Fine drop recovery in batch gas-agitated liquid-liquid systems

The hydrodynamics of batch gas-agitated liquid-liquid dispersions has received comparatively little attention in the open literature even though such systems arise in diverse contexts: batch pyrometallurgical processes, radioactive extraction processes and oil spills. Initially the two immiscible liquids form stratified layers. Liquid from the lower phase is then entrained into the upper phase in the form of small and large drops by gas bubbles passing through the liquid- liquid interface. At the end of a batch, gas agitation is 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. Both of these effects have sparked interest in slag cleaning and other remedial measures.; In this work, mechanisms for the recovery of fine water drops suspended in a sunflower oil + decane solution by injecting large nitrogen gas bubbles into the dispersion at a low flux rate are assessed and 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. Such loops are best generated through gas injection at or above the liquid-liquid interface. The primary mechanism for enhanced fine drop recovery, resulting from bubble injection, arises from improving drop liquid-liquid interface coalescence. Large drop-small drop coalescence is shown to be a secondary drop recovery mechanism. Bubble injection below the liquid-liquid interface, a frequently used industrial practice is not an optimal design option for exploiting these effects.; Measurements of drop concentration in the upper liquid phase reveal that there is an optimum gas flux corresponding to the best fine drop recovery rate and that the optimum gas flux is a function of fine drop concentration. At the optimum gas flux 20--100 mum diameter water drops are recovered at the approximately same rate. The overall rate of fine drop recovery is up to 4 times the rate obtained by gravity settling.; The physics of fine drop movement and capture in the experimental flow field are modeled numerically. Key features such as the spacial distribution of fine drops and the evolution of a complex concentration distribution pattern over time are modeled qualitatively. The model provides clear explanations for the drop concentration maxima arising remote from the bubble street and near the centroid of the circulation loops, and provides a basis for the development of a more general fluid mechanic model for such flows. The importance of mixing in the bubble street, and the flow profile adjacent to the liquid-liquid interface is also elucidated.