Impeller power draw across the full Reynolds number spectrum
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. Compared to uniform suspension in which the solids are dispersed throughout a 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 different parameters, such as solid and liquid properties and impeller features, 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 Zwietering's correlation. Also, effort was devoted to exploring the influence of scale-up in coal slurry just-suspended agitation. The solids loading exponents of this study were higher than those found by Zwietering, and the solids loading exponent increased at higher solids loadings. The solids loading exponent was studied specifically in three ranges of Zwietering's solids loading from low (0.25 to 5), moderate (5 to 33) to high (33 to 100) and the higher the solids loading, the higher the exponent. Additionally, the solids loading exponent was observed to depend on scale (i.e. -- vessel size), with the solids loading exponent decreasing with increasing scale. This indicates that, contrary to the Zwietering correlation of the just-suspended speed, the effects of different parameters on the just-suspended speed are not independent. 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.