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.