Development of experimental design and image processing methods for drops diameter measurement in an annular centrifugal solvent extraction contactor (ECRAN)

International audienceLiquid-liquid extraction is a well-known method used to separate compounds based on their relative solubilities in two different immiscible liquids. Among the different types of liquid-liquid extraction apparatus, centrifugal extractors have advantages of low liquid hold-up and high mass transfer efficiency. As a consequence, many types of industrial monostage or multistage centrifugal extractors are currently used in nuclear but also pharmaceutical, food, chemical or hydrometallurgy industries with capacities from a few liters per hour to 100 cubic meters per hour.In order to determine these industrial applications, testing must generally be performed on small scale equipment to evaluate liquid-liquid extraction relevant parameters such as drop size distribution which is the objective of this investigation.Among the miniature extraction contactors developed since 1970, CEA has conceived a new annular centrifugal contactor referred as ECRAN with two original propertiesThere are two rotors driven by the same shaft and positioned the one under the second; the upper accomplishes the separation of the phases, the lower the mixtureThe two phases are fed at the bottom of the mixing zone by means of two channels and discharge into the separation rotor after mixing; so the emulsion fills the entire mixing zone and its volume is constant at any experimental conditions.The CEA chose the ECRAN to measure the drop size distribution of liquid/liquid systems with the shear rate which depends on the rotor speed and the gap in the annular mixing zone. Compared to a conventional ECRAN, the prototype of this investigation is composed of a stationary cylinder and a mixing rotor made of transparent materials (Plexiglas or borosilicate glass). The experimental setup is completed by a high speed camera and quartz tungsten halogen lamps to ensure clear sharp images of projected area of droplets inside the annular mixing zone.Images have been processed using three different image analysis software ImageJ created at the National Institutes of Health, MATLAB developed by MathWorks, DaVis developed by LaVision.In order to ensure best possible drop detection, every processing technique has involved several steps crop, denoising, background removal, thresholding, contour detection, particle analysis.Results are presented by histograms of the drop diameter and descriptive statistics (mean diameter,). The number of detected drops by the three image processing methods is generally different, and can be, in some cases, twice. That is because, for example, whether or not the drops on the borders of the image or drops overlapping are taken into account, the size or circularity ranges are too restrictive. Despite these differences, the deviation of the mean diameters in a same test calculated by the three techniques is of around $\pm$20 per cent