Multiphase Phenomena and Design of Liquid-Liquid Stirred Tanks

Liquid-liquid stirred tanks are used as various unit operations, such as separation (liquid-liquid extraction), as multiphase reactors, or to formulate or homogenize emulsion products. Among multiphase systems in stirred tank operations, liquid-liquid dispersions are perhaps the most challenging. This is partly due to their inherently nonlinear phase equilibria and sensitivity of operation to small amounts of impurities. Reliable design of liquid-liquid stirred tanks and related operations requires both well-established simple correlations and rigorous mech¬anistic models, in connection with qualitative experience. Fluid flow in stirred tanks can be modeled with varying level of detail, from an ideal mixing assump¬tion up to a numerical solution of flow fields with multiphase computational fluid dynamics. Optimal approach should be selected based on the design needs, keeping in mind that the fluid flow pattern alone is rarely the primary design goal. Often the final design criteria are related to mass transfer and drop size distributions, and thus, particular attention needs to be paid to model these properly. Combining qualitative understanding of liquid-liquid stirred tank sys¬tems first with simple correlations and then with rigorous mechanistic modeling builds confidence to design.