fluidized bed reactor.ppt
TRANSCRIPT
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