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Title: Micromixing studies in turbulent stirred baffled and unbaffled vessels agitated by a Rushton turbine : an experimental study
Author: Assirelli, Melissa
Awarding Body: University of Birmingham
Current Institution: University of Birmingham
Date of Award: 2004
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The parallel-competing iodide-iodate reaction scheme has been used to study micromixing phenomena in baffled and unbaffled vessel of 0.29 m diameter (T) agitated by a Rushton turbine (D=1/3T) under turbulent conditions in a semi-batch mode. Firstly it was confirmed that by using successive injections, this reaction scheme is very efficient for such a study. Using this technique for the rest of the programme of work the first experiments were undertaken in the “standard baffled geometry”. Four agitator speeds giving mean specific energy dissipation rates, T ε , from ~0.2 W/kg to ~1.2 W/kg were used. For a given speed, addition at five different static feed locations, one just above the liquid surface and four submerged, were investigated. The four sub-surface feeding positions were associated with different local specific energy dissipation rates, εT, ranging from less than T ε , 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 (εT)max. For the maximum speed, the segregation index, Xs, as a measure of the amount of 'waste product', was ~20% with feed onto the top of the liquid (as preferred industrially because of its convenience) or if slightly submerged. This 'waste' was reduced to ~5% by feeding at (εT)max. The distribution of εT was estimated from numerical solutions of the incorporation model combined with relationships arising from turbulence concept. 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 T ε of ~1.2 W/kg. It was estimated that the ratio of T T ε /ε (=φ) gave a maximum circumferentially averaged value of φ of ~90. The iodide-iodate technique was then used to ascertain the effectiveness of micromixing in a standard baffled reactor when feeding with pipes rotating with the impeller and discharging continuously into the region of (εT)max. The results when compared with those obtained using the fixed pipe at the equivalent position showed a significant intensification. Overall, a seven-fold reduction in ‘waste product’ was found between the most convenient fixed feed pipe position above the liquid surface and the rotating pipes, even at the modest T ε of ∼1.0 W/kg. This study is the first to use feeding continuously into the region of (εT)max. Thus an angular resolved (εT)max could be estimated from the incorporation model compared to a circumferentially averaged (εT)max obtained with a fixed pipe. The former was ∼3.0 times the latter, in close agreement with recent PIV based measurements. Finally the same geometry but without baffles was studied. The power number Po was measured and found to be Po=0.8, much less than Po=4.3 for the baffled tank. Visual decolourisation to give the macromixing time θm revealed evidence of a poor mixing region corresponding to the forced vortex region. The poor mixing in this region was confirmed by micromixing experiments. At the same T ε of 0.18 W/kg in the two cases, the values of segregation index, Xs, were found to be from ∼50% to 40% less than those III obtained with the baffled configuration except in the region close to the impeller where similar Xs values were found. These results are considered surprising, the unbaffled vessel proving much more effective for micromixing then expected. However, these improvements are limited by the very low maximum specific power input of 0.18 W/kg achievable with this configuration. Using the same micromixing model, all local (εT) values were > T ε . Clearly, this result is wrong. It is postulated that the swirling vortex flow past the fixed feed tube may give rise to a high very localised εT value. Clearly, further experimental and modelling work is required to gain a better understanding of the flow structure and micromixing in unbaffled vessels.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available