Research On Hydrodynamic Characteristics Of A New Type Cross-flow Extraction Column With Conical Bubble Caps

As an important chemical separation technology,liquid-liquid extraction has been used for a range of applications in the nuclear energy,petroleum,medicine,biology,new materials,environmental protection and other areas for many years.As an important research in the extraction and separation field,the improvement and enhancement of liquid-liquid extraction equipment is increasingly the focus of public attention.Therefore,this paper creatively put forward a new concept "cross-flow extraction using conical bubble cap",and a new type of extraction column without downcomer,named "A cross-flow extraction column with conical bubble caps" was designed and manufactured,and hydrodynamic characteristics of this new kind of extraction column were studied in three aspects:operation characteristics,axial mixing characteristics and computational fluid dynamics(CFD)numerical simulation,aiming to provide an extraction equipment with high operational elasticity and high stability in the chemical separation field.The study of operation characteristics included four parts:determination of droplet diameter in dispersed phase,determination of the dispersed phase hold-up,characteristic velocity calculation and flooding velocity measurement.The main experimental results and conclusions were as follow:(1)The droplet diameter of the dispersed phase in the extraction column satisfied the normal distribution,and sauter-mean droplet diameter was not only related to the physical parameters of the system,but also to the velocity of the dispersed phase.(2)Dispersed phase hold-up was positively correlated with the velocity of the dispersed phase and the velocity of the continuous phase,and the influence of the velocity of the dispersed phase was greater than that of the continuous phase.(3)The extraction column had certain ranges of operating flexibility for the extraction systems with medium and high interfacial tension,especially for the extraction system with medium interfacial tension.Due to the special structure of the conical bubble cap with four-sided openings,this paper also introduced the concept "effective opening area ratio" to observe and study the formation of dispersed phase droplets,the concept as an important parameter also was applied to the study of sauter-mean droplet diameter and dispersed phase hold-up.The study of axial mixing characteristics included two parts:determination of continuous phase residence time distribution(RTD)and calculation of axial mixing coefficient of continuous phase.The axial mixing of continuous phase in different extraction systems under different operating conditions was discussed.The results showed that the axial mixing of the continuous phase was mainly affected by the velocity of the continuous phase,while the velocity of the dispersed phase had little influence on it.Additionally,in the study of two-phase operation characteristics and axial mixing characteristics,the empirical correlation and dimension analysis,were used to correlate the hydrodynamics parameters.Computational fluid dynamics(CFD)numerical simulation research includes two parts:simulation of small-scale experiment devices and industrial scale devices for cross-flow extraction columns with conical bubble caps,by analyzing the simulation results of velocity distribution and velocity vector diagram of continuous phase flow in the two-dimensional structure model of extraction columns,the conclusions were as follow:(1)For the conical bubble cap,too high conical baffle will waste column's space and increase the axial mixing degree,too low conical baffle was not conducive to the "dispersion-coalescence-dispersion" process in the extraction column.And the continuous phase in the conical bubble cap with vertical guide plate had better continuous phase flow performance and less backflow dead zone compared to the conical bubble cap with inclined guide plate.It was also found that the continuous phase flow patterns of different conical bubble caps were different,but overall,all of them had the characteristics of smooth continuous phase flow,uniform velocity distribution,small backflow dead area and weak axial mixing.(2)The continuous phase flow performances of the industrial scale devices were similar to that of the small-scale devices.Futhermore,compared with the arrangement of single conical bubble cap on each tray in the small-scale device,the arrangement of multiple conical bubble caps on each tray in the industrial scale device can also optimize the flow pattern in the column,reduce the axial mixing,to some extent,it showed the feasibility of industrial scale-up study in the later stage of the new type of extraction column.