| As a separation unit operation, liquid-liquid extraction is a good method for the separation of liquid mixtures which cannot be treated by distillation. Mixer-settlers have a wide range of applications in solvent extraction such as separation of rare earth elements. With the development of computational fluid dynamics (CFD), more and more researchers have applied this method in the study and development of chemical processes and equipment. Delta impeller used in mixing chamber has been designed to meet the characteristics of mixer-settlers, and has many industrial applications in extraction of rare earth elements for its excellent performance. But with the scale up of mixer-settlers, the efficiency of inter-stage liquid transport in multi-stage mixer-settlers with delta impellers has come into doubt.In this paper, hydrodynamic characteristics study of delta impeller pump-mixers using computational fluid dynamics simulations has been carried out. Sliding mesh approach is used for the modeling of interaction between static tank and rotating impeller, and standard k-s turbulence model is adopted. With the effects of various geometric parameters and operation conditions on impeller performance being studied, new structures of delta impeller have been proposed to improve the suction head. The main contents and results are as follows:First, with simulation method being validated by comparison of simulation results with experimental data, optimization of delta impeller used in industrial multi-stage mixer-settlers with CFD simulations to improve pumping capacity has been conducted. The results show that, on the basis of sufficient mixing, low impeller installation height is good for suction, and will not bring an increase in power consumption. Highest suction head can obtained with optimal diameter for the outlet orifice (Do/T=5/24). Increase in blade number and impeller diameter can lead to increment in the suction head, but will also bring significant increase in power consumption at the same time. However, increasing the height of mixing tank suction orifice can increase pumping capacity without power consumption increment, which provides a direction for modification of delta impeller.Secondly, based on the former study on the original delta impeller, several new geometric structures of the impeller have proposed to improve suction performance, and simulations of three newly designed delta impellers were conducted. The structure of Design-1 is a cylindrical structure draft tube mounted at the bottom of delta impeller, while Design-2 is a taper shaped draft tube and Design-3 is a shrouded disc turbine. The results show that under the same agitation speed, these new designs exhibit no significant difference in power consumption from the original delta impeller, but Design-3 which consists of a shrouded disc turbine at the bottom of delta impeller can provide the highest suction head.Finally, in order to guarantee normal operation of the newly designed delta impeller meet in larger scale industrial mixer-settlers, scale up study has been conducted at two different vessel sizes, and experimental verification has also been carried out at the scaled-up bench device. The results show a close agreement in power number and head number of different geometric scales that confirms the validity of CFD simulations in scale-up of the delta impeller for industrial-scale application. Compared with the original delta impeller at the same power consumption level P/V= 0.5 kW/m3,13.5% increase of the suction head can be obtained with Design-3 in the scaled-up mixer (T=430 mm). And mixing performance has also been improved. Results of CFD simulations have been validated and agree with scale up experimental data.Results of this study can provide a theoretical guidance to the structure optimization of mixing tank impeller, improvement of inter-stage transport capability in multi-stage mixer-settlers and enhancement of equipment processing throughput. |
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