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CHAPTER 9. AGITATION AND MIXING

PRESENTATION OUTLINE

-Definitions

-Purposes of agitation

-Devices to produce agitation

-Power consumption of agitators

-Blending and Mixing

-Suspension of solid particles

-Dispersion Operations-Agitator selection and Scale up

ABRAHAM ROGELIO MARTIN GARCIA

Copyright 2003 A. R. Martin

DEFINITIONS

Agitation: It refers to the induced motion of ahomogenous material in a specified way

Mixing: It is the random distribution, into and through oneanother, of two or more initially separate phases

PURPOSES OF AGITATION

Suspending solid particles

Blending miscible liquids

Dispersing a gas through the liquid

Dispersing a second liquid to form an emulsion orsuspension

Promoting heat transfer

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DEVICES TO PRODUCE AGITATION

IMPELLERS

Propellers (high-speed, lowviscosity)

Turbines (Moderate viscosity)

High-efficiency impellers

Three-blade

marine

propeller

Straight-blade

turbine

Disk turbine

Concave-blade CD-6 impellerPitched-blade turbine

Source: McCabe et al. 2001Copyright 2003 A. R. Martin

DESIGN OF TURBINES

6

5

4

3

2

1

4

1

12

1

5

1

1

3

1

3

1

SD

L

SD

J

SD

W

SD

H

SD

E

SD

D

a

t

a

t

t

t

a

Source: McCabe et al. 2001

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POWER CONSUMPTION OF AGITATORS

> @

number)(Flow

tan1

3

3

a

Q

a

aa

nD

qN

nDq

WDknDKq

v

SES

It is a function of the volumetric flow rate and the kinetic energy

Values of NQ

0.47HE-3 high-efficiency impeller

1.3Disk turbine

0.87Four-blade 45q

turbine (W/Da)=1/6

0.5Marine propellers (square pitch)

NQ

Impeller

number)(Power2

2

22

53

22

53

2253

2

2

a

pQ

a

Qa

k

a

t

k

Dn

PNN

Dn

P

NDnP

EqP

nDV

E

U

SD

U

SD

U

D S

U

U

Copyright 2003 A. R. Martin

np SSSFrN ,...,,,Re, 21\

Source: McCabe et al. 2001

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U

P

U

53

32

53

10000Re

10Re

aT

aL

ap

DnKP

For

DnKP

For

DnNP

!

Source: McCabe et al. 2001Copyright 2003 A. R. Martin

BLENDING AND MIXING

Source: McCabe et al. 2001

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Source: McCabe et al. 2001Copyright 2003 A. R. Martin

SUSPENSION OF SOLID PARTICLES

13.0

45.0

2.01.085.0BgDSDn pac

'

U

U

Q

Shape factor S

6.5

8.5

9.5

4

4

2.5

3

4

4

Three-blade

propeller

Np=0.5

4.8

8

12.5

4

4

4

2

3

4

Two-blade paddle

Da/W=4

Np=2.5

4.1

7.5

11.5

4

4

4

2

3

4

Six-blade turbine

Da/W=5

Np=6.2

SDt/

E

Dt/D

aImpeller type

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DISPERSION OPERATIONS

6.032

4.51058.0

V

U

\

aC

a

S Dn

D

D

Source: McCabe et al. 2001Copyright 2003 A. R. Martin

AGITATOR SELECTION AND SCALE UP

AGITATOR SELECTION

Depends on the viscosity of the liquid

For specific time of mixing, the best mixer is the onethat mixes in the required time with the smallestamount of power

SCALE UP

Maintaining constant power per unit volume and

geometric similarity

3

2

2

1

1

2

a

a

D

D

n

n

Copyright 2003 A. R. Martin