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Research Doctorate - Doctor of Philosophy (PhD) / Determining the operating parameters and design considerations for dense phase (non-suspension) conveying of fine powders in pneumatic systems typically use empirical, steady-state modelling techniques, as the mechanisms of the flow behaviour are still not fully understood. However, this necessary simplification in the modelling of the dense phase flow still presents significant challenges in ensuring that the predicted outcomes adequately reflect the physical nature of the flow, and therefore provide good design guidance. This thesis represents an examination and development of techniques required for designing dense phase systems of fine powders in three specific areas; prediction of a materials potential to dense phase convey, solids friction correlations and their subsequent effect on pressure drop prediction, and modelling the solids flow from a local perspective. The dense phase capability analysis was conducted by reviewing the current predictive techniques utilising known dense phase material data. It was apparent in the thesis that there were distinct strong predictive regions in all the diagrams; however some diagrams showed areas with weak predictive regions. This work also illustrated the difficulties in comparing different de-aeration rate techniques and significantly, a new mode of flow predictive chart was developed which eliminated the need to determine de-aeration rates while still maintaining distinctly strong dense phase predictive capability. Solids friction based pressure models invariably use a power law which require determination of co-efficient/s and exponent/s. Detailed in this thesis is the research which shows why solutions do not always occur in these power law based friction models and defines a method of determining stable and meaningful values for the exponents. Furthermore, a generic air/particle parameter based solids friction model was developed which is a clear advancement in defining the frictional resistance of dense phase pneumatic conveying of powder. This thesis also proposes a new continuum model which calculates the force balance between the conveying air flow, the resistance of the particles and geometrical effects, like bends. The solution to this model provides qualitative information on fine powder dense phase flow velocity from a solids flow perspective and represents a major step in advancing dense phase modelling from a particle flow basis.

Research Doctorate - Doctor of Philosophy (PhD) / Determining the operating parameters and design considerations for dense phase (non-suspension) conveying of fine powders in pneumatic systems typically use empirical, steady-state modelling techniques, as the mechanisms of the flow behaviour are still not fully understood. However, this necessary simplification in the modelling of the dense phase flow still presents significant challenges in ensuring that the predicted outcomes adequately reflect the physical nature of the flow, and therefore provide good design guidance. This thesis represents an examination and development of techniques required for designing dense phase systems of fine powders in three specific areas; prediction of a materials potential to dense phase convey, solids friction correlations and their subsequent effect on pressure drop prediction, and modelling the solids flow from a local perspective. The dense phase capability analysis was conducted by reviewing the current predictive techniques utilising known dense phase material data. It was apparent in the thesis that there were distinct strong predictive regions in all the diagrams; however some diagrams showed areas with weak predictive regions. This work also illustrated the difficulties in comparing different de-aeration rate techniques and significantly, a new mode of flow predictive chart was developed which eliminated the need to determine de-aeration rates while still maintaining distinctly strong dense phase predictive capability. Solids friction based pressure models invariably use a power law which require determination of co-efficient/s and exponent/s. Detailed in this thesis is the research which shows why solutions do not always occur in these power law based friction models and defines a method of determining stable and meaningful values for the exponents. Furthermore, a generic air/particle parameter based solids friction model was developed which is a clear advancement in defining the frictional resistance of dense phase pneumatic conveying of powder. This thesis also proposes a new continuum model which calculates the force balance between the conveying air flow, the resistance of the particles and geometrical effects, like bends. The solution to this model provides qualitative information on fine powder dense phase flow velocity from a solids flow perspective and represents a major step in advancing dense phase modelling from a particle flow basis.

Electrostatic charging is a widely known natural phenomenon that has been observed sinceancient times. This phenomenon is also reported in solids handling and processing industrieswith varying extent of its interference in established processes and operations. Particlecollisions lead to electrostatic charge generation through triboelectrification. Sustainedcharging leads to particle agglomeration and adhesion, or even explosions. Hence, the presenceof charged particles are seen as a hindrance and a risk in certain processes. The focus of thisthesis is directed at gas-solid fluidization and pneumatic conveying where this holds true.Polyethylene is commonly manufactured in catalytic gas-solid fluidized bed reactors. Theinsulative nature of the polymer particles, the catalyst particles and the surrounding gas set upa conducive environment for electrostatic charge generation. Charged particles adhere on tonearby surfaces forming fused masses of polymer sheets. Presence of sheets in the reactorhinders the reactor productivity, thus warranting reactor shutdown and maintenance. On theother hand, catalyst introduction into the polyethylene reactor is performed through pneumaticconveying systems. In general, solids pneumatic conveying is known to cause the largestdegree of triboelectrification among many gas-solid systems. Therefore, the charging ofcatalyst particles may also contribute to operational challenges faced by this industry.Numerous studies have attributed particle characteristics, system variables, and operatingparameters as probable sources contributing to electrostatic charging in both fluidizationprocess and pneumatic conveying systems. However, a comprehensive consensus explainingparticle charging in real-world scenarios and suitable methods to mitigate or prevent chargingstill require further investigation. Thus, desirable control of charging in affected industrialsectors is still not present.Beyond the scope of fluidization and pneumatic conveying, certain studies have investigatedthe influence of dielectric strength of gases on the charging behaviour of solids. The worksclaim that gases with low dielectric strength perform better in minimizing electrostatic chargeof solids in controlled environments due to gas discharge and subsequent charge dissipation.Thus, applicability of such gases in dynamic processes like fluidization and pneumaticconveying must be investigated in hopes of reproducing similar observations. The principalaim of this thesis was to uncover a functional method to limit charging and particle adhesionin fluidization and other solids handling systems. As a means of accomplishing this, theobjective of this thesis was directed to study the efficacy of argon, which has a low dielectricstrength, against nitrogen in reducing triboelectrification of polyethylene particles influidization and pneumatic conveying operations.A stainless-steel fluidization apparatus was used to study the charging behaviour of acommercially produced polyethylene resin at atmospheric pressure. Results were drawn forpure argon and compared against pure nitrogen. Aiming to minimize the quantity of argonwhile simultaneously retaining as much efficacy as possible, binary mixtures of nitrogen andargon were also tested along with successive fluidization trials. Pure argon resulted in 90%reduction in fouling compared to pure nitrogen. Even binary mixture of 10 vol % argon showeda reduction of 50% in fouling values. Successive fluidization resulted in fouling valuescomparable to pure argon trials. Multiple pulse pneumatic conveying was carried out in astainless-steel tube with dehydrated amorphous silica that is a commonly used catalyst base inpolyethylene process. The net specific charge of the particles and the fouling inside the tubewere smaller under argon in the first injections. Subsequent injections were not as significant.Results from the above operations were validated through bench-scale shake tests performedunder controlled gaseous environment. Single large polyethylene particle charging was firsttested in nitrogen and argon atmosphere followed by multiple smaller particles. Bench-scaleshake tests showed argon influenced the saturation charges, reducing it and reaching it earliercompared to nitrogen. However, the degree of charge and fouling reduction was not assignificant as observed in fluidization trials.The thesis concludes that argon is indeed influential in reducing particle charge and particleadhesion in applicable systems. Influence of argon was observed in all operations withfluidization exhibiting the greatest degree of reduction in charging and fouling values.Furthermore, even small quantities of argon can make a non-linear impact on said parameters.The results also suggest that the majority of gas discharge and subsequent charge dissipationoccurs in areas of considerable electric fields. These are observed to entertain large number ofparticles contact and separation, providing plenty of opportunities for gas molecules to ionize.

A wide range of industrial processes involve multiphase granular flows. These include catalytic reactions in fluidized beds, the pneumatic conveying of raw materials and gas-particle separators. Due to the complex nature of multiphase flows and the lack of fundamental understanding of the phenomena in a multiphase system, appropriate design and optimized operation of such systems has remained a challenging field of research. Design of these processes is hampered by difficulties in upscaling pilot scale results, the difficulties involved in experimental measurements and in finding reliable numerical modelling methods. Significant work has been carried out on numerical modelling of multiphase systems but challenges remain, notably computational time, appropriate definition of boundary conditions, relative significance of effects such as lift and turbulence and the availability of reliable model validation. The work presented in this thesis encompasses experimental and numerical investigations of horizontal pneumatic conveying. In the experimental work, carefully controlled experiments were carried out in a 6.5 m long, 0.075 m diameter horizontal conveying line with the aid of the laser Doppler anemometry (LDA). Initially, LDA measurements were performed to measure the gas velocity in clear flow. Good agreement was observed between the theory and experimental measurements. For two-phase experiments, spherical and non-spherical particles with different sizes and densities were used to study the effect of particle size and solid loading ratio on the mean axial particle velocity. Three different sizes of spherical glass beads, ranging from 0.9 mm to 2 mm and cylindrical shaped particle of size 1x1.5 mm were employed. It was found that by increasing the particle size and solid loading ratios, the mean axial particle velocity decreased. Turbulence modulation of the carrier phase due to the presence of spherical particles was also investigated by measuring fluctuating gas velocity for clear gas flow and particle laden flow with different particle sizes and solid loading ratios. Results suggested that for the size ranges of particles tested, the level of gas turbulence intensity increased significantly by adding particles, and the higher the solid loading ratio, the higher the turbulence intensity. With the rapid advancement of computer resources and hardware, it is now possible to perform simulations for multiphase flows. For a fundamental understanding of the underlying phenomena in pneumatic conveying, the coupled Reynolds averaged Navier-Stokes and discrete element method (RANS-DEM) was selected. The aim of the modelling section of this study was to evaluate the abilities of coupled RANSDEM to predict the phenomena occurring in a research-sized pneumatic conveying line. Simulations for both one-way and two-way RANS-DEM coupling were performed using the commercial coupled software FLUENT-EDEM in an Eulerian- Lagrangian framework, where the gas is simulated as a continuum medium, while solid phase is treated as a discrete phase. In one-way coupling simulations, a considerable discrepancy in mean axial particle velocity was observed compared to the experimental results, meaning two-way coupling was required. It was further found that the inclusion of Magnus lift force due to particle rotation was essential to reproduce the general behaviour observed in the experiments. Turbulence modulation also was investigated numerically. Experimental and simulation results of gas and particle velocities were compared showing that the RANS-DEM method is a promising method to simulate pneumatic conveying. However, some discrepancy between simulation and experimental results was observed. Most studies in two-phase flow fields have focused on spherical particles. However the majority of particles encountered in industry involve non-spherical granules which show considerably different transportation behaviour compared with spherical particles. Further modelling of cylindrical particles was conducted using a multisphere model to represent cylindrical particles in the DEM code. Drag and lift forces and torque equations were modified in the code to take the effect of particle orientation into account. The framework developed was evaluated for two test cases, indicating a good agreement with the analytical and experimental results. The transportation of isometric (low-aspect-ratio) non-spherical particles in pneumatic conveying was also modelled. The simulation results of mean axial particle velocity agreed well with the experimental measurements with the LDA technique.

The rotary valve is a widely used mechanical device in many solids-handling industrial processes. However, it may also be responsible for most of the attrition effects occurring in a typical process. In this study, the attrition effects occurring in a rotary valve operating as a stand-alone device and as part of a pneumatic conveying system were investigated. In the former case granular attrition was carried out at three different rotary valve speeds and the experimental results obtained were found to be in good agreement with the Gwyn correlation. In the latter case three typical air flow rates were used in the pneumatic conveying system. The size distribution of the attrition product obtained at the lowest air flow rate used was not adequately described by the Gwyn correlation. The attrition process and mechanisms involved were analysed and the minimum size of the attrition product obtained from both modes of operations was found to be similar. / Singapore-MIT Alliance (SMA)

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In order to mitigate the risk of global warming by reducing CO2 emissions, the co-firing technique, burning pulverized coal and granular biomass together in conventional pulverised fuel power station boilers, has been advocated to generate “greener” electricity to satisfy energy demand while continuing to utilize existing rich coal resources. A major problem is controllably distributing fuel mixtures of pulverized coal and granular biomass in a common pipeline, thus saving much investment. This is still under development in many co-firing studies. This research into particle dynamics in pipe flow was undertaken in order to address the problem of controllable distribution in co-firing techniques and gain an improved understanding of pneumatic conveying mechanisms. The objectives of this research were, firstly, to numerically evaluate the influence of various factors on the behaviour of particles of the different materials in a horizontal pipe gas-solid flow, secondly, to develop an extended technique of Laser Doppler Anemometry in order to determine cross-sectional characteristics of the solid phase flow in the horizontal and vertical legs of a pneumatic conveying system, and, thirdly, to develop a novel imaging system for visualizing particle trajectories by using a high definition camcorder on a cross-section illuminated by a white halogen light sheet. Finally, a comparison was made of cross-sectional flow characteristics established by experiments and those simulated by using a commercial Computational Fluid Dynamics code (Fluent) and the coupling calculations of Fluent & EDEM (a commercial code of Discrete Element Method) respectively. Particle dynamic behaviour of the solid phase in a dilute horizontal pipe flow was investigated numerically by using the Discrete Phase Model (DPM) in Fluent 6.2.16. The numerical results indicate that the Saffman force plays an important role in re-suspending particles at the lower pipe boundary and that three critical parameters of the critical air: conveying velocity, the critical particle size and the critical pipe roughness, exist in pneumatic conveying systems. The Stokes number can be used as a similarity criterion to classify the dimensionless mean particle velocity of the different materials in the fully developed region. An extended Laser Doppler Anemometry (LDA) technique has been developed to measure the distributions of particle velocities and particle number over a whole pipe cross section in a dilute pneumatic conveying system. The first extension concentrates on a transform matrix for predicting the refracted laser beams’ crossing point in a pipe according to the shift coordinate of the 3D computer-controlled traverse system on which the probes of the LDA system were mounted. Another part focussed on the proper sampling rate of LDA for measurements on the gas-solid pipe flow with polydispersing particles. A suitable LDA sampling rate should ensure that enough data is recorded in the measurement interval to precisely calculate the particle mean velocity or other statistical values at every sample point. The present study explores the methodology as well as fundamentals of measurements of the local instantaneous density of particles as a primary standard using a laser facility. The extended LDA technique has also been applied to quantitatively investigate particle dynamic behaviour in the horizontal and vertical pipes of a dilute pneumatic conveying system. Three kinds of glass beads were selected to simulate the pulverized coal and biomass pellets transported in a dilute pneumatic conveying system. Detailed information on the cross-sectional spatial distributions of the axial particle velocity and particle number rate is reported. In the horizontal pipe section, experimental data on a series of cross-sections clearly illustrate two uniform fluid patterns of solid phase: an annular structure describing the cross-sectional distribution of the axial particle velocity and a stratified configuration describing particle number rate. In the vertical pipe downstream of an elbow R/D=1.3, a horseshoe-shaped feature, which shows that the axial particle velocity is highest in wall regions of the pipe on the outside of the bend for all three types of glass beads on the section 0D close to the elbow outlet. The developments of cross-sectional distributions of particle number rate indicate that the horseshoe-shaped feature of particle flow pattern is rapidly dispersed for particles with high inertia. A video & image processing system has been built using a high definition camcorder and a light sheet from a source consisting of a halogen lamp. A set of video and image processing algorithms has been developed to extract particle information from each frame in a video. The experimental results suggest that the gas-solid flow in a dilute pneumatic conveying system is always heterogeneous and unsteady. The parameter of particle mass mean size is superior to particle number mean size for statistically describing the unsteady properties of gas-solid pipe flow. It is also demonstrated that the local data of particle number rate or concentration are represented by a stratified structure of the flow pattern over a horizontal pipe cross-section. Finally, comparisons of numerically predicated flow patterns and experimental ones show that there is reasonable agreement at pipe cross-sections located at horizontal positions less than half the product of particle mean velocity and mean free fall time in the pipe from the particle inlet. Further away from the inlet, the numerical results show flow patterns which are increasingly divergent from the experimental results along the pipe in the direction of flow. This discrepancy indicates that particles’ spatial distribution in the pipe is not accurately predicted by the Discrete Phase Model or Fluent coupled with EDEM.

In this work the electrostatics of the pneumatic conveying of granular materials in a non-conducting (PVC) vertical pipe is studied using Electrical Capacitance Tomography (ECT) system. The non-conducting wall in general attains static charges arising from particle-wall collisions in the initial periods of conveying process and then reaches equilibrium with the surroundings. The polarity of particles and conveying pipe inner wall agrees reasonably well with the contact potential difference measurements. The perturbations in the capacitance signal due to charge accumulation are larger with smaller air superficial velocity. The denser flow regimes give larger wall residual charge. Wall charging process shows similar trend by surface potential and ECT measurements. The addition of small amount (0.5% by weight) of anti-static agent (Larostat-519) in the powder form decreases the electrostatic charge generation by altering the patterns for particle-particle and particle-wall collisions. / Singapore-MIT Alliance (SMA)