Understanding Particle Flows in Sub-Fluidized Horizontal Stirred Bed Reactors By Radioactive Particle Tracking

Horizontal stirred bed reactors are commercially employed in polypropylene manufacturing processes. The reactors generally contain a bed that is mildly stirred by a series of paddles attached to a center shaft. Via nozzles located at the bottom of the reactor gaseous propylene enters the reactor at controlled flowrate to prevent bed fluidization, bringing the bed in sub-fluidized state. It is crucial to have a well-mixed system with a narrow particle cycle time distribution throughout the reactor to avoid the formation of hotspots that could lead to expensive reactor shutdowns due to lump formation. It is therefore of great importance to understand the particle flows in horizontal stirred bed reactors.

In this research, we developed a lab-scale horizontal stirred bed reactor that enables particle flow studies in non-reactive environments. Since optical techniques are inadequate to study the dense flows, we study the reactor hydrodynamics with X-ray imaging and radioactive particle tracking (RPT). Gas-solid distribution maps acquired with X-ray imaging showed that at higher inlet velocities bed spouting occurs, leading to substantial gas by-pass and poor gas-solid interaction. Now we use RPT to characterize particle trajectories in the reactor. The RPT setup consists of three scintillation detectors that form a field of view around the reactor. Within this field of view the location of the tracer particle is reconstructed with sub-millimetre and sub-second accuracy. We studied the effect of the reactor filling, gas inlet flowrate and agitator rotation speed on the particle cycle time distribution and axial dispersion for industrial-grade polypropylene (Geldart B type). We found that at higher reactor fillings and higher rotation speeds the particle has a shorter cycle time, which would lead to improved heat removal in the process. The results from this study contribute to better understanding of the flow behaviour in horizontal stirred bed reactors.