Abstract

Design equations have been developed to estimate liquid velocities and mixing times in air agitated tanks. Determination of the gas rate necessary for adequate agitation in a given geometry is possible with this information. Air agitation offers benefits of increased dissolved oxygen and cost effective mixing for some waste water treatment applications.

Empirical expressions for surface and bottom velocities, as a function of gas flow rate and tank geometry have been developed from laboratory measurements. Since neither statistical nor dimensional analysis of the laboratory results could prove conclusively the correct form of the velocity correlations, the different correlation forms were used to verify large scale velocity measurements. Only one of the three trial correlations correctly predicted large scale velocities. The importance of these velocity correlations is evident from experience with mechanical agitator design, which shows that liquid velocity is the appropriate design criterion for most similar applications.

Mixing times were measured experimentally in the laboratory and studied with a mathematical model. The model was an unsteady state mass balance containing convective flow terms with turbulent dispersion super-imposed on the flow. The velocities for the convective flow terms were calculated from the empirical velocity correlations. Estimates of the turbulent dispersion coefficients were investigated experimentally.

Because multiple velocity correlations and a computer model for mixing time are difficult to use when performing design calculations, empirical correlations for bulk velocity and mixing time were derived. Combined with a relationship for power input, the design correlations provide information necessary to determine operating conditions in large scale, air agitated tanks. The effects of tank geometry on air agitated design have been explored within a range of typical construction dimensions. Thus, the principal elements of a complete design approach to air agitated rectangular tanks are presented.

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