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

An aerated vessel with Simcar type surface aerator was studied to estimate its significant performance characteristics i.e., oxygen transfer rate, mixing time and power consumption. The oxygen transfer and mixing characteristics were investigated with variations in the operating variables such as rotational speed, diameter, bottom clearance of impeller, liquid height and power consumption in single and dual impeller systems. The bottom clearance of the top impeller was found to affect largely the capacity of the impeller 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 under ungassed conditions. The dual impeller system was employed for the purpose 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 rate was independent of the lower impeller mainly depending on the capacity of the top impeller though mixing time was sharply reduced by the lower impeller. The effect of the bottom clearance of the lower impeller was related to the liquid flow pattern inside the vessel, which was presented by computational fluid dynamics (CFD). The oxygen transfer rate and the mixing time were correlated with power consumption within reliable error range.