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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

Muller Mixer

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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

Pugmills

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Lighter machines are there for dry powders and thin pastes

Ribbon blender Tumbling mixer Vertical screws Impact wheel / rotating disc

Mixers for free flowing solids

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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

Ribbon Blender

Ribbon Blender

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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

Internal Screw Mixer

Internal Screw Mixer

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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

1. 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

Double Cone Mixer

Twin Shell Blender

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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

Impact Wheels

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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 criteriao Time required for mixing

o Power load of mixer

o Properties of product from mixer

Effective mixing objectiveso 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

Quiz no. 1course: chapter no. 28 from 5th

edition date: 6th December, 2012

time: 12:00 pmvenue: seminar hall

marks: 10fill in the blanks, mcq’s,

true/false, short questionsno. Of minutes = no. Of

questions32