One-, two- and three-phase mixing systems have been extensively studied. The experiments were performed in a range of standard (baffled) vessel geometries of diameter varying between 0.22 - 1.83 m. Rushton disc turbines and mixed flow impellers (both pumping directions) were fully investigated. Water and glucose solution (~ - 120 mPas) were employed. For single phase systems, the work has shown that the power number of disc turbines depends on disc thickness and scale of vessel. For the mixed flow agitators the power number is dependent upon the blade thickness and (D/T) ratio. Correlations enabling the ungassed power number to be calculated as a function of these parameters are given. For gassed systems, the power drawn by each type of impeller is explained by local impeller hyarodynamics (cavity structure) and the bulk flow pattern. The fiooding-Ioading transition (NF) and the complet~ dispersion condition (NeD) have also been studied. A large mixed flow impeller (6MFU45 ; D - T/2) with a large ring sparger is the most energy efficient at NF and NCD speeds as compared with the other geometries studied and correlations enabling the prediction of NF and NCD for all geometries studied are presented. Hold-up correlations are also given for each impeller firstly as a function of specific energy dissipation rate and superficial gas velocity and secondly as' a function of agitator speed and volumetric gassing rate. For each impeller, each method is equally good statistically for scale-up but the latter is more explicit. All impellers give approximately the same hold-up under equal specific power inputs and superficial gas velocity but there are small but statistically significant differences. These differences are discussed. For solid-liquid systems, correlations in the literature for the calculation of the minimum speed to just suspend solids, NJS ' are tested for each system geometry with glass Ballotini particles. The correlation proposed by Chapman et al. is shown to fit the present experimental data best. The specific power input per unit mass (ET)JS - constant, is proposed as a scale-up criterion for solids suspension. Large 6MFD45 (D - T/2) is the most energy efficient for suspension but 6MFU45 (D - T/2) is only slightly worse. In the three-phase mixing systems, the 6MFD 45 , D - T/2, is most energy efficient for solid suspension (ET)JSg' at low gassing rates (up to 1 vvm) but exhibits large flow pattern and torque fluctuations. Above _ 1 vvm, 6MFU45 (D = T/2) becomes the most energy efficient for solid suspension. In addition the minimum impeller speed for solid suspension NJSg for this impeller is almost independent of gassing rate and gives very stable flow patterns and torque. output throughout the whole gassing range. Again (eT)JSg - const is the recommended scale-up criterion for solids suspension under gassed conditions. Large (D - T/2) impellers are found to be more energy efficient and correlations for predicting N 45 45 . JSg for 6DT. 6MFD and 6MFU are obta~ned. Increase in liquid viscosity has a rather small effect on gas dispersion. Up to 120 mPas: (N) Q:I (N ) and (N ) Q:I F viscous F water CD viscous (NCD) water uO.06 On the other hand, viscosity has a significant effect on NJS and 3 to 5 times more energy is required for solid suspension at 120 ,mPas.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:323109 |
| Date | January 1986 |
| Creators | Bujalski, Waldemar |
| Publisher | University of Birmingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
Page generated in 0.002 seconds