Mass Transfer Enhancement And Modelling Of Coalescence-Dispersion Pulsed-Sieve-Plate Extraction Column

The reliable design and scale-up of extraction columns as well as theextraction process intensification are strongly depended on the fundamentalunderstanding of liquid-liquid two-phase flow and mass transfer characteristics.In this work the hydrodynamic and mass transfer performances of acoalescence-dispersion pulsed-sieve-plate extraction column (CDPSEC) withdiameter in 150mm and plate spacing in 25mm have been studied. And theperformances of this column were compared with those of other extractioncolumns. The influence of coalescence and dispersion of the dispersed phase onthe mass transfer performance was discussed. It was found that the two-phase flow characteristics in the CDPSECcolumn with coalescence-dispersion cartridge are controlled by two kinds ofmechanisms. One of the mechanisms is that the two-phase flow characteristicsare mainly controlled by the dispersion performance of the dispersed phasewhen the holdup of the dispersed phase is higher. Under the action of thismechanism the two-phase flow can be described with the modified Pratt'sequation. The other one is that the two-phase flow is controlled by thecoalescence behavior when the holdup is lower. In this case the modified Pratt'sequation can't be applied, so an empirical equation is suggested, which canpredict the holdup very well. The comparison of mass transfer performances ofthree types of sieve plate cartridges (standard cartridge coalescence-dispersioncartridge with plate spacing in 25mm and coalescence- dispersion cartridgewith plate spacing in 50mm) showed that the extraction efficiencies mightdecrease if the dispersed phase is excessive dispersed or coalesced. In the senseof mass transfer enhancement of CDPSEC, only when the dispersed phase hasbeen dispersed well enough to provide high mass transfer interface area, themass transfer can be enhanced by the interface renewal effect caused byintroducing moderate droplet coalescence. - II -英 文 摘 要 A parameter of coalescence ratio of the dispersed phase was defined toexpress the coalescence degree, and a quantitative method based on the "areaprinciple" was developed to calculate the coalescence ratio in the CDPSEC. Byanalyzing the values of the coalescence ratio and mass transfer performance, itwas concluded that the coalescence ratio should be less than 10% to avoid theexcessive coalescence of droplets. A new measurement technique, named dual optic probe, was developed tomeasure the droplet velocity and drop size distribution at higher holdupcondition in the CDPSEC. The mechanism of the new technique is based on thedifference of refractive indexes of the two phases to infrared laser. Animportant fact was obtained that the drop velocities are uniform even if the dropsizes are not the same. The velocities are mainly decided by the pulsationintensity. A correlation for predicting the mean drop size at higher holdupcondition was developed. Considering the influence of droplet dispersion and coalescence on theextraction column performances, a dynamic combined model was developed. Atwo-point dynamic method based on the dynamic combined model wassuggested to evaluate the mass transfer performances of the CDPSEC. Thedynamic combined model is much closer to real flow condition in extractioncolumns than the traditional diffusion model. In the new model themathematical equations and the boundary conditions are much simpler, onlyone parameter is optimized. Therefore the design and scale-up of the CDPSECwith the new model will be more reliable.