Abstract
Previous work (Bao et al., 2005) reported the influence of solid concentration and agitator selection on the just-drawdown agitation speed and gas holdup in three-phase reactors containing nonwetting buoyant solids. This paper reports experimental results for the unaerated and aerated aqueous suspension of buoyant (polypropylene and polyethylene) particles in a tall vessel of 0.476 m diameter. The composite agitators used a concave blade dispersing impeller surmounted by one or two up-pumping wide-blade hydrofoils. To find the influence of the density and size of particles on the drawdown of particles, all solids were cleaned up to four times with pure alcohol until they were hydrophilic. The influence on suspension and gas retention of solid density (900-955 kg.m -3 ) and size (0.5-4 mm), baffle width, and agitator centering have been studied. There is little difference in the gas retention behavior in the presence of similarly sized different particles. Gas holdup is relatively unaffected by particle size, though particles <1 mm tend to reduce gas retention. Baffle width has significant effects: drawdown is easier with either narrow-or full-width baffles than with midwidth baffles, which need higher agitator speeds. In a fully baffled three-phase system, the drawdown of buoyant particles results from the bulk velocity and turbulence intensity, while with narrow baffles, conditions appear to be controlled by the development of a central vortex. However, gas dispersion in such a partially baffled system is not as effective as when fully baffled, so the fully baffled conditions are recommended for the sparged drawdown of buoyant particles. Despite the possible mechanical drawbacks, off-centered agitators achieve effective drawdown of buoyant particles in both sparged and unaerated systems. Off-centering the agitator does not affect the retained void fraction except at low gassing rates and specific power input.
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