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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
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Muller Mixer
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
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Pugmills
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
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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
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Ribbon Blender
Ribbon Blender
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
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Internal Screw
Mixer
Internal Screw Mixer
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
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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
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Double Cone Mixer
Twin Shell Blender
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
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Impact Wheels
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
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Criteria of Mixing Effectiveness: Mixing Index
 Performance criteria
o Time required for mixing
o Power load of mixer
o Properties of product from mixer
 Effective mixing objectives
o Rapid mixing action with less time
o Minimum power required
o High degree of uniformity (homogeneous product)
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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
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 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’)
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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
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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’
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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:
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 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.
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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
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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
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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
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 Standard deviation for completely random mix
 For granular solids mixing index is defined as
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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;
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Rate of Mixing
 Rate is proportional to driving force
 Time calculated for given degree of mixing
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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
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Quiz no. 1
th
course: chapter no. 28 from 5
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
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