fcc stripper cold flow cfd modeling using an eulerian...
TRANSCRIPT
FCC Stripper Cold Flow CFD Modeling using
an Eulerian-Granular Approach
Wilson Kenzo Huziwara
Celso Murilo
Waldir Martignoni
Fusco Mozart Daniel Ribeiro
Contents
• Introduction
• Mathematical Model
• Results
• Conclusions
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)
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
Introduction
• Stripper in the FCC Unit
context
Reactor
Regenerator
Stripper
Riser
Stand
Pipe
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
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
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
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)
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
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
Results
• Instantaneous catalyst volume fraction at 7 s
Gibilaro C=1.0 GidaspowGibilaro C=0.6 Gibilaro C=0.7
Experiment
bed height
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)
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
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)
• Vertical analysis planes
Results
Plane XZ
Plane XZ
Plane XZ
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)
• 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
Bubble size distribution• Image analysis
– Chimera
– Volume fraction limit?
• Bubble size distribution
• Etc...• Etc...
Binarization Characterization
• 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
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]
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]
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]
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]
He concentration
• Normalized He concentration
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
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)
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.
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.