Evaluating Mixing in Stirred Reactors by 3-D Visualization: Partial Segregation for Dual-Feed Semi-Batch Operation

Stirred vessel reactors, when operated semi-batchwise, will exhibit departures from perfect mixing if the reactions being carried out are faster than the rates of mixing induced by stirring. This gives rise to partial segregation between the primary reagents, which will have consequences for selectivity from complex sets of chemical reactions. The extent and intensity of this partial segregation can be visualized in 3-D by single point acid/ alkali additions in conjunction with coloured pH indicators. The role of impeller type and design in determing this reactive partial mixing remains obscure. Results for a standard angled blade impeller and an A320 are presented using this approach. The greater convection induced by the Lightnin’ impeller produces a less marked, though still significant segregation, when simultaneous ‘front’ elevation and ‘underneath’ plan images from a glass semi-tech scale vessel are compared. Surprisingly, there is extreme segregation in the tangential direction, which is difficult to deduce from front viewimages alone. The 3-D visualizations are supported by CFD simulations using PHOENICS which are presented in see-through solid-body graphics. These theoretical image reconstructions confirm that the degree of partial segregation is more intense for the angled-blade as compared to the A320 impeller. However, the use of an assumption of perfect micromixing in each CFD volume element possibly exaggerates the predicted absolute levels of segregation between acid and alkali.