Stirred vessels large scale flow

This means that there are no draft tubes, tube brmdles, or packings. A large-scale flow usually turbulent is maintained by the stirrer. In many cases solid or fluid particles (brrbbles or drops) are more or less evenly distributed in a liquid continrrous phase. In Fig. 3.7-1 often rrsed stirred vessels are depicted ... 

First of all, the increased computer power makes it possible to switch to transient simulations and to increase spatial resolution. One no longer has to be content with steady flow simulations on relatively coarse grids comprising 104-105 nodes. Full-scale Large Eddy Simulations (LES) on fine grids of 106—107 nodes currently belong to the possibilities and deliver realistic reproductions of transient flow and transport phenomena. Comparisons with quantitative experimental data have increased the confidence in LES. The present review stresses that this does not only apply to the hydrodynamics but relates to the physical operations and chemical processes carried out in stirred vessels as well. Examples of LES-based simulations of such operations and processes are due to Flollander et al. (2001a,b, 2003), Venneker et al. (2002), Van Vliet et al. (2005, 2006), and Flartmann et al. (2006). 

A particular problem that frequently arises with suspension duties in very large stirred vessels with several side-entry agitators installed below the liquid surface is that on scale-up the volume increases in proportion to the cube of the linear scale-up factor whereas the surface area increases in proportion to its square. If the arrangement of the impellers found to give a certain rotary flow pattern for optimum suspension of the suspended solids at the model scale is retained at the industrial scale, a fundamentally different flow pattern would result that is totally unsuitable for suspension purposes. Particular account must be taken here of the fact that the secondary flow patterns in the large vessel are scale dependent on account of changed ratio of inertial to frictional forces. Other scale-up criteria derived from considerations of a theoretical free jet are required to achieve an optimum suspension of the particles. 

It must be emphasized that even though these results seem reasonable and that extensive validation of stirred-tank simulations has been done for various continuous flow and semibatch cases, only isothermal validation has been done, and no large ratio scale-up validations have been done. The feed time of 50 s for a 3.8 m vessel is very short and exaggerates the heat effects. These aspects of the problem need to be studied before complete confidence may be placed on the CFD simulation of highly exothermic reactions. 

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