Applied Science, Faculty of
Chemical and Biological Engineering, Department of
DSpace
UBCV
Hou, Jiang Tian
Doctor of Philosophy - PhD
University of British Columbia
Electrostatic charging is a pervasive challenge in gas–solid fluidized beds, arising from continuous contacts among fluidizing particles and between particles and the column wall. In commercial operations such as polyolefin polymerization, pharmaceutical manufacturing, and dust handling, uncontrolled electrostatics can lead to wall sheeting, particle agglomeration, and even safety hazards. Despite decades of research, the interplay between particle properties, gas-phase conditions, and boundary effects remains insufficiently understood, limiting the effectiveness of mitigation strategies. This thesis systematically investigates the mechanisms and controlling factors of electrostatic charging in gas–solid fluidized beds. Fluidization experiments were conducted with high-density polyethylene (HDPE) and glass particles under different fluidizing gases and varying relative humidity levels. The results demonstrate that gas composition regulates the saturation charge via its dielectric breakdown strength, with argon in particular shown to effectively suppress charge accumulation of HDPE particles and the associated wall fouling and agglomeration. Humidity, in turn, critically influences both charge generation and dissipation. Its effectiveness is strongly material-dependent and is especially pronounced for glass beads, which are more hydrophilic and possess relatively lower electrical resistivity. Additional studies highlight the role of wall material and grounding in determining charge polarity and accumulation within the bed. To isolate interparticle charging, a custom-designed acoustic levitation platform was developed to enable controlled collisions between particles of the same or different sizes and materials. These experiments confirmed that for HDPE particles, collisions between dissimilar-sized particles consistently led to size-dependent bipolar charging, while collisions between similar-sized particles produced random polarity outcomes. In contrast, material pairings with large effective work function differences exhibit strongly directional charge transfer, consistent with observations in the fluidized bed system. Pre-charging experiments further revealed that when one particle carried a substantial initial charge, the size-dependent tendency was suppressed, and charge was redistributed to reduce the initial imbalance. A case study on wood dust handling provides additional insights into charge generation and dissipation mechanisms in practical industrial contexts, highlighting materials commonly encountered in wood processing. These findings help explain electrostatic hazards in dust-handling operations and provide guidance for reducing the risks of fires and explosions in industrial environments.
ELECTROSTATIC CHARGING IN GAS–SOLID FLUIDIZED BEDS: EFFECTS OF GAS COMPOSITION, HUMIDITY, AND INTERPARTICLE CHARGING by Jiang Tian Hou B.Eng., Imperial College London, London, UK, 2008 M. Eng., University of British Columbia, Vancouver, Canada, 2012 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemical and Biological Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) November 2025 © Jiang Tian Hou, 2025 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled: Electrostatic Charging in Gas–Solid Fluidized Beds: Effects of Gas Composition, Humidity, and Interparticle Charging submitted by Jiang Tian Hou in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemical and Biological Engineering Examining Committee: Tony (Xiaotao) Bi, Professor, Chemical and Biological Engineering, UBC Supervisor John R. Grace, Professor, Chemical and Biological Engineering, UBC Co-supervisor James J. Feng, Professor, Chemical and Biological Engineering, UBC Supervisory Committee Member Fariborz Taghipour, Professor, Chemical and Biological Engineering, UBC Supervisory Committee Member Mark Martinez, Professor, Chemical and Biological Engineering, UBC University Examiner Rizhi Wang, Professor, Materials Engineering, UBC University Examiner Jun Yao, Professor, College of Mechanical and Transportation Engineering, China University of Petroleum-Beijing External Examiner iii Abstract Electrostatic charging is a pervasive challenge in gas–solid fluidized beds, arising from continuous contacts among fluidizing particles and between particles and the column wall. In commercial operations such as polyolefin polymerization, pharmaceutical manufacturing, and dust handling, uncontrolled electrostatics can lead to wall sheeting, particle agglomeration, and even safety hazards. Despite decades of research, the interplay between particle properties, gas-phase conditions, and boundary effects remains insufficiently understood, limiting the effectiveness of mitigation strategies. This thesis systematically investigates the mechanisms and controlling factors of electrostatic charging in gas–solid fluidized beds. Fluidization experiments were conducted with high-density polyethylene (HDPE) and glass particles under different fluidizing gases and varying relative humidity levels. The results demonstrate that gas composition regulates the saturation charge via its dielectric breakdown strength, with argon in particular shown to effectively suppress charge accumulation of HDPE particles and the associated wall fouling and agglomeration. Humidity, in turn, critically influences both charge generation and dissipation. Its effectiveness is strongly material-dependent and is especially pronounced for glass beads, which are more hydrophilic and possess relatively lower electrical resistivity. Additional studies highlight the role of wall material and grounding in determining charge polarity and accumulation within the bed. To isolate interparticle charging, a custom-designed acoustic levitation platform was developed to enable controlled collisions between particles of the same or different sizes and materials. These experiments confirmed that for HDPE particles, collisions between dissimilar-sized particles consistently led to size-dependent bipolar charging, while collisions between similar-sized particles produced random polarity outcomes. In contrast, iv material pairings with large effective work function differences exhibit strongly directional charge transfer, consistent with observations in the fluidized bed system. Pre-charging experiments further revealed that when one particle carried a substantial initial charge, the size-dependent tendency was suppressed, and charge was redistributed to reduce the initial imbalance. A case study on wood dust handling provides additional insights into charge generation and dissipation mechanisms in practical industrial contexts, highlighting materials commonly encountered in wood processing. These findings help explain electrostatic hazards in dust-handling operations and provide guidance for reducing the risks of fires and explosions in industrial environments. v Lay Summary Electrostatics occurs when particles gain or lose electric charge as they move and collide with each other or with container walls. In industrial processes such as making plastics, handling powders, or processing wood, this buildup can cause serious problems. It can make particles stick to walls, form clumps, block pipelines, or even trigger fires and explosions. This research examined how different conditions affect electrostatics in systems where particles are kept in motion by flowing gas. Experiments showed that the type of gas and the amount of moisture in the air are very important. For example, using argon gas helped reduce charging of plastic particles, while humidity played a stronger role for glass particles, which dissipated charge more readily under moist conditions. A new experiment method using sound waves was also developed to study interparticle collisions. The findings provide practical ideas to make industrial processes safer and more reliable. vi Preface The work presented in this thesis was carried out by the author under the supervision of Professor Xiaotao Bi and Professor John R. Grace at the University of British Columbia. The author was responsible for all stages of the research, including experimental design and execution, data collection and analysis, and manuscript preparation. The supervisors provided guidance primarily through conceptual discussions, critical evaluation of results, and editorial feedback on manuscripts. In Chapter 2, M. H. Rahman and H. Li contributed to the initial setup of the experimental apparatus and the development of preliminary experimental concepts. X. Liu and S. Sundaresan assisted in the construction of the breakdown model. In Chapter 5, G. Hu helped with the execution of some experimental procedures. The use of Generative AI in this thesis was strictly limited to language-related assistance. Specifically, ChatGPT-5 was employed for proof-reading to ensure correctness of grammar, spelling, and clarity. No AI tools were used for generating, analyzing, or interpreting research data, nor for drafting the scientific content of the thesis. At the time of writing this thesis, four research papers have been published based on the work conducted during this doctoral study. These publications correspond to the content presented in Chapters 2, 3, and 5. The work described in Chapter 4 has been completed and is being prepared for publication. This thesis therefore offers an integrated account of the author’s original research on electrostatic phenomena in gas–solid fluidized beds, with key findings corroborated by peer-reviewed publications. vii Publications: J. Hou, X. Liu, M.H. Rahman, H. Li, S. Sundaresan, J.R. Grace, X. Bi, Effect of gas properties and wall materials on particle charging in gas–solid fluidized beds, Can J Chem Eng 101 (2023) 244–255. https://doi.org/10.1002/cjce.24568. J. Hou, J.R. Grace, X. Bi, Effect of gas properties on particle charging and wall fouling in gas-solids fluidized beds, Powder Technol 421 (2023) 118419. https://doi.org/10.1016/J.POWTEC.2023.118419. J. Hou, G. Hu, J.R. Grace, X. Bi, Experimental study of electrostatic charging related to prevention of fire and dust explosions in wood processing facilities, J Electrostatics 132 (2024) 103983. https://doi.org/10.1016/J.ELSTAT.2024.103983. J. Hou, J.R. Grace, X. Bi, Effect of fluidizing gases, humidity, and grounding on particle charging dynamics in gas-solids fluidized beds, Chem Eng Sci 306 (2025) 121279. https://doi.org/10.1016/J.CES.2025.121279. viii Table of Contents Abstract ......................................................................................................................... iii Lay Summary ................................................................................................................ v Preface ........................................................................................................................... vi Table of Contents ....................................................................................................... viii List of Tables ................................................................................................................ xii List of Figures ............................................................................................................. xiii List of Symbols ......................................................................................................... xviii List of Abbreviations ................................................................................................... xx Acknowledgements .................................................................................................... xxi Dedication .................................................................................................................. xxii Chapter 1 Introduction ........................................................................................... 1 1.1 Electrostatic Charge Generation ........................................................................... 2 1.1.1 Charge transfer models .................................................................................. 2 1.1.2 Triboelectric series and effective work function ............................................ 6 1.2 Electrostatics in Gas-solid Fluidized Beds ........................................................... 8 1.2.1 Gas-solid fluidization ..................................................................................... 8 1.2.2 Charge generation and dissipation in fluidized beds .................................... 11 1.2.3 Effect of dielectric strength of fluidizing gas .............................................. 13 1.2.4 Effect of humidity of fluidizing gas ............................................................. 16 1.2.5 Adding fine powders in fluidized beds ........................................................ 18 1.2.6 Size-dependent charging .............................................................................. 19 1.2.7 Acoustic levitation ....................................................................................... 21 1.3 Thesis Objectives ................................................................................................ 24 1.4 Thesis Outline ..................................................................................................... 26 ix Chapter 2 Effect of Fluidization Gas on Saturation Charges and Wall Fouling in Fluidized Beds ......................................................................................................... 27 2.1 Introduction ......................................................................................................... 27 2.2 Experimental Setup ............................................................................................. 27 2.2.1 Fluidization unit ........................................................................................... 27 2.2.2 Tested particles ............................................................................................. 29 2.2.3 Charge measurement .................................................................................... 30 2.2.4 Characterization of wall fouling .................................................................. 31 2.3 Results and Discussion ....................................................................................... 34 2.3.1 Charge polarity of fluidized particles .......................................................... 34 2.3.1.1 HDPE particles ...................................................................................... 34 2.3.1.2 Glass beads ............................................................................................ 34 2.3.2 Saturation charge level in different columns ............................................... 36 2.3.2.1 HDPE particles ...................................................................................... 36 2.3.2.2 Glass beads ............................................................................................ 38 2.3.3 Effect of fluidizing gas composition ............................................................ 39 2.3.3.1 HDPE particles ...................................................................................... 39 2.3.3.2 Glass beads ............................................................................................ 44 2.3.4 Effect of humidity ........................................................................................ 45 2.3.4.1 HDPE particles ...................................................................................... 45 2.3.4.2 Glass beads ............................................................................................ 48 2.3.5 Wall fouling .................................................................................................. 50 2.3.5.1 HDPE particles ...................................................................................... 50 2.3.5.2 Glass beads ............................................................................................ 55 2.3.6 Size analysis and agglomeration of wall coated particles ............................ 57 2.4 Summary ............................................................................................................. 61 x Chapter 3 Effect of Fluidization Gas on Particle Charging Dynamics in Fluidized Beds ............................................................................................................. 65 3.1 Introduction ......................................................................................................... 65 3.2 Experimental Setup ............................................................................................. 66 3.3 Results and Discussion ....................................................................................... 69 3.3.1 Charging dynamics of fluidized particles .................................................... 69 3.3.2 Effect of fluidizing gas composition ............................................................ 73 3.3.3 Effect of humidity ........................................................................................ 76 3.3.4 Effect of grounding ...................................................................................... 81 3.4 Summary ............................................................................................................. 84 Chapter 4 Interparticle Charging Behavior ....................................................... 87 4.1 Introduction ......................................................................................................... 87 4.2 Experimental Setup ............................................................................................. 87 4.2.1 Acoustic levitation and controlled collision ................................................. 87 4.2.2 Charge polarity measurement ...................................................................... 92 4.2.3 Tested particles ............................................................................................. 94 4.3 Results and Discussion ....................................................................................... 95 4.3.1 Initial charge ................................................................................................ 95 4.3.2 Collision and charging between similar sized particles ............................... 97 4.3.3 Collision and charging between binary sized particles .............................. 100 4.3.4 Effect of pre-existing charge ...................................................................... 105 4.4 Summary ........................................................................................................... 108 Chapter 5 A Case Study: Electrostatic Charging of Sawdust in Wood-Processing Facilities ................................................................................................... 110 5.1 Introduction ........................................................................................................ 110 5.2 Experimental Setup ............................................................................................ 113 xi 5.2.1 Vibration tests of air hose charging ............................................................. 113 5.2.2 Pneumatic conveying tests .......................................................................... 114 5.2.3 Charge decay tests ....................................................................................... 117 5.3 Results and Discussion ...................................................................................... 118 5.3.1 Vibration tests of air hoses .......................................................................... 118 5.3.2 Pneumatic conveying tests ......................................................................... 120 5.3.2.1 Effect of particle size .......................................................................... 120 5.3.2.2 Effect of air velocity and humidity ..................................................... 121 5.3.2.3 Effect of coil material and grounding ................................................. 122 5.3.2.4 Effect of wood species ........................................................................ 123 5.3.2.5 Sawdust from different wood-processing equipment ......................... 125 5.3.3 Charge decay .............................................................................................. 128 5.4 Summary ........................................................................................................... 132 Chapter 6 Conclusions and Future Works ....................................................... 135 6.1 Conclusions ....................................................................................................... 135 6.2 Future Works ..................................................................................................... 142 References .................................................................................................................. 144 Appendix .................................................................................................................... 162 xii List of Tables Table 2.1 Estimated Umf for each test group using the Wen and Yu [83] correlation. ...... 28 Table 2.2 Key physical properties of tested particles. ...................................................... 30 Table 2.3 Comparison of estimated and measured |Q/m| of HDPE particles. .................. 41 Table 2.4 Comparison of estimated and measured |Q/m| of glass beads. ......................... 45 Table 2.5 Reynolds number, gravity, buoyancy, and drag force acting on a single HDPE particle (260 μm in diameter) in different gases. .............................................................. 53 Table 3.1 Key physical properties of the tested particles. ................................................. 68 Table 3.2 Estimated minimum fluidization velocity (Umf) for each test group. ............... 68 Table 4.1 Key physical properties of tested particles. ...................................................... 95 Table 5.1 Details of the sample holders used in the charge decay tests. .......................... 118 Table 5.2 Summary of hemlock wood sawdust samples collected near different wood-processing equipment..................................................................................................... 127 Table 5.3 Mass-based particle size distributions of hemlock wood sawdust collected near different wood-processing equipment. ............................................................................ 127 Table 5.4 Predicted half-life values and percentages of charge remaining on dry hemlock sawdust placed on different sample holders after 10 and 120 hours. ............................. 131 Table 5.5 Predicted half-life values and percentages of charge remaining on hemlock sawdust with different moisture contents (MC) placed on grounded and ungrounded metal holders after 10 and 120 hours. ....................................................................................... 131 xiii List of Figures Figure 1.1 Schematic of electron transfer (adapted from Lacks et al. [10]). As two metal surfaces are brought into contact, metal 1, with higher Fermi energy (EFermi1) and lower work function (Φ1), transfers electrons to metal 2, with lower Fermi energy (EFermi2) and higher work function (Φ2; top). This leads to an accumulation of electrons on metal 2. ... 4 Figure 1.2 Schematic of the adsorbed water layer in the ion-transfer model of triboelectric charging for polymers containing covalently bound ions and mobile counterions (adapted from McCarty et al. [32]). The mobile ions may originate from the polymer itself or from aqueous species present in the environment, such as H⁺ or OH⁻. ....................................... 6 Figure 1.3 a) An example of conventional triboelectric series; b) A set of five materials that form a cyclic triboelectric series (adapted from McCarty et al. [32]). ............................... 8 Figure 1.4 Schematic of a typical gas–solid fluidized bed. .............................................. 10 Figure 1.5 Fluidization flow regimes as fluidizing gas velocity increases (adapted from Grace [48]). ....................................................................................................................... 10 Figure 1.6 Geldart solids classification (adapted from Cocco et al. [47]). ........................ 11 Figure 1.7 Schematic diagrams of (a) charge generation, where charges are produced by triboelectrification during particle–wall contact, and (b) charge dissipation, where charges are transferred to the wall during particle–wall contact. ................................................... 13 Figure 1.8 Gaseous discharge when electric field (E) around a charged particle exceeds the dielectric strength of the fluidizing gas (i.e., E>EBreakdown). Under these conditions, gas molecules become ionized, generating electron–ion pairs. Some positively charged ions subsequently collide with the particle and neutralize the negative charges present on its surface. .............................................................................................................................. 15 Figure 1.9 Acoustic levitation of a particle near the pressure node of a plane standing wave field, established between a transducer and a reflector. The particle is trapped at the acoustic potential well formed near the pressure node, where the acoustic radiation force balances gravity, allowing stable levitation. .................................................................................... 23 Figure 1.10 Schematic of the acoustic levitation device (adapted from Kline et al. [35])............................................................................................................................................ 24 xiv Figure 2.1 Schematic of fluidization unit and camera setup. ............................................ 29 Figure 2.2 (A) Backlit wall-coating image after fluidization of HDPE particles; (B) image processed with Watershed Segmentation; (C) same image processed without segmentation............................................................................................................................................ 32 Figure 2.3 Saturation charge per unit mass, Q/m, acquired by HDPE particles fluidized in the (A) acrylic column and (B) steel column with various dry fluidizing gases (RH < 0.1%)............................................................................................................................................ 34 Figure 2.4 Saturation charge per unit mass, Q/m, acquired by glass beads fluidized in the (A) acrylic column and (B) steel column with various dry fluidizing gases (RH < 0.1%)............................................................................................................................................ 36 Figure 2.5 Saturation charge per unit mass, Q/m, acquired by HDPE particles fluidized in the (A) acrylic column and (B) steel column at different relative humidity (RH) levels. 46 Figure 2.6 Saturation charge per unit mass, Q/m, acquired by glass beads fluidized in the (A) acrylic column and (B) the steel column at different relative humidity (RH) levels. For RH=10% and RH=24% in steel column, solid bars denote data obtained in April 2019, the checkerboard-patterned bar represents a repeat measurement at RH = 10% in March 2022, and the diagonally striped bar represents a repeat measurement at RH = 24% in March 2022................................................................................................................................... 49 Figure 2.7 Percentage wall coverage of HDPE particles fluidized in an acrylic column by argon, air, and nitrogen. The corresponding saturation charge densities are provided in the inset for direct comparison................................................................................................ 51 Figure 2.8 Force balance on a single spherical particle near a fluidization column wall. 53 Figure 2.9 Percentage wall coverage of HDPE particles fluidized in an acrylic column with argon at 0.317 m/s and 0.267 m/s. .................................................................................... 54 Figure 2.10 Percentage wall coverage of glass beads fluidized in an acrylic column by argon, air, and nitrogen. The corresponding saturation charge densities are provided in the inset for direct comparison................................................................................................ 57 Figure 2.11 Median diameters d(0.5) of (A) HDPE particles and (B) glass beads coated to an acrylic column after fluidized with argon, air and nitrogen. ........................................ 59 xv Figure 2.12 Agglomerate fraction index of HDPE particles coated to an acrylic column after fluidized with argon, air, and nitrogen. ..................................................................... 61 Figure 3.1 The copper fluidization column housed inside an acrylic outer shell. ............ 68 Figure 3.2 Charge per unit mass in absolute value, |Q/m|, acquired by HDPE particles and glass beads fluidized in the isolated and grounded copper columns by dry air at different fluidization time intervals. The dotted lines (blue for HDPE and orange for glass) represent the estimated maximum charge of the particles when gas breakdown occurs.................. 70 Figure 3.3 Charge per unit mass, Q/m, acquired by HDPE particles fluidized in isolated and grounded copper columns with argon and dry air at different time intervals. The dotted lines (blue for air and orange for argon) represent the estimated maximum charge when gas breakdown occurs. The enlarged view highlights the early-stage charging behavior. ...... 74 Figure 3.4 Charge per unit mass, Q/m, acquired by HDPE particles fluidized in the isolated and grounded copper columns with dry and humidified air as a function of fluidization time. The dotted line represents the estimated maximum specific charge when gas breakdown occurs. ............................................................................................................................... 77 Figure 3.5 Charge per unit mass, Q/m, acquired by glass particles fluidized in the isolated and grounded copper columns with dry and humidified air as a function of fluidization time. The dotted line represents the estimated maximum specific charge when gas breakdown occurs. ............................................................................................................................... 79 Figure 3.6 Schematic illustration of charge dissipation pathways in isolated and grounded fluidization columns. In grounded systems, charges accumulated on the wall are rapidly neutralized through conduction to ground, maintaining a nearly neutral wall surface. In isolated systems, charges remain on the wall, generating a counteracting electric field that can partially suppress further triboelectric charge transfer. Despite these differences in wall behavior, particle charge dissipation remains limited primarily by the high particle–particle contact resistance (Rp), which dominates over particle–wall (Rc), wall resistance (Rw), and grounding resistance (Rg). ................................................................................................. 84 Figure 4.1 Schematic of the acoustic levitation device and charge measurement system. Two particles are levitated and confined in an acoustic potential well. A horizontal electric field is applied across the levitation region to separate the particles according to their charge polarity. ............................................................................................................................. 88 Figure 4.2 HDPE particles trapped within acoustic potential wells. ................................ 88 xvi Figure 4.3 Representative still frames from a slow motion video, showing the controlled collision process between two HDPE particles levitated in the same acoustic potential well. (A) The particles adhere and rotate together within the well. (B–C) The amplifier’s pulsed mode periodically interrupts the acoustic field for a few milliseconds, causing the particles to temporarily lose their attractive forces and briefly separate under centrifugal motion. (D) Once the field resumes, the particles return to close contact and continue their coupled rotation. ............................................................................................................................. 91 Figure 4.4 Initial charge density (Q/m) of the tested particles measured using a Faraday cup. .................................................................................................................................... 96 Figure 4.5 Distribution of initial particle charge polarity (positive, negative, neutral) expressed as the percentage of total particles tested. Polarity measurements were obtained using acoustic levitation combined with a horizontal electric field. ................................. 97 Figure 4.6 Distribution of charge polarity combinations between HDPE (PE-M) and glass particles (GB-M) after acoustic levitation collisions, expressed as the percentage of total measurements. ................................................................................................................... 98 Figure 4.7 Distribution of charge polarity combinations between similarly sized HDPE particles (PE-M) after acoustic levitation collisions, expressed as the percentage of total measurements. ................................................................................................................. 100 Figure 4.8 Charge polarity distribution of large HDPE particles (PE-L) after collisions with smaller HDPE particles (PE-S). 84% of the large particles became positively charged, while 16% showed no measurable deflection under the applied electric field. .............. 102 Figure 5.1 Schematic (top) and photograph (bottom) of the vibration test unit. ............. 114 Figure 5.2 Schematic of the pneumatic conveying unit. .................................................. 116 Figure 5.3 Photographs showing the copper coiled tube used for pneumatic conveying (left) and the Faraday vessel for charge measurement (right). ................................................. 116 Figure 5.4 Schematic of the charge decay test unit. ........................................................ 118 Figure 5.5 Charge-to-length ratios of PU, PVC, and rubber air hoses after rubbing against different vibrating surfaces. ............................................................................................ 120 Figure 5.6 Comparison of charge per unit mass (Q/m) for pine wood sawdust particles of different size ranges conveyed by compressed air. ......................................................... 121 xvii Figure 5.7 Charge per unit mass (Q/m) of sawdust from different wood species, conveyed through a PVC tube, in size ranges of 150–300 μm (left) and <150 μm (right). ............ 125 Figure 5.8 Photographs of sawdust particles from different wood species in the <150 μm size range. ....................................................................................................................... 125 Figure 5.9 Charge per unit mass (Q/m) of unsieved hemlock wood sawdust collected at different locations within a wood-processing facility. .................................................... 127 Figure 5.10 Charge per unit mass (Q/m) of hemlock wood sawdust collected near different wood-processing equipment, for size ranges of 150–300 μm and <150 μm. ................. 128 Figure 5.11 Charge decay curves and fitted exponential decay trendlines for pre-dried hemlock sawdust placed on different sample holders. .................................................... 131 Figure A.1 Size distribution of (A) HDPE particles and (B) glass beads. ...................... 162 Figure A.2 Photos from optical coherence tomography to show the surface roughness of (A) PU hose (B) PVC hose and (C) rubber hose. ........................................................... 163 xviii List of Symbols Ap Cross-sectional area of particle, m2 Cd Drag coefficient d(0.5) Mass median diameter, μm E Electric field, V/m EBreakdown Dielectric strength, kV/mm EFermi Fermi energy, eV g Gravitational acceleration, m/s2 mp Mass of particle, g Q Electrostatic charge, C Q/m Electrostatic charge per unit mass, μC/kg Rp Particle-particle contact resistance, Ω Rc Particle–wall contact resistance, Ω Rw Wall material resistance, Ω Rg Wall-to-ground resistance, Ω Re Reynolds number RH Relative humidity, % Umf Minimum fluidization velocity, m/s Up Particle velocity, m/s Vp Volume of particle, m3 Greek letters αg Void fraction, % 𝜀0 Permittivity of free space, F/m 𝜀g Relative permittivity of gas, F/m xix 𝜀m Mixture permittivity of the bed, F/m 𝜀s Relative permittivity of the solid, F/m λ Sound wavelength, m ρg Density of the fluidization gas, kg/m3 Φ Work function, eV xx List of Abbreviations EWF Effective work function GB soda-lime glass bead HDPE High-density polyethylene PMMA Polymethyl methacrylate PU Polyurethane PVC Polyvinyl chloride xxi Acknowledgements First and foremost, I would like to express my sincere gratitude to my supervisors, Prof. Xiaotao Bi and Prof. John R. Grace, for their excellent guidance and continuous support throughout my research journey. I am deeply thankful to Prof. Bi for his inspiration, assistance, and encouragement, which motivated me to return to UBC to fulfill one of the most important aspirations of my life. I also wish to express my heartfelt appreciation and remembrance for Prof. Grace, who continued to guide me even during his illness. His rigorous pursuit of academic excellence, optimism in life, and remarkable perseverance and dedication to work have profoundly inspired and influenced me. I would like to thank my research committee members, Prof. James Feng and Prof. Fariborz Taghipour, for their long-term support, encouragement, and valuable advice on my research. My sincere thanks also go to Prof. Sankaran Sundaresan, Prof. Guilin Hu, Prof. Simcha Srebnik, Dr. Xiaoyu Liu, Dr. Hafiz Rahman, Dr. Qun Zhou, Pu Yang, and Bill Cheng for their continuous help, insightful discussions, and encouragement throughout my studies. I am also grateful to the workshop, store, and administrative staff of the Department of Chemical and Biological Engineering for their technical and logistical support. Finally, I would like to acknowledge NOVA Chemicals Inc. and Potters Industries Inc. for providing the particle samples used in this research, as well as the financial support from the WorkSafeBC Research Grants Program and the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Program. xxii Dedication This thesis is dedicated to my mother, Long Bai. Without her lifelong guidance, selfless devotion, and unconditional love, I would never have been able to reach this pinnacle of academic pursuit and achieve such a milestone in my life. Though this period of study may seem but a fleeting moment in the long journey of life, it carries profound meaning for me. I am deeply grateful to my mother for giving me the strength and encouragement to pursue this meaningful journey of learning and growth at UBC.1 Chapter 1 Introduction Gas-solid fluidization is a process in which solid particles are suspended and transformed into a fluid-like state by a flowing gas [1]. It is widely employed in many industries such as oil and gas, chemical and pharmaceutical sectors due to its excellent solids mixing, efficient and uniform heat transfer, and temperature uniformity. However, a long-standing problem in fluidized beds is the generation of triboelectric charges arising from continuous collisions among fluidizing particles and between particles and the column wall [2]. Charge accumulation on particles can lead to operational challenges, including particle agglomeration, disruption of particle motion, and hazardous electrostatic discharge [2,3]. For instance, during gas-phase polymerization of ethylene in gas–solid fluidized beds, charged particles may adhere to the reactor wall, forming sheets of fused material. As these sheets grow thicker, they can detach and fall, potentially blocking the gas distributor, leading to reactor shutdown for cleaning and resulting in significant economic losses [4]. The accumulation of electrostatic charges on particles and reactor walls, along with the associated high-voltage electric fields, can significantly affect hydrodynamics and promote the formation of undesired byproducts in chemical reactors [5]. Furthermore, electrostatic effects may interfere with sensors and internal components of the bed, thereby inducing malfunctions in measurement and monitoring instruments as well as control devices, ultimately leading to operational disruptions [6]. Despite extensive research aimed at understanding particle charging in gas–particle flows [7–12] and efforts to develop mitigation strategies for fluidized beds [13–15], many aspects of triboelectric charging remain poorly understood, particularly in the case of dielectric materials such as polymers [9,10]. Electrostatic charging in fluidized beds is a complex phenomenon influenced by a wide range of factors, including the material properties of particles and walls, the type and velocity of the fluidizing gas, relative humidity, pressure, 2 temperature, and other environmental conditions [15–21]. Moreover, the charge on particles may be non-uniformly distributed, making the estimation of the electrostatic charge levels and process control more difficult [11]. For example, as reactor scale increases, particle–particle interactions become more dominant than particle–wall collisions. Bipolar charging can occur through these interparticle collisions, wherein larger particles tend to acquire charges of one polarity while finer particles acquire the opposite [22,23]. This phenomenon not only alters the net charge level of the bed but also promotes fine–coarse agglomeration, which in turn affects the hydrodynamics of the system [24]. The overall goal of this thesis is to improve the understanding of electrostatic charging mechanisms in gas–solid fluidized beds, with particular attention to the influence of fluidizing gas properties and size-dependent charging behavior. A deeper understanding of these mechanisms will facilitate the development of effective strategies for controlling electrostatic charging in fluidized systems, with a focus on approaches that are practical for industrial implementation. 1.1 Electrostatic Charge Generation 1.1.1 Charge transfer models Static electrification is one of the earliest studied phenomena in science and is well known for its widespread and sometimes severe consequences [10]. When two neutral surfaces are brought into contact and then separated, transfer of charge generally occurs such that one surface becomes charged positively and the other surface becomes charged negatively. Yet, a comprehensive understanding of the mechanisms behind charge transfer remains elusive. For instance, the identity of the charge carriers involved in triboelectric charging remains under debate. 3 It is well established that when two metals come into contact, electrons transfer until their Fermi levels equilibrate [25]. The driving force for this process is the difference in surface work function (ϕ), which quantifies the energy required to lift an electron from the Fermi level to the vacuum level: ϕ = Evacuum − EFermi (1.1) where Evacuum represents the potential energy of an electron just outside the surface. As illustrated in Figure 1.1, when two dissimilar metals are brought into contact, electrons flow from the metal with the lower work function (higher EFermi) to the one with the higher work function (lower EFermi). This electron transfer continues until the Fermi levels equalize through the establishment of an interfacial potential difference that offsets the original work function difference. The resulting potential difference, often referred to as the contact potential, satisfies e∆V = ϕ2 − ϕ1 (1.2) where e is the elementary charge, and ϕ1 and ϕ2 are the work functions of the two metals. The established interfacial potential prevents further electron transfer, and opposite surface charges appear on the two metals. Therefore, the amount and polarity of charge transferred during contact are determined by the difference in work function between the two surfaces, with electrons flowing from the lower- ϕ metal to the higher- ϕ metal until the electrochemical potentials are equalized. 4 Figure 1.1 Schematic of electron transfer (adapted from Lacks et al. [10]). As two metal surfaces are brought into contact, metal 1, with higher Fermi energy (EFermi1) and lower work function (Φ1), transfers electrons to metal 2, with lower Fermi energy (EFermi2) and higher work function (Φ2; top). This leads to an accumulation of electrons on metal 2. However, electron transfer between insulators does not seem possible due to the large band gap that prevents transferring electrons without a significant energy requirement [10]. This therefore makes contact charging involving insulators a more complicated phenomenon. Lowell [26] and Lacks [9,27] proposed that surface electrons trapped in intermediate energy states (often introduced by structural defects such as strained bonds or vacancies) may be responsible for the finite conductivity observed in insulating materials. These trap states, which lie energetically between the valence and conduction bands, provide a semi-bound environment where electrons are not fully mobile but can still participate in charge transfer processes under certain conditions. When two materials come into contact, the relative electron affinity between them plays a critical role in driving the direction of charge transfer. Specifically, electrons tend to migrate from the material with lower electron affinity to the one with higher electron affinity to minimize the system's free energy. In this context, the defect-induced trap states act as an electron reservoir, while the difference in 5 electron affinity provides the driving force for triboelectric charging. This mechanism is supported by several experimental studies on oxides and polymers [28,29], which suggest that both surface defect states and material-specific electron affinity are key factors governing charge transfer at interfaces. In contrast, a different theory was proposed in which triboelectric charging occurs through transfer of mobile ions rather than electrons [30–32]. Certain materials consist of molecules that easily dissociate as ions. Ions of one polarity could be more strongly bound than those of the opposite polarity. Experimental findings indicate that, upon contact with a second surface, triboelectric charging arises from the transfer of the weakly bound ions to the opposing surface, while the more strongly bound ions remain on the original surface, causing a net transfer of charges between the two surfaces [32–34]. Furthermore, experimental evidence suggests that for materials incapable of contributing mobile ions, charge carriers may originate from adsorbed environmental ions, particularly those residing in surface water layers formed under ambient humidity [30,32]. It has been proposed that when two such surfaces come into contact under humid conditions, a transient 'water bridge' forms between them, facilitating ion transfer [32,33]. Figure 1.2 presents a schematic of the ion-transfer model, illustrating the role of surface-adsorbed water. Once the water bridge is established, mobile ions (either from the material itself or from environmental sources) can move freely within this liquid region. This hypothesis is further supported by observations showing that electrostatic charge levels vary with surface hydrophilicity [35], temperature [36] and relative humidity [37–39], as these parameters influence the thermodynamics of water adsorption and, consequently, ion transfer between the surfaces. Both models are supported by reproducible experimental results obtained using the latest techniques available; however, they differ in their conclusions about which material 6 properties influence charge transfer. Despite this divergence, the conclusions may not be inherently contradictory, as distinct mechanisms could dominate under different conditions, such as the presence or absence of ambient humidity, or material-specific properties. Figure 1.2 Schematic of the adsorbed water layer in the ion-transfer model of triboelectric charging for polymers containing covalently bound ions and mobile counterions (adapted from McCarty et al. [32]). The mobile ions may originate from the polymer itself or from aqueous species present in the environment, such as H⁺ or OH⁻. 1.1.2 Triboelectric series and effective work function In the absence of definitive knowledge regarding the identity of the charge transfer species, the direction and magnitude of charge transfer remain difficult to predict theoretically. As 7 a result, empirical approaches have long been employed to correlate charge transfer behavior with the chemical composition and surface properties of materials [40]. One such approach is the triboelectric series: an empirically derived ordered list of materials based on their tendency to acquire positive or negative charge upon contact [41]. In such a series, materials tend to acquire a positive charge when coming into contact with another material positioned lower in the sequence, as illustrated in Figure 1.3a. However, triboelectric series are purely experimental constructs and often suffer from poor reproducibility. For example, certain groups of materials form what is known as a so-called cyclic triboelectric series (see Figure 1.3b) [25]. The existence of these cyclic relationships suggests that no single physical property can fully explain all instances of contact electrification. Instead, it indicates that multiple mechanisms may be involved, influenced by variations in experimental conditions (e.g., humidity) and material characteristics (e.g., impurity states at the surface) during the generation of the series. For metals, the work function has traditionally been used to determine their position in the triboelectric series. The magnitude of charge transfer between metals is generally well described by the difference in their work function values [25]. However, as previously discussed, work function correlates poorly with the triboelectric behavior of insulators, primarily due to their large band gaps, which impose a substantial energy barrier to electron transfer. To address this limitation, some studies have introduced the concept of an effective work function (EWF) for insulators [7,11]. The EWF is a phenomenological, lumped parameter that captures surface properties relevant to charge transfer, thereby enabling the extension of charge transfer models originally developed for metals to insulating materials. In these models, the charge transfer rate is governed by the difference in EWF between the contacting surfaces, as well as the local electric field at the contact location. This electric field is influenced not only by the charges on the interacting surfaces but also by the 8 surrounding space charges, a phenomenon commonly referred to as the space-charge effect [42–44]. Figure 1.3 a) An example of conventional triboelectric series; b) A set of five materials that form a cyclic triboelectric series (adapted from McCarty et al. [32]). 1.2 Electrostatics in Gas-solid Fluidized Beds 1.2.1 Gas-solid fluidization Gas–solid fluidization is a widely utilized process in various industrial applications, including catalytic gas–solid reactions, particle mixing, drying, and combustion. A schematic diagram of a typical gas–solid fluidized bed is shown in Figure 1.4. In this 9 system, fluidizing gas is introduced from the bottom of the column through a distributor plate and flows upward through a bed of solid particles. Fluidization occurs when the upward drag force exerted by the gas balances the gravitational force acting on the particles. The corresponding gas velocity is defined as the minimum fluidization velocity, Umf. Below Umf, the gas primarily flows through the voids of the packed bed. Once the gas velocity exceeds Umf, excess gas forms bubbles that rise through the bed, leading to the development of heterogeneous flow structures and enhanced mixing [45]. As the superficial gas velocity continues to increase, the bed transitions through various flow regimes, as illustrated in Figure 1.5. If the gas velocity exceeds the terminal velocity of certain particles during fluidization, these particles, known as entrained particles, will be carried out of the column along with the gas stream. To minimize such entrainment, an expanded section is typically installed at the top of the column to reduce the gas velocity and allow most of the entrained particles to fall back into the bed. The ease of fluidization depends strongly on the properties of both the particles (e.g., size, density) and the fluidizing gas (e.g., density, viscosity). To characterize fluidization behavior, Geldart [46] proposed a widely used classification scheme based on particle size and density, as shown in Figure 1.6. Particles are divided into four groups (A, B, C, and D), each with distinct fluidization characteristics. Group A particles are typically small and relatively low-density, and exhibit smooth fluidization with noticeable bed expansion before the onset of bubbling. Group B particles are larger and/or denser, and tend to fluidize with vigorous bubbling and minimal bed expansion. They represent typical fluid-like behavior and are widely used in industrial fluidized beds. Group C particles are very fine and cohesive, often exhibiting poor fluidization due to strong interparticle forces. Group D particles are large and/or very dense, and may exhibit spouting or channeling rather than uniform fluidization. High gas velocities are required to suspend them [47]. 10 Figure 1.4 Schematic of a typical gas–solid fluidized bed. Figure 1.5 Fluidization flow regimes as fluidizing gas velocity increases (adapted from Grace [48]). 11 Figure 1.6 Geldart solids classification (adapted from Cocco et al. [47]). 1.2.2 Charge generation and dissipation in fluidized beds Due to the inherently dynamic nature of gas–solid fluidized beds, where particles are in continuous contact with each other and with the column walls, triboelectric charging is essentially unavoidable. In practice, electrostatic charging in such systems is influenced by a wide range of factors, including the materials of the particles and column walls, the type and velocity of the fluidizing gas, and environmental conditions such as relative humidity, pressure, and temperature [15–21]. When all fluidized particles are composed of the same material, charge generation is primarily driven by particle–wall interactions, while particle–particle collisions serve mainly to redistribute the charge. In these cases, as illustrated in Figure 1.7a, the difference in EWF between the particles and the wall is commonly considered the primary driving force of charge generation. Therefore, the relative positions of the particle and wall materials in the triboelectric series are often used to predict both the polarity and the magnitude of the charges generated. However, the net charge level on particles is governed not only by charge generation but also by concurrent charge dissipation processes [49]. 12 In gas–solid fluidized beds, particles can dissipate charge through contact with the column wall. The rate of charge dissipation depends on two key factors: the local electric field at the contact point and the overall electrical resistance along the conductive path. The local electric field, determined by the charge levels on contacting surfaces and the surrounding electrostatic environment, can either facilitate or hinder charge transfer depending on its orientation. Meanwhile, the overall resistance is composed of several components, including particle-particle contact resistance (Rp), particle–wall contact resistance (Rc), and wall material resistance (Rw) as well as wall-to-ground resistance, as illustrated in Figure 1.7b. Higher resistance in this path slows charge dissipation because it limits the movement of charges from particles to the wall or ground. A key parameter for evaluating electrostatic charging in gas–solid fluidized beds is the charge saturation level, defined as the steady-state charge reached when the rates of charge generation and dissipation are balanced. Most previous studies have focused primarily on measuring this saturation level as an indicator of electrostatic behavior. It has been shown that the time required to reach charge saturation varies from 5 to 60 minutes, depending on factors such as particle type, column material, gas type and velocity, relative humidity, and the measurement technique used [50–53]. In summary, the buildup of electrostatic charges on particles in gas–solid fluidized beds arises from the continuous competition between charge generation and dissipation processes. The overall charge accumulation within the bed can be described as a dynamic balance between these two opposing effects: (dQdt)total= (dQdt)gen − (dQdt)dis (1.3) Here, (dQdt)gen and (dQdt)dis denote the rates of charge generation and dissipation, respectively. The instantaneous charge level in the bed is determined by the balance 13 between these two competing processes, and a steady-state charge saturation is reached when the two rates become equal. Figure 1.7 Schematic diagrams of (a) charge generation, where charges are produced by triboelectrification during particle–wall contact, and (b) charge dissipation, where charges are transferred to the wall during particle–wall contact. 1.2.3 Effect of dielectric strength of fluidizing gas Since the saturation charge level in a gas–solid fluidized bed results from the dynamic balance between charge generation and dissipation, the inherently high resistivity of insulating particles can lead to minimal charge dissipation. Nevertheless, in practice, the charge on such particles does not increase indefinitely. This limitation is primarily attributed to the dielectric breakdown of the surrounding gas, which imposes an upper limit on charge accumulation in fluidized beds [54,55]. As shown in Figure 1.8, a fluidized particle accumulates charge and generates an electric field in its vicinity. When this local electric field becomes sufficiently strong, it can initiate dielectric breakdown of the gas. Free electrons are accelerated by the field and collide with gas molecules, causing ionization and producing electron–ion pairs. While many of these 14 pairs quickly recombine, some positive ions migrate toward the negatively charged particle surface and partially neutralize the accumulated charge. In addition, the formation of conductive ionization channels can facilitate the direct emission of surface charges from the particles, further contributing to charge loss. The overall charge evolution in the bed can thus be expressed as the competition among charge generation via particle-wall contact, dissipation via particle-wall conduction, and gas breakdown-induced charge loss: (dQdt)total= (dQdt)gen − (dQdt)dis(p−w)− (dQdt)breakdown (1.4) Here, the third term on the right side represents charge loss associated with dielectric breakdown of the gas. This process is only triggered when sufficient charge is accumulated on particles such that the local electric field exceeds the dielectric breakdown threshold. Prior to breakdown, (dQdt)breakdown is negligible. Once breakdown is initiated, gas ionization creates a conductive path, causing this charge-loss term to rise sharply. This accelerated discharge process shifts the overall charge balance toward greater dissipation, limiting the steady-state saturation charge level that particles can attain. Several studies have demonstrated that electrostatic charge accumulation is significantly lower in an argon atmosphere than in air or nitrogen, primarily due to argon’s lower dielectric breakdown strength. For instance, Matsuyama and Yamamoto [54] reported that polymer spheres acquired lower charge levels when repeatedly impacted against a metal plate in argon compared to air. They attributed this reduction to gaseous discharge occurring during the particle’s rebound and interpreted the results using the Paschen curve. Similarly, Liu and Sundaresan [56] examined the effect of gas composition on particle charging by measuring the total charge acquired by soda-lime glass and polyethylene particles in a vibrated bed under either nitrogen or argon atmospheres. They found that the 15 use of argon, which has a lower dielectric breakdown strength, resulted in reduced charge accumulation. Although those studies have explored the use of argon for mitigating electrostatic charging, research on the role of different fluidizing gases in fluidized bed systems remains limited. There is a significant need for further investigation into how various particles charge in different fluidizing gases and how these charging behaviors relate to issues such as wall fouling under different operating conditions. Moreover, while gases are often treated as passive carriers in fluidized systems, their physical properties, such as polarizability, adsorption affinity, and ion mobility, may influence both the efficiency of triboelectric charge generation and the rate of charge dissipation, corresponding to (dQdt)gen and (dQdt)dis(p−w) in Eq. (1.4). These potential gas-phase effects remain poorly understood and are not explicitly addressed in existing research. Figure 1.8 Gaseous discharge when electric field (E) around a charged particle exceeds the dielectric strength of the fluidizing gas (i.e., E>EBreakdown). Under these conditions, gas molecules become ionized, generating electron–ion pairs. Some positively charged ions subsequently collide with the particle and neutralize the negative charges present on its surface. 16 1.2.4 Effect of humidity of fluidizing gas The complexity of electrostatic charging in gas–solid fluidized beds increases in the presence of water, as water can simultaneously influence both charge generation and dissipation through distinct mechanisms. Many previous studies have shown that, at high relative humidity (RH), a thin water film typically forms on particle surfaces, reducing overall electrical resistance and creating a more conductive pathway to the column wall. This enhanced conductivity facilitates charge dissipation, thereby lowering the net charge buildup in the system [8,15,19,57]. For example, Moughrabiah et al. [57] and Al-Smari [58] investigated the effect of RH on the charging behavior of glass beads and polyethylene particles fluidized in a steel column, finding that electrostatic charge accumulation decreased as RH increased from 3% to 30%. Conversely, other studies have demonstrated that adsorbed water can, under certain conditions, enhance triboelectric charging [30,36,59]. These researchers attribute this phenomenon to discrete water patches formed by ambient humidity, which can create “water bridges” at contact points, facilitating charge transfer via OH⁻ or H⁺ ion exchange. Under such circumstances, increasing humidity can lead to higher particle charge levels due to enhanced charge transfer. Kolehmainen et al. [38] investigated the triboelectric charging of polyethylene particles in a glass container subjected to vertical vibration across different humidity levels. They revealed that the EWF difference between contacting surfaces varies with RH, indicating that charge generation is indeed humidity-dependent and material-specific. Similarly, Schella et al. [37] observed that the charge levels of polymer spheres shaken in a stainless steel container first increased and then decreased as RH rose, with this transition between charging and dissipating regimes strongly influenced by the material’s hydrophobicity. 17 Furthermore, because the dielectric strength of the fluidizing gas is a key factor determining the saturation charge of fluidized particles, it is important to recognize that variations in humidity can also affect this parameter. Previous studies have shown that the relationship between humidity and breakdown strength is complex and can vary based on multiple factors, including gas type, humidity level, electrode configuration, gap distance, and other environmental conditions. In some cases, higher humidity has been reported to increase dielectric strength by reducing the number of free electrons, thus raising the breakdown voltage [60,61]. In other cases, its effect on breakdown strength has been found to be negligible [62,63]. Those findings highlight that controlling ambient humidity can be a promising strategy for managing electrostatic charging in gas–solid fluidized beds. However, the effectiveness of humidity-based mitigation is highly dependent on system-specific factors. From the perspective of the charge balance described in Eq. (1.4), humidity can simultaneously modify all three right-hand-side terms governing the evolution of particle charge. It may enhance (dQdt)dis(p−w) by promoting charge dissipation through the formation of surface water films, while also increasing (dQdt)gen via ion-transfer mechanisms such as water bridging. The dominant effect depends on the material properties of both the particles and the wall surfaces. In addition, changes in gas humidity can potentially alter the dielectric strength of the fluidizing gas, thereby affecting (dQdt)breakdown and ultimately modifying the saturation charge levels attained by fluidized particles. Despite the importance of these coupled effects, their combined influence on electrostatic charging in gas–solid fluidized beds has not yet been systematically investigated. 18 1.2.5 Adding fine powders in fluidized beds Adding fine powders has been shown to significantly influence the hydrodynamics of gas–solid fluidized beds by reducing particle cohesiveness and agglomeration, thereby transforming cohesive powders into more free-flowing materials [64]. These fine particles act as spacers, increasing the separation distance between host particles, thereby weakening van der Waals and other interparticle forces, ultimately improving bed flowability. In a subsequent study, Xu et al. [65] reported that the effectiveness of ultra-fine coatings in enhancing flowability depends on the material properties of both the host and guest particles, as well as the guest-to-host mass ratio. In addition to their hydrodynamic effects, fine antistatic additives have also been found to influence the electrostatic behavior of fluidized beds. However, their role in mitigating electrostatic charging has been interpreted differently across various studies. For example, Wolny and Opaliński [66] introduced a small fraction of fine particles into a bed of fluidized polystyrene beads and observed a reduction in charge accumulation. They attributed this effect to the alteration of contact conditions induced by the fines, which enhanced charge transfer and redistribution within the system. In contrast, Wu and Bi [13] demonstrated that incorporating fine carbon nanotubes into coarse glass and polyethylene powders enhanced charge dissipation by reducing the overall resistivity in a mixing unit. Beyond electrical resistivity, the EWF of fine additives also plays a critical role in determining their triboelectric charging behavior. In fluidized beds, these fine additives may acquire a charge of opposite polarity through contact with the column wall. For instance, Ghori et al. [67] observed in powder mixing experiments that fine theophylline particles acquired charges via particle-wall interactions and subsequently adhered to oppositely charged methylcellulose particles, effectively reducing the net charge of the bulk system. As the concentration of fine additives increases, the frequency of their 19 interactions with the column wall also rises, potentially altering both the magnitude and polarity of the overall bed charge. Current techniques involving the addition of fine powders to reduce electrostatic charges in gas–solid fluidized beds still present several limitations. For instance, the effectiveness of charge mitigation, whether through the particles generating opposite charges or enhanced charge dissipation, can be strongly influenced by operating conditions. As previously discussed, factors such as the type of fluidizing gas and ambient humidity can significantly impact the performance of fine additives, thereby limiting their consistency and reliability across different environments. Moreover, it is important to note that, in certain applications, the introduction of foreign particulate materials may not be permissible due to the risk of contamination or adverse effects on product quality. In such cases, these additives could interfere with downstream processing, alter the physical or chemical properties of the final product, or violate regulatory or purity specifications. 1.2.6 Size-dependent charging Numerous laboratory experiments have demonstrated size-dependent charging (also called bipolar charging) among particles of identical chemical composition, specifically, a tendency for larger particles to charge with one polarity and smaller particles with the opposite polarity [18,23,68]. This phenomenon was first observed in natural dust storms, where studies revealed bipolar electric fields oriented perpendicular to the Earth's surface. It is widely hypothesized that such charge separation arises from the preferential charging of particles based on size, with gravitational settling subsequently segregating oppositely charged particles spatially [69,70]. The particle-size dependent polarity of charging leads to important consequences. For example, the magnitude of charge separation becomes greater when the particle size 20 distribution is broader. Several hypotheses have been proposed to explain the origin of this phenomenon. Lowell and Truscott [71] proposed a theory based on asymmetric contact during collisions between particles of different sizes. In this model, a relatively small area on a small particle may rub against a much larger area on a large particle, analogous to the interaction between a violin string and a bow. The bow interacts with the string along a line, whereas the string contacts the bow at only a single point. This contact asymmetry leads to the depletion of charge carriers from the larger surface and their accumulation on the smaller one, resulting in net charge transfer. However, the underlying driving force responsible for the movement of charge carriers was not clearly identified in their study. Subsequently, Pähtz et al. [72] demonstrated that particle collisions in the presence of an external electric field can break the symmetry between identical materials, leading to charge transfer. The electric field induced by charged particles causes polarization in neighboring particles, producing surface charge distributions with oppositely charged hemispheres. When the positive side of one particle contacts the negative side of another particle, partial neutralization occurs, resulting in net charge transfer upon separation. Moreover, SEM analysis conducted by Zhao et al. [23] revealed that larger polymer particles typically exhibit smoother surfaces than smaller ones. Microscopic differences in surface characteristics, such as roughness, curvature, and geometry, can influence the EWF and contact mechanics between particles of different sizes. Finally, due to their higher specific surface area, smaller particles offer more surface area per unit mass for the adsorption of fine additives or environmental contaminants. This enhanced adsorption capability may further modify the surface properties, influencing the EWF and thereby affecting the charge transfer behavior during collisions. Experiments investigating the particle-size dependence of triboelectric charging have been conducted across a variety of materials. In most cases, results show that smaller particles tend to acquire negative charges, whereas larger particles become positively charged 21 [23,73–75]. In contrast, Mehrani et al. [18] and Al-Smari [58] reported opposite behavior in fluidized beds using binary mixtures of large and fine glass beads—entrained fines were positively charged, while the larger in-bed particles were negatively charged. These discrepancies can be attributed to differences in particle sizes, surface properties, gas humidity, and most importantly, the dominant charging mechanism between particle–particle and particle–wall interactions. Conducting controlled experiments to isolate these charging mechanisms remains challenging, as unintended contact with surrounding surfaces of fluidized beds often introduces unwanted charge transfer. An ideal experimental platform would eliminate the influence of wall effects and allow for independent evaluation of interparticle interactions. One such approach is the use of an acoustic levitation system, which provides a wall-free environment and enables fundamental investigation into the direction of charge transfer during collisions between particles of the same material but different sizes. Insights gained from such studies could further inform practical charge-mitigation strategies. For example, adjusting the mass ratio of fine to coarse particles, given their opposite charging tendencies, could offer a practical method to reduce net charge accumulation in fluidized beds. Unlike antistatic additives, this approach introduces no foreign substances, making it particularly suitable for applications requiring high product purity. 1.2.7 Acoustic levitation As a form of mechanical wave, sound carries momentum, which can exert force on objects through acoustic radiation forces [76]. Acoustic levitation harnesses these forces to trap and suspend objects in mid-air, preventing any physical contact with surrounding surfaces. This technique enables the stable levitation of various materials ranging from millimeter to sub-millimeter in size. Due to these unique features, acoustic levitation serves as an 22 excellent experimental platform for eliminating wall effects and enabling the independent evaluation of interparticle interactions. The most widely adopted technique for airborne particle levitation is the resonant single-axis standing wave method [77]. In this approach, a transducer emits sinusoidal sound waves toward a reflector, forming an acoustic cavity between the two surfaces. When the spacing between the transducer and the reflector is precisely adjusted to an integer multiple of half the sound wavelength (i.e., n·λ/2, where n is a positive integer), the incident and reflected waves constructively interfere to form a stable standing wave. This standing wave exhibits a series of pressure nodes (regions of minimal pressure amplitude) and antinodes (regions of maximal amplitude) along the propagation axis, as shown in Figure 1.9. Although particles are often described as being trapped at the pressure nodes, it is more accurate to state that they are suspended in acoustic potential wells formed near these nodes [78–80]. These wells arise from the acoustic radiation force, a nonlinear, time-averaged force exerted by the oscillating sound field. For solid particles suspended in air, this force tends to drive them toward regions of minimum acoustic potential energy, i.e., toward the acoustic potential wells, resulting in a stable and contactless particle confinement. The simplest configuration for a single-axis acoustic levitator consists of a planar transducer and a planar reflector. However, it has been demonstrated that concave reflectors generate a three-dimensional acoustic potential well structure. The lateral confinement arises from inward-directed pressure gradients, thereby reducing lateral drift and improving overall stability [79,81,82]. With an appropriately chosen curvature radius and section radius, the required driving voltage or current to levitate a given particle can be substantially reduced, thereby enhancing the efficiency and performance of the levitator [77,79]. 23 Figure 1.9 Acoustic levitation of a particle near the pressure node of a plane standing wave field, established between a transducer and a reflector. The particle is trapped at the acoustic potential well formed near the pressure node, where the acoustic radiation force balances gravity, allowing stable levitation. Kline et al. [35] recently developed an apparatus based on acoustic levitation to controllably and repeatedly collide two levitated sub-millimeter particles and measure the charge on individual particles. A schematic of their experimental setup is shown in Figure 1.10. A pair of sub-millimeter particles (black circles) are levitated at the midpoint between a grounded aluminum transducer and an aluminum reflector. The reflector is mounted on a lab jack for precise control of the cavity height and is connected to an AC voltage supply to generate a vertical electric field across the levitation region, enabling charge measurement after particle collisions. An acrylic "hand" (blue rectangle) is positioned between the particles to induce separation via acoustic scattering. The particles are backlit and the movement is recorded using a high-speed camera. Although their study focused solely on collisions between two similar-sized particles, it successfully demonstrated that the acoustic levitation–based technique can effectively isolate the particle–particle system from any external surfaces. This complete electrical isolation makes it an ideal platform for investigating size-dependent charging behavior. 24 Figure 1.10 Schematic of the acoustic levitation device (adapted from Kline et al. [35]). 1.3 Thesis Objectives Electrostatic charging in gas–solid fluidized beds presents persistent challenges for both fundamental understanding and industrial practice. Despite decades of research, there are still knowledge gaps regarding (i) how gas-phase properties such as gas composition and humidity influence saturation charge and the overall charging process in fluidized beds, and (ii) how interparticle collisions contribute to the charging process of fluidized particles. In industrial contexts, gas properties and particle size distribution can strongly influence reactor wall fouling, particle agglomeration, and hazardous discharges, all of which compromise product quality and process safety. Addressing these unresolved issues requires a systematic investigation of gas–particle–wall interactions, time-resolved charging dynamics, and size-dependent interparticle charging mechanisms under well-controlled experimental conditions. The main goal of this thesis is to improve the understanding of electrostatic charging mechanisms in gas–solid fluidized beds, with particular attention to the influence of 25 fluidizing gas properties and size-dependent charging behavior. This knowledge will provide a foundation for developing effective mitigation strategies for electrostatic-related issues, with a focus on solutions that are practical and easy to implement in industrial settings. Specific objectives contributing to this goal are: 1. To investigate the role of fluidizing gas composition and relative humidity on saturation charge levels and wall fouling under various combinations of particle and wall materials. The results will help demonstrate that adjusting the type of fluidizing gas and controlling humidity are effective approaches for reducing charge accumulation and mitigating wall fouling in gas–solid fluidized beds. 2. To analyze the charging dynamics of fluidized particles under controlled gas composition and humidity conditions, with a focus on the temporal evolution of charge accumulation. This analysis enables a clearer understanding of how specific gas-phase properties independently influence charge generation and dissipation processes. 3. To isolate and examine size-dependent interparticle charging mechanisms using an acoustic levitation device. This novel approach enables controlled collisions between particles of different sizes without wall interference, providing insights into bipolar charging behavior and its potential role in charge mitigation within fluidized beds. 4. To assess electrostatic charging hazards in real industrial contexts by focusing on charge retention, dissipation timescales, and the influence of processing conditions in wood-processing facilities. Key factors considered include equipment material, particle size distribution, and moisture content. This objective aims to evaluate whether the charging mechanisms identified in this study are applicable to other industrial powder handling environments. 26 1.4 Thesis Outline Chapter 2 investigates the influence of fluidizing gas type (e.g., air, argon) and relative humidity on saturation charge levels, wall fouling, and particle agglomeration in gas–solid fluidized beds. The charging behaviors of HDPE and glass particles are compared under these conditions, and the study further explores how adjusting gas composition (particularly its dielectric breakdown strength) can reduce charge accumulation and mitigate wall fouling by particles. Chapter 3 examines the time-resolved dynamics of charge accumulation and dissipation in fluidized beds under various gas compositions and humidity levels. The roles of electrostatic boundary conditions, such as wall grounding, are systematically evaluated. The study also clarifies how humidity independently affects charge generation and dissipation across different particle systems. Chapter 4 introduces a custom-designed acoustic levitation platform to isolate interparticle charge transfer mechanisms. By allowing controlled collisions between particles of different sizes without wall interference, the experiments reveal the direction of charge transfer and provide new insights into bipolar charging phenomena in fluidized beds. Chapter 5 presents a case study examining electrostatic charging in wood-processing facilities, where triboelectric charging of sawdust is linked to operational hazards such as fire and dust explosions. The study also evaluates long-term charge retention behavior and explores how the findings from earlier chapters translate to industrial environments. Chapter 6 concludes the thesis by summarizing key findings and providing recommendations for future work, particularly focusing on mitigation strategies for industrial fluidized bed processes. 27 Chapter 2 Effect of Fluidization Gas on Saturation Charges and Wall Fouling in Fluidized Beds 2.1 Introduction This chapter investigates how the properties of fluidizing gases influence electrostatic charging behavior in gas–solid fluidized beds. The primary objective is to assess whether gas selection can serve as a viable strategy to control saturated particle charge levels, suppress adverse electrostatic phenomena, and reduce wall fouling. By adjusting the type and humidity of the fluidizing gas, and comparing behaviors across different particle and column materials, this study aims to investigate the underlying mechanisms that govern charge generation, dissipation, and accumulation in practical fluidization systems. 2.2 Experimental Setup 2.2.1 Fluidization unit The experiments were conducted using two interchangeable, grounded cylindrical fluidization columns, one constructed from stainless steel and the other from transparent acrylic (PMMA). The schematic of the fluidization unit is illustrated in Figure 2.1. Both columns had identical dimensions, with an inner diameter of 0.15 m and a height of 2 m. The steel column was equipped with three sight glasses located at 0.23 m, 0.48 m, and 0.63 m above the distributor to enable visual monitoring of bed behavior. The gas distributor consisted of two aligned perforated steel plates: the top plate featured 50 holes of 4 mm diameter, while the bottom plate contained 50 holes of 5.6 mm diameter. A 15 μm stainless-steel screen was sandwiched between the plates to prevent fine particles from falling into the windbox. The distributor provided an open area ratio of 3.8%, suitable for uniform gas distribution at high velocities. 28 Air, nitrogen, and argon were used as fluidizing gases. Air was supplied by a rotary screw compressor (KAESER Model SK19), with its relative humidity adjusted between 7% and 24%. Alternatively, dry air, nitrogen, and argon (RH < 0.1%) were supplied from gas cylinders. The inlet humidity was continuously monitored using a Vaisala HMT 238 humidity transmitter to ensure stability throughout each run. All experiments were carried out at ambient temperature (20 ± 2°C). The minimum fluidization velocity (Umf) for each gas–particle combination was estimated using the Wen and Yu correlation [83] and is summarized in Table 2.1. All tests were conducted at superficial gas velocities of Umf + 0.3 m/s, and fluidization was visually confirmed at the start of each experiment. Due to the low velocities required for direct Umf measurement (0.015–0.021 m/s), the pressure drop across the distributor was insufficient to ensure uniform flow, making accurate experimental determination impractical. However, given that the operational velocities were significantly higher than Umf, the use of the Wen and Yu correlation introduces minimal error into the experimental design. Table 2.1 Estimated Umf for each test group using the Wen and Yu [83] correlation. Bed materials and fluidizing gases Estimated Umf (m/s) HDPE with air 0.021 HDPE with nitrogen 0.021 HDPE with argon 0.017 GB with air 0.019 GB with nitrogen 0.019 GB with argon 0.015 29 Figure 2.1 Schematic of fluidization unit and camera setup. 2.2.2 Tested particles High-density polyethylene (HDPE) particles and soda-lime glass beads (GB) were used as bed materials. The HDPE particles were provided by NOVA Chemicals Inc. and were directly collected from their industrial fluidized-bed reactors, thereby reflecting the characteristics of particles typically encountered in practical operations (e.g., intrinsic material properties and particle size distribution). These HDPE particles served as the primary focus of the present study. In addition, soda-lime glass beads supplied by Potters 30 Industries Inc. were employed as a reference material. Compared with HDPE, the glass beads exhibit a lower electrical resistivity and a more hydrophilic surface, thus providing a useful contrast for charge generation and dissipation. Key physical properties of both materials are summarized in Table 2.2. Material densities were provided by the manufacturers. Particle size distributions (see Appendix Figure A.1) and mass median diameters d(0.5) were measured using a Malvern Mastersizer 2000. Due to the substantially higher density of glass beads compared with HDPE, smaller-sized GB particles were selected to ensure that both materials exhibited comparable minimum fluidization gas velocities under the tested conditions. Table 2.2 Key physical properties of tested particles. Bed particles Size range (μm) Mass median diameter (μm) Geldart group Particle density (kg/m3) HDPE 75-600 260 B 940 Glass 75-300 150 B 2500 2.2.3 Charge measurement A fresh batch of particles was used for each experiment to minimize variability and ensure consistency across all tests. The static bed height was maintained at 0.3 m above the distributor across all tests. Preliminary tests with varying fluidization durations confirmed that, under all operating conditions investigated, particles reached charge saturation within 30 min. Therefore, a fluidization time of 30 min was adopted in all experiments prior to sampling to ensure saturation charging. A particle extraction port was located 0.15 m above the distributor, equipped with ball valves made of the same material as the fluidization column to avoid additional charging caused by contact with dissimilar materials. To prevent additional handling-induced charge, samples were directly ejected into a Faraday cup connected to an electrometer (Keithley Model 6514), allowing direct measurement of particle charge. 31 The charge density (Q/m) was calculated from the measured charge and the sample mass. For statistical reliability, each measurement was repeated at least six times, and the reported value represents the average, with error bars denoting the standard deviation. The extracted sample mass ranged from 8–30 g for HDPE particles and 20–100 g for glass beads. Prior to fluidization, the initial charge of particles was measured in the Faraday cup. In all cases, the initial specific charge was at least 20 times smaller than the value measured after fluidization. Since sampling occurred only after particles had reached charge saturation, the influence of initial charge was considered negligible for all experiments. 2.2.4 Characterization of wall fouling After completing the charge measurements, fluidization was stopped by cutting off the gas supply. To quantify particle adhesion on the transparent acrylic wall, a digital camera and a backlight source were positioned on opposite sides of the fluidized bed. Differences in transparency under illumination enabled the visualization of wall-coated particles. The imaging setup is illustrated in Figure 2.1. An example image of wall coating after fluidization with HDPE particles is shown in Figure 2.2A. A 6 × 6 mm area was selected as the region of interest. This image was then processed using ImageJ software (version 1.53f51), as shown in Figure 2.2B. The software identified adhered particles within the selected area and provided quantitative data on total particle count, size distribution, and wall coverage percentage. 32 Figure 2.2 (A) Backlit wall-coating image after fluidization of HDPE particles; (B) image processed with Watershed Segmentation; (C) same image processed without segmentation. Wall images were captured at five vertical sampling points located at 0.45, 0.55, 0.65, 0.75, and 0.85 m above the gas distributor. Sampling point 1 was located in the transition zone, while the remaining points were in the freeboard region. Figure 2.2A illustrates several particles adhering together to form visible clusters. To quantify the degree of particle agglomeration, ImageJ was used to analyze the images. In Figure 2.2B, the “Watershed Segmentation” function was applied to divide clustered particles into individual particles, allowing them to be counted separately. In contrast, Figure 2.2C presents the same image without segmentation, where clusters were counted as single large particles. As a result, the total particle count in Figure 2.2C is significantly lower than that in Figure 2.2B due to the presence of agglomerates. (A) (B) (C) 33 To characterize the extent of agglomeration, we define an agglomerate fraction index as the ratio of the total number of particles in the segmented image (Figure 2.2B) to that in the unsegmented image (Figure 2.2C): Agglomerate Fraction Index =Number of particles (clusters segmented) Number of particles (clusters counted as one) In the absence of agglomeration, this index would equal 1. Values greater than 1 indicate the presence of clusters, with higher values corresponding to a greater degree of particle agglomeration. Due to their smaller size and transparent appearance, glass beads were excluded from agglomeration analysis, which was only conducted for HDPE particles. It should be acknowledged that the Watershed Segmentation algorithm may not perfectly distinguish between particles that are merely close to each other and those that are actually in contact, particularly when the interparticle spacing is below the image resolution. The segmentation outcome therefore depends on the chosen threshold and contrast parameters. As a result, the calculated agglomerate fraction index may not represent the actual degree of agglomeration. However, because the same image-processing parameters were applied consistently across all experimental conditions, the relative comparison among different gases remains valid. The index thus provides a reliable basis for assessing the influence of gas composition on the extent of particle agglomeration. To ensure reproducibility, three images were captured at each sampling location. The average values of wall coverage percentage, particle size, and agglomerate fraction index were calculated from these replicates, and the standard deviation was used to represent measurement variability. 34 2.3 Results and Discussion 2.3.1 Charge polarity of fluidized particles 2.3.1.1 HDPE particles Figure 2.3 presents the saturation charge densities acquired by HDPE particles when fluidized in acrylic and steel columns under various dry gases. In both cases, the particles consistently acquired negative charges, aligning well with expectations from the published triboelectric series [32]. HDPE tends to acquire more negative charges than both acrylic (PMMA) and stainless steel, so negative charging is expected regardless of the column material. These experimental results confirm that the HDPE particles behave predictably in terms of polarity across all tests. While the overall charge levels varied with gas type, the polarity remained negative in all cases. Figure 2.3 Saturation charge per unit mass, Q/m, acquired by HDPE particles fluidized in the (A) acrylic column and (B) steel column with various dry fluidizing gases (RH < 0.1%). 2.3.1.2 Glass beads For glass beads, the results were more complex and deviated from the predictions of the triboelectric series. As shown in Figure 2.4A, when fluidized in the acrylic column, glass beads acquired positive charges, which is consistent with the series (glass tends to become more positively charged than acrylic). However, this behavior shifted unexpectedly in the 35 steel column (Figure 2.4B), where glass beads acquired small but consistently negative charges under all dry gas conditions. This contradicts the expected outcome, as glass is generally considered more positive than stainless steel and would therefore be expected to acquire positive charges. Although the charge density of the glass particles was relatively low in magnitude, it is difficult to dismiss the consistent observation of negative polarity as merely experimental noise. Repeated measurements across multiple tests confirmed this trend, indicating a systematic deviation rather than random variation. Interestingly, conflicting results were obtained for glass beads in humid environments across experiments conducted at different times, as discussed later in Section 2.3.4.1. One plausible explanation for the observed negative polarity in dry conditions is bipolar charging, a phenomenon in which fine and coarse particles acquire opposite charge polarities. Previous studies by Alsmari et al. [19] using the same steel column observed a similar effect: fines in the freeboard carried positive charges, while coarse particles in the dense bed carried negative charges. Because bipolar charging primarily arises from particle-particle interactions, it becomes more pronounced when particles acquire very low charges from particle–wall contacts, as was the case in these experiments. This effect could be further amplified by axial segregation within the fluidized bed. This size-dependent segregation causes larger particles to accumulate near the bottom of the bed, while finer particles tend to migrate upward, creating local variations in particle concentration along the bed height [84]. Under such conditions, coarse particles in the lower regions may carry charges of opposite polarity to those located higher up. As a result, samples extracted from the bottom of the bed may not accurately reflect the average charge state of the entire column due to this segregation-driven spatial charge distribution. In later experiments presented in Chapter 3, where a narrower particle size distribution was used and all particles were extracted and measured after fluidization, no polarity reversal was observed. 36 This suggests that the interplay between bipolar charging and axial segregation likely accounts for the unexpected polarity measurements seen in the earlier tests. Figure 2.4 Saturation charge per unit mass, Q/m, acquired by glass beads fluidized in the (A) acrylic column and (B) steel column with various dry fluidizing gases (RH < 0.1%). 2.3.2 Saturation charge level in different columns 2.3.2.1 HDPE particles In dry environments, HDPE particles exhibited higher charge levels in the steel column compared to the acrylic column, as shown in Figure 2.3. This behavior can be partially explained by the triboelectric series [32], in which polyethylene is positioned farther from steel than from acrylic (PMMA). This greater separation suggests a stronger propensity for charge transfer when HDPE particles contact the steel wall. Expressed in terms of the effective work function (EWF), this implies that the EWF difference between HDPE and steel is larger than that between HDPE and acrylic, leading to more robust charge generation during particle–wall collisions. In systems where all particles are composed of the same material, as in this study, the EWF difference between particle and wall materials becomes the dominant factor governing both the direction and magnitude of charge transfer during particle–wall interactions. A larger EWF difference not only determines the polarity of the transferred charge but also enhances the overall rate of charge generation. 37 However, it is important to recognize that the net charge on particles is not solely determined by charge generation; charge dissipation and gas breakdown also play critical roles. As described by the charge balance relationship introduced in Chapter 1 (see Eq. (1.4)), the overall charge evolution in a fluidized bed is determined by charge generation and dissipation. Among the processes contributing to charge dissipation, two main pathways are typically involved: (1) dissipation through contact between particles and the column wall, and (2) gaseous discharge that occurs when the local electric field exceeds the dielectric breakdown threshold of the fluidizing gas. The wall-contact-based dissipation rate is influenced by the electrical resistivity of both the particles and the wall [85]. Given the extremely high resistivity of HDPE (~1015 Ω·m) [86], significant charge dissipation through wall contact is unlikely under dry conditions, regardless of whether the wall is made of steel or acrylic. In addition, the conductivity of the wall material plays a key role in defining the electrostatic boundary conditions at the particle–wall interface. For insulating materials like acrylic, transferred charges remain largely localized due to limited mobility over the surface, resulting in stronger and highly non-uniform electric fields near the wall. These localized fields can inhibit further charge transfer during particle–wall interactions, especially when the particle and wall carry opposite charges. In contrast, conductive materials such as steel allow rapid redistribution of surface charges via free electron movement. Moreover, because the steel wall is grounded, it maintains a near neutral surface and therefore exhibits lower boundary field intensity than acrylic. These contrasting boundary behaviors affect the local electric environment surrounding fluidizing particles and likely contribute to the observed differences in charge accumulation between the two columns. 38 Consequently, in dry environments where charge dissipation through wall contact is minimal, dielectric breakdown of the fluidizing gas likely serves as the dominant mechanism limiting the saturation charge level of HDPE particles. Under this condition, the maximum charge density attainable by the particles should correlate with the dielectric breakdown strength of the gas. This relationship is consistent with the trend observed in our experimental results: gases with lower breakdown strength, such as argon, consistently yielded lower particle charge levels compared to air and nitrogen. A more detailed analysis of this correlation and its implications is presented in Section 2.3.3. 2.3.2.2 Glass beads In contrast, glass particles exhibited the opposite column dependence: the measured |Q/m| values were significantly lower in the steel column than in the acrylic column (see Figure 2.4). This behavior is likely governed by a combination of reduced charge generation and, to a lesser extent, enhanced charge dissipation. Specifically, the effective work function (EWF) difference between glass and steel is likely smaller than that between glass and acrylic. Since triboelectric charge transfer during particle–wall collisions is fundamentally governed by this EWF difference, a smaller difference would result in less efficient charge generation. This provides the most compelling explanation for the reduced charge accumulation observed in the steel column. Although the conductive nature of the steel wall could in principle promote greater charge dissipation, its influence is likely limited in this case. The effective resistivity at the particle–wall interface is primarily governed by the glass particle itself, which has a high resistivity in the range of 109-1012 Ω ·m [87]. For glass–acrylic contacts, the limiting resistivity instead arises from the acrylic wall (~1013 Ω·m [86]). Therefore, the difference in interfacial resistivity between the two column materials is modest. While localized charge leakage through the grounded steel wall may contribute marginally to dissipation, 39 this mechanism alone is unlikely to account for the substantial drop in |Q/m| values. Hence, the primary cause remains the diminished charge generation efficiency due to the smaller EWF difference. Furthermore, unlike the strong gas dependence seen in the HDPE cases, gas breakdown is unlikely to have played a significant role in limiting the charge levels of glass particles, as no clear correlation was observed between |Q/m| and the dielectric breakdown strength of the fluidizing gas. This further supports the conclusion that, for glass particles, charge generation, primarily driven by the EWF difference, is the controlling mechanism, with only minor influence from charge dissipation process. 2.3.3 Effect of fluidizing gas composition 2.3.3.1 HDPE particles In both columns, the magnitude of the charge density of the HDPE particles fluidized by argon was considerably smaller than that observed for the other fluidizing gases (Figure 2.3). When the field potential on a highly charged particle exceeds the threshold value for gas breakdown, gaseous discharge occurs, and the surface charge is neutralized by ions or electrons generated during the discharge. The dielectric strength of argon, air, nitrogen at 1 atm are 0.6 kV/mm, 3.0 kV/mm, and 3.4 kV/mm, respectively [88]. Therefore, dielectric breakdown, which does occur at much lower electric field strength in argon than in the other gases, emerges as the likely cause. To validate our hypothesis, we applied a simple model to estimate the charge saturation levels in various gases. The model assumes that charge saturation occurs when the electric field at the column wall reaches the dielectric breakdown limit of the gas, and that particles in a homogeneous fluidized bed are uniformly charged. Based on these assumptions, Gauss’s law gives the governing equation: 40 Qε0εm= ∯ n̂S ∙ Eds (2.1) Here, Q represents the total charge within the fluidized bed, S denotes a closed surface enclosing the homogeneous fluidized bed. n̂ is the unit outward normal to this surface, E represents the electric field, and ds is a differential surface area. ε0 is permittivity of free space, while εm denotes the mixture permittivity of the bed, which can be estimated using the Bruggeman equation [89]: αg = (εs − εm εs − εg) (εgεm)13 (2.2) Here, εs, εg, αg denote the relative permittivity of the solid, relative permittivity of gas, and void fraction, respectively. For the materials used in this study, εs is 2.3 for HDPE and 10 for glass, while εg is approximately 1 for all the gases. Consequently, we assume εg = 1 in the subsequent analysis. Using Eq. (2.1), the magnitude of the average electric field on the surface can be estimated as |Q|Sε0εm, where S is the total surface area. Although this is a simplified approximation, it enables an estimation of the maximum |Q/m| in the bed, as expressed in the equation below: |Qm|max=Sε0εmEBreakdownm (2.3) where EBreakdown is the dielectric strength of the gas. The maximum |Q/m| values estimated using Eq. (2.3), along with the experimentally measured |Q/m| values for HDPE particles, are summarized in Table 2.3. These estimates were calculated under conditions consistent with the experimental setup, including the same particle mass and column diameter. 41 As the fluidized bed expands, the average solid volume fraction decreases while the total surface area (S) increases, potentially allowing particles to acquire higher levels of charge before reaching the dielectric breakdown threshold. Under the tested gas velocities, the bed operated in a vigorous bubbling regime. In this regime, the frequent eruption of rising bubbles caused the bed surface to fluctuate significantly, making it difficult to determine a precise bed height. For the purpose of these calculations, an expanded bed height of 0.5 m was used, corresponding to the average of the fluctuating bed height observed during the experiments. The estimated maximum |Q/m| represents the theoretical upper limit of particle charge density attainable when gas breakdown occurs. In other words, it corresponds to the condition where the electric field generated by the accumulated charge reaches the dielectric breakdown strength of the fluidizing gas. The purpose of these estimations is to compare the theoretical charge limits imposed by gas breakdown with the experimentally measured saturation charges of the particles. Such comparison allows assessment of whether the observed saturation levels are primarily governed by the gas breakdown constraint or by other charge dissipation mechanisms. Table 2.3 Comparison of estimated and measured |Q/m| of HDPE particles. Fluidizing gas Estimated max |Q/m| (μC/kg) Average of measured |Q/m| in acrylic column (μC/kg) Average of measured |Q/m| in steel column (μC/kg) Argon 0.81 0.78 1.40 Air 3.49 2.72 4.22 Nitrogen 3.96 3.61 4.42 Before discussing the results, it is important to acknowledge the limitations of the simple model used for charge estimation. Under vigorous bubbling conditions, dynamic fluctuations in bed height can introduce uncertainty into the calculated volume and, consequently, into the estimated total charge. Additionally, the model assumes the fluidized bed behaves as a spatially homogeneous bulk with uniform charge distribution. However, 42 previous studies [90] suggest that charge distribution in fluidized beds is typically non-uniform. For instance, smaller particles tend to acquire higher charge densities (per unit mass) and reach the dielectric breakdown threshold more readily than larger particles [91]. Moreover, size-dependent segregation, where larger or denser particles accumulate near the bottom of the bed and smaller or lighter particles preferentially migrate to the top, can lead to local variations in particle concentration and electric field intensity [84]. Such particle segregation may induce localized gas breakdown events at charge densities that differ from those predicted by bulk-average models. With these limitations in mind, we note that the estimated maximum |Q/m| values for HDPE particles are lowest in argon, consistent with its reduced dielectric strength that restricts charge accumulation before breakdown. The measured values follow the same trend and, despite some discrepancies, show good agreement with the estimates, confirming dielectric breakdown as the primary mechanism limiting particle charging under the tested conditions. An additional noteworthy observation is the contrasting relationship between estimated and measured |Q/m| across the two column materials. In the steel column, the measured |Q/m| values for all gases, especially argon, tend to exceed the estimated values. Conversely, in the acrylic column, the measured values are slightly lower or comparable to the estimates. As discussed in Section 3.2.2.1, the effective work function (EWF) difference between HDPE and steel is larger than that between HDPE and acrylic, which leads to a higher rate of charge generation during particle–wall collisions in the steel column. However, it is important to recognize that a larger EWF difference primarily accelerates the approach to saturation, rather than increasing the saturation charge level itself. Because the theoretical |Q/m| limit is determined solely by gas breakdown strength, it should remain identical across both columns. The consistently higher |Q/m| measured in the steel column therefore indicates the presence of additional mechanisms that delay or locally suppress breakdown. 43 One such factor is the difference in electrostatic boundary conditions imposed by the two wall materials, as discussed earlier in Section 3.2.2.1. Acrylic, being an insulator, restricts surface charge mobility, allowing charges to remain localized near the point of contact. This leads to the development of strong, short-range electric fields adjacent to the wall. Such localized fields not only suppress further charge transfer during particle–wall interactions but also lower the threshold particle charge for gas breakdown, particularly when particle and wall charges are of opposite polarity. In contrast, the conductive nature of the steel column results in more uniform charge distribution along the wall surface. Being grounded, the steel column maintains a relatively neutral potential, which helps reduce the boundary field intensity. This condition delays the onset of dielectric breakdown, allowing particles to acquire higher charges before discharge occurs. This hypothesis helps explain why higher |Q/m| values were consistently measured in the steel column, even when the gas environment was the same. Another possible contributing factor is the strength of charge generation. Although it does not directly determine the final saturation level, which is primarily constrained by gas breakdown, it may still exert a secondary influence. In real fluidized beds, charge distribution is typically non-uniform, and gas breakdown is likely to occur locally rather than uniformly. Moreover, neutralization through gaseous discharge is not instantaneous. The recombination of surface charges with ionized species requires finite time. Under conditions of strong charge generation, such as in the steel column with its larger EWF difference, particles may transiently accumulate excess charge before gaseous discharge processes fully equilibrate, slightly elevating the apparent saturation level. Furthermore, while gas breakdown likely plays a significant role in determining the saturation charge level, other factors such as the influence of gas properties on charge generation and dissipation through wall contacts, cannot be simply dismissed. Although gases are often considered passive carriers in fluidized systems, their physical properties 44 (e.g., polarizability, adsorption affinity, ion mobility) may influence both the efficiency of triboelectric charge transfer and the rate of charge dissipation at the wall interface. These effects remain poorly understood and are not explicitly captured in the current experimental design. Since these experiments primarily focused on the final steady state of particle charging, whether the charging process is also influenced by those gases remains an open question. A more detailed investigation of these factors is presented in Chapter 3. Despite these complexities, the relatively close agreement between the measured and estimated |Q/m| values is encouraging. It supports the conclusion that the saturation charge levels of fluidized HDPE particles are primarily governed by the dielectric breakdown strength of the fluidizing gas, while still being affected, to a lesser extent, with secondary influences from wall material properties and other system-specific factors. 2.3.3.2 Glass beads Figure 2.4 reveals that the fluidizing gas had a much weaker influence on the charge density of glass beads compared to HDPE particles. While HDPE particles exhibited substantial variation in charge accumulation depending on the gas used, the charge levels acquired by glass beads remained relatively consistent across argon, air, and nitrogen. Notably, the charge densities of HDPE particles and glass beads were comparable only in the case of argon; in all other cases, the glass beads exhibited significantly lower charge levels than the HDPE particles. Table 2.4 presents a comparison between the estimated maximum |Q/m| values, obtained from Eq. (2.3), and the experimentally measured saturation charge density for glass beads fluidized in acrylic and steel columns. Except for the case of argon in the acrylic column, the measured |Q/m| values fall significantly below the theoretical estimates. This consistently lower |Q/m| suggests that glass beads, unlike HDPE particles, were unable to accumulate charge levels sufficient to approach the dielectric breakdown threshold of the 45 tested gases, and thus gas breakdown was unlikely to serve as the dominant limiting mechanism in these experiments. The lack of correlation between the measured Q/m values and the dielectric breakdown strengths of the fluidizing gases further supports this conclusion. Except for the argon–acrylic case, the saturation charge levels of glass beads appear to be governed primarily by material-dependent charge generation and dissipation behaviors, rather than by gas breakdown constraints. Therefore, as discussed in section 2.3.2.2, glass beads exhibited the opposite column dependence to that of HDPE particles: the measured |Q/m| values were significantly lower in the steel column than in the acrylic column. This behavior is attributed to a combination of weaker charge generation, due to a smaller EWF difference between glass and steel. These findings emphasize the fundamentally different electrostatic charging behavior of glass beads and highlight the importance of tailoring electrostatic mitigation or control strategies to the material-specific properties of particles in fluidized bed systems. Table 2.4 Comparison of estimated and measured |Q/m| of glass beads. Fluidizing gas Estimated max |Q/m| (μC/kg) Average of measured |Q/m| in acrylic column (μC/kg) Average of measured |Q/m| in steel column (μC/kg) Argon 0.50 0.73 0.08 Air 2.15 0.69 0.16 Nitrogen 2.44 0.90 0.13 2.3.4 Effect of humidity 2.3.4.1 HDPE particles This section examines the saturation charge levels of HDPE particles in both columns under dry and humidified air conditions. The relative humidity (RH) of the humidified air entering the columns was controlled at three levels: dry (<0.1%), 10% ± 2%, and 24% ± 2%. As previously discussed, humidity can simultaneously influence charge generation and 46 dissipation, with the final charge level determined by the interplay between these two competing mechanisms. Our experimental findings, illustrated in Figure 2.5, highlight this complex behavior. Within the tested humidity range, the saturation charge of HDPE particles increased monotonically with RH in the acrylic column. By contrast, the steel column displayed a non-monotonic dependence, reaching a maximum charge density at RH = 10% and subsequently decreasing at RH = 24%. Figure 2.5 Saturation charge per unit mass, Q/m, acquired by HDPE particles fluidized in the (A) acrylic column and (B) steel column at different relative humidity (RH) levels. Previous studies have suggested that contact electrification in dielectric materials is associated with the transfer of H⁺ and OH⁻ ions through adsorbed water patches at the interface between contacting surfaces [31,32,36,92]. At low humidity levels, the increased availability of these ions enhances charge generation. However, at higher RH, these discrete water patches may coalesce into a continuous water film, especially on hydrophilic surfaces, providing a conductive pathway for charge dissipation. Consequently, the observed Q/m is governed by the competing effects of ion-assisted charge generation and water-film-mediated charge dissipation. The intermediate maximum observed in the steel column supports this interpretation, indicating that both mechanisms are active and competing over the tested humidity range. 47 In contrast, the slight and monotonic increase in Q/m with rising RH observed in the acrylic column suggests that, under the tested conditions, humidity primarily enhanced charge generation rather than dissipation, albeit to a limited extent. This may be attributed to the greater hydrophobicity of acrylic (PMMA) compared to steel, as evidenced by its higher water contact angle and lower propensity to form a continuous water film. It is possible that an intermediate maximum also exists for the acrylic column, but it may occur at higher RH levels beyond the range tested in this study, where the balance between generation and dissipation would eventually shift. Another possible factor influencing the saturation charge is the impact of RH on the dielectric breakdown strength of the fluidizing gas. The measured Q/m values under different humidity levels are close to the estimated maximum |Q/m| values calculated using the model in Section 2.3.3.1, except for the steel column at RH = 24%, suggesting that gas breakdown may still act as a limiting mechanism in most cases. Previous studies have shown that the relationship between humidity and breakdown strength is complex and influenced by several variables, including gas type, humidity level, electrode configuration, gap distance, and environmental conditions. In some configurations, higher humidity has been reported to increase dielectric strength by reducing the number of free electrons in the gas phase, thereby raising the breakdown threshold [60,61]. The observed increase in Q/m from dry to RH = 24% in the acrylic column and from dry to RH = 10% in the steel column may be attributed, at least in part, to this effect. However, the decline in Q/m observed for HDPE particles fluidized in the steel column at RH = 24% suggests that under this condition, charge dissipation became the dominant factor. It is likely that the effective surface conductivity of the system increased with higher humidity, facilitating rapid charge leakage before the electric field could reach the breakdown threshold. As a result, the particle charge remained below the maximum charge level predicted by the gas breakdown model, indicating that charge 48 dissipation, rather than breakdown strength, ultimately governed the final charge state in this case. To clearly differentiate the roles of humidity in promoting charge generation and dissipation versus altering the dielectric breakdown threshold, one must move beyond steady-state measurements and examine the charging dynamics directly. In Chapter 3, time-resolved experiments capturing the evolution of particle charge under controlled humidity conditions are presented to offer deeper insight into these underlying mechanisms. 2.3.4.2 Glass beads Here we extend the investigation to glass beads, which exhibit different electrostatic behavior under varied humidity. Figure 2.6A presents the saturation charge density of glass beads fluidized in the acrylic column under different humidity conditions. As the relative humidity increased from dry air to 10%, the measured Q/m increased noticeably from 0.69 to 0.90 μC/kg, suggesting that moderate humidity enhances charge generation, likely due to increased availability of mobile H⁺ and OH⁻ ions on the particle and wall surfaces. When the RH was further increased to 24%, the charge density slightly decreased to 0.87 μC/kg. This minor reduction may indicate the onset of enhanced charge dissipation via surface water films, although the overall effect remains relatively weak. Notably, the estimated maximum |Q/m| for glass beads in dry air is approximately 2.15 μC/kg (see Table 2.4), substantially higher than any of the measured values, suggesting that dielectric breakdown was not the limiting mechanism in these tests. Instead, the saturation charge levels were more likely governed by the dynamic balance between charge generation and dissipation. 49 Figure 2.6 Saturation charge per unit mass, Q/m, acquired by glass beads fluidized in the (A) acrylic column and (B) the steel column at different relative humidity (RH) levels. For RH=10% and RH=24% in steel column, solid bars denote data obtained in April 2019, the checkerboard-patterned bar represents a repeat measurement at RH = 10% in March 2022, and the diagonally striped bar represents a repeat measurement at RH = 24% in March 2022. The results obtained for glass beads fluidized in the steel column under different relative humidity conditions are presented in Figure 2.6B. An intriguing observation is the reversal in the polarity of the acquired charge between two sets of experiments conducted three years apart. Under dry air conditions, the glass beads consistently acquired negative charges. However, in humid air, qualitatively different results were observed. In the 2019 experiments, the glass beads acquired small negative charges at both RH = 10% and 24%. In contrast, in the more recent 2022 experiments, the same humidity levels yielded small positive charges, consistent with predictions from the triboelectric series [32]. Interestingly, even under dry conditions, the observed polarity was inconsistent with expectations based on the triboelectric series, further complicating interpretation of glass charging behavior. In the 2022 dataset, a clear polarity reversal was observed. As relative humidity increased, the net bed charge shifted from negative to positive. This was accompanied by an increase in the absolute charge magnitude, suggesting a trend toward more positive net charging with increasing RH. In contrast, the 2019 data showed a decrease in the magnitude of negative charge with increasing humidity, indicating that a polarity reversal might have occurred at higher RH levels beyond those tested. Taken 50 together, the two datasets suggest a common trend—an overall shift in bed polarity from negative to positive with increasing relative humidity. A similar polarity reversal with increasing humidity was previously reported by Sippola et al. [93] for polyethylene particles fluidized in a glass column, indicating that such behavior is not unique to the present study. However, their work did not offer a definitive explanation for the underlying mechanism. We hypothesize that this phenomenon is associated with the wide particle size distribution and the resulting bipolar charging effects, which will be further examined in Chapters 3 and 4. 2.3.5 Wall fouling 2.3.5.1 HDPE particles This section examines the relationship between particle charge density and wall fouling in the acrylic column. The percentage of wall coverage by HDPE particles at different sampling heights after fluidization with various gases is shown in Figure 2.7, along with the corresponding saturation charge density of the particles for direct comparison. A clear correlation is observed between the extent of wall fouling and the particle charge density. Among the tested gases, argon generated the lowest charge levels on HDPE particles, and was therefore expected to result in weaker electrostatic attraction between the particles and the wall. Accordingly, the lowest wall coverage was observed for argon. In contrast, air and nitrogen produced similar charge densities and led to comparable degrees of wall fouling. 51 Figure 2.7 Percentage wall coverage of HDPE particles fluidized in an acrylic column by argon, air, and nitrogen. The corresponding saturation charge densities are provided in the inset for direct comparison. The adhesion of charged particles to the column wall is primarily determined by the interplay between the forces acting in the vertical and horizontal directions. In the vertical direction, gravitational, buoyancy, and drag forces tend to move particles upward or downward, whereas in the horizontal direction, attractive interactions, such as van der Waals, capillary, and electrostatic forces, draw the particles toward the wall. As a particle approaches the wall, these attractive forces pull it into contact with the surface, initiating adhesion. Upon contact, the normal component of the attraction acts as an effective pressing load between the particle and the wall, which in turn gives rise to interfacial friction. This frictional resistance counteracts any vertical motion of the particle, inhibiting its tendency to slide upward or downward under the influence of gravity or drag. Consequently, particles experiencing stronger attraction are more stably retained on the wall surface and less likely to be detached. 52 A force balance for a single spherical particle near a fluidization column wall is shown in Figure 2.8. In the vertical direction, three forces are considered, gravity, buoyancy, and drag force. It should be noted that the frictional force between the particle and the wall is not considered in this analysis, because the relationship between horizontal loading and particle–wall friction is not well established. The present force balance focuses solely on the vertical components of gravity, buoyancy, and drag to assess how their relative magnitudes vary with different fluidizing gases. Fnet,vertical = mg − ρgVPg −12CdUp2ρgAP (2.4) where m is the mass of the particle, g is the gravitational acceleration, ρg is the density of the fluidization gas, Vp is the volume of the particle, Cd is the drag coefficient, Up is the particle velocity and Ap is the cross-sectional area of the particle. The drag coefficient is estimated using the correlation by Clift, Grace, and Weber [94] for 0.01 < Re ≤ 20: Cd =24𝑅𝑒 [1 + 0.1315Re(0.82−0.05w)], w = Log10Re (2.5) where Re is Reynolds number of that particle. Table 2.5 summarizes the calculated Reynolds number, gravitational force, buoyancy force, and drag force acting on a single HDPE particle with a diameter of 260 μm, which matches the mean particle size used in our experiments, under fluidization with different gases. 53 Figure 2.8 Force balance on a single spherical particle near a fluidization column wall. Table 2.5 Reynolds number, gravity, buoyancy, and drag force acting on a single HDPE particle (260 μm in diameter) in different gases. Air Nitrogen Argon Argon (lower velocity) Gas velocity (m/s) 0.321 0.321 0.317 0.267 Reynolds number 5.56 5.53 6.14 5.17 Gravity (10-8 N) 8.48 8.48 8.48 8.48 Buoyancy (10-10 N) 1.09 1.05 1.50 1.50 Drag force (10-8 N) 2.14 2.08 2.67 2.16 At the same excess velocity of Umf + 0.3 m/s (0.321 m/s, 0.321 m/s, and 0.317 m/s for air, nitrogen and argon respectively), the drag force in argon was about 25%–30% greater than that in air or nitrogen, primarily due to argon’s higher density and viscosity. Buoyancy forces were also greater in argon, but their magnitude was two orders of magnitude lower than gravity and drag, and thus negligible in the overall force balance. This elevated drag force introduces uncertainty in attributing the observed reduction in wall fouling under argon. Specifically, it becomes difficult to discern whether the decreased fouling results primarily from weaker electrostatic attraction due to lower particle charge, or from increased drag-induced particle removal. 54 To resolve this ambiguity, additional experiments were conducted using argon at a lower velocity (0.267 m/s), which produced a drag force comparable to that in air and nitrogen at higher velocities (see Table 2.5). If drag force were the dominant factor governing wall adhesion, one would expect an increase in wall fouling at the reduced velocity. However, as shown in Figure 2.9, wall coverage further decreased under these conditions. This outcome strongly suggests that the observed reduction in wall fouling in argon is primarily driven by weaker electrostatic forces rather than by hydrodynamic removal. Figure 2.9 Percentage wall coverage of HDPE particles fluidized in an acrylic column with argon at 0.317 m/s and 0.267 m/s. Beyond hydrodynamic considerations, it is important to evaluate the relative significance of different attractive forces contributing to wall fouling. Among van der Waals, capillary, and electrostatic interactions, the latter appears to be the dominant mechanism under our test conditions. Jiang et al. [95] demonstrated that electrostatic forces in fluidized beds exceed van der Waals forces for particles larger than 150 µm. Given that the HDPE particles used in our study had a mean diameter of 260 µm, van der Waals interactions were likely minimal. Likewise, capillary forces were negligible, as tests conducted with dry and humidified air yielded similar wall coverage, and relative humidity remained below the ~65% threshold typically required for capillary bridge formation [96–99]. Taken together, 55 these observations support the conclusion that electrostatic attraction is the principal adhesive force responsible for particle coating onto the wall. The reduced wall fouling observed under argon is therefore best explained by the lower charge density of HDPE particles, rather than by competing attractive mechanisms. Furthermore, Figure 2.7 shows that wall fouling decreased with increasing column height. Fouling in the lower freeboard region (points 2-3) and transition zone (points 1) is primarily caused by particles ejected from the dense bed by erupting bubbles. In contrast, fouling in the upper freeboard region (points 4–5) is mainly attributed to entrained fine particles. These entrained fines are typically smaller and more highly charged per unit mass, making them more likely to adhere to the wall [100,101]. Additional size analysis of wall-coated particles is provided in Section 2.3.6. As elevation increases, both the concentration of particles and their residence time near the wall decrease, leading to a reduction in wall fouling. Below the transport disengagement height, particles that are not fully entrained tend to fall back, either returning to the dense phase or adhering to the wall due to electrostatic forces. Therefore, the combined effects of decreasing particle size, reduced contact probability, and lower particle flux with height explain the observed vertical gradient in wall fouling. 2.3.5.2 Glass beads Figure 2.10 shows that, for glass beads, no clear correlation exists between the measured in-bed charge density and the extent of wall coverage. Although the in-bed charge densities were similar across gases, wall coverage in dry air and nitrogen was substantially higher than in argon. This discrepancy suggests that the charge densities shown in the figure primarily represent particles in the dense phase, while wall fouling is dominated by entrained fine particles, which generally carry much higher specific charge than bulk particles. As discussed in the next section, particles adhered to the column wall were 56 consistently smaller than the as-received glass beads, indicating that wall deposition was dominated by fine particles. These fines, owing to their high surface-area-to-mass ratio, are more likely to acquire and retain significantly higher charge densities. This observation aligns with the findings of Lu et al. [102], who reported that the charge density of wall-adhered glass particles can exceed that of in-bed particles by up to two orders of magnitude. In this context, coarse glass beads in the dense bed are unlikely to reach the charge threshold required for gas breakdown. However, entrained fines, especially those in argon, could reach this threshold more easily, triggering gas breakdown and thus reducing their tendency to adhere to the wall. This mechanism likely explains the lower wall fouling observed in argon for glass beads. Another important observation from Figure 2.10 is the significant reduction in wall fouling when glass particles were fluidized with humidified air (RH = 24%) compared to dry air. This trend contrasts with the negligible humidity effect on wall fouling observed for HDPE particles. The difference can be attributed to material-dependent moisture adsorption behavior. As a hydrophilic material, glass more readily adsorbs water, allowing the formation of continuous surface water films that lower surface resistivity and enhance charge dissipation. In contrast, the hydrophobic nature of polyethylene inhibits the formation of such conductive films, resulting in minimal humidity-induced charge dissipation. While the Q/m values of in-bed glass particles showed little change with humidity, it is plausible that humidity had a greater impact on the entrained fines (the particles primarily responsible for wall fouling) whose charge densities are significantly higher. These findings suggest that humidity control may be an effective strategy for mitigating electrostatic wall fouling in fluidized systems using hydrophilic materials such as glass, due to their ability to form conductive surface water films that promote charge dissipation. In contrast, for hydrophobic materials like HDPE, the effect of humidity was negligible within the tested RH range (see Figure 2.7), possibly because significantly 57 higher humidity levels are required to initiate meaningful surface conduction. This highlights the material-dependent nature of humidity-based charge control strategies and underscores the need for tailored electrostatic mitigation approaches based on particle surface properties. Figure 2.10 Percentage wall coverage of glass beads fluidized in an acrylic column by argon, air, and nitrogen. The corresponding saturation charge densities are provided in the inset for direct comparison. 2.3.6 Size analysis and agglomeration of wall coated particles Figure 2.11A presents the median diameters of HDPE particles adhered to the acrylic column wall after fluidization with argon, air, and nitrogen. For air and nitrogen, particle size decreased with increasing elevation, reflecting a progressive increase in the concentration of fines in the upper regions. In contrast, the wall-coated particles from the argon tests exhibited consistently small diameters from sampling points 2 to 5, indicating that wall fouling under argon was dominated by entrained fines. This is likely because the 58 weaker electrostatic forces in argon were insufficient for larger particles to adhere to the wall. Unlike HDPE, the glass particles adhering to the wall (Figure 2.11B) were consistently much smaller than the as-received particles across all tested gases and sampling points, indicating that wall fouling was dominated by fine glass beads. This behavior can be explained by the relatively low charge accumulation observed for glass particles in the dense bed across all tested gas types and humidity levels. As discussed earlier, the small effective work function difference between glass and acrylic, combined with glass’s inherently lower surface resistivity, limited the ability of larger particles to reach high charge levels. Consequently, only fine particles, more easily entrained and capable of acquiring higher surface charge densities due to their large surface area-to-mass ratios, contributed significantly to wall adhesion. This interpretation is consistent with both the observed particle size distributions and the elevated |Q/m| values previously reported for wall-adhered fines in similar systems [102]. 59 Figure 2.11 Median diameters d(0.5) of (A) HDPE particles and (B) glass beads coated to an acrylic column after fluidized with argon, air and nitrogen. Figure 2.12 shows the agglomerate fraction index of HDPE particles adhering to the wall under different gas environments. A higher index indicates a greater extent of agglomeration. Across all gases, this index generally decreased with increasing elevation, except at point 2. This trend aligns with previous observations by Cocco et al. [103], which suggest that large agglomerates predominantly form in the dense bed and grow as they are transported upward by erupting bubbles. At the top surface of the bed, these clusters are either ejected into the freeboard or fall back into the lower dense phase. After multiple 60 circulation cycles, large clusters tend to accumulate in the transition region (point 1), as their size and mass typically prevent them from being carried to higher elevations by the drag force. Furthermore, once ejected into the freeboard, these clusters can easily break apart into smaller clusters or individual particles due to collisions and turbulence. The higher agglomeration level observed at point 1 is likely due to accumulation of such large clusters. Point 3, near the transport disengaging height, represents the highest elevation that large agglomerates can reach. Clusters adhering to the wall at this location are less likely to be disturbed by subsequent bubbling events and thus remain attached. In contrast, clusters at lower elevations, such as point 2, are more frequently dislodged due to collisions with newly ejected particles, leading to a lower agglomeration level observed at that point. At points 2 to 5, the agglomerate fraction index was consistently lower for particles fluidized with argon. This is consistent with their lower charge level, attributable to argon’s lower dielectric breakdown strength. According to Cocco et al. [103], electrostatic, capillary, and van der Waals forces all contribute to particle cohesion. In our case, electrostatics forces appear to dominate, as evidenced by the strong correlation between in-bed charge levels and agglomeration extent. Capillary forces are expected to be minimal under the dry and moderately humid conditions tested, a conclusion further supported by the negligible difference in agglomeration between dry and humidified air runs. 61 Figure 2.12 Agglomerate fraction index of HDPE particles coated to an acrylic column after fluidized with argon, air, and nitrogen. 2.4 Summary This chapter systematically investigated how different fluidizing gases influence particle charge accumulation, wall fouling, and particle agglomeration in gas–solid fluidized beds. Our primary objective was to explore whether electrostatic charging in such systems can be controlled by modifying the gas-phase environment. Through comparative analysis of HDPE particles and glass beads fluidized in acrylic and steel columns under various gas types and humidity levels, we have identified key trends and underlying mechanisms that govern saturation charge levels and the extent of wall fouling. For HDPE particles, the results demonstrated that the saturation charge level is largely governed by the dielectric breakdown strength of the fluidizing gas, as evidenced by the close agreement between the estimated and measured |Q/m| values. The data also revealed notable effects of wall material properties, attributable both to differences in electrostatic boundary conditions between insulating (acrylic) and conductive (steel) columns and to variations in effective work function (EWF) difference, which are particularly pronounced for the steel walls. These secondary effects contributed to deviations in charge saturation levels, with the steel column generally allowing higher |Q/m| values due to its ability to 62 maintain a more uniform electric field and robust charge generation. In contrast, glass beads, due to their lower surface resistivity and smaller work function difference relative to the walls, did not accumulate sufficient charge to trigger gas breakdown under most conditions. Instead, their saturation charge was governed by the dynamic balance between charge generation and dissipation, rather than by gas-phase breakdown limitations. Humidity introduced additional complexity into the electrostatic behavior of fluidized particles. For HDPE particles, increasing humidity slightly raised the saturation charge in acrylic column but produced an intermediate maximum in steel column, reflecting a competition between enhanced ionic charge transfer (via H⁺ and OH⁻ ions in adsorbed water layers) and increased charge dissipation through conductive water films. In contrast, glass beads in the dense bed (acrylic column) exhibited weaker sensitivity to humidity in terms of their measured charge density. However, wall fouling was substantially reduced under humidified air, despite minimal changes in in-bed charge levels. This suggests that for glass, humidity primarily enhanced surface conductivity and promoted charge dissipation, especially for entrained fine particles responsible for wall fouling. Moreover, repeated experiments in steel columns revealed a puzzling polarity reversal of charged glass particles between tests conducted in 2019 and 2022, with the net charge shifting from negative to positive. A similar polarity shift was previously observed by Sippola et al. [93] for polyethylene particles fluidized in a glass column, suggesting that this behavior is not unique to our system. However, the underlying mechanism was not clearly identified in that study. We hypothesize that this phenomenon is associated with the wide particle size distribution and the resulting bipolar charging effects, which will be examined in detail in Chapters 3 and 4. The observed differences in wall fouling behavior further highlight the practical implications of electrostatic charging in gas–solid fluidized systems. For HDPE particles, a strong correlation was observed between saturation charge and the extent of wall fouling, 63 confirming that electrostatic forces are the dominant adhesion mechanism under the tested conditions. Among all gases tested, argon, due to its lower dielectric breakdown strength, produced the lowest saturation charge levels and correspondingly the least wall fouling. This trend was supported not only by wall coverage measurements but also by particle size analysis, which showed that wall fouling in argon was primarily caused by entrained fine particles, while larger particles did not acquire sufficient charge to adhere to the column wall. For glass beads, wall fouling across all gases was dominated by entrained fine particles, which possessed significantly higher specific charge than those in the dense bed. Interestingly, humidity had little effect on wall fouling of HDPE particles but significantly reduced wall fouling for glass beads, once again highlighting the material-dependent nature of electrostatic responses to moisture in the fluidizing gas. Despite these insights, the study has several limitations. While the dielectric breakdown-based model provides a useful framework for estimating saturation charge levels, it does not fully account for the influence of gas properties beyond breakdown strength, such as ion mobility, polarizability, and gas–surface interactions, which may impact both the dynamics of charge generation and dissipation. Furthermore, because the experiments were primarily based on steady-state measurements, they do not capture the transient nature of charge accumulation. This limitation is particularly critical when interpreting the effects of humidity: determining whether humidity alters the balance between charge generation and dissipation, or modifies the breakdown strength of the gas, requires time-resolved transient analysis. To address these gaps, Chapter 3 investigates the dynamic evolution of particle charge density under various fluidizing gas compositions and humidity levels to gain a deeper understanding of the underlying mechanisms. Overall, this chapter provides a detailed understanding of how gas-phase properties can be strategically selected to control electrostatic charging and wall fouling in gas–solid fluidized beds. The insights gained not only enhance the predictive capability of 64 electrostatic charging models but also support the development of targeted mitigation strategies for industrial applications. For example, in polyolefin production, excessive electrostatic charge and wall fouling can lead to polymer sheeting, process interruptions, and inconsistent product quality. This study demonstrates that selecting a fluidizing gas with a lower dielectric breakdown strength, such as argon, can effectively reduce particle charging and wall fouling. Furthermore, particle agglomeration was also suppressed when using argon, which may benefit other processes, such as pharmaceutical manufacturing or granulation, where agglomeration control is critical. These findings offer practical and cost-effective strategies that can be readily implemented in industrial fluidized bed operations to improve process reliability, enhance safety, and maintain consistent product quality, particularly when handling dielectric materials. 65 Chapter 3 Effect of Fluidization Gas on Particle Charging Dynamics in Fluidized Beds 3.1 Introduction Chapter 2 identified several factors influencing steady-state charge accumulation in gas–solid fluidized beds, including particle material, wall conductivity, fluidizing gas, and ambient humidity. While these findings revealed clear trends and correlations, several open questions remained. For example, it was unclear whether the observed differences in saturation charge between steel and acrylic columns were primarily caused by differences in effective work function (EWF), or by electrostatic boundary conditions such as wall grounding. Additionally, the polarity reversal observed for glass beads raised the possibility of size-related bipolar charging effects, though the exact mechanism remained speculative. Furthermore, while different gases appeared to affect final charge levels, it was not evident whether this influence extended to the rate of charge generation itself. To address these open questions, this chapter investigates the time-resolved dynamics of particle charging in fluidized beds under systematically varied conditions. Specifically, we examine how gas type, humidity, wall grounding, and particle properties influence both the rate and extent of charge accumulation over time. By capturing the full temporal profile of charging behavior, these experiments aim to distinguish between competing mechanisms, such as humidity-enhanced charge generation versus humidity-enhanced dissipation, and to clarify the role of gas breakdown in limiting charge buildup. This dynamic perspective provides critical insights that complement the steady-state findings in Chapter 2 and lays the foundation for more robust electrostatic control strategies in industrial applications. 66 3.2 Experimental Setup The experiments in this chapter were conducted using a cylindrical copper fluidization column, housed within an acrylic outer shell (see Figure 3.1). The inner column had a diameter of 0.1 m and a height of 0.7 m. This small system was selected for easy handling, particularly because frequent loading and unloading of particles was required. The copper column was electrically insulated from the rest of the apparatus and could be grounded as needed for specific tests. Air and argon were used as the fluidizing gases. Oil-free compressed air supplied from the building, had a temperature of 20 ± 2 °C and a relative humidity (RH) of approximately 1%. Dry argon gas (RH < 0.1%) was supplied from a high-purity gas cylinder connected upstream of the fluidization unit. For experiments requiring humidified air, one air stream was passed through a water-bath humidifier and then mixed with a dry air stream before entering the column. The RH level at the column inlet was controlled by adjusting the mixing ratio and was continuously monitored using a RH meter (Vaisala Model HMT 238). The particles used in this study were high-density polyethylene (HDPE) resin and soda-lime glass beads (GB), identical to those used in Chapter 2. Their physical properties are summarized in Table 3.1. Unlike the wide size distributions used previously, the particles here were sieved into narrower size ranges using a mechanical sieving device (Gilson Company Inc., Ohio) to minimize bipolar charging effects. In this chapter, the experimental design required unloading all particles after each run to measure the average charge density over time. As discussed in Chapter 2, electrostatic forces are the dominant contributor to wall fouling in fluidized beds, and fouling increases with operation time as particle charge accumulates. Since wall-adhered particles typically carry high charges but are difficult to recover, excluding them from measurements, 67 especially in long-duration tests, can lead to substantial underestimation of charge density. To minimize this issue, we used particles of large mean diameters, which reduced both bipolar charging and wall fouling. Experimental results confirmed negligible wall fouling across all runs, and glass beads exhibited consistent charge polarity under all tested conditions. The minimum fluidization velocities (Umf) for each particle–gas combination were estimated using the Wen and Yu correlation [83], and the corresponding values are listed in Table 3.2. Due to the larger particle sizes employed in this chapter, the Umf values were higher than those used in Chapter 2. All experiments were performed at a fixed excess velocity of Umf + 0.2 m/s to ensure consistent fluidization conditions. Bubbling fluidization was confirmed through visual observation. After each run, all particles were poured directly into a Faraday cup for charge measurement without any additional handling. An electrometer (Keithley Model 6514) was used to record the total charge, and the charge density (Q/m) was calculated based on the sample mass. These Q/m values were then plotted as a function of fluidization time to analyze the temporal evolution of particle charging. To ensure consistency, the column was thoroughly cleaned between runs using a vacuum cleaner and a wet cloth, and then dried with compressed air. A fresh batch of particles was used for each run. Initial charge levels were measured before each experiment using the Faraday cup. The variation in initial Q/m values was within 8%, allowing the assumption of equivalent initial charge across all runs. Each experiment was repeated at least three times to ensure reproducibility. The reported Q/m values represent the average of these tests, with error bars corresponding to the standard deviation. 68 Figure 3.1 The copper fluidization column housed inside an acrylic outer shell. Table 3.1 Key physical properties of the tested particles. Bed particles Size range (μm) Mass median diameter (μm) Geldart group Particle density (kg/m3) HDPE 600-850 730 B 940 Glass 300-550 400 B 2500 Table 3.2 Estimated minimum fluidization velocity (Umf) for each test group. Bed materials and fluidization gases Estimated Umf (m/s) HDPE with compressed air 0.15 HDPE with argon 0.12 GB with compressed air 0.12 GB with argon 0.10 69 3.3 Results and Discussion 3.3.1 Charging dynamics of fluidized particles This section examines the time-resolved electrostatic charging behavior of HDPE particles and glass beads in the copper column. The experiments were conducted using dry air under both grounded and electrically isolated column conditions. As expected from the published triboelectric series [32], HDPE particles consistently acquired negative charges, whereas glass beads acquired positive charges. The polarity reversal previously observed for glass beads in the steel column (Chapter 2) did not occur in these experiments, likely because the narrower particle size distribution minimized bipolar charging effects. Additionally, all particles were extracted and measured after fluidization, eliminating spatial charge segregation associated with axial particle segregation in the bed. Figure 3.2 presents the measured specific charge (Q/m) of both particle types as a function of fluidization time. Charge densities are plotted in absolute value to enable direct comparison between the two particle types. The maximum charge levels estimated using the gas breakdown model in Chapter 2 are shown as dotted lines. For HDPE particles, a rapid increase in charge density was observed within the first 5 minutes of fluidization, followed by a gradual approach to saturation. The charge appeared to stabilize after approximately 18 minutes, reaching a maximum of about 6.5 μC/kg, very close to the breakdown-limited maximum predicted by the model. In contrast, glass beads exhibited a different charging pattern. Their charge densities remained consistently low across all fluidization durations and column configurations, reaching a plateau within approximately 1.5 minutes. These values stayed below 1.5 μC/kg, far below the theoretical breakdown threshold, indicating that the equilibrium charge density in this case was reached from the balance between charge generation and dissipation rather than from the gas breakdown. 70 Figure 3.2 Charge per unit mass in absolute value, |Q/m|, acquired by HDPE particles and glass beads fluidized in the isolated and grounded copper columns by dry air at different fluidization time intervals. The dotted lines (blue for HDPE and orange for glass) represent the estimated maximum charge of the particles when gas breakdown occurs. Although both grounded and isolated columns were tested, the resulting differences in charge accumulation were relatively modest for both particle types. For HDPE, grounded columns exhibited slightly higher charge levels at some time points, indicating a modest influence of wall grounding. However, the general trend and saturation timing were largely unaffected. For glass beads, the impact of grounding was negligible, with nearly overlapping charging curves observed between the two configurations. A more detailed evaluation of wall grounding effects is provided in Section 3.3.4. The observed charging dynamics can be explained by the interplay between triboelectric charge generation and various dissipation pathways. As discussed in previous chapters, charges are generated as particles repeatedly collide with the column wall, driven primarily by the difference in EWF between the particle and wall materials. Simultaneously, charge dissipation occurs via two main routes: (1) surface conduction through the wall, and (2) 71 gaseous discharge once the local electric field exceeds the dielectric strength of the fluidizing gas. The efficiency of the first dissipation route depends on the total resistance along the dissipation path, comprising particle–particle contact resistance (Rp), particle–wall contact resistance (Rc) and wall material resistance (Rw). Notably, Rp can increase by up to two orders of magnitude when particles are fluidized [104], significantly reducing dissipation efficiency. For the second route, breakdown-induced dissipation is activated only when the accumulated charge produces an electric field exceeding the gas’s breakdown threshold, which is gas-specific. Furthermore, both charge generation and dissipation rates are influenced by the local electric field at the point of contact. This field is determined by the accumulated charge on the particle and wall surfaces, as well as the geometry and electrical properties of the surrounding system. At early stages of fluidization, the electric field is relatively weak due to minimal charge buildup, allowing charge generation to dominate. As charge accumulates, the local field intensifies, which suppresses further charge generation and enhances dissipation. If the field strength remains below the breakdown threshold, the system eventually reaches a steady-state equilibrium between generation and dissipation. If the breakdown threshold is exceeded, additional charge is dissipated through gaseous discharge, producing a breakdown-limited form of charge stabilization. These mechanisms are clearly reflected in the charge evolution of HDPE particles, which can be described by the charge balance relationship introduced in Chapter 1: (dQdt)total= (dQdt)gen− (dQdt)dis(p−w)− (dQdt)breakdown (1.4 reproduced) 72 The large EWF difference between HDPE and the copper wall facilitates strong triboelectric charge generation, resulting in a dominant (dQdt)gen term. In contrast, the high effective resistivity of the system, primarily governed by Rp and Rc, restricts charge dissipation through the wall and yields a comparatively weak (dQdt)dis(p−w) term. This imbalance drives the rapid increase in the net charge observed during the first 5 minutes of fluidization. As charge accumulates and the local electric field strengthens, the wall-contact dissipation term (dQdt)dis(p−w) does increase, but its growth remains insufficient to counterbalance the generation term (dQdt)gen. Consequently, charge accumulation persists until the field strength approaches the dielectric breakdown threshold of the gas. At that point, gaseous discharge is triggered, activating the third term in the charge balance equation, (dQdt)breakdown. The onset of breakdown effectively restores the charge balance by offsetting the excess generation rate and halting further charge accumulation. Consequently, saturation is achieved after approximately 18 minutes, not through a steady balance between generation and dissipation, but through breakdown-limitation. In contrast, glass beads exhibit much lower saturation levels, which are well below the predicted breakdown thresholds, indicating that dielectric breakdown does not occur in these systems. The charge evolution can still be interpreted using the charge balance relationship (Eq. (1.4)), in which the breakdown-loss term is negligible. Under these conditions, the system rapidly reaches a steady state as the charge dissipation rate (dQdt)dis(p−w) becomes comparable to the generation rate (dQdt)gen . This balance arises from weaker charge generation (due to smaller EWF difference) and more efficient dissipation (enabled by lower Rp and Rc), leading to rapid charge stabilization (within 1.5 minutes). 73 These findings, consistent with the steady-state observations in Chapter 2, underscore the material-dependent nature of charging behavior in fluidized beds. For insulating particles such as HDPE, high resistivity and large EWF differences promote strong charge generation and breakdown-limited saturation; using a gas with lower dielectric strength (e.g., argon) can therefore effectively reduce saturation charge. In contrast, for more conductive and hydrophilic materials such as glass beads, breakdown is rarely reached, and gas composition plays a minimal role. For these systems, humidity control offers a more promising route for charge mitigation—an aspect explored further in the following sections. 3.3.2 Effect of fluidizing gas composition Building on the results of Section 3.3.1, this section examines how the composition of the fluidizing gas affects the dynamics of particle charging, with a particular focus on HDPE particles (Figure 3.3). Dry air and argon were selected because they possess markedly different dielectric breakdown strengths and exhibited substantial differences in steady-state saturation charge in Chapter 2. Glass beads were not included here, as their charge levels remained well below the breakdown threshold under all tested conditions in Section 3.3.1, indicating minimal sensitivity to gas-phase properties. The objective of this section is to determine whether, and to what extent, the fluidizing gas composition influences the rates of charge generation and dissipation during fluidization. By analyzing the time-resolved evolution of Q/m, rather than only the final saturation levels, we aim to clarify how gas composition contributes to early-stage charging behavior and to the onset of gas-breakdown-limited saturation. From the enlarged section of Figure 3.3, the Q/m values in both dry air and argon are nearly identical during the early stage of fluidization, particularly within the first 1.5 minutes. This suggests that HDPE particles experience similar rates of charge generation, (dQdt)gen, and 74 particle–wall dissipation rates, (dQdt)dis(p−w), in both gases before breakdown is triggered. This similarity indicates that gas composition does not significantly influence the underlying mechanisms of charge generation and particle–wall dissipation at this stage. Figure 3.3 Charge per unit mass, Q/m, acquired by HDPE particles fluidized in isolated and grounded copper columns with argon and dry air at different time intervals. The dotted lines (blue for air and orange for argon) represent the estimated maximum charge when gas breakdown occurs. The enlarged view highlights the early-stage charging behavior. This observation is consistent with the triboelectric charging theory, where charge generation is primarily governed by the EWF difference between the contacting materials. In this case, the large EWF difference between HDPE and the copper wall provides a strong, gas-independent driving force for tribocharging. The nearly identical early-stage Q/m growth rates in both gases support the hypothesis that gas-phase properties play a negligible role in charge generation until the electric field becomes strong enough to activate gas-phase processes such as breakdown. 75 Dissipation mechanisms also remain largely unaffected during the early stages. As described in Section 3.3.1, charge loss occurs primarily through conduction via particle–wall contacts and, eventually, through gaseous discharge once the electric field exceeds the breakdown threshold. At low charge levels, the local electric field remains weak, making gaseous discharge highly unlikely. Under these conditions, dissipation is governed mainly by the system’s high effective resistivity, which is dominated by particle–particle (Rp) and particle–wall (Rc) contact resistances. Since these resistances are not significantly influenced by gas composition in the absence of adsorbed moisture or surface conduction layers, the early-stage charging behavior remains consistent between dry air and argon. As fluidization continues, however, distinct differences emerge between the two gases. In air, charge saturation is reached after approximately 18 minutes, while in argon, saturation occurs much earlier, within 3 minutes, and at a significantly lower charge level. This sharp contrast is consistent with their respective dielectric strengths: approximately 3 × 106 V/m for air and 0.7 × 106 V/m for argon at 1 atm [88]. The lower dielectric strength of argon means it undergoes dielectric breakdown sooner, limiting further charge accumulation and resulting in an earlier and lower saturation charge level. The estimated maximum charge levels based on the gas breakdown model mentioned in Chapter 2 are shown as dotted lines in Figure 3.3. For particles fluidized in air, the estimated level slightly exceeds the measured value, whereas for particles fluidized in argon, the estimated level is slightly lower than the measured value. These small deviations likely arise from the simplifying assumptions underlying the model presented in Chapter 2. For instance, the model assumes a uniform bed height and homogeneous charge distribution, whereas real fluidized beds experience dynamic fluctuations in bed expansion due to bubbling and localized instabilities. In addition, particle segregation, where smaller, more highly charged particles tend to rise while larger particles settle, can lead to spatial variations in local electric field strength. These non-uniformities may cause gas breakdown 76 to occur at charge densities somewhat above or below the predicted threshold for a bulk-averaged system. Despite these limitations, the close alignment between experimental and predicted saturation values reinforces the conclusion that dielectric breakdown is the dominant mechanism limiting charge buildup in HDPE particles. In summary, the matching Q/m trends between dry air and argon during the early stages of fluidization indicate that charge generation is fundamentally material-dependent and largely independent of gas properties until breakdown conditions are approached. Differences in final saturation levels arise not from differences in the charging process itself, but from gas-specific limits on how much charge the system can sustain before breakdown-induced discharge occurs. These findings highlight the value of adjusting gas composition as a practical strategy for electrostatic control in fluidized bed systems, particularly for insulating particles such as polyethylene. 3.3.3 Effect of humidity This section examines the influence of humidity on the charging dynamics of HDPE particles and glass beads. The relative humidity (RH) of the humidified air entering the column was maintained at 30% ± 3%. As shown in Figure 3.4, HDPE particles exhibited similar final saturation charge levels in both dry and humidified air, but their charging dynamics differed substantially. Particles fluidized in humidified air reached saturation more rapidly, within roughly 12 minutes compared with 18 minutes in dry air, and displayed higher instantaneous charge levels throughout the charging phase. 77 Figure 3.4 Charge per unit mass, Q/m, acquired by HDPE particles fluidized in the isolated and grounded copper columns with dry and humidified air as a function of fluidization time. The dotted line represents the estimated maximum specific charge when gas breakdown occurs. As discussed in previous chapters, humidity influences both charge generation and dissipation. At moderate RH, adsorbed water molecules on particle or wall surfaces can form localized water patches that act as ion-transfer pathways (via H⁺ and OH⁻), thereby enhancing triboelectric charge generation. At higher RH, these patches may merge into a continuous surface film, particularly on hydrophilic materials, providing conductive pathways that facilitate charge dissipation. This material dependence is significant: the surface resistivity of hydrophobic polyethylene decreases by less than one order of magnitude when RH increases from 0% to 60% [105], whereas hydrophilic glass experiences a reduction of 3–4 orders of magnitude over the same range [106]. As a result of these competing effects, Q/m often exhibits a non-monotonic dependence on RH, such as the intermediate peak observed in steady-state measurements in Chapter 2. The RH at which this intermediate peak occurs depends strongly on surface properties, appearing at lower RH for hydrophilic materials such as glass and at higher RH for more hydrophobic materials such as polyethylene. 78 In the charging curves shown in Figure 3.4, the steeper initial charging slope and earlier onset of saturation in humidified air indicate that humidity enhances charge generation throughout the charging phase, whereas any humidity-induced increase in charge dissipation is likely minimal under the tested conditions. This behavior is consistent with the high resistivity and hydrophobic surface of HDPE, which restrict the formation of continuous conductive water films, thereby minimizing humidity-driven conductive dissipation. Although humidity clearly accelerates the early-stage charging process, it does not appear to significantly alter the final saturation level. The measured Q/m values at saturation in both dry and humidified air closely match the theoretical maximum charge levels predicted by the breakdown model. This indicates that gas breakdown is the limiting mechanism for final charge accumulation under both conditions. Once particle charge approaches the breakdown threshold of air, further accumulation is limited by gaseous discharge. Because increasing RH from 0% to 40% changes the breakdown voltage of air by less than 3% [107,108], humidity exerts only a minor influence on gas breakdown. This explains why the final saturation charge levels remain essentially unchanged between dry and humidified air. Figure 3.5 presents the charging dynamics of glass beads fluidized in dry and humidified air. In contrast to HDPE, the glass beads showed slower charge accumulation in humid air, with Q/m values consistently lower across all fluidization times, including the final saturation level. This behavior suggests that for glass beads, humidity primarily promotes charge dissipation rather than generation. Unlike HDPE, the hydrophilic surface of glass readily adsorbs water vapor, enabling the formation of continuous water films even at moderate RH. These films provide conductive pathways that enhance charge dissipation through surface conduction to the column wall. 79 Under both dry and humid conditions, the final charge levels remained far below the saturation thresholds predicted by the gas breakdown model, indicating that dielectric breakdown was not triggered. The differences in saturation Q/m between dry and humidified air therefore arise from changes in the balance between charge generation and dissipation, influenced by the presence of adsorbed water films. Figure 3.5 Charge per unit mass, Q/m, acquired by glass particles fluidized in the isolated and grounded copper columns with dry and humidified air as a function of fluidization time. The dotted line represents the estimated maximum specific charge when gas breakdown occurs. Notably, the polarity reversal induced by humidity in the steady-state experiments of Chapter 2 did not appear in the present time-resolved tests. In all cases, the glass beads acquired positive charges, consistent with the triboelectric series. This difference is likely due to the narrower particle size distributions and larger mean particle sizes used here, which substantially reduced the presence of fines and therefore minimized bipolar charging. As a result, the overall bed polarity remained stable across humidity conditions. Nevertheless, the observed trend of decreasing Q/m with increasing humidity for glass beads is consistently observed here and in Chapter 2. It raises a plausible hypothesis: In systems with broader particle size distributions, bipolar charging may arise from 80 interparticle collisions, where smaller and larger particles acquire opposite polarities due to differences in surface area, contact mechanics, and charge transfer dynamics. In such cases, larger particles, which acquire lower |Q/m|, may carry charges of one polarity. Finer particles, possessing higher surface-area-to-mass ratios, may accumulate higher specific charges, but with opposite polarity. This mechanism could explain the consistently negative charges measured on glass beads in earlier tests under dry conditions (see Section 2.3.1.2), as the samples were predominantly collected from the lower regions of the bed, where larger particles were concentrated. Thus, if humidity disproportionately affects these two particle groups (e.g., due to greater water film formation on fine particles), it could shift the net charge balance of the bed, potentially causing the polarity reversals observed under humidified conditions in Chapter 2. By contrast, no polarity reversal was observed for HDPE in Chapter 2 despite its broad size distribution. This difference can be attributed to the much higher charge densities carried by HDPE particles, including the larger ones. Their high resistivity and large EWF difference with the wall allow substantial charge buildup before breakdown. Thus, although fine particles may acquire opposite polarity, their small mass fraction and limited total charge do not significantly affect the overall bed polarity. In conclusion, the time-resolved charging dynamics confirm that the influence of humidity in gas–solid fluidized beds is strongly material-dependent and governed by the interplay between charge generation and charge dissipation. For hydrophobic and highly resistive particles such as HDPE, moderate humidity enhances ion-mediated charge transfer, accelerating early-stage charging, but has little effect on charge dissipation or on the dielectric breakdown strength of the gas. As a result, the final saturation level remains breakdown-limited and essentially unchanged. In contrast, for hydrophilic and less 81 resistive particles such as glass beads, humidity primarily promotes charge dissipation through surface conduction, particularly via continuous water films connecting particles to the column wall, thereby suppressing charge accumulation and reducing the saturation level. These findings highlight the importance of tailoring electrostatic control strategies to specific particle and system properties. Future studies across a wider humidity range with diverse material combinations are needed to develop robust, humidity-based methods for managing electrostatics in fluidized bed systems 3.3.4 Effect of grounding This section examines the effect of grounding based on experimental results presented in Figures 3.2–3.5. Grounding the column wall is a commonly adopted strategy for mitigating electrostatic charging in gas–solid fluidized beds, primarily used to reduce electrostatic hazards. However, numerous studies [8,109,110] have shown that grounding is often ineffective in reducing particle charge buildup. Therefore, this study investigates how grounding influences charge dynamics and saturation levels, to clarify its actual impact on electrostatic behavior in fluidized beds. In Chapter 2, we observed that HDPE particles fluidized in grounded steel column and acrylic column reached different saturation charge levels, despite both systems being limited by gas breakdown. Since the dielectric breakdown strength of the gas was the same in both cases, the observed difference is likely due to two factors: (1) a larger EWF difference between HDPE and the steel wall may allow greater charge accumulation before breakdown occurs; and (2) different electrostatic boundary conditions determined by the wall conductivity (i.e., grounded conductor vs. insulating surface). To isolate the second factor, grounding experiments were conducted using the same wall material. The difference in wall charging behavior was expected to influence particle charging by modifying the local electric field at the particle–wall interface. 82 However, our results show that grounding has minimal impact under all tested conditions (see Figures 3.2–3.5). The charging dynamics and saturation charge levels of both HDPE and glass particles show negligible differences between grounded and isolated columns, suggesting that wall grounding has a negligible effect on particle charging, particularly for particles with high electrical resistivity. As illustrated in Figure 3.6, triboelectric charging originates from repeated collisions between particles and the column wall, resulting in charge transfer and the buildup of opposite charges on both surfaces. In grounded columns, charges generated on the wall are rapidly neutralized through conduction to the ground, maintaining a nominally neutral wall surface. In contrast, in isolated columns, charges of opposite polarity accumulate on the wall, which leads to the development of a stronger local electric field at the particle–wall interface due to the increased potential difference. This intensified field may inhibit further charge transfer, effectively self-limiting the charging process. From this perspective, while grounding the wall effectively prevents charge buildup on the wall itself, it does little to reduce, and may even slightly increase, the net charge acquired by fluidized particles due to the weaker counteracting electric field. Our experiments on pneumatic conveying of insulating particles (sawdust) in a copper pipe revealed that particles conveyed through an isolated pipe acquired lower charge densities than those in a grounded pipe (details provided in Chapter 5). A similar observation was reported by Sowinski et al. [109] for polyethylene particles in fluidized steel columns. These findings further suggest that grounding conductive walls does not necessarily reduce particle charge accumulation, and in some cases may even increase it because of a weaker electric field at the interface. The insensitivity of particle charging to grounding can also be explained by the dominant charge dissipation pathway, as illustrated in Figure 3.6. For dielectric particles like HDPE and glass beads, the primary bottleneck for charge dissipation is the high particle–particle contact resistance (Rp), rather than particle–wall contact resistance (Rc) or the wall 83 resistance (Rw). Even when the wall is well grounded, charges on the particles cannot be efficiently transferred unless Rp is sufficiently low, which is typically not the case with these insulating particles. Furthermore, it is important to consider the electrical resistance between the column wall and ground (Rg), which depends on the supporting structure. In our setup, Rg exceeded the upper measurement limit of the electrometer (>2×1011 Ω), primarily due to the 5 mm thick Plexiglas outer shell of the column, which alone contributes a resistance of approximately 1010–1011 Ω. This is comparable to the estimated Rp of 108-1011 Ω for GB and ~1014 Ω for HDPE in a 0.1 m diameter column, based on an assumed effective conduction distance of 0.05 m. Given these extremely high estimated resistances, charge leakage through the supporting structure may still play a significant role in dissipating charges from the non-conductive wall, as achieving Rg ≫ Rp (i.e., true electrical isolation) is inherently challenging. This highlights the importance of measuring and reporting Rg in future studies. We strongly recommend that future research include precise measurements and reporting of Rg values to better understand their role in charge dissipation mechanisms. In summary, although grounding effectively neutralizes charge accumulation on the column wall, it does not substantially reduce the net charge acquired by fluidized dielectric particles. This limited effect is attributed to the dominant role of particle–particle contact resistance (Rp) in governing charge dissipation, which remains unaffected by wall grounding. Additionally, while ungrounded walls may develop counteracting electric fields that slightly suppress further triboelectric charging, our results show that this influence is minimal across the tested conditions and materials. The overall insensitivity of charging behavior to grounding suggests that electrostatic boundary conditions imposed by wall conductivity play a minor role under typical fluidization conditions. 84 This outcome helps to distinguish between the two hypotheses proposed in Chapter 2 regarding the observed differences in HDPE saturation charge across columns with different wall materials. The grounding experiments in this chapter indicate that the observed differences were primarily driven by the EWF difference between the particles and the wall materials, rather than by electrostatic boundary conditions at the particle–wall interface. Figure 3.6 Schematic illustration of charge dissipation pathways in isolated and grounded fluidization columns. In grounded systems, charges accumulated on the wall are rapidly neutralized through conduction to ground, maintaining a nearly neutral wall surface. In isolated systems, charges remain on the wall, generating a counteracting electric field that can partially suppress further triboelectric charge transfer. Despite these differences in wall behavior, particle charge dissipation remains limited primarily by the high particle–particle contact resistance (Rp), which dominates over particle–wall (Rc), wall resistance (Rw), and grounding resistance (Rg). 3.4 Summary This chapter systematically investigated the time-resolved charging dynamics of dielectric particles in gas–solid fluidized beds, focusing on the effects of fluidizing gas composition, humidity, and wall grounding. The results confirmed that triboelectric charging behavior is strongly material-dependent and governed by the interplay between charge generation, dissipation through column walls, and gas breakdown processes. 85 HDPE particles exhibited progressive charge accumulation over time, reaching saturation levels governed by the dielectric breakdown strength of the fluidizing gas. In contrast, glass beads showed much lower charge levels, with rapid stabilization driven by the balance of generation and dissipation, independent of gas breakdown. Time-resolved measurements in both dry air and argon revealed nearly identical charging behavior during the early stages of fluidization, indicating that the charge accumulation rate is largely insensitive to gas composition. However, the final saturation levels diverged, reflecting the differing breakdown strengths of the two gases. These results demonstrate that gas composition primarily influences the saturation charge level through its dielectric strength, while exerting minimal influence on the underlying charge accumulation process. The role of humidity was also found to be strongly material-dependent. For hydrophobic and high-resistivity HDPE particles, moderate humidity accelerated charge accumulation but did not alter final charge levels, indicating enhanced charge generation without significantly affecting dissipation or gas breakdown strength. In contrast, for hydrophilic and low-resistivity glass beads, humidity slowed down charge accumulation and reduced charge saturation levels, primarily due to increased dissipation via water-assisted surface conduction to the column wall. In Section 3.3.3, we also introduced a plausible explanation for the humidity-induced polarity reversal observed in Chapter 2: bipolar charging effects arising from a wide particle size distribution may cause fine and coarse particles to acquire opposite polarities. Under certain humidity conditions, differential charge dissipation between these particle groups could shift the net bed polarity. The time-resolved results in this chapter, obtained by using narrow size fractions, did not reproduce the reversal phenomenon, suggesting that polydisperse particle systems may be responsible for such polarity shifts. These effects will be further examined in Chapter 4. Grounding experiments demonstrated that the wall potential has little influence on particle charging, particularly in the case of dielectric particles. Despite clear differences in wall 86 charging between grounded and isolated columns, the overall particle charging trends and saturation levels were nearly identical. This insensitivity is attributed to the dominant role of particle–particle contact resistance (Rp), which constrains charge dissipation and is unaffected by whether the wall is grounded. These findings address the competing hypotheses proposed in Chapter 2 and suggest that the differences in HDPE charge levels observed across columns with different wall materials arise primarily from variations in EWF rather than electrostatic boundary conditions imposed by wall conductivity. Overall, this chapter offers a detailed understanding of how particle charging evolves over time in gas–solid fluidized beds, highlighting the critical roles of gas dielectric strength, particle resistivity, and particle–wall EWF differences in determining charge accumulation and saturation. The findings provide a strong basis for developing effective electrostatic control strategies through careful selection of fluidizing gas, particle–wall material combinations, and humidity conditions in industrial applications. Building on these insights, the subsequent chapter will extend the discussion to explore how particle size influences charging behavior, with a particular emphasis on bipolar charging phenomena arising from wide size distributions. 87 Chapter 4 Interparticle Charging Behavior 4.1 Introduction This chapter aims to investigate the fundamental mechanisms of interparticle charge transfer through controlled collisions between dielectric particles. To achieve this, a custom-designed acoustic levitation platform was developed, enabling precise manipulation of particle-particle interactions without interference from container walls. Combined with a parallel-plate electric field for post-collision charge polarity analysis, this setup was used to systematically explore how particle size, material composition, and initial charge state influence interparticle triboelectric charging behavior. 4.2 Experimental Setup 4.2.1 Acoustic levitation and controlled collision In the experiment, particles were suspended in air using a resonant single‑axis standing‑wave acoustic levitation system. A schematic of the setup are shown in Figure 4.1. An ultrasonic transducer (Hesentec Model HS‑8SH‑3840) was driven sinusoidally at its resonance frequency (~40 kHz) by a power amplifier (PiezoDrive Model PDUS210). A concave glass reflector with a focal length of 5 cm was positioned beneath the transducer. Its geometry produced a three‑dimensional acoustic potential well with lateral confinement due to an inward‑directed acoustic pressure gradient, thereby preventing particle drift. The spacing between the transducer and reflector was set to 10 or 12 times the sound half wavelength (i.e., h = n·λ/2, where n = 10 or 12). At a sound speed of 343 m/s in air (20 °C) and a frequency of 40 kHz, the wavelength is 8.6 mm, giving a spacing of approximately 43–52 mm. This setup allows the formation of multiple acoustic potential wells along the axis, enabling simultaneous levitation of multiple particles and providing sufficient space 88 for experimental manipulation (see Figure 4.2). In our experiments, particles were typically trapped in the well closest to the centerline of the electrode plates to ensure exposure to a uniform electric field. Figure 4.1 Schematic of the acoustic levitation device and charge measurement system. Two particles are levitated and confined in an acoustic potential well. A horizontal electric field is applied across the levitation region to separate the particles according to their charge polarity. Figure 4.2 HDPE particles trapped within acoustic potential wells. Stable levitation required sufficiently high acoustic pressure and precise alignment of the acoustic focus. To achieve this, the levitator assembly was mounted on a microscope stand equipped with a focus block, which allowed fine adjustment of the transducer and reflector spacing to maintain the standing wave. The transducer was driven at a peak-to-peak voltage of 150-180 V. At such high power levels, overheating was common, and the generated heat 89 dissipated into the surrounding air. Because the acoustic wavelength depends on the speed of sound in air, and the speed of sound varies with temperature, any fluctuation in ambient temperature could shift the resonance condition of the standing wave. To mitigate this issue, the transducer was enclosed in an aluminum shell connected to a water-cooling loop, which helped prevent overheating and reduce temperature-induced drift. The focus block also allowed real-time readjustments to compensate for minor temperature-induced changes in spacing. In acoustic levitation, trapped particles often exhibit spontaneous rotation. This rotational behavior arises from slight asymmetries in the acoustic field, which may stem from misalignment between the transducer and reflector, imperfections in reflector curvature, non-uniform vibration modes, or ambient airflow. Such asymmetries generate weak tangential acoustic forces that drive the rotational motion. For non-spherical particles with irregular shapes or surface variations, the coupling between acoustic radiation pressure and particle surface anisotropy could produce additional torque [111]. The concave reflector further enhances these rotational effects. When two particles are trapped within the same acoustic potential well, they tend to move toward each other and rotate as a coupled pair. In this scenario, each particle experiences tangential acoustic forces arising from asymmetries in the acoustic field and differences in particle shape. In addition, the particles interact through sound scattered from their neighbors, a phenomenon known as secondary acoustic radiation forces (or second-scattering forces). When a particle is placed in an acoustic field, it scatters the incident sound waves and alters the surrounding pressure field. A nearby particle is thus subjected not only to the primary standing wave, but also to the scattered field generated by the first particle. This scattered field creates local pressure gradients that induce short-range attractive forces between the particles. As a result, the particles tend to aggregate and form stable clusters that rotate collectively around the central axis [112]. Moreover, interparticle 90 interactions, particularly electrostatic forces in our experiments, could reinforce the aggregation, stabilize their relative orientation, and maintain persistent rotational motion. To achieve controlled collisions between two particles, we deliberately utilized their spontaneous rotational behavior. Specifically, both particles were levitated simultaneously within the same acoustic potential well, allowing them to adhere and rotate as a bound pair. Controlled collisions were induced by switching the amplifier to a pulsed output mode, which introduced periodic brief pauses (lasting a few milliseconds) in the acoustic field. During each pause, the acoustic potential weakened or momentarily vanished, causing the particles to temporarily lose their inward-directed attractive forces and separate slightly under centrifugal motion, illustrated in Figure 4.3. When the acoustic field resumed, the particles quickly reattached and resumed their coupled rotation. By tuning the pulse interval, we controlled the collision dynamics and investigated different particle combinations and their associated charging behavior. We also observed that when the pause duration exceeded 2 ms, the levitated particles began to oscillate noticeably along the vertical axis. This vertical motion increased frictional contact between the particles, thereby enhancing contact electrification. However, excessively long pauses destabilized the standing wave, occasionally leading to particle ejection from the acoustic potential well and requiring the experiment to be reset. To balance charging efficiency and system stability, the pause duration was maintained within a range of 1-3 ms. Additionally, the pulsed mode was periodically deactivated to allow the particles to re-stabilize within the acoustic potential well. Under these conditions, an average collision frequency of approximately 15 collisions per minute was achieved for a pair of similar-sized particles. It should be noted that controlled collision charging in acoustic levitation may not exactly replicate the interparticle charging in fluidized beds. In a fluidized bed, each particle 91 collides with many different neighboring particles within a short time frame, rather than undergoing repeated contacts with the same particle. Moreover, variations in collision angle, impact velocity, and surface condition introduce subtle differences in charge transfer with every encounter. While such complex collision dynamics cannot be fully reproduced in the levitation system, the accumulated charging outcomes from sustained two-particle collisions still provide valuable insight into the fundamental mechanisms of interparticle triboelectrification. These mechanistic insights, in turn, help interpret and rationalize the more complex charging behaviors observed in fluidized beds. Figure 4.3 Representative still frames from a slow motion video, showing the controlled collision process between two HDPE particles levitated in the same acoustic potential well. (A) The particles adhere and rotate together within the well. (B–C) The amplifier’s pulsed mode periodically interrupts the acoustic field for a few milliseconds, causing the particles to temporarily lose their attractive forces and briefly separate under centrifugal motion. (D) Once the field resumes, the particles return to close contact and continue their coupled rotation. (A) (B) (C) (D) 92 4.2.2 Charge polarity measurement At the beginning of each experiment, the distance between the transducer and the reflector was adjusted to establish a resonant standing wave. Once the acoustic field was properly focused and stabilized, two particles were manually introduced into a single acoustic potential well using metal tweezers and levitated. The amplifier was then switched to pulsed mode to induce repeated collisions between the levitated particles. Each experiment was conducted for five minutes, during which approximately 70-80 collisions occurred between the particle pair. Particles occasionally escaped from the acoustic well during collisions, and the likelihood of ejection increased as the test duration increased. Therefore, each experiment was limited to five minutes to balance charging efficiency and system stability. Validation tests confirmed that this number of collisions was sufficient to produce measurable charge separation and to yield statistically meaningful polarity distributions across different particle combinations. An electric field was applied across the levitation region using two parallel copper plates (circular, 2 cm diameter) positioned on either side of the gap between the transducer and reflector. The plates were connected to a high-voltage power supply (Matsusada Precision, AU series), and a voltage of 20 kV was applied across a 4 cm gap, producing a uniform electric field of 5 kV/cm. Approximately 1–2 seconds after the electric field was activated, the acoustic levitation system was turned off. Once released from the acoustic potential well, the particles moved laterally under the applied electric field according to their charge polarity. Positively charged particles were attracted to the negative electrode, while negatively charged particles were deflected toward the positive electrode. The entire process was recorded in slow motion video. In this way, the post-collision charge polarity of different particle materials could be clearly identified. 93 For the binary-sized particle experiments, the size difference made it challenging to retain the small particles within the acoustic potential well, as they were much more susceptible to ejection, especially during the pause periods in the pulsed mode. To address this, a single large particle was continuously levitated throughout the entire experiment, while several small particles were introduced into the trap at the beginning of each run. As these small particles were gradually ejected, additional particles were added as needed to replenish the system and maintain repeated collisions with the same large particle. Each experiment was conducted for a duration of at least five minutes to ensure a sufficient number of collisions for meaningful charge transfer analysis. After the collision phase, the pause duration in pulsed mode was briefly increased to induce strong vertical oscillations, which helped expel any remaining small particles from the acoustic well. The charge polarity of the large particle was then measured using the electric field. Based on tribocharging principles and established charge transfer mechanisms, the small particles are expected to acquire a polarity opposite to that of the large particle. Therefore, the charge polarity of the small particles could be inferred accordingly. To ensure experimental repeatability, each collision experiment was repeated at least 15 times under identical conditions. All trials were conducted using the same procedure, and the laboratory environment (temperature = 22 ± 2 °C, RH = 25% ± 5%) was kept constant throughout the experiments. For each experimental group, results were reported using percentage distributions to reflect the statistical probability of a given particle acquiring positive, negative, or neutral charge after collision. The initial charge of the test particles was measured using two complementary methods. First, bulk samples of each particle group were measured using a Faraday cup connected to an electrometer (Keithley Model 6514), to determine their net charge prior to levitation. This provided an average charge polarity and magnitude. To ensure measurement repeatability, each Faraday cup test was repeated at least eight times, and the reported value 94 represents the average, with error bars denoting the standard deviation. For individual particle measurements, a single particle was levitated in an acoustic potential well without any collisions. The horizontal electric field was then activated, and after 1–2 seconds, the levitation system was turned off to allow the particle to move freely under the electric field. The resulting deflection was recorded to determine the particle's initial charge polarity. Each electric field measurement was repeated at least 15 times to ensure statistical reliability. Results were reported using percentage distributions to reflect the statistical probability of a given particle acquiring positive, negative, or neutral charge. 4.2.3 Tested particles In this chapter, high-density polyethylene (HDPE) particles were selected as the primary material of interest. These particles were identical to those used in Chapters 2 and 3. They were sieved into three distinct size ranges. In addition to HDPE, particles made of other materials, including soda lime glass beads and silica sand, were also used in selected experiments. The purpose of using those additional particles was to test the performance and validity of the experimental design, including the controlled-collision approach and the parallel-plate electric field setup for measuring particle charge polarity. Table 4.1 summarizes the physical properties of the selected particles. Each particle was used only once to minimize the risk of contaminants, such as dust or other impurities, affecting the charging behavior. 95 Table 4.1 Key physical properties of tested particles. Sample Material Mass median diameter (μm) Density (kg/m3) PE-L HDPE 1000 940 PE-M HDPE 610 940 PE-S HDPE 245 940 GB-M Glass 560 2500 SS-S Silica Sand 250 2650 4.3 Results and Discussion 4.3.1 Initial charge The initial specific charge (i.e., charge per unit mass) of the five particle groups was measured using a Faraday cup. As shown in Figure 4.4, all three HDPE groups exhibited a net negative charge, likely resulting from triboelectric charging during the sieving process, where particles repeatedly contacted the metal mesh. Among them, smaller particles displayed higher charge densities, attributable to their greater specific surface area and thus higher charge per unit mass. Overall, the measured charge levels were relatively low, less than one-tenth of the saturation charge observed for HDPE particles in the fluidized bed experiments discussed in Chapter 3 (approximately -6.5 μC/kg). In contrast, both the glass and silica particles were not subjected to sieving and showed significantly lower initial charges, with values close to neutral. It should be noted that Faraday cup measurements capture the bulk charge of a particle ensemble; therefore, a small number of particles with opposite polarity may still be present. To further assess the initial charge polarity of individual particles, acoustic levitation combined with a horizontal electric field was employed, and the results are presented in Figure 4.5. Due to their small size and high density, silica particles could not be stably levitated with the current setup and were therefore excluded from this test. 96 The results indicate that the particles did not carry identical polarity but instead exhibited a distribution of charge polarities. For the three HDPE groups, most particles appeared electrically neutral, while the remaining particles were predominantly negatively charged. It is important to note that the electric field strength was fixed in all tests; therefore, some particles classified as “neutral” may have carried weak positive or negative charges whose magnitudes were too small to produce visible deflection. For the GB-M group, approximately 80% of the particles appeared electrically neutral. Given their low initial charge measured by the Faraday cup, it is likely that the applied field was insufficient to induce visible deflection in most cases. Figure 4.4 Initial charge density (Q/m) of the tested particles measured using a Faraday cup. 97 Figure 4.5 Distribution of initial particle charge polarity (positive, negative, neutral) expressed as the percentage of total particles tested. Polarity measurements were obtained using acoustic levitation combined with a horizontal electric field. 4.3.2 Collision and charging between similar sized particles In this section, we investigated the charging behavior between particles of similar size. To validate the experimental setup, we first examined collisions between similar-sized particles of different materials: PE-M (HDPE) and GB-M (glass beads). Figure 4.6 shows the percentage distribution of charge-polarity combinations observed after collision in the levitation device. The polarity of each particle was determined based on its deflection under a horizontally applied electric field. The dominant outcome (80%) was negatively charged HDPE paired with positively charged glass beads, indicating a clear and consistent polarity separation during contact. This trend agrees well with predictions based on the triboelectric 98 series [32]. A small fraction of particles appeared electrically neutral after collision; however, their counterparts consistently exhibited the expected polarity. This suggests that the “neutral” particles likely carried insufficient charge to produce detectable deflection under the applied field. Furthermore, when compared to their initial charge states, both HDPE and GB particles exhibited increased charge magnitudes after collision. Prior to interaction, both particle types were mostly neutral and showed negligible deflection in the electric field. After collisions, the majority exhibited measurable deflection, and the particle pairs consistently carried opposite charge polarity, indicating the particles consistently transferred charge in the expected direction. These results confirm the validity of the apparatus. Figure 4.6 Distribution of charge polarity combinations between HDPE (PE-M) and glass particles (GB-M) after acoustic levitation collisions, expressed as the percentage of total measurements. Next, we examined collisions between two HDPE particles (PE-M), as shown in Figure 4.7. In approximately 54% of the cases, the particle pairs acquired opposite polarities, meaning that one particle became positively charged and the other negatively charged. These cases were not further differentiated into positive/negative or negative/positive outcomes, as the 99 HDPE particles were identical in material and size and thus lacked any consistent charging tendency, such as an EWF difference, that could determine which particle became positive and which became negative. In 29% of the cases, one particle became negatively charged while the other remained neutral. In 12% of cases, both particles carried negative charge, and in only 5% of cases one particle became positively charged while the other remained neutral. Those results suggest that when particles of the same material and similar size collide, the resulting charge distribution is relatively random. While the polarity separation observed in most particle pairs clearly indicates that charge transfer occurred, the underlying driving force remains unclear. Due to the high surface resistivity of insulating materials, charge migration is restricted to the contact region during collision. Consequently, the direction of charge transfer is likely governed by localized variations at the contact interface, such as pre-existing charge, contamination, adsorbed moisture, oxide layers, and mechanical deformation. These factors could introduce subtle asymmetries that influence individual collision outcomes. Therefore, the driving force for charge transfer between same-material particles appears relatively weak and random, insufficient to produce consistent or unidirectional polarity. This behavior stands in sharp contrast to collisions between dissimilar materials, where charge polarity is generally predictable and consistent with the triboelectric series. Kline et al. [35] previously investigated same-material particle collisions using an acoustic levitation system with similarly sized HDPE particles (710–850 μm in diameter). By analyzing the oscillatory motion of charged particles under an AC electric field, they estimated the charge transfer per collision to be less than 3,000 elementary charges (approximately 4.5 × 10-16 C), indicating a very small transfer magnitude. This low charge transfer rate may explain why some particles in our experiments remained electrically neutral even after repeated collisions, as the accumulated charge was insufficient to cause 100 measurable deflection. Similarly, observations in which both particles were negatively charged or only one acquired positive charge may reflect incomplete or asymmetric charge redistribution, likely resulting from slow or inefficient charge transfer processes. Figure 4.7 Distribution of charge polarity combinations between similarly sized HDPE particles (PE-M) after acoustic levitation collisions, expressed as the percentage of total measurements. 4.3.3 Collision and charging between binary sized particles In this section, we examine the charge transfer resulting from collisions between particles of different sizes. To validate the reliability of the experimental design, collisions were first conducted between large HDPE particles (PD-L) and small silica particles (SS-S). Following controlled collisions in the acoustic levitation system, the charge polarity of the large HDPE particle was directly determined directly from its lateral deflection in the applied electric field. Although the polarity of the small silica particles was not measured individually, it can be reasonably inferred that they acquired the opposite polarity to the large particles, based on charge-conservation considerations. 101 The results show that all HDPE particles consistently acquired negative charge following collisions, implying that the small silica particles became positively charged. This outcome aligns with expectations based on the triboelectric series [32], in which silica is positioned more positive relative to HDPE. Compared with their initially near-neutral states, the post-collision HDPE particles exhibited clear deflection under the applied electric field, indicating a significant increase in charge magnitude. These findings confirm that strong and unidirectional charge transfer occurred during the collisions, leading to consistent charge accumulation on the large HDPE particles. Moreover, these observations suggest that in systems involving dissimilar materials, the size difference between colliding particles is unlikely to determine the charge polarity. Rather, the direction and extent of charge transfer are primarily governed by the difference in EWF between the materials. Accordingly, the triboelectric series serves as a reliable framework for anticipating charge polarity in such systems. We next investigated charge transfer between HDPE particles of identical material but different sizes (PE-L and PE-S). As shown in Figure 4.8, 84% of the measured cases indicate that the large particles acquired positive charges following collisions, implying that the smaller particles became negatively charged. In the remaining 16%, the large particles exhibited no observable deflection, likely because their charge magnitude fell below the detection threshold. The clear post-collision deflection observed in most large particles confirms that substantial charge transfer occurred, resulting in charge accumulation sufficient to induce measurable motion under the test field. Compared with their initial charge states, where most HDPE particles appeared neutral or negatively charged in electric field testing, and slightly negatively charged in Faraday-cup measurements, none of the large particles were observed to carry negative charge after binary-sized collisions. These results provide compelling evidence that collisions between 102 HDPE particles of binary sizes consistently lead to unidirectional charge transfer, with large particles acquiring positive charge and small particles acquiring negative charge. Figure 4.8 Charge polarity distribution of large HDPE particles (PE-L) after collisions with smaller HDPE particles (PE-S). 84% of the large particles became positively charged, while 16% showed no measurable deflection under the applied electric field. This trend is consistent with numerous prior observations [22,23,75,113]. The contact asymmetry between particles of different sizes is widely considered the primary mechanism responsible for bipolar charging. Such asymmetry promotes the depletion of charge carriers from the larger particle’s surface and their accumulation on the smaller one, resulting in net charge transfer. However, the fundamental driving force behind the motion of these charge carriers, and even the identity of the carriers themselves, remains under debate. Studies by Lee et al. [92] and Harris et al. [36] have implicated adsorbed OH⁻ ions as the primary species transferred during contact. Under such a mechanism, larger particles donate more OH⁻ ions than smaller ones and therefore tend to acquire a net positive charge. More recently, Sobarzo et al. [114] demonstrated that the polarity of contact electrification between nominally identical materials is strongly influenced by their contact history. Using low-energy ion scattering (LEIS), which probes only the outermost atomic layers of the 103 surface, they found that polymer surfaces subjected to repeated contact become progressively smoother and tend to charge negatively. This provides a plausible explanation for why, in binary-sized particle systems, the smaller particles, which experience more frequent collisions, are more likely to carry negative charge due to enhanced surface smoothing. This surface smoothing effect may also have implications for the EWF of the particles. Although the relationship between surface roughness and work function has been examined in several studies, most have focused on metals, owing to the relative ease of experimentally measuring their work function. Reported results show that some metals (e.g., copper) exhibit lower work functions with increasing surface roughness [115], whereas others (e.g., magnesium and aluminum alloys) display the opposite trend [116]. Such divergent behavior is likely attributable to differences in surface chemistry, electronic structure, and the characteristics of the roughness, including asperity size and morphology. From an electron-transfer perspective, processes such as surface polishing, repeated friction, or micro-cutting can induce localized plastic deformation and transient heating, leading to atomic-scale rearrangement at the surface. Such rearrangement can alter the density and nature of surface defects, thereby modifying the average electron-binding energy and, consequently, the EWF. From an ion-transfer perspective, surface roughness can alter surface wettability and the amount of adsorbed moisture, which in turn affects the direction and efficiency of ion transfer during triboelectric charging. Taken together, these factors indicate a strong link between surface roughness and EWF, suggesting that differences in roughness between large and small particles could lead to measurable variations in their EWF. This aspect warrants further investigation in future studies. From an industrial perspective, the larger dimensions of fluidized bed reactors substantially reduce wall effects, making interparticle interactions increasingly important. Under these conditions, bipolar charging can induce size-dependent charge separation between large 104 and small particles. Due to axial segregation within the bed, larger particles tend to stay in the dense bed, while finer particles are entrained upward to the freeboard region, leading to opposite net charges at different bed heights. More importantly, partial elutriation of fine particles from the bed can further result in an overall net charge on the entire system. A similar phenomenon was observed in Chapter 2, where polarity reversal occurred for glass beads, which may have been caused by these mechanisms. Under such circumstances, the general charge balance relationship described in Chapter 1 can be extended as: (dQdt)total= (dQdt)gen(p−w) + (dQdt)gen(p−p) − (dQdt)dis(p−w)− (dQdt)breakdown (5.1) Here, (dQdt)gen(p−w) corresponds charge generation from particle–wall contacts, while the new term (dQdt)gen(p−p) represents charge generation arising from interparticle bipolar charging. In large-scale systems, the first term may be comparatively weak and can often be considered negligible. It can therefore be anticipated that interparticle charging, rather than particle-wall charging, will play an increasingly important role in determining the overall electrostatic behavior of industrial fluidized bed reactors—a topic that requires further investigation in the future. Moreover, bipolar charging between same-material particles can lead to charge separation and the development of long-range electrostatic forces. These forces are considered a key driver of particle clustering. In the experiments presented in Chapter 2, particles responsible for wall fouling were consistently smaller than the average in-bed particle size, suggesting that fine particles are the primary contributors to fouling. Building on this, future studies could investigate whether engineered binary-sized mixtures, with optimized size ratios, can mitigate wall fouling by promoting interparticle clustering and reducing the mobility of fine particles toward the freeboard. 105 4.3.4 Effect of pre-existing charge In an early study, Chowdhury et al. [117] reported that in collision experiments involving small spheres of identical material, the direction of charge transfer was largely insensitive to the initial charge polarity when the initial charge magnitude was small. Consistent with this observation, our results in Sections 4.3.2 and 4.3.3 demonstrated that, across all tested particle combinations, the initial charge polarity had no significant influence on the direction of charge transfer. We then asked a related question: what happens when two same-material, similar-sized particles collide and one of them already carries a substantial initial charge? This scenario mirrors conditions in our earlier fluidized-bed studies, where HDPE particles in contact with the metallic wall accumulated substantial negative charge due to the large EWF difference. We therefore asked how such highly charged particles would behave upon colliding with nearly neutral particles located closer to the bed interior. Furthermore, if the neutral particles are smaller (as in the binary‑sized experiments discussed in Section 4.3.3), will size-dependent bipolar charging still persist? To address these questions, we pre-charged HDPE particles and conducted levitation-based collision experiments under the same conditions as before. PE-L particles were selected as the test group. Approximately 5 g of the particles were placed in a copper container and manually shaken for one minute. The resulting charge was measured using a Faraday cup, yielding an average specific charge of -4.3 μC/kg. Extending the shaking duration beyond one minute produced no additional charge accumulation, indicating that the particles had reached their saturation charge. This charge level is comparable to the saturation charge (approximately -6.5 μC/kg) observed for HDPE in the fluidized-bed experiments of Chapter 3, confirming the effectiveness of the pre-charging procedure. 106 Next, the pre-charged PE-L particles were collided with uncharged PE-L particles of similar size in the levitation device. After collision, both particles consistently exhibited negative polarity, indicating transfer of negative charge from the pre-charged to the initially neutral particle. Collisions were then performed between pre-charged PE-L and uncharged PE-S particles. In this case, all pre-charged large particles remained negatively charged, demonstrating that size-dependent bipolar charging did not produce enough positive charge to reverse or neutralize the strong initial polarity of the large particles. These results offer several important insights. First, when two similar‑sized, same-material particles collide and only one carries a substantial initial charge, charge transfer tends to occur in a direction that reduces the initial charge imbalance. In our experiments, charge moved from the negatively pre-charged particle to the initially neutral one, causing both particles to become negatively charged. A larger initial charge difference would be expected to further strengthen this equilibrium-driven behavior. This behavior may also explain the charge redistribution observed in fluidized beds, where wall-contacting particles that acquire substantial charge subsequently transfer charge to relatively neutral particles in the bed interior. Over time, this process leads to a more uniform charge distribution across the particle bed. Furthermore, the findings indicate that bipolar charging can be overshadowed by the dominant transfer of charge from a highly charged particle to a relatively neutral one. This mechanism may help explain the observations in Chapter 2, where polarity reversal attributed to bipolar charging was observed for glass particles but not for HDPE particles, despite similar size distributions. In fluidized beds, the presence of the column wall introduces additional complexity to charge generation and transfer processes. Particles can continuously acquire charge through repeated collisions with the wall. As observed for HDPE in our previous fluidization tests, such wall contacts can generate substantial charge on wall-contacting particles, which is subsequently transferred to less-charged interior 107 particles, regardless of particle size. When wall–particle collisions dominate the charging process, the overall charge of the bed becomes largely dictated by wall-derived charging, which can overshadow size-dependent bipolar charging between fine and coarse particles. For HDPE particles, all size groups consistently acquired substantial negative charge through collisions with the metal wall. As a result, both coarse and fine particles became negatively charged at equilibrium, although fine particles tended to carry more negative charge, reflecting the unidirectional charge transfer inherent to bipolar charging. In contrast, glass particles acquired far less charge from wall collisions, leading to much weaker initial charge asymmetry. Under these conditions, size-dependent bipolar charging became the dominant mechanism, potentially explaining the polarity reversal observed in Chapter 2. Previous studies by Alsmari et al. [58] and Poupak et al. [18] using metal columns (steel and copper) also reported polarity reversal, with fine glass beads in the freeboard becoming positively charged and coarse particles in the dense bed becoming negatively charged. This outcome contradicts the conventional expectation for bipolar charging, in which fine particles typically charge negatively and coarse particles positively. The underlying causes of these discrepancies were not explicitly addressed in prior studies, and because the fundamental origins of bipolar charging remain unresolved, a definitive explanation is not yet possible. Nevertheless, the presence of walls in fluidized beds undeniably complicates the charging process, and the observed polarity reversal is likely the combined result of both particle–wall and interparticle charging. Because walls are unavoidable in fluidized beds, it remains difficult to determine which mechanism dominates. These results highlight the importance of eliminating wall effects in levitation-based studies to enable cleaner, more definitive investigation of bipolar charging mechanisms in future work. 108 4.4 Summary In this chapter, we have demonstrated a novel experimental method combining acoustic levitation with a parallel-plate electric field to investigate bipolar charging of single particles. This approach proved reliable and simple to operate, and it allowed precise control of particle–particle interactions without wall interference. As a result, it enabled effective investigation of interparticle charging behavior. Although the present study focused on qualitatively determining post-collision charge polarity, the setup can be readily extended for future quantitative analyses. For example, integrating a high-speed camera would help track particle trajectories under an electric field, thus enabling estimation of the charge magnitude. Overall, the simplicity and versatility of this experimental design make it a valuable tool for systematically exploring interparticle charging mechanisms. Experiments involving collisions between similar-sized HDPE particles revealed relatively random charge polarity outcomes, indicating weaker and less predictable charge transfer compared to collisions between similar-sized particles of different materials (e.g., HDPE and glass beads). In the latter case, the EWF difference between the materials led to strong and unidirectional charge transfer, resulting in charge polarities that aligned with triboelectric series predictions. Moreover, binary-sized collision experiments clearly and consistently demonstrated bipolar charging for HDPE particles of different sizes, with larger particles acquiring positive charge and smaller particles becoming negatively charged. This trend agrees with theoretical models and previous findings, confirming the sensitivity and reliability of the experimental setup in capturing size-dependent charging effects. Additionally, pre-charging experiments revealed that substantial initial charge on one particle could dominate the charge transfer process and overshadow size-dependent bipolar charging. When one particle carried a high initial charge, collisions consistently led to charge redistribution that reduced the charge imbalance between the particles. 109 Taken together, these findings show that the acoustic levitation system provides a practical and flexible tool for exploring interparticle charging under well-controlled conditions. Its ability to isolate particle–particle interactions from wall effects offers valuable insights into how charge is generated and transferred during collisions. These insights lay the foundation for developing effective strategies to address electrostatic problems in fluidized beds, including charge buildup, wall fouling, and particle agglomeration. 110 Chapter 5 A Case Study: Electrostatic Charging of Sawdust in Wood-Processing Facilities 5.1 Introduction Building on the fundamental understanding of particle charging behavior in fluidized beds presented in the previous chapters, this chapter investigates a real-world problem as a case study to address electrostatic hazards in wood-processing facilities: a sector where dust explosions present a critical safety concern. By applying the understanding of charge generation, accumulation, and dissipation developed earlier, this chapter explores how these electrostatic phenomena manifest in practical gas–solid flow systems. Canada, with its abundant forest resources, is a major global supplier of high-quality wood products. In 2020, the wood products industry contributed 6% to the country’s total manufactured goods revenue [118]. In recent years, growing concerns over climate change and fossil fuel dependency have further boosted demand for renewable energy sources, such as wood pellets. As one of the world’s leading exporters, Canada’s wood pellet production has surged by 133% over the past decade, reaching 3.5 million tonnes in 2021 [119]. However, alongside this rapid industrial growth, the risks associated with fire and dust explosions in wood-processing facilities have become increasingly severe. Numerous incidents reported in the past decade have resulted in devastating damage to equipment and infrastructure, loss of life, environmental contamination, and long-term health impacts. For example, a dust explosion at the Babine Forest Products sawmill in British Columbia in January 2012 killed two workers and injured 20 others [120]. Just months later, a similar explosion occurred at Lakeland Mills in the same province, again claiming two lives and injuring many more [121]. 111 Dust explosions are not only a concern in Canada but also a significant issue worldwide, causing substantial economic losses and human casualties. For example, on May 3, 2021, an explosion at a particleboard facility in Klaipėda, Lithuania, resulted in multiple injuries and one fatality. According to recent statistics, a total of 225 dust-related fires and explosions were recorded globally in 2020, with 28% involving wood dust [122]. These incidents highlight the urgent need for comprehensive safety measures, including explosion vents, suppression systems, routine housekeeping, and electrostatic control protocols. In Europe, regulatory frameworks like the “Equipment for Potentially Explosive Atmospheres” (ATEX) directive have been established to mitigate these risks and enhance safety in industrial environments. Dust explosions require five key elements: fuel, oxidant, ignition source, dispersion, and confinement [123]. In wood-processing facilities, combustible dust is generated throughout the entire process from sawing and milling to pelletizing, conveying, and storage. Therefore, if flammable atmospheres cannot be fully eliminated, mitigating ignition sources becomes critical [124]. Among these, electrostatic discharge is a well-documented and potentially catastrophic ignition source [125]. For instance, a reported explosion in a Chinese furniture factory occurred when dry wood dust, transported at high velocity through a plastic air hose, generated significant electrostatic charge, which eventually ignited the dust cloud via electrostatic spark [126]. Air-blow cleaning using high-pressure hoses is a common practice in wood-processing facilities. However, this method may inadvertently increase the risk of electrostatic discharge. Prolonged dragging of plastic hoses across the floor may lead to charge accumulation due to friction between the hose and flooring materials. Additionally, during cleaning, airborne dust particles may acquire or transfer additional charge through repeated contact with various building surfaces. These processes can substantially elevate the overall charge level, thereby increasing the likelihood of a hazardous discharge. 112 Triboelectric charging is the primary mechanism responsible for electrostatic charge accumulation on dust particles and processing equipment. As discussed in earlier chapters, this accumulation is influenced by several factors, including the effective work function (EWF) differences between materials, the area of contact, the electrical resistivity of the contacting surfaces, and the surrounding environmental conditions. When charge accumulation surpasses a critical threshold, spark discharge can occur, initiating a dust explosion if the energy exceeds the dust cloud's minimum ignition energy [127]. Zhang et al. [128] reported that wood dust can reach electrostatic charge levels comparable to or even exceeding those of well-studied dusts such as coal and wheat flour. Moreover, the lower minimum ignition temperature and minimum ignition energy of wood dust make it more susceptible to ignition [129,130]. Despite these risks, research on the electrostatic charging behavior and associated hazards of wood dust remains limited. To address this knowledge gap, this study systematically investigates electrostatic charge generation and accumulation in common materials and processes used in wood-processing facilities. Specifically, we examine charge buildup on air hoses of different materials when rubbed against typical flooring surfaces using a vibration test device. A pneumatic conveying unit is employed to simulate airborne wood dust transport and evaluate charge accumulation from contact with both conductive and insulating surfaces. Additionally, we assess the charge decay behavior of charged wood dust to better understand dissipation rates on various surfaces under different environmental conditions. The objective of this chapter is to examine how specific materials and operational conditions, with a particular focus on air-blow cleaning practices, contribute to electrostatic charging. It also aims to identify scenarios and material combinations that pose the greatest hazard. These insights are essential for developing targeted mitigation strategies that can reduce the risk of electrostatic discharge and ultimately improve fire and explosion safety in wood-processing facilities. 113 5.2 Experimental Setup 5.2.1 Vibration tests of air hose charging The experimental apparatus for vibration tests is illustrated in Figure 5.1. A vibratory feeder equipped with a stainless-steel tray (Eriez Model 15A) served as the charging device. The tray was electrically insulated from the rest of the unit to prevent unintended charge leakage. To simulate conditions encountered during air-blow cleaning operations, where air hoses are dragged across different floor surfaces, various materials including bare stainless steel, fir wood, painted fir wood, and cement board, were affixed to the tray surface. The vibration amplitude, tray length, and angle of inclination were kept constant throughout all tests to ensure consistent mechanical conditions. An electrometer (Keithley Model 6514) was connected to a Faraday cup for measuring the electrostatic charge generated on test samples through contact with the vibrating surface. Three types of air hoses, polyvinyl chloride (PVC), natural rubber, and abrasion-resistant polyurethane (PU), were tested in this study. All hoses had an identical outer diameter of 15 mm and were cut into uniform 20-mm segments. The surface roughness of each hose material was characterized using optical coherence tomography (Thorlabs TELESTO-II). For each test, hose segments were first placed in the Faraday cup to measure any pre-existing charge. Subsequently, the segments were positioned at the closed end of the vibrating tray and transported to the open end by vibration, after which they fell into the Faraday cup. The net charge acquired by the hose segments was recorded, and the charge-to-length ratio was calculated by dividing the total measured charge by the combined length of the tested segments (e.g., 50 pieces × 20 mm = 1 m total length). This ratio, expressed in μC/m, represents the electrostatic charge density generated per unit length of hose during sliding over the vibrating surface. Each experimental condition was repeated seven times 114 to ensure reproducibility. The average charge-to-length ratio for each hose material was calculated from these repeated measurements, and error bars represent the standard deviation across the seven runs. Figure 5.1 Schematic (top) and photograph (bottom) of the vibration test unit. 5.2.2 Pneumatic conveying tests A schematic of the pneumatic conveying unit is presented in Figure 5.2. Two coiled tubes (one made of copper and the other of PVC) were employed as tribo-chargers to investigate the electrostatic charging behavior of airborne wood dust. Both tubes had identical dimensions: an inner diameter of 3.75 mm, a total length of 5 m, a coil diameter of 0.2 m, 115 and eight coils. The copper tube was electrically insulated from the rest of the apparatus and could be either grounded or left ungrounded, depending on the experimental condition. Oil-free compressed air was used as the conveying medium and was maintained at approximately 3% relative humidity (RH) and a temperature of 20 ± 2 °C. Humidity level could be adjusted by varying the fraction of air passed through a water-bath humidifier. The RH of the gas stream entering the coiled tubes was continuously monitored using a humidity meter (Vaisala HMT238). Air flow rates were regulated between 65 and 102 L/min, corresponding to exit air velocities ranging from 4-6 m/s. Several types of wood dust were tested in this study. Douglas fir (Pseudotsuga menziesii) and pine (Pinus spp.) sawdust were provided by Tolko Industries Ltd., while hemlock (Tsuga spp.) sawdust was sourced from Teal Jones Group. All sawdust samples were sieved for 15 minutes using a mechanical sieving device to separate the particles into the size ranges of <150 μm, 150–300 μm, 300–425 μm, 425–600 μm, and >600 μm. The sieved samples were oven-dried overnight at 100 °C and subsequently stored in sealed containers until testing. Initial charge level of the sawdust particles were measured in a Faraday cup prior to each test. The initial charge magnitude was at least two orders of magnitude lower than the charges measured after conveying, confirming that initial charge had negligible influence on the final results. In each experiment, approximately 3.5 g of sawdust was injected into the compressed air stream via a sample injector. The particles were conveyed through the coiled tube and collected in a filtered Faraday vessel, as shown in Figure 5.3. The vessel was connected to the electrometer to measure the net electrostatic charge of the transported particles. Unless otherwise specified, all tests were performed at 3% RH and an air velocity of 5 m/s. To ensure repeatability, each test was conducted at least seven times. The charge per unit mass (Q/m) of the sawdust was calculated based on the measured charge and the 116 mass of the sample, with error bars representing the standard deviation across repeated runs. Between tests, the tubes were thoroughly cleaned using water and compressed air to prevent any residual sawdust from affecting subsequent measurements. Figure 5.2 Schematic of the pneumatic conveying unit. Figure 5.3 Photographs showing the copper coiled tube used for pneumatic conveying (left) and the Faraday vessel for charge measurement (right). 117 5.2.3 Charge decay tests The charge decay behavior of hemlock wood sawdust in the 150–300 μm size range was investigated using a contactless Faraday cup method [131,132]. Pre-charged particles were placed on a sample holder positioned inside the inner cup of the Faraday cup, electrically isolated by a Teflon spacer beneath the holder, as illustrated in Figure 5.4. When the particles were dropped into the cup, the electrostatic charge on the particles induced an equal but opposite charge on the inner cup, which was detected by the connected electrometer. As the charge on the sample gradually dissipated, the induced charge on the inner cup diminished accordingly, allowing real-time monitoring of the decay process. To simulate typical surfaces encountered in wood-processing facilities, four sample holders made of different materials were fabricated. The details of these holders are summarized in Table 5.1. In each test, 15 g of sawdust particles were tribo-charged using the vibratory feeder under fixed vibration amplitude, tray length, and tray angle. The charged particles were then deposited onto the sample holder and left undisturbed for a 10-hour monitoring period, during which the charge level was continuously recorded by the electrometer. The acquired data were used to plot decay curves of the residual charge as a percentage of the initial value. Exponential decay trendlines were fitted to these curves to estimate the decay half-life and the residual charge remaining over time. To investigate the effect of moisture content on charge dissipation, two batches of sawdust were prepared. One batch was oven-dried overnight at 100 °C to reduce the moisture content to below 1% by weight. The second batch, after drying, was subsequently exposed to ambient air for 10 days, allowing moisture absorption until it reached and stabilized at approximately 15%. All moisture contents reported in this study are expressed on a dry basis and were determined by oven-drying at 100 °C for 24 hours. 118 Table 5.1 Details of the sample holders used in the charge decay tests. Simulated surface Sample holder material Grounded Ungrounded metal Aluminum No Grounded metal Aluminum Yes Painted metal Aluminum painted with an oil-base enamel paint Yes Concrete/Cement Portland cement Yes Figure 5.4 Schematic of the charge decay test unit. 5.3 Results and Discussion 5.3.1 Vibration tests of air hoses The results for the charge densities acquired by PU, PVC, and rubber hoses after being rubbed against different vibrating surfaces are presented in Figure 5.5. Among the tested hoses, the PU hose consistently exhibited the lowest charge density across all surfaces, while the rubber hose exhibited the highest. These differences can be partly explained by variations in EWF among the materials and are further supported by their positions in the published triboelectric series [41,133,134]. PVC, PU, and rubber are all positioned on the negative side of the series, whereas the vibrating surface materials lie closer to the center. Thus, all hoses would be expected to acquire negative charges when rubbed against these surfaces, which aligns with our observations. Based on its position furthest on the negative side of the triboelectric series, PVC would be expected to exhibit the highest charge density ElectrometerSample HolderCharged SawdustFaraday Cup119 among the hoses. However, the experimental results show that the rubber hose consistently accumulated higher charge densities than the PVC hose. This discrepancy may be attributed to the rubber hose’s rougher surface finish compared to that of the PVC hose (see Appendix Figure A.2). A rougher surface can increase the actual contact area during rubbing, leading to enhanced triboelectric charge generation. Another contributing factor is the material density. The weights of 20-mm segments of PU, PVC, and rubber hoses were measured at 2.1 g, 3.0 g, and 4.8 g, respectively. The higher density of the rubber hose likely increases the normal force exerted at the contact points during vibration, resulting in greater frictional energy and thus more significant charge generation. This observation aligns with previous studies showing that charge generation increases with both rubbing velocity and contact force [135]. Conversely, the lighter weight of the PU hose, combined with its smoother surface and occasional jumping motions observed during the vibration tests, reduced the effective contact area and limited charge buildup. Among the four vibrating surfaces, the wood surface generally produced lower charge densities, while the cement surface produced the highest. Again, this difference can be attributed both to variations in the EWF differences between the surfaces and the hoses, and to differences in surface roughness. The cement sheet displayed a notably rougher surface compared to the other materials, increasing the effective contact area and thereby promoting higher triboelectric charging. Interestingly, although painted wood surfaces are typically smoother and thus expected to produce lower charge densities than unpainted wood, the experimental results showed the opposite trend. This could be due to the specific chemical composition of the acrylic paint used, which might increase the EWF difference between the hoses and the painted surface, enhancing charge transfer during contact. 120 Figure 5.5 Charge-to-length ratios of PU, PVC, and rubber air hoses after rubbing against different vibrating surfaces. 5.3.2 Pneumatic conveying tests 5.3.2.1 Effect of particle size The charge densities (Q/m) of pine wood sawdust particles across different size ranges conveyed by compressed air are presented in Figure 5.6. The results show that the charge density is inversely proportional to particle size, aligning with findings from our previous work [128]. This behavior can be attributed to the increase in specific surface area involved in triboelectric charging as particle size decreases. Consequently, particles of the same weight but smaller size possess a higher surface-area-to-mass ratio, leading to greater charge accumulation per unit mass, as confirmed by the experimental data. 121 Figure 5.6 Comparison of charge per unit mass (Q/m) for pine wood sawdust particles of different size ranges conveyed by compressed air. 5.3.2.2 Effect of air velocity and humidity In this section, pine wood sawdust in the size range of 150–300 μm was conveyed through compressed air at different velocities. The Q/m values obtained by the sawdust at gas velocities of 4 m/s, 5 m/s, and 6 m/s were measured as 194 μC/kg, 268 μC/kg, and 419 μC/kg, respectively, indicating that the charge generated on the particles increased with rising air velocity. Pneumatic conveying involves continuous contact and separation between particles and the tube wall, as well as friction from particles rubbing against the wall. The observed increase in charge levels can be attributed, on one hand, to higher gas velocities enhancing the frequency of particle–wall collisions within the tube, thereby increasing triboelectric charging due to more frequent contact events. On the other hand, higher gas velocities also raise the kinetic energy of particles during collisions, leading to greater frictional forces and, consequently, higher levels of triboelectric charge generation on the contact surfaces. To investigate the effect of in-line air humidity on the charging behavior of pine wood sawdust, two sets of tests were conducted: one at 3% RH and another at 70% RH. Results revealed that the group tested at 70% RH exhibited a 17% reduction in charge density. As 122 discussed in previous chapters, humidity can influence both charge generation and dissipation. In this case, the primary effect of higher humidity appears to be enhanced charge dissipation rather than increased charge generation. This outcome is consistent with the unique properties of wood. Although dry wood typically exhibits high electrical resistivity in the range of 10¹⁴ to 10¹⁶ Ω·m, wood is far more prone to moisture adsorption than materials such as HDPE or glass, owing to its porous, hydrophilic fiber structure. The presence of hydroxyl groups and an extensive internal surface area enable wood to adsorb moisture more readily from the surrounding air, dramatically lowering its electrical resistivity and facilitating faster charge dissipation. Literature reports indicate that wood’s resistivity can decrease by several orders of magnitude—from around 10¹⁴–10¹⁶ Ω·m in dry conditions to as low as 10³–10⁴ Ω·m at fiber saturation [136]. However, the reduction in charge density observed in this study was relatively modest. This may be attributed to the short residence time of particles in the conveying system, which might have limited the extent of moisture uptake during the process. A longer conveying duration could potentially allow more substantial moisture adsorption and, consequently, a greater reduction in electrostatic charge levels. 5.3.2.3 Effect of coil material and grounding The measured Q/m values for pine wood sawdust (150–300 μm) conveyed through different coil materials were 268 μC/kg in the PVC tube, –94 μC/kg in the ungrounded copper tube, and –138 μC/kg in the grounded copper tube. These results reveal two key findings: first, the charge polarity of the sawdust particles reversed from positive in the PVC tube to negative in the copper tubes; second, the overall charge magnitude was significantly lower in the copper tubes than in the PVC tube. 123 These observations can be attributed primarily to differences in the EWF between wood particles and the tube materials. As discussed in previous chapters, the extent and polarity of triboelectric charge transfer are strongly influenced by the EWF difference between contacting materials. Since PVC and copper occupy distinct positions in the triboelectric series, wood particles acquire not only different charge magnitudes but also opposite polarities when conveyed through these materials. The lower charge magnitude observed in copper tubes reflects the smaller EWF difference between wood and copper compared to that between wood and PVC. Interestingly, the grounded copper tube resulted in higher Q/m values than the ungrounded copper tube, contrary to the traditional expectation that grounding reduces electrostatic charging. Grounding provides a pathway for charge dissipation, effectively keeping the tube wall near electrical neutrality. However, as discussed in section 3.3.4, for insulating particles like HDPE and wood dust, charge transfer to the grounded wall remains limited because the particles are less prone to lose their acquired charges due to high particle–particle contact resistance (Rp). Meanwhile, in the ungrounded copper tube, residual charges of opposite polarity may accumulate on the wall, creating an opposing electric field that suppresses further charge transfer from the particles. Thus, in the ungrounded case, this counteracting electric field likely reduces the net charge acquired by the conveyed particles. This explanation mirrors the conclusions drawn in earlier chapters regarding the limited influence of grounding in systems involving dielectric particles. 5.3.2.4 Effect of wood species Figure 5.7 presents the Q/m values of sawdust particles from different wood species in the size ranges of 150–300 μm and <150 μm. In both size categories, hemlock particles exhibited the highest charge-to-mass ratios, while Douglas fir particles showed the lowest. 124 Several factors may contribute to the species-dependent differences in triboelectric charging behavior. First, slight differences in chemical composition among wood species could affect the EWF, thereby increasing or decreasing the extent of charge transfer when particles contact the tube wall. Second, morphological differences also play an important role. Visual observations revealed that hemlock particles tended to be longer and slimmer than the more rounded Douglas fir and pine particles. This elongated shape could enhance aerodynamic mobility during pneumatic conveying, potentially leading to more frequent particle–wall collisions and higher charge accumulation. Additionally, differences in particle shape may influence the effective contact surface area available during collisions. The variation in particle shape could be affected by factors such as material density, fiber length, and fracture behavior during particle formation. Third, as shown in Figure 5.8, hemlock particles were the least prone to agglomeration, remaining mostly as individual particles that could move freely and maximize contact with the tube wall. In contrast, Douglas fir particles tended to clump together into large agglomerates, significantly reducing their available contact surface area and, consequently, their ability to acquire charge. Pine particles exhibited moderate agglomeration, forming smaller clusters. Overall, stronger agglomeration leads to less effective particle–wall interaction, thereby diminishing triboelectric charging efficiency. In summary, while the experimental results reveal clear differences in charging behavior among wood species, the underlying mechanisms remain complex and are likely governed by a combination of chemical composition, density, fiber structure, and other intrinsic material properties. Further research into the material characteristics of different wood species is warranted to clarify their roles in triboelectric charge generation. 125 Figure 5.7 Charge per unit mass (Q/m) of sawdust from different wood species, conveyed through a PVC tube, in size ranges of 150–300 μm (left) and <150 μm (right). Figure 5.8 Photographs of sawdust particles from different wood species in the <150 μm size range. 5.3.2.5 Sawdust from different wood-processing equipment In this section, four sawdust samples were collected from a local sawmill near different pieces of wood-processing equipment, all sourced from the same batch of hemlock logs on the same day. Table 5.2 lists the sampling locations, and Table 5.3 summarizes their mass-based particle size distributions. Sample A, collected near a wood trimmer, contained a large fraction of coarse particles, which may have been influenced by factors such as saw blade type, cutting speed, or 126 differences between heartwood and sapwood. Samples B and C exhibited similar particle size distributions, whereas Sample D, gathered from the top surfaces of beams and machinery, consisted predominantly of fine dust that is readily suspended in air and may originate from various wood-processing operations. The darker color observed in Samples B and D suggests a greater presence of wood bark or potential contaminants from processing equipment. Figure 5.9 illustrates the Q/m values measured for these samples in pneumatic conveying tests. All samples were oven-dried overnight but were not sieved prior to testing. As discussed earlier, particle size has a substantial influence on charge density. Sample A exhibited the lowest Q/m among the four, likely due to its higher proportion of coarse particles. Interestingly, despite its fine particle size, Sample D did not show a significantly higher charge density compared to Samples B and C. This indicates that other factors, such as particle shape, agglomeration behavior, or the presence of processing-related contaminants, may also affect triboelectric charging. Figure 5.10 presents Q/m results for the same samples after sieving into two narrower size fractions, ensuring that any differences observed were not due to variations in particle size distribution. The results show that Sample C, collected near the vertical edger, consistently exhibited lower charge densities in both size ranges than the other samples. No visible differences in particle shape or agglomeration were observed that might explain this outcome. However, differences in wood-processing equipment and operating conditions could affect the material properties of the sawdust. For example, distinctions between heartwood and bark, or the presence of contaminants like oil or rust introduced by specific machinery, may contribute to variations in triboelectric charging behavior. Overall, these observations suggest that the relationship between wood-processing operations, sawdust material properties, and triboelectric charging is complex and shaped 127 by multiple interacting factors. To fully understand these interactions, further studies involving detailed material characterization and analysis of processing conditions are necessary. Table 5.2 Summary of hemlock wood sawdust samples collected near different wood-processing equipment. Table 5.3 Mass-based particle size distributions of hemlock wood sawdust collected near different wood-processing equipment. Size (μm) Sample A Sample B Sample C Sample D <150 5% 9% 11% 23% 150-300 19% 32% 30% 44% 300-425 21% 22% 20% 22% 425-600 48% 28% 29% 9% >600 7% 9% 10% 1% Figure 5.9 Charge per unit mass (Q/m) of unsieved hemlock wood sawdust collected at different locations within a wood-processing facility. Sample Collection Location Color A Near wood trimmer Light B Near dust screen Dark C Near vertical edger (VDA) Light D Beams & equipment surfaces Dark 128 Figure 5.10 Charge per unit mass (Q/m) of hemlock wood sawdust collected near different wood-processing equipment, for size ranges of 150–300 μm and <150 μm. 5.3.3 Charge decay Figure 5.11 shows charge decay curves and fitted trendlines for pre-dried hemlock wood sawdust placed on different sample holders. The results indicate that the charges on the sawdust decay faster when resting on a grounded metal holder than on an ungrounded one. In addition to the metal holders, grounded cement and painted metal surfaces were evaluated to simulate common materials found in wood-processing facilities, such as concrete floors and painted structural beams. As shown in Figure 5.11, charge decay on these surfaces is faster than on ungrounded metal, although still slower than on grounded bare metal. Table 5.4 summarizes the half-life values derived from exponential fits to the decay curves, along with the percentages of charge remaining on the sawdust after 10 and 120 hours. Unlike in the previous fluidization experiments, the grounded metal holder did indeed accelerate charge dissipation from the contacting particles, although only to a limited extent. As discussed earlier, the high surface resistivity of dry sawdust hinders efficient charge 129 transfer from the particle bulk to the grounded holder. Under fluidization or pneumatic conveying conditions, particles have only brief contact with the wall, so charge dissipation through the wall is expected to be minimal. Moreover, in this decay test, both the grounded and ungrounded holders were electrically neutral because the particles were pre-charged upstream in the vibrating feeder. This means that the electric field between the particles and the holders was comparable in both cases, and there was no opposing field that might suppress charge transfer as seen in isolated columns during fluidization. Previous studies have shown that the rate of charge decay depends primarily on the resistance along the conductive path to ground [85]. The results of the charge decay tests align well with this principle. The ungrounded metal holder exhibits the highest resistance to ground, due to the high resistivity of the Teflon spacer beneath it, while the grounded metal holder offers the lowest resistance. Among the other two holders, cement has a slightly lower resistivity than painted metal, where the enamel paint contributes to higher surface resistivity. Consequently, charge decay on cement surfaces occurs slightly faster than on painted metal. However, Table 5.4 highlights that charge dissipation remains slow overall. Even under the best conditions tested, sawdust retains more than 50% of its initial charge after 120 hours (five days). This slow dissipation occurs because, in stationary sawdust beds, only the bottom layer of particles is in direct contact with the holder, while charges on upper layers face significant resistance in transferring downward. The primary bottleneck for charge decay is the high particle–particle contact resistance (Rp), which hinders effective charge flow between particles. As noted earlier, wood-processing operations such as sawing, sanding, and screening generate triboelectric background charges on sawdust. If these background charges decay slowly, subsequent cleaning activities like floor or equipment blowdowns can add further 130 charges, elevating the total charge level and increasing the risk of electrostatic discharges. Therefore, effective dissipation of background charges is essential for mitigating electrostatic hazards during cleaning operations. Improving charge dissipation through grounded surfaces requires strategies to lower Rp, such as increasing the moisture content of sawdust, which has been shown to enhance charge dissipation in section 5.3.2.2. These results show that sawdust particles are highly sensitive to relative humidity, particularly in terms of charge dissipation. When water vapor is present, it adsorbs onto the surface of sawdust, forming a thin, slightly conductive layer that lowers surface resistivity and promotes faster charge decay. Table 5.5 summarizes the half-life values and residual charge percentages after 10 and 120 hours for sawdust samples with 1% and 15% moisture content, placed on both ungrounded and grounded metal surfaces. The data confirm that higher moisture content generally accelerates charge decay. However, the improvement is modest in the ungrounded tests, where the percentage of charge remaining after 120 hours drops only slightly from 67% to 60%, and the half-life shortens from 206 to 162 hours. This limited effect is due to the persistently high resistance of the path to ground in the ungrounded configuration, which still impedes effective charge flow despite reduced particle surface resistivity. In contrast, for grounded metal holders, increasing the sawdust moisture content significantly enhances charge decay. The charge remaining after 120 hours decreases substantially from 53% for the dry sample to 32% for the humidified sample, and the half-life shortens from 130 to 72 hours. These results indicate that when a conductive path to ground is available, moisture plays a more pronounced role in facilitating charge dissipation from the particle surfaces. This trend suggests that further increasing the moisture content of sawdust could potentially accelerate charge decay even more, offering a practical strategy for mitigating electrostatic hazards in wood-processing environments, especially where grounded surfaces are present. 131 Figure 5.11 Charge decay curves and fitted exponential decay trendlines for pre-dried hemlock sawdust placed on different sample holders. Table 5.4 Predicted half-life values and percentages of charge remaining on dry hemlock sawdust placed on different sample holders after 10 and 120 hours. Sample holder Charge remaining after 10 hours Charge remaining after 120 hours Half-life of charge decay (hours) Ungrounded metal 97% 68% 208 Painted metal 96% 65% 196 Cement 95% 57% 147 Grounded metal 95% 53% 130 Table 5.5 Predicted half-life values and percentages of charge remaining on hemlock sawdust with different moisture contents (MC) placed on grounded and ungrounded metal holders after 10 and 120 hours. Sawdust and holder combinations Charge remaining after 10 hours Charge remaining after 120 hours Half-life of decay (hours) 1% MC on ungrounded metal 97% 68% 208 15% MC on ungrounded metal 96% 60% 162 1% MC on grounded metal 95% 53% 130 15% MC on grounded metal 91% 32% 72 132 5.4 Summary In summary, this study provides critical insights into the mechanisms of charge generation and dissipation in wood dust and other materials commonly encountered in wood-processing facilities. These findings, together with the results from earlier chapters of this thesis, help build a comprehensive understanding of electrostatic phenomena in gas–solid systems and are pivotal for improving safety practices and mitigating the risks of fires and dust explosions in industrial wood-processing environments. The vibration tests revealed that the effective work function difference, unit weight, and surface roughness are key parameters influencing the charging behavior of air hoses. Although it is challenging in practice to select a single hose material with a minimal EWF difference relative to all types of floor materials present in wood facilities, a practical approach may involve choosing hose materials optimized for the dominant flooring type, such as concrete or wood. Moreover, among the hoses tested, polyurethane (PU) consistently generated the lowest charge density against all vibrating surfaces, mainly because of its smaller EWF difference, lower mass, and smoother surface finish, which reduce frictional energy and contact area. The pneumatic conveying experiments demonstrated that using lower air velocities can reduce charge generation on airborne wood dust, thereby lowering the risk of electrostatic discharges during cleaning operations. However, reduced air velocities may compromise cleaning efficiency, necessitating a balance between safety and operational performance. Increasing the moisture content in compressed air offers a practical alternative: our results show that conveying wood dust in humidified air (70% RH) resulted in a 17% reduction in charge density compared to dry air. This reduction could be even more significant with longer exposure times to humid air. Importantly, fine particles are found to acquire much 133 higher specific charges than coarser fractions, underlining the need for heightened vigilance when handling fine wood dust. Species-specific effects were also observed. Hemlock wood dust, for instance, exhibits higher charge densities than Douglas fir, highlighting the influence of chemical composition, fiber structure, and particle morphology on triboelectric behavior. Furthermore, different processing equipment can produce wood dust with varying particle sizes, fiber content, and chemical properties, leading to significant differences in charge accumulation. These observations align with earlier findings from Chapters 2 and 3, where particle material properties and size distributions were shown to critically influence electrostatic charging behavior. Consequently, tailored safety protocols should be developed for high-risk areas within wood-processing facilities, taking into account the specific materials and processes involved. Future systematic laboratory studies and field investigations are necessary to further elucidate these species- and equipment-dependent effects. Regarding charge dissipation, the results from the charge decay tests confirmed observations from earlier chapters: the dissipation rate is strongly influenced by the overall electrical resistance along the path to ground. Metal surfaces in wood-processing facilities should therefore be properly grounded to enable charge transfer from accumulated sawdust. Applying antistatic coatings on non-metal surfaces can also reduce surface resistivity and improve dissipation. However, even on grounded metal surfaces, dry wood dust exhibited slow charge decay because of its inherently high surface resistivity. Typically, only the bottom layer of particles is in direct contact with the grounded surface, restricting overall dissipation. This challenge mirrors the findings in Chapter 3, where grounding was shown to have limited effectiveness in dissipating charge from high-resistivity materials such as HDPE in fluidized beds. 134 Moisture, however, offers a practical way to improve charge dissipation under such conditions. Higher moisture content reduces surface resistivity of sawdust and accelerates charge decay. Thus, maintaining higher ambient relative humidity (e.g., during rainy weather or by introducing controlled humidification) can be a practical strategy to reduce static hazards during cleaning operations. Additionally, frequent cleaning may be necessary to prevent excessive accumulation of charged dust, especially on insulating surfaces. Overall, the combined insights from this chapter and the preceding experimental studies contribute valuable knowledge for developing effective electrostatic control strategies in wood-processing environments. Such strategies should integrate material selection, operational parameters (e.g., airflow velocity and humidity), and rigorous grounding protocols to mitigate the risk of electrostatic discharges and ensure safer industrial practices. 135 Chapter 6 Conclusions and Future Works 6.1 Conclusions Electrostatic charging in gas–solid fluidized beds is a well-recognized challenge in both research and industry. In commercial operations such as polyolefin polymerization, pharmaceutical manufacturing, and dust handling, uncontrolled electrostatics can lead to wall sheeting, agglomeration, and even process interruptions or safety hazards. While the fundamental mechanisms of triboelectric charging in particulate systems have been studied for decades, many industrial problems persist because the interplay between particle properties, gas-phase conditions, wall materials, and particle–particle interactions remains insufficiently understood. The primary objective of this thesis was to improve the understanding of electrostatic charging mechanisms in gas–solid fluidized beds, particularly regarding the influence of fluidizing gas properties and size-dependent charging behavior, and to use this knowledge as a basis for developing effective mitigation strategies for electrostatic-related issues such as reactor wall fouling, particle agglomeration, and hazardous discharge. The research began with a systematic investigation of gas effects on particle charging and wall fouling in gas–solid fluidized beds (Chapter 2). HDPE and glass beads were fluidized in acrylic and steel columns under varying gas types and humidity levels. For HDPE particles, the results showed that the saturation charge level was largely governed by the dielectric strength of the fluidizing gas. This was demonstrated by the close agreement between estimated and measured |Q/m| values across different gases. Wall material also played a secondary role, as conductive steel walls tended to allow a higher charge saturation level than insulating acrylic walls. In contrast, glass beads, due to their lower surface resistivity and smaller work function difference relative to the wall materials, did not reach breakdown-limited charge levels under most conditions. 136 Humidity introduced an additional layer of complexity to the charging behavior. For HDPE particles, increasing humidity slightly raised the saturation charge in acrylic columns but produced an intermediate maximum in steel columns. This pattern reflects the competing effects between enhanced charge transfer through H⁺ and OH⁻ ions in adsorbed water layers and increased dissipation via conductive water films on particle surfaces. For glass beads in acrylic columns, bulk charge density showed only weak sensitivity to humidity. However, humidified air substantially reduced wall fouling, despite minimal changes in the measured bulk charge level. This suggests that for more conductive and hydrophilic materials such as glass, humidity primarily enhances surface conductivity through adsorbed water layers and thereby accelerates charge dissipation, especially in entrained fine particles. Repeated tests in steel columns also revealed a puzzling polarity reversal of glass beads over time, shifting from net negative to net positive charge. This behavior is hypothesized to arise from bipolar charging effects in particles of wide size distributions—a mechanism examined in Chapter 4. We also evaluated wall fouling behavior from wall images to quantify surface coverage, combined with particle size analysis of adhered particles. Differences in wall fouling patterns further highlight the significance of electrostatics in fluidized beds. For HDPE, a strong correlation was found between saturation charge and wall coverage, confirming that electrostatic forces were the dominant adhesion mechanism under the tested conditions. Among all gases tested, argon produced the lowest saturation charge levels and the least fouling, owing to its low dielectric breakdown strength. For glass beads, fouling across all gases was dominated by highly charged entrained fines, while particles in the dense bed, particularly the larger ones, carried very low charge levels and thus contributed little to wall fouling. While humidity had little effect on HDPE fouling, it significantly reduced fouling in glass beads, reinforcing the material-dependent nature of electrostatic responses to gas-phase moisture. 137 Chapter 3 extended these findings through a systematic investigation of the temporal evolution of particle charging under different gas compositions, humidity levels, and wall grounding conditions. The results confirmed that triboelectric charging behavior is strongly material-dependent and governed by the interplay among charge generation, dissipation, and gas breakdown processes. For HDPE particles, charge accumulated progressively until reaching saturation levels set by the dielectric strength of the fluidizing gas. In contrast, glass beads exhibited much lower charge levels, with rapid stabilization controlled by the balance between charge generation and dissipation and independent of gas breakdown. Time-resolved measurements in both dry air and argon revealed nearly identical charge generation rates during the initial stages of fluidization, indicating that gas composition had little influence on the underlying charge generation mechanism. In contrast, the final saturation levels differed significantly, with argon yielding lower values due to its reduced dielectric strength. These results show that gas composition affects the saturation charge level primarily through its dielectric strength, while having minimal impact on the intrinsic rate of charge accumulation. Humidity effects in time-resolved measurements were again strongly material-specific. For hydrophobic, high-resistivity HDPE particles, moderate humidity accelerated charge build-up but left the final saturation level essentially unchanged, indicating enhanced charge generation without significant changes in charge dissipation or breakdown limit. In contrast, for hydrophilic, lower-resistivity glass beads, humidity lowered the final charge, primarily due to increased dissipation via water-assisted surface conduction to the column walls. Grounding experiments in fluidized beds showed that wall grounding had minimal influence on the charging of the tested particles. For both HDPE and glass beads, the charging trends and final saturation levels were nearly identical under grounded and isolated conditions. This suggests that high particle–particle contact resistance dominates 138 charge retention, making grounding alone ineffective for controlling electrostatic charging in high-resistivity particles in fluidized beds. Overall, the studies of Chapters 2 and 3 confirm that both gas composition and humidity play critical roles in governing electrostatic charge accumulation and wall fouling in fluidized beds, with direct implications for industrial gas–solid processes. The results demonstrate that selecting fluidizing gases with lower dielectric breakdown strength, such as argon, can effectively suppress charge buildup and reduce wall fouling. At the same time, controlling humidity offers an additional lever to tune charge dissipation pathways, especially for hydrophilic particles. Together, these approaches provide a simple and practical means of controlling electrostatics without requiring changes to other operational parameters in industrial processes. For certain gas-sensitive applications, such as commercial polyolefin production, complete replacement of the fluidizing gas would require major modifications to the existing process and equipment, which may not be economically viable. However, introducing a fraction of an inert gas with lower breakdown strength into the process gas could provide a practical means to mitigate electrostatic charging while avoiding large-scale process redesign. This provides a promising direction for future research, where mixed-gas systems (e.g., air–argon mixtures) can be systematically tested for their impact on particle charging and wall fouling. In contrast, for applications that are less sensitive to gas composition, such as pharmaceutical powder processing or pneumatic conveying processes, switching the fluidizing gas imposes minimal disruption to existing operations and can therefore be implemented more readily. In Chapter 4, we developed a novel experimental platform combining acoustic levitation with a parallel-plate electric field to investigate particle–particle charging mechanisms. 139 This approach proved reliable and simple to operate, and it allowed precise control of particle–particle interactions without wall interference. Collisions between particles of different materials produced strong and unidirectional charge transfer, driven by the EWF difference between the colliding surfaces. These results, consistent with triboelectric series predictions, confirmed the reliability of the experimental setup. In contrast, collisions between similar-sized HDPE particles yielded random polarity outcomes, indicating not only weaker charge transfer but also an unpredictable transfer direction among particles of the same material. When binary-sized HDPE particles were used, collisions consistently produced unidirectional charge transfer, with larger particles acquiring positive charge and smaller particles negative charge, in agreement with most previous studies. Pre-charging experiments further showed that when one particle carried a substantial initial charge, this size-dependent tendency could be overshadowed, with charge redistributed to reduce the initial imbalance. These findings shed new light on several phenomena observed in the fluidized bed studies of earlier chapters. They clarify how particles that gain substantial charge from wall contact can redistribute it to other particles through subsequent collisions, thereby producing a more uniform charge distribution across the bed. They also help explain unique behaviors in systems with wide particle size distributions, such as polarity reversal, particularly under conditions where interparticle charging dominates particle–wall interactions. From an industrial perspective, these results carry several important implications. In large-scale commercial fluidized-bed reactors, wall effects become relatively weaker with increasing reactor size, thereby amplifying the role of interparticle charging. Under such conditions, bipolar charging may cause particles of different sizes to acquire opposite polarities. When fine particles are preferentially entrained from the bed, their removal can leave behind a net charge imbalance, increasing the risk of electrostatic hazards. Moreover, 140 charge separation between large and small particles can promote particle agglomeration, which in some processes may significantly compromise product quality. At the same time, agglomeration can also be exploited beneficially: deliberately engineered binary-sized mixtures with optimized size ratios may mitigate wall fouling by promoting interparticle clustering and reducing the mobility of fine particles toward the freeboard. Taken together, these observations suggest that bipolar charging requires careful management in industrial systems, particularly in operations where particle size distributions are broad and wall effects are minimal. Chapter 5 presents a case study on dust handling in wood-processing facilities, offering important insights into the mechanisms of charge generation and dissipation in materials commonly encountered in wood processing. Combined with the understanding developed in earlier chapters, these results provide practical guidance for mitigating electrostatic hazards and reducing the risk of fires and dust explosions in industrial environments. The vibration tests, designed to mimic the repeated contact and sliding of flexible air hoses over flooring surfaces during cleaning, showed that the EWF difference, material density, and surface roughness are key factors controlling charge generation. Among the tested materials, polyurethane hoses generated the lowest charge density due to their smooth surfaces and relatively low mass, which together reduced frictional energy and effective contact area. While no single hose material can minimize EWF differences for all flooring types, selecting hoses that best match the predominant floor material in a facility can help mitigate static hazards during cleaning. The pneumatic conveying experiments showed that adjusting operating parameters, such as air velocity and humidity, can effectively reduce dust charging. Fine wood dust consistently carried much higher specific charges than coarse particles, echoing the fines-driven fouling observed in fluidized beds. We also found that dust from certain wood 141 species and from some types of processing equipment tended to accumulate much higher charges than others. Such high-risk dust types may therefore require tailored mitigation strategies, including targeted humidity control, optimized airflow settings, and enhanced cleaning protocols in critical areas. The charge decay tests confirmed earlier observations that the dissipation rate is strongly influenced by the overall electrical resistance along the path to ground. Dry wood dust retained high charge levels for long periods even on grounded metal, mirroring the negligible effect of grounding observed for HDPE particles in fluidized bed experiments. These persistent background charges decay slowly, and subsequent cleaning activities, such as floor sweeping or equipment blowdowns, can add further charge to the system, raising the total charge level and increasing the likelihood of electrostatic discharges. Test results also showed that humid conditions greatly accelerated dissipation, indicating that maintaining higher ambient humidity in processing areas is an effective and practical way to reduce static hazards during wood dust handling and cleaning. Taken together, the work presented in this thesis provides a coherent framework for understanding and controlling electrostatics in fluidized particle systems. The findings demonstrate that electrostatic charging is governed by the interplay of gas dielectric strength, particle electrical resistivity, surface hydrophobicity, EWF differences, and particle size distribution, with secondary effects from wall material and conductivity. The mechanistic understanding developed here enables targeted, material-specific mitigation strategies, ranging from fluidizing gas selection and humidity control to engineered particle size distributions, that can be implemented to improve safety, reduce fouling, and maintain product quality in industrial gas–solid systems. Overall, the integrated insights from this thesis bridge fundamental electrostatic science and practical engineering solutions, advancing both academic understanding and industrial application. 142 6.2 Future Works While the present study has provided substantial advances in understanding electrostatics in fluidized beds, several aspects still require further investigation to fully bridge the gap between laboratory findings and industrial applications. One promising direction for future research is the study of mixed-gas systems as a practical means of electrostatic control. Completely replacing the fluidizing gas in industrial fluidized-bed operations would require substantial modifications to reactor design and process operation, making it economically impractical. In such cases, introducing a fraction of an inert gas with a lower breakdown strength into the fluidizing gases could offer a feasible solution to electrostatic charging mitigation while avoiding large-scale process redesign. Systematic studies on mixed-gas environments, for example, air–argon mixtures, could provide valuable insights into how partial substitution influences charge levels and wall fouling, thereby assessing the effectiveness of this strategy as a practical electrostatic-control measure. The acoustic levitation platform, used here primarily for qualitative determination of particle charge polarity after collisions, could be extended for quantitative analysis. For example, integrating a high-speed camera to track particle trajectories under a calibrated electric field would allow estimation of charge magnitude. With quantitative charge measurements, it would become possible to systematically investigate how different particle size ratios influence the extent of charge separation. Such analysis would provide deeper insights into the mechanisms of size-dependent bipolar charging and clarify how particle size distribution governs overall charge dynamics in particulate systems. Given the strong influence of humidity on particle charging observed in Chapter 2 for HDPE particles, incorporating the acoustic levitation system into a humidity-controlled 143 environment would allow direct examination of humidity effects on size-dependent bipolar charging. Such tests could critically validate the hypothesis that adsorbed ions serve as the primary charge carriers during dielectric–dielectric contact. Confirming this ion-based process would significantly deepen our understanding of how ambient humidity and surface chemistry govern triboelectric charging for dielectric materials. In commercial-scale fluidized beds, reduced wall-to-particle interactions shift the dominant charge generation pathway toward interparticle collisions. Under such conditions, size-dependent bipolar charging may play a larger role in determining the bed’s overall net charge. This was demonstrated in Chapter 2, where polarity reversal occurred for glass beads, likely due to weak wall charging and low net particle charge, making bipolar effects more important. Future studies should investigate whether adjusting particle size distributions in large-scale systems, where wall effects are inherently weak, can serve as a deliberate strategy to lower the overall charge levels in the bed. Bipolar charging between particles of the same material can create charge separation and long-range electrostatic forces, driving particle clustering. In Chapter 2, particles responsible for wall fouling were consistently smaller than the average in-bed particle size, suggesting that fines are the primary contributors to fouling. Building on this, future studies could investigate whether deliberately engineered binary sized mixtures with optimized size ratios can mitigate wall fouling by promoting interparticle clustering and reducing the mobility of fine particles toward the freeboard or the wall. Through these research directions, future studies can deepen our understanding of electrostatic mechanisms in particulate systems, strengthen the predictive capability of charging models, and develop robust, cost-effective strategies for electrostatic control. Such advances will ultimately support safer and more efficient operation across a broad range of industrial gas–solid fluidization processes. 144 References [1] J.R. Grace, Introduction, History, and Applications, Essentials of Fluidization Technology (2020) 1–9. https://doi.org/https://doi.org/10.1002/9783527699483.ch1. [2] P. Mehrani, M. Murtomaa, D.J. Lacks, An overview of advances in understanding electrostatic charge buildup in gas-solid fluidized beds, J Electrostat 87 (2017) 64–78. https://doi.org/10.1016/j.elstat.2017.03.005. [3] F. Fotovat, X.T. Bi, J.R. Grace, Electrostatics in gas-solid fluidized beds: A review, Chem Eng Sci 173 (2017) 303–334. https://doi.org/10.1016/j.ces.2017.08.001. [4] G. Hendrickson, Electrostatics and gas phase fluidized bed polymerization reactor wall sheeting, Chem Eng Sci 61 (2006) 1041–1064. https://doi.org/10.1016/j.ces.2005.07.029. [5] Y. Cheng, E.W.C. Lim, C.H. Wang, G. Guan, C. Fushimi, M. Ishizuka, A. Tsutsumi, Electrostatic characteristics in a large-scale triple-bed circulating fluidized bed system for coal gasification, Chem Eng Sci 75 (2012) 435–444. https://doi.org/10.1016/J.CES.2012.03.040. [6] W.B. Zhang, C. Wang, Y.L. Wang, Parameters selection in the cross-correlation based velocimetry using circular electrostatic sensors, IEEE Trans. Instrum. Meas. 59 (2010) 1268. [7] X. Liu, J. Kolehmainen, I. Nwogbaga, A. Ozel, S. Sundaresan, Effect of particle size on tribocharging, Powder Technol 375 (2020) 199–209. https://doi.org/10.1016/j.powtec.2020.07.078. 145 [8] A.H. Park, H. Bi, J.R. Grace, Reduction of electrostatic charges in gas-solid fluidized beds, Chem Eng Sci 57 (2002) 153–162. https://doi.org/10.1016/S0009-2509(01)00352-9. [9] D.J. Lacks, R. Mohan Sankaran, Contact electrification of insulating materials, J Phys D Appl Phys 44 (2011). https://doi.org/10.1088/0022-3727/44/45/453001. [10] D.J. Lacks, T. Shinbrot, Long-standing and unresolved issues in triboelectric charging, Nat Rev Chem 3 (2019) 465–476. https://doi.org/10.1038/s41570-019-0115-1. [11] S. Matsusaka, H. Maruyama, T. Matsuyama, M. Ghadiri, Triboelectric charging of powders: A review, Chem Eng Sci 65 (2010) 5781–5807. https://doi.org/10.1016/j.ces.2010.07.005. [12] D. Carter, C. Hartzell, Effect of mixture properties on size-dependent charging of same-material dielectric grains, J Electrostat 107 (2020) 103475. https://doi.org/10.1016/j.elstat.2020.103475. [13] J. Wu, H.T. Bi, Addition of fines for the reduction of powder charging in particle mixers, Advanced Powder Technology 22 (2011) 332–335. https://doi.org/10.1016/j.apt.2010.04.009. [14] H. Wang, F. Fotovat, X.T. Bi, J.R. Grace, Tribo-charging of binary mixtures composed of coarse and fine particles in gas–solid pipe flow, Particuology 43 (2019) 101–109. https://doi.org/10.1016/j.partic.2018.07.001. [15] A. Giffin, P. Mehrani, Effect of gas relative humidity on reactor wall fouling generated due to bed electrification in gas-solid fluidized beds, Powder Technol 235 (2013) 368–375. https://doi.org/10.1016/j.powtec.2012.10.037. 146 [16] L. Yao, H.T. Bi, A.H. Park, Characterization of electrostatic charges in freely bubbling fluidized beds with dielectric particles, J Electrostat 56 (2002) 183–197. https://doi.org/10.1016/S0304-3886(02)00062-1. [17] A. Sowinski, A. Mayne, P. Mehrani, Effect of fluidizing particle size on electrostatic charge generation and reactor wall fouling in gas-solid fluidized beds, Chem Eng Sci 71 (2012) 552–563. https://doi.org/10.1016/j.ces.2011.11.031. [18] P. Mehrani, H.T. Bi, J.R. Grace, Electrostatic charge generation in gas-solid fluidized beds, J Electrostat 63 (2005) 165–173. https://doi.org/10.1016/j.elstat.2004.10.003. [19] T.A. Alsmari, J.R. Grace, X.T. Bi, Effects of particle properties on entrainment and electrostatics in gas–solid fluidized beds, Powder Technol 290 (2016) 2–10. https://doi.org/https://doi.org/10.1016/j.powtec.2015.06.074. [20] F. Salama, D. Song, P. Mehrani, Characterizing Electrostatic Charges in High-Pressure Gas- Solid Fluidized Beds : Experimental Design and Preliminary Results, in: Proceedings of the 14th International Conference on Fluidization, 2013. [21] W.O. Moughrabiah, J.R. Grace, X.T. Bi, Effects of pressure, temperature, and gas velocity on electrostatics in gas-solid fluidized beds, Ind Eng Chem Res 48 (2009) 320–325. https://doi.org/10.1021/ie800556y. [22] K.M. Forward, D.J. Lacks, R. Mohan Sankaran, Particle-size dependent bipolar charging of Martian regolith simulant, Geophys Res Lett 36 (2009) 1–5. https://doi.org/10.1029/2009GL038589. 147 [23] H. Zhao, G.S.P. Castle, I.I. Inculet, A.G. Bailey, Bipolar charging of poly-disperse polymer powders in fluidized beds, IEEE Trans Ind Appl 39 (2003) 612–618. https://doi.org/10.1109/TIA.2003.810663. [24] V. Lee, S.R. Waitukaitis, M.Z. Miskin, H.M. Jaeger, Direct observation of particle interactions and clustering in charged granular streams, Nat Phys 11 (2015) 733–737. https://doi.org/10.1038/nphys3396. [25] W.R. Harper, Contact and frictional electrification, Clarendon Press, Oxford, 1967. file://catalog.hathitrust.org/Record/001484523. [26] J. Lowell, A.C. Rose-Innes, Contact electrification, Adv Phys 29 (1980) 947–1023. https://doi.org/10.1080/00018738000101466. [27] D.J. Lacks, A. Levandovsky, Effect of particle size distribution on the polarity of triboelectric charging in granular insulator systems, J Electrostat 65 (2007) 107–112. https://doi.org/10.1016/j.elstat.2006.07.010. [28] K.-E. Byun, Y. Cho, M. Seol, S. Kim, S.-W. Kim, H.-J. Shin, S. Park, S. Hwang, Control of Triboelectrification by Engineering Surface Dipole and Surface Electronic State, ACS Appl Mater Interfaces 8 (2016) 18519–18525. https://doi.org/10.1021/acsami.6b02802. [29] S.-H. Shin, Y.E. Bae, H.K. Moon, J. Kim, S.-H. Choi, Y. Kim, H.J. Yoon, M.H. Lee, J. Nah, Formation of Triboelectric Series via Atomic-Level Surface Functionalization for Triboelectric Energy Harvesting, ACS Nano 11 (2017) 6131–6138. https://doi.org/10.1021/acsnano.7b02156. [30] J.A. Wiles, M. Fialkowski, M.R. Radowski, G.M. Whitesides, B.A. Grzybowski, Effects of surface modification and moisture on the rates of charge transfer 148 between metals and organic materials, Journal of Physical Chemistry B 108 (2004) 20296–20302. https://doi.org/10.1021/jp0457904. [31] L.S. McCarty, A. Winkleman, G.M. Whitesides, Ionic electrets: Electrostatic charging of surfaces by transferring mobile ions upon contact, J Am Chem Soc 129 (2007) 4075–4088. https://doi.org/10.1021/ja067301e. [32] L.S. McCarty, G.M. Whitesides, Electrostatic charging due to separation of ions at interfaces: Contact electrification of ionic electrets, Angewandte Chemie - International Edition 47 (2008) 2188–2207. https://doi.org/10.1002/anie.200701812. [33] A.F. Diaz, D. Wollmann, D. Dreblow, Contact electrification: ion transfer to metals and polymers, Chemistry of Materials 3 (1991) 997–999. https://doi.org/10.1021/cm00018a006. [34] L.S. McCarty, A. Winkleman, G.M. Whitesides, Electrostatic Self-Assembly of Polystyrene Microspheres by Using Chemically Directed Contact Electrification, Angewandte Chemie International Edition 46 (2007) 206–209. https://doi.org/https://doi.org/10.1002/anie.200602914. [35] A.G. Kline, M.X. Lim, H.M. Jaeger, Precision measurement of tribocharging in acoustically levitated sub-millimeter grains, Review of Scientific Instruments 91 (2020). https://doi.org/10.1063/1.5133049. [36] I.A. Harris, M.X. Lim, H.M. Jaeger, Temperature dependence of nylon and PTFE triboelectrification, Phys Rev Mater 3 (2019). https://doi.org/10.1103/PhysRevMaterials.3.085603. 149 [37] A. Schella, S. Herminghaus, M. Schröter, Influence of humidity on tribo-electric charging and segregation in shaken granular media, Soft Matter 13 (2017) 394–401. https://doi.org/10.1039/C6SM02041K. [38] J. Kolehmainen, P. Sippola, O. Raitanen, A. Ozel, C.M. Boyce, P. Saarenrinne, S. Sundaresan, Effect of humidity on triboelectric charging in a vertically vibrated granular bed: Experiments and modeling, Chem Eng Sci 173 (2017) 363–373. https://doi.org/10.1016/j.ces.2017.08.006. [39] J.R. Toth, A.K. Phillips, S. Rajupet, R.M. Sankaran, D.J. Lacks, Particle-Size-Dependent Triboelectric Charging in Single-Component Granular Materials: Role of Humidity, Ind Eng Chem Res 56 (2017) 9839–9845. https://doi.org/10.1021/acs.iecr.7b02328. [40] P.E. Shaw, Experiments on Tribo-Electricity. I. The Tribo-Electric Series, Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character 94 (1917) 16–33. [41] A.F. Diaz, R.M. Felix-Navarro, A semi-quantitative tribo-electric series for polymeric materials: the influence of chemical structure and properties, J Electrostat 62 (2004) 277–290. https://doi.org/https://doi.org/10.1016/j.elstat.2004.05.005. [42] G.S.P. Castle, L.B. Schein, General model of sphere-sphere insulator contact electrification, J Electrostat 36 (1995) 165–173. https://doi.org/10.1016/0304-3886(95)00043-7. [43] J.C. Laurentie, P. Traoré, L. Dascalescu, Discrete element modeling of triboelectric charging of insulating materials in vibrated granular beds, J Electrostat 71 (2013) 951–957. https://doi.org/https://doi.org/10.1016/j.elstat.2013.08.001. 150 [44] B.N. Cole, M.R. Baum, F.R. Mobbs, An Investigation of Electrostatic Charging Effects in High-Speed Gas-Solids Pipe Flows, Proceedings of the Institution of Mechanical Engineers, Conference Proceedings 184 (1969) 77–83. https://doi.org/10.1243/PIME_CONF_1969_184_082_02. [45] J.R. Grace, Properties, Minimum Fluidization, and Geldart Groups, Essentials of Fluidization Technology (2020) 11–31. https://doi.org/https://doi.org/10.1002/9783527699483.ch2. [46] D. Geldart, Types of gas fluidization, Powder Technol 7 (1973). https://doi.org/10.1016/0032-5910(73)80037-3. [47] R. Cocco, J.W. Chew, 50 years of Geldart classification, Powder Technol 428 (2023) 118861. https://doi.org/10.1016/J.POWTEC.2023.118861. [48] J.R. Grace, Contacting modes and behaviour classification of gas—solid and other two-phase suspensions, Can J Chem Eng 64 (1986) 353–363. https://doi.org/https://doi.org/10.1002/cjce.5450640301. [49] H.T. Bi, Electrostatic phenomena in gas-solids fluidized beds, China Particuology 3 (2005) 395–399. https://doi.org/10.1016/s1672-2515(07)60220-6. [50] A. Wolny, W. Kaźmierczak, Triboelectrification in fluidized bed of polystyrene, Chem Eng Sci 44 (1989) 2607–2610. https://doi.org/10.1016/0009-2509(89)85204-2. [51] F. Sharmene Ali, I.I. Inculet, A. Tedoldi, Charging of polymer powder inside a metallic fluidized bed, J Electrostat 45 (1999) 199–211. https://doi.org/10.1016/S0304-3886(98)00050-3. 151 [52] M. Murtomaa, E. Räsänen, J. Rantanen, A. Bailey, E. Laine, J.P. Mannermaa, J. Yliruusi, Electrostatic measurements on a miniaturized fluidized bed, J Electrostat 57 (2003) 91–106. https://doi.org/10.1016/S0304-3886(02)00121-3. [53] J. Revel, C. Gatumel, J.A. Dodds, J. Taillet, Generation of static electricity during fluidisation of polyethylene and its elimination by air ionisation, Powder Technol 135–136 (2003) 192–200. https://doi.org/10.1016/J.POWTEC.2003.08.015. [54] T. Matsuyama, H. Yamamoto, Charge relaxation process dominates contact charging of a particle in atmospheric conditions, J Phys D Appl Phys 28 (1995) 2418–2423. https://doi.org/10.1088/0022-3727/28/12/005. [55] T. Matsuyama, H. Yamamoto, Charge-relaxation process dominates contact charging of a particle in atmospheric condition: II. the general model, J Phys D Appl Phys 30 (1997) 2170–2175. https://doi.org/10.1088/0022-3727/30/15/008. [56] X. Liu, S. Sundaresan, The effect of gas on tribocharging of particles in a vibrated bed, Powder Technol (2022) 117272. https://doi.org/https://doi.org/10.1016/j.powtec.2022.117272. [57] W.O. Moughrabiah, Ph.D. Thesis, University of British Columbia, 2009. [58] T.A. Al-Smari, Ph.D. Thesis, University of British Columbia, 2014. [59] T.R.D. Ducati, L.H. Simões, F. Galembeck, Charge Partitioning at Gas−Solid Interfaces: Humidity Causes Electricity Buildup on Metals, Langmuir 26 (2010) 13763–13766. https://doi.org/10.1021/la102494k. [60] E. Kuffel, Influence of humidity on the breakdown voltage of sphere-gaps and uniform-field gaps, Proceedings of the IEE Part A: Power Engineering 108 (1961). https://doi.org/10.1049/pi-a.1961.0060. 152 [61] H.F.A. Verhaart, P.C.T. van der Laan, The influence of water vapor on avalanches in air, J Appl Phys 55 (1984) 3286–3292. https://doi.org/10.1063/1.333364. [62] D. Rodriguez, R.S. Gorur, P.M. Hansen, Effect of humidity on the breakdown characteristics of air in non-uniform fields at 30 kHz, IEEE Transactions on Dielectrics and Electrical Insulation 17 (2010). https://doi.org/10.1109/TDEI.2010.5412001. [63] Y. Zheng, J. Ren, S. Shu, J. Zhang, Effect of Air Humidity on Breakdown Characteristics of Short Gaps Under Alternating Non-uniform Electric Fields, Gaodianya Jishu/High Voltage Engineering 45 (2019). https://doi.org/10.13336/j.1003-6520.hve.20190410013. [64] J. Zhu, H. Zhang, Fluidization additives to fine powders, US6833185B2, 2004. https://patents.google.com/patent/US6833185B2/en. [65] C.C. Xu, H. Zhang, J. Zhu, Improving flowability of cohesive particles by partial coating on the surfaces, Can J Chem Eng 87 (2009) 403–414. https://doi.org/https://doi.org/10.1002/cjce.20179. [66] A. Wolny, I. Opaliǹski, Electric charge neutralization by addition of fines to a fluidized bed composed of coarse dielectric particles, J Electrostat 14 (1983) 279–289. https://doi.org/10.1016/0304-3886(83)90026-8. [67] M.U. Ghori, E. Šupuk, B.R. Conway, Tribo-electrification and powder adhesion studies in the development of polymeric hydrophilic drug matrices, Materials 8 (2015) 1482–1498. https://doi.org/10.3390/ma8041482. 153 [68] A. Sowinski, L. Miller, P. Mehrani, Investigation of electrostatic charge distribution in gas–solid fluidized beds, Chem Eng Sci 65 (2010) 2771–2781. https://doi.org/https://doi.org/10.1016/j.ces.2010.01.008. [69] W.D. Crozier, The electric field of a New Mexico dust devil, Journal of Geophysical Research (1896-1977) 69 (1964) 5427–5429. https://doi.org/https://doi.org/10.1029/JZ069i024p05427. [70] G.D. Freier, The electric field of a large dust devil, Journal of Geophysical Research (1896-1977) 65 (1960) 3504. https://doi.org/https://doi.org/10.1029/JZ065i010p03504. [71] J. Lowell, W.S. Truscott, Triboelectrification of identical insulators. II. Theory and further experiments, J Phys D Appl Phys 19 (1986) 1281–1298. https://doi.org/10.1088/0022-3727/19/7/018. [72] T. Pähtz, H.J. Herrmann, T. Shinbrot, Why do particle clouds generate electric charges?, Nat Phys 6 (2010) 364–368. https://doi.org/10.1038/nphys1631. [73] K.M. Forward, D.J. Lacks, R.M. Sankaran, Charge segregation depends on particle size in triboelectrically charged granular materials, Phys Rev Lett 102 (2009) 1–4. https://doi.org/10.1103/PhysRevLett.102.028001. [74] K.M. Forward, D.J. Lacks, R.M. Sankaran, Triboelectric charging of granular insulator mixtures due solely to particle - Particle interactions, Ind Eng Chem Res 48 (2009) 2309–2314. https://doi.org/10.1021/ie8004786. [75] S.R. Waitukaitis, V. Lee, J.M. Pierson, S.L. Forman, H.M. Jaeger, Size-dependent same-material tribocharging in insulating grains, Phys Rev Lett 112 (2014) 1–5. https://doi.org/10.1103/PhysRevLett.112.218001. 154 [76] L.V. King, On the acoustic radiation pressure on spheres, Proc R Soc Lond A Math Phys Sci 147 (1934). https://doi.org/10.1098/rspa.1934.0215. [77] M.A.B. Andrade, N. Pérez, J.C. Adamowski, Review of Progress in Acoustic Levitation, Brazilian Journal of Physics 48 (2018) 190–213. https://doi.org/10.1007/s13538-017-0552-6. [78] Z.Y. Hong, J.F. Yin, W. Zhai, N. Yan, W.L. Wang, J. Zhang, B.W. Drinkwater, Dynamics of levitated objects in acoustic vortex fields, Sci Rep 7 (2017). https://doi.org/10.1038/s41598-017-07477-1. [79] W.J. Xie, B. Wei, Parametric study of single-axis acoustic levitation, Appl Phys Lett 79 (2001). https://doi.org/10.1063/1.1391398. [80] L. P. Gor’kov, Forces acting on a small particle in an acoustic field within an ideal fluid, Dokl. Akad. Nauk SSSR, 140:1 (1961). [81] C.R. Field, A. Scheeline, Design and implementation of an efficient acoustically levitated drop reactor for in stillo measurements, Review of Scientific Instruments 78 (2007). https://doi.org/10.1063/1.2818798. [82] R.R. Boullosa, A. Pérez-López, R. Dorantes-Escamilla, An ultrasonic levitator, Journal of Applied Research and Technology 11 (2013). https://doi.org/10.1016/S1665-6423(13)71592-X. [83] C.Y. Wen, Y.H. Yu, A generalized method for predicting the minimum fluidization velocity, AIChE Journal 12 (1966) 610–612. https://doi.org/https://doi.org/10.1002/aic.690120343. 155 [84] L. Huilin, Z. Yunhua, J. Ding, D. Gidaspow, L. Wei, Investigation of mixing/segregation of mixture particles in gas–solid fluidized beds, Chem Eng Sci 62 (2007) 301–317. https://doi.org/10.1016/J.CES.2006.08.031. [85] T. Itakura, H. Masuda, C. Ohtsuka, S. Matsusaka, The contact potential difference of powder and the tribo-charge, J Electrostat 38 (1996) 213–226. https://doi.org/10.1016/S0304-3886(96)00026-5. [86] J.G. Drobny, Polymers for Electricity and Electronics: Materials, Properties, and Applications, John Wiley & Sons, Inc., Hoboken, NJ, 2012. https://doi.org/10.1002/9781118160121.app1. [87] D.C. Giancoli, Physics: Principles with Applications, 7th ed., Pearson, London, 2013. [88] K.P. Brand, Dielectric Strength, Boiling Point and Toxicity of Gases - Different: Aspects of the Same Basic Molecular Properties, IEEE Transactions on Electrical Insulation EI-17 (1982) 451–456. https://doi.org/10.1109/TEI.1982.298489. [89] R. Rokkam, R. Fox, M. Muhle, Computational fluid dynamics and electrostatic modeling of polymerization fluidized-bed reactors, Powder Technol 203 (2010) 109–124. https://doi.org/10.1016/j.powtec.2010.04.002. [90] M. Fujino, S. Ogata, H. Shinohara, The electric potential distribution profile in a naturally charged fluidized bed and its effects, 25:1 (1985). https://www.osti.gov/biblio/5036475. [91] H. MASUDA, S. MATSUSAKA, Maximum Electrostatic Force acting on a Particle, Journal of the Society of Powder Technology, Japan 30 (1993) 713–716. https://doi.org/10.4164/sptj.30.10_713. 156 [92] V. Lee, N.M. James, S.R. Waitukaitis, H.M. Jaeger, Collisional charging of individual submillimeter particles: Using ultrasonic levitation to initiate and track charge transfer, Phys Rev Mater 2 (2018) 35602. https://doi.org/10.1103/PhysRevMaterials.2.035602. [93] P. Sippola, J. Kolehmainen, A. Ozel, X. Liu, P. Saarenrinne, S. Sundaresan, Experimental and numerical study of wall layer development in a tribocharged fluidized bed, J Fluid Mech 849 (2018) 860–884. https://doi.org/DOI: 10.1017/jfm.2018.412. [94] R. Clift Grace, John R., Weber, Martin E., Bubbles, drops, and particles, (2005). [95] P. Jiang, H. Bi, S. Liang, L. Fan, Hydrodynamic behavior of circulating fluidized bed with polymeric particles, AIChE Journal 40 (1994) 193–206. https://doi.org/10.1002/aic.690400202. [96] M.C. Coelho, N. Harnby, The effect of humidity on the form of water retention in a powder, Powder Technol 20 (1978) 197–200. https://doi.org/https://doi.org/10.1016/0032-5910(78)80048-5. [97] G.A. Turner, M. Balasubramanian, Investigations of the Contributions to the Tensile Strength of Weak Particulate Masses, Powder Technol 10 (1974) 121–127. https://doi.org/https://doi.org/10.1016/0032-5910(74)80037-9. [98] J. Li, K. Kato, Effect of electrostatic and capillary forces on the elutriation of fine particles from a fluidized bed, Advanced Powder Technology 12 (2001) 187–205. https://doi.org/https://doi.org/10.1163/15685520052385014. [99] J.A. Wiles, M. Fialkowski, M.R. Radowski, G.M. Whitesides, B.A. Grzybowski, Effects of surface modification and moisture on the rates of charge transfer 157 between metals and organic materials, Journal of Physical Chemistry B 108 (2004) 20296–20302. https://doi.org/10.1021/jp0457904. [100] D. Song, P. Mehrani, Mechanism of particle build-up on gas-solid fluidization column wall due to electrostatic charge generation, Powder Technol 316 (2017) 166–170. https://doi.org/https://doi.org/10.1016/j.powtec.2017.01.031. [101] Y. Tian, P. Mehrani, Effect of particle size in fluidization of polyethylene particle mixtures on the extent of bed electrification and wall coating, J Electrostat 76 (2015) 138–144. https://doi.org/10.1016/j.elstat.2015.05.020. [102] J. Lu, S. Fujii, M. Yasuda, S. Matsusaka, Analysis of wall fouling and electrostatic charging in gas-solid fluidized beds, Advanced Powder Technology 31 (2020) 3485–3491. https://doi.org/10.1016/j.apt.2020.06.034. [103] R. Cocco, F. Shaffer, R. Hays, S.B. Reddy Karri, T. Knowlton, Particle clusters in and above fluidized beds, Powder Technol 203 (2010) 3–11. https://doi.org/https://doi.org/10.1016/j.powtec.2010.03.023. [104] A.H. Pulsifer, T. Wheelock, The Electrical Resistance of Gas Fluidized Beds, in: J.F. Davidson (Ed.), Fluidization: Proceedings of the Second Engineering Foundation Conference, Cambridge University Press, Cambridge, 1978: p. 76. [105] H. Takahashi, K. Sato, S. Sakata, T. Okada, Charge leakage characteristic of glass substrate for liquid crystal display, J Electrostat 35 (1995). https://doi.org/10.1016/0304-3886(95)00004-T. [106] M. Glor, K. Moritz, Effectiveness of increase of relative humidity as a measure to reduce the ignition probability of explosive atmospheres by static electricity, Chem Eng Trans 48 (2016). https://doi.org/10.3303/CET1648056. 158 [107] K.R. Allen, K. Phillips, Effect of humidity on the spark breakdown voltage, Nature 183 (1959). https://doi.org/10.1038/183174a0. [108] Y. An, M. Su, Y. Hu, S. Hu, T. Huang, B. He, M. Yang, K. Yin, Y. Lin, The Influence of Humidity on Electron Transport Parameters and Insulation Performance of Air, Front Energy Res 9 (2022). https://doi.org/10.3389/fenrg.2021.806595. [109] A. Sowinski, A. Mayne, B. Javed, P. Mehrani, Comparison of the effect of grounding the column wall in gas-solid fluidized beds on electrostatic charge generation, J Phys Conf Ser 301 (2011) 012068. https://doi.org/10.1088/1742-6596/301/1/012068. [110] T. Servais, C. Bernot, Measurement of electrostatic effects in a fluidized bed reactor, in: First European Conference on the Reaction Engineering of Polyolefins., Lyons, 2000. [111] Michal.W. Kepa, T. Tomizaki, Y. Sato, D. Ozerov, H. Sekiguchi, N. Yasuda, K. Aoyama, P. Skopintsev, J. Standfuss, R. Cheng, M. Hennig, S. Tsujino, Acoustic levitation and rotation of thin films and their application for room temperature protein crystallography, Sci Rep 12 (2022) 5349. https://doi.org/10.1038/s41598-022-09167-z. [112] N.M. Brown, B. VanSaders, J.M. Kronenfeld, J.M. DeSimone, H.M. Jaeger, Direct measurement of forces in air-based acoustic levitation systems, Review of Scientific Instruments 95 (2024) 094901. https://doi.org/10.1063/5.0225745. [113] F. Chowdhury, M. Ray, A. Passalacqua, P. Mehrani, A. Sowinski, Electrostatic charging due to individual particle-particle collisions, Powder Technol 381 (2021) 352–365. https://doi.org/10.1016/j.powtec.2020.12.012. 159 [114] J.C. Sobarzo, F. Pertl, D.M. Balazs, T. Costanzo, M. Sauer, A. Foelske, M. Ostermann, C.M. Pichler, Y. Wang, Y. Nagata, M. Bonn, S. Waitukaitis, Spontaneous ordering of identical materials into a triboelectric series, Nature 638 (2025) 664–669. https://doi.org/10.1038/s41586-024-08530-6. [115] W. Li, D.Y. Li, On the correlation between surface roughness and work function in copper, J Chem Phys 122 (2005) 064708. https://doi.org/10.1063/1.1849135. [116] M. Xue, S. Peng, F. Wang, J. Ou, C. Li, W. Li, Linear relation between surface roughness and work function of light alloys, J Alloys Compd 692 (2017) 903–907. https://doi.org/https://doi.org/10.1016/j.jallcom.2016.09.102. [117] F. Chowdhury, B. Elchamaa, M. Ray, A. Sowinski, A. Passalacqua, P. Mehrani, Apparatus design for measuring electrostatic charge transfer due to particle-particle collisions, Powder Technol 361 (2020) 860–866. https://doi.org/10.1016/j.powtec.2019.11.013. [118] Statistics Canada, Sawmill industry in Canada: 15 years in review, 2022. [119] A. Watters, Canada Wood Pellet Annual, Ottawa, 2023. [120] WorkSafeBC, Incident Investigation Report. Explosion at Babine Forest Products, January 20, 2012, British Columbia, Canada, 2013. [121] WorkSafeBC, Incident Investigation Report. Explosion at Lakeland Mills, April 23, 2012, British Columbia, Canada, 2014. [122] C. Cloney, 2020 Combustible Dust Incident Report - Version #1, 2021. 160 [123] P.R. Amyotte, R.K. Eckhoff, Dust explosion causation, prevention and mitigation: An overview, J Chem Health Saf 17 (2010) 15–28. https://doi.org/10.1016/J.JCHAS.2009.05.002. [124] I.D. Pavey, Electrostatic Hazards in the Process Industries, Process Safety and Environmental Protection 82 (2004) 132–141. https://doi.org/10.1205/095758204322972771. [125] X. Zhou, X. Li, Z. Cui, L. Wu, H. Zhou, X. Lu, Combustible wood dust explosions and impacts on environments and health - A review, Environ Res 216 (2023) 114658. https://doi.org/https://doi.org/10.1016/j.envres.2022.114658. [126] X. Zhou, H. Zhou, T. Ding, C. Wu, D. Guo, Analysis of causes of wood dust explosion in wood processing enterprises and its prevention measures, China Forest Products Industry 42 (2015) 49–52. [127] R.K. Eckhoff, Chapter 1 - Dust Explosions—Origin, Propagation, Prevention, and Mitigation: An Overview, in: R.K. Eckhoff (Ed.), Dust Explosions in the Process Industries (Third Edition), Gulf Professional Publishing, Burlington, 2003: pp. 1–156. https://doi.org/https://doi.org/10.1016/B978-075067602-1/50002-0. [128] L. Zhang, J. Hou, X. Bi, Triboelectric charging behavior of wood particles during pellet handling processes, J Loss Prev Process Ind 26 (2013) 1328–1334. https://doi.org/https://doi.org/10.1016/j.jlp.2013.08.004. [129] S. Melin, Determination of explosibility of dust layers in pellet manufacturing plants, WPAC, Wood Pellet Association of Canada (2012). [130] R.K. Eckhoff, ed., Appendix - Ignitability and Explosibility Data for Dusts from Laboratory Tests, in: Dust Explosions in the Process Industries (Third Edition), 161 Gulf Professional Publishing, Burlington, 2003: pp. 681–704. https://doi.org/https://doi.org/10.1016/B978-075067602-1/50011-1. [131] M. Kachi, M. Nemamcha, B. Tabti, L. Dascalescu, Comparison between three measurement methods for characterizing the charge state of granular insulating materials, J Electrostat 69 (2011) 394–400. https://doi.org/https://doi.org/10.1016/j.elstat.2011.05.002. [132] R.S. Sigmond, Faraday cage profiles giving self-similar charge drift and conduction in insulator samples, IEEE Transactions on Dielectrics and Electrical Insulation 7 (2000) 197–203. https://doi.org/10.1109/94.841809. [133] Y. Liu, J. Mo, Q. Fu, Y. Lu, N. Zhang, S. Wang, S. Nie, Enhancement of Triboelectric Charge Density by Chemical Functionalization, Adv Funct Mater (2020). https://doi.org/10.1002/adfm.202004714. [134] Y.J. Kim, J. Lee, S. Park, C. Park, C. Park, H.-J. Choi, Effect of the relative permittivity of oxides on the performance of triboelectric nanogenerators, RSC Adv 7 (2017) 49368–49373. https://doi.org/10.1039/C7RA07274K. [135] J. Cross, Electrostatics : principles, problems and applications, Hilger, Bristol, 1987. [136] W. Simpson, a TenWolde, Physical properties and moisture relations of wood, in: Wood Handbook: Wood as an Engineering Material, 1999: pp. 3.1-3.24. 162 Appendix Figure A.1 Size distribution of (A) HDPE particles and (B) glass beads. (A) (A) (B) 163 Figure A.2 Photos from optical coherence tomography to show the surface roughness of (A) PU hose (B) PVC hose and (C) rubber hose. (A) (B) (C)
Thesis/Dissertation
eng
Chemical and Biological Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
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Bi, Xiaotao
Grace, John R.
Electrostatic charging in gas–solid fluidized beds : effects of gas composition, humidity, and interparticle charging
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