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Electrostatic charge generation is unavoidable in gas-solid fluidized beds due to the repeated particle-particle and particle reactor wall contacts and separations. In industrial operations such as in polyethylene production this phenomenon results in the significant problem of reactor wall fouling, known as “sheeting”. To better understand the underlying charging mechanisms involved in gas-solid fluidized beds in an attempt to eliminate and/or reduce the effect a novel on-line electrostatic charge measurement technique was developed, which concurrently provided information on both the degree of fluidized bed electrification and reactor wall fouling. A Faraday cup replaced the windbox of the fluidized bed while another cup was placed at the top of the column. The distributor plate was uniquely designed for the systematic removal of bed particles and those adhered to the column wall for their charge measurement with the bottom Faraday cup, and the charge of the entrained particles was measured by the top Faraday cup. This is the first study which allowed the charge measurement of particles in the bulk of the bed, particles adhered to the column wall, and those entrained, simultaneously. In addition, this method uniquely permitted the evaluations of the degree of reactor wall fouling under different operating conditions. An experimental program was designed to investigate the influence of bed hydrodynamics (fluidizing gas velocity and particle size), fluidization column wall material, and the addition of different solid additives. Fluidizing particles were polyethylene resin from an industrial reactor. Bi-polar charging was observed where the elutriated particles were oppositely charged compared to those in the bulk of the bed and those adhered to the column wall. Particles within the wall coating were also found to be bi-polarly charged. With the resin tested as received, a certain sized particles (350-575 µm) adhered to the column wall. The specific charge of the particles near the column wall was found not to be a definite indication of the amount of wall fouling. Increasing the gas velocity promoted wall fouling and elevated the charge density of the particles within both bubbling and slugging flow regimes. The effect of solid additive injection was examined with two static drivers known to reduce wall fouling in industrial operations, a deactivated catalyst, and the catalyst support. It was found that the catalyst promoted, while one of the static drivers reduced wall fouling.

In gas-solid fluidized beds, the generation of electrostatic charges due to continuous contacts between fluidizing particles, and the particles and the fluidization vessel wall, is unavoidable. Industrial operations, such as the production of polyethylene, are susceptible to significant operational challenges caused by electrostatics including reactor wall fouling, a problem known as “sheeting”. The formation of particle sheets can require shutdown periods for clean-up which results in significant economic losses. To gain a better understanding of the underlying mechanisms of electrostatic charging in gas-solid fluidized beds, in an attempt to eliminate or minimize this problem, a pilot-scale pressurized gas-solid fluidization system was designed and built, housing an online electrostatic charge measurement technique consisting of two Faraday cups. The system permits the study of the degree of particle wall fouling at pressures and temperatures up to 2600 kPa and 100°C, respectively, and gas velocities up to 1 m/s (covering a range including turbulent flow regime). The system also allowed, for the first time, the measurement of the fluidizing particles’ mass, net charge and size distribution in various regions of the bed, especially those related to the wall coating under the industrially relevant operating conditions of high pressures and gas velocities. Experimental trials were carried out using polyethylene resin received from commercial reactors to investigate the influence of pressure and gas velocity on the bed hydrodynamics and in turn, the degree of bed electrification. Mechanisms for particle charging, migration and adherence to the column wall were proposed. The size distribution of the gas bubbles shifted towards smaller bubbles as the operating pressure was raised. Thus, higher pressures lead to greater mixing within the bulk of the bed and resulted in a higher degree of particle wall fouling. Moreover, the extent of wall fouling increased linearly with the increase in gas velocity and as the bed transitioned to turbulent regime, due to the increase in particle-wall contacts. Bipolar charging was observed especially within the wall coating with smaller particles being negatively charged. Overall, particle-wall contacts generated negatively charged particles resulting in a net negative charge in the bed, whereas particle-particle contacts generated positively and negatively charged particles resulting in no net charge when entrainment was negligible. The formation of the wall layer and its extent was influenced by the gravitational and drag forces balancing the image force and Coulomb forces (created by the net charge of the bed and the metallic column wall as the attraction between oppositely charged particles).

Cascade impactors, routinely used for in vitro particle size characterization of pharmaceutical aerosols, are calibrated using dilute, charge-neutralized, monodisperse aerosols. But pharmaceutical aerosols are known to generate concentrated, inherently charged, polydisperse aerosol clouds. A computational model of the Andersen Cascade Impactor (ACI) suggested that the presence of charge on aerosol particles may influence their deposition within the ACI, but experimental validation of the model is warranted. This dissertation investigates the influence of electrostatic charge upon the deposition behavior of aerosols within cascade impactors, to address the impact of charge on particle size characterization. The influence of applied charge upon the deposition pattern and aerodynamic particle size distribution (APSD) of commercially available pressurized metered dose inhalers (pMDIs) within the Electrical Low Pressure Impactor (ELPI) was examined. Electrostatic properties were modified using an external voltage source in conjunction with the ELPI corona charger and observed to be dependent on the formulation and device packaging. Induced artificial charge on the aerosol particles influenced the deposition pattern within the impactor, but did not result in a significant change in the apparent APSD. An experimental apparatus capable of producing charge neutralized and charged aerosol, with targeted deposition on the CFD predicted ‘charge sensitive’ ACI stages, was developed. In vitro results were observed to be in partial agreement with the CFD predictions. While charge influenced the deposition pattern in the ACI with increased deposition observed in the charger and on the upper stages of the ACI, it did not influence the apparent APSD of the aerosol. Electrostatic charge effects on deposition behavior within cascade impactors were delineated with respect to space charge and image charge effects by investigating the influence of impactor grounding, particle size, stage coating and loading. While the deposition pattern within the ACI was influenced by charge, only stage coating and stage loading resulted in a small, significant difference in the apparent APSD, which may not be practically relevant due to the variability associated with in vitro aerosol testing. Similar trends were observed in the deposition behavior of charge neutralized and charged aerosol within an abbreviated ACI system compared to the full resolution ACI.

The dynamics of particle laden flows are integral to the analysis of toxic particle deposition and medical respiratory aerosol delivery. Computational fluid-particle dynamics (CFPD) can play a critical role in developing a better understanding of particle laden flows, especially in a number of under-explored areas. The applications considered in this study include both the numerical aspects and the physical phenomena of respiratory aerosol transport. Objective I: Considering the effects of mesh type and grid convergence, four commonly implemented mesh styles were applied to a double bifurcation respiratory geometry and tested for flow patterns and aerosol deposition. Results indicated that the mesh style employed had a significant effect on the transport and deposition of aerosols with hexahedral meshes being most accurate. Objective II: In order to evaluate the effects of bronchoconstriction under exhalation conditions, normal and constricted pediatric airway models were considered. Results include (i) a significant increase in deposition for constricted airways, and (ii) a novel correlation for deposition during exhalation based on the Dean and Stokes numbers. Objective IIIa: Considering evaluation of an aerosol size sampler, an eight-stage Andersen cascade impactor (ACI) was numerically analyzed. The numerical simulations indicated high non-uniformity and recirculation in the flow field. Numerical predictions of retention fraction matched well with existing experiments (0.5 – 11% error). Objective IIIb: As an extension to this study, numerical predictions of electrostatic charge effects on aerosol transport and deposition in the ACI were presented. Charges consistent with standard pharmaceutical pressurized metered dose inhalers and dry powder inhalers were considered. The numerical predictions indicated that charged aerosols deposit as if they were 5 – 85% larger due to electrostatic effects. Applications of the studies considered include (i) quantitative guidance in selecting numerical mesh styles and development of standard grid convergence criteria, (ii) the development of more accurate whole-lung deposition models that better evaluate exhalation conditions,(iii) improvements in the design of pharmaceutical assessment and delivery devices, and (vi) correction values to account for electrostatic charge on pharmaceutical aerosols.

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