1. Different mixing action
Muller mixers
Different mixing action
Mulling is smearing or rubbing action similar to that in mortar and pestle
Wide, heavy wheels of the mixer did the same job
Pan is stationary & central vertical shaft is driven – causing the muller wheels to roll in circular path on solid
Rubbing action results from slip of the wheel on solids
Plows – guide solids under wheels or to discharge opening
Axis of the wheels is stationary & pan is rotated
Good mixer for batches of heavy solids and pastes
Effective in coating the granular particles with liquid

2. Muller Mixer

3. Pug Mills
Mixing is done by blades or knives set in helical pattern on a horizontal shaft.
Open trough or closed cylinder
Cut, mixed and moved forward
closed mixing chamber - Single shaft
Open trough – double shaft for more rapid & thorough mixing
Mostly cylindrical in shape
Heating or cooling jackets
Blend and homogenize clays, mix liquids with solids to form thick heavy slurries

4. Pugmills

5. Mixers for free flowing solids
Lighter machines are there for dry powders and thin pastes
Ribbon blender
Tumbling mixer
Vertical screws
Impact wheel / rotating disc

6. Horizontal trough – central shaft and a helical ribbon agitator
Ribbon Blenders
Horizontal trough – central shaft and a helical ribbon agitator
Two counteracting ribbon mounted on same shaft
One moving in one direction
Second in other direction
Ribbon – continuous or interrupted
Mixing – turbulence by counteracting agitators
Mode of operation – batch or continuous
Trough – open or closed
Moderate power consumption

7. Ribbon Blender

8. Ribbon Blender

9. Internal screw mixers
Vertical tank containing a helical conveyor that elevates and circulates the material
For free flowing grains and light solids
Double motion helix orbits about the central axis of the conical vessel visiting all parts of the vessel
Mixing is slower than ribbon blenders but power requirement is less

10. Internal Screw Mixer

11. Internal Screw Mixer

12. Partly filled container rotating about horizontal axis
Tumbling mixer
Partly filled container rotating about horizontal axis
Mostly no grinding element
Effectively mix – suspension of dry solid in liquid, heavy dry powders
Wide size range and material of construction
Double cone mixer
Batch – charged from above – 50 to 60 %full
Free flowing dry powders
Close end of vessel – operated 5 to 20 min
2. Twin shell blender
Two cylinder joined to form a V
rotated about horizontal axis
More effective than double cone mixer

13. Double Cone Mixer

14. Twin Shell Blender

15. Impact wheels
Operating continuously by spreading them out in a thin layer under centrifugal action
Several dry ingredients are fed continuously near the high speed spinning disk 10 to 27 in. in diameter throwing it in a stationary casing.
Intense shear cause mixing
1750 to 3500 rpm
Several passes through same or in series
1 to 25 tons/hr
Fine light powders like insecticides

16. Impact Wheels

17. Power Requirement for mixing
Mechanical Energy is required for mixing
Large for heavy plastics masses
Relatively small for dry solids
Only part of the energy supplied is directly useful and this part is small
Mixers
Work intensively on small quantities
Work slowly on large quantities
Light machines waste less energy than heavier one
The shorter the mixing time required to bring the material to homogeneity, larger the useful fraction of energy supplied
Major portion of energy supplied appears as heat

18. Criteria of Mixing Effectiveness: Mixing Index
Performance criteria
Time required for mixing
Power load of mixer
Properties of product from mixer
Effective mixing objectives
Rapid mixing action with less time
Minimum power required
High degree of uniformity (homogeneous product)

19. Mixing index for cohesive solids/pastes
The degree of uniformity by sample analysis is a measure of mixing effectiveness
Sampling – number of spot samples
A – tracer
B – tracer free
μ – overall concentration of tracer in mixture
N – number of spot samples
xi – conc. of tracer in ith sample
x’ – average concentration of tracer in all spot samples

20. If N is very large, i.e; N infinite
average conc. will be equal to overall conc. of tracer (x’ = µ)
If N is very small, i.e; N zero
average conc. and overall conc. of tracer will be appreciably different ((x’ ≠ µ)
If the mixture is perfectly mixed
conc. of each sample is same as average conc. (xi = x’)
If the mixture is not completely mixed
conc. of each sample is different from average conc. (xi ≠ x’)

21. Statistical method/procedure to find out quality of mixing
Assumption – methods used for determining the conc. of tracer are highly accurate
Standard deviation of xi about the average value of x’ is a measure of quality of mixing i.e. xi – x’
Mean deviation of conc.
Mean square value of deviation
Root mean square value – standard deviation
Population standard deviation - σ
Sample standard deviation – s
Bessel’s correction

22. So the sample standard deviation
low value of s  Good mixing
High value of s  Poor mixing
More general measure of mixer effectiveness is given by ‘Mixing Index’

23. Mixing index is the ratio of standard deviation at zero time to the standard deviation at any time
At t = 0, there will be two layers in the mixer; one containing tracer material and the other containing tracer free material.
Standard deviation at zero time is given by:

24. Mixing index for pastes
Ratio of max standard deviation to the instantaneous standard deviation
Ip is unity at the start and increases as mixing
Theoretically Ip would become infinity at long mixing times but actually it does not occur.

25. Mixing index for granular / non cohesive solids
As for granular solids
Intense agitation is not required
Less power load
Relatively less heat load
Mixing index for granular solids based
Not on zero mixing condition
But on standard deviation that would be observed with completely random, fully blended mixture
At t = 0, there is some mixing for these type of solids
For granular solids – conc. is expressed as number fraction of tracer particles

26. Mixing index for granular solids
Sampling – number of spot samples
A – tracer
B – tracer free
μp – overall concentration of tracer in mix
N – number of spot samples
n – average no. of particles per sample
xi – conc. of tracer in ith sample
x’ – average no. fraction of tracer in each sample

27. Statistical method/procedure to find out quality of mixing
Standard deviation is measure of quality of mixing
Mean deviation of conc.
Mean square value of deviation
Root mean square value – standard deviation
Sample standard deviation - s
Population standard deviation – σ
Bessel’s correction factor

28. Standard deviation for completely random mix
For granular solids mixing index is defined as

29. Mixing Index at zero time for granular solids
Standard deviation at complete mixing – granular solid
Standard deviation at zero mixing - paste
For n = 1 , two relations are identical
For a sample of one particle, taken from a mixture of granular solids, the analysis shows either xi = 0 or xi = 1 i.e. the same as with completely unmixed material at zero time, So, S.D. at zero mixing can be used for granular solids when n = 1
So, mixing index at zero time for granular solids is;

30. Rate of Mixing
Rate is proportional to driving force
Time calculated for given degree of mixing

31. Axial Mixing
Mixing
Radial
Axial
Degree of axial mixing is measured by injecting the small amount of tracer into feed and check the conc. of tracer at outlet
Max conc. Of tracer
Length of time

32. Quiz no. 1 course: chapter no
Quiz no. 1 course: chapter no. 28 from 5th edition date: 6th December, 2012 time: 12:00 pm venue: seminar hall marks: 10 fill in the blanks, mcq’s, true/false, short questions no. Of minutes = no. Of questions