(239d) Investigation of the Dushman Reaction At Concentrations Relevant to Mixing Studies In Stirred Tank Reactors

Mixing processes both in stirred laboratory vessels and continuous flow mixers are frequently characterized with competing chemical reaction schemes. The most popular of these is the Iodide Iodate Reaction Method (also referred to as the Villermaux-Dushman method). Despite its popularity, the reaction kinetics of the Dushman reaction, which serves as a time scale for the examined mixing processes is poorly understood [Guichardon2000, Bourne2008]; it has only been investigated at concentrations far below those relevant to mixing studies. Experiments at concentrations relevant to mixing studies are rarely published [Kölbl2008, 2010]. We report on examinations in a 5L baffled laboratory tank equipped with a six-bladed Rushton turbine. Our aim is to provide more reliable data for the quantitative examination of mixing processes in stirred laboratory vessels at reactant concentrations relevant to mixing studies. Experimental results and investigation strategies are discussed.

Determination of the reaction order of the compounds

Through a systematic variation of the reactant concentrations, their relative influence on the velocity of the Dushman reaction can be examined. For continuous flow mixers the suggestions for the reaction orders of earlier examinations (H⁺: 2; I^-: 2; IO₃^-: 1) [Guichardon2000] have been substantiated. For stirred laboratory tanks we suggest the following reaction orders: (H⁺: 0.6; I^-: 1.5; IO₃^-: 0.6). The different reaction orders point to different reaction mechanisms effective during the investigation of continuous flow mixers and stirred laboratory vessels. Experimental results suggest that the reaction zone in a stirred laboratory tank is close to the injection needle, where high acid concentrations are found. The experimental characterization of continuous flow mixers are carried out at low acid concentrations. The difference in H⁺ concentration might result in different reaction mechanisms.

Investigation of the influence of the acid employed

Experiments have been carried out with sulfuric acid (typically employed with the Iodide Iodate Reaction Method) and perchloric acid as a very strong acid. Opposed to previous findings [Kölbl2010], the choice of the acid in the case of stirred laboratory vessels has a considerable effect on the experimental results. Using perchloric acid, the triiodide concentrations obtained are 1.5 – 2 times higher than those obtained with sulfuric acid at the same experimental conditions. For the derivation of quantitative measures such as mixing times this experimental finding has to be considered.

Investigation of the influence of the ionic strength

Previous publications suggest a severe influence of the ionic strength on the reaction rate of the Dushman reaction [Guichardon2000]. We did not find any influence of the ionic strength.

We attribute the different findings to the use of potassium sulfate in the original examinations of a possible influence of the ionic strength. Sulfate acts a base especially in strong acid media, reducing the effective local H⁺ concentration, thus resulting in reduced final triiodide concentrations.

[Bourne2008]

J.R. Bourne:

“Comments on the iodide/iodate method for characterising micromixing”

Chemical Engineering Journal 140(2008)638-641.

[Guichardon2000]

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

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

Chemical Engineering Science 55(2000)4245-4253.

[Kölbl2008]

A. Kölbl:

„Further Comments on the Iodide Iodate Reaction Method for Characterising Micromixing“

Chemical Engineering Journal 145(2008)176-177.

[Kölbl2010]

A. Kölbl and S. Schmidt-Lehr:

“The Iodide Iodate Reaction Method: The Choice of the Acid”

Chemical Engineering Science 65(2010)1897-1901.