Pseudoplastic Slurries

For suspension of free-settling particles, circulation of pseudoplastic slurries, and heat transfer or mixing of miscible liqiiids to obtain uniformity, a speed of 3.50 or 420 r/min should be stipulated. For dispersion of dry particles in hquids or for rapid initial mixing of hquid reactants in a vessel, an 11.50- or 1750- r/min propeller should be used at a distance Df/4 above the vessel bottom. A second propeller can be added to the shaft at a depth below the hquid surface if the submergence of floating hquids or particiilate solids is other wise inadequate. Such propeller mixers are readily available up to 2.2 kW (3 hp) for off-center sloped-shaft mounting. 

For suspension of rapidly setthng particles, the impeller turbine diameter should be Df/3 to Dfl2. A clearance of less than one-seventh of the fluid depth in the vessel should be used between the lower edge of the turbine blade tips and the vessel bottom. As the viscosity of a suspension increases, the impeller diameter should be increased. This diameter may be increased to 0.6 Df and a second impeller added to avoid stagnant regions in pseudoplastic slurries. Moving the baffles halfway between the impeller periphery and the vessel wall will also help avoid stagnant fluid near the baffles. 

This means that there is a much greater variety of shear rates in the larger tank, and in dealing with pseudoplastic slurry it will have a quite different viscosity relationship around the tank in the big system compared to the smaller system. 

The effects of polymer concentration, shear rate, and temperature were also investigated. As the polymer concentration increases (power law exponent n decreases, that is, the viscosity of the carrier fluid or suspending liquid increases), the relative viscosity of the pseudoplastic slurry decreases. It is important to know that the more viscous the fluid (lower n value), its relative viscosity will deviate further from the Newtonian predictions. Relative viscosity decreases as the shear rate increases. Again, the effect is more pronounced at higher solid concentration than at low solid concentration. Temperature also has a dramatic effect on the relative viscosity of slurries. This effect is partly due to the reduction in carrier fluid viscosity because of thermal effects. The relative viscosity increases as the temperature increases. Overall, Shah s experimental results agree more closely with Keck s than with any other reported study. 

The apparent viscosity, / app, is equal to the slope of a line from the origin to a point on the shear stress-shear rate curve it decreases or increases as the shear rate increases. Hence, the term viscosity for a non-Newtonian fluid has no meaning unless the shear rate is specified. In shear-thinning (or pseudoplastic) slurries, the apparent viscosity decreases as the shear rate increases and the value for n is less than one. In shear-thickening (or dilatant) slurries the apparent viscosity increases as the shear rate increases and the value of n is greater than 1 (Figure 4.2). 

The behavior of pseudoplastic fluids is difficult to define accurately. Various empirical equations have been developed over the years and involve at least two empirical factors, one of which is an exponent. For these reasons, pseudoplastic slurries are often called power-law slurries. The shear stress is defined in terms of the shear rate by the following equation ... 

For a pseudoplastic slurry or power law fluid, the shear stress is expressed by Equation 3-43. By analogy with the method developed for a Bingham flow in a tube, the following equation is expressed ... 

FIGURE 5-4 Values of critical Reynolds number and critical fanning factor versus the flow index n for pseudoplastic slurries based on Equation 5-27. 

FIGURE 5-5 Values of the critical value of the Hanks and Ricks Reynolds number versus the flow index n for yield pseudoplastic slurries. 

Slatter [4] defined Rer as the roughness Reynolds number for yield pseudoplastic slurry... 

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