Stirred vessels: Computational modeling of multiphase flows and mixing

Stirred vessels are widely used in the pharmaceutical industry to carry out a large number of reactions and recipes involving multiple phases. Stirred vessels offer unmatched flexibility in operation and many degrees of freedom for design engineers. The large number of options available in selecting the shape and size of a vessel, type, size, number, and locations of impellers, spargers, and inlet/outlet nozzles often complicates the scale-up and design procedures and leads to uncertainty about the optimum selection. This is mainly because despite the widespread use of stirred vessels, their scale-up and design are still mainly based on empirical methods. Recent advances in understanding of multiphase flows, numerical techniques/algorithms, and computing resources provide opportunity to establish a direct link between configuration and operating protocols of stirred vessels and their performance via detailed simulations of fluid dynamics and other processes occurring in these vessels. Multiphase flows and mixing in stirred reactor exhibit different regimes. Often the dispersed phases in stirred vessels form specific structures especially around rotating impeller blades. It is important to critically evaluate the current understanding of multiphase flows in stirred vessels and translate this understanding into ready-to-apply computational models for simulating complex multiphase flows in stirred vessels. Such an attempt is made here. In this chapter, we critically review the state of the art of computational modeling of multiphase flows and mixing in stirred vessels, demonstrate the extent of applicability of this with some examples, and provide our views on the path forward. Role of turbulence, multiphase flow, interphase interactions (drag, lift, virtual mass, coalescence and breakup, etc.), and flow regimes are critically analyzed. The second part of the chapter discussed application of computational models to address variety of industrial processes. An attempt has been made to evolve (i) recommendations for industrial applications based on today's state of the art and (ii) recommendations for the path forward in enhancing application horizons of computational models. The computational models and results discussed in this work are useful for simulating industrial multiphase stirred vessels and for formulating further research in this area.