Stirrer Power Requirements for Non-Newtonian Liquids

Industrial fermentors, as well as pipings, pipe fittings, and valves, and all parts that come into contact with the culture media and sterilized air are usually constructed from stainless steel. All ofthe inside surfaces should be smooth and easily polished in order to help maintain aseptic conditions. All fermentors (other than the glass type) must incorporate glass windows for visual observation. Naturally, all fermentors should have a variety of fluid inlets and outlets, as well as ports for sampling and instrument insertion. Live steam is often used to sterilize the inside surfaces of the fermentor, pipings, fittings, and valves. 

Instrumentation for measuring and controlling the temperature, pressure, flow rates, and fluid compositions, including oxygen partial pressure, is necessary for fermentor operation. (Details of instrumentation and control for fermentation are provided in Chapter 13.) 

Although local values of the shear rate in a stirred liquid may not be uniform, the effective shear rate (S- ) was found to be a sole function of the impeller 

Values of the consistency index K and the flow behavior index n for a non-Newtonian fluid can be determined experimentally. For pseudoplastic fluids, n 1. 

Estimate the stirrer power requirement P for a tank fermentor, 1.8 m in diameter, containing a viscous non-Newtonian broth, of which the consistency index A = 124, flow behavior index n = 0.537, density p = 1050 kg m , stirred by a pitched-blade, turbine-type impeller of diameter d = 0.6 m, with a rotational speed AT of 1 s .  

Although local values of the shear rate in a stirred liquid may not be uniform, the effective shear rate Seff (s ) was found to be a sole function of the impeller rotational speed N (s 1), regardless of the ratio of the impeller diameter to tank diameter (d/D), and was given by the following empirical equation [1]  

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Details of stirrer power requirement for non-Newtonian liquids are provided in Section 12.2. 

When estimating the stirrer power requirements for non-Newtonian liquids, correlations of the power number versus the Reynolds number (Re see Figure 7.8) for Newtonian hquids are very useful. In fact, Figure 7.8 for Newtonian liquids can be used at least for the laminar range, if appropriate values of the apparent viscosity are used in calculating the Reynolds number. Experimental data for various non-Newtonian fluids with the six-blade turbine for the range of (Re) below 10 were correlated by the following empirical Equation 12.1 [1] ... 

It is worth remembering that the power requirements of geometrically similar stirred tanks are proportional to N3 ds in the turbulent regime, and to N2 d3 in the laminar regime. Equal power per unit liquid volume is sometimes used as a criterion for scale-up. Details of stirrer power requirements for non-Newtonian liquids are provided in Section 12.2. 

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