(226c) Mixing Time in Partially Filled Stirred Vessels

Mixing time, the time required to achieve a predefined level of homogeneity of a tracer in a mechanically stirred vessel, is an important parameter to evaluate the mixing efficiency of the system. In this work, mixing time was obtained for the case in which the liquid height-to-tank diameter ratio, H/T, is less than 1, i.e., for partially filled vessels.  The time, t₉₅, required to achieve a 95% level of tracer homogeneity a stirred vessel with a disk turbine with D/T=0.31, C_b/T=0.30 and for different H/T values (corresponding to different values of the impeller submergence ratio, S_b/D) was measured experimentally using a colorimetric technique coupled with image processing and predicted computationally using CFD simulations.  In addition, a correlation between mixing time and t_BR, defined as the ratio between the liquid volume in the tank and the flow rate generated by the impeller, was developed.  The experimental results show that there is a critical impeller submergence ratio S_bc/D below which mixing time increases suddenly and significantly (implying that the mixing efficiency drops significantly) compared the mixing time at higher submergence ratios.  Above the critical impeller submergence ratio t₉₅ decreases with decreasing impeller submergence ratio although the non-dimensional mixing time, t₉₅N, remains constant irrespective of the agitation speeds for each impeller submergence ratio. Lower values for t₉₅N can be observed for lower submergence ratios.  In general, good agreement between the experimental data at different S_b/D ratios, the predicted results from CFD simulations, and the proposed correlation for mixing time was obtained.