Document Type

Thesis

Date of Award

1-31-1991

Degree Name

Master of Science in Chemical Engineering - (M.S.)

Department

Chemical Engineering, Chemistry and Environmental Science

First Advisor

Piero M. Armenante

Second Advisor

Dana E. Knox

Third Advisor

Demetri P. Petrides

Abstract

A large number of references can be found in the literature on the effect of mixing parameters on the achievement of the minimum agitation speed to just suspend solid particles. In the vast majority of these studies the agitation system consisted of just one centrally mounted impeller. However, the determination of the minimum agitation speed to achieve the just suspended state when the additional impellers are used have received very little attention. It is the intent of this work to investigate the role of multiple impeller agitation system on the achievement of the complete particle suspension state. The effect of a number of variables such as impeller type and size, the impeller clearance off the tank base, the spacing between impellers and the other geometric parameters were studied in detail.

The results obtained here indicate that, contrary to intuition, the presence of the additional impellers may not necessarily be beneficial to the achievement of the just suspended state. In general, it appears that the minimum agitation speed is only slightly affected by the presence of the additional impellers. In particular, if the flow pattern of the additional impellers contrasts with the flow pattern which would be established by a single impeller, then the just suspended state may be achieved at an agitation speed higher for multiple impeller systems than for single impeller systems. Furthermore, the power required to achieve the just suspended state is considerably higher whenever multiple impellers are used. This implies that the bottom impeller is primarily responsible for generating turbulence near the tank bottom (i.e. where the particles become suspended), and for producing particle suspension, independently of the number of impellers used.

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