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The just-suspended condition is often regarded as the optimum criterion for liquid-solid agitation in chemical industries for processes such as crystallizing and dissolution. Literarily it is defined as the minimum speed that all particles are in motion and no particle remains on the vessel base for more than 1 to 2 seconds. Compared to uniform suspension in which the solids are dispersed throughout the liquid phase, just-suspended agitation reduces power consumption and equipment investment while exposing the entire solid surface to surrounding liquid. In this research, the characteristics of coal powders just suspended in water prior to transportation in pipelines were studied. Zwietering developed a correlation for just-suspended speed that indicated that it was affected by various parameters, such as solid and liquid properties and impeller characteristics, with each effect being presumed to be independent. However, this correlation was developed based on limited data such as low solid loading. In the current study, the solids loading exponent reflecting the effect of high solids concentration on just suspended speed is contrasted with Zwieterings' correlation. Also, effort was devoted to exploring the influence of scale-up in coal slurry just-suspended agitation. Additionally, the unsuspended solids fraction at stirrer speeds below just-suspended speed was studied. The solids loading exponents of this study were higher than those found by Zwietering, and the higher the solids loading, the higher the exponent. The scale-up exponent that describes the effect of scale on the just-suspended speed was found to depend on solids loading, with the scale-up exponent increasing with increasing solids loading. It was found that the suspended solids fraction did not fall below 98% until the stirrer speed was decreased to 70% of the just-suspended speed. Specifically, reduction of the speed by 30% reduces power requirements by 65% while keeping 98% of the solids in suspension.
Kevin J. Myers
Primary Advisor's Department
Chemical and Materials Engineering
Stander Symposium poster
"The Influence of High Solids Loading Concentration and Scaling-up Operation in Coal Slurry Just-Suspended Agitation by High-Efficiency Impellers" (2013). Stander Symposium Projects. 294.