Mixing studies of non-Newtonian fluids in an anchor agitated vessel

Abstract The success of any mixing operation involving liquid–liquid, gas–liquid and gas–liquid–solid systems depends mainly on the geometry of the vessel and impeller, operating conditions and properties of the system. Transformation of laboratory results to commercial scale unit is very difficult due to the complexity of flow phenomena and the scale up is being done by adopting a conservative approach which is based on the geometric, kinematic and dynamic similarities. This approach does not take into account the non-ideal flow behavior of the fluid and the design of commercial unit will be more rational if this information is included in the design of the unit. An attempt has been made to generate the data on non-ideal flow by carrying out the tracer experiments in an anchor agitated vessel. The fluids studied include water, castor oil, castor oil methyl esters and carboxy methyl cellulose (0.5 and 1 wt%), paper-pulp suspension (0.5 and 2 wt%) and starch suspension (2 and 4 wt%) in presence and absence of aeration. The data is analyzed for characterizing the flow by employing mixed model and dispersion model. Increase in the fraction of well-mixed zone ‘g’ from 0.7 to 0.95 with increase in impeller speed has been observed for both Newtonian and non-Newtonian systems but the increase is small for viscous fluids. A correlation between the model parameter ‘g’ and impeller speed, aeration and properties of system has been developed. Incorporating the value of ‘g’ in different scale up rules a commercial scale stirred vessel is designed. Constant tip speed resulted in large well-mixed zone with minimum power consumption.

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