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PERFORMANCE STUDIES OF TRICKLE BED REACTORS

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. Trickle Bed Reactors.

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PERFORMANCE STUDIES OF TRICKLE BED REACTORS

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  1. 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

  2. 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) CREL

  3. FLOW REGIMES AND CATALYST WETTING EFFECTSDOWNFLOW (TRICKLE BED REACTOR) UPFLOW (PACKED BUBBLE COLUMN) CREL

  4. 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

  5. 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

  6. 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 CREL

  7. Experimental Setup CREL

  8. 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 CREL

  9. 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 CREL

  10. 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) CREL

  11. 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) CREL

  12. Effect of Pressure on Downflow Performance CREL

  13. Effect of Pressure (as transition to liquid limitation occurs) on Upflow Reactor Performance. CREL

  14. Slurry Kinetics CREL

  15. 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. CREL

  16. 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 CREL

  17. Upflow and Downflow Performance at Low Pressure (Gas Limited Condition)Experimental Data and Model Predictions CREL

  18. Upflow and Downflow Performance at High Pressure (Liquid Limited Conditions): Experimental Data and Model Predictions CREL

  19. 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) CREL

  20. Unsteady State Operation in Trickle Bed Reactors“Modulation 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 CREL

  21. 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 CREL

  22. 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 CREL

  23. 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 CREL

  24. 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 CREL

  25. Experimental Results Gas Limited Conditions (g = 20) Low Pressure, High Liquid Feed Concentration Liquid Limited Conditions (g = 2) High Pressure, Low Liquid Feed Concentration CREL

  26. Effect of Cycle Split on Performance Enhancement Steady State Gas Limited Conditions (g = 20) Operating Conditions : Pressure=30 psig Liquid Reactant Feed Concentration= 1484 mol/m3 Cycle Split (St)= Liquid ON Period/Total Cycle Period(T) CREL

  27. Phenomena occurring under unsteady state operation with flow modulation in a trickle-bed reactor GOAL: To Predict Velocity, Holdup, Concentration and Temperature Profiles CREL

  28. SOLID LIQUID GAS NiGS NiLS EGS z=0 ELS NiGL C1G C2G . . CnG C1L C2L . . CnL EGL NiLS NiGS EGS ELS NiGL EGL The Model Structure Bulk Phase Equations Species Energy z=L CREL

  29. Advantages of Maxwell-Stefan Multicomponent Transport Equations over Conventional Models • Multicomponent effects are considered for individual component transport [k]’s 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] = [k][F] [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

  30. Flow Model Equations Momentum uiL,uiG Continuity eL,eG,P Pressure Z Staggered 1-D Grid CREL

  31. 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 CREL

  32. 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 G L L G G Type III: Both Sides Externally Dry Type I: Both Sides Externally Wetted Type II: Half Wetted CREL

  33. Holdup and Liquid Velocity Profiles Operating Conditions: Liquid ON time= 15 s, OFF time=65 s Liquid ON Mass Velocity : 1.4 kg/m2s Liquid OFF Mass Velocity: 0.067 kg/m2s Gas Mass Velocity : 0.0192 kg/m2s CREL

  34. Pseudo-Transient Simulation ResultsAlpha-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) CREL

  35. 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 CREL

  36. Alpha-methylstyrene and Cumene Concentration Profiles During Flow Modulation Supply and Consumption of AMS and Corresponding Rise in Cumene Concentration Operating Conditions: Cycle period=40 sec, Split=0.5 (Liquid ON=20 s) Liquid ON Mass Velocity : 1.01 kg/m2s Liquid OFF Mass Velocity: 0.05 kg/m2s Gas Mass Velocity : 0.0172 kg/m2s CREL

  37. Catalyst Level Hydrogen and Alpha-methylstyrene Concentration Profiles During Flow Modulation Concentration of Alpha-MS in previously dry pellets during Liquid ON (1:20s, Wetted Catalyst ) and Liquid OFF(20:40 s, Dry catalyst) Concentration of Hydrogen during Liquid ON (1:20s, Wetted Catalyst ) and Liquid OFF(20:40 s, Dry catalyst) for negligible reaction test case CREL

  38. 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. CREL

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