Mixing patterns in Rushton-turbine-agitated reactors

This Ph.D. Thesis concerns the study of mixing in Rushton turbine agitated reactors. In industrial (bio)reactors, mixing is recognised as a limiting factor that significantly affects the performance of the process. This limitation has motivated the present study.

It is first shown how planar laser-induced fluorescence (PLIF) can be used to investigate mixing in a model reactor. The principles of laser-induced fluorescence are presented, followed by the calibration methodologies developed to analyse the fluorescence images. Advanced mathematical concepts are then developed for the quantitative description of mixing. This approach is based on the description of large eddies that transport the fluorescent tracer in the reactor. The results presented are aimed at a better understanding of the effect of the location of the injection port and that of the turbine agitation speed on the size, the shape and the spatial instability of coherent mixing structures.

The results of mixing experiments carried out in industrial stirred and gassed reactors are then presented. A new empirical correlation for estimating the mixing time in production tanks has been developed, tested and validated for several reactors. The results are related to the flow structure in two gas-liquid phases. The strong compartmentalisation of the reactors is evidenced both by the mixing model and by the experimental data. The formation of concentration gradients, overshoots and oscillations has been further investigated in one of the production reactor studied. The results obtained show that two types of flow structures are possible and, that the flow structure seriously influences the mixing process. It is furthermore demonstrated that it is possible to modify the mixing by adjusting the agitation and aeration parameters. The potential of such results is further discussed in the final chapter.