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Variety of Agitation Equipment have been developed

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Agitation is the key to many heat and mass transfer operations that rely on mixing. Process requirements vary widely, some applications requiring homogenization at near molecular level while other objectives can be met as long as large scale convective flows sweep through the whole vessel volume. Performance is crucially affected both by the nature of the fluids concerned and on how quickly the mixing or dispersion operation must be completed. For these reasons a wide variety of Agitation Equipment have been developed. Conventional, mechanically agitated, stirred tank reactors may be used for either batch or continuous processes, though the design and operating constraints are different in the two cases. Low viscosity fluids can usually be mixed effectively in baffled tanks with relatively small high speed impellers generating turbulent flows, while high viscosity (typically above about 10 Pa s) and non-Newtonian materials require larger, slow moving agitators that work in the laminar or transitional flow regimes. It is convenient to classify impellers as radial or axial pumping depending on the flow they generate in baffled tanks.

Mixing rates in agitated vessels are predicted through measurement of the flow patterns which determine them. These measurements suggest the use of a model that assumes that nearly all the mixing occurs in a small “perfectly mixed” region near the impeller, with flow throughout the remainder of the tank serving primarily to bring the fluid into this region of the impeller. On the basis of this model, equations were developed for relating volumetric flow rates, hence the mixing rates, to the operating variables. While the theory could be checked directly only to Reynolds numbers of slightly over 600 (owing to limitations of the experimental technique employed in this part of the mixing‐rate studies), the volumetric flow rates could be measured from Reynolds numbers of 36 to 1.7 × 104. The times required for completion of an acid‐base neutralization (terminal mixing) were also measured from Reynolds numbers of 1.6 to 1.8 × 105. Flat‐blade, dimensionally similar turbines with diameters of 2, 4, and 6 in. were used. Tank diameters ranged from 5.76 to 15.5 in. The baffle width equaled one tenth of the tank diameter in all runs. All the data were for Newtonian fluid systems, but the extension of this work to non‐Newtonian materials is discussed briefly.

The nonstandard design problem that has to be solved to be able to build such equipment was solved using a step-by-step approach: the first step was to build a large laboratory pressure-Thickening Equipment unit and determine the parameters of the process under laboratory conditions; the second step entailed constructing a three-dimensional hydrodynamic model that could be used to build a specimen suitable for factory tests. Data obtained from these tests would then be used to validate the hydrodynamic model, which would in turn make it possible to choose the best of several variants for modernizing the existing leaching equipment without having to make a large capital investment.

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