CFD procedure for studying dispersion flows and design optimization of the solvent extraction settler

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Doctoral thesis (monograph)
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Helsinki University of Technology doctoral theses in materials and earth sciences, 7
Computational fluid dynamics (CFD) modelling has been applied to study the behaviour of organic-aqueous dispersion and flow patterns in the copper solvent extraction settler with the aim of developing a complementary method for traditional physical experiments in settler design work. The simulations have been carried out with an Eulerian-Eulerian two-phase model of the commercial CFX-4.4 software in conjunction with the incorporated MUSIG model, which is based on a population balance equation and takes into account the break-up and coalescence models of droplets. Due to the complicated process phenomena of the phase separation including droplets deformation, variable collision forces, the drop-drop and drop-interface coalescences, film thinning of the continuous phase between droplets or a droplet and its homophase, high-volume fractions of the separated phases and numerical convergence difficulties, a new six-item CFD calculation procedure and the specified boundary conditions were developed. During the developing work, the relative velocity of the phases versus drag force correlations, the dispersed droplet size range, inner iteration of the MUSIG model, the drag force in a dense dispersion, the calibration coefficient of the coalescence model, the discretisation schemes, and steady-state versus transient simulation were tested. The developed calculation procedure and the temperature-related phase separation correlation coefficient, together with the specified boundary conditions, were applied to the pilot settler with the aim of testing and showing the possibilities of the CFD in the settler design, and gaining new information about dispersion flows and force balances on the droplet level in the settler. The simulation cases took into account the thickness of the dispersion layer versus the specific volume flow rate, temperature and number of picket fences. Furthermore, the pressure drop over the picket fence, the droplet size distribution, the linear velocity of the organic phase, and the hydrodynamic force balances between the viscous, inertia, surface and buoyancy forces were studied. The developed new calculation procedure proved to be very useful in complicated multiphase modelling. It ensures that all simulation cases can be carried out in the same way, and thus the obtained results are comparable. In addition, use of the procedure improves the overall trust of CFD simulations. The simulated results agree well with experimental data obtained with a pilot settler. The model predicts that the phase separation is achieved more effectively when the picket fences are set into the settler, the temperature is increased or the specific volume flow rate is decreased. These phenomena could also be obtained from the results of the force balances. The inertia forces decrease and, respectively, the buoyancy and surface forces increase when the picket fences are set, and when the dispersion disengages and flows from the front end of the settler towards the rear end. Furthermore, an effective separation requires that the buoyancy forces become stronger than the surface forces. It can be concluded that settler operations with picket fences can be optimized with the aid of CFD modelling.
solvent extraction, settler, picket fence, CFD, two-phase, break-up, coalescence, dispersion, phase separation
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