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https://hdl.handle.net/2142/66881
Description
Title
Mixing for Suspension of Solids
Author(s)
Boening, Paul Henrik
Issue Date
1980
Department of Study
Civil Engineering
Discipline
Environmental Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Sanitary and Municipal
Language
eng
Abstract
The power required to suspend several solid materials in a circular fully baffled tank was studied. A single 45 degree pitched blade impeller was used to produce downward flow. Sand, wheat straw, corn stover, activated carbon, and activated carbon-sand mixture were studied in this system. Several concentrations of the wheat straw, corn stover, and activated carbon were examined. The liquid level was equal to one tank diameter and the impeller diameter was one-half the tank diameter.
The degree of suspension reported quantitatively as the Mixing Index which is defined as, the sample concentration at the sample point divided by the theoretical average concentration. A mathematical model for the Mixing Index and power number was derived by linear multiple regression based on particle and fluid properties. Sampling procedures, particle property and fluid property determination techniques are covered in the thesis. The model equations for Mixing Index and power number are coupled by the use of the impeller Reynolds number and effective slurry viscosity. The model equations were restricted to conditions of partial initial suspension, for impellers effectively located above the settled solids. Confidence limits were determined for the final Mixing Index and power number equations.
It was found that a slow large axial flow impeller was more efficient than a small fast radial turbine impeller, based on the power number versus impeller Reynolds number relationship. The power required for mixing increased as the impeller was located closer to the bottom of the tank. The Mixing Index increased as the impeller was located closer to the bottom of the tank. The effect of impeller location decreased with increased impeller speed, as the fluid velocities increased in the tank. As the density of the solids increased relative to the fluid, the impeller location became more important.
The slurry apparent kinematic viscosity was found to be a function of slurry concentration expressed in terms of porosity. Hindered settling velocities, both measured and theoretically determined, were used to compute slurry apparent kinematic viscosity. As porosity approached 0.5, settling motion of a particle was of the compression type. Slurry apparent kinematic viscosity increased dramatically as particles lost their liquid lubrication under compression conditions.
The model was adapted to handle solids of two different densities. It was shown that the degree of suspension of the heavier material was dependent on the total porosity of the system due to both solids.
Using the equations developed in this study the power required to suspend a given solid to a given level of uniformity at a desired location in a circular baffled tank may be found subject to the limits placed on the equations.
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