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Mixing of fluids and suspensions in a laminar stirred tank
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posted on 16.02.2017by Wang, Jian Ke Steven
Mechanically agitated vessels are widely used unit operations in the modern processing industry. Mixing vessels are critical for bulk materials chemical slurry processing such as leaching, digestion, crystallization, adsorption, ion-exchange and polymerization. Homogeneous mixing is vital for product blending consistency and purity of the reaction products. Inconsistent solute distribution and non-homogeneous blending due to poor mixing can lead to undesirable by-product which is often unacceptable in terms of product quality. One distinctive phenomenon related to poor mixing in a stirred vessel is the formation of isolated mixing regions. Isolated mixing regions or IMRs refer to regions of confined mixed zone, segregated by well-defined boundary layer. The present work is driven by a need to eliminate IMRs at high viscosities with practical engineering designs. Experiments were carried out in a small perspex tank. The features of the IMRs formed in the viscous Newtonian systems in a stirred tank have been studied. Homogeneous mixing can be achieved via some geometrical modifications including baffle installation and addition of particles, via completely eliminating the isolated mixing regions. The experimental and CFD results confirmed that the breaking spatial symmetry in the angled-shaft system leads to destruction of the isolated mixing regions. A new particle-localization phenomenon has been discovered in a solid-liquid laminar mixing tank, and the mechanism has been uncovered from the prospective of dynamical systems. The results demonstrated that sufficiently straining flow combined with the presence of separated flow regions (i.e IMRs) are the necessary conditions for particle clustering and the particles cluster into the IMRs when the total fluid strain on a particle exceeds a critical value that depends on the particle and fluid inertia. A facile separation strategy using a highly novel particle-location-manipulation physics has been successfully developed, and several methods have been proposed to improve the mixing in the solid-liquid slurry system.