Macro-mixing in the Drais-reactor

During semi-batch polycondensation, the viscosity of the reaction medium increases dramatically, which can cause a change in flow condition from turbulent to laminar. This change implies that two types of agitation are needed for mixing and heat removal. A reactor that is especially suitable for fluid mixing under high viscous conditions is the Drais-reactor. This reactor is a horizontally positioned cylindrical vessel with a heavily designed stirrer. Its mixing performance in general, and the macro mixing in particular, were studied under turbulent conditions, in order to investigate its application in a polycondensation process. The goal was to provide scaling rules that relate macro-mixing times to process parameters such as the reactor's length/diameter-ratio L/D, Reynolds-number Re, and degree of filling x. Mixing times in water were measured with the experimental technique Planar Laser Induced Fluorescence (PLIF). With PLIF it is possible to visualise the mixing process of fluorescent dyes in a translucent scale model of the Drais-reactor. A high-speed camera recorded this mixing process, providing a digital film. By relating the greyscale in the images with the recorded time, it was possible to create response curves from which mixing times were obtained. Simulations have also been carried out by predicting the mixing of scalar concentrations with Computational Fluid Dynamics (CFD). From the PLIF data it was shown that the dimensionless mixing time correlated with the (L/D)1.4, Re0.39 and x0.25. The value 1.4 indicates that the mixing mechanism is a combination of dispersion and convection. The positive power in Re indicates that the stirrers become less effective when the number of revolutions increases. The CFD simluations didn't produce satisfying results yet because the difficult geometry resulted in large numerical problems.