1 Muller mixers - PowerPoint Presentation

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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 liquidSlide2

Muller MixerSlide3

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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 slurriesSlide4

PugmillsSlide5

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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 solidsSlide6

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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 consumptionSlide7

Ribbon BlenderSlide8

Ribbon BlenderSlide9

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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 lessSlide10

Internal Screw MixerSlide11

Internal Screw MixerSlide12

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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

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 mixerSlide13

Double Cone MixerSlide14

Twin Shell BlenderSlide15

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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 insecticidesSlide16

Impact WheelsSlide17

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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 Slide18

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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)Slide19

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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

x

i

– conc. of tracer in i

th

sample

x’ – average concentration of tracer in all spot samplesSlide20

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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. (x

i

= x’)

If the mixture is not completely mixed

conc. of each sample is

different from average

conc. (x

i

≠

x

’)Slide21

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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 x

i

about the average value of x’ is a measure of quality of mixing i.e. x

i

– 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 correctionSlide22

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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’Slide23

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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:Slide24

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Mixing index for pastes

Ratio of max standard deviation to the instantaneous

standard deviation

I

p

is unity at the start and increases as mixing

Theoretically

I

p

would become infinity at long mixing times but actually it does not occur. Slide25

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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 particlesSlide26

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Mixing index for granular solids

Sampling – number of spot samples

A – tracer

B – tracer free

μ

p

– overall concentration of tracer in mix

N – number of spot samples

n – average no. of particles per sample

x

i

– conc. of tracer in i

th

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 Slide28

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Standard deviation for completely random mix

For granular solids mixing index is defined as Slide29

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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 x

i

= 0 or x

i

= 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;Slide30

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Rate of Mixing

Rate is proportional to driving force

Time calculated for given degree of mixingSlide31

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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 timeSlide32

Quiz no. 1

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

no. Of minutes = no. Of questions

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