Suspension and liquid homogenization in high solids concentration stirred chemical reactors

In many crystallization processes and other solid-liquid reactors, solids a re present in high concentrations. Often these reactors are operated in the semi-batch mode and the purity, productivity and selectivity of the reacti on (and in the case of precipitation and crystallization, the size distribu tion and morphology too) depends on the relative rates of mixing (or homoge nization) compared to chemical reaction. It has usually been assumed that, provided the solids are all fully suspended, the rate of mixing is similar to that found in single-phase systems. Recent extensive work carried out by the above team at The University of Birmingham with support from DuPont ha s analysed the processes of homogenization and particle suspension. It has shown that, for typical industrial conditions, there is a range of operatin g conditions in which the solid particles at high concentration are fully s uspended but have a clear liquid layer above them in which the local specif ic energy dissipation rate appears to be very low. Under these conditions, the mixing time may be two or more orders of magnitude longer than in the s ingle phase case. This paper describes the phenomenon, analyses its importa nce, develops a physical model and proposes ways of overcoming the problem.