Droplet Break-Up and Coalescence in Rotary Agitated Column Extractors

Abstract

The systematic design of agitated continuous countercurrent extractors requires an understanding of the influence of contactor variables, viz. rotor speed, geometry and volumetric throughput, on droplet size distributions, residence times and interactions. There= fore the literature pertaining to droplet break-up and coalescence, droplet phenomena in agitated systems and the design, characteristics and uses of agitated columns has been critically examined and the important phenomena studied experimentally. The most common commercial columns are the Rotating Dise Contactor, the Scheibel Column and the Oldshue-Rushton Column. A column section having interchangeable internal components was constructed and used to study droplet hydrodynemics in these three designs. Systema with a range of interraetel tension from 9.7 to 55 dynes per cm. were employed in the absence of mass trensfer. High and normal speed cine, high speed still photography and the injection of organic soluble dye were among the techniques used. The menner in which capacity and holé=up vary with opereting paramsters was confirmed. Interdroplet coalescence was showmto be largely absent in the ReDC. <A critical speed was established and consideration was given to the applicability cf drop size correlations above and below this speed. At low speeds an approximate form of correlation is, <FORMULA> where k is approximately 0.55. Mathematical modals have been proposed for break=up in the latter case and for the case of a dispersed phase wetted disc. Interdroplet coalescence has been found to predominate in the Scheibel column but, due to globule formation from the packing, mean interfacial area is unpredictable. Above & definable critical speed, a coalescence-redispersion mechanism has been found in the Oldshue-Rushton column; solute trans= fer rates should therefore approximate to those for fully circulating droplets. At reduced speeds, operation was acollisive and the form of drop size correlation epproximetes to thet for the R.D.C. with k approximately equal to 0.34 or, at very low speeds, 0.57. For normal operation, the proposed form is, <FORMULA> Effects arising from. dispersed phase wetting of colum rotors, stators or packing were determined, Reliable estimation of efficiency or capacity was found to be impracticable. Conclusions have been drewn regarding the potential uses and practical limitetions of the designs and current design procedures. Two illustrative cine films have been compiled and further work has been initiated in several of the areas recommended