On stirring and mixing (S&M) effects in CSTR studies of chemical instabilities

Abstract

The unifying theme of the two studies reported here are the mixing-induced heterogeneities in a CSTR: Part (A) deals with their characterization in the chlorite/iodide reaction. A spatial microelectrode scan reveals pronounced and persistent macroscopic gradients in the neighbourhood of the stirrer. These gradients are related to the local intensity of micromixing and its coupling to the chemical mechanism. Any spatial coherence of the global dynamics arises, in the light of this intrinsic spatial dependence of control parameters, from the mutual entrainment of dynamically distinct but strongly coupled spatial regions. (B) In the second part the bistability hysteresis E(ko/S,M) of the BrO 3 . /Br/Ce+++ system is studied, whith the inverse residence time ko as control parameter, as a function of stirring rate S and mixing mode. In contrast to the ‘classical’ chlorite/iodide system, premixing has the opposite dynamical effect from enhanced stirring (i.e. stabilization vs. destabilization of the thermodynamic branch). Furthermore, great sensitivity of the hysteresis to stirring is observed: in the non-premixed NPM mode, low S broadens the loop and shifts it to a domain that does not overlap that obtained at high S. These results are counterintuitive and surprising. They demonstrate the different dynamical roles of stirring and premixing and the importance of heterogeneity-induced effects.