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Effects of impeller design, baffle, and gas flow rate on distributions of floating solid particles were examined in a stirred tank of 0.2 m i.d. for solid concentrations up to 50 vol %. Dual small cross-section impeller systems, i.e., dual four-flat blade disk turbines and dual four-pitched blade downflow disk turbines, and large cross-section impellers, i.e., Maxblend impeller and Fullzone impeller, were used. The minimum impeller speeds for off-surface floating-particle suspension decreased with aeration because bubbles rising near the tank wall enhanced the breakup of the floating-particle stagnant layer formed on the liquid surface and then the dispersion of floating particles into the liquid. The minimum impeller speeds for ultimately homogeneous floating-particle suspension also decreased with aeration. These results are contrary to those for the settling particles. The local solid particle concentrations at different heights in the stirred tank were measured. The axial solid particles concentration profiles were examined using the proposed one-dimensional floating-particle dispersion model. The Peclet numbers for floating-particle dispersion in the model were reasonably correlated in terms of impeller speed, power consumption, and forces exerted on floating particles. It was found that the large cross-section impellers could disperse completely floating particles into the liquid with less agitated speed and power consumption as compared with the dual small cross-section impeller systems used in this work.

or efficient design of the slurry stirred tank reactors, it is essential to know the solid particle distributions in stirred tank reactors.F However, a number of studies have focused on the just-suspended condition. In many slurry processes, it is important to operate a t an impeller speed that ensures that the particles are well mixed and completely incorporated into the liquid. For industrial stirred tank reactors, therefore, there is need to understand the distribution states of solid particles besides the just-suspended condition.Several investigators (Bourne and Sharma, 1974;Fajner et al., 1985;Brucato et al ., 1991;Yamazaki et al ., 1991; MacTaggart et al., 1993; Hicks et al., 1997) have examined the solid particle distributions in stirred tanks with solid-liquid two-phase systems. Nevertheless, the published literature does not give sufficient elucidation regarding hydrodynamics in gas-liquid-solid three-phase stirred tank reactors. Solid suspension in a gassed system may be primarily determined by gas-liquid hydrodynamics in the impeller region. Therefore, it is definitely desirable to study the solid suspension in aerated systems.The use of multiple impellers has been justified to obtain good distribution of solids in high aspect ratio stirred tank reactors (Hicks et al., 1997). Despite this fact, available information on solid suspension in stirred tank reactors with multiple impellers is limited (Harnby et al., 1985; Tatterson, 1994; Dutta and Pangarkar, 1995).The objective of this work is to examine solids suspensions in slurry stirred tank reactors with multiple impellers and to develop correlations for solid dispersion. We measured the minimum impeller speeds for ultimately homogeneous solid suspension defined as the speeds beyond which more uniform or homogenous solid suspension are not obtained. . While the two four-pitched blade downflow disk turbines were chosen to provide good gas dispersion, the Pfaudler type impeller was installed exclusively to *Author to whom correspondence may be addressed. E-mail address: bc&awase@ eng. foyo.ac. j p Solids suspension characteristics in gas-liquid-solid three-phase stirred tanks with multi-impellers were experimentally examined. Minimum impeller speeds for ultimately homogeneous solid suspension have been measured stirred tank reactors. Three impellers were installed: two four-pitched blade downflow disk turbines and one Pfaudler type impeller chosen to provide good gas dispersion and to accomplish off-bottom suspension of solid particles, respectively. Gas dispersion causes an increase in particle sedimentation associated with a decrease in power consumption and as a result, minimum impeller speeds for ultimately homogeneous solid suspension increase with increasing gas flow rates. A correlation was developed to predict minimum impeller speeds for ultimately homogeneous solid suspension. The proposed correlation, which agrees satisfactorily with the experimental results, is expected to be useful in design and scale-up.On a examine exp6rimentalement le...

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