Investigation on the mixing performance in a superblend coaxial mixer.

摘要

Mixing is a ubiquitous unit operation in the process industries with numerous applications in reaction (polymerization, fermentation), distribution of solids and liquids, and blending (manufacturing of formulated products). In many cases of single-phase and multi-phase systems, widely and rapidly varying viscosities over the processing time occur along with the development of complex rheological behaviors. Since there is a huge gap between the design principle of the standard mixing approach and the mixing mechanism in rheologically complex systems, the resulting mixing inefficiencies have roused the innovations in mixer design and optimization.;New definition of characteristic speed proposed by Farhat et al. (2008) for coaxial mixers were extended to some other multi-shaft mixers not considered in previous works, and the applicability and limitations were discussed. Based on the comparison and analysis of existing resource, the power consumption, mixing time and mixing energy of different mixers were presented and a general approach to predict the power consumption in multi-shaft mixer was introduced. The Superblend mixer outperforms all the other mixers from the perspective of mixing time and mixing energy despite the lack of power-efficiency.;Among a variety of equipments and geometries designed to fill this gap, a recently emerged mixer concept Superblend coaxial mixer is one of the very promising candidates. Superblend coaxial mixer consists of two impellers: a helical ribbon as outer impeller and a Maxblend impeller as inner impeller. This combination allows a synergy between impeller geometries in different operating conditions in a single vessel to tackle the problems in rheologically complex mixing. Aiming at providing comprehensive scientific information on mixing performance, process design principles and scale-up guidelines, a full characterization of the single-phase and multiphase hydrodynamics in the Superblend mixer with both Newtonian and non-Newtonian fluids was carried out. Results exhibited that various experimental parameters such as rheological behavior, particle size and concentration, speed ratio and rotating mode have significant influence on the mixing performance in terms of flow pattern, power consumption, mixing time and evolution, contribution of each impeller and the optimal operating conditions.