CHAPTER 9. AGITATION AND MIXING - Chemical Engineering

CHAPTER 9. AGITATION AND MIXING

ABRAHAM ROGELIO MARTIN GARCIA

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

Copyright ©2003 A. R. Martin

DEFINITIONS
• Agitation: It refers to the induced motion of a

“homogenous” material in a specified way
• Mixing: It is the random distribution, into and through one

another, 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 or

suspension
• Promoting heat transfer

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DEVICES TO PRODUCE AGITATION

IMPELLERS Three-blade Straight-blade Disk turbine
• Propellers (high-speed, low marine turbine
propeller
viscosity)
• Turbines (Moderate viscosity)
• High-efficiency impellers

Pitched-blade turbine
Concave-blade CD-6 impeller

Source: McCabe et al. 2001

Copyright ©2003 A. R. Martin

DESIGN OF TURBINES

Da 1 S1
Dt 3

E 1 S2
Dt 3

H
Dt 1 S3

W1
Da 5 S4

J 1 S5
Dt 12

L 1 S6
Da 4

Source: McCabe et al. 2001

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POWER CONSUMPTION OF AGITATORS

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

q K >SDa n1 k tan E SDaW @ Values of NQ NQ

q v nDa3 Impeller 0.5
Marine propellers (square pitch) 0.87
NQ q (Flow number) Four-blade 45q turbine (W/Da)=1/6 1.3
nDa3 Disk turbine 0.47
HE-3 high-efficiency impeller

Ek U Vt 2 U DSnDa 2

2
2

P q ˜ Ek

©¨¨§ D 2S 2 ·¹¸¸

P Un 3 Da5 2 NQ

P D 2S 2 Np P (Power number)

Un3 Da5 2 NQ Un3 Da5

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\N p Re, Fr, S1, S2 ,..., Sn

Source: McCabe et al. 2001

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P N p n3 Da5 U
For Re 10
P K L n2 Da3 P
For Re ! 10000
P KT n3 Da5 U

Source: McCabe et al. 2001

Copyright ©2003 A. R. Martin

BLENDING AND MIXING

Source: McCabe et al. 2001

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Source: McCabe et al. 2001

Copyright ©2003 A. R. Martin

SUSPENSION OF SOLID PARTICLES

nc D 0.85 SQ D0.1 0.2 ¨§¨© g 'U ·¹¸¸0.45 B 0.13
a p U

Shape factor S

Impeller type Dt/D Dt/ S
E
Six-blade turbine a 4.1
Da/W=5 4 7.5
Np=6.2 2 4 11.5
3 4
4 4.8
8
Two-blade paddle 2 4 12.5
3 4
Da/W=4 4 4 6.5
Np=2.5 8.5
9.5
Three-blade 34
propeller 44
Np=0.5 4 2.5

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DISPERSION OPERATIONS

0.0581 5.4\ ¨¨©§ UC n2 Da3 ·¸¹¸ 0.6
V
DS

Da

Source: McCabe et al. 2001

Copyright ©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 one

that mixes in the required time with the smallest
amount of power

SCALE UP

• Maintaining constant power per unit volume and
geometric similarity

2

n2 ¨¨©§ Da1 ¸·¹¸ 3
n1 Da2

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