Mixing Efficiency in Draft Tube Column and Stirred Tanks

Hydrodynamics and mixing efficiency in bubble columns and stirred tanks are important for the design of many industrial processes such as food production and oil refinery. Lack of fundamental understanding of mixing mechanisms in these systems leads to huge economic losses due to non-optimal energy utilisation. It is known that the economics of such processes depend on the performance of a bubble column or stirred tank with respect to mixing, power consumption, rate of reactions and the product quality. In both bubble column and stirred tanks, draft tube improves mixing efficiency, which results in energy saving. It is therefore necessary that an optimal design of the draft tube be determined for both bubble columns and stirred tanks. In the present study, both experimental and simulation methods have been employed to determine the flow fields in mixing tanks and columns. The laser Doppler velocimetry (LDV) and computational fluid dynamics (CFD) techniques were used to determine the flow fields in a system with and without a draft tube. The flow fields revealed dead zones, which reduce the mixing efficiency. These dead zones were suppressed or completely eliminated when a draft tube was employed. It was further shown that the homogeneity of the system depended on the diameter, height and clearance of the draft tube. Results showed that an optimal design of a draft tube column or draft tube stirred tank could result in energy saving.