Suspension of Solids with Gas Dispersion

Three-phase (gas-liquid-solid) systems such as gaseous slurry reactions in stirred vessels are common in the chemical industry. They present special mixing challenges. The presence of gas tends to disturb the liquid flow patterns established 

In a stndy of gassed solid suspension in an agitated vessel, Chapman (1981) found that for small-diameter (D = T/4) 45° pitched blade impellers, a sudden collapse of the snspension occurs at some critical gas rate. This is when the flow pattern becomes dominated by gas flow as opposed to impeller flow. The gas flow decreases the eddies and the upward velocities that maintain the suspension. 

A theoretical correlation for Njs by Baldi et al. (1978) implies that Njs for gassed slnrry systems is higher than for ungassed systems. This has been confirmed (Chapman et al., 1983) in experiments performed in 0.56 m-diameter vessels using particles of size greater than 80 mm and particle density greater than 1.2 g/cm in distilled water. Chapman found that as the gas rate is increased, substantial increases in Njs are required to achieve a complete suspension of the solids. He also found that the impeller speed required for the just suspended state is always higher than that required for a complete dispersion of the gas bubbles. At low gas rates (volume of gas per minute per volume of liquid, vvm, less than 0.75), he found 45° pitched blade impellers to be more efficient than disk or Rushton turbines for solid suspension. 

Shamlou and Zolfagharian (1990) have studied liquid-jet stirred suspension and found the mechanism of suspension to be similar to impeller-stirred suspension. The preferred design consists of a downward-pointing feeder nozzle centrally mounted with the tip fully submerged in the slurry. They found that to achieve an acceptable cloud height, the tip of the nozzle should be below half the slurry height. They showed also that  

There is no significant effect of the jet clearance—the distance between the tip of the jet and the vessel bottom—on the minimum jet velocity for solid suspension. The recommendation is to use the smallest practical jet clearance, but greater than eight jet nozzle diameters, to avoid erosion of the tank base. 

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