Studies on the overall oxygen transfer rate and mixing time in pilot-scalesurface aeration vessel

An aerated vessel with Simcar type surface aerator was studied to estimate its significant performance characteristics i.e., oxygen transfer rate, mix ing time and power consumption. The oxygen transfer and mixing characterist ics were investigated with variations in the operating variables such as ro tational speed, diameter, bottom clearance of impeller, liquid height and p ower consumption in single and dual impeller systems. The bottom clearance of the top impeller was found to affect largely the capacity of the impelle r by varying the quantity of liquid loaded to it. It was also ensured that surface aeration caused a slight increase in mixing time compared to it und er ungassed conditions. The dual impeller system was employed for the purpo se of improving oxygen transfer and mixing at T/H < 1 varying the types and the bottom clearance of the lower impeller. However, the oxygen transfer r ate was independent of the lower impeller mainly depending on the capacity of the top impeller though mixing time was sharply reduced by the lower imp eller. The effect of the bottom clearance of the lower impeller was related to the liquid flow pattern inside the vessel, which was presented by compu tational fluid dynamics (CFD). The oxygen transfer rate and the mixing time were correlated with power consumption within reliable error range.