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

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

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

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

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.

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 )

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

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

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

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 4.8: Solids Loading Versus Slip Velocity__________________________________133
Figure 4.10: Solids Loading Versus Slip Velocity_________________________________135

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 _________________________________________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
114 um Glass Ballotini / 10° Blade Set _________________________________________158
-181um Glass Ballotini / 10° Blade Set _________________________________________158
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
Smelter Grade Alumina / 30° Blade Set _________________________________________169
Smelter Grade Alumina / 30° Blade Set _________________________________________171
Figure 4.35: Horizontal Gas Velocity Versus Slip Velocity-Smelter Grade Alumina / 20°
Blades___________________________________________________________________172
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

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

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

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”.

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)

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.

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.

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

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

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.

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

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.

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.

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

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.

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

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.

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.

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

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.

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

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

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.

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

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.

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.

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)

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,

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.

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

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.

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.

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.

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]

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.

32
Figure 2.3: Typical Fast-Fluidised Bed Arrangement
Downcomer
Gas-Solid
Separator
Exhaust
Riser
Process
Gas

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]).

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.

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

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

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)

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

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.

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

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

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,

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

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

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