263791 Mixing Time in Partially Filled Stirred Vessels

Tuesday, October 30, 2012: 9:20 AM
Frick (Omni )
Shilan Motamedvaziri and Piero Armenante, Otto H. York Department of Chemical, Biological and Pharmaceutical Engineering, New Jersey Institute of Technology, Newark, NJ

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, Cb/T=0.30 and for different H/T values (corresponding to different values of the impeller submergence ratio, Sb/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 tBR, 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 Sbc/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 Sb/D ratios, the predicted results from CFD simulations, and the proposed correlation for mixing time was obtained.

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See more of this Session: Mixing In Single Phase Systems
See more of this Group/Topical: North American Mixing Forum