Design of Unbaffled Stirred Tank with Concave Blade Impeller

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The wide applicability of mechanically stirred tanks in industry demands a comprehensive understanding of hydrodynamics controlling the performance of these fundamental units. Standard Rushton impellers in baffled tanks are widely used in mechanically agitated reactors because of the high dispersion efficiency and its flexibility of operation. However, unbaffled tanks are used in many industrial stirred tank systems where the presence of baffles is undesirable. Both experimental investigations and computational studies to analyze the fluid flow hydrodynamics which often shows good potential for industrial use of unbaffled of stirred tanks. Unbaffled tanks are also advisable in crystallizers, where the presence of baffles may promote the particle attrition phenomenon. Unbaffled tanks give rise to higher fluid particle mass transfer rates for a given power consumption, which may be desirable in a number of processes. Regarding Rushton impeller, several researchers argue its inferior efficiency as compared with other types of impeller. Compared to the Rushton impeller, Concave blade impellers were reported to provide at least the same gas handling capacity, equivalent fluid homogenization performance, similar gas holdup characteristics and less power fluctuations. Impeller position (impeller clearance depth, C, when single impeller is used and bottom impeller clearance depth, C1 and impeller spacing, C2 when dual impeller is used) should be the major concern as an influencing parameter in impeller driven stirred system apart from other affecting parameters as the flow pattern generated inside the vessel is dependent on it. Not only the flow pattern but also the purpose of stirred tank is also dependent on it. Compared with single impeller, dual combination has more advantages, such as efficient gas distribution, longer gas phase residence time, increased gas hold-up, superior liquid flow characteristics and lower power consumption per impeller. The aim of this research is to establish the design criteria for Concave blade impeller stirred unbaffled tank. This was accomplished through experimentation on unbaffled stirred tank in single and dual impeller with and without sparging system. The influence of impeller diameter (ratio of d/D is taken as 0.2, 0.3 & 0.4), impeller clearance depth (ratio of C/D is taken from 0.3 to 0.9 with 0.1 interval for single impeller and C1/D of 0.15 to 0.85 with 0.05 interval for dual impeller) and spacing between two impeller (C2/D of 0.2 to 0.8) are studied for the governing process dynamics at various impeller rotational speed (N=100 to 1200rpm).
Supervisor: Bimlesh Kumar