Inhomogeneities of CSTR on a Macroscale Due to Spatial Dependence of Micromixing Time: The BZ Reaction Peter Strizhak, † Fathei Ali, and Michael Menzinger* Department of Chemistry, UniVersity of Toronto, Toronto, Ontario M5S 3H6, Canada ReceiVed: September 26, 1997 X Macroscopic concentration gradients are measured through spatial microelectrode scans in the most turbulently stirred zone of a CSTR on both steady states of the bistable Belousov-Zhabotinsky reaction. The concentration distribution that exists on the macroscale is shown to arise indirectly from the spatial dependence of the micromixing time through the coupling of chemical reaction with micromixing. 1. Introduction Stirring effects on nonlinear reactions in continuous flow stirred tank reactors (CSTR) demonstrate that the notion of the “well-stirred reactor” is an idealization and that the rate and dynamics of nonlinear systems may depend sensitively on reactor inhomogeneities. 1,2 The nature, origin, and dynamical consequences of reactor inhomogeneities are important for the correct interpretation of CSTR and rapid flow kinetics experi- ments 3 and for practical applications, since stirring and mixing are crucial and nontrivial steps in industrial processes. Inhomogeneities in CSTRs are thought to arise passively through the incomplete, hydrodynamic mixing of the feed- stream(s) into the reactor bulk, and they cover a wide range of length scales: large-scale mixing or macromixing entrains the feedstream(s), blends them into the reactor bulk, and reduces concentration gradients on the macroscale of the reactor. Concurrently, the feedstream packets undergo a turbulent stretching-and-folding process known as micromixing that reduces them to successively smaller, fractal eddies down to the Kolmogorov-length scale. The final dissipation of these eddies occurs by molecular diffusion. 4 The purpose of the present work is to show that there exists also an indirect, reactive mode by which macroscopic concen- tration gradients arise in reactive flows through the coupling of chemical reaction with the inhomogeneous hydrodynamic flow field that always exists in a CSTR. 5 In a recent CSTR experiment 6,7 a microelectrode was scanned across the reactor just below the stirrer, using the bistable chlorite-iodide system. Surprisingly large, 1 stationary concentration gradients were observed on the macroscopic scale of the reactor in the most intensely stirred, turbulent zone of the CSTR, where the circulation and macromixing rates are sufficiently high to let one expect a much higher degree of homogeneity than what is actually observed. At a stirring rate of 880 rpm the concentra- tion monitored by a Pt-electrode changed by 800% between the axis and the tip of the propeller stirrer. To rationalize this finding, 6 two facts have to be taken into account: first, the turbulent flow field is highly inhomogeneous and anisotropic, 2 as expressed for instance by the scalar field of energy dissipation density ǫ(r). Its highest values are near the tip and edge of the stirrer blade, and the lowest values are found in stagnant dead zones in the far corners of the reactor. Second, the potential of a spatially fixed Pt-microelectrode E(S), which monitors the logarithm of the concentration of a readily oxidized species, shows a marked dependence on stirring rate S, the usual macroscopic stirring effect. Consequently, the inhomogeneous turbulence makes the local micromixing time τ mix (r) dependent on spatial position and gives rise to a spatial dependence of the stirring effect. We possess now a quantitative description of stirring effects in bistable systems with one variable. 9-11 In such systems the hysteresis contracts, in response to reduced stirring, inside the high-S loop, and the steady states approach each other. This “stirring effect of the first kind” is also observed in some multivariable systems which may be termed “effectively one- dimensional”, among them the Belousov-Zhabotinsky (BZ) reaction, 9 the chlorite/iodide reaction, 12,13 and the arsenite/iodate system. 10,11 To show the connection between the stirring dependence of the steady states and concentration gradients in nonlinear chemical systems, we chose here the bistable Belousov-Zhabotinsky reaction, using a single, premixed feedstream, to be able to analyze the results by the one- dimensional theory. 9,11 2. Experiments The experiments were conducted in a cylindrical plexiglass CSTR described elsewhere 9,10 at T ) 36 °C. To maximize turbulence, the reactor was fitted with four vertical, 5 mm wide baffles and with a rectangular impeller (8 × 15 mm, stainless steel coated with Teflon), positioned 30 mm above the bottom of the reactor. The spatial dependence of the system state in the CSTR was monitored by a Pt-microelectrode (20 µm diameter Pt-wire fused in glass and trimmed to ∼30 µm length) which was mounted radially, 25 mm above the bottom of the reactor on a micrometer for spatial scans, relative to a Hg/HgSO 4 reference electrode. The microelectrode could be moved 20 mm in a radial direction. In addition, the state of the system was monitored by a space-fixed Pt electrode (0.1 mm Pt-wire fused in glass) relative to a Hg/HgSO 4 reference electrode. The impedance-matched electrode signals were fed via an A/D converter into a personal computer. The single feedstream was peristaltically pumped into the high-turbulence zone of the reactor through a single tube located just below the impeller blade, at a rate of 5.97 mL/min, † L.V. Pisarzhevskii Institute of Physical Chemistry, National Ukrainian Academy of Sciences, pr. Nauki 31, Kiev, Ukraine, 252038. * Corresponding author. E-mail: mmenzing@alchemy.chem.utoronto.ca. X Abstract published in AdVance ACS Abstracts, December 1, 1997. 188 J. Phys. Chem. A 1998, 102, 188-191 S1089-5639(97)03129-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/01/1998