Investigation and analysis of flows in flotation cells using experimentation, computational fluid dynamics simulations, and mathematical models
Creators: name: Basavarajappa, Manjunath
Publisher: name: University of Utah
Year: 2018
Subjects: Bubble Size Distribution
CFD
Flotation
Multiphase Flows
Population Balance Modeling
Turbulence

description: Froth flotation is a highly complex, multiphase, and multiscale process that is usually performed in large tanks called mechanical flotation cells. The aim of this research is to investigate the single and multiphase flow hydrodynamics in lab scale flotation cells by decoupling the hydrodynamics from physicochemical effects. Both experimental and numerical approaches are used to study the behavior of flows in lab and pilot scale flotation cells. Nonintrusive experimental techniques such as particle image velocity (PIV) and electrical resistance tomography (ERT) techniques are used to measure flow velocities, solids holdup, mixing efficiency, and to interpret flow pattern. Eulerian-Eulerian computational fluid dynamics (CFD) models are developed and tested for solid-liquid (slurry) and gas-liquid flows in stirred tanks and flotation cells.    Using single phase CFD simulations, the effect of flotation specific impeller blade shape and impeller size on mean flow and pumping behavior is tested in lab scale flotation cells for the first time. In the absence of a stator, the mean flow is found to transition from radial to axial type flow when the off-bottom clearance is below the critical value. This prediction is experimentally verified using time averaged PIV data. Based on the analysis of pumping and power number data, the rectangular shaped blade design is found to be the most efficient. The impeller blade shape is found to critically affect the flow in the vicinity of the impeller and a design with the largest surface area is needed to create an intense turbulence zone, needed for mixing and dispersion of incoming air. Eulerian-Eulerian CFD model is used to study the solid phase suspension and mixing characteristics for monosized silica particles. Experimental comparison with the results from the literature for stirred tanks and in-house ERT measurements suggest that the model performs reasonably well.      Population balance equation model (PBM) is coupled with CFD to study gas dispersion, mixing, and local bubble size distribution in the stirred tank and flotation cell using quadrature method of moments (QMOM) approach in ANSYS Fluent solver. The default QMOM model in Fluent is found to be inaccurate due to independent solution of moment transport equations and therefore is supplied with a moment correction algorithm from the literature to successfully identify and correct the invalid moment sequence during the CFD simulation. The new model is found to be superior to the current models in its ability to satisfactorily predict the overall gas holdup and local bubble size distribution for stirred tanks under moderate aeration and agitation rates. This model is extended to study the development of flow regimes based on the gas dispersion pattern in a generic flotation cell. Though highly useful, the coupled CFD-PBM approach is computationally intensive and requires considerable effort to achieve an accurate solution. This motivated us to develop a PBM based on the high-order moment conserving method of classes (HMMC) approach for a pilot scale XCELL flotation cell for frother concentration over critical coalescence concentration, thus, only considering breakage of bubbles. Nonlinear optimization solvers in Matlab are used to calculate the point estimates of adjustable parameters in breakage models. The 95% bootstrap calculated using empirical bootstrap indicates very high confidence in estimated parameters. The HMMC model provides an accurate description of steady state bubble size distribution and the mean number diameters only using overall gas holdup and specific energy as inputs.