Study Of Micromixing in a Stirred Tank Using a Rushton Turbine

• Published in: Chemical engineering research & design 2002-11, Vol.80 (8), p.855-863
• Main Authors: Assirelli, M., Bujalski, W., Eaglesham, A., Nienow, A.W.
• Format: Article
• Language: English
• Subjects: feed location; micromixing time; Rushton turbine; segregation index; stirred vessel
• ISSN: 0263-8762
• DOI: 10.1205/026387602321143390

The consecutive-competing iodide–iodate reaction scheme has been used to study micromixing phenomena in a baffled vessel of 0.29 m diameter agitated by a Rushton turbine. It has been confirmed that, by using successive injections, this reaction scheme is very efficient for such a study. Four agitator speeds giving mean specific energy dissipation rates, ε T from ∼0.2 to ∼ 1.2 Wkg −1 have been used, with sub-surface feeding at one of four points. For a given speed, addition at each of these four points gave different local values of ε ¯ T, ranging from less than ε ¯ T very close to the top of the liquid to much greater close to the impeller. The point closest to the impeller was chosen to be such that feeding was estimated to be at the point of maximum ε T. For the maximum speed, the segregation index, as a measure of the amount of ’waste product’, was ∼20% with feed at the top of the liquid (as preferred industrially because of its convenience). This ’waste’ was reduced to ∼5% by feeding at the point of maximum ε T close to the impeller. A comparison was also made with results reported in the literature using the same reaction for two new devices developed for improved micromixing. By feeding at the carefully chosen position close to the impeller, the results with the Rushton turbine were as good as or better than with the special devices even at the comparatively low ε ¯ T of ∼ l.2 Wkg −1. It was estimated that the maximum local specific energy dissipation rate was ∼70 times the mean, in reasonable agreement with a very recent study where the same pair of reactions and LDV were both used.