Determination of the minimum agitation speed to attain the just dispersed state in solid-liquid and liquid-liquid reactors provided with multiple impellers

The determination of the minimum agitation conditions to achieve the just suspended state of the solid particles or the just dispersed state of an immiscible liquid in a mechanically agitated liquid in tank reactors is a problem of considerable industrial importance. Previous investigations on this subject have been limited to single-impeller systems only. In the present work the role of multiple impeller agitation systems on the achievement of the complete suspension of dispersion state was investigated both solid-liquid and liquid-liquid systems. The effects of a number of variables such as impeller type and position, clearance of the bottom impeller off the reactor bottom, distance between the impellers and other geometric and parameters were studied. The results indicate that, contrary to intuition, the presence of multiple impellers may not necessarily be beneficial to the achievement of the just dispersed state. In particular, it appears that if the flow pattern of the additional impellers contrasts with the flow pattern which would be established by a single impeller, then the just dispersed state may be achieved at agitation speed higher for the multiple impeller system than for the single impeller case. Even in the majority of cases in which the agitation speed actually decreases with the number of impeller on the shaft, it appears that the power required to achieved the just dispersed state is higher when multiple impellers are used.