Experimental Residence Time Distribution (Rtd) Studies on Effects of Axial and Radial Flow Impellers on Hydrodynamic Parameters of Stirred Vessels

Abstract

In many industrial and biotechnological processes, stirring is achieved by rotating an impeller in a vessel containing a fluid (stirred tank). Many operations depend to a great extent on the effective mixing of fluids. In stirred vessels, this is achieved by the use of impellers. The vessel is usually a cylindrical tank equipped with an axial or radial impeller which does the actual stirring/ mixing. However, perfect mixing cannot be achieved due to certain malfunctions such as insufficient or excessive mixing, stagnant regions, bypassing and recycling etc. in the vessel. Industries are mostly confronted with the challenge of which flow impeller to use to achieve better mixing. Information on hydrodynamics such as mixing efficiency, Mean Residence Time (MRT) and the flow structure in the vessel can be used to solve the problem of improper reactor design, understand mixing process etc. Residence Time Distribution (RTD) analysis has been recognized as the top experimental and classical tool to monitor the behavior of non-ideal chemical reactors and industrial circuits. In this current study, the aim of the research is to use the experimental RTD to determine the effect of the impeller type ( axial or radial) on the Mean Residence time(MRT) and mixing, then also which model best describes the flow structure of the vessels with various impeller configurations. The method of moments was used in determining the MRT and variance from the RTD data. The MRT for the two radial flow impellers and the two axial flow impellers exceeded the theoretical MRT, however the experimental MRT of the one axial flow impeller tank and tank with no stirrers was lower than the theoretical MRT with effective volumes of 80.4% and 77.3% respectively. The two axial flow impellers showed higher variance hence better mixing than the two radial flow impellers. the and subsequent modelling of the data using DTSPRO software to determine which flow model best describes the flow structure in the vessels when the various impellers are used. The model of best fit was the perfect mixers in series with back - mixing model which described the flow structure.