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Hydrodynamic Behaviour of the Torbed® Reactor Operating in Fine Particle Mode.

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Hydrodynamic Behaviour of the Torbed® Reactor Operating in Fine Particle Mode.

  1. 1. HYDRODYNAMIC BEHAVIOUR OF THE TORBED® REACTOR OPERATING IN FINE PARTICLE MODE by Grant Ashley Wellwood BE (Chem.) (with distinction) (Royal Melbourne Institute of Technology) A thesis submitted to The University of Queensland as a requirement for admission to the degree of DOCTOR OF PHILOSOPHY Department of Chemical Engineering University of Queensland Queensland 4072 AUSTRALIA 2000
  2. 2. ii
  3. 3. iii STATEMENT OF ORIGINALITY To the best of my knowledge and belief the work presented in this thesis, with the exception of acknowledged text references, is original and has not been submitted for a degree at any university either in whole or in part. Grant Ashley Wellwood
  4. 4. iv
  5. 5. v University of Queensland Abstract HYDRODYNAMIC BEHAVIOUR OF THE TORBED® REACTOR OPERATING IN FINE PARTICLE MODE by Grant Ashley Wellwood In “expanded-bed” mode, the Torbed® Reactor unit offers a unique gas-solid contacting capability by virtue of the enhanced transport (heat and mass transfer) and lower pressure drop environment provided. However a detailed understanding of both these process aspects was found to be lacking, which in-turn was retarding development. The focus of the following study was therefore to understand both these characteristics with a view to facilitating the important process development activities of technology selection, physical optimisation and scale-up. In many situations, particularly those involving fine powders, the magnitude of the system differential gas-solid or slip velocity controls the rate of transport phenomena. Although a relatively dilute gas-solid system, steady state slip velocities many times those of the single particle terminal velocities involved have been recorded in an expanded Torbed reactor. A detailed study of the Torbed reactor operated in this mode was therefore undertaken to identify the fundamentals underlying this slip velocity behaviour and therefore enable the effect to be optimised. Qualitative results indicate that the hydrodynamic regime prevalent during fine particle mode is conducive to a particular form of particle clustering known as streaming. Particle streamers increase the effective terminal velocity of the solids involved, which increases their effective slip velocity. The potential for using an equation analogous to the Ideal gas equation of state, to predict this behaviour was identified and investigated. The subsequent empirical study found general support for thermodynamic analogue, which provides a simple yet sound framework upon which to predict system performance as a function of easily determined inputs. The study also found that the value analogous to the Ideal gas constant, while not
  6. 6. vi an absolute constant across all systems, might be a simple function of particle geometry. To test the “robustness” of the thermodynamic analogue slip velocity model, preliminary validation was undertaken with a non-ideal solid (smelter grade alumina). While the initial objective of the study was to simply predict trends in slip velocity as a function of system configuration, the validation tests indicated the model is capable of predicting absolute values of slip velocity within ±30%. In the context of gas-solids contacting systems, this is an encouraging outcome. Further research is recommended to confirm these findings and refining the relationships developed. Slip velocity data generated as a result of this research was also used to define the operating window associated with the Torbed reactor operated in fine particle mode. Superimposing this information onto a conventional fluidisation map indicates the range of particle and gas characteristics over which the “expanded-bed” mode is stable. This information provides process developers with a clearer picture of the capabilities of the Torbed reactor in fine particle mode. The fluidisation map also helps identify the main points of difference that exist between the Torbed reactor and more traditional gas- solid contactors. Such information is essential if an informed decision regarding the most appropriate unit operation for a give duty is to be made. The other important characteristic of the Torbed reactor that was investigated is that of pressure drop. In terms of the overall cost of using the Torbed reactor, there is a link between slip velocity and system pressure drop. Intuitively, reactor modifications aimed at increasing slip velocity could be expected to impact on the velocity of gas passing through the distributor and therefore the pressure drop energy penalty. Pressure drop is a particularly important consideration in applications involving large gas flow rates. The relationship between pressure drop, reactor geometry and process conditions was therefore investigated. The findings indicate that pressure drop associated with the Torbed reactor is around an order of magnitude lower than most conventional gas-solid contactors and is controlled by kinetic energy losses through the distributor. As such, the Torbed reactor pressure drop is well described by a power law model. Although pressure drop across the Torbed reactor distributor is a function of velocity squared and therefore sensitive to velocity changes, its magnitude is an order less than those of conventional gas-solid contact devices. A simple pressure drop model was devised which enables the distributor pressure drop to be determined for a given reactor design and gas throughput.
  7. 7. vii The culmination of this research was a practical application of the slip velocity and pressure drop findings to the duty of dry scrubbing. Following successful piloting of the reactor, designed primarily on the findings of this research, commercial units with 6 metre internal diameters capable of processing 320,000 Nm3 /h.unit were commissioned. At present, there are 19 such units in service and their performance has proven to be consistent with model predictions. (Keywords: gas diffusion limited; slip velocity, streamers, clusters, thermodynamic analogy, pressure drop, dry scrubbing )
  8. 8. viii
  9. 9. ix TABLE OF CONTENTS TITLE PAGE I STATEMENT OF ORIGINALITY III ABSTRACT V TABLE OF CONTENTS IX LIST OF FIGURES XIII LIST OF TABLES XVII ACKNOWLEDGMENTS XIX TECHNICAL PAPERS AND PATENTS ARAISING FROM THIS RESEARCH XXI INTRODUCTION........................................................................................................................1 1.0 SCOPE AND PURPOSE..........................................................................................................1 1.1 WHAT IS A TORBED REACTOR?.........................................................................................1 1.2 ATTRIBUTES OF THE TORBED REACTOR AND POTENTIAL APPLICATIONS..........................7 1.3 FACTORS LIMITING DEVELOPMENT..................................................................................11 1.4 SCOPE AND DEVELOPMENT ..............................................................................................15 LITERATURE REVIEW .........................................................................................................22 2.0 SCOPE AND PURPOSE........................................................................................................22 2.1 ENHANCED SLIP VELOCITY CHARACTERISTICS - A DIFFERENTIATING FEATURE OF THE TORBED REACTOR UNIT...................................................23 2.2 TECHNICAL LITERATURE REGARDING THE TORBED REACTOR........................................26 2.3 SELECTION OF AN APPROPRIATE SCIENCE........................................................................27 2.3.1 Conventional Fluidisation Technology ...................................................................29 2.3.2 Fast-Fluidisation......................................................................................................31 2.3.2.1 Streamers......................................................................................................................... 36 2.3.2.1.1 Summary-Streamers ............................................................................................... 40 2.3.3 Dilute Phase Pneumatic Conveying........................................................................41 2.3.4 Batch Fluid Bed .......................................................................................................44 2.3.5 Air Slides..................................................................................................................46 2.3.6 Conclusion Regarding the Applicability of Existing Gas-Solid Hydrodynamic Theories..............................................................................................................................47 2.4 THE THERMODYNAMIC ANALOGY ...................................................................................48 2.4.1 Equations of State. ...................................................................................................49 2.4.1.1 Summary-Equations of State .......................................................................................... 54 2.4.2 Utilisation of the Ideal Gas Equation of State.........................................................54 2.4.3 Recent Developments...............................................................................................57 2.5 AREAS REQUIRING FURTHER RESEARCH .........................................................................59 2.6 SUMMARY.........................................................................................................................61 EXPERIMENTAL TECHNIQUE...........................................................................................64 3.0 SCOPE AND PURPOSE........................................................................................................64 3.1 EXPERIMENTAL OBJECTIVES ............................................................................................64 3.1.1 Success Criteria .......................................................................................................66 3.2 QUALITATIVE STUDY........................................................................................................66 3.2.1 Design Details-400mm Test Unit.............................................................................67 3.2.1.1 System Air Flow Rate Measurement.............................................................................. 67 3.2.2 Gas / Solid System....................................................................................................69 3.2.3 Chamber Pressure Measurement ............................................................................71 3.2.4 Operating Procedure...............................................................................................72 3.3 QUANTITATIVE STUDY......................................................................................................74
  10. 10. x 3.3.1 Detailed Design-Linear Track Reactor...................................................................74 3.3.1.1 Plenum and Freeboard Sections ..................................................................................... 74 3.3.1.2 Process Gas Distributor................................................................................................... 76 3.3.1.3 Air Moving Device ......................................................................................................... 78 3.3.1.4 Solids Feeder................................................................................................................... 78 3.3.1.5 Solids Disengagement..................................................................................................... 81 3.3.2 Characterisation of the Study Gas-Solid System.....................................................81 3.3.2.1 Gas Phase. ....................................................................................................................... 82 3.3.2.2 Solid Phase...................................................................................................................... 83 3.3.3 Instrumentation........................................................................................................88 3.3.3.1 Air Flow Rate.................................................................................................................. 89 3.3.3.2 Bed Profile ...................................................................................................................... 91 3.3.3.3 Differential Pressure Measurements............................................................................... 93 3.3.3.4 Measurement of Particulate Velocity ............................................................................. 94 3.3.4 Experimental Plan ...................................................................................................95 3.3.5 Test Procedure.........................................................................................................96 3.3.6 Sources of Error.......................................................................................................97 3.3.6.1 Human Error ................................................................................................................... 97 3.3.6.2 Instrument Error.............................................................................................................. 97 3.3.6.3 Process Error................................................................................................................... 98 3.3.7 Data Processing-Spreadsheet Calculations............................................................99 3.3.7.1 “Laboratory Book Reference” ........................................................................................ 99 3.3.7.2 “Blade Angle”................................................................................................................. 99 3.3.7.3 “Solid Type”.................................................................................................................... 99 3.3.7.4 “Access Point” (Elevation) ............................................................................................. 99 3.3.7.5 “Cut Plate Opening” ..................................................................................................... 100 3.3.7.6 “Nominal Solids Flow Rate” ........................................................................................ 102 3.3.7.7 “Test Duration”............................................................................................................. 102 3.3.7.8 “Mass Collected” .......................................................................................................... 102 3.3.7.9 “Solids Velocity”(Us ) and Standard Deviation............................................................ 102 3.3.7.10 “Pressure Drop” .......................................................................................................... 102 3.3.7.11 “Bed Height”............................................................................................................... 103 3.3.7.12 “Bed LO” (lift off)....................................................................................................... 103 3.3.7.13 “Mass Flow Rate”....................................................................................................... 103 3.3.7.14 “Superficial Gas Velocity-Horizontal” (Ugh)............................................................. 103 3.3.7 15 “Superficial Gas Velocity-Vertical” (Ugv) ................................................................. 104 3.3.7.16 “Slip Velocity” (Usl).................................................................................................... 104 3.3.7.17 “Bed Depth”................................................................................................................ 104 3.3.7.18 “Calculated Us”........................................................................................................... 104 3.3.7.19 “% Difference Up”....................................................................................................... 105 3.3.7.20 “Area Voidage”........................................................................................................... 105 3.3.7.21 “Volumetric Flow Rate”............................................................................................. 105 3.3.7.22 “Solids Loading”......................................................................................................... 105 RESULTS OF INVESTIGATIONS.......................................................................................108 4.0 SCOPE AND PURPOSE..............................................................................................108 4.2 QUALITATIVE STUDY ..............................................................................................108 4.1.1 Observations Relating to Gas-Solid Behaviour in the Torbed Reactor................109 4.1.1.1 Transitional Flow Regimes........................................................................................... 109 4.1.1.2 Particles Travel in Straight Lines.................................................................................. 113 4.1.1.3 Distributor Losses Dominate System Pressure Drop ................................................... 114 4.1.1.4 Distributor Pressure Drop Follows Power Law............................................................ 116 4.1.1.5 General Characteristics are Not Scale Sensitive........................................................... 123 4.1.2 Simplifying Assumptions........................................................................................124 4.1.3 Summary-Qualitative Assessment..........................................................................124 4.2 QUANTITATIVE STUDY....................................................................................................125 4.2.1 Base Model Configuration-30 o Blades.................................................................126 4.2.1.1 GB114um Ballotini / 30° Blades.................................................................................. 128 4.2.1.2 GB181um Ballotini / 30o Blades .................................................................................. 132 4.2.1.3 GB633um / 30o Blade System...................................................................................... 133
  11. 11. xi 4.2.1.4 Comparison of Proportionality Constant R*................................................................ 135 4.2.1.5 Implications................................................................................................................... 143 4.2.1.6 Conclusions................................................................................................................... 146 4.2.2 Reduced Blade Angle (20o ) Blade Set ...................................................................147 4.2.2.1 GB114um Ballotini / 20o Blade System....................................................................... 148 4.2.2.2 GB181um / 20o Blade System...................................................................................... 149 4.2.2.3 GB633um / 20o Blade System...................................................................................... 151 4.2.2.4 Comparison of R* Values............................................................................................. 153 4.2.3 Low Angle (10o ) Blade Set.....................................................................................154 4.2.3.1 Ballotini (GB114um, GB181um,GB633um) Investigated on 10o Blades................... 155 4.2.3.2 Comparison of Proportionality Constant R*-10o Blade Set......................................... 159 4.2.3.3 Conclusions................................................................................................................... 160 4.2.4 Slip Velocity Improvement Strategy Based on Blade Angle Reductions. .............160 4.3 NON SPHERICAL SOLIDS (ALUMINA) TRIALS .................................................................167 4.3.1 Base (30 o ) Blade Set - Smelter Grade Alumina System ......................................168 4.3.2 20o Blade Set..........................................................................................................171 4.3.3 Comparison of Slip Velocity Behaviour - Smelter Grade Alumina System..........173 4.3.4 Summary of Alumina/Air Tests..............................................................................174 4.3.5 Predictive Power and Functionality of the Model ................................................175 4.5 FIELD TRIALS ..........................................................................................................177 4.4.1 The Process Objective............................................................................................178 4.4.2 Current Best Practice Techniques.........................................................................178 4.4.2.1 Transport Reactors........................................................................................................ 179 4.4.2.2 Fluid Bed Dry Scrubbing Systems................................................................................ 180 4.4.3 Drivers for Improvement........................................................................................180 4.4.4 Torbed Reactor Pilot Plant Design and Application............................................183 4.4.5 Model Predictions Versus Actual Performance....................................................183 4.4.6 Post Study Developments.......................................................................................185 4.6 CLASSIFICATION OF THE TORBED REACTOR...........................................................189 4.6.1 Mapping of Experimental Results..........................................................................190 CONCLUSIONS AND IMPLICATIONS.............................................................................203 5.0 INTRODUCTION. ..............................................................................................................203 5.1 QUALITATIVE ASSESSMENT............................................................................................203 5.2 QUALITATIVE RESULTS...................................................................................................203 5.3 CLASSIFICATION OF THE TORBED REACTOR...................................................................203 5.4 AREAS REQUIRING FURTHER RESEARCH .......................................................................200 BIBLIOGRAPHY....................................................................................................................208 NOMENCLATURE ................................................................................................................218 APPENDIX 1 MATERIAL CHARACTERISTICS................................................................3 APPENDIX 2 EQUIPMENT SPECIFICATIONS 4 APPENDIX 3 QUALITATIVE ASSESSMENT DATA..........................................................5 APPENDIX 4 QUANTITATIVE ASSESSMENT DATA.......................................................6 APPENDIX 5 PUBLISHED SLIP VELOCITY DATA ..........................................................7
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  13. 13. xiii LIST OF FIGURES Number Page Figure 1.1: Function of Torbed Reactor Gas Distributor with Respect to Gas Flow.________2 Figure 1.2 a/b: Torbed Reactor Operating Modes __________________________________3 Figure 1.3: Overall (a) and Typical (b) Arrangements of a Torbed Reactor Unit __________6 Figure 1.4: Rate Limiting Steps-Shrinking Core Model ______________________________9 Figure 1.5: Gas-Solid Contactor Regime Diagram ________________________________14 Figure 2.1: Impact of Slip Velocity on Gas Phase Boundary Layer Thickness____________24 Figure 2.2: Typical Relationship Between Mass Transfer Coefficient and Slip Velocity. ____25 Figure 2.3: Typical Fast-Fluidised Bed Arrangement ______________________________32 Figure 2.4a: Single Particle in Vertical Free-Fall _________________________________36 Figure 2.4b: Vertical Streamer Formation _______________________________________36 Figure 2.4c: Horizontal Streamer Formation _____________________________________36 Figure 2.5: Horizontal Streamer Propulsion _____________________________________40 Figure 2.6: Arrangement of Gill Plate Distributor _________________________________45 Figure 2.7: General Arrangement of Air Slide Conveyor Segment_____________________47 Figure 2.8: Ideal Gas Equation Analogue Constants R* Calculated from Literature Data. _57 Figure 3.1: General Arrangement of the Torbed Reactor 400 mm ID Model_____________69 Figure 3.2: Plan View of Torbed Reactor 400mm Distributor With Respect to Pressure Sensing Points _____________________________________________________________72 Figure 3.3: General Arrangement of Linear Track Analogue_________________________75 Figure 3.4: Geometry of Torbed Reactor Distributor Blades _________________________77 Figure 3.5: Arrangement for Delivering Air to the Plenum __________________________79 Figure 3.6: General Arrangement of Solids Metering System Employed in Quantitative Linear Track Reactor Trials. ________________________________________________________80 Figure 3.7: Particle Size Distributions for Experimental Feedstocks ___________________87 Figure 3.8: Definition of Zones Defined in the Linear Track Reactor __________________89 Figure 3.9: Typical Blower/Slide Gate Valve Calibration Curve ______________________91 Figure 3.10: Example of Flash Photography Detection of the “Torbed” Regime Upper Interface. _________________________________________________________________93 Figure 3.11: Specimen Primary Data Collection Sheet ____________________________101 Figure 4.1: Transitional Gas-Solid Flow Regimes Observed in the Torbed Reactor.______112 Figure 4.2: Observed Trajectory of Fine Particles in a “Torbed” Regime______________114 Figure 4.3: Relationship between Distributor and Bed Pressure Drop -T400 Pilot Plant over a range of conditions- ________________________________________________________116 Figure 4.4a: Comparison of Torbed Reactor Distributor Pressure Drop Data __________122 Figure 4.4b: Comparison of Torbed Reactor Distributor Pressure Drop Data __________123 Figure 4.5: Horizontal Gas Velocity Versus Slip Velocity __________________________129 114um Glass Ballotini / 30° Blade Set __________________________________________129 Figure 4.6: Solids Loading Versus Slip Velocity__________________________________130 114 um Glass Ballotini / 30° Blades____________________________________________130 Figure 4.7: Horizontal Gas Velocity Versus Slip Velocity __________________________132 181um Glass Ballotini / 30° Blade Set __________________________________________132 Figure 4.8: Solids Loading Versus Slip Velocity__________________________________133 181um Glass Ballotini / 30° Blade Set __________________________________________133 Figure 4.9: Horizontal Gas Velocity Versus Slip Velocity __________________________134 633um Glass Ballotini / 30° Blade Set __________________________________________134 Figure 4.10: Solids Loading Versus Slip Velocity_________________________________135 633um Glass Ballotini / 30° Blade Set __________________________________________135
  14. 14. xiv Figure 4.11: Relationship Between Calculated R* and Mean Particle Diameter for 30° Blade Set Glass Ballotini Trials.____________________________________________________137 Figure 4.12: Comparison of Calculated R* (for 30° Blade Set Glass Ballotini Trials) and Published Data Versus Mean Particle Diameter. _________________________________138 Figure 4.13: Sensitivity of Calculated Slip Velocity to Errors in the Thermodynamic Analogue Proportionality Constant R* _________________________________________________140 Figure 4.14: Comparison of Actual and Predicted R* Values With Respect to Mean Particle Diameter_________________________________________________________________142 Figure 4.15: Sensitivity of Calculated Slip Velocity to Errors in the Input Parameters ____143 Figure 4.16: Predicted Change in System Slip Velocity Versus Angle of Blade Inclination-for a given gas flow rate. ________________________________________________________145 Figure 4.17: Horizontal Gas Velocity Versus Slip Velocity-_________________________148 114um Glass Ballotini / 20° Blade Set __________________________________________148 Figure 4.18: Solids Loading Versus Slip Velocity_________________________________149 -114um Glass Ballotini / 20° Blade Set _________________________________________149 Figure 4.19: Horizontal Gas Velocity Versus Slip Velocity-181um Glass Ballotini / 20° Blade Set______________________________________________________________________150 Figure 4.20: Solids Loading Versus Slip Velocity-181um Glass Ballotini / 20° Blade Set __151 Figure 4.21: Horizontal Gas Velocity Versus Slip Velocity-633um Glass Ballotini / 20° Blade Set______________________________________________________________________152 Figure 4.22: Solids Loading Versus Slip Velocity- ________________________________153 633um Glass Ballotini / 20° Blade Set __________________________________________153 Figure 4.23: Horizontal Gas Velocity Versus Slip Velocity-_________________________155 114um Glass Ballotini / 10° Blade Set __________________________________________155 Figure 4.24: Horizontal Gas Velocity Versus Slip Velocity-_________________________156 181um Glass Ballotini / 10° Blade Set __________________________________________156 Figure 4.25: Horizontal Gas Velocity Versus Slip Velocity-_________________________157 633um Glass Ballotini / 10° Blade Set __________________________________________157 Figure 4.26: Solids Loading Versus Slip Velocity- ________________________________158 114 um Glass Ballotini / 10° Blade Set _________________________________________158 Figure 4.27: Solids Loading Versus Slip Velocity_________________________________158 -181um Glass Ballotini / 10° Blade Set _________________________________________158 Figure 4.28: Solids Loading Versus Slip Velocity- ________________________________159 633um Glass Ballotini / 10° Blade Set __________________________________________159 Figure 4.29: Slip Velocity Versus Specific Gas Throughput- ________________________162 114um Glass Ballotini-All Blade Angles ________________________________________162 Figure 4.30: Actual Versus Predicted Horizontal Gas Velocity ______________________163 All Blade Angles___________________________________________________________163 Figure 4.31: Actual Versus Predicted* Slip Velocity- _____________________________165 114um Glass Ballotini, All Blade Angles ________________________________________165 Figure 4.32: Ideas For Optimising the Horizontal Gas Velocity Component From Torbed Reactor Gas Distributor. ____________________________________________________166 Figure 4.33: Horizontal Gas Velocity Versus Slip Velocity-_________________________169 Smelter Grade Alumina / 30° Blade Set _________________________________________169 Figure 4.34: Solids Loading Versus Slip Velocity- ________________________________171 Smelter Grade Alumina / 30° Blade Set _________________________________________171 Figure 4.35: Horizontal Gas Velocity Versus Slip Velocity-Smelter Grade Alumina / 20° Blades___________________________________________________________________172 Figure 4.36: Solids Loading Versus Slip Velocity_________________________________173 Smelter Grade Alumina / 20° Blades ___________________________________________173 Figure 4.37: Comparison of Measured Slip Velocity Velocities Versus Blade Angle for Given Specific Gas Flow Rates_____________________________________________________174 Figure 4.38: Blind Comparison of Actual verses Predicted Slip Velocity for Non-Ideal Gas- Solid Systems _____________________________________________________________176
  15. 15. xv Figure 4.39: Gas Moving Power Consumption vs System Pressure Drop-Typical Smelter (basis 4x106 Am3 /h, motor efficiency of 60%). ____________________________________182 Figure 4.40: Torbed Reactor Based Dry Scrubbing Pilot Plant-1000mm Internal Diameter.183 Figure 4.41: Adsorption Isotherm for Hydrogen Fluoride on Smelter Grade Alumina.____186 Figure 4.42: Production Scale Dry Scrubbing Modules Based on Torbed Reactor Technology.187 Figure 4.43: Fluidisation Map _______________________________________________194 -including Torbed Reactor Operating Window.___________________________________194
  16. 16. xvi
  17. 17. xvii LIST OF TABLES Table 2.1: Comparison of Key Characteristic Dense-Bed Fluidisation, Fast-Fluidisation and Torbed Reactor Behaviour .___________________________________________________30 Table 2.2: Observed Gas-Solid Behaviour in the Torbed Reactor-as a Function of Superficial Gas Velocity- reference Wellwood [1992] Air-Ballotini (associated solid properties given in table 3.2) _________________________________________________________________31 Table 2.3: Distinction between Dilute and Dense Phase Pneumatic Conveying (reference Duckworth-[1982])._________________________________________________________42 Table 2.4: Comparison of Actual Slip Velocity Versus that Predicted by the technique proposed by Rahemen and Jindal [1993].________________________________________43 Table 2.5: Analogous Parameters between Solid/Liquid/Vapour and Gas/ Solid Systems (Reference - Zenz [1987]) ____________________________________________________49 Table 2.6: Definition of j Factors and their Analogous Parameter (Reference- Wallis [1969], Tuba et al [1981]) __________________________________________________________55 Table 2.7: Key Differences Between Pneumatic Conveying and Torbed Reactor Systems. __60 Table 3.1: Summary of Key Dimensions-T400 Pilot Scale Unit _______________________67 Table 3.2: Characteristics of Ballotini Solids Used For Qualitative Analysis ____________71 Table 3.3: Dimensions of Linear Track Reactor Unit _______________________________75 Table 3.4: Details of Linear Track Reactor Cyclone Unit____________________________81 Table 3.5: Properties of Gas Phase Used in Linear Track Reactor Experiments __________82 Table 3.6: Summary of Physical Properties of the Solids Used in the Quantitative Investigation. ______________________________________________________________86 Table 3.7: Experimental Program Matrix________________________________________96 Table 4.1: Comparison of Pressure Drop Distributions ____________________________115 Table 4.2: Independent Variables Reported for Gas-Solid Systems with Respect to Slip Velocity__________________________________________________________________128 Table 4.3: Plot Schedule to Test Thermodynamic Analogy Hypothesis ________________128 Table 4.4: Comparison of Average R* Values for 30o Blade Set. _____________________136 Table 4.5: Group Summary of Published R* Data for - ____________________________139 Dilute Phase Systems (<50 kg/m2.s) ___________________________________________139 Table 4.6: Predicted Slip Velocity Values for GB181um (181um) Ballotini as a Function of Blade Angle ______________________________________________________________146 Table 4.7: Comparison of Actual Versus Predicted Ideal Gas-Solid Transport Coefficient (R*) for all Ballotini Feedstocks and the 20° and 30° Blade Sets _________________________154 Table 4.8: Summary of Results from Low Blade Angle (10o ) Tests ____________________160 Table 4.9: Predicted Verses Actual Slip Velocity based on Reductions in Blade Angle ____161 Table 4.10: Comparison of Key Particle Characteristics ; Ballotini / Smelter Grade Alumina (determinations from appendix 1) _____________________________________________168 Table 4.11: Comparison of Fluoride Loading on Alumina From Various Gas-Solid Contact Systems__________________________________________________________________184 Table 4.12: Comparison of the Scale Associated with the Three Steps taken to Apply Torbed Reactor Technology to Dry Scrubbing of Aluminium Smelter Exhausts. ________________189 Table 4.13: Abscissa Values for Grace [1986] Style Fluidisation Map Based on Tests Reported in this Thesis. _____________________________________________________191 Table 4.14: Selection of Data from the Torbed Reactor Operating in Fine Particle Mode _192
  18. 18. xviii
  19. 19. xix ACKNOWLEDGMENTS The author wishes to acknowledge (in no particular order) the assistance of the following individuals and institutions in helping to facilitate this research program: • Dr Victor Rudolph, for his advice and encouragement during the course of this project. • Comalco Aluminium Limited, for their support in kind. • Dr. Christopher Goodes, for his moral support during the formative stages of the project. • Mr Roger Marks and Mr Franco Provenzale, for their mechanical assistance in fabrication of some of the physical models used. • The Post Graduate Research Fund, for their ongoing financial support. • Torftech Limited (UK) for their support in kind. • Mrs Sandra Wellwood, for her support throughout the project, especially through the “doldrums”.
  20. 20. xx
  21. 21. xxi TECHNICAL PAPERS AND PATENTS ARISING FROM THIS RESEARCH Technical Papers Wellwood, G.A.; "Hydrodynamic Behaviour of the Torbed Gas-Solid Contactor - A Qualitative Assessment", Fluidization VII Conference 1992 (Written and Presented by G.A.Wellwood). Bolt, N., Konings, T., Notebaart, C., Oudenhoven, B., and Wellwood, G.A., [1996]. “New Reactor Provides Effective Means of Processing Alternative Fuels for Electrical Power Generation”. in 9th International Conference and Exhibition for the Power Generating Industries (pp. 291-300). Houston, Texas: PennWell Conferences and Exhibitions. Koopersmith, C., Wellwood, G.A.; "New Gas Solid Reactor for Improving Gas Scrubbing Processes", PowerGen96-Conference 1996 (Written and Presented by G.A.Wellwood). Wellwood, G.A.; "Predicting the Slip Velocity in a Torbed Reactor Unit Using an Analogy to Thermodynamics”, 14th International ASME Conference of Fluidized Bed Combustion, May 1997- (Written and Presented by G.A.Wellwood). Wellwood, G.A.; " Behaviour of the Torbed Reactor Unit in Expanded Bed Mode.”, 15th International ASME Conference of Fluidized Bed Combustion, May 1999-(in preparation) (Written and to be Presented by G.A.Wellwood). Patents "A Process and Apparatus for Treating Particulate Matter” (WO 99/16541) “Counter-Current Gas-Solid Contacting” (US 5,718,873) "Gas-Solid Contacting Method" (WO 92/02289) "Scrubbing of Gaseous Fluorides from Process Exhausts" (WO 93/02772)
  22. 22. C h a p t e r 1 INTRODUCTION 1.0 Scope and Purpose The purpose of this chapter is to firstly introduce the Torbed® reactor concept, its key processing attributes and then outline both the focus and thesis of this investigation. 1.1 What Is A Torbed Reactor? “Torbed” is the name coined to describe the gas-solid contact device developed by Torftech Limited (United Kingdom) in 1981 for the exfoliation of perlite and vermiculite. The main distinguishing feature of the reactor unit with respect to other gas- solid contact devices is an annular gas distributor, which consists of an array of angled stators or “blades”. These blades, which are set in the horizontal plane, serve to deflect the process gas, which enters the slots tangentially on the underside. The deflection causes the gas to present to the solids, which reside in the region above the blades, at a shallow angle with respect to the horizontal (Figure 1.1). Because the vertical velocity component of the gas has been reduced, there is in most cases no entrainment and only minimal elutriation from the reaction zone. The gas-solid regime formed is homogeneous, consisting of a continuous gas phase with no bubbles. Such a system fits Kunii and Levenspiel’s [1969] description of “smooth” fluidisation.
  23. 23. 2 Ug,or Ugh Ugv Figure 1.1: Function of Torbed Reactor Gas Distributor with Respect to Gas Flow. The Torbed reactor has two basic operating modes; • “Compact Bed” characterised by a very distinct region of solids with a voidage not much above that of incipient fluidisation (Figure 1.2a). • “Expanded Bed” (also known as “Fine Particle” mode) featuring a nebulous or cloud like region of solids in which the voidage is close to that found in dilute phase pneumatic conveying systems (ie ε > 0.98) (Figure 1.2b). The prevailing operating mode is dictated by particulate characteristics and to a lesser extent the gas velocities and physical reactor configuration involved.
  24. 24. 3 “Compact” Bed Mode (a) “Expanded” Bed Mode (b) Figure 1.2 a/b: Torbed Reactor Operating Modes These two basic modes of operation are however quite distinct and although there is a lack of fundamental understanding associated with both, in the interests of a focused investigation it was necessary to narrow the scope of the study. The main criteria considered in the selection of the operating mode for detailed study was the future processing trends and requirements of industry. There is a definite trend in industry towards the use of fine dry powders being driven by a number of factors including benefits in: • Quality (for example; producing a more stable mixtures of constituents)- Williams [1990] • Reactivity (for example; a higher specific surface area and enhanced access to internal reaction sites) • Product formulations • Energy efficiency, Fine powders are generally defined as solids with diameters greater than 0.1µm (below which Brownian motion dominates solids movement) and less than
  25. 25. 4 3000µm (above which the influence of the interstitial gas diminishes). Fine powders feature prominently in the process industries. For example, it is estimated that 80% of the products produced by DuPont, one of the worlds largest process companies and typical of the industry in general, involves fine powders (Rhodes [1990]). Unfortunately, a fundamental understanding of the behaviour of the fine powders, including the gas-solid contacting activities possible in the Torbed reactor, lag behind industrial practice. The result is that an alarmingly high number of powder handling units operations fail to meet their design capacities. Merrow [1985] reports that in a survey of 37 processing projects, two-thirds of the plants processing fine powders operated at less than 80% of design capacity even after one year, while around one quarter failed to even achieve 40% design capacity after this period. This failure to meet design capacity even up to one year after commissioning can be attributed in the main to a lack of understanding regarding the behaviour of fine powders. Given the established trend within many processing technologies towards the use of fine particles and the importance of accurate design techniques, this investigation focuses on the study of “Expanded” mode, which caters particularly for fine powder processing. The dissertation is therefore limited to “expanded bed” mode only. During stable operation in “expanded-bed” mode, the particles in the Torbed reactor form a dilute yet distinct homogeneous phase with the process gas. Due to the annular arrangement of the distributor, the gas-solid phase assumes a toroidal geometry, hence the origin of the acronym Torbed (Toroidal Bed). In a qualitative assessment of the unit, Wellwood [1992] likened this Torbed reactor gas-solid regime to horizontal fast-fluidisation. While not strictly true according to all definitions (see-Table 2.1), the fast-fluidised bed is probably the Torbed reactors’ closest relation in terms of existing unit operations and provides a general visual picture of the mode of fluidisation within the Torbed reactor gas-solid system. The research reported in this thesis furnishes an
  26. 26. 5 additional and significantly more detailed description of the expanded “Torbed” regime. The “Torbed” regime is typically generated and contained within a vertically orientated cylindrically or conically shaped body, with the section below the blades forming the plenum and the upper section the reaction and freeboard zones (Figure 1.3a). Some commercial designs feature more elaborate cross- sections based on diverging and converging wall profiles, which are designed to promote internal particle circulation and/or gas-solid disengagement. However, these embellishments usually only relate to the upper freeboard of the unit and therefore as far as the particle bed is concerned, the containing walls are generally vertical. Arrangements for admitting and removing the process streams tend to be customised to the application being considered. The detailed design of reactor internals is also application specific but within empirically derived guidelines (Groszek [1990]). To illustrate the principles behind the technology, a generic arrangement of the reactor is given in Figure 1.3b.
  27. 27. 6 Exhaust Gas Feed Distribution Cone Blade Gas Distributor Freeboard Process Gas Processed Solids Discharge (central option) Gas Distribution Plenum Feed Entry Point Plenum (below distributor) Freeboard (above distributor) Distributor Process Gas Main System Components Generic Reactor Arrangement Figure 1.3: Overall (a) and Typical (b) Arrangements of a Torbed Reactor Unit
  28. 28. 7 1.2 Attributes of the Torbed Reactor and Potential Applications. The basic principle underlying the Torbed reactor, in either mode of operation, is somewhat different from that of existing gas-solid contact devices. The key process attributes of the Torbed reactor unit that make it unique are : 1. Potential for increased differential velocity between the solid and gaseous phases leading to more intensive reaction(s). 2. Low pressure drop over the distributor and bed hence lower system pressure drop. 3. Reduced particulate carry-over for a given specific gas throughput. 4. Stability over a wide range of superficial gas-solid ratios (ie. high turndown capability, particularly with respect to solids throughput). 5. Relative insensitivity to the physical attributes of the particle population including shape, geometry and particle size distribution. Of particular interest from a process engineering perspective is the ability to increase the system “slip velocity”, which is defined as the velocity differential between the gas and solid phases. To identify situations where slip velocity (Usl) increases would be beneficial, it is helpful to consider a basic model of the dynamic gas-solid interactions occurring. The “Shrinking Core” model describes the mass transfer characteristics of many important gas-solid reaction systems and Levenspiel [1972] identified five steps that characterise practically all such gas-solid reaction systems viz: 1. Diffusion of the gaseous reactant through the gas phase surrounding each particle. 2. Diffusion of the gaseous reactant through the ash layer surrounding the particle.
  29. 29. 8 3. Reaction rate of the gaseous reactant at the active surface. 4. Diffusion of the gaseous products of reaction through the particle ash layer. 5. Diffusion of the gaseous products of reaction through the gas boundary layer that surrounds each particle. These steps are shown graphically in Figure 1.4. Although not every gas-solid reaction involves all five of these events, those steps that are part of a given reaction sequence contribute in an additive manner to the overall reaction rate. Typically, the contribution of one step is much higher than that of the others. In this situation, the step with the lowest rate is referred to as the rate-limiting step, because of its domination of the overall rate. Slip velocity is a significant factor in systems where the rate of diffusion of gaseous reactants/products or heat energy through the gas boundary layer (steps 1 and 5) controls the overall process. Boundary layer diffusion also tends to be rate limiting in systems where the reaction between the gas and the particle at the active site (step 3) is fast (Szekely et al [1976]). This situation is often encountered in systems where the particles are finely divided and porous. In such systems, slip velocity can influence both heat and mass transfer rates. The focus of this study is concentrated on mass transfer aspects.
  30. 30. 9 Gaseous Reactant Gaseous Products 1 2 4 5 3 Unreacted Particle Core Figure 1.4: Rate Limiting Steps-Shrinking Core Model Although the rate of gas phase diffusion controlled reactions can be influenced to some extent by modifying the particle size and/or the gas properties, these parameters are often beyond the influence of the process engineer at the design stage. Therefore, the scope to increase the rate of reactions in such systems, through the selection and operation of the unit operation, is attractive. Pressure drop is also an important consideration in the design of gas-contact devices. The shaft power consumed in overcoming the large system pressure drop associated with many traditional gas-solid contact devices, can in some cases even negate the advantages of fluidisation as a processing technique (Kunii and Levenspiel [1991]). Consequently, any technique capable of
  31. 31. 10 decoupling the advantages of fluidised gas-solid contact from a pressure drop penalty will be of interest to process developers. Many industrially significant processes could benefit from the attributes associated with the Torbed reactor. The intensity of pulverised coal combustion for example, is enhanced by increased slip velocity. In this system, a high slip velocity environment increases the supply of oxygen to, and removal of carbon dioxide from, reacting particles (Nieh and Yang, [1987]). Another example where the Torbed reactor attributes of enhanced slip velocity, low pressure drop and reduced elutriation characteristics are all of high value is the increasingly important duty of scrubbing process exhausts using a dry solid sorbent. In this application, gas diffusion often limits the overall rate because of the fine particle size and high internal surface area characteristics of the solid media usually employed. In addition, the volumes of gas to be treated are usually large thus making process pressure drop and specific throughput important considerations. Low particle elutriation rates from the reactor can also be important in terms of minimising or eliminating the requirement for downstream gas conditioning prior to exhaust, therefore enabling environmental constraints to be satisfied at minimal cost. This thesis has been undertaken with a gas scrubbing duty in mind. As a practical case study, the findings of this thesis have been applied to an operating dry scrubbing unit used in the aluminium smelting industry.
  32. 32. 11 1.3 Factors Limiting Development Despite its attributes (section 1.2), development and subsequent application of the Torbed reactor concept has been somewhat limited during the 16 years since its inception. The two main factors inhibiting development appear to be: • A lack of fundamental understanding regarding its operation. • Inability to profile the unit within the standard gas-solid reactor characterisation framework, which in turn is limited by the lack of a simple descriptive model. In the absence of a fundamental understanding and therefore the ability to construct a simple descriptive model, application of the technology has relied almost exclusively on physical modelling. This incremental empirical and evolutionary approach is both resource intensive and time consuming. If uncoordinated, such an approach also does little to address the underlying lack of understanding necessary to extrapolate performance and allow the technology to be applied in novel applications without piloting. The development of the Torbed reactor mirrors, in many respects, the history of applied fluidisation. Grace and Berruti [1995] observed that practical application of fluid–particle reactors most often preceded a full understanding of the underlying fundamentals. However, as also pointed out by Grace and Berruti [1995], a degree of fundamental understanding is critical to avoid failures on scale-up, improve design, optimise operation and allow the development of efficient control strategies. A good illustration of this point occurred in the 1940’s when circulating fluidised beds were first introduced. Driven by competitive pressures and the high profit potential associated with fluidised catalytic cracking, circulating fluidised beds (CFB’s) were scaled up for this duty ahead of a fundamental understanding of their operation. Because of the complex fluid mechanics associated with the gas-solid systems to which it was applied, the behaviour of
  33. 33. 12 a given CFB configuration was not predictable with sufficient accuracy. This lack of understanding often resulted in costly post-commissioning design modifications based on observed behaviour of the initial design in the field (Rhodes [1990]). To some extent, this situation describes the development of the Torbed reactor. In common with CFB’s, the solution involves the development of simple yet reasonably accurate description of the gas-solid interactions involved. The ability of technology developers to identify the process capabilities of the Torbed reactor in relation to those of other more traditional gas-solid technologies is also an important requirement. In principle, this is a pre-cursor stage in the development of a new process and should ideally precede the more detailed development stage outlined above. Identification of potential reactor types is often based on the assessment of graphical aids, like a fluidisation map, along the lines of that shown in figure 1.5. This particular framework was developed by Grace [1986] after Reh [1968], and has since become somewhat of a standard, being used by many practitioners in the field to relate their findings, because of its direct relevance to design (Kunii and Levenspiel [1991]). To date, the Torbed reactor has not been profiled in this manner and therefore many applications for which it may have offered superior performance may have been missed.
  34. 34. 13 Two things considered necessary to advance future development of the Torbed process are: • development of a simple mathematical model capable of describing the key hydrodynamic characteristics of the Torbed reactor unit. • a concise characterisation of the abilities of Torbed reactors using a recognised framework. These two objectives represent the basis for the research undertaken in this study. The basic requirement of the model is to predict the behaviour of the main process attribute, namely differential gas-solid velocity, as a function of feed material properties and design parameters. Such a model would enable the boundaries of the Torbed reactor concept to be explored and its development accelerated. The data generated in the process of formulating and validating the model is then applied to build a profile of the Torbed reactor’s fluidisation behaviour in relation to other reactor options, using a standard framework. The profile provides a basis for process developers to identify situations where the technology may have an advantage.
  35. 35. 14 Figure 1.5: Gas-Solid Contactor Regime Diagram (adapted from Grace [1986], Kunii and Levenspiel [1991]) A Bubbling fluidised beds Spouted beds Pneumatic transport ut * dp * B D ut umf ut Fast fluidised bed
  36. 36. 15 1.4 Scope and Development of this Thesis The Torbed reactor is a new class of gas-solid contacting device offering a number of attractive process attributes, however to date, utilisation of this potential has been spasmodic. A lack of fundamental understanding regarding its basic operating principles appears to be the root cause. This inability to predict performance in key areas, as a function of basic design and operating conditions, has translated into the need for extensive and costly piloting as a precursor to nearly every new application considered. The absence of a fundamental model has also hindered the definition of a generic operating window for the technology. Reactor specific operating windows, as those superimposed onto fluidisation maps, form the basis of most technology assessment and selection exercises. Without a predictive modelling tool, definition of the operating window associated with the Torbed reactor in fine particle mode must rely almost totally on empirical data, making it a very drawn out exercise. The net result is that the Torbed reactor technology continues to suffer from a low profile. These factors, combined with the general apprehension associated with new technologies, constitute a powerful barrier to its utilisation and perhaps explain the relatively slow utilisation of the technology. The most important contribution of this thesis is the construction and validation of a simple yet robust mathematical model, capable of predicting the Torbed reactors most important process attributes of slip velocity and pressure drop as a function of basic reactor configuration and operating parameters.
  37. 37. 16 The thesis comprises of six main themes viz: • Identification of process attributes differentiating the Torbed reactor from other gas-solid contact devices. • Assessment of existing knowledge and identification of the most appropriate theoretical framework upon which to construct a model. • Rationalisation of the study system based on a qualitative assessment of the Torbed reactor in fine particle mode. • Development and validation of the model via a quantitative assessment. • Use of the model in an industrial application. • Use of a recognised framework to compare the performance of the Torbed reactor in fine particle mode with the performance of other gas-solid reactors. The first task of this study was to identify the attributes differentiating the Torbed reactor from other gas-solid contact devices. Although it was observed that in many situations, processing in the Torbed reactor provided better quality gas-solid contact, the physical explanation(s) for the improved performance were not well understood. This study identified enhanced slip velocity between the gas and solid phases in the Torbed reaction zone as the primary reason for its superior heat and mass transfer characteristics. The second differentiating feature of the reactor, identified as part of this study, was the low pressure drop nature of the gas distributor. In common with its enhanced transport abilities, little was known of the relationship between reactor geometry, process conditions and pressure drop prior to this investigation. These two aspects of the technology therefore become the focus of the detailed study. Being a new genre of gas-solid contact device from empirical origins, information describing the fundamentals underpinning behaviour of the Torbed reactor operated in fine particle mode was virtually non-existent. A review of available knowledge and literature (Chapter 2) confirmed this situation. The
  38. 38. 17 review exercise was therefore broadened to cover literature associated with more mature gas-solid contact devices and techniques. The most likely cause of the enhanced slip velocity behaviour observed in the Torbed reactor was attributed to horizontal streamer formation. Although the phenomenon of streamer formation in vertical systems is well covered in terms of fundamental understanding, a gap was identified in the theory describing its manifestation in horizontal gas-solid systems, like those found in the Torbed reactor. An alternative approach, involving the use of thermodynamic equations of state to describe interactions between fine particles and gases in horizontal systems, was identified. If applicable, this approach has potential as a means of overcoming the gap in fundamental understanding, therefore enabling the development of a slip velocity model of the Torbed reactor in fine particle mode. Utilisation of this well established thermodynamic framework to describe the slip velocity characteristics of the Torbed reactor relied upon establishing analogous behaviour between the fine particles and gases in the Torbed reactor, and solid-liquid-vapour in thermodynamic systems. The ideal equation of state was selected as the most appropriate basis for the slip velocity model and a strategy to test its validity was devised. These elements become the broad focus of the applied sections of the thesis. Although simple in essence, the gas-solid contacting dynamics associated with commercially configured Torbed reactors is complicated by non-contributing effects included to enhanced operability rather than process performance. The aim of the qualitative section of this thesis (reported in chapter 4) was to identify any factors redundant to the objectives of the study. The resulting rationalisation of the study system enabled a more focussed quantitative investigation and a simpler model as a result. This important element of the thesis established that the circular arrangement of the reactors gas distributor, despite being one of the Torbed reactor technologies main distinguishing and marketed features, is in effect non- contributing with respect to the quality of gas-solid contact in fine particle
  39. 39. 18 mode. The valuable insight facilitated a simplification of the study system, thereby avoiding treatment of the complicated particle trajectories observed in commercial Torbed reactors. Consequently, a special linear version of the Torbed reactor distributor was constructed (details chapter 3) for the quantitative aspects of the study. The assumption regarding the non- contributing nature of the circular distributor arrangement was indirectly confirmed towards the end of the investigation by application of the model to a large-scale commercial unit (section 4.4). Having identified a theoretical framework for the descriptive model and simplified the study system, it was necessary to make quantitative measurements to firstly validate the model and then refine its capabilities. A testing protocol and experimental schedule designed to assess the applicability of the thermodynamic analogy was developed in chapter 3, with a discussion of the outcomes given in chapter 4. The first phase of the quantitative study confirmed the overall system pressure drop is dominated by losses across the gas distributor, which in turn are due to kinetic losses. A model for pressure drop constructed on this basis gave an accurate fit with data from both the abstract study system constructed for this investigation and Torbed reactors in commercial operation. Consequently, it is now possible to accurately predict the pressure drop across the Torbed reactor distributor as a function of its slot velocity (Ug,or ), a parameter easily determined from blade geometry and the prevailing process gas characteristics. The second phase of the quantitative study focused on the slip velocity characteristics of the system and development of a simple descriptive model based on the thermodynamic analogy. The initial series of tests, using standard distributor geometry and a controlled gas-solid system, found that the model based on a modified version of the Ideal gas equation exhibited a good correlation with the experimental results. The investigation also identified a correlation between particle diameter and the proportionality constant in the Ideal gas equation analogue. This finding is quite significant as it enables the slip velocity model to be used without any experimental input at all.
  40. 40. 19 The main insight resulting from the validation of the thermodynamic model was that the prevailing slip velocity was heavily dependent on the magnitude of the horizontal gas velocity. This is also an important finding. For a specific gas flow rate, the horizontal gas velocity within the gas-solid contacting zone can be influenced by the angle of blade inclination in the distributor design, which is an easily modified and inexpensive design variable. A series of additional tests were subsequently undertaken, using distributors with different inclination angles designed to explore this model prediction. The relationship between slip velocity and horizontal gas velocity predicted by the model was confirmed on the lower angled blade sets. However, the magnitude of the horizontal gas velocity and therefore slip velocity increase was not as great as predicted by a simple trigonometric consideration of the distributor. This deviation is attributed to the aerodynamic profile of the current blade design, which may be inducing vortex shedding/back-mixing and therefore producing a lower than predicted horizontal velocity. Additional work is required to confirm this hypothesis and optimise design of the individual blades that constitute the distributor. The model was also tested on a non-ideal gas-solid system involving smelter grade alumina in air. Despite some significant physical differences in the solid phases involved, the model was capable of predicting slip velocity characteristics within 30% of the experimental values. Having validated and refined the concept on the purpose built study distributor, the model was then used in the design of an industrial dry scrubbing unit being based on the Torbed reactor. Aluminium smelters rely on dry scrubbers to remove trace amounts of the acid gas hydrogen fluoride (HF) from their large exhaust volumes. The fluoride removal capacity of the plant is determined by the amount of fluoride that can be loaded into a unit mass of alumina, which in this context is a finite resource. The actual removal mechanism involves the chemisorption of the HF onto the alumina particles. Under normal circumstances the overall reaction is limited by the presentation of fluoride the reactive sites on the alumina and is therefore sensitive to gas boundary layer diffusion. Simply reducing the inclination angle of the blades within the
  41. 41. 20 Torbed distributor from the standard 25° with respect to the horizontal to 10° increased the fluoride loading of the alumina by 30%. This very significant outcome, which equates to a 30% improvement in scrubber capacity, was however in line with model predictions regarding slip velocity enhancements. The experimental data gathered during the course of this investigation together with predictions from the slip velocity model were then used to construct an operating window for the Torbed reactor operated in fine particle mode. This window was then superimposed onto an accepted fluidisation map framework to show its performance envelope with respect to those associated with other technologies. As a result of this study, it is now possible for process developers to assess the suitability of the Torbed reactor during the formative stages of the design process. Having established a common basis for comparison, it is now possible to publish material relating to transfer properties imparted by the Torbed reactor and therefore lift its technical profile. The thesis concludes (chapter 5) with a summary of the salient features of the investigation together with the conclusions drawn. Being the first qualitative investigation of the Torbed reactor focusing on its operating fundamentals, there is obviously scope for additional research to refine the model developed and explore the influential design aspects identified. Recommendations regarding these research opportunities are presented in chapter 5.
  42. 42. 21
  43. 43. 22 C h a p t e r 2 LITERATURE REVIEW 2.0 Scope and Purpose The purpose of this chapter is to review the body of literature considered relevant to the development of a simple model to describe the differential gas- solid or “Slip” velocity characteristics of the Torbed reactor. Open literature addressing the technical fundamentals of any aspect of the Torbed reactor unit is virtually non-existent. This literature review therefore focussed on like sciences and their applicability to the differential velocity focus of this investigation. The selection of like sciences was based primarily on qualitative observations of the Torbed reactor unit (section 4.1). Conventional fluidisation is a commercially significant means of contacting gas and solid phases, and as such has been the subject of intense study. It is a science relatively well defined in terms of fundamentals and thus was the first area considered. Literature in the associated fields of fast-fluidisation, batch fluid bed processing and fluidised conveying (air slides) were also examined. Dilute phase pneumatic conveying is a technique used primarily for the transporting of solids. Although less well developed in terms of its underlying fundamentals, it has a number of hydrodynamic similarities to those observed in the qualitative study of the Torbed reactor, and was thus also considered relevant. The third and main focus of this review is the literature associated with the similarity between gas-solid systems and thermodynamics. In particular, investigations where the concept has been developed to describe the behaviour of dilute phase pneumatic conveying systems.
  44. 44. 23 2.1 Enhanced Slip Velocity Characteristics - A Differentiating Feature of the Torbed Reactor. In many important gas-solid reactions, the actual rate of chemical reaction is fast and the transfer of reactants and/or heat energy through the gas phase (Section1.2) limits the overall rate. Such systems are said to be gas phase diffusion controlled. Gas phase diffusion resistance is a function of a number of factors including: • Fluid Properties -viscosity -diffusivity of reactant phase -density • Particle Size • Differential Gas-Solid Velocity (Slip Velocity) Most of these factors are fixed by the definition of the system, hence beyond exploitation by the design engineer. An important exception is slip velocity, which is defined as the relative velocity between a particle and the surrounding gas phase. Slip velocity is a function of the design and operation of the gas- solid contact device and hence provides a potential leverage point. Slip velocity increases the rate of mass transfer in systems where gas phase diffusion resistance is limiting, by reducing the thickness of the gas boundary layer around the particles (Figure 2.1). In addition to improved mass transfer hence overall reaction rates, slip velocity also impacts a number of other system characteristics (Tanner et al [1994]) including: • heat transfer rate • solids hold-up (particle segregation)
  45. 45. 24 • fluid dynamics (momentum balance) Szekely, Evans and Sohn [1976] report there is general consensus amongst investigators regarding the relationship between mass transfer coefficients and slip velocity in diffusion controlled systems. vs vg vs= vtp < vg vs << vg vs <<< vg vg =vs ta tb ( < ta) tc ( <tb << ta) Figure 2.1: Impact of Slip Velocity on Gas Phase Boundary Layer Thickness The general relationship existing between mass transfer coefficient (kd) and slip velocity is illustrated in Figure 2.2, which is based on a commonly accepted predictive correlation given by Ranz and Marshall [1952]. Characteristically, the relationship shows a relatively large increase in mass transfer rate from the initial increase in slip velocity. This is due to the fact that in this situations, gas phase diffusion is controlling mass transfer. With further increases in slip velocity, diffusion no longer limits mass transfer. Therefore,
  46. 46. 25 the associated increase in mass transfer rate decreases and the relationship eventually becomes one of diminishing return. 0 20 40 60 80 100 120 140 160 180 200 0 100 200 300 400 500 Slip Velocity (cm/s) MassTransferCoefficient[kD](cm/s) 250 microns 500 microns 750 microns Figure 2.2: Typical Relationship Between Mass Transfer Coefficient and Slip Velocity. Note: This figure illustrates the general sensitivity between the mass transfer characteristic and slip velocity, particularly at the lower values of slip velocity usually encountered in non-packed bed devices. None the less, there is significant scope to increase the rate of gas-solid reactions controlled by diffusion, by increasing slip velocity. In most conventional gas-solid reactors, the magnitude of slip velocity is limited to a value close to the terminal velocity (vtp )of the particles being processed, which can be quite low for fine discrete particles. Yet because of its operating characteristics, the slip velocities possible in the Torbed reactor unit are not subject to this constraint (as detailed in section 4.1.1.1). To capitalise on this attribute, an understanding of the relationship between the design and operation of the Torbed reactor unit and slip velocity is required.
  47. 47. 26 2.2 Technical Literature Regarding the Torbed Reactor. Although the basic concept underlying the Torbed reactor unit is simple, the design engineer is faced with numerous physical arrangements and operating options when considering an application of the technology. Current design of the Torbed reactor is based more on empirical necessities, developed from operational experience, rather than an understanding of the underlying fundamentals. As a first step, identification of the key parameters and their relationship with slip velocity would provide a basis for a more rational design in circumstances where enhanced transport properties are desirable. Because of the proprietary nature of its applications, quality technical literature covering the performance of Torbed is virtually non-existent. Patents (Dodson [1987]) aside, the only public domain literature regarding the Torbed reactor tends to be quasi-scientific marketing orientated material. Flint’s [1992] study of the Torbed reactor unit alluded to its high intensity heat and mass transfer characteristics but instead focussed on the elutriation characteristics of fine particulate processed using a resident bed of larger particles. Although superior to many other gas-solid contact units in this regard, the issue of elutriation from the Torbed reactor is of little practical importance when considering operation in fine particle mode. In most cases this function can be easily and more efficiently carried out by downstream devices like cyclones, bag filters, electrostatic precipitators and alike if required. The specific issue of mass transfer rates, as a function of unit design and operation, was not addressed by Flint. Wellwood [1992] presented a qualitative assessment of the Torbed reactor process and highlighted a number of its unique characteristics. Experimental data relating to the relatively low pressure drop of the distributor, which is an important consideration in applications involving high gas throughput, was
  48. 48. 27 presented. The issue of enhanced mass transfer was identified as one requiring further study. The remaining available reference addressing the Torbed reactor (Clarke [1984]) contains little technical information or experimental data, focussing instead on commercial aspects. Other groups have now commenced scientific research of the technology, however like Flint [1992], the focus still relates to elutriation and freeboard behaviour (Shu et al [1999]). Given the lack of a specific literature foundation, this review also includes a more general body of relevant gas-solid contacting knowledge. 2.3 Selection of an Appropriate Science. Flint [1992] reported that the particle motion within the reaction zone of the Torbed reactor was chaotic and thus not amenable to detailed modelling. Qualitative assessments, made as a precursor to this investigation (section 4.1), support the observations of Flint, but also led to the conclusion that the circular geometry of the Torbed reactor was the main factor responsible for the observed chaotic behaviour within the reaction zone. It was postulated that the main mass/heat transfer enhancing attribute of the system, the slip velocity, was not a particular characteristic of the circular geometry. It was concluded that the circular distributor arrangement associated with commercial Torbed reactor designs improves the functionality of the unit in a production environment by: • Helping maintain a solids inventory thus providing extended residence time for the particles. • Encouraging/inducing radial mixing of the bed solids.
  49. 49. 28 • Creating a cyclone like motion in the gas phase as it passes through the freeboard zone thus assisting gas-solid disengagement. • Enabling the unit to be squatter for a given space time However, the circular distributor arrangement does not significantly impact on the magnitude of slip velocity achieved. The conclusions and postulates from the qualitative assessment (section 4.1), upon which the analysis above is based, were combined into the following simplifying assumption: As far as slip velocity is concerned, the Torbed reactor gas-solid contact system can be studied in the simpler linear geometry without loss of accuracy. The justification for this assumption is strengthened by the fact that in commercial practice, the trend is to increase the diameter of the Torbed reactor to leverage economies of scale. As the diameter and hence radius of curvature of the Torbed reactor is increased, the geometry of the distributor track approaches linearity. This arrangement avoids the complexities associated with centrifugal flows, including for example the “rope” behaviour often observed in cyclone separators. In a linear arrangement, the reaction zone of the Torbed reactor becomes a conduit or track along which the materials flow, and as such has a lot of behavioural similarities to dilute phase pneumatic conveying. Nevertheless, before reviewing literature relating to dilute phase pneumatic conveying, a review of the relatively mature science of conventional fluidisation, as it applies to the Torbed reactor, was undertaken. For completeness, fluidised bed dryers and air slide conveyors were also considered.
  50. 50. 29 2.3.1 Conventional Fluidisation Technology Much research effort has been given to the commercially significant technology of dense-bed gas-solid fluidisation, as many commercial operations are based upon this phenomenon. As a consequence, relatively accurate (+/- 25%), fundamentally based design equations are available. Unfortunately, the gas-solid regime in the Torbed reactor differs significantly from the regime associated with the fluidised beds. The extent of difference is such that the fundamentals developed to describe "Dense-bed Fluidisation" (according to its common definition-Kunii and Levenspiel [1991]), are not considered applicable to the behaviour of the Torbed reactor. The key points of difference supporting this position are presented in table 2.1. Observations made by Wellwood [1992] indicate that the Torbed reactor regime of most interest, namely fine particle mode, only exists at relatively high gas velocities. Also observed was the fact that both fixed and dense bubbling-bed type regimes can be encountered in the Torbed reactor at lower gas velocities. A description of the behaviour observed in a 400mm annular Torbed reactor with a ballotini-air system is presented in Table 2.2. In general terms, these transitions are the same as those experienced in a conventional fluidised bed. By extending the analogy, the "Torbed” Regime, which is the one of most interest from a slip velocity maximisation point-of- view, could be considered as fast-fluidisation in the horizontal plane. A review of the literature associated with fast-fluidised beds was therefore undertaken and findings are presented in the next section.
  51. 51. 30 Characteristic REGIME “Dense Fluidised Bed” (Bubbling) "Fast-Fluidised Bed" (raiser section) "Torbed Reactor" (fine particle mode) Bed Voidage 0.5-0.6 1 0.75-0.99 2 +0.99 Behaviour With Velocities >vmf Bubbles/ Entrainment Homogeneous/ Elutriation Homogeneous/ Elutriation Orientation of Particulate Travel Vertical Vertical Horizontal ContinuousPhase Gas-solid Emulsion Gas Gas Differential Gas-Solid (Slip) Velocity < Terminal Velocity >Terminal Velocity >( Ug>20vt ) 3 > Terminal Velocity Load Change Response Limited by elutriation/ defluidisation4 Excellent4 Excellent Particle Size Distribution Narrow4 Wide4 Wider5 Tolerance of Particle Geometry Low4 Good4 Excellent5 Gas-Solid Mixing Fair4 Excellent4 Excellent6 Gas/Gas Mixing Poor-due to dense-bed and slow freeboard4 Good-due to “expanded-bed” and fast- freeboard4 Good-due “expanded-bed” and fast-freeboard Process Controllability Limited- diminishes on scale-up Excellent Excellent Pressure Drop High Low Low7 Bed Depth Deep Shallow Shallow Heat/Mass Transfer High Higher Higher Particulate Mixing Back mixing Back Mixing8 Plug flow Gas Mixing Torturous/ Back mix Plug Flow Plug Flow Particle Motion Non-Linear Linear Linear Gas-Solid Contact Cross Current Co-Current Cross Current Throughput:Investment Low:Higher4 Higher:Lower4 Higher:Lower Particulate Flux Normal to Solids Flow (kg/s. m2 ) n/a 100-200 20 Clustering/Streamers Limited to bubble tails Yes3 Yes Table 2.1: Comparison of Key Characteristic Dense-Bed Fluidisation, Fast-Fluidisation and Torbed Reactor Behaviour . Observed Behaviour of Solids System Gas Velocity Ratio (Uo/vmf) 1 Reh[1971] 2 Dry et al [1987] 3 Kunii and Levenspiel [1991] 4 Peinemann et al [1992] 5 Bolt et al [1996] 6 Flint [1992] 7 Wellwood [1992] 8 Yerushalmi [1978]
  52. 52. 31 No observable solids movement ("Fixed Bed") 3.0 Localised bubbling above each blade slot in the distributor ring ("Bubbling Bed") 3.9 Vigorous boiling of the entire bed but no horizontal travel ("Boiling Bed") 6.2 Segment of de-fluidised material travels around the annulus followed by a region of exposed distributor ("Travelling Slug") 7.7 Distinct, homogeneous air-solid phase covering the entire distributor and travelling in a horizontal plane ("Torbed” regime") 8.5-9.2 Onset of stratification and loss of bed definition >9.2 Table 2.2: Observed Gas-Solid Behaviour in the Torbed Reactor-as a Function of Superficial Gas Velocity- reference Wellwood [1992] Air- Ballotini (associated solid properties given in table 3.2) 2.3.2 Fast-Fluidisation Increasing the specific gas velocity in a conventional fluidised bed to try and boost the slip velocity leads to increased elutriation of solids. However, by passing the exhaust gas through a cyclone separator prior to discharge, it is possible to recover the solids for recycle back to the bed (Figure 2.3), thus resulting in a viable gas-solid contacting system. This approach leads to enhanced heat and mass transfer and is the concept behind the circulating or fast-fluidised bed.
  53. 53. 32 Figure 2.3: Typical Fast-Fluidised Bed Arrangement Downcomer Gas-Solid Separator Exhaust Riser Process Gas
  54. 54. 33 As shown in Table 2.1, fast-fluidisation exhibits many characteristics found in the fine particle mode “Torbed” regime, in particular the existence of a continuous gas phase in which the particles have slip velocities greater than their single particle terminal velocity. However, fast-fluidisation literature was considered to have little to offer in terms of describing the linkage between slip velocity and operating parameters in the Torbed reactor, as discussed below. In isolation, the maximum differential or slip velocity between an individual particle and the surrounding air is, by definition, its single particle terminal velocity (vtp ). In vertical systems, this value can be determined by solving the balance of gravity, buoyancy and drag forces acting on the particle (Figure 2.4a), and techniques like the one developed by Kunii and Levenspiel [1991] are proven in this regard. The question then arises, how then can particles in regimes like those found in fast-fluidisation and Torbed reactors, exhibit slip velocities greater that that of the constituent particles? In the case of fast-fluidised systems, the existence of gas-solid slip velocities in excess of the constituent particle terminal velocity, is attributed to the formation of streamers or strands of individual particles. In the immediate wake of moving particles, there is a region of turbulence. Particles travelling within the turbulent wake of predecessor particles experience lower resistance to flow (Figure 2.4b). Therefore, there is a driving force, based on energy minimisation, for downstream particles to travel in the wake of upstream particles. Once formed, streamers will continue to grow in length until equilibrium is achieved. The more particles in the strand at steady state, the higher the effective terminal or strand velocity, hence the greater the differential or slip velocity of the particle constituents (Sobocinski et al [1995]). In systems like fast-fluidised beds and circulating fluidised bed downcomers, the tendency for streamer formation and their length is a function of the concentration of solids within the gas phase (Molerus and Wirth [1991]).
  55. 55. 34 Kunii and Levenspiel [1991] concluded that in dilute systems, like pneumatic conveying, where the voidage is of the order of 0.980 to 0.999, there will be no interaction between particles, hence the velocity of individual particles will approach their inherent terminal velocity value(s). While intuitively this seems a reasonable position supported by data from pneumatic conveying systems, it does not describe the observed behaviour of the Torbed reactor. Data obtained from Torbed reactors operating in fine particle mode (Wellwood [1997a], plus new data presented in section 4 of this thesis) appear to contradict the conclusions of Kunii and Levenspiel [1991]. The voidage of this expanded regime is very high and on par with that associated with dilute pneumatic conveying systems, yet the slip velocities are many times the single particle terminal velocity. The reason for this apparent discrepancy appears to be the fact that although there are no bubbles in the expanded “Torbed” regime, the solids are not evenly distributed across the bed cross-section but are present as streamers. Molerus and Wirth [1991] refer to this stable gas-solid regime, which exists between fully dispersed flow and saltation, as partial phase separation. It is therefore proposed that the “Torbed” regime is conducive to the formation of particle strands or “streamers” (Figure 2.4c), even though the solids concentration within the “bed” region is relatively low when compared to fast- fluidised systems (Table 2.1). Consequently, it was necessary to consider the literature addressing the phenomenon of streamers.
  56. 56. 35 Single Particle at Steady-State (acceleration=0)Fb Fd Fg vs vs= vtp Fb FdFg = + Figure 2.4a: Single Particle in Vertical Free-Fall vs vs vs vs Vertical Particle Cluster vs= vc > vtp Fb Fd Fg vs Figure 2.4b: Vertical Streamer Formation vs vs vs vs vs Horizontal Particle Cluster vgh-vs= vsl > vtp vgh vs vs Figure 2.4c: Horizontal Streamer Formation
  57. 57. 36 2.3.2.1 Streamers Published research on streamer formation is heavily biased towards vertical conveying systems, as they represent the commercially significant area of Fluid Catalytic Cracking (FCC) operations, which are based on circulating fluid bed units. An example of this research is that reported by Sobocinski et al [1995]. In their study of strands, Sobocinski et al [1995] used a time domain analyser fitted with a specially modified probe to measure the velocities of individual strands of FCC particles formed in an experimental “downcomer”, which is generally defined as the vertical solids return line of a CFB (see Figure 2.3). Using the measured strand velocity and other independently measured information about the system, including voidage and gas phase velocity, Sobocinski et al [1995] calculated the associated strand slip velocity. By assuming that the strand slip velocity represents the effective terminal velocity of the strand and solving the force balance, Sobocinski et al [1995] developed expressions determining the geometry of the strands involved. Although fundamentally based and in good agreement with the experimental data, the model is only strictly applicable for the case of vertical travel. In this case, the drag force acting on the particle is balanced by the net downward force, which inturn is the difference between the gravitational and buoyancy forces (Figure 2.4b). To be of use in the study of particles in the “Torbed” regime, an expression describing the horizontal gas-solid conveying system is required. Little fundamental research is available on the slip velocity behaviour of particles in horizontal conveying systems. As noted by Wirth and Molerus [1985], and Raheman and Jindal [1993], the mechanism involved in horizontal conveying is extremely complex with respect to the vertical system and no simple relationship exists. In their study of dilute phase horizontal pneumatic conveying systems, Molerus and Wirth [1991] proposed that the energetic optimum for such systems occurred in the transition between strand and fully developed flow. Because of
  58. 58. 37 the complicated structures associated with transitional flow, Molerus and Wirth [1991] concluded that models involving empirical correlations fail, or are of limited value, as they simply ignore the complexities. The failure of more sophisticated modelling techniques to predict the behaviour of horizontal systems is also linked back to the fact that transitional behaviour prevails; yet most models are based on the assumption of non-transitional flow regimes. Based mainly on visual observations, Molerus and Wirth [1991] constructed a simple descriptive model for horizontal pneumatic systems based on momentum exchange between the phases observed. From a consideration of observed pressure drop behaviour, Molerus and Wirth [1991] first constructed a plot over a range of superficial gas velocities (Ug ) for various mass fluxes. The plot emerged looking similar in nature to the familiar temperature-volume diagram used in thermodynamics to describe the behaviour of pure compressive substances. Based on observations made on their study system, which consisted of a dilute horizontal pneumatic conveyor, Molerus and Wirth [1991] developed a mechanistic model to describe the phase interactions. Firstly, it was observed that the system consisted of two solid phases: • Individual particles travelling at the single particle terminal velocity (vs = vtp ), which is less than the superficial gas velocity (Ugh ) of the system but greater than the strand velocity (vc ). • Strands of particle travelling at velocity (vc ) mainly along the bottom of the pipe. This transitional, yet stable state, was referred to as partial phase separation. Secondly, upon impact with the strand, the faster moving single particles were observed to decelerate and become part of the strand thus transferring their momentum (J) according to ∆J = mp (vs-vc)
  59. 59. 38 At steady state, every particle-strand impact results in another particle being expelled from the strand into the surrounding gas at the strand velocity vc, from where it is then accelerated to the single particle terminal velocity vtp,as illustrated in Figure 2.5. On this basis, it was concluded that the horizontal strands were being propelled primarily by impinging particles and retarded by wall friction. Molerus and Wirth [1991] then divided the system up according to the phases observed and then constructed a force balance for each region. Linking the regional systems via a mass balance, it was then possible to describe the entire system in terms of six non-dimensional numbers. Plotting non-dimensional pressure drop against the system friction number Fri (Fri), produced a useful phase diagram showing the operating window associated with each flow regime associated with the horizontal system being studied. The diagram has many similarities to the phase diagrams used in thermodynamics, which supports the existence of a analogy between the behaviour of dilute gas-solid systems and the thermodynamic behaviour of compressible substances identified by other researchers (for example Wallis [1969], Tuba et al [1981], Zenz [1987]….). Molerus and Wirth [1991] reported a good correlation between their model and the behaviour of their study system. Although the mechanism of strand formation and propulsion proposed by Molerus and Wirth [1991] provides a useful mechanistic hypothesis for the behaviour of fine powders in an expanded Torbed reactor system, the associated mathematical analysis was not considered to be directly applicable. The main deviations between the observed behaviour of the Torbed reactor in fine particle mode and the horizontal system studied by Molerus and Wirth [1991] that precluded direct application of the model included: 1. Molerus and Wirth [1991] based their model force balance on the observed travel of clusters along the bottom of the horizontal pipe. As no material has been observed sliding along the high gas velocity distributor region of
  60. 60. 39 the Torbed reactor when in fine particle mode, wall friction is not considered to be a significant retarding force. 2. The model also assumes the geometry of the containing vessel and therefore the wetted perimeter is circular. This situation does not adequately describe the more complex geometry assumed by the gas-solid region in an expanded “Torbed” regime. 3. The Molerus and Wirth [1991] model requires an estimate of the system pressure gradient, defined as the pressure drop per unit length of particle travel, to assist in determining the system force balance. While there is a pressure drop across the expanded “Torbed” regime, the geometry of the system means that the bed is an endless toroid with a common freeboard. Therefore, it is impossible to physically determine the pressure gradient input information required by the model. Despite being unable to utilise the model directly to describe the behaviour of the Torbed reactor in fine particle mode, some elements of the mechanism observed proposed by Molerus and Wirth [1991] are potentially useful. In particular the observed mechanism of horizontal strand propulsion provides a useful working hypothesis to describe the movement of strands within the expanded Torbed reactor environment. The findings of Molerus and Wirth [1991] are also useful in that they provide an empirically derived and independent link between the behaviour dilute horizontal gas-solid systems and thermodynamics.
  61. 61. 40 vs= vtp v*c vgh Strand Velocity Single particle travelling at the single particle terminal velocity (vtp) APPROACHES a strand of particles travelling slower at strand velocity (vc) vc Single particle COLLIDES with and becomes part of the strand. vc vgh >> vs > vc Single particle is EJECTED from the strand to maintain the momentum balance. The ejected particle ACCELERATES from velocity vc towards vtp. The ejected particle ASSUMES single particle terminal velocity (vtp). vc vs= vtp Figure 2.5: Horizontal Streamer Propulsion 2.3.2.1.1 Summary-Streamers In summary, the most plausible explanation for the fact that slip velocity measurements in the expanded “Torbed” regime are well in excess of the
  62. 62. 41 single particle terminal velocity of the constituent particles, is the existence of streamers. The models developed to describe the phenomenon of streamers in vertical systems, like those proposed by Sobocinski et al [1995] and Molerus and Wirth [1991], are not directly applicable to the Torbed reactor in fine particle mode. The fact that the prevailing gas-solid regime in the Torbed reactor is horizontally orientated alters the underlying force balance, which in the case of horizontal systems is more complex. Molerus and Wirth [1991] also developed a model to describe the behaviour of dilute horizontal gas-solid conveying systems. However, as was the case with the vertical models, fundamental differences between the Torbed reactor and the study system preclude the direct application of the model. The mechanism proposed by Molerus and Wirth [1991] to describe the formation and propulsion of streamers does however provide a useful hypothesis for the dynamics within the “Torbed” regime. An alternate approach capable of accounting for the complexities associated with horizontal gas-solid systems, with respect to streamer formation as found in the “Torbed” regime, is required. Slip velocity also has some importance in the field of pneumatic conveying, particularly in the horizontal plane, and a review of this literature is presented next. 2.3.3 Dilute Phase Pneumatic Conveying. In the study of pneumatic conveying, the issue of particle velocity relative to gas velocity is important from a perspective of saltation, solids hold-up, pressure drop and to a lesser extent particle attrition. Researchers in this field refer to two basic pneumatic conveying regimes and Duckworth [1982], made the distinction between “dilute” and “dense” phase conveying as follows (Table 2.3) : CHARACTERISTIC REGIME
  63. 63. 42 “Dilute” Phase “Dense” Phase M*- Solids to Gas Mass Ratio (also referred to as “Phase Density”) (kg Solid / kg air) < 20:1 >100:1 Volumetric Concentration (volume % solids) < 2 % >10 % Table 2.3: Distinction between Dilute and Dense Phase Pneumatic Conveying (reference Duckworth-[1982]). Measurements in the quantitative assessment of the linear Torbed reactor analogue (Section 4.1) confirm that, according to Duckworth’s definition above, the fine particle mode “Torbed” regime is dilute phase. Unlike conventional fluidisation, the design of pneumatic conveying systems which, in many cases involve horizontal arrangements, is usually governed by simplified empirical equations based on one or two easily measured parameters. The capability of this approach is however restricted to predicting behaviour only slightly different from the base case used to formulate the governing equations (Duckworth [1982]). Performance on extrapolated or new systems requires reformulation of the governing equations via empirical testing thus reducing the value of the model to the design engineer. As concluded in the previous section, the requirement for this study was a more comprehensive technique for estimating strand formation and therefore slip velocity behaviour of horizontal gas-solid systems. The general approach taken in the treatment of pneumatic conveying systems is typified by the work of Rahemen and Jindal [1993], who developed a procedure to predict the slip velocity of agricultural grains being conveyed in a dilute phase pneumatic conveying system. In their investigation, Rahemen and Jindal [1993] assumed that the complexities associated with horizontal conveying could be accounted for in the determination of the drag coefficient,

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