Fluid Bed Reactors

Chapter (Not in book)

CH EN 4393

Terry A. Ring

Fluidization

• Minimum Fluidization– Void Fraction– Superficial Velocity

• Bubbling Bed Expansion

• Prevent Slugging– Poor gas/solid contact

Fluidization

• Fluid Bed– Particles– mean particle size, Angular

• Shape Factor• Void fraction = 0.4 (bulk density)

Geldart, D. Powder Technology 7,285(1973), 19,133(1978)

FluidizationRegimes

Fluidization Regimes

• Packed Bed

• Minimum Fluidization

• Bubbling Fluidization

• Slugging (in some cases)

• Turbulent Fluidization

Minimum Fluidization

• Bed Void Fraction at Minimum Fluidization

Overlap of phenomenon

• Kinetics– Depend upon solid content in bed

• Mass Transfer– Depends upon particle Re number

• Heat Transfer– Depends upon solid content in bed and gas Re

• Fluid Dynamics– Fluidization – function of particle Re– Particle elution rate – terminal settling rate vs gas

velocity– Distribution Plate Design to prevent channeling

Packed Bed

• Pressure Drop

P vo LR

vo

Dp

1

3

150 1 ( )

Dp

1.75 vo

Void Fraction, ε=0.2-0.4, Fixed

0 0.2 0.4 0.6 0.810

100

1 103

1 104

1 105

P vft

s

psi

v

Now if particles are free to move?

• Void Fraction

0 0.2 0.40

0.2

0.4

0.6

0.8

Superficial Gas Velocity (ft/s)

Bed

Voi

d F

ract

ion f vo

ft

s

mf

f vR

vo

Gmf

ft

s

vR

ft

s

P f vo if vo

Gmf

LR

vo

Dp

1 f vo

f vo 3

150 1 f vo

Dp

1.75 vo

LR

vo

Dp

1

3

150 1 ( ) Dp

1.75 vo

Void Fraction, ε=0.2-0.4 packed BecomesεMF=0.19 to εF=0.8.

MF Pressure drop equals the weight of Bed

015 2 1 ( )

3

vo Dp

1.75

3

vo Dp

2

Dp

3 S g

2

Fluid Bed Pressure Drop

• Lower Pressure Drop @ higher gas velocity

• Highest Pressure Drop at onset of fluidization

0 0.2 0.40

20

40

60

Superficial Gas Velocity (ft/s)

Pre

ssur

e D

rop

(psi

)

P f voft

s

psi

P mf

psi

P f vR psi

vo

Gmf

ft

s

vR

ft

s

Bed at Fluidization Conditions

• Void Fraction is High

• Solids Content is Low

• Surface Area for Reaction is Low

• Pressure Drop is Low

• Good Heat Transfer

• Good Mass Transfer

Distributor Plate Design

• Pressure Drop over the Distributor Plate should be 30% of Total Pressure Drop ( bed and distributor) – Pressure drop at distributor is ½ bed pressure

drop.

• Bubble Cap Design is often used

Bubble Caps

• Advantages– Weeping is reduced or totally avoided

• Sbc controls weeping– Good turndown ratio– Caps stiffen distributor plate– Number easily modified

• Disadvantages– Expensive– Difficult to avoid stagnant regions– More subject to bubble coalescence– Difficult to clean– Difficult to modify

From Handbook of Fluidization and Fluid-Particle Systems By Wen-Ching Yang

Bubble Cap Design

• Pressure drop controlled by – number of caps– stand pipe diameter– number of holes

• Large number of caps– Good Gas/Solid Contact

• Minimize dead zones• Less bubble coalescence

– Low Pressure Drop

Pressure Drop in Bubble Caps

• Pressure Drop Calculation Method• Compressible Fluid• Turbulent Flow

– Sudden Contraction from Plenum to Bottom of Distributor Plate

– Flow through Pipe– Sudden Contraction from Pipe to hole– Flow through hole– Sudden Expansion into Cap

Elution of Particles from Bed

• Particle Terminal Setting Velocity

• When particles are small they leave bed

Terminal Settling Velocity

0 50 100 150 2000

1

2

3

4

Particle Diameter (microns)

Term

inal S

ettlin

g Velo

city (

ft/s)

Gas Velocity

vt4

3

g Dp

f

S

2Dp

2

2

S g

9

Cyclone

• Used to capture eluted particles and return to fluid bed

• Design to capture most of eluted particles

• Pressure Drop

Big particles

P i V( ) 0.24 V2

Cyclone Design

• Inlet Velocity as a function of Cyclone Size

• Cut Size (D50%)

Cyclone EquationsPerry's HB 5th ed, P 20-85+7th ed, 17-28

Vin Dc QR

Dc2

4 2

D50 Dc 9

Dc

4

N Vin Dc Vin Dc Si

1

2

D50 Dc 9

Dc

4

N Vin Dc Vin Dc Si

1

2

Dc = Cyclone diameter

Cyclone Cut Size

• Diameter where 50% leave, 50% captured

0 1 2 3 40

10

20

30

40

50

60

70

80

90

100

Cyclone Diameter(ft)

Cut

Siz

e P

artic

le D

iam

eter

(m

icro

ns)

D50

9 Dc

4

N Vin S

1

2

Size Selectivity Curve

20 40 600

0.2

0.4

0.6

0.8

24 in cyclone14 in cycloneD50 for 24 in Cyclone20 in cycloneDiameter of Eluted Particles

Particle Diameter (microns)

Siz

e S

elec

tivity

SS D( ) 1 exp 0.693D

D50

3.12

Mass Transfer

• Particle Mass Transfer– Sh= KMTD/DAB = 2.0 + 0.6 Re1/2 Sc1/3

• Bed Mass Transfer– Complicated function of

• Gas flow• Particles influence turbulence• Particles may shorten BL• Particles may be inert to MT

Fluid Bed Reactor Conclusions

• The hard part is to get the fluid dynamics correct

• Kinetics, MT and HT are done within the context of the fluid dynamics

Heat Transfer

• Particle Heat Transfer– Nu= hD/k = 2.0 + 0.6 Re1/2 Pr1/3

• Bed Heat Transfer– Complicated function of

• Gas flow• Particle contacts