Batch blend time in square stirred tanks

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Abstract

Batch mixing in cylindrical and square stirred tanks was studied using a spectrophotometer. A number of different impeller types and different impeller diameters were tested. The results for the cylindrical tank compared very well with the Grenville correlation for blend time. A nearly identical correlation equation can be used for the square tank if the “diameter” of the square tank is based on the side dimension of the tank and the power number is assumed to be the same as that for a cylindrical tank. The square tank geometry is widely used in the water treatment industry, where this correlation is expected to find useful application.

Introduction

Stirred tanks are often used in industry for blending and for homogeneous liquid phase reactions. In both cases, impeller and motor design requires calculation of blend time based on the impeller characteristics and the tank geometry. Many workers have measured and correlated blend time, particularly for cylindrical tanks. A well-accepted correlation is available for this geometry; however, almost no information is available for the square tank which is widely used in the water treatment industry (Clark et al., 1994). The objective of this work is to validate the blend time correlation for a range of impeller geometries in cylindrical tanks, and to define a blend time correlation equation for square tanks.

Section snippets

Experimental setup

A diagram of the equipment used to measure the blend time in the stirred tank is shown in Fig. 1. Experiments were carried out in cylindrical tanks with tank diameters of 0.14 and 0.24m, and in square tanks with side lengths of 0.178 and 0.28m. Four baffles of standard width (W=T/10) were used in the cylindrical stirred tank with flat bottom. No baffles were used in the square tanks. The corners act as virtual baffles, as shown by the bold lines in Fig. 2. This prevents formation of a surface

Results and discussion

For all impellers tested, Nθ, the product of blend time and impeller rotational speed, was averaged from 12 runs in the big tanks and 6 runs in the small tanks. The power numbers in the square tank are taken to be the same as those for the cylindrical stirred tank. The power number for the paddle impeller is calculated from Sano and Usui (1985), others are standard literature values which were verified by Zhou and Kresta (1996). The results, shown in Fig. 5, agree very well with the published

Conclusions

The blend time correlation for cylindrical tanks can also be used for square tanks if the side dimension of the tank is used as the “diameter” and the correlation constant is increased from 5.8 to 6.0. This suggests that the square tank is a somewhat less efficient mixing geometry than the cylindrical tank.

Notation

Coff bottom clearance of the impeller,
measured to the bottom of the blades, m
Cinormalized concentration at probe i
Dimpeller diameter, m
Hheight of fluid, m
nbnumber of blades
Nrotational speed of the impeller, rps
Nppower number
Reimpeller Reynolds number, dimensionless
ttime, s
Tdiameter of cylindrical tank, m
Tslength of the side of the square tank, m
Wwidth of impeller blade, m
Greek letters
αblade angle, deg
θblend time, s
σvariance of normalized concentration

Acknowledgements

The authors would like to acknowledge Dallas Chapple, who provided technical support for the blend time measurements, as well as Lightnin and NSERC, who funded this project through a Strategic Grant.

References (6)

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