Agitation in Suspension Polymerizers: Comparison of Impeller Configurations Using LES-CFD

Suspension polymerization is a common process for making products such as polyvinyl chloride and polyacrylonitrile. Droplets of monomer are dispersed in water in the presence of a surfactant and once the desired droplet size is achieved an initiator is added to start the reaction. The size of the polymer particles produced is governed by the size of the monomer droplets formed at the start of the process.

As the reaction proceeds, coagulum is formed which can coat the impellers and vessel walls or form large masses which can damage the agitator if they impact the blades or shaft. The formation of coagulum can be minimized by using an impeller that generates a high flow but, at the same time, it must provide enough shear to generate the desired monomer drop size distribution.

Droplet sizes in stabilized liquid-liquid dispersions have been measured to identify which impellers can be characterized as “high or low shear”. The droplet sizes are correlated in terms of the kinetic energy dissipation rate (KEDR) in the impellers’ trailing vortices (see Grenville et al., Chem. Engg., Aug 2017). The measured relationship between the impellers’ characteristics has been used to successfully validate prediction of the trailing vortex KEDR for single impellers made with large-eddy simulation computational fluid dynamics.

This presentation will describe CFD models of a polymerization reactor in which a retrofit was made and resulted in a reduction of coagulum fouling and associated mechanical issues. But the retrofit changed the particle size distribution of the polymer to the extent that the retrofit was replaced with the original impeller configuration.

The validated impeller models have been combined to develop multi-impeller models of the full-scale reactor configurations in order to examine how differences in the flow / shear characteristics of the original and retrofit impeller configurations may affect the result of the polymerization, particularly the particle size distribution.

Finally, recommendations regarding a potential retrofit which increases flow while maintaining shear characteristics will be discussed.