CHAPTER 9. AGITATION AND MIXING

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
• Propellers (high-speed, low
viscosity)
• Turbines (Moderate viscosity)
• High-efficiency impellers
Three-blade
marine
propeller
Straight-blade
turbine
Disk turbine
Pitched-blade turbine
Concave-blade CD-6 impeller
Source: McCabe et al. 2001
Copyright ©2003 A. R. Martin
DESIGN OF TURBINES
Da
Dt
1
3
S1
E
Dt
1
3
S2
H
Dt
1
S3
W
Da
1
5
J
Dt
1
12
S5
L
Da
1
4
S6
S4
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
Values of NQ
NQ
q
Impeller
K >SDa n1 k tan E SDaW @
q v nDa3
NQ
Ek
P
P
q
(Flow number)
nDa3
U Vt 2
U
2
q ˜ Ek
2
©
P
Un 3 Da5
2
D 2S 2
2
0.5
Four-blade 45q turbine (W/Da)=1/6
0.87
Disk turbine
1.3
HE-3 high-efficiency impeller
0.47
DSnDa 2
§ D 2S 2
Un 3 Da5 ¨¨
Marine propellers (square pitch)
·
N Q ¸¸
¹
NQ
Np
P
Un 3 Da5
(Power number)
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Np
\ Re, Fr, S1 , S 2 ,..., S n Source: McCabe et al. 2001
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P
N p n 3 Da5 U
For Re 10
P
K L n 2 Da3 P
For Re ! 10000
P
K T n 3 Da5 U
Source: McCabe et al. 2001
Copyright ©2003 A. R. Martin
BLENDING AND MIXING
Source: McCabe et al. 2001
Copyright ©2003 A. R. Martin
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Source: McCabe et al. 2001
Copyright ©2003 A. R. Martin
SUSPENSION OF SOLID PARTICLES
nc Da0.85
§ 'U ·
¸¸
© U ¹
0.45
SQ 0.1 D p0.2 ¨¨ g
B 0.13
Shape factor S
a
Dt/
E
S
Impeller type
Dt/D
Six-blade turbine
Da/W=5
Np=6.2
2
3
4
4
4
4
4.1
7.5
11.5
Two-blade paddle
Da/W=4
Np=2.5
2
3
4
4
4
4
4.8
8
12.5
Three-blade
propeller
Np=0.5
3
4
4
4
4
2.5
6.5
8.5
9.5
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DISPERSION OPERATIONS
DS
Da
§ U C n 2 Da3 ·
¸¸
© V
¹
0.6
0.0581 5.4\ ¨¨
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
n1
§ Da1 · 3
¨¨
¸¸
© Da 2 ¹
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