3D Modelling by Computational Fluid Dynamics of Local Interactions ofMomentum, Mass and Heat Transfers with Catalyst Deactivation in Gas-SolidCatalytic Reactors of Low Aspect RatiosByFaris Abdullah AlzahraniA thesis submitted to Lancaster University in partial fulfilmentof the requirements for the degree of Doctor of Philosophyin Chemical EngineeringOctober 2016Engineering DepartmentFaculty of Science and TechnologyLancaster UniversityI
DeclarationI declare that this thesis consists of original work undertaken solely by myself at LancasterUniversity between the years 2013 and 2016. In case where contributions by other authors arereferred to, they have been properly referenced.October 2016II
AcknowledgementThis project could not have been finished without the support of many people. I would like toextend my sincerest thanks to my supervisor, Dr. Farid Aiouache, for his guidance throughout thecourse of this work. His advices, comments and support during this research work werefundamental for its development. I appreciate greatly his time, continuous patience, priceless helpand encouragement, enthusiasm and truly motivating introduction to the world of chemicalengineering research. It has been a great pleasure and an honour to work for him. I would not havebeen contemplated such an undertaking without him. The support from the Chemical engineeringDepartment of Lancaster University is gratefully acknowledged. Besides the academics, there weremany people who defined my life during my work, I am very grateful to them. A huge thank yougoes to my wife who has tolerated me while undertaking this academic and research life. At last,but not least, I would like to thank my mother and my brothers and my sisters for their support inmy choices; they made me who I am today.Thanks everybodyIII
AbstractPacked beds of gas-solid systems are extensively used as reactors, separators, dryers, filters, heatexchangers and combustors. The design of packed beds requires a detailed knowledge of localdynamics of flow, composition and temperature. Unfortunately, investigations for the developmentof 3D modelling codes by computational fluid dynamics are still not sufficiently mature comparedwith those relying on 2D modelling or simplified pseudo-homogenous models. This projectinvestigates non-uniform catalyst deactivation in packed bed reactors of low aspect ratios understeady-state and dynamic operations. Low aspect ratio packed beds were selected as they are knownto generate non-uniform distribution of local flow. Detailed knowledge of flow dynamics in termsof local structure of the packed bed, pressure drops, interstitial flow, heat and mass rate distributionswas examined. The discrete element method was used to generate various packing configurationsand the results of profiles of porosity were in a good agreement with the semi-analytical models,especially, in the vicinity of the wall. Similar oscillation trends with damping profiles towards thecentre of the packed beds were observed. Flow heterogeneity was assessed by tests of mass transferdispersion through a Lagrangian approach. Interactions of fluid flow, mass and heat transfers, andlocal deactivation of alumina catalyst Al2O3 of CO oxidation were investigated under design andoperating conditions. An increase in the activation energy of deactivation promoted the deactivationby accelerating the reaction rate and releasing additional thermal energy, which in turn acceleratedthe deactivation. The 3D modelling allowed observation of local catalyst deactivation at packingpore level which is typically not accessible by the 2D modelling or pseudo-homogeneous models.In addition, the deactivation was quite asymmetrical along axial and radial directions, leading touneven rates of thermal expansion and contraction and causing local deactivation associated withtemperature runaways.IV
ContentsList of figures IXList of tables .XIIAbbreviations .XIIINomenclature .XIV1. Introduction .11.1Problem statement .11.2 Research motivation .21.3 Objectives 31.4 Thesis outline .41.5 Methodology (work plan) 61.6 Contribution of the thesis 61.7 Publications &Presentations & trainings related to this work .72. Literature survey 102.1 Engineering reactors for catalytic reactions 102.2 Packed bed reactors (PBRs) .112.2.1Introduction .112.2.2Modelling and Design of packed bed reactors (PBRs) .122.2.3Aspect ratio (AR) .142.2.3.1 Low aspect ratios (ARs) of Packed bed reactors .142.2.3.2 Some previous studies on low aspect ratios (AR) of PBRs .162.3 Catalyst deactivation in packed bed reactors .172.3.1Mechanisms (types) of catalyst deactivation 182.3.2Kinetics of catalyst deactivation .232.3.3Influence of composition and temperature gradients on deactivation .262.3.3.1 Internal heat and mass resistances 262.3.3.2 Mass and heat dispersions .272.3.3.3 Process design and reaction engineering . .28Process dynamics and catalyst monitoring. .292.3.4.1 Packed bed reactors with spatially variable activity .292.3.4.2 Dynamic instability of PBR in presence of spatially variable activity .302.4 Challenges and limitations of modelling PBRs of low aspect ratio AR .31Modelling transport properties .322.3.42.4.1V
2.4.2Computational fluid dynamics (CFD) simulation and PBRs of low AR .342.4.3Derivative framework for CFD modelling of PBRs .352.4.4 Characteristics of computational fluid dynamic (CFD) 38Packing geometry .392.4.5.1 Discrete element method (DEM) .402.4.5.2 Particle -flow code of three –dimensional (PFC3D) .40Commonly used CFD codes .422.4.6.1 PHOENICS .422.4.6.2 ANSYS Fluent .432.4.6.3 COMSOL Multiphysics 432.5 Packed bed structure and flow dynamics 442.5.1 Packing structure and porosity distribution . .442.5.1.1 Average porosity (Bed porosity).442.5.1.2 Locally radial porosity .452.5.1.3 Effect of confining wall of low (AR) of PBRs .472.5.2 Pressure drop in low aspect ratio (AR) of PBRs. .472.5.3 Local velocity distribution in low aspect ratio (AR) of PBRs. .482.5.4 Particle tracking of mass dispersion (PTM) in PBRs .493. Experimental and Numerical Methods . .533.1 Pressure drop tests .533.2 CFD Simulation Part 563.2.1 Model development .563.2.1.1 Packing generation by DEM .563.2.1.2 Meshing modulation 594.0 Analysis of fluid flow .624.1 Introduction . 624.2 DEM and CFD (Brief review) .634.3 Description of 3D Fluid flow model .644.4 Fluid flow profiles by 3D modelling .674.4.1Structural porosity profiles .674.4.2Pressure drop profiles .734.4.3Velocity field profiles 794.5Mass dispersion model .834.5.1Mass dispersion profiles by CFD .852.4.52.4.6VI
4.6 Summary of the chapter .885.0 Analysis of mass and heat transfer in PBRs .905.1 Introduction 905.2 Carbon monoxide (CO) oxidation .925.2.1 Theoretical background 925.3 3D Modelling description .945.3.1 Model equations .945.3.2 Boundary conditions and solver details .97Results and Discussions .985.4.1 Validation of CFD model for CO oxidation .985.4.1.1 Effect of inlet temperature on total conversion rate of CO oxidation .995.4.1.2 Effect of flow on conversion of CO oxidation .1025.4.1.3 Effect the Size of ARs on the CO oxidation .1035.4.2 Temperature profiles inside the packed bed reactor .1055.4.3 Summary of the chapter .1156.0 Analysis of Catalyst Deactivation in Packed bed reactor 1166.1 introduction .1166.2 Model development of deactivation 1176.2.1 Model description by 2D modelling .1176.2.2 Model description by 3D modelling .1206.3 Results and Discussions .1226.3.1By 2D Model 1226.3.1.1 Effect of dimensionless activation energy of deactivation (ϒD) .1226.3.1.2 Effect of the Damköhler number of deactivation reaction (DaD) .1266.3.1.3 Effect of local deactivation on the temperature runaway .1276.3.2By 3D Model .1296.3.2.1Effect of dimensionless activation energy of deactivation (ϒD) 1296.3.2.2Effect of the Damköhler number (DaD) .1356.3.2.3Investigation of wrong way behaviour .1366.3.2.4Effect of peclet number of axial dispersion (Pe) 1386.3.2.4.1 Effect of Peclet Number of Mass Dispersion (Pe1) .1386.3.2.4.2 Effect of Peclet Number of Heat Dispersion (Pe2) 1406.3.2.5 Effect of internal mass transfer coefficient on deactivation profiles 1416.3.2.6 The effect of thermal inertia of the solid catalytic phase .1435.4VII
6.3.3 Comparison of ϒD and DaD profiles obtained by 2D and 3D modelling .1436.3.4 Summary of the chapter .1457.0 Conclusions and Recommendations .1477.1 Conclusions .1477.2 Recommendations .150References .151VIII
Lists of FiguresFigure 2.1: Effect of catalyst during chemical reaction .11Figure 2.2: Schematic illustration of a packed bed reactor .13Figure 2.3: Different sizes of Packed Bed Reactors .14Figure 2.4: Major types of deactivation in heterogeneous catalysis .20Figure 2.5: Schematic of the different stages in the formation and growth of particlesfrom a monomer dispersion .21Figure 2.6: Derivative framework for CFD modelling of a PBR .36Figure 2.7: Example of unstructured 3D mesh applied to a cylindrical tube 38Figure 2.8: Generation of the random packing: raining process (a) and the resultingsphere packing (b) .39Figure 2.9: Calculation cycle in PFC3D, ITASCA, 2008 .42Figure 2.10: Time loop of the particle tracking .51Figure 3.1: Experimental pressure drops setup .54Figure 3.2:Random arrangements of various AR of the packed bed were builtexperimentally:(a) AR2, (b)AR2.5, (c) AR3.3 and (d) AR5 for tube size 10 mm .55Figure 3.3: Three randomly generated packings by DEM (PFC3d) for (a) AR: 3, (b)AR: 6 and (c) AR: 9 .56Figure 3.4: Building steps of a random packed bed reactor for COMSOL modelling(herein AR 3 as example) .58Figure 3.5: Different sizes of meshes applied for a random packed bed reactor byCOMSOL, (a) Large mesh, (b) small mesh for AR: 1.5 .60Figure 3.6: Calculation procedure of CFD modelling .61Figure 4.1: Comparison of the packing: (a) by DEM and (b) by experimental forAR3 63Figure 4.2: Reduction procedure of volumetric 3D domain index data. (a) Crosssectional slicing of domain index, (b) irregular distribution of domain index dataretrieved and (c) averaging procedure from 3D domain index to 2D and to 1Ddata .69Figure 4.3: Spatial distribution of surface fraction εs of the solid particles for packedbeds of ARs from 1.5 to 5 .71Figure 4.4: Averaged porosity variation along the radial coordinates at (a) AR: 2, (b)AR: 3, and (c) AR: 5. (Dots: simulation data; lines: Mueller’s model) .Figure 4.5: Pressure drop profiles for (a) AR: 1.5 and (b) AR: 5; dot: CFDIX73
simulation .76Figure 4.6: Ratio of pressure drop to pressure drop of infinite packing profiles, (a)simulation, and experimental data, and (b1 b4) Carman, Zhavoronkov, Ergun, andReichelt models 77Figure 4.7: Cross-sectional and vertical cuts of the velocity (m s-1) inside a packed bedreactor: (a) AR 1.5, Re 375 and (b) AR3, Re 187 .79Figure 4.8: Velocity distribution in 2D profile in a packed bed reactor: (a) AR1.5, (b)AR2, (c) AR 2.5, (d) AR3, and (e) AR5 81Figure 4.9: Pressure drop contours for AR 2: (a) 3D modelling and (b) 2D modelling(Re 284) .83Figure 4.10: 2D vertical slices of particle tracers. AR 2 (a1: Pe 0.01 and a2: Pe 100) and AR 4 (b1: Pe 0.01 and b2: Pe 100) 86Figure 4.11: Axial and radial dispersion coefficients along with flow dynamics,respectively: (a1, a2) for AR of 2 and (b1, b2) for AR of 4 .87Figure 5.1: Conversion extent of CO versus temperature for (AR3) .100Figure 5.2: Conversion distribution of CO obtained by CFD for AR 3: (a1) 3D view onslices and (a2) 2D contour plot .101Figure 5.3: Temperature profiles of CO obtained by CFD for AR 3: (b1) 3D view onslices and (b2) 2D contour plot .102Figure 5.4: The simulated and measured conversions as function of Re. AR3 .103Figure 5.5: Effect of size of aspect ratio AR on steady-state conversion for carbonoxidation of CO, inlet velocity u0 3.11 (m s-1) .104Figure 5.6: Axial temperature distributions at different values of Re for AR: (a) AR2.5, (b) AR 3 and (C) AR5, inlet temperature T0 458 K 107Figure 5.7: (a) 2D contour plot of temperature on the mid-plane. (b) 2D contour plotof temperature on cross section, Re 240 108Figure 5.8: Temperature distributions at different bed depths for AR4. (Cross sign:simulation data; square shape: literature data by Behnam et al.) .110Figure 5.9: Compression of the simulated Nusselt number, 𝑁𝑢 with publishedcorrelations, for AR 4 111Figure 5.10: Temperature profiles for different, inlet feed temperature, T0; (lines:simulation data; Dots: Experimental data by Jaree et al 112Figure 5.11: 2D map of temperature distribution obtained by CFD with time at; T0 458K and Re 187: (a) AR 2.5 and (b) AR 5 .X115
Figure 6.1: Effect of deactivation energy (ϒD) on the catalytic activity profile by (2D)for AR 3; (a) and (c) by CFD simulation: (b) and (d) are Jaree’s results [59]: (e)Temperature by CFD, (DaD 4 10-6, Pe1 300, Pe2 40, Da 0.4, ϒ 15 and at τ 50,000) .125Figure 6.2: Effect of DaD on the catalytic activity profile (2D) for AR 3; (a) by CFDsimulation and (b) Jaree’s results [59] :( Pe1 300, Pe2 40, Da 0.4, ϒ 15, ϒD 15 andat τ 50,000) .127Figure 6.3: Temperature at different axial positions; (DaD 4 10 6, Da 0.4, ϒ 15,ϒD 5.5 and τtotal 5000) .128Figure 6.4: Effect of ϒD on catalyst activity profile by CFD (3D), (DaD 4 10-6, Da 0.4, ϒ 15 and at τ 50,000) .131Figure 6.5: 3D Map of activity coefficient under effect of different values of ϒD forAR3 by 3D CFD; (5, 13 and 17), (a, b and c) respectively .133Figure 6.6: 2D Cross- section profiles of deactivation coefficient by CFD ; (a) AR3and (b) AR 4 at ϒD 17 .134Figure 6.7 Effect of DaD on the catalytic activity profile by 3D CFD, :( Da 0.4, ϒ 15, ϒD 15 and at τ 50,000) .135Figure 6.8: History plot of temperature ratio (T/T0) at different axial positions by 3D(DaD 4 10 6, Da 0.4, ϒ 15, ϒD 5.5 and τ total 50,000 136Figure 6.9: History plot of temperature ratio (T/T0) at different axial positions; byCFD (3D) :( DaD 0.004, Da 0.4, ϒ 15, ϒD 4 and τ total 3000) .137Figure 6.10: Effect of Pe1 on the catalytic activity profile (3D CFD); (ϒ 15, ϒD 15,DaD 4 10 6, Da 0.4 and at τ 50,000) .139Figure 6.11: Effect of Pe2 on the catalytic activity profile (3D CFD) ;(ϒ 15, ϒD 15,DaD 4 10 6, Da 0.4 and at τ 50,000) .141Figure 6.12: Plot of internal mass transfer effect (3D CFD); (DaD 4 10 6, Da 0.4,ϒ 15, ϒD 15 and at τ 50,000) .142Figure 6.13: Axially cross-sectional profiles of activity coefficient for AR 3 .142Figure 6.14: Effect of CpS on the catalytic activity profile by 3D CDF 143Figure 6.15 : Plot of ϒD (a , b) and DaD ( c ) effect on the catalytic activity by 2D(solid line) and 3D (dotted line) modelling for AR3 .XI145
List of TablesTable 2.1 Main packed bed catalytic processes .10Table 3.1 Setting Parameters of DEM Based Modelling .57Table 3.2 Packing Parameters for different AR (mm) .59Table 4.1 Porosity trends for ARs from 1.5 to 5 .70Table 5.1 Properties of particles used .98XII
AbbreviationsAR: tube-to-particle diameter Aspect RatioCFD: Computational Fluid DynamicsDEM: Discrete Element MethodFD: Finite DifferencesFE: Finite ElementsFV: Finite VolumesGCI: Grid Convergence IndexGMRES: Generalized Minimal Residual SolverI.D: Internal DiameterLDV: Laser Doppler VelocimetryMRI: Magnetic Resonance ImagingPBR: Packed Bed ReactorPFC3D: Particle -Flow Code of Three –DimensionalPT: Particle TrackingPTM: Patrick Tracking MethodXIII
NomenclatureAw non-dimensional pressure drops model constant(-)Bw non-dimensional pressure drops model constant(-)C concentration (mol m-3)C0 inlet concentration of reactant (mol m-3)D tube diameter (m)DI domain index (-)Dm molecular diffusion coefficient (m2 s-1)dp particle diameter (m)Dax/rad axial or radial dispersion coefficient (m2 s 1)ea21 relative error (-)F drag force per unit of volume (N m 3)K permeability of the packed bed (m2)h grid size (-)m apparent order (-)L length of bed (m)n particle indexNP number of Lagrangian tracer particles (-)N number of cells (-)p pressure drops (Pa)Pe1 Peclet number for mass dispersion (-)Pe2 Peclet number for heat dispersion (-)r radial coordinate (m)r̅ averaged displacements of all particles along the radial coordinates (m)r21 refinement factor (-)Re Reynolds number (-)t time (s)XIV
u fluid flow velocity (m s 1)x̅ averaged displacements of all particles along the axial coordinates (m)𝑟̅ averaged displacements of all particles along the radial coordinates (m)a catalyst activity coefficient (-)A pre-exponential constant of the main reaction (s-1)AD pre-exponential constant of the deactivation reaction (s-1)Cpg heat capacity of gas (J g-1 K-1)Cps heat capacity of solid (J g-1 K-1)D diffusion coefficient (m2s-1)Da Damkoehler number of the main reaction (-)DaD Damkoehler number of the deactivation reaction (-)Deff or Die effective diffusion coefficient (m2s-1)E activation energy (J mol K-1)ED activation energy of deactivation (J mol-1)ΔH heat of reaction (J mol-1)k effective axial conductivity (Wm-1 K-1)Δp pressure drop (Pa)r radial variable (m)re effective radius (m)rr radial position parameter (m)rs sphere center radial position (m)rse effective sphe
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