Muller mixers

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