Study On The Performance Of Hydrodynamics And Mass Transfer In Pulsed Extraction Columns

Pulsed extraction columns are important separation equipments in chemicalengineering, and have been being industrially applied in spent fuel reprocessing fromnuclear fuel cycle. The operation and control of the extraction columns can often bevery problematic, partially due to their multiphase nature, and partially because of thedifficulty of on-line measurements of output variables. The measures to ensure astable and safe plant operation can only be designed when the dynamic and staticbehavior of the apparatus is known. Of the mathematical models for extractioncolumns, the uniform drop diameter model is adopted in the paper on account of itssimple math formula, less number of parameters to be regressed, and convenience tobe calculated. The dynamic behavior of mass transfer and holdup is simulated withthe help of simplifying the specific model, that is, the motion of the dispersed phase isassumed to be plug flow while axial dispersion is considered to be imposed on thecontinuous phase flow only. For the static behavior of mass transfer, the numericalresults are obtained by finite central difference method according to differentboundary conditions for ordinary differential equations (ODE). Optimization methodsare applied for the regression of the three essential parameters which determine theperformance of liquid liquid extraction columns, i.e. the continuous Peclet numberPe_x, the dispersed Peclet number Pe_yand the overall number of mass transfer unit Nox.The main conclusions of the thesis are as follows:As for dynamic simulation for mass transfer, based on axial dispersion model,the simplified parabolic differential equations depicting mass transfer are established.It is acceptable to neglect the axial dispersion for the dispersed phase throughanalyzing the response to a disturbance suddenly imposing higher continuous inletconcentration on the column.With respect to steady simulation for mass transfer, the ODE for static masstransfer can be numerically solved to get the concentration distribution of both phasesusing varieties of types of boundary conditions. It is demonstrated that the both phaseinlet concentration in effective extraction part of the columns, and the feedconcentration of the dispersed phase have directly grate effect on the accuracy of the numerical results.About dispersed phase holdup simulation, the dynamic behavior of holdup inextraction column is approximately reflected by the uniform drop diameter modelwith axial dispersion. And some aspects of the limitation for this model equation areanalyzed and discussed later.Concerning the regression of the three essential parameters, two differentmethods are developed to optimize dimensionless number Pe_x, Pe_yand Nox. One is toindependently optimize Pe_xand Pe_y, and the initial valve problems for ODE have tobe solved in the object function, the other is to simultaneously optimize those threenumbers with solving pentadiagonal linear algebra equations in its object function.The shortcomings of sensitivity to the initial value of Pe_xand Pe_yset for optimizationin the former method are overcome in the later. And the effect of axial dispersion ofthe dispersed phase on mass transfer is proved to be neglectable under someconditions.