Conventional fluidization of cohesive powders is challenging due to their strong interparticle forces, often requiring assistance methods. In this study, the hydrodynamics of pulsed and vibrated beds of cohesive Geldart C silica powder (Sauter mean diameter d₃₂=7.9μm) in a 19.2cm diameter column were investigated using X-ray imaging. The results show that low-frequency, moderate-amplitude gas pulsation improves fluidization by disrupting long, persistent gas channels. Higher-frequency pulsation is dampened throughout the bed, resulting in negligible improvement over unassisted systems. When coupled with mechanical vibration, gas pulsation slightly mitigates solid compaction at the bottom section, but the overall flow pattern remains largely unchanged compared to vibration alone. The findings highlight the potential of integrating gas pulsation with other assistance methods to enhance fluidization in practical applications.

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Polypropylene (PP) is a versatile polymer extensively used in industries such as food packaging, automotive, healthcare, and textiles. Industrially, PP is produced via gas-phase solid-catalyzed polymerization in horizontal stirred bed, vertical stirred bed, or fluidized bed reactors. These reactors operate under controlled conditions to polymerize propylene monomers into solid PP particles. Despite their widespread application, operating these reactors is challenging due to a lack of fundamental understanding and modeling capabilities, which leads to reduced production capacity and lower quality of the final product. This gap in understanding is primarily due to the scarcity of detailed experimental data, which is difficult to obtain because of the opaqueness of the flow and the rapidly evolving gas-solids distribution, necessitating non-optical measurements with high temporal resolution.

In this dissertation, a deeper understanding of granular flow behavior in these reactors was achieved through detailed experimental measurements using radiation-based imaging. Recognizing the direct link between macro-scale flow behavior and particle-scale phenomena, this research spanned both scales. Although the primary focus of this thesis is on a horizontal stirred bed, experiments were also conducted using two additional lab-scale reactor configurations: a vertical stirred bed and a fluidized bed. High-quality data on flow patterns, phase holdup, and particle dynamics were obtained using X-ray imaging and single-photon emission radioactive particle tracking. A key novelty of this research was the use of industrial-grade powders, such as polypropylene reactor powder, as encountered in horizontal stirred bed reactors. The collected data were thoroughly analyzed to identify the key parameters influencing granular flow behavior, utilizing statistical methods and visualization tools to uncover critical insights.

First, the flow behavior of polypropylene reactor powder in a laboratoryscale horizontal stirred bed reactor (HSBR) was investigated using X-ray imaging. It was observed that agitation significantly dictates overall flow behavior and phase holdup in the HSBR. Gas injection through inlet points at the bottom resulted in spouting behavior, and the gas holdup at fixed agitator positions remained highly consistent across successive revolutions. The presence of liquid was found to deteriorate the flow behavior due to liquid bridging at particle contact points, with particle size and surface morphology influencing the powders’ susceptibility to liquid.

Subsequently, a single-photon emission radioactive particle tracking method was presented, allowing the tracking of individual photon-emitting particles to evaluate the hydrodynamics of multiphase flows. This method directly utilized detected photon hit locations to reconstruct the three-dimensional position of the tracer particle, avoiding assumptions in count rate fluctuations. The tracer particle’s position was determined by finding the intersection point of three two-dimensional planes from the detectors, achieving a spatial accuracy of approximately 1 mm through a subsequent calibration experimentation procedure.

Thereafter, the method was employed to characterize the particle dynamics in the HSBR. It was found that, besides the agitator rotation speed, the flow behavior is significantly influenced by the reactor fill level. At low rotation speeds and fill levels, solids motion was primarily induced by impeller blade passage, resulting in semi-static bed motion and poor solids distribution. Increased fill levels and rotation speeds led to continuous solids motion and uniform distribution. Solids circulation, quantified by a dimensionless cycle number, increased with higher fill levels and rotation speeds. The axial dispersion coefficient ranged from 10^-6 to 10^-5 m² s^-1, increasing with rotation speed, although no conclusive relationship with fill level was observed.

Thereafter, the fluidization behavior of Geldart B particles in a vertical stirred bed reactor was investigated using X-ray imaging, pressure drop measurements, and numerical simulations via Computational Fluid Dynamics (CFD) coupled with Discrete Element Method (DEM) and Immersed Boundary Method (IBM). The experimentally obtained minimum fluidization curve and time-averaged pressure drop showed good qualitative agreement with simulations. Visual observations indicated that increasing the agitator’s angular velocity reduced bubble size and improved bed homogeneity, as evidenced by reduced pressure fluctuations. Simulations revealed that while the impeller enhances solids agitation, a proper design study is essential, as static immersed bodies like the stirrer shaft can adversely impact solids motion.

Finally, the correlation between the fluidization behavior and flow properties of 10 commercially available cohesive powders was experimentally investigated. The fluidization quality of the powders in a laboratory-scale fluidized bed was assessed using a Fluidization Quality Index (FQI), computed by integrating gas holdup and its temporal variation acquired through X-ray imaging. Flowability was measured in a rotating drum operated at high speeds, which aerated the powder bed, a critical factor in correlating fluidization behavior with flow properties. This study established a positive correlation between cohesive powders’ flowability and fluidization quality, suggesting that fast and user-friendly flowability measurements in a rotating drum instrument can predict fluidization potential, aiding in process optimization and enhancing fluidization studies for cohesive powders.

In summary, the insights acquired from this thesis enhance the understanding of flow behavior and phase holdup in stirred bed reactors and cohesive fluidized beds. These findings can serve as a valuable foundation for designing, optimizing, and intensifying systems for the industrial-scale manufacturing of high-quality PP resins.@en

Horizontal stirred bed reactors are widely used in the commercial manufacturing of polypropylene. However, a comprehensive understanding of the particle dynamics in horizontal stirred bed reactors remains elusive, primarily due to the lack of detailed experimental data. In this work, we studied the influence of operating parameters on the particle flow dynamics in a laboratory-scale horizontal stirred bed reactor using single-photon emission radioactive particle tracking. The results show that the general solids flow behavior is strongly affected by both the agitator rotation speed and reactor fill level. Operation at low rotation speed and low fill level results in solids flow with poor radial and circumferential distribution due to internal bed circulation. On the contrary, at increased rotation speeds and fill levels, solids motion throughout the bed is continuous resulting in excellent solids distribution. The solids circulation was found to increase for both an increase in rotation speed and reactor fill level. The axial dispersion coefficient, on the other hand, shows a linear relation with the rotation speed, but no conclusive relation between the axial dispersion coefficient and the reactor fill level was found.

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Vibro-assisted fluidization of cohesive micro-silica has been studied by means of X-ray imaging, pressure drop measurements, and off-line determination of the agglomerate size. Pressure drop and bed height development could be explained by observable phenomena taking place in the bed; slugging, channeling, fluidization or densification. It was observed that channeling is the main cause of poor fluidization of the micro-silica, resulting in poor gas-solid contact and little internal mixing. Improvement in fluidization upon starting the mechanical vibration was achieved by disrupting the channels. Agglomerate sizes were found to not significantly change during experiments.

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Stirred gas–solid fluidized bed reactors are commercially employed in polyolefin manufacturing, but the complex gas–solid contacting dynamics pose challenges in design, scale-up, and operation. In this study, the influence of agitation on the fluidization performance of Geldart B particles was investigated experimentally by X-ray imaging and pressure drop measurements and numerically by Computational Fluid Dynamics (CFD) - Discrete Element Method (DEM) - Immersed Boundary Method (IBM). The experimentally obtained minimum fluidization curve and time-averaged pressure drop show good qualitative agreement with the simulation results. Visual observations underscore that an increase in the angular velocity of the agitator results in reduced bubble size and improved bed homogeneity, as further evidenced by reduced pressure fluctuations. Furthermore, the simulations reveal that while the impeller enhances solids agitation, a proper design study is imperative, considering that static immersed bodies, such as the stirrer shaft, can adversely impact solids motion.@en

This study introduces a mathematical model to predict the dynamic angle of repose exhibited by granular materials in a rotating drum. The model accounts for the effect of particle properties, particularly sliding and rolling friction, as well as process conditions, i.e., the Froude number. We show that the effects of particle properties can be modeled independently of the process conditions, resulting in a multiplicative model. The model is validated using experimental data taken at different Froude numbers, yielding favorable agreement between the predicted dynamic angle of repose and the experimental observations. The findings of this study hold significant implications for engineering disciplines, as they provide crucial insights for optimizing processes involving granular materials in various engineering applications.@en

Horizontal stirred bed reactors (HSBRs) are widely used in the commercial production of polypropylene (PP). Despite their commercial significance, a comprehensive understanding of the flow behavior in HSBRs remains elusive, primarily due to the lack of detailed experimental data. This study investigates the influence of operating parameters on the particle flow behavior of two types of PP reactor powder in a laboratory-scale HSBR using X-ray imaging. Our results indicate that the overall flow behavior and phase holdup in the HSBR are dominated by agitation. Moreover, gas injection through the inlet points at the bottom of the HSBR results in spouting behavior, which can lead to reduced gas–solid contacting and, in extreme cases, complete bypass. Finally, the presence of liquid (in this study, isopropyl alcohol) adversely affects the flow behavior of the PP reactor powder due to liquid bridging at the contact points of particles. Powders that comprise particles with relatively small sizes and dense surface morphology are particularly prone to reduced flow behavior when exposed to liquid.@en

In this study, we investigate the correlation between fluidization behavior and flow properties of 10 commercially available cohesive powders through fluidization and rotating drum experiments. The rotating drum was operated at high speeds to aerate the powder, creating flow fields and stress conditions comparable to those in gas–solid fluidized beds. We introduce a fluidization quality index (FQI) computed from X-ray imaging, which addresses the limitations of conventional analyses for assessing fluidization quality of cohesive powders. By analyzing flow patterns, the FQI integrates gas holdup and its temporal variation, providing a reliable measure of fluidization quality. The findings establish a positive correlation between flowability and fluidization quality, demonstrating how flowability measurements can predict the effectiveness of mechanical vibration in enhancing fluidization characteristics. These results suggest that fast, user-friendly flowability assessments in a rotating drum can effectively predict fluidization potential, contributing to process optimization and advancing fluidization studies for cohesive powders.@en