Segregated regions in continuous laminar stirred tank reactors

Using visualization techniques, including acid/base reactions and UV fluorescence, we provide experimental evidence of segregated regions (islands) during mixing of viscous Newtonian fluids under laminar flow conditions in continuous stirred tank reactors (CSTRs). The effect of inlet/outlet stream position and Reynolds number on the dynamics of the mixing processes is examined. Numerical experiments in 3-D map were able to capture the main features of the CSTR flow by perturbing a Batch system using an imposed axial flow. Asymmetric flow patterns produced by off-center positioning of inlet and outlet pipes cause a reduction in size of the segregated region, enlarging the chaotic region and leading to more efficient mixing. Under dynamic inlet flow conditions, the laminar steady flow is perturbed, giving rise to an asymmetric flow pattern that is able to destroy toroidal segregated regions. Counter-intuitively, higher agitation speed (higher Re) did not enhance overall mixing efficiency. Faster agitation stabilized the toroidal regions, making it harder to destroy them. In addition, dynamic mixing protocols are investigated to enhance mixing performance. We demonstrate that time-dependent pumping and stirring protocols are able to efficiently destroy long-lasting toroidal regions.

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