PERFORMANCE STUDIES OF TRICKLE BED REACTORS

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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

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Trickle Bed Reactors
Catalyst Wetting Conditions in Trickle Bed Reactor
Cocurrent Downflow of Gas and Liquid on a Fixed
Catalyst Bed
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)
Limiting Reactant criterion
Gas limited reaction if
Liquid limited reaction if
Flow Map (Fukushima et al., 1977)
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FLOW REGIMES AND CATALYST WETTING
EFFECTSDOWNFLOW (TRICKLE BED REACTOR)
UPFLOW (PACKED BUBBLE COLUMN)
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Motivation

• A clear understanding of the differences between
  the two modes of operation is needed,
  particularly for high pressure 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 regarding the preferred
  mode of operation for scale-up/scale-down of
  reactors for gas or liquid reactant limited
  reactions

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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
  and GAS VELOCITY on the performance of both
  modes of operation
• Study the effect of FINES on the performance of
  the two modes at different feed concentrations
  and pressures
• Compare MODEL PREDICTIONS with experimental data
  at different pressures

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Reaction Scheme
Catalyst 2.5 Pd on Alumina
(cylindrical 0.13 cm dia.) Fines Silicon
carbide 0.02 cm Range of Experimentation
Alpha-methylstyrene cumene
B (l) A(g) P(l)

• 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 3.1 - 7.8 (230-600
  mol/m3)
• Operating Pressure 30 - 200 psig (3-15
  atm)
• Feed Temperature 24 oC

Limiting Reactant criterion
Gas limited reaction if
Liquid limited reaction if
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Experimental Setup
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Downflow and Upflow Experimental Results at Low
Pressure (Gas limited Reaction) without Fines
DOWNFLOW OUTPERFORMS UPFLOW DUE TO PARTIAL
EXTERNAL WETTING LEADING TO IMPROVED GAS
REACTANT ACCESS TO PARTICLES
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Downflow and Upflow Experimental Results at High
Pressure (Liquid limited Reaction) without Fines
UPFLOW OUTPERFORMS DOWNFLOW DUE TO MORE COMPLETE
EXTERNAL WETTING LEADING TO BETTER TRANSPORT OF
LIQUID REACTANT TO THE CATALYST
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Downflow and Upflow Experimental Results at Low
Pressure (Gas limited Reaction) with Fines
ABOUT EQUAL PERFORMANCE DUE TO COMPLETE WETTING
Fines Packing Procedure Vol. of Fines Void
volume (Al-Dahhan et al. 1995)
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Downflow and Upflow Experimental Results at High
Pressure (Liquid limited Reaction) with Fines
SAME PERFORMANCE DUE TO COMPLETE WETTING
Fines Packing Procedure Vol. of Fines Void
volume (Al-Dahhan et al. 1995)
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Effect of Pressure on Downflow Performance
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Effect of Pressure (as transition to liquid
limitation occurs) on Upflow Reactor Performance.
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Slurry Kinetics
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El- Hisnawi (1982) model

• Reactor scale plug flow equations
• Liquid phase gas reactant concentration
• Constant effectiveness factor
• Modified by external contacting efficiency
• Allowance for rate enhancement on
• externally dry catalyst
• Direct access of gas on inactively wetted
  pellets.

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Beaudry (1987) model

• Pellet scale reaction diffusion equations
• For fully wetted and partially wetted slabs
• Effectiveness factor weighted based on
• contacting efficiency
• Overall effectiveness factor changes along
• the bed length
• Evaluation of overall effectiveness with change
  in
• concentration and contacting
• Overall Effectiveness factor at any location

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Upflow and Downflow Performance at Low Pressure
(Gas Limited Condition)Experimental Data and
Model Predictions
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Upflow and Downflow Performance at High Pressure
(Liquid Limited Conditions) Experimental Data
and Model Predictions
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Summary

• DOWNFLOW PERFORMS BETTER AT LOW PRESSURE.
• (Hydrogenation of alpha-methylstyrene is a gas
  limited reaction.
• Partial wetting is helpful in this situation.)
• UPFLOW PERFORMS BETTER 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)

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Unsteady State Operation in Trickle Bed
ReactorsModulation of input variables or
parameters to create unsteady state conditions to
achieve performance better than that attainable
with steady state operation Motivation

• Performance enhancement in existing reactors
• Design and operation of new reactors
• Lack of systematic experimental or rigorous
  modeling studies in lab reactors necessary for
  industrial application
• Two Scenarios
• Gas Limited Reactions
• Liquid Limited Reactions

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Objectives

• To experimentally investigate trickle bed
  performance under unsteady state operation (flow
  modulation) for gas and liquid limited conditions
  for a test hydrogenation system
• To develop model equations for unsteady state
  phenomena occurring in trickle-bed reactors
• To simulate unsteady state transport processes in
  trickle-bed reactors including bulk and
  interphase momentum, mass, and energy transport
  for the test reaction system

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Strategies for Unsteady State Operation

• Flow Modulation (Gupta, 1985 Haure, 1990 Lee
  and Silveston, 1995)
• Liquid or Gas Flow
• Isothermal/Non-Isothermal
• Adiabatic
• Composition Modulation (Lange, 1993)
• Pure or Diluted Liquid/Gas
• Isothermal/Non-Isothermal
• Adiabatic
• Activity Modulation (Chanchlani, 1994 Haure,
  1994)
• Enhance activity due to pulsed component
• Removal of product from catalyst site
• Catalyst regeneration due to pulse

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Possible Advantages of Unsteady State Operation

• Gas Limited Reactions
• Partial Wetting of Catalyst Pellets -Desirable
• Internal wetting of catalyst
• Externally dry pellets for direct access of gas
• Replenishment of reactant and periodic product
  removal
• Catalyst reactivation
• Liquid Limited Reactions
• Partial Wetting of Catalyst Pellets-Undesirable
• Achievement of complete catalyst wetting
• Controlled temperature rise and hotspot removal

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Test Reaction and Operating Conditions
Alpha-methylstyrene hydrogenation to isopropyl
benzene (cumene)
Operating Conditions

• Superficial Liquid Mass Velocity 0.1-3.0
  kg/m2s
• Superficial Gas Mass Velocity
  3.3x10-3-15x10-3 kg/m2
• Feed Concentration 2 .7 - 20 (200-1500
  mol/m3)
• Cycle time (Total Period) 40-900 s
• Cycle split (ON Flow Fraction) 0.1-0.6
• Max. Allowed temperature rise 25 oC
• Operating Pressure 30 -200 psig (3-15
  atm)
• Feed Temperature 20-35 oC

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Experimental Results
Gas Limited Conditions (g 20) Low Pressure,
High Liquid Feed Concentration
Liquid Limited Conditions (g 2) High Pressure,
Low Liquid Feed Concentration
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Effect of Cycle Split on Performance Enhancement
Steady State
Gas Limited Conditions (g 20) Operating
Conditions Pressure30 psig Liquid Reactant
Feed Concentration 1484 mol/m3 Cycle Split (St)
Liquid ON Period/Total Cycle Period(T)
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Phenomena occurring under unsteady state
operation with flow modulation in a trickle-bed
reactor
GOAL To Predict Velocity, Holdup, Concentration
and Temperature Profiles
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The Model Structure
Bulk Phase Equations
Species
Energy
zL
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Advantages of Maxwell-Stefan Multicomponent
Transport Equations over Conventional Models

• Multicomponent effects are considered for
  individual component transport ks are
  matrices
• Bulk transport across the interface is
  considered
• Nt coupled to energy balance (non zero)
• Transport coefficients are corrected for high
  fluxes
• k corrected to ko kF
  exp(F)-I-1
• Concentration effects and individual pair binary
  mass transfer coefficients considered
• Thermodynamic non-idealities are considered by
  activity correction of transport coefficients
• Holdups and velocities are affected by interphase
  mass transport and corrected while solving
  continuity and momentum equations

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Flow Model Equations
Momentum
uiL,uiG
Continuity
eL,eG,P
Pressure
Z
Staggered 1-D Grid
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Stefan-Maxwell Flux Equations for Interphase Mass
and Energy Transport
Gas-Liquid Fluxes
Liquid-Solid and Gas-Solid Fluxes

• Bootstrap Condition for Multicomponent Transport
• Interphase Energy Flux for the Gas-Liquid
  Transport and Bulk to Catalyst
• Interface Transport
• Net Zero Volumetric Flux for Liquid-Solid and
  Gas-Liquid Interface for
• Intracatalyst Flux

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Catalyst Level Equations
Approach I Rigorous Single Pellet Solution of
Intrapellet Profiles along with
Liquid-Solid and Gas-Solid Equations
CiCP
L
G
xc
Approach II Apparent Rate Multipellet Model
Solution of Liquid-Solid and Gas-Solid
Equations
CiCP
CiCP
CiCP
L
L
G
L
G
G
Type III Both Sides Externally Dry
Type I Both Sides Externally Wetted
Type II Half Wetted
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Holdup and Liquid Velocity Profiles
Operating Conditions Liquid ON time 15 s, OFF
time65 s Liquid ON Mass Velocity 1.4
kg/m2s Liquid OFF Mass Velocity 0.067
kg/m2s Gas Mass Velocity
0.0192 kg/m2s
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Pseudo-Transient Simulation Results
Alpha-methylstyrene Concentration Profiles
time,s
Alpha-methylstyrene Concentration buildup in the
reactor to steady state or during ON
cycle of flow modulation Feed Concentration
200 mol/m3 Pressure 1 atm. Reaction
Conditions Gas Limited (g 10)
(Intrinsic Rate Zero order w.r.t. Alpha-MS)
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Pseudo-Transient Cumene and Hydrogen
Concentration Profiles
Profiles show build up of Cumene and Hydrogen
profiles to steady state or during ON part of
the pulse
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Alpha-methylstyrene and Cumene Concentration
Profiles During Flow Modulation
Supply and Consumption of AMS and Corresponding
Rise in Cumene Concentration Operating
Conditions Cycle period40 sec, Split0.5
(Liquid ON20 s) Liquid ON Mass Velocity
1.01 kg/m2s Liquid OFF Mass Velocity 0.05
kg/m2s Gas Mass Velocity
0.0172 kg/m2s
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Catalyst Level Hydrogen and Alpha-methylstyrene
Concentration Profiles During Flow Modulation
Concentration of Alpha-MS in previously dry
pellets during Liquid ON (120s, Wetted Catalyst
) and Liquid OFF(2040 s, Dry catalyst)
Concentration of Hydrogen during Liquid ON
(120s, Wetted Catalyst ) and Liquid OFF(2040
s, Dry catalyst) for negligible reaction test
case
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Conclusions

• Performance enhancement under unsteady state
  operation is demonstrated to be
• significantly dependent on reaction and
  operating conditions
• Rigorous modeling of mass and energy transport
  by Maxwell-Stefan equations
• and solution of momentum equations needed to
  simulate unsteady state flow,
• transport and reaction occurring in a trickle
  bed reactor has been accomplished.
• This algorithm can be used as a generalized
  simulator for any multicomponent,
• multi-reaction system and converted to a
  multidimensional code for large scale
• industrial reactors.
• Pseudo-transient and transient operation is
  simulated for the case of liquid flow
• modulation to demonstrate performance
  enhancement under unsteady state
• conditions. Product formation rate is enhanced
  due to increased supply of liquid
• reactant to dry pellets (during ON cycle) and
  gaseous reactant to previously
• wetted pellets (during OFF cycle). Exothermic
  enhancement and higher
• hydrogen solubility can also be taken
  advantage of in the OFF cycle due to
• systematic quenching during the ON cycle.

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