| Because the mechanically agitated tanks are efficient gas-liquid contactors, they are widely used in the process industry. According to the liquid viscosity processed, two types of mixer are commonly used. On the one hand, with low viscosity liquids, we use small open impellers with high shearing action in a baffled tank. On the other hand, when rheologically complex liquids are mixed (viscous or non-Newtonian), close clearance impellers appear necessary to efficiently distribute a gas in the liquid and baffles have to be avoided due to the poor mixing behind them.;The main objective of this thesis is to characterize the power consumption, the hydrodynamics and the gas-liquid mass transfer performances of the unbaffled dual off-centered shaft mixer.;The first part of this work deals with the improvement of the color-discoloration technique to determine the mixing time. By using digital image analysis, the new technique appears reproducible and gives information on mixing kinetics. Thus, phenomena like flow compartmentalization or segregated region can be quantify.;The second part studies the effects of shaft eccentricity on power consumption and mixing efficiency in laminar flow. By using the digital image analysis technique, it was shown that Reynolds number and shaft eccentricity have both important effects on the segregated region breakage in laminar flow. Concerning the power consumption, shaft eccentricity has shown to have no effect in laminar flow. Results conclude that a complete mixing can be obtained in laminar flow when Reynolds number and shaft eccentricity are above critical values.;The third part is focused on hydrodynamics performances of the dual shaft mixer in laminar flow. By comparing this reactor with some others unbaffled configurations, it appears that the use of two off-centered shafts prevent the formation of compartmentalization and improve axial mixing. The two rotating modes of the dual shaft mixer have been compared and it appears that the counter-rotating mode of the impellers improves the interaction between them. For the power consumption, it appears that the two impellers do not interact due to their distance.;Finally, the last part deals with the mass transfer performances of the dual shaft mixer. When low to high viscosity Newtonian liquids are used, it appears that the dual shaft mixer operating in counter-rotating mode prevents the vortex formation. Thus, with Newtonian liquids, good mass transfer can be obtained on a wide range of viscosity. Moreover, this configuration appears flexible to control the mass transfer axial homogenisation in the tank. However, the use of eccentric shaft must be avoided to disperse gas in non-Newtonian shear-thinning fluids. Indeed, it appears that a centered shaft gives far better mass transfer. (Abstract shortened by UMI.). |
