Dr. Philippe A. TANGUY
Total Professor of Process, Technology and Engineering URPEI, Department of Chemical Engineering
Ecole Polytechnique
Montreal, CANADA
ABSTRACT: Fluid mixing is a major chemical engineering unit operation in the natural resource sector and the process industries with numerous applications in reactions (polymerization, fermentation), dispersing of solids and liquids, contacting (flotation), and blending (manufacturing of formulated products). In many cases of practical interest, widely and rapidly varying viscosities over the processing time occur along with the development of complex rheological behaviors like shear-thinning, yield stress, and even elasticity. When a standard mixing approach is used (typified by a single or a cascade of impellers axially mounted), the processing efficiency of these rheologically complex systems is dramatically poor. The main reason is the use of impeller shape/design guidelines established for turbulent mixing and blindly applied to the case of viscous laminar and transition regimes. As a result, mixing mal-distribution, stagnancy, fouling, build-up, by-passing and segregations are observed in the process vessels, all of which contributing to a drastic decrease of selectivity, an increase in waste and by-products, cleaning difficulties, etc. One technology that has been developed to overcome these issues is coaxial mixing. A coaxial mixer is composed of 2 concentric shafts, namely a fast shaft driving a turbine (dispersing or distributing) and a slow shaft supporting a wall scraping arm like an anchor. When dealing with viscous and non-Newtonian fluids, the selection of the right set of impellers and the operating conditions (speed, speed ratio and rotation mode) is of primary importance. Although coaxial mixing is a fairly common technology in large sectors of the process industries for the manufacturing of coatings, cosmetics and foodstuff, it has received little attention in terms of the understanding of its real performance in the past. Recent progress has been made on the knowledge on the hydrodynamics and mixing characteristics of coaxial mixers. The performance of these systems has been investigated both experimentally and using advanced CFD tools in the case of Newtonian and shear-thinning fluids. The objective of the presentation will be to summarize the body of knowledge on coaxial mixers. A thorough review of the literature will be presented, from the first systems that were studied in the mid 90’s to the latest findings found in published work. Several examples will be given to illustrate the mixing mechanisms generated by these mixers, the design concepts and the possible scaleup guidelines. The presentation will also address the use of 3D finite element to simulate the fluid mechanics induced by these mixers and predict the performance. It will be shown that CFD can provide a lot of insight on the fluid mechanics in the reactor, and give access to mixer design parameters like the power consumption, the mixing time and the effective rate-of-deformation. Finally, a brief outloo
