Effect of dispersed holdup on drop size distribution in oil–water dispersions: Experimental observations and population balance modeling

highlights
• Population balance equations are used to predict drop size distributions in stirred vessels.
• A two-region approach is followed with different breakage and coalescence correlations.
• Drop size distributions measured with a FBRM instrument is compared.
• Both experiments and PBEs modeling are compared up to 40% holdup in unstable systems.

The drop diameter distribution (DSD) of dispersed Exxsol D80 oil-in-water in a lab scale stirred tank was predicted numerically using population balance equations (PBEs) modeling. It was assumed that the flow area was split into two zones, around the impeller with high energy dissipation rate and further away from the impeller with low energy dissipation rate. The dispersed phase chord length (CLD) distribution was experimentally measured with a focused beam reflectance method (FBRM) probe, and was then converted to the drop diameter distribution (DSD) numerically using a backward transform. Comparisons between the PBEs model results against experimental data from the current work and from previous literature showed that the PBEs predicted well the data at low dispersed phase fractions but overpredicted them at high ones. The predictions were slightly better when an increased power number was used. In addition, it was found that improved predictions could be obtained when the region of high energy dissipation rate around the impeller was enhanced.