fcc stripper cold flow cfd modeling using an eulerian...
TRANSCRIPT
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FCC Stripper Cold Flow CFD Modeling using
an Eulerian-Granular Approach
Wilson Kenzo Huziwara
Celso Murilo
Waldir Martignoni
Fusco Mozart Daniel Ribeiro
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Contents
• Introduction
• Mathematical Model
• Results
• Conclusions
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Introduction
• The aim of catalyst stripping is remove
residual hydrocarbons from catalyst surface
after cracking reaction
• Higher catalyst fluxes demand bring flow • Higher catalyst fluxes demand bring flow
pattern problem in strippers (McKeen &
Pugsley, 2003)
• Higher efficiency -> lower HC entrainment ->
lower delta coke (Batista et al., 2002)
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Introduction
• Fluidization and mass transfer fundamental principles lead to the conclusion that systems which
– Limit the size of bubbles
– Limit the bubble rise velocity– Limit the bubble rise velocity
– Limit bubble life
– Enhance horizontal motion of the catalyst
– Disperse the bubbles
– Maximize the number of stages
• Will have improved performance
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Introduction
• Stripper in the FCC Unit
context
Reactor
Regenerator
Stripper
Riser
Stand
Pipe
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Introduction
• This study had two main objectives
– Test different internals configuration and analyse
the important variables
– Test several operation conditions in proposed
internals configurationinternals configuration
• This presentation will show the first item
above
– Just two configurations will be compared due to
time frame
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Mathematical Model
• Fluent 6.3
• Granular modification of Two-Fluid Model
(Ishii, 1975; Enwald et al., 1996)
• Continuous (Gas) phase: all properties • Continuous (Gas) phase: all properties
constant
• Granular phase: stress tensor calculated by
means of the Kinetic Theory
– Using equilibrium hypothesis for granular energy
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Mathematical Model
• Granular phase shear viscosity takes into
account three terms
– Collisional (particle-particle collisions)
– Kinetic (fluctuating motion)– Kinetic (fluctuating motion)
– Frictional (particle-particle friction)
• Laminar flow
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Results
• In order to obtain the right drag curve, tests
were made with a free bubbling 2D case and
compared to experimental results from
Pugsley & McKeen (2003a)
• Several drag laws were used
– Wen-Yu
– Gidaspow (Wen-Yu + Ergun)
– Gibilaro et al (1985)
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Results
• 2D model schematic
1 m
Pressure Prescribed
Vg=Vmf=
0,35cm/s
Catalyst level
0,5 m
1 m
ε = 0.45
Vs=0
Vg=Vmf/ε
Uniform gas velocity = 1 cm/s
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Results
• Bed expansion
90
100
Gibilaro C=0.6
Gidaspow
Gibilaro C=0.5
Gibilaro C=0.3
40
50
60
70
80
0 2 4 6 8 10 12 14 16
Altura [cm]
Tempo [s]
Gibilaro C=0.3
Gibilaro C=1
Gibilaro C=0.7
Wen-Yu
Gidaspow KT
Experimental
Bed height
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Results
• Instantaneous catalyst volume fraction at 7 s
Gibilaro C=1.0 GidaspowGibilaro C=0.6 Gibilaro C=0.7
Experiment
bed height
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Results
• Actual pilot scale 3D geometry
• Comparison between two different internal
baffles configuration
• Main issues• Main issues
– Large internal volume and fine details (large
meshes)
– High velocities (small timesteps)
– Long transient simulations (long CPU demand)
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Results
• Boundary Conditions
– Top outlet (1): pressure outlet
– Catalyst inlet (2): cyclones diplegs
• solids velocity calculated from catalyst flux
1
2
4
– Air inlet (3): pipegrid
• Velocity calculated from superficial velocity
– Bottom outlet (4): standpipe
• Solids mass flow prescribed from inlet
• Gas exits with zero slip velocity
3
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Results
• 4 radial profiles
– Above Cone 4
– Below Cone 4
– Below Cone 3
– Below Cone 2
He Entrance
– Below Cone 2
Baffles
(takes thickness
into account)
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• Vertical analysis planes
Results
Plane XZ
Plane XZ
Plane XZ
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Catalyst VOF – Case Base
• Averaged catalyst volume
fraction profile
• A big amount of gas is
captured by internals
• Huge amount of catalyst
mass flows through the mass flows through the
top (flooding)
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• With the modified
geometry, it is still
possible to see gas below
baffles but less than Case
Base
• Less flooding is observed
Catalyst VOF – Case 02
• Less flooding is observed
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Bubble size distribution• Image analysis
– Chimera
– Volume fraction limit?
• Bubble size distribution
• Etc...• Etc...
Binarization Characterization
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• Chimera Image Analysis
– Based on vertical analysis planes
– Error bars mean distribution standard deviation at each time sample
Bubble size distribution
3.5
4
4.5
Caso Base
Caso 02
0
0.5
1
1.5
2
2.5
3
3.5
15.5 16 16.5 17 17.5 18 18.5 19 19.5
Tempo [s]
Raio Hidráulico [cm]
Caso 02
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Time averaged catalyst radial velocity
0.4
0.9
1.4
Velocidade Radial do Catalisador [m/s]
Acima Cone 4
Abaixo Cone 4
Abaixo Cone 3
Abaixo Cone 2
Cat radial vel - Case Base
-1.1
-0.6
-0.1
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
r/R
Velocidade Radial do Catalisador [m/s]
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Time averaged catalyst radial velocity
0.4
0.9
1.4
Velocidade Radial [m/s]
Acima Cone 4
Abaixo Cone 3
Abaixo Cone 1
Cat radial vel - Case 02
-1.1
-0.6
-0.1
0.4
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
r/R
Velocidade Radial [m/s]
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Time averaged catalyst axial velocity
Cat axial vel - Caso Base
0.5
1
1.5
2
Velocidade Axial [m/s]
Acima Cone 4
Abaixo Cone 4
Abaixo Cone 3
Abaixo Cone 2
-1.5
-1
-0.5
0
0.5
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
r/R
Velocidade Axial [m/s]
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Time averaged catalyst axial velocity
0.5
1
1.5
2
Velocidade Axial [m/s]
Acima Cone 4
Abaixo Cone 3
Abaixo Cone 1
Cat axial vel - Case 02
-1.5
-1
-0.5
0
0.5
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
r/R
Velocidade Axial [m/s]
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He concentration
• Normalized He concentration
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Conclusions
• CFD results reveal opposite effects from
modified baffles
– They decrease bubbles size but in the other hand
increase bubbles rise velocity
• Horizontal motion is qualitatively equivalent in
both cases
• Case 02 allowed deeper He penetration which
is not desirable for stripping efficiency
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Case Base Case 02Bubble size Bubble size
Catalyst horizontal
movement
Catalyst horizontal
movement
Conclusions
Bubble rise velocity Bubble rise velocity
Stripping efficieny
(He penetration)
Stripping efficiency
(He penetration)
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References
• McKeen, T.; Pugsley, T.S. (2003a) “Simulation
and experimental validation of a freely
bubbling bed of FCC catalyst”, Powder Tech.,
v.129, pp 139-152.
• McKeen, T.; Pugsley, T.S. (2003b) “Simulation
of cold flow FCC stripper hydrodynamics at
small scale using computational fluid
dynamics”, Int. J. Chem. Reactor Eng., v.1, A18.
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References
• Gibilaro, L.G.; Di Felice, R.; Waldram, S.P.
(1985) “Generalized friction factor and drag
coefficient correlations for fluid-particle
interactions”, Chem. Eng. Sci., v.40, i.10, pp
1817-18231817-1823
• Gidaspow, D. (1994) “Multiphase flow and
fluidization: continuum and kinetic theory
descriptions”, Academic Press, USA.