화학공학소재연구정보센터
Chemical Engineering Science, Vol.55, No.20, 4719-4735, 2000
Fine drop recovery in batch gas-agitated liquid-liquid systems
The hydrodynamics of fine dense liquid drop recovery from batch gas-agitated liquid-liquid dispersions has received comparatively little attention in the open literature even though such systems arise in diverse contexts. At the end of a batch, gas agitation is typically stopped and the lower liquid phase drops suspended inside the upper-liquid phase, separate under the influence of gravity. Fine drops separate slowly and consequently a small amount of the more-dense liquid phase remains dispersed in the upper phase. For pyrometallurgical processes such as slag cleaning, long settling periods reduce equipment productivity and metal drops entrained in slag reduce metal yields and pose waste disposal problems. Anecdotal industrial data suggest that the injection of gas at low fluxes into such vessels improves metal drop recovery rates. In this work, mechanisms for the recovery of fine water drops suspended in a sunflower oil + decane solution, arising from the injection of gas bubbles in the form of slow continual streams, are assessed experimentally and modelled mathematically. It is shown how net rates of fine drop recovery can be enhanced by imposing circulation loops within the upper liquid phase that are oriented perpendicular to the liquid-liquid interface. The maximum rate of fine drop recovery observed experimentally is up to 35 times the rate obtained by gravity settling alone for 10-30 mu m diameter drops and up to Four times greater for 90-110 mu m diameter drops. Consequently, the recovery rates for drops with diameters in the 10-110 mu m range are largely independent of size. The mathematical model describes the movement of fine drops in the central plane of the experimental flow field and explains the evolution of the spatial drop concentration distribution with time as well as the enhancement vis-a-vis gravity settling and the size independence of drop recovery rates.