PERFORMANCE STUDIES OF TRICKLE BED REACTORS
Mohan R. Khadilkar
Thesis Advisors: M. P. Dudukovic and M. H. Al-Dahhan
Chemical Reaction Engineering Laboratory
Department of Chemical Engineering
Washington University
St. Louis, Missouri
CREL
Objectives and Accomplishments
 Examined the current state of the art in experimentation and
modeling of trickle beds critically and successfully answered
the often asked questions
1. Do upflow and downflow differ? When and Why ?
2. How to get reproducible scale-up data from small scale
reactors independent of flow mode?
3. To what extent can current models predict the observed
behavior?
 Formulated the rigorous approach to trickle bed modeling on
pellet and reactor scale and illustrated the effectiveness of this
approach for prediction of steady and unsteady state performance
 Examined experimentally and via models, unsteady state operation
in trickle beds and identified regions of, and, reasons for
performance enhancement
CREL
Performance of Trickle Bed Reactors
1. Comparison of Trickle Bed and Upflow
Effect of Bed Dilution
Model Evaluation
2. Rigorous Steady State Model
3. Unsteady State Performance Experiments
4. Rigorous Unsteady State Model
Chemical Kinetics
Fluid Dynamics
Phase Interaction
& Contacting
Transport
Coefficients
5. Transient Fluid Dynamic Simulation
Reactor Design
& Scale-up
Performance
CREL
Trickle Bed Reactors
Cocurrent Downflow of Gas and Liquid
on a Fixed Catalyst Bed
Catalyst Wetting Conditions in Trickle Bed Reactor
...
......
........ ......
Operating Pressures up to 20 MPa
Operating Flow Ranges:
High Liquid Mass Velocity (Fully Wetted Catalyst)
(Suitable for Liquid Limited Reactions)
Low Liquid Mass Velocity (Partially Wetted Catalyst)
(Suitable for Gas Limited Reactions)
.......
L i q u id F il m
o r R i v u le
L i q u id F il le d p o r e s
D r y P e lle t
C a p ill a r y
Limiting Reactant criterion:
DeB C Bi
1
DeAC A*
D C
  eB Bi
1
DeA C A*
 
Gas limited reaction if
L IQ U ID
G AS
C o n d e n s a ti
Liquid limited reaction if
Flow Map (Fukushima et al., 1977)
10000
C AT AL Y S T
SPRAY
B ED
Re(Gas)
WAVY
G AS
1000
PULSE
TRICKLE
100
DISP .
BUBBLE
10
1
L IQ U ID
10
100
1000
Re (Liquid)
CREL
FLOW REGIMES AND CATALYST WETTING EFFECTS
DOWNFLOW (TRICKLE BED REACTOR)
UPFLOW (PACKED BUBBLE COLUMN)
PARTIAL WETTING
COMPLETE WETTING
CATALYST
LIQUID
GAS
(Trickle Flow Regime)
(Bubble Flow Regime)
CREL
Motivation



To understand the differences between downflow and upflow operation.
Are upflow reactors indicative of trickle bed performance under different
reaction conditions?
To understand the effects of bed dilution with fines on reactor
performance
To develop guidelines for scale-up/scale-down of reactors for gas or
liquid reactant limited reactions
Objectives



Experimentally investigate the performance of downflow (Trickle Bed)
and upflow (Packed Bubble Column) reactors for a test hydrogenation
reaction
Study the effects of pressure, feed concentration, gas velocity and bed
dilution on the performance of both modes of operation
Evaluate available reactor models in comparison with experimental
data
Reaction Scheme:
CH3
C
CH2
+ H2
Catalyst : 2.5 % Pd on Alumina
(cylindrical 0.13 cm dia.)
Fines
: Silicon carbide 0.02 cm
CH3
HC CH3
Pd/Alumina
Alpha-methylstyrene
B (l) + A(g)
cumene
P(l)
Range of Experimentation :
• Superficial Liquid Velocity (Mass Velocity) : 0.09 - 0.5 cm/s (0.63-3.85 kg/m2s)
• Superficial Gas Velocity (Mass Velocity) : 3.8 -14.4 cm/s (3.3x10-3-12.8x10-3 kg/m2s)
• Feed Concentration
• Operating Pressure
• Feed Temperature
: 3.1 - 7.8 % (230-600 mol/m3)
: 30 - 200 psig (3-15 atm)
: 24 oC
Limiting Reactant criterion:
Gas limited reaction if
 
DeB CBi
 1
DeA C*A
Liquid limited reaction if
 
DeB CBi
 1
DeA C*A
CREL
Experimental Setup
High Pressure
Gas Supply
PC
Timer
PC
PC
Distributor
Rotameter
Damper
TT
High Pressure
Gas Supply
PT
Feed Tank
LT
LT
Cooling
Jacket
LT
Saturators
HP Feed
Tank
DPT
Reactor
Vent
Solvent
PC
TT
High Pressure
Diaphragm Pump
Vent
PT
TT
PT
PC
PC
Rotameter
LT
LC
Demister
Waste Tank
Gas-Liquid
Separator
PC
PC
Gas
Chromatograph
Computer
CREL
Downflow and Upflow Experimental Results under Gas and Liquid
Limited Conditions without Fines
1
UPFLOW
DOWNFLOW
Conversion(X)
Conversion(X)
0.8
CBi=7.8%v/v, P=30psig
0.6
Gas Limited 8.8
0.4
0.2
0
0
100
200
300
Space time , s
Downflow outperforms upflow due
to partial external wetting and
improved gas reactant access to
particles
400
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
DOWNFLOW
UPFLOW
CBi=3.1(v/v)%,P=200psig
Liquid Limited 0.8
0
50
100
Space time, s
150
200
Upflow outperforms downflow due
to more complete external wetting and
better transport of liquid reactant to
the catalyst
CREL
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
DOWNFLOW
Conversion(X)
Conversion(X)
Downflow and Upflow Experimental Results under Gas and Liquid
Limited Conditions with Fines
UPFLOW
CBi=6.7 %(v/v), P=30 psig
7.5
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
DOWNFLOW
UPFLOW
CBi=3.18%(v/v), P=200 psig.
0.8
0
0
50
100
Space time,s
150
200
50
100
150
200
Space time,s
ABOUT EQUAL PERFORMANCE DUE TO COMPLETE WETTING
Fines Packing Procedure: Vol. of Fines ~Void volume (Al-Dahhan et al. 1995)
CREL
Effect of Pressure and Gas Velocity on Performance
1
=4.0
p=100psig =1.5
p=30psig
0.8
p=200psig
Conversion (X)
Conversion(X)
1
=0.8
0.6
0.4
0.2
Downflow
Upflow
P=100 psig,  = 1
0.8
0.6
0.4
0.2
CBi=3.1%,Ug=3.8 cm/s
0
0
0
50
100
150
Space time,s
Transition to Liquid
Limited Conditions
200
0
3
6
9
12
Superficial Gas Velocity, cm/s
Negligible Effect of Gas Velocity
CREL
Conversion(X)
Slurry Kinetics
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
# 1 p =3 0 p s ig , C B i=3 . 9 %
# 2 p =1 0 0 p s ig , C B i=3 . 9 9 %
# 3 p =2 0 0 p s ig , C B i=4 %
# 4 p =3 0 0 p s ig , C B i=3 . 4 5 %
0
100
200
300
Time (min)
LHHW FORM
r
kvsCams Ch2
( 1  K1Cams  K2Ccume )
Pressure (psig)
kvs
3
(m iq./m3cat./s)
*(mol/m3 liq)r-1
K1
K2

30
100
200
0.0814
1.14
0.022
0
4.41
0.0273
0
11.48
0.021
0
1
2
CREL
El- Hisnawi (1982) model
REACTOR SCALE
•Reactor scale plug flow equations
L
G
Liquid phase gas reactant concentration
Direct Access
.
Access of Gas
of gas
.
.
.
.
.
.
.
.
.
via Liquid .....................to Dry Areas
...................
..............................
. ....................
.....................
.
•Constant effectiveness factor
Modified by external contacting efficiency
•Allowance for rate enhancement on
externally dry catalyst
Direct access of gas on inactively wetted pellets.
Liquid Dry
Film
L
G
CREL
Beaudry (1987) model
• Pellet scale reaction diffusion equations
DRY
HALF-WET
FULLY WET
For fully wetted and partially wetted slabs
d 2C ' A
 (1   ) 2  A2C ' A  0, 0  x  1;
2
dx
d 2C ' A
  A2C ' A  0, 0  y  1
2
dy
• Effectiveness factor weighted based on
contacting efficiency
Catalyst Pellet
Flowing Liquid
• Overall effectiveness factor changes along
the bed length
Evaluation of overall effectiveness with change in
concentration and contacting
Overall Effectiveness factor at any location
o  ( 1  ce )2 od  2 ce ( 1  ce )odw  2ce ow
CA
CB
CB
CA
2V/S
0
1
x
0
0
y
1
CREL
Conversion(X)
Upflow and Downflow Performance at Low Pressure
(Gas Limited Condition)
Experimental Data and Model Predictions
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
down,El-Hisnawi
upflow,El-Hisnawi
downflow,Beaudry
upflow,Beaudryi
downflow,exp
upflow,exp
Ug=4.4cm/s,Co=7.6%(v/v),p=30psig
0
100
200
300
400
Space time(s)
CREL
Conversion(X)
Upflow and Downflow Performance at High Pressure
(Liquid Limited Conditions):
Experimental Data and Model Predictions
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Ug=3.8cm/s,Co=3.1%(v/v),p=200psig
down,El-Hisnawi
up, El-Hisnawi
down, Beaudry
up, Beaudry
downflow,exp
upflow,exp
0
50
100
150
200
250
space time(s)
CREL
Summary

DOWNFLOW OUTPERFORMS UPFLOW AT LOW PRESSURE.
(Hydrogenation of alpha-methylstyrene is a gas limited reaction.
Partial wetting is helpful in this situation.)

UPFLOW OUTPERFORMS DOWNFLOW AT HIGH PRESSURE.
(Hydrogenation of alpha-methylstyrene becomes a liquid limited
reaction. Complete wetting is beneficial to this situation.)

THE PREFERRED MODE FOR SCALE-UP (UPFLOW OR DOWNFLOW)
DEPENDS ON THE TYPE OF REACTION SYSTEM AS WELL AS ON THE
RANGE OF OPERATING CONDITIONS THAT AFFECT CATALYST WETTING.

FINES NEUTRALIZE PERFORMANCE DIFFERENCES DUE TO MODE OF
OPERATION AND REACTION SYSTEM TYPE , DECOUPLE
HYDRODYNAMICS AND KINETICS, AND HENCE ARE TO BE PREFERRED
AS SCALE-UP TOOLS.

THE TESTED MODELS PREDICT PERFORMANCE WELL
(although improvements in mass transfer correlations are necessary)
CREL
Drawbacks of Evaluated Models






Isothermal Operation
Liquid Volatility Effects not Considered
Coupling of Hydrodynamics and Transport Ignored
Fully Internally Wetted Pellets Assumed
Single Component, Dilute Solution Transport Assumed
Multicomponent Effects not Considered
Simplified Models Accounting for Some of the Above Effects


Pellet Scale (Level I) Model (Kim and Kim, 1981; Harold,1988)
Reactor Scale (Level II) Model (LaVopa and Satterfield,1988; Kheshgi
et al., 1992)
Extended Steady State Rigorous (Level III) Model
Test Reaction System: Hydrogenation of Cyclohexene
CREL
L-III Model Reactor Scale Equations (1-D)
Continuity
d L u IL  L 
   N iGL a GL M i   N iLSa LSM i
dz
d   G u IG G 
  N iGL aGL M i   N iGS aGS M i
dz
Species
d
u IL LCiL   N iGL aGL  N iLS aLS
dz
d
u IG G CiG    N iGL aGL  N iGS aGS
dz
Momentum
d
dp
(  G  G u IG u IG )   G  G g   G
 FD ,Gas  K GL (u IL  u IG )
dz
dz G
I
 u IG
( N iGL aGL M i   N iGS aGS M i )
Energy
d ( L u IL  L H L )
 E GL aGL  E LS aSL  E LA aLA
dz
d ( G u IG  G H G )
  E GL aGL  E GS aGS  E GA aGA
dz
Fluxes Modeled by Multicomponent Stefan-Maxwell Formulation
CREL
L-III Model Catalyst Scale Equations
(Extension of Harold,1988)
Liquid Filled Zone
Fully Externally Wetted,
Partially Liquid Filled Pellet
d
N    R
C
iL
dx
NiLC 
i ,liq
 
CiLC C nc 1
d C
NtL  {[ BL ]1[]
CiL }i , j
C
CtL
dx
j 1
2
keL
i
 
d
TLC 
2
dx
d  N iLC H iLC
dx
| Wet Zone
| Dry Zone |
0 
0Lc
0
Gas Filled Zone
 
d
C
N iG
  i Ri , gas
dx
C
NiG

keG
 
C
nc
CiG
d C
C
N

{[ BG ]1
CiG }i , j

tG
C
CtG
dx
j 1
d 2 C d  NiG H iG
TG 
0
dx2
dx
 
C
C
Intra-catalyst G-L Interface
Continuity of temperature,
mass and energy fluxes,
and equilibrium relations
between compositions
CREL
Simulation Results: Multiplicity Effects
1.2
460
1
T emperature, K
440
Conversion
0.8
0.6
Wet Branch(Exp t)
0.4
Dry Branch (Exp t)
Dry Branch (L-II)
Dry Branch (L-III)
380
N=6 .3 , Dry (L-III)
N=8 , Dry (L-III)
360
N=1 1 , Dry (L-III)
300
0
5
10
Hydrogen Feed Ratio, N
N=4 .5 , Wet (L-III)
320
Wet Branch, (L-III)
0
N=2 .8 , Wet (L-III)
400
340
Wet Branch (L-II)
0.2
N=1 .4 , Wet (L-III)
420
15
0
0.05
0.1
0.15
Axial Location, m
0.2
• Hysteresis Predicted
• Wet Branch Temperature Rise (~10-
• Two Distinct Rate Branches Predicted
(As Observed by Hanika, 1975)
• Branch Continuation, Ignition and
Extinction Points
• Wet Branch Conversion (~30 %)
• Dry Branch Conversion (> 95 %)
15 oC)
• Dry Branch Temperature Rise
(~140-160 oC)
CREL
Wet Branch Simulation
0.045
0.35
0.04
0.035
0.3
0.03
Gas Velocity
0.25
0.2
0.15
Gas Holdup
0.025
Liquid Velocity
Liquid Holdup
0.02
0.015
0.1
0.01
N=2.8, Wet Branch (L-III)
0.05
0.005
0
0.012
0
0
0.05
0.1
0.15
Axial Location,m
0.2
6500
16
6000
14
5500
12
5000
cyclohexene
10
4500
cyclohexane
8
hydrogen
4000
6
N=2.8, Wet Branch (L-III)
3500
4
3000
2
2500
0
0
0.05
0.1
Axial Location,m
0.15
0.2
0.008
N(H2 ),z=
0 .0 1
N(H2 ),z=
0 .0 5
N(H2 ),z=
0 .1 5
N(ene),z
=0 .0 1
N(ene),z
=0 .0 5
N(ene),z
=0 .1 5
N(ane),z
=0 .0 1
N(ane),z
=0 .0 5
N(ane),z
=0 .1 5
0.006
0.004
0.002
0
-0.002
Concentration, mol/m3
Concentration, mol/m3
Reactor Scale Species Concentrations
0.01
Intracatalyst Flux, mol/m2s
0.4
Liquid Velocity (m/s) and Holdup
Gas Velocity (m/s) and Holdup
Reactor Scale Hydrodynamics
-0.004
N=2 .8 , Wet Branch (L-III)
-0.006
0
0.5
1
Intracatalyst Coordinate, (x/)
Pellet Scale Multicomponent
Flux Profiles
CREL
Dry Branch Simulation
Pellet Scale Pressure Profiles
Reactor Scale Hydrodynamics
0.25
1.900E+05
99990
1.800E+05
99980
1.700E+05
99970
1.600E+05
99960
99950
0.2
99940
Gas Velocity, m/s
Pressure, N/m2
0.15
Pressure, N/m2
Gas Velocity, m/s
0.3
100000
99930
99920
99910
N=8, Dry Branch, (LIII)
0.1
Pressure, N/m2
0.35
1.400E+05
1.300E+05
1.200E+05
0.05
0.1
0.15
Axial Location
m
m
m
m
N=8, Dry Branch (LIII)
1.100E+05
1.000E+05
9.000E+04
99900
0
z=0.01
z=0.03
z=0.06
z=0.12
1.500E+05
0
0.2
0.2
0.4
0.6
0.8
1
Intracatalyst Coordinate, ((x-)/(Lc-))
Reactor Scale Species Concentrations Pellet Scale Temperature Profiles
N=8, Dry Branch (LIII)
550
500
20
T emperature, K
Concentration, mol/m3
25
15
10
z=0 .0 1
400
z=0 .0 3
z=0 .0 6
hydrogen
cyclohexene
cyclohexane
5
450
350
z=0 .1 2
N=8, Dry Branch (LIII)
300
0
0
0
0.05
0.1
0.15
Axial Location,m
0.2
0.2
0.4
0.6
0.8
1
Intracatalyst Coordinate, ((x-)/(Lc-))
CREL
Intra-reactor Wet-Dry Transition
0.0003
420
1
FL (liquid flow)
T emperature, K)
400
0.0002
380
0.00015
360
0.0001
340
0.00005
320
0
300
0
0.02
0.04
0.06
0.08
Axial Location, m
0.1
Mole Fractions, Wetting Fraction
0.00025
T emperature, K
Liquid and Vapor Flow, mol/s
Internal T ransition, N=7 (LII)
Fw (wetting)
YA (cyclohexene)
YB (hydrogen)
YC (cyclohexane)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
Internal T ransition, N=7 (L II)
0.1
0
0
0.05
0.1
Axial Location, m
0.15
• Abrupt drop in liquid flow
• Abrupt change in catalyst wetting
• Temperature rise after liquidgas transition
• Cyclohexene and cyclohexane
mole fraction shows evaporation
and reaction
CREL
Summary




The reactor scale variation of phase holdups and velocities, multiplicity,
and temperature rise were successfully simulated by the developed
(LIII) model more rigorously than other models developed earlier
Both wet and dry branch temperature, conversion, and corresponding
fluxes were successfully modeled by the set of equations developed
Intra-pellet reaction-transport equations for the wet and dry zones in
presence of multicomponent interactions, evaporation, and
condensation were successfully modeled
The wet to dry transition prediction requires robust numerical
techniques to yield stable solution at pellet scale (for the LIII model),
but depicts the observed abrupt transition with a reactor scale (LII)
model
CREL