全逆流混合澄清槽中萃取体系料液流场CFD模拟研究

CFD Simulation Study of Full Countercurrent Mixer-settler

为了研究全逆流混合澄清槽内气-液(水相)-液(有机相)三相流体在微观层面的行为,本文采用CFD方法,以全逆流混合澄清槽为对象进行了研究。结果表明,搅拌会在混合室内产生一个V型液位分布,桨叶后方出现负压区域,且上部会出现空化现象;正方形混合室的四角方向会出现压力滞止死区;混合室内有机相会出现围绕搅拌轴的包覆层,水相主要从桨叶正下方吸入后与有机相在桨叶区域进行混合;混合室内存在多个局部速度旋涡场,桨叶下部是四周液体向桨叶正下方汇集,桨叶上部则是先向上后向旋转轴汇集;两相的混合与液滴的破碎与并聚主要发生在桨叶处及混合室附近壁面上。

The mixer-settler is a stepwise contact liquid-liquid extraction equipment widely used in the spent fuel reprocessing and rare earth industry.The materials and solvents are mixed with each other in the mixing chamber through stirring for mass transfer,and then enter the clarification chamber for separation by gravity.The fluid mechanics performance of the mixer-settler has an important impact on the hydraulics stability,treatment capacity and mass transfer efficiency of the equipment operation.Many scholars conducted research on mixer-settler based on fluid experiments,and focused on simulation calculations of single parts such as mixing chambers,clarification chambers,or stirring paddles.There were few reports on CFD simulation of the entire production scale full countercurrent mixer-settler,which inevitably lacked the research about the influence of the combination of mixing chambers and clarification chambers.Moreover,existing research mostly focused on single-phase systems or liquid-liquid two-phase systems,there is limited research on the simulation of gas-liquid(aqueous phase)-liquid(organic phase)three-phase systems.In this paper,the behavior of gas-liquid(water phase)-liquid(oil phase)three-phase fluid at the micro level in the industrial full countercurrent mixer-settler was studied by CFD method.The mixer-settler is divided into two grid structures,the validation on grid independence analysis was carried out,and the simulation results were analyzed and discussed.The results show that mixing will produce a V-shaped liquid level distribution in the mixing chamber,and there will be a negative pressure area behind the blade and cavitation in the upper part.The pressure stagnation dead zone will appear at the four corners of the square mixing chamber.The oil phase in the mixing chamber will have a coating layer around the mixing shaft,and the water phase will be mixed with the oil phase in the blade area after being inhaled directly below the blade.There are multiple local velocity vortex fields in the mixing chamber.The lower part of the blade is the flow of the surrounding liquid directly below the blade,and the upper part of the blade is the flow of the upward and backward rotating shaft.The mixing of two phases and the breakup and coalescence of liquid droplets mainly occur at the blade and on the wall near the mixing chamber.Besides,when using Eulerian multiphase flow simulation,there may be a certain degree of discrete error due to software calculations.Reducing the volume fraction relaxation factor can reduce the error,but it can’t be avoided.When considering computational efficiency,the gas phase can be set as the main phase to reduce the error.At the end of this paper,some optimization methods and suggestions were given to improve the operation of the mixer-settler.