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Published: Kem. Ind. 52 (9) (2003) 433–441
Paper reference number: KUI-16/2003
Paper type: Conference paper
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Determination of the Mixing Time and Power Consumption for Floating Solid-Liquid Dispersion in Mechanically Agitated Tank Provided with Multiple Impellers

N. Kuzmanić and R. Žanetić+


In the proces industry contact between solids and liquids is frequently obtained by the use of the mechanically agitated vesels. Today there is a considerable body of research activities on the suspension of solids heavier than the liquid medium. However, only a few studies have been reported on the complete drawdown of floating solids in liquid by agitation, although dispersion of floating particles is a proces of significant industrial importance. The predominant motivation for the present work is to investigate capability of the agitated system provided with multiple impellers on the suspension of floating solid material in liquid. Experiments were performed in cylindrical flat bottom vesel with internal diameter of 0.32 m. The tank was equipped with four baffles placed at 90o around the vesel periphery. The suspension of floating solids (PEHD particles) was stirred by two down-pumping three-pitched blade turbines (3-PTD type of impeller); (D/T = 0.27-0.45; blade angle -30o). The influence of floating solids concentrations, surface tension, impeller diameter, off-bottom impeller clearance as well as the spacing between impellers on the mixing time, values of nJS and (P/m)JS were examined in detail. Mixing time values were measured by conductivity measurements using a pulse technique, whereas, the power consumption was determined by the torque table. The critical impeller speed for the complete suspension of floating solid particles, nJS, was determined using Joosten visual method. In this case the value of nJS was defined as the speed at which all stagnant zones of floating solids at the liquid surface had just disappeared. On the basis of the experimental results and observations, could be concluded that the application of the second 3-PTD impeller on the same shaft improves the performance of the agitation system for the proces of floating solids suspension. It was observed that the presence of additional (second) impeller actually produced the reduction in the power consumption to achieve complete suspension of floating particles, in the minimum impeller speed for the complete suspension of floating solid particles, nJS, as well as decrease of mixing time of system examined.

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suspension, floating particles, mixing, multiple impellers, just suspended state, power consumption, missing time