Liquid-liquid mixing in stirred tanks with varying liquid depths
Otto H. York Department of Chemical Engineering
Master of Science
Armenante, Piero M.
Barat, Robert Benedict
Considerable attention has been devoted in the past to the determination of the minimum agitation speed, Ncd, required for the complete dispersion of two immiscible liquids in mechanically stirred tanks. When this situation is achieved the dispersed phase is no longer present as a distinct layer, such as a light oil phase above an aqueous solution, but becomes completely dispersed, in the form of droplets, throughout the continuous phase. The achievement of the dispersed state is of significant importance in many industrial operations. Nevertheless, the effect on Ncd of a number of operating variables remains poorly understood. In particular, the effect on Ncd of the liquid depth, H, has not been established, especially at different values of the impeller off-bottom clearances, C. This situation is especially common, and potentially critical, in a number of processes in the pharmaceutical and food industries, when the completely dispersed state must be maintained at all times as the vessel is either charged with a liquid or emptied.
This investigation is focused on the experimental determination of the minimum agitation speed and power dissipation required to completely disperse two immiscible liquids at different liquid heights and impeller off-bottom clearances. Two types of impellers were used here: a six-blade disc turbine and a six-blade (45º) pitched-blade turbine. The minimum agitation speed was first experimentally determined using a visual approach. In order to validate visual observation method, a previously developed sampling method was also used (Armenante, P.M. and Huang, Y.T., Ind. Eng. Chem. Res., 31: 1395-1406, 1992). This method is based on sampling the liquid-liquid mixture at different agitation speeds, determining the content of the dispersed phase in each sample, and analyzing the data so obtained using a mathematical model.
Ncd and the corresponding power, P, drawn by the impeller at Ncd were found to be strongly affected by the impeller type, as expected. However, they were also significantly affected by both the liquid height and the impeller off-bottom clearance. Typically, both Ncd and P decreased with decreasing H. The effect of C was more complex. Of even greater significance, it was observed that, for specific combinations of H and C, and especially when the liquid head above the impeller was below a critical value, the state of complete liquid-liquid dispersion was not achievable, irrespective of the agitation speed. This implies that operating in regions where Ncd cannot be achieved should be avoided if complete liquid-liquid dispersion is to be maintained. The results of this investigation are expected to be of significant importance in the industrial practice.
njit-etd2003-010 (124 pages ~ 4,156 KB pdf)
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Created October 29, 2003