Study of micromixing in a stirred tank using a Rushton turbine - Comparison of feed positions and other mixing devices

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) over bar (T) from similar to 0.2 to similar to 1.2 W kg(-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 f (ε) over bar (T), ranging from less than (ε) over bar (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 epsilon(T). For the maximum speed, the segregation index, as a measure of the amount of 'waste product', was similar to20% with feed at the top of the liquid (as preferred industrially because of its convenience). This 'waste' was reduced to similar to5% by feeding at the point of maximum epsilon(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) over bar (T) of similar to 1.2 W kg(-1). It was estimated that the maximum local specific energy dissipation rate was similar to70 times the mean, in reasonable agreement with a very recent study where the same pair of reactions and LDV were both used.