Power Characteristics in Coaxial Mixing: Newtonian and Non-Newtonian Fluids Ste ´ phane Foucault, Gabriel Ascanio, and Philippe A. Tanguy* URPEI, Department of Chemical Engineering, Andre ´ Aisenstadt Building, Room M-5505, Ecole Polytechnique of Montreal, P. O. Box 6079, Station Centre-ville, Montreal QC H3C 3A7, Canada The power consumption of a coaxial mixer consisting of a wall-scraping anchor and different dispersion impellers (radial discharge) operating in co- and counterrotating modes has been experimentally characterized in the case of viscous Newtonian and non-Newtonian fluids. It was found that the anchor speed did not affect the power consumption of the dispersion turbines. The power consumption of the anchor was found to increase when the dispersion impellers were used in counterrotating mode and to decrease when they were used in corotating mode. Following the Metzner and Otto approach [AIChE J. 1957, 3 (1), 3-10], new correlations based on the impeller geometry for the generalized Reynolds number and the power number are proposed, and it is shown that a power master curve can be generated for speed ratios larger than 10. Introduction The importance of the mixing quality during a chemi- cal reaction in a batch or continuous process is well recognized. The major design task is the selection of the most appropriate mixing system according to the pro- cess specifications. The selection criteria are often based on the mixture quality (homogenization) and the power consumption. 1 The selection of the hydrodynamic condi- tions for a specific process can be particularly challeng- ing, especially when dealing with time-dependent rhe- ology products. In some cases, the product viscosity, initially close to that of water, can reach several pascal- seconds, thus changing the flow regime and the fluid dynamics in the vessel. It then becomes harder to control the reaction in the tank, especially in poorly agitated zones or the dead zones of the environment. 2,3 Many examples of this particular case can be observed in the field of polymerization and in fermentation processes in which the agitation becomes critical to ensure both good productivity and high selectivity. The evolution of the fluid rheology can also be caused by the physical changes of the solution microstructure (flocculation, gelation, dispersion, etc.). The final viscos- ity of the product is often higher than the viscosity of each individual ingredient composing the mixture. Depending on the nature of the products to be dispersed, non-Newtonian properties can also develop (nonlinear viscosity, thixotropic behavior, etc.). Several innovative strategies have been proposed to control or eliminate segregated regions in stirred ves- sels, based, for instance, on coaxial mixers, planetary mixers, conical mixers, and dual mixers comprising several impellers rotating at different speeds. Both numerical and experimental work has been carried out to characterize planetary mixers, 4 conical mixers, 5 and several types of coaxial mixers. 6-15 A classical example of a coaxial mixer is the combination of a high-speed turbine and a low-speed anchor scraper. It has been shown that these technologies are capable of adequately mixing and kneading high-solids-content media such as coating fluids and pastes and of dispersing gas in aerated mixing vessels with a low power consumption, even in the presence of a non-Newtonian rheology. In practice, however, the design of coaxial mixers is based on empirical considerations and industrial experience, and limited knowledge is available for optimizing the system. Because of the complex arrangement of a coaxial mixer (several different impellers, two agitation speeds, two possible rotation modes), it is quite difficult to characterize the mixing system in terms of the power consumption. Tanguy and Thibault 12,13 determined the power consumption of a new coaxial mixer consisting of a wall-scraping arm (anchor) and a series of rods and a pitched-blade turbine operating in counterrotating mode. They found that the power was a function of the speed ratio of the fast agitator to the slow agitator in both the laminar and turbulent regimes. Their correla- tions allowed for the prediction of the total power consumption of the system, but they did not allow for the independent evaluation of the influence of each agitator on the total power consumption. Recently, Foucault et al. 15 have shown that the power consump- tion of radial discharge turbines is not affected by the anchor rotation whether in co- or counterrotating mode. Contrary to these findings, Ko ¨hler et al. 14 observed a large influence of the anchor on the power consumption of a central agitator provided with flat paddles in counterrotating mode, for impeller tip speed ratios between 0 and 10. Vanhove et al. 6 studied the power consumption of a double coaxial mixer for the prepara- tion of a silica gel in counterrotating mode. Using two Mixel TTP propellers on the central shaft, they dem- onstrated that the power consumption of the propellers in the turbulent regime was not affected by the anchor for speed ratios larger than 7.67. All of the above studies clearly indicate that the interaction between the central agitator and the scraping agitator affects the power consumption of each impeller, the degree of which depends on the system geometry and the speed ratio selected. Because the various correlations available were unfortunately developed for specific geometries, it is not * To whom correspondence should be addressed. E-mail: philippe.tanguy@polymtl.ca. Tel.: 1-514-340-4040. Fax: 1-514- 340-4105. 5036 Ind. Eng. Chem. Res. 2005, 44, 5036-5043 10.1021/ie049654x CCC: $30.25 © 2005 American Chemical Society Published on Web 07/28/2004