Study On CFD-PBM Simulation Of Stirred Sieve-plate Extraction Column

In this work,the fluid dynamics software FLUENT was used to simulate the fluid flow in the stirred sieve-plate extraction column.The simulation for systems without mass transfer was carried out in a three-stage stirred sieve-plate extraction column.A user-defined function(UDF)was used to load the breakage and coalescence model into FLUENT.A computational fluid dynamics(CFD)-population equilibrium model(PBM)coupled simulation method was established to simulate the average droplet diameter and the dispersed phase holdup under different operating conditions.Furthermore,the CFD-PBM combined mass transfer model was used to simulate the effects of flow rates,stirring speeds and properties of the system on the droplet diameter and distribution for systems with mass transfer.In addition,the extraction efficiency and the dispersed phase holdup were predicted.The conclusions of this work are as follows:(1)The CFD-PBM model can be used to simulate the droplet size distribution in the stirred sieve-plate extraction column.The droplet size distribution simulated by Coulaloglou&Tavlarides(CT)model could be fitted to lognormal distribution,which is in good agreement with the experimental results,so the CT model can be used to predict the droplets’ breakage and coalescence in the stirred sieve-plate extraction column.When using CT model to simulate the average droplet diameter,there is a deviation between the simulation results and the experimental results.On this basis,for different dispersed phases,a modified model considering the viscosity of the dispersed phase was used,in which the model parameter for the dispersed phase is obtained by fitting one simulation data and experimental data.The results show that the simulation results are in good agreement with the experimental results for different systems,which indicates that the modified model has good applicability.For the systems with the same dispersed phase and different continuous phase,the same model parameter is used,and the simulation results are in good agreement with the experimental results,indicating that the model parameter is only related to the properties of the dispersed phase,and have nothing to do with the continuous phase.(2)It could be seen that with the increase of stirring speed,the droplet diameter decreases,while the dispersed phase holdup increases.When the flow rate of the dispersed phase increases,the droplet size gets bigger,and the dispersed phase holdup increases.Under the same operating conditions,the dispersed phase holdup in each stage in the extraction column decreases from top to bottom,while the droplet diameter increases slightly from top to bottom.It can be seen that the continuous phase flow rate has no obvious effect on the droplet size and the dispersed phase holdup.The simulation results are in good agreement with the experimental results,much better than the results of previous empirical correlation in prediction of droplet diameter.(3)In the system with mass transfer,the droplet diameter decreases and the distribution becomes narrower as the stirring speed increases.When the flow rate of dispersed phase increases,the droplet diameter increases and the distribution becomes wider.The droplet diameter of the system with mass transfer is smaller than that of the system without mass transfer,because of the low interfacial tension caused by the solute.The extraction efficiency increased with the increase of stirring speed and decreased with the increase of flow rate and the deviation between the simulation results and the experimental results was within ±2%.In simulation of the droplet diameter,there is a slight deviation between the simulation results and the experimental results.This could be explained by the fact that the properties of the two phases,especially the interfacial tension,would change with the change of solute concentration,while this was not considered in the simulation.In addition,the simulation method in this work has some limitations when predicting undesirable operations such as flooding.In this work,CFD-PBM simulation of the stirred sieve-plate extraction column was carried out by CFD software FLUENT,which would greatly reduce the experimental work and effort.Accurate hydrodynamic data could be obtained,which would be helpful for structural optimization and industrial design of the stirred sieve-plate extraction column.