Micromixing energy

Guichardon etal. (1994) studied the energy dissipation in liquid-solid suspensions and did not observe any effect of the particles on micromixing for solids concentrations up to 5 per cent. Precipitation experiments in research are often carried out at solids concentrations in the range from 0.1 to 5 per cent. Therefore, the stirred tank can then be modelled as a single-phase isothermal system, i.e. only the hydrodynamics of the reactor are simulated. At higher slurry densities, however, the interaction of the solids with the flow must be taken into account. 

The conventional scale-up criteria scale-up with constant stirrer speed , scale-up with constant tip speed and scale-up with constant specific energy input are all based on the assumption that only one mixing process is limiting. If, for example, the specific energy input is kept constant with scale-up, the same micromixing behaviour could be expected on different scales. The mesomixing time, however, will change with scale-up as a result, the kinetic rates and particle properties will be different and scale-up will fail. 

For a reactive process, the reactants must be brought into contact by mixing before a reaction can occur. In a motionless mixer in turbulent flow, the pressure drop defines the turbulent energy dissipation rate, which then determines the macro-, meso-, and micromixing rates. 

The reason for the decrease in the micromixing time with the increase in impinging velocity is clear a higher impinging velocity implies an increased energy dissipation rate and thus more efficient micromixing. 

The micromixing state in an agitated vessel is connected with coalescence of bubbles. Coalescence occurs mainly in the stirrer zone, where the recirculated gas bubbles partially mix with fresh gas in the cavities. It also occurs to a lesser extent in the highly turbulent stream leaving the stirrer, but it is virtually absent in other parts of the vessel because the low kinetic energy of the bubbles cannot stretch out the liquid film between a pair of bubbles to reach the coalescence thickness. 

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