No abstract

Despite much research on gas-liquid-solid systems and their widespread application in industry, gas dispersion with solid suspension in multistage stirred reactors equipped with multiple impellers has received little attention. We report here the critical just-suspension impeller speed for different concentrations of solid particles, gas holdup, and shaft power in a vessel of 0.48 m diameter with four baffles and dished base. Five agitator configurations, each with three impellers mounted on a single shaft, have been used in the experiments. Two novel impeller designs were used, a deep hollow blade (semi-ellipse) disc turbine (HEDT) and four-wide-blade hydrofoil impellers. The hydrofoils were used in both up-pumping (WH U ) and down-pumping (WH D ) modes. Glass beads of 50 $ 150 lm diameter and density 2500 kg Á m À3 were suspended at solid volumetric concentrations of 1.5, 3, 6, 9, and 15%. Results show that these suspended solids have little effect on the relative power demand. Agitators using the HEDT radial dispersing impeller at the bottom have a higher relative power demand (RPD ¼ P G =P U ) than those with WH D or WH U as the lowest one. For all impeller combinations there is little or no effect on gas holdup with increasing solid concentrations. Of the five different impeller combinations, those with an axial flow bottom impeller have significantly higher just-suspension agitation speeds and power consumption, so mounting the hydrofoil impeller at the bottom is not the optimal configuration for particle suspension. Of these impeller combinations, at a given gas flow rate the arrangement of HEDT þ 2WH U has the highest relative power demand, gas holdup, and power input for both the suspension of settling particles and gas dispersion.

Many industries use gas-liquid stirred-tank reactors with a third phase of buoyant particles. Few studies have looked at the effects of buoyant particle characteristics on solid-liquid suspension and gas-liquid dispersion, and even less have considered operation at elevated temperatures. This paper reports power consumption and gas holdup measurements made in a hot-sparged three-phase system. The vessel was a dished-base reactor of diameter 0.476 m (T) holding 0.145 m 3 of liquid, agitated by a multi-impeller agitator (a hollow half-elliptical blade dispersing turbine below two up-pumping wide-blade hydrofoils, identified as HEDT+2WH U ). This configuration has been recommended in previous work. Air, deionized water, and polypropylene (PP) particles were used in this work. The temperature of the water was in the range 80.5-82.5 °C. The results show that, as in a cold system, the effect of the solid concentration on aerated agitator power demand is negligible. There is little difference between the agitator power demand in cold-and hotsparged systems, except that the relative (gassed-to-ungassed ratio) power demand, RPD, is a little higher in hot-sparged conditions. The influence of power consumption and gas rate on gas holdup is slightly less when hot than when cold. Gas holdup in the hot-sparged system, which, with similar total gas rates and power input, is only about one-half or two-thirds of that at room temperature, is slightly increased at higher solids concentrations. The reasons for the differences in gas dispersion between cold-and hot-sparged systems are analyzed and discussed in this paper, with further experimental and CFD studies planned for future work.

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.