CFD investigations of particle segregation and dispersion mechanisms inside a polyolefin 8-leg loop reactor of industrial scale

• Reveal the slurry flow inside an industrial-scale loop reactor
• Demonstrate the solid segregation and dispersion mechanisms
• Explain the fluctuated profiles observed during the reactor operations
• Compare the effects of particle sizes on reactor performances

The formation of polymer slugs inside loop reactors has long been a troubling issue for the polyolefin industry using slurry-phase process. The mechanism of solid phase segregation occurring in the slurry flow has been reported as one of the root causes of the formation of large polymer slugs. However, the mechanism of solid phase dispersion, which counters the solid phase segregation and retards slug formation, has not been fully understood yet especially from the aspect of fluid dynamics. Therefore, in this study we apply computational fluid dynamic (CFD) simulations to provide insightful details about these two competing mechanisms inside an 8-leg loop reactor of industrial scale. The simulations adopt the transient Eulerian–Eulerian two fluid model incorporated with the kinetic theory of granular flow to describe the slurry flow consisting of propylene in liquid state and solid polypropylene particles. The solid particles with an average diameter of 2.5 mm are found to segregate from the slurry mixture due to the centrifugal force induced by the bend geometry, forming thick particulate ropes close to the outer pipe wall. These particulate ropes are dispersed inside the vertical legs by the secondary flow, which exhibits the flow structure as a single vortex on the cross sections. The competition between the segregation and dispersion mechanisms results in numerous slurry clusters of varying solid contents. While they are circulated by the axial flow pump, the loop reactor shows fluctuating profiles of solid volume fractions and liquid velocity as well as the pump pressure output with respect to time. The simulation results reveal that the variations of pump pressure output, which is equivalent to the pump power consumption, are resultants from the varying frictions exerted by the slurry clusters. In addition, our simulation results suggest that operating loop reactors with small particle sizes can suppress the solid segregation mechanism as well as improve the uniformity of particle distributions, which consequently retards the formation of large slugs.

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