420527 CFD Investigations of Particle Segregation and Dispersion Mechanisms inside a Polyolefin 8-Leg Loop Reactor of Industrial Scale

Wednesday, November 11, 2015: 4:40 PM
250B (Salt Palace Convention Center)
Yuehao Li, Krishnaswamy Nandakumar and Rupesh K. Reddy, Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA

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 insight details about these two competing mechanisms inside an 8–leg loop reactor of industrial scale. The simulations adopt 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 slurry flow consisting of solid particles of averaged diameter as 2.5 and 0.5 millimeter are investigated by CFD simulations.

            The simulation results indicated that the centrifugal force induced by the bend geometry causes the segregation of the solid particles inside the bottom bends, resulting in thick particulate ropes close to the outer pipe wall. The segregation extents are different depending on the location of the bends: no clear particulate layers are found inside the top bends or the bends close to the axial flow pump. As the slurry flow is forced to change its axial flow direction inside the 2nd and 6th bends, the sudden change of flow directions induces “double–bend” effect. Governed by such effect, the secondary flow on the cross sections of vertical legs exhibits the “single–vortex” structure. This type of flow structure enhances the solid dispersion mechanism inside vertical legs. While rotating slowly inside vertical legs, particulate ropes are gradually dispersed and mixed with the liquid phase by the secondary flow as well as the turbulent dispersion. In the initial stage prior to the formation of large slugs, the competition between the segregation and dispersion mechanisms generates multiple slurry clusters of various solid volume fractions. When these clusters are circulated inside the loop reactor by the axial flow pump, the monitors on the cross sections of the loop reactor observe fluctuating profiles of the operating parameters such as the solid volume fraction and velocity magnitudes. When these slurry clusters pass through the pump region, fluctuating pump pressure output, in other words pump power consumption, is observed due to the varying frictional force exerted to the pump impeller. For example, if clusters containing large amounts of solid particles pass through the pump impeller, the frequent interactions between the solid particles and pump impeller generates strong frictional force. In order to maintain its constant rotational speed, the pump has to consume significant amount of power to overcome the friction force. In contrast, the pump requires less amount of power to drive the flow when clusters having smaller amount of particles present in the impeller region. If the solid segregation mechanism is not controlled properly, the particulate ropes can eventually develop into large slugs as the segregation phenomenon proceeds for 6 to 8 hours. The movements of large slugs inside loop reactors cause rapid and violent pump power fluctuations that the safety interlocks has to shut down the entire production process.

            The simulation results suggested that the solid segregation can be suppressed effectively if particle sizes are reduced. Small particles do not stratify into thick particulate ropes inside bottom bends, and they are distributed more uniformly in loop reactors compared to large particles. Although reducing particle sizes can retard the formation of large slugs, such technique is challenging to adopt as particle sizes are generally determined by reaction conditions. Other mitigation techniques, i.e., using bypass line to loop reactors, are more feasible to control the formation of large slugs in the loop reactor.

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