Dispersion of immiscible liquids in a static mixer without inserted elements: A CFD-PBM study on droplet size reduction and distribution

A CFD-PBM (Computational Fluid Dynamics-Population Balance Model) coupled model has been employed to investigate the turbulent dispersion of immiscible liquids through a static swirl mixer without inserted elements, aiming to mitigate the disadvantages of existing static mixers, e.g., high energy consumption and maintenance costs. The model was firstly validated in a Kenics static mixer with experimental data of a benzene-water system with a benzene volume fraction of 0.5 % and empirical correlations, and was then used to evaluate the performance of the proposed static swirl mixer in pressure losses, Sauter mean diameter, droplet size distribution, effect of number of swirl sections, and PD (pitch to diameter) ratios. The static swirl mixer demonstrated a clear effect in the dispersion of immiscible benzene into water producing fine droplets, with its dispersive mixing efficiency being more pronounced in flows with higher Weber numbers. Its droplet size reduction effect is relatively less effective than the Kenics mixer for the same Weber numbers, attributed to the absence of inserted elements, but at the expense of a much lower pressure loss than the Kenics mixer. In producing the same droplet sizes, the static swirl mixers cause a lower pressure loss than the Kenics mixer to a certain extent depending on different the Weber numbers and PD ratios, demonstrating an energy-saving potential. A decreasing PD ratio produces smaller droplet sizes but at higher pressure losses. A PD ratio between 1.5 and 3 may be optimal. Ten swirl sections would appear to be sufficient to produce the equilibrium droplet size distribution despite the different PD ratios.