(36d) The Iodide Iodate Reaction Method: On the Determination of Mixing Times from UV Measurements

Introduction

The Iodide Iodate Reaction Method is a reliable method for mixing parameter studies. The method was originally developed for macroscopic stirred tanks [Fournier1996a&b] by the working group of J. Villermaux at the CRNS in Nancy, France. With help of kinetic data published by the same group [Guichardon2000], numerous ?mixing times' were calculated from experimentally determined product selectivities utilizing the Iodide Iodate Reaction Method. Those ?mixing times' were derived from a simple mixing model (the incorporation model). These values are conceptual model parameters and should not be confused with physical mixing times, which are defined as the time it takes to achieve a specific degree of homogeneity [Zlokarnik1999]. The applicability of the Iodide Iodate Reaction Method as a quantitative method is a matter of recent discussion in scientific literature [Bourne 2008, Kölbl2008a], mainly because the kinetic data were collected at reaction conditions far different from those relevant to mixing studies.

The Iodide Iodate Reaction Method was later adjusted to the needs of continuous flow mixers [Panic2004]. In the mixer two solutions are contacted: one contains diluted sulfuric acid; the other one contains the other essential reactants. In a recent publication [Kölbl2008b] the authors elucidated the need to use appropriate reactant concentrations in order to obtain mixing sensitive results. This finding was shown on the example of micromachined multichannel V-type mixers, which possess the same geometry of each single channel but differ in the number of channels for each mixing device (figure 1).

Figure 1: Construction principle of microstructured multichannel V-type mixers: (a) stainless steel foils with micro channels and (b) stack of stainless steel foils.

Common to all used mixing devices is that both solutions (solution 1 and 2) which are mixed are multilaminated in geometrically identical channel systems (the same number of channels per passage for the respective solution) prior to mixing. Further information on the microstructured mixers can be found in literature [Kölbl2008b]. ?Mixing times' based on the same model (incorporation model) and the same chemical system were derived by the working group of Lee for a Kenics static mixer [Fang2001].

Experiments

In order to test the Iodide Iodate Reaction Method for its suitability to derive mixing times, V-type mixers, similar to those described above are employed. But unlike to those, the channel systems for the inlet feeds are different. They differ in the number of channels per inlet. Othrwise the geometry of the single channels is identical (100 µm · 70 µm cross section and 1414 mm length). The number of channels in the mixers used for this study are given in table 1.

Table 1: Number of microchannels in the respective mixers and inlets.

A competing chemical reaction system that is possibly suitable for the derivation of mixing times should not influence the experimental results. Hence it should have no influence on the experimental results whether solution 1 is fed in inlet 1 and solution 2 is fed in inlet 2 or vice versa. Experimental results for mixer I and mixer II are depicted in figure 2 and 3. The red color indicates the connection of inlet 1 with solution 1 (RO: regular orientation); the blue color the connection of inlet 1 with solution 2 (IO: irregular orientation). Different symbols (circles, crosses, diamonds, and triangles) represent experimental runs at different days. Different days for experiments were chosen in order to show reproducibility of the presented experiments. In all depicted experiments equal volume flows were mixed (dV₁/dt = dV₂/dt), which results in different exit velocities of the respective flows. The hydrodynamic situation is in both configurations the same and any differences of the experimental results obtained with both configurations reflect the influence of the chemical reaction system. Different experimental results lead to different mixing times, when calculating them from the simple mixing models, although the hydrodynamic situation is the same in both cases

Figure 2: Experimentally obtained optical densities over total mass flow; mixer I.

Figure 3: Experimentally obtained optical densities over total mass flow; mixer II.

Figure 2 and 3 show that the experimental results depend upon the configuration of inlet and feed. The differences are less apparent with mixer I, where the number of channels in the respective inlets differs less (30/20) than with mixer II (50/5). With mixer II the experimental results differ over the whole mass flow range, while with mixer I the difference are only apparent at low mass flow rates.

Conclusions

In this contribution new experimental findings are presented, which indicate mixing times cannot be calculated solely from selectivity measurements in the resulting mixtures for the case of continuous flow mixers. For quantitative evaluation of the experimental results, mixing models more complex than the incorporation model or the IEM (Interaction by Exchange with the Mean) model are needed, because as demonstrated in this work, the geometry of the mixer cannot be ignored.

Cited Literature

[Fournier1996a]

M.C. Fournier, L. Falk, J. Villermaux:

?A new parallel competing reaction system for assessing micromixing efficiency ? experimental approach?

Chemical Engineering Science 51(1996)5053-5064.

[Fournier1996b]

M.C. Fournier, L. Falk, J. Villermaux:

?A new parallel competing reaction system for assessing micromixing efficiency ? determination of micromixing by a simple mixing model?

Chemical Engineering Science 51(1996)5187-5192.

[Guichardon2000]

P. Guichardon, L. Falk, J, Villermaux:

?Characterisation of micromixing efficiency by the iodide-iodate reaction system. Part II: kinetic study?

Chemical Engineering Science 55(2000)4245-4253.

[Bourne 2008]

J.R. Bourne:

?Comments on the iodide/iodate method for characterising micromixing?

Chemical Engineering Journal 140(2008)638-641.

[Kölbl2008a]

A. Kölbl:

?Further Comments on the Iodide Iodate Reaction Method for Characterising Micromixing?

Chemical Engineering Journal 145(2008)176-177.

[Kölbl2008b]

A. Kölbl, M. Kraut, K. Schubert:

?The iodide iodate method to characterize microstructured mixing devices?

AIChE Journal 542008, 54, 639-645.

[Panic2004]

S. Panic, S. Loebbecke, T. Tuercke, J. Antes, D. Boskovic:

?Experimental approaches to a better understanding of mixing performance of microfluidic devices?

Chemical Engineering Journal 101(2004)409-419.

[Zlokarnik1999]

M. Zlokarnik:

?Rührtechnik Theorie und Praxis?

Springer, Berlin u.a., Germany, 1999.

English Translation: ?Stirring Theory and Practice?, Wiley-VCH, Weinheim, Germany, 2001.

[Fang2001]

J.Z. Fang and D.J. Lee:

?Micromixing efficiency in static mixer?

Chemical Engineering Science 56(2001)3797-3802.