Effect of Solids Concentration on Cloud Height in a Stirred Tank
In mechanically stirred vessels containing solids, the most important process results are off bottom suspension and vertical distribution of the solids. There is a large literature on off bottom suspension but little on the vertical distribution of the solids. Some studies have looked at concentration distribution with height; however a simpler, visual and useful way is to look at the interface between the solids rich area and the solid lean area called the cloud height. This paper will discuss some of the parameters that control this cloud height. Two tank sizes and one impeller type were used with the chief variables being physical properties of the fluids and solids and concentration. It was found that the liquid level, if above the cloud height, had no effect on cloud height. In addition the minimum speed to suspend solids also had no effect. The biggest effects were speed and vessel size. Physical properties had a small but significant effect. At low solids concentration below about five percent by weight there was no effect on cloud height while above that value under certain conditions increasing solids lowered the cloud height significantly.
Correlations are given for cloud height at low solids concentrations and high solids concentrations.
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