Two-phase mixing processes : some liquid-liquid and liquid-solid systems design

Mechanically stirred vessels are widely used in the materials processing industry for mixing immiscible phases. The basic phenomena and principles of multiphase systems (liquid-liquid, liquid-solid, gas-liquid) were described in the article [J 3 801] . The aim of this article is to present several examples of calculations which illustrate the basic concepts described above.

The first two examples concern the agitation of liquid-solid and liquid-liquid systems. First of all, the purpose of agitation must be clearly defined. For liquid-solid systems, it means keeping solid particles in suspension in the vessel. For liquid-liquid systems, it's to obtain a dispersion of droplets of a given diameter. Next, we need to calculate the operating conditions of the agitation system, characterized by the agitator speed and power dissipation.

Having selected the agitator(s) best suited to the desired mixing operation, the proposed calculations show the influence of the various parameters on the conditions for maintaining suspension or obtaining an emulsion of fixed size. For example, for liquid-liquid systems, the crucial role of interfacial tension is clearly demonstrated. The material transfer aspect is addressed by calculating the values of material transfer coefficients. Theoretical approaches are also used to select the invariant in the case of large-scale extrapolation of geometrically similar systems.

The third example involves estimating the dissolution time of solid particles in a stirred tank. It is based on a non-steady-state material balance. An analytical solution is proposed for specific conditions.