Numerical simulation of industrial vinyl chloride suspension polymerization reactor by CFD-guided downscaling: From liquid-liquid dispersion to solid-liquid suspension

Numerical scale-up plays a key role in industrial production of commercial polymers. However, direct simulation of industrial reactor is generally challenging due to its large volume. In this contribution, a 106 m3 industrial vinyl chloride (VC) suspension polymerization reactor equipped with flat impeller is scaled down to a 31.4 L geometrically similar reactor by a factor of 14.2 based on the scale-up rule of constant power input per volume (P/V). Multiphase phenomena including both liquid-liquid dispersion (i.e., insoluble monomer is shaped into micrometer-sized droplets with a certain distribution by turbulence shear) and solid-liquid suspension inside the small reactor are studied via computational fluid dynamics-population balance model (CFD-PBM) and CFDkinetic theory of granular flow (CFD-KTGF) simulations, respectively. Flow characteristics, droplet size distributions (DSDs) and impeller performances with different stirring speeds N, impeller diameters D, blade widths b and off-bottom clearances C are elaborated. Formulas correlating power number Np, flow number Nqc with D and b are obtained, providing guidelines for selecting proper impeller sizes and operation conditions. Simulation results indicate that an impeller with low off-bottom clearance is better for suspension polymerization process. For impeller optimization, a dual axial impeller with 45 degrees-pitched blades is more suitable as confirmed by improved homogeneity inside the reactor, narrower DSD and lower power input. The present study tracks the multiphase flows inside the suspension polymerization reactor and offers insights beneficial for its scale-up and optimization.