Fluid Velocities in Mixing Equipment

Fluid Velocities in Mixing Equipment Fluid velocities have been measured for various turbines in baffled and unbaffled vessels. Typical data are summarized in Uhl and Gray, op. cit., vol. I, chap. 4. Velocity data have been used for calculating impeller discharge and circulation rates but are not employed directly in the design of mixing equipment. 

The Monte Carlo approach was extended to reactors with a plug flow macro-mixing RTD by Rattan and Adler [124]. Here, the coalescing fluid elements are moved through the reactor at a speed corresponding to the constant mean fluid velocity. Rattan and Adler [124] were able to simulate experimental results of Vassilatos and Toor [125]. The coalescence frequency was found from data for extremely rapid reactions, where the observed rate is essentially completely controlled by the micromixing in this situation with a flat velocity profile. Then, these coalescence rates were used to predict the experimental results for rapid and slow reactions taking place in the same equipment. 

In the introduction to this chapter it has been already pointed out that for the design of chemical process equipment rules of thumb exist. Upon closer examination, these rules provide conditions which unconsciously accept partial similarity. Actually, one cannot expect that complicated processes of fluid dynamics occurring during mass and heat transfer can be adequately described by criterions such as power per unit of volume, P/V, for mixing vessels and superficial gas velocity, v = q/S, for bubble columns. 

In many multiphase (gas-liquid, gas-solid, liquid-liquid and gas-liquid-solid) contactors, a large degree of circulation of both discrete and continuous phases occurs. This circulation causes a good degree of mixing and enhances heat and mass transfer between fluid and walls. The degree of circulation depends on a number of parameters such as the size of equipment, the nature of the phases involved, velocities of various phases, nature of the internals within the equipment and many others. 

---