Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Solid > Flow > Flashcards

Flow Flashcards

1
Q

What are the causes of poor powder flow?

A
  • surface forces (smaller particles have larger surfaces, creates more attractive forces)
  • inter-particle friction (may require lubrication)
  • interlocking of particles (may be present in fibrous materials)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What factors results in the cohesion and adhesion of surface forces?

A
  • non-specific VDW forces
  • moisture
  • electrical charge
  • particle size
  • contact sites
  • time of contact
  • crystalline bridges
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is the difference btw cohesion and adhesion?

A

Cohesion: particles of same substance stick together

Adhesion: particles of dissimilar particles or surfaces stick together

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

How to overcome flow issues?

A
  • Container (hopper) - improve design
  • Conveyance - paddle, auger, suction: type, design
  • Formulation - add glidant, lubricant
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What are some powder flow problems (esp in orifices)?

A

a) Arching (bridging): arch-shaped obstruction forms above discharge outlet and prevents flow; may be due to mechanical interlocking of large particles/ cohesive arch when particles bond together; potential problem significant when outlet <6-10 times the maximum particle size of particles
b) Ratholing: discharge only in central flow channel located above outlet; stuck at sides; cohesive material outside channel will not flow and may cake or agglomerate. (solution = need a steeper design)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

When does arching occurs?

A

Opening too small

funnel diameter needs to be >6-10x the largest particle of the powder

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is the purpose of glidants?

A
  • additive; improves flowability of powders or granules

- require certain conc range for optimal activity, excess may be detrimental

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the mechanism of action of glidants?

A
  • ball bearing effect: correct surface irregularity; rolling action
  • reduce interparticulate friction
  • reduce surface charge

glidants can also go in btw the particles and fill the irregular spaces –> reduce surface irregularity –> thus, reduce friction and charge

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What are some examples of glidants?

A 
talc (1-5%)
corn starch (5-10%)
colloidal silica (0.5-3%)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

How does the ball bearing effect works?

A

Rolling effect of small spherical particles, decreasing friction due to rough surfaces by physical separation

e.g. starch

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

How does reducing friction in powders work?

A

Fine glidant powders will coat larger host particles, thus increasing the separation distance btw the large particles, and hence, reduce interparticle attractive forces (VDW forces)

e.g. fumed silica

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What are the types of angles of repose (flow test methods)?

A

a) Angle of repose: angle of inclination of a heap of powder carefully built up by dropping material (usually through appropriate sieve) through a funnel onto a horizontal plate.
b) Angle of fall: angle of repose of a heap of material obtained after application of certain shock (movement)
c) Angle of difference: diff btw angle of repose and angle of fall

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is an angle of spatula?

A

angle of slope made by powder heap formed on spatula; insert spatula in powder bed and lift spatula OR form powder bed over spatula and lower the base or lift the spatula

the angle of repose is a qualitative assessment of the internal cohesive and frictional effects under low levels of external loading in die filling or mixing

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

How to measure angle of repose?

A

a) fixed funnel and free-standing cone: tan (tether) = Height / Radius (1/2 of base diameter, 2R)
b) Fixed bed cone: tan (tether) = Height / Radius (1/2 of base diameter, 2R)

c) fixed height: correction of heap height needed for apex in funnel stem if fixed funnel method is employed;
H = H’ + h = the heap height + heap contact with the funnel)

tan (tether) = H / R

the lower value of angle of repose –> better flowability

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

How to compare the angles of repose?

A

Angle of repose: max angle of inclination

<35o = free flowing
>45o = poorly flowing

Use: for comparative flow studies, e.g. btw granules of different formulations or process conditions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What are some precautions for angle of repose measurements?

A
  • no vibration at measurement site
  • ensure level and horizontal plane
  • conditioned surface characteristics of base
  • funnel type and diameter (there should not be arching)
  • constant material moisture content
  • controlled ambient humidity
  • no particle fragmentation
  • accuracy of measurement (< +2%)
17
Q

What are the 2 types of orifice flow (flow test methods)?

A

A) Volumetric flow (use fixed volume of powder)

B) Gravimetric flow (use fixed weight of powder) - preferred

18
Q

How to measure orifice flow?

A

Flow rate = Weight or Vol / Flow time

Laser will be blocked by powder once orifice is open
Detector stops once there is no longer change in laser blocking

  • flow a known qty of material through an orifice of known opening size
  • flowability reported as time required for known qty (mass/vol) of material to flow through the orifice
  • Orifice flow can be adopted as a quick ‘go’; ‘no go’ indicator, particularly for free flowing particulates
  • no general scale is available, comparison of materials under the same conditions
19
Q

What is the method for tapping studies?

A
  1. Sieve powder into cylinder
  2. Remove excess, level
  3. Weigh
  4. Put on tapping machine
  5. Tap to constant volume
20
Q

How do we read the compressibility index and the hausner ratio?

A

Tapped density: final volume after tapping (constant volume)
Bulk density: quotient of weight of powder and volume of cylinder

Hausner ratio (HR): Tapped density/ Bulk density (<1.26-1.34)

Carr index (Compressibility Index) (CI): (Tapped - Bulk density) / Tapped density x 100 (<21-25% to pass)

(V0-Vf)/V0 = when value is low, little rearrangement of the particles, indicates good initial packing due to good flow

(V0-Vf)/V0 = when value is high, much rearrangement of the particles, indicates initial packing is loose with void spaces, material is sticky/ cohesive with poor flow

21
Q

What is the function of powder rheometer in tapping studies/ flow measurements?

A

Eradication of powder history - conditioning process
gentle loosening and slight aeration of powder; disturb and gently drop each particle in order to construct a homogenously packed powder bed, removing any precompaction or excess air and ensuring the results from the following test are independent of powder history

22
Q

How does the powder rheometer works?

A

Rotate impeller down and upwards to measure the resistance of powder –> cause particles to interact, or flow relative to one another

resistance experienced by blade –> difficulty of this relative particle movement/ bulk flow properties

The more particles resist motion and the harder it is to get the powder to flow, the more difficult it is to move the blade.

Work done = energy = (Resistance x Dist travelled)

‘Resistance’ = combined torque and force
Energy gradient = energy per mm of blade travel (calculated from measurements of torque and force)

23
Q

What is basic flowability energy (BFE) for forced (or confined) flow in powder rheometer?

A

BFE measured during downward blade movement. Powder confined by closed bottom end of test vessel

Push powder at the side
Shows resistance
Measure the powder flowability; hard to remove from sides or not

e.g. through a screw feeder / active feed frame

24
Q

What is Specific Energy (SE) for low stress (or unconfined) flow in powder rheometer?

A

SE: resistance to flow is measured as blade traverses from bottom of vessel to top.

e.g. during low stress filling / low shear blending

As there is no solid surface at the top of the vessel preventing the powder from dilating and moving upwards, the powder is unconfined during the test

25
Q

What is the revolution powder tester?

A
  • determine avalanche flow characteristics
  • images collected due to drum rotation used to determine powder behaviour over time.
  • images are analysed by software algorithm, and data generated used to calculate various parameters representing powder flowability.
26
Q

What the revolution powder tester measures?

A

Angle it tumbles
Consistency: time btw the avalanche

  • can calculate power (potential energy) of powder for every image, determine powder flow behaviour

measures:

a) flowability of powders in LOW STRESS situations
b) study how the powder behaves once it is flowing
c) determine condition of powder as it moves through a process

27
Q

What is a shear cell?

A
  • Shear cell splits horizontally, forming a shear plane btw the lower stationary base and the upper moveable portion and could provide information regarding the flowability via a number of indices
  • Shear cell data is also necessary to design of the hoppers for powder flow patterns and outlet dimensions
28
Q

Which type of powders are suitable for shear cells?

A

ONLY FOR POOR FLOW POWDER

good flow powders uses repose and tapping studies (mainly)

29
Q

How do we read the shear cell data

A

Measurement: flow factor (ff)
More suitable for cohesive material

ff < 1  non-flowing
ff 1-2 very cohesive
ff 2-4 cohesive
ff 4-10 easy flowing
ff >10 free flowing
30
Q

What are the 3 types of rotational shear cell testers?

A
  1. Schulz: annular type (twisting), well-established test method
  2. Brookfield: annular type, most recent commercial shear tester, most automated with min. operator contact

Schulze and Brookfield same shear testers but diff in physical size and operations

  1. FT4: rotational type shear tester; new equipment; other capabilities for bulk characterisation, quick operation
31
Q

Why do we need to design particles? (RECAP)

A

Particles responsible for: flow, packing, mechanical strength, solubility, stability, etc.

larger particles flows better (but dissolution poorer, etc)

Impt: optimise and need to balance for flow, stability and dissolution

32
Q

To size reduce the particle, what is the first important step? (RECAP)

A

Milling

Hammer mill, Pin mill, Cone mill, Fluid energy or Jet mill (<10um)

33
Q

What are ways we can produce particles? (RECAP)

A 
crystallisation
milling
sprat drying/ spray congealing
compact and mill
fluid bed coating others
34
Q

Powder flow is related to the interparticulate _____ and surface __________

A

Powder flow is related to the interparticulate adhesion and surface friction

35
Q

______ are important for erratically flowing powder

A

Glidants

36
Q

Due to the importance of powder flow, many _________ are available

A

flow characterisation methods

37
Q

____________ eliminates operator variability

A

Automation in powder testing methodology will eliminate operator variability

38
Q

________ powders will require flow measurement using shear cells

A

Poor flowing powders will require flow measurement using shear cells

39
Q

Particles are key to ________ products

A

Particles are key to good products

Solid (16 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions