Granular flow in stirred bed reactors

Abstract

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.