(158b) Single and Multiphase Mixing in Partially Filled Stirred Vessels Using Computational Fluid Dynamics and Particle Image Velocimetry

In many industrial applications, mixing vessels have liquid height-to-tank diameter ratio H/T equal to, or larger than, 1. However, there are many instances where this ratio is lower than 1, as in all those cases in which the vessel is emptied. Even when H/T<1, sufficient agitation must still be provided in order to attain the process objectives. When the impeller submergence is reduced as a result of lowering the liquid level, the fluid dynamics of even a single-phase stirred liquid can become quite complex, with different regimes possibly existing depending on the geometric characteristics of the system (such as impeller clearance, liquid height, or liquid head above the impeller). The objective of this work is to determine the minimum liquid level and the critical impeller submergence for a standard impeller off-bottom distance (C/T=1/3) where adequate mixing process can still be achieved both in a single liquid phase and solid-liquid suspension.

Computational Fluid Dynamic (CFD) simulation as well as Particle Image Velocimetry (PIV) was used here to study the velocity profile in flat-bottom baffled vessels equipped with a single Disk Turbine (DT) for different height-to-tank diameter ratios when the liquid level is smaller than the tank diameter. A commercial pre-CFD mesh generator (Gambit 2.4.6) coupled with a CFD software package (Fluent 6.3.26) was used. A standard k-ε model as well as a realizable k-ε model coupled with enhanced wall treatment was used to model the turbulence flow. Results were obtained using a multiple references of frames (MRF) approach. Transient simulations based on a sliding mesh technique were additionally conducted. Experimental work was also carried out with a single liquid phase as well as in solid-liquid suspensions with 0.5% dispersed solids.

In general, good agreement between the experimental data and the predicted results for the velocities distribution, power number, and pumping number were obtained. Both the experimental and the computational results show that there is a minimum liquid level (or minimum impeller submergence) below which: (1) the macroscopic flow pattern changes substantially; (2) the power number and the pumping number drop significantly; and (3) air entrainment occurs and solids suspension becomes problematic.