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Questions and Answers
What is the term used to describe the relative amount of air entrapped between particles in a powder?
What is the term used to describe the relative amount of air entrapped between particles in a powder?
Which packing arrangement has the highest porosity?
Which packing arrangement has the highest porosity?
What particle distribution gives the densest packing arrangement?
What particle distribution gives the densest packing arrangement?
How is porosity influenced when using powders of different sizes?
How is porosity influenced when using powders of different sizes?
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In the measurement of powder porosity, what is the result of placing powder in a measuring cylinder without any disturbance?
In the measurement of powder porosity, what is the result of placing powder in a measuring cylinder without any disturbance?
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What is the significance of the term 'poured bulk volume'?
What is the significance of the term 'poured bulk volume'?
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Which formula accurately represents porosity?
Which formula accurately represents porosity?
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What does the term 'tapped density' refer to?
What does the term 'tapped density' refer to?
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What does a large difference between poured and tapped densities indicate about powder flow?
What does a large difference between poured and tapped densities indicate about powder flow?
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What is the purpose of determining 'tapped density'?
What is the purpose of determining 'tapped density'?
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How is apparent porosity best described?
How is apparent porosity best described?
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Which Carr’s compressibility index percentage categorizes powder flow as 'Excellent'?
Which Carr’s compressibility index percentage categorizes powder flow as 'Excellent'?
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What is the relationship between packing fraction 'k' and porosity 'ε'?
What is the relationship between packing fraction 'k' and porosity 'ε'?
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What is the Hausner ratio's role in assessing powder flow?
What is the Hausner ratio's role in assessing powder flow?
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What is the main action during the tapping process in a tap volumeter?
What is the main action during the tapping process in a tap volumeter?
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If a powder has a Hausner ratio greater than 1.5, what does it indicate about its flow?
If a powder has a Hausner ratio greater than 1.5, what does it indicate about its flow?
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Which of the following defines the 'void volume' of a powder?
Which of the following defines the 'void volume' of a powder?
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What percentage of Carr's index suggests a 'Fair to passable' flow?
What percentage of Carr's index suggests a 'Fair to passable' flow?
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According to the data presented, what can be inferred about corn starch's behavior during tapping?
According to the data presented, what can be inferred about corn starch's behavior during tapping?
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Which statement is true regarding lactose’s flow properties in the context provided?
Which statement is true regarding lactose’s flow properties in the context provided?
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What effect can glidants have on powders with a Carr’s index in the range of 23-35?
What effect can glidants have on powders with a Carr’s index in the range of 23-35?
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Study Notes
Preformulation - Bulk Powder Properties
- Powders exist in a variety of packing states, from loosely packed (maximum airspace, minimum contact points) to tightly packed (minimum air entrapped, maximum interparticulate contact).
- Porosity refers to the relative amount of air entrapped between particles. Porosity depends on particle shape and size. Smaller particles fill voids between larger ones.
- A packing arrangement of roughly 75% coarse particles and 25% fine particles results in the densest packing.
- Different packing arrangements (like cubic and rhombohedral packing) have different porosities. Cubic packing has an approximate porosity of 48%, and rhombohedral packing has an approximate porosity of 26%. Shapes like cubes exhibit significant differences, with porosity ranging from 0% (for perfect cubic packing) to approximately 55%.
- Coarse and fine particles influence packing density. Increasing the amount of fine particles generally results in an increase in packing density.
- Determining porosity involves placing powder in a measuring cylinder without disturbance (loosest packing state). This volume is known as poured bulk volume. The reciprocal is referred to as poured bulk density. A related calculation considers void volume. Porosity (ε) is calculated as (Vb - Vp)/Vb = 1-(Vp/Vb), where Vb is bulk volume and Vp is particle volume. Porosity is sometimes expressed as a percentage.
- Determining tapped density involves using a tapping volumeter to measure the minimum bulk volume after repeated tapping. This volume is known as tapped volume (Vt). The reciprocal is tapped density ( ρt).
- Tapping, especially, helps determine dynamic powder packing by monitoring density changes with time, i.e., how the number of taps affects powder density.
- Packing and flow are closely related. A larger difference between poured and tapped densities indicates more interparticulate forces. The number of taps needed to reach a tight packing state also increases with interparticulate forces.
- Carr's compressibility index and Hausner ratio use powder packing data to estimate powder flow. Carr's compressibility index = ((Tapped density-bulk density)/Tapped density) x 100. A lower index indicates better flow.
- Carr's index values correlate to flow types. Values between 5-15 indicate excellent flow, 12-16 good, 18-21 fair to passable, 23-35 poor, 33-38 very poor, and above 40 extremely poor. Flow can be improved with glidants (e.g., 0.2% Aerosil).
- Hausner ratio = tapped density/poured density. A ratio less than 1.25 indicates good flow, 1.25-1.5 indicates fair to passable flow, and greater than 1.5 suggests poor flow. Glidants often improve flow between 1.25 and 1.5, but not always over 1.5.
- Examples (like corn starch, lactose, and MCC) exhibit various flow properties and characteristics, and these properties depend on particle size, shape, aspect ratio, etc. This illustrates how the particle properties relate to flow properties.
- Angle of repose is the maximum angle between powder surface and a horizontal plane. Lower values indicate better flow, as it corresponds to fewer interparticulate attractive forces affecting flow characteristics. The angle of repose is influenced considerably by the rougher and more irregular the surface of the particles. Different flow conditions can be determined based on the angle.
- Hopper design influences flow. Two fundamental flow types are funnel flow and mass flow. Funnel flow often displays rat-holing and other difficulties; mass flow tends to avoid this but often requires specific manufacturing designs.
- Improper emptying (inadequate emptying) is often a result of insufficient cone angles in funnel flow in hoppers.
- Critical orifice diameter is crucial to preventing arching and rat holing in hoppers. Flow rate is linked to the ratio of orifice diameter to mean particle diameter (Do/Dp). A ratio of less than 6 often indicates good flow, while a ratio close to 20-30 can lead to high flow rates. However, reducing the ratio below this point will likely result in the reduced flow or arching forming.
- Glidants, aeration, hoppers, and equipment (like screw feeders and vacuum systems) are used to improve powder flow.
Flow improvement
- Particle Properties: Size, shape, charge, moisture
- Formulation: Glidants
- Equipment: Hoppers, aeration, force feeders, vacuum
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Description
Explore the complex properties of bulk powders, focusing on factors like packing states, porosity, and the influence of particle shape and size. Understand how different packing arrangements affect density and how the combination of coarse and fine particles plays a critical role in packing efficiency.
