(40a) Transitional Flow Behaviour in the Shear and Sedimentation Test Cell (SSTC) | AIChE

(40a) Transitional Flow Behaviour in the Shear and Sedimentation Test Cell (SSTC)


Machado, M. B. - Presenter, University of Alberta
Kresta, S., University of Alberta

The dispersion and dissolution of chemical additives into liquids needs to be efficient in order to achieve reasonable industrial yields. It is known that current bench scale tests do not always represent industrial scale behavior, and turbulence inhomogeneity in bench scale stirred tanks can lead to significant uncertainties about the process mixing requirements. In order to better represent the industrial scale, a new bench scale stirred tank has been developed: the Shear and Sedimentation Test Cell (SSTC). The SSTC provides more uniform intensity of mixing over the volume of the tank, uses a much smaller volume of sample than the traditional stirred tank, and allows for the possibility of a sedimentation step after the mixing step with no sample transfer required. The unique geometry of the SSTC allows a more uniform turbulence distribution, which is not obtained in the other bench scale tests. The SSTC is a narrow and tall tank (T=7.6 cm, H=3T) running with multiple impellers. In order to check the limit of fully turbulent flow in the SSTC, three sets of impellers were used:  the A310, Rushton and Intermig. Sets of five (Rushton and A310) or six (Intermig) impellers of a relatively large diameter (D=T/2 for Rushton and A310 and D=2T/3 for Intermig) were used. Two fluids were used in the tank: water (1cp) and triethylene glycol (6 cp); to check the effect of the fluid viscosity. The Reynolds number was varied from 1100 to 30000. The power consumption of all sets of impellers was determined over a range of Reynolds numbers. The dissolution of solids and blending of liquids performance was also assessed. Mean and rms velocities were measured throughout the tank using an LDV. When Rushton impellers were used, the impeller discharge stream was still operating in turbulent conditions at Re = 1 800. The A310 and Intermig were still operating in turbulent flow at Re = 4200-7500. As the impeller to tank diameter ratio was large, the ratio between the maximum local dissipation and the bulk dissipation was smaller than the ratio obtained in a regular mixing tank. The maximum local energy dissipation was located in the impeller discharge stream for all configurations. The SSTC is able to provide a more uniform mixing for the bitumen extraction operation and is proposed as a test cell for additive chemicals in a wide range of applications.
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See more of this Group/Topical: North American Mixing Forum