Dave Dickey Forum Moderator 286 Posts
Re: Can I use any correlation factor of liquid mixing for solid suspension?12 December 2011 at 1:29pm
Some correlations for liquid mixing can be used for solids suspension, but none of them will directly predict whether solids can be suspended or not. Liquid blending depends primarily on the liquid density and viscosity. Solids suspension involves variables such as particle size, particle density, liquid density, and solids concentration. If the particles are small (settle slowly) and the concentration is high, the slurry may behave like a viscous fluid, in which case liquid blending correlations may predict mixing behavior.
An impeller power number only correlates the relationship between impeller diameter, rotational speed, and liquid density, it does not predict the success for failure of mixer performance. The impeller power number is a constant for turbulent conditions, but requires a correction for the effect of viscosity if the impeller Reynolds number is less than 20,000. The impeller power number is effectively unchanged whether a given (diameter and speed) impeller is applied in a small or large vessel, even though the mixing intensity will be drastically changed, to the point of mixing failure in the large vessel.
A power number of 1.37 applies to a four-blade, pitched-blade, 45-degree angle, blade-width to impeller-diameter ratio of one-fifth (w/D=0.2) impeller. If the number of blades, blade angle, or blade width to diameter are different, the impeller power number will be different. An impeller with a w/D = 1.25 is unusual, since the blade width is greater than the impeller diameter. Some impellers, described as elephant-ear impellers, may have blades that wide, but often have only two or three blades.
Correlations for solids suspension can be found in several references, such as the Handbook of Industrial Mixing, John Wiley, 2004 and "The high-efficiency road to liquid-solid agitation", Corpstein, et. al., Chemical Engineering, October 1994. Neither of these references provide guidance for solids suspension with unusual impeller geometries. Solids suspension with unique impeller designs or vessel geometries often require small-scale testing and scale-up. CFD analysis is not effective in predicting solids suspension or scale-up.
A second 4PBT impeller has little effect on solids suspension, because the lower impeller defines more than 90% of the off-bottom suspension. The second impeller only reduces the concentration in the lower portion of the vessel. The general effect of concentration is important, but the effect of concentration uniformity in a single vessel is small.
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