Chapter 6 Conclusions and further works
6 Conclusions and further works
As pointed out along this thesis, the cold modelling of non-chlorine cleaning of aluminium in furnaces is a tricky problem; in fact, the geometries are not the standard geometries usually found in mixing processes. To solve this problem a standard impeller, the 3PBT30, has been used to compare the performances of the industrial impeller produced by S.T.A.S.-Unigec 2002. Furthermore to evaluate the mixing and power capabilities, STAS impellers have been compared also with a simple lance system, which is one of the most used systems in aluminium industrial plants.
In usual mixing processes the direction of pumping for the same impeller does not influence a lot the properties of mixing and power consumption; on the contrary in this vessel shape the STAS impeller shows completely different behaviour if it is acting downward or upward pumping. In fact, concerning for example the mixing time the STAS UP has worse results, with mixing times of 15-30% longer. This is due substantially to the position of the impeller; in fact angled at 45° relatively close at one side, the STAS UP generates a very good mixing in that area, while its pumping capacity is not enough to reach the opposite side. This is a big limit for this kind of process, where a good liquid recirculation is fundamental for chemical reaction; so this impeller takes on a secondary importance compared to the STAS DOWN.
Chapter 6 Conclusions and further works
Despite the fact that the STAS DOWN and the 3PBT30 impellers are geometrically completely different, they have shown very similar behaviour in simple liquid phase; while in two-phase configuration more differences are present. In fact, the two impellers for ungassed system have matching results for all specific energy dissipation rates, while for increasing gas flow rates the 3PBT30 shows shorter mixing times. This fact can be related both the power draw and gas dispersion; because for STAS DOWN the drop of power for gassed condition is higher than the 3PBT30, at the same time, its spreading capacity generates smaller bubbles. It means that in two-phase system the 3PBT30 maintains high pumping quality while is not particularly good for gas dispersion, as shown in bubble size analysis. Finally the lance mixing has shown to be inferior compared with impeller systems; in fact, at the same mean energy dissipation rate, it shows larger mean bubble size and develops an inferior liquid velocity field along the vessel.
In conclusion, the STAS impeller has been considered a good solution for this kind of cleaning process, in fact, despite the strange shape (very thick blades), which is due to the high temperature and the material, the impeller shows nice property as like a standard impeller,
Chapter 6 Conclusions and further works
asymmetrical flows and finding an optimum position to suggest to the industrial partners. At last, a CFD (Computational Flow Dynamic) work is being developed using experimental results for both the scales (mixing time and 3D-PIV).
References
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