Carr index via Tapping studies
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7
Q

Describe Angle of Spatula method to determine flowability

A
  • Angle of slope made by powder heap formed on spatula
  • Qualitative assessment of internal cohesive and frictional effects under low levels of external loading in die filling or mixing
  • Simple and practical technique to measure resistance to particle movement
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8
Q

Distinguish between angle of repose and angle of fall

A
  • Angle of repose: Angle of inclination of heap of powder built by passing material through a sieve onto a horizontal plate
  • Angle of fall: angle of heap of material after application of certain shock (E.g. tapping)
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9
Q

What is angle of difference and what is it commonly applied in? What is an advantage of using angle of difference in determining flowability?

A

Angle of difference = Angle of repose - angle of fall

  • Commonly used to determine flow of poorly flowing micronised particles
  • Angle of difference is more consistent compared to angle of repose and angle of fall
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10
Q

For angle of repose, measuring the height and base diameter is difficult in a fixed-funnel and free-standing cone. What are the 2 methods to allow easier measurement of height and diameter respectively?

A
  1. To measure height easily: Fixed bed cone (fixed base diameter)
  2. To measure diameter easily: Fixed height (fixing height of funnel hence height of cone)
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11
Q

Advantage of using fixed height cone to determine powder flowability

A

Can fix the height of cone to be very low such that very little powder is needed to measure flowability. Good for powder in R&D stage

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12
Q

“The angle of repose of this solid is 60º, hence it is a solid with bad flowability”. Comment on this statement

A

60º does not mean it is a solid with bad flowability as it depends on the context.

Angle of repose is mainly used for comparative flow studies.
E.g. between different type of printer ink.

Hence the statement may be true only if the solid was compared with another another solid for the same purpose but made differently

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13
Q

Name some precautions when carrying out angle of repose measurements

A
  • No vibration during measurement
  • Ensure horizontal, levelled plane
  • Conditioned surface characteristics of base (e.g. put powder and condition it first)
  • Ensure funnel orifice large enough to prevent arching
  • Constant material moisture content (e.g. can store in humidity-controlled room)
  • Controlled ambient humidity (related to moisture content)
  • No particle fragmentation
  • Accurate measurement (should be <2% discrepancy, do at least 5x replicated measurement for angle of repose)
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14
Q

What are the two types of orifice flow method used to determine flowability?

A
  1. Volumetric flow (i.e. fixed volume of powder)

2. Gravimetric flow (i.e. fixed weight of powder)

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15
Q

Formula used in orifice flow

A

Flow rate = Weight or Volume of powder/Flow time

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16
Q

Can orifice flow be used to conclude that a powder is good or bad flowing?

A

No, there is no general scale for that. Orifice flow is used to compare flow of materials under the same condition

17
Q

Describe the steps to carrying out tapping studies to determine flowability

A 
- Weigh cylinder
Sieve powder into measuring cylinder
- Remove excess and level
- Weigh cylinder + powder
- Measure W/V
- Transfer to tapping machine
- Tap, and measure V over number of taps
18
Q

Using tapping studies, how can we compare flowability of powders?

A

The powder with less change in volume after the same number of tap as the other is the powder with greater flowability

This is because the better flowing powder has good initial packing hence closer to its optimal volume

19
Q

Formula for Hausner ratio and Carr index from tapping studies

A

Hausner ratio = Tapped density/ Bulk density

Carr index = (tapped - bulk)/tap x 100

  • Tapped density: Final volume after tapping (constant volume)
  • Bulk density: Weight of powder/Volume of cylinder
20
Q

Largest source of error from tapping studies, and how to overcome it

A

Bulk density, due to agitation of poorly flowing powder to get it to flow and fill the cylinder which causes wide variability. This leads to inaccurate Hausner ratio and Carr index

Solution: Use a sieve to allow powder flow into measuring cylinder

21
Q

What must be done before starting the powder rheometer to measure flowability?

A

Condition powder by moving stirrer gently, which helps to remove compression and excess air to ensure that results from tests are independent of powder history

22
Q

What does powder rheometer measure?

A

Resistance of powder to flow using a rotating impeller both downwards and upwards.

This resistance is represented by work done required by the combined torque and force of the impeller to penetrate the powder bed

23
Q

Distinguish between Basic Flowability Energy (BFE) and Specific Energy (SE) obtained from using powder rheometer to determine flow

A

BFE: measured from forced/confined flow, when the blade of the powder rheometer moves downwards.

SE: Measure of powder’s flowability when unconfined, when blade moves from bottom upwards. Powder is unconfined as there is no solid surface on top of the powder

24
Q

The graph obtained from powder rheometer is energy gradient against height. What does the AUC represent?

A

Total Flow Energy

25
Q

Two main measurements that can be obtained using the revolution powder tester?

A
  1. Avalanche angle (angle in which the solid topples)

2. Frequency of avalanche

26
Q

Describe the process of Shear cell to test powder flowability

A

For a poor flowing powder:

  • Powder inside cell is consolidated via stress
  • Then cell will attempt to break the consolidated power
  • The torque/force required to break the powder is measured
27
Q

What is the graph that can be obtained from Shear cell data, and the measurement that can be obtained

A

Graph: Unconfined failure strength against major principal consolidation stress

Measurement obtained: Flow factor (1/gradient of graph)

28
Q

Name three types of rotational shear cell testers

A
  1. Schulz
  2. Brookfield
  3. FT4
29
Q

Why is particle design important in the pharma industry

A

In pharma industry: Particles responsible for the characteristics of solid dosage forms such as:

  • Flow
  • Packing
  • Mechanical strength
  • Solubility, stability, etc.

Important to optimise particles while balancing for flow, stability and dissolution

E.g. larger particles can flow better, but has poorer dissolution

Form & Tech CA3 Rev (10 decks)

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